Triflate anion and ligand influences in silver(I) coordination polymers of four isomeric dipyridyl ketone oximes

Abstract

The synthesis and X-ray characterisation of four isomeric dipyridyl ketone oxime ligands, 4,4′-dipyridyl ketone oxime, L⁴⁴, 3,3′-dipyridyl ketone oxime, L³³, 2,4-dipyridyl ketone oxime, L²⁴ and 2,3-dipyridyl ketone oxime, L²³ is described. X-ray structural characterisation of the precursor ketones 3,3′-dipyridyl ketone, L1, and 2,4-dipyridyl ketone, L2, is also reported for comparison. Four AgCF₃SO₃ complexes of the dipyridyl ketone oxime ligands were prepared and structurally characterised, [Ag(L⁴⁴)₂](CF₃SO₃), 1, {[AgL³³](CF₃SO₃)}_∞, 2, [AgL²⁴(CF₃SO₃)]_∞, 3, and [Ag₂L²³(CF₃SO₃)₂]_∞, 4. The structural role of the CF₃SO₃⁻ anion in the formation of Ag(I) coordination polymer networks, together with the effect of the varying pyridyl substitution patterns of L⁴⁴, L³³, L²⁴ and L²³ was investigated. In 1 the CF₃SO₃⁻ anion was unbound and disordered. It was encapsulated in the channels of a honeycomb 3D H-bonded structure. In 2 the CF₃SO₃⁻ anion remained unbound, being encapsulated within the centre of a helical polymer chain. In 3 the CF₃SO₃⁻ anion was weakly bound to the Ag(I) ion and as such decorated the side of a bowed 1-D chain. The two different CF₃SO₃⁻ anions in 4 adopted a range of binding modes from monodentate, bis-monodentate to tris-monodentate with the Ag(I) ions and gave rise to the formation of an unusual CF₃SO₃⁻ bridged 2-D network.

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Introduction

The chemistry of extended network structures and coordination polymers is an extremely topical and active area of research.^1–4 For example, the area of 1D coordination polymers has been thoroughly reviewed recently.⁵ Judicious choice of organic-linker ligands and transition metal ion nodes has allowed the engineering of a wide variety of coordination networks in recent years.^6–10 Thus the main focus has largely been on the binding arrangements of the bridging ligand entities and their interactions with the metal ions. However, the balance between the formations of different coordination networks is often subtle, such that metal-to-ligand ratio,^11–13 anion-based interactions^{11,12,14–24} and solvent effects^{11,12,14,16,17,19,20,22} can all have an influence on the final structural outcomes. While not unimportant the structural role of a specific counter anion is often of secondary consideration as many studies have probed the comparative effects of a range of different counterions.^20,25–32 In part this is due to the fact that many anions are deliberately selected for their “non-coordinating” ability and it is hoped that their necessary incorporation into a coordination complex will have limited consequences for the final array.³³ Thus understanding the role played by a specific counterion, especially one which has varying binding abilities and differing binding modes, in concert with subtle changes in the binding arrangements of the ligand entities is of importance in gaining an insight into the formation of network arrays.^34,35

There have been numerous studies of the coordinating ability of polyatomic monoanions.^{26,27,36–38} While the results differ slightly depending on the systems in question the following generalisation can be made, that NO₃⁻ and CF₃CO₂⁻ are coordinating anions while ClO₄⁻, BF₄⁻ and PF₆⁻ are non-coordinating anions. The coordinating ability of the CF₃SO₃⁻ anion is ambiguous as it appears to sit somewhere in the middle of the series. This gives rise to its varying coordination abilities^39–42 and hence its interest in this study. Ag(I) is an ideal metal ion in this context and Ag-coordination polymers are prominent in early investigations of coordination polymers.^43–46 A key property is that the Ag(I) ion's d¹⁰ configuration imposes few structural requirements on the surrounding ligand and anion entities. A search of the Cambridge Structural Database (CSD) (version 5.33),^47,48 provided details of typical Ag(I) cation and CF₃SO₃⁻ anion interactions. Table 1 displays the mean values and range of bond distances for different types of Ag(I)–OSO₂CF₃ interactions. From these results it can be concluded that while monodentate bonding interactions between Ag(I) cations and CF₃SO₃⁻ anions were relatively common, bridging through a single O atom was relatively rare.⁴⁹ Based on the results in Table 1 a convenient and slightly generous cut-off limit for Ag–O bonding interactions would seem to be 2.7 Å (vide infra).

Table 1 Mean values and range for selected Ag(I)–O bond distances involving the CF₃SO₃⁻ anion

Bond/interaction types	Complexes	Observations	Mean/Å	Range/Å
a Bonding arrangement involves two different monodentate O donors from the same CF3SO3^−. b Bonding arrangement involves three different monodentate O donors from the same CF3SO3^−. c Bonding arrangement involves two different O donors from the same CF3SO3^−.
Ag(I)–O	230	303	2.482	2.140–2.960
2 × Ag(I)–O^a	77	186	2.457	2.204–2.917
3 × Ag(I)–O^b	8	30	2.438	2.207–2.630
Ag(I)–O–Ag(I)	13	40	2.444	2.251–2.799
Ag(I)–O–Ag(I) and Ag(I)–O^c	6	21	2.482	2.317–2.629

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In our study of the role of the CF₃SO₃⁻ anion in association with the subtle changes that different binding arrangements of ligand entities bring about, we chose to investigate the reaction of AgCF₃SO₃ with four isomeric dipyridyl ketone oxime ligands, namely 4,4′-, 3,3′-, 2,4- and 2,3-dipyridyl ketone oxime (L⁴⁴, L³³, L²⁴, and L²³, respectively). These ligands consisted of two pyridyl rings of varying substitution, bound to a central oxime {C=N–OH} moiety and are structural isomers of the commercially available 2,2′-dipyridyl ketone oxime. While this latter ligand has a rich coordination chemistry with a variety of metal ions, including Ag(I),⁵⁰ the metal chemistry of its isomers is non-existent.

The oxime functional group is a very versatile group capable of binding to metal ions as well as forming a variety of H-bonding interactions, which may be useful in extending coordination polymer arrays.⁵⁰ Similarly, the pyridyl rings are capable of binding to metal ions and also participating in a variety of H-bonding interactions, for example (pyridine)N⋯H–O(oxime). Within the structure of each ligand there is a degree of conformational freedom about the C_py–C_ox (py = pyridine; ox = oxime) bond. This feature allows the formation of two-bladed chiral propellers in the solid state, whereby each ring is twisted relative to each other by as much as 90°.⁵¹ As such, chirality can, in principle, be introduced into the resulting complexes. In the case of both the 2,3- and 2,4-diypridyl oxime, this degree of conformational freedom can be somewhat constrained by the formation of a five-membered chelate ring between the 2-substituted pyridyl ring and the N atom of the oxime moiety. However, the formation of a chelate ring provides a strong anchor from which to build extended arrays and provides an elegant example of the effect of varying substitution patterns within a ligand series.

Herein we report the synthesis and characterisation of four isomeric dipyridyl ketone oxime ligands L⁴⁴, L³³, L²⁴ and L²³ and their AgCF₃SO₃ coordination polymers. X-ray structural analysis showed the differing role played by the CF₃SO₃⁻ anion in these coordination polymers. In [Ag(L⁴⁴)₂](CF₃SO₃), 1 the CF₃SO₃⁻ anion was unbound and disordered. It was encapsulated in the channels of a honeycomb 3D H-bonded network. In {[AgL³³](CF₃SO₃)}_∞, 2 the CF₃SO₃⁻ anion was unbound and encapsulated within a helical polymer chain while in [AgL²⁴(CF₃SO₃)]_∞, 3 the CF₃SO₃⁻ anion was weakly bound to the Ag(I) ion and decorated the side of an L-shaped 1-D chain. The complex [Ag₂L²³(CF₃SO₃)₂]_∞, 4 formed an unusual CF₃SO₃⁻ bridged 2-D network.

Experimental section

General

Commercially available 4,4′-dipyridyl ketone was acquired from Chem Bridge and all other precursors were obtained from Sigma Aldrich. Methyl nicotinate was dried by azeotropic distillation using toluene. Methyl isonicotinate was dried by vacuum distillation at 92 °C and 30 mm Hg. Diethyl ether and THF were obtained from a Pure Solv MD-6 solvent purification system. All other reagents were used without purification. Column chromatography was performed using silica gel. Elemental microanalyses were carried out at the Campbell Microanalytical Laboratory, University of Otago. All ¹H, ¹³C and two-dimensional NMR spectra were measured on a 500 MHz Varian UNITYINOVA spectrometer at 298 K. NMR spectra were collected in DMSO and referenced to the solvent signal. High resolution mass spectrometry was recorded on a Bruker micrOTOF-Q spectrometer. Data were presented as m/z values for the parent molecular ion. Infrared (IR) spectra were measured with either a Perkin Elmer Spectrum BX FT-IR System using KBr disks or a Bruker Optics Alpha FT-IR spectrometer (with a diamond Attenuated Total Reflectance (ATR) top-plate).

