Coordination polymers and discrete complexes of Ag(I)

Journal of Coordination Chemistry

ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20

Coordination polymers and discrete complexes of Ag(I)-N-(pyridylmethylene)anilines: Synthesis, crystal structures and photophysical properties Eric M. Njogu, Bernard Omondi & Vincent O. Nyamori To cite this article: Eric M. Njogu, Bernard Omondi & Vincent O. Nyamori (2017): Coordination polymers and discrete complexes of Ag(I)-N-(pyridylmethylene)anilines: Synthesis, crystal structures and photophysical properties, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2017.1370088 To link to this article: http://dx.doi.org/10.1080/00958972.2017.1370088

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Date: 29 August 2017, At: 05:52

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Publisher: Taylor & Francis Journal: Journal of Coordination Chemistry DOI: http://doi.org/10.1080/00958972.2017.1370088

Coordination polymers and discrete complexes of Ag(I)-N-(pyridylmethylene)anilines: Synthesis, crystal structures and photophysical properties ERIC M. NJOGU, BERNARD OMONDI* and VINCENT O. NYAMORI School of Chemistry and Physics, University of KwaZulu-Natal, South Africa, Private bag X54001, Durban, 4000

Reactions of AgO2C2F3 with (E)-N-(pyridylmethylene)aniline in which the pyridyl N is in the por m-position yielded two 1-D coordination polymers, [(AgO2C2F3)2(La)2]n (La = (E)-2,6diisopropyl-N-(pyrid-3-ylmethylene)aniline) (1) and [(AgOC2F3)2(Ld)2]n (Ld = (E)-2,6diisopropyl-N-(pyrid-4-ylmethylene)aniline) (5), and three discrete complexes, [(AgO2C2F3)2(La)4] (2), [AgO2C2F3(Lb)2] (Lb = (E)-N-(pyrid-4-ylmethylene)aniline) (3) and [(AgOC2F3)2(Lc)4] (Lc = (E)-2,6-dimethyl-N-(pyrid-4-ylmethylene)aniline) (4). The structures were determined by MS, FT-IR and NMR spectroscopies, elemental analysis and single crystal XRD. 1 is an organometallic coordination polymer with silver in η1-arene coordination, but is a discrete dimeric complex 2 when crystallized from warm diethylether. The geometries around silver(I) in 1 and 4 are tetrahedral, ‘inverted seesaw’ in 2 and T-shaped in 3 and in all the anion seems to play a role. Ag(I) centers in 5 have distorted trigonal bipyramid and inverted seesaw geometries. The trifluoroacetate anions in these complexes display variable monodentate and short bridging coordination patterns. All complexes absorb and strongly emit UV-Vis radiation at room temperature. Keywords: Silver(I) organometallic compounds; 1-D Coordination polymers; Pyridyl ligands; Schiff base; Argentophilic interactions 1. Introduction The synthesis of silver(I) coordination complexes with structural diversities has progressively *Corresponding author. Email: [email protected]

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attracted attention. Principally, silver(I) complexes are sought after due to intriguing architectures which are obtainable courtesy of the high flexibility of the Ag(I) coordination sphere [1, 2]. The silver(I) arrays show vital physicochemical properties that make them promising antimicrobial [3-6] and anticancer agents [7], luminescent materials [8-10] and


efficient catalysts for various reactions [11-13]. Several examples of silver(I) supramolecular assemblies generated through secondary interactions such as hydrogen-bonding [14], π-π interactions [15, 16], hydrophilic-hydrophobic interactions [17, 18], closed d10 argentophilic interactions (Ag…Ag) [19, 20] as well as anion bridging the metal centers have been reported. The overall structural networks in these assemblies are influenced by several factors such as: (i) the coordination geometries of silver(I), (ii) the solvent system used in synthesis [21, 22], (iii) the nature and structure of the organic ligands [23], (iv) the nature of counter-anions [24-26] and (v) argentophilic interactions [27]. Silver(I) supramolecules of N heterocyclic ligands are among the most studied [28, 29] and generally, the supramolecular arrays have repeating units generated by Ag…Ag (argentophilic), π···π, Ag…C, Ag…π, C—H…Ag, anion bridging and coulombic interactions as reviewed by Khlobystov et al. [30]. The inter- and intra-chain Ag…Ag interactions are fundamental in yielding coordination complexes with intriguing structural dimensionality and enhanced photoluminescence [31, 32]. Moreover, coordination of silver(I) to pyridyl N prompts withdrawal of electrons from the pyridyl ring, escalating electrostatic component of intermolecular π-π interactions in stacked pyridyl rings, often in ‘head-to-tail’ fashion [33]. Also, the phenyl rings have favorable π-π interactions whose energies are to a great extent influenced by the substituents on the phenyl ring [30]. In this work, we investigate the role of the pyridyl N and the imine N donors in (E)-N(pyridylmethylene)aniline ligands in coordinating silver(I) centers. One set of ligands has the pyridyl N in the 3-position and the other set has N in the 4-position, creating possibilities of a variety of topologies in the structures generated. We anticipated supramolecular networks of silver(I) complexes or coordination polymers with interesting argentophilic interactions, π···π interactions of phenyl and pyridyl rings as well as hydrogen bonding interactions as a means of increasing dimensionality [34] and hopefully interesting physical properties. We also sought to investigate the electronic and steric influence of the methyl and isopropyl substituents on the phenyl rings on the coordination and the structure of the anticipated silver(I) complexes or coordination polymers.

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2. Experimental 2.1. Materials and instrumentation Reagent grade chemicals were purchased from local suppliers and used without purification.


