Central Gold Atom Research Articles

A 35Cl NQR Study on Cs2[AUICl2] [AuIIICl4

Abstract A pair of 35Cl NQR spin echo signals has been observed for the mixed valence complex Cs2 [AuICl2] [AuIIICl4] between 77 and 243 K. At 77 K, two resonance lines with the half widths ΔvQ ~ 50 kHz were located at vQ1 = 17.28 MHz for the AuI-Cl chlorine and at vQ2 = 27.10 MHz for the AuIII -Cl chlorine in accordance with the crystal structure. The chlorine ionic characters of the AuI-Cl and AuIII-Cl bonds are estimated as 0.63 and 0.42, respectively. The central gold atom carries a fractional protonic charge of 0.26 in [AuICl2]- and 0.68 in [AuIIICl4]-. The charge distributions in the complex anions differ insignificantly from those in the isolated [AuCl2]- and [AuCl4]- for ordinary complexes, indicating that the charge transfer interactions between the anions are weak in the mixed valence complex. The observed linear temperature dependencies of VQ and log TlQ are well explained by the lattice vibration. When the temperature was increased from 77 K, the resonance lines became gradually weak without changing ΔvQ and immeasurable above 215 K. ESR spectra taken at various temperatures revealed the presence of paramagnetic sites of ca. 5 x 1020 mol- 1 arising from Au(II). The small but finite concentration of Au(II) or some other reason should be responsible for the fade out phenomenon and the large ΔvQ observed.

Aurophilicity at Sulfur Centers. Synthesis and Reactivity of the Complex [S(Au2dppf)]; Formation of Polynuclear Sulfur-Centered Complexes. Crystal Structures of [S(Au2dppf)]·2CHCl3, [(μ-Au2dppf){S(Au2dppf)}2](OTf)2· 8CHCl3, and [S(AuPPh2Me)2(Au2dppf)](ClO4)2·3CH2Cl2

Treatment of [Au2Cl2(μ-dppf)] (dppf = 1,1‘-bis(diphenylphosphino)ferrocene) with Li2S (molar ratio 1:1) in ethanol gives a yellow solid [S(Au2dppf)] (1). The sulfur atom in complex 1 can coordinate several neutral or cationic gold(I) fragments giving the trinuclear neutral [S{Au(C6F5)}(Au2dppf)] (2) or cationic derivatives [S(AuL)(Au2dppf)]ClO4 [L = CH2PPh3 (3), PPh3 (4), PPh2Me (5)]. A pentanuclear complex [Au{S(Au2dppf)2}]ClO4 (6) is obtained by reaction of 2 equiv of complex 1 with 1 equiv of [Au(tht)2]ClO4, in which the central gold atom is bonded to two sulfur atoms. Treatment of compound 1 with [Au2(OTf)2(μ-dppf)] (OTf = trifluoromethylsulfonate) in a molar ratio 2:1 affords the hexanuclear complex [(μ-Au2dppf){S(Au2dppf)}2](OTf)2 (7), in which two SAu3 units are joined through the dppf ligand. The reaction of 1 with 2 equiv of [Au(OClO3)PR3] leads to the quadruply bridging derivatives [S(AuPR3)2(Au2dppf)](ClO4)2 [PR3 = PPh3 (8), PPh2Me (9)]. Crystal structure determinations were performed for complexes 1, 7, and 9. [S(Au2dppf)] (1) crystallizes in the monoclinic space group P2/n, with a = 12.571(4) Å, b = 10.579(4) Å, c = 15.212(4) Å, β = 107.84(3)°, Z = 2, T = −130 °C. [(μ-Au2dppf){S(Au2dppf)}2](OTf)2 (7) is triclinic, P1̄, with a = 15.177(3), b = 18.408(4), c = 27.894(8) Å, α = 88.83(2), β = 84.46(2), γ = 67.78(2), Z = 2, T = −100 °C. [S(AuPPh2Me)2(Au2dppf)](ClO4)2 (9) is triclinic, P1̄, with a = 13.727(2) Å, b = 15.952(3) Å, c = 17.763(2) Å, α = 71.738(12)°, β = 73.264(14)°, γ = 75.96(2)°, Z = 2, T = −100 °C. All three complexes display short gold(I)−gold(I) interactions.

