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Organometallics 2010, 29, 4210–4212 DOI: 10.1021/om100630g
Extraction of Hydrogen from Alcohols by a Methylene-Bridged Iridium(I) Dinuclear Complex Having a Short Ir-Ir Double Bond Hidetaka Nakai,* Saori Nakano, Shunsuke Imai, and Kiyoshi Isobe* Department of Chemistry, Graduate School of Natural Science & Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan Received June 30, 2010 Summary: Reduction of [Cp*2Ir2(μ-CH2)2Cl2] (Cp* = η5C5Me5) with Na in benzene leads to the formation of a methylene-bridged iridium(I) dinuclear complex with a considerably short Ir-Ir double bond (2.4375(2) A˚), [Cp*2Ir2(μ-CH2)2], which reacts with alcohols (RCH2OH, R = H, Me, Et, Ph) to afford a unique cis-dihydride complex and the respective aldehyde (RCHO). Transition metal complexes with a metal-metal multiple bond are of fundamental importance and have always fascinated chemists.1 Among them, the carbonyl-bridged dinuclear complexes of [(η5-C5R5)2M2(μ-CO)2] (M = Ir, Rh, Co; R = H, Me) (Figure 1a), which possess the metalmetal double bond, have received a great deal of attention in the past four decades because of their unique electronic structures and reactivities.2 The corresponding μ-CH2 complexes of [(η5-C5R5)2M2(μ-CH2)2], where the μ-CH2 is isolobal with the μ-CO, have not previously been found. The replacement of the μ-CO with the μ-CH2, being a less π-acidic ligand, may enhance electron density in the metalmetal double bond and its reactivity. In our efforts to synthesize the methylene-bridged analogues, we now have successfully isolated a desired iridium(I) dinuclear complex, [Cp*2Ir2(μ-CH2)2] (1) (Cp* = η5-C5Me5, Figure 1b), having a considerably short Ir-Ir double bond. Herein we report the synthesis and structure of 1 and its unique reactivity toward alcohols. Treatment of the dichloro diiridium(II) complex [Cp*2Ir2(μ-CH2)2Cl2]3 with sodium metal in benzene at room temperature (rt) for 5 h under a N2 atmosphere yielded the double-bonded complex 1 as a deep red powder (>90%). The 1H NMR spectrum of 1 in benzene-d6 exhibited resonances *To whom correspondence should be addressed. E-mail: nakai@ cacheibm.s.kanazawa-u.ac.jp;
[email protected]. (1) Cotton, F. A.; Murillo, C. A.; Walton, R. A. Multiple Bonds between Metal Atoms, 3rd ed.; Springer: New York, 2005. (2) (a) Winter, M. J. Adv. Organomet. Chem. 1989, 29, 101. (b) Ball, R. G.; Graham, W. A. G.; Heinekey, D. M.; Hoyano, J. K.; McMaster, A. D.; Mattson, B. M.; Michel, S. T. Inorg. Chem. 1990, 29, 2023. (c) Heinekey, D. M.; Fine, D. A.; Barnhart, D. Organometallics 1997, 16, 2530. (d) Wang, H.; Xie, Y.; King, R. B.; Schaefer, H. F., III. J. Am. Chem. Soc. 2005, 127, 11646. (e) Phillips, A. D.; Ienco, A.; Reinhold, J.; B€ottcher, H.-C.; Mealli, C. Chem.;Eur. J. 2006, 12, 4691. (3) Nishioka, T.; Kitayama, H.; Breedlove, B. K.; Shiomi, K.; Kinoshita, I.; Isobe, K. Inorg. Chem. 2004, 43, 5688. (4) There is an electron-counting ambiguity for the bridging methylene ligand (μ-CH2) depending on whether it is considered as a neutral (two-electron donor) or dianionic (four-electron donor) ligand. We will take the methylene as a neutral ligand throughout the paper, on the basis of our NMR results of 1 and its isolobality with the carbonyl ligand. Thus, for instance, the formal oxidation state of iridium atoms in 1 is “þ1”: (a) Mealli, C.; Proserpio, D. M. J. Organomet. Chem. 1990, 386, 203. (b) Poduska, A.; Hoffmann, R.; Ienco, A.; Mealli, C. Chem. Asian J. 2009, 4, 302. pubs.acs.org/Organometallics
Published on Web 09/08/2010
Figure 1. (a) Carbonyl-bridged dinuclear complexes. (b) Synthesis of the methylene-bridged iridium(I) dinuclear complex 1.
