Role and Effect of Anions in the Construction of Silver Complexes

Article pubs.acs.org/crystal

Role and Effect of Anions in the Construction of Silver Complexes Based on a Pyridylimidazole Ligand with L‑Type Coordination Vectors and Their Photoluminescence Properties Jieun Lee,† Youngjin Kang,*,† Nam Sung Cho,§ and Ki-Min Park*,‡ †

Division of Science Education, Kangwon National University, Chuncheon 24341, Republic of Korea Electronics and Telecommunications Research Institute (ETRI), 161 Gajeong-dong, Yuseong-gu, Daejeon 305-350, Republic of Korea ‡ Research Institute of Natural Science, Gyeongsang National University, Jinju 52828, Republic of Korea §

S Supporting Information *

ABSTRACT: Three anion-dependent Ag(I) coordination complexesspecifically, [Ag2(pyim)2(NO3)2] (1), {[Ag(pyim)2]· ClO4·CH3OH·(H2O)1.25}n (2), and [Ag4(pyim)4]·(CF3SO3)4 (3)were prepared by the reaction of the corresponding silver salts with a rigid ditopic N-terphenyl-substituted 2-(4-pyridyl)imidazole (pyim) ligand possessing an “L”-type coordination vector. Complex 1, in which the nitrate anion acts as a monodentate terminal ligand, exhibits a discrete cyclic dimer structure, whereas complex 2, incorporating a perchlorate anion with weak coordination ability, displays an anion-free one-dimensional (1D) looped chain structure resulting from the Ag sharing of consecutive cyclic dimers. When using a trifluoromethanesulfonate (triflate) as a counteranion with moderate affinity toward the metal center, the resulting complex 3 exhibits an unusual cyclic tetramer structure. In 3, the triflate anions act as bridges between adjacent cyclic tetramers via the weak interaction with the Ag(I) ions, yielding a parquet-like two-dimensional (2D) framework. All three complexes display violet-blue emission, with maxima ranging from 388 to 396 nm. Furthermore, in solution, complex 2 exhibits a substantial emission enhancement, resulting in an emission intensity nearly 2 orders of magnitude greater than those of both the free ligand and the two other Ag(I) complexes, 1 and 3. Counteranions possessing different abilities to coordinate to Ag(I) play important roles in the structural diversity and photoluminescence properties of 1−3.

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ion.12−14 Moreover, the various coordination environments around the Ag(I) center are generally constructed by the ligands, solvent molecules, and counteranions. Irrespective of the coordination of solvent molecules to the Ag(I) center, the formation of Ag(I) coordination polymers substantially influences the counteranions.15−19 Therefore, we envision that the polymeric structures and photoluminescent properties of pyridyl-imidazole-based silver(I) complexes would be highly dependent upon simple variations in the silver counteranion.

INTRODUCTION

Recently, great attention has been devoted to phenyl-imidazolebased metal complexes by many researchers in both industry and academia because of their vast applicability to molecular electronics.1−10 In particular, phenyl-imidazole-based iridium(III) complexes are considered strong candidates as triplet emitters in blue phosphorescent organic light-emitting diodes (PHOLEDs).9,10 Despite these broad investigations, reports concerning the structural diversity and photoluminescence of pyridyl-imidazole-based ligands in group-11 metal complexes are scarce, and related research is limited. 11 Among coordination polymers of group-11 elements, Ag(I) coordination polymers have been demonstrated to exhibit structural diversity as a result of the d10 configuration of the Ag(I) © XXXX American Chemical Society

Received: October 31, 2015 Revised: December 7, 2015

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DOI: 10.1021/acs.cgd.5b01544 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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because the two nitrogen donor atoms are positioned at ca. 70° angle with respect to each other. In this study, we report the preparation, structural characterization, and photophysical properties of three supramolecular Ag(I) complexes of the pyim ligand, whose structures depend on the counteranions used, as depicted in Scheme 1. The resulting Ag(I) complexes were also characterized by thermogravimetric analysis (TGA).

