Synthesis, spectroscopic, and x-ray characterization of rhenium carbonyl complexes with different silsesquioxanes, as models that mimic the chemical behavior and the topology of the silica surface

Reactions of [Re2(CO)8(THF)2] with various silsesquioxanes ([(c-C6H11)7Si8O12(OH)] (1), [(c-C6H11)8Si8O11(OH)2] (2), and endo-[(c-C6H11)8Si8O11(OH)2] (3)), which mimic different kinds of silanol sites of the silica surface, afford new molecular models of the silica-anchored [Re2(CO)2(μ-H)(μ-OSi=)] surface species, which is proposed as the first step of the thermal interaction of [Re2(CO)10] with silica. In particular, reaction of [Re2(CO)8(THF)2] with 1, a completely condensed silsesquioxane bearing one silanol group in one vertex instead of the cyclohexyl substituent, gives [Re2(CO)8(μ-H)(μ-O)]O12Si8(c-C6H11)7 (6), model of the surface [Re2(CO)8(μ-H)(μ-OSi=)] species anchored to an isolated silanol. Reaction of 2, bearing two silanol groups sufficiently far from each other to react independently, with an equimolar amount of [Re2(CO)8(THF)2] gives a mixture of the disubstituted [Re2(CO)8(μ-H)(μ-O)]2O11Si8(c-C6H11)8 (7) and monosubstituted [Re2(CO)8(μ-H) (μ-O)](OH)O11Si8(c-C6H11)8 (8) silsesquioxanes. An excess of [Re2(CO)2(THF)2] is necessary to allow the complete conversion of 8 into 7; 8 can be obtained in a pure form by hydrolysis of 7. In addition, silsesquioxane 3, with two pseudovicinal silanol groups in endo positions, reacts with an equimolar amount of [Re2(CO)8(THF)2] to give the monosubstituted silsesquioxane [Re2(CO)8(μ-H)(μ-O)](OH)O11Si8(c-C6H11)8 (9) along with the new and unexpected species [Re2(CO)7(μ-H)(μ-O)O(H)] O11Si8(c-C6H11)8 (10). The structure of 10 suggests that the close arrangement of the two vicinal silanols of 3 can induce further interaction of the free silanol of complex 9 with its Re2(CO)2core, replacing one carbonyl ligand; it appears thus in the case of silsesquioxane 3, with the endo arrangement of the silanol groups, that the formation of the disubstituted derivative is inhibited even in the presence of an excess of [Re2(CO)8(THF)2]. The crystal structures of 6 and 7 and partially of 10, obtained by X-ray diffraction, are reported and discussed. © 2009 American Chemical Society.