# Fluorescence-based measurement of the Lewis acidities of lanthanide triflates in solution

Paul Demay-Drouhard, Joshua R Gaffen, Christopher B Caputo, Thomas Baumgartner

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Paul Demay-Drouhard, Joshua R Gaffen, Christopher B Caputo, Thomas Baumgartner. Fluorescence-based measurement of the Lewis acidities of lanthanide triflates in solution. Canadian Journal of Chemistry, 2022, 101 (3), pp.146-153. 10.1139/cjc-2022-0115. hal-05365354

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# Fluorescence-based measurement of the Lewis acidities of lanthanide triflates in solution

Paul Demay-Drouhard, Joshua R. Gaffen, Christopher B. Caputo, and Thomas Baumgartner

Department of Chemistry, York University, 4700 Keele Street, Toronto, ON M3J 1P3, Canada

Corresponding authors: Thomas Baumgartner (email: tbaumgar@yorku.ca); Christopher B. Caputo (email: caputo@yorku.ca)

## Abstract

Despite the prominence of rare-earth complexes to act as Lewis acid catalysts in organic synthesis, the comprehensive measure of the Lewis acid strength of such compounds has yet to be performed due to incompatibilities with existing methods. We report our results in measurement of a sequence of lanthanide triflates via our recently established fluorescent Lewis adduct (FLA) method. The persistence in solution of these Lewis acids as solvated coordination complexes is accurately measurable by the FLA method. However, several of the rare-earth species exhibit fluorescence quenching, which may potentially inhibit the measurement. Nevertheless, meaningful FLA measurements were still possible, and the results correspond to both periodic trends and were even consistent with previous correlated reported data.

**Key words:** Lewis acids, lanthanides, fluorescence, dithienophosphole, catalysis

## Résumé

Malgré l'importance des complexes de terres rares agissant comme acides de Lewis catalytiques en synthèse organique, on n'a pas encore quantifié de manière précise le degré d'acidité de Lewis de tels composés en raison d'incompatibilités avec les méthodes actuelles. Nous présentons nos résultats, qui consistent en des mesures réalisées sur une séquence de triflates de lanthanides au moyen d'une méthode de fluorimétrie de l'adduit de Lewis (FLA, pour Fluorescent Lewis Adduct) que nous avons récemment mise au point. La persistance en solution de ces acides de Lewis comme complexes de coordination solvatés est mesurable avec précision par la méthode FLA. Cependant, plusieurs espèces de terres rares présentent une extinction de fluorescence qui a le potentiel de fausser les mesures. Néanmoins, il nous a tout de même été possible de réaliser des mesures significatives par FLA. Les résultats obtenus correspondaient aux tendances périodiques et concordaient même avec des données corrélées publiées antérieurement. [Traduit par la Rédaction]

**Mots-clés :** acides de Lewis, lanthanides, fluorescence, dithienophosphole, catalyse

## Introduction

Being simply defined as electron-pair acceptors, Lewis acids are key reagents in organic synthesis. Among many applications, they have proven particularly useful as catalysts and are known to enable a variety of chemical transformations to occur under very mild conditions and often with unique selectivities.¹ A diverse selection of Lewis acids is available with compositions spanning the whole periodic table, ranging from main group compounds to metal salts. Among the latter, rare-earth metal triflates [RE(OTf)₃] have been intensively used as catalysts since the beginning of the 1990s as they display numerous advantageous features.² Compared to other metal-based Lewis acids, they are stable in water, enabling organic chemical transformations to occur in aqueous medium. Catalytic loadings are usually low, and catalysts can usually be easily recovered and reused with minimal loss of activity. However, the exact nature of the rare-earth metal often plays a prominent role in the efficiency of RE(OTf)₃

