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

Abstract

The preparations are described of the hexahapto-mesitylene complex [OsCl2(η-C6H3Me3)]2 (1) and of range of monomeric tertiary phosphine (L) derivatives containing halide, hydride, alkyl or aryl ligands of the general types OsCl2(η-C6H3Me3)(L), OsHCl(η-C6H3Me3)(L), OsCl(R)(η-C6H3Me3)(L) (R = CH3, C6H5), OsH2(η-C6H3Me3)(L) and Os(CH3)2(η-C6H3Me3)(L). Heating OsCl2(η-C6H3Me3)(PPh3) (2) with propan-2-ol and Na2CO3 gives initially OsHCl(η-C6H3Me3)(PPh3) (7) and finally the ortho-metallated complex OsH(o-C6H4PPh2)(η-C6H3Me3) (12). Similar treatment of OsCl2(η-C6H3Me3)(P-t-BuPh2) (4) gives OsHCl(η-C6H3Me3)(P-t-BuPh2) (9) and the diastereomeric ortho-metallated complexes OsX(o-C6H4P-t-BuPh)(η-C6H3Me3) (X = H (13a, 13b), Cl (14a, bd14b)). Reduction of OsClR(η-C6H3Me3)(L) with NaBH4 in propan-2-ol gives thermally stable hydrido(alkyl) or hydrido(phenyl) complexes OsH(R)(η-C6H3Me3)(L) (R = CH3, L = PPh3 (31), PMe3 (32); R = C6H5, L = PPh3 (33), PMe3 (34). Treatment of OsCl2(η-C6H3Me3)(L) (L = PPh3 (2), PMe3 (3)) with neopentyllithium gives the osmacycles Os(CH2CMe2CH2)(η-C6H3Me3)(L) (L = PPh3 (27), PMe3 (28). Complex 28 reacts with HCl to give OsCl(CH2CMe3)(η-C6H3Me3)(PMe3) (30), which is reduced by LiAlH4 to the thermally stable hydrido(neopentyl) complex OsH(CH2CMe3)(η-C6H3Me3)(PMe3) (35). Unlike their isoelectronic (η-C5Me5)Ir counterparts, the hydrido(alkyl) complexes OsH(R)(η-C6H3Me3)(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-C6H3Me3)(L) are, therefore, less accessible thermally than Ir(η-C5Me5)(L). The alkanes RH are, however, eliminated quantitatively from 31, 32, and 35 at room temperature in the presence of arenes (benzene, benzene-d6, toluene, naphthalene) and of a trace of alumina or silica. Under these conditions, 32 and 35 give OsH(aryl)(η-C6H3Me3)(PMe3) and 31 gives a mixture (ca. 5/1) of 12 and OsH(aryl)(η-C6H3Me3)(PPh3). There is no reaction with alkanes, ethylene, norbornene or CO. The fragments Os(η-C6H3Me3)(L) (L = PPh3, PMe3) can also be generated by reduction of 2 or 3 with NaC10H8 and detected either by formation of 12 (L = PPh3) 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-C6H3Me3)(L) (L = PPh3 (2), PMe3 (3)) has a substantial kinetic barrier even though it should be thermodynamically favourable. Show more