Syntheses, crystal structures, circular dichroism, and magnetic properties of chiral dinuclear and polymeric nickel(II) compounds

Chiral dinuclear nickel(II) complexes, [Ni(L^{R, R})(C₂O₄)Ni(L^{R, R})](ClO₄)₂·4CH₃CN (3) and [Ni(L^S,S)(C₂O₄)Ni(L ^S,S)](ClO₄)₂· 4CH₃CN (4) and chiral polymeric compounds, [Ni(L^{R ,R})(CrO₄)]_n·2H ₂O·CH₃CN (5) and [Ni(L^{S, S})(CrO₄)]_n·2H₂O· CH₃CN (6) have been synthesized and characterized (L^{R ,R/S,S} = 1,8-di((R/S)-α-methylbenzyl)-1,3,6,8,10,13-hexaazacyclotetradecane). These chiral compounds were characterized by X-ray crystallography, circular dichroism, and molecular magnetism. The nickel(II) ions in 3 and 4 have a distorted octahedral geometry by coordination with four nitrogens of a macrocyclic ligand with chiral pendents in a folded conformation and two oxygens of an oxalate ion in the cis positions. The nickel(II) ions in 5 and 6 have a distorted octahedral geometry by coordination with four nitrogens of a macrocyclic ligand in a planar conformation and two oxygens of two chromate ions in the axial positions. Complexes 3 and 4 show strong antiferromagnetic interactions [3: g = 2.36, J/k_B = -29.9 K (-20.8 cm^-1); 4: g = 2.18, J/k_B = -25.5 K (-17.7 cm^-1)], while 5 and 6 exhibit weak antiferromagnetic couplings [5: g = 2.25, J/k_B = -1.20 K (-0.83 cm^-1); 6: g = 2.25, J/k_B = -0.68 K (-0.47 cm ^-1)]. The former complexes occur strong antiferromagnetic interactions via the oxalato bridges within the nickel(II) dimers, the latter compounds are weak antiferromagnetic interactions through the chromate ions within the 1D polymers. The circular dichroism (CD) spectrum of 3 has exhibited two negative peaks at 336 and 533 nm, and that of 4 has displayed an enantiomeric pattern. The CD spectrum of 5 has appeared a negative absorption above ca. 550 nm, while that of 6 has shown an enantiomeric pattern in the same wavelength region.