Peraurated ruthenium hydride carbonyl clusters: aurophilicity, isolobal analogy, structural isomerism, and fluxionality†

Cristiana Cesari *^a, Marco Bortoluzzi ^b, Cristina Femoni ^a, Francesca Forti ^a, Maria Carmela Iapalucci ^a and Stefano Zacchini ^a
aDipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
^bDipartimento di Scienze Molecolari e Nanosistemi, Ca’ Foscari University of Venice, Via Torino 155, 30175 Mestre, Ve, Italy

First published on 19th January 2024

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Introduction

Molecular compounds containing two or more [AuPPh₃]⁺ fragments have considerably contributed to the development of the aurophilicity concept.^1–5 Indeed, closed-shell d¹⁰ Au(I) ions tend to attract themselves, resulting in weak aurophilic interactions, whose strength is somehow comparable to hydrogen bonding.^6,7 It has been shown that aurophilic attractions can have electronic and/or dispersive (London forces) nature, which are both due to pronounced relativistic effects on gold atoms.^8–10

Attractive aurophilic interactions span a wide range of distances, that is ca. 2.75–3.50 Å.¹¹ For comparison, the closest contact of two Au atoms in fcc gold metal is 2.88 Å, the covalent Au–Au bond in dinuclear Au(II) (d⁹) complexes is ca. 2.57 Å, and the sum of two van der Waals radii of Au(I) is ca. 3.60 Å.^2,4,12

Both inter-molecular and intra-molecular aurophilic contacts are well-represented. Inter-molecular interactions have a considerable relevance in supramolecular chemistry and self-assembly phenomena based on Au(I) compounds, with applications in materials science.^11,13 Intra-molecular aurophilic contacts contribute to the overall structures of peraurated complexes, which may also show structural isomerism.^14–17 In this respect, several molecular metal carbonyl clusters decorated on the surface by [AuPPh₃]⁺ fragments have been reported.^18–22

Protons and [AuPPh₃]⁺ fragments are isolobal, and often metal carbonyl clusters containing a single [AuPPh₃]⁺ fragment are isostructural to related hydride derivatives.^23–27 However, when two or more [AuPPh₃]⁺ fragments are present, the structural analogy with hydride derivatives fades, due to the formation of weak Au⋯Au aurophilic contacts.^1,2,5,6,14 Indeed, often polyhydride and related peraurated carbonyl clusters differ in the geometrical arrangement of the H and AuPPh₃ fragments around the metal carbonyl cage of the cluster. This, of course, is more a structural consideration than a general statement. If one ignores for a moment charges and redox states and just looks at H and Au(PPh₃), the presence of multiple Au(PPh₃) units in a cluster does not invalidate their similarity to a cluster containing the corresponding number of H atoms. Furthermore, it is known that hydrido clusters can be viewed, at least in a limiting form, as dihydrogen complexes (two H atoms close to each other forming H₂, like two Au(PPh₃) units close to each other forming P–Au–Au–P).²⁸ Such molecules are also often fluxional. The isolobal analogy was not meant to address metallophilic interactions and it still applies to complexes with multiple d¹⁰ centres, even if their detailed structure may not be predicted.

Thus, peraurated metal carbonyl clusters are good platform compounds suitable to experimentally test aurophilicity.^29–31 Indeed, they contain stronger core interactions, whereas weaker forces operate on their surface. The overall arrangement of the [AuPPh₃]⁺ fragments on the surface of the cluster is, therefore, often the result of a subtle balance between several different weak intramolecular forces, such as Au⋯Au aurophilic contacts, π–π and π–H interactions due to the presence of aromatic groups, as well as steric effects.³² Packing effects may also be relevant in the solid state. As a consequence, structural isomerism and fluxionality are not rare phenomena in the case of peraurated metal carbonyl clusters. In addition, (polarized) heterometallic M–Au bonds may contribute to enhanced catalytic properties.^33–38

A few neutral tetrahedral Ru₄ carbonyl clusters containing 1–3 [AuPPh₃]⁺ fragments are known, that is, H₃Ru₄(CO)₁₂(AuPPh₃), H₂Ru₄(CO)₁₂(AuPPh₃)₂, and HRu₄(CO)₁₂(AuPPh₃)₃.^39–42 These are closely related to the tetra-hydride H₄Ru₄(CO)₁₂, even though structural differences occur upon replacement of hydrides with [AuPPh₃]⁺ fragments, due to the formation of Au⋯Au aurophilic contacts. Also in the case of Os, only neutral tetrahedral clusters have been reported so far, that is H₃Os₄(CO)₁₂(AuPPh₃), H₂Os₄(CO)₁₂(AuPPh₃)₂, H₃Os₄(CO)₁₁(AuPPh₃)₃, and H₂Os₄(CO)₁₁(AuPPh₃)₄.^40–45 Conversely, both neutral and anionic clusters are known for Fe, that is HFe₄(CO)₁₂(AuPPh₃)₃ and [HFe₄(CO)₁₂(AuPPh₃)₂]⁻, respectively.⁴⁶ It is worth noting that HFe₄(CO)₁₂(AuPPh₃)₃ represents a unique case of a metal carbonyl cluster containing μ₄-H within a tetrahedral metal cage. This should be in contrast to HRu₄(CO)₁₂(AuPPh₃)₃, whose hydride is located on the surface of the cluster.

