Magnetocaloric effect in 1D-polymers bearing 15-metallacrown-5 {GdCu 5 } 3+ units and anionic oxalate complexes.

Two complexes {[GdCu 5 (GlyHA) 5 (H 2 O) 7 Cr(C 2 O 4 ) 3 ]·11.02H 2 O} n (1) and {{[GdCu 5 (GlyHA) 5 (H 2 O) 6 ]μ 2 -[Cu(C 2 O 4 ) 2 (H 2 O)]} 2 μ 4 -[Cu(C 2 O 4 ) 2 ]·15.8H 2 O} n (2), were obtained as outcomes of the reactions between the cationic hexanuclear {GdCu 5 (GlyHA) 5 } 3+ 15-metallacrown-5 complex (where GlyHA 2- = glycinehydroxamate) and the anionic oxalate complexes K 3 [Cr(C 2 O 4 ) 3 ] or K 2 [Cu(C 2 O 4 ) 2 ]. Both 1 and 2 possess polymeric 1D-chain structures according to X-ray structural analysis. As a consequence of the geometric orientations of the donor atoms in the oxalates from [Cr(C 2 O 4 ) 3 ] 3- , the Cu 5 mean planes of neighboring 15-metallacrown-5 units {GdCu 5 (GlyHA) 5 } 3+ are angled at 75.5° to each other, which leads to formation of a zig-zag motif in the 1D-chains of complex 1. The centrosymmetric complex 2 contains two structurally different bis(oxalato)cuprate anions μ 2 -[Cu(C 2 O 4 ) 2 (H 2 O)] 2- , for one of which, coordination to two adjacent {GdCu 5 (GlyHA) 5 } 3+ units leads to formation of linear 1D-chains in 2, while the second type, μ 4 -[Cu(C 2 O 4 ) 2 ] 2- , is coordinated to four {GdCu 5 (GlyHA) 5 } 3+ units, causing the cross-linking of single 1D-chains into a double-chain 1D coordination polymer. Studies of χ M T vs. T data for 1 and 2 in a 2-300 K temperature range revealed the presence of both ferromagnetic and antiferromagnetic interactions amongst paramagnetic centres. The experimental χ M T vs. T data for 1 were fitted using a model which takes into account exchange interactions between adjacent copper(II) ions, the Gd-Cu exchange interactions within {GdCu 5 (GlyHA) 5 } 3+ units and additionally Gd-Cr exchange interactions. Fitting of the χ M T vs. T data for 2 was not possible, since coordination of μ 4 -[Cu(C 2 O 4 ) 2 ] 2- to {GdCu 5 (GlyHA) 5 } 3+ led to the non-equivalence of several Cu-Cu exchange interactions within the metallacrown units and hence a superfluity of fittable parameters. Complexes 1 and 2 are the first examples of 15-metallacrown-5 complexes demonstrating a magnetocaloric effect (-Δ S M at 13 T reaches 24.26 J K -1 kg -1 at 5 K and 19.14 J K -1 kg -1 at 4 K for 1 and 2, respectively).