Noncovalent Interactions in Coordination Chemistry of Cyclic Trinuclear Copper(I) and Silver(I) Pyrazolates

Abstract

Group 11 metals form with pyrazolate ligand complexes with a general formula of [MPz]_n. The value of “n” varies depending on the type of substituent in the ligand and the metal atom. Copper(I) and silver(I) ions mainly form cyclic di-, tri-, and tetra-nuclear complexes or polymeric structures. Cyclic trinuclear d¹⁰ metal pyrazolates [MPz^m]₃ (M = Cu(I) and Ag(I); Pz = substituted pyrazolate ligand) are of particular interest because their planar structure allows them to form supramolecular aggregates via noncovalent metal–metal, metal–π, and metal–electron donor interactions. Designing complexes based on these interactions has been a focus of research for the last two decades. The ability of cyclic trinuclear copper(I) and silver(I) pyrazolates to form coordination and supramolecular structures determines their properties and potential applications in catalysis, gas sensing, molecular recognition, and photoluminescence. In this review, we discuss noncovalent interactions between cyclic trinuclear silver(I) and copper(I) complexes with various types of ligands.

1. Introduction

The coordination chemistry of group 11 metals, particularly copper(I) and silver(I), has seen rapid development in the last two decades. The main interest in this class of compounds, based on monovalent metal ions, is focused on the design of luminescent complexes because of their wide range of potential practical applications [1,2,3]. The group 11 metal complexes can be used in light-emitting diodes (LEDs) [4,5,6,7,8,9,10], light-emitting electrochemical cells (LECs) [11,12,13], sensors for small molecules and pollution sources [14], bioimaging [15,16,17], photocatalytic reactions [18,19,20,21], and dye-sensitized solar cells [22,23,24]. The main advantages of using copper(I) and silver(I) complexes over commonly used noble metal complexes, such as Ir(III), Pt(II), and Ru(II), are their lower cost and environmental safety. In light of the need for sustainable development and a green economy, it is a pressing task to develop alternative approaches to efficient light-emitting molecular systems and materials [25,26]. On the other hand, the efficiency of d¹⁰ metal complexes is related to thermally activated delayed fluorescence (TADF). This phenomenon occurs due to a small energy gap between the excited singlet and triplet states, which allows for equilibrium between them at room temperature [27,28,29,30,31,32,33,34]. The nitrogen-containing heterocycles are popular ligands in coinage metal chemistry [35,36,37]. Deprotonated pyrazole derivatives act as bridging ligands and counterions, forming neutral cyclic or polymeric complexes with group 11 metals, as well as metal–organic frameworks (MOFs) in the case of using bipyrazoles. The most common types of complexes formed by copper(I) and silver(I) cations with pyrazolate ligands are cyclic compounds with the general formula [MPzⁿ]_m (m = 2–4), where n depends on the substituents on the Pz units and/or the metal ion (Scheme 1 and Scheme 2) [38].

Of particular interest are the trinuclear cyclic pyrazolate complexes with the formula [MPz]₃, which have a planar structure in the solid state. Some of these complexes form infinite columns through intermolecular metallophilic interactions in this form, leading to photoluminescence through the formation of exciplexes [39,40]. The position of the emission maximum depends on the substituents in the pyrazolate ligand, the metal atom, temperature, solvent, and concentration. The influence of the solvent and concentration suggests that the emission behavior is dependent on the presence of intermolecular interactions [41,42,43,44,45,46,47]. From another viewpoint, the formation of stable mixed-ligand complexes with N- and P-containing ligands leads to the rearrangement of the central core, obtaining new complexes with different properties [48,49,50,51,52,53]. A systematic approach, including the study of complexation in solution and the solid state, allows us to establish the relationship between complexes’ composition, structure, and properties [54,55,56]. Obtaining new complexes and analyzing their structures helps to determine the factors that have the greatest influence on useful properties. This, in turn, opens the door to creating materials with controlled behavior. Therefore, the importance and relevance of this area of research—the study of the formation of copper(I) and silver(I) cyclic pyrazolate, intermolecular, noncovalent “host–guest” complexes—is emphasized.

2. The General Principles of the Self-Assembly of Cyclic Trinuclear Copper(I) and Silver(I) Pyrazolates

2.1. Intermolecular Metallophilic Interactions

The presence of substituents in pyrazolate ligands that cannot participate in coordination with Lewis acidic centers allows the formation of metal–metal interactions. These interactions are one of the most common pathways for the formation of supramolecular packing in [MPz]₃ crystals. Silver pyrazolate adduct was first reported by Buchner in 1889. The first crystal structure of coinage metal pyrazolates was established one century later, using the example of cyclic trinuclear gold(I) 3,5-bis(trifluoromethyl)pyrazolate [57]. Then, several other groups published their results on the synthesis and characterization of this class of compounds [58,59,60]. The first example of copper pyrazolate complexes demonstrating metallophilic interactions was obtained from 3,5-dimethylpyrazole (HPz¹) [61]. Soon after, this group described a similar Cu(I) cyclic trinuclear complex with 3,4,5-trimethylpyrazole (HPz²) [62]. The authors found Cu…Cu intermolecular interactions and noted that they were significantly longer in the case of the trisubstituted Pz ligand (3.070(1) Å vs. 2.947(1) Å; Figure 1). It is worth noting that in the crystal, trimers form dimers, and packing is mainly realized due to multiple H…H and H…π^Pz contacts. Both complexes demonstrate a head-to-tail arrangement.

The first and second structures of trinuclear silver complexes with non-substituted [AgPz]₃ and 3,5-dimethylpyrazolate [AgPz]₃ possessing intermolecular Ag–Ag interactions were determined using ab initio calculations based on X-ray powder diffraction. Silver-containing analogues demonstrated a head-to-tail orientation of trimers, similar to that observed in copper complexes. The intermolecular Ag…Ag contacts in [AgPz]₃ were 3.431 Å, and in [AgPz¹]₃ they ranged from 3.279 to 3.358 Å, forming infinite stacks of “folded-ribbon-like” Ag–Ag interactions (Figure 2) [63,64].

In 1999, Dias and co-workers demonstrated a convenient synthesis of [MPz³]₃ complexes with the 3,5-bis(trifluorometyl)pyrazolate ligand via a simple reaction of metal oxide with pyrazole [65]. The complexes obtained possessed good solubility in organic solvents. The copper-containing complex [CuPz³]₃ demonstrated distances of 3.879(1) and 3.893(1) Å in the crystal Cu–Cu intertrimer, which were significantly longer than those of the non-fluorinated complexes [CuPz¹]₃ and [CuPz²]₃. In contrast, the silver-containing analogue formed dimers with shorter Ag–Ag intertrimer distance of 3.307(1) Å. The distance to the other trinuclear units was 3.830(1) Å (Figure 3).

This research became a starting point for a series of studies that have demonstrated the role of intermolecular metal–metal interactions in the photophysics of cyclic trinuclear metal pyrazolates [39,40,41,66]. It was shown that the emission properties correspond to the formation of exciplexes (Figure 4) [67].

2.2. Intermolecular Interactions with Halogen Atom in Pyrazolate Fragment

Presence of halogen substituents in Pz ligands allows the self-organization of the cyclic metal pyrazolates due to formation of intermolecular, noncovalent M–Hal interactions. For example, for the cyclic silver(I) 4-chloro/iodo-3,5-diphenyl pyrazolates (R = Cl, [AgPz⁴]₃; R = I, [AgPz⁵]₃), it has been shown that the Ag…Cl/I bond distances and the geometry of the C-Cl/I bonds, as well as the crystal packing diagrams, confirmed the existence of Ag₃…Cl/I interactions in some structures. These interactions were strong enough to significantly affect the spatial arrangement of trimers in the solid state (Figure 5) [68,69]. According to DFT calculations, these interactions not only have an electrostatic character, but also a weak “covalent” component, due to the overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). It should be noted that the largest C-Cl-Ag and C-I-Ag bond angles were 150.6(2)° and 140.0(2)°, respectively, demonstrating the orientation of the halogen atom to the one metal. For chlorine, this contact was the shortest (3.185(2) Å), but for iodine it was the longest (3.818(1) Å). The difference is more likely due to the significantly different radii of chlorine and iodine atoms.

Similar M₃…Hal interactions have been observed for fluorinated silver(I) and copper(I) trinuclear pyrazolate adducts (Scheme 3) [70].

In this case, another type of arrangement of trinuclear cores was observed. The neighboring complexes demonstrated practically parallel M3N6 central cores. As shown in Figure 6, one chlorine atom of each of the trinuclear [CuPz⁶]₃ showed three such Cu···Cl interactions of 3.604(3), 4.432(3), and 4.647(3) Å. According to Batsanov and Alvarez [71,72], such separations could be assigned to the rather strong interactions. In addition, there were no other driving forces for crystal packing because the shortest Cu–Cu distance was 5.079(3) Å, and for Cu–C^Pz was 3.827(6) Å. In the case of the silver-containing complex [AgPz⁶]₃, two halogens were coordinated with Ag3 on both sides of the macrocycle plane. The intermolecular Ag…Cl contacts were significantly closer, ranging from 3.658(1) to 3.719(1) Å for one set of coordination, and from 3.746(1) to 4.159(1) Å for another set. The formation of stronger interactions led to the non-significant distortion of the Ag3N6 core. In the case of bromine, shortened M3…Hal contacts have been observed (ranging from 3.555(1) to 4.129(1) Å for Cu, and from 3.5678(5) to 3.9164(5) Å for Ag), which is in agreement with stronger interactions. The bromine substituent had a non-significant influence on the crystal packing.

Complex [CuPz⁶]₃ demonstrated bright phosphorescence with maxima near 570−580 nm, with lifetimes in the 50−70 μs range. The authors have demonstrated an associative excited state behavior. Complexes [AgPz⁶]₃ and [AgPz⁷]₃ exhibited emission in a blue region (450–480 nm) (Figure 7). It was concluded that depending on the halogen atom, the emission maxima could be fine-tuned.

Using bromine-containing pyrazole (4-(4-bromophenyl)-3,5-dimethylpyrazole, HPz⁸) under solvothermal conditions allows obtaining two polymorphs and a “pseudo-polymorph” of a trinuclear copper(I) pyrazolate complex [CuPz⁸]₃ [73]. One of them showed a noticeable bend in the shape of the molecule, which was caused by the short intermolecular Cu…Cu (2.817(1) Å) interactions and halogen bonding between bromine atoms (3.8579(9) Å, ∠C-Br-Br = 164.9(2)°). It should be noted that another set of shortened Br–Br interactions also occurred in the structure (3.3967(9) and 4.0407(9) Å), but they were not halogen bonds due to the non-linear geometry of the C-Br-Br fragments. The angle between the C-Br and the Br atoms was 145.1(2)° in one case and 135.9(2)° in the other. These noncovalent interactions occurred in different directions and did not directly influence each other. These interactions led to the formation of an insulating layer in one polymorph of the crystal structure. The distance between copper atoms in this polymorph increased in the range of 2.8, 3.0, and 3.2 Å. The complexes obtained demonstrated phosphorescence due to the formation of exciplexes, which is typical for copper pyrazolate trimers.

2.3. Self-Organization Through Coordination Interactions with N- or O-Donor Substituents in the Pyrazolate Moiety

Supramolecular aggregates are formed due to coordination interactions between substituents in pyrazolate ligands containing N-donor groups, such as pyridinyl. The main strategy used to produce new trinuclear copper(I) pyrazolate complexes involves the reaction of CuX salts (X = Br⁻, I⁻, ClO₄⁻, NO₃⁻) with the corresponding pyrazolyl ligand, resulting in the formation of the [CuPz⁹]₃(CuX)_m complexes (Figure 8) [74]. In these cases, planar cyclic metal pyrazolates form extended structures through the binding of Cu ions to the Py fragment. There are also intermolecular Cu…Cu interactions in these types of complexes.

On the other hand, under solvothermal conditions (140 °C), five pseudo-polymorphic complexes were obtained, starting from copper(II) salts (Cu(NO₃)₂·3H₂O, Cu(ClO₄)₂·6H₂O, and CuCl₂·2H₂O) and 4-(pyridin-4-ylthio)-3,5-dimethyl-1H-pyrazole (HPz⁹) [75]. The use of a labile ligand resulted in the presence of different orientations of the Py fragments, corresponding to the M₃Pz₃ triangle plane, leading to the formation of anti- and syn-conformers (Scheme 4).

The intermolecular Cu…N^Py interactions (ranging from 2.308(6) to 2.67(1) Å) allow the formation of self-aggregated [CuPz⁹]₃ units to form supramolecular packing. Depending on the orientation of the Py fragment and the solvent, the four general dimerization paths have been established (Figure 9).

It should be noted that, typically, for cyclic trinuclear metal pyrazolates, the intermolecular Cu…Cu contacts were also observed in anti-[CuPz⁹]₃ and syn-[CuPz⁹]₃∙(C₂H₅OH). For the anti-conformer, such interactions are weak at 3.748(2) Å, but for the syn-complex they are significantly shorter at 3.094(1) Å.

The presence of C=O groups in pyrazolate ligand substituents, such as ethyl-4′-benzoate-3,5-dimethylpyrazole (HPz¹⁰) and methyl-4′-benzoate-3,5-dimethylpyrazole (HPz¹¹), allows for intermolecular M…O interactions, which play a significant role in the supramolecular organization of these complexes [76]. The variation in the crystal packing of dimers of trimers formed through Cu…Cu interactions resulted in the formation of several polymorphs (Figure 10). It has been shown that this type of compound exhibits dual-emission behavior depending on whether it is aggregated in a crystalline or grounded state.

On the other hand, the presence of halogen ions in a solution containing [AgPzⁿ]₃ leads to the immediate formation of AgCl. In the case of copper-containing complexes, oxidation of Cu centers is typically observed, accompanied with the rearrangement of the trinuclear core [77,78]. We demonstrated the reversible non-destructive complexation of [AgPz³]₃ and [CuPz³]₃ with the halogen atom X = Cl and Br of η³-allyliron tricarbonyl halides in the solution by means of IR and NMR spectroscopy [79].

3. Intermolecular Complexes of Cyclic Trinuclear Copper(I) and Silver(I) Pyrazolates with Bases of Different Natures

3.1. Intermolecular Interactions with π-Electron Systems

The high affinity of cyclic trinuclear silver(I) and copper(I) pyrazolate adducts to aromatic compounds is well known. According to Dias’s suggested synthetic procedure, the aromatic solvent acts as a template during the reaction, allowing the formation of a trinuclear planar structure. After the reaction, it is necessary to recrystallize [MPzⁿ]₃ in order to remove the coordinated solvate molecule [40,65,66,80,81,82]. These types of interactions were formulated as π-acid/π-base (A/B) interactions. The cyclic trinuclear silver(I) and copper(I) pyrazolate complexes with electron-withdrawing substituents, such as fluorinated alkyl groups or NO₂, are Lewis acids. Dias and Gamage concluded that, depending on the crystallization conditions, there are five main principles for the formation of supramolecular packing of cyclic metal pyrazolates with aromatic compounds. This was comprehensively represented by the example of interaction of cyclic silver(I) 3,5-bis(trifluoromethyl)pyrazolate [AgPz³]₃ with benzene, mesytylene, and toluene [80]. The first one is a complex, containing two molecules of benzene per one [AgPz³]₃, which is formulated as {BAB} (Figure 11a). The Ag…C intermolecular contacts ranged from 3.109(3) to 3.403(4) Å, and further packing was realized due to multiple H…H, H…π, and H…F forced contacts. On the other hand, the use of an additional solvent, such as CH₂Cl₂, allowed the formation of {BAAB}-type structures (Figure 11b). In this case, the intermolecular Ag…C contacts were in a narrower range of 3.187(4) Å to 3.346(4) Å, but they were slightly weaker. Further packing was realized due to rather strong CH^Pz…π^B hydrogen bonds. The coordination of benzene molecules resulted in a slight increase in the Ag…Ag intertrimer separation, from 3.307(1) Å in free [AgPz³]₃ [65] to 3.289(4) Å in the {BAAB} complex. The use of a free mesitylene as a solvent for the crystallization of [AgPz³]₃ resulted in the formation of the {BAB}_∞ complex (Figure 11c). Despite the presence of the Me substituents, the Ag…C contacts were practically the same as that observed for the complex with benzene (3.196–3.356 Å). Further aggregation was realized due to π–π interactions between the mesitylenes in neighboring {BAB} units (the shortened C–C distance was 3.660(4) Å, see Figure 11c). Crystallization from mesitylene/CH₂Cl₂ solution led to the formation of infinite stack of alternating A/B molecules {AB}_∞ (Figure 11d). Interestingly, using toluene as a coordinating solvent led to the formation of {BAA}_∞ (Figure 11e), even in the case of cyclic gold(I) pyrazolate complex [40].

Based on the example of silver complexes, it has been shown that trinuclear pyrazolates could be used as sensors for benzene and its methylated derivatives. When a sample is exposed to vapors of aromatic solvents, the emission intensity significantly increases. The maximum emission varies depending on the type of arene. After exposure to benzene or toluene vapor, a non-luminescent, dry substance begins to emit bright green fluorescence, while mesitylene vapor causes bright blue fluorescence. The peak maxima for these emissions are approximately 520, 515, and 410 nm, respectively. After evaporation, the complex reverts to its original state, and these cycles can be repeated more than 20 times without any degradation of the material. Solvates can also be obtained by slow evaporation of solutions with these solvents. At room temperature, these crystals exhibit the same bright green and blue fluorescence as when they interact with the respective vapor [83]. Crystallization from the benzene or toluene solution of the mixture of [MPz³]₃ (M = Cu(I) or Ag(I)) with naphthalene led to the formation of infinite stacks (Figure 12) [84,85]. The guest molecule could not be removed from the complex, even at low pressure and high temperature. The intermolecular M…C interactions were stronger for the silver-containing complexes, ranging from 3.0035 to 3.4168 Å. The same interactions in [CuPz³]₃ were in the range 3.090(2)–3.417(2) Å. Moreover, in the case of the complexation of [AgPz³]₃ with naphthalene, there were significantly more interactions compared to the [CuPz³]₃ complex. This was due to variations in the orientation of the naphthalene molecules in these complexes, which led to differences in the arrangement of M3N6 planes. The neighboring trimers were practically parallel for the [CuPz³]₃, with an angle of 0° between the Cu3N6 planes, while they were wedge-shaped for [AgPz³]₃, with an angle of 24.82° between the Ag3N6 planes. The {BA}_∞-type structures have been observed for the complexes of [CuPz³]₃ and [AgPz³]₃ with Fe(II) and Ru(II) sandwich compounds and cyclopentadienyl and indenyl ligands [86,87].

In the frozen CH₂Cl₂ solution containing equal amounts of [AgPz³]₃ and naphthalene, only a narrow band in the green region was observed, which corresponded to the phosphorescence of naphthalene. The intensity of this band was higher in the complex compared to the free form of naphthalene. In the crystalline form, both at room temperature and at low temperatures, the same emission band was observed. This emission corresponded to phosphorescence, with a quantum yield at room temperature of 15% and at 77 K of 45% [84]. Complex {[CuPz³]₃∙(naphtalene)} also demonstrated the green phosphorescence, with maxima at 497, 526, and 564 nm.

By increasing the number of condensed systems in a guest molecule to seven, even stronger intermolecular interactions were observed. The interaction of coronene with [MPz³]₃∙(M = Cu(I), Ag(I)) led to the poorly soluble complex formation, which could also be represented by an infinite stack {AB}_∞ [88]. The reaction can be performed mechanically in the solid state by triturating in the presence of a few drops of CH₂Cl₂ or THF (Figure 13). Complexes of coronene with trinuclear silver(I) 3,5-(CF₃)₂-pyrazolate ([AgPz³]₃) or 3,5-(NO₂)₂-pyrazolate ([AgPz¹²]₃) were practically non-emissive at ambient temperature in the solid state. Evidently, complex formation resulted in the quenching of emission from the parent compound due to strong intermolecular interactions. However, complex [CuPz³]₃∙(coronene) demonstrated emission of comparable intensity, with maxima at 625 and 505 nm. These values were slightly shifted toward the blue region compared to the parent compounds’ emissions [88].

In 2019, Zhan and Li et al. proposed using o-terphenyl as a guest molecule to form a complex with [AgPz³]₃ [89]. Several polymorphs were formed by the slow evaporation of an equimolar solution of reagents in hexane, and all of them exhibited a supramolecular structure similar to that described for the complexes with simple aromatic derivatives of the general formula {BA} (Figure 14). The main driving force of the supramolecular packing was a network of Ag–π interactions that led to the formation of infinite stacks. The near arrangement of phenyl fragments led to steric repulsion. Each molecule of o-terphenyl interacted with two Ag3 units through its central and side phenyl groups, with centroid distances ranging from 3.3310(2) to 3.8989(3) Å, which was much larger than that of {[AgPz³]₃·(arene)} analogues. This resulted in the twisting of these fragments, obstructing the formation of strong aggregates. Interestingly, Ag…C contacts were within the range of 3.06(1)–3.41(1) Å, which is typical for complexes of cyclic silver pyrazolate adducts with arenes.

At 77 K, the complex could be emissive in the entire visible color region. Depending on the excitation energy, different bands were observed in the photoluminescence spectra (Figure 15). A low-energy broadband appeared when the excitation occurred between 320 and 330 nanometers. And within this range, white light was observed according to the CIE diagram.

Recently, we have shown that similar {AB}_∞ adducts could be obtained for 1,1′-biphenyls. The series of complexes of [AgPz³]₃ with 1,1′–biphenyl and their 4,4′-halogenated derivatives have been obtained. Complexes were formed by slow solvent evaporation from an equimolar mixture in CH₂Cl₂, resulting in the formation of infinite stacks via the alternating arrangement of trimers and ligands (Figure 16) [90]. The shortest Ag…C contact of 3.09(1) Å was observed for the {[AgPz³]₃∙(1,1′–biphenyl)} complex, which was only slightly larger than in the complex with naphthalene (3.0035 Å).

This type of packing is typical for all halogenated biphenyls, except for the iodine derivative (4,4′–diiodo–1,1′–biphenyl), which is coordinated by a halogen atom to a silver atom via I…Ag₃ (3.3010(8)–3.5513(7) Å) coordination mode, similar to the self-assembled adducts described above (Figure 17) [68,69]. This adduct did not exhibit luminescent properties.

The luminescence of the [AgPz³]₃∙(4,4′-dihalo-1,1′-biphenyl) adducts in the solid state exhibited a dual-emission behavior (Figure 18a–c). Green phosphorescence with a maximum wavelength of 480–540 nm was observed upon excitation at 310 nm, and a blue fluorescence band with a maximum at 400–420 nm upon excitation at 360 nm. Using intermediate excitation energies allowed for the generation of two emission bands that covered the entire visible spectrum, enabling the production of white light (Figure 18d). The obtainment of room-temperature phosphorescence or dual-emissive behavior may have potential application in anti-counterfeiting.

To improve the supramolecular packing effect of cyclic trinuclear metal complexes, G.-H. Ning, D. Li, and co-workers proposed using coordination cages [Cu₂Pz¹³]₃ (Pz = 4,4′-thiophene-bisethylene-bis(3,5-diethyl)pyrazolate-4-yl) with a large cavity [91]. This allowed for the encapsulation of aromatic guest molecules and further improvement of different photophysical properties. Crystals were obtained under solvothermal conditions. The distances between the Cu3N6 core and aromatic rings ranged from 3.411 to 3.982 Å, and the intramolecular Cu–Cu interactions were absent. The coordination cage formed infinite stacks through intermolecular Cu…Cu interactions, with the shortest Cu–Cu contact being 2.969(2) Å (see, for example, the complex with benzene in Figure 19). The size of the cavity varied depending on the steric effects of the bulk of the guest molecule.

The obtained complexes exhibited red phosphorescence, with maxima ranging from 636 to 694 nm (Figure 20). Only the pyridine-containing complex showed a slight shift to the high-energy region, compared to other adducts. Dual-emission bands were observed at room temperature and 77 K. As the number of substituents decreased, the emission maximum shifted to the low-energy region (from 636 to 694 nm). This can be explained by the increasing strength of the intermolecular interactions. These interactions significantly contributed to the phosphorescence band. Then, cages with a similar supramolecular arrangement containing halogen-substituted benzenes were obtained. As the size of the halogen increased, the distance between the copper atoms also increased, which affected the luminescent properties. It has been shown that the stronger the metal–metal interactions were, the more significant the shift toward the lower-energy region was [92]. The quantum yield (QY) increased with the increasing size of the halogen atom, from 12.6% for Ph-F to 74.3% for Ph-I.

The presence of several coordination centers in the base molecules, such as pyridine-containing chalcones (3-(anthracen-9-yl)-1-(pyridin-2-yl)prop-2-en-1-one, A-PyC; 3-phenyl-1-(pyridin-2-yl)prop-2-en-1-one, P-PyC; Scheme 5), mainly led to the formation of complexes with [AgPz³]₃ due to multiple M…π interactions [93].

Anthracene-containing chalcones, A-PyC, undergo cis–trans isomerization under the influence of light. Crystallization of an equimolar solution of A-PyC and [AgPz³]₃ in the absence of light protection resulted in a more stable complex with a cis-form of A-PyC, in which the carbonyl and pyridine groups coordinated with the Ag atom. In the dark, an adduct with the trans-form of chalcone was formed, which was stabilized by the π-electron density of the fused aromatic rings (Figure 21A). Phenyl-containing chalcones with [AgPz³]₃ also formed the same structure via multiple Ag–π interactions (Figure 21B). However, it should be noted that the contact between the trimer, carbonyl group, and nitrogen atom of the pyridine fragment was also established through metal–π coordination. In both cases, more typical {AB}_∞ supramolecular packing was realized. Complexation had a minimal effect on the emission maximum in the case of A-PyC, and only an effect of silver heavy metal was observed. On the other hand, P-PyC was non-emissive in its solid state and solutions, but the complex {[AgPz⁶]₃∙(P-PyC)} exhibited red phosphorescence.

Trinuclear cyclic silver(I) and copper(I) pyrazolates coordinate sulfur-containing heterocycles, such as thiophene derivatives (benzotiophene, dibenzothiophene), via intermolecular contacts [94]. In Figure 22, the structures of complexes {[AgPz³]₃∙(2,5-dimethylthiophene)} ({[AgPz³]₃∙(dmt)}) and {[AgPz³]₃∙(4,6-dimethyldibenzothiophene)} ({[AgPz³]₃∙(dmdbt)}) are presented. In a crystal, 1:1 adducts were formed, stabilized by Ag…C and Ag…S contacts and packed into {AB}_∞ infinite stacks via multiple Ag…π interactions. The Ag…C contacts, which ranged from 3.137(8) to 3.323(9) Å for {[AgPz³]₃∙(dmt)} and 3.112(8) to 3.389(8) Å for {[AgPz³]₃·(dmdbt)}, were shorter than Ag…S (3.242(3)–3.265(3) Å and 3.392(2)–3.457(2) Å, respectively). Benzothiophene (BT) and its substituted analogues have a nearly parallel arrangement relative to the macrocycle core. In the structure of the complex with 2,5-dimethylthiophene, the neighboring Ag3N6 planes formed a wedge-shaped sandwich, with the Ag atoms pointing toward the sulfur atoms, forming shorter contacts. This suggests a tendency toward the possibility of coordination of silver ions with a lone pair of sulfur atoms, although it is not strong enough to be formulated as such.

Replacing the -CF₃ substituents with -C₂H₅ in the 3,5-positions and introducing the -NO₂ group to the 4th position of the pyrazolate ligand ([MPz¹⁴]₃, M = Cu, Ag) increased the solubility of the corresponding trinuclear metal pyrazolate compounds in hydrocarbons [95]. The complexes with thiophene derivatives exhibited a similar supramolecular arrangement in the form of {AB}_∞ columns. The M…S contacts in these complexes were slightly shorter than those in similar complexes containing [AgPz³]₃. Due to the ease of forming intermolecular adducts with [MPzⁿ]₃, the potential to use this approach to remove heterocyclic aromatic sulfur compounds from fuels has been demonstrated [94,95,96]. The use of more donating compounds, such as tetrathiafulvalene derivatives with multiple sulfur atoms, led to the formation of an infinite stack with [MPz³]₃ of alternating molecules [97]. Despite the presence of several possible coordination M-S bonds, the shortest contacts were found to be 3.265(2) Å for [AgPz³]₃ and 3.211(2) Å for the [CuPz³]₃, further confirming the preference for metal–π interactions in the crystal packing.

Recently, Wang, Guang, and colleagues have successfully synthesized a new trinuclear copper complex with 4-nitro-3,5-bis(trifluoromethyl)pyrazolate ([CuPz¹⁵]₃), and its interaction with thiophene compounds has been studied [98]. It was shown that the Cu…S distances in the crystal structure ranged from 2.3266(9) to 2.4216(9) Å (Figure 23), indicating the involvement of the sulfur lone pairs in coordination with the copper atoms. This was attributed to the highly acidic nature of the pyrazolate ligand, which allowed for the formation of such a strong Cu…S interaction.

An interesting observation in the study of host–guest complexes based on trinuclear coinage metal pyrazolates was the isolation of their adducts with fullerene. Treatment of C60 with [MPz³]₃ in a 1:1 molar ratio in CS₂ resulted in the formation of an air-stable, violet crystalline {([MPz³]₃)₄∙(C60)} adduct (Figure 24) [99].

A three-dimensional {([MPz³]₃)₄∙(C60)}_∞ network with tetrahedral symmetry has been established in the crystal structure. The crystal packing was realized due to metallophilic bonding between triangles in neighboring {([MPz³]₃)₄∙(C60)} moieties. The intermolecular Ag…Ag and Cu…Cu contacts were shorter than in the packing of the initial metal pyrazolate adduct, [MPz³]₃.

3.2. Intermolecular Interactions with O-, N-Donor Sites in Bases

Trinuclear metal complexes of silver(I) and copper(I), with pyrazolate ligands containing electron-withdrawing substituents, act as polydentate Lewis acids, and could interact with the electron densities of bases of various structures. In 2013, Galassi, Burini, and Omary et al. presented the results of the investigation into the chemisorption of small volatile molecule vapors by the known [AgPz³]₃ (Pz³ = 3,5-(CF₃)₂Pz⁻) and a newly synthesized [AgPz¹²]₃ (Pz¹² = 3,5-(NO₂)₂Pz⁻) [100]. It was shown that the complex containing NO₂ substituents was capable of adsorbing molecules like acetone, acetylacetone, ammonia, pyridine, acetonitrile, triethylamine, dimethyl sulfide, and tetrahydrothiophene at room temperature and atmospheric pressure. The formed adducts of macrocycles with row of compounds were characterized by elemental analysis, NMR and FTIR spectroscopy, and thermogravimetric analysis. At the same time, both compared macrocycles did not show sorption of CO, tetrahydrofuran, C₂H₅OH, CH₃OH, (C₂H₅)₂O. Using the Brunauer–Emmett–Teller (BET) theory, as well as spectroscopy methods, it was proven that the formation of adducts was caused not by physical adsorption of small molecules in the pores of the trinuclear macrocycles [MPzⁿ]₃, but due to chemical binding of small molecules, i.e., chemisorption. Based on the DFT, the lack of sorption properties in [AgPz³]₃ was attributed to its low electrostatic potential. In contrast, the new analogue [AgPz¹²]₃, which contains NO₂ substituents, had a significantly higher electrostatic potential due to the Lewis acidity of the silver ions. Crystallization of the [AgPz¹²]₃ was carried out with hot acetonitrile in the presence of diethyl ether vapor. A complex of the composition {[AgPz¹²]₃∙2(CH₃CN)} was isolated (Figure 25). It possessed symmetrical coordination of two acetonitriles with silver ions above and below the plane of the Ag3N6 core.

Interestingly, the interaction of the above-described metal pyrazolate complexes with halogen substituents also formed crystals with solvate molecules. The crystals of [AgPz⁴]₃ obtained from CH₂Cl₂ or benzonitrile contained solvent molecules coordinated with the metal centers, retaining the ability for intermolecular interactions between the chlorine atom of the pyrazolate ligand and the Ag3N6 core of another molecule. Similar to the silver complex with strong acceptor CF₃ groups [AgPz⁶]₃, the crystal packing was organized according to the principle of Ag…Cl intermolecular contacts, as observed in previous studies [68,69,70]. Using acetonitrile as a coordinating solvent, the principle of Ag…Cl intermolecular interactions disappeared. The adducts of the general formula {[AgPz⁴]₃∙CH₃CN}, in contrast to complexes with CH₂Cl₂ or benzonitrile, exhibiting an interaction between silver ions and nitrogen atoms of acetonitrile (Ag…N = 2.939(5)–3.003(4) Å), did not possess any Ag…Cl interactions (Figure 26). The coordination of an acetonitrile molecule with [AgPz⁴]₃ resulted in the silver ions becoming less acidic due to the donation of an electron lone pair from the nitrogen atom. The shortest distance between the Ag and Cl atoms was 4.798(2) Å, indicating the absence of interaction between them (Figure 26). This confirmed that the silver ions lost their ability to coordinate with other molecules.

Weaker coordination of a benzonitrile and CH₂Cl₂ resulted in a lower donating ability, which was not sufficient for complete “neutralization” of the silver ion’s Lewis acidity. For example, the shortest Ag…Cl (CH₂Cl₂) contact was 3.433(6) Å, indicating a weak coordination between the chlorine and silver ions (Figure 27). Interaction between PhCN and [AgPz⁴]₃ was also realized due to the weak π coordination of Ag(I) ions with the C≡N group. The contacts Ag… C and Ag…N were 3.358(9) and 3.498(9) Å, respectively. In this case, the ability for noncovalent intermolecular interactions between Ag and Cl was preserved.

The same tendency has been demonstrated by the example of [CuPz⁴]₃ in the presence of CH₃CN molecules [101,102]. The shortest Cu…N contacts ranged from 2.783(3) to 2.853(3) Å, and packing was realized due to H…Cl contacts (2.9854 Å) (Figure 28). There were no Cu…Cl intermolecular interactions.

These examples support the conclusion that the donor/acceptor properties of the substituents in the pyrazole ligand influenced the acidity of the metal ion, which in turn determined the possibility of noncovalent interactions with the basic centers.

Recently, a new and environmentally friendly method has been proposed for the synthesis of [AgPz³]₃, which does not require the use of aromatic hydrocarbons or an inert atmosphere, as is required by traditional methods [103]. Mixing the initial pyrazole with a silver salt (AgNO₃ in the presence of Et₃N or PhCOOAg), in methanol or tetrahydrofuran, resulted in the formation of complexes containing a trinuclear metal pyrazolate moiety with triethylammonium nitrate or benzoic acid dimers. The pure [AgPz³]₃ could then be easily obtained from these adducts by extracting or washing with water. In the crystal structure of the adduct {[AgPz³]₃∙(NO₃)₂}, two nitrate anions were symmetrically located on both sides of the Ag3N6 plane through rather strong Ag…O contacts (2.676(4)–2.953(4) Å; Figure 29A). Supramolecular packing was realized due to CH…O hydrogen bonds (2.607 Å) between the proton at the 4th position of the pyrazolate ligand in the neighboring triangle and the nitrate anion. The adduct of [AgPz³]₃ with benzoic acid (BnzA) {[AgPz³]₃ (BnzA)} formed due to relatively strong Ag…π (3.25(1)–3.40(1) Å) and rather weak Ag…O contacts. There was a simple dimer of benzoic acid via OH…O hydrogen bonds in the structure, which additionally explained the weak coordination of oxygen atoms to silver ions (Figure 29B). Moreover, one contact (3.282(7) Å) could be described as an interaction with the lone pair of oxygen atoms in the C-O-H fragment (∠COAg = 116.6(6)°), and the second one was a coordination to the lone pair of the oxygen atom of the CO double bond.

The use of ketones as Lewis bases allows the formation of various types of structures. It is worth noting that the interaction of [MPz³]₃ (M = Cu, Ag) with butanone-2 has been established in the solution by spectroscopic methods [104]. In the case of complexation of macrocycles with benzophenone (Ph₂CO), different types of complexes have been observed depending on the metal atom. The copper-containing analogue formed only a wedge-shaped sandwich {([CuPz³]₃)₂∙(Ph₂CO)} via coordination of oxygen atoms to only one metal in both triangles (Cu…O = 2.879 Å) (Figure 30). Coordination of the oxygen atom with copper did not result in a noticeable elongation of the bond. In the crystal, the CO bond length was 1.221 Å, compared to 1.222 Å in the uncoordinated benzophenone molecule. In addition to the Cu…O contacts, shortened bonds were also found between copper ions and phenyl substituents of benzophenone in the sandwich complex (Cu–π^Ph interaction). The Cu…C contacts ranged from 3.194(2) Å to 3.941(3) Å.

The interaction of [AgPz³]₃ with benzophenone led to two types of complexes. The first type was a 1:1 complex, denoted as {[AgPz³]₃·(Ph₂CO)} (Figure 31). The second type involved two macrocycles interacting with one benzophenone molecule, which could be formulated as {[AgPz³]₃)₂·(Ph₂CO)}, being similar to that obtained for [CuPz³]₃ (Figure 32). In the structure of {[AgPz³]₃∙(Ph₂CO)}, three shortened Ag…O (2.768(3)–2.952(3) Å) contacts were observed (Figure 31A). Argentophilic interactions between neighboring Ag3N6 moieties led to the dimers’ formation. The Ag…Ag distance was 3.4337(7) Å, being longer than that for the free [AgPz³]₃ dimers (Figure 31B).

Complex {[AgPz³]₃)₂·(Ph₂CO)} had an unsymmetric structure, in which the oxygen atom of the carbonyl group was coordinated with only one silver ion out of two in the [AgPz³]₃. This is in accordance with the Ag…O distances of 2.909(6) and 3.524(6) Å (Figure 32A). The Ag…C contacts (2.909(6) and 3.524(6) Å), similar to those found in the copper complex, were one of the main driving forces for crystal packing. Complex {[AgPz³]₃)₂·(Ph₂CO)} had an asymmetric structure, in which the oxygen atom of the carbonyl group was bonded to only one silver ion out of two [AgPz³]₃. This is consistent with Ag…O bond distances of 2.909(6) and 3.524(6) Å, as shown in Figure 32A. Ag…C bonds (3.229(6)–3.420(6) Å) were similar to those observed in copper complexes and were one of the main driving forces for the crystal packing. However, unlike the copper analogue, this complex relied on Ag…Ag interactions (3.2945(8) and 3.4566(8) Å) for its structure (Figure 32B).

The ferrocene-containing ketones also formed stable complexes with trinuclear copper and silver pyrazolates via multiple O…M₃ contacts [105]. Complexes of [MPz³]₃ with acetylferrocene (AcFc) containing one trinuclear pyrazolate per two molecules of ketone ({[MPz³]₃∙(AcFc)₂}) did not exhibit intermolecular interactions between the metals and the π-electron density of the ferrocenyl fragments. In the copper complex, the O…Cu contacts were different above and below the metal plane, ranging from 2.587(3) to 2.664(3) Å, with an average of 2.614(3) Å (Figure 33A). The silver-containing complex possessed symmetric coordination of the carbonyl groups from both sides of the M3N6 plane (Ag…O = 2.615(1), 2.674(0), and 2.763(2) Å; Figure 33B).

The presence of an additional aromatic ring in phenylacetylferrocene (PhAcFc) also led to the formation of Ag–π interactions. The intermolecular Ag…O contacts ranged from 2.676(2) to 2.908(2) Å, and Ag…C contacts ranged from 3.057(5) to 3.367(4) Å (Figure 34). Similar triple coordination of the C=O group has also been observed in the case of the complexation of [AgPz³]₃ with isocoumarins [106].

Based on the compounds provided above, which ranged from simple arenes like benzene to more complex structures, such as fullerene and phenylacetylferrocene, it was shown that cyclic trinuclear silver(I) and copper(I) pyrazolates can be described as crystalline “sponges” for guest molecules. Recently, D. Li et al. demonstrated this concept by using natural compounds isolated from a crude extract, which were combined with [AgPz³]₃ in a random order [107]. Most of these compounds contained oxygen-containing functional groups, such as carbonyls, aldehydes, and esters (Figure 35). Co-crystals were formed with five flavonoids, allowing their identification, including two novel ones whose structures and absolute configurations had not previously been established. In addition to the 25 compounds studied, the team also investigated other compounds that contained functional groups and formed complexes with trinuclear cyclic silver(I) pyrazolate. It was shown that the presence of oxygen atoms allowed crystallization with macrocycles in single, double, or multiple binding modes, depending on the number of coordination sites and the resulting interactions with the π-acidity of [AgPz³]₃. Furthermore, numerous C–H/F interactions occurred between the molecules and [AgPz³]₃, leading to reduced mobility and more efficient crystal packing. To maximize efficient packing, the Ag3N6 core often lies out of plane, allowing [MPz³]₃ to co-crystalize with a variety of organic molecules containing functional groups, providing insight into their structures.

3.3. Intermolecular Interactions with Hydride Ligands

Intermolecular complexes of cyclic trinuclear metal pyrazolates with hydrides are limited to boron derivatives. The first example is complexes with polyhedral [B₁₀H₁₀]²⁻ and [B₁₂H₁₂]²⁻ anions [108], and the second is with aminoborane (BH₃NEt₃) [109]. It is important to note that the reactivity of the hydride compound is a key requirement for the formation of the complex. Basically, when [MPz³]₃ interacts with hydride donor compounds, a reduction reaction takes place. The interactions of cyclic trinuclear copper and silver pyrazolates [MPz³]₃ with polyhedral borate anions, such as [B₁₀H₁₀]² and [B₁₂H₁₂]²⁻, were established only in the solutions in solvents with low polarity using FTIR spectroscopy in a wide temperature range (190–290 K). Two complexes of two compositions were identified in solution: {([MPz³]₃)[B_nH_n]}²⁻ (1:1) and {([MPz³]₃)₂[B_nH_n]}²⁻ (2:1; M = Ag, Cu; n = 10, 12). The stability constants for these complexes were determined based on the infrared spectra. Complexation of BH₃NEt₃ with [AgPz³]₃ has been studied in solution and the solid state. It has been shown that in both the solution and the solid phase, hydride atoms from aminoborane coordinated with the metallocycle. This was confirmed by the complex obtained in the solid phase. The coordination of amine–borane occurred through two short Ag…H contacts of 2.56(3) and 2.32(4) (Figure 36). The third hydride ion did not participate in coordination, which is in agreement with the appearance of a new high-frequency band in the IR spectra in both solution and solid form, indicating similar structures in both media. The complex, consisting of one molecule of base per one macrocycle, exhibited identical shortened Ag…Ag (3.272(3) Å). Interestingly, complexation resulted in a shortening of intertrimeric Ag–Ag interactions compared to free [AgPz³]₃ dimers (3.307(1) Å).

4. Conclusions

This review summarized the main data on the self-organization of cyclic trinuclear copper(I) and silver(I) pyrazolate complexes [MPz]₃, as well as the formation of intermolecular aggregates based on them. It also showed the main types of noncovalent interactions that determine the supramolecular packing of these compounds in the solid state. The presence of metallophilic interactions in the crystal packing of free MPz3 allowed the formation of excimers in the excited state, which led to their photoluminescence. Changing the donor–acceptor properties of substituents or their steric hindrance in the pyrazolate ligand can influence the supramolecular packing and, consequently, the emission. On the other hand, introducing substituents into the pyrazolate ligand, which can coordinate with a metal atom (halogen or carbonyl group), made this coordination site more favorable. This led to the formation of different types of supramolecular organizations. Fine-tuning of crystal packing then resulted in a change in the excited state, allowing influence on the emission properties. For example, the trinuclear silver(I) 3,5-bis(trifluoromethyl)pyrazolate [AgPz³]₃, which formed dimers through short Ag…Ag contacts, did not emit light at room temperature. In contrast, its analogues [AgPz⁶]₃ and [AgPz⁷]₃, with chlorine or bromine atoms in the fourth position of the ligand, emitted light at 298 K. In these cases, the halogen atom coordinated to the metal centers, leading to a different type of crystal packing. On the other hand, the role of acid–base properties in metal complexes became clearer based on the described intermolecular interactions. For example, π-electron donors, such as aromatic hydrocarbons, being weak bases, did not interact with [MPz]₃ complexes containing donor substituents, and vice versa. Additionally, the nature and availability of the base determined which coordination interactions occurred with the metal center. The strength of these interactions correlates with the acidity of the macrocycles and must also match the coordinated base. For instance, when forming strong sigma complexes with acetonitrile, it became impossible to coordinate other types of bases, such as halogens or pi-dense atoms. Replacing CH₃CN with PhCN led to the formation of a π-coordination. This interaction minimally satisfied the macrocycle’s coordination capacity and did not prevent other types of noncovalent intermolecular interactions. Thus, the acidity of metal centers plays a crucial role in the formation of supramolecular structures. On the other hand, the nature of the base is also essential for a specific type of coordination. The data presented here provide qualitative insights into the nature of these interactions on the selected platform and can be valuable in the future design of similar supramolecular systems.