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Published in final edited form as: Nat Catal. 2024 Jan 9;7(3):242–251. doi: 10.1038/s41929-023-01089-x

Ring expansion of indene by photoredox-enabled functionalized carbon-atom insertion

Fu-Peng Wu 1, Chetan C Chintawar 1, Remy Lalisse 2,3, Poulami Mukherjee 2,3, Subhabrata Dutta 1, Jasper Tyler 1, Constantin G Daniliuc 1, Osvaldo Gutierrez 2,✉, Frank Glorius 1,✉
PMCID: PMC11540421  NIHMSID: NIHMS1990327  PMID: 39512751

Abstract

Skeletal editing has received unprecedented attention as an emerging technology for the late-stage manipulation of molecular scaffolds. The direct achievement of functionalized carbon-atom insertion in aromatic rings is challenging. Despite ring-expanding carbon-atom insertion reactions, such as the Ciamician–Dennstedt re-arrangement, being performed for more than 140 years, only a few relevant examples of such transformations have been reported, with these limited to the installation of halogen, ester and phenyl groups. Here we describe a photoredox-enabled functionalized carbon-atom insertion reaction into indene. We disclose the utilization of a radical carbyne precursor that facilitates the insertion of carbon atoms bearing a variety of functional groups, including trifluoromethyl, ester, phosphate ester, sulfonate ester, sulfone, nitrile, amide, aryl ketone and aliphatic ketone fragments to access a library of 2-substituted naphthalenes. The application of this methodology to the skeletal editing of molecules of pharmaceutical relevance highlights its utility.

Single-atom skeletal editing has emerged as a vital synthetic tool in contemporary structural modification, enabling the targeted reshaping of molecules through atom insertion, deletion and replacement (Fig. 1a)1–8. Of particular interest are those transformations in which ring expansion is achieved through the insertion of a carbon atom. Compared with other single-atom insertion reactions9–14, the advantage of the carbon atom is that it contains four covalent bonds, allowing the introduction of a functional group and providing more strategies for molecular diversification. The utilization of single-atom insertion reactions into aromatic ring systems is a formidable challenge given the high energy barriers associated with de-aromatization and cleavage of carbon–carbon bonds15,16. In this aspect, the Ciamician–Dennstedt re-arrangement17 represents one of the earliest known examples of carbon-atom insertion where a dichlorocarbene, generated from chloroform, undergoes cycloaddition to the indole ring system to provide a dichlorocyclopropane intermediate (Fig. 1b). Further re-arrangement upon the addition of a strong base yields the desired ring expansion product. Despite its underlying mechanism offering great potential for modifying cyclic motifs, this carbon-atom insertion reaction has been rarely applied as a tool for late-stage modification18 due to practical limitations such as the competing Reimer–Tiemann reaction19, harsh reaction conditions, low yield and the fact that only halogenated products are accessible. Recently, several exciting methods have been disclosed to achieve the conversion of indole to quinoline systems using carbynyl cation equivalent reagents such as α-halo diazoalkanes20 and α-chlorordiazirines (Fig. 1c)21,22. Conversely, little attention has been given to the related ring expansion of indenes to generate naphthalenes, in spite of the fact that these carbocycles and their related derivatives are valued structural motifs found in many products and biologically active molecules23–26. As only a single example of such a carbon-atom insertion. reaction was developed by Reiff and co-workers in 1955 (refs. 27,28), we determined that accessing a mild and general method to achieve this transformation would be of great synthetic value.

Fig. 1 |. Skeletal editing via single-atom insertion.

Fig. 1 |

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a, Ring expansion by single-atom insertion. b, Mechanism of previously reported carbon-atom insertion reaction. c, Carbon-atom insertion reagents. d, Possible reaction mechanisms and challenges for carbyne radical precursors. e, Indene ring expansion via functionalized carbon-atom insertion.

