# Stabilization of Luminescent Mononuclear Three-Coordinate CuI Complexes by Two Distinct Cavity-Shaped Diphosphanes Obtained from a Single α-Cyclodextrin Precursor

Tuan-Anh Phan, Matthieu Jouffroy, Dominique Matt, Nicola Armaroli, Alejandra Saavedra Moncada, Elisa Bandini, Béatrice Delavaux-Nicot, Jean-Francois Nierengarten, Dominique Armspach

► To cite this version:

Tuan-Anh Phan, Matthieu Jouffroy, Dominique Matt, Nicola Armaroli, Alejandra Saavedra Moncada, et al. Stabilization of Luminescent Mononuclear Three-Coordinate CuI Complexes by Two Distinct Cavity-Shaped Diphosphanes Obtained from a Single α-Cyclodextrin Precursor. Chemistry - A European Journal, 2024, 30 (7), pp.e202302750. 10.1002/chem.202302750. [https://doi.org/10.1002/chem.202302752](https://doi.org/10.1002/chem.202302752)

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# Stabilization of Luminescent Mononuclear Three-Coordinate Cu¹ Complexes by Two Distinct Cavity-Shaped Diphosphanes Obtained from a Single α-Cyclodextrin Precursor

Tuan-Anh Phan,[a, e] Matthieu Jouffroy,[b] Dominique Matt,[b] Nicola Armaroli,*[c]
Alejandra Saavedra Moncada,[c] Elisa Bandini,[c] Béatrice Delavaux-Nicot,[*d]
Jean-François Nierengarten,[*e] and Dominique Armspach*[a]

This article is dedicated to Prof. Catherine Housecroft and Prof. Edwin Constable on the occasion of their retirement

Slightly different reaction conditions afforded two distinct cavity-shaped cis-chelating diphosphanes from the same starting materials, namely diphenyl(2-phosphanylphenyl)phosphane and an α-cyclodextrin-derived dimesylate. Thanks to their metal-confining properties, the two diphosphanes form only mononuclear [CuX(PP)] complexes (X=Cl, Br, or I) with the tricoordinated metal ion located just above the center of the cavity. The two series of Cu¹ complexes display markedly different luminescence properties that are both influenced by

the electronic properties of the ligand and the unique steric environment provided by the cyclodextrin (CD) cavity. The excited state lifetimes of all complexes are significantly longer than those of the cavity-free analogues suggesting peculiar electronic effects that affect radiative deactivation constants. The overall picture stemming from absorption and emission data suggests close-lying charge-transfer (MLCT, XLCT) and triplet ligand-centered (LC) excited states.

## Introduction

Ruthenium and iridium metal complexes are nowadays widely used as photocatalysts and photosensitizers as a result of their exceptional photophysical properties, namely strong absorption in the visible range, high reduction and oxidation potentials, as well as long-lived excited states.[1] However, the scarcity of these second row transition metals and their high toxicity have prompted chemists to look for more benign and abundant first row alternatives.[2] Among all non-noble metals, copper stands

on its own since copper(I) complexes have a closed shell d¹⁰ ground state configuration that prevents them from exhibiting fast nonradiative deactivation of the excited state via low-lying metal-centered (MC) states as observed for other first row transition metal complexes.[3] Even if this is clearly a major asset, nonradiative deactivation processes are still present and need to be addressed for copper(I) complexes to compete with photoactive noble metal analogues. Tetracoordinated copper(I) complexes[4] are by far the most common ones, and absorption bands located in the visible part of their spectrum correspond

[a] Dr. T.-A. Phan, Prof. D. Armspach
Equipe Confinement Moléculaire et Catalyse
Institut de Chimie de Strasbourg, UMR 7177 CNRS
Université de Strasbourg
4 rue Blaise Pascal, CS90032
67081 Strasbourg cedex (France)
E-mail: d.armspach@unistra.fr
Homepage: http://armspach.com/

[b] Dr. M. Jouffroy, Dr. D. Matt
Laboratoire de Chimie Inorganique Moléculaire et Catalyse
Institut de Chimie de Strasbourg, UMR 7177 CNRS
Université de Strasbourg
4 rue Blaise Pascal, CS90032
67081 Strasbourg cedex (France)

[c] Dr. N. Armaroli, Dr. A. S. Moncada, Dr. E. Bandini
Istituto per la Sintesi Organica e la Fotoreattività
Consiglio Nazionale delle Ricerche
Via P. Gobetti 101, 40129 Bologna (Italy)
E-mail: nicola.armaroli@isof.cnr.it
Homepage: http://www.isof.cnr.it/armaroli_nicola/

[d] Dr. B. Delavaux-Nicot
Laboratoire de Chimie de Coordination du CNRS (LCC), UPR 8241
Université de Toulouse (UPS)
205 route de Narbonne, 31077 Toulouse cedex 4 (France)
E-mail: beatrice.delavaux-nicot@lcc-toulouse.fr
Homepage: http://www.lcc-toulouse.fr/dendrimeres-et-heterochimie-equipe-m/

