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

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.


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, C 3 H 3 N 2 − ) [4][5][6][7]. Many substituents have been introduced at the three, four, and five positions of the five-membered ring ( Figure 1).
Molecules 2023, 28,2936 2 of 13 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-5tertiary 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  We have been interested in modeling the structure and function of transition metalcontaining proteins [8]. The active sites of some copper-containing proteins have been investigated by X-ray structural analysis, which revealed N 2 S and N 3 donor ligands coordinating to the metal center [9]. We similarly used N 3 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.
Powder X-ray diffraction analysis of the white powders matched the sin structures, indicating phase purity (Figures S1 and S2 from Supplementary Ma

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

Structures
Single-crystal X-ray structures of coordination polymers (

Solution-State Properties
The 1 H-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
The Ag(I) atoms in [Ag(CF 3 SO 3 )(Me 4 bpzH 2 )] (Figure 7) were coordinated by two pyrazole N atoms of two Me 4 bpzH 2 and one O atom of the CF 3 SO 3 − 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 Me 4 bpzH 2 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 CF 3 SO 3 − 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(CF 3 SO 3 )(Me 4 bpzH 2 )] forms a double-chain structure (Figure 9). ] was observed at almost the same energy, but with a different molecular extinction coefficient. Therefore, the structure of [Ag(CF 3 SO 3 )(Me 4 bpzH 2 )] in the solution remains intact. However, we did not measure concentration dependences in the NMR or UV-Vis experiments. Unfortunately, the solubility of [Ag(CF 3 CO 2 )(Me 4 bpzH 2 )] was poor, and we could not obtain a UV-Vis spectrum in the MeOH solution.

Material and General Techniques
The preparation and handling of the two silver(I) complexes were perfo an argon atmosphere using standard Schlenk tube techniques under li conditions. Ultra-dry methanol was purchased from Wako Pure Chemical I deoxygenated by purging with argon gas. Deuteriochloroform was ob Cambridge Isotope Laboratories, Inc. Other reagents were commercially a used without further purification. The 3,3′,5,5′-tetramethyl-4,4′-bipyrazole In addition to the most intense 420 nm emission band of [Ag(CF 3 CO 2 )(Me 4 bpzH 2 )] and the 397 nm emission of [Ag(CF 3 SO 3 )(Me 4 bpzH 2 )], the corresponding measurements at 83 K revealed an additional band around~330 nm, which was also observed in the ligand Me 4 bpzH 2 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 d 10 -d 10 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(CF 3 SO 3 )(Me 4 bpzH 2 )] 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 [Ag 2 (SO 4 )(Me 4 bpzH 2 ) 2 ]·3H 2 O [25]. We are now in the process of probing the origin of this behavior through theoretical and more detailed physicochemical research.

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 (Me 4 bpzH 2 ) was prepared by published methods [19,28]. The purity of the ligand was checked by 1 H-NMR spectroscopy.

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. 1 H-NMR (500 MHz) and 13 C-NMR spectra (125 MHz) were obtained on a Bruker AVANCE III-500 NMR spectrometer at room temperature (298 K) in CDCl 3 -d 1 or CD 3 OD-d 3 (Bruker Japan, Yokohama, Japan). 1 H and 13 C 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. The bispyrazole ligand was prepared by published methods [19,28]. The purity of the ligand was checked by 1 H-NMR spectroscopy and characterized as indicated below.

Preparation of Ligand and Complexes
Calcd for C 10

X-ray Crystal Structure Determination
The diffraction data of [Ag(CF 3 CO 2 )(Me 4

bpzH 2 )] and [Ag(CF 3 SO 3 )(Me 4 bpzH 2 )]
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 nonhydrogen 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 F 2 . 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.

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(CF 3 CO 2 )(Me 4 bpzH 2 )] and [Ag(CF 3 SO 3 )(Me 4 bpzH 2 )]. We determined both solid-state structures at a low temperature. The Ag(I) atoms in [Ag(CF 3 CO 2 )(Me 4 bpzH 2 )] were coordinated by two pyrazole N atoms of two Me 4 bpzH 2 and one O atom of a CF 3 CO 2 − anion, giving a distorted trigonal pyramidal geometry. In the 1-D polynuclear structure, a zig-zag configuration was formed. Likewise, the coordinated CF 3 CO 2 − anions were also located in a zig-zag pattern. By comparison, the Ag(I) atoms in [Ag(CF 3 SO 3 )(Me 4 bpzH 2 )] were coordinated by two pyrazole N atoms of two Me 4 bpzH 2 and one O atom of the CF 3 SO 3 − anion, giving a distorted trigonal pyramidal geometry. In the 1-D polynuclear structure, a linear configuration was formed. The coordinated anions CF 3 SO 3 − 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 1 H-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.  Figure S6: IR spectra of the ligand and silver(I) complexes, Figure S7: FT-Raman spectra of the ligand and silver(I) complexes, Figure S8: Photoluminescence spectra of the ligand and silver(I) complexes at 298 K, Figure S9: Temperature dependent photoluminescence spectra of the ligand Me 4 bpzH 2 , Figure S10: Temperature dependent photoluminescence spectra of [Ag(CF 3 CO 2 )(Me 4 bpzH 2 )], Figure  S11: Temperature dependent photoluminescence spectra of [Ag(CF 3 SO 3 )(Me 4 bpzH 2 )], Figure S12: Solid-state photoluminescence spectra of the ligand and silver(I) complexes at 173 K, Figure S13: Solid-state photoluminescence spectra of the ligand and silver(I) complexes at 298 K.