Silver(I) Coordination Polymer Ligated by Bipyrazole Me₄bpzH₂, [Ag(X)(Me₄bpzH₂)] (X = CF₃CO₂⁻ and CF₃SO₃⁻, Me₄bpzH₂ = 3,3′,5,5′-Tetramethyl-4,4′-bipyrazole): Anion Dependent Structures and Photoluminescence Properties

Abstract

Coordination polymers of transition metal ions are fascinating and important to coordination chemistry. One of the ligands known to form particularly interesting coordination polymers is 3,3′,5,5′-tetramethyl-4,4′-bipyrazole (Me₄bpzH₂). Group 11 metal(I) ion coordination polymers, other than those of copper(I), are relatively easy to handle because of their low reactivity towards dioxygen and moisture. However, the known silver(I) coordination polymers often have poor solubility in common solvents and so cannot be easily analyzed in solution. By using a tetramethyl substituted bipyrazole ligand, we have synthesized more soluble silver(I) complexes that contain the trifluoromethyl group in the coordinated ions CF₃CO₂⁻ and CF₃SO₃⁻ in [Ag(CF₃CO₂)(Me₄bpzH₂)] and [Ag(CF₃SO₃)(Me₄bpzH₂)]. We determined both structures by single-crystal X-ray analysis at low temperatures and compared them in detail. Moreover, we investigated the solution behavior of these coordination polymers by ¹H-NMR, IR, Raman, UV–Vis spectroscopies, and their low-temperature, solid-state photoluminescence. The high-energy band at ~330 nm corresponded to ligand-centered (bipyrazole) fluorescence, and the low-energy band at ~400 nm to ligand-centered phosphorescence resulting from the heavy atom effect.

1. Introduction

Cyclic trinuclear complexes (CTCs) with coinage metal(I) ions are of theoretical and practical interest to inorganic and coordination chemists [1,2,3]. A useful class of ligands for the formation of these CTCs is pyrazolate, which is known to act as a linking ligand. The simple, neutral 1H-pyrazoles and their deprotonated pyrazolate anions have two adjoining nitrogen donors in the five-membered aromatic rings; thus, they can coordinate and bridge metal ions with an Npz–M–Npz linear coordination mode (pz = pyrazolate anion, C₃H₃N₂⁻) [4,5,6,7]. Many substituents have been introduced at the three, four, and five positions of the five-membered ring (Figure 1).

We have been interested in modeling the structure and function of transition metal-containing proteins [8]. The active sites of some copper-containing proteins have been investigated by X-ray structural analysis, which revealed N₂S and N₃ donor ligands coordinating to the metal center [9]. We similarly used N₃ tripodal ligands in which three pyrazoles linked by a boron atom in hydridotris(pyrazolyl)borate gave copper(II) dioxygen complexes as simple hemocyanin models [8,10,11] and copper(II) thiolato complexes for copper-containing electron transfer model complexes [12]. As part of these investigations, we made numerous pyrazoles, varying in their steric and electronic properties. In the present work, we have explored the use of pyrazole to make new CTC compounds.

Our first publication reported silver(I) CTCs with 3,5-diisopropyl, 3-isopropyl-5-tertiary butyl, and 3,5-ditertiary butyl pyrazoles (Figure 2). We showed that the geometries of these complexes were greatly influenced by the steric influence exerted by the substituent groups on the pyrazolyl rings, and the differences in the central metal(I) ionic radius in trinuclear complexes [Ag(μ-3,5-iPr₂pz)]₃, [Ag(μ-3-tBu-5-iPrpz)]₃, and tetranuclear [Ag(μ-3,5-tBu₂pz)]₄ [13]. Halogen atoms were introduced using N-halosuccinimides, and the electronegativity of the halogen substituent could be correlated with the strength of the Ag⋯Ag interaction and the wavelength of solid-state photoluminescence in dimeric trinuclear (hexanuclear) complexes {[Ag(μ-4-X-3,5-R₂pz)]₃}₂ (R = iPr, X = Cl, Br, and I) and trinuclear [Ag(μ-4-X-3,5-R₂pz)]₃ (R = iPr, X = I; R = Ph, X = Cl, R = Ph, X = Br) [14]. Phenyl substituents in [Ag(μ-4-Ph-3,5-iPr₂pz)]₃ altered the solid-state crystal packing to a stair-type structure, which was quite distinct from that observed for the parent [Ag(μ-3,5-iPr₂pz)]₃ [15]. Employing the less hindered ethyl group gave a dimeric trinuclear (hexanuclear) complex with two intermolecular argentophilic interactions {[Ag(μ-4-Ph-3,5-Et₂pz)]₃}₂ [16]. This complex easily incorporated aromatic guests to form arene-sandwiched, π acid/base complexes, [Ag(μ-4-Ph-3,5-Et₂pz)]₃(toluene), and [Ag(μ-4-Ph-3,5-Et₂pz)]₃(mesitylene). An unexpected synthetic outcome yielded a silver(I) coordination polymer [Ag(µ-4-Cl-3,5-iPr₂pz)]_n from the reaction of {[Ag(µ-4-Cl-3,5-iPr₂pz)]}₂ with (ⁿBu₄N)[Ag(CN)₂] [17]. We have expanded this study to make a silver(I) coordination polymer with 3,3′,5,5′-tetramethyl-4,4′-bipyrazole (Me₄bpzH₂).

