Conformational isomers of bis(tert-butylphosphine)osmium complexes

Abstract

Reaction of [OsI₂(C₆H₆)(PMeBu^t₂)] with Na[ONCMe₂] in methanol, in presence of KPF_6′, afforded the oximato compound [Os(C₆H₆)(η²-ONCMe₂)(PMeBu^t₂)]PF₆ which on further reaction with HNCPh₂ gave the azavinylidene complex [Os(C₆H₆)(NCPh₂)(PMeBu^t₂)]PF_6′. By using a similar synthetic route, the bis(tert-butylphosphine)osmium(II) derivatives [OsX₂(arene)(PHBu^t₂)], [Os(arene)(η²-ONCMe₂)(PHBu^t₂)]PF₆ and [Os(arene)(NCPh₂)(PHBu^t₂)]PF₆(arene = C₆H₆ or C₆H₃Me₃-1,3,5; X = Cl or I) were obtained, in most cases almost quantitatively. The NMR spectra of all the latter complexes and of [Os(C₆H₆)(η²-ONCMe₂)(PMeBu^t₂)]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)(NCPh₂)(PHBu^t₂)]PF₆ higher than in [OsX₂(arene)(PHBu^t₂)] and in the mesitylene complexes higher than in the benzene derivatives. For [Os(C₆H₆)(NCPh₂)(PHBu^t₂)]PF₆, a value of ΔG^‡ for the rotational barrier of ca. 60 kJ mol^–1 has been calculated.