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. 2020 Feb 28;5(9):4668–4672. doi: 10.1021/acsomega.0c00083

Hydrothermal Synthesis and Structure of a Dinuclear Molybdenum(III) Hydroxy Squarate with a Mo–Mo Bond

Greg D Barber †, Christy George ‡, Kathryn Hogg †, Shae T Johnstone †, Carlos N Pacheco ‡, Hemant P Yennawar §, William G Van Der Sluys †,*
PMCID: PMC7066654  PMID: 32175513

Abstract

graphic file with name ao0c00083_0001.jpg

The reaction of molybdenum(II) and chromium(II) acetates with squaric acid in degassed and deionized water under hydrothermal conditions at 150 °C is described. The products have been formulated as M2(μ-OH)2(μ-C4O4)2(H2O)4·2H2O, where M = Cr (1) and Mo (2), based on combustion elemental analysis, infrared spectroscopy, magic angle spinning (MAS) solid-state carbon-13 nuclear magnetic resonance (NMR), and single-crystal X-ray diffraction. The edge-shared bioctahedral structures involve doubly bridging hydroxide ligands and μ-squarate ligands. The chromium compound lacks a direct metal–metal-bonding interaction, while in contrast the molybdenum compound contains a Mo–Mo bond [2.491(2) Å]. The nature of the Mo–Mo-bonding interaction is compared with that of other similar d3–d3 dimers.

Introduction

As part of our search for metal–organic framework (MOF)-type materials containing metal–metal multiple bonds,1−7 we have been investigating the reaction of quadruply bonded chromium(II) and molybdenum(II) compounds with simple diprotic carboxy acids, such as oxalic acid or squaric acid.3 Chisholm and Cotton have shown that it is possible to link quadruply bonded dinuclear units using bridging oxalate and squarate ligands, to form dimers of dimers that facilitate varying degrees of electronic communication through the π-system of the carboxy anion linker.2,3,7 Therefore, we reasoned that it may be possible to prepare two-dimensional polymeric species containing metal–metal quadruple bonds linked by squarate, which might have interesting electrical conducting properties.1 Unfortunately, the compounds we have obtained thus far and described in this paper are more like traditional molecular species and do not fit the typical description of an MOF-type material. However, the products are of interest from a theoretical point of view with respect to the electronic structure of transition-metal complexes and the nature of metal–metal-bonding interactions.2 In addition, this work points out the limitations of the hydrothermal synthetic approach, which generally produces the most thermodynamically stable product, rather than kinetically stable materials that might be produced under less thermally demanding conditions.4

Results and Discussion

We have investigated the reaction of chromium(II) acetate dihydrate and molybdenum(II) acetate with squaric acid under anaerobic hydrothermal reaction conditions. We obtained an air-stable, light dichroic purple/green crystalline material in the case of chromium and an air-stable, light green/brown crystalline material in the case of molybdenum. The empirical formula of these products is best characterized as MC4H7O8 [M = Cr (1) and Mo (2)] based on combustion elemental analysis. This formulation requires the metal atoms to have undergone a one-electron oxidation per metal atom. The compounds were sparingly soluble and relatively stable in degassed deionized (DI) water and dimethylformamide (DMF), but virtually insoluble in organic solvents such as methanol, acetone, diethyl ether, dichloromethane, and hexane. The KBr pellet infrared spectrum clearly indicates the presence of both squarate and hydrogen-bonded hydroxyl groups and/or water. Compound 1 appears to be the same as the product obtained by other investigators, in which chromium(III) starting materials were reacted with excess squaric acid, forming the dinuclear species Cr2(OH)2(C4O4)2(H2O)4·2H2O.8,9 We have also been able to prepare 1 by exposure of chromium(II) squarate hydrate to air10 and have been able to prepare 2 from potassium octachlorodimolybdate,2 using the hydrothermal approach. Similar tungsten hydroxide formamidinate complexes have been prepared by the serendipitous exposure of quadruply bonded ditungsten tetraformamidinate compounds to air and moisture.11 We cannot rule out the possibility that the metal ions reduce the squaric acid during preparation.12

