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Multifunctional Triple-Decker Inverse 12-Metallacrown-4 Sandwiching Halides Ling-Yu Guo, Hai-Feng Su, Mohamedally Kurmoo, Xingpo Wang, QuanQin Zhao, Shui-Chao Lin, Chen-Ho Tung, Di Sun, and Lan-Sun Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 28, 2017

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ACS Applied Materials & Interfaces

Multifunctional Triple-Decker Inverse 12-Metallacrown-4 Sandwiching Halides Ling-Yu Guo,†,# Hai-Feng Su,‡,# Mohamedally Kurmoo,§ Xing-Po Wang,† Quan-Qin Zhao,† Shui-Chao Lin,† Chen-Ho Tung,† Di Sun*,† Lan-Sun Zheng‡ †

Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, P. R. China. ‡ State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China. § Institut de Chimie de Strasbourg, Université de Strasbourg, CNRS-UMR 7177, 4 rue Blaise Pascal, 67008 Strasbourg Cedex, France. #

These authors contributed equally to this work.

ABSTRACT: A family of six triple-decker complexes, {(MX)2[CuI4(pz)4]3}, (Hpz = 4-nitropyrazole, MX = NaCl, 1; NaBr, 2; NaI, 3; KCl, 4; KBr, 5 and KI, 6, displaying inclusion of halides into inverse 12-metallacrown-4 (inv-[12-MCCu(I), pz-4]) array has been realized. Single-crystal X-ray crystallography of each compound reveals a common structural feature consisting of a four CuI ions are bonded by four pz to form a square metallomacrocycle comprising four metal centers and eight N atoms, thus giving an inv-[12MCCu(I), pz-4] motif. Two halides are sandwiched by three inv-[12-MCCu(I), pz-4] to form triple-deckers, which are further extended in an offset stacking mode by ligand-unsupported cuprophilicity interactions to form a 1D chain structure. Halides are attached to six CuI centers with weak CuI···halogen interaction, resembling anion templates. High-resolution electrospray ionization mass spectrometry (HRESI-MS) reveals that the predominant fragments corresponding to a half of the triple-decker structures of 1-3 exist in solution. Compound 4, 5 and 6 showed excellent electrocatalytic activities toward reduction of nitrite and can be also used as selective ‘turn-off’ sensors for Ag(I) in water. The present results will be helpful for the future design and synthesis of functional inverse metallacrowns as well as their multiple-decker complexes. KEYWORDS: metallacrowns, inclusion compound, solution stability, luminescence sensor, electrocatalysis yashi groups have reported several series of tetrapyrrolesandwiched metal complexes from double- and triple-decker to the new quadruple- and quintuple-decker, and even largest sextuple-decker in this family to date.13 They attributed the formation of such complex multiple-deckers to the effective intramolecular π···π orbital overlapping from aromatic macrocyclic ligands, which largely limited the development of multiple-decker molecules due to the scarcity of organic conjugate ligands. Although the advances of metallacrowns have cast new light on this research area, multiple-decker molecules built from them are still extremely rare.14 Burini and Fackler et. al., first unmasked a double-decker Ag{[Au(bzim)]3}2 (bzim = 1-benzylimidazolate) with one Ag atom sandwiched by two 9MCAu-3 units.15 Fujita et. al., reported a cage-trapped tripledecker cluster constructed from three 9-MCAu-3 units followed by the uptake of two silver atoms between them.16 From the above results, we speculate that larger metallacrowns may not favor the formation of multiple-decker arrays because of the larger macrocyclic motifs result in more distortion from planarity, as supported by some saddle-like 12-MCCu-4 geometries. In this work, based on larger inverse 12-MCCu(I),pz-4 ([CuI(pz)]4), we realized the synthesis of triple-decker complex from novel planar metallocrown and halides (Scheme 1). Its 4-fold symmetric structure highly resembles an inverse triple-decker Ln(III)-phthalocyaninato/porphyrinato sandwich complexes with the replacement of Ln(III) by halide and organic ligand by metallocrown. The synergetic effects from both electron-withdrawing nitro group and electron-donating halide template are responsible for the formation of such unusual triple-decker complexes. Moreover, solution stability, electrochemistry behavior, electrocatalysis performance and the silver ion sensing property were studied in details.