catalysts. For instance, a screening of RE(OTf)₃ catalysts for a model aldol reaction gave yields varying between 8% (La) and 91% (Yb).³ Likewise, yields ranging from 24% (Pr) to 89% (Eu) were observed in a Friedel-Crafts aromatic sulfonylation reaction.⁴ The nature of the rare-earth species in bimetallic nickel catalysts was also found to govern the chemo- and regioselectivity of alkyne semihydrogenation.⁵ This complicates the optimal choice of RE(OTf)₃ catalyst for a given chemical transformation. The catalytic efficiency of RE(OTf)₃ compounds has usually been rationalized by considering their ionic radii, which decrease steadily along the lanthanide series. High catalytic activity (and thus high Lewis acidity) is then associated with species with a small ionic radius, such as Sc(III) or Yb(III).² The Lewis acidity of RE(OTf)₃ salts also correlates well with the decrease of the pKₐ of the associated metal-aquo complexes along the lanthanide (Ln) series.⁶ Nevertheless, many results do not support these trends, with reaction yields that do not correlate with the ionic radius of
the RE(OTf)₃ catalyst employed.⁷,⁸ A quantitative evaluation of the Lewis acidity of RE(OTf)₃ catalysts would thus be highly desirable to gain proper insight into their reactivities. Surprisingly, very few studies have been performed to this end to date. Prominent examples include the work of Yamaguchi and Imamoto who have compared the competitive ligand dissociation of Ln(III) compounds using tandem mass spectrometry.⁹ However, this set of data did not reflect the behaviour of RE(OTf)₃ catalysts in solution, which would be better to emulate the environment in which these compounds are used. More recently, Agapie and co-workers observed a linear relationship between the reduction potential of mixed metal oxide clusters of Mn and Ln and the pKₐ of the metal-aquo Ln complex or the radius of the Ln(III) ion.¹⁰ A similar correlation was also observed by Lu and co-workers.⁵

We recently reported¹¹ and refined¹² a new method for evaluating the relative acidity of a vast series of Lewis acids in solution. This method uses a set of luminescent Lewis basic dithienophosphole oxide probes whose emission is redshifted upon coordination to Lewis acids via their P=O functional group. We found that the chromaticity of the resulting fluorescent Lewis adducts (FLAs), assessed by their Commission Internationale de l'Éclairage (CIE) coordinates, correlates very precisely with the strength of the Lewis acids. By using the (x, y) coordinates of the CIE diagram, we were able to attribute a specific Lewis acid unit (LAU) value to more than 50 Lewis acids belonging to very different classes of compounds (neutral boranes, heavy group 13 elements, cationic and metallic species). Unlike other strategies, the FLA method delivers a solution-based measurement of the effective Lewis acidity of a species in a given environment. Consequently, this method seemed particularly well suited for assessing the Lewis acidity of rare-earth metal triflates. It is worth noting that this represents a challenging task as lanthanides generally display similar physicochemical properties and detectable changes are often minimal. The relevance of this study is also reinforced by the various applications of rare-earth phosphine oxide complexes, especially in catalysis and photoluminescence.¹³ The scope and limitations of the FLA method applied to RE(OTf)₃ compounds are the subject of this paper.

## Materials and methods

All solvents were dried with an MБraun Solvent Purification System. Flash column chromatography was performed using a SiliaFlash M60 silica gel (60–120 µm). NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer. Chemical shifts were referenced to residual non-deuterated solvent peaks (7.26 ppm for ¹H and 77.16 ppm for ¹³C for CHCl₃). Optical spectroscopy was performed on an Agilent Cary 5000 UV–vis spectrophotometer. Fluorescence spectroscopy was performed on an Edinburgh Instruments FS5 spectrofluorometer. Lanthanide triflates Ln(OTf)₃ were purchased from Strem, stored in a glovebox, and used without further purification. All dithienophosphole oxide probes except 2* were synthesized according to literature procedures.¹⁴⁻¹⁶ Probe 2* was synthesized as

