Chemistry Department, Brock University, 500 Glenridge Avenue, St. Catharines, ON, L2S 3A1, Canada.
Chemistry. 2013 Jun 24;19(26):8573-90. doi: 10.1002/chem.201300376. Epub 2013 May 13.
The reactions of bis(borohydride) complexes [(RN=)Mo(BH4)2(PMe3)2] (4: R = 2,6-Me2C6H3; 5: R = 2,6-iPr2C6H3) with hydrosilanes afford new silyl hydride derivatives [(RN=)Mo(H)(SiR'3)(PMe3)3] (3: R = Ar, R'3 = H2Ph; 8: R = Ar', R'3 = H2Ph; 9: R = Ar, R'3 = (OEt)3; 10: R = Ar, R'3 = HMePh). These compounds can also be conveniently prepared by reacting [(RN=)Mo(H)(Cl)(PMe3)3] with one equivalent of LiBH4 in the presence of a silane. Complex 3 undergoes intramolecular and intermolecular phosphine exchange, as well as exchange between the silyl ligand and the free silane. Kinetic and DFT studies show that the intermolecular phosphine exchange occurs through the predissociation of a PMe3 group, which, surprisingly, is facilitated by the silane. The intramolecular exchange proceeds through a new non-Bailar-twist pathway. The silyl/silane exchange proceeds through an unusual Mo(VI) intermediate, [(ArN=)Mo(H)2(SiH2Ph)2(PMe3)2] (19). Complex 3 was found to be the catalyst of a variety of hydrosilylation reactions of carbonyl compounds (aldehydes and ketones) and nitriles, as well as of silane alcoholysis. Stoichiometric mechanistic studies of the hydrosilylation of acetone, supported by DFT calculations, suggest the operation of an unexpected mechanism, in that the silyl ligand of compound 3 plays an unusual role as a spectator ligand. The addition of acetone to compound 3 leads to the formation of [trans-(ArN)Mo(OiPr)(SiH2Ph)(PMe3)2] (18). This latter species does not undergo the elimination of a Si-O group (which corresponds to the conventional Ojima's mechanism of hydrosilylation). Rather, complex 18 undergoes unusual reversible β-CH activation of the isopropoxy ligand. In the hydrosilylation of benzaldehyde, the reaction proceeds through the formation of a new intermediate bis(benzaldehyde) adduct, [(ArN=)Mo(η(2)-PhC(O)H)2(PMe3)], which reacts further with hydrosilane through a η(1)-silane complex, as studied by DFT calculations.
双（硼氢化物）配合物[(RN=)Mo(BH4)2(PMe3)2]（4：R=2,6-Me2C6H3；5：R=2,6-iPr2C6H3）与硅烷的反应生成新的硅烷氢化物衍生物[(RN=)Mo(H)(SiR'3)(PMe3)3]（3：R=Ar，R'3=H2Ph；8：R=Ar'，R'3=H2Ph；9：R=Ar，R'3=(OEt)3；10：R=Ar，R'3=HMePh）。这些化合物也可以通过在硅烷存在下，用[（RN=）Mo（H）（Cl）（PMe3）3]与一当量的 LiBH4 反应方便地制备。化合物 3 经历了分子内和分子间膦交换，以及硅烷配体与游离硅烷之间的交换。动力学和 DFT 研究表明，分子间膦交换是通过 PMe3 基团的预解离发生的，令人惊讶的是，硅烷促进了这种预解离。分子内交换通过一种新的非 Bailar-扭曲途径进行。硅烷/硅烷交换通过一种不寻常的 Mo(VI)中间体进行，[(ArN=)Mo(H)2(SiH2Ph)2(PMe3)2]（19）。研究发现，3 是多种羰基化合物（醛和酮）和腈的硅氢化反应以及硅烷醇解反应的催化剂。DFT 计算支持的酮的硅氢化的化学计量学机理研究表明，该反应采用了一种意想不到的机制，即化合物 3 的硅烷配体作为一种旁观配体发挥了不寻常的作用。丙酮与化合物 3 的加成导致[反式-(ArN)Mo(OiPr)(SiH2Ph)(PMe3)2]（18）的形成。后一种物质不会经历 Si-O 基团的消除（这对应于传统 Ojima 的硅氢化反应机制）。相反，18 经历了异丙氧基配体的不寻常的可逆β-CH 活化。在苯甲醛的硅氢化反应中，反应通过形成新的二苯甲醛加合物[(ArN=)Mo(η(2)-PhC(O)H)2(PMe3)]进行，该加合物通过 DFT 计算进一步与硅烷通过η(1)-硅烷配合物反应。
