Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Iridium(III)-Based Metal−Organic Frameworks as Multiresponsive Luminescent Sensors for Fe3+, Cr2O72−, and ATP2− in Aqueous Media Kun Fan, Song-Song Bao, Wei-Xuan Nie, Chwen-Haw Liao, and Li-Min Zheng* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, People’s Republic of China S Supporting Information *
ABSTRACT: Three iridium(III)-based metal−organic frameworks (MOFs), namely [Cd3{Ir(ppy-COO)3}2(DMF)2(H2O)4]·6H2O·2DMF (1), [Cd3{Ir(ppy-COO)3}2(DMA)2(H2O)2]·0.5H2O·2DMA (2), and [Cd3{Ir(ppyCOO)3}2(DEF)2(H2O)2]·8H2O·2DEF (3) (ppy-COOH = methyl-3-(pyridin-2-yl)benzoic acid, DMF = N,N-dimethylformamide, DMA = N,Ndimethylacetamide, DEF = N,N-diethylformamide), have been synthesized and characterized. Single-crystal structural determinations reveal that compounds 1−3 are isostructural, showing a three-dimensional framework structure with (3,6) connected rtl topologyin whose trimers of {Cd3(COO)6} are cross-linked by Ir(ppy-COO)33−. The structures are completely different from those of other Ir(III)-based MOFs. Compound 1 was selected for a detailed study on sensing properties. The excellent luminescence as well as good water stability of 1 makes it a highly selective and sensitive multiresponsive luminescent sensor for Fe3+ and Cr2O72−. The detection limits are 67.8 and 145.1 ppb, respectively. Compound 1 can also be used as an optical sensor for selective sensing of adenosine triphosphate (ATP2−) over adenosine diphosphate (ADP2−) and adenosine monophosphate (AMP2−) in aqueous solution. This is the first example of iridium(III)-based MOFs for the optical detection of Fe3+, Cr2O72−, and ATP2−. More interestingly, the luminescent composite film doped with 1% (w/w) of compound 1, 1@PMMA (PMMA = poly(methyl methacrylate)), can be successfully prepared, which endows efficient sensitivity for Fe3+ and Cr2O72− detection and thus provides great potential for future applications.
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INTRODUCTION Metal−organic frameworks (MOFs)1 are crystalline materials composed of organic linkers and secondary building units. Due to their well-defined structures and tunable porosities, a great number of innovative MOFs have been extensively studied for potential applications in numerous areas, including but not limited to gas storage and separation,2 sensing,3 ionic conductivity,4 catalysis,5 and other functional materials.6 In this regard, luminescent MOFs acting as optical sensors have attracted much interest on account of their obvious advantages of high sensitivity, low cost, and facile manipulation. In previous studies, the luminous property of MOFs has been shown to typically originate from organic ligands7 or lanthanide-based emissions (such as Eu3+ or Tb3+).8 Recently, great efforts have been made to improve the fluorescence or phosphorescence properties of MOFs through employment of functional molecular building blocks of the frameworks.9 In this regard, metalloligands with efficient light harvesting and high quantum yield are the desired candidates for this strategy.10 The luminescent iridium complexes have gained great attention owing to their high quantum efficiency, tunable emission energy, and long lifetime. The highly efficient emission is attributed to strong spin−orbit coupling of the Ir3+ ion with a 5d configuration, which leads to an efficient intersystem crossing of the singlet to the triplet excited state. The triplet metal to ligand charge transfer (3MLCT) results in © XXXX American Chemical Society
the phosphorescence. The long-lived phosphorescence and high quantum yield of the iridium complexes, especially Ir(ppy)3 (ppy = 2-phenylpyridine) and its derivatives, give them distinct advantages for applications in organic lightemitting diodes,11 chemosensors,12 bioimaging,13 and photocatalysis.14 In contrast, MOFs containing iridium complexes as linkers are extremely rare,15−18 and only a handful of them have been used as luminescent sensors to detect gas or vapors. The first example of this kind is [Zn4(μ4-O){Ir(ppy-COO)3}2] (ppy-COOH = 3-(pyridin-2-yl)benzoic acid) (abbreviated as ZnIr-MOF),15 which shows a two-dimensional bilayer structure and can act as an effective sensor for O2. By use of Ir(ppy)2(L) (L = organic dicarboxylate), a few luminescent MOFs were also obtained, and their selective detection of nitroaromatic explosives or organic solvents was explored.16,17 Selective sensing of ions and small biomolecules is very important in the life and environmental sciences. Much attention has been paid to the detection of ferric (Fe3+) and hexavalent chromium (Cr2O72−, CrO42−) ions.19−21 The Fe3+ ion influences electron transfer and oxygen metabolism processes in DNA and RNA synthesis. Both excess and deficiency from the normal permissible limit can induce serious disorders.22 Meanwhile, hexavalent chromium ions are potent Received: September 29, 2017
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DOI: 10.1021/acs.inorgchem.7b02513 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
ATP2− in aqueous media. More interestingly, its sensing ability toward Fe 3+ and Cr 2 O 7 2− ions remains in a poly(methylmethacrylate) (PMMA) polymer film doped with a 1% concentration of compound 1, which could pave the way for future applications.
