Synthesis and characterisation of arylazoimidazolium

Indian Journal of Chemistry Vol. 51A, July 2012, pp. 917-926

Synthesis and characterisation of arylazoimidazolium iodide and tetraiodocadmium(II) compounds D Mallicka, R Sahab, K K Sarkerc, P Dattad & C Sinhae, * a

Department of Chemistry, Mrinalini Datta Mahavidyapith, Birati, Kolkata 700 051, West Bengal, India b Department of Physics, Jadavpur University, Kolkata 700 032, West Bengal, India c Department of Chemistry, Mahadevananda Mahavidyalaya, Barrackpore, Monirampore, Kolkata 700 120, West Bengal, India d Calcutta Institute of Engineering and Management, 24/1A Chandi Ghosh Road, Kolkata 700 040, West Bengal, India e Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Raja Suboth Mullick Road, Kolkata 700 032, West Bengal, India Email: [email protected] Received 9 January 2012; revised and accepted 22 June 2012 Sixteen new inorganic-organic hybrid compounds of 1,3-dialkyl-2-(arylazo)imidazolium, [RaaiR/R//]+ (R, alkyl group), and I- or [CdI4]2- as counter ion are synthesized and characterized by spectroscopic (IR, UV-vis, 1H NMR) techniques. The single crystal X-ray diffraction studies of representative compounds, viz., 1,3-dimethyl-2-(p-tolylazo)imidazolium iodide [MeaaiMe2+]I- and bis- [1-methyl-3-benzyl-2-(p-tolylazo)imidazolium]tetraiodocadmium(II) [Meaai(Me)CH2Ph+]2 [CdI4]2-, have confirmed the proposed structures. DFT computation has been used to explain the electronic structure and the electronic spectra of the compounds. Keywords: Coordination compounds, Inorganic-organic Arylazoimidazolium salts, Iodometallates,

Inorganic–organic hybrid compounds have attracted attention to generate high conducting useful ionic materials of high mechanical and dimensional stability1-6. The transition and non-transition metal-halides show various coordination numbers and structures7-9 and have generated halometallates which are building agents to bring organic cations together to develop sustainable hybrid materials. Iodide (I-) has generated polymeric network with most of the metal ions and form myriad of bridged dimers, cubanes, chains, strands, etc.7-12 The N-heterocycle organic cations like dialkyl imidazolium iodides are interesting class of ionic liquids with low vapor pressure and are important media for organic and inorganic compound synthesis and in electrochemistry for large potential window and separation processes13-15. We have extensively studied the functionalization of imidazole and have characterized arylazoimidazoles and their derivatives16-20. Alkylation of arylazoimidazoles has synthesized 1,3-dialkyl-2-(arylazo)imidazolium salts of different anions like PF6¯ (ref. 21), ReO4¯ (ref. 22,23),

hybrids, Cadmium, Dialkyl-arylazoimidazolium salts, X-ray structures, Density functional calculations

Cl¯ , [ZnCl4]2–, [PtCl6]2– (ref. 24), [PbI3¯ ]n (ref. 25). Studies on the structural diversity of arylazoimidazolium ions with different oxo and non-oxo anions have been reported by other groups also26,27. In this work we report arylazoimidazolium salts of iodide and tetraiodocadmium(II) derivatives. The compounds are characterized by different spectroscopic studies and structural confirmation has been carried out in representative cases by single crystal X-ray diffraction measurements. Materials and Methods 2-(Arylazo)imidazoles were prepared by the reported procedure28. All other chemicals and solvents were reagent grade and used as received. Cadmium iodide was purchased from SD Fine Chemicals. Microanalytical data (C, H, N) were collected on Perkin–Elmer 2400 CHNS/O elemental analyzer. Spectroscopic data were recorded using the following instruments: UV-vis spectra from Perkin-Elmer Lambda 25 spectrophotometer, IR spectra (KBr disk, 4000–450 cm−1) on Perkin-Elmer

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INDIAN J CHEM, SEC A, JULY 2012

RX-1 FTIR spectrophotometer and 1H NMR spectra on Bruker (AC) 300 MHz FTNMR spectrometer. Molar conductance was measured using Systronics-304 conductivity meter. Preparation of compounds

/

The 1-alkyl-2-(arylazo)imidazoles (RaaiR ) were synthesized from the corresponding 2-(arylazo)imidazoles (RaaiH, 1-3) by reacting the latter with the respective alkyl halide in the presence of sodium hydride in dry THF16. Preparation of 1,3-dimethyl-2 (tolaylazo)imidazolium iodide (MeaaiMe2)+I ¯ (4b)