Until now, only three examples of pyridyl-imidazole-based silver(I) complexes have been reported in the literature.11,20,21 To study the effect of the counteranion on the structure and photophysical properties of pyridyl-imidazole-based silver(I) complexes, we chose N-terphenyl substituted 2-(4-pyridyl)imidazole (pyim) as a rigid ditopic ligand. As shown in Scheme 1, the pyim ligand possesses an “L”-type coordination vector

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Scheme 1. Anion-Dependent Silver(I) Complexes Prepared in This Work

RESULTS AND DISCUSSION We have designed and synthesized N-terphenyl substituted pyridylimidazole, pyim, in which a bulky terphenyl substituent renders the simple pyridyl-imidazole ligand the restriction of conformational flexibility. In addition, a variety of intermolecular interactions such as π−π stacking and C(π)···H between substituents of neighboring pyim ligands can be expected. These facts can be led to unusual assembly of silver(I) containing architectures. The ligand was synthesized by a coupling reaction. Synthetic details are described in Experimental Section. To investigate the anion-dependent structures and photoluminescent properties of the Ag(I) complexes based on a rigid ditopic N-terphenyl-substituted 2-(4-pyridyl)imidazole (pyim), we prepared three Ag(I) complexes (1−3) of pyim through the self-assembly reactions of pyim with AgX (X = NO3−, ClO4−, and CF3SO3−) in acetonitrile/methanol. The structures of these three complexes are dependent on the coordination abilities of the anions. The complexes were isolated by slow evaporation of the solutions and were allowed to crystallize. Complexes 1 and

Table 1. Crystal Data and Structure Refinement for 1−3 identification codes

1

2

3

empirical formula formula weight temperature (K) wavelength (Å) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z Dcalc (mg/m3) μ (mm−1) F(000) crystal size (mm) θmin, θmax (deg) reflections collected independent reflections absorption correction max. and min transmission data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)]

C58H50Ag2N8O6 1170.80 173(2) 0.71073 triclinic P1̅ 8.9217(1) 11.8291(1) 12.9303(2) 111.3800(10) 103.1940(10) 93.8150(10) 1220.16(3) 1 1.593 0.866 596 0.19 × 0.13 × 0.12 1.757, 27.498 21827 5594 [R(int) = 0.0254]

C59H56.5AgClN6O6.25 1092.92 173(2) 0.71073 triclinic P1̅ 13.3879(3) 13.9279(3) 16.6273(4) 75.3080(10) 71.5250(10) 69.9840(10) 2726.11(11) 2 1.331 0.475 1133 0.37 × 0.18 × 0.10 1.577, 28.224 49126 13393 [R(int) = 0.0306] semiempirical from equivalents 0.9541, 0.8439 13393/30/694 1.040 R1 = 0.0404 wR2 = 0.1070 R1 = 0.0504 wR2 = 0.1135 full-matrix least-squares on F2

C120H100Ag4F12N12O12S4 2689.83 173(2) 0.71073 monoclinic P21/c 17.9811(3) 14.9253(2) 23.2695(4) 90 110.2960(10) 90 5857.18(16) 2 1.525 0.815 2720 0.26 × 0.18 × 0.14 1.653, 28.293 56838 14494 [R(int) = 0.0445]

R indices (all data)

0.9032, 0.8527 5594/0/334 1.049 R1 = 0.0238 wR2 = 0.0590 R1 = 0.0258 wR2 = 0.0603

refinement method B

0.8945, 0.8161 14494/123/802 1.011 R1 = 0.0405 wR2 = 0.0889 R1 = 0.0614 wR2 = 0.0998



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2 crystallized in the triclinic space group P1,̅ whereas complex 3 crystallized in the monoclinic space group P21/c. The crystallographic data and selected geometric parameters for 1−3 are summarized in Tables 1 and 2, respectively.

Ag1−N3i 2.2184(14) Å). The remaining coordination site is occupied by one oxygen atom of a nitrate anion (Ag1−O1 2.3592(12) Å). Thus, the coordination geometry of the Ag(I) atom is a considerably distorted, trigonal planar configuration with N1−Ag1−N3i, N1−Ag1−O1, and N3i−Ag1−O1 bond angles of 122.03(5)°, 90.13(5)°, and 147.83(5)°, respectively. As shown in Figure 1, two symmetry-related pyim ligands bind two symmetry-related Ag atoms, leading to the formation of a 14-membered cyclic dimer, which may be stabilized via a π−π stacking interaction between two parallel pyridine rings (centroid-to-centroid distance = 3.774(1) Å). The Ag(I) atoms within the cyclic dimer are separated by 6.9469(3) Å. In the crystal structure of 1, the adjacent cyclic dimers are connected by intermolecular Ag···Ag interactions (3.3395(3) Å) to form a one-dimensional (1D) pseudolooped chain propagating along the a-axis direction (Figure 2a). Further-

Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for 1−3 Complex 1 Ag1−N1 2.3034(14) Ag1−O1 2.3592(12) N1−Ag1−N3i 122.03(5) O1−Ag1−N3i 147.83(5) Symmetry operation: (i) −x + 1, −y + Complex 2 Ag1−N1i 2.2809(18) Ag1−N4ii 2.2868(19) N1i−Ag1−N3 123.68(7) N3−Ag1−N4ii 100.42(7) N6−Ag1−N1i 101.57(7) Symmetry operation: (i) −x + 1, −y + 1, −z + 1; (ii) −x + Complex 3 Ag1−N1 Ag2−N3i Ag2···Ag2i N1−Ag1−N4 Symmetry operation:

Ag1−N3i Ag1···Ag1ii N1−Ag1−O1

2.2184(14) 3.3395(3) 90.13(5)

1, −z + 1; (ii) −x, −y + 1, −z + 1. Ag1−N3 Ag1−N6 N1i−Ag1−N4ii N3−Ag1−N6 N6−Ag1−N4ii

2.3211(18) 2.3455(19) 109.72(7) 101.82(7) 120.99(7)

2, −y + 1, −z + 1.

2.109(2) Ag1−N4 2.192(2) Ag2−N6 3.0851(5) 173.16(8) N6−Ag2−N3i (i) −x + 1, −y + 1, −z.

2.112(2) 2.179(2) 148.59(9)

Synthesis and Crystal Structures of the Silver Complexes Incorporating Different Anions. The reaction of pyim with silver nitrate in acetonitrile/methanol produced a colorless X-ray-quality crystalline product 1. The X-ray crystal structure of 1 is presented in Figure 1 and shows that this

Figure 2. (a) The 1D pseudolooped chain structure formed by Ag···Ag interactions (black dashed lines). (b) The 2D supramolecular network formed by intermolecular π−π stacking interaction (red dashed lines) between 1D pseudolooped chains.

more, the 1D chains are packed by π−π interactions between the phenyl rings of the pyim ligands (centroid-centroid distance = 3.669(1) Å), resulting in the formation of a two-dimensional (2D) supramolecular network extending parallel to the ab plane (Figure 2b). Several C−H···O/N hydrogen bonds between the cyclic dimers and anions and C−H···π and π−π interactions between the cyclic dimers contribute to the stabilization of the crystal structure (Table S1, Supporting Information). To understand the influence of the counteranion with a noncoordinating anion on the formation of silver complexes of pyim, as outlined in Scheme 1, we repeated the complexation using AgClO4 instead of AgNO3. When silver(I) perchlorate was used under the same reaction conditions as those employed for silver(I) nitrate, a colorless crystalline product, 2, suitable for X-ray analysis was obtained. The X-ray crystal structure of 2

Figure 1. Discrete cyclic dimer complex 1, [Ag2(pyim)2(NO3)2]. The yellow dashed line represents intramolecular π−π interaction. H atoms have been omitted for clarity. [Symmetry code: (i) 1 − x, 1 − y, 1 − z].

complex is an anion-coordinated cyclic dimer of formula [Ag2(pyim)2(NO3)2] with inversion symmetry. Therefore, the asymmetric unit consists of one Ag(I) ion, one pyim ligand, and one nitrate anion. In the coordinated ligand L, the dihedral angles between the imidazolyl and pyridyl rings are 48.09(5)°. In 1, the Ag atom is three-coordinated by two individual nitrogen atoms of the imidazole and pyridine groups of two individual pyim ligands (Figure 1, Ag1−N1 2.3034(14) Å, C



