Self-Assembly of Manganese (I)-Based Molecular Squares: Synthesis

Feb 22, 2012 - Department of Chemistry, Pondicherry University, Puducherry, 605014, India. ‡. Solid State and Structural Chemistry Unit, Indian Inst...
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Self-Assembly of Manganese(I)-Based Molecular Squares: Synthesis and Spectroscopic and Structural Characterization S. Karthikeyan,† K. Velavan,† Ranganathan Sathishkumar,‡ Babu Varghese,§ and Bala. Manimaran*,† †

Department of Chemistry, Pondicherry University, Puducherry, 605014, India Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, 560012, India § Sophisticated Analytical Instruments Facility, Indian Institute of Technology-Madras, Chennai, 600036, India ‡

S Supporting Information *

ABSTRACT: Syntheses of manganese(I)-based molecular squares have been accomplished in facile one-pot reaction conditions at room temperature. Self-assembly of eight components has resulted in the formation of M4L4-type metallacyclophanes [Mn(CO)3Br(μ-L)]4 (1−3) using pentacarbonylbromomanganese as metal precursor and rigid azine ligands such as pyrazine, 4,4′-bipyridine, and trans-1,2-bis(4pyridyl)ethylene, respectively, as bridging ligands. The metallacyclophanes have been characterized on the basis of IR, NMR, and UV−vis spectroscopic techniques and single-crystal X-ray diffraction methods.



INTRODUCTION There have been stupendous efforts made in the past few decades toward the synthesis of large macrocyclic compounds using noncovalent interactions that afford metal-based supramolecules such as molecular triangles, squares, rectangles, prisms, and cages via transition metal-directed self-assembly processes.1−6 Among all supramolecular systems, molecular squares assembled from metallacorners and linear bridging ligands have highly ordered structures of discrete size and shape and were found to exhibit enormous applications in molecular recognition, chemical sensing, gas separation, and storage.7 Fujita et al. have pioneered a novel concept for the construction of cis-protected palladium-based molecular squares, which had the ability to bind guest in aqueous media.8 Stang and coworkers established the synthesis of platinum- and palladiumbased molecular squares with chelating bis-phosphino ligands that displayed noncovalent host−guest interactions.9 Luminescent and neutral molecular squares with octahedral rhenium centers bridged by ditopic ligands were synthesized by Hupp et al., and the molecular squares have been shown to work as molecular sensors for volatile organic compounds.10 Jeong and co-workers synthesized neutral and octahedral osmium(IV)containing molecular squares that have a well-defined geometry and cavity.11 Würthner et al. reported Pd- and Pt-based molecular squares containing a perylene skeleton exhibiting fluorescence properties with good quantum yields and interesting redox behavior.12 The first paramagnetic square macrocyclic assemblies based on Ru25+ corners and dicarboxylate linkers were reported by Cotton and co-workers.13 Mirkin et al. demonstrated the anion-dependent route for the design of rhodium-based molecular squares assembled via hemilabile ligands.14 Chiral molecular squares constructed by Lin et al. displayed enantioselective luminescence quenching by chiral amino alcohols.15 Maverick and co-workers reported copper β© 2012 American Chemical Society

diketonate molecular squares, in which bifunctional organic moieties serve as the corners bonded to the metal center in the middle of the edges.16 Theilmann et al. synthesized titaniumbased molecular squares with azobispyridines showing cis−trans isomerism.17 Although, there have been several reports on many transition metal-containing molecular squares in the literature,18,19 the synthesis of Mn(I)-based molecular squares with a tunable cavity dimension by a self-assembly process is still an existing challenge. Herein, we report on the novel onepot synthesis of manganese-based molecular squares [Mn(CO)3Br(μ-L)]4 (1−3) via self-assembly of four units of Mn(CO)5Br and four units of azine ligands to afford M4L4-type metallacyclophanes. The Mn(I)-based molecular squares have been synthesized in facile reaction conditions at room temperature and characterized by spectroscopic techniques. Molecular structures of the metallacyclophanes 1 and 2 were determined by the single-crystal X-ray diffraction method. The self-assembly of molecular squares 2 and 3 was monitored by 1 H NMR spectroscopy to evidence the exclusive formation of a single product from the reaction of eight components.



