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. 2020 Jan 31;59(4):2228–2240. doi: 10.1021/acs.inorgchem.9b02912

Thermal Growth of Au–Fe Heterometallic Carbonyl Clusters Containing N-Heterocyclic Carbene and Phosphine Ligands

Beatrice Berti †, Marco Bortoluzzi ‡, Cristiana Cesari †, Cristina Femoni †, Maria Carmela Iapalucci †, Rita Mazzoni †, Federico Vacca †, Stefano Zacchini †,*
PMCID: PMC7997394  PMID: 32003563

Abstract

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The thermal reactions of [NEt4][Fe(CO)4(AuNHC)] [NHC = IMes ([NEt4][1]) or IPr ([NEt4][2]); IMes = C3N2H2(C6H2Me3)2; IPr = C3N2H2(C6H3iPr2)2], Fe(CO)4(AuNHC)2 [NHC = IMes (3) or IPr (4)], Fe(CO)4(AuIMes)(AuIPr) (5), and Fe(CO)4(AuNHC)(AuPPh3) [NHC = IMes (6) or IPr (7)] were investigated in different solvents [CH2Cl2, CH3CN, dimethylformamide, and dimethyl sulfoxide (dmso)] and at different temperatures (50–160 °C) in an attempt to obtain higher-nuclearity clusters. 1 and 2 completely decomposed in refluxing CH2Cl2, resulting in [Fe2(CO)8(AuNHC)]− [NHC = IMes (10) or IPr (11)]. Traces of [Fe3(CO)10(CCH3)]− (12) were obtained as a side product. Conversely, 6 decomposed in refluxing CH3CN, affording the new cluster [Au3{Fe(CO)4}2(PPh3)2]− (15). The relative stability of the two isomers found in the solid state structure of 15 was computationally investigated. 4 was very stable, and only after prolonged heating above 150 °C in dmso was limited decomposition observed, affording small amounts of [Fe3S(CO)9]2– (9), [HFe(CO)4]− (16), and [Au16S{Fe(CO)4}4(IPr)4]n+ (17). A dicationic nature for 17 was proposed on the basis of density functional theory calculations. All of the other reactions examined led to species that were previously reported. The molecular structures of the new clusters 11, 12, 15, and 17 were determined by single-crystal X-ray diffraction as their [NEt4][11]·1.5toluene, [Au(IMes)2][15]·0.67CH2Cl2, [NEt4][12], and [17][BF4]n·solvent salts, respectively.

Short abstract

The nuclearity of Au−Fe molecular clusters has been increased by thermal methods, resulting in heterometallic species stabilized by CO, NHC, and phosphine ligands.

1. Introduction

The Fe(CO)4 group was a very versatile fragment for stabilizing gold clusters.1 These included low-nuclearity complexes such as Fe(CO)4(AuPPh3)2,2 Fe(CO)4(AuNHC)2 [NHC = IMes, IPr, or IBu; IMes = C3N2H2(C6H2Me3)2; IPr = C3N2H2(C6H3iPr2)2; IBu = C3N2H2(CMe3)2],3 and [Fe(CO)4(AuNHC)]−,4 as well as one- and two-dimensional gold clusters, such as [Au3Fe2(CO)8(IMes)2]−,3 [Au3Fe(CO)4(dppm)2]+ (dppm = Ph2PCH2PPh2),5 [Au3Fe2(CO)8(dppm)]−,6 [Au3{Fe(CO)4}3]3–,7 [Au4{Fe(CO)4}4]4–,8 [Au5Fe4(CO)16]3–,9 [Au5Fe2(CO)8(dppm)2]+,6 and Au8Fe4(CO)16(dppe)4 (dppe = Ph2PCH2CH2PPh2).10 These clusters could be viewed as being composed of [Fe(CO)4]2– moieties and Au(I) ions, containing in some cases additional NHC and/or phosphine ligands. The general strategy for their syntheses was the reaction of Colman’s reagent Na2[Fe(CO)4]·2thf with Au(I) complexes such as Au(Et2S)Cl, [AuBr2]−, Au(PPh3)Cl, Au(NHC)Cl, Au2(dppm)Cl2, and Au2(dppe)Cl2.

In addition, three-dimensional metalloid Fe–Au clusters were obtained from the redox condensation of [Fe3(CO)11]2– and [AuCl4]−. This category included Au–Fe–CO molecular nanoclusters such as [Au21Fe10(CO)40]5–, [Au22Fe12(CO)48]6–, [Au28Fe14(CO)52]8–, and [Au34Fe14(CO)50]8–, stabilized by Fe(CO)4 and Fe(CO)3 groups present on their surface.9 The Au atoms within their Aun core displayed a formal oxidation state between +1 and 0. Linear Fe–Au–Fe staple motifs, reminiscent of the very well-known S–Au–S staple motifs found in Au–thiolate nanoclusters, were present on the surface of these organometallic Au–Fe carbonyl clusters.9,11,12

The desire for atomically precise (molecular) gold nanoclusters has incredibly grown in recent years, because of their fundamental aspects and properties as well as possible applications.13−19 Thiolate and phosphine ligands were widely employed for the preparation of molecular gold nanoclusters, but other ligands,20−22 including organometallic fragments, might be employed. In this sense, the combination of Fe(CO)4, NHC, and phosphine ligands might be an interesting approach for the growth of new gold clusters. In addition, Au–Fe carbonyl clusters were also useful platforms for the study of intramolecular aurophilic interactions.1,7,11,23−25 In light of this broad interest in gold clusters and aurophilic interactions, the preparation of gold-containing molecular clusters of increasing sizes is still a fascinating challenge.

Thermal reactions of low-nuclearity precursors might be an alternative to redox condensation for the preparation of higher-nuclearity Fe–Au carbonyl clusters. Indeed, it was recently reported that the thermal treatment of Fe(CO)4(AuIMes)2 resulted in [Au3{Fe(CO)4}3]3– or [Au3Fe2(CO)8(IMes)2]− depending of the experimental conditions.3,7 In both cases, the reactions involved ionization of the neutral precursors and rearrangement of the ligands, with retention of the original −2 and +1 oxidation states for Fe and Au, respectively. It is also worth noting that thermal reaction of Fe(CO)4(AuIMes)2 was the only synthetic approach viable for the [Au3{Fe(CO)4}3]3– trinuclear compound, whereas the direct reaction of Na2[Fe(CO)4]·2thf with [AuBr2]− afforded selectively the [Au4{Fe(CO)4}4]4– tetranuclear compound.8 These preliminary results prompted us to systematically study the thermal reactions of low-nuclearity monoanionic and neutral precursors such as [NEt4][Fe(CO)4(AuNHC)] [NHC = IMes ([NEt4][1]) or IPr ([NEt4][2]); IMes = C3N2H2(C6H2Me3)2; IPr = C3N2H2(C6H3iPr2)2 (Scheme 1)], Fe(CO)4(AuNHC)2 [NHC = IMes (3) or IPr (4)], Fe(CO)4(AuIMes)(AuIPr) (5), and Fe(CO)4(AuNHC)(AuPPh3) [NHC = IMes (6) or IPr (7)]. The outcomes of the different reactions are reported herein, supported by a computational investigation of new products characterized by unusual isomerism and bond structure.

