Intermolecular and intramolecular CH activation reactions of (η-mesitylene)osmium complexes containing tertiary phosphines

Abstract

The preparations are described of the hexahapto-mesitylene complex [OsCl 2(η-C 6H 3Me 3)] 2 ( 1) and of range of monomeric tertiary phosphine (L) derivatives containing halide, hydride, alkyl or aryl ligands of the general types OsCl 2(η-C 6H 3Me 3)(L), OsHCl(η-C 6H 3Me 3)(L), OsCl(R)(η-C 6H 3Me 3)(L) (R = CH 3, C 6H 5), OsH 2(η-C 6H 3Me 3)(L) and Os(CH 3) 2(η-C 6H 3Me 3)(L). Heating OsCl 2(η-C 6H 3Me 3)(PPh 3) ( 2) with propan-2-ol and Na 2CO 3 gives initially OsHCl(η-C 6H 3Me 3)(PPh 3) ( 7) and finally the ortho-metallated complex OsH( o-C 6H 4P Ph 2)(η-C 6H 3Me 3) ( 12). Similar treatment of OsCl 2(η-C 6H 3Me 3)(P-t-BuPh 2) ( 4) gives OsHCl(η-C 6H 3Me 3)(P-t-BuPh 2) ( 9) and the diastereomeric ortho-metallated complexes OsX( o-C 6H 4P -t-BuPh)(η-C 6H 3Me 3) (X = H ( 13a, 13b), Cl ( 14a, bd14b)). Reduction of OsClR(η-C 6H 3Me 3)(L) with NaBH 4 in propan-2-ol gives thermally stable hydrido(alkyl) or hydrido(phenyl) complexes OsH(R)(η-C 6H 3Me 3)(L) (R = CH 3, L = PPh 3 ( 31), PMe 3 ( 32); R = C 6H 5, L = PPh 3 ( 33), PMe 3 ( 34). Treatment of OsCl 2(η-C 6H 3Me 3)(L) (L = PPh 3 ( 2), PMe 3 ( 3)) with neopentyllithium gives the osmacycles Os(CH 2CMe 2C H 2)(η-C 6H 3Me 3)(L) (L = PPh 3 ( 27), PMe 3 ( 28). Complex 28 reacts with HCl to give OsCl(CH 2CMe 3)(η-C 6H 3Me 3)(PMe 3) ( 30), which is reduced by LiAlH 4 to the thermally stable hydrido(neopentyl) complex OsH(CH 2CMe 3)(η-C 6H 3Me 3)(PMe 3) ( 35). Unlike their isoelectronic (η-C 5Me 5)Ir counterparts, the hydrido(alkyl) complexes OsH(R)(η-C 6H 3Me 3)(L) ( 31, 32, 35) do not undergo exchange with alkanes or arenes on heating, although some exchange, accompanied by loss of coordinated mesitylene, occurs on UV irradiation in the presence of arenes. The 16-electron fragments Os(η-C 6H 3Me 3)(L) are, therefore, less accessible thermally than Ir(η-C 5Me 5)(L). The alkanes RH are, however, eliminated quantitatively from 31, 32, and 35 at room temperature in the presence of arenes (benzene, benzene- d 6, toluene, naphthalene) and of a trace of alumina or silica. Under these conditions, 32 and 35 give OsH(aryl)(η-C 6H 3Me 3)(PMe 3) and 31 gives a mixture (ca. 5/1) of 12 and OsH(aryl)(η-C 6H 3Me 3)(PPh 3). There is no reaction with alkanes, ethylene, norbornene or CO. The fragments Os(η-C 6H 3Me 3)(L) (L = PPh 3, PMe 3) can also be generated by reduction of 2 or 3 with NaC 10H 8 and detected either by formation of 12 (L = PPh 3) or by their reaction with arenes; there is, however, no reaction with cyclohexane. These observations suggest that the oxidative addition of alkanes to Os(η-C 6H 3Me 3)(L) (L = PPh 3 ( 2), PMe 3 ( 3)) has a substantial kinetic barrier even though it should be thermodynamically favourable.