and Photochromic Zn(II) Complexes - ACS Publications - American

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Arylazoimidazole Coordinated and Naphthalene- dicarboxylato Bridged Polymers of Co(II) and Photochromic Zn(II) Complexes Kaushik Naskar, Suvendu Maity, Srikanta Jana, Basudeb Dutta, Shinnosuke Tanaka, Debasish Mallick, Takashiro Akitsu, and CHITTARANJAN SINHA Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00111 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Crystal Growth & Design

Arylazoimidazole dicarboxylato

Coordinated

Bridged

Polymers

and

Naphthalene-

of

Co(II)

and

Photochromic Zn(II) Complexes Kaushik Naskar,†Suvendu Maity,† Srikanta Jana,† Basudeb Dutta,♣Shinnosuke Tanaka,¥ Debashis Mallick,# Takashiro Akitsu¥ and Chittaranjan Sinha†*

†

Department of Chemistry, Jadavpur University, Kolkata - 700 032, India; E-mail :

[email protected] ♣

Departmentof Chemistry, Aliah University, New Town, Kolkata - 700156, India.

#

Department of Chemistry, Mrinalini Dutta Mahavidyapith, Kolkata - 700051, India.

¥

Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-

ku, Tokyo 162-8601, Japan KEYWORDS Coordination Polymer; helical; photochromism; arylazoimidazole

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Naphtheledicarboxylato ((NDC2-) bridged Coordination Polymers (CPs) along with (E)-1-methyl-2-(pchlorophenylazo)imidazole (ClPai-Me) coordination to Co(II), [Co(∝-NDC)0.5(∝4-NDC)0.5(ClPaiMe)]·0.5H2O (1) and to Zn(II), [Zn(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (2) have been characterized. In the single crystal X-ray structure of 1, ClPai-Me chelates to Co(II) ion by N(azo) and N(imidazolyl), whereas in compound 2, it acts as monodentate N(imidazolyl) donor to Zn(II) ion. The coordination atmosphere around Co(II) in 1 ion is distorted octahedral CoN2O4, whereas in the case of 2, it is distorted square pyramidal ZnNO4. The compounds 1 and 2 exhibit the right handed (P) and lefthanded (M) 1D helical chain. NDC-2 is serving as a bridge between two M(II) ions to constitute µ-NDC and four M(II) ions to construct µ4-NDC to assemble 3D polymers. Upon UV light (369 nm) irradiation, the compound 2 shows trans-to-cis isomerisation of -N=N-C6H4-Cl-p both in solid and solution state but 1 remains silent. Prolong light irradiation in solid state (film phase) does not change the coordinated ClPai-Me in the complexes 1 and 2, whereas the free stage of ClPai-Me undergoes photoreduction of – N=N– bond and forms azo radicals with concomitant permanent colour change. The persistent of radical has been characterised by EPR spectra in solid state at g=2.009. The effective magnetic moment of 1 is 4.17 B.M. at 300 K, Co(II) ion of S=3/2.

Introduction The coordination polymers (CPs) are useful in the devices of gas storage and gas separation, catalysis, drug delivery, electrical conductivity, sensing studies, energy strategy, and oxygen balance.1-14 Interests to the design of task-specific materials as a response to external stimuli, such as light, heat, magnetic field, or mechanical effects are of highest importance.15However, the optical stimulation to CPs is of the most effective because the light is one of the most simple and practical signals and the material may absorb at a specific wavelength and subsequent performance, such as emission of longer wavelength, chemical reaction, energy or electron transfer, sensitization or transformation may have many applications. Optical


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signal is very faithful to the user and switching off the source causes immediate off the optical signal. Hence, does not induce any change of material structure and may be remote controlled over long distances, useful in ultra-high–density optical memories, optical information storage, light control, and optical switches. The photoactive CP-materials, either in bulk solution, incorporated in suitable matrices or on surfaces may be used for various applications, such as molecular nanomachine, photochromic glasses, high-capacity undistorted data storage devices,16 etc. The photoresponsive materials17 are sensitive to nature of substituents (steric and electronic effects), strength of covalent/noncovalent interactions, and the external factors, such as solvent (polarity, viscosity, dipole moment, etc.), presence of innocent (to be chemically non-interacting), and non-innocent (chemically interacting) ions or molecules. Azobenzene undergoes optically stimulated reversible cis/trans isomerisation.18-32 The conformational change is coupled with a significant change of volume; trans-azobenzene is linear, rod shaped (length, 9 Å), and less space demanding, whereas cis-azobenzene (length, 5.5 Å) is bent shaped and demands larger space.33-35 Combination of such kind of smart moieties in the CPs or Metal Organic Frameworks (MOFs)36-37 may construct a fascinating and challenging class of materials. The conformational change of azo benzene in the MOF cavity shows light-stimulated sorption behaviour, such as absorption of the significant amount of CO2 at dark, and upon light irradiation, CO2 is released.38-40 In search of new azo functionalized photochromes, 1-alkyl-2-(arylazo)imidazoles exhibit potential optical stimulated structural isomerisation and has been employed to examine the effect of substituents of different electronic activity, nature of solvent, presence of foreign innocent molecules and micelles, coordination with metal ions preferably d8 (low spin) and d10 electronic structure.25-32 This has motivated us to synthesise arylazoimidazole bonded coordination polymer. Optically induced structural change of 1alkyl-2-(arylazo)imidazoles (RaaiR/)25-29,41-43 (Photochromism, Scheme 1) has been examined vastly in solution phase.30-32 However, the inclusion of photo-switching molecule, azoimidazole motif to organic linker, or binding to the metal node as an integrated part of CPs (film phase) have not been studied.



