Evidence for intramolecular permutation (pseudorotation) at the central antimony atom and strong equatophilicity of an iron and a ruthenium ligand inpentacoordinate hypervalent antimony compounds

Diastereomeric pentacoordinate hypervalent stiboranes with an Sb-Fe bond (4 a and 4b: RfRfm*Sb*FeCp(CO)(2) {Rf = o-C6H4C(CF3)(2)O-, Rfm* = o-C6H4C*(CF3 )(Me)O-} were synthesized by the reaction of stiboranide anion, RfRfm*Sb*-L i+ (3-Li), with CpFeI(CO)(2) in the presence of AgBF4. The carbonyl group o f 4 was replaced with triphenylphosphine by irradiation with a tungten lamp to give a mixture of four diastereomers (5a-5d: RfRfm*Sb*Fe*Cp(CO)(PPh3)}. Each of the diastereomers was separated by TLC, and the relative stereoche mistry was determined by X-ray crystallographic analysis. The thermal equil ibration from the pure diastereomer of 5 indicated that the isomerization t ook place through inversion (pseudorotation) at the central antimony atom. The pseudorotational barriers of 5 were much higher than those of Rf(2)Sb*C l and RfRfm*Sb*(p-CH3C6H4). These results are consistent with the electron- donating properties of the group 8 transition metal fragment. Hypervalent s tiboranes {6, Rf(2)Sb*Fe*Cp(CO)(PMe3); 7, Rf(2)Sb*Fe*Cp(CO)(PEt3)} were als o prepared by similar procedures. The order of pseudorotational barriers [2 {Rf(2)Sb*Fe*Cp(CO)(PPh3)} (32.8, 33.2 kcal/mol) > 7 (32.5, 32.9 kcal/mol) > 6 (32.2, 32.7 kcal/mol)] suggests that the steric effect of the iron liga nd also played a role. The pseudorotational barriers of the corresponding r uthenium compounds, RfRfm*Sb*'RuCp(CO)(2) (12a and 12b), were slightly high er than those of the corresponding iron compounds (4a and 4b).