Introduction

Quinoline moieties are important in anticancer drug improvement, as their derivatives show great results through different operations such as growth inhibitors by cell cycle arrest, apoptosis, inhibition of angiogenesis, disruption of cell migration, and modulation of nuclear receptor responsiveness [1]. The fused pyranoquinoline moiety is an extremely common structural motif, existing in many naturally occurring or biologically active alkaloids [2,3,4]. The natural product Haplamine (Fig. 1), extracted from Haplophyllum perforatum, is commonly used in central Asia to treat various diseases, including testicular cancer. Researchers evaluated the haplamine-induced cell death and its major metabolites trans/cis-3,4-dihydroxyhaplamine (1 and 2, Fig. 1). The IC50 values were 52.5, 24.3, 59.7, 41.5, 72, 32 μM in human pancreatic cancer (Capan1 and Capan2), hepatic cancer (HepG2), and colorectal cancer (LS174T, HT29, and SW620) cell lines, respectively. Meanwhile, the IC50 values of trans/cis-3,4-dihydroxyhaplamine metabolites 1 and 2 were both > 200 μM [5].

Fig. 1
figure 1

Structure of anticancer pyranoquinolones 1, 2, 3a–3j, 4a–4f, and 5a, 5b

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Various 2,5-dialkyloxazolopyrano[3,2-c]quinolone derivatives 3a–3j were evaluated for antitumor activity against three human cancer cell lines, namely MCF-7 (breast carcinoma), HepG-2 cells (human hepatocellular carcinoma), and HCT-116 (colon carcinoma) using 5-fluorouracil as a standard drug [6]. Compounds 3c and 3f showed higher inhibitory activity against all three tumor cell lines with having IC50 values in between 6.2–28.3 µg/cm3 and 28.7–43.2 µg/cm3, respectively (Fig. 1) [6]. Interested results were obtained among the synthesized and assigned compounds of 2'-amino-2,7-dibromo-5'-oxo-5',6'-dihydrospiro[fluorene-9,4'-pyrano[3,2-c]quinoline]-3'-carbonitriles 4a–4f (Fig. 1), the derivatives of 4b, 4c, and 4d showed an inhibition towards Src kinase activity with IC50's of 4.9, 5.9, and 0.9 μM, respectively [7].

Kumar et al. [8] developed fused quinolone derivatives 5a and 5b (Fig. 1). The obtained compounds were evaluated for their in vitro cytotoxic potential colon (HT-29, HCT-116), human lung (A549), breast (MCF-7), and prostate (PC-3 and DU145) cancer cell lines. Compound 5a showed promising anti-proliferative activity against lung (A549) cancer cell line with an IC50 value of 3.17 ± 0.52 µM. Flow cytometric analyses showed that 5a, in a dose-dependent manner, arrested both the Sub G1 and G2/M phases of the cell cycle. Also, 5b revealed significant inhibition of tubulin polymerization and disruption of the microtubule network with an IC50 value of 5.15 ± 0.15 µM [8].

Previously, it was reported that pyrano[3,2-c]quinolin-5-one derivatives could be obtained via a three-component reaction of 4-hydroxyquinolin-2(1H)-ones with aldehydes and malononitrile. This reaction can be catalyzed by piperidine, TEBA, ammonium acetate, or triethylamine [9,10,11,12]. Also, Gunasekaran et al. showed that 6-methyl-2-(methylamino)-3-nitro-4H-pyrano[3,2-c]quinolin-5(6H)-ones were obtained by one-pot reaction of quinolone, (E)-N-methyl-1-(methylthio)-2-nitro-ethenamine, and aromatic aldehydes in the presence of anhydrous ZnCl2 [12]. In addition, Zhu and co-workers [13] reported the synthesis of pyranoquinolinones when mixtures of quinolin-2-ones, Meldrum's acid, and aromatic aldehydes in the presence of L-proline were allowed to react in refluxing ethanol [13]. Aly et al. reported the preparation of ethyl 5,6-dihydro-2,5-dioxo-6,9-disubstituted-2H-pyrano[3,2-c]quinoline-4-carboxylates by the reaction of equimolar amounts of quinolin-2-ones and diethyl acetylenedicarboxylate in absolute ethanol containing catalytic amounts of triethylamine (Et3N) [14]. The reaction of the β-keto acid derivatives with isatine was carried out under Knoevenagel reaction conditions using fused sodium acetate and glacial acetic acid. The product of this reaction was characterized as 2-(indol-3-ylidene)propanoic acid [15]. This cyclization occurred when that product was treated with concentrated sulfuric acid and formed the pyranoquinolone [15]. Furthermore, Aly et al. synthesized spiro(indoline-3,4'-pyrano[3,2-c]quinoline)-3'-carbonitrile by refluxing equimolar amounts of quinolin-2-ones with 2-(2-oxo-1,2-dihydroindol-3-ylidene)malononitrile in dry pyridine solution [16]. Upon refluxing quinolin-2-ones in benzene containing tributyltin(IV) chloride (Bu3SnCl) and sodium cyanoborohydride in the presence of azobisisobutyronitrile, the reaction proceeded to give the tetracyclic pyranoquinolin-7(8H)-ones [17]. Bu3SnH-mediated the radical cyclizations to the regioselective synthesis of tetracyclic heterocycles 2H-benzopyrano[3,2-c]quinolin-7(8H)-ones were described as general and attractive procedure due to its simplicity [17].