The recent discovery of a straightforward synthesis of [HRu₄(CO)₁₂]³⁻ (1)⁴⁷ prompted a detailed study of its reactivity toward Au(PPh₃)Cl. This resulted in the isolation of the first anionic Ru₄ carbonyl clusters containing [AuPPh₃]⁺ fragments, that is [HRu₄(CO)₁₂(AuPPh₃)]²⁻ (2) and [HRu₄(CO)₁₂(AuPPh₃)₂]⁻ (3), an improved synthesis of HRu₄(CO)₁₂(AuPPh₃)₃ (4), as well as the synthesis of tetra-aurated Ru₄(CO)₁₂(AuPPh₃)₄ (5). Details of their preparation, spectroscopic and structural characterization, structural isomerism, fluxional behaviour and theoretical investigation are herein reported.

Results and discussion

The reaction of [HRu₄(CO)₁₂]³⁻ (1) with Au(PPh₃)Cl: general features and characterization of [HRu₄(CO)₁₂(AuPPh₃)]²⁻ (2)

The stepwise addition of increasing amounts of Au(PPh₃)Cl to [HRu₄(CO)₁₂]³⁻ (1), as [NEt₄]⁺ salt, results in the sequential formation of [HRu₄(CO)₁₂(AuPPh₃)]²⁻ (2), [HRu₄(CO)₁₂(AuPPh₃)₂]⁻ (3), and HRu₄(CO)₁₂(AuPPh₃)₃ (4), as evidenced by IR analyses (Scheme 1, Table 1 and Fig. 1). Alternatively, 4 can be obtained upon addition of HBF₄·Et₂O (two mole equivalents) to 3. In this case, H₄Ru₄(CO)₁₂, [H₃Ru₄(CO)₁₂]⁻ and [H₂Ru₄(CO)₁₂]²⁻ are obtained as side-products,^48–51 as evidenced by IR and ¹H NMR spectroscopy. The formation of these homometallic Ru hydride carbonyls is in agreement with the fact that 4 contains more Au than the precursor 3. Addition of a slight excess of acid to 3 (three mole equivalents) results in the formation of the tetra-aurated cluster Ru₄(CO)₁₂(AuPPh₃)₄ (5). As above, H₄Ru₄(CO)₁₂ and [H₃Ru₄(CO)₁₂]⁻ are the main side products observed during the synthesis of 5. It is likely that the formation of 4 and 5 upon addition of acids to 3 proceeds via a general rearrangement, producing 4 and 5, which are richer in Au than 3, as well as Au-free clusters H₄Ru₄(CO)₁₂, [H₃Ru₄(CO)₁₂]⁻ and [H₂Ru₄(CO)₁₂]²⁻, guaranteeing the mass balance.

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Mono-aurated 2 is very elusive and has been only spectroscopically identified (IR, ¹H and ³¹P{¹H} NMR). Conversely, 3–5 have been fully characterized by IR, ¹H and ³¹P{¹H} NMR spectroscopies (Fig. S1–S27 in the ESI†), and their molecular structures are determined by SC-XRD as [NEt₄][3]·2CH₂Cl₂, 4-b·2CH₂Cl₂, 4-a, 5·0.5CH₂Cl₂·solv (monoclinic, C2/c), and 5·solv (monoclinic, P2₁/c). The labels 4-a and 4-b refer to the two structurally characterized isomers of 4 (see below for further details).

Two alternative syntheses were previously reported for 4, that is (a) the reaction of H₄Ru₄(CO)₁₂ or H₂Ru₄(CO)₁₂(AuPPh₃)₂ with Au(CH₃)(PPh₃) and (b) the reaction of [H₃Ru₄(CO)₁₂]⁻ with [{Au(PPh₃)}₃O][BF₄].^41,42 The latter synthesis afforded 4 in combination with H₃Ru₄(CO)₁₂(AuPPh₃) and H₂Ru₄(CO)₁₂(AuPPh₃)₂, which were separated by chromatography. It has also been reported that the reaction of [HFe₄(CO)₁₂]³⁻ with two mole equivalents of Au(PPh₃)Cl affords [HFe₄(CO)₁₂(AuPPh₃)₂]⁻, which is transformed into HFe₄(CO)₁₂(AuPPh₃)₃ upon addition of HBF₄·Et₂O.⁴⁶

The ν_CO bands are moved 37–40 cm⁻¹ towards higher wavenumbers upon each addition of one [AuPPh₃]⁺ fragment to the 1–4 series of clusters, a result in line with the computational IR simulations (Fig. S28 in the ESI†). This point strongly supports the formation of 2, even if it has not been possible to isolate and structurally characterize it. Moreover, the ¹H NMR spectrum of 2 displays a doublet at δ_H −19.51 ppm with J_H–P = 2.0 Hz, in agreement with the presence of one hydride and one AuPPh₃ group. This is further supported by the presence of a resonance at δ_P 62.14 ppm in the ³¹P{¹H} NMR spectrum.

