A Study of the Equilibrium and Kinetics of Urea Binding by a Biomimetic Dinickel(II) Complex
A Study of the Equilibrium and Kinetics of Urea Binding by a Biomimetic Dinickel(II) Complex
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仿生二镍(II)络合物尿素结合的平衡和动力学研究
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发表时间：
2003
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通讯作者：
H. Pritzkow
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作者：
S. V. Kryatov;E. Rybak;F. Meyer;H. Pritzkow
本文研究了模型镍配合物与尿素结合的平衡和动力学，这与尿素酶的活性有关。吡唑基、O2 H3桥连的双核镍（II）配合物[LNi 2（OH）（H2O）]2+（1）在有机溶剂中可逆地结合尿素，形成N，O桥连的尿素阴离子配合物[LNi 2 [OC（NH 2）NH）]2+（2）和两个水分子。在25 °C下，在丙酮中测得的平衡常数为4.3（4），在乙腈中测得的平衡常数为2.7（5），在甲醇中测得的平衡常数为3（1）。将化合物1溶于无水甲醇中，O2 H3桥取代O2 Me 2 H桥，得到[LNi 2（OMe）（MeOH）]2+（4），其已结晶为4·（ClO 4）2，并通过X射线衍射表征。4中的两个NiII离子都是五配位的，其几何形状介于正方锥体和三角双锥体之间。Me 2 O2 H桥连单元中的H原子位于不对称位置。络合物1和4中的水和甲醇配体的交换在25 °C下在溶液中非常快（kobsd. > 103 s−1）。通过络合物1和4结合尿素是较慢的反应（kobsd.在所用浓度条件下为10 - 1至101 s-1），并可通过停流技术进行监测。详细的动力学研究表明，尿素的结合是一个多步骤的过程。在快速预平衡中分别与起始配合物1（R = H）和4（H = Me）形成了实验式为[LNi 2（OR）（urea）n]2+（n = 1，2）的稳态中间体。双齿N，O-配位和去质子化的O-结合的脲配体构成的整体限速步骤。动力学数据表明，单和双（脲）配合物参与形成的螯合物2，后者是实质上更活泼。双（脲）途径是出乎意料的，因为只有一个脲分子掺入最终产物2中。反应性中间体[LNi 2（OH）（脲）n]2+是脲酶反应性中间体的类似物，其也具有结合在二镍核上的氢氧根和脲配体。然而，中间体的反应性不同。在我们的模型配合物中的氢氧化物配体作为对尿素的碱，并形成尿素阴离子配合物2。在尿素酶中，氢氧化物配体作为亲核试剂攻击尿素，导致尿素水解。（© Wiley-VCH Verlag GmbH & Co. KGaA，69451魏因海姆，德国，2003）
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, O2H3-bridged dinuclear nickel(II) complex [LNi2(OH)(H2O)]2+ (1) binds urea reversibly in organic solvents with the formation of an N,O-bridged urea anion complex [LNi2[OC(NH2)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 O2H3 bridge is substituted for an O2Me2H bridge to give [LNi2(OMe)(MeOH)]2+ (4),which has been crystallized as 4·(ClO4)2 and characterized by X-ray diffraction. Both NiII ions in 4 are five-coordinate with geometries intermediate between square-pyramidal and trigonal-bipyramidal. The H atom in the Me2O2H 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 (kobsd. > 103 s−1). Binding of urea by complexes 1 and 4 are slower reactions (kobsd. ≈ 10−1 to 101 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 [LNi2(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 [LNi2(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)