Development of such an optically sensitive new class of switching material is subject of present research interest.

Scheme 1 Photochromismof 1-alkyl-2-(arylazo)imidazole In this study, the ClPai-Me ((E), 1-methyl-2-(p-chlorophenylazo)imidazole), a photochrome, is coordinated to M(II) (Co(II), Zn(II)) along with coordination polymer forming ligand, 2,6naphthalenedicarboxylic acid (H2NDC) to synthesize a high spin Co(II) coordination polymer, [Co(∝NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (1) and [Zn(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (2) (Scheme 2). Although large number of 1-alkyl-2-(arylazo)imidazoles (RaaiR/) are known but Clsubstituted arylazo motif in ClPai-Me crystallizes metal complex in suitable shape and size; so, ClPai-Me derivative has been synthesized. In 1, ClPai-Me serves as N(azo), N(imidazolyl) chelator to Co(II) and in 2, it acts as monodentate N(imidazolyl) donor to Zn(II)44,45 and assist π-interactions to improve dimensional stability. The structures of the complexes are supported by single crystal X-ray diffraction measurements. Photo responsive properties are examined in solution and in solid state (film phase). As far as literature is concerned, this is the first example of CPs of Co(II) and Zn(II) ions attached to an arylazoimidazole photochoromic motif with 2,6-naphthalenedicarboxylate. The arylazoimidazole performs conformational isomerisation by light-stimulated trans-cis isomerisation and thermal process assists the cis-trans conversion, which has been authenticated by UV-Vis spectroscopy in both solution and solid state. The bonded ClPai-Me in the compound 2 shows active photochromism but 1 remains


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inert. Solid phase light irradiation to ClPai-Me in free ligand phase shows azo radical formation (EPR active) while in the complexes (1 and 2) no such effect is observed. The redoxproperties of these compounds were obtained by cyclic voltammetry. The magnetic behavior of 1 was authenticated by SQUID.

Experimental Section Materials and Physical Measurements Reagents and solvents were commercially available (Sigma-Aldrich) and were used as received. The Fourier transform (FT) IR spectra (KBr pellet) were taken on a Perkin-Elmer RX1 spectrometer. Powder X-ray diffraction (PXRD) patterns were recorded with a Phillips PANalytical diffractometer Cu-Kα radiation (λ=1.5406 Å) at room temperature, with a scan speed of 2 min−1 and a step size of 0.02 in 2θ.

Scheme 2 Synthesis of CPs, [Co(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (1), and [Zn(∝NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (2).



The polymer films were prepared on quartz supports by casting from tetrahydrofuran solution by mixing equivalent portions of a 5 wt% polymer solution (polyvinyl chloride, Aldrich, Mw = 43,000) and a 0.2 wt% solution of low-mass compound 2 and ClPai-Me. The films were dried under reduced pressure at 50 °C for 24 h. TGA experiments were carried out in the temperature range of 25-600°C on a SDT Q600 TG-DTA analyzer under the N2 atmosphere at a heating rate of 10°C min−1. Magnetic properties were investigated using a Quantum Design MPMS-XL superconducting quantum interference device magnetometer (SQUID) at an applied field 0.5 T in a temperature range 5-300 K. The diamagnetic correction was carried out by using Pascal constants.46 A Bruker (AC) 400 MHz FT-NMR spectrometer using TMS as an internal standard was available for 1H spectra recording. Solar simulator ISS P110 Lamp Power Supply was used as light source (λ, 360 nm). EPR data were collected Magnettech GmbH Mini Scope MS400 spectrometer. X-ray Data Collection and Structure Solution Two good-shaped single crystals (0.361×0.225×0.083 mm) (1) and (0.215×0.115×0.081 mm) (2) were used for data collection via Bruker SMART APEX II diffractometer, having graphite-monochromated Mo-Kα radiation (λ= 0.71073 Å). Least squares refinements of all reflections within hkl range −18 ≤ h ≤ 18, −24 ≤ k ≤ 24, −19 ≤ l ≤ 19 (1) and −18 ≤h ≤18, −25 ≤k ≤25,−20 ≤l ≤20 (2)were used to determine the unit cell parameters and crystal-orientation matrices. The intensity data were corrected for Lorentz and polarization effects.47 The collected data (I >2σ (I)) were integrated using SAINT program and the absorption correction were made with SADABS. Full matrix least-squares refinements on F2 were carried out using SHELXL-9748 with anisotropic displacement parameters for all non-hydrogen atoms. Hydrogen atoms were constrained to ride on the respective carbon or nitrogen atoms with isotropic displacement parameters equal to 1.2 times the equivalent isotropic displacement of their parent atom in all cases. All calculations were carried out using SHELXL 97,48 SHELXS 97,49 PLATON 9950, and ORTEP-351 program. Crystal data and experimental details for data collection and structure refinement are reported in Table 1.


