CrystEngCommwww.rsc.org/crystengcomm Volume 15 | Number 46 | 14 December 2013 | Pages 9823–10144

COVER ARTICLEKubicki et al. Pseudosymmetry, polymorphism and weak interactions: 4,4”-difluoro-5´-hydroxy-1,1´:3´,1”-terphenyl-4´-carboxylic acid and its derivatives

CrystEngComm

PAPER

a Faculty of Chemistry, Adam Mickiewicz University, Umultowska 90b, 61–614

Poznań, Poland. E-mail: mkubicki@amu.edu.pl; Tel: +48618291256bDepartment of Studies in Chemistry, Mangalore University, Mangalagangotri-574

199, Indiac Department of Studies in Chemistry, University of Mysore, Manasagangotri,

Mysore-570 006, India

† CCDC numbers 933612 (1), 933613 (2A), 933614 (2B) and 933615 (3). Forcrystallographic data in CIF or other electronic format See DOI: 10.1039/c3ce41639a

CrystEngComm,This journal is © The Royal Society of Chemistry 2013

Cite this: CrystEngComm, 2013, 15,9893

Received 17th August 2013,Accepted 29th August 2013

DOI: 10.1039/c3ce41639a

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Pseudosymmetry, polymorphism and weakinteractions: 4,4′′-difluoro-5′-hydroxy-1,1′:3′,1′′-terphenyl-4′-carboxylic acid and its derivatives†

Anita M. Owczarzak,a Seranthimata Samshuddin,b Badiadka Narayana,b

Hemmige S. Yathirajanbc and Maciej Kubicki*a

The crystal structures of three derivatives of 4,4′′-difluoro-5′-hydroxy-1,1′:3′,1′′-terphenyl-4′-carboxylic acid are

discussed. The acid itself (1), its ethyl ester (2)and hydrazide (3) have been chosen to study the influence of the

hydrogen bonding potential on the crystal packing. In 1 and 2 short intramolecular O–H⋯O hydrogen bonds

between the hydroxyl and carbonyl groups engage the strong hydrogen bond donors and acceptors, and both

these compounds show the effects of packing conflicts. In 1 almost centrosymmetric, stable hydrogen-bonded

dimers form between symmetry independent molecules, but the crystal structure is non-centrosymmetric and con-

tains altogether four symmetry-independent molecules (two independent dimers), which show different pseudo-

symmetries. In 2 dimer formation is impossible but two different crystal forms of this compound have been found.

Both polymorphs crystallize in the P1 space group and differ mainly in the orientation of the OEt group. In turn in

3 there are no intramolecular hydrogen bonds and the crystal structure is determined mainly by the open motifs

created by classical hydrogen bonds and by the complementarity of the respective hydrophilic and hydrophobic

parts of the molecule.

Introduction

It has been generally accepted that multiple molecules in theasymmetric part of the unit cell are often, among other fac-tors like modulation, equi-energetic conformations, crystalli-zation kinetics etc., the result of a conflict between differentfactors influencing the crystal packing such as the tendencytowards close packing, space group constraints, intermolecularinteractions etc.1 This phenomenon has been widely discussed2

as it seems to be crucial for e.g. crystal structure predictionand there is even a website (http://www.dur.ac.uk/zprime). Forinstance, we have reported the strange case of two polymor-phic forms of a carbazole derivative, each having foursymmetry-independent molecules (Z′ = 4)3; recently we havealso reported the experimental charge-density analysis for thestructure with Z′=2.4 Anderson et al.,5 on the basis of anexhaustive analysis of the Cambridge Crystallographic

Database, concluded that “(…) formation of co-crystals orstructures with Z′ > 1 is strongly linked to small, rigid, awk-wardly shaped molecules that have more constraints on theircrystal packing requirements.”

Quite often the existence of multiple molecules is accom-panied by pseudosymmetry, the approximate symmetry oper-ations which do not belong to the space group of a givencrystal form, between the symmetry-independent molecules;6

this might be connected for instance with the presence of dif-ferent enantiomers in the crystal structure in the case whenthe intermolecular interactions prefer the centrosymmetricmotifs (e.g. carboxylic acid dimers).7 Gavezzotti8 found out that83% of the Z′ = 2 crystals show some form of pseudosymmetry.Also this phenomenon was recently studied by means of theexperimental charge density analysis for 3-isopropyl-4-thiomethyl-N6-benzoylsidnone imine)9 which crystallizes withZ′ = 4 and displays a high degree of pseudosymmetry.

