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

Two complexes {[GdCu(GlyHA)(HO)Cr(CO)]·11.02HO} (1) and {{[GdCu(GlyHA)(HO)]μ-[Cu(CO)(HO)]}μ-[Cu(CO)]·15.8HO} (2), were obtained as outcomes of the reactions between the cationic hexanuclear {GdCu(GlyHA)} 15-metallacrown-5 complex (where GlyHA = glycinehydroxamate) and the anionic oxalate complexes K[Cr(CO)] or K[Cu(CO)]. 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(CO)], the Cu mean planes of neighboring 15-metallacrown-5 units {GdCu(GlyHA)} 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-[Cu(CO)(HO)], for one of which, coordination to two adjacent {GdCu(GlyHA)} units leads to formation of linear 1D-chains in 2, while the second type, μ-[Cu(CO)], is coordinated to four {GdCu(GlyHA)} units, causing the cross-linking of single 1D-chains into a double-chain 1D coordination polymer. Studies of 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 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(GlyHA)} units and additionally Gd-Cr exchange interactions. Fitting of the data for 2 was not possible, since coordination of μ-[Cu(CO)] to {GdCu(GlyHA)} 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 at 13 T reaches 24.26 J K kg at 5 K and 19.14 J K kg at 4 K for 1 and 2, respectively).