New copper carboxylate pyrene dimers: synthesis, crystal structure, Hirshfeld surface analysis and electrochemical characterization

Two new copper dimers, [Cu2(pyr-COO–)4(DMSO)2] (1) and [Cu2(pyr-COO–)4(DMF)2] (2) (pyr = pyrene) were synthesized from the reaction of pyrene-1-carboxylic acid, copper(II) nitrate and triethylamine from solvents DMSO and DMF, respectively. Electrochemical characterization and Hirshfeld surface analysis was carried out.


Chemical context
Copper(II) carboxylate complexes with paddle-like structure have been proposed in solar energy conversion and storage, redox mediators, magnetism, dyes and in catalysis, among other applications (Benesperi et al. 2020;Kozlevc ˇar et al., 2004;Rajakannu et al., 2019;Murugavel et al., 2000;Rao et al., 2004;Boulsourani et al., 2017;Baldoma ´et al., 2006;Seo et al., 2000).The unique characteristics of copper(II) carboxylate complexes of general formula [Cu 2 (RCOO À ) 4 (L) 2 ] are based on their easy synthesis, the relative abundance of the starting materials, their stability, and their low toxicity, which enables a vast number of research directions to be performed from such materials.The structural features of these compounds are related to the coordinating aspect of the ligands: the two possible coordination sites through the carboxylate oxygen atoms result in various modes of coordination, such as monodentate, bidentate and bridging, offering a variability of polynuclear metal complexes (Rajakannu et al., 2019;Murugavel et al., 2000;Rao et al., 2004), and a degree of stability for dinuclear and trinuclear complexes.Additionally, the carboxylate group could participate in hydrogen bonds, leading to a supramolecular network (Aakero ¨y et al., 2006).Moreover, dinuclear copper(II) carboxylate complexes may have switchable electronic properties such as intermetal magnetic exchange and electron transfer (Vishnoi et al., 2017).The electrochemical properties of copper(II) carboxylate complexes are reported to be highly influenced by the redoxactive nature of copper(II/I) and subjected to potential changes due to the presence of substituents in the carboxylate ligands (Wang et al., 2013), thereby influencing the stability of its oxidation state (Modec et al., 2020).
In this work we report the structure of two new copper(II) carboxylate complexes from pyrene-1-carboxylic acid.The structure of pyrene is based on four fused benzene rings, thus it belongs to the group of polycyclic aromatic hydrocarbons (PAH) that have been well studied since their remarkable fluorescence and phosphorescence properties were noted (Haldar et al., 2020).Carboxylic acid from 1-pyrene has been proposed in supercapacitor devices by functionalization of graphene and it is also used to design and synthesize luminescent metal-organic complexes for sensing applications.
In addition, pyrene ligands have been used as an organic linker or as building blocks for the design of new classes of metal-organic frameworks (MOFs).The functionalization of pyrene with phosphonates, sulfonates, and carboxylates allows metal coordination to yield MOF structures exhibiting new photophysical and photochemical properties.MOF structures with pyrene ligands result in promising optical properties such as luminescence sensing, photocatalysis, electrochemistry, adsorption and separation applications, and biomedical applications (Kinik et al., 2021).Derivatives of carboxylate pyrene ligands have been studied because of their extraordinary photophysical properties, chemical stability, �-� stacking interactions, and high-charge-delocalized systems (Guan et al., 2019).The planar �-conjugated surface of pyrene and its molecular rearrangement is favorable for the detection of guest molecules in molecular tweezer hosts, for example with platinum, ruthenium, and copper complexes.Another application of pyrene can be found in the functionalization of carbon nanotubes (CNTs) as a result of its �-� interactions with polycyclic aromatic molecules (Zhao & Stoddart, 2009).
Here, we report the novel synthesis, characterization, and crystal structure of two copper dimers with tetracarboxylate pyrene and two solvent molecules in axial positions, [Cu 2 (pyr-COO À ) 4 (DMSO) 2 ] (1) and [Cu 2 (pyr-COO À ) 4 (DMF) 2 ] (2).Structural characterization from single-crystal X-ray diffraction experiments show crystallization under two crystal systems, which translates into different extended contacts, such as �-� stacking interactions, among others.In terms of the chemistry of these copper structures, they are very promising because the axial positions can be substituted by bridging ligands, which can form coordination polymers such as the 1D, 2D, and 3D polymeric architectures that have been proposed in molecular sensing, gas storage, and separation (Karmakar et al., 2021).Hirshfeld surface analysis was undertaken to show the contributions from intermolecular interactions in the crystal-packing array.The pyrene rings participate in �-� interactions, yet some rings have weaker interactions based on their position in the crystal structure.DMSO (1) and DMF (2) axial ligands, play a crucial role in the crystal packing by participating in interactions with the rest of the molecule.In addition, Hirshfeld surface analysis showed that compound 2 has shorter distances for most interactions.Electrochemical characterization of compound 2 was performed by cyclic voltammetry at varying scan rates (50-2000 mV s À 1 ), revealing a diffusion-controlled Cu 2+ /Cu 1+ quasi-reversible process that may involve an electron reduction at an E 1/2 potential around À 0.52 V vs F c /F c + (Iqbal et al., 2013;Bonomo et al., 2000).

