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. 2026 May 1;32(27):e71090. doi: 10.1002/chem.71090

Trigonal Phosphine Umpolung: Electrophilicity Driven by a Redox‐Active Boron Cluster

Bryce C Nussbaum 1, Mark D Smith 1, Dmitry V Peryshkov 1,✉
PMCID: PMC13380368  PMID: 42065873

ABSTRACT

Trivalent phosphines are classically defined by their nucleophilic character. The reversal of polarity, or umpolung, at phosphorus is a conceptually significant transformation leading to new reactivity. While geometrically constrained phosphines have been demonstrated to exhibit electrophilic character, similar behavior for trigonal phosphines is rare. In this study, we report direct experimental evidence of electrophilic behavior in a non‐constrained trigonal phosphine enabled by the attachment of a redox‐active boron cluster. The diphosphine 1‐PtBu2‐2‐PiPr2‐closo‐C2B10H10 undergoes selective addition of anionic nucleophiles, including nBu− and CN−, at its PtBu2 group. These reactions are accompanied by a two‐electron reduction of the carborane cage and its conversion from a neutral closo‐ to a dianionic nido‐ structure, demonstrating that cluster‐centered redox activity drives phosphorus‐centered electrophilicity. The resulting nido‐carboranyl phosphines exhibit enhanced nucleophilicity at the remaining phosphine arm, enabling subsequent trapping with alkyl halides, carbon disulfide, and electron‐deficient fluoroarenes. These findings establish redox‐active boron clusters as platforms for inducing umpolung and provide a new strategy for accessing ambiphilic reactivity at phosphorus.

Keywords: ambiphilicity, boron clusters, phosphines, umpolung

Coupling of a phosphine to a redox‐active cluster enables polarity reversal, or umpolung, at phosphorus. Phosphines are classically known for their nucleophilicity, and while electrophilic reactivity has been observed in geometrically constrained, non‐trigonal systems, similar behavior in ordinary trigonal phosphines is rare. Non‐constrained, trigonal phosphines exhibit clear electrophilic behavior when attached to a redox‐active boron cluster.

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1. Introduction

The chemistry of trivalent phosphines is primarily determined by their nucleophilicity. Phosphines are ubiquitous ligands and organocatalysts notable for their strongly electron‐donating character [1, 2]. However, some examples of electrophilic behavior for phosphines have been recognized in the literature. The reversal in polarity of functional groups through specialized interconversions is referred to as “umpolung” in organic chemistry [3]. This type of intentional modification has been extended to the reversal in Lewis acidity/basicity of main group elements such as nitrogen, boron, and phosphorus [4, 5, 6]. Such transformations promote unique reactivity pathways toward the synthesis of complex organic compounds, often in a selective manner.

One such mode of reactivity can be found in a series of recent reports of ambiphilic behavior through metallomimetic bond activation by geometrically constrained, non‐trigonal phosphines [7, 8, 9, 10]. Theoretical calculations and x‐ray absorption near edge structure (XANES) data have indicated an unusual electrophilic behavior in the coordination and activation of O─H, N─H, and B─H bonds by these non‐trigonal phosphines [11, 12, 13]. Furthermore, the binding of anionic nucleophiles such as alkoxides, amides, and acetylides to the phosphorus atom constrained by a rigid pincer‐type trianionic ONO ligand led to the isolation of anionic [(ONO)P–X]− compounds (X = NPh2, OtBu, and CCPh), demonstrating electrophilic behavior for phosphorus(III) in a non‐trigonal environment (Scheme 1) [14, 15].

SCHEME 1.

SCHEME 1

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(A) Previous studies with constrained, non‐trigonal phosphines acting as electrophiles. (B) A non‐constrained, trigonal phosphine acting as an electrophile in the reaction with ammonia. (C) The cluster‐promoted electrophilicity of non‐constrained, trigonal phosphines toward nucleophiles and subsequent quenching with electrophiles.

In the realm of non‐constrained, trigonal phosphines, nucleophilic behavior is dominant. However, incorporation of strongly electron‐withdrawing groups has led to observable changes toward electrophilicity. For example, fluoro‐ and fluoroalkylphosphines have been demonstrated to exhibit significant π‐accepting character as ligands for transition metals [16]. In addition, utilization of positively charged substituents in the phosphine ligand framework has led to the development of α‐cationic phosphines, which are also strong π‐acids [17]. There are also a handful of examples of the nucleophilic addition of anions to neutral non‐constrained P(III) compounds, with all of them limited to highly electron‐deficient halo‐, cyano‐, or perfluoroalkylphosphines [18, 19, 20].

