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Synthesis, Structural Characterization, and Host−Guest Studies of Aminoquinonato-Bridged Re(I) Supramolecular Rectangles R. Govindarajan, R. Nagarajaprakash, and Bala. Manimaran* Department of Chemistry, Pondicherry University, Puducherry 605014, India S Supporting Information *

ABSTRACT: Aminoquinonato bridged Re(I)-based metallarectangles have been constructed via an orthogonal bonding approach. Self-assembly of Re2(CO)10 and aminoquinone ligands in the presence of ditopic linear pyridyl ligands has resulted in the formation of metallarectangles of the general formula [{(CO)3Re(μ-η4-L)Re(CO)3}2(μ-N-L′-N)2] (1−4), wherein 1, L = 2,5-bis(n-butylamino)-1,4-benzoquinonato (bbbq) and N-L′-N = 4,4′-bipyridine (bpy); 2, L = 2,5-bis(phenethylamino)-1,4benzoquinonato (bpbq) and N-L′-N = 4,4′-bipyridine; 3, L = 2,5-bis(n-butylamino)-1,4-benzoquinonato (bbbq) and N-L′-N = trans-1,2-bis(4-pyridyl)ethylene (bpe) and 4, L = 2,5-bis(phenethylamino)-1,4-benzoquinonato (bpbq) and N-L′-N = trans-1,2bis(4-pyridyl)ethylene (bpe). Metallarectangles 1−4 have been characterized by elemental analysis, IR, NMR, and UV−vis absorption spectroscopic techniques. The molecular structures of 1 and 4 were determined by single-crystal X-ray diffraction methods. The molecular recognition capability of 1 and 3 with pyrene and triphenylene has been investigated using UV−vis absorption and emission spectroscopic techniques. The formation of host−guest complex has been further corroborated by the single-crystal X-ray structural evidence of carceplex system (3⊃pyrene)·(DMF).

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cytotoxic behavior against certain cancer cell lines.13−16 In particular, significant interest in fac-Re(CO)3-core containing supramolecular architectures stem from their tunable luminescence properties and usefulness in sensing applications.17 We have focused our attention on developing diverse classes of Re(I)-based supramolecular rectangles, and recently, we have reported two different series of selenato-bridged and oxamidatobridged Re(I)-rectangles that demonstrated their guest binding properties.18 The cavities of these rectangles were relatively smaller to accommodate guest species, although some of them exhibited out of cavity interactions with aromatic guest species. In our continued efforts to generate larger metallarectangles that could encapsulate guest species inside the cavity, we have chosen aminoquinone ligands to act as organic pillars in the present study. To the best of our knowledge, the aminoquinonato bridging motif has not been employed in the construction of rhenium-based supramolecular architectures, although a few reports on Ru-, Rh-, and Ir-based compounds are available.19,20 Herein, we report on the one-step self-assembly of Re(I)-based novel supramolecular rectangles of general formula [{(CO)3Re(μ-η 4-L)Re(CO) 3} 2(μ-N-L′-N) 2] (1−4) from dirhenium

INTRODUCTION Over the past two decades, a wide spectrum of two-dimensional (2-D) and three-dimensional (3-D) metallasupramolecular architectures have been developed owing to their interesting structural features and intriguing electronic, magnetic, host− guest, and catalytic properties.1,2 Especially, favorable host− guest properties of metallacycles and metallacages have gained profound applications in filtration tasks, entrapping hazardous chemicals, sensors, and as molecular reaction flasks.2k−t Supramolecular compounds of rectangular topologies of lower symmetry were shown to exhibit unique binding abilities and selectivity for planar aromatic guests and small organic molecules over the higher symmetry architectures.3 Supramolecular rectangles now comprise a subset of a large collection of discrete cavity-containing supramolecular assemblies.4−7 Following the seminal work carried out on the design and synthesis of rectangular supramolecules by the research groups of Stang,8 Hupp,9 Sullivan,10 and Lu,11 several other research groups have been involved in expanding the library of discrete metallarectangles using different transition metals and ligand motifs.12 Lately, arene-Ru/Os and Cp*-Rh/Ir complexes have been used as effective building blocks in the construction of several tetranuclear rectangles that showed prominent guest-binding properties toward biologically relevant molecules and promising © XXXX American Chemical Society

