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• W2029041099 abstract "Abstract The equilibrium and kinetics of urea binding by model dinickel complexes have been studied, which is relevant to the activity of the urease enzyme. The pyrazolate‐based, O 2 H 3 ‐bridged dinuclear nickel( II ) complex [LNi 2 (OH)(H 2 O)] 2+ ( 1 ) binds urea reversibly in organic solvents with the formation of an N , O ‐bridged urea anion complex [LNi 2 [OC(NH 2 )NH)] 2+ ( 2 ) and two water molecules. The equilibrium constant has been measured as 4.3(4) in acetone, 2.7(5) in acetonitrile, and 3(1) in methanol at 25 °C. Upon dissolving 1 in anhydrous methanol, the O 2 H 3 bridge is substituted for an O 2 Me 2 H bridge to give [LNi 2 (OMe)(MeOH)] 2+ ( 4 ),which has been crystallized as 4· (ClO 4 ) 2 and characterized by X‐ray diffraction. Both Ni II ions in 4 are five‐coordinate with geometries intermediate between square‐pyramidal and trigonal‐bipyramidal. The H atom in the Me 2 O 2 H bridging unit is located in an asymmetric position. The exchange of water and methanol ligands in complexes 1 and 4 is very fast in solution at 25 °C ( k obsd. > 10 3 s −1 ). Binding of urea by complexes 1 and 4 are slower reactions ( k obsd. ≈︁ 10 −1 to 10 1 s −1 under the concentration conditions used) and can be monitored by stopped‐flow techniques. Detailed kinetic studies indicate that binding of urea is a multi‐step process. Steady‐state intermediates of the tentative formula [LNi 2 (OR)(urea) n ] 2+ ( n = 1, 2) are formed in fast preequilibrium with the starting complexes 1 (R = H) and 4 (H = Me), respectively. The bidentate N , O ‐coordination and deprotonation of an O ‐bound urea ligand constitute the overall rate‐limiting step. The kinetic data suggest that both mono‐ and bis(urea) complexes participate in the formation of the chelate 2 , and that the latter are substantially more reactive. The bis(urea) pathway was unexpected because only one urea molecule is incorporated into the final product 2 . Reactive intermediates [LNi 2 (OH)(urea) n ] 2+ are close analogs of the reactive intermediate of urease, which also has the hydroxide and urea ligands bound at a dinickel core. However, the reactivities of the intermediates are different. The hydroxide ligand in our model complex acts as a base towards urea, and the urea anion complex 2 is formed. In urease, the hydroxide ligand attacks urea as a nucleophile leading to the hydrolysis of urea. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)" @default.
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• W2029041099 date "2003-04-01" @default.
• W2029041099 modified "2023-09-23" @default.
• W2029041099 title "A Study of the Equilibrium and Kinetics of Urea Binding by a Biomimetic Dinickel(<scp>II</scp>) Complex" @default.
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• W2029041099 doi "https://doi.org/10.1002/ejic.200390208" @default.
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