Spectroscopic Evaluation of the Molecular Structures of di-μ-Chlorobis(1,5-Cyclooctadiene) Iridium (I) and Rhodium (I) Complexes ()
1. Introduction
Di-μ-chlorobis(1,5-cyclooctadiene) of iridium (I) and rhodium (I) ions are popular complexes which are employed in several homogenous catalytic reactions. They are mostly used as precursor catalysts in the development of catalytic systems for applications in different reactions well-reported in the literature [1]-[3]. The importance of these two complexes has attracted the attention to explore their structural and vibrational properties over the past years. A crystallographic study showed that both complexes retain D2h symmetry in the solid phase [4]-[6]. An infrared study on the assignments of ν(C=C), ν(M-Cl) and ν(M-olefin) vibrations [7] as well as the strength of metal-olefin bond interaction was also reported [8]. These studies showed that the molecular symmetry plays a role in understanding the electronic, structural properties as well as the catalytic activity. In the same sense, the symmetry of the diolefinic ligands used in complex build-up is essential in determining the symmetry of resulting complex. 1,5-cyclooctadiene (COD) was reported to have C2v symmetry in tub form but Ci in chair configuration [7] [8]. It commonly exits as C2v tub configuration in both liquid and solid states [7] for complexation, which results in D2h symmetry (Figure 1(a)) explained in literature for di-μ-chlorobis(1,5-cyclooctadiene) of both iridium (I), [Ir(µ-Cl)COD]2, and rhodium (I), [Rh(µ-Cl)COD]2 [7] [8]. Further analysis of the infrared and H-NMR spectra of the two complexes has been carried out, and the Raman spectra of the solid phase have been collected and interpreted. In this paper we highlight some of our findings which will be detailed in future in another article.
2. Experimental
The complex synthesis was carried out under an inert atmosphere with use of Schlenk-tube technique. All the chemicals used for this research work were purchased from Sigma Aldrich Chemicals and used without further purification while the solvents used in the synthesis were first purified using standard procedures and distilled under argon before use.
2.1. Synthesis of [Ir(µ-Cl)COD]2
6 mL of distilled water, 2.2 mL of isopropanol and 1.2 mL of 1,5-cyclooctadiene were measured into 50 mL two neck round bottomed flask. The solvent mixture was first purged with argon after which a condenser was connected to the flask. Then, the mixture was refluxed for 1hour prior to the addition of 1 g of IrCl3 and further refluxed for 8 hours after the addition under strong stirring. The suspended liquid was removed via cannula and the cold methanol was added to red oil left to effect formation of red crystalline solid. The solid was later filtered, washed with cold methanol and dried under vacuum.
2.2. Synthesis of [Rh(µ-Cl)COD]2
1 g of RhCl3 was weighed in 50 mL two neck round bottomed flask fitted with condenser, then vacuumed and filled with argon. Then, 10 mL of ethanol, 2 mL of distilled water and 1.4 mL of 1,5-cyclooctadiene were added into the flask. Thereafter, the reaction mixture was refluxed for 8 hours under strong stirring. The resulting yellow precipitate was filtered, washed with distilled water and ethanol and dried under high vacuum.
2.3. Characterization
The NMR analysis of the deuterated chloroform solution of the two synthesized complexes was carried. The infrared spectrum of 1,5-cyclooctadiene was recorded through smart orbit technique while two complexes were sampled in KBr pellets. Raman spectra for the cyclooctadiene and the two complexes were recorded for sample in a 1 mm glass tube using a laser source of 1064 nm with 0.3 W power.
2.4. Computation
Ab initio density functional theory (DFT) calculations at B3LYP level were performed on the molecules using Gaussian 09 program [9]. Optimization was carried out using basis set of 6 - 311 ++ G(d, p) for the nonmetals and SDD for the transition metals. The same basis sets were used for vibrational frequency calculations with the population analysis being undertaken through full natural bond orbital analysis calculation while the NMR was computed using Gauge-Independent Atomic Orbital (GIAO) method. The Polarizable Continuum Model (PCM) using the integral equation formalism variant (IEF) was implemented to predict the solvent effect on the theoretical spectral data for both complexes.