Scheme 1. IMes and IPr Ligands.

Scheme 1

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2. Results and Discussion

The thermal reactions of complexes 1–7 were investigated with the goal of obtaining higher-nuclearity species. As a general strategy, 1–7 were heated in different solvents [CH2Cl2, CH3CN, dimethylformamide (dmf), and dimethyl sulfoxide (dmso)] at temperatures in the range of 50–160 °C monitoring the evolution of the reactions over time by infrared (IR) spectroscopy in the νCO region. Anionic compounds were examined as [NEt4]+ salts. The crude reaction mixtures were recovered after removal of the organic solvent under reduced pressure or, in the case of dmf or dmso as the solvent, by precipitation with H2O in the presence of a suitable tetraalkyl-ammonium salt. The solid residue was extracted with solvents of increasing polarity in the attempt to separate the products from the crude reaction mixtures. Further details can be found in the Experimental Section. All of the results obtained are summarized in Scheme 2. The new results herein obtained can be summarized as follows.

Scheme 2. Thermal Reactions of 1–7.

Scheme 2

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(1) Heating monoanions 1 and 2 as [NEt4]+ salts in CH2Cl2 at refluxing temperature resulted in the formation of [Fe2(CO)8(AuNHC)]− [NHC = IMes (10) or IPr (11)]. Traces of [Fe3(CO)10(CCH3)]− (12) were obtained as a side product. These represented an interesting addition to the limited number of compounds with the Fe2(CO)6(μ-CO)2 unit (section 2.1).

(2) The thermal decomposition of 6 in CH3CN at 80 °C afforded the larger [Au3{Fe(CO)4}2(PPh3)2]− (15) cluster that was present as two isomers in the solid state structure (section 2.2).

(3) 4 was only partially decomposed after prolonged heating in dmso at 130–160 °C, resulting in a mixture of [Fe3S(CO)9]2– (9), [HFe(CO)4]− (16), and [Au16S{Fe(CO)4}4(IPr)4]n+ (17). Compound 17 was rather interesting because it was a high-nuclearity Au cluster containing an interstitial μ12-S atom stabilized on the surface by Fe(CO)4 fragments and IPr ligands (section 2.3).

The other reactions studied led to compounds that were previously published, and therefore, they will not be discussed further.3,4,7 These included [Au3{Fe(CO)4}3]3– (8),7 [Fe3S(CO)9]2– (9),26 and [Au3Fe2(CO)8(IMes)2]− (13).3 In particular, 8 was obtained as the main product of several reactions. We reported the synthesis of 8 by the thermal decomposition of 3 in dmso at 130 °C in a previous communication.7 As summarized in Scheme 2, 8 could also be obtained by thermal treatment of 1, 2, and 5. 13 was obtained by the thermal reaction of 3 at lower temperatures (≤100 °C) in dmf or dmso,3 whereas 8 was formed at higher temperatures. The computed Gibbs energy variations (C-PCM/PBEh-3c calculations) for reactions 1 and 2 were −13.1 and −23.1 kcal mol–1, respectively, and suggested that the formation of [Au(IMes)2]+ was the driving force. As a general comment on Scheme 2, IPr-containing species were far more thermally stable than IMes-containing species.

2. 1
2. 2

2.1. Syntheses and Characterization of [Fe2(CO)8(AuNHC)]− [NHC = IMes (10) or IPr (11)] and [Fe3(CO)10(CCH3)]− (12)

Anionic species 1 and 2 were not stable in chlorinated solvents such as CH2Cl2 at room temperature. Complete decomposition occurred after heating at 50 °C, resulting in the formation of the new species [Fe2(CO)8(AuNHC)]− [NHC = IMes (10) or IPr (11)]. Formation of 10 and 11 required the formal oxidation of iron from −2, as in 1 and 2, to −1, as in the final products. Because this reaction did not occur in nonchlorinated solvents even after heating for several hours, we could rule out the possibility that adventitious oxygen was the oxidizing species. Thus, the oxidant should be CH2Cl2 itself.27 Unfortunately, all attempts to identify the products of the reduction of CH2Cl2 by GC-MS analyses failed. Therefore, it was not possible to deduce the mechanism of the reaction.

Compounds 10 and 11 were characterized by means of IR and multinuclear nuclear magnetic resonance (NMR) spectroscopy, and the molecular structure of 11 was crystallographycally determined as its [NEt4][11]·1.5toluene salt (Figure 1). The molecular structure of 11 may be viewed as the result of the addition of a [AuIPr]+ fragment to [Fe2(CO)8]2–. It displayed six terminal and two edge-bridging carbonyl ligands, as previously found in the PPh3 derivative [Fe2(CO)8(AuPPh3)]−.28 Conversely, the related copper species [Fe2(CO)8(CuPCy3)]−29 displayed only terminal carbonyls. 11 displayed also some short sub-van der Waals Au···C(O) contacts. The structure of 11 was an interesting addition to the limited number of compounds with the Fe2(CO)6(μ-CO)2 unit.30,31 The Fe–Fe bond distance of such compounds spanned a very large range (2.39–2.62 Å). In the case of 11, the Fe–Fe distance [2.573(4) Å] was between those of Fe2(CO)9 (2.52 Å)32 and [Fe2(CO)8(AuPPh3)]− (2.605 Å).28

Figure 1.

Figure 1

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Molecular structure of 11. Au–C(O) contacts [2.830(19)–2.977(19) Å] are represented as dashed lines. Hydrogen atoms have been omitted for the sake of clarity (green, Fe; yellow, Au; blue, N; red, O; gray, C). Selected bond lengths (angstroms): Fe–Fe, 2.573(4); Fe–Au, 2.665(3) and 2.677(3); Au–Ccarbene, 2.013(18); Fe–C(O)bridge, 1.90(2)–1.969(18); Fe–C(O)terminal, 1.73(2)–1.83(3).

