# Impact of Cis – Trans Isomerism on Crystal Packing and Photo-Activity of a Cobalt(III) Room-Temperature Photoswitch

Patryk Borowski, Katarzyna Jarzembska, Radoslaw Kamiński, Krzysztof

Durka, Dominik Schaniel

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Patryk Borowski, Katarzyna Jarzembska, Radoslaw Kamiński, Krzysztof Durka, Dominik Schaniel. *Impact of Cis – Trans Isomerism on Crystal Packing and Photo-Activity of a Cobalt(III) Room-Temperature Photoswitch*. Crystal Growth & Design, 2023, 23 (8), pp.5986-5997.
10.1021/acs.cgd.3c00544. hal-04183776

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# Impact of Cis–Trans Isomerism on Crystal Packing and Photo-Activity of a Cobalt(III) Room-Temperature Photoswitch

Patryk Borowski, Katarzyna N. Jarzembska,* Radosław Kamiński,* Krzysztof Durka, and Dominik Schaniel

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Cite This: Cryst. Growth Des. 2023, 23, 5986–5997

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## ACCESS

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Supporting Information

**ABSTRACT:** A bis(acetylacetonato-O,O') (imidazole)nitrocobalt(III) coordination compound was synthesized and characterized toward photoswitching properties. The compound exists as two isomers, *cis* and *trans*, depending on the relative arrangement of the two acetylacetone ligands chelating the metal center. Crystals of both isomers were obtained. The *cis* form was proven to be photoswitchable in the solid state at room temperature and also to a lesser extent at lower temperatures down to 10 K, upon near-UV irradiation (365–405 nm LED light). Based on the IR spectroscopic results, the photo-reaction is very efficient under ambient conditions with the maximum nitro-to-nitrito conversion reaching ca. 80%. The reaction is reversible. The photo-induced isomer is stable at 100 K, whereas at room temperature, full sample

relaxation is observed 7 h after irradiation. Photocrystallographic experiments indicated the formation of the *exo*-nitrito isomer under continuous illumination of the sample with the 405 nm LED light at room temperature. The achieved conversion amounted to around 60%. In turn, the *trans* analogue does not exhibit photoswitching properties under such conditions. The experimental solid-state spectroscopic and X-ray diffraction results were supported by theoretical modeling and physicochemical analyses. The differences in photoactivity of the examined *cis* and *trans* isomers of the studied compound in the solid state can be explained based on steric and energetic factors.

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## 1. INTRODUCTION

Conscious design of functional photoactive materials with desired properties, which are at the same time inexpensive and easy to synthesize, remains a scientific and technological challenge. In this respect, transition-metal coordination compounds constitute an interesting group of systems thanks to their rich electronic properties which can be relatively easily tuned by structural changes, e.g., ligand modifications, or exchanging metal centers. Some of such compounds have already found important real-life applications as photocatalysts, biological markers, or in optoelectronics.<sup>1–9</sup>

The photoactivity of various compounds can be manifested in many different ways. Some of them may exhibit luminescence, and others may undergo chemical transformations upon light irradiation etc.<sup>10–15</sup> For instance, molecular photoswitches are materials in which chemical reactions, such as isomerization, cyclization, etc., can be triggered by light of specific energy.<sup>16–21</sup> Transition-metal complexes possessing ambidentate ligands (e.g., NO, NO<sub>2</sub>, or SO<sub>2</sub>) constitute a good example of such systems<sup>17,19,22–28</sup> and a promising and readily modifiable group of compounds.<sup>29</sup> The industrially desirable photoswitchable materials should work under ambient conditions, be stable, relatively cheap, nontoxic,

and easy to synthesize in high-yield. In turn, the photo-induced transformation should be selective, efficient, controllable, and reproducible.

As far as the ambidentate ligands are concerned, the NO<sub>2</sub> group has caught particular attention. It may adopt up to nine different binding modes,<sup>30</sup> and some of which can interconvert from one to another upon certain conditions.<sup>31–33</sup> Up-to-date, numerous photoswitchable nitro transition-metal coordination systems have been reported.<sup>23,34–38</sup> Most of them, however, are characterized by partial nitro-to-nitrito photoconversion, which is observed at rather low temperatures, i.e., below 250 K.<sup>22,32,34,39–42</sup> There are although a few examples of high conversion materials of this kind, or complexes which work at higher temperatures. For instance, Hatcher<sup>43</sup> introduced a 100%-photoswitchable [Pd(Bu<sub>4</sub>dien)(η<sup>1</sup>-NO<sub>2</sub>)]/[BPh<sub>4</sub>] complex for which the photo-induced linkage isomer (PLI) was

Received: May 4, 2023

Revised: July 6, 2023

Published: July 20, 2023

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<figure><img src="image_9.png" /><figcaption>**Figure 1.** (a) Two types of crystals present in the acetonitrile solution of the [Co(acac)₂(imH)NO₂] complex while being slowly evaporated: cis (cis-Co), large spindle-shaped crystals) and trans (trans-Co, small needle-shaped crystals) isomers are specifically depicted as blue solid-line and green dashed-line circles, respectively. (b, c) Molecular structures of the studied isomers of the [Co(acac)₂(imH)NO₂] complex (100 K data sets): (b) cis isomer (cis-Co), (c) trans isomer (trans-Co). Atomic thermal motion is represented as ellipsoids at the 50% probability level; selected hydrogen atoms are omitted for clarity; for trans-Co, the disorder is also removed for clarity (the molecule lies on a special position on a 2-fold axis).</figcaption></figure>

detected in a single crystal up to 260 K under continuous illumination. A Pt¹¹ nitro complex⁴⁴ for which PLI was observed at 300 K, though with a rather moderate conversion level (30%), constitutes another distinguished example. Nevertheless, to the best of our knowledge, no highly efficient room-temperature (RT) photoswitches based on nitro transition-metal complexes have been reported to date.

