Photoinduced Ni—CF3 Cleavage Enabled by Ligand-Centered Radicals

Abstract

Traditional approaches for improving the reactivity and selectivity of homogenous molecular catalysts center on fine tuning the steric and electronic properties of the ancillary ligands. Beyond shaping the catalytic environment, various research groups have pursued the incorporation of non-covalent interactions within the secondary coordination sphere of transition metal catalysts to facilitate challenging chemical transformations. Conventional strategies for modifying the secondary coordination sphere have explored the use of hydrogen bond donors, Lewis acidic and basic functional groups, and cation sequestration sites within the ligand architecture. However, the design of latent ligand-centered radicals offers a distinct approach for the activation of persistent organometallic complexes. In particular, this research presents a strategy for activating kinetically inert nickel—trifluoromethyl (Ni—CF3) bonds, as CF3 functional groups are valuable structural motifs in medicinal chemistry and drug discovery processes. Notably, homolytic cleavage of both Ni—CF3 bonds within a fluorenone-appended bis(pyridine-2-ylmethyl)amine bis(trifluoromethyl)nickel(II) complex was observed under visible light irradiation, leading to controlled CF3 radical generation and Csp²—CF3 bond formation. Through a combination of electrochemical, experimental, and spectroscopic studies, the reactivity of the fluorenone-appended bis(trifluoromethyl)nickel(II) complex under visible light irradiation is demonstrated to be fundamentally distinct from that of the intermolecular system employing exogenous diaryl ketone under light irradiation and from reactions with stoichiometric oxidant under thermal heating. The ability to activate Ni—CF3 bonds formally at Ni(II) was found to be dictated by the photophysical properties of the diaryl ketone, the choice of solvent, and whether the organic chromophore is appended or not. Moreover, insights into the effect of coordination number on the reactivity of the fluorenone-appended system in the presence of coordinating solvent were further obtained through chemical oxidation of [(MeNNN)NiII(CF₃)₂] to a paramagnetic [(MeNNN)NiIII(CF3)2(MeCN)]BF4 complex.