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Paramagnetic NMR Investigations of High-Spin Nickel(II) Complexes. Controlled Synthesis, Structural, Electronic, and Magnetic Properties of Dinuclear vs Mononuclear Species

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posted on 28.07.2001, 00:00 by Catherine Belle, Catherine Bougault, Marie-Thérèse Averbuch, André Durif, Jean-Louis Pierre, Jean-Marc Latour, Laurent Le Pape
New dissymmetric tertiary amines (N3SR) with varying N/S donor sets have been synthesized to provide mono- and dinuclear complexes. Acetate ions are used to complete the octahedral coordination sphere around nickel(II) atom(s). The facile conversion of mononuclear to dinuclear systems can be controlled to produce either mono- or dinuclear complexes from the same ligand. The dinuclear complex a(BPh4)2 ([Ni2(N3SSN3)(OAc)2](BPh4)2) has been characterized in the solid state by X-ray diffraction techniques as solvate:  a(BPh4)2·1/2[5(CH3OH)·(CH3CN)·(CH3CH2OH)]. The two Ni atoms are six-coordinated and bridged by a disulfide group and two bidentate acetates. Magnetic susceptibility reveals a weak ferromagnetic exchange interaction between the two Ni atoms with J = 2.5(7) cm-1. UV−vis studies suggest that the six-coordinated structure persists in solution. The 1H NMR spectrum of a(BPh4)2 exhibits sharp significantly hyperfine shifted ligand signals. A complete assignment of resonances is accomplished by a combination of methods:  2D-COSY experiments, selective chemical substitution, and analysis of proton relaxation data. Proton isotropic hyperfine shifts are shown to originate mainly from contact interactions and to intrinsically contain a small J-magnetic coupling and/or zero-field splitting contribution. A temperature dependence study of longitudinal relaxation times indicates that a very unusual paramagnetic Curie dipolar mechanism is the dominant relaxation pathway in these weakly ferromagnetically spin-coupled dinickel(II) centers. The mononuclear nickel(II) analogue exhibits extremely broader 1H NMR signals and only partial analysis could be performed. These data are consistent with a shortening of electronic relaxation times in homodinuclear compounds with respect to the corresponding mononuclear species.