Chromotropic Behavior of Lophine Nitro-Derivatives - Crystal Growth

CRYSTAL GROWTH & DESIGN

Chromotropic Behavior of Lophine Nitro-Derivatives

2006 VOL. 6, NO. 10 2281-2288

Natalya Fridman, Shammai Speiser, and Menahem Kaftory* Department of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel ReceiVed February 22, 2006; ReVised Manuscript ReceiVed July 25, 2006

W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: The crystal and molecular structures of two novel derivatives of the lophine chromophore with different solvents of crystallization are presented and discussed. Phenol, 2-[4,5-bis(4-methoxyphenyl)-1H-imidazole-2-yl]-4-nitro (1) and phenol, 2-[4,5bis(4-methyl-phenyl)-1H-imidazole-2-yl]-4-nitro (2) form inclusion compounds with various guest molecules. The guest molecules interconnect host molecules of 1 and 2 through hydrogen bonds. The inclusion compounds showed chromotropic properties upon trituration, heating, or using different solvents for crystallization. The thermochromic properties have been determined by the removal of the guest molecules. To clarify the relations between the color, thermochromic properties, and the role of host in crystal packing, we have determined and compared a single-crystal X-ray analysis of six crystal structures of the two compounds. Thermal behavior in the solid state and spectroscopic behavior at different pH values and at various solvents are presented and discussed. Introduction The design of novel chromotropic compounds that exhibit sensitive color changes upon complexation with different guest molecules has attracted increasing attention because of their potential applications in analytical chemistry and materials science.1 Scott et al.2 demonstrated potochromism for 2-propynylallene derivatives in the solid state. 2-[2,7-Dibromo-9-(3-oxo-3-phenylprop-1-ynyl)-9H-fluoren-9-yl]-3-fluoren-9-ylidene-1-phenyl propenone shows crystalline photochromism from pale yellow to green upon exposure to sunlight and the green color fades gradually in the dark. The green crystals reverted gradually into pale yellow crystals on storage in the dark for more than two weeks at room temperature or quickly on heating at around 60 °C or dissolving in solvents. Scott et al.3 showed that photochromism of 2-propynylallene derivatives depends largely on the substituents on the aromatic rings. Scott et al.4,5 prepared novel chromotropic host compounds based on fluorenone or fluorene cores with bulky end groups and hydrogen-bonding capability; the compounds exhibit a fluorescence color change on complexation with guest molecules. A number of inclusion complexes are formed on recrystallization of 2,7-bis-(3-hydroxy-3,3-diphenylprop-1-ynyl)fluoren-9-one from solution, such as pyridine, acetylpyridine, DMF, and triethylamine. These complexes range in color from a very bright acid-yellow to orange and exhibit various host: guest ratios. No solid-state fluorescence enhancement is detected in inclusion compounds that have crystal structures that exhibit hydrogen bonds to the carbonyl oxygen, whereas the stacked non-hydrogen-bonded pyridine complex shows significant fluorescence enhancement because of excimer formation combined with the lack of close intermolecular interactions, which would allow relaxation by nonradiative pathways. Fluorescence inclusion compounds of 2,7-bis-(3-hydroxy-3,3-diphenylprop-1ynyl)fluorene change color from yellow to blue depending on the guest molecules. Dimers of 2,4,5-triarylimidazolyl radicals, which are obtained by the oxidation of 2,4,5-triarylimidazoles, have chromotropic properties.6 The dimers dissociate to the radicals upon irradiation, heating, and trituration. Similar properties have been * To whom correspondence should be addressed. E-mail: kaftory@ tx.technion.ac.il.

