Pursuit of an Electron Deficient Titanium Nitride

The nitride salt [(PN)₂Ti≡N{μ₂-K(OEt₂)}]₂ (1) (PN^- = (N-(2-PPr₂-4-methylphenyl)-2,4,6-Me₃C₆H₂) can be oxidized with two equiv of I₂ or four equiv of ClCPh₃ to produce the phosphinimide-halide complexes (NPN')(PN)Ti(X) (X^- = I (2), Cl (3); NPN' = N-(2-NPPr₂-4-methylphenyl)-2,4,6-Me₃C₆H₂^2-), respectively. In the case of 2, H₂ was found to be one of the other products; whereas, HCPh₃ and Gomberg's dimer were observed upon the formation of 3. Independent studies suggest that the oxidation of 1 could imply the formation of the transient nitridyl species [(PN)₂Ti(≡N•)] (A), which can either oxidize the proximal phosphine atom to produce the Ti(III) intermediate [(NPN')(PN)Ti] (B) or, alternatively, engage in H atom abstraction to form the parent imido (PN)₂Ti≡NH (4). The latter was independently prepared and was found to photochemically convert to the titanium-hydride, (NPN')(PN)Ti(H) (5). Isotopic labeling studies using (PN)₂Ti≡ND (4-d₁) as well as reactivity studies of 5 with a hydride abstractor demonstrate the presence of the hydride ligand in 5. An alternative route to putative A was observed via a photochemically promoted incomplete reduction of the azide ligand in (PN)₂Ti(N₃) (6) to 4. This process was accompanied by some formation of 5. Frozen matrix X-band EPR studies of 6, performed under photolytic conditions, were consistent with species B being formed under these reaction conditions, originating from a low barrier N-insertion into the phosphine group in the putative nitridyl species A. Computational studies were also undertaken to discover the mechanism and plausibility of the divergent pathways (via intermediates A and B) in the formation of 2 and 3, and to characterize the bonding and electronic structure of the elusive nitrogen-centered radical in A.