Many transition metals ligated by 3,3′,5,5′-tetramethyl-4,4′-bipyrazole (Me₄bpzH₂) have been reported [18]. The geometry of this bipyrazole is presumably controlled by the steric repulsion of the four-methyl groups, which influence the configuration of the two pyrazole rings and interplanar angle (φ), which is also controlled by the metal ion and its coordination environment (Figure 3). Single-crystal structures reported for silver(I) complexes ligated by bipyrazoles include the following: [Ag(NO₃)(Me₄bpzH₂)]·MeOH [19], [Ag(Me₄bpzH₂)](ClO₄) [20], [Ag(Me₄bpzH₂)](PO₂F₂) [20], [Ag₄(NO₃)₄(Me₄bpzH₂)₅]·2H₂O [20], [Ag(CF₃SO₃)(Me₄bpzH₂)] [20], [Ag₂(CF₃CO₂)₂(Me₄bpzH₂)₃] [20], [Ag(C₂F₅CO₂)(Me₄bpzH₂)] [20], [Ag₂(Me₄bpz)] [21,22], [Ag₃₀(Me₄bpz)₁₅]·10(C₆H₆) [21,22], [Ag₃₀(Me₄bpz)₁₅]·9(C₆H₅CH₃) [21,22], [Ag(p-HO₂C₆H₄CO₂)(Me₄bpzH₂)] [23], [Ag₂(m-O₂C₆H₄CO₂)(Me₄bpzH₂)₂] [23], [Ag(CH₃CO₂)(Me₄bpzH₂)]·5.4H₂O [23], [Ag₆(Ph₄bpz)₃] (Ph₂bpz = 3,3′,5,5′-tetraphenyl-4,4′-bipyrazole dianion) [24], and [Ag₂(SO₄)(Me₄bpzH₂)₂]·3H₂O [25]. Depending on the metal-to-ligand ratio, and other factors, it is possible to form many structures, such as coordination polymers with trinuclear structures. However, silver(I) coordination polymers are insoluble in most solvents once formed. To overcome this disadvantage, anions with trifluoromethyl groups such as CF₃CO₂⁻ and CF₃SO₃⁻ were used in the present study as coordinated ions. We have previously reported the use of the trifluoromethyl group to make the manganese(II) complex [Mn^II{HB(3-CF₃-5-Mepz)₃}₂], where HB(3-CF₃-5-Mepz)₃⁻ = hydridotris(3-trifluoromethyl-5-methylpyrazolyl-1-yl)borate anion [26] and copper(I) complexes [Cu^I{HB(3-CF₃-5-Mepz)₃}(CO)] and [Cu^I{HB(3-CF₃-5-Mepz)₃}(PPh₃)] [27]. The trifluoromethyl group has unique electronegativity, hydrophobicity, metabolic stability, and bioavailability. It is therefore widely employed in medicine, agrochemicals, and organic materials [28]. In the present work, we report the synthesis of silver(I) coordination polymers, [Ag(CF₃CO₂)(Me₄bpzH₂)] and [Ag(CF₃SO₃)(Me₄bpzH₂)], and their characterization by ¹H-NMR, IR, Raman, UV–Vis, and photoluminescence spectroscopies. The reported structure of [Ag(CF₃SO₃)(Me₄bpzH₂)] [20] had a severe disorder in the trifluoromethyl groups, and this problem was avoided in the present study by acquiring the diffraction data at −95 °C.