follows: 2,6-dibromo-dithieno[3,2-**b**:2',3'-**d**]phosphole oxide¹⁵ (500 mg, 1.12 mmol, 1.0 equiv) and 4-tert-butylphenylboronic acid (399 mg, 2.24 mmol, 2.0 equiv) were dissolved in 30 mL toluene. To this solution, 15 mL aqueous 2 mol/L Na₂CO₃ was added, followed by Pd(PPh₃)₄ (130 mg, 0.11 mmol, 0.1 equiv). The mixture was stirred at 100 °C for 15 h and then cooled down to 20 °C. Dichloromethane was added, and the mixture was washed with water (1×) and brine (1×), dried over MgSO₄, filtered, and concentrated. Column chromatography (SiO₂, 100% CH₂Cl₂ to 9/1 CH₂Cl₂/EtOAc) followed by recrystallization from CHCl₃/acetone gave the product (433 mg, 0.78 mmol, 70%) as an orange solid. ¹H NMR (400 MHz, CDCl₃) δ 7.81 (ddd, J=13.5, 8.3, 1.2 Hz, 2H, P-Ph), 7.55 (tq, J=7.4, 1.6 Hz, 1H, P-Ph), 7.49 (d, J=8.5 Hz, 4H, tBu-Ph), 7.47–7.43 (m, 2H, P-Ph), 7.41 (d, J=8.5 Hz, 4H, tBu-Ph), 7.32 (d, J=2.7 Hz, 2H, Th), 1.33 (s, 18H, tBu) ppm. ³¹P{¹H} NMR (162 MHz, CDCl₃) δ 19.3 ppm. ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 151.8 (tBu-Ph), 148.6 (d, J=14.5 Hz, Th), 144.2 (d, J=23.3 Hz, Th), 139.1 (d, J=112.0 Hz, Th), 132.6 (d, J=2.9 Hz, P-Ph), 131.1 (d, J=11.5 Hz, Th), 130.7 (tBu-Ph), 129.9 (d, J=107.6 Hz, P-Ph), 129.1 (d, J=13.0 Hz, P-Ph), 126.2 (tBu-Ph), 125.6 (tBu-Ph), 121.2 (d, J=14.4 Hz, P-Ph), 34.9 (tBu), 31.3 (tBu) ppm. Anal. Calcd. for C₃₄H₃₃OPS₂: C, 73.88; H, 6.02; S, 11.60. Found C, 73.93; H, 6.30; S, 11.77.

The samples for the FLA method were prepared and measured as follows: The corresponding RE(OTf)₃ salt (200 or 500 equiv) was mixed with a 2.5 × 10⁻⁵ mol/L solution of a dithienophosphole oxide probe (3 mL). The resulting mixture was sonicated for 1 min to ensure complete dissolution. Its UV–vis spectrum was measured immediately, followed by its fluorescence spectrum (excitation at the maximum wavelength of absorption).

## Results

The FLA method utilizes a series of emissive dithieno[3,2-**b**:3',2'-**d**]phosphole oxide Lewis-basic probes.¹¹,¹² A single FLA measure of a Lewis acid requires measuring the emission of the corresponding Lewis acid–base adduct. The chromaticity of the emission of each adduct is plotted in CIE space where a parabolic trend can be established. This trend is compared against the chromaticity of the emission of the free phosphole oxide probes in solution. Our original reports¹¹,¹² used toluene as a solvent due to its relatively low reactivity toward strong Lewis acids; however, for measuring the rare-earth Lewis acids, tetrahydrofuran (THF) was used. THF was chosen both for its ability to solubilize the majority of the rare-earth species tested, as well as its propensity to generate a rare-earth complex with a single dithienophosphole ligand in addition to several coordinated THF molecules. To minimize potential error, a new solvent fit function was generated for THF. To generate this function, the chromaticity of the emission of each of a total of 6 probes from our previous studies was measured. As shown in Fig. 1A, the plot of the chromaticities of emissions fit very well with a parabolic function, which was then used for THF-based FLA measurements. A new probe, 2*, was also developed. Probe 2* contains the same core phosphole oxide as probe 2, but with a pendant
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** (A): CIE diagram overlaid with the parabolic fit in THF for the six dithienophosphole oxide probes. (B): Sample fit functions of each quenching, weakly quenching, and non-quenching rare-earth Lewis acid in CIE space. [Colour online.]</figcaption></figure>

<figure><img src="image_3.png" /></figure>

*p*-tert-butyl group bound to the 2,6-substituted phenyl rings. This additional weak donor affords a marginal redshift in the emission spectrum in an attempt to circumvent some of the reabsorption that leads to emission quenching in these species. The fluorescence spectra of the FLAs formed upon addition of 200 equiv of a RE(OTf)₃ salt to probes **1**, **2**, and **8** in THF were then recorded (Fig. 2, see Supplementary material for other probes). The LAU values of the RE(OTf)₃ salts were then extracted based on the fit (Fig. 1B).

