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) Å; β = 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 Å) to the secondary amines of the macrocyclic ligand and slightly longer distances to the pendant
primary amine donors (2.03 Å); 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.