Abstract
The reactivity of aryl monocarboxylic acids (benzoic, 1‐ or 2‐naphtoic, 4’‐methylbiphenyl‐4‐carboxylic, and anthracene‐9‐carboxylic acids) as complexing agents for the ethoxide niobium(V) (Nb(OEt)5 precursor has been investigated. A total of eight coordination complexes were isolated with distinct niobium(V) nuclearities as well as carboxylate complexation states. The use of benzoic acid gives a tetranuclear core Nb4(μ2‐O)4(L)4(OEt)8] (L=benzoate (1)) with four Nb−(μ2‐O)−Nb linkages in a square plane configuration. A similar tetramer, 7, was obtained with 2‐naphtoic acid by using a 55 % humid atmosphere synthetic route. Two types of dinuclear brick were identified with one central Nb−(μ2‐O)−Nb linkage; they differ in their complexation state, with one bridging carboxylate ([Nb2(μ2‐O)(μ2‐OEt)(L)(OEt)6], with L=1‐naphtoate (3) or anthracene‐9‐carboxylate (5)) or two bridging carboxylate groups ([Nb2(μ2‐O)(L)2(OEt)6], with L=4’‐methylbiphenyl‐4‐carboxylic (4) or anthracene‐9‐carboxylate (6)). An octanuclear moiety [Nb8(μ2‐O)12(L)8(η1‐L)4−x (OEt)4+x ] (with L=2‐naphtoate, x=0 or 2; 8) was obtained by using a solvothermal route in acetonitrile; it has a cubic configuration with niobium centers at each node, linked by 12 μ2‐O groups. The formation of the niobium oxo clusters was characterized by infrared and liquid 1H NMR spectroscopy in order to analyze the esterification reaction, which induces the release of water molecules that further react through oxolation with niobium atoms, in different {Nb2O}, {Nb4O4} and {Nb8O12} nuclearities.
Keywords: carboxylic acids, coordination complexes, infrared spectroscopy, liquid 1H NMR spectroscopy, niobium, poly-oxo clusters
Polynuclear niobium(V)‐based carboxylate oxo clusters: A series of aryl monocarboxylate ligands was used to isolate molecular coordination complexes corresponding to dinuclear {Nb2O}, tetranuclear {Nb4O4}, or octanuclear {Nb8O12} niobium(V)‐centered poly‐oxo clusters related to different hydrolysis rates.
Introduction
The remarkable development of the porous metal‐organic framework (MOF) materials has drastically renewed the chemistry of metal carboxylates, as most of these crystalline solids are built up from basic assemblies consisting of inorganic cores linked to each other via organic ligands through M−O−C bonding. Indeed, the chemical richness of such associations lies in the library of the large number of various polytopic organic molecules, despite the rather limited choice of metallic elements, related to their availability to produce stable metal carboxylate linkages for generating three‐dimensional open frameworks. The majority of MOF compounds involve the use of divalent (e. g., Zn2+, Cu2+, …), trivalent (Al3+, Fe3+, Cr3+, … – including lanthanides) or tetravalent cations (Ti4+, Zr4+, Ce4+… – including actinides), related to higher‐valence cations. [1] We may quote the quite unique case encountered for the uranyl chemistry related to the oxidation state +6 for uranium. But due to its specific configuration of double trans oxo ligands, this 5d element behaves rather as a formal divalent cationic species corresponding to the UO2 2+ moiety, giving rise to a myriad of organic‐inorganic networks when associated with organic linkers, as we observed for other divalent transition metals. [2] In fact, the reactivity of the high valence transition metals with carboxylic acids has been hardly investigated, despite the occurrence of some coordination complexes involving carboxylate ligands.
Among the candidates with high‐valence cations, niobium(V) has drawn our attention because its chemistry was reported in very rare cases related to the context of the elaboration of MOF materials. Thus, some recent works describe the incorporation of niobium as a doping element, into various MOF well‐known archetypes such as MOF‐5 (NbIV), [3] UiO‐66 (NbV), [4] NU‐1000 (NbV), [5] or CPO‐27(Co) (NbV), [6] with the aim to study properties in catalysis field or modification in electronic structures. To our knowledge, the synthesis of niobium‐based MOF structures has not been explored with carboxylate species as organic linkers. However, niobium has been encountered in MOF‐like compounds as an oxyfluorinated (NbOF5)2− basic building brick in a mixed Nb−Ni square network (KAUST‐7) with pyrazine ligands [7] or as mononuclear Nb node in a layered honeycomb lattice with 3,6‐dichloro 2,5‐dihydroxybenzoquinone ligand. [8]
If extended networks are not considered, there nevertheless exist several coordination complexes of niobium stabilized by carboxylate molecules in molecular assemblies. It can be found as a commercial precursor in a discrete Nb‐centered mononuclear unit, when precipitated by oxalic acid to give either identified [A][NbO(C2O4)2(H2O)2]⋅x H2O (A=NH4 +[9,10] or Cs+[11]) or [A]3[NbO(C2O4)3]⋅x H2O (A=NH4 +[12] or Rb+[13]) salts, which can be further associated with various pyridine‐based molecules (phenanthroline, bipyridine, etc…) [14] or divalent transition metals complexes with [Zn(bpy)]2+, [15] [Co(terpy)]2+, [16] including a series [(Fe−Zn)(bpy)]2+[17] (bpy=2,2’‐bipyridine, terpy=2,2’:6’,2’’‐terpyridine), or with rare‐earth cations. [18] Other niobium oxalates are been identified in the presence of barium in [Ba2(H2O)5][NbO(C2O4)3][HC2O4]⋅H2O [19] or with peroxide groups. [12] The occurrence of peroxo‐niobium centers are also encountered in a series of complexes involving citrate, tartrate, ethylenediaminetetra‐acetate (edta) and propylenediaminetetra‐acetate (pdta) ligands.[ 20 , 21 , 22 ] The other types of niobium‐centered coordination complexes have been isolated starting from niobium pentachloride or niobium alkoxides precursors.
For the first family, the reactivity of benzoic acid (HO2C−C6H5) has been tested with the halide precursor, which gave rise to the formation of molecular dinuclear complexes, in which the niobium centers are bridged through the carboxylate arm of the monotopic benzoate molecules. In one case, the dimer [Nb2Cl6(O2C−C6H5)2O] [23] also showed additional μ2‐oxo group linking the two adjacent niobium centers, whereas it does not exist in a second dimer [Nb2Cl4(OEt)4(O2C−C6H5)2]. [24] Other benzoate derivatives (O2C−R where R=FC6H4, p‐ClC6H4, p‐IC6H4, p‐MeC6H4) are also mentioned in the work of Brown et al.[ 24 , 25 , 26 ] In this series, the addition of ethanol solvent to the initial solution led to the exchange of some chloride anions by ethoxy (OEt) groups, which stabilized this specific configuration without any central bridging μ2‐oxo group. A related dinuclear complex has also been identified with fluoro‐substituted benzoate linker (O2C−C6F5) for the purely chloride salt [Nb2Cl8(O2C−C6F5)2]. [25]
The chemical synthetical pathway using of metal alkoxides (M(OR)n, where R=Me for methoxide, OEt for ethoxide, etc) has been more intensely studied, since these metallo‐organic compounds are known for their utilization as precursors for the elaboration of ceramics[ 27 , 28 , 29 , 30 ] and the substitution of a bidentate ligand can attribute to the alteration and modification of its properties..[ 31 , 32 , 33 , 34 , 35 , 36 ] Regarding the chemistry of niobium alkoxides, many coordination complexes have been identified with monocarboxylic acids and are related to different niobium nuclearities. The alkoxide route allowed for the identification of a dinuclear niobium‐centered complex [Nb2O(ONep)6(piv)2] (where ONep=OCH2CMe3 or neopentanoxy) in the presence of pivalic acid (Me3C−CO2H=Hpiv). Its structure is related to the dimer [Nb2Cl6(O2C−C6H5)2O] [26] obtained from the halide route, with two carboxylate arms from pivalate ligand and one bridging μ2‐oxo group. Here, the chloro anions are substituted by the neopentanoxy groups. However, in the same system, when using the terbutylacetic acid (Me3C−CH2−CO2H=HtBAc), a higher condensed coordination complex has been isolated with four niobium centers placed in a square configuration, and connected to four μ2‐oxo groups [Nb4O4(ONep)8(tBAc)4]. This dimeric moiety can be seen as the association of the two previously described dinuclear bricks [Nb2O(Oalkoxide)3(carboxylate)2] linked to other by two additional μ2‐oxo groups. Indeed, this tetranuclear unit has been reported by Hubert‐Pfalzgraf and co‐workers with acetic acid [Nb4O4(Ac)4(OiPr)8] [37] (ac=acetate) or methacrylic acid [Nb4O4(MAA)4(OiPr)8] [38] (MAA=methacrylate) from niobium isopropoxide. For this series, starting niobium alkoxides plays a key‐role in the synthesis of niobium(V) carboxylate coordination complexes as the esterification process that is induced by the alkoxy groups (leaving as alcohol) and the carboxylic groups results in the production of water molecules and the creation of μ2‐oxo‐bridges between niobium(V) atoms. A variant of these two configurations has been described by using bis(hydroxymethyl)propionic acid (H 2BHMP), in which two niobium‐centered dimers containing one central μ2‐oxo groups are connected by both the carboxylate and hydroxy functions of the ligand. [39] This arrangement generates through the BHMP molecule, a tetranuclear species [(BHMP){Nb2O(OEt)5}2], without any additional μ2‐oxo groups linking the niobium atoms. Another example of such an association of niobium carboxylate from the alkoxide route is also illustrated in a mixed bismuth‐niobium moiety, [40] which has been synthesized by means of salicylic acid (or 2‐hydroxybenzoic acid) leading to a dinuclear [(Bi2Nb2O(OEt)2(sal)4(Hsal)4)] or tetranuclear [(Bi2Nb4O4(OiPr)4(sal)4(Hsal)3)] (where sal=O2C−C6H4−2‐O; Hsal=O2C−C6H4−2‐OH) coordination complexes. In both species, the presence of μ2‐oxo‐bridges between niobium(V) atoms are observed. Related to the nuclearity in molecular niobium carboxylates, the largest poly‐oxo cluster was recently identified from the solvothermal reaction of niobium ethoxide with pivalic acid in an acetonitrile medium, which yielded to the formation of the species [Nb16O28(OEt)12(piv)12]. [41]
In this contribution, our work deals with the investigation of the reactivity of aromatic monocarboxylic acid, by using the alkoxide route from niobium penta‐ethoxide (Nb(OEt)5) as metallic precursor. Indeed, this synthetic pathway was previously reported with NbCl5 combined to derivatives of benzoic acids.[ 23 , 24 , 42 ] Therefore, the first study was focused on the reaction of benzoic acid with Nb(OEt)5 (compound 1), then followed by the reaction of other aromatic carboxylic acids such as 2‐naphthoic acid (compounds 2, 7, and 8), 1‐naphthoic acid (compound 3), 4’‐methylbiphenyl‐4‐carboxylic acid (compound 4) and anthracene‐9‐carboxylic acid (compounds 5 and 6). By changing the nature of ligands and synthetic conditions, a total of eight coordination complexes have been isolated with nuclearities varying from 2 {Nb2O} up to 4 {Nb4O4} and 8 {Nb8O12}. An understanding of condensation mechanisms is proposed and based on infrared and 1H liquid‐state NMR, 1H,13C HMBC 2D correlation spectroscopies, in order to examine the esterification reaction occurring during the crystal growth of such compounds.