INTRODUCTION Since the macrocyclic crown ethers was first synthesized by the Nobel Laureate Pederson,1 cavity-accessible molecules have captivated chemists due to their capability to bind cations and anions, trap transient species, and act as a molecular flask for chemical reactions.2 Their inorganic analogues, metallacrowns, have similar skeletons and adjustable cavities, but also incorporate metal atoms into them, which, as a result, produced several additional functional properties such as singlemolecule magnet, bioactivity, catalysis and molecular recognition from either cavity or metal center, or both.3 Over recent years, abundant metallacrowns constructed using various metal and ligands have been reported and were comprehensively reviewed by Pecoraro in 2007.4 Among the ligands used in the construction of metallacrowns, pyrazole and its derivatives are the least studied, although they have electronically similar neighboring N atoms that can replace adjacent methylene carbon atoms of a crown ether, that could clamp more than one metal atoms in close proximity.4 Although some polynuclear coinage complexes with pyrazole or its derivatives have been reported,5 most of them are the trinuclear 9-MCCu/Ag/Au-3 complexes.6 There are sporadic tetranuclear 12-MCCu-4 complexes, such as [Cu(dppz)]4 (Hdppz = 3,5-diphenylpyrazole) observed,7 but the majority are not planar molecule but adopt a saddle-like conformation (D2 point symmetry),8 and planar 4fold 12-MCCu-4 complexes. Multiple-decker structures in organometallic chemistry9 with a simplest example of ferrocene10 have been considerably developed for Ln(III)-phthalocyaninato/porphyrinato sandwich complexes,11 which exhibited great potential in the fields of molecular magnets such as single-molecule magnets and single-ion magnets due to their unique coordination symmetry resulting in very large magnetic anisotropy.12 Jiang and Koba- 1-

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RESULTS AND DISCUSSION Synthesis The metallacrown complexes were synthesized in mixed DMF-ethanol (v:v = 1:5) solution by using a one-pot solvothermal reaction at 160 °C from CuII(BF4)2 and 4nitropyrazole in the presence of different MX salts (MX = NaCl, 1; NaBr, 2; NaI, 3; KCl, 4; KBr, 5 and KI, 6). The resulting complexes were isolated in moderate yields (50~60 %). In situ reduction of CuII to CuI is not uncommon as seen in many CuI coordination complexes. The MX salts have important influences on the final oxidation state of the Cu in the products. A hexanuclear CuII cluster II ([F@Cu 6(pz)9(OH)2(DMF)4(H2O)]·2DMF, 7) was isolated when NaF was used. Different substituted pyrazole ligands, such as pyrazole, 3,5-dimethyl-pyrazole, 3-methyl-pyrazole, 4-methyl-pyrazole, 4-chloro-pyrazole, 4-bromo-pyrazole and 4-iodo-pyrazole, have been used in this system, but only unknown precipitates formed after solvothermal reactions, which indicated the substituent groups also impact the formation of triple-decker complexes.

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metallacrown and gives an approximately square window. If weak CuI···Cl interaction is included, each CuI is in T-shaped three-coordinate geometry completed by one Cl- and two N atoms of two different pzligands making an average N-Cu-N angle of 158.2o. Such deviation from linearity should be pushed by Cl- in the perpendicular direction with respect to NCu-N orientation. T-shaped coordinate geometry of Cu(I) has been widely observed in biological protein such as hemocyanin18 and also several Cu(I) coordination compounds.19 In contrast, the N-Cu-N angles in 7 tend to linearity (165.15(10) 179.60(11)°), which indicates that the stronger interaction between Cu and Cl/Br/I helps stabilize the otherwise more constrained and thus less stable 12-MCCu(I), pz-4. The Cu-N bond distances vary from 1.868(5) to 1.882(5) Å. Four pzligands are oriented in almost the same plane coincided with that built from four CuI centers. This geometry is substantially different from that of [Cu(dppz)]4 (Hdppz = 3,5diphenylpyrazole)7 with every two dppz ligands project above and below the Cu4 ring, forming a saddle-like molecule. Four nitro groups point to the outside of the metallacrown and thus giving a potential coordination site (see below).