The results of the measurements were, somewhat expect-
edly, impacted by the specific lanthanide being measured,
due to the ability of some of the rare-earth compounds to
quench the fluorescence.¹⁷ Nonetheless, the data could be
categorized into three general groups, demonstrated in Fig. 2:
the RE(OTf)₃ salts that are (1) non-quenching, consisting of Sc,
Y, La, Ce, Gd, and Lu; (2) partially quenching, consisting of Tb,
Dy, and Yb; and (3) strongly quenching, consisting of Pr, Nd,
Sm, Eu, Ho, Er, and Tm. The degree to which a fluorescent
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Emission spectra of probe 2 and its FLA with Eu³⁺ and Gd³⁺ in various solvents. [Colour online.]</figcaption></figure>

<figure><img src="image_5.png" /></figure>

Lewis adduct was quenched varied by choice of phosphole oxide probe, but in general the property of non-, moderately, or strongly quenching was retained. The quenching systems are, unsurprisingly, those that typically exhibited additional absorption events in the UV–vis absorption spectra (see the Supplementary material).

## Non-quenching species

Pleasingly, the FLA method can easily differentiate between elements of group 3 (Sc, Y, and La): in this case, the difference in the fluorescence of the adducts can even be seen by the naked eye (see the Supplementary material). LAU values for Sc³⁺, Y³⁺, and La³⁺ are 36.82, 31.68, and 26.67, respectively, showing a considerable decrease in terms of Lewis acidity when going down group 3. This is in accordance with the pronounced increase in atomic radii for these elements, the concomitant shielding of the atomic nucleus, and the reduction potential of rare-earth compounds, for instance in the case of mixed metal oxide clusters of Mn and Ln.¹⁰ The FLA method can also distinguish the neighbouring lanthanides La³⁺ and Ce³⁺ (LAU of 27.18 for the latter). The LAU value of Lu³⁺ (32.45) is close to the one found for Y³⁺, which is again in agreement considering the similarity of the ionic radii. With the margin of error for the FLA measurement being 0.25 LAU,¹² this means that Y³⁺ and Lu³⁺ have a measurable difference

in terms of Lewis acid strength. Interestingly, the LAU value of Gd³⁺ (27.46) is lower than expected. This may reflect the so-called gadolinium break and is in agreement with the fact that Gd(OTf)₃ displays a poor catalytic efficiency in some reactions.⁴

The LAU value for Sc³⁺ in THF (Dimroth-Reichardt Eₜ(30) parameter 37.4 kcal/mol and donor number 20.0 kcal/mol)¹⁸,¹⁹ is much higher than the one found in a solvent of similar polarity and donicity, such as Et₂O. This may be attributable to THF generating only mono-coordination complexes with rare-earth species, which does not hinder binding of the Lewis base.

## Partially quenching species

For these three compounds, a noticeable amount of fluorescence quenching can be deduced from the emission spectra of the FLAs. Nevertheless, the corresponding CIE coordinates fit the probe parabola quite well, which indicates that the LAU values are reliable and were found to be 30.40, 28.70, and 30.45 for Tb³⁺, Dy³⁺, and Yb³⁺, respectively. The reliability may be due to the fact that chromaticity is composed of hue, saturation, and intensity, and that the solution quenching may impact intensity or saturation while maintaining hue; i.e., spectral features such as emission maxima are unaffected. This, in addition to the fact that shifts in CIE coor-
**Fig. 4.** Blue bars: Lewis acidity scale (LAU values) for RE(OTf)₃ salts in THF. Orange curve: pure oxophilicity of RE(OTf)₃ salts as defined by Yamaguchi and Imamoto and obtained from tandem mass spectrometry.⁹ Full table of values is provided in the Supplementary material. [Colour online.]

