Stereochemical investigation of bis(bidentate)-palladium(II) complexes. Transmission of ring chiralities between chelating amine ligands via their prochiral N-methyl substituents

Abstract

Three diastereomeric palladium(II) complex cations containing the orthometalated (S)-dimethyl(1-(α-naphthyl)-ethyl)amine together with the (R,R)-, (S,S)- and (R,S)- forms of N,N,N′,N′-tetramethyl-2,3-butanediamine have been prepared separately as their perchlorate salts. The (S,R,R) isomer with [α]D+167° (c 1.0, CH2Cl2) crystallizes in the space group P212121 with a = 7.872 (3) Å, b = 15.843 (5) Å, c = 20.273 (6) Å, Z = 4, and R = 0.0372. The square-planar geometry around palladium is slightly distorted. The (S,S,S) isomer with [α]D +87° (c 1.0, CH2Cl2) crystallizes in the space group P21 with a = 7.978 (2) Å, b = 20.856 (2) A,̊ c = 15.218 (2) Å, β = 90.06 (2)°, Z = 4, and R = 0.0362. The coordination geometry around palladium is distorted seriously from square-planar due to severe ligand-ligand interactions. The (S,R,S) isomer with [α]D +158° (c 1.0, CH2Cl2) crystallizes stereoselectively as a sole conformer in the space group P21 with a = 11.661 (3) Å, b = 9.982 (2) Å, c = 13.227 (4) Å, β = 105.29 (2)°, Z = 2, and R = 0.0319. In solution, the diamine ligands of these complexes are found to be labile under ambient conditions, as evidenced by facile intermolecular ligand redistributions between diastereomers. For the (S,R,S) isomer, a marked temperature dependent asymmetric equilibration occurred between two possible internal diastereomeric structures. Furthermore, it has been shown that although steric factors govern the relative thermodynamic stabilities in this series of complexes, these effects are frequently outweighed by lattice effects in isolated solids, where a sterically unfavorable isomer can be isolated readily from a diastereomeric equilibrium mixture in the crystallization process. © 1994 American Chemical Society.