CARBOXY AND PHOSPHATE ESTERS CLEAVAGE WITH MONO- AND DINUCLEAR ZINC(II) MACROCYCLIC COMPLEXES IN AQUEOUS SOLUTION, CRYSTAL STRUCTURE OF [ZN(2)L1(MU-PP)(2)(MEOH)(2)](CLO4)(2) (L1=[30]ANEN(6)O(4), PP- EQUALS DIPHENYL PHOSPHATE)

摘要

The ligand [30]aneN(6)O(4) (L1) binds two Zn(II) in aqueous solution. The stability constants of the L1 complexes have been determined at 308.1 K by means of potentiometric measurements. Dinuclear monohydroxo [Zn(2)L1OH](3+) and dihydroxo [Zn(2)L1(OH)(2)](2+) complexes are formed in aqueous solution from neutral to alkaline pH. The kinetics of promoted hydrolysis of p-nitrophenyl acetate (NA) was studied. Both hydroxo species promote p-nitrophenyl acetate (NA) hydrolysis at 298.1 M with second-order kinetics. The activity of these species in NA hydrolysis is similar to that found for the mononuclear L2-Zn-OH+ complex (L2 = [15]aneN(3)O(2)), indicating that the hydrolytic process takes place via a simple bimolecular mechanism. The hydrolysis rate of bis(p-nitrophenyl) phosphate (BNP) was measured in aqueous solution at 308.1 K in the presence of the LB and L2 zinc complexes, The hydrolysis rate of BNP is increased almost 10-fold by the dinuclear [Zn(2)L1(OH)(2)](2+) complex with respect to the mononuclear L2-Zn-OH+ one. This result indicates a cooperative role of the two metals in the hydrolytic mechanism. A bridging coordination of the phosphate ester to the two Zn(II) ions can be suggested. The crystal structure of [Zn(2)L1(mu-PP)(2)(MeOH)(2)](ClO4)(2) (PP- = diphenyl phosphate) (space group , a = 10.681(5) Angstrom, b = 12.042(1) Angstrom, c = 13.191(3) Angstrom, alpha = 74.63(2)degrees, beta = 71.74(3)degrees, gamma = 68.41(2)degrees, V = 1476.4(8) Angstrom(3), Z = 1, R = 0.0472, R-w(2) = 0.1166) strongly supports this hypothesis, since in the [Zn(2)L1(mu-PP)(2)-(MeOH)(2)](2+) cation the diphosphate anions bridge the two metals. The dinuclear Zn(II) complexes of L1 provide a simple model system for hydrolytic dizinc enzymes. [References: 54]