Trinuclear clusters containing 2-aminopyridinate/pyrimidinate ligands as electrocatalysts for proton reduction

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 Fe3(CO)9(μ3-pyNH)(μ-H) (1), Fe3(CO)9(μ3-pymNH)(μ-H) (2), Ru3(CO)9(μ3-pyNH)(μ-H) (3) and Ru3(CO)9(μ3-pymNH)(μ-H) (4) (pyNH = 2-aminopyridinate, pymNH = 2-aminopyrimidinate) are prepared from reactions of M3(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 Å). The triiron 2-aminopyridinate cluster 1 does not protonate by TsOH·H2O (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 (Ep = −1.61 V for 1; Ep = −1.47 V for 2) in the cathodic region of their CVs, while the triruthenium clusters display a pair of irreversible cathodic waves (Ep = −2.01 and −2.15 V for 1; Ep = −1.93 and −2.09 V for 2). All catalyze proton reduction in the presence of TsOH·H2O 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.