Facile assembly of 1D multinuclear Ag_n (n = 11, 11, 12) alkynyl chains with CF₃COO⁻/CH₃COO⁻ as the auxiliary ligand

Abstract

Three 1D multinuclear silver(I) alkynyl compounds, [Ag₁₁(C≡C^tBu)₆(CF₃COO)₅(CH₃OH)]_n (1), [Ag₁₁(C≡C^tBu)₇(CH₃COO)₄]_n (2) and [Ag₁₂(C≡C^tBu)₇(CF₃COO)₅·H₂O]_n (3), constructed from AgC≡C^tBu ligand and CF₃COO⁻/CH₃COO⁻ auxiliary ligand, have been obtained by changing the type and source of the auxiliary ligand as well as adjusting the pH value in different solvents. The synthesis of 1–3 indicates that this kind of 1D multinuclear Ag_n (n = 11, 11, 12) alkynyl chain can be isolated through a facile one-pot process. Among these, an infinitely extended 2D supramolecular network is fabricated from 1D chains of 1 via hydrogen bonds, while 3 crystallizes in the chiral space group P2₁. In addition, the solid state optical diffuse reflection spectra and luminescence spectra of 1–3 have been obtained. Compound 2 is taken as an example to study the electrochemical behaviour of this kind of compound, and the results indicate that 2-CPE possesses good electrocatalytic activity toward the reduction of nitrite.

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Introduction

The simple coinage-metal (Cu, Ag, and Au) alkynyl compounds originally studied by Nast and Schindel et al. half a century ago have been widely discussed up to now.¹ Simple coinage-metal (Cu, Ag, and Au) alkynyl compounds attract a lot of attention, which not only stems from the structural diversity that they show,² but also originates from their promising applications as luminescent materials and rigid-rod molecular wires.³ Alkynyl ligands displaying flexible coordination environments can serve as good σ-donors and modest π-acceptors⁴ and metal–alkynyl σ, π and mixed (σ, π) bonds will be formed between metal centres and alkynyl ligands.⁵ Silver(Ι) alkynyl compounds have been actively researched among coinage-metal (Cu, Ag, and Au) alkynyl compounds because silver metal has an acceptable price, versatile coordination modes, and a high binding ability to donor ligands.⁶ Meanwhile, multiple argentophilic interactions exist in the central Ag skeletons during the formation of multinuclear silver(Ι) alkynyl compounds.⁷ Currently, several active groups, Mak, Wang, Zhao, and Jansen et al. are making contributions to the development and flourishing of multinuclear silver(Ι) alkynyl compounds. Many cases of multinuclear silver(Ι) alkynyl compounds constructed from ^tBuC≡C⁻ ligand and stabilised by other auxiliary ligands with a great diversity of configurations have been documented.^8–11

The crystal engineering of supramolecular architectures constructed by metal and organic building blocks has also been rapidly researched.¹² Among these, the rational design and synthesis of new extended supramolecular networks with higher dimensionalities by the assembly of low dimensional coordination complexes/polymers built by coordinative covalent bonds via hydrogen bonding interactions are of great interest.¹³ In fact, silver(Ι) alkynyl compounds have also been proven to be a better system for building supramolecular architectures through hydrogen bonding interactions. It is worth mentioning that Mak and Zhao et al. have reported eight coordination and hydrogen-bonded networks with the tert-Bu-C≡C⊃Ag_n (n = 4, 5) supramolecular synthon and so on.¹⁴

Initially, in order to construct multinuclear silver(Ι) alkynyl clusters induced by polyanions in our group,^15,16 many attempts were made through changing the type and source of the auxiliary ligand CF₃COO⁻/CH₃COO⁻ along with introducing several polyoxometalates (POMs) as template reagents in different solutions. Contrary to expectations, three 1D multinuclear silver(I) alkynyl compounds, [Ag₁₁(C≡C^tBu)₆(CF₃COO)₅(CH₃OH)]_n (1), [Ag₁₁(C≡C^tBu)₇(CH₃COO)₄]_n (2) and [Ag₁₂(C≡C^tBu)₇(CF₃COO)₅·H₂O]_n (3) have been obtained through a facile one-pot process. An extended 2D supramolecular network is fabricated from 1D covalently-bonded chains of 1 through hydrogen bonds between the coordinated ^tBuC≡C⁻ ligands and CF₃COO⁻ auxiliary ligands, while compound 3 crystallizes in the chiral space group P2₁. Herein, the synthesis, structures, characteristics and properties of 1–3 have been discussed.

Experimental section

Materials and measurements

In this paper, all reagents and solvents for synthesis were obtained from commercial sources and used without further purification, such as 3,3-dimethyl-1-butyne (Aladdin, >95%). All other reagents were of analytical grade and were used as received. [AgC≡C^tBu]_n,¹⁷ [Mn₁₂(CH₃COO)₁₆(H₂O)₄O₁₂]·2CH₃COOH·4H₂O, (NH₄)₃[CrMo₆H₆O₂₄]·7H₂O and α-K₆P₂W₁₈O₆₂·14H₂O were synthesized according to the literature.¹⁸ Elemental analyses (C and H) were performed on a Perkin-Elmer 2400 CHN elemental analyzer. Ag was analyzed on a PLASMA-SPEC(I) ICP atomic emission spectrometer. The FT-IR spectra were recorded from KBr pellets in the range of 4000–400 cm⁻¹ on a Mattson Alpha-Centauri spectrometer. PXRD patterns were recorded on a Siemens D 5005 diffractometer with Cu-Kα (λ = 1.5418 Å) radiation in the range of 3–50 °C. Luminescence was measured on an F-7000 FL Spectrophotometer. Diffuse reflectivity was measured from 200 to 800 nm using barium sulfate (BaSO₄) as a standard with 100% reflectance on a Varian Cary 500 UV-Vis spectrophotometer. A CHI 440 Electrochemical Quartz Crystal Microbalance was used for the electrochemical experiments. A conventional three-electrode cell was used at room temperature. The modified electrode was used as the working electrode. A SCE and a platinum wire were 0used as the reference and auxiliary electrodes, respectively.

Caution! Owing to the potential explosive nature of silver(I) alkynyls, great care should be taken, they should be handled with care and only small amounts should be used.

