Polyhedron 31 (2012) 506–514

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Polyhedron

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The synthesis, structure and photochromism of mercury(II)-iodide complexesof 1-CnH2n+1-2-(arylazo)imidazoles (n = 4, 6, 8)

Debashis Mallick 1, Avijit Nandi, Shilpi Datta, Kamal Krishna Sarker 2, Tapan Kumar Mondal,Chittaranjan Sinha ⇑Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 August 2011Accepted 4 October 2011Available online 12 October 2011

Keywords:ArylazoimidazoleMercury-iodide complexesSpectral studyPhotochromism

0277-5387/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.poly.2011.10.002

⇑ Corresponding author. Fax: +91 33 2414 6584.E-mail address: c_r_sinha@yahoo.com (C. Sinha).

1 Address: Department of Chemistry, MrinaliniKolkata 700051, India.

2 Present address: Department of Chemistry, MaBarrackpore, Monirampore, Kolkata 700120, India.

The reaction of HgI2 with 1-CnH2n+1-2-(arylazo)imidazole (Raai-CnH2n+1 where n = 4, 6, 8) has isolatediodide bridging dimeric complexes, [Hg(RaaiCnH2n+1)(l-I)(I)]2. The structures of the ligand and the com-plexes have been established by spectral (UV–Vis, IR, 1H NMR) data. One of these complexes [Hg(1-hexyl-2-(p-tolylazo)imidazole)(l-I)(I)]2 has been structurally confirmed by single crystal X-ray diffractionstudy. The ligand, Raai-CnH2n+1 exists at ambient condition in trans geometry about azo (–N@N–) group;the UV light irradiation in MeOH solution shows E-to-Z isomerisation. The reverse transformation, Z-to-E,is very slow with visible light irradiation while isomerises rapidly on heating. The coordinated ligand,Raai-CnH2n+1 in the complexes exhibit similar behaviour in DMF solution. Quantum yields (/E?Z) of E-to-Z isomerisation are higher for free ligands than that of their metal complexes. The Z-to-E isomerisationis a thermally induced process. The activation energy (Ea) is calculated by controlled temperatureexperiment.

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1. Introduction

Organic–inorganic hybrid materials are of current researchinterest [1–7]. The properties of the materials can be varied bychanging the ligand types, presence of substituents, different car-bocyclic and heterocyclic rings, and also by using different metalions. Towards the study of properties of the complexes we havebeen interested to examine the photochromism of azoheterocyclesand the effect on metal coordination thereof. Photochromism is areversible photo-induced transformation between two molecularstates whose absorption spectra differ significantly [8–11].Azo-conjugated metal complexes exhibit unique properties uponlight irradiation in the area of photon-mode high-density informa-tion storage photoswitching devices [12–14]. Arylazoimidazolesconstitute an interesting class of heterocyclic azo compounds,since imidazole is a ubiquitous and essential group in chemistryand biology, especially as a metal coordinating site. The azohetero-cycles have been extensively used as ligands for metal ions by us[15–17] and others [18,19]. However, very few reports concerningthe photochromic property (Scheme 1) of arylazoimidazole dyes

ll rights reserved.

Datta Mahavidyapith, Birati,

hadevananda Mahavidyalaya,

are found in the literature [19–25]. The photochromism of1-alkyl-2-(arylazo)imidazole [20,21] and their Cu(I) [22], Cd(II)[23], Hg(II) [24] and Pd(II) [25] complexes are so far reported inthe literature. An important advantage of imidazole is its chemicaland biological ubiquity and alkylation of N(1) and/or N(3) centreswith number of carbon centres as per one’s requirement. Doublealkylation has synthesised imidazolium salts those are potent ionicliquids [26] and at present are using as green solvents in chemicalreactions. Presence of long chain alkyl group is useful in the disper-sion of organic–inorganic hybrid material in organic media, forma-tion of thin film, soft matter, etc. In this work we use, first time,long chain alkyl group consisting of four (n-butyl), six (n-hexyl)and eight (n-octyl) carbons in the alkylation of 2-(arylazo)imida-zoles to synthesise 1-CnH2n+1-2-(arylazo)imidazole (Raai-CnH2n+1

where n = 4, 6, 8). The ligands so-obtained are used to synthesisehitherto unknown mercury(II) complexes. Both ligands and com-plexes are characterised by spectroscopic studies and in one caseby single crystal X-ray diffraction study. The photochromic activityof the ligands and complexes are examined.

2. Results and discussion

2.1. Synthesis and formulation of compounds

2-(Arylazo)imidazoles were synthesised by the diazotisation ofarylamine in NaNO2/HCl and followed by coupling with imidazolein aqueous sodium carbonate solution (pH �7). The [Hg(Haai-

N N

NN

R

R

N N

NN

RR

trans- (E) cis- (Z)

Scheme 1. Isomerisation of 1-alkyl-2-(arylazo)imidazole.