<figure><img src="image_6.png" /></figure>

dinates are linear combinations of the individual changes in solution, allows us to still determine chromaticity and use it when emission quenching is minor.

## Strongly quenching species

The fluorescence of the probe is strongly quenched in the case of the remaining seven lanthanides. This translates to CIE coordinates that start to deviate from the probe parabola. Adduct formation with the Lewis base probes is still observed, due to the clear redshift in emission maxima of the probe; however, the emission intensity is significantly reduced. For this reason, the attribution of a LAU value becomes challenging. This is especially the case for Eu³⁺ (LAU = 5.81*) and Pr³⁺ (LAU = 23.53*) since their CIE coordinates show the strongest deviation from the probe parabola, meaning that the LAU value cannot be safely extracted from the chromaticity (indicated by an asterisk). The other lanthanides, whose CIE coordinates still roughly fit the probe parabola, display reasonable LAU values (28.28, 28.51, 31.25, 32.58, and 29.45 for Nd³⁺, Sm³⁺, Ho³⁺, Er³⁺, and Tm³⁺, respectively) that follow the general upward trend between La³⁺ and Lu³⁺.

## Solvent influence

In hopes of reducing the fluorescence quenching, a set of solvents (acetone, chloroform, dimethylsulfoxide (DMSO), acetonitrile, methanol, and toluene) was screened. The absorption and emission spectra of the FLAs with a strong quencher (Eu³⁺) and with a non-quencher (Gd³⁺) were recorded (Fig. 3). As the solubility of RE(OTf)₃ salts was limited in chloroform and toluene, a saturated solution was used instead. Depending on the solvent, different behaviours were observed: acetonitrile showed the same behaviour as THF, with Eu³⁺ being totally quenched, while Gd³⁺ is not. No difference in terms of absorption and emission could be detected when using DMSO and methanol: this was at-

tributed to the strong coordinating ability of these solvents, meaning that the formation of a lanthanide-solvent adduct is preferred and no FLA is formed. In fact, similar behaviour was observed with other Lewis acids for acetonitrile.²⁰ On the contrary, Eu³⁺ exhibited weaker quenching in acetone and particularly in toluene. However, the maxima of the emission spectra of the FLA of Gd³⁺ and Eu³⁺ are still very different, which leads to unrealistic LAU values for Eu³⁺. Finally, the behaviour of Eu³⁺ and Gd³⁺ was reversed in chloroform, which may be attributed to the very low solubility of the FLAs in this solvent.

## Discussion

As a whole, and despite some shortcomings, the FLA method is able to quantitatively assess the Lewis acidities of RE(OTf)₃ compounds. It is thus possible to distinguish between the minute differences of rare-earth compounds by means of Lewis acid strength. Our results also coincide with the investigation of Yamaguchi and Imamoto,⁹ suggesting that FLA measurements are fully appropriate and accurate. Indeed, in both cases, a sharp decrease in Lewis acidity between Sc and La, followed by a slow increase between La and Lu, was observed (Fig. 4). This trend also aligns well with the progression of ionic radii in the periodic table.

The main issue here is the strong fluorescence quenching of the probe, which hampers the attribution of a meaningful LAU value for the very strong quenching rare-earth elements (Eu³⁺ and Pr³⁺). The fluorescence quenching mechanism can be either static or dynamic: in the former case, a non-fluorescent ground-state complex is formed before excitation occurs, whereas in the latter, interaction of the fluorophore in an excited state with the quencher causes radiation-less deactivation to the ground state. Examination of the absorption spectra of probe 2 alone and in
**Fig. 5.** UV–vis spectra of probe 2 in THF (2.5 × 10⁻⁵ mol/L) alone and in the presence of 200 equiv Eu(OTf)₃. The shoulder at 394 nm is attributed to an f–f transition of Eu(OTf)₃. [Colour online.]

