Recent developments in hydrogen evolving molecular cobalt(II)–polypyridyl catalysts

• New hydrogen evolving molecular cobalt–polypyridyl catalysts have recently emerged.
• Their evaluation for electro- and photocatalytic hydrogen production is discussed.
• These robust catalysts display high activities under fully aqueous conditions.
• Mechanistic considerations and structure–activity relationships are highlighted.

The search for efficient noble metal-free hydrogen-evolving catalysts is the subject of intense research activity. A new family of molecular cobalt(II)–polypyridyl catalysts has recently emerged. These catalysts prove more robust under reductive conditions than other cobalt-based systems and display high activities under fully aqueous conditions. This review discusses the design, characterization, and evaluation of these catalysts for electrocatalytic and light-driven hydrogen production. Mechanistic considerations are addressed and structure–catalytic activity relationships identified in order to guide the future design of more efficient catalytic systems.