
==== Front
Nat Commun
Nat Commun
Nature Communications
2041-1723
Nature Publishing Group UK London

39256342
51506
10.1038/s41467-024-51506-3
Article
Couple-close construction of non-classical boron cluster-phosphonium conjugates
Sun Zhaofeng 1
Zong Jibo 1
Ren Hongyuan 1
Lu Changsheng 1
http://orcid.org/0000-0002-9685-6289
Tu Deshuang tudeshuang@126.com

1
http://orcid.org/0000-0002-0814-5074
Poater Jordi jordi.poater@ub.edu

23
http://orcid.org/0000-0002-1917-7450
Solà Miquel miquel.sola@udg.edu

4
http://orcid.org/0000-0003-4571-4413
Shi Zhuangzhi shiz@nju.edu.cn

1
http://orcid.org/0000-0003-3993-0013
Yan Hong hyan1965@nju.edu.cn

1
1 grid.41156.37 0000 0001 2314 964X State Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023 China
2 https://ror.org/021018s57 grid.5841.8 0000 0004 1937 0247 Departament de Química Inorgànica i Orgànica & IQTCUB, Universitat de Barcelona, Martí i Franquès 1-11, Barcelona, 08028 Spain
3 grid.425902.8 0000 0000 9601 989X ICREA, Pg. Lluís Companys 23, Barcelona, 08010 Spain
4 https://ror.org/01xdxns91 grid.5319.e 0000 0001 2179 7512 Institut de Química Computacional i Catàlisi, Universitat de Girona, C/ Maria Aurèlia Capmany, 69, Girona, 17003 Catalonia Spain
11 9 2024
11 9 2024
2024
15 793425 3 2024
9 8 2024
© The Author(s) 2024
2024
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Heteropolycyclic molecular systems, which are essential components in the fields of materials and pharmacology, frequently consist of 2D extended organic aromatic rings. Here, we introduce a type of inorganic-organic hybrid 3D conjugates by merging an aromatic boron cluster with a phosphine and a π-conjugated unit. To achieve this, a couple-close synthetic strategy via B–H activation of nido-carboranes with alkynes has been developed, which leads to diverse boron cluster-extended phosphoniums in a twisted structure with high yields under mild conditions. Experimental and theoretical results reveal that the fusion between the boron cluster and the formed borophosphonium heterocycle facilitates electron delocalization throughout the structure. The unusual framework demonstrates distinct properties from bare boron clusters and pure aromatic ring-extended counterparts, such as improved thermal/chemical stability and photophysical properties. Thus, the boron cluster-based 3D conjugates expand the library of aromatic-based heterocyclics, showcasing great potential in functional materials.

2D π-conjugated systems incorporated with heteroatoms can suffer from stability issues and aggregation-induced quenching effects. Here, the authors develop a methodology for the synthesis of cluster-ring-fused 3D aromatic heterocycles, which could find use in functional materials, via the palladium-catalyzed B–H activation of boron clusters.

Subject terms

Optical materials
Organic-inorganic nanostructures
Organic-inorganic nanostructures
Synthetic chemistry methodology
https://doi.org/10.13039/501100001809 National Natural Science Foundation of China (National Science Foundation of China) 92261202, 22025104 Yan Hong https://doi.org/10.13039/501100004608 Natural Science Foundation of Jiangsu Province (Jiangsu Provincial Natural Science Foundation) BZ2022007 Yan Hong the Ministry of Science and Technology (2021YFE0114800)Fundamental Research Funds for the Central Universities (2024300362)https://doi.org/10.13039/501100002809 Generalitat de Catalunya (Government of Catalonia) 2021SGR623 and 2021SGR442 Solà Miquel the Ministerio de Ciencia e Innovación of Spain (PID2020-113711GB-I00, PID2022-138861NB-I00, PID2019-106830GB-I00, CEX2021-001202-M)issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