X-ray crystallographic analysis

Single-crystal X-ray diffraction data for compounds 1–3 were recorded on a Bruker Apex CCD II area-detector diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at room temperature. Absorption corrections were applied using a multi-scan technique and were performed by using the SADABS program. Their structures were solved by the direct method of SHELXS-97 and refined by full-matrix least-square techniques using the SHELXL-97 program within WINGX.¹⁹ All of the crystal data and structure refinement details for 1–3 are given in Table 1. Comparative experiments without POMs for the synthesis of 1–3 were performed as shown in Table S1.† Selected bond lengths (Å) and angles (°) for 1–3 are listed in Tables S2–S4.† Powder X-ray diffractions of 1–3 are included in Fig. S1–S3.†

Table 1 Crystal data and structure refinements for compounds 1–3

Compound	1	2	3
a R1 = ∑||Fo| − |Fc||/∑|Fo.b wR2 = {∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]}^1/2.
Empirical formula	C47H58F15O11Ag11	C50H75O8Ag11	C52H65F15O11Ag12
Formula weight	2266.47	1990.67	2445.48
Crystal system	Triclinic	Triclinic	Monoclinic
Space group	P1̄	P1̄	P21
a (Å)	14.551(4)	13.983(5)	11.490(5)
b (Å)	14.805(4)	14.005(5)	23.129(5)
c (Å)	18.520(7)	18.653(5)	14.609(5)
α (°)	110.957(4)	99.434(5)	90.000(5)
β (°)	92.602(4)	99.090(5)	106.035(5)
γ (°)	117.093(3)	116.326(5)	90.000(5)
V (Å^3)	3211.2(17)	3117.9(18)	3731(2)
Z	2	2	2
D/g cm^−3	2.344	2.120	2.177
μ/mm^−1	3.367	3.415	3.156
F(000)	2152	1912	2328
Reflection collected	15 742	15 420	18 864
Unique reflections	11 016	10 520	9684
Parameters	492	404	470
Rint	0.0573	0.0523	0.0385
GOF	0.950	0.993	1.036
R1^a [I > 2σ(I)]	0.0781	0.0639	0.0607
wR2^b (all data)	0.2233	0.1799	0.1663

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Synthesis

[Ag₁₁(C≡C^tBu)₆(CF₃COO)₅(CH₃OH)]_n (1). [AgC≡C^tBu]_n (0.0759 g, 0.4016 mmol) was dissolved in methanol (10 mL) under stirring. Then, CF₃COOH (0.2 mL, 0.0027 mmol) and [Mn₁₂(CH₃COO)₁₆(H₂O)₄O₁₂]·2CH₃COOH·4H₂O (0.0219 g, 0.0106 mmol) were added in turn under stirring to form a brown suspension. After stirring for 10 h, the suspension (pH ≈ 1.2) was filtered and the filtrate was evaporated slowly at room temperature in an Erlenmeyer flask. A few weeks later, we unexpectedly achieved compound 1, which was deposited as colorless block crystals. Yield: ca. 16% (based on Ag). Elemental analysis (%) calcd for C₄₇H₅₈F₁₅O₁₁Ag₁₁: C, 24.86; H, 2.57; Ag, 52.26. Found: C, 23.18; H, 2.97; Ag, 53.82. IR (KBr): ν = 1988 cm⁻¹ (vs, C≡C); 1683 cm⁻¹ (w, C=O).

[Ag₁₁(C≡C^tBu)₇(CH₃COO)₄]_n (2).
Method 1. [AgC≡C^tBu]_n (0.0588 g, 0.3111 mmol), (NH₄)₃[CrMo₆H₆O₂₄]·7H₂O (0.0206 g, 0.0172 mmol), Ni(CH₃COO)₂·4H₂O (0.0175 g, 0.0703 mmol), NaClO₄·H₂O (0.0083 g, 0.0591 mmol) and AgBF₄ (0.0173 g, 0.0889 mmol) were added in turn into ethanol–acetonitrile–dimethylformamide (v : v : v = 1 : 1 : 1) (15 mL) under ultrasonication. The mixture was stirred for 28 h to give a white suspension (pH ≈ 7.4), which was filtered and the filtrate was evaporated slowly at room temperature in an Erlenmeyer flask. A few days later, we had successfully achieved compound 2, which was deposited as colorless block crystals. Yield: ca. 29% (based on Ag).
Method 2. [AgC≡C^tBu]_n (0.0106 g, 0.0561 mmol) was dissolved in a solution of Co(CH₃COO)₂·4H₂O (0.0327 g, 0.1313 mmol) in ethanol (25 mL) under ultrasonication. Then, 1,10-phenanthroline monohydrate (0.0256 g, 0.1291 mmol) was added to the above resulting solution. The mixture was stirred for 20 h to give a light pink solution (pH ≈ 8.5). The solution was evaporated slowly at room temperature in an Erlenmeyer flask. A few days later, we had successfully achieved compound 2, which was deposited as colorless block crystals. Yield: ca. 21% (based on Ag). Elemental analysis (%) calcd for C₅₀H₇₅O₈Ag₁₁: C, 30.17; H, 3.80; Ag, 59.61. Found: C, 28.75; H, 4.08; Ag, 58.26. IR (KBr): ν = 2022 cm⁻¹ (vs, C≡C); 1661 cm⁻¹ (w, C=O).

[Ag₁₂(C≡C^tBu)₇(CF₃COO)₅·H₂O]_n (3). [AgC≡C^tBu]_n (0.0565 g, 0.2989 mmol) was dissolved in a solution of CF₃COOAg (0.1099 g, 0.4975 mmol) in methanol (10 mL) under ultrasonication. Then α-K₆P₂W₁₈O₆₂·14H₂O (0.0218 g, 0.0045 mmol) and H₂O (2 mL) were added to the above resulting solution. The mixture was stirred for 3 h to give a light yellow suspension. The reaction mixture was transferred to a Teflon-lined stainless autoclave (25 mL) and kept at 85 °C for 22 h. After cooling to room temperature, the solution (pH ≈ 4.7) was filtered and the filtrate was evaporated slowly at room temperature in an Erlenmeyer flask. A few weeks later, we had successfully achieved compound 3, which was deposited as colorless rectangular crystals. Yield: ca. 21% (based on Ag). Elemental analysis (%) calcd for C₅₂H₆₅F₁₅O₁₁Ag₁₂: C, 25.54; H, 2.68; Ag, 52.93. Found: C, 25.88; H, 2.49; Ag, 50.97. IR (KBr): ν = 2007 cm⁻¹ (vs, C≡C); 1677 cm⁻¹ (m, C=O).

