Previously reported mono- and dinuclear Zn(II), Cu(II), and Ni(II) complexes of 1,4,7,10-tetrazacyclododecane (12aneN4 or cyclen) with different heterocyclic spacers (triazine, pyridine, 2,2'-bipyridine), optionally contg. pendant azacrown ether moiety, exhibit catalytic activity in hydrolysis of bis(4-nitrophenyl) phosphate (BNPP) in physiol. conditions (pH 7-9, 25°). All Zn(II) complexes promote the hydrolysis of BNPP under physiol. conditions, while those of Cu(II) and Ni(II) do not have a significant effect on the hydrolysis reaction. The hydrolysis rate consts. in buffered solns. (0.05 M Bis/Tris, TRIS, HEPES, or CHES, I = 0.1 M, NaCl) at 25° were detd. by the initial slope method at product conversion less than 5\%. Comparison of the second-order pH-independent rate consts. (kBNPP, M-1 s-1) for the zinc 12aneN4 mononuclear complexes indicate that the pendant heterocyclic moiety, 4,6-dimethoxy-1,3,5-triazin-2-yl, 2-pyridinyl, 4-azacrown-6-methoxy-1,3,5-triazin-2-yl, improves the rate of hydrolysis up to six times over the parent Zn(12aneN4) complex (kBNPP = 1.1 $\times$ 10-5 M-1 s-1). The reactive species is the Zn(II)-OH- complex, in which the Zn(II)-bound OH- acts as a nucleophile. For zinc dinuclear complexes, in which the 12aneN4 moieties are bridged by 1,3,5-triazin-2,4-diyl or 2,2'-bipyridine-6,6'-diyl spacers, the rate of reaction is defined by the degree of cooperation between the metal centers, which is detd. by the spacer length. The complexes possessing shorter 1,3,5-triazin-2,4-diyl spacers are able to hydrolyze BNPP 1 to 2 orders of magnitudes faster than 2,2'-bipyridine-6,6'-diyl-bridged complex. The high BNPP hydrolytic activity may be related to p-stacking and hydrophobic interactions between the arom. spacer moieties and the substrate. The tested complexes show hydrolytic activity at pH 7 and 8, which allows for the hydrolysis of activated phosphate esters under physiol. conditions. on SciFinder(R)