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Notably, the replacement of nitrate anions with perchlorate ions led to a change of the coordination geometry around the Ag atoms as a consequence of the difference of coordination abilities of the counter-anions toward Ag(I). In the case of 2, as shown in Figure 3, the Ag atom is in a distorted tetrahedral coordination environment defined by two imidazole nitrogen atoms and two pyridine nitrogen atoms from four individual pyim ligands. The tetrahedral angles around Ag(I) atoms fall in the range from 100.42(7) to 123.68(7)°. The average Ag−N distance is 2.309(2) Å (Table 2). As expected, the noncoordinated perchlorate anion with weak coordination ability lies between the silver centers with the shortest separation distance (Ag(I)···O-ClO3−) of 4.616(3) Å. Similar to 1, two silver(I) atoms in 2 are connected by two pyim ligands to form a 14-membered cyclic dimer with Ag···Ag separation distances of 6.9811(3) and 7.1193(4) Å and with π−π interactions between the pyridyl rings (Figure 3, centroidto-centroid distances = 3.9671(12) and 3.9493(13) Å). In comparison with 1, however, a notable structural difference of 2 is that these cyclic dimers are connected to each other by sharing Ag(I) atoms, leading to the formation of an infinite looped chain structure extending parallel to the a-axis. Unlike the anion-coordinated cyclic dimer of 1, the formation of the looped chain in 2 may be due to the open coordination environment of the Ag(I) ion resulting from the absence of a coordinating anion. In 2, the π−π interactions (centroid-to-centroid distance = 3.7705(15) Å) between the phenyl rings of 1D looped chains give rise to the formation of a 2D supramolecular network extending parallel to the ab plane (Figure 4). No notable interlayer interactions were identified. The perchlorate anions and lattice solvent molecules occupy the void volume between the 2D layers. The crystal structure of 2 is stabilized by weak C−H···O hydrogen bonds between the looped chains and the lattice solvent molecules and by C−H···π and π−π interactions between the looped chains (Table S2, Supporting Information). By contrast, when a moderately coordinating anion, CF3SO3−, was used in the formation of the silver complex of pyim under the same reaction conditions previously described, a discrete complex, 3, with weak interactions between the Ag(I)

is shown in Figure 3. Unlike the anion-coordinated discrete complex 1, X-ray analysis revealed that 2 is an anion-

Figure 3. 1D looped-chain complex 2, {[Ag(pyim)2]·ClO4·CH3OH· (H2O)1.25}n. The Ag···Ag distances are 6.9811(3) for Ag1···Ag1i and 7.1193(4) for Ag1···Ag1ii. Yellow dashed lines represent intramolecular π−π interaction, where the distances between N3- and N6-containing pyridine rings are 3.9671(12) and 3.9493(13) Å, respectively. H atoms and the disordered oxygen atom of perchlorate anion have been omitted for clarity. [Symmetry codes: (i) 1 − x, 1 − y, 1 − z; (ii) 2 − x, 1 − y, 1 − z].

noncoordinated polymeric complex of formula {[Ag(pyim)2]· ClO4·CH3OH·(H2O)1.25}n. The asymmetric unit of 2 contains one Ag(I) ion, two crystallographically independent pyim ligands, one perchlorate anion, one methanol molecule, and one and one-fourth water molecules. Two crystallographically independent ligands adopt very similar conformations, such that the dihedral angles between the imidazolyl and pyridyl rings in the two ligands are 40.64(11)° and 41.06(9)°, respectively.

Figure 4. 2D supramolecular network formed by intermolecular π−π interaction (yellow dashed lines) between 1D looped chains in 2: (a) front view and (b) side view. D



