Purine-derived N-heterocyclic carbene metal complexes: catalytic applications and reactivity

Caffeine and other purine bases are naturally occurring, inexpensive, and readily available heterocycles composed of a fused imidazole-pyrimidine framework, whose five-membered imidazole core corresponds to an N-heterocyclic carbene (NHC) precursor. These features make purine scaffolds ideal precursors for the design of metal-NHC ligands that combine biological relevance, structural diversity, and tunable electronic properties. Over the past two decades (2004-2025), an expanding family of purine-derived NHC complexes has been developed across the periodic table, including those of Ru, Ir, Rh, Ni, Pd, Pt, Cu, Ag and Au, revealing distinctive coordination modes, redox behaviors, and catalytic activities. This review provides a comprehensive overview of their synthetic strategies, structural features, and catalytic applications, emphasizing how the intrinsic anisotropy and multifunctionality of the purine framework enable unique metal-ligand interactions and reactivity patterns relevant to sustainable catalysis, photophysics, and bio-organometallic chemistry.