Results and discussion

The three 1D chain compounds 1–3 were similarly synthesized by a facile one-pot process and their formations were strongly influenced by reaction conditions such as pH value, solvent, and the type and source of auxiliary ligand.

Effect of the pH value

It is well known that the pH can have an impact on the resulting coordination architectures, and optimization of the pH condition may also be considered as a structure-directing factor from the viewpoint of forming hydrogen-bonding supramolecular interactions.²⁰ In this work, the final mixture pH values were approximately 1.0–1.2, 7.4–8.5 and 3.7–4.7 for compounds 1, 2 and 3, respectively (as shown in Scheme 1). The lower pH value for 1 leads to an extended 2D supramolecular network, which is fabricated from the 1D covalently bonded chains of 1 via hydrogen bonds between the coordinated ^tBuC≡C⁻ ligands and CF₃COO⁻ auxiliary ligands.

Scheme 1 Synthetic procedures for compounds 1–3.

Effect of the solvent

Solvents also have significant effects on the final coordination architectures.²¹ CH₃OH not only serves as an organic solvent which provides the reaction medium but also acts as a ligand by attaching to the central silver skeletons in 1. Additionally, H₂O also plays the role of a ligand which coordinates to the central silver skeletons in 3.

Effect of the type and source of auxiliary ligand

The structural differences among the three multinuclear silver(I) alkynyl compounds 1–3 can also be ascribed to the effect of the type and source of auxiliary ligand. In detail, the type of carboxylate ligand is different between 1, 3 (CF₃COO⁻) and 2 (CH₃COO⁻), while the source of the CF₃COO⁻ auxiliary ligands is also different between 1 (CF₃COOH) and 3 (CF₃COOAg). Although similar Ag₁₁ skeletons of the columnar structures exist in 1 and 2, they finally display different configurations that are not only due to the different pH values and solvents used but also because of the difference between the CF₃COO⁻ and CH₃COO⁻ auxiliary ligands. Compound 2 provides a simple synthetic protocol for the preparation of stable silver(Ι) alkynyl compounds with higher yield by introducing CH₃COO⁻ auxiliary ligands instead of CF₃COO⁻.

Effect of the POMs

POMs were originally introduced as template reagents to induce the formation of core–shell silver(Ι) alkynyl clusters. However, three POMs in different configurations, namely, [Mn₁₂(CH₃COO)₁₆(H₂O)₄O₁₂]·2CH₃COOH·4H₂O for the synthesis of 1, (NH₄)₃[CrMo₆H₆O₂₄]·7H₂O for 2, and α-K₆P₂W₁₈O₆₂·14H₂O for 3, were not involved in building the target compounds. In order to understand and confirm what role the three POMs play in constructing compounds 1–3, comparative experiments were performed in the absence of POMs. Fortunately, compounds 1–3 can be obtained in the absence of POMs, because little change was observed in the pH values of the three reaction systems without POMs (as depicted in Table S1†). Furthermore, compound 2 can be synthesized not only by the self-assembly of [AgC≡C^tBu]_n and CH₃COO⁻ with/without (NH₄)₃[CrMo₆H₆O₂₄]·7H₂O (pH ≈ 7.4/8.0) as shown in method 1, but also through the reaction of [AgC≡C^tBu]_n and CH₃COO⁻ accompanied by other unreacted reagents replacing (NH₄)₃[CrMo₆H₆O₂₄]·7H₂O (pH ≈ 8.5) as illustrated in method 2. These results indicate that POMs are not necessary for the preparation of compounds 1–3 since they can be ignored or replaced if the pH values of the reaction systems are well controlled in the appropriate range.

Effect of cobalt/nickel metal cations

Compound 2 can be obtained through the two methods mentioned above, in which two different acetates, Ni(CH₃COO)₂·4H₂O and Co(CH₃COO)₂·4H₂O were used in method 1 and method 2, respectively. Ni(CH₃COO)₂·4H₂O and Co(CH₃COO)₂·4H₂O only act as acetates to provide the CH₃COO⁻ auxiliary ligand. Hence, Ni²⁺ and Co²⁺ make no difference in the synthesis of 2.

Effect of other factors

The coordination from η² to η⁶ of silver ions and the μ₂- to μ₄-coordination modes of ^tBuC≡C⁻ ligands together with argentophilic interactions also play important roles in the construction of the three silver(Ι) alkynyl compounds 1–3.

From what has been discussed above, we can see that this work implies that the crystallization conditions are important, because the synthesis of these compounds is affected not only by the pH value but also by the solvent, and the type and source of auxiliary ligand in the reaction systems. These results further provide us with a useful and important guide to the synthesis of other silver(Ι) alkynyl compounds by a facile one-pot process.

Powder X-ray diffraction analyses

To characterize the phase purities of compounds 1–3, powder X-ray diffraction (PXRD) patterns have been obtained at ambient temperature. The experimental PXRD patterns of compounds 1–3 are in agreement with the corresponding simulated ones, which demonstrate that the synthesized bulk materials and the measured single crystals are the same (Fig. S1–S3†). By contrast, the slight differences in intensities may be attributed to the preferred orientation of the crystalline powder samples, and the tiny peak shifts may be assigned to the baseline drift of the PXRD diffractometer and different measuring temperatures for single-crystal and powder diffraction.²²

