Base Hydrolysis of Coordinated Thiophene in (C₅R₅)Rh(C₄Me₄S)²⁺: Nucleophilic Attack at Divalent Sulfur and the Reversible Cleavage of C-S Bonds

This report describes studies on the base hydrolysis of [(C₅Me₅)Rh(C₄Me₄S)]²⁺ (1) which gives an acyl thiolate (C₅Me₅)Rh(η⁴-SC₃Me₃C(O)Me) (2) concomitant with cleavage of one C-S bond (C₄Me₄S = 2,3,4,5-tetramethylthiophene). The C-S cleavage event involves deprotonation of [(C₅Me₅)Rh(C₄Me₄S-2-OH)]⁺ (3). This cation was prepared by protonation of 2 using HOTf or NH₄PF₆. The conversion of 2 to 3 is proposed to involve protonation of the carbonyl oxygen followed by intramolecular alkylation of the thiolate ligand. This alkylation was modeled by the addition of MeOTf to 2 to give [(C₅Me₅)Rh(MeSC₃Me₃C(O)Me)]OTf (4). Single-crystal X-ray diffraction studies show that both 3 and 4 can be described as η⁴-thioether allyl complexes of Rh^III. Earlier steps in the base hydrolysis reaction were uncovered following the discovery that addition of excess OH⁻ to the tetramethylcyclopentadienyl complex [(C₅Me₄H)Rh(C₄Me₄S)]²⁺ gave the S-oxide (C₅Me₄H)Rh(C₄Me₄S-1-O), not the acyl thiolate. This S-oxide species reacts with NH₄PF₆ to give [(C₅Me₄H)Rh(C₄Me₄S-2-OH)]OTf, an analog of 3, which in turn deprotonates to give the acyl thiolate (C₅Me₄H)Rh(SC₃Me₃C(O)Me), an analog of 2. This conversion establishes that the S-oxide is a kinetic product, less stable thermodynamically than the acyl thiolate isomer. Thus the S-oxide (C₅Me₅)Rh(η⁴-C₄Me₄S-1-O) reacts with NH₄PF₆ to give 2 and high [OH⁻] favors the conversion of 1 to (C₅Me₅)Rh(η⁴-C₄Me₄S-1-O).