Syntheses of ligands

The procedure for all four ligands L⁴⁴, L³³, L²⁴ and L²³ was modified from that described by Blicke and Maxwell.⁵²

4,4′-Dipyridyl ketone oxime, L⁴⁴

Hydroxylamine hydrochloride (475 mg, 6.83 mmol) dissolved in water (10 mL) was added to 4,4′-dipyridyl ketone (311 mg, 1.69 mmol) dissolved in ethanol (5 mL). NaOH (276 mg, 6.91 mmol) was slowly added to the solution with vigorous stirring at 0 °C. After 10 min the solution was warmed to room temperature and heated at reflux overnight. The solution was cooled to room temperature, filtered and washed with water to give the product as a white solid (306 mg, 91%). Anal. found: C, 66.39; H, 4.71; N, 20.24. Calc. for C₁₁H₉N₃O: C, 66.32; H, 4.55; N, 21.09. ¹H NMR (500 MHz, DMSO): δ 12.21 (s, 1H, OH), 8.72 (dd, J = 2.0, 4.5 Hz, 2H, H2/6 or H9/13), 8.59 (dd, J = 2.0, 4.5 Hz, 2H, H2/6 or H9/13), 7.34 (ddd, J = 2.0, 4.5, 4.5, 4H, H3, H5, H10, H12). ¹³C NMR (500 MHz, DMSO): δ 151.667 (C7), 150.077 (C6/2 or C9/13) 149.940 (C6/2 or C9/13), 142.314 (C4 or 11), 139.852 (C4 or C11), 123.566 (C3/5 or C10/12), 120.762 (C3/5 or C10/12). ESMS (+ve, ESI) m/z calc. for C₁₁H₁₀N₃O 200 [M + H⁺], found 200. Selected IR/cm⁻¹ 1847 (m, br), 1596 (m, sh), 1410 (m, sh), 1011 (s, sh). Colourless crystals of L⁴⁴·2H₂O suitable for X-ray determination were grown by slow evaporation a 5 : 1 solution of H₂O : MeOH.

3,3′-Dipyridyl ketone oxime, L³³

The precursor 3,3′-dipyridyl ketone L1 was prepared by the literature procedure.⁵³ Colourless crystals suitable for X-ray determination were grown by slow evaporation of a MeOH solution of the ketone L1. Hydroxylamine hydrochloride (550 mg, 7.91 mmol) dissolved in water (20 mL) was added to L1 (364 mg, 1.98 mmol) dissolved in EtOH (5 mL). NaOH (324 mg, 8.10 mmol) was added slowly to the solution with vigorous stirring at 0 °C. The solution was warmed to room temperature and heated at reflux overnight. The reaction was allowed to cool to room temperature, diluted with water and extracted with DCM. The organic extracts were combined and dried over anhydrous Na₂SO₄ and concentrated in vacuo. The product was purified by column chromatography [ethyl acetate (EA)/triethylamine (TEA) 5%] to give the oxime as a white solid (388 mg, 98%). Anal. found: C, 66.22; H, 4.57; N, 20.92. Calc. for C₁₁H₉N₃O: C, 66.32; H, 4.55; N, 21.09. ¹H NMR (500 MHz, DMSO): δ 11.92 (s, 1H, OH), 8.64 (dd, J = 1.7, 4.9 Hz, 1H, H6), 8.60 (dd, J = 0.8, 2.3 Hz, 1H, H9), 8.59 (dd, J = 1.6, 4.8 Hz, 1H, H13), 8.56 (dd, J = 0.9, 2.2 Hz, 1H, H2), 7.79 (dt, J = 2.0, 2.0, 8.0 Hz, 1H, H4), 7.74 (ddd, J = 1.7, 2.2, 8.0 Hz, 1H, H11), 7.52 (ddd, J = 0.9, 4.9, 7.8 Hz, 1H, H5), 7.43 (ddd, J = 0.9, 4.8, 8.0 Hz, 1H, H12). ¹³C NMR (DMSO): δ 150.749 (C7), 149.904 (C13), 149.693 (C6), 149.394 (C2), 147.640 (C9), 136.737 (C4), 134.330 (C11), 131.966 (C10), 128.369 (C3), 123.611 (C5), 123.537 (C12). HRMS (+ve, ESI) m/z calc. for C₁₁H₁₀N₃O 200.2199 [M + H⁺], found 200.0818. Selected IR (KBr)/cm⁻¹: 3076 (m, sh), 3030 (m, sh), 2983 (m, sh), 2755 (s, sh), 1615 (w, sh), 1595 (m, sh), 1481 (m, sh), 1455 (m, sh), 1417 (m, sh), 1401 (m, sh), 1315 (w, sh), 955 (m, sh), 938 (s, sh), 929 (m, sh), 835 (m, sh), 816 (m, sh), 754 (w, sh), 707 (s, sh). UV-Vis (MeOH)/cm⁻¹ (ε/L mol⁻¹ cm⁻¹): 45 045 (13 846), 40 000 (12 939), 6109 (256), 5194 (118). Colourless X-ray quality crystals of two different morphologies were isolated. Crystals of L³³ were obtained by cooling of a 5 : 1 H₂O : MeOH solution and crystals of L³³·2H₂O were obtained by the slow diffusion of MeCN into a MeOH solution of L³³ through a layer of EtOH.

2,4-Dipyridyl ketone oxime, L²⁴

The precursor 2,4-dipyridyl ketone L2 was prepared by the literature procedure.⁵⁴ Colourless crystals suitable for X-ray determination were grown by slow evaporation of a CDCl₃ solution of the ketone L2. Hydroxylamine hydrochloride (357 mg, 5.13 mmol) dissolved in water (16 mL) was added to L2 (236 mg, 1.28 mmol) dissolved in EtOH (5 mL). NaOH (210 mg, 5.26 mmol) was added slowly to the solution with vigorous stirring at 0 °C. The solution was warmed to room temperature and heated at reflux overnight. The reaction was cooled to room temperature, filtered and washed with water to give the isolated isomer A (46.0 mg, 18%). The filtrate was diluted with water and extracted with DCM. The organic extracts were combined and dried over anhydrous Na₂SO₄ and concentrated in vacuo. The product was purified by column chromatography (EA/TEA 5%) to give a mixture of isomers of the oxime as a cream solid (205 mg, 80%). Anal. found: C, 66.35; H, 4.86; N, 20.93. Calc. for C₁₁H₉N₃O: C, 66.32; H, 4.55; N, 21.09. ¹H NMR (500 MHz, DMSO): single isomer A δ 11.98 (s, 1H, OH), 8.64 (dd, J = 1.6, 4.4 Hz, 2H, H9, H13), 8.47 (dd, J = 1.6, 4.4 Hz, 1H, H6), 7.95 (dt, J = 1.1, 1.1, 7.9 Hz, 1H, H3), 7.87 (td, J = 1.7, 7.8, 7.8 Hz, 1H, H4), 7.38 (ddd, J = 1.2, 4.8, 7.6 Hz, 1H, H5), 7.30 (dd, 1.7, 4.4 Hz, 2H, H10, H12). ¹³C NMR (500 MHz, DMSO): single isomer A δ 153.789 (C7), 153.709 (C2), 149.156 (C9, C13), 148.795 (C6), 140.808 (C11), 136.826 (C4), 124.084 (C10, C12), 123.825 (C5), 120.962 (C3). ¹H NMR (500 MHz, DMSO): mixture: δ 12.07 (s, 1H, OH-b), 11.97 (s, 1H, OH-a), 8.68 (ddd, J = 1.0, 1.5, 5 Hz, 1H, H6b), 8.63 (dd, J = 2.0, 4.5 Hz, 1H, H9, H9a, H13a), 8.55 (dd, J = 1.5, 4.5 Hz, 2H, H9b, H13b), 8.47 (ddd, J = 1.0, 1.5, 5.0 Hz, 0.5H, H6a), 7.94 (m, 1.5H, H3a, H4b), 7.87 (m, 0.5H, H4a), 7.63 (dt, J = 1.0, 7.5 Hz, 1H, H3b), 7.47 (ddd, J = 1.0, 5.0, 7.5 Hz, 1H, H5b), 7.39 (ddd, J = 1.5, 5.0, 7.5 Hz, 0.5H, H5a), 7.31 (m, 2H, H10b, H12b), 7.29 (m, 1H, H10a, H12a). ¹³C NMR (500 MHz, DMSO): mixture δ 153.749 (C7a), 153.699 (C2a), 152.830 (C7b), 150.867 (C2b), 149.768 (C9b, C13b), 149.497 (C6b), 149.136 (C9a, C13a), 148.776 (C6a), 142.980 (C11b), 140.779 (C11a), 136.821 (C4a), 136.468 (C4b), 125.630 (C3b), 124.057 (C10a, C12a), 123.860 (C5b), 123.815 (C5a), 121.082 (C10b, C12b), 120.940 (C3a). HRMS (+ve, ESI) m/z calc. for C₁₁H₁₀N₃O 200.2199 [M + H⁺], found 200.0818. Selected IR (KBr)/cm⁻¹: 3421 (m, br), 3085 (m, sh), 2999 (s, sh), 1604 (s, sh), 1583 (m, sh), 1563 (m, sh), 1550 (w, sh), 1334 (m, sh), 1007 (s, sh), 993 (s, sh), 958 (s, sh), 836 (m, sh), 790 (m, sh), 741 (w, sh), 684 (s, sh). UV-Vis (MeOH)/cm⁻¹ (ε/L mol⁻¹ cm⁻¹): 43 101 (12 371), 37 594 (11 534), 6365 (179), 6305 (219), 5845 (179). Colourless crystals suitable for X-ray determination were grown by slow evaporation of a H₂O : MeOH (1 : 1) solution of the isolated isomer A.

