Solvent- and heat-induced polymorphic transformation with single-crystal integrity in Cu(II) paddle wheel metal complexes

Abstract

We report the single-crystal-to-single-crystal (SCSC) transformation of the Cu(II) paddle wheel complex [Cu₂(benzoate)₄(24F-4spy)₂] involving two novel polymorphs from form II to form I driven by both through solvent induced and thermal activation. In form I, the 24F-4spy ligands are aligned in a head-to-tail fashion, leading to photoreactivity, whereas form II is photostable. Single crystals of form II gradually transform into form I at room temperature, as confirmed by single-crystal X-ray diffraction and powder X-ray diffraction. In addition, two related Cu(II) paddle-wheel complexes, [Cu₂(benzoate)₄(26F-4spy)₂] and [Cu₂(benzoate)₄(34F-4spy)₂], are also described for comparison.

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Polymorphism and solid-state reactivity constitute fundamental principles in the design of functional crystalline materials, enabling precise tuning of structural and physicochemical properties without modification of the molecular composition. Single-crystal-to-single-crystal (SCSC) transformations, in particular, afford a unique opportunity to elucidate molecular rearrangements and phase transitions at the atomic level while maintaining crystallinity, thereby providing direct structural characterization of the transformation pathway. These processes are of paramount importance for elucidating structure–property relationships and developing advanced stimuli-responsive materials for applications in actuation, sensing, and optoelectronics.^1–7 This enables direct structural insight into the transformation mechanisms. Such transformations are crucial not only for understanding structure–property relationships but also for developing stimuli-responsive materials for actuation, sensing, and optoelectronic applications. 1971 Gerhard Schmidt showed a very classical work of solid-state organic photochemistry and stated that reactions can be easily facilitated in the solid state when the reactive functional groups are close and in the correct orientation.⁸ In the context of coordination photochemistry, transformations coupled with topochemical [2+2] cycloaddition reactions (CAR) have opened exciting avenues for developing crystalline materials that convert molecular-level reactions into macroscopic responses. However, understanding the interplay between metal–ligand conformation, polymorphism, and photoreactivity remains limited.⁹ Later on, these developments are extended with variety of applications like topochemical transformations, photoreactive (PR) materials especially simple metal complexes wherein they would form new bond in the crystalline state which is the only way to attain C–C bond formation, and several polymorphs to get advanced smart materials with significant applications like actuation, soft robotics, sensing, self-healing crystals, optoelectronics and flexible photonics.^10–14

Among SCSC processes, those involving polymorphic transitions are especially valuable because they reveal how subtle differences in molecular packing, intermolecular interactions, or conformation can lead to distinctly different physical behaviours. Polymorphs can interconvert in response to external stimuli such as heat, light, solvent, or mechanical stress, often resulting in phase-selective reactivity or changes in mechanical properties. Despite extensive studies on pharmaceutical solids, controlled SCSC polymorphic interconversions in metal–organic and coordination complexes remain comparatively rare, particularly in systems that maintain structural integrity during transformation. However, for many molecular crystals that change structure under external stimuli or even at room temperature, the underlying mechanisms are not yet fully understood.^15–21 In fact polymorphic transformations of SCSC of materials, pharmaceutical drugs,^22–24 smart materials^25–29 and simple metal complexes,^{20,21,30–33} has gain huge interest due to their significant applications and solid-state purity. This growing attention highlights the need for deeper mechanistic understanding to better design responsive crystalline systems.

