Inorganica Chimica Acta 380 (2012) 104–113

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Inorganica Chimica Acta

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N-Salicylidene anil anions as thermo-sensitive components of organic–inorganichybrid materials

François Robert, Anil D. Naik, Bernard Tinant, Yann Garcia ⇑Institute of Condensed Matter and Nanosciences, Université Catholique de Louvain, Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium

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

Article history:Available online 8 September 2011

Young Investigator Award Special Issue

Keywords:N-Salicylidene anilineThermochromic anionsHybrid materialsFluorimetryProton transfer

0020-1693/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.ica.2011.08.040

⇑ Corresponding author. Tel.: +32 10472831; fax: +E-mail address: yann.garcia@uclouvain.be (Y. Garc

New N-salicylidene meta and para aminobenzenesulfonate anions (1 and 2) presenting both reversiblethermochromic properties and high thermal stability have been synthesized. These functional organiccomponents have been successfully inserted into inorganic cationic [CuII(phen)3] and [NiII(phen)3]frameworks, as a proof of concept, affording [Cu(phen)3](1)2 (C1), [Ni(phen)3](1)2 (C2), [Cu(phen)3](2)Cl�5H2O�2CH3CH2OH (C3) and [Ni(phen)3](2)Cl�7H2O�CH3OH (C4), that have all been structurally charac-terized by single crystal X-ray diffraction. Temperature dependent fluorimetry demonstrates, for the firsttime, that keto-enol tautomerization of the anion is effective in the organic–inorganic hybrid materials,even though no thermochromism is observed. This insertion method paves the way towards the synthe-sis of a panel of hybrid multifunctional materials.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

N-Salicylidene anils have been used in diverse fields such as op-tics [1–3], catalysis [4], magnetism [5] and biology [6]. Variousapplications have been proposed from chemical sensing [7], stim-uli responsive polymers [8], to molecular machines [9], but themost promising one is presumably the data storage and displayability of these molecules thanks to their thermo- and photochro-mic switching properties [1,10,11]. Interestingly, these phenomenaoccur in the solid state and without fatigue [12] which allow toforesee potential applications. Thermochromism, which corre-sponds to a temperature-induced reversible colour change [13],is observed thanks to a thermal tautomeric equilibrium betweenuncoloured enol and yellow cis-keto forms (Fig. 1). Photochro-mism, which is a similar optical phenomenon induced by light irra-diation [13], is encountered for few N-salicylidene anils in the solidstate [14–16] and arises from a photo-isomerisation from bothenol and cis-keto forms to a red trans-keto form.

A challenging perspective is the preparation of hybrid molecu-lar materials using the thermo- or/and the photo-sensibility of thisclass of molecules that could be incorporated as guest anionic sen-sors in various cationic host functional matrices. For this purpose, asulfonate group was selected because of its low coordination abil-ity and its permanent charge. Up to now, N-salicylidene anilinederivatives have been introduced into host matrices as clathrates[17–22] or thanks to the ‘ship-in-a-bottle’ synthesis [23]. Introduc-ing an anion in a host charged matrix would afford a better control

ll rights reserved.

32 10472330.ia).

of the number of incorporated functional molecules compared tothe above insertion modes that provide a random composition ofneutral guest molecules. Because the host material can influenceoptical properties of N-salicylidene anils [17,23–27], it is also ofgreat importance to control both molecular environment and com-position of host materials. This controlled approach may leads tomultifunctional hybrid materials that could benefit from bothproperties brought by the anil guest molecule and by the hostmaterial.

In this work, we have prepared and studied optical properties oftwo novel thermochromic sulfonate anions 1 (N-salicylidene m-aminobenzene sulfonate) and 2 (N-salicylidene p-aminobenzenesulfonate) that have been introduced into [Cu(phen)3] and[Ni(phen)3] inorganic cationic frameworks, as a proof of concept.This host network was selected because it can easily crystallizeand it is very stable thanks to the chelating effect of 1,10-phenan-throline. This methodology afforded [Cu(phen)3](1)2 (C1),[Ni(phen)3](1)2 (C2), [Cu(phen)3](2)Cl�5H2O�2CH3CH2OH (C3) and[Ni(phen)3](2)Cl�7H2O�CH3OH (C4), which have been all structur-ally characterized by single crystal X-ray diffraction. The opticalproperties of the hybrid inorganic–organic materials C1 and C2

are discussed in the light of the keto-enol tautomerization ofN-salicylidene derivatives.

2. Materials and methods

2.1. Chemicals

Solvents (methanol HPLC grade from Prolabo; DMSO-d6 99.9atom% D from Aldrich) and reagents (salicylaldehyde, 99%;

Fig. 1. Thermochromism of N-salicylidene aniline.

F. Robert et al. / Inorganica Chimica Acta 380 (2012) 104–113 105

1,10-phenanthroline, 99+% and copper (II) chloride dihydrate, 98%from Acros Organics; 3-aminobenzenesulfonic acid, >98% from Flu-ka; sodium hydroxide, analytical reagent from Fisher Scientific;copper(II) sulphate pentahydrate from Riedel-de-Haïn; sulfanilicacid, 99% from Sigma–Aldrich; nickel(II) chloride from UCB) wereobtained commercially and used as received.

2.2. Instrumentation

1H and 13C NMR spectra were recorded at 300 and 75 MHz on aBrüker AC300 instrument with DMSO protons peak as internalstandard. Mass spectral data were obtained on ThermofinniganLCQ ion trap spectrometer using an ESI ionization mode. TheTGA/DTA instrument used was a TA instrument SDT2960 Simulta-neous DSC-TGA with alumina crucibles filled with �10 mg of sam-ple. Approximately 10 mg of dried aluminium oxide were used asreference for DTA measurements. Optical experiments were car-ried out with Na1, Na2, C1 and C2 because of the limited availableamount of C3 and C4. Diffuse reflectance spectra were obtained on aVarian Cary 5E spectrometer using PTFE as a reference. Solid stateemission and excitation spectra were obtained with a Fluorolog-3(Jobin-Yvon-Spex Company) spectrometer combined with an Ox-ford Optistat DN. Reproducibility of temperature dependent emis-sion and excitation spectra was checked on fresh samples of Na1,Na2, C1 and C2 that provided the same spectroscopic results. Noemission or excitation spectra were recorded on cooling becauseno particular hysteresis effect was expected.

