Assembly of photoluminescent [Cu_nI_n] (n = 4, 6 and 8) clusters by clickable hybrid [N,S] ligands

Abstract

Three new 1,2,3-triazole-based NS ligands, 2-((4-((benzylthio)methyl)-1H-1,2,3-triazol-1-yl)methyl)pyridine (L1), 2-((4-(2-(cyclopentylthio)ethyl)-1H-1,2,3-triazol-1-yl)methyl)pyridine (L2) and 2-((4-(2-(cyclopentylthio)ethyl)-1H-1,2,3-triazol-1-yl)methyl)quinoline (L3) and the corresponding copper(I)-iodide complexes [Cu₄I₄(L1)₂] (1), [Cu₆I₆(L2)₂] (2) and [Cu₆I₆(L3)₂] (3A) have been prepared and characterized by single-crystal X-ray diffraction (XRD), powder XRD, photoluminescence spectroscopy and thermogravimetric analysis. Complexes 1, 2 and 3A exhibit stair-step [Cu_nI_n] (n = 4 and 6) cluster structures with supporting ligands L1, L2 and L3, respectively. Ligand L1 coordinates with a bidentate/monodentate binding mode in the [Cu₄I₄] cluster complex 1 using the pyridyl–triazole moiety and with a pendant –CH₂SCH₂Ph group. Increasing the length of the bridge from –CH₂– in L1 to –C₂H₄– in L2 and L3 engages the S donor and these ligands coordinate using a bidentate/monodentate/monodentate mode supporting larger [Cu₆I₆] cluster complexes 2 and 3A. A kinetic product [Cu₈I₈(L3)₂(CH₃CN)₂] (3B) was isolated from the reaction of L3 with CuI in CH₃CN and the single-crystal X-ray structure indicates a rare discrete stair-step [Cu₈I₈] core supported by two L3 and two coordinated CH₃CN solvates. The structure is further stabilized by intermolecular π⋯π stacking interactions in the lattice. Isolation of 1–3 provides a good demonstration of the use of multidentate and multifunctional hybrid ligands in supporting [Cu_nI_n] clusters of different sizes (n = 4, 6, 8). Ligands L1–L3 are blue emissive molecules. The corresponding complexes display strong blue (1 and 2) or remarkable yellow (3A) emissions between 500 and 700 nm in the solid state. The structures of sulfur-containing ligands and their copper-iodide complexes are described and discussed.

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Introduction

Metal cluster complexes, with unique structures and properties blending mononuclear complexes and bulk solids, have attracted considerable attention in view of their myriad of structures and potential applications as conductive, catalytic, magnetic or optical materials.^1–4 Copper(I)-iodide based complexes are particularly appealing because of their diverse cluster motifs and rich photoluminescence behaviour.^5–8 Many copper(I) luminescent complexes with various supporting ligands have been reported, but the precisely controlled synthesis of such species is yet unrealized because of the numerous structural possibilities of copper(I)-iodide clusters.^9–13 Systematic design and synthesis of supporting ligands continues to be an important strategy towards controlled assembly of functional coordination complexes and our tool of choice is the copper-catalyzed azide–alkyne cycloaddition (CuAAC) click reaction. The major benefit is ready access to hybrid 1,2,3-triazole ligands under mild conditions. The excellent tolerance of this reaction towards diverse alkyne and azide substrates permits access to a range of functional molecules.^14–18 Sulfur-containing ligands have been applied to the construction of copper(I) iodide complexes with interesting crystal structures and promising luminescence properties.^19–22 We have contributed to this field with a systematic study of luminescent and/or catalytically active copper(I) iodide complexes supported by hybrid ligands including pincer SNS and chelate-bridging NS-N ligands.^23–31 In the present work, three new NS ligands with different substituent groups have been synthesized using CuAAC click reactions and then used in the tunable assembly of emissive copper(I) iodide complexes (Scheme 1). The sulfur donor groups can be pendant or coordinating, thus making them adaptive in supporting neutral stair-step [Cu₄I₄], [Cu₆I₆] and [Cu₈I₈] cluster complexes (1–3B).

