Formation of Phosphorus−Nitrogen Bonds by Reduction of a Titanium Phosphine Complex under Molecular Nitrogen

The reduction of high oxidation state metal complexes in the presence of molecular nitrogen is one of the most common methods to synthesize a dinitrogen complex. However, the presence of strong reducing agents combined with the poor binding ability of N₂ can lead to unanticipated outcomes. For example, the reduction of [NPN]ZrCl₂(THF) (where NPN = PhP(CH₂SiMe₂NPh)₂) with KC₈ under N₂ leads to the formation of the side-on bridged dinuclear dinitrogen complex ([NPN]Zr(THF))₂(μ-η²:η²-N₂) with an N−N bond distance of 1.503(3) Å; however, reduction of the corresponding titanium precursor, [NPN]TiCl₂, under N₂ does not generate a dinitrogen complex, rather the bis(phosphinimide) derivative, ([N(PN)N]Ti)₂, is isolated in which the added N₂ is incorporated between the titanium and phosphine centers. Performing the reaction under ¹⁵N₂ results in the ¹⁵N label being incorporated in the phosphinimide unit. A suggested mechanism for this process involves an initially formed dinitrogen complex being over reduced to generate a species with bridging nitrides that undergoes nucleophilic attack by the coordinated phosphine ligands and formation of the PN bond of the phosphinimide.