Communication pubs.acs.org/IC
Sonogashira Cross-Coupling as a Route to Tunable Hybrid Organic− Inorganic Rods with a Polyoxometalate Backbone Long-Sheng Wang, Yue Lu, Gearóid M. Ó Máille, S. Philip Anthony, Deanne Nolan, and Sylvia M. Draper* Department of Chemistry, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland S Supporting Information *
organic transformations, such as esterification,17 amidation,18 and imidization.13,19 Such versatile strategies have been used to build organic-functionalized POMs. Peng et al. have reported a hybrid dumbbell comprising two terminal POMs generated via Sonogashira coupling,20 Wei et al. have reported one chiral rigidrod-like triad of [Mo6O18NC(OCH2)3(MMo6O18)(OCH2)3CNMo6O18]7− (M = MnIII, FeIII) using the imidization reaction of [Mo6O19]2− and [MMo6O18{(OCH2)3CNH2}2]3−,21 and Cronin et al. have reported a series of adjustable nanosized metal oxide oligomers through click reactions on POMs bearing alkyne and azide groups.24 In this work, we demonstrate one protocol for postfunctionalization to build hybrid organic−inorganic rods, with a POM backbone, through Sonogashira cross-coupling between a new diiodo-bifunctionalized Anderson−Evans POM and rigid alkynyl moieties. We herein reported the syntheses, structures, and characterizations of (TBA)3[MnMo6O18{(OCH2)3CNHCO(C 6 H 4 -p-I)} 2 ] (1; TBA = [(C 4 H 9 ) 4 N] + ) and (TBA) 3 [MnMo6O18{(OCH2)3CNHCO(C6H4CCC4H3S)}2] (2a), (TBA)3[MnMo6O18{(OCH2)3CNHCO(C6H4CCC6H4-p-tBu)}2] (2b), (TBA)2·[(i-Pr)2NH2]·[MnMo6O18{(OCH2)3CNHCO(C6H4CCC16H9)}2] (2c). The ligand Tris-I, containing a triol moiety and one iodo atom, was synthesized following a revision of Hill’s procedure.25 Compound 1 was prepared via the reflux of Tris-I, (TBA)4[Mo8O26], and MnIII(CH3CO2)3 in dry acetonitrile using a procedure similar to that of Hasenknopf et al.12,13 1 was obtained in 55% yield by recrystallization from a N,Ndimethylformamide (DMF) solution. In its IR spectrum (Figure S1), a strong sharp absorption at 1652 cm−1 shows the presence of the amido group and strong absorptions at 937, 918, and 901 cm−1 are characteristic of Anderson−Evans POMs.12,13 Two singlets at 7.81 and 7.57 ppm in the 1H NMR spectrum (Figure S5) are consistent with the presence of two 1,4-disubstituted phenyl rings and appear in a region similar to that of the unreacted Tris-I (7.81 and 7.61 ppm). The molecular ion peak at m/z 2584.3755 in the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) agrees with that calculated for [M + TBA]+ (m/z 2584.3699) and provides further support for the molecular formula of 1. Single-crystal X-ray diffraction structural analysis of 1 revealed that it crystallizes in a monoclinic P2(1)/c space group. Half of the cluster [TBA]3[MnMo6O18{(OCH2)3CNHCO(C6H4-pI)}2]3− is contained in the asymmetric unit. 1 has typical
ABSTRACT: A new diiodo-bifunctionalized Anderson− Evans polyoxometalate (TBA)3[MnMo6O18{(OCH2)3CNHCO(C6H4-p-I)}2] (1; TBA = [(C4H9)4N]+) was prepared and used as a new platform to generate tunable rigid-molecular rods (2a−2c) via Sonogashira crosscoupling. Single-crystal X-ray diffraction analysis of 1 and 2c reveals that they are type B Anderson−Evans structures with molecular lengths of 23 and 38 Å, respectively.
