Synthesis and characterization of bis- and tris(carbene)borate complexes. Investigation of hydrogen atom transfer in iron(IV) imido complexes

A new synthetic route to bulky tris(carbene)borate ligands that incorporate strongly donating imidazol-2-ylidine donors is reported, allowing for the preparation of low coordinate iron and nickel complexes. The key feature of this new synthetic route is the integration of bulky N-substituents (e.g. tBu, Mes) into the imidazol-2-ylidine donors. This synthetic strategy also allows for the preparation of tris(carbene)borates that have different groups on the boron atom. The influence of tris(carbene)borate substituents on the donor strength of the ligand have been elucidated from IR spectroscopic investigations of {NiNO}10 derivatives.;A modified synthetic strategy allows for the preparation of a bulky bis(carbene)borate ligand from 1-tert-butylimidazole. This ligand coordinates to nickel(II) to yield a diamagnetic square-planar complex whose structure differs from that of the related bis(3-tert-butylpyrazolyl)borate nickel(II) complex (which is octahedral and paramagnetic). Density functional theory has been used to evaluate the structural differences between these two complexes.;The bulky tris(carbene)borate ligands allow stable high valent iron imido complexes, [PhBCMes3FeIVNR] + (R = Ad, tBu), to be prepared. The thermal stability of these complexes has allowed us to carry out studies relevant to hydrogen atom transfer (HAT) to the imido ligands. These thermodynamic investigations determine the propensity for HAT and show that the high basicity of the reduced imido complexes provides the thermodynamic driving force. The effect of solvent on the thermodynamics of HAT to the iron imido complexes was also investigated.