Nickel Complexes as Artificial Urea Lyases with 1012-Fold Rate Enhancement

Catalytic acceleration of urea decomposition offers a route to artificial urea lyases. Because urease-inspired complexes do not reproduce the enzymatic hydrolysis pathway but instead promote urea decomposition, we developed four dinuclear nickel catalysts for urea lyase activity. The complexes are phenolate-bridged in the solid state, with triazole units bearing butyl ((Ni,Ni)L1), n-propanol ((Ni,Ni)L2), or phenyl ((Ni,Ni)L3 and (Ni,Ni)L3″) groups at C4 to modulate the electronic and secondary coordination environment. Characterization by microanalysis, MALDI-TOF, FTIR, UV-Vis, 1H NMR, TGA, conductivity, and magnetic susceptibility indicates high-spin Ni(II) centers in (Ni,Ni)L1-L3 and a nearly diamagnetic species in (Ni,Ni)L3″. All complexes catalyzed urea decomposition to isocyanate and ammonia. Stopped-flow pH monitoring showed that (Ni,Ni)L1 and (Ni,Ni)L3 complexes gave the greatest rate enhancements (kcat/knon-cat = 7.7 × 1011 for (Ni,Ni)L1 and 4.6 × 1012 for (Ni,Ni)L3). These differences were linked to distinct active species in solution in which aquation breaks the phenolate bridge and generates species with different electrolyte compositions. The most active complexes formed 2:1 electrolytes, whereas the less active ones formed 1:1 or 3:1 electrolytes. These results relate solution-state speciation to urea lyase activity and highlight the role of coordination-sphere reorganization in controlling catalytic urea decomposition.