Scheme 1 Synthesis of ligands L1–L3 from CuAAC reactions.

Experimental

Materials and instrumentation

All starting chemicals were used as received. Elemental analyses were performed on a thermo electron corporation flash EA 1112 series analyzer. Infrared spectra were obtained on a Perkin Elmer Spectrum 2000 FT-IR spectrometer from samples in KBr discs. Electrospray ionization mass spectra (ESI-MS) were recorded in the positive ion mode using a Shimadzu LCMS-IT-TOF mass spectrometer. NMR spectra were measured at room temperature using a JEOL 500 MHz NMR spectrometer. Powder X-ray diffraction data were collected on a Bruker AXS GADDS X-ray diffractometer with Cu-Kα radiation (λ = 1.54056 Å). Thermogravimetric analyses (TGA) were carried out under an air stream using a TA Instruments TGA Q500 analyzer with a heating rate of 20 °C min⁻¹. Photoluminescence was measured using a Perkin Elmer LS55 luminescence spectrometer. An attenuation of 1% T was used for complexes 1–3A due to their strong emission.

Ligand synthesis

Sodium azide is potentially explosive. Only micro-scaled reactions may be performed, and all operations must be handled with necessary precautionary measures. The alkyne precursors benzyl(prop-2-yn-1-yl)sulfane and but-3-yn-1-yl(cyclopentyl)sulfane were prepared following literature procedures for thioether formation.²³ These crude alkynes benzyl(prop-2-yn-1-yl)sulfane (for L1) or but-3-yn-1-yl(cyclopentyl)sulfane (for L2 and L3) were placed in a round bottom flask containing 2-(chloromethyl)pyridine hydrochloride (328 mg, 2 mmol, for L1 and L2) or 2-(chloromethyl)quinoline hydrochloride (428 mg, 2 mmol, for L3), Na₂CO₃ (210 mg, 2 mmol), NaN₃ (156 mg, 2.4 mmol), CuI (23 mg, 0.12 mmol) and CH₃OH/H₂O (1 : 1 v : v, 6 mL) (Scheme 1). The reaction was stirred at 50 °C for 24 h. The residue was extracted with ethyl acetate (3 × 30 mL) and the combined organic layers were washed with water (3 × 10 mL) and dried over anhydrous Na₂SO₄. The product was purified on silica gel.

2-((4-((Benzylthio)methyl)-1H-1,2,3-triazol-1-yl)methyl)pyridine (L1), (C₁₆H₁₆N₄S, MW 296.39). Yield: 350 mg, 59%. ESI-MS (m/z, %): [L1 + H]⁺ (297, 100). ¹H NMR (CDCl₃, 500.2 MHz) δ: 8.59–8.58 (m, 1H, pyridine, NCH), 7.73–7.69 (m, 1H), 7.56 (s, 1H, triazole), 7.30–7.257 (m, 5H), 7.22–7.20 (m, 2H), 5.63 (s, 2H, pyridine–CH₂–triazole), 3.71 (s, 2H) and 3.69 (s, 2H) for CH₂SCH₂. ¹³C NMR (CDCl₃, 125.8 MHz): 154.3, 149.4, 146.2, 138.0, 137.9, 129.1, 128.6, 127.1, 123.7, 122.8 and 122.7 (C in pyridine, triazole and benzene groups), 55.3 (pyridine–CH₂–triazole), 36.1 and 25.7 (CH₂SCH₂).