Description of the crystal structures of 1–3

[Ag₁₁(C≡C^tBu)₆(CF₃COO)₅(CH₃OH)]_n (1). Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the triclinic P1̄ space group with a columnar structure. In a [Ag₁₁(C≡C^tBu)₆(CF₃COO)₅(CH₃OH)] asymmetric unit (Fig. 1a), it consists of eleven Ag(I) ions, six ^tBuC≡C⁻ ligands, five CF₃COO⁻ auxiliary ligands and one CH₃OH ligand. Herein, Ag1, Ag2, Ag4, Ag5, Ag6 and Ag7 are chelated by ^tBuC≡C⁻, CF₃COO⁻ and CH₃OH ligands, while Ag3, Ag8, Ag9, Ag10 and Ag11 are left open for connection to neighbouring asymmetric units through ^tBuC≡C⁻ and CF₃COO⁻ ligands. Consequently, as shown in Fig. 1b, a 1D chain in the shape of a columnar structure along the c axis is formed depending on Ag⋯Ag connections (2.849(2)–3.368(2) Å), Ag–C bonds (2.063(15)–2.707(15) Å) and Ag–O bonds (2.212(13)–2.561(13) Å). It can be seen that Ag⋯Ag connections lying in the range 2.849(2)–3.368(2) Å (partly shorter than twice the van der Waals radius for Ag: 3.44 Å) powerfully demonstrate that the skeleton of the columnar structure in 1 is promoted by the existence of important argentophilic Ag⋯Ag connections, which are clearly indicative of the presence of d¹⁰–d¹⁰ interactions.²³ The eleven silver ions display η² to η⁶ coordination modes. The ethynide moieties, C11≡C12 and C29≡C30 adopt the μ₃–η¹, η¹, η² mode; C17≡C18, C35≡C36 and C41≡C42 take the μ₄–η¹, η¹, η¹, η² mode; and C23≡C24 is in the μ₄–η¹, η¹, η², η² mode. Among the five bridging trifluoroacetate ligands, O1–O2, O5–O6 and O7–O8 are in the syn-μ₃-O, O′, O′ mode, and O3–O4 and O9–O10 are in the μ₂-O, O′ mode. In addition, the oxygen atom of one CH₃OH molecule is linked to Ag9 with the Ag9–O11 bond length of 2.561(15) Å. Finally, these 1D [Ag₁₁(C≡C^tBu)₆(CF₃COO)₅(CH₃OH)]_n chains are further connected by relatively weak hydrogen bonds C28–H28C⋯F4 (H⋯F 2.54 Å) between the coordinated ^tBuC≡C⁻ ligands and CF₃COO⁻ auxiliary ligands along the b direction to yield a hydrogen-bonded layer parallel to the bc plane, as shown in Fig. 2.

Fig. 1 (a) Atom labeling ball-and-stick model of the asymmetric unit in 1. (b) Standard ball-and-stick model of a 1D chain in 1. Hydrogen atoms are omitted for clarity. Color code: Ag, sea green; C, gray-25%; O, red; F, bright green.

Fig. 2 Polyhedron/standard ball-and-stick model of the 2D layer in 1 interwoven by hydrogen bonds parallel to the bc plane. Hydrogen atoms are omitted for clarity. Color code: Ag polyhedron, teal; C, gray-40%; O, red; F, bright green.

[Ag₁₁(C≡C^tBu)₇(CH₃COO)₄]_n (2). Single-crystal X-ray diffraction analysis reveals that compound 2 crystallizes in the triclinic P1̄ space group, and eleven Ag(I) ions, seven ^tBuC≡C⁻ ligands and four CH₃COO⁻ auxiliary ligands can be seen in an asymmetric unit. Herein, the η²–η⁵ coordination modes of the silver ions and five types of coordination modes for the ethynide moieties are observed in 2. The ethynide moieties; C27≡C28 bridges two silver ions in the μ₂–η¹, η¹ mode; C9≡C10 and C21≡C22 connect three silver ions in theμ₃–η¹, η¹, η¹ mode; C12≡C13 and C18≡C19 link three silver ions in the μ₃–η¹, η¹, η² mode; C15≡C16 concatenates four silver ions in the μ₄–η¹, η¹, η¹, η¹ mode; and C24≡C25 is bound to four silver ions in the μ₄–η¹, η¹, η¹, η² mode to take the shape of tert-Bu-C≡C⊃Ag_n (n = 2, 3, 4) units. Then, these units approach each other so that they coalesce by sharing Ag⋯Ag connections (2.8986(18)–3.376(2) Å) to generate a [Ag₁₁(C≡C^tBu)₇]⁴⁺ segment. This [Ag₁₁(C≡C^tBu)₇]⁴⁺ segment is further stabilized by four bridging CH₃COO⁻ ligands in the syn-μ₃-O, O′, O′ mode to make up a [Ag₁₁(C≡C^tBu)₇(CH₃COO)₄] aggregation (Fig. 3a). Such Ag₁₁ aggregations are fused through Ag⋯Ag connections, Ag–O bonds (2.247(13)–2.596(14) Å) and Ag–C bonds (2.046(15)–2.703(15) Å) to produce an infinite 1D coordination column along the c axis (Fig. 3b). Although ^tBuC≡C⁻ and CH₃COO⁻ ligands are coordinated to Ag₁₁ skeletons and warp the column up, tert-butyl groups and the CH₃ moieties protrude on both sides of this 1D chain to prevent further linkage.

Fig. 3 (a) Atom labeling ball-and-stick model of the asymmetric unit in 2. (b) Standard ball-and-stick model of a 1D chain in 2. Hydrogen atoms are omitted for clarity. Color code: Ag, sea green; C, gray-25%; O, red.

[Ag₁₂(C≡C^tBu)₇(CF₃COO)₅·H₂O]_n (3). Compound 3 crystallizes in the monoclinic P2₁ space group. An asymmetric unit comprises a [Ag₁₂(C≡C^tBu)₇(CF₃COO)₅·H₂O] aggregation (Fig. 4a), which is made up of a central Ag₁₂ skeleton surrounded by seven ^tBuC≡C⁻ ligands, five CF₃COO⁻ ligands and one aqua ligand with Ag⋯Ag connections ranging from 2.788(2) to 3.334(2) Å. The twelve silver ions are in the η²–η⁵ coordination modes. The ethynide moieties, C11≡C12, C23≡C24, C47≡C48 adopt the μ₃–η¹, η¹, η¹ ligation mode; C17≡C18, C35≡C36 take the μ₃–η¹, η¹, η² ligation mode; C41≡C42 exhibits the μ₄–η¹, η¹, η¹, η¹ ligation mode; and C29≡C30 is in the μ₄–η¹, η¹, η¹, η² ligation mode to make connections around the Ag₁₂ skeleton. As for the five CF₃COO⁻ ligands, except for the O9–O10 group in the syn-μ₃-O, O′, O′ mode and the O7–O8 group in the μ₁-O mode, the other groups lying above and below the Ag₁₂ skeleton span the Ag⋯Ag edges through the μ₂-O, O′ mode. The Ag–C bonds range from 1.959(15) to 2.708(18) Å, and the Ag–O bonds are in the range of 2.128(16)–2.584(17) Å. In addition, an aqua ligand coordinates with Ag12 with the Ag12–O11W bond length of 2.52(3) Å. Then, a 1D chain along the a axis made up of inversion-related [Ag₁₂(C≡C^tBu)₇(CF₃COO)₅·H₂O] aggregations bridged pairwise is formed (Fig. 4b).

