ARTICLE pubs.acs.org/Organometallics
Adaptation toward Restricted Conformational Dynamics: From the Series of Neutral Molecular Rotors Palanisamy Rajakannu,† Bhaskaran Shankar,† Anju Yadav,† Ramasamy Shanmugam,† Deepak Gupta,† Firasat Hussain,*,† Che-Hao Chang,‡ Malaichamy Sathiyendiran,*,† and Kuang-Lieh Lu*,‡ † ‡
Department of Chemistry, University of Delhi, Delhi 110 007, India Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan, Republic of China
bS Supporting Information ABSTRACT: A semirigid ligand, 1,4-bis(2-nonylbenzimidazol1-ylmethyl)benzene (Nbenzbix), containing a long alkyl chain substituted stator and a rotating unit was designed and synthesized. Three dinuclear metallacycles [(Re(CO)3)(μ-L) (μ-Nbenzbix)(Re(CO)3)] (1, H2-L = H2-dhnq = 6,11-dihydroxy-5,12-naphthacenedione; 2, H2-L = H2-dhaq = 1,4-dihydroxy-9,10-anthraquinone; and 3, H2-L = H2-CA = 2,5dichloro-3,6-dihydroxy-p-benzoquinone) were synthesized from Re2(CO)10, Nbenzbix, and H2-L units. Compounds 13 and the ligand Nbenzbix were characterized by elemental analysis, FT-IR, and 1H NMR spectroscopy. Compound 1 was further characterized by a single-crystal X-ray diffraction analysis. The dynamic properties of 13 in solution were studied using variable-temperature 1H NMR spectroscopy, and the findings indicate that the p-phenylene unit in the metallacycle rotates in solution. To prevent the stator units from undergoing conformational changes due to the syn/anti arrangement of the benzimidazolyl units, a long-chain alkyl group was introduced at the 2-position of the benzimidazolyl unit. Molecular modeling calculations indicate that the energy barrier for the p-phenylene rotating unit in the metallacycle would be very low. Hence, these neutral metallacycles can be regarded as surface-mounted altitudinal rotors in which the bischelating unit is related to the surface, the nonylbenzimidazolyl units are related to the stators, and the p-phenylene is related to the rotating unit.
’ INTRODUCTION Research directed toward the design, synthesis, and characterization of molecules possessing a rotating unit has attracted considerable attention, particularly with respect to the development of artificial molecular rotors and motors, in an attempt to mimic the intelligent and useful functions of biological molecular rotors and motors.14 In addition, it can provide future functional materials in the field of information storage, molecular electronics, and mechanics.5 A variety of organic molecules as rotors and motors have been synthesized,69 whereas the design and synthesis of discrete metallacyclic rotors and motors are in a primary state.7,1015 Rotor-shaped metallacycles were recently synthesized using supramolecular metal-directed approaches. The cis-protected Pt(II)-directed self-assembly of organic units containing both stators and rotators resulted in the formation of ionic metallacyclic rotors,16 and a fac-Re(CO)3-directed orthogonal bonding approach resulted in neutral metallacyclic rotors.17 The above-mentioned synthetical approaches for the production of neutral and ionic metallacyclic rotors are workable. However, only a small number of studies have been reported thus far, and better results are surely yet to come. This young field provides many opportunities for developing new novel molecules or improving the properties of existing ones. r 2011 American Chemical Society
Herein, we report on the synthesis and characterization of a semirigid ligand, 1,4-bis(2-nonylbenzimidazol-1-ylmethyl)benzene (Nbenzbix), and three neutral, altitudinal rotor-shaped metallacycles (13). These metallacycles were synthesized by a one-pot orthogonal bonding approach using a Re2(CO)10, rigid dianionic bischelating ligand and semirigid ditopic donors containing long alkyl chain substituted bulky stators and a p-phenylene rotating unit. Various conformers of the semirigid ligand due to the orientation of the benzimidazolyl unit were prevented by introducing the long alkyl chain as steric bulk at the 2-position of the benzimidazolyl unit. The ligand Nbenzbix and the metallacycles were characterized by elemental analysis, FTIR, and 1H NMR spectroscopy. The structure of compound 1 was further confirmed by single-crystal X-ray crystallography. The solution dynamics of 13 were examined by using variabletemperature 1H NMR spectroscopy.
