METAL ION-INDUCED SELF-ASSEMBLY OF FUNCTIONALIZED 2,6-OLIGOPYRIDINES .3. METAL-METAL INTERACTION AND REDOX STATE-INDUCED TRANSFORMATIONS IN DOUBLE-STRANDED HELICATES DERIVED FROM FUNCTIONALIZED QUINQUEPYRIDINE AND SEXIPYRIDINE

4',4'''-Bis(alkylthio)-2,2':6',2'':6'',2''':6''',2''''-quinquepyridine yielded at ambient temperature double-stranded, bimetallic helical complexes of composition [L2M2OAc]3+ with Fe(II), Co(II), Ni(II), and Zn(II); with Pd(II) the complex composition was [L2M2]4+. X-ray data for the Ni(II) complex showed the two ligand strands intertwined about each other and around the two metal ions in a double helical fashion, the metal ions having distorted octahedral geometry and different chemical environments (N6 and N4O2) with a supplementary bidentate acetate ligand completing the coordination shell of one of the Ni(II) ions. 4',4''''-Bis(alkylthio)-2,2':6',2'':6'',2''':6''',2'''':6'''',2''''' sexipyridines yielded analogous but symmetrical double-stranded bimetallic complexes with Fe(II), Co(II), Ni(II), and Cu(II), the X-ray structure of the Cu(II) complex showing the metals' distorted octahedral geometry and identical chemical environments (N6, N6). The S-n-propyl substituents in diamagnetic complexes were used as an H-1 NMR probe for demonstrating helical chirality and establishing the solution integrity of the complexes in different solvents. Electrochemical techniques and measured values of DELTAE-degrees' (difference in formal potential) were used to show that metal-metal interaction occurred in the symmetrical homobimetallic systems; e.g., in the Co(II) complex the DELTAE-degrees' for the Co centers' oxidation [i.e., Co(II/III)] and reduction [i.e., Co(II/I)] were 390 and 130 mV respectively. The sexipyridine ligand also gave a trimetallic Cu(I) helical complex [L2Cu(I)3] with the Cu centers having tetrahedral geometry and the same chemical environments (N4, N4, N4). Redox-induced transformations between trimetallic Cu(I) and bimetallic Cu(II) complexes were demonstrated chemically and verified spectroelectrochemically.