RESULTS AND DISCUSSION When Mn(CO)5Br was treated with rigid azine ligands in acetone medium at room temperature, the manganese-based molecular squares [Mn(CO)3Br(μ-L)]4 (1−3), L = pz (1), bpy (2), and bpe (3), were formed in a single step via a selfassembly process (Scheme 1). Molecular squares 1−3 have been characterized by spectroscopic techniques. The 1H NMR spectra of molecular squares 1−3 displayed appropriate signals for the bidentate Received: December 15, 2011 Published: February 22, 2012 1953

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Scheme 1. Synthetic Route to the Self-Assembled Molecular Squares 1−3

azine ligands bonded to a metal center, and the spectral data are given in the Experimental Section. The proton signals of 1−3 were shifted downfield when compared to free ligands due to the formation of new coordination bonds between the azine ligands and the manganese metal centers.20 Similarly, the 13C NMR spectra of 1−3 displayed signals for the ligands complexed with metal centers. The IR spectra of 1−3 exhibited three strong bands in the region of ν(CO) 2038−1907 cm−1, characteristic of a fac-Mn(CO)3 moiety.21 The electronic absorption spectra of 1−3 showed intense higher energy bands between λmax 228 and 295 nm, for ligand-centered π−π* transitions, and lower energy transitions in the range λmax 381− 388 nm due to metal-to-ligand charge transfer.10 The molecular squares 1−3 were obtained as a single product via a self-assembly process. To evidence this, the selfassembly of manganese(I) molecular square [Mn(CO)3Br(μbpy)]4 (2) was monitored by in situ 1H NMR spectroscopy.22 Reaction between Mn(CO)5Br and 4,4′-bipyridine was conducted in an NMR tube in acetone-d6 at 25 °C. The course of the reaction was monitored by recording 1H NMR spectra of reaction mixture at one-hour intervals. Initially the signals of H2 and H3 protons of the free 4,4′-bipyridine ligand appeared at δ 8.71 and 7.74 ppm. After one hour, the proton signals corresponding to the manganese(I) molecular square 2 started appearing at δ 8.95 and 7.88 ppm. The intensities of free ligand signals were found to decrease, while those of molecular square 2 were found to increase, indicating product formation. The self-assembly process was nearly completed in nine hours, at which time free 4,4′-bipyridine proton signals had almost disappeared. The stack plot of time-dependent 1H NMR spectra is given in Figure 1. An in situ 1H NMR spectral study supports the formation of exclusively single product from the self-assembly of eight components. Similarly, the self-assembly of [Mn(CO)3Br(μ-bpe)]4 (3) was also monitored by in situ 1H NMR spectroscopy (Figure S1). The molecular square [Mn(CO)3Br(μ-pz)]4 (1) was obtained as orange crystals suitable for single-crystal X-ray structure analysis from a solution of 1 in acetone at 5 °C. Compound 1 crystallized in the tetragonal space group I41/a. The ORTEP diagram of 1 is shown in Figure 2, and selected bond lengths and bond angles are given in Table 1. The crystal structure of [Mn(CO)3Br(μ-pz)]4 (1) adopted a square architecture with a manganese center at the four corners. Each manganese atom is bonded with two pyrazine molecules through nitrogen atoms, three carbonyl groups, and one bromine atom to give the metal center a distorted octahedral geometry. Each pyrazine ligand is bridging two manganese

Figure 1. Stack plot of time-dependent 1H NMR spectra showing selfassembly of [Mn(CO)3Br(μ-bpy)]4 (2).

