Isolation, bonding and reactivity of a monomeric stibine oxide

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.

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, Dipp 3 SbO (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 Dipp 3 SbO allows the chemistry of the stiboryl group to be explored. Here we show that Dipp 3 SbO can act as a Brønsted base, a hydrogenbond acceptor and a transition-metal ligand, in addition engaging in 1,2-addition, O-for-F 2 exchange and O-atom transfer. In all cases, the reactivity of Dipp 3 SbO differed from that of the lighter congeners Dipp 3 AsO and Dipp 3 PO.
The stability of the pnictogen-oxygen bond in phosphine oxides has been used for over a century to drive chemical reactions such as those discovered by Wittig 1 , Mitsunobu 2 , Appel 3 and Staudinger 4 . The electronic structure that gives rise to the stability of the P=O/P + -O − bond was once a topic of intense debate, but the currently accepted model features a single covalent bond between the P and O atoms strengthened by electrostatic attraction between the P + and O − centres as well as donation from O-centred lone pairs into P-C antibonding orbitals 5 . As the Group 15 element increases in atomic number, however, the pnictogen valence orbitals become more diffuse, overlap with O-based orbitals decreases, and the pnictogen atom becomes increasingly able to expand its coordination sphere. These trends suggest that the heavier congeners of phosphine oxides could exhibit distinct and interesting reactivity 6 . The behaviour and properties of these heavier congeners would also provide a means of validating the bonding model currently used to describe the Pn=O/Pn + -O − bond, where Pn is a pnictogen 5 . For As, the variations from P are small enough, possibly as a result of the scandide contraction 7 , that arsine oxides are generally analogous to phosphine oxides: they are monomeric species with As=O/As + -O − polar covalent bonds. For example, oxidation of either Ph 3 P or Ph 3 As with H 2 O 2 readily affords monomeric Ph 3 PnO (Pn = P, As; Fig. 1a). The situation changes substantially for Sb: no molecules containing an unperturbed Sb=O/Sb + -O − bond have ever been isolated.
A substance described as triphenylstibine oxide was first reported in 1938 8 , and many other investigators subsequently purported to produce 'Ph 3 SbO' by treating Ph 3 Sb with H 2 O 2 (ref. 9 ). Melting-point measurements and careful molecular-weight determinations showed these different substances to be dimeric or polymeric compounds and their structures were ultimately established with single-crystal X-ray diffraction and Sb extended X-ray absorption fine structure analysis (EXAFS; Fig. 1b) 10-12 . Although Ph 3 SbO is not stable as a monomer, disaggregation of the polymer can be achieved with the Lewis acid B(C 6 F 5 ) 3 to afford the Lewis acid-base adduct Ph 3 SbOB(C 6 F 5 ) 3 (Fig. 1d) 13 . Another Lewis-acid-stabilized stibine oxide was obtained with a biphenylene-bridged system featuring a stibine oxide intramolecularly coordinated to a stiborane (Fig. 1e) 14 . A final example of Lewis-acid stabilization comprises [(3,5-F 2 C 6 H 3 ) 4 SbOSbEt 3 ][B(C 6 F 5 ) 4 ], which formed when a mixture of Et 3 Sb and [(3,5-F 2 C 6 H 3 ) 4 Sb][B(C 6 F 5 ) 4 ] was exposed to oxygen 15 . In these compounds, interaction with a Lewis acid stabilizes Article https://doi.org/10.1038/s41557-023-01160-x by Sasaki and colleagues 26,27 , whereby the aryl group is installed on the Sb centre with an organocopper(I) species. In this way, 1a was isolated as a colourless, crystalline, air-stable solid (Fig. 2a). Although the 1 H NMR spectrum of 1a indicates that rotation about the Sb-C and C Ar -C iPr bonds is rapid on the NMR timescale at room temperature, the X-ray crystal structure highlights the extremely crowded environment around the Sb atom (Fig. 2b,c). The corresponding arsine (1b) and phosphine (1c) were similarly prepared (Fig. 2).
Addition of 1a to a suspension of PhIO in CH 2 Cl 2 led to rapid consumption of the solid. Solvent was stripped from the reaction mixture and the residue was washed with pentane to yield a colourless solid, 2a (Fig. 3a). The infrared spectrum of 2a shows a new band at 779 cm −1 , which we assign as a ν SbO stretching frequency. This value is greater than any of the ν SbO values of (Ph 3 SbO) 2 (643/651 cm −1 ) 10 , trans-Sb(OH) 2 Mes 3 (520 cm −1 ) 22,28  The oxidation state of 2a was probed with Sb X-ray absorption spectroscopy (XAS), which we have recently used to shed light on the structures of Sb-containing compounds 29 . The Sb K edge of 2a is 2 eV higher in energy than that of 1a (Fig. 3c). A similar shift was seen for a variety of Sb(V) compounds, including a dimeric stibine oxide (Ph 3 SbO) 2 (A), a dihydroxystiborane trans-Sb(OH) 2 Mes 3 (B) and a hydroxystibonium salt [Dipp 3 SbOH][O 3 SPh] (C, vide infra), indicating that 2a also contains Sb(V) (Fig. 3c). The K-edge EXAFS data were collected to high resolution to gain further insight into the structure of 2a. Similar data were collected from 1a, A, B and C for comparison (Fig. 3d). The Fourier transform of the data from A shows a distinct Sb···Sb scattering at 3.148(3) Å (superimposed on an outer-shell carbon backscattering), which is absent for B and C as well as 2a, indicating that 2a does not feature a dioxadistibetane. A detailed fit of the data from B shows two O scatters at 2.128(3) Å, whereas C is better fit by a single O scatterer at 1.905(1) Å. These values are in excellent agreement with the crystallographically determined structures of these compounds. In contrast, the data from 2a are best fit with a single the stibine oxide but also perturbs the Sb-O bonding interaction, preventing direct analysis of the periodic bonding trend across the pnictine oxides. These examples highlight that stibine oxides can be non-aggregated, but they raise the question of whether a monomeric stibine oxide is isolable in the absence of a Lewis acid interacting with and stabilizing the Sb=O/Sb + -O − bond. Matrix isolation studies afford evidence for the existence of monomeric H 3 SbO (Sb-O stretching frequency (ν SbO ) = 825 cm −1 ), but only in solid argon at 12 K (ref. 16 ).
We sought to explore a kinetic stabilization approach in which the reactive Sb=O/Sb + -O − bond is protected by sterically bulky groups, a strategy that has been used with great success in the stabilization of other reactive main-group bonds [17][18][19][20][21] . The bulky mesityl groups of Mes 3 Sb prevent polymerization upon treatment with H 2 O 2 , but not coordination sphere expansion: the product is the stiborane trans-Sb(OH) 2 Mes 3 (Fig. 1c) 22 . Our re-investigation of reports of Mes 3 SbO showed that the reported species is, in fact, a hydroxystibonium cation (Fig. 1f) 23,24 . This work similarly called into question the previously reported (2,6-(MeO) 2 Ph) 3 SbO (ref. 25 ).

