Proton Activation in the Presence of a Weak Acid Facilitated by Second Coordination Sphere Effects in Iron Porphyrins**

Abstract

The catalytic activity of two iron‐based porphyrin complexes containing pyridine‐functionalized second coordination spheres, referred to as Py2XPFe and CuPy2XPFe, have been investigated for the hydrogen evolution reaction (HER) and compared with the unsubstituted analog TMPFe in acetonitrile. The CuPy2XPFe incorporates a second metal center within the pyridine residues and represents a heterodinuclear system, while the structurally analogous Py2XPFe lacks an additional metal in the second coordination sphere. Both the Py2XFe and CuPy2XPFe complexes are observed to activate the weak acid, acetic acid (AcOH) at the FeII/I couple rather than at the more energy intensive FeI/0 couple observed for the unfunctionalized TMPFe complex at low acid concentrations. The ability of the monometallic Py2XPFe to activate the weak proton source at the FeII/I couple manifests as an ECEC(E)′ type electrochemical mechanism, rather than an EECC(E)′ type mechanism as observed for TMPFe. The CuPy2XPFe displays improved reactivity compared with the Py2XPFe and TMPFe under the same substrate conditions, however the mechanistic nuances into bimetallic CuPy2XPFe system are currently unclear. The concentration of AcOH was incrementally increased and the rate constants of the initial protonation step i. e. formation of a hydridic species (k1,app), as well as of the protonation of the hydridic species to produce dihydrogen (kglobal) were calculated using the Foot‐of‐the‐wave and plateau current rate equation methodologies, respectively. The kinetic analysis indicates that the activation of protons through the ECEC(E)′ type mechanism for the Py2XPFe results in a system in which k1,app< kglobal. As both the monometallic Py2XPFe and bimetallic CuPy2XPFe activate the AcOH at less cathodic potentials than TMPFe under identical conditions, the overpotentials (ηTOF/2) for these complexes are thus dramatically lower. The reactivity difference of the Py2XPFe when compared to non‐substituted iron porphyrin analogs is due to the hanging group's influence, which is believed to either act as hydrogen bond promoters for the active site or aids to stabilize a catalytic intermediate species during catalysis.