Figure 2. ORTEP diagram of [Mn(CO)3Br(μ-pz)]4 (1) with thermal ellipsoids at the 50% probability level. Solvent (acetone) molecules were omitted for clarity.

corners to form a molecular square architecture. Atom Br(1) is disordered with the trans CO group at 26/74 occupancy.23 The dimensions of the molecular square 1 are ∼6.92 × 6.92 Å. The distance between two diagonal manganese atoms is ∼9.79 Å. The solvent molecules of acetone were present above and below the plane of 1 with C−O···H (2.595 Å) and C−O···C (3.169 Å) interactions (Figure S2).24 The packing diagram of 1 along the c axis showed infinite channels containing acetone molecules (Figure S3). 1954

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with two 4,4′-bipyridine moieties, three carbonyl groups, and one bromine atom to give the metal center a distorted octahedral geometry. Each 4,4′-bipyridine ligand is bridging two manganese corners to form a molecular square architecture. Atom Br(1) is disordered with the trans CO group at 11/89 occupancy.23 The molecular dimensions of molecular square 2 are ∼11.27 × 11.27 Å. When the molecular square is viewed laterally along any two manganese centers, the structure is observed to be puckered (Figure S4). The distance between two diagonal manganese centers is ∼15.07 Å. The packing diagram of 2 along the ab plane is viewed as an infinite doublestranded zigzag arrangement linked via intermolecular soft interactions such as CH···Br (2.973 Å), CO···H (2.700 Å), and CO···C (3.208 Å) (Figure S5).24,25 Also the packing diagram of 2 along the b axis displayed infinite channels with dimensions of ∼9.18 × 4.27 Å (Figure S6). The crystallographic data and structure refinement of 1 and 2 are given in Table 3.

Table 1. Selected Bond Distances and Bond Angles for [Mn(CO)3Br(μ-pz)]4 (1) (a) Bond Distances (Å) Mn(1)−C(1) Mn(1)−C(2) Mn(1)−C(3) Mn(1)−N(1) Mn(1)−N(2) N(1)−Mn(1)−N(2) C(1)−Mn(1)−N(1) C(2)−Mn(1)−N(1)

1.784(9) Mn(1)−Br(1) 1.804(9) C(1)−O(1) 1.823(7) C(2)−O(2) 2.070(6) C(3)−O(3) 2.070(6) N(1)−C(4) (b) Bond Angles (deg) 86.5(2) 176.8(3) 92.4(3)

C(3)−Mn(1)−N(1) N(1)−Mn(1)−Br(1) C(4)−N(1)−Mn(1)

2.488(2) 1.164(9) 1.158(9) 1.186(8) 1.326(9) 94.3(4) 89.49(18) 121.8(5)

The molecular square [Mn(CO)3Br(μ-bpy)]4 (2) was obtained as brown crystals suitable for single-crystal X-ray structure analysis from a solution of 2 in acetone at 5 °C. Compound 2 crystallized in the tetragonal space group I41/a. The ORTEP diagram is shown in Figure 3, and selected bond

Table 3. Crystallographic Data and Structure Refinement of 1 and 2 1 empirical formula fw cryst syst temp (K) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z F(000) Dcalc (mg m−3) μ (mm−1) theta range for data collection (deg) cryst size (mm) reflns collected/unique Rint data/restraints/params goodness-of-fit on F2 final R indices [I > 2σ(I)]

0.21 × 0.18 × 0.13 17 716/2966 0.1223 2966/8/146 0.928 R1 = 0.0763, wR2 = 0.1667 R indices (all data) R1 = 0.1518, wR2 = 0.1942 largest diff peak and hole (e 1.000 and −0.625 Å3)

Figure 3. ORTEP diagram of [Mn(CO)3Br(μ-bpy)]4 (2) with thermal ellipsoids at the 50% probability level.