To a THF solution (10 ml) of 1-methyl-2(p-tolaylazo)imidazole (0.2 g, 1.00 mmol), MeI was added and kept at low temperature for 30 min and then reflux for 4 h. The solution was evaporated under reduced pressure and the dry mass was extracted in water and filtered. The filtrate was evaporated slowly in air and orange crystalline products were isolated. The orange-red needle shaped crystals were then recrystallised from methanol-water (1:1, v/v) yield: 0.24 g (68 %). All other compounds were prepared by identical procedure in the range of 65-75 % yield. Microanalytical data of the complexes are as follows: [HaaiMe2]+I- (4a), m. pt. 185±1 °C; Anal. (%): Found: C, 40.31; H, 4.01; N, 17.03. Calc.: C, 40.24; H, 3.96; N, 17.07. FT-IR (KBr disc, cm-1), ν(N=N), 1428; ν(C=N), 1590. UV-vis spectral data in CH3CN (λmax(nm) (10-4 ∈ (dm3 mol-1cm-1): 239(0.05), 369(2.7), 380(2.5), 452(1.6). [MeaaiMe2]+I- (4b), m. pt. 168±1 °C; Anal. (%): Found: C, 42.13; H, 4.43 N, 16.43. Calc.: C, 42.10; H, 4.39; N, 16.37. FT-IR (KBr disc, cm-1); ν(N=N), 1424; ν(C=N), 1592. UV-vis spectral data in CH3CN (λmax(nm) (10-4 ∈ (dm3 mol-1cm-1) 242 (0.06), 368 (2.6), 384 (2.4), 454(1.8). [HaaiEt2]+I- (5a), m. pt. 176 ±1 °C; Anal. (%): Found: C, 43.90; H, 4.83 N, 15.70. Calc.: C, 43.82; H, 4.78; N, 15.73. FT-IR (KBr disc, cm-1), ν(N=N), 1420; ν(C=N), 1590. UV-vis spectral data in CH3CN (λmax(nm) (10-4 ∈ (dm3 mol-1cm-1 ) 243 (0.04), 367 (2.5), 379 (2.2), 449(1.6). [MeaaiEt2]+I- (5b), m. pt. 173±2 °C; Anal. (%): Found: C, 42.13; H, 4.43 N, 16.43. Calc.: C, 45.78; H,

4.36; N, 15.26. FT-IR (KBr disc, cm-1), ν(N=N), 1420; ν(C=N), 1594. UV-vis spectral data in CH3CN (λmax(nm)(10-4 ∈ (dm3 mol-1cm-1) 241 (0.06), 366(2.7), 378 (2.2), 450 (1.5). [Haai(CH2Ph)2]+I- (6a) , m. pt. 230±1°C; Anal. (%): Found: C, 57.43; H, 4.32; N, 11.58. Calc.: C, 57.50; H, 4.38; N, 11.67. FT-IR (KBr disc, cm-1), ν(N=N), 1422; ν(C=N), 1592. UV-vis spectral data in CH3CN (λmax(nm) (10-4 ∈ (dm3 mol-1cm-1): 244 (0.06), 364 (2.3), 381 (2.1), 451 (1.5). [Meaai(CH2Ph)2]+I- (6b) , m. pt. 245±1°C; Anal. (%): Found: C, 58.21; H, 4.60; N, 11.26. Calc.: C, 58.29; H, 4.66; N, 11.34. FT-IR (KBr disc, cm-1), ν(N=N), 1425; ν(C=N), 1590. UV-vis spectral data in CH3CN (λmax(nm) (10-4 ∈ (dm3 mol-1cm-1): 245 (0.07), 365 (2.3), 379 (2.0), 448 (1.7). [HaaiMe(CH2Ph)]+I- (7a) m. pt. 235±1 °C. Anal. (%): Found: C, 50.54; H, 4.21; N, 13.82. Calc. C, 50.51; H, 4.24; N, 13.86. FT-IR (KBr disc, cm-1), ν(N=N), 1428; ν(C=N), 1595. UV-vis spectral data in CH3CN (λmax(nm) (10-4∈(dm3 mol-1cm-1): 238 (0.04), 368 (2.7), 378 (2.3), 449 (1.4). [MeaaiMe(CH2Ph)]+I- (7b), m. pt. 248±1 °C Anal. (%): Found: C, 51.75; H, 4.54; N, 13.46. Calc. C, 51.69; H, 4.58; N, 13.40 %. FT-IR (KBr disc, cm-1), ν(N=N), 1430; ν(C=N), 1588. UV-vis spectral data in CH3CN (λmax(nm) (10-4 ∈ (dm3 mol-1cm-1): 240 (0.05), 366 (2.4), 380 (2.1), 450 (1.6). Synthesis of [MeaaiMe(CH2Ph)+]2 [CdI4]2- (11b)