2,3-Dipyridyl ketone oxime, L²³

Hydroxylamine hydrochloride (214 mg, 3.08 mmol) dissolved in water (10 mL) was added to 2,3-dipyridyl ketone (142 mg, 0.770 mmol) dissolved in EtOH (5 mL). NaOH (126 mg, 3.16 mmol) was added slowly to the solution with vigorous stirring at 0 °C. The solution was warmed to room temperature and heated at reflux overnight. The reaction was cooled to room temperature, filtered and washed with water to give the isolated isomer A (14.0 mg, 9%). The filtrate was diluted with water and extracted with DCM. The organic extracts were combined and dried over anhydrous Na₂SO₄ and concentrated in vacuo. The product was purified by column chromatography (EA/TEA 5%) to give a mixture of the oxime isomers as a cream solid (123 mg, 80%). Anal. found: C, 66.54; H, 4.72; N, 20.84. Calc. for C₁₁H₉N₃O: C, 66.32; H, 4.55; N, 21.09. ¹H NMR (500 MHz, DMSO): single isomer A δ 11.94 (s, 1H, OH), 8.56 (dd, J = 1.7, 4.9 Hz, 1H, H13), 8.51 (dd, J = 0.9, 2.2 Hz, 1H, H9), 8.48 (ddd, J = 0.9, 1.8, 4.8 Hz, 1H, H6), 7.97 (dt, J = 1.1, 1.1, 8.0 Hz, 1H, H3), 7.87 (td, J = 1.5, 8.0 Hz, 1H, H4), 7.75 (ddd, J = 2.9, 4.9, 8.0 Hz, 1H, H11), 7.46 (ddd, J = 0.9, 4.9, 7.8 Hz, 1H, H12), 7.39 (ddd, J = 1.1, 4.8, 7.5 Hz, 1H, H5). ¹³C NMR (500 MHz, DMSO): single isomer A δ 154.132 (C2), 153.195 (C7), 149.678 (C9), 148.859 (C13), 148.751 (C6), 137.098 (C11), 136.804 (C4), 128.643 (C10), 123.763 (C5), 122.954 (C12), 121.077 (C3). ¹H NMR (500 MHz, DMSO): mixture: δ 11.93 (s, 0.5H, OH-a), 11.87 (s, 1.0H, OH-b), 8.67 (ddd, J = 1.0, 2.0, 5.0 Hz, 1.0H, H6b), 8.55 (m, 2.5H, H13a, H9b, H13b), 8.49 (m, 1.0H, H6a, H9a), 7.94 (m, 1.5H, H3a, H4b), 7.88 (ddd, J = 1.5, 7.5, 7.5 Hz, 0.5H, H4a), 7.74 (m, 1.5H, H11a, H11b), 7.69 (dt, J = 1.0, 1.0, 8.0 Hz, 1H, H3b), 7.46 (m, 1.5H, H12a, H12b), 7.39 (m, 1.5H, H5a, H5b). ¹³C NMR (500 MHz, DMSO): mixture δ 154.11 (C2a), 153.172 (C7a), 152.436 (C2b), 151.089 (C7b), 149.650 (C9a), 149.505 (C6b), 149.447 (C13b or 9b), 148.842 (C13a), 148.732 (C6a), 147.742 (C9b or 13b), 137.065 (C11a), 136.790 (C4a), 136.404 (C4b), 134.308 (C11b), 131.824 (C10b), 128.618 (C10a), 125.782 (C3b), 123.821 (C12b), 123.748 (C5a), 123.389 (C5b), 123.935 (C12a), 121.059 (C3a). HRMS (+ve, ESI) m/z calc. for C₁₁H₁₀N₃O 200.2199 [M + H⁺], found 200.0841. Selected IR (KBr)/cm⁻¹: 3159 (s, br), 3049 (s, sh), 1636 (w, sh), 1589 (s, sh), 1498 (s, sh), 1472 (s, sh), 1415 (s, sh), 1311 (w, sh), 998 (s, sh), 963 (m, sh), 941 (s, sh), 812 (s, sh), 790 (m, sh), 743 (m, sh). UV-Vis (MeOH)/cm⁻¹ (ε/L mol⁻¹ cm⁻¹): 44 444 (13 829), 38 168 (11 895), 7037 (222), 5147 (191), 5045 (213). Colourless crystals suitable for X-ray determination were grown by slow evaporation of a MeOH solution of the isolated isomer A.

Syntheses of complexes

[Ag(L⁴⁴)₂](CF₃SO₃) 1. AgCF₃SO₃ (12.6 mg, 0.0493 mmol) and L⁴⁴ (23.3 mg, 0.117 mmol) was dissolved in MeOH and MeCN (1 : 1). The solution was allowed to evaporate to give colourless X-ray quality crystals (5 mg, 32.7%). Anal. found: C, 42.35; H, 3.01; N, 12.52; F, 8.73. Calc. for C₂₃H₁₈N₆O₅AgF₃S: C, 42.15; H, 2.77; N, 12.82; F, 8.70. Selected IR (ATR)/cm⁻¹: 3439 (w, br) (L⁴⁴), 3051–2870 (w, sh) (L⁴⁴), 1600 (w, sh) (L⁴⁴), 1424 (w, sh) (L⁴⁴), 1239 (s, sh) (CF₃SO₃⁻), 1220 (vs, sh) (CF₃SO₃⁻), 1166 (s, sh) (CF₃SO₃⁻), 1021 (vs, sh) (CF₃SO₃⁻), 955 (m, sh) (CF₃SO₃⁻), 835 (m, sh) (CF₃SO₃⁻), 672 (s, sh) (CF₃SO₃⁻).

{[AgL³³](CF₃SO₃)}_∞, 2. AgCF₃SO₃ (131 mg, 0.510 mmol) dissolved in degassed MeCN (10 mL) was added via cannula to L³³ (101 mg, 0.510 mmol) dissolved in degassed MeOH (10 mL). The resultant solution was stirred for 7 h during which time the solution was concentrated in volume to form a brown oil. Addition of a minimal volume of MeOH yielded a cream solid which was filtered washed with diethyl ether and dried in vacuo (57.0 mg, 25%). Colourless X-ray quality crystals were grown from the slow diffusion of a MeOH solution of L³³ (20.0 mg, 0.100 mmol) layered with EtOH into a MeCN solution of AgCF₃SO₃ (25.0 mg, 0.100 mmol). Anal. found: C, 31.53; H, 2.02; N, 9.06. Calc. for C₁₂H₉N₃O₄AgF₃S: C, 31.60; H, 1.99; N, 9.21. Selected IR (KBr)/cm⁻¹: 3419 (m, br) (L³³), 3038–2849 (w, sh) (L³³), 1616 (w, sh) (L³³), 1596 (w, sh) (L³³), 1483 (w, sh) (L³³), 1422 (m, sh) (L³³), 1282 (vs, sh) (CF₃SO₃⁻), 1247 (vs, sh) (CF₃SO₃⁻), 1162 (s, sh) (CF₃SO₃⁻), 1028 (s, sh) (CF₃SO₃⁻), 963–948 (w, sh) (L³³), 813–777 (w, sh) (L³³), 636 (s, sh) (CF₃SO₃⁻).

[AgL²⁴(CF₃SO₃)]_∞, 3. AgCF₃SO₃ (130 mg, 0.500 mmol) dissolved in degassed MeCN (10 mL) was added via cannula to L²⁴ (100 mg, 0.500 mmol) dissolved in degassed MeOH (10 mL). The resultant solution was stirred for 4 h during which time a cream precipitate formed. This was filtered, washed with diethyl ether and dried in vacuo (76.0 mg, 33%). Colourless X-ray quality crystals were grown from the slow diffusion of a MeOH solution of L²⁴ (20.0 mg, 0.100 mmol) layered with EtOH into a MeCN solution of AgCF₃SO₃ (27.0 mg, 0.100 mmol). Anal. found: C, 31.88; H, 2.11; N, 9.22. Calc. for C₁₂H₉N₃O₄AgF₃S: C, 31.60; H, 1.99; N, 9.21. Selected IR (KBr)/cm⁻¹: 3447 (m, br) (L²⁴), 3083–3000 (w, sh) (L²⁴), 1607 (m, sh) (L²⁴), 1582 (w, sh) (L²⁴), 1562 (w, sh) (L²⁴), 1477–1420 (m, sh) (L²⁴), 1339 (w, sh) (L²⁴), 1261 (vs, sh) (CF₃SO₃⁻), 1179 (m, sh) (CF₃SO₃⁻), 1034 (s, sh) (CF₃SO₃⁻), 967 (m, sh) (L²⁴), 832–737 (w, sh) (L²⁴), 642 (m, sh) (CF₃SO₃⁻).