The 1H and 13C{1H} NMR spectra of 11 (Figures S1 and S2) displayed all of the expected resonances due to the IPr group. Conversely, in the carbonyl region of the 13C{1H} NMR spectra recorded at 298 and 273 K, only a single sharp resonance at 230.5 ppm was detected. Coalescence was, then, observed at 213 K (Figure S3), suggesting the presence of a fluxional behavior that made the eight CO ligands equivalent at higher temperatures. The structures of 10 and 11 were also optimized by means of density functional theory (DFT) calculations. The root-mean-square deviation (RMSD) between the experimental and computed structures of the anion of 11 was quite low (0.311 Å), and the value decreased to 0.183 Å upon removal of the substituents on the nitrogen atoms from the comparison. The computed structure of 10 strongly resembled that of 11 (Figure S17), with negligible variations in bond lengths and angles. This indicated the scarce influence of the different substituents on the NHC ligands.

Besides 11 which was the major product, a few crystals of [NEt4][Fe3(CO)10(CCH3)] were isolated as side products of the thermal decomposition of 2 in CH2Cl2, and their nature was completely revealed by X-ray crystallography. These crystals contained the μ3-ethylidyne cluster [Fe3(CO)10(CCH3)]− (12) (Figure 2), whose synthesis was previously reported, whereas its structure, to the best of our knowledge, has not been described previously.33 The molecular structure of 12 was composed of a triangular Fe3 core, bonded to nine terminal CO ligands (three per Fe atom), one μ3-ethylidyne, and one μ3-CO. The μ3-ethylidyne ligand was previously found on related triiron carbonyl clusters, such as Fe3(CO)8(Cp)(CCH3),34 H3Fe3(CO)9(CCH3),35 Fe3(CO)9(COCH3)(CCH3),36 and Fe3(CO)10(CuPPh3)(CCH3),37 as well as the tetrairon cluster [Fe4(CO)12(CCH3)]−.38 It is noteworthy that the closely related Fe3(CO)10(CuPPh3)(CCH3),37 which formally arose from the addition of a [CuPPh3]+ fragment to 12, displayed nine terminal ligands and one edge-bridging μ-CO ligand, instead of a face-bridging μ3-CO. A similar stereochemistry of the carbonyl ligands was found in the μ3-methylidyne cluster [Fe3(CO)10(CH)]−.39 The bonding parameters of 12 (see the legend of Figure 2) were similar to those previously reported for related clusters.34−38 The μ3-CO [Fe–C(O)bridging, 2.015(2)–2.077(2) Å] and μ3-CCH3 [Fe–Cethylidyne, 1.940(2)–1.960(2) Å] ligands were symmetrically bonded to the Fe3 triangle, and the C–Cethylidyne distance [1.497(3) Å] was as expected for a single bond.

Figure 2.

Figure 2

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Molecular structure of 12 (green, Fe; red, O; gray, C; white, H). Selected bond lengths (angstroms): Fe–Fe, 2.5285(5)–2.5458(5); Fe–Cethylidyne, 1.940(2)–1.960(2); Fe–C(O)bridging, 2.015(2)–2.077(2); Fe–C(O)terminal, 1.766(3)–1.811(3); C–Cethylidyne, 1.497(3).

12 was previously synthesized from the reaction of [HFe3(CO)11]− with acetylene.33 The mechanism for the formation of 12 as a side product along with the thermal decomposition of 2, which afforded 11 as the major product, was not clear. It probably involved the oxidation of 2 by means of CH2Cl2 as described above followed by removal of the AuIPr fragment and rearrangement of the cluster core. Unfortunately, due to the very low yields, it was not possible to further elucidate the mechanism.

2.2. Syntheses and Characterization of [Au3{Fe(CO)4}2(PPh3)2]− (15)

Complex 6, which contained mixed IMes/PPh3 ligands, was not very stable in polar solvents at room temperature.4 Indeed, its 31P{1H} NMR spectrum in a CD3COCD3 solution displayed a major resonance (δP) 40.8 ppm attributable to 6, accompanied by minor resonances at 40.1 and 38.5 ppm (Figures S8 and S9). These resonances corresponded to Fe(CO)4(AuPPh3)2 (14) and a new species 15, respectively. The former was a byproduct of the synthesis of 6 as previously reported,4 whereas the formation of 15 arose from partial decomposition (ionization) of 6. Indeed, after this mixture had been heated in CH3CN at 80 °C for 3 h, the intensity of the resonance at 40.8 ppm (δP) considerably decreased, whereas the resonance at 38.5 ppm (δP) became the major one (Figure S9). This indicated an almost complete conversion of 6 into 15. This new compound was completely characterized by IR, 1H, 13C{1H}, and 31P{1H} NMR spectroscopy (Figures S10–S12), and its structure was determined by single-crystal X-ray diffraction as its [Au(IMes)2][15]·0.67CH2Cl2 salt (Figures 3 and 4 and Table S1). The latter was composed of [Au(IMes)2]+ cations and [Au3{Fe(CO)4}2(PPh3)2]− anions (15), according to eq 3.

2.2. 3

Within the crystals of [Au(IMes)2][15]·0.67CH2Cl2, two isomers of anion 15 were present in a 2:1 ratio (termed isomers 15a and 15b, respectively). Both isomers were composed of a Au3 core bonded to two μ-Fe(CO)4 units and two terminal PPh3 ligands. The Au3 core of 15a displayed a V-shaped geometry [∠Au–Au–Au, 132.00(4)°], whereas it adopted a linear arrangement in 15b with the central Au atom located on an inversion center [∠Au–Au–Au, 180.00(10)°]. The structure of isomer 15b was similar to that previously reported for 13.3 Both isomers displayed two aurophilic Au···Au contacts [2.9353(13) and 2.8855(14) Å for 15a and 2.9177(14) and 2.9177(14) Å for 15b] as well as sub-van der Waals Au···C(O) contacts [2.34(6)–2.87(8) Å for 15a and 2.56(3)–2.89(3) Å for 15b].

Figure 3.

Figure 3

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Molecular structures of the two isomers of 15. The two isomers were present within the crystal in a 2:1 15a:15b ratio. Two views of isomer 15a are shown in panels a and b, and two views of isomer 15b are shown in panels c and d. Au–C(O) contacts [2.34(6)–2.87(8)Å for 15a and 2.56(3)–2.89(3) Å for 15b] are represented as dashed lines. Hydrogen atoms have been omitted for the sake of clarity (green, Fe; yellow, Au; purple, P; blue, N; red, O; gray, C).

Figure 4.

Figure 4

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Au3Fe2(CO)8P2 cores of (a) 15a and (b) 15b (green, Fe; yellow, Au; purple, P; red, O; gray, C).

The presence in the solid state of two isomers of 15 prompted a variable-temperature 31P{1H} NMR investigation. Unfortunately, a single resonance was observed at all of the temperatures examined (193–298 K), suggesting a fast exchange between 15a and 15b in solution.

