The Role of Excited States of LNiII/III(Aryl)(Halide) Complexes in Ni–Halide Bond Homolysis in the Arylation of Csp3–H Bonds

LNiII(Ar)X (L = bipyridine-type ligand; X = Br and Cl) complexes are fundamental intermediates in photoredox-Ni catalysis for the activation of Csp3–H bonds in Csp3–Csp2 cross-coupling reactions. Their interaction with a light-excited Ir-photocatalyst is assumed to promote the Ni-complex to an excited state from which the Ni–X bond homolysis event occurs. Debate is open on this interaction occurring via single electron transfer (SET) or via energy transfer (EnT), which is the indirect excitation of the Ni-complex by the Ir-photocatalyst. Aiming at addressing this question, we report an electronic structure analysis of these complexes and their 1e– oxidized form using multireference electronic structure methods, which were needed because of the highly multiconfigurational nature of some of the species involved in these reactions. We identified the excited states of LNiII(Ar)X relevant to Ni–X homolysis within the energy range of the irradiating light source, and we propose that they are accessible via triplet Dexter EnT. This is a spin-conserved process with simultaneous exchange of ground- and excited-state electrons between the light-excited Ir-photocatalyst and the Ni-complex. Conversely, SET from LNiII(Ar)X to the Ir-photocatalyst followed by light excitation of the resulting [LNiIII(Ar)X]+ complex is unlikely, as no excited state corresponding to Ni–X dissociation was found in the energy range consistent with the irradiating light source.