Communication pubs.acs.org/Organometallics
Photochemical Reaction of Diruthenium Tetrahydride-Bridged Complexes with Carbon Dioxide: Insertion of CO2 into a Ru−H Bond versus CO Double-Bond Cleavage Ryuichi Shimogawa, Toshiro Takao, Gen-ichi Konishi, and Hiroharu Suzuki* Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo, 152-8552 Japan S Supporting Information *
ABSTRACT: Photochemical reactions of diruthenium tetrahydride complexes containing cyclopentadienyls as auxiliary ligands with carbon dioxide were studied for the effective fixation and reduction of CO2. Whereas the reactions of CpsRu(μ-H)4RuCps (Cps = Cp*, C5Me5; CpEt, C5Me4Et; Cp‡, 1,2,4-C5(tBu)3H2) with CO2 did not proceed under dark and mild conditions, the photochemical reactions under UV (365 nm) irradiation smoothly proceeded to afford two types of products, (i) a μformato complex and (ii) a μ-carbonyl-μ-oxo complex, according to the width of the space for the reaction stretching between the two auxiliary cyclopentadienyl ligands.
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formations. Consequently, activation of CO2 via coordination to a transition metal has become a subject of great interest.7 Coordination patterns are primarily controlled by the electronic situation at the metal center, and thus, electron-rich transition metal centers can perform a nucleophilic attack at the carbon atom of CO2.8 In contrast, electron-poor metal centers act as electrophiles and react with CO2 via one of the oxygen atoms.9 We report herein that the reactions of CpsRu(μ-H)4RuCps with CO2 smoothly proceed with preservation of dinuclear structure under UV irradiation to produce a μ-formato complex or a μ-carbonyl-μ-oxo complex selectively, according to the bulkiness of the auxiliary ligands. UV light (365 nm) irradiation of Cp*Ru(μ-H)4RuCp* (1a) (48.0 mg, 101.5 μmol) in THF under atmospheric CO2 at 0 °C for 24 h changed the solution color from orange to brown. Removal of the solvent under reduced pressure, followed by extraction from the brown residual solid with n-pentane at −78 °C, afforded a novel bridging-formato complex, Cp*Ru(μOCHO)(μ-H)3RuCp* (2a) (eq 1). Triruthenium pentahy-
he photochemical behavior of mononuclear polyhydrido transition metal complexes has been intensively studied since the early 1970s.1 Reductive elimination of H2 is the most salient process observed in the initial stages of photolysis of mononuclear polyhydride species.2 The coordinatively unsaturated, highly reactive species generated by dehydrogenation often promote bond activation of relatively inert hydrocarbons.3 In contrast, polynuclear polyhydride-bridged complexes tend to be highly resistant toward reductive elimination of H2, because each hydride ligand is bound to multiple metal centers.4 Initial reactions observed for the photolysis of mononuclear polyhydride complexes, i.e., the formation of vacant sites via liberation of H2, would, therefore, be impeded upon photoexcitation of a bridging polyhydride cluster. At the same time, preservation of the bridging hydrides would prevent fragmentation of the cluster framework. Recently, we reported photoinduced C−H bond cleavage of acetone, resulting in the formation of a dinuclear bridging oxatrimethylenemethane complex of ruthenium.5 This reaction is considered to be very typical of polyhydride-bridged cluster complexes, because the reaction mechanism does not involve either initial H2 elimination or fragmentation of the cluster framework. This result strongly stimulated us to examine the reaction of the polyhydride-bridged clusters with carbon dioxide under photoirradiation to activate less-reactive CO2 in a bimetallic way. Carbon dioxide has long been considered to be an abundantly available feedstock for C1 chemistry as well as carbon monoxide, methanol, and methane.6 However, the chemical inertness of CO2, originating from its highly negative reduction potential, has limited its use in organic trans© 2014 American Chemical Society
Received: June 10, 2014 Published: August 28, 2014 5066