The structures of 15a and 15b were also optimized by DFT calculations. The RMSD values of the computed [Fe2Au3] cores with respect to the experimental data were 0.129 and 0.064 Å for 15a and 15b, respectively. The deviations could mainly be attributed to a slight overestimation of the Au–Au distances, caused by the known weakness of DFT methods in predicting dispersion interactions such as the aurophilic one.40 Despite this limit, the computed energy difference between the two isomers was 0.9 kcal mol–1, 15a being slightly more stable than 15b, in agreement with the observed fast exchange. In both clusters, no (3, −1) bond critical point (bcp) for Au–Au interactions was found, the gradient norm of electron density being greater than zero along the Au–Au bonds (minimum gradient values were 0.005 and 0.004 au for 15a and 15b, respectively). This result, which suggested a delocalized dispersion interaction, was in line with the data previously reported for 8.7 The (3, −1) bcp was instead found for the Fe–Au bonds, and relevant data are listed in Table S2 and compared with those obtained for compounds 10 and 11. All Fe–Au bcp’s were characterized by negative energy density (E) values, while the Laplacian of electron density (∇2ρ) was positive, in agreement with Bianchi’s definition of M–M bonds.41 ρ and V values of 15a and 15b were closely comparable, and the bonds with terminal Au atoms were stronger than those with the central Au. The data listed in Table S2 indicated that the different mode of binding of Au in 10 and 11 caused a slight decrease in the Fe–Au bond strength. With respect to the charge distribution, the three Au atoms in 15a and 15b had very similar Hirshfeld partial charges, in the ranges of 0.060–0.064 au for 15a and 0.057–0.065 au for 15b, as expected considering the formal homogeneity of the oxidation states.

2.3. Syntheses and Characterization of [Au16S{Fe(CO)4}4(IPr)4]n+ (17)

4 was recovered almost intact also after heating in dmso at 140 °C for 5 h. It started to show a partial decomposition only after prolonged heating above 150 °C in dmso. Among the decomposition products, it was possible to isolate a few crystals of [NEt4]2[Fe3S(CO)9] ([NEt4][9]), [Au(IPr)2][HFe(CO)4] ([Au(IPr)2][16]), and [Au16S{Fe(CO)4}4(IPr)4][BF4]n·solvent ([17][BF4]n·solvent). The presence of [BF4]− anions in the latter salt was due to the use of [NEt4][BF4] during workup of the reaction mixture.

Anions 9 and 16 were previously reported, and26,42 therefore, their structures will not be discussed further. Their crystal data were deposited within the Cambridge Crystallographic Data Centre, and a representation of the molecular structure of 9 is included as Figure S16.

Formation of 9 was rather interesting because it suggested that S atoms were somehow generated from dmso after the prolonged thermal treatment of 4. This was in keeping with the formation of the new species 17, which contained an interstitial sulfur atom. Compound 17 was formed in only trace amounts, and because of this, only very few small crystals were grown. This allowed the complete determination of the molecular structure of the cluster molecule (Figure 5 and Table S3), which occupied 78% of the unit cell volume. The remaining 22% of the volume of the unit cell was likely to be occupied by cations/anions and/or solvent molecules (Figure S17), whose nature was not determined.

Figure 5.

Figure 5

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Molecular structure of 17: (a and b) two different views as well as (c) the space filling model. Au–C(O) contacts [2.636(4)–2.723(4) Å] are represented as dashed lines. Hydrogen atoms have been included only in the space filling model (green, Fe; yellow, Au; orange, S; blue, N; red, O; gray, C; white, H).

Despite the fact that 17 was obtained in low yields, it was also possible to characterize it by multinuclear NMR techniques. 1H and 13C{1H} NMR analyses were in agreement with the presence of CO and IPr ligands (Figures S13 and S14). More interestingly, the 19F NMR spectrum of 17 (Figure S15) displayed the typical resonance of the [BF4]− anion. Therefore, 17 was better formulated as a cationic species, and because of this, its crystals were denoted [17][BF4]n·solvent.

17 consisted of a Au12-cubeoctahedron centered by a μ12-S atom, whose surface was decorated with four μ3-Fe(CO)4 and four μ3-AuIPr fragments with a pseudo-Td symmetry (Figure 6). A related structure, where a μ12-S atom was encapsulated within a Cu12-cubeoctahedral cage, was recently reported for the neutral [Cu12(μ12-S)(S2CNnBu2)6(C≡CPh)4] cluster.43 As in the case of [Cu12(μ12-S)(S2CNnBu2)6(C≡CPh)4], the Au–S distances [2.7641(13)–2.7995(16) Å, average of 2.777(3) Å] of 17 were rather elongated in light of the high coordination number of the interstitial μ12-S atom. For comparison, the sums of the covalent and van der Waals radii of Au and S were 2.38 and 3.46 Å, respectively.44 Prior of the isolation of 17, the highest coordination number observed for S with Au was four, and the corresponding Au–S distances were considerably shorter (2.30–2.42 Å).45,46

Figure 6.

Figure 6

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Three different views of the Au16S core of 17 (green, Fe; yellow, Au atoms of the Au12 cubeoctahedron; blue, Au atoms of the μ3-AuIPr fragments; orange, S).

The tangential Au–Au contacts [2.702(2)–2.874(2) Å, average of 2.753(6) Å] were more dispersed compared to the more localized Au–Au contacts involving the μ3-AuIPr fragments [2.724(2)–2.733(2) Å, average of 2.728(3) Å]. Similarly, the Au–Fe distances [2.625(5)–2.650(5) Å, average of 2.636(9) Å] displayed by 17 that presented μ3-Fe(CO)4 groups were significantly longer than those found in clusters containing μ2-Fe(CO)4 fragments such as 15 [2.529(3)–2.601(11) Å, average of 2.564(8) Å].

Molecular gold nanoclusters stabilized by ligands have been extensively studied in recent years.11−22 Au13 and Au12M cages often adopted icosahedral structures, and a few clusters displaying a cubeoctahedral structure were reported.15 This point was also computationally investigated, showing that, depending on the central atom, Au12M clusters could adopt Ih (icosahedron) or Oh (cubeoctahedron) symmetry.47

DFT calculations were carried out on models of compound 17. The substituents on the nitrogen atoms of the NHC ligands were replaced by methyl groups to reduce the computational effort. The coordinates of the other atoms were obtained from X-ray data. The singlet multiplicity was always maintained, and the charge was varied from 2+ to 6+. The most stable electronic structure resulted in the most reduced one, that is 2+. The 4+ and 6+ cations were less stable by 0.9 and 2.2 au, respectively. For this reason, the formula [Au16S{Fe(CO)4}4(IPr)4]2+ was proposed. The computed energy gap between frontier orbitals in the model compound was quite high, 3.9 eV.

