Hydrogen Bonding and Conformational Analysis of Chelate‐Stabilized Alkoxopalladium(II) Complexes Derived from Amino Alcohol Ligands

Abstract The reaction of palladium acetate with two equivalents of di‐ and triethanolamines RN(CH 2 CH 2 OH) 2 in the presence of a base affords the new chelate‐stabilized alkoxo Pd(II) complexes [Pd(OCH 2 CH 2 N(R)CH 2 CH 2 OH) 2 ] [R = Me ( 1 ), Et ( 2 ), n ‐Bu ( 3 ), benzyl ( 4 ) or CH 2 CH 2 OH ( 5 )]. These N,O ‐ligated complexes are isolated in high yield as yellow, crystalline solids and are thermally stable despite the presence of several β‐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 S N S N , 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 1 H‐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.

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