Organotitanium Complexes Supported by a Dianionic Pentadentate Ligand

The tetrapyralzolylpyridyl diborate pentadentate ligand B2Pz4Py has been complexed to Ti(III) to form the chloro complex (B2Pz4Py)Ti(III)Cl, which is a convenient starting material for preparing alkyl and hydride derivatives of this ligand. The former (R = CH3 and CH2SiMe3) are highly thermally stable and do not react with dihydrogen to form (B2Pz4Py)Ti(III)H. Rather, treatment of the chloro starting material with NaHBEt3 affords the desired hydrido complex in 85% yield. This Ti(III) hydride was fully characterized and exists in both solution and the solid state as a dimeric species; dissociation into monomers faces a high barrier of over 40 kcal/mol, according to Density Function Theory computations. This is due to stabilization of the dimer by dispersion forces. The computations show that the dimer has an S = 1 ground state, but in solution, partial dissociation into an intermediate dimer which is an open shell singlet is possible, which accounts for the lower than expected magnetic susceptibility of 1.93 per dimer. Full dissociation into reactive monomers does not occur, based on the observed lack of reactivity with carbon dioxide. All of these com-pounds react with water to form a mu-oxo dinuclear species, which reacts further with dioxygen to form oxidized peroxo and oxo Ti(IV) complexes. All three of these compounds were fully characterized.