The approximate Td symmetry was confirmed by all of the population analyses, and the four C3 axes are reported in Figure 7 for the sake of clarity. The compound can be considered to be composed of four [FeAu3] tetrahedra, each one forming three bonds with the central sulfur. One of the [FeAu3] tetrahedra and its bonds with S are colored red in Figure 7. The [FeAu3] tetrahedra were interconnected by Au–Au bonds, and each [AuNHC] fragment (NHC = 1,3-dimethylimidazol-2-ylidene) was bonded to three Au atoms belonging to different [FeAu3] tetrahedra, with the formation of [Au4] tetrahedra, one of them highlighted in Figure 7. The bonds involving the Au centers can therefore be grouped into six types, as depicted in Figure 7: (a) Au–S, (b) Au–Fe, (c) Au–Au in [FeAu3], (d) Au–Au in [Au4], between iron-bonded centers, (e) Au–Au in [Au4] involving the [AuNHC] fragment, and (f) Au-NHC. Average values concerning the (3, −1) bcp are listed in Table S4. It is worth noting that the AIM analysis was unable to find the (3, −1) bcp associated with Au–CO interactions.

Figure 7.

Figure 7

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Structure of 17 with one [FeAu3] and one [Au4] tetrahedron highlighted. The CO ligands have been removed for the sake of clarity. Only the donor atoms of the NHC ligands are depicted. The four C3 axes are shown. Different types of bonds involving the Au centers are labeled. Color map: Au, yellow; S, orange; Fe, green; C, gray.

As for the previously discussed compounds, all of the bcp’s considered in Table S4 were characterized by negative E and positive ∇2ρ values, in agreement with the definition of M–M and dative bonds.41 The Au–Au bonds in the [Au4] tetrahedra had similar ρ and V values at bcp, thus indicating comparable strength. The Au–Au interactions in the [FeAu3] fragments were comparatively slightly weaker. Considering the Au–S, Au–Fe, and Au–Au bonds, the V values fell in a quite limited range, between −0.034 and −0.062 au, while the average V value related to the Au-NHC bcp was meaningfully more negative, −0.193 au. The picture coming from the AIM analysis was that the Au, S, and Fe atoms in 17 formed a network of bonds having roughly comparable strength.

The partial charges on the Au atoms obtained from the Hirshfeld population analysis were between 0.059 and 0.126 au. The less positive values were related to the NHC-bonded Au atoms, probably because of the donation from the ligands. The maximum charge variation among the other Au centers was 0.03 au, supporting a homogeneous distribution of electron density. Quite interestingly, also the Hirshfeld charge on sulfur was slightly positive (0.045 au). Therefore, AIM and Hirshfeld data suggested that the behavior of the central sulfur was roughly comparable to that of the surrounding Au atoms. Finally, as expected, the Hirshfeld charge on Fe atoms was negative, −0.176 au.

The electron count of 17 was based on the following assumptions. The μ3-AuIPr fragments were considered to contribute one electron each, being isolobal to μ3-H. The μ3-Fe(CO)4 groups were usually described in the literature as four-electron donors.48 The interstitial μ6-S atom was considered to contribute with all of its six valence electrons. Therefore, if 17 was a dication, as inferred from DFT calculations, it should possess 156 [11 × 12 (Au) + 6 × 1 (μ6-S) + 4 × 1 (μ3-AuIPr) + 4 × 4 (μ3-Fe(CO)4) – 2 (charge +2)] cluster valence electrons (CVEs). The expected CVEs depended of the model adopted. According to the EAN (effective atomic number) rule, a cubeoctahedron should have 168 CVE. PSEPT (polyhedral skeletal electron pair theory) predicted 170 CVE by interpreting a cubeoctahedron as a four-connected polyhedron. Conversely, assuming that radial bonding predominates, on the basis of Mingos rules a cubeoctahedron should have 162 CVE.48,49 In this respect, 17 appeared to be electron poor, as often happened for gold clusters.49

3. Conclusions

Low-nuclearity Fe–Au compounds 1–7 thermally decomposed to high-nuclearity species. The obtained Fe–Au products could be grouped within the following categories. (1) Products 8, 13, and 15 were the result of ionization and rearrangement of the starting species. Thus, they retained the original oxidation states of the metals, that is, Au(+1) and Fe(−2). (2) 10 and 11 resulted from oxidation of iron from −2 to −1, whereas gold retained the original +1 oxidation state. (3) The unique species 17 (even if obtained in very low yields) formally contained Fe(−2), whereas the oxidation state of Au was between 0 and +1. This assignment was based on the assumption that, as usually found in Au–Fe carbonyl clusters,1,3−8 the Fe(CO)4 fragments retained their original dianionic nature.

All of the heterometallic clusters reported contained strong Fe–CO, Fe–Au, Au–P, and Au–NHC bonds as well as weak Au···Au interactions. AIM analyses and DFT studies pointed out that the Au···Au interactions in such heterometallic clusters were mainly dispersion-driven. In addition, the different behavior of IMes and IPr derivatives was essentially due to steric effects, because no appreciable electronic difference is evidenced by population analyses based on DFT calculations, as previously reported.4

IPr-containing species were in general more stable than IMes-containing species. In all cases, even when 1–7 were heated to 160 °C, the formation of carbido clusters was not observed. This was probably due to the presence of the AuNHC fragments, because in their absence, anions of iron carbonyls afforded Fe5 and Fe6 carbido clusters.50,51

4. Experimental Section

4.1. General Experimental Procedures

All reactions and sample manipulations were carried out using standard Schlenk techniques under nitrogen and in dried solvents. All of the reagents were commercial products (Aldrich) of the highest available purity and used as received, except 1–7, which were prepared according to the literature.3,4 Analyses of C, H, and N were obtained with a Thermo Quest Flash EA 1112NC instrument. IR spectra were recorded on a PerkinElmer Spectrum One interferometer in CaF2 cells. Structure drawings were performed with SCHAKAL99.52

4.2. Thermal Decomposition of [NEt4][Fe(CO)4(AuNHC)] [NHC = IMes (1) and IPr (2)] in Nonchlorinated Solvents

A solution of [NEt4][1] (0.530 g, 0.663 mmol) in dmso (10 mL) was heated at 130 °C for 3 h, and the reaction monitored by IR spectroscopy. Then, a saturated solution of [NEt4]Br in H2O (40 mL) was added to complete precipitation. The resulting solid was recovered by filtration, washed with H2O (3 × 15 mL) and toluene (3 × 15 mL), and extracted with acetone (15 mL). A microcrystalline powder of [NEt4]3[8] was obtained after removal of the solvent under reduced pressure (0.134 g yield, 41% based on Fe, 41% based on Au). The compound was identified by comparison of its IR data with those reported in the literature.7

Decomposition of [NEt4][2] to produce 8 occurred at 150 °C in dmso. With a further increase in the temperature to 160–170 °C, a complex mixture of decomposition products was formed, among which 9 was the major species detected by IR spectroscopy.