[Ag₂L²³(CF₃SO₃)₂]_∞, 4. AgCF₃SO₃ (150 mg, 0.600 mmol) was dissolved in MeCN (3 mL) and layered with MeCN. L²³ (20.0 mg, 0.100 mmol) was dissolved in MeOH (2 mL) and layered with MeCN. These solutions were allowed to diffuse together to give colourless X-ray quality crystals (10.0 mg, 14%). Anal. found: C, 22.06; H, 1.62; N, 5.59. Calc. for C₁₃H₉N₃O₇Ag₂F₆S₂: C, 21.95; H, 1.28; N, 5.91. In a bulk reaction AgCF₃SO₃ (260 mg, 1.00 mmol) dissolved in degassed MeCN (10 mL) was added via cannula to L²³ (100 mg, 0.500 mmol) dissolved in degassed MeOH (10 mL). The resultant solution was stirred for 7 h during which time the solution concentrated in volume to form a brown oil. Addition of a minimal volume of MeOH yielded a cream solid which was filtered, washed with diethyl ether and dried in vacuo (188 mg, 82% based on L²³). For the bulk reaction [AgL²³(CF₃SO₃)]_∞ showed Anal. found: C, 31.57; H, 1.95; N, 8.93. Calc. for C₁₂H₉N₃O₄AgF₃S: C, 31.60; H, 1.99; N, 9.21. Selected IR (KBr)/cm⁻¹: 3442 (m, br) (L²³), 3088–2850 (w, sh) (L²³), 1618 (w, sh) (L²³), 1596 (w, sh) (L²³), 1566 (w, sh) (L²³), 1487–1434 (m, sh) (L²³), 1344 (w, sh) (L²³), 1288 (vs, sh) (CF₃SO₃⁻), 1234 (vs, sh) (CF₃SO₃⁻), 1174 (s, sh) (CF₃SO₃⁻), 1159 (s, sh) (CF₃SO₃⁻), 1032 (s, sh) (CF₃SO₃⁻), 952 (w, sh) (L²³), 818–744 (w, sh) (L²³), 639–631 (s, sh) (CF₃SO₃⁻).

X-ray crystallography

X-ray diffraction data were collected on a Bruker APEX II CCD diffractometer, with graphite monochromated Mo Kα (λ = 0.71073 Å) radiation. Intensities were corrected for Lorentz polarisation effects^55,56 and a multiscan absorption correction⁵⁷ was applied. The structures were solved by direct methods (SHELXS⁵⁸ or SIR-97⁵⁹) and refined on F² using all data by full-matrix least-squares procedures (SHELXL 97⁶⁰). Non-hydrogen atoms were refined with anisotropic thermal parameters. H atoms were placed in ideal positions except for the H atoms of H₂O in L⁴⁴·2H₂O, L³³·2H₂O, and 3, and the oxime units in L⁴⁴, L³³, L²³, L²⁴, 2, 3 and 4 which were located from the Fourier synthesis maps. The absolute configuration of L1 could not be determined because the compound was a weak anomalous scatterer. The data for L²⁴ were de-twinned using the RLATT procedure of SHELXTL.⁶⁰ In 1 the disordered CF₃SO₃⁻ anion could not be modelled and the electron density associated with it was removed using the SQUEEZE procedure in PLATON.⁶¹ Crystals of 3 were very thin and diffracted weakly at high angles. All calculations were performed using the WinGX interface.⁶² Detailed analyses of the extended structure were carried out using PLATON⁶³ and MERCURY^64,65 (Version 1.4.2). In both L³³·2H₂O and 2 the oxime units were disordered over two sites having site occupancy factors of 0.69 and 0.31. L⁴⁴·2H₂O and L1 belonged to non-centrosymmetric space groups and as they contained no heavy atoms the assignment of absolute configuration through the Flack parameter was ambiguous.^66–68 Crystal data and refinement details are summarised in Table 2.

Table 2 Summary of crystallographic data

	L^44·2H2O	L1	L^33	L^33·2H2O
Empirical formula	C11H13N3O3	C11H8N2O	C11H9N3O	C11H13N3O3
M	235.24	184.19	199.21	235.24
Crystal system	Monoclinic	Orthorhombic	Orthorhombic	Monoclinic
Space group	P21	Pna21	Pbca	P21/c
a/Å	4.0517(4)	21.0949(17)	11.4954(17)	13.9556(14)
b/Å	13.9045(14)	10.5185(8)	10.6744(17)	10.0763(11)
c/Å	10.2601(11)	3.8315(4)	15.506(3)	8.6383(8)
β/°	94.720(4)	—	—	101.106(3)
V/Å^3	576.06(10)	850.16(13)	1902.5(6)	1192.0(3)
Z	2	4	8	4
T/K	90(2)	90(2)	90(2)	90(2)
μ/mm^−1	0.078	0.096	0.094	0.098
Reflections collected	2430	7927	6102	12 219
Unique reflections (Rint)	2430 (0.0000)	1360 (0.0297)	1708 (0.0289)	2348 (0.0243)
R 1 indices [I > 2σ (I)]	0.0396	0.0299	0.0377	0.0347
wR2 (all data)	0.0995	0.0759	0.0987	0.0939
	L2	L^24	L^23
Empirical formula	C11H8N2O	C11H9N3O	C11H9N3O
M	184.19	199.21	199.21
Crystal system	Monoclinic	Orthorhombic	Monoclinic
Space group	C2/c	Pnma	P21/c
a/Å	21.6741(18)	14.2517(8)	9.1149(2)
b/Å	3.7762(3)	8.1077(6)	10.1937(2)
c/Å	21.1734(18)	8.9124(6)	10.1280(2)
β/°	102.254(5)	—	91.9285(12)
V/Å^3	1639.5(2)	1029.81(12)	940.51(3)
Z	8	4	4
T/K	90(2)	90(2)	90(2)
μ/mm^−1	0.096	0.087	0.095
Reflections collected	12 280	19 954	10 203
Unique reflections (Rint)	1587 (0.0289)	859(0.0352)	1752 (0.0195)
R 1 indices [I > 2σ (I)]	0.0349	0.0384	0.0307
wR2 (all data)	0.0951	0.0952	0.0789
	1	2	3	4
Empirical formula	C44H36N12O4Ag2 F0S0	C12H9N3O4F3SAg	C12H11N3O5F3SAg	C13H9N3O7F6S2Ag2
M	1012.59	456.15	474.17	713.11
Crystal system	Monoclinic	Monoclinic	Monoclinic	Triclinic
Space group	C2/c	P21/n	P21/n	P1̄
a/Å	34.844(3)	11.3883(12)	12.0134(6)	10.2591(8)
b/Å	13.5225(11)	9.4818(10)	9.9549(5)	10.2823(8)
c/Å	6.7362(5)	15.1208(16)	13.2918(7)	11.0872(15)
α/°	—	—	—	96.288(5)
β/°	91.412(3)	110.395(4)	90.9902(14)	103.286(5)
γ/°	—	—	—	116.555(4)
V/Å^3	1029.81(12)	1530.4(4)	1589.3(2)	987.45(18)
Z	2	4	4	2
T/K	90(2)	90(2)	90(2)	90(2)
μ/mm^−1	0.656	1.510	1.463	2.296
Reflections collected	10 121	25 891	5492	10 790
Unique reflections (Rint)	2660 (0.0377)	2847 (0.0226)	2578 (0.0250)	3668 (0.0288)
R 1 indices [I > 2σ (I)]	0.0578	0.0291	0.0485	0.0373
wR2 (all data)	0.1621	0.0725	0.1009	0.1080

---

Results and discussion

The isomeric dipyridyl ketone oximes were prepared in a two-step synthesis from the precursor materials, except for the 4,4′-dipyridyl ketone which was commercially available. Methyl nicotinate was reacted with the product of the lithiation of 2- or 3-bromopyridine to give the respective 2,3-dipyridyl or 3,3′-dipyridyl (L1) ketones as solids in moderate yields. Methyl isonicotinate was used to prepare 2,4-dipyridyl ketone (L2) in a similar manner. Subsequent reaction of the respective ketones with hydroxylamine hydrochloride gave the required oximes L⁴⁴, L²³, L³³, and L²⁴ in high yield. Isolation of one isomer of each of the non-symmetrical oximes L²³ and L²⁴ was achieved by initial recrystallisation. Further work up produced a mixture of the E and Z isomers for L²³ (Fig. 1). Symmetrical oximes L³³ and L⁴⁴ were isolated under different conditions; L⁴⁴ precipitated out of solution on cooling, whereas work up of the solution was required to form L³³. These compounds have been briefly mentioned in the literature before, typically as intermediates in pharmaceutical applications. However, details of preparation and characterisation are rather vague.^69–71 All four oximes were thoroughly characterised by ¹H, ¹³C and 2-D NMR techniques, microanalysis, mass spectrometry and X-ray structural analysis. In the case of L³³ a pseudo-polymorph L³³·2H₂O was also isolated. In addition, the precursor ketones L1 and L2 were characterised by X-ray crystallography to provide comparisons with the dipyridyl ketone oximes.