Hence, quite recently, we have got interested in the design of new photoswitchable complexes of this kind which would be efficient and active at RT, or close. It has turned out that in the case of Ni complexes working, temperatures are usually rather low, i.e., below 240 K.²²,³⁴,³⁹,⁴³ On the other hand, the less studied Co-based photoswitches appeared to be a promising alternative. To date, only a couple of Co¹¹ nitrite complexes have been studied, mainly spectroscopically, toward linkage isomerism.⁴⁵⁻⁵⁰ Some of them, however, exhibited RT photoswitching behavior, such as the lately reported by us simple trinitro cobalt(III) complex.³⁷ In the mentioned case, it was possible to achieve about 55% population of the PLI under ambient conditions in a single crystal without losing its crystallinity (the conversion could have been even higher, roughly 90%, but then a significant crystal degradation was observed). Therefore, a new photoswitchable bis-(acetylacetonato-O,O') (imidazole)nitrocobalt(III) complex (hereafter [Co(acac)₂(imH)NO₂]; acac = acetylacetonate ligand, imH = imidazole) is reported along with its structural and physicochemical characterization in the solid state. The experimental data have been supported by theoretical computations.

## 2. EXPERIMENTAL SECTION

### 2.1. Synthesis and Crystallization.
The studied compound was obtained according to the literature procedure.⁵¹ The 3.2 g of NaOH were added to a solution of 15 g of Na₃Co(NO₂)₆ in 50 mL of distilled water and further combined with 8 mL of acacH (acetylacetone) in 50 mL of distilled water. The obtained solution was stirred for 10 min, and immediately afterward, blue crystals were removed by filtration. The filtrate was left for 24 h. During this time, a reddish solid was formed. The obtained product was collected by filtration and washed with acetone and ether to remove the remains of [α-Co(acac)₃]. Pure [Na[Co(acac)₂(NO₂)₂]] product was left for 2 h to dry. The product was dissolved in water and the obtained solution was filtered into a saturated solution of sodium nitrite, whereupon

crystals precipitated. The material was collected on a filter, washed with acetone and ether, and finally, air-dried. The obtained crystals did not need recrystallization. Finally, a solution of 0.5 g of the obtained [Na[Co(acac)₂(NO₂)₂]] complex dissolved in 10 mL of water was mixed with 4 mmol of imidazole (imH). The resulting solution was stirred for 10 min. The obtained product was collected by filtration and washed successively with water, methanol, and diethyl ether. Yield: 54%.

The nuclear magnetic resonance (NMR) spectrum was recorded with an Agilent NMR 400 MHz Varian spectrometer; ¹H chemical shifts are given relative to TMS using residual solvent resonances. Elemental analysis was carried out with an Elementar Vario EL III analyzer; ¹H NMR (400 MHz, DMSO-d₆): δ = 12.77 (bs, 1H, NH), 7.68 (s, 1H), 7.24 (s, 1H), 6.81 (s, 1H), 5.36 (s, 2H, acac), 1.96 (s, 12H, CH₃) ppm; ¹H NMR (400 MHz, acetone-d₆): δ = 7.85 (s, 1H), 7.28 (s, 1H), 7.04 (s, 1H), 5.38 (s, 2H, acac), 1.98 (s, 12H, CH₃) ppm; ¹H NMR (400 MHz, acetonitrile-d₆): δ = 7.64 (s, 1H), 7.14 (s, 1H), 6.95 (s, 1H), 5.42 (s, 2H, acac), 2.09 (s, 12H, CH₃) ppm. Elemental analysis: C₁₃H₁₈CoN₃O₆ (371.2), calcd: C 42.17%, H 4.63%, N 11.35%, found: C 41.59%, H 4.72%, N 11.17%.

Crystals of [Co(acac)₂(imH)NO₂] suitable for X-ray diffraction experiments were grown from acetonitrile solution, in which the reaction product was dissolved upon mild heating for 5 min. During slow crystallization, two crystal forms are observed—small needle-shaped and large spindle-shaped crystals (Figure 1a). When the reaction product is dissolved in acetonitrile without heating of the solution, only one type of crystal morphology is observed. Neither the evaporation crystallization from various other solvents (e.g., hexane, ethanol, acetone, dimethylformamide) nor vapor diffusion technique did result in growing crystals suitable for X-ray diffraction measurements.

### 2.2. X-ray Diffraction.
All X-ray diffraction experiments were carried out on a Rigaku Oxford Diffraction SuperNova single-crystal diffractometer equipped with a CCD detector, a copper microfocus X-ray source, a low-temperature nitrogen gas flow Oxford Cryosystems device, and our home-made light-delivery device, which allows *in situ* photocrystallographic experiments.⁵² The optimal data-collection strategy took into account the mounted light-delivery device and was prepared using the native diffractometer software. For photo-crystallographic experiments, the same strategy (in which only the exposure time was adjusted for various temperatures) was used for all data collected for a given crystal. All data collections were carried out in complete darkness (the sample mounting and centering were done at RT prior to any further data collection, and all experiments were performed with all the diffractometer lights permanently switched off). For details concerning data collection codes and the measure-
ment sequence for each sample, see the Supporting Information (Table 1S). Data processing (i.e., unit-cell determination, raw diffraction-frame integration, absorption correction, and scaling) was the same for all data collected. All structures were solved using an intrinsic phasing method as implemented in the SHELXT program⁵³ and refined with the JANA package⁵⁴ within the independent atom model approximation. The CIF files can be retrieved from the Cambridge Structural Database⁵⁵,⁵⁶ (deposition numbers: CCDC 2233240-1 and 2256610-12) or the Supporting Information.

**2.3. Spectroscopy.** All infrared (IR) measurements were performed using a Nicolet 5700 FTIR spectrometer (spectral resolution of 2 cm⁻¹ in the range of 360–4000 cm⁻¹) equipped with a closed-cycle cryostat (Oxford Optistat V01). The sample was ground, mixed with spectroscopic-grade KBr, pressed into pellets, and glued to the cold finger of the cryostat using a silver-paste thermal adhesive. During measurements, the sample was kept in a vacuum inside the cryostat. Irradiation of the sample was achieved through the cryostat window using various light-emitting diodes (LEDs; Thorlabs L and LP series), the central wavelengths of which covered the range from violet to red (from 385 to 735 nm).

