cis–trans isomerism and rotational isomerism in triazene-1-oxide bis-chelates of palladium(ii)

Abstract

Triazene-1-oxide bis-chelates of palladium(II) reveal the cis–trans isomerism. The molecular structures of five trans and two cis isomers are presented. The cis complexes reveal the intramolecular interligand π–π interactions. The intra- and inter-molecular C–H⋯O interactions stabilise the trans and cis complexes, respectively. The cis complexes are usually obtained by thermal isomerisation of the corresponding trans complexes in acetonitrile followed by chromatographic separation. The isomerisation occurs only for complexes bearing at least one alkyl substituent in the ortho position of the ligand aromatic ring. This phenomenon is interpreted in terms of steric crowding and intermolecular C–H⋯O hydrogen bonding. One- and two-dimensional ¹H-NMR studies reveal that Pd(II) triazene-1-oxide cis-bis(chelates) bearing at least one ligand with heteromorphically substituted ortho positions of the phenyl ring display dynamic rotational isomerism in solution due to rotation of the ligand phenyl ring around the N3–C1 bond. For homoleptic systems this isomerism results in formation of two diastereoisomers: achiral C_s with ligands having opposite axial chirality and a chiral one, C₂, corresponding to those with homomorphic ortho groups located on the same or opposite sides of the planar co-ordination core, respectively. For heteroleptic systems either the dynamic racemization of the complex or the diastereoisomeric interconversion is possible depending on whether one or both ligands reveal the axial chirality for non-co-planar conformation. The corresponding processes are observed for the highly sterically congested trans complexes.