’ RESULTS AND DISCUSSION Synthesis and Characterization of the Nbenzbix Ligand. The ligand Nbenzbix was synthesized by reacting one equivalent Received: March 26, 2011 Published: May 09, 2011 3168
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Figure 1. Benzimidazolyl and 2-nonylbenzimidazolyl units in benzbix and Nbenzbix are highlighted.
Figure 2. Ball-and-stick representation of the optimized structure of the Nbenzbix ligand.
of 1,4-bis(bromomethyl)benzene with two equivalents of 2-nonylbenzimidazole in the presence of a strong KOH solution. The isolated yellow powder was air and moisture stable and was soluble in organic solvents. The 1H NMR spectrum of the Nbenzbix contained five sets of resonances in the aromatic region. Among these, two sets of doublets and a multiplet, corresponding to the benzimidazolyl protons, are indicative of the asymmetrical nature of the benzimidazolyl unit and the similar chemical environment of the two benzimidazolyl units in Nbenzbix. A singlet at 7.09 ppm was assigned to the p-phenylene protons. The protons of the methylene group that connects both the p-phenylene and the 2-nonylbenzimidazolyl moieties appeared as a singlet at 5.44 ppm, indicating that Nbenzbix is flexible in solution. Although the new 1,4-bis(2-nonylbenzimidazol-1-ylmethyl)benzene ligand is similar to the reported 1,4-bis(benzimidazol-1ylmethyl)benzene (benzbix) and 1,4-bis(imidazol-1-ylmethyl)benzene (bix) ligands,17 the main feature of this ligand is its steric bulkiness at the 2-position of the imidazolyl unit due to the length of the alkyl chain. Molecular modeling showed that the distance between the terminal alkyl chain and the center of half of the imidazolyl unit in Nbenzbix (∼13.6 Å) is longer than the value calculated for the benzbix and bix ligands (∼2.3 Å, Figures 1, 2). This steric bulkiness prevents the 2-nonylbenzimidazolyl unit from rotating 180° in the expected metallacyclic cavity (∼5.88.1 Å) formed using Nbenzbix and similar types of ligands,17 thereby restricting syn/anti interconversion of the benzimidazolyl units. However, the introduction of lengthy alkyl
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chains can be used to stabilize the metallacycle on the carboncoated surface for further materials studies.20 Synthesis and Characterization of Dinuclear Complexes 1, 2, and 3. Metallacycles 1 and 2 were self-assembled from equimolar amounts of Re2(CO)10, Nbenzbix, and H2-dhnq (6,11-dihydroxy-5,12-naphthacenedione)/H2-dhaq (1,4-dihydroxy-9,10-anthraquinone) by an orthogonal bonding approach under solvothermal conditions (Scheme 1).17,18 The resulting products are air and moisture stable and were soluble in organic polar solvents. The FT-IR spectrum shows strong stretching bands at 2008, 1903, and 1875 cm1 for 1 and at 2009, 1899, and 1881 cm1 for 2. These values are within the expected range for other complexes with the fac-Re(CO)3 unit.17,18 Compound 1 was further characterized by 1H,1H1H COSY and 1H1H NOESY NMR spectroscopy. The 1H NMR spectrum of 1 at room temperature in acetone-d6 showed well-separated peaks (Figure 3). The dhnq (H2-dhnq = 6,11-dihydroxy-5,12naphthacenedione) protons appeared as four multiplets with equal intensity at 8.6, 8.45, 7.9, and 7.75 ppm. The 1H1H COSY spectrum of 1 confirmed that these