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

The dinuclear nickel(II) complex of the asymmetric ligand 1-[N,N-bis(2-pyridylmethyl)amino]-3-[2-(3,5dimethyl-1H-pyrazol-1-yl)ethoxy]-2-hydroxypropane (HL1) was prepared as a model for the active site of urease. The novel complex [Ni2(L1)(MeCOO)(ClO4)(EtOH)2](ClO4) · 0.5 Et2O (1) crystallizes in the triclinic space group P 1 with a = 11.639(2) Å, b = 12.571(3) Å, c = 16.341(3) Å, α = 92.29°, β = 106.54°, and γ = 113.73°. 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-water (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.