ARTICLE pubs.acs.org/Organometallics
Characterization of the E Isomer of Tetrasubstituted [5]Cumulene and Trapping of the Z Isomer as a Zirconocene Complex Noriyuki Suzuki,*,†,‡ Nozomu Ohara,† Kosuke Nishimura,† Yoshio Sakaguchi,‡ Shinkoh Nanbu,† Sohei Fukui,† Hirotaka Nagao,† and Yoshiro Masuyama† †
Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan ‡ Chemcal Analysis Team, Advanced Technology Support Division, RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan
bS Supporting Information ABSTRACT: The E isomer of a [5]cumulene derivative, 2,2, 9,9-tetramethyl-3,8-diphenyldeca-3,4,5,6,7-pentaene (1), which was previously believed to be unisolable owing to very fast E/Z isomerization, was isolated and structurally characterized. The Z isomer was trapped as the transition-metal complex 5, and the molecular structure was determined. DFT calculations and an electrochemical study on 1 are also described.
’ INTRODUCTION Compounds with cumulative double bonds, such as [n]cumulenes and their derivatives, have received much attention because of the attractive properties caused by their small HOMOLUMO gap.1 It is known that higher [n]cumulenes (n = odd numbers) have a low isomerization barrier between their E and Z forms.2 Some examples of synthetic routes to symmetrical [5]cumulenes have been reported.3 However, the isolation and characterization of either the E or Z isomer of tetrasubstituted [5]cumulenes have not been achieved due to fast E/Z isomerization.2d Kuhn and co-workers have obtained crystals of 2,2,9, 9-tetramethyl-3,8-diphenyldeca-3,4,5,6,7-pentaene (1), although the 1H NMR spectrum of the solution of this solid showed a pair of signals ascribed to a mixture of the E and Z isomers (eq 1).2a It is worth reexamining their results, to clarify if it was one of the isomers or a mixture of the E and Z isomers. Moreover, it remains a challenge to catch the other isomer of this compound.
We recently reported the synthesis and characterization of [5]cumulene complexes of zirconium that have a five-membered alkyne structure and revealed their reactivity.4 These results prompted us to study the reactivity of [5]cumulene (1) toward zirconium. Metal complexation is one of the promising methods for trapping unstable organic species.5 Herein, we report the structural characterization of the E isomer of tetrasubstituted [5]cumulene 1 and the trapping of the Z isomer as a zirconium complex. r 2011 American Chemical Society
’ RESULTS AND DISCUSSION The [5]cumulene derivative 1 was prepared from the 2, 4-diyne-1,6-diol 2 by a modified literature method.2,3 Recrystallization from diethyl ether solution afforded pale yellow crystals of 1 in 38% yield (Scheme 1). Its melting point was 120 °C.2a,c It is of interest that an X-ray diffraction study revealed that these crystals were (E)-1 (Figure 1). To the best of our knowledge, this is the first example of the structural characterization of the E isomer of substituted [5]cumulenes. Table 1 summarizes the selected bond lengths and angles of (E)-1 in comparison with the results calculated by a DFT study. The molecular structure of (E)-1 was fundamentally similar to those of known [5]cumulene compounds.6 The lengths of the cumulative double bonds were in the range 1.27 1.33 Å. It is noteworthy that the terminal double bond (C1C2) was slightly longer than the others. The two phenyl groups are twisted away from the C1C4C8 plane by 54°, probably due to the steric demand of the tert-butyl groups. (E)-1 was stable in the crystalline state at room temperature for a few months under an inert atmosphere, although it gradually degraded in air. The solid was dissolved in cold chloroform-d, and the solution was observed by 1H NMR at low temperature. Only one singlet for tert-butyl groups was observed at 1.30 ppm at 20 °C, indicating that the crystals were those of the single isomer. At higher temperature, a new signal assignable to the tert-butyl moieties of the Z isomer appeared at 1.34 ppm.2c It isomerized at room temperature within a few minutes to reach an equilibrium (51/49 of (E)-/(Z)-1). We observed two sets of signals corresponding to (E)and (Z)-1, which coalesced at 125 °C in 1,2-dichlorobenzene-d4 Received: March 21, 2011 Published: June 06, 2011 3544
