Magnetic and EPR spectroscopic studies of heterometallic rings featuring Cr6 chains and rare-earth metal centres

A rare example of a seven-membered heterometallic ring [CrIII 6 CeIIIF7(O2CtBu)14(THF)2] (MeCN)2 (1) and five eight-membered heterometallic rings, [nPr2NH2][CrIII = Y, L = H2O, x = 1, 4, Ln = Gd, L = HO2CtBu, x = 1; 5, Ln = Tb, L = HO2CtBu, x = 1; 6, Ln = Yb, no L) have been synthesized and structurally characterized through X-ray diffraction. The structures consist of eight metals in an octagon, with Cr...Cr and Cr...Ln edges bridged by a fluoride and two carboxylates, while the Ln...Ln edges are bridged by a fluoride and three carboxylates. The magnetisation and susceptibility of these compounds were measured using SQUID magnetometry and electron paramagnetic resonance (EPR) spectroscopy. The magnetic data were fitted with antiferromagnetic exchange interactions between chromium(III) ions, which can be fitted in the {Cr6Y2} complex 3 and these parameters were then used to fit the magnetic properties of the {Cr6Gd2} complex 4 adding in exchange interactions between the CrIII and GdIII The magnetisation and susceptibility below 80 K of 1 and 2 were fitted on the basis of CASSCF-SO calculations at the CeIII site, and showed a weak ferromagnetic interaction between CrIII and CeIII. For 5 and 6 the magnetisation data was fitted by subtracting the data for 3 and treating the residual data as a {Tb2} and {Yb2} dimer respectively. The EPR spectra are rich, and for 3 can be modelled as due to S = 1 and S = 2 states of the {Cr6} chain. The spectra of 1 and 2 are similar, consistent with very weak interactions between the CeIII and the {Cr6} chain, while the spectra of 5 and 6 are different to that of 3, suggesting that the low temperature spectroscopy is due to a spin system in which the LnIII ions interact with the {Cr6} chain.