Carboxy and Diphosphate Ester Hydrolysis by a Dizinc Complex with a New Alcohol-Pendant Macrocycle

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 [Zn2L2]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.