RT UV–vis absorption measurements were performed with a Shimadzu UV-2600i spectrometer equipped with the ISR-2600Plus integrating sphere using the diffuse reflectance method (300–800 nm wavelength range in 1.0 nm intervals). The sample holder was loaded with the uniform mixture of the title compound and BaSO₄ as the white reference.

**2.4. Computations.** All computations were carried out using the GAUSSIAN package (ver. 16).⁵⁷ In the case of the quantum-mechanics/molecular-mechanics (QM/MM) method, the crystal environment was modeled by cutting out a shell with a radius of 15 Å around the central molecule from the examined experimental crystal structure, the C–H distances of which were set to the neutron-normalized values.⁵⁸,⁵⁹ Density functional theory (DFT) at the DFT(B3LYP)/6-311++G** level of theory was applied for the optimization of the central molecule,⁶⁰–⁶⁴ whereas the molecular shell was kept fixed and approximated with the Universal Force Field⁶⁵ using Hirshfeld atomic charges⁶⁶ initially derived at the same level of theory. Interaction energies, isolated-molecule geometry optimizations, vertical electronic transitions (time-dependent DFT method; TDDFT), and normal-mode frequencies were also calculated at the DFT(B3LYP)/6-311++G** level of theory. For harmonic-mode computations, no imaginary frequencies were found. In the case of interaction energy calculations, the Grimme empirical dispersion correction⁶⁷,⁶⁸ modified by the Becke-Johnson damping function,⁶⁹,⁷⁰ and a correction for basis set superposition error⁷¹,⁷² was applied. The derived molecular energies include the zero-point energy (ZPE) contribution. The semiautomatic generation of input files was accomplished with the CLUSTERGEN program.⁷³

## 3. RESULTS AND DISCUSSION

**3.1. Crystal Structures.** Metal acetylacetonates, i.e., metal complexes coordinated by the acetylacetone ligand (acac), have already found applications in analytical chemistry, organic synthesis, electrocatalysis, and industry.⁷⁴,⁷⁵ Furthermore, acetylacetonates can be easily modified by the substitution of hydrogen atoms attached to the middle carbon atom or the terminal methyl fragments. Such changes in the molecular structure may strongly affect physicochemical properties of the compound⁷⁶ (e.g., color, melting point, reactivity, solubility, etc.). Moreover, for octahedral complexes of the M(acac)₂XY type (M = metal, X, Y = ligands), the cis-trans isomerization can be observed. (Cis and trans prefixes are used to display the mutual arrangement of the acac ligands in the complex.) Indeed, in the case of the already reported complexes,⁷⁷,⁷⁸ crystal structures of cis and/or trans isomers were formed. In turn, as far as nitro complexes of this kind are concerned, to

the best of our knowledge, only trans-acac isomers were already described in the literature.⁷⁹,⁸⁰

In the case of the examined [Co(acac)₂(imH)NO₂] complex, recrystallization of the synthesis product via dissolving of the sample upon mild heating in acetonitrile and subsequent slow solvent evaporation leads to two types of crystals in the crystallization batch—small red needles and notably larger red spindle-shaped crystals (Figure 1a). Structural studies revealed that the former type of crystals was formed by the [trans-Co(acac)₂(imH)NO₂] isomer of the studied compound (hereafter trans-Co), whereas the latter by the [cis-Co(acac)₂(imH)NO₂] equivalent (cis-Co). It should also be noted that several recrystallization attempts were carried out to isolate the two isomers. It has appeared that the trans-Co isomer crystals can be obtained as the major crystalline product from acetonitrile solution when the reaction product is initially dissolved without heating, as confirmed by the X-ray diffraction and ¹H NMR spectroscopy experiments. Molecular structures of both isomers are presented in Figure 1b,c. Selected crystal structure parameters, data collection, and refinement details are summarized in Table 1. Since more

<table><caption>**Table 1.** Selected Crystal Structure and Refinement Parameters for Both Crystallized Cis and Trans Isomers of the [Co(acac)₂(imH)NO₂] Complex Measured at 100 Kᵃ</caption><thead><tr><th>complex</th><th>cis-Co</th><th>trans-Co</th></tr></thead><tbody><tr><td>moiety formula</td><td>C<sub>13</sub>H<sub>18</sub>CoN<sub>3</sub>O<sub>6</sub></td><td>C<sub>13</sub>H<sub>18</sub>CoN<sub>3</sub>O<sub>6</sub></td></tr><tr><td>moiety formula mass, M<sub>n</sub>, a.u.</td><td>371.2</td><td>371.2</td></tr><tr><td>crystal system</td><td>monoclinic</td><td>monoclinic</td></tr><tr><td>space group</td><td>P2<sub>1</sub>/n (No. 14)</td><td>C2/c (No. 15)</td></tr><tr><td>Z</td><td>4</td><td>4</td></tr><tr><td>F<sub>000</sub></td><td>768</td><td>768</td></tr><tr><td>crystal color &amp; shape</td><td>large red spindle-shaped</td><td>small red needle-shaped</td></tr><tr><td>a, Å</td><td>9.6574(15)</td><td>8.0540(10)</td></tr><tr><td>b, Å</td><td>12.8255(15)</td><td>13.3870(14)</td></tr><tr><td>c, Å</td><td>13.533(2)</td><td>14.1141(9)</td></tr><tr><td>β, °</td><td>107.603(15)</td><td>93.554(8)</td></tr><tr><td>V, Å<sup>3</sup></td><td>1597.7(4)<br/>[1626.2(2)]</td><td>1518.8(3)<br/>[1557.9(6)]</td></tr><tr><td>d<sub>calc</sub> g·cm<sup>-3</sup></td><td>1.5433 [1.5163]</td><td>1.6191 [1.5828]</td></tr><tr><td>R<sub>int</sub></td><td>16.99%</td><td>7.67%</td></tr><tr><td>R[F] (I > 3σ(I))</td><td>9.49%</td><td>5.19%</td></tr><tr><td>R[F] (all data)</td><td>14.38%</td><td>5.88%</td></tr><tr><td>q<sup>min/max</sup> (e·Å<sup>-3</sup>)</td><td>-1.25/+1.64</td><td>-0.38/+0.63</td></tr><tr><td>CCDC code</td><td>2233241</td><td>2233240</td></tr></tbody></table>

accurate structural data are available at low temperature, most of the results presented in this part relate to the 100 K experiment. Nevertheless, RT X-ray diffraction data sets were also collected and discussed when appropriate.

