Solid state halogen bonded networks vs. dynamic assemblies in solution: explaining N···X interactions of multivalent building blocks

Tetrapyridine functionalized resorcinarene macrocycles were used as multivalent building blocks for the construction of halogen bonded networks with aryl halide linkers. In the solid state, resorcinarene macrocycles and aryl halide linker molecules assembled into interpenetrated, multidimensional halogen bonded networks with porous structure caused by the 3D block scaffold of the resorcinarenes. 19 F NMR spectroscopy proved halogen bond formation also in solution, as either upfield or downfield shifts were observed depending on the bivalent or monovalent halogen bond binding mode. The binding mode in solution was explained by density functional theory computations.


Introduction
In recent years one of the compelling interactions in supramolecular chemistry has been halogen bonding (XB), 1 which is a noncovalent interaction between an electrophilic halogen atom and a Lewis base. 2 Particularly, the directionality and predictability of halogen bonds 3 have already achieved considerable attention in the construction of, for example, magnetic materials 4 and liquid crystals, 5 as well as other types of solid state networks. The simplest networks consist of linear building blocks, 6 but also more complex and multidimensional building blocks having several XB binding sites are known. Examples of more complicated structures include branched tectons 7 macrocyclic porphyrin structures, 8 piperazine cyclophanes 9 and bis-urea macrocycles 10 to mention a few.
One attractive approach to construct more diverse and multidimensional halogen bonded networks is to use a wellknown group of concave macrocycles, namely calixarenes or closely related resorcinarene, 11 as three-dimensional preorganized scaffolds. Previously, the macrocyclic cavity and halogen bonding interactions have mainly been utilized in guest binding 12  with a handful of examples reporting a capsular and deep cavity cavitand-like halogen bonded assemblies. 13 The boat conformation of resorcinarenes, in particular, has shown to be extremely versatile in the construction of multidimensional hydrogen bonded networks.
14 The well-defined and preorganized macrocyclic structure provides intrinsic dimensionality and order to the network assembly. Majority of the research of the halogen bonded assemblies has focused solely on the solid state structures, but understanding the properties and behavior of halogen bonding in solution has recently gained increasing interest. 15 To a great extent, NMR spectroscopy has proven to be a convenient and informative tool to study halogen bonding in solution. 6b,15,16 For smaller halogen bond systems even association constants reflecting the strength of the interaction have been determined.

15,16
Profound discussion of halogen bond formation and properties of multivalent XB donors and acceptors in solution, however, are still lacking, and phenomena, such as hierarchy, the number of halogen bonds formed in the multivalent systems (fully vs partially XB bonded) and the comparison to binding behavior in solid state structures, are still scarcely explained and poorly understood. Only recently, Jungbauer et al. 17 reported strong molecular recognition of a finite three-point halogen bonded assembly both in solution and in the solid state, where the receptor/substrate match was nearly perfect when the number of the donor and acceptor sites was equal but less optimal with substrates having fewer acceptor sites than the ideal three. Our earlier study explaining the properties of halogenated resorcinarenes and their solid state packing that was driven by intermolecular halogen bonds 18 intrigued us to explore the potential of tetrapyridine functionalized resorcinarenes in the construction of halogen bonded networks with aryl halides as connecting linkers. Herein, we present the synthesis and solution dynamics of two multidimensional resorcinarene based halogen bond acceptors, as well as, a successful construction of two network structures stabilized by halogen bonds, in which the dimensionality and architecture was obtained by the resorcinarene core, directionality by the podand pyridine arms and cohesion by the aryl halide linkers. Solution behavior of the halogen bond donor-acceptor systems were additionally investigated in detail using NMR spectroscopic techniques and the challenges imposed by the multiple XB sites, such as several possible simultaneous interactions in solution, were resolved by computational methods using density functional theory (DFT) that provided important insight into the changes observed in the 19 F NMR spectra and into the behavior of these complex systems in solution.

Results and discussion
Design and structural properties of the halogen bond acceptors 1 and 2 In order to form multidimensional halogen bonded networks expanding to four different directions, C-methyl tetramethoxy resorcinarene was selected as a platform and pyridine podand arms as the halogen bond accepting substituents on the upper rim (Fig. 1a). Since halogen bonds are highly directional and linear in nature, 3 two different constitutional isomers of tetravalent halogen bond acceptors differing only by the position of the nitrogen atom in the pyridine rings (3-or 4pyridyl; 1 and 2, respectively) were synthesized to control the node angles of the networks. The choice of the well-defined Cmethyl tetramethoxy resorcinarene 19 as a platform minimized the unwanted influence of the longer lower rim alkyl chains 20 on the crystal packing and on the network assembly.
