DIASTEREOISOMERIC 4-COORDINATE COMPLEXES .V. PSEUDO-TETRAHEDRAL COMPLEXES OF CONTROLLED ABSOLUTE CONFIGURATION . CONFIGURATIONAL INTERCONVERSION OF NICKEL(2) COMPLEXES WITHOUT RACEMIZATION

The stereochemistry of four-coordinate metal(II) complexes derived from 2,2′-bis(salicylideneamino)-biphenyl [M(X-sal)2bp, M = Co, Ni], 2,2′-bis(salicylideneamino)-6,6′-dimethylbiphenyl [M(X-sal)2bmp, M = Ni, Pd, Zn], and 2,2′-bis(salicylideneamino)-1,1′-binaphthyl [Ni(X-sal)2bn] has been investigated in solution. A previous claim that ligands of the first two types can, by virtue of steric constraints, form only pseudo-tetrahedral species has been vitiated by the isolation of Pd(3sBu-sal)2bmp and a variety of crystalline, diamagnetic Ni(II) complexes. In solution it is demonstrated by ligand-field spectral and proton contact shift studies that Ni(II) complexes of the three series undergo a planar * tetrahedral equilibrium which lies heavily toward the planar side. When these complexes are prepared from the racemic diamine and racemic 3-sec-butyl-5-methylsalicylaldehyde, three azomethine proton and 5-methyl signals are observed in every case. By controlling the absolute configurations of the diamine and 3-sec-butyl group, it is shown that these signals arise from the diastereoisomers Δ(+,+) = Δ(-,-), Δ(-,-) = *(+,+) (active), and Δ(+,-) = *(+,-) (meso) of the pseudo-tetrahedral form. The absolute configuration (Δ, *) at the pseudo-tetrahedral metal is controlled entirely by the configuration (R, S) of the diamine with the following stereochemical correlations operative: R. Preparation of pseudo-tetrahedral complexes from resolved diamines therefore results in completely stereospecific formation of a given absolute configuration at the metal. Proton resonance measurements up to 90° reveal that the signals of the two active forms of the Ni(II) complexes (e.g., Δ(+,+) and *(+,+)) are not averaged, indicating that the dynamic structural conversion occurs without racemization at the metal. This behavior is contrasted to that of analogous bis-chelate Ni(II) complexes for which two signals of the corresponding active isomers cannot be observed. The contribution of the entropy change to the values of ΔF for the planar ↦AR tetrahedral conversion of Ni(II) complexes is discussed. © 1968, American Chemical Society. All rights reserved.