Molecular botanical garden: assembly of supramolecular silver(I) and mercury(II) complexes of NS₂-donor macrocycles with flower-, leaf- and tree-shaped structures

Abstract

Isomeric NS₂macrocycles (ortho: L¹, meta: L² and para: L³) were synthesised and their complexes (1–4) were isolated; L¹ and L² afforded flower-type cyclic hexamer (1) and tree-shaped 3 : 4 (M/L) complex (2), respectively, in the reactions of AgPF₆, meanwhile L² and L³ afforded a respective leaf-like 1D chain (3a) and a brick-wall type 2D network (4) in the reactions of HgBr₂.

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The recent upsurge in construction of metal–organic hybrid materials is motivated not only by their potential properties for magnetism, catalysis, and luminescence but also because of their interesting structural topologies.¹ By employing specially designed ligands and carefully selected metal salts, specific coordination polymers can be synthesised.²Ag(I) and Hg(II) have been known for their versatile coordination modes, which can give discrete and polymeric complexes.³ In particular, the Ag(I) and Hg(II) have weak stereochemical demands, often producing complex geometries dictated by the conformational preferences of the preorganised ligand rather than the geometric bonding demands of the metal ion.³ Recently, we reported the exocoordination⁴ behaviour of dithiamacrocycles, which afforded diverse types of metallosupramolecules upon variation of the donor atoms, counter-ions and solvents.⁵ In this regard, we are interested in the synthesis of small NS₂-donor macrocyclic isomers L¹–L³ rigidified with one xylyl subunit (Scheme 1). In this work, a change of ring rigidity influences the frameworks of their products in the reactions with Ag(I) and Hg(II). The versatility of these results is highlighted by the formation of plant-shaped products such as flower, leaf and tree.

Scheme 1 Synthesis of isomeric NS₂-donor macrocycles, L¹–L³.

The ligands L¹–L³ were synthesised by coupling reactions from N-Boc-protected dithiol 8⁶ and corresponding α,α′-dibromo-xylene followed by deprotection of the Boc-group from macrocycles 9a–9c (Scheme 1).

The reaction of L¹ with AgPF₆ afforded the flower-shaped product 1 with formula [Ag₆(L¹)₆(PF₆)](PF₆)₅, which crystallises in the cubic space groupIa-3d (Fig. 1). The asymmetric unit of 1 contains one L¹, one Ag atom and three independent PF₆⁻ moieties with each P atom on a special position. Interestingly, the simplicity of this structure and symmetry operation lead to an elegant cyclic hexamer due to the discriminated coordination mode of L¹ towards the Ag atom. The Ag atom is bonded to two S atoms and one N atom from one L¹ and to one S atom from adjacent L¹, forming a distorted tetrahedral environment with the bond angles falling in the range 79.8–144.2°. It is notable that two facial Ag–S bond lengths [Ag1–S1 2.614(1), Ag1–S2 2.613(1) Å] are apparently longer than that of linking the Ag–S bond [Ag1–S1A 2.452(1) Å]. Consequently, the repeating Ag–S–Ag–S bonding gives a cavity formed with an Ag₆(L¹)₆ core. The resulting structure of 1 has an outer diameter of approximately 15 Å and a thickness of 7.3 Å. The six ligand molecules consisting of the cyclic core are arranged in an alternating up-anddown fashion (see Fig. S7 of the ESI).† Notably, this nanosized metallocycle affords a cavity in the centre which is completely sealed by one PF₆⁻ ion (Fig. 1b). The closest distance between Ag and F atoms is 2.987 Å (dashed lines). Other five PF₆⁻ ions are found external to the cavity (not shown). There are three crystallographically different PF₆⁻ ions, with different symmetry requirements. Two types of anions are located in a distinct chain with a separation of 5.225 Å [see Fig. S8(a) of the ESI]†. In the packing structure, the intermolecular π–π stacking interaction in the distance of 3.392 Å was observed [dashed lines in Fig. S8(b) of the ESI].† The particular flower-type cyclic hexamer 1 seems to be obtained because of the relatively flexible nature of L¹ together with, at least in part, the anion stabilisation effect.

Fig. 1 Flower-shaped cyclic hexamer structure of 1, [Ag₆(L¹)₆(PF₆)](PF₆)₅: (a) ball-and-stick and (b) space-filling views. Noncoordinating anions are omitted except for one PF₆⁻ ion in the cavity.