We have been able to characterize both 1 and 2 by single-crystal X-ray diffraction and have found that they adopt similar structures (Figure 1), with one major difference. Both adopt edge-shared bioctahedral structures, in which there are bridging hydroxide ligands between the metal atoms. The trans-squarate ligands bridge the two metal atoms to form a six-membered ring, rather than coordinating to a single metal atom, which would result in a five-membered ring with significant ring strain and an unfavorable bite angle. We note that squarate is a versatile ligand, having a wide variety of coordination modes.13,14 Both 1 and 2 have four terminally ligated water molecules, resulting in roughly D2h symmetry, which is lowered by the presence of the additional waters of crystallization. We were unable to locate all of the hydrogen atoms in the difference map of 2, but have included a computer-generated approximation for the hydrogen-bonding interactions in Figure 1. The structure of 1 is consistent with that observed by Chesick and Doany.15 The major difference between the structures of 1 and 2 is the distance between the metal atoms and the subsequent effect on bond angles. The Cr–Cr distance (3.0 Å) is typical for a chromium(III) paramagnetic dimer having antiferromagnetic coupling.16−18 In contrast, the Mo–Mo distance in 2 is significantly shorter [2.491(2) Å] and consistent with a diamagnetic compound having a metal–metal bond, which causes a decrease in the Mo(1)–O(4)–Mo(1′) angle, 74.9(1)°, for the bridging hydroxide ligands, as compared with the same angle in 1, 128.6(1)°. In addition, the trans-squarate O(1)–Mo(1)–O(5′) angles of 172(1)° versus 160.9(1)° for 1 and 2, respectively, are consistent with a distortion that would occur due to the presence of the Mo–Mo bond and the relatively large bite angle of the inflexible squarate ligand [O(1)–C(2)–C(1), 134.0(4)°]. Similar to what was previously observed for 1, the C–O bond lengths for the squarate in 2 are approximately 0.03 Å longer when the oxygen atoms are coordinated to the molybdenum atoms than the C–O bond lengths for the oxygen atoms that are not coordinated to the metal atoms. The latter are involved in hydrogen-bonding interactions with water molecules. There are no significant differences between the C–O and C–C bond lengths within the squarate ligands for 1 as compared with 2, suggesting that any π back-bonding from metal-based d-orbitals into the C–O π* orbitals and carbon ring is not significantly altered by the presence of the metal–metal bond. The Mo–O bond lengths for the squarate ligands, the bridging hydroxide ligands, and the coordinated water molecules are all remarkably similar (avg. 2.09 ± 0.06 Å)19 and slightly longer than the average Cr–O bond lengths (avg. 1.97 ± 0.02 Å), consistent with the increased ionic radius of molybdenum. The relative Mo–O bond lengths follow the order squarate, which is less than the bridging hydroxide, followed by coordinated waters, consistent with the relative charge interactions.

Figure 1.

