High-spin Ni₃Fe₂(CN)₆ and Cu ₃Cr₂(CN)₆ clusters based on a trigonal bipyramidal geometry

The synthesis and characterization of two new cyanobridged cluster compounds, [Tp₂(cyclen)₃Ni^II₃Fe ^III₂(CN)₆](BF₄)₄ (1) and [(Me₃tacn)₅Cu^II₃Cr ^III₂(CN)₆](ClO₄)₆ (2), are reported. The structure of each cluster is based upon a trigonal bipyramidal geometry, and ferromagnetic coupling between constituent metal ions is shown to give rise to S = 4 and S = 9/2 ground states, respectively. A room-temperature assembly reaction generates a purple form of compound 2, wherein two cyanide stretching frequencies in the infrared spectrum (v_CN = 2094, 2156 cm^-1) indicate partial isomerism of the cyanide bridging ligands. The X-ray crystal structure confirms this isomerism via examination of the C and N thermal parameters. At -40 °C, a metastable green form of the com-pound is instead isolated, for which a single cyanide stretch at v_CN = 2156 cm^_1 is consistent with an unisomerized cluster. The temperature-dependence of the magnetization under varying applied field reveals significant axial anisotropy in compound 1, with a significant axial zero-field splitting while compound 2 shows no zero-field splitting in either isolated form. In line with the observed anisotropy, AC magnetic susceptibility measurements performed on compound 1 reveal a frequency-dependent out-of-phase signal suggestive of single-molecule magnet behavior.