Predictable Electronic Tuning By Choice of Azine Substituent in Five Iron(II) Triazoles: Redox Properties and DFT Calculations.

Five new mononuclear iron(II) tris-ligand complexes, and four solvatomorphs, have been made from the azine-substituted 1,2,4-triazole ligands (L ): [Fe (L ) ](BF ) (1), [Fe (L ) ](BF ) (2), [Fe (L ) ](BF ) (3), [Fe (L ) ](BF ) (4), and [Fe (L ) ](BF ) (5). Single-crystal XRD and solid-state magnetometry reveal that all of them are low-spin (LS) iron(II), except for solvatomorph 5⋅4 H O. Evans method NMR studies in CD Cl , (CD ) CO and CD CN show that all are LS in these solvents, except 5 in CD Cl (consistent with L imposing the weakest field). Cyclic voltammetry in CH CN vs. Ag/0.01 m AgNO reveals an, at best quasi-reversible, Fe redox process, with E increasing from 0.69 to 0.99 V as the azine changes: pyridine< pyridazine<2-pyrimidine<4-pyrimidine< pyrazine. The observed E values correlate linearly with the DFT calculated HOMO energies for the LS complexes.