Syntheses and characterization of aqua-bridged dimetallic complexes, M2(mu-H2O)(mu-OAc)2(Im)4(OAc)2(M=Mg2+, Mn2+, and Ni2+). Structural models for the active sites of dimetallic hydrolases.

Four novel aqua-bridged dinuclear complexes with a formula M2(mu-H2O)(mu-OAc)2(Im)4(OAc)2(Im)4(OAc)2 (where Im=imidazole, M=Mg2+ 1, Mn2+ 2, Ni2+ 3 and Co2+ 4) have been synthesized and characterized. Complexes 1, 2 and 3 have been characterized by X-ray crystallography. Two M2+ ions are bridged by an aqua molecule and two carboxylate anion with M...M=3.635-3.777 A, M-OH(2)=2.109-2.246 A and M-OH(2)-M=114.4-119.0 degrees, respectively. Each complex is further stabilized by two intramolecular hydrogen bonds between the hydrogens of the bridging aqua and the oxygens of the terminal monodentate acetates with a distance of O...O=2.6 A. The terminal monodentate acetates display "reversed" C-O distances, namely the C-O(free) distances are actually longer than the C-O(coordinating) distances. This abnormal geometry of a monodentate carboxylate would be caused by the strong "pulling effect" on the terminal carboxylates by intra- and intermolecular hydrogen bonds. The O-H stretching vibration of the bridging water was identified at ca. 2328 cm(-1) in IR spectra based on the deuterium isotope shift. The solid state 13C and 15N NMR spectra of 1 displayed two sets of peaks for acetate and Im ligands, respectively, consistent with the presence of two types of coordination modes of acetate and the two symmetrically non-equivalent Im as revealed by X-ray structure. 15N chemical shift of NH in Im ligands underwent about 6 ppm downfield shift due to its involvement in an intermolecular hydrogen bond.