Herein, we report a rare case of Cu(II) paddle wheel complexes exhibiting solvent and thermally induced polymorphic SCSC transformation between two distinct forms. The metastable form II gradually converts to the stable form I, at room temperature, in solution and upon heating, accompanied by remarkable changes in conformation and alignment of the olefinic ligands. The stable form I, subsequently undergoes a solid-state [2+2] cycloaddition under UV irradiation (CAR), yielding a topochemical photoproduct. This study establishes a clear structure–conformation-reactivity correlation in Cu(II) coordination complexes³⁴ providing valuable insights for the rational design of photoresponsive and polymorphically adaptive crystalline materials. We present a rare structural characterization of Cu(II) paddle wheel complexes with difluorostyrylpyridine ligands, which elucidates the conformational changes critical for understanding topochemical [2+2] CAR, and thermosalient behaviour. All these are supported by powder X-ray diffraction (PXRD) (Fig. S1 in SI), thermogravimetric analysis TGA (Fig. S2 in SI) differential scanning calorimetry (DSC), (Fig. 3 and Fig. S3 in SI) and the crystal structures are confirmed by single crystal X-ray diffraction (SCXRD) (Table S1, SI). [Cu₂(benzoate)₄(24F-4spy)₂] of form I as 1 and form II as 2 (where as photoproduct of form I/1 is labelled as 3) are studied here in detail whereas [Cu₂(benzoate)₄(26F-4spy)₂] 4 (where as photoproduct of 4 is labelled as 6), [Cu₂(benzoate)₄(34F-4spy)₂] 5 are shown in SI. The phase purity was confirmed by comparing the PXRD (Fig. S1 and S2, in SI) patterns of the bulk with those generated from the single crystal data.

[Cu₂(benzoate)₄(24F-4spy)₂] crystallizes as two morphologically distinct concomitant polymorphs, form I (1) appeared as light green, plates/rods of P1̄ space group (triclinic, two molecules in the asymmetric unit) whereas form II (2) crystallizes dark green hexagonal, block morphology crystals of P1̄ space group (triclinic, one molecule in the asymmetric unit) (Scheme 1). Single crystals of the form I and II are obtained by slow evaporation of methanol solution of Cu(NO₃)₂·6H₂O, sodium benzoate and the ligand (E)-4-(2,4-difluorostyryl)pyridine, (24F-4spy) in the molar ratio of 1 : 2 : 1 to result in [Cu₂(benzoate)₄(24F-4spy)₂] with 75% yield of form I and 60–70% of form II from slow evaporation (all the details are at Experimental section, SI). The structure and packing of these compounds were determined by SCXRD (Table S1, Fig. S4 in SI).

Scheme 1 General representation of the polymorphs in current study.

Variable temperature powder X-ray diffraction (VT-PXRD) experiments and the powder XRD experiments with time inter wells are additional proof for the polymorphic transition. Single-crystal XRD study confirmed that form II to form I transition happened SCSC fashion not only under thermal activation, even in solution and indeed spontaneously at room temperature. Hence form I is more stable polymorph. Single crystal structure analysis of the form I exhibited one half unit of the paddle wheel complex (Z = 1) in asymmetric unit, whereas form II showed two half units of paddle wheel complex (Z = 2) and interestingly both the polymorphs are existing triclinic crystal system, P1̄ (No. 2) space group. A crystallographic inversion centre is present in the middle of the paddle-wheel structure (Fig. 1a) where half of the formula unit of form I is located in the asymmetric unit and next to these are extended through weak C–H⋯F and π⋯π auxiliary interactions through head-to-tail fashion gain aligned C=C bonds of distance 3.75 Å and the packing in 2D view showed (Fig. 1b and c). Which suggests it can undergo [2+2] CAR. Two F atoms of 24F-4spy are form C–H⋯F interactions, para F atom form bifurcated with benzoate H atoms (Fig. S5a and b in SI) with distances 3.37 Å, 3.11 Å, 3.19 Å. Form I of 24F-4spy is very planar and the angle between them is 14° (Fig. S5c in SI).

Fig. 1 Crystal structure of form I. (a) basic paddle wheel complex. (b) 1D packing view, where the ligands are head–tail manner placed. (c) 2D packing view.