2-((4-(2-(Cyclopentylthio)ethyl)-1H-1,2,3-triazol-1-yl)methyl)pyridine (L2), (C₁₅H₂₀N₄S, MW 288.41). Yield: 440 mg, 76%. ESI-MS (m/z, %): [L2 + H]⁺ (289, 100). ¹H NMR (CDCl₃, 500.2 MHz) δ: 8.603 and 8.595 (d, 1H, J = 4 Hz, pyridine, NCH), 7.77–7.74 (m, 1H), 7.60 (s, 1H, triazole), 7.34–7.31 (m, 1H), 7.25 and 7.24 (d, 1H, J = 8 Hz), 5.69 (s, 2H, pyridine–CH₂–triazole), 3.11–3.05 (m, 1H, SCH), 3.02–2.99 (t, 2H, J = 8 Hz), 2.87–2.84 (t, 2H), 2.00–1.93(m, 2H), 1.74–1.68(m, 2H) and 1.58–1.44(m, 4H). ¹³C NMR (CDCl₃, 125.8 MHz): 154.3, 148.8, 147.2, 138.5, 123.8, 123.1 and 122.1 (C in pyridine and triazole groups), 54.9 (pyridine–CH₂–triazole), 44.1 (SCH), 34.0, 31.5, 26.6 and 24.9.

2-((4-(2-(Cyclopentylthio)ethyl)-1H-1,2,3-triazol-1-yl)methyl)quinoline (L3). (C₁₉H₂₂N₄S, MW 338.47). Yield: 460 mg, 68%. ESI-MS (m/z, %): [L3 + H]⁺ (339, 100). ¹H NMR (CDCl₃, 500.2 MHz) δ: 8.19 and 8.18 (d, 1H, J = 9 Hz), 8.14 and 8.12 (d, 1H, J = 9 Hz), 7.84 and 7.82 (d, 1H, J = 8 Hz), 7.78–7.75 (m, 1H), 7.60–7.57 (m, 2H), 7.31 and 7.29 (d, 1H, J = 9 Hz), 5.85 (s, 2H, pyridine–CH₂–triazole), 3.09–3.04 (s, 1H, SCH), 3.01, 3.00 and 2.98 (t, 2H, J = 7 Hz), 2.86–2.83 (m, 2H), 1.97–1.92 (m, 2H), 1.70–1.66 (m, 2H) and 1.54–1.44 (m, 4H). ¹³C NMR (CDCl₃, 125.8 MHz): 154.7, 147.3, 147.0, 138.5, 130.7, 128.7, 127.9, 127.7, 127.5, 122.1 and 119.9 (C in quinoline and triazole groups), 55.8 (quinoline–CH₂–triazole), 44.0 (SCH), 34.0, 31.5, 26.6 and 24.9.

Complex synthesis

Complexes 1–3 were prepared by mixing a CH₃CN (2 mL) solution of the NS ligand (L1–L3, 0.1 mmol) with a CH₃CN (5 mL) solution of CuI (0.2 mmol, 38 mg for 1; 0.3 mmol, 57 mg for 2 and 3) in a test tube, respectively. These tubes were then sealed and left to stand at room temperature to give crystals of Cu(I) complexes 1–3 within two weeks. Two kinds of crystals were observed in the reaction of L3 with CuI. The majority were yellow plate crystals of 3A (thermodynamic product) with a smaller amount of block crystals of 3B (kinetic product). The latter was characterized by single-crystal XRD characterization but limited sample amount did not permit other types of measurements.

[Cu₄I₄(L1)₂] (1), yield: 30 mg, 45%. Anal. Calcd for C₃₂H₃₂Cu₄I₄N₈S₂ (1354.54): C, 28.37; H, 2.38; N, 8.27%. Found: C, 28.35; H, 2.20; N, 8.29%. Main IR bands (cm⁻¹): 3119(m), 3025(m), 2960(m), 2916(m), 1632(m), 1600(m), 1536(m), 1494(m), 1477(m), 1452(m), 1417(m), 1349(m), 1306(m), 1235(m), 1155(m), 1146(m), 1099(m), 1070(m), 1042(m), 1016(m), 935(m), 920(m), 898(m), 817(m), 759(s), 732(m), 705(m), 648(m), 562(m), 466(m) and 416(m). ESI-MS (m/z, %): [L1 + H]⁺ (297, 100) and [Cu(L1)₂]⁺ (655, 12).

