Experimental Evidence for the Solid-State
Nitrite-Ligand Photoisomerization Mechanism in
Nickel(II) Square-Planar Complexes

Krystyna A Deresz, Artem Mikhailov, Joanna Jankowska, Lorenzo Donà,
Bartolomeo Civalleri, Adam Krówczyński, Radosław Kamiński, Dominik
Schaniel, Katarzyna N Jarzembska

► To cite this version:

Krystyna A Deresz, Artem Mikhailov, Joanna Jankowska, Lorenzo Donà, Bartolomeo Civalleri, et al. Experimental Evidence for the Solid-State Nitrite-Ligand Photoisomerization Mechanism in Nickel(II) Square-Planar Complexes. ACS Omega, 2025, 10 (43), pp.51940-51954. 10.1021/acsomega.5c08267. hal-05352008

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# Experimental Evidence for the Solid-State Nitrite-Ligand Photoisomerization Mechanism in Nickel(II) Square-Planar Complexes

Krystyna A. Deresz, Artem Mikhailov, Joanna Jankowska, Lorenzo Donà, Bartolomeo Civalleri, Adam Krówczyński, Radosław Kamiński, Dominik Schaniel, and Katarzyna N. Jarzemska*

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Cite This: ACS Omega 2025, 10, 51940–51954

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## ACCESS

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Supporting Information

**ABSTRACT:** Two square-planar nitrite nickel(II) photoswitches were designed and synthesized, referred to as **Ni-4d** and the related oxime **Ni-4d'**. In the ground-state single crystals of both compounds, the nitro form (Ni–N(O)₂) is the dominant one. For the two systems, it was possible to generate and detect both *endo*-nitrito and *exo*-nitrito linkage isomers using 530 nm light-emitting diode (LED) light at 100 K. Over time, the *endo*-nitrito form takes over, and almost 100% conversion can be achieved, as confirmed by photocrystallographic and infrared (IR) spectroscopic experiments. The stability of this isomer is similar for both systems, which is confirmed by the experimentally determined decay temperature (*T*₉) values. Furthermore, when 660 nm LED light is applied at 90 K, the *exo*-nitrito form can be generated up to about 25%

population with no admixture of the *endo*-nitrito isomer for **Ni-4d**. Based on the kinetic parameters, the *exo*-nitrito isomer is slightly less stable for **Ni-4d'**. In this work, we report for the first time the experimental evidence of the earlier theoretically predicted nitro-to-*endo*-nitrito mechanism via the *exo*-nitrito form for square-planar nickel(II) nitrite coordination compounds in the solid state.

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## 1. INTRODUCTION

The term photoswitchable compounds refers to materials that undergo various types of transformations under light

<figure><img src="image_9.png" /><figcaption>**Scheme 1.** Selected Possible Coordination Modes of the Nitrite Group in Mononuclear Metal Complexes</figcaption></figure>

irradiation, such as cyclization, cis-trans isomerization, or linkage isomerization.¹ Thanks to the ability to transform from one form to another upon exposure to light, photoswitches may find wide applications in materials science,² optoelectronics,³ or medicine.⁴ Transition-metal complexes containing ambidentate ligands, e.g., SO₂, NO, and NO₂,⁵⁻⁹ constitute an interesting and readily modifiable group of molecular photo-switches. The ambidentate ligands are key fragments in such molecules, as they may bind to a metal center in more than one way, depending on the conditions. For instance, the nitrite group may adopt four different binding modes in mononuclear complexes (*Scheme 1*): nitro-(*η*¹-N(O)₂) (further referred to as nitro), *exo*-nitrito-(*η*¹-ONO) (further referred to as *exo*-nitrito), *endo*-nitrito-(*η*¹-ONO) (further referred to as *endo*-

nitrito), and *κ*-nitrito-(*η*²-O₂N),¹⁰ which, under some circumstances, may be interchanged.

Understanding of the influence of supporting ligands, metal centers, or crystal packing on the photoswitchable properties, as well as the isomerization reaction mechanism itself, is crucial in the context of designing new efficient photoswitches. To the best of our knowledge, to date, there is no direct experimental confirmation of the nitro-to-*endo*-nitrito reaction mechanism in square-planar nickel complexes. However, quite recently, Warren et al. proposed a mechanism according to which the nitro isomer transforms to the *exo*-nitrito form *first*, and subsequently converts to the final photoproduct, the *endo*-nitrito isomer.¹¹ More recently, a possible isomerization reaction pathway for square-planar complexes was discussed in two other articles. The isomerization reaction mechanism fully consistent with the one described above was reported by us¹² for crystals of the nickel nitro complex with the 1-phenyl-2-hydroxyimino-3-[(2'-dimethylamino)ethyl]imino-1-propa none moiety as an (N,N,O)-donor supporting ligand (Ni-1d'). The computationally evaluated mechanism (nitro → *exo*-

Received: August 29, 2025

Revised: September 17, 2025

Accepted: October 8, 2025

Published: October 26, 2025

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Scheme 2. Schematic Representation of the Studied Systems: Ni-4d and Ni-4d'ᵃ

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ᵃCorresponding molecular fragments are colored in blue, oxime fragment is shown in magenta; note the numbering of atoms underscores the difference between the two compounds.

nitrito → *endo*-nitrito) sheds light on the experimental observations. In contrast, Mikhailov et al.¹³ presented two possible reaction pathways based on the experiments performed for a series of complex salts of the [Pd(NH₃)₄][Pd(NH₃)₃NO₂] [M(Ox)₃].y H₂O type (M = Cr, Rh, Co; Ox = oxalate). In the first considered mechanism, the *endo*-nitrito isomer served as an intermediate product, while in the other potential pathway, the transformation went through the *exo*-nitrito isomer, similarly as suggested by the above-discussed papers.

Hence, in this paper, two related photoswitchable square-planar Ni¹¹ nitrite complexes, namely Ni-4d and Ni-4d', are introduced (Scheme 2). These compounds were designed based on the nickel nitro complexes with (N,N,O)-donor supporting ligands previously studied by some of us.¹²,¹⁴,¹⁵ The two compounds were crystallized and completely characterized, both experimentally and theoretically, in order to shed more light on the nitro-to-*endo*-nitrito photoreaction mechanism.

## 2. EXPERIMENTAL SECTION

**2.1. Synthesis.** The compounds Ni-4d and Ni-4d' were synthesized in the same manner as in literature procedures¹²,¹⁴ using the following compounds to create a chelating ligand: 4-dimethyloaminoacetophenone, ethyl formate, and N,N-dimethyloethylenediamine. It is worth stressing that the Ni-4d compound is obtained when using a deficiency of LiNO₂ in the reaction, and when an excess of LiNO₂ is used, Ni-4d' is derived. The reaction steps are schematically presented in Figure S1 (Supporting Information). The yields of the reaction for Ni-4d and Ni-4d' were 55 and 49%, respectively.

**2.2. X-ray Diffraction.** All X-ray diffraction experiments (including the preliminary ones) were carried out on a Rigaku Oxford Diffraction SuperNova single-crystal diffractometer equipped with an Atlas CCD detector, a copper microfocus X-ray source (Cu-Kα radiation, λ = 1.54184 Å) coupled with the multilayer mirror optics, a low-temperature nitrogen gas flow Oxford Cryosystems device, and our homemade light-delivery device,¹⁶ which allows *in situ* photocrystallographic experiments. Single crystals were irradiated with 530 and 660 nm LED light (LED = light-emitting diode). The data-collection strategy took into account the mounted light-delivery device and was automatically prepared by using the native diffractometer software. In the case of photocrystallographic experiments, the same strategy was used for all data collected for a given crystal (only the exposure time was adjusted for various temperature points). All data collections were carried out in complete darkness (the sample mounting and centering were done at room temperature prior to any further data collection, and all experiments were performed

with all of the diffractometer lights permanently switched off). For details concerning photocrystallographic data-collection codes and the measurement sequence for each sample, see the Supporting Information (Tables S5 and S6). Further data processing (i.e., unit-cell determination, raw diffraction-frame integration, absorption correction, and scaling) was the same for all data sets collected. All structures were solved using an intrinsic phasing method as implemented in the SHELXT program¹⁷ and refined with the SHELXL program¹⁸ in the OLEX2 package¹⁹ within the independent atom model (IAM) approximation.

**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 (referred to as thin-film samples) via a mechanical press, and glued to the coldfinger 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 LEDs (Thorlabs L and LP series), the central wavelengths of which covered the range from violet to red (from 385 to 735 nm). LED powers are presented in the Supporting Information (Table S2).

