Energetics and mechanism of alkylamine N-H bond cleavage by palladium hydroxides: N-H activation by unusual acid-base chemistry

The dimeric palladium hydroxide complex [(PPh3)(Ph)Pd(mu-OH)](2) underwent proton exchange reactions to cleave the N-H bonds of both alkyl and aryl primary amines to generate dimeric palladium amido complexes. This acid-base chemistry was reversible and provided thermodynamic information on the relative bond energies of the bridging amido and bridging hydroxo ligands. The mechanism of the N-H activation reaction was studied in both the forward and reverse directions to gain a detailed picture of the reaction pathway. Our data indicate that the reaction proceeds via cleavage of the hydroxide dimer by association of 2 equiv of amine followed by reversible proton transfer to create the amido ligand and a bound water ligand. Reassociation of the amido monomer with one of the original hydroxide monomers with displacement of water and amine gives the final amido hydroxo dimer product. Microscopic reversibility leads to the prediction that the reaction of water with the amido complexes is first order in product amine and is, therefore, auto-catalytic. This unusual kinetic behavior was observed.