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anoscale molecular rods are of interest because of their extensive application in supramolecular assemblies, liquid crystal materials, and molecular devices.1 Polyoxometalates (POMs), a class of discrete metal oxide clusters, have many potential applications in the fields of nanochemistry, analytical chemistry, catalysis, medicine, and materials science owing to their immense structural variety and diverse properties.2 POMbased molecular rods are an emerging new class of materials possessing both the virtues of POMs and nanorods, although they remain virtually unstudied because of the difficulties inherent in their synthesis. This difficulty arises because of the fact that common POMs with one or two free vertices are inclined to adopt quasi-spherical/spherical, quasi-cyclic/cyclic, or tire-like structures3−5 owing to the Lipscomb restriction.2b Only a few examples of cigar-like, 1D chain, and rigid-rod POMs have been reported.6 Two types of organic−inorganic hybrid molecular rods involving POMs have been successfully constructed to date, with their development arising from recent advances in POM functionalization chemistry.7−21 The first consists of two terminal POMs that are covalently attached to one rigid bifunctional linker unit to form a molecular dumbbell.8 The second comprises a linear bifunctionalized POM backbone tethered to two organic ligands on each side.9−11 Such systems demonstrate wide potential application in the fields of chiral synthesis,21 luminescence,11 coordination chemistry,22 catalysis,22b and self-assembly in solution.23 Molecular rods of Anderson−Evans POMs containing remote groups such as pyridine,13 ferrocene,14 pyrene,11 terpyridine,15 and diazo linkers16 have been constructed via prefunctionalization. In each case, the corresponding ligands comprising a triol group are presynthesized before being attached to the POM. Improvements to the syntheses are still possible, e.g., to the time scale or the processes and yields and to address the current linear limitations in ligand design. Postfunctionalization involves an organic derivatized POM with active groups that can be further modified by common © XXXX American Chemical Society
Received: June 9, 2016
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DOI: 10.1021/acs.inorgchem.6b01395 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Importantly, only two TBA anions are observed in the 1H NMR spectrum of 2c, with the remaining counterion provided by a protonated diisopropylamine. This is evidenced by the methyl doublet at 1.19 ppm, which is close to the signal of the methyl group in (i-Pr)2NH·HCl (1.24 ppm in DMSO-d6, 400 MHz; see Figure S9) but is downfield compared to that of free diisopropylamine, which gives a doublet signal at 1.00 ppm. 2a was obtained as a yellow powder, from which it was difficult to obtain single-crystal data. 2b and 2c were obtained as crystals; however, crystals of 2b lost solvent even at low temperature, and crystal data could not be collected. The single-crystal X-ray data of 2c revealed that it crystallized in a monoclinic, P2(1)/c space group. There is half an anion cluster, one TBA cation, and one DMF molecule in the asymmetric unit. From the NMR, we assume that there is half a protonated diisopropylamine in the asymmetric unit, as is required to balance the charge. This counterion was seriously disordered and treated with SQUEEZE. The anionic cluster of 2c, [MnMo6O18{(OCH2)3CNHCO(C6H4CCC16H9)}2]3−, retains a type B Anderson−Evans structure (Figure 2). The central MnIII atom sits on the inversion
structural features of type B Anderson−Evans POMs. Its central MnIII atom sits on the inversion center of the cluster and is surrounded by six Mo atoms to form a planar hexagon via edgesharing connections. Two ligands of Tris-I cap opposite sides of the Anderson−Evans cluster to form a molecular rod, which is 23 Å in length (Figure 1).
Figure 1. ORTEP drawing of the anionic cluster of 1 with 50% probability thermal ellipsoids (solvent and counter-ions omitted for clarity).
Using a typical Sonogashira coupling procedure,26−28 1 was then coupled with 2-ethynylthiophene, 4-tert-butylphenylacetylene, and 1-ethynylpyrene, to give compounds 2a−2c respectively. A mixture of dry solvents (tetrahydrofuran/ acetonitrile with 1:1 volume ratio) was used to aid solubility, and the reactions were complete after 24 h. The reaction mixtures were filtered. The crude products were obtained after removal of the solvent under vacuum and purified via recrystallization in DMF and ether (Table 1).
Figure 2. ORTEP drawing of the asymmetric unit of the anionic cluster of 2c with 40% probability thermal ellipsoids (solvent and counter-ions omitted for clarity).
Table 1. Synthesis of Compounds 2a−2c
R
product
yield [%]
2-ethynylthiophene 4-tert-butylphenylacetylene 1-ethynylpyrene
2a 2b 2c
72 72 68