Received: July 10, 2015

A

DOI: 10.1021/acs.inorgchem.5b01543 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Self-Assembly of Aminoquinonato-Bridged Re(I)-Rectangles 1−4

absorption spectra of 1−4 displayed two higher energy bands in the range λmax 229−312 nm due to ligand-centered transitions and a lower energy band in the range λmax 464−475 nm due to MLCT transitions.16d,22 Upon excitation at MLCT absorption wavelength region, the metallarectangles 1−4 displayed broad fluorescence emission centered around λmax 560−570 nm, consistent with known rhenium bipyridyl complexes.23 Molecular structures of 1·(DMF)4 and 4 were determined by single-crystal X-ray diffraction analysis. Good quality single-crystals of 1·(DMF)4 were grown from a DMF solution of 1 at 5 °C, whereas single-crystals of 4 were obtained by slow evaporation of chloroform solution of 4 at −5 °C. ORTEP diagrams of 1·(DMF)4 and 4 are given in Figures 1a and 2a, and the selected bond distances and bond angles are listed in Tables 1 and 2 respectively. The crystallographic data and structure refinement details are given in Table S1 (Supporting Information). Both compounds 1·(DMF)4 and 4 displayed rectangular architectures with four fac-Re(CO)3 moieties as corners, two aminoquinonato bridges as shorter edges, and two linear ditopic pyridyl linkers as longer edges. Each rhenium center was present in a distorted octahedral geometry with three terminal CO groups, one nitrogen atom from pyridyl ligand, and one nitrogen atom and an oxygen atom from aminoquinonato bridge. The overall dimensions of metallarectangle 1·(DMF)4 as defined by Re···Re distances were found to be ∼8.14 × 11.51 Å. The longer edges of the rectangle 1 were slightly bent inward, reducing the centroid···centroid distances between the pyridyl rings of bpy units to 7.585 Å from 8.140 Å that was observed for Re···Re separation along the aminoquinonato pillars. In rectangle 1, the two aminoquinonato bridges were oriented syn with respect to each other. The corners of the rectangle 1·(DMF)4 occupied by four rhenium centers were perfectly coplanar with zero torsional strain. Interestingly, a disordered molecule of DMF was trapped inside the rectangular cavity of 1, and three more DMF molecules were present in its periphery. Several CH···π interactions were observed between the π clouds of organic bridges of rectangle 1 and the methyl hydrogens of DMF molecule present in its cavity. The DMF molecules present

decacarbonyl, aminoquinones (H2L = bbbq, bpbq), and bispyridyl ligands (N-L′-N = bpy, bpe). Metallarectangles 1−4 have been characterized using spectroscopic techniques, and the molecular structures of 1 and 4 have been elucidated using singlecrystal X-ray diffraction methods. The guest binding properties of rectangles 1 and 3 with electron-rich planar aromatic guests such as pyrene and triphenylene have been studied using UV−vis absorption and emission spectroscopic techniques. The host− guest complexation has been further substantiated from the single-crystal X-ray structure of (3⊃pyrene)·(DMF) (3a).

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RESULTS AND DISCUSSION Self-assembly of Re2(CO)10, aminoquinone ligands (H2L = 2,5-bis(n-butylamino)-1,4-benzoquinone (bbbq), 2,5-bis(phenethylamino)-1,4-benzoquinone (bpbq), and dinucleating linear pyridyl ligands (N-L′-N = 4,4′-bipyridine (bpy) and trans1,2-bis(4-pyridyl)ethylene (bpe)) in an equimolar ratio afforded tetrarhenium rectangles [{(CO)3Re(μ-η4-L)Re(CO)3}2(μ-NL′-N) 2] (1−4). The reactions were carried out under solvothermal conditions in mesitylene medium, and the products obtained were air, light, and moisture stable and soluble in polar organic solvents. Compounds 1−4 were characterized using NMR, IR, and UV−vis spectroscopic methods. In the 1H NMR spectra of metallarectangles 1−4, signals corresponding to aminoquinonato bridges were shifted downfield, while those of ditopic pyridyl ligands were shifted upfield in comparison to the signals of free ligands. The 13C NMR spectra of 1, 2, and 4 showed signals pertinent to various types of carbons present in the rectangular architecture. Further, IR spectra of 1−4 displayed four strong bands in the range ν 2020−1890 cm−1, and the observed patterns were typical of related fac-Re(CO)3-cornered tetranuclear rectangles.18b,21 The CO stretching frequency of aminoquinonato bridges appeared as a strong band in the region ν 1536−1533 cm−1, whereas the CO stretching frequency of free aminoquinone ligands (H2L) appeared at ν 1583 cm−1. The NH stretching bands observed for free ligands (H2L) were absent in 1−4. These spectral observations supported the complexation of organic ligands with rhenium metal centers. The electronic B