To a methanol solution (20 ml) of [MeaaiMe(CH2 Ph)]+ I (1-methyl-3-benzyl-2-(p-tolylazo) imidazolium iodide) (0.105 g, 0.36 mmol), was added CdI2 (0.132 g, 0.36 mmol) pinchwise, with stirring and then refluxed for 8 h. The hot solution was filtered through G4 crucible and allowed to evaporate slowly in air. The block shaped orange-red crystals deposited on the wall of beaker were collected by filtration. These crystals were ground and the 2-methoxyethanol solution was prepared. Methanol (2 vol.) was added into this solution and after a week, bright orange-red colored block shaped crystals were deposited at the base of the container, which were collected and dried over CaCl2 in a desiccator (yield 0.18 g, 62 %).

MALLICK et al.: SYNTHESIS OF ARYLAZOIMIDAZOLIUM SALTS OF I¯ & [CdI4]2– DERIVATIVES

The other complexes were prepared by the same procedure. In all the cases, crystalline products were obtained. The yield varied from 60-70 %. Microanalytical data of these complexes are as follows: [HaaiMe2+]2 [CdI4]2- (8a): Anal. (%): Found: C, 25.85; H, 2.4; N, 11.03. Calc.: C, 25.84; H, 2.6; N, 11.0. FTIR (KBr disc, cm-1), ν(N=N), 1432; ν(C=N), 1593 cm-1. UV-vis spectral data in CH3CN (λmax(nm) (10-4 ∈ (dm3 mol-1cm-1): 366 (7.6), 380 (7.1), 467 (4.6). [MeaaiMe2+]2 [CdI4]2- (8b): Anal. (%): Found: C, 27.78; H, 3.02; N, 10.73. Calc.: C, 27.80; H, 2.91; N, 10.81. FT-IR (KBr disc, cm-1), ν(N=N), 1442; ν(C=N), 1596. UV-vis spectral data in CH3CN (λmax(nm)(10-4 ∈ (dm3 mol-1cm-1): 370 (7.7), 382 (7.2), 464 (4.8). [HaaiEt2+]2 [CdI4]2- (9a): Anal. (%): Found: C, 29.83; H, 3.24; N, 10.57. Calc.: C, 29.72; H, 3.26; N, 10.67. FT-IR (KBr disc, cm-1), ν(N=N), 1436; ν(C=N), 1597. UV-vis spectral data in CH3CN (λmax(nm) (10-4 ∈ (dm3 mol-1cm-1): 368 (7.9), 379 (7.1), 465 (5.0). [MeaaiEt2+]2 [CdI4]2- (9b): Anal. (%): Found: C, 31.63; H, 3.63; N, 10.51. Calc: C, 31.59; H, 3.59; N, 10.53. FT-IR (KBr disc, cm-1), ν(N=N), 1438; ν(C=N), 1601. UV-vis spectral data in CH3CN (λmax(nm) (10-4 ∈ (dm3 mol-1cm-1): 366 (7.4), 384 (6.7), 468 (4.7). [Haai(CH2Ph)2+]2 [CdI4]2- (10a): Anal. (%): Found: C, 41.58; H, 3.32; N, 8.43. Calc.: C, 41.64; H, 3.20; N, 8.4. FT-IR (KBr disc, cm-1), ν(N=N), 1435; ν(C=N), 1592. UV-vis spectral data in CH3CN (λmax(nm) (10-4∈ (dm3 mol-1cm-1): 368 (7.7), 378 (6.9), 470 (4.9). [Meaai(CH2Ph)2+]2[CdI4]2- (10b): Anal. (%): Found: C, 43.06; H, 3.57; N, 8.29. Calc.: C, 43.0; H, 3.5; N, 8.3. FT-IR (KBr disc, cm-1), ν(N=N), 1440; ν(C=N), 1600. UV-vis spectral data in CH3CN (λmax(nm) (10-4∈ (dm3 mol-1cm-1): 374 (7.6), 385 (6.8), 469 (5.2). [HaaiMe(CH2Ph)+]2[CdI4]2- (11a): Anal. (%): Found: C, 34.38; H, 2.85; N, 9.4. Calc. C, 34.35; H, 2.9; N, 9.43. FT-IR (KBr disc, cm-1), ν(N=N), 1435; ν(C=N), 1594. UV-vis spectral data in CH3CN (λmax(nm) (10-4∈(dm3 mol-1cm-1): 371 (7.4), 386 (7.0), 462 (5.1).