Figure 1

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(a) View of 2, with atom numbering scheme, emphasizing the edge-shared bioctahedral structure, and (b) view of the unit cell emphasizing hydrogen-bonding interactions. Selected bond lengths and angles include Mo(1)–Mo(1), 2.491(2); Mo(1)–O(1), 2.090(3); Mo(1)–O(2), 2.130(4); Mo(1)–O(3), 2.154(4); Mo(1)–O(4), 2.046(4); Mo(1)–O(4), 2.049(4); Mo(1)–O(5), 2.094(3); O(4)–Mo(1)′, 2.049(4); O(1)–C(2), 1.268(5); O(5)–C(1), 1.275(5); O(6)–C(4), 1.238(5); O(7)–C(3), 1.240(5); C(1)–C(2), 1.425(6); C(1)–C(4), 1.461(5); C(2)–C(3), 1.456(5); C(3)–C(4) 1.496(6); O(1)–Mo(1)–Mo(1′), 99.60(11)°; O(1)–Mo(1)–O(5′), 160.93(12)°; O(4′)–Mo(1)–O(2), 169.38(14)°; O(4)–Mo(1)–O(3), 171.73(12)°; O(4)–Mo(1)–O(4′), 105.06(10)°; C(2)–O(1)–Mo(1), 126.7(3)°; O(5)–C(1)–C(2), 133.5(4)°; O(5)–C(1)–C(4), 134.6(4)°; C(2)–C(1)–C(4), 91.7(3); O(1)–C(2)–C(1), 134.0(4)°; O(1)–C(2)–C(3), 134.8(4)°; C(1)–C(2)–C(3), 91.1(3)°; O(7)–C(3)–C(2), 135.4(4)°; O(7)–C(3)–C(4), 135.4(4)°; C(2)–C(3)–C(4), 89.1(3)°; O(6)–C(4)–C(1), 136.2(4)°; O(6)–C(4)–C(3), 135.6(4)°; C(1)–C(4)–C(3), 88.1(3)°.

We have also measured the solid-state carbon-13 magic angle spinning nuclear magnetic resonance (MAS NMR) spectrum of a powdered sample of 2 and have observed two well-resolved signals at 192.0 and 194.7 ppm, with a nearly 1:1 integral ratio,20 consistent with a diamagnetic species, with approximate D2h symmetry. We would like to assign the former signal to the proximal carbons and the latter signal to the distal carbons, respectively, based on comparison with the reported solid-state data for squaric acid.21 The chemical shifts for squaric acid were observed at 187.4 and 194.1 ppm, with the former being attributed to the hydroxyl carbons and the latter being attributed to the carbonyl carbons. We suggest that the proximal carbon in 2 experiences a deshielding effect, as compared to the hydroxyl carbons of squaric acid, while the distal carbonyl in 2 appears to be very similar to the carbonyls in squaric acid and is minimally affected by coordination to the Mo2(μ-OH)22+ unit. This is consistent with a slight withdrawal of electron density from the squarate, due to columbic interactions with the metal ion and minimal π-back-bonding from the metal d-orbitals that form the metal–metal bond. This would also be consistent with the observed solution carbon-13 NMR spectra, recorded in H2O/D2O, in which a single time-averaged signal was observed for squaric acid at 195.3 ppm, while the sodium squarate signal was observed at 204.2 ppm. The time-averaged solution chemical shift compares well with the chemical shifts of the solid-state MAS signals that we have observed for sodium squarate (see the Supporting Information). It should also be noted that an increase in ring size in going from squarate, to croconate (189.3 ppm), to rhodizonate (174.2 ppm) resulted in a shielding effect in which the C–O bonds become less polarized.22

A number of dinuclear molybdenum and tungsten d3–d3 compounds having edge-shared bioctahedral structures are known and have been found to have metal–metal bond lengths ranging from 2.35 to 2.8 Å.23 Furthermore, Zubieta and co-workers have described a series of Mo2O4(C4O4)L4 compounds, where L = pyridine and 3,5-lutidine, that adopt edge-shared bioctahedral structures having Mo–Mo single bonds with an average distance of 2.55 Å.24 The nature of the metal–metal bonding in these compounds can be complicated.25 In some cases, the d3–d3 dimers can be described as a relatively weak triple bond, having a σ2π2δ2 configuration, where the sideways overlap of dx2–y2 orbitals forming the δ-bond does not contribute to a significant attractive force between the metal atoms. This type of metal–metal triple bond is generally longer than unbridged triple bonds having σ2π4 configurations, whose bond lengths range from 2.2 to 2.4 Å.2 However, it has been argued that, in some examples of edge-shared bioctahedra, ligand–metal π interactions can destabilize the δ-orbital, resulting in a formal metal–metal single bond, with a σ2π2δ*2 configuration, where the antibonding character of the δ* orbital minimally cancels some of the metal–metal-bonding interactions.