It was hypothesized that the key to achieving a synthesis of 2-functionalized naphthalenes would be to design a carbyne precursor reagent that does not require the use of elevated temperatures and strong base. In this regard, we identified masked carbynyl radical species such as classical carbyne equivalents α-iodonium diazo compounds29. Such reagents can be synthesized bearing a variety of functional groups but despite their inimitable reactivity have never been applied to the field of skeletal editing. Several applications of such compounds have been previously reported. For example, the Suero group has reported the synthesis of exceptionally stable and isolable α-iodonium diazo compounds and demonstrated their ability to facilitate arene C–H diazomethylation reactions30. Simiarly, Wang31 and co-workers utilized α-diazo sulfonium triflates as carbyne precursors in their photocatalysed synthesis of 1,4-dicarbonyl Z-alkenes. Furthermore, α-iodonium diazo reagents can be directly attacked by nucleophiles in SN2-type reactions32,33. We hypothesized that, after diazo insertion, photoredox-enabled generation of the corresponding radical intermediate would trigger ring expansion and rearomatization to access the targeted functionalized naphthalene products. However, it must be noted that the carbynyl radical equivalent has three potential intermediates that could be generated upon activation, namely, the carbon radical [∙C(N2)-R] (ref. 30), the carbene [X-C(:)-R] (ref. 34) and the free carbyne species [:Ċ-R] (refs. 31,35,36) (Fig. 1d). Therefore, to harness the utility of this reagent, it is essential to achieve the selective formation of a single reactive intermediate through modulating the relative rates of N2 loss and radical generation.

In this Article, we report the successful development of a photocatalysed ring expansion method for the insertion of a carbon atom into indene to access 2-functionalized naphthalenes. Depending on the substitution of the α-iodonium diazo compounds, various 2-functionalized naphthalenes can be effectively accessed. This mild and operationally simple reaction tolerates a broad range of functional groups, and experimental and computational mechanistic studies reveal that a radical chain mechanism is in operation.

Results

Reaction development

To create an efficient functionalized carbon-atom insertion reaction for indene, the key challenge was the development of an easily accessible and bench-stable carbyne reagent that is compatible with a wide range of functional groups. In most cases, metal catalysts are employed to facilitate the production of metal carbene intermediates. However, this often necessitates undesirable procedures such as slow reagent addition and low temperatures35,37. To circumvent this, we focused our efforts on using visible light to facilitate the generation of a carbyne intermediate. Initially, we chose unsubstituted indene (1a) as the model substrate and Ru(dtbbpy)3(PF6)2 as the photocatalyst under visible light irradiation (blue light-emitting diodes (LEDs)) with Na2CO3 in CH3CN (Fig. 2a). Firstly, carbynyl cation equivalent α-chloro, α-bromo and α-iododiazoacetate (2a–2c)38 were tested, but the desired naphthalene was not observed, presumably due to the inability of the cyclopropane intermediate to eliminate. Pleasingly, when compounds 2d and 2e were employed, good yields of the desired product were observed. Alternative hypervalent iodonium diazo compounds 2f and 2i were also shown to deliver 3a in 75% and 26% yield, respectively. Electron-withdrawing groups on hypervalent iodonium diazo compounds (2g and 2h) slightly reduce the yields. However, the hypervalent iodonium diazo compound with electron-donating groups on aromatic ring such as OMe was rapidly decomposed due to its highly unstable nature. Despite α-diazo sulfonium triflate 2j (ref. 39) providing access to the naphthalene product in good yield, α-diazo dimethylsulfonium 2k (ref. 29) failed to deliver any observable 3a. In addition, thermally stable α-diazo pyridine 2l (ref. 40) and α-diazo quaternary ammonium salt 2m (ref. 29) both gave no indication that ring expansion had occurred. These results suggested that the identity of the photolabile group is highly important to the outcome of the carbon-atom insertion reaction, with 2d taken forward as the most promising candidate. Subsequently, a set of control studies revealed that light and photocatalyst are crucial for the reaction (Fig. 2b). The base was also shown to have a substantial effect on this transformation, with the absence of Na2CO3 severely compromising product formation. Other photocatalysts such as [Ir(dF(CF3)ppy)2(dtbbpy)](PF6) and thioxanthone decreased the reaction efficiency, providing 3a in 16% and 4% yield, respectively. Notably, the reaction was only sensitive towards high oxygen concentration, whereas solvent moisture content, temperature fluctuation, light intensity and concentration had only a minimal effect on the reaction outcome (Fig. 2c).

Fig. 2 |. Development of the functionalized carbon-atom insertion reaction.