3. Synthesis

3.1. Synthesis of anions

3.1.1. Sodium salt of N-salicylidene meta-aminobenzenesulfonate(Na1)

3-Aminobenzene sulfonic acid (5 g, 28.8 mmol, 1 equiv.) wassuspended in hot methanol (55 mL) and deprotonated by addingsolid sodium hydroxide (1.18 g, 28.8 mmol, 1 equiv.) under refluxfor 1 h. Afterwards, salicylaldehyde (3 mL, 28.8 mmol, 1 equiv.)was added to the mixture to give a yellow solution which was stir-red for 2 h at r.t. and concentrated. The crystalline yellow powderis filtered and washed with 10 mL of methanol (7.09 g,yield = 82%). 1H NMR (300 MHz, d6-DMSO, 298 K, ppm): 7.01 (m,2H), 7.43 (m, 3H), 7.59 (m, 2H), 7.73 (m, 1H), 9.01 (s, 1H), 13.07(s, 1H). 13C NMR (75 MHz, d6-DMSO, 298 K, ppm): 117.3, 119.0,119.9, 120.0, 122.4, 124.8, 129.7, 133.4, 134.1, 148.2, 150.2,161.0, 164.4. Elemental Anal. Calc. for C13H10NSO4Na (Mr =299.28 g mol�1): C, 52.17; H, 3.37; N, 4.68. Found: C, 51.79; H,3.31; N, 4.66%. MS: m/z = 276 (M�), 212 (M� – SO2), 196 (M� –SO3), 118 (C7H4NO�), 574 (NaM2

�). X-ray powder diffraction (�):7.26 (m), 10.92 (s), 14.52 (w), 14.46 (w), 18.12 (w), 21.82 (m),25.06 (w), 25.54 (w), 26.44 (w), 27.26 (w), 29.28 (m), 30.20 (w),35.62 (w), 36.74 (w), 38.88 (w), 43.34 (w). FTIR (KBr disk, cm�1):445 (w), 467 (w), 498 (w), 534 (m), 548 (w), 573 (w), 611 (m),629 (m), 662 (w), 692 (m), 725 (m), 742 (m), 752 (s), 810 (m),839 (w), 885 (w), 904 (w), 912 (w), 928 (w), 984 (w), 995 (w),1032 (w), 1047 (m), 1092 (w), 1111 (m), 1148 (m), 1186 (s),

1219 (s), 1281 (m), 1362 (w), 1385 (m), 1414 (w), 1429 (w),1460 (w), 1474 (m), 1498 (m), 1572 (s), 1593 (s), 1622 (s), 3466(w, br). Degradation temperature by TGA/DTA: 340(1) �C.

3.1.2. Sodium salt of N-salicylidene para-aminobenzenesulfonate(Na2)

This salt was prepared following our reported procedure [16].1H NMR (300 MHz, d6-DMSO, in ppm, 298 K): d = 7.00 (m, 2H),7.42 (m, 3H), 7.70 (m, 3H), 9.00 (s, 1H), 13.06 (s, 1H). Degradationtemperature by TGA–DTA: 384(1) �C.

3.2. Synthesis of complexes

3.2.1. [Cu(phen)3](1)2 (C1)Na1 (0.2 g, 0.7 mmol, 2 equiv.) was dissolved in hot methanol

(60 mL) to give a yellow solution. CuSO4�5H2O (0.06 g, 0.5 mmol,1 equiv.) dissolved in methanol (5 mL) was added dropwise tothe Na1 solution to give a clear brown solution. 1,10-Phenantho-line (0.18 g, 1 mmol, 3 equiv.) was dissolved in methanol (5 mL)and then added to the mixture to give a clear green solution. Themixture was allowed to slowly evaporate in darkness during15 days. Green crystals of good quality were formed and freshly ta-ken out from the mother solution for analyses. Elemental Anal.Calc. for C62H44CuN8O8S2�H2O (Mr = 1174.77 g mol�1): C, 63.39; H,3.95; N, 9.54. Found: C, 63.12; H, 3.68; N, 9.43%. FTIR (KBr disk,cm�1): 418 (w), 500 (w), 523 (m), 542 (w), 559 (w), 617 (s), 658(m), 685 (m), 719 (s), 750 (m), 781 (m), 795 (m), 854 (s), 864(m), 914 (m), 957 (w), 995 (m), 1032 (s), 1082 (m), 1103 (s),1146 (s), 1209 (s, br), 1278 (s), 1342 (w), 1358 (w), 1421 (m),1454 (m), 1470 (s), 1470 (s), 1516 (s), 1568 (s), 1587 (s), 1616(s), 3053 (w, br). Degradation temperature by TGA/DTA: 309(1) �C.

3.2.2. [Ni(phen)3](1)2 (C2)Na1 (0.15 g, 0.5 mmol, 2 equiv.) was dissolved in hot methanol

(50 mL) at 80 �C to give a yellow solution. NiCl2 (0.06 g, 0.25 mmol,1 equiv.), dissolved in methanol (5 mL) was added dropwise to aNa1 solution to give a clear orange solution. 1,10-Phenantholine(0.14 g, 0.8 mmol, 3 equiv.) was dissolved in methanol (5 mL) andthen added to the mixture to give a red solution. The mixturewas allowed to slowly evaporate in darkness during 15 days. Or-ange crystals of good quality were formed and freshly taken outfrom the mother solution for analyses (0.213 g, yield = 22%). Ele-mental Anal. Calc. (%) for C62H44NiN8O8S2 (Mr = 1151.89 g mol�1):C, 64.65; H, 3.85; N, 9.73. Found: C, 64.52; H, 3.81; N, 9.73%. FTIR(KBr disk, cm�1): 422 (w), 442 (w), 507 (w), 523 (m), 542 (m),559 (w), 573 (w), 617 (s), 644 (w), 658 (w), 687 (m), 723 (s), 752(m), 781 (w), 795 (w), 841 (w), 854 (s), 866 (m), 914 (m), 966(w), 995 (m), 1032 (s), 1092 (w), 1105 (s), 1134 (m), 1145 (s),1211 (s), 1281 (m), 1344 (w), 1360 (w), 1396 (m), 1420 (s), 1431(s), 1454 (m), 1470 (m), 1493 (m), 1518 (s), 1568 (s), 1587 (m),1616 (m), 3053 (w), 3433 (w, br). Degradation temperature byTGA–TDA: 370(1) �C.