In an effort to get a sense of the electron configuration in our d3–d3 dimolybdenum hydroxy squarate complex, we have performed density functional theory molecular orbital (DFT MO) calculations. The unrestricted calculations produce an energetically minimized structure that is a very good approximation of the observed structure, with a calculated Mo–Mo distance of 2.415 Å.3 We have deposited a figure in the Supporting Information, showing an overlapping plot of the experimentally observed X-ray structure and the calculated structure. The only significant difference between these structures is the position of the hydrogen atoms for the coordinated water molecules, which is presumably due to slight differences in the energy minimum resulting from the hydrogen-bonding interactions in the solid-state structure. Figure 2 shows a picture of the resulting highest occupied molecular orbital (HOMO, au) and lowest unoccupied molecular orbital (LUMO, b2g). It is evident that the HOMO primarily represents the Mo–Mo δ* orbital, with minor antibonding contributions from the squarate ligand orbitals, while the LUMO primarily represents the Mo–Mo δ orbital, also with antibonding contributions from the squarate ligand orbitals. However, the LUMO has significant π-antibonding interactions with the oxygen lone-pair orbitals on the bridging hydroxide ligands, which energetically destabilizes this MO. This suggests that the best description for the metal–metal bond is a formalized single bond with a σ2π2δ*2 configuration. As we have observed, this electron configuration should result in a relatively long, formal Mo–Mo single bond, comparable with the Mo–Mo single bond distances reported by Zubieta et al. for the Mo2O4(C4O4)L4 compounds, which are Mo(V)–Mo(V) dimers.24

Figure 2.

Figure 2

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(a) HOMO (−4.639 eV) and (b) LUMO (−4.094 eV) of 2 appearing perpendicular to the virtual plane of symmetry that includes the squarate ligands.

Weak antiferromagnetic interactions for edge-shared bioctahedral chromium dimers can be rationalized based on the relatively small radial extension of the d-orbitals, making the formation of metal–metal bonds less thermodynamically favorable. Poli and Gordon26 have discussed the competing factors for edge-shared bioctahedral halide phosphine compounds for the group VI metals, highlighting the preference for metal–metal-bonded structures in the molybdenum and tungsten compounds. The stabilization of the nonbonded structures for chromium was rationalized based on preferable coulombic metal–ligand interactions and exchange interactions involving the three unpaired electrons per metal atom located in the t2g-type d-orbitals.27

Conclusions

In summary, the reaction of quadruply bonded M2(O2CCH3)4, where M = Cr or Mo, with an excess of squaric acid under hydrothermal conditions, results in a product formulated as M2(μ-OH)2(μ-C4O4)2(H2O)4·2H2O. Both compounds adopt similar edge-shared bioctahedral structures, but the metal–metal bond distances indicate that the chromium complex has a nonbonded antiferromagnetic interaction, whereas the molybdenum complex is diamagnetic with a metal–metal single bond of σ2π2δ*2 configuration.

Experimental Section

General Methods

Starting materials and solvents were either purchased from VWR or prepared by slight variations of literature methods, as described in the Supporting Information.

Infrared spectra were recorded as potassium bromide pellets, by grinding small portions of sample with a few milligrams of KBr in air, using an agate mortar and pestle, and pressed using a stainless steel Little-Press nut-and-bolt-type pellet die kit. The spectroscopy-grade KBr was purchased from VWR. The Fourier-transform infrared (FT-IR) spectra were recorded on a Thermo Scientific Nicolet iS10 FT-IR spectrometer. A background spectrum of air was subtracted to produce the sample’s percent transmittance spectrum. Residual gaseous carbon dioxide asymmetric vibrations were often observed in the 2400 cm–1 region, due to incomplete background subtraction. The spectrometer’s calibration was regularly checked using a film of polystyrene. Solid-state carbon-13 NMR spectra were recorded on a Bruker Avance-III-HD spectrometer using a double-resonance 4 mm HR-MAS probe, by collecting ∼4k scans, at a spin rate of 13 kHz and a temperature of 298 K, with an interpulse delay of 15 s. The sample was ground to a fine powder using an agate mortar and pestle, packed into a rotor, fitted with a cap, and mounted in the MAS device. Spinning sidebands were observed and confirmed as sidebands, by altering the spin rate. Combustion elemental analyses were performed by Atlantic Microlabs Inc., using standard procedures for air-stable inorganic compounds. Details of the DFT calculations can be found in the Supporting Information. The X-ray crystallographic data has been submitted to the Cambridge Structural Database.

