High‐Spin Ni₃Fe₂(CN)₆ and Cu₃Cr₂(CN)₆ Clusters Based on a Trigonal Bipyramidal Geometry

Abstract

Abstract The synthesis and characterization of two new cyano‐bridged cluster compounds, [Tp 2 (cyclen) 3 Ni II 3 Fe III 2 (CN) 6 ](BF 4 ) 4 ( 1 ) and [(Me 3 tacn) 5 Cu II 3 Cr III 2 (CN) 6 ](ClO 4 ) 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 = $\rm ^{9}/_{2}$ ground states, respectively. A room‐temperature assembly reaction generates a purple form of compound 2 , wherein two cyanide stretching frequencies in the infrared spectrum ( ν 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 compound is instead isolated, for which a single cyanide stretch at ν 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.