[e] Dr. T.-A. Phan, Dr. J.-F. Nierengarten
Laboratoire de Chimie des Matériaux Moléculaires
Université de Strasbourg et CNRS (LIMA - UMR 7042)
Ecole Européenne de Chimie, Polymères et Matériaux
25 rue Becquerel, 67087 Strasbourg cedex 2 (France)
E-mail: nierengarten@unistra.fr
Homepage: http://nierengartengroup.com/

[f] Supporting information for this article is available on the WWW under https://doi.org/10.1002/chem.202302750

[g] © 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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to d<sub>π–π<sup>+</sup></sub> metal to ligand charge transfer (MLCT) transitions useful for photochemical applications. In the MLCT excited state, the copper(I) ion is formally oxidized to copper(II), while one of the ligands is reduced. This copper(II) cation undergoes a pseudo-Jahn–Teller flattening and the resulting transient state is prone to nonradiative deactivation, also via interaction with coordinat- ing solvent molecules.<sup>[3a]</sup> On the other hand, luminescent three coordinate Cu<sup>I</sup> metal complexes are relatively rare, even if some of them display remarkable photophysical properties.<sup>[3g]</sup> In particular, [CuX(PP)] (X=halide) complexes<sup>[5a,b]</sup> in which the PP ligand is a bulky diphosphane, may display long excited state lifetimes and unusually high photoluminescence quantum yields (PLQYs) even in solution (Figure 1).<sup>[5a,b]</sup> Their highly emissive nature is a result of their 3-coordinate geometry, which lessens the effect of Jahn–Teller distortion in the excited state. However, even in these systems, nonradiative deactivation processes may still occur if a T-shaped geometry is accessible in the excited state.<sup>[6]</sup> As for tetrahedral d<sup>10</sup> Cu<sup>I</sup> complexes, controlling the metal coordination sphere is therefore of paramount importance for improving their photophysical properties. This can be achieved sterically by using bulky ligands, notably cavity-shaped ones that can wrap themselves around the metal center. There is a clear need for designing new macrocyclic ligands capable of imposing different steric constraints on the metal to evaluate the effect of metal encapsulation on the luminescent properties of the complexes. Recently, we have demonstrated that a cavity-shaped ligand in which a 1,2-bis(phenylphosphanyl)benzene unit is embedded in a permethylated β-CD unit, can enhance the luminescent properties of a tetrahedral [Cu(NN)(PP)]<sup>+</sup> complex by limiting the coordination sphere distortion of the excited state.<sup>[7]</sup> In the present article, we describe two diphosphanes (1a and 2) with distinct inner-cavity environments derived from the smaller α-CD that can be elegantly synthesized from the same starting materials using slightly different reaction conditions. We then

demonstrate their ability to form exclusively mononuclear trigonal [CuX(PP)] complexes by ensuring that the CuX unit (X = halide anion) remains inside the CD cavity. Finally, we provide an analysis of the electrochemical and photophysical properties of the new Cu<sup>I</sup> complexes.

## Results and Discussion

Diphosphane **1a** was synthesized in a fashion similar to that for the β-CD analog **3a**.[7] The synthesis takes advantage of a Smiles-like rearrangement of diphenyl(2-phosphanylphenyl)phosphane (4) in the presence of excess n-BuLi. The 1:1 mixture of the two resulting diastereomeric dianions **5a,b** (Scheme 1) was reacted with α-CD-derived dimesylate **6a** in THF to afford diphosphane-capped CDs as a 6:4 mixture consisting of, respectively, the desired confining cis diphosphane **1a** (Scheme 2) and its two *trans* diastereomers **1b,c** (Figure S2). As for the β-CD series, a single cis diastereomer was detected. Notably, heating the mixture of diphosphanes in boiling mesitylene (160 °C) for 4 h converted most of the unwanted *trans* diastereomers **1b,c** into the desired cis diastereomer **1a**, which obviously is sterically much more favored than the **1b/1c** pair (1a/(1b + 1c) ratio = 24). It is worth mentioning that these observations contrast with those made

<figure><img src="image_4.png" /><figcaption>Scheme 1. Synthesis of dianions 5a,b from diphosphane 4 with excess n-BuLi.</figcaption></figure>

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

<figure><img src="image_6.png" /><figcaption>**Figure 1.** P,P ligands present in luminescent [CuX(PP)] complexes.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Scheme 2. Synthesis of diphosphanes 1a, 2 and 3a.[9]</figcaption></figure>
<figure><img src="image_8.png" /></figure>

for the β-CD series (3a/(3b+3c)) ratio = 1.5; 3b,c: trans diastereomers of 3a). Remarkably, the two trans diastereomers 1b,c could be easily distinguished from 1a in the ³¹P NMR spectrum of the mixture since, for the latter, the close spatial proximity of the phosphorus lone pairs causes ³Jₚₚ' values to be rather large in the AB system (³Jₚₚ' = 176 Hz), whereas in the formers, all phosphorus atoms resonate as singlets (Figure S2).[8] Finally, residual amounts of 1b,c were separated from the targeted cis counterpart 1a by column chromatography to afford pure 1a in 47% yield.[9]