carcinogens, which can cause serious damage to human health and the environment.23 On the other hand, great efforts have also been devoted to the selective detection of nucleoside phosphates such as adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP),24 which play key roles in many cellular functions: for example, DNA synthesis, cell signaling, and membrane transport.25 Although a few luminescent MOFs have been developed in recent years to efficiently sense Fe3+, Cr2O72−, and ATP2−,26 those using iridium(III)-based MOFs have never been described. Herein we report for the first time that iridium(III)-based MOFs can serve as multiresponsive selective sensors to detect Fe3+, Cr2O72−, and ATP2− in aqueous media. Three new compounds with the formulas [Cd3{Ir(ppyCOO)3}2(DMF)2(H2O)4]·6H2O·2DMF (DMF = N,N-dimethylformamide) (1), [Cd3{Ir(ppy-COO)3}2(DMA)2(H2O)2]· 0.5H2O·2DMA (DMA = N,N-dimethylacetamide) (2), and [Cd3{Ir(ppy-COO)3}2(DEF)2(H2O)2]·8H2O·2DEF (DEF = N,N-diethylformamide) (3) have been obtained via solvothermal reactions (Scheme 1). The compounds are isostructural
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EXPERIMENTAL SECTION
Materials and Physical Measurements. Methyl 3-(pyridin-2yl)benzoate (ppy-COOMe), [Ir(ppy-COOMe)2(μ-Cl)]2, Ir(ppyCOOMe)3, and Ir(ppy-COOH)3 were prepared according to the literature.15 All other starting materials were of reagent grade quality and were obtained from commercial sources without further purification. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα) at room temperature. Infrared spectra were measured as KBr pellets on a Bruker Tensor 27 spectrometer in the range of 400−4000 cm−1. Thermogravimetric analysis (TGA) was performed in nitrogen in the temperature range 25−500 °C at a heating rate of 5 °C/min on a Mettler Toledo TGA/DSC 1 instrument. Elemental analyses for C, H, and N were determined with a PerkinElmer 240C elemental analyzer. Adsorption/desorption isotherms were measured using a BELSORPmax instrument (BEL Japan, Inc.). The UV−vis absorption and luminescence spectra were recorded on a PerkinElmer Lambda 950 UV/vis/NIR spectrometer and a PerkinElmer LS55 fluorescence spectrometer at room temperature, respectively. The emission lifetime in the solid state was determined on a Horiba Scientific FL3-1iHR1 spectrometer. Inductively coupled plasma (ICP) experiments were performed on PE Optima 5300 DV. Synthesis of 1. Compound 1 was obtained by a solvothermal reaction of Cd(NO3)2·4H2O (0.1 mmol, 30.8 mg) and Ir(ppyCOOH)3 (0.045 mmol, 34.6 mg) in a solution mixture of 12 mL of DMF and 4 mL of water at 100 °C. After 24 h, the solution was cooled to room temperature and orange hexagonal-plate-shaped crystals were obtained. Anal. Calcd for C84H90Cd3Ir2N10O26: C, 42.43; H, 3.81; N, 5.89. Found: C, 42.81; H, 3.98; N, 5.92. IR data (KBr, cm−1): 3435 (s), 1656 (m), 1589 (s), 1556 (s), 1508 (m), 1475 (m), 1456 (m), 1407 (s), 1375 (s), 1315 (m), 1271 (m), 1226 (m), 1056 (m), 1028 (m), 779 (s). Synthesis of 2. The synthetic procedure for 2 was the same as that for 1 except that the solvent DMF was replaced by DMA. Anal. Calcd for C88H84Cd3Ir2N10O18.5: C, 45.97; H, 3.68; N, 6.09. Found: C, 46.55; H, 4.40; N, 5.69. IR data (KBr, cm−1): 3413 (s), 3101 (w), 3041 (w),
Scheme 1. Synthetic Routes for Compounds 1−3
and show a new type of three-dimensional open framework structure among the Ir(III)-based MOFs. Compound 1 is chosen as a representative to detect a series of metal ions and anions as well as nucleoside phosphates by taking the advantage of its excellent luminescence and good water stability. The results demonstrate that compound 1 is an efficient multiresponsive sensor for optical detection of Fe3+, Cr2O72−, and
Table 1. Crystallographic Data for Ir(ppy-COOMe)3, Ir(ppy-COOH)3·DMF, and Compounds 1−3
empirical formula formula wt cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z ρcalcd (g cm−3) μ (mm−1) F(000) Rint Tmax, Tmin R1, wR2 (I > 2σ(I)) R1, wR2 (all data) goodness of fit (Δρ)max, (Δρ)min (e Å−3) CCDC no.