The ¹H and ³¹P{¹H} NMR spectra clearly indicate that 2 is always formed in combination with 3 and [H₂Ru₄(CO)₁₂]²⁻. This is probably the reason why it was not possible to crystallize 2. Thus, its structure has been predicted and optimized by DFT methods. The ground-state geometry, shown in Fig. 2, is composed of a Ru₄ tetrahedron capped on a triangular face by an AuPPh₃ fragment, while on another face, a μ₃-coordinated hydride is present. The coordination sphere of the Ru centres is completed by nine terminal and three bridging CO ligands. The AIM analysis on the electron density revealed the presence of a (3,−1) bond critical point (BCP) between the Au centre and one of the terminal carbonyl ligands. Selected computed properties at the BCP are provided in the caption of Fig. 2. The negative value of the energy density (E) and the positive value of the Laplacian of electron density (∇²ρ) are in agreement with Bianchi's definition of a dative bond.⁵² Even if the presence of different attractors does not allow a direct comparison, it is worth noting that the potential energy density (V) at the Au⋯C BCP is about one tenth of the value calculated for the Ru–C BCP involving the same carbonyl ligand, in agreement with a quite weak interaction.

Molecular structure, spectroscopic characterization and DFT analysis of [HRu₄(CO)₁₂(AuPPh₃)₂]⁻ (3)

Cluster 3 was obtained in good yield (78%) upon addition of a further mole equivalent of Au(PPh₃)Cl to 2. The molecular structure of 3 (Fig. 3 and Table 2) closely resembles that previously reported for [HFe₄(CO)₁₂(AuPPh₃)₂]⁻.⁴⁶ It consists of a Ru₄ tetrahedron capped on a triangular face by one AuPPh₃ fragment adjacent to a Ru₃ triangle capped by the μ₃-H ligand. The second AuPPh₃ fragment is located on the Ru₂Au triangle near the hydride ligand. Alternatively, 3 may be viewed as a Ru₄Au trigonal bipyramid capped on a Ru₃ and a Ru₂Au face, sharing a Ru–Ru edge, by μ₃-H and μ₃-AuPPh₃, respectively. Among the 12 CO ligands bonded to the four Ru atoms, 10 are terminal and 2 are edge bridging. The unique hydride ligand has been located in the final Fourier difference map and refined isotropically. The hydride location is the same in 3 and [HFe₄(CO)₁₂(AuPPh₃)₂]⁻.⁴⁶

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The Ru–Ru bonds are rather spread [2.8156(7)–3.0483(6) Å, average 2.9002(15) Å] compared to the parent 1 [2.8001(11)–2.8113(11) Å, average 2.805(3) Å] in view of the coordination of two [AuPPh₃]⁺ fragments, which causes swelling of the Ru₄ tetrahedron. It must be remarked that this phenomenon is less marked for [H₃Ru₄(CO)₁₂]⁻ [2.7614(5)–2.9423(4) Å, average 2.8504(12) Å for isomer C₂; 2.7733(5)–2.9380(5) Å, average 2.8519(12) Å for isomer C_3v], which arises from 1 upon addition of two H⁺ ligands instead of two [AuPPh₃]⁺ fragments. This supports the fact that the swelling of 3 is mainly due to the presence of bulky AuPPh₃ groups, rather than a charge effect. Two isomers of [H₃Ru₄(CO)₁₂]⁻ are known, and their molecular structures display only edge bridging hydride ligands.^49,50 The different stereochemistry observed for the unique hydride and the two [AuPPh₃]⁺ ligands of 3 further supports the evidence that their isolobal analogy fades when two or more Au(I) centers are present, due to the insurgence of aurophilicity.^1,2,5,6,14

Cluster 3 displays five Ru–Au bonding contacts [2.7808(5)–2.9408(5) Å, average 2.8231(11) Å] and one aurophilic Au–Au contact [2.8393(3) Å]. Some sub-van der Waals Au⋯CO contacts [2.76–3.06 Å] are also present. As previously reported, the latter Au⋯CO contacts are mainly sterically driven and not attractive bonding interactions.^53,54

The ³¹P{¹H} NMR spectrum of 3 recorded in acetone-d₆ at 298 K displays a singlet at δ_P 63.41 ppm, indicating a fluxional behaviour which makes the two AuPPh₃ fragments equivalent in the timescale of NMR. This point is further corroborated by the fact that the unique hydride resonates as a triplet at δ_H −13.74 ppm (J_H–P 0.9 Hz) in the ¹H NMR spectrum under the same conditions. ¹H and ³¹P{¹H} NMR spectra of 3 do not change down to 223 K, indicating a very fast exchange even at a low temperature.

