Synthesis, molecular structure, and reactivity of neutral and cationic areneosmium(II) complexes with diarylcarbenes as ligands

While the dinuclear compounds [(eta(6)-mes)Os{kappa(1)-OC(O)CF3}(mu-Cl)](2) (2) and [(eta(6)-mes)OS{kappa(1)-OS(O)(2)CF3}(mu-Cl)](2) (3), prepared from [(eta(6)-mes)OsCl(eta(3)-C(3)H(5)A] (1) and CF3CO2H or CF3SO3H by elimination of propene, are not suitable precursors for the synthesis of osmium carbenes, the bis(trifluoroacetato) derivatives [(eta(6)-arene)Os{kappa(1)-OC(O)CF3}(kappa(2)-O2CCF3)] (4, 9, 10) are useful starting materials. They react with diaryldiazomethanes R2CN2 to give the half-sandwich-type complexes [(eta(6)-arene)Os{kappa(1)-OC(O)CF3}(2)(=CR2)] (11-17) in good to excellent yields. The bis(tosylato)osmium(II) compounds [(eta(6)-arene)Os{kappa(1)-OS(O)(2)R}{kappa(2)-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 [(eta(6)-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 [(eta(6)-mes)OsCl2(=CPh2)] (28) with M(acac-[F-n]) (M = Na, Tl; n = 0, 3, or 6) affords the chelate compounds [(eta(6)-mes)OsCl(kappa(2)-acac-[F-n])] (33-35) via elimination of the carbene ligand. Compounds 33-35 are also accessible from [(eta(6)-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 eta(3)-allyl complex [(eta(6)-mes)OsBr(eta(3)-CH2CHCPh2)] (36). A C-C coupling also takes place upon treatment of 14 with CH2=CHOEt, resulting in the formation of the metallacyclic compound [(eta(6)-mes)Os{kappa(2)(C,O)-Ph2CCH=CHOEt}{kappa(1)-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 [(eta(6)-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 [(eta(6)-mes)OsCl(PPh3)(=CR2)]PF6 (45, 46) in nearly quantitative yields. The molecular structures of 2, 15, 28, 35, and 46 were determined crystallographically.