Scheme 1. (a) Organic 12-crown-4 ether and its inorganic analogue, inverse 12-metallacrown-4. (b) Schematic representation for the assembly of triple-decker array from 12-MCCu-4 (gray disk) and halogen ions (green ball).

X-ray Structures of 1-6. The compounds 1–6 have similar one-dimensional structure based on centrosymmetric triple-decker subunits, as evidenced by X-ray single-crystal determinations. Due to their structural similarity only the structure of 1 is described in detail. For others, only where significant differences are present will be noted. Some key structural parameters of 1–6 are listed in Table 1 for comparisons. Selected details of the data collection and structure refinements are listed in Table S1. Selected bond lengths and angles are in Table S2. The bond valence sum (BVS) analysis for the Cu atoms in 1-6 indicated that their charge states are +1 (Table S3). The formula of 1 was written as {(NaCl)2[CuI4(pz)4]3} to facilitate the understanding of its structure. The metallacrown [CuI4(pz)4] has a neutral charge and the sandwiched Cl- is charge balanced by the linked Na+ at the periphery of metallacrown to keep the overall charge neutrality. As shown in Figure 1a, four CuI and four pz- comprise a 12-memebered metallomacrocycle, and the CuI atoms orient toward the center of a cavity, thus defining an inverse metallacrown 12-MCCu(I), pz4,17 which has pseudo 4-fold axis passing through the center of -2-


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Figure 1. (a) The structure of inverse metallacrown 12-MCCu(I), pz-4 in 1. (b) Side view of the triple-decker structure built from three metallacrowns and the two sandwiched Cl- ions (Color legend: purple, CuI; green, Cl; red, O; blue: N; black: C). (c) Top view of the triple-decker structure along the pseudo 4-fold axis with each metallacrown differed by colors.

Interestingly, two Cl- ions are separately sandwiched by every neighboring pair of 12-MCCu(I), pz-4 disks to form a triple-decker structure (Figure 1b). The interior Cl- is unsymmetrically encapsulated, forming six CuI···Cl bonds with the interatomic distances of 2.593(2)-2.920(3) Å, which are significantly longer than the sums of copper and chlorine ionic radii (1.79 Å), but shorter than the sum of their van der Waals radii (3.15 Å).20 The distance of two Cl- ions in one triple-decker is 4.29 Å, whereas each Cl- ion is not in the center of two layers with two different Cg···Cl distances of 1.07 and 2.15 Å (Cg is the centroid of four CuI in one metallacrown). Three 12MCCu(I), pz-4 disks are parallel to each other without noticeable offset and arranged in an equidistance fashion (d ≈ 3.2 Å) imposed by the crystallographic inversion center. From top to the bottom, three layers are related to each other by clockwise rotation of 38.5o (top→middle) and 51.2o (middle→bottom), respectively, which also indicates the bottom layer is almost eclipsed with the top one (Figure 1c) with a very small rotation angle of 0.3o. Formation of such planar square metallacrowns should be induced by the spherical Cl- template.21

Figure 2. (a) The 1D chain structure built from triple-decker units through cuprophilic interaction highlighted by purple dashed lines. (b) The 3D framework from Na linked 1D chains viewed along the a axis. (Color legend: purple, CuI; green, Cl; red, O; blue: N; black: C; yellow: Na).