Fig. 2 |

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a, Identification of carbyne reagent. Reaction conditions: indene (0.1 mmol), 2 (0.12 mmol), Ru(dtbbpy)3(PF6)2 (1 mol%) and Na2CO3 (0.2 mmol) in CH3CN (0.05 M), irradiation with a 30 W blue LED (λmax = 450 nm) under an argon atmosphere at room temperature (r.t.) for 12 h. The yields were determined by GC-FID analysis using hexadecane as an internal standard. b, Control experiments. c, Sensitivity assessment of reaction conditions. GC-FID, gas chromatography-flame ionization detector; T, temperature; C, concentration; I, light intensity.

Substrate scope

With the optimized reaction conditions in hand, the scope of the indene skeletal ring expansion reaction was explored using this protocol (Fig. 3). 2-Ethyl naphthoate products featuring electron-withdrawing substituents, such as halogens (3b-c, 3e–3f, and 3h), triflate (3g), trifluoromethyl (3k), nitro (3n), cyano (3o) and phenylsulfonyl (3p), as well as electron-donating groups such as thiomethyl (3l) were all obtained in moderate to good yields. In addition, indenes bearing heterocyclic rings like thiophene (3j) and pyridine (3m) were efficiently converted to the corresponding ring-expanded products. 2-Substituted indenes (3q and 3r) were successfully transformed into the desired naphthalenes; however, 3-substituted indenes were observed to only yield fewer products and partial starting materials remained (Supplementary Fig. 11). We speculate that, after the radical [∙C(N2)COOEt] addition into the indene and SET, the addition of the resulting cation to the carbene is disfavoured due to steric effects. Additionally, indenes containing aliphatic chains bearing halogen (3w and 3x), cyano (3y), silyl (3z), ester (3aa), phosphate (3ab), boronic ester (3ad) and alkene (3ae) groups were well tolerated and afforded the corresponding product in moderate to good yields. This method also accessed the trisubstituted naphthalene (3af) from tetrasubstituted indene in 31% yield. Despite the success of the indene substrates, subjecting indole to identical reaction conditions yielded only 10% of the corresponding quinoline product. We speculate that this may be due to competing direct nucleophilic addition of indole to 2d.

Fig. 3 |. Substrate scope for the functionalized carbon-atom insertion reaction.

Fig. 3 |

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Isolated yields on a 0.2 mmol scale unless stated otherwise. Reaction conditions: indene (0.2 mmol), 2d (0.24 mmol), Ru(dtbbpy)3(PF6)2 (1 mol%) and Na2CO3 (0.4 mmol) in CH3CN (4 ml, 0.05 M), irradiation with a 30 W blue LED (λmax = 450 nm) under an argon atmosphere at room temperature (r.t.) for 12 h. a2f was used instead of 2d. Isolated after oxidation to alcohol. r.s.m., remaining starting material. bIsolated after oxidation to alcohol.

The reaction was further examined by exploring alternative α-iodonium diazo compounds containing a variety of functional groups (Fig. 4). Trifluoromethyl naphthalenes (4a–4e) could be obtained in moderate to good yields. Despite their structural simplicity, trifluoromethyl arenes are often tedious to synthesize and are essential functional materials across several scientific fields41,42. Primary, secondary and tertiary esters were also accessed from the corresponding α-iodonium diazo compounds. Remarkably, terminal alkene (4h), alkyne (4i) and phenol (4m) groups were compatible in the transformation and produced moderate yields of the corresponding products. As well as this, a range of other synthetically useful functional groups could be incorporated into the final product including phosphate ester, sulfonate ester, sulfone, nitrile, amide, aryl ketone and aliphatic ketone groups.

Fig. 4 |. α-Iodonium diazo compounds scope for the functionalized carbon-atom insertion reaction.

Fig. 4 |

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Isolated yields on a 0.2 mmol scale unless stated otherwise. Reaction conditions: indene (0.2 mmol), α-iodonium diazo reagent (0.24 mmol), Ru(dtbbpy)3(PF6)2 (1 mol%) and Na2CO3 (0.4 mmol) in CH3CN (4 ml, 0.05 M), irradiation with a 30 W blue LED (λmax = 450 nm) under an argon atmosphere at room temperature (r.t.) for 12 h. aα-Iodonium diazomethylnitrile (0.3 mmol), stirred at room temperature for 1 h.