The pyridine functionalized tetramethoxy resorcinarenes 1 and 2 were synthesized using a similar protocol as reported earlier for pyridine functionalized resorcinarenes by McIldowie et al. 21 resulting in the desired resorcinarene derivatives 1 and 2 with 4749% yields. Full structural characterization was done by NMR spectroscopy, high-resolution mass spectrometry and X-ray crystallography.
As expected, tetrapyridine functionalized C-methyl tetramethoxy resorcinarenes 1 and 2 adopt a boat conformation both in solution and in the solid state as a result of the lack of crown conformation stabilizing intramolecular upper rim hydrogen bonds. In solution, the boat conformation usually interconverts between boat 1 and boat 2 conformers ( Fig. 1a) but the conversion is typically too fast to be observed on the NMR time scale at 30 °C, and the proton signals appear as one sharp and averaged set of signals instead of the expected two for the C 2v symmetrical boat conformation. 22 The pyridine functionalized resorcinarenes 1 and 2, however, showed slower dynamic properties than anticipated, which was seen as very broad proton signals at 30 °C (Fig. 2). Therefore, variable temperature NMR measurements in deuterated chloroform starting at 30 °C and decreasing in 10 degree decrements to -30 °C were done in order to investigate the unexpected dynamic behavior in more detail. At -30 °C the proton spectra of the resorcinarene derivatives 1 and 2 indeed showed two sets of very sharp signals consistent with the C 2v symmetry of the boat conformation. Possible reasons for the unusually slow boat-to-boat interconversion were reasoned to be either the interaction of the pyridine substituents with solvent or the effect of the short lower rim alkyl chain length on the interconversion dynamics. To explore the possible solvent effects on the dynamic properties, a proton spectrum of the benzyl analogue 3 23 without Lewis basic pyridine nitrogens was recorded for comparison. The reference compound 3 showed the same broadening of the proton signals at 30 °C, which proved that intermolecular interactions between pyridine nitrogen and solvent played no role in dynamics leaving the flexibility of the macrocyclic ring caused by the short lower rim alkyl chains as a possible reason. This reasoning is also supported by the earlier reported resorcinarene tetrapodand derivatives 18 resorcinarene also indicate the conformational flexibility of the resorcinarene core. Therefore, it appears that larger substituents at the lower rim restrict the conformational mobility of the resorcinarene core more, whereas the smaller methyl groups allow extended movement thus slowing down the interconversion rate. Further evidence of the conformational mobility was provided by the crystal structures of the pyridine functionalized resorcinarene derivatives 1 and 2, which crystallized as four different solvate structures from chloroform (2-CHCl3) solution or from chloroform-methanol (1-MeOH), chloroform-acetonitrile (2-MeCN) and chloroformethanol (2-EtOH) mixtures of the respective resorcinarene derivatives (see crystallization details in ESI). In all of these structures, the resorcinarene skeleton adopts a boat conformation and in three of them (1-MeOH, 2-MeCN, and 2-EtOH) the horizontal aromatic rings are bent over the plane formed by the methine bridges (183.9-187.7). This overbending might be the cause for the slower boat-to-boat interconversion of resorcinarenes 1 and 2 in solution at higher temperature as seen by the broadened NMR signals. The crystal structures also confirmed the ability of the pyridine functionalized resorcinarene tetrapodands 1 and 2 to function as potential multidimensional halogen bond acceptors, which was seen in the orientation and weak interactions of the pyridine rings with the solvent molecules. Depending on the surrounding interactions or the lack of them the podand arms showed a clear tendency to orient in certain directions. The pyridine arm can settle according to the plane formed by its parent aromatic ring (anti-orientation, ArC-O-CH 2 -PyC) or perpendicular to the plane (gauche-orientation, Table S1). In the structures 1-MeOH and 2-EtOH the arms are hydrogen bonded to the solvent molecules and they all orient to the same direction as their parent aromatic ring (anti) expanding the cavity dimensions to the full, both vertically and horizontally (Type I, Fig. 3a and Fig. S12). On the other hand, without any solvent interactions present (structures 2-MeCN and 2-CHCl 3 ), one of the pyridine arms attached to the horizontal aromatic ring orients along the vertical plane (gauche) blocking the expansion of the cavity dimensions from that end (Type II, Fig. 3 and Fig. S13). This indicates that the anti-orientation is induced by the solvent coordination. The same two trends were also seen in earlier structures of pyridine functionalized resorcinarenes. 18,21 In all structures of compounds 1 and 2, regardless of the 3-or 4-position, the pyridine nitrogen atoms point away from the cavity being properly aligned for forming intermolecular interactions.