**2.4. Theoretical Calculations.** Isolated molecule, dimer interaction energies, and normal-mode frequencies were calculated using the density functional theory (DFT) at the DFT(B3LYP)/6-311++G** level of theory²⁰⁻²³ using the GAUSSIAN package (ver. 16, rev. C.01).²⁴ For harmonic-mode computations, no imaginary frequencies were found. All optimized molecular energies were corrected for the zero-point energy (ZPE). For all of the calculations performed with the GAUSSIAN package, the Grimme empirical dispersion correction²⁵,²⁶ modified by the Becke-Johnson damping function was applied;²⁷,²⁸ in the case of the interaction energy calculations, a correction for basis set superposition error²⁹,³⁰ (BSSE) was also included. The semiautomatic generation of input files was accomplished with the CLUSTERGEN program.³¹ Additionally, unit-cell parameters and atomic positions were optimized using the periodic approach implemented in the CRYSTAL program (ver. 17)³² applying the PBESOL0-3C functional and the sol-def2-mSVP basis set for all atoms.³³

Potential-energy profiles (PEPs) calculations in the ground (S₀) and in the lowest excited (S₁) singlet states, as well as the approximate minimum-energy conical intersection (MECI) optimizations were performed at the time-dependent density functional theory level with the Tamm-Dancoff approximation (TDA-TDDFT).³⁴ In these calculations, the B3LYP functional
<figure><img src="image_13.png" /><figcaption>**Figure 1.** Molecular structures of Ni-4d (left) and Ni-4d' (right) derived from the X-ray diffraction data sets collected in complete darkness at 100 K (atomic thermal motion is represented as ellipsoids at the 50% probability level; hydrogen atoms are omitted for clarity; note the disorder, shown in gold, at the NO₂ group in Ni-4d').</figcaption></figure>

<table><caption>**Table 1.** Selected Crystal-Structure Parameters of the Studied Complexes at 100 K Prior to Any Light Exposure (“Dark” Structures)ᵃ</caption><thead><tr><th>structure/compound</th><th>Ni-4d</th><th>Ni-4d'</th></tr></thead><tbody><tr><td>data set</td><td>Ni-4d-100 K-dark-01-xtal1</td><td>Ni-4d-prim-100 K-dark-18-xtal1</td></tr><tr><td>moiety formula</td><td>C<sub>15</sub>H<sub>22</sub>N<sub>4</sub>Ni<sub>1</sub>O<sub>3</sub></td><td>C<sub>15</sub>H<sub>21</sub>N<sub>5</sub>Ni<sub>1</sub>O<sub>4</sub></td></tr><tr><td>moiety formula mass, M/a.u.</td><td>365.06</td><td>394.06</td></tr><tr><td>crystal system</td><td>monoclinic</td><td>triclinic</td></tr><tr><td>space group</td><td>P2<sub>1</sub>/c (no. 14)</td><td>P<span style="text-decoration: overline">1</span>/c (no. 2)</td></tr><tr><td>Z</td><td>4</td><td>2</td></tr><tr><td>F<sub>000</sub></td><td>768</td><td>412</td></tr><tr><td>crystal color and shape</td><td>orange block</td><td>orange block</td></tr><tr><td>T/K</td><td>100</td><td>100</td></tr><tr><td>a/Å</td><td>11.0495(5)</td><td>7.3612(2)</td></tr><tr><td>b/Å</td><td>10.4673(4)</td><td>10.0034(3)</td></tr><tr><td>c/Å</td><td>14.7449(6)</td><td>11.8479(4)</td></tr><tr><td>α/°</td><td>90</td><td>85.320(3)</td></tr><tr><td>β/°</td><td>107.461(5)</td><td>89.590(2)</td></tr><tr><td>γ/°</td><td>90</td><td>72.842(3)</td></tr><tr><td>V/Å<sup>3</sup></td><td>1626.79(12)</td><td>830.71(5)</td></tr><tr><td>d<sub>calc.</sub>/g·cm<sup>-3</sup></td><td>1.490</td><td>1.575</td></tr><tr><td>R[F] (I > 3σ(I))</td><td>4.45%</td><td>3.47%</td></tr><tr><td>wR[F<sup>2</sup>] (all data)</td><td>11.8%</td><td>9.03%</td></tr><tr><td>Q<sub>res</sub><sup>min/max</sup>/e·Å<sup>-3</sup></td><td>-0.41/+0.59</td><td>-0.69/+0.47</td></tr><tr><td>CCDC code</td><td>2433376</td><td>2433395</td></tr></tbody></table>

with the three-body Grimme dispersion correction was employed, with the 6–311++G** basis set applied for all atoms but the nickel, for which the 6–31G** basis set³⁵ was used. The PEP scans have been performed with the TURBOMOLE suite of programs (ver. 7.1),³⁶ while the CIOPT code³⁷ interfaced with TURBOMOLE was used for the MECI optimization calculations.

### 3. RESULTS AND DISCUSSION

The examined Ni-4d and Ni-4d' were synthesized using the same synthetic protocol as in our previous articles.¹²,¹⁴,¹⁵ It appears that selection of an (N,N,O)-donor ligand, thus the amine and ketone used in the synthesis, may affect the synthetic route, leading to an expected complex described in a series of our other articles,¹⁴,¹⁵,³⁸ or to a rarer oxime-type product, as, e.g., the reported Ni-1d'.¹² In the latter case, it was the use of a more basic aliphatic amine that resulted in a nontypical synthesis product. The obtained compound appeared to be an efficient photoswitch in the solid state and can be successfully isomerized from the ground-state nitro form both to the endo- and exo-nitrito isomers upon light irradiation. The direct experimental confirmation of the proposed reaction pathway was, however, challenging in this case since the endo- and exo-nitrito isomers had a very similar

stability, and there was a rather low-energy barrier for transformation in both ways. Hence, for the purpose of the current study, we have decided to use similar ligands in the synthesis so as to possibly obtain both kinds of compounds, a typical product and the oxime-type one, in order to further investigate experimentally the isomerization mechanism and compare the behavior of both systems in the solid state.

Consequently, when compared to the synthesis of the previous Ni-1d' compound, the same aliphatic amine substrate was used, but the ketone substrate was changed from acetophenone to a more basic N,N-dimethyl acetophenone. In such a case, depending on the amount of LiNO₂ added during the reaction, different products are obtained. These are the two desired coordination compounds, Ni-4d and an oxime-type Ni-4d', which differ in the way the carbonyl fragment is attached to the metal center as shown in Scheme 2. In the case of Ni-4d', an additional NO fragment is incorporated in the coordination sphere of Ni which is the result of the excess of nitrite salt in the reaction environment along with the basicity of the amine used. Molecular structures of Ni-4d and Ni-4d' are presented in Figure 1. Similarly to the photoswitching nickel nitro complexes previously studied by us, the metal center is coordinated by the (N,N,O)-donor supporting ligand

<footnote>ᵃFor more information see Supporting Information.</footnote>
<figure><img src="image_14.png" /><figcaption>**Figure 2.** Hirshfeld fingerprint plots generated for (a) Ni-4d and (b) Ni-4d', based on the 100 K ground-state (GS) crystal structures. The intensity of the blue color reflects the contribution of specific intermolecular interactions: the brighter the blue, the greater the contribution. The presence of diffuse regions at *d*<sub>i</sub> and *d*<sub>e</sub> values (normalized "internal" and "external" surface-nearest-atom distances) around 2.0 Å and higher indicates less efficient crystal packing. The more defined and sharp features are in the plots, the stronger and more directional the corresponding interactions.<sup>39</sup></figcaption></figure>

<table><caption>**Table 2.** Interatomic Contact Contributions to the Hirshfeld Surface Generated for the Ni-4d and Ni-4d' Molecules in the GS Crystal Structures, Computed with the CRYSTALEXPLORER Program<sup>40, a</sup></caption><thead><tr><th></th><th>Ni···H</th><th>N···O</th><th>N···C</th><th>N···H</th><th>O···O</th><th>O···C</th><th>O···H</th><th>C···C</th><th>C···H</th><th>H···H</th></tr></thead><tbody><tr><th>Ni-4d</th><td>2.1%</td><td>0.2%</td><td>0.2%</td><td>2.9%</td><td></td><td>0.1%</td><td>23.2%</td><td></td><td>19.7%</td><td>51.5%</td></tr><tr><th>Ni-4d'</th><td>1.2%</td><td>0.7%</td><td>0.3%</td><td>10.3%</td><td>0.4%</td><td>1.2%</td><td>30.1%</td><td>0.7%</td><td>17%</td><td>38%</td></tr></tbody></table>

