A thermochromic europium(iii) room temperature ionic

Showcasing research from the Photochemistry and Supramolecular Chemistry Group, Department of Chemistry, Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa, Portugal.

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A thermochromic europium(III) room temperature ionic liquid with thermally activated anion–cation interactions Rare-earth β-diketonates, one of the most popular and intensively investigated rare-earth coordination compounds, still surprise us. An observable and reversible case of thermochromism due to an unusual interaction between a tetraalkyl phosponium and the β-diketone of a europium(III) room temperature ionic liquid is reported. See João Paulo Leal, César A. T. Laia, Cláudia C. L. Pereira et al., Chem. Commun., 2017, 53, 850.

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Cite this: Chem. Commun., 2017, 53, 850 Received 25th October 2016, Accepted 29th November 2016 DOI: 10.1039/c6cc08593h

A thermochromic europium(III) room temperature ionic liquid with thermally activated anion–cation interactions† Bernardo Monteiro,a Mani Outis,b Hugo Cruz,b Joaõ Paulo Leal,*ac b b Ce ´sar A. T. Laia* and Claúdia C. L. Pereira*

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We report the first example of an observable and reversible case of thermochromism due to the interaction of an alkylphosphonium (P6,6,6,14)+ with a b-diketonate (1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dionate-fod) of an europium(III) tetrakis-b-diketonate room temperature ionic liquid. This thermochromism is characterized by the conversion of a light yellow viscous liquid, at room temperature, to a reddish substance close to 80 8C. The reversibility of this optical effect was highlighted by the thermal stability of the Eu(III) complex.

The first paper that reports ionic liquids based on lanthanide complexes dates from 2006 and refers to lanthanide-containing ionic liquids of general formula [BMIM]x 3[Ln(NCS)x(H2O)y], Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, and Yb.1 One year later the first review of the properties and applications of f-elements in room-temperature ionic liquids was published with a detailed overview of the research possibilities of this new class of compounds.2 An example present in global and modern world is the use of ILs as very effective solvents in reprocessing of spent nuclear waste, being considered as an alternative for traditional organic solvents used in liquid–liquid extraction systems.3 Generally for lanthanide, and particularly for europium, the chemistry is highly focused on their optical properties due to the sharp nature of the inner 4f transitions. Tris- and tetrakis lanthanide(III) b-diketonate complexes are the most widely studied complexes of the 4f-element series, with the weakest

a

Centro de Cieˆncias e Tecnologias Nucleares (C2TN), Instituto Superior Tećnico, Universidade de Lisboa, Campus Tecnolo´gico e Nuclear, Estrada Nacional 10, ao km 139,7, 2695-066 Bobadela, Portugal. E-mail: [email protected] b REQUIMTE, Dep. de Quı´mica, Universidade Nova de Lisboa, 2829-516 Monte de Caparica, Portugal. E-mail: [email protected], [email protected] c Centro de Quı´mica e Bioquı´mica–Faculdade de Cieˆncias, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal † Electronic supplementary information (ESI) available: Details of nuclear magnetic resonance, thermophysical analysis, electrochemical characterization, infra-red spectroscopy, ESI-MS characterization, spectrophotometry and elemental analysis. See DOI: 10.1039/c6cc08593h

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luminescence observed for the tris complexes, while Lewis base adducts give higher intensities, with the tetrakis b-diketonate complexes giving the highest luminescence intensity.4 One particular and interesting type of this bidentate oxo-ligands, that prompted us for the study presented here, are highly fluorinated b-diketones that show unique features regarding the extraction of lanthanide ions.5 The electron-withdrawing effect of the fluorine atoms reduces the charge density on the oxygen atoms and hence the complexa¨nzli and co-workers assumed tion strength to the metal centre. Bu for such ligands that fluorine contribute to the formation of a repulsive shell around europium.5 In the present work we combined 1,1,1,2,2,3,3-heptafluoro7,7-dimethyloctane-4,6-dionate (fod), a highly fluorinated betadiketone, with Eu(fod)3 to yield a symmetric tetrakis europate anion with four fod units bonded to the Eu(III) centre. The Eu(fod)4 complex, as a sodium salt, was made to react with P6,6,6,14Cl, a well known alkylphosphonium ionic liquid to yield the room temperature luminescent Eu(III) ionic liquid (RTIL), [P6,6,6,14][Eu(fod)4] (1) (experimental details in ESI†). During heating of 1 a gradual colour change was observed, from the familiar light yellow to a deep red at temperatures close to 80 1C. This phenomenon displayed visible colour reversibility, a fact that was somehow unexpected and, according with literature, nothing similar was ever reported. This fascinating effect of temperature was studied first by a thorough thermal and photophysical characterization in an attempt to clarify the structural change behind the thermochromism. The UV-vis absorption spectra of 1 at room temperature, and after heating at 80 1C for 1 hour and with the compound still hot, referred from here as compound 2 (1 heated) exhibited for both an intense band centred at 292 nm, assigned to the ligands of the europium complex and two broad bands between 500 and 600 nm for complex 2 as a reflect of its red colour (Fig. S1, ESI†). The change in the coordination sphere, attributed to a decrease of symmetry caused by heating, was evidenced by an increment in band intensity that corresponds to the 5D0 - 7F0 strictly forbidden transition at 579 nm (Fig. 1).6