[Cu₆I₆(L2)₂] (2), yield: 42 mg, 49%. Anal. Calcd for C₃₀H₄₀Cu₆I₆N₈S₂ (1719.46): C, 20.96; H, 2.34; N, 6.52%. Found: C, 20.65; H, 2.23; N, 6.46%. Main IR bands (cm⁻¹): 3125(m), 2955(m), 2863(m), 1634(m), 1597(m), 1544(m), 1476(m), 1440(m), 1421(m), 1377(m), 1346(m), 1304(m), 1242(m), 1226(m), 1153(m), 1102(m), 1071(m), 1053(m), 1013(m), 966(m), 936(m), 889(m), 834(m), 798(m), 757(s), 724(m), 670(m), 637(m), 603(m), 458(m) and 412(m). ESI-MS (m/z, %): [Cu(L2)₂]⁺ (639, 100).

[Cu₆I₆(L3)₂] (3A), yield: 52 mg, 57%. Anal. Calcd for C₃₈H₄₄Cu₆I₆N₈S₂ (1819.65): C, 25.08; H, 2.44; N, 6.16%. Found: C, 25.24; H, 2.34; N, 6.18%. Main IR bands (cm⁻¹): 3119(m), 3063(m), 2955(m), 2864(m), 1620(m), 1598(m), 1567(m), 1544(m), 1509(m), 1422(m), 1380(m), 1350(m), 1311(m), 1246(m), 1221(m), 1148(m), 1126(m), 1072(m), 1044(m), 973(m), 946(m), 904(m), 869(m), 826(m), 803(s), 778(m), 755(m), 733(m), 652(m), 627(m), 496(m), 482(m) and 427(m). ESI-MS (m/z, %): [L3 + H]⁺ (339, 7) and [Cu(L3)₂]⁺ (739, 100).

X-ray crystallography

All measurements were conducted on a Bruker AXS APEX diffractometer equipped with a CCD area-detector using Mo-K_α radiation (λ = 0.71073 Å).(Table 1) The collecting frames of data, indexing reflection and determination of lattice parameters and polarization effects were performed with the Bruker SMART programs.³² The integration of intensity of reflections and scaling was carried out using Bruker SAINT.³² The empirical absorption correction was performed using SADABS.³³ The space group determination, structure solution and least-squares refinements on |F|² were carried out using Bruker SHELXL.³⁴ The structures were solved by direct methods to locate the heavy atoms, followed by difference maps for the light non-hydrogen atoms. For complex 1, the –SCH₂Ph group was disordered into two positions with an occupancy ratio of 50 : 50. Restraints in bond lengths and thermal parameters were applied on the disordered atoms. Anisotropic thermal parameters were refined for the rest of the non-hydrogen atoms. Hydrogen atoms were placed geometrically and refined isotropically. CCDC reference numbers: 1038927 (1), 1038928 (2), 1038929 (3A) and 1038930 (3B).

Table 1 Summary of crystallographic data for complexes 1–3

Complex	1	2	3A	3B
Formula	C32H32Cu4I4N8S2	C30H40Cu6I6N8S2	C38H44Cu6I6N8S2	C42H50Cu8I8N10S2
M W	1354.54	1719.46	1819.57	2282.56
T/K	100(2)	100(2)	100(2)	100(2)
Crystal size/mm^3	0.17 × 0.16 × 0.10	0.50 × 0.25 × 0.03	0.30 × 0.20 × 0.05	0.15 × 0.12 × 0.05
Crystal system	Triclinic	Triclinic	Triclinic	Triclinic
Space group	P1̄	P1̄	P1̄	P1̄
a/Å	9.1116(5)	8.712(2)	8.7035(7)	9.3044(6)
b/Å	9.1187(5)	9.519(2)	9.4819(8)	12.4409(8)
c/Å	13.5556(7)	14.794(4)	15.507(1)	26.221(2)
α/°	76.481(2)	84.748(5)	86.797(2)	103.477(1)
β/°	85.819(2)	80.363(6)	82.541(2)	95.743(2)
γ/°	60.208(1)	65.779(5)	71.277(2)	96.496(1)
V/Å^3	949.09(9)	1102.8(5)	1201.6(2)	2907.4(3)
Z	1	1	1	2
D calc/g cm^−3	2.370	2.589	2.515	2.607
μ/mm^−1	5.610	7.168	6.587	7.227
θ range /°	2.579–28.318	1.397–30.703	1.325–33.240	0.805–26.410
Reflections collected	23 795	38 504	58 285	89 730
Independent reflections [Rint]	4709 [0.0276]	6701 [0.0438]	9129 [0.0365]	11 909 [0.0508]
Parameters	270	235	271	631
GOF	1.058	1.196	1.060	1.045
R 1(I > 2σ(I))	0.0217	0.0452	0.0281	0.0564
wR2(all data)	0.0414	0.1181	0.0714	0.1530