2. Results and Discussion

2.1. Synthesis

The reactions of 3,3′,5,5′-tetramethyl-4,4′-bipyrazole (Me₄bpzH₂) [19,29] with one equivalent of silver(I) ions, Ag(CF₃CO₂) and Ag(CF₃SO₃), were carried out at room temperature (Figure 4), and they were given white powders after 48 h. The yields were modest (50–60%). Single crystals were obtained from the filtrate by slow evaporation at room temperature.

Powder X-ray diffraction analysis of the white powders matched the single-crystal structures, indicating phase purity (Figures S1 and S2 from Supplementary Materials).

2.2. Structures

Single-crystal X-ray structures of coordination polymers (Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9), [Ag(CF₃CO₂)(Me₄bpzH₂)] and [Ag(CF₃SO₃)(Me₄bpzH₂)], are shown in Figure 5 and Figure 7, respectively. The 1-D polynuclear structures of [Ag(CF₃CO₂)(Me₄bpzH₂)] and [Ag(CF₃SO₃)(Me₄bpzH₂)] are presented in Figure 6 and Figure 8, respectively. Fragments of the double-chain structures of [Ag(CF₃SO₃)(Me₄bpzH₂)] are shown in Figure 9.

The Ag(I) atoms in [Ag(CF₃CO₂)(Me₄bpzH₂)] (Figure 5) were coordinated by two pyrazole N atoms of two Me₄bpzH₂ and one O atom of a CF₃CO₂⁻ anion, giving a distorted trigonal pyramidal geometry with 0.27 Å distance between the Ag(I) ion and the plane created by the coordinated atoms of the N₂O ligand donor set. The coordinated pyrazoles’ dihedral angle in Me₄bpzH₂ was 42.9°, and the shortest Ag⋯Ag distance was 9.9857(4) Å. The dihedral angle of the bipyrazole (φ) in Figure 3 is 62.7°, which is within the range of the reported values. Therefore, in the 1-D polynuclear structure, a zig-zag configuration was formed (Figure 6). Likewise, the coordinated CF₃CO₂⁻ anions were also located in a zig-zag pattern. The distance to the next Ag(I) ion was 18.5775(4) Å, and the dihedral angle between these pyrazoles was 0°. The carboxylate oxygen was coordinated to the Ag(I) ions at a relatively long distance of Ag1–O1, 2.544(2) Å with a very weak Ag1···O2 interaction of 3.349(2) Å. This conformation was stabilized by two intramolecular hydrogen bonds of 2.801(3) Å N12⋯O1 and 2.738(3) N22⋯O2. The interdimer Ag⋯Ag distances were 3.4250(4) and 8.6779(3) Å (Figure 6). The former is almost the same as the sum of twice Bondi’s van der Waals radius (3.44 Å) [30], indicating small argentophilic interactions [31].

The Ag(I) atoms in [Ag(CF₃SO₃)(Me₄bpzH₂)] (Figure 7) were coordinated by two pyrazole N atoms of two Me₄bpzH₂ and one O atom of the CF₃SO₃⁻ anion, giving a distorted trigonal pyramidal geometry with 0.08 Å in distance between the Ag(I) ion and the plane created by the coordinated atoms. The coordinated pyrazoles’ dihedral angle in Me₄bpzH₂ was 77.05°, and the shortest Ag⋯Ag distance was 9.9158(4) Å. The dihedral angle of the bipyrazole (φ) was 77.05° (Figure 3), which is in the range of the reported values. Therefore, in the 1-D polynuclear structure, a linear configuration was formed (Figure 8). The coordinated anions CF₃SO₃⁻ were oriented in the same direction. The distance to the next Ag(I) ion was 19.815(5) Å, this value is twice the Ag1···Ag1 distance of 9.9158(4) Å, so that each Ag(I) ion was linear. The dihedral angle between these pyrazoles was 0°. The carboxylate oxygen was coordinated to the Ag(I) ions at a relatively long distance of Ag1···O1, 2.678(3) Å with no interaction between Ag1···O2, 4.233(2) Å. This conformation was stabilized by two intermolecular hydrogen bonds of 2.844(4) Å N12⋯O3 and 2.865(4) N22⋯O2. Moreover, the interdimer Ag⋯Ag distance was 4.4592(4), which is longer than the sum of twice the Bondi’s van der Waals radius (3.44 Å) [30], indicating almost no argentophilic interaction [31] (Figure 9). However, [Ag(CF₃SO₃)(Me₄bpzH₂)] forms a double-chain structure (Figure 9).