D. Mallick et al. / Polyhedron 31 (2012) 506–514 507

C4H9)(l-I)(I)]2 (4a), [Hg(Meaai-C4H9)(l-I)(I)]2 (4b) [Hg(Haai-C6H13)(l-I)(I)]2 (5a), [Hg(Meaai-C6H13)(l-I)(I)]2 (5b), [Hg(Haai-C8H17)(l-I)(I)]2 (6a), [Hg(Meaai-C8H17)(l-I)(I)]2 (6b) alkylationwas carried out by adding alkyl halide in dry THF solution to thecorresponding 2-(arylazo)imidazole in presence of NaH (Scheme2). The n-butyl (C4H9–), n-hexyl (C6H13–) and n-octyl (C8H17–)are appended at N(1) of imidazole unit of 2-(arylazo)imidazole tosynthesise present series of molecules, Raai-CnH2n+1 (n = 4, 6, 8).The ligands are unsymmetrical N,N0-bidentate chelator where Nand N0 refer to N(imidazole) and N(azo) donor centres, respec-tively. The reaction between Raai-R0 and HgI2 in 1:1 mole ratio inMeOH and ethyleneglycol monomethyl ether (EGME) mixturehas isolated compounds of chemical formula [Hg(Raai-R0)(l-I)I]2.

µ

µ

µ

µ

[Hg(Ha

Scheme 2. Preparation of ligands

2.2. Molecular structure of [Hg(Meaai-C6H13)(l-I)(I)]2 (5b)

The crystals of [Hg(Meaai-C6H13)(l-I)(I)]2 (5b) are grown byslow evaporation of the reaction mixture in MeOH–DMF at ambi-ent condition for a week. The molecular structure is shown inFig. 1. The bond parameters are listed in Table 1. Each discretemolecular unit consists of dinuclear iodide bridging Hg2I2 frag-ment. Meaai-C6H13 acts as N,N0-donor end capping agent whereN and N0 refer to N(imidazole) and N(azo) donor centres and anon-bridged-I atom lies in a semi-axial position. The Hg2I2 bridgehas an asymmetric tetra-atomic puckered rhombohedral geome-try: Hg(1)–I(2), 2.6818(6) Å; Hg(1)–I(2a), 3.421(6) Å (symmetrycode (a) = �x, y, 1/2 � z). Atoms Hg(1), N(4), N(3), C(8), N(1) consti-tute the chelate plane with a maximum deviation <0.04 Å. The pen-dant aryl ring makes a small dihedral of 4.7� with the chelatedazoimidazole ring. The bond angles I(1)–Hg(1)–I(2), 135.36(3)�and I(1)–Hg–I(2a), 97.85(3)� also support this distorted geometry.The structural demand of long chain 1-C6H13– group may be thereason for larger deviation of both bridging and non-bridging met-ric parameters compared to previously reported data [24]. Interest-ingly two 1-C6H13– groups in Hg2I2 dimmer stereochemicallydisposed to make 34.33� torsion angle. The small chelate angle(64.52(3)�) may be one of the reasons for geometrical distortion.The Hg–N(imidazole) (Hg(1)–N(1/1a), 2.306(8) Å) is shorter thanHg–N(azo) (Hg(1)–N(4/4a), 2.717(2) Å, which reflects strongerinteraction between Hg(II) and N(imidazole). Although Hg–N(azo) bond length is very long but it is less than the sum of van

µ

µ

and Hg(II)-iodide complexes.

Fig. 1. The crystal structure of [Hg(Meaai-C6H13)(l-I)(I)]2 (5b).

Table 1Selected bond distances (Å) and angles (�) for [Hg(Meaai-C6H13)(l-I)(I)]2 (5b).

Bond distances (Å) Bond angles (�)

Hg(1)–I(1) 2.6476(9) I(1)–Hg(1)–I(2) 135.36(3)Hg(1)–I(2) 2.6818(9) I(1)–Hg(1)–I(2a)a 97.85(3)Hg(1)–I(2a)a 3.421(6) N(1)–Hg(1)–I(1) 114.9(2)Hg(1)–N(1/1a) 2.306(8) N(4)–Hg(1)–I(2) 93.98(3)Hg(1)–N(4) 2.717(2) N(4)–Hg(1)–I(2)b 148.74(12)N(1)–C(8) 1.352(12) I(2a)a–Hg(1)–I(2)b 92.23(10)N(2)–C(8) 1.363(13) Hg(1)–I(2a)Hg(1a) 83.86(1)N(4)–N(3) 1.263(11) N(1)–Hg(1)–N(4) 64.52(3)

N(4)–Hg(1)–I(1) 98.84(4)N(1)–Hg(1)–I(2) 109.3(2)

a Symmetry: �x, �y, �z.b Symmetry: 1 � x, 2 � y, 1 � z.

