The bi- and trinuclear iron(III) complexes [1,3-{Cp*(dppe)Fe(C⋮C−)}2(C6H4)][PF6]2 (2 2+ ) and [1,3,5-{Cp*(dppe)Fe(C⋮C−)}3(C6H3)] [PF6] (3 3+ ) were prepared by oxidation of [1,3-{Cp*(dppe)Fe(C⋮C−)}2(C6H4)] or [1,3,5-{Cp*(dppe)Fe(C⋮C−)}3(C6H3)] with 2 or 3 equiv of [(C5H5)2Fe][PF6], respectively. Complexes 2 2+ and 3 3+ were isolated as thermally and air stable blue microcrystalline solids in 95 and 80% yield, respectively. These paramagnetic compounds were characterized by cyclic voltammetry, IR, UV−vis, 1H NMR, Mössbauer, and ESR spectroscopies. The three organoiron groups of 3 3+ are not located on the same side of the molecule, and its two faces are therefore magnetically nonequivalent. The 1H NMR isotropic shifts are expected to be essentially contact shifts and the zfs (zero field splitting) parameter D is expected to be small for 2 2+ and 3 3+ since the Curie law is accurately obeyed for the proton resonances of the π-bound Cp* ligand. ESR spectra of the bi- and triradicals showed broad and unresolved signals at g = 2.10 (2 2+ , ΔH pp = 550 G) and 2.13 (3 3+ , ΔH pp = 170 G) in addition to signals at g = 4.55 and 4.46, respectively due to Δm s = 2 transition. The Δm s = 3 transition was observed at g = 7.97 in the spectrum of 3 3+ . The temperature dependence of molar susceptibility obtained by SQUID measurements on microcrystalline samples suggested that the ferromagnetic interaction produces a triplet ground state in biradical 2 2+ (2J = 130.6 ± 0.2 cm-1) and a quartet ground state for triradical 3 3+ . The two doublet states lie above the quartet by 18.7 ± 0.2 and 28.8 ± 0.2 cm-1. These results constitute the first examples of magnetic exchange interactions in a three-spin organometallic system with a triangular topology and the ferromagnetic coupling occurs at nanoscale distances between the metal spin carriers. The geometries of 2 2+ and 3 3+ were optimized using DFT calculations. High spin species were computed to be energetically favored with the spin density mainly localized on the iron centers supporting the experimental results.

URu2Si2 is surely one of the most mysterious of the heavy-fermion compounds. Despite more than twenty years of experimental and theoretical works, the order parameter of the transition at T0 = 17.5 K is still unknown. The state below T0 remains called "hidden-order phase" and the stakes are still to identify the energy scales driving the system to this phase. We present new magnetoresistivity and magnetization measurements performed on very-high-quality single crystals in pulsed magnetic fields up to 60 T. We show that the transition to the hidden-order state in URu2Si2 is initially driven by a high-temperature crossover at around 40-50 K, which is a fingerprint of intersite electronic correlations. In a magnetic field H applied along the easy-axis c, the vanishing of this high-temperature scale precedes the polarization of the magnetic moments, as well as it drives the destabilization of the hidden-order phase. Strongly impurity-dependent magnetoresistivity confirms that the Fermi surface is reconstructed below T0 and is strongly modified in a high magnetic field applied along c, i.e. at a sufficiently-high magnetic polarization. The possibility of a sharp crossover in the hidden-order state controlled by a field-induced change of the Fermi surface is pointed out.

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