Crystal structure of the form II exhibited two half units of the paddle wheel complex (Z = 2) in asymmetric unit (Fig. 2a). Unlike form I, here in form II one unit of 24F-4spy are almost planar (8°), but in second unit 24F-4spy are slightly angular (16°), the whole paddle wheel complex of this unit is bent type rather than planar (Fig. 2c). Therefore, the whole structure changed and hence may be the whole units are slightly moved apart in the final there dimensional packing. Due to that the ligands are moved out, the distance 4.65 Å (Fig. 2b) and finally it is photostable. Next, both the units are extended through weak C–H⋯F interactions with benzoate H atoms (Fig. S6a, in SI) with distances 3.37 Å, 3.27 Å, 3.46 Å. Form II is less stable and always transform to form I. This is also confirmed by DSC (Fig. 3) and further by PXRD experiments at room temperature, high temperature (heated crystals).

Fig. 2 Crystal structure of form II. (a) basic paddle wheel complex. (b) 2D packing view, where the ligands are head–tail manner placed but they are far each other. (c) conformation overlay of the three units of both the forms. Yellow is form I, green and blue are two units of form II.

Fig. 3 DSC of the form II, the phase transition of the form II to I, was observed at 162 °C. Confirmed irreversible phase transformation.

DSC experiments clearly showed that, there is phase change at 162 °C (Fig. 3) while heating cycle, but in cooling cycle, no change was observed which confirm the enantiotropic relationship of the form II with form I. Variable temperature powder X-ray diffraction (VT-PXRD) experiments and the powder XRD experiments with time interval are additional proof for the polymorphic transition. Single-crystal XRD study confirmed that form II to form I transition happened SCSC fashion not only under thermal activation, even in solution and indeed at room temperature. Hence form I is more stable polymorph. Stability relation between form I and II are further understood by PXRD (Fig. 4). Crystallization in MeOH after 24 hrs all the crystal are confirmed as form II, by PXRD. After two days, in same solution there are two different morphologies of crystals are observed and confirmed polymorph transformation, which was further conformed by PXRD. Where in 2 theta value of form II at 6° along with peak at 7° which is form I. Which suggests both the forms are presented in solution (Fig. 5a and Fig. S4). We have continued the same experiment, collected crystals after 3–4 days periodically and proved that after 4-5 days all are form I (Fig. 4a).

Fig. 4 (a) Solution mediated phase transformation of the form II to I with time interval 24 h. It showed that initially form II then it was transformed to form I in MeOH solution over the time period. (b) Phase transformation of the form II to I during the heating at 160 °C. It showed that initially form II, which was then transformed to form I during heat.

Fig. 5 (a) Crystallization experiments in MeOH at 24 h, 48 h, 72 h, suggests that, at first form II and then form I formed. (b) Photoreactive study of the form I under weak UV. (c) Crystal jumped under the heat in form I. Scale bar is 1 mm.

Next, we found that even at room temperature after two weeks all the form II crystals are transformed to form I (confirmed by PXRD and SCXRD). After confirmation in MeOH solution, at room temperature we have extended at high temperature. DSC of the form II proved the same (Fig. S3a, in SI), which revealed that, after 160 °C, which is phase change temperature (DSC, Fig. 3), form II to form I. This confirmed irreversible phase transition and the same is further confirmed by PXRD. Jumping of the few crystals, phase change of the form I, after 160 °C was observed (Fig. S3b and S3c in SI). During cooling cycle, we did not see any significant change. Occasionally, form I crystals showed jumping of the few crystals (Fig. S3b and S3c in SI) may be due to thermal expansion. Phase change and thermosalient are discussed below. PXRD was conducted with 10 min intervals. After 10 min heating, form I started appearing 2 theta value at 7° and after 30 min whole the crystals are converted to form I (Fig. 4b). Finally, we proved that in solution, or during heating, or at room temperature all the cases form II was transformed to form I and it is irreversible transformation. SCSC further confirmed by SCXRD as the unit cell, and the whole molecular packing in crystal structure matched with form I (Table S1).