Auriophilicity in Polynuclear (Phosphane)gold(I) Thiolates {Me 3 CS[Au(PPh 3 )] 2 } + BF ‐ 4 and {(CH 2 S) 2 [Au(PPh 3 )] 3 } + BF ‐ 4

The reaction of tert-butanethiol with tris[(triphenylphosphane)gold(I)]oxonium tetrafluoroborate in dichloromethane in the presence of sodium tetrafluoroborate gave quantitative yields of a product {Me3CS[Au(PPh3)]2}+ BF-4 (1). In the crystal lattice of this salt, the sulfonium cations are associated into dimers through close Au…Au contacts. The centrosym-metrical tetranuclear unit has Au…Au distances of 3.292(1) and 3.206(1) A, and the Au-S-Au angles are exactly 90.0(1)°. Both data are indicative of significant intra- and intercationic Au…Au attractions. The analogous reaction of ethane-1, 2–dithiol afforded high yields of a product {(CH2S)2[Au(PPh3)]3}+ BF-4 (2). The compound is fluxional in CD2Cl2 solution and shows only one 31P-NMR signal at -60°C. In the crystal, the quasi C2-symmetrical trinuclear cations feature the sulfur atoms in bridging positions between two gold atoms. The central gold atom is thus three-coordinate; the peripheral gold atoms are two-coordinate with Au…Au contacts at 3.284(1) and 3.129(1) A and Au-S-Au angles as small as 86.5(1) and 81.0(1)°. This provides clear evidence for “auriophilic” bonding.

Mechanism of Sulfur Inversion in Sulfide Complexes of Gold(I) and Gold(III)

Abstract Kinetic parameters for the sulfur inversion in various gold chloride complexes of alkyl benzyl sulfides were measured by the dynamic NMR method in order to obtain conclusive evidence for the inversion mechanism. A feature of kinetic data is that entropies of activation are nearly zero both in the gold(III) and gold(I) complexes, this suggesting that the sulfur inversion takes place via pyramidal inversion mechanism without breakage of the Au–S coordination bonds but not the dissociative mechanism. The negligible solvent effect on the inversion rates in the gold(III) and gold(I) complexes supports this mechanism as well. The barrier to sulfur inversion is higher by 2.9 kcal mol−1 in the gold(III) complexes than in the corresponding gold(I) complexes. The formal gold(II) complex exists as 1 : 1 mixture of gold(I) and gold(III) complexes in solution, each of them showing the dynamic behavior independently. The effect of oxidation state of the central gold atom on the sulfur inversion is discussed on the basis of MO calculation.

Liquid-crystalline (lsonitrile)gold(I) acetylide complexes

The mesomorphic properties of three types of gold(i) acetylide complexes, [(ArNC)Au-CCAr, (ArNC)Au-CCR and (RNC)Au-CCAr, where R and Ar represent alkyl and aryl groups, respectively] were systematically examined and it was found that of the three, (RNC)Au-CCAr (complex C) is the choice for liquid-crystalline materials, and that introduction of an appropriate lateral group on the isonitrile and/or acetylene ligands led to the formation of nematic and smectic A phases without any decomposition of the complexes. The linear geometry around the central gold atom was confirmed by X-ray crystallographic analysis of complex C-5b. The relation between the thermal behaviour and the molecular structure of the complexes is discussed.

Synthesis and microcrystal structure determination of [Au10(PPh3)7{S2C2(CN)2}2] with monochromatic synchrotron radiation

The structure of this high nuclearity gold cluster has been determined from a single microcrystal of dimensions 10 × 10 × 30 µm (the X-ray diffraction data were recorded with synchrotron radiation using an Enraf-Nonius area detector diffractometer at Daresbury Laboratory); in the cluster three trigonal bipyramidal Au5 groups share a common central gold atom and share edges; the group often gold atoms has symmetry near to D3h, but when the seven triphenyl phosphine and two dithiolene ligands are taken into account the molecular symmetry is very approximately C2.

