A Focused Review on Bio-inspired Multi-site Water Oxidation Catalysis by Ruthenium Complexes

The creation of efficient water oxidation catalysts (WOCs) is a key area of advancement in sustainable energy researches, particularly for enabling large-scale hydrogen production through water electrolysis. Recent advances in molecular catalyst design have highlighted the prominence of multi-site ruthenium-based systems, with Ru(bda) (bda = 2,2-bipyridine-6,6-dicarboxylate) complexes emerging as a benchmark due to their mechanistic versatility and tunable reactivity. Substantial progress has been achieved through strategic modifications of Ru(bda) catalysts, including dimerization, macrocyclization, and integration into covalent organic frameworks (COFs). Dimeric Ru(bda) systems, for instance, exhibit exceptional turnover frequencies (TOFs) by leveraging intramolecular radical coupling to circumvent the concentration limitations typical of bimolecular O-O bond formation pathways. Meanwhile, macrocyclic Ru(bda) derivatives, inspired by the natural oxygen-evolving complex (OEC), demonstrate enhanced proton-coupled electron transfer (PCET) kinetics, with TOFs reaching 7.9 s-1, attributed to the preorganization of water molecules within their confined cavities. Further innovations include embedding Ru(bda) units into COFs or immobilizing them on carbon nanotubes (CNTs). These hybrid architectures combine the precision of molecular catalysis with the stability of heterogeneous systems, enabling efficient water oxidation via the I2M mechanism. Notably, Ru(bda)-CNT assemblies achieve record-breaking performance with TOFs exceeding 3200 s-1. This review underscores the transformative potential of multi-site Ru(bda) catalysts and outlines a roadmap for rational catalyst design. However, challenges such as long-term stability under oxidative conditions and the scalability of noble-metal-based systems remain to be addressed. Future research directions may focus on elucidating structure-activity relationships and exploring earth-abundant alternatives to further advance practical applications in renewable energy technologies.