The reaction of L² with AgPF₆ afforded the tree-shaped 3 : 4 (M/L) complex 2 with formula [Ag₃(L²)₄](PF₆)₃·C₆H₅CH₃ (Fig. 2). The mass spectrum for 2 showed a peak at m/z 426.1 (calculated value; 425.7) corresponding to [Ag₃(L²)₄]³⁺ (see Fig. S10 of the ESI).† In complex 2, all four ligands and three Ag atoms adopting crystallographically independent environments. Each Ag atom is face-coordinated by NS₂-donors from each terminal L² and the fourth site is occupied by one S atom (for Ag1 or Ag3) or one N atom (for Ag2) from the linker (stem) ligand. Two linking Ag–S bonds are relatively short [Ag1–S4 2.486(2), Ag3–S3 2.486(2) Å] and other six facial Ag–S bonds are relatively longer [2.669(2)–2.928(2) Å]. No anions and solvent are included in the coordination spheres. As we understand, this is the first characterised example of a macrocyclic complex with a 3 : 4 stoichiometry. The endo- and exocyclic coordination in one multinuclear macrocyclic complex is also very novel. L³ gave no Ag(I) complex suitable for X-ray analysis.

Fig. 2 Tree-shaped 3 : 4 (M/L) complex 2, [Ag₃(L²)₄](PF₆)₃·C₆H₅CH₃. Noncoordinating anions and solvent molecule are omitted.

Having successfully obtained two Ag(I) complexes, we proceeded to the preparation of the corresponding complexes by employing Hg(II) salts. Reactions of L² with HgX₂ (X = Br and I) afforded infinite 1D coordination polymers of formula [Hg₂(L²)X₄]_n (3a; X = Br and 3b; X = I), produced by the linkage of (Hg–X)_n (Fig. 3). Since the results of the crystallographic analyses indicate that 3a and 3b have the same structure, only the structure of 3a is described as a representative example (see Fig. S12 of the ESI for 3b).† Interestingly, compound 3a resembles a model of leaves on a branch with a –Hg–Br–Hg–Br– backbone as the branch and macrocyclic Hg(II) complex units as leaves. In 3a, there are two crystallographically independent Hg atoms which are bridged by Br4 with the Hg1–Br4–Hg2 angle of 118.0(1)°. The Hg1 atom has a distorted trigonal bipyramidal environment with coordination sites occupied by the facial mode of NS₂-donors from one L² in a bent arrangement. The two remaining sites are occupied by two Br atoms (Br1 and Br4). The S1, S2, and Br4 atoms define the equatorial plane, with the axial positions occupied by N1 and Br1 atom [N1–Hg1–Br1, 170.8(2)°]. Meanwhile, the Hg2 atom, which lies outside the cavity, is situated in a roughly tetrahedral coordination environment [Br–Hg2–Br, 71.3–103.9°], bonding to one terminal Br and three bridging μ₂-Br atoms to provide a continuous Br2–Hg2–Br2′–Hg2′ chain. The Hg–S bond length [2.524(4) Å] in 4 is shorter than those in 3a [Hg1–S1 2.928(3), Hg1–S2 2.864(3) Å]. No solvent molecules are found in the crystal lattice. In the packing structures of 3a and 3b, there are π⋯π stacking interactions between two adjacent parallel chains (see Fig. S13 of the ESI).†

Fig. 3 (a) Leaf-shaped infinite 1D structure and (b) asymmetric unit of 3a, [Hg₂(L²)Br₄]_n.

In case of the reaction of HgBr₂ with L³, a colourless crystalline product 4 was obtained. Unlike 3, complex 4 was revealed as a 2D polymeric array of formula [Hg₂(L³)Br₄]_n (Fig. 4). The gross framework of 2D architecture for 4 can be described as an infinite brick-wall pattern. The asymmetric unit consists of one L³, two Hg atoms and two Br atoms. A single brick unit of 4 contains six asymmetric units where each ligand is interconnected by square-type Hg–Br2–Hg linkers alternately. There are two crystallographically independent Hg atoms which are bridged by Br2 with the Hg1–Br2–Hg2 angle of 148.92(11)°. The Hg1 atom is four-coordinated by NS-donors from one L³ and the two remaining sites are occupied by two Br atoms. Unlike the Hg1, the Hg2 atom that lies outside the cavity is five-coordinated by four Br atoms and one S donor from L³. In the crystal lattice, none of the solvent molecules is included. In this case, the preference of a 2D brick-wall structure is probably due to the longer sulfur-to-sulfur distance which allows L³ as a bridging ligand. Consequently, comparing the structures between 3 and 4 afforded a good case for the ligand conformation effect. No X-ray-quality single crystals were obtained for the Hg(II) complex of L¹.

