Novel thiones-containing borate hybrids: synthesis, characterization, and alkylation reactivity of their zinc(II) complexes

Abstract

Three new sulfur-rich “scorpionate” ligands containing thione groups have been prepared: potassium hydrobis(pyrazolyl)-N-(2,6-dimethoxyphenyl)-2-thioimidazolylborate (KN2S), potassium hydro{bis-[N-(2,6-dimethoxyphenyl)-2-thioimidazol-1-yl]-[5-phenyl-3-methyl-pyrazol-1-yl]}borate (KNS2), and potassium hydrotris[N-(2,6-dimethoxyphenyl)-2-thioimidazolyl]borate (KS3). These ligands offer progressively higher sulfur content (N2S, NS2, S3 donor sets), which is known to strongly influence zinc–thiolate reactivity in biomimetic systems. Each ligand has been fully characterized by elemental analysis, Fourier-transform infrared (FT-IR), and 1H-13C NMR spectroscopes. To probe their functional behavior, the alkylation reactivity of zinc(II)-benzylthiolato complex S3Zn–SBz as a representative example was evaluated using methyl iodide. Methylation initiated an intramolecular nucleophilic attack by the coordinated thiolate on the methyl group, followed by iodide binding at zinc, cleanly converting S3Zn–SBz into the corresponding zinc(II) iodide complex, S3Zn–I, and the benzyl methyl. Thioether kinetic measurements show that this transformation follows a second-order rate law – first order in zinc complex and first order in methyl iodide – with a rate constant of 5.9 × 10−2 M−1 s−1. This value is in the range reported for intramolecular SN2-type alkylation at zinc-bound thiolates in related tripod systems, underscoring the strong activating effect of an S3 donor environment on sulfur nucleophilicity.