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Table 1.Crystallographic data of 1 and 2

1

2

Formulae

C44H31Cl2Co2N8O9

C44H31Cl2N8O9Zn2

Formula weight

1004.53

1017.45

Crystal system

Monoclinic

Monoclinic

Space group

C 2/c

C 2/c

a (Å)

14.5276(5)

14.487(3)

b (Å)

19.2757(6)

19.330(4)

c (Å)

15.4603(5)

15.350(3)

α (o)

90.00

90.00

β (o)

99.352(2)

99.619(5)

γ (o)

90.00

90.00

V (Å3)

4271.8(2)

4238.0(16)

T (K)

273(2)

293(2)

Z

4

4

Dcalcd (Mg/m3)

1.562

1.595

µ (mm-1)

0.969

1.326

λ (Å)

0.71073

0.71073

θ range (°)

1.770−27.362

1.773−28.450

Total reflections

36235

24450

Unique reflections

3960

3901

Refine parameters

304

295

R1a [ I > 2σ (I) ]

0.0390

0.0371

wR2b

0.1061

0.0997

Goodness-of-fit

1.047

1.041

Difference between peak and 0.684, −0.658 hole (e Å-3)


0.791, -0.638


Photometric Measurements Absorption spectra were taken with a Perkin Elmer Lambda 25 UV-VIS Spectrophotometer in a 1x1 cm quartz optical cell maintained at 25°C. The light source of ISS P110 Lamp Power Supply was used as an excitation light. Quantum yields (φ) were obtained by measuring initial trans-to-cis isomerization rates (ν) in a wellstirred solution within the above instrument by means of the Eq.1:

ν= (φ I0 /V)(1−10−Abs)

….. (1)

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 at the irradiation wavelength. The value of I0 was obtained by using azobenzene (φ=0.11 for

π-π* excitation) under the same irradiation conditions. The rates of thermal cis-to-trans isomerisation were collected by monitoring absorption changes for a cis-rich solution kept in the dark at constant temperatures (T) in the range from 298-313 K. The Arrhenius plot, lnk = lnA – Ea/RT (k, the measured rate constant; R, the gas constant, and T, temperature) is used to calculate the activation energy (Ea), the activation free energy (∆G*), activation entropy (∆S*), and the frequency factor (A) from Eq. 2: ∆G* = Ea –RT-T∆S* and ∆S* = [ln A -1- ln(kBT/h)/R] ….. (2) where, kB and h are Boltzmann’s and Plank’s constants, respectively. Syntheses (E)-1-Methyl-2-(p-chlorophenylazo)imidazole, (ClPai-Me). (E)-1-Methyl-2-(p-chlorophenylazo)imidazole (ClPai-Me) had been synthesized and characterized by literature method.44,45 [Co(∝ ∝-NDC)0.5(∝ ∝4-NDC)0.5(ClPai-Me)]·0.5H2O (1). The ClPai-Me (0.1 mmol), H2NDC (0.1 mmol), and Co(NO3)2.6H2O (0.1 mmol) were mixed in a 30 mL vial. Then mixed solution (9 mL) of methanol:


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water: DMF was added at an equivalent ratio (1:1:1). The vial was capped well and heated to 100°C for 72 h; and cooling at the rate of 5°C/h. The mother liquor was decanted and the crystals were washed with hexane (15 mL) thrice. Dark black-colored crystals were collected by filtration and dried in air for X-ray study (10 min). Yield: 33.9 mg (~67%). [Co(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (1) was isolated by filtration. Micro-analytical data for (C44H31Cl2Co2N8O9)∞(1): C, 52.61; H, 3.11; N, 11.15, Found: C, 52.56; H,3.03; N 11.09, IR (KBr pellet, cm-1): 1609 νas(COO−), 1331 νs(COO−) and 1396 ν(-N=N-) (ESI†, Figure S1). TGA plot of compound 1 confirmed the number of crystallization water molecule and it is the moderately stable upto 330°C (ESI†, Figure S2). Powder X-ray diffraction patterns of 1 as-synthesized are added in Figure S3. [Zn(∝ ∝-NDC)0.5(∝ ∝4-NDC)0.5(ClPai-Me)]·0.5H2O (2).The compound 2 was also isolated above identical procedure. Yield: 40.6 mg (~79%). Micro-analytical data for (C44H31Cl2N8O9Zn2)∞(2): C,51.94; H, 3.07; N, 11.01, Found: C, 51.89; H,3.01; N 10.98, IR (KBr pellet, cm−1): 1615 νas(COO−), 1361 νs(COO−) and 1408 ν(-N=N-) (ESI†, Figure S1).By the thermal analysis of compound 2 had been confirmed the number of crystallization water molecule and it is moderately stable up to 290 °C (ESI†, Figure S2).Powder X-ray diffraction patterns of 2,as-synthesized is added in Figure S4.