These included: 4-pyridinecarboxaldehyde 97% (Aldrich, USA), 3-pyridinecarboxaldehyde 98% (Aldrich, USA), silver trifluoroacetate 98% (Aldrich, USA), diethyl ether 99.8% (Aldrich, USA), ethanol 99.5% (Aldrich, USA), DMSO–d6 99.8% (Merck, Germany), argon gas, 5.0 technical grade (Airflex Industrial Gases, South Africa). 1H

and 13C NMR spectra were recorded on a BRUKER 400 MHz spectrometer in

DMSO-d6. 1H NMR chemical shifts are reported in parts per million (ppm) relative to the DMSO-d6 residual peak, δ = 2.5 ppm. 13C NMR chemical shifts are reported relative to the DMSO-d6 residual peak, δ = 39.5 ppm. Infrared data are reported as percentage transmittances at given wavenumbers (cm-1) between 4000 and 400 cm-1 and were recorded using a Perkin Elmer spectrum 100 FT-IR spectrometer. High and low resolution electro-spray ionization (ESI) mass spectrometry spectra were recorded using a Waters Micromass LCT Premier TOF-Ms instrument with only molecular ion peaks (M+), and major fragmentation peaks being reported with intensities quoted as percentages of the base peak. UV–Vis spectra were recorded on a Perkin Elmer Lambda 35 UV/VIS spectrophotometer in acetonitrile. The emissions were recorded on a Perkin Elmer LS55 fluorescence spectrometer. Elemental analyses were performed on a Thermal-Scientific Flash 2000 CHNS/O analyser. All melting points were determined using a Stuart Scientific melting point apparatus SMP10. 2.2. Synthesis of ligands The ligands reported here are numbered as La–d (figure 1) for the convenience of discussion. The ligands were synthesized by grinding the respective aniline with the appropriate pyridinecarboxaldehyde. Grinding for 2–10 min resulted in a paste (La, Lb and Ld) while Lc was obtained as an amber colored oil. Physical and spectral data for the known ligands [38, 73] i.e. appearance, melting points, IR, 1H and 13C NMR were in line with the ones previously reported (see Supporting Information). 2.3. Synthesis of complexes

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Complexes 1-5 were synthesized by reacting La–d with silver trifluoroacetate in anhydrous ethanol (ca. 20 ml) under dry argon. The reactions were carried out by magnetically stirring ethanol solution of the reactants for 12 h. The solvent was removed under reduced pressure; the resulting solids were each suspended in anhydrous hexane and filtered using a Buchner system.


The residue was rinsed with cold diethyl ether (10 ml x 2) and then dried in-vacuo. Crystals suitable for single crystal X-ray diffraction analysis were obtained by re-dissolving the solids in dichloromethane and slowly diffusing hexane or layering with diethyl ether. The experimental data for each of the complexes are given in Sections 2.3.1 to 2.3.5. 2.3.1. Complex 1, [AgO2C2F3La)2]. Synthesized by reaction of (E)-2,6-diisopropyl-N-(pyridin3-ylmethylene)aniline La (0.2934 g, 1.10 mmol) and silver trifluoroacetate (0.2126 g, 0.96 mmol). The lemon yellow solid obtained was washed with hexane and vacuum dried. X-ray quality single crystals were obtained by slow diffusion of hexane in dichloromethane solution of the product. Yield: 0.4170 g, 89%. Melting point: 165–167 °C. 1H NMR (400 MHz, DMSO–d6, 25 °C): δ = 1.11 (d, 3JH,H = 6.9 Hz, 24 H, CH3), 2.85 (d, 3JH,H = 6.9 Hz, H 4, CH), 7.10 (q, 3JH,H = 8.8 Hz, 3JH,H = 6.3 Hz, 2 H, Hg-C6H3), 7.16 (d, 3JH,H = 6.7 Hz, 4 H, Hf-C6H3), 7.63 (q, 3JH,H = 7.8 Hz, 3JH,H = 4.8 Hz, 2 H, Hc-C5H4N), 8.40 (dt, 3JH,H = 7.9 Hz, 3JH,H = 1.9 Hz, 2 H, Hd-C5H4N), 8.43 (s, 2 H, He-CH=N), 8.76 (dd, 3JH,H = 4.8 Hz, 3JH,H = 1.6 Hz, 2 H, Hb-C5H4N), 9.11 (s, 2 H, Ha-C5H4N) ppm. 13C NMR (400 MHz, DMSO–d6, 25 °C): δ = 23.14 (CH3), 27.43 (CH), 122.82 (C9-C6H3N), 124.21 (C4-C5H4N), 124.45 (C9-C6H3), 131.32 (C1-C5H4N), 135.38 (C5-C5H4N), 136.70 (C8-C6H3), 148.60 (C7-C6H3), 150.27 (C2-C5H4N), 152.48 (C3-C5H4N), 160.66 (C6CH=N) ppm. FT-IR: 3068 cm-1 (C-H), 2962 cm-1 (C-H), 2871 cm-1 (C-H), 1660 cm-1 (C=O), 1598 cm-1 (C=O), 1319 cm-1 (C-N), 1175 cm-1 (C-F), 1134 cm-1 (C-F). UV/Vis (CH3CN): λmax 234, 345 nm. Fluorescence (CH3CN): 1 λEX 345 nm, λEM 385 and 422 nm. MS (ESI-TOF) m/z (%): Calcd. for [AgO2C2F3.La] 373.0834; found: 373.0841 (100). Anal. Calcd. (%) for [(AgO2C2F3La)2]: C, 49.30; H, 4.55; N, 5.75, O, 6.57; found: C, 49.63; H, 4.23; N, 5.68, O, 6.77. 2.3.2. Complex 2, [AgO2C2F3La)2]. Complex 1 (0.210 g, 0.205 mmol) was dissolved in warm diethyl ether (15 ml) and solvent allowed to evaporate slowly obtaining pale yellow X-ray quality crystals after two days. Yield: 0.1651 g, 85%. Melting point: 145–146 °C. 1H NMR (400 MHz, DMSO–d6, 25°C): δ = 1.10 (d, 3JH,H = 6.9 Hz, 24 H, CH3), 2.85 (d, 3JH,H = 6.9 Hz, H 4,

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CH), 7.10 (d, 3JH,H = 6.2 Hz, 2 H, Hg-C6H3), 7.14 (d, 3JH,H = 6.6 Hz, 4 H, Hf-C6H3), 7.57 (q, 3J H,H

= 7.8 Hz, 3JH,H = 4.8 Hz, 2 H, Hc-C5H4N), 8.35 (dt, 3JH,H = 7.9 Hz, 3JH,H = 1.9 Hz, 2 H, Hd-

C5H4N), 8.41 (s, 2 H, He-CH=N), 8.74 (dd, 3JH,H = 4.8 Hz, 3JH,H = 1.6 Hz, 2 H, Hb-C5H4N), 9.08 (s, 2 H, Ha-C5H4N) ppm. 13C NMR (400 MHz, DMSO–d6, 25 °C): δ = 23.14 (CH3), 27.43 (CH),


