Photochemistry of tris(pyrazolyl)borate titanium(IV) complexes

The electronic features and photochemistry of TpTiCl3 (1) (Tp = hydrotris(pyrazol-1-yl)borate) and Tp*TiCl3 (2) (Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate) were studied in THF. Reactive decay of the excited states produced either View the MathML sourceHBpz3 (or View the MathML sourceHBpz3∗) and metal center Ti(III) radicals via homolytic cleavage of the Tp → Ti (Tp* → Ti) bond. Cleavage of the Tp → Ti and the Tp* → Ti bond as a primary photoprocess is shown to be consistent with LMCT Tp → Ti and Tp* → Ti excitation. TpTiCl2(THF) (3) and Tp*TiCl2(THF) (4) were also prepared by stoichiometric reduction of 1 and 2 with Li3N. The THF ligand in 3 and 4 was replaced by the stable nitroxyl radical TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) to provide the new complexes TpTiCl2(TEMPO) (5) and Tp*TiCl2(TEMPO) (6) in which the TEMPO ligand is η1 coordinated to Ti(IV). Photolysis of 5 and 6 generate Ti(III) and the TEMPO radical in the primary photochemical step.