Ordered binary crystals via cocrystallisation of quasiracemic Pd(ii) complexes in a borderline regime between inertness and lability

Abstract

Square planar complexes of the type [2-(1-aminoethyl)phenyl-kappa(2)C(1),N]chloro(py-kappaN)palladium(II)(py = pyridine derivative) have been prepared by cyclopalladation of 1-phenylethylamine. When opposite enantiomers of the chiral amine and different pyridine ligands were coordinated to the metal, quasiracemic complexes were obtained. The racemisation speed of solutions containing equimolar amounts of such quasiracemic complexes was used to test the compounds for inertness: slow ligand exchange was found for methylated pyridines. Enantiomerically pure complexes of opposite chirality with the electron rich ligands 2,6-collidine and 2,4,6-lutidine were successfully cocrystallised: they form a well-ordered binary solid in which four independent molecules, two of each constituent, are related by pseudo symmetry operations. This quasiracemate as well as its components and the true racemates of the latter were characterised by single crystal X-ray diffraction. The heterochiral crystals, both the cocrystal as well as the racemates, show better space filling than the homochiral solids.

Unexpected solids from enantiopure cationic palladium complexes and racemic anions: A structural study of chiral non-discrimination

“…One might perhaps argue that our cations are ''not very chiral'' as their enantiomers ''only'' differ in the orientation of the methyl substituent in the cyclopalladated amine. However, in the organopalladium residues presented in this work as well as in a number of quasiracemic crystals which were synthesized in an entirely different context [27,28] and contain neutral molecules with related ligands, the assignment of chirality was always unambiguous, and the difference between the enantiomers proved sufficient to ensure ordered structures.…”

New principle for palladacycle resolution: diastereoselective monomer to dimer conversion

“…As for the reason of such instability, it could arise from a rather weak bonding of hard donor atom(s) of auxiliary ligands of this kind with the very soft metal centre of the palladacycle with assistance of known palladium(II) metal lability. [50][51][52] Decomplexation of monodentate ligand Q from its adduct with palladacycle on the sorbent is reduced to the shift of the equilibrium between mononuclear (M 1 ) and dimeric species (D) to the latter (Scheme 5).…”

(S)-[2-(1-Aminoethyl)phenyl]chloro(3-chloropyridine)palladium(II) acetone 0.25-solvate

“…The dinuclear intermediates in the scheme have been prepared according to the procedures of Fuchita et al (1997) and Vicente et al (1997). Structural characterizations of the products (2) and (3) of the cyclopalladation reaction pathway for unsubstituted and parasubstituted primary amines have already been published (Calmuschi & Englert, 2002;Calmuschi, Alesi & Englert, 2004;Calmuschi, Jonas & Englert, 2004). Reaction of the dinuclear chloro complex (2) (0.5 mmol, 0.262 g) with a Lewis base (in this case 3-chloropyridine, 1.1 mmol, 0.125 g) in dichloromethane (40 ml) results in precipitation of the monomeric complex (3) as a light-yellow crystalline powder.…”

“…The mean planes through the atoms directly bonded to palladium and the heteroaromatic ring subtend a dihedral angle which is quite variable, with values of 57.51 (13), 52.47 (12), 53.14 (12) and 66.86 (12) . The large number of symmetrically independent residues per asymmetric unit indicates inefficient crystal packing in the complex; indeed, the overall space filling amounts to only 68.4%, which is low by comparison with chemically related complexes (Calmuschi, Alesi & Englert, 2004;Calmuschi & Englert, 2005). Attempts to cocrystallize this compound with related compounds of opposite chirality have, to date, resulted only in disordered binary crystal structures.…”