Homochiral Cobalt Phosphonates Containing Δ-Type Chains with a Tunable Interlayer Distance and a Field-Induced Phase Transition

Two pairs of enantiomeric Co-II compounds with formulas [Co-2(mu(3)-OH)(cyamp)(CnH(2n+1)COO)] (cyampH(2)=(S)-or (R)-[(1-cyclohexylethyl) amino] methylphosphonic acid; n= 1 (1); n= 7 (2)) were synthesized. The structures of S-1 and S-2 were determined by single-crystal structural analyses. Both crystallize in a monoclinic chiral space group P2(1), and exhibit layered structures in which the Delta-type chains of corner-sharing Co-3(mu(3)-OH) triangles are connected by the phosphonate groups. The interlayer spaces are filled with the organic groups of the phosphonate and carboxylate ligands. Therefore, the distances between the layers can be manipulated by the length of the alkyl chain of the carboxylate ligands, from 14.6 angstrom in 1 to 20.0 angstrom in 2. Magnetic studies were carried out for compounds S-1 and S-2. Both show metamagnetism at low temperature. The critical field decreases with increasing interlayer distance from 8.18 kOe for S-1 to 7.01 kOe for S-2 at 1.8 K. The optical properties were also studied.