Pyrazolylborate-zinc-hydrosulfide complexes and their reactions

Four new hydrosulfide complexes Tp*Zn-SH of substituted pyrazolylborate ligands (Tp*) were prepared by reactions of Tp*Zn-OH with H2S, and three of them were structurally characterized. Unlike the Tp*Zn OH complexes they do not react with esters, phosphates, or CO2, e.g. for thiolytic cleavage reactions. They also cannot be deprotonated, as various bases induce precipitation of ZnS and release of anionic Tp*. Acidic organic X-OH compounds (carboxylic acids, trinitrophenol, hexafluoroacetylacetone) replace the SH groups with formation of Tp*Zn-OX. Thiols undergo an entropy-driven SH substitution to yield the Tp*Zn-SR complexes. Like the Tp*Zn-SR complexes the Tp*Zn-SH complexes are quite reactive toward alkylation with methyl iodide, yielding Tp*Zn-I and CH3SH. The kinetic investigation of the methylation of Tp(Ph),Zn-Me-SH has shown it to be a clean second-order reaction, thereby indicating that the SH group is alkylated in the zinc-bound state.