Pure diphosphane 2 was also synthesized from dimesylate 6a (Scheme 2) by totally avoiding the Smiles-like rearrangement of 4. This could be achieved by using excess lithium diisopropylamide (LDA) instead of n-BuLi as a base, which does not lead to the rearranged dianions 5a,b, but instead, gives a phosphide nucleophile, most probably a monoanionic species given the lower pKₐ of LDA. Such monoanion can undergo a first nucleophilic substitution with dimesylate 6a, and the resulting CD secondary phosphane can then cyclize upon deprotonation by the remaining LDA to give the P-bridged diphosphane 2 in 57% yield (Scheme 2). Similar cyclizations have been previously performed with phenylphosphane[10] and N,N-dimethyl-2-phosphanylaniline,[11] which, unlike 4, do not rearrange in the presence of excess n-BuLi. As for the latter primary phosphanes, the cyclization reaction is 100% stereoselective, the second possible diastereomer in which the phosphorus lone pair is pointing outside the cavity was not detected in the reaction mixture.

Moreover, using other strong bases for deprotonation of 4 (t-BuLi, PhLi, lithium bis(trimethylsilyl)amide) led to either significant amounts of rearranged products 5a,b, even in the presence of only 2 equiv. of base or the formation of small amounts of a tetraphosphane side product (Table S1 and Figure S1). Although some of the P(III) atoms of 1a and 2 are seemingly quite basic (δ₃₁p = -27.2 and -19 ppm for 1a and -28.7 and -14.7 ppm for 2), these ligands are considerably more stable against air oxidation than their PhP-bridged counterpart (δ₃₁p = -16.2 ppm) and can be purified by standard column chromatography as in the case of an analogous cavity-shaped P,N ligand without noticeable formation of phosphane oxides.[11] The presence of through-space PC couplings involving the triarylphosphane P-atom and C-6 carbon atoms of the glucose units C and F (T⁵Jₚ,ₐ = 7.7 Hz and 3 Hz, respectively) is in agreement with a triarylphosphane unit sitting just above the CD unit (Figure S4e). These rare through-space couplings usually occur in rigid and constrained systems and reveal the close proximity between phosphorus and carbon atoms.[12] As for 1a, the ³Jₚₚ' coupling is rather large (³Jₚₚ' = 167 Hz), reflecting some degree of overlap between the phosphorus lone pairs (Figure S4b). Clearly, both P donor atoms in 2 are ideally placed for chelating a metal center within the α-CD cavity. Both ligands 1a and 2 have a cavity big enough to host a CuX unit with variable halide size. Indeed, the reaction between either 1a or 2 with copper halides (X = chloride, bromide, or iodide) afforded respectively the mononuclear Cu¹ complexes 7a-c and 8a-c in solution (Scheme 3). The DOSY diffusion coefficients of both complexes (Figures S6g and S9g) turned out to be very close to

those of their respective ligands **1a** and **2** (Figures S3i, S4k), which is a clear indication of the absence of any dinuclear species comprising bridging halides[13] or tetrahedral, homoleptic [[Cu(PP)₂]⁺ complexes[5d,14] that usually form when the metal center is not sterically protected. This result also points to a CD-encapsulated CuX unit that is prevented from interacting with a second metal center. As expected, cis-chelation of Cu¹ by either diphosphane causes the PP coupling constant to increase significantly (260 Hz ≤ Jₚₚ ≤ 308 Hz vs. 176 Hz and 167 Hz for ligands **1a** and **2**, respectively) and all through-space couplings to disappear. None of the inner cavity H-5 protons of either ligand undergo significant downfield shifts upon complexation. This means that the halido ligand does not interact in a significant way with inner-cavity protons in solution.[15]

X-ray diffraction (XRD) studies on single crystals of 7a,b, and 8b[16] confirmed the presence of a chelated metal center within the α-CD cavity for both types of complexes (Figures 2 and S25-S27). All CD cavities have a familiar circular shape, with all glucose units adopting the standard ⁴C₁ conformation. The

<caption>**Figure 2.** X-ray crystal structures of complexes **7b** and **8b** (solvent molecules and H atoms omitted for clarity; C: pale brown, O: red, P: orange, Cu: cyan, Br: dark red).</caption>

<figure><img src="image_9.png" /><figcaption>**Scheme 3.** Synthesis of trigonal Cu¹ complexes 7a-c and 8a-c from, respectively, diphosphanes 1a and 2.</figcaption></figure>
<figure><img src="image_10.png" /></figure>

molecular structures of 7a,b as well as that of 8b, show a trigonal geometry with the CuX rod pointing toward the cavity interior (angle between CD O-4 atoms and the PCuXP' planes = 26.7° for 7a and 27° for 7b). The CuX rod of 8b is more tilted toward the cavity interior (angle between CD O-4 atoms and the Cu–Br bond = 36.6°) than in 7a,b. In 7a, one of the Cu–P bonds is significantly longer than the other (2.30 Å and 2.24 Å for 7a vs. 2.26 Å for 7b). This is also the case for 8b, in which the more electron-rich phosphorus P(alk)$_2$Ar atom forms a shorter CuP bond (2.23 Å) than the one involving the PPh$_2$Ar unit (2.27 Å).[17] Moreover, the PP'CuBr unit in 8b is no longer planar as in 7a,b, the CuBr rod forming an angle of 10.5° with the PCuP' plane. In none of the complexes, the distance between the inner-cavity H-5 protons and the halido exogenous ligand is shorter than 3 Å, in agreement with solution NMR studies which also did not reveal the presence of CH...X interactions. Finally, complexes 7a,b display a coordination sphere very close to that of mononuclear bis(triarylphosphane) analogues described by Osawa.[5a,b]