Scheme 1

obtained in crystals of 4,5-bis-(4-methoxyphenyl)-2-(4-nitrophenyl) imidazolium acetate dihydrate that show rapid color change upon drying, trituration, or heating.7,8 4,5-Bis(methoxyphenyl)-2-(4-nitrophenyl)-1H-imidazole and 4,5-bis(methoxyphenyl)-2-(3-nitrophenyl)-1H-imidazole9 form, with different guest molecules, a wide variety of inclusion compounds of different colors, changing from yellow to black. Kaftory et al.10 described the thermochromic properties, crystal structure, and thermal behavior of inclusion compounds of 4,5-diphenyl-2(4-nitrophenyl)-1H-imidazole and 4,5-diphenyl-2-(2-hydroxy5-nitrophenyl)-1H-imidazole. We have recently prepared new derivatives of imidazole, phenol, 2-[4,5-bis(4-methoxyphenyl)-1H-imidazole-2-yl]-4-nitro (1) and phenol, 2-[4,5-bis(4-methylphenyl)-1H-imidazole-2-yl]4-nitro (2) (see Scheme 1) , that crystallize with guest molecules in various colors. The crystal structure of these compounds showed that the solvent of crystallization and intermolecular hydrogen bonds play an essential role in the chromotropic behavior and in the packing of molecules in the crystals. We were interested in finding the reason for the color change in the solid state and comparing the chromotropic properties in solution. In all the compounds, there is intramolecular hydrogen bond between the nitrogen atom of the imidazole ring and the ortho-OH of the 2-phenyl group in all the chromotropic compounds. We describe here the crystal structure and thermal behavior of six crystals: dark orange nonsolvated crystals of 1 from acetonitrile 1a, yellow solvate prisms from 1,4-dioxane 1b, yellow solvate needles from 2-salicylaldehyde 1c, yellow solvate ovals 1d from a 1:1 ethanol:water solution, bright yellow nonsolvated needles of 2 from acetonitrile 2a, and lemon solvate prisms from 1,4-dioxane 2b. Some of them show thermochromic properties. No similarity was observed between the crystal

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2282 Crystal Growth & Design, Vol. 6, No. 10, 2006

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Table 1. Crystallographic Data and Parameters for Compounds 1a-d, 2a, and 2b 1a

1b

1c

1d

2a

2b

formula fw cryst color, habit

C23H19N3O5 417.41 orange needle

C23H19N3O5‚C4H8O2 505.52 yellow plate

C23H19N3O5‚C7H6O2 539.53 yellow needle

C23H19N3O5‚H2O 435.41 yellow oval

C23H19N3O3‚1/2C4H8O2 429.46 lemon prism

cryst syst space group a (Å) b (Å0 c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (g cm-3) θ range (deg) data/restraints /params T (K) R1 wR2 stability upon ambient storage

orthorhombic P212121 4.535(2) 17.557(3) 24.498(5) 90 90 90 1950.6(10) 4 1.421 1.66-25.1 2370/0/284

triclinic P1h 8.954(2) 9.943(2) 14.841(3) 90.99(2) 97.60(2) 105.38(2) 1260.9(5) 2 1.331 1.60-25.05 4458/2/365

monoclinic Cc 20.256(4) 17.013(3) 7.942(2) 90 107.56(2) 90 2609.4(10) 4 1.373 1.39-25.05 4433/0/337

monoclinic P21/c 13.486(3) 12.280(3) 12.404(3) 90 92.69(2) 90 2051.9(8) 4 1.351 1.51-25.05 3628/0/301

C23H19N3O3 385.41 bright yellow needle triclinic P1h 6.630(2) 8.975(2) 16.408(3) 88.730(2) 89.67(2) 79.51(2) 959.8(4) 2 1.334 2.48-25.05 3168/0/276

triclinic P1h 8.095(2) 9.525(2) 14.306(3) 93.67(2) 96.60 (2) 96.91(2) 1084.4(4) 2 1.315 1.44-25.04 3827/0/312

298(2) 0.0358 0.1207 stable

298(2) 0.0479 0.1011 unstable

298(2) 0.0463 0.0709 unstable

298(2) 0.0402 0.1026 unstable

298(2) 0.0489 0.0980 stable

298(2) 0.0494 0.0741 unstable

structure of the inclusion compounds 1a and 2a and 1b and 2b. 2b, 4,5-bis-(4-methoxyphenyl)-2-(3-nitrophenyl)-1H-imidazole,9 and 4,5-diphenyl-2-(2-hydroxy-5-nitro-phenyl)-1H-imidazole10 crystallize from dioxane; the dioxane molecule lies on an inversion center and links two host molecules by hydrogen bonds with NH of the imidazole, whereas 1b and 4,5-bis(4methoxyphenyl)-2-(4-nitrophenyl)-1H-imidazole7 form inclusion compounds with dioxane and have different packing. One molecule of dioxane is hydrogen bonded to the HN of the imidazole, whereas a second dioxane molecule in a disordered state lies in a cavity encircled by four p-methoxyphenyl groups.

