Reactivity and Mechanisms of Photoactivated Heterometallic [RuIINiII] and [RuIINiIIRuII] Catalysts for Dihydrogen Generation from Water.

Two heterometallic photocatalysts were designed and probed for water reduction. Both [(bpy)2RuIINiII(L1)](ClO4)2 (1) and [(bpy)2RuIINiII(L2)2RuII(bpy)2](ClO4)2 (2) can generate the low‐valent precursor involved in hydride formation prior to dihydrogen generation. However, while the bimetallic [RuIINiII] (1) requires the presence of an external photosensitizer to trigger catalytic activity, the trimetallic [RuIINiIIRuII] (2) displays significant coupling between the catalytic and light‐harvesting units to promote intramolecular multielectron transfer and perform photocatalysis at the Ni center. A concerted experimental and theoretical effort proposes mechanisms to explain why 1 is unable to achieve self‐supported catalysis, while 2 is fully photocatalytic. [ABSTRACT FROM AUTHOR]