Redistribution at silicon by ruthenium complexes. Bonding mode of the bridging silanes in Ru2H4(μ-η2:η2:η2:η2-SiH4)(PCy3)4 and Ru2H2(μ-η2:η2-H2Si(OMe)2)3(PCy3)2
The bis(dihydrogen) complex RuH2(η2-H2)2(PCy3)2 (1) reacts with 2 equiv. of H2SiMePh to produce a mixture of Ru2H4(μ-η2:η2:η2:η2-SiH4)(PCy3)4 (2) and RuH2(η2-H2)(η2-HSiPh3)(PCy3)2 (4) together with HSiMePh2, HSiMe2Ph and traces of HMe2SiSiMe2H as a result of redistribution at silicon. The bridging SiH4 ligand in 2 is coordinated to the two ruthenium via four σ-Si–H bonds in agreement with NMR, X-ray data (on 2, and 2′ the analogous PiPr3 complex) and DFT calculations. Each interaction involves σ-donation to a ruthenium and back-bonding from the other ruthenium. Elimination of SiH4 and formation of RuH2(CO)2(PCy3)2 (5), RuH2(tBuNC)2(PCy3)2 (6) or RuH(η2-H2)Cl(PCy3)2 (7) were observed upon the reaction of 2 with CO, tBuNC, CH2Cl2, respectively. No reaction occurred in the presence of H2, but H/D exchange was observed under D2 atmosphere. Another redistribution reaction at silicon can be obtained by adding 4 equiv. of HSi(OMe)3 to 2 to produce Si(OMe)4 and Ru2H2(μ-η2:η2-H2Si(OMe)2)3(PCy3)2 (3) displaying three bridging (μ-η2:η2 alkoxysilane) ligands. Complex 3 is characterized by multinuclear NMR spectroscopies and by a crystal structure. DFT calculations show that the model complex Ru2H2(μ-η2:η2-H2Si(OR)2)3(PR3)2 (R = H, Me) is a minimum on the potential energy surface, and support the dihydride formulation with three bridging H2Si(OMe)2 ligands coordinated to the two ruthenium through σ-Si–H bonds.