Zn−O2H3−Zn: a Coordination Mode of the Hydrolytic Zinc−Aqua Function and a Possible Structural Motif for Oligozinc Enzymes

Reactions of the pyrazolylborates KTp‘ with Zn(ClO 4 ) 2 and KOH in a 2:2:1 ratio yield the complexes [Tp‘ 2 Zn 2 O 2 H 3 ]ClO 4 ( 1 ) for Tp‘ = hydrotris(3-R-5-methylpyrazolyl)borate with R = p -isopropylphenyl ( a ), 3-pyridyl ( b ), and 3-(6-methyl)pyridyl ( c ). Their structure determinations have revealed a Zn-OH-H···OH-Zn array (i.e., a bridging H 3 O 2 - ligand) in the dinuclear complex cations. The zinc ions in complex 1a are in a distorted tetrahedral ZnN 3 O environment, and those in 1b and 1c are in a distorted trigonal bipyramidal ZnN 4 O environment due to additional coordination of one pyridyl nitrogen from the opposing Tp‘ ligand. In 1b, one Zn-OH function is involved in a hydrogen bridge to a cocrystallized methanol molecule. The complexes represent a mode of stabilization for the zinc−aqua function in the cationic Tp‘Zn-OH 2 species, and they are intermediates in the formation of the stable Tp‘Zn-OH complexes. It is suggested that the Zn-O 2 H 3 -Zn unit is present as a structural and possibly functional motif in oligozinc enzymes like phospholipase C or P1 nuclease.