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Results and discussion

Synthesis and characterization

Ligands L1–L3 were synthesized from one-pot CuAAC reactions. The Cu(I) complexes 1–3 were prepared by the reaction of the corresponding ligand (L1–L3) and CuI in CH₃CN solvent. The bulk powder samples of 1–3A were collected from filtration under vacuum and washed sequentially with CH₃CN, ethanol and diethyl ether. The crystal and powder samples of complexes 1–3A were measured and confirmed by single-crystal and powder XRD characterization and microanalyses. All complexes 1–3A are thermally stable below 220 °C in air.

Crystal structures

Ligand L1 coordinates to the stair-step [Cu₄I₄] cluster in 1 using a bidentate/monodentate binding mode of the pyridyl–triazole moiety with the S atom remaining unbound. Ligands L2 and L3 exhibit the more common NS mode of bidentate/monodentate (for N) and monodentate (for S) coordination in complexes 2 and 3 (3A and 3B). Both complexes 2 and 3A are stair-step [Cu₆I₆] cluster structures, while 3B is a stair-step [Cu₈I₈] cluster. Complexes 1–3 crystallize in the triclinic crystal system with a space group of P1̄. All of the Cu(I) centers in complexes 1–3 are four-coordinate tetrahedral. The [Cu_nI_n] (n = 4, 6 and 8) stair-stepped core structures are supported by two ligands in 1–3B and by additional coordinating CH₃CN molecules in 3B.

The asymmetric unit of complex 1 contains two crystallographically independent Cu(I) centers, two I⁻ and one L1 ligand. The Cu1 is coordinated by two N donors from the pyridyl–triazole moiety of ligand L1, one μ₂-I and one μ₃-I bridges, giving a [CuN₂I₂] environment (Fig. 1). The Cu2 is coordinated by the 3′-N_Tri donor of ligand L1, one μ₂-I and two μ₃-I bridges, forming a [CuNI₃] environment. The geometry index³⁵τ4 for Cu1 and Cu2 is 0.80 and 0.84, respectively, which indicates the distortion of the tetrahedron. The Cu1⋯Cu2 and Cu2⋯Cu2 distances are 2.62 and 2.78 Å, respectively. The shortest intermolecular Cu⋯Cu distance is 6.2 Å in 1.

Fig. 1 Molecular structure of 1.

The asymmetric unit of 2 contains three crystallographically independent Cu(I) centers, three I⁻ ions and one ligand L2. The Cu1 is coordinated by two N donors from the pyridyl–triazole moiety of ligand L2, one μ₂-I and one μ₃-I bridges, giving a [CuN₂I₂] environment (Fig. 2). The Cu2 is coordinated by one S donor from another ligand L2, one μ₂-I and two μ₃-I bridges, forming a [CuSI₃] environment. The Cu3 is coordinated by one 3′-N_Tri atom of ligand L2 and three μ₃-I bridges, forming a [CuNI₃] environment. The tetrahedral geometry index³⁵τ4 for Cu1, Cu2 and Cu3 is 0.82, 0.90 and 0.89, respectively. The Cu1⋯Cu2, Cu2⋯Cu3 and Cu3⋯Cu3 distances are 2.65, 3.08 and 2.64 Å, respectively. The shortest intermolecular Cu⋯Cu distance is 7.0 Å in 2.

