Acidity/hydridicity evaluation of Sb–H bonds and formation of a new group 15 metal cluster topology

Abstract

We report the first estimates for the proton transfer ability and hydride donor ability of a fifth-row E–H bond besides Sn–H bonds. A new D_2d-symmetric cluster type for M₈ species (M = pnictogen) is shown, along with the first amido-stibanide. Sterically determined deprotonation and redox outcomes are demonstrated.

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The chemistry of main group element-hydrogen (E–H) bonds is an important area of study due to its relevance for synthesis (e.g., reduction),¹ catalysis (e.g., hydrogenation),² energy storage (e.g., hydrogen surrogates),³ and materials science (e.g., metal deposition).^4,5 In contrast to E–H bonds involving second-, third-, and fourth-row p-block elements, the chemistry of the heavier fifth- or sixth-row elements remains underexplored due to synthetic challenges.^6,7

The reactivity of light E–H bonds is generally well-predicted by polarity considerations.^6,7 For example, compounds containing B–H, Al–H, or Ga–H bonds generally behave as hydride sources and compounds containing N–H, P–H, or As–H bonds generally behave as proton sources. Only in rare cases, namely, when either very Lewis basic or very Lewis acidic co-reactants are present, or very strong π-donor groups flank protic E–H bonds, does the reactivity deviate from these trends.^8–10

For heavier elements, simple bond polarity arguments are not always good predictors of reactivity as radical processes like bond homolysis can also be accessible for their long and weak E–H bonds.^6,7 This feature underpins the widespread use of light or radical-activated organotin hydrides such as ⁿBu₃SnH in organic synthesis.¹¹ Thus, the ability to access diverse fission pathways offers a rich bouquet of chemical reactivity for heavy main group E–H species with demonstrated practical applications. A deeper understanding of fundamental parameters for heavy E–H bonds is needed to enable the rational development of their multifaceted chemistry, but such data remain elusive. For example, widely employed acidity and hydridicity references lack values for fifth- and sixth-row main group elements (with the exception of tin), and, to the best of our knowledge, such values have not been experimentally reported for antimony or bismuth.^1,12,13 Nevertheless, species with reactive Sb–H bonds are implicated in fascinating stoichiometric and catalytic processes,^14–19 motivating the pursuit of fundamental data about this functional group and a deeper understanding of its reaction modes.

We have been studying the chemistry of Sb–H bonds in derivatives of 1RH, starting with the discovery of hydrostibination in 2019 (Fig. 1).²⁰ Our original report showed that 1EtH is a source of hydride, rapidly yielding Ph₃CH in reactions with the Ph₃C⁺ cation (with presumed formation of unisolable [1R]⁺). Subsequently, we found that 1EtH is also a source of protons towards M–N or M–C bonds, giving the first examples of bismuthanylstibanes 1RBi and Sb–Fe bimetallics.^21,22 Finally, we found evidence of Sb–H homolysis and a radical mechanism in the reaction of 1RH with phenylacetylene to give styrene 1RSty.²³ Thus, derivatives of 1RH show diverse reactivity modes within a single molecular platform. Recognizing this feature and the abovementioned scarcity of reactivity data for heavy E–H bonds,^24,25 here we provide the first estimates of proton and hydride transfer ability for 1RH (R = Et, iPr) by bracketing experiments and product isolation studies. We also reveal a new main group cluster topology. Our results deepen the fundamental understanding of Sb–H bonds and metallocluster formation to guide further developments in antimony chemistry, while providing insights that may also translate to other heavy main group E–H bonds.

Fig. 1 Observed modes of reactivity for derivatives of 1RH.

As compounds 1EtH and 1iPrH are both highly air and moisture sensitive, standard titration experiments using protic solvents are unsuitable for establishing their proton transfer ability. Moreover, as we show later, the anions formed by deprotonation only have a fleeting existence, precluding the establishment of a true equilibrium between the derivatives of 1RH, H⁺, and [1R]⁻. Given these limitations, we adopted a bracketing approach involving the combination of 1RH with organic bases whose pK_a values are known in THF to look for evidence of proton transfer. The pK_a of the most basic species that fails to react with 1RH forms one limit of the pK_a bracket, while the pK_a of the least basic species that deprotonates 1RH forms the other limit. An additional limitation arises from the possibility of base coordination to the metal rather than deprotonation as we have previously shown the derivatives of 1RH exhibit antimony-centred Lewis acidity and coordination.^20,23 To thwart such coordination and promote reactivity at the Sb–H bond, only bases of moderate to high steric bulk that were soluble in non-protic solvents were suitable.