Ir(ppy-COOMe)3
Ir(ppy-COOH)3·DMF
1
2
3
C39H30IrN3O6 828.88 trigonal R3̅ 14.5228(3) 14.5228(3) 26.5540(13) 90 4850.2(4) 6 1.703 4.184 2460.0 0.0512 0.818, 0.665 0.0290, 0.0556 0.0421, 0.0595 1.000 0.492, −0.488 1572531
C39H31IrN4O7 859.85 trigonal R3̅c 14.6948(3) 14.6948(3) 55.655(3) 90 10407.9(7) 12 1.646 3.906 5111.7 0.0887 0.696, 0.683 0.0326, 0.0443 0.0829, 0.0504 0.999 0.903, −0.590 1572532
C84H90Cd3Ir2N10O24 2345.3 monoclinic P21/n 14.2240(7) 14.4669(7) 21.1188(11) 100.9633(16) 4266.5(4) 2 1.826 3.925 2308 0.0606 0.827, 0.680 0.0710, 0.1727 0.1039, 0.1902 1.01 4.497, −1.292 1572533
C80H64Cd3Ir2N8O16 2115.07 monoclinic P21/n 14.3245(8) 14.2690(9) 21.2590(12) 97.545(2) 4307.6(4) 2 1.631 3.871 2052.0 0.0604 0.698, 0.500 0.0648, 0.1651 0.1086, 0.1886 1.037 2.273, −3.053 1572535
C92H94Cd3Ir2 N10O20 2381.45 monoclinic P21/n 14.0958(12) 14.6981(13) 21.3773(19) 101.130(1) 4345.7(7) 2 1.820 3.852 2348.0 0.0312 0.831, 0.250 0.0452, 0.1107 0.0588, 0.1185 1.033 4.494, −1.799 1572534
B
DOI: 10.1021/acs.inorgchem.7b02513 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. (a) Building unit of 1 at 30% probability. Symmetry codes are the same as those given in Table S3. (b) Secondary building unit containing the {Cd3(COO)6} trimer in the structure of 1. (c) Perspective view of the 3D structure of 1, showing the channels along the b axis (lattice solvent molecules are removed for clarity). (d) Topological presentation of structure 1. 2933 (w), 2788 (w), 1623 (m), 1589 (s), 1556(s), 1510 (s), 1475 (m), 1456 (m), 1407 (s), 1375 (s), 1315 (m), 1269 (m), 1224 (m), 1055 (m), 1028 (m), 779 (s), 748 (w). Synthesis of 3. The synthetic procedure for 3 was the same as that for 1 except that the solvent DMF was replaced by DEF and the mixture was heated at 90 °C. Anal. Calcd for C92H106Cd3Ir2N10O26: C, 44.38; H, 4.29; N, 5.63. Found: C, 43.85; H, 4.00; N, 4.96. IR data (KBr, cm−1): 3404 (s), 2974 (w), 2935 (w), 2877 (w), 1643 (m), 1589 (s), 1558 (s), 1510 (s), 1465 (m), 1373 (s), 1267 (m), 1220 (m), 1166 (m), 1112 (m), 1056 (m), 1024 (m), 885 (s), 777 (s). Preparation of 1@PMMA Film. PMMA was doped with 1 powder in a proportion of 1% (w/w). The PMMA powder (500 mg) was dissolved in 5 mL of DMF, followed by the addition of the required amount of 1 (5 mg) in DMF, and the resulting mixture was heated at 35 °C for 60 min. The polymer film was obtained after evaporating the excess solvent at 40 °C. Luminescence Sensing Experiments. A suspension of compound 1 in water was prepared by dispersing 10 mg of 1 powder in 1000 mL of ultrapure water under ultrasonic agitation for 90 min. The final concentration was 10 mg L−1. To explore the ability of the luminescent probe for ion detection, 2 mL of the above suspension was put in a 1 cm × 1 cm quartz cell, different volumes of detection solution were sequentially added into the cell, and the luminescence intensity was monitored at ca. 515 nm. Sample Preparation for PXRD Studies. A 5 mg portion of powder 1 was dispersed in 5 mL of Fe3+, Cr2O72−, AMP2−, ADP2−, and ATP2− solution (1 and 10 mM, respectively) for 12 h. The suspension was then centrifuged at 10000 rpm for 5 min to collect the powder. After washing 10 times with ultrapure water, the resulting powder was kept at 40 °C for 12 h and then subjected to PXRD measurements. Single-Crystal Structure Determination. Single crystals of 1−3 were selected for indexing and intensity data collection on a Bruker SMART APEX CCD diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 123 K. The data were integrated using the Siemens SAINT program,27a with the intensities corrected