The electrochemical properties of complexes 7a–c and 8a–c were studied by cyclic voltammetry (CV) and Oster young square wave voltammetry (OSWV). All experiments were performed at room temperature in CH$_2$Cl$_2$ solutions containing n-Bu$_4$NBF$_4$ (0.1 M) as supporting electrolyte, with a Pt wire as the working electrode and a saturated calomel electrode (SCE) as the reference electrode. Selected potential data for all the compounds are collected in Table 1, and the voltammograms are shown in Figures S11–S18. The oxidation potential of ligand 2 ($E_{ox}^{-1}$) is close to that found for the dppb ligand (+1.04 V),[18] under our conditions, while that of the more electron-rich ligand 1a is significantly lower (around 80 mV). Both values are close to those of triaryl-substituted phosphanes reported by Osawa,[5b] which according to their $^{31}$P chemical shifts, should have similar basicity. To the best of our knowledge, electrochemical data dealing with tricoordinate [CuX(PP)] complexes

are rather scarce, particularly their voltammograms, which have not been reported so far. Their electrochemical behavior is fairly complex (see Supporting Information) and very sensitive to the conditions. The OSWV voltammograms of all complexes exhibit a first major and broad oxidation process located in the 0.9–1.0 V range. A minor small oxidation process was also observed at +0.56 V and +0.72 V for 7c and 8c, respectively. These minor peaks are ascribed to the presence of small amounts of [(1a)Cu]$^+$ and [(2)Cu]$^+$ resulting from a dynamic coordination/decoordination of the weakly bound iodine ligand of 7c and 8c in solution.[19] For all the complexes, the first oxidation peak corresponds to an irreversible process and is likely metal-centered. It can be noted that the observed Cu$^{II}$ oxidation potentials of 7a–c and 8a–c are affected by the nature of the halido ligand. In both series, the $E_{ox}^{-1}$ values are shifted to lower values on going from Cl to Br and from Br to I. This trend is in full agreement with the decreased electronegativity of halogens down the group 7. Further examination by cyclic voltammetry suggests that the initial Cu$^{II}$-centered oxidation is likely followed by a fast partial decomplexation process of the resulting Cu$^{II}$ cation. The second oxidation ($E_{ox}^{-2}$) is therefore ascribed to the decoordinated phosphane ligand generated upon the initial metal-centered oxidation.

The absorption spectra of ligands 1a and 2 and the corresponding complexes 7a–c and 8a–c are depicted in Figure 3. Both ligands exhibit intense absorption bands in the UV region with a maximum at ~270 nm ($\epsilon=5600–6300 M^{-1} cm^{-1}$ respectively), which are entirely attributed to $\pi\to\pi^*$ transitions of the diphosphane units,[13,20] as α-CDs are

<table><caption>**Table 1.** Selected electrochemical data[a] of ligands 1a, 2, and complexes 7a–c, 8a–c in CH$_2$Cl$_2$, 0.1 M n-Bu$_4$NBF$_4$ at room temperature.</caption><thead><tr><th>Compound</th><th>Halido ligand</th><th><i>E</i><sub>ox</sub><sup>1(b)</sup></th><th><i>E</i><sub>ox</sub><sup>2</sup></th></tr></thead><tbody><tr><td>1a</td><td></td><td>+0.96 (0.41)</td><td></td></tr><tr><td>7a</td><td>Cl</td><td>+0.98 (0.43)</td><td>+1.05<sup>[c]</sup> (0.50)</td></tr><tr><td>7b</td><td>Br</td><td>+0.92 (0.37)</td><td>+1.06 (0.51)</td></tr><tr><td>7c</td><td>I</td><td>+0.90<sup>[d]</sup> (0.35)</td><td>+1.04 (0.49)</td></tr><tr><td>2</td><td></td><td>+1.02 (0.47)</td><td></td></tr><tr><td>8a</td><td>Cl</td><td>+0.99<sup>[e]</sup> (0.44)</td><td></td></tr><tr><td>8b</td><td>Br</td><td>+0.92 (0.37)</td><td>+1.09<sup>[f]</sup> (0.54)</td></tr><tr><td>8c</td><td>I</td><td>+0.91<sup>[g]</sup> (0.36)</td><td>+1.09 (0.54)</td></tr></tbody></table>

[a] Selected data refer to OSWV experiments; ferrocene was used as internal reference (Fc$^{+}$/Fc is observed at +0.55 ± 0.01 V vs. SCE). [b] $E_{ox}^{-1}$ = first major oxidation potential expressed vs. SCE ($E_{ox}^{-n}$ values in brackets are expressed vs. Fc/Fc$^{+}$). [c] shoulder of $E_{ox}^{-1}$ not easy to reproduce very accurately. [d] small additional oxidation process detected at +0.56 V. [e] In this case, $E_{ox}^{-1}$ and $E_{ox}^{-2}$ could not be distinguished. [f] Intense shoulder of the peak situated at 0.92 V. [g] minor oxidation process detected at +0.72 V.