Structural commentary
The crystal structures of complexes [Cu 2 (pyr-COO À ) 4 -(DMSO) 2 ] (1), space group P1, and [Cu 2 (pyr-COO À ) 4 -(DMF) 2 ], space group P2 1 /n (2), are presented in Fig. 1.The copper atoms have octahedral geometries with four oxygen atoms from the pyrene-1-carboxylate ligand at equatorial positions, one axial ligand from the solvent molecule and the remaining axial coordination occupied by a metal-metal copper contact.The asymmetric unit contains half the molecule in both structures.The Cu� � �Cu contact distance in 1 is 2.5934 (3) A ˚in comparison with the structure of 2 for which it is 2.6295 (5) A ˚. Likewise, the Cu-O5 bond distance in the axial position is shorter in 1 than in 2, with values of 2.1441 (12) and 2.1769 (13) A ˚, respectively.The difference in the elongation of these bond distances could be the result of the influence of the axial ligand (DMSO vs DMF) with stronger �-back-bonding character, thus better binding (Deacon & Phillips, 1980).The Cu-O bonds in equatorial positions are shorter than those in axial positions in both structures, with distances ranging from 1.9530 (13) to 1.9593 (13) A ˚, which may be indicative of Jahn-Teller effects on Cu II centers.All the other structural features in the two Cu dimers do not change significantly.Structural disorder of four carbon atoms from the pyrene (C29-C32-C33-C34) unit is observed in complex 2 as well as in one of the carbon atoms from the DMF molecule, precisely on C37, for which atoms had to be modeled in two parts.

Supramolecular features
Long-range interactions for 1 and 2 are different in terms of their �-� stacking, as well as the axial hydrogen interactions with � rings.In the case of complex 1, the most important �-� interactions is observed for C22� � �C16 at 3.393 (3) A ˚. C-H to �-ring interactions are observed between C28� � �H15 and C27� � �H15 at 2.87 and 2.90 A ˚, respectively, and the solvent oxygen interaction �-ring end hydrogen is observed through O5� � �H4 at a distance of 2.56 A ˚.In complex 2 however, �-� interactions are present from C4� � �C4 of neighboring rings with a distance 3.178 (4) A ˚; other interactions are attributed to C-H end to �-ring for C16� � �H37B, C19� � �H16, and C24� � �H16 with distances of 2.87, 2.85, and 2.70 A ˚, respectively.The packing for 1 and 2 is shown in shown in Fig. 2.