(2) Crystal Structure Determination. Single-crystal X-ray diffraction data was collected at 298 K with a Nonius Kappa CCD diffractometer using Mo KR radiation (λ ) 0.71073 Å). Details of crystallographic data collection and crystal structure determination and refinement for 1a-d, 2a, and 2b are given in Table 1. All the nonhydrogen atoms of the inclusion compounds were refined with anisotropic displacement parameters. The hydrogen atoms were located in a Fourier difference map and then refined isotropically, riding on the atoms to which they are bonded. The software programs used for data collection and reduction were KappaCCD12 and DENZO SMN;13 for structure solution and refinement, SHELXS-97 and SHELXL-97;14 and for graphic presentations, Mercury 1.4.1 for Windows. The crystal structure data of compounds 1a-d, 2a, and 2b are provided in the Supporting Information and are deposited in the Cambridge Crystallographic Data Centre.

Experimental Section Preparation of Materials. Commercially available reagents were purchased from Aldrich and used without further purification. The host molecules 1 and 2 were synthesized according to the Davidson method11 and characterized by 1H NMR. Nuclear magnetic resonance spectra were recorded on a Bruker AC-400 spectrometer at 298 K. The inclusion compounds 1a-d, 2a, and 2b were obtained by crystallization from the guest solvents. Thermal analyses and melting points were carried out using a DSC apparatus. The differential scanning calorimetry (DSC) experiments were carried out with a model DSC PL (Polymer Laboratories) fitted with a standard aluminum sample pan. The conditions of DSC were as follows: sample masses of about 2 mg and the heating rate of 10 K min-1 under a nitrogen atmosphere. A sample of indium was used as the reference. Absorption spectra were recorded on a Shimadzu UV-1601 UVvis spectrophotometer. (1a) Synthesis. Phenol, 2-[4,5-Bis(4-methoxyphenyl)-1H-imidazole2-yl]-4-nitro (1) and phenol, 2-[4,5-bis(4-methylphenyl)-1H-imidazole2-yl]-4-nitro (2) were synthesized in good yield according to the Davidson method.11 4,4′-Dimethoxybenzil or 4,4′-dimethylbenzil (0.5 mmol), 5-nitro-2-salycilaldehyde (0.5 mmol), and ammonium acetate (0.8 g) were dissolved in boiling glacial acetic acid (8 mL) and refluxed for ca. 3 h. The progression of the reaction was monitored by TLC. The reaction mixture was poured into ice-water and the solids collected on a filter, washed by cold water, dried, and recrystallized from the suitable solvent. (1b) Characterization. Nonsolvate crystals of 1 and 2 were originally isolated and characterized. 1: mp: 256 °C; 1H NMR (400 MHz, CDCl3) δ 8.54 (s, 1H), 8.13 (d, 1H), 7.47 (d, 4H), 7.11 (d, 4H), 3.84 (s, 6H), 1.23 (s, 1H). 2: mp 267 °C; 1H NMR (400 MHz, CDCl3) δ 8.51 (s, 1H), 8.13 (d, 1H), 7.43 (d, 4H), 7.18 (d, 1H), 7.10 (s, 4H), 2.37 (s, 6H), 1.23 (s, 1H).

Results and Discussion Crystal Structure and Thermal Properties. Nonsolvate Phenol, 2-[4,5-Bis(4-methoxyphenyl)-1H-imidazole-2-yl]-4nitro (1a). Crystallization of 1 from an alcohol like MeOH, EtOH, or i-PrOH yields small dark orange needles. Good dark orange nonsolvate crystals were obtained from MeCN. The hydrogen-bond geometry is given in Table 2. The molecular arrangement showing the hydrogen bonds in 1a is shown in Figure 1. The crystal structure of 1a shows a hydrogen bond of the type -NH‚‚‚Od. The NH hydrogen of one molecule bonds to the nitro oxygen of another. The 2-hydroxy-5-nitrophenyl is nearly coplanar with the imidazole ring, and the dihedral angle between the two planes is 10.6(2)°. The other p-methoxyphenyl rings are rotated by 9.9(2) and 50.0(1)° with respect to the imidazole ring. The intramolecular hydrogen-bond distance between the nitrogen atom of the imidazole ring and the OH group (N1‚‚‚H3O3) of the 2-hydroxy-5-nitrophenyl ring is 1.864 Å, and the O3-H3O3‚‚‚N1 angle is 137.5°. The intermolecular hydrogen-bond distance between the NH hydrogen of one molecule bonds and an oxygen of a nitro group of another one (O4‚‚‚H2N2) is 2.316 Å and the N2-H2N2‚‚‚O4 angle is 153.4°. The thermal behavior of 1a is shown by the differential scanning calorimeter (DSC) thermograph given in Figure 1. The endotherm at 529 K is assigned to the melting of dark orange compound 1 (∆H ) 12.0 kJ mol-1). 2-[4,5-Bis(4-methoxyphenyl)-1H-imidazole-2-yl]-4-nitro1,4-dioxane (1b). Crystallization of 1 from 1,4-dioxane yields