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[MeaaiMe(CH2Ph )+]2[CdI4]2- (11b): Anal. (%): Found: C, 35.93; H, 3.16; N, 9.29. Calc. C, 35.95; H, 3.12; N, 9.32 %. FT-IR (KBr disc, cm-1), ν(N=N), 1464; ν(C=N), 1602. UV-vis spectral data in CH3CN (λmax(nm)(10-4 ∈ (dm3 mol-1cm-1): 373 (7.8), 390 (7.3), 463 (4.9). X-ray crystal structure analyses

Crystals suitable for X-ray diffraction study of (MeaaiMe2)+I- (4b) (0.12 × 0.14 × 0.22 mm) were prepared by slow evaporation of aqueous solution in air and for [MeaaiMe(CH2Ph)+]2 [CdI4]2- (11b) (0.05 × 0.10 × 0.20 mm) were crystallized from a mixture of methanol and 2-methoxy-ethanol (2:1, v/v) at ambient condition. Diffraction data were collected on a Bruker SMART 1K CCD area-detector diffractometer using fine focused sealed tube graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å). Unit cell parameters were determined from least-squares method in the range of -9 ≤ h ≤ 9, -12 ≤ k ≤ 9, -13 ≤ l ≤ 13 and angle variation 3.98 < 2θ < 55.00° for 4b and in the case of 11b, the parameters are -26 ≤ h ≤ 27, -17 ≤ k ≤ 19, -18 ≤ l ≤ 12 and angle variation 1.92< 2θ < 56.76°. A summary of the crystallographic data and structure refinement parameters are given in Table 1. Data were corrected for Lorentz and polarization effects. Data reduction was carried out by using SAINT programme and the structure was solved by direct method using SHELXS-9729 and successive difference Fourier syntheses. All non-hydrogen atoms were refined anisotropically. Full matrix least squares refinements on F02 were carried out using SHELXL-9730 with anisotropic displacement parameters for all non-hydrogen atoms. Hydrogen atoms were constrained to ride on the respective carbon or nitrogen atoms with anisotropic displacement parameters equal to 1.2 times the equivalent isotropic displacement of their parent atom in all cases. The maximum (∆max, e Å-3) and minimum (∆min, e Å-3) electron densities were 0.572, −0.263 (4b) and 0.680, −0.807 (11b). All calculations were carried out using SHELXS 9729, PLATON 9931, ORTEP-332 programs. Computation details

All the calculations were carried out with the density functional theory (DFT) method as implemented in GAUSSIAN 03 (G03) program package33 and the calculations were performed using the B3LYP exchange correlation functional34.

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Table 1 – Crystallographic data for [MeaaiMe2+]I- (4b) and [MeaaiMe(CH2Ph) +] 2 [CdI42-] (11b) [MeaaiMe2+]IEmpirical formula Formula weight Temperature (K) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å)

[MeaaiMe(CH2Ph)+]2 [CdI42-]

C12H15I N4 342.18 293(2) triclinic P -1

C36H38I4N8Cd 1202.74 294(2) monoclinic P 21/c 21.157(3) 14.3035(17) 13.6334(16)

α (°)

7.2434(14) 9.6969(19) 10.474(2) 77.69(3)

β (°)

87.59(3)

92.135(3)

γ (°) V (Å)3 Z

74.43(3)

λ (Å)

692.3(2) 2 0.71073

µ (Mo-Kα) (mm-1) Dcalc (mg m-3) Refine parameters Total reflections Unique data [I > 2σ (I) ]

2.298

3.560

1.642 166 11663 3105

1.938 442 27623 10042

0.0289

0.0442

0.0651 0.925

0.0842 0.878

R1a [ I > 2σ (I) ] wR2b Goodness of fit

4122.9(9) 4 0.71073

a

R = ΣF0 –Fc / Σ F0. bwR = [Σ w(F02 – Fc2)/ Σ w F04]1/2 are general but w are different. w = 1/ [σ2 (Fo2) + (0.0396P)2 + 0.3036P] for (4b); w = 1/[σ2(F2) + ( 0.0100P)2 + (0.0000P)] for (11b) where P = (Fo2+2Fc2)/3.

The geometry of the compounds (4b) and (11b) were fully optimized in gas phase using 6-31G* basis set for all elements except cadmium and iodine. The Los Alamos effective core potential plus double zeta (LanL2DZ)35 basis set was employed along with the corresponding pseudo-potential of cadmium and iodine without any symmetry constrain. The vibrational frequency calculation was also performed for both compounds to ensure that the optimized geometries represent the local minima and there are only positive Eigen values. The calculated and experimental metric parameters agree well to support proper selection of methods and functions used. To assign the low lying electronic transitions in the experimental spectra, TDDFT36 calculations of the complexes were done in acetonitrile using conductorlike polarizable continuum model (CPCM)37-39 with

larger basis set 6-31g+(d) for H, C, N and LanL2DZ for cadmium and iodine. Results and Discussion Synthesis and characterisation of arylazoimidazolium iodides

