Pyrazolylborate ligand / Zinc hydroxide complex / Enzyme model
An L3ZnOH Complex as a Functional Model of the Enzyme Carbonic Anhydrase
The tridentate ligand hydrotris(3-tert-butyl-5-methylpyrazo-1yl)borate (L3) reacts with zinc perchlorate hexahydrate to form L3ZnOH (1) which is the first mononuclear neutral zinc hydroxide complex. 1 is a structural model of the enzyme carbonic anhydrase due to its tetrahedral coordination with three N-heterocycles and one OH ion as ligands. Its functional analogy to the enzyme involves (a) reversible uptake of COz forming the unstable bicarbonate complex L3ZnOCOOH (4) in solution from which the dinuclear carbonate complex L3ZnOC(0)OZnL3 (3) crystallizes, (b) reaction with dialkyl pyrocarbonates forming the complexes L3ZnOC(0)OR (5) which are stable esters of the bicarbonate complex and which are easily hydrolyzed to ROH, COz, and L3ZnOH, (c) thermal decarboxylation of the ethyl carbonate complex 5b resulting in the ethanolate complex L3ZnOEt (6) which is extremely sensitive to hydrolysis, (d) "inhibition" by small anions due to conversion of 1 into the complexes L3ZnX (?: X = C1, CN, N3, OAc). The identity of the various L3Zn derivatives is established by NMR methods and structure determinations.