Halogen bonding
In order to achieve multidimensional halogen bonded networks, three different bivalent halogen bond donors with either linear 180 angle (A and B) or 120 angle (C) between the donor sites were selected as the linker molecules (Fig. 1b). Aryl halides suit well for the network formation because they are structurally rigid and not too elongated to impede the close packing in the solid state. Since the bivalent linkers A-C are able to form interactions from either one or two donor sites, two monovalent linkers (D and E, Fig. 1b) were additionally selected as reference due to their unambiguous interactions. Solid state networks. The study was started with crystallization experiments of tetravalent resorcinarenes 1 and 2 with the linkers A-E. So far only the crystallizations with the strongest halogen bond donor, namely linear bivalent iodo linker B, gave suitable crystals for crystallographic studies, affording a linear chain structure of the 3-pyridine functionalized resorcinarene 1 and a tangled grid structure of the 4-pyridine functionalized resorcinarene 2. In both of the structures, the resorcinarene core adopts a boat conformation and the torsion angles (ArC-O-CH 2 -PyC) of the pyridine arms are anti-oriented i.e. type I (Fig. 3a), and the pyridine rings are horizontally aligned. As such, the halogen bonds are aligned 2dimensionally but with respect to the resorcinarene platform, the bivalent linker B can interconnect via halogen bond interactions between the neighboring resorcinarene at two or more levels creating multidimensional nanostructures, which are discussed in more detail below.   (Fig. 4, orange and elemental color). The double chains are then piled on top of each other and the two crystallographically independent linker molecules stack diagonally in a 44.5 angle resembling a structure of a "leaning tower" (Fig. 4, blue). The piled double chains are horizontally packed side by side via the fourth pyridine arm (N1) with chloroform molecules filling the voids in between (Fig. S14). Resorcinarene 2 forms a 2:4 (resorcinarene:linker) network with the linker B when crystallized from a mixture of chloroform-methanol (structure 2-B). The structure is highly symmetrical showing only a half of the resorcinarene 2 and one linker molecule B in an asymmetric unit. Unlike in any of the other structures (2-MeCN, 2-CHCl 3 , 1-MeOH, 2-EtOH and 1-B) the boat conformation of the resorcinarene skeleton is highly twisted 28 causing a curved shape to the macrocycle core (Fig. S11). The repeating unit in this network structure, the grid mesh, is composed of four linker B molecules adjoining four resorcinarene 2 molecules at the nodes via N1···I104 (2.81 Å, 178.4°) and N2···I102 (2.82 Å, 175.6°) halogen bonds (mesh area 7.4 nm 2 , Fig. 6). All four pyridine arms are horizontally aligned at two different levels, similarly to resorcinarene derivative 1 in structure 1-B. This, together with the highly twisted and curved resorcinarene core, enables meshes of three consecutive individual grids ( Fig. 6c: red, blue and orange) to go under and over each other thus forming an interpenetrated structure, where the crossing points of the grids are held together by a linker-pyridine - stacking interaction (3.65 Å). Each tangled grid layer is composed of resorcinarene molecules with the same inherent chirality. The layers are stacked diagonally on top of one another alternating with the inherent chirality of resorcinarenes (Fig. S15). The stacked layers form porous channels filled with chloroform molecules (cross-section area 64 Å 2 , Fig. 7). Thus, unlike in structure 1-B, where the different inherently chiral resorcinarenes are part of the same structural entity, in structure 2-B, the self-assembly shows enantiomeric separation.
