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 [Fe^III(H₃ntb)Cl₂]ClO₄, 1, with 3 equiv of sec-butylamine in methanol led to the formation of mononuclear cis-dimethoxo octahedral Fe^III(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 Hntb^2- that stabilized the dinuclear μ-oxo Fe^III₂O(Hntb)₂, 3, adopting a five-coordinate trigonal bipyramidal geometry. The magnetic data for 3 are consistent with the antiferromagnetically coupled Fe^III (S = ⁵/₂) sites with the coupling constant J = −127 cm^-1.