Evidence for Intramolecular Permutation (Pseudorotation) at the Central Antimony Atom and Strong Equatophilicity of an Iron and a Ruthenium Ligand in Pentacoordinate Hypervalent Antimony Compounds

Abstract

Diastereomeric pentacoordinate hypervalent stiboranes with an Sb−Fe bond {4a and 4b: RfRfm*Sb*FeCp(CO)2 {Rf = o-C6H4C(CF3)2O-, Rfm* = o-C6H4C*(CF3)(Me)O-} were synthesized by the reaction of stiboranide anion, RfRfm*Sb*-Li+ (3-Li), with CpFeI(CO)2 in the presence of AgBF4. The carbonyl group of 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 stereochemistry was determined by X-ray crystallographic analysis. The thermal equilibration from the pure diastereomer of 5 indicated that the isomerization took place through inversion (pseudorotation) at the central antimony atom. The pseudorotational barriers of 5 were much higher than those of Rf2Sb*Cl and RfRfm*Sb*(p-CH3C6H4). These results are consistent with the electron-donating properties of the group 8 transition metal fragment. Hypervalent stiboranes {6, Rf2Sb*Fe*Cp(CO)(PMe3); 7, Rf2Sb*Fe*Cp(CO)(PEt3)} were also prepared by similar procedures. The order of pseudorotational barriers [2 {Rf2Sb*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 ligand also played a role. The pseudorotational barriers of the corresponding ruthenium compounds, RfRfm*Sb*RuCp(CO)2 (12a and 12b), were slightly higher than those of the corresponding iron compounds (4a and 4b).

Synthesis, Structure, and Properties of a T-Shaped 14-Electron Stiboranyl-Gold Complex

“…In search of such ligands, we were drawn by the fact that antimony species such as Ph 4 SbX (X = halogen) become increasingly ionic as the size of the halogen increases . This behavior, which is promoted by the electropositive character of antimony, suggested to us that a tetraorganostiboranyl (R 4 Sb) ligand may possess strong σ-donating properties and may be well-adapted for the formation of T-shaped 14-electron transition metal complexes.…”

Synthesis and Structures of Hypervalent Antimony Compounds Bearing an Antimony−Group 6 Transition Metal Bond

“…At 110 °C (383 K), the pseudorotational barriers were 31.3 ( 11a to 11b ) and 31.7 kcal/mol ( 11b to 11a ), which were slightly higher than those of RfRfm*Sb*FeCp(CO) 2 (30.5 ( 15a to 15b ) and 30.2 kcal/mol ( 15b to 15a )) and RfRfm*Sb*RuCp(CO) 2 (30.7 ( 16a to 16b ) and 30.4 kcal/mol ( 16b to 16a )) 1 and much higher than that of Rf 2 Sb*Cl ( 13 ; 14.6 kcal/mol) 10 and those of RfRfm*Sb*( p -CH 3 C 6 H 4 ) (28.1 kcal/mol ( 14a to 14b ), 27.7 kcal/mol ( 14b to 14a )) 12 (Table ). Therefore, the strong “equatophilicity”, which was observed in the iron and ruthenium compounds, was comfirmed in these cases, too. Although the energy difference between the group 6 (Mo) and the group 8 transition metals (Fe and Ru) was small, we might explain the difference on the basis of the electronic effect.…”

“…In the preceding paper we reported the synthesis and isomerization of diastereomeric RfRfm*Sb*FeCp(CO) 2 ( 15 ) and RfRfm*Sb*Fe*Cp(CO)(PPh 3 ) ( 15-PPh 3 ) and confirmed that isomerization of these complexes took place through a Berry pseudorotation process at the Sb atom. The energy barriers of the pseudorotation at the Sb atom in these compounds were determined to be high (30−33 kcal/mol); thus, the apicophilicity of the transition metal (group 8 element) fragments should be considered to be less than that of the p -tolyl group.…”

“…In other words, the electron-donating effect of molybdenum is the highest; therefore, the CpMo(CO) 3 ligand would prefer the equatorial position strongly. In addition, on the basis of the discussion in our preceding paper, steric effects of the phosphine ligand in Rf 2 Sb*Fe*Cp(CO)(PR 3 ) played some role in determining the pseudorotational barriers. Therefore, we think that the difference between (Fe and Ru) and Mo would be partly due to the steric bulkiness.…”

121Sb Mössbauer Spectra of Hypervalent Complexes Having an Antimony-transition Metal Bond and Partial Quadrupole Coupling Constants

“…This is well explained by the increased equatophilicity for the iron fragment resulting from the strong electron-donating power. The large barriers for RfRfm*SbMoCp(CO) 3 and RfRfm*SbMCp(CO) 2 (M = Fe, Ru) [12,13] are also explained by the high equatophilicity. There is, however, a discrepancy between the order of the energy barrier and the donor power implied from the Mössbauer spectra.…”