Fig. 4 (a) Atom labeling ball-and-stick model of the asymmetric unit in 3. (b) Standard ball-and-stick model of a 1D chain in 3. Hydrogen atoms are omitted for clarity. Color code: Ag, sea green; C, gray-25%; O, red; F, bright green.

Solid state optical diffuse reflection spectra of 1–3

Some multinuclear silver clusters have been reported to be promising semiconductive materials.¹⁶ Therefore, the solid state optical diffuse reflection spectra of 1–3 were measured at room temperature to obtain their band gaps (E_g, photoresponse wavelength region). The E_g term is defined as the intersection point between the energy axis and the line extrapolated from the linear portion of the adsorption edge in a plot of the Kubelka–Munk function F versus energy E.²⁴ Absorption data were calculated from the reflectance using the Kubelka–Munk function. As depicted in Fig. S4–S6,† the UV-vis-NIR absorption spectra of 1–3 in the solid state display absorption peaks at ca. 425, 442 and 438 nm, and the energy gaps of 1–3 (see inset of Fig. S4–S6†) were determined to be 3.60, 2.73 and 2.92 eV, respectively. The results indicate that compounds 1–3 are a kind of potential wide band gap semiconductor material.

Luminescence properties of 1–3

Because compounds with d¹⁰-transition metal centers, such as copper, silver and gold, have excellent luminescence properties,²⁵ the luminescence spectra of 1–3 in the solid state have been obtained at room temperature. Compounds 1–3 show similar luminescence spectra with a maximum emission band at ca. 421 nm and several shoulder peaks upon excitation at 365 nm (Fig. S7–S9†). With reference to our previous spectroscopic work on related polynuclear silver clusters²⁶ and the luminescence spectra of the AgC≡C^tBu ligand in the solid state (Fig. S10†), the emissions of 1–3 are probably ligand-to-metal-charge-transfer (LMCT) transitions.

Electrochemical behavior of 2

Because compounds 1–3 have similar structures as well as consisting of the same silver(Ι) ions and ^tBuC≡C⁻ ligands, compound 2 is taken as an example to study in detail the electrochemical behaviour of this kind of compound. Hence, a 2-bulk-modified carbon paste electrode (2-CPE) was fabricated as the working electrode to study the electrochemical properties of this type of silver(I) alkynyl compound.²⁷ As shown in Fig. 5, the voltammetric behavior of 2-CPE in a 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution at a scan rate of 0.1 V s⁻¹ displays an obvious redox couple in the potential range of 0 to 600 mV. The mean peak potential E_1/2 = (E_pa + E_pc)/2 is 267 mV for 2-CPE, which is similar to the AgC≡C^tBu ligand (E_1/2 = 285 mV) (vs. SCE). Thus, the redox peak for 2 is attributed to Ag⁺/Ag⁰ couples, but the slightly different peak potentials between 2 and the AgC≡C^tBu ligand should be affected by the final composition of 2.^9f,10a

Fig. 5 Cyclic voltammogram of 2-CPE (0 to 600 mV) in a 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution. Scan rate: 100 mV s⁻¹. The inset shows the cyclic voltammogram of the AgC≡C^tBu ligand (−250 to 1000 mV) in a 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution.

The effect of scan rate on the electrochemical behavior of 2-CPE was investigated in the potential range of 0 to 600 mV in a 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution. As shown in Fig. 6, with the scan rate increasing from 120 to 280 mV s⁻¹, the peak potential changes gradually: the cathodic peak potential gradually shifts in the negative direction and the corresponding anodic peak potential shifts in the positive direction. The inset of Fig. 6 shows that the peak currents are proportional to the scan rate up to 280 mV s⁻¹ for 2-CPE, suggesting that the redox process for 2-CPE is surface-controlled.²⁸

Fig. 6 Cyclic voltammograms of the 2-CPE in the 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution at different scan rates (from inner to outer: 120, 140, 160, 180, 200, 220, 240, 260, 280 mV s⁻¹). The inset shows the plots of the anodic and cathodic peak currents against scan rates.

As we know, the electroreduction of nitrite requires a large overpotential and no response has been observed in the potential range of 0 to 600 mV on a bare CPE in a solution containing nitrite. However, 2-CPE displays excellent electrocatalytic activity toward the reduction of nitrite, as shown in Fig. 7.²⁹ With the addition of nitrite, the reduction peak current increased while the corresponding oxidation peak current decreased, suggesting that 2-CPE possesses good electrocatalytic activity toward the reduction of nitrite.

Fig. 7 Cyclic voltammograms of 2-CPE in a 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution containing 0.0–8.0 mM KNO₂ and a bare CPE in a 1.0 mM KNO₂ + 0.01 M H₂SO₄ + 0.5 M Na₂SO₄ aqueous solution (scan rate: 100 mV s⁻¹).

Conclusions

In summary, the preparation of multinuclear silver(Ι) alkynyl compounds constructed from ^tBuC≡C⁻ ligand with a wide variety of structures is affected by many factors, such as the pH value of the reaction system, solvent, the type and source of auxiliary ligand, and so on. In this paper, three 1D silver(Ι) alkynyl compounds, [Ag₁₁(C≡C^tBu)₆(CF₃COO)₅(CH₃OH)]_n (1), [Ag₁₁(C≡C^tBu)₇(CH₃COO)₄]_n (2) and [Ag₁₂(C≡C^tBu)₇(CF₃COO)₅·H₂O]_n (3) have been acquired by changing the type (CF₃COO⁻/CH₃COO⁻) and source of auxiliary ligand along with adjusting the pH values in different solvents. Notably, an extended 2D supramolecular network is fabricated from 1D covalently-bonded chains of 1 via hydrogen bonds. Furthermore, the synthesis, structures, characteristics and properties of 1–3 have been discussed. Further work on the construction of multinuclear silver(I) alkynyl compounds is underway in our group.

