Organotitanium Complexes Supported by a Dianionic Pentadentate Ligand

The pentadentate tetrapyrazolylpyridyl diborate ligand B2Pz4Py has been complexed to TiIII to form the chloro complex (B2Pz4Py)TiIIICl, 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)TiIIIH. Rather, treatment of the chloro starting material with NaHBEt3 affords the desired hydrido complex in 85% yield. This TiIII 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 60 kcal/mol, according to density functional theory computations. This is due to stabilization of the dimer by dispersion forces. The computations show that the dimer has an open-shell singlet (S = 1) ground state, which is consistent with the magnetic susceptibility of 1.73 mu B per titanium center measured by the Evans method. However, the triplet state is very close in energy. The large barrier to dissociation into reactive monomers is reflected in the observed lack of reactivity with carbon dioxide to form the expected formato derivative, which can be prepared separately from (B2Pz4Py)TiIIICl and sodium formate. Both the hydrido and alkyl TiIII compounds react with water to form a mu-oxo dinuclear species, which reacts further with dioxygen to form oxidized peroxo and oxo TiIV complexes. All three of these compounds were fully characterized.