Elementary Graph-Set Descriptors in Crystal Structure Comparison of

Article pubs.acs.org/crystal

Elementary Graph-Set Descriptors in Crystal Structure Comparison of 2‑Methyl-4-Nitroanilinium Hexachloridostannate(IV), Bromide and Two Noncentrosymmertic Chlorides. X‑Ray, Vibrational and Theoretical Studies Marek Daszkiewicz* Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna str. 2, P.O. Box 1410, 50-950 Wrocław, Poland S Supporting Information *

ABSTRACT: Crystal structures of two polymorphs of [H2m4na] Cl (1α,1β) and (H2m4na)2SnCl6·H2O (2), where 2m4na = 2methyl-4-nitroaniline are shown and comparison of these crystal structures with the bromide salt reported earlier is presented. In all the presented crystal structures, alternatively arranged cations and anions form chain and ring hydrogen-bonding patterns of weak unconventional N−H···Cl hydrogen bonds. Interrelations among the elementary graph-set descriptors and descriptors of the hydrogen-bonding patterns are presented. Nonhydrogen bonding interactions between the nitro groups are also described using graph-set descriptors. Comparison of the experimental, also for deuterated 1α polymorph, and theoretical, for H2m4na+ ion with B3LYP/6-31G(d,p), spectra showed good agreement among the frequencies due to very weak interactions existing in studied compounds. Detailed analysis of the spectra revealed that the interaction between adjacent −NO2 groups in 1α is stronger than the other types involving the nitro group. The bands were assigned on the basis of theoretical calculations of vibrational frequencies for H2m4na+ ion and PED analysis.

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INTRODUCTION The 2-methyl-4-nitroaniline (2m4na) is studied due to nonlinear optical properties, which are related to the noncentrosymmetric crystal structure studied both experimentally and theoretically.1−6 The analysis of the X-ray diffraction data revealed a small contribution from the quinonoid resonance form to the molecular structure of 2m4na.2 Charge density analysis of X-ray diffraction data showed enhancement of the dipole moment of the 2m4na molecule caused most probably by the crystal field.3 The in-crystal dipole moment was assessed, being over two times larger in the crystal than in the gas phase. The result was reassessed fourteen years later, and the enhancement of the in-crystal dipole moment was established at a much lower level (i.e., ∼30%).4 The neutron diffraction results showed that the aromatic ring and nitro group are coplanar, but the amino group is slightly pyramidal.4 The hydrogen atoms of the amino group are displaced from the plane of the aromatic ring and lie at the same side of the ring’s plane. Large vibrational motion of hydrogen atoms of the methyl group at low temperature was also reported.4 FT-IR and 1 H NMR data showed small rotational barriers of all the −NH2, −NO2, and −CH3 groups.6 It was shown that reorientation of the methyl group in 2m4na and also in its deuterated analog © XXXX American Chemical Society

becomes the dominant motion above 90 K, and it plays a positive role toward NLO properties.6,7 In the case of the protonated form of 2m4na, a survey of the Cambridge Structural Database and literature revealed the only six crystal structures containing H2m4na+ ion.8−14 All of these are centrosymmetric, and the hydrogen-bonding patterns are exclusively constructed by the −NH3+ group and anion. Among these, isomorphic bromide and iodide salts were also reported.12 However, up until now, the crystal structure of the chloride has not been reported in literature. In this work, a comparison of the crystal structures of two polymorphs of [H2m4na]Cl (1α,1β) and (H2m4na)2SnCl6·H2O (2) is presented. A discussion also includes the bromide salt (3) reported earlier.12 Crystal packing and hydrogen bonding patterns found in the crystals are discussed. All of the example structures were intentionally synthesized because no strong hydrogen bonds exist between the organic and inorganic species in the crystal structure. Thus, an influence of hydrogen bonds on the frequency of normal Received: November 26, 2012 Revised: March 26, 2013

