Steric Tuning of Spin States and Redox Potentials in Tris(imidazole) Triazacyclononane Complexes of Fe2+/3+ and Co2+/3+

A series of Co^2+/3+ and Fe^2+/3+ complexes is prepared using three variants of a hexadentate tris(imidazole)triazacyclononane ligand bearing different 4-alkyl substituents on the imidazole rings. The steric bulk of the alkyl substituent (R=H, ⁱPr, or ^tBu) alters the preferred size of the ligand binding cavity by inhibiting close approach of the imidazole donors with bulky substituents. The resulting changes in geometry, redox potentials, spin states, and optical properties are catalogued across the series, demonstrating redox potential tuning over at least 670 mV as well as spin state switching based on the choice of substituent. The ligand field splitting of the complexes decreases with increasing bulk of the substituents. Tuning of the steric bulk of the substituents in these positions therefore allows for the electronic properties of the complexes to be fine-tuned in a manner orthogonal to the donor properties of the substituents.