INTRAMOLECULAR MAGNETIC AND ELECTROSTATIC INTERACTIONS IN STEPWISE STACKED TRINUCLEAR PARAMAGNETIC METALLOCENES WITH THE METAL SEQUENCES FEM'FE AND NIM'NI(M'=V, CR, CO, NI) AND COCRCO

A building block concept was used for the synthesis of paramagnetic trinuclear metallocenes which are Cp-bridged by two adjacent Me(2)Si groups. The series of compounds abbreviated by the metal sequence FeM'Fe contained one central paramagnetic metallocene with M' = V, Cr, Co, and Ni and two terminal ferrocenes, while three paramagnetic metallocenes were assembled in CoCrCo and the series NiM'Ni with M' = V, Cr, Co, and Ni. Two paramagnetic centers were present in NiFeNi. The X-ray crystal structure analysis of FeVFe showed a folding of the bridging ligand so that the axes of adjacent metallocenes are inclined by 39.9 degrees and the CH3 groups are not equivalent (triclinic, space group P ($) over bar 1, a = 10.075(2) Angstrom, b = 10.268(1) Angstrom, c = 9.019(1) Angstrom, alpha = 96.93(1)degrees, beta = 95.78(1)degrees, gamma = 95.15(1)degrees, Z = 1). Seven species MM'M were investigated by cyclic voltammetry. They underwent up to five oxidations and two reductions. From the potential splittings Coulomb interactions of 140 and 40 mV were estimated between next-neighbor and terminal metallocenes, respectively. H-1 and C-13 NMR studies of the series FeM'Fe established the orbital which is preferred by the unpaired spin and that appreciable spin density is transferred within the bridging ligand from the paramagnetic to the diamagnetic metallocene depending on the folding angle. In the series NiM'Ni and in CoCrCo the spin densities add and subtract depending on the metals. From temperature-dependent H-1 NMR studies of NiVNi and NiNiNi it was concluded that in solution the bending of the molecules decreased with increasing temperature. The H-1 NMR signal widths disclosed magnetic interactions in NiM'Ni. Solid state magnetic measurements on NiVNi, NiCrNi, and NiNiNi showed that the interaction is antiferromagnetic with J = -3.08, -1.95, and -10.5 cm(-1), respectively; the Hamiltonian describing the interaction between two local spins is written in the form -JS(A)S(B). The ground state of these molecules was discussed in terms of a model which relates them to localized trimethylenediyl diradicals.