122.83 (C9-C6H3N), 124.24 (C4-C5H4N), 124.55 (C9-C6H3), 131.40 (C1-C5H4N), 135.59 (C5-C5H4N), 136.71 (C8-C6H3), 148.57 (C7-C6H3), 150.33 (C2-C5H4N), 152.58 (C3-C5H4N), 160.64 (C6-CH=N) ppm. FT-IR: 3068 cm-1 (C-H), 2962 cm-1 (C-H), 2871 cm-1 (C-H), 1660 cm-1 (C=O), 1598 cm-1 (C=O), 1319 cm-1 (C-N), 1175 cm-1 (C-F), 1134 cm-1 (C-F). MS (ESI-TOF) m/z (%): Calcd. for [AgO2C2F3.La] 373.0834; found: 373.0841 (100). Anal. Calcd. (%) for [(AgO2C2F3La)2]: C, 49.30; H, 4.55; N, 5.75, O, 6.57; found: C, 49.63; H, 4.23; N, 5.68, O, 6.77. 2.3.3. Complex 3, [AgO2C2F3(Lb)2]. Synthesized by reaction of (E)-N-(pyridin-4ylmethylene)aniline Lb (0.3640 g, 2.20 mmol) and silver trifluoroacetate (0.2210 g, 1.00 mmol). The yellow residue obtained was dissolved in dichloromethane and precipitated using hexane. The precipitate was filtered, washed with hexane and dried in-vacuo. Diffusion of hexane into dichloromethane solution of product yielded quality X-ray single crystals within three days. Yield: 0.4278 g, 73%. Melting point: 114–115 °C. 1H NMR (400 MHz, DMSO–d6, 25 °C): δ = 7.33 (m, 6 H, Hd-, He-C6H5), 7.46 (t, 3JH,H = 15.5 Hz, 3JH,H = 8.1 Hz, 4 H, Hc C6H5), 7.91 (d, 3JH,H = 6.0 Hz, 4 H, Hb-C5H4N), 8.71 (s, 2 H, CH=N), 8.76 (d, 3JH,H = 5.8 Hz, 4 H, Ha-C5H4N). 13CNMR (400 MHz, DMSO–d6, 25 °C): δ = 121.20 (C2, C6-C5H4N), 122.45 (C8, C12-C6H5), 128.75 (C10-C6H5), 129.29 (C9, C11-C6H5), 142.83 (C1-C5H4N), 150.44 (C7-C6H5), 150.80 (C3, C4-C5H4N), 159.12 (CH=N) ppm. FT-IR: 3041 cm-1 (C-H), 1661 cm-1 (C=O), 1613 cm-1 (C=N), 486 cm-1 (C-H), 1423 cm-1 (C=O), 1328 cm-1 (C-N), 1165 cm-1 (C-F), 1113 cm-1 (C-F). UV/Vis (CH3CN): λmax 228, 260 nm. MS (ESI-TOF) m/z (%): Calcd. for [Ag(Lb)2] 471.0739; found 471.0760 (100%). Anal. Calcd. (%) for [(AgO2C2F3Lb)2]: C, 53.35; H, 3.44; N, 9.57, O, 5.47; found C, 53.49; H, 3.42; N, 9.50, O, 5.71. 2.3.4. Complex 4, [(AgO2C2F3)2(Lc)4]. Synthesized by reaction of (E)-2,6-dimethyl-N-(pyridin4-ylmethylene)aniline Lc (0.2486 g, 1.18 mmol) and silver trifluoroacetate (0.2548 g, 1.15 mmol). A yellow powder was obtained, re-dissolved in dichloromethane and layered with diethyl ether from which X-ray quality crystalline blocks were obtained after three days. Yield: 0.4227

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g, 85%. Melting point: 144–145 °C. 1H NMR: (400 MHz, DMSO–d6, 25 °C): δ = 2.06 (s, 24 H, Hf-CH3), 6.96 (t, 3JH,H = 7.5 Hz, 4 H, He-C6H3), 7.08 (d, 3JH,H = 9.7 Hz, 8 H, Hd-C6H3), 7.92 (d, 3J H,H

= 6.0 Hz, 8 H, Hb-C5H4N) 8.4 (s, 4 H, Hc-CH=N), 8.78 (d, 3JH,H = 6.0 Hz, 8 H, Ha-C5H4N)

ppm. 13C-NMR: (400 MHz, DMSO–d6, 25 °C): δ = 17.82 (C12, C13-CH3), 150.09 (C3-C5H4N),


150.86 (C7-C6H3), 151.28 (C1, C4-C5H4N), 158.27 (C6-CH=N), 162.05 (C15-O2C2F3) ppm. FTIR ʋ: 3054 (C-N) cm-1, 2921 cm-1 (C-H), 1663 cm-1 (C=O), 1640 cm-1 (C=N), 1467 cm-1 (C=O), 1322 cm-1 (C-N), 1178 cm-1 (C-F), 1133 cm-1 (C-F). UV/Vis (CH3CN): λmax 234, 345 nm. Fluorescence (CH3CN): 1 λEX 345 nm, λEM 385 and 422 nm. MS (ESI-TOF) m/z (%): Calcd. for [Ag(Lc)2] 529.1362; found: 529.1370 (100); [AgO2C2F3(Lc)2] 749.034 (70), 751.0349 (30). Anal. Calcd. (%) for [(AgO2C2F3Lc)2]: C, 44.57; H, 3.27; N, 6.50; O, 7.42; found: C, 44.78; H, 3.32; N, 6.40; O, 7.64.