Synthesis of 1

A typical synthesis involved placing chromium(II) acetate hydrate (0.135 g, 0.359 mmol) and slightly greater than 2 equiv of squaric acid (0.085 g, 0.745 mmol) in the Teflon liner of an autoclave, which was then placed in a glovebag along with a squirt bottle filled with degassed DI water. The bag was filled with ultra high purity (UHP) nitrogen, several times, to displace the air. Approximately 25 mL of the freshly degassed water was added, and the lid was placed on a Teflon container, which was then removed from the glovebag and sealed in the stainless steel jacket of an autoclave. The autoclave was placed in a laboratory convection oven at 150 °C for 24 h. After slowly cooling the autoclave to room temperature, the liner was removed and opened in air. The solution was vacuum-filtered using a preweighed medium-porosity sintered glass Buchner funnel and washed with several portions of DI water, acetone, and diethyl ether. After drying in vacuo for several hours, the mass of purple crystals was 0.120 g, 71.3% yield. Combustion elemental analysis was based on an empirical formula of CrC4H7O8, theoretical C = 20.43% and H = 3.00%, found C = 20.31% and H = 3.09%, and IR data (KBr cm–1) 1795 (vw), 1620 (m), and 1502 (vs), as well as broad O–H stretches at 3000–3500 (m).

Synthesis of 2

A procedure similar to the synthesis of 1 was used, in which molybdenum(II) acetate (0.501 g, 1.17 mmol) was combined with squaric acid (0.271 g, 2.38 mmol), producing 0.551 g of yellow/green-brown crystals, 84.3% yield. Combustion elemental analysis was based on an empirical formula of MoC4H7O8, theoretical C = 17.22% and H = 2.53%, found C = 17.20% and H = 2.46%, and IR data (KBr cm–1) 1801 (w), 1618 (m), and 1506 (vs), as well as broad hydrogen-bonded O–H stretches at 3450 (m) and 3150 (m).

Acknowledgments

The authors would like to thank the Pennsylvania State University, Altoona, Office of Research and Sponsored Programs for their support through the Undergraduate Research and Development Grant program.

Glossary

Abbreviations

MOF

metal–organic framework

NMR

nuclear magnetic resonance

MAS

magic angle spinning

HOMO

highest occupied molecular orbital

LUMO

lowest unoccupied molecular orbital

MO

molecular orbital

DFT

density functional theory

DMF

dimethylformamide

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c00083.

  • Synthetic methods for starting materials; crystallographic data; MO calculation details; IR and MAS NMR spectra; and overlapping plots of the X-ray and DFT structures (PDF)

  • Crystallographic data (CIF)

Author Present Address

∥ Department of Chemistry and Environmental Science, Tiernan Hall - B006, New Jersey Institute of Technology, University Heights, Newark, New Jersey 07102, United States (C.N.P.)

Author Contributions

The manuscript was written through contributions of all authors.

The authors declare no competing financial interest.

Supplementary Material

ao0c00083_si_001.pdf (474.9KB, pdf)
ao0c00083_si_002.cif (26.2KB, cif)

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Associated Data

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Supplementary Materials

ao0c00083_si_001.pdf (474.9KB, pdf)
ao0c00083_si_002.cif (26.2KB, cif)

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