Conjugated systems bearing π-electron delocalization have become indispensable elements in the fields of chemistry, biology, and materials science owing to their modifiable skeletons and structural diversity as well as valuable applications. By incorporating heteroatoms into π-conjugated systems, conjugated heterocyclic molecular architectures that possess distinctive physicochemical properties could be produced (Fig. 1a)1,2. In general, heterocyclic extension was elicited based on a planar π-conjugated system1–6 like BODIPY and Rhodamine. Nonetheless, the two-dimentional (2D) conjugated planar molecules frequently have issues such as photo/chemical instability and aggregation-induced quenching (ACQ) effect7, which impede their applications. In contrast, a conjugated heterocyclic system via the extension of three-dimensional (3D) skeleton is underexplored8. Boron clusters, a class of nanoscale entities, possess a 3D aromatic structure9,10, which have been extensively utilized in different fields9–24. We anticipate that the construction of a hybrid conjugated heterocyclic system via the extension of 3D aromatic boron clusters not only allows for the generation of a type of 3D aromatic cluster-extended heterocycles but also enables to overcome the issues associated with 2D planar heterocyclic systems (Fig. 1b). Therefore, the development of a methodology to produce such a class of heterocycles is valuable, yet remains unexplored.Fig. 1 Background and development.

a The introduction of a heteroatom to π-conjugated systems. b Three-in-one molecular design strategy for the construction of a boron cluster hybrid conjugate by merging a nido-carborane cage, a phosphine, and a π-conjugated unit (this study). c Palladium-catalyzed synthesis of 2D phosphacycles by C–P metathesis and C–P coupling. d This work: Pd-catalyzed P(III)-directed selective B–H activation of nido-carboranyl phosphines and cascade annulation with alkynes.

Phosphorus(P)-containing 2D heterocycles have garnered considerable attention from both academic and industrial communities due to their fascinating structures, prominent properties, and myriad applications25–34. However, the synthesis of P-doped heterocycles is much less developed, in sharp contrast to the overwhelming nitrogen-containing heterocycles, primarily because of their intrinsic instability32–34. In 2017, the Morandi group made a breakthrough by introduction of the palladium-catalyzed intramolecular ring-closing C–P bond metathesis (Fig. 1c)35. Subsequently, we presented a versatile method for the synthesis of PN-heterocycles where the central phosphorus atoms are derived from phosphanamines via palladium-catalyzed intramolecular C–P coupling of aryl halides36. Despite these advancements, the C–P coupling events in the reactions necessitated pre-installation of a leaving group35–37. It would be of significance in finding a facile strategy for straightforward construction of 3D aromatic carborane cluster-based phosphorus-containing heterocycles. The aromatic polyhedral carboranes, such as C2B10H12, a class of boron clusters, bear ten B–H bonds that share similar chemical environments and feature high bond dissociation energy (~100 kcal/mol)9. Consequently, the straightforward and selective activation of B–H bonds at a specific vertex poses a great challenge. Over the past decades, metal-mediation has been the main-stream strategy in the B–H functionalization of carboranes38–50. In particular, the combination of a directing group could target the metal center to activate a specific B–H bond via a cyclometallated species, presenting a streamlined approach for the precise functionalization of carboranes. This strategy offers an efficient means for constructing diverse carborane derivatives42,46–50. Despite these advances, the exploration of synthetic strategies for function-oriented carborane compounds remains of paramount significance.

In this work, we design a couple-close strategy that nido-carboranes bearing a C–PR2 motif were utilized to undergo cyclization with alkynes (Fig. 1d). A series of boron cluster-phosphonium-fused 3D conjugates have been produced through selective B–H activation and simultaneous formation of a C–B bond and a C–P bond. Notably, the pathway involves a rare dehydrogenation process under palladium catalysis, thereby avoiding the use of an exogenous oxidant. These are distinct from the reported carborane-based phospholes generated from a non-metal-catalyzed pathway and not involving B–H activation51,52. The formed 3D heterocycles show impressive thermal stability and chemical stability towards acid or base in air, which are distinct from the reported π-conjugated aromatic ring-based phosphoniums53. In sharp contrast to the photo-inert boron cage9, the cluster-based framework exhibits diversified emissive behaviors, including aggregation-induced emission (AIE), highly efficient color-tunable solid-state luminescence and aggregation-induced circular polarization luminescence (AICPL). The hybrid nature of this framework not only surpasses the properties of individual boron cages and 2D-conjugated phosphoniums but also broadens the range of 2D aromatic-based heterocycles. Furthermore, it exhibits great promise for applications in functional materials.

