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

A new Gd 3+ coordination polymer (CP), namely, poly[diaqua[μ 4 -1′-carboxy-3,3′-(diazene-1,2-diyl)dibenzene-1,2,2′-tricarboxylato]gadolinium(III)], [Gd(C 16 H 7 N 2 O 8 )(H 2 O) 2 ] n , (I), has been synthesized hydrothermally from Gd(NO 3 ) 3 ·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 C 2/ c . The structure features a 4-connected topology in which Gd 3+ 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\overline{1}) plane. A very short hydrogen bond [O...O = 2.4393 (4) Å] links neighbouring layers into a three-dimensional network. A magnetic study revealed antiferromagnetic Gd...Gd coupling within the chain direction. CP (I) displays a significant magnetocaloric effect (MCE), with a maximum −Δ S m of 27.26 J kg −1 K −1 for Δ H = 7 T at 3.0 K. As the MCE in (I) exceeds that of the commercial magnetic refrigerant GGG (Gd 3 Ga 5 O 12 , −Δ S m = 24 J kg −1 K −1 , Δ H = 30 kG), CP (I) can be regarded as a potential cryogenic material for low-temperature magnetic refrigeration.