Cyclophosphazenes as Nodal Ligands in Coordination Polymers

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Abstract
Herein, we show that cyclotriphosphazenes carrying organo amino side chains, (RNH)6P3N3 {R = n-propyl (1), cyclohexyl (2), benzyl (3)}, and (C4H8N)6P3N3 (4) produce supramolecular coordination compounds in conjunction with silver salts by formation of linear N−Ag−N connections via nitrogen centers of the phosphazene ring. Crystalline materials were obtained by layering methanol solutions containing phosphazene ligands with methanol solutions of AgClO4 and AgNO3. The donor ability of the anion and the steric demand of the lipophilic ligand sphere R control the topology of the coordination network: (1)2(AgClO4)3 forms a graphite-type (6,3) network. All three N(ring) atoms of the phosphazene ligand coordinate to silver ions, which, in return, linearly bridge two phosphazene ligands. The phosphazene−Ag(I) arrangement in 1(AgNO3)2 exists of zigzag chains featuring one bridging silver ion and one terminally coordinated silver ion per ligand molecule. The terminally located Ag(I) ions of neighboring chains are bridged by nitrate ions, resulting in a 2D network. Both 2(AgClO4) and 4(AgClO4) contain only one bridging silver ion per phosphazene ligand, which leaves one N(ring) site vacant and gives 1D zigzag chain arrangements. The crystal structures of 3(AgClO4)2 and 3(AgNO3)2 resemble that of 1(AgNO3)2, but show additional Ag−π(aryl) interactions between the terminally arranged silver ions and benzyl groups. Treatment of 3 with a methanol solution containing both AgNO3 and AgClO4 leads to the heteroanion derivative 3(AgNO3)(AgClO4). Phosphazene ligands 1−3 have the ability to undergo hydrogen bonding to anions via the six NH groups, and the coordination polymers containing these ligands feature dense networks of NH···O bonds.

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