Synthetic utility

To illustrate the potential of the transformation in expanding accessible chemical space, we applied it to polycyclic compounds and drug molecules. Biindene was conveniently transformed into the corresponding binaphthalene-3,3′-phosphate in 32% yield (Fig. 5a). Moreover, the skeletal core of aldosterone was directly modified to afford the corresponding naphthalene in a single synthetic step (Fig. 5b). This functionalized carbon-atom insertion reaction provides opportunities to design more concise synthetic routes for natural products. For example, a derivative of the naphthalene-containing pharmaceutical adapalene43 could be readily accessed in high yield via the established ring expansion of indene followed by borylation and subsequent Suzuki–Miyaura reaction (Fig. 5c).

Fig. 5 |. Synthetic applications of the functionalized carbon-atom insertion reaction.

Fig. 5 |

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a, Functionalized carbon-atom insertion of biindene. b, Skeletal modification of aldosterone synthase inhibitor. c, Application to synthesis of adapalene derivative 5d.

Mechanistic investigations

After developing a suitable method for indene editing via carbon-atom insertion, subsequent efforts were directed towards exploring the reaction mechanism (Fig. 6). First, ultraviolet–visible absorption spectroscopy indicated that the photocatalyst Ru(dtbbpy)3(PF6)2 and 2d are both light-absorbing species near the excitation wavelength (λmax = 450 nm, Fig. 6a). To rule out the possibility of direct excitation of 2d being responsible for reactivity, we conducted control experiments. The control experiments showed that no desired product was observed when the reaction mixture was irradiated with a higher-energy light source (λmax = 450 nm, λmax = 405 nm and λmax = 365 nm) in the absence of photocatalyst (Fig. 6b). Low levels of indene conversion were detected, and no characterizable species other than o-iodobenzoate could be isolated from the control experiments. We speculate that any carbene species generated under these conditions will react with the solvent or dimerize to form the corresponding azine. Stern–Volmer analysis revealed that the luminescence emission of Ru(dtbbpy)3(PF6)2 was quenched efficiently by 2d, whereas no quenching was observed with indene, which indicates that an interaction between photocatalyst and α-iodonium diazo reagent exists under the conditions of the reaction (Fig. 6c). Additionally, a cyclic voltammetry measurement suggests that reagent 2d (Ered = −0.46 V versus SCE, CH3CN) undergoes a single-electron transfer with the highly reducing photocatalyst excited state *[Ru]II(E1/2(ii)*/(iii)=−0.81v)44 to generate radical ·C(N2)COOEt, aryl iodide and a triflate anion (Fig. 6d,e). Moreover, quantum yield measurements (Φ = 11.5) suggest that this transformation occurs via an efficient chain process (Supplementary Methods). These results strongly indicate that this transformation is a redox process. Next, deuterium-labelled substrates 6a and 6c were subjected to the reaction conditions, providing the desired products in good yield without any abatement of deuterium (Fig. 6f). Thus, we speculate that no proton elimination process occurs in the reaction process. The reaction of 1a with 2d was largely inhibited by the addition TEMPO and 2,6-DTBP as radical scavengers, and intermediates 6e and 6f were directly observed by gas chromatography–mass spectrometry (GC–MS) analysis, indicating the potential existence of a radical intermediate (Fig. 6g). Furthermore, by conducting the reaction with 1-allyl-indene 6g, 3a and 6h were both afforded in 24% yield (Fig. 6h). Additionally, when 1,1-dimethylindene 6i was employed in this synthesis, unexpected product 6j was isolated in 22% yield (Fig. 6i). We speculate that these species (3a and 6j) arise via a re-arrangement of an allyl cation intermediate.

Fig. 6 |. Mechanistic studies.

Fig. 6 |

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a, The ultraviolet–visible absorption spectrum of 1a (5 × 10−3 mol l−1), 2d (6 × 10−3 mol l−1) and Ru(dtbbpy)3(PF6)2 (5 × 10−5 mol l−1) in MeCN was collected, respectively. Accordingly, the photocatalyst Ru(dtbbpy)3(PF6)2 and 2d were found to be absorbing species near the excitation wavelength (λmax = 450 nm). b, Infeasibility of direct photosensitization excludes the possibility of an energy transfer pathway. c, Stern–Volmer quenching studies. Stern–Volmer analysis revealed that the luminescence emission of Ru(dtbbpy)3(PF6)2 was quenched efficiently by α-iodonium diazo 2d, whereas no quenching was observed with indene. d, Cyclic voltammetry measurements of indene. e, Cyclic voltammetry measurements of 2d. f, Deuterium labelling experiments. g, Radical capture experiments. h. Deallylation in the carbon insertion of substrate 6g. i, Methyl migration in the carbon insertion of 1,1-dimethylindene 6i. r.t., room temperature. TEMPO, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl; 2,6-DTBP, butylhydroxytoluol.