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with the relevant bond lengths and angles. Figure 1 unambiguously shows a dinuclear ruthenium structure bridged by a formato group. The molecule has two crystallographic mirror planes, and the two Cp* groups occupy “cis” sites with respect to the Ru1−Ru2 vector. The analogous diruthenium tetrahydrido complex 1b with C5Me4Et groups as auxiliary ligands underwent a similar photochemical insertion of CO2 into a Ru−H bond. This insertion was much more facile in comparison to 1a because of the increased solubility in hydrocarbons. In contrast, a diruthenium tetrahydrido complex with a sterically demanding 1,2,4-tri-tert-butylcyclopentadienyl group, Cp‡Ru(μ-H)4RuCp‡ (1c), showed a dramatic change in its reactivity toward CO2 under photochemical conditions, although the UV−vis spectrum of 1c was very similar to that of 1a (see Supporting Information). UV (365 nm) irradiation of 1c under 1 atm of CO2 in benzene at ambient temperature led to the formation of a μcarbonyl-μ-oxo complex, Cp‡Ru(μ-CO)(μ-O)RuCp‡ (5), via cleavage of a CO double bond (eq 2). After 50 h, the
dride complex (Cp*Ru)3(μ-H)3(μ3-H)2 (3a), which was inevitably formed via the reaction of formed 2a with 1a, was removed by O2 oxidation before the extraction. As mono-μformato complex 2a readily releases CO2 to result in the formation of 1a under an Ar atmosphere or reduced pressure, the yield of 2a was determined based on the 1H NMR spectroscopy recorded under an atmosphere of CO2. The photoreaction of 1a with 1 atm of CO2 was monitored in THF-d8 at 0 °C using an NMR tube equipped with a J. Young valve. The selectivity of 2a was 59% at the moment of 96% conversion of 1a, and 3a was formed in 23% yield, as the byproduct. Under dark conditions, the reaction of 1a with CO2 did not proceed at ambient temperature. In the 1H NMR spectrum of 2a recorded in THF-d8 at −80 °C, a singlet peak (30H) for the Cp* groups appears at δ 1.83. Signals for the hydride ligands appear at δ −15.07 (d, J = 6.0 Hz, 2H) and δ 0.86 (t, J = 6.0 Hz, 1H), respectively. The latter was observed significantly low field and was assigned to the hydride ligand lying on the same plane as the μ-formato group. The hydride ligand, therefore, lies on the nodal plane for the πbonding systems of the μ-formato ligand, and its 1H NMR signal is, therefore, shifted significantly downfield. The validity of the notably lower shift of the hydride ligand was demonstrated by gauge-independent atomic orbital (GIAO) calculations (see Supporting Information).10 A signal for the proton attached to the μ-formato carbon appears at δ 6.80, which is comparable to the chemical shifts observed in the analogous μ-formato complexes Cp*Ru(μ-OCHO) 2 (μH)2RuCp* (δ 6.75)11 and Cn*Ru(μ-OCHO)2(μ-H)RuCp* (δ 7.85).12 The proton-coupled 13C NMR spectrum of 2a recorded at 25 °C exhibits a set of doublet signals characteristic of the μ-formato group at δ 171.1. The shift is also similar to those of the above-mentioned μ-formato complexes.11,12 Although the dinuclear μ-formato complex 2a could not be obtained in pure form from column chromatography because of its lability, a few orange single crystals suitable for X-ray diffraction precipitated from a cold (−30 °C) pentane solution, fortunately. The molecular structure of 2a is shown in Figure 1
conversion of 1c reached 92% and the selectivity to 5 was ca. 90%. Complex 5 is stable both in solution and in the solid state, and analytically pure 5 was obtained as a red-purple crystalline solid by column chromatography on acidic Al2O3 with npentane and toluene. The 1H NMR spectrum of 5, recorded in C6D6 at 25 °C, revealed three kinds of singlet peaks for protons directly bonded to the C5 rings and a set of tert-butyl groups on the C5 rings at δ 4.91 (s, 4H), 1.41 (s, 18H), and 1.02 (s, 36H), respectively. The 13C NMR signal for the bridging carbonyl group appeared at δ 255.5. A red single crystal suitable for X-ray diffraction study was obtained from a solution of 5 in cold (−30 °C) n-pentane. The molecular structure of 5 is shown in Figure 2, along with relevant bond lengths and angles. Figure 2 shows the dinuclear structure of 5 containing a bridging oxygen atom and a bridging carbonyl group, which are derived from CO2 by way of reductive cleavage of a CO double bond. The interatomic distance between C1 and O2, 2.969(4) Å, is long enough to be judged that there is no bonding interaction between these atoms. The short Ru1−Ru2 distance, 2.5355(3) Å, clearly reflects the highly unsaturated character of 5. To the best of our knowledge, isolation of dinuclear μ-oxo-μcarbonyl complex derived from the reaction of a transition metal complex and CO2 is rare, although there have been many examples of transition metal-mediated reductive cleavage of the CO double bond of CO2.13 Herein, we came to a conclusion that dinuclear tetrahydrido complexes containing Cp*, CpEt, or Cp‡ auxiliary ligands react smoothly with CO2 under photoirradiation at ambient temperatures, although the thermal reaction of these complexes with CO2 do not proceed irrespective of substituents on the C5 ring of the auxiliary ligands. We have also demonstrated that two types of reactions proceed with CO2 under photoirradiation: (i) insertion of CO2 into a Ru−H bond, affording