Through pairwise reactions between equimolar amounts of 1EtH and >20 bases, we successively narrowed the limits of the pK_a range spanned by the limiting bases. This process culminated with the finding that piperidine (pK_a of conjugated acid in THF = 14.3)²⁶ failed to deprotonate 1EtH while 1,1,3,3-tetramethylguanidine (TMG, pK_a of conjugated acid in THF = 15.5)²⁶ succeeded, bracketing the proton transfer ability of 1EtH to within the 14.3–15.5 pK_a range in THF (Fig. S1–S5). No other suitable base could be found within this range to further narrow the pK_a bracket. The analogous sequence of experiments with 1iPrH indicated a proton transfer ability value in the 20.2–21.0 pK_a range, with the phosphazene base ^tbutylimino-tri(pyrrolidino)phosphorane (BTTP, pK_a of conjugate acid in THF = 20.2)²⁶ being unable to significantly deprotonate this compound, while 7-methyl-1,5,7-trizabicyclodecene (TBD, pK_a of conjugate acid in THF = 21.0)²⁶ was successful (Fig. S6–S9). To the best of our knowledge, despite being ranges rather than absolute values, these represent the first experimental acidity measurements for any fifth- or sixth-row main group E–H bond, besides Sn–H bonds.

The significant reduction in acidity going from 1EtH to 1iPrH was surprising, given that the Sb–H group is well-separated from the substitutional changes occurring in the silyl groups. We hypothesized that this may have a kinetic origin, namely, that the bulky triisopropylsilyl groups prevent approach of the base to the Sb–H bond, thereby suppressing deprotonation that may otherwise readily occur in a less hindered case. To evaluate this possibility, we calculated the energies for transferring a proton from each compound to the di-isopropylamide anion using density functional theory and a polarization continuum model for THF. The ΔG for this reaction (−34.07 kcal mol⁻¹) captures the relative acidity of 1EtH and iPr₂NH, indicating that the latter is 24.99 pK_a units less acidic than the former. Based on the experimentally known pK_a of iPr₂NH (35.7 in THF), a DFT-calculated value of 10.71 is obtained for 1EtH.²⁷ The analogous calculation for 1iPrH yielded a calculated pK_a of 9.06. These calculated values, which reflect thermodynamic, not kinetic, outcomes by ignoring counterion, solvation, and steric effects, are quite close to one another (within 2 pK_a units). By comparison, the experimental brackets for the two compounds are separated by almost 6 pK_a units, supporting our hypothesis of kinetic inhibition for 1iPrH.

Next, we sought to isolate the stibanide anions resulting from deprotonation of 1RH. Breunig has previously characterized derivatives of R₂SbLi(THF)₃ (R = Ph, Mes; Mes = 2,4,6-trimethylphenyl) by deprotonation of the corresponding R₂SbH precursor with nBuLi in THF.²⁸ Power also generated the Ph₂Sb⁻ anion in a crown ether solvated lithium salt via deprotonation of Ph₂SbH with nBuLi.²⁹ More recently, von Hänisch isolated alkali metal salts of the parent stibanide, [SbH₂]⁻, via reaction between SbH₃ and nBuLi or MO^tBu salts (M = Na, K, Rb, Cs).³⁰ In contrast to such compounds, derivatives of 1RH feature π-electron-rich amido groups and stibanides featuring such groups remain unknown, further motivating our attempts to isolate them.

Combining 1EtH with LiHMDS resulted in an immediate colour change to dark red/orange and formation of hexamethyldisilazane, as per ¹H NMR spectroscopy. The prominent ¹H NMR resonance of the Sb–H group at ca. 10 ppm also disappeared, as did the associated Sb–H stretch at 1883 cm⁻¹ in the IR spectrum (Fig. S20). Storage of the solution at −30 °C yielded red crystals identified by a diffraction experiment as compound 2, which has the overall formula (LSb-Sb)₄, where L is the triethylsilyl naphthalenediamine ligand (Fig. 2). This outcome confirms the deprotonation but indicates that the subsequent anion is unstable towards redox rearrangement and ligand loss, instead yielding polystibanes. Such redox chemistry has previously only been observed in reactions of antimony centres containing a single organic group [e.g. RSbCl₂ or cyclo-(RSb)_n] and reducing agents such as alkali metals.^28,31 Cluster formation has, however, not been observed following deprotonation of Ar₂SbH or SbH₃,³⁰ highlighting the distinct reactivity of amido-substituted secondary stibanes. It is likely that the increased lability of the very polar Sb–N bonds compared to Sb–C or Sb–H bonds permits reaction trajectories involving the loss of even chelating amido ligands. The electron-rich nature of the naphthalenediamine ligand also makes it more susceptible to oxidation.^32–34

Fig. 2 Formation of compounds 2 and 3 and oxidation product 4 detected by ESI-MS.