4.3. Synthesis of [Fe2(CO)8(AuNHC)]− [NHC = IMes (10) or IPr (11)]

A solution of [NEt4][2] (0.530 g, 0.600 mmol) in CH2Cl2 (20 mL) was heated at 40 °C for 4 h, and the reaction monitored by IR spectroscopy. Then, the solvent was removed under reduced pressure, and the residue washed with H2O (3 × 15 mL) and extracted with toluene (10 mL). Crystals of [NEt4][11]·1.5toluene suitable for X-ray crystallography were obtained by slow diffusion of n-pentane (25 mL) on the toluene solution (0.207 g yield, 58% based on Fe, 29% based on Au).

A few crystals of [NEt4][12] were isolated as side products of the thermal decomposition of 2 in CH2Cl2, and their nature was completely revealed by X-ray crystallography.

[NEt4][11]·1.5toluene. C53.5H68AuFe2N3O8 (1189.77). Calcd (%): C, 59.98; H, 5.76; N, 3.53. Found: C, 60.12; H, 5.38; N, 3.21. IR (nujol, 293 K): νCO 2004(w), 1956(s), 1923(ms), 1895(vs), 1880(sh) cm–1. IR (dmso, 293 K): νCO 2004(w), 1958(s), 1913(sh), 1900(vs), 1728(ms) cm–1. IR (CH3CN, 293 K): νCO 2006(w), 1960(s), 1903(vs), 1727(ms) cm–1. IR (acetone, 293 K): νCO 2004(w), 1958(s), 1912(sh), 1902(vs) cm–1. IR (toluene, 293 K): νCO 2005(w), 1963(s), 1900(vs), 1721(m) cm–1. IR (CH2Cl2, 293 K): νCO 2007(w), 1961(s), 1905(vs), 1715(ms) cm–1. IR (thf, 293 K): νCO 2002(w), 1959(s), 1904(vs), 1720(m) cm–1. 1H NMR (CD3COCD3, 298 K): δ 7.50–7.24 (m, 8H, CHAr + CHimid), 3.44 (q, 2JHH = 6.2 Hz, 8H, NCH2CH3), 2.95 [sept, 2JHH = 6.8 Hz, 4H, CH(CH3)2], 1.36 [d, 2JHH = 6.8 Hz, 12H, CH(CH3)2], 1.35 (t, 2JHH = 6.2 Hz, 12H, NCH2CH3), 1.15 [d, 2JHH = 6.8 Hz, 12H, CH(CH3)2]. 13C{1H} NMR (CD3COCD3, 298 K): δ 231.5 (CO), 200.4 (C–Au), 145.4, 135.9, 129.4, 123.7, 123.3 (CAr and CHimid), 51.9 (NCH2CH3), 28.2 [CH(CH3)2], 23.9, 23.3 [CH(CH3)2], 6.7 (NCH2CH3).

The thermal decomposition of 1 under the same experimental conditions described above afforded [Fe2(CO)8(AuIMes)]− (10). IR (CH2Cl2, 293 K): νCO 2000(w), 1959(s), 1899(vs), 1712(ms) cm–1.

4.4. Synthesis of [NBu4][Au3Fe2(CO)8(IMes)2]·CH3COCH3 ([NBu4][13]·CH3COCH3)

A large excess of [NBu4][BF4] was added as a solid to a solution of 3 (0.190 g, 0.531 mmol) in dmf (20 mL), and the mixture stirred at 100 °C for 1 h. Then, the orange solution was cooled to room temperature, and H2O (60 mL) was added until complete precipitation occurred. The solid was recovered by filtration, washed with H2O (40 mL), and extracted in acetone (10 mL). Needle-like pale yellow crystals of [NBu4][13]·CH3COCH3 suitable for X-ray analyses were obtained by slow diffusion of n-hexane (30 mL) on the acetone solution (122 g yield, 25% based on Fe).3

C69H90Au3Fe2N5O9 (1836.06). Calcd (%): C, 45.11; H, 4.94; N, 3.81; Fe, 6.09; Au, 32.19. Found: C, 45.41; H, 5.12; N, 3.62; Fe, 6.31; Au, 31.85. IR (nujol, 293 K): νCO 1948(vs), 1877(sh), 1867(s), 1836(sh), 1712(m) cm–1. IR (acetone, 293 K): νCO 1968(sh), 1947(m), 1924(m), 1872(s) cm–1. 1H NMR (CD2Cl2, 298 K): δ 7.12 (s, 8H, CHimid), 6.94 (s, 16H, CHAr), 3.20 (br, 8H, NCH2CH2CH2CH3), 2.47 (s, 24H, CH3), 1.74 (s, 48H, CH3), 1.65 (br, 8H, NCH2CH2CH2CH3), 1.47 (br, 8H, NCH2CH2CH2CH3), 1.01 (br, 12H, NCH2CH2CH2CH3). 13C{1H} NMR (CD2Cl2, 298 K): δ 220.9 (CO), 185.3 (C–Au), 139.4, 134.6, 134.1, 129.0 (CAr), 122.8 (CHimid), 58.6 (NCH2CH2CH2CH3), 23.7 (NCH2CH2CH2CH3), 19.6 (NCH2CH2CH2CH3), 20.9, 16.9 (CH3), 13.3 (NCH2CH2CH2CH3).

4.5. Synthesis of [NMe4]2[Au(IMes)2][Au3{Fe(CO)4}3] ([NMe4]2[Au(IMes)2][8])

A solution of 3 (0.450 g, 0.384 mmol) in dmso (15 mL) was heated at 130 °C for 0.5 h, and the reaction monitored by IR spectroscopy. Then, a saturated solution of [NMe4]Cl in H2O (40 mL) was added to complete precipitation. The resulting solid was recovered by filtration, washed with H2O (3 × 15 mL) and toluene (3 × 15 mL), and extracted with acetone (15 mL). Crystals of [NMe4]2[Au(IMes)2][8] suitable for X-ray crystallography were obtained by slow diffusion of n-hexane (35 mL) on the acetone solution (0.14 g yield, 52% based on Fe, 36% based on Au).7

[NEt4]2[Au(IMes)2][8]·CH3COCH3 was obtained following a similar procedure and employing [NEt4]Br instead of [NMe4]Cl.

