# Self-Assembled Tetrahedral $[Cr^{III}_{4}L_{6}]^{12+}$ Cage Displaying Near-Infrared Spin-Flip Photoluminescence

Yating Ye, Carlos M. Cruz, Benjamin Doistau, Enrique Colacio, Claude Piguet, Juan Manuel Herrera, and Juan-Ramón Jiménez*

<figure><img src="image_1.png" /></figure>

Cite This: https://doi.org/10.1021/acs.inorgchem.4c04180

<figure><img src="image_2.png" /></figure>

Read Online

## ACCESS

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

Metrics & More

<figure><img src="image_4.png" /></figure>

Article Recommendations

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

Supporting Information

**ABSTRACT:** The thermodynamically controlled self-assembly of bis-bidentate quaterpyridine ligand, L = 2,2':5',5":2",2"'-quaterpyridine, with Cr<sup>II</sup> and subsequent oxidation to Cr<sup>III</sup> yields the first photoluminescent tetrahedral $[Cr^{III}_{4}L_{6}]^{12+}$ molecular cage. Single-crystal X-ray diffraction reveals the presence of two homochiral cages (ΛΛΛΛ and ΔΔΔΔ) in the unit cell that crystallize as a racemic mixture. Additionally, a PF<sub>6</sub> anion is observed inside the cavity, in line with isostructural cages built with Ni<sup>II</sup> or Fe<sup>II</sup>. Each corner of the polyhedron is occupied by weakly antiferromagnetically coupled {Cr(bipy)<sub>3</sub>}<sup>3+</sup> (bipy = 2,2'-bipyridine) patterns, as revealed by magnetometry. Upon light excitation in the UV-vis region, spin-flip luminescence from the <sup>2</sup>E/<sup>2</sup>T<sub>1</sub> excited states with a maximum at 727 nm (13755 cm<sup>-1</sup>) was detected at room temperature. The measured excited state lifetime of 183 µs is longer than the 102 µs recorded for the mononuclear $[Cr(bipy)<sub>3</sub>]^{3+}$ complex under anaerobic conditions, whereas the luminescence quantum yields are in the same order of magnitude and amount to 10<sup>-2</sup> %. The photoluminescence brightness, B, calculated using the maxima of the absorption spectra for both species, goes from 14 M<sup>-1</sup>.cm<sup>-1</sup> for the mononuclear compound to 90 M<sup>-1</sup>.cm<sup>-1</sup> for the tetrahedral cage. This 6-fold improvement is observed across the entire excitation wavelength range, and it is due to the incorporation of four light-harvester units in the molecular cage.

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

## INTRODUCTION

Chromium(III) polypyridyl complexes have garnered special attention in the past years in the field of photophysics and photochemistry.<sup>1–8</sup> Cr<sup>III</sup> ions present an efficient Ruby-like spin-flip emission,<sup>9</sup> arising from the <sup>2</sup>E/<sup>2</sup>T<sub>1</sub> doublet excited state to the <sup>4</sup>A<sub>2</sub> quartet ground state,<sup>10–17</sup> when they are embedded in a strong ligand field environment with a slightly distorted octahedral geometry.<sup>18</sup> This radiative transition typically occurs in the 700–800 nm range of the electromagnetic spectrum. However, recent advancements in the fine-tuning of the nephelauxetic effect in Cr<sup>III</sup> complexes have shifted this emission into the NIR-II region, opening new avenues for applications.<sup>19–21</sup> A notable characteristic of the spin-flip transition is its intraconfigurational nature, involving electronic reorganization within the t<sub>2g</sub> orbitals, which results in very sharp emissions akin to the f-f transitions in lanthanides. Additionally, these emissions are long-lived, reaching the millisecond range, due to their spin and Laporte forbidden character.<sup>22,23</sup> These unique properties have led to these complexes to be used for molecular upconversion,<sup>24–28</sup> molecular thermometry,<sup>17</sup> circularly polarized luminescence (CPL)<sup>27,29–32</sup> and photocatalysis.<sup>5,33</sup> Despite these intriguing properties, there are limited examples of Cr<sup>III</sup>-based metallosupramolecular species, likely due to the complex synthetic

procedures required by the inert nature of Cr<sup>III</sup> ions (ligand self-exchange kinetic constant: $k_{H_2O} = 2.4 \times 10^{-6}$ s<sup>-1</sup> for $[Cr(H_2O)_6]^{3+}$).<sup>34</sup> Nonetheless, remarkable examples have been reported by Winnpenny and co-workers in which they employed high-temperature solvothermal synthesis to overcome kinetic barriers leading to the serendipitous formation of supramolecular rings and metallacrown wheels.<sup>35–37</sup> Alternatively, a more rational synthetic approach has been recently developed involving the use of inert Cr<sup>III</sup> complexes as molecular building blocks to construct photoactive d–d and d–f dinuclear and trinuclear species.<sup>38–40</sup> Back at the turn of the century, Piguet and co-workers employed the labile Cr<sup>II</sup> ion to thermodynamically control the self-assembly of metallosupramolecular architectures. Subsequent oxidation of the Cr<sup>II</sup> to Cr<sup>III</sup> “locks” the assembly, allowing the obtention of inert Cr<sup>III</sup>-based helicates.<sup>24,41–43</sup> Analogous to Cr<sup>III</sup> complexes, low-spin Co<sup>III</sup> also forms kinetically inert complexes

Received: October 1, 2024

Revised: November 15, 2024

Accepted: November 20, 2024
due to the maximized crystal-field splitting stabilization, and they have been employed for the preparation of tetrahedral capsules and helicates using an “assembly-followed-by-oxidation” protocol from a CoII precursor.44,45 This approach that possesses advantages of reversible and self-healing self-assembly reactions gives structures that possess a covalent-like level of kinetic robustness. This is usually achieved by using nonlabile metal ions, most commonly the scarce and expensive 4d- and 5d-block elements,46 which become dynamic only at high temperatures. Within this context, Lusby and co-workers encapsulates the γ-emitting [99mTc]TcO4⁻ anion within the kinetically inert CoIII₄L₆ tetrahedral cage.47 *In vivo* imaging of the molecular composite shows a significant change in biodistribution compared to the free oxo-anion, suggesting the potential applications of supramolecule species in clinical treatments. Other discrete supramolecular cationic tetrahedral cages or capsules with the general formula M₄L₆, where L represents 2,2' : S', S":2",2'''-quaterpyridine and derivatives and M denotes various metal ions such as NiII, CoIII, and FeII, have been extensively studied.47–52 Metal ions occupy the vertices of the polyhedral cage, creating central cavities that enable host–guest chemistry and selective anion binding within these systems. For instance, Brechin and co-workers demonstrated some fine-tuning of the magnetic exchange interactions between a NiI₄L₆ cage and encapsulated paramagnetic tetrahalometallates.51 Other examples based on the labile FeII was shown to encapsulate selectively PF₆⁻ over BF₄⁻.49 However, the use of luminescent and inert CrIII ions to form metallosupramolecular capsules or cages remains, to the best of our knowledge, unexplored. The versatile coordination chemistry of CrIII together with its cheap character can be an alternative to luminescent supramolecular 4d-5d/4f materials53,54 and might open new avenues for the construction of luminescent cages or capsules with sensing and/or bioimaging applications.55–57 Thus, we report on what we believe to be the first synthesis and characterization of a tetrahedral CrIII₄L₆ (L = 2,2' : S', S":2",2'''-quaterpyridine) cage displaying long-lived NIR luminescence.