We have recently disclosed metal‐free bond activation by carboranyl diphosphines [21]. Polyhedral dicarbadodecaborane (C2B10H12) clusters are neutral three‐dimensional organomimetic molecules exhibiting relative chemical stability and increased steric profile. Boron clusters have been utilized in ligand design, electrochemical energy storage, medicine, and polymers [22, 23, 24, 25, 26, 27, 28, 29, 30]. The ortho‐{C2B10} cluster isomer with two neighboring carbon atoms exhibits a strongly electron‐withdrawing ability exceeding that of perfluoroaryl groups when connected through its carbon atoms [31, 32]. The carbon‐connected ortho‐{C2B10} cluster can also act as a π‐acceptor for the exohedral substituents via negative hyperconjugation [33, 34, 35].

The relatively low‐lying LUMO of ortho‐{C2B10} has significant σ*C─C antibonding character and renders it redox‐active [36, 37]. These clusters can accept two electrons, which leads to the cleavage of the intracluster C─C bond, transforming them from closed neutral closo‐{C2B10} to open dianionic nido‐{C2B10}2− cages, in some instances reversibly [38, 39].

The electron‐withdrawing nature and redox activity of boron clusters have been harnessed to promote unusual chemical reactivity at exohedral substituents [40, 41, 42, 43, 44]. In our recent report of the N─H bond activation of ammonia by diphosphine 1‐PtBu2‐2‐PiPr2‐closo‐C2B10H10 (1), theoretical calculations of the reaction mechanism revealed that an ammonia molecule initially binds through its nitrogen atom to the PtBu2 group [45]. In the corresponding transition state, the cluster C─C bond is broken, indicating its reduction. It has been proposed that the phosphine group in 1 acts as an electrophile driven by the reduction of the electron‐accepting boron cluster as the initial mechanistic step (Scheme 1).

In the present work, we provide the first direct experimental evidence of reactivity umpolung for the non‐constrained, trigonal phosphine group in 1, which exhibits electrophilic behavior. We found that strong anionic nucleophiles such as alkyllithium or cyanide selectively bind to its PtBu2 group. As hypothesized in our earlier works, the electrophilicity at phosphorus is driven by the two‐electron reduction of the boron cluster (Scheme 1).

2. Results and Discussion

The reaction of 1 and one equivalent of nBuLi in dimethoxyethane (DME) at room temperature led to an immediate color change from yellow to red–orange. The 31P NMR spectrum of the mixture exhibited two new singlets at 27.0 and 44.8 ppm corresponding to a single product. As the starting compound 1 has two doublets in its 31P NMR spectrum with 3 J PP of 90 Hz, the presence of two singlets indicated the occurrence of reductive cluster opening from the neutral closo‐ to the dianionic nido‐ form with breaking of the intracluster C─C bond. The 11B{1H} NMR spectrum of 2 contained signals corresponding to a single open boron cage. Crystallization of 2 proved difficult, with multiple attempts producing single crystals of a new product 7‐P(nBu)tBu2‐10‐P(H)iPr2‐nido‐C2B10H10 (3). Compound 3 is a zwitterion containing the boron cluster in the open dianionic form with a broken C─C bond, one alkylated phosphonium group P(nBu)tBu2, and another protonated phosphonium group P(H)iPr2 (Figure 1).

FIGURE 1.

FIGURE 1

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Synthesis and displacement ellipsoid plot (50% probability) of 3. Hydrogen atoms of alkyl groups are omitted for clarity.

The crystal structure of 3 features short exohedral bonds to phosphorus (1.765(1) Å for C1–P1 and 1.754(1) Å for C2–P2) indicating their partial ylide character. The carbon atoms of the cluster are well separated with a C1···C2 distance of 2.954(2) Å. The C 2‐symmetric nido‐ cluster geometry corresponds to the previously reported diphosphonium nido‐carborates [21, 46, 47].

Product 3 can be synthesized by exposing a solution of 2 in THF to air overnight. Consistent with the crystal structure, the 31P{1H} NMR spectrum of 3 exhibited two singlets at 40.7 and 44.5 ppm with the signal at 40.7 ppm converting into a doublet in the proton‐coupled 31P NMR spectrum with 1 J PH of 434 Hz. Importantly, only one product corresponding to a single isomer of 3 was observed with the alkylated nBu–PtBu2 and protonated H–PiPr2 groups.

