Consecutive Ligand‐Based Electron Transfer in New Molecular Copper‐Based Water Oxidation Catalysts

Abstract Water oxidation to dioxygen is one of the key reactions that need to be mastered for the design of practical devices based on water splitting with sunlight. In this context, water oxidation catalysts based on first‐row transition metal complexes are highly desirable due to their low cost and their synthetic versatility and tunability through rational ligand design. A new family of dianionic bpy‐amidate ligands of general formula H2LNn− (LN is [2,2′‐bipyridine]‐6,6′‐dicarboxamide) substituted with phenyl or naphthyl redox non‐innocent moieties is described. A detailed electrochemical analysis of [(L4)Cu]2− (L4=4,4′‐(([2,2′‐bipyridine]‐6,6′‐dicarbonyl)bis(azanediyl))dibenzenesulfonate) at pH 11.6 shows the presence of a large electrocatalytic wave for water oxidation catalysis at an η=830 mV. Combined experimental and computational evidence, support an all ligand‐based process with redox events taking place at the aryl‐amide groups and at the hydroxido ligands.


O2 evolution experiments
Controlled Potential Electrolysis (CPE) experiments were performed at different potentials and different pH values to catalyze the water oxidation reaction by the complexes by using a two-compartment cell closed with a septum. As working electrode large surface BDD electrodes (rectangular shape with 1.5 cm 2 surface) were used together with a silver/silver chloride (KCl sat.) as a reference electrode. These ones were placed in one of the compartments that was filled with a 1.5 mM solution of the complex (phosphate buffer pH 7, borate buffer pH 9 or phosphate buffer pH 11.6, of 0.1 M ionic strength). In the other compartment, containing only the buffer solution, a mesh platinum counter electrode was used. The oxygen evolution was monitored with an OXNP type Clark electrode in gas phase (from Unisense Company). The CPE was carried out using an IJ-Cambria CHI-660 potentiostat and was started as soon as the oxygen sensor signal was stable under air atmosphere. During the experiment, solutions of both compartments were vigorously stirred. Calibration of the oxygen sensor was performed after each experiment by adding known amounts of pure oxygen into the cell using a Hamilton syringe. The results of the water oxidation catalysis with the complexes were compared with blank experiments under the same conditions but in the absence of the complex. The Faradaic efficiency was determined according to the total charge passed during the CPE and the total amount of generated oxygen by considering that water oxidation is a 4eoxidation process.

Foot of the Wave Analysis (FOWA). kobs Calculation
Under catalytic conditions FOWA equation is operative.
Equation (1) where kobs is the apparent WNA pseudo-rate constant (k[H2O]), E 0 cat corresponds to the standard potential for the catalytic wave (E 0 cat according to the DPVs shown in Figure S46), i is the current in the presence of substrate, ip corresponds to the peak current of oneelectron redox process of the catalyst (extracted from the Cu II /Cu I couple when available), F is the faradaic constant (96485 C mol -1 ), T is the temperature (298 K), v is the scan rate (100 mV s -1 ) and R is 8.314 J mol -1 K -1 . 3

TON Calculation
The total TON values can be obtained from the oxygen evolution experiment taking in account the total amount of catalyst present in the solution by using equation (2). However, since only the catalyst present in the layer of the solution in contact with the electrode is involved in the water oxidation reaction, this TON value is underestimated. Lin and co-workers adapted it to the formula (3) based on the previous methodology developed by Savéant and co-workers, which gives a more realistic TON value based on the amount catalyst in contact with the electrode. 4 = µ 2 µ .  [2,2'-bipyridine]-6,6'-dicarboxylic acid were suspended in 20 mL of SOCl2 and the mixture was refluxed at 85 °C under a nitrogen atmosphere during 6 hours. After complete dissolution of the reactant, SOCl2 was completely removed under vacuum, yielding a white powder corresponding to the acyl chloride derivative. The white solid was re-suspended in 40 mL of dry DCM and the temperature was decreased until 0 °C using an ice bath. Then, 4 eq. of NEt3 were added dropwise and stirred for 10 minutes. Finally, a previously prepared dispersion of the corresponding phenylamine or naphthylamine (4.1 mmol, 2.0 eq.) in 40 mL of dry DCM were added dropwise to the reaction volume and the mixture was vigorous stirred for 72 h at room temperature.