center, and the Mn−O bond lengths are in the range of 1.917(6)−2.022(7) Å. The bond lengths of Mo−μ3-O, Mo−μ2O, and Mo−t-O are in the ranges of 2.322(6)−2.420(8), 1.893(8)−1.956(8), and 1.673(8)−1.718(7) Å, respectively. The anion cluster can be viewed as that of two (HOCH2)3CNHCOC6H4CCC16H9 ligands capping opposite sides of a [MnMo6] cluster skeleton, yielding an Anderson−Evans clusterbridged molecular rod with an overall rod length of 38 Å. Using the POM core molecular structure data from 1 and 2c, estimates of the molecular lengths of compounds 2a and 2b give values of 34 Å for 2a and 39 Å for 2b (the molecular lengths of 2ethynylthiophene and 4-tert-butylphenylacetylene are 5.8029 and 8.46 Å,30 respectively). C−H···O hydrogen bonds play a key role in the cell packing of 2c. Two adjacent anion clusters are connected via two C−H···O hydrogen bonds (C34−H34A···O3#, 2.51 Å, 149.7°; C37− H37A···O3#, 2.42 Å, 151.9°; #, x − 1, −y + 3/2, z + 1/2) to form a five-membered ring with a hydrogen-bonding building block [R21(5)].31 The anion cluster can be viewed as a four-connected building block, which is connected to a rhombus 2D grid of 18 × 25 Å (Figure 3). The voids are filled with counter cations and solvent molecules. The space-filling diagram is presented in Figure S14. Compound 1 shows an absorption band with λmax = 246 nm (Figure 4). An analogous but bathochromically shifted band was observed at 292 nm in 2a and 302 nm in 2b and is consistent with the extension of conjugation across the CC spacer. The planar, rigid chromophore of 2c imparts a more structured spectrum, with shouldered absorption peaks at around 387 and 395 nm that are similar to those of pyrene (319 and 334 nm; Figure S13) but
A strong absorption due to the stretching vibration of the amide group is found in the IR spectra of 2a−2c (1657 cm−1 for 2a, 1657 cm−1 for 2b, and 1654 cm−1 for 2c, as shown in Figures S2−S4). The strong MoO stretching vibrations around 940, 919, and 904 cm−1 in compounds 2a−2c confirm the presence of a Anderson−Evans POM cluster.12 1 H NMR spectra of 2a−2c of dilute DMSO-d6 solutions (Figures S6−S8) were obtained. The sharp signals associated with the aryl protons of Tris-I are found at 7.82 and 7.58 ppm in 2a, 7.83 and 7.59 ppm in 2b, and 7.86 ppm in 2c, respectively. In addition, the spectrum of 2a contains two new signals at 7.67 and 7.31 ppm associated with the thiophene moiety. The spectrum of 2b contains two signals at 7.52 and 7.46 ppm arising from the terminal phenyl ring and one new singlet at 1.29 ppm arising from the t-Bu group. The 1H NMR spectrum of 2c has a set of signals at 8.68, 8.42, 8.38, 8.34, 8.27, and 8.16 ppm, which correspond to those of the pyrene but are shifted downfield by about 0.5 ppm from those of 1-ethynylpyrene (8.16, 8.05, and 7.99 ppm). The NMR data reveal the success of the various coupling reactions. B
DOI: 10.1021/acs.inorgchem.6b01395 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
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Experimental materials and physical measurements, crystallographic data, TGA diagrams, IR spectra, 1H NMR spectra of compounds 1 and 2a−2c, a UV−vis and luminescent spectrum, and space-filling diagram of 2c (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +353 1 896 2026. Fax: +353 1 671 2826.
Figure 3. 2D hydrogen-bonding network with a grid of 18 × 25 Å in 2c. The cations and solvents were omitted for clarity. Color code: purple, Mn; green, Mo; red, O; blue, N; gray, H; cyan, C.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was supported financially by Science Foundation Ireland (Grants 05PICAI819 and 15IA3046) and a Marie-Curie Fellowship (ToK 6-1447).
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Figure 4. UV−vis (left) and luminescence (right) spectra of compounds 1 and 2a−2c in a CH3CN solution [1 × 10−5 mol/L]. The feature at 317 nm in 2a is the solvent Raman scattering band.
bathochromically shifted by about 60 nm because of extended conjugation. Compound 1 exhibits a weak emission band with λmax = 394 nm. Compounds 2a and 2b display intense broad emission bands at around 377 and 364 nm, respectively, which are blue-shifted by about 17 and 30 nm compared to those of 1. Compound 2c displays the most bathochromically shifted emission, consisting of double emission bands at 427 and 447 nm, which are similar to those of pyrene (375 and 395 nm) with a red shift of 52 nm. The thermal stability of compounds 2a−2c was explored via thermogravimetric analysis (TGA) under a N2 atmosphere (Figures S10−S12). After the loss of two DMF molecules, compounds 2a and 2b are stable up to 260 and 280 °C, respectively, and then they decompose rapidly (weight losses of 29.4% for 2a and 31.5% for 2b) and lose three TBA groups each (calcd: 29.7% for 2a and 28.1% for 2b). It is notable that compound 2c has a more pronounced weight loss of approximately 9.80% before 150 °C, which corresponds to the loss of two DMF molecules and one [(i-Pr)2NH2]+ (9.7%). Rapid weight loss (15.3%) is found between 230 and 320 °C, corresponding to the loss of two TBA groups (18.0%). In conclusion, Sonogashira cross-couplings based on our new diiodo-bifunctionalized Anderson−Evans POM 1 were successfully developed. Three novel organic−inorganic hybrid rigid rods (2a−2c) with tunable rod lengths were obtained in desirable yields. Compounds 1 and 2a−2c were characterized using a combination of single-crystal X-ray diffraction, Fourier transform infrared, 1H NMR, MALDI-TOF MS, elemental analysis, and TGA. Their electronic absorption and luminescence in solution depend on the attached organic ligands. Further studies of their supramolecular properties are in progress through collaboration.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01395. C
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DOI: 10.1021/acs.inorgchem.6b01395 Inorg. Chem. XXXX, XXX, XXX−XXX