four multiplets belong to one dhnq unit, assigned to the protons of rings A and D, as shown in Figure 3 (Figure S1). A 1H1H NOESY measurement provided the correlation between the Ha of the dhnq unit (ring A) to the H of the nonyl alkyl chain (C1H2) connected to the imidazolyl unit (Figure S2). These findings suggest that the dhnq2 unit in 1 is unsymmetrical. A single set of two doublets and two triplets was observed for the benzimidazolyl protons, indicating that the two units are in the same environment in 1. In addition, well-separate positions for the H5 and H6 in 1, which were merged together in the free Nbenzbix, indicate that both protons experience different chemical environments in the metallacycle. As expected, the H4 signal of Nbenzbix was shifted downfield (δ 0.76) due to the loss of electron density after metal coordination.17,22a The single resonance at δ 6.05 corresponds to the p-phenylene protons, which appeared in the upfield region in comparison to the free ligand (δ 7.09). From the solid-state structure, two upfield signals for the p-phenylene protons due to two different edge-to-face CH 3 3 3 π interactions between the dhnq units and the p-phenylene moiety would be expected. Hence, the moderate upfield shift of the singlet for the pphenylene protons in 1 suggests that the p-phenylene ring rotates in solution and the observed singlet for the p-phenylene group represents an average value for various rotational conformers. Compound 2 shows the simple 1H NMR pattern with a 1:1 (dhaq:Nbenzbix) proton ratio (Figure 3). The proton signal of the p-phenylene unit in 2 was shifted upfield (6.12), similar to those found in 1. To confirm the role of bis-chelating unit on the p-phenylene unit, metallacycle 3, possessing the less conjugative bis-chelating unit CA2, was prepared. The absence of absorption around 1665 and 1632 cm1 and the presence of strong absorption at 1506 cm1 in 3 indicate that the CA unit binds with the metal ions in a bidentate fashion.19a The 1H NMR spectrum shows a slightly downfield shift (7.24 ppm) for the p-phenylene unit in 3 compared to the free ligand Nbenzbix (7.01 ppm) in DMSO-d6 (Figure 4). This clearly confirms that the p-phenylene units in 13 rotate in solution and also experience a ring current effect by the bis-chelating unit. It is noteworthy that 1H NMR spectra for the first neutral metallacyclic rotors [(Re(CO)3) (μ-L)(μ-L0 )(Re(CO)3)] (4, L0 = benzbix and L = dhnq; 5, L0 = bix and L = dhnq; 6, L0 = benzbix and L = dhaq; and 7, L0 = bix and L = dhaq) possessing benzimidazolyl/imidazolyl and dhnq/ dhaq stators and a p-phenylene rotator, reported by Sathiyendiran 3169
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Scheme 1. Self-Assembly of Metallacycles 1 and 2
Figure 3. Partial 1H NMR spectra of 1 (top), 2 (middle), and free Nbenzbix (bottom), in acetone-d6.
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Figure 4. Partial 1H NMR spectra of 3 (top) and free Nbenzbix (bottom), in DMSO-d6.
Figure 5. Metallacycle 1 in the crystal form. Front view (ball-and-stick model): the rotator unit is indicated by dark lines (left). The side view (ball-and-stick model) shows the arrangement of alkyl chains (right). Space-filling representation of 1 3 toluene (bottom). C gray, H white, N blue, O red, Re green.