The DFT-optimized structure of 3 is in good agreement with the X-ray data (RMSD = 0.416 Å) and the μ₃ coordination mode of the hydride is confirmed by the simulations. As stated before, the HRu₄Au core resembles the HFe₄Au one in the analogous iron cluster. The best superposition of the metal hydride fragments in 3 and [HFe₄(CO)₁₂(AuPPh₃)₂]⁻, optimized at the same theoretical level, is shown in Fig. S29 in the ESI.† In contrast to the observation of 2, the AIM analysis of 3 did not localize any (3,−1) BCP related to the Au⋯CO contacts. On the other hand, a (3,−1) BCP between the two Au centres was found, as shown in Fig. 4. The computed properties, summarized in the caption of Fig. 4, are in line with a weak Au⋯Au metal–metal interaction.⁴⁷ The computed Au⋯Au distance, 2.861 Å, is in excellent agreement with the experimental value, 2.8393(3) Å. Upon comparison, the Au⋯Au interaction in [HFe₄(CO)₁₂(AuPPh₃)₂]⁻, optimized at the same theoretical level, was found to be slightly stronger (ρ = 0.042 a.u., V = −0.036 a.u.), despite the fact that the computed Au⋯Au distance is the same as in 3, 2.861 Å.

Attempts to understand the fluxional behaviour of 3 were carried out considering the possible presence of more symmetric structures in solution. In particular, starting geometries with the C_s symmetry of the metal hydride fragment were considered, and quite a symmetric stationary point was achieved (Fig. S30 in the ESI†). The optimized geometry, where the hydride is μ₃-coordinated to one triangular face of the Ru₄ tetrahedron and the two AuPPh₃ fragments are symmetrically capping other two faces, was however meaningfully less stable with respect to the observed isomer of 3, with the Gibbs energy difference around 21.2 kcal mol⁻¹. It is therefore unlikely that such species could have a role in the apparent symmetry observed by means of NMR spectroscopy; thus, the fluxional behaviour appears to be ascribed to the fast exchange of the fragments capping the Ru₄ tetrahedron.

Structural, spectroscopic and computational studies of the two isomers of HRu₄(CO)₁₂(AuPPh₃)₃ (4)

Neutral cluster 4 contains one additional [AuPPh₃]⁺ fragment compared to 3. Two different isomers have been characterized in the solid state, that is, isomer 4-a and isomer 4-b (Fig. 5, 6 and Table 1). The latter isomer has been found in the solvate crystals 4-b·2CH₂Cl₂, whereas isomer 4-a has been found in solid solvent-free 4-a (triclinic, P). The structure of isomer 4-b was previously determined in solid solvent-free crystals (monoclinic, P2₁/n),^41,42 suggesting that crystallization of 4-a or 4-b does not depend on the presence/absence of co-crystallized solvent molecules. The two isomers are rapidly exchanging in solution, as indicated by VT ¹H and ³¹P{¹H} NMR experiments (see below). The structures of both 4-a and 4-b differ from that of H₄Ru₄(CO)₁₂⁴⁸ due to aurophilic interactions.

Isomer 4-a formally arises from the addition of the third [AuPPh₃]⁺ fragment on the triangular face of 3 originally capped by μ₃-H, with concomitant migration of the hydride on an adjacent Ru₃ face (Scheme 2). In contrast, isomer 4-b originates from the addition of the third [AuPPh₃]⁺ fragment onto a Ru₂Au triangular face of 3 without migration of the hydride ligand.

The Ru₄Au₃ metal cage of 4-a may be described as composed of five tetrahedra (Ru₄, Ru₃Au, Ru₂Au₂, Ru₂Au₂, Ru₃Au) sharing five triangular faces, resulting in a pentagonal bipyramid. The same metal cage was previously found in HFe₄(CO)₁₂(AuPPh₃)₃,⁴⁶ even though the unique hydride was located within the tetrahedral Fe₄ cage, rather than on a triangular face.

Isomer 4-b is composed of a trigonal bipyramidal Ru₄Au core capped on two Ru₂Au faces by two further Au atoms. The unique hydride ligand is face capping a triangular Ru₃ face.

In both isomers, all the CO ligands are essentially bonded to the four Ru atoms, showing only some weak sub-van der Waals Au⋯C(O) contacts. The nature of the latter contacts is rather debated, as previously discussed.^53,54 Disregarding the Au⋯C(O) contacts, all CO ligands are terminally bonded in isomer 4-b, three per Ru atom. Conversely, isomer 4-a contains 10 terminal and two edge bridging CO ligands on two Ru–Ru edges.

