Sulfur-Centered Mechanism in Catalytic Methanolysis of Hydrosilanes Mediated by Air-Stable Mo3S4 Clusters.

Methanolysis of hydrosilanes is catalyzed by incomplete cubane-type Mo3(μ3-S)(μ-S)3 clusters functionalized with diamino and imidazolyl amino ligands under mild conditions. Silane activation mediated by the air-stable [Mo3(μ3-S)(μ-S)3Cl3(ImNH2)3]Cl (ImNH2 = (1-methyl-1H-imidazol-2-yl)methanamine) ([3]Cl) cluster salt has been elucidated through a comprehensive experimental and theoretical study. Our results support the operation of a sulfur-centered mechanism without direct participation of the metals in clear contrast with all previously reported mechanisms catalyzed by transition metal complexes. The reaction proceeds in two steps, with the first one being the rate-determining step. The process starts with the hydrosilane Si-H bond activation, which occurs at one of the bridging sulfur atoms of the Mo3(μ3-S)(μ-S)3 cluster unit. This step takes place through a concerted and asynchronous transition state with the participation of one methanol molecule to yield the silyl ether product and a bis(hydrosulfido) intermediate. Analysis of this transition state reveals that its imaginary frequency is basically associated with the silane hydride transfer and the formation of the Si-O bond in agreement with the observed KIE results. The second step consists in the hydrogen release from the bis(hydrosulfido) intermediate, from which the cluster catalyst is recovered. The same mechanism operates for the diamino [Mo3S4Cl3(en)3]Cl (en = ethylenediamine) ([1]Cl) and [Mo3S4Cl3(dmen)3]+ (dmen = N,N'-dimethylethylenediamine) ([2]Cl) cluster salts. The calculated free energy barriers for those cluster catalysts agree with the observed catalytic activities, giving further support to our mechanistic proposal.