Fig. 2 Molecular structure of 2.

The asymmetric unit of 3A contains three crystallographically independent Cu(I) centers, three I⁻ and one L3 ligand. The Cu1 is coordinated by two N donors from the quinolyl–triazole moiety, one μ₂-I and one μ₃-I bridges, giving a [CuN₂I₂] environment (Fig. 3). The Cu2 is coordinated by one S donor from another ligand L3, one μ₂-I and two μ₃-I bridges, forming a [CuSI₃] environment. The Cu3 is coordinated by one 3′-N_Tri donor of ligand L3 and three μ₃-I bridges, forming a [CuNI₃] environment. The tetrahedral geometry index³⁵τ4 for Cu1, Cu2 and Cu3 is 0.81, 0.88 and 0.92, respectively. The Cu1⋯Cu2, Cu2⋯Cu3 and Cu3⋯Cu3 distances are 2.72, 2.93 and 2.77 Å, respectively. The shortest intermolecular Cu⋯Cu distance is 6.9 Å in 3A.

Fig. 3 Molecular structure of 3A.

The asymmetric unit of 3B contains eight crystallographically independent Cu(I) centers, eight I⁻ ions, two L3 ligands and two coordinating CH₃CN molecules. Both Cu1 and Cu8 are coordinated by two N donors from the quinolyl–triazole moiety, one μ₂-I and one μ₃-I bridges, giving a [CuN₂I₂] environment (Fig. 4). Both Cu2 and Cu7 are coordinated by one N donor from the CH₃CN molecule, one μ₂-I and two μ₃-I bridges, forming a [CuNI₃] environment. Both Cu3 and Cu6 are coordinated by one 3′-N_Tri donor from ligand L3 and three μ₃-I bridges, forming a [CuNI₃] environment. Both Cu4 and Cu5 are coordinated by one S donor and three μ₃-I bridges to form a [CuSI₃] core. This is a rare example of a [M₈] (M = Cu) open cluster assembled by two multidentate hybrid ligands (with bridging iodides and coordinating CH₃CN molecules providing additional support). The tetrahedral geometry index³⁵τ4 for Cu1, Cu2, Cu3, Cu4, Cu5, Cu6, Cu7 and Cu8 is 0.82, 0.94, 0.91, 0.93, 0.93, 0.91, 0.91 and 0.86, respectively. The [Cu₈I₈] stair-stepped core is supported by two bidentate/monodentate/monodentate ligand L3 and two coordinating CH₃CN molecules. The Cu1⋯Cu2, Cu2⋯Cu3, Cu3⋯Cu4, Cu4⋯Cu5, Cu5⋯Cu6, Cu6⋯Cu7 and Cu7⋯Cu8 distances are 2.67, 3.04, 2.91, 2.89, 2.94, 3.23 and 2.70 Å, respectively. There are intermolecular π⋯π interactions between the quinoline groups of neighbouring ligands with a centroid–centroid distance of 3.6 Å. The shortest intermolecular Cu⋯Cu distance is 6.7 Å in 3B.

Fig. 4 Molecular structure of 3B.

Structural discussion

Many types of sulfur-containing ligands have been synthesized for the construction of copper(I)-iodide complexes, because the soft sulfur donor has an affinity for soft metals. In our previous study, ligand 1-(2-picolyl)-4-(2-(methylthio)-pyridine)-1H-1,2,3-triazole was used to support a [Cu₆I₆]-cluster based complex (I) and all of the potential N atoms were involved in coordination, while the S atom of the 2-(methylthio)-pyridine substituent was not needed (Scheme 2).²⁹ This contrasts a similar S donor ligand 2-(4-pyridylsulfanylmethyl)pyridine that is coordinated to Cu(I) in a 2-D coordination polymer.²⁷

Scheme 2 Coordination mode of triazole NS ligands and the resultant chemical structures of copper(I)-iodide complexes.

