Model Complexes for Ureases: A Dinickel(II) Complex with a Novel Asymmetric Ligand and Comparative Kinetic Studies on Catalytically Active Zinc, Cobalt, and Nickel Complexes

摘要

The dinuclear nickel(II) complex of the asymmetric ligand 1-[N,N-bis(2-pyridylmethyl)amino]-3-[2-(3,5-dimethyl-1H-pyrazol-1-yl)ethoxy]-2-hydroxypropane (HL1) was prepared as a model for the active site of urease. The novel complex [Ni_2(L1)(MeCOO)(ClO_4)(EtOH)_2](ClO_4). 0.5Et_2O (1) crystallizes in the triclinic space group P1 with a = 11.639(2) A, b = 12.571(3) A, c = 16.341(3) A, #alpha# = 92.29 deg, #beta# = 106.54 deg, and #gamma# = 113.73 deg. The nickel ions (c.n. 6) are bridged by the alkoxy donor substituent of the ligand and an acetate anion. The dinuclear nickel(II), cobalt(II), and zinc(II) complexes of the ligands 1-[N,N-bis(2-benzimidazolylmethyl)amino]-3-[2-(3,5-dimethyl-1 H-pyrazol-1-yl)ethoxy]-2-hydroxypropane (HL2), N-methyl-N,N',N'-tris(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane (HL3), and N,N,N,N'-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane (HL4) were investigated for their activity towards the hydrolysis of the test substrate p-nitrophenyl acetate (npa) in ethanol-waser (1:1). The second-order rate constants for the cleavage of npa were determined for all complexes. The profile of the pH dependence indicates that a hydroxide initially binds to the metal ion. The bound nucleophile subsequently attacks the test substrate. The results are discussed in terms of a refined model for the structure activity relationships of the dinuclear active site of urease.