2.3. Solution-State Properties

The ¹H-NMR spectrum of the obtained white powder [Ag(CF₃SO₃)(Me₄bpzH₂)] in CDCl₃ revealed only a broad 1.61 ppm signal (Figure S3 from Supplementary Materials), which was different from that of the ligand, Me₄bpzH₂ at 2.10 ppm (Figure S4 from Supplementary Materials). This observation is also supported by its solution-state UV–Vis spectra in MeOH (Figure S5 from Supplementary Materials). A broad absorption of Me₄bpzH₂ in the UV region was observed at around 230 nm, and the shoulder of [Ag(CF₃SO₃)(Me₄bpzH₂)] was observed at almost the same energy, but with a different molecular extinction coefficient. Therefore, the structure of [Ag(CF₃SO₃)(Me₄bpzH₂)] in the solution remains intact. However, we did not measure concentration dependences in the NMR or UV–Vis experiments. Unfortunately, the solubility of [Ag(CF₃CO₂)(Me₄bpzH₂)] was poor, and we could not obtain a UV–Vis spectrum in the MeOH solution.

2.4. Solid-State Properties

In [Ag(CF₃CO₂)(Me₄bpzH₂)], the characteristic CO₂ stretching vibrations could be observed in the IR region at 1683 cm⁻¹, and in [Ag(CF₃CO₂)(Me₄bpzH₂)], the characteristic stretching vibrations from the CF₃SO₃⁻ group were observed at 1260 cm⁻¹ ν_as(SO₃), 1175 cm⁻¹ ν_as(CF₃), 1026 cm⁻¹ ν_s(SO₃) in the IR spectrum, and 1027 cm⁻¹ ν_s(SO₃) in the Raman spectrum [32,33]. Strong peaks in the far-IR region were assigned to the ν(C–C) of bipyrazole, which was observed at 627 cm⁻¹ (IR) and 625 cm⁻¹ (Raman) in [Ag(CF₃CO₂)(Me₄bpzH₂)], and at 627 cm⁻¹ (shoulder) (IR) and 624 cm⁻¹ (Raman) in [Ag(CF₃SO₃)(Me₄bpzH₂)], and at 628 cm⁻¹ (IR) and 619 cm⁻¹ (Raman) in Me₄bpzH₂. An additional peak at 644 cm⁻¹ was assigned to δ_s(SO₃) (Figure 10, Figures S6 and S7 from Supplementary Materials).

The Ag–N stretching vibration has been previously reported at ~500 cm⁻¹ [13,14,15,16,17,32]. However, the ligand Me₄bpzH₂ exhibited some peaks in this region. Therefore, we cannot conclusively assign this vibration as ν(Ag–N). The Ag–O stretching vibration could be assigned at 519 cm⁻¹ (IR) in [Ag(CF₃CO₂)(Me₄bpzH₂)] and 520 cm⁻¹ (IR) in [Ag(CF₃SO₃)(Me₄bpzH₂)], compared with the Ag–O stretching vibration of its precursors, 518 cm⁻¹ (IR) in [Ag(CF₃CO₂)] and 519 cm⁻¹ (IR) in [Ag(CF₃SO₃)]. These vibration data confirm the solid-state structure observed by X-ray crystallography.

The emission spectra of the silver(I) complexes [Ag(CF₃CO₂)(Me₄bpzH₂)], [Ag(CF₃SO₃)(Me₄bpzH₂)], and Me₄bpzH₂ are shown in Figure S8 from Supplementary Materials (solid-state and solution-state at 298 K), Figure S9 from Supplementary Materials (temperature dependence, Me₄bpzH₂), Figure S10 from Supplementary Materials (temperature dependence, [Ag(CF₃CO₂)(Me₄bpzH₂)]), Figure S11 from Supplementary Materials (temperature dependence, [Ag(CF₃SO₃)(Me₄bpzH₂)]), Figure S12 from Supplementary Materials (solid-state at 173 K, comparison), and Figure S13 from Supplementary Materials (solid-state at 298 K, comparison).

