Electrocatalytic CO2 reduction by aminopyridine cobalt complexes: electronic effect of substituents on the pyridyl ring and mechanistic insights

In the last decade, several earth-abundant metal-based molecular catalysts have been found highly active for the photochemical or electrochemical CO2 reduction. However, despite their efficiency for the light-driven CO2-to-CO process,1 the electrocatalytic performances of Co complexes containing N4 or N5 ligands are still generally affected by catalyst deactivation,2 large overpotentials3 and low faradaic yields, due to either ligand decomposition or a preferential H2 evolution pathway under acidic conditions.4 To overcome these barriers, design of novel Co catalysts should be coupled to a deep understanding of the electrocatalytic mechanism, based on the characterization of key intermediates formed during the process. 5 Herein, we present a series of novel synthesized [CoII(Y,XPyMetacn)(OTf)2] complexes (1R, Scheme 1) containing N4 tetradentate ligands with general formula Y,XPyMetacn (1-[2′-(4-Y-6-X-pyridyl)methyl]-4,7-dialkyl-1,4,7-triazacyclononane),6 employed as catalysts for the electrochemical reduction of CO2. The introduction of different substituents at the - and -positions of the pyridine allowed us to systematically evaluate the effect of the electronic properties of the ligand on the catalytic activity. As highlighted by the electrochemical data, the redox non-innocent character of the Y,XPyMetacn ligand is extremely sensitive to the substitution at the pyridyl ring, and influences not only the E1/2(CoII/I) value, but also the nature of the reduction event itself, thus leading to different reactivity of the electrochemically generated CoI species towards CO2. Moreover, extensive spectroscopic (NMR) and spectroelectrochemical (IR and UV-Vis) studies were carried out to investigate the intermediates produced in the course of the catalytic process. Theoretical modelling provided also key mechanistic details for the CO2 reduction reaction. Scheme 1. General structures of the 1R complexes under study References 1 Z. Guo, S. Cheng, C. Cometto, E. Anxolabéhère-Mallart, S.-M. Ng, C.-C. Ko, G. Liu, L. Chen, M. Robert,T.-C. Lau, J. Am. Chem. Soc. 2016, 138, 9413−9416 2 K.-M. Lam, K.-Y. Wong, S.-M. Yang, C.-M. Che, Dalton Trans. 1995, 1103−1107 3 A. Chapovetsky, T. H. Do, R. Haiges, M. K. Takase, S. C. Marinescu, J. Am. Chem. Soc. 2016, 138, 5765−5768 4 D. C. Lacy, C. C. L. McCrory, J. C. Peters, Inorg. Chem. 2014, 53, 4980−4988 5 H. Sheng, H. Frei, J. Am. Chem. Soc. 2016, 138, 9959−9967 6 a) A. Call, F. Franco, S. Fernandez, N. Kandoth, J. M. Lluis J. Lloret-Fillol, Chem. Sci. 2016, submitted; b) A. Call, Z. Codola, F. Acuna-Pares, J. Lloret-Fillol, Chem. Eur. J. 2014, 20, 6171 – 6183