Isomerism of Aurated Phosphine Sulfides, Thiophosphinates, Thiophosphonates, and Thiophosphates: Structural and Quantum Chemical Studies

Abstract

The reaction of disodium phenyltrithiophosphonate with (triorganophosphine)gold(I) chlorides in the molar ratio 1:2 in dichloromethane gives high yields of the corresponding dinuclear complexes PhP(S)[SAu(PR3)]2 with R = Ph (1), Me, Et, i-Pr, and o-Tol. The crystal structure of complex 1 has been determined by single crystal X-ray methods. The two gold atoms are bound to separate sulfur atoms, leaving only one sulfur atom uncoordinated. The compound is stabilized by an intramolecular Au···Au contact. This result shows that individual auration of sulfur atoms is energetically preferred over double auration of only one sulfur atom. An analogous product is obtained from trisodium tetrathiophosphate hydrate and (Ph3P)AuCl in CH2Cl2/MeOH: The reaction is accompanied by partial methanolysis to give MeOP(S)[SAu(PPh3)]2. Again each gold atom is bound to a different sulfur atom. In the crystal structure of this compound, intermolecular Au···Au contacts lead to one-dimensional aggregates. All attempts to aurate the above products further with excess (R3P)AuCl or {[(R3P)Au]3O}+BF4- reagents were unsuccessful. P−S cleavage occurred instead to give tris[(triorganophosphine)gold)]sulfonium salts, {[(R3P)Au]3S}+X-. The origin of this P−S cleavage has been traced by ab initio quantum chemical calculations of the mono-, bis-, and trisauration of the model compound H3PS (phosphine sulfide) with [(H3P)Au]+ units. Only the first step was found to be strongly exothermic (−74.5 kcal/mol), and to lead to a significant lengthening of the P−S bond. The second step has an energy balance of only −1.1 kcal/mol, but induces a further lengthening of the P−S bond, while the third step is strongly endothermic (+67.3 kcal/mol). Bond rupture probably sets in at the second auration step. Quantum chemical calculations have also confirmed the experimental finding (for 1) that auration of the dithiophosphinate anion [H2PS2]- with [(H3P)Au]+ gives the cation [H2P(SAuPH3)2]+ (with the gold atoms bound to different sulfur atoms), the structure of which is preferred over the isomer [H2P(S)S(AuPH3)2]+ (with both gold atoms bound to the same sulfur atom).