At 298 K, there were no significant differences between silver(I) complexes [Ag(CF₃CO₂)(Me₄bpzH₂)] and [Ag(CF₃SO₃)(Me₄bpzH₂)] and the ligand Me₄bpzH₂. However, some shift was observed between the solid-state and solution-state spectra of [Ag(CF₃SO₃)(Me₄bpzH₂)] (Figures S8 and S13 from Supplementary Materials). This may be caused by the dissociation of [Ag(CF₃SO₃)(Me₄bpzH₂)] in the solution. At lower temperatures of 173 K and 83 K, a new broad emission band was observed at 420 nm in [Ag(CF₃CO₂)(Me₄bpzH₂)] and at 397 nm in [Ag(CF₃SO₃)(Me₄bpzH₂)] (Figure 11, Figure S10–S12 from Supplementary Materials).

In addition to the most intense 420 nm emission band of [Ag(CF₃CO₂)(Me₄bpzH₂)] and the 397 nm emission of [Ag(CF₃SO₃)(Me₄bpzH₂)], the corresponding measurements at 83 K revealed an additional band around ~330 nm, which was also observed in the ligand Me₄bpzH₂ at the same temperature (Figure 11). This higher energy emission band may be from ligand-based phosphorescence [25]. The lower energy emission band was attributed to metal-based phosphorescence arising from closed shell d¹⁰–d¹⁰ intermolecular argentophilic (Ag···Ag) interactions [13,14,15,16,17,34,35,36]. Both ~330 nm and ~400 nm bands were ascribed to ligand-based phosphorescence, since [Ag(CF₃SO₃)(Me₄bpzH₂)] has no argentophilic interaction, as indicated by the interdimer Ag⋯Ag distance of 4.4592(4) Å. The latter emission was also attributed to the heavy metal effect [1,2,3]. This explanation has been proposed based on experimental observations of the previously reported [Ag₂(SO₄)(Me₄bpzH₂)₂]·3H₂O [25]. We are now in the process of probing the origin of this behavior through theoretical and more detailed physicochemical research.

3. Materials and Methods

3.1. Material and General Techniques

The preparation and handling of the two silver(I) complexes were performed under an argon atmosphere using standard Schlenk tube techniques under light-shielded conditions. Ultra-dry methanol was purchased from Wako Pure Chemical Ind. Ltd. and deoxygenated by purging with argon gas. Deuteriochloroform was obtained from Cambridge Isotope Laboratories, Inc. Other reagents were commercially available and used without further purification. The 3,3′,5,5′-tetramethyl-4,4′-bipyrazole (Me₄bpzH₂) was prepared by published methods [19,28]. The purity of the ligand was checked by ¹H-NMR spectroscopy.

3.2. Instrumentation

IR spectra (4000–400 cm⁻¹) and far-IR spectra (680–150 cm⁻¹) were recorded as KBr pellets using a JASCO FT/IR-6300 spectrophotometer under ambient conditions (JASCO, Tokyo, Japan) and as CsI pellets using a JASCO FT/IR 6700 spectrophotometer under vacuum (JASCO, Tokyo, Japan), respectively. Raman spectra (4000–200 cm⁻¹) were measured as powders on a JASCO RFT600 spectrophotometer with a YAG laser 600 mW (JASCO, Tokyo, Japan). Abbreviations used in the description of vibration data are as follows: s, strong; m, medium; and w, weak. ¹H-NMR (500 MHz) and ¹³C-NMR spectra (125 MHz) were obtained on a Bruker AVANCE III-500 NMR spectrometer at room temperature (298 K) in CDCl₃-d₁ or CD₃OD-d₃ (Bruker Japan, Yokohama, Japan). ¹H and ¹³C chemical shifts were reported as δ values relative to residual solvent peaks. UV–Vis spectra (solution and solid, 1000–200 nm) were recorded on a JASCO V-570 spectrophotometer (JASCO, Tokyo, Japan). The values of ε were calculated per silver(I) ion. Solid samples (mulls) for UV–Vis spectroscopy were prepared by finely grinding microcrystalline material into powders with a mortar and pestle and then adding mulling agents (nujol, poly(dimethylsiloxane), viscosity 10,000) (Aldrich)) before uniformly spreading between quartz plates. Luminescence spectra were recorded on a JASCO FP-6500 (solution and solid, 600–300 nm) spectrofluorometer (JASCO, Tokyo, Japan). Low-temperature luminescence spectra were recorded using solid samples, which were prepared by finely grinding microcrystalline material into powders with a mortar between quartz plates cooled with a liquid nitrogen cryostat (CoolSpeK USP-203) from Unisoku Scientific Instruments (Osaka, Japan). Powder X-ray diffraction (XRD) measurements were conducted on a Rigaku SmartLab-SP/IUA X-ray diffractometer (Rigaku, Tokyo, Japan) with a Cu Kα radiation (λ = 1.54 Å) source (40 kV, 30 mA) and a high-speed one-dimensional detector D/teX Ultra 250. The 2θ was measured in the range of 5–90° with a scan step of 0.02° and scan speed of 10° min⁻¹. Solid samples for XRD were prepared by finely grinding microcrystalline materials into powders with a mortar and pestle and then placing them on an aluminum dish (0.2 mm thickness). Simulated powdered XRD patterns were calculated from single-crystal data using the MERCURY software suite from CCDC. The elemental analyses (C, H, and N) were performed by the Chemical Analysis Center of Ibaraki University.