■ RESULTS AND DISCUSSION

**Synthesis and Structural Analysis.** The tetrahedral CrIII
cage, abbreviated as CrIII₄L₆ (L = 2,2':S',S":2",2'''-quaterpy-
ridine), was prepared in a glovebox by reacting 4 equiv of the
labile CrII (as CF₃SO₃ salt) with 6 equiv of L in dry
acetonitrile. Subsequent oxidation of CrII to CrIII was
accomplished using a stoichiometric amount of silver triflate.
After completion of the oxidation process, the mixture was
filtered to remove the reduced metallic silver particles and
concentrated under a vacuum. Diethyl ether was added to the
concentrated solution, and an orange precipitate appeared,
which was filtered and washed with dichloromethane and dried
with diethyl ether. The solid was dissolved in water and
filtered. A saturated solution of KPF₆ was added obtaining an
orange precipitate (see the SI for details). Slow diffusion of
diethyl ether into a concentrated acetonitrile solution gave
poor quality crystals after a few days but was suitable for single-
crystal X-ray diffraction. The CrIII₄L₆ species crystallized in the
rhombohedral space group R3c (Table S1). Similar to other
reported tetrahedral cages, the four vertices are occupied by
the CrIII ions, which are linked to each other through
coordinated bis-bidentate ligands (Figure 1). Each of the two
bypiridyl (bipy) units of a given ditopic ligand is twisted with
respect to the neighbor bipy unit by approximately 66°. The

<figure><img src="image_7.png" /><figcaption>**Figure 1.** (a) Schematic synthetic pathway for the preparation of the Cr<sup>III</sup><sub>4</sub>L<sub>6</sub> cage. (b) X-ray molecular structure showing the ΔΔΔΔ enantiomer of the *rac*-Cr<sup>III</sup><sub>4</sub>L<sub>6</sub> encapsulating PF<sub>6</sub><sup>−</sup>. The rest of the anions and the hydrogen atoms have been omitted for clarity. Color codes: Cr (orange), N (blue), C (gray), F (green), and P (red).</figcaption></figure>

distance between each of the Cr<sup>III</sup> centers is 9.76(3) Å, which
corresponds to an encapsulated volume of approximately 99
Å<sup>3</sup>. Each of the vertices of the assembly is formed by a
{Cr(bipy)<sub>3</sub>}<sup>3+</sup> moiety, with a typical Cr<sup>III</sup>–N bond length
range of 1.95(4)–2.06(3) Å (Table S2) and bite angles of
80.7(12)° according to the five-membered chelate ring nature
of the bipy scaffolds.<sup>34</sup> The compound crystallized as a racemic
mixture of the two homochiral ΔΔΔΔ and ΛΛΛΛ
enantiomeric cages with a PF<sub>6</sub><sup>−</sup> anion located inside the
cavity, a feature also observed in related cages (Figure 1b).<sup>51</sup>

The ¹⁹F-NMR spectrum of the cage in CD₃CN displayed broad features due to the paramagnetic and slow nuclear relaxation of Cr³⁺ ions in its ground state (Figure S1).⁵⁸ However, three signals located at -63 and -70 ppm and at -74 ppm can be distinguished (Figure S1b). The intense doublet corresponds to the noninteracting PF₆ anions in the outer sphere of the cage, and the weaker one corresponds to strongly broadened doublet that corresponds to the PF₆ anion
<figure><img src="image_8.png" /><figcaption>**Figure 2.** Temperature dependence of $\chi_M T$ (experimental: red circles; fit: blue line). Inset: $\chi_M T$ vs log T scale for highlighting the low temperature region (left) and field dependence of the magnetization at the indicated temperatures (inset). Solid lines represent the best fit to equation.</figcaption></figure>

inside the cavity. If the $PF_6^-$ anion would be in a single environment, we would expect to observe only one set of two signals due to the $^{19}$F-$^{31}$P nuclei coupling (Figure S1a). In fact, it is reasonable to infer that these environments would correspond to endo and exo $PF_6^-$ anions, a feature also observed in similar cages.⁴⁹ The high-resolution electrospray ionization mass spectrum (HR-ESI-MS) showed +5, +6 ions with masses corresponding to those calculated for successive losses of $PF_6^-$ anions from the parent species of formula $Cr_4L_6(PF_6)_{12}$ (Figure S2).

**Magnetic Properties.** The dc magnetic properties of $Cr^{III}_4L_6$ were studied in the 2–300 K temperature range with an applied magnetic field of 1000 Oe (Figure 2). The $\chi_M T$ value at room temperature of 7.75 cm$^3$.mol$^{-1}$.K matches that expected for four noninteracting $Cr^{III}$ ions with S = 3/2 and g = 2 (7.75 cm$^3$.mol$^{-1}$.K). Upon cooling, the $\chi_M T$ product remains almost constant until about 20 K and then sharply decreases to reach a value of 6.15 K cm$^3$.mol$^{-1}$.K at 2 K. The decrease of $\chi_M T$ at a very low temperature is due to zero field splitting (ZFS) single-ion anisotropy and eventually weak intra- and intramolecular antiferromagnetic interactions. The field dependence of the magnetization in the 2–6 K range is given in the inset of Figure 2. The isotherm at 2 K is almost saturated at the maximum applied field of 7 T, reaching a value of 11.94 $\mu_B$ ($M_{sat}$ = 12 $\mu_B$ for S = 3/2 and g = 2). The magnetic data were analyzed by the phenomenological approach based on the ZFS of an S = 3/2 through the following anisotropic spin-Hamiltonian:

$$ \hat{H} = -J_{ij} \sum_{i=1, j>i}^{4} S_i S_j + \sum_{i=1}^{i=4} D_i [\hat{S}_{zi}^2 - S(S+1)/3] + \mu_B \sum_{i=1}^{i=4} \overrightarrow{g_i \hat{H}_i} \hat{S}_i \quad (1) $$

where the first term represents the intramolecular magnetic exchange interactions between the $Cr^{III}$ ions and the second and third terms correspond to the single ion axial magnetic anisotropy and the Zeeman interaction, respectively. $J_{ij}$ represents the corresponding magnetic coupling constants, D is the axial magnetic anisotropy parameter, $\mu_B$ is the Bohr magneton, and H is the applied magnetic field.

