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-pyri dylmethyl) amino]-3-[2-(3,5-dimethyl-1H-pyrazol-1-yl) ethoxy]-2-hydroxyprop ane (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 i n the triclinic space group <P(1)over bar> with a = 11.639(2) A, b = 12.571 (3) Angstrom, c = 16.341(3) A, alpha = 92.29 degrees, beta = 106.54 degrees , and gamma = 113.73 degrees. The nickel ions (c.n.6) are bridged by the al koxy donor substituent of the ligand and an acetate anion. The dinuclear ni ckel(II), cobalt(II), and zinc(II) complexes of the ligands 1-[N,N-bis(2-be nzimidazolylmethyl)amino]-3-[2-(3,5-dimethyl-1H-pyrazol-1-yl)ethoxy]-2-hydr oxypropane (HL2), N-methyl-N,N',N'-tris(2-benzimidazolylmethyl)-2-hydroxy-1 ,3-diaminopropane (HL3), and N,N,N',N'-tetrakis(2-benzimidazolylmethyl)-2-h ydroxy-1,3-diaminopropane (HL4) were investigated for their activity toward s the hydrolysis of the test substrate p-nitrophenyl acetate (npa) in ethan ol-water (1:1). The second-order rate constants for the cleavage of npa wer e determined for all complexes. The profile of the pH dependence indicates that a hydroxide initially binds to the metal ion. The bound nucleophile su bsequently attacks the test substrate. The results are discussed in terms o f a refined model for the structure activity relationships of the dinuclear active site of urease.