2.3.5. Complex 5, [(AgO2C2F3Ld)2]. Synthesized by reaction of (E)-2,6-diisopropyl-N-(pyridin4-ylmethylene)aniline Ld (0.3100 g, 1.12 mmol) and silver trifluoroacetate (0.2210g, 1 mmol). The lemon yellow solid obtained was washed with hexane, then with cold diethyl ether and filtered. X-ray quality crystals were obtained by slow diffusion of hexane into dichloromethane solution of the solid. Yield: 0.4318 g, 87%. Melting point: 147 – 148 °C. 1H NMR (400 MHz, DMSO–d6, 25 °C: δ = 1.19 (d, 3JH,H = 6.8 Hz, 24 Hg, CH3), 2.89 (t, 3JH,H = 6.8 Hz, 4 H, Hf-iPr), 7.18 (d, 3JH,H = 6.5 Hz, 2 H, He-C6H3), 7.24 (d, 3JH,H = 6.5 Hz, 4 H, Hd-C6H3), 7.98 (dd, 3JH,H = 6.0 Hz, 3JH,H = 1.4, 4 H, Hb-C5H4N), 8.49 (s, 2 H, Hc-CH=N), 8.90 (dd, 3JH,H = 4.5 Hz, 3JH,H = 1.4 Hz, 4 H, Ha-C5H4N) ppm. 13C NMR (400 MHz, DMSO–d6, 25 °C): δ = 17.82 (C10-C6H3), 30.63 (C9-C6H3), 126.18 (C2-C5H4N), 127.70 (C7-C6H3), 142.6 (C6-C6H3), 150.86 (C5-C6H3), 151.26 (C3=C5H4N), 158.29 (C1-C4H5N), 162.05 (C4-CH=N) ppm. FT-IR: 3074 cm-1(C-H), 2963 cm-1 (C-H), 1659 cm-1 (C=O), 1609 cm-1 (C=N), 1560 cm-1 (C=O), 1461 cm-1 (C-H), 1421 cm-1 (C-H), 1320 cm-1 (C-N), 1179 cm-1 (C-F), 1136 cm-1 (C-F). UV/Vis (CH3CN): λmax 236, 350 nm. Fluorescence (CH3CN): 1 λEX 349 nm, λEM 390 and 427 nm. MS (ESI-TOF) m/z (%), Calcd. for [AgLd] 373.0834; found 373.1126 (98); [Ag(Ld)2] 639.3119. Anal. Calcd. (%) for [(AgO2C2F3Ld)2]: C, 49.30; H, 4.55; N, 5.75; O, 6.57; found: C, 49.64; H, 4.63; N, 5.70; O, 6.79. 2.4. Single-crystal X-ray diffraction Single crystals of 1-5 were each selected, glued onto the tips of glass fibers using epoxy and

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centered in the X-ray beam by the aid of a video camera. Crystal evaluation and data collection were done on a Bruker Smart APEX2 diffractometer with Mo Kα radiation (λ = 0.71073 Å) equipped with an Oxford Cryostream low temperature apparatus operating at 100(1) K. The initial cell matrix was determined from three series of scans consisting of twelve frames


collected at intervals of 0.5° in a 6° range with exposure time of ten seconds per frame. Each of the three series of scans was collected at different starting angles and indexing of the reflections done by APEXII [74] program suite. Data collection involved using omega scans of 0.5° width with an exposure time of 20 seconds per frame. The total number of images was based on results from the program COSMO [75] whereby the expected redundancy was to be 4.0 and completeness of 100% out to 0.75 Å. Cell parameters were retrieved using APEXII [74] and refined using SAINT [75] on all observed reflections. Data reduction was performed using the SAINT software and the scaling and absorption corrections were applied using SADABS [76] multi-scan technique. The structures were solved by direct methods using the SHELXS [77] program and refined using SHELXL [78]. Non-hydrogen atoms were first refined isotropically and then by anisotropic refinement with full-matrix least squares based on F2 using SHELXL [78]. All hydrogens were positioned geometrically, allowed to ride on their parent atoms and refined isotropically. The visual crystal structure information were performed using ORTEP-3 [79], MERCURY [80] and DIAMOND [81] software. In 2 one of the trifluoroacetate groups was disordered and was resolved using PART instructions and refined over two positions with the major component having an occupancy of 57%. Also, in 4 disorder was found in one of the trifluoroacetate groups which was resolved using PART instructions and refined over two positions with the major component having occupancy of 56%. A second disorder was also found in the pyridyl moieties including the imine bond with the minor component (occupying ≈ 49%) twisting away from the other by about 16.63(4). One third of a water molecule was also found and seems to play a role in the disorder. A summary of crystal data and structural refinement information is given in table 1. 3. Results and discussion 3.1. Synthesis Pyridyl Schiff base ligands La–d (figure 1) were obtained in excellent yields by single step condensation of anilines with pyridinecarboxaldehydes [35-37] through solvent-free grinding of

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the reactants for five minutes. Progress of each reaction was monitored by FT-IR while 1H and 13C

nuclear magnetic resonance spectroscopy were used to confirm product formation. The

synthesis method employed is more efficient, faster and higher yielding in comparison to other conventional methods for the synthesis of pyridyl Schiff bases [38-44]. Lc was obtained as a


yellow oil whereas the rest of pyridyl Schiff bases were obtained as off-white solids. Complexes 1-5 were prepared by mixing La–d with silver(I) trifluoroacetate in ethanol at room temperature. The complexes were isolated in good to excellent yields (73–89%) as crystalline yellow solids and their molecular structures established using microanalysis, NMR, IR and UV spectroscopic techniques. 3.2. Spectroscopic and analytical studies All infrared spectra of the complexes show characteristic stretching bands for the carboxylate of the trifluoroacetate anion synonymous with the nature of anion coordination. In 1 the asymmetric and symmetric stretching bands are 1660 and 1598 cm-1 and Δν between νOCO(asym) and νOCO(sym) is 62 cm-1. Complex 2 has bands at 1671 and 1437 cm-1 with a Δν of 234 cm-1. In 2 coordination of the anion is very weak as confirmed by single crystal X-ray diffraction studies. Complexes 3 and 4 have asymmetric and symmetric stretching bands at 1661 and 1423 cm-1 and at 1663 and νOCO(sym) 1417 cm-1, respectively. In both complexes the Δνs are 238 cm-1 and 246 cm-1, indicating a disproportionate coordination of the trifluoroacetate [9]. Complex 5 shows asymmetric (νOCO) and symmetric (νOCO) stretches at 1659 and 1560 cm-1, respectively, where the Δν is 99 cm-1. The Δν in 1 and 5 are consistent with short bridging of the trifluoroacetate anions [45]. In 5, the C=N stretching frequency (1659 cm-1) is blue shifted relative to that of free ligand (1637 cm-1) and this could be attributed to coordination of the imine N [45], a postulate confirmed by single X-ray diffraction (Section 3.3). The 1H NMR spectra of 1-5 in deuterated DMSO show differences from those of La-d, consistent with coordination to silver(I) observed especially with the chemical shifts of the protons in the vicinity of N. Generally, when imine N atoms are not involved in coordination to Ag(I) as in 2-4, they resonate relatively at the same positions as in the free ligands. On the other hand, when they are coordinated as in 5, the sp2 imine protons resonate further downfield when compared to those of the free ligands. The alpha protons on the carbons bonded to pyridyl N show chemical shifts at 9.11, 9.08, 8.76, 8.78 and 8.90 ppm in 1-5, respectively, shifted further