Other sulfur-containing ligands similarly display a variety of coordination modes in copper(I)-iodide metal complexes. Tetrahydrothiophene, for example, exhibits μ₂-bridging in cluster-based coordination polymers and mono-coordination in [Cu₄] and [Cu₂] clusters, depending on the temperature and substrate content.³⁶ Very recently, dialkyl sulfide ligands displaying mono-coordination and μ₂-bridging modes were used in the construction of copper(I)-iodide clusters and coordination polymers.³⁷ The anionic sulfur donor of ligand 1-(4-pyridyl)-4-thiopyridine displays a μ₂-bridging mode in a cluster-based coordination polymer.³⁸ Likewise, the podal ligand 1,12-diphenyl-5,8-dioxa-2,11-dithiadodecane shows bridges with two S donors to construct a [Cu₄I₄] cluster-based coordination polymer.³⁹ The ditopic xylyl-bridged NO₂S₂ macrocycle ligand (1,4-bis((7,8,10,11,19,20-hexahydro-5H,9H,13H-dibenzo[e,p][1,4]dioxa[8,14]dithia[11]azacycloheptadecin-9-yl)methyl)benzene) forms a [Cu₄I₄]-based one-dimensional coordination polymer.⁴⁰ The asymmetric dithioether ligands (2-(benzylthio)-1-thiomorpholinoethan-1-one) and (2-((cyclohexylmethyl)thio)-1-thiomorpholinoethan-1-one) adopt bridging modes using their sulfur donors in [Cu₂I₂] and [Cu₄I₄]-cluster based coordination polymers.⁴¹ The bridging ligand 1,4-bis((cyclohexylthio)acetyl)piperazine using sulfur donors yields a ‘three-runged ladder’ [Cu₃I₃] subunit based coordination polymer.⁴² The potentially chelating calix[4]bis(thiacrown-5) instead assembles copper(I)-iodide cluster-based coordination polymers bridged by sulfur donors.⁴³ Less complicated is the heterocyclic thione benz-1,3-imidazole-2-thione which forms a simple copper(I)-iodide coordination polymer using μ₂-bridging mode.⁴⁴ Dithioether ligands PhS(CH₂)_nSPh (n = 1–5) exhibit tunable bridging coordination in a similar system.^19,22,45 Interestingly, tribenzo-macrocycle ligands yield endo- and exocyclic copper(I)-iodide supramolecular complexes,²⁰ while tri- and di-thioethers tris(2-tert-butyl-4-methylphenylthiomethyl)amine and bis(2,4-dimethylphenylthio)methane exhibit chelating and bridging modes of binding, respectively.²¹

These previous studies indicate that both the assembly conditions (temperature, solvent and reaction stoichiometry) and nature of the ligand (stereogeometry and electron density) influence the structural outcome in these systems.

The present work illustrates the coordination flexibility of hybrid ligands. Ligand L1 with a benzyl substituent on S displays only N coordination. Increasing the flexibility of the substituent from –py to –CH₂Ph is not sufficient to promote S coordination. However, increasing the length of the bridge from –CH₂– to –C₂H₄– in L2 and L3 enables binding of the thioether.

Stairstep [Cu₄I₄] and [Cu₆I₆] structures are observed in the biimidazole (1,4-di(2-methyl-imidazol-1-yl)butane) supported 1-D and 2-D coordination polymers.⁴⁶ The double-cubane [Cu₈I₈] cluster can be stabilized with support from ligand 4-(diphenylphosphino)-N,N-dimethylaniline.⁴⁷ Quinolyl-triazoles promote [Cu₄I₄] and [Cu₃I₃] cluster-based copper(I)-iodide complexes.³¹ More interesting is that the subtle change from pyridyl in L2 to quinolyl in L3 enables the formation of a discrete stairstep [Cu₈I₈] cluster structure. This rare motif arises from the additional stability imparted by weak π⋯π stacking and solvate (CH₃CN) coordination interactions that allow this (presumably) kinetic intermediate to be observed. Acetonitrile stabilization of [Cu₄I₄] structures has been observed previously.²¹