The structure of compound 2 is remarkable for two reasons. First, the open D_2d-symmetric metal core (Fig. 3b) is unprecedented for pnictogens. Reported molecular Sb₈ compounds form S₈-like eight-membered rings,³⁵ realgar-like closed polycycles,³⁶ or interconnected 2 × Sb₄ rings.³⁷ Second, the planarity of the Sb₄ core (D_4h) is unexpected in the absence of any obvious multiple bonding. Indeed, four-membered pnictogen rings form folded structures (Fig. 3c), whereas the Sb₄ core of 2 is more reminiscent of σ-aromatic [M₄]⁴⁺ rings.³⁸

Fig. 3 (a) Structure of 2 in the solid state as determined by single crystal diffraction. Hydrogen atoms have been omitted, and ellipsoids are drawn at 50% probability. (b) The Sb₈ core of compound 2 with amido ligands omitted for clarity. (c) The folded structure of the Sb₈ core, as found in the calculated structure of N–Me substituted derivative 2′. (d) Structure of 3 in the solid state as determined by single crystal diffraction. Hydrogen atoms have been omitted, and ellipsoids are drawn at 50% probability. Lithium aryl interactions are omitted but are comprised by an η⁴ interaction with the naphthalene ring.

Multiple bonding can be ruled out as the cause of planarity since the Sb–Sb bond distances within the Sb₄ core [2.8452(4)–2.8683(4) Å] are comparable to those between the core and peripheral Sb atoms [2.8264(4)–2.8342(4) Å]. Lattice packing effects can also be discounted since the DFT-calculated structure of an isolated molecule closely resembles the one that is experimentally observed. And although we have previously shown that silylnaphthalene diamine ligands are exceptional dispersion donors,³⁹ this feature appears to have only a minor effect in terms of promoting planarity – DFT calculations including or omitting dispersion corrections gave very similar geometries, matching the experimental structure (Fig. S23). Instead, it appears that the high steric repulsion between the silyl groups forces the ring to be flat such that the distance between the ligands is maximized. Consistently, the only other molecule showing such an open D_2d M₈ arrangement (M = main group metal) is [(Me₃Si)₃Ge–Ge]₄, which features the very large (Me₃Si)₃Ge group.⁴⁰ Further supporting this steric repulsion argument, the DFT-optimized geometry of a hypothetical, sterically unhindered, N–Me substituted derivative showed a heavily puckered ring (metal portion shown in Fig. 3c, see Fig. S22) that reflects the expected shape for such a species.

The analogous anion isolation experiment of 1iPrH with LiHMDS also showed complete loss of the Sb–H functional group by NMR and IR spectroscopy. However, recrystallization of the reaction mixture in this case gave a lithium tristibanide salt 3 as a THF solvate (Fig. 3d). Tristibanides have previously been prepared by reduction of cyclotetrastibanes or triarylstibanes with alkali metals.^29,41 Formation of an Sb₃ rather than Sb₈ unit from 1iPrH is likely due to the high steric bulk of the triisopropylsilyl groups, which prevents the more extensive aggregation needed to make the Sb₈ cluster found in 2. In this context, compound 3 can be considered a kinetic snapshot of a stabilized intermediate en route to larger clusters. The Sb–Sb bond distances in the tristibanide 3 [2.7704(9) and 2.7546(8) Å] are on average 0.07 Å shorter than the corresponding values in the neutral polystibane 2, while the average Sb–N distance in 3 (2.083 Å) is slightly longer than in 2 (2.042 Å). We ascribe these differences to hyperconjugation between the lone pairs at Sb2 and the Sb1–N σ* acceptor orbitals, which would increase Sb2–Sb1 covalency at the expense of Sb1/3-N bonding.