Polymorphism has been also connected with the packingconflicts, i.e. the possibility of the (almost) equi-energeticaldisposition of molecules in the crystal.

Here we report the crystal structures of three compounds(Scheme 1), 4,4′′-difluoro-5′-hydroxy-1,1′:3′,1′′-terphenyl-4′-carboxylic acid (1) and its derivatives the ethyl ester (2) andhydrazide (3) which differ in terms of their intra- orintermolecular hydrogen bonding possibilities. To our surprise,structural reports on such compounds are quite rare. In the

2013, 15, 9893–9898 | 9893

Scheme 1 The studied compounds: 1: R = OH, 2: R = OC2H5, 3: R = NH–NH2.

Fig. 1 The perspective view of the A–B dimer of molecule 1; the ellipsoids are

drawn at the 50% probability level, the hydrogen atoms are represented by the

spheres of arbitrary radii and the hydrogen bonds are shown as thin blue lines.

Fig. 2 The perspective view of molecule 2A; the ellipsoids are drawn at the 50%

probability level, the hydrogen atoms are represented by the spheres of arbitrary

radii and the intramolecular hydrogen bond is shown as a thin blue line.

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Cambridge Structural Database10 (ver. 5.34, last update May2013) there are no structures of 1,1′:3′,1′′-terphenyl deriva-tives with both hydroxyl and carboxyl functions (in disposi-tion like that in 1), and there is only one ester (methyl2′-fluoro-5′-hydroxy-1,1′;3′,1′′-terphenyl-4′-carboxy-late11) andone carboxamide (N-(1,3-dioxo-2,3-dihydro-1H-isoindol-2-yl)-4,4′′-difluoro-5′-hydroxy-1,1′:3′,1′′-terphenyl-4′-carboxamide12).The database analysis prompted to another interesting fact:for compounds with a benzene ring with hydroxyl and car-boxylic acid functions in the neighbouring positions (onlyorganic, no fused rings) we have found over the averagenumber of the structures with multiple molecules (Z′ > 1).Such a situation was observed in 41 out of 305 single struc-tures (the re-determinations were rejected), that is for almost13.5% of structures, the mean value for all of the structures isaround 8.8% and is claimed to not change significantly withthe growth of the CDB.2 This can suggest that this certain dis-position of the substituents may be connected with the higherprobability of structures with Z′ > 1.

Results and discussionPseudosymmetry and Z′ = 4 in 1

In the structure of carboxylic acid, 1, there are foursymmetry-independent molecules in the asymmetric part ofthe unit cell. These molecules do not differ significantly; forthe bond lengths the maximum difference is 5 standard devi-ations (where σ is defined as the root square of the sum ofσ1

2 and σ22, individual e.s.d.'s) and for the bond angles, it is

only 4 similarly defined standard deviations. The more seri-ous differences are seen at the conformation level; forinstance, the dihedral angle between the mean planes of thetermina rings are in the range from 58.86(6)° to 76.28(7)°, i.e.188 standard deviations. Furthermore, the four molecules aregrouped in two pairs of centrosymmetric dimers (typical forcarboxylic acids), connected by strong O–H⋯O hydrogenbonds, and these dimers are nearly centrosymmetric (the per-spective view of one of the dimers is presented in Fig. 1).

We propose to regard as the measure of the degree ofpseudosymmetry, in case of a pseudo-center of symmetry, themean values of the sums of appropriate coordinates, i.e. <x> =1/2<xA + xB>, <y> = 1/2<yA + yB> and <z> = 1/2<zA + zB>,where xA, xB etc. are the coordinates of related atoms and the

9894 | CrystEngComm, 2013, 15, 9893–9898

standard deviations of such mean values. In the case of 1 thevalues are: for the pair AB: <x> = 0.8935(8), <y> = 0.126(5),<z> = 0.1651(10) and for the pair CD: <x> = 0.8928(4), <y> =0.379(4), and <z> = 0.6703(16). One can realize also, thatthese pseudo-centers of symmetry are further connected by apseudo-c-glide, which is parallel and lies midway between thecrystallographic exact c-glides, perpendicular to b with thesymmetry operation x, 1/2 − y, 1/2 + z. The different pseudo-symmetry combinations can be also found for other pairs ofmolecules, but in general the quality of these elements, asmeasured by the standard deviations, are at least an order ofmagnitude worse than for the pseudo-centers of symmetrydescribed above. The pairs AC and BD are connected bypseudo-c-glide (as above) while the pairs AD and BC areconnected by something like a two-fold axis with the transla-tion by ¼ (pseudosymmetry operations: 1.8 − x, 1/4 + y, 0.84 − zand 1.8 − x, −1/4 + y, 0.8 − z, respectively).

