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, [<i>t</i>Bu₃Si­(NHC)­(H)­Si:→Fe­(CO)₄] (<b>2</b>), was synthesized starting from the corresponding hydrosilylene [<i>t</i>Bu₃Si­(NHC)­(H)­Si:] (<b>1</b>). Complex <b>2</b> 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 [<i>t</i>Bu₃Si­(NHC)­(H)­Si:→W­(CO)₅] (<b>3</b>). A series of NHC-stabilized silylene iron complexes ([R₂(NHC)­Si:→Fe­(CO)₄], where R = Cl (<b>4</b>), H (<b>5</b>), and Me (<b>6</b>)) were synthesized and fully characterized to investigate the influence of different substituents. The dihydrosilylene iron complex [H₂(NHC)­Si:→Fe­(CO)₄] (<b>5</b>) 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 <b>2