Table 2. Selected Bond Distances and Bond Angles for [Mn(CO)3Br(μ-bpy)]4 (2) (a) Bond Distances (Å) C(11)−Mn(1) C(12)−Mn(1) C(13)−Mn(1) N(1)−Mn(1) Mn(1)−N(2) N(1)−Mn(1)−N(2) C(11)−Mn(1)−N(1) C(12)−Mn(1)−N(1)

1.809(6) Br(1)−Mn(1) 1.821(6) C(11)−O(1) 1.808(6) C(12)−O(2) 2.086(4) C(13)−O(3) 2.092(4) C(1)−N(1) (b) Bond Angles (deg) 85.05(17) 175.4(2) 94.5(4)

C(13)−Mn(1)−N(1) N(1)−Mn(1)−Br(1) C(1)−N(1)−Mn(1)

C34H28N8Br4O14Mn4 1312.04 tetragonal 150(2) I41/a 20.972(3) 20.972(3) 15.313(4) 90 90 90 6735(2) 4 2560 1.294 3.154 1.65 to 25.00

2.5288(13) 1.142(7) 1.146(9) 1.144(7) 1.339(7)



2 C58H44N8Br4O14Mn4 1616.41 tetragonal 150(2) I41/a 15.4691(7) 15.4691(7) 27.759(3) 90 90 90 6642.6(7) 4 3200 1.616 3.215 1.51 to 24.99 0.23 × 0.16 × 0.13 31 512/2919 0.0581 2919/39/237 1.178 R1 = 0.0555, wR2 = 0.1460 R1 = 0.0674, wR2 = 0.1523 1.876 and −0.507

CONCLUSION

In conclusion, the synthesis of novel manganese(I)-based molecular squares with tunable cavity dimensions has been achieved in a facile one-pot reaction condition at room temperature. The molecular squares were characterized by spectroscopic techniques, and their molecular structures were determined using the single-crystal X-ray diffraction method. The self-assembly of molecular squares was monitored by 1H NMR spectroscopy to evidence the exclusive formation of a single product from the reaction of eight components. We are

89.9(2) 89.93(12) 122.2(4)

lengths and bond angles are given in Table 2. The crystal structure of [Mn(CO)3Br(μ-bpy)]4 (2) adopted a square architecture like [Mn(CO)3Br(μ-pz)]4 (1) with a manganese center at the four corners. Each manganese atom is bonded 1955

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picture was generated using ORTEP-3.24.33 Crystallographic details are given in Table 3.

currently exploiting the metal-induced self-assembly for the facile synthesis of Mn(I)-based, larger molecular squares using functionalized ditopic linkers to extend their molecular recognition capabilities. Further work is in progress to explore the applications of these manganese(I)-based molecular squares in our laboratory.





ASSOCIATED CONTENT

S Supporting Information *

Experimental procedure, spectroscopic characterization, figures, and CIF files giving crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

