Reduction of Tetravalent Group 4 Metal Complexes Supported by an Extremely Bulky, Unsymmetrically Substituted β-Diketiminato Ligand Leading to the Regioselective CN Bond Cleavage Giving Ring-Contracted Metal-Imido Complexes

Reductions of tetravalent group 4 metal β-diketiminates, [MIVCl3(nacnac)(thf)n] (2a, M = Ti; 3b, M = Zr; 3c, M = Hf; nacnac = {N(Tbt)C(Me)CHC(Me)N(Mes)}-, Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes = 2,4,6-trimethylphenyl, n = 0, 1), with 2 equiv of KC8 in the presence of LiCl and tmeda afforded the metal-imido complexes [MNTbt{C(Me)CHC(Me)N(Mes)}(μ-Cl)2Li(tmeda)] (4a, M = Ti; 4b, M = Zr; 4c, M = Hf; tmeda = Me2NCH2CH2NMe2). The formation of imido complexes 4a−c can be explained in terms of the generation of the corresponding divalent complexes [MIICl(nacnac)(L)n] (6a, M = Ti; 6b, M = Zr; 6c, M = Hf; L = thf or Li(tmeda), etc.), followed by the reductive, regioselective cleavage of the CN bond tethered to the Tbt group. The intermediacy of the divalent titanium complex 6a was supported by the alternative formation of 4a in the reaction of [Li(nacnac)] (1) with [TiIICl2(tmeda)2]. Interestingly, 4a−c reacted with H2O to give the free ligand nacnacH (8).