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.