Two decanuclear cyclic Cr(III) complexes have been synthesised in high yield by solvothermal techniques: magnetic susceptibility studies reveal ferromagnetic Cr…Cr exchange in one, and antiferromagnetic Cr…Cr exchange in the other.

We use the strategy of diamagnetic substitution for obtaining information on the crystal field effects in paramagnetic rare earth ions using the homologous series of compounds with the diamagnetic tropolonato ligand, Ln(Trp)(HBPz(3))(2), and the paramagnetic semiquinone ligand, Ln(DTBSQ)(HBPz(3))(2), (DTBSQ = 3,5-di-tert-butylsemiquinonato, Trp = tropolonate, HBPz(3)= hydrotrispyrazolylborate) for Ln = Sm(iii), Eu(iii), Gd(iii), Tb(iii), Dy(iii), Ho(iii), Er(iii) or Yb(iii). The X-ray crystal structure of a new form of tropolonate derivative is presented, which shows, as expected, a marked similarity with the structure of the semiquinonate derivative. The Ln(Trp)(HBPz(3))(2) derivatives were then used as a reference for the qualitative determination of crystal field effects in the exchange coupled semiquinone derivatives. Through magnetisation and susceptibility measurements this empirical diamagnetic substitution method evidenced for Er(iii), Tb(iii), Dy(iii) and Yb(iii) derivatives a dominating antiferromagnetic coupling. The increased antiferromagnetic contribution compared to other radical-rare earth metal complexes formed by nitronyl nitroxide ligands may be related to the increased donor strength of the semiquinone ligand.

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The complex [L(H2O)Fe(μ-O)Fe(OH)L](ClO4)3·H2O, where L = N,N‘-dimethyl-N,N‘-bis(2-pyridylmethyl)ethane-1,2-diamine, was synthesized. It crystallizes in the orthorhombic space group P212121 with a = 13.283(5) Å, b = 16.050(9) Å, c = 20.050(9) Å, V = 4476(6) Å3, and Z = 4. It presents the hydrogen-bonded [(H2O)Fe(μ-O)Fe(OH)]3+ core unit characterized by an Fe−O−Fe angle of 137.5(2)° and an Fe−Fe distance of 3.396(1) Å. The measurement of the magnetic susceptibility as a function of the temperature indicated an antiferromagnetic coupling between the two high-spin Fe(III) ions J = −184 cm-1 (H = −J S 1 ·S 2 ). In the solid state the symmetric stretching vibration was observed at 438 cm-1. Upon dissolution in dry acetonitrile, this vibration was no longer detected and an intense band was observed at 600 cm-1. This frequency can be correlated with an Fe−O−Fe angle of 111°. This value suggests that the species which exists in these conditions is the protonated diamond core [Fe(μ-O)(μ-OH)Fe]3+, analogous to that identified by Zang et al. (J. Am. Chem. Soc. 1994, 116, 3653) and by Hazell et al. (J. Chem. Soc., Dalton Trans. 1995, 707). Upon addition of water, the original aquated species is observed in equilibrium with the protonated diamond unit. By analysis of the changes in UV−vis spectra as a function of the amount of water added, the equilibrium constant of the formation of the aquated species was found to be 5.4 M-1.