[NMe4]2[Au(IMes)2][8]. C62H72Au4Fe3N6O12 (2048.67). Calcd (%): C, 36.32; H, 3.54; N, 4.10. Found: C, 36.14; H, 3.71; N, 3.89. IR (nujol, 293 K): νCO 1970(m), 1932(s), 1843(s) cm–1. IR (dmso, 293 K): νCO 1974(w), 1930(s), 1879(s) cm–1. IR (CH2Cl2, 293 K): νCO 1975(w), 1929(s), 1877(s) cm–1. IR (CH3CN, 293 K): νCO 1929(s), 1867(s) cm–1. IR (acetone, 293 K): νCO 1969(w), 1928(s), 1864(s) cm–1. 1H NMR (CD3CN, 298 K): δ 7.25 (s, 4H, CHimid), 6.98 (s, 8H, CHAr), 3.17 (s, 24H, NMe4), 2.45 (s, 12H, CH3), 1.72 (s, 24H, CH3). 13C{1H} NMR (CD2Cl2, 298 K): δ 224.4 (CO), 185.3 (C–Au), 139.7, 135.0, 134.6, 129.2 (CAr), 123.4 (CHimid), 55.6 (1JCN = 3.9 Hz, NMe4), 20.6, 16.7 (CH3).

4.6. Synthesis of [Au(IMes)2][Au3{Fe(CO)4}2(PPh3)2]·0.67CH2Cl2 ([Au(IMes)2][15]·0.67CH2Cl2)

A solution of 6 (0.220 g, 0.188 mmol) in CH3CN (15 mL) was heated at 80 °C for 3 h, and the reaction monitored by IR spectroscopy. Then, a saturated solution of [NEt4]Br in H2O (40 mL) was added to complete precipitation. The resulting solid was recovered by filtration, washed with H2O (3 × 15 mL) and toluene (3 × 15 mL), and extraced with CH2Cl2 (15 mL). Crystals of [Au(IMes)2][15]·0.67CH2Cl2 suitable fon X-ray crystallography were obtained by slow diffusion of n-pentane (35 mL) on the CH2Cl2 solution (0.110 g yield, 51% based on Fe, 51% based on Au).

C86.67H79.33Au4Cl1.33Fe2N4O8P2 (2313.64). Calcd (%): C, 44.98; H, 3.46; N, 2.42. Found: C, 45.12; H, 3.71; N, 2.14. IR (nujol, 293 K): νCO 1977(w), 1953(s), 1887(s), 1864(sh), 1843(sh) cm–1. IR (CH3CN, 293 K): νCO 1989(w), 1965(m), 1891(s) cm–1. IR (acetone, 293 K): νCO 1988(w), 1963(m), 1891(s) cm–1. 1H NMR (CD3COCD3, 298 K): δ 7.85–6.98 (m, 42 H, CHAr + CHimid + Ph), 2.46 (s, 12H, CH3), 1.76 (s, 24H, CH3). 13C{1H} NMR (CD3COCD3, 298 K): δ 220.8 (CO), 185.1 (C–Au), 139.3, 134.6, 134.4, 134.2, 130.6, 129.0, 128.9, 128.8, 123.3 (CHAr + CHimid + Ph), 20.3, 16.4 (CH3). 31P{1H} NMR (CD3COCD3, 298 K): δ 38.5.

4.7. Thermal Decomposition of Fe(CO)4(AuIPr)2 (4)

4 was very stable in solution even after being heated at 130–150 °C in dmso. The reactions were periodically monitored by IR spectroscopy, and even after 12–24 h, the main νCO bands present in the spectra were those attributable to the starting 4. Then, a saturated solution of [NEt4][BF4] in H2O (40 mL) was added to complete precipitation. The resulting solid was recovered by filtration, washed with H2O (3 × 15 mL) and toluene (3 × 15 mL), and extracted with solvents of increasing polarity: CH2Cl2 (15 mL), thf (15 mL), acetone (15 mL), CH3CN (15 mL), and dmso (15 mL). 4 was the main product recovered independent of the experimental conditions. Nonetheless, several attempts at crystallization were made by layering suitable solvents on the solutions mentioned above. Besides the crystals of 4, these attempts resulted in a few crystals of [Au(IPr)2][16], [NEt4]2[9], and [17][BF4]n·solvent. These were likely to arise from partial decomposition of 4, which also involved dmso activation and formation of sulfide ions. The crystals of [Au(IPr)2][16], [NEt4]2[9], and [17][BF4]n·solvent were separated from the reaction mixtures and analyzed by X-ray crystallography, as well as IR spectroscopy ([NEt4]2[9] and [17][BF4]n·solvent) and 1H, 19F, and 13C{1H} NMR spectroscopy ([17][BF4]n·solvent).

[NEt4]2[9]. IR (nujol, 293 K): νCO 1999(m), 1820(s), 1892(s), 1865(m) cm–1. IR (CH3CN, 293 K): νCO 1988(m), 1932(s), 1904(m), 1873(w) cm–1.

[17][BF4]n·solvent. IR (nujol, 293 K): νCO 1975(s), 1903(m), 1856(w) cm–1. IR (CH2Cl2, 293 K): νCO 2039(m), 1974(s), 1883(s), 1863(m) cm–1. IR (thf, 293 K): νCO 2037(m), 1975(s), 1899(s), 1885(s), 1867(m) cm–1. IR (acetone, 293 K): νCO 2037(m), 1973(s), 1885(s), 1869(m) cm–1. 1H NMR (CD3COCD3, 298 K): δ 7.51 (s, 8H, CHimid), 7.43 (t, 2JHH = 7.7 Hz, 8H, CHAr), 7.26 (d, 2JHH = 7.7 Hz, 16H, CHAr), 2.67 [sept, 2JHH = 7.4 Hz, 16H, CH(CH3)2], 1.25 [d, 2JHH = 7.4 Hz, 48H, CH(CH3)2], 1.17 [d, 2JHH = 7.4 Hz, 48H, CH(CH3)2]. 13C{1H} NMR (CD3COCD3, 298 K): δ 222.3 (CO), 199.2 (C–Au), 150.6, 140.2, 135.0, 128.9, 128.1 (CAr and CHimid), 33.7 [CH(CH3)2], 28.9, 28.7 [CH(CH3)2]. 19F NMR (CD3COCD3, 298 K): δ −151.76, −151.81 ([BF4]−).

4.8. Thermal Decomposition of Fe(CO)4(AuIMes)(AuIPr) (5)

A solution of 5 (0.450 g, 0.359 mmol) in dmso (15 mL) was heated at 130 °C and the reaction monitored by IR spectroscopy. After 3 h, the IR spectrum showed the typical νCO absorptions of 8 and the reaction was stopped without any further workup.

4.9. Thermal decomposition of Fe(CO)4(AuIPr)(AuPPh3) (7)

A solution of 7 (0.450 g, 0.371 mmol) in dmso (15 mL) was heated at 130 °C, and the reaction monitored by IR spectroscopy. After 5 h, the IR spectrum showed the typical νCO absorptions of the starting compound 7. The temperature was increased to 150 °C without any clear evidence of decomposition.

