Crystal and molecular structures of 1:2 crystalline salts of two isomeric double betaines with hydrobromic acid and nitric acid

Journal of Molecular Structure 693 (2004) 1–10 www.elsevier.com/locate/molstruc

Crystal and molecular structures of 1:2 crystalline salts of two isomeric double betaines with hydrobromic acid and nitric acid Lin-Ping Zhang, Thomas C.W. Mak* Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China Received 20 May 2003; revised 11 December 2003; accepted 11 December 2003

Abstract Four 1:2 crystalline salts of the isomeric flexible double betaines 1,4-bis(2-picolyloxyl)benzene-N,N0 -diacetate (L1) and 1,4-bis(3picolyloxyl)benzene-N,N0 -diacetate (L2) with HBr and HNO3, [H2L1Br2] 1, [H2L1(NO3)2] 2, [H2L2Br2] 3 and [H2L2(NO3)2]·2H2O 4, have been synthesized and characterized by single-crystal X-ray analysis. They are all hydrogen-bonded networks consolidated by strong intermolecular interactions involving the bromide or nitrate ion, together with p– p stacking between the betaine molecules. The isostructural bromide – nitrate pair of compounds 1 and 2 provide an interesting corollary to the well-known methyl-chloro and benzene – thiophene exchange rules in crystal engineering. q 2003 Elsevier B.V. All rights reserved. Keywords: Double betaine; Bromide–nitrate exchange rule; X-ray diffraction; IR spectra; Hydrogen bond; p–p stacking

1. Introduction The design and synthesis of substrates for the ultimate preparation of supramolecular species has received considerable interest in recent years [1]. A double betaine comprises two betaine moieties in the same molecule and accordingly possesses pairs of carboxylate groups and quaternary ammonium or pyridinium moieties [2]. The anionic carboxylate terminals are of great utility for linkage to a broad array of metal ions and organic molecules, generating a variety of supramolecular entities, ranging from discrete moieties, through networks linked by hydrogen bonding, to metallo-supramolecular systems [3]. The carboxylate group is basic, so betaines are good proton acceptors that easily form complexes with Brønsted acids [4 – 10]. Although double betaines can form either 1:1 or 1:2 crystalline salts, to our knowledge only a few 1:1 salts have been reported [11]. Here we report the synthesis of 1:2 salts of the isomeric double betaines 1,4-bis(2-picolyloxyl)benzene-N,N0 -diacetate (L1) and 1,4-bis(3-picolyloxyl)benzene-N,N0 -diacetate (L2) (Scheme 1) [12] with the inorganic acids HBr and HNO3, namely [H2L1Br2] 1, [H2L1(NO3)2] 2, [H2L2Br2] 3 and [H2L2(NO3)2]·2H2O 4, * Corresponding author. Tel.: þ 852-2609-6279; fax: þ 852-2603-5057. E-mail address: [email protected] (T.C.W. Mak). 0022-2860/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2003.12.010

and their structural characterization using IR spectral data and single-crystal X-ray analysis. The results shed light on details of the molecular structure, hydrogen bonding, and p – p interaction and that give rise to three-dimensional network structures. 2. Experimental 2.1. Synthesis Synthesis of 1,4-bis(2-picolyloxyl)benzene-N,N 0 -diacetic acid dibromide, 1. 1,4-bis(2-Picolyloxyl)benzene (2.92 g, 10 mmol) [12a] was dissolved in methanol (30 ml) to give a light yellow solution, to which ethyl bromoacetate (3 ml, 27 mmol, Aldrich) was added. The resulting solution was refluxed for 3 days. After the methanol was removed by rotary evaporation under reduced pressure, hydrobromic acid (25 ml, 4.8% (w/v)) was added to the yellow residue. The mixture was refluxed for 24 h to give a yellow solution. Removal of solvent afforded a light yellow powdery product 1 (3.1 g, yield 54%). It was re-crystallized in water to obtain suitable single crystals for X-ray analysis. Melting point: 249.4 – 250.2 8C. Elemental analysis calc. (%) for C22H22N2O6Br2 (570.23): C 46.34, H 3.89, N 4.91; found: C 46.64, H 4.08, N 4.89. nmax =cm21 (KBr) 3401, 3079, 1735, 1639, 1508, 1456, 1369, 1230, 1061, 905, 809, 773, 713.

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Scheme 1.