Within the metallacrown and triple-decker, we found the CuI···CuI separations are substantially longer than the sum of the van der Waals radii of CuI center (2.8 Å), indicating the absence of cuprophilic interaction.22 However, this supramolecular interaction was clearly visible between adjacent tripledecker units with CuI···CuI separation of 2.838(1) Å (Cu3···Cu4), which are very closed to the double of the van der Waals radii of CuI center, indicating the typical cuprophilic interaction. The triple-decker units are offset in the stacking in the crystalline state to give 1D extended columnar structure with only two of four CuI ion in one metallacrown involved in such cuprophilic interaction (Figure 2a). The noninnocent nitro groups also coordinate to Na+ (Na-O: 2.762(6)2.963(7) Å) to extend the 1D chains to 3D framework (Figure 2b). Although 1-6 crystallized into triclinic P 1 space group and each unit cell contains one triple-decker molecule, the different radius and electronegativity of the halides imposed significant effects on the geometry of the triple-decker unit. As shown in Table 1, from NaCl to NaI (1-3) and KCl to KI (4-6), the interplanar distance (d1) gradually becomes larger from 3.2 to 3.4 Å, consistent with ionic radii of the halides. We also observed the consistent evolution rule in the CuI···CuI separation in metallacrown 12-MCCu(I), pz-4 and averaged Cu-N distances, that is, larger radius of halide leads to the larger size of metallacrown 12-MCCu(I), pz-4. All the variations of metric parameters with respect to the halide radius clearly demonstrates the anion template effect. The formation of such anioncentered triple-decker structure should be dominated by the electronic properties of substituents on the pyrazolyl moiety. The peripheral electron-withdrawing nitro groups create the electron-deficient centers, therefore the halides are sandwiched between them through electrostatic interactions to neutralize local positive charges. This case is completely opposite in double-decker {Ag[(AuL)3]2}+ (HL = 3-(2-thienyl)-5-phenyl1H-pyrazole) in which metallacrown 9-MCAu(I)-3 is electronrich and can act as π-base to sandwich acidic AgI cation through AgI···AuI metallophilic interaction.23 On the other hand, the existence of halides also makes tetranuclear 12MCCu-4 crown planar instead of saddle-like.8 Due the strongest electronegativity and smallest radius of Famong the halides, we could not isolate the F--centered tripledecker motif but only obtain a hexanuclear cluster ([F@CuII6(pz)9(OH)2(DMF)4(H2O)]·2DMF, 7) in the presence of NaF, in which all Cu atoms have a +2 oxidation state. The structure of 7 revealed by X-ray crystallography on a single crystal shows a hexanuclear trigonal prism motif with a pair of parallel [CuII3(pz)3(OH)] trigons clamped by three pz bridges. A F- ion is sandwiched between a pair of [CuII3(pz)3(OH)] trigons with Cu-F distances of 2.218(4)-2.784(4) Å. This structure is very similar to that of [F@CuII6(L1)6(L2)3(OH)2] (HL1 = pyrazole; L2 = 3,5-diphenyl-pyrazole).24 We therefore will not discuss too much about the structure of 7 due to such similarities and the structure figure for 7 is shown in Figure S1, however, this result illustrated the size and electronegativity of the halides have important influence on the formation of analogues to 1-6.

Table 1. Selected Metric Parameters in the Crystal Structures of 1-6. Complex

1

2

3

4

-3-


5

6


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Halogen atom Definition of geometric parameters

Cl

Br

I

Cl

Br

I

Av. Cu-N (Å)

1.87

1.88

1.89

1.87

1.88

1.89

Av. Cu-X (Å)

2.76

2.88

2.96

2.76

2.88

2.93

Av. M-O (Å)

2.81

2.81

2.80

2.83

2.88

2.88

Av. N-Cu-N (o) CuI···CuI separation in metallacrown (Å) CuI···CuI separation between triple-deckers (Å) θ1/ θ2(o) d1 / d2 / d3 / d4 (Å)

158.2 3.49 - 3.53 Av 3.51 2.84

158.7 3.52 - 3.54 Av 3.53 2.86

159.0 3.55 - 3.56 Av 3.56 2.81

158.6 3.49 – 3.53 Av 3.51 2.84

158.7 3.52 – 3.53 Av 3.53 2.85

159.0 3.53 – 3.54 Av 3.54 2.81

38.5 / 51.2 38.9 / 51.1 39.3 / 50.8 38.6 / 51.1 38.2 / 51.9 3.20 / 1.07 / 2.15 / 3.21 / 1.25 / 1.97 / 3.34 / 1.36 / 1.98 / 3.20 / 1.04 / 2.17 / 3.21 / 1.25 / 1.97 / 4.29 3.93 3.97 4.35 3.94 Note: The black ball is the centroid of every four CuI ions in one metallacrown. Green ball is halogen ion and purple ball is CuI ion.