and Lu et al., showed two types of signals for both the benzbix/ bix and dhaq ligands in 6 and 7, indicating the presence of two isomers in solution.17 In the case of 2, however, only one type of signal for both ligand units was observed. In addition, the
chemical shift of the protons of the p-phenylene unit in the metallacycles did not change much upon varying the polarity of the solvents (Figure S4S6). These data clearly suggest that the nonyl group on the imidazolyl ring is an obstacle for the rotation 3171
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Table 1. Crystallographic Data of 1 3 C7H8
cryst syst
monoclinic
space group
P21/c
a (Å) b (Å)
16.7355(8) 19.0723(11)
c (Å)
20.2933(8)
β (deg)
129.512(2)
V (Å3)
6477.3(5)
Z
4
T (K)
183(2)
λ (Å)
0.71073
Dcalc (g cm3) μ (mm1)
1.550 3.796
F(000)
3016
goodness-of-fit
1.0099
R1a/wR2b [I > 2σ(I)]
0.0660/0.1985
R1a/wR2b (all data)
0.0802/0.2031
largest residuals (e Å3)
4.834/4.881
R1 = ∑ Fo||Fc /∑|Fo|. b wR2 = {∑[w(Fo2 Fc2)2]/∑[w(Fo2)2]}1/2. )
a
of the benzimidazolyl unit, thereby preventing the formation of various conformations due to the orientation of the benzimidazolyl units. Molecular Structure. A single-crystal X-ray diffraction analysis at a temperature of 183(2) K shows that metallacycle 1 adopts a M2LL0 -type architecture, which is made up of two facRe(CO)3 cores, one Nbenzbix, and one dhnq, as shown in Figure 5. The Re(I) adopts a distorted octahedral arrangement with a C3NO2-donor environment. The dianionic tetradentate dhnq2 chelates two rhenium atoms through the four oxygen atoms, resulting in a ReRe distance of 8.6 Å. The dhnq2 unit is planar with π-electron delocalization confined to the bischelating units and the two terminal arene units.17,19 The Nbenzbix ligand adopts a syn-conformation mode, with both 2-nonylbenzimidazolyl arms on the same side with a dihedral angle of 31°, and serves as a molecular clip. The ditopic clip coordinates with two rhenium ions using its benzimidazolyl nitrogen, and the ReN bond distances of 1 are very similar and consistent with values reported for other fac-Re(CO)3 complexes containing an imidazolyl/benzimidazolyl unit.17 The p-phenylene unit is perpendicular to the dhnq unit (dihedral angle = ∼77°) with an estimated distance of 3.7 Å separating the mean plane of the dhnq and the p-phenylene carbons. These values are less than 4.1 Å and suggest that the p-phenylene hydrogen is involved in weak CH 3 3 3 π interactions with the dhnq unit.21 In addition, the remaining half of the p-phenylene unit interacts with the dhnq unit of the neighboring metallacycle through edge-to-face CH 3 3 3 π interactions (Figure 6). The nonyl groups are oriented in an outward direction with respect to the interior of the backbone. The conformations of the two nonyl chains are different from each other, and one chain displays more twisting and/or kinking than the other. Furthermore, because the face-to-face distance of the two benzimidazolyl ring ranges from 6.8 to 8.1 Å, 1 can function as a molecular host to accommodate small guest molecules/planar aromatics in the empty space. The guest toluene interacts with the cavity walls by three strong CH(sp3) 3 3 3 π and edge-to-face CH(sp2) 3 3 3 π interactions, thus stabilizing the hostguest moiety. Variable-Temperature 1H NMR Studies of 1 and 3. Cyclic compounds containing benzbix and related ligands in solution
C71H70N4O10Re2 1511.71
)
Figure 6. The p-phenylene and dhnq units in the two neighboring metallacycles 1 are shown as a ball-and-stick model. C gray, H white, N blue, O red, Re green.
formula Mr
Figure 7. Partial variable-temperature 1H NMR spectra of 1 in acetone-d6.
