Balance between Metal and Ligand Reduction in Diiminepyridine Complexes of Ti

Reaction of the diiminepyridine ligand EtDIP (2,6-Et2- C6H3NCMe)2C5H3N) with TiCl3(THF)3 gave the corresponding Ti(III) complex (EtDIP)TiCl3 (1). Reduction of 1 with 1 equiv of KC8 produced the formally Ti(II) complex (EtDIP)TiCl2 (2). From this, (EtDIP)TiClR complexes (R = Me (3a), Me3SiCH2 (3b), Ph (3c)) were obtained by addition of 1 equiv of RLi. Similarly, dialkyl complexes (EtDIP)TiR2 (R = Me (4a), Me3SiCH2 (4b)) were obtained with 2 equiv of RLi. All new complexes except 3b were characterized by single-crystal X-ray diffraction. EPR studies indicate that complex 1 is best regarded as a true Ti(III) complex with an “innocent” DIP ligand. Complexes 2−4 are all diamagnetic. In contrast to DIP complexes of the late transition metals Fe and Co, the new complexes 2−4 show strong upfield 1H NMR shifts for the pyridine β and γ protons caused by transfer of negative charge to the DIP ligand. On the basis of this and the CN and Cimine−CPy bond lengths, a description involving Ti(IV) and a dianionic ligand seems most appropriate, and DFT calculations support this interpretation. This means that reduction of Ti(III) complex 1 results in oxidation of the metal center to Ti(IV). VT-NMR studies of 4a suggest a small and temperature-dependent thermal population of a triplet state, and indeed calculations indicate that 4a has the lowest singlet− triplet energy difference of the systems studied.