<table><caption>**Table 3.** Dimeric Motifs Engaging the NO₂ Group and Respective Interaction Energies Based on the Experimental Ground-State Crystal Structure of the Ni-4d Compound (Figure S14, Supporting Information)<sup>a,b</sup></caption><thead><tr><th>motif</th><th>E<sub>int</sub>/kJ·mol<sup>-1</sup></th><th>selected interactions</th><th>d<sub>H···A</sub>/Å</th><th>d<sub>D···A</sub>/Å</th><th>θ<sub>D···H···A</sub>/Å</th></tr></thead><tbody><tr><td>D1</td><td>-33.82</td><td>O2···H7b-C7<sup>#1</sup></td><td>2.50</td><td>3.113(4)</td><td>121.74</td></tr><tr><td rowspan="3">D2</td><td rowspan="3">-107.05</td><td>O1···H1a-C1<sup>#1</sup></td><td>2.53</td><td>3.293(4)</td><td>136.61</td></tr><tr><td>C3···H6b-C6<sup>#2</sup></td><td>2.86</td><td>3.693(5)</td><td>145.13</td></tr><tr><td>C4···H6b-C6<sup>#2</sup></td><td>2.80</td><td>3.500(5)</td><td>130.74</td></tr><tr><td rowspan="2">D3</td><td rowspan="2">-59.86</td><td>O1···H1b-C1<sup>#3</sup></td><td>2.78</td><td>3.460(4)</td><td>128.83</td></tr><tr><td>C10-H10···C3<sup>#3</sup></td><td>2.92</td><td>3.693(5)</td><td>138.51</td></tr></tbody></table>

<table><caption>**Table 4.** Dimeric Motifs Engaging the NO₂ Group and Respective Interaction Energies Based on the Experimental Ground-State Crystal Structure of Ni-4d' Structure (Figure S15, Supporting Information)<sup>a,b</sup></caption><thead><tr><th>motif</th><th>E<sub>int</sub>/kJ·mol<sup>-1</sup></th><th>selected interactions</th><th>d<sub>H···A</sub>/Å</th><th>d<sub>D···A</sub>/Å</th><th>θ<sub>D···H···A</sub>/Å</th></tr></thead><tbody><tr><td>D1</td><td>-26.08</td><td>O2···H15c-C15<sup>#1</sup></td><td>2.63</td><td>3.494(13)</td><td>150.42</td></tr><tr><td>D1'</td><td>-25.79</td><td>O2···H15c-C15<sup>#1</sup></td><td>2.51</td><td>3.379(11)</td><td>150.54</td></tr><tr><td>D3</td><td>-49.66</td><td>C6-H6c···O2<sup>#2</sup></td><td>3.84</td><td>3.740(16)</td><td>76.68</td></tr><tr><td>D3'</td><td>-37.29</td><td>N1···H5a-C5<sup>#2</sup></td><td>3.84</td><td>3.813(15)</td><td>81.45</td></tr><tr><td></td><td></td><td>O2···H6c-C6<sup>#2</sup></td><td>2.39</td><td>2.640(11)</td><td>94.10</td></tr></tbody></table>

and the ambivalent nitrite group, while the metal center coordination geometry is close to square-planar.<sup>12,14</sup>

**3.1. Crystal Structures.** Ni-4d crystallizes in the monoclinic P2<sub>1</sub>/c space group, with one molecule in the asymmetric unit (ASU) (Table 1). The nitrite group in the Ni-

4d crystal structure adopts the nitro binding mode in the ground state (GS) (Figure 1).

In turn, Ni-4d' crystallizes in the triclinic P<span style="text-decoration: overline">1</span> space group with one molecule in ASU. In the Ni-4d' crystal structure, the nitrite group exists in two isomeric forms, i.e., nitro and endo-

<footnote><sup>a</sup>Total reciprocal (i.e., X···Y and Y···X) contributions are always shown.</footnote>

<footnote><sup>a</sup>Geometries with the X-H neutron-normalized distances were used for computations. <sup>b</sup>Symmetry operations: (#1) −*x*, −*y*, −*z* − 1, (#2) −*x*, −*y* + 1, −*z* − 1, (#3) *x*, −*y* + 1/2, *z* − 1/2.</footnote>

<footnote><sup>a</sup>Geometries with the X-H neutron-normalized distances were used for computations. Dx denotes motifs formed by the nitro isomer, whereas Dx' the ones formed by the endo-nitrito isomer (*x* − motif number). <sup>b</sup>Symmetry operations: (#1) *x* − 1, *y* + 1, *z*, (#2) −*x* − 2, −*y* + 2, −*z* + 1.</footnote>
<figure><img src="image_15.png" /><figcaption>**Figure 3.** IR spectra were collected for the **Ni-4d** sample. Upper panel: before (green line) and after (red line) optimal irradiation time with 530 nm LED at 100 K for generating *endo*-nitrito form; gray lines correspond to subsequent irradiation points (1–5 min); bottom panel: before (green line) and after irradiation (blue line) with 530 nm LED at 100 K and during temperature relaxation (cyan and red line) of *endo*-nitrito form; gray lines correspond to subsequent temperature points during sample heating.</figcaption></figure>

nitrite in the GS with the nitro binding mode predominating in the crystal structure (population of ca. 70%). Multitemperature measurements of **Ni-4d′** crystal structures were performed in the 280–100 K range, showing that the *endo*-nitrito population remains generally stable (Table S7, Supporting Information). The small deviations are within experimental error.

Crystal packings of **Ni-4d** and **Ni-4d′** are different. Molecules in the crystal structure of **Ni-4d** are arranged in a herringbone-manner along the [010] direction (Figure S4, Supporting Information), which is mainly supported by C–H···π interactions and hydrogen bonds involving nitrite ligands. Furthermore, the molecules are arranged into a ribbon-like structure along the [001] direction (Figure S5, Supporting Information) which is formed mainly by C–H···π interactions and hydrogen bonds involving the nitrite ligand as in the former case. Main motif in the **Ni-4d′** structure is a dimeric plane along the [100] direction (Figure S6, Supporting Information), majorly supported by hydrogen bonds involving nitrite group as well as O3, O4, and N4 atoms from the supporting ligand along with C–H···π interactions.

Differences and similarities in the crystal packing of the examined compounds can be well illustrated and analyzed using the concept of Hirshfeld surfaces and further – fingerprint plots (Figure 2 and Table 2).⁴¹⁻⁴⁵ The calculated interatomic contact contributions to the Hirshfeld surfaces

<figure><img src="image_16.png" /><figcaption>**Figure 4.** IR spectra collected for the **Ni-4d′** sample. Upper panel: before (green line) and after (red line) optimal irradiation time with 530 nm LED light at 100 K for generating the *endo*-nitrito form; gray lines correspond to subsequent irradiation point (5 min), bottom panel: before (green line), after irradiation (blue line) at 100 K, and during temperature relaxation (cyan and red line) of the *endo*-nitrito form; gray lines correspond to subsequent temperature points during sample heating.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Figure 5.** IR spectra collected before (green line) and after (red line) optimal irradiation time at 10 K for generating the *exo*-nitrito form in the **Ni-4d′** sample with 660 nm LED; gray lines correspond to subsequent irradiation points (1–60 min).</figcaption></figure>

generated for the **Ni-4d** and **Ni-4d′** crystal structures show that both of them are dominated by H···H interactions, which is typical for molecular crystals of organic compounds. Interestingly, the percentage contribution of these contacts is significantly smaller for **Ni-4d′**. In this case, this is balanced by
<table><caption>**Table 5. Kinetic Parameters of the Endo-Nitrito and Exo-Nitrito Form Relaxation Were Determined by IR Spectroscopy**</caption><thead><tr><th rowspan="2"></th><th colspan="2">endo-nitrito</th><th colspan="2">exo-nitrito</th></tr><tr><th>Ni-4d</th><th>Ni-4d'</th><th>Ni-4d</th><th>Ni-4d'</th></tr></thead><tbody><tr><td>E<sub>a</sub>/kJ·mol<sup>-1</sup></td><td>66(2)</td><td>77 (8)</td><td>30(3)</td><td>11.4(8)</td></tr><tr><td>log(k<sub>0</sub>)</td><td>13.5(5)</td><td>15.8(2)</td><td>12.77(13)</td><td>3.4(5)</td></tr><tr><td>T<sub>d</sub>/K</td><td>211(7)</td><td>214 (2)</td><td>100(9)</td><td>93(7)</td></tr></tbody></table>

notably greater contributions of the N···H and O···H type contacts. The increased impact of these interactions results from the presence of the oxime moiety (N4–O3) and the exposed carbonyl oxygen engaged in hydrogen-bond-type contacts in the crystal structure of Ni-4d'. This kind of interactions are naturally also formed by the nitrite group, involving its exposed oxygen atoms which contribute the most to the total number of hydrogen-bond-like contacts in both systems. From the respective fingerprint plots, it can be concluded that hydrogen bonds are stronger and more distinct in the Ni-4d' structure compared to Ni-4d. However, theoretical calculations of dimeric motifs (Tables 3 and 4 and Figures S14 and S15, Supporting Information) reveal that the nitrite group in Ni-4d is better stabilized by these interactions. This suggests that the strong hydrogen bonding interactions observed in the fingerprint plot for Ni-4d' involve other parts of the molecule rather than the nitrite group. In turn, the structure of Ni-4d exhibits more effective C–H···π edge-to-face interactions between the phenyl ring, the nickel-containing aromatic ring, and hydrogen atoms from methyl groups, the phenyl ring, and the nonaromatic ring as well. Similar interactions occur in the Ni-4d' structure; however, there is fewer of them, and the molecules adopt less favorable orientations. Based on the obtained fingerprint plots, it can also be concluded that packing efficiency is higher in the case of the Ni-4d' crystal structure.