508 D. Mallick et al. / Polyhedron 31 (2012) 506–514

der Waals radii of Hg(II) (1.55 Å) and N(sp2) (1.53 Å). This impliessignificant bonding interaction between these components. Thebond lengths of Hg–N and Hg–I are slightly longer than that of pre-viously reported results [24]. The stronger coordination of imidaz-ole-N to Hg(II) compared to azo-N has significant biochemicalimplication and explains strong toxicity of Hg(II) [27]. Because oflong Hg(II)–N(azo) distance, the molecule becomes useful forphoto-activation via cleavage of this bond followed by rotation tointroduce photoisomerisation (vide infra). The N@N distance is1.263(11) Å and is comparable to previously reported result(1.262(6) Å) [20,21]. The Hg–I(bridge) (Hg(1)–I(2), 2.6818(9) Å) islonger than Hg–I(non-bridge) (Hg(1)–I(1), 2.6476(9) Å). The Hg–I(1/1a) lies at semi-axial position to their respective plane. The che-late ring around Hg is twisted in a manner so that the Hg–N(azo)inclines to axial Hg–I bond, N(4)–Hg(1)–I(1), 99.03(4)�. Two aro-matic rings (p-tolyl and imidazole) of each coordinated Meaai-

C6H13 show p–p interactions with aromatic groups of neighbour-ing molecule leading to the formation of 1D chain (Fig. 2). Theinteraction units are p-tolyl (Cg(4) of molecule A)–imidazole(Cg(3) of molecule B), 3.764 Å (symmetry, 1/2 � x, 1/2 � y, �z)and p-tolyl (Cg(4) of molecule A)–p-tolyl (Cg(4) of molecule B),3.865 Å (symmetry, �x, y, 1/2 � z). A weak hydrogen bond is alsoobserved at C(11)–H(11B)� � �N(3) where distance of H(11B)� � �N(3)and C(11)� � �N(3) are 2.50 and 2.92 Å, respectively, and angle is106� (Fig. 2). Although iodine is not sufficiently electronegativeto form hydrogen bonded structure but coordinated semi-axial io-dine, Hg(1)–I(1), has ability to form hydrogen bond with stereo-chemically close hydrogen of –C(13)H2– of neighbouringdimmer: C(13)–H(13b)� � �I(1), H(13b)� � �I(1), 3.60 Å, C(13)� � �I(1),4.084 Å and \C(13)–H(13b)� � �I(1), 159.51� which makes dim-mer–dimmer interaction to constitute 1D chain. The disposal ofdimmer lies in a manner such that 1-C6H13 groups of two dimmersare interpenetrated to constitute a tetrameric motif that serves ascrystal building motif (Fig. 3).

2.3. Spectral studies

The bands in the FT-IR spectra of the ligands (1–3) and the com-plexes (4–6) are assigned based on comparing with reported work[22,27]. The point of interest is the band due to the azo (–N@N–)and imine (–C@N–) groups in compounds. The complexes, 4–6,show moderately intense stretching at 1580–1695 and 1370–1385 cm�1 for m(C@N) and m(N@N), respectively, and free ligand,1–3, values appear at higher frequency, 1620–1625 and 1400–1410 cm�1, respectively. In the complexes, the frequencies areshifted to lower value than free ligand which support the coordina-tion of azo-N and imine-N to Hg(II).

Fig. 2. 1D chain structure of [Hg(Meaai-C6H13)(l-I)(I)]2 (5b) formed by C–H� � �N/I hydrogen bonding and p–p interactions.

Fig. 3. View of interdigitated tetramer that serves as motif to pack crystal in 2D geometry.

D. Mallick et al. / Polyhedron 31 (2012) 506–514 509

Fig. 4. UV–Vis spectra of (a) Meaai-C6H13 (2b) in MeOH and (b) [Hg(Meaai-C6H13)(l-I)(I)]2 (5b) in DMF solution.

510 D. Mallick et al. / Polyhedron 31 (2012) 506–514

The absorption spectra were recorded in MeOH solution for theligands and in DMF solution (because of sparing solubility of thecomplexes in MeOH) for the complexes, in the wavelength range200–700 nm (Fig. 4). The spectra of the ligands show absorptionband at 340–380 nm with a molar absorption coefficient in the or-der of 103 M�1 cm2 and a weak band at 450–455 nm. The intenseabsorption band is assigned to p–p⁄ transitions, while the tail cor-responds to n-p⁄ transition. The p–p⁄ band exhibits bathochromicshifts by �30 nm compared with azobenzene [28], while the n-p⁄

band shows little shift. As a consequence, the energy separationbetween the p–p⁄ and n-p⁄ transitions in arylazoimidazoles is nar-rower than that of azobenzene.

The characteristics common to the complexes are a structuredabsorption band around 360–390 nm with a molar absorptioncoefficient on the order of 103 M�1 cm2 and a weak band at 450–460 nm (e � 103 M�1 cm2). From the analogy with the absorptionspectra of ligands (1–3) it is likely that the large absorption bandaround 360–380 nm corresponds to p–p⁄ transitions, while the tailcorresponds to n-p⁄ transition. The transitions are shifted to longerwavelength by av. 8 nm. This may due to the overlapping of MLCTtransition from Hg(II) ? p⁄(azoimine) (Fig. 4). The p–p⁄ absorptionpeaks (kmax) for derivatives of (2-phenylazo)imidazole are within arange of 365–385 nm, which is between the p–p⁄ absorption bandsof azobenzene (313 nm) and 4-N,N-dimethylaminoazobenzene(390 nm) [29,30].

The 1H NMR spectra of ligands (1–3) are recorded in CDCl3 andthose of complexes [Hg(Raai-CnH2n+1)(l-I)(I)]2 (4–6) are recorded

Table 21H NMR spectral data in DMSO-d6 at room temperature.

