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 μB 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 μ-oxo dinuclear species, which reacts
further with dioxygen to form oxidized peroxo and oxo TiIV complexes. All three of these compounds were fully characterized.