dx.doi.org/10.1021/om2002427 | Organometallics 2011, 30, 3544–3548
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Table 2. Cyclic Voltammetry of [5]Cumulene Derivativesa
Scheme 1. Preparation of [5]Cumulene 1
entry
redn
oxidn
(E)-1
1.93
þ0.78, þ1.00
3
n.d. ( 2.00σ(I))
0.0589
0.0536
R1 (all rflns)
0.0848
0.0685
residuals
wR2 (all rflns) goodness of
0.1822
0.1376
1.169
1.042
fit indicator Flack param max shift/
0.01(7) 0.001
0.001
0.22
1.38
0.26
1.21
error in final cycle max peak in final diff map, e/Å3 min peak in final diff map, e/Å3
11
techniques. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were refined using the riding model. The molecule has a symmetry center at the middle of linear six carbon atoms. The final cycle of full-matrix least-squares refinement on F2 was based on 1646 observed reflections and 123 variable parameters. All calculations were performed using the CrystalStructure12 crystallographic software package,
except for refinement, which was performed using SHELXL-97.13 Crystallographic data are summarized in Table 3. The CIF data have been deposited with the Cambridge Structural Database (CCDC810469). Theoretical Methods: DFT Study on 1. The molecular structures of the isomers of 1 were fully optimized by density functional theory (DFT) at the Becke and Lee, Yang, and Parr (B3LYP) level of theory.14 Dunning’s augmented cc-pVTZ (correlation consistent, polarized valence, triple-ζ) basis set was used.15 All these ab initio calculations were performed with use of the electronic structure program Gaussian 03.1. Using 16 CPUs of Fujitsu Primergy RX200S3 at the Computer Centre of Kyushu University, it took 2 weeks to get each result for the isomers. The MOL files of the compounds are available in the Supporting Information. X-ray Diffraction Study of 5. Single crystals were obtained by recrystallization from a hexane solution. A yellow crystal (0.13 0.05 0.01 mm) was mounted in a loop and coated with paraffin. All measurements were made on a Rigaku Saturn CCD area detector with graphite-monochromated Mo KR radiation (λ = 0.710 70 Å) at 90 K. The structure was solved by direct methods16 and expanded using Fourier techniques.11 The non-hydrogen atoms were refined anisotropically. Hydrogen atoms on the cyclopentadienyl rings were refined isotropically, and the rest were located at calculated positions and refined using the riding model. The final cycle of full-matrix least-squares refinement on F2 was based on 3046 observed reflections and 170 variable parameters. All calculations were performed using the CrystalStructure12 crystallographic software package, except for refinement, which was performed using SHELXL-97.13 Crystallographic data are summarized in Table 3. The CIF data have been deposited with the Cambridge Structural Database (CCDC-810468).
’ ASSOCIATED CONTENT
bS
Supporting Information. Text, figures, tables, and CIF and MOL files giving details of the preparation, spectroscopic data, and CIF files for 1 and 5, spectroscopic data for 6, an NMR study of 1, electrochemical studies of 1, 3, and 4, details of the DFT calculations, and structures of (E)- and (Z)-1. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel/fax: þ81-3-3238-3452. E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank Ms. K. Yamada (RIKEN) for assistance in elemental analysis. Dr. D. Hashizume (RIKEN) is acknowledged for assistance in X-ray diffraction studies. We are grateful to Nichia Corp. for providing zirconocene dichloride. This work was financially supported by a Grant-in-Aid for Scientific Research (B, 22350022) and by the Asahi Glass Foundation. ’ REFERENCES (1) (a) Ogasawara, M. In Cumulenes and Allenes; Krause, N., Ed.; Thieme: Stuttgart, Germany, 2007; Science of Synthesis Vol. 44, pp 970. (b) Hino, S.; Okada, Y.; Iwasaki, K.; Kijima, M.; Shirakawa, H. Chem. Phys. Lett. 2003, 372, 59–65. (2) (a) Kuhn, R.; Schulz, B.; Jochims, J. C. Angew. Chem., Int. Ed. 1966, 5, 420–420. (b) Dewar, M. J. S.; Haselbach, E. J. Am. Chem. Soc. 1970, 92, 590–598. (c) Bertsch, K.; Karich, G.; Jochims, J. C. Chem. Ber. 1977, 110, 3304–3313. (d) Skibar, W.; Kopacka, H.; Wurst, K.; 3547
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