The synthesis and characterization of a hetero‐dinuclear compound is presented, in which a copper(I) trishistidine type coordination unit is positioned directly above a zinc porphyrin unit. The close distance between the two coordination fragments is secured by a rigid xanthene backbone, and a unique (intramolecular) copper porphyrin‐π‐bond was determined for the first time in the molecular structure. This structural motif was further analyzed by temperature‐dependent NMR studies: In solution at room temperature the coordinative bond fluctuates, while it can be frozen at low temperatures. Preliminary reactivity studies revealed a reduced reactivity of the copper(I) moiety towards dioxygen. The results adumbrate why nature is avoiding metal porphyrin‐π‐bonds by fixing reactive metal centers in a predetermined distance to each other within multimetallic enzymatic reaction centers.

A bispyridylamine-based hanging unit within the ligand framework of a newly synthesized iron porphyrin complex (Py 2 XPFe) can act, on the one hand, as a hydrogen bonding site to facilitate proton transfer in catalysis and, on the other hand, as coordination site for a second Lewis acidic metal center. The bispyridylamine group in close proximity of the iron porphyrin center is able to mediate electrocatalytic CO 2 reduction in anhydrous MeCN. The hydrogen bonding interactions within the hanging group affect the kinetics of catalysis likely through stabilization of the [Fe I (CO 2 H)] À intermediate, increasing the overall rate of catalysis when compared to the non-functionalized analog, TMPFe (TMP = tetramesitylporphyrin). The rate constants (k app ) of the reduction reaction were calculated using the FOWA method which resulted in a higher TOF max for the complex Py 2 XPFe compared with TMPFe in neat MeCN (1.7 × 10 2 vs. 1.1 × 10 1 s À 1 ). The addition of weak Brønsted acids to the reaction mixture (TFE or PhOH) shows an increase in the rate of catalysis for both complexes, yet the Py 2 XPFe analog displays higher TOF max at each relative acid concentration, suggesting the hanging group beneficially impacts the rate of catalysis in the presence of these proton sources. The addition of Lewis acidic Sc 3 + to Py 2 XPFe also results in an increase in current density of the CO 2 reduction reaction. Resonance Raman as well as 1 H-NMR spectroscopy indicates coordination to the pyridine substituents.

The iron-based porphyrin complex containing a bispyridine-based hanging unit termed Py 2 XPFe was previously used as an effective catalyst for the reduction of protons to molecular hydrogen in solution. Here, the molecular compound was immobilized on a modified gold electrode surface and investigated by spectroelectrochemical methods under catalytic conditions. Immobilization of the Py 2 XPFe was facilitated using a pyridine-based amine linker molecule grafted to the gold electrode by electrochemical amine oxidation. The linker molecule denoted in this report as Pyr-1 allows for effective coordination of the iron porphyrin compound to the modified gold surface through axial coordination of the pyridine component to the Fe center. Resonance Raman spectroelectrochemistry was performed on the immobilized catalyst in pH 7 buffer at increasing cathodic potentials. This facilitates the electrochemical hydrogen evolution reaction (HER) while concurrently allowing for the observation of the v 4 , v 3 , and v 2 porphyrin marker bands, which are sensitive to oxidation and spin state changes at the metal center. The observed changes in these bands at decreasing potential indicate that the immobilized Py 2 XPFe exists in the formal highspin Fe III state before being reduced to the low-spin Fe II state resulting from axial interaction with the linker moiety. This Fe II state likely acts as the precatalyst for the HER reaction. Surfaced enhanced Raman spectroelectrochemistry was also conducted on the system as the gold electrode provides a sufficient surface enhancement effect so as to observe the bonding nature of the pyridine substituents within the second coordination sphere. As the potential is lowered cathodically, the pyridine ring breathing modes at 999 cm −1 are shown to increase in intensity due to protonation, which reach an intensity saturated limit whereat HER is conducted. This suggests that in pH 7 buffer, the increase in cathodic potentials facilitates protonation of the pyridine-based second coordination sphere. The extent to which protonation occurs can be viewed as a function of decreasing potential due to an increase in proton flux at the immobilized catalyst which, at the required onset potential for catalysis, aids in the reduction of protons to molecular hydrogen.

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.

Synthesis, characterization and reactivity studies of a hetero‐dicopper complex, particularly towards oxygen reduction are presented. A bischlorido copper(II) trishistidine‐type coordination unit is positioned directly above a copper porphyrin unit. The close distance between the two coordination fragments is secured by a rigid xanthene backbone. Surprisingly, the dinuclear complex is not active towards oxygen reduction unlike the earlier published mononuclear analogues with a bispyridylamine copper center. However, the compound architecture of this multinuclear metal complex is interesting and can play an important role in the development of new catalysts for ORR.