DOI: 10.1021/acs.inorgchem.5b01543 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

whose concentration was maintained constant (5 × 10−6 M) was titrated against a solution of compound 1, whose concentration was varied (1 × 10−7 to 1.0 × 10−6 M). The absorbance of pyrene was enhanced progressively as the concentration of host rectangle 1 increased. This observation led us to conclude that rectangle 1 and pyrene were able to form host−guest complex, probably aided by the electron-rich nature of pyrene and the electron-withdrawing influence of the pyridyl groups bonded with Re metal centers. Such electron donor−acceptor interactions (D−A interactions) are known to favor the formation of stable host−guest complexes.25 A double reciprocal plot of change in absorbance (ΔA) of guest versus change in the concentration of host [H] gave a linear correlation. The linear Benesi−Hildebrand plot suggests a 1:1 host/guest binding ratio, and the binding constant was calculated as 1.3 × 105 M−1.26 Similar UV−vis spectral titration studies carried out between 1 and triphenylene guest gave a binding constant value of 3.1 × 105 M−1. The evidence for the formation of host−guest complexes between 1 and aromatic guests (pyrene and triphenylene) was further augmented by the fluorescence titration experiments in which the initial emission intensity of guest species was quenched upon incremental addition of host 1. A Stern−Volmer plot of I0/I versus [H], wherein I0 and I were emission intensities of guests in the absence and presence of host rectangle 1, and [H] was the concentration of host 1, gave a linear relation. The Stern−Volmer constants (Ksv) calculated from the slope of linear plots were found to be 1.9 × 105 M−1 and 1.3 × 105 M−1 for pyrene and triphenylene respectively (Figures S4 and S5, Supporting Information).27 The binding constants (Kb) calculated from UV−vis spectral titrations of 3 with pyrene and triphenylene guests were found to be 2.6 × 105 M−1 and 1.1 × 105 M−1, respectively. Fluorescence titration experiments for host 3 with pyrene and triphenylene furnished the Stern−Volmer constants (Ksv) of 1.5 × 105 M−1 and 1.3 × 105 M−1, respectively (Figures S6 and S7, Supporting Information). To gain further insights into host−guest complexation of metallarectangles with planar aromatic guests, we attempted to grow single-crystals of 1−4 in different organic solvents such as dichloromethane, chloroform, acetone, DMF, and DMSO in the presence of pyrene and triphenylene. However, in most of the cases, the crystals obtained were of low quality and diffracted poorly, and hence they were of little use to corroborate the formation of host−guest complex. After several attempts, good quality single-crystals of (3⊃pyrene)·(DMF) were obtained from a nearsaturated DMF solution of 3 in the presence of pyrene at 5 °C. Single-crystal X-ray analysis of (3⊃pyrene)·(DMF) confirmed the formation of 1:1 donor−acceptor complex between 3 and pyrene. Remarkably, a pyrene guest molecule was stacked inside the porous cavity of tetrarhenium metallarectangle 3, and a solvent DMF molecule was present outside its cavity. The molecular structure of (3⊃pyrene)·(DMF) revealed a tetrarhenium architecture with dimensions of about ∼8.15 × 13.74 Å (Figure S3, Supporting Information). The overall structure of (3⊃pyrene)·(DMF) differed considerably from the rectangular topology observed for 1·(DMF)4 and 4, implying that accommodation of large planar guest pyrene imposed significant conformational strain on the host 3. Four Re centers present at the vertices of 3 were not in a plane as indicated by a torsional strain of 22.49° due to canting of two {(CO)3Re(μ-η4-bbbq)Re(CO)3} pillar units in opposite directions. The longer edges of the rectangle occupied by two trans-1,2-bis(4-pyridyl)ethylene (bpe) bridging ligands were also twisted due to the canting of {(CO)3Re(μ-η4-bbbq)Re(CO)3} units. Furthermore, the bpe

