Electronic and Steric Tuning of Catalytic H2 Evolution by Cobalt Complexes with Pentadentate Polypyridyl-Amine Ligands

Structural modifications of molecular cobalt catalysts have provided important insights into the structure–function relationship for the hydrogen evolution reaction. We have shown that replacement of equatorial pyridines with more basic and conjugate isoquinoline groups of a pentadentate ligand results in lower overpotential and higher catalytic activity for electro- and photolytic H₂ production in aqueous solutions. To fully understand the electronic and steric effects of the axial group that lies trans to the proposed cobalt hydride intermediate, isoquinoline groups were introduced in two new pentadentate ligands, N,N-bis(2-pyridinylmethyl)[3-(2-pyridinyl)isoquinoline)]-1-methanamine (DPA-1-MPI) and N,N-bis(2-pyridinylmethyl)[1-(2-pyridinyl)-isoquinoline)]-3-methanamine (DPA-3-MPI). Despite a slight structural difference of the introduced isoquinoline group, the resulting cobalt complexes display drastic changes in their electro- and photochemical properties. There are positive shifts of 290 and 260 mV, respectively, for the Coᴵᴵ/Coᴵ and Coᴵᴵᴵ–H/Coᴵᴵ–H couples from [Co(DPA-1-MPI)(H₂O)](PF₆)₃ to [Co(DPA-3-MPI)(H₂O)](PF₆)₃, with the former being ∼32 times as active as the latter in photocatalytic H₂ production. Density functional theory (DFT) calculations show that the protonation of Coᴵ to yield the Coᴵᴵᴵ–H species is energetically more favorable for [Co(DPA-1-MPI)(H₂O)](PF₆)₃ than that of [Co(DPA-3-MPI)(H₂O)](PF₆)₃. Both experimental results and DFT computations suggest that the presence of a planar conjugate bipyridyl unit or its isoquinoline derivative is a key feature for stabilizing low valent Coᴵ species toward proton binding. The incorporation of an electron-donating group trans to the proposed Co–H species also facilitates proton binding and H–H bond formation, which is proposed to occur by the heterolytic coupling of Coᴵᴵ–H species. The overall catalytic H₂ evolution is presented as the modified electron transfer (E)–proton transfer (C)–electron transfer (E)–proton transfer (C) (mod-ECEC) pathway. This study provides important new insight into the electronic and steric factors controlling catalytic H₂ production by Co complexes with pentadentate ligands.