Crystal Growth & Design, Vol. 6, No. 10, 2006 2283

Figure 1. Molecular arrangement of 1a showing the hydrogen bonding (drawn by dashed lines; left) and DSC thermograph of 1a, with a heating rate of 10°/min (right). W A 3D rotatable image in Mercury format is available.

Figure 2. Molecular arrangement of 1b showing the hydrogen bonding (drawn by dashed lines; left) and DSC thermograph of 1b, with a heating rate of 10°/min (right). W A 3D rotatable image in Mercury format is available. Table 2. Intermolecular Contacts (distances in Å and angles in deg) Operating in the Crystal Structures of 1a-d, 2a, and 2b D-H‚‚‚A O3-H3O3‚‚‚N1 N2-H2N2‚‚‚O4

d(D-H) 0.820 0.860

∠D-H‚‚‚A

symmetry

1.864 2.316

1a 2.530 3.109

137.5 153.4

[x + 1/2, -y + 1/2, -z + 1]

150.0 174.6

[-x + 1, -y + 1, -z + 1]

d(H‚‚‚A)

d(D‚‚‚A)

O3-H3‚‚‚N1 N2-H2N2‚‚‚O2s

0.820 0.860

1.810 2.002

1b 2.554 2.859

O3-H3‚‚‚N1 N2-H2N2‚‚‚O2s

0.820 0.860

1.821 2.076

1c 2.568 2.933

150.7 173.8

[x + 1/2, -y + 1/2, z + 1/2]

1.947 1.810

1d 2.552 2.668

126.3 175.4

[x, -y + 1/2, z + 1/2]

149.1 167.2

[-x - 1, -y, -z + 1]

149.3 171.8

[x - 1, y, z]

N1-H1N1‚‚‚O3 N2-H2N2‚‚‚O3

0.860 0.860

O1-H1O1‚‚‚N1 N2-H2N2‚‚‚O2

0.820 0.860

1.818 2.219

2a 2.556 3.064

O1-H1O1‚‚‚N1 N2-H2N2‚‚‚O1s

0.820 0.860

1.850 2.043

2b 2.589 2.897

large yellow solvate prisms 1b with a 1:1 host:guest ratio. The hydrogen-bond geometry is given in Table 2. The molecular arrangement of 1b is shown in Figure 2. The crystal structure of 1b shows a hydrogen bond of the type -NH‚‚‚Od. The imidazole and the 2-hydroxy-5-nitrophenyl rings are nearly coplanar, making a dihedral angle of 3.3(1)°, slightly smaller than the dihedral angle found in 1a. The coplanarity is attributed to the intramolecular hydrogen bond between the hydroxy and the nitrogen atom of the imidazole ring. The intramolecular hydrogen-bond distance (N1‚‚‚H3O3) is 1.810 Å, and the O3-H3O3‚‚‚N1 angle is 150.0°. The two

dioxane molecules lie on an inversion center, and one of them is connected to the host molecule by a hydrogen bond with NH of the imidazole; a second dioxane molecule in a disordered state lies in a plane encircled by two p-methoxyphenyl rings. The strong intermolecular hydrogen-bond distance (H2‚‚‚O2S) is 2.002 Å, and the N2-H2‚‚‚O2S angle is 174.6°. The other p-methoxyphenyl rings are rotated by 42.2(1) and 34.3(1)° with respect to the imidazole ring. The thermal behavior of 1b is shown by the differential scanning calorimeter (DSC) thermograph given in Figure 2. The first broad endotherm between 338 and 443 K is assigned to the removal of two nonequivalent


Fridman et al.

Figure 3. Molecular arrangement of 1c showing the hydrogen bonding (drawn by dashed lines; left) and DSC thermograph of 1c, with a heating rate of 10°/min (right). W A 3D rotatable image in Mercury format is available.

