Dynamic Equilibria in Solvent-Mediated Anion, Cation and Ligand Exchange in Transition-Metal Coordination Polymers: Solid-State Transfer or Recrystallisation?

The solution properties of a series of transition-metal-ligand coordination polymers [ML(X)(n)](infinity) [M=Ag-I, Zn-II, Hg-II and Cd-II; L=4,4'-bipyridine (4,4'-bipy), pyrazine (pyz), 3,4'-bipyridine (3,4'-bipy), 4-(10-(pyridin-4-yl)anthracen-9-yl)pyridine (anbp); X = NO3-, CH3COO-, CF3SO3-, Cl-, BF3-; n=1 or 21 in the presence of competing anions, metal cations and ligands have been investigated systematically. Providing that the solubility of the starting complex is sufficiently high, all the components of the coordination polymer, namely the anion, the cation and the ligand, can be exchanged on contact with a solution phase of a competing component. The solubility of coordination polymers is a key factor in the analysis of their reactivity and this solubility depends strongly on the physical properties of the solvent and on its ability to bind metal cations constituting the backbone of the coordination polymer. The degree of reversibility of these solvent-induced anion-exchange transform at ions is determined by the ratio of the solubility product constants for the starting and resultant complexes, which in turn depend upon the choice of solvent and the temperature. The extent of anion exchange is controlled effectively by the ratio of the concentrations of incoming ions to outgoing ions in the liquid phase and the solvation of various constituent components comprising the coordination polymer. These observations can be rationalised in terms of a dynamic equilibrium of ion exchange reactions coupled with Ostwald ripening of crystalline products. The single-crystal X-ray structures of [Ag(pyz)ClO4](infinity) (1), {{Ag(4,4'-bipy)(CF3SO3)]center dot CH3CN}(infinity) (2), [[Ag(4,4'-bipy)(CH3CN)]ClO4 center dot 0.5CH(3)CN}(infinity) (3), metal-free anbp (4), [Ag(anbp)NO3(H2O)](infinity) (5), {[Cd(4,4'-bipy)(2)(H2O)(2)](NO3)(2)center dot 4H(2)O}(infinity), (6) and {[Zn(4,4'-bipy)SO4(H2O)(3)]center dot 2H(2)O}(infinity) (7) are reported.