These two network structures show that the pyridine functionalized resorcinarenes 1 and 2 have an ideal geometry to form multilevel 2-dimensional frameworks with the linear bivalent linker molecules in the solid state, which is mainly due to the halogen bond accepting pyridine arms that settle into different levels provided by the resorcinarene cavity in the boat conformation. The rigidity and concave shape of the resorcinarene core combined with the planar linkers furthermore enable the formation of porous structures as seen by the solvent filled interstice. In addition, varying the position of the pyridine nitrogen atoms controls the direction of the Please do not adjust margins Please do not adjust margins  Dynamic solution assemblies. In addition to the explicit solid state networks, the solution behavior between the tetravalent resorcinarenes 1 and 2 and the bi-and monovalent linkers AE in terms of halogen bonding were complemented by means of 1 H, 19 F and diffusion ordered (DOSY) NMR spectroscopy.
Deuterated chloroform was used as a solvent because of the poor solubility of resorcinarenes 1 and 2 in other solvents. In solution there are a multitude of different possible interactions between resorcinarenes (R), linkers (L) and the surrounding solvent molecules (S). Thus, in addition to halogen bonding, also hydrogen bonding and halogen-halogen interactions have to be taken into consideration (Fig. 8). 19 F NMR spectroscopy is ideal for studying the interactions of the aryl halide linkers as the changes in the chemical shifts of the linker molecules can be followed directly, and the information is also conveyed about the nature and the strength of the interaction. Therefore, in order to study halogen bonding between resorcinarenes 12 and the linker molecules AE, samples with different concentrations of the interacting species (1:0 to 1:1) were prepared and the chemical shift differences then compared with the signals of the pure linker molecule. It was contemplated that increasing the equivalency of the halogen bond accepting resorcinarenes the probability for halogen bond formation will also increase.
With monovalent linkers D and E increasing the concentration of the halogen bond acceptor resorcinarenes 1 and 2 showed a downfield shift of the linker fluorine signals up to 1:1 mixtures, consistent to the previously reported literature 16c (Fig. 9a and S16-19). The iodo linker E showed a much larger downfield shift (up to 0.15 ppm) than the bromo linker D, which showed only a minimal shift of 0.01 ppm. The bivalent meta-substituted linker C showed similar changes in the chemical shifts to the monovalent linkers E and D (Fig. 9d): increasing the resorcinarene concentration caused a downfield shift (avg. 0.01-0.04 ppm), and at 1:1 mixtures the shift became more significant (avg. 0.11 ppm). In the case of the bivalent linear para-substituted linkers A and B, increasing the resorcinarene concentration caused at first a downfield shift of the fluorine signals (0.02-0.05 ppm, Fig. 9b-c), but when the concentration of resorcinarene was increased up to 1:1 ratio the fluorine signals shifted dramatically upfield (-0.21 ppm for A and -0.03 ppm for B, Fig. 9b and c). Contrary to monolinkers D and E, the largest changes in the fluorine shifts were observed with the bromo linker A (0.21 ppm) instead of the linker B with iodo atoms as XB donors (0.03 ppm).
In order to explore the reasons behind the bivalent linkers A and B showing both downfield and upfield shifts in the 19 F NMR studies, DFT calculations of the model systems composed of halogen bond donating linkers A and B, halogen bond accepting pyridine ring and solvent chloroform were performed (Fig. S24, Table S3). As a starting point the relative 19 F NMR shift of the fluorine atoms of the different interactions summarized in Fig. 8 were calculated (Fig. 10). In 1:0 mixture (L:R) the possible prevailing interaction of the linkers in solution is hydrogen bonds (HB) with the solvent, and thus the signal of a linker is most likely an average of these interactions (L···S). The linker has the total of four fluorine atoms meaning that in theory each linker could form four HBs. Therefore, 19 F NMR shifts were calculated for combinations of pure L, L+S, L+2S and L+4S which shows a shift trend toward upfield, respectively. The upward shift takes also place in experimental measurements since the chemical shifts of linkers A and B in noncoordinating solvent, pentane, are downfield (-132.89 ppm for A and -119.44 ppm for B) 16b compared to the chemical shifts in deuterated chloroform (-134.48 ppm for A and -121.20 ppm for B). When a halogen bond acceptor, pyridine, is added to the model system, the linker A (L···R) shows a downfield shift compared to the average L+S shift. With the linker B, however, the signal shifts slightly upfield, but it is downfield compared to L+4S signal. When the concentration of the bivalent para linkers A and B is decreased, the probability of forming two halogen bonds from the opposite XB donor sites (R···L···R) becomes the prevailing type of interaction instead of single halogen bonds. In the presence of the double halogen bonding, the calculated signals of the linkers clearly shift upfield. Therefore, with a large excess of halogen bond donating linkers, it is more likely Please do not adjust margins Please do not adjust margins that only a single halogen bond forms in solution, which is seen as a downfield shift of the fluorine signals. When the halogen bond acceptor concentration exceeds the available donor sites, the probability of double halogen bonding to the bivalent linker molecule increases, which would explain the experimentally observed upfield shift of the fluorine signal upon the increased concentration of resorcinarene (1:1). An intriguing observation is that the bivalent meta-linker C seems to behave similarly to the monovalent linkers D and E, as observed by downfield shifts of the fluorine signals in the NMR spectrum. This suggests that the angle of the metasubstitution is not ideal for double halogen bonding to the neighboring resorcinarene derivatives.
