Photochemistry of Ni(II) tolyl chlorides supported by bidentate ligand frameworks

Herein, we investigate the photoactivity of four NiII tolyl chloride complexes supported by either the bulky, bidentate [2.2]pyridinophane (HN2) ligand or the traditional 2,2′-bipyridine (tBubpy) ligand. Despite a change in ligand framework, we observe comparable quantum yields for the photodegradation of all four NiII complexes but do see changes in their affinity for side reactivity and stabilization of photogenerated NiI monomeric species. Additionally, we show that tBubpyNi(tolyl)Cl compounds are not bench-stable, while also observing side reactivity that leads to C-O bond formation and C-C bond formation. By varying the location of the methyl on the tolyl group, we can further perturb the quantum yield of the compounds and the extent of their side reactivity. Time-dependent density functional theory (TDDFT) and ab initio modeling (CASSCF) reveal that a smaller HOMO/LUMO gap and a more energetically accessible tetrahedral-geometry triplet state correlates with increased quantum yields and O2 side-reactivity. By leveraging our HN2 ligand, a bidentate ligand that hinders axial interactions around the nickel center, the radical side reactivity is limited. This study of this new bidentate pyridinophane ligand highlights how photoactivity is affected by the steric environment around the Ni center, and that such photoactivity is not unique to bipyridyl-supported Ni compounds.