On the Synthesis and Chemical Behaviour of the Elusive Bis(hydrosulfido)-Bridged Dinuclear Rhodium(I) Complexes [{Rh(μ-SH)(CO)(PR3)}2]

Abstract

Several bis(hydrosulfido)-bridged dinuclear rhodium(I) compounds, [{Rh(μ-SH)(L)2}2], have been prepared from rhodium(I) acetylacetonato complexes, [Rh(acac)(L)2], and H2S(g). Reaction of [Rh(acac){P(OPh)3}2] with H2S(g) affords the dinuclear bis(hydrosulfido)-bridged compound [{Rh(μ-SH){P(OPh)3}2}2] (1). However, reaction of complexes [Rh(acac)(CO)(PR3)] with H2S(g) gives the dinuclear compound [{Rh(μ-SH)(CO)(PR3)}2] (R=Cy, 2; R=Ph, 4) and the trinuclear cluster [Rh3(μ-H)(μ3-S)2(CO)3(PR3)3] (R=Cy, 3; R=Ph, 5). The selective synthesis of both type of compounds has been carried out by control of the H2S(g) concentration in the reaction media. The trinuclear hydrido–sulfido cluster [Rh3(μ-H)(μ3-S)2(CO)3(PPh3)3] (5) has been also obtained by reaction of [{Rh(μ-SH)(CO)(PPh3)}2] (4) with [Rh(acac)(CO)(PPh3)], proceeding through the trinuclear hydrosulfido–sulfido intermediate [Rh3(μ3-SH)(μ3-S)(CO)3(PPh3)3]. The molecular structures of complexes 1 and 3 have been determined by X-ray diffraction methods. Compound 1 is stable in solution, but complexes 2 and 4 slowly transform in solution into the trinuclear hydrido–sulfido clusters 3 and 5, respectively, with the release of H2S(g) in a reversible way. 1H NMR kinetic experiments for the transformation of 4 into 5 have revealed that this transformation follows second-order-type kinetic. The following activation parameters, ΔH≠=24±3 kJ mol−1 and of ΔS≠=−223±8 J K−1 mol−1, have been calculated from the determination of the second-order rate constants in the temperature range 30–45 °C. The large negative value of the activation entropy is consistent with an associative character of the rate-determining step. A plausible multistep mechanism based on the chemical behaviour of hydrosulfido–metal complexes and compatible with the kinetic behaviour has been proposed.