Copper(I) 2-Isopropylimidazolate: Supramolecular Isomerism

DOI: 10.1021/cg501737j. Publication Date (Web): February 25, 2015 ... Zhao-Di Liu , Rui Qiao , and Song Yang. Crystal Growth & Design 2016 16 (1), 229...
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Copper(I) 2‑Isopropylimidazolate: Supramolecular Isomerism, Isomerization, and Luminescent Properties Yu-Jun Su, Yi-Lan Cui, Yu Wang, Rui-Biao Lin, Wei-Xiong Zhang, Jie-Peng Zhang,* and Xiao-Ming Chen MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: Solvothermal reactions of Cu(NO3)2 and 2isopropylimidazole (Hipim) in different solvents/templates gave three supramolecular isomers: α-[Cu(ipim)] (1), β-[Cu(ipim)] (2), and γ-[Cu(ipim)]·1/3C6H6 (3). Single-crystal X-ray diffraction analyses showed that 1, 2, and 3 are parallel-packed wavy chains, triple-stranded helices, and cubic−orthogonal-packed helical chains consisting of alternatively interconnected twocoordinated Cu(I) ions and ipim− ligands. Because of the different intermolecular interactions, 1, 2, and 3 show different phosphorescence properties with emission maxima at 555, 540, and 530 nm, respectively. More interestingly, [Cu(ipim)] can melt at high temperature without decomposition. Compounds 1 and 2 melt above 185 and 146 °C, respectively; this liquid solidifies below 117 °C to form 1 again. However, 3 loses the benzene guest molecules above 90 °C and transforms to 1 in a crystal-to-crystal manner.

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INTRODUCTION Being a fascinating phenomenon in coordination chemistry, supramolecular isomerism has attracted much attention.1−19 With predictable coordination geometries and chemical compositions, metal azolate frameworks (MAFs) can be used for rational construction of supramolecular isomers.20 For example, univalent coinage-metal imidazolates usually form simple one-dimensional (1D) zigzag chains because the ligand is bent exobidentate while the univalent coinage-metal ions are forced to be linearly coordinated.21−30 However, 0D polygons and more complicated 1D chains can be synthesized by finely adjusting the synthesis conditions and/or templates.19,20,27−34 For example, under different synthetic conditions, [Cu(im)] and [Ag(im)] (Him = imidazole) zigzag chains can form four different packing patterns.27−30 By adding small organic molecules (e.g., benzene and xylene) as templates, [Cu(mim)] (Hmim = 2-methylimidazole) can form octagons and decagons,31 whereas [Ag(mim)] can form helical and sinusoidal chains.32 In the case of [Cu(eim)] (Heim = 2-ethylimidazole), triple helix was obtained in the high-polarity solvent.33 For [Ag(ipim)] (Hipim = 2-isopropylimidazole), parallel-packed sinusoidal chains, quintuple helices, and chicken-wire-woven helical chains were obtained by extensively varied synthesis conditions, and no simple zigzag chain was observed.34 It can be concluded that besides alkyl groups and synthesis conditions, the type of univalent coinage-metal ion also plays a significant role in the supramolecular isomerism of this type of compounds. As an extension of our previous study on crystal engineering and properties of univalent coinage-metal 2-alkylimidazolates, © XXXX American Chemical Society

here we report a new supramolecular isomerism system, Cu(I) 2-isopropylimidazolate, that shows not only new supramolecular structures but also interesting physicochemical properties.

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EXPERIMENTAL SECTION

Materials and Methods. Commercially available reagents were used as received without further purification. Elemental analyses (C, H, N) were performed with a Vario EL elemental analyzer. Thermogravimetry (TG) analysis was performed under N2 using a TA Q50 instrument with a heating rate of 10 °C/min. Powder X-ray diffraction (PXRD) patterns were measured using a Bruker D8 Advance diffractometer (Cu Kα). Photoluminescence spectra were collected on an Edinburgh FLS920 spectrometer equipped with a continuous Xe900 xenon lamp. Differential scanning calorimetry (DSC) was performed under N2 using a Netzsch DSC 204F1 system. Synthesis of α-[Cu(ipim)] (1). A mixture of Cu(NO3)2·3H2O (0.242 g, 1.0 mmol), Hipim (0.220 g, 2.0 mmol), and aqueous ammonia (25%, 6 mL) was stirred for 15 min under air, then transferred and sealed in a 10 mL Teflon-lined vessel, which was heated in an oven to 160 °C for 80 h, and then cooled to room temperature at a rate of 5 °C h−1. The resulting crystals were filtered, washed by ethanol, and dried under N2 (yield ca. 46%). EA calcd (%) for C6H9CuN2: C, 41.73; H, 5.22; N, 16.23. Found: C, 41.93; H, 5.16; N, 16.24. Synthesis of β-[Cu(ipim)] (2). The reaction was carried out using the same method as for 1, except that aqueous ammonia (25%, 6 mL) was replaced by a mixture of aqueous ammonia (25%, 3 mL) and Received: November 28, 2014 Revised: February 4, 2015

