Synthesis and Spectroscopic and Structural Characterization of

R. Nagarajaprakash, D. Divya, Buthanapalli Ramakrishna, and Bala. Manimaran*. Department of Chemistry, Pondicherry University, Puducherry 605014, Indi...
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Synthesis and Spectroscopic and Structural Characterization of Oxamidato-Bridged Rhenium(I) Supramolecular Rectangles with Ester Functionalization R. Nagarajaprakash, D. Divya, Buthanapalli Ramakrishna, and Bala. Manimaran* Department of Chemistry, Pondicherry University, Puducherry 605014, India S Supporting Information *

ABSTRACT: Oxamidato-bridged Re(I)-based supramolecular rectangles with an ester functionality have been synthesized via an orthogonal bonding approach under solvothermal conditions. Self-assembly of Re2(CO)10 and oxamide ligands (H2L1 = N,N′-dibutyloxamide, H2L2 = N,N′-dioctyloxamide, H2L3 = N,N′-didodecyloxamide, and H2L4 = N,N′-dibenzyloxamide) with the pyridyl ligand phenyl-1,4-bis(isonicotinate) (pbin) has resulted in the formation of metallarectangles of general formula [{(CO)3Re(μ-η4-L)Re(CO)3}2(μ-pbin)2] (1− 4), wherein L = N,N′-dibutyloxamidato (1), N,N′-dioctyloxamidato (2), N,N′-didodecyloxamidato (3), N,N′-dibenzyloxamidato (4) and pbin = phenyl-1,4-bis(isonicotinate). The metallarectangles have been characterized using spectroscopic techniques, and single-crystal X-ray structures have been obtained for 1 and 4. The guest binding ability of 2 has been investigated with a few aromatic amines and an amino ketone using electronic absorption and fluorescence emission spectroscopy, and the results revealed a strong binding interaction between host−guest species. The luminescence properties of 2 and 3 have been tuned using organic−aqueous solvent mixtures.

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binding properties.16a−d Therrien et al. have demonstrated the synthesis of various ruthenium- and osmium-based metallarectangles containing organic bridges with O,O/O,N donors and linear ditopic azine ligands. The Ru(II) and Os(II) metallarectangles are cationic in nature and exhibit promising anticancer activity.8b,16e−i The research groups of Stang and Chi have prepared several arene−ruthenium-containing molecular rectangles, [(p-cymene) 4 Ru 4 (μ-OO∩OO/NN∩NN) 2 (μN∩N)2](OTf)4 (OO∩OO = oxalato, quinonato; NN∩NN = 2,2′-bisbenzimidazolate; N∩N = amide/alkyne functionalized linear ditopic azine ligands), and some of them are capable of sensing multicarboxylate anions selectively.16j−o Few research groups have utilized fac-Re(CO)3 cores for the construction of metallarectangles and studied their photophysical properties.17 Hupp and co-workers have developed several rhenium-based rectangles via stepwise synthesis by treating a rhenium−thiolate dimer with linear ditopic pyridyl ligands. In a continuation of this strategy, they have also synthesized a few more interesting molecular rectangles utilizing bimetallic edges such as [{(CO) 3 Re 2 }(bipyrimidine)]2+/[{(CO)4Re2}(BiBzIm)] (BiBzIm = bisbenzimidazolate) with linear ditopic pyridyl ligands.17a,c,d Lu et al. have systematically designed a series of rhenium(I) rectangles [{Re(CO)3(μ-bpy)Br}{Re(CO)3(μ-A)Br}]2 (A = pyrazine, trans-1,2-bis(4-pyridyl)ethylene, 4,4′-dipyridylacetylene, 4,4′dipyridylbutadiyne) that exhibited promising luminescent

