A diazadiphospholenium cation featuring a reactive P[double bond, length half m-dash]P bond: synthesis and reversible main-group bond activation

Abstract

We report a high-yielding synthesis of a novel diazadiphospholenium cation featuring a sterically exposed, reactive P[double bond, length half m-dash]P bond. It displays aromatic properties and greater reactivity and selectivity than its parent diphosphene, enabling selective [4+2] cycloaddition and unusual, reversible E–E bond scission in diphenyldichalcogenides, highlighting its potential for main-group bond activation.

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Organic chemistry is dominated by the versatile bonding modes of carbon, particularly its ability to form multiple bonds to other carbon- or heteroatoms. In comparison, multiple bonds between heavier main-group elements are far less common as they are challenged by less efficient orbital overlap, diminished hybridisation and core–core repulsion, resulting in higher reactivity.^1–3 The pioneering isolation of compounds containing formal multiple bonds between P,⁴ Si,⁵ Sn,⁶ and S,⁷ has led to the synthesis of a wide range of formally multiple-bonded heavier main-group species, many of which have shown great potential in bond activation processes.^8–12

The generation of such highly reactive multiply-bonded species usually necessitates the incorporation of sterically demanding substituents, providing kinetic stabilisation. Insufficient steric protection generally results in oligo- or polymerisation (cf. cyclic single-bonded (PhP)₅ and P[double bond, length half m-dash]P double-bond-containing Mes*₂P₂) (Mes* = 2,4,6-tritertbutyphenyl).^4,13,14 A distinct class of low-valent, formally multiply-bonded phosphorus species is found in respective heterocycles. These compounds can exhibit aromatic properties comparable to those of their carbon analogues. Five-membered heterocycles isolobal to the cyclopentadienide anion (Cp⁻) are of particular relevance to this work.¹⁵ The aromatic pentaphospholide anion, for example,¹⁶ has been synthesised in solution as the respective alkali metal [cyclo-P₅]⁻ salts,¹⁷ and has been employed as a 6-π aromatic ligand in organometallic chemistry (e.g. pentaphospaferrocene, [η⁵-P₅]Fe[η⁵-(CH)₅]).^18,19

The isolobal substitution of four “CH” fragments in Cp⁻ with two “N” and two “P” units results in diazadiphospholes.²⁰

Three examples of diazadiphospholes containing a direct P–P connection are known to date (see Fig. 1a). Mathey and coworkers reported the [3+2] cycloaddition between lithium (trimethylsilyl)-diazomethanide and white phosphorus to obtain heterocyclic anion I⁻ in 1996.²¹ Twenty-five years later, Cummins and coworkers described the synthesis of heterocyclic anion II⁻ by [3+2] cycloaddition between dibenzo-7-phosphabornadiene-substituted diazene with phospha-ethynolate.²² Recently, the groups of Wolf and Hansmann reported the [3+2] cycloaddition between white phosphorus and diazoalkenes bearing 1,2,3-triazole or imidazole substituents, affording two neutral diazadiphospholes III.²³ All of these heterocycles, in contrast to the above-described concepts, share a sterically exposed P–P moiety. Furthermore, the interatomic distances in the structurally authenticated examples II⁻ and III attest to this P–P unit's significant double-bond character. Evidently, the delocalisation in these heterocycles provides significant thermodynamic stabilisation, allowing for their isolation. The further reactivity of these compounds has only scarcely been investigated. Protonation of the structurally uncharacterized I⁻ yields a neutral 2H-1,2,3,4-diazadiphosphole I-H,²¹ while III was shown to coordinate to main-group Lewis acids via an endocyclic nitrogen atom.²³ We therefore became interested in synthesising a yet unknown 1,4,2,3-diazaphospholenium cation featuring a similarly sterically exposed, formal P[double bond, length half m-dash]P double bond to explore the untapped reactivity of this compound class concerning cycloaddition and E–E bond activation reactions.