Thus, based on NMR spectroscopic data of 2 and its subsequent conversion to 3 upon slow hydrolysis, compound 2 was assigned as Li[7‐P(nBu)tBu2‐10‐PiPr2‐nido‐C2B10H10] (2). In the reaction of the diphosphine 1 and nBuLi, the PtBu2 group acts as an electrophilic center for the addition of the anionic nBu− alkyl group. In general, non‐constrained non‐halogenated P(III) centers do not exhibit significant electrophilicity. In contrast, constrained, non‐trigonal P(III) compounds have been demonstrated to bind to anionic nucleophiles. In this work, the driving force for the unusual electrophilic reactivity of trivalent phosphine 1 is the redox behavior of the boron cluster that accepts two electrons and converts from the neutral closo‐ to the open dianionic nido‐ form. In the related chemistry, lithiated alkyls are capable of nucleophilic substitution with phosphites and aminophosphines, which presumably proceeds via an initial SN2 attack [48].

The addition of nBu− to a non‐constrained trigonal phosphine group in 1 serves as direct evidence for electrophilicity of carboranyl phosphines. The occurrence of addition and its selectivity for the PtBu2 group also serves as indirect evidence for an electrophilic mechanism of N─H bond activation of ammonia by 1 which we recently reported [45]. In that work, our computational studies indicated that the initial step of the reaction is the selective binding of the nitrogen atom of ammonia to the PtBu2 group with concomitant reductive opening of the boron cluster. The selectivity of ammonia binding to the PtBu2 group and not to the PiPr2 group in 1 was attributed mostly to the steric effects that stabilized the transition state with a planarized electrophilic phosphorus center and open reduced cluster.

Interestingly, a nido‐carboranyl diphosphonium compound similar to product 3 has been isolated previously as a side product in the preparation of carboranyl diphosphines [46]. Its formation has been attributed by the authors to be nucleophilic reactivity of the phosphine with a putative halobutane impurity in the nBuLi solution that was used for the deprotonation of the cluster during the synthesis. In light of the results presented in this study, we can now confidently assign the formation of 3 and its analogs to the direct electrophilic reactivity of 1 with nBuLi, with subsequent hydrolysis.

The PiPr2 group in compound 2 is connected to the dianionic nido‐ cluster, thus making it significantly electron‐rich at phosphorus [49, 50]. The ease of protonation of 2 to produce 3 confirms this. As product 2 eluded crystallization, we aimed to stabilize it by quenching with other electrophiles. The sequential addition of nBuLi and nBuBr to a solution of 1 in DME produced a new set of singlets for a product 4, one of them shifted downfield from those of 2 from 27.0 to 34.6 ppm corresponding to a P(nBu)iPr2 group and another singlet essentially unchanged at 44.9 ppm corresponding to a P(nBu)tBu2 group. Single crystals were grown from dichloromethane and hexanes, confirming the expected structure of 4 as 7‐P(nBu)tBu2‐10‐P(nBu)iPr2‐nido‐C2B10H10 (Figure 2). The open cluster adopts the same C 2‐symmetric geometry as in 3 with short C1–P1 (1.761(7) Å) and C2–P2 (1.766(7) Å) bonds. A similar product 7‐P(nBu)tBu2‐10‐P(p‐Xyl)iPr2‐nido‐C2B10H10 (5) formed in the sequential reaction of 1 with nBuLi and p‐xylyl bromide (Figure 2). It is important to note that starting 1 does not react with p‐xylyl bromide under the same conditions (1 h at room temperature in DME), only showing unreacted 1 in the 31P NMR spectrum, which highlights the enhanced nucleophilicity of the PiPr2 group in 2 due to the addition of the anionic nBu− fragment.

FIGURE 2.

FIGURE 2

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(a) Synthesis of 4. (b) Synthesis of 5. (c) Displacement ellipsoid plot (50% probability) of 4. Hydrogen atoms of alkyl groups are omitted for clarity. One of the two crystallographically independent molecules in the asymmetric unit is shown. A co‐crystallized dichloromethane solvent molecule is not shown. (d) Displacement ellipsoid plot (50% probability) of 5. Hydrogen atoms of alkyl and aryl groups are omitted for clarity.