The Ru–Ru contacts in isomers 4-a [2.7887(6)–2.9888(6) Å; average 2.8865(15) Å] and 4-b [2.770(2)–3.090(2) Å; average 2.902(5) Å] are rather similar and also comparable to those of 3. This indicates that the addition of a further AuPPh₃ fragment does not significantly alter the Ru₄ tetrahedron from 3 to 4. Isomer 4-a displays eight Ru–Au bonding contacts [2.7730(5)–3.0487(5) Å; average 2.8689(13) Å], whereas only seven Ru–Au bonds are present in 4-b [2.7637(18)–2.9022(18) Å; average 2.826(5) Å], in view of the different coordination modes of the three AuPPh₃ fragments to the Ru₄ tetrahedron in the two isomers.

Two aurophilic Au⋯Au contacts are present in both 4-a [2.8288(3)–2.8623(3) Å; average 2.8456(4) Å] and 4-b [2.9247(11)–2.9515(12) Å; average 2.9381(17) Å].

The behavior in solution of 4 has been investigated by VT ¹H and ³¹P{¹H} NMR spectroscopy (Fig. 7, 8 and Fig. S17–S26 in the ESI†). Two major differences with the related HFe₄(CO)₁₂(AuPPh₃)₃ cluster are evident. First of all, dissociation of 4 into 3 and [AuPPh₃]⁺ does not occur even in polar solvents such as CH₃CN and DMSO, whereas HFe₄(CO)₁₂(AuPPh₃)₃ is stable only in CH₂Cl₂ and rapidly dissociates into [HFe₄(CO)₁₂(AuPPh₃)₂]⁻ and [AuPPh₃]⁺ in acetone. Moreover, HFe₄(CO)₁₂(AuPPh₃)₃ is fluxional at all temperatures and only one isomer has been detected, whereas the two isomers of 4 rapidly interconvert at room temperature but the process is sensibly slowed down at lower temperatures.

The ¹H NMR spectrum of 4 in CD₂Cl₂ at 298 K shows a broad resonance at δ_H −13.01 ppm and almost complete coalescence is observed at 273 K. A quartet at δ_H −12.80 ppm with J_H–P = 5.2 Hz appears, then, at 248 K, indicating that fluxionality makes the three AuPPh₃ groups equivalent from the point of view of the unique hydride. This resonance is further broadened at 223 K, and eventually, two equally intense and closely spaced resonances are observed at 198 K and 173 K. These may be interpreted as two singlets due to the presence of a 1:1 mixture of two isomers, or as a doublet where the hydride strongly couples with just one AuPPh₃ group. In order to shed more light on this point, VT ³¹P{¹H} NMR experiments have been performed. A broad resonance is observed at 298 K with δ_P 60.37 ppm and coalescence occurs at 273 K. Two resonances in a 2:1 ratio appear at lower temperatures, which become well resolved at 223 K, with δ_P 60.91 (2P) and 51.95 (1P) ppm. At 198 K, the lower frequency resonance is further split into two 1:1 resonances at δ_P 51.96 and 51.92 ppm, whereas that at a higher frequency remains a singlet at δ_P 61.00 ppm. This observation may be interpreted assuming the presence in solution of two isomers each containing two equivalent AuPPh₃ groups and one unique AuPPh₃ group, as experimentally found in 4-a and 4-b. For these isomers, the resonances of the unique AuPPh₃ group are resolved at 198 K, whereas the resonances of the two equivalent AuPPh₃ groups are almost superimposed for the two isomers also at 198 K. This is in agreement with the observation of two isomers in the solid state, which very rapidly exchange in solution.

The DFT-optimized structures of 4-a and 4-b agree with the X-ray data, with RMSD values of 0.928 Å for 4-a and 0.369 Å for 4-b. The localization of the hydrides is confirmed, as observable from the superposition of the HRu₄Au₃ fragments in Fig. S31 in the ESI.† Gas-phase calculations indicate that 4-b is more stable than 4-a by 4.9 kcal mol⁻¹, and thus, the isolation of both the species is to be ascribed almost in part to different packing forces. Another isomer (4-c) was optimized by changing the position of the hydride in 4-b. The disposition of the metal centres is comparable with respect to 4-b, but the hydride is μ₂-coordinated to two Ru centres at the equatorial position of the Ru₄Au trigonal bipyramid at the opposite side with respect to the AuPPh₃ fragments capping two Ru₂Au faces. 4-c was more stable than 4-b by about 4.2 kcal mol⁻¹ in the gas phase. The three isomers are depicted in Fig. 9. It is worth noting that the relative energy values were strongly dependent upon the surrounding medium. The introduction of DMSO as an implicit solvent inverted the stability order of 4-a and 4-b, and the Gibbs energy interval between the less stable isomer (4-b) and the most stable one (4-c) was reduced to only 2.8 kcal mol⁻¹. The position of the hydride is also influenced by the surrounding medium, since the addition of the solvation model caused the change in the coordination mode in 4-a from μ₃-H to μ₂-H (Fig. S32 in the ESI†). The AIM analyses allowed the localization of all the isomers of (3,−1) BCP related to the Au⋯Au interactions, roughly comparable to that described for 3. Selected data, provided in the caption of Fig. 9, indicate that the Au⋯Au interactions are slightly stronger in 4-b and 4-c, perhaps thanks to the disposition of the Au centres. No (3,−1) BCP was instead localized for the Au⋯C contacts.