Results and Discussion
Structure description
Compound [Nb4(μ2‐O)4(C7H5O2)4(OEt)8] (1): The crystal structure of compound 1 is described from a centrosymmetric molecular tetranuclear core (Figure 1) bearing two crystallographically independent niobium atoms, Nb1 and Nb2. Both cationic centers are sixfold coordinated with two μ2‐oxo groups (Nb1−O3=1.913(3) Å; Nb1−O4=1.899(3) Å; Nb2−O3=1.918(3); Nb2−O4=1.885(3) Å), two carboxyl oxygen atoms from benzoate ligands (Nb1−Oc=2.172(3)–2.176(3) Å; Nb2−Oc=2.154(3)–2.178(3)Å) and two oxygen atoms from ethoxy groups (N1−OEt=1.833(4)‐1.888(3) Å; Nb2−OEt=1.850(4)–1.883(3) Å). The presence of the bridging μ2‐oxo group agrees well with bond valence calculations 1.976 for O3; 2.106 for O4). [43] The μ2‐oxo groups are bridging two adjacent niobium atoms and are located in cis positions in the {NbO6} octahedra, resulting in a square ring of Nb1−O−Nb2 bonding within the tetrameric brick. The Nb1⋅⋅⋅Nb2 bond lengths are in the range 3.649(5)–3.664(4) Å, with Nb−O−Nb angles of 146.0(2)° (concave bending of Nb1−O3−Nb2 bond) and 149.3(2)° (convex bending of Nb1−O4−Nb2 bond). For one Nb1−O3−Nb2 sub‐unit, the carboxylate arms from the benzoate molecule occupy a cis position, leading to one benzene group located along the Nb1−O3−Nb2 square plane, and a second benzene group placed either up or down perpendicularly to Nb1−O3−Nb2 square plane. The carboxylate group adopts a syn‐syn bidentate bridging fashion with two neighboring niobium centers Nb1 and Nb2, linked through the μ2‐O3 groups only. The neutral tetranuclear core is decorated by ethoxy groups attached to the niobium atoms. One of them (O8) linked to Nb1, is found to be disordered on two crystallographic positions. This tetrameric building block {Nb4O4} can be seen as the association of two dinuclear subunit of the type [Nb2(μ2‐O)(C7H5O2)2(OEt)6], closely structurally related to that of coordination complexes 2 and 3 (see hereafter) for which two sets of two ethoxy groups has been formally substituted by one additional μ2‐oxo group (here O4) in order the generate the Nb−O−Nb linkage within the square ring system. It is the fourth example of such a brick with niobium(V) synthesized in the presence of carboxylic acids. Previous studies reported a similar core, with different monocarboxylate linkers, such as methacrylate, [38] acetate, [44] and tert‐butylacetate. [45] A related tetranuclear coordination complex was also described with salicylate groups associated with one hetero‐element bismuth. [40] Due to the presence of bismuth, the off‐planar benzene rings of the salicylate molecules are all located in one side of the tetranuclear core, and the alkoxyl molecules (isopropanol) are located on the opposite side. At last, this tetramer {Nb4O4} is suggested in the work reported by Brown et al. as a possible analog of the parent tantalum(V) benzoate derivatives (benzoate, para‐methylbenzoate and para‐fluorobenzoate), but without any structural evidences for the niobium(V)‐based complex. [26]
Figure 1.
Ball and stick (top) and polyhedral (bottom) representations of the molecular tetranuclear coordination complex 1. One of the ethoxy groups linked to the Nb1 site (through Nb1−O8 bonding) is disordered on two equivalent positions. Only one of these ethoxy groups is shown for clarity.
Compound [Nb2(μ2‐O)(C11H7O2)2(OEt)6] (2): The crystal structure of compound 2 is built up from one neutral molecular dinuclear species, containing two different crystallographically independent niobium(V) centers (Nb1 and Nb2), with a distorted octahedral geometry (Figure 2). Each of them is coordinated with the same configuration, three oxo groups from ethoxy molecules (Nb1−OEt in the range 1.873(1)–1.880(1) Å; Nb2−OEt in the range 1.865(2)–1.908(2) Å), two oxo groups from the carboxylate arm of the 2‐naphtoate ligand (Nb1−Oc in the range 2.179(1)–2.195(1) Å; Nb2−Oc in the range 2.137(2)–2.156(1) Å) and one bridging μ2‐oxo groups with Nb1−O5 bond length of 1.930(1) Å and Nb2−O5 bond length of 1.908(1) Å. The occurrence of bridging μ2‐oxo species agrees well with bond valence calculations (1.958 for expected value of 2 for oxo). [43] Within the dinuclear core, the niobium‐centered octahedral polyhedra are sharing one corner, with a Nb1⋅⋅⋅Nb2 distance of 3.667(4) Å. The niobium atoms are further linked to each other through two bidentate carboxylate arms of the two distinct 2‐naphtoate ligands, with a syn‐syn bridging mode. The two 2‐naphtoate molecules are located in a cis position, creating a tilting angle 145.7(8)° for the Nb1−O5−Nb2 bonding. This arrangement is closely related to those encountered in the niobium(V) oxo‐chloride [{NbCl3(C7H5O2)}2(μ2‐O)] stabilized by two benzoate ligands [23] or in the niobium oxo‐neopentyl alcohol [{Nb(ONep)3(C5H10O2)}2(μ2‐O)] stabilized by two pivalate ligands. [45] Both monocarboxylate linkers adopt a similar bidentate bridging cis fashion: for the benzoate case, the Nb−O−Nb bridge was found to be bent with an angle of 143.2°, as it is 144.0(4)° for the pivalate case.
Figure 2.
Ball and stick (top) and polyhedral (bottom) representations of the molecular dinuclear coordination complex 2. One of the ethoxy groups linked to the Nb2 site (through Nb2−O10 bonding) is disordered on two equivalent positions. Only one of these ethoxy groups is shown for clarity.
Compound [Nb2(μ2‐O)(μ2‐OEt)(C11H7O2)(OEt)6] (3): Compound 3 crystallizes in a non‐centric space group P212121, and exhibits a structure built up from two crystallographically independent niobium(V) centers, Nb1 and Nb2 (Figure 3). Both cations are octahedrally coordinated with three oxo groups from ethoxy molecules (Nb1−OEt in the range 1.848(3)–1.903(4) Å; Nb2−OEt in the range 1.878(4)–1.901(4) Å), one oxo groups from the carboxylate arm of the 1‐naphtoate ligand (Nb1−O1c=2.216(3) Å; Nb2−O2c=2.204(2) Å), one bridging μ2‐ethoxy group with Nb1−O6 bond length of 2.130(4) Å and Nb2−O6 bond length of 2.128(4) Å, and one bridging μ2‐oxo groups with Nb1−O10 bond length of 1.952(4) Å and Nb2−O10 bond length of 1.918(5) Å. The bond valence of the μ2‐oxo species is 1.72, close to that of 2 expected for such species. [43] In this coordination complex, only one 1‐naphthoate molecule is found and the carboxylate arm adopts a syn‐syn bidentate bridging connecting mode with the two distinct niobium atoms Nb1 and Nb2. This configuration differs from the previous complex 2, for which two 1‐naphthoate ligands were observed in the cis position. The occurrence of the single carboxylate bridge together with the bridging μ2‐ethoxy group induces a shortening of the Nb1⋅⋅⋅Nb2 distance, which is 3.2277(10) Å, instead of 3.667(4) Å in the dinuclear core of compound 2, and resulting in a tilting angle 113.0(2)° for the Nb1−O10−Nb2 bonding. The niobium‐centered octahedral polyhedra are therefore linked through an edge‐sharing mode via the μ2‐oxo and μ2‐ethoxy groups. The metric values of Nb⋅⋅⋅Nb lengths are smaller than those observed in the niobium‐based dimeric moieties bearing two bridging alkoxides species, such as a μ2‐methoxy bridge (Nb⋅⋅⋅Nb=3.493 Å; [46] Nb⋅⋅⋅Nb=3.450 Å [47] ) or a μ2‐ethoxy bridge (Nb⋅⋅⋅Nb=3.518 Å [39] ). Indeed, the occurrence of the bridging μ2‐oxo group with smaller Nb−O bond distance decreases the Nb⋅⋅⋅Nb lengths in compound 3, when an additional bridging μ2‐ethoxy group is present. At the opposite, the Nb⋅⋅⋅Nb lengths are increased when lacking the μ2‐ethoxy group, which are substituted by a second carboxylate pincer as found in the compound 2. One may also notice that the Nb⋅⋅⋅Nb distance from edge‐sharing {NbO6} octahedra in the dense purely inorganic B form of Nb2O5, [48] is 3.404 Å, and close to the value reported in the dimeric niobium alkoxides. It is interesting to notice that the position of carboxylate arm on the naphthalene group induces either a monocarboxylate coordination complex in compound 3 (position 1) or a dicarboxylate coordination complex in compound 2 (position 2).
Figure 3.
Ball and stick (top) and polyhedral (bottom) representations of the molecular dinuclear coordination complex 3.
Compound [Nb2(μ2‐O)(C14H11O2)2(OEt)6] (4): The crystal structure of compound 4 exhibits the same dinuclear motif (Figure 4) as found for compound 2, except that the centrosymmetric molecular moiety results in only one crystallographically independent niobium(V) center (Nb1). The latter is octahedrally coordinated through three oxo groups from ethoxy molecules, with Nb1−OEt bond distances varying from 1.868(2) to 1.887(2) Å, two oxo groups from carboxylate arms (Nb1−O1c=2.167(2) Å and Nb1−O2c=2.169(2) Å), and one bridging μ2‐oxo group (Nb1−O3=1.914(1) Å). The bond valence calculation for this oxygen O3 atom gives the value of 1.984, in agreement with the occurrence of an oxo group. [43] The oxygen O3 atom sits on the special position 4e (axis 2 point symmetry), resulting in two equivalent niobium atoms forming the dimeric unit, in which the Nb1⋅⋅⋅Nb1 distance is 3.645(2) Å, related to the tilting angle of 144.42(2)° for the Nb1−O3−Nb1 bonding. As for compound 2, two carboxylate groups from two distinct 4‐methyl‐biphenylmonocarboxylate ligands are linked to the two adjacent niobium (Nb1) centers, in a syn–syn bidentate bridging fashion, and adopting a cis position.