For the synthesis of the targeted cation, we took inspiration from a report by Jones and coworkers.²⁴ Here, the reduction of dichloropnictogen-amidinates/guanidinates led to the isolation of amidinato- or guanidinato-bridged diarsenes (IV, Fig. 1b). The removal of one of the ligands from such species should result in heterocyclic cations as relatives to the herein targeted diazadiphospholenium cations. However, in the same report, the synthesis of phosphorus analogues of IV was attempted but proved unfruitful. For the synthesis of the diphosphene precursor, we adapted a synthesis described by Schulz and coworkers for the generation of a masked phosphinidene (V, Fig. 1b),²⁵ albeit using a much less sterically crowded amidinate ligand to facilitate dimerisation of the intermediary-generated phosphinidene. Hence, a solution of PCl₂(NCN^Dipp,Tol) (NCN^Dipp,Tol = [(DippN)₂(CTol)], Dipp = 2,6-diisopropylphenyl) (1) in THF was added to magnesium turnings and sonicated (35 kHz) for 48 h with concomitant warming of the sonicator bath to 50 °C. NMR spectroscopy revealed complete consumption of starting material 1, accompanied by formation of a new species exhibiting triplet splitting of its ³¹P NMR resonances at δ(³¹P) = −81.9 and −321.2 ppm (Fig. 2a). Single-crystal X-ray diffraction (scXRD) analysis (see Fig. S7; SI), revealed a P₄-butterfly motif bound by two amidinate ligands at the wing-tip phosphorus atoms (2). The structural metrics are similar to previously reported P₄-butterflies bearing amido ligands.²⁶ Proton NMR spectra of the isolated crude product revealed the presence of two further amidinate-containing yet ³¹P NMR silent species, which were tentatively assigned to MgCl(NCN^Dipp,Tol) and Mg(NCN^Dipp,Tol)₂. One of the two species, presumably the former, was removed from the product mixture dissolved in THF by washing with degassed water and extraction with Et₂O. Yet, the second impurity could not be removed. As Mg⁰ was overreducing to form 2 instead of the targeted bisamidinato-diphosphene 3, we investigated different reducing agents: Surprisingly, KC₈ did not show appreciable reactivity with 1 in THF, benzene or hexane. The dimeric Mg(I) compound {NacNac^DippMg}₂ cleanly and swiftly forms a single new resonance at δ(³¹P) = 402.5 ppm, closely related to the chemical shift for a masked amidinato-phosphinidene as reported by Schulz and coworkers (cf.Vδ(³¹P) = 407.9 ppm²⁵). However, the reaction byproduct NacNac^DippMgCl has proven difficult to remove. Analytically pure samples of 3 were obtained when the mild reducing agent CoCp₂ was employed, as the formed [CoCp₂]Cl is quantitatively precipitated during the reaction in n-hexane (Fig. 2b) while 3 readily crystallises from the reaction mixture at −35 °C. The obtained solid-state molecular structure of 3 (see Fig. S12; SI) shows structural metrics in line with those of previously reported diphosphenes bearing C- or N-bound ligands.^4,27 Notably, the bicyclic structure observed in IV is not present in 3, yet it seems that there is a weak interaction (d(P1–N4) = 241.2(1) pm) between one of the imine moieties and the respective more distal phosphorus atom.

Fig. 1 (a) Literature known diazadiphosphole derivatives containing P–P moieties. (b) Relevant literature examples for the synthesis of targeted 1,3,2,4-diazadisphospholenium cations.

Fig. 2 Synthesis of (a) amidinato-P₄ butterfly 2, (b) amidinato-coordinated diphosphene 3, (c) and (d) synthesis of diazadiphospholenium cation via ligand abstraction from 3 or 2-electron oxidation of 2 respectively. (e) Reduction of 4⁺ yielding 2. Solid-state molecular structure of 4⁺, showcasing (f) PPNCN heterocycle and (g) closest counter-ion contact as well side-on view emphasizing planarity of the heterocycle as well as steric exposure of central P[double bond, length half m-dash]P unit. (f) Dipp and Tolyl moieties drawn as wireframe and counter-ion [OTf]⁻ omitted. (f) and (g) protons omitted and thermal displacement ellipsoids drawn at 50% probability.

The targeted diazadiphospholenium cation 4⁺ is successfully generated by a reaction between 3 and a Lewis-acidic (LA⁺) reagent (e.g. Li[AlOR^F₄], [CPh₃][SbF₆], [CPh₃][BAr^F₄]; OR^F = OC(CF₃)₃, Ar^F = C₆F₅) and is stable with these weakly coordinating anions (WCA). The most convenient synthesis method involved using TMS-OTf as the ligand-abstracting agent (Fig. 2c). The formed N-silylated amidinate is facilely removed by trituration of the reaction mixture with n-hexane, providing access to analytically pure samples of 4[OTf] in 91% yield (378 mg, 0.389 mmol). This synthesis was successfully scaled up to receive 800 mg of analytically pure 4[OTf] with a still very good percentage yield of 77%. Alternatively, 4⁺ can also be synthesised by the two-electron oxidation of 2 with [FeCp₂][BAr^F₄] or less selectively [CPh₃][AlOR^F₄] (Fig. 2d), presumably via a retro-cycloaddition in the intermediary formed 22⁺. Vice versa, the reduction of 4⁺ with CoCp₂ results in the formation of 2 (Fig. 2e).

