Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
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Zwitterionic nido-Carborane-Fused Phospholes Guanyu Tao, Zheng Duan,* and Francois Mathey College of Chemistry and Molecular Engineering, International Phosphorus Laboratory, International Joint Research Laboratory for Functional Organophosphorus Materials of Henan Province, Zhengzhou University, Zhengzhou 450001, P.R. China
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S Supporting Information *
ABSTRACT: Using readily available and easy-to-handle reagents, a novel type of nido-carborane-fused phosphole was obtained from a simple procedure. Its crystallographic structure was determined, and DFT calculations have been performed, unraveling the key influence of the zwitterionic structure on the photophysical properties.
T
Scheme 2. Unexpected nido-Carborane-Fused Phosphole
here has been an explosion of interest in the incorporation of a carborane moiety into π-conjugated systems for the production of novel three-dimensional (3D) molecules and materials with unusual properties and applications.1 The search for new carborane-fused π-conjugated molecules is of great interest for new organic light-emitting diodes,2 biopharmaceuticals,3 and probes.4 The nido-carboranes (dicarba-nido-undecaboranes) are important and interesting anionic open-cage analogues of closo-carboranes. These boron clusters with pronounced electron-donating ability have attracted increasing attention as a core scaffold for coordination chemistry5 and intramolecular charge transfer systems.6 Compared with the well-developed closo-carborane-based luminescent materials, the nido-carborane-based luminophores are rare,7 especially the nido-carborane-fused π-conjugated heterocycles. Very recently, our group8 and others9 became interested in reactions involving the open-chain acetylenic phosphines. Such reactions were used to rapidly access various two-dimensional (2D) benzophospholes, which have attracted much attention owing to its intrinsic optoelectronic properties.10 We envisaged that transformation of phosphino-o-carboranylalkyne should provide a series of unknown 3D carborane-fused phospholes11 (Scheme 1). First, we attempted to synthesize phosphino-o-carboranylalkyne 3a (Scheme 2) by considering the synthetic utility of P−N bond transformations.12 Following a modified literature procedure,13 n-BuLi (1.2 equiv) was added dropwise to 1a14 in THF at −78 °C. The reaction mixture was stirred for 20 min at this temperature and then for 1 h at 0 °C before being cooled to −78 °C. Chlorophosphin (1.2 equiv) was added to this mixture. The solution was slowly warmed to room temperature
Figure 1. X-ray crystal structure of 4a. The level set for thermal ellipsoids of all atoms is 30%. Except for the hydrogen atom on the carborane, it has been omitted for clarity. Main bond lengths (Å) and angles (deg): C1−C2 1.570(3), C1−P1 1.798(2), P1−C4 1.799(2), P1−N1 1.626(2), P1−N2 1.632(2), C3−C4 1.339(3), C1−P1−C4 96.17(10), C1−P1−N1 114.65(11), C1−P1−N2 111.43(11), N1− P1−N2 109.89(11), N2−P1−C4 112.25(11), N1−P1−C4 111.91(11). CCDC reference number 1837420.
and stirred for 2 h. Then, the reaction was quenched with water and extracted with dichloromethane. Chromatography and recrystallization in air afforded a colorless crystal 4a. In addition to the NMR studies, the structure of 4a was confirmed by X-ray crystallography (Figure 1). Most strikingly, a novel zwitterionic 3D nido-carborane-fused phosphole was obtained.15 Even though various reaction conditions were examined, the desired 3a could not be obtained. To the best of our knowledge, this is the first example of a nido-carborane-fused phosphole.16
Scheme 1. Proposed 3D Carborane-Fused Phosphole
Received: February 12, 2019
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.9b00562 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 3. Synthesis of Zwitterionic nido-Carborane-Fused Phospholes
Figure 3. Calculated HOMO (bottom) and LUMO (top) of 4a.
C4 (1.339(3) Å) is in the range of a benzophosphole-type double bond. The angle between the planar phosphole moiety (C1−C2−C3−C4, 2.0(3)°; C2−C1−P1−C4, 3.94(15)°; C2− C3−C4−P1, 1.1(3)°) and C2B3 plane is 141.5°. Next, we tried to evaluate the generality of this reaction between various o-carboranylalkyne derivatives 1a−e and chlorophosphines (Scheme 3). The cycloaddition proceeded smoothly with electron-neutral (4a−d), -rich (4e, 4g), or sterically bulky (4h) aromatic substituted alkynes. The comparatively lower yields were obtained with the electronpoor substrates 4f (23%) and 4j (20%). Importantly, the nature of the substituents on phosphorus had an impact on the cycloaddition process. Diaminophosphines provided results better than those of diphenylphosphines 4a (43%) vs 4c(25%), 4e (48%) vs 4i (29%). When unsymmetric phosphine was used, two isomers (4b, 35%) were obtained in almost 1:1 ratio. The corresponding diethylphenylphosphonite and dicyclohexylphenylphosphine derivatives were found to be unsuitable substrates. When R1 = SiMe3(TMS) substituted alkyne was used, the TMS group was removed during the reaction and 4j was formed. It should be noted that around 20−30% of staring carboranylalkyne 1a was recovered after the reactions. Deuteriolysis of the reaction mixture, which provided 4d, with CD3OD afforded a mixture of 4d and monodeuterated 4dD with D (62% deuterium) at the β position of the phosphole moiety. According to this result, we proposed that the formation of nido-carboranefused phosphole goes through the following procedure: (1) formation of phosphino-o-carboranylalkyne 3, (2) intramolecular attack of alkyne by the phosphine moiety18 and the degradation of the closo-carborane skeleton, affording a zwitterionic phosphole 5,19 and (3) hydrolization providing the final product 4. The absorption and fluorescence spectra of nido-carboranefused phospholes 4a−c, 4g, and 4h were recorded in CH2Cl2 (Figure 2). All of these compounds exhibit strong absorption bands between 323 and 365 nm and excellent fluorescent properties. The spectra show broad emissions in the blue region (λem = 445−469 nm) with fluorescence quantum yields (Φf) of
Figure 2. (a) UV−vis absorption (top) and PL spectra (bottom) of 4a, 4b, 4c, 4g, and 4h (1 × 10−5 M) in CH2Cl2 at 298 K. (b) Normalized PL spectra of 4g (1 × 10−5 M) in various solvents.
