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.