Polymorphism in 2

For the ethanolate (2, Fig. 2) we have found two polymorphicforms, both crystallizing in the triclinic P1 space group. The

This journal is © The Royal Society of Chemistry 2013

Fig. 4 The perspective view of molecule 3; the ellipsoids are drawn at the 50%

probability level, the hydrogen atoms are represented by the spheres of arbitrary radii.

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bond lengths in the two forms are very similar; using thecomparison as described earlier, they are within the 3.5 σ

limit. However, contrary to the pseudosymmetric case above,the maximum difference in the bond angles is almost 8 σ,and the conformation of the two molecules is significantlydifferent (up to 56 σ).

A comparison of the two molecules is shown in Fig. 3. Thelargest differences are connected with the orientation of theCOOEt substituent, and also, to a lesser degree, with the ori-entation of the rings.

The principal difference is that while in 2A the ethyl groupis twisted with respect to the COO group (the displacementof the C22 atom from the COO plane is 0.257(3) Å), in 2B it isalmost coplanar (C22 is only 0.042(4) Å out of the plane ofCOO group).

In the crystal structure two most important specific,directional interactions are also the same in both polymorphs:centrosymmetric dimers are created by intermolecularC8–H8⋯O9 and C12–H12⋯F16 hydrogen bonds. The differ-ences are seen in the weaker interactions, including C–H⋯π

directional contacts. No π⋯π contacts are observed.

Conformation of the molecules

The overall conformation of the molecules can be describedby the dihedral angles between the planar fragments. Theappropriate values are listed in Table 1.

The overall twist, as defined by the angle between the ter-minal rings, is quite similar in all 7 molecules (Fig. 4 showsmolecule 3) It might be however noted that this twist isobtained differently in 1 and 3, where the A/C angle is largerthan the other two, and in 2, where the largest value is for

Fig. 3 A comparison of the molecules 2A and 2B, one of the terminal phenyl

rings were fitted one onto another.

Table 1 The dihedral angles between the least-squares planes (°); A: ringC1⋯C6, B: C7⋯C12, C: C13⋯C18

A/B B/C A/C B/COO (CON)

1A 19.26(13) 69.18(8) 76.30(8) 4.1(4)1B 22.13(13) 66.11(8) 71.93(8) 4.8(4)1C 12.20(15) 51.99(7) 60.22(7) 16.0(4)1D 18.67(13) 50.21(7) 58.86(7) 16.9(4)2A 26.23(11) 70.33(6) 65.18(7) 6.5(4)2B 31.55(9) 70.00(6) 60.48(7) 19.8(2)3 29.63(8) 46.16(6) 69.62(6) 64.94(9)

This journal is © The Royal Society of Chemistry 2013

the angle A/B, which means that the sense of twist along theC11–C13 angle is reversed. The orientation of the substituentis closely related with the intramolecular O–H⋯O hydrogenbond, which causes a small twist of COO group with respectto the ring plane in 1 and 2 (15–30°), while the lack of thisbond in 3 is connected with the twist of the CON plane by64.94(9)°.

Hydrogen bonds and other weak interaction

The geometrical data of the interactions are listed in Table 2.In 1 and 2 there are intramolecular O–H⋯O hydrogen bonds,in 1 additionally there are, as mentioned above, relativelystrong intermolecular O–H⋯O hydrogen bonds, that connecttwo symmetry-independent molecules into a typical carboxylic

