A step further in the comprehension of the magnetic coupling in gadolinium(III)-based carboxylate complexes

Three new gadolinium(III) complexes of formula [Gd4(bta)3(H2O)16]n·12nH2O (1), [Gd4(bta)3(H2O)12]n·18nH2O (2) and [Gd2(H2bta)(bta)(H2O)2]n·4nH2O (3) (H4bta = 1,2,4,5-benzenetetracarboxylic acid) have been synthesized and their structures determined by X-ray diffraction. 1 and 3 are three-dimensional compounds whereas 2 exhibits a two-dimensional structure. The ability of the bta4− to adopt different coordination modes accounts for these high dimensionalities although it precludes a rational structural design. The structures of 1–3 have in common the double oxo-carboxylate bridge between gadolinium(III) ions (μ-O:κ2O,O′) either as a discrete units (1 and 2) or as a chain (3) and one (3) or two (2) syn–syn carboxylate bridges that reinforce this bridging pathway in 2 and 3. The values of the gadolinium-gadolinium separation through the double μ-O:κ2O,O′ skeleton are 4.1801(4) (1), 3.9345(7) (2) and 3.93279(13) Å (3) and those of the angle at the oxo-carboxylate bridge (θ) are 113.29(10) (1), 106.65 (2) and 104.42(6) and 108.22(6)° (3). The magnetic properties of 1 and 3 have been investigated in the temperature range 1.9–300 K and they show the occurrence of very weak ferro- (1) and weak antiferromagnetic (3) interactions [J = +0.00120(4) (1) and −0.0149(1) cm−1 (3), the Hamiltonians being defined as H = −JS1. S2 and H = −JΣi=1Si. Si+1, respectively]. The magneto–structural data of 1 and 3 show that the nature of the magnetic coupling between two gadolinium(III) ions is governed by the geometrical parameters at the μ-O:κ2O,O′ digadolinium(III) core. Although a quantitative magneto–structural correlation for this family cannot be established because the small number of reported examples nowadays, the present data do not follow the semi-empirical law proposed by Baggio et al. [1].

The determination of the geometrical parameters that govern the magnetic interchange between gadolinium(III) ions, in particular, the systematic studies of the magnetic behaviour as a function of different bridges between gadolinium ions, is one of the most extended studies. In this work three new gadolinium(III) complexes with 1,2,4,5-benzenetetracarboxylate ligand are presented and two of them are included in the global magneto–structural study based on the influence of the type bridge in gadolinium(III) molecular-based compounds.Figure optionsDownload as PowerPoint slide