Positional effects of second-sphere amide pendants on electrochemical CO2 reduction catalyzed by iron porphyrins

Abstract

The development of catalysts for electrochemical reduction of carbon dioxide offers an attractive approach to transforming this greenhouse gas into value-added carbon products with sustainable energy input. Inspired by natural bioinorganic systems that feature precisely positioned hydrogen-bond donors in the secondary coordination sphere to direct chemical transformations occurring at redox-active metal centers, we now report the design, synthesis, and characterization of a series of iron tetraphenylporphyrin (Fe-TPP) derivatives bearing amide pendants at various positions at the periphery of the metal core. Proper positioning of the amide pendants greatly affects the electrocatalytic activity for carbon dioxide reduction to carbon monoxide. In particular, derivatives bearing proximal and distal amide pendants on the ortho position of the phenyl ring exhibit significantly larger turnover frequencies (TOF) compared to the analogous para-functionalized amide isomers or unfunctionalized Fe-TPP. Analysis of TOF as a function of catalyst standard reduction potential enables first-sphere electronic effects to be disentangled from second-sphere through-space interactions, suggesting that the ortho-functionalized porphyrins can utilize the latter second-sphere property to promote CO₂ reduction. Indeed, the distally-functionalized ortho-amide isomer shows a significantly larger through-space interaction than its proximal ortho-amide analogue. These data establish that proper positioning of secondary coordination sphere groups is an effective design element for breaking electronic scaling relationships that are often observed in electrochemical CO₂ reduction.

Scheme 1. The active site of Ni–Fe CO dehydrogenase, showing activated CO₂ in red and the nearby network of hydrogen bond donor amino acid residues in blue (left), inspires the design of ortho-amide-functionalized Fe porphyrins examined in this work (right).

Fig. 1. Titration of 3,5-[bis(trifluoromethyl)phenyl]amide to Fe-TPP under CO₂ showing current increases with increasing concentrations of amide. Conditions: 0.1 M TBAPF₆ in DMF, 1 mM Fe-TPP.

Scheme 2. Synthesis of ortho- and para-functionalized tetraphenylporphyrin ligands ortho-1-amide, ortho-2-amide, para-1-amide, and para-2-amide.

Fig. 2. Solid-state structure of Zn–ortho-1-amide. Non-coordinated solvent molecules and non-amide hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% level.

Scheme 3. Positional isomers of amide-functionalized iron tetraphenylporphyrins examined in this work.

Fig. 3. Cyclic voltammograms of amide-functionalized porphyrins and unfunctionalized Fe-TPP under CO₂ atmosphere. Conditions: 1 mM catalyst, 100 mM phenol, 0.1 M TBAPF₆ in DMF, saturated with CO₂ (0.23 M); scan rate 100 mV s^–1. i represents current under catalytic conditions; i0p represents the cathodic peak height of the formal Fe^II/I couple under inert atmosphere.

Fig. 4. Observed rate constants (s^–1) for catalytic CO₂ reduction as a function of PhOH concentration for Fe–ortho-1-amide (blue diamonds), Fe–ortho-2-amide (red squares), Fe–para-1-amide (purple upward triangles), and Fe–para-2-amide (orange downward triangles). Conditions: 1 mM catalyst, 0.1 M TBAPF₆ in DMF; 0.23 M CO₂; scan rate is 100 mV s^–1. k_obs values were determined by FOWA.

Fig. 5. Catalytic Tafel plots for Fe–ortho-1-amide (blue diamond), Fe–ortho-2-amide (red square), Fe–para-1-amide (purple upward triangle), Fe–para-2-amide (orange downward triangle), and Fe-TPP (black circle). Conditions: 0.1 M TBAPF₆ in DMF; 0.23 M CO₂; 100 mM PhOH; scan rate is 100 mV s^–1. TOF = k_obs and was determined by FOWA.

Fig. 6. Correlation between log(TOF_max) and E0cat illustrating the through-space interactions that promote catalysis in the case of Fe–ortho-1-amide (blue diamond) and Fe–ortho-2-amide (red square). Porphyrin catalysts with no through-space interactions are shown in black: Fe–para-(CF₃)₄ (white circle), Fe-TPP (black circle), and Fe–para-(OMe)₄ (gray circle). Fe–para-2-amide (orange downward triangle) and Fe–para-1-amide (purple upward triangle) show minimal through-space effects. TOF_max determined by FOWA in the presence of 100 mM phenol and 0.23 M CO₂.