The susceptibility and magnetization data were simultaneously fitted using the PHI software⁵⁹ to the above Hamiltonian. To avoid overparameterization, all the J coupling constants and all the local D values were considered equal. Moreover, a term accounting for temperature-independent paramagnetism (TIP) was also included in the Hamiltonian. A very good quality fit was obtained with the following parameters: $J = -0.058(7)$ cm$^{-1}$, $g = 2.003(1)$, $|D| = 0.58(3)$ cm$^{-1}$, and TIP = 0.0015 cm$^3$ mol$^{-1}$. The extracted parameters confirm that the magnetic exchange interaction between the $Cr^{III}$ ions is antiferromagnetic in nature and is very weak in magnitude. It should be mentioned that the low sensitivity of the powder magnetic measurements does not allow the accurate determination of the sign of the D parameter. However, the value remains in line with other $Cr^{III}$ complexes (<1 cm$^{-1}$).⁶⁰⁻⁶⁷ When either J or D were fixed to zero, the fits led to the following magnetic parameters: $g = 1.990 (2)$, $|D| = 1.14 (2)$ cm$^{-1}$, and TIP = 0.0016 (1) cm$^3$ mol$^{-1}$ and $g = 2.006 (1)$, $J = -0.072 (1)$ cm$^{-1}$, and TIP = 0.0012 (1) cm$^3$ mol$^{-1}$. Therefore, the J and D parameters are correlated and cannot be accurately determined from the magnetic data. Nevertheless, the values of J or D extracted when D or J was, respectively, fixed to zero can be considered as limits for these parameters. It is worth noting that the quality of the fit with D = 0 was slightly worse than that obtained when both J and D were allowed to freely vary. However, the fit with J = 0 is significantly worse than the other two fits, particularly below 10 K, thus indicating that J plays an essential role in the magnetic behavior of $Cr^{III}_4L_6$. Although the dipolar magnetic
interaction between metal ions with very small *D* values, like Cr<sup>III</sup>, is expected to be very weak, one might wonder if the very feeble magnetic coupling observed for Cr<sup>III</sup><sub>4</sub>L<sub>6</sub> could be dipolar in origin. To answer this question, we have estimated a value of the maximum dipolar contribution to the antiferromagnetic interaction in Cr<sup>III</sup><sub>4</sub>L<sub>6</sub> (when the magnetic anisotropic axes of two neighboring metal ions are parallel and the anisotropic axis and the Cr–Cr direction form an angle of 90°) of *J*<sub>dip</sub> = 0.002 cm<sup>-1</sup> (see the SI). This value is an order of magnitude smaller than the experimental *J* value, and therefore, the antiferromagnetic interaction is mainly due to magnetic exchange. Experimental magneto-structural studies on dinuclear Cr<sup>III</sup> complexes with bis(terpyridine)-type bridging ligands<sup>64</sup> seem to support this conclusion because the complex [(tpy)Cr(bbt)Cr(tpy)](PF<sub>6</sub>)<sub>6</sub>·8CH<sub>3</sub>CN·(CH<sub>3</sub>CH<sub>2</sub>)<sub>2</sub>O (where tpy = 2,2′:6′,2″-terpyridine and bbt = 4,4″-bis(2,2′:6′,2″-terpyridine) with a Cr···Cr distance of 10.9 Å presents a magnetic coupling constant (*J* = −2.37 cm<sup>-1</sup>) smaller than that found for the complex [(tpy)Cr(ebbt)Cr(tpy)](PF<sub>6</sub>)<sub>6</sub>·10 CH<sub>3</sub>CN (ebbt = 4,4″-(ethynyl)-bis(2,2′:6′,2″-terpyridine) of *J* = −2.73 cm<sup>-1</sup>) with a Cr···Cr distance of 13.5 Å. Therefore, the dipolar contribution to the magnetic coupling should be comparatively irrelevant in these types of complexes.

The magnetic exchange propagates across the π-system of the bridging ligand via overlap of its p(π) orbitals and the t₂g(π) orbitals of Cr<sup>III</sup> ions, probably also involving a spin-polarization mechanism. In such a mechanism, the nitrogen donor atoms hold the same spin density as the Cr<sup>III</sup> ions due to the spin delocalization in the Cr–N bonds, whereas the carbon atoms of the aromatic spacer possess an alternating sign of spin density owing to spin polarization, leading to the opposite sign of spin density on the Cr<sup>III</sup> ions (antiferromagnetic interaction) when, like in this case, there is an even number of aromatic carbon atoms involving the magnetic exchange pathway.<sup>68,69</sup> The very weak antiferromagnetic coupling found in the cage is a consequence of the long intramolecular distance between the Cr<sup>III</sup> ions (9.76(3) Å) and the dihedral angle between the bipyridine units of the bridging ligand (twist angle). It is worth noting that the extracted *J* value is of the same order of magnitude as that previously reported for the Ni₄ counterpart.<sup>51</sup> The *J* value for this latter compound is larger than that found for the Cr<sup>III</sup><sub>4</sub>L<sub>6</sub> cage, which is mainly due to the higher number of unpaired electrons and the larger twist angle for the Cr<sup>III</sup> counterpart (66° vs 44°). The fact that the *J* value for the dinuclear complex [(tpy)Cr(bbt)Cr(tpy)](PF<sub>6</sub>)<sub>6</sub>·8CH<sub>3</sub>CN·(CH<sub>3</sub>CH<sub>2</sub>)<sub>2</sub>O is much larger than that observed for the Cr<sup>III</sup><sub>4</sub>L<sub>6</sub> cage (−2.37 cm<sup>−1</sup> vs −0.058 cm<sup>−1</sup>), even though the Cr···Cr distance and the twist angle are very close for both compounds (around 10 Å and 60°), could be due to the larger aromaticity of the terpyridine moiety. A more extended delocalized π-system lowers the energy of the LUMO and increases the π-back-donation from the metal to this orbital, and the ligand becomes a better π acceptor. This leads to a decrease in energy of the Cr-centered t₂g magnetic orbitals, thus favoring their overlap with the occupied molecular orbital of the bis(terpyridine) bridging ligand and consequently the increase in the antiferromagnetic interaction. A similar increase in the antiferromagnetic magnetic interaction has been observed for bis(terpyridine) bridged complexes when end-cap anionic π-donor chloride ligands are replaced by π-acceptor terpyridine ligands.<sup>64</sup>

**Absorption and Emission Properties.** The absorption spectrum of the Cr<sup>III</sup> cage in acetonitrile shows maxima

between 300 and 350 nm (ε > 140 × 10³ M⁻¹·cm⁻¹), which can be attributed to standard intense π* ← π transitions, together with bands of lower intensities between 300 and 350 nm with (ε < 10³ M⁻¹cm⁻¹) assigned to mixed metal-centered and ligand-to-metal charge transfer MC/LMCT transitions (Figure 3). The absorption spectrum of the cage is slightly red-

<figure><img src="image_9.png" /><figcaption>**Figure 3.** Absorption spectrum and emission spectrum of [Cr<sup>III</sup><sub>4</sub>L<sub>6</sub>]<sup>12+</sup> (red solid traces) and absorption spectrum and emission spectrum of [Cr(bipy)<sub>3</sub>]<sup>3+</sup> (dashed black traces) upon excitation at 330 nm at room temperature in acetonitrile with their respective band assignation.</figcaption></figure>

shifted compared to that of the mononuclear Cr<sup>III</sup> compound, likely due to a bathochromic effect from the partially extended conjugation of the quarterpyridine ligands.

