Pendant Hydrogen-Bond Donors in Cobalt Catalysts Independently Enhance CO₂ Reduction

Abstract

The bioinspired incorporation of pendant proton donors into transition metal catalysts is a promising strategy for converting environmentally deleterious CO2 to higher energy products. However, the mechanism of proton transfer in these systems is poorly understood. Herein, we present a series of cobalt complexes with varying pendant secondary and tertiary amines in the ligand framework with the aim of disentangling the roles of the first and second coordination spheres in CO2 reduction catalysis. Electrochemical and kinetic studies indicate that the rate of catalysis shows a first-order dependence on acid, CO2, and the number of pendant secondary amines, respectively. Density functional theory studies explain the experimentally observed trends and indicate that pendant secondary amines do not directly transfer protons to CO2, but instead bind acid molecules from solution. Taken together, these results suggest a mechanism in which noncooperative pendant amines facilitate a hydrogen-bonding network that enables direct proton transfer from acid to the activated CO2 substrate.

Second‐Sphere Biomimetic Multipoint Hydrogen‐Bonding Patterns to Boost CO2 Reduction of Iron Porphyrins

“…[5] An escape from this iron law was exemplified with catalysts having a local proton source or favorable second-sphere interactions. [13][14][15][16][17][18][19][20]22] In the case of FeTPP-Am, in which hydrogen-bond interaction with amide groups can be expected, only a small positive deviation is observed. This positive deviation is much more pronounced in the case of FeTPP-Ur catalyst which has comparable TOF to that of FeTPP at ca.…”

“…[10][11][12] Implementing these functionalities in the second coordination sphere [5] such as local proton source, (i.e. phenols, [13][14][15] carboxylic acids [16] ), H-bond donors (amines, [17] amides, [18] guanidine [19] ) and reaction intermediate stabilizers (ammonium cations, [20] and imidazolium moieties [21,22] ) have successfully lead to decrease significantly the overpotential while improving the catalytic turnover numbers (TONs) and frequencies (TOFs).…”

Even-handed subsystem selection in projection-based embedding

“…Intramolecular hydrogen bonding in a CO2 reduction complex As a first example of even-handed partitioning, we consider a cobalt aminopyridine complex that has been shown to catalyze the two-electron two-proton reduction of carbon dioxide to carbon monoxide. 58,59 Experimental and DFT studies on analogous Co, Fe, and Ni complexes suggest that CO 2 is stabilized by intramolecular hydrogen bonds from pendant protons, and that these protons may transfer to the CO 2 during the reduction process. [60][61][62] However, crystallography on this particular aminopyridine complex suggests that, prior to CO 2 binding, the pendant protons point away from the binding pocket, necessitating a conformational rearrangement in order for intramolecular hydrogen bonds to form.…”

Analytical gradients for projection-based wavefunction-in-DFT embedding

“…Projection-based embedding has proven to be a useful tool in a wide range of chemical contexts including transition-metal complexes, 19,20,24,27 protein aca) tfm@caltech.edu tive sites, 21,23 excited states [28][29][30] and condensed phase systems, 16 among others. [31][32][33][34][35][36][37] The development of analytical nuclear gradients for projection-based embedding will expand its applicability to include geometry optimization, transition state searches, and potentially ab initio molecular dynamics.…”