Figure 4.
Ball and stick (top) and polyhedral (bottom) representations of the molecular dinuclear coordination complex 4. One of the ethoxy groups linked to the Nb1 site (through Nb1−O6 bonding) is disordered on two equivalent positions. Only one of these ethoxy groups is shown for clarity.
Compound [Nb2(μ2‐O)(μ2‐OEt)(C15H9O2)(OEt)6]2 (5): The crystal structure of compound 5 is built up from two crystallographically independent motifs containing two distinct niobium(V) sites (Nb1 and Nb2, Nb3 and Nb4), linked to each other through one μ2‐oxo and one μ2‐ethoxy groups, and one carboxylate arm of the 9‐anthracenecarboxylate ligand (Figure 5). The existence of only one carboxylate molecule within such a dinuclear niobium core is similar to that encountered in compound 3, obtained with 1‐naphthoic acid. The 9‐anthracenecarboxylate ligand acts as syn‐syn bidentate linker with two adjacent niobium atoms (either Nb1 and Nb2 or Nb3 and Nb4). The four niobium centers are typically octahedrally coordinated with an identical geometrical environment. They are bound to one μ2‐oxo group (Nb1−O3=1.934(1) Å; Nb2−O3=1.937(1) Å; Nb3−O13=1.939(2) Å; Nb4−O13=1.936(2) Å), one bridging μ2‐ethoxy group (Nb1−O4Et=2.128(1) Å; Nb2−O4Et=2.121(1) Å; Nb3−O14Et=2.126(1) Å; Nb4−O14Et=2.1148(1) Å), one oxo group from the carboxylate arm of the 9‐anthracenecarboxylate ligand (Nb1−O1c=2.196(1) Å; Nb2−O2c=2.181(1) Å; Nb3−O12c=2.205(1) Å; Nb4−O11c=2.191(1) Å), and three oxo groups from ethoxy molecules (Nb−OEt in the range 1.855(2)–1.917(2) Å). The calculated bond valences of the μ2‐oxo group bridging the two niobium centers are 1.872 for O3 and 1.862 for O13, in agreement with the occurrence of an oxo group. [43] As observed in the dinuclear unit in compound 3, we observe that the niobium atoms are connected through only one carboxylate pincer in compound 5, resulting in an additional bridging μ2‐ethoxy group. In compounds 2–4, this μ2‐ethoxy group was not present and substituted by a second carboxylate bridge from a second organic linker. For 5, the niobium‐centered octahedral polyhedra are therefore linked through an edge‐sharing mode via the two μ2‐oxo ligands (oxo and ethoxy), corresponding to a shorter Nb1⋅⋅⋅Nb2 and Nb3⋅⋅⋅Nb4 distances of 3.2283(3) and 3.2561(3) Å, respectively. Related Nb−O−Nb bond angles are also lower, with value of 113.01(7)° and 114.37(8)° for the set Nb1−Nb2 and Nb3−Nb4, respectively. Similar metric values were observed in compound 3. The crystal structure packing of compound 5 results from the molecular assemblies of two neutral dinuclear units with 9‐anthracenecarboxylate, which differ only by the disorder of some ethoxy groups linked to the niobium centers. There exist one disordered ethoxy group for the Nb1−Nb2 set whereas there are three disordered ethoxy groups for the Nb3−Nb4 set.
Figure 5.
Ball and stick (top) and polyhedral (bottom) representations of the molecular coordination complex 5. For one molecule, one of the ethoxy groups linked to the Nb2 site (through Nb2−O6 bonding) is disordered on two equivalent positions. The same configuration occurs for the Nb3 and Nb4 sites of the second molecule, with three disordered ethoxy groups (through Nb3−O17, Nb4−O18, and Nb4−O20 bonding). Only one of these ethoxy groups is shown for clarity.
Compound [Nb2(μ2‐O)(C15H9O2)2(OiPr)6]2 (6): With 9‐anthracenecarboxylic acid (with a starting molar ratio of 0.375 carboxylic acid ligand for 1 niobium precursor; 0.333 carboxylic acid for 1 niobium in compound 5), a new type of crystalline product appeared, as compound 6, when adding isopropanol as solvent in the reaction medium. Its crystal structure is closely related to those observed in compounds 2 and 4, and contains a centrosymmetric dinuclear core with one crystallographic unique niobium(V) atom (Nb1) and two bridging 9‐anthracenecarboxylate species (Figure 6). This cation is octahedrally coordinated with three oxo groups from isopropoxy molecules, with Nb1−OPr bond distances varying from 1.863(2) Å to 1.880(2) Å, two oxo groups from carboxylate arms (Nb1−O1c=2.175(2) Å and Nb1−O2c=2.219(2) Å), and one bridging μ2‐oxo group (Nb1−O3=1.9103(6) Å). The bond valence calculation for this oxygen O3 atom gives the value of 2.038, in agreement with the occurrence of an oxo group. [43] Within the dinuclear core, two niobium centers are linked through two carboxylate arms from two distinct 9‐anthracenecarboxylate ligands and one bridging μ2‐oxo group (on special position 4e), generating a Nb1⋅⋅⋅Nb1 distance of 3.664(5) Å and a tilting Nb1−O3−Nb1 angle of 147.1(1)°, as found in the similar coordination complexes 2 and 4. The two 9‐anthracenecarboxylate ligands bridge two niobium atoms in a syn–syn bidentate fashion. However, the terminal ethoxy groups occurring in compounds 1–5 have been substituted by isopropoxy ligands, some of which occupy two disordering positions (through O6 atom; Figure 6).
Figure 6.
Ball and stick (top) and polyhedral (bottom) representations of the molecular dinuclear coordination complex 6. One of the isopropoxy groups linked to the Nb1 site (through Nb1−O6 bonding) is disordered on two equivalent positions. For them, the methyl groups is located on two close equivalent positions with occupancy 53/47. Only two of the four methyl groups are shown for clarity.
From the observation in compound 6, we were able to synthesize different coordination complexes with a dinuclear niobium core associating bulky anthracene moiety, with two distinct Nb/L ratio of 2 : 1 and 2 : 2 by playing on the addition of alcohol solvent. These results seem to indicate that the Nb/L ratio would be controlled by the initial carboxylic acid content rather that its steric hindrance.
Compound [Nb4(μ2‐O)4(C11H7O2)4(OEt)8] (7): Compound 7 is the second complex synthesized with 2‐naphthoic acid (see compound 2). Its crystal structure is closely related to that of the tetranuclear niobium(V)‐centered {Nb4O4} motif stabilized by benzoates species (compound 2), except that the latter ligand is substituted by the 2‐naphthoate molecules (Figure 7). It is built up from two inequivalent niobium(V) atoms (Nb1 and Nb2) linked to each other via μ2‐oxo groups (O7 and O8), within a square‐ring unit. Each niobium center is octahedrally coordinated by two μ2‐oxo groups (Nb1−O7=1.908(4) Å, Nb1−O8=1.919(4) Å; Nb2−O7=1.895(4) Å, Nb2−O8=1.921(3) Å), two oxygen atoms from ethoxy groups (Nb1−OEt=1.858(4)–1.879(4) Å; Nb2−OEt=1.869(4)–1.885(3) Å), and two carboxyl oxygen atoms belonging to the 2‐naphthoate ligand (Nb1−Oc=2.157(3)–2.163(4) Å; Nb2−Oc=2.167(4)–2.176(4) Å). The bond valence calculation for the μ2‐oxygen atoms O7 and O8 gives the value of 1.987 and 2.017, respectively, in agreement with the occurrence of an oxo group. [43] There occur two types of 2‐naphthoate molecules acting through the carboxylate arm as a syn‐syn bidentate bridging connecting mode with the two distinct niobium atoms Nb1 and Nb2. They are located in cis position related to the Nb1−O8−Nb2 linkage, which is characterized by a bonding angle of 145.9(2)°. The other Nb1−O7−Nb2 bond angle is slightly larger with a value of 152.5(2)°. These metric values are similar to those found the other tetrameric brick observed with benzoate ligands (compound 1), with the same alternation of concave and convex bending of Nb1−(μ2‐O)−Nb2 bonding. Within the tetramer, the Nb1⋅⋅⋅Nb2 bond lengths are in the range 3.672(2)–3.694(1) Å. The dihedral angle of the two naphthalene rings is nearly perpendicular, with a value of 89.7(2)°, with one naphthalene pointing toward along the {Nb4O4} tetramer plane and the second one upward and downward to this {Nb4O4} plane. In this series, this value is the lowest one, since all other complexes involving dinuclear niobium‐centered units with two carboxylate ligands, or tetranuclear niobium‐centered units, exhibit an angle ranging from 100.92(4)° (compound 4) up to 118.92(5)° (compound 2), indicating a more opened angle for cis conformation of the carboxylate linkers. Indeed, this dihedral value is much lower that reported in the tetramer (112.8°) with benzoate ligands (compound 1). The {Nb4O4} motif is then further decorated with eight ethoxy groups pointing perpendicularly to the plane formed by the naphthalene groups.
Figure 7.
Ball and stick (top) and polyhedral (bottom) representations of the molecular tetranuclear coordination complex 7. One of the ethoxy groups linked to the Nb1 site (through Nb1−O10 bonding) and Nb2 site (through Nb2−O7 bonding) is disordered on two equivalent positions. Only one of these ethoxy groups is shown for clarity.