Acknowledgements

This work was financially supported by the NSFC of China (no. 21471027, 21171033, 21131001, 21222105), the National Key Basic Research Program of China (no. 2013CB834802), the Foundation for Author of National Excellent Doctoral Dissertation of P.R. China (FANEDD) (no. 201022), The Science and Technology Development Planning of Jilin Province (no. 20140203006GX), Changbai mountain scholars of Jilin Province and FangWu distinguished young scholar of NENU, and the Fundamental Research Funds for the Central Universities (12SSXT142).

Notes and references

1. (a) R. Nast and H. Schindel, Z. Anorg. Allg. Chem., 1963, 326, 201; (b) R. Nast, Coord. Chem. Rev., 1982, 47, 89; (c) H. Schmidbaur, Chem. Soc. Rev., 1995, 24, 391; (d) V. W.-W. Yam, W. K.-M. Fung and K.-K. Cheung, Angew. Chem., Int. Ed. Engl., 1996, 35, 1110; (e) T. C. W. Mak and L. Zhao, Chem.–Asian J., 2007, 2, 456; (f) Y.-P. Xie and T. C. W. Mak, J. Am. Chem. Soc., 2011, 133, 3760; (g) C.-Y. Gao, L. Zhao and M.-X. Wang, J. Am. Chem. Soc., 2011, 133, 8448; (h) C. S. Ma, C. T.-L. Chan, W.-M. Kwok and C.-M. Che, Chem. Sci., 2012, 3, 1883; (i) P. Maity, T. Wakabayashi, N. Ichikuni, H. Tsunoyama, S. H. Xie, M. Yamauchi and T. Tsukuda, Chem. Commun., 2012, 48, 6085; (j) L. Rodríguez, M. Ferrer, R. Crehuet, J. Anglada and J. C. Lima, Inorg. Chem., 2012, 51, 7636.
2. (a) G.-C. Guo, G.-D. Zhou and T. C. W. Mak, J. Am. Chem. Soc., 1999, 121, 3136; (b) H. Lang, D. S. A. George and G. Rheinwald, Coord. Chem. Rev., 2000, 206–207, 101; (c) L. Zhao and T. C. W. Mak, J. Am. Chem. Soc., 2005, 127, 14966; (d) T. C. W. Mak, X.-L. Zhao, Q.-M. Wang and G.-C. Guo, Coord. Chem. Rev., 2007, 251, 2311; (e) B. Li, S.-Q. Zang, R. Liang, Y.-J. Wu and T. C. W. Mak, Organometallics, 2011, 30, 1710; (f) P.-S. Cheng, S. Marivel, S.-Q. Zang, G.-G. Gao and T. C. W. Mak, Cryst. Growth Des., 2012, 12, 4519.
3. (a) Y.-Y. Lin, S.-W. Lai, C.-M. Che, K.-K. Cheung and Z.-Y. Zhou, Organometallics, 2002, 21, 2275; (b) M. P. Cifuentes and M. G. Humphrey, J. Organomet. Chem., 2004, 689, 3968; (c) V. W.-W. Yam and K. M.-C. Wong, Top. Curr. Chem., 2005, 257, 1; (d) Q.-H. Wei, G.-Q. Yin, L.-Y. Zhang and Z.-N. Chen, Inorg. Chem., 2006, 45, 10371; (e) Z.-N. Chen, N. Zhao, Y. Fan and J. Ni, Coord. Chem. Rev., 2009, 253, 1; (f) B. R. Buckley, A. N. Khan and H. Heaney, Chem.–Eur. J., 2012, 18, 3855; (g) I. O. Koshevoy, Y.-C. Chang, A. J. Karttunen, S. I. Selivanov, J. Jänis, M. Haukka, T. Pakkanen, S. P. Tunik and P.-T. Chou, Inorg. Chem., 2012, 51, 7392; (h) M. C. Blanco, J. Cámara, M. C. Gimeno, A. Laguna, S. L. James, M. C. Lagunas and M. D. Villacampa, Angew. Chem., Int. Ed., 2012, 51, 9777.
4. (a) D. Rais, D. M. P. Mingos, R. Vilar, A. J. P. White and D. J. Williams, J. Organomet. Chem., 2002, 652, 87; (b) S.-Q. Zang and T. C. W. Mak, Inorg. Chem., 2008, 47, 7094; (c) L. Zhao and T. C. W. Mak, Inorg. Chem., 2009, 48, 6480.
5. (a) L. Zhao, W.-Y. Wong and T. C. W. Mak, Chem.–Eur. J., 2006, 12, 4865; (b) S. C. K. Hau and T. C. W. Mak, Chem.–Eur. J., 2013, 19, 5387.
6. (a) G.-C. Guo and T. C. W. Mak, Angew. Chem. Int. Ed., 1998, 37, 3183; (b) C.-M. Che, M.-C. Tse, M. C. W. Chan, K.-K. Cheung, D. L. Phillips and K.-H. Leung, J. Am. Chem. Soc., 2000, 122, 2464; (c) P.-S. Cheng, S. Marivel, S.-Q. Zang, G.-G. Gao and T. C. W. Mak, Cryst. Growth Des., 2012, 12, 4519.
7. (a) G.-G. Gao, P.-S. Cheng and T. C. W. Mak, J. Am. Chem. Soc., 2009, 131, 18257; (b) J. Qiao, K. Shi and Q.-M. Wang, Angew. Chem., Int. Ed., 2010, 49, 1765; (c) G. Li, Z. Lei and Q.-M. Wang, J. Am. Chem. Soc., 2010, 132, 17678.
8. (a) D. Rais, D. M. P. Mingos, R. Vilar, A. J. P. White and D. J. Williams, Organometallics, 2000, 19, 5209; (b) X.-D. Chen, M. Du and T. C. W. Mak, Chem. Commun., 2005, 4417; (c) S.-D. Bian, J.-H. Jia and Q.-M. Wang, J. Am. Chem. Soc., 2009, 131, 3422; (d) F. Gruber, M. Schulz-Dobrick and M. Jansen, Chem.–Eur. J., 2010, 16, 1464; (e) C.-Y. Gao, L. Zhao and M.-X. Wang, J. Am. Chem. Soc., 2012, 134, 824.