INTRODUCTION Metal-mediated self-assembly has evolved as a powerful strategy for the generation of well-defined and complex metallasupramolecular architectures with nanoscopic dimensions.1−5 Owing to the availability of a plethora of organic linkers and transition-metal precursors with interesting geometries at their disposal, several research groups have been involved in the design of wide range of discrete metallacyclophanes with twoand three-dimensional topologies.6−8 Apart from their aesthetic appeal, metallasupramolecules find applications in various fields such as molecular recognition, photophysics/chemistry, molecular devices, catalysis, and host−guest chemistry.9−11 Since the discovery of metallasupramolecular squares by Fujita, the square-planar Pd(II)- and Pt(II)-based metal precursors have dominated as the favorite building blocks for the construction of a myriad of shapes such as triangles, squares, rhomboids, rectangles, and several higher order structures.12,13 Among the diverse shapes of metallasupramolecules, syntheses of molecular rectangles are not straightforward, despite their simple topology. Stang et al. have developed a strategy for the synthesis of molecular rectangles, wherein a predesigned Pt-based molecular clip with parallel coordination sites has been used in combination with linear ditopic ligands.14 Bosnich and co-workers have reported larger rectangles with a terpyridyl-based dipalladium clip and linear ligands such as 4,4′bipyridine.15 Supramolecular rectangles incorporating halfsandwich complexes of Ru, Rh, and Ir have been developed by the research groups of Suss-Fink and Jin in a two-step process, and some of these rectangles display interesting guest © 2014 American Chemical Society

Received: August 2, 2013 Published: March 7, 2014 1367

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Scheme 1. Self-Assembly of Metallarectangles 1−4

Figure 1. (a) ORTEP diagram of [{(CO)3Re(μ-η4-dibutyloxamidato)Re(CO)3}2(μ-pbin)2] (1) with thermal ellipsoids drawn at the 30% probability level. (b) Stick representation of 1 displaying intramolecular CH···O hydrogen bonding (green dotted line) and CH···π (orange dotted lines) interactions. (c) Packing diagram of 1 viewed along the c axis showing zigzag chain formation.

context, oxamide-based ligands form an important class of compounds, with N and O donor atoms that can chelate as well as bridge two metal centers.21 Recent literature evinced the synthesis of Ru(II)-, Rh(I)-, and Ir(I)-based cationic metallarectangles with oxamidato bridges in a stepwise manner.22 However, the self-assembly of neutral Re(I)-based metallarectangles containing oxamidato bridges with tunable cavity dimensions and solubility in organic solvents for extended applications exists as a challenging task. Herein, we report on the one-pot synthesis of novel Re(I)-based neutral supramolecular rectangles [{(CO)3Re(μ-η4-L)Re(CO)3}2(μ-pbin)2] (1−4) via self-assembly of rudimentary rhenium carbonyl, bischelating oxamidato ligands and ditopic ester containing pyridyl ligands. The rhenium-based metallacyclophanes have

properties. In addition, they have also devised a one-step synthetic strategy called the “orthogonal bonding approach” for self-assembly of Re(I)-based metallacycles and cages. The unique electronic and geometric features of the Re(I) metal center aided this straightforward synthetic methodology.18 Specific interest in Re(CO)3 core containing complexes emanates from their intriguing photophysical/photochemical properties and related applications in light harvesting, solar energy conversion, and luminescence sensing.19 Although Re(I)-based rectangles with linear ditopic pyridyl ligands as longer edges and alkoxide, thiolate, and bisbenzimidazolate/ quinonoid bridges as shorter edges are known, bischelating ligands with N,O donor atoms have rarely been used as shorter edges in the design of metallacyclophanes.11h,13a,17a−d,20 In this 1368

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Figure 2. (a) ORTEP diagram of [{(CO)3Re(μ-η4-dibenzyloxamidato)Re(CO)3}2(μ-pbin)2] (4) with thermal ellipsoids drawn at the 30% probability level. (b) Stick representation of 4 displaying intramolecular CH···O hydrogen bonding (green dotted line) and CH···π (orange dotted lines) interactions. (c) Packing diagram of 4 viewed along the a axis showing soft interactions.