at 1.72 (s, C5(CH3)5, 30H) and 9.01 (s, CH2, 4H) ppm. In the C spectrum, the resonance of the bridging methylene was observed at 88.1 ppm. These NMR results indicate that the methylene ligands in 1 have a neutral carbenoid character (:CH2).4 Crystals of 1 suitable for X-ray diffraction analysis were grown from a saturated THF solution at -30 °C (Figure 2).5 Complex 1 has a crystallographic center of inversion. Therefore, the Cp* rings of 1 are in a mutually staggered conformation and the four-membered ring Ir1-C11-Ir1*C11* is planar. The Ir1-C11-Ir1* and C11-Ir1-C11* angles are 73.76(16)° and 106.2(2)°, respectively. Intriguingly, the Ir-Ir bond length (2.4375(2) A˚) in 1 is much shorter than those previously reported for the formal Ir-Ir double bonds found in the carbonyl-bridged analogues [Cp*2Ir2(μ-CO)2] (2.554(1) A˚),2b [Ir2(μ-PPh2)2(CO)2(PPh3)2] (2.551(1) A˚),6a and [Cp*2Ir2(μ-H)2(μ2-η1,η1-N2C3H3)]þ (2.663(1) A˚)6b and is similar to those reported for the formal Ir-Ir triple bonds found in [Cp*2Ir2(μ-H)2] (2.4157(6) A˚)7a 13
(5) Crystal data for 1: C22H34Ir2, M = 682.94, red plate, T = 123 ( 1 K, triclinic, space group P1, a = 6.7259(3) A˚, b = 8.2968(3) A˚, c = 10.4452(5) A˚, R = 68.933(5)°, β = 79.531(6)°, γ = 67.621(5)°, V = 502.28(4) A˚3, Z = 1, 5548 reflections measured, 2229 unique (Rint = 0.026), R1 = 0.0239 [I > 2σ(I)], wR2 = 0.0535 (all data), GOF = 1.100. CCDC 782294. Crystal data for 2: C22H36Ir2, M = 684.96, yellow block, T = 123 ( 1 K, monoclinic, space group P21/n, a = 11.1666(13) A˚, b = 13.0931(15) A˚, c = 14.6548(19) A˚, β = 101.245(3)°, V = 2101.5(4) A˚3, Z = 4, 22 691 reflections measured, 4799 unique (Rint = 0.052), R1 = 0.0564 [I > 2σ(I)], wR2 = 0.1595 (all data), GOF = 1.075. CCDC 782295. (6) (a) Bellon, P. L.; Benedicenti, C.; Caglio, G.; Manassero, M. J. Chem. Soc., Chem. Commun. 1973, 946. (b) Oro, L. A.; Carmona, D.; Puebla, M. P.; Lamata, M. P.; Foces-Foces, C.; Cano, F. H. Inorg. Chim. Acta 1986, 112, L11. (7) (a) Hou, Z.; Fijita, A.; Koizumi, T.-a.; Yamazaki, H.; Wakatsuki, Y. Organometallics 1999, 18, 1979. (b) Bau, R.; Teller, R. G.; Kirtley, S. W.; Koetzle, T. F. Acc. Chem. Res. 1979, 12, 176. (c) Ogo, S.; Nakai, H.; Watanabe, Y. J. Am. Chem. Soc. 2002, 124, 597. r 2010 American Chemical Society
Communication
Figure 2. Molecular structure of 1 with 50% probability ellipsoids. Front view (left) and side view (right). Hydrogen atoms are omitted for clarity. Selected bond distances (A˚) and angles (deg): Ir1-Ir1* = 2.4375(2), Ir1-C11 = 2.031(6), Ir1-C11* = 2.030(3); Ir1-C11Ir1* = 73.76(16), C11-Ir1-C11* = 106.2(2).