Fig. 1 E and Z isomers of L²³.

4,4′-Dipyridyl ketone oxime L⁴⁴·2H₂O

X-ray structural analysis of L⁴⁴·2H₂O revealed that it crystallised in the monoclinic space group P2₁. The asymmetric unit consisted of one complete ligand molecule and two water molecules (Fig. 2). The angle between the plane of each pyridyl rings was 56.66° giving a two-bladed chiral propeller conformation to the ligand.

Fig. 2 (Top) View of L⁴⁴ with crystallographic numbering. Thermal ellipsoids are drawn at 50% probability level. Selected bond lengths (Å) and angles (°): C(3)–C(6) 1.482(3), C(6)–C(7) 1.497(3), C(6)–N(2) 1.295(3), N(2)–O(1) 1.387(2); C(2)–N(2)–O(1) 114.09(16), C(3)–C(6)–C(7) 121.12(17), C(7)–C(6)–N(2) 124.77(17), C(3)–C(6)–N(2) 114.11(17). (Bottom) View down c axis of double chain showing interaction between water chain and ligands (H-bonds shown in green).

A series of extensive H-bonding was observed in this structure. Individual water molecules were H-bonded to each other forming zigzag chains along the a axis (Fig. 2). The H–O⋯H distances were found to be 1.96 and 1.78 Å respectively, with corresponding O⋯O distances of 2.7740(19) and 2.6821(19) Å respectively. Such chains of water molecules are increasingly being recognised as important in supramolecular systems because they may provide insight into the properties of bulk water and its role in chemical and possibly biological systems.^72–78

The first two H-bonding interactions between water molecules and the ligand were between each of the N_py atoms in L⁴⁴ and two distinct water molecules, generating an infinite chain along the b axis. The N_py⋯H–O distances were found to be 1.93 and 1.95 Å for N1 and N3 respectively, with corresponding N⋯O distances of 2.800(2) and 2.760(2) Å. The third H-bonding interaction between the water chain and ligands was to the oxime unit of L⁴⁴, generating a chain along the a axis. The O–H_ox⋯O distance was found to be 1.70 Å with an O⋯O distance of 2.650(2) Å. Overall these H-bonding interactions generated a 2-D network in the ab plane.

3,3′-Dipyridyl ketone L1

X-ray structural analysis of L1 showed that it crystallised in the orthorhombic space group Pna2₁. The asymmetric unit consisted of one complete molecule (Fig. 3). The angle between the planes of each of the pyridyl rings was found to be 42.71° giving a two-bladed chiral propeller conformation of the ligand.⁵¹ Notably, the 3-substituted pyridyl rings were not arranged in a symmetrical fashion. Analysis of the structure suggested the respective orientation of the pyridyl rings may have been a consequence of the nature of the packing.

Fig. 3 View of the asymmetric unit of L1 with crystallographic numbering. Thermal ellipsoids are drawn at 50% probability level. Selected bond lengths (Å) and angles (°): C(1)–C(6) 1.496(2), C(6)–C(7) 1.498(2), C(6)–O(1) 1.228(2); C(1)–C(6)–C(7) 121.73(13).

The packing of L1 involved only non-classical H-bonds which formed an infinite 3-D network. Each molecule within this network was H-bound to four others due to two crystallographically distinct H-bonds (Fig. 4). These H-bonds existed between the N_py donor atoms and nearby H atoms of adjacent pyridyl rings. The N_py⋯H–C bond distances were 2.59 and 2.46 Å, respectively, with corresponding N_py⋯C distances of 3.497(5) and 3.353(5) Å, respectively. The stronger H-bond generates a helix running along the c axis. No other significant interactions existed between individual molecules.

Fig. 4 View of L1 showing the four H-bonds for each molecule and the helix running along the c axis.

3,3′-Dipyridyl ketone oxime L³³

The X-ray structure of L³³ revealed that it crystallised in the orthorhombic space group Pbca. The asymmetric unit consisted of one complete ligand entity (Fig. 5). The arrangement of the pyridyl rings were such that a two-bladed chiral propeller⁵¹ was formed, with the angle between the planes of each ring being 84.93°. This was almost twice the value found for the angle between the planes of the pyridyl rings for the related carbonyl precursor. Once again the N_py atoms were not arranged in a symmetrical fashion.

Fig. 5 View of asymmetric unit of L³³ with crystallographic numbering. Thermal ellipsoids are drawn at 50% probability level. Selected bond lengths (Å) and angles (°): C(4)–C(6) 1.476(2), C(6)–C(7) 1.490(2), C(6)–N(3) 1.293(2), N(3)–O(1) 1.3903 (17); C(6)–N(3)–O(1) 111.47(12), C(4)–C(6)–C(7) 118.79(12), C(4)–C(6)–N(3) 116.48(13), C(7)–C(6)–N(3) 124.68(14), C(5)–C(4)–C(6) 120.11(13), C(8)–C(7)–C(6) 120.09(13).

The oxime L³³ formed an infinite 1-D zigzag chain running along the a axis. The zigzag chains contained only one enantiomer. However other zigzag chains in the structure were formed with the opposite enantiomer. These chains were formed as a result of a single H-bond between the oxime unit and donor N2 on the pyridyl ring of an adjacent ligand. The N_py⋯H–O distance was found to be 1.77 Å (corresponding to a N_py⋯O distance of 2.737(2) Å). Individual chains were linked into a 2-D sheet consisting of chains of alternating enantiomers by a T-shaped C–H⋯π interaction between H11 and the ring containing N1 (Fig. 6). The C–H⋯ring centroid distance was 2.72 Å.⁷⁹

Fig. 6 View of L³³ in the ab plane of two zigzag chains formed as a result of classical H-bonding interactions (shown in green) and C–H⋯π interactions (shown in black).

3,3′-Dipyridyl ketone oxime L³³·2H₂O

X-ray structural analysis of L³³·2H₂O revealed that it crystallised in the monoclinic space group P2₁/c. The asymmetric unit of this structure contained a complete ligand molecule in which the oxime group was disordered in a 69 : 31 ratio (Fig. 7) and two water molecules. The disorder arose from the possibility of two conformations of L³³ existing in the same crystal (Fig. 1). In the crystal analysed the dominant conformation had the oxime unit pointing towards the pyridyl ring containing N2. The other conformation had the oxime unit pointing towards the pyridyl ring containing N1. Analysis will focus on the main component of disorder. As with the pseudo polymorph L³³, the pyridyl rings were arranged in such a way as to form a two-bladed chiral propeller⁵¹ with an angle of 69.72° between the planes of each ring. This angle was smaller than that found in L³³. Interestingly, the individual N_py atoms were arranged symmetrically in that they were on the same side of the molecule as the oxime functional group [C6, N3, O1].

Fig. 7 View of asymmetric unit of L³³·2H₂O with crystallographic numbering, and showing oxime disorder. Thermal ellipsoids are drawn at 50% probability level. Solvent molecules have been omitted for clarity. Selected bond lengths (Å) and angles (°): C(1)–C(6) 1.4833(18), C(6)–C(7) 1.4857(17), C(6)–N(3) 1.300(3), N(3)–O(1) 1.398(2), C(6)–N(3′) 1.360(6), N(3′)–O(1′) 1.388(5); C(6)–N(3)–O(1) 108.6(2), C(1)–C(6)–C(7) 118.61(10), C(1)–C(6)–N(3) 108.85(14), C(7)–C(6)–N(3) 132.54(15), C(2)–C(1)–C(6) 121.91(11), C(8)–C(7)–C(6) 120.32(10), C(6)–N(3′)–O(1′) 105.5(5), C(1)–C(6)–N(3′) 139.9(3), C(7)–C(6)–N(3′) 101.2(3).

In this structure an extensive series of H-bonding interactions was observed. The individual water molecules were H-bound to each other to form zigzag chains (Fig. 8) in a manner similar to L⁴⁴·2H₂O. These chains ran along the c axis and lay in the cavities between individual ligand entities. The H–O⋯H distances were found to be 1.91 and 1.80 Å, respectively, with corresponding O⋯O distances of 2.751(2) and 2.639(2) Å, respectively. These chains of water molecules were also H-bound to individual ligand entities through three separate H-bonding interactions to generate an infinite 3-D network (Fig. 8). The first two H-bonding interactions were between two water molecules and each of the N_py atoms of L³³. The N_py⋯H–O distances were found to be 1.93 and 1.98 Å for N1 and N2, respectively, with corresponding N⋯O distances of 2.791(3) and 2.807(2) Å, respectively. The third H-bonding interaction was between one water molecule and the oxime unit of L³³. The O–H_ox⋯O distance was found to be 1.85 Å with an O⋯O distance of 2.657(4) Å. One weak π–π interaction, with a centroid–centroid distance of 3.82 Å, was observed between two N2 containing rings on adjacent ligands. No other significant interactions were observed between individual ligand entities.

