Synthesis, structure and magnetocaloric properties of a new two-dimensional gaaounium(III) coordination polymer based on azobenzene-2,2′,3,3′-tetracarboxylic acid

A new Gd3+ coordination polymer (CP), namely, poly[diaqua[mu(4)-1'-carboxy-3,3'-(diazene-1,2-diy1)dibenzene-1,2,2'-tricarboxylatolgadolinium(III)1, [Gd-(C16H7N2O8)(H2O)(2)](n), (I), has been synthesized hydrothermally from Gd(NO3)(3)center dot 6H(2)O and azobenzene-2,2',3,3'-tetracarboxylic acid (H(4)abtc). The target solid has been characterized by single-crystal and powder X-ray diffraction, elemental analysis, IR spectroscopy and susceptibility measurements. CP (I) crystallizes in the monoclinic space group C2/c. The structure features a 4-connected topology in which Gd3+ ions are connected by carboxylate groups into a linear chain along the monoclinic symmetry direction. Adjacent one-dimensional aggregates are bridged by Habtc(3-) ligands to form a two-dimensional CP in the (10 (1) over bar) plane. A very short hydrogen bond [O center dot center dot center dot O = 2.4393 (4) angstrom] links neighbouring layers into a three-dimensional network. A magnetic study revealed antiferromagnetic Gd center dot center dot center dot Gd coupling within the chain direction. CP (I) displays a significant magnetocaloric effect (MCE), with a maximum -Delta S-m of 27.26 J kg(-1) K-1 for Delta H = 7 T at 3.0 K. As the MCE in (I) exceeds that of the commercial magnetic refrigerant GGG (Gd3Ga5O12, = 24 J kg(-1) K-1, Delta H= 30 kG), CP (I) can be regarded as a potential cryogenic material for low-temperature magnetic refrigeration.