The total ε values of the cage complex are around four times more important than those of mononuclear [Cr(bipy)<sub>3</sub>]<sup>3+</sup> due to the presence of four [Cr(bipy)<sub>3</sub>]<sup>3+</sup> units per molecular entity. Additionally, a similar band shape is observed in the whole spectra for both compounds. The weak transitions located at even lower energies (450–500 nm) are ascribed to the spin-forbidden ³π* ← π transitions.<sup>28,31</sup> According to TD-DFT calculations on the model [Cr(bipy)<sub>3</sub>]<sup>3+</sup>, the bands located between 400 and 430 nm can be attributed to the spin-allowed Cr(<sup>4</sup>T<sub>2</sub> ← <sup>4</sup>A<sub>2</sub>) transition (Figures S4, S6, and S7 and Table S5). At lower energies, in the spin-flip region, no spin-forbidden ²E ← <sup>4</sup>A<sub>2</sub> and ²T<sub>1</sub> ← <sup>4</sup>A<sub>2</sub> transitions could be identified even by using highly concentrated solutions. This may be attributed to the band tail of the LMCT, which obscures the spin-flip absorptions. At room temperature and upon excitation at 330 nm, the Cr<sup>III</sup> cage exhibits a sharp emission centered at 727 nm (13755 cm<sup>-1</sup>) together with a small band located at 697 nm (14347 cm<sup>-1</sup>). The emission profile of the cage is identical to that of mononuclear [Cr(bipy)<sub>3</sub>]<sup>3+</sup>, suggesting that both systems share the same low-lying excited state landscape (Figure 3). This similarity indicates that the coordination environment and electronic structure of the chromium centers in both the cage and the mononuclear complexes are similar, leading to indistinguishable emission spectra. This implies that any differences in the physical arrangement or macroscopic properties of the cage do not affect the electronic excited states of the chromium centers. According to theoretical calculations on the mononuclear complex (CASSCF(7,12)/FIC-NEVPT2), the energies of the lowest ²E and ²T<sub>1</sub> states are similar and suggest that the lowest energy emission band could be attributed to a ²T<sub>1</sub> → <sup>4</sup>A<sub>2</sub> transition (Tables S3 and S4 Figure S5). The luminescence quantum yield (φ) of the Cr<sup>III</sup> cage and [Cr(bipy)<sub>3</sub>]<sup>3+</sup> in
CH₃CN was determined by a relative method using [Cr-(ddpd)₂]³⁺ (N,N' dimethyl-N,N' dipyridine-2-yl-pyridine-2,6-diamine) as reference (see the SI for details).¹⁶ The value obtained in aerated conditions is 0.044%, while in deaerated conditions, it increases to 0.067%. In both cases, these values are slightly higher compared to the mononuclear [Cr(bipy)₃]³⁺ (φ = 0.030% (aerated), φ = 0.057% (deaerated)). Similarly, the excited state lifetime of the Cr³⁺ cage shows a monoexponential decay with a value of τ = 79 µs (aerated) and increases almost twice to reach τ = 183 µs in a deaerated solution (Figure S3). In the case of [Cr(bipy)₃]³⁺, this value increases from 40 (aerated) to 102 µs (deaerated) (Figure S3). The longer excited-state lifetime observed in the metallosupramolecular species is likely attributed to increased rigidity, which hinders certain nonradiative relaxation pathways. Finally, the presence of four [Cr(bipy)₃]³⁺ units in the cage together with the little increase in the luminescence quantum yield led to an enhancement of the photoluminescence brightness, B (determined as B = ε·φ), that goes from 14 M⁻¹·cm⁻¹ for the mononuclear complex to 90 M⁻¹·cm⁻¹ for the cage, calculated by excitation at λ<sub>abs,max</sub> for each complex. This represents nearly a 6-fold improvement. Thus, increasing the number of chromophores within the same molecular structure can be a remedy for increasing the photoluminescence performance of poor spin-flip emitters such as the archetypal [Cr(bipy)₃]³⁺ complex.

## CONCLUSIONS

The first tetrahedral Cr³⁺ cage was prepared using an “assembly followed by oxidation” approach. Initially, labile CrII reacts with the appropriate amount of ligand, which is subsequently oxidized to produce the kinetically inert [Cr₄III·L₆]¹²⁺ cage, which encapsulates a PF₆⁻ anion as confirmed by single-crystal X-ray diffraction. The four CrIII ions located at the corners of the tetrahedron exhibit antiferromagnetic exchange interactions at low temperatures. Upon light excitation, spin-flip emission from the radiative relaxation of doublet excited states ²E/²T₁ to ground state ⁴A₂ was detected at room temperature. Both the luminescence quantum yield and the excited-state lifetime under aerobic and anaerobic conditions at room temperature are slightly higher compared with the mononuclear analogue [Cr(bipy)₃]³⁺. This improvement is attributed to the more rigid structure of the assembled cage, which prevents structurally related nonradiative relaxation pathways. Additionally, the presence of up to four [Cr(bipy)₃]³⁺ units in the cage significantly enhances the photoluminescence brightness of the cage compared to the mononuclear complex.

## EXPERIMENTAL PART

"No uncommon hazards are noted."

Elemental analyses were carried out on a Fisons-Carlo Erba analyzer model EA 1108. ¹H nuclear magnetic resonance spectroscopy (RMN) data were recorded on a 400 MHz BRUKER Nanobay Avance III HD High-Definition spectrometer, and the spectra were internally referenced to solvent signals. Absorption spectra in acetonitrile solution were recorded using a Jasco Cary (Agilent Technologies) spectrometer (quartz cell path length 1 cm or 1 mm, 290–800 nm domain, 2 × 10⁻⁴ M and 650–800 nm domain, 7.7 mM). Emission and excitation spectra were measured on a UV–vis–PTI QuantaMaster 8000 spectrofluorometer equipped with a Picosecond Photon Detector (230–850 nm, PPD-850, HORIBA Scientific) and a continuous Xenon Short Arc Lamp (190–2000 nm, USHIO). All of the spectra (emission and excitation) were corrected with real-time correction functions. TCSPC lifetime measurements

were performed using a flash lamp (1 µs pulse, HORIBA Scientific). All emission quantum yields were measured according to a relative method using the [Cr(ddpd)₂]³⁺ as reference (λ<sub>exc</sub> = 435 nm; φ = 11% in deaerated acetonitrile; estimated uncertainty ±10%). Deaerated solutions were prepared in a 1 cm cuvette inside a glovebox filled with Ar. Variable-temperature (2–300 K) magnetic susceptibility measurements were carried out on polycrystalline samples under an applied field of 1000 Oe using DynaCool PPMS-9 physical measurement equipment. High-resolution ESIMS of the cage was recorded on a ThermoFischer Exactive Orbitrap using acetonitrile as the solvent.