Results and discussion

Design principle

We initiated our scientific study on the activation of B–H bonds and the annulation process using nido-carboranyl phosphine (1a) and diphenyl acetylene (2a) as model substrates (Fig. 2a). Following a thorough screening process, we obtained the optimized conditions: 10 mol% of Pd(OAc)2 as catalyst and DCE as solvent under an argon atmosphere for a duration of 6 hours at a temperature of 35 oC. These curated conditions led to a yield of 83% of desired product of 3aa. Notably, the 11B NMR spectra of 3aa exhibited a diagnosable resonance with an evident downfield shift (δ = 4.2 ppm), indicating the formation of an exopolyhedral B–C bond. This proposed structure for 3aa was further confirmed by single-crystal X-ray diffraction (SC-XRD) analysis. When we explored the reaction with alternative transition metal catalysts such as Co(OAc)2, [Cp*IrCl2]2, or [RuCl2(p-cymene)]2, no product was generated. Furthermore, different bases or raising the temperature to 60 °C failed to increase yield. The indispensability of DCE as the solvent was underscored by the diminishing yields of 3aa when alternative solvents like THF and DCM were used owing to the solubility issue.Fig. 2 Reaction discovery and mechanistic studies.

a General reaction conditions: 1a (0.1 mmol, 1.0 equiv.), 2a (0.12 mmol, 1.2 equiv.), Pd(OAc)2 (10 mol%), DCE (2.0 mL), 35 oC, 6 h, Ar atmosphere, yield of isolated product. b HRMS detection of the palladacyclic intermediate A; synthesis and isolation of B(9) palladation intermediate B; verification of reactivity of B toward alkyne. c Proposed reaction mechanism. The hydrogen atoms including B–H–B bridging hydride are omitted for clarity.

Mechanistic investigation

Once the optimized reaction conditions were established, we conducted mechanistic studies to elucidate the reaction pathway (Fig. 2b). To detect the formation of a Pd–B intermediate, we initially used the catalyst Pd(PPh3)2Cl2 instead of Pd(OAc)2 to slow down the reaction. Such a strategy only led to the observation of a diagnostic HRMS signal for an intermediate A, which could further react with alkyne to yield 3aa. In order to isolate such a crucial intermediate, we designed a stoichiometric reaction by mixing 1b, Pd(OAc)2 and the chelating ligand of 1,10-phenanthroline. Fortunately, a yellow complex B was isolated in 70% yield, and characterized by SC-XRD, multinuclear NMR, and HRMS. The crystal structure revealed the formation of a type of four-membered Pd–B–C–P palladacycle, generated by P(III)-directed palladium-mediated B–H activation at B(9) site. The chelating coordination of 1,10-phenanthroline assists in stabilizing such a structure and makes this provable species isolatable. Complex B exhibits the further transformation toward alkyne to lead to 3ba in 37% yield. This observation implies that a four-membered-ring palladacyclic species should be an intermediate. It is worth noting that previous studies have demonstrated the B–H functionalization of carborane via the formation of a kinetically favored five- or six-membered cyclometalated intermediate with a transition metal40–42,46–50. Here, we obtained the crystal structure of a rare four-membered Pd–B–C–P cyclometalated intermediate for verification of both B–H activation and its subsequent transformation. This finding provides insights into both understanding the reaction mechanism and the design of B–H activation modes for carborane functionalization.