Apart from alkyl halides, carbon disulfide also proved successful in nucleophilic attack by the reactive PiPr2 group in 2. Trapping the reactive dithiocarboxylate species with methyl iodide, we isolated 7‐P(nBu)tBu2‐10‐P(CS2Me)iPr2‐nido‐C2B10H10 (6) (Figure 3). In the crystal structure of 6, the cluster─phosphorus bonds are short (C1–P1 = 1.783(3) Å and C2–P2 = 1.751(3) Å). Bond lengths in the dithiocarboxylate methyl ester fragment are within the expected range (P2–C7 = 1.837(3) Å, C7–S1 = 1.636(3) Å, and C7–S2 = 1.717(3) Å).

FIGURE 3.

FIGURE 3

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Synthesis and displacement ellipsoid plot (50% probability) of 6. Hydrogen atoms of alkyl groups are omitted for clarity.

We also explored whether the electron‐rich phosphine center in 2 could participate in the C─F bond activation of aromatic compounds. We found that while addition of hexafluorobenzene to a solution of freshly generated 2 did not cause any changes in the 31P NMR spectrum, the use of other electron‐deficient fluoroarenes, such as pentafluorobenzonitrile, pentafluoropyridine, and decafluorobiphenyl resulted in the clean formation of single products at room temperature, according to 31P NMR spectroscopy (see Figures S24 and S25).

Addition of decafluorobiphenyl to a solution of 2 in DME caused a gradual color change from red–orange to a dark amber along with the appearance of a broadened signal at 52.7 ppm and a sharp signal at 44.8 ppm in the 31P NMR spectrum. The 19F NMR spectrum of the product contained one broad signal at –122.0 ppm and one sharp signal at –134.2 ppm. Single crystal x‐ray diffraction studies revealed the molecular structure of (7‐P(nBu)tBu2‐10‐P(C6F4)iPr2‐nido‐C2B10H10)2 (7). In the zwitterionic product 7, a 4,4’‐octafluorobiphenyl fragment bridges two PiPr2 groups of the parent 2 (Figure 4).

FIGURE 4.

FIGURE 4

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(a) Synthesis of 7. (b) Displacement ellipsoid plot (50% probability) of 7. Hydrogen atoms of alkyl groups are omitted for clarity. Co‐crystallized acetonitrile solvent molecules are not shown. (c) Another view of the molecular structure showing half of the molecule.

Metal‐free activation of C─F bonds has attracted significant attention recently in light of the use of defluorination chemistry for the synthesis of fluorine‐containing compounds and environmental remediation [51]. Geometrically constrained phosphorus(III) compounds have been shown to activate C─F bonds of perfluoroarenes and promote defluorination reactivity [52, 53]. For non‐constrained phosphines, C─F bond activation normally involves higher reaction temperatures [54, 55, 56] or significantly activated bonds and stabilized products, for example, in the reactions of electron‐rich trialkylphosphines and B(C6F5)3 with the formation of zwitterionic phosphonium‐borates [57, 58, 59, 60]. The generation of the anionic phosphine 2 from 1 significantly increases its nucleophilicity, as can be demonstrated by the facile activation of (C6F5)2, C5NF5, and C6F5CN at room temperature. In contrast, the parent 1 shows no reaction with these substrates under the same conditions (1 h at room temperature in DME).

We probed the basicity of the phosphine group in 2 through the generation of its selenide. The magnitude of the J P–Se coupling constant in NMR spectra has been correlated with the s‐ character of the P─Se bond and the basicity of the parent phosphine [61, 62]. Reactions of triarylphopshines with selenocyanate salts have been documented to produce corresponding phosphine selenides in a facile manner [63]. The reaction of 2 and KSeCN at room temperature led to the formation of a single product according to 31P NMR spectroscopy, with two new singlets, one sharp at 44.2 ppm and another broad at 71.9 ppm. The single crystal structure of the product was determined to be K(dme)2[7‐P(nBu)tBu2‐10‐P(Se)iPr2‐nido‐C2B10H10] (Figure 5, 8·dme). Its structure features a rather long P═Se bond (2.154(1) Å) with potassium coordinated to the selenium atom (K–Se distance is 3.326(1) Å), as well as three B─H bonds of the cluster and two DME molecules. The fluxional coordination of the alkali metal cation to the anionic phosphine selenide for the solution of 8 in DME is likely the reason for the broadening of the 31P NMR signal at 71.9 ppm and the absence of observable 77Se satellites.

FIGURE 5.