Figure 1. (a) ORTEP diagram of [{(CO)3Re(μ-η4-bbbq)Re(CO)3}2(μ-bpy)2]·(DMF)4 (1) with thermal ellipsoids drawn at the 40% probability level. DMF molecules disordered in two positions are shown in one position. (b) Packing diagram of 1 viewed along the c axis depicting the formation of infinite rectangular channels in which DMF molecules (space filling representation, blue colored) are entrapped. DMF molecules present outside the cavity are shown in yellow color in space filling representation.

outside the cavity of 1 were also stabilized by similar soft interactions with the host molecules. The CH···π interaction distances were in the range 3.378−3.384 Å.24 The molecular structure of 4 revealed a larger rectangular architecture with dimensions of about ∼8.15 × 13.74 Å due to the presence of trans-1,2-bis(4-pyridyl)ethylene (bpe) bridging ligand along the longer edges. The two aminoquinonato moieties at the shorter edges were oriented anti with respect to each other. In rectangle 4, the centroid···centroid distance between the pyridyl moieties was 7.40 Å, whereas the centroid···centroid distance between the ethylene bridges was 7.31 Å. These distances indicate that the bpe ligands in 4 were bent inward to a greater degree than bpy ligands in 1·(DMF)4 owing to the flexibility of ethylene bridges in bpe. The torsional angle between four rhenium atoms present at the corners of the rectangle was 0.75° indicating that all rhenium atoms are positioned in a nearly coplanar fashion. Encouraged by the porous rectangular cavity evinced from single-crystal X-ray structures of 1 and 4, we instigated the studies related to guest binding properties of select metallarectangles (1 and 3) toward planar aromatic guests such as pyrene and triphenylene. Initially, we carried out UV−vis spectral titration experiments, in which a solution of pyrene, C

DOI: 10.1021/acs.inorgchem.5b01543 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) ORTEP diagram of [{(CO)3Re(μ-η4-bpbq)Re(CO)3}2(μ-bpe)2] (4) with thermal ellipsoids drawn at the 30% probability level. (b) Packing diagram of 4 viewed along the c axis showing the formation of rectangular channels in space filling representation.

Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1 Re1−N1 Re1−N3 Re1−O7 Re1−C1 Re1−C2 Re1−C3

2.147(5) 2.215(4) 2.119(4) 1.905(6) 1.921(6) 1.923(6)

N1−Re1−N3 O7−Re1− N1 O7−Re1−N3 C1−Re1−N1 C1−Re1−N3 C1−Re1−O7

Table 2. Selected Bond Lengths (Å) and Angles (deg) for 4

84.71(17) 75.41(15) 81.04(15) 93.60(2) 178.20(2) 99.15(2)

Re1−N1 Re1−N3 Re1−O7 Re1−C1 Re1−C2 Re1−C3

2.043(17) 2.193(7) 2.226(12) 1.95(2) 2.035(17) 1.847(18)

N1−Re1−N3 N1−Re1−O7 O7−Re1−N3 C1−Re1−N1 C1−Re1−N3 C1−Re1−O7

82.8(7) 76.7(4) 81.2(5) 92.80(9) 175.57(9) 97.70(7)

at the edges was ∼8.15 Å (as defined by Re···Re separation), whereas at the center it was observed to be ∼6.96 Å. The bpe edge moieties were bent inward to enhance the π···π stacking

ligands were bent inward significantly, reducing cavity width at the center. The two aminoquinonato moieties at the shorter edges were oriented anti with respect to each other. The cavity width D

