Synthesis and Study of Photoactive Heterobimetallic Complexes Bearing Ditopic Imidazophenanthroline Ligands: Evaluation of Viability as Unimolecular Metallaphotoredox Catalysts

Abstract

A small library of heterobimetallic ruthenium(II)/iridium(III)–palladium(II) complexes with ditopic imidazophenanthroline ligands was synthesized to explore their potential as unimolecular metallaphotoredox catalysts for photoassisted Suzuki–Miyaura cross-coupling reactions. To this end, a concise and modular synthetic approach for the construction of imidazol{[4,5-f][1,10]-phenanthrolin}-2-ylidenes with bulky secondary alkyl N-substituents was developed, creating access to these previously inaccessible systems. All complexes were characterized via solid-state, photophysical, spectroscopic, and electrochemical methods to elucidate structure–function relationships. The ditopic ligand was designed to facilitate electron transfer from the photoactive ruthenium(II)/iridium(III) unit to a palladium(II) center to potentially enable C–C cross-couplings of weak electrophiles, under mild conditions. However, these heterobimetallic species did not promote photoassisted Suzuki–Miyaura reactions. Transient absorption spectroscopy and computational studies suggest that the absence of electronic communication between the photosensitizer unit and the palladium(II) center precludes catalytic activity. These findings reveal critical ligand-specific limitations to facilitating effective couplings in such systems.