Kinetics and Mechanism of the Nucleophilic Substitution of Tellurium(II) Dialkanethiolates, Te(SR¹)₂ with Thiols, HSR²

The equilibrium reaction between tellurium(II) dithiolates and thiols, Te(SR¹)₂ + 2 HSR² ⇌ Te(SR²)₂ + 2 HSR¹ was studied by means of ¹H- and ¹²⁵Te NMR spectroscopy and ab initio quantum chemical methods. It was found that the reaction is catalyzed by Brønsted acids and bases, the catalytic activity corresponding to the strength of the respective acid or base. Investigation of the initial step of the reaction, Te(SR¹)₂ + HSR² ⇌ Te(SR¹)(SR²) + HSR¹, showed it to proceed according to first order kinetics for Te(SR¹)₂, HSR² and for the catalyst. Ab initio geometry optimizations and frequency calculations suggest [Te(SR¹)(HSR¹)(HSR²)]⁺ and [Te(SR¹)₂(SR²)]⁻ to be stable intermediates and not transition states in the acid and base catalyzed reactions, respectively. The reaction hence proceeds via an additional elimination rather than an S _N 2 mechanism. The catalytic activity displayed by acids and bases can be applied to reduce the temperature in synthesis of thermally labile tellurium(II) dithiolates.

Keywords:

• Ab initio
• tellurium
• ¹²⁵Te NMR
• thiolate

Notes

^a Substochiometric amounts of NMI or pyridine were used to catalyze the exchange reactions.

^b Initial concentrations, c ₀ are molalities, given in mol kg⁻¹.

^{a a} B = pyridine.

^b B = imidazole (imidazol (pK_b = 6.97) was used as model compound for N-methylimidazol.

^c HA = CH₃COOH

^d HA = CF₃COOH.