Iron complexes of tris(pyrazolyl)ethane ligands methylated in the 3-, 4-, and 5-positions

摘要

Four tris(pyrazolyl)ethane (tpe) ligands were synthesized from tris(pyrazolyl)methane (tpm) starting materials. The new tpe ligands differ only in the placement of methyl groups on the pyrazole rings. For each tpe ligand, the 2:1 complex with Fe(II) was readily synthesized and the structures were determined by X-ray crystallography and characterized more completely by Fe-57 Mossbauer spectroscopy, H-1 NMR, and UV-Vis spectroscopy. The complexes all contain Fe(II) in the low spin (LS) state at low temperatures, but differ in Fe-N bond lengths and spectroscopic parameters. Methyl groups in the 5-position of the pyrazoles of the tpe ligand close the bite of the ligand, strengthening the ligand field and shifting the visible absorbance band of the complex to higher energies. Methyls in the 4-position had almost no effect on the ligand, making the iron(II) complex almost indistinguishable from that of the parent tpe ligand. Methyl groups in the 3-position of the tpe ligand cause an opening of the bite of the ligand, which is evidenced by increased Fe-N bond lengths in the complex and a shift of the visible absorbance to lower energies. The complex with methyls in the 3-position also starts to display spin crossover behavior near room temperature, whereas the other tpe complexes do not. (C) 2014 Elsevier B.V. All rights reserved.