Compounds 4-Hs 5-Hs 7,11-Hd 8,10-

Haai-C4H9 (1a) 7.29 7.16 7.86 7.40Meaai-C4H9 (1b) 7.26 7.13 7.79 7.35Haai-C6H13 (2a) 7.32 7.17 7.97 (8.2) 7.41Meaai-C6H13 (2b) 7.31 7.15 7.89 (8.6) 7.29Haai-C8H17 (3a) 7.33 7.18 7.99 (8.0) 7.42Haai-C8H17 (3b) 7.32 7.15 7.95 (7.5) 7.30[Hg(Haai-C4H9)(l-I)(I)]2 (4a) 7.75 7.33 7.95 (8.2) 7.55[Hg(Meaai-C4H9)(l-I)I]2 (4b) 7.73 7.30 7.82 (7.6) 7.40[Hg(Haai-C6H13)(l-I)(I)]2 (5a) 7.78 7.35 7.98 (8.2) 7.58[Hg(Meaai-C6H13)(l-I)(I)]2 (5b) 7.75 7.32 7.86 (7.6) 7.42[Hg(Haai-C8H17)(l-I)(I)]2 (6a) 7.79 7.36 7.99 (8.5) 7.61Hg(Meaai-C8H17)(l-I)(I)]2 (6b) 7.77 7.34 7.90 (8.7) 7.60

s, singlet; d, doublet; t, triplet; bs, broad singlet; L, last –CH3 of respective N-alkyl grou* All Hs of middle ‘ACH2A’ of CnH2n+1.

in DMSO-d6 (Table 2) because of solubility problem in former sol-vent. The atom numbering pattern is shown in Scheme 1. Thealkylation of imidazole is supported by the disappearance ofd(N–H) at �10.30 ppm and the appearance of N(1)-alkyl signal at0.85–4.40 ppm for 1, 2 and, 3; –N–CH2–(CH2)n–CH3 shows a tripletfor –N–CH2– at 4.39–4.46 ppm, a triplet at 0.85–89 ppm for –CH3

group and a multiplet for –(CH2)n– at 1.30–1.90 ppm. Imidazole4- and 5-H appears as broad singlet at 7.26–7.33 and 7.13–7.18 ppm, respectively. Broadening may be due to rapid proton ex-change between these imidazole protons The aryl protons (7-H to11-H) are upfield shifted on going from phenylazo (a) to p-tolylazo(b) which may be due to +I effect of –Me group. Data (Table 2) re-veal that the signals in the spectra of the complexes are shifted todownfield side relative to free ligand values at 6.5–8.5 ppm. Thesignals at 1.00–5.00 ppm hardly exhibit any change. Importantfeatures of the spectra are the shifting of imidazole protons 4-Hand 5-H to lower d-values, in general, relative to perturbation ofaryl protons (7-H–11-H). Imidazole protons suffer downfield shift-ing by 0.2–0.5 ppm compared to the free ligand signal position.This supports the strong preference of binding of imidazole-N toHg(II). Aryl signals shift to the lower field side on Me-substitutionto the aryl ring. This is due to electron donating effect of the Me-group. This conclusion is also supported by single crystal X-raystructure of one of the complexes (Fig. 1).

2.4. Photochromism

Upon UV-light irradiation at fixed time interval at kmax to aMeOH solution of the ligands shows the changes of absorptionspectrum that is corresponding to the structural change of the li-gand from trans-Raai-CnH2n+1 (E-isomer) to cis-Raai-CnH2n+1 (Z-iso-mer) (Fig. 5). The intense peak at kmax decreases, which isaccompanied by a slight increase at the tail portion of the spectrumuntil a stationary state is reached. Subsequent irradiation at thenewly appeared longer wavelength peak reverses the course ofthe reaction and the original spectrum is recovered up to a point,which is another photostationary state under irradiation at thelonger wavelength peak. The quantum yields of the E-to-Z photo-isomerisation are determined using those of azobenzene [24,28]as a standard and the results are tabulated in Table 3.

The E-to-Z isomerisation of Hg(II) complexes of Raai-CnH2n+1

(n = 4, 6, 8) in presence of I� are carried out in DMF solution be-cause of sparing solubility in MeOH. It is observed that upon irra-diation with UV light E-to-Z change proceeds and the Z molar ratiois reached to �75%. The absorption spectra of the coordinated Raai-CnH2n+1 in E-form have changed with isosbestic points upon excita-tion (Fig. 6) into the Z-isomer. The ligands and the complexes showlittle sign of degradation upon repeated irradiation at least up to 15cycles in each case. The quantum yields were measured for the

Hd 9-Rs 12-CH2t LCH3

t LCH3—ðCH2Þ*– CH2�bs

7.40 4.40 0.87 1.85–1.312.40 4.39 0.85 1.84–1.307.41 4.45 (6.9) 0.88 (5.8) 1.86–1.302.42 4.41 (7.0) 0.86 (6.1) 1.87–1.317.42 4.46 (7.1) 0.89 (6.5) 1.89–1.332.43 4.43 (6.4) 0.87 (6.2) 1.88–1.327.55 4.40 (6.45) 0.81 (6.0) 1.82–1.232.38 4.39 (7.0) 0.79 (6.4) 1.80–1.227.58 4.42 (6.45) 0.82 (6.0) 1.82–1.232.40 4.45 (7.0) 0.80 (6.4) 1.82–1.237.60 4.43 (7.4) 0.84 (6.1) 1.83–1.242.42 4.46 (7.5) 0.85 (7.0) 1.85–1.29

p.