Synthesis and characterization
We sought to prepare the even more sterically hindered stibine Dipp 3 Sb, 1a, where Dipp = 2,6-diisopropylphenyl. Although many R 3 Sb species are readily accessed from SbCl 3 and either RMgBr or RLi, these strategies do not afford 1a. We therefore adapted a synthetic strategy developed   Ultimately, we were successful in growing diffraction-quality single crystals of 2a. The asymmetric unit of the crystal structure features a single molecule of Dipp 3 SbO, which we unambiguously assign as the identity of 2a (Fig. 4a). Hirshfeld atom refinement (HAR) afforded a Sb-O bond length of 1.8372(5) Å, which is in excellent agreement with the EXAFS distance. The next-nearest Sb···O distance is 9.0791(4) Å; space-filling diagrams highlight the steric shielding provided by the Dipp groups ( Supplementary Fig. 99). One of the i Pr C-H units is directed at the stiboryl O atom with an O···H distance of 2.132(9) Å (note: HAR affords freely refined H-atom positions similar to those given by neutron diffraction) 30 . The C-H···O bond angle of 148.1(8)° suggests a strong electrostatic contribution to the interaction relative to weaker C-H···O H-bonds, in which isotropic van der Waals forces play a larger role 31 .
The molecular geometry of 2a from our X-ray crystal structure is in excellent agreement with the one from a theoretical geometry optimization (PBE0/def2-TZVPP), which features an Sb-O bond length of 1.827 Å. Notably, the scaled theoretical ν SbO of 781 cm −1 at the gas-phase optimized geometry is in excellent agreement with the experimental value for 2a (779 cm −1 ). These results combine to allow us to conclude that we have isolated an example of a monomeric stibine oxide. For comparison, we similarly synthesized and characterized the lighter congeners Dipp 3 AsO (2b) and Dipp 3 PO (2c).