Synthesis of Cu-complexes
The general procedure for the synthesis of the copper complexes was adapted from a previous published work. 6 Typically, 0.2 mmol of the corresponding ligand were suspended in 8 mL of MeOH and stirred during 15 minutes. Afterwards, 7.2 mL (4 eq.) of 0.1 M NaOHaq. were added to the reaction mixture and vigorously stirred during 30 minutes at room temperature. After complete dissolution of the ligand, 0.2 mmol of copper perchlorate hexahydrate dissolved in 8 mL of MeOH were added dropwise to the mixture, which were allowed to react overnight (16 h) at room temperature. Then, the reaction mixture was filtrated and MeOH was evaporated under vacuum, and the remaining solution was diffused with the corresponding solvent (further details about crystallization are explained in Table S1), yielding the corresponding pure copper complexes.

[(L1)Cu]·1.5 H2O·MeOH
Yield                 can be potentially assigned to molecular Cu degradation products based on the isotopic cracking pattern. The peak at m/z= 712.0 is assigned in Figure S28.

X-Ray Crystallography
Crystal preparation: Table S1. Summary of the conditions employed for the crystallization of the complexes studied in this work.
Compound Methodology H2L5 2-Crystals were grown in water by slow diffusion of EtOH.

[(L4)Cu] 2-
Crystals were grown in water by slow diffusion of acetone.

[(L5)Cu] 2-
Crystals were grown in water by slow diffusion of acetone.

[(L7)Cu] 2-
Crystals were grown in water by slow diffusion of acetone.

Computational Details
All calculations were carried out with the Gaussian09 program package 13 using DFT methodology. We used B3LYP as the functional, including D3 empirical dispersion correction developed by Grimme (B3LYP-D3). 14,1514,15 The basis set was split into 6-31+G(d) for C, N, S, O and H 16,17,18 and LANL2TZ(f) for Cu. 19,20,21,22,23 Implicit solvation was introduced through the SMD model, 24 with water as the solvent. All geometry optimizations were computed in solution without symmetry restrictions. The nature of all computed stationary points as minima or transition states was confirmed through vibrational frequency calculations. Free energy corrections were calculated at 298.15 K and 105 Pa pressure, including zero point energy corrections (ZPE). In addition, a correction term of 1.89 kcal/mol (at 298 K) was added when necessary to account for the standard state concentration of 1 M, except for water, whose concentration was considered to be 55.6 M and its correction term 4.3 kcal/mol. Unless otherwise mentioned, all reported energy values are free energies in solution. In addition, stability of the wave function was checked for the calculations (stable option in G16).
The reaction energy barriers of the Minimum Energy Crossing Points (MECP) were estimated from potential energy relaxed scan from the crossing point of the quartet and doublet potential energy surfaces, along the O-O internal reaction coordinate, when the transition states could be found (or do not exist), applying entropic corrections from the minima to compute an estimated free energy change.
In the transformation from free energies to electrochemical magnitudes the values of 4.28 V for the absolute potential of the standard hydrogen electrode 25 and -11.72 eV for the free energy of the proton in aqueous solution at pH=0 were taken from the literature. 26 The value for the free energy of the proton was translated to the experimental pH value by adding a correction term of -0.059*pH, following the same procedure described elsewhere. 27 The functional for the DFT calculations was B3LYP-D3 based on the calibration carried out in a previous work on related systems, 6,28 where its performance was compared with that of M06, M06-D3, M06L, M06-2X, B97xD and B97D. In order to validate this DFT methodology, the calculated optimized structures were compared to the X-Ray ones. Table S4 summarizes all the main metrics for the coordination environment of the copper metal center. In addition, we have recalculated as single points all the species involved in Figure 3 using a larger basis set (6-311++G(3d,2p) for all the atoms except Cu/LANL2TZ(f) for Cu) and no significant differences were found (see Figure S65).      . 29 This results suggest that the energy of solvation for OHmight be underestimated by SMD calculations, leading to less favorable coordination of the OHto the Cu center. However, the calculated speciation in the different oxidation states is not substantially altered from that proposed using SMD, with the exception of formation of complex [(L4 ·· -k-N 3 )Cu(OH)2] 2that is now slightly uphill but still accessible at room temperature towards formation of O2. Therefore, the proposed reaction pathways are still supported by DFT but the calculated barrier for the O-O bond formation step might be overestimated when using the SMD model.