4.10. X-ray Crystallographic Study

Crystal data and collection details for [Au(IMes)2][15]·0.67CH2Cl2, [Au(IPr)2][16], [NEt4]2[9], [NEt4][12], [17][BF4]n·solvent, and [NEt4][11]·1.5toluene are reported in Table S5. The diffraction experiments were carried out on a Bruker APEX II diffractometer equipped with a CCD ([17][BF4]n·solvent) or a PHOTON100 ([Au(IMes)2][15]·0.67CH2Cl2, [Au(IPr)2][16], [NEt4]2[9], [NEt4][12], and [NEt4][11]·1.5toluene) detector using Mo Kα radiation. Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction SADABS).53 Structures were determined by direct methods and refined by full-matrix least squares based on all data using F2.54 Hydrogen atoms were fixed at calculated positions and refined by a riding model. All non-hydrogen atoms were refined with anisotropic displacement parameters, unless otherwise stated.

4.10.1. [Au(IMes)2][15]·0.67CH2Cl2

The asymmetric unit of the unit cell contains one cluster anion located on a general position, half of a cluster anion located on an inversion center, one [Au(IMes)2]+ cation located on a general position, half of a [Au(IMes)2]+ cation located on an inversion center, and one CH2Cl2 molecule located at a general position. The CO ligands of the cluster anion located on an inversion center as well as the IMes ligands of the [Au(IMes)2]+ cation located on an inversion center are disordered. Thus, they have been split into two positions and refined using one occupancy factor per disordered group. The disordered CO ligands and the CH2Cl2 molecule have been refined isotropically. All C, N, and O atoms have been restrained to have similar U parameters (SIMU line in SHELXL, s.u. 0.01) and to isotropic behavior (ISOR line in SHELXL, s.u. 0.01). All of the aromatic C atoms have been constrained to fit regular hexagons (AFIX 66 line in SHELXL). Mainly because of the disorder issues mentioned above, the refined R1 factor was 0.1276.

4.10.2. [Au(IPr)2][16]

The asymmetric unit of the unit cell contains half of a [HFe(CO)4]− anion and half of a [Au(IPr)2]+ cation both located on the 2-fold axis. The N and C atoms of the IPr ligands have been restrained to have similar U parameters (SIMU line in SHELXL, s.u. 0.02). The [HFe(CO)4]− anion is disordered over two equally populated and symmetry-related positions. Because of this disorder, it has not been possible to locate the hydride ligand.

4.10.3. [NEt4]2[9]

The asymmetric unit of the unit cell contains two cluster anions and four [NEt4]+ cations located on general positions. The crystals are racemically twinned with a refined batch factor of 0.32(2). Two [NEt4]+ cations are disordered, and thus, they have been split into two positions each and refined anisotropically with one occupancy factor per disordered unit. The disordered cations have been restrained to have similar U parameters (SIMU line in SHELXL, s.u. 0.01), similar geometries (SAME line in SHELXL, s.u. 0.02), and isotropic behavior (ISOR line in SHELXL, s.u. 0.01).

4.10.4. [NEt4][12]

The asymmetric unit of the unit cell contains one cluster anion and one [NEt4]+ cation both located on general positions.

4.10.5. [17][BF4]n·Solvent

The asymmetric unit of the unit cell contains one-fourth of a cluster molecule located on 4. A total potential solvent accessible void of 1794 Å3 (∼22% of the cell volume) remains within the unit cell after refinement. These voids are organized in infinite channels parallel to the crystallographic c axis. In view of the fact that the crystals are very small, even if the data have been collected at 100 K with 120 s per frame, it has not been possible to crystallographically identify any molecule within these channels. For the same reasons, the final R1 factor was 0.1799. It must be remarked that, even if hundreds of Fourier peaks are included during refinement (PLAN 200 or even higher in SHELXL), all of them are located close to the cluster molecule and not within the void channels. In addition, 1H and 13C NMR analyses of the crystals dissolved in d6-acetone did not show any significant peaks apart those attributable to the cluster molecule. Conversely, 19F NMR analyses clearly pointed out the presence of [BF4]− anions. Nonetheless, because it was not possible to locate and refine such anions within the crystal structure, these voids were treated using the SQUEEZE routine of PLATON.55 All phenyl rings have been constrained to fit regular hexagons (AFIX 66 line in SHELXL).

4.10.6. [NEt4][11]·1.5Toluene

The asymmetric unit of the unit cell contains one cluster anion located at a general position, one [NEt4]+ cation located at a general position, one toluene molecule located at a general position, and one toluene molecule located at an inversion center disordered over two symmetry-related positions (occupancy factor of 0.5). All of the C, O, and N atoms have been restrained to have similar U parameters (SIMU line in SHELXL, s.u. 0.01). The C–N and C–C distances of the [NEt4]+ cation have been restrained to be similar (SADI line in SHELXL, s.u. 0.02). The aromatic rings of the toluene molecules have been constrained to fit regular hexagons (AFIX 66 line in SHELXL), and all of the C atoms of the toluene molecules have been restrained to isotropic behavior (ISOR line in SHELXL, s.u. 0.01). The overall quality of the crystals was rather low, leading to a final R1 factor of 0.2778.

4.11. Computational Details

Geometry optimizations of clusters 10, 11, 15a, and 15b were performed in the gas phase using the range-separated hybrid DFT functional ωB97X.56 The basis set used was the Ahlrichs’ def2 split-valence, with polarization and diffusion functions and relativistic ECP for Au.57 Single-point calculations on the optimized structures of 10, 11, 15a, and 15b and on the models for compound 17 were carried out at the same theoretical level, including nonlocal correlation by the VV10 functional (wB97X-v).58 Geometry optimizations of 3, 8, and 13 were carried out using the PBEh-3c method, which is a reparameterized version of PBE0 (with 42% HF exchange) that uses a split-valence double-ζ basis set (def2-mSVP) and adds three corrections that consider dispersion, basis set superposition, and other basis set incompleteness effects.59 The C-PCM solvation model was added to PBEh-3c calculations,60 considering a dielectric constant of 41.7 and a refractive index of 1.45544, intermediate between the values reported for dmso and dmf. The “restricted” approach was used in all cases. Calculations were performed with ORCA version 4.0.1.2.61 The output, converted in .molden format, was used for AIM and Hirshfeld analyses,62 performed with Multiwfn version 3.5.63 Cartesian coordinates of the DFT-optimized structures are collected in a separate .xyz file.

Acknowledgments

The authors thank the University of Bologna for financial support and the referees for the useful suggestions in revising the manuscript.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b02912.

  • Crystals and experimental details (PDF)

  • Optimized coordinates (XYZ)

Accession Codes

CCDC 1955604–1955609 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

The authors declare no competing financial interest.

Supplementary Material

ic9b02912_si_001.pdf (3.1MB, pdf)
ic9b02912_si_002.xyz (53.1KB, xyz)

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