Electronic structure
To gain insight into the nature of the Sb=O/Sb + -O − bonding motif, we analysed the topology of the theoretical electron density of 2a (DKH-PBE0/old-DKH-TZVPP) (Fig. 4c-e). This analysis shows the locations of critical points in the electron density (ρ), that is, points in space where the derivative of ρ is zero in three mutually orthogonal directions. These critical points are characterized with a pair of numbers (ω, σ), where ω is the number of non-zero eigenvalues of the Hessian and σ is the sum of the signs of those eigenvalues 32 . As expected, a (3, −3) critical point is present near the nuclear position of each atom. We also identified (3, −1) critical points, also known as bond critical points, between each of the covalently bonded atoms ( Supplementary Fig. 76). Although the values of various real-space functions at a (3, −1) critical point are frequently used to describe the nature of that bonding interaction 32 , for polar covalent bonds, like the Sb + -O − bond in a stibine oxide, these functions are more informative when evaluated along the length of the bond path ( Fig. 4e) 33  Further insight into the Sb-O bonding in 2a was obtained from molecular orbital analyses. The canonical molecular orbitals (CMOs) are, as expected, highly delocalized across the molecule (Fig. 5b). The frontier CMOs feature substantial π or π* character from the Dipp substituents. The nearly degenerate highest occupied molecular orbital (HOMO) and HOMO-1 feature a substantial contribution from the lone pairs on the O atom. The lowest unoccupied molecular orbital More detailed information was obtained by analysing the natural localized molecular orbitals (NLMOs) of 2a ( Fig. 4b and Supplementary  Figs. 91-93). An Sb-O bonding NLMO is present and is polarized 74:25 toward the more electronegative O atom, which uses a hybrid atomic orbital enriched in p character (79%) to interact with the Sb. The Sb-O antibonding orbital is correspondingly polarized toward the Sb and exhibits the large lobe opposite the Sb-O bond that was observed in the LUMO CMO. There are two O-centred lone pair natural bond orbitals (NBOs) with nearly pure p character and a second-order perturbation theory analysis uncovered donor-acceptor interactions that delocalize electron density from these lone pairs into Sb-C antibonding orbitals (Supplementary Table 14). Similar delocalizations were observed for 2b and 2c, and deletion calculations showed that the non-covalent interactions between the O and Dipp 3 Pn fragments decreased from 2c to 2b to 2a. These donor-acceptor interactions strengthen the Pn-O bonds, and the decreased delocalization in 2a affords the lowest Wiberg Pn-O bond order of the three, but the O atom consequently retains the greatest natural atomic charge (Supplementary Table 15). The variation in charge accumulation is also reflected in the magnitude of the electrostatic surface potential minimum, for which 2c < 2b < 2a (Fig. 5c). The decrease in Pn + -O − bond strength (PO > AsO > SbO), is also reflected in the Pn + -O − stretch force constants (Supplementary Fig. 75) and the ratio of ΔE orb /ΔE total from an energy decomposition analysis of O and Dipp 3 Pn fragments (Supplementary Tables 8-10). Deformation density analyses show a redistribution of electron density from the Dipp 3 Pn fragment to the O atom to an extent that decreases from Sb to As to P (Supplementary Fig. 90).
Donor-acceptor interactions were also observed from the O-centred lone pairs to the i Pr C-H antibonding orbitals for 2a-c (Supplementary Fig. 93), consistent with the presence of the O···H bond paths noted above. Non-covalent interaction analysis of 2a (Supplementary Fig. 89) uncovered a region with a negative product of ρ and the sign of the second-largest eigenvalue of the Hessian of ρ, sign(λ 2 )ρ, between the O and i Pr C-H; the value of sign(λ 2 )ρ was less negative for 2b and 2c, indicating that this interaction, which may help to stabilize the Sb + -O − bond, is present in 2a and weakens for 2b and 2c. The presence of this hydrogen-bonding interaction in 2a was further confirmed by NBO perturbation theory and deletion calculations ( Supplementary  Fig. 93).