DOI: 10.1021/acs.inorgchem.5b01543 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a). Molecular structure of (3⊃pyrene)·(DMF) showing the presence of pyrene guest in rectangular cavity (DMF molecule is not shown for clarity). (b) Top view of (3⊃pyrene)·(DMF) showing the distorted structure of the host. (c) Stick representation of (3⊃pyrene)·(DMF) indicating the centroid···centroid distances between aromatic rings of host and pyrene guest. (d) Packing diagram of (3⊃pyrene)·(DMF) viewed along the a axis (DMF molecules are not shown for clarity).

severe structural distortions in order to provide a “best-fit” for the guest pyrene. The longer cross section of pyrene guest molecule present inside the cavity was aligned in nearly parallel fashion to

interactions with the guest pyrene. The π···π interaction distances between the bpe bridges and pyrene were in the range 3.52− 4.27 Å.15b,28 Apparently, the host rectangle 3 tolerated these E

DOI: 10.1021/acs.inorgchem.5b01543 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 3. Binding Constants (Kb) and Stern−Volmer Quenching Constants (Ksv) for the Host−Guest Systems of Molecular Rectangles 1 and 3 with Pyrene and Triphenylene with pyrene Kb, M 1 3

−1

1.3 × 10 2.6 × 105 5

Ksv, M

1.9 × 10 1.5 × 105 5

1 3

Kb, M−1

Ksv, M−1

3.1 × 10 1.1 × 105

1.3 × 105 1.3 × 105

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EXPERIMENTAL SECTION

Materials and General Methods. Solvothermal reaction methods were adopted for the syntheses of metallarectangles 1−4. Re2(CO)10, 4,4′-bipyridine, and trans-1,2-bis(4-pyridyl)ethylene were purchased from Sigma-Aldrich Chemicals. The aminoquinone ligands (bbbq and bpbq) were synthesized by literature methods.30 Solvents were dried using standard methods and freshly distilled prior to use.31 IR spectra were recorded on a Nicolet iS10 FT-IR spectrometer. Electronic absorption spectra were obtained on a Shimadzu UV-2450 spectrophotometer. Emission spectra were recorded on a Fluoromax-4 spectrofluorometer. Solvents used for UV−vis and emission titration experiments were of spectral grade. 1H and 13C NMR spectra were recorded on a Bruker Avance 400 MHz NMR spectrometer. Elemental analyses were performed using Thermo Scientific Flash 2000 CHNS analyzer. Synthesis of [{(CO)3Re(μ-η4-bbbq)Re(CO)3}2(μ-bpy)2] (1). A mixture of Re 2(CO)10 (68 mg, 0.1 mmol), 2,5-bis(n-butylamino)-1,4benzoquinone (25 mg, 0.1 mmol), and 4,4′-bipyridine (16 mg, 0.1 mmol) in mesitylene (6 mL) were taken in a 23 mL PTFE flask and placed inside a steel bomb. The bomb was kept in an oven maintained at 160 °C for 10 h and then cooled to room temperature. Good quality maroon colored crystals were obtained. The crystals were separated, washed with hexane, and dried under a vacuum. Yield: 67 mg, 68%. Anal. Calcd for C60H56N8O24Re4: C, 38.13; H, 2.99; N, 5.93. Found: C, 39.21; H, 3.11; N, 5.71. IR (CH2Cl2): νCO 2020 (s), 2014 (s), 1917 (s), 1891 (s), νaminoquinonato C=O 1536 (m). 