Flexible pyridyloxy-substituted cyclotetraphosphazene platforms linked by silver(i)

Abstract

The pyridyloxy-substituted cyclotetraphosphazene ligands, octakis(2-pyridyloxy)cyclotetraphosphazene (L), and octakis(4-methyl-2-pyridyloxy)cyclotetraphosphazene (MeL), react with [Ag(CH₃CN)₄]PF₆ and [Ag(CF₃SO₃)] to form the complexes {[AgL](PF₆)·0.5CH₃CN·0.5C₄H₁₀O}_n (1), {[Ag₂L](PF₆)₂}_n (2), [Ag₂L](CF₃SO₃)₂ (3), {[Ag₂L](CF₃SO₃)₂·C₄H₁₀O}_n (4), {[Ag₃(MeL)(CH₃CN)₂](PF₆)₃·2CH₃CN}_n (5), and [Ag₄(MeL)(CF₃SO₃)(CH₃CN)₃](CF₃SO₃)₃ (6), which have been characterized by single crystal X-ray crystallography. The structure of (1) is a coordination polymer containing repeating [AgL]⁺ units that form 1-D chains. The PF₆⁻ anions lie between the sheets formed when the individual chains approach each other such that the pendant pyridyloxy rings have numerous close intermolecular contacts. The triflate solvated salt (4) also is a 1-D coordination polymer, with the individual chains packed close together due to multiple hydrogen bonding contacts between the coordinated triflate fluorine atoms and aromatic hydrogen atoms on adjacent chains, whereas the unsolvated form (3) contains discrete molecules of a dimetallic complex. For the latter two complexes the influence of the solvent on the structures is notable and shows the flexibility of the ligand system. Complex (5) forms a 1-D coordination polymer with the chain being propagated by a silver bound in a near linear manner by a pyridyloxy pendant from one molecule, and by a pyridyloxy from an adjacent molecule. Apart from (4) and (6) all the compounds show this or a similar way of linking the units. Complex (6) forms discrete molecules of a tetra-metallic complex but with one silver involved in an argentophilic interaction at 3.408 Å from its symmetry equivalent. This complex is the most metal-rich phosphazene reported for this ligand type, with all eight of the pyridine ligand arms involved in binding the four Ag(I) centres. The formation of silver-ligand bonds, hydrogen bonds, π–π stacking, argentophilic and anionic interactions along with subtle kinetic factors influence the self-assembly process.