The 1,3-dialkyl-2-(arylazo)imidazolium iodides ([RaaiR/2]+ I- , where R = H (a), Me (b); R/ = Me (4), Et (5), CH2Ph (6)] were synthesized by addition of R/I to THF solution of 1-alkyl-2--(arylazo)imidazoles (RaaiR/) and purified by crystallization from methanol-water. Unsymmetrical imidazolium iodides, [Raai(Me)CH2Ph]+I- (7) were synthesized by benzylation of 1-methyl-2-(arylazo)imidazoles (RaaiMe). Ionic compounds of the composition, [RaaiR/2+]2[CdI4]2- (R/ = Me (8), Et (9), CH2Ph (10)) and [Raai(CH3)(CH2Ph)+]2[CdI4]2- (11) were obtained as brown-red precipitate by mixing the aqueous suspension of CdI2 to the solution of [RaaiR/R//+]I- in 1:2 molar ratio at room temperature, filtered and washed with cold water and dried in vacuuo. These compounds were recrystallised from 2-methoxyethanol-methanol (1:2, v/v) by slow evaporation in air. The composition of the compounds is supported by elemental analyses (C, H, N) and the structures established by spectroscopic data. The single crystal X-ray diffraction data confirmed the structure of the compounds as shown in Scheme 1. The compounds are conducting both in aqueous and alcohol solutions. Methanol solution of the compounds, (4 – 7) shows 1:1 conductivity (60 - 80 Ω-1 mol-1), while the tetraiodocadmium(II) salts (8 – 11) are also conducting molecules and show 2 : 1 conductivity. Spectral studies of arylazoimidazolium iodides

The IR frequencies of the compounds have been assigned on comparing data with those of the free ligand16. Moderately intense stretching at 1595–1600 and 1435–1445 cm-1 is due to υ(C=N) and υ (N=N), respectively, for these compounds. Since the compounds are soluble in acetonitrile, the solution electronic spectra were recorded in acetonitrile in the wavelength range of 200−900 nm. There are three bands and of these, two are high intense (ε, 104 mol-1 dm3 cm-1) bands at 360–370 and 375–385 nm. On comparing with free ligand spectra16, it may be concluded that these bands are due to π-π* charge transfer transitions. A weak band (ε, 103 mol-1 dm3 cm-1) appears at 460–475 nm. The electronic spectra may be assigned as the


N

N H

(i) NaH / THF

R

5

4

N 1

N 3

+ IR

N

N N

(ii)

R/I,

R

R RaaiR/

5

+ I-

R

R

+ N 3

N 1

R

N

N

N

N

2

8

RaaiR/ 2+I-, 4 - 7

RaaiH, 1 - 3

N 3

N

9 R

R

N 1

+ 7

10

4

R

N

11

5

N

N R/

N

Reflux

921

[CdI4 ]2-

+ CdI2

11

7

10

8

7

11 10

8 9 R

9 R

2n

8 -11 R = H (a), Me (b), R/ = -12CH3 (1, 4, 8); -12CH2 13CH3 (2, 5 ,9), -12 CH2Ph (3, 6, 10); R/ = Me, R// = CH2Ph (7, 11) Syntheses of ligands RaaiR/ (1-3), [RaaiR/R//]+I- (4-7) and [RaaiR/R//+]2 [CdI4]2- (8-11) Scheme 1

admixture of charge transfer transitions of π(imidazolium)/π(iodide)/π(aryl)→π*(azo)/π*(aryl) transition and has been supported by DFT computation (vide infra) of optimized geometry of the compounds. The 1H NMR spectra of 1, 3-dialkyl2-(arylazo)imidazolium iodide, [RaaiR/2]+I- (4-6) were recorded in CDCl3. The atom numbering pattern is shown in Scheme 1. The proton signals are assigned on comparing with RaaiR/ 16, spin-spin interaction and effect of substitution. 1,3-Dialkyl-2-(arylazo)imidazolium iodides show a single resonance for two R/; 1,3-Me2 of (4) shows a sharp singlet at 4.25 ppm; 1,3-Et2 of [RaaiEt2]+I- (5) exhibits 1-(CH2)-3-(CH2)as quartet at ~4.85 ppm and methyl (-Me) shows triplet splitting pattern at ~1.5 ppm. Asymmetric dialkyl

azoimidazolium salts, [Raai (Me)(CH2Ph)]+I- (7) and [Raai(Me)(CH2Ph)+]2[CdI4]2- show a sharp singlet at 5.70-5.80 ppm that refers to N-CH2(Ph) signal. Imidazole protons (4-, 5-H) appear as broad single resonance at 7.0-7.2 ppm. The broadening may be due to rapid proton exchange between two positions. The aryl protons (7-H to 11-H) are perturbed by 9-R substituent and 9-Me group shifts the proton signal upfield due to +I effect. Molecular structure of [MeaaiMe2]+ I- (4b)

