Symmetry-based magnetic anisotropy in the trigonal bipyramidal cluster [Tp₂(Me₃tacn)₃Cu₃Fe ₂(CN)₆]⁴⁺

Reaction of [(Me₃tacn)Cu(H₂O)₂]₂+ (Me₃tacn = N,N′,N″-trimethyl-1,4,7-triazacyclononane) with [TpFe(CN)₃]^- (Tp^- = hydrotris(pyrazolyl)borate) in a mixture of ethanol and acetonitrile affords the pentanuclear cluster [Tp₂(Me₃tacn)₃Cu₃Fe₂(CN)₆]⁴⁺. Single-crystal X-ray analysis reveals a trigonal bipyramidal structure featuring a D_3h-symmetry core in which two opposing Fe^III (S = 1/2) centers are linked through cyanide bridges to an equatorial triangle of three Cu^II (S = 1/2) centers. Fits to variable-temperature dc magnetic susceptibility data are consistent with ferromagnetic coupling to give an S = 5/2 ground state, while fits to low-temperature magnetization data indicate the presence of a large axial zero-field splitting (D = -5.7 cm^-1). Frequency dependence observed in the ac magnetic susceptibility data confirms single-molecule magnet behavior, with an effective spin reversal barrier of U_eff = 16 cm^-1. When compared with the much lower anisotropy barrier previously observed for the face-centered cubic cluster [Tp₈(H₂O)₆Cu₆Fe₈(CN)₆]⁴⁺, the results demonstrate the enormous influence of the geometry in which a given set of metal ions are arranged.