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downfield by ca. 0.18 ppm when compared to those of the corresponding metal free ligands. High resolution mass spectra of 1-5 showed peaks at 373.08, 639.26, 471.08, 529.14 and 373.11, respectively, as the most intense peaks corresponding to [Ag(L)]+ for 1 and 5 and [Ag(L)2]+ for 2-4, indicating that in solution the complexes are mononuclear. In the solid state,


all five complexes are stable in air. The complexes are highly soluble in polar solvents and moderately polar solvents. From X-ray crystallographic studies, the complexes are di- or multinuclear. The composition of bulk crystalline samples of 1-5 was further confirmed by elemental analysis and correspond to their solid-state structures. 3.3. Crystallographic analysis of 1-5 Crystals suitable for structural analysis of 1, 3 and 5 were obtained by diffusion of hexane into dichloromethane solution of the complexes. Crystals for 4 were obtained by layering diethyl ether onto dichloromethane solution of the complex while those of 2 were obtained from hot diethyl ether solution of 1. This probably is an indication that there is an initial formation of linear mononuclear species of all five complexes which depending the crystallization process results in multinuclear complexes as observed in 1 and 3-5. Figures 3-7 show the molecular structures for all five complexes. While 1 and 5 are coordination polymers, 2-4 are discrete complexes. In 1-4, coordination of ligands is monodentate through the pyridyl N only, akin to what is described elsewhere [46-48], while in 5 the ligand coordinates through both pyridyl and imine N [49, 50]. 3.3.1. The role of the trifluoroacetate and the ligand conformation in the molecular arrangement in 1-5. The highly-flexible trifluoroacetate anions also act as auxiliary ligands and contribute to the various coordination modes observed in all five complexes (figure 2) [51-54]. In 1, two trifluoroacetates bridge two silver(I) centers in a µ2-κ2O:O' fashion and forms a sixmember metallacycle, (O—C—O—Ag)2, figure 2(i). In 2 the trifluoroacetates bridge two Ag(I) centers to form a four-member metallacycle, (O—Ag)2, through a single O in a µ2-κO fashion, figure 2(ii). In 3, the anion appears to interact remotely through a single O with the silver(I) center which connects two ligands via the pyridyl Npy in a linear fashion (figure 2(iii)). In 4, the trifluoroacetate anions bridge two silver(I) centers in a µ2-κO fashion through a single O to form a four-member metallacycle ((O—Ag)2) (figure 2(ii)), similar to 2, an arrangement also reported

9

in the literature. The trifluoroacetates play a more interesting role in coordination polymer 5 where O4 forms a four-member metallacycle (Ag2-O4-Ag2-O4) with different Ag—O bond distances (table 2) in a µ2-κO fashion while O1 and O2 bridge the two silver(I) centers in a µ2κ2O:O' fashion to form two five-member metallacycles at approximate right angles (table 2).


Two sets of the five-member metallacycles are connected by the four-member ring and form what could be described as a “paddle mixer” (figure 2(iv)). The Ag—O bond lengths within the dinuclear units range from to 2.226(5) to 2.479(4) Å while those bridging the dinuclear units are 2.592(4) Å (table 2), which is consistent with those reported for similar complexes with bridging trifluoroacetates. The shortest Ag–O bonds are observed in 1 and 5, where they are involved in short bridging coordination of Ag(I) centers in tetrahedral geometry. In the asymmetric unit of 4, where the Ag(I) is in a distorted trigonal geometry, the Ag–O bond distance is 2.377(3) Å. In the asymmetric units of 2 and 3, the silver centers are in T-shaped geometry and the Ag–O bond lengths are 2.5592(16) and 2.555(2) Å, respectively. 3.3.2. Molecular structures of 1-5. The asymmetric unit of 1 consists of one (E)-2,6diisopropyl-N-(pyridylmethylene)aniline molecule, one trifluoroacetate and one silver(I). Selected bond distances and angles for 1-5 are given in table 3. The silver(I) is coordinated to two centrosymmetrically related ligands through a pyridyl N on one and through C of an arene ring in the other, making it an organometallic complex. The silver(I) is further coordinated to two oxygens from two, also centrosymmetrically related anions completing distorted tetrahedral coordination. The angles around the silver(I) range from 93.83(10) and 130.83(11). The second set of oxygens coordinate to a silver(I) in the adjacent asymmetric unit completing the µ2-κ2O:O' bridge. The Ag…Ag distance between adjacent silver(I) centers that are within the metallacycle is 4.250 Å, slightly long to be considered argentophilic. This arrangement results in an infinite 1D polymeric chain shown in figure 3b. In 1, the Ag—N bond distance is 2.234(3) Å, similar to others involving pyridyl N donors [55]. The Ag—C, 2.586(4) Å, is significantly smaller than the sum of van der Waals radii of C and Ag (3.42 Å) [56] and also lies perfectly within 2.36 to 2.77 Å limits set for silver(I)-arene ligated (Ag—η1 distance) complexes [57-59]. The two Ag—O bond distances from the bridging trifluoroacetate are 2.262(3) and 2.491(3) Å. The polymeric chains described above run diagonally across the ab face (figure 3c).

