SYNTHESIS AND CHARACTERIZATION OF TRINUCLEAR IRON(II) AND MANGANESE(II) CARBOXYLATE COMPLEXES - STRUCTURAL TRENDS IN LOW VALENT IRON AND MANGANESE CARBOXYLATES

Reaction of Fe(O2CCH3)2 OT Mn(O2CCH3)2.4H2O with bidentate nitrogen donor ligands affords the trinuclear complexes [M3(O2CCH3)6L2] [M = Fe, L = BIPhMe (1); M = Mn, L = BiPhMe (2) or 1,10-phenanthroline (3)] in high yields. As judged from X-ray diffraction studies, these complexes adopt a novel linear structure, with one monodentate and two bidentate bridging carboxylates spanning each pair of metal atoms. Within this motif there are two geometric isomers that exist, designated ''syn'' or ''anti'' depending upon the orientation of the bidentate nitrogen donor ligands with respect to one another across the plane defined by the three metal atoms and the two monodentate bridging oxygen atoms. The bidentate and monodentate bridging modes are related by a carboxylate shift mechanism, proposed on the basis of observed variations in the interaction of the nonbridging, or dangling, oxygen atoms of the monodentate carboxylates with the terminal metal atoms. Such a carboxylate shift has recently been observed for the bridged dimetallic center in ribonucleotide reductase. Structural characterization of three isomers of compound 2 revealed a considerable degree of flexibility in the tricarboxylate-bridged dimetallic unit, with M...M distances ranging from 3.370 (3) to 3.715 (2) angstrom. From temperature-dependent magnetic susceptibility studies, compound 1 was found to be a ferromagnetically coupled triiron(II) complex with bridging oxygen atoms. A theoretical fit of the magnetic susceptibility data for 1 revealed the J value for ferromagnetic exchange coupling between adjacent iron atoms to be in the range +5 to +10 cm-1. Magnetic susceptibility results for 2 revealed more typical antiferromagnetic coupling, with J = -2.8 (1) cm-1 for adjacent manganese atoms. The relevance of these results to the properties of carboxylate-bridged dimetallic iron and manganese centers in metalloproteins and the chemical reactivity of the complexes are briefly discussed.