Electron-rich metal centres (Au, Pt) as sources of organometallic complexes with unusual features

The central gold(I) atoms in binuclear complexes of the type[AuR 2] + (R = C 6F 5, C 6Cl 5 CH 2PR 3) are able to donate electron density to silver(I) cations to form polymeric or oligomeric compounds containing the cyclic Au 2Ag 2 moiety, with direct AuAg bonds unsupported by any other bridging ligand. One of the gold atoms in binuclear (μ,μ′-bis(diphenylphosphinium)bis(methylido)digold(I) also donates electron density to the gold(III) atom in Au(C 6F 5) 3 to form the first unbridged direct Au IAu III bond. Anionic Pt II complexes [PtR 4] 2−, [PT 2(μ-X) 2R 4 2−, [Pt 2(μ-R 2) 2R 4] 2−, etc. (R = C 6F 5) react with silver salts or complexes to form a variety of unusual heteronuclear derivatives, most of which display o-X…Ag short contacts, which seem tobe responsible for the stability of the clusters. The homoleptic anion [Pt 2(μ-C 6F 5) 2(C 6F 5) 4] − containing Pt in the average oxidation state of +2.5 can also be prepared by oxidation of the electron-rich binuclear Pt II precursor.

Gold-197 Mössbauer spectra and the bonding of some gold–gold and gold–platinum clusters

The 197Au Mossbauer parameters of the AuPt3, Au2Pt2, and Au2Pt3 clusters [AuPt3(µ-CO)3L4]+, [Au2Pt2L4(CNR)4]2+, [AuPt3(µ-SO2)2(µ-Cl)L4] and [Au2Pt3(µ-SO2)2(µ-Cl)L4][L = P(C6H11)3 or P(C6H4F-p)3, R = xylyl] are consistent with the bonding schemes previously proposed, showing a diminution with increasing delocalisation of the bonding electrons and increase in the metal-metal bond lengths. Separate signals are resolved for the non-equivalent gold atoms in [Au6{P(C6H11)Ph2}6]2+, the parameters and intensities of which correlate with the expected electron densities and observed connectivities of the gold atoms. The centred-crown cluster [Au9L8]3+[L = P(C6H4OMe-p)3] gives slightly greater parameters than its icosahedral-fragment analogues, consistently with the observed shortening in Au–Au contacts. No separate signal is resolved for the central gold atom.

Acid induced oligomerization of aurocyanide adsorbed on carbon

Aspects of the adsorption of aurocyanide anions Au(CN) 2 − onto an activated carbon from alkaline media and the effects of subsequent acid reaction have been examined using X-ray photoelectron spectroscopy (XPS). The results indicate that the linear Au(CN) 2 − anion adsorbed intact on and parallel to the graphitic planes of the carbon, with the graphitic π-electrons taking part in a donor bond to the central gold atom, stabilized by charge transfer to the terminal nitrogen atoms. This bonding mechanism was maintained after acid reaction induced oligomerization of the adsorbed Au(CN) 2 − producing the tetramer Au 4(CN) 5 − which attached to the graphitic plane via four identical π-donor bonds. These additional bonds necessitated the terminal nitrogen atoms to accept even further charge transfer so as to accommodate the larger carbon-anion complex.

Synthesis and structural characterisation of a novel high-nuclearity gold–tin cluster compound, [Au8(PPh3)7(SnCl3)]2[SnCl6

The compound [Au8(PPh3)7(SnCl3)]2[SnCl6] has been synthesised from either [Au8(PPh3)7][NO3]2 or [Au9(PPh3)8][NO3]3 and excess of SnCl2·2H2O in acetone solution. Alternatively it can be synthesised in high yield from [Au8(PPh3)8][NO3]2 and NEt4SnCl3 in acetone. The compound has been characterised by 31P-{1H} n.m.r. and 119Sn Mossbauer spectroscopy and a single-crystal X-ray crystallographic determination. It crystallises in the monoclinic space group C2/c with a= 48.230(14), b= 16.831(7), c= 32.240(10)A, and β= 96.50(3)°. The geometry of the gold atoms in the cluster closely resembles that observed previously in [Au8(PPh3)7]2+ and corresponds to a fragment of a centred icosahedron. The SnCl3 ligand is co-ordinated only to the central gold atom, with Au–Sn 2.625(3)A, and does not interact significantly with the surface gold atoms to complete a hemispherical polyhedron. The distances from the central gold atom to those gold atoms on the surface lie in the range 2.633(2)–2.679(2)A and are shorter than the distances between the surface gold atoms [2.847(2)–2.965(3)A]. The relationship between the structure of [Au8(PPh3)7(SnCl3)]+ and those of related high-nuclearity gold cluster compounds with toroidal and spherical topologies has been explored using molecular-orbital calculations.