As shown in the crystal structure (Figure 1), the central phosphorus atom adopts a distorted tetrahedral geometry. The two intracyclic P−C bonds lengths are strictly identical (P1−C1, 1.798(2) Å; P1−C4, 1.799(2) Å) and in agreement with the P− C single bond.17 These bonds are shorter than those in bis(carboranyl)phosphines.11 The C−P bond between the nidocarborane and phosphorus (P1−C1) is only marginally longer than that of uncyclized nido-carborane phosphonium zwitterions (1.7867(14) Å).16 The C1−C2 (1.570(3) Å) bond falls in the range of a typical C−C single bond. The bond length of C3− B
DOI: 10.1021/acs.orglett.9b00562 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 1. Photophysical and Electrochemical Data of nido-Carborane-Fused Phospholes compound
λmax (nm)a
4a 4b 4c 4g 4h
323 332 331 365 353
λem (nm)a 446 445 451 469 455
Stokes shift (cm−1)
Φ fb
8487 7947 7648 6075 6350
0.67 0.61 0.34 0.99 0.77
Eox (eV)c
Ered (eV)c
Eox,onset (eV)
Eg (eV)d
1.46 1.53 1.51 0.99e 1.47
−1.0 −1.02e −0.97e −0.4 −0.4
0.94 1.01 1.01 0.49 0.83
3.83 3.73 3.73 3.39 3.55
e
a Measured in CH2Cl2 (1 × 10−5 M). Emission maxima upon excitation at the absorption maximum wavelengths. bMeasured relative to quinine sulfate (H2SO4, 0.1 M), ±15%. cIn degassed CH3CN with Bu4NPF6 (0.2M) at a scan rate of 50 mV s−1 at 298 K. dEg = 1240/λabs. eReversible process.
34−99%. They also have large Stokes shifts of 8487 cm−1 (122 nm, 4a), 7947 cm−1 (119 nm, 4b), 7648 cm−1 (113 nm, 4c), 6075 cm−1 (104 nm, 4g), and 6350 cm−1 (102 nm, 4h). Both λabs (365 nm) and λem (469 nm) of diphenylaniline-extended 4g are longest among these compounds. This result indicates that the photochemical properties of these nido-carborane-fused phospholes can be modulated by the aryl substituents of the alkynes and offer a new opportunity to develop high-performing blue emitters. The photoluminescence spectra of 4g solution bathochromically shifted with an increase of solvent polarity (λem = 450 nm in toluene, λem = 463 nm in THF, λem = 469 nm in DCM, λem = 500 nm in CH3CN, λem = 501 nm in DMSO). However, the absorption spectra do not show obvious solvent effects (see Supporting Information). The Stokes shift of 4g is larger in a polar solvent than that in an apolar solvent. Density functional theory calculations were performed with compound 4a at the B3LYP/6-31+G(d,p) level20 (Figure 3). The HOMO (highest occupied molecular orbital) is mainly localized on the carborane moiety, and the LUMO (lowest occupied molecular orbital) is mostly localized on the πconjugated phosphole unit. The absorption band is attributed to a π−π* transition. The electrochemical properties of this series of nidocarborane-fused phospholes were also studied by cyclic voltammetry (Table 1). 4a−c show irreversible oxidation waves at around 1.5 V and reversible reduction waves at around −1 V (vs ferrocene/ferrocenium). Compound 4g is much easier to oxidize (Eox = 0.99 V vs Fc+/Fc). Furthermore, both diphenylaniline-extended 4g and 1-pyrene-substituted 4h are easier to reduce than others. In conclusion, a new type of annulated phosphole was prepared from a simple procedure, and the photoluminescent properties of these compounds can be tuned by the substituent effect. To our knowledge, this is the first example of nidocarborane-fused π-conjugated phosphacycles. We anticipate that this study opens a door to a new avenue for exploring the potential of the zwitterionic nido-carborane chemistry.
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Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zheng Duan: 0000-0002-0173-663X Francois Mathey: 0000-0002-1637-5259 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to the National Science Foundation of China (Nos. 21672193 and 21272218) and Zhengzhou University of China for financial support of this research. We thank Dr. Bingxin Yuan for her assistance in revision.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00562. Experimental procedure and characterization of all new compounds (PDF) Accession Codes
CCDC 1832497 and 1837420 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The C
DOI: 10.1021/acs.orglett.9b00562 Org. Lett. XXXX, XXX, XXX−XXX
Letter
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DOI: 10.1021/acs.orglett.9b00562 Org. Lett. XXXX, XXX, XXX−XXX