Figure 1. Molecular structure of 2a with thermal ellipsoids at the 30% probability level. Selected bond lengths [Å] and angles [deg]: Ru(1)− Ru(#1) 2.6699(9), Ru(1)−O(1) 2.130(5), O(1)−C(1) 1.246(6), C(1)−H(1) 0.91(9), Ru(1)−H(2) 1.65(6), Ru(1)−H(#2) 1.83(6), Ru(1)−H(3) 1.74(7), O(1)−Ru(1)−Ru(#1) 84.33(10), C(1)− O(1)−Ru(1) 121.1(5), O(1#)−C(1)−O(1) 129.0(9). 5067
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excited to generate a transient species A; the excited species A would then interact with CO2 to form an intermediary complex B. There are two possible reaction paths for B, namely, (1) nucleophilic attack of the hydride at the central carbon of CO2 (CO insertion) and (2) nucleophilic attack of the ruthenium center (CO cleavage). The mode of the subsequent reactions, CO insertion into the Ru−H bond versus CO double bond cleavage, would be controlled by sterical situations rather than electronic ones. In the reactions of 1a and 1b, the hydride attacks at the central carbon atom of carbon dioxide (intermediate C) to yield the μ-formato complex 2, finally, as a result of CO insertion into the H−Ru bond. In contrast, a μformato complex corresponding to 2c could not be formed in the reaction of 1c with CO2 probably due to severe steric repulsion among the Cp‡ groups laid cis position with respect to the Ru−Ru vector in 2c. As mentioned above, the μ-formato complex 2 readily liberates CO2 in solution. Therefore, paths from B to 2 via C should be reversible. In the reaction of 1c that has sterically demanding Cp‡ groups, another reaction route to 5 by way of D would be forced.
Figure 2. Molecular structure of 5 with thermal ellipsoids at the 30% probability level. Selected bond lengths [Å] and angles [deg]: Ru(1)− Ru(2) 2.5355(3), Ru(1)−C(1) 2.011(3), Ru(2)−C(1) 1.999(3), Ru(1)−O(2) 1.903(2), Ru(2)−O(2) 1.901(2), C(1)−O(1) 1.188(3), C(1)−O(2) 2.969(4), Ru(1)−CEN(1) 1.825, Ru(2)− CEN(2) 1.822, Ru(1)−Ru(2)−O(2) 48.22(7), Ru(1)−Ru(2)−C(1) 50.99(8), O(2)−Ru(2)−C(1) 99.13(10), O(2)−Ru(1)−Ru(2) 48.17(6), C(1)−Ru(1)−Ru(2) 50.57(8), C(2)−Ru(1)−Ru(2) 145.44(7), O(2)−Ru(1)−C(1) 98.65(10), O(2)−Ru(1)−C(2) 133.25(10), C(1)−Ru(1)−C(2) 110.68(11), O(1)−C(1)−Ru(2) 140.8(2), O(1)−C(1)−Ru(1) 140.7(2), Ru(1)−C(1)−Ru(2) 78.44(10), Ru(1)−O(2)−Ru(2) 83.61(9).
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ASSOCIATED CONTENT
* Supporting Information S
Experimental details, UV−vis spectra of 1a, 1b, and 1c, spectral data for 2a, 2b, and 5, summary of crystal data and results of XRD studies, CIF files giving X-ray crystallographic data for 2a and 5, and DFT calculations for the structure and GIAO calculation of 2a. This material is available free of charge via the Internet at http://pubs.acs.org.
a μ-formato complex, and (ii) reductive cleavage of a CO double bond to yield a μ-oxo-μ-carbonyl complex. The electronic properties of the three cyclopentadienyls, Cp*, CpEt, and Cp‡, can be assessed on the basis of the wavenumbers correspond to the stretching vibration of carbonyl group of LRu(CO)2Br (L = substituted cyclopentadienyl) as Tolman showed an important relationship between wavenumber νCO (A1 mode) of (PR1R2R3)Ni(CO)3 and the substituent R.14 Consequently, the electron-donating ability of these ligands was in the order CpEt > Cp* > Cp‡.15 In contrast, the bulkiness of the ligands decreases in the order Cp‡ > CpEt > Cp*. In the reaction of 1c with CO2, dimerization yielding a tetranuclear cluster is completely restrained, while the reaction of 1a and 1b with CO2 inevitably produces small amounts of the tetranuclear clusters as byproducts. Plausible reaction pathways are shown in Scheme 1. Upon irradiation (365 nm), the diruthenium tetrahydride complex is
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for JSPS Fellows grant number 26009727. R.S. thanks the JSPS Research Fellowship for Young Scientists for financial support.
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REFERENCES
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Scheme 1. Plausible Mechanism for Reaction of 1 with Carbon Dioxide
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