## X-RAY CRYSTALLOGRAPHY

Single-crystal X-ray diffraction studies were performed on a Bruker D8 VENTURE equipped with a Photon 3 detector and graphite monochromated Mo Kα radiation (λ = 0.71073 Å) diffractometer. The temperature during data collection was controlled by means of a dry N₂(g) cryostream (Oxford Cryostream 800). All data was collected using Mo Kα (λ = 0.711 Å) radiation. Suitable single crystals were selected and mounted on MiTeGen polymer loops. APEX 3 software was used to collect and reduce the data. Adsorption corrections were applied using empirical methods using symmetry equivalent reflections combined with measurements at different azimuthal angles implemented using SABADS. The structure was initially solved using the SHELXT (Supporting Information, S1) program using an intrinsic phasing method implemented through OLEX 2 (Supporting Information, S2) (v1.5) and refined using SHELXL (Supporting Information, S3) least-squares refinement procedures against all *F*² values. The solvent was particularly disordered, and the solvent mask protocol implemented in the OLEX 2 procedure was applied to estimate the electrons within the void space, accounting with a PF₆⁻ anion and 4.5 molecules of acetonitrile. All nonmetal atoms were refined isotropically to maximize the data/parameter ratio. Hydrogen atoms were placed in calculated positions and refined with idealized geometries and assigned fixed occupancies and isotropic displacement parameters. PF₆⁻ molecules were located after completion of the framework structure by inspection of residual electron density and were modeled by fitting a rigid body model to electron density. X-ray diffraction data for crystals of [Cr<sub>4</sub>III·L₆](PF₆)₁₂ presented a resolution of 1.35 Å, so the data was trimmed accordingly. This resolution is typical of these cages, and traditionally synchrotron X-ray source is used to elucidate the structure of the macromolecules (Supporting Information, S4 and S5). The high R1 and wR2 values are a consequence of the poor resolution, which intrinsically produced ill-shaped electron density maps. Fitting and refine of the atomic independent model built based on the ill-shaped electron density produce unusually high R1 and wR2 values. Crystallographic information files CCDC 2368147 contain full details for the crystal structure reported.

## THEORETICAL STUDIES

The Orca ((Supporting Information, S6) (version 5.0.4) software package was used to investigate the structural and electronic properties of [Cr(bipy)₃]³⁺. Starting from the X-ray diffraction structures, the ground, ⁴A₂ state, structure of [Cr(bipy)₃]³⁺ was obtained from DFT optimizations using the unrestricted version of the Becke three-parameter exchange function in combination with the Lee–Yang–Parr correlation functional (UB3LYP). The Ahlrichs' polarized valence triple-ζ basis set def2-TZVPP was used for these optimizations. The
D3 version of Grimme's dispersion with Becke-Johnson damping (GD3BJ) was applied. Solvent effects were included via the conductor-like polarizable continuum model (CPCM) as implemented in Orca 5.0.4 with the dielectric constant of acetonitrile. Optimized geometries were confirmed to be stationary points by the analysis of their vibrational frequencies. Tight convergence criteria were selected for the optimization step. The resolution of identity approach for the Coulomb term in combination with the chain-of-spheres approximation for the exchange term (RIJCOSX) was applied. The zero-order relativistic approximation (ZORA) was used to describe relativistic effects for $[Cr(bipy)₃]³⁺$. Spin density information was extracted from the optimized geometries.

To accurately model the ligand field, the complete-active-space self-consistent field method (CASSCF) together with the fully internally contracted N-electron valence perturbation theory to second order (FIC-NEVPT2) was used. Dominant bonding/antibonding orbitals formed between the ligand and chromium and a second d shell were considered, creating an active space of 7 electrons and 12 orbitals (CASSCF(7,12)/FIC-NEVPT2). Ten quartet and nine doublet roots were computed to calculate the energies of the excited states.

The 100 lowest energetic transitions were calculated by TD-DFT as implemented in Orca 5.0.4, using the unrestricted version of the B3LYP functional, selecting the def2-TZVPP basis set, and considering solvent effects. Electronic transitions were corrected by -0.2 eV to better fit the experimental results.

## ASSOCIATED CONTENT

### Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c04180.

Complete experimental details, complex syntheses, characterizations, and photophysical data (DOCX)

### Accession Codes

Deposition Number 2368147 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via the joint Cambridge Crystallographic Data Centre (CCDC) and Fachinformationszentrum Karlsruhe Access Structures service.

## AUTHOR INFORMATION

### Corresponding Author

Juan-Ramón Jiménez - Department of Inorganic Chemistry, University of Granada and "Unidad de Excelencia en Química (UEQ)", Granada 18071, Spain; ● orcid.org/0000-0003-3871-3594; Email: jrjimenez@ugr.es

### Authors

Yating Ye - Department of Inorganic Chemistry, University of Granada and "Unidad de Excelencia en Química (UEQ)", Granada 18071, Spain; ● orcid.org/0000-0002-1003-4285

Carlos M. Cruz - Department of Organic Chemistry, University of Granada and "Unidad de Excelencia en Química (UEQ)", Granada 18071, Spain; ● orcid.org/0000-0002-0676-5210

Benjamin Doistau - Laboratoire de Chimie et de Biochimie Pharmacologiques et Toxicologiques (UMR 8601), Université Paris Cité, CNRS, Paris F-75006, France

Enrique Colacio - Department of Inorganic Chemistry, University of Granada and "Unidad de Excelencia en Química (UEQ)", Granada 18071, Spain; ● orcid.org/0000-0002-6745-9241

Claude Piguet - Department of Inorganic and Analytical Chemistry, University of Geneva, Geneva CH-1211, Switzerland; ● orcid.org/0000-0001-7064-8548

Juan Manuel Herrera - Department of Inorganic Chemistry, University of Granada and "Unidad de Excelencia en Química (UEQ)", Granada 18071, Spain; ● orcid.org/0000-0002-9255-227X

Complete contact information is available at: https://pubs.acs.org/10.1021/acs.inorgchem.4c04180

## Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

### Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

The authors acknowledge the funding received from the grant TED2021.129598A.I00 funded by MCIN/AEI/10.13039/501100011033 and by European Union NextGenerationEU/PRTR and Junta de Andalucía. J.R.J. thanks Ministerio de Ciencia Innovación y Universidades for a Ramón y Cajal contract (grant RYC2022-037255-I) funded by MCIN/AEI/10.13039/501100011033 and FSE+. Financial support from the Ministerio de Ciencia e Innovación, Grant PID2022-138090NB-C21 funded by MICIU/AEI/10.13039/501100011033 and by ERDF/EU. C.M.C. acknowledges Junta de Andalucía for a Postdoctoral Grant (POST-DOC_21_00139), grant PID2022-137403NA-I00 funded by MICIU/AEI/10.13039/501100011033 and by ERDF/EU, and grant PPJIA2023.057 funded by University of Granada. We thank Centro de Instrumentación científica (CIC) and the Centro de Servicios de Informática y Redes de Comunicaciones (CSIRC), Universidad de Granada, for providing the technical facilities and the computing time. Author B.D. thanks the Agence Nationale de la Recherche for financial support (ANR JCJC: ANR-23-CE07-0019-01). Authors acknowledge Iñigo J. Vitorica-Yrezabal for the single-crystal X-ray data acquisition and treatment.

## REFERENCES

(1) Wegeberg, C.; Wenger, O. S. Luminescent First-Row Transition Metal Complexes. JACS Au. **2021**, *1*, 1860–1876.

(2) Glaser, F.; Wenger, O. S. Recent Progress in the Development of Transition-Metal Based Photoredox Catalysts. Coord. Chem. Rev. **2020**, *405*, No. 213129.

(3) Wenger, O. S. Photoactive Complexes with Earth-Abundant Metals. J. Am. Chem. Soc. **2018**, *140* (42), 13522–13533.

(4) Büldt, L. A.; Wenger, O. S. Chromium Complexes for Luminescence, Solar Cells, Photoredox Catalysis, Upconversion, and Phototriggered NO Release. Chem. Sci. **2017**, *8* (11), 7359–7367.

