Remarkable Enhancement of Catalytic Activity of Cu‐Complexes in the Electrochemical Hydrogen Evolution Reaction by Using Triply Fused Porphyrin

Abstract A bimetallic triply fused copper(II) porphyrin complex (1) was prepared, comprising two monomeric porphyrin units linked through β–β, meso–meso, β′–β′ triple covalent linkages and exhibiting remarkable catalytic activity for the electrochemical hydrogen evolution reaction in comparison to the analogous monomeric copper(II) porphyrin complex (2). Electrochemical investigations in the presence of a proton source (trifluoroacetic acid) confirmed that the catalytic activity of the fused metalloporphyrin occurred at a significantly lower overpotential (≈320 mV) compared to the non‐fused monomer. Controlled potential electrolysis combined with kinetic analysis of catalysts 1 and 2 confirmed production of hydrogen, with 96 and 71 % faradaic efficiencies and turnover numbers of 102 and 18, respectively, with an observed rate constant of around 107 s−1 for the dicopper complex. The results thus firmly establish triply fused porphyrin ligands as outstanding candidates for generating highly stable and efficient molecular electrocatalysts in combination with earth‐abundant 3d transition metals.

equipped with molecular-sieve columns and dual TCD and FID detectors using helium as a carrier gas.

Control experiments:
Reduction by CoCp2: The reduction of 1 by CoCp2 was monitored via UV-Vis-NIR spectroscopy at room temperature. One electron reduced species, 1  was generated by a treatment of 10 mL solution of 1 (20 mg (0.011 mmol) in dry CH2Cl2 with one equivalent of CoCp2 (2.07 mg, 0.011 mmol) under inert atmosphere. The reaction was allowed to stir for 30 min followed by removal of a solvent under reduced pressure. The residue was dissolved again in dry CH2Cl2 and UV-Vis-NIR spectra of the species was recorded immediately. The resulting solution was further treated with excess TFA and UV-Vis-NIR spectra of the resulting species was also recorded.
Reduction by KC8: Two electron reduced species, 1 2 was generated by treating 10 mL solution of 1 (20 mg (0.011 mmol) in dry CH2Cl2 with 2 equivalents of KC8 under inert reaction conditions. The color of the solution changed immediately from violet to deep green and the reaction was allowed to stir for another 10 minutes at room temperature. The solvent was then removed under reduced pressure followed by a filtration to remove unreacted KC8. UV-Vis-NIR spectra of the resulting solution was then recorded immediately. Further reaction of the doubly reduced species with excess TFA was also monitored by UV-Vis-NIR spectroscopy.

Spectroelectrochemistry:
Spectro-electrochemical measurements were carried out in an optically transparent thin-layer electrochemical (OTTLE) [3] cell (CaF2 windows) with a platinum mesh working electrode, a platinum-mesh counter electrode, and a silver-foil pseudoreference electrode.

Electron Paramagnetic Resonance:
Electron Paramagnetic Resonance. EPR spectra at X-band frequency (ca. 9.5 GHz) were obtained with a Magnettech MS-5000 benchtop EPR spectrometer equipped with a rectangular TE 102 cavity and TC HO4 temperature controller. The measurements were performed in synthetic quartz glass tubes. Spectral simulations were performed using the EasySpin package [4] running in Matlab R2018b.

Density Functional Theory:
All calculations were performed with the ORCA program package, versions 4.0.1.2 and 4.2.8 [5] The geometries of all species were optimized using the PBE0 functional, [6] the def2-SVP basis sets on all atoms except for Cu, for which the def2-TZVP basis set was used. [7] Solvation was taken into account using the using the SMD method together with the CPCM model [8] using DMF as solvent, and dispersion corrections were included using the D3 dispersion correction model. [9] The resolution-of-the identity (RI) approximation, [10] with matching basis sets, [11] as well as the RIJCOSX approximation (combination of RI and chain-of-spheres algorithm for exchange integrals) were used to reduce the time of calculations. Numerical frequencies calculations were used in order to check that the optimized structures were local minima and to obtain Gibbs free enthalpies. To obtain more reliable energetics single-point calculations were performed using the optimized geometries, the PBE0 functional and def2-TZVP basis sets on all atoms. Low-lying excitation energies were calculated with time-dependent DFT (TD-DFT).
For all calculations spin densities were calculated according to the Löwdin population analysis. [12] Broken-symmetry calculations [13] were carried out using optimized geometry to evaluate the exchange coupling constants. Plots of spin-densities and optimized geometries were performed using Chemcraft. [14] SUPPORTING INFORMATION

Overview of synthesis
Step -I: Step -II: Synthetic Procedure: Methyl 4-formylbenzoate were synthesized following a reported procedure, [15] by reacting 4formylbenzoic acid (1.0 eq.) with methyl iodide (1.2 eq.) and K2CO3 (1.0 eq.) in DMF at 60 ºC for 2-4 hours. The reaction was quenched with H2O. The crude products were extracted with DCM, the organic phase was washed with H2O and dried over Na2SO4. Chromatographic purification in a small silica column (DCM) was performed to remove yellow impurities. The identity and purity of the product was confirmed by NMR.

S11
For Cu(II) metallation, a solution of Cu(OAc)2 (14 mg, 0.073 mmol) in MeOH was added to a solution of 6 (50 mg, 0.049 mmol) in CHCl3, and the resulting mixture was stirred for 1 h. After the complete metalation was confirmed by TLC, the mixture was poured into water, and the porphyrin products were extracted with CHCl3. The organic layer was separated, and the combined extracts were washed with water and brine and dried over anhydrous Na2SO4. The solvent was removed and the residue was purified by silica gel column chromatography. The desired red colored pure product was eluted by DCM/hexane (1:1) solvent mixture followed by a recrystallization from chloroform/methanol yielded pure 2. For Cu(II) metallation, a saturated solution of Cu(OAc)2 in MeOH was added to a solution of free base fused porphyrin in CHCl3, and the resulting mixture was refluxed for overnight. After the complete metallation was confirmed by TLC, the mixture was poured into water, and the porphyrin products were extracted with CHCl3. The organic layer was separated, and the combined extracts were washed with water, brine soluton and dried over anhydrous Na2SO4.
The solvent was removed and the residue was purified via silica gel column chromatography.
Purfification on a silica gel (60M) column by using DCM/hexane (7:3) solvent mixture as the eluent resulted in the isolation of violet colored pure compound. Removal of solvent followed by a crystallization from the mixture of DCM and methanol yielded pure complex 1.

Calculation of observed rate-constant (kobs).
The kinetic observed rate constant kobs (also referred to as TOF) was calculated using the following relation