Fig. 8 (Top) View of the network formed in the ab plane as a result of H-bonding between L³³ and individual water molecules with one example of the weak π–π interaction shown by the solid line between two N2 pyridyl rings. View is along the c axis. (Bottom) Zigzag chain of water molecules formed as a result of two H-bonding interactions (shown in green). View is along the b axis.

2,4-Dipyridyl ketone L2

X-ray structural analysis revealed that L2 crystallised in the monoclinic space group C2/c. The asymmetric unit consisted of one complete molecule (Fig. 9). Each pyridyl ring was twisted relative to the other giving rise to an angle of 48.41° between them. The N2 atom was arranged such that it was in a trans relationship to the ketone functional group [C6, O1].

Fig. 9 (Left) View of the asymmetric unit of L2 with crystallographic numbering. Thermal ellipsoids are drawn at 50% probability level. Selected bond lengths (Å) and angles (°): C(5)–C(6) 1.5038(16), C(6)–C(7) 1.5015(16), C(6)–O(1) 1.2200(14); C(5)–C(6)–C(7) 120.01(10). (Right) Stacked nature of 1-D chains formed as a result of π–π interactions and intermolecular H-bonds (shown in green). View is in the bc plane.

Infinite 1-D π-stacked chains of the same enantiomer were formed as a result of π–π interactions between identical rings of adjacent ligand entities. The centroid–centroid distances were 3.78 Å. Each π-stacked chain was linked to one other chain, containing the opposite enantiomer to form a double chain as a result of two complementary non-classical H-bonds between N2 of the 2-substituted pyridyl ring and H8 of adjacent ligands (Fig. 9). The N_py⋯H–C distance was found to be 2.71 Å (corresponding to a N_py⋯C distance of 3.537(2) Å). As a further consequence of these H-bonds, the carbonyl units were positioned on the outside edges of each double chain and therefore pointed in opposite directions.

2,4-Dipyridyl ketone oxime L²⁴

X-ray structural analysis of L²⁴ revealed that it crystalised in the orthorhombic space group Pnma. The asymmetric unit consisted of one half of a ligand molecule (Fig. 10). The ligand had a mirror plane of symmetry such that the oxime unit and one ring (N1) lay in the plane while the other ring (N2) was bisected by the plane. Given this imposed crystallographic symmetry L²⁴ cannot act as a two bladed chiral propeller like the other ligands. The N2 atom was in a Z relationship to the oxime functional group while the N1 atom was in an E relationship.

Fig. 10 (Top) View of L²⁴ with crystallographic numbering. Thermal ellipsoids are drawn at 50% probability level. Selected bond lengths (Å) and angles (°): C(1)–C(6) 1.480(3), C(6)–C(7) 1.495(3), C(6)–N(3) 1.283(2), N(3)–O(1) 1.390(2); C(6)–N(3)–O(1) 112.83(19), C(7)–C(6)–C(1) 121.59(15), C(7)–C(6)–N(3) 122.22(16), C(1)–C(6)–N(3) 116.67(16), C(8)–C(7)–C(6) 120.98(9), N(1)–C(1)–C(6) 115.90(16). (Bottom) View along b axis showing the 1-D chain (H-bonds shown in green) and T shaped C–H⋯π interaction (black) linking chains together.

Individual ligand molecules were held together with H-bonding forming an infinite 1-D chain running parallel to the a axis (Fig. 10). The H-bonding interaction occurred between the H atom of the oxime group and the N2 of the 4-substituted ring. The N_py⋯H–O distance was found to be 1.77(3) Å with a corresponding N⋯O distance of 2.695(2) Å. These individual chains were then further linked into a 2-D sheet in the ac plane by a T shaped C–H⋯π interaction between H4 and the N2 ring. The C–H⋯ring distance was 2.77 Å, corresponding to a C⋯ring distance of 3.586(3).⁷⁹

2,3-Dipyridyl ketone oxime L²³

L²³ crystallised in the monoclinic space group P2₁/c. The asymmetric unit consisted of one complete ligand molecule (Fig. 11). The angle between the plane of each of the pyridyl rings was 59.94° giving rise to the formation of a two-bladed chiral propeller.⁵¹ The N2 atom of the 2-substituted pyridyl ring was in a trans relationship to the oxime functional group while the N1 atom of the 3-substituted pyridyl ring was in a cis relationship.

Fig. 11 (Top left) View of asymmetric unit of L²³ with crystallographic numbering. Thermal ellipsoids are drawn at 50% probability level. Selected bond lengths (Å) and angles (°): C(2)–C(6) 1.4891(15), C(6)–C(7) 1.4919(15), C(6)–N(3) 1.2873(10), N(3)–O(1) 1.3960(12); C(6)–N(3)–O(1) 114.59(9), C(2)–C(6)–C(7) 119.46(10), C(2)–C(6)–N(3) 127.18(10), C(7)–C(6)–N(3) 113.35(10), C(3)–C(2)–C(6) 120.62(10), N(2)–C(7)–C(6) 116.35(10). (Top right) View down c axis of the 1-D double chain showing the chains offset from one another. (Bottom) View of 1-D double chains running parallel to the c axis (H-bonds shown in green).

Individual ligand molecules were held together through one main H-bonding interaction to form an infinite 1-D double chain running parallel to the c axis (Fig. 11). Each side of the double chain contained only one enantiomer. This key H-bonding interaction occurred between the H atom of the oxime unit and N1 of the 3-substituted pyridyl ring. The N_py⋯H–O distance was found to be 1.79 Å (corresponding to a N⋯O distance of 2.7281(12) Å). Individual chains were offset from one another and no significant interactions existed between them (Fig. 11).

[Ag(L⁴⁴)₂](CF₃SO₃) 1. Colourless X-ray quality crystals of [Ag(L⁴⁴)₂](CF₃SO₃) 1 were grown by evaporation of a 2 : 1 molar ratio of L⁴⁴ and AgCF₃SO₃, respectively, in a 1 : 1 mixture of MeCN and MeOH. Microanalysis of the crystalline sample was consistent with a 2 : 1 ligand-to-metal ratio. Despite many attempts, ligand to metal ratios of 1 : 1 and 1 : 2 only gave the 2 : 1 product upon crystallisation. Analysis of the IR spectrum confirmed the presence of L⁴⁴ and the CF₃SO₃⁻ (Experimental section) and suggested that the symmetry of the CF₃SO₃⁻ anion was unchanged upon formation of the complex.

X-ray structural analysis of 1 showed that it crystallised in the monoclinic C2/c space group as a discrete cation forming a 2-D hydrogen-bonded network. The asymmetric unit comprises of one complete L⁴⁴ ligand, half an Ag(I) cation and half a very disordered CF₃SO₃⁻ anion (Fig. 12). The disordered CF₃SO₃⁻ anion could not be appropriately modelled and the electron density associated with the CF₃SO₃⁻ anion was removed using the SQUEEZE procedure in PLATON.⁶¹ However, the presence of the CF₃SO₃⁻ anion was confirmed from the microanalysis and the IR spectrum (Experimental section). The Ag(I) ion was two-coordinate and formed a centrosymmetric dimer through binding to two symmetry related ligands through the N_py donors of the N1 atom with a N–Ag distance of 2.134(4) Å. The ligand acted as a chiral two bladed propeller with an angle of 72.65° with the dimer consisting of both enantiomers. Hydrogen bonding between the N(2) and O(1) formed an infinite planar 2-D honey-comb network. The O–H⋯N distance was found to be 1.87 Å (corresponding to an O⋯N distance of 2.677(5) Å). This extended into an infinite 3-D network via π–π stacking between the N1 rings of two ligands with a centroid distance of 3.575(3) Å. This interaction was reinforced by weak argentophilic interactions with an Ag⋯Ag distance of 3.3680 Å. The overall stacking was further reinforced by π–π stacking of N2 rings with distance of 3.948(3) Å between the centroids (Fig. 13). The 2-D sheets were relatively well aligned generating channels running along c axis. This left no place for the CF₃SO₃⁻ anion to bind or interact with the complex, as all significant functional groups were already involved in some type of interaction. This essentially left the CF₃SO₃⁻ anion to fill the space in the channels, causing it to be highly disordered.

Fig. 12 View of the cationic complex 1 with crystallographic numbering. Thermal ellipsoids are drawn at 50% probability level. Selected bond lengths (Å) and angles (°): N(1)–Ag(1) 2.134(4), C(6)–N(3) 1.276(6), N(3)–O(1) 1.384(5); N(1)–Ag(1)–N(1A) 180.00, C(6)–N(3)–O(1) 112.1(4). (Symmetry code: A − x, −y, 1 − z).

Fig. 13 (Top) view of 1 along b axis showing π–π stacking of N1 rings and argentophilic interactions. (Bottom) View along c axis showing infinite planar 2D honeycomb in ab plane (H-bonding shown in green, π–π stacking shown in black).