The multicomponent pathway describes the formation of pyrano[3,2-c]quinolin-5-ones has been shed light due to their decent yields coupled with easy isolation of the products and avoidance of conventional purification methods. A recent approach described that was established by the reaction of isatins with phenyl (or alkyl) sulfonyl acetonitrile and 4-hydroxy-N-methylquinoline-2-one [18].

As a part of our ongoing research, we aim in this paper to synthesize pyrano[3,2-c]quinolones 8a–8g via the reaction of 4-hydroxy quinolin-2-ones 6a–6g with ethene-1,2,3,4-tetracarbonitrile (TCNE, 7).

Results and discussion

Initially, the reaction between 4-hydroxy-2-quinolin-2-ones 6a–6g and ethene-1,2,3,4-tetracarbonitrile (7) was conducted in dry THF at room temperature. After 8–12 h, the desired pyrano[3,2-c]quinolones 8a–8g were obtained in 75–85% yields (Scheme 1).

scheme 1

We carried out the reaction of 6a with 7 under different conditions with the optimized reaction conditions in hand. On refluxing the two starting substances (entry 2, Table 1), the yield of 8a was decreased; however, the reaction time was low. Increasing the reaction temperature might increase the oxidation of TCNE and therefore increase the side products. Similarly, adding a few drops of triethylamine or piperidine to the reaction mixture (entries 3 and 4, Table 1) did not increase the yield of 8a, and the yields were decreased to 74 and 76%, respectively. Upon carrying out the reaction in polar solvents such as EtOH (entry 5, Table 1), the time taken to obtain 8a was increased, whereas its yield was decreased.

Table 1 The reaction conditions and the yields for the formation of 8a
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Furthermore, adding a few drops of Et3N or piperidine (entries 6 and 7, Table 1) did not increase the resulting yield of 8a. Interestingly, using DMF gave a good yield of 8a, but it was still low than THF, and the reaction took more time. In general, the best condition can be described as a high yield of pyrano[3,2-c]quinolones 8a–8g using dry THF at room temperature (entry 1, Table 1). In addition, it was found that increasing the amount of the starting material 7 was not necessary to obtain the products 8a–8g in high yield. Pyrano[3,2-c]quinolone derivatives 8a–8g were produced by adding only equal equivalent of TCNE. High amounts of the products were obtained under standard ambient conditions, whereas the same reactions under inert atmosphere produced low yields of the products.

It appears that one of the two nucleophilic sites – C-3 and OH of the quinolone – attacks the C=C bond of TCNE, and the other attacks a nitrile group, to yield two possible products, 8a or 8a' (Fig. 2). To differentiate between these two suggested structures, their mass spectrometry, 1H NMR, 1H-1H COSY, 13C NMR, HMBC, 15 N NMR, and IR spectra were studied.