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(a) (b)
Figure 1. The two symmetries of [M(µ-Cl)COD]2; M = Ir or Rh (blue); Cl (green), C (gray). (a) D2h; (b) D2.
3. Results and Discussion
3.1. Geometry
Two symmetries D2 and D2h were fully optimized and their relative energies are shown in Table 1. The D2 configuration is predicted to be more stable in the gas phase compared to D2h by 5.88 and 5.04 kcal/mol, respectively for [Rh(µ-Cl)(COD)]2 and [Ir(µ-Cl)(COD)]2. The D2 symmetry is obtained from the Ci COD, while the D2h complex is associated with C2 COD [7] [8].
The 1H NMR calculations were carried out for the two configurations of both complexes in gas phase and in CHCl3 solvent using the PCM model and compared to the experimental ones. Three sets of 8H peaks for D2h symmetry but six sets of 4H peaks for D2 symmetry were predicted (Table 2). These sets have different chemical environments. The D2h 1H NMR result is in agreement with the theoretical 1H NMR spectra for both complexes confirming the higher level symmetry even in solution phase. It is also interesting to notice that the average shielding values for the D2 configuration matches with shielding value of each set in that of D2h as depicted in Table 2.
3.2. Vibration
Selected modes of vibrations associated with C=C and C-H bonds of COD were followed theoretically and experimentally for the free configuration and coordinated ones in the complex as indicated in Table 3. There is a quite agreement between the two methods for the % shift in frequency depicted in Table 4. All the corresponding frequencies of mode of vibrations in the free diolefin shift to lower values in the complex with the exception of out-of-plane wagging in both complexes. The sum of shift in C=C stretch (in-phase) and C-H in-plane wag (in- plane) frequencies between the free olefin and coordinated one reflects the total (σ + π) metal-olefin interaction [8]. There is agreement between the sums obtained from theoretical calculation and experiment with [Ir(µ-Cl)(COD)]2 (13%) having slightly higher value than [Rh(µ-Cl)(COD)]2 (12%) while its % shift in out of plane wag almost twice that of Rh complex. These results indicate a stronger metal-olefin interaction for Ir complex. This conclusion is ascertained by results obtained from natural bond orbital (NBO) calculation with hybridization values of carbons in C=C and C-H of the Ir complex being higher than those in the Rh complex as seen in Table 5. Iridium
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Table 1. Optimization energy for different configurations of the complexes.
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Table 2. Shielding values for 1H NMR peaks in ppm.
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Table 3. Vibrational assignment for some selected modes of vibration around C=C in cm−1.
Mode of Vibrations: a: asymmetric stretch; b: symmetric stretch; c: in-plane wag; d: in-plane wag; e: out-of-plane wag.
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Table 4. Percent (%) shift in frequency between free 1,5-cyclooctadiene and complex.
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Table 5. Natural bond orbital analysis for 1,5-cyclooctadiene and complex.
is more electropositive than Rh and as such it populates carbon with more electron than Rh, thus decreasing the bond order which in return increases the bond length. This consequently resulted in shift to lower wavenumbers for such vibrational modes for coordinated olefinic C=C with Ir than Rh.
4. Conclusion
Both vibrational and H-NMR spectroscopic investigation reveals complex of di-μ-chlorobis(1,5-cyclooctadiene) of iridium (I) and rhodium (I) to be of the same geometry. The single-molecule vapor-phase density functional theory (DFT) calculation for the fully optimized structures of both complexes shows that D2 structure is more stable than D2h by 5 - 6 kcal/mol. In contrast, the spectroscopic analysis study affirms that the two complexes retain their higher D2h symmetry in the solid phase. Theoretical calculations as well as experiment show greater total (σ + π) metal-olefin interaction for iridium complex.
Acknowledgements
The authors express their sincere appreciation of the financial support provided by King Abdulaziz City of Science and Technology (KACST) under the funded project (T-K-11-630).
NOTES
*Corresponding author.