Electronic Structure and Reactivity of a Well-Defined Mononuclear Complex of Ti(II)

A facile and high-yielding protocol
to the known Ti­(II) complex trans-[(py)4­TiCl2] (py = pyridine) has been developed. Its electronic
structure has been probed experimentally using magnetic susceptibility,
magnetic circular dichroism, and high-frequency and high-field electron
paramagnetic resonance spectroscopies in conjunction with ligand-field
theory and computational methods (density functional theory and ab initio methods). These studies demonstrated that trans-[(py)4­TiCl2] has a 3Eg ground state (dxy1dxz,yz1 orbital occupancy), which, as a result of spin–orbit
coupling, yields a ground-state spinor doublet that is EPR active,
a first excited-state doublet at ∼60 cm–1, and two next excited states at ∼120 cm–1. Reactivity studies with various unsaturated substrates are also
presented in this study, which show that the Ti­(II) center allows
oxidative addition likely via formation of [Ti­(η2-R2E2)­Cl2(py)n] E = C, N intermediates. A new Ti­(IV) compound, mer-[(py)3(η2-Ph2C2)­TiCl2], was prepared by reaction with Ph2C2, along with the previously reported complex trans-(py)3TiNPh­(Cl)2, from reaction with
Ph2N2. Reaction with Ph2CN2 also yielded a new dinuclear Ti­(IV) complex, [(py)2(Cl)2­Ti­(μ2:η2-N2CPh2)2­Ti­(Cl)2], in
which the two Ti­(IV) ions are inequivalently coordinated. Reaction
with cyclooctatetraene (COT) yielded a new Ti­(III) complex, [(py)2­Ti­(η8-COT)­Cl], which is a rare
example of a mononuclear “piano-stool” titanium complex.
The complex trans-[(py)4­TiCl2] has thus been shown to be synthetically accessible, have
an interesting electronic structure, and be reactive toward oxidation
chemistry.