The possible formation of isomers with the hydride inside the metal cage was also considered, starting from the X-ray structure of HFe₄(CO)₁₂(AuPPh₃)₃ and replacing the metal centres. On the other hand, an isomer of HFe₄(CO)₁₂(AuPPh₃)₃ with the hydride on the surface of the metal cage was optimized, starting from the geometry of 4-b. The metal hydride cores are compared in Fig. S33 in the ESI.† From a thermodynamic point of view, the migration of the hydride from an M₃ triangular face to the M₄ cage is more favoured for the Fe cluster with respect to the Ru analogue by about 4.3 kcal mol⁻¹, in agreement with the experimental localizations of the hydride ligands. This is probably due to the steric problems on the surface of the iron cluster. Indeed, Fe being smaller than Ru, it is likely that there is not enough space on the surface of an Fe₄ tetrahedron in order to locate 12 CO ligands, three AuPPh₃ fragments and a hydride. Thus, the H-atom is forced to migrate inside the Fe₄ tetrahedron, whereas there is enough space to remain on the surface of the Ru₄ tetrahedron of 4.

Molecular structure, spectroscopic characterization and DFT analysis of Ru₄(CO)₁₂(AuPPh₃)₄ (5)

The molecular structure of the tetra-aurated cluster 5 has been determined on two solvates, that is, 5·0.5CH₂Cl₂·solv (monoclinic C2/c) and 5·solv (monoclinic P2₁/c), displaying almost identical structures and bonding parameters (Fig. 10 and Table 2). Its metal cage may be viewed as a Ru₄Au trigonal bipyramid capped on all three Ru₂Au faces by Au atoms. The resulting Ru₄Au₃ core possesses idealized C_3v symmetry and is closely related to the metal cage of 4-b. In agreement with this, the ³¹P{¹H} NMR spectrum of 5, recorded in CD₂Cl₂ at 298 K, displays two resonances at δ_P 68.46 and 67.15 ppm, with relative intensities at 3:1.

Coordination of four AuPPh₃ groups to the Ru₄ tetrahedron significantly swells the Ru–Ru contacts [2.870(2)–3.0838(16) Å, average 2.977(5) Å for 5·0.5CH₂Cl₂·solv (monoclinic C2/c); 2.8899(18)–3.213(2) Å, average 3.018(5) Å for 5·solv (monoclinic P2₁/c)] compared to 3 and 4. Considering only interactions with Ru atoms, the CO ligands are all terminal (three per Ru), even though some sub-van der Waals Au⋯C(O) contacts are present.

Cluster 5 displays nine Ru–Au bonding contacts [2.7861(18)–2.9838(14) Å, average 2.860(4) Å for 5·0.5CH₂Cl₂·solv (monoclinic C2/c); 2.8107(16)–3.1396(18) Å, average 2.893(5) Å for 5·solv (monoclinic P2₁/c)] as well as three aurophilic Au⋯Au contacts [2.8180(8)–2.8671(8) Å, average 2.8405(14) Å for 5·0.5CH₂Cl₂·solv (monoclinic C2/c); 2.8531(14)–2.8966(14) Å, average 2.880(2) Å for 5·solv (monoclinic P2₁/c)].

The only other species of the general formula M₄(CO)₁₂(M'PPh₃)₄ (M = Fe, Ru, Os; M′ = Cu, Ag, Au) reported prior to this work was Ru₄(CO)₁₂(CuPPh₃)₄ where the four CuPPh₃ groups are capping the four triangular faces of the Ru₄ tetrahedron without any Cu⋯Cu interaction.⁵⁴

The DFT-optimized structure of 5 is in good agreement with the X-ray data, the RMSD being 0.588 Å. The Ru₄Au₄ fragments are superimposed in Fig. S34 in the ESI.† The AIM analysis on 5 revealed the presence of three (3,−1) Au–Au BCPs between the Au centre in the axial position of the Ru₄Au trigonal bipyramid and the three AuPPh₃ fragments capping the Ru₂Au triangular faces (Fig. 11). The properties at the three BCPs are roughly the same (see the caption of Fig. 11), a result in line with the approximate C_3v symmetry of the Ru₄Au₄ fragment (R = 0.077). The values of ρ and V are similar to those obtained for 4-c, suggesting comparable strength of the Au–Au interactions. The presence of bonding overlaps among the Au atoms in 5 can be observed in Fig. 12, where the HOMO is plotted, limited to the contributions of the Au centres.

