One of the key challenges of modern chemistry is to couple renewable energy sources, such as sunlight, to energy storage, by efficiently converting energy-poor molecules into energy-rich ones. Nature accomplishes these intricate multi-electron, multi-proton transformations using enzymes deriving from earth-abundant elements. As chemists, we must take inspiration from Nature and create viable and robust catalysts. The work herein describes progress towards electrocatalytic hydrogen evolution, electrocatalytic carbon dioxide reduction, and small molecule activation using earth abundant transition metal complexes. These investigations focus on first-row transition metal complexes with bio-inspired, weak ligand field polypyridine ligands.
To introduce this work, Chapter 1 surveys recent work undertaken by the Long and Chang laboratories towards electrochemical and photochemical hydrogen evolution using cobalt- and molybdenum- polypyridine complexes. In particular, the Chapter highlights a key advantage of synthetic molecular catalysts—through judicious design, molecular catalysts can retain the small, functional units of their biological or materials counterparts. This is accomplished with the use of semi-rigid polypyridine ligands that stabilize the transition metal ion. In conjunction with the appropriate anions, these catalyst systems are soluble in water, which is the most abundant proton source and solvent on this planet. Furthermore, the activities of these catalysts are sensitive to electronic tuning of the peripheral ligand, thus providing another synthetic opportunity to optimize catalytic performance.
Taking into account the tunability of molecular catalysts, Chapter 2 focuses on electronic modifications in a family of isostructural pentapyridine cobalt complexes for hydrogen evolution. Using a combination of synthetic methods, crystallography, and electrochemical methods, it is identified that the redox-active ligand scaffold PY4IqMe2 (PY4IqMe2 = 1,3-bis(1,1-di(pyridin-2-yl)ethyl)isoquinoline) affords a low-valent cobalt(I) ion that is protonated to form a cobalt(III) hydride fastest. This kinetic advantage is due to the levelling of two one-electron redox couples, which enables a single metal center to better perform two-electron reactions. Moreover, electrochemical kinetic analyses identify an ECEC′ mechanism as the dominant pathway for hydrogen evolution catalysis by pentapyridine cobalt complexes.
Chapter 3 applies the concepts established in proton reduction chemistry in Chapters 1 and 2 towards noble metal-free electrocatalytic carbon dioxide reduction. A series of iron(II) complexes of the functionalized tetrapyridine ligands bpyRPY2Me (bpyPY2Me = 6-(1,1-di(pyridin-2-yl)ethyl)-2,2′-bipyridine) is systematically studied in pursuit of water-stable molecular Fe complexes that are selective for the electrocatalytic formation of carbon monoxide from carbon dioxide. Taking advantage of the inherently high degree of tunability of this ligand manifold, protic functional groups of varying acidities (–H, –OH, –OMe, –NHEt, and –NEt2) were installed into the ligand framework to systematically modify the second coordination sphere of the Fe center. Comparative catalytic evaluation of this set of compounds via voltammetry and electrolysis experiments identified [(bpyNHEtPY2Me)Fe]2+ in particular as an efficient, iron-based, non-heme CO2 electro-reduction catalyst that displays significant selectivity for the conversion of CO2 to CO in acetonitrile solutions with 11 M water. We propose that the NH group either acts as a local proton source or is involved in CO2 capture. Interestingly, the complex with the most acidic functional group in the second coordination sphere, [(bpyOHPY2Me)Fe]2+, favors formation of H2 over CO. These results highlight the sensitivity of catalytic metrics to adjustment of the acidity of the second coordination sphere functional group and emphasize the continued untapped potential that synthetic molecular chemistry offers in the pursuit of next-generation CO2 reduction electrocatalysts.
Leveraging the electrophilicity of divalent metal–polypyridine complexes, Chapter 4 presents the synthesis of coordinatively-unsaturated, cationic metal complexes towards the binding and activation of weak σ donors such as N2, CO, H2, and ethylene. Using a weakly coordinating borate anion, pentapyridine–metal chloride complexes of the form [(PY5Me2)MCl]+ (PY5Me2 = 2,6-bis[1,1-bis(2-pyridyl)ethyl]pyridine) are synthesized for the first-row transition metals, with the exception of Mn(II). Notably, the complex [(PY5Me2)TiCl]+ represents a rare, well-defined titanium(II) cation without π-acidic ligands. Using Grignard reagents, the chloride ligand in Cr(II), Fe(II), Co(II), and Ni(II) complexes can be exchanged for alkyl and aryl ligands to form cationic [(PY5Me2)MCH3]+ and [(PY5Me2)MC6H5]+ complexes. Protonolysis of these complexes in solvents with low donor strength result in highly electrophilic, coordinatively-unsaturated coordination fragments that rapidly scavenge trace water or bind ethereal donors.
