Tuning of Cr-Cr Magnetic Exchange through Chalcogenide Linkers in Cr 2 Molecular Dimers.

A set of three Cr-dimer compounds, Cr 2 Q 2 (en) 4 X 2 ( Q : S, Se; X : Br, Cl; en : ethylenediamine), with monoatomic chalcogenide bridges have been synthesized via a single-step solvothermal route. Chalcogenide linkers mediate magnetic exchange between Cr 3+ centers, while bidentate ethylenediamine ligands complete the distorted octahedral coordination of Cr centers. Unlike the compounds previously reported, none of the chalcogenide atoms are connected to extra ligands. Magnetic susceptibility studies indicate antiferromagnetic coupling between Cr 3+ centers, which are moderate in Cr 2 Se 2 (en) 4 X 2 and stronger in Cr 2 S 2 (en) 4 Cl 2 . Fitting the magnetic data requires a biquadratic exchange term. High-frequency EPR spectra showing characteristic signals due to coupled S = 1 spin states could be interpreted in terms of the "giant spin" Hamiltonian. A fourth compound, Cr 2 Se 8 (en) 4 , has a single diatomic Se bridge connecting the two Cr 3+ centers and shows weak ferromagnetic exchange interactions. This work demonstrates the tunability in strength and type of exchange interactions between metal centers by manipulating the interatomic distances and number of bridging chalcogenide linkers.