Multi-modal bridging ligands; effects of ligand functionality, anion and crystallisation solvent 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.