The examined isomers crystallize in the monoclinic crystal system. The cis form adopts the P2<sub>1</sub>/n space group symmetry with one molecule in the asymmetric unit (ASU), whereas the trans isomer crystallizes in the C2/c group with half of the molecule in ASU. In the latter case, the molecule is located on the 2-fold axis of symmetry, which results in a disorder of the imidazole ring (C7 and N3 atoms are refined as superimposed). It appears that in the ground-state crystal structures of cis-Co and trans-Co isomers, the NO₂ group adopts solely

<footnote>ᵃSelected values for RT crystal structures are given in square brackets. For more information, see the Supporting Information.</footnote>
<figure><img src="image_10.png" /><figcaption>**Figure 2.** Schematic representation of (a, b) a single molecular layer and crystal packing of *trans*-Co in the crystal structure (light-gray areas in panel (b) represent distinct (001) layers from panel (a)), (c) *cis*-Co crystal structure. Hirshfeld surface fingerprint plots generated for the 100 K crystal structures: (d) *trans*-Co and (e) *cis*-Co using the *CRYSTALEXPLORER* software.⁸⁴</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 3.** Main structural motifs encountered in the analyzed crystal structures. Dimers' symbols are the same as in Table 2; dimers M1–3 are formed by the *trans* isomer, M4–7 by *cis*. Only relevant hydrogen atoms are shown for clarity.</figcaption></figure>

the nitro binding mode (i.e., Co–N(O)₂). No fractions of other linkage isomers of the nitrite ligand are observed. Furthermore, a series of multitemperature single crystal X-ray diffraction experiments performed for both complexes showed no phase transition in the 100–290 K temperature range. They also proved a lack of temperature-induced *exo*- and *endo*-nitrito forms (i.e., Co–O–N=O) (which were observed for some other transition-metal nitro complexes³²⁻³⁵,³⁹,⁸¹⁻⁸³).

In both cases, the metal center is chelated by two acetylacetonate ligands, a 5-membered imidazole ring and an ambidentate nitrite moiety. The coordination geometry around the cobalt(III) center can be described as a distorted octahedron involving all oxygen atoms of acetylacetone ligands along with the N1 and N2 nitrogen atoms of the nitro group and heterocyclic ring, respectively (Figure 1c, Supporting

Information). In the case of the *trans* isomer, the top and bottom ligands in the square bipyramid (Figure 1c, Supporting Information), namely the nitro moiety and the imidazole ring, are rotated by almost 90° relative to each other. For the *cis* isomer, these ligands are located in the neighboring apexes of the octahedron. It should be noted that the Co–N bond lengths are on average slightly longer than the Co–O ones, which is in accordance with literature reports on similar compounds.⁴⁶,⁷⁹ In turn, regarding the N-donor ligands, the bond between the metal center and nitro moiety is shorter than that with the imidazole ring; however, both bonds are longer for the *trans* isomer with respect to *cis*. Altogether this shows that the coordination sphere of the *cis* isomer is tighter than that of *trans*-Co (Supporting Information). The longest Co–O distance, 1.915(7) Å, is observed in the *trans* isomer for
<table><caption>Table 2. Selected Dimeric Motifs Present in the Examined Crystal Structures: Geometrical Parameters and Interaction Energy Values ($E_{\text{int}}$; DFT(B3LYP)/6-311++G** Level of Theory with the Grimme Dispersion Correction Applied; Geometries with the X-H Neutron-Normalized Distances Were Used for Computations)</caption><thead><tr><th>complex motif</th><th><i>E</i><sub>int</sub>/kJ·mol<sup>-1</sup></th><th>selected interactions</th><th><i>d</i><sub>H...A</sub>/Å</th><th><i>d</i><sub>D...A</sub>/Å</th><th>θ<sub>D-H...A</sub>/°</th></tr></thead><tbody><tr><td colspan="6"><strong><i>trans-Co</i></strong></td></tr><tr><td rowspan="3"><b>M1</b><sup>b,c</sup></td><td rowspan="3">-33.1/-64.6</td><td>C6-H6...O1<sup>#1</sup></td><td>2.528</td><td>3.106(5)</td><td>118.80</td></tr><tr><td>C7/N3-H7...O1</td><td>2.657</td><td>3.153(5)</td><td>112.58</td></tr><tr><td>C7/N3-H7...O3</td><td>2.187</td><td>3.142(5)</td><td>173.28</td></tr><tr><td><b>M2</b><sup>a,c</sup></td><td>-39.5/-41.7</td><td>C1-H1a...C3<sup>#2</sup></td><td>2.811</td><td>3.536(6)</td><td>132.99</td></tr><tr><td><b>M3</b><sup>a,c</sup></td><td>-37.1/-40.3</td><td>C1-H1b...O2<sup>#3</sup></td><td>2.643</td><td>3.460(5)</td><td>143.11</td></tr><tr><td colspan="6"><strong><i>cis-Co</i></strong></td></tr><tr><td rowspan="3"><b>M4</b><sup>b</sup></td><td rowspan="3">-78.1</td><td>N3-H14...O1<sup>#4</sup></td><td>2.545</td><td>2.98(1)</td><td>111.80</td></tr><tr><td>N3-H14...O4</td><td>2.028</td><td>2.890(8)</td><td>171.40</td></tr><tr><td>C3-H3...O1</td><td>2.486</td><td>2.98(1)</td><td>112.01</td></tr><tr><td><b>M5</b></td><td>-57.6</td><td>C8-H8b...C2<sup>#5</sup></td><td>2.750</td><td>3.59(2)</td><td>145.71</td></tr><tr><td rowspan="3"><b>M6</b><sup>a,b</sup></td><td rowspan="3">-52.8</td><td>C1-H1...C9</td><td>2.868</td><td>3.49(1)</td><td>123.69</td></tr><tr><td>C6-H6...O1<sup>#2</sup></td><td>2.692</td><td>3.56(1)</td><td>150.46</td></tr><tr><td>C8-H8a...O2</td><td>2.613</td><td>3.34(1)</td><td>132.80</td></tr><tr><td><b>M7</b><sup>b</sup></td><td>-25.2</td><td>C5-H5c...O2<sup>#6</sup></td><td>2.470</td><td>3.23(1)</td><td>136.00</td></tr></tbody></table>

