Outer Sphere Urea Hydrolysis by Bis-Nickel Complexes: Questioning the Urea Activation by the Urease Enzyme

Abstract

Urease enzyme has a dinuclear nickel active centre that hydrolyze urea into carbon dioxide and ammonia. In this work, two bis-nickel urease models were synthesized, [Ni<sub>2</sub>L(OAc)] and [Ni<sub>2</sub>L(Cl)(Et<sub>3</sub>N)<sub>2</sub>], based on the Trost bis-Pro0Phenol ligand (L). Interestingly, both complexes produced ammonia from urea, in which the [Ni<sub>2</sub>L(OAc)] complex was ten times slower than urease, whereas the more labile complex [Ni<sub>2</sub>L(Cl)(Et<sub>3</sub>N)<sub>2</sub>],was only four times slower. The intermediates were evaluated both experimentally and theoretically, indicating that the [Ni<sub>2</sub>L(H<sub>2</sub>O)<sub>2</sub>]<sup>+</sup> intermediate is the most important to activate urea via an outersphere mechanism. Isocyanate was produced in a self-elimination mechanism. The reaction performed with different substrates indicated that the biomimetic complexes were able to hydrolyze isocyanate. The outersphere activation of urea by these complexes reals an alternative activation mechanism to be considered for the urease enzyme, not yet reported in the literature. . <br>