Spontaneous symmetry breaking in the formation of a dinuclear gadolinium semiquinonato complex: synthesis, high-field EPR studies, and magnetic properties.

The synthesis and characterisation of an asymmetric dinuclear gadolinium(III) semiquinonato complex, namely [Gd2(HBPz3)2(dtbsq)4] CHCl3 (1; HBPz3 = hydrotris(pyrazolyl)borate, dtbsq = 3,5-di-tert-butyl-O-semiquinone), is reported. The crystal structure of 1 was determined at room temperature. It crystallises in the triclinic system P1, with a = 16.735(5) A, b = 17.705(5) A, c = 19.553(5) A, alpha = 99.680(5) degrees, beta = 109.960(5), gamma = 107.350(5) degrees, Z = 2 and R = 9.96. The structure of 1 consists of a dinuclear asymmetric unit in which the two gadolinium(III) ions have coordination numbers of eight and nine. Three of the dioxolene molecules act as asymmetric bridging ligands, while the fourth molecule behaves as a bidentate ligand towards a single metal ion. The magnetic properties of 1 were investigated by means of susceptibility measurements and high-field electron paramagnetic resonance (HF-EPR) spectroscopy. They revealed an S = 0 ground spin state with excited states of higher spin very close in energy and a small negative zero-field splitting with a transverse anisotropy term for a S = 7 state.