3.2.3. [Cu(phen)3](2)Cl�5H2O�2CH3CH2OH (C3)Na2 (0.2 g, 0.7 mmol, 2 equiv.) was dissolved in technical grade

methanol (20 mL) at 80 �C to give a yellow solution. CuCl2�2H2O(0.06 g, 0.4 mmol, 1 equiv.), dissolved in methanol (5 mL), wasadded dropwise to a Na2 solution (0.3 mmol, 1 equiv.) to give aclear brown solution. 1,10-Phenantholine (0.18 g, 1 mmol,3 equiv.) was dissolved in methanol (5 mL) and then added tothe mixture to give a clear green solution. The mixture was allowedto slowly evaporate in darkness during 15 days. Green crystals(few mg) of good quality were formed and freshly taken out fromthe mother solution for X-ray diffraction analysis.

106 F. Robert et al. / Inorganica Chimica Acta 380 (2012) 104–113

3.2.4. [Ni(phen)3](2)Cl�7H2O�CH3OH (C4)Na2 (0.2 g, 0.7 mmol, 2 equiv.) was dissolved in technical grade

methanol (20 mL) at 80 �C to give a yellow solution. NiCl2 (0.08 g,0.3 mmol, 1 equiv.), dissolved in methanol (5 mL), was added drop-wise to the Na2 solution to give a clear green solution. 1,10-Phe-nantholine (0.18 g, 1 mmol, 3 equiv.) was dissolved in methanol(5 mL) and then added to the mixture to give a clear orange solu-tion. The mixture was allowed to slowly evaporate in darknessduring 10 days. Long orange/red crystals (few mg) of good qualitywere formed and freshly taken out from the mother solution for X-ray diffraction analysis.

4. Single crystal X-ray diffraction

X-ray intensity data were collected for C1, C2 and C4 at 120(2) K,and at 120(2) and 250(2) K for C3 with a MAR345 image plate usingMo Ka (k = 0.71069 Å) radiation. The crystal was chosen, mounted ininert oil and transferred quickly to the cold gas stream for flash cool-ing. The unit cell parameters were refined using all collected spotsafter the integration process. The data were not corrected for absorp-tion, but the data collection mode partially takes the absorption phe-nomena into account. The structure was solved by direct methodswith SHELX97 [28]. All the structures were refined by full-matrixleast-squares on F2 using SHELX97 [28]. All non-hydrogen atoms wererefined with anisotropic temperature factors. Tables of distancesand angles were obtained using PLATON software [29]. Only the Hatom of O–H or N–H functions was localized by Fourier difference,the other ones were calculated with AFIX. Crystal data for C1:C62H44CuN8O8S2, Mr = 1156.71 g mol�1; green colour; crystaldimensions 0.44 � 0.32 � 0.30 mm; monoclinic, space group C2/c

(no 15), a = 26.553(7), b = 10.812(2), c = 21.889(6) Å, b =124.59(2)�, V = 5173(2) Å3, Z = 4; qcalc = 1.485 mg m�3; T = 120(2)K; F(000) = 2388; l = 0.5272 mm�1; collected/unique reflections =106674/5868; hmax = 27.50�; Rint = 0.04; R1 = 0.0442, I > 2r(I) =5594, wR2 = 0.1208; largest peak = 0.481, hole = �0.668, complete-ness: 98.5%. Crystal data for C2: C62H44NiN8O8S2, Mr =1151.88 g mol�1; orange colour; crystal dimensions 0.55 � 0.50 �0.30 mm; monoclinic, space group C2/c (no 15), a = 26.475(7), b =10.747(2), c = 21.930(6) Å, b = 124.24(2)�, V = 5158(2) Å3, Z = 4;qcalc = 1.483 mg m�3; T = 120(2) K; F(000) = 2384; l =0.527 mm�1; collected/unique reflections = 91450/5328; hmax =26.73�; Rint = 0.04; R1 = 0.0359, I > 2r(I) = 5204, wR2 = 0.1010; larg-est peak = 0.451, hole = �0.549, completeness: 97.3%. Crystal datafor C3 at 120(2) K: C53H56ClCuN7O11S, Mr = 1098.10 g mol�1; greencolour; crystal dimensions 0.32 � 0.18 � 0.08 mm; triclinic, space

Fig. 2. (a) Diffuse reflectance spectra of pure solid samples of Na1, Na2, C1 and C2 at

group P�1 (no 2), a = 11.198(3), b = 12.969(4), c = 16.759(5) Å,a = 86.69(2)�, b = 83.75(2)�, c = 81.74(2)�, V = 2392(1) Å3, Z = 2;qcalc = 1.525 mg m�3; F(000) = 1146; l = 0.63 mm�1; collected/un-ique reflections = 20500/6398; hmax = 23.25�; Rint = 0.0409; R1 =0.0518, I > 2r(I) = 5304, wR2 = 0.1436; largest peak = 1.43,hole = �0.788, completeness: 92.9%. Crystal data for C3 at250(2) K: green colour; triclinic, space group P�1 (no 2),a = 11.302(3), b = 13.011(4), c = 16.949(5) Å, a = 86.62(2)�,b = 83.48(2)�, c = 82.18(2)�, V = 2451(1) Å3, Z = 2; qcalc =1.488 mg m�3; F(000) = 1146; l = 0.614 mm�1; collected/uniquereflections = 17370/5660; hmax = 21.96�; Rint = 0.061; R1 = 0.049,I > 2r(I) = 4592, wR2 = 0.1321; largest peak = 0.50, hole = �0.49,completeness: 94.7%. Crystal data for C4: C50H52ClNiN7O12S,Mr = 1069.21 g mol�1; orange/red colour; crystal dimensions0.25 � 0.20 � 0.20 mm; triclinic, space group P�1 (no 2),a = 11.185(3), b = 12.935(4), c = 16.700(5) Å, a = 86.66(2)�, b =83.82(2)�, c = 82.13(2)�, V = 2377(1) Å3, Z = 2; qcalc = 1.494 mg m�3;T = 120(2) K; F(000) = 1116; l = 0.582 mm�1; collected/uniquereflections = 29231/7634; hmax = 24.43�; Rint = 0.055; R1 = 0.047,I > 2r(I) = 6450, wR2 = 0.1228; largest peak = 0.77, hole = �0.59,completeness: 92.8%.

5. Results

5.1. Synthesis

Sodium salt of the anions 1 and 2 (Na1 and Na2) were synthe-sized as powders. These anions both have a very high thermal sta-bility for organic salts as confirmed by TGA/DTA (degradationtemperature = 340(1) �C for 1 and 384(1) �C for 2). These anionsmaterials both reveal thermochromic properties on cooling fromdark yellow (Na1) or pale yellow (Na2), at room temperature, towhite at 77 K (Fig. 2). The hybrid materials C1–4 have been success-fully crystallized without modifying the nature of the inserted or-ganic guest as found by single crystal X-ray diffraction (see nextsections). The hybrid C1 and C2 also present a very good thermalstability (degradation temperatures = 309(1) and 370(1) �C,respectively). Single crystals C3 and C4 were only obtained in en-ough amount for an X-ray diffraction study.