(5) Sittel, S.; Naumann, R.; Heinze, K. Molecular Rubies in Photoredox Catalysis. Front. Chem. **2022**, *10*, 340.

(6) Förster, C.; Heinze, K. Photophysics and Photochemistry with Earth-Abundant Metals-Fundamentals and Concepts. Chem. Soc. Rev. **2020**, *49* (4), 1057–1070.

(7) Kitzmann, W. R.; Moll, J.; Heinze, K. Spin-Flip Luminescence. Photochem. Photobiol. Sci. **2022**, *21*, 1309–1331.
(8) Förster, C.; Heinze, K. The Photophysics and Applications of Molecular Rubies. *Adv. Inorg. Chem.* **2024**, *83*, 111.

(9) Maiman, T. H. Stimulated Optical Radiation in Ruby. *Nature* **1960**, *187*, 493–494.

(10) Jiménez, J. R.; Doistau, B.; Besnard, C.; Piguet, C. Versatile Heteroleptic Bis-Terdentate Cr(III) Chromophores Displaying Room Temperature Millisecond Excited State Lifetimes. *Chem. Commun.* **2018**, *54* (94), 13228–13231.

(11) Jiménez, J. R.; Poncet, M.; Doistau, B.; Besnard, C.; Piguet, C. Luminescent Polypyridyl Heteroleptic CrII complexes with High Quantum Yields and Long Excited State Lifetimes. *Dalt. Trans.* **2020**, *49* (39), 13528–13532.

(12) Chong, J.; Besnard, C.; Cruz, C. M.; Piguet, C.; Jiménez, J. R. Heteroleptic Mer-[Cr(N∩N∩N)(CN)3] Complexes: Synthetic Challenge, Structural Characterization and Photophysical Properties. *Dalt. Trans.* **2022**, *S1* (11), 4297–4309.

(13) Otto, S.; Förster, C.; Wang, C.; Resch-Genger, U.; Heinze, K. A Strongly Luminescent Chromium(III) Complex Acid. *Chem. - A Eur. J.* **2018**, *24* (48), 12555–12563.

(14) Treiling, S.; Wang, C.; Förster, C.; Reichenauer, F.; Kalmbach, J.; Boden, P.; Harris, J. P.; Carrella, L. M.; Rentschler, E.; Resch-Genger, U.; Reber, C.; Seitz, M.; Gerhards, M.; Heinze, K. Luminescence and Light-Driven Energy and Electron Transfer from an Exceptionally Long-Lived Excited State of a Non-Innocent Chromium(III) Complex. *Angew. Chem., Int. Ed.* **2019**, *58* (50), 18075–18085.

(15) Reichenauer, F.; Wang, C.; Christoph, F.; Boden, P.; Ugur, N.; Ricardo, B.; Kalmbach, J.; Carrella, L. M.; Rentschler, E.; Ramanan, C.; Niedner-schatteburg, G.; Gerhards, M.; Seitz, M.; Resch-genger, U.; Heinze, K. Strongly Red-Emissive Molecular Ruby [Cr(Bpmp)2]3+ Surpasses [Ru(Bpy)3]2+. *J. Am. Chem. Soc.* **2021**, *143*, 11843–11855.

(16) Otto, S.; Grabolle, M.; Förster, C.; Kreitner, C.; Resch-Genger, U.; Heinze, K. [Cr(Ddpd)2]3+: A Molecular, Water-Soluble, Highly NIR-Emissive Ruby Analogue. *Angew. Chem., Int. Ed.* **2015**, *54* (39), 11572–11576.

(17) Otto, S.; Scholz, N.; Behnke, T.; Resch-Genger, U.; Heinze, K. Thermo-Chromium: A Contactless Optical Molecular Thermometer. *Chem. - A Eur. J.* **2017**, *23* (50), 12131–12135.

(18) Otto, S.; Dorn, M.; Förster, C.; Bauer, M.; Seitz, M.; Heinze, K. Understanding and Exploiting Long-Lived near-Infrared Emission of a Molecular Ruby. *Coord. Chem. Rev.* **2018**, *359*, 102–111.

(19) Sinha, N.; Jiménez, J. R.; Pfund, B.; Prescimone, A.; Piguet, C.; Wenger, O. S. A Near-Infrared-II Emissive Chromium(III) Complex. *Angew. Chem., Int. Ed.* **2021**, *60* (44), 23722–23728.

(20) Sinha, N.; Yaltseva, P.; Wenger, O. S. The Nephelauxetic Effect Becomes an Important Design Factor for Photoactive First-Row Transition Metal Complexes. *Angew. Chem., Int. Ed.* **2023**, *62*, No. e202303864.

(21) Sawicka, N.; Craze, C. J.; Horton, P. N.; Coles, S. J.; Richards, E.; Pope, S. J. A. Long-Lived, near-IR Emission from Cr(III) under Ambient Conditions. *Chem. Commun.* **2022**, *58*, 5733.

(22) Laporte, O.; Meggers, W. F. Some rules of spectral structure. *J. Opt. Soc. Am.* **1925**, *11* (5), 459–463.

(23) Harris, D. C.; Bertolucci, M. D. Symmetry and Spectroscopy: An Introduction to Vibrational and Electronical Spectroscopy; Courier Corporation. 1978.

(24) Aboshyan-Sorgho, L.; Besnard, C.; Pattison, P.; Kittilstved, K. R.; Aebischer, A.; Bünzli, J. C. G.; Hauser, A.; Piguet, C. Near-Infrared Visible Light Upconversion in a Molecular Trinuclear d-f-d Complex. *Angew. Chem., Int. Ed.* **2011**, *50* (18), 4108–4112.

(25) Suffren, Y.; Zare, D.; Eliseeva, S. V.; Guénée, L.; Nozary, H.; Lathion, T.; Aboshyan-Sorgho, L.; Petoud, S.; Hauser, A.; Piguet, C. Near-Infrared to Visible Light-Upconversion in Molecules: From Dream to Reality. *J. Phys. Chem. C* **2013**, *117* (S1), 26957–26963.

(26) Golesorkhi, B.; Taarit, I.; Bolvin, H.; Nozary, H.; Jiménez, J. R.; Besnard, C.; Guénée, L.; Fürstenberg, A.; Piguet, C. Molecular Light-Upconversion: We Have Had a Problem! When Excited State

Absorption (ESA) Overcomes Energy Transfer Upconversion (ETU) in Cr(III)/Er(III) Complexes. *Dalton Trans.* **2021**, *50*, 7955–7968.

(27) Dee, C.; Zinna, F.; Kitzmann, W. R.; Pescitelli, G.; Heinze, K.; Di Bari, L.; Seitz, M. Strong Circularly Polarized Luminescence of an Octahedral Chromium(III) Complex. *Chem. Commun.* **2019**, *55* (87), 13078–13081.

(28) Kalmbach, J.; Wang, C.; You, Y.; Förster, C.; Schubert, H.; Heinze, K.; Resch-Genger, U.; Seitz, M. Near-IR to Near-IR Upconversion Luminescence in Molecular Chromium Ytterbium Salts. *Angew. Chem., Int. Ed.* **2020**, *59* (42), 18804–18808.

