SYNTHESIS, CRYSTAL-STRUCTURES AND PHYSICAL-PROPERTIES OF MU-OXO-MU-CARBOXYLATO-DIIRON(III) COMPLEXES WITH TRIPODAL POLYBENZIMIDAZOLE LIGANDS CONTAINING DISTINCT IRON SITES

mu-Oxo-mu-carboxylato-diiron complexes [Fe2O(O(2)CR)L(2)]X(3) (R = Me or Ph, X = ClO4 or NO3) with the tripodal polybenzimidazole tris[(benzimidazol-2-yl)methyl]amine (L(1)), tris[(N-methylbenzimidazol-2 yl)methyl]amine (L(2)) and tris [(N-ethylbenzimidazol-2-yl)methyl] amine. (L(3)) have been synthesized and characterized by elemental analysis. IR, electronic, resonance-Raman and H-1 NMR spectroscopies and cyclic voltammetry. The crystal structures of [Fe2O(O(2)CMe)L(2)(2)][ClO4]3H2O . 3MeCN and [Fe2O(O(2)CPh)L(2)(2)][ClO4](3) . H2O . 2MeCN have been determined. They show that the complexes contain two distinct iron sites. At one the benzimidazole nitrogen atom is trans to the oxo-bridge, while at the other the tertiary amine nitrogen atom is trans to the ore-bridge. That this solid-state structure persists in solution was confirmed by 1(H) NMR spectroscopy. The asymmetry is manifested in the resonance Raman spectra as an enhancement of v(asym)(Fe-O-Fe) and-a large intensity ratio (l(asym)/l(sym) 0.19-0.26:1) of v(asym)(Fe-O-Fe) to v(sym)(Fe-O-Fe). The first complex exhibits;strongly antiferromagetic exchange with a J-value of -108.22 cm(-1). The Mossbauer spectra of these two complexes and of [Fe2O(O(2)CMe)L(2)(3)] [ClO4](3) . 0.5H(2)O have been recorded. The electrochemical results show that the complexes exhibit completely irreversible redox behaviour which is different from that found in mu-oxo-diiron proteins.