Rh–POP Pincer Xantphos Complexes for C–S and C–H Activation. Implications for Carbothiolation Catalysis

The neutral Rh­(I)–Xantphos complex [Rh­(κ³-_P,O,P-Xantphos)­Cl]_n, 4, and cationic Rh­(III) [Rh­(κ³-_P,O,P-Xantphos)­(H)₂]­[BAr^F₄], 2a, and [Rh­(κ³-_P,O,P-Xantphos-3,5-C₆H₃(CF₃)₂)­(H)₂]­[BAr^F₄], 2b, are described [Ar^F = 3,5-(CF₃)₂C₆H₃; Xantphos = 4,5-bis­(diphenylphosphino)-9,9-dimethylxanthene; Xantphos-3,5-C₆H₃(CF₃)₂ = 9,9-dimethylxanthene-4,5-bis­(bis­(3,5-bis­(trifluoromethyl)­phenyl)­phosphine]. A solid-state structure of 2b isolated from C₆H₅Cl solution shows a κ¹-chlorobenzene adduct, [Rh­(κ³-_P,O,P-Xantphos-3,5-C₆H₃(CF₃)₂)­(H)₂(κ¹-ClC₆H₅)]­[BAr^F₄], 3. Addition of H₂ to 4 affords, crystallographically characterized, [Rh­(κ³-_P,O,P-Xantphos)­(H)₂Cl], 5. Addition of diphenyl acetylene to 2a results in the formation of the C–H activated metallacyclopentadiene [Rh­(κ³-_P,O,P-Xantphos)­(ClCH₂Cl)­(σ,σ-(C₆H₄)­C­(H)CPh)]­[BAr^F₄], 7, a rare example of a crystallographically characterized Rh–dichloromethane complex, alongside the Rh­(I) complex mer-[Rh­(κ³-_P,O,P-Xantphos)­(η²-PhCCPh)]­[BAr^F₄], 6. Halide abstraction from [Rh­(κ³-_P,O,P-Xantphos)­Cl]_n in the presence of diphenylacetylene affords 6 as the only product, which in the solid state shows that the alkyne binds perpendicular to the κ³-POP Xantphos ligand plane. This complex acts as a latent source of the [Rh­(κ³-_P,O,P-Xantphos)]⁺ fragment and facilitates ortho-directed C–S activation in a number of 2-arylsulfides to give mer-[Rh­(κ³-_P,O,P-Xantphos)­(σ,κ¹-Ar)­(SMe)]­[BAr^F₄] (Ar = C₆H₄COMe, 8; C₆H₄(CO)­OMe, 9; C₆H₄NO₂, 10; C₆H₄CNCH₂CH₂O, 11; C₆H₄C₅H₄N, 12). Similar C–S bond cleavage is observed with allyl sulfide, to give fac-[Rh­(κ³-_P,O,P-Xantphos)­(η³-C₃H₅)­(SPh)]­[BAr^F₄], 13. These products of C–S activation have been crystallographically characterized. For 8 in situ monitoring of the reaction by NMR spectroscopy reveals the initial formation of fac-κ³-8, which then proceeds to isomerize to the mer-isomer. With the para-ketone aryl sulfide, 4-SMeC ₆H₄COMe, C–H activation ortho to the ketone occurs to give mer-[Rh­(κ³-_P,O,P-Xantphos)­(σ,κ¹-4-(COMe)­C₆H₃SMe)­(H)]­[BAr^F₄], 14. The temporal evolution of carbothiolation catalysis using mer-κ³-8, and phenyl acetylene and 2-(methylthio)­acetophenone substrates shows initial fast catalysis and then a considerably slower evolution of the product. We suggest that the initially formed fac-isomer of the C–S activation product is considerably more active than the mer-isomer (i.e., mer-8), the latter of which is formed rapidly by isomerization, and this accounts for the observed difference in rates. A likely mechanism is proposed based upon these data.