Interaction of urea with a hydroxide-bridged dinuclear nickel center: An alternative model for the mechanism of urease

A hydroxide-bridged dinuclear nickel complex with a urea molecule linking t he two metal ions through its carbonyl oxygen atom has been prepared as a m odel for the metalloenzyme urease. This complex, [Ni-2(mu-OH)(mu-urea)(bdpt z)(urea) (CH3CN)](ClO4)(3), where bdptz is the dinucleating ligand 1,4-bis( 2,2'-dipyridylmethyl)phthalazine, effects the hydrolysis of urea upon heati ng in a two-step reaction. In the first step, a molecule of ammonia is elim inated from urea with concomitant production of cyanate, the first-order ra te constant in acetonitrile being (7.7 +/- 0.5) x 10(-4) h(-1). This reacti on is at least 500 times faster than the spontaneous decomposition of urea under the same conditions. When the cyanate-containing product is further h eated in the presence of water, the cyanate is hydrolyzed with a second-ord er rate constant of (9.5 +/- 1) x 10(-4) M-1 h(-1). Reaction of [Ni-2(mu-OH )(mu-urea)(bdptz)(urea)(CH3CN)](ClO4)(3) in 50% aqueous acetonitrile afford ed ammonia with no appreciable buildup of the cyanate-containing species. A possible analogue of the cyanate-containing product, [Ni-2(mu-OH)(mu-H2O)( bdptz)(mu-OCN)](2)(OTs)(4) was independently synthesized and structurally c haracterized. These results establish the precedence for hydrolysis of urea via a cyanate intermediate as an alternative mechanism for the urease-cata lyzed hydrolysis of urea.