title: Stereoselective association of binuclear metallacycles in coordination polymers.
abstract: A series of structurally related binuclear metallacycles [Cd(NO(3))(2)L](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(NO(3))(2)L](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(NO(3))(2)(2,4'-pyacph)](2) (2,4'-pyacph = 2,4'-(4-ethynylphenyl)bipyridyl). Homochiral association results in the formation of helical [Cd(NO(3))]( infinity ) chains which link the binuclear [Cd(NO(3))(2)(2,4'-pyacph)](2) metallacycles into racemic two-dimensional sheets which contain both P and M [Cd(NO(3))]( infinity ) 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.
authors.affiliation: School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, U.K.
authors.forename: Andrei N
authors.initials: AN
authors.lastname: Khlobystov
authors.forename: Matthew T
authors.initials: MT
authors.lastname: Brett
authors.forename: Alexander J
authors.initials: AJ
authors.lastname: Blake
authors.forename: Neil R
authors.initials: NR
authors.lastname: Champness
authors.forename: Peter M W
authors.initials: PM
authors.lastname: Gill
authors.forename: Darragh P
authors.initials: DP
authors.lastname: O'Neill
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Teat
authors.forename: Claire
authors.initials: C
authors.lastname: Wilson
authors.forename: Martin
authors.initials: M
authors.lastname: Schröder