Conclusions

The mono-hydride trianionic cluster 1 is a suitable starting material for the addition of one up to four [AuPPh₃]⁺ fragments, resulting in the formation of 2–5. These Ru–Au clusters retain the tetrahedral Ru₄ core of 1, which is decorated on the surface by an increasing number of [AuPPh₃]⁺ fragments. Clusters 3–5, which contain two or more Au(I) centres, display aurophilic Au⋯Au contacts, as previously found in other peraurated carbonyl clusters.^{18–22,29–31} These are soft interactions and thus, rearrangement of the [AuPPh₃]⁺ fragments can be observed. Indeed, two isomers, 4-a and 4-b, have been structurally characterized in the solid state for 4, and their rapid exchange in solution was revealed by VT multinuclear NMR spectroscopy. Interconversion of 4-a and 4-b requires both rearrangement of the three [AuPPh₃]⁺ fragments as well as migration of the unique hydride ligand.

The molecular structures of 3–5 differ from the isolobally related hydrides [H₃Ru₄(CO)₁₂]⁻ and H₄Ru₄(CO)₁₂^48–50 due to the different locations of H and AuPPh₃ fragments, because of aurophilicity. Thus, the study of peraurated hydride carbonyl clusters offers the possibility to study at the molecular level the interplay and dynamic behaviour of [AuPPh₃]⁺ and hydride ligands on the surface of the same metal core.

Experimental

General procedures

All reactions and sample manipulations were carried out using standard Schlenk techniques under nitrogen and in dried solvents. All the reagents were commercial products (Aldrich) of the highest purity available and used as received, except for [NEt₄]₃[1]⁴⁷ and Au(PPh₃)Cl,⁵⁵ which have been prepared according to the literature. Analyses of C, H and N were performed with a Thermo Quest Flash EA 1112NC instrument. IR spectra were recorded on a PerkinElmer Spectrum One interferometer in CaF₂ cells. ¹H and ³¹P{¹H} NMR measurements were performed on a Varian Mercury Plus 400 MHz and a Varian Inova 600 MHz instrument. The proton chemical shifts were referenced to the non-deuterated aliquot of the solvent. The phosphorus chemical shifts were referenced to external H₃PO₄ (85% in D₂O). Structure drawings have been performed with SCHAKAL99.⁵⁶

Synthesis of [NEt₄]₂[HRu₄(CO)₁₂(AuPPh₃)] ([NEt₄]₂[2])

Au(PPh₃)Cl (0.135 g, 0.274 mmol) was added as a solid in a small portion to a solution of [NEt₄]₃[1] (0.310 g, 0.274 mmol) in CH₃CN (15 mL). The mixture was stirred at room temperature for 1 h and the reaction was monitored by IR spectroscopy. At the end of the reaction, the solvent was removed in vacuo, and the residue was washed with water (2 × 20 mL) and toluene (2 × 10 mL) and extracted with CH₂Cl₂ (15 mL). IR and NMR analyses on the extraction revealed the presence of [NEt₄]₂[2] in a mixture with [NEt₄][3] (1:0.68 ratio by ¹H NMR).

IR (CH₂Cl₂, 298 K) ν_CO: 2034(m), 1953(m), 1929(s), 1744(m) cm⁻¹. ¹H NMR (CD₂Cl₂, 298 K) δ: −19.51 (d, J_H–P = 2.0 Hz). ³¹P{¹H} NMR (CD₂Cl₂, 298 K) δ: 62.15 ppm.

Synthesis of [NEt₄][HRu₄(CO)₁₂(AuPPh₃)₂]·2CH₂Cl₂ ([NEt₄][3]·2CH₂Cl₂)

Au(PPh₃)Cl (0.280 g, 0.566 mmol) was added as a solid in a small portion to a solution of [NEt₄]₃[1] (0.320 g, 0.283 mmol) in CH₃CN (15 mL). The mixture was stirred at room temperature for 2 h and the reaction was monitored by IR spectroscopy. At the end of the reaction, the solvent was removed in vacuo, and the residue was washed with water (2 × 20 mL) and toluene (2 × 10 mL) and extracted with CH₂Cl₂ (15 mL). Crystals of [NEt₄][3]·2CH₂Cl₂ suitable for SC-XRD were obtained by layering n-pentane on the CH₂Cl₂ solution (yield 78%).