Fig. 2
figure 2

Suggested structure of the product 8a and 8a'

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The IR spectroscopy of 8a appears several peaks characteristic for the following functional groups, two bands at \(\overline{V}\) = 3296 and 3280 cm−1 due to NH2 group, at 2202 cm−1 for the nitrile group, while at 1671 cm−1 for the C=O and at 1627 cm−1 for the Ar–C=N group. The mass spectrometry of 8a showed a molecular ion at m/z = 304.1 ([M+ + H], 60%) indicated the formation of the product via the reaction of 6a and 7 without loss of any molecules. By studying the NMR spectrum of 8a, the quinolone substructures can be interpreted identically, whether the structure is 8a or 8a'. The methyl protons H-6b are distinctive at δH = 3.72 ppm; their attached carbon appears at δC = 29.8 ppm. H-6b gives HMBC correlation with nitrogen at δN = 141.6 ppm, assigned as N-6, and carbons at δC = 158.1, 139.6, and 115.6 ppm, assigned as C-5, C-6a, and C-7 in that order; they also give weak HMBC correlation with carbon at δC = 96.2 ppm, assigned as C-4a; its upfield chemical shift reflects its position in a push–pull system (Table 2). N-6 also gives HMBC correlation with a 1H doublet at δH = 7.73 ppm, assigned as H-7; this proton gives HSQC correlation with C-7. COSY and HSQC correlations lead straightforwardly to the assignments of H-8, H-9, H-10, C-8, C-9, and C-10, as shown in Table 2. C-6a gives HMBC correlation with all four protonated aromatic carbons. H-7, H-8, and H-10 give HMBC correlation with carbon at δC = 111.7 ppm, assigned as C-10a; H-10 and H-7 give HMBC correlation with carbon at δC = 151.7 ppm, assigned as C-10b. These assignments are the same in either 8a or 8a'. The third ring is a pyran in either 8a or 8a'; it contains three carbons not shared with the quinolone substructure, three nitrile carbons (two equivalent), and an amino group. At δH = 8.54 ppm, the amino protons give HSQC correlation with their attached nitrogen at δN = 85.0 ppm, and HMBC correlation with all three ring carbons just mentioned as well as C-10b, which would be four bonds from the amino protons in either 8a or 8a'. The two equivalent nitrile carbons appear upfield of the unique nitrile, at δC = 116.4 vs. 113.8 ppm; the latter is α,β-unsaturated. These assignments, too, do not differentiate the structures. The calculated 13C shifts for 8a are considerably closer to observation than those for 8a' (rms deviation 11.6 vs. 20.0 for the five carbons with different shifts). In particular, the carbon bearing two nitriles is farther upfield in 8a, in which it is attached only to carbons as C-4, than in 8a', in which it is attached to oxygen as C-2. The other large change is in C-3, which is in a push–pull system in either structure, but receives electron donation from both O and N in 8a (Table 2).

Table 2 NMR spectroscopic assignments of compound 8a
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Single-crystal X-ray analysis provided strong support for the structure of 8a (Fig. 3). The same structure was suggested for the other derivatives 8b-8 g based on their similarities in NMR spectroscopic analysis.

Fig. 3
figure 3

X-ray structure analysis of compound 8a (displacement parameters are drawn at 30% probability level)

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The mechanism describing the product's formation was based upon nucleophilic addition of C-3 in compounds 6a–6g to the electrophilic carbon of 7, which would give intermediate 9 (Scheme 2). The intermediate 9 would then exist in tautomerism with intermediate 10. After that, cyclization was occurred by the nucleophilic attack of the OH lone pair to the carbonitrile-carbon to give the intermediate 11. Finally, hydrogen shift accompanied with aromatization would give compounds 8a–8g (Scheme 2).

scheme 2

Conclusion

This work focused on synthesizing new heteroannulated pyrano[3,2-c]quinolones. By applying reaction between 4-hydroxyquinolin-2-one derivatives 6a–6g and ethene-1,2,3,4-tetracarbonitrile (TCNE, 7), pyrano[3,2-c]quinolones 8a–8g were obtained in good yield (75–85%). Non-polar and neutral conditions were considered the best ones to obtain high yields of the target products. Therefore, prospective work in our lab involving the reactions of quinolone derivatives with bi-electrophilic compounds under similar conditions would be interesting.