have been previously observed to exhibit dynamic behavior due to the interconversion between the syn and anti conformations of the two heterocyclic units.17,22 Our interest in 1, 2, and 3 was to explore the nature of the rotational behavior of the p-phenylene unit, i.e., changes in the 1H NMR pattern either due to the rotation of the p-phenylene unit or a conformation equilibrium involving syn/anti forms of the benzimidazolyl units in the metallacycle, because it is always important to exclude other possible mechanisms for dynamic NMR phenomena. Metallacycle 1 was subjected to a VT 1H NMR study from 300 to 182 K (Figure 7). The proton signal corresponding to the p-phenylene rotating unit was very broad at a temperature below 208 K, whereas the benzimidazolyl and dhnq protons showed no broadening or any additional peak except for a chemical shift until an accessible temperature (182 K) was reached. This indicates that the p-phenylene in 1 rotates in solution, even at low temperature. Three possibilities exist for the shift in proton signals for the benzimidazolyl and dhnq units: (1) slight changes in the orientation of the benzimidazolyl rings, (2) rapid 3172
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Figure 8. Schematic representation of various possible conformers of 4 (IIV) and 1 (VVII) due to different orientations of the two benzimidazolyl units and the nonyl benzimidazolyl units (i for in, o for out).
equilibrium established between various conformers due to the syn/anti orientation of the nonylbenzimidazolyl unit, and (3) rapid dissociationrecoordination of Nbenzbix ligand in the metallacycle on the NMR time scale. The latter possibility can be ruled out because the 1H NMR spectrum of a solution of 1 and free Nbenzbix showed two sets of signals that correspond to those of 1 and free Nbenzbix, indicating the stability of metallacycles in solution; that is, a fast dissociationrecoordination of Nbenzbix does not occur in 1, since this would lead to the observation of a single pattern (Figure S3). The possibility of syn/anti conformation dynamics was verified by the closely similar spectra of complexes 47 reported by Sathiyendiran and Lu et al. along with 1, 2, and 3. The metallacycles 47 may exist as four major conformers in solution (IIV) owing to the syn/anti form of benzbix due to different orientations of the benzimidazolyl units (Figures 8, S7). The tail-to-tail syn conformer IV is less likely in solution, since this conformer would involve unfavorable steric hindrance between the fused arene of the benzimidazolyl and dhnq units. Molecular modeling of 1 and 2 in which the p-phenylene unit was maintained parallel to the dhnq2 clearly shows the intramolecular cavity, in which the fused arene of the benzimidazolyl group cannot pass in by pivoting the ring about its NN axis (Figures 9, S8, S9). The
Figure 9. DFT energy calculated structure of 1 by fixing the pphenylene unit parallel to the dhnq plane: C gray 40%, H gray 25%, N blue, Re green. Rotator bonds are indicated by black lines.
remaining three isomers (IIII) may be present in solution. However, the available intramolecular cavity while p-phenylene is parallel to the dhnq unit is sufficient to pass through the edge of the imidazolyl unit, which may facilitate a rapid interconversion among isomers IIII. In contrast, metallacycles 13 are able to exist in two conformers (V or VII) in solution. The remaining conformer, VI, is not possible due to the steric bulkiness of the lengthy alkyl chain (∼10.2 Å), as no significant upfield shift was observed for nonyl alkyl chains. Most importantly, conformers V and VII cannot interconvert, due to the lengthy nonyl group and 3173