as a medium-intensity band at 1751 cm−1. The electronic absorption spectra of 1−4 displayed four bands. The higher energy bands in the range λmax 220−275 nm were due to ligand-centered transitions, and the lower energy bands in the range λmax 320−391 nm were assigned to MLCT.18a,24 Photoexcitation of 1−4 at λmax 388−391 nm resulted in a broad emission centered at λmax 609−612 nm, and the emission quantum yields (Φem) calculated with reference to [Ru(bpy)3]2+ were found to be in the range (5.75−8.84) × 10−4. 1 H NMR spectra of 1−4 displayed appropriate signals corresponding to the protons of oxamidato ligands and an ester-containing pyridyl ligand, and the spectral data are given in the Experimental Section. In comparison to the signals for free ligands, the oxamidato proton signals were shifted downfield, while those of the ditopic pyridyl ligand were shifted upfield in 1−4.6e The 13C NMR spectra of 2 and 3 showed signals relevant to various types of carbons present in the rectangular architecture. The m olecular structures of [{(CO) 3 Re(μ -η 4 dibutyloxamidato)Re(CO)3}2(μ-pbin)2] (1) and [{(CO)3Re(μ-η4-dibenzyloxamidato)Re(CO)3}2(μ-pbin)2] (4) have been obtained using single-crystal X-ray diffraction methods. The ORTEP diagrams of 1 and 4 are shown in Figures 1a and 2a, and selected bond lengths and bond angles are given in Tables 1 and 2, respectively. The crystallographic data and structure refinement details of 1 and 4 are given in Table S4 (Supporting Information). Compound 1 crystallized in the monoclinic crystal system in the P21/c space group, and compound 4 crystallized in the triclinic crystal system in the P1̅ space group. The ORTEP diagrams revealed a rectangular architecture for 1 and 4 with similar structural features. Each Re atom is bonded to three terminal CO groups, one N atom from the pyridyl moiety of pbin, and one N atom and one O atom from the oxamidato bridge, giving rise to a distorted-octahedral geometry

been synthesized under facile solvothermal reaction conditions and characterized by spectroscopic techniques. Structural elucidation has been accomplished for 1 and 4 using singlecrystal X-ray diffraction methods. Investigations pertaining to aggregation induced enhancement in the emission properties of metallacyclophanes 2 and 3 have been carried out. The guest binding ability of 2 with a few aromatic amines and an amino ketone has been assessed.

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RESULTS AND DISCUSSION Supramolecular rectangles [{(CO)3Re(μ-η4-L)Re(CO)3}2(μpbin)2] (1−4) were self-assembled from an equimolar ratio of Re2(CO)10, oxamide ligands (H2L1 = N,N′-dibutyloxamide, H2L2 = N,N′-dioctyloxamide, H2L3 = N,N′-didodecyloxamide, and H2L4 = N,N′-dibenzyloxamide) and the ester-containing pyridyl ligand phenyl-1,4-bis(isonicotinate) (pbin) in mesitylene medium via the orthogonal bonding approach under solvothermal conditions (Scheme 1). The resulting products were air-, light-, and moisture-stable and soluble in common organic solvents. The solubility of rectangles 2 and 3 has been enhanced by introducing long alkyl substituents in oxamidato bridges. Compounds 1−4 have been characterized using spectroscopic techniques. IR spectra of 1−4 displayed strong bands in the range of 1896−2025 cm−1 characteristic of the facRe(CO)3 core.23 The CO stretching frequency of the oxamidato bridging moiety in the rectangles appeared as a strong band at 1601 cm−1, whereas the CO stretching frequency of the oxamide group in H2L1−H2L4 appeared at 1650 cm−1. The oxamide NH stretching bands were absent in 1−4. The shifting of the CO stretching frequency from 1650 to 1601 cm−1 and the disappearance of NH stretching bands in rectangles in comparison to the signals of free ligands (H2L1− H2L4) indicated coordination of oxamide ligands with rhenium centers. The ester CO stretching frequency of pbin appeared 1369

