Zinc(II) complexes with intramolecular amide oxygen coordination as models of metalloamidases

Polydentate ligands (6- R-1-2-pyridylmethyl) -R-2 (R-1 = (NHCOBu)-Bu-t, R-2 = bis(2-pyridylmethyl) amine L-1, bis(2-(methylthio)ethyl) amine L-2 and N(CH2CH2)(2)S L-3) form mononuclear zinc(II) complexes with intramolecular amide oxygen coordination and a range of coordination environments. Thus, the reaction of Zn(ClO4)(2) . 6H(2)O with L1-3 in acetonitrile affords [(L)Zn](ClO4)(2) (L = L-1, 1; L-2, 2) and [(L-3) Zn(H2O)(NCCH3)](ClO4)(2) 3. The simultaneous amide/water binding in 3 resembles the motif that has been proposed to be involved in the double substrate/ nucleophile Lewis acidic activation and positioning mechanism of amide bond hydrolysis in metallopeptidases. X-ray diffraction, H-1 and C-13 NMR and IR data suggests that the strength of amide oxygen coordination follows the trend 1 > 2 > 3. L1-3 and 1-3 undergo cleavage of the tert-butylamide upon addition of Me4NOH . 5H(2)O (1 equiv.) in methanol at 50(1) degreesC. The rate of amide cleavage follows the order 1 > 2 >> 3, L1-3. The extent by which the amide cleavage reaction is accelerated in 1 - 3 relative to the free ligands, L1-3, is correlated with the strength of amide oxygen binding and Lewis acidity of the zinc(II) centre in 1 - 3 deduced from the X-ray, NMR and IR studies.