Synthesis of 1,4-bis(2-picolyloxyl)benzene-N,N 0 -diacetic acid dinitrate, 2. 1,4-bis(2-Picolyloxyl)benzene-N,N0 -diacetate (80 mg, 0.2 mmol) [12a] was dissolved in water (7 ml), and nitric acid (10%) was added until the pH reached 1 –2. Several days later, light yellow block-like crystals of 2 were obtained for X-ray analysis. Melting point: 172.9– 173.4 8C. Elemental analysis calc. (%) for C22H22N2O6·(NO3)2 (534.44): C 49.44, H 4.15, N 10.48; found: C 49.32, H 3.98, N 10.91. nmax =cm21 (KBr) 3469, 3086, 1746, 1635, 1521, 1447, 1385, 1320, 1217, 1071, 829, 789, 708. Synthesis of 1,4-bis(3-picolyloxyl)benzene-N,N 0 -diacetic acid dibromide, 3. This compound was synthesized as light yellow block-like crystals using 1,4-bis(3-picolyloxyl)benzene [12b] and ethyl bromoacetate by the procedure used for 1. Melting point: 197.4– 198.2 8C. Elemental analysis calc. (%) for C22H22N2O6Br2 (570.23): C 46.34, H 3.89, N 4.91; found: C 46.88, H 4.11, N 4.92. nmax =cm21 (KBr) 3456, 3053, 1750, 1622, 1508, 1461, 1400, 1224, 1058, 836, 809, 789, 695. Synthesis of 1,4-bis(3-picolyloxyl)benzene-N,N 0 -diacetic acid dinitrate dihydrate, 4. In a similar way as for 2, 1,4-bis (3-picolyloxyl)benzene-N,N0 -diacetate (80 mg, 0.2 mmol) [12b] was dissolved in water (7 ml), and nitric acid (10%) was added until the pH reached 1– 2. Several days later, yellow block-like crystals of 4 were obtained. Melting point: 165.4 – 166.2 8C. Elemental analysis calc. (%) for [C22H22N2O6·(NO3)2]·2H2O (570.47): C 46.32, H 4.59, N 9.82; found: C 46.36, H 4.52, N 9.77. nmax =cm21 (KBr) 3486, 3059, 1727, 1629, 1509, 1385, 1329, 1220, 1051, 836, 689, 695. 2.2. Crystal data X-ray data for 1 were collected on a Siemens P4 and a Rigaku AFC7R four-circle diffractometer, respectively, in

the variable v-scan mode using Mo Ka radiation ˚ ) in the range 1:5 , u , 268 at 293 K (l ¼ 0:71073 A [13]. Intensity data for 2, 3 and 4 were collected on a Bruker SMART 1000 CCD diffractometer with graphite ˚ ) in the monochromatized Mo Ka radiation (l ¼ 0:71073 A range 1:5 , u , 288 at 293 K [14]. All structures were solved by direct methods and refined by full-matrix least squares based on F 2 using the SHELXTL program package [15]. Information concerning crystallographic data and structure refinement of all four compounds is summarized in Table 1, and selected bond lengths and angles are shown in Table 2. For all compounds, the non-hydrogen atoms were treated as anisotropic. All H atoms were introduced based on idealized molecular geometry, and their Uiso s were fixed at 1.2 times the Ueq s of their parent atoms, except for the carboxyl H(1) atom and the hydrogen atoms of water molecules, which were located from DF maps and not refined. The crystallographic data have been deposited at the Cambridge Data Center as CCDC reference numbers 210862 – 210865.

3. Results and discussion 3.1. Synthesis The carboxylate group in zwitterionic betaine compounds can easily abstract a hydrogen atom from an inorganic acid. Accordingly the bromide and nitrate salts of betaines L1 and L2 were obtained by acidifying the zwitter ions to pH 1 –2 with the corresponding aqueous acid. Since all these salts can be crystallized, a single-crystal X-ray diffraction study was undertaken to investigate their molecular structures and the role of hydrogen bonding in molecular packing.

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Table 1 Cystal data and structure refinement parameters for the series of double betaine salts 1–4 Complex

Empirical formula Formula weight Crystal size Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a (8) b (8) g (8) ˚ 3) V (A Z Dcalc (g/cm3) m (Mo Ka) (mm21) Fð000Þ Reflections collected Independent reflections Observed reflections ½I . 2sðIÞ Parameters Goodness-of-fit R1 ½I . 2sðIÞa wR2 (all data)b P P a R1 ¼ llFo l 2 lFc ll= lFo l: P P b 2 wR2 ¼ ½ ½wðFo 2 Fc2 Þ2 = ½wðFo2 Þ2 1=2 :

1

2

3

4

C22H22Br2N2O6 570.24 0.42 £ 0.38 £ 0.30 Monoclinic P21 =n (no. 14) 11.777(1) 8.464(1) 12.288(2) 90 114.73(1) 90 1112.6(3) 2 1.702 3.685 572 2826 2157 1361 145 1.028 0.0521 0.1226

