Experimental and Theoretical Study of a Tungsten Dihydride Silyl Complex: New Insight into Its Bonding Nature and Fluxional Behavior

Tungsten dihydride silyl complex Cp*(CO)2W(H)2[SiH(OMe){C(SiMe3)3}] (R1) was synthesized by the reaction of silylene complex Cp*(CO)2(H)WSi(H){C(SiMe3)3} (R2) with MeOH. Complex R1 was characterized by spectroscopic data as well as elemental analysis, and its geometry was supported by X-ray crystallography. Interestingly, the variable-temperature 1H NMR spectroscopy revealed that R1 shows highly fluxional behavior, in which two diastereotopic hydrides rapidly exchange their positions in the NMR time scale. The DFT-optimized geometry of Cp(CO)2W(H)2[SiH(OMe){C(SiH3)3}] (1a), which was employed as a model of R1, indicates the presence of a W−silyl and two W−hydride bonds in addition to two weak Si---H interactions. Based on the analyses of molecular orbitals and electron populations, a theoretical understanding of 1a is presented, as follows: (i) 1a is neither a pure dihydride silyl complex of W(IV) bearing d2 electron configuration nor a pure silicate complex of W(II) bearing d4 electron configuration, (ii) the W center takes a +II oxidation state rather than a +IV oxidation state, (iii) the {(H)2[SiH(OMe){C(SiH3)3}]} moiety has a negative charge like silicate, but its two Si−H interactions are very weak and nonclassical, and (iv) the W center takes eight-coordination structure including one W−Si and two W−H bonds. Also, theoretical investigations reveal that 1a exhibits fluxional behavior via two processes: (1) Up−down position change of the [SiH(OMe){C(SiH3)3}] group with respect to the WHH plane easily occurs with a very small activation barrier of 2.9/3.8 kcal/mol through a new transition state, where the DFT-calculated activation barrier and activation free energy change are given before and after the slash, respectively. The silicate character of the {(H)2[SiH(OMe){C(SiH3)3}]} moiety increases when going from 1a to the transition state, though it is not a pure silicate in both. (2) Two hydrides easily exchange their positions with a moderate activation barrier of 7.6/5.5 kcal/mol through a new transition state, in which neither a silane nor a dihydrogen is involved. The reasons for the moderate activation barrier are discussed in terms of the bonding interaction in detail.