Tripodal bisphenolate-bisphosphine ligands with hybrid hard/soft donors: titanium complexes and dinitrogen activation

The design of multidentate ligands incorporating both hard and soft donors is of fundamental interest and importance in coordination chemistry. Here, we report a novel class of tetradentate dianionic bisphenolate-bisphosphine (P₂O₂) ligands featuring hybrid hard (phenolate) and soft (phosphine) donor atoms. Titanium(IV) and titanium(III) chloride complexes of the P₂O₂ ligands were synthesized and characterized by X-ray crystallography, NMR spectroscopy, solution magnetic susceptibility measurements (Evans method) and EPR spectroscopy, revealing distorted octahedral geometries and providing insight into coordination modes and spin states. Reduction of the P₂O₂-ligated titanium chloride complexes yielded dinitrogen-bridged dititanium species with a [NN]^2- unit, unprecedented for complexes bearing a tripodal ligand with hybrid equatorial donor atoms. Subsequent protonation afforded ammonia, demonstrating the feasibility of N₂ activation and functionalization under mild conditions. These findings underscore the adaptability of the P₂O₂ ligand framework in stabilizing multiple oxidation states and modulating metal coordination, offering potential for applications in small-molecule activation and catalysis.