10

The asymmetric unit of 2 consists of a neutral species of the complex with two (E)-2,6diisopropyl-N-(pyridin-3-ylmethylene)aniline ligands, a Ag(I) and a trifluoroacetate (figure 4a) which develops into a centrosymmetric dimer of four (E)-2,6-diisopropyl-N-(pyridin-4ylmethylene)aniline ligands and two silver trifluoroacetate molecules in the full molecule


(figure 4b). Each silver(I) coordinates to two (E)-2,6-diisopropyl-N-(pyridin-3ylmethylene)aniline ligands through the pyridyl N in a linear fashion. The trifluoroacetate coordinates weakly to silver(I) via O1, with a Ag–O distance of 2.559(2) Å. The dimer is generated by a center of inversion and shows ligand supported argentophilic interaction (figure 4b) where the Ag…Ag distance is 3.0475(3) Å, a distance lower than the sum of van der Waals distance of two silver(I) ions. The O1 interacts weakly with the second silver(I) center in the dimer (Ag–O = 2.770(2) Å), hence helping stabilize the dimer (figure 4b). Silver(I) is in an ‘inverted seesaw’ geometry given the Npy—Ag—Npy angle of 163.25(7)° (figure 4b), which is comparable to dimeric silver(I) complexes previously reported [60, 61]. The asymmetric unit of 3 consists of two (E)-N-(pyridin-4-ylmethylene)aniline ligands, one silver(I) and a trifluoroacetate. The Ag(I) is coordinated to two pyridyl nitrogens from two N-(pyridin-4-ylmethylene)aniline ligands in a linear fashion like in 2. The Npy—Ag—Npy bond angle in 3 is 169.14(8). In 3, the acetate weakly coordinates to silver(I) via one O (Ag—O = 2.555(2) Å), resulting in T-shaped geometry [25, 62-65] (figure 5). In both 2 and 3, the Ag—N bond distances are 2.1600(2) – 2.1976(17) Å while the Ag—O bond distances lie between 2.555(2) and 2.770(2) Å (table 2), confirming moderate anion coordination and the influence on geometries around Ag(I). In 3, where the Ag—O bond is shorter, the N—Ag—N deviates more significantly from linearity than in 2. The Ag—O bond distances are significantly longer than the Ag—N bond distances, indicating the relative weaker coordination by trifluoroacetate, which is not unusual. The asymmetric unit of 4 comprises half of the complex i.e. two 2,6-dimethyl-N-(pyridi4-ylmethylene)aniline ligands, one trifluoroacetate and one Ag(I) in a trigonal geometry, where the coordination sites are occupied by two Npy atoms and one O atom, figure 6a. The N—Ag—N bond angle is 139.48(12)° deviating significantly from linearity courtesy of the short Ag—O bonds. The other half of the complex is completed through a center of inversion lying at the center of an O2Ag2 metallacycle (figure 6b). The geometry around the Ag(I) centers is distorted tetrahedral with two Npy atoms and two bridging oxygens occupying the coordination sites The

11

angles around silver(I) range from 139.43(12) to 85.20(10). In 4, the Ag—N bond distances are 2.214(3) and 2.258(3) Å while the Ag—O bond distance is 2.376 Å; all lie in the normal range for such coordination [63, 66-68]. The asymmetric unit of 5 has two monodentate (E)-2,6-diisopropyl-N-(pyridin-4-


ylmethylene)aniline ligands, two different Ag(I) centers and two trifluoroacetates. Ag1 is coordinated to a pyridyl N from the first ligand, an imine N from the second ligand and two oxygens from two different trifluoroacetate. The two O atoms and the pyridyl N seem to be on a distorted trigonal plane around Ag1 with angles around the Ag center being 145(16), 118.72(14) and 94.59(15). The imine N4 coordinates to the same Ag(I) center to complete a distorted trigonal bipyramid geometry with angles between N4 and the three atoms on the trigonal plane through Ag1 being 98.75(14), 89.08(14) and 91.83(14). Ag2 is coordinated to three oxygens and the imine N. The imine N and one O coordinate in a linear fashion to Ag2 (N3—Ag2—O2 = 161.63(16)). The other two O atoms, both O4 with different bond distances to Ag2 (2.592(4) and 2.480(4) Å), are at an angle of 83.57(13) with the symmetry relation between them being -x + 1, -y + 1, -z + 1. They all complete a geometry that can best be described as distorted seesaw (see table 2 for the rest of the angles around Ag2). In the arrangement, the two Ag(I) centers are in close proximity with an argentophilic distance of 3.0807(5) Å. If this argentophilic interaction is included, we have distorted trigonal bipyramid coordination geometry around Ag1 and square pyramidal coordination geometry around Ag2. The bridging by the trifluoroacetate anions best described as a paddle mixer, together with the ligands, yields an infinite 1-D coordination polymer that runs along the crystallographic a axis (figure 7b), several of which are connected through a series of weak C—H…Ag and face-edge C—H…π interactions to yield a 2-D network. A view of the packing diagram of 5 down the crystallographic a axis reveals large voids (figure 7c). This kind of coordination by the trifluoroacetate anion has been reported elsewhere in silver(I) trifluoroacetate with tridentate N'NN'-ligands [69]. The Ag—Npy bond distances are 2.201(5) and 2.253(4) Å while the Ag—Nim distance is 2.562(4) Å, longer than Ag—Npy. These bond distances are similar to those of related silver(I) coordination polymers. The Ag—O bond distances are longer than all the Ag—N distances, indicating weak coordination ability of O from trifluoroacetate anion in comparison to nitrogen coordination. The independent silver(I) centers in the asymmetric unit of 5 show significant argentophilic interactions supported by trifluoroacetate with Ag(1)…Ag(2) bond 12

length of 3.0807(5) Å, comparable to similar ligand supported Ag…Ag interactions [69]. The pyridyl and phenyl rings in 1-5 form different planes that are nonplanar. In 1, the pyridyl ring including imine carbon form one plane while the phenyl ring including the imine carbon form another plane where the dihedral angle between the two planes is 48.24(2). In 2,