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Table 1. Crystal Data and Structure Refinement for the (H2m4na)Cl (1α,1β) and [H2m4na]2SnCl6·H2O (2) chemical formula Mr crystal system, space group a, b, c (Å) α, β, γ (°) V (Å3) Z, dcalc (g cm−3) μ (mm−1) crystal size (mm) absorption correction Tmin, Tmax no. of measured, independent and observed [I > 2σ(I)] reflections Rint (sin θ/λ)max (Å−1) R[F2 > 2σ(F2)], wR(F2), S no. of reflections, parameters, restraints Δ⟩max, Δ⟩min (e Å−3) flack parameter

C7H9N2O2Cl

(C7H9N2O2)2SnCl6·H2O

188.61 orthorhombic, P212121 5.5223(4), 7.7292(6), 20.8328(16) 90, 90, 90 889.21(12) 4, 1.409 0.39 0.36 × 0.32 × 0.21 numerical 0.853, 0.928 12732, 2118, 1835

655.73 monoclinic, C2/c 12.5392(3), 9.2956(2), 20.6018(4) 90, 99.435(2), 90 2368.85(9) 4, 1.839 1.79 0.31 × 0.26 × 0.20 numerical 0.606, 0.725 15073, 2705, 2448

0.071 0.649 0.065, 0.168, 1.08 2115, 111, 0 1.16, −0.31 0.0(16)

0.062 0.658 0.035, 0.092, 1.08 2118, 111, 0 0.18, −0.21 −0.01(7)

0.019 0.649 0.021, 0.054, 1.07 2705, 144, 0 0.60, −0.58 −

modes is weak.15 Besides, the bands corresponding to the frequencies of the organic and inorganic part of the hybrid compound occur in separate regions. Therefore, a comparison between calculated and measured spectra can easily be done. The assignment of the bands was done with the help of the spectra of the deuterated analog of 1α.

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C7H9N2O2Cl

188.61 tetragonal, P4̅21c 8.0420(3), 8.0420(3), 28.3749(12) 90, 90, 90 1835.11(12) 8, 1.375 0.38 0.43 × 0.33 × 0.06 numerical 0.847, 0.958 22766, 2115, 1523

Single Crystal X-ray Diffraction Studies. X-ray diffraction data were collected on a KUMA Diffraction KM-4 four-circle single-crystal diffractometer equipped with a CCD detector, using graphitemonochromatized Mo Kα radiation (λ = 0.71073 Å). The raw data were treated with the CrysAlis Data Reduction Program (version 1.172.32.6), taking into account an absorption correction. The intensities of the reflections were corrected for Lorentz and polarization effects. The crystal structures were solved for by direct methods16 and refined by the full-matrix least-squares method using SHELXL-9716 (Table 1). Nonhydrogen atoms were refined using anisotropic displacement parameters. The chlorides Cl1 and Cl2 in 1α lie at the inversion axis 4̅ and two-fold axis respective of the Wyckoff positions 4k and 4m. The water molecule in 2 is disordered over two symmetry-dependent positions, related by the two-fold axis. This axis also passes through the SnCl62− anion−Wyckoff position 4e. The other atoms occupy general positions. H atoms were visible on the Fourier difference maps but placed by geometry and allowed to refine riding on the parent atom. The hydrogen atoms were not localized on the water molecule in 2 probably because of disorder. The diffraction pattern for 1α was checked with respect to twinning, but no twin domains were observed. The Flack parameter obtained initially was equal to 0.45(11) and, therefore, inversion twinning was taken into account in the refinement. The crystal structure was checked by the Platon program, which suggested a centrosymmetric space group, P42/nmc. However, hk0 = 2n general reflection condition was not fulfilled because of the presence of as many as 1410 reflections violating general condition among all of the 24172 measured ones. CCDC-875783 for 2-methyl-4-nitroanilinium chloride (1α), CCDC-875782 for 2-methyl-4-nitroanilinium chloride (1β), and CCDC-905995 for bis(2-amino-4-nitroanilinium) hexachloridostannate(IV) monohydrate (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: +44 1223 336033; e-mail: deposit@ ccdc.cam.uk) Spectroscopic Measurements. Room temperature FT-IR spectra in the 4000−400 cm−1 range were measured on the Bruker IFS-88 spectrometer with a 2 cm−1 resolution. Nujol and fluorolube mull techniques have been used in the measurements. Far-infrared spectra in the 550−50 cm−1 region were measured in Nujol suspension between polyethylene plates. Room temperature FTRaman spectra were measured in the 4000−80 cm−1 range using the Bruker FRA-106 attachment with 2 cm−1 resolution.

