Paramagnetic NMR investigations of high-spin nickel(II) complexes. Controlled synthesis, structural, electronic, and magnetic properties of dinuclearvs mononuclear species

New dissymmetric tertiary amines (N3SR) with varying N/S donor sets have be en synthesized to provide mono- and dinuclear complexes. Acetate ions are u sed to complete the octahedral coordination sphere around nickel(II) atom(s ). The facile conversion of mononuclear to dinuclear systems can be control led to produce either mono- or dinuclear complexes from the same ligand. Th e dinuclear complex a(BPh4)(2) ([Ni-2(N-3-SSN3)(OAc)(2)](BPh4)(2)) has been characterized in the solid state by X-ray diffraction techniques as solvat e: 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 acetate s. Magnetic susceptibility reveals a weak ferromagnetic exchange interactio n between the two Ni atoms with J = 2.5(7) cm(-1). UV-vis studies suggest t hat the six-coordinated structure persists in solution. The H-1 NMR spectru m of a(BPh4)(2) exhibits sharp significantly hyperfine shifted ligand signa ls. A complete assignment of resonances is accomplished by a combination of methods: 2D-COSY experiments, selective chemical substitution, and analysi s 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 tempe rature dependence study of longitudinal relaxation times indicates that a v ery unusual paramagnetic Curie dipolar mechanism is the dominant relaxation pathway in these weakly ferromagnetically spin-coupled dinickel(II) center s. The mononuclear nickel(II) analogue exhibits extremely broader H-1 NMR s ignals and only partial analysis could be performed. These data are consist ent with a shortening of electronic relaxation times in homodinuclear compo unds with respect to the corresponding mononuclear species.