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 the two metal ions through its carbonyl oxygen atom has been prepared as a model for the metalloenzyme urease. This complex, Ni2(μ-OH)(μ-urea)(bdptz)(urea)(CH3CN)(ClO4)3, where bdptz is the dinucleating ligand 1,4-bis(2,2‘-dipyridylmethyl)phthalazine, effects the hydrolysis of urea upon heating in a two-step reaction. In the first step, a molecule of ammonia is eliminated from urea with concomitant production of cyanate, the first-order rate constant in acetonitrile being (7.7 ± 0.5) × 10-4 h-1. This reaction is at least 500 times faster than the spontaneous decomposition of urea under the same conditions. When the cyanate-containing product is further heated in the presence of water, the cyanate is hydrolyzed with a second-order rate constant of (9.5 ± 1) × 10-4 M-1 h-1. Reaction of Ni2(μ-OH)(μ-urea)(bdptz)(urea)(CH3CN)(ClO4)3 in 50% aqueous acetonitrile afforded ammonia with no appreciable buildup of the cyanate-containing species. A possible analogue of the cyanate-containing product, Ni2(μ-OH)(μ-H2O)(bdptz)(μ-OCN)2(OTs)4, was independently synthesized and structurally characterized. These results establish the precedence for hydrolysis of urea via a cyanate intermediate as an alternative mechanism for the urease-catalyzed hydrolysis of urea.