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unusual.11,32−35 The cyclic tetramer is located around a crystallographic inversion center. Therefore, the asymmetric unit is composed of two Ag(I) ions (Ag1 and Ag2), two pyim ligands, and two trifluorosulfonate anions. In two crystallographically independent pyim ligands of the asymmetric unit, the dihedral angles between the imidazole and pyridine rings are 44.45(12)° for the N3-containing ligand and 31.55(14)° for the N6-containing ligand (Figure 5). In 3, the Ag1 atom is coordinated by two imidazole nitrogen atoms from different ligands, giving a geometry that is slightly distorted from linear [N1−Ag1−N4 = 173.16(8)°], whereas the Ag2 atom is coordinated by two pyridine nitrogen atoms from different ligands in a bent arrangement [N6−Ag2−N3i = 148.59(9)°] (Figure 5). This bent geometry around the Ag2 atom originates from the presence of not only the electrostatic interactions between an Ag atom and a bridging(μ)-type oxygen atom of the CF3SO3− anion [Ag2···O4 2.648(6) Å, Ag2···O4i 3.125(5) Å] but also the argentophilic interaction [Ag2···Ag2 = 3.0851(5) Å], which is shorter than the sum of the van der Waals radii of A(I) ions (3.44 Å).36 Such transannular Ag···Ag interactions impart the overall molecular structure of 3 with a goggles-like appearance and provides a striking contrast between this cyclic tetramer and the previously reported molecular rectangle comprising four silver(I) ions and four nonsubstituent 2-(4-pyridyl)imidazole ligands with L-type coordination vectors.3 Within the cyclic tetramer, the Ag1···Ag1 separation is 14.4651(4) Å. The Ag1 atoms also interact with oxygen atoms of the CF3SO3− anion [Ag1···O1 2.673(2) Å]. Notably, the goggles-like cyclic tetramer of 3, induced by the interactions between Ag(I) ions and the CF3SO3− anions, is clearly distinguishable from the anion-coordinated cyclic dimer of 1 and the anion-noncoordinated looped chain of 2. In the crystal structure of 3, as shown in Figure 6, the adjacent cyclic tetramers are connected by the Ag···O interactions [Ag1···O1 2.673(2) Å, Ag2···O2ii 2.960(2) Å,

ions and triflate anions was isolated. The X-ray crystal structure of 3 is shown in Figure 5.

Figure 5. Discrete cyclic tetramer 3, [Ag4(pyim)4]·(CF3SO3)4. Black and yellow dashed lines present Ag···Ag and Ag···O interactions, respectively. H atoms and disordered atoms of trifluoromethanesulfonate anions have been omitted for clarity. [Symmetry code: (i) 1 − x, 1 − y, −z].

The single-crystal structure analysis showed that 3 is a fascinating cyclic tetramer of formula [Ag4(pyim)4]·(CF3SO3)4 that is built up from the alternating connection of four Ag(I) ions to four ligands. Although examples of cyclic dimers consisting of two ligands and two Ag(I) ions, as in the case of complex 1, are common in the literature,22−31 cyclic tetramers composed of four ligands and four silver(I) ions are

Figure 6. 2D parquet-like framework formed by Ag···O interactions between tetramers and anions in 3. The bulky aryl substituents in the cyclic tetramers are represented by sky-blue spheres. E



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symmetry code (ii) −x + 1, y − 0.5, −z + 0.5] between the cyclic tetramers and CF3SO3− anions, resulting in the formation of a 2D network in which the cyclic tetramers display a parquetlike arrangement. These parquet layers are extended parallel to the bc plane (Figure 6), with interlayer separation distances of approximately 18.0 Å. Neighboring 2D layers further interact with each other via π−π interactions (centroid-to-centroid distance = 3.807(2) Å) between the phenyl rings of each layer, resulting in the formation of a three-dimensional (3D) supramolecular framework. In addition, several C−H···O/F hydrogen bonds between the cyclic tetramers and the anions, and C−H···π and π−π interactions between the cyclic tetramers, are observed (Table S3, Supporting Information). Thermal Properties. To investigate the thermal stabilities of 1−3, we characterized them by TGA. As shown in Figure 7,

Figure 8. Emission spectra of free ligand and complexes 1−3 in CH3CN at ambient temperature.

Figure 9a. The observed emission is attributed to intraligand (IL) π−π* transition.37 Interestingly, a significant emission enhancement for 2, which is nearly 2 orders of magnitude greater than those of both free ligand and the other complexes, was observed under the same conditions (Figure 9). This enhancement of emission intensity can be due to the coordination of the ligand to the Ag(I) ion, which effectively increases the rigidity of the free ligands, thus suppressing nonradiative decay process in the ligand-based excited state.38 The same trends were observed in the solid states of all complexes (Figure S1, Supporting Information). These results clearly support the hypothesis that few differences exist between the solid and fluid states of 1−3. To investigate possible structural dynamics in the fluid state, we characterized all of the complexes using NMR. The 1H NMR spectra of 1−3 have been recorded in CD3CN. The resonances and relative chemical shift changes further, confirming the luminescence proposed for 1−3, as shown in Figure 10. As compared to chemical shifts of free ligand in the region of phenyl and heterocyclic aromatic ring, those of all complexes appear upfields.(see Figure S2, Supporting Information) These observations also provide an evidence of maintaining complexation in solution. The chemical shifts for pyridine (Py) and imidazole (Im) in 2 appear further downfield compared to those for pyridine and imidazole in 1 and 3. This shift is attributable to the strong interactions between the N(py, im) atoms and Ag(I) ions. The differences of the Ag−N(py, im) interactions may originate from binding properties of Ag(I) and its counteranion. To further confirm this observation, we added L in CH3CN in the presence of CF3COOH (H+) because of the minimal interactions between the cation and its counteranion in the fluid state. As expected, a strong emission enhancement was also observed. On the basis of these results, we concluded that the binding of counteranions with Ag(I) ions plays an important role in changing the emission properties of the peripheral imidazole ligand.

