LAUSR

Abstract Triiron and triruthenium clusters containing capping 2-aminopyridinate/pyrimidinate ligands are developed as functional models of the [FeFe]-hydrogenase for electrocatalytic reduction of protons to hydrogen. The 48-electron clusters Fe 3 (CO) 9 (μ 3 -pyNH)(μ-H) ( 1 ), Fe 3 (CO) 9 (μ 3 -pymNH)(μ-H) ( 2 ), Ru 3 (CO) 9 (μ 3 -pyNH)(μ-H) ( 3 ) and Ru 3 (CO) 9 (μ 3 -pymNH)(μ-H) ( 4 ) (pyNH = 2-aminopyridinate, pymNH = 2-aminopyrimidinate) are prepared from reactions of M 3 (CO) 12 (M = Fe or Ru) with the corresponding heterocyclic amine at elevated temperatures. Each contains a hydride and a residual amino hydrogen in close proximity ( ca. 2.8 A). The triiron 2-aminopyridinate cluster 1 does not protonate by TsOH·H 2 O (used as the proton source during catalysis), whereas its ruthenium analogue 3 undergoes slow protonation across a ruthenium-ruthenium bond. The 2-aminopyrimidinate clusters 2 and 4 undergo rapid protonation at the ring nitrogen. In MeCN, the triiron clusters show a single irreversible reduction wave ( E p  = −1.61 V for 1 ; E p  = −1.47 V for 2 ) in the cathodic region of their CVs, while the triruthenium clusters display a pair of irreversible cathodic waves ( E p  = −2.01 and −2.15 V for 1 ; E p  = −1.93 and −2.09 V for 2 ). All catalyze proton reduction in the presence of TsOH·H 2 O but different mechanisms are proposed. The triiron clusters are more efficient toward proton reduction and operate at reduced overpotentials as compared to their triruthenium analogues. Introduction of a potential proton relay in these clusters neither increases the efficiency nor reduces the overpotential of catalysis which is in sharp contrast with the results observed for hydrogenase biomimetics.