Powder XRD, TGA and photoluminescence

The powder X-ray diffraction patterns of complexes 1–3A showed good agreement with their simulated patterns derived from single-crystal XRD, supporting their phase purity (Fig. 5). The thermogravimetric analysis (TGA) curves of these complexes from room temperature to 600 °C are given in Fig. 6. Complexes 1–3A were stable in air up to ∼220 (for 1) and 250 °C (for 2 and 3A), followed by steady decompositions until reaching residual weights at ∼580, 550 and 560 °C in 1–3A, respectively. Complexes 2 and 3A exhibited slightly higher thermal stabilities (∼30 °C) than 1, pointing to the thermal robustness of a larger [Cu₆I₆] core.

Fig. 5 Powder XRD patterns of 1–3A (T = theoretical profile referenced to the experimentally determined structure by single-crystal XRD; E = experimental data).

Fig. 6 TGA curves of 1–3A.

The excitation spectra of ligands L1–L3 (Fig. 7) contained absorption peaks between 327 and 400 nm with maxima at ∼345, 343 and 353 nm and broad emission spectra between 360 and 570 nm with maxima at ∼431, 433 and 443 nm, respectively. These emissions probably originate from intramolecular π–π* transitions. The excitation spectra of complexes 1–3A (Fig. 8) displayed two main absorption peaks between 325 and 420 nm with maxima at ∼326, 326 and 394 nm, respectively. Complexes 1 and 2 indicated blue emissions with maxima at ∼483 nm. Complex 3A exhibited a remarkable red-shift (∼101 nm) relative to 1 and 2 with broad yellow emission between 500 and 700 nm with maxima at 584 nm, which is comparable to the emissions (543–592 nm) in the quinolyl–triazole ligand supported copper(I)-iodide clusters and coordination polymers.³¹

Fig. 7 Normalized solution excitation (dotted line) and emission (solid line) spectra of ligands L1–L3.

Fig. 8 Normalized solid state excitation (dotted line) and emission (solid line) spectra of complexes 1–3A.

It is reasonable to assign this difference to the extended conjugation of the quinolyl moiety in L3 compared to pyridyl. The origins of the emissions in complexes 1–3A can be assigned to triplet metal-to-ligand charge transfer (MLCT), iodide-to-ligand charge transfer (XLCT) and/or cluster-centered (CC) excited states.^48,49 The different emission behaviours of complexes 1–3A are due to the combined substituent effects of the different supporting ligands, Cu(I) geometries, size difference of [Cu_nI_n] stair-step clusters and the different intramolecular Cu⋯Cu distances.

Conclusions

We have extended the application of CuAAC click chemistry to the preparation of hybrid NS ligands L1–L3 that provide diverse and stable coordination environments around Cu(I) centers in complexes 1–3. The variation of the substituent groups leads to stair-step cluster structures from a [Cu₄I₄] (1) to [Cu₆I₆] (2 and 3A) and a [Cu₈I₈] (3B) with remarkably different photoluminescence behavior between 3A and its pyridyl counterparts 1 and 2. Extending the bridge to the sulfur donor moiety brings this coordinating group into an active role in stabilizing large clusters. The isolation of complexes 3A and 3B raises the possibility for material growth and modification by introduction and integration of additional supporting interactions. It also raises the possibility of controlled and stepwise growth of metal clusters.

Acknowledgements

We are grateful to Dr L. L. Koh, G. K. Tan and Y. M. Hong for X-ray diffractometry measurement assistance. We acknowledge the financial support (IMRE/12-1P0907, IMRE/14-1C0248 & IMRE/15-1C0202) provided by the Institute of Materials Research and Engineering, A*STAR and the National University of Singapore.

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Footnote

† CCDC 1038927–1038930. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5qi00030k

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