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)₂O}_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 ¹H 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 [Co₂(3)₂][PF₆]₄·6MeCN and [Co₂(4)₂][PF₆]₅·2MeCN (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 [Co₃(2)₃][PF₆]₉·2MeCN·3.5H₂O, [Co₂(3)₂][PF₆]₆·10MeCN and [Co₂(4)₂][PF₆]₆·6MeCN 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.