Tailoring Function-Integrated Multinuclear Au-Ag Complex for Visible-Light Energy Transfer Catalysis

As one of the classical representatives of precious metals, gold-based photocatalysis has traditionally relied on single-electron transfer (SET) mechanisms, with few systems capable of engaging in triplet energy transfer (EnT) catalysis due to poor visible-light absorption and short-lived excited states. Herein, we rationally design and prepare a series of functionalized heteromultinuclear Au-Ag complexes by integrating a photosensitizing ligand strategy. It is interesting to find that the number of metal centers can be precisely controlled via the anion effect, where the nitrate anion benefits the formation of Au-Ag-Au-Ag tetranuclear complexes while trifluoromethanesulfonate facilitates the production of Au-Ag-Au trinuclear complexes. This structural fine-tunes over metal-metal interactions, would enhance molecular rigidity and consequently prolong triplet-state lifetimes. Remarkably, these structurally diverse Au-Ag complexes can exhibit broad ligand-to-ligand charge transfer (LLCT) absorption bands, long triplet lifetimes, and tunable emission properties, thus serving as efficient EnT photocatalyst under visible light. They can enable a robust intermolecular [2 + 2] cycloaddition of different kinds of alkenes under mild conditions by means of visible light irradiation. A wide range of general alkenes can be successfully employed without the installation of chromophore functional groups, affording a rich library of structurally diverse multisubstituted cyclobutanes in moderate to good yields (up to 90%) with consistent diastereoselectivity. Density functional theory (DFT) calculations support a stepwise EnT pathway in which the first C-C bond formation governs both the rate and stereoselectivity.