Intramolecular ferro- and antiferromagnetic interactions in oxo-carboxylate bridged digadolinium(III) complexes

Two new digadolinium(III) complexes with monocarboxylate ligands, [Gd-2(pac)(6)(H2O)(4)] (1) and [Gd-2(tpac)(6)(H2O)(4)] (2) (Hpac = pentanoic acid and Htpac = 3-thiopheneacetic acid), have been prepared and their structures determined by X-ray diffraction on single crystals. Their structures consist of neutral and isolated digadolinium(III) units, containing six monocarboxylate ligands and four coordinated water molecules, the bridging skeleton being built by a mu O(1):kappa O-2(1)O(2) framework. This structural pattern has already been observed in the parent acetate-containing compound [Gd-2(ac)(6)(H2O)(4)]center dot 4H(2)O (3) whose structure and magnetic properties were reported elsewhere (L. Canadillas-Delgado, O. Fabelo, J. Cano, J. Pasan, F. S. Delgado, M. Julve, F. Lloret and C. Ruiz-Perez, CrystEngComm, 2009, 11, 2131). Each gadolinium(III) ion in 1 and 2 is nine-coordinated with seven carboxylate-oxygen atoms from four pac (1)/tpac (2) ligands and two water molecules (1 and 2) building a distorted monocapped square antiprism. The values of the intramolecular gadolinium-gadolinium separation are 4.1215(5) (1), 4.1255(6) (2) and 4.1589(3) angstrom (3) and those of the angle at the oxo-carboxylate bridge (theta) are 113.16(13) (1), 112.5(2) (2) and 115.47(7)degrees (3). Magnetic susceptibility measurements in the temperature range 1.9-300 K reveal the occurrence of a weak intramolecular antiferromagnetic interaction [J = -0.032(1) (1) and -0.012(1) cm(-1) (2), the Hamiltonian being defined as (H) over cap = -J (S) over cap (A)center dot(S) over cap (B)] in contrast with the intramolecular ferromagnetic coupling which occurs in 3 (J = +0.031(1) cm(-1)). The magneto-structural data of 1-3 show the relevance of the geometrical parameters at the mu O(1):kappa O-2(1)O(2) bridge on the nature of the magnetic coupling between two gadolinium(III) ions.