When is a metallopolymer not a metallopolymer? : When it is a metallomacrocycle

Abstract

We report reactions of cobalt(II) acetate with a series of ditopic bis(tpy) ligands (tpy = 2,2`:6`,2 ``-terpyridine) containing flexible polyethyleneoxy spacers (tpy-4`-O(CH2)(2)O(n)-4`-tpy, n = 2, 3, 4 or 6 ligands 1-4, respectively) which result in the formation of complicated mixtures of species, presumed to be both open chain and cyclic species. Well resolved paramagnetically shifted H-1 NMR spectra are a powerful tool for the analysis of these solution systems. Upon equilibration, [n + n] metallomacrocycles are isolated as the dominant (thermodynamic) species in some cases, and the single crystal X-ray structures of [Co-2(3)(2)][PF6](4)center dot 6MeCN and [Co-2(4)(2)][PF6](5)center dot 2MeCN (a mixed cobalt(II)/cobalt(III) species) are presented. Oxidation of the equilibrated cobalt(II) mixtures to kinetically inert cobalt(III) species provides additional evidence for the formation of metallomacrocycles as the thermodynamic products. Single crystal structural data for [Co-3(2())3][PF6](9)center dot 2MeCN center dot 3.5H(2)O, [Co-2(3)(2)][PF6](6)center dot 10MeCN and [Co-2(4)(2)][PF6](6)center dot 6MeCN confirm the assembly of [2 + 2]and [3 + 3] metallomacrocycles. PGSE NMR spectroscopy has been used to determine the hydrodynamic radii of the solution species