Table 2 Hydrogen bond data

1

D H A D–H H⋯A D⋯A D–H⋯AO9A H9A O20A 0.91(4) 1.71(4) 2.548(2) 153(4)O9B H9B O20B 0.85(3) 1.80(3) 2.541(2) 144(3)O9C H9C O20C 0.99(5) 1.64(5) 2.564(2) 154(4)O9D H9D O20D 0.96(4) 1.75(4) 2.589(2) 143(3)O21A H21A O20B 1.25(7) 1.42(7) 2.636(2) 163(6)O21B H21B O20A 0.89(3) 1.75(3) 2.644(2) 178(3)O21C H21C O20D 1.03(5) 1.60(5) 2.625(2) 174(4)O21D H21D O20C 0.88(3) 1.73(3) 2.610(2) 171(3)C17A H17A F4Ca 0.95 2.25 3.126(3) 154C17B H17B F4Bb 0.91 2.34 3.104(2) 142

2AO9 H9 O20 0.94(3) 1.65(3) 2.536(2) 155(3)C8 H8 O9c 0.96(2) 2.58(2) 3.518(2) 164(2)C12 H12 F16d 1.00(2) 2.50(2) 3.494(2) 175(1)

2BO9 H9 O20 0.93(3) 1.69(3) 2.567(2) 156(2)C8 H8 O9c 0.97(2) 2.66(2) 3.551(3) 154(2)C12 H12 F16e 0.93(2) 2.62(2) 3.524(2) 164(2)

3O9 H9 N22f 0.87(3) 2.07(3) 2.917(2) 164(2)N21 H21 O20g 0.90(2) 1.93(2) 2.730(2) 148(2)C18 H18 F4h 0.99(2) 2.44(2) 3.385(2) 159(1)

Symmetry codes:a −1 + x, y, z. b x, −1 − y, −1/2 + z. c 1 − x, 1 − y,1 − z. d

1 − x, 2 − y, −z. e 3 − x, 2 − y, 1 − z. f 1/2 + x, 1/2 − y, −1/2 + z. g −1/2 +x,1/2 − y, −1/2 + z. h 2 − x, −y, −z.

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Fig. 6 The packing scheme of 1; the symmetry-independent molecules are shown

with different colours.

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group dimer. The positions of the hydrogen atoms in thesebonds are different, from being well localized at the oxy-gen atom to being displaced towards the acceptor oxygenatom. In the difference Fourier maps, calculated for themodels without the H21 atoms, these differences areclearly seen (Fig. 5).

In these two compounds the strong hydrogen bond donorsare involved in the intramolecular and intra-dimer hydrogenbonds, so the packing of the molecules is determined byweaker interactions. In 3 in turn there are no intramolecularhydrogen bonds. The hydroxyl group acts as the donor for theO–H⋯N intermolecular hydrogen bonds, but three N–Hgroups are involved only in one weak N–H⋯O(carbonyl)hydrogen bond and two very weak contacts.

In 1 there are some specific although weak interactions:five relatively short C–H⋯F hydrogen bonds (cf. Table 2),some weaker C–H⋯F (C⋯F distances longer than 3.3 Å) andC–H⋯O contacts, as well as a few relatively short π⋯π stack-ing contacts (centroid⋯centroid distances of 3.7–3.8 Å,interplanar distances of ca. 3.6 Å) and C–H⋯π interactionswith H⋯centroid distances below 2.8 Å. In the crystal struc-ture one could therefore recognize the columns of alternatedimers (Fig. 6).

In both polymorphs of 2 there are two relatively direc-tional contacts, mentioned above (cf. Table 2) and almost no

Fig. 5 Fragments of the difference Fourier maps after removing the H21

hydrogen atoms. The contour level is 0.1 e Å−3

(blue-negative, green-positive, dashed

red-zero level).

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other short contacts (H⋯F distances larger than 2.5 Å), andonly very vague C–H⋯π interactions. So, both of these forms,due to the involvement of both the strong hydrogen bonddonor and acceptor in the intramolecular hydrogen bond, aredetermined mainly by the adhesion forces and close packingrequirements; this may be regarded as an explanation of theeasily formed different crystalline forms.

In 3 in turn there are no intramolecular hydrogen bonds,and the structure is determined by classical hydrogen bonds(Fig. 7). One can observe the hydrophobic and hydrophilicregions sticking towards similar regions of neighbouringmolecules. Interestingly, only one out of three NH hydrogenatoms is involved in medium strong hydrogen bonds asthe other two are far from any potential hydrogen bondacceptors.

In all three cases the F atoms tend to take part in weakC–F⋯H–C contacts rather than to stick away from the rest ofthe structure; therefore fluorophilic rather than fluorophobiceffects are involved in the crystal structures.13

Fig. 7 The packing of 3 as seen along the z-direction; thin blue lines denote the

hydrogen bonds.

