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Highly substituted pyrazolylborate ligands TpRrMe were used to limit the coordination number of zinc towards cysteineand histidine-derived coligands. Monodentate thiolate attachment (-+ la-f) was achieved with N-and C-protected cysteine, monodentate carboxylate attachment (-+ 3) with Nprotected histidine. C-protected cysteine was found to form a five-membered N,S-chelate ring (2). While imidazole coordination with N-and C-protected histidine could not be achieved, cationic pyrazolylborate-zinc-coligand complexes [TpZn-L]+ (4a, b) were obtained for L = 2-methylimidazole. The new complexes were characterized by their spectra and three structure determinations.Zinc exercises its biological functions in a chemical environment consisting mainly of the thiolate groups of cysteine and the imidazole groups of histidine in the protein [2]. Accordingly, the investigation of its coordination chemistry with these two amino acids as well as simple derivatives and small peptides thereof is an important aspect of the bioinorganic chemistry of z i n~ [~,~] .We are contributing to this by the determination of stabilities and structures of cysteine-and histidine-containing zinc complexe~[~~.It is a general problem of model studies of this kind that the two amino acids are polyfunctional, even in the form of their derivatives. Therefore, in contrast to the situation in a protein, they normally occupy more than one coordination position on zinc, which often results in the formation of coordination polymers. This problem can be avoided in model compounds only in a signular case, namely by providing the most advantageous electronic situation in the form of i1 ZnN2S2 coordination. The problem does not exist in the case of the protein because the functionality of cysteine and hstidine is reduced to that of their side chain donors thiolate and imidazole, and because a favorable geometrical situation is preformed. This paper reports on our attempts to imitate the natural case by controlling the geometrical situation and by reducing the donor abilities of cysteine and histidine derivatives to monodentate coordination. Our approach was based on the use of highly substituted pyrazolylborate ligands. The target compounds were the complexes of type A in which the substituents R at the 3-positions of the pyrazole units encapsulate the zinc ion and its coligands. The coligands X were meant to be cysleine and histidine derivatives bound in a monodentate fashion via their thiolate or imidazolr: functions. The neutral complexes with X = OH were used as starting compounds, based on our experien~e[~,~I that the OH-group is the most efficient leaving group in this class of complexes.

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The Tp ligand tris(3-cumenyl-5-methylpyrazolyl)borate was found to stabilize zinc complexes of nucleobases, of their natural precursors, and of nucleoside and nucleotide derivatives. Dihydroorotic acid and orotic acid are bound as monodentate carboxylate ligands. Uracil is coordinated via its deprotonated N1; 6-methylthiouracil acts as a bidentate ligand via N1 and S. Analogously, xanthine is a monodentate ligand bound by its deprotonated N7, while 6-mercaptopurine seems to bind in a bidentate fashion via N7 and S. Spectroscopic evidence indicates coordination of uridine and 2',3'-O-isopropylideneuridine via their deprotonated N3, as well as of xanthosine via N7. The hydrolytic cleavage of 2',3'-O-isopropylideneuridine 5'-(bis(p-nitrophenyl) phosphate) by TpZn-OH is preceded by an attachment of one TpZn unit to the deprotonated uracil base, presumably via N3.

The zinc hydroxide complexes Tp*Zn-OH of highly substi-ents, could be incorporated. Thus, the arylmethoxides tuted pyrazolylborate ligands react with phenols, and alco-OCH2C6F, and OCJ-I,C6H4N0,-p, as well as the alkoxides hols, of sufficient acidity, in a condensation reaction with re-OCH2CF3 and OCH2CC13, were attached.

Three (pyrazolylborate)zinc hydroxide complexes Tp*Zn−OH were used as hydrolytic reagents to cleave the P−O−P, P−O−S, and S−O−S linkages of organic diphosphates, sulfonatophosphates, and disulfonates. The resulting complexes of the types Tp*Zn−OPO(OR)2 and Tp*Zn−OSO2R could also be obtained by condensation reactions between Tp*Zn−OH and HO−PO(OR)2 or HO−SO2R. Two of them were characterized by structure determinations.

Five different (pyrazolylborate)zinc hydroxide complexes Tp*Zn−OH (1) were used as hydrolytic reagents towards esters of various acids of phosphorus. Trimethyl phosphate and trimethyl phosphite could not be cleaved. Dimethyl and diphenyl phosphite yielded TptBu,MeZn−OPHO(OR) (2, 3). Triphenyl phosphate reacted slowly producing moderate yields of Tp*Zn−OPO(OPh)2 (4). Tris(p‐nitrophenyl) phosphate was cleaved rapidly, forming Tp*Zn−OPO(OC6H4NO2)2 (5) and Tp*Zn−OC6H4NO2 (6). Alkylbis(p‐nitrophenyl) phosphates showed intermediate reactivity, losing p‐nitrophenolate upon hydrolysis and producing Tp*Zn−OPO(OR)(OC6H4NO2) (7, 8). When phosphorus acid diesters were employed, condensation between the Zn−OH and P−OH functions occurred. This proved to be the convenient way of preparing the organophosphate complexes Tp*Zn−OPO(Ph)2 (9), Tp*Zn−OPO(OPh)2 (4), and Tp*Zn−OPO(OC6H4NO2)2 (5). Six structure determinations showed the structural variability of the resulting complexes.