Iron-oxo aggregates. Binuclear and tetranuclear complexes of N,N,N′,N′-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane

Qiuhao Chen, John B. Lynch, Lawrence Que, Pedro Gomez-Romero, Ahmed Ben-Hussein, Geoffrey B. Jameson, Charles J. O’Connor

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Two types of iron(III) complexes of N,N,N′,N′-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane (L) and its 1-ethylbenzimidazole (L-Et) and 5,6-dimethyl (L-Me2) derivatives have been isolated from organic solvents and characterized by NMR, IR, and magnetic susceptibility data. Two tetranuclear compounds have been structurally characterized in the space group P̅1, [Fe4O2L2(O2CC6H5)2](ClO4)2(OTs)2.6CH3CN.2CH3OH (a = 14.719 (11) Å, b = 14.784 (11) Å, c = 16.716 (25) Å, α = 66.53 (9)°, β = 65.03 (9)°, γ = 85.70 (6)°) and [Fe4O2(L-Et)2(O2CCH3)2](BF4)4.x(solvate) (a = 14.825 (5) Å,b = 15.481 (5) Å, c = 12.794 (3) Å, α = 94.21 (2)°, β = 106.25 (2)°, γ = 92.10 (3)°). Both complexes consist of two [Fe2L(O2CR)] units connected by nearly linear oxo bridges to form a nearly square array of iron atoms. The iron atoms in the [Fe2L(O2CR)] unit are bridged by the alkoxo group from the septadentate ligand and the carboxylate. These complexes and similar derivatives exhibit strong antiferromagnetic coupling presumably via the oxo bridges as reflected by the weak magnetic susceptibility and the relatively small NMR isotropic shifts. The other type of complex obtained has the general formulation [Fe2L(O2CR)2]X3. The iron(III) centers are coupled antiferromagnetically via the alkoxo bridge with J = -25 cm-1. On the basis of IR and NMR observations, the iron coordination in this complex is postulated to consist of the N3O donor groups from L and the oxygen atoms from bridging carboxylates, which are coordinated quite unsymmetrically. This unsymmetric bridging is attributed to the stereochemical constraints imposed by the short length of the hydroxypropane unit that links the two N3 units of L in the (μ-alkoxo)bis(μ-benzoato)diiron(III) unit. The factors that govern formation of the binuclear complex or the tetranuclear complex are not understood.

Original languageEnglish (US)
Pages (from-to)2673-2681
Number of pages9
JournalInorganic chemistry
Issue number15
StatePublished - Jul 1 1988


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