Preparation, structure, and electronic properties of a low-spin iron(II) hexaamine compound

[Fe(trans-diammac)]2+ (trans-diammac) ) exo-6,13-diamino-6,13-dimethyl-1,4,8,11-tetraazatetradecane) is one of the very few fully characterized examples of a low-spin iron(II) compound with saturated amine ligands. The crystal structure analysis (monoclinic, P21/c; a ) 9.547(8), b ) 14.631(13), c ) 16.91(2) A; â ) 98.92(7)°; Z ) 4) defines the iron(II) coordination geometry as distorted octahedral with very short in-plane Fe-N distances (average of 2.01 A) to the secondary amines of the macrocyclic ligand and slightly longer distances to the pendant primary amine donors (2.03 A); there is a considerable tilt of the vector involving the axial donors with respect to the plane defined by the secondary amines and the metal center (θ ) 11.5°). The metal-donor distances are shorter than those for other low-spin iron(II) hexaamines, and consequently, the redox potential (Fe3+/2+) is very small (0.45 V vs SHE) and the ligand field splitting is very large (Dq ) 1785 cm-1). The structural, magnetic, and spectroscopic properties are discussed on the basis of the experimental data in comparison with model studies.