It is evident from the single crystal structure packing that the olefin pairs presented in form I are aligned and CAR in the formation of 1D CP. At first, form I was placed between glass slides and kept under UV, then checked ¹H-NMR and confirm the cyclobutane formation (Fig. S12a and b, SI). Crystal structures and packing of the 4, 5 are shown in Fig. S7 and S8 in SI, photoproduct of 4, named as 6. Crystals of 4 showed very slow photoreactive under UV light (Fig. S10, SI) whereas 5 showed photostability. Our aim is to show that, changing the F atom position of 4spy ligands can have a significant impact in structure and finally to give the stimuli responsive effects. Hence, we would like 4, 5 to be part of this manuscript and not to remove as it is part of the ligand design. Few handpicked crystals of form I are placed under weak UV, up on UV irradiation, after 30 min topochemical reaction occurred and caused cracks on the crystals (Fig. 5b and Fig. S9a, b in SI). Similar cracks are also observed for 4 (Fig. S9 in SI), hence form I and 4 have showed slow photoreactivity. Thermosalient studies are performed to understand the stimuli responsive nature of the form I, crystals of the different size/shape are selected to perform the thermosalient effect, few crystals jumped during heating, it might be due to the anisotropic thermal expansion (Fig. 5c). The crystal which jumped was retested unit cell and structure analysis conformed that there is no change in crystal structure and both showed 100% match (Table S1 and Fig. S11, SI). DSC of the form I, single crystals batch repeated for four cycles (Fig. S3b SI), which showed there is disturbance in DSC after 160 °C which would be due to jumping of the few crystals.³⁴ Next, we have performed DSC of the crystals from different batches with various sizes to check how the jumping of the form I crystals occurs, (Fig. S3c, SI) Jumping of the crystals after 160 °C was observed, which confirmed the reproducibility in several batches. Density measurements are in Table S2, SI, which Showed volume expansion is less after [2+2] CAR.

In summary, we report a rare case of polymorphs of Cu(II) complexes [Cu₂(benzoate)₄(24F-4spy)₂] form I, (photoreactive), form II, (photoinert), which can be transformed from II to I both applying heat and through ageing in solution. While polymorphism is often observed in functional organic molecules and pharmaceutical drugs, it is rare in metal complexes. Conformational study of the form I and II suggested that, metastable polymorph is not planar and they tend to adopt a stable planar conformation with planar 4spy ligands, eventually leading to stable form I. This conformational adjustment phenomenon is very interesting to understand. Our results prove that the conformation of the paddle wheel complex and the planar nature of the 4spy ligands are major factors enabling topochemical [2+2] CAR and SCSC transformations. PXRD confirmed the SCSC transformation from form II to I. Some crystals exhibited jumping during heating, likely due to thermal expansion; as, there was no phase change observed in form I. This study provides a good example of how SCSC transformations occur during the transition from metastable to stable complexes. In addition, current study also highlights the importance of position of fluorine in the photoreactive linker, as we couldn’t obtain the Cu(II) complexes where there is an alignment in olefins while we change the position of fluorine from 24F-4spy to 26F-4spy and 34F-4spy.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). SI contains materials synthesis of complexes PXRD, TGA, DSC, NMR, Photo reactivity of the crystal under weak UV irradiation and thermosalient experiments. See DOI: https://doi.org/10.1039/d6cc00385k.

CCDC 2484745, 2484746, 2486473, 2486474 and 2487112 contain the supplementary crystallographic data for this paper.^35a–e

Acknowledgements

This work was financially supported by the Ministry of Education, Singapore (Grant No. Tier 1 WBS R-143-000-A12-114 and R-143-000-B13-114). R. M. acknowledges IBITF (Grant IBITF/PRAYAS/Note/2023-24/0009) for financial support. G. B. and R. M. thank Prof J. J. Vittal for lab facilities and funding support. We acknowledge the help of Ms Geok Kheng for collecting the single crystal X-ray intensity data. Department of chemistry, CMMAC, NUS for all facilities.

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35. (a) CCDC 2484745: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc2pdl2s; (b) CCDC 2484746: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc2pdl3t; (c) CCDC 2486473: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc2pgctc; (d) CCDC 2486474: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc2pgcvd; (e) CCDC 2487112: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc2ph1fp.

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