Figure 7. TGA curves for complexes 1, 2, and 3.

the TGA curves indicate that cyclic complexes 1 and 3 are thermally stable to 220 and 315 °C, respectively. However, upon further heating, the structures begin to decompose, presumably reflecting the loss of organic molecules. These thermal analysis results suggest that the cyclic tetramer, 3, is more stable than the cyclic dimer, 1. The high thermal stability of 3 may originate from the Ag···O interactions between the cyclic tetramers and the anions, as previously mentioned. By contrast, crystals of 2 lose solvate methanol and water molecules immediately after the crystals are removed from the mother liquor. Therefore, the TGA trace of 2 shows a weight loss of 4.3% (calcd 5.0%) in the temperature range from 25 to 120 °C, corresponding to the loss of solvate methanol and water molecules. After a stable plateau, complex 2 begins to decompose at 225 °C, suggesting that the thermal stability of the looped-chain motif of 2 is similar to that of the cyclic dimer, 1. Photophysical Properties. In the solution state, the free ligand exhibits a weak emission band with λmax = 426 nm. This band may arise from π−π* transitions involving pyridylimidazole, with contributions from the terphenyl unit. However, upon the complexation of ligand with an Ag(I) salt (i.e., AgNO3, AgClO4, or AgCF3SO3), significant blue-shifted emissions (>30 nm) were observed in all cases. As shown in Figure 8, complexes 1−3 have broad fluorescence emission bands at room temperature, with λmax = 388, 396, and 390 nm for 1, 2, and 3, respectively. The emission enhancement of 1−3 was also observed compared to that of free ligand, as shown in

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CONCLUSION Three silver(I) coordination complexes, 1−3, of pyim with Ltype coordination vectors were characterized by single-crystal X-ray analysis to elucidate how counteranions affect their structure and photoluminescence properties, which are strongly related to the coordination nature of the counteranions. When nitrate anions, which exhibit strong coordinating ability, were used in the assembly of Ag(I) coordination complexes, discrete cyclic dimers containing two Ag(I) ions and two pyim ligands were obtained. Furthermore, when noncoordinating perchloF



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Figure 9. (a) Comparison of the photoluminescent efficiencies for 1−3 (I0 = intensity of free ligand). (b) Emission photo of free ligand and 1−3 in CH3CN.

Figure 10. NMR spectra in CD3CN showing the dependence of chemical shifts on the counteranions of Ag(I).

the subtle effect of counteranion identity. This work reveals that the coordination nature of the counteranion plays a subtle yet discernible role in the final structure and ligand-based photoluminescence properties of Ag(I) coordination complexes. The luminescent intensities of both the free ligand and its Ag(I) complexes are quite sensitive to pH and metal counteranion identity. Further investigation of pH and anion sensors using this ligand and complexes is currently in progress.

rate anions were used, 1D looped chains were obtained, in which cyclic dimers contained two Ag(I) ions and two pyim ligands connected to each other by shared Ag atoms,. Finally, when triflate ions, which exhibit moderate coordination ability, were used as the counteranions, an unusual goggles-like cyclic tetramer consisting of four Ag(I) ions and four pyim ligands was obtained. The photoluminescence properties of 1−3 are also influenced by the coordination abilities of the counteranions. The coordination of Ag(I) ion to the free ligand resulted in enhanced fluorescence intensity and in a hypsochromic shift originating from intraligand (IL) π−π* transition. In particular, the emission intensity of the Ag(I) complex with the noncoordinating perchlorate anion was substantially enhanced compared to those of the Ag(I) complexes with the strongly coordinating nitrate or moderately coordinating triflate anions. This result reveals that the binding ability of the counteranions toward Ag(I) ions plays an important role in governing the emission properties of the peripheral imidazole ligand. The chemical shifts in the 1H NMR spectra of complexes 1−3 also demonstrated structural and emission differences induced by