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Organometallics as well as steric hindrance imposed by the fused arene group of the benzimidazolyl units, respectively. The available intramolecular cavity when p-phenylene is parallal to the dhnq unit is not sufficient to permit passage through the nonyl chain. On the basis of these arguments, together with the simple single 1H NMR patterns obtained at these temperatures, we conclude that the proton NMR shift of the benzimidazolyl unit is likely due to slight changes in the orientation of the benzimidazolyl rings (Va(ii) h Vc(oo)) in 1. The isomer Vb(io) can be ruled out because two types of patterns for the benzimidazolyl units would be expected. The shift in dhnq protons may be due to the slight bending of the benzimidazolyl units at low temperatures, causing the C1H2 unit to move away from dhnq and the C2H2 unit to approach the dhnq unit. This would result in a similar chemical environment for Ha and Ha0 and different chemical environments for Hb and Hb0 of the dhnq unit, respectively. The bending movement of the benzimidazolyl units is further supported by the upfield shift of H6 of the benzimidazolyl unit, which may experience a small ring current effect from the p-phenylene unit. These data indicate that the metallacycle 1 is present as a single conformer with a p-phenylene rotating unit.17b Hence, metallacycles 13 can be regarded as surface-mounted altitudinal rotors, in which the dianionic dhnq/dhaq/CA unit is related to the surface, the nonyl benzimidazolyl units are related to the stators, the p-xylene is related to the axle, and the p-phenylene is related to the rotating unit. Molecular Modeling. A DFT experiment was performed for both 1 and 2 to determine the intermolecular distance between the p-phenylene and dhnq/dhaq units, the ground-state conformation of the metallacycles, and the rotation energy barrier for the p-phenylene unit in the metallacycle. The global minima of the energy structure showed that Nbenzbix adopts a syn conformation and the p-phenylene moitey lies perpendicular to the dhnq/dhaq plane in 1 and 2, respectively (Figures 9, S7, S8). This is in good agreement with the crystal structure of 1 (Table S2). The major difference between the energy-minimized structure and the single-crystal X-ray structure is the distance between the p-phenylene carbons and the mean dhnq plane. The former shows a distance of 4.4 Å and the latter 3.7 Å. The calculated relative energy difference between the metallacycle in which p-phenylene is perpendicular to the dhnq plane and the metallacycle in which p-phenylene is parallel to the dhnq plane is very low. These results indicate that the p-phenylene moiety is able to rotate in the metallacyclic framework (Table S1).
’ CONCLUSION A semirigid ligand, Nbenzbix, possessing two bulky stators and a rotating unit was designed and synthesized. Three metallacycles were self-assembled by reacting Nbenzbix, a bis-chelating unit, and Re2(CO)10 using an orthogonal bonding approach. Various conformers of the semirigid ligand due to the syn/anti forms of Nbenzbix were avoided by introducing a long alkyl chain at the 2-position of the benzimidazolyl unit, thus leading to a single species with a p-phenylene rotating unit. The dynamic properties of 13 were studied using variable-temperature 1H NMR methods. Comparison of these results with recently reported findings and the experimental data obtained for compounds reported in this paper clearly confirms that the rotation of the p-phenylene unit (but not the nonylbenzimidazolyl units) is facilitated in the metallacycle. This work would be very helpful