{[AgL³³](CF₃SO₃)}_∞2. Reaction of L³³ with AgCF₃SO₃ in a 1 : 1 molar ratio produced a tan coloured precipitate in low yield and microanalysis was consistent with a 1 : 1 metal-to-ligand ratio. Analysis of the IR spectrum confirmed the presence of L³³ in the bulk material and peaks corresponding to the CF₃SO₃⁻ anion suggested the symmetry of the anion remained unchanged upon formation of the complex (Experimental section).⁸⁰ Colourless X-ray quality crystals were grown by the slow diffusion of L³³ in MeOH into a solution of AgCF₃SO₃ in MeCN.

X-ray structural analysis of 2 showed that it crystallised in the monoclinic space group P2₁/n and formed a tightly wound 1-D helical polymer (Fig. 14) encapsulating very weakly interacting CF₃SO₃⁻ anions. The asymmetric unit contained a Ag(I) cation, a CF₃SO₃⁻ anion and a complete L³³ ligand disordered around the oxime group in a 69 : 31 ratio in a similar fashion to the free ligand, vide supra (Fig. 14). The main component of the disordered oxime was used in the structural analysis of 2. The Ag(I) was bound to two symmetry related ligands through the N_py donors and adopted an essentially linear arrangement with a N–Ag–N angle of 174.84(9)°. A CF₃SO₃⁻ anion sat at a distance of 2.745(3) Å from the Ag(I) ion. This distance was well above the mean for Ag–OSO₂CF₃ distances (Table 1). The small deviation from linearity in the N–Ag–N bond angle can be attributed to the very weak interaction of the CF₃SO₃⁻ anion.⁴

Fig. 14 (Top) View of asymmetric unit of 2 showing disorder of the oxime unit (crystallographic numbering). The CF₃SO₃⁻ anion is omitted for clarity. Thermal ellipsoids are drawn at 50% probability level. Selected bond lengths (Å) and angles (°): N(1)–Ag(1) 2.150(3), N(2)–Ag(1) 2.142(3), C(6)–N(3) 1.276(6), C(6)–N(3′) 1.404(13), N(3)–O(1) 1.390(6), N(3′)–O(1′) 1.377(14); N(1)–Ag(1)–N(2) 174.84(9), C(6)–N(3)–O(1) 109.5(6), C(6)–N(3′)–O(1′) 101.9(11). (Bottom) View showing the coordination environment of the Ag(I) ion within the 1-D helical polymer chain (crystallographic numbering) (symmetry code: A ½ − x, −½ + y, ½ − z).

The L³³ ligand was bound to two symmetry related Ag(I) ions through the 3-substituted N_py donor atoms and thus acted as a bridge between individual Ag(I) ions. It adopted a conformation in which each ring was approximately orthogonal to the other at 71.12°. As a consequence the ligand acted as a two-bladed chiral propeller forming the 1-D helical polymer. The centrosymmetric space group meant that both M- and P-helices were present within the structure. The 1-D helix ran parallel to the crystallographic b axis and had a helical pitch of 9.4818(10) Å, corresponding to the length of the b axis.

The CF₃SO₃⁻ anion sat within the helix (Fig. 15) by virtue of the very weak Ag(I)⋯OSO₂CF₃ interaction. Also holding it in place was a weak H-bonding interaction between an O atom of the CF₃SO₃⁻ anion and a pyridyl H atom of the helix. The C–H⋯O distance was found to be 2.54 Å (corresponding to a C⋯O distance of 3.219(5) Å).

Fig. 15 View showing how the CF₃SO₃⁻ anions sit within the 1-D helices running along the crystallographic b axis.

The strongest interaction between individual helices was an H-bonding interaction between an O atom of a CF₃SO₃⁻ encapsulated in one helix and the oxime OH group of an adjacent helix. The O–H⋯O distance was found to be 1.94 Å (corresponding to a O⋯O distance of 2.683(7) Å). As a result of this H-bonding interaction the helices involved were slightly interdigitated and in this way chirality was transferred between adjacent oxime H-bonded helices. This gave rise to H-bonded sheets composed of helices with the same chirality. Each helix was surrounded by six others (Fig. 16).

Fig. 16 View in the ac plane showing the chirality of adjacent helices.

{[Ag(L²⁴)CF₃SO₃](H₂O)}_∞3. Reaction of L²⁴ with AgCF₃SO₃ in a 1 : 1 molar ratio afforded off-white precipitates in low yield which gave analyses consistent with 1 : 1 metal-to-ligand ratios. Analysis of the IR spectrum confirmed the presence of both CF₃SO₃⁻ and L²⁴ in the complex (Experimental section). The peaks corresponding to the CF₃SO₃⁻ anion were consistent with the symmetry of CF₃SO₃⁻ remaining essentially unchanged upon formation of the complex.⁸⁰ Colourless X-ray quality crystals were grown by the slow diffusion of L²⁴ in MeOH into a solution of AgCF₃SO₃ in MeCN.

X-ray structural analysis of 3 revealed the complex to be a 1-D polymer chain decorated on one side with weakly bound CF₃SO₃⁻ anions (Fig. 17). The complex crystallised in the monoclinic space group P2₁/n. The asymmetric unit contained one Ag(I) ion, one ligand, one CF₃SO₃⁻ counterion and a H₂O molecule of crystallisation. The Ag(I) ion was in a distorted trigonal pyramidal arrangement. The trigonal plane was formed with one chelating ligand involving 2-substituted N_py and N_ox donors and a 4-substituted N_py donor from an adjacent symmetry related ligand. The remaining site was occupied by a weakly bound CF₃SO₃⁻ anion at a Ag–O distance of 2.698(4) Å. This distance was near the limit of what would be considered a Ag–OSO₂CF₃ bond (Table 1). The Ag(I) ion was 0.108 Å below the plane and pointing away from the CF₃SO₃⁻ anion. The ligand L²⁴ adopted a conformation in which the 4-substituted pyridine ring was approximately orthogonal to the remainder of the ligand at 70.49°. This twist allowed L²⁴ to behave as a two-bladed chiral molecular propeller.

Fig. 17 View of a 1-D chain of 3 (crystallographic numbering) running along the [1 0 1] direction with H₂O molecules omitted for clarity (50% probability ellipsoids). Selected bond lengths (Å) and angles (°): Ag1–N1 2.174(5), Ag1–N2A 2.247(5), Ag1–N3A 2.467(5), Ag1–O11 2.698(4); N1–Ag1–N2A 160.12(18), N1–Ag1–N3A 128.42(19), N2A–Ag1–N3A 70.31(17), N1–Ag1–O11 88.56(15), N2A–Ag1–O11 84.60(15), N3A–Ag1–O11 89.74(14) (symmetry code: A ½ + x, ½ − y, −½ + z).

The complex formed a 1-D chain running parallel to the [1 0 1] diagonal axis. All the ligands pointed in one direction along the chain and adjacent ligands were oriented at 113.7° with respect to each other forming an L-shaped chain. The CF₃SO₃⁻ anions sat within the bend of the L shaped chain and point outwards. The ligands and CF₃SO₃⁻ anions, respectively, were related by a glide plane which bisected the dihedral angle of the L-shaped chain. As a consequence of the glide plane the chirality of individual ligands alternated along the chain giving rise to an overall achiral structure. Each L-shaped chain interacted with two adjacent chains forming a zigzag pattern through a series of H-bonding interactions. The H-bonding interactions were responsible for forming a series of 18-membered rings (Fig. 18). These rings involved Ag1–N3–O1H1A⋯O2H2A⋯O12–S1–O11–Ag1–N3–O1H1A⋯O2H2A⋯O12–S1–O11 at H-bonding distances of 1.72 and 1.85 Å, respectively [corresponding to an O1⋯O2 separation of 2.614(5) and a O2⋯O12 separation of 2.806(7) Å, respectively]. The chains were arranged in such a way that the chelated oxime unit of one chain registered with the monodentate 4-pyridyl unit of an adjacent chain. These adjacent chains were bowed such that the Ag⋯N_py and Ag⋯Ag distances were 3.157(6) and 3.3236(18) Å, respectively (Fig. 18). This bowing appeared to arise as a consequence of steric distortion of the chains rather than any weak bonding interactions involving the Ag(I) ions. Similarly the influence of π-bonding interactions was ruled out as they were too weak at 4.04 Å. Therefore, the steric distortion was caused by the presence of the 18-membered rings pulling adjacent chains into close proximity resulting in the formation of an overall 3-D network. Interactions within this network structure were augmented by inter-chain π–π interactions at 3.78 Å.

Fig. 18 (Left) View down the diagonal ac axis showing the L-shaped nature of the chain. (Right) View of two adjacent 1-D chains of 3 showing the 18-membered H-bonded rings and the bowing of the chains.

[Ag₂L²³(CF₃SO₃)₂]_∞4

Colourless X-ray quality crystals of [Ag₂L²³(CF₃SO₃)₂]_∞4 were grown by the slow diffusion of a solution of AgCF₃SO₃ in MeCN into a solution of L²³ in MeOH through blank layers of EtOH and MeCN. Microanalysis of the crystalline sample was consistent with a 1 : 2 ligand-to-metal ratio. Analysis of the IR spectrum confirmed the presence of L²³ and the CF₃SO₃⁻ (Experimental Section).

