Proton activation in the presence of a weak acid facilitated via second coordination effects in iron porphyrins

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 MeCN. 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 TMPFe species at low acid concentrations. The ability of the monometallic Py2XPFe complex 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 the unsubstituted TMPFe. The CuPy2XPFe displays improved reactivity compared with the Py2XPFe and TMPFe under the same substrate conditions; however the mechanistic nuances into the 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 hydric species to produce dihydrogen (k2,app) were calculated using the Foot-of-the-wave and KS zone rate equation methodologies, respectively. The kinetic analysis indicates that the activation of protons through the ECEC(E) type mechanism results in a system in which k1,app < k2,app. As both the monometallic Py2XPFe and bimetallic CuPy2XPFe activate the AcOH at less cathodic potentials than the TMPFe under identical conditions, the overpotential (TOF/2) for these complexes is thus dramatically lower. The reactivity difference of the Py2XPFe when compared to the TMPFe due to the hanging groups influence is postulated to be either a result of the pyridine residues acting as hydrogen bonding promoters to the active site or aiding to stabilize a catalytic intermediate species during catalysis. Interestingly, a mechanistic change for the TMPFe is also observed at higher AcOH concentrations, underlying perhaps an increased HER reactivity of iron-porphyrins in MeCN.