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for future rational design of novel molecular rotors with restricted conformational dynamics.
’ EXPERIMENTAL SECTION General Data. All starting materials and products were found to be stable toward moisture and air, and no specific precautions were taken to rigorously exclude air when solvothermal methods were used. Starting materials such as Re2(CO)10, H2-dhnq, H2-dhaq, H2-CA, 2-nonylbenzimidazole, 1,4-bis(bromomethyl)benzene, and KOH were procured from commercial sources and used as received. Aromatic solvents were purified by conventional procedures and were distilled prior to use. Conventional synthetic methods were routinely carried out in room atmosphere. Elemental analyses were performed on a Elementar Analysensysteme GmbH Vario EL-III instrument. FT-IR spectra were recorded on a Perkin-Elmer FT-IR-2000 spectrometer. 1H NMR spectra were recorded on Bruker AMX-400 FT-NMR and Jeol JNMECX-400P spectrometers. Synthesis of 1,4-Bis(2-nonylbenzimidazol-1-ylmethyl)benzene (Nbenzbix). A mixture of powdered KOH (1.50 g, 26 mmol) and 2-nonylbenzimidazole (4.89 g, 20 mmol) in THF (100 mL) was placed in a round-bottom flask, and the suspension was stirred for 2 h. A solution of p-xylylenedibromide (2.64 g, 10 mmol) in THF (50 mL) was slowly added to the yellowish solution, and the reaction mixture was then stirred continuously overnight. The solvent was removed under reduced pressure, the residue was poured into 100 mL of water, and the resulting solution was extracted three times with CH2Cl2 (3 100 mL). The combined organic extracts were washed with water, dried (Na2CO3), and concentrated. The crude products were obtained as yellow-colored powders. Yield: 88%. (5.20 g, 8.8 mmol). Anal. Calcd for C40H54N4: C, 81.31; H, 9.21; N, 9.48. Found: C, 79.99; H, 9.02; N, 8.92. ESI-MS (TOF) (m/z): 592.060 [Mþ]. 1H NMR (400 MHz, acetoned6): δ 7.56 (dd, 2H, J = 2.5 Hz, H4), 7.32 (dd, 2H, J = 3.2 Hz, H7), 7.11 (m, 4H, H5,6), 7.06 (s, 4H, H9-arene), 5.44 (s, 4H, H8-methylene), 2.81 (m, 4H, -C1H2, nonyl), 1.76 (m, 4H, -C2H2, nonyl), 1.23 (m, 24H, (C3H2C8H2), nonyl), 0.84 (t, 6H, C9H3, nonyl). Synthesis of [(CO)3Re(μ-dhnq)(μ-Nbenzbix)Re(CO)3] (1). A mixture of Re2(CO)10 (195.2 mg, 0.299 mmol), Nbenzbix (182.6 mg, 0.309 mmol), and H2-dhnq (90.7 mg, 0.312 mmol) in toluene (10 mL) in a Teflon flask was placed in a steel bomb. The bomb was placed in an oven maintained at 160 °C for 48 h and then cooled to 25 °C. Goodquality, dark rose-colored single crystals of 1 were obtained. The crystals were separated by filtration and washed with toluene. Yield: 83% (353 mg). Anal. Calcd for C64H62N4O10Re2 3 (C7H8)2: C, 56.41; H, 4.67; N, 3.71. Found: C, 56.78; H, 4.76; N, 3.62. IR (CH3COCH3, cm1): 2008 (CtO), 1903 (CtO), 1875 (CtO). 1H NMR (400 MHz, acetoned6): δ 8.55 (q, 2H, Ha, dhnq), 8.42 (q, 2H, Ha0 , dhnq), 8.32 (d, 2H, J = 8.24 Hz, H4, Nbenzbix), 7.85 (q, 2H, Hb, dhnq), 7.73 (q, 2H, Hb0 , dhnq), 7.16 (t, 2H, H5, Nbenzbix), 7.05 (d, 2H, J = 8.04 Hz, H7, Nbenzbix), 6.81 (t, 2H, H6, Nbenzbix), 5.98 (s, 4H, H9-arene, Nbenzbix), 5.50 (dd, J = 16.96 Hz, 4H, H8-methylene, Nbenzbix), 4.12 (t, 4H, C1H2, nonyl), 2.67 (m, 4H, C2H2, nonyl), 1.29 (m, 24H, (C3H2C8H2), nonyl), 0.86 (t, 6H, C9H3, nonyl). Synthesis of [(CO)3Re(μ-dhaq)(μ-Nbenzbix)Re(CO)3] (2). A mixture of Re2(CO)10 (197.3 mg, 0.302 mmol), Nbenzbix (182.5 mg, 0.309 mmol), and H2-dhaq (86.7 mg, 0.377 mmol) in mesitylene (10 mL) in a Teflon flask was placed in a steel bomb. The bomb was placed in an oven maintained at 160 °C for 48 h and then cooled to 25 °C. Good-quality, dark green-colored powders of 2 were obtained. The powders were separated by filtration and washed with distilled mesitylene. Yield: 83% (344.5 mg). Anal. Calcd for C60H60N4O10Re2 3 C9H12: C, 54.19; H, 4.65; N, 3.92. Found: C, 53.98; H, 4.62; N, 3.93. IR (CH3COCH3, cm1): 2009 (CtO), 1899 (CtO), 1881 (CtO). 1H NMR (400 MHz, acetone-d6): δ 8.39 (q, 2H, Ha, dhaq), 8.24 (d, 2H, 3174