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 3.** Absorption spectra of ligands 1a (a) and 2 (b) and their halide complexes 7a–c (a) and 8a–c (b) in CH$_2$Cl$_2$.</figcaption></figure>
<figure><img src="image_13.png" /></figure>

optically transparent in the observed spectral window.[21]
Among complexes **7a-c** and **8a-c**, only the iodine-containing ones (**7c** and **8c**) have a distinct feature at around 270 nm, corresponding to the maximum of the free ligand, but with higher $\varepsilon$ values (8760–9110 $M^{-1}$ cm$^{-1}$ respectively). All complexes exhibit two extra absorption bands with respect to their ligands, in line with reported analogues.[5a] The first one shows a maximum in the range 306–312 nm with molar absorption coefficients for **8a-c** lower ($\varepsilon$ = 3900–5220 $M^{-1}$ cm$^{-1}$) than those of **7a-c** ($\varepsilon$ = 8300–10400 $M^{-1}$ cm$^{-1}$) and with the latter exhibiting more defined and intense features. The lowest energy band is weaker, broader, and extends up to the visible region above 400 nm. The two absorption features characteristic of the complexes are similar to those observed in trigonal Cu¹ analogues. They can be assigned to a mixed transition ($\sigma \to \pi^*$ and $\pi \to \pi^*$) and to an $X \to \pi^*$ transition (where X is Cl, Br, or I).[5a,c,f,13,20] The absorption spectral shapes and intensities of the free ligands in CH₂Cl₂ do not vary significantly over time. On the contrary, the analogous spectra of the Cu¹ complexes show significant changes after 24 h, especially for the **7a-c** series (Figures S19 and S20). However, these data suggest that, within 30 min after solubilization, spectroscopic measurements can be safely made.

Upon excitation at 300 nm at room temperature in CH₂Cl₂ solution, **1a** and **2** exhibit a weak emission band at ~365 nm (Figure S24). The luminescence signal of **1a** is stronger (PLQY = 0.5%) compared to that of **2** (0.1%); both exhibit a lifetime of about 35 ns, with **2** showing a longer-lived component, which is hard to attribute. The luminescence of phosphane ligands is a scarcely investigated area of research, with only scattered papers across time.[5b,22] For instance, we have reported that POP (i.e., bis[(2-diphenylphosphanyl)phenyl]ether)) exhibits luminescence at room temperature, whereas dppb (1,2-bis(diphenylphosphanyl)benzene) does not. On the other hand, they both emit at 77 K.[18] Here, the shape of the excitation spectra of **1a** and **2** (Figure S21), in comparison with the absorption profiles, suggests that the observed luminescence is genuine, despite the very weak signal in an unfavorable spectral window (240–340 nm). At 77 K, both diphosphanes show a strong phosphorescence (Figure 4b) at 520 (1a) and 510 nm (2),

with excitation profiles (Figure S21) showing a neat signal at 280 nm. Excited state decays are best fitted with bi-exponential functions, dominated by the longer-lived component with a lifetime of around 350 µs. The luminescence spectra of the six Cu¹ complexes in CH₂Cl₂ solution at 298 K have been recorded by excitation at 380 nm, in order to avoid light absorption from any traces of free ligands that might occur. The spectra are similar in shape and width (Figure 4a). However, emission bands of complexes **7a-c** are blue-shifted with respect to those of **8a-c** (Table 2), which might underpin a lower tendency to undergo T-type distortion upon light excitation.[6] This is corroborated by the fact that **2** can more easily form T-shaped [AuCl(PP)] complexes than **1a** (see below).[23] Moreover, a trend can be observed for the **7a-c** series, where the wavelength emission maximum ($\lambda_{em}$) increases in the order: **7a > 7b > 7c**. This behavior is in agreement with other studies, showing that the maximum $\lambda_{em}$ is affected by the ligand field strength effect induced by the halogen ions in the complex ($Cl^- > Br^- > I^-$).[5a,b,13,20,24] The nature of the halide ions, which are directly bound to the copper center, notably affects the luminescent excited state. The latter has a charge transfer character, where the donor orbital can be both on the Cu center and on the halogen atom, whereas the acceptor is an antibonding orbital on the phosphane ligand (Cu$\to\pi^*$, MLCT; X$\to\pi^*$, XLCT).[5c,f] In particular, a decrease in the ligand-field strength reduces the splitting of the d-orbitals on the Cu center, leading to a larger MLCT energy and a shorter emission wavelength.[20,24d,e] The weakest ligand field occurs with the iodido ligand. Accordingly, **7c** exhibits an emission maximum at 530 nm, whereas the bands of the chloride (**7a**) and bromide analogues (**7b**) are shifted to 550 nm. In the case of the **8a-c** series, the halide ions do not significantly affect the $\lambda_{em}$ of the complex, which is centered at about 600 nm. This behavior has been explained by the predominant contribution of the phosphane or halogen donor orbitals to the HOMO.[24d,25] All six Cu¹ complexes exhibit low photoluminescence quantum yields (PLQYs) in CH₂Cl₂ solution, between 0.1 and 0.6%. Despite very low signal intensities, the excitation spectra are well-matched with the absorption profiles, corroborating the attribution.

