Synthesis, Molecular Structure, and Reactivity of Neutral and Cationic Areneosmium(II) Complexes with Diarylcarbenes as Ligands^§

While the dinuclear compounds [(η⁶-mes)Os{κ¹-OC(O)CF₃}(μ-Cl)]₂ (2) and [(η⁶-mes)Os{κ¹-OS(O)₂CF₃}(μ-Cl)]₂ (3), prepared from [(η⁶-mes)OsCl(η³-C₃H₅)] (1) and CF₃CO₂H or CF₃SO₃H by elimination of propene, are not suitable precursors for the synthesis of osmium carbenes, the bis(trifluoroacetato) derivatives [(η⁶-arene)Os{κ¹-OC(O)CF₃}(κ²-O₂CCF₃)] (4, 9, 10) are useful starting materials. They react with diaryldiazomethanes R₂CN₂ to give the half-sandwich-type complexes [(η⁶-arene)Os{κ¹-OC(O)CF₃}₂(CR₂)] (11−17) in good to excellent yields. The bis(tosylato)osmium(II) compounds [(η⁶-arene)Os{κ¹-OS(O)₂R}{κ²-O₂S(O)R}] (20, 21; R = p-C₆H₄CH₃) behave differently and upon treatment with Ph₂CN₂ and PhCHN₂ afford tetraphenylethene and a mixture of (E)- and (Z)-stilbene. The reaction of 12−15 and 17 with either Me₃SiX or NH₄X (X = Cl, Br, I) leads to the replacement of the trifluoroacetato ligands and the formation of the corresponding carbene complexes [(η⁶-arene)OsX₂(CR₂)] (22−31) in 67−91% yield. An exchange of trifluoroacetate for chloride or bromide by using HCl or HBr is also possible. Treatment of [(η⁶-mes)OsCl₂(CPh₂)] (28) with M(acac-[F_n]) (M = Na, Tl; n = 0, 3, or 6) affords the chelate compounds [(η⁶-mes)OsCl(κ²-acac-[F_n])] (33−35) via elimination of the carbene ligand. Compounds 33−35 are also accessible from [(η⁶-mes)OsCl₂]₂ (19) and Hacac/NEt₃ or Na(acac-[F_n]) (n = 3 or 6), respectively. While 14 (arene = mes; R = Ph) and 28 react with C₆H₅MgBr and CH₃MgI only by displacement of trifluoroacetate or chloride for bromide or iodide, the reaction of 14 with CH₂CHMgBr gives the η³-allyl complex [(η⁶-mes)OsBr(η³-CH₂CHCPh₂)] (36). A C−C coupling also takes place upon treatment of 14 with CH₂CHOEt, resulting in the formation of the metallacyclic compound [(η⁶-mes)Os{κ²(C,O)-Ph₂CCHCHOEt}{κ¹-OC(O)CF₃}] (38). The bis(trifluoroacetato) derivatives 14, 15, and 17 react in acetone with water to afford the diaryl(carbonyl)osmium(II) complexes [(η⁶-mes)OsR₂(CO)] (39−41). On the basis of a labeling experiment, a mechanism for this unusual C−C cleavage reaction is proposed. The reaction of the dichloro compounds 28 and 29 with PPh₃ in the presence of AgPF₆ gives the cationic carbene complexes [(η⁶-mes)OsCl(PPh₃)(CR₂)]PF₆ (45, 46) in nearly quantitative yields. The molecular structures of 2, 15, 28, 35, and 46 were determined crystallographically.