5.2. Diffuse reflectance spectroscopy

Spectra were recorded on pure powder samples and a KubelkaMunk (KM) treatment was applied (Fig. 2). It was not relevant todilute these samples, as it is routinely done for other compounds,

298 K. (b) Photographs of tubes containing Na1, Na2, C1 and C2 at 298 and 77 K.

F. Robert et al. / Inorganica Chimica Acta 380 (2012) 104–113 107

because matrix effects can dramatically modify the keto-enol equi-librium that is effective for N-salicylidene aniline molecules[17,24–27], thus preventing any reliable comparative study. Forboth salts, an important contribution, coming from the enol form,is observed from UV to �425 nm. This broad contribution appearsin Na1 with a bathochromic shift of �19 nm with respect to Na2.An intense band is also observed in the visible range, from 425to �530 nm, that is due to a significant amount of the cis-ketoform, which appears to be only present as a trace in Na2 at298 K (Fig. 2). This assignment agrees with reported band rangeson diluted samples of N-salicylidene aniline [1]. The intensity ofthe cis-keto band is much lower than the enol band, this latterband being expected to decrease with temperature due to a ther-mally induced proton transfer (Fig. 1). UV irradiation of Na1 andNa2 did not change the diffuse reflectance spectrum which con-firms the absence of trans-keto form [16] and photochromism atroom temperature for these salts.

The diffuse reflectance spectrum of C1, also displayed in Fig. 2,contains three intense bands which can be attributed to the enolform (in the UV range), the cis-keto form (to approximately530 nm) and to a broad charge transfer band (between phenan-throline and copper ion, kmax �637 nm). The signal of the cis-ketoform is more intense than in Na1. The green colour of C1, whichoriginates from the bands observed in the visible range of theKM spectrum, is maintained on cooling to 77 K (Fig. 2). The diffusereflectance spectrum of C2 is very similar to the one of C1 withthree intense non-resolved bands: the enol and cis-keto bands(from UV to approximately 570 nm) and a broad metal to ligand

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Fig. 3. Temperature dependent emission spectra of pure solid samples of Na1 (a) and Nawith a kexc = 350 nm. The arrows indicate the major a, b, c and d emission bands as disc

charge transfer band (kmax �647 nm). No thermochromism is ob-served on cooling to 77 K too for C2.

5.3. Emission spectroscopy

Solid state emission spectra were recorded for Na1, Na2, C1 andC2 at kexc = 350 nm, which fall in the band of the enol form in dif-fuse reflectance spectra (see Fig. 2). These emission spectra werealso recorded on pure samples in order to avoid matrix effectwhich can strongly modify the population of keto/enol forms[17,24–27]. Temperature dependent fluorimetry analyses werecarried out on warming from 77 to 300 K in order to study theinfluence of temperature on the photochemical processes. Na1 pre-sents an emission spectrum in agreement with literature data ofthis class of molecules (Fig. 3a) [30]. A major broad contribution(b, kmax = 547 nm) is observed between �490 and �650 nm overthe whole investigated temperature range, which is assigned tothe radiative relaxation of the excited cis-keto⁄ form (pathway Bin Fig. 4a). A sharper band (a, kmax = 529 nm), attributed to a cis-keto⁄ planar conformation [30], is also noted at 77 K but disappearon warming. The temperature dependent emission spectrum alsocontains a weak, badly resolved contribution between �370 nmand 490 nm (c and d), which is attributed to the radiative relaxa-tion of the excited enol⁄ form [15] (pathway A in Fig. 4a). This con-tribution grows at higher temperatures compared to the cis-ketoband. As the radiative emissions are temperature independent, thisunexpected temperature dependent behaviour could result from:(i) the presence of less reactive enol conformers that could

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2 (b), C1 (c), C2 (d) recorded on warming over 77–300 K and 77–270 K respectively,ussed in the text.

Reaction coordinate

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Fig. 4. Photochemical process evidencing two radiative relaxation pathways inemission mode (a) or two absorption pathways in excitation mode (b) for N-salicylidene aniline derivatives.

108 F. Robert et al. / Inorganica Chimica Acta 380 (2012) 104–113

decrease the proton transfer efficiency in the excited state (whichwould lead to a smaller population of the cis-keto⁄ form, Fig. 4a)thus affording less cis-keto⁄ emission. (ii) the presence of enol con-formers for which the excited state proton transfer reaction, be-tween enol⁄ and cis-keto⁄ forms, does not occur at all.

A similar profile was found for Na2 with an intense emissioncentred at approximately 543 nm, attributed to the radiative relax-ation of the cis-keto⁄ form, and a weak emission approximatelycentred at 470 nm, attributed to the radiative relaxation of the en-ol⁄ form (Fig. 4b). Similar conclusions were made with the emis-sion spectra at 400 nm (corresponding to the excitation of thecis-keto form) where broad emission bands, attributed to the radi-ative desexcitation of cis⁄, are observed at kmax of about 578 nm forNa1, and 553 nm for Na2. In all cases, bands appear to be verybroad which could be due to several N-salicylidene anils conforma-tions, closed in energy in the ground and excited states, as sug-gested by Ref. [27]. Temperature dependent fluorescenceexperiments over the range 77–270 K support this hypothesis be-cause a modification of the emission bands shape is observed(Fig 3b). A stabilisation of some conformations with respect to oth-ers leading to a change in emission intensities is assumed. More-over the presence of various emission maxima in the lowesttemperature spectra (from 130 to 77 K) in the cis-keto⁄ emissionrange suggests the formation of a vibrational substructure.

