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 (PO) ligands featuring hybrid hard (phenolate) and soft (phosphine) donor atoms. Titanium(IV) and titanium(III) chloride complexes of the PO 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 PO-ligated titanium chloride complexes yielded dinitrogen-bridged dititanium species with a [NN] unit, unprecedented for complexes bearing a tripodal ligand with hybrid equatorial donor atoms. Subsequent protonation afforded ammonia, demonstrating the feasibility of N activation and functionalization under mild conditions. These findings underscore the adaptability of the PO ligand framework in stabilizing multiple oxidation states and modulating metal coordination, offering potential for applications in small-molecule activation and catalysis.