Figure 4. Molecular arrangement of 1d showing the hydrogen bonding (drawn by dashed lines; left) and DSC thermograph of 1d, with a heating rate of 10°/min (right). W A 3D rotatable image in Mercury format is available.

molecules of dioxane. Upon removal of the guest molecules, the yellow crystals turned completely dark orange. The second endotherm at 529 K is assigned to the melting temperature of host compound 1 (∆H ) 9.8 kJ mol-1). 2-[4,5-Bis(4-methoxyphenyl)-1H-imidazole-2-yl]-4-nitro-2salicylaldehyde (1c). Crystallization of 1 from 2-salicylaldehyde yields long solvated yellow needles 1c with a 1:1 host:guest ratio. The hydrogen-bond geometry is given in Table 2. The molecular arrangement of 1c is shown in Figure 3. The crystal structure of 1c shows a hydrogen bond of the type -NH‚‚‚Od. The NH hydrogen of the host molecule bonds to the carbonyl oxygen of the guest molecule. The dihedral angle between the imidazole ring and the 2-hydroxy-5-nitrophenyl is 4.9(2)°, slightly smaller than the dihedral angle found in 1a. The p-methoxyphenyl rings are rotated by 30.1(2) and 34.5(2)° with respect to the imidazole ring. The intramolecular hydrogen-bond distance between the nitrogen atom of the imidazole ring and the hydroxyl of the 2-phenyl ring (N1‚‚‚ H3) is 1.821 Å, and the O3-H3‚‚‚N1 angle is 150.71°. The intermolecular hydrogen-bond distance between the NH hydrogen of the host molecule and the oxygen of a carbonyl group of the guest molecule (H2N2‚‚‚O2S) is 2.076 Å, and the N2H2N2‚‚‚O2S angle is 173.8°. The thermal behavior of 1c is shown by the differential scanning calorimeter (DSC) thermograph given in Figure 3. Between 333 and 447 K, the solvent molecules are removed. Upon removal of the guest molecules the yellow crystals turned completely dark orange. The second endotherm at 528 K is assigned to the melting temperature of host compound 1 (∆H ) 9.8 kJ mol-1). 2-[4,5-Bis(4-methoxyphenyl)-imidazolium-2-yl]-4-nitrowater (1d). Crystallization of 1 from a 1:1 ethanol:water

solution yields a mixture of two compounds: dark orange needles of 1a and yellow solvate ovals 1d with water molecule. The hydrogen-bond geometry is given in Table 2. The molecular arrangement of 1d is shown in Figure 4. The crystal structure of 1d shows a hydrogen bond of the type -NH‚‚‚Od. The imidazole ring is protonated and in the same plane as the 2-hydroxy-5-nitrophenyl ring, making a dihedral angle of 2.9(1)°, slightly smaller than the dihedral angle found in 1a. The coplanarity is attributed to the intramolecular hydrogen bond between the oxygen and NH of the imidazole ring. The intramolecular hydrogen-bond distance (O3‚‚‚H1N1) is 1.949 Å, and the N1-H1N1‚‚‚O3 angle is 126.29°. The strong intermolecular hydrogen-bond distance (O3‚‚‚H2N2) is 1.812 Å, and the N2-H2N2‚‚‚O3 angle is 175.55°. The other p-methoxyphenyl rings are rotated by 10.7(1) and 51.4(1)° with respect to the imidazole ring. The solvate compound 1d consists of a molecule of water with 18% occupancy. The thermal behavior of 1d is shown by the differential scanning calorimeter (DSC) thermograph given in Figure 4. The first broad endotherm between 338 and 443 K is assigned to removal of the molecules of water. Upon removal of the guest molecules, the yellow crystals turned completely dark orange. The second endotherm at 530 K is assigned to the melting temperature of host compound 1 (∆H ) 10.8 kJ mol-1). Nonsolvate Phenol, 2-[4,5-Bis(4-methylphenyl)-1H-imidazole-2-yl]-4-nitro (2a). Crystallization of 2 from MeCN yields nonsolvate bright yellow needles. The hydrogen-bond geometry is given in Table 2. The molecular arrangement of 2a is shown in Figure 5. The crystal structure of 2a shows a hydrogen bond of the type -NH‚‚‚Od. The NH hydrogen of one molecule bonds to


Crystal Growth & Design, Vol. 6, No. 10, 2006 2285

Figure 5. Molecular arrangement of 2a showing the hydrogen bonding (drawn by dashed lines; left) and DSC thermograph of 2a, with a heating rate of 10°/min (right). W A 3D rotatable image in Mercury format is available.