9. (a) D. Rais, J. Yau, D. M. P. Mingos, R. Vilar, A. J. P. White and D. J. Williams, Angew. Chem., Int. Ed., 2001, 40, 3464; (b) O. M. Abu-Salah, M. H. Ja’far, A. R. Al-Ohaly, K. A. Al-Farhan, H. S. Al-Enzi, O. V. Dolomanov and J. A. K. Howard, Eur. J. Inorg. Chem., 2006, 2353; (c) M.-L. Chen, X.-F. Xu, Z.-X. Cao and Q.-M. Wang, Inorg. Chem., 2008, 47, 1877; (d) S.-D. Bian, H.-B. Wu and Q.-M. Wang, Angew. Chem., Int. Ed., 2009, 48, 5363; (e) G.-G. Gao, P.-S. Cheng and T. C. W. Mak, J. Am. Chem. Soc., 2009, 131, 18257; (f) J. Qiao, K. Shi and Q.-M. Wang, Angew. Chem., Int. Ed., 2010, 49, 1765; (g) F. Gruber and M. Jansen, Angew. Chem., Int. Ed., 2010, 49, 4924.
10. (a) Y.-P. Xie and T. C. W. Mak, Chem. Commun., 2012, 48, 1123; (b) S. C. K. Hau, P.-S. Cheng and T. C. W. Mak, J. Am. Chem. Soc., 2012, 134, 2922; (c) Y.-P. Xie and T. C. W. Mak, Inorg. Chem., 2012, 51, 8640; (d) Y.-P. Xie and T. C. W. Mak, Angew. Chem., Int. Ed., 2012, 51, 8783; (e) Y.-Y. Li, F. Gao, J. E. Beves, Y.-Z. Li and J.-L. Zuo, Chem. Commun., 2013, 49, 3658; (f) Z.-G. Jiang, K. Shi, Y.-M. Lin and Q.-M. Wang, Chem. Commun., 2014, 50, 2353.
11. S.-D. Bian and Q.-M. Wang, Chem. Commun., 2008, 5586.
12. (a) M. Fujita, Y. J. Kwon, S. Washizu and K. Ogura, J. Am. Chem. Soc., 1994, 116, 1151; (b) H. Oshio, Y. Saito and T. Ito, Angew. Chem., Int. Ed. Engl., 1997, 36, 2673; (c) P. J. Hagrman, D. Hagrman and J. Zubieta, Angew. Chem., Int. Ed., 1999, 38, 2638; (d) J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon and K. Kim, Nature, 2000, 404, 982; (e) E. Coronado, J. R. Galán-Mascarós, C. J. Gómez-García and V. Laukhin, Nature, 2000, 408, 447; (f) Y. H. Wang, L. Y. Feng, Y. G. Li, C. W. Hu, E. B. Wang, N. H. Hu and H. Q. Jia, Inorg. Chem., 2002, 41, 6351.
13. (a) D. Braga, F. Grepioni and G. R. Desiraju, Chem. Rev., 1998, 98, 1375; (b) H. L. Li, M. Eddaoudi, M. O’Keeffe and O. M. Yaghi, Nature, 1999, 402, 276; (c) S. S.-Y. Chui, S. M.-F. Lo, J. P. H. Charmant, A. G. Orpen and I. D. Williams, Science, 1999, 283, 1148; (d) D. Braga and F. Grepioni, Acc. Chem. Res., 2000, 33, 601; (e) J. C. MacDonald, P. C. Dorrestein, M. M. Pilley, M. M. Foote, J. L. Lundburg, R. W. Henning, A. J. Schultz and J. L. Manson, J. Am. Chem. Soc., 2000, 122, 11692; (f) B. J. Holliday and C. A. Mirkin, Angew. Chem., Int. Ed., 2001, 40, 2022; (g) D. Armentano, G. D. Munno, F. Lloret, A. V. Palii and M. Julve, Inorg. Chem., 2002, 41, 2007; (h) J.-M. Lehn, Science, 2002, 295, 2400.
14. (a) S.-Q. Zang, L. Zhao and T. C. W. Mak, Organometallics, 2008, 27, 2396; (b) L. Zhao, C.-Q. Wan, J. Han, X.-D. Chen and T. C. W. Mak, Chem.–Eur. J., 2008, 14, 10437; (c) B. Li, R.-W. Huang, H.-C. Yao, S.-Q. Zang and T. C. W. Mak, CrystEngComm, 2014, 16, 723.
15. (a) K. Zhou, C. Qin, H.-B. Li, L.-K. Yan, X.-L. Wang, G.-G. Shan, Z.-M. Su, C. Xu and X.-L. Wang, Chem. Commun., 2012, 48, 5844; (b) K. Zhou, X.-L. Wang, C. Qin, H.-N. Wang, G.-S. Yang, Y.-Q. Jiao, P. Huang, K.-Z. Shao and Z.-M. Su, Dalton Trans., 2013, 42, 1352; (c) K. Zhou, C. Qin, X.-L. Wang, L.-K. Yan, K.-Z. Shao and Z.-M. Su, CrystEngComm, 2014, 16, 10376.
16. (a) K. Zhou, C. Qin, X.-L. Wang, K.-Z. Shao, L.-K. Yan and Z.-M. Su, CrystEngComm, 2014, 16, 7860; (b) K. Zhou, C. Qin, X.-L. Wang, K.-Z. Shao, L.-K. Yan and Z.-M. Su, Dalton Trans., 2014, 43, 10695; (c) K. Zhou, Y. Geng, L.-K. Yan, X.-L. Wang, X.-C. Liu, G.-S. Shan, K.-Z. Shao, Z.-M. Su and Y.-N. Yu, Chem. Commun., 2014, 50, 11934; (d) K. Zhou, C. Qin, L.-K. Yan, W.-E. Li, X.-L. Wang, H.-N. Wang, K.-Z. Shao and Z.-M. Su, Dyes and Pigments, 2015, 113, 299.
17. (a) L. Zhao, C.-Q. Wan, J. Han, X.-D. Chen and T. C. W. Mak, Chem.–Eur. J., 2008, 14, 10437; (b) Z.-G. Jiang, K. Shi, Y.-M. Lin and Q.-M. Wang, Chem. Commun., 2014, 50, 2353.