Compound [Nb8(μ2‐O)12(C11H7O2)8( η1‐C11H7O2)4(OEt)4] ⋅ [Nb8(μ2‐O)12(C11H7O2)8(η1‐C11H7O2)2‐(OEt)6] (8): Compound 8 is the third niobium‐centered complex isolated by means of the 2‐naphthoate ligand (see compounds 2 and 7). Its crystal structure is built up from two distinct neutral molecular entities (denoted A and B), containing both eight niobium centers, with similar connection modes consisting of twelve bridging μ2‐oxo groups (Figure 8). The two arrangements can be viewed as the condensation of two niobium‐centered tetranuclear bricks (related to those encountered in compounds 7 and 1) through four additional μ2‐oxo groups, located perpendicularly to its square plane of Nb−O−Nb bonding. This results in a cubic octanuclear core {Nb8O12}, in which the niobium atoms occupy the eight corners, and the bridging μ2‐oxo groups are located nearby the twelve edges. For each octameric entities A and B, there exist four crystallographic inequivalent niobium atoms, which are octahedrally coordinated to three μ2‐oxo groups, two carboxyl oxygen atoms belonging to the 2‐naphthoate ligand (in bidentate syn–syn bridging mode between two adjacent Nb) and one remaining oxygen atoms from either ethoxy groups or monodentate 2‐naphthoate ligand. The Nb−Ooxo bond distances are in the range 1.825(9)–1.967(9) Å for core A and 1.835(7)–1.969(6) Å for core B, whereas the Nb−Oc (from the bidentate 2‐naphthoate ligand) are in the range 2.092(7)–2.196(10) Å for core A and 2.132(8)–2.175(11) Å for core B. The bond valence calculations for the twelve μ2‐oxygen atoms O1 A−O6 A and O1B−O6B, from the molecules A and B, corresponds to the values range of 2.022–2.122 and 1.979–2.083, respectively, in agreement with the occurrence of an oxo group. [43] The last Nb−O bond lengths related to the ethoxy or monodentate naphthoate (η1) ligands are observed in the range 1.941(14)–2.009(15) Å for core A and 1.927(7)–2.022(13) Å for core B. For molecule A, the Nb−O−Nb bonding angles vary from 143.5(4)° to 147.7(4)°, whereas they vary from 142.6(4)° to 150.6(4)° for molecule B. Within the octamer A, the Nb⋅⋅⋅Nb bond distances are in the range 3.672(2)‐3.694(1) Å and they are in the range 3.672(2)–3.694(1) Å for the octamer B. For the bidentate 2‐naphtoate ligands, we observe dihedral angles between the naphthalene rings, nearly perpendicular, with values 85.2(6)–86.6(6)° for molecule A and 84.1(2)–86.3(4)° for molecule B. It results that the naphthalene rings pointed out in the (a,c) plane, with pairs of adjacent aromatic molecules, which exhibit typical π–π interactions, as reveal the Car⋅⋅⋅Car (Car=aromatic carbon) distances close to 3.6–3.7 Å. Moreover, weaker π–π interactions also occur between the naphthalene rings (Car⋅⋅⋅Car of 3.7–4.0 Å length) belonging to two distinct neighboring octanuclear cores, which ensure the structural cohesion in the [100] and [001] directions. The two entities A and B only differ by the nature of terminal ligands through oxo groups attached to niobium centers, which are four ethoxy groups and four monodentate 2‐naphtoate groups for A, and six ethoxy groups and two monodentate 2‐naphtoate groups for A. These naphthalene monocarboxyl species exist under its nonprotonated form, with short nonbonded C−O distances in the range ≈1.16(3)–1.24(2) Å, relating to the C=O linkage. An infrared vibration band at 1670 cm−1 (Table 1, below; Figure S3h in the Supporting Information) is visible and attributed to such configuration of monodentate naphthoate linker. Indeed, it is the first example in our work of the substitution of ethoxy groups by monodentate carboxylic acids, with variable contents (either 4 EtO/4 naphthoate or 6 EtO/2 naphthoate), leading to a complex molecular assembly of octanuclear niobium cores. The other originality of this polynuclear niobium(V) brick is the number of the eight metallic 4d centers, engaged in a cubic configuration, which has not previously been reported in literature. Only a closely related octanuclear building block was described with the parent pentavalent tantalum(V), in the complex [Ta8(O)12(O2CNEt)2)16]. [49] The latter brick is stabilized by the 16 N,N‐diethylcarbamato ligands which act either as monodentate and bridging bidentate connection mode.
Figure 8.
Ball and stick (top) and polyhedral (bottom) representations of the molecular octanuclear coordination complex 8. The metal center is shown in greater detail in the middle. The molecule consists of two distinct inequivalent entities which contain either 4 ethoxy groups/4 monodentate 2‐naphthoate groups (molecule A, left) or 6 ethoxy groups/2 monodentate 2‐naphthoate groups (molecule B, right). These terminal ethoxy or monodentate 2‐naphthoate groups are indicated in blue for clarity.
Table 1.
Comparison of the ν(C=O) vibrations (from carboxylic acid function) and ν(COO) vibrations (from carboxylate function) for the complexes 1–8.
|
|
|
Vibrations [cm−1] |
||
|---|---|---|---|---|
|
|
|
ν(C=O)acid |
ν asym(COO) |
ν sym(COO) |
|
Complex 1 |
benzoic acid |
1680 |
/ |
/ |
|
supernatant (1 h) |
/ |
1598 |
1556 |
|
|
supernatant (1 month) |
/ |
1599 |
1560 |
|
|
crystalline product |
/ |
1598 |
1560 |
|
|
Complex 2 |
2‐naphtoic acid |
1683 |
/ |
/ |
|
supernatant (1 h) |
/ |
1599 |
1551 |
|
|
supernatant (5 days) |
/ |
1600–1587 |
1560 |
|
|
crystalline product |
/ |
1598 |
1560 |
|
|
Complex 3 |
1‐naphtoic acid |
1670 |
/ |
/ |
|
supernatant (1 h) |
/ |
1596 |
1543 |
|
|
supernatant (1 months) |
/ |
n.o.[a] |
n.o.[a] |
|
|
crystalline product |
/ |
1597 |
1537 |
|
|
Complex 4 |
4‐methylbiphenylcarboxylic acid |
1678 |
/ |
/ |
|
supernatant (1 h) |
/ |
1592 |
1537 |
|
|
supernatant (3 weeks) |
/ |
1591 (weak) |
1535 (weak) |
|
|
crystalline product |
/ |
1598 |
1561 |
|
|
Complex 5 |
anthracene‐9‐carboxylic acid |
1678 |
/ |
/ |
|
supernatant (1 h) |
/ |
1594 |
1540 |
|
|
supernatant (2 weeks) |
/ |
n.o.* |
n.o.[a] |
|
|
crystalline product |
/ |
1597 |
1561 |
|
|
Complex 6 |
anthracene‐9‐carboxylic acid |
1682 |
/ |
/ |
|
supernatant (1 h) |
/ |
1554 |
n.o.[a] |
|
|
supernatant (2 weeks) |
/ |
1557 (weak) |
n.o.[a] |
|
|
crystalline product |
/ |
1584 |
1527 |
|
|
Complex 7 |
2‐naphtoic acid |
1683 |
/ |
/ |
|
|
supernatant (1 h) |
/ |
1597 |
1550 |
|
|
crystalline product |
/ |
1598 |
1557 |
|
Complex 8 |
2‐naphtoic acid |
1683 |
/ |
/ |
|
|
crystalline product |
/ |
1670,[b] 1586 |
1548 |
[a] n.o.: not observed. [b] From monodentate naphthoate species.
Infrared spectroscopy
We investigated the different niobium(V) ethoxide‐carboxylic acid systems by infrared spectroscopy, in order to characterize the crystallization of the complexes 1–8. After the thermal treatment for the formation of complexes 7 and 8, no supernatant was obtained due to the large amount of organic ligand that recrystallized once they cooled down. For each batch (except for compounds 7 and 8, for which no supernatant is obtained), the supernatant solutions have been studied just after the starting reactants mixing at t=1 h at room temperature, and then after the crystallization of the compounds (variable times depending on the crystallization duration for a given complex). Distinct zones are of interest and attributed to the ethoxide (or isopropoxide for complex 5) group. [50] The 3050–2750 cm−1 range is of interest and related to the ν(C−H) stretching vibrations of the ethyl groups (with typical bands at ≈2970(asym) and ≈2860(sym) cm−1), and the 1480–1370 cm−1 zone related to the ν(C−H) bending vibrations of the methyl groups (with typical bands at 1470–1404 and 1375 cm−1). The bands at around 1200–1000 cm−1 are assigned to the ν(C−O) vibrations coming from the ethoxide group linked to niobium center, through Nb−O−CEt linkage: typical peaks are found at 1100–1090 and 1060−1050 cm−1. The broad peak at around 573–529 cm−1 are related to ν(Nb−O) stretching modes. [51] These different signatures are observed either in the supernatant solution or in crystallized niobium complexes, as expected.
For our interest in the formation of niobium carboxylate complexes, we focused our attention on the 1750–1550 cm−1 region, where free carboxylic acid ν(C=O), and bonded carboxylate ν asym(COO) and ν sym(COO) stretching vibrations are typically found.
For complexes 2–6 (and 7 for only before thermal treatment), we observed the same trends (Figure S3b–g), since unprotonated carboxylate ligands are always visible in the supernatant solutions after mixing the chemical reactants (Table 1). Indeed, for the complex 2 system, the band at 1683 cm−1, is related to the ν(C=O) stretching from 2‐naphtoic acid, but disappears when it is mixed with the niobium ethoxide solution. The corresponding vibration stretching is shifted to 1599 and 1551 cm−1, related to the ν asym(COO) and ν sym(COO), respectively, revealing the bonding between carboxylate group and niobium centers [44] in supernatant solution, just after 1 h of mixing. This observation indicates the relative rapid complexation of the starting niobium ethoxide by the carboxylate group occurring at room temperature in less than an hour, and therefore the presence of niobium‐ethoxide‐carboxylate in solution. This group of two peaks are also found in the solid‐state form of the crystalline coordination complex of 2 (at 1598 and 1560 cm−1). This reactivity of the carboxylic acid toward the niobium ethoxide is observed for the different crystallization processes for complexes 2–7. However, infrared signatures differ for some supernatant solution samples after crystallization, since some of them only show very weak bands related to ν(C‐OO) vibrations, corresponding to a smaller content of niobium carboxylate complex in the final solution. This evolution is attributed to the transfer of “soluble” niobium‐ethoxide‐carboxylate complex into the solid crystalline form. This is clearly the case for the crystallization of complex 3 (with 1‐naphthoic acid, after 6 months), or in a less extend, for complexes 4 (with 4‐methylbiphenylcarboxylic acid, after 3 weeks) and 5 (with anthracene‐9‐carboxylic acid, after 2 weeks), for instance.
The formation of the complex 1 with benzoic acid, shows the same behavior, with the band of the carboxylic function at 1680 cm−1 (ν(C=O)) shifting to 1598 and 1560–1556 cm−1 (ν(COO)) when mixed with the niobium ethoxide precursor. It is related to its unprotonated state, either in supernatant solution and solid‐state crystallized form, indicating the carboxylate complexation with niobium cations. However, a new group of bands is appeared in the range 1722–1698 cm−1 in the supernatant solution (Figure S3a), either after 1 h of mixing or 1 month's crystallization, and is assigned to the ν(C=O) coming from the formation of ethyl‐benzoate ester. Similar reaction has been previously described for the synthesis of the tetranuclear niobium acetate complex. [44] The formation of such basic ester organic molecule is expected in such a mixture involving the reaction between a carboxylic acid and an alcohol. The latter will come from proton exchange between the benzoic acid and the attached ethoxide group from niobium alkoxide precursor. This results in the complexation of benzoate with niobium (as indicated by the IR bands of the ν(COO) vibrations) and a leaving group of ethanol species, which further react with benzoic acid, present in excess in the starting mixture. It is noticed that the bands attributed to the ethyl benzoate ester is not visible for the crystalline complex 1, and occurs only in the solution. However, such ester molecules have not been detected in the other niobium‐carboxylic acid system, which could be due the sensitivity of the IR technique. Thus, the formation of the ester molecules with the systems involving other carboxylic acids has been then investigated by liquid 1H NMR of the supernatant solution.