Experimental

Melting points were taken in open capillaries on a Gallenkamp melting point apparatus (Weiss–Gallenkamp, Loughborough, UK). The IR spectra were recorded from potassium bromide disks with an FT device (Germany). Elemental analyses were carried out at the Perkin-Elmer Elemental analyzer (Germany). The NMR spectra were measured in DMSO-d6 on a Bruker AV-400 spectrometer (400 MHz for 1H, 100 MHz for 13C, and 40.55 MHz for 15 N); and the chemical shifts are expressed in δ (ppm), versus internal tetramethylsilane (TMS) = 0 ppm for 1H and 13C, and external liquid ammonia = 0 ppm for 15 N. Coupling constants are stated in Hz. Using 1H-1H COSY, 1H-13C, and 1H-15 N HSQC and HMBC experiments, correlations were established. Mass spectra were recorded on a Finnigan Fab 70 eV (Germany), Institute of Organic Chemistry, Karlsruhe University, Karlsruhe, Germany. TLC was performed on analytical Merck 9385 silica aluminum sheets (Kieselgel 60) with Pf254 indicator; TLC's were viewed at λmax = 254 nm. Elemental analyses for C, H, N were carried out with Elementar 306.

1,6-Disubstituted-quinoline-2,4-(1H,3H)-diones 6a–6g were prepared according to the literature [19]. TCNE (7) was bought from Aldrich.

The reaction of 6a–6g with TCNE (7): synthesis of compounds 8a–8g

A suspension of 1,6-disubstituted quinoline-2,4-(1H,3H)-diones 6a–6g (1 mmol) in 20 cm3 dry tetrahydrofuran (THF) was added to a solution of 0.128 g TCNE (7, 1 mmol) in 15 cm3 dry THF. The reaction mixture was stirred for 20–25 h until the reactants disappeared (monitored by TLC). The resulting precipitates of 8a–8g, obtained on cold, was filtered off and dried. The precipitates were recrystallized from the stated solvents.

2-Amino-5,6-dihydro-6-methyl-5-oxo-4H-pyrano[3,2-c]quinoline-3,4,4-tricarbonitrile (8a, C16H9N5O2)

Yellow crystals (DMF); yield: 0.280 g (85%); m.p.: 336–338 °C; IR (KBr): \(\overline{V}\) = 3296, 3280 (NH2), 2202 (CN), 1671 (CO), 1627 (Ar–C = N) cm−1; 1H NMR (400 MHz, DMSO-d6): δ = 8.54 (bs, 2H, H-2a), 8.00 (d, J = 8.0 Hz, 1H, H-10), 7.87 (ddd, J = 8.5, 7.3, 1.2 Hz, 1H, H-8), 7.73 (d, J = 8.6 Hz, 1H, H-7), 7.49 (dd, J = 7.6, 7.6 Hz, 1H, H-9), 3.72 (s, 3H, H-6b) ppm; 13C NMR (100 MHz, DMSO-d6): δ = 159.0 (C-2), 158.1 (C-5), 151.7 (C-10a), 139.6 (C-6a), 134.0 (C-8), 123.1 (C-10), 123.0 (C-9), 116.4 (C-3a), 115.6 (C-7), 113.8 (C-4b), 111.7 (C-10a), 96.2 (C-4a), 50.0 (C-3), 32.1 (C-4), 29.8 (C-6b) ppm; 15 N NMR (40.55 MHz, DMSO-d6): δ = 141.6 (N-6), 85.0 (N-2a) ppm; MS (FAB, 70 eV): m/z (%) = 304.1 ([M + H]+, 65).

4-Amino-5,6-dihydro-6-ethyl-5-oxo-4H-pyrano[3,2-c]quinoline-3,4,4-tricarbonitrile (8b, C17H11N5O2)

Brown crystals (DMF/EtOH); yield: 0.252 g (80%); m.p.: 342–344 °C; IR (KBr): \(\overline{V}\)= 3346, 3334 (NH2), 2201 (CN), 1644 (CO), 1622 (Ar–C = N) cm−1; 1H NMR (400 MHz, DMSO-d6): δ = 8.53 (s, 2H, H-2a), 8.02 (dd, J = 8.1, 1.1 Hz, 1H, H-10), 7.87 (ddd, J = 8.5, 7.2, 1.4 Hz, 1H, H-8), 7.79 (d, J = 8.6 Hz, 1H, H-7), 7.48 (dd, J = 7.7, 7.3 Hz, 1H, H-9), 4.38 (q, J = 7.0 Hz, 2H, H-6b), 1.27 (t, J = 7.0 Hz, 3H, H-6c) ppm; 13C NMR (100 MHz, DMSO-d6): δ = 159.1 (C-2), 157.8 (C-5), 151.8 (C-10b), 138.5 (C-6a), 134.1 (C-8), 123.4 (C-10), 122.9 (C-9), 116.4 (C-3a), 115.3 (C-7), 113.8 (2C-4b), 111.9 (C-10a), 96.1 (C-4a), 49.9 (C-3), 37.5 (C-6b), 32.0 (C-4), 12.7 (C-6b) ppm; 15 N NMR (40.55 MHz, DMSO-d6): δ = 155.0 (N-6), 85.3 (N-2a) ppm; MS (FAB, 70 eV): m/z (%) = 318.1 ([M + H]+, 60).

