Structural Insight into the Reaction Mechanism of Pd-Catalyzed Nitrile Hydration: Trapping the [Pd(H2O)4]2+ Cation Through a Supramolecular Complex

Four new bis(oxamato)palladate(II) complexes of formula (n-Bu4N)2[Pd(2,4,6-Me3pma)2]2CH3CN (1),(n-Bu4N)2[Pd(2,4,6-Me3pma)2]2CH3CONH2 (2) and (n-Bu4N)4[Pd(H2O)4][Pd(4-Xpma)2]32CH3CONH2 withX = Br (3) and Cl (4) (2,4,6-Me3pma = N-2,4,6-trimethylphenyloxamate, 4-Brpma = N-4-bromophenyloxamate,N-4-chlorophenyloxamate and n-Bu4N+ = tetra-n-butylammonium) have been obtained and characterizedby single crystal X-ray diffraction. All of them contain bis(oxamato)palladate(II) anions andbulky n-Bu4N+ cations, but compounds 3 and 4 have also the out of the ordinary [Pd(H2O)4]2+ inorganiccation. Acetonitrile and appealing acetamide are present as lattice molecules in compounds (1) and (2–4),respectively. While 1 was prepared in a neutral solution, species 2–4 were obtained in a basic medium.The acetamide molecules are the natural consequence of the acetonitrile hydration reaction during theoxamate-palladate(II) complexation. Interestingly, the use of a different oxamate ligand supports(through a molecular-recognition interaction) the structural grasping of the [Pd(H2O)4]2+ species, whichcan be considered as a reaction intermediate for the nitrile hydration.