C22H22N4O12 534.44 0.40 £ 0.30 £ 0.30 Monoclinic P21 =n (no. 14) 12.264(3) 8.6879(9) 12.364(2) 90 114.29(3) 90 1200.7(3) 2 1.478 0.122 556 5207 2291 1004 173 1.017 0.0752 0.2320

C22H22Br2N2O6 570.24 0.30 £ 0.20 £ 0.20 Monoclinic P21 =c (no. 14) 5.9448(4) 16.592(1) 11.5071(8) 90 101.389(1) 90 1112.7(1) 2 1.702 3.685 572 7737 2688 1593 145 0.964 0.0606 0.1888

C22H26N4O14 570.47 0.47 £ 0.27 £ 0.24 Monoclinic P21 =c (no. 14) 7.645(2) 12.790(3) 13.185(3) 90 92.714(5) 90 1287.7(5) 2 1.471 0.125 596 8656 3104 1488 194 0.951 0.0474 0.1467

3.2. IR absorption spectra The n(OH) band occurs at ca. 3050– 3090 cm21 in the IR spectra of all the compounds, the intensities being strong for compound 1 and 3 but weak for 2 and 4. Two absorption bands of n(CO) were observed in bromide complexes 1 and 3: strong in the range 1630– 1640 cm21 and weak in the range 1730 – 1750 cm21. In nitrate compounds 2 and 4, similar splitting of the n(CO) absorption band was found, but the intensities of the bands were reversed, being strong in the range 1720 –1750 cm21 and weak in the range 1630 – 1650 cm21; in addition, a very strong n(NO) absorption at 1380– 1385 cm21 also appeared. 3.3. X-ray crystallography In the crystal structure of 1 (H2L1Br2), the phenylene ring of the fully protonated double betaine H2L1 is located at an inversion center. The two pyridyl rings are necessarily parallel to each other, each making a dihedral angle of 11.38 with the phenylene ring. The cationic betaine molecule H2L1 adopts its fully extended configuration so that the two carboxylate groups and two nitrogen atoms are at maximum separation from each other. The plane of the carboxymethyl group is nearly perpendicular to its parent pyridyl ring with a dihedral angle of 88.48. The close proximity of O2 to N1 facilitates internal charge compensation (Table 2).

The bromide ion is connected to the betaine molecule through hydrogen bonding at O1 – H1· · ·Br1 168.58, ˚ . The betaine molecules are linked by O1· · ·Br1 3.182(4) A weak hydrogen bonds of the type C10 –H· · ·O2a 151.28, ˚ (a: 2x þ 1; 2y þ 1; 2z þ 1) [16], C10· · ·O2a 3.329(6) A generating an infinite chain along the b axis (Fig. 1). In this chain, the betaine molecules exhibit p – p interactions between each phenylene ring and a pair of pyridine rings Table 2 ˚ ) and bond angles (8) of compounds 1–4 Bond lengths (A 1 C(1)–O(1) C(1)–O(2) C(1)–C(2) N(1) –C(2) C(3)–C(8) O(3) –C(8) O(3) –C(9) N(1)· · ·O(1) N(1)· · ·O(2) O(2) –C(1)–O(1) O(2) –C(1)–C(2) O(1) –C(1)–C(2) N(1) –C(2)–C(1) O(3) –C(8)–C(3) C(9)–O(3) –C(8)

2 1.302(6) 1.179(6) 1.524(7) 1.484(6) 1.497(7) 1.416(5) 1.394(5) 3.601 2.724

125.7(5) 124.3(5) 110.0(5) 109.8(4) 107.1(4) 116.5(4)

3 1.306(6) 1.181(6) 1.500(7) 1.489(6) 1.485(7) 1.423(5) 1.392(6) 3.608 2.706

124.9(6) 123.6(5) 111.5(5) 110.4(4) 107.2(4) 115.6(4)

4 1.319(5) 1.204(5) 1.500(7) 1.480(6) 1.497(6) 1.417(5) 1.376(5) 3.613 2.701

124.9(4) 123.6(4) 111.5(4) 110.1(3) 106.1(3) 119.0(3)

1.317(3) 1.195(3) 1.504(3) 1.471(3) 1.495(3) 1.416(2) 1.376(3) 3.593 2.697 125.8(2) 123.6(2) 110.5(2) 109.9(2) 108.4(2) 116.7(2)

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Fig. 1. Infinite chain in 1 along the [0 1 0] direction, showing H2L1(Br)2 aggregates connected by C –H· · ·O hydrogen bonds together with p–p interactions. Hydrogen atoms not involved in hydrogen bonding are omitted for clarity.

