Among molecular water oxidation catalysts (WOCs), those featuring a reactive set of four multi-redox transition metals can leverage an extraordinary interplay of electronic and structural properties. These are of particular interest, owing to their close structural, and possibly functional, relationship to the oxygen evolving complex of natural photosynthesis. In this review, special attention is given to two classes of tetrametallic molecular WOCs: (i) M₄O₄ cubane-type structures stabilized by simple organic ligands, and (ii) systems in which a tetranuclear metal core is stabilized by coordination of two polyoxometalate (POM) ligands. Recent work in this rapidly evolving field is reviewed, with particular emphasis on photocatalytic aspects. Special attention is given to studies addressing the mechanistic complexity of these systems, sometimes overlooked in the rush for oxygen evolving performance. The complementary role of molecular WOCs and their relationship with bulk oxides and heterogeneous catalysis are discussed.

Photoinduced water oxidation to molecular oxygen takes place in systems made of [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) as the photosensitizer, [Co₄O₄(O₂CMe)₄(py)₄ ] (py = pyridine) as the molecular catalyst and Na₂S₂O₈ as the sacrificial electron acceptor. The photochemical quantum yield of the process reaches the outstanding value of 30% and depends on pH and catalyst concentration. Transient absorption spectroscopy experiments aimed to clarify the first events of the photocatalytic process are also reported.

Water oxidation is the crucial stage in the chemical and molecular sequence of photosynthesis, designed by Nature to convert solar light into chemical energy. The artificial “off-leaf” transposition is a major goal of energy research, aiming at the continuous production of hydrogen as a solar fuel, through the photo-catalytic splitting of water. Success in this task primarily depends on the interplay of light-activated multi-electron oxidation and reduction cycles and on the invention of stable and robust water oxidation catalysts, liberating oxygen with fast rates, high quantum yield, and long-term activity. A promising perspective is herein envisaged in the molecular design of functional metal-oxide cores and composite nano-materials.

A salophen cobalt(II) complex enables water oxidation at neutral pH in photoactivated sacrificial cycles under visible light, thus confirming the high appeal of earth abundant single site catalysis for artificial photosynthesis.

Water oxidation catalysts: evolution of [Co₄(H₂O)₂(α-PW₉O₃₄)₂]¹⁰⁻ to catalytically active species is assessed by laser flash photolysis in sacrificial photocatalytic cycles with Ru(bpy)₃ ²⁺ as a photosensitizer.