1H NMR (400 MHz, (CD3)2CO, ppm): δ 8.72 (d, J = 8.0 Hz, 8H, H2, bpy), 7.88 (d, J = 8.0 Hz, 8H, H3, bpy), 5.72 (s, 4H, quinone-H), 4.01 (m, 4H, H1, CH2−butyl), 3.90 (m, 4H, H1′, CH2−butyl), 1.90 (m, 4H, H2, CH2−butyl), 1.75 (m, 4H, H2′, CH2−butyl), 1.53 (m, 8H, H3, CH2−butyl), 1.02 (m, 12H, H4, CH3butyl). 13C NMR (100 MHz, (CD3)2CO, ppm): δ 198.7, 198.6, 194.3 (Re(CO)3), 178.6 (aminoquinonato CO), 167.4 (C2, quinone-C), 153.6 (C2, bpy), 146.2 (C4, bpy), 124.4 (C3, bpy), 96.2 (C3, quinone-C), 56.5, (C1, butyl), 32.6 (C2, butyl), 14.2 (C3 and C4, butyl). UV−vis {λmax (CH2Cl2)/(nm) (ε/M−1 cm−1)}: 229 (1.2 × 105), 303 (4.6 × 104) (LIG); 464 (8.3 × 104) (MLCT). Emission {λmax (CH2Cl2)/(nm)}: 568. Synthesis of [{(CO)3Re(μ-η4-bpbq)Re(CO)3}2(μ-bpy)2] (2). Compound 2 was synthesized by following the procedure adopted for 1 using Re2(CO)10 (68 mg, 0.1 mmol), 2,5-bis(phenethylamino)-1,4-benzoquinone (34 mg, 0.1 mmol), and bipyridine (16 mg, 0.1 mmol) and 2 was obtained as maroon colored crystals. Yield: 68 mg, 63%. Anal. Calcd for C76H78N8O16Re4: C, 43.84; H, 2.71; N, 5.38. Found: C, 45.06; H, 2.35; N, 5.46. IR (CH2Cl2): νCO 2019 (s), 2012 (s), 1918 (s), 1893 (s), νaminoquinonato C=O 1534 (s) cm−1. 1H NMR (400 MHz, CDCl3, ppm): δ 8.66 (d, J = 6.8 Hz, 8H, H2, bpy), 7.48 (d, J = 6.8 Hz, 8H, H3, bpy), 7.36 (m, 20H, phenethyl), 5.75 (s, 4H, quinone-H), 4.04 (m, 4H, H1, CH2−phenethyl), 3.93 (m, 4H, H1′, CH2−phenethyl), 3.23 (m, 4H, H2, CH2−phenethyl), 2.89 (m, 4H, H2′, CH2−phenethyl). 13C NMR (100 MHz, (CDCl3), ppm): δ 198.3, 197.3, 192.0 (Re(CO)3), 177.6 (aminoquinonato CO), 166.5 (C2, quinone-C), 153.6 (C2, bpy), 145.1 (C4, bpy), 138.0 (C1, phenethyl), 129.1 (C3, phenethyl), 129.0 (C2, phenethyl), 127.0 (C4, phenethyl), 124.4 (C3, bpy), 96.2 (C3, quinoneC), 58.9 (C1, CH2−phenethyl), 36.0 (C2, CH2−phenethyl). UV−vis {λmax (CH2Cl2)/(nm) (ε/M−1 cm−1)}: 229 (1.3 × 105), 312 (5 × 104) (LIG); 474 (8.5 × 104) (MLCT). Emission {λmax (CH2Cl2)/(nm)}: 567. Synthesis of [{(CO)3Re(μ-η4-bbbq)Re(CO)3}2(μ-bpe)2] (3). Compound 3 was synthesized by following the procedure adopted for 1 using Re2(CO)10 (68 mg, 0.1 mmol), 2,5-bis(n-butylamino)-1,4-benzoquinone (25 mg, 0.1 mmol), and trans-1,2-bis(4-pyridyl)ethylene (18 mg, 0.1 mmol) and 3 was obtained as a maroon crystalline product. Yield: 63 mg, 62%. Anal. Calcd for C64H60N8O16Re4: C, 39.58; H, 3.11; N, 5.77. Found: C, 41.26; H, 3.21; N, 5.71. IR (CH2Cl2): νCO 2018 (s), 2011 (s), 1912 (s), 1890 (s), νaminoquinonato C=O 1533 (s) cm−1. 1H NMR (400 MHz, CDCl3, ppm): δ 8.52 (d, J = 8.0 Hz, 8H, H2, bpe), 7.38 (d, J = 8.0 Hz, 8H, H3, bpe), 7.16 (s, 4H, ethylenic CH, bpe), 5.64 (s, 4H, quinone-H), 3.86 (m, 4H, H1, CH2−butyl), 3.75 (m, 4H, H1′, CH2− butyl), 1.92 (m, 4H, H2, CH2−butyl), 1.73 (m, 4H, H2′, CH2−butyl), 1.47 (m, 8H, H3, CH2−butyl), 1.02 (m, 12H, H4, CH3-butyl). UV−vis {λmax(CH2Cl2)/(nm) (ε/M−1 cm−1)}: 230 (8.5 × 104), 301 (7.9 × 104) (LIG); 468 (7.1 × 104) (MLCT). Emission {λmax (CH2Cl2)/(nm)}: 560.