belonging to adjacent molecules, with ring center-to-ring ˚ and dihedral angle 11.38. These center distance 4.010 A chains are further connected through weak hydrogen bonds: ˚ ; C7 –H· · ·Br1c C6 – H· · ·Br1b 149.18, C6· · ·Br1b 3.888(5) A ˚ (b: 2x þ 3=2; y 2 1=2; 2z þ 1548, C7· · ·Br1c 3.795(5) A 1=2; c: x 2 1=2; 2y þ 1=2; z 2 1=2), generating an infinite three-dimensional network (Figs. 2 and 3). The data in Table 1 clearly shows that compound 2 is isomorphous to 1, and the corresponding structural parameters of the centrosymmetric dication H2L1 are: dihedral angles 7.3 and 94.38 for the phenylene ring with each pyridyl ring and for the plane of the carboxymethyl

group with its parent pyridyl ring, respectively. The nitrate ion is bonded to the betaine molecule through the hydrogen ˚ . The bond O1 – H1· · ·O4 161.18, O1· · ·O4 2.694(7) A betaine molecules are bridged by weak hydrogen bonds of ˚ (a: the type C10 – H· · ·O2a 152.78, C10· · ·O2a 3.363(7) A 2x þ 1; 2y þ 1; 2z), generating a chain similar to that in 1 (Fig. 4). The betaine molecules also have p –p interactions between adjacent phenylene and pyridyl rings, with center˚ and dihedral angle 7.38. These to-center distance 4.13 A chains are further connected through weak hydrogen ˚; bonds C6 – H· · ·O5b 161.88, C6· · ·O5b 3.344(9) A ˚ C7 – H· · ·O4c 140.88, C7· · ·O4c 3.257(8) A (b: 2x þ 1=2;

Fig. 2. Perspective view of compound 1 showing a layer corresponding to the ac plane.

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Fig. 3. Three-dimensional hydrogen-bonded network in 1, viewed along the b direction.

y 2 1=2; 2z þ 1=2; c: x þ 1=2; 2y þ 1=2; z þ 1=2) to generate an infinite three-dimensional network similar to that in 1 (Fig. 5). The pair of compounds 1 and 2 provide an interesting example of the isostructurality relationship [17], and the substitution of the nitrate ion for the bromide ion without causing a change in crystal packing may be compared with the well-established methyl –chloro [18] and benzene –thiophene (or phenyl –thienyl) [19] exchange rules in the crystal engineering of organic solids. In compound 3, the phenylene ring of the double betaine H2L2 is also located at an inversion center, making a dihedral

angle of 10.98 with each pyridyl ring. The dihedral angle between the carboxymethyl group and its parent pyridine plane is 73.38. The bromine ion is connected to the betaine molecule through a hydrogen bond (O1– H1· · ·Br1 176.68, ˚ ). The betaine molecules are linked by O1· · ·Br1 3.140(7) A weak hydrogen bonds of the type C7 – H· · ·O2a 140.18, ˚ (a: x 2 1; y; z) to generate an infinite C7· · ·O2a 3.16(1) A chain (Fig. 6). In this chain, the betaine molecules are parallel to one another and exhibit intermolecular p –p interactions between each phenylene ring and a pair of ˚ adjacent pyridyl rings, with center-to-center distance 4.40 A

Fig. 4. Chain structure in 2 along the [0 1 0] direction, showing its similarity with that in 1.

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Fig. 5. Perspective view of compound 2 showing a layer corresponding to the ac plane.

Fig. 6. Hydrogen-bonded chain in 3 along the [1 0 0] direction, showing H2L2(Br)2 aggregates connected by C–H· · ·O hydrogen bonds and p –p interactions.

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Fig. 7. Hydrogen-bonded three-dimensional network in 3, viewed along the a direction.

and dihedral angle 11.48. These chains are further connected through weak hydrogen bonds C3 – H· · ·Br1b 154.48, ˚ ; C6 – H· · ·O2c 126.78, C6· · ·O2c C3· · ·Br1b 3.871(8) A ˚ (b: x; 2y þ 3=2; z þ 1=2; c: 2x; 2y þ 1; 3.22(1) A 2z þ 1), generating three-dimensional network (Fig. 7). In compound 4, the protonated double betaine H2L2 is located at an inversion center with each pyridyl ring making

a dihedral angle 3.78 with the central phenylene ring. The plane of the carboxymethyl group is nearly perpendicular to the parent pyridine plane, making a dihedral angle of 91.3. The independent nitrate ion forms an acceptor hydrogen bond with the protonated double betaine molecule ˚ ), and adjacent (O1 – H· · ·O5 1478, O1· · ·O5 2.851(3) A neutral H2L2(NO3)2 aggregates are bridged by the lattice

2 Fig. 8. Hydrogen-bonded zigzag chains [H2O·NO2 3 ]1 running parallel to the c axis, which are cross-linked by H2L cations to form a porous two-dimensional network in 4.