Liquid NMR 1H (and 2D 13C,1H) of supernatants for complexes 1, 2 and 4
Nuclear magnetic resonance was chosen as a more informative technique in order to investigate the presence of the ester in the supernatant of each reaction batch. As mentioned before, the presence of these ester functions is a condition to form, in situ, water molecules able to condense, in a controlled way, the niobium cations to each other through bridging oxo ligands, and form polynuclear species. Since the niobium precursor {Nb(OEt)5} is a liquid, its 1H NMR spectrum has been recorded too, revealing a quartet centered at 5.035 ppm which is attributed to the two protons of −CH2− and a triplet at 1.83 ppm attributed to the three protons of −CH3 (Figure S4a). At 4.82 ppm another broad peak is observed which can derive from the existence of two different −CH2− groups, either terminal or bridging in the dimeric {Nb2(μ2‐OEt)2(OEt)8} precursor. Indeed, the integration matches with this hypothesis and we can assume that the 8 terminal −CH2− give the quartet at 5.035 ppm and the 2 bridging ones the broad signal at 4.82 ppm. However, in the case of −CH3 groups belonging to the bridging ethoxy ligand, a signal is not clearly observed. One can assume that, due to the higher distances of the −CH3 groups from the niobium centers, there are no significant differences in the chemical shifts those belonging to terminal and bridging ethoxy ligand so that both signals appear overlapped.
Among the eight systems giving rise to crystalline products, we selected three of them (1, 2 and 4), for which clear liquid 1H NMR signatures can be extracted from the supernatant solution, without significant perturbation of overlapping peaks due to the complexity of some carboxylic acid ligands in the aromatic region. For each reaction leading to the compounds 1, 2 and 4, supernatant 1H NMR experiments were conducted before and after the crystallization process to follow the formation of the ethyl ester species. As “before crystallization” we define the time for which the ligand solubilizes totally into the niobium(V)ethoxide precursor (ca. 1 h) and as “after crystallization”, we define the time after the appearance of crystals (reaction time ranging from 5 days (2), 7 days (4) and 1 month (1)).
We first describe in detail the results for compound 2 (involving 2‐naphthoic acid) as the best resolved system. Before crystallization, we observe expected 1H NMR signals located in the aromatic region (from 7 to 9 ppm), and aliphatic region (from 0 to 5 ppm; Figure 9, top). For the for 2‐naphthoic acid, the protons H1 and H2 show a singlet at 9.20 ppm and a doublet at 8.70 ppm respectively. The rest of the aromatic protons resonates at 8.46 ppm (doublet, H3), 8.40 ppm (multiplet, H4 and H7) and at 8.07 ppm (multiplet, H5 and H6). The signature of the ethoxy group of the niobium ethoxide precursor, appears with a weak shielding of the chemical shifts (at 4.98 ppm for −CH2− groups and 1.77 ppm for −CH3 groups), compared to the free niobium ethoxide precursor (Figure S4a). It is worth noting that we still observed the presence of the terminal and bridging ethoxide groups generating the broad signal at 4.98 ppm and 4.76 ppm respectively
Figure 9.
1H NMR spectra focused on the aromatic region for the supernatant of complex 2 before (top) and after (bottom) crystallization. Green box: hydroxylic proton of ethanol, red box: −CH2 group of ester form B and blue box: −CH3 group of ester form B.
After the crystallization, the 1H NMR spectrum of supernatant is more complex indicating that a reaction occurred between the 2‐naphtoic ligand and the niobium ethoxide precursor (Figure 9, bottom). It was clearly observed a mixture of 2‐naphthoic acid species (denoted A), an ethyl‐naphthoate ester form (denoted B) and likely naphthoate ligand interacting with niobium centers prior to their crystallization (denoted C). For these three species, the aromatic part of the ligand remains unchanged and should generate an identical number of 1H signals but, different chemical shifts are observed due to the different chemical configurations. In the aromatic region, the H1 atom is firstly considered as isolated (one singlet for the starting solution (Figure 9, top), but it is seen as three singlet signals corresponding to the three different configurations of naphthoates species, A, B and C.
To discriminate them, a 2D 13C,1H HMBC experiment was performed (Figure 10). This method gives correlations between carbons and protons that are separated by two, three, and, sometimes in conjugated systems, four bonds. From this analysis, we assign the signal located at 8.85 ppm to the ester form (B) as it is correlated to the carbon atom C1B (carbon in 1 position for the naphthalene part of molecule B) bearing the H1B proton with the −CH2− group of the ethyl group, centered at 4.71 ppm. The two remaining singlet signals correspond to the naphthoic acid, H1A at 8.94 ppm and H1C at 9.06 ppm, respectively. The latter has a weaker intensity because it is involved during the solid‐state crystal growth of complex 2, and thus a lower concentration in supernatant solution. This observation regarding the differentiation between A and C species is confirmed by a DOSY experiment performed on the supernatant after crystallization (Figure S4b). For the aromatic region of the spectrum, we clearly see the presence of the three different forms with diffusion values of 4.95×10−10, 6.27×10−10, and 2.55×10−10 m2 s−1 related to species A, B and C, respectively. As diffusion parameter is related to the size of a given molecule in solution, the lower diffusion value (2.55×10−10 m2 s−1) is related to the naphthoate ligand interacting with the niobium centers (C). The remaining naphthoic acid (A) has thus a diffusion value of 4.95×10−10 m2 s−1. The latter appears to be smaller than the ester form (B) probably because of dimerization by COOH⋅⋅⋅COOH interactions in the supernatant, resulting in a bigger hindrance for the (A) form than that of ester form (B).
Figure 10.
13C HMBC spectrum for the supernatant of compound 2.
The second set of signals between 8.25 ppm and 8.6 ppm appears as three doublets standing for the H2 hydrogen. Accordingly, with the determination of the three singlet H1, the three H2 doublet signals centered at 8.29, 8.40 and 8.54 ppm are attributed to H2B, H2A and H2C, with the same C, A, B order, respectively. H2B is further confirmed by the 13C,1H HMBC measurement (Figure 10) as it interacts with the −CH2− ethyl signal located at 4.71 ppm. The signals corresponding to the other protons (H3‐7) are heavily overlapped and resonate as two multiplets centered at 8.18 and 7.81 ppm. It may be caused by their location, far from the carboxylate function. No HMBC correlation is observed for these protons. The free niobium ethoxide precursor pics are also visible at 5.88, 3.98 and 1.54 ppm.
For the aliphatic region, signals of the protons of the ester, −CH2− that resonate at 4.72 ppm (quartet circled in red) and −CH3− that resonate at 1.71 ppm (triplet circled in blue) belong also to the 2‐ethylnaphthoate ester form (B). Moreover, we observe a broad singlet deriving at 5.88 ppm coming from the released ethanol molecules (green box in Figure 9).
The same procedure was tentatively followed for the systems 1 (involving benzoic acid) and 4 (involving 4’‐methylbiphenyl‐4‐carboxylic acid).
The benzoic ligand in system 1 appears to be more reactive than the naphthoic ligand in system 2. Indeed, even prior to crystallization, several intense signals are already visible on the 1H NMR spectrum. These signals are quite different and much less resolved than the free ligand spectrum in CDCl3 (Figure S4c). In the aromatic region, three main signals (P1, P2 and P3) centered at 7.53, 8.26, and 8.35 ppm, respectively, are visible and two very small signals (P4 and P5) centered at 8.55 and 8.1 ppm are present, respectively. In the aliphatic region, several peaks (labeled E1 to E4) centered at 4.22, 4.60, 4.75, and 4.36 ppm respectively, and corresponding to −CH2− groups of esters compounds are observed. It is difficult to assign these signals to the molecules occurring in the supernatant. To get more information on the size of the various components of the mixture and based on our interpretation on the diffusion values, we performed a DOSY measurement (Figure S4e). The E1 signal possesses a high diffusion value (compared to other systems; see Figure S4f) indicating that it is a small molecule and could therefore be interpreted as being an ethanol or ethanolate molecule. It is more probably assign to a species in equilibrium between these two forms causing the broadening of the E1 signal. This ethanol/ethanolate moiety comes from the niobium ethoxide precursor after the first complexation by a benzoic acid ligand. The acid ligand exchanges his proton with the ethanolate to form the ethanol and consequently, the benzoate ligand starts to complex the niobium centers. DOSY experiments also show that the E2 and E3 signals correspond to bigger molecules in solution, probably the unreacted niobium ethoxide precursor or benzoate ligand coordinated to the niobium centers. P4 and E3 signals diffuse at the same speed on the DOSY spectrum (Figure S4e). Diffusion values of the P2, P3 and P1 signals suggest that the associated molecules are relatively large and could correspond to benzoate ligands linked to niobium centers.
After crystallization, the signals are well defined and can be interpreted by following the same way as we found for system 2. For the aromatic region, the signals are visible for the H1 and H5 protons of the benzoate groups. They correspond to the acid form (denoted A), the ethyl ester form (denoted B) and the complex form (denoted C; Figure S4g). The HMBC 2D measurement confirms that the B signals corresponds to an ethyl ester form as correlations exist between H1B/H5B with the −CH2− groups of the ethyl ester (Figure S4 h). Based on the observation made with system 2, the two remaining signals centered at 8.03 ppm and 8.14 ppm should correspond to free benzoic acid (A) and complex ligand (B), respectively.
Before crystallization in system 4, four signals centered at 8.71, 8.16, 8.05, and 7.71 ppm are assigned to H1, H2, H3, and H4, respectively, and are related to the aromatic region (Figure S4i, top). They correspond to the acid form of the ligand (4’‐methylbiphenyl‐4‐carboxylic acid). The −CH3 group is also present in the aliphatic region at 2.87 ppm alongside with the niobium ethoxide precursor (signals at 4.96 and 1.77 ppm). After crystallization, some modifications appear in the spectrum (Figure S4i, bottom). In the aromatic region, four main doublets are visible at 8.59, 8.07, 7.94, and 7.60 ppm for protons H1A, H2A, H3A and H4A, respectively. These protons belong to the acid form (A) of the ligand. Two small doublets are also visible on each side of the H1A signal at 8.65 and 8.46 ppm. The HMBC 2D measurement reveals that the doublet located at 8.46 ppm belongs to the ethyl ester form of the ligand (B) as it is correlated to the −CH2− group (small signal located at 5.05 ppm) of the ethyl ester. Consequently, the second doublet at 8.65 ppm stands for the presence of a small amount of acid ligand complexed to the niobium centers (C) prior to crystallization.
Discussion
The systematic reaction of the niobium ethoxide precursor with different benzene‐based monocarboxylic acids gave rise to the identification of eight distinct coordination complexes, with nuclearities rising from 2 to 8.