This journal is © The Royal Society of Chemistry 2013

‡ Crystal data: 1: C19H12F2O3, Mr = 326.29, monoclinic, Pc, a = 13.4533(5) Å, b =9.8330(4) Å, c = 22.8428(9) Å, β = 101.296(9)°, V = 3017.0(2) Å

3, Z = 8, dx = 1.44 g cm

−3,

F(000) = 1344, μ = 0.96mm−1, 12858 reflections collected up to θ = 73.7°, 7066

unique reflections (Rint = 0.022), 6851 with I > 2σ(I). Final R(I > 2σ(I)) = 0.033,wR2(I > 2σ(I)) = 0.091, R(all data) = 0.034, wR2(all data) = 0.092, S = 1.02, max/minΔρ 0.22/−0.19 e Å

−3. 2A: C21H16F2O3, Mr = 354.34, triclinic, P1, a = 7.3019(6) Å,

b = 10.2526(9) Å, c = 13.0430(10) Å, α = 94.613(7)°, β = 103.996(7)°, γ = 110.709(8)°,V = 871.30(14) Å

3, Z = 2, dx = 1.35 g cm

−3, F(000) = 368, μ = 0.10 mm

−1, 5343

reflections collected up to θ = 27°, 3409 unique reflections (Rint = 0.014), 2401with I > 2σ(I). Final R(I > 2σ(I)) = 0.050, wR2(I > 2σ(I)) = 0.116, R(all data) = 0.074,wR2(all data) = 0.132, S = 1.05, max/min Δρ 0.15/−0.14 e Å

−3. 2B: C21H16F2O3,

Mr = 354.34, triclinic, P1, a = 6.9778(3) Å, b = 11.1289(5) Å, c = 11.5883(5) Å, α =96.213(3)°, β = 98.252(4)°, γ = 93.542(4)°, V = 878.80(7) Å

3, Z = 2, dx = 1.34 g cm

−3, F

(000) = 368, μ = 0.10 mm−1, 9504 reflections collected up to θ = 25°, 3080 unique

reflections (Rint = 0.024), 1988 with I > 2σ(I). Final R(I > 2σ(I)) = 0.048, wR2(I >

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ExperimentalChemistry

The synthetic strategies adopted to obtain the target com-pounds are depicted in Scheme 2. The key intermediate inthe present study is ethyl 4,4′′-difluoro-5′-hydroxy-1,1′:3′,1′′-terphenyl-4′-carboxylate 2, which is prepared from 4,4′-difluoro chalcone according to the method described in ourprevious work.14 The ester thus obtained is hydrolyzed to giveits acid derivative 1. Also, the reaction of the ester with hydra-zine hydrate resulted in the formation of its carbohydrazidederivative 3.

The recrystallization of the key intermediate ethyl 4,4′′-difluoro-5′-hydroxy-1,1′:3′,1′′-terphenyl-4′-carboxylate 2 in dif-ferent solvents (ethanol and DMF) resulted in two polymor-phic forms of the compound (2A-ethanol, 2B-DMF).

General procedures for the synthesis

The synthesis of ethyl 4,4′′-difluoro-5′-hydroxy-1,1′:3′,1′′-terphenyl-4′-carboxylate (2). Ethyl 4,4′′-difluoro-5′-hydroxy-1,1′:3′,1′′-terphenyl-4′-carboxylate (1) was synthesized by theoxidative aromatization of ethyl 4,6-bis(4-fluorophenyl)-2-oxocyclohex-3-ene-1-carboxylate using chloramine–T, whichin turn was prepared by the condensation of ethylacetoacetate to 4,4′-difluoro chalcone according to themethod described in our previous work.4

The synthesis of 4,4′′-difluoro-5′-hydroxy-1,1′:3′,1′′-terphenyl-4′-carboxylic acid (1). A mixture of ester 1 (1.77 g, 0.005 mol)and sodium hydroxide (0.4 g, 0.1 mol) in ethanol (60 mL) washeated under reflux for 20 h and then cooled to roomtemperature. The reaction mixture was extracted from etherfollowed by sodium bicarbonate solution. The aqueous layerwas then neutralized with dilute hydrochloric acid to yield thepure acid derivative. Yield 78%; mp 190–193 °C.