300 350 400 450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

1.2

300 350 400 450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

1.2

Isobestic point

323 nm

433 nm

Trans-isomer

cis-isomer

Abs

orba

nce

Wavelength(nm)

323 nm

433 nm

Isobestic pointIncreasing

Decreasing

Abs

orba

nce

Wavelength(nm)

Fig. 5. Spectral changes of Meaai-C6H13 (2b) in MeOH upon repeated irradiation at364 nm at 3 min interval at 25 �C.

D. Mallick et al. / Polyhedron 31 (2012) 506–514 511

E-to-Z (/E?Z) photoisomerisation of these ligands in MeOH andthat of the complexes in DMF on irradiation of UV wavelength (Ta-ble 3). The /E?Z values are significantly depend on nature of sub-stituents; the Me substituent at azoaryl group and thesubstituents (CnH2n+1: n = 4, butyl; n = 6, hexyl; n = 8, octyl) atN(1)-position both reduce /E?Z values. The photoisomerisationrate and quantum yields of coordinated ligand are decreased com-pared to free ligands and in general, increase in mass of the mole-cule reduces the rate and quantum yield of isomerisation.

The lowering of /E?Z in the complexes may be due to the pres-ence of coordinated HgI2 that increases molecular weight and se-verely interferes the motion of the –N@N–Ar moiety and photobleaching efficiency of halide [31] may snatch out energy fromp–p⁄ excited state. These may cause very fast deactivation otherthan photochromic route. Both rotor mass and volume are in-creased upon coordination of ligand to HgI2. These two factors havesignificant influence on the isomerisation rate and quantumyields.

The Z-to-E isomerisation of Raai-CnH2n+1 (1–3) and their Hg(II)complexes (4–6) are followed by spectra measurements in MeOHand DMF, respectively, at varied temperatures, 298–313 K. TheEyring plots in the range 298–313 K gave a linear graph fromwhich the activation parameters DS⁄ and DH⁄ are calculated (Table4 and Fig. 7). In the complexes, the Eas are severely reduced which

Table 3Results of photochromism, rate of conversion and quantum yields upon UV light irradiati

Compounds kp;p� (nm) Isobestic point (nm)

1a 363 328, 4371b 362 327, 4302a 363 325, 4352b 364 323, 4333a 364 330, 4273b 365 324, 4354a 367 335, 4394b 366 331, 4405a 367 336, 4385b 368 332, 4366a 368 333, 4386b 369 332, 448

means faster Z-to-E thermal isomerisation of the complexes. Theentropy of activation (DS⁄) are highly negative in the complexesthan that of free ligands. This is also in support of increase in rotorvolume of the complexes.

3. Experimental

3.1. Materials

HgI2 was prepared by adding KI solution to Hg(NO3)2 solution,filtered and washed with profuse amount of water and dried in aCaCl2 desiccator. 2-(Arylazo)imidazoles were synthesised by re-ported procedure [29]. All other chemicals and solvents were re-agent grade as received.

3.2. Physical measurements

Microanalytical data (C, H, N) were collected on Perkin-Elmer2400 CHNS/O elemental analyzer. Spectroscopic data wereobtained using the following instruments: UV–Vis spectra from aPerkin-Elmer Lambda 25 spectrophotometer; IR spectra (KBr disk,4000–400 cm�1) from a Perkin-Elmer RX-1 FT-IR spectrophotome-ter; photo excitation has been carried out using a Perkin-Elmer LS-55 spectrofluorimeter and 1H NMR spectra were recorded from aBruker (AC) 300 MHz FTNMR spectrometer.

3.3. Preparation of compounds

3.3.1. Synthesis of [N(1)-hexyl-2-(p-tolylazo)imidazole], Meaai-C6H13

(2b)To dry THF solution (75 ml) of 2-(p-tolylazo)imidazole (2 g,

10.8 mmol), NaH (50% paraffin) (0.56 g) was added in small portionand stirred at cold condition on ice bath for 0.5 h. 1-Bromohexane(3.57 g, 11.69 mmol) was added slowly through pressure equalis-ing system under stirring condition for a period of 1 h and thenwarm for another 1 h on steam bath. The solution was evaporatedto dryness, extracted with CH2Cl2, washed with NaOH solution(10%, 3� 10 ml) and finally by distilled water (3� 20 ml). TheCH2Cl2 extract was chromatographed over silica gel column (45�1 ml) prepared in benzene. The elution was performed by ben-zene–acetonitrile (10:1 v/v) mixture. On slow evaporation in airorange crystalline product was obtained and then dried overP4O10 under vacuum.