Since the nitrite ligand in Ni-4d' exists in two isomeric forms in the ground state, namely the nitro and endo-nitrito binding modes, intermolecular interactions formed by the nitro and endo-nitrito isomers can be compared. Analyzing the fingerprint plot (Figure S7, Supporting Information), it can be clearly seen that the endo-nitrito isomer is engaged in shorter and better-defined hydrogen bonds. However, theoretical calculations of dimeric motifs (Table 4) indicate that this does not improve the stability of this isomer, which might be due to the noticeably fewer interactions formed with oxygen in favor of interactions involving nitrogen. Still, part of the intermolecular interactions are similar for both endo-nitrito and nitro form. The packing for Ni-4d' in endo-nitrito form is visibly less efficient than in the case of the nitro form (Figure S7, Supporting Information).

A very useful tool in the analysis of light-induced transformations in crystals is the concept of the reaction cavity. The term and first computations were introduced by

Ohashi and co-workers⁴⁶ with a great deal of success in explaining many subtle phenomena occurring when molecules change in crystals. The method is relatively simple. It requires only “cutting out” the photoactive part from the crystal structure (in our case this would be the NO₂ group) and analyzing the shape and volume of the artificially created “void.” Nowadays several programs can compute the crystal voids and related properties. The most well-known are PLATON⁴⁷ (grid search with the van der Waals radii criteria⁴⁸), MERCURY⁴⁹ (“rolling-ball” method⁵⁰), or CRYSTALTEXPLORER (promolecule electron density cutoff criterion⁵¹). Here, we used MERCURY, with the settings identical to those in our previous work for consistency.

The reaction-cavity volumes calculated for both compounds amount to 26.3 Å³ for Ni-4d and 33.9 Å³ for Ni-4d'. Although both values are within the volume range reported for compounds exhibiting photoswitching behavior of the nitro group, the cavity in the Ni-4d crystal structure is at the lower limit.¹⁴,⁵²,⁵³ In the context of isomerization reaction, the cavity shape in the Ni-4d' crystal structure, which already hosts two linkage isomers, appears to be more favorable as it is rather evenly distributed in all directions (Figures S8 and S9, Supporting Information) providing enough space for rearrangement of the ambidentate ligand.

In view of the above, both compounds have an isomerization potential. The nitro group in both structures mainly forms hydrogen-bond-like interactions with the neighboring molecules, which, unlike strong hydrogen-bond interactions involving more electronegative atoms, may promote the linkage isomerism reaction by stabilizing the resulting isomers while also being easy to break when exposed to visible light.⁵²,⁵⁴ Furthermore, the reaction cavities in both crystal structures are sufficient to accommodate the ambidextrinated ligand transformation.

**3.2. Formation and Decay of Photoinduced Linkage Isomers.** 3.2.1. *Infrared Spectroscopy.* A series of solid-state infrared spectroscopic measurements were conducted for samples just after being exposed to different LEDs' light for varying periods of time at selected temperatures. Such an approach enables characterization of the photoswitching behavior of the examined material and defines optimal conditions of the analyzed photoinduced transformation, based on intensity changes of bands assigned to the vibrations of the nitrite group. The bands' intensity changes with irradiation time and during sample relaxation provide us also with some information about the kinetics of the process. Thermal behavior of nonirradiated samples was also investigated.

For both compounds, the respective vibration ranges for each linkage isomer under consideration are comparable. In all cases, the bands were assigned to specific vibrations based on the calculated frequencies (Table S3, Supporting Information) and available literature.¹²⁻¹⁴ For the nitro and endo-nitrito binding modes, these are bands located from 1300 to 1400

<table><caption>**Table 6. Populations of Linkage Isomers before and after Irradiation with a 660 nm LED for 30 Min at 90 K for Ni-4d and Ni-4d'**</caption><thead><tr><th rowspan="2">irradiation time</th><th colspan="3">Ni-4d</th><th colspan="3">Ni-4d'</th></tr><tr><th>nitro</th><th>endo-nitrito</th><th>exo-nitrito</th><th>nitro</th><th>endo-nitrito</th><th>exo-nitrito</th></tr></thead><tbody><tr><td>before irradiation</td><td>100%</td><td>0%</td><td>0%</td><td>69%</td><td>31%</td><td>0%</td></tr><tr><td>after 30 min irradiation</td><td>75%</td><td>0%</td><td>25%</td><td>43%</td><td>29%</td><td>28%</td></tr></tbody></table>
<figure><img src="image_18.png" /><figcaption>**Figure 6.** Molecular structures of Ni-4d (left) and Ni-4d' (right) derived from the X-ray diffraction data sets collected after 30 min of irradiation with 660 nm LED at 90 K. Note the disorder on the NO₂ site (nitro isomer - violet color, *exo*-nitrito - orange, *endo*-nitrito - yellow, atomic thermal motion is represented as ellipsoids at the 50% probability level, hydrogen atoms are omitted for clarity, only selected atom labels are shown - disorder labels, A/B/C are not shown for clarity, atoms N1, O1, N2 are approximately shown).</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>**Figure 7.** Average populations estimated for each isomer (green line - nitro, blue line - *endo*-nitrito) after irradiation with 530 nm LED at 100 K and during temperature relaxation. Upper panel presents results obtained for Ni-4d while bottom panel presents results obtained for Ni-4d'. These population values are averaged, accounting for the concurrent decay occurring during the data-collection period at each temperature point.</figcaption></figure>

cm⁻¹ and from 1070 to around 1140 cm⁻¹, respectively. Upon 530 nm LED irradiation, in the IR spectrum of **Ni-4d** (Figure 3), the intensity of the 1068, 1105, and 1121 cm⁻¹ bands associated with the symmetrical stretching vibrations of the *endo*-nitrito group are increasing, which is accompanied by lowering of the 1352, 1358, and 1372 cm⁻¹ bands corresponding to symmetrical stretching (1352 cm⁻¹) and asymmetrical stretching (1358 and 1372 cm⁻¹) vibrations of the nitro isomer, respectively. The presence of the *endo*-nitrito binding mode is also evidenced by the appearance of the 1404 and 1409 cm⁻¹ bands, which can be attributed to their symmetrical stretching. For **Ni-4d'**, the nitro-to-nitrito isomerization process is manifested by the intensity rise of the following bands upon 530 nm LED irradiation: 1065, 1085, 1104, and 1128 cm⁻¹ related to symmetrical vibrations of the *endo*-nitrito binding mode (Figure 4). Furthermore, spectral changes accompanying this transformation are also visible in the higher vibrational frequency regions, i.e., around 1410 and 1426 cm⁻¹. In turn, the nitro isomer gradual depopulation can be monitored by observation of the following bands 1321, 1330, 1361, and 1386 cm⁻¹, which are assigned to asymmetrical stretching vibrations.

In order to investigate the temperature effect on the samples, multitemperature measurements were conducted in the RT → 10 K range. The temperature was gradually reduced, and an IR spectrum was collected every 20 K. For both compounds, no spectral changes were observed during these experiments. However, it can be noticed that in the **Ni-4d'** sample, bands associated with the *endo*-nitrito binding mode are already visible without any irradiation, which is in agreement with the single-crystal structure of this compound.

The most significant spectral changes associated with the nitro-to-*endo*-nitrito isomerization for the examined compounds were observed after sample irradiation with a 530 nm LED light. The optimal irradiation time was determined at 100 K. It appears that maximal population of the *endo*-nitrito form is reached faster for **Ni-4d**, for which 5 min of LED irradiation, equal to 21.3 J·cm⁻² fluence (or energy density; Table S2, Supporting Information), of thin-film sample is sufficient. In turn, the **Ni-4d'** sample requires 10 min of irradiation (42.5 J·cm⁻² fluence) to reach the maximum conversion level. Photoinduced linkage isomers in both cases are stable up to 220 K at which their populations visibly decrease with time. At 240 K, in both cases, full relaxation to the initial state is noted.
<table><caption>Table 7. Populations of Different Linkage Isomers before and during Subsequent Steps of Irradiation with 530 nm LED at 100 K for Ni-4d and Ni-4d'</caption><thead><tr><th rowspan="2">irradiation time</th><th colspan="3">Ni-4d</th><th colspan="3">Ni-4d'</th></tr><tr><th>nitro</th><th>endo-nitrito</th><th>exo-nitrito</th><th>nitro</th><th>endo-nitrito</th><th>exo-nitrito</th></tr></thead><tbody><tr><td>before irradiation</td><td>100%</td><td>0%</td><td>0%</td><td>70%</td><td>30%</td><td>0%</td></tr><tr><td>after 1 h irradiation</td><td>57%</td><td>35%</td><td>8%</td><td><sup>a</sup></td><td><sup>a</sup></td><td><sup>a</sup></td></tr><tr><td>after 2 h irradiation</td><td>26%</td><td>74%</td><td>0%</td><td>19%</td><td>82%</td><td>0%</td></tr><tr><td>after 4 h irradiation</td><td>8%</td><td>92%</td><td>0%</td><td>7%</td><td>93%</td><td>0%</td></tr></tbody><tfoot><tr><td colspan="7"><sup>a</sup>Data not collected after this irradiation time.</td></tr></tfoot></table>