the dihedral angle between equivalent planes is 56.317(5) in one molecule and 85.586(5) in the second molecule. Similarly, in 3, the dihedral angles are 41.327(6) and 33.438(8), in 4, 88.65(2) and 67.66(1), and 62.32(7) in 5. It is apparent that the substituents on the phenyl rings contribute to the orientation of the phenyl and pyridyl ring to each other. In 5, where there is no substitution on the ligand, the dihedral angles are smaller compared to the other complexes where the phenyl ring is substituted. The pyridyl rings directly coordinated to the same Ag(I) are twisted from each other at an angle of 16.041(6)° in 2, 9.954(2)° in 3 and 62.304(5) in 4. 3.3.3. Intermolecular interactions in 2, 3 and 5. Crystals of 2 contain intramolecular C—H…O hydrogen bonds between the uncoordinated O of trifluoroacetate and pyridyl H (2.478 Å, 159.08 and 2.704 Å, 130.91) that help stabilize the dimer. In the crystal of 3 there are significant intramolecular π…π and C—H…π interactions between pyridine rings and the pyridine and an aromatic proton of the phenyl ring, respectively (figure 8). In these interactions the centroid-centroid distance between pyridyl rings is 3.973 Å, while the C—H…π is 3.268 Å and has a C—H…π angle of 155.16. This complex also shows hydrogen bonds involving a pyridyl H and trifruoroacetate O, C—H…O (2.428 Å 164.7 and 2.500 Å, 131.6), that help stabilize the packing (figure 8). Adjacent chains are joined by C—H…F interactions (2.614 Å, 133.56) and C—H…O (2.521 Å, 165.08) interactions involving the pyridyl ring facilitating the formation of a 2-D wavelike structure. Similar wavelike architectures have been reported in other silver(I) complexes with N-donors [66]. In the crystal of 5 there are C—H…O interactions between the uncoordinated O and phenyl H (2.662 Å, 124.20) and (2.705 Å, 122.43) within the 1-D polymeric chain. This network is further stabilized by C—H…π (2.497 Å) and π…π (3.365(9) Å) interactions. The C— H…π interactions (2.723 Å, 149.62) between adjacent chains generate a 2-D network. Also, H…π (2.497 Å) involving pyridyl H and phenyl rings stabilize the 2-D supramolecular structure. These 2-D networks are joined by multiple hydrogen bonds ranging from 2.656 to 2.723 Å to

13

generate a 3-D crystal structure. 3.4. Photophysical studies The electronic absorption spectra for all the free ligands and their complexes were recorded in


acetonitrile. The UV-Vis absorption spectra of 1, 3 and 4 and their corresponding ligands are shown in figure 9. The ligands show two absorption bands between 230 and 240 nm and between 310 and 351 nm. These bands can be assigned to π-π* in the phenyl and pyridyl rings and n-π* transitions in the pyridyl and imine functions, respectively. The complexes show similar absorption patterns albeit with marginal blue shift in the absorption maxima. The absorption bands in the complexes can be assigned to intra-ligand n-π* and π-π* and/or metal to ligand charge transfer transitions (MLCT) since no d-d transitions are expected for d10 silver(I) complexes [32, 70]. The absorption patterns of all the complexes are similar having two absorption bands at 330-350 nm, which may be attributed to the n-π* transition and 230-240 nm allied to π-π* transition in the ligand. This observation indicates that the absorption pattern of the ligand is independent of complex formation. The UV-Vis absorption involving the n-π* transitions in the complexes are red shifted when compared to those of the free ligands, a trend reported elsewhere for silver(I) complexes [71]. The red shifts could be attributed to decrease in energy of the excited states upon coordination to silver(I) [72]. Complexes 1, 4 and 5 show two emission bands in the UV-Vis region as shown in figure 10. The emissions are assignable to ligand-ligand charge transfer of the complexes. As observed in figure 10(c), 4 has two emission bands whereas its ligand has only one emission band. The emission bands associated with π-π* in the complexes are red shifted compared to those of free ligands. This is also attributable to the lower energy in excited states of the complexes thus the longer wavelengths [72]. 4. Conclusion Silver(I) coordination complexes of 3- and 4-pyridyl ligands were synthesized and characterized using NMR, IR, MS and elemental analysis. The structures of the complexes were determined by single-crystal X-ray diffraction. Three discrete complexes, 2-4, and two coordination polymers, 1 and 5, were obtained. The silver(I) in 1 and 4 are all four-coordinate in distorted tetrahedral geometry. Complex 2 is four-coordinate too but in ‘inverted see saw’ geometry, while 3 has

14

silver(I) in a T-shaped geometry. Complex 5 has two types of geometries around silver(I), with Ag1 in distorted trigonal bipyramid environment and Ag2 in square pyramidal geometry. Complex 1 is an organometallic 1-D coordination polymer. The 1-D chains in 1 form a 2-D supramolecular structure generated by various non classical interactions between adjacent


chains. Complex 5 on the other hand is a linear coordination polymer where the infinite chains form 2-D polymeric structures through non-classical interactions; the 2-D polymeric sheets are joined by multiple π-π interactions and hydrogen-bonding, resulting in a 3-D multi-layered architecture. Complexes 2, 3 and 4 are discrete complexes with supramolecular structures generated by π-π interactions and hydrogen bonding. From this study, it follows that substitutions on the organic skeleton of the ligands influence the architecture of the complexes. The ability of the trifluoroacetate to have numerous coordination modes aided in tuning the structure of the complexes and controlling the geometry of silver(I). The complexes show stable photoluminescence at room temperature, a rare observation since most silver(I) complexes are known to emit at low temperatures. Appendix A: Supplementary material CCDC numbers 1500190–1500194 contain the supplementary crystallographic data for 1-5, respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK. Fax: +44 1223 336033). Acknowledgements The authors wish to express their gratitude to the University of KwaZulu-Natal and National Research Foundation South Africa for the financial support. Authors acknowledge the assistance of Ms. Unathi Bongoza and Mr. Sizwe J. Zamisa in single-crystal XRD experiments. References [1]

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Figure captions Figure 1. The molecular structures of La-d.