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Organometallics J = 8.02 Hz, H4, Nbenzbix), 7.74 (q, 2H, Hb, dhaq), 7.15 (s, 2H, Hc, dhaq), 7.12 (t, 2H, H5, Nbenzbix), 7.08 (d, 2H, H7, Nbenzbix), 6.79 (t, 2H, H6, Nbenzbix), 6.12 (s, 4H, H9-arene, Nbenzbix), 5.58 (s, 4H, H8methylene, Nbenzbix), 3.99 (t, 4H, C1H2, nonyl), 2.02 (m, 4H, C2H2, nonyl), 1.28 (m, 24H, (C3H2C8H2), nonyl), 0.86 (t, 6H, C9H3, nonyl). Synthesis of [(CO)3Re(μ-CA)(μ-Nbenzbix)Re(CO)3] (3). A mixture of Re2(CO)10 (195.4 mg, 0.299 mmol), Nbenzbix (184.3 mg, 0.312 mmol), and H2-CA (66.0 mg, 0.316 mmol) in toluene (10 mL) in a Teflon flask was placed in a steel bomb. The bomb was placed in an oven maintained at 160 °C for 48 h and then cooled to 25 °C. Goodquality, light purple-colored powder of 3 was obtained. The powder was separated by filtration and washed with distilled toluene. Yield: 68% (272.8 mg, 0.2038 mmol). Anal. Calcd for C52H54Cl2N4O10Re2 3 C7H8: C, 49.54; H, 4.37; N, 3.92. Found: C, 49.72; H, 4.67; N, 4.61. IR (CH3COCH3, cm1): 2012 (CtO), 1899 (CtO), 1902 (CtO). 1H NMR (400 MHz, DMSO-d6): 7.79 (d, 2H, J = 8.08 Hz, H4, Nbenzbix), 7.72 (d, 2H, J = 8.08 Hz, H7, Nbenzbix), 7.48 (m, 4H, H5,6, Nbenzbix), 7.24 (s, 4H, H9-arene), 5.71 (s, 4H, H8-methylene, Nbenzbix), 3.12 (t, 4H, C1H2, nonyl), 1.67 (m, 4H, C2H2, nonyl), 1.18 (m, 24H, (C3H2C8H2), nonyl), 0.83 (t, 6H, C9H3, nonyl). X-ray Crystallography. A single crystal of metallacycle 1 was mounted on a Cryoloop for indexing and intensity data collection at 183(2) K on an Oxford Xcalibur Ruby CCD single-crystal diffractometer (Mo KR radiation, λ = 0.71073 Å).23 Routine Lorentz and polarization corrections were applied, and an absorption correction was performed using the ABSCALE 3 program. Direct methods were used to locate the heavy metal atoms (SHELXS-97). The remaining atoms were located from successive Fourier maps (SHELXL-97).23 All heavy atoms were refined anisotropically. All hydrogen atoms were calculated after each cycle of refinement using a riding model, with CH = 0.93 Å þ Uiso(H) = 1.2Ueq(C) for aromatic H atoms, with CH = 0.97 Å þ Uiso(H) = 1.2Ueq(C) for methylene H atoms. The crystallographic data of 1 are summerized in Table 1. Computational Section. The singlet ground-state geometry optimizations of Nbenzbix, 1, and 2 were performed in the gas phase using the B3LYP24 functional and the basis set SDD25 for Re and 6-311G*26 for other atoms, available in the Gaussian 03 program package. The initial geometry of 1 was obtained from the crystal structure coordinates. For 2, the initial geometry was obtained from the optimized structure of metallacycle 1.
’ ASSOCIATED CONTENT
bS
Supporting Information. Spectral data of Nbenzbix, 1, 2, and 3 and optimized structures for 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected] (M.S.);
[email protected] (F.H.);
[email protected]. edu.tw (K.L.L.).
’ ACKNOWLEDGMENT We thank the Department of Science and Technology (SR/ S1/IC-22/2008), New Delhi, for financial support. We also thank USIC, University of Delhi, and SAIF, CDRI, Lucknow, for providing the NMR facility.
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