The molecular structure of [MeaaiMe2]+ I- (4b) is shown in Fig. 1 and bond parameters are listed in Table 2. The p-tolyl and imidazole rings are virtually coplanar as indicated by the torsion angle, C(1)-N(3)N(4)-C(6) of 178.64(12)° with the dihedral angle between two planes being 8.80(8)°. The N(3)-N(4)

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Fig. 1 – ORTEP diagram of [MeaaiMe2+] I- (4b) with 30 % atomic probability. Table 2 – Selected bond lengths and bond angles of [MeaaiMe2+]I− (4b) and [MeaaiMe(CH2Ph)+]2 [CdI42−] (11b) Bond lengths (Å)

Bond angles (°)

[MeaaiMe2+]I− (4b) N(2)-C(5) N(1)-C(3) N(1)-C(4) N(3)-N(4) N(4)-C(6)

1.462(5) 1.361(4) 1.471(4) 1.237(4) 1.439(4)

C(1)-N(1) C(4) C(1)-N(2)-C(5) N(4)-N(3)-C(1) N(3)-N(4)-C(6)

128.1(3) 125.7(3) 113.1(3) 112.3(3)

[MeaaiMe(CH2Ph)+]2 [CdI42−] (11b) Cd – I(1) Cd – I(2) Cd – I(3) Cd – I(4) N(3) – N(4) N(7) – N(8) N(3) – C(1) N(4) – C(5)

2.7672(9) 2.7507(9) 2.7975(10) 2.7851(10) 1.215(8) 1.231(8) 1.469(10) 1.462(11)

I(2) – Cd – I(1) 109.79(3) I(2) – Cd – I(4) 114.17(3) I(1) – Cd – I(4) 102.85(3) I(2) – Cd – I(3) 102.03(3) I(1) – Cd – I(3) 109.28(3) I(4) – Cd – I(3) 118.69(3) N(3)-N(4)-C(5) 111.2(10) N(4)-N(3)-C(1) 110.4(10) N(7)-N(8)-C(23) 110.5(9) N(8)-N(7)-C(19) 114.8(10) N(2)-C(12) C(13) 114.9(7) N(5)-C(30)-C(31) 112.8(8)

bond length is 1.237(4) Å, which is shorter than that earlier reported23-25 for 1-methyl-2-(phenylazo) imidazolium perchlorate (1.252(2) Å) and 1-ethyl-2(naphthyl-α-azo)imidazolium hexafluorophosphate (1.267(3) Å). The N(azo)-C(imidazole) bond, N(3)-C(1) (1.398(4) Å), is shorter than the N(azo)C(phenyl) bond N(4)-C(6) (1.439(4) Å), which indicates a stronger bonding between azo and imidazole group than between azo and phenyl group. Imidazolium ion is formed by N(1) and N(2) methylation of imidazole. Presence of I− balances the

Fig. 2 – The 1D supramolecular chain formed by π…π interactions along crystallographic a-axis of (4b).

charge of the species. The hydrogen bonding (D–H---I, D–H---N(azo) and π---π interactions build up the 1D-chain (Fig. 2). I− forms three hydrogen bonds with three MeaaiMe2+ in a triangular arrangement where C(11)–H(11)---I(1): H(11)---I(1), 3.114 Å; C(11)----I(1), 4.031 Å; ∠C(11)–H(11)---I(1), 169.16°. C(5)–H(5B)---I(1): H(5B)---I(1), 3.125 Å; C(5)----I(1), 4.042 Å; ∠C(5)–H(5B)---I(1), 160.25°. C(3)–H(3)---I(1): H(3)---I(1), 2.966 Å; C(3)----I(1), 3.875 Å and ∠C(3)–H(3)---I(1), 165.91°. The presence of intermolecular hydrogen bond between azo-N of one molecular ion with C–H of imidazole-N-CH3 of neighboring ion increases the contact and enhances the strength of interaction. The hydrogen bond exists as N(3)---H(4B)–C(4), N(3)---H(4B) (2.682 Å); N(3)---C(4) (3.534 Å) and ∠N(3)–H(4B)–C(4) (148.22°). The packing view shows that the compound forms 1D chain along crystallographic a-axis through π----π interactions (Fig. 3) between phenyl and imidazolyl rings. The Cg(1), C(1)-N(1)-C(3)-C(2)-N(2) (imidazolyl ring) interacts with Cg(2) C(6)-C(7)-C(8)C(9)-C(10)-C(11) (phenyl ring) at -1-x, 2-y, -z. The distance between the centroids is 3.7254(19) Å and the dihedral angle is 4.47(16)° Similarly, the Cg(1) interacts with another Cg(2′) (neighbouring molecule) at -x, 2-y, -z symmetry. The distances between the centroids is 3.8085(19) Å and the dihedral angle is 4.47(16)°. In this way, 1D chains are