Figure 6. Molecular arrangement of 2b showing the hydrogen bonding (drawn by dashed lines; left) and DSC thermograph of 2b, with a heating rate of 10°/min (right). W A 3D rotatable image in Mercury format is available.

Figure 7. Color and crystal form of compounds 1a-d, 2a, and 2b.

the nitro oxygen of another. The 2-hydroxy-5-nitrophenyl ring and the imidazole ring are nearly coplanar; the dihedral angle between the two planes is 6.3(1)°. The intramolecular hydrogen-bond distance between the nitrogen atom of the imidazole ring and the OH group of the 2-hydroxy-5-nitrophenyl ring (N1‚‚‚H1N1) is 1.818 Å, and the O1-H1O1‚‚‚N1 angle is 149.1°. The intermolecular hydrogen-bond distance between the NH hydrogen of one molecule bonded to an oxygen of a nitro group of another one is (O2‚‚‚H2N2) 2.219 Å, and the N2H2N2‚‚‚O2 angle is 167.2°. The other p-tolyl rings are rotated by 30.2(2) and 42.3(2)° with respect to the imidazole ring. The thermal behavior of 2a is shown by the differential scanning calorimeter (DSC) thermograph given in Figure 5. The endotherm at 540 K is assigned to the melting of 2 (∆H ) 36.0 kJ mol-1). Phenol, 2-[4,5-Bis(4-methylphenyl)-1H-imidazole-2-yl]-4nitro-1,4-dioxane (2b). Crystallization of 2b from 1,4-dioxane yields large lemon solvate prisms with a 2:1 host:guest ratio. The hydrogen-bond geometry is given in Table 2. The molecular arrangement of 2b is shown in Figure 6. The imidazole and the 2-hydroxy-5-nitrophenyl rings are nearly coplanar, making a dihedral angle of 8.5(2)°, slight-

ly larger than the dihedral angle found in 2a. In 2b, the dioxane molecule lies on an inversion center and links two host molecules by the hydrogen bonds with NH of the imidazole. The intramolecular hydrogen-bond distance is (N1‚‚‚H1O1) 1.850 Å, and the O1-H1O1‚‚‚N1 angle is 149.3°; the intermolecular hydrogen-bond distance (O1S‚‚‚H2N2) is 2.043 Å, and the N2-H2N2‚‚‚O1S angle is 171.8°. Three hydrogen atoms of the methyl group (C7) are disordered between two states with equal occupancy. The other p-tolyl rings are rotated by 36.5(2) and 34.3(2)° with respect to the imidazole ring. The thermal behavior of 2b is shown by the differential scanning calorimeter (DSC) thermograph given in Figure 6. Between 483 and 503 K, the dioxane is removed. Upon removal of the guest molecules, the lemon crystals turned completely bright yellow. The second endotherm at 541 K is assigned to the melting temperature of host compound 2 (∆H ) 36.6 kJ mol-1). The geometry of all imidazole molecules are normal and need no comments; however, the different C-O bond length in the 2-carboxy-5-nitrophenyl ring in 1d is worth noting. The C-O bond length is significantly shorter (1.305 Å) compared with the bond in all other compounds (1.339-1.348 Å). The shor-


Fridman et al.

Table 3. Comparison of Rotation Angles (notation in Scheme 1) compd

R (deg)

β (deg)

γ (deg)