Temperature dependent emission spectrum of C1 looks different(Fig. 3c). An intense contribution of the radiative relaxation of thecis-keto⁄ form is clearly identified between �490 and �650 nm(kmax = 530 nm at 300 K) (a and b) (pathway B in Fig. 4a). A hypso-chromic shift of about 17 nm is observed between the cis-ketoemissions in Na1 and C1. This difference can be explained by themodification of the supramolecular network around anions butalso by the change of the medium polarisability induced by thereplacement of the sodium cation by the inorganic copper com-plex. The emission spectrum of C1 also contains an intense contri-bution, between �370 and �490 nm, coming from the radiativerelaxation of enol⁄ form of the anion (pathway A in Fig. 4a). Thisemission band appears to be constituted by two badly resolvedcontributions (kmax = 418 nm (c) and 435 nm (d), at 210 K). The ori-gin of these emissions could come from the presence of two differ-ent excited states conformations, as seen for the cis-keto⁄ form. Thetemperature has also a large impact on the proportional intensityof the enol⁄ and cis-keto⁄ emissions. High temperatures strongly fa-vour the emission of the enol⁄ form at the expense of the emissionof the cis-keto⁄ form for C1. It suggests that the energy of both enol⁄

and cis-keto⁄ forms are very similar in C1, in contrast to Na1 as wereported for N-salicylidene aminopyridines [15]. Similar observa-tions can be done for C2 where the emission from the enol⁄ and

cis-keto⁄ forms are also observed but with significant maximumemission wavelengths shifts (536, 580, 413 and 435 nm for a, b,c and d, respectively) (Fig. 3d). This shift could be explained eitherby a slight modification of steric hindrance, which is however notobserved by X-ray crystallography (see below), or by the slightpolarity change of the molecular environment induced by thereplacement of CuII by NiII complexes. Indeed, medium polarisabil-ity (and polarity) has already been reported to have a strong influ-ence on the optical behaviours of N-salicylidene anils [23].

5.4. Excitation spectroscopy

The thermo-induced tautomerizations of the Na1 and Na2 an-ions as well as on complexes C1 and C2 were followed by tempera-ture dependent fluorimetry in excitation mode from 77(1) K toroom temperature (Fig. 5). We did not use diffuse reflectance spec-troscopy to track the tautomerization [1] because of the very elab-orate diffuse reflectance spectrum of C1 and C2 (Fig. 2), that unableto fully resolve the enol and cis-keto bands at room temperature forthe hybrids. Instead, we have focused on the cis-keto⁄? cis-ketoemission pathway (B in Fig. 4a) and did not consider the enol emis-sion because its absorption band is known to not vary very muchwith temperature [1]. Thus, the emission wavelength was fixed at�550–560 nm. For Na1, a signal ranging from UV to �430 nm isattributed to the absorption of the enol form (kmax = 384 nm) andits associated fluorescence (Fig. 5a). This signal is very intense be-cause the enol form should be present in majority compared tothe cis-keto form (Fig. 2) and this situation is expected to be muchfavoured on cooling. Thus, pathway A on Fig. 4b is favoured. Onwarming, it is seen that the intensity of the enol form decreasesas expected with the growing of the cis-keto form. Actually, thecis-keto absorption range (from�430 to �520 nm) shows no signalat low temperature (77–150 K). This form appears at 170 K andslowly increases with temperature to reach an intensity value of�27000 at 250 K (Fig. 5a). The proton transfer (leading to theenol/cis-keto tautomerization) at high temperature is thus clearlyobserved. A decrease of the proton transfer efficiency in the excitedstate or the presence of other non radiative relaxation pathwayscould also explain the decrease of the enol excitation band. A sim-ilar decrease of the excitation signal was observed for Na2 in the UVrange. Moreover, in the visible range, an increase of the excitationsignal with temperature could also be distinguished (Fig. 5b).

Temperature dependent excitation spectra were also recordedon pure samples of C1 and C2 (Fig. 5c and d). Using excitation spec-tra allow to hide the broad charge transfer band observed in diffusereflectance spectra (Fig. 2). The enol excitation band is clearly iden-tified (kmax�400 nm) but appears, for compound C1, to be shifted of�16 nm to lower energies in comparison to Na1. The enol excitationband decreases on warming from 77 K, for both C1 and C2, whichcould be due to a smaller population of the enol form in the groundstate (Fig. 4) and to other reasons invoked above. As for Na1, theexcitation band intensity of the cis-keto form (kmax �490 nm) sur-prisingly increases with temperature whereas non radiative relax-ation is expected to increase with temperature. This phenomenoncan only be explained by an increase of the cis-keto population in-duced by the tautomeric equilibrium. Thus, these excitation spectrademonstrate that the tautomeric equilibrium is still observed in thehybrids C1 and C2 thanks to the cis-keto excitation band that growson warming from 77 K to room temperature.

5.5. Crystallography

5.5.1. Crystal structures of [Cu(phen)3](1)2 (C1) and [Ni(phen)3(1)2

(C2)The asymmetric part of the unit cell (Fig. 6) shows CuII and NiII

mononuclear complexes, respectively, with a distorted octahedral

Fig. 6. ORTEP view of the asymmetric part of the unit cell of C1 (a) and C2 (b), showing 50% probability displacement ellipsoids.

1 104

3 104

5 104

7 104

9 104

300 350 400 450 500 550

78 K90 K110 K130 K150 K170 K190 K210 K230 K250 K

Inte

nsity

(a.u

.)

λ [nm]

Enol

Cis-keto

4 104

8 104

1.2 105

1.6 105

2 105

2.4 105

2.8 105

3.2 105

300 350 400 450 500 550

77 K90 K130 K110 K170 K190 K210 K230 K250 K270 K300 K

Inte

nsity

(a.u

.)

λ [nm]

Enol

Cis-keto

0

1 105

2 105

3 105

4 105

5 105

6 105

7 105

300 350 400 450 500 550

77 K90 K110 K130 K150 K170 K190 K210 K230 K250 K270 K

Inte

nsity

(a.u

.)

λ [nm]

Enol

Cis-keto

0

5 104

1 105

1.5 105

2 105

2.5 105

3 105

3.5 105

300 350 400 450 500 550

80 K110 K130 K170 K190 K210 K230 K250 K270 K300 K

Inte

nsity

(a.u

.)

λ [nm]

Cis-keto

Enol

(a)

(b)

(c)

(d)

Fig. 5. Temperature dependent excitation spectra of pure solid samples recorded on warming from 77 to 300 K for Na1, kem = 560 nm (a); Na2, kem = 550 nm (b); C1,kem = 550 nm (c) and C2, kem = 535 nm (d).