Fig. 4 (a) Brick-wall-type infinite 2D structure and (b) asymmetric unit of 4, [Hg₂(L³)Br₄]_n.

The photoluminescence properties were studied in the solid state (Fig. 5). No clear photoluminescence was observed for each ligand and its complexes 1, 2 and 4. Only mercury(II) halide complexes of L², 3a (Br-form) and 3b (I-form), exhibit emission at 360 and 366 nm, respectively. No emissions originating from metal-centred excited states are expected for the d¹⁰ metal ion species such as Hg(II) complexes.⁷ It is clear that the emission band for the bromide clusters occurs at a lower energy than that of the iodide one, although the red shift of the iodide complex is difficult to explain. Thus, the emissions of 3a and 3b seem to be attributed to a halide-to-ligand charge-transfer (XLCT).⁸

Fig. 5 Emission spectra of L¹–L³ and their complexes 1–4 at room temperature (excitation at 290 nm).

In conclusion, three isomeric NS₂-macrocycles were synthesised and their exocoordination-based Ag(I) and Hg(II) complexes with different structural topologies were isolated and structurally characterised as conclusive evidences for the influence of the positional isomerism of the ligands on their complexation. The tendency to adopt these respective arrangements with different connectivity patterns is a reflection of the rigidity variation of the macrocyclic isomer system. Thus, we realised that the rigidity control of the macrocyclic ligand system is an attractive and versatile tool, at least in the metallo-supramolecular construction based on the exocoordination.

Acknowledgements

This work was supported by the Korean Science and Engineering Foundation (R01-2007-000-20245-0).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: NMR spectra of L¹–L³, crystal structures of 1 and 3b, mass spectrum of 2, and packing structures of 3a and 3b. CCDC reference numbers 682706–682710. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b814051k

‡ Synthetic details for L¹: KOH (6.4 g, 115 mmol) was dissolved in DMF (1 L). N-Boc-protected 2,2′-iminobis(ethanethiol) (8) (5.54 g, 23.0 mmol) and α,α′-dibromo-o-xylene (6.16 g, 23.0 mmol) was dissolved in DMF and this solution was added to a syringe. The contents of the syringe was added dropwise into a DMF solution of KOH at 45–50 °C for 24 h. The reaction mixture was kept for a further 10 h with rapid stirring, allowed to cool to room temperature, then filtered. The filtrate was evaporated and the residue was portioned between water and dichloromethane. The aqueous phase was separated and extracted with dichloromethane. The combined organic phase was dried with anhydrous sodium sulfate and then evaporated to dryness. Flash column chromatography on silica gel using 20% ethyl acetate/n-hexane led to the isolation of 9a as a crystalline product in a 41% yield. 9a was added to a stirred mixture of dichloromethane (60 mL) and trifluoroacetic acid (40 mL). The reaction mixture was stirred at room temperature for 2 h. After removal of the solvent, methanol was added and then evaporated again to dryness. Aqueous sodium carbonate (15%, 50 mL) was added to the residue and this mixture was extracted with dichloromethane. Flash column chromatography on silica gel using ethyl acetate led to the isolation of L¹ as a colourless solid in 96% yield. Mp 101 °C. IR (KBr, cm⁻¹): 3425, 3339, 2912, 1659, 1458, 1142, 982, 862, 764, 707. ¹H NMR (300 MHz, CDCl₃, δ): 7.29 (t, 2H, Ar), 7.20 (t, 2H, Ar), 4.11 (s, 4H, ArCH₂S), 2.95 (t, 4H, NCH₂CH₂), 2.81 (t, 4H, CH₂CH₂S), 2.30 (s, 1H, NH). ¹³C NMR (75 MHz, CDCl₃, δ): 136.3, 130.8, 127.7, 47.7, 33.1, 32.6. EI-MS: m/z 239 [C₁₂H₁₆S₂N]⁺. Anal. Calc. for C₁₂H₁₇NS₂: C, 60.20; H, 7.16; N, 5.85; S, 26.79. Found: C, 60.41; H, 7.58; N, 5.71; S, 26.31%. Synthetic details for L²: General procedures are same as for L¹ except using of α,α′-dibromo-m-xylene and 9b. Mp 92 °C. IR (KBr, cm⁻¹): 3136, 2906, 2345, 1701, 1510, 1431, 1085, 983, 894, 765. ¹H NMR (300 MHz, CDCl₃, δ): 8.10 (s, 1H, Ar), 7.16 (m, 3H, Ar), 3.76 (s, 4H, ArCH₂S), 2.76 (s, 1H, NH), 2.62 (t, 4H, NCH₂CH₂), 2.07 (t, 4H, CH₂CH₂S). ¹³C NMR (75 MHz, CDCl₃, δ): 140.1, 129.1, 128.2, 127.5, 46.6, 37.1, 31.5. EI-MS: m/z 239 [C₁₂H₁₆S₂N]⁺. Anal. Calc. for C₁₂H₁₇NS₂: C, 60.20; H, 7.16; N, 5.85; S, 26.79. Found: C, 60.58; H, 7.61; N, 5.44; S, 27.06%. Synthetic details for L³: General procedures are same as for L¹ except using of α,α′-dibromo-p-xylene and 9c. Mp 85 °C. IR (KBr, cm⁻¹): 3259, 2912, 2833, 1436, 1408, 1094, 825, 739. ¹H NMR (300 MHz, CDCl₃, δ): 7.27 (s, 4H, Ar), 3.74 (s, 4H, ArCH₂S), 2.34 (t, 4H, NCH₂CH₂), 1.96 (t, 4H, CH₂CH₂S), 0.75 (s, 1H, NH). ¹³C NMR (75 MHz, CDCl₃, δ): 137.8, 130.3, 49.5, 36.9, 28.7. EI-MS: m/z 239 [C₁₂H₁₆S₂N]⁺. Anal. Calc. for C₁₂H₁₇NS₂: C, 60.20; H, 7.16; N, 5.85; S, 26.79. Found: C, 59.92; H, 7.23; N, 5.91; S, 26.93%.