Results and Discussion (E)-1-Methyl-2-(p-chlorophenylazo)imidazole (ClPai-Me) has been synthesized and characterized by literature method.44,45 It has two eligible donor centres, N(azo) and N(imidazolyl). Solvothermal synthesis using ClPai-Me, H2NDC and Co(NO3)2. 6H2O /Zn(NO3)2. 6H2O has isolated coordination polymers. The structures of [Co(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (1) and [Zn(∝-NDC)0.5(∝4-NDC)0.5(ClPaiMe)]·0.5H2O (2), are confirmed by single crystal X-ray structure determination.52,53The complexes 1 and 2 both crystallizes in the Monoclinic space group C2/c with Z=4 and structure is shown in Scheme 1. In 1, each Co(II) ion adopts a CoN2O4 distorted octahedral geometry by chelating two nitrogen donor centres of ClPai-Me (N(azo) and N(imidazolyl)) and four carboxylato-O centres from three naphthyl-2,6-



dicarboxylates (NDC). Two NDC-2s act as monodentate carboxylato-O donors and third NDC-2 is a carboxylato-O, O chelator to Co(II) and -COO– of another end of the linker serves as a bridging agent to generate polymeric framework (Scheme 2, Figure 1). Thus, coordination polymer is constituted by bridging two adjacent Co(II) centres by two chelated carboxylate-O,O of NDC-2 (µ) and bridging four adjacent Co(II) centres by four carboxylate-O of second NDC-2 (µ4) (Figure S7), which leads to generate chair form Co2O4C2 repeating unit. Thus, eight-member Co2O4C2 distorted chair geometry repeats throughout the polymer (Figure 1).The Co-N(imidazolyl) bond distance is 2.041(3) Å that is comparable to reported structure of Co-arylazoimidazole complexes.54 The Co-N(azo) bond length 2.376(3) Å is longer than Co-N(imidazolyl), which accounts that Co(II) prefers to bind N(imidazolyl) and this is very common in biology.55 The Co-O bond lengths vary noticeably in the bridging partner: NDC bridged Co4 motif shows 2.008(3) and 2.016(3) Å, whereas, carboxylato chelated Co2 motif determines 2.235(3) and 2.128(3) Å (Table 2). In the chelated motif, both steric and angular strains may cause elongation of bond length.56 In 2, the coordination arrangement is [ZnNO4] and central Zn(II) lies in a distorted square pyramidal centre (τ5=0.1603)[For trigonal bipyramidal structure τ5 = 1, while a square pyramidal structure τ5= 0].57ClPai-Me acts as a monodentate imidazolyl-N donor, Zn-N distance is 2.024(3) Å, which is comparable with reported data59 and ClC6H4-N=N- group is freely suspended. There are three NDC units about the coordination sphere around Zn(II). One NDC unit bridges two Zn(II) centres forming chelated unsymmetrical ZnO2 motif (Zn-O lengths: 2.454(2) and 1.994(2) Å); each of the two NDCs is monodentate carboxylato-O donor and bridges four Zn(II) centres and thus forms eight-member Zn2O4C2 distorted chair geometry (Figure S7) to generate the coordination polymer (Figure 1, Table 2). The 2D polymers of 1 and 2 have enhanced supramolecular strength by the π•••π and C-H•••π interactions. In 1 the π•••π interaction between two different azoimidazolyl units from each 1D chain (Figure 1) has constructed the 2D supramolecule at a distance 3.972 Å and in Zn-CP (2) the distance is 3.914 Å. There are also extensive edge-to-face C-H•••π interactions in 1 and 2 at a distance of 3.094 Å


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and 3.111 Å, respectively to construct the helical 2Dsupramolecule. (Figure 2) However, NDC-2 linker connects the metal ions in the compounds 1 and 2 and has been expanded to 3D supramolecular structures.

1

2

Figure 1.1D Chain of CPs 1 and 2, blue circle shows M2O4C2 eight member distorted chair conformation.

