Iron(II) Tris-[N4-substituted-3,5-di(2-pyridyl)-1,2,4-triazole] Complexes: Structural, Magnetic, NMR, and Density Functional Theory Studies

Eight mononuclear iron(II) complexes of N-4-3,5-di(2-pyridyl)-1,2,4-triazole (Rdpt) ligands have been prepared and characterized. In all cases the iron(II)/ligand ratio used is 1:3, giving red complexes of the general formula [Fe-II(RdPt)(3)]-(BF4)(2) center dot solvents, in 55-89% yield. The ligands differ only in the nature of the N-4-substituent (amino, pyrrolyl, iso-butyl, methyl, phenyl, para-tolyl, 3,5-dichlorophenyl, and 4-pyridyl; for ligands adpt, pldpt, ibdpt, medpt, phdpt, ptdpt, Cldpt, and pydpt, respectively) allowing substituent effects on the properties of the resulting iron(II) complexes to be probed. The low temperature crystal structures of seven of the complexes reveal low spin iron(II) environments. Packing analyses reveal anion-pi and acetonitrile-pi interactions involving the tetrafluoroborate counteranions and interstitial acetonitrile molecules, respectively. Both "pi-pockets" and "pi-sandwiches" are observed. Solid state magnetic susceptibility measurements (4-300 K) indicate the iron(II) is low spin (LS) in all complexes at all temperatures studied, except for [Fe-II(pldpt)(3)](BF4)2 center dot 1(1)/2H(2)O which has the beginnings of spin crossover (SCO) at elevated temperatures. Downfield shifts and peak broadening observed in the variable temperature H-1 NMR studies indicate that in d(3)-nitromethane solution the LS [Fe-II(RdPt)(3))(2+) complexes are in equilibrium with a trace of a high spin (HS) species. N-15 NMR spectra (measured and calculated) of the ligands reveal that altering the N-4-substituent changes the chemical shift of the N-1 triazole and pyridine nitrogen atoms, allowing probing of the relationship between ligand substituent and the nature of the coordinating nitrogen atoms.