The microanalytical data of the complexes are as follows: Haai-C4H9 (1a), Anal. Calc. for C13H16N4: C, 68.39; H, 7.06; N, 24.54.Found: C, 68.35; H, 7.08; N, 24.50%. FT-IR (KBr disc, cm�1),m(N@N), 1404; m(C@N), 1621 cm�1. UV–Vis spectroscopic data inMeOH (kmax (nm) (10�3 e (dm3 mol�1 cm�1): 363 (13.1), 376(12.4), 448 (2.3). Meaai-C4H9 (1b), Anal. Calc. for C14H18N4: C,69.39; H, 7.49; N, 23.12. Found: C, 69.34; H, 7.47; N, 23.14%. FT-IR

on.

Rate of E ? Z conversion � 109 (s�1) /E?Z conversion � 109

36.01 1.90 ± 0.00535.71 1.87 ± 0.00234.92 1.81 ± 0.00134.09 1.79 ± 0.00333.76 1.69 ± 0.00432.14 1.65 ± 0.00232.69 1.75 ± 0.00232.01 1.71 ± 0.00231.09 1.65 ± 0.00130.11 1.61 ± 0.00328.12 1.57 ± 0.00327.03 1.55 ± 0.001

Table 4Rate and activation parameters for Z(c) ? E(t) thermal isomerisation.

Compounds T (K) Rate of thermal Z ?E conversion � 105 (s�1)

Ea (kJ mol�1) DH⁄ (kJ mol�1) DS⁄ (J mol�1 K�1) DG⁄c (kJ mol�1)

1a 298 3.62 88.20 85.66 �43.27 98.9303 5.10308 10.5313 19.0

1b 298 6.01 88.47 85.93 �37.96 97.53303 9.02308 18.04313 32.01

2a 298 4.20 91.73 89.19 �30.09 98.36303 6.20308 13.2313 23.5

2b 298 6.09 92.11 89.57 �25.12 98.38303 11.21308 19.01313 37.03

3a 298 4.40 93.38 90.85 �23.83 98.13303 7.60308 13.20313 27.30

3b 298 7.11 93.66 91.12 �19.45 97.07303 10.21308 21.01313 42.03

4a 298 24.00 29.65 27.12 �204.24 89.5303 27.33308 36.50313 41.20

4b 298 52.55 28.51 25.97 �220.90 93.46303 58.36308 69.07313 91.87

5a 298 34.55 28.36 25.82 �224.82 94.51303 39.36308 45.07313 60.87

5b 298 55.55 27.55 25.01 �223.53 93.3303 64.36308 72.07313 96.87

6a 298 39.57 25.62 23.80 �232.39 94.01303 49.61308 57.64313 65.21

6b 298 59.15 25.43 22.90 �230.12 93.12303 67.36308 74.52313 98.97

512 D. Mallick et al. / Polyhedron 31 (2012) 506–514

(KBr disc, cm�1), m(N@N), 1402; m(C@N), 1622 cm�1. UV–Vis spec-troscopic data in MeOH (kmax (nm) (10�3 e (dm3 mol�1 cm�1): 362(12.9), 379 (11.9), 447 (1.9). Haai-C6H13 (2a), Anal. Calc. forC15H20N4: C, 70.28; H, 7.81; N, 21.86. Found: C, 70.32; H, 7.81; N,22.00%. FT-IR (KBr disc, cm�1), m(N@N), 1408; m(C@N), 1625 cm�1.UV–Vis spectroscopic data in MeOH (kmax (nm) (10�3 e(dm3 mol�1 cm�1): 363 (13.5), 380 (11.4), 451 (2.1). Meaai-C6H13

(2b), Anal. Calc. for C16H22N4: C, 71.08; H, 8.20; N, 20.72. Found:C, 71.20; H, 8.30; N, 20.78%. FT-IR (KBr disc, cm�1), m(N@N), 1403;m(C@N), 1627 cm�1. UV–Vis spectroscopic data in MeOH (kmax

(nm) (10�3 e (dm3 mol�1 cm�1): 364 (13.9), 379 (12.9), 447 (1.9).Haai-C8H17 (3a), Anal. Calc. for C17H24N4: C, 71.80; H, 8.51; N,19.70. Found: C, 71.76; H, 8.45; N, 19.77%. FT-IR (KBr disc, cm�1),m(N@N), 1405; m(C@N), 1620 cm�1. UV–Vis spectroscopic data inMeOH (km (nm) (10�3 e (dm3 mol�1 cm�1): 360 (12.7), 380 (11.5),451 (2.2). Meaai-C8H17 (3b), Anal. Calc. for C18H26N4: C, 72.44; H,8.78; N, 18.77. Found: C, 72.47; H, 8.73; N, 18.72%. FT-IR (KBr disc,cm�1), m(N@N), 1407; m(C@N), 1615 cm�1. UV–Vis spectroscopicdata in MeOH (kmax (nm) (10�3 e (dm3 mol�1 cm�1): 365 (13.3),380 (12.03), 449 (1.9).