with triphenylene −1

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one of the bpe bridges of rectangle 3. These observations led us to suggest that the host 3 showed greater selectivity in encapsulating pyrene over DMF, which was present outside its cavity. It is worth noting that in the absence of pyrene, one of the rectangles encapsulated a DMF molecule in its cavity as observed in 1·(DMF)4. The factors such as complementary size and shape of 3 and pyrene, donor−acceptor interactions, hydrophobic nature of host cavity, and the flexibility of host that aided in enduring the conformational strain during guest encapsulation might have been critical for effective binding of pyrene.7a,29 In addition to UV−vis and fluorescence titration experiments, the host−guest complexation between 3 and pyrene/triphenylene in solution was further supported by 1H NMR spectroscopic studies. The solution-state 1H NMR spectra of mixtures of host 3 and pyrene (0.5−1.5 equiv) in acetone-d6 at room temperature revealed that the signals pertaining to pyrene were significantly shifted upfield {Δδ ≈ 0.33 ppm (Hapyrene), Δδ ≈ 0.60 ppm (Hbpyrene) and Δδ ≈ 0.40 ppm (Hcpyrene)} in comparison to the signals of free pyrene for the 1:1 ratio of host−guest complex (Figure S8, Supporting Information). Furthermore, the signals corresponding to the protons of bpe bridges of the host showed considerable upfield shifts {Δδ ≈ 0.10 ppm (H2py), Δδ ≈ 0.35 ppm (H3py) and Δδ ≈ 0.58 ppm (Hethylenic)}, while protons of the aminoquinonato ring and N-alkyl groups displayed a marginal downfield shift (Δδ ≈ 0.04−0.12 ppm) in comparison with the signals of free host 3. Similar trends were observed in the 1H NMR studies carried out for the mixtures of host 3 and triphenylene guest (Figure S9, Supporting Information). The prominent upfield shifts observed for the guest species in the presence of 3 indicate the formation of inclusion complexes between 3 and the aromatic guests. These spectroscopic results undoubtedly substantiate that these metallarectangles can effectively bind planar aromatic guests.29d−f

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CONCLUSIONS We have demonstrated the one-step self-assembly of Re(I)supramolecular rectangles from rudimentary rhenium carbonyl, bis-chelating aminoquinonato organic pillars, and linear ditopic pyridyl linkers. Spectroscopic evidences and single-crystal X-ray structural analyses have established a rectangular architecture for compounds 1−4. UV−vis absorption and fluorescence spectral investigations for assessing the host capability of rectangles 1 and 3 toward polycyclic aromatic compounds such as pyrene and triphenylene have revealed strong interactions between the respective host and guest species. Formation of a 1:1 host−guest complex between 3 and pyrene has been explicitly supported by the single-crystal X-ray structure of (3⊃pyrene)·(DMF). The inclination to provide a “best fit” for incoming guest pyrene by the rectangle 3, as seen from its molecular structure of (3⊃pyrene)· (DMF), has demonstrated the advantages of developing flexible, yet robust, metallasupramolecules. Current attempts are directed toward developing flexible-Re(I)-supramolecular assemblies with specific organic bridges that could potentially control the presence/ absence of guests inside their cavities, triggered by external stimuli. F