C₅₈H₅₅Au₂Cl₄NO₁₂P₂Ru₄ (1959.98): calcd (%): C 35.54, H 2.83, N 0.71; found: C 35.37, H 3.05, N 0.84. IR (CH₂Cl₂, 298 K) ν_CO: 2033(w), 1986(m), 1967(s), 1749(m) cm⁻¹. IR (acetone, 298 K) ν_CO: 2031(w), 1985(m), 1967(s) cm⁻¹. IR (Nujol, 298 K) ν_CO: 2029(w), 1983(s), 1961(m), 1941(m), 1908(w) cm⁻¹. ¹H NMR (Acetone-d₆, 298 K) δ: −13.74 ppm (t, J_H–P = 0.9 Hz). ³¹P{¹H} NMR (Acetone-d₆, 298 K) δ: 63.41 ppm.

Synthesis of HRu₄(CO)₁₂(AuPPh₃)₃ (4)

Synthesis of Ru₄(CO)₁₂(AuPPh₃)₄ (5)

HBF₄·Et₂O (79.6 μL, 0.585 mmol) was added dropwise to a solution of [NEt₄][3] (0.350 g, 0.195 mmol) in CH₃CN (10 mL). The mixture was stirred at room temperature for 1 h and the reaction was monitored by IR spectroscopy. At the end of the reaction, the solvent was removed in vacuo, and the residue was washed with water (2 × 20 mL) and toluene (2 × 10 mL) and extracted with CH₂Cl₂ (15 mL). Crystals of 5·0.5CH₂Cl₂·solv (monoclinic, C2/c)* suitable for X-ray analyses were obtained by layering n-pentane on the CH₂Cl₂ solution (yield 28%).

*Sometimes crystals of 5·solv (Monoclinic, P2₁/c) were obtained instead of 5·0.5CH₂Cl₂·solv (monoclinic, C2/c).C_84.5H₆₁Au₄ClO₁₂P₄Ru₄ (2619.81): calcd (%): C 38.74, H 2.35; found: C 39.03, H 2.61. IR (CH₂Cl₂, 298 K) ν_CO: 2082(w), 2072(m), 2029(s), 1970(m) cm⁻¹. IR (Nujol, 298 K) ν_CO: 2078(m), 2063(m), 2030(s), 2020(vs), 1979(w), 1952(w) cm⁻¹. ³¹P{¹H} NMR (CD₂Cl₂, 298 K) δ: 68.46 (3P), 67.15 (1P) ppm.

X-ray crystallographic study

Crystal data and collection details for [NEt₄][3]·2CH₂Cl₂, 4-b·2CH₂Cl₂, 4-a, 5·0.5CH₂Cl₂·solv, and 5·solv are reported in Table S1 in the ESI.† The diffraction experiments were carried out on a Bruker APEX II diffractometer equipped with a PHOTON2 detector using Mo-Kα radiation. Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction SADABS).⁵⁷ Structures were solved by direct methods and refined by full-matrix least-squares based on all data using F².⁵⁸ Hydrogen atoms were fixed at calculated positions and refined using a riding model, except for hydride ligands which were tentatively located in the Fourier difference map and refined isotropically using the 1.2-fold U value of the parent metal atom. All non-hydrogen atoms were refined with anisotropic displacement parameters, unless otherwise stated. The unit cells of Ru₄(CO)₁₂(AuPPh₃)₄·0.5CH₂Cl₂·solv and Ru₄(CO)₁₂(AuPPh₃)₄·solv contain additional total potential solvent accessible voids of 1801 and 2115 ų (ca. 10 and 22% of the cell volume), respectively, which are likely to be occupied by highly disordered solvent molecules. These voids have been treated using the SQUEEZE routine of PLATON.^59,60

Computational details

Geometry optimizations were carried out without symmetry constraints using the r²-SCAN-3c method,⁶¹ based on the meta-GGA r²SCAN functional⁶² combined with a tailor-made triple-ζ Gaussian atomic orbital basis set, with ECPs for Ru and Au. The method also includes refitted D4 and geometrical counter-poise corrections for London-dispersion and basis set superposition error.^63–65 The C-PCM implicit solvation model was added considering DMSO as a continuous medium.^66,67 Calculations were carried out using ORCA 5.0.3,^68,69 and the output files were analysed with Multiwfn, version 3.8.⁷⁰ Cartesian coordinates of the DFT-optimized structures are provided as ESI .xyz file.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The financial support of the University of Bologna is gratefully acknowledged. CINECA (Bologna) is acknowledged for the availability of high-performance computer resources (class C project INLIGHT). We thank the referees for very useful suggestions in revising the manuscript.

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Footnote

† Electronic supplementary information (ESI) available: Supplementary experimental and computational figures and tables. Crystal data and collection details (PDF). DFT-optimized coordinates in the XYZ format (.xyz). CCDC 2305339 ([NEt4][3]·2CH2Cl2), 2305340 (4-b·2CH2Cl2), 2305341 (4-a), 2305342 (5·0.5CH2Cl2·solv), and 2305343 (5·solv). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt04282k

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