EXPERIMENTAL SECTION

Synthesis. The starting compounds, 2-methyl-4-nitroaniline [Aldrich, purum, > 98% (NT)], hydrochloric acid (Aldrich, 35 wt % in H2O, 99.95%), hydrobromic acid (Aldrich, 40 wt % in H2O, 99.95%), and tin(II) chloride (POCh Gliwice, 99.95%) were used as supplied. The 2-methyl-4-nitroaniline (1 mmol, 0.1522g) and 20 mL of 35% HCl were mixed and a small amount of 2m4na was dissolved by heating. The clear brown solution was allowed to stand for several weeks. Thin colorless square-plate crystals of 2-methyl-4-nitroanilinium chloride (1α) were obtained after three weeks by slow evaporation of the solution. The second polymorph of 2-methyl-4nitroanilinium chloride (1β) was crystallized one week later, yellow to brown prismatic crystals. The compound 1α can also be obtained using 1 mmol of 2m4na and the following mixtures: (a) 10 mL 35% HCl + 10 mL H2O + 10 mL of methanol, (b) 0.2 mL 35% HCl + 3 mL methanol, or (c) 0.2 mL 35% HCl + 3 mL methanol + 3 mL 2propanol. In these cases, nice colorless square plate crystals of 1α were obtained, but the crystals of compound 1β were not formed. The SnCl2 (2 mmol, 0.7012 g) and 2-methyl-4-nitroaniline (2 mmol, 0.3043 g) were dissolved in a mixture of 10 mL HCl and 10 mL H2O. The solution was slowly evaporated at room temperature, and two kinds of crystals were obtained after three weeks. Plate and prismatic crystals were identified as compound 1α and bis(2-amino-4nitroanilinium) hexachloridostannate(IV) monohydrate (2), respectively, using X-ray diffraction. It was tried to obtain the deuterated analog of 1α, but recrystallization from D2O was unsuccessful because of precipitation of pure 2-methyl-4-nitroaniline. Therefore, the deuterated analog of 1α was synthesized using 2-methyl-4-nitroaniline (1 mmol, 0.1522g), 5 mL of CH3OD, and 0.5 mL HCl. The D:H ratio in the obtained crystals was assessed at ∼40%. Plate crystals were obtained, and the lattice parameters were checked using X-ray diffraction. The 2-methyl-4-nitroanilinium bromide (3) was obtained dissolving 1 mmol of 2m4na in a mixture of 1 mL of 40% HBr and 5 mL of methanol. Brownish long-plate crystals were checked using X-ray diffraction, comparing the parameters of the unit cell reported by Lemmerer and Billing.12 B

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Figure 1. Crystal packing of (a) (H2m4na)Cl (1α), (b) (H2m4na)Cl (1β), (c) (H2m4na)Br, and (d) (H2m4na)2SnCl6·H2O along the b axis.

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Computational Details. All the computations were performed for the singlet electronic ground state of 2-methyl-4-nitroanilinium cation using density-functional theory (DFT) and hybrid Becke’s threeparameter Lee−Yang−Parr correlation functionals (B3LYP).17−21 The 6-31G(d,p) basis set was used because it gives satisfactory results for vibrational data with respect to the computational cost.22 Since the ammonium and the methyl groups can rotate around the N−C and C−C bonds, respectively, the geometry of four conformers were checked (see the Supporting Information). The VEDA program23−25 was used for potential energy distribution (PED) analysis for the conformer of the lowest energy H2m4na+ cation. Frequencies were scaled using the scaling equation, νsc = 0.9543·νcalc + 22.1 introduced for benzene and aniline.22 The definitions of local modes calculated for the H2m4na+ cation are deposited as Supporting Information. The calculated IR intensities of the bands were normalized with the highest peak absorption equal to 100. The calculated Raman scattering activities (Si) were converted to relative Raman intensities (Ii) using the following relationship:26−28

