Systematic Study of N-Heterocyclic Carbene Coordinate Hydrosilylene Transition-Metal Complexes

An in-depth study of the synthesis and structures of N-heterocyclic carbene (NHC)-stabilized silylene transition-metal complexes is reported. An iron hydrosilylene complex, [tBu₃Si(NHC)(H)Si:→Fe(CO)₄] (2), was synthesized starting from the corresponding hydrosilylene [tBu₃Si(NHC)(H)Si:] (1). Complex 2 was fully characterized, including X-ray diffraction analysis, which showed an unusual long Si-Fe bond length. A very long bond length was also observed in the novel hydrosilylene tungsten complex [tBu₃Si(NHC)(H)Si:→W(CO)₅] (3). A series of NHC-stabilized silylene iron complexes ([R₂(NHC)Si:→Fe(CO)₄], where R = Cl (4), H (5), and Me (6)) were synthesized and fully characterized to investigate the influence of different substituents. The dihydrosilylene iron complex [H₂(NHC)Si:→Fe(CO)₄] (5) represents a new example of a donor-acceptor-stabilized parent silylene (H₂Si:). Density functional theory calculations were utilized to understand the influence of the electronic and steric effects of the silylene unit and its substituents on the Si-Fe bond in these iron complexes, in particular to rationalize the long Si-Fe bond in 2.