A series of new pyrazolylborate-zinc-thiolate complexes Tp(Ph,Me)Zn-SR and Tp(Me,Me)Zn-SR, including two homocysteine derivatives, were prepared and structurally characterized. Their reactions with methyl iodide in nonpolar media resulted in the formation of the thioethers MeSR, including two methionine derivatives, and Tp(R',Me)Zn-I in all cases. Methylation of the thiolates could also be achieved with dimethyl sulfate and trimethylsulfonium iodide but not with trimethyl phosphate or N-methylpyridinium salts. The accumulated evidence indicates that the methylation occurs intramolecularly, i.e., at the zinc-bound thiolates: (i) The reactions occur readily in nonpolar media. (ii) Thiolate exchange at Tp(Ph,Me)Zn-SR with [PPN]SR' is slower than thiolate alkylation. (iii) The methylation of Tp(Ph,Me)Zn-SBn with MeI is a clean second-order reaction with k'' = 1.75 x 10(-2) M(-1) s(-1) at 300 K.

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.

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The reaction of [PPh4]3[Re7C(CO)21] (1) with 1 or more equiv of Hg(OAc)2 in dichloromethane provides the monomercury derivative [PPh4]2[Re7C(CO)21HgOAc] (2) in high yield. However, in the presence of methanol the reaction of 1 with 2 equiv of Hg(OAc)2 yields the dimercury hexarhenium cluster compound [PPh4]2[Re6C(CO)18(HgOAc)2] (3) together with the dirhenium complex [PPh4][Re2(CO)6(mu-OMe)2(mu-OAc)] (4). The dimercury compound 3 reacts with various thiols HS-Z to form thiolate-substituted derivatives [PPh4]2[Re6C(CO)18(HgSZ)2] [Z = C6H4Br (5); C5H4N (6); C2H4COOH (7)]. All new compounds have been characterized by a combination of analytical and spectroscopic data, and the molecular structures of compounds 3-6 have been determined by X-ray crystallography.

The zinc complex chemistry of the tridentate thiol ligand N-(2-mercaptophenyl)(2-picolyl)amine (MPPAH) differs considerably from that of the aliphatic analogue N-(2-mercaptoethyl)(2-picolyl)amine reported previously. This is due to the lower basicity and hence bridging tendency of its aromatic thiolate function. With zinc salts of 0x0 anions the 1:l complexes (MPPA)ZnX (la-c: X = ONO,, OAc, OBz) are formed which according to spectroscopic data are monomeric while the halide (MPPA)ZnBr (2) seems to be polymeric. In the absence of coordinating anions the monomeric 2 : l complex (MPPA),Zn (3) results which according to a crystal structure determination contains tetrahedral zinc coordinated by one MPPA ligand in a tridentate fashion while the other is only monodentate and sulfur-bound. A similar NzS2 coordination can be deduced for the 1:l:l complex (MPPA)Zn(STrt) (4) resulting from ZnEt,, MPPAH and triphenylmethanethiol.The purpose of our investigations into the zinc complex chemistry of chelating ligands with N and S donors is twofold. Firstly we want to gain an understanding of the factors controlling their coordinative behaviour (stoichiometries, coordination numbers, thiolate bridging, stabilities, etc.). Secondly, we want to learn how to construct structural and possibly functional models of enzymes containing zinc in a ZnN,S, environment like liver alcohol dehydrogenase"], spinach carbonic anhydraseI21, or bovine aminolaevulinate dehydrataset31. In order to do t h s we screen known or newly synthesized polydentate amine-thiol ligands for their ligation behavior toward various zinc salts.In the previous paper of this seriesL41 we outlined our main objectivcs and the synthetic approach used to obtain zinc complexes in which the metal is placed into the right steric and electronic environment of N and S donors, the desirable ligand combinations being ZnN2SX, ZnN3SX, or ZnNS2X. In these combinations the N and S donors are meant to be provided by one chelating ligand while the group X represents a labile coligand or the substrate of a possible enzyme model. While many chelating ligands exist with the appropriate number of N and S donor functions it must be said that their use for the modelling of the abovementioned enzymes is still an open challenge, despite some impressive results obtained with alcohol dehydrogenase models r5 3 6].This paper reports on our investigations with another ligand suitable for the ZnN2SX combination. This ligand, N-(2-mercaptophenyl)(2-picolyl)amine (MPPAH) is the aromatic analogue of the aliphatic thiol N-(2-mercaptoethyl)(2-picoly1)amine (MEPAHjr4'. We foundL41 that MEPAH yields a puzzling variety of zinc complexes, a dominating feature of which is the tendency of thiolate bridging and hence oligomerization. We hoped that the more rigid nature of the aromatic NCCS unit in MPPAH and the lower basicity of its thiolate function would help to suppress this unfavorable property.

The hydroxo complex [Re7C(CO)21HgOH]2- reacts at room temperature with H2S to give the hydrosulfide complex [Re7C(CO)21HgSH]2- and with various thiols HS-Z to give the derivatives [Re7C(CO)21HgS-Z]2-, where Z contains a free amine or carboxyl group.

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