METAL ION-INDUCED SELF-ASSEMBLY OF FUNCTIONALIZED 2,6-OLIGOPYRIDINES .2. COPPER-CONTAINING DOUBLE-STRANDED HELICATES DERIVED FROM FUNCTIONALIZED QUATERPYRIDINE AND QUINQUEPYRIDINE - REDOX STATE-INDUCED TRANSFORMATIONS AND ELECTRON COMMUNICATION IN MIXED-VALENCE SYSTEMS

Spontaneous self-assembly of functionalized quaterpyridines and quinquepyridines at ambient temperature in the presence of an appropriate copper ion resulted in a versatile group of multimetallic, double-stranded helical complexes. Methylthio-substituted quaterpyridine and copper ion gave a bimetallic complex with Cu(I) and a monometallic complex with Cu(II). Redox state-induced transformations between these complexes were demonstrated chemically and verified spectroelectrochemically. The [LCu(I)] 2[PF6]2 complex crystallized in the monoclinic space group I2/a (No. 15) with unit cell dimensions a = 17.021(4) angstrom, b = 16.212(4) angstrom, c = 20.303(4) angstrom, beta = 103.09(2)-degrees, V = 5457(2) angstrom3, and Z = 2. The X-ray data showed that the two ligand strands intertwined about each other and around the two distorted tetrahedral metal ions in a double-helical fashion, with the Cu-Cu distance being 3.32 angstrom. Alkylthio-substituted quinquepyridines formed homo- and mixed-valence, bimetallic and also trimetallic helical complexes in quantitative yield at ambient temperature, depending on the nature of the metal ion and that of a supplementary ligand and the reaction conditions. Redox state-induced transformations between these complexes were also demonstrated chemically and verified spectroelectrochemically. The trimetallic complex in acetonitrile showed three reversible, one-electron oxidations at +1.03(70), +0.50(50), and -0.04(60) V attributed to three Cu(I)/Cu(II) couples. The significant difference in oxidation potentials (DELTAE-degrees' = 530 and 540 mV) is indicative of metal-metal interactions in this homometallic complex, and H-1 NMR studies established its high symmetry and chiral nature. The homovalence [L2Cu(II)2OAc] [PF6]3 complex crystallized in the orthorhombic space group Pbnn (No. 52) with unit cell dimensions a = 16.558 (3) angstrom, b = 16.684(3) angstrom, c = 25.577(4) angstrom, alpha = beta = gamma = 90-degrees, V = 7066(2) angstrom3, and Z = 4, and the X-ray data showed its double-stranded helical nature, the metal ions having distorted octahedral geometry and different chemical environments (N6 and N4O2)with a supplementary bidentate acetate ligand completing the coordination sphere of one of the Cu(II) ions. The Cu-Cu distance was 4.44 A. The [L2Cu(II)/Cu(I)][PF6]3 mixed-valence complex crystallized in the monoclinic space group I2/a (No. 15) with unit cell dimensions a = 25.002(6) angstrom, b = 15.501(4) angstrom, c = 33.560(6) angstrom, beta = 91.09(2)-degrees, V = 13004(4) angstrom3, and Z = 8, and the X-ray data showed a similar helical arrangement of the ligand about the two copper ions, which had distorted octahedral and distorted tetrahedral geometry, respectively, with a Cu-Cu distance being 4.25 angstrom. An intervalence electron-transfer (IT) transition between the mixed-valence copper centers showed broad absorption centered at 1414 nm [epsilon = 70 (cm M)-1] is clear evidence for electron delocalization and metal-metal coupling between the two copper centers in this complex.