<sup>a</sup>Centrosymmetric dimer. <sup>b</sup>Interaction involving the NO₂ moiety. <sup>c</sup>Two dimers of this kind per molecule. Symmetry operations: (#1) x - 1/2, y - 1/2, z; (#2) -x + 1, -y + 1, -z + 1; (#3) -x + 1.5, -y + 1/2, -z + 1; (#4) x + 1/2, -y + 1.5, z + 1/2; (#5) -x + 1.5, y - 1/2, -z + 1/2; (#6) -x + 1/2, y + 1/2, -z + 1/2. Data for 100 K crystal structures are shown; for numerical data computed for the RT structures see Supporting Information.

the bond opposite to the Co-N bond between the metal center and nitrite moiety. Similarly to most of the d⁶ cobalt complexes, the analyzed systems are low spin as confirmed by ¹H NMR spectra and theoretical calculations.

As far as the supramolecular features are concerned, molecules of **trans-Co** form a layered crystal architecture (Figure 2a,b). The 2-dimensional chessboard-type layers, in which each nitrite ligand is situated between two imidazole rings of the adjacent molecules as shown in Figure 2a, are parallel to the (001) crystal plane and are held mainly by the N3/C7-H7...O3 interactions between the imidazole fragment and the acac oxygen atom, as well as by lateral weaker N3/C7-H7...O1 and C6-H6...O1 contacts involving the nitro ligand. It should be noted that the dimeric motif stabilized by these contacts, **M1** (Figure 3, Table 2), is characterized by the most advantageous interaction energy value, -64.6 kJ·mol⁻¹, when the N3-H7...O1 hydrogen bond is formed (**M1** exists in four different versions depending on the mutual arrangement of the disordered C7 and N3 atoms). The mentioned hydrogen bonds are visible as sharp “spikes” in the respective Hirshfeld fingerprint plot⁸⁴⁻⁸⁷ (Figure 2d). The described layers interact further one with another via weak C1-H1b...O2 interactions (centrosymmetric **M3** motif) and also by some dispersive intermolecular contacts, such as rather distant edge-to-face contacts between methyl hydrogen and imidazole ring, and C1-H1a...C3 present in the centrosymmetric dimer **M2**. The percentage contribution of various interatomic contacts to the generated Hirshfeld surface is presented in the Supporting Information.

In turn, the *cis* isomer creates a significantly different crystal architecture than its *trans* equivalent (Figure 2c). No molecular layers can be distinguished. The crystal structure is mainly stabilized by the C3-H3...O1 interactions and N3-H14...O4 hydrogen bonds forming the major dimeric motif, **M4**, with a total energy value equal to -78.1 kJ·mol⁻¹. The former interaction is created between the imidazole ring and the NO₂ group, whereas the latter one between imidazole and acac. Apart from this interaction, the nitrite ligand is also involved in the C6-H6...O1 and C8-H8a...O2 intermolecular contacts

formed with acac, which lead to the centrosymmetric molecular dimer, motif **M6**, characterized by -57.6 kJ·mol⁻¹ total interaction energy. The last notable intermolecular contact created by the nitro moiety is the C5-H5c...O2 interaction present in **M7**. Otherwise, some C-H...π interactions (namely C8-H8b...C2) and weak dispersive C1-H1...C9 contacts resulting in motif **M5** should be noted.

As far as the potential photo-reactivity of the examined systems is concerned, two factors are of major importance: the above-mentioned interactions involving the ambidentate ligand and the reaction cavity volume.³⁷,⁸²,⁸⁸ Regarding the interactions of the nitro group, they should neither be too strong thereby hampering the photoreaction, nor too weak so as to contribute sufficiently to the crystal stability. Furthermore, the reaction product should also be comparably well-stabilized in a crystal. Taking into consideration these factors, both *cis* and *trans* isomers of [Co(acac)₂(imH)NO₂] seem to be promising candidates for solid-state photoswitches. The *trans*-Co crystal is packed more tightly than that of *cis*-Co, as reflected by the Hirshfeld fingerprint plots and crystal density (Table 1). Nevertheless, despite a greater percentage contribution of O-H interactions to the Hirshfeld surface for *trans*-Co (Supporting Information), major interactions involving the ambidentate ligand seem to be stronger on average in the case of *cis*-Co. In turn, regarding the reaction cavity volume, not only the size of the cavity is relevant, but also its shape so as to assure sufficient space for molecular rearrangement along with the smallest possible changes in the unit cell upon molecular transformation. A notably larger reaction cavity volume⁸⁹ (*V*<sub>cav</sub>) was computed for the *cis*-Co crystal structure at 100 K when compared to that of *trans*-Co (33.1 Å³ for *cis*-Co vs 27.8 Å³ for *trans*-Co; standard deviation on *V*<sub>cav</sub> is estimated to be ca. 0.5 Å³⁹). At RT the respective cavity volumes amount to 31.8 for *cis*-Co and 34.3 Å³ for *trans*-Co. These results indicate an almost constant cavity volume in the examined temperature range for *cis*-Co and a notable decrease of this value for *trans*-Co between RT and 100 K. The shape of the reaction cavities is illustrated in Figure 4 and the Supporting Information. In the case of the *trans* isomer, the
<figure><img src="image_12.png" /><figcaption>**Figure 4.** Visualization of the reaction cavities for the examined crystal structures (MERCURY program,⁸⁹ probe radius of 1.2 Å, grid spacing of 0.1 Å, hydrogen atoms omitted for clarity): (a) view along the Y axis in the *cis-Co* crystal structure; (b) view along the X axis in the *trans-Co* crystal structure.</figcaption></figure>

reaction cavity reflects rather strictly the shape of the nitrite ligand adopting the nitro binding mode, which may hamper molecular rearrangement leading to the formation of sterically different linkage isomers. In turn, for the *cis* isomer, the shape of the cavity is more ball-like which increases its relative photoisomerization reaction potential. All the calculated cavity volumes are comparable to those reported for other nitro complexes undergoing nitro-to-nitrito photoisomerization reaction.²²,²³,³⁴,³⁷,³⁹,⁴³,⁸³