<table><caption>**Table 2.** Photophysical data of ligands **1a** and **2** and complexes **7a-c** and **8a-c** in CH₂Cl₂ at 298 K, in frozen solution at 77 K, and in 1 wt % PMMA at 298 K.</caption><thead><tr><th rowspan="2">Complex</th><th colspan="3">CH₂Cl₂, 298 K</th><th colspan="3">CH₂Cl₂, 77 K</th><th colspan="2">PMMA (1%), K</th></tr><tr><th>λ<sub>em</sub> [nm]</th><th>PLQY<sup>[a]</sup> [%]</th><th>τ [ns]</th><th>λ<sub>em</sub> [nm]</th><th>τ [μs]<sup>[h]</sup></th><th>λ<sub>em</sub> [nm]</th><th>PLQY<sup>[e]</sup> [%]</th><th>τ [μs]<sup>[h]</sup></th></tr></thead><tbody><tr><td>1a</td><td>~365<sup>[b]</sup></td><td>0.5<sup>[b]</sup></td><td>37<sup>[f]</sup></td><td>~520<sup>[b]</sup></td><td>49 (11%) 339 (89%)</td><td>~500</td><td>14.5<sup>[c]</sup></td><td>16 (18%) 250 (82%)</td></tr><tr><td>7a</td><td>~550<sup>[d]</sup></td><td>0.1<sup>[d]</sup></td><td>38<sup>[g]</sup> (81%) 139 (19%)</td><td>~480<sup>[d]</sup></td><td>492</td><td>~528</td><td>7.5<sup>[d]</sup></td><td>247</td></tr><tr><td>7b</td><td>~550<sup>[d]</sup></td><td>0.1<sup>[d]</sup></td><td>40<sup>[g]</sup> (13%) 514 (87%)</td><td>~470<sup>[d]</sup></td><td>122 (51%) 369 (49%)</td><td>~500</td><td>6.8<sup>[d]</sup></td><td>17 (15%) 275 (85%)</td></tr><tr><td>7c</td><td>~530<sup>[d]</sup></td><td>0.3<sup>[d]</sup></td><td>30<sup>[g]</sup> (31%) 196 (69%)</td><td>~490<sup>[d]</sup></td><td>26 (34%) 293 (66%)</td><td>~480</td><td>5.1<sup>[d]</sup></td><td>11 (53%) 252 (47%)</td></tr><tr><td>2</td><td>~365<sup>[b]</sup></td><td>0.1<sup>[b]</sup></td><td>33<sup>[f]</sup> (24%) 302 (76%)</td><td>~510<sup>[b]</sup></td><td>48 (9%) 359 (91%)</td><td>~500</td><td>2.2<sup>[c]</sup></td><td>216</td></tr><tr><td>8a</td><td>~600<sup>[d]</sup></td><td>0.6<sup>[d]</sup></td><td>46<sup>[g]</sup> (5%) 221 (95%)</td><td>~488<sup>[d]</sup></td><td>501</td><td>~535</td><td>4.5<sup>[d]</sup></td><td>65 (14%) 350 (86%)</td></tr><tr><td>8b</td><td>~600<sup>[d]</sup></td><td>0.4<sup>[d]</sup></td><td>62<sup>[g]</sup> (8%) 232 (92%)</td><td>~485<sup>[d]</sup></td><td>135 (58%) 259 (42%)</td><td>~545</td><td>1.0<sup>[d]</sup></td><td>20 (7%) 260 (93%)</td></tr><tr><td>8c</td><td>~600<sup>[d]</sup></td><td>0.4<sup>[d]</sup></td><td>63<sup>[g]</sup> (11%) 215 (89%)</td><td>~480<sup>[d]</sup></td><td>39 (47%) 343 (53%)</td><td>~495</td><td>5.2<sup>[d]</sup></td><td>241</td></tr></tbody></table>

[a] Measured with respect to [Ru(bpy)₃]²⁺ as standard (PLQY = 0.028 in air equilibrated water); [b] $\lambda_{exc} = 300$ nm; [c] $\lambda_{exc} = 320$ nm; [d] $\lambda_{exc} = 380$ nm;

[e] Determined using an integrating sphere; [f] $\lambda_{exc} = 331$ nm NanoLED; [g] $\lambda_{exc} = 373$ nm NanoLED; [h] $\lambda_{exc} = 370$ nm SpectraLED.
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>**Figure 4.** Emission spectra of ligand **1a** and complexes **7a–c** (column left), and ligand **2** and complexes **8a–c** (column right) in (a) CH₂Cl₂ at 298 K, λem=380 nm; (b) in CH₂Cl₂ at 77 K, λem=300 nm for ligands and λex=380 nm for complexes; (c) in PMMA 1 wt% at 298 K, λex=320 nm for ligands and λex=380 nm for complexes.</figcaption></figure>