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Crystal Growth & Design water (3 mL). Pale-yellow needle-like crystals were obtained in ca. 50% yield. EA calcd (%) for C6H9CuN2: C, 41.73; H, 5.22; N, 16.23. Found: C, 41.92; H, 5.18; N, 16.15. Synthesis of γ-[Cu(ipim)]·1/3C6H6 (3). The reaction was carried out using the same method as for 1, except that additional benzene (0.5 mL) and methanol (0.5 mL) were added to the reaction mixture. Pale-yellow polyhedral crystals were obtained in ca. 53% yield. EA calcd (%) for C24H33Cu3N6: C, 48.35; H, 5.54; N, 14.10. Found: C, 48.10; H, 5.60; N, 14.03. Crystallography. Diffraction intensities were collected on a Rigaku XtaLAB P300DS PILATUS area-detector diffractometer (Mo Kα). The structures were solved with the direct method and refined with a full-matrix least-squares technique with the SHELXTL program package. Anisotropic thermal parameters were applied to all nonhydrogen atoms except the disordered guest molecules. Hydrogen atoms were generated geometrically and refined by the riding mode. The crystal data and structure refinement results are listed in Table S1.

Figure 1. Perspective views of 1: (a) single chain (symmetry codes: A = −x, −y, −1 − z, B = 2 − x, 3 − y, 3 − z), (b) 3D packing structure, and (c) simplified packing pattern of chains (different colors are used to distinguish adjacent chains).

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RESULTS AND DISCUSSION Synthesis and Structure. Solvothermal reactions of Cu(NO3)2 and Hipim in concentrated and dilute aqueous ammonia produced colorless, blocky (1) or needle-like (2) crystals of [Cu(ipim)], respectively. High concentrations of ammonia may slow down the crystallization process and favor the formation of the thermodynamically stable isomer, although the density of 1 (1.534 g·cm−3) is a just little higher than that of 2 (1.523 g·cm−3). When methanol and benzene were added to the reaction system, colorless polyhedral crystals of [Cu(ipim)]·1/3C6H6 (3) were obtained. Obviously, Cu(II) ions were reduced in situ under the solvothermal reactions; this is similar to many other analogous systems in which the organic ligand and/or NH3 were usually assigned as the reducing agent.27,31,35 We found that increasing the feeding amount of Hipim can lead to higher yields of the crystals, and the optimized feeding ratio is Cu(NO3)2/Hipim = 1:2, implying that Hipim is the main reducing agent. In addition, all three isomers are insoluble in common solvents. Single-crystal X-ray diffraction analyses revealed that 1−3 are supramolecular isomers, in which Cu(I) and ipim− are both two-coordinated and interconnect with each other to form typical 1D chains, although their configurations, packing, and interweaving fashions are distinctly different. Obviously, solvent and/or template molecules play an important role in controlling the supramolecular isomerism of [Cu(ipim)]. Compound 1 crystallizes in the triclinic space group P1̅, containing 6.5 crystallographically independent Cu(ipim) units. The conformation of a univalent coinage-metal imidazolate chain can be described by the dihedral angle (DA) between adjacent imidazolato rings (quoted by the metal ion they share).34 As reflected by the large asymmetric unit, the DA values in 1 vary significantly (Cu1 180°, Cu2 34.6(4)°, Cu3 128.8(4)°, Cu4 68.7(4)°, Cu5 155.0(4)°, Cu6 167.0(4)°, and Cu7 13.7(5)/166.3(6)°), giving a wavy and irregular shape for the coordination chain. These chains pack parallel to one another through van der Waals interactions (shortest Cu···Cu 4.763(3) Å) to form the 3D supramolecular structure (Figure 1). Compound 2 crystallizes in the monoclinic space group P21/ n, containing 4 crystallographically independent Cu(ipim) units. The Cu(ipim) units with nearly trans conformations (DA: Cu1 155.4(3)° and Cu3 148.4(2)°) and semi-cis conformations (DA: Cu2 78.6(3)° and Cu4 82.7(3)°) connect alternately to give a 21 helical chain with a long pitch of 37.49(2) Å. Every three helices intertwine to generate a triple-

stranded helix with the isopropyl groups gathering inside. The shortest Cu···Cu distance (6.313(2) Å) inside the triple helix is much longer than the sum of the van der Waals radii of copper atoms. The triple helixes with different chirality pack parallel to one another through van der Waals interactions (shortest Cu··· Cu 4.066(2) Å) to form the final 3D supramolecular structure (Figure 2).