Solution Behaviors of 1-6. We studied the solution behaviors of such interesting family by high-resolution electrospray ionization mass spectrometry (HRESI-MS). Crystals of 1-6 were dissolved in DMF (N,N’dimethylformamide) and diluted with methanol, then transferred to the gas phase by electrospray ionization (ESI). As shown in Figure 3, there are a series of singly charged envelopes in the m/z range of 1000-3000 in negative-mode ESI-MS of 1-3, whereas no signals were observed for 4-6, which may be dictated by the mass spectrometry conditions. Comparing the simulated and experimentally observed isotope distributions (Figure S2), we were able to attribute these predominant species in ESI-MS of 1-3 to a general formula of [Cux(pz)y] (x = 5, 6 or 7, y = x-1, x or x+1). For 1, the most dominated peak at m/z = 1343.6977 (1a) was assigned to [Cu6(pz)6Cl2H(DMF)3]-, which could be seen as a complete 12MCCu(I), pz-4 metallacrown plus another half of it, that is a half of the parent triple-decker metallacrown. Similarly, the dominated peaks of 2 and 3 at m/z = 1353.6189 (2a) and 1401.6108 (3b) can be also assigned to a half of triple-decker metallacrowns with formulae of [NaCu6(pz)7Br(CH3OH)(H2O)3]and [NaCu6(pz)7I(CH3OH)(H2O)3]-, respectively. The detailed assignments for other species in solutions of 1-3 were listed in Table S4. Based on the X-ray structures and ESI-MS analysis, we rationalized the embryonic 12-MCCu(I), pz-4 metallacrowns in solution are the most possible building blocks for the assembly of triple-decker motifs under the induction of alkali halide.

38.4 / 51.5 3.30 / 1.35 / 1.95 / 3.91

Electrochemistry and Electrocatalytic Properties of 4, 5 and 6. The electrochemical properties of the compounds 4, 5 and 6 were investigated (scan rate: 100 mV s−1) in 0.5 M H2SO4 aqueous solution by fabricating them as carbon paste electrodes (CPEs). As shown in Figure 4, the cyclic voltammetry (CV) of 4-, 5- and 6-CPEs reveal a couple of similar redox peak (I/I’) in the sweep range between -0.3 and 0.3 V, which should be attributed to the redox process of CuI /CuII.25 And their redox peak potentials are located at 0.082 and -0.063 V, 0.103 and -0.056 V, and 0.118 and -0.049 V for 4-, 5- and 6CPE, respectively, indicating that the oxidation ability should follow an order of 6 > 5 > 4. The redox potential of 4-6 should be exclusively governed by the electronegativity of the halide (Cl, Br and I). For example, chloride has the strongest electronegativity that causes the CuI easier to be oxidized, thus, compound 4 has the weakest oxidation ability.26 Furthermore the CV of 6-CPE also exhibits another pair of redox peaks in the potential range from 0.3 to 0.8 V, which is ascribed to the redox process I-/I3-.27

Figure 4. Successive cyclic voltammograms of 4-, 5- and 6-CPE in 0.5 M H2SO4 aqueous solution at scan rate of 100 mV s−1.

With the knowledge that direct electroreduction of nitrite requires a large overpotential at most electrode surfaces,28 it is essential to seek appropriate intermediates to catalyze the reduction of nitrite. As shown in Figure 5, the 4, 5 and 6 exhibit excellent performances for the electrochemical reduction of nitrite. The electrocatalytic currents in continuous CVs of 4-,

Figure 3. Negative-mode HRESI-MS of 1-6 dissolved in DMF-CH3OH.

-4-


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5- and 6-CPE in 0.5 M H2SO4 aqueous solution increase stepwise with successive additions of nitrite from 0 to 40 mM (scan rate: 100 mV s−1), corresponding to the sensitive and rapid nitrite reduction at the electrodes. A large amount of gas bubbles (NO), according to eq 1: 3HNO2 = HNO3 + 2NO ↑+ H2O, were generated on the electrode surface (Figure S3), indicating a superior electrocatalytic activity of 4, 5 and 6 toward the reduction of nitrite. We also compared the electrocatalytic activities of 4, 5 and 6 for nitrite reduction by calcaulating the catalytic efficiency (CAT).29 The results showed that the electrocatalytic activity followed an order of 5 > 4 > 6 (Figure S4).