3.3.2. Synthesis of [Hg(Meaai-C6H13)(l-I)(I)]2 (5b)To methanol (25 ml) solution of Meaai-C6H13 (0.025 g,

0.093 mmol), HgI2 (0.040 g, 0.087 mmol) in ethyleneglycol mono-methylether (EGME, 10 ml) was added and refluxed for 2 h. Or-ange-yellow precipitate appeared. The precipitate was collectedby filtration, washed with cold MeOH and dried over CaCl2 in vac-uum. The yield was 0.046 g (72%). Other complexes were preparedunder identical conditions and the yield varied in the range 65–75%.

The microanalytical data of the complexes are as follows:[Hg(Haai-C4H9)(l-I)(I)]2 (4a), Anal. Calc. for C13H16N4HgI2: C,22.85; H, 2.34; N, 8.21. Found: C, 22.89; H, 2.37; N, 8.19%. FT-IR(KBr disc, cm�1), m(N@N), 1372; m(C@N), 1583 cm�1. UV–Vis spec-troscopic data in DMF (kmax (nm) (10�3 e (dm3 mol�1 cm�1): 285(8.0), 366 (11.21), 381 (9.7), 455 (2.4). [Hg(Meaai-C4H9)(l-I)(I)]2

(4b), Anal. Calc. for C14H18N4HgI2: C, 24.11; H, 2.60; N, 8.04. Found:C, 24.16; H, 2.55; N, 8.00%. FT-IR (KBr disc, cm�1), m(N@N), 1374;m(C@N), 1591 cm�1. UV–Vis spectroscopic data in DMF (km (nm)(10�3 e (dm3 mol�1 cm�1): 280 (8.2), 367 (11.1), 383 (9.7), 460(1.9). [Hg(Haai-C6H13)(l-I)(I)]2 (5a), Anal. Calc. for C15H20N4HgI2:

300 350 400 450 500 550 600 650 700 750 8000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

300 350 400 450 500 550 600 650 700 750 8000.000.050.100.150.200.250.300.350.400.450.500.55

Cis-isomer

Trans-isomer

Abs

orba

nce

Wavelength(nm)

332 nm

439nm

Increasing

Decreasing

Abs

orba

nce

Wavelength(nm)

Fig. 6. Spectral changes of [Hg(Meaai-C6H13)(l-I)(I)]2 (5b) in DMF upon repeatedirradiation at 368 nm at 5 min interval at 25 �C.

Table 5Summarised crystallographic data for [Hg(Meaai-C6H13)(l-I)(I)]2 (5b).

Compound 5b

Empirical formula C32H44N8I4Hg2

Formula weight 1449.53T (K) 293(2)Crystal system monoclinicSpace group C2/cCrystal size (mm)3 0.34 � 0.30 � 0.29a (Å) 14.6328(4)b (Å) 19.0566(6)c (Å) 15.0012(5)a (�) 90.00b (�) 91.152(2)c (�) 90.00V (Å3) 4182.3(2)Z 4l (Mo Ka) (mm�1) 10.317h range 1.75–30.68hkl range �20 < h < 20; �20 < k < 27; �21 < l < 20Dcalc (mg m�3) 2.1302Refine parameters 208Total reflections 32102Unique reflections 6360R1

a [I > 2r(I)] 0.0423wR2

b 0.1143Goodness-of-fit (GOF) on F2 0.994

w = 1/[r2(Fo)2 + (0.0482P)2 + (4.8416P)] for 5b; where P ¼ ððF2o þ 2F2

c Þ=3.a R =

P||Fo| � |Fc||/

P|Fo|.

b wR2 ¼ ½PðF2

o � F2c Þ

2=P

wðF2oÞ

2�1=2.

D. Mallick et al. / Polyhedron 31 (2012) 506–514 513

C, 25.35; H, 2.84; N, 7.88. Found: C, 25.42; H, 2.81; N, 7.80%. FT-IR(KBr disc, cm�1), m(N@N), 1373; m(C@N), 1580 cm�1. UV–Vis spec-troscopic data in DMF (km (nm) (10�3 e (dm3 mol�1 cm�1): 290(7.8), 367 (11.4), 383 (9.9), 458 (2.4). [Hg(Meaai-C6H13)(l-I)(I)]2

(5b), Anal. Calc. for C16H22N4HgI2: C, 26.49; H, 3.04; N, 7.73. Found:C, 26.57; H, 3.00; N, 7.6%. FT-IR (KBr disc, cm�1), m(N@N), 1375;m(C@N), 1597 cm�1. UV–Vis spectroscopic data in DMF (km (nm)(10�3 e (dm3 mol�1 cm�1): 288 (7.9), 368 (11.30), 381 (9.8), 454(1.9). [Hg(Haai-C8H17)(l-I)(I)]2 (6a), Anal. Calc. for C17H24N4HgI2:C, 27.62; H, 3.25; N, 7.58. Found: C, 27.73; H, 3.29; N, 7.63%. FT-IR (KBr disc, cm�1), m(N@N), 1374; m(C@N), 1599 cm�1. UV–Visspectroscopic data in DMF (km (nm) (10�3 e (dm3 mol�1 cm�1):287 (7.82), 368 (11.2), 387 (9.9), 457 (2.1). [Hg(Meaai-C8H17)(l-I)(I)]2 (6b), Anal. Calc. for C18H26N4HgI2: C, 28.70; H, 3.49; N,7.44. Found: C, 28.69; H, 3.49; N, 7.41%. FT-IR (KBr disc, cm�1),m(N@N), 1379; m(C@N), 1590 cm�1. UV–Vis spectroscopic data inDMF (km (nm) (10�3 e (dm3 mol�1 cm�1): 288 (8.1), 369 (10.8),386 (9.7), 455 (2.3).

