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

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.