(a)

(b)



(c)

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(d)

Figure 2. (a,c) π•••π interaction constructs the 2D supramolecular aggregate and (b,d) edge-to-face CH•••π interactions in Compound 1 and 2,respectively. In 1 and 2, metal ions, Co(II) and Zn(II), are bridging through 2,6-naphthalenedicarboxylate groups to form the 1D homochiral helical chain60(M and P)along the b-axis. The helical structures are clearly viewed in Figure 3(c,d).Ligand to metal coordination interaction is common and efficient supramolecularsynthon for the constructions of helical coordination compounds. The crystallographic structures of 1 and 2, show the screw axis with a pitch of b 22.994(3) and 22.992(3) Å respectively. Paramagnetism of [Co(∝ ∝-NDC)0.5(∝ ∝4-NDC)0.5(ClPai-Me)]·0.5H2O (1) Paramagnetic Co(II) (d7) complexes are common and experimental magnetic moment is greater than spin-only value, which is due to orbital contribution.61 For last few years, magnetic moment and temperature-dependent properties of polynuclear Co(II) or Coordination Polymers/MOF are intensively studied.62,63Figure 4 shows the χMT vs T plot of the variable-temperature magnetic properties under an external field of 5000 Oe for [Co(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (1). The χMT value of 2.25 cm3 K mol−1 at 5 K increases to the maximum value of 35.5 cm3Kmol−1 at 160 K and slightly decreases to 34.7 cm3Kmol−1 at 300 K for four Co(II) in a NDC2- bonded core of 1. Experimental magnetic moment


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(µ) per Co(II) in SBU is 4.17 BM at 300 K, which suggests isolated Co(II) ion of S=3/2. The increased magnetic moment in addition to orbital contribution may be due to spin-superexchange in the dinuclear and tetranuclear carboxylate bridging motif.62

1

2

Figure 3. (a,b) Helical aspect of 1 and 2, respectively through π•••π stacking and C-H•••π interactions viewed along the ac-plane; (c,d) Side view of 1D chain, a pair of enantiomeric helicates (M and P) along the b-axis.



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Table 2. Selective Bond Distances and Angles of 1 and 2 Compound 1

Compound 2

Bond angle (º)

Bond angle (º)

O(3)–Co(1)–O(4)

102.55(13)

O(3)–Zn(1)–O(2)

108.69(8)

O(3)–Co(1)–N(1)

98.39(12)

O(3)–Zn(1)–O(4)

106.50(7)

O(2)–Co(1)–O(1)

59.91(10)

O(2)–Zn(1)–O(1)

58.08(9)

N(1)–Co(1)–O(1)

155.25(16)

O(3)–Zn(1)–O(1)

158.74(8)

Bond length(Å)

Bond length(Å)

Co(1)–O(3)

2.008(3)

Zn(1)–O(3)

1.9828(16)

Co(1)–O(4)

2.016(3)

Zn(1)–O(2)

1.994(2)

Co(1)–N(1)

2.041(3)

Zn(1)–O(4)

2.0082(17)

Co(1)–O(1)

2.235(3)

Zn(1)–O(1)

2.454(2)

Co(1)–O(2)

2.128(3)

Zn(1)–N(1)

2.020(2)

N(3)–N(4)

1.269(3)

N(3)–N(4)

1.257(3)


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Figure 4.The χMT vs. T plots for compound [Co(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (1) in the temperature range 5-300 K. Electrochemistry The redox activities of the coordination polymers (1, 2) in DMF were investigated by cyclic voltammetry at 296 K. The redox potential data referenced to ferrocenium/ferrocene (Fc+/Fc) couple and the cyclic voltammograms are shown in Figure 5. The Complex 1 exhibits one quasi-reversible voltammogram at E1/2, 0.72 V (∆Ep>200 mV), whereas the complex 2 is silent within a potential window at the positive potential side. The complex 1 bears redox-sensitive Co(II), whereas in 2 it is Zn(II), which has not participated in redox process. Thus, voltammogram in Figure 5(a) refers to the Co(III)/Co(II) redox couple.64 An irreversible cathodic peak observed at −1.65 V for 1 and −1.20 V for 2 but in the case of free ClPai-Me ligand the reversible reduction potential at -1.72 V (Figure S8) which is assigned to reduction of azo group65 [-N=N-]/[-N–N-]•-.

Figure 5. Cyclic voltammograms of (a) 1 and (b) 2 in DMF at 296 K. Conditions: 0.2 M [N(nBu)4]PF6 supporting

electrolyte;

scan

rate,

100

mV

s–1;


platinum

working

electrode.