Figure 2. Perspective views of 2: (a) single-stranded 21 helix (symmetry codes: A = 0.5 − x, −1.5 + y, 0.5 − z, B = 0.5 − x, 1.5 + y, 0.5 − z), (b) triple helix, and (c) simplified packing pattern of chains (different colors are used to distinguish adjacent chains).

Compound 3 crystallizes in a high-symmetry cubic space group Ia3d̅ , containing 0.5 crystallographically independent Cu(ipim) unit. Each pair of adjacent ipim− ligands are in a nearly orthogonal conformation (DA: 85.9(2)°), giving a single-stranded 41 helix structure with a long pitch of 22.204(7) Å (equal to the a-axis length) and a small diameter of 2.322(1) Å. Interestingly, the chains run along three orthogonal directions parallel to the a, b, and c axes, respectively (Figure 3).36,37 For crystal structures consisting of 1D chains, most examples possess the simple parallel packing pattern. Packing patterns of 1D chains running along two or more directions are rare.29,30,32 The cubic−orthogonal packing pattern of 3 has been only observed in two coordination polymers.38,39 Because the cubic−orthogonal packing cannot fully use the space, there are isolated voids in 3 that are filled with benzene. If the benzene molecules were removed from 3, then the crystal contains voids of 24.8% volume ratio. In contrast with 1 and 2, the shortest interchain Cu···Cu separation in 3 is relatively short (3.228(2) Å). It can be seen that the supramolecular structures of [Cu(ipim)] isomers are quite different from other univalent B

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Crystal Growth & Design

curve of 3 shows a weight-loss of 12.9% (calc. 13.1%) from 90 to 140 °C, followed by plateaus until decomposition above 300 °C (Figure S2, Supporting Information), and finally the formation of elemental copper (Figure S4, Supporting Information). PXRD patterns show that after removal of all guest molecules (prolonged heating at 90 °C), 3 transforms into nonporous isomer 1 (Figure S3, Supporting Information), which illustrates that the structure of 3 is indeed maintained by the guest benzene and that 1 should be the thermodynamically stable isomer. Nonporous isomer 2 can also transform into 1. After being heated to 200 °C and cooled down to room temperature, 2 lost its original shape and turned into an opaque waxlike solid, whose PXRD pattern (Figure S3, Supporting Information) matches that of 1, showing that 2 melts at high temperature and then solidifies and crystallizes as the more stable structure 1 when the liquid was cooled down. The phase transition behavior of 2 was further investigated by DSC for two heating− cooling cycles. The first cycle (Figure 4) shows an endothermic

Figure 3. Perspective views of 3: (a) single-stranded 41 helix (symmetry codes: A = −0.25 + y, 0.75 − x, 0.25 + z, B = 0.75 − y, 0.25 + x, −0.25 + z), (b) 3D packing structure, and (c) simplified packing pattern of chains. Different colors are used to distinguish chains running along the three crystallography axes, and template molecules are shown by yellow spheres.

coinage-metal imidazolates. Here, we compare these structures and discuss how different metals and alkyl groups affect their chain conformations and packing patterns. Compared with methyl and ethyl groups, the larger isopropyl groups tend to approach one another, which makes it difficult for [Ag(ipim)]34 and [Cu(ipim)] to form simple zigzag chains like [Ag(mim)]32 and [Cu(eim)].33 Additionally, because of the greater steric hindrance effects of isopropyl groups, they cannot form rings like [Cu8(mim)8] or [Cu10(mim)10].31 With the same ligand ipim−, the steric hindrance effect in [Cu(ipim)] is greater than those in [Ag(ipim)] because the smaller radius of Cu(I) makes the isopropyl groups closer to each other. Therefore, 1 forms a more irregular wavy chain instead of a sinusoidal chain as in [Ag(ipim)]. The structural differences among triple-helix isomer 2 and other multiple-helix structures of univalent coinage-metal imidazolates can be also explained by similar reasons. [Cu(eim)] (triple helix), [Ag(ipim)] (quintuple helix), and 2 all have their alkyls gather inside the helix (Figure S1, Supporting Information), illustrating the structural-directing effects of the alkyl chains. Compared with [Cu(eim)] (triple helix, cross-section area ca. 50.5 Å2, helical pitch = 33.28(1) Å), 2 has a larger cross-section area (ca. 63.2 Å2) and a longer helical pitch (37.49(2) Å) because the isopropyl groups take up more space than the ethyl ones. Compared with [Ag(ipim)] (quintuple helix, cross-section area ca. 74.1 Å2, helical pitch 60.502(5) Å), the lower strand number and smaller size of 2 could be attributed to the shorter and more irregular chain of [Cu(ipim)]. It is difficult to explain why there is no cubic−orthogonal packing mode (as in 3) observed in any other univalent coinage-metal imidazolates. Obviously, the semilinear and single-strand helical chain in 3 is beneficial for the formation of cubic−orthogonal packing, and the remaining voids can be perfectly filled by benzene. First, other univalent coinage-metal imidazolate chains tend to form cis or trans conformations rather than the nearly orthogonal conformation of [Cu(ipim)]. Second, if the coordination chain in 3 is replaced by other compositions, then the shape and size of the voids will significantly change, which may not be easily fitted by common template molecules. Supramolecular Isomerization. Under appropriate conditions, some univalent coinage-metal imidazolates may be converted from one isomer to another.34,40 Thermal stability, structural transformation, and host−guest behavior of the [Cu(ipim)] isomers were studied by TG, PXRD, and DSC. TG