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Fig. 9. Interlocking of two two-dimensional networks in 4.

water molecule [O1W – H· · ·O6 1678, O1W· · ·O6 ˚ ; O1W –H· · ·O5a 1628, O1W· · ·O5a 2.959(4) A ˚ 3.044(4) A (a: x; 2y þ 3=2; z þ 1=2)], yielding an infinite zigzag chain along the b direction that differs from that in 3. These chains are further cross-linked by hydrogen bonds of the type O1Wa– H· · ·O6a and O1Wa –H· · ·O5b (b: x; y; z þ 1), which generate infinite [H2O·NO2 3 ]1 zigzag chains running parallel to the c axis, leading to a porous layer structure (Fig. 8). These hydrogen-bonded layers are interlocked in two-fold interpenetration (Fig. 9). Further connection between adjacent layers by C – H· · ·O hydrogen bonding between the betaine molecules and nitrate ions ˚ ; C7 – (C6 – H· · ·O4c 126.68, C6· · ·O4c 3.184(3) A ˚ H· · ·O1Wc 158.28, C7· · ·O1Wc 3.234(3) A; c: 2x þ 2; y 2 1=2; 2z þ 3=2) leads to a three-dimensional network (Fig. 10). The adjacent H2L2 aggregates overlap through p – p interactions between the phenylene and pyridyl rings, ˚ and dihedral angle with center-to-center distance 3.669 A

3.78. The existence of the lattice water molecules in 4 accounts for the fact that 3 and 4 have different crystal structures.

4. Summary and concluding remarks According to the X-ray data, the present 1:2 bis-betaine salts are stabilized by COOH· · ·X2 (X ¼ Br, NO3), with additional weak C – H· · ·X2 (X ¼ Br, O, NO3) hydrogen bonding and p – p interactions between adjacent fully protonated betaine molecules, as in the case of the 1:1 mono-betaine salts [20]. However, in the 1:1 bis-betaine salts, the betaine molecules are bridged by protons to form strong and symmetrical O· · ·O hydrogen bonds, leading to an infinite chain. In the 2:1 mono-betaine salts, two betaine molecules are strongly bound by a proton to form a dimeric cation [11,21].

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Fig. 10. Hydrogen-boned three-dimensional network in 4.

The conformations of H2L1 and H2L2 can be conveniently described in terms of the angular parameters listed in Table 3. In all four compounds, the three rings of each double betaine are roughly co-planar, with dihedral angles dpy in the range 3.7 –10.98, and the relationship between the pair of O – CH2 bonds are all anti ðltl < 1808Þ: As might be expected, in each double betaine the molecular backbone is fully extended so that the two betaine groups are at maximum separation. The plane of the carboxymethyl group is nearly perpendicular to its parent pyridyl ring, with a dihedral angle dCO2 that falls in the range 73.3– 94.38. Each double betaine molecule is protonated by accepting two hydrogen atoms, which form donor hydrogen bonds with the bromide or nitrate ions, leading to chain-like structures. Further connection by weak C – H· · ·O hydrogen bonds, then generates a threedimensional framework. Although a few examples of isomorphous inorganic crystals containing exchangeable halogeno and nitrato

ligand sites are known in the literature, for example AgX·As(C6H11)3 (X ¼ Cl, NO3) [22] and CuX·3SbPh3·n L (X ¼ Br, NO3; L ¼ solvate) [23], we are unaware of any precedent of exchange of bromide with nitrate anions that gives rise to a pair of isostructural crystalline salts, as in the case of compounds 1 and 2. Table 3 Conformational parameters of double betaines L1 and L2 in protonated forms (8) Compound

Cation

t

dpy

dCO2

1 2 3 4

H2L1 H2L1 H2L2 H2L2

180 180 180 180

11.3 7.3 10.9 3.7

88.4 94.3 73.3 91.3

t is the torsion angle CH2 –O· · ·O–CH2. dpy is the dihedral angle between the central phenylene ring and the pyridyl ring. dCO2 is the dihedral angle between a pyridyl ring and its adjacent carboxymethyl group.

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L.-P. Zhang, T.C.W. Mak / Journal of Molecular Structure 693 (2004) 1–10

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