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EXPERIMENTAL SECTION

General Consideration and Measurement. All experiments were performed under a dry N2 atmosphere using standard Schlenk techniques. All solvents were freshly distilled over appropriate drying reagents prior to use unless otherwise stated. The FT-IR spectra were measured with a Nicolet iS10 spectrometer. The NMR spectra were recorded on a Bruker Avance 600 NMR spectrometer, and chemical shifts were referenced to residual solvent peak. For photoluminescent measurements, complexes 1−3 were dissolved in CH3CN after the isolation of all complexes from the mixture of CH3CN/MeOH. All solutions were degassed with nitrogen for 1−2 min. Emission spectra for all samples with concentrations in the range of 10−50 μM were obtained from the PerkinElmer Luminescence spectrometer LS 50B. G



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factors of 0.50, 0.25, and 0.25, respectively. One oxygen atom of the perchlorate anion was also disordered over two positions (O4 and O4′) with equal site occupancy factors of 0.5. In 3, one trifluorosulfonate anion was disordered over two sites with equal site occupancy actors of 0.5. Displacement parameters of the disordered atoms in 2 and 3 were restrained to be approximately isotropic (ISOR). In all cases, all non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in calculated positions and refined isotropically in a riding manner along with their respective parent atoms. However, no hydrogen atoms were located for the disordered lattice water molecules in 2. The figures were prepared using the Diamond program.43 Relevant crystal collection data and refinement data for the crystal structures of 1−3 are summarized in Table 1.

