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

Abstract

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 [MNTbt{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 CN 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).