FIGURE 5

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Synthesis and displacement ellipsoid plot (50% probability) of 8·dme. Hydrogen atoms are omitted for clarity.

To mitigate the influence of the cation, two equivalents of 12‐crown‐4 were added to the solution of 8 in DME, resulting in a slight shift and sharpening of the signal at 70.8 ppm in the 31P NMR spectrum (8·12‐crown‐4). The 77Se satellites were prominent, corresponding to the value of J P─Se = 649 Hz. While we have not been able to obtain single crystals of 8·12‐crown‐4, we note that the use of crown ethers has been reported to encapsulate alkali‐metal cations and prevent their direct bonding to strongly coordinating anions [64, 65]. The value of J P─Se for 8·12‐crown‐4 is lower than that for typical triaryl and trialkyl phosphines, indicating its enhanced basicity due to the negative charge of the boron cluster [62]. Interestingly, the magnitude of J P ─ Se for 8·12‐crown‐4 approaches the range for that of the most electron‐rich phosphazenyl and B‐carboranyl phosphines [66, 67]. These results corroborate the strong nucleophilicity of the phosphine group in 2 and its associated reactivity with fluoroarenes.

We explored some other anionic nucleophiles in reactions with 1 to probe the nucleophilicity required to reductively open the cluster. Of the reagents tested, sodium methoxide, potassium tert‐butoxide, and sodium azide did not produce the expected results. Each of these led to numerous decomposition products rather than the signals with known chemical shifts for –P(OR)tBu2 and –P(N3)tBu2, which we isolated in previous studies [21, 68]. However, tetraethylammonium cyanide in the presence of p‐xylyl bromide led to the formation of the target nido‐carboranyl diphosphonium zwitterionic compound 9 (Figure 6). In the crystal structure of 9, the open cluster adopts the same geometry as in 3. The C1─P1 bond (1.736(2) Å) in 9 is slightly shorter than that in 3. In addition, the P1─C3 bond to the cyano group lies in the relatively short range at 1.795(3) Å, while the C3─N1 bond is 1.149(3) Å. Notably, cyanophosphonium cations have been previously generated using electrophilic cyanation reagents and not the nucleophilic cyanide anion, which provides another example of the umpolung reactivity of 1 [69].

FIGURE 6.

FIGURE 6

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Synthesis and displacement ellipsoid plot (50% probability) of 9. Hydrogen atoms are omitted for clarity. One of the two crystallographically independent molecules in the asymmetric unit is shown.

3. Conclusion

In summary, we demonstrated that non‐constrained, trigonal phosphines exhibit clear electrophilic behavior when coupled to a redox‐active carborane cluster. Anionic nucleophiles selectively add to the PtBu2 group of 1, rather than the PiPr2 group, leading to reductive cluster opening to the dianionic nido‐ cage. This process constitutes direct experimental evidence of phosphine reactivity umpolung in this non‐constrained P(III) system. The resulting nido‐carboranyl phosphines are highly nucleophilic at the remaining PiPr2 arm, allowing further functionalization with electrophiles, such as alkyl halides, carbon disulfide, and electron‐deficient fluoroarenes. These results highlight the ability of redox‐active boron clusters to modulate the reactivity of main‐group centers through electronic control. The strategy described in this work may open new opportunities for designing main‐group systems that access ambiphilic reactivity profiles.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: The file contains additional experimental details, NMR spectra, and references. The authors have cited additional references within the Supporting Information [21, 70, 71, 72, 73, 74, 75, 76]. Single crystal structural data is available in Ref. [77].

CHEM-32-e71090-s002.pdf (2.5MB, pdf)

Supporting File 2: chem71090‐sup‐0002‐cif.zip.

CHEM-32-e71090-s001.zip (4.2MB, zip)

Acknowledgments

This material is based upon work supported by the National Science Foundation Award CHE‐2453775.

Data Availability Statement

The data that supports the findings of this study are available in the Supporting Information of this article.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supporting File 1: The file contains additional experimental details, NMR spectra, and references. The authors have cited additional references within the Supporting Information [21, 70, 71, 72, 73, 74, 75, 76]. Single crystal structural data is available in Ref. [77].

CHEM-32-e71090-s002.pdf (2.5MB, pdf)

Supporting File 2: chem71090‐sup‐0002‐cif.zip.

CHEM-32-e71090-s001.zip (4.2MB, zip)

Data Availability Statement

The data that supports the findings of this study are available in the Supporting Information of this article.

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