To determine the fate of the lost ligand equivalents in formation of 2 and 3, we conducted mass spectrometric analyses of reaction mixtures from which these clusters were obtained (Fig. S24–S30). The spectra for the reaction involving 1iPrH showed formation of tetraaminoperylene 4 (Fig. S28), which is the expected oxidation product in line with Gade's previous report that 1,8-bis(silylamino)naphthalene complexes of thallium (a fifth-row metal like antimony) undergo loss of reduced thallium metal with concomitant formation of tetraaminoperylenes.³²

To estimate the hydride ion affinity of [1R]⁺, a similar bracketing methodology was applied. After testing >20 boron, carbon, and silicon-based Lewis acids, we observed that (i) boron Lewis acids were unsuitable due to formation of multiple B–C/Sb–H metathesis products, and (ii) both stibanes readily transferred a hydride to the [Ph₃C]⁺ cation (calculated hydride ion affinity = 471.5 kJ mol⁻¹ in dichlorobenzene).⁴² The latter reactions result in the clean disappearance of the Sb–H resonance and appearance of the C–H resonance of triphenylmethane. Moreover, we found that neither stibane transferred a hydride to the [Et₃Si]⁺ cation (calculated hydride ion affinity = 463.6 kJ mol⁻¹ in dichlorobenzene).⁴² This allows us to bracket the hydride ion affinity of [1R]⁺ as being in the 463.6–471.5 kJ mol⁻¹ range, which almost perfectly encapsulates the DFT-calculated HIA values of 461 kJ mol⁻¹ for 1EtH and 460 kJ mol⁻¹ for 1iPrH, indicating minimal steric modulation. Diarylstibenium cations have previously been prepared by hydride abstraction from secondary stibanes.^43,44 N-heterocyclic stibenium ions, including those featuring a naphthalenediamine core, have been structurally authenticated previously.^45–48 However, several attempts showed that isolation of these highly reactive cations with N-silyl ligands was not possible in our hands. Once generated, they decomposed with half-lives on the order of hours in solvent, precluding access to pure samples. Crystallization attempts gave oils that continued to degrade even under inert atmosphere. Nevertheless, in line with the bracketing above, we confirmed that addition of Et₃SiH to the putative [1R]⁺ product in the reaction between 1RH and [Ph₃C]⁺ immediately regenerated the resonances for 1RH (Fig. S10–S13), supporting the intermediacy of a stibenium ion abstracting hydride from Et₃SiH.

In summary, we have determined the first estimates for the proton transfer ability and hydride donor ability of two stibanes, providing rare fundamental data about fifth-row E–H bonds. The proton transfer ability estimates reflect effective acidity rather than absolute acidity because true equilibrium between 1RH and [1R]⁻ is prevented by the onward redox reactivity of the stibanides to give clusters and chains. Nevertheless, observations employing a broad range of bases reproducibly and unambiguously indicate that diamidostibanes are comparable in acidity to protonated guanidines (pK_a ∼15), except when sterically very hindered. The latter point is made by 1iPrH, which shows comparably low acidity to protonated phosphazene superbases (pK_a ∼20). As part of attempts to isolate stable stibanides, we additionally isolated the first D_2d-symmetric M₈ cluster for the pnictogens, evidencing a topology that has only been observed in the case of one germanium species.⁴⁰ The first amido-substituted tristibanide was also isolated. The isolation of these clusters evidences divergent behaviour towards bases as a function of substituent at Sb (aryl vs. amido), and steric bulk (NSiEt₃ vs. NSi(ⁱPr)₃). On the other end of the spectrum, bracketing experiments indicate that the stibanes are slightly weaker hydride donors (HIA ∼460 kJ mol⁻¹) than trialkylsilanes like Et₃SiH. Here, the stibenium products could not be isolated due to instability, but strong evidence of their formation and persistence was observed by reverse titration with Et₃SiH. Collectively, these results provide insights into Sb–H bonds, which may have implications for future applications in areas such as catalysis, main group nanocluster formation, and metal deposition.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5qi02115d.

CCDC 2495433 (2) and 2495434 (3) contain the supplementary crystallographic data for this paper.^49a,b

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49. (a) CCDC 2495433: Experimental Crystal Structure Determination, 2025,  DOI:10.5517/ccdc.csd.cc2prpv0; (b) CCDC 2495434: Experimental Crystal Structure Determination, 2025,  DOI:10.5517/ccdc.csd.cc2prpw1.

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