Fig. 7. The Eyring plots of rate constants of Z-to-E isomerisation of (a) Meaai-C6H13

(2b) and (b) [Hg(Meaai-C6H13)(l-I)(I)]2 (5b) at 298–313 K.

3.4. X-ray diffraction study

The crystallographic data are shown in Table 5. Suitable singlecrystal of 5b was mounted on a Siemens CCD diffractometerequipped with graphite monochromated Mo Ka (k = 0.71073 Å)radiation. The unit cell parameters and crystal-orientation matri-ces were determined for two complexes by least squares refine-ments of all reflections. The intensity data were corrected forLorentz and polarisation effects and an empirical absorption cor-rection were also employed using the SAINT program. Data were col-lected applying the condition I > 2r(I). All these structures weresolved by direct methods and followed by successive Fourier anddifference Fourier syntheses. Full matrix least squares refinementson F2 were carried out using SADBAS [32] with anisotropic displace-ment parameters for all non-hydrogen atoms. Hydrogen atomswere constrained to ride on the respective carbon or nitrogenatoms with an isotropic displacement parameters equal to 1.2times the equivalent isotropic displacement of their parent atomin all cases. Complex neutral atom scattering factors were usedthroughout for all cases. All calculations were carried out usingSHELXS97 [33], SHELXL97 [33], PLATON99 [34], ORTEP-3 [35] program.

3.5. Photometric measurements

Absorption spectra were taken with a Perkin-Elmer Lambda 25UV–Vis Spectrophotometer in a 1 � 1 cm quartz optical cell main-tained at 25 �C with a Peltier thermostat. The light source of a Per-kin-Elmer LS-55 spectrofluorimeter was used as an excitation light,with a slit width of 10 nm. An optical filter was used to cut offovertones when necessary. The absorption spectra of the cis iso-mers were obtained by extrapolation of the absorption spectra ofa cis-rich mixture for which the composition is known from 1HNMR integration. Quantum yields (/) were obtained by measuringinitial trans-to-cis isomerisation rates (m) in a well-stirred solutionwithin the above instrument using the equation, m = (/I0/V)(1 � 10�Abs) where I0 is the photon flux at the front of the cell,V is the volume of the solution, and Abs is the initial absorbance

514 D. Mallick et al. / Polyhedron 31 (2012) 506–514

at the irradiation wavelength. The value of I0 was obtained by usingazobenzene (/ = 0.11 for p–p⁄ excitation [28]) under the same irra-diation conditions.

The thermal cis-to-trans isomerisation rates were obtained bymonitoring absorption changes intermittently for a cis-rich solu-tion kept in the dark at constant temperatures (T) in the range from298 to 313 K. The activation energy (Ea) and the frequency factor(A) were obtained from the Arrhenius plot, lnk = lnA � Ea/RT,where k is the measured rate constant, R is the gas constant, andT is temperature. The values of activation free energy (DG⁄) andactivation entropy (DS⁄) were obtained through the relationships,DG⁄ = Ea � RT � TDS⁄ and DS⁄ = [lnA � 1 � ln(kBT/h)/R where kB

and h are Boltzmann’s and Plank’s constants, respectively.

4. Conclusion

1-Alkyl-2-(arylazo)imidazoles, Raai-CnH2n+1 where n = 4, 6 and8 are used in this study. Mercury(II)-iodide complexes of theseligands exist in iodide-bridge dimeric structure, [Hg(Raai-CnH2n+1)(l-I)(I)]2. The complexes are characterised by spectro-scopic techniques and in one case the structure is confirmed bysingle crystal X-ray diffraction study. Photochromism of thecomplexes are examined by repetitive UV light irradiation in meth-anol solution for the ligands and the DMF solution is used for thecomplexes. The rate and quantum yields of E-to-Z isomerisationof the complexes are less than that of free ligand data. The rotormass and volume may be the regulating agents for this observa-tion. The E-to-Z isomerisation is thermally driven process. The acti-vation energies (Eas) of isomerisation of the free ligands are threetimes greater than that of the complexes that implies the loweringof rate in the complexes. Besides, the higher rotor volume andmass of the complexes may support the slow rate of isomerisationthan that of free ligands.

Acknowledgements

Financial support from Department of Science & Technology,New Delhi is thankfully acknowledged. One of us (A. Nandi) thanksthe University Grants Commission, New Delhi for fellowship.

Appendix A. Supplementary data

CCDC 835026 contains the supplementary crystallographic datafor [Hg(Meaai-C6H13)(l-I)(I)]2 (5b). These data can be obtained freeof charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or

from the Cambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:deposit@ccdc.cam.ac.uk.

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