Furthermore, the electrocatalytic activity of 4, 5 and 6 to the reduction of H2O2 were also tested under similar conditions. As depicted in Figure S5, the gradual addition of H2O2 to the electrolyte results in a slow increase of the cathodic peak currents for 4-, 5- and 6-CPE, whereas the corresponding anodic peak current decrease tardily. Moreover, upon addition of H2O2, the reduction peak currents were shifted toward more negative potentials, suggesting a kinetic limitation exists in the reaction between 4, 5 and 6 and H2O2.30 The CATs of 4, 5 and 6 showed that 5 has better electrocatalytic performance than 4 and 6.

Figure 5. Successive cyclic voltammograms of (a) 4-CPE, (b) 5-CPE and (c) 6-CPE in 0.5 M H2SO4 aqueous solution containing 0 - 40 mM NaNO2 solution. Scan rate: 100 mV s-1.

Electrochemical Stability. Regarding the electrochemical stability of 4, 5 and 6, we selected 5 as a representative to elucidate the stability during electrochemical process in 0.5 M H2SO4 aqueous solution. (1) We studied the electrochemical stability of 5 in CPE by using 35 electrochemical cycles between -0.3 and 0.8 V in 0.5 M H2SO4 aqueous solution at a scan rate of 100 mV s-1 and under N2 atmosphere. As shown in Figure S6, the redox peaks corresponding to CuI/CuII and I-/I3- remain unchanged including the potential and current after 35 electrochemical cycles, which clearly indicates 5 in carbon paste electrode is rather stable during CV process when using 0.5 M H2SO4 aqueous solution as electrolyte. (2) After 35 electrochemical cycles, we re-collected the CV of the electrolyte solution by changing 5-CPE to a bare carbon paste electrode (Figure S7a) and bare glassy carbon electrode (GCE) (Figure S7b), and compared them with the CV of blank electrolyte solution. The CVs of blank electrolyte solution and that of electrolyte solution after 35 cycles are very similar (Figure S7c) which suggests there are no additional active species dissolved into solution from 5-CPE. We also compared the CV of 5-CPE with the above results and the characteristic redox peaks of 5-CPE were not observed in them (Figure S7d). All tests consistently supported the stability of 5CPE in 0.5 M H2SO4 aqueous solution. (3) To further prove the electrochemical stability of 5 under continuous CV scans in 0.5 M H2SO4 aqueous solution, we performed long-term chronoamperometric test using 5-CPE. Figure S8 shows that the amperometric response of 5-CPE versus time recorded at 1.3 V for 6200 s. In this time scale, no noticeable change of the oxidation current is detected for 5CPE, but only very slight oxidation current decay (< 5%) was

found after 6200 s, which demonstrated that 5-CPE is fairly stable in 0.5 M H2SO4 aqueous solution. (4) In order to prove the redox signals are coming from 5 rather than 'free' copper ion in 0.5 M H2SO4 aqueous solution, we recorded the CV of 0.256 mM CuSO4 in 0.5 M H2SO4 aqueous solution (scan rate : 100 mV s-1) using bare CPE. Only one anodic peak located at 0.048 was detected, which was clearly different from that of 5-CPE (Figure S9), illustrating that the redox peaks in CV of 5-CPE are not electrochemically responded from 'free' copper ion. (5) We also immersed 5 in 0.5 M H2SO4 aqueous solution for 5 h while taking pictures periodically. As depicted in Figure S10, the color and morphology of microcrystalline sample of 5 remain unchanged after 5 h, and especially, the H2SO4 aqueous solution was pellucid and colorless. The stability of 5 in 0.5 M H2SO4 aqueous solution was also proved by comparing powder X-ray diffraction patterns (Figure S11) and IR results (Figure S12). In all, 5 should be fairly stable in 0.5 M H2SO4 aqueous solution, which was in accordance with the results obtained above. Luminescent Behaviors and Sensing Properties. The UV/Vis absorption spectra for compound 6 was characterized in the solid state by using the diffuse reflectance mode (Figure S13). The absorption maximum at 302 nm should originate from ligand-centered (LC) π→π* transitions, and that around 450 nm may be from a halogen-to-metal charge transfer (XLCT) process.31 The solid state photoluminescence spectra for 6 and Hpz were recorded at room temperature and shown in Figure 6a. The emission maximum centered at 464 nm was detected under the 410 nm UV irradiation and displays characteristics similar to Hpz (λem = 470 nm) with a 6

-5-



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nm blue-shift, indicating the photoluminescence of 6 is mainly from the ligand-based excitation state. The absolute quantum yield of 6 in solid state is ca. 0.6%.