To further investigate the reaction mechanism, dispersion-corrected density functional theory (DFT) calculations were carried out. As shown in Fig. 7b, homolytic I–C bond cleavage takes place after single-electron transfer (SET) between the triplet [RuII] (T1) and the hypervalent iodonium diazo compound 2f to generate [RuIII] and loosely coordinated 22f. The next step involves the loss of aryl iodide and a triflate anion resulting in the generation of C-centred radical I•, favoured by 15.0 kcal mol−1. From here, as shown in Fig. 7a, I• can undergo N2 extrusion via I-TS-N2 (with a barrier of 27.0 kcal mol−1) to form a carbyne. However, regioselective radical addition of I• to the indene 1a proceeds via a much lower energy barrier (8.4 kcal mol−1 via I-TS-II•) to generate stabilized indene radical II•, which is thermodynamically favoured by 19.0 kcal mol−1. Thus, these calculations show that formation of free carbyne is kinetically and thermodynamically unfavourable and is probably not a reaction intermediate in this protocol. As shown in blue, we considered a radical cyclization process in which intermediate II• could then undergo intramolecular cyclopropanation by means of II-TS-III• with a barrier of 17.6 kcal mol−1 and 4.6 kcal mol−1 uphill in energy to generate the iminyl radical III•. In turn, nitrogen extrusion from III• can occur via III-TS-IV• with a low energetic barrier (0.7 kcal mol−1) to generate IV• downhill in energy by −27.6 kcal mol−1 with respect to III•. After the formation of intermediate IV• the re-arrangement to the six-membered ring allyl radical intermediate VIII• may proceed via IV-TS-VIII• with a barrier of 7.2 kcal mol−1 and downhill by 41.2 kcal mol−1.

Fig. 7 |. Proposed reaction mechanism and DFT calculations.

Fig. 7 |

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a, Computed Gibbs energy profiles for competing pathways for indene ring expansion from C-centred radical I• intermediate. Calculated Gibbs free energies (uB3LYP-D3/def2tzvpp//uB3LYP/def2svp–CPCM (acetonitrile)) are given in kcal mol−1. b, Energetics for the formation of intermediate• promoted by single electron transfer between the triplet [RuII] and reagent 2f. For computational details, see Supplementary Methods.

Since the high quantum yield measurements implied a radical chain mechanism, we explored the potential possible radical chain pathway by DFT simulations (Supplementary Fig. 16). In particular, we considered competing SET pathways from several radical intermediates II•, III•, IV• and VIII•. We determined that there are two competing SET pathways. The first pathway (path 1, Fig. 7, shown as red) involves the formation of II• followed by single electron transfer with reagent 2f. We found that SET1 to form cationic species II+ (and concomitant formation of OTf-, Ph-I and radical chain carrier I•) is thermodynamically favourable (downhill by 16.3 kcal mol−1). In this pathway, subsequent cationic-promoted ring closure to form three-membered cyclic intermediate III+ proceeds via a much lower barrier (6.4 kcal mol−1 via II-TS-III+ versus 17.6 kcal mol−1 via radical II-TS-III•). In turn, nitrogen extrusion from III+ occurs via III-TS-V+ with a barrier (11.5 kcal mol−1) to generate the bicyclic intermediate V+ with a ΔG of −14.9 kcal mol−1. Finally, favourable coordination of NaCO3− to V+ followed by deprotonation via VI-TS-VII proceeded with a barrier of 20.2 kcal mol−1 and is thermodynamically favoured by 10.7 kcal mol−1. From here, VII can undergo C–C bond cleavage to generate 3a by means of VII-TS-3a with a small barrier of 6 kcal mol−1. Thus, calculations show that the most favourable pathway is formation of radical chain carrier I• from SET of II• with 2f followed by cationic cyclization via intermediate III+. We also considered formation of the six-membered ring intermediate (VIII•) followed by SET4 downhill by 16.1 kcal mol−1 to generate the naphthalene cation VIII+ (path 2, Fig. 7, shown as blue), but this process was ruled out on the basis of the alternative cationic pathway (red).