<figure><img src="image_7.png" /></figure>

**Fig. 6.** Emission spectra of probe 1 (2.5 × 10⁻⁵ mol/L) in the absence and presence of varying concentrations of Eu(OTf)₃ in THF, and Stern–Volmer plot of the fluorescence quenching of probe 1 with Eu(OTf)₃. I₀ is the fluorescence intensity without quencher (i.e., probe 1 alone), I is the fluorescence intensity with quencher (Eu³⁺), KSV is the Stern–Volmer constant, and Q is the concentration. [Colour online.]

<figure><img src="image_8.png" /></figure>

the presence of 200 equiv Eu(OTf)₃ (the strongest quencher) revealed a bathochromic shift of 14 nm induced by the addition of Eu³⁺ (Fig. 5). This strong perturbation of the absorption spectrum points toward a static fluorescence quenching.²¹

To assess the quenching quantitatively, probe 1 was titrated in THF with Eu(OTf)₃ (Fig. 6). Fluorescence intensity decreased progressively upon addition of Eu(OTf)₃ until the emission nearly disappeared. We applied the Stern–Volmer relationship²² and obtained a linear I/I₀ versus Q plot (Fig. 6). Along with the fact that the absorption spectrum of dithienophosphole oxide probes is strongly modified in the presence of Ln³⁺ salts, this supports a static fluorescence quenching mechanism. The Stern–Volmer constant (KSV = 1.66 × 10⁵ L/mol) for

Eu³⁺ is very high. Such a strong quenching ability for Eu³⁺ was already observed for other fluorophores, especially indole derivatives.²³⁻²⁷

The fluorescence quenching of aromatic systems by lanthanide ions is a known phenomenon and has been already studied for indole ring systems,²³⁻²⁷ coumarins,²⁸ porphyrins,²⁹ and fluoresceins.³⁰ Qualitatively, the emission of the solutions of 500 equiv of RE(OTf)₃ exhibited reduced intensity, and it was more difficult to measure an accurate LAU value; however, simply reducing the concentration of the lanthanide Lewis acid risks not generating a saturated solution of Lewis adduct. The use of 200 equiv of RE(OTf)₃ appears to be an acceptable balance between intensity and saturation.
## Conclusion

Lanthanide triflates were successfully measured with the FLA method, despite the propensity for several of the rare-earth complexes to quench fluorescence. Lanthanides that afforded little to no fluorescence quenching could be measured and even be readily discerned from one another. For the species that afforded more significant fluorescence quenching, an LAU value could still be generated, albeit with far less certainty. Nonetheless, the resulting effective Lewis strengths are consistent with both periodic trends and available literature measures. For the strong quenching species, spectroscopic data point toward a static fluorescence quenching mechanism. These data, and the method in general, may afford a means by which to study the fluorescence quenching of these rare-earth species, and alternatively may provide insight into a way the FLA method can handle independently fluorescent species, or fluorescence quenching species, with as high a precision as typical for FLA measurements. In addition, the use of CIE, as a linear combination of all changes to the emission in solution, means that the quenching titrations likely provide, in a future time, a means by which to separate emission or quenching events from the full emission spectrum. A preprint of this article has been published on ChemRxiv.<sup>31</sup>

## Acknowledgements

We are grateful to the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation for Innovation, Mitacs, and the Canada Research Chairs Program for funding of this work.

## Article information

### History dates

Received: 29 April 2022

Accepted: 25 May 2022

Accepted manuscript online: 23 August 2022

Version of record online: 23 August 2022

Corrected: 4 June 2024

### Notes

The article was made Open Access following publication, the license statement has been updated.

### Copyright

© 2022 The Author(s). This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.

## Author information

### Author ORCIDs

Paul Demay-Drouhard https://orcid.org/0000-0003-2270-1177

Joshua R. Gaffen https://orcid.org/0000-0002-6178-8853

Christopher B. Caputo https://orcid.org/0000-0002-7523-0324

Thomas Baumgartner https://orcid.org/0000-0001-8066-0559

## Author notes

Dedicated to the memory of Suning Wang - a good friend and pioneer of our research field.

This paper is part of a special issue to honour Professor Suning Wang.

## Competing interests

The authors declare no competing interest.

## Supplementary material

Supplementary data are available with the article at https://doi.org/10.1139/cjc-2022-0115.

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