Recent applications of [Cu(dap)2]Cl in visible light-mediated photoredox catalysis
Olding, A and Nicholls, TP and Bissember, AC, Recent applications of [Cu(dap)2]Cl in visible light-mediated photoredox catalysis, Australian Journal of Chemistry, 71, (7) pp. 547-548. ISSN 0004-9425 (2018) [Contribution to Refereed Journal]
Since 2008, visible light-mediated photoredox catalysis has enjoyed a renaissance in organic synthesis.[1–5] The most commonly employed metal-based photoredox catalysts typically utilise second- and third-row transition metals featuring polypyridyl ligand systems such as [Ru(bpy)3]Cl2 (1). In contrast, the synthetic applications of copper-based photoredox catalysts have received much less attention. A class of photoactive homoleptic, phenanthroline-based copper(I) complexes, including [Cu (dap)2]Cl (2), were first prepared some 35 years ago (Chart 1).[7,8] This class of complexes share several interesting properties that can be exploited to facilitate chemical transformations. The absorption of a photon in the visible range leads to a metal-to-ligand charge transfer (MLCT) excited state. Subsequent intersystem crossing (ISC) results in spin inversion and allows access to long-lived triplet photoexcited states. In its photoexcited state, complex 2 is a strong reductant (-1.43 V versus saturated calomel electrode) capable of reducing organic acceptor molecules via single electron transfer (SET) processes. The ensuing CuIIspecies can undergo a second SET process to oxidise an organic donor molecule, which reduces the catalyst back to its ground state and closes the catalytic cycle. The coordinatively unsaturated nature of these copper(I) complexes typically provides lower coordinative stability relative to saturated octahedral complexes (e.g. 1). Also, in the ground state, these copper(I) complexes typically exhibit a distorted tetrahedral coordination geometry. Upon photoexcitation, oxidation of the metal centre occurs as a result of the population of the MLCT excited state. This causes a flattening distortion that results in a more square-planar coordination geometry. These two factors are primarily responsible for reducing excited state lifetimes. The present focus article aims to highlight recent applications of [Cu(dap)2]Cl in organic synthesis.