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

Abstract

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