### 3.2. Photoswitching Properties via IR Spectroscopy.

Prior to IR spectroscopic experiments, the product of the synthesis was examined for its chemical composition, purity, and atom connectivity using elemental analysis and ¹H NMR spectroscopy. The sample formula was confirmed. The ¹H NMR spectra were collected in three deuterated solvents (DMSO-d₆, acetone-d₆, and acetonitrile-d₃) for samples dissolved with and without mild heating (Figures S7–S9, Supporting Information). The latter set of samples gave similar ¹H NMR spectra with relatively narrow signals (e.g., the acac ligand singlets display only 1.1 Hz fwhm), which are assigned to the pure *trans*-Co derivative. In contrast, after heating of the solution, a number of signals appear in all of the solvents. The major new signals—a second set of imidazole peaks and the two extra resonances—are attributed to the *cis*-Co isomer formation. Some additional signals presumably result from the nitro ⇌ nitrito equilibration or degradation of the sample. Their intensities increase after prolonged heating. It should be noted that all the forms remain in the solution after its cooling. Hence, the respective solid-state sample contains both *trans* and *cis* isomers of the studied compound.

The subsequent solid-state infrared (IR) spectroscopy experiments were conducted to explore the effect of temperature and light on the studied sample. All of them were performed using thin-film samples prepared based on the synthesis product after sample dissolution upon mild heating and slow recrystallization from acetonitrile. The interpretation of the collected IR absorption spectra is supported by literature findings and normal-mode frequency calculations for the nitro and potentially photo-generated *exo-* and *endo*-nitrito forms (Table 3). According to these results and available literature,¹⁹,²²,³⁰,³³,³⁴,³⁷,⁹⁰⁻⁹³ vibrational bands around 600, 850, 1050, and 1385 cm⁻¹ should change the most during the formation of the ONO linkage isomers and the NO₂ population decrease. As shown in Table 3, in the case of the *cis* isomer, most of the selected bands are shifted toward lower energies when compared to the *trans* analogue. In the experimental spectrum, two well-distinguished separated signals are visible around 600 cm⁻¹, which can be assigned to wagging vibrations of the nitro group in *cis-Co* and *trans-Co*, respectively (Figure 5a, Supporting Information). These peaks should best reflect any binding mode changes of the ambidentate ligand in the case of either form of the examined compound.

As far as the thermal effects on the sample are concerned, it appeared that in the 10–290 K temperature range, only some minor spectral changes are observed (Supporting Information), which indicates that no isomerization takes place under such conditions. In turn, irradiation of the sample with different LEDs showed that the UV or near-UV visible light (365–405 nm) causes the most significant changes to the IR spectra. The effective excitation wavelengths agree well with the lowest-energy band in the UV–vis absorption spectrum (Supporting Information). Importantly, the isomerization reaction is more effective at RT than at lower temperatures (Figure 16S in the Supporting Information shows a comparison of respective photo-IR spectra at RT, 100, and 10 K), which is rather uncommon for the nitro complexes of other transition metals reported in the literature.²²,³⁴,⁴⁴,⁴⁵,⁹⁴

The clearest intensity changes are observed for bands assigned to the wagging vibrational mode of NO₂ (Figure 5a). The 595 cm⁻¹ band (assigned to the wagging vibration of the NO₂-group for the *cis* form) almost completely disappears upon irradiation, whereas the 620 cm⁻¹ one (wagging vibration of the NO₂-group for the *trans* form) displays only some minor changes, if any. A band observed at 1013 cm⁻¹ rises significantly, which can be assigned to the asymmetric stretching vibrations of the generated nitrito binding mode. Based on the theoretical results, this is most probably the sign

<table><caption>**Table 3.** Calculated Normal-Mode Frequencies of Key Vibrational Bands Relevant for the NO₂-Group Switching (QM/MM-Optimized Geometries—Upper Values, Isolated-Molecule-Optimized Geometries—Bottom Values in Square Brackets; all Values Given in cm⁻¹)</caption><thead><tr><th rowspan="2">vibration type</th><th colspan="3">cis-Co</th><th colspan="3">trans-Co</th></tr><tr><th>nitro</th><th>exo-nitrito</th><th>endo-nitrito</th><th>nitro</th><th>exo-nitrito</th><th>endo-nitrito</th></tr></thead><tbody><tr><td>wagging (ω)</td><td>594.57 [598.44]</td><td></td><td></td><td>599.38 [610.70]</td><td></td><td></td></tr><tr><td>scissoring (δ)</td><td>850.74 [837.86]</td><td>862.54 [862.39]</td><td>852.25 [839.48]</td><td>841.95 [842.69]</td><td>926.96 [865.70]</td><td>862.58 [847.09]</td></tr><tr><td>symmetric stretching (ν<sub>s</sub>)</td><td>1385.29 [1377.38]</td><td></td><td></td><td>1392.73 [1389.78]</td><td></td><td></td></tr><tr><td>asymmetric stretching (ν<sub>as</sub>)</td><td>1518.36 [1539.57]</td><td>1049.05 (N=O)<br>[1022.83]</td><td>1143.43 (N=O)<br>[1087.47]</td><td>1497.31 [1535.71]</td><td>1143.54<br>(N=O) [1031.50]</td><td>1158.91 (N=O)<br>[1095.97]</td></tr><tr><td></td><td></td><td>1559.32 (N=O)<br>[1571.17]</td><td>1509.51 (N=O)<br>[1519.28]</td><td></td><td>1579.36 (N=O)<br>[1572.47]</td><td>1532.95 (N=O)<br>[1524.39]</td></tr></tbody></table>
<figure><img src="image_13.png" /><figcaption>**Figure 5.** (a) Solid-state IR absorption spectra of the studied sample before (black) and after 405 nm LED irradiation (red) at RT. (b) Time-evolution of two crucial IR bands observed after sample irradiation with the 405 nm LED light for 30 min at RT.</figcaption></figure>

of the *exo*-nitrito isomer formation. Finally, the band around 1322 cm⁻¹ (the symmetric stretching vibration of the nitro group) diminishes significantly, whereas the band at 1461 cm⁻¹ rises (N=O fragment oscillation in the nitrito form) which also supports the formation of the nitrito isomer. Nevertheless, most of the key vibrations severely overlap with the neighboring bands which smears out the picture and hampers the estimation of the reaction conversion. The intensity analysis of the best-separated bands around 600 cm⁻¹ indicates that **trans-Co** almost does not isomerize, while the highest level of conversion which can be achieved for the **cis** isomer can be estimated at ca. 80%. According to theoretical outcomes, the formation of the *exo-* and *endo*-nitrito isomers leads to rather similar spectral changes, which makes it hard to identify the photoproducts unambiguously (Supporting Information).