According to the above mechanistic studies and literature40–42,46–50,54–57, a plausible mechanism was proposed (Fig. 2c). The Pd(OAc)2 catalyst first binds to 1a and then leads to B(9)–H activation with the assistance of P(III)-directing to liberate the key intermediate I, which contains a cyclometalated Pd–B–C–P four-membered ring, similar to the detected or isolated intermediate A or B. Subsequently, ligand exchange between acetic acid and diphenyl acetylene occurs, yielding intermediate II. Next, followed by alkyne insertion into the Pd–B bond, intermediate III is inclined to undergo reductive elimination to give rise to the desired product 3aa and Pd(0) species. Lastly, the oxidation of Pd(0) by HOAc affords catalytic species Pd(OAc)2 and H2, as confirmed by GC analysis (Supplementary Fig. 50).

Scope study

Next, we set out to expand this catalytic strategy to the exploration of the variety of carboranyl phosphines with alkyne 2a (Fig. 3a). The introduction of aryl (3ba) or alkyl (3ca) moiety to the other carbon site of the carborane cage led to comparable outcomes with 3aa. Furthermore, the group of PR2 anchored at one carbon site of the carborane cage proved to be crucial in driving this transformation. For example, cyclohexyl (3da) and isopropyl (3ea) could provide the desired products in moderate yields. However, the reaction hardly occurred when tBu was employed owing to the large obstruction imposed by PtBu2. Nevertheless, the situation was released by replacing one of the tBu groups with a phenyl group (3ga). Notably, the introduction of an electron-donating group, such as a methoxy substituent, to the phenyl group (3 ha) greatly enhanced the efficiency of the reaction, leading to a yield as high as 91%. Conversely, when an electron-withdrawing group, specifically –CF3 (3ia) was introduced, a moderate level of yield (53%) was generated. These observations demonstrate the susceptibility of the alkyne cycle-closing process to the electronic and steric factors at the phosphorus site. Furthermore, the methodology was expanded to different types of alkyne substrates, encompassing a multitude of diverse substituents, generating the corresponding boron cluster-extended phosphoniums with impressive yields (Fig. 3b). Diarylacetylene substrates carrying electron-donating groups on the aryl rings, such as –CH3 (3ab), –OCH3 (3ac), –tBu (3ad), –NH2 (3ae) and –Ph2N (3af), gave rise to higher yields (82–96%). However, the reaction was severely influenced by the steric environment of the alkyne, as evidenced by the fact that diarylacetylene bearing a methyl group in the ortho-position yielded only a moderate yield of 3ag. The diarylacetylene hosting multiple methyl substituents (3ah) is compatible with this method. Interestingly, substrates featuring fluoro groups in ortho-, meta-, and para-positions of the aryl alkyne, respectively, exhibited satisfactory performance (3ai, 3aj, and 3ak) in this reaction, yielding the desired products in good yields (68–78%). The structure of compound 3ai was further confirmed through SC-XRD analysis. Moreover, if substrates bear electron-withdrawing groups in the para position, including –CN (3ao), –NO2 (3ap), and –CF3 (3aq), we were delighted to isolate the desired products in yields ranging from 66 to 79% (3al–3aq). It is worth noting that substrates with reactive functional groups, such as –NH2 (3ae, 95%) and –CHO (3an, 76%), exhibited good tolerance. These intriguing results manifest the influence of the electronic and steric nature of the alkyne on the reactivity.Fig. 3 Substrate scope.

General reaction conditions: 1 (0.1 mmol, 1.0 equiv.), 2 (0.12 mmol, 1.2 equiv.), Pd(OAc)2 (10 mol%), DCE (2.0 ml), 35 °C, 6 h, Ar atmosphere, yield of isolated product. aReaction time is 10 h. The hydrogen atoms of crystal structures are omitted for clarity. a Scope of carboranyl phosphines. b Scope of alkyne symmetrically substrates. c Scope of non-symmetrically disubstituted alkynes.