color

1a 1b 1c 1d 2a 2b

10.6 3.3 4.9 2.9 6.3 8.5

50.0 42.2 34.5 51.4 42.3 34.3

9.9 34.3 30.1 10.7 30.2 36.5

orange yellow yellow yellow yellow yellow

tening of the bond is explained by the fact that the proton was transferred to the imidazole ring; this is therefore equivalent to a C-O- bond.10 There are differences between the color of the free solvent crystals and those containing solvent molecules (Figure 7). Crystals of phenol, 2-[4,5-bis(4-methoxyphenyl)-1H-imidazole-2-yl]-4nitro (1a) are dark orange, whereas crystals of all solvates of 1 are yellow. The same is true for crystals of phenol, 2-[4,5-bis(4-methylphenyl)-1H-imidazole-2-yl]-4-nitro (2a), which are bright-yellow, whereas the crystal of the solvate (2b) has a lemon color. The presence of the nitro group is essential for having a colored compound. The imidazoles lacking the nitro group are colorless, as will be seen in ref 15. The colors of the solvates crystals turn into the color of the free solvent crystals upon loss of the solvent molecules. It is therefore evident that the presence of solvent molecules is responsible for the color change. In determining the cause for the color change, three major factors should be considered: (1) changes in the molecular conformation as a result of the presence of the solvent molecules, (2) changes in the packing of molecules in the crystal, and (3) changes in the intermolecular interactions caused by the presence of the solvent molecules. The molecular conformation is determined by the rotation of the three phenyl rings relative to the imidazole ring (given in Table 3). The range of rotation of the nitrophenol ring with respect to the imidazole ring (R) is 3.3-10.6°; this difference cannot account for the color change. Comparison of the two other

Figure 10. Absorption spectra of 2 at different apparent pH values in MeCN.


Figure 12. λmax of absorption (left) and transition energies of absorption (νabs) (right) vs apparent pH for 1 (blue), 2 (red), and 3 (green) in acetonitrile (C ≈ 1 × 10-5 M).

Figure 8. Absorption spectra of 1 in various solvents.


rotation angles (β and γ) shows that in two compounds, the solvent-free 1a and the protonated imidazole 1d, the rotation angle of one of the phenyl rings is almost coplanar with the imidazole ring (9.9 and 10.7°, respectively). In all other compounds, the rotation angle range is 30.1-51.4°. The meaning of this difference is that the effective molecular planarity increases in 1a, thus increasing the delocalization conjugation. This difference may account for the color difference of 1a. The packing of the molecules in the crystals are different in each of the compounds and therefore does not provide some common features to enable an understanding of the effect of packing on coloration. The major intermolecular interactions in all the crystals are hydrogen bonds. In all of them, two types of hydrogen bonds may be characterized. The first type is an intramolecular hydrogen bond between the imidazole nitrogen atom and the hydroxyl group of the 2-hydroxy-5-nitrophenyl ring. The second type is an intermolecular hydrogen bond between the imidazole donor for hydrogen (NH group) and a solvent molecule. The involvement of the nitro group in a hydrogen bond with the same molecule (as seen in 1a and 2a) effects the withdrawing


Crystal Growth & Design, Vol. 6, No. 10, 2006 2287 Scheme 2

properties of the nitro group. This may explain the color change. At the moment, we cannot distinguish between the effect of conformation and hydrogen bonding on the color change and this should be further investigated. Absorption Spectroscopy of Compounds 1 and 2 in Solution. Electronic absorption spectra of 1 at about 1 × 10-5 M, in different solvents, were almost identical for all derivatives. The absorption maximum is located around 257 nm for acetic acid and 265 nm for ethyl acetate and 1,4-dioxane. The absorption spectra in selected solvents are shown in Figure 8. We have measured the absorption spectra of 1 and 2 in an aprotic solvent, acetonitrile, at different pH values. These lophine derivatives show halochromic behavior dependent on the pH and have absorption spectra very similar to the ones measured in protic solutions such as ethanol. The dependence of the absorption spectra of each of the three compounds in MeCN on the pH (see Figures 9-11) was measured by adding diluted solutions of HCl and NaOH to MeCN. Figure 12 depicts a plot of the absorption maximum (λmax) and the transition energies of absorption (νabs) as a function of the apparent pH for 1-3 in acetonitrile. The absorption band was red shifted with increasing pH. In acidic solutions, the absorption peak (λmax) was located at 300 and 297 nm for 1 and 2, respectively. As the pH of the solution increased, these absorption bands disappeared, and new absorption bands appeared at 350 and 450 nm. There are two isobestic points, approximately at 330 and 380 nm for compounds 1 and 2. In the acidic pH regime of the experiment, the systems seem to be neutral. The first deprotonation step around pH 4 is attributed to the loss of the phenolic proton, forming [1-H]and [2-H]- species of 1 and 2, respectively. The second deprotonation step occurs at around pH 9.5. This process is attributed to the formation of the doubly deprotonated system [1-2H]2- and [2-2H]2- species of 1 and 2, respectively. The second deprotonation step occurs at around pH 9.5. This process is attributed to the formation of the doubly deprotonated system [1-2H]2- and [2-2H]2-. Comparing systems 1 and 2 with a system that lacks the phenolic proton, one can clearly see that the first deprotonation step of 3 is almost identical to the second deprotonation step of 1 and 2, supporting our assumption that the deprotonation step at basic pH is of the imidazole ring. Scheme 2 depicts the proposed acid-base processes for 1-3. Conclusions In this study, we have shown that phenol, 2-[4,5-bis(4methoxyphenyl)-1H-imidazole-2-yl]-4-nitro (1) and phenol,

