CONFORMATIONAL ISOMERS OF BIS(TERT-BUTYLPHOSPHINE)OSMIUM COMPLEXES

Reaction of [Osl(2)(C6H6)(PMeBu(2)(t))] with Na[ON=CMe(2)] in methanol, in presence of KPF6, afforded the oximato compound [Os(C6H6)(eta(2)-ON=CMe(2))(PMeBU(2)(t))]PF6 which on further reaction with HN=CPh(2) gave the azavinylidene complex [Os(C6H6)(=N=CPh(2))(PMeBu(2)(t))]PF6. By using a similar synthetic route, the bis(tert-butylphosphine) osmium(II) derivatives [OsX(2)(arene)(PHBu(2)(t))], [Os(arene)(eta(2)-ON=CMe(2))-(PHBu(2)(t))]PF6 and [Os(arene)(=N=CPh(2))(PHBu(2)(t))]PF6 (arene = C6H6 or C(6)H(3)Me(3)-1,3,5; X = Cl or l) were obtained, in most cases almost quantitatively. The NMR spectra of all the latter complexes and of [Os(C6H6) (eta(2)-ON=CMe(2)) (PMeBu(2)(t))]PF6 are temperature-dependent and thus indicative of a dynamic behaviour in solution. The most reasonable explanation for these observations is that due to a hindered rotation around the Os-P axis conformational isomers are formed. The energy barrier for the conversion significantly depends on the type of the non-phosphine ligands co-ordinated to osmium and is, for arene = C6H6, in [Os(arene)(=N=CPh(2))(PHBu(2)(t))] PF6 higher than in [OsX(2)(arene)(PHBu(2)(t))] and in the mesitylene complexes higher than in the benzene derivatives. For [Os(C6H6)(=N=CPh(2))(PHBu(2)(t))]PF6,a value of Delta G double dagger for the rotational barrier of ca. 60 kJ mol(-1) has been calculated.