Studies on the Metal Amide Bond 21. Metal Complexes of Chiral Phenyl-Substituted Bis-Picolinamide Tetradentates

Abstract

Two new C-phenyl bis-picolinamide tetradentates, N', N,'-bis(2-pyridinecarboxamide)-1,2-diphenyIethane (bpstienH₂) and N,N'-bis(2-pyridinecarboxamide)phenylethane (bpstyenH₂) together with their deproto-nated complexes of bivalent Cu, Ni and Pd have been prepared and characterised. A crystallographic analysis of the two copper(II) complexes is reported, with the S-bpstyenH₂ ligand found to be susceptible to racemisation under the warm basic conditions used in the syntheses.

{N,N'-Bis(2-pyridinecarboxamido)-S',S-1,2-diphenylethane]copper(II)}, C₂₆H₂₀N₄O₂Cu is mono-clinic, spacegroup P2₁ (No. 4), with a = 11.170(3), b = 10.853(4), c = 18.570(7) Å, β = 103.74(3)°, Z = 4, with two molecules in the asymmetric unit. The structure was refined to R = 0.039 for 2519 reflexions. Each molecule is five coordinate with the fifth position occupied by an amide oxygen atom of a neighbouring molecule. These Cu-O(amide) distances are 2.554(7) and 2.757(6) Å. The stereochemistry of molecule 1 closely resembles a square-based pyramid, whereas that for molecule 2 approximates a distorted trigonal bipyramid. In each case the central five-membered chelate ring has a γ conformation with the two phenyl substituents occupying the axial positions in a rare configuration.

{AquaN,N'-bis(2-pyridinecarboxamido)phenylethane]-copper(II)}-sesquihydrate, C₂₀H₂₁N₄O₄₅Cu, is monoclinic, space group P2₁/c (No. 14), with a= 12.122(4), b= 10.691(2), c= 15.952(4)Å, β = 110.76(2)°, Z = 4. The structure was refined to R = 0.046 for 1939 reflexions. The stereochemistry of the five-coordinate copper atom approximates a square pyramid with the copper atom displaced 0.26 Å towards the apical water molecule (Cu-O 2.251(5) Å). In the enantiomer containing S-bpstyen the central five-membered chelate ring has a Δ conformation with the phenyl substituent occupying an equatorial position.

In solution the palladium(II) and nickel(II) deprotonated metal complexes of the S,S- and S- ligand forms are shown by ¹H NMR analysis, in conjunction with their chiroptical properties, to each have a X gauche central chelate ring conformation with the phenyl substituent(s) occupying axial positions.