Hydrogen bonding and conformational analysis of chelate-stabilized alkoxopalladium(II) complexes derived from amino alcohol ligands

The reaction of palladium acetate with two equivalents of di- and triethanolamines RN(CH2CH2OH)(2) in the presence of a base affords the new chelate-stabilized alkoxo Pd(II) complexes [Pd(OCH2CH2N(R)CH2CH2OH)(2)] [R = Me (1), Et (2), n-Bu (3), benzyl (4) or CH2CH2OH (5)]. These N,O-ligated complexes are isolated in high yield as yellow, crystalline solids and are thermally stable despite the presence of several beta-hydrogen atoms in the ligand system. Both complexes possess a square-planar palladium coordination geometry with the two oxygen atoms positioned mutually trans. The most notable difference in the molecular structures is that 1 forms a two dimensional network of intermolecular O-H ... O hydrogen bonds, whereas 5 forms intramolecular O-H ... O hydrogen bonds, which cage the palladium center. In solution 1-4 exist as a diastereoisomeric mixture (a racemic enantiomeric pair SNSN, R(N)R(N) and a mesomeric form R(N)S(N)) in a 1:1 molar ratio, and this ratio is independent of temperature in nonalcoholic solvents. When complexes 1-4 are dissolved in protic solvents (e.g. MeOH) the diastereomeric excess is temperature-dependent due to an exchange process between the meso diastereoisomer and the (racemic) enantiomeric pair. Thermodynamic parameters for this process in a mixture of MeOH-toluene have been determined with NMR and show this process to be influenced by the steric nature of the alkyl substituent (R) on nitrogen. A conformational analysis based on H-1-NMR coupling constants within the N,O-chelate ring of complexes 1-4 provides details on the solution structure of the ring in both diastereoisomers.