In rigid CH₂Cl₂ matrix at 77 K (Figure 4b), the emission bands of the six complexes are more intense and remarkably blue-shifted with respect to 298 K, i.e., by about 120 nm for the **8a–c** series (λem = ~480 nm), and 70 nm for the **7a–c** one (λem = 470–490 nm). This behavior has been observed previously in Cu¹ complexes[26] and highlights the predominance of rigidochromic effects in the glass matrix on electronic effects. The excited state lifetimes (also measured by exciting outside the free ligand absorption window) are generally biexponential, as it often happens with Cu¹ complexes with phosphane ligands.[5f,26b] At 298 K, the longer component (in the range 220–230 ns) is largely predominant for **8a–8c**. For **7a–7c**, the situation is more complicated with the shorter component nearly identical for all compounds (30–40 ns)

and the longer one rather scattered. 77 K lifetimes are much longer. The two chloride complexes **7a** and **8a** exhibit mono-exponential decays around 500 µs. In contrast, the two iodine and bromine-based systems show bi-exponential decays with similar contributions, made of a longer component in the range ca. 260–370 µs and a shorter lifetime, which, in the case of the iodido complexes is as short as 26 (7c) and 39 µs (8c). In the related bromide complexes (7b and 8b), such a component is substantially longer (122 and 135 µs).

The luminescence properties were also investigated in PMMA matrix at 298 K (Figure 4c), and the emission intensity is significantly stronger than in CH₂Cl₂ solution at 298 K. The **7a–c** complexes exhibit more intense emissions (PLQYs = 5–7%) than
<figure><img src="image_16.png" /></figure>

their **8a–c** counterparts (PLQYs = 1–5%), most likely because T-type distortion upon light excitation is facilitated in **8a–c** as revealed by the X-ray crystal structure of the [AuCl(2)] gold(I) analog of **8a** which is clearly more T-shaped than complex [AuCl(1a)] in the solid state.[23] The **7a–c** series also displays the same blue spectral shift observed in CH₂Cl₂ at 77 K, with λem increasing in the following order: **7a** > **7b** > **7c** (Cl⁻ > Br⁻ > I⁻). This trend can be rationalized by ligand field arguments, as suggested above. However, such a trend is not observed for **8a–c**, even if the two iodido complexes are the most blue-shifted and exhibit similar PLQYs at around 5%. This behavior can be ascribed to the relatively weaker ligand field and larger atomic number of the iodide ion.[24e]

Compared to analogous cavity-free trigonal [CuX(PP)] complexes studied by Osawa et al.,[5a,b] Daly et al.,[5d] and Du, Zhang et al.,[5f] the PLQYs of the systems presented here are significantly lower, whether in solution or in the solid state, and more in line with those observed by Liu, Zhong, Wang, Wong, Cong et al.[5c] On the other hand, the excited state lifetimes in amorphous films at room temperature of all of our Cu¹ complexes are about two-orders of magnitude longer than those observed by Osawa.[5a] This result might indicate a different nature of the excited state of the present system compared to Osawa's, also considering the rather different radiative constant (kᵯ = Φ/τ) observed for **7a** in solution (7.2×10³ s⁻¹), compared to Osawa's Cu¹ chlorido analog (8.8×10⁴ s⁻¹)[5a] which comprises the dtpb = 1,2-bis(o-ditolylphosphanyl)benzene ligand. It cannot be ruled out that the luminescence bands of the metal complexes presented here are - at least partially - attributable to ligand-centered (LC) triplet excited states. As a matter of fact, the onset of the 77 K phosphorescence spectra of **7a** and **8** at around 400 nm is well matched with the end of the charge-transfer absorption bands of the metal complexes. Therefore LC, MLCT and XLCT excited states are expected to be very close in energy, which makes the interpretation of the photophysical behavior particularly challenging. The complex pattern of experimental data is compelling in PMMA rigid matrix, where the emission spectrum of the ligand **1a** is fully overlapped with the bromide complex **7b**, whereas the chloride and iodide systems (**7a**, **7c**) – despite virtually identical onsets – have a substantially different spectral width and/or shape. Another interesting difference between our cavity-shaped complexes and Osawa's systems is that the former have emission maxima that tend to be red-shifted by up to 80 nm in fluid solution, and this effect is particularly marked in the case of the **8a–c** series.

The intricate picture that emerged in the present study is prompting us to deepen our investigation and carry out extensive DFT and TD-DFT studies to clarify the complex pattern of the photophysical results, also in relation to (i) the likely interplay between charge-transfer states and ligand centered triplets; (ii) the properties observed for the bare phosphane ligands, to be further investigated; (iii) the need to clarify any influence of the appended CD cage. Other d¹⁰ metal ions are also being considered in order to get an even deeper insight into the electronic and excited state properties of this intriguing family of trigonal complexes.

