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
The stepwise addition of increasing amounts of Au(PPh3)Cl to [HRu4(CO)12]3− (1) 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). Alternatively, 4 can be obtained upon addition of HBF4·Et2O (two mole equivalents) to 3. Further addition of acid to 3 (three mole equivalents) results in the formation of the tetra-aurated cluster Ru4(CO)12(AuPPh3)4 (5). Compounds 2–5 have been characterized by IR, 1H and 31P{1H} NMR spectroscopies. Moreover, the molecular structures of 3–5 have been determined by single crystal X-ray diffraction as [NEt4][3]·2CH2Cl2, 4-b·2CH2Cl2, 4-a, 5·0.5CH2Cl2·solv, and 5·solv crystalline solids. Two different isomers of 4, that is 4-a and 4-b, have been crystallographically characterized and their rapid interconversion in solution was studied by variable temperature 1H and 31P{1H} NMR spectroscopies. Weak aurophilic Au⋯Au contacts have been detected in the solid state structures of 3–5. Computational studies have been performed in order to elucidate bonding and isomerism, as well as to predict the possible structure of the elusive species 2.
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.
Fig. 1 IR spectra in the νCO region of 2–4 obtained by stepwise addition of Au(PPh3)Cl to 1. All spectra have been recorded in CH3CN solution at 298 K. |
IRa (cm−1) | 1H NMR (ppm) | 31P{1H} NMR (ppm) | |
---|---|---|---|
a The strongest absorption was used for each νCO value. b From ref. 50 and 51. c From ref. 49 and 50. d From ref. 48. | |||
1 | 1928(s), 1898(vs), 1855(m), 1714(m) | −17.65 (br) | — |
[H 2 Ru 4 (CO) 12 ] 2− b | 2035(w), 1992(s), 1956(vs), 1750(m) | −19.48 | — |
[H 3 Ru 4 (CO) 12 ] −c | 2036(s), 2016(s), 1997(vs), 1975(m) | −17.12 | — |
H 4 Ru 4 (CO) 12 d | 2080(s), 2065(vs), 2021(s) | −17.70 | — |
2 | 2034(m), 1953(m), 1929(s), 1744(m) | −19.51 (d) | 62.14 |
3 | 2033(w), 1986(m), 1967(s), 1749(m) | −13.97 (t) | 63.13 |
4 | 2053(s), 2007(vs), 1989(m), 1953 (w) | −13.01 (br) | 60.38 |
5 | 2082(w), 2072(m), 2029(s), 1970(m) | — | 68.46 (3P), 67.15 (1P) |
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 νCO 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 δH −19.51 ppm with JH–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 δP 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.
Ru–Ru | Ru–H | Ru–Au | Au⋯Au | |
---|---|---|---|---|
a From ref. 47. b As [NEt4][3]·2CH2Cl2 (triclinic, P). c As 4-a (triclinic, P) (isomer 4-a). d As 4-b·2CH2Cl2 (monoclinic, P21/c) (isomer 4-b). e As 5·0.5CH2Cl2·solv (monoclinic, C2/c). f As 5·solv (monoclinic, P21/c). g From ref. 48. h Isomer C2, from ref. 49 and 50. i Isomer C3v, from ref. 49 and 50. j From ref. 49 and 51. | ||||
1 | 2.8001(11)–2.8113(11), average 2.805(3) | 1.77(4)–1.78(4), average 1.78(7) | — | — |
3 | 2.8156(7)–3.0483(6), average 2.9002(15) | 1.73(6)–1.89(6), average 1.82(10) | 2.7808(5)–2.9408(5), average 2.8231(11) | 2.8393(3) |
4-ac | 2.770(2)–3.090(2), average 2.902(5) | 1.80(2) | 2.7637(18)–2.9022(18), average 2.826(5) | 2.9247(11)–2.9515(12), average 2.9381(17) |
4-bd | 2.7887(6)–2.9888(6), average 2.8865(15) | 1.88(3) | 2.7730(5)–3.0487(5), average 2.8689(13) | 2.8288(3)–2.8623(3), average 2.8456(4) |
5 | 2.870(2)–3.0838(16), average 2.977(5) | — | 2.7861(18)–2.9838(14), average 2.860(4) | 2.8180(8)–2.8671(8), average 2.8405(14) |
5 | 2.8899(18)–3.213(2), average 3.018(5) | — | 2.8107(16)–3.1396(18), average 2.893(5) | 2.8531(14)–2.8966(14), average 2.880(2) |
H 4 Ru 4 (CO) 12 g | 2.7839(8)–2.9565(7), average 2.895(2) | — | — | — |
[H 3 Ru 4 (CO) 12 ] −h | 2.7614(5)–2.9423(4), average 2.8504(12) | 1.72(4)–1.80(4), average 1.76(8) | — | — |
[H 3 Ru 4 (CO) 12 ] −i | 2.7733(5)–2.9380(5), average 2.8519(12) | 1.72(4)–1.83(4), average 1.76(9) | ||
[H 2 Ru 4 (CO) 12 ] 2− j | 2.7526(4)–2.9771(4), average 2.8436(10) | 1.64(4)–1.87(4), average 1.76(6) | — | — |
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 C3v], 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 δP 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 δH −13.74 ppm (JH–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 Cs 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.
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.
