COMPOSITIONAL AND GEOMETRICAL-ISOMERS OF 15-METALLACROWN-5 COMPLEXES

Two mixed valent, hexanuclear manganese clusters of general composition Mn(acetate)2[Mn(III)(salicylhydroximate)]5(pyridine)6, Mn(II)(OAC)2(15-MC(Mn3+N)-5) have been prepared in almost-equal-to 70% yield. Structural characterization of these complexes using X-ray crystallography provides the first examples of 15 membered ring metallacrowns. A 15 membered chelate ring is constructed of five [Mn(III)-N-O] linkages. The five manganese(III) ions are designated Mn(1)-Mn(5). The Mn(1), Mn(2) and Mn(3) atoms adopt propeller configurations with LAMBDA,DELTA,LAMBDA absolute stereochemistry, respectively. The Mn(4) and Mn(5) atoms are restricted to planar configurations. Both complexes contain a seven coordinate manganese(II) [Mn(6)] encapsulated in the core of this 15 membered ring. The Mn(6) polyhedron is best described using an 8 coordinate reference frame in which one of the vertices of a trigonal faced dodecahedron is vacant. The Mn(6) coordination sphere contains five hydroximate oxygen donors, provided by the metallacrown ring, and two oxygens from acetates that bridge between ring manganese(III) and the encapsulated manganese(II). Two geometrical isomers are formed that differ in the bridging acetate modes. In 1, the bridging acetate forms a bifurcated bridge between Mn(2) and Mn(6) while complex 2 has a bifurcated acetate bridge between Mn(5) and Mn(6). A compositional isomer, Mn(II)(acetate)(HSal)(15-MC(Mn3+N)-5), 4, which substitutes the carboxylate function of salicylate for one of the acetates has also been prepared. This isomer adopts the same bifurcation mode, using the salicylate carboxylate, as complex 2. Studies to define the solution integrity of the metallacrowns are presented in addition to the solid state structures.