Intersystem Crossing Enables Photoredox Oxidation of Alkanes by a Cu(II) Complex: Insights from MS-CASPT2 and DFT Calculations

Abstract

The photophysical processes and photochemical reactions of Cu(II) complexes remain poorly understood due to their intrinsically electronic characteristics and pronounced relativistic effects, especially in photooxidation reactions. In this study, a combined multi-state complete active space second-order perturbation theory (MS-CASPT2), density functional theory (DFT), and hydrogen atom abstraction (HAA) rate calculations are employed to elucidate the photoactivation and subsequent oxidation of alkanes mediated by the 2TpCF3CuII−OClO2 complex. MS-CASPT2 calculations reveal that visible-light excitation populates a doublet ligand-to-metal charge transfer (2LMCT) state, followed by efficient intersystem crossing to a quartet intra-ligand charge transfer (4ILCT) state that enables ClO2· elimination, thereby forming the triplet product 3TpCF3CuII−O·. The resulting 3TpCF3CuII−O· species abstracts a hydrogen atom from ethane to yield 2TpCF3CuII−OH and an ethyl radical, a step identified as the rate-limiting process. Branching from the 2TpCF3CuII−OH, the reaction diverges into two pathways: a rebound process affording ethanol and a proton transfer leading to ethene. Kinetic analysis further demonstrates that electron-withdrawing substituents on the Tp ligand accelerate both steps and selectively form ethanol. The calculations elucidate the pivotal character of intersystem crossing in the current Cu(II) complex and provide a design strategy for optimizing and developing Cu(II)-catalyzed photooxidation of alkanes.