<figure><img src="image_20.png" /><figcaption>**Figure 8.** Molecular structures of Ni-4d (left) and Ni-4d' (right) derived from the X-ray diffraction data sets collected after 4 h of irradiation with 530 nm LED at 100 K. Note the disorder on NO₂ group (nitro isomer - violet color, exo-nitrito - orange, endo-nitrito - yellow, atomic thermal motion is represented as ellipsoids at the 50% probability level, hydrogen atoms are omitted for clarity, only selected atom labels are shown - disorder labels, A/B/C are not shown for clarity, atoms N1, O1, N2 are approximately shown).</figcaption></figure>

Throughout the IR experiments, the generation of the exo-nitrito isomer under 530 and 660 nm LED light irradiation was detected for both Ni-4d and Ni-4d'. For Ni-4d', the existence of this metastable form was clearly evidenced by the distinct appearance of 1044, 1065, and 1460 cm⁻¹ bands after irradiation. These spectral changes were particularly well-defined, allowing for definitive identification of the formation of the exo-nitrito form. For the Ni-4d complex, the transition to the exo-nitrito form was confirmed by an increase of the 1068 and 1436 cm⁻¹ bands. However, the 1068 cm⁻¹ band substantially overlaps with that from the endo-nitrito binding mode vibrations, complicating the precise identification of the exo-nitrito isomer. This contrasts with the Ni-4d' complex, where the spectral changes allow for more reliable characterization of the isomeric transition.

The optimal irradiation wavelengths for inducing the formation of the exo-nitrito isomer were determined at 10 K, where this isomer can be more effectively generated. For Ni-4d', short irradiation with 530 nm LED (5 min, 4.3 J·cm⁻² fluence) produced excellent results, with clearly visible spectral bands corresponding to the exo-nitrito form. Notably, Ni-4d' requires significantly lower LED power compared to Ni-4d to achieve the exo-nitrito form with minimal admixture of the endo-nitrito isomer. This difference is attributed to the rapid formation of the endo-nitrito binding mode when Ni-4d is exposed to 530 nm LED light (Figures S12 and S13, Supporting Information).

Further experiments revealed that 660 nm LED light could generate even higher populations of the exo-nitrito form in both compounds with lower conversion to the endo-nitrito isomer (Figure 5). The optimal irradiation time for Ni-4d' was longer (1 h, 451.7 J·cm⁻² fluence) compared to that of Ni-4d (Figure S12, Supporting Information). While 530 nm proved to be the energy-optimal wavelength for generating the exo-nitrito isomer in both complexes, the 660 nm LED provided better conditions for monitoring the isomerization process due

to the slower course of the photoinduced transformation in this case.

### 3.2.2. Relaxation Kinetics of the Photoinduced Linkage Isomers.
The kinetics of the relaxation of the metastable endo-nitrito and exo-nitrito isomers in the solid state was examined for both compounds (Table 5). For this purpose, a series of isothermal IR spectroscopic measurements during the relaxation process were conducted. In order to determine the decay constants (k) at each temperature, the first-order kinetic equation was employed. The activation energy (Eₐ) and frequency factor (k₀) were calculated based on the Arrhenius equation. The decay temperature (Tₐ), which corresponds to the point at which k = 10⁻³ s⁻¹, was calculated based on the obtained kinetic parameters. Despite the fact that the activation energy and frequency factor differ between the two compounds, the resulting decay temperatures, derived for both the exo-nitrito and endo-nitrito metastable forms, are comparable. For the endo-nitrito isomers, Tₐ is almost equal; hence, it can be concluded that their stability in the Ni-4d and Ni-4d' thin films is very similar. In turn, Tₐ determined for the exo-nitrito forms differs more significantly between the studied compounds, which indicates slightly lower stability of this linkage isomer in the Ni-4d' solid-state sample.

### 3.2.3. Photocrystallography.
Having the basic knowledge about the Ni-4d and Ni-4d' crystal structures along with the results of IR spectroscopic experiments, photocrystallographic studies on single crystals were conducted. In accordance with the spectroscopic findings, single crystals were irradiated with 530 and 660 nm LED light and examined toward formation of the nitrito linkage isomers. Such experiments allow us to evaluate the actual structures of the photoinduced linkage isomers, to relatively accurately estimate their populations, and to contrast the outcomes with those deduced from the spectroscopic studies for thin-film samples. The metastable state population is determined from structural refinement of the disordered nitrite group. Based on our rough analysis
<figure><img src="image_21.png" /><figcaption>**Figure 9.** IR spectra collected for the Ni-4d' sample during: (i) top panel: subsequent cycles of irradiation with 530 nm LED at 10 K, (ii) middle panel: exo-nitrito isomer relaxation over time at 90 K, (iii) bottom panel: thermal relaxation of the exo-nitrito isomer in the 10–220 K temperature range. Green line indicates the initial state, red line represents spectra collected after irradiation, and blue line signifies the final state. Gray lines represent spectra collected between irradiation and final state.</figcaption></figure>

(Supporting Information), the estimated standard deviations do not exceed 3%. Thus, in the following discussion, the populations are rounded and analyzed, focusing mainly on the observed trends and/or changes, taking into account possible slight deviations from the given values.

**3.2.3.1. Photocrystallography at 90 K.** Based on the above-described kinetic measurements for the exo-nitrito form in the Ni-4d compound, it is evident that achieving a substantial

<figure><img src="image_22.png" /><figcaption>**Figure 10.** Intensity evolution of the characteristic bands for the exo- and endo-nitrito binding modes during irradiation of Ni-4d' with 530 nm LED at 10 K.</figcaption></figure>

population of this isomer would be a challenging task at 100 K and above. This is because the estimated lifetime of this isomer at 100 K is about 16 min, as calculated applying the Arrhenius equation with the determined and above-described kinetic parameters. Thus, during a crystallographic measurement, a significant portion of this form may undergo thermal relaxation. In turn, at 90 K, the exo-nitrito form's lifetime extends to approximately 10 h, which facilitates obtaining more reliable results. The highest conversion to the exo-nitrito form, 25%, was reached after irradiation for 30 min with a 660 nm LED (1.86 J·cm⁻² fluence) (Table 6). The same experiment was conducted for Ni-4d'—in this case, the predicted lifetime of the exo-nitrito form at 90 K reaches about 27 min. Nevertheless, the reaction was stopped after 30 min irradiation with a 660 nm LED (2.59 J·cm⁻² fluence); it was possible to generate the exo-nitrito isomer and estimate its population to 28%, while the endo-nitrito population remained unchanged within the estimation error. Crystal structures obtained after irradiation are listed in Figure 6.

**3.2.3.2. Photocrystallography at 100 K.** The ground-state structure of Ni-4d at 100 K contained exclusively the nitro form. In order to determine the optimal irradiation time to generate the metastable states, the sample was irradiated within three time steps with 530 nm LED (Figure 7 and Table 7). After 1 h (1.05 J·cm⁻² fluence), a population of 35% of the endo-nitrito and 8% of the exo-nitrito linkage isomer was generated. During the subsequent 1 h of light exposure (2.10 J·cm⁻² fluence), the exo-nitrito isomer completely vanished, while the endo-nitrito binding mode's population increased to 74%. The maximal conversion to this latter form was achieved after 2 additional hours of irradiation (4.21 J·cm⁻² fluence) and reached about 92%. Regarding the relaxation of the endo-nitrito linkage isomer in Ni-4d crystals, it appears that up to 180 K, no population changes are observed. At 200 K, there is a small reduction of the endo-nitrito (by 7%) population, which sharply decreases to 6% at 220 K. At 240 K, complete relaxation to the ground-state structure was noted.

Similar investigations were conducted for the Ni-4d' crystal structure (Figure 7 and Table 7). As already mentioned, in the ground-state structure at 100 K, it consists of ca. 30% of the endo-nitrito form and ca. 70% of the nitro form. The crystal
<figure><img src="image_23.png" /><figcaption>**Figure 11.** Populations estimated for each isomer (green line - nitro, red line - *exo*-nitrito, blue line - *endo*-nitrito) before irradiation at 90 K (filled points), after irradiation with 660 nm LED for 30 min at 90 K, and during temperature relaxation. Upper panel presents results obtained for **Ni-4d** compound while bottom panel presents results obtained for the **Ni-4d'** compound.</figcaption></figure>

was irradiated with a 530 nm LED in two time steps, each lasting for 2 h. The initial 2 h light exposure (16.94 J·cm⁻² fluence) led to an increase of the *endo*-nitrito population to 82%, while during the subsequent 2 h of irradiation (33.88 J·cm⁻² fluence), the population of the *endo*-nitrito isomer further increased reaching 93%. Due to high population of the *endo*-nitrito isomer in the ground state, the relaxation of the photoinduced *endo*-nitrito form is particularly interesting. The population of *endo*-nitrito isomer remains stable up to 150 K and decreases to 79% at 200 K. As the temperature rises to 220 K, the population further diminishes to 28%, reaching a thermally invariant state at this point—the population of the *endo*-nitrito isomer remains constant at higher temperatures. Crystal structures obtained after irradiation are listed in Figure 8.