(29) Jiménez, J. R.; Doistau, B.; Cruz, C. M.; Besnard, C.; Cuerva, J. M.; Campaña, A. G.; Piguet, C. Chiral Molecular Ruby [Cr(Dqp)-2]3+ with Long-Lived Circularly Polarized Luminescence. *J. Am. Chem. Soc.* **2019**, *141* (33), 13244–13252.

(30) Doistau, B.; Jiménez, J. R.; Piguet, C. Beyond Chiral Organic (p-Block) Chromophores for Circularly Polarized Luminescence: The Success of d-Block and f-Block Chiral Complexes. *Front. Chem.* **2020**, *8* (July), 1–27.

(31) Jiménez, J. R.; Míguez-Lago, S.; Poncet, M.; Ye, Y.; Ruiz, C. L.; Cruz, C. M.; Campaña, A. G.; Colacio, E.; Piguet, C.; Herrera, J. M. Eu III Functionalized Silica Nanoparticles Encapsulating Chiral Cr III Complexes with Simultaneous Unpolarized Red and Polarized NIR-I Luminescence. *J. Mater. Chem. C* **2023**, *11*, 2582–2590.

(32) Cheng, Y.; He, J.; Zou, W.; Chang, X.; Yang, Q.; Lu, W. Circularly Polarized Near-Infrared Phosphorescence of Chiral Chromium(Ii) Complexes. *Chem. Commun.* **2023**, *59* (13), 1781–1784.

(33) Bürgin, T. H.; Glaser, F.; Wenger, O. S. Shedding Light on the Oxidizing Properties of Spin-Flip Excited States in a CrIIIPolypyridine Complex and Their Use in Photoredox Catalysis. *J. Am. Chem. Soc.* **2022**, *144* (31), 14181–14194.

(34) Jiménez, J. R.; Doistau, B.; Poncet, M.; Piguet, C. Heteroleptic Trivalent Chromium in Coordination Chemistry: Novel Building Blocks for Addressing Old Challenges in Multimetallic Luminescent Complexes. *Coord. Chem. Rev.* **2021**, *434*, No. 213750.

(35) McInnes, E. J. L.; Piligkos, S.; Timco, G. A.; Winpenny, R. E. P. Studies of Chromium Cages and Wheels. *Coord. Chem. Rev.* **2005**, *249* (23), 2577–2590.

(36) Alotaibi, R.; Fowler, J. M.; Lockyer, S. J.; Timco, G. A.; Collison, D.; Schnack, J.; Winpenny, R. E. The Synthesis and Characterisation of a Molecular Sea-Serpent: Studies of a {Cr24Cu7} Chain. *Angew. Chem., Int. Ed.* **2021**, *60*, 9489.

(37) Timco, G. A.; Mc Innes, E. J. L.; Winpenny, R. E. P. Physical Studies of Heterometallic Rings: An Ideal System for Studying Magnetically-Coupled Systems. *Chem. Soc. Rev.* **2013**, *42* (4), 1796–1806.

(38) Doistau, B.; Jiménez, J. R.; Guerra, S.; Besnard, C.; Piguet, C. Key Strategy for the Rational Incorporation of Long-Lived NIR Emissive Cr(III) Chromophores into Polymetallic Architectures. *Inorg. Chem.* **2020**, *59* (2), 1424–1435.

(39) Doistau, B.; Jiménez, J. R.; Lawson Daku, L. M.; Piguet, C. Complex-As-Ligand Strategy as a Tool for the Design of a Binuclear Nonsymmetrical Chromium(III) Assembly: Near-Infrared Double Emission and Intramolecular Energy Transfer. *Inorg. Chem.* **2022**, *61* (29), 11023–11031.

(40) Poncet, M.; Besnard, C.; Jiménez, J. R.; Piguet, C. Maximizing Nanoscale Downshifting Energy Transfer in a Metallosupramolecular Cr(III)-Er(III) Assembly. *Inorg. Chem.* **2024**, *63*, 18345.

(41) Cantuel, M.; Bernardinelli, G.; Imbert, D.; Bünzli, J. C. G.; Hopfgartner, G.; Piguet, C. A. Kinetically Inert and Optically Active CrIII Partner in Thermodynamically Self-Assembled Heterodimetallic Non-Covalent d-f Podates. *J. Chem. Soc., Dalton Trans.* **2002**, *9*, 1929–1940.

(42) Imbert, D.; Cantuel, M.; Bünzli, J. C. G.; Bernardinelli, G.; Piguet, C. Extending Lifetimes of Lanthanide-Based Near-Infrared Emitters (Nd, Yb) in the Millisecond Range through Cr(III) Sensitization in Discrete Bimetallic Edifices. *J. Am. Chem. Soc.* **2003**, *125* (S1), 15698–15699.
(43) Cantuel, M.; Bernardinelli, G.; Muller, G.; Riehl, J. P.; Piguet, C. The First Enantiomerically Pure Helical Noncovalent Tripod for Assembling Nine-Coordinate Lanthanide(III) Podates. *Inorg. Chem.* **2004**, *43* (6), 1840–1849.

(44) Burke, M. J.; Nichol, G. S.; Lusby, P. J. Orthogonal Selection and Fixing of Coordination Self-Assembly Pathways for Robust Metallo-Organic Ensemble Construction. *J. Am. Chem. Soc.* **2016**, *138* (29), 9308–9315.

(45) Symmers, P. R.; Burke, M. J.; August, D. P.; Thomson, P. I. T.; Nichol, G. S.; Warren, M. R.; Campbell, C. J.; Lusby, P. J. Non-Equilibrium Cobalt(III) "Click" Capsules. *Chem. Sci.* **2015**, *6* (1), 756–760.

(46) Ibukuro, F.; Kusukawa, T.; Fujita, M. A Thermally Switchable Molecular Lock. Guest-Templated Synthesis of a Kinetically Stable Nanosized Cage. *J. Am. Chem. Soc.* **1998**, *120* (33), 8561–8562.

(47) Burke, B. P.; Grantham, W.; Burke, M. J.; Nichol, G. S.; Roberts, D.; Renard, I.; Hargreaves, R.; Cawthorne, C.; Archibald, S. J.; Lusby, P. J. Visualizing Kinetically Robust Co III4 L 6 Assemblies in Vivo: SPECT Imaging of the Encapsulated [99m Tc]TcO 4-Anion. *J. Am. Chem. Soc.* **2018**, *140* (49), 16877–16881.

(48) Glasson, C. R. K.; Clegg, J. K.; McMurtrie, J. C.; Meehan, G. V.; Lindoy, L. F.; Motti, C. A.; Moubaraki, B.; Murray, K. S.; Cashion, J. D. Unprecedented Encapsulation of a [FeIIICl4]- Anion in a Cationic [FeII4L6]8+ Tetrahedral Cage Derived from 5,5"'-Dimethyl-2,2',5',5"',2",2'''-Quaterpyridine. *Chem. Sci.* **2011**, *2* (3), 540–543.

(49) Glasson, C. R. K.; Meehan, G. V.; Clegg, J. K.; Lindoy, L. F.; Turner, P.; Duriska, M. B.; Willis, R. A New FeII Quaterpyridyl M4L6 Tetrahedron Exhibiting Selective Anion Binding. *Chem. Commun.* **2008**, *10*, 1190–1192.