Figure 3. Reaction of 1 with various alcohols.
and [Cp*2Ir2(μ-H)3]þ (2.458(6)7b and 2.4677(4)7c A˚). This unusually short, electron-rich Ir-Ir double bond can extract hydrogen from alcohols as described below. The reaction of 1 with RCH2OH (R = H, Me, Et, Ph) as a solvent at rt led to the formation of a dihydride iridium(II) complex, [Ir2Cp*2(μ-CH2)2H2] (2), that could be isolated as a yellow powder in excellent yield (>85%, Figure 3). Complex 2 was characterized by 1H NMR, 13C NMR, and FT-IR spectroscopy as well as X-ray diffraction analysis. The coordination of the hydride ligands to the iridium centers in 2 was confirmed by 1H NMR and FT-IR spectroscopy. In the 1H NMR spectrum (benzene-d6), the characteristic resonances of the hydride ligands are observed at -18.9 ppm. The two 1H signals (6.54 and 7.94 ppm) from the μ-CH2 indicate a cis-structure. IR absorption bands at 2168 and 2154 cm-1, which are assigned to the νIr-H band, are also evidence of the Ir-hydride coordination. Since terminal hydride ligands generally give rise to the IR bands of ν(M-H) in the range from 2200 to 1600 cm-1 and bridging hydride ligands from 1600 to 800 cm-1,8 the ν(Ir-H) of 2 supports a terminal hydride description. Crystals of 2 suitable for X-ray diffraction analysis were grown from a saturated MeOH solution at rt. The molecular structure of 2 is depicted in Figure 4.5 The four-membered ring Ir1C21-Ir2-C22 in 2 is puckered (the angle between the Ir2C planes is 25.958°). The Ir-C distances in 2 are 2.054(9), 2.057(10), 2.046(11), and 2.075(9) A˚. The Ir1-C21-Ir2, Ir1-C22-Ir2, C21-Ir1-C22, and C21-Ir2-C22 angles (8) (a) Heinekey, D. M.; Fine, D. A.; Harper, T. G. P.; Michel, S. T. Can. J. Chem. 1995, 73, 1116. (b) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987; p 83.
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Figure 4. Molecular structure of 2 with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. The hydride ligands were not located. Selected bond distances (A˚) and angles (deg): Ir1-Ir2 = 2.6447(6), Ir1-C21 = 2.054(9), Ir1-C22 = 2.057(10), Ir2-C21 = 2.046(11), Ir2-C22 = 2.075(9); Ir1-C21Ir2 = 80.3(3), Ir1-C22-Ir2 = 79.6(3), C21-Ir1-C22 = 96.8(4), C21-Ir2-C22 = 96.5(4).
