Electroactive Co(iii) salen metal complexes and the electrophoretic deposition of their porous organic polymers onto glassy carbon

This paper reports the CO2 electroreduction properties of three bis-bromo Co(iii) salen metal complexes and their Porous Organic Polymers (POPs) as a platform for using the salen core as a multi-electron reducing agent. Although Co(iii) salen metal complexes have been studied extensively for their chemical catalysis with CO2, their electrochemical behaviour, particularly their reduction, in the presence of CO2 is much less explored. The discrete Co(iii) complexes enabled the reduction of CO2 to CO in faradaic efficiencies of up to 20%. The reductive electrochemical processes of Co(iii) salen complexes are relatively unknown; therefore, the mechanism of reduction for the complexes was investigated using IR and UV-Vis-NIR spectroelectrochemical (SEC) techniques. The discrete bis-bromo salen complexes were incorporated into POPs with tris-(p-ethynyl)-triphenylamine as a co-ligand and were characterised using solid state NMR, IR, UV-Vis-NIR and Field Emission Scanning Electron Microscopy (FE-SEM). The POP materials were electrophoretically deposited onto glassy carbon under milder conditions than those previously reported in the literature. Direct attachment of the POP materials to glassy carbon enabled improved solid state electrochemical analysis of the samples. The POP materials were also analysed via SEC techniques, where a Co(ii/i) process could be observed, but further reductions associated with the imine reduction compromised the stability of the POPs.

The solution was stirred at room temperature for 2 h, prior to the removal of N 2 and the addition of LiCl (0.136 g, 3.21 mmol). The mixture was allowed to stir in air at room temperature for 2 d.

POPs containing Co(III) salen complexes.
General procedure for the synthesis of POPs.

Calculations
If an electrochemical system obeys pseudo-first order kinetics (proof that the reaction is first order in the analyte and the concentration of CO 2 , and that the concentration of CO 2 is large in comparison to that of the analyte), it is possible to calculate kinetic data for the interaction of salen complexes with CO 2 . Upon comparison of the peak currents for salen complexes in the presence of CO 2 with the absence of CO 2 , an expression for k cat [Q], or TOF, in terms of the ratio, which can be directly examined from CV. (1) where F = Faraday constant (C mol -1 ), ν = scan rate (V s -1 ), n p = number of electrons facilitated by the redox process, R = universal gas constant (J mol -1 K -1 ), T = temperature (K), n cat = number of electrons facilitated in the catalytic transformation, i cat = peak catalytic current (mA/cm 2 ), i p = peak current under N 2 (mA/cm 2 ).
Low pressure CO 2 measurements (up to 1 bar) were carried out at three temperatures (typically 288, 298 and 308 K) on the ASAP2020 or 3-Flex as described above. The data were modelled using a virial equation or the interpolation function before applying the Clausius-Clapeyron relation. The heat of adsorption for CO 2 was determined by comparing CO 2 isotherms at 288, 298 and 308 K. Isosteric heat of adsorption calculations (Q st ) for CO 2 at these temperatures were undertaken using the Clausius-Clapeyron equation (2) (ln ) = -( )( 1 ) + where P = pressure (mbar), n = amount of gas adsorbed (mol/mol), T = temperature (K), R = universal gas constant (J mol -1 K -1 ) and C = constant).
The selectivity (S) for adsorption of CO 2 over N 2 was estimated from the single-component N 2 and CO 2 room temperature isotherm data. The values for this approximation are derived from an approximate flue gas composition of 15% CO 2 , 75% N 2 and 10% other gases, at a total pressure of 1 bar. (3) where q = quantity of gas adsorbed (mmol g -1 ), p = partial pressure at which each gas is adsorbed). Figure S1: TGA of POPCo1 (black), POPCo2 (red) and POPCo3 (blue) taken from 25 to 650 °C. The temperature was ramped at 1 °C min -1 . Figure S2: Solid State ATR-IR measurements of (i) Co1 (ii) Co2 (iii) Co3 (iv)POPCo1 (v) POPCo2 and (vi) POPCo3. * denotes the shift in the ν C=N stretch from the discrete complexes to the POPs, while the ν C≡C stretch appears in the polymers but not in the discrete complexes. Figure S3: Solid State UV-Vis-NIR measurements of (i) Co1 (ii) Co2 (iii) Co3 (iv) POPCo1 (vi) POPCo2 (v) POPCo3. * denotes the shift in bands from the discrete complexes to the POPs, while the T denotes the band that appears from the TPA co-ligand.  Figure S4: 13 C CPTOSS of Co2 (above) and POPCo2 (below). The full 13 C NMR spectra are plotted in blue, while the spectra in red are the non-protonated or methyl carbon species detected after 40 μs of dipolar dephasing. Residual ethanol is denoted with a #, while residual triethylamine is noted with a * Figure S5: 13 C CPTOSS of Co3 (above) and POPCo3 (below) the aromatic salen POPs, as well as the POP made in the absence of salen metal complex. The full 13 C NMR spectra are plotted in blue, while the spectra in red are the nonprotonated or methyl carbon species detected after 40 μs of dipolar dephasing. Residual ethanol is denoted with a #, while residual triethylamine is noted with a * Figure S6: 13 C CPTOSS of POPTPA. The full 13 C NMR spectra are plotted in blue, while the spectra in red are the nonprotonated or methyl carbon species detected after 40 μs of dipolar dephasing. Residual triethylamine is noted with a * Figure S7: DFT pore size distributions for POPCo1 (black), POPCo2 (red) and POPCo3 (blue).      Figure S17: 1 H NMR of the Co2 bulk electrolysis solution after work up from CPE in D 2 O at 300 MHz under CO 2 saturation after 30 min (black), 60 min (red), 90 min (blue), 120 min (cyan) and in the absence of CO 2 after 120 min (orange). The peak at δ = 8.00 ppm corresponds to the generation of formic acid. Spectra were referenced to D 2 O. E pc = 1.85 V vs. Ag/Ag + . Figure S18: Solid state CV of A POPCo1 B POPCo2 and C POPCo3 under N 2 (black) and under CO 2 (red) with TFE (0.14 mmol-blue, 0.28 mmol-green) (0.1 M LiBF 4 /MeCN as the supporting electrolyte, scan rate: 0.025 V s -1 , Fc (1 mM) was used as an internal standard). N 1 denotes the new