Reactivity Patterns of O2, CO2, Carboxylic Acids, and Triflic Acid with Molybdenum Silyl Hydrido Complexes Bearing Polydentate Phosphinoalkyl-Silyl Ligands: Pronounced Effects of Silyl Ligands on Reactions

The reactivity patterns of a series of molybdenum silyl hydrido complexes, [MoH3{[Ph2PCH2CH2P(Ph)-C6H4-o](2)(R)Si-P,P,P,P,Si}] (2a, R = Ph; 2b, R = C6F5; 2c, R = 4-Me2NC6H4; 2d, R = cyclohexyl; 2e, R = n-C6H13), toward electrophilic materials, including dioxygen, carbon dioxide, carboxylic acids, and triflic acid, have been explored. The influence of substituents directly attached to the Si atom in the quintuply chelating diphosphine-silyl ligands during the course of the reactions has been studied in detail. Complexes 2a-c react readily with dioxygen at ambient temperature, yielding eta(2)-O-2 complexes 3a-c, in which the dioxygen ligand ligates at the site opposite to the Si atom. Single-crystal X-ray studies have established that the Mo-O distance of 3a, which contains a phenyl-substituted Si fragment, is significantly longer than that found in common eta(2)-O-2 complexes, in which the sizes of the central atoms are nearly equal to that of Mo as a consequence of the strong trans influence of the silyl ligand. In contrast, the replacement of the phenyl group on a Si fragment by an electron-withdrawing perfluoro phenyl group, as in 3b, gives rise to a dramatic decrease in the trans influence. Treatment of 2a-c with 2 equiv of carboxylic acids, such as formic acid, acetic acid, and benzoic acid, readily leads to the formation of carboxylato molybdenum complexes, in which the carboxylate ligand coordinates to the metal in a monodentate mode. The resulting formato complex 4a1 can also be derived from the insertion of CO2 into the Mo-H bond of 2a. In addition, complexes 2a,c,e mediate the catalytic carbon dioxide hydrogenation. The activity of the catalysts depends dramatically on the electronic effects of the substituents attached to the Si atom; electron-donating substituents lead to an increase in the catalytic activity. Reactions of complexes 2a,d with triflic acid yield highly labile triflato complexes, which can be further treated with aqueous NaOH to give hydroxo complexes 5a,d, respectively. The molecular structure of 5d has been determined by single-crystal X-ray diffraction. The reactivities of 2a-e are discussed in comparison with those of the parent complex [MoH4(dppe)(2)] (1; dppe = Ph2PCH2CH2PPh2).