A Mononuclear and High-Spin Tetrahedral Ti^II Complex

A high-spin, mononuclear Ti^II complex, [(Tp)TiCl] [Tp = hydridotris(3-tert-butyl-5-methylpyrazol-1-yl)borate], confined to a tetrahedral ligand-field environment, has been prepared by reduction of the precursor [(Tp)TiCl₂] with KC₈. Complex [(Tp)TiCl] has a ³A₂ ground state (assuming C₃ symmetry based on structural studies), established via a combination of high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy, solution and solid-state magnetic studies, Ti K-edge X-ray absorption spectroscopy (XAS), and both density functional theory and ab initio (complete-active-space self-consistent-field, CASSCF) calculations. The formally and physically defined Ti^II complex readily binds tetrahydrofuran (THF) to form the paramagnetic adduct [(Tp)TiCl(THF)], which is impervious to N₂ binding. However, in the absence of THF, the Ti^II complex captures N₂ to produce the diamagnetic complex [(Tp)TiCl]₂(η¹,η¹;μ₂-N₂), with a linear Ti═N═N═Ti topology, established by single-crystal X-ray diffraction. The N₂ complex was characterized using XAS as well as IR and Raman spectroscopies, thus establishing this complex to possess two Ti^III centers covalently bridged by an N₂^2- unit. A π acid such as CNAd (Ad = 1-adamantyl) coordinates to [(Tp)TiCl] without inducing spin pairing of the d electrons, thereby forming a unique high-spin and five-coordinate Ti^II complex, namely, [(Tp)TiCl(CNAd)]. The reducing power of the coordinatively unsaturated Ti^II-containing [(Τp)TiCl] species, quantified by electrochemistry, provides access to a family of mononuclear Ti^IV complexes of the type [(Tp)Ti═E(Cl)] (with E^2- = NSiMe₃, N₂CPh₂, O, and NH) by virtue of atom- or group-transfer reactions using various small molecules such as N₃SiMe₃, N₂CPh₂, N₂O, and the bicyclic amine 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene.