Trinuclear {M1}CN{M2}2 complexes (M1 = CrIII, FeIII, CoIII; M2 = CuII, NiII, MnII).: Are single molecule magnets predictable?

The reaction of the hexacyanometalates K-3[M-1(CN)(6)] (M-1 = Cr-III, Fe-III, Co-III) with the bispidine complexes [M-2(L-1)(X)](n+) and [M-2(L-2)(X)](n+) (M-2 = Mn-II, Ni-II, Cu-II; L-1 = 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; L-2 = 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 = Ni-II, Cu-II) with S = 3/2 (Cu-II) and S = 5/2 (Ni-II) ground states are analyzed with an extended Heisenberg Hamiltonian which accounts for anisotropy and zero-field splitting, and the data of the Cull 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 Fe-III(T-2(2g)) ground state.