(50) Taylor, L. L. K.; Andrews, R.; Sung, A. C. Y.; Vitorica-Yrezabal, I. J.; Riddell, I. A. Synthesis and Characterisation of an Integratively Self-Sorted [Fe4L6]8+ Tetrahedron. *Chem. Commun.* **2022**, *58* (88), 12301–12304.

(51) Scott, A. J.; Vallejo, J.; Sarkar, A.; Smythe, L.; Regincós Martí, E.; Nichol, G. S.; Klooster, W. T.; Coles, S. J.; Murrie, M.; Rajaraman, G.; Piligkos, S.; Lusby, P. J.; Brechin, E. K. Exploiting Host-Guest Chemistry to Manipulate Magnetic Interactions in Metallosupramolecular M4L6tetrahedral Cages. *Chem. Sci.* **2021**, *12* (14), 5134–5142.

(52) Nielsen, H. H.; Vilariño, P.; Rodríguez, G.; Trepard, F.; Roubeau, O.; Aromí, G.; Aguilà, D. Self-Assembly of a Supramolecular Spin-Crossover Tetrahedron. *Dalt. Trans.* **2024**, *53* (23), 9792–9797.

(53) Luis, E. T.; Iranmanesh, H.; Arachchige, K. S. A.; Donald, W. A.; Quach, G.; Moore, E. G.; Beves, J. E. Luminescent Tetrahedral Molecular Cages Containing Ruthenium(II) Chromophores. *Inorg. Chem.* **2018**, *57* (14), 8476–8486.

(54) Yan, L. L.; Tan, C. H.; Zhang, G. L.; Zhou, L. P.; Bünzli, J. C.; Sun, Q. F. Stereocontrolled Self-Assembly and Self-Sorting of Luminescent Europium Tetrahedral Cages. *J. Am. Chem. Soc.* **2015**, *137* (26), 8550–8555.

(55) Riddell, I. A.; Smulders, M. M. J.; Clegg, J. K.; Hristova, Y. R.; Breiner, B.; Thoburn, J. D.; Nitschke, J. R. Anion-Induced Reconstitution of a Self-Assembling System to Express a Chloride-Binding Co 10 L 15 Pentagonal Prism. *Nat. Chem.* **2012**, *4* (9), 751–756.

(56) Taylor, L. L. K.; Vitorica-Yrezabal, I. J.; Borilović, I.; Tuna, F.; Riddell, I. A. Self-Assembly of a Trigonal Bipyramidal Architecture with Stabilisation of Iron in Three Spin States. *Chem. Commun.* **2021**, *57* (85), 11252–11255.

(57) Dissanayake, A.; Spernyak, J. A.; Morrow, J. R. An Octahedral Coordination Cage with Six Fe(III) Centers as a T1MRI Probe. *Chem. Commun.* **2024**, *60*, 12249.

(58) Bertini, I.; Fragai, M.; Luchinat, C.; Parigi, G. Solvent 1H NMRD Study of Hexaaquochromium(III): Inferences on Hydration and Electron Relaxation. *Inorg. Chem.* **2001**, *40* (16), 4030–4035.

(59) Chilton, N. F.; Anderson, R. P.; Turner, L. D.; Soncini, A.; Murray, K. S. PHI: A Powerful New Program for the Analysis of Anisotropic Monomeric and Exchange-Coupled Polynuclear d- and f-Block Complexes. *J. Comput. Chem.* **2013**, *34* (13), 1164–1175.

(60) Lenz, S.; Bamberger, H.; Hallmen, P. P.; Thiebes, Y.; Otto, S.; Heinze, K.; Van Slageren, J. Chromium(III)-Based Potential Molecular Quantum Bits with Long Coherence Times. *Phys. Chem. Chem. Phys.* **2019**, *21* (13), 6976–6983.

(61) Fataftah, M. S.; Zadrozny, J. M.; Coste, S. C.; Graham, M. J.; Rogers, D. M.; Freedman, D. E. Employing Forbidden Transitions as Qubits in a Nuclear Spin-Free Chromium Complex. *J. Am. Chem. Soc.* **2016**, *138* (4), 1344–1348.

(62) Graham, M. J.; Zadrozny, J. M.; Shiddiq, M.; Anderson, J. S.; Fataftah, M. S.; Hill, S.; Freedman, D. E. Influence of Electronic Spin and Spin-Orbit Coupling on Decoherence in Mononuclear Transition Metal Complexes. *J. Am. Chem. Soc.* **2014**, *136* (21), 7623–7626.

(63) Gabbani, A.; Poncet, M.; Pescitelli, G.; Carbonaro, L.; Krzystek, J.; Colacio, E.; Piguet, C.; Pineider, F.; Di Bari, L.; Jiménez, J. R.; Zinna, F. Magnetic Circularly Polarized Luminescence from Spin-Flip Transitions in a Molecular Ruby. *Chem. Sci.* **2024**, *15*, 17217.

(64) Zare, D.; Doistau, B.; Nozary, H.; Besnard, C.; Guénée, L.; Suffren, Y.; Pelé, A. L.; Hauser, A.; Piguet, C. CrIII as an Alternative to RuII in Metallo-Supramolecular Chemistry. *Dalt. Trans.* **2017**, *46* (28), 8992–9009.

(65) Elbers, G.; Remme, S.; Lehmann, G. EPR of Cr³⁺ in Tris(Acetylacetonato)Gallium(III) Single Crystals. *Inorg. Chem.* **1986**, *25* (7), 896–897.

(66) Barbour, J. C.; Kim, A. J. I.; DeVries, E.; Shaner, S. E.; Lovaasen, B. M. Chromium(III) Bis-Arylterpyridyl Complexes with Enhanced Visible Absorption via Incorporation of Intraligand Charge-Transfer Transitions. *Inorg. Chem.* **2017**, *56* (14), 8212–8222.

(67) Liakos, D. G.; Ganyushin, D.; Neese, F. A Multiconfigurational Ab Initio Study of the Zero-Field Splitting in the Di- and Trivalent Hexaquo-Chromium Complexes. *Inorg. Chem.* **2009**, *48* (22), 10572–10580.

(68) Castellano, M.; Ruiz-García, R.; Cano, J.; Ferrando-Soria, J.; Pardo, E.; Fortea-Pérez, F. R.; Stiriba, S. E.; Julve, M.; Lloret, F. Dicopper(II) Metallacyclophanes as Multifunctional Magnetic Devices: A Joint Experimental and Computational Study. *Acc. Chem. Res.* **2015**, *48* (3), 510–520.

(69) Meseguer, C.; Titos-Padilla, S.; Hänninen, M. M.; Navarrete, R.; Mota, A. J.; Evangelisti, M.; Ruiz, J.; Colacio, E. Single-Molecule Magnet Behavior and Magnetocaloric Effect in Ferromagnetically Coupled LnSingle-Molecule Magnet Behavior and Magnetocaloric Effect in Ferromagnetically Coupled Ln<sup>III</sup>-Ni<sup>II</sup>-Ni<sup>II</sup>-Ln<sup>III</sup> (Ln<sup>III</sup> = Dy<sup>III</sup> and Gd<sup>III</sup>) Linear Complexes. *Inorg. Chem.* **2014**, *53*, 12092–12099.