F. Robert et al. / Inorganica Chimica Acta 380 (2012) 104–113 109

coordination sphere built with three phenanthroline ligands(Table 1). The structure contains two non coordinated N-salicylid-ene anil anions per metal ion. The anions are in the enol form andgenerated by symmetry thanks to the C2/c space group (the metalions are localised on the twofold axis). The enol form is confirmedby single and double bonds (C15–O1 = 1.341(3) Å and 1.344(2) Å,C21–N11 = 1.278(3) Å and 1.282(2) Å, N11���H1 = 1.88 Å and1.73 Å, O1–H1 = 0.84 Å and 0.96 Å, for C1 and C2, respectively)[31]. The molecule appears to be almost planar with a dihedral an-gle between aromatic rings of 13(1)� and 12(1)�, for C1 and C2,

respectively. The anions in C1 and C2 seem to be similar in anglesand bond lengths (except for the H-bonds O1–H1���N11). The dis-tances between the centroid of the anions and the nearest metalion (A���M = 5.79(1)–Å and 5.74(1) Å for C1 and C2, respectively)are relatively close. The formation of a dense supramolecular net-work involving the anions and the copper complexes in hydrogenbonding, p–p and CH���p interactions is favoured. The crystal pack-ing (Fig. 7), organized by planes which contain either the sulfonategroups or the organic part of anions and the metal complexes, issimilar for C1 and C2. It is governed by the combination of strong

Table 1Selected distances and angles in C1–4.

C1 C2 C3 C4

T (K) 120(2) 120(2) 120(2) 250(2) 120(2)M���M (Å) 10.81(1) 10.75(1) 9.09(1) 10.34(1) 9.15(1)Anion���M (Å)a 5.79(1) 5.74(1) 5.07(1) 5.12(1) 5.05(1)U1

b (�) 13(1) 12(1) 32(1) 32(1) 32(1)C1–N2 (Å)c 1.431(3) 1.427(2) 1.418(5) 1.418(6) 1.407(3)N2–C3 (Å)c 1.278(3) 1.282(2) 1.284(5) 1.276(6) 1.268(3)C3–C4 (Å)c 1.456(3) 1.453(3) 1.453(6) 1.456(7) 1.439(3)C4–C5 (Å)c 1.419(3) 1.417(3) 1.412(6) 1.411(6) 1.415(3)C5–O6 (Å)c 1.341(3) 1.344(2) 1.350(5) 1.350(6) 1.347(3)O6–H7 (Å)c 0.84 0.96 0.82(6) 0.82(6) 0.83(4)H7���N2 (Å)c 1.88 1.73 1.87(7) 1.84(7) 1.85(3)H7���Hb (Å)c 2.12 2.22 2.22(7) 2.25(7) 2.30(4)M–N1 (Å) 2.024(2) 2.080(2) 2.136(4) 2.146(4) 2.085(2)M–N10 (Å)d 2.170(2) 2.094(2) 2.067(4) 2.107(4) 2.108(2)M–N101 (Å) 2.189(2) 2.099(2) 2.240(3) 2.194(4) 2.087(2)M–N110 (Å)d – – 2.117(4) 2.122(4) 2.061(2)M–N201 (Å) – – 2.042(4) 2.073(4) 2.088(2)M–N210 (Å)d – – 2.226(4) 2.178(5) 2.081(2)P

(�)e 72(1) 65(1) 74(2) 80(2) 62(1)

a Distance between M and the centroid of 1 or C1.b Dihedral angle between phenolic and benzoic rings of 1 or C1.c Atoms numbering in respect with Scheme 1.d N10, N110 and N210 are called N4, N104 and N204 respectively in the case of C3

and C4.e Distortion parameter following the equation

P¼P12

i¼1ðjui � 90jÞ where ucorresponds to N–M–N angle.

110 F. Robert et al. / Inorganica Chimica Acta 380 (2012) 104–113

electrostatic interactions and supramolecular interactions such asH-bonds and p–p stacking.

5.5.2. Crystal structures of [Cu(phen)3](2)Cl�5H2O�2CH3CH2OH (C3)and [Ni(phen)3](2)Cl�7H2O�CH3OH (C4)

[Cu(phen)3](2)Cl�5H2O�2CH3CH2OH (C3) crystallizes in the tri-clinic Pı̄ space group. At 120(2) K and 250(2) K, the asymmetricpart of the unit cell (Fig. 8a) contains a CuII mononuclear complexbuilt with three bidentate phen ligands. The metal ion is sur-rounded by six nitrogen atoms forming a distorted octahedral

Fig. 7. Projection of the crystal structure

coordination sphere as shown in Table 1. The complex is sur-rounded by one anion 2 and by a chloride ion which ensure theelectro-neutrality of this material. The distance 2���Cu is rathershort (5.07(1) and 5.12(1) Å at 120(2) and 250(2) K respectively).2 is observed in the twisted enol form (U = 32(1)� which is higherthan the angle obtained for anion 1 in C1 and C2, U = 12(1)�) asindicated by the O–H distance (0.80(6) and 0.83(6) Å), the singlebonds C3–C4 (1.453(6) and 1.456(7) Å) and C5–O6 (1.350(5) and1.350(6) Å) and the double bond N2–C3 (1.284(6) Å and1.277(6) Å at 120(2) and 250(2) K respectively) (Table 1) [31].Crystal structure of C3 is characterized by the presence of plentifulsolvent guest molecules (five water and two ethanol molecules,one of which presents a C01a/C01b disorder). Thanks to these mol-ecules, the packing is highly directed by the strong supramolecularnetwork built with 11 or nine H-bonds, four p–p and three or twoC–H���p interactions at 120(2) or 250(2) K, respectively. Anarrangement by planes is clearly noted (Fig. 9). Hydrophilic planes,containing chloride ions, sulfonate groups and solvent molecules,are packed with hydrophobic planes containing inorganic com-plexes and organic parts of 2. Interestingly, H-bonds, which arestrongly developed in hydrophilic planes, bind 2 to the inorganicstructure through to the sulfonate group. The temperature has avery low influence on the crystal structure of C3. A normal cell dila-tation is observed in combination with a slight decrease of thenumber of significant supramolecular interactions. As expected,no influence on the bond lengths in 2 is observed (Table 1) [14].

[Ni(phen)3](2)Cl�7H2O�CH3OH (C4), crystallizes in the triclinicP�1 space group. At 120(2) K, the asymmetric part of the unit cell(Fig. 8b) contains a NiII mononuclear built with the same architec-ture than the CuII complex observed in C3. The NiII complex is sur-rounded by one anion 2 and one chloride ion. The distance 2���Ni isshort (5.05(1) Å). 2 is observed in a twisted enol form (U = 32 (1)�)as indicated by the O–H distance (0.83(4) Å), the single bonds C3–C4 (1.439(3) Å) and C5–O6 (1.347(3) Å) and the double bond N2–C3 (1.268(3) Å) (Table 1) [31]. In C4, seven water molecules andone disordered methanol are incorporated in the structure. Thecrystal packing in C4 is similar to the one of C3 (Fig. 9) with a supra-

of C1 (a) and C2 (b) along the b axis.