Figure 2. The diverse coordination modes of trifluoroacetate observed in 1-5. Figure 3. (a) The asymmetric unit of 1. All hydrogens have been omitted for clarity and ellipsoids are drawn at 30% probability; (b) a perspective of 1 showing the 1D coordination polymer and (c) packing diagram of 1 as viewed down the crystallographic c axis (the symmetry elements used to generate the coordination polymer are -x + 2, -y, -z and –x + 1, -y + 1, -z). Figure 4. ORTEP diagram for (a) asymmetric unit of 2 and (b) the dimeric structure of 2 showing argentophilic interaction. All hydrogens have been omitted for clarity and ellipsoids are drawn at 50% probability (symmetry elements used to generate equivalent atoms: (i) = 2 - x, 1 - y, -z). Figure 5. The asymmetric unit of 3. All hydrogens have been omitted for clarity and ellipsoids are drawn at 50% probability. Figure 6. (a) The asymmetric unit of 4. All hydrogens have been omitted for clarity and ellipsoids are drawn at 50% probability, (b) the dimeric structure of 4 showing the anion bridging two Ag(I) centers in a µ2-κO fashion and also the disorder in one of the 2,6-dimethyl-N-(pyridin4-ylmethylene)aniline (symmetry used to generate equivalent atoms: –x + 2, -y + 1, -z + 1). Figure 7. (a) The asymmetric unit of coordination polymer 5. (b) A section of the coordination chain in 5. (c) The packing diagram of the coordination polymer 5 showing the solvent voids. All hydrogens have been omitted for clarity and ellipsoids are drawn at 50% probability (symmetry elements used to generate equivalent atoms: (i) -x,-y+2,-z+1, (ii) -x+1,-y+2,-z+1). Figure 8. The C—H…π in the phenyl rings, C—H…O hydrogen bonds and pyridyl π…π in 3. Figure 9. UV-Vis absorbance spectra for La-d and their complexes; (a) La (λmax 240, 339 nm) and 1 (λmax 234, 345 nm), (b) Lb (λmax 229, 258 nm) and 3 (λmax 228, 260 nm), (c) Lc (λmax 234, 310) and 4 (λmax 234, 345 nm), (d) Ld (λmax 234, 349 nm) and 5 (λmax 236, 350 nm). Figure 10. Emission spectra for complexes and their corresponding free ligands (a) La λEX 339 nm, λEM 379 and 428 nm, 1 λEX 345 nm, λEM 385 and 422 nm, (b) Lc λEX 310 nm, λEM 419 nm, 4 λEX 345 nm, λEM 385 and 427 nm, and (c) Ld λEX 351 nm, λEM 393 and 428 nm, 5 λEX 349 nm, λEM 390 and 427 nm.

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Graphical abstract

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Table 1. Crystal data and structure refinement for 1-5.

Empirical formula Formula weight Temperature (K) Wavelength (Å Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å)

 (°)  (°)  (°) Å3

V Z ρcalc / g cm-3 µ / mm-1 F(000) Crystal size (mm3) Theta range for data collection /° Index ranges

Reflections collected Independent reflections Data / restraints / parameters Goodness-of-fit on F2 Final R indices [I>2sigma(I)]

1

2

3

4

5

C20H21AgF3N2O2 486.26 173(2) 0.71073 Triclinic P1

C74H22Ag2F6N8O4 376.82 173(2) 0.71073 Monoclinic P21/n

C52H40Ag2F6N8O4 1170.66 173(2) 0.71073 Monoclinic P21/c

C60H56Ag2F6N8O4 641.43 173(2) 0.71073 Triclinic P1

C20H22AgF3N2O2 487.26 173(2) 0.71073 Triclinic P1

9.5415(4) 10.8520(4) 11.0638(4) 67.685(2)

9.9568(1) 24.8232(4) 14.6503(2) 90

14.1700(5) 21.0120(8) 8.0759(3) 90

8.424(5) 12.402(5) 14.519(5) 68.518(5)

10.7085(6) 15.0773(8) 15.5307(9) 87.364(3)

81.911(2)

90.276(1)

96.308(2)

83.418(5)

71.529(3)

88.576(2)

90

90

81.456(5)

85.576(3)

1048.75(7) 2 1.540 1.003 490 0.23×0.13×0.05 2.01 to 28.54

3620.92(8) 8 1.381 0.609 1560 0.31×0.27×0.22 1.61 to 24.44

2390.0(2) 2 1.627 0.899 1176 0.42×0.28×0.25 1.74 to 28.30

1392.8(11) 2 1.529 0.778 652 0.38×0.31×0.23 1.78 to 28.50

2370.6(2) 2 1.360 0.888 976 0.46×0.20×0.18 1.38 to 28.46

-12≤h≤12 -14≤k≤14 -14≤l≤14 25601 5250 [R(int) = 0.0240] 5250 / 0 / 257 1.257 0.0535, 0.1505

-13≤h≤12 -33≤k≤32 -19≤l≤19 52525 8723 [R(int) = 0.0182] 8723 / 0 / 469 1.096 0.0286, 0.0756

-18≤h≤18 -27≤k≤27 -10≤l≤10 55356 5922 [R(int) = 0.0398] 5922 / 0 / 325 1.047 0.0367, 0.0848

--11≤h≤11 -16≤k≤16 -16≤l≤19 27969 6970 [R(int) = 0.0178] 6970 / 15 / 457 1.400 0.0419, 0.1193

-14≤h≤14 -20≤k≤20 -20≤l≤20 67284 11757 [R(int) = 0.0864] 11757 / 0 / 513 1.102 0.0653, 0.2000

All refined by full-matrix least-squares on F2

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Table 2. The Ag–O bond distances and angles in 1-5. Parameters

1

2

3

4

5

2.262(3) 2.491(3)

2.5592(16)

2.555(2)

2.377(3) 2.587(3)

2.243(4) 2.405(4) 2.226(4) 2.480(4) 2.592(4) 2.592(4)

-

85.20(10)

94.35(15) 93.34(14) 83.57(13)

Bond distances O–Ag


Bond angles O–Ag–O

103.55(12)

-

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Table 3. Selected bond lengths and angles in 1-5. Parameters

1

2

3

4

5

2.234(3)

2.1772(14)

2.160(2)

2.215(3)

2.253(4)

2.1817(14)

2.175(2)

2.259(3)

2.201(5)

-

-

2.562(4)

-

-

-

3.0807(5)

123.72(10)

94.59(15)

95.58(11)

161.63(16)

96.74(11)

103.84(15)

Bond distances


Npy—Ag Nim—Ag

-

-

C—Ag

2.586(4)

-

Ag—Ag

3.0475(3)

3.0538(3)

130.83(11)

89.70(5)

93.83(10)

97.61(5)

-

Bond angles Npy—Ag—O

123.72(10) 96.74(11) 90.35(11)

91.22(15)

Npy—Ag—Npy

-

163.17(6)

95.58(11) 169.14(8)

Npy—Ag—C

112.82(13)

-

-

-

O—Ag—C

111.09(13)

-

-

-

-

-

-

139.48(12)

-

94.75(11) O—Ag—Npy

-

91.83(14) 89.08(14)

Npy—Ag—Nim

-

-

-

-

98.75(14)

Symmetry transformations used to generate equivalent atoms: 1 #1 -x+2,-y,-z; 2 #1 -x+2,-y+1,-z; 4 #1 -x+2,-y+1,-z+1; 5 #1 -x,-y+2,-z+1

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