A convenient high yield synthesis of 2,2′:6′,2″:6″,2‴:6‴,2″″ :6″″,2″‴-sexipyridine and helical transition-metal complexes of substituted sexipyridines

Abstract

The compound 2,2′:6′,2″:6″,2‴:6‴,2″″:6″″,2″‴-sexipyridine (spy) has been synthesised in high yield by the coupling of 6-bromo-2,2′:6′,2″-terpyridine with a nickel(0) reagent, followed by demetallation of the resultant double-helical [Ni₂(spy)₂]⁴⁺ complex with KCN. In order to probe the effects of substitution of spy upon helication processes, the substituted compounds 4′,4″″-bis(methylsulfanyl)-2,2′:6′,2″:6″,2‴:6‴,2″″:6″″,2″‴-sexipyridine (msspy), 4′,4″″-diphenyl-2,2′:6′,2″:6″,2‴:6‴,2″″:6″″,2″‴-sexipyridine (pspy) and 4′,4″″-bis(4-tert-butylphenyl)-2,2′:6′,2″:6″,2‴:6‴,2″″:6″″,2″‴-sexipyridine (tbspy) were also prepared and studied. The reaction of each of these compounds with transition-metal ions capable of adopting an octahedral geometry results in the self-assembly of dinuclear double-helical complexes of the type [M₂L₂]⁴⁺[M = iron(II), cobalt(II), nickel(II), copper(II), zinc(II), cadmium(II) or mercury(II); L = spy, msspy, pspy or tbspy]; the double-helical topology of the zinc complex of pspy was confirmed by a crystal-structure analysis of the salt [Zn₂(pspy)₂][PF₆]₄·2MeCN·2H₂O [space group P4₂2₁2, a= 15.518(2), c= 18.443(1)Å, R= 0.0638, R′= 0.0735].