DIASTEREOISOMERIC 4-COORDINATE COMPLEXES .6. PARAMAGNETIC NICKEL (2) COMPLEXES WITH 4 ASYMMETRIC LIGAND CENTERS

A series of bis-chelate nickel(II) complexes containing four asymmetric ligand centers occurring as two inequivalent pairs has been synthesized, and their proton resonance spectra have been investigated in order to determine to what extent the contact shift method can detect all possible diastereoisomers. The complexes examined are of the bis(salicylaldimine) type having the general formulation Ni(X-R-sal)2, in which X = 3sBu, 5Me and R = CH3CHCH2Ph (Amp), CH3CHPh (PhEt), and sec-Bu (= sBu). All complexes are shown to exist in the dynamic planar tetrahedral equilibrium in chloroform solution. It is demonstrated that, apart from the absolute configuration at the metal in the tetrahedral form, six pmr-distinguishable diastereoisomers are possible for complexes prepared from racemic ligand components. Signals corresponding to the six isomers were observed in the pmr spectra of the three groups of complexes. A general method of signal assignments for the Ni(X-R-sal)2 isomers is described and applied to Ni(3sBu-Amp-sal)2, resulting in unequivocal assignment of certain signals to the six possible isomers. The marked sensitivity of the pmr method in detecting diastereoisomers arises from the contact interaction and the presence of the structural equilibrium, for which the ΔF value of each diastereoisomer is measurably different. These values are considered to differ principally because of inequalities in the free energies of the paramagnetic, tetrahedral forms which in turn are produced by the various sets of intramolecular (R-R, R-X) interactions. Intrinsic differences in electron-nuclear coupling constants are shown to be unimportant in producing the primary chemical shift separation between R-active and R-meso sets of isomers with R = Amp. The sets of thermodynamic parameters (ΔF, ΔH, ΔS) describing the planar-tetrahedral structural change of the six diastereoisomers of Ni(3sBu,5Me-Amp-sal)2 have been derived from the temperature dependence of the contact shifts of each isomer. © 1968, American Chemical Society. All rights reserved.