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Fig. 3 – The 1D supramolecular chains packed along crystallographic b-axis to form supramolecular channel where guest I− gets stabilized by van der Waals interactions in (4b).

formed and packed to form channel along a-axis where guest iodide ions are stabilized (Fig. 3) by weak van der Waals interactions. Azoimidazolium tetraiodocadmium(II)

The 1,3-di-alkyl-2-(arylazo)imidazolium iodides (symmetrical: RaaiR/2+I-, R/ = Me, Me (4); Et, Et (5); CH2Ph, CH2Ph (6) and R = H (a), Me (b); unsymmetrical: RaaiMe(CH2Ph)+I- (7)) were used to synthesize [RaaiR/2+]2[CdI4]2- (8-11). The molar conductance measurements show that cadmium complexes are 1:2 electrolyte. Moderately intense stretching at 1595-1600 and 1435-1445 cm-1 are due to ν(C=N) and ν(N=N), respectively for the complexes. Other stretching frequencies are shifted to lower frequency in the iodometallates (8-11) as compared to the free ligand values. The solution spectra of (8-11) were recorded in acetonitrile in the wavelength range 230-600 nm. The compounds show transitions at 235-245, 365-375 and 450-460 nm. On comparing with free ligand spectra16 and that of [RaaiR/2]+I-, it may be concluded that the bands are associated with intramolecular charge transfer transitions (π–π*), π(Imz/p-Tolyl/Iodine) → π* (azo/Imz) transitions 25. The 1H NMR spectra of [RaaiR/2+]2 [CdI4]2− (8-11) recorded in CDCl3 reveal that the signals in the spectra of the complexes are comparable with those of [RaaiR/2+]I− Aryl signals shift downfield on Me-substitution on the aryl ring. This is due to the electron donating effect of the Me-group.

Fig. 4 – ORTEP diagram of [MeaaiMe(CH2Ph)]+2 CdI42- (11b). Molecular structure of [MeaaiMe(CH2Ph)+]2 [CdI4]2− (11b)

The molecular unit of (11b) is shown in Fig. 4 and bond parameters are listed in Table 2. The structure is constituted from [CdI4]2− and two units of [MeaaiMe(CH2Ph)]+. Anionic moieties [CdI4]2− are intercalated between the layers of organic cations, where Cd is present at the centre of a distorted tetrahedron constituted by Cd and four I atoms. Bond lengths are in the range of 2.7507(9) — 2.7975(10) Å. In [MeaaiMe(CH2Ph)]+, the p-tolylazo and imidazolyl rings are virtually coplanar as indicated by the torsion angle, C(1)-N(3)-N(4)-C(5) (179.0(6)°) and C(19)–N(7)–N(8)–C(23) (178.7(2)°) and the dihedral angle of two planes 7.6(4)° and 6.8(5)°. Nonplanarity of benzylic group (C(13) to C(18) and C(31) to C(36)) with imidazole ring is due to –CH2 group and makes an angle of 68.1(5)° or 67.1(6)° with the imidazole unit. The N(3)-N(4) bond length is 1.215(8) Å and that of N(7)–N(8) is 1.231(8) Å, which are lower than in the free ligand (1.237(4) Å) for 1-methyl-2-(phenylazo)imidazolium iodide (4b) (Fig. 1 and Table 2). There are three types of aromatic rings, viz., (i) imidazolyl (Imz: Cg(1), N(1)-C(1)N(2)-C(3)-C(2)) and Cg(2), N(5)-C(19)-N(6)-C(21)C(20)), (ii) p-tolyl (p-Tol: Cg(3), C(5)-C(6)-C(7)C(8)-C(9)-C(10) and Cg(5), C(23)-C(24)-C(25)C(26)-C(27)-C(28)) and (iii) benzyl (Bz: Cg(4), C(13)-C(14)-C(15)-C(16)-C(17)-C(18) and Cg(6), C(31)-C(32)-C(33)-C(34)-C(35)-C(36)) and a total of six rings in two [MeaaiMe(CH2Ph)]+ units. These rings are connected through π---π interactions

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between Imz---p-Tol, Imz---Bz and p-Tol---Bz groups with neighbouring cations. The Cg(1) is embedded between two Cg(3) rings of neighbouring molecular units, i. e., Cg(1)---Cg(3), 3.743 Å (symmetry: x, y, z

Fig. 5 – The 3D supramolecular structure formed by π…π interactions in [MeaaiMe(CH2Ph) +] 2 [CdI4 ]2− (11b).