The shapes of the fluorine NMR signals are much broader for linker B than for linker A, which could indicate that there are more simultaneous, and competing interactions between the iodo linker B and resorcinarene acceptors and solvent molecules. Therefore, the halogen bonding process could be more dynamic in nature than that of the bromo linker A. The sharper fluorine signals of the bromo linker A at higher resorcinarene concentrations, on the other hand, indicate most likely that the system reaches equilibrium faster and majority of the linker molecules are double halogen bonded. 1 H NMR measurements, which disclose the information of the halogen bonding interaction with respect to the halogen bond accepting resorcinarenes, were done at -30 °C because of the dynamic behavior of the resorcinarene macrocycle broadening the proton signals at higher temperatures. In the proton spectra the changes in the chemical shifts of the pyridine arms of resorcinarenes 1-2 in the presence of linkers A-E were not as significant as the shifts in the fluorine spectra, but the changes were nonetheless distinguishable (0.01-0.05 ppm Fig. S23). Due to the C 2v symmetry of the boat conformation two sets of signals of the pyridine arms are visible corresponding to the pyridine podand arms attached to either on the horizontal or vertical aromatic rings. Only one set of signals was affected upon halogen bonding suggesting that the tetravalent resorcinarenes 1 and 2 do not form more than two halogen bonds in solution unlike in the solid state, where three or all four halogen bond accepting sites were occupied.
In order to identify possible network formation in solution, DOSY NMR measurements were performed in deuterated chloroform for 1:1 mixtures (L:R) of the resorcinarenes 1 and 2 and the bivalent halogen bond donating linkers AC. The average diffusion coefficients obtained for the linkerresorcinarene mixtures (5.82-6.08x10 -10 m 2 s -1 ), however, did not differ notably from the diffusion coefficients of the pure resorcinarene solutions (5.93-5.94x10 -10 m 2 s -1 , Table S3). This indicates that in solution stable larger structures are not formed even though halogen bonds are present. This is most likely caused by the more complex and competing interactions prevailing in solution compared to the solid state. In addition, the complex dynamic behavior of C-methyl resorcinarene skeleton shown in variable temperature NMR could prevent the optimal preorganization of the resorcinarene framework and self-assembly into larger network in solution.

Conclusions
Tetramethoxy resorcinarene derivatives with 3-(1) and 4pyridine (2) podand arms were synthesized in order to use them as tetravalent halogen bond acceptors to construct multidimensional network structures with monovalent bromoand iodopentafluorobenzene (D-E) and bivalent para-and meta-substituted dibromo-and diiodotetrafluorobenzene (A-C) halogen bond donors in the solid state and in solution. In the solid state, multidimensional halogen bonded networks with a potential to porosity were successfully crystallized with the strongest halogen bond forming 1,4diiodotetrafluorobenzene linker B. 3-Pyridyl functionalized resorcinarene 1 assembled into a linear chain structure, whereas, interlocked tangled grid structure was obtained with the 4-pyridyl substituted resorcinarene 2, with enantiomeric separation of the left and right handed resorcinarene isomers. The halogen bonds expand 2-dimensionally but the multidimensionality of the structures are achieved by the 3-D structure of the resorcinarene.
