A Mononuclear and High-Spin Tetrahedral Ti II Complex.

A high-spin, mononuclear Ti ^II complex, [(Tp ^{t Bu,Me} )TiCl] [Tp ^{t Bu,Me-} = 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 ^{t Bu,Me} )TiCl ₂ ] with KC ₈ . Complex [(Tp ^{t Bu,Me} )TiCl] has a ³ A ₂ ground state (assuming C _{3 v} 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 ^{t Bu,Me} )TiCl(THF)], which is impervious to N ₂ binding. However, in the absence of THF, the Ti ^II complex captures N ₂ to produce the diamagnetic complex [(Tp ^{t Bu,Me} )TiCl] ₂ (η ¹ ,η ¹ ;μ ₂ -N ₂ ), with a linear Ti═N═N═Ti topology, established by single-crystal X-ray diffraction. The N ₂ 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 N ₂ ^2- unit. A π acid such as CNAd (Ad = 1-adamantyl) coordinates to [(Tp ^{t Bu,Me} )TiCl] without inducing spin pairing of the d electrons, thereby forming a unique high-spin and five-coordinate Ti ^II complex, namely, [(Tp ^{t Bu,Me} )TiCl(CNAd)]. The reducing power of the coordinatively unsaturated Ti ^II -containing [(Τp ^{t Bu,Me} )TiCl] species, quantified by electrochemistry, provides access to a family of mononuclear Ti ^IV complexes of the type [(Tp ^{t Bu,Me} )Ti═E(Cl)] (with E ^2- = NSiMe ₃ , N ₂ CPh ₂ , O, and NH) by virtue of atom- or group-transfer reactions using various small molecules such as N ₃ SiMe ₃ , N ₂ CPh ₂ , N ₂ O, and the bicyclic amine 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene.