Structural diversity and photoluminescence of copper(I) carboxylates: From discrete complexes to infinite metal-based wires and helices

Coordination Chemistry Reviews 295 (2015) 125–138

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Review

Structural diversity and photoluminescence of copper(I) carboxylates: From discrete complexes to infinite metal-based wires and helices Oleksandr Hietsoi, Alexander S. Filatov, Cristina Dubceac, Marina A. Petrukhina ∗ Department of Chemistry, University at Albany, State University of New York, 1400 Washington Avenue, Albany, NY 12222, USA

Contents 1. 2. 3. 4.

5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of copper(I) carboxylates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoluminescence of copper(I) carboxylates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structural variations of copper(I) carboxylates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Dinuclear complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Tetranuclear complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Hexanuclear complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Copper(I) “wires” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Copper-based helices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

a r t i c l e

i n f o

Article history: Received 28 January 2015 Accepted 10 March 2015 Available online 20 March 2015 Keywords: Copper(I) carboxylate Solid state structure Cuprophilic interactions Photoluminescence X-ray crystallography

a b s t r a c t Copper(I) carboxylates have rich photoluminescent properties and broad applications in catalysis, microelectronics, and bioinorganic chemistry. They also exhibit a remarkable structural diversity ranging from discrete complexes to infinite networks based on mono-, di-, tetra-, or hexanuclear copper(I) cores, including unique double-helical arrangements. These structural variations should have important implications on physical and chemical behavior of the resulting crystalline solids, but no structural analysis or structure–property correlations have been established for this family. This work provides the first overview of the types of structures exhibited by crystallographically characterized copper(I) carboxylates having no exogenous ligands, opening pathways for a better understanding of their properties and further advancement of their practical applications. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Copper(I) carboxylates exhibit a diverse range of interesting properties and practical applications. One of the major draws toward the study of copper(I) carboxylates is their ability to catalyze a great variety of chemical transformations, thus making them useful and effective catalysts and initiators [1]. Copper(I) carboxylates play an important role in biochemical processes and enzyme modeling as probes of the binuclear sites of oxygenase,

∗ Corresponding author. Tel.: +1 518 442 4406; fax: +1 518 442 3462. E-mail address: [email protected] (M.A. Petrukhina). http://dx.doi.org/10.1016/j.ccr.2015.03.009 0010-8545/© 2015 Elsevier B.V. All rights reserved.

125 126 127 127 127 128 131 132 133 135 137 137 137 137

nitrous oxide reductase, and heme-copper oxidase [2]. They can serve as versatile building blocks in supramolecular chemistry [3]. Copper(I) carboxylates are also broadly used as volatile precursors in the chemical vapor deposition manufacture of integrated circuits in the microelectronic industry [4]. Very recently, fluorinated copper(I) carboxylates showed effective p-doping of organic hole transport layers in single carrier devices to afford organic lightemitting diodes with high luminous efficacies [5]. In addition, polynuclear copper(I) complexes have been broadly studied as potential optoelectronic materials [6] due to their remarkable photophysical properties [7]. The attractive interactions between the closed d10 shells in various clusters of copper(I), silver(I), and gold(I) are subjects of considerable academic

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interest [8] since the arrangement of metal centers and, in particular, the number of those in close proximity affect the photo-optical behavior of the resulting polynuclear products. Therefore, controlled formation of small clusters of these metals having defined nuclearities and studies of their structures and properties are of great importance for tailored design of functional electronic and optoelectronic materials. In this regard, polynuclear copper(I) carboxylates, being inexpensive and exhibiting rich photoluminescent properties along with a remarkable structural diversity, have attracted our attention. The synthetic routes toward copper(I) carboxylate complexes have been well developed [9], but the ease of disproportionation, hydrolysis, and oxidation of these compounds in solution [10] thwarted the isolation of single crystalline products and studies of their physical and chemical properties. Thus, the first reports on fluorescence of aliphatic and aromatic copper(I) carboxylates appeared in the literature more than three decades ago [11], but there have been no follow-up studies on the origin of photoluminescence or structure–property relationships for this class of compounds. Furthermore, in spite of the well-known synthetic routes to access various copper(I) carboxylate complexes, only four compounds of the general formula [Cu(O2 CR)], where R = CH3 [12], C6 H5 [13], CF3 [14], and C(CH3 )3 [15], have been structurally characterized prior to our work. Interestingly, each complex exhibits a unique structural type with specific architectural and functional features. With the goal of examining the role of bridging carboxylate ligands on the resulting solid state structures, we have significantly expanded the number of crystallographically characterized copper(I) complexes in the last several years [16–21]. We have specifically targeted volatile copper(I) carboxylates that allowed the effective use of sublimation–deposition procedures for crystal growth of products having no exogenous ligands. Changing the degree of fluorination and bulkiness of carboxylate bridges was used as a tool to varying the copper(I) core structures and intermolecular interactions in the solid state. The addition of new members to this interesting class of compounds has further reaffirmed a fascinating structural diversity exhibited by copper(I) carboxylates. Next, rationalization of the observed structural types should be used as a key to understanding of their properties and reactivity. Herein, we provide the first overview of the crystallographically characterized “unligated” (having no exogenous ligands) copper(I) carboxylates known to date and classify them based on their core nuclearity and solid state structures. 2. Synthesis of copper(I) carboxylates Detailed synthetic methods for the preparation of various copper(I) carboxylates were reported in the early 70s by the groups of Edwards and Fernando [9]. Brief descriptions of four major strategies are presented below. The first one relies on thermal decomposition in vacuo of a copper(II) carboxylate to yield the corresponding copper(I) product (Eq. (1)). Copper(I) acetate was first isolated by this method back in 1902 [22]. Extremely low product yields make this reaction unsuitable for large-scale practical applications: T

Cu(O2 CR)2 −→Cu(O2 CR)

(1)

The second method is based on the reduction of copper(II) carboxylates (Eqs. (2) and (3)). For example, copper(II) acetate and trifluoroacetate can be reduced by copper(0) metal in acetonitrile in the presence of corresponding carboxylic acid [23]. Copper(I) formate and acetate were prepared by using sulfur dioxide and hydroxylammonium sulfate as reducing agents. The drawbacks of the latter route are low yields and sulfur contamination. The yields

can be improved to 60–80% when copper(0), hydrazine hydrate, or hydroxylammonium carboxylates are used as reductants for reactions performed in a solution of carboxylic acid. However, long reaction times and the need to use coordinating solvents (acetonitrile, pyridine) are not appropriate for the isolation of “unligated” carboxylates. Hydroquinone was proposed as a suitable reducing agent when subsequent benzoquinone coordination is not an issue [24]. Later, tin(II) 2-ethylhexanoate was suggested for the preparation of copper(I) acetate, stearate, and benzoate (Eq. (3)) [25]: Cu(O2 CR)2 + Cu → 2Cu(O2 CR)

(2)

2Cu(O2 CR)2 + Sn(O2 CR  )2 → 2Cu(O2 CR) + Sn(O2 CR  )2 (O2 CR)2 (3) The latter reaction (Eq. (3)) takes place in the non-coordinating solvent dichloromethane. However, this route is not suitable for copper(II) carboxylates that are sparingly soluble in dichloromethane. The reduction of copper(II) trifluoroacetate by copper(0) metal in the presence of trifluoroacetic acid vapors in a sealed ampule at 140 ◦ C has also been investigated. As a result, copper(I) trifluoroacetate was formed in a liquid form, which crystallized upon cooling [9b]. The third approach, based on the ligand exchange reaction of copper(I) carboxylates and the corresponding carboxylic acids, was suggested by Edwards for the synthesis of copper(I) dichloroacetate, trifluoroacetate, and cyanoacetate (Eq. (4)) [9a]. This reaction route resulted in 95–98% product yields, when triethyl orthoformate was used as a solvent: Cu(O2 CR) + HO2 CR  → Cu(O2 CR  ) + HO2 CR

(4)

In 1998, a hydrothermal synthesis of the monofumarate copper(I) complex was published by Schultz. The exchange reaction between acetate and fumarate ligands (Eq. (4)) was set up in a Teflon-lined autoclave, which was sealed and placed in a furnace at 155 ◦ C [26]. Copper(I) halobenzoates of the general formula Cu(O2 C(2X)C6 H4 ) (X = Cl, Br) were also prepared starting with mesitylcopper(I) and the corresponding halobenzoic acid (Eq. (5)) [27]: Cu((2,4,6-Me)3 C6 H2 ) + HO2 CC6 H4 X → Cu(O2 CC6 H4 X) + (2,4,6-Me)3 C6 H3

(5)

The fourth synthetic procedure includes a reaction between copper(I) oxide and a carboxylic acid in an aromatic solvent with subsequent removal of the water by-product. This method was used by Edwards and Richards in 1973 to synthesize copper(I) benzoate (Eq. (6)) [9a]. Xylene was used as a solvent and water formed in the course of reaction was removed using a Dean-Stark apparatus: Cu2 O + 2HO2 CR → 2Cu(O2 CR) + H2 O

(6)

The presence of water in the reaction mixture and long reaction times prompted further modifications of the above approach. Carboxylic acid/carboxylic anhydride mixture was used by the Sheppard group in 1970 [28]. In 1974, Amma [29] reported that the use of only carboxylic anhydride is sufficient for this reaction to proceed (Eq. (7)): Cu2 O + O(OCR)2 → 2Cu(O2 CR)

(7)

In benzene, the insoluble copper(I) oxide is transformed into the soluble copper(I) carboxylate, which can be readily separated from the reaction mixture. The procedure, applied for the first time to a synthesis of the highly electrophilic copper(I) trifluoroacetate, resulted in the isolation of its benzene adduct [Cu4 (O2 CCF3 )4 ](C6 H6 )2 [29]. Heating of the reaction mixture at ca. 100 ◦ C in vacuo for a few days is required to obtain an unligated

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copper(I) product [14]. This synthetic method is preferred when the corresponding anhydride is available. It is important to emphasize that copper(I) carboxylates easily disproportionate producing copper(II) salts and metallic copper. This process is facilitated by elevated temperatures, prolonged reaction times, and coordinating solvents. In addition, copper(I) carboxylates tend to get oxidized to copper(II) products in the presence of oxygen and moisture [9a,30]. Copper(I) aryl carboxylates are prone to decarboxylation at temperatures as low as 60 ◦ C. All these factors can significantly complicate the isolation of target copper(I) complexes in their pure forms. Nevertheless, copper(I) carboxylates are viable synthetic targets with the use of an inert atmosphere environment, standard Schlenk and glove box techniques, and careful experimental design. A variety of synthetic techniques targeting the preparation of specific copper(I) carboxylates without exogenous ligands have been adapted and further developed in our laboratory. The preferential methods include the copper(I) oxide/carboxylic anhydride reaction and ligand exchange procedures. The latter is more applicable, since in many cases carboxylic acids are commercially available, while their anhydrides are not. This method usually affords copper(I) carboxylates in high yields, although the final product is always contaminated with the unreacted carboxylic acid, which is used in excess. Some traces of copper(II) carboxylate can also be present. The use of gas phase sublimation procedures under reduced pressure is very effective in purifying the target copper(I) carboxylates from all impurities. At the same time, it allows the efficient growth of single crystals suitable for X-ray diffraction characterization. Importantly, the “solvent-free” technique excludes any inclusion or interference of solvents during crystallization and affords products without additional ligands [31], allowing one to evaluate the role of carboxylate bridges on the resulting structures. Using these sublimation-deposition procedures we have successfully added multiple new members to the family of copper(I) carboxylates in the last several years [16–21,32–36].

3. Photoluminescence of copper(I) carboxylates The first reports on fluorescence of a series of aliphatic and aromatic copper(I) carboxylates are going back to 1978 and 1981 [11]. The spectral behavior at two different temperatures, namely 300 K and 77 K, has been investigated (Table S1 in Supporting Information). The emission maxima of copper(I) formate, acetate, propionate, butyrate, valerate, hexanoate, and heptanoate vary in the broad range of 535–660 nm at room temperature (ex = 305 − 350 nm) [11b]. It is worth mentioning that among aliphatic copper(I) carboxylates only copper(I) acetate was structurally characterized at that time [12]. It exhibits a 1D polymeric structure based on dicopper(I) units that are further linked by axial Cu· · ·O interactions (Scheme 1). The structures of other aliphatic copper(I) carboxylates remained unknown, but since several mass spectroscopic studies showed the similarity of mass fragments found in the vapor phase [9,37], other members of this family were assumed to have similar dinuclear solid state structures. This assumption persisted in the literature as even the recent mass spectroscopic investigation of copper(I) pivalate [38] has also used indirect structural correlations. However, by now it is crystallographically confirmed that the above-mentioned carboxylates belong to different structural types in the solid state. Thus, the direct correlation of vapor phase and solid state structures for copper(I) carboxylates is proved to be erroneous. Therefore, caution must be exercised when massspectroscopy data collected on the fragments existing in the gas phase are used for direct structural assignments of solids in the absence of X-ray crystallographic studies. In addition, broad

127

Scheme 1. Schematic representation of dinuclear core structure of copper(I) acetate (R = CH3 ).

variations in photoluminescence (PL) exhibited by aliphatic copper(I) carboxylates [11b] could also point to a structural diversity of this series. However, the photoluminescent properties of copper(I) carboxylates have never been correlated with their solid state structures, leaving this field fully open for discussion. 4. Structural variations of copper(I) carboxylates 4.1. Dinuclear complexes Copper(I) acetate has been known since 1812 [39] and broadly investigated over the years [22]. Nevertheless, its structure was elucidated by single-crystal X-ray diffraction only in 1973 [12], being the first among the entire copper(I) carboxylate family. Copper(I) acetate was prepared by reduction of copper(II) acetate with copper metal in pyridine and crystallized from acetonitrile by slow solvent evaporation [12c]. Copper(I) acetate is built of dimetal units that are further linked by intermolecular Cu· · ·O interactions to form a 1D polymer, [Cu2 (O2 CCH3 )2 ]∞ (1) (Scheme 1, Fig. 1a). Notably, the crystal structure of copper(I) acetate obtained by gas phase deposition at 160 ◦ C is the same as the above, as confirmed by X-ray powder diffraction [33]. Despite the fact that the dinuclear structural type is the most common for copper(II) carboxylates [40], for more than three decades copper(I) acetate remained the only structurally characterized example having a dimetal core in the copper(I) carboxylate family. Only recently the crystallographic characterization of copper(I) 2,6-bis(trifluoromethyl)benzoate revealed a similar structural motif based on dicopper units (Fig. 1b) [32]. [Cu2 (O2 C(2,6CF3 )2 C6 H3 )2 ] (2) was synthesized by the ligand exchange reaction starting from copper(I) trifluoroacetate and crystallized from the gas phase at 260 ◦ C. A direct comparison of two structural analogs, 1 and 2, reveals similarity in their Cu· · ·Cu and Cu Oeq distances. The O Cu O angles are also close (169.8(3)◦ for 1 vs. 168.2(3)◦ for 2), while the intermolecular Cu· · ·O contacts are shorter in 2 (average ˚ than those in 1 (2.310(7) A). ˚ 2.229(6) A) The crystalline sample of 2 displays green photoluminescence at room temperature upon exposure to UV-radiation (ex = 350 nm) with the emission maxima centered at 558 nm. Interestingly, Weber et al. [11b] reported PL for two modifications of copper(I) acetate. The emission maxima for the room temperature modification of 1 was detected at 660 nm (ex = 350 nm), while for the second modification it was observed at 560 nm (ex = 310 nm) [11b,41]. We recollected the PL data on freshly sublimed crystalline

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Fig. 1. Fragments of polymeric chains in [Cu2 (O2 CCH3 )2 ]∞ (1) (a) and [Cu2 (O2 C(2,6-CF3 )2 C6 H3 )2 ]∞ (2) (b). Axial Cu· · ·O interactions are shown by dashed lines. Cu-blue, O-red, C-gray, H-light gray, F-green (this color scheme is used in all figures).

copper(I) acetate, the structure of which was confirmed to be the same as previously reported [12]. The room temperature PL measurement (ex = 350 nm) for 1 revealed the emission maxima at 663 nm [33], consistent with that reported for the room temperature modification. We have not seen any evidence of the existence of the second structural modification of copper(I) acetate in our work. Surprisingly, the dinuclear-based structural type seems rather rare based on crystallographically characterized copper(I) carboxylates known to date. We have seen only two additional examples of such arrangement for the entire family. First, we have confirmed by single crystal X-ray diffraction that copper(I) heptafluorobutyrate, crystallized either from gas phase or from solution, possesses a 1D polymeric structure consisting of dicopper units, [Cu2 (O2 CCF2 CF2 CF3 )2 ]∞ (3) (Fig. 2a) [33]. Second, the structure of copper(I) 2-fluoro-6-(trifluoromethyl)benzoate (4) [33], a close analog of 2, is also based on dicopper units connected by intermolecular Cu· · ·O interactions to form a 1D chain (Fig. 2b), similar to that in 1–3. Notably, in contrast to copper(I) acetate, propionate and butyrate complexes of Cu(I) do not exhibit the dinuclear core type of structures, as shown below. 4.2. Tetranuclear complexes The tetranuclear core structural type was first seen in copper(I) benzoate, [Cu4 (O2 CC6 H5 )4 ] (5), which was prepared using the reaction of copper(I) oxide with benzoic acid back in 1973 [9a] and structurally characterized a few years later [13]. Copper(I) benzoate crystallizes with two closely aligned crystallographically

Fig. 2. Fragments of polymeric chains in [Cu2 (O2 CCF2 CF2 CF3 )2 ]∞ (3) (a) and [Cu2 (O2 C(2-F)(6-CF3 )C6 H3 )2 ]∞ (4) (b). Hydrogen atoms are omitted for clarity. Axial Cu· · ·O interactions are shown by dashed lines.

independent tetramers in the asymmetric unit with the shortest ˚ The tetramers are intermolecular Cu· · ·Cu contact of 3.239(2) A. similar but not identical; both have a planar core comprised of four copper atoms bridged by four benzoate ligands alternating above and below the Cu4 -plane (Fig. 3a). The intramolecular Cu· · ·Cu distances within the tetranuclear units range from 2.713(2) to ˚ The average value of Cu· · ·Cucarb-bridged separations in 5 2.748(2) A. ˚ is within the sum of the van der Waals radii of copper (2.728(2) A) ˚ [42]. (rvdW (Cu) = 1.92 A) In the solid state, very weak intermolecular Cu· · ·O ˚ interactions ˚ and Cu· · ·(C6 H5 ) (3.21–3.70 A) (3.098(8)–3.468(8) A) can be identified between the two tetramers. The PL measurements for crystalline copper(I) benzoate, [Cu4 (O2 CC6 H5 )4 ] (5), revealed an emission centered at 676 nm (ex = 418 nm). The tetranuclear core was then found in copper(I) trifluoroacetate (6), which was prepared by reaction of Cu2 O with trifluoroacetic anhydride in anhydrous benzene at 60–70 ◦ C. Single crystals were obtained by sublimation–deposition of the crude solid at 110–120 ◦ C [14]. In contrast to 5, copper(I) trifluoroacetate is composed of tetrameric molecules that form a polymeric zigzag ribbon in the solid state, [Cu4 (O2 CCF3 )4 ]∞ (6) (Fig. 4a). This structural arrangement was related to the great electron-withdrawing ability of the trifluoroacetate groups which enhances the electrophilicity of copper(I) centers and enforces additional, rather ˚ intermolecular Cu· · ·O interactions strong (averaged to 2.621 A), between the Cu4 -units. The intramolecular Cu· · ·Cu interactions ˚ The PL meain [Cu4 (O2 CCF3 )4 ] range from 2.719(1) to 2.833(1) A. surements (ex = 350 nm) carried out at room temperature on the crystalline sample of 6 revealed max = 583 nm. Fluorination of carboxylate ligands holding a specific polynuclear copper(I) core together has been an important structure-controlling factor that allows switching on and off the intermolecular forces between clusters. We have observed the effect of fluorination for the series of tetranuclear complexes consisting of copper(I) 3-fluorobenzoate (7), copper(I) 2,3,4-trifluorobenzoate (8), and mixed ligand copper(I) trifluoroacetate/pentafluorobenzoate (9). These products were synthesized by ligand exchange procedures based on refluxing copper(I) trifluoroacetate with excess carboxylic acid in benzene and crystallized from the gas phase in the 220–240 ◦ C range for 7, 160–220 ◦ C for 8, and at ca. 150 ◦ C for 9 [16]. The X-ray structural characterizations of 7–9 revealed that all complexes contain the planar Cu4 -core with carboxylate bridges positioned above and below the plane (Fig. 4). In contrast to 6, the solid state structures of 7 and 8 are based on discrete tetracopper units. At the same time, the mixed carboxylate compound containing very electrophilic trifluoroacetate ligands and fully fluorinated benzoate groups, [Cu4 (O2 CCF3 )2 (O2 CC6 F5 )2 ]∞ (9), is structurally similar to copper(I) trifluoroacetate, [Cu4 (O2 CCF3 )4 ]∞ (6), and exhibits a 1D polymeric structure built on intermolecular Cu· · ·O interactions between the tetramers (Fig. 4d).

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Fig. 3. The asymmetric unit in [Cu4 (O2 CC6 H5 )4 ] (5) (a) and schematic representation of intermolecular contacts in the solid state structure of 5 (b).

The Cu· · ·Cu distances vary to some extent within this [Cu4 (O2 CR)4 ] series and are longer for complexes containing trifluoroacetate ligands and having extended polymeric structures. Thus, the longest Cu· · ·Cu separation between the carboxylate bridged copper(I) centers is observed in 9 and 6 (2.8314(10) and ˚ respectively). The intermolecular Cu· · ·O contacts in 2.833(1) A, 9 in the range of 2.405(4)–2.746(4) A˚ are comparable to those of 2.621(6) A˚ in 6. All copper(I) carboxylates 7–9 exhibit yellow-to-green photoluminescence upon exposure to UV radiation in the solid state. The PL measurements (ex = 350 nm) carried out at room temperature on crystalline samples revealed max at 502 nm for 7 and 507 nm for 8, while max for 9 is shifted to 583 nm. Thus, the emission wavelengths are very close for the structurally similar complexes [Cu4 (O2 C(3-F)C6 H4 )4 ] (7) and [Cu4 (O2 C(2,3,4-F)3 C6 H2 )4 ] (8), having discrete tetranuclear structures. At the same time, the tetramers [Cu4 (O2 CC6 F5 )2 (O2 CCF3 )2 ]∞ (9) and [Cu4 (O2 CCF3 )4 ]∞ (6), having analogous polymeric structures in the solid state, both display yellow emission at 583 nm. The fully substituted copper(I) pentafluorobenzoate complex has also been prepared in the single crystalline form. However, crystals grew as extremely thin needles, unsuitable for X-ray diffraction analysis. Interestingly, the crystalline sample of copper(I) pentafluorobenzoate shows emission at 589 nm (ex = 350 nm) close to that of 6 and 9. We have speculated that this fact suggests that copper(I) pentafluorobenzoate may exhibit

the Cu4 -based extended structure built on Cu· · ·O intermolecular interactions, similar to those of 6 and 9. Another series of tetranuclear copper(I) complexes that show intriguing photoluminescent properties is comprised of several aliphatic copper(I) carboxylates such as copper(I) propionate, [Cu4 (O2 CCH2 CH3 )4 ]∞ (10), copper(I) trifluoroacetate/propionate, [Cu4 (O2 CCF3 )(O2 CCH2 CH3 )3 ]∞ (11), and copper(I) pentafluoropropionate, [Cu4 (O2 CCF2 CF3 )4 ]∞ (12) [17]. Copper(I) propionate (10) and pentafluoropropionate (12) were prepared by refluxing copper(I) oxide with the corresponding acid/anhydrides mixtures, while copper(I) trifluoroacetate/propionate (11) was synthesized by partial ligand exchange starting from copper(I) trifluoroacetate. Single crystals were isolated by sublimation–deposition procedures at 110 ◦ C for 10 and at 95 ◦ C for 11 and 12. For 10, single crystals were also grown from o-dichlorobenzene/hexanes and those were identical to crystals obtained from the gas phase. The X-ray structural characterization revealed that complexes 10–12 all have the planar Cu4 -cores supported by the corresponding carboxylate ligands positioned above and below the plane (Fig. 5). The average values of Cu· · ·Cucarb-bridged distances are equal to 2.7425(5) A˚ in 10, 2.7651(12) A˚ in 11, and 2.7918(9) A˚ in 12. The Cu· · ·Cu distances vary to some extent within the series of complexes 10–12 and are longer for complexes containing trifluoroacetate and pentafluoropropionate ligands. Thus, the longest Cu· · ·Cu separation between the carboxylate-bridged copper(I) atoms is observed in 11 and 12 (2.8028(12) and

Fig. 4. The molecular structures of [Cu4 (O2 CCF3 )4 ]∞ (6) (a), [Cu4 (O2 C(3-F)C6 H4 )4 ] (7) (b), [Cu4 (O2 C(2,3,4-F)3 C6 H2 )4 ] (8) (c), and [Cu4 (O2 CCF3 )2 (O2 CC6 F5 )2 ]∞ (9) (d).

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Fig. 5. The molecular structures of [Cu4 (O2 CCH2 CH3 )4 ] (10) (a), [Cu4 (O2 CCF3 )(O2 CCH2 CH3 )3 ] (11) (b), and [Cu4 (O2 CCF2 CF3 )4 ] (12) (c).

˚ respectively), however these distances are still within 2.8127(9) A, the sum of van der Waals radius for copper [42]. In the solid state, complexes 10 and 11 form infinite 1D chains based on intermolecular Cu· · ·O interactions between the tetramers. Notably, these 1D chains further interact with the neighboring chains via longer intermolecular Cu· · ·O contacts, thus forming 2D networks (Scheme 2) that were not previously seen in the copper(I) carboxylate family. In contrast to 10 and 11 having 2D tetramer-based structures, copper(I) carboxylate containing fully fluorinated propionate ligands, [Cu4 (O2 CCF2 CF3 )4 ] (12), is structurally similar to copper(I) trifluoroacetate, [Cu4 (O2 CCF3 )4 ] (6). Complex 12 has the 1D polymeric structure built from tetramers having intermolecular Cu· · ·O interactions in the solid state (Scheme 2). The average intermolec˚ are close ular Cu· · ·O interactions within 1D chains in 12 (2.561 A) ˚ but shorter than in 10 (2.756 A) ˚ and to those observed in 6 (2.621 A) ˚ 11 (2.759 A). The aliphatic copper(I) carboxylates 10–12 exhibit yellow photoluminescence upon exposure to UV radiation in the solid state. The room temperature PL measurements (ex = 350 nm) revealed broad emission bands centered at 560 nm (10) and 577 nm (11), while for 12 max is shifted to 588 nm. It seems that with increasing the electrophilicity of bridging carboxylates, going from 10 to 12, the PL emission maxima become more red-shifted. Copper(I) butyrate is a new member of the family of tetramers that was prepared by reaction of Cu2 O with butyric acid/anhydride mixture and crystallized from benzene by slow cooling [34]. The X-ray crystallographic study revealed that [Cu4 (O2 CCH2 CH2 CH3 )4 ]∞ (13) has a planar core comprised of four copper atoms bridged by four butyrate ligands in an alternating

fashion. In the solid state, the Cu4 -units are engaged in intermolecular copper-oxygen interactions to form a 1D polymeric chain (similar to 6 and 12) with average Cu· · ·O contacts of ca. 2.427(2) A˚ (Scheme 2). Notably, the average intramolecular Cu· · ·Cu distance between the bridged metal centers in 13 ˚ is significantly shorter than those observed in cop(2.7258(5) A) ˚ trifluoroacetate (2.7825(1) A), ˚ and per(I) propionate (2.7425(5) A), ˚ Copper(I) butyrate exhibits pentafluoropropionate (2.7918(9) A). red photoluminescence with broad emission band centered at 625 nm upon exposure to UV radiation (ex = 350 nm) at room temperature in the solid state. In addition, we have also synthesized copper(I) 4[Cu4 (O2 CC6 H4 CO2 C2 H5 )4 ]∞ (14) (ethoxycarbonyl)benzoate, [35], and crystallized it from the gas phase at 149 ◦ C in order to investigate how additional Cu· · ·O intermolecular interactions introduced through the coordinatively active carboxylate ligand affect the product structure and photoluminescent properties. The X-ray crystallographic study revealed that 14 consists of tetranuclear units that are engaged in intermolecular Cu· · ·O interactions ˚ and through carbonyl oxygen atoms from ester (2.749–2.754 A) ˚ to form a 2D polymeric network in the carboxylic groups (2.562 A) solid state (Fig. 6). No photoluminescence was detected for the crystalline sample of 14. However, 14 is emissive in toluene solution at room temperature (max = 543 nm) upon 350 nm excitation, which could be related to the breakage of intermolecular Cu· · ·O interactions existing in the solid state. So far, the tetranuclear-based copper(I) carboxylates constitute the most abundant and diverse set within this family. Structural variations include discrete copper(I) tetramers, as well as 1D and 2D extended networks formed by Cu4 -units (Scheme 3).

Scheme 2. Schematic representation of the extended structures in 10–13.

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Scheme 3. Schematic representation of tetranuclear core structures: R = (3-F)C6 H4 (7), (2,3,4-F)3 C6 H2 (8) (a); R = C6 H5 (5) (b); R = CF3 (6), CF3 /C6 F5 (9), CF2 CF3 (12), CH2 CH2 CH3 (13) (c); R = CH2 CH3 (10), CF3 /CH2 CH3 (11) (d).

4.3. Hexanuclear complexes The hexanuclear core structure was first seen in copper(I) 3,5-difluorobenzoate, [Cu6 (O2 C(3,5-F)2 C6 H3 )6 ] (15) [18], and was later found in the analogous copper(I) 2,4-difluorobenzoate, [Cu6 (O2 C(2,4-F)2 C6 H3 )6 ] (16) [33] as well as in the polyarene adducts of copper(I) 3,5-bis(trifluoromethyl)benzoate [19,36]. Copper(I) difluorobenzoates were prepared by ligand exchange reactions between copper(I) trifluoroacetate and the corresponding carboxylic acids. Crystals of 15 and 16 were obtained by gas phase

Fig. 6. Molecular structure of [Cu4 (O2 CC6 H4 CO2 C2 H5 )4 ]∞ (14) (a) and schematic representation of intermolecular contacts between Cu4 -units in the solid state structure (b). H atoms are omitted for clarity.

sublimation–deposition procedures at 210–250 ◦ C and 160–190 ◦ C, respectively. Both complexes have a planar hexanuclear core comprised of six copper atoms bridged by six fluorinated benzoate ligands alternating above and below the Cu6 -plane (Fig. 7). The Cu· · ·Cucarb-bridged distances range over 2.7064(8)–2.8259(8) A˚ in 15 and 2.6905(10)–2.7487(11) A˚ in 16. In the solid state, the Cu6 -units of 15 and 16 are not involved in any cuprophilic or Cu· · ·O intermolecular interactions but show very weak Cu· · · contacts (3.11–3.72 A˚ in 15 and 3.20–3.46 A˚ in 16). Notably, the hexanuclear copper units remain intact upon gas phase co-deposition of 15 with coronene (C24 H12 ) to give the [Cu6 (O2 C(3,5-F)2 C6 H3 )6 ](C24 H12 ) adduct in the solid state [18]. Upon exposure to UV radiation (ex = 350 nm) at room temperature, the crystalline complexes 15 and 16 exhibit bright green photoluminescence at close values of 554 and 557 nm. In contrast, the above coronene adduct is non-luminescent in the visible region, showing that incorporation of polyarenes into the crystalline lattice causes dramatic changes of the photophysical properties of the resulting product. A very interesting example of the hexanuclear copper(I) core was found when crystals of the long-known benzoate [Cu4 (O2 CC6 H5 )4 ] [13] were subjected to gas phase sublimation–deposition procedures at 230 ◦ C. In contrast to solution crystallization, which affords the tetranuclear copper(I) benzoate (5), sublimation at elevated temperature results in crystallization of [Cu6 (O2 CC6 H5 )6 ] (17) units (Fig. 8) [20]. The Cu· · ·Cu distances within the Cu6 core in 17 range from 2.6797(6) to ˚ Similarly to 15 and 16, only weak Cu· · · interactions 2.7231(6) A. ˚ can be found in the solid between the copper(I) units (3.01–3.54 A) state structure of 17. The emission maxima of the crystalline solid sample of 17 is 577 nm at room temperature (ex = 350 nm), which is very different from that of 5 (676 nm) (Fig. S1). Copper(I) benzoate provides a remarkable example of polymorphism for discrete polynuclear copper(I) complexes [20]. Two products of different nuclearity, [Cu4 (O2 CC6 H5 )4 ] and

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Fig. 7. Molecular structures of [Cu6 (O2 C(3,5-F)2 C6 H3 )6 ] (15) (a) and [Cu6 (O2 C(2,4-F)2 C6 H3 )6 ] (16) (b).

[Cu6 (O2 CC6 H5 )6 ], have been individually prepared in high yield and fully characterized by X-ray diffraction and spectroscopic techniques. The tetranuclear polymorph (5) crystallizes from solution (xylene, o-dichlorobenzene) at room temperature, while the hexanuclear one (17) deposits from the gas phase at 230 ◦ C under reduced pressure. Importantly, this system has provided the first tetranuclear/hexanuclear core interconversion for the unligated copper(I) carboxylate family that can be induced by different crystallization conditions (Fig. 8). Single crystals of 5 can be completely converted to 17 by employing sublimation–deposition procedures. Similarly, when crystals of 17 are dissolved and then recrystallized from o-dichlorobenzene, they quantitatively transform back to 5. These procedures can be repeated multiple times, confirming a reversible transformation between 5 and 17. In addition, a reversible single-crystal-to-single-crystal temperature-induced phase transition was revealed for the hexanuclear copper(I) benzoate polymorph 17, which manifests itself in slightly different crystal packing of the Cu6 -units at 173 K and 293 K [20]. As a result, the copper(I) benzoate provided a unique system to examine the fine effects of core structures and intermolecular interactions for the same bridging carboxylate ligand on the resulting spectroscopic properties. Another example of hexanuclear core in the mixed ligand copper(I) 4-(ethoxycarbonyl)benzoate/trifluoroacetate, [Cu6 (O2 CC6 H4 CO2 C2 H5 )4 (O2 CCF3 )2 ]∞ (18), can be briefly mentioned here [35]. The parameters of the copper core in 18 are

close to those in 16 and 17. However, its solid state structure is different as all Cu6 units are engaged in additional intermolecular Cu· · ·O interactions through carbonyl oxygen atoms from both the ˚ and carboxylic (2.513 A) ˚ groups to form a 2D ester (2.510–2.672 A) extended network (Fig. 9) [35]. This resulted in quenching of the solid state photoluminescence of 18. 4.4. Copper(I) “wires” An unprecedented “molecular wire” type of copper(I) carboxylate has been synthesized in our group by using the sterically bulky 2,4,6-triisopropylbenzoate ligand that allowed to switch off axial Cu· · ·O interactions and instead to utilize cuprophilic interactions for the chain propagation [21]. The target complex was prepared by ligand exchange reaction between copper(I) acetate and 2,4,6-triisopropylbenzoic acid. Sublimation of the crude product at 186 ◦ C under reduced pressure yielded very thin colorless needles. Their X-ray diffraction study revealed an essentially linear “wire” chain structure [Cu(O2 C16 H23 )]∞,linear (19) comprised of copper(I) centers bridged by carboxylate groups and further supported by cuprophilic interactions (Fig. 10a). By increasing the sublimation temperature to 215 ◦ C, the second structural polymorph of copper(I) 2,4,6-triisopropylbenzoate has been isolated [21]. Its X-ray diffraction analysis revealed the formation of an infinite zigzag “wire” [Cu(O2 C16 H23 )]∞,zigzag (20),

Fig. 8. Reversible transformation between [Cu4 ] and [Cu6 ] units in copper(I) benzoates, [Cu4 (O2 CC6 H5 )4 ] (5) and [Cu6 (O2 CC6 H5 )6 ] (17).

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The zigzag chain 20 is characterized by two Cu Cu Cu angles of 96.22(1)◦ and 180.00(1)◦ , while all angles in 19 are 159.43(5)◦ . The above two infinite “wire” type polymers stand out in the entire copper(I) carboxylate family, although for other transition metals the ligand supported metal atom chains of various length are well known [43]. The PL measurements (ex = 350 nm) carried out at room temperature on crystalline samples revealed emission max at 563 nm (green region) for 19 and at 610 nm (orange region) for 20 (Fig. S2). Plus, the emission intensity is substantially greater in the case of the linear “copper wire” compared to the zigzag polymorphic form. Such a drastic difference in PL properties of 19 and 20 (including both the emission wavelength and intensity) shows that relatively small variations in positions of copper(I) centers held together by the same bridging ligand may lead to significant changes in photoluminescence. 4.5. Copper-based helices

Fig. 9. Molecular structure of [Cu6 (O2 CC6 H4 CO2 C2 H5 )4 (O2 CCF3 )2 ]∞ (18) (a) and schematic representation of intermolecular contacts between Cu6 -units in the solid state structure of 18 (b). H atoms are omitted for clarity.

having a different spatial distribution of the same structural unit as in 19 (Fig. 10b). The two polymorphic forms of copper(I) 2,4,6triisopropylbenzoate have essentially the same cell volumes normalized by the number of formula units (382 and 380 A˚ 3 per [Cu(O2 C16 H23 )] in 19 and 20, respectively). The Cu· · ·Cu distance ˚ than that in 19 (2.9397(5) A), ˚ but in in 20 is shorter (2.8843(3) A) both cases they are significantly shorter than the sum of their van der Waals radii (rvdW (Cu) = 1.92 A˚ [42]) and fall into the category of cuprophilic interactions. The average Cu Ocarb distances in 19 ˚ respectively). and 20 are very similar (1.8445(3) and 1.8483(12) A,

The first representative of this class of copper(I) carboxylate compounds was reported by Awaga and co-workers [15] in 2006 for copper(I) pivalate prepared by a thermal decomposition method. Its structure consists of five unique copper(I) ions and five pivalate ligands that form a remarkable structural arrangement based on cylindrical right- and left-handed infinite chains [Cu(O2 CC(CH3 )3 )]∞ (21) (Fig. 11). The double-helical chain structure of 21 was suggested to be a result of balancing the cuprophilicity and the steric effect of bulky tert-butyl groups. Copper(I) pivalate 21 shows no photoluminescence in the solid state, and its unique infinite helical structure held together by cuprophilicity was considered responsible for such non-radiative decay [15]. The second example of a copper-based helix was reported later for copper(I) 3,5-bis(trifluoromethyl)benzoate, [Cu(O2 C(3,5CF3 )2 C6 H3 )]∞ (22) [19]. It was prepared by ligand exchange reaction starting with copper(I) trifluoroacetate and crystallized from the gas phase at 140–200 ◦ C. The X-ray crystallographic study of 22 revealed a remarkable structure based on seven copper(I) atoms in an asymmetric unit bridged by carboxylate groups in a consecutive fashion (Fig. 12).

Fig. 10. Fragments of molecular structure of [Cu(O2 C16 H23 )]∞,linear (19) (a) and [Cu(O2 C16 H23 )]∞,zigzag (20) (b). Isopropyl groups and hydrogen atoms are omitted for clarity.

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Fig. 11. A fragment of the double-helical polymeric structure in [Cu(O2 CC(CH3 )3 )]∞ (21).

Scheme 4. Temperature-dependent fragmentation of the CF3 )2 C6 H3 )]∞ (22) helix into copper(I) units of different nuclearity.

Fig. 12. A fragment of the double-helical polymeric structure in [Cu(O2 C(3,5CF3 )2 C6 H3 )]∞ (22).

Similarly to copper(I) pivalate (21), copper(I) 3,5-bis(trifluoromethyl)benzoate (22) exhibits an infinite double-helical polymeric arrangement (Fig. 13). The Cu· · ·Cu distances between the 2 -bridged copper centers in 22 vary broadly from 2.693(2) to ˚ A close comparison of 21 ˚ averaging to 2.803(2) A. 3.143(2) A, and 22 shows that the range of Cu· · ·Cu carboxylate-bridged dis˚ than in 21 (2.850(2)–2.897(2), tances is greater in 22 ( = 0.45 A) ˚ In both 21 and 22 the above-mentioned Cu· · ·Cu dis ≈ 0.05 A). tances are within with the sum of van der Waals radii for Cu [42].

Fig. 13. Fragments of the infinite double-helical structures in copper(I) pivalate (21) (a) and copper(I) 3,5-bis(trifluoromethyl)benzoate (22) (b).

[Cu(O2 C(3,5-

In contrast to the non-luminescent 21, solid 22 exhibits photoluminescence with an emission maximum centered at 594 nm (yellow color) upon exposure to UV radiation (ex = 350 nm) at room temperature. The addition of this new helix to the family of structurally characterized copper(I) carboxylates has ruled out the earlier assumption that cuprophilic interactions along the chain provide some path for non-radiative decay. Interestingly, the gas phase reactions of 22 with fluoranthene, pyrene, and coronene performed at 137, 162 and 220 ◦ C, respectively, resulted in the controlled cleavage of the infinite helix into discrete hexa-, tetra-, and dinuclear copper(I) fragments (Scheme 4) crystallized as adducts with the above polyarenes, namely [Cu6 (O2 C(3,5-CF3 )2 C6 H3 )6 ](C16 H10 )2 , [Cu4 (O2 C(3,5CF3 )2 C6 H3 )4 ](C16 H10 ), and [Cu2 (O2 C(3,5-CF3 )2 C6 H3 )2 ](C24 H12 ) [19]. Notably, the copper unit of the highest nuclearity is formed at the lowest reaction temperature, while temperature increase results in further cleavage of the extended structure into smaller copper(I) fragments. None of the copper(I) 3,5bis(trifluoromethyl)benzoate adducts with polyaromatic ligands exhibit detectable emission in the solid state. For the next step, the aryl carboxylate product with two different ring substituents, namely 3-fluoro-5-(trifluoromethyl)benzoate of copper(I), was prepared by ligand exchange reaction starting from copper(I) trifluoroacetate. Variation of the crystal growth conditions in the gas phase resulted in the isolation of three different polymeric structures for [Cu(O2 C(3-F)(5-CF3 )C6 H3 )], illustrating its remarkable coordination flexibility. This is the first such example for the copper(I) carboxylate family [33]. The first product (23) was obtained by sublimation-deposition procedures of copper(I) 3-fluoro-5-(trifluoromethyl)benzoate at 120 ◦ C. Complex 23 crystallizes in the monoclinic P2/c space group with 15 independent copper(I) atoms in the asymmetric unit. Its structure consists of a long polymeric chain containing 11 Cu(I) centers bridged by carboxylate groups and two additional dinuclear copper(I) units, which are connected to the main chain by copper-copper and copper–oxygen interactions (Fig. 14a; Fig. S3). The Cu· · ·Cucarb-bridged distances within the Cu11 -chain are ranging

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Fig. 14. Fragments of the infinite double-helical structures of copper(I) 3-fluoro-5-(trifluoromethyl)-benzoate in 23 (a), 24 (b), and 25 (c).

Table 1 Selected distances of [Cu(O2 C(3-F)(5-CF3 )C6 H3 )] in 23, 24, and 25. Cu· · ·Cu (Å) 23 24 25

2.767(2)–3.610(2) 2.843(4)–3.174(4) 2.753(15)–3.367(15)

Cu· · ·Cucarb-bridged (Å) 2.679(2)–2.876(2) 2.635(4)–2.801(4) 2.652(15)–2.808(15)

Helix periodicity (Å) 15.87 19.67 19.49

˚ (Table 1) [33]. from 2.679(2) to 2.876(2) A˚ (averaged at 2.770(2) A) Also, each of the eleven copper(I) atoms of the main chain in 23, besides being in proximity with the adjacent carboxylate-bridged Cu(I) centers, has close contacts to the non-bridged copper atoms, ˚ Such conwith the contacts ranging from 2.767(2) to 3.610(2) A. tacts were first observed in the solid state structure of copper(I) pivalate (21) [15] and later identified in the helical structure of copper(I) 3,5-bis(trifluoromethyl)benzoate (22) [19]. The extended structure of 23 can also be viewed as a double-helix (Fig. 14a). Interestingly, the periodicity of these helices is increasing from ˚ to [Cu(O2 C(3,5-CF3 )2 C6 H3 )]∞ (22) [Cu(O2 CC(CH3 )3 )]∞ (21) (5.61 A) ˚ and [Cu(O2 C(3-F)(5-CF3 )C6 H3 )]∞ (23) (15.87 A). ˚ (8.13 A) Increasing the sublimation temperature to 140 ◦ C allowed the isolation of the second product (24) from the same system. Complex 24 crystallizes in the triclinic P 1¯ space group with 23 independent copper(I) atoms in the asymmetric unit. The double-helix in 24 consists of a longer polymeric chain containing 15 Cu(I) centers bridged by carboxylate ligands in an alternating fashion. In addition, two carboxylate-supported copper dimers and one tetramer are connected to the main chain by multiple Cu· · ·Cu and Cu· · ·O interactions (Fig. 14b; Fig. S4). Finally, gas phase sublimation-deposition at 180 ◦ C afforded crystals of 25. Complex 25 crystallizes in the monoclinic P2/c space group with 25 independent copper(I) atoms in the asymmetric unit. The structure of 25 also consists of 15 Cu(I) centers forming an infinite double-helical chain supported by carboxylate bridges. In contrast to 24, one dimer and two tetramers are connected to the main chain by Cu· · ·Cu and Cu· · ·O interactions (Fig. 14c; Fig. S5). Next, these chains are further connected by bridging tetracopper units to form a 2D network in the solid state of 25. Notably, the co-crystallization of double-helical chains with different combinations of discrete copper units in the solid state (two Cu2 in 23, two Cu2 and one Cu4 in 24 vs. one Cu2 and two Cu4 in 25) show the inherent structural flexibility of this system.

The PL measurements (ex = 400 nm) carried out at room temperature on the crystalline sample of 23 revealed a broad emission band centered at 622 nm along with a shoulder at 660 nm. In the solid state, both 24 and 25 exhibit green photoluminescence entirely different from that of 23. Thus, upon exposure to UV radiation complex 24 emits at 530 nm (ex = 350 nm), while 25 at 512 nm (ex = 325 nm) [33]. 5. General trends Based on the above structural considerations, the following major structural types can be identified for the copper(I) carboxylate family: dinuclear, tetranuclear, hexanuclear, and polynuclear complexes. Among those, tetranuclear core complexes show the greatest structural variations as they exist as discrete units and extended assemblies having 1D or 2D structures based on copper(I) tetramers linked by additional Cu· · ·Cu or Cu· · ·O contacts. Polynuclear complexes show interesting variations as well, ranging from single chain infinite “wire-type” polymers to double-chain helical structures of different helicity. In contrast, the known dinuclear copper(I) carboxylates belong to the same structural type. They all consist of dicopper units, [Cu2 (O2 CR)2 ], that are further linked into 1D chains by axial Cu· · ·O interactions (Scheme 1, Figs. 1 and 2). Interestingly, only very few complexes were crystallographically confirmed to have the dinuclear core structure, similar to that of the well-known copper(I) acetate (1) [12]. Those include copper(I) 2,6-bis(trifluoromethyl)benzoate (2) [32], 2-fluoro-6(trifluoromethyl)-benzoate (4) [33], and heptafluorobutyrate (3) [33]. Despite close similarity of their structures, the PL properties of these products in the solid state are very different, ranging from 660 nm emission for the aliphatic copper(I) acetate to non-luminescent heptafluorobutyrate (Table 2). The two rather close fluorinated benzoates 2 and 4 emit at 558 and 546 nm, respectively. By now, the tetranuclear motif has appeared as the most common among the copper(I) carboxylate family. As for the dinuclear core members, the tetranuclear type of structure is also characteristic for carboxylates with both aliphatic and aromatic substituents. Overall, tetranuclear copper(I) carboxylates exhibit four different types of packing in the solid state, which can be

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Table 2 Structurally characterized copper(I) carboxylates and their solid state PL emission maxima, max .. Structural type

Complex formula

max (nm)

Ref.

[Cu2 (O2 CCH3 )2 ]∞ (1) [Cu2 (O2 C(2,6-CF3 )2 C6 H3 )2 ]∞ (2) [Cu2 (O2 C(2-F)(6-CF3 )C6 H3 )2 ]∞ (4) [Cu2 (O2 CCF2 CF2 CF3 )2 ]∞ (3)

660 558 546 Quenched

[11,12] [32] [33] [33]

[Cu4 (O2 C(3-F)C6 H4 )4 ] (7) [Cu4 (O2 C(2,3,4-F)3 C6 H2 )4 ] (8) [Cu4 (O2 CC6 H5 )4 ] (5)

502 507 676

[16] [16] [13,20]

[Cu4 (O2 CCF3 )4 ]∞ (6) [Cu4 (O2 CCF3 )2 (O2 CC6 F5 )2 ]∞ (9) [Cu4 (O2 CCF2 CF3 )4 ]∞ (12) [Cu4 (O2 CCH2 CH2 CH3 )4 ]∞ (13)

583 583 588 625

[14] [16] [17] [34]

[Cu4 (O2 CCH2 CH3 )4 ]∞ (10) [Cu4 (O2 CCF3 )(O2 CCH2 CH3 )3 ]∞ (11) [Cu4 (O2 CC6 H4 CO2 C2 H5 )4 ]∞ (14)

560 577 Quenched

[17] [17] [35]

[Cu6 (O2 C(3,5-F)2 C6 H3 )6 ] (15) [Cu6 (O2 C(2,4-F)2 C6 H3 )6 ] (16) [Cu6 (O2 CC6 H5 )6 ] (17) [Cu6 (O2 CC6 H4 CO2 C2 H5 )4 (O2 CCF3 )2 ]∞ (18)

554 557 577 Quenched

[18] [33] [20] [35]

[Cu(O2 C16 H23 )]∞,linear (19) [Cu(O2 C16 H23 )]∞,zigzag (20)

563 610

[21] [21]

[Cu(O2 CC(CH3 )3 ]∞ (21) [Cu(O2 C(3,5-CF3 )2 C6 H3 )]∞ (22) [Cu(O2 C(3-F)(5-CF3 )C6 H3 )]∞ (23) [Cu(O2 C(3-F)(5-CF3 )C6 H3 )]∞ (24) [Cu(O2 C(3-F)(5-CF3 )C6 H3 )]∞ (25)

Quenched 594 622,660 530 512

[15] [19] [33] [33] [33]

Dinuclear

Tetranuclear

Hexanuclear

Polynuclear – wires

Polynuclear–helices

ex is 350 nm for all compounds, except for 5 (ex = 418 nm), 23 (ex = 400 nm), and 25 (ex = 325 nm).

considered separately. First, in the solid state structures of tetranuclear copper(I) 3-fluorobenzoate (7) and 2,3,4-trifluorobenzoate (8), no intermolecular Cu· · ·Cu or Cu· · ·O interactions are observed ˚ can be and only very weak Cu· · · interactions (3.23–3.48 A) identified between the tetramers (Scheme 3a, Fig. 4b and c). These complexes display intense photoluminescence at 502 nm (7) and 507 nm (8) [16]. Second, weak intermolecular Cu· · ·Cu, Cu· · ·O, and Cu· · · interactions can be identified in the solid state structure of tetranuclear copper(I) benzoate (5) that exhibits emission maximum at 676 nm (Scheme 3b, Fig. 3) [20]. Third, copper(I) butyrate (13) [34], trifluoroacetate (6) [14], pentafluoropropionate (12) [17], and the mixed ligand trifluoroacetate/pentafluorobenzoate (9) [16] can be described as tetramers further linked by additional axial Cu· · ·O contacts, forming the 1D polymeric zigzag ribbon structures (Scheme 3c, Figs. 4a,d and 5c). The latter three fluorinated copper(I)

complexes (6, 9, and 12) exhibit emission within the close range of 583–588 nm, while for 13 the corresponding maximum appears at 625 nm. Next, copper(I) propionate (10) and the mixed ligand trifluoroacetate/propionate (11), consisting of tetrameric units that are linked by Cu· · ·O contacts to form infinite 2D networks in the solid state, have their emissive bands at 560 and 577 nm, respectively (Scheme 3d, Fig. 5a and b) [17]. And finally, the non-luminescent in the solid state copper(I) 4-(ethoxycarbonyl)benzoate (14) features a 2D structure based on tetranuclear units connected through rather strong Cu· · ·O interactions involving the carbonyl O-atoms of the ester groups (Fig. 6). Although the first hexanuclear copper(I) carboxylate complex was reported only in 2007 [18], it was quickly confirmed that such core complexes [20,35] are not less common than dinuclear core compounds. Three closely related copper(I) benzoates, namely the

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becomes very broad for several double-helical products. In order to pinpoint additional trends, further crystallographic studies are clearly needed, as the number of structurally characterized copper(I) carboxylate examples in each sub-group is still very limited. 6. Conclusions

Fig. 15. Emission maxima (max , nm) for copper(I) carboxylates 1–25 of various nuclearity and structural types.

high temperature polymorph of unsubstituted benzoate (17), along with copper(I) 3,5-difluorobenzoate (15) [18] and copper(I) 2,4difluorobenzoate (16) [33], have been structurally characterized to confirm a discrete hexagonal core in the solid state (Figs. 7 and 8). The PL measurements (ex = 350 nm) revealed emission max at 577 nm for the parent benzoate (17), while two fluorinated analogs 15 and 16 emit at 554 nm and 557 nm, respectively (Table 2). The mixed ligand 4-(ethoxycarbonyl)benzoate/trifluoroacetate (18) features a 2D network of hexanuclear copper units linked through strong intermolecular Cu· · ·O interactions (Fig. 9) and shows no emission [35]. It is noteworthy that two structural polymorphs of copper(I) 2,4,6-triisopropylbenzoate, [Cu(O2 C16 H23 )]∞,linear (19) and [Cu(O2 C16 H23 )]∞,zigzag (20), having infinite single chain structures composed of copper(I) ions bridged by carboxylate groups, show rather different PL emission upon exposure to UV radiation in the solid state (563 nm for 19 and 610 nm for 20, Fig. S2) [21]. Although a substantial difference in PL properties of two different polynuclear core complexes, Cu4 and Cu6 , bridged by the same benzoate ligand has been observed for 5 and 17 (Fig. S1) [20], complexes 19 and 20 provided the first instance when a significant difference in PL emission is observed for structural polymorphs having a similar 1D arrangement which differs only by the Cu Cu Cu angles (Fig. 10). The emission of a series of copper(I) 3-fluoro-5(trifluoromethyl)benzoate complexes having the double-helical structures in the solid state (Fig. 14) spans a very broad range from 660/622 nm (23) to 530 nm (24), and 512 nm (25) [33]. While the double-helical copper(I) 3,5-bis(trifluoromethyl)benzoate (22) falls into that range with max of 594 nm [19], the non-luminescent behavior of copper(I) pivalate (21) still requires some explanation [15]. The comparison of emission maxima of all structurally characterized copper(I) carboxylates (Fig. 15) reveals some interesting observations. The discrete hexanuclear core complexes with the related benzoate bridges emit in the close range of 554–577 nm. In contrast, the dinuclear and especially tetranuclear core complexes cover the very broad emission range of ca. 545–660 nm and 500–680 nm, respectively. Furthermore, the tetranuclear family demonstrates a very strong dependence of photoluminescent properties on the nature of carboxylate ligands and the overall structural type. The two infinite “wire”-type polymorphs show rather different emission maxima (563 and 610 nm) for such closely related structures, and the range of emission values

Luminescent copper(I) complexes being cheaper than precious silver(I) and gold(I) analogs are considered as potentially valuable materials in “alternative energy” devices such as organic lightemitting diodes (OLEDs) and light-emitting electrochemical cells (LECs). The experimental results accumulated to date for copper(I) carboxylates showed fascinating structural variations for only about two dozen structurally characterized members. This diverse family also shows interesting photoluminescent properties and underlines the importance of correlation of the measured PL properties with the solid state structures. The crucial role of the copper core, the nature of carboxylate bridges, as well as that of intermolecular interactions in the solid state on the resulting luminescent behavior is clearly observed. Moreover, the consideration of PL properties for close analogs demonstrated that emission wavelengths and intensities depend not only on the overall structural type but show great sensitivity to subtle differences in spatial distribution of copper(I) centers bridged by the same carboxylate ligands. Next, in order to rationalize these effects the use of theoretical modeling applied to the structurally characterized copper(I) carboxylate systems is needed. Unfortunately, up to now there are no reliable theoretical models that describe photoluminescence of cuprophilic compounds properly. With this overview we aim to attract broad attention of physicists and theoreticians to the revealed empirical structure-function correlations shown by a unique family of copper(I) carboxylates. We also would like to underline that fundamental understanding of the complexity of these systems and the effects of the solid state structures on photoluminescent properties are critically important. Acknowledgments The authors thank the National Science Foundation for funding and instrumentation (CHE-1212441 and CHE-1337594). The contribution to this work of all graduate and undergraduate students, postdoctoral associates and collaborators listed in the references cited is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ccr.2015.03.009. References [1] (a) D.J. Darensbourg, E.M. Longridge, M.W. Holtcamp, K.K. Klausmeyer, J.H. Reibenspies, J. Am. Chem. Soc. 115 (1993) 8839; (b) D.J. Darensbourg, E.M. Longridge, B. Khandelwal, J.H. Reibenspies, J. Coord. Chem. 32 (1994) 27; (c) D.J. Darensbourg, M.W. Holtcamp, E.M. Longridge, B. Khandelwal, K.K. Klausmeyer, J.H. Reibenspies, J. Am. Chem. Soc. 117 (1995) 318; (d) A. Alexakis, C. Benhaim, S. Rosset, M. Humam, J. Am. Chem. Soc. 124 (2002) 5262; (e) I.P. Beletskaya, A.V. Cheprakov, Coord. Chem. Rev. 248 (2004) 2337; (f) G. Evano, N. Blanchard, M. Toumi, Chem. Rev. 108 (2008) 3054; (g) S.R. Harutyunyan, T. den Hartog, K. Geurts, A.J. Minnaard, B.L. Feringa, Chem. Rev. 108 (2008) 2824; (h) A. Alexakis, J.E. Bäckvall, N. Krause, O. Pàmies, M. Diéguez, Chem. Rev. 108 (2008) 2796; (i) J.S. Alford, H.M.L. Davies, J. Am. Chem. Soc. 136 (2014) 10266.

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