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ions is induced by more acidic AgNO3 aqueous solution (pH = 5.47 for 10 mM AgNO3 aqueous solution). We also checked the recyclability of this sensor, unfortunately, it cannot work in the second cycle, which may be caused by strong interaction between silver ions and nitro group (Figure S16). The UV/Vis spectrum for the emulsion of 6 was measured by adding different concentration of Ag+ (Figure S17), and the redshift from 425 to 485 nm were detected with the addition of Ag+ ions from 0 to 0.25 mM, implying the mechanism of 6 quenching effect by Ag+ may be ligand-to-metal charge transfer process, induced by the specific coordination interaction of Ag+ and the peripheral NO2 groups.34 We also considered the halogen effects on the Ag+ sensing behaviors by the fluorescence titration for 4 and 5 under the same conditions (Figure S20). Based on the calculated LOD (Figure S21), we found the 4 and 5 have the much lower respective LOD of 65.6 and 74.4 µM.

CONCLUSIONS

Figure 6. (a) The solid state photoluminescence spectra of 6 and Hpz. (b) Fluorescence intensity of 6 upon addition the different concentration of Ag+ (from top to bottom: 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60 mM) in H2O.

Being inspired by a previous work on the fluorescence sensing properties of metallacrowns,32 the samples of 6 (5 mg) were immersed into the 5 mL aqueous solutions containing different metal ions (Zn2+, Mg2+, K+, Hg2+, Cr3+, Cd2+, Al3+, Co2+ and Ag+; 1 mM), then ultrasonicated for 5 minutes to form a suspension for luminescence sensing studies. We found that the addition of Co2+ ions hardly change the emission intensity, whereas the addition of Hg2+ ions can slightly enhance the emission intensity. For Zn2+, Mg2+, K+, Cr3+, Al3+, and Cd2+, only marginal quenching effect on the emission intensity was detected (Figure S14). In contrast, we found that Ag+ ions can dramatically quench the luminescence at 508 nm, which indicates that 6 can selectively sense Ag+ ions through luminescence quenching. It should be noted that red-shifted emission of 6 in water suspension with respect to that in solid state is most likely caused by the formation of hydrogen bonds between –NO2 group and water molecule. We thus examined the quantitative response of 6 toward Ag+ by fluorometric titrations (Figure 6b). The emission intensity of emulsion of 6 linearly decreased upon increasing the Ag+ concentration from 0 to 0.6 mM (Figure S15). The limit of detection (LOD) was determined to be 114 µM for probe 6 on the basis of the signal-to-noise ratio, S/N = 3.33 The pH values of the suspension are 6.31 and 5.81 before and after the fluorometric titrations, respectively. The decreased pH value after the addition of Ag+

In summary, we have isolated and structurally characterized a series of 12-metallacrown-4 (12-MCCu(I), pz-4) based tripledecker complexes with different trapped alkali halides acting as templates. They have very similar motif but with some structural variations dependent on the radius and electronegativity of halides. HRESI-MS revealed the possible building blocks for such triple-decker motifs are 12-MCCu(I), pz-4 metallacrowns and the assembly should be in a stepwise fashion. Moreover, compound 4, 5 and 6 proved to be excellent bifunctional materials as electrocatalysts toward the reduction of nitrite and selective ‘turn-off’ sensors for Ag(I) in water. These findings deepen our understanding of the design and synthesis of multifunctional metallacrown-type compounds as well as their multiple-decker complexes.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Detailed synthesis procedure, additonal figures and tables, IR spectroscopy data, powder Xray diffractions, cyclic voltammetries, TGA, HR-ESI-MS, and UV-Vis spectra (PDF). X-ray crystallographic files (CIF)

AUTHOR INFORMATION Corresponding Author Email: [email protected]

ACKNOWLEDGMENT This work was supported by the NSFC (Grant Nos. 21571115, 21227001, and 21201110), the Natural Science Foundation of Shandong Province (No. ZR2014BM027), Young Scholars Program of Shandong University (2015WLJH24), and the Fundamental Research Funds of Shandong University (104.205.2.5 and 2015JC045). MK is funded by the CNRSFrance.

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