Proton activation in the presence of a weak acid facilitated via second coordination 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 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.

Potential Induced Protonation of the Second Coordination Sphere in a Hangman-type Iron Porphyrin Complex Promotes HER: Insights via in Situ Raman Spectroelectrochemistry

“…The applied cathodic potential increases the local pH of the electrochemical double layer at the working electrode due to the Au-promoted HER. , However, the presence of the linker molecule aids to extend the catalyst beyond this layer (electrochemical double layer at an electrode on the order of 1 nm at 0.1 M electrolyte) and possibly hinder ion mobility to the Au surface altogether . The electric field, however, likely extends through the grafted Pyr-1 layer as exemplified on Au surfaces with thiol-based SAMs. , Therefore, the electron transfer from the electrode to the Py 2 XPFe is suggestive of an outer sphere electron transfer, , observed as classical behavior in MeCN in solution . In this case, the immobilized catalyst is located beyond the boundary layer at the Au electrode yet is affected by the electric field from the electrode through the grafted linker.…”

“…If the reduction from the Fe II to Fe I species is kinetically slow and the follow-up chemical steps of the reactive Lewis-basic Fe I with a proton are sufficiently fast, then catalysis likely occurs rapidly enough that the short-lived intermediates involved in the reaction mechanism are unable to be observed on the spectroelectrochemical time scale. The Fe III –H to Fe II –H reduction is presumably thermodynamically easier to achieve than the Fe II/I couple due to the protonation of the hanging group and thus occurs at more anodic potentials ( E 2 < E 3 ), which implies little to no accumulation of either the Fe I or the Fe III –H species in the spectroelectrochemical experiments on the chronoamperiometric time scale. , Although the formation of Fe III –H may be the rate limiting step, at least in solution, electron transfer may be hindered from the electrode to the analyte due to the grafted Pyr-1. Alternatively, direct protonation of the hanging group in this medium could facilitate a different reaction mechanism compared with the hanging group acting as only a hydrogen bond donor in the presence of weak acids in solution.…”

“…Previous investigations in organic solution indicate the Py 2 XPFe complex is capable of activating weak acids already at the Fe II/I couple compared with the unsubstituted TMPFe complex which activates weak acids at the more energy-intensive Fe I/0 couple . The improved reactivity of the Py 2 XPFe complex at dramatically earlier onset potentials allows for a more facile spectroelectrochemical investigation at lower cathodic potentials.…”

“…Previous investigations in organic solution indicate the Py 2 XPFe complex is capable of activating weak acids already at the Fe II/I couple compared with the unsubstituted TMPFe complex which activates weak acids at the more energyintensive Fe I/0 couple. 22 The improved reactivity of the Py 2 XPFe complex at dramatically earlier onset potentials allows for a more facile spectroelectrochemical investigation at lower cathodic potentials. The spectroelectrochemical investigations conducted using Raman spectroelectrochemistry were performed on a roughened gold electrode with the complex Py 2 XPFe coordinated to the roughened surface via an organic linker molecule.…”

“…36,52 Therefore, the electron transfer from the electrode to the Py 2 XPFe is suggestive of an outer sphere electron transfer, 53,54 observed as classical behavior in MeCN in solution. 22 In this case, the immobilized catalyst is located beyond the boundary layer at the Au electrode yet is affected by the electric field from the electrode through the grafted linker. The induced cathodic flux from the electric field promotes the reduction of the Fe center and affects the protonation state of the pyridine-based hanging group near the active site, where the amount of protonation and therefore the increase in intensity of the 999 and 1061 cm −1 bands is more prevalent in pH 7 compared to pH ∼ 13 solution.…”