From the applicative point of view, photoswitching performance under ambient conditions is most interesting. At RT, the maximum conversion was achieved after 5 min of irradiation with the 365 nm LED light, while using the 405 nm LED, the same effect was obtained after 30 min. Full sample relaxation at RT was reached 7 h after its exposure to light (Figure 5b), whereas at 100 K, the light-induced product is stable.

**3.3. Photocrystallography.** In view of the above, a series of photocrystallographic experiments were performed for both **cis-Co** and **trans-Co** crystals. According to the spectroscopic results, **cis-Co** undergoes rather efficient nitro-to-nitrito transformation at RT while to some lesser extent at lower temperatures (ca. 30 and 20% at 100 and 10 K, respectively). Therefore, a series of photocrystallographic experiments were performed under ambient and 100 K conditions. First, the respective single crystals were irradiated using the 365 and 405 nm LED light for 30 and 60 min, respectively, but no structural changes were observed afterward. Hence, in the next step, the crystal was irradiated continuously before and during the entire crystallographic data collection at RT with the 405 nm LED light, ensuring the maximal average population of the excited state (irradiation with the 365 nm LED light causes partial crystal degradation). This time the nitro binding mode was visibly transformed into the *exo*-nitrito linkage isomer (Figure 6), with quite a decent population of about 62%. The geometry

<figure><img src="image_14.png" /><figcaption>**Figure 6.** Photodifference Fourier map ($F^{\text{ON}} - F^{\text{OFF}}$) indicating structural reorganization of the parent nitro linkage isomer to the *exo*-nitrito form upon light irradiation in the **cis-Co** crystal structure at RT. The map is superimposed onto the refined model of both nitro and *exo*-nitrito isomers; +1.0 e·Å⁻³ blue isosurfaces indicate the formation of the nitrito binding mode, -1.0 e·Å⁻³ red one shows diminishing of the nitro isomer population.</figcaption></figure>

of the generated metastable state is out-of-plane of the parent nitro form. It should also be noted that at lower temperatures no structural changes were observed for **cis-Co** in both steady-state and constant-illumination experiments. Analogous experiments were performed for **trans-Co** single crystals and, as expected, no structural changes were detected. These findings are in agreement with the spectroscopic results.

**3.4. Computational Analysis.** In order to justify the formation of any other PLI, theoretical computations were conducted. For that purpose, geometry optimizations for the most common NO₂ linkage isomers⁴²,⁸²,⁸³ (i.e., nitro, *exo-* and *endo*-nitrito) of **cis-Co** and **trans-Co** were performed for the isolated molecules and by exploiting the QM/MM approach, which relatively well mimics the crystal environment.²²,⁹⁵⁻¹⁰⁰ All optimizations were done starting from the crystal structures measured at 100 K. Relative molecular energies computed for all optimized molecular geometries are gathered in Table 4. As expected, the nitro isomer turns out to be the most energetically stable form of the NO₂ group for both the **trans-Co** and **cis-Co** compounds. Furthermore, the nitro isomers of **trans-Co** and **cis-Co** have equal molecular energies
<table><caption>**Table 4. Relative Energy Differences (ΔE<sub>rel</sub>) between the Ground (Nitro) and Hypothetical Metastable (Exo- and Endo-Nitrito) PLIs Computed Using QM/MM Approach and the Optimized Isolated-Molecule Geometries<sup>a</sup>**</caption><thead><tr><th rowspan="2">crystal structure</th><th rowspan="2">linkage isomer</th><th colspan="2">ΔE<sub>rel</sub>/kJ·mol<sup>-1</sup></th></tr><tr><th>QM/MM</th><th>isolated molecule</th></tr></thead><tbody><tr><td rowspan="3"><i>cis</i>-Co</td><td>nitro</td><td>0.0</td><td>0.0</td></tr><tr><td>exo-nitrito</td><td>6.8</td><td>23.7</td></tr><tr><td>endo-nitrito</td><td>27.1</td><td>19.2</td></tr><tr><td rowspan="3"><i>trans</i>-Co</td><td>nitro</td><td>0.0</td><td>0.0</td></tr><tr><td>exo-nitrito</td><td>65.8</td><td>29.2</td></tr><tr><td>endo-nitrito</td><td>36.9</td><td>26.7</td></tr></tbody></table>

taking into account the methods accuracy. According to the isolated-molecule optimization results the *endo-* and *exo-nitrito* forms of the two compounds are characterized by very comparable molecular energies. It should be noted, however, that the energy difference between these hypothetical isomers and the ground-state nitro form are smaller in the case of *cis-Co*. As far as the crystal environment is concerned in general the difference between molecular energies of considered linkage isomers is more pronounced. However, for *cis-Co*, the *exo-nitrito* isomer is only about 7 kJ·mol⁻¹ less energetically stable than the parent nitro form, whereas the *endo-nitrito* isomer is clearly the least energetically favorable. The nitro-*(*endo*-nitrito) energy difference is indeed larger when the crystal environment is taken into account, but it still does not exceed 30 kJ·mol⁻¹. In turn, for *trans-Co*, the energy differences between the hypothetically generated photo-induced linkage isomers are way more significant (+38.92 and +63.75 kJ·mol⁻¹, for the *endo-* and *exo-nitrito* forms with respect to the nitro isomer). In the case of the *trans-Co* crystal structure, the energetic differences seem to be more emphasized due to the previously mentioned more restricted shape of the reaction cavity. Such energetic results additionally support the better photoswitching potential of *cis-Co*. Based on the computational outcomes, it also seems that the formation of both linkage isomers of *cis-Co* could be possible, but the *exo-nitrito* binding mode is energetically favored. Nevertheless, to draw binding conclusions, also other factors should be analyzed here, such as the entropy contribution and energy barriers of molecular transformations.

