Isolation, bonding and reactivity of a monomeric stibine oxide

Abstract

In contrast to phosphine oxides and arsine oxides, which are common and exist as stable monomeric species featuring the corresponding pnictoryl functional group (Pn=O/Pn+–O−; Pn = P, As), stibine oxides are generally polymeric, and the properties of the unperturbed stiboryl group (Sb=O/Sb+–O−) remain unexplored. We now report the isolation of the monomeric stibine oxide, Dipp3SbO (where Dipp = 2,6-diisopropylphenyl). Spectroscopic, crystallographic and computational studies provide insight into the nature of the Sb=O/Sb+–O− bond. Moreover, isolation of Dipp3SbO allows the chemistry of the stiboryl group to be explored. Here we show that Dipp3SbO can act as a Brønsted base, a hydrogen-bond acceptor and a transition-metal ligand, in addition engaging in 1,2-addition, O-for-F2 exchange and O-atom transfer. In all cases, the reactivity of Dipp3SbO differed from that of the lighter congeners Dipp3AsO and Dipp3PO.

Variation in pnictogen–oxygen bonding unlocks greatly enhanced Brønsted basicity for the monomeric stibine oxide

“…S15†), consistent with formation of hydroxypnictonium triflate salts 2a–c . 53 In the 31 P NMR spectrum of a mixture of 1c and triflic acid, a single broad signal is observed at 50.60 ppm, which is shifted significantly downfield from that of free 1c (25.33 ppm), further consistent with formation of the hydroxyphosphonium cation. The hydroxystibonium triflate ( 2a ) crystallized preparatively from a mixture of DCM and hexanes.…”

“…Our preliminary investigation into the reactivity of these species with Brønsted acids confirmed this hypothesis. 53 We previously observed that addition of 1 equiv of benzenesulfonic acid to 1a or 1b cleanly affords the hydroxypnictonium benzenesulfonate salts. In the case of 1c , titration with excess benzenesulfonic acid was needed to observe the formation of hydroxyphosphonium in solution.…”

“…We previously performed a detailed theoretical investigation of 1a–c (DKH-PBE0/old-DKH-TZVPP//PBE0/def2-TZVPP); select results are reproduced in Table 1. 53 To facilitate the subsequent discussion, we briefly recapitulate some of the findings from that prior work. Topological analysis revealed a bond critical point on each Pn–O interatomic vector.…”

“…aThese values are reproduced from a prior report. 53 bElectron density at the Pn–O bond critical point.cEnergy of HOMO.dEnergy of LUMO.eNatural charge of the O atom.fNatural charge of the pnictogen atom.gEnergy of destabilization upon deletion of dative interactions between the O atom and Dipp 3 Pn molecular fragments.…”

“…We recently succeeded in preparing the first unperturbed monomeric stibine oxide, Dipp 3 SbO (where Dipp = 2,6-diisopropylphenyl) ( 1a ), by capitalizing on the kinetic stabilization afforded by the bulky aryl substituents (Scheme 1). 53 We also isolated its lighter congeners Dipp 3 AsO ( 1b ) and Dipp 3 PO ( 1c ) for direct comparison. We note that monomeric bismuthine oxides are an unknown class of molecule, preventing the extension of periodic trends in pnictoryl bonding to bismuth.…”