The halogen bond formation between the resorcinarenes and all of the halogen bond donors was also observed in solution by NMR spectroscopic studies, but the formation of larger networks was not observed, likely due to complex dynamics of the resorcinarene core and competing multiple interactions. The number of donor sites (mono vs bivalent linkers) and the position of the donor atom (para-vs. metasubstitution) were all observed to affect the halogen bond formation in solution either as a downfield shift (monovalent and meta-substituted linkers) or as an upfield shift (bivalent para-substituted linkers) in the linker 19 F NMR signals. DFT calculations of respective model systems supported the observed relationship between the direction of the chemical shift and binding mode. The combination of DFT calculations Please do not adjust margins Please do not adjust margins and experimental methods is thus a convenient tool for predicting and interpreting halogen bonding in solution, which is still poorly understood phenomenon with larger building blocks. Pyridine functionalized resorcinarenes 1 and 2 with preorganized structural features of the core and podand arms have proven to be an inspiring example of how predefined 3D macrocyclic structures can be exploited in the design and construction of multilevel nanosized halogen bond networks with a potential to porosity.

Experimental
General 1 H, 13 C, 19 F, COSY, HMQC and HMBC NMR spectra were recorded on a Bruker Avance DRX 500 MHz spectrometer and DOSY measurements on a Bruker Avance DRX 400 MHz spectrometer. Accurate ESI mass spectra were measured with a Micromass LCT ESI-TOF instrument. Melting points were obtained with a Stuart melting point apparatus SMP3. Tetramethoxy resorcinarene was prepared according to a literature procedure. 19 All other reagents were commercial and used as received. Detailed synthetic procedures and characterizations are provided in ESI.

3-Pyridyl Functionalized Resorcinarene Tetrapodand 1.
A mixture of tetramethoxy resorcinarene 19 (0.25 g, 0.42 mmol) and Cs 2 CO 3 (1.13 g, 3.47 mmol) in dry DMF (20 ml) at 90 °C (bath temperature) was stirred for 60 min under nitrogen before the dropwise addition of 3-(chloromethyl)pyridine hydrochloride (0.31 g, 1.88 mmol) in DMF (10 ml). The dark brown suspension was stirred at 90 °C for 21 hours, after which it was filtered through a pad of Hyflo Super®. Solvent was evaporated under reduced pressure and the residue was dissolved in dichloromethane (70 ml). The organic layer was washed once with deionized water (70 ml) and once with brine (70 ml) before drying it with MgSO 4 . Solvent was evaporated under reduced pressure. Recrystallization of the residue from a methanol-chloroform solution afforded an off-white powder (yield 0. 19

4-Pyridyl Functionalized Resorcinarene Tetrapodand 2.
A mixture of tetramethoxy resorcinarene 19 (0.25 g, 0.42 mmol) and Cs 2 CO 3 (1.18 g, 3.62 mmol) in dry DMF (20 ml) at 90 °C (bath temperature) was stirred for 60 min under nitrogen before the dropwise addition of 4-(chloromethyl)pyridine hydrochloride (0.31 g, 1.88 mmol) in DMF (10 ml). The brown suspension was stirred at 90 °C for 22 hours, after which it was filtered through a pad of Hyflo Super®. Solvent was evaporated under reduced pressure. The residue was dissolved in dichloromethane (50 ml) and it was washed twice with brine (2×50 ml). The organic layer was then dried over found 965.4465. Benzyl Functionalized Resorcinarene Tetrapodand 3. A white suspension of tetramethoxy resorcinarene 19 (0.25 g, 0.42 mmol), benzyl bromide (210 µl, 1.77 mmol), dibenzo-18crown-6 (0.04 g, 0.12 mmol) and Cs 2 CO 3 (0.98 g, 2.99 mmol) was refluxed in acetonitrile (30 ml) under nitrogen for 24 h. The warm reaction mixture was filtered by suction through a pad of Hyflo Super®. Chloroform was filtered through the same pad of Hyflo Super® in case the product had crystallized during filtration. The solvents were then evaporated under vacuum and the resulting off-white residue was dissolved in chloroform (50 ml). The organic layer was washed once with deionized water (50 ml) and once with brine (50 ml) after which it was dried over MgSO 4 . The solvent was evaporated to dryness under vacuum. The product was recrystallized twice from a methanol-chloroform solution. Yield: 0.34 g (85%) of white crystalline solid; m.p. 235-236 °C;