Electrochemical measurements
Electrochemical properties were measured in DMF for complex 2; complex 1 was not soluble therein, and thus was not characterized electrochemically.The cyclic voltammograms (CV) of compound 2 at multiple scan rates are shown in Fig. 3.The main feature presented by compound 2 exhibits a redox couple at ca À 0.5 V vs F c /F c + associated with the Cu 2+ /Cu 1+ couple (Iqbal et al., 2013;Bonomo et al., 2000).This redox process was found to be quasi-reversible because as the scan rate increased, the peak-to-peak separation increased, indicating that this process is not reversible.Another indication of the quasi-reversible nature of compound 2 is that the ratio between the cathodic and anodic peak current is less than 1.According to the Randles-Sevcik equation, the observed linear relationship between the square root of the  scan rate and the peak current confirms that the quasireversible process is diffusion-controlled (Fig. 4) (Elgrishi et al., 2018).It was observed that on increasing the scan rates to 750 mV s À 1 , two irreversible oxidation processes appeared at 0.05 and 0.50 V vs F c /F c + .In summary, complex 2 possesses a quasi-reversible diffusion-controlled redox process corresponding to the Cu 2+ /Cu 1+ couple.
Short interactions are better perceived in Fig. 6, both on the pyrene moiety (core and edges) and ligand positions.The role of pyrene rings in the crystal packing is evident, as well as for solvent molecules, even though their contributions are different (Fig. 7).Important �-� interactions are observed in the shape-index surface, represented by characteristic adjacent red-yellow and blue-green triangles (and back-to-back diamonds) on pyrene rings (Fig. 7a) and in the axial ligand    Cyclic voltammograms of 1 mM of compound 2 at 50-2000 mV s À 1 .Cyclic voltammograms were obtained in a 0.1 M TBAPF 6 in DMF with a glassy carbon working electrode, a graphite rod counter-electrode, and 0.01 M AgNO 3 silver wire as the pseudo-reference electrode corrected with ferrocene.region (Fig. 7c).Interestingly, not all pyrene cores have the same degree of interactions within the crystal packing.The pyrene rings are either engaged in strong �-� interactions or in other interactions, predominantly of C� � �H/H� � �C(core) and H� � �H(edges) type.The intercentroid distance for rings that exhibit strong �-� interactions is 3.75 A ˚and these rings greatly overlap.Fig. 7b defines hollows toward the center of the molecule and bumps on the pyrene edges, confirming that intermolecular interactions allow molecules to interlock for the crystal packing, Evaluation of the curvedness reflects the planarity of the pyrene rings, specifically for those exhibiting strong �-� interactions (Fig. 8a), while the other rings and solvent ligands have both flat and positive curvatures.Hence, compound 1 has diverse interactions that give way to the resulting array.Fig. 8a portrays a superposition of where the pyrene ring is located below the generated surface; only a green color (i.e., flatness) is observed in this region.Going from Fig. 8b to Fig. 8c, the red boxes are localized to fit together interlocking pyrene moieties towards the innermost region of the molecule, highlighting complementarity in the crystal-packing array.
The fingerprint plot for compound 1 is symmetric, and contacts occur over a long range of distances (i.e., d e and d i scale) for C� � �H/H� � �C (39.8%),H� � �H (44.2%), and H� � �O/ O� � �H (7.4%) type primarily.H� � �H contacts make up almost half the total interactions (44.2%).A large concentration of points is centered around 1.6-2.0A ˚, linked to �-� stacking; contacts of C� � �C (7.3%) type comprise DMSO-pyrene and pyrene-pyrene.Furthermore, characteristic traits are distinguished: both peaks and wings are demarcated in the C� � �H/ H� � �C, H� � �H, and H� � �O/O� � �H plots, so different contacts are present; they all incorporate the pyrene moieties and solvent ligands.Upon further analysis, it was found that DMSO participates in each type of contact in Fig. 9, either from the sulfur, oxygen, or methyl groups.Contacts of the C� � �O/O� � �C (0.1%), C� � �S/S� � �C (1.0%), and H� � �S/S� � �H (0.3%) types are not made out from short-contact interaction analysis because their distances are very long.Contacts of the C� � �O/O� � �C type arise from carboxylate and DMSO oxygen atoms to pyrene ring carbons, the C� � �S/S� � �C type go from the DMSO sulfur atom to pyrene ring carbons, and H� � �S/ S� � �H from the DMSO sulfur atom to pyrene ring hydrogens.
The Hirshfeld surface generated for title compound 2 evaluated over d norm shows the significance of the axial ligands as well as the pyrene moieties, like in compound 1.As seen in Fig. 10, adjacent molecules surrounding the generated surface deliver multiple interactions, which are distributed from the innermost region near the oxygen atoms (e.g., O2� � �H34A at 2.79 A ˚, O2� � �H34B at 2.61 A ˚), equatorial pyrene moieties (core and edges) (e.g., C4� � �C4 at 3.18 A ˚, C24� � �H16 at 2.70 A ˚), to the axial ligands (e.g., H37B� � �C16 at 2.87 A ˚).
Most of the red spots are intense (i.e., short distance), mainly C� � �C, H� � �H, H� � �O/O� � �H, and C� � �H/H� � �C type interactions.However, just a few light-red spots (i.e., longer distance) color are recognized as additional contacts, primarily C� � �H/H� � �C type.In Fig. 11, a Cu 2 (C 17 H 9 O 2 ) 4 (C 2 H 6 OS) 2 ] and [Cu 2 (C 17 H 9 O 2 ) 4 (C 3 H 7 NO) 2 ] 5   Figure 9 Fingerprint plot analysis for title compound 1.   qualitatively greater amount of blue regions; however, the red spots are more intense, implying compound 1 has strong interactions distributed over more parts of the surface but compound 2 has shorter distances in most of its interactions.
Pyrene ring surfaces with red-yellow and blue-green adjacent triangles, as displayed in Fig. 12a, are characteristic of �-� interactions, which are expected due to the nature of the PAHs.Similar to compound 1, not all rings show this degree of interaction because of the position of each pyrene ring with respect to other moieties in the crystal-packing array and corresponding interactions.Different from Fig. 7a, Fig. 12a has a less uniform pattern of �-� interactions than for title compound 1, as a result of the less overlapping pyrene rings.Rings with weak �-� interactions have more C� � �H/ H� � �C(core) and H� � �H(edges) contacts, analogous to compound 1.
The planarity of the pyrene moieties is depicted by the curvedness (Fig. 13a) where most of the surface is flat.However, even pyrene rings that exhibit strong �-� interactions do not possess a completely flat surface region (unlike in compound 1), and the other rings have alternating regions of flatness.The intercentroid distance for rings that exhibit strong �-� interactions is 5.83 A ˚, farther apart than for compound 1.In addition, the red box in Fig. 13b can be translated into the one in Fig. 13c; thus, complementarity is observed within the generated surface, coming from intermolecular interactions that follow the screw axes and glide planes present in title compound 2 (P2 1 /n).Both compounds achieve complementarity in their crystal packing, but each arises from different intermolecular interactions.
The 2D fingerprint plot for compound 2 (Fig. 14) has the following features: it is quasi-symmetric, C� � �H/H� � �C interactions account for almost half of the contacts (44.9%) followed by H� � �H (40.5%), with fewer contributions from H� � �O/O� � �H (10.7%) and C� � �C (3.4%) interactions.C� � �H/ H� � �C contacts have broad peaks spread out over most of the plot, H� � �H contacts also cover a broad range of distances and several types of interactions, and H� � �O/O� � �H contacts have wide peaks and fewer weak contacts.In the same way as for compound 1, all contacts in Fig. 14 include atoms from the solvent, DMF in the case of compound 2. Likewise, contacts of the C� � �O/O� � �C (0.1%), C� � �N/N� � �C (0.4%), and H� � �N/ N� � �H (0.1%) types are not identified from short-contact interaction analysis because the distances are long.Contacts of the C� � �O/O� � �C type arise from carboxylate and DMF oxygen atoms to pyrene ring carbons (as in compound 1), the C� � �N/N� � �C type go from the DMF nitrogen atom to pyrene ring carbons, and H� � �N/N� � �H from the DMF nitrogen atom to pyrene ring hydrogens.