for Lorentz factor, polarization, air absorption, and absorption due to variation in the path length through the detector face plate. Absorption corrections were applied. The structures were solved by direct methods and refined on F2 by full-matrix least-squares using SHELXTL.27b All of the non-hydrogen atoms were located from the Fourier maps and were refined anisotropically. All H atoms were refined isotropically with the isotropic vibration parameters related to the non-hydrogen atoms to which they are bonded. Guest molecules in the pores were highly disordered and could not be modeled. To account for this electron density, the program SQUEEZE was employed for compound 2. The number of free solvent molecules was confirmed by thermal analyses. The crystallographic data of 1−3 as well as monomers Ir(ppy-COOMe)3 and Ir(ppy-COOH)3·DMF are given in Table 1, and the selected bond lengths and angles are given in Tables S1−S5.
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RESULTS AND DISCUSSION Crystal Structures. Compounds 1−3 were obtained as yellow hexagonal crystals by solvothermal reactions of Ir(ppyCOOH)3 and Cd(NO3)2·4H2O in sol/H2O (sol = DMF, DMA, DEF) solution at 90−100 °C. According to the powder X-ray diffraction (PXRD) and infrared (IR) measurements, these compounds are isostructural, which was further confirmed by single-crystal structure analyses (Figures S1 and S2 and Table 1). Therefore, only the structure of compound 1 is described in detail. Compound 1 crystallizes in the monoclinic system space group P21/n. The asymmetric unit contains one and a half Cd2+ ions, one Ir(ppy-COO)33−, two coordinated water molecules, one coordinated DMF, and one lattice DMF molecule. Other lattice solvent molecules cannot be accurately determined due to the severe disorder. The total number of lattice solvent molecules in the molecular formula C
DOI: 10.1021/acs.inorgchem.7b02513 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry was confirmed on the basis of both elemental and thermal analyses (Figure S3). As shown in Figure 1a, the metallo-ligand of Ir(ppyCOO)33− within the framework structure exhibits geometrical parameters (Ir−C, 1.983(12)−2.013(11) Å; Ir−N, 2.126(10)− 2.137(12) Å) similar to those for discrete Ir(ppy-COOMe)3 (Ir−C, 2.014(3) Å; Ir−N, 2.137(3) Å) or Ir(ppy-COOH)3 (Ir−C, 1.996(3) Å; Ir−N, 2.126(3) Å) complexes (Tables S1− S5 and Figures S4 and S5). Each Ir(ppy-COO)33− chelates and/or bridges three Cd2+ ions via its six carboxylate oxygen atoms. Two crystallographically distinct Cd atoms are found, both of which have octahedral environments. The Cd1 lies on the special position (0.5, 0.5, 0), surrounded by four carboxylate oxygen atoms and two oxygen atoms from DMF (Cd−O, 2.249(9)−2.302(12) Å), while Cd2 is located at a general position with the six sites occupied by four carboxylate oxygen atoms and two water molecules (Cd−O, 2.218(8)− 2.461(14) Å). The neighboring {CdO6} octahedra are connected by O−C−O units, forming a trimer of {Cd3(COO)6} (Figure 1b). The Cd···Cd distance within the trimer is 4.051 Å. Each trimer is linked to six Ir(ppy-COO)33− ligands, and each Ir(ppy-COO)33− is connected by three {Cd3(COO)6} trimers. Consequently, a three-dimensional framework structure is constructed (Figure 1c), which shows a (3,6) connected rtl topology (Figure 1d). The framework structure contains a 1D channel along the b axis, where the lattice