3.3. Preparation of Ligand and Complexes

• 3,3′,5,5′-Tetramethyl-4,4′-bipyrazole (Me₄bpzH₂)

The bispyrazole ligand was prepared by published methods [19,28]. The purity of the ligand was checked by ¹H-NMR spectroscopy and characterized as indicated below.

Calcd for C₁₀H₁₆N₄O = Me₄bpzH₂•H₂O: C, 57.67; H, 7.74; N, 26.90. Found: C, 57.95; H, 7.82; N 27.13. ¹H-NMR (CDCl₃, 500 MHz): δ/ppm (assignments): 2.18 (s, 12 H, Me). ¹H-NMR (CD₃OD, 500 MHz): δ/ppm (assignments): 2.05 (s, br, 12 H, Me). ¹³C-NMR (CD₃OD, 125 MHz): δ/ppm (assignments): 9.8 (3- or 5-Me), 12.2 (3- or 5-Me), 109.8 (pz-4C), 139.6 (3- or 5-pzC), 149.4 (3- or 5-pzC). IR (KBr, cm⁻¹): 3200 s ν(N–H), 3082 s ν(N–H), 2925 s ν(C–H), 2824 s ν(C–H), 1614 w, 1568 m, 1545 m, 1416 s, 1371 w, 1309 m, 1291 m, 1256 m, 1172 w, 1062 w, 1041 w, 1016 s, 842 m, 786 s, 625 w, 519 w, 479 w. Far–IR (CsI, cm⁻¹): 662 w, 628 s ν(C–C), 591 w, 521 s, 480 s, 429 m, 351 m, 338 m, 277 s, 180 s. Far–IR (CsI, cm⁻¹): 662 w, 628 s ν(C–C), 591 w, 521 s, 480 s, 429 m, 351 m, 338 m, 277 s, 180 s. Raman (solid, cm⁻¹): 2928 s ν(C–H), 1623 m, 1539 w, 1473 m, 1421 m, 1375 w, 1307 w, 1156 w, 1139 w, 973 w, 783 w, 710 w, 619 s ν(C–C), 592 m, 518 w, 486 w, 423 w, 343 m. UV–Vis (solution, methanol, λ_max/nm (ε/cm⁻¹ mol⁻¹ dm³)): 223 (5100). Emission (solid, ex. 250 nm, λ_max/nm): 83 K, 327; 173 K, 328, 83 K, 328.

• [Ag(CF₃CO₂)(Me₄bpzH₂)]

A solution of 3,3′,5,5′-tetramethyl-4,4′-bipyrazole (Me₄bpzH₂) (388 mg, 2.04 mmol) in methanol (10 cm³) was added to a solution of silver(I) trifluoroacetate (446 mg, 2.02 mmol) in methanol (10 cm³). The mixture was stirred for 48 h, and the resulting powder was filtered and dried under vacuum. The colorless powder was obtained by filtration (561 mg, 1.36 mmol, 67%). Colorless crystals for X-ray analysis were obtained from the filtrate.

Calcd for C₁₂H₁₄AgF₃N₄O₂: C, 35.06; H, 3.43; N, 13.63. Found: C, 34.94; H, 3.51; N 13.67.