Fig. 6 – [CdI4]2- units placed within the supramolecular channel of [MeaaiMe(CH2Ph) +] in (11b).

Fig. 7 - Surface plots of some selected MOs of [MeaaiMe2]+I- (4b) and [MeaaiMe(CH2Ph)+]2[CdI4]2- (11b).


of Cg(1) and –x, 1-y, 1-z of Cg(3)) and 3.543 Å (symmetry of Cg(3) is x, 3/2-y, -1/2+z). The Cg(2) (symmetry, x, y, z) is interconnected with Cg(4) (symmetry, x, y, z) at 3.573 Å and Cg(5) (symmetry, 1-x, 1-y, 1-z) at 3.562 Å. The Cg(5) is again significantly bonded with Cg(6) (symmetry, x, ½-y, ½+z) at 3.694 Å. All these π-interactions between aromatic rings form 1D channel along crystallographic a-axis followed by the construction of 3D structure (Fig. 5). The [CdI4]2- units get stabilized within the supramolecular channel through van der Waals interactions (Fig. 6). DFT and TD-DFT calculations

DFT calculations were carried out to establish the electronic structure, spectral transitions, and redox properties of the studied compounds. The calculated structures correlate well with the results of their X-ray analysis. The theoretical N=N, C=N, C=C and single bonds C–C, N–N, C–N, Cd–I are slightly longer (0.01 – 0.05 Å) than the experimental data. The DFT calculations have been done using optimized geometry of (4b) and (11b). The surface plots of the MOs are given in Fig. 7. The composition of MOs of [MeaaiMe2+]I- (4b) shows participation of imidazolyl (Imz), p-tolyl (p-Tol), azo (-N=N-) and iodide groups. The HOMO is a mixture of Imz (5 %), p-Tol (48 %), azo (5 %) and I (42%) with HOMO-1 having 33 % I and 67 % p-tolyl function. The lower energy occupied MOs (HOMO-2, HOMO-3, etc.) have 70-90 % contribution from imidazolyl group. In [MeaaiMe(CH2Ph)+]2[CdI4]2- (11b), the occupied MOs (HOMO, HOMO-1, HOMO-2 etc.) are mainly constituted by orbitals of iodine (>80 %) and the unoccupied MOs are made up of functions from organic cation (MeaaiMe(CH2Ph)+) (>90 %). The organic cation consists of azo, p-tolyl and imidazolium motifs. The LUMO and LUMO+1 carry these three parts in significant amounts. LUMO carries 38 % azo, 18 % p-tolyl and 35 % imidazolium ion and the LUMO+1 has 40 % azo, 21 % p-tolyl and 29 % imidazolium ion while the higher energy unoccupied MOs (LUMO+2, LUMO+3 etc) are dominated by only imidazolium ion function (>95 %). The solution electronic spectra are explained on the basis of the composition of occupied and unoccupied MOs and their energies. The [MeaaiMe2+]I- (4b) and [MeaaiMe(CH2Ph)+]2 [CdI4]2- (11b) exhibit intense transitions at 350 – 380 nm and 250 – 300 nm

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respectively and a weak band at 400 – 420 nm. In (5a), the transition at 400 – 410 nm may be assigned to HOMO→ LUMO, LUMO+1 which is due to π(I/Ar) → π*(azo), π*(Imz) and π*(p-Tol)40,41. The band at 300 – 350 nm is due to n/π(I/Imz) → π*(N=N), while the 250 – 200 nm transitions is assigned to π(Imz) → π*(p-Tol) /π*(imz). In (11b), the HOMO-n/HOMO → LUMO/LUMO+1 may be observed at >400 nm, which are assigned to I (iodine) → π*(azo) and the higher energy transitions, HOMO-n/HOMO → LUMO+2, LUMO+3, etc., to I (iodine) → π*(imidazole). Thus, two classes of transitions are possible in [RaaiR/R// +]2 [CdI4]2−, both of which are indeed observed. Acknowledgement Financial support from Council of Scientific and Industrial Research (CSIR), New Delhi, India, is gratefully acknowledged. Thanks are due to Prof T–H Lu and Mr J Chang, Department of Physics, National Tsing Hua University, Taiwan, ROC, for initial crystallographic work. Supplementary Data Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, under CCDC Nos 827648 for (4b) and 654252 for (11b). Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2 1FZ, UK, Email: [email protected] or http://www.ccdc.cam.ac.uk. Other supplementary data, i. e., DFT calculations and characterization data are available in electronic form at http://www.niscair.res.in/jinfo/ijca/IJCA 51A(07) 917-926_Suppl Data.pdf. References 1 2 3 4 5 6 7 8

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