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Fig. 2 FODP6,6,6,14, a keto-phosphorane like compound, 3, derived upon heating the ionic liquid [P6,6,6,14][fod] at 80 1C. Fig. 1 Partial luminescence spectrum of [P6,6,6,14][Eu(fod)4], at 25 1C (blue line) and red complex generated upon heating (red line) with excitation wavelength of 395 nm.

The total intensity of the 5D0 - 7F1 transition, dominated by the magnetic dipole, has apparently the same integrated intensity that is usually used to calibrate the intensity of the europium(III) luminescence spectra.6 The most intense hypersensitive 5 D0 - 7F2 transition maintains the intensity during heating (Fig. S2, ESI†), although a shoulder around 620 nm appears (the emission spectra are normalized at 612 nm for comparison) which is also consistent with a decrease of complex symmetry around metallic centre.7 The photophysical analysis drove us to believe that heating the light yellow viscous liquid [P6,6,6,14][Eu(fod)4], from room temperature to 80 1C, produced a asymmetric europium(III) viscous compound. Despite this assumption, the new complex geometry around Eu(III) centre didn’t explain the temperaturedependent yellow-to-red colour change. We focused the attention on possible interactions between the coutercation [P6,6,6,14]+ and the [fod] ligand, that as was reported previously for this ligand and tetrabutylphosphonium cation, [P4,4,4,4]+, produced a light pink solid after drying under low pressure at 60 1C.8 The ionic liquid [P6,6,6,14][fod] was then prepared by a simple metathesis reaction between P6,6,6,14Cl and Nafod salts and the effect of temperature on the structure evaluated. Upon heating, this colourless liquid moved, irreversibly, to a deep red viscous liquid at 80 1C, compound 3. Characterizing this compound we can, indirectly, understand the structural changes behind the reversible thermochromism of 1, since 1 and 3 evolved similarly with temperature increase. ESI/MS analysis of 3 (Fig. S11, ESI†) was fundamental to enlighten the structure behind this colour. The negative mode of ESI/MS detects the presence of the uncharged compound (FODP6,6,6,14) associated with a fod unit. The aggregation of a neutral complex and a charged specie is commonly observed in ESI-MS analysis of ionic liquids.9 Besides this strong evidence, 1D- and 2D-NMR spectroscopy in CD2Cl2 (1H, 13C, HMBC and HSQC) was consistent with the structure depicted in Fig. 2, although only discrete changes were detected upon heating of the [P6,6,6,14][fod] compound.


This conversion is associated with a rapid and irreversible change of phosphorus from a four to a five bonded atom, changing from tetrahedral to trigonal bipyramid geometry, with a pair of delocalized electron circulating between P, O and C atoms, to which we attribute the reddish colour of these compounds. Similarly to conventional tetraalkylphosphoranes of the type R4PX, where the non carbon X is stable as an anion, there is a strong tendency of 3 to dissociate to the phosphonic salt, especially in polar solvents.10 This was verified by ESI-MS analysis of a solution in methanol, where in the negative mode only the fod specie was detected. No aggregates of P6,6,6,14 FOD (3) were detected after its solubilisation in methanol. Actually dissolving this red viscous liquid in solvents like methanol or ethanol the solution had a light yellow colour, indicating that P–O bond may have been broken to yield the specie prior to heating, [P6,6,6,14][fod], although after solvent evaporation the red colour is regained. (Fig. S18, ESI†). This organic compound, a keto-phosphorane like compound presented a relevant and uncommon robustness towards heating, with decomposition starting close to 250 1C. This feature may be attributed to some level of P–O covalent bond since other phosphonium-fluorinated betadiketonates start to decompose before 200 1C, except when fod anion is used.8 Hereupon, it seemed possible that trihexyl(tetradecyl)phosphonium interaction with the Eu(III) anionic component may go beyond a simple electrostatic mechanism. Thermophysical characterization of complex 1 includes thermogravimetric analysis (TGA), specific heat capacity calculation (Cp) and differential scanning calorimetry (DSC). Fig. 3 represents the measured Cp while heating compound 1 at different rates (1 1C min 1 and 20 1C min 1). For the higher rate (purple line) the compound has no time to achieve transformation into the new specie, and thus presents a very small increase in the heat capacity (Cp). When the heating rate is 1 1C min 1 (green line), the compound has time to rearrange, to which corresponds the formation of a different inorganic Eu(III) complex, presenting a Cp with a significant increment, going from 1.67 J kg 1 K 1 at room temperature to 2.12 J kg 1 K 1 at 80 1C. The effect of temperature on partial decomplexation of a highly fluorinated betadiketonate ligand was already reported

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Fig. 3 Specific heat capacity, Cp, evolution for Eu(III) complex at two different heating rates.

by Ezequiel Wolcan and co-workers,7 that refer chemical interaction of a Cu(II) macrocycle with the dissociated fragment of fod ligand, with delocalized electrical charge between Eu(III) and Cu(II) metal centres. Furthermore, phosphonium cation fluorophilicity is also known to have cooperative effects in b-diketonate

Fig. 4 TGA results obtained for [P6,6,6,14][Eu(fod)4] for three heating/cooling cycles.