### 3.3. Mechanism of the Nitro-to-*Endo*-Nitrito Transformation.
As previously described, irradiation of the samples with a 530 nm LED can result in the generation of both *endo-* and *exo*-nitrito forms. For both compounds, the isomerization reaction is notably slower at 10 than at 100 K. Thus, IR

<figure><img src="image_24.png" /><figcaption>**Figure 12.** Potential-energy profiles (PEPs) calculated for the S₀ state along the O-N-Ni valence angle (nitro → *exo*-nitrito, left panel), and the O-N-O-Ni torsion angle (*exo*-nitrito → *endo*-nitrito, right panel), respectively (black curve - **Ni-4d**, red curve - **Ni-4d'**).</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>**Figure 13.** Potential-energy profiles along the O-N-Ni reaction coordinate optimized in the S₁ state for the **Ni-4d** (black lines) and the **Ni-4d'** (red lines) systems. The optimized state in each set has been marked with circles: full for **Ni-4d** and empty for **Ni-4d'**, while the other states, calculated vertically on the S₁ geometries, are marked with solid/dashed lines for **Ni-4d**/*Ni-4d'*, respectively. The black-full/red-empty diamonds mark positions of the optimized MECI points of **Ni-4d** and **Ni-4d'**.</figcaption></figure>

experiments with gradual irradiation with 530 nm LED at 10 K were conducted in order to shed light on the photo-isomerization reaction mechanism.

Formation of both photogenerated binding modes is easier to trace for **Ni-4d'**; since, in this case, it is possible to differentiate bands related to both metastable forms. It can be clearly seen that after the first cycle of irradiation for 5 min, the bands associated with the *exo*-nitrito and *endo*-nitrito form appear (Figure 9). After the second irradiation step (additional 10 min of irradiation), there is a noticeable decrease of the *exo*-nitrito population with a concurrent increase in the population of the *endo*-nitrito linkage isomer. During further light exposure (for 20, 30, 60, 90, and 120 min), the *exo*-nitrito isomers' population gradually diminishes until it completely disappears, while the population of the *endo*-nitrito binding mode continuously increases (Figure 10).

The reaction mechanism can also be investigated via tracing of the *exo*-nitrito state thermal relaxation (Figure 11). Thus,
after reaching maximal intensity of the band associated with the *exo*-nitrito form at 10 K with 530 nm, the sample was heated and IR spectra were collected with a 20 K step until the complete disappearance of the *endo*-nitrito isomer at 220 K. Additionally, it was observed that at 90 K, the *exo*-nitrito binding mode, once generated, transforms into the *endo*-nitrito isomer in time when the irradiation is stopped. This clearly suggests that the *exo*-nitrito isomer converts directly to the *endo*-nitrito binding mode by thermal relaxation.

In the IR experiments for Ni-4d, an identical pattern was observed under irradiation with a 530 nm LED at 10 K and during thermal and time relaxation of the *exo*-nitrito form.

Finally, related photocrystallographic experiments were conducted in order to verify the applicability of spectroscopic observations to single-crystal samples. Experimental conditions were selected based on kinetic considerations and the available lowest temperature for the X-ray diffraction studies at our home laboratory. Hence, single crystals of Ni-4d and Ni-4d' were irradiated with a 660 nm LED light for 30 min at 90 K so as to generate the *exo*-nitrito binding mode. 25% of this isomer was generated in the case of the Ni-4d system, whereas for Ni-4d', such population reached 29% (the admixture of around 30% of the *endo*-nitrito form remained the same as for the ground-state structure). In order to verify the isomerization mechanism, i.e., the *endo*-nitrito isomer formation through the *exo*-nitrito form as an intermediate photoproduct, the X-ray diffraction measurements were conducted during heating at the following temperatures: 100, 120, 140, 180, and 220 K. This way the relaxation channel of the process was probed. Based on these experiments, it is clearly visible that during relaxation, the *exo*-nitrito form fully transforms to the *endo*-nitrito linkage isomer. Starting at 100 K, partial conversion of the *exo*-nitrito form to the *endo*-nitrito binding mode was observed. In the Ni-4d crystal, population of the *exo*-nitrito species decreased to about 16% concurrently generating 9% of the *endo*-nitrito form. Similarly, in Ni-4d', the relaxation of the *exo*-nitrito isomer to 19% resulted in 12% increase of the *endo*-nitrito form's population. Full conversion to the *endo*-nitrito isomer occurred at 120 K in both systems and its population remained stable up to 220 K, at which the *endo*-nitrito isomer fully relaxed to the nitro form in Ni-4d, while in Ni-4d', the population of this isomer decreased to 26%—which is comparable to its initial value.

The theoretical modeling of the mechanism coincides with the results obtained in our previous paper concerning the Ni-1d' compound in the solid state. First, to get a notion of a possible source of differences in the interisomer transformation dynamics observed in the electronic ground state for Ni-4d and Ni-4d', we had a look at the S₀ potential-energy profiles obtained for respective isolated molecules. The S₀ profiles between the nitro, *exo*-nitrito, and *endo*-nitrito forms, calculated along the O-N-Ni angle (nitro → *exo*-nitrito), and O-N-O-Ni torsion angle (*exo*-nitrito → *endo*-nitrito) coordinates, are shown in Figure 12. One can notice that, in both cases, the barriers for the nitro → *exo*-nitrito transformation, estimated to ca. 0.936 (90.3 kJ·mol⁻¹) and 0.910 eV (87.8 kJ·mol⁻¹) for Ni-4d and Ni-4d', respectively, significantly exceed the corresponding *exo*-nitrito → *endo*-nitrito reaction barriers of 0.321 (31.0 kJ·mol⁻¹) and 0.293 eV (28.3 kJ·mol⁻¹). Moreover, the slightly smaller values obtained for the Ni-4d' system stay in line with the observed lower thermal stability of its nitrito forms, on the one hand, and the faster *exo*-nitrito to *endo*-nitrito transformation, on the other.

In the next step, to investigate the possible mechanism of the photoinduced nitro → *exo*-nitrito transformation expected to be the primary photoswitching channel in the studied compounds, we calculated the S₁-optimized potential-energy profiles (Figure 13) along the O-N-Ni angle: a reaction coordinate used also in the S₀ PEP calculations. Both for Ni-4d and Ni-4d', it can be observed that after the S₀ → S₁ excitation, initially the system ends up in a rather flat S₁ potential-energy surface area, protected from the reactive transformation by a small energy barrier of 0.061 (5.9 kJ·mol⁻¹) and 0.054 eV (5.2 kJ·mol⁻¹) for Ni-4d and Ni-4d', respectively. At the same time, the expected kinetic energy excess gained at the Franck-Condon vicinity due to the initial relaxation from the respective vertically excited GS structures amounts for Ni-4d and Ni-4d' to 0.392 (37.8 kJ·mol⁻¹) and 0.403 eV (38.9 kJ·mol⁻¹). Thus, it can be expected that after some time spent in the Franck-Condon area, the excited nitro isomer may easily reach a potential-energy surface region characterized with a very small S₁-S₀ energy gap and, eventually, relax through a nearby, energy-accessible conical intersection. Such minimum-energy conical intersection points have been identified for both studied systems; their geometrical structures can be found in the Supporting Information (Figure S18). Energy-wise, both found MECIs lie below the scanned S₁ profiles, characterized by relative energies of 1.780 (171.7 kJ·mol⁻¹) and 1.848 eV (178.3 kJ·mol⁻¹) for Ni-4d and Ni-4d', respectively.