Ii =

RESULTS AND DISCUSSION

Crystal Packing. All the presented structures are constructed by cationic (C) and anionic (A) layers, where the H2m4na+ ion penetrates more or less the anionic layer. The layers are arranged in the sequence ACCA in 1α, 1β, and 3, whereas the ACC sequence exists in 2 because the bivalent SnCl62− anion balance two monovalent H2m4na+ ions (Figure 1). In the crystal structure of (H2m4na)Br (3), the aromatic rings of the adjacent H2m4na+ ions are parallel to each other (Figure 1c). They are also parallel to the H2m4na+ ions positioned in the next layer. However the NNH2−NNO2 vectors of neighboring ions are close to orthogonal. Parallel arrangement of the aromatic rings also exists in (H2m4na)2SnCl6·H2O (2), but one C layer penetrates the neighboring C layer to a great extent (Figure 1d). In contrast, compound 2 has a headto-tail packing arrangement. In the case of the tetragonal polymorph 1α, H2m4na+ ions are perpendicular to each other in one ACCA sequence. On the other hand, the relative position of the H2m4na+ ions lying at the opposite sites of the anionic layer are parallel (Figure 1a). In 1β, two adjacent H2m4na+ ions from two adjacent C layers are positioned at 31.9°. It appears that such mutual arrangement of the H2m4na+ ions is favorable concerning the crystal packing because the density of the polymorph 1β is higher than for 1α (Table 1). In both polymorphs of (H2m4na)Cl, the cations realize a head-tohead packing arrangement.

f (ν0 − νi)4 Si νi[1 − exp(− hcνi /kT )]

where ν0 = 9398.5 cm−1 is the exciting frequency (λ = 1064 nm, Nd:YAG laser), νi is the vibrational wavenumber of the ith normal mode, h, c, and k are universal constants, and f is the suitably chosen common scaling factor for all the peak intensities. Then, the intensities were normalized with the highest peak equal to 100. C

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Table 2. Hydrogen bonds (Å, deg) for (H2m4na)Cl (1α,1β) and (H2m4na)2SnCl6·H2O (2)a D−H···A (H2m4na)Cl (1α) N1AH1A1···Cl1b N1AH1A1···Cl2c N1AH1A2···Cl1 N1AH1A3···Cl2 (H2m4na)Cl (1β) N1H1A···Cl N1H1B···Cld N1H1C···Cle (H2m4na)2SnCl6·H2O (2) N1H1A···Cl3f N1H1A···Cl2g N1H1B···O3W N1H1B···O3Wh N1H1C···Cl3 N1H1C···Cl1 O3W−H2W3···Cl1 O3W−H1W3···Cl2I

d(D−H)

d(H···A)

d(D···A)

0.89 0.89 0.89 0.89

2.54 2.56 2.30 2.29

3.177(3) 3.178(3) 3.133(3) 3.141(3)

128.9 127.1 155.9 159.4

0.89 0.89 0.89

2.35 2.26 2.31

3.1880(15) 3.1382(15) 3.1498(15)

156.4 170.0 156.6

0.89 0.89 0.89 0.89 0.89 0.89 0.81 0.79

2.65 2.71 2.14 1.92 2.67 2.73 2.54 2.89

3.3586(19) 3.4195(19) 2.970(4) 2.796(4) 3.3997(19) 3.2770(18) 3.221(3) 3.661(4)

137.2 138.0 154.7 166.4 140.1 121.1 142.6 166.0

O···πring and C−Hring···O in 1β > C−HCH3···O and πring···πring in 3 > C−HCH3···O and πNO2···πring in 2.

group (i.e., O···πring and C−Hring···O in 1β, C−HCH3···O and πNO2···πring in 2, and C−HCH3···O and πring···πring in 3).

