Reactivity and Mechanisms of Photoactivated Heterometallic [RuII NiII ] and [RuII NiII RuII ] Catalysts for Dihydrogen Generation from Water.

Two heterometallic photocatalysts were designed and probed for water reduction. Both [(bpy)2 RuII NiII (L1 )](ClO4 )2 (1) and [(bpy)2 RuII NiII (L2 )2 RuII (bpy)2 ](ClO4 )2 (2) can generate the low-valent precursor involved in hydride formation prior to dihydrogen generation. However, while the bimetallic [RuII NiII ] (1) requires the presence of an external photosensitizer to trigger catalytic activity, the trimetallic [RuII NiII RuII ] (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.