Photochromism Optically stimulated reversible cis/trans isomerisationof 1-alkyl-2-(arylazo)imidazoles(RaaiR/)in solution in free ligand phase and mononuclear coordination complexesis known.25-32 However, the inclusion of azoimidazole motif in CPs or MOF to act as photo-switching unit has not been reported previously. The photochromism of free ClPai-Me, the complexes1 and 2 has been studied in solid and solution phases. ClPai-Me and 2 show photoisomerisation and 1 is silent to light induced structural change. The property of photochrome is due to configurational change about –N=N– group either by torsion or cleavage of double bond. IR spectral analysis in particular to stretching of –N=N- (ν-N=N-) which appears at 1410 cm-1 of free ligand, ClPai-Me, is shifted to 1396 cm-1 in 1 [Co(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O) and 1408 cm-1 in [Zn(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (2). In the compound 1, ClPai-Me serves as N(imidazolyl),N(azo) chelating agent which may be the reason of lowering of –N=N- stretching frequency. At the same time, the stretching frequency of –N=N- in the compound 2 remains almost unshifted which implies non-coordination of N(azo) centre and p-Cl-C6H4-N=N- remains suspended in space (Figure. S1). The crystal structure of 2 also shows the presence of pendant free –N=N-C6H5-Clp(N-azo group) which takes part in active photochromism. The inertness of optical irradiation of 1 may be explained on considering the energy scavenging process of excited species and out of them two main reasons may be accounted : (i) paramagnetism of Co(II) complex and (ii) chelation stability of [Co(-N=NC=N-)] motif. Because of unpaired spin of paramagnetic centre, Co(II), the excited energy may be released by spin inversion followed by large number of L-S coupled states. The chelate structure makes photochromic function stable in one stereochemical form so torsional motion or cleavage of azo bond followed by inversion is restricted.66-69

Solution Phase Study The absorption spectra of the cis isomers (ClPai-Me and 2) have been obtained by extrapolation of the absorption spectra of a cis-rich mixture. We have collected 1H NMR spectra of ClPai-Me (Figure S5


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andTable S1) upon light irradiation (Figure6) but the spectrum of 2 was not readable to collect the percentage of conversion. It is observed that some new signals have been generated (Figure 6) those are due to cis-configuration about -N=N- bond of the photochrome, ClPai-Me.30-32

Figure 6. Detection of trans-to-cis isomerisation of 2 by 1H NMR spectral study in DMSO-d6.



Figure 7. Spectral changes of ClPai-Me in DMF due to E (trans) → Z (cis) isomerisation upon repeated irradiation (362 nm) at 3 min interval at 25°C. Inset figure shows spectra of trans and cis-isomer of the complexes.

Figure 8. Spectral changes of ClPai-Me in DMF due to Z (cis) →E (trans) isomerisation upon repeated irradiation (362 nm) at 3 min interval at 25°C. Inset figure shows spectra of cis and trans isomer of the complexes.


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Figure 9. Spectral changes of [Zn(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (2) in DMF due to E (trans) → Z (cis) isomerisation upon repeated irradiation (369 nm) at 3 min interval at 25°C. Inset figure shows spectra of trans and cis isomer of the complexes.

Figure 10. Spectral changes of [Zn(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (2) in DMF due to Z (cis) → E (trans) isomerisation upon repeated irradiation (369 nm) at 25°C. Inset figure shows spectra of cis and trans isomer of the complexes.



Because of sparing solubility of 2 in dichloromethane and acetonitrile, the absorption spectrum is recorded in DMF solution and shows bands at 369 nm and 455 nm. The solution intensity and the absorption band position of 2 remain unchanged keeping the solution for a period of a month or so, which predicts that the structure of the complex remains unperturbed and stable in the experimental condition. The electronic spectral bands in 2 appear in a longer wavelength than that of free ligand, ClPai-Me, 362 nm and 450 nm, in the same solvent. The bands are consigned to π-π* and n-π* transitions, respectively. The solution of ClPai-Me is then irradiated for the fixed time with UV light at 362 nm followed by a collection of absorption spectrum (Figure 7); the spectral change is similar to that observed for the transto-cis isomerization of 1-methyl-2-(phenylazo)imidazole.25-32 Intense peak at 362 nm decreases along with increase at the annex portion at 450 nm until a stationary state is reached (Photostationary-I, PS-I). Reversal of experiment, i.e., irradiation of visible light of 420 nm to the cis-rich solution increases the intensity of band at 362 nm followed by small decrement at 462 nm and restoration of trans-configuration is recommended (Figure 8; Photostationary-II, PS-II). Similarly, DMF solution of 2 is irradiated at 369 nm and the spectral change is recorded in Figure 9, which shows a decrease in intensity and subsequent enhancement of band at 456 nm. Further, cis-rich component undergoes irradiation at the visible region at 450 nm and recovers its original spectrum (Figure 10). Thus, reversibility of isomerisation process upon optical influence is satisfied.The structure of 2 (ESI†, Figure S7) shows pendant -N=N-C6H4-Cl, which is supposed to undergo free rotation and isomerizes to cis-form. The rate of isomerisation is faster for ClPaiMe (3.612 × 10−8 s−1) than 2 (0.879 × 10−8 s−1). The quantum yields for the trans-to-cis (φt→c) photo isomerisation, 0.201± 0.003 (ClPai-Me) and 0.071± 0.001 (2) are comparable with reported results.25-32,7072

In general, increase in mass of the photochrome reduces the rate of isomerisation.


