No abstract

The synthesis of the new alcohol-pendant macrocycle 4-(2-hydroxyethyl)-1,4,7,16,19,22-hexaaza-10,13,25,28-tetraoxacyclotriacontane (L2) is reported. This ligand contains two different triamine moieties, one of them bearing an ethanolic sidearm. L2 binds two Zn(II) ions in aqueous solution. The stability constants of the L2 complexes have been determined at 298.1 and 308.1 K by means of potentiometric measurements. Besides a [Zn2 L2]4+ species, a deprotonated [Zn2(L2-H)]3+ complex and a hydroxo [Zn2(L2-H)(OH)]2+ complex are formed in aqueous solution. Zn(II)-assisted deprotonation of the alcoholic group takes place at neutral pH, giving the [Zn2(L2-H)]3+ complex. In [Zn2(L2-H)]3+, the deprotonated R−O- function bridges the two metals, as shown by the crystal structure of [Zn2(L2-H)Br2]BPh4·MeOH. The hydroxo species [Zn2(L2-H)(OH)]2+ is formed at slightly alkaline pH's. This complex contains both a Zn(II)-bound alkoxide and a Zn(II)−OH nucleophilic function. Therefore, it may provide a simple model system for alkaline phosphatases, where both a deprotonated serine and a Zn−OH function are involved in phosphate ester hydrolysis. Indeed, this complex promotes the hydrolysis of the carboxy ester p-nitrophenyl acetate (NA) as well as the cleavage of phosphate ester bis(p-nitrophenyl) phosphate (BNP). The kinetics of promoted hydrolysis of NA and BNP were studied by means of UV and 1H and 31P NMR measurements. In NA hydrolysis, the R−O-−Zn(II) function acts as nucleophile in the first step of the hydrolytic mechanism, to give an acetyl derivative, which is subsequently hydrolyzed to acetate by a Zn−OH group. Similarly, in BNP cleavage, the nucleophilic attack of alkoxide on phosphorus gives a pendant-alcohol phosphorylated intermediate, which undergoes subsequent intramolecular nucleophilic attack of a Zn(II)-bound hydroxide to yield a phosphomonoester product.

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 K 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 L1and 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(&mgr;-PP)(2)(MeOH)(2)](ClO(4))(2) (PP(-) = diphenyl phosphate) (space group P&onemacr;, a = 10.681(5) Å, b = 12.042(1) Å, c = 13.191(3) Å, alpha = 74.63(2) degrees, beta = 71.74(3) degrees, gamma = 68.41(2) degrees, V = 1476.4(8) Å(3), Z = 1, R = 0.0472, R(w)(2) = 0.1166) strongly supports this hypothesis, since in the [Zn(2)L1(&mgr;-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.