DOI: 10.1021/acs.inorgchem.5b01543 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry [{(CO)3Re(μ-η4-bpbq)Re(CO)3}2(μ-bpe)2] (4). Compound 4 was synthesized by following the procedure adopted for 1 using Re2(CO)10 (68 mg, 0.1 mmol), 2,5-bis(phenethylamino)-1,4-benzoquinone (34 mg, 0.1 mmol), and trans-1,2-bis(4-pyridyl)ethylene (18 mg, 0.1 mmol) and 4 was obtained as a maroon crystalline product. Yield: 73 mg, 66%. Anal. Calcd for C80H60N8O16Re4: C, 45.02; H, 2.83; N, 5.25. Found: C, 45.01; H, 2.70; N, 5.25. IR (CH2Cl2): νCO 2019 (s), 2012 (s), 1915 (s), 1890 (s), νaminoquinonato C=O 1533 cm−1. 1H NMR (400 MHz, (CD3)2CO, ppm): δ 8.60 (dd, J = 5.4, 1.4 Hz, 8H, H2, bpe), 7.65 (dd, J = 5.4, 1.4 Hz, 8H, H3, bpe), 7.52 (s, 4H, ethylenic CH, bpe), 7.36 (m, 20H, phenethyl), 5.73 (s, 4H, quinone-H), 4.15 (m, 4H, H1, CH2−phenethyl), 4.05 (m, 4H, H1′, CH2−phenethyl), 3.24 (m, 4H, H2, CH2−phenethyl), 3.01 (m, 4H, H2′, CH2−phenethyl). 13C NMR (100 MHz, (CD3)2CO, ppm): δ 198.9, 198.7, 194.4 (Re(CO)3), 178.8 (aminoquinonato CO), 167.4 (C2, quinone-C), 153.5 (C2, bpe), 146.7 (C4, bpe), 139.5 (C1, phenethyl), 132.3 (C5, ethylenic, bpe), 129.8 (C3, phenethyl), 129.6 (C2, phenethyl), 127.6 (C4, phenethyl), 124.3 (C3, bpe), 95.8 (C3, quinoneC), 58.9 (C1, CH2−phenethyl), 36.3 (C2, CH2−phenethyl). UV−vis {λmax (CH2Cl2)/(nm) (ε/M−1 cm−1)}: 229 (8 × 104), 300 (7.1 × 104) (LIG); 475 (6.2 × 104) (MLCT). Emission {λmax (CH2Cl2)/(nm)}: 556. Crystallographic Structure Determination. Single-crystal X-ray structural studies of 1, 4, and 3a were performed on an Oxford Diffraction XCALIBUR-EOS CCD equipped diffractometer, with an Oxford Instrument low-temperature attachment. Data were collected at 150 K using graphite-monochromated Mo Kα radiation (λα = 0.71073 Å). The strategy for data collection was evaluated using CrysAlisPro CCD software. The crystal data were collected by standard “ψ−ω scan” techniques and were scaled and reduced using CrysAlisPro RED software. The structures were solved by direct methods using SHELXS and refined by full matrix least-squares with SHELXL and SHELXH refining on F2.32 Disordered CHCl3 solvent molecules were identified (per asymmetric unit of 4), and their contribution to the scattering values was removed using the SQUEEZE algorithm in PLATON due to the unstable refinement.33 Positions of all the atoms were obtained by direct methods. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed in geometrically constrained positions and refined with isotropic temperature factors, generally 1.2 Ueq of their parent atoms.

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REFERENCES

Dedicated to Professor Pradeep Mathur on the occasion of his 60th birthday.

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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.5b01543. UV−vis absorption and emission spectra of compounds 1−4. ORTEP diagram of compound 3a. Experimental details for UV−vis and fluorescence titration experiments, NMR titration experiments and figures (PDF) CIF files giving the crystallographic data and structure refinement details of 1 (CCDC No. 1403878), 4 (CCDC No. 1403880), and 3a (CCDC No. 1403879) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; fax: 91 4132656740; tel: 91 413 2654414. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Department of Science and Technology, Government of India, for financial support. R.G. gratefully acknowledges the University Grant Commission for the award of Junior Research Fellowship. We are grateful to the Central Instrumentation Facility, Pondicherry University, for providing spectral data. We are thankful to the DST-FIST program sponsored Single-crystal X-ray diffraction facility to the Department of Chemistry, Pondicherry University. G

DOI: 10.1021/acs.inorgchem.5b01543 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.5b01543 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b01543 Inorg. Chem. XXXX, XXX, XXX−XXX