Base Hydrolysis of Ruthenium(II) Thiophene Complexes and Reactions of the Coordinated Ligands

Described are reactions of aqueous base with dicationic thiophene complexes of the type [Ru(C₆R₆)(C₄R₄S)]²⁺ (1) and the acid/base reactivity of these products. Attack of OH- on [Ru(C₆R₆)(C₄R₄S)]²⁺ (1) occurs at sulfur to give [Ru(C₆R₆)(C₄R₄S-1-OH)]⁺ (2H⁺) which was observed by ¹H NMR spectroscopy. This species displays two forms of reactivity: (i) further reaction with OH- to give the S-oxides (sulfoxides) Ru(C₆R₆)(C₄R₄S-1-O) (2) and (ii) rearrangement to give 2-hydroxythiophene-yl derivatives [Ru(C₆R₆)(C₄R₄S-2-OH)]⁺ (3). In solution the S-oxides rearrange to give acyl thiolate complexes such as Ru(cymene) (SC₃MeH₂-COMe) (4d). The acyl thiolato complexes exist in two isomeric forms which were separated in the case of 4d and are labeled 4d_kin and 4d_therm- The conversion of 4d_kin to 4d_therm proceeds via a first order process with t_1/2 = 4.5 h and K_eq ∼ 30. In contrast the acyls derived from tetramethylthiophene are dynamic on the NMR time scale until low temperatures. [Ru(cymene)-(MeSC₃Me₃COMe)]⁺ prepared by methylation of 4a is static. Similarly, methylation of the two isomers of 4d gave the corresponding S-methylated isomers 4d_thermMe⁺ and 4d_kinMe⁺ whose isomerization is catalyzed by HOTf. Solutions of H4d_thermMe²⁺ react with dimethylthiophene to regenerate 1d. Base hydrolysis of [Ru(arene)(C₄H₄S)]²⁺ affords a pair of isomeric formyl thiolates Ru(arene) (SC₃H₃CHO), which were shown not to undergo H/D exchange with aqueous base. The isomerization of the species with arene = cymene isomerized very slowly (t_1/2 = 84 h) vs 4d. Basification of [Ru(C₄Me₄S)(C₄R₄S)]²⁺ (R₄ = Me₄, 2,5-Me₂H₂) results in the addition of O²- to one ring, in the case of the mixed thiophene complex the addition occurs at the less methylated ring. The base hydrolysis reactions are reversible such that reprotonation affords the starting dicationic thiophene sandwich complexes. However it was found that protonation of 4d_therm affords the vinyl-thiolato complex [Ru(C₆R₆)(μ-SCMe=CHCH₂COMe)] ₂²⁺, not thiophene complex 1d. Deprotonation of this diruthenium species with KOH gives the dimetallic thioaldehyde complex [Ru(C₆R₆)(μ-S=CMeCH=CHCOMe)]₂ which rapidly (minutes, 25°C) isomerizes to 4d_therm. Treatment of 4d_therm with BH(OMe)₃^- affords one stereoisomer of Ru(cymene)(SC₃MeH₂CHOHMe).