At the same time, we note that no direct photoreaction path between the nitro and *endo*-nitrito isomers was established in the course of the present study, which is in agreement with our previous study on a closely related system.¹²

## 4. MOLECULAR STABILITY AND INTERMOLECULAR INTERACTIONS ANALYSIS

To support the experimental findings and possibly gain deeper insights into the examined processes, structure optimizations

<table><caption>**Table 8.** Energy Differences (*E*<sub>diff</sub>) between the Ground (Nitro) and Metastable (*Exo-* and *Endo*-Nitrito) Linkage Isomers Computed for the Optimized Isolated-Molecule Geometries and for the Molecular Geometries from the CRYSTAL-Optimized Ni-4d and Ni-4d' Crystal Structures</caption><thead><tr><th rowspan="3">isomer</th><th colspan="4">E<sub>diff</sub>/kJ·mol<sup>-1</sup></th></tr><tr><th colspan="2">Ni-4d</th><th colspan="2">Ni-4d'</th></tr><tr><th>isolated mol.</th><th>CRYSTAL</th><th>isolated mol.</th><th>CRYSTAL</th></tr></thead><tbody><tr><td>nitro</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>endo-nitrito</td><td>8.16</td><td>16.79</td><td>5.78</td><td>5.66</td></tr><tr><td>exo-nitrito</td><td>16.44</td><td>30.93</td><td>17.69</td><td>17.50</td></tr></tbody></table>

<table><caption>**Table 9.** Deformation Energy (*E*<sub>def</sub>) between Optimized Isolated Molecule and the Molecule Optimized in the Crystal Environment (*E*<sub>def</sub> = *E*<sub>cry</sub> − *E*<sub>isol</sub>) Presented for the Nitro, *Endo*-Nitrito, and *Exo*-Nitrito Isomers Both for Ni-4d and Ni-4d'</caption><thead><tr><th rowspan="2">isomer</th><th colspan="2">E<sub>def</sub>/kJ·mol<sup>-1</sup></th></tr><tr><th>Ni-4d</th><th>Ni-4d'</th></tr></thead><tbody><tr><td>nitro</td><td>21.12</td><td>28.74</td></tr><tr><td>endo-nitrito</td><td>17.06</td><td>29.22</td></tr><tr><td>exo-nitrito</td><td>22.54</td><td>30.44</td></tr></tbody></table>
<table><caption>Table 10. Dimeric Motifs Engaging the NO₂ Group and the Respective Interaction Energies Based on the CRYSTAL- Optimized Crystal Structures for Each Linkage Isomer in the Ni-4d Structure (Figure S14, Supporting Information)ᵃ,ᵇ</caption><thead><tr><th>motif</th><th>E<sub>int</sub>/kJ·mol<sup>-1</sup></th><th>selected interactions</th><th>d<sub>H···A</sub>/Å</th><th>d<sub>D···A</sub>/Å</th><th>θ<sub>D···H···A</sub>/Å</th></tr></thead><tbody><tr><td>D1</td><td>-30.69</td><td>O2···H7b-C7#1</td><td>2.50</td><td>3.113</td><td>121.74</td></tr><tr><td>D2</td><td>-92.91</td><td>O1···H1a-C1#2<br>C3···H6b-C6#2<br>C4···H6b-C6#2</td><td>2.53<br>2.86<br>2.80</td><td>3.293<br>3.693<br>3.500</td><td>136.61<br>145.13<br>130.74</td></tr><tr><td>D2'</td><td>-91.90</td><td>O1···H1b-C1#2<br>C3···H6c-C6#2<br>C4···H6c-C6#2</td><td>2.46<br>2.87<br>2.82</td><td>3.193<br>3.717<br>3.497</td><td>132.74<br>147.95<br>128.44</td></tr><tr><td>D2"</td><td>-112.83</td><td>N1···H1a-C1#2<br>C4···H6b-C6#2<br>C5···H6b-C6#2</td><td>2.52<br>2.79<br>2.73</td><td>3.36<br>3.507<br>3.454</td><td>146.72<br>131.86<br>132.87</td></tr><tr><td>D3</td><td>-37.30</td><td>C11···H6a-C6#3<br>N4···H6a-C6#3</td><td>3.00<br>2.85</td><td>3.638<br>3.729</td><td>125.18<br>152.03</td></tr><tr><td>D3'</td><td>-36.76</td><td>C11···H6a-C6#3<br>N4···H6a-C6#3</td><td>2.96<br>2.78</td><td>3.604<br>3.663</td><td>125.44<br>152.59</td></tr><tr><td>D3"</td><td>-35.25</td><td>C14···H14b···O1#3<br>C11···H6a-C6#3<br>C12···H6a-C6#3</td><td>3.17<br>3.00<br>3.57</td><td>3.772<br>3.638<br>4.049</td><td>122.03<br>125.18<br>113.08</td></tr><tr><td>D4</td><td>-60.96</td><td>O1···H1b-C1#4<br>C10···H10···C3#4</td><td>2.78<br>2.92</td><td>3.460<br>3.693</td><td>128.83<br>138.51</td></tr><tr><td>D4'</td><td>-55.27</td><td>N1···H1a-C1#4<br>N1···H2b-C2#4<br>C10···H10···C3#4</td><td>2.63<br>2.70<br>2.92</td><td>3.325<br>3.191</td><td>129.25<br>112.55<br>147.92</td></tr><tr><td>D4"</td><td>-48.73</td><td>O2···H1b-C1#4<br>C10···H10···C3#4</td><td>2.20<br>2.92</td><td>3.014<br>3.715</td><td>141.56<br>141.26</td></tr></tbody></table>

ᵃDx denotes motifs formed by the nitro isomer, Dx' formed by the endo-nitrito isomer, and Dx" formed by the exo-nitrito isomer (x - motif number). ᵇSymmetry operations: (#1) -x, -y, -z - 1, (#2) -x, -y + 1, -z - 1, (#3) x - 1, -y + 1/2, z - 1/2, (#4) x, -y + 1/2, z - 1/2.

and interaction energy computations were executed. Isolated-molecule geometry optimizations were performed in the GAUSSIAN program, as well as crystal-structure optimizations in CRYSTAL.³² The obtained results are summarized in Tables 8 and 9. Based on the calculations and experimental outcomes, it is evident that the nitro form constitutes the most stable linkage isomer in the case of both compounds, whereas the exo-nitrito binding mode itself is the least energetically favored one. The stability of the exo- and endo-nitrito isomers is comparable for the Ni-4d and Ni-4d' compounds as far as the isolated molecules are concerned. However, after optimizing the whole crystal structure, it becomes clear that packing and intermolecular interactions play an important role in the stability of different forms of the nitrite ligand, especially in the case of the Ni-4d crystal structure. The calculated energy differences between the considered linkage isomers become more significant in this case, which is in contrast to Ni-4d' for which they remain similar to the ones calculated for the isolated molecules. The minor role of intermolecular interactions for Ni-4d', thus very similarly energetically stabilized nitro and endo-nitrito isomers in the crystal environment (<6 kJ·mol⁻¹ energy difference), supports their coexistence in the crystal structure. In turn, the deformation energies provided in Table 9 appear significantly larger for Ni-4d'-this might, to some extent, explain a greater difficulty in

<table><caption>Table 11. Dimeric Motifs Engaging the NO₂ Group and the Respective Interaction Energies Based on the CRYSTAL- Optimized Crystal Structures for Each Linkage Isomer in the Ni-4d' Structure (Figure S15, Supporting Information)ᵃ,ᵇ</caption><thead><tr><th>motif</th><th>E<sub>int</sub>/kJ·mol<sup>-1</sup></th><th>selected interactions</th><th>d<sub>H···A</sub>/Å</th><th>d<sub>D···A</sub>/Å</th><th>θ<sub>D···H···A</sub>/Å</th></tr></thead><tbody><tr><td>D1</td><td>-25.67</td><td>O2···H15c-C15#1</td><td>2.63</td><td>3.494</td><td>150.42</td></tr><tr><td>D1'</td><td>-25.87</td><td>O2···H15c-C15#1</td><td>2.50</td><td>3.38</td><td>151.2</td></tr><tr><td>D1"</td><td>-24.79</td><td>N1···H15c-C15#1</td><td>2.63</td><td>3.494</td><td>150.42</td></tr><tr><td>D2</td><td>-86.16</td><td>O3···H1a-C1#2<br>N4···H2b-C2#2<br>C9···H2b-C2#2<br>C14-H14c···O4#2</td><td>2.62<br>2.67<br>2.81<br>2.32</td><td>3.191<br>3.670<br>3.136</td><td>118.39<br>113.93<br>148.95<br>142.89</td></tr><tr><td>D2'</td><td>-84.53</td><td>O3···H1a-C1#2<br>N4···H2b-C2#2<br>C9···H2b-C2#2<br>C14-H14c···O4#2</td><td>2.62<br>2.67<br>2.81<br>2.32</td><td>3.191<br>3.670<br>3.136</td><td>118.39<br>113.93<br>148.95<br>142.89</td></tr><tr><td>D2"</td><td>-87.34</td><td>O2···H6a-C6#2<br>O3···H1a-C1#2<br>N4···H2b-C2#2<br>C9···H2b-C2#2<br>C14-H14c···O4#2</td><td>2.50<br>2.62<br>2.67<br>2.81<br>2.32</td><td>3.415<br>3.191<br>3.187<br>3.670<br>3.136</td><td>160.37<br>118.39<br>113.93<br>148.95<br>142.89</td></tr><tr><td>D3</td><td>-47.27</td><td>C6-H6c···O2#3</td><td>3.84</td><td>3.740</td><td>76.68</td></tr><tr><td>D3'</td><td>-46.77</td><td>N1···H5a-C5#3<br>O2···H6c-C6#3</td><td>3.84<br>2.39</td><td>3.813<br>2.640</td><td>81.45<br>94.10</td></tr><tr><td>D3"</td><td>-51.00</td><td>O2···H5a-C5#3<br>O2···H6c-C6#3</td><td>2.88<br>2.71</td><td>2.920<br>3.481</td><td>82.94<br>91.35</td></tr></tbody></table>

ᵃDx denotes motifs formed by the nitro isomer, Dx' formed by the endo-nitrito isomer, and Dx" formed by the exo-nitrito isomer (x - motif number). ᵇSymmetry operations: (#1) x - 1, y + 1, z, (#2) x + 1, y, z, (#3) -x - 2, -y + 2, -z + 1.

forming crystals which was noted during crystallization in the experimental process.