A bulk reaction of L²³ with AgCF₃SO₃ was consequently carried out with a 1 : 2 ligand-to-metal ratio and a pale cream precipitate was obtained in good yield. Microanalysis of this solid was consistent with a 1 : 1 ligand-to-metal ratio. The solid was found to be insoluble in all common organic solvents. X-ray quality crystals of the 1 : 1 product could not be obtained, despite many attempts at growing crystals of this material. The bulk reaction was repeated a number of times using different molar ratios. However the same result of a 1 : 1 metal-to-ligand complex was always obtained.

X-ray analysis of 4 revealed it to be an infinite neutral 2-D network lying in the ab plane. The complex crystallised in the triclinic space group P1̄ and the asymmetric unit consisted of one L²³ entity, two Ag(I) cations and two CF₃SO₃⁻ anions (Fig. 19). The Ag1 cation was bound to L²³ through the chelating N_py, N_ox donor set and through O atoms from three different CF₃SO₃⁻ anions and adopted a distorted square-based pyramidal geometry with a τ₅ value of 0.28.⁸¹ The Ag1 ion sat 0.15 Å above the basal plane (N1, N2, O2, O4B). The second Ag2 cation was in a distorted tetrahedral coordination environment being bound to L²³ through the 3-substituted N_py donor atom and through O atoms from three different CF₃SO₃⁻ anions. The τ₄ value was found to be 0.87⁸² as the bond angles around Ag2 varied from 88.11° to 149.67°. The Ag–O distances of each of the Ag(I) ions were found to be in the range of 2.239(4) to 2.584(3) Å (Table 1). The ligand itself acted as a two-bladed chiral propeller with an angle of 74.47° between the plane of each ring. The centrosymmetric nature of the space group meant that both enantiomers of L²³ were present within the 2-D network and were found to alternate along the ligand ribbons (vide infra).

Fig. 19 View of the coordination environment of each unique Ag(I) ion in 4 (crystallographic numbering). Thermal ellipsoids are drawn at 50% probability level. Selected bond lengths (Å) and angles (°): Ag(1)–N(1) 2.298(4), Ag(1)–N(2) 2.433(4), Ag(1)–O(2) 2.344(3), Ag(1)–O(4B) 2.447(3), Ag(1)–O(5) 2.584(3), Ag(2)–N(3B) 2.239(4), Ag(2)–O(3) 2.495(3), Ag(2)–O(5) 2.561(3), Ag(2)–O(7B) 2.350(3), N(2)–O(1) 1.387(5); N(1)–Ag(1)–O(2) 142.95(12), N(1)–Ag(1)–O(4B) 112.47(12), N(1)–Ag(1)–O(5) 113.79(11), N(1)–Ag(1)–N(2) 70.06(12), N(2)–Ag(1)–O(2) 108.54(12), N(2)–Ag(1)–O(4B) 159.82(11), N(2)–Ag(1)–O(5) 86.23(11), O(2)–Ag(1)–O(4B) 81.76(11), O(2)–Ag(1)–O(5) 102.85(11), O(4B)–Ag(1)–O(5) 74.38(10), N(3B)–Ag(2)–O(3) 99.70(12), N(3B)–Ag(2)–O(5) 109.60(12), N(3B)–Ag(2)–O(7B) 149.67(12), O(3)–Ag(2)–O(7B) 89.99(11), O(3)–Ag(2)–O(5) 123.04(10), O(5)–Ag(2)–O(7B) 88.11(11) (symmetry codes: A 1 + x, 1 + y, z; B −1+ x, −1 + y, z).

The infinite 2-D network was formed as a result of the bonding interactions observed between the O atoms of the CF₃SO₃⁻ anions and the Ag(I) ions. These interactions played a key role in the formation of the overall structure. The first CF₃SO₃⁻ anion was bound to two Ag(I) ions, one in a monodentate fashion and the other in a bis-monodentate fashion. The second CF₃SO₃⁻ anion was bound to two Ag(I) ions in a tris-monodentate fashion. Thus each acted as a bridge between two Ag(I) ions in differing binding modes. There are only a few examples of AgCF₃SO₃ complexes where the CF₃SO₃⁻ anion is bound in a bridging fashion through a single O atom (Table 1). The presence of these bonding interactions resulted in the formation of AgCF₃SO₃ units which then assembled to form threads along the a axis (Fig. 20). The L²³ ligand then acted as a bridge between each Ag(I) ion within the threads and in this way linked the individual AgCF₃SO₃ threads to each other. The only H-bonding interaction present within the structure was an H-bond between the oxime OH and an O atom of the CF₃SO₃⁻ anion. The O–H⋯O distance was found to be 1.81 Å (corresponding to an O⋯O distance of 2.629(4) Å).

Fig. 20 View of the ab plane showing how the L²³ and AgCF₃SO₃ threads run parallel to each other along the a axis.

Comparison

The resulting polymeric Ag(I) structures were a subtle balance between the influences of the CF₃SO₃⁻ anion and the N_py substitution pattern of the isomeric dipyridyl ketone oxime ligands. This balance appeared to dictate the stoichiometry in the complexes formed. The network of 1 was formed from the Ag(I) ion binding exclusively to one pyridyl ring of the ligand. This entity then essentially formed an infinite 2-D structure of closed circles through H-bonding between the remaining pyridyl ring and the oxime of an adjacent ligand. An infinite 3-D honeycomb network with large channels throughout the structure was generated by π–π stacking combined with weak argentophilic interactions. The CF₃SO₃⁻ anion was seriously disordered as no significant functional groups remained to interact with thus leaving it to occupy the channels in a random fashion.

The helical nature of 2 arose in part from the 3,3′-substitution pattern of L³³ and the propeller twist of pyridyl rings relative to each other. Crucial to the formation of a helix was the need to link these chiral propellers through a linear Ag(I) ion. Such a coordination environment precluded any strong interaction with the counterion. Thus the CF₃SO₃⁻ anion remained unbound and by virtue of a very weak interaction was encapsulated, essentially filling space, within the tightly wound helical polymer chain. A strong H-bonding interaction between the encapsulated CF₃SO₃⁻ anion of one chain and the oxime OH group of an adjacent chain assisted with the transfer of chirality to the neighbouring chain.

In contrast, the achiral L-shaped nature of the 1-D chain in 3 arose from the essentially linear-linking nature of L²⁴. Formation of a chelate ring with Ag(I) locked one pyridyl ring into a set conformation and, although the other pyridyl ring remained free to rotate, the N donor atom of the 4-substituted pyridyl ring essentially remained static as the ring rotated about its axis. In addition, the monodentate binding by the CF₃SO₃⁻ anion to the Ag(I) ion limited the possible structural outcomes with a trigonal pyramidal coordination environment being adopted by the Ag(I) ion and as a consequence the L²⁴ ligands all pointed in the same direction along the chain. Positioning the CF₃SO₃⁻ anion in the bend of the L-shaped chain allowed H-bonding interactions which were responsible for the formation of 18-membered rings between adjacent 1-D polymer chains. These 18-membered rings contained solvent H₂O molecules and were approximately orthogonal giving rise to a 3-D H-bonding network.

Finally, in 4 a 2-D network was formed as a consequence of the CF₃SO₃⁻ anions strongly binding to the two different Ag(I) ions through two rare bridging modes. These bridging interactions in 4 were facilitated by a metal rich 2 : 1 metal-to-ligand ratio, compared to the 1 : 1 ratio in 2 and 3. Again formation of a 5-membered chelate ring by L²³ restricts the possible ways this ligand can contribute to a structural outcome for this polymer.

The twisting of the pyridyl rings in the complex arrays relative to each other was about the same at 72°. In contrast, the twists observed in the free ligands and related precursor ketones varied markedly. This suggested there was a reasonable degree of conformational freedom in the ligand structures which was probably influenced by crystal packing forces. The approximate uniformity of the twist in each of the complexes suggested that the conformational freedom inherent in the C_py–C_ox bond was not being fully utilised in these structures. These effects were compounded in 2 and 3 by the fact that in 2 each polymer chain was composed of only one enantiomeric form of L³³ whereas in 3 the handedness of the enantiomers alternate along the chain.

The strongly coordinating and structurally complex role of the CF₃SO₃⁻ anion in 4 necessitated such a high metal salt ratio and generated the final 2-D structure. In 4 the two Ag(I) ions each took on different coordination geometries, namely distorted tetrahedral and distorted square-based pyramidal. In particular the varying N_py substitution patterns and the ability of L²³ and L²⁴ to form chelate rings, have an effect on the nature of the final polymer formed. Formation of a chelate ring in both L²³ and L²⁴ locks one pyridyl ring into a set conformation upon binding to a Ag(I) ion. However, given that one pyridyl ring remains free to rotate, sufficient conformational freedom remains allowing the Ag(I) ions together with the anion to have an impact on the overall structure.

Acknowledgements

We acknowledge the Tertiary Education Commission New Zealand for the award of a Top Achiever Doctoral Scholarship (VJA) and thank the University of Otago Research Committee (VJA) and the University of Otago for financial support. We thank Dr Scott A. Cameron for assistance with the twinning in the ligand structure L²⁴.

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Footnote

† CCDC reference numbers 899902–899912. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ce26449h

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