§ Crystal data for 1: C₇₂H₁₀₂Ag₆F₃₆N₆P₆S₁₂, M = 2953.36, cubic, a = 36.322(3) Å, b = 36.322(3) Å, c = 36.322(3) Å, U = 47921(8) ų, T = 173(2) K, space group Ia-3d, Z = 16, µ(Mo Kα) = 1.344 mm⁻¹, 145662 reflections measured, 5010 unique (R_int = 0.1461) which were used in all calculations. The final wR(F₂) was 0.1266 (all data). Crystal data for 2: C₅₅H₇₆Ag₃F₁₈N₄P₃S₈, M = 1808.20, triclinic, a = 11.4605(5) Å, b = 11.6121(6) Å, c = 13.1785(6) Å, α = 90.294(1)°, β = 99.549(1)°, γ = 94.075(1)°, U = 1724.87(14) ų, T = 173(2) K, space group P1, Z = 1, µ(Mo Kα) = 1.244 mm⁻¹, 10668 reflections measured, 8741 unique (R_int = 0.0118) which were used in all calculations. The final wR(F₂) was 0.1397 (all data). Crystal data for 3a: C₁₂H₁₇Br₄Hg₂NS₂, M = 960.21, orthorhombic, a = 18.2813(8) Å, b = 8.1124(4) Å, c = 26.4524(12) Å, U = 3923.0(3) ų, T = 173(2) K, space groupPbca, Z = 8, µ(Mo Kα) = 23.988 mm⁻¹, 23045 reflections measured, 4740 unique (R_int = 0.1028) which were used in all calculations. The final wR(F₂) was 0.1191 (all data). Crystal data for 3b: C₁₂H₁₇Hg₂I₄NS₂, M = 1148.17, orthorhombic, a = 18.9518(19) Å, b = 8.5212(9) Å, c = 27.755(3) Å, U = 4482.2(8) ų, T = 173(2) K, space groupPbca, Z = 8, µ(Mo Kα) = 19.375 mm⁻¹, 25670 reflections measured, 4888 unique (R_int = 0.0589) which were used in all calculations. The final wR(F₂) was 0.1331 (all data). Crystal data for 4: C₁₂H₁₇Br₄Hg₂NS₂, M = 960.21, monoclinic, a = 13.5645(11) Å, b = 11.9094(10) Å, c = 12.4053(10) Å, β = 104.114(2)°, U = 1943.5(3) ų, T = 173(2) K, space groupP2₁/c, Z = 4, µ(Mo Kα) = 24.210 mm⁻¹, 8437 reflections measured, 3943 unique (R_int = 0.0857) which were used in all calculations. The final wR(F₂) was 0.2120 (all data). CCDC reference numbers 682706 (1), 682707 (2), 682708 (3a), 682709 (3b) and 682710 (4).

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