Fig. 8. ORTEP view of the asymmetric part of the unit cell of C3 (a) and C4 (b), showing 50% probability displacement ellipsoids.

Fig. 9. Projection of the crystal structure packing of C3 (a) and C4 (b) along the b axis.

C

C4

O6H

5

C3

N2 C1

R2

R1

R1 = H and R2 = SO3 (1)R1 = SO3 and R2 = H (2)

Scheme 1. Molecular topology of 1 and 2.

F. Robert et al. / Inorganica Chimica Acta 380 (2012) 104–113 111

molecular architecture also built by numerous supramolecularinteractions but their number appears to be significantly higherin C4 than in C3 (same p–p stacking and C-H���p interactions but18 H bonds in contrast to 11 in C3).

6. Discussion

The preservation of the keto-enol equilibrium after insertion ofa functional non coordinated anion (1) into an inorganic frame-work, built from [CuII(phen)3] and [NiII(phen)3] complexes, wasnot a priori guaranteed. Firstly, from a synthetic point of view,the insertion has been successful as demonstrated by the absenceof both hydrolysis of the N-salicylidene derivative [32] and coordi-nation to the metal neither by the hydroxy group [5] nor by thesulfonate (as shown in Figs. 6 and 8). Secondly, although tautomer-ization in N-salicylidene anilines derivatives can occur in solution

[3,33], the situation in the solid state is different because the ma-trix can strongly influence the energy levels of the enol and cis-ketoforms [15–17,24–27], which thereby can modify optical propertiesand, in a few cases, even leads to the absence of thermochromism[15,16]. In the present hybrid materials, the thermally inducedketo-enol equilibrium could be tracked by the analysis of temper-ature dependent excitation spectra, for the first time, in the ab-sence of visible chromic properties, due to the visible absorptionof the host. This study thus demonstrates the usefulness of fluo-rimetry (in excitation mode) to follow the thermo-induced tauto-merization in such hybrids compared to diffuse reflectancespectroscopy and to single crystal X-ray diffraction that was notsuitable too, because when the cis-keto form is present in low con-centration, only the enol form is detected [14], as confirmed byvariable temperature dependent X-ray measurements performedon C3.

The influence of the insertion of a N-salicylidene aniline deriv-ative into an inorganic framework, on their optical properties wasnoted through drastic changes in the energy levels of the enol andketo forms. The Kubelka Munk spectra of C1 and C2 are indeedelaborate thanks to an intense signal in the UV range (that orig-inates from the enol form of 1 and the phenanthroline ligand),a broad band between approximately 400 and 550 nm (fromthe cis-keto form) and a very large charge transfer phenomenonbetween approximately 550 and 700–800 nm (Fig 2a). Fluores-cence measurements also show a large change induced by the

112 F. Robert et al. / Inorganica Chimica Acta 380 (2012) 104–113

incorporation of 1 in C1 and C2, with for instance a shift of themaximum wavelength of the cis-keto⁄ emission in C2. These dif-ferences between optical properties of C1 and C2, which are notedalthough the crystal structure and molecular formula are almostidentical, point out to a strong sensibility of the anion to the nat-ure of inorganic complex. In addition, variable temperature fluo-rimetry also shows that several conformations of 1 are presentin ground and excited states of the hybrid materials, leading tobroad emission bands which change their shape and nature withtemperature.

The insertion into the [M(phen)3]2+ network of anion 2 differsslightly from anion 1. Indeed, crystal structures of hybrid com-pounds C3 and C4 contain a very large number of solvent guestmolecules (until eight moles per mole of metal ion) which leadsto the formation of hydrophilic and hydrophobic planes in thecrystal packing (Fig. 9). In addition, compared to C1 and C2 thatdo not contain any solvent molecules, single crystals of C3 and C4

could not be grown in large quantity. The preparation of a singlecrystal of C3 was carried out in methanol but ethanol molecules,present as impurity, were included too as guest molecules whichconfirms that these crystals behave as molecular sponges [34].Interestingly, one anion 2 instead of two for 1, are found for C3

and C4, presumably due to the crystal packing differences governedby the sulfonate position. Anion 2 in the inorganic framework[MII(phen)3] induces a too large steric hindrance and is too widelyinvolved in its dense supramolecular network to include a secondnon coordinated organic anion in the structure, thus welcomingguest chloride ions.

The synthesis of crystalline inorganic materials, such as C1–2,where the number of inserted N-salicylidene anil anions (1) canbe controlled through the charge of the metal ion, properly vali-dates our claim to use N-salicylidene anils as thermo- or photo-sensibilisators into multifunctional materials, compared to previ-ous studies where the incorporation of the chromophores wasstatistical [17–23]. The remarkable chemical and thermal stabilityof N-salicylidene anil anions preclude future studies where thesemolecular thermo and photo-switches could be inserted in a widerange of charged materials such as cationic functional molecules(conductors, spin crossover switches and magnets, luminescentprobes, etc. . . .) or polymer supramolecular matrixes, zeolites,metal organic frameworks, coordination polymers, to name afew, which could lead to a novel generation of hybrid functionalmaterials. As a promising perspective, we have successfully in-cluded anion 2 into a trinuclear FeII spin crossover complex with1,2,4-triazole ligands [35]. Although the crystal structure of the hy-brid could not yet been obtained, the influence of the tautomericequilibrium of the anion on the complex was unambiguouslysensed by temperature dependent 57Fe Mössbauer spectroscopy[36]. Such a step opens the route towards the observation of anAnion Driven-Temperature Induced Spin Change (AD-TISC) thanksto the cis-keto equilibrium of the N-salicylidene derivative, andmost presumably to an Anion Driven-Light Induced Spin Change(AD-LISC), where the spin state of the metal center could beindirectly modified thanks to the cis–trans isomerisation of the an-ion [37].

7. Conclusion

In this work, we have been able to successfully include suitablethermochromic anil molecules into CuII and NiII complexes. In asecond step, we have studied their optical properties and discov-ered that their keto-enol tautomerization is preserved in the inor-ganic complexes, even though no thermochromism was observedfor the hybrid complexes. This insertion method paves the way

towards the synthesis of a panel of hybrid multifunctional materi-als thanks to their remarkable stability.