4-Amino-5,6-dihydro-8-methyl-5-oxo-4H-pyrano[3,2-c]quinoline-3,4,4-tricarbonitrile (8c, C16H9N5O2)

Brown crystals (DMF/EtOH); yield: 0.273 g (83%); m.p.: 320–322 °C; IR (KBr): \(\overline{V}\) = 3333, 3320 (NH2), 3174 (NH), 2209 (CN), 1671 (CO), 1646 (Ar–C=N) cm−1; 1H NMR (400 MHz, DMSO-d6): δ = 12.45 (b, 1H, NH-6), 8.53 (b, 2H, H-2a), 7.73 (bs, 1H, H-10), 7.50 (bd, J = 8.5 Hz, 1H, H-9), 7.25 (d, J = 8.4 Hz, 1H, H-7), 2.45 (s, 3H, H-8a) ppm; 13C NMR (100 MHz, DMSO-d6): δ = 159.2 (C-2), 158.3 (C-5), 152.6 (C-10b), 136.9 (C-6a), 134.7 (C-9), 132.2 (C-8), 121.7 (C-10), 116.5 (C-3a), 115.8 (C-7), 113.9 (2C-4b), 111.9 (C-10a), 96.5 (C-4a), 49.1 (C-3), 31.3 (C-4), 20.5 (C-8a) ppm; MS (FAB, 70 eV): m/z (%) = 304.2 ([M + H]+, 45).

4-Amino-5,6-dihydro-9-methyl-5-oxo-4H-pyrano[3,2-c]quinoline-3,4,4-tricarbonitrile (8d, C16H9N5O2)

Brown crystals (DMF); yield: 0.260 g (79%); m.p.: 308–310 °C; IR (KBr): \(\overline{V}\)= 3334, 3322 (NH2), 3174 (NH), 2219 (CN), 1672 (CO), 1649 (Ar–C = N) cm−1; 1H NMR (400 MHz, DMSO-d6): δ = 12.43 (b, 1H, NH-6), 8.49 (b, 2H, H-2a), 7.70 (bs, 1H, H-10), 7.58 (bd, J = 8.5 Hz, 1H, H-8), 7.36 (d, J = 8.4 Hz, 1H, H-7), 2.40 (s, 3H, H-9a) ppm; 13C NMR (100 MHz, DMSO-d6): δ = 159.1 (C-2), 158.5 (C-5), 152.5 (C-10b), 136.9 (C-6a), 134.9 (C-8), 132.1 (C-9), 121.9 (C-10), 116.4 (C-3a), 115.9 (C-7), 113.8 (2C-4b), 110.8 (C-10a), 96.4 (C-4a), 49.9 (C-3), 31.5 (C-4), 20.6 (C-9a) ppm; 15 N NMR (40.55 MHz, DMSO-d6): δ = 146.8 (N-6), 85.5 (N-2a) ppm; MS (FAB, 70 eV): m/z (%) = 304.2 ([M + H]+, 35).

2-Amino-8-chloro-5,6-dihydro-5-oxo-4H-pyrano[3,2-c]quinoline-3,4,4-tricarbonitrile (8e, C15H6 ClN5O2)

Brown crystals (DMF/H2O); yield: 0.232 g (75%); m.p.: 336–338 °C; IR (KBr): \(\overline{V}\) = 3333, 3324 (NH2), 3193 (NH), 2213 (CN), 1691 (CO), 1650 (Ar–C = N) cm−1; 1H NMR (400 MHz, DMSO-d6): δ = 12.50 (b, 1H, NH-6), 8.88 (b, 2H, H-2a), 7.35 (bs, 1H, H-10), 7.10 (bd, J = 8.5 Hz, 1H, H-9), 7.70 (d, J = 8.4 Hz, 1H, H-7) ppm; 13C NMR (100 MHz, DMSO-d6): δ = 159.1 (C-2), 158.2 (C-5), 152.5 (C-10b), 136.9 (C-6a), 134.2 (C-8), 129.7 (C-10), 125.8 (C-9), 119.8 (C-7), 116.5 (C-3a), 113.9 (2C-4b), 111.8 (C-10a), 96.5 (C-4a), 50.1 (C-3), 31.3 (C-4) ppm; MS (FAB, 70 eV): m/z (%) = 324.1 ([M + H]+, 45).