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partial release from the lanthanide centre.11 Assuming a similar effect of temperature in complex 1, together with compound 3 photophysical details and thermophysical information we can now propose the formation of the new europium(III) b-diketonate complex (Fig. 5, compound 2) bonded to the non-ionic ligand FODP6,6,6,14, a keto-phosphorane like compound with ketofunctional groups capable of establishing a bond with Eu(III) centre. Once cooled to room temperature, 2 returns to 1, as the P–O bond breaks, with negative charge centred again between the carbonyl of the b-diketone, which is capable of restoring the symmetric octacoordinated Eu(III) tetrakis-b-diketonate anion, 1. This assumption is reinforced after analysis of compound 2 absorption spectra during cooling while it turns from red to light yellow (Fig. S3, ESI†), with the broad absorption band between 500 and 600 nm almost disappearing with temperature decrease. Thermogravimetric analysis of 1 (Fig. S5, ESI†) shows that it only starts to decompose near 250 1C. Even so, a series of heating/cooling cycles were made between room temperature and 80 1C (Fig. 4). The data clearly shows that in the first cycle a small decrease in the mass can be detected (probably due to some minor amount of solvent remaining, only 0.6% mass loss) but in the subsequent cycles no variation is observed, meaning that heating and cooling does not affect the composition of the sample, just changes the conformation. A electrochemical characterization of 1 in solution was performed as a way of discarding any redox process evolving Eu(III) with temperature variation (Results and discussion in ESI†) In respect to NMR analysis of 1, the most significant variation was observed for phosphorus-31 (Fig. S9, ESI†). Increasing temperature from 25 1C to 80 1C, besides a reasonable upfield shift, the original single pick gave rise to three distinct broad peaks that correspond to different phosphorus environments. This means that a temperature increase implies changes in the coordination on P with clear modification of its chemical shift. Also that 31P is more shielded, feeling a weaker magnetic field due to increased electron density around P, consistent with a

Possible structure of the complex Eu(III) formed upon heating to 80 1C. Pictures were taken under daylight (left) and under 366 nm UV irradiation (right).

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stronger interaction between P and O atoms. Restoring temperature to 25 1C, 31P recovers the initial chemical shift of near 51 ppm. 1 H-NMR spectroscopic results present an intense decrease of signals intensity at 80 1C that may be related with an increase of the Eu(III) magnetic moment (mB) of 2.12 These signals shifting only occur when the heating rate is slow enough to yield compound 2 (Fig. S7 and S8 of ESI†). This observation is in agreement with the one previously presented respecting specific heat capacity (Cp) determination; a slow heating rate is necessary to achieve conversion of 1 in 2. Thermochromism of ionic liquids was already reported for uranyl thiocyanate complexes with 1-alkyl-3-methylimidazolium cations. Temperature-dependent yellow-to-red colour was attributed to changes in the local environment of the uranyl ion, including the coordination number, as well as to cation–anion interactions.13 Another curious feature of these type of complexes, that will be presented in further publications, was observed for the mixed b-diketonate europium ionic liquid [P6,6,6,14][Eu(DBM)(fod)3], where DBM is dibenzoylmethanate ligand. This complex doesn’t present the optical thermochromism of [P6,6,6,14][Eu(fod)4] (1) which is probably related with the electron-donating properties of the aromatic dibenzoylmethanate that may annul some of the electron-withdrawing effect of the other 3 fod units. Their outstanding photophysical properties inspire us to further investigate other Lnfod-containing ionic liquids. Combination of trihexyltetradecyl phosphonium cation with 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dionate anion provided the synthesis of not only a new organic visible light solvatochromic compound but also allowed the design of a Eu(III) switching complex with an on/off Eu–O bond, triggered by temperature variation.


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ˆncia This work has been supported by Funda¸ c˜ ao para a Cie e a Tecnologia through the grants SFRH/BPD/102705/2014 (H. Cruz), SFRH/BPD/108959/2015 (C. C. L. Pereira) and SFRH/ BPD/47087/2008 (B. Monteiro). C2TN/IST authors gratefully acknowledge the FCT support through the UID/Multi/04349/2013 project. The NMR spectrometers are part of The National NMR õ para a Cie ˆncia e a Tecnologia Facility, supported by Fundaça (RECI/BBB-BQB/0230/2012).

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