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interaction along the π cloud of the pyridyl moiety of pbin with an interaction distance of 3.325 Å (Figure 3b). The packing diagram of 4 revealed the formation of a three-dimensional network via CO···π, CO···H, CO···π, and CO···H soft interactions (Figure 2c). The fascinating structural features of metallarectangles with ester functional groups incorporated in the architecture provide opportunity for utilization as molecular receptors for binding relevant aromatic molecules. Investigations pertaining to the guest binding ability of 2 with 3,3′-methylenedianiline were performed using electronic absorption and fluorescence spectroscopic methods. Compound 2 was selected for spectroscopic titration experiments, owing to its good solubility in organic solvents. Titration of 3,3′-methylenedianiline with increasing concentrations of 2 was carried out and followed up by UV−visible absorption and emission spectral measurements (Figure S2, Supporting Information). Interestingly, an increase in absorbance and quenching in the emission pattern of 3,3′methylenedianiline was observed while increasing the concentration of 2, due to host−guest complex formation. The binding constant (Ka) was calculated as 2.2 × 105 M−1 from the slope and intercept of a linear Benesi−Hildebrand plot obtained using UV−vis spectroscopic titration data.27 In addition, the Stern−Volmer constant (KSV) was calculated as 3.9 × 105 M−1 from the slope of a linear Stern−Volmer plot acquired from fluorescence titration data.28 We further investigated the binding ability of 2 with few more aromatic guest species such as 4,4′-methylenedianiline and 1,5-diaminoanthraquinone, which revealed strong host− guest interactions (Table S1 and Figures S3 and S4, Supporting Information). The host−guest interactions between 2 and the aromatic amines and amino ketone were presumably due to NH···O hydrogen-bonding interactions between amino groups of guest molecules and O atoms of ester groups present in rectangle 2, though other possible intermolecular interactions such as CH···π and π···π interactions could not be ruled out. It is worthwhile to mention that Re(I)-based metallacycles containing long alkyl substituents form self-aggregates in organic−aqueous solvent mixtures and thereby lead to enhancement in the luminescence properties of metallacycles.29 The absorption and emission spectra of rectangles 2 and 3 that have long alkyl substituents were recorded in various ratios of THF/H2O solvent mixtures (Figures S5−S8, Supporting Information). In the UV−vis spectra, the absorption intensity of 2 and 3 was found to be enhanced upon increasing the water content. The MLCT band at λmaxab 391 nm was red-shifted by 5 nm, while there was no significant shift in the other bands up to 90% water. In 95% water content, the band at λmaxab 227 nm was blue-shifted by 10 nm for 2 and 3. In the emission spectra, a significant enhancement in emission intensity corresponding to MLCT absorption (λmaxab 391 nm) was observed beyond 70% water content for both 2 and 3. A concomitant blue shift was also observed in the emission maxima (λmaxem 609 to 568 nm for 2; λmaxem 612 to 555 nm for 3) with increase in emission intensity, characteristic of rigidochromism.29c The emission quantum yields (Φ) of 2 and 3 were found to be increased 10fold and 100-fold, respectively, when the medium was changed from organic to aqueous (Tables S2 and S3, Supporting Information). The significant enhancement in the luminescence properties of 2 and 3 could be attributed to their ability to aggregate in an organic−aqueous solvent mixture, facilitated by the hydrophobic nature of long alkyl chain substituents.29d

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

2.096(14) 2.223(11) 1.856(8) 83.7(4) 76.6(6) 87.3(5)

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

1.889(8) 1.888(8) 2.217(5) 171.1(5) 95.0(6) 93.1(3)

Table 2. Selected Bond Lengths (Å) and Angles (deg) for 4 Re1−O7 Re1−N1 Re1−N3 O7−Re1−N3 N1−Re1−O7 N1−Re1−N3

2.166(7) 2.151(9) 2.210(8) 82.6(3) 75.7(3) 86.3(3)

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

1.876(15) 1.915(17) 1.915(12) 174.2(5) 100.4(6) 92.9(4)