2-Amino-9-chloro-5,6-dihydro-5-oxo-4H-pyrano[3,2-c]quinoline-3,4,4-tricarbonitrile (8f, C15H6 ClN5O2)

Brown crystals (DMF/MeOH); yield: 0.247 g (80%); m.p.: 342–344 °C; IR (KBr): \(\overline{V}\) = 3360, 3345 (NH2), 3279 (NH), 2215 (CN), 1674 (CO), 1605 (Ar–C = N) cm−1; 1H NMR (400 MHz, DMSO-d6): δ = 11.88 (s, 1H, NH-6), 8.55 (b, 2H, H-2a), 7.72–7.61 (m, 1H, H-10), 7.60–7.53 (m, 1H, H-8), 7.45–7.31 (m, 1H, H-7) ppm; 13C NMR (100 MHz, DMSO-d6): δ = 168.3 (C-2), 164.8 (C-5), 164.2 (C-10b), 162.9 (C-6a), 139.6 (C-9), 137.9 (C-10), 133.1 (C-8), 122.9 (C-7), 116.3 (C-3a), 113.3 (2C-4b), 100.5 (C-10a), 98.1 (C-4a), 51.3 (C-3), 35.7 (C-4) ppm; MS (FAB, 70 eV): m/z (%) = 324.1 ([M + H]+, 40).

2-Amino-9-bromo-5,6-dihydro-5-oxo-4H-pyrano[3,2-c]quinoline-3,4,4-tricarbonitrile (8 g, C15H6 BrN5O2)

Brown crystals (DMF/MeOH); yield: 0.215 g (78%); m.p.: 304–306 °C; IR (KBr): \(\overline{V}\) = 3376, 3350 (NH2), 3179 (NH), 2297 (CN), 1678 (CO), 1638 (Ar–C = N) cm−1; 1H NMR (400 MHz, DMSO-d6): δ = 12.62 (s, 1H, NH-6), 8.51 (b, 2H, H-2a), 8.07 (d, J = 2.3 Hz, 1H, H-10), 7.91 (dd, J = 8.9, 2.3 Hz, 1H, H-8), 7.40 (d, J = 8.9 Hz, 1H, H-7) ppm; 13C NMR (100 MHz, DMSO-d6): δ = 159.5 (C-2), 159.0 (C-5), 152.4 (C-10b), 138.3 (C-6a), 136.6 (C-9), 125.4 (C-10), 118.8 (C-8), 116.8 (C-7), 115.1 (C-3a), 114.1 (2C-4b), 113.3 (C-10a), 98.2 (C-4a), 50.3 (C-3), 34.7 (C-4) ppm; MS (FAB, 70 eV): m/z (%) = 368.1 ([M]+, 45).

Crystal structure determination of 8a

The single-crystal X-ray diffraction study was carried out on a Bruker D8 Venture diffractometer with PhotonII detector at 298(2) K using Cu-Ka radiation (λ = 1.54178 Å). Dual space methods (SHELXT) [20] were used for structure solution, and refinement was carried out using SHELXL-2014 (full-matrix least-squares on F2) [21]. Hydrogen atoms were localized by difference electron density determination and refined using a riding model (H(N) free). Semi-empirical absorption corrections and extinction corrections were applied.

8a: yellow crystals, C16H9N3O2, Mr = 303.28, crystal size 0.18 × 0.14 × 0.04 mm, monoclinic, space group P21/c (No. 14), a = 5.9873(1) Å, b = 14.9471(2) Å, c = 15.3227(2) Å, β = 91.016(1)°, V = 1371.06(3) Å3, Z = 4, ρ = 1.469 Mg m−3, µ(Cu-Kα) = 0.85 mm−1, F(000) = 624, T = 298 K, 2θmax = 144.2°, 16,614 reflections, of which 2708 were independent (Rint = 0.057), 216 parameters, 2 restraints, R1 = 0.042 (for 2491 I > 2σ(I)), wR2 = 0.118 (all data), S = 1.03, largest diff. peak / hole = 0.22 / -0.19 e Å−3.

CCDC 2,115,414 (8a) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.