Dynamic behavior of hydrogen in transition metal bis(silyl) hydride complexes

Abstract

A series of rhodium complexes CpRh(SiMe2X)2(SiMe 3)(H) (X = Me, Cl, Br, I), Cp1-Cp4, CpRh(SiMe2X) 2(PMe3)(H)+ (X = Me, Cl, Br, I), Cp5-Cp8, CpRh(SiMe3)2(SiF3)(H), Cp9, CpRh(SiMe 3)2(SiH3)(H), Cp10, TpRh(SiH3) 2(SiMe3)(H), Tp1, TpRh(SiH3) 2(PMe3)(H)+, Tp2, and TpRh(SiF 3)2(PMe3)(H)+, Tp3, were studied computationally to understand the hydrogen behavior in the Si·· ·H···Si moiety. The hydride ligand interacts with at least one of the silyls, and in many cases with both, but is located asymmetrically with regard to them, giving rise to a double-well potential energy surface (PES) for hydrogen motion. The hydrogen transfer barriers ΔE vary from 0.03 to 3 kcal·mol-1. For selected complexes Tp1, Tp2, Tp3, and Cp9 the three-dimensional PESs were constructed and the vibrational Schrödinger equation was solved. The PES is highly anharmonic in all four cases. The hydrogen is delocalized between two silicons in complexes Tp1, Tp3, and Cp9, but localized around the energy minima in complex Tp2. Complex Tp3 is an intermediate case with a substantial tunneling. The delocalized behavior is pertinent to systems with ΔE < 0.25 kcal·mol-1. For complexes Tp1, Tp2, Tp3, and Cp9 the J(Si-H) spin-spin coupling constants were calculated taking into account the vibrational motion of hydride. For Tp1, Tp3, and Cp9 both J(Si1-H) and J(Si 2-H) are negative due to simultaneous Si 1···H···Si2 interactions, while for Tp2J(Si2-H) is positive, indicating a single Si···H interaction only. Negative J(Si-H) values were obtained even for Si···H distances as large as 2.3 Å (complex Tp3). A possible effect of vibrations on the J(Si-H) values is also discussed