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 C_imine–C_Py 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.