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((benz imidazol-2-yl)methyl)amine(ntb) have been synthesized and characterize d. The mu-oxo bridged dinuclear complex, [Fe2O(ntb)2Cl2](PF6)2.4THF, h as been isolated-and structurally characterized by X-ray crystallograp hy. The complex crystallizes in the triclinic space group P1BAR with o ne 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 = 9 0.56(2), gamma = 60.65(2)-degrees and V = 1765.2 angstrom3. The struct ure refined to a final agreement factor (R) of 0.057 using 5092 reflec tions with I>3sigma(I). The complex is a centrosymmetric dimer and the coordination environment around each iron(III) center is pseudooctahe dral. One benzimidazole pendant is bonded trans to tbe bridging oxo gr oup, while the other benzimidazole pendants, the tertiary amine nitrog en atom and Cl- ion are bonded cis to the oxo ligand. The Fe-0-Fe brid ging angle is 180-degrees, as required by the crystallographically imp osed 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 be en prepared and shown to form 1 in aqueous ethanol solutions containin g NEt4Cl. Compounds 1 and 2 can be used to prepare compound 3 which co ntains a mu-oxo-mu-acetato bridged core structure, [Fe2O(ntb)2(OAc)](C lO4)3. Compound 3 was recrystallized from an acetonitrile solution con taining THF. The dinuclear complex 1 exhibits antiferromagnetic exchan ge with a J value of -100 cm-1. The room temperature solution effectiv e magnetic moments for compounds 1 (mu(eff) = 2.0) and 3 (mu(eff) = 1. 85 BM) are consistent with antiferromagnetically coupled diiron(III) c omplexes 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 th ose observed for related complexes. The cyclic voltammetry of compound 1 indicates that the dinuclear complex can stabilize redox levels hig her than +3. Compound 2 exhibits a single one electron redox process a t E1/2=0.468 V NHE. The proton NMR spectra of compounds 1 and 3 confir m the stability of tbe mu-oxo bridged structures in solution. Inequiva lent benzimidazole NH proton signals are observed for compounds 1 and 3, consistent with the X-ray crystal structure of 1.