Michael Rombach

Coordination modes of aminoacids to zinc

Two new binary zinc aminoacid complexes were crystallized: Zn(Ile) 2 (H 2 O) 2 is octahedral with two O,N-chelating aminoacid ligands; Zn(Phe) 2 is a one-dimensional coordination polymer with η 2 -carboxylate bridges. The pyrazolylborate zinc complex Tp P h , M e Zn-OH reacted with simple aminoacids to form the complexes Tp P h , M e Zn-AA (AA = Gly, Val, Leu, Met. Phe, Trp) in which AA is a O.N-chelate ligand. In a similar fashion aminoacid-nitroanilides were deprotonated at the N atom yielding the Tp P h , M e Zn complexes with the N.N-chelating ligands Gly-Nit. Ala-Nit and Leu-Nit. When terminal amine functions were missing, pure carboxylate coordination resulted, as in Tp P h , M e Zn-GlyPheBoc and Tp P h , M e Zn-AspPheOMe, Exclusive side-chain coordination was enforced in Bz-Tyr-OEt resulting in the Tp P h , M e Zn complex with the phenolate-bound aminoacid. Combinations of the binding types (O,N-chelate + carboxylate, O,N-chelate + phenolate, O,N-chelate - thiolate) were realized in the binuclear complexes Tp P h , M e -Zn-AA-ZnTp P h , M e for AA = Asp, Glu. Tyr and Cys.

The Insertion of Heterocumulenes into Zn−H and Zn−OH Bonds of Pyrazolylborate−Zinc Complexes

The pyrazolylborate−zinc complexes TpPh,MeZn−H and TpPh,MeZn−OH undergo insertion reactions with heterocumulenes. CO2, CS2 and RNCS insert into the Zn−H function to yield the corresponding Zn−OCH(O), Zn−SCH(S) and Zn−SCH(NR) complexes. The primary products resulting from the insertion of CS2, COS and RNCS into the Zn−OH function undergo rearrangements and subsequent reactions. TpPh,MeZn−OH and CS2 or COS yield TpPh,MeZn−SH and COS or CO2, whereas with RNCS the complexes TpPh,MeZn−SC(O)NHR are formed. In the presence of alcohols these are incorporated in the Zn−OH insertion products: CS2/methanol yields TpPh,MeZn−SC(S)OMe and RNCS/ethanol yields TpPh,MeZn−SC(NR)OEt. All product types were characterized by X-ray crystallography. The reactions and structures support the four-centre reaction mechanism proposed for hydrolytic zinc enzymes.

Evidence for a Trajectory of Hydrolytic Reactions brought about by [L3Zn−OH] Species

A proposal for a detailed geometrical pathway for the hydrolysis of substrates Y2PO−X and RCO−X, under the influence of zinc complexes or hydrolytic zinc enzymes, has been yielded by the combination of a kinetic study and a structure correlation analysis (see diagram). The essentials of this pathway are a four-center interaction between the Zn−OH and P−X or C−X units in the activation process and a Berry pseudorotation at zinc during the bond-breaking/bond-making process.

Pyrazolylborate)zinc Organophosphate Complexes Resulting from Hydrolytic Cleavage of Phosphate Esters

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.

Methylation of zinc bound thiolates; a model for cobalamine independent methionine synthase

Pyrazolylborate–zinc–thiolate complexes react under mild conditions with methyl iodide, dimethylsulfate and trimethylsulfonium iodide, liberating the corresponding methyl thioethers; the driving force for these reactions lies in the high nucleophilicity of the zinc-bound thiolates and the low donor quality of thioethers toward zinc.

Synthesis, optical nonlinearity and crystal structure of a novel mixed-chalcogen, thiolato-bridged complex [(η5-C5H5)2Mo2(μ-S)(μ-Te)(μ-SPh)2]

Abstract A novel dimolybdenum complex in which the metal atoms are bridged by two different chalcogens and two thiolato ligands has been synthesised and structurally characterised. This complex shows a large third-order optical nonlinearity as measured by degenerate four-wave mixing with picosecond excitation at 532 nm. A hyperpolarisability (γ) of 1.2×10−30 esu has been obtained. The structure of the complex was established by the single-crystal X-ray diffraction method. It consists of a Mo–Mo unit, which is bridged by one S, one Te and two (SPh) groups. Each Mo atom also has a (η5-C5H5) group attached to it.

The TpZn–OH/CS2 reaction: theoretical and preparative visualization of an essential bioinorganic reaction path

The reaction between L3Zn–OH complexes (L = N3 based ligand) and CS2 which is an analogue of the carbonic anhydrase functionality was computed at the B3LYP/6-311 + G* level to proceed via stable four-center intermediates [L3Zn–SC(S)OH or L3Zn–SC(O)SH] to give L3Zn–SH and COS; in agreement with these calculations, the chemical reaction of TpPh,MeZn–OH with CS2 resulted in the quantitative formation of TpPh,MeZn–SH and COS; in the presence of 1 equivalent of MeOH the reaction yielded TpPh,MeZn–SC(S)OMe, thus also supporting the existence of the four-center intermediate L3Zn–SC(S)OH.