Coordination-induced bond weakening and small molecule activation by low-valent titanium complexes

Bond activation of small molecules through coordination to low valent metal complexes in MMIDLINE HORIZONTAL ELLIPSISX-H type interactions (where X = O, N, B, Si, etc.) leads to the formation of unusually weak X-H bonds and provides a powerful approach for the synthesis of target compounds under very mild conditions. Coordination of small molecules like water, amides, silanes, boranes, and dinitrogen to Ti(iii) or Ti(ii) complexes results in the synergetic redistribution of electrons between the metal orbitals and the ligand orbitals which weakens and enables the facile cleavage of the X-H or N-N bonds of the ligands. This review presents an overview of coordination-induced bond activation of small molecules by low valent titanium complexes. In particular, the applications of low valent titanium-induced bond weakening in nitrogen fixation are presented. The review concludes with potential future directions for work in this area including low-valent Ti-based PCET systems, photocatalytic nitrogen reduction, and approaches to tailoring complexes for optimal bond activation. The coordination of small molecules to low valent titanium complexes provides a powerful platform for the transformation of challenging substrates either through PCET reactions or bond-weakening induced by pi-back donation of electrons from Ti.