Scheme 2 Formal transformation of 3 into 4-a and 4-b, the two isomers of 4 (orange, Ru; yellow, Au; white, H). CO and PPh3 ligands have been omitted for clarity. |
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 δH −13.01 ppm and almost complete coalescence is observed at 273 K. A quartet at δH −12.80 ppm with JH–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 δP 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 δP 60.91 (2P) and 51.95 (1P) ppm. At 198 K, the lower frequency resonance is further split into two 1:1 resonances at δP 51.96 and 51.92 ppm, whereas that at a higher frequency remains a singlet at δP 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.
Fig. 10 Two views of the molecular structure of Ru4(CO)12(AuPPh3)4 (5) (orange Ru; yellow, Au; purple, P; red O; grey C; white H). Au⋯C(O) contacts [2.70–3.11 Å] are represented as fragmented lines. |
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 C3v 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.
Fig. 12 Two views of the Ru4Au4 fragment of 5 (orange Ru; Au yellow) with the HOMO plotted (blue tones), limited to the contributions of the Au centres. Surface isovalue = 0.005 a.u. |
The molecular structures of 3–5 differ from the isolobally related hydrides [H3Ru4(CO)12]− and H4Ru4(CO)1248–50 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.
IR (CH2Cl2, 298 K) νCO: 2034(m), 1953(m), 1929(s), 1744(m) cm−1. 1H NMR (CD2Cl2, 298 K) δ: −19.51 (d, JH–P = 2.0 Hz). 31P{1H} NMR (CD2Cl2, 298 K) δ: 62.15 ppm.
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) νCO: 2033(w), 1986(m), 1967(s), 1749(m) cm−1. IR (acetone, 298 K) νCO: 2031(w), 1985(m), 1967(s) cm−1. IR (Nujol, 298 K) νCO: 2029(w), 1983(s), 1961(m), 1941(m), 1908(w) cm−1. 1H NMR (Acetone-d6, 298 K) δ: −13.74 ppm (t, JH–P = 0.9 Hz). 31P{1H} NMR (Acetone-d6, 298 K) δ: 63.41 ppm.
*Sometimes a second isomer crystallized out of the solution as 4-a solvent-free crystals
*Sometimes a second isomer crystallized out of the solution as 4-a solvent-free.
C68H50Au3Cl4O12P3Ru4 (2288.97): calcd (%): C 35.68, H 2.20; found: C 35.41, H 2.37. IR (CH2Cl2, 298 K) νCO: 2053(s), 2007(vs), 1989(m), 1953 (w) cm−1. IR (Nujol, 298 K) νCO: 2048(s), 2004(vs), 1981(m), 1960(w), 1924(w) cm−1. 1H NMR (CD2Cl2, 298 K, 400 MHz) δ: −13.01 (br) ppm. 1H NMR (CD2Cl2, 273 K, 400 MHz) δ: −12.84 (br) ppm. 1H NMR (CD2Cl2, 248 K, 400 MHz) δ: −12.80 (q, JH–P = 5.2 Hz) ppm. 1H NMR (CD2Cl2, 223 K, 400 MHz) δ: −12.72 (br), −12.75 (br) ppm. 31P{1H} NMR (CD2Cl2, 298 K, 400 MHz) δ: 60.37 ppm. 31P{1H} NMR (CD2Cl2, 273 K, 400 MHz) δ: 59.80 (br) ppm. 31P{1H} NMR (CD2Cl2, 248 K, 400 MHz) δ: 62.1 (br), 54.1 (br) ppm. 31P{1H} NMR (CD2Cl2, 223 K, 400 MHz) δ: 61.54 (2P), 52.59 (1P) ppm. 1H NMR (CD2Cl2, 298 K, 600 MHz) δ: −13.00 (br) ppm. 1H NMR (CD2Cl2, 273 K, 600 MHz) δ: −12.91 (br) ppm. 1H NMR (CD2Cl2, 248 K, 600 MHz) δ: −12.77 (br) ppm. 1H NMR (CD2Cl2, 223 K, 600 MHz) δ: −12.74 (br) ppm. 1H NMR (CD2Cl2, 198 K, 600 MHz) δ: −12.70 (s), −12.72 (s) or −12.71 (d, J = 15 Hz) ppm. 1H NMR (CD2Cl2, 173 K, 600 MHz) δ: −12.75 (s), −12.77 (s) or −12.76 (d, J = 15 Hz) ppm. 31P{1H} NMR (CD2Cl2, 298 K, 600 MHz) δ: 60.35 (br) ppm. 31P{1H} NMR (CD2Cl2, 248 K, 600 MHz) δ: 61.93 (br, 2P), 53.26 (br, 1P) ppm. 31P{1H} NMR (CD2Cl2, 223 K, 600 MHz) δ: 60.91 (2P), 51.95 (1P) ppm. 31P{1H} NMR (CD2Cl2, 198 K, 600 MHz) δ: 61.00 (2P), 51.96, 51.92 (1P) ppm.
*Sometimes crystals of 5·solv (Monoclinic, P21/c) were obtained instead of 5·0.5CH2Cl2·solv (monoclinic, C2/c).C84.5H61Au4ClO12P4Ru4 (2619.81): calcd (%): C 38.74, H 2.35; found: C 39.03, H 2.61. IR (CH2Cl2, 298 K) νCO: 2082(w), 2072(m), 2029(s), 1970(m) cm−1. IR (Nujol, 298 K) νCO: 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.
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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