Amide hydrolysis effected by a hydroxo-bridged dinickel(II) complex: Insights into the mechanism of urease

A series of dinuclear nickel complexes, [Ni-2(H2O)(4)(bdptz)](OTs)(4), [Ni-2(OH)(H2O)(3)(bdptz)](OTs)(3), and [Ni-2(H2O)(OH)(2)(bdptz)](2)(OTs)(4), where bdptz is the dinucleating ligand 1,4-bis(2,2'-dipyridylmethyl)phthalazine and OTs is p-toluenesulfonate, were prepared as models for urease. Potentiometric titration of [Ni-2(H2O)(4)(bdptz)](OTs)(4) revealed two deproronation constants with pK(a) values of 4.38 +/- 0.02 and 8.51 +/- 0.02. The product of the first deprotonation, [Ni-2(OH)(H2O)(3)(bdptz)](OTs)(3), effected the stoichiometric hydrolysis of picolinamide in ethanol, a heretofore unprecedented transformation in a dinuclear metallohydrolase model complex. Picolinamide hydrolysis by [Ni-2(H2O)(4)(bdptz)](OTs)(4) or [Ni(teTpy)(H2O)(3)](OTs)(2) (terpy = 2,2':6',2 "-terpyridine) under the same conditions was not observed. This amide hydrolysis mimics the hydrolysis of urea by the metalloenzyme urease in that a hydroxide nucleophile is produced by the dinuclear metal center and the amide is activated for nucleophilic attack by coordination to the dinuclear nickel center.