2-[4,5-bis(4-methyl-phenyl)-1H-imidazole-2-yl]-4-nitro (2) form inclusion compounds with various guest molecules that possess hydrogen donor or hydrogen acceptor groups. The guest molecules play a crucial role in building up the threedimensional structure. The guest molecules link host molecules of 1 and 2 through hydrogen bonds. The thermochromic properties have been determined by the removal of the guest molecules. To clarify the relations between the color, thermochromic properties, and the role of host in crystal packing, we have determined and compared a single-crystal X-ray analysis of six crystal structures of the two compounds. Spectroscopic behavior of 1 and 2 at different pH and various solvents were presented. Comparison of 1 and 2 with 3 showed that systems 1 and 2 undergo deprotonation in basic pH. The observed features in the solution are determined by specific interactions of the NH hydrogen. The color change in the solid state is attributed to the variation in the hydrogen bonding. Acknowledgment. This research was supported by the Fund for the Promotion of Research at the Technion and the Technion VPR Fund. We are grateful to Professor Eichen for many stimulating discussions. Supporting Information Available: X-ray crystallographic information files (CIF) are available for compounds 1a-d, 2a, and 2b. The CIF files have been deposited in the Cambridge Crystallographic Centre with the numbers CCDC 298985-298990. This material is also available free of charge via the Internet at http://pubs.acs.org.

References (1) Bishop, R. Chem. Soc. ReV. 1996, 25, 311-320. (2) Tanaka, K.; Tomomori, A.; Scott, J. L. CrystEngComm 2003, 5 (27), 147-149. (3) Tanaka, K.; Tomomori, A.; Scott, J. L. Bull. Chem. Soc. Jpn. 2005, 78, 294-299. (4) Scott, J. L.; Yamada, T.; Tanaka, K. Bull. Chem. Soc. Jpn. 2004, 77, 1697-1701. (5) Scott, J. L.; Yamada, T.; Tanaka, K. New J. Chem. 2004, 28, 447450. (6) (a) Maeda, K.; Hayashi, T. Bull. Chem. Soc. Jpn. 1969, 42, 35093514; (b) Maeda, K.; Hayashi, T. Bull. Chem. Soc. Jpn. 1970, 43, 429-438. (7) Sakaino, Y.; Takizawa, T.; Inouye, Y.; Kakisawa, H. J. Chem. Soc., Perkin Trans. 2 1986, 1623-1629. (8) (a) Inouye, Y.; Sakaino, Y. Acta Crystallogr., Sect. C 1985, 41, 1106-1108. (b) Inouye, Y.; Sakaino, Y. Bull. Chem. Soc. Jpn. 1986, 59, 3295-3296. (c) Sakaino, Y.; Fujii, R.; Fujiwara, T. J. Chem. Soc., Perkin Trans. 1 1990, 2852-2854. (9) Inouye, Y.; Sakaino, Y. Acta Crystallogr., Sect C 2000, 56, 884887. (10) Kaftory, M.; Taycher, H.; Botoshansky, M. J. Chem. Soc., Perkin Trans. 2 1998, 407-412.

2288 Crystal Growth & Design, Vol. 6, No. 10, 2006 (11) Davidson, D.; Weiss, M.; Jelling, M. J. Org. Chem. 1937, 2, 319-327. (12) COLLECT; Nonius BV: Delft, The Netherlands, 2000. (13) Otwinowski, Z.; Minor, W. In Methods in Enzymology; Carter, C. W., Jr., Sweet, R. M., Eds.; Academic Press: New York, 1997; Vol. 276: Molecular Crystallography, Part A, pp 307-326.

Fridman et al. (14) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (15) Fridman, N.; Kaftory, M.; Speiser, S. To be published.

CG0600952

Chromotropic Behavior of Lophine Nitro-Derivatives - Crystal Growth

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