Trinuclear {M¹}CN{M²}2 Complexes (M¹ = CrIII, FeIII, CoII; M² = CuII NiII MnII). Are Single Molecule Magnets Predictable?

The reaction of the hexacyanometalates K3[M¹(CN)6] (M¹ = CrIII, FeIII, CoIII) with the bispidine complexes [M²-(L¹)(X)]n+ and [M²(L²)(X)]n+ (M² = MnII, NiII, CuII; L¹ = 3-methyl-9-oxo-2,4-di-(2-pyridyl)-7-(2-pyridylmethyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylic acid dimethyl ester; L² = 3-methyl-9-oxo-7-(2-pyridylmethyl)-2,4-di-(2-quinolyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylic acid dimethyl ester; X = anion or solvent) in water-methanol mixtures affords trinuclear complexes with cis- or trans-arrangement of the bispidine-capped divalent metal centers around the hexacyanometalate. X-ray structural analyses of five members of this family of complexes (cis-Fe[CuL²]2, trans-Fe[CuL¹]2, cis-Co[CuL²]2, trans-Cr[MnL¹]2, trans-Fe[MnL¹]2) and the magnetic data of the entire series are reported. The magnetic data of the cyanide bridged, ferromagnetically coupled cis- and trans-Fe[ML]2 compounds (M = NiII, CuII) with S = 3/2 (CuII) and S = 5/2 (NiII) ground states are analyzed with an extended Heisenberg Hamiltonian which accounts for anisotropy and zero-field splitting, and the data of the CuII systems, for which structures are available, are thoroughly analyzed in terms of an orbital-dependent Heisenberg Hamiltonian, in which both spin-orbit coupling and low-symmetry ligand fields are taken into account. It is shown that the absence of single-molecule magnetic behavior in all spin clusters reported here is due to a large angular distortion of the [Fe(CN)6]3- center and the concomitant quenching of orbital angular momentum of the FeIII (²T2g) ground state. [ABSTRACT FROM AUTHOR]