The synthesis of 4,4′′-difluoro-5′-hydroxy-1,1′:3′,1′′-terphenyl-4′-carbohydrazide (3). A mixture of ester 1 (1.77 g, 0.005 mol)and hydrazine hydrate (1 mL, 99%) in ethanol (20 mL) washeated under reflux for 16 h and cooled to room temperature.The resultant solid was filtered and recrystallized from ethanolto give colorless crystals. Yield 84%; mp 193–196 °C.

X-Ray diffraction

The diffraction data were collected at room temperature onan Agilent Technologies Xcalibur diffractometer with an EOSCCD area detector (fine-focus tube, MoKα, λ = 0.71073 Å) for2A and 2B, and on an Agilent Technologies SuperNova diffrac-tometer with an Atlas CCD area detector (microfocus tube,CuKα, λ = 1.54178 Å) for 1 at 130(1) K and for 3 at 295(1) K.The data collections for the crystals were performed with

Scheme 2 Synthetic pathway.

This journal is © The Royal Society of Chemistry 2013

CrysAlisPro.15a Unit cell parameters were determined by theleast-squares procedure for the reflections chosen from thewhole datasets on the basis of their intensities (10084 for 1,2341 for 2A, 2961 for 2B and 6967 for 3). The data werecorrected for Lorentz and polarization effects, as well as forabsorption, by empirical correction using spherical harmonics,implemented in the SCALE3 ABSPACK scaling algorithm. Allof the structures were determined by direct methods usingSIR9215b and refined by the full-matrix least-squares methodwith SHELXL-9715c (these procedures were performed withinthe WinGX suite of programs15d). All of the non-hydrogenatoms were refined with anisotropic displacement parameters.In 2A and 2B hydrogen atoms from the CH3 groups were gen-erated geometrically and refined as ‘riding’ on their parentatoms; their isotropic displacement parameters were set at1.5 times Ueq of their parent atoms. All other hydrogen atomsin these two structures, as well as all of the hydrogen atoms in3 were located from Fourier maps and isotropically refined. In1 hydrogen atoms were found in the difference Fourier mapsand freely refined (OH) or refined as the ‘riding model’ in thefound positions; their isotropic displacement parameters wereset at 1.2 times Ueq of their parent atoms.‡

Conclusions

The crystal structures of three derivatives of 4,4′′-difluoro-5′-hydroxy-1,1′:3′,1′′-terphenyl-4′-carboxylic acid – the acid itself (1),its ethyl ester (2)and hydrazide (3) show different effects ofpacking conflicts. In 1 there are four symmetry-independentmolecules in the asymmetric part of the unit cell, which formtwo almost centrosymmetric, stable hydrogen-bonded dimers.Different pseudosymmetries, from almost exact centers ofsymmetry within the dimers to less obvious, nonstandardsymmetry operations are observed between the independentmolecules. In 2 two different crystal forms of this compoundhave been found, both polymorphs are triclinic, and the dif-ferences between the two forms are small, both at the

2σ(I)) = 0.101, R(all data) = 0.085, wR2(all data) = 0.113, S = 0.98, max/min Δρ

0.11/−0.15 e Å−3. 3: C19H14N2F2O2, Mr = 340.32, monoclinic, P21/n, a = 5.6775(3) Å,

b = 30.6798(9) Å, c = 9.5451(4) Å, β = 105.898(4)°, V = 1599.02(12) Å3, Z = 4,

dx = 1.41 g cm−3, F(000) = 704, μ = 0.92 mm

-1, 9401 reflections collected up

to θ = 73.8°, 3108 unique reflections (Rint = 0.023), 2879 with I > 2σ(I). FinalR(I > 2σ(I)) = 0.040, wR2(I > 2σ(I)) = 0.106, R(all data) = 0.043, wR2(all data) =0.108, S = 1.04, max/min Δρ 0.18/−0.22 e Å

−3.

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intramolecular (mainly in the orientation of the OEt group)and intermolecular (similar principal specific interactions, dif-ferent details of weak contacts) levels. In 3 there are no intra-molecular hydrogen bonds, which were present in 1 and 2,and the crystal structure is determined mainly by classicalhydrogen bonds. However, even in this case only one out ofthree NH hydrogen atoms is involved in real hydrogen bonds.

Acknowledgements

B. N. thanks the UGC SAP for financial assistance for the pur-chase of chemicals. H. S. Y. thanks University of Mysore forresearch facilities.

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