solvent molecules reside. When the van der Waals radii are accounted for, the size of the channel is estimated as ca. 2.5 × 5.0 Å2. The structures of compounds 2 and 3 are similar to that of 1. The main difference is that the DMF molecule coordinated to the Cd1 atom in compound 1 is replaced by DMA in 2 and DEF in 3, respectively. Meanwhile, only one coordination water molecule is found in the asymmetric unit of compounds 2 and 3, and thus the Cd2 atom becomes five-coordinated in these two cases. Such a coordination mode could be a consequence of steric hindrance of the coordinated DMA or DEF molecules, which contain more −CH3 groups in comparison to DMF. As shown clearly in Figures S6 and S7, the organic groups of the DMA and DEF molecules protrude out to the center of the channels and fill in the channel cavities. It is worth mentioning that [Zn4(μ4-O){Ir(ppy-COO)3}2] (ZnIr-MOF) is the only known MOF-containing Ir(ppyCOO)33− metallo ligand so far.15 It shows a two-dimensional layer structure in which the Zn4(μ4-O) nodes are connected by Ir(ppy-COO)33− linkers. Therefore, compounds 1−3 are the second examples of MOFs containing Ir(ppy-COO)33− linkers and provide an entirely new type of structure among the iridium(III)-based MOFs. A detailed investigation on the gas sorption and ion detection has been performed for compound 1 as a representative. Gas Sorption Isotherms. Before the gas sorption isotherms measurement, compound 1 was activated at 100 °C under vacuum for 2 h. The PXRD pattern of the activated sample is identical with that simulated from the single-crystal data (Figure 2), suggesting that the framework structure of 1 is maintained after activation. As shown in Figure 3, the N2 adsorption and desorption isotherm at 77 K shows a type I curve, which is typical for microporous materials. The Langmuir surface area is 194.6 m2 g−1. The pore size distribution determined by the Horvath−Kawazoe (HK) method is 0.54 nm for 1 (Figure 3, inset), close to that found for ZnIr-MOF (0.51 nm).15,28 The total pore volume is
Figure 2. Powder X-ray diffraction patterns of compound 1: (a) as simulated from single-crystal data, (b) after thermal activation at 100 °C under vacuum for 2 h, (c) after immersing in water for 2 months, and (d) after suspension in water under ultrasonication for 1.5 h.
Figure 3. N2 adsorption (solid squares) and desorption (open squares) isotherms at 77 K for compound 1. Inset: Horvath−Kawazoe (HK) pore size distributions of 1.
0.089 cm3/g, which is slightly larger than the calculated value (0.076 cm3/g), attributed to the partial loss of the coordinated water molecules during the thermal activation process. The CO2 adsorption and desorption isotherms were also measured at 273 and 298 K (Figure S8). The maximum CO2 uptakes at 100 kPa are 18.86 cm3 g−1 at 273 K and 11.9 cm3 g−1 at 298 K, respectively. The adsorption enthalpy (Qst) of CO2 at low coverage was calculated to be 31.2 kJ mol−1 and remains steady at 26−28 kJ mol−1 throughout the adsorption process (Figure S9). Clearly, the Qst value of 1 for CO2 is much higher than that of ZnIr-MOF (21.8 kJ mol−1),28 attributed to the interactions between CO2 and exposed Cd sites in the framework host after thermal activation. Optical Properties. Figure 4a shows the solid state UV−vis absorption spectra of both compound 1 and metallo-ligand Ir(ppy-COOH)3 at room temperature, translated from the diffuse reflectance spectra by the Kubelka−Munk equation: F(R) = (1 − R)2/2R. The high-energy bands (