We conducted further investigations on alkynes bearing polycyclic and heterocyclic substrates, which also led to satisfactory yields (64–75%, 4aa–4ae). However, the alkyne featuring two strongly coordinated pyridinyl groups (4af) gave rise to a modest yield, which should be ascribed to the disturbance of pyridinyl group on catalyst. The dialkyl-substituted internal alkynes such as hex-3-yne, oct-4-yne, and 1,4-dimethoxybut-2-yne exhibited moderate reactivity, producing acceptable yields of 66% (4ag), 63% (4ah), and 70% (4ai), respectively. In an effort to expand the repertoire of boron cluster-extended phosphoniums, we delved into the realm of non-symmetrically disubstituted alkynes (Fig. 3c). We employed the phenylpropyl alkyne carrying a comparable steric effect for methyl and phenyl. As anticipated, two isomers, 5aa1 and 5aa2, were obtained with a ratio of 3:5, and their structures were also confirmed by SC-XRD analysis (Supplementary Figs. 5 and 6). Alkynes containing two groups with distinct steric hindrance such as trimethylsilyl and phenyl resulted in the formation of two isomers, namely 5ab1 and 5ab2, with a ratio of (5:1). The larger trimethylsilyl group displays a tendency to position itself far away from the phosphonium unit. The precise structures of 5ab1 and 5ab2 were confirmed by SC-XRD analysis, aligning with the proposed structural formulation (Supplementary Figs. 7 and 8). In addition, we also examined electron-rich acetylenes (5ac1–5ad2), which reacted and yielded two isomeric products. Interestingly, terminal alkynes featuring aryl (5ae–5ag) and triisopropylsilyl (5ah) groups showed exclusive regio-selectivity in this annulation event owing to the sensitivity of annulation on steric hindrance.

Synthetic applications

The compatibility of the reaction prompted us to explore its practicality (Fig. 4). Firstly, we incorporated photo-functional groups into the carboranyl phosphine (1a) by employing this synthetic strategy. Materials science has witnessed great interest in carbazole-based systems possessing pure π bonding structures owing to their one-of-a-kind light-emitting properties. To obtain carborane-based conjugates, the alkyne substrate 4,4’-bis(N-carbazoly)tolan (BCT) was used to lead to 6aa in a yield of 82%. Notably, BCT suffered from the ACQ effect, however, the conjugate 6aa exhibited distinct AIE property and showed high luminous efficiency of an absolute quantum yield of 70% in the solid state (Supplementary Figs. 19–21). Based on the same synthetic strategy, the acridine group was introduced to the carborane-based phosphine (1a) to generate two isomers (6ab1 and 6ab2). Secondly, we extended this methodology to the late-stage modification of drug molecules, such as Erlotinib, a small-molecule epidermal growth factor receptor tyrosine kinase inhibitor. Under the standard conditions, Erlotinib could directly react with 1a to afford the exclusive drug-modified molecule 6ac, which may be potential in the targeting drug for lung cancer on the basis of boron neutron capture therapy (BNCT)58,59. This protocol was also applicable to the modification of the drug tazarotene to produce 6ad1 and 6ad2. To our delight, dibenzocyclooctyne-COOH (DBCO-COOH), a classical reagent studied in biorthogonal chemistry, could be coupled with 1a to generate 6ae in a yield of 61%. Notably, 6ae is not only an instance bearing a large pentacyclic conjugation but also provides the possibility for further applications in materials and bioconjugation for BNCT owing to the reserved reactive carboxylic acid. Moreover, when 1a reacted with a polyalkyne such as 1,4-bis(phenylethynyl)benzene, 6af was produced containing two carborane-fused heterocycles, whose crystallographic structure was confirmed by SC-XRD. Thus, this protocol has indicated valuable application potential in materials and drug development.Fig. 4 The modification of functional molecules.

General reaction conditions: 1a (0.1 mmol, 1.0 equiv.), 2 (0.12 mmol, 1.2 equiv.), Pd(OAc)2 (10 mol%), DCE (2.0 mL), 35 °C, 6 h, Ar atmosphere, yield of isolated product.

