Rich or poor: the impact of electron donation and withdrawal on the photophysical and photocatalytic properties of copper(i) complexes

Abstract

Motivated by the need to design efficient photocatalysts based on earth-abundant metals and to elucidate essential structure–property relationships, this study deals with heteroleptic copper(I) complexes. A systematic series of four novel diimine–diphosphine copper(I) complexes of the type [Cu(N^N)(P^P)]⁺ with different electron donating and withdrawing substituents in the 4 and 7-positions of the phenanthroline moiety has been prepared and compared to those with substituents in the 5 and 6-positions. In addition, two crystal structures were obtained, including that of the important synthesis intermediate [Cu(MeCN)₂(P^P)]⁺. Accompanying DFT calculations revealed the orbital energies and HOMO–LUMO gaps, which are strongly dependent on the substituents. Most strikingly, substitution at the 4 and 7-positions leads to better absorption properties of the resulting complexes than that at the 5 and 6-positions. Within the series, the bistrifluoromethyl groups (CF₃)₂ play a special role, e.g. the Cu(CF₃)₂ complex is the only one that can be reduced twice. Moreover, Cu(CF₃)₂ represents the strongest excited state oxidant, whose potential is shifted by 90 mV compared to the reference complex CuH lacking substituents. The production of reactive singlet oxygen (¹O₂), including the catalytic oxygenation of 2,5-diphenylfuran (DPF), and the photocatalytic reductive dehalogenation of aryl halides were then chosen to evaluate the activity of these complexes. In fact, all complexes are good sensitizers generating ¹O₂ with singlet oxygen quantum yields of up to 54% and a DPF oxygenation efficiency that depends on the electron donation ability of the respective substituents. A screening of different substrates, including the challenging 2-chloropyridine, and catalysis parameters demonstrated that these copper(I) complexes are very potent photocatalysts for reductive dehalogenations. They enable high yields >90% and fast reactions with maximum turnover frequencies of 1180 h⁻¹ (Ar–Br) and 2939 h⁻¹ (Ar–I), exceeding previous benchmark complexes.