IR (KBr, cm⁻¹): 3305 s, 3079 s, 2929 s, 1683 s ν(C=O), 1558 m, 1542 m, 1496 m, 1462 m, 1429 m, 1374 w, 1281 m, 1261 m, 1206 s, 1132 s, 1042 m, 835 m, 798 m, 780 m, 720 m, 708 m, 616 w, 597 w, 566 w. Far–IR (CsI, cm⁻¹): 627 s ν(C–C), 597 w, 519 s ν(Ag–O), 496 w, 479 m ν(Ag–N), 429 w, 351 m, 266 s, 179 s. Raman (solid, cm⁻¹): 2970 m ν(C–H), 2932 s ν(C–H), 1619 s ν(C=O), 1544 w, 1489 m, 1450 m, 1428 s, 1388 m, 1303 w, 1188 w, 835 w, 625 s ν(C–C), 592 w, 533 w, 454 m ν(Ag–N), 412 w, 349 w, 300 w. Emission (solid, ex. 250 nm, λ_max/nm): 83 K, 420; 173 K, 423; 298 K, 331.

• [Ag(CF₃SO₃)(Me₄bpzH₂)]

A solution of 3,3′,5,5′-tetramethyl-4,4′-bipyrazole (Me₄bpzH₂) (271 mg, 1.43 mmol) in methanol (10 cm³) was added to a solution of silver(I) trifluoromethanesulfonate (366 mg, 1.43 mmol) in methanol (10 cm³). The mixture was stirred for 48 h. A colorless powder was obtained (349 mg, 0.78 mmol, 55%) by slow evaporation of the transparent solution. Colorless crystals for X-ray analysis were obtained by recrystallization from methanol at room temperature.

Calcd for C₁₁H₁₄AgF₃N₄O₃S: C, 29.54; H, 3.16; N, 12.53. Found: C, 29.52; H, 3.19; N, 12.56.

IR (KBr, cm⁻¹): 3315 s, 3245 s, 3099 m, 2964 m, 2927 m, 1628 w, 1598 m, 1563 m, 1545 m, 1463 m, 1420 m, 1378 w, 1377 w, 1260 s ν_as(SO₃), 1227 s, 1175 s ν_as(CF₃), 1157 m, 1104 w, 1026 s ν_s(SO₃), 800 m, 784 w, 732 m, 707 w, 638 s, 576 w, 519 m. Far-IR (CsI, cm⁻¹): 689 w, 644 s δ_s(SO₃), 627 sh ν(C–C), 596 w, 578 m, 520 s ν(Ag–O), 480 w ν(Ag–N), 429 w, 351 w, 267 m, 180 m. Raman (solid, cm⁻¹): 2932 s, 1628 s, 1544 m, 1484 m, 1424 m, 1385 m, 1307 w, 1226 w, 1170 w, 1154 w, 1027 s ν_s(SO₃), 761 m, 707 w, 625 s ν(C–C), 593 m, 577 w, 523 w, 442 w, 353 m, 340 w, 319 m. ¹H-NMR (CDCl₃, 500 MHz): δ/ppm (assignments): 1.61 (s, br, 12 H, Me). UV–Vis (solution, MeOH, λ_max/nm (ε/cm⁻¹ mol⁻¹ dm³)): 230 (shoulder, 8300). Emission (solution, MeOH, ex. 260 nm, λ_max/nm): 337. Emission (solid, ex. 250 nm, λ_max/nm): 83 K, 397; 173 K, 393; 298 K, 324.

3.4. X-ray Crystal Structure Determination

The diffraction data of [Ag(CF₃CO₂)(Me₄bpzH₂)] and [Ag(CF₃SO₃)(Me₄bpzH₂)] were obtained on a Rigaku XtaLAB P200 diffractometer using multi-layer mirror monochromated Mo Kα (λ = 0.71073 Å) radiation at –95 ± 2 °C. A crystal of suitable size and quality was coated with Paratone-N oil (Hampton Research, Aliso Viejo, CA, USA) and mounted on a Dual-Thickness MicroLoop LD (200 μM) (MiTeGen, New York, NY, USA). The unit cell parameters were determined using CrystalClear from 18 images [37]. The crystal to detector distance was ca. 45 mm. Data were collected at 0.5° intervals in φ and ω to a maximum 2θ value of 55.0°. The highly redundant data sets were reduced using CrysAlisPro [38]. An empirical absorption correction was applied for each complex. Structures were solved by direct methods (SIR2008 [39] and SIR2004 [40]). The position of the silver ions and their first coordination sphere were located using a direct method (E-map). Other non-hydrogen atoms were found in alternating difference Fourier syntheses, and least squares refinement cycles. During the final refinement cycles, the temperature factors were refined anisotropically. Refinement was carried out by a full matrix least-squares method on F². All calculations were performed with the CrystalStructure [41] crystallographic software package except for refinement, which was performed using SHELXL 2013 [42]. Hydrogen atoms were placed in calculated positions. Crystallographic data and structure refinement parameters, including the final discrepancies (R and Rw), are listed in