EXPERIMENTAL SECTION



General Details. All manipulations were carried out under a nitrogen atmosphere using standard Schlenk techniques. All solvents were dried and distilled prior to use according to standard methods. Mn(CO)5Br was prepared as reported in the literature.26 IR spectra were taken on a Thermo Nicolet 6700 FT-IR spectrophotometer. 1H NMR and 13C NMR spectra were recorded on Bruker 400 and 500 MHz spectrometers. The electronic absorption spectra were obtained on a Shimadzu 2450 spectrophotometer at room temperature in a 1 cm quartz cell. Elemental analyses were performed using an Elementar Micro Cube CHN analyzer. Synthesis of [Mn(CO)3Br(μ-pz)]4 (1). A mixture of Mn(CO)5Br (55 mg, 0.2 mmol) and pyrazine (16 mg, 0.2 mmol) was placed in a 100 mL round-bottom Schlenk flask equipped with a magnetic stirring bar. The system was evacuated and purged with nitrogen using a vacuum Schlenk line. To this was added acetone (40 mL), and the reaction mixture was stirred at room temperature (25 °C) for 24 h under dark conditions. An orange solid was obtained by removing the solvent using vacuum, washed with hexane, and dried under vacuum. Yield: 55 mg, 95%. Anal. Calcd for C28H16N8O12Br4Mn4: C, 28.12; H, 1.35; N, 9.37. Found: C, 28.70; H, 1.36; N, 9.30. IR (KBr): ν(CO) 2038 (s), 1953 (s), 1927 (s) cm−1. 1H NMR (400 MHz, (CD3)2CO, ppm): δ 8.93 (s, 16H). 13C NMR (125 MHz, (CD3)2SO, ppm): δ 220.7 (s, CO), 147.2 (C2, pz). UV−vis {λmaxab (CH2Cl2)/(nm)}: 229 (LIG) and 381 (MLCT). Synthesis of [Mn(CO)3Br(μ-bpy)]4 (2). The molecular square 2 was synthesized by following the procedure adopted for 1, using Mn(CO)5Br (55 mg, 0.2 mmol) and 4,4′-bipyridine (32 mg, 0.2 mmol), and compound 2 was obtained as a bright yellow solid. Yield: 69 mg, 93%. Anal. Calcd for C52H32N8O12Br4Mn4: C, 41.63; H, 2.15, N, 7.47. Found: C, 42.14; H, 2.19, N, 7.35. IR (KBr): ν(CO) 2027 (s), 1937 (s), 1909 (s) cm−1. 1H NMR (400 MHz, (CD3)2CO, ppm): δ 8.95 (s, 16H, H2, py), 7.88 (s, 16H, H3, py). 13C NMR (125 MHz, (CD3)2SO, ppm): δ 223.6 (s, CO), 156.7 (C2, py), 146.8 (C4, py), 123.7 (C3, py). UV−vis {λmaxab (CH2Cl2)/(nm)}: 228 (LIG) and 388 (MLCT). Synthesis of [Mn(CO)3Br(μ-bpe)]4 (3). The molecular square 3 was synthesized by following the procedure adopted for 1, using Mn(CO)5Br (55 mg, 0.2 mmol) and trans-1,2-bis(4-pyridyl)ethylene (36 mg, 0.2 mmol), and compound 3 was obtained as a bright orangeyellow solid. Yield: 73 mg, 92%. Anal. Calcd for C60H40N8O12Br4Mn4: C, 44.92; H, 2.51; N, 6.98. Found: C, 44.31; H, 2.48; N, 6.89. IR (KBr): ν(CO) 2025 (s), 1935 (s), 1907 (s) cm−1. 1H NMR (400 MHz, (CD3)2CO, ppm): δ 8.81 (s, 16H, H2, py), 7.72 (s, 16H, H3, py and 8H ethylenic). 13C NMR (125 MHz, (CD3)2SO, ppm): δ 223.7 (s, CO), 156.1 (C2, py), 146.4 (C4, py), 132.5 (C5, ethylenic), 123.3 (C3, py). UV−vis {λmaxab (CH2Cl2)/(nm)}: 228, 295 (LIG) and 388 (MLCT). In Situ 1H NMR Spectral Study of 2. Mn(CO)5Br (0.0075 mmol, 2.0 mg) and 4,4′-bipyridine (0.0075 mmol, 1.1 mg) were dissolved in acetone-d6 (0.5 mL) in an NMR tube under a nitrogen atmosphere, and the self-assembly of molecular square [Mn(CO)3Br(μ-bpy)]4 (2) was monitored by in situ 1H NMR techniques at 25 °C. 1H NMR spectra of the sample were recorded every hour on a Bruker AMX-400 FT-NMR spectrometer. Single-Crystal X-ray Diffraction Study. Single-crystal experiments were performed on a Bruker AXS SMART APEX II CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The data collection was performed with APEX2,27 data reduction with SAINT,28 absorption correction with SADABS,29 structure solution with SHELXS97,30 and structure refinement by fullmatrix least-squares against F2 using SHELXL-9731 and WinGX.32 The

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We thank the Department of Science and Technology, Government of India, for financial support. K.V. acknowledges CSIR (13(8399-A)/Pool/2010), Government of India, for the award of a Senior Research Associateship. We are grateful to the NMR Research Centre, Indian Institute of Science, Bangalore, and Central Instrumentation Facility, Pondicherry University, for providing spectral data. We are also thankful to Prof. P. Sambasiva Rao and Dr. R. Venkatesan for valuable discussions.



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dx.doi.org/10.1021/om201244a | Organometallics 2012, 31, 1953−1957