Database survey
A search of the Cambridge Structural Database (CSD Version 5.44, June 2023 update; Groom et al., 2016) for the two

Synthesis and crystallization
All the chemicals were purchased from Sigma-Aldrich.The chemicals and solvents were used as supplied without further purification.IR spectra were recorded on a FT-IR Frontier Perkin Elmer spectrophotometer with ATR modality in the region 4000-600 cm À 1 .UV-vis spectra were recorded on a UV-1900 spectrophotometer in the range 200-1000 nm using a 1 cm path-length cell for solution in DMSO or DMF.The CVs were recorded in a BioLogic potentiostat using a solution of 0.1 M TBAPF 6 with a glassy carbon working electrode, a graphite rod counter-electrode, and a 0.01 M AgNO 3 silver wire pseudo-reference electrode corrected with ferrocene.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1.H atoms were included in geometrically calculated positions and refined as riding atoms with C-H = 0.93 A ˚and U iso (H) = 1.2U eq (C).

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.

Figure 2
Figure 2Crystal packing of Cu complexes 1 and 2 along the a axis with ellipsoids at the 50% probability level.

Figure 1
Figure 1 Asymmetric units of Cu complexes 1 (a) and 2 (b) with labels for non-C/H atoms and ellipsoids at the 50% probability level.(c) and (d) views of the complete molecules from the b axis.

Figure 6
Figure 6Hirshfeld surface evaluated over d norm for title compound 1.

Figure 4
Figure 4Square root of scan rate versus peak current plot (a) anodic peak and (b) cathodic peak of compound 2.

Figure 3
Figure 3 few red spots are present on the pyrene aromatic core and most are located near the edges.In contrast, the DMF region has strong red spots.When comparing Fig. 6 and Fig. 11, the latter surface contains a research communications Acta Cryst.(2024).E80, 1-9 Nogue ´-Guzma ´n et al. � [

Figure 7
Figure 7Hirshfeld surface evaluated over shape-index for title compound 1, viewed from the side (a), (b) and top (c).

Figure 8
Figure 8Hirshfeld surface evaluated over curvedness for title compound 1, viewed from the side (a) and top (b), (c).

Figure 13
Figure 13Hirshfeld surface evaluated over curvedness for title compound 2, viewed from the side (a) and top (b), (c).

Figure 11
Figure 11Hirshfeld surface evaluated over d norm for title compound 2.

Figure 12
Figure 12Hirshfeld surface evaluated over shape-index for title compound 2, viewed from the side (a), (b) and top (c).