around the metal center. Two ester carbonyl groups on each longer edge of the rectangle are oriented anti with respect to each other. The faces of two central phenyl rings on opposite longer edges are slipped away from one another. In rectangle 1, four corners are occupied by four Re atoms, while the longer edges are occupied by the pbin ligand and the shorter edges are occupied by the dibutyloxamidato bridge. The dimensions of rectangle 1 as defined by Re atoms are ∼5.67 × 19.61 Å. On each longer edge of the rectangle, two pyridyl rings of the pbin bridge are tilted out of the plane of the central phenyl ring with dihedral angles of 56 and 79°. On each shorter edge of the rectangle constituted by the oxamidato bridge, there are two Nsubstituted butyl groups. One butyl group on each oxamidato edge is directed toward the cavity of the rectangle, while the other group is directed away from the cavity. In the butyl group positioned toward the cavity of the rectangle, H(11A) of the third methylene group is placed between π clouds of two pyridyl moieties of pbin bridges. The CH(11A)···π(centroid) interaction distance with the closest pyridyl moiety of pbin is 3.728 Å (Figure 1b).25 In addition, one of the hydrogen atoms of the methylene groups bonded to N atoms of each oxamidato bridge is involved in intramolecular hydrogen bonding with the oxygen atom of the oxamidato carbonyl group. The hydrogenbonding interaction distances of CH(13B)···O(8) and CH(9A)···O(7) are found to be 2.445 and 2.468 Å, respectively.26 Atoms N1, N2 and O7, O8 of the oxamidato bridge are substitutionally disordered respectively with occupancies of 70/ 30, and hence the butyl groups substituted at N1 and N2 are disordered accordingly. The packing diagram of 1 showed zigzag chain formation via CO···π, CO···H, and CO···π soft interactions (Figure 1c). The molecular structure of [{(CO)3Re(μ-η4dibenzyloxamidato)Re(CO)3}2(μ-pbin)2] (4) revealed structural features similar to those of 1 with the four corners of the rectangle being occupied by four Re atoms, longer edges by pbin, and shorter edges by dibenzyloxamidato bridges (Figure 2a). The dimensions of rectangle 4 as defined by Re atoms are ∼5.70 × 19.46 Å. On each longer edge of the rectangle, two pyridyl rings of the pbin bridge are tilted out of the plane of the central phenyl ring with dihedral angles of 44 and 72°. Intramolecular hydrogen bonding interactions are observed between the benzyl CH2 hydrogen atom H(9B) and oxygen atom O(8) and between H(16B) and oxygen atom O(7) of oxamidato carbonyl groups with interaction distances of 2.387 and 2.366 Å, respectively. Apart from this, H(22) of the aryl ring of the benzyl group is involved in an intramolecular CH···π 1370