Beside the carboxylate complexation, the common feature is the occurrence of one or several μ2‐oxo groups bridging two adjacent niobium centers, which are present as one or multiple Nb pairs in these molecular species (1×Nb2 (2–6), 2×Nb2 (1 and 7), 4×Nb2 (8)). Indeed, this bridging oxygen atom is not present in the initial niobium ethoxide source Nb(OEt)5, but the latter is able to undergo dimerization reaction, generating a dinuclear niobium precursor. It is thus observed by liquid 1H NMR (Figure S4a), the formation of dimeric species (Nb2(μ2‐OEt)2(OEt)8) stabilized by two bridging μ2‐ethoxy groups together with eight terminal ethoxy groups. Such a dinuclear moiety has been structurally reported for niobium alkoxides with bridging μ2‐isopropoxy, [52] μ2‐methoxy, [47] and μ2‐ethoxy ligands. [39] The reaction of a carboxylic acid with the Nb2(μ2‐OEt)2(OEt)8 unit generates dinuclear units in which one or two central bridging μ2‐OEt groups are replaced by the carboxylate arms of the aromatic ligand (Figure 11 [A]). Indeed, one observes the complexation by one carboxylate in compounds 3 and 5, but still a bridging μ2‐OEt group is present (as well as a μ2‐oxo group) in [Nb2(μ2‐O)(μ2‐OEt)(L)(OEt)6] (L=bidentate aryl monocarboxylate). When two carboxylate bridges occurs in the dimer, only one central μ2‐oxo group is found in compounds 2, 4 and 6 of the type [Nb2(μ2‐O)(L)2(OEt)6]. The substitution of the μ2‐OEt groups by one carboxylate arm, releases ethanol molecule (coming from H exchange between R‐CO2 H and Nb−OEt → R‐CO2−Nb and Et‐OH) in the medium which is able to further react with free carboxylic acid through an esterification process with the in‐situ production of water, leading to the hydrolysis and Nb−O−Nb condensation [44] (Figure 11 [B]). This esterification reaction has been observed in the supernatant liquor by infrared spectroscopy for the crystallization of compound 1 (Figures 9 and 10) and by 1H NMR for compounds 1, 2 and 4 (Figure S4g–m). This carboxylate complexation is relatively rapid since IR spectra clearly indicates the formation of such niobium carboxylate species in solution after 1‐hour reaction. Therefore, the monocarboxylate niobium dimeric complexes 3 and 5 can be viewed as a first step of the carboxylate complexation, with one bridging μ2‐ethoxy leaving groups substituted by one bidentate carboxylato bridging group. The presence of the additional bridging μ2‐oxo group requires the withdrawal of two terminal ethoxy groups, which contributes to the release of extra ethanol molecules in the reaction medium (from H exchange between H 2O and 2×Nb−OEt → Nb−O−Nb + 2×Et‐OH; (Figure 11 [B]).). The second step of the carboxylate complexation would then lead to the formation of a dinuclear species containing two carboxylate linkers, as observed in compounds 2, 4, and 6, with the substitution of the second bridging μ2‐OEt group by an additional carboxylate ligand (Figure 11 [C]).
Figure 11.
Schematic proposed reactional pathways for the formation of the di‐, tetra‐, and octanuclear cores of niobium(V) monocarboxylate coordination complexes 1–8. For compound 8, only bidentate 2‐naphtoate linkers are shown; terminal monodentate 2‐naphtoate ligands are not shown for clarity. Green: niobium, dark gray: carbon, red: oxygen from ethoxy group, purple: bridging oxygen between Nb centers, orange: oxygen from bidentate carboxylate groups.
Moreover, these two distinct configurations of Nb/ligand ratios of 2 : 1 or 2 : 2 might differ from steric hindrance of the bulky aromatic molecule, as illustrated with the position of the bridging carboxylates in ortho or meta (or para), relative to the position of adjacent neighboring benzene ring. Thus, the ortho position of carboxylate group in 1‐napthoate (3) or anthracene‐9‐carboxylate (5) ligands gives rise to the monocarboxylate niobium dimeric complexes, whereas the meta (2‐napthoate; 2) or para (4‐methyl‐biphenylmonocarboxylate; 4) positions of the carboxylate group leads to the formation to the dicarboxylate niobium dimeric complexes. However, all these coordination complexes have been synthesized directly from the mixture of the liquid phase of niobium ethoxide with a monocarboxylic acid source, with quite low yields (<20 %Nb). One limiting step of such reaction could be the relatively low solubility of such aromatic monocarboxylic acids in the pure niobium ethoxide solution, which may result in Nb/ligand ratio of 2 : 1 or 2 : 2 in dinuclear cores, independently of the steric hindrance or position of the carboxylate function, of the different monocarboxylate molecules. Indeed, in the different synthesis batches, we observed some difficulties to dissolve the monocarboxylic acid. Unreacted starting ligands were often visible as crystallizing as by‐product together with the niobium coordination complexes. Depending on the solubility of the carboxylic acid in niobium ethoxide, undissolved starting ligand sometimes remains on the bottom of the vial without reacting, while the ligand placed on the top that comes first in contact with niobium ethoxide is dissolved and reacts to form crystals of the present products. After mild heating (as alkoxides are reported not to be stable in great temperatures, subliming around 100 °C), [39] we can observe solubilization (clear solution), but no crystallization.
In order to investigate the influence of the monocarboxylic acid concentration, we thus investigated a second series of syntheses involving the addition of alcohol solvent. When isopropanol (HOiPr) was used in molar ratio 0.8 Nb(OEt)5:13.1 HOiPr (volume ratio 0.2 Nb(OEt)5:1 HOiPr), a clear solution was systematically obtained after mixing the different source reactants (niobium ethoxide, monocarboxylic acid, isopropanol). However, when using anthracene‐9‐carboxylic acid, we were able to isolate in such isopropanol/niobium ethoxide mixture, a second dinuclear complex (6) containing two carboxylate ligands instead of one in complex 5 (synthesized in pure niobium ethoxide solution). The isopropanol solvent is assumed to promote the dissolution of the poly‐benzene carboxylic acid in order to enhance the carboxylate complexation. Therefore, the formation of the monocarboxylate or dicarboxylate dimeric niobium‐centered complexes is not directly correlated to the steric hindrance of the organic linker in this case.
Another aspect of the production niobium carboxylate from metal alkoxide source is the occurrence of multiple nuclearities for niobium, which have been isolated in a tetranuclear core with benzoate and 2‐naphtoate bidentate bridging linkers, or octanuclear core with 2‐naphtaote one. With benzoic acid (compound 1), only a tetrameric moiety is observed with four niobium centers, located at each node of square geometry, and linked to each other through μ2‐oxo groups, resulting in a [Nb4(μ2‐O)4(L)4(OEt)8] core. The benzene rings of the four bidentate carboxylate linkers are pointed along the square plane direction for two of them, and up and downward the square plane for the two others. It is interesting to observe that the in‐situ generation of water from esterification process is particularly efficient with the benzoic acid, leading directly to a tetranuclear species, instead of the dinuclear one, as observed with other poly‐benzene‐based monocarboxylic acids (compounds 2–6). Indeed, the ethyl benzoate ester is visible just after one‐hour reaction in the mother liquor (see infrared analysis, Figure S3a), whereas it not observed for the other aryl acids, for which ester forms are seen when crystallized phase is produced (see 1H NMR, for the systems 2 and 4, for instance Figure S3b and d). In fact, a previous study has reported the occurrence of such dinuclear species containing a central μ2‐oxo group in a benzoate complex [Nb2Cl6(O2C−C6H5)2O], [23] when the niobium source was NbCl5, and mixed in chloroform or carbon tetrachloride, which was then heated at 60–110 °C. For the latter, the origin of the bridging oxygen atom comes from the formation of acyl chloride (3R−CO2H+2NbCl5→R−COCl+Nb2Cl6(O2C−C6H5)2O), which releases one available oxo species for the Nb−O−Nb condensation. The yield of acyl chloride reaction seems to be less efficient for the generation of dinuclear unit [Nb2(μ2‐O)(L)2Cl6] compared to that giving rise straightly to a higher condensed species as found in the [Nb4(μ2‐O)4(L)4(OEt)8] core. As suggested by different authors,[ 38 , 44 , 45 ] the tetranuclear entity [Nb4(μ2‐O)4(L)4(OEt)8] can be viewed as the assembly of two dinuclear [Nb2(μ2‐O)(L)2(OEt)6] bricks through two additional μ2‐oxo groups, coming from the removal of four terminal ethoxy groups (Figures 11 [D] and S5a). This tetrameric entity can thus be considered as a second step of condensation with the formation of additional double Nb−O−Nb linkages.
With benzoic acid, it was only possible to isolate either a crystalline dinuclear or tetranuclear species by changing the niobium source (NbCl5 or Nb(OEt)5). It seems that multiple nuclearity was not reported in literature for a given niobium precursor. Indeed, it was proposed that steric hindrance of the ligand might orientate the nuclearity of the complex as illustrated in the work of Boyle, [45] for which a dimer was obtained with a bulky molecule (pivalic acid), whereas a tetramer is obtained with a less bulky molecule (terbutylacetic acid), prepared in the same synthetic conditions. In fact, by using 2‐naphthoic acid, we show here that the nuclearity of the inorganic core is controlled by the hydrolysis rate. For a low water content (coming from esterification reaction at room temperature), the dinuclear core [Nb2(μ2‐O)(L)2(OEt)6] (compound 2) is observed, whereas the tetranuclear core [Nb4(μ2‐O)4(L)4(OEt)8] (compound 7) has been identified in a closed reaction medium saturated with 55 % water humidity. This suggests that an increasing water content promotes the Nb−O−Nb condensation process, which can favor the formation of higher nuclearity in niobium carboxylate complexes. The last coordination complex observed in our system involving 2‐naphthoic acid, is the isolation of poly‐oxo cluster containing eight niobium centers located at each node of a cubic geometry (compound 8). Indeed, its crystal structure exhibits one octanuclear moiety which is stabilized under two slightly different forms, with a general formula of [Nb8(μ2‐O)12(L)8(OEt)4+x (η1‐L)4‐x ] (x=0 or 2), with varying numbers of terminal ethoxy/naphthoic acid groups. Apart that this octamer is new in chemistry of niobium carboxylates poly‐oxo clusters, it is a good illustration of a third step for the condensation Nb−O−Nb process, by considering the association of two tetramers [Nb4(μ2‐O)4(L)4(OEt)8] through the addition of quadruple μ2‐oxo ligand between the niobium centers (Figures 11 [E] and S5a). For compound 8, the hydrolysis rate is enhanced by using a solvothermal synthetic route (here heating at 100 °C for 4 days in acetonitrile), which allows for the production of higher water content through esterification increasing yield. Similar reaction conditions thus led to the synthesis of a 16‐nuclearity poly‐oxo cluster in [Nb16O28(OEt)12(piv)12]. [41]
From this series, it was possible to play with the hydrolysis rate by isolating different steps of Nb−O−Nb condensation from non‐oxo {Nb} species to oxo species with {NbO0.5} (compounds 2–6), {NbO1.0} (compounds 1 and 7), {NbO1.5} (compound 8) and {NbO1.75}, [41] knowing that the ultimate member would be the infinite dense niobium oxide network {NbO2.5}∞ as found in Nb2O5. It is noticeable to observe that the octanuclear {Nb8O12} brick, as a corner‐sharing octahedral arrangement of perovskite‐related ReO3 type structure, can be considered as a fragment of the Nb2O5 network. Although its crystal chemistry offers many complex polymorphic stackings, [53] one may identify some blocks with such ReO3 configurations, in some of them, related to different forms identified at various temperatures (Figure S5b).