Structural study

Next, we investigated the structural characteristics of the boron cluster-fused phosphoniums as they represent a type of framework bearing a B, P-containing heterocycle. Accordingly, the crystal structures of 3aa and the control compounds nido-K and Model-1 were carefully studied (Fig. 5a, b). 3aa shows a folded molecular structure with a dihedral angle of 35.07 (6)o between the generated five-membered ring (plane I) and the C2B3 plane of nido-carborane (plane II). In sharp contrast, Model-1 shows a nearly planar bicyclic structure with a dihedral angle of 2.05 (5)o. Such a large difference could be attributed to the  3D steric effect of the carborane cage. Furthermore, the carborane fusion also contributes to the structural deformation in comparison to Model-1 and nido-K. The size of the five-membered ring P1C1C2B9C8 in 3aa is larger than the corresponding P1C1C2C3C4 ring in Model-1, as demonstrated by the selected bond lengths (Fig. 5b). For example, B9–C8 (1.616 (3) Å) in 3aa is much longer than C3–C4 (1.398 (2) Å) in Model-1, and C2–B9 (1.576 (3) Å) in 3aa vs C2–C3 (1.447 (2) Å) in Model-1 is longer, which could be interpreted by the larger atomic radius of boron than carbon. In the case of planes II, in general, the deformation in 3aa vs nido-K is much less.Fig. 5 Structural investigations.

a Crystal structures of 3aa, nido-K, and Model-1. Insets are the selected dihedral angles between planes I and II. b The selected bond lengths for planes I and the planes II of 3aa and the control compounds. The estimated standard deviation values are shown in parentheses. The H atoms in crystal structures are omitted for clarity. c Left, Comparison of 3aa and Model-1 enclosing geometries with NICS (top value, in ppm) and MCI (bottom value, in au); Middle, VDD charges (in a.u.); and right, molecular electrostatic potential isosurfaces (electronic density isovalue = 0.03 a.u.). d The chemical stability comparison of 3aa and the control compounds.

To further understand the electronic structure of the fused system, DFT computations at the ZORA-BLYP-D3(BJ)/TZ2P were performed on 3aa, 1a, and Model-1 (see Supplementary information for additional computational details). The charges were evaluated with the Voronoi Deformation Density (VDD) method, but other methods gave the same trends (Supplementary Fig. 58). Firstly, the computed equilibrium geometries are in good agreement with the X-ray data (Fig. 5c and Supplementary Fig. 56). The carborane cage keeps its aromaticity when fused to the five-membered heterocycle (magnetic-based NICS in the center of the cage is –22.0 for 1a and –19.8 ppm for 3aa, respectively). The aromaticity of the cage is further confirmed by Wade-Mingos’ rule with a total of 26 valence electrons, thus following the 2·(11 + 2) rule for a nido cluster with 11 vertices. At difference, the B, P-heterocycle in 3aa is non-aromatic as indicated by both NICS (–1.9 ppm) and electronic-based MCI (0.001 a.u.), due to the localization of the 2π-electrons in the C–C double bond in this ring, with the P atom not allowing for the delocalization (Fig. 5c and Supplementary Fig. 52). Next, evaluation of the charges shows an important charge transfer from carborane to the heterocycle in 3aa (0.512 e) compared to Model-1 (0.170 e). Despite the charge transfer, the P atom remains positively charged, in part due to the electron-withdrawing phenyl substituents (Supplementary Figs. 53–58). The charge transfer due to the formation of the fused ring can be observed through the electrostatic potential isosurfaces (Fig. 5c).

Study of properties

Then we examined the stability of the fused framework. 3aa shows a higher decomposition temperature of 323.6 °C according to thermogravimetric analysis in comparison to 273.6 °C for Model-1 (Supplementary Fig. 18). In particular, 3aa demonstrates satisfactory chemical stability towards acid or base in air (Fig. 5d and Supplementary Figs. 12–17). However, the 2D aromatic ring-based phosphoniums of Model-1 and Model-2 underwent facile ring-opening via C–P bond cleavage53. This is attributed to the more reduced electron density at P site in 3aa vs Model-1 (Fig. 5c). Therefore, the 3D aromatic nido-carborane enables to stabilize P-containing heterocycle relative to the corresponding 2D π-conjugated aromatic system.

