Nickel complexes of carboxylate-containing polydentate ligands as models for the active site of urease

Two new carboxylate-containing polydentate ligands have been synthesized, the symmetric ligand 2,6-bis[N-(N(carboxyl methyl)-N-((1-methylimidazol) methyl)amine)methyl]-4-methylphenolate (BCIMP) and the corresponding asymmetric ligand 2-(N-isopropyl-N-((1-aminomethyl)-4-methylphenol (ICIMP). The ligands have been used to prepare model complexes for the active site of the dinuclear nickel enzyme urease, viz. [Ni-2(BCIMP)Ac-2](-) (6), [Ni-2(BCIMP)(Ph2Ac)(2)](-) (7), [Ni-2(ICIMP)(Ph2Ac)(2)] (14), [Ni-4(ICIMP)(2)(Ph2Ac)(2)][ClO4](2) (15), [Ni-4(ICIMP)(2)(Ph2Ac)(2)(DMF)(2)][ClO4](2) (16), and [Ni-4(ICIMP)(2)(Ph2Ac)(2)(urea)(H2O)][ClO4](2) (17), where the latter complex contains urea coordinated in a unidentate fashion through the carbonyl oxygen. The N2O-N2O2 donor set of ICIMP provides a good framework for the preparation of urease models, but in some cases tetranuclear nickel complexes are formed due to coordination of the carboxylate moiety of one dinickel-ICIMP unit to one or both of the nickels of a second Ni-2 unit. Reactivity and kinetics studies of 7 and 15 show that these model complexes catalyze hydrolysis of 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) at basic pH. In this assay, complexes based on the asymmetric ligand ICIMP exhibit a significantly faster rate of hydrolysis than the corresponding BCIMP complexes. Magnetic measurements indicate that there are weak antiferromagnetic interactions between the nickel ions in complex 16.