Standard redox potential of [Co(bpy)₃]2+/3+ and [Co(phen)₃]2+/3+ redox couples using a density functional theory protocol

The electronic structures and redox properties of the [Co(bpy) _3 ] ^2+/3+ and [Co(phen) _3 ] ^2+/3+ redox couple were investigated using the two different exchange–correlation functional, namely CAM-B3LYP and TPSSh, with the def2-SVP and def2-TZVP basis sets. Our results indicate that [Co(bpy) _3 ] ^2+ and [Co(phen) _3 ] ^2+ complexes exhibit a high-spin ground state, whereas [Co(bpy) _3 ] ^3+ [Co(phen) _3 ] ^3+ complexes adopt a low-spin ground state. Both basis sets and exchange–correlation functionals consistently predict the same ground-state spin configurations for these complexes. For [ Co(bpy)_3 ]^2+ , the energy gap between the doublet and quartet spin states is relatively small when using the TPSSh functional, amounting to 2.21 kcal/mol with the Def2-SVP basis set and 0.09 kcal/mol with the Def2-TZVP basis set. In contrast, when the CAM-B3LYP functional is employed, the energy splitting becomes significantly larger, with values of 7.91 kcal/mol and 6.12 kcal/mol for the Def2-SVP and Def2-TZVP basis sets, respectively. We also observed similar trends for [ Co(phen)_3 ]^2+ . In contrast, [ Co(bpy)_3 ]^3+ exhibits a significantly larger energy separation between spin states. Using the CAM-B3LYP functional, the energy difference between the singlet ground state and the quintet excited state is calculated to be 41.42 kcal/mol and 42.07 kcal/mol with the Def2-SVP and Def2-TZVP basis sets, respectively. When the TPSSh functional is employed, this singlet-quintet energy gap becomes slightly larger, further reinforcing the strong preference for the low-spin singlet configuration in the oxidized complex. Additionally, we have observed that adiabatic ionization potentials are less affected by the choice of basis when the CAM-B3LYP functional is used. In contrast, vertical ionization potential exhibits a more pronounced basis set dependence. However, ionization potentials calculated with the TPSSh functional exhibit significantly reduced sensitivity to basis set choice. The calculated oxidation potentials ( E^∘ ) using CAM-B3LYP functional for the [Co(bpy) _3 ] ^2+/3+ couple are 5.32 V and 5.26 V employing the def2-SVP and def2-TZVP basis sets, respectively, whereas ( E^∘ ) calculated at the TPSSh functional is 4.62 V and 4.55 V. We also have observed that the standard oxidation potential of [ Co(phen)_3 ]^2+/3+ is relatively higher than the values of [Co(bpy) _3 ] ^2+/3+ . Computational analyses of [Co(bpy) _3 ] ^3+/2+ and [Co(phen) _3 ] ^3+/2+ establish a unified redox picture grounded in Gaussian MO compositions and NBO donor–acceptor metrics. In both families, Co(III) displays ligand– π HOMOs with a Co(3d) block immediately below, while Co-centered d acceptors comprise the LUMO/LUMO+1. One-electron reduction populates a metal-centered acceptor, yielding Co(II) doublets with a two-long/four-short Co–N pattern and quartets with uniformly longer Co–N bonds. NBO E^(2) values for LP(N) → LV(Co) σ donation decrease systematically from Co(III) to Co(II), rationalizing bond elongation and modest increases in Co natural charges. Phenanthroline narrows the d/π ^* gap relative to bipyridine, making ligand-centered pathways slightly more competitive; nevertheless, the primary Co(III) → Co(II) event remains metal-centered across spin states examined. These findings contribute to a deeper understanding of redox property calculations via DFT and may support the potential application of cobalt-containing redox couples in thermoelectric materials.