The crystal and molecular structure of tetrabutylammonium tetraphenylaurate(III)

An X-ray structure determination of tetrabutylammonium tetraphenylaurate(III) has shown that the tetraphenylaurate(III) anion consists of four phenyl rings σ-bonded to the central gold atom in a square-planar arrangement. The phenyl groups trans to each other are in the same plane. The crystals are monoclinic, space group, I2/ c, a 17.15(3), b 10.31(2), c 22.76(5) Å, β 115.4(2)°, V 3635.2 Å 3, Z = 4, ϱ c 1.37 g cm −3, μ(Cu- K α) 79.12 cm −1, R = 0.0957.

Some theoretical and structural aspects of gold cluster chemistry

The bonding in tertiary phosphine cluster compounds of gold is sufficiently straightforward to permit an effective interaction between theoretical concepts developed from semi-empirical molecular orbital calculations and synthetic and structural chemistry. At the simplest conceptual level the isolobal nature of the Au(PR 3 ) fragment and either the CH 3 or H radicals provides a basis for understanding the structures of a wide range of homonuclear and heteronuclear clusters, e.g. Os 3 (CO) 10 - H(AuPPh 3 ) and (OG) 5 VAu 3 (PPh 3 ) 3 . However, this simplified approach neglects some secondary gold-gold interactions between adjacent gold atoms, which arise from the availability of the higher-lying gold 6p orbitals. In low-nuclearity clusters tetrahedral fragments, which permit the effective formation of four-centre two electron bonds between the Au(PR 3 ) fragments, are preferred to larger deltahedra. In higher-nuclearity clusters the stabilities of the clusters depend on the presence of a central gold atom that provides strong radial gold-gold bonding. The relative importance of the radial and tangential components to the total bonding has been effectively demonstrated by a structural comparison of alternative Au 9 (PR 3 )3/8+ clusters. The predictive capability of the theoretical approach has been demonstrated by the synthesis and structural characterization of the icosahedral cluster [Au 13 Cl 2 (PMe 2 Ph) 10 ]3+.

Mössbauer investigation of gold cluster compounds

AbstractThe Mössbauer spectra of Au11L7X3, [Au11 L8X2]PF6, [Au11(dppp)5](SCN)3, [Au8L8](PF6)2 and [Au9L8](PF6)3 (L = PPh3, X = SCN, Cl or CN, dppp = 1,3‐bis(diphenyl‐phosphino)propane) are presented and interpreted on the basis of three different gold sites, namely: (1) a central gold atom, (2) peripheral gold atoms coordinated to phosphines and (3) peripheral gold atoms coordinated to (pseudo)halides. The peripheral gold atoms in all the clusters behave like linearly hybridized atoms. The zero quadrupole splitting of the central gold atom reflects the almost spherical environment of this atom in the clusters, with [Au9L8]3+ being a remarkable exception.

A new class of gold cluster compounds. Synthesis and X-ray structure of the octakis(triphenylphosphinegold) dializarinsulphonate, [Au8(PPh3)8](aliz)2

[Au9L8]3+(L = PPh3) reacts with L to give the new dication [Au8L8]2+; an X-ray analysis of [Au8(PPh3)8](aliz)2(aliz = alizarinsulphonate) shows that the central gold atom, while still displaying the usual short interactions with all the peripheral metal atoms, is no longer confined in an entirely metallic cage, but is also directly bonded to a phosphine ligand.

Nuclear quadrupole resonance of halogens in tetrahaloaurates(III)

The nuclear quadrupole resonance of halogens in a variety of anhydrous and hydrated tetrahaloaurates(III) was observed between 77 and 400°K. From the number of resonance lines, the number of different kinds of crystallographically nonequivalent halogen atoms was determined. The ionic character of AuCl and AuBr bonds is estimated as 41% and 34%, respectively. The central gold atom in the tetracoordinated complex ion carries a fraction of protonic charge (0.64ϵ in [AuCI 4] − and 0.36ϵ in [AuBr 4] −, respectively). The positive temperature coefficient of resonance frequencies observed for some tetrahaloaurates(III) is attributable to hydrogen bond formation. The anomalous temperature dependence of bromine resonance frequency of cesium tetrabromoaurate(III) is presumed to be due to the torsional oscillation of tetrabromoaurate(III) ions in crystals involving a considerable anharmonicity.