Multi-modal bridging ligands; effects of ligand functionality, anion and crystallisationsolvent in silver(I) co-ordination polymers

Abstract

The influence of solvent and anion on the formation of co-ordination polymers of silver(I) and the multi-modal ligands 2,2′-bipyrazine (bpyz) and pyrazino[2,3-f]quinoxaline (pyq) has been studied. Reaction of AgX (X = BF₄⁻ or PF₆⁻) with bpyz or pyq in MeNO₂ affords three-dimensional co-ordination networks, {[Ag(bpyz)]X}_∞ or {[Ag(pyq)]X}_∞. Whereas {[Ag(bpyz)]X}_∞ form diamond-like networks, which adopt a chiral structure due to the different bridging modes of the bpyz ligand, {[Ag(pyq)]X}_∞ form unusual achiral three-dimensional frameworks which are constructed from bridged [Ag(pyq)]_∞ tubes. In the case of {[Ag(bpyz)]X}_∞ an increase in anion volume from BF₄⁻ to PF₆⁻ leads to a corresponding increase in helix volume and concomitant contraction of the helical pitch of the diamondoid framework. The use of co-ordinating solvents of crystallisation in the reactions of AgBF₄ with bpyz results in the formation of an undulating two-dimensional sheet, {[Ag(bpyz)(MeCN)]BF₄}_∞, when MeCN replaces MeNO₂, or a one-dimensional polymer {[Ag₂(bpyz)₂(PhCN)][BF₄]₂}_∞, when PhCN is used. Reaction of AgBF₄ with pyq in either MeCN or PhCN affords the discrete molecular complex [Ag(pyq)₂]BF₄ in which only the chelating donors of the pyq ligand are co-ordinated to the silver(I) ion. [Ag(pyq)₂]BF₄ exhibits dimorphism with the two structures observed differing in the nature of their π–π interactions.