The thermogravimetric analysis (TGA) was carried out on a TA Instruments TGA-Q50 thermogravimetric anlayzer under nitrogen environment at a heating rate of 10 °C/min from room temperature to 25−900 °C. The elemental analysis was carried out on a Thermo Scientific Flash 2000 Series elemental analyzer. The proper starting materials were purchased from either Aldrich or 4Chem Laboratory. CAUTION! Although no problems were encountered in this work, the perchlorate-containing complexes are potentially explosive and appropriate precautions should be taken during their preparation, handling, and storage. Preparation of Ligand (L). Phenylboronic acid (0.54 g, 4.4 mmol), 4-[1-(2,6-dibromo-4-isopropylphenyl)-1H-imidazol-2-yl]pyridine, (0.76 g, 1.8 mmol), Pd(acac)2 (0.025 g, 0.11 mmol), triphenylphosphine (0.12 g, 0.44 mmol), and sodium carbonate (1.26 g, 11.9 mmol) were dissolved in dimethoxyethane (DME, 20 mL) and H2O (10 mL). The mixture was refluxed with stirring for 12 h under a nitrogen atmosphere. After being cooled, the mixture was extracted with ethyl acetate. The organic layer was dried over MgSO4. The solvent was removed under reduced pressure to give a crude residue. The crude product was purified by column chromatography on silica gel to obtain a colorless solid. Yield: 45%. 1H NMR (CD3CN): δ 8.30 (m, 2H), 7.40 (s, 2H), 7.21 (m, 2H), 7.14 (m, 4H), 6.98−6.94 (m, 7H), 6.91 (d, J = 1.5 Hz, 1H), 3.10 (sept, J = 6.8 Hz, 1H), 1.36 (d, J = 6.8 Hz, 6H). Preparation of [Ag2L2(NO3)2] (1). To a stirred solution of L (0.10 g, 0.24 mmol) was slowly added corresponding AgNO3 (0.041 g, 0.24 mmol) in either CH3CN or CH3CN/CH3OH (5 mL each) at ambient temperature under the dark. Slow evaporation of acetonitrile (and methanol mixture) solution of L and silver nitrate gave colorless X-ray quality crystalline products of 1. Yield: 76% (based on ligand). Mp: 219−221 °C (decomp.). 1H NMR (600 MHz, CD3CN): δ 8.20 (m, 2H), 7.39 (s, 2H), 7.24 (m, 2H), 7.16 (t, J = 6.2 Hz, 4H), 7.06 (s, 1H), 6.92−6.87(m, 7H), 3.09 (sept, J = 6.8 Hz, 1H), 1.24 (d, J = 6.8 Hz, 6H). IR (KBr pellet): 3113, 3053, 2962, 2868, 1065, 1529, 1497, 1472, 1385, 1298, 1217, 1140, 1070, 1032, 1003, 972, 918, 887, 825, 783, 762, 702, 651, 613, 561, 515 cm−1. Anal. Calcd for C58H50Ag2N8O6: C, 59.50; H, 4.30; N, 9.57%. Found: C, 59.74; H, 4.41; N, 9.41%. Preparation of {[AgL2]·ClO4·CH3OH·(H2O)1.25}n (2). The preparation of 2 was similar to that of 1 except that AgClO4 (0.050 g, 0.24 mmol) was used instead of AgNO3. Yield: 68% (based on ligand). The element analysis was carried out after the solvent molecules were dried under a vacuum. Mp: 220−222 °C (decomp.). 1H NMR(600 MHz, CD3CN): δ 8.28 (m, 2H), 7.40(s, 2H), 7.23 (m, 2H), 7.16 (m, 4H), 7.03 (s, 1H), 6.94−6.92 (m, 7H), 3.10 (sept, J = 6.8 Hz, 1H), 1.35 (d, J = 6.8 Hz, 6H). IR (KBr pellet): 3415, 3051, 2962, 2872, 1607, 1527, 1495, 1470, 1443, 1298, 1219, 1097, 1001, 972, 926, 887, 827, 781, 762, 702, 852, 621, 563. 515 cm−1. Anal. Calcd for C58H50AgClN6O4: C, 67.09; H, 4.85; N, 8.09%. Found: C, 66.94; H, 4.97; N, 8.14%. Preparation of [Ag4L4]·(CF3SO3)4 (3). The preparation of 3 was similar to that of 1 except that AgCF3SO3 (0.062 g, 0.24 mmol) was used instead of AgNO3. Yield: 43% (based on ligand). Mp: 315−317 °C (decomp.). 1H NMR (600 MHz, CD3CN): δ δ 8.11 (m, 2H), 7.39(s, 2H), 7.26 (m, 2H), 7.18(t, J = 7.5 Hz, 4H), 6.93(d, J = 1.5 Hz, 1H), 6.90 (d, J = 7.5 Hz, 4H), 6.82 (m, 2H), 3.09 (sept, J = 6.6 Hz, 1H), 1.33 (d, J = 7.5 Hz, 6H). IR (KBr pellet): 3146, 3055, 2962, 2874, 1612, 1543, 1497, 1470, 1443, 1277, 1250, 1163, 1028, 978, 920, 889, 829, 760, 702, 636, 569, 515 cm −1 . Anal. Calcd for C120H100Ag4F12N12O12S4: C, 53.58; H, 3.75; N, 6.25%. Found: C, 53.71; H, 3.82; N, 6.33%. X-ray Crystallographic Analysis. The X-ray data were collected on a Bruker SMART APEX II ULTRA diffractometer equipped with a graphite monochromated Mo Kα (λ = 0.71073 Å) radiation generated by a rotating anode and a CCD detector. The cell parameters for the complexes were obtained from a least-squares refinement of the spots (from 36 collected frames). Data collection, data reduction, and semiempirical absorption correction (SADABS39) were carried out using the software package of APEX2.40 The structures were determined with direct methods using the SHELXS-2014 program,41 and refinement was performed against F2 using SHELXL-2014 program.42 In 2, one water molecule was disordered over three positions (O2WA, O2WB, and O2WC) with the sites occupancy

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01544. Emission photo in solid state, Inter- and intramolecular C−H···O, C−H···N, C−H···π, and π−π interactions for 1, 2, and 3 (PDF) Accession Codes

CCDC 1440812−1440814 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*(Y.K.) E-mail: [email protected]. *(K.-M.P.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported from NRF (2012R1A4A1027750, 2015R1D1A3A01020410, and 2011-0010518) and the industrial strategic technology development program (10039141), funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

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REFERENCES

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Role and Effect of Anions in the Construction of Silver Complexes

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