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An L3ZnOH Complex as a Functional Model of the Enzyme Carbonic AnhydraseThe 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. Carboanhydrase ist eines der am besten untersuchten Enzyme[']. Ihre bemerkenswert einfache Funktion besteht in der Katalyse der hydrolytischen Umwandlung zwischen Kohlendioxid und Hydrogencarbonat, die normalerweise langsam, da kinetisch gehemmt, ist. An vielen Stellen im Korper, so auch in den roten Blutkorperchen als das biologische Gegenstuck des Hamoglobins, sorgt sie damit fur den raschen Umsatz bzw. die rasche Entfernung des korpereigenen Verbrennungsprodukts C02. Ihr katalytisch aktives Zentrum besteht aus einem Zink-Ion, das tetraedrisch von drei Histidin-Imidazol-Gruppen und einem Wassermolekul umgeben ist. Als Teilschritte des Katalysecyclus werden die Deprotonierung des H20-Liganden und der Angriff des starken Nucleophils [His3ZnOH] + an C 0 2 unter Bildung eines Hydrogencarbonat-Liganden diskutiert, der dann unter Substitution durch ein neues Wassermolekiil abgelost wird[',21. Da jedoch bis jetzt weder der Enzym-Substrat-(C02)-Komplex noch andere Intermediate des katalytischen Cyclus durch Strukturanalysen identifiziert sind, beruhen die Vorschlage zu seinem Verlauf bisher nur auf kinetischen, spektroskopischen und theoretischen StudienL3]. Ansatze zur strukturellen Modellierung des Enzyms konnten in der Vergangenheit nicht die tetraedrische N3-ZnOH2-Koordination reprod~zieren[~], und Studien zur funktionellen Modellierung rnit anderen Zink-Komplexen blieben zumeist auf Ester-Hydrolysen beschrankt [' I. Die beste Annaherung an den Zustand im Enzym gelang bisher Kimura et al. rnit dem Komplex [(1,5,9-Triazacyclododecan)ZnOH2I2+, der nahe dem Neutralpunkt deprotoniert wird und die CO,-Hydratisierung katalysiert [61. Einen ein deutig charakterisierten einkernigen tetraedrischen L3ZnOH-Komplex gab es zu Beginn unserer Untersuchungen nicht, obgleich basische Salze eine normale Erscheinung in der anorganischen Chemie von Zink sind"] und obgle...

Monofunctional Tetrahedral Zinc Complexes L3ZnX [L3 = Tris(pyrazolyl)borate]The pyrazolyl borates K [HBpzz] (abbreviated KL3, pz* = 3-phenyl-, 3-tolyl-, 3-anisyl-, 3,5-diphenyl pyrazolyl) react with zinc salts ZnX, (X = C1, Br, I, NO3) to form the mononuclear neutral tetrahedral zinc complexes L3ZnX (1 -4). The corresponding alkylzinc complexes L3ZnR (5-8; R = Me, Et, tBu, Ph) result either from L3ZnC1 and LiR or from ZnEt, and KL3. These alkylzinc compounds are remarkably stable towards oxidation and hydrolysis. For specific cases the reaction of L3ZnR with carboxylic acids is found to yield the carboxylates L3ZnOCOR (9, lo), and likewise with thioacetic acid to yield the thioacetates L3ZnSCOMe (11). Facile cleavage of L3ZnR with thiols and selenophenol produces the thiolates L3ZnSR (12, 13) and the selenolates L'ZnSePh (14). The complexes L3ZntBu which are the most reactive of the alkylzinc compounds are cleaved by H20, H2S, NH3 and various OH and NH compounds with formation of the bis(1igand) complexes Zn(L3), (15). Crystal structure determinations of one L3ZnX complex each for X = NO3 (4a), CH3 (5a), SCOMe ( l l a ) , and SEt (12a) confirm the nature of the compounds and the relation of 4a and 12a to the active centers of zinc-containing enzymes.

The pH-dependent photophysical properties of a series of polypyridyl ruthenium-substituted amino acids were investigated by steady-state and time-resolved luminescence spectroscopy. [H(3)N-DAPA(Rub(2)m)-OH](PF(6))(3) (1), [H(3)N-DABA(Rub(2)m)-OH](PF(6))(3) (2), [H(3)N-Orn(Rub(2)m)-OH](PF(6))(3) (3), and [H(3)N-Lys(Rub(2)m)-OH](PF(6))(3) (4) were obtained by formation of an amide link between the omega-NH(2) group of the respective commercially available amino acid and [Rub(2)(m-OH)](2+) (b = bipyridine, m-OH = 4'-methyl-2,2'-bipyridine-4-carboxylic acid). Due to the absence of significant electronic interactions between the ruthenium chromophore and the amino acid moieties, the energetics and extinction coefficients of the absorption spectra of 1-4 do not change as a function of pH. The luminescence intensities of these complexes, however, show a marked dependence on pH. At low pH (<2), quenching via excited-state protonation of the amide link leads to short lifetimes. In the pH 2-8 range, the lifetimes depend on the amino acid side chain length of the complex. At high pH (>9), lifetimes are approaching that of [Ru(bpy)(3)](2+), suggesting that the amino acid moiety has a negligible effect on nonadiabatic pathways in the excited-state decay of the ruthenium moiety. Our results are discussed with respect to the rapidly growing interest in ruthenium-substituted amino acids as spectroscopic and mechanistic tools in biological systems.

90 3C/0.001 Torr) 23 b affords the products 25a-c in almost statistical I : 2: 1 distribution (67 % yield). If the distillation is conducted at 180 "C/26 mbar, the thermodynamically favored Z,Z isomer 25a predominates (56 % yield of 2 5 8 : 1 : 1 distribution). Apparently at least the second rearrangement step, 24 + 25, is reversible at higher temperatures.As anticipated, on treatment with tetracyanoethylene (TCNE) 25a reacts considerably faster to 26a than its stereoisomer 25b to the cycloadduct 26b.Isothiocyanate-substituted 1,3-butadienes can be used as well as the allenes 4 for the synthesis of thiazoles, but also for the preparation of other heterocycles.