18. (a) T. Lis, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1980, 36, 2042; (b) K. Nomiya, T. Takahashi, T. Shirai and M. Miwa, Polyhedron, 1987, 6, 213; (c) W. G. Klemperer and O. M. Yaghi, Inorg. Synth., 1990, 27, 105.
19. G. M. Sheldrick, SHELXL-97, Program for X-ray Crystal Structure Refinement, University of Göttingen, Göttingen, Germany, 1997.
20. (a) S.-L. Li, K. Tan, Y.-Q. Lan, J.-S. Qin, M.-N. Li, D.-Y. Du, H.-Y. Zang and Z.-M. Su, Cryst. Growth Des., 2010, 10, 1699; (b) X. X. Xu, X. Zhang, X. X. Liu, T. Sun and E. B. Wang, Cryst. Growth Des., 2010, 10, 2272; (c) Z. Su, J. Fan, T. Okamura, W.-Y. Sun and N. Ueyama, Cryst. Growth Des., 2010, 10, 3515; (d) X.-L. Wang, J. Luan, F.-F. Sui, H.-Y. Lin, G.-C. Liu and C. Xu, Cryst. Growth Des., 2013, 13, 3561; (e) H.-M. Zhang, Y.-C. He, J. Yang, Y.-Y. Liu and J.-F. Ma, Cryst. Growth Des., 2014, 14, 2307.
21. (a) G.-S. Yang, Z.-L. Lang, H.-Y. Zang, Y.-Q. Lan, W.-W. He, X.-L. Zhao, L.-K. Yan, X.-L. Wang and Z.-M. Su, Chem. Commun., 2013, 49, 1088; (b) Y.-C. He, J. Guo, H.-M. Zhang, J.-F. Ma and Y.-Y. Liu, CrystEngComm, 2014, 16, 5450.
22. X.-L. Wang, F.-F. Sui, H.-Y. Lin, J.-W. Zhang and G.-C. Liu, Cryst. Growth Des., 2014, 14, 3438.
23. (a) T. L. Zhang, J. X. Kong, Y. J. Hu, X. G. Meng, H. B. Yin, D. S. Hu and C. P. Ji, Inorg. Chem., 2008, 47, 3144; (b) D. Sun, Z.-H. Wei, D.-F. Wang, N. Zhang, R.-B. Huang and L.-S. Zheng, Cryst. Growth Des., 2011, 11, 1427; (c) D. Sun, F.-J. Liu, R.-B. Huang and L.-S. Zheng, Inorg. Chem., 2011, 50, 12393; (d) S. Yuan, Y.-K. Deng, X.-P. Wang and D. Sun, New J. Chem., 2013, 37, 2973; (e) B. Li, J.-H. Liao, Y.-J. Li and C. W. Liu, CrystEngComm, 2013, 15, 6140; (f) D. Sun, H. Wang, H.-F. Lu, S.-Y. Feng, Z.-W. Zhang, G.-X. Sun and D.-F. Sun, Dalton Trans., 2013, 42, 6281; (g) H. Wang, C.-Q. Wan and T. C. W. Mak, Dalton Trans., 2014, 43, 7254.
24. (a) G. D. Santis, L. Fabbrizzi, M. Licchelli, A. Poggi and A. Taglietti, Angew. Chem., Int. Ed. Engl., 1996, 35, 202; (b) Q. G. Wu, M. Esteghamatian, N.-X. Hu, Z. Popovic, G. Enright, Y. Tao, M. D’Iorio and S. N. Wang, Chem. Mater., 2000, 12, 79; (c) J. E. McGarrah, Y.-J. Kim, M. Hissler and R. Eisenberg, Inorg. Chem., 2001, 40, 4510.
25. (a) M.-L. Chen, X.-F. Xu, Z.-X. Cao and Q.-M. Wang, Inorg. Chem., 2008, 47, 1877; (b) M.-X. Yang, L.-J. Chen, S. Lin, X.-H. Chen and H. Huang, Dalton Trans., 2011, 40, 1866.
26. (a) V. W.-W. Yam, K. K.-W. Lo, W. K.-M. Fung and C.-R. Wang, Coord. Chem. Rev., 1998, 171, 17; (b) J.-H. Jia and Q.-M. Wang, J. Am. Chem. Soc., 2009, 131, 16634; (c) S. Perruchas, C. Tard, X. F. L. Goff, A. Fargues, A. Garcia, S. Kahlal, J.-Y. Saillard, T. Gacoin and J.-P. Boilot, Inorg. Chem., 2011, 50, 10682; (d) D. Sun, L. L. Zhang, H. F. Lu, S. Y. Feng and D. F. Sun, Dalton Trans., 2013, 42, 3528; (e) B. Li, R.-W. Huang, J.-H. Qin, S.-Q. Zang, G.-G. Gao, H.-W. Hou and T. C. W. Mak, Chem.–Eur. J., 2014, 20, 12416.
27. (a) X.-L. Wang, H.-Y. Lin, B. Mu, A.-X. Tian, G.-C. Liu and N.-H. Hu, CrystEngComm, 2011, 13, 1990; (b) X.-L. Wang, B. Mu, H.-Y. Lin, G.-C. Liu, A.-X. Tian and S. Yang, CrystEngComm, 2012, 14, 1001; (c) X.-L. Wang, B. Mu, H.-Y. Lin, S. Yang, G.-C. Liu, A.-X. Tian and J.-W. Zhang, Dalton Trans., 2012, 41, 11074; (d) X. L. Wang, C. Xu, H. Y. Lin, G. C. Liu, J. Luan and Z. H. Chang, RSC Adv., 2013, 3, 3592.
28. (a) E. H. Kim, D. I. Kim, H. S. Lee, J. C. Byun, J. H. Choi and Y. C. Park, Polyhedron, 2007, 26, 85; (b) X. L. Wang, H. Y. Zhao, H. Y. Lin, G. C. Liu, J. N. Fang and B. K. Chen, Electroanalysis, 2008, 20, 1055.
29. (a) B. Keita and L. Nadjo, J. Electroanal. Chem., 1987, 227, 77; (b) X.-L. Wang, B. Mu, H.-Y. Lin and G.-C. Liu, J. Organomet. Chem., 2011, 696, 2313; (c) X. L. Wang, J. Luan, H. Y. Lin, Q. L. Lu, C. Xu and G. C. Liu, Dalton Trans., 2013, 42, 8375.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 926484, 926485 and 926482. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra09876e

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