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 (Lazine): [FeII(Lpyridazine)3](BF4)2 (1), [FeII(Lpyrazine)3](BF4)2 (2), [FeII(Lpyridine)3](BF4)2 (3), [FeII(L2pyrimidine)3](BF4)2 (4), and [FeII(L4pyrimidine)3](BF4)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 H2O. Evans method NMR studies in CD2Cl2, (CD3)2CO and CD3CN show that all are LS in these solvents, except 5 in CD2Cl2 (consistent with L4pyrimidine imposing the weakest field). Cyclic voltammetry in CH3CN vs. Ag/0.01 m AgNO3 reveals an, at best quasi‐reversible, FeIII/II redox process, with Epa increasing from 0.69 to 0.99 V as the azine changes: pyridine< pyridazine