Reactivity
With an isolated stibine oxide in hand, we next explored its chemistry. The bonding characteristics outlined above suggest that 2a should exhibit O-centred Lewis-basic behaviour. Cooling a solution of 2a in neat 4-fluoroaniline affords colourless blocks, which X-ray diffraction analysis confirmed to contain the stibine oxide-aniline hydrogen-bonded adduct 3 (Fig. 6(i)). In the HAR model, the hydrogen-bonding H atom of 3 is located on the N atom with a N-H distance of 1.04(2) Å. The N···O distance of 2.858(1) Å implies that the hydrogen-bonding interaction is of moderate strength. The Sb-O bond remains short at 1.8421(7) Å, but is statistically significantly lengthened as compared to 2a. Consistent with this bond lengthening, the Sb-O IR stretching frequency decreases slightly from 779 cm −1 for 2a to 762 cm −1 for 3. Neither 2b nor 2c affords a similar product, consistent with the lower nucleophilicity of the O atoms in these species.
We next sought to determine whether this Lewis basicity would also manifest in metal ion coordination. Combination of 2a with 1 equiv. of CuCl yielded the complex (Dipp 3 SbO)CuCl (4; Fig. 6(ii)),  Table 18). Solution of the structure of a second polymorph of 6 showed, however, that the complex can also take on a rigorously linear configuration. The Sb-O-M bending is most probably driven by crystal packing forces. We note that, in all cases, the geometry about the metal centres in 4-6 is nearly perfectly linear, as expected. Neither 2b nor 2c was able to form analogous complexes; the NMR resonances of these lighter pnictine oxides exhibited only minor shifts upon mixing with the metal precursors ( Supplementary Fig. 49-51). We note that the strength of the intramolecular CH iPr ···O interaction decreases upon coordination of 2a (Supplementary Tables 13 and 15). Room-temperature treatment of 2a with a strong Brønsted acid, PhSO 3 H, resulted in clean formation of the hydroxystibonium salt [Dipp 3 Sb(OH)][O 3 SPh] (7a; Fig. 6(v)). Crystallographic analysis of the salt confirmed protonation at the Sb-bound O atom, which lengthens the Sb-O bond to 1.9119(7) Å and decreases ν SbO to 611 cm −1 . Compound 2b can be similarly protonated to yield [Dipp 3 As(OH)][O 3 SPh] (7b; Supplementary Fig. 106). Compound 2c interacts much more weakly with PhSO 3 H, but titration with up to 10 equiv. of the acid results in a systematic shift in the NMR resonances of 2c. This behaviour may arise from reversible formation of a hydrogen-bonded adduct in equilibrium with the dissociated species (Supplementary Figs. 59 and 60).
We were surprised to find that, in contrast, acetic acid not only protonates the O atom of 2a, but adds across the Sb-O bond at room temperature, affording the neutral stiborane cis-Sb(OH)(OAc)Dipp 3 (8; Fig. 6(vi)). This 1,2-addition chemistry highlights the unsaturated nature of the stiboryl (Sb=O/Sb + -O − ) group. The cis isomer forms despite the expectation that the more sterically bulky Dipp groups would assume the less-crowded equatorial positions and that the more apicophilic hydroxy and acetoxy groups would assume the trans-disposed axial positions. An intramolecular hydrogen-bonding interaction is present between the hydroxy and acetoxy groups (O···O = 2.630(2) Å), which may be responsible for the cis configuration. Neither 2b nor 2c reacts in this manner with acetic acid ( Supplementary  Figs. 64-66). We have yet to observe any cycloaddition chemistry (Supplementary Fig. 74), but substrates continue to be explored.
Combination of 2a and BF 3 ·OEt 2 at −78 °C results in rapid and clean conversion to 9, which does not feature an 11 B NMR signal, but does exhibit a single sharp 19  Finally, we observed that PhSiH 3 is able to abstract the O atom from 2a to cleanly afford 1a (Fig. 6(viii)). The reaction does not proceed at room temperature, but readily reaches completion within 1 h at 50 °C. Under these mild conditions, neither 2b nor 2c reacts with PhSiH 3 (Supplementary Figs. 72 and 73).

Discussion
The isolation of a monomeric stibine oxide, 2a, was achieved using a kinetic stabilization approach in which the unsaturated Sb + -O − bond is protected by the sterically bulky Dipp groups bound to the Sb centre. The isolation of 2a has permitted the spectroscopic and crystallographic characterization of this functional group. In combination with these experimental measurements, theoretical calculations provide insight into the nature of the Pn-O bonding interaction, and the variation in this bonding as the pnictogen is varied from Sb to As to P. The increased accumulation of charge on the O atom confers upon 2a reactivity that differs notably from that of 2b and 2c. We have described examples of 2a acting as a hydrogen-bond acceptor, a transition-metal ligand and a Brønsted base. The unsaturated nature of the Sb + -O − bond also allows it to engage in addition chemistry, as exemplified by the reaction with acetic acid. Finally, the Sb-O bond can be cleaved, either with maintenance of the Sb(V) oxidation state, as in the reaction with BF 3 , or with reduction to Sb(III), as in the reaction with PhSiH 3 . We will continue to investigate in greater depth each of these classes of reactions, and others, with an emphasis on comparing and contrasting the reactivity of stibine oxides with that of phosphine and arsine oxides.

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