Magnetic measurements on dimeric and tetrameric fluoro-bridged cobalt and manganese compounds

Abstract

Measurements of the magnetic susceptibility (2 K< T<100 K), high-field magnetization ( B max≈30 T) and specific heat (0.1 K T<100 K) are reported on the dimetric compounds [Co F L 3] 2(BF 4) 2, with L=3,5-dimethyl-pyrazole (DMPz); 3,4,5-trimethylpyrazole (TMPz); and 4-ethyl-3,5-dimethyl-pyrazole (EDMPz); as well as on the tetrameric compounds [Co F L 3] 4(BF 4) 4 and [Mn F L 3] 4(BF 4) 4 with L=N-ethylimidazole (NEtIz) and lastly on [Co F L 3] 4(BF 4) 4 with L=N-propylimidazole. In these materials the 3 d-metal ions are magnetically coupled via the fluor ligands with M-F-M-bond angles of about 100° to form dimeric or tetrameric antiferromagnetic clusters. In the dimeric and tetrameric compounds the metal ions are in 5 and in 6 coordination, respectively. The analysis of the experiments in terms of magnetic cluster models yields an estimate of the exchange constants in the materials. For [Co F (DMPz) 3] 2(BF 4) 2, [Co F (EDMPz) 3] 2(BF 4) 2 and [Co F (TMPz) 3] 2(BF 4) 2 we obtain J/K⋍−1 K , −4± 1 K, and −4±1 K, respectively. For [Co F (NEtIz) 3] 4(BF 4) 4 and [Mn F (NEtIz) 3] 4(BF 4) 4 the v values are J/K⋍−10 K and −1.2±0.5 K. Comparing these results with typical values for 180° fluoro-bridged exchange paths, namely 70–100 K for cobalt salts and 3–4 K for manganese compounds, one observes a drastic reduction in the strength of the antiferromagnetic exchange constant. This reduction is predicted by recent theoretical models for the superexchange interaction.