Acknowledgements

This work was partly funded by the Fonds National de laRecherche Scientifique-FNRS (FRFC No. 2.4508.08, IISN4.4507.10), the IAP-VI (P6/17) INANOMAT program and by a Con-certed Research Action of the ‘Communauté Française de Belgique’allotted by the Académie Universitaire Louvain. We also thank theF.R.I.A. for a doctoral scholarship allocated to F.R.

Appendix A. Supplementary material

CCDC 752244, 827696, 827697, 827698 and 827699 contain thesupplementary crystallographic data for C1, C2, C3 and C4, respec-tively. These data can be obtained free of charge from The Cam-bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this articlecan be found, in the online version, at doi:10.1016/j.ica.2011.08.040.

References

[1] T. Fujiwara, J. Hadara, K. Ogawa, J. Phys. Chem. B 108 (2004) 4035.[2] A. Plaquet, M. Guillaume, B. Champagne, L. Rougier, F. Mançois, V. Rodriguez,

J.-L. Pozzo, L. Ducasse, F. Castet, J. Phys. Chem. C 112 (2008) 5638.[3] M. Sliwa, N. Mouton, C. Ruckebusch, L. Poisson, A. Idrissi, S. Aloise, L. Potier, J.

Dubois, O. Poizat, G. Buntinx, Photochem. Photobiol. Sci. 9 (2010) 661.[4] C. Baleizão, H. Garcia, Chem. Rev. 106 (2006) 3987.[5] F. Robert, B. Tinant, R. Clérac, P.-L. Jacquemin, Y. Garcia, Polyhedron 29 (2010)

2739.[6] R.K. Parashar, R.C. Sharma, A. Kumar, G. Mohan, Inorg. Chim. Acta 151 (1988)

201.[7] Z. Cimmerman, N. Galic, B. Bosner, Anal. Chim. Acta 343 (1997) 145.[8] F.D. Jochum, P. Theato, Macromolecules 42 (2009) 5941.[9] B.L. Feringa, Acc. Chem. Res. 34 (2001) 504.

[10] S. Kawata, Y. Kawata, Chem. Rev. 100 (2000) 1777.[11] B.L. Feringa, W.F. Jager, B. De Lange, Tetrahedron 49 (1993) 8267.[12] D.S. Lo, D.M. Manikowski, M.M. Hanson, Appl. Optics 10 (1971) 918.[13] Y. Fukuda, in: Inorganic Chromotropism, Basic Concepts and Applications of

Colored Materials, Kodansha, Tokyo, Springer, New York, 2007;G.H. Brown, in: Photochromism, Wiley-Interscience, New York, 1971;H. Dürr, B. Laurent, in: Photochromism: Molecules and Systems, second ed.,Elsevier, Amsterdam, The Netherlands, 2003;J.C. Crano, R.J. Guglielmetti, in: Organic Photochromic and ThermochromicCompounds, Eds Plenum, New York, 1998.

[14] (a) E. Hadjoudis, I. Moustakali-Mavridis, Chem. Soc. Rev. 3 (2004) 579;(b) E. Hadjoudis, S.D. Chatziefthimiou, I.M. Mavridis, Curr. Org. Chem. 13(2009) 269.

[15] F. Robert, A.D. Naik, B. Tinant, R. Robiette, Y. Garcia, Chem. Eur. J. 15 (2009)4327.

[16] F. Robert, A.D. Naik, F. Hidara, B. Tinant, R. Robiette, J. Wouters, Y. Garcia, Eur. J.Org. Chem. (2010) 621.

[17] E. Hadjoudis, V. Verganelakis, C. Trapalis, G. Kordas, Mol. Eng. 8 (1999) 459.[18] P. Xue, R. Lu, G. Chen, Y. Zhang, H. Nomoto, M. Takafuji, H. Ihara, Chem. Eur. J.

13 (2007) 8231.[19] T. Kawato, H. Koyama, H. Kanatomi, K. Yonetani, H. Matsushita, Chem. Lett.

(1994) 665.[20] G. Pistolis, D. Gegiou, E. Hadjoudis, J. Photochem. Photobiol., A. Chem. 93

(1996) 179.[21] E. Hadjoudis, T. Dziembowska, Z. Rozwadowski, J. Photochem. Photobiol., A.

Chem. 28 (1999) 97.[22] (a) T. Haneda, M. Kawano, T. Kojima, M. Fujita, Angew. Chem., Int. Ed. 46

(2007) 6643;(b) Y. Inokuma, M. Kawano, M. Fujita, Nat. Chem. 3 (2011) 349.

[23] I. Casades, M. Alvaro, H. Garcia, M.N. Pillai, Eur. J. Org. Chem. (2002)2074.

[24] K. Amimoto, T. Kawato, J. Photochem. Photobiol. C 6 (2005) 207.[25] L.Z. Zhang, Y. Xiang, P. Cheng, G.Q. Tang, D.Z. Liao, Chem. Phys. Lett. 358 (2002)

278.[26] S. Sakagami, T. Koga, A. Takase, Mol. Cryst. Liq. Cryst. 373 (2002) 71.[27] M. Ziółek, I. Sobczak, J. Inclusion Phenom. Macrocyclic Chem. 63 (2009)

211.[28] G.M. Sheldrick, SHELX97: Program System for Crystal Structure Determination,

University of Göttingen, Germany, 1997.[29] A.L. Spek, Acta Crystallogr., Sect. D65 (2009) 148.

F. Robert et al. / Inorganica Chimica Acta 380 (2012) 104–113 113

[30] J. Harada, T. Fujiwara, K. Ogawa, J. Am. Chem. Soc. 129 (2007)16216.

[31] K. Ogawa, Y. Kasashara, Y. Otani, J. Harada, J. Am. Chem. Soc. 120 (1998) 7107.[32] I.R. Bellobono, G. Favini, J. Chem. Phys. 48 (1968) 5738.[33] K. Ogawa, J. Harada, T. Fujiwara, S. Yoshida, J. Phys. Chem. A 105 (2001) 3425.

[34] L.J. Barbour, Chem. Commun. 11 (2006) 1163.[35] Y. Garcia, P. Guionneau, G. Bravic, D. Chasseau, J.A.K. Howard, O. Kahn, V.

Ksenofontov, S. Reiman, P. Gütlich, Eur. J. Inorg. Chem. (2000) 1531.[36] F. Robert, A.D. Naik, Y. Garcia, J. Phys. Conf. Ser. 217 (2010) 012031.[37] P. Gütlich, Y. Garcia, T. Woike, Coord. Chem. Rev. 219–221 (2001) 839.