Table 1. Crystal data and structure refinement of [Ag(CF₃SO₃)(Me₄bpzH₂)] and [Ag(CF₃CO₂)(Me₄bpzH₂)].

Complex	[Ag(CF3CO2)(Me4bpzH2)]	[Ag(CF3SO3)(Me4bpzH2)]
CCDC number	2,227,168	2,227,169
Empirical formula	C12H14AgF3N4O2	C11H14AgF3N4O3S
Formula weight	411.13	447.18
Crystal system	Monoclinic	Triclinic
Space group	P21/n (#14)	P$\overline{1}$ (#2)
a/Å	13.2724(2)	8.66665(15)
b/Å	8.67316(15)	9.91576(18)
c/Å	13.3047(2)	10.2913(2)
α/°	90	111.6690(19)
β/°	91.3059(17)	102.4538(17)
γ/°	90	90.8501(14)
V/Å3	1531.15(4)	798.20(3)
Z	4	2
Dcalc/g cm−3	1.783	1.860
μ(MoKα)/cm−1	13.560	14.391
2θ range, °	6–55	6–55
Reflections collected	23895	25699
Unique reflections	3516	3666
Rint	0.0304	0.0270
Number of variables	199	208
Refls./Para. ratio	17.67	17.63
Residuals: R1 (I > 2 σ (I))	0.0337	0.0320
Residuals: R (All refl.)	0.0359	0.0351
Residuals: wR2 (All refl.)	0.0999	0.0965
Goodness of fit ind.	1.054	1.078
Max/min peak,/e Å−3	1.27/–0.73	1.21/–0.45

^a R = Σ ||Fo| − |Fc||/Σ |Fo |; wR2 = [(Σ (w (|Fo|² − |Fc|²)²)/Σ w (Fo²))²]^1/2.

.

4. Conclusions

Silver(I) coordination polymers are important in coordination chemistry, but they often have very poor solubility in common solvents. To overcome this disadvantage, we synthesized silver(I) complexes with a trifluoromethyl group, viz [Ag(CF₃CO₂)(Me₄bpzH₂)] and [Ag(CF₃SO₃)(Me₄bpzH₂)]. We determined both solid-state structures at a low temperature. The Ag(I) atoms in [Ag(CF₃CO₂)(Me₄bpzH₂)] were coordinated by two pyrazole N atoms of two Me₄bpzH₂ and one O atom of a CF₃CO₂⁻ anion, giving a distorted trigonal pyramidal geometry. In the 1-D polynuclear structure, a zig-zag configuration was formed. Likewise, the coordinated CF₃CO₂⁻ anions were also located in a zig-zag pattern. By comparison, the Ag(I) atoms in [Ag(CF₃SO₃)(Me₄bpzH₂)] were coordinated by two pyrazole N atoms of two Me₄bpzH₂ and one O atom of the CF₃SO₃⁻ anion, giving a distorted trigonal pyramidal geometry. In the 1-D polynuclear structure, a linear configuration was formed. The coordinated anions CF₃SO₃⁻ were oriented in the same direction. This conformation was stabilized by two intermolecular hydrogen bonds, forming a double-chain structure. Solution properties were measured by ¹H-NMR, UV–Vis absorption, and photoluminescence spectroscopies. These silver(I) coordination polymers exhibited interesting photoluminescence properties resulting from the presence of intermolecular argentophilic (Ag···Ag) interactions and/or ligand-based phosphorescence with the heavy atom effect. Further efforts to probe how the structures of coinage silver(I) coordination polymers are affected by ligand and coordination environments are in progress in our laboratory.