SYNTHESIS AND PROPERTIES OF A BINUCLEAR (MU-OXO) DIIRON(III) COMPLEX CONTAINING A TRIPODAL POLYBENZIMIDAZOLE LIGAND

Several iron(III) complexes of the polybenzimidazole ligand tris((benzimidazol-2-yl)methyl)amine(ntb) have been synthesized and characterized. The mu-oxo bridged dinuclear complex, [Fe2O(ntb)2Cl2](PF6)2.4THF, has been isolated-and structurally characterized by X-ray crystallography. The complex crystallizes in the triclinic space group P1BAR with one complex molecule per unit cell. The cell dimensions are a = 13.758(3), b = 13.913(2), c = 11.526(1) angstrom, alpha = 110.52(1), beta = 90.56(2), gamma = 60.65(2)-degrees and V = 1765.2 angstrom3. The structure refined to a final agreement factor (R) of 0.057 using 5092 reflections with I>3sigma(I). The complex is a centrosymmetric dimer and the coordination environment around each iron(III) center is pseudooctahedral. One benzimidazole pendant is bonded trans to tbe bridging oxo group, while the other benzimidazole pendants, the tertiary amine nitrogen atom and Cl- ion are bonded cis to the oxo ligand. The Fe-0-Fe bridging angle is 180-degrees, as required by the crystallographically imposed center of symmetry. The Fe-Fe separation is 3.610(1) angstrom. A mononuclear form of complex 1, [Fe(ntb)Cl2](PF6)0.5(Cl)0.5 (2), has been prepared and shown to form 1 in aqueous ethanol solutions containing NEt4Cl. Compounds 1 and 2 can be used to prepare compound 3 which contains a mu-oxo-mu-acetato bridged core structure, [Fe2O(ntb)2(OAc)](ClO4)3. Compound 3 was recrystallized from an acetonitrile solution containing THF. The dinuclear complex 1 exhibits antiferromagnetic exchange with a J value of -100 cm-1. The room temperature solution effective magnetic moments for compounds 1 (mu(eff) = 2.0) and 3 (mu(eff) = 1.85 BM) are consistent with antiferromagnetically coupled diiron(III) complexes and indicate that the mu-oxo bridged structure is retained in solution. The room temperature effective magnetic moment of compound 2 is 5.9 BM. The electronic spectra of compounds 1-3 are similar to those observed for related complexes. The cyclic voltammetry of compound 1 indicates that the dinuclear complex can stabilize redox levels higher than +3. Compound 2 exhibits a single one electron redox process at E1/2=0.468 V NHE. The proton NMR spectra of compounds 1 and 3 confirm the stability of tbe mu-oxo bridged structures in solution. Inequivalent benzimidazole NH proton signals are observed for compounds 1 and 3, consistent with the X-ray crystal structure of 1.