Stereoselective Association of Binuclear Metallacycles in Coordination Polymers

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Abstract
A series of structurally related binuclear metallacycles [Cd(NO3)2L]2, where L is an angular exo-bidentate ligand, have been synthesized. Each metallacycle contains two coordinatively unsaturated, chiral metal centers within a single molecule, and the assembly of these metallacycles into polymeric framework structures has been studied systematically for the first time. Stereoselective homochiral association of [Cd(NO3)2L]2 leads to the formation of helical coordination polymers, whereas meso type association results in nonhelical chain structures. The type of stereoselective aggregation depends on the conditions of self-assembly as well as on ligand functionality. Both helical and nonhelical polymeric complexes have been isolated for the metallacycle [Cd(NO3)2(2,4‘-pyacph)]2 (2,4‘-pyacph = 2,4‘-(4-ethynylphenyl)bipyridyl). Homochiral association results in the formation of helical [Cd(NO3)]∞ chains which link the binuclear [Cd(NO3)2(2,4‘-pyacph)]2 metallacycles into racemic two-dimensional sheets which contain both P and M [Cd(NO3)]∞ helices. In contrast, meso-association leads to the formation of nonhelical one-dimensional chains. It is shown that the product of homochiral association is predominately formed at room temperature and that of meso-association is generated at elevated temperatures. Thus, it may be concluded that the homochiral association appears to be energetically less favorable than the meso-association, a conclusion that has been confirmed by theoretical calculations of the crystal lattice energy. Several high-yield syntheses of bipyridyl-type ligands used for metallacyclic assembly are also reported.