³¹P NMR spectroscopic analysis of 4[OTf] shows a singlet resonance at δ = 386.4 ppm, while collected ¹H NMR spectra attest to the high symmetry of the generated cation. The nature of the counter ion subtly influences the electronic environment at phosphorus, which is reflected in minor variations of the ³¹P NMR chemical shift (Δδ = −4.7 to −2.4 ppm) depending on the employed 4[WCA] salt (WCA = [AlOR^F₄]⁻, [BAr^F₄]⁻, [SbF₆]⁻). The by scXRD analysis obtained solid-state molecular structure of 4[OTf] is shown in Fig. 2(f) and (g). The P1–P2 interatomic distance of d = 205.68(6) pm agrees with presence of a P[double bond, length half m-dash]P double bond (cf. Mes*P[double bond, length half m-dash]PMes* d(P–P) = 203.4(2) pm⁴, I-Hd(PP) = 207.06(6) pm²¹) and is significantly shorter than in structurally characterised 1,2,3,4-diazadiphospholes II⁻ (d(P–P) = 210.29(6) pm²²) and III (d(P–P) = 208.68(6) to 209.27(14) pm²³). The observed C–N and P–N interatomic distances (d(C–N)_av. = 134.3 pm and d(P–N)_av. = 174.2 pm) fall between typical single and double bond lengths for these atom pairs (CN: 146 pm and 127 pm; PN: 182 pm and 162 pm).²⁸ This suggests partial double-bond character and delocalisation within the planar ring. The performed DFT calculations and determination of NICS(1) = −6.9 ppm and NICS_zz(1) = −20.6 ppm of 4⁺ values equally suggest delocalisation and aromatic properties comparable to II⁻ but less pronounced than in I⁻ and III (see Section S5; SI).

The flanking diisopropylphenyl groups are arched backwards (∢ P–N–C 116.6(1)°, see Fig. 2g), leaving the P–P unit somewhat sterically exposed (for a more detailed analysis of the steric encumbrance and the buried volume see Section S6, SI).^29–31 The closest contact between the triflate counter ion and a non-H atom in 4⁺ is with d(P1–O2) = 273 pm, 100 pm longer than the sum of the covalent radii,²⁸ indicating at most a weak interaction.

Initial investigations into the reactivity of 4⁺ focused on non-polar E–E and E[double bond, length half m-dash]E bond-containing substrates. Accordingly, 4⁺ was reacted with 2,3-dimethylbutadiene in o-DFB at ambient temperature, resulting in the immediate formation of a single product resonating at δ(³¹P) = 66.0 ppm. The solid-state molecular structure obtained by scXRD analysis (Fig. 3g) shows the expected cycloaddition product (5⁺[OTf]), with a P–P single bond (d(P1–P2) = 220.95(7) pm). Comparative reactivity studies were performed with the parent diphosphene 3, revealing a significantly slower and less selective reaction towards the corresponding cycloaddition product, among other species (see Section S4, SI). We then wondered if 4⁺ could function as a P₂ transfer agent. A two-electron reduction of 5⁺ could result in the release of an elusive dihydrodiphosphinine, trappable by further [4+2] cycloaddition with a suitable substrate. Hence, isolated 5⁺ was reacted with 2 equiv. of CoCp₂ in the presence of 2,3-dimethylbutadiene (see Fig. 3b). The collected ³¹P NMR spectra indeed show the formation of minor quantities of the literature-known double cycloaddition product 6 resonating at ³¹P NMR = −54.0 ppm.³² The major product, however, showed a more complex AA′MM′ splitting pattern with corresponding multiplets centred at δ(³¹P) = 58.2 ppm and −33.78 ppm. The obtained solid-state molecular structure (Fig. 3h) revealed the formation of a catenated tetraphosphane 7, presumably as the result of the dimerisation of the neutral radical 5˙. Vice versa, oxidation of 7 with 2 eq. of Ag[SbF₆] reforms 5⁺ (Fig. 3c).

Fig. 3 (a) Reactivity of 4[OTf] towards 2,3-dimethylbutadiene furnishing the cycloaddition product 5[OTf]. (b) Subsequent reduction of 5[OTf] in presence of 2,3-dimethylbutadiene results in dicycloaddition product 6 as the minor and dimerization product 7 major compound. (c) Oxidation of 7 results in the reisolation of 5⁺. (d) Reactivity of 4[OTf] towards diphenyldichalcogenides PhCh–ChPh (Ch = S, Se) with formation of corresponding addition products 8[OTf] and 9[OTf]. The addition was found to be reversible for Ch = Se which is further shown by (e) formation of 5[OTf] from 9[OTf] upon addition of 2,3-dimethylbutadiene. Solid state molecular structures of (f) 9[OTf], (g) 5[OTf] and (h) 7. Protons and counter ion [OTf]⁻ are omitted and Dipp and Tolyl moieties (f) and (g) or amidinate ligand (h) are drawn as wireframe for clarity. Thermal displacement ellipsoids are drawn at 50% probability.