Figure 4. DSC profiles of 1 and 2.

peak at 185 °C in the heating process and an exothermic peak at 117 °C in the cooling process. In the second cycle, these values are 146 and 118 °C, respectively, which match well with those of 1. The final products of isomer 1 and 2 after DSC analysis were both isomer 1, as is proven by PXRD (Figure S5, Supporting Information). To the best of our knowledge, neither the organic-polymer-like melting behavior nor the melting−solidification isomerization process has been reported for any other coordination polymers, which is probably due to the large isopropyl groups that weaken the intermolecular interactions and reduce the long-range order of the chain. For comparison, heating also transforms the chicken-wire isomer of [Ag(ipim)] into the quintuple helix one; however, this occurs through a solid-state crystal-to-crystal structural transformation mechanism. Besides, the [Ag(ipim)] sinusoidal chain and quintuple helix isomers do not melt or transform to other structure upon heating below the decomposition temperature.34 Photoluminescence Property. Although the π conjugate system of imidazolate is small, Cu(I) imidazolates usually show 3 [MLCT] phosphorescence.25,33 Under UV irradiation at room temperature, 1, 2, and 3 show orange, yellow, and green photoluminescence with emission maxima at 555, 540, and 530 nm, respectively (Figure 5). The emissive lifetimes of 1, 2, and 3 were determined as ca. 10−20 μs (Figure S7, Supporting Information). The broad and structureless emission spectra, the C

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “973 Project” (2012CB821706 and 2014CB845602), NSFC (21225105 and 21290173), and NSF of Guangdong (S2012030006240).

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Figure 5. (a) Photographs and (b) photoluminescence spectra of 1, 2, and 3.

large Stokes shifts, and the relatively long emissive lifetimes meet the characteristics of 3[MLCT] excited state of common Cu(I) complexes.41 The difference of emission maxima among the three isomers of [Cu(ipim)] (25 nm) is larger than that of other coinage-metal imidazolates, such as [Cu(mim)] (20 nm),31 [Cu(eim)] (8 nm),32 and [Ag(ipim)] (12 nm).34 Because these isomers have similar local coordination environments, the variations of emission maxima of these isomers may be explained by the different intermolecular interactions. Because the shortest interchain Cu···Cu distances (3.2−4.8 Å) are too long in these structures, the interligand contacts should be the main effect on the 3[MLCT]-excited-state energy. Careful analyses showed that the shortest interatomic distances between imidazolate rings in 1, 2, and 3 are 3.42(2), 3.62(1), and 3.74(1) Å, respectively (Table S2 and Figure S8, Supporting Information), meaning that the sequential blueshift of emission peaks can be explained by the decreasing intermolecular interactions of the π systems involved in the 3 [MLCT] excited states.

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CONCLUSIONS By subtle variation of the synthesis conditions, we synthesized three supramolecular isomers of Cu(I) 2-isopropylimidazolate, possessing parallel-packed wavy chains, triple-stranded helices, and benzene-including cubic−orthogonal-packed helical chains, respectively. Beside the unique chain conformations and packing fashions compared with other coordination polymers including the known univalent coinage-metal imidazoltes, the Cu(I) 2-isopropylimidazolate isomers also show distinct photoluminescence properties related with their supramolecular structures. Furthermore, they can undergo interesting melting−solidifying isomerization, which is unprecedented for coordination polymers. This work highlighted the important roles of both metal ion and side groups in directing the selfassembly and crystallization processes as well as the diversified structures and dynamics of these simple coordination polymers that should be also valid for more complicated supramolecular systems.

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