Additionally, interaction energies between dimeric motifs' components involving different ambidicate ligand's binding modes were analyzed (Tables 10 and 11, and Figures S14 and S15, Supporting Information). Generally, the nitrite ligand is engaged mostly in weak, hydrogen-bond-type interaction, such as O···H-C and N···H-C. The dimeric motifs in the Ni-4d structure are overall better energetically stabilized than those in Ni-4d'. The energies and interactions of the corresponding dimeric motifs of Ni-4d noticeably vary between isomeric forms in some cases. Primarily, the nitro isomer in the Ni-4d structure creates an additional interaction relative to the exo- and endo-nitrito isomers, which is present in the D4 motif. The exo-nitrito form is best stabilized in dimer D6, where its interaction energy is lower by ca. 20 kJ·mol⁻¹ when compared to other isomers, which results from the formation of a hydrogen bond with the nitrogen atom from the NO₂ group. All of the isomers are similarly stabilized in motif D7. In motif D8, each isomer is engaged in different intermolecular interactions, which results in noticeable differences in their energies with the nitro group being the most stabilized and the exo-nitrito form the least. In turn, the stabilization energies for the corresponding dimeric motifs formed by various isomers in Ni-4d' are quite similar. In motif D1, the exo-nitrito form is slightly less stabilized due to the formation of hydrogen bonds with the nitrogen instead of the oxygen atom in the case of the remaining isomers. However, D2 and D8 engaging the exo-nitrito form are characterized by the lowest energy. The overall stabilization energies of the endo-nitrito and nitro forms are
very similar, which, combined with the small energy gap between the optimized molecules of these isomers, calculated both for the isolated molecule and within the crystal environment (Table 8), could explain the initial high population of the *endo*-nitrito isomer in the ground state of the *Ni-4d'* crystal.

## 5. CONCLUSIONS

Two related square-planar nickel(II) complexes—*Ni-4d* and *Ni-4d'*—were designed in order to investigate the mechanism of photoinduced nitro-to-*endo*-nitrito transformation. For both *Ni-4d* and *Ni-4d'*, we were able to generate and experimentally observe all three forms of the nitrite ligand—nitro, *endo*-nitrito, and *exo*-nitrito isomers—which made these complexes promising for tracking the photoisomerization pathway. For both systems, the *endo*-nitrito isomer can be generated most efficiently using 530 nm LED light with nearly 100% conversion at 100 K. This isomer showed comparable stability for both compounds ($T_d \sim 210–215$ K) in the solid state. In turn, the *exo*-nitrito isomer is formed under 530 or 660 nm LED light irradiation. However, its observation is facilitated at temperatures below 100 K, due to its lower thermal stability ($T_d$ amounts to 100 K for *Ni-4d* and 93 K for *Ni-4d'*). The maximal obtained populations of this form based on the photo-XRD experiments amounted to ca. 25% at 90 K in single crystals of both compounds when irradiated with 660 nm LED light. The lower stability of the *exo*-nitrito isomer compared to the *endo*-nitrito form in these compounds and slower kinetics at lower temperatures enabled experimental tracking of the nitro-to-*endo*-nitrito isomerization pathway.

We report the first experimental evidence of the previously predicted nitro-to-nitrito isomerization mechanism for square-planar nickel(II) nitrite coordination compounds in the solid state. The formation of the *exo*-nitrito linkage isomer in the first stage of the photoreaction has been confirmed spectroscopically and structurally during extended irradiation with 530 nm LED, as well as via monitoring of both thermal and time relaxation processes. The experimental findings are supported by theoretical calculations performed for both compounds. The barriers for the nitro → *exo*-nitrito transformation (ca. 90.3 and 87.8 kJ·mol⁻¹ for *Ni-4d* and *Ni-4d'*, respectively) significantly exceed the corresponding *exo*-nitrito → *endo*-nitrito reaction barriers (31.0 for *Ni-4d* and 28.3 kJ·mol⁻¹ for *Ni-4d'*). Finally, the photoswitching reactivity of the studied systems is confirmed by determination of low-energy $S_1–S_0$ conical intersection points, easily accessible after the respective nitro-form excitation. Importantly, no direct photo-reaction path between the nitro and *endo*-nitrito isomers was established, which indicates the sequential mechanism via the *exo*-nitrito intermediate.

In summary, the presented experimental evidence clearly shows that the nitrite group photoisomerization in square-planar $Ni^{II}$ nitro complexes can be described as the nitro → *exo*-nitrito → *endo*-nitrito transformation in the solid state. This is in agreement with theoretical modeling and indicates that the mechanism *differs* from that proposed for the Ni nitro *octahedral* complexes. Nevertheless, there is still an open question whether an analogous mechanism of the nitrite group isomerization applies for other transition-metal square-planar nitro complexes. This will be further investigated.

## ■ ASSOCIATED CONTENT

### Data Availability Statement

High-volume raw and processed data deposited in the UW Research Data Repository under the following DOI: 10.58132/izx4cu.

### ⚙️ Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c08267.

Comprehensive synthesis and compound characterization data, structure, crystal packing figures, supporting spectroscopic plots, cavity volume data, and additional computational data (PDF)

* Ni-4d-series1 (CIF)

* Ni-4d-series2 (CIF)

* Ni-4d-prim-series3 (CIF)

* Ni-4d-prim-series4 (CIF)

* Ni-4d-prim-series5 (CIF)

### Accession Codes

CCDC 2433376–406, 2433713–5, and 2481487–93 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/ or by emailing data_request@ccdc.cam.ac.uk or by contacting the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.

## ■ AUTHOR INFORMATION

### Corresponding Author

Katarzyna N. Jarzembska — University of Warsaw, Faculty of Chemistry, 02-089 Warsaw, Poland; ● orcid.org/0000-0003-4026-1849; Email: katarzyna.jarzembska@uw.edu.pl

### Authors

Krystyna A. Deresz — University of Warsaw, Faculty of Chemistry, 02-089 Warsaw, Poland

Artem Mikhailov — Université de Lorraine, CNRS, CRM2, 54500 Nancy, France; ● orcid.org/0000-0002-3230-1137

Joanna Jankowska — University of Warsaw, Faculty of Chemistry, 02-089 Warsaw, Poland; ● orcid.org/0000-0002-7045-1478

Lorenzo Donà — Dipartimento di Chimica, Università di Torino, 10125 Torino, Italy; ● orcid.org/0000-0001-7735-3881

Bartolomeo Civalleri — Dipartimento di Chimica, Università di Torino, 10125 Torino, Italy; ● orcid.org/0000-0003-3198-3161

Adam Krówczyński — University of Warsaw, Faculty of Chemistry, 02-089 Warsaw, Poland

Radosław Kamiński — University of Warsaw, Faculty of Chemistry, 02-089 Warsaw, Poland; ● orcid.org/0000-0002-8450-0955

Dominik Schaniel — Université de Lorraine, CNRS, CRM2, 54500 Nancy, France; ● orcid.org/0000-0003-2440-9540

Complete contact information is available at: https://pubs.acs.org/10.1021/acsomega.5c08267

### Notes

The authors declare no competing financial interest.
## ACKNOWLEDGMENTS

K.A.D. and K.N.J. thank both the National Science Centre (Poland; grant No. 2019/35/O/ST4/04197, PRELUDIUM BIS program) and Polish National Agency for Academic Exchange (grant No. PPN/STA/2021/1/00050/U/00001) for financial support. Furthermore, financial support by Agence Nationale de la Recherche (France; grant Nos. ANR-21-CE30-0045-01 and LUE-ANR-15-IDEX-0004) is gratefully acknowledged. The in-house X-ray diffraction experiments were carried out at the Department of Physics, University of Warsaw, on a Rigaku Oxford Diffraction SuperNova diffractometer, which was cofinanced by the European Union within the European Regional Development Fund (POIG.02.01.00-14.122/09). The Wrocław Centre for Networking and Supercomputing (Poland; grant No. 285) and the PLGrid high-performance computing infrastructure (Academic Computer Centre Cyfro-net AGH, Poland; grant No. PLG/2024/017760) are gratefully acknowledged for providing computational facilities. For the purpose of *open access* (OA), the Authors applied a CC-BY public copyright license to any Author Accepted Manuscript (AAM) version arising from this submission.

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