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(a)

(b)

Figure 11. Spectral changes of (a) ClPai-Me (Inset : irradiation of film phase at 374 nm) and (b) compound 2 in solid state due to E (trans) → Z (cis) photo-isomerisation upon repeated irradiation (374 nm) at 3 min interval at 25°C. The UV-light irradiation experiment of 1 does not show any tractable observation and is silent to photo isomerisation. There may be several reasons based on structure, electronic structure, and energy ordering of the participating functions. ClPai-Me is chelated to Co(II) and the complex is paramagnetic (µ, 4.17 BM at 300 K). Thus, structural rigidity and paramagnetism centre may resist photoisomerisation.63 The cis-to-trans isomerisation of ClPai-Me and 2 are carried out at varying temperature, 298-313 K and the Eyring plots determine the activation energy (Ea) 87.95 and 20.18 kJ mol−1 and the activation entropy (∆S#) −38.06 and −257.20 J mol−1K−1, respectively for ClPai-Me and 2 (Table 3, Figure 12). In the complex, the Eas are severely decreased which means faster cis-to-trans thermal isomerisation of the complex. The entropy of activation (∆S*) is highly negative in the complex than that of free ligand. This is also in support of an increase in molar mass and rotor volume in the complexes.



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Table 3. Rate and activation parameters for Z (cis)→E (trans) thermal isomerisation in solution and solid state Temp (K)

Rate of thermal c→t conversion x 104 (s–1)

Ea (kJ mol−1)

∆H*kJ

ClPaiMe

298

0.721

87.95

( in Solution)

303

1.051

308

2.452

313

3.601

ClPaiMe

298

0.6745

( in Solid)

303

0.9785

308

2.1410

313

3.1521

2

298

1.8294

( in Solution)

303

2.1444

308

2.3539

313

2.7364

2

298

1.7240

( in Solid)

303

1.9024

308

2.1231

313

2.4510

Compounds

−1

∆S*J mol−1

∆G*ckJ

K

mol−1

85.41

−38.06

97.03

83.86

81.32

−52.30

97.29

20.18

17.64

−257.20

96.21

18.04

15.51

−265.00

96.46

mol

1

Solid (film) phase study The photostabilities of the compound ClPai-Me in the solid phase were investigated by visual color change and spectroscopic (IR, UV-Vis, PXRD, and EPR) measurements. The compounds were exposed in a solar-simulator test chamber for 30 min. Then, the IR spectra of the irradiated compounds were recorded and compared with those of not exposed sample (Figure S9). The irradiation does not cause pronounced changes in the IR spectrum of 2 while IR spectrum of ClPai-Me shows substantial change of

ν(-N=N-) at 1410 cm-1 before irradiation and a new stretching appears at 1245 cm-1 after irradiation


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which may be due to ν(-N–N-) and may be obtained by photoexcited reduction process. This modification could be assigned to change in the skeletal vibrations of the -N=N- bonds. To investigate further the photostabilities of the complexes, the powder XRD (PXRD) patterns of the compounds were recorded. The PXRD patterns of 2 remain unchanged which support photostability of the complexes at CP phase but in the case of ClPai-Me ligand shows the significant change in PXRD pattern after the formation of azo anion radical (Figure S10).In solid state after irradiation with light, ClPai-Me shows EPR signal (Figure S11) at gav= 2.009 (298 K) which may be due to the formation of azo anion radical and the colour changes from yellow to dark red (Figure S12). The EPR signal disappears in solution in DCM. But compound 2 does not show any EPR signal in solid and solution.

(a)

(b)

Figure 12.The Eyring plots of rate constants ofE (cis) →Z (trans) thermal isomerisation of (x) ClPai-Me and (y) 2 in both solution (a) and solid state (b) at different temperatures. 298-313 K.

Figure 11(a,b) illustrates the changes in the electronic absorption spectra of the ClPai-Me and 2,respectively in polystyrene media during irradiation with UV radiation at 360 nm. We see that the polymer films exhibit slightly higher absorption maxima than that of liquid samples, namely at around 374 and 378 nm, assigned to the π-π* transitions, respectively. Theπ-π*absorption band decreases in intensity with the irradiation time due to the trans-cis photo-isomerization process of azobenzene



moieties. The photo-stationary state is achieved after 32 min when the conversion is about 60%. It is realized that the cis content of the stationary state was a little lower in polymer film than in solution. A photo-isomerization should be suppressed in solid samples because the free volume around the chromophore is smaller than that of the liquid samples due to a closer packing of the polymeric film.