The different polynuclear niobium oxo‐clusters 1–8 are systematically constructed from octahedral NbO6 polyhedra connected through a corner‐sharing mode. This particular linkage fashion is surely promoted by the double bidentate carboxylate bridge, which acts as a pincer between two adjacent niobium atoms, and thus favors the oxo‐corner fashion. This specific connection mode is nevertheless not observed in the chemistry of niobium‐based polyoxometalates species, since the basic bricks are composed of the hexanuclear entity [Nb6O19]8− of Lindqvist type, or decaniobiate [Nb10O28]6−, which both are characterized by edge‐sharing NbO6 octahedra. [54] These anionic moieties are able to further react to generate a large number of highly condensed oxo clusters such as the giant {Nb288O768(OH)48(CO3)12}, [55] for instance. The edge‐sharing mode is also observed in purely niobium alkoxides such as the dimeric precursors,[ 39 , 47 , 52 ] hexanuclear [Nb6O8(OiPr)14(HOiPr)2] species with isopropoxy groups [56] or octanuclear species [Nb8O10(OEt)20] with ethoxy groups. [57] Another structural difference between the niobium‐based polyoxometalates and the niobium alkoxides molecules is the distinct nature of the terminal oxo groups. The polyoxometalates are usually characterized by the occurrence of terminal niobyl‐oxo groups, which are not observed in the niobium mixed alkoxide‐carboxylates, since the terminal oxo groups belong to alkoxo species decorating the external periphery of the polynuclear moiety.
Conclusion
In conclusion, various niobium(V)‐centered cores have been identified by the use of arene or polyarene monocarboxylic acids. Indeed, by starting from the niobium ethoxide as precursor, eight different discrete molecules of niobium carboxylates poly‐oxo clusters have been obtained with nuclearities varying from {Nb2} via {Nb4} up to {Nb8}, with various fashions of oxo bridges connecting the Nb centers. The synthetic condition associating carboxylic acid with leaving ethanol groups allows the slow production of ester species with the release of water molecules, which are at the origin of the oxo‐condensation bridge between the niobium centers. Indeed, the occurrence of moisture atmosphere or/and elevated temperature (up 100 °C in our study) favors the generation of water molecules, which induce the formation of higher nuclearities for the resulting niobium‐based poly‐oxo clusters ({Nb4} or {Nb8}). From the structural point of view, the addition of monocarboxylic acid to the niobium ethoxide (Nb2(OEt)10) led to the formation of dinuclear unit (Nb2O(carboxylate)1 or 2), which can be assembled twice to give a tetranuclear unit (Nb4O4(carboxylate)4). The latter can be further condensed twice more, to produce an octanuclear brick ((Nb8O12(carboxylate)8). These different well‐defined niobium(V)‐centered cores can be considered as potential inorganic building units for the generation of extended networks by using polydentate carboxylic acid ligands. Indeed, considering the niobium(V)‐oxygen bond length, we observed that Nb−Ooxo distances are quite a lot shorter (≈1.9 Å) than those of Zr‐O observed in well‐known MOFs such as UiO‐66 (2.23 Å), [58] but metal−Ocarboxylate bond lengths are identical (Nb−Oc ≈2.10–2.20 Å). With this purely metric view, we might expect a better chemical stability of the niobium‐MOF compounds. Work exploring the structural flexibility of using such polynuclear units to form MOF‐like architectures is currently underway.
Experimental Section
Reagents: The following reactants were used: niobium(V) ethoxide complex (Nb(OCH2CH3)5, 99.95 %, Sigma‐Aldrich), benzoic acid (C7H6O2, 99.5 %, Sigma‐Aldrich), 1‐naphthoic acid (C11H8O2, 96 %, Sigma‐Aldrich), 2‐naphthoic acid (C11H8O2, 98 %, Sigma‐Aldrich), 4’‐methylbiphenyl‐4‐carboxylic acid (C14H12O2, 96 %, Sigma‐Aldrich), anthracene‐9‐carboxylic acid (C15H10O2, 99 % Sigma‐Aldrich), anhydrous isopropanol (C3H8O, HOiPr, 99.5 %, Sigma‐Aldrich). The chemical reactants were commercially available, were used without any further purification and manipulated in a glove box under argon to prevent any hydrolysis reaction of the niobium(V) alkoxide into niobium(V) oxide.
Syntheses
Compound 1 [Nb4(μ2‐O)4(C7H5O2)4(OEt)8]: A mixture of 0.75 mL (3 mmol) Nb(OCH2CH3)5 and 756.4 mg (6.2 mmol) benzoic acid was placed in a 2 mL glass tube, sealed with a phenolic cap, and left at room temperature. The formation of brick‐like colorless crystals, was observed after one month. Compound 1 was analyzed by optical microscope showing large elongated parallelepiped‐shape crystals up to 1 mm size. The resulting colorless crystals filtered off, washed with ethanol and dried at room temperature. Crystallization yield was 24.5 %Nb. (Figure S1a).
Compound 2 [Nb2(μ2‐O)(C11H7O2)2(OEt)6]: A mixture of 0.75 mL (3 mmol) Nb(OCH2CH3)5 and 1.033 g (6 mmol) 2‐naphthoic acid was placed in a 2 mL glass tube, sealed with a phenolic cap, and left at room temperature. Baguette‐like transparent crystals appeared after 5 days. Compound 2 was analyzed by optical microscope showing large elongated parallelepiped‐shape crystals up to 300 μm size. The resulting colorless crystals filtered off, washed with ethanol and dried at room temperature. Crystallization yield was 6.1 %Nb. (Figure S1a).
Compound 3 [Nb2(μ2‐O)(μ2‐OEt)(C11H7O2)(OEt)6]: A mixture of 0.75 mL (3 mmol) Nb(OCH2CH3)5 and 1.033 g (6 mmol) 1‐naphthoic acid was placed in a 2 mL glass tube, sealed with a phenolic cap, and left at room temperature. Baguette‐like transparent crystals appeared after 6 months. Compound 3 was analyzed by optical microscope showing large elongated parallelepiped‐shape crystals up to 300 μm size. The resulting colorless crystals filtered off, washed with ethanol and dried at room temperature. Crystallization yield was 2.4 %Nb. (Figure S1b).
Compound 4 [Nb2(μ2‐O)(C14H11O2)2(OEt)6]: A mixture of 0.75 mL (3 mmol) Nb(OCH2CH3)5 and 1.273 g (6 mmol) 4’‐methylbiphenyl‐4‐carboxylic acid was placed in a 2 mL glass tube, sealed with a phenolic cap, and left at room temperature. Plate‐like crystals appeared after 3 weeks. Compound 4 was analyzed by optical showing elongated parallelepiped‐shape crystals up to 40 μm size. The resulting colorless crystals filtered off, washed with ethanol and dried at room temperature. Crystallization yield was 19.1 %Nb. (Figure S1b).
Compound 5 [Nb2(μ2‐O)(μ2‐OEt)(C15H9O2)(OEt)6]2 : A mixture of 0.75 mL (3 mmol) Nb(OCH2CH3)5 and 222.24 mg (1 mmol) anthracene‐9‐carboxylic acid was placed in a 2 mL glass tube, sealed with a phenolic cap, and left at room temperature. Brick‐like crystals appeared after 2 weeks. Compound 5 was analyzed by optical microscope showing block‐shape crystals up to 30 μm size. The resulting pale yellowish crystals filtered off, washed with ethanol and dried at room temperature. Crystallization yield was 13.1 %Nb. (Figure S1c).
Compound 6 [Nb2(μ2‐O)(C15H9O2)2(OiPr)6]: A mixture of 0.2 mL (0.8 mmol) Nb(OCH2CH3)5 and 85.3 mg (0.3 mmol) anthracene‐9‐carboxylic acid, isopropanol 1 mL (13.1 mmol) was placed in a 2 mL glass tube, sealed with a phenolic cap, and left at room temperature. Brick‐like transparent crystals appeared after 2 weeks. Compound 6 was analyzed by optical microscope showing large block‐shape crystals up to 50 μm size. The resulting colorless crystals filtered off, washed with ethanol and dried at room temperature. Crystallization yield was 4.8 %Nb. (Figure S1c).
Compound 7 [Nb4(μ2‐O)4(C11H7O2)4(OEt)8]: A mixture of 0.2 mL (0.8 mmol) Nb(OCH2CH3)5 and 145 mg (0.85 mmol) 2‐naphthoic acid was placed in a 2 mL glass tube, sealed with a phenolic cap in inert atmosphere. The vial was placed in a desiccator with a saturated solution of Na2CO3 (55 % humidity) with an open cap and after 24 h was closed and the vial was placed in the oven at 50 °C. Only a powdered sample is obtained, in which needle‐like transparent crystals are observed after 3 weeks. Compound 7 was analyzed by optical microscope showing large elongated parallelepiped‐shape crystals up to 150 μm size. The resulting colorless crystals filtered off, washed with ethanol and dried at room temperature. Crystallization yield was 9.7 %Nb (Figure S1d).
Compound 8 [Nb8(μ2‐O)12(C11H7O2)8(η1‐C11H7O2)4(OEt)4] ⋅ [Nb8(μ2‐O)12(C11H7O2)8(η1‐C11H7O2)2‐(OEt)6]: A mixture of 60 μL (0.24 mmol) Nb(OCH2CH3)5, 460 mg (2.7 mmol) 2‐naphthoic acid and acetonitrile (1 mL, 19.1 mmol)) was placed in a 2 mL glass tube, sealed with a phenolic cap, and placed in an oven at 100 °C. After 4 days dark red parallelepiped blocks appeared and as the temperature decreases we observe the recrystallisation of 2‐naphthoic acid and finally no supernatant is obtained. Compound 8 was analyzed by optical microscope showing large parallelepiped‐shape crystals up to 40 μm size. The resulting dark red crystals filtered off, washed with acetonitrile and dried at room temperature. Crystallization yield was 11.6 %Nb. (Figure S1d).
