Linkage Isomerism and Spin Frustration in Heterometallic Rings: Synthesis, Structural Characterization, and Magnetic and EPR Spectroscopic Studies of Cr7Ni, Cr6Ni2, and Cr7Ni2 Rings Templated About Imidazolium Cations

The synthesis and structural characterization of three heterometallic rings templated about imidazolium cations is reported. The compounds are [2,4-DiMe-ImidH][(Cr7NiF8)-F-II(O(2)CtBu)(16)] 1 (2,4-DiMe-ImidH = the cation of 2,4-dimethylimidazole), [ImidH](2)[Cr6Ni2II-F-8(O(2)CCtBu)(16)] 2 (ImidH = the cation of imidazole), and [1-Bz-ImidH](2) [(Cr7Ni2F9)-F-II(O(2)CtBu)(18)] 3 (1-Bz-Imid H = the cation of 1-benzylimidazole). The structures show the formation of octagonal arrays of metals for 1 and 2 and a nonagon of metal centers for 3. In all cases the edges of the polygon are bridged by a single fluoride and two pivalate ligands, and the position of the divalent metal centers cannot be distinguished by X-ray diffraction. Magnetic studies combined with EPR spectroscopy allow the characterization of the magnetic states of the compounds. In each case the exchange is antiferromagnetic with a magnetic exchange parameter J approximate to -5.8 cm(-1), and it is not possible to differentiate the exchange between two Cr-III centers (J(CrCr)) from the exchange between a Cr-III and a Ni-II center (J(CrNi)). For 2 there is evidence for the presence of at least two, possibly four, linkage isomers of the heterometallic ring, caused by the presence of two divalent metal centers in the ring. The EPR spectroscopy of 3 suggests an S=1/2 ground state of the ring and that it is likely that only one linkage isomer is present.