Conclusions 2,6-Naphthalene dicarboxylic acid (H2NDC) serves as organic linker to Co(II) and Zn(II) ions along with coordination of (E)-1-methyl-2-(p-chlorophenylazo)imidazole, (ClPai-Me), to generate helical Coordination Polymers, [Co(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (1) and [Zn(∝-NDC)0.5(∝4NDC)0.5(ClPai-Me)]·0.5H2O (2).ClPai-Me is a monodentate N(imidazolyl) donor to Zn(II) (2) and the pendant

(-N=N-C6H4-Cl) undergoes UV light-induced trans-to-cisisomerisation, whereas Co(II)

compound is paramagnetic and optically silent. Photochromism of bonded azoimidazole is reported first time for Zn(II)- coordination polymer in solution phase. Light irradiation in film phase (solid) generates azo radical to free ligand, ClPai-Me while the complex 2 remains silent in solid state.

ASSOCIATED CONTENT Supporting Information (ESI†). The spectral data of 1 and 2 (FT-IR), Figure S1; TGA, Figure S2; PXRD of 1,Figure S3; PXRD of 2, Figure S4; 1H-NMR spectra of ClPai-Me before and after UV light irradiation, Figure S5; 1H-NMR spectrum of [Zn(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (2) in DMSO-d6, Figure S6;2,6-NDC as a chelate and bridging four adjacent metal centre in compound [Co(∝NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (1) and [Zn(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (2), Figure S7; Cyclovoltamerty of ClPai-Me, Figure S8; 1H-NMR signal assignment before and after UV irradiation to ClPai-Me in DMSO-d6, Table S1); IR spectra of ClPaiMe before and after irradiation, Figure S9;PXRD of ClPai-Me before irradiation and after irradiation, Figure S10; Solid state EPR of


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ClPai-Me after irradiation, Figure S11; Photoirradiation of free azo ligand (ClPaiMe) in solid mass and polymeric film, Figure S12. CCDC contains [Co(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (1) (CCDC No. 1556980); and [Zn(∝-NDC)0.5(∝4-NDC)0.5(ClPai-Me)]·0.5H2O (2) (CCDC No. 1556979). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. AUTHOR INFORMATION

Corresponding Author E-mail :[email protected] Present Addresses †Department of Chemistry, Jadavpur University, Kolkata - 700 032, India Author Contributions These authors contributed equally. Funding Sources The authors acknowledge for the financial support from the Council of Scientific and Industrial Research (CSIR, Sanction No. 01(2894)/17/EMR-II) New Delhi, India. ACKNOWLEDGMENT The authors acknowledge for the financial support from the Council of Scientific and Industrial Research (CSIR, Sanction No. 01(2894)/17/EMR-II) New Delhi, India and SERB (New Delhi) PDF/2016/001813. ABBREVIATIONS



CPs,Coordination Polymers; ClPai-Me, (E)-1-methyl-2-(p-chlorophenylazo)imidazole; 2,6-NDC, 2,6naphthalenedicarboxylic acid.

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For Table of Contents Use Only

Arylazoimidazole Coordinated and Naphthalene- dicarboxylato Bridged Polymers of Co(II) and Photochromic Zn(II) Complexes Kaushik Naskar, Suvendu Maity, Srikanta Jana, Basudeb Dutta, Shinnosuke Tanaka, Debashis Mallick, Takashiro Akitsu and Chittaranjan Sinha*

SYNOPSIS. Two novel Co(II) and Zn(II) helical coordination polymers (CPs) bridged by 2,6naphthalenedicarboxylic acetate (NDC-2) in collaboration with with a photoswitchable ligand, (E)-2-((4-chlorophenyl)diazenyl)-1-methyl-imidazole (ClPai-Me) are characterized. Interestingly Zn-CP only undergoes light induced trans-cis isomerization about -N=N-C6H4-Cl both in solution phase and solid phase.


35


Table of Contents (TOC)

Arylazoimidazole Coordinated and Naphthalene- dicarboxylato Bridged Polymers of Co(II) and Photochromic Zn(II) Complexes Kaushik Naskar, Suvendu Maity, Srikanta Jana, Basudeb Dutta, Shinnosuke Tanaka, Debashis Mallick, Takashiro Akitsu and Chittaranjan Sinha*

SYNOPSIS. Two novel Co(II) and Zn(II) helical coordination polymers (CPs) bridged by 2,6naphthalenedicarboxylic acetate (NDC-2) in collaboration with with a photoswitchable ligand, (E)-2-((4-chlorophenyl)diazenyl)-1-methyl-imidazole (ClPai-Me) are characterized. Interestingly Zn-CP only undergoes light induced trans-cis isomerization about -N=N-C6H4-Clboth in solution phase and solid phase.


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and Photochromic Zn(II) Complexes - ACS Publications - American

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