Now we turn to investigate the photophysical properties associated with such a fused framework. Firstly, the UV-Vis spectra reveal an absorption peak around 370 nm for 3aa in contrast to 1a (Fig. 6a). This should be attributed to the charge transfer transition, as demonstrated by the DFT calculations (Supplementary Fig. 59). Thus, the electronic communication between the 3D carborane and the B, P-heterocycle was further experimentally confirmed. The computed UV-Vis spectrum is also consistent with the measured one. In contrast, 3aa shows undetectable emission in solution in sharp contrast to the strong luminescence observed for Model-1 (Fig. 6b). This must be caused by the facile molecular motions in the excited state triggered by the folded structure in contrast to a rigid aromatic structure in Model-1, thus leading to severe non-radiative decay in solution7,60,61. To our delight, 3aa exhibits typical AIE effect. As shown in Fig. 6c, no emissions were observed when the water fractions (fw) for the THF/water mixtures were less than 70%. When fw was reached to 80%, a faint emission could be observed due to the initial formation of molecular aggregates. At fw = 90% the greenish-yellow luminescence showed a sharp rise (Fig. 6d). This turn-on emission behavior should be attributed to the restriction of molecular motion caused by molecular aggregation7,60,61. On the other hand, the bulky size of carborane and the twisted molecular structure of the framework can effectively inhibit the π···π interactions in the aggregate state, thus further improving the aggregate-state luminescence62–67. To quantify the AIE effect, the αAIE = ϕ (fw = 99%)/ϕ (fw = 0%) value was used by measuring the quantum yields in THF solution and 99% water fraction, respectively. As a result, the αAIE value reached 3000 for 3aa, surpassing many classical 2D π-conjugated AIE systems7,60,61. Other carborane-based phosphoniums in this study exhibited more conspicuous AIE performance (Fig. 6e, and Supplementary Figs. 22–27, 34–45), in particular, 5aa2 showed αAIE > 7000. Hence these AIE molecules might be useful in the design of advanced molecular systems for bioimaging applications. Moreover, in the solid state, the color-tunable emissions ranging from blue to red have been achieved in the framework by simply altering the electronic effect of the substituents on the alkyne aryl groups, demonstrating the electronic sensitivity of the core framework on periphery environment (Fig. 6f and Supplementary Figs. 28, 29). The solid-state luminescence efficiency could reach up to 70% (3ad), in sharp contrast to the faint emission in Model-1 (Supplementary Figs. 22, 30–33) and non-emissive 1a. The distinctive photophysical phenomena between the 3D aromatic boron cluster-based phosphoniums and the 2D aromatic π-conjugated phosphoniums (i.e., Model-1) are attributed to the unusual electronic and geometric structures of the framework. These properties may have promising potentials in photo-functional materials.Fig. 6 Photophysical properties of the selected compounds.

a The UV/Vis absorption spectra in THF solution (c = 10 μM). b The photoluminescence (PL) spectra in THF solution (c = 10 μM, λex = 380 nm). c The emission intensity of 3aa in a THF/H2O mixture with increased H2O volume fraction (fw) to 99% (c = 10 μM, λex = 380 nm). d Relative emission intensity of 3aa. e The αAIE of 3aa, 3ac, 4ad, 5aa2, 3ba, 3ea and Model-1. f The solid-state luminescence of the selected products. Insets are the absolute luminescence efficiency, CIE (Commission Internationle de l’Eclairage) coordinates, and emission peaks. g The chiral crystal structures of 3aa-Rp and 3aa-Sp. h The CD spectra of 3aa-Rp and 3aa-Sp in THF. i The CPL spectra of 3aa-Rp and 3aa-Sp in PMMA with a weight ratio of 1:100.