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CONCLUSIONS Strong second-order nonlinear optical activity has been reported for the noncentrosymmetric crystals of 2m4na1. Both the polymorphs α and β of [H2m4na]Cl are also promising materials for nonlinear optics because they crystallize without an inversion center. It is worth noting that the tetragonal phase α tends to crystallize as an inversion twin. However, vanishing of the crystal polarity and a canceling of the nonlinear optical properties are expected but only along the boundaries of the twin domains.41 Although the packing in presented crystal structures is different, H2m4na+ and Cl−/Br− ions assemble in cationic and anionic layers, respectively. They alternate in pairs in a sequence ACCA. In the crystal structure of the hexachloridostannate salt (2), the only anionic layer exists because the anion is bivalent and it corresponds to a double-anionic layer in chloride and bromide salts. Weak unconventional N−H···Cl/Br hydrogen bonds exist between the layers. In all the presented crystal structures, alternatively arranged cations and anions form chain and ring hydrogen-bonding patterns. A superposition of the rings and chains results in a two-dimensional pattern in 1α, a ladder of hydrogen bonds in 1β and 3, and a chain of separate rings in 2. Overall, the only possible hydrogen-bonding patterns constructed by H2m4na+ and Cl− ions are connected with the interaction between the −NH 3+ and Cl − species. Construction of the simple hydrogen-bonding patterns is reflected in the summation of the elementary graph-set descriptors GNH3 + GCl = GHB, where GHB = {D(2); C(2); R12(4); C12(4)}. Superposition of these graph-set descriptors gives descriptors of the complex hydrogen-bonding patterns. Thus, the relations among the elementary graph-set descriptors and those of the simple and complex hydrogen-bonding patterns were established for presented structures, for instance E02(3)NH3 + E10(1)Cl + E01(1)NH3 + E10(1)Cl = R12(4) + D(2) = R23(6). Comparative analysis of one-dimensional patterns found in 2 (chain of separate rings) and 1α, 3 (ladder structure) showed that the difference between the C22(6) and C12(4) chains gives a E10(2) elementary graph-set descriptor which refers to twoatomic pathway Sn−Cl. The same result is obtained for the ring R21(4) and discrete pattern D(2). Thus, comparing the hydrogen-bonding patterns, the smaller the difference between the graph-set descriptors is, the greater the similarity. However, if many hydrogen-bonding patterns exist in one arrangement of the cations and anions, for comparison purposes, it is better to juxtapose all the graph-set descriptors for two structures. In those example structures, the ladder consists of two chains C12(4) and two additional chains C24(8) (Figures 3ab), whereas the arrangement of H2m4na+ and SnCl62− ions contains two chains C22(6). Thus, the difference of the arrangement of the ions is significant because of the substantial difference between the set of patterns, despite the fact that the comparison of particular chains C22(6) and C12(4) results in the short twoatomic pathway E10(2)Sn−Cl, indicating significant similarity. Comparison of the presented crystal structures shows that the nitro group of the H2m4na+ ion interacts with the adjacent species in many modes. This group participates in weak hydrogen bonds C−HCH3···O and C−Hring···O and also in a very weak ONO2···π(N)NO2 interaction. To the best of the

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ASSOCIATED CONTENT

S Supporting Information *

Scheme of four conformers and energy of the H2m4na+ ion calculated using the B3LYP/6-31G(d,p) level in the Gaussian program; infrared and Raman spectra for compounds 1−3; definitions of local modes calculated for the H2m4na+ ion used in calculations by VEDA program; comparison of theoretical and experimental IR and Raman spectra; PED and assignment of the bands; and X-ray crystallographic information files (CIF) are available for compounds 1α, 1β, and 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +48 71 39 54 145. Fax: +48 71 34 410 29. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Calculations have been carried out in the Wroclaw Centre for Networking and Supercomputing (http://www.wcss.wroc.pl). ABBREVIATIONS 2m4na, 2-methyl-4-nitroaniline; PED, potential energy distribution REFERENCES

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H

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