are 80.3(3)°, 79.6(3)°, 96.8(4)°, and 96.5(4)°, respectively. Unfortunately, we could not locate the H atoms of the hydrides by X-ray diffraction, but the arrangement of the two Cp* groups also supports that 2 has a cis-structure. The Ir-Ir bond length (2.6447(6) A˚) in 2 is consistent with those previously reported for the methylene-bridged Ir-Ir single bonds found in [Cp*2Ir2(μ-CH)2(μ-S2)] (2.642(1) A˚)3 and [Cp*2Ir2(μ-CH)2(μ-SSO2)] (2.6373(6) A˚)3 and is much shorter than that reported for the unbridged Ir-Ir single bond found in dihydride complex [Cp*2Ir2(CO)2H2] (2.730(1) A˚).2c The reaction with alcohols affords specifically the cis-isomer and not any trans-isomer. Although n-propanol (EtCH2OH) reacts with 1 to give 2, 2-propanol ((CH3)2CHOH), which is known as a typical hydride source for transition metal complexes, is unreactive. This difference in their reactivity probably occurs from the ease of the approach of the two H atoms, one of which is the H of the OH group and the other is the H of the C nearest to the OH, toward the Ir-Ir double bond. Hence, the steric bulkiness of the substrates is a key feature for the reaction of the alcohols with 1. The destiny of the alcohols in this reaction was clarified by GC. Ethanol (MeCH2OH) and benzyl alcohol (PhCH2OH) were selected as model alcohols because of easy detection of the corresponding organic products. Only acetaldehyde (CH3CHO) or benzaldehyde (PhCHO) was found in both the headspace of the reaction vessel and/or the solvent phase. No other organic product was detected. Thus, it is confirmed that 2 reacts with the alcohols to afford a unique cisdihydride complex and the respective aldehyde. In order to confirm the origin of the two hydride ligands in 2, we carried out labeling experiments by using CD3OH, CH3OD, and CD3OD (Figure 3). The 1H NMR and FT-IR spectroscopic data of each resulting complex of 2 show that the deuterium atoms are incorporated into 2 as the deuterio ligand. Meanwhile, the H/D exchange reaction of nondeuterated 2 in CD3OD for >24 h does not occur. These labeling experiments suggest that double-bonded complex 1 extracts hydrogen from both C-H and O-H bonds of CH3OH. Furthermore, the spectroscopic measurements suggest that the μ-CH2 in 2 was also partly deuterated through the reaction, and hence the C-H and O-H activations of the alcohols cooperatively take place on both the iridium metal centers and the μ-CH2 units.9 The cis-structure of 2 is probably due to a kinetic control that is initially derived from a double (or multiple) interaction between the Ir-Ir double bond and the alcohol substrate at one site of the Ir2C2 plane of 1 (Figure 3, inset).10 A similar interaction is also proposed in the heterogeneous catalytic aldehyde formation
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from alcohols for the initial step of the alcohol activation.11 The elucidation of the detailed reaction mechanism is in progress. In conclusion, an iridium(I) dinuclear complex having an electron-rich and highly reactive Ir-Ir double bond was successfully synthesized and structurally characterized. The novel double-bonded complex extracted hydrogen from alcohols and was transformed to a cis-dihydride complex with the generation of an aldehyde. Selective oxidation from alcohol to aldehyde using transition metal catalysts, espe(9) The variable-temperature 1H NMR experiments of the dihydride complex 2 show no significant temperature dependence of the signals of the μ-CH2 and hydride ligands up to 75 °C, which means no H-atom exchange between them: once 2 was formed in the reaction system, it exists as a stable species and does not perform the H-atom exchange between these ligands, for example, through formation of CH3 species. (10) It has been computationally demonstrated for the carbonylbridged complexes that the isomers with bridging or terminal CO ligands are very close in energy.2e The conjunction of one bridged and one terminal CH2 ligand may cause some electronic asymmetry between the two metal atoms at a certain activation stage. (11) A similar multiple interaction is proposed for selective oxidation of alcohols: Mori, K.; Hara, T.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem. Soc. 2004, 126, 10657.
Nakai et al.
cially a primary alcohol to an aldehyde, is of industrial importance.12 We believe that our findings offer attractive new insight into the construction of transition metal catalysts for selective oxidation of alcohols and hydrogen gas production from alcohols.
Acknowledgment. This work was financially supported by Grant-in-Aid “20550060” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We thank Prof. Z. Hou (RIKEN) for his kindness in obtaining elemental analysis data of air- and moisturesensitive complexes 1 and 2. Supporting Information Available: Text and table containing experimental and characterization details of 1 and 2 as well as a CIF file giving crystallographic details of 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org. (12) (a) Weissermel, K.; Arpe, H.-J. Industrial Organic Chemistry, 4th ed.; Wiley-VHC: Weinheim, 2003; p 38. (b) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Science 2006, 311, 362. (c) Johnson, T. C.; Morris, D. J.; Wills, M. Chem. Soc. Rev. 2010, 39, 81.
