Adaptive Ligand Coordination Enables Orthogonal Catalysis in Titanium and Tantalum Complexes

A tridentate pyridyl-amidate ligand was used to prepare titanium(IV) and tantalum(V) complexes which exhibit distinct coordination geometries and divergent reactivity profiles. While the titanium complex adopts a meridional NNN coordination mode, the tantalum complex favors an ONO binding motif. Density functional theory (DFT) calculations, including natural bond orbital (NBO), atomic dipole-corrected Hirshfeld (ADCH), Wiberg bond index (WBI), and buried-volume analyses, were used to compare the relative energetics, steric environments, and electronic structures of alternative coordination modes. Catalytically, the titanium complex displays high efficiency in hydroamination of phenylacetylene with aniline, facilitated by its accessible coordination site and five-coordinate geometry. In contrast, the coordinatively saturated six-coordinate tantalum complex is inactive in hydroamination but shows excellent activity in the hydroboration of nitriles and imines with HBpin, consistent with ligand-assisted B-H bond activation involving the NMe2 ligands. This study demonstrates how an adaptable ligand framework can respond to different metal centers through distinct steric and electronic interactions, resulting in divergent coordination modes and complementary catalytic reactivities.