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× 104), 388 (0.9 × 104) (MLCT). Emission (λmaxem (CH2Cl2), nm): 609 (Φem = 7.09 × 10−4). Synthesis of [{(CO) 3 Re(μ-η 4 -didodecyloxamidato)Re(CO)3}2(μ-pbin)2] (3). Compound 3 was synthesized by following the procedure adopted for 1 using Re2(CO)10 (65 mg, 0.1 mmol), N,N′-didodecyloxamide (42 mg, 0.1 mmol), and phenyl-1,4-bis(isonicotinate) (32 mg, 0.1 mmol), and 3 was obtained as a yellow solid. Yield: 82 mg, 64%. Anal. Calcd for C100H124N8O24Re4: C, 47.83; H, 4.98; N, 2.23. Found: C, 47.70; H, 4.89; N, 2.27. IR (KBr, cm−1): ν(CO) 2024 (s), 2017 (s), 1909 (s), 1896 (s), ν(ester CO) 1750 (m), ν(oxamidato CO) 1602 (s). 1H NMR (400 MHz, (CD3)2CO, ppm): δ 8.71 (m, 8H, H2py, pbin), 8.01 (m, 8H, H3py, pbin), 7.30 (d, 8H, Hph, pbin), 4.17 (m, 4H, H1, CH2-oxamidato), 3.36 (m, 4H, H1′, CH2-oxamidato), 1.77 (quintet, 8H, H2, CH2-oxamidato), 1.34 (m, 72H, H3−11, CH2-oxamidato), 0.88 (m, 12H, H12, CH3-oxamidato). 13 C NMR (100 MHz, (CD3)2CO, ppm): δ 198.0, 197.6, 196.1 (Re(CO)3), 173.2 (oxamidato CO), 163.7 (ester CO), 154.2, (C2, pypbin), 149.2, (C1, ph-pbin), 139.6 (C4, py-pbin), 126.12 (C2, ph-pbin), 123.9 (C3, py-pbin), 50.1, (C1, CH2-oxamidato), 29.4−19.1 (C2−11, CH2 -oxamidato), 14.4 (C 12 , CH 3 -oxamidato). UV−vis (λ max ab (CH2Cl2), nm (ε, M−1 cm−1)): 227 (4.6× 104), 268 (3.2 × 104) (LIG); 320 (2.9 × 104), 390 (0.9 × 104) (MLCT). Emission (λmaxem (CH2Cl2), nm): 612 (Φem = 8.84 × 10−4). [{(CO)3Re(μ-η4-dibenzyloxamidato)Re(CO)3}2(μ-pbin)2] (4). Compound 4 was synthesized by following the procedure adopted for 1 using Re2(CO)10 (65 mg, 0.1 mmol), N,N′-dibenzyloxamide (27 mg, 0.1 mmol), and phenyl-1,4-bis(isonicotinate) (32 mg, 0.1 mmol), and 4 was obtained as a yellow crystalline material. Yield: 78 mg, 69%. Anal. Calcd for C80H52N8O24Re4: C, 42.63; H, 2.33; N, 4.97. Found: C, 42.76; H, 2.24; N, 4.85. IR (KBr, cm−1): ν(CO) 2023 (s), 2017 (s), 1912 (s), 1890 (s), ν(ester CO) 1750 (m), ν(oxamidato CO) 1598 (s). 1H NMR (400 MHz, (CD3)2CO, ppm): δ 8.54 (m, 8H, H2py, pbin), 7.89 (m, 8H, H3py, pbin), 7.33 (m, 20H, H2, H3 and H4, CH-oxamidato), 7.28 (s, 8H, Hph, pbin), 5.27 (d, 4H, H, CH2oxamidato), 4.57 (d, 4H, H′, CH2-oxamidato). UV−vis (λmaxab (CH2Cl2), nm (ε, M−1 cm−1): 227 (3.6 × 104), 268 (2.3 × 104) (LIG); 324 (2.0 × 104), 390 (0.7 × 104) (MLCT). Emission (λmaxem (CH2Cl2), nm): 611 (Φem = 7.47 × 10−4). Crystallographic Structure Determination. Single-crystal X-ray structural studies of 1 and 4 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 = 0.7107 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 refining on F2.31 Positions on all the atoms were obtained by direct methods. All nonhydrogen atoms were refined anisotropically. The hydrogen atoms were placed in geometrically constrained positions and refined with isotropic temperature factors, generally 1.2 times the Ueq value of their parent atoms.

CONCLUSION In conclusion, we have synthesized neutral Re(I)-based supramolecular rectangles 1−4 with oxamidato and pbin bridging ligands via an orthogonal bonding approach in a one-step process. Single-crystal X-ray diffraction studies of 1 and 4 confirmed the tetrarhenium rectangular architecture for these metallacyclophanes. These rectangles feature tunable size, solubility, and luminescence properties. Investigations pertaining to the guest binding ability of rectangle 2 indicated strong host−guest interactions with a few aromatic amines and an amino ketone. Extending these novel 2D rectangles to 3D prismatic cages would lead to useful materials with improved selectivity in molecular recognition, and efforts toward realizing such 3D prismatic architectures with oxamidato bridges are underway.