Another aspect that could be important regarding differences in the photoactivity of the *trans-Co* and *cis-Co* compounds in the solid state is the close vicinity of the imidazole ligand with respect to the nitrite moiety in *cis-Co*. In *cis-Co*, it can relatively easily rotate within some angular range and adapt its orientation to the changes in the NO₂ site which is not the case for *trans-Co* (Supporting Information). For *trans-Co*, the preferred and adopted conformation is more strictly defined.

Finally, based on the TDDFT computational results obtained for the *cis* and *trans* forms, in both cases, there are low-energy electronic transitions, similar in nature to that reported in the literature,³⁸,⁴³,¹⁰¹ which may lead to the Co-N bond breaking, thus to further light-induced switching. For example, in the case of the *cis-Co*, the HOMO-2, HOMO-1 and LUMO, LUMO+1 pairs of occupied and unoccupied orbitals exhibit bonding and antibonding character, respectively (Supporting Information). A similar situation is observed for the *trans-Co* HOMO-2, HOMO-1, and LUMO orbitals.

Therefore, judging from the electronic factors alone both isomers are prospective to switch when excited by light. Therefore, clearly other factors govern the compounds' behavior in the solid state.

## 4. CONCLUSIONS

A new bis(acetylacetonato-O,O') (imidazole)nitrocobalt(III) coordination compound is reported. The compound exists as two isomers, *cis* and *trans*, depending on the relative arrangement of the two acetylacetone ligands chelating the metal center. It was possible to grow crystals of both isomeric forms. However, crystals of the *trans-Co* form seem to be more thermodynamically stable. Solid-state IR spectroscopy indicated that the *cis-Co* isomer exhibits photoswitchable properties in the 10-290 K temperature range upon near UV-light irradiation (365-405 nm LED light) with the maximum nitro-to-nitrito conversion achieved at RT reaching ~80%. The reaction is fully reversible. Full sample relaxation is achieved 7 h after irradiation at RT. At 100 K, the light-induced species are stable. In turn, the *trans-Co* form is almost photoinert. Photocrystallographic studies showed that notable reorganization of the nitro site is observed in a single crystal only for *cis-Co*. Continuous *in situ* illumination of the crystal with the 405 nm LED light at RT results in a relatively efficient formation of the *exo-nitrito* linkage isomer (conversion ~60%).

Based on the gathered results, the photoswitching properties of the *cis-Co* crystals can be to some extent explained by the small energy difference between nitro and *exo-nitrito* forms in a crystal environment along with the steric factors. The former is much more pronounced for the *trans-Co* crystal structure. The latter is the spacious and ball-shaped reaction cavity and the vicinity of the relatively mobile imidazole moiety to the nitrite ligand in *cis-Co*. A large reaction cavity minimizes the amount of atomic or molecular movement accompanying the isomerization process and its shape facilitates the nitrite group isomerization in contrast to the nitro-group-shaped cavity in the *trans-Co* crystal structure. In turn, the imidazole ring may, within a certain angular range adopt different binding modes of the ambidentate ligand in *cis-Co* which is not the case for *trans-Co*. The differences in the cavity volume and intermolecular interactions seem to be secondary factors in the current study.

The outcomes of this article along with our recent findings³⁷ show that cobalt nitro complexes can be interesting compounds in search for effective RT photoswitches. The study also provides some insights toward a better understanding of factors governing the nitro-to-nitrito linkage isomerization reaction.

### ■ ASSOCIATED CONTENT

**● Supporting Information**

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.cgi.3c00544.

Additional data collection information, structural information, DFT computations (orbitals, molecular vibrations, TDDFT results, bond rotations scans), PXRD pattern, reaction cavity visualizations, NMR spectra, TGA-DSC traces, additional IR absorption spectra under various conditions before and after irradiation, solid-state UV-vis spectrum (PDF)

<footnote><sup>a</sup>Computations performed at the DFT(B3LYP)/6-311++G** level of theory; ZPE corrections included.</footnote>
## Accession Codes

CCDC 2233240–2233241 and 2256610–2256612 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

## AUTHOR INFORMATION

### Corresponding Authors

Katarzyna N. Jarzembska – Department of Chemistry, University of Warsaw, 02-089 Warsaw, Poland;

orcid.org/0000-0003-4026-1849;

Email: katarzyna.jarzembska@uw.edu.pl

Radosław Kamiński – Department of Chemistry, University of Warsaw, 02-089 Warsaw, Poland;

orcid.org/0000-0002-8450-0955; Email: rkaminski85@uw.edu.pl

### Authors

Patryk Borowski – Department of Chemistry, University of Warsaw, 02-089 Warsaw, Poland;

Krzysztof Durka – Department of Chemistry, Warsaw University of Technology, 00-664 Warsaw, Poland;

orcid.org/0000-0002-6113-4841

Dominik Schaniel – Université de Lorraine, CNRS, CRM², 54500 Nancy, France

Complete contact information is available at: https://pubs.acs.org/10.1021/acs.cgd.3c00544

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

P.B. and K.N.J. would like to acknowledge the Polish Ministry of Education and Science for financial support (“The Best of the Best” program No. MEiN/2021/236/DIR/NN4). Authors thank A. Gorski and J. Waluk (Warsaw, Poland) for allowing preliminary IR studies. Financial support by ANR (grant No. ANR-21-CE30-0045-01) is gratefully acknowledged. The Wroclaw Centre for Networking and Supercomputing (grant No. 285) is acknowledged for providing computational facilities. The in-house X-ray diffraction experiments were carried out at the Department of Physics, University of Warsaw, on Rigaku Oxford Diffraction SuperNova diffractometer, which was co-financed by the European Union within the European Regional Development Fund (program No. POIG.02.01.00-14.122/09).

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