Conclusions

In summary, we have developed a photoredox ring expansion reaction for the rapid insertion of functionalized carbon atoms into indene. Crucial to realizing this reactivity was the application of α-iodonium diazo compounds as masked carbyne equivalents and the simultaneous implementation of photoredox catalysis. This mild method exhibits remarkable tolerance towards various functional groups, enabling access to an extensive library of 2-functionalized naphthalenes. DFT simulations further explored possible radical chain pathways, as well as indicating that the mechanism probably proceeds via an initial addition of a diazomethyl radical to indene. We anticipate that this broadly applicable approach will provide a strategy for skeletal editing and help alleviate the need to redesign synthetic routes when targeting core scaffold modification.

Methods

To an oven-dried 10-ml Schlenk tube equipped with a polytetrafluoroethylene-coated oval stirring bar was added Ru(dtbbpy)3(PF6)2 (1 mol%, 2.4 mg), hypervalent iodine reagents (0.24 mmol, 1.2 equiv.) and Na2CO3 (0.4 mmol, 0.2 equiv.) under air, and then the vessel was evacuated and re-filled with argon four times. Dry acetonitrile (4.0 ml, 0.05 M) and indene (0.2 mmol, 1.0 equiv.) were added using a syringe under argon counterflow. The vessel was sealed with a screw cap and then irradiated at 30 W blue LED (λmax = 450 nm) using the photochemical set-up for 12 h. After irradiation, the resulting solution was transferred to a 25 ml round-bottom flask and volatiles were removed under reduced pressure. Purification by flash column chromatography on SiO2 (pentane/EtOAc) afforded the corresponding naphthalene products.

Supplementary Material

Ring expansion of indene by photoredox-enabled functionalized carbon-atom insertion
Cartesian coordinates for all calculated structures
Crystallographic data of compound 3y
CheckCIF data of compound 3y

Acknowledgements

Generous financial support by the Deutsche Forschungsgemeinschaft (Leibniz Award) and the Alexander von Humboldt Foundation (C.C.C. and J.T.) is gratefully acknowledged. O.G. gratefully acknowledges financial support from the National Institutes of Health (R35GM137797), the Camille and Henry Dreyfus Foundation and the Welch Foundation (A-2102-20220331) for supporting this work. O.G. also acknowledges the Texas A&M University HPRC resources (https://hprc.tamu.edu) for computational resources. We sincerely thank G. Tan, H. Wang, R. Kleinmans and A. Heusler for help in preparing the manuscript and many helpful discussions.

Footnotes

Competing interests

The authors declare no competing interests.

Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41929-023-01089-x.

Data availability

The data that support the findings of this study are available within the main text and its Supplementary Information file. Source data are provided as Source Data file. Data are also available from the corresponding author upon request. Crystallographic information data files and xyz coordinates of the optimized structures are available as supplementary files. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre under deposition no. CCDC 2262556 (3y). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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Associated Data

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

Supplementary Materials

Ring expansion of indene by photoredox-enabled functionalized carbon-atom insertion
NIHMS1990327-supplement-Ring_expansion_of_indene_by_photoredox-enabled_functionalized_carbon-atom_insertion.pdf (21.9MB, pdf)
Cartesian coordinates for all calculated structures
NIHMS1990327-supplement-Cartesian_coordinates_for_all_calculated_structures.txt (69.7KB, txt)
Crystallographic data of compound 3y
NIHMS1990327-supplement-Crystallographic_data_of_compound_3y.cif (355KB, cif)
CheckCIF data of compound 3y
NIHMS1990327-supplement-CheckCIF_data_of_compound_3y.pdf (70KB, pdf)

Data Availability Statement

The data that support the findings of this study are available within the main text and its Supplementary Information file. Source data are provided as Source Data file. Data are also available from the corresponding author upon request. Crystallographic information data files and xyz coordinates of the optimized structures are available as supplementary files. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre under deposition no. CCDC 2262556 (3y). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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