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)-Ph2 CCH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.