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

Reaction of [Re2(CO)8(THF)2] with various silsesquioxanes ([(c-C6H11)7Si8O12(OH)] (1), [(c-C6H11)8Si8O11(OH)2] (2), endo-[(c-C6H11)8Si8O11(OH)2] (3)), which mimic the different kinds of silanol sites of the silica surface, affords new molecular models of silica-anchored [Re2(CO)8(μ-H)(μ-OSi≡)] species, proposed as product by thermal interaction of [Re2(CO)10] with silica. Thus, reaction of [Re2(CO)8(THF)2] with (1), a silsesquioxane having only one silanol group, gives [Re2(CO)8(μ-H)(μ-O)O12Si8(c-C6H11)7] (6), model of a surface [Re2(CO)8(μ-H)(μ-OSi≡)] specie anchored to an isolated silanol. Reaction of (2), silsesquioxane with two silanol groups far from each other, and one equimolar amount of [Re2(CO)8(THF)2] gives a mixture of the di-substituted {[Re2(CO)8(μ-H)]2(μ-O)O11Si8(c-C6H11)8]} (7) and mono-substituted {[Re2(CO)8(μ-H)[(μ-O)(OH)O11Si8(c-C6H11)8]} (8) silsesquioxanes; the use of an excess of [Re2(CO)8(THF)2] allows the complete conversion of (8) into (7). Besides, the silsesquioxane (3), with two vicinal silanol groups, reacts with one equimolar amount of [Re2(CO)8(THF)2] to give the monosubstituted silsesquioxane {Re2(CO)8(μ-H)[endo-(μ-O)(OH)O11Si8(c-C6H11)8]} (9) along with the new and unexpected complex {[Re2(CO)7(μ-H)(μ-O)O(H)]O11Si8(c-C6H11)8]} (10). The structure of (10) is of particular interest, it suggests that the close arrangement of the two vicinal silanols of (3) can easily induce further interaction of the free silanol of complex (9) with the Re2(CO)8 moiety, labilizing one carbonyl ligand. With silsesquioxane (3) the formation of the di-substituted derivative is inhibited even in the presence of excess of [Re2(CO)8(THF)2]. The crystal structure of (6), (7) and (10) was obtained by X-ray diffraction.