Moreover, carborane clusters are an unconventional source for chirality investigations, including both cage and planar chirality. Until now, boron cluster-based CPL molecules are rarely documented68,69. In this study, the carborane-fused phosphoniums provide a promising avenue to investigate boron cluster-based CPL molecules. Interestingly, we have successfully separated two chiral carborane-fused phosphoniums, 3aa-Rp and 3aa-Sp, through chiral high-performance liquid chromatography, as confirmed by SC-XRD analysis (Fig. 6g). Their CD spectra showed a mirror-image relationship with the alternating positive and negative Cotton effect (Fig. 6h). Surprisingly, no CPL was observed for 3aa-Rp and 3aa-Sp in solution (Supplementary Fig. 46). However, in a PMMA film they exhibited enhanced CPL with a photoluminescence dissymmetry factor |gPL| of 9.9 × 10–3 (Fig. 6i), indicating an aggregation-induced enlargeable CPL property. Such a high value of |gPL| represents impressive performance among the reported CPL-active organic small molecules. Additionally, we have observed an unexpected phenomenon of isomer-dependent luminescence, detailed in Supplementary Figs. 62–64. These results also indicate the potential to develop a type of luminescent materials using the 3D boron cluster-fused framework.

We have demonstrated a couple-close synthetic strategy to obtain a type of 3D aromatic boron cluster-fused phosphoniums through B–H activation of carboranyl phosphines with alkynes. The dehydrogenative process avoids the use of extra oxidant during the reaction. Mechanistic studies have revealed a rare four-membered palladacyclic intermediate, which is distinct from the previously reported five/six-membered cyclometalated intermediates for B–H activation. The resulting type of conjugated framework shows high thermal and chemical stability in contrast to 2D aromatic ring-based analogs, in particular, interesting photophysical properties, such as unusual AIE effect with a αAIE value of up to 7000, readily tuned emission color spanning the entire visible light region and highly efficient solid-state emission with Φ up to 70%, as well as AICPL with a gPL value of 1.0 × 10–2. This study is devoted to developing synthetic strategies, gaining structural insights, and exploring valuable physicochemical properties of the 3D boron cluster-based heterocyclic systems. We hope these results would expand scopes of both boron clusters and heterocycles and demonstrate the application prospect in materials and drug development.

Methods

General procedure for the synthesis of boron cluster-phosphonium-fused 3D conjugates

Carborane substrate 1 (0.1 mmol), alkyne substrate 2 (0.2 mmol), and Pd(OAc)2 (0.01 mmol) were added into a 10 mL Schlenk tube under argon atmosphere. Then, anhydrous DCE (2.0 mL) was added via a syringe under argon atmosphere. The stirring was maintained at 35 °C for 6 hours. Upon completion, as monitored by TLC, the reaction mixture was concentrated under vacuum. The crude mixture was purified on silica gel column chromatography (eluting with DCM/petroleum) to afford the desired products.

Supplementary information

Supplementary Information

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Supplementary information

The online version contains supplementary material available at 10.1038/s41467-024-51506-3.

Acknowledgements

This work is supported by the Ministry of Science and Technology (2021YFE0114800), the National Natural Science Foundation of China (92261202 and 22025104), the Natural Science Foundation of Jiangsu Province (BZ2022007), the Fundamental Research Funds for the Central Universities (2024300362), Henan Normal University, TaiShan Industrial Experts Programme, the Ministerio de Ciencia e Innovación (MCIN/AEI/10.13039/50110001103, projects PID2020-113711GB-I00, PID2023-147424NB-I00, PID2022-138861NB-I00, and CEX2021-001202-M), and the Generalitat de Catalunya (2021SGR623 and 2021SGR442). The high-performance computing center of Nanjing University is acknowledged.

Author contributions

Z.F.S. and J.Z. performed the experiments and analyzed the data. J.P. and M.S. performed the DFT calculations. C.L. supervised the mechanistic study. H.R. performed the crystallographic studies. D.T., Z.Z.S. and H.Y. conceived and designed the research.

Peer review

Peer review information

Nature Communications thanks the anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

All data that support the findings of this study are available within the paper and its supplementary information files, and also available from the corresponding author upon request. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers CCDC 2309466−8, 2309620, 2309541, 2309569, 2309570, 2309590, 2309651, 2327211, 2327213, 2337774, 2358184. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. All other data supporting the findings of the study, including experimental procedures and compound characterization, are available within the paper and its Supplementary Information, or from the corresponding author upon request. Coordinates of the optimized structures are provided in the source data file. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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