Self-Assembly of Homochiral Double Helix and Side-by-Side Helix Conformers of Double-Stranded Disilver(I)− and Digold(I)−Tetra(tertiary phosphine) Helicates

Abstract

The enantiomers of the (R*,R*)-(±) diastereomer of the chelating C₂-tetra(tertiary phosphine) (R*,R*)-(±)-1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane, (R*,R*)-(±)/(R*,S*)-1, spontaneously self-assemble into homochiral double-stranded disilver(I) and digold(I) helicates of the type [M₂(tetraphos)₂]X₂ upon reaction with appropriate silver(I) and gold(I) salts. The corresponding copper(I) complex is mononuclear. Crystal and molecular structures of Δ-(−)-[Cu{(R,R)-1}]PF₆·EtOH, Λ-(−)-[Ag₂{(R,R)-1}₂](PF₆)₂, and Λ-(−)-[Au₂{(R,R)-1}₂](PF₆)₂·CH₂Cl₂ are reported. The structure determinations on the silver and gold complexes are the first to be performed on enantiomerically pure dimetal helicates with chiral ligands. The dinuclear silver complex crystallizes with one molecule each of the left-handed (Λ) D₂-double helix and C₂-side-by-side helix conformers of the cation and associated anions in each unit cell, whereas crystals of the analogous gold complex contain only the side-by-side helix and associated anions. The absolute configuration(s) of the metal stereocenter(s) in each complex is S. Conductance measurements in acetonitrile indicated considerable rearrangement of the mononuclear copper complex into the dinuclear helicate complex, whereas the silver and gold complexes conducted as di-univalent salts under similar conditions. Energy minimization calculations of the structures of the disilver complex cation with use of the program SPARTAN 3.0 predicted the structures observed with considerable accuracy, especially the conformations of the chiral central ten-membered ring in the complex and the relationship of the helicity of this ring to the stereoselective formation of the double helix and side-by-side helix structures. The resolution of (R*,R*)-(±)-1 is the first on a tetra(tertiary phosphine). The more-soluble (R*,R*)-(±) form of the ligand was separated in high yield from the less-soluble (R*,S*) form by selective extraction with tetrahydrofuran, whereupon it was resolved by the method of metal complexation with the readily prepared homochiral complex (+)-di(μ-chloro)bis[(R)-1-[1-(dimethylamino)ethyl]-2-phenyl-C²,N]dipalladium(II)−1-dichloromethane, (R)-2·CH₂Cl₂. The enantiomers of the phosphine were obtained by liberation from the diastereomeric complexes (R_C),(R_P,R_P)- and (R_C),(S_P,S_P)-3 (X = PF₆) and brought to optical purity by crystallization from acetone−ethanol, giving colorless needles having mp 88 °C and [α]²¹_D +20.5 (c 1.0, CH₂Cl₂) (S,S enantiomer) and [α]²¹_D −20.5 (c 1.0, CH₂Cl₂) (R,R enantiomer). The crystal and molecular structures of (R_C),(R_P,R_P)-3 (X = PF₆) have been determined. The complete optical purity of each enantiomer of the tetra(tertiary phosphine) was confirmed in each case by the quantitative repreparation of the diastereomeric palladium complex from which it was liberated.