## Conclusions

By avoiding the previously described Smiles-like rearrangement using the weaker base LDA for the deprotonation of diphenyl(2-phosphanylphenyl)phosphane and subsequent cyclization with the α-CD-based dimesylate **6**, access to a new type of *cis*-chelating diphosphane (2) with metal-confining properties was made possible. When using the stronger *n*-BuLi base instead of LDA, the smaller α-CD analog (1a) of previously reported diphosphane **3a** was obtained with excellent stereo-selectivity. Both diphosphanes, which display somewhat different electronic properties, are able to force a single chelated metal center to reside inside the CD cavity but with a distinct steric environment around the metal. The two types of trigonal [CuX(PP)] complexes obtained give rise to different photophysical properties, but both with unusually long-lived excited states. Theoretical studies are currently in progress to rationalize these differences and have a complete elucidation of the ground and excited state properties of these intriguing classes of metal complexes that incorporate a simple trigonal coordination motif within a complex, cage-like architecture. The ability of these new cavity-shaped, *cis*-chelating P,P ligands to promote the exclusive formation of mononuclear complexes showing metal encapsulation is also currently exploited for catalytic purposes,[27] notably in ethylene oligomerization.[11]

## Experimental Section

Procedure for the synthesis of diphosphane **1a**. A solution of *n*-BuLi in hexane (1.6 M, 2.77 mL, 4.33 mmol) was added dropwise to a stirred solution of **4** (0.326 g, 1.11 mmol) in **thf** (16 mL) at –78 °C. After 10 min, the reaction mixture was allowed to reach room temperature and then kept at this temperature for 8 h under stirring. The resulting red suspension was cannulated within 15 min into a stirred solution of dimesylate **6a** (0.75 g, 0.55 mmol) in **thf** (45 mL). The reaction mixture was stirred for 12 h at room temperature. The solvent was then removed *in vacuo* and excess (LiPPh)₂Ar was protonated with MeOH (5 mL). After removal of the solvent *in vacuo*, mesitylene (20 mL) was added and the resulting solution was heated at reflux for 4 h. Removal of the solvent *in vacuo*, afforded a colorless solid, which was filtered over a short plug of silica using CH₂Cl₂/MeOH (500 mL, 90:10, v/v) as solvent. The resulting colorless residue was then subjected to column chromatography (SiO₂, CH₂Cl₂/MeOH, 97:3, v/v) to afford **1a** (yield: 0.38 g, 47%) as a colorless solid.

Procedure for the synthesis of diphosphane **2**. A solution of LDA in **thf/heptane/ethyl benzene** (2.0 M, 1.11 mL, 2.22 mmol) was added dropwise to a stirred solution of **4** (0.262 g, 0.89 mmol) in **thf** (10 mL) at –78 °C. The red solution was stirred at –78 °C for 10 min before being allowed to reach 0 °C over 20 min. The resulting red suspension maintained at 0 °C was cannulated within 5 min into a stirred solution of dimesylate **6a** (1.00 g, 0.74 mmol) in **thf** (24 mL) at 0 °C. The reaction mixture was stirred for 12 h at room temperature. The solvent was then removed *in vacuo* and excess LiPHArPPh₂ was protonated with MeOH (5 mL). Removal of the solvent *in vacuo*, afforded a colorless solid, which was filtered over a short plug of silica using CH₂Cl₂/MeOH (500 mL, 90:10, v/v) as solvent. After removal of the solvent *in vacuo*, the resulting colorless residue was subjected to column chromatography (SiO₂,
<figure><img src="image_17.png" /></figure>

CH₂Cl₂/MeOH, 97:3, v/v) to afford 2 (yield: 0.61 g, 57%) as a
colorless solid.

General procedure for the synthesis of complexes 7a-c and 8a-c
Anhydrous CuX (X=Cl, Br or I) (0.06 mmol) was added to a solution
of 1a or 2 (0.06 mmol) in CH₂Cl₂ (3 mL) under stirring. After 2 h, the
reaction mixture was filtered over Celite® and the filtrate was
evaporated under vacuum. The resulting solid was washed with
pentane to afford the corresponding [CuX(PP)] complex (93–97%)
as a pale yellow solid.

Supporting Information

Detailed experimental materials and methods as well as
supplementary figures and tables, experimental spectra and X-
ray crystallographic data are available in the Supporting
Information. The authors have cited additional references within
the Supporting Information.[28]

Acknowledgements

This work was supported by the CNR through the CNR-CNRS joint lab D10-GREEN “d¹⁰ Metal Architectures for Green Lighting, Photoredox Catalysis and Remote Sensing” and the Interdisciplinarity Thematic Institute SysChem, via the IdEx Unistra (ANR 10-IDEX-0002) within the French Investments for the Future Program. T.-A.P. further thanks the Fondation Jean-Marie Lehn (University of Strasbourg – Frontier Research in Chemistry) for his PhD fellowship. We also thank Dr. A. Sournia-Saquet (LCC) for electrochemical measurements.

Conflict of Interests

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available
from the corresponding authors, upon reasonable request.

Keywords: copper • cyclodextrin • halide • luminescence •
phosphane

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Manuscript received: August 22, 2023

Accepted manuscript online: November 23, 2023

Version of record online: December 18, 2023