Next, 4⁺ was reacted with PhCh–ChPh (Ch = S or Se). In both cases, 4⁺ is selectively transformed into novel species 8⁺ (Ch = S) and 9⁺ (Ch = Se) resonating at δ(³¹P) = 105.2 ppm and 106.0 ppm (¹J_P–Se = 171.0 Hz; ²J_P–Se = 24.2 Hz) respectively (Fig. 3d). The solid-state molecular structure of 8[OTf] (Fig. 3f) shows successful splitting of the S–S bond over the two phosphorus centres, with concomitant oxidation of the P[double bond, length half m-dash]P double bond (d(P1–P2) = 220.62(6) pm). The obtained NMR spectroscopic data for 9[OTf] showed 100% turnover within minutes at ambient temperature and strongly imply a structural relation with 8⁺. Yet crystallisation attempts only resulted in the reisolation of starting material 4[OTf] in 60% crystalline yield. This was seen as an indication for a reversible addition of PhSe−SePh. To test this hypothesis, we repeated the synthesis of 7⁺ and monitored complete turnover by ³¹P NMR spectroscopy. Next, 2,3-dimethylbutadiene was added to the NMR sample, resulting in the formation of 5⁺ over time, which proves the reversible activation of PhSe−SePh. We subsequently followed a similar protocol for the disulfide product 8⁺ and observed some conversion to 5⁺ in the presence of 2,3-dimethylbutadiene after heating to 90 °C. The regeneration of delocalisation upon reductive elimination of the dichalcogenides likely provides the driving force for this process. It could potentially prove to be a generalisable concept for this compound class in the reversible activation of other substrates. In contrast, neutral diphosphene 3, again reacts much more sluggishly and less selectively with both substrates, giving a broader product scope (see Section S4; SI).

In conclusion, we have developed a synthetic protocol that provides facile, high-yielding access to a novel diazadiphospholenium cation. The formal P[double bond, length half m-dash]P double bond contained herein is sterically exposed and exhibits high reactivity, surpassing its parent neutral diphosphene in both reactivity and selectivity of the formed products. The heterocyclic cation was successfully employed in a model [4+2] cycloaddition with 2,3-dimethylbutadiene. Furthermore, we were able to demonstrate that subsequent reduction of the obtained cycloaddition product could provide access to elusive carbo-cyclic diphosphenes, which are subsequently trapped by a [4+2] cycloaddition reaction. This proof-of-principle transformation is currently under investigation for optimisation (i.e., suppression of dimerisation product), extension of scope, and the generality of this method. The diazadiphospholenium cation exhibits reactivity towards apolar E–E bonds in diphenyldisulfide and -diselenide. Both of which were shown to be somewhat or entirely reversible processes, respectively. This could potentially enable catalytic transfer of the Ch–Ph moiety to a suitable substrate. In this context, preliminary investigations have suggested the transformation of phenylacetylene and diphenyldiselenide in the presence of the heterocyclic cation into a yet-to-be-identified product. This, as well as the activation of other substrates, is under current investigation in our group.

We thank Prof. Dr. Simon Aldridge and Prof. Dr. Robert Wolf for their invaluable support and guidance. Funding from the Deutsche Forschungsgemeinschaft (DFG) RTG 2620 - 426795949 and the Fonds der chemischen Industrie (Material Allowance Grant) is gratefully acknowledged. The Leibniz Supercomputing Centre is gratefully acknowledged for providing computing time on its Linux cluster.

J.W.: investigation (experiment design and execution), validation, data curation, writing – original draft, F.C.: investigation (DFT-calculations), M.S.: conceptualisation, supervision, writing – review and editing, funding acquisition.

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/d5cc04820f.

CCDC 2480362 (2), 2480363 (3), 2480364 (4[OTf]), 2480365 (5[OTf]), 2480366 (7), 2480367 (8[OTf]) and 2480368 (S2) contain the supplementary crystallographic data for this paper.^33a–g

Notes and references

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33. (a) CCDC 2480362: Experimental Crystal Structure Determination, 2025 DOI:10.5517/ccdc.csd.cc2p80pp; (b) CCDC 2480363: Experimental Crystal Structure Determination, 2025 DOI:10.5517/ccdc.csd.cc2p80qq; (c) CCDC 2480364: Experimental Crystal Structure Determination, 2025 DOI:10.5517/ccdc.csd.cc2p80rr; (d) CCDC 2480365: Experimental Crystal Structure Determination, 2025 DOI:10.5517/ccdc.csd.cc2p80ss; (e) CCDC 2480366: Experimental Crystal Structure Determination, 2025 DOI:10.5517/ccdc.csd.cc2p80tt; (f) CCDC 2480367: Experimental Crystal Structure Determination, 2025 DOI:10.5517/ccdc.csd.cc2p80vv; (g) CCDC 2480368: Experimental Crystal Structure Determination, 2025 DOI:10.5517/ccdc.csd.cc2p80ww.

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