All niobium‐based coordination complexes 1–8 were found to be stable under air for several weeks and manipulated without any specific precaution.
Single‐crystal X‐ray diffraction: Crystals of compounds 1–7 were selected under polarizing optical microscope and were mounted on MicroMount patented by MiTeGen, inserted into a goniometer base to perform single‐crystal X‐ray diffraction experiment. X‐ray intensity data were collected on a Bruker DUO‐APEX2 diffractometer using MoKa radiation (λ=0.71073 Å) with an optical fiber as collimator. Several sets of narrow data frames (20s per frame for 1, 20s for 2, 20s for 3, 20s for 4, 20s for 5, 20s for 6 and 15 s for 7) were collected at different values of q for two initial values of ϕ and ω, respectively, using 0.3° increments of ϕ with ω scans.<
Due to its very small crystal size (≈ 20×20×10 μm), the X‐ray diffraction intensities data of compound 8 have been collected from a Synchrotron radiation facility (PROXIMA‐2 A beamline, SOLEIL, France), by using a wavelength of λ=0.774901 Å, at 100 K (under liquid nitrogen gas flow). The structural analysis shows high values for many thermal parameters, related to organic ligands. Consequently, most of the naphthoate ligands were constrained with the AFIX function (with m=11 to get idealized naphthoic groups) and Exyz constraint to get ideal aromatic rings with averages C−C distances of 1.39 Å. SQUEEZE procedure implemented in the olex2 Software was used to remove residual electronic densities corresponding to disordered solvent molecules, which are too difficult to model. The procedure removed 392 electrons per cell units in the lattice.
Data reduction was accomplished using SAINT V8.34a. [59] The substantial redundancy in data allowed a semi‐empirical absorption correction (SADABS V2014/5) to be applied, on the basis of multiple measurements of equivalent reflections. [60] The structures were solved by direct methods, developed by successive difference Fourier syntheses, and refined by full‐matrix least‐squares on all F 2 data using SHELX program suites, implemented in the OLEX2 graphical tool. [61]
The crystal data are given in Table 2. Supporting Information is available in CIF format. Deposition Numbers 2172037 (for 1), 2172039 (for 2), 2172034 (for 3), 2172040 (for 4), 2172038 (for 5), 2172035 (for 6), 2172036 (for 7), and 2172041 (for 8) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.
Table 2.
Crystal data and structure refinements for compounds 1–8.
|
|
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
||
|---|---|---|---|---|---|---|---|---|---|---|
|
formula |
C11H15NbO5 |
C34H44Nb2O11 |
C25H42Nb2O10 |
C40H52Nb2O11 |
C58H88Nb4O20 |
C192H236.5Nb8O44 |
C60H68Nb4O20 |
C262H204Nb16O78 |
||
|
formula weight |
320.14 |
814.51 |
688.4 |
894.64 |
1 476.92 |
3 991.58 |
1 214.54 |
6 086.80 |
||
|
T/K |
296 |
102 |
100 |
100 |
100 |
100 |
100 |
100 |
||
|
crystal type |
colorless parallelepiped |
colorless parallelepiped |
colorless parallelepiped |
colorless parallelepiped |
colorless parallelepiped |
yellowish plate |
colorless needle |
colorless needle |
||
|
crystal size/mm |
0.20×0.19×0.13 |
0.99×0.66×0.30 |
0.58×0.26×0.10 |
0.13×0.12×0.11 |
0.31×0.26×0.13 |
0.28×0.16×0.10 |
0.42×0.10×0.09 |
0.02×0.02×0.01 |
||
|
crystal system |
triclinic |
monoclinic |
orthorhombic |
monoclinic |
triclinic |
monoclinic |
monoclinic |
monoclinic |
||
|
space group |
P
|
P21 /c |
P212121 |
C2/c |
P
|
C2/c |
P21 /c |
C2/c |
||
|
a/Å |
10.8589(6) |
20.228(2) |
8.1182(14) |
25.4759(7) |
11.7194(6) |
21.839(2) |
10.9286(10) |
67.5185(14) |
||
|
b/Å |
11.9022(6) |
8.5199(8) |
19.040(3) |
9.0694(3) |
14.1832(7) |
9.9717(11) |
20.1535(15) |
10.7915(2) |
||
|
c/Å |
12.4716(6) |
20.738(2) |
19.081(3) |
17.9087(4) |
20.3412(10) |
21.376(2) |
16.3901(13) |
47.1089(9) |
||
|
α/° |
72.281(2) |
90 |
90 |
90 |
102.816(3) |
90 |
90 |
90 |
||
|
β/° |
70.918(2) |
97.487(3) |
90 |
101.834(1) |
95.187(2) |
95.901(3) |
120.679(3) |
134.142(1) |
||
|
γ° |
67.815(2) |
90 |
90 |
90 |
99.809(3) |
90 |
90 |
90 |
||
|
V/Å3 |
1380.08(13) |
3543.5(6) |
2946.4(9) |
4049.9(2) |
3219.6(3) |
4630.6(8) |
3104.7(5) |
24595.5(9) |
||
|
Z, ρ calcd/g cm−3 |
4, 1.541 |
4, 1.527 |
4, 1.550 |
4, 1.467 |
2, 1.523 |
1, 1.431 |
2, 1.584 |
4, 1.644 |
||
|
μ/mm−1 |
0.878 |
0.704 |
0.827 |
0.623 |
0.764 |
0.553 |
0.793 |
1.114 |
||
|
Θ range/° |
1.77–26.40 |
2.11–30.53 |
1.51–26.42 |
1.63–26.39 |
1.50–30.53 |
1.92–26.42 |
1.76–26.39 |
0.92–25.00 |
||
|
limiting indices |
−13≤h≤13 −14≤k≤14 −15≤l≤15 |
−28≤h≤28 −12≤k≤10 −21≤l≤29 |
−9≤h≤10 −23≤k≤22 −23≤l≤23 |
−31≤h≤31 −11≤k≤11 −22≤l≤19 |
−16≤h≤16 −16≤k≤20 −29≤l≤29 |
−27≤h≤27 −12≤k≤12 −26≤l≤26 |
−13≤h≤13 −23≤k≤25 −18≤l≤20 |
−77≤h≤76 −10≤k≤10 −55≤l≤55 |
||
|
collected reflections |
36 059 |
60 031 |
20 882 |
30 431 |
8 430 |
36 900 |
24 721 |
84 196 |
||
|
unique reflections |
5 642 [R(int)=0.0245] |
10 808 [R(int)=0.0267] |
5 810 [R(int)=0.0581] |
4 132 [R(int)=0.0396] |
19 179 [R(int)=0.0259] |
4 753 [R(int)=0.0287] |
6 330 [R(int)=0.0964] |
10 386 [R(int)=0.0506] |
||
|
parameters |
301 |
466 |
342 |
254 |
814 |
303 |
383 |
1296 |
||
|
goodness‐of‐fit on F2 |
1.080 |
1.044 |
1.053 |
1.048 |
1.067 |
1.046 |
1.007 |
1.107 |
||
|
final R indices [I>2σ(I)] |
R1=0.0430 wR2=0.1193 |
R1=0.0332 wR2=0.0772 |
R1=0.0387 wR2=0.0661 |
R1=0.0272 wR2=0.0647 |
R1=0.0319 wR2=0.0780 |
R1=0.0292 wR2=0.0695 |
R1=0.0539 wR2=0.1081 |
R1=0.1008 wR2=0.3164 |
||
|
R indices (all data) |
R1=0.0563 wR2=0.1364 |
R1=0.0360 wR2=0.0799 |
R1=0.0504 wR2=0.0691 |
R1=0.0366 wR2=0.0687 |
R1=0.0393 wR2=0.0820 |
R1=0.0325 wR2=0.07151 |
R1=0.1072 wR2=0.1292 |
R1=0.1280 wR2=0.3328 |
||
|
largest diff. peak and hole/e Å−3 |
1.48 and −0.97 |
1.58 and −3.39 |
0.77 and −0.68 |
0.41 and −0.42 |
1.57 and −1.14 |
0.93 and −0.54 |
0.71 and −1.17 |
1.38 and −0.94 |
Powder X‐ray diffraction: X‐ray powder diffraction was performed on Bruker D8 Advance diffractometer (LynxEye detector) in a Bragg‐Brentano θ‐θ mode using CuKα radiation. Each powder pattern was recorded by using a classical glass specimen holder, within an angular range of 5–50° in 2θ, with steps of 0.02° and counting time of 0.5 s per step.
Infrared spectroscopy: Infrared spectrum was measured on Perkin Elmer Spectrum TwoTM spectrometer between 4000 and 400 cm−1, equipped with a diamond Attenuated Total Reflectance (ATR) accessory. No ATR correction was applied on the spectrum.
Liquid NMR spectroscopy: The supernatant solutions obtained after the crystallization of compounds 1, 2 and 4 have been analyzed by solution 1H NMR at 300 K and 9.4 T, on a Bruker Avance NEO 400 spectrometer equipped with a z‐gradiented TBI 5 mm probe. In order to prevent any modifications by adding any substances into the samples, the solution samples have been introduced in standard 5 mm tubes with a coaxial insert (Eurisotop, Saint‐Aubin, France) containing deuterated chloroform (CDCl3, isotopic enrichment 99.8 %, Eurisotop, Saint‐Aubin, France) used for the lock and shims settings. Chemical shifts in parts per million are referenced to the residual solvent peak of CHCl3 at 7.26 ppm.
Conflict of interest
The authors declare no conflict of interest.
1.
Supporting information
As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.
Supporting Information
Acknowledgements
The authors would like to thank Nora Djelal, Laurence Burylo and Philippe Devaux for their assistance with the synthesis and XRD powder patterns measurements (UCCS). Dr. Clément Falaise (Université of Versailles Saint Quentin‐en‐Yvelines) is thanked for his help with the single‐crystal XRD data collection (compound 8) at synchrotron SOLEIL (France). The “Fonds Européen de Développement Régional (FEDER)”, “CNRS”, “Région Hauts de France”, and “Ministère de l′Education Nationale de l′Enseignement Supérieur et de la Recherche” are acknowledged for the funding of X‐ray diffractometers from the Chevreul Institute platform.
D. Andriotou, S. Duval, C. Volkringer, X. Trivelli, W. E. Shepard, T. Loiseau, Chem. Eur. J. 2022, 28, e202201464.
Data Availability Statement
The data associated with the findings of this study can be found in the online version (optical microscope photographs, powder XRD pattern, IR spectra, liquid 1H NMR summary of the representation of polynuclear niobium‐centered cores).
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.
Supporting Information
Data Availability Statement
The data associated with the findings of this study can be found in the online version (optical microscope photographs, powder XRD pattern, IR spectra, liquid 1H NMR summary of the representation of polynuclear niobium‐centered cores).