Digging the Sigma-Hole of Organoantimony Lewis Acids by Oxidation

The development of group 15 Lewis acids is an area of active investigation that has led to numerous advances in anion sensing and catalysis. While phosphorus has drawn considerable attention, emerging research shows that organoantimony(III) reagents may also act as potent Lewis acids. Comparison of the properties of SbPh₃ , Sb(C₆ F₅ )₃ , and SbAr^F ₃ with those of their tetrachlorocatecholate analogues SbPh₃ Cat, Sb(C₆ F₅ )₃ Cat, and SbAr^F ₃ Cat (Cat=o-O₂ C₆ Cl₄ , Ar^F =3,5-(CF₃ )₂ C₆ H₃ ) demonstrates that the Lewis acidity of electron deficient organoantimony(III) reagents can be readily enhanced by oxidation to the +V state-as verified by binding studies, organic reaction catalysis, and computational studies. The results are rationalized by explaining that oxidation of the antimony center leads to a lowering of the accepting σ* orbital and a deeper carving of the associated σ-hole.