A ladder coordination polymer based on Ca2+ and (4,5‐dicyano‐1,2‐phenylene)bis(phosphonic acid): crystal structure and solution‐state NMR study

The preparation of coordination polymers (CPs) based on either transition metal centres or rare‐earth cations has grown considerably in recent decades. The different coordination chemistry of these metals allied to the use of a large variety of organic linkers has led to an amazing structural diversity. Most of these compounds are based on carboxylic acids or nitrogen‐containing ligands. More recently, a wide range of molecules containing phosphonic acid groups have been reported. For the particular case of Ca²⁺‐based CPs, some interesting functional materials have been reported. A novel one‐dimensional Ca²⁺‐based coordination polymer with a new organic linker, namely polydiaquaμ₄‐(4,5‐dicyano‐1,2‐phenylene)bis(phosphonato)]μ₃‐(4,5‐dicyano‐1,2‐phenylene)bis(phosphonato)]dicalcium(II)] tetrahydrate], {Ca₂(C₈H₄N₂O₆P₂)₂(H₂O)₂]·4H₂O}_n, has been prepared at ambient temperature. The crystal structure features one‐dimensional ladder‐like _∞¹Ca₂(H₂cpp)₂(H₂O)₂] polymers H₂cpp is (4,5‐dicyano‐1,2‐phenylene)bis(phosphonate)], which are created by two distinct coordination modes of the anionic H₂cpp^2? cyanophosphonate organic linkers: while one molecule is only bound to Ca²⁺ cations via the phosphonate groups, the other establishes an extra single connection via a cyano group. Ladders close pack with water molecules through an extensive network of strong and highly directional O—H…O and O—H…N hydrogen bonds; the observed donor–acceptor distances range from 2.499 (5) to 3.004 (6) Å and the interaction angles were found in the range 135–178°. One water molecule was found to be disordered over three distinct crystallographic positions. A detailed solution‐state NMR study of the organic linker is also provided.