The Effect of Ligand Charge on the Coordination Geometry of an Fe(III) Ion: Five- and Six-Coordinate Fe(III) Complexes of Tris(2-benzimidazolylmethyl)amine

By using the tripodal tetradentate ligand tris(2-benzimidazolylmethyl)amine (H3ntb), which can have several charge
states depending on the number of secondary amine protons, mononuclear octahedral and dinuclear trigonal
bipyramidal Fe(III) complexes were prepared. The reaction of mononuclear octahedral [FeIII(H3ntb)Cl2]ClO4, 1, with
3 equiv of sec-butylamine in methanol led to the formation of mononuclear cis-dimethoxo octahedral FeIII(H2ntb)(OMe)2, 2. One equivalent of the sec-butylamine was used to generate the monoanionic H2ntb- ligand where one
of the three amines in the benzimidazolyl groups was deprotonated. The remaining 2 equiv were used to generate
two methoxides that were coordinated to the octahedral Fe(III) ion in a cis fashion as demonstrated by the chlorides
in 1. Reaction of 1 with excess (7 equiv) sec-butylamine generated the doubly deprotonated dianionic Hntb2- that
stabilized the dinuclear μ-oxo FeIII2O(Hntb)2, 3, adopting a five-coordinate trigonal bipyramidal geometry. The magnetic
data for 3 are consistent with the antiferromagnetically coupled FeIII (S = 5/2) sites with the coupling constant J =
−127 cm-1.