Photochemistry of Ni(II) Tolyl Chlorides Supported by Bidentate Ligand Frameworks

Herein, we investigate the photoactivity of four Ni^II tolyl chloride complexes supported by either the new bidentate [2.2]pyridinophane (^HN2) ligand or the traditional 4,4′-di-tert-butyl-2,2′-dipyridyl (^tBubpy) ligand. Despite a change in the ligand framework, we observe similar quantum yields for the photodegradation of all four Ni^II complexes, while noting changes in their affinity for radical side reactivity and ability to stabilize the photogenerated mononuclear Ni^I species. Furthermore, changing from an ortho-tolyl to a para-tolyl group affects the geometry of the complexes and makes the Ni center more susceptible to side reactivity. By leveraging the newly developed ^HN2 ligand, a bidentate ligand that hinders axial interactions with the Ni center, we limit the radical side reactivity. Time-dependent density functional theory (TDDFT) and complete active space self-consistent field (CASSCF) calculations predict that all four complexes have accessible MLCTs that excite an electron from a Ni-aryl bonding orbital into a Ni-aryl antibonding orbital, initiating photolysis. By decreasing this energy gap and stabilizing the tetrahedral triplet excited state, we increase quantum yields of photoexcitation. Importantly, we characterize the photogenerated mononuclear Ni^I chloride species using X-band EPR spectroscopy and show that the ^HN2-supported Ni^I complexes do not undergo the deleterious dimerization and tetramerization observed for the (bpy)Ni^ICl species. Overall, this study provides valuable insight into how the steric environment around the Ni center affects its photoactivity and demonstrates that such photoactivity is not unique to bipyridyl-supported Ni compounds.