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EXPERIMENTAL SECTION

General Details. Solvothermal reaction methods were adopted for the syntheses of complexes 1−4. Re2(CO)10 was purchased from Sigma Aldrich. The oxamide ligands (H2L1−H2L4) and phenyl-1,4bis(isonicotinate) (pbin) were synthesized by literature methods.30 The solvents were dried using standard methods and freshly distilled prior to use. IR spectra were recorded on a Nicolet-6700 FT-IR spectrophotometer. 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 fluorescence spectroscopic titration experiments were of spectral grade. 1H and 13C NMR spectra were recorded on an Avans Bruker 400 MHz NMR spectrometer. Elemental analyses were performed in a Micro Cube CHN analyzer. Synthesis of [{(CO)3Re(μ-η4-dibutyloxamidato)Re(CO)3}2(μpbin)2] (1). A mixture of Re2(CO)10 (65 mg, 0.1 mmol), N,N′dibutyloxamide (20 mg, 0.1 mmol), and phenyl-1,4-bis(isonicotinate) (32 mg, 0.1 mmol) in mesitylene (5 mL) was 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 4 h and then cooled to room temperature. Good-quality yellow crystals were obtained. The crystals were separated, washed with hexane, and dried under vacuum. Yield: 70 mg, 66%. Anal. Calcd for C68H60N8O24Re4: C, 38.56; H, 2.86; N, 5.29. Found: C, 38.78; H, 2.90; N, 5.35. IR (KBr, cm−1): ν(CO) 2025 (s), 2017 (s), 1909 (s), 1896 (s), ν(ester CO) 1750 (m), ν(oxamidato CO) 1602 (s). 1H NMR (400 MHz, (CD3)2CO, ppm): δ 8.70 (m, 8H, H2py, pbin), 8.03 (m, 8H, H3py, pbin), 7.30 (d, 8H, Hph, pbin), 4.17 (m, 4H, H1, CH2-oxamidato), 3.34 (m, 4H, H1′, CH2-oxamidato), 1.75 (quintet, 8H, H2, CH2-oxamidato), 1.45 (m, 4H, H3, CH2oxamidato), 1.34 (m, 4H, H3′, CH2-oxamidato), 1.01 (m, 12H, H4, CH3-oxamidato). UV−vis (λmaxab (CH2Cl2), nm (ε, M−1 cm−1)): 227 (4.5 × 104), 268 (3.1 × 104) (LIG); 321 (2.9 × 104), 391 (1.0 × 104) (MLCT). Emission (λmaxem (CH2Cl2), nm): 611 (Φem = 5.75 × 10−4). Synthesis of [{(CO)3Re(μ-η4-dioctyloxamidato)Re(CO)3}2(μpbin)2] (2). Compound 2 was synthesized by following the procedure adopted for 1 using Re2(CO)10 (65 mg, 0.1 mmol), N,N′dioctyloxamide (31 mg, 0.1 mmol), and phenyl-1,4-bis(isonicotinate) (32 mg, 0.1 mmol), and 2 was obtained as a yellow crystalline material. Yield: 66 mg, 63%. Anal. Calcd for C84H92N8O24Re4: C, 43.07; H, 3.96; N, 4.78. Found: C, 43.27; H, 3.92; N, 4.70. IR (KBr, cm−1): ν(CO) 2024 (s), 2017 (s), 1909 (s), 1896 (s), ν(ester CO) 1750 (m), ν(oxamidato CO) 1601 (s). 1H NMR (400 MHz, (CD3)2CO, ppm): δ 8.70 (m, 8H, H2py, pbin), 8.03 (m, 8H, H3py, pbin), 7.30 (d, 8H, Hph, pbin), 4.17 (m, 4H, H1, CH2-oxamidato), 3.36 (m, 4H, H1′, CH2-oxamidato), 1.79 (quintet, 8H, H2, CH2-oxamidato), 1.34 (m, 40H, H3−7, CH2-oxamidato), 0.89 (m, 12H, H8, CH3-oxamidato). 13C NMR (100 MHz, (CD3)2CO, ppm): δ 198.1, 197.9, 196.1 (Re(CO)3), 173.2 (oxamidato CO), 163.7 (ester CO), 154.1 (C2, py-pbin), 149.3 (C1, ph-pbin), 139.6 (C4, py-pbin), 126.12 (C2, ph-pbin), 123.9 (C3, py-pbin), 50.2 (C1, CH2-oxamidato), 29.7−14.4 (C2−7, CH2oxamidato), 14.4 (C8, CH3-oxamidato). UV−vis (λmaxab (CH2Cl2), nm (ε, M−1 cm−1)): 227 (4.9 × 104), 268 (3.2 × 104) (LIG); 321 (2.6

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF files giving UV−vis spectra of 1− 4, experimental procedures for molecular recognition and molecular aggregation studies, and crystallographic data for 1 and 4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for B.M.: [email protected]. Notes

The authors declare no competing financial interest. 1371

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ACKNOWLEDGMENTS We thank the Council of Scientific and Industrial Research, Government of India, and the Department of Science and Technology, Government of India, for financial support. We are grateful to the Central Instrumentation Facility, Pondicherry University, for providing spectral data.

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