Chemistry and catalysis advances in organometallic chemistry and catalysis
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35.3.1 Concave- η
-Bound Cyclopentadienyl Iron Cation Complex of Sumanene Coordination chemistry of sumanene (1) has been limited to the computational study. For example, the coordination of M(PH 3
2 (M = Pd or Pt) was investigated to compare sumanene (1) with fullerene and corannulene (2) [46]. In this context, we started from the coordination of CpFe + by taking stability into consideration. The complexation of 1 with CpFe + was performed by ligand exchange with a Cp group of ferrocene [47]. The reaction proceeded in the presence of aluminum powder and aluminum chloride without a solvent under argon at 120 ◦ C for 19 h. The counteranion of the crude complex was replaced by hexafluorophosphate, giving [CpFe(sumanene)]PF 6 (15) as an orange solid. The use of excess ferrocene and aluminum chloride selectively afforded the desired monometallated complex in 91% yield (Scheme 35.2). However, the reaction in decahydronaphthalene as a solvent did not yield 15 [47]. The complex 15 was fully characterized by fast atom bombardment (FAB) mass spectrometry, 1 H and 13 C NMR spectroscopy, and X-ray crystallography. In the NMR study, the symmetry of a series of protons showed the coordination of CpFe + at a flank benzene ring rather than the central (hub) benzene ring [47]. The benzylic protons appeared as two pairs of doublets in a 2 : 1 ratio (J = 19.1 Hz, 4H, H d
= 20.1 Hz, 2H, H e ). The high field endo-benzylic protons (4.96 and 3.92 ppm, H d,endo and H
e,endo ) and low field exo-benzylic protons (5.21 and 4.94 ppm, H d,exo
and H e,exo
) support the bowl-shaped structure, as observed for 1. The signal of the Cp protons H f at 3.26 ppm was shifted approximately by δ = 1.3 ppm to a higher field than the corresponding protons of [CpFe( η 6
6 and [CpFe( η 6
6 . This strong ring-current effect also supports the concave binding. Furthermore, the concave binding structure of 15 was confirmed by nuclear Overhauser effect (NOE) experiments (Fig. 35.9) [47]. Irradiation of the Cp protons resulted in NOE signals at the endo-benzylic protons (2.6% for each H d,endo and 3.0% for H e,endo ), whereas no NOE signals were observed for the corresponding exo-benzylic protons. η 6 -COORDINATION COMPLEXES OF SUMANENE 479 C6 C7 C19 C20
C26 C21
Fe 0.99 Å
1.07 -1.13 Å (a)
(b) Figure 35.10 ORTEP drawings of 15 with thermal ellipsoids set at 50% probability: (a) top and (b) side views. PF 6 anion is omitted for clarity. X-ray crystallographic analysis clearly showed the concave binding of the CpFe + moiety to a flank benzene ring of 15 (Fig. 35.10) [47]. The Fe–C bonds from the iron center to the six-membered ring (C6, C7, C19, and C20) range from 2.01 to 2.07 ˚
A. Although the distances to the hub carbon atoms (C21 and C26) are a little longer (2.136 and 2.135 ˚ A, respectively), the coordination can be considered almost as η 6 . Such a selective concave η 6 coordination was thus achieved for the first time in the curved π-conjugated carbon system. The Cp ring is tilted 6 ◦ to the iron-bound benzene ring, perhaps because of steric repulsion with the sumanene skeleton. A small amount of bowl flattening was observed around the coordinated side (0.99 ˚
A); in contrast, the bowl depth at the other side (1.07–1.13 ˚ A) was almost the same as found for 1 (1.11 ˚ A). Columnar stacking was observed in the crystal packing (Fig. 35.11). Partial π –π orbital overlap is probably present between the Cp and hub benzene rings of adjacent molecules. The redox properties of 15 were investigated through electrochemical study. The complex 15 exhibited the Fe(II)/Fe(I) reduction, which displayed features of partial chemical reversibility [48]. Furthermore, the methyl-substituted complex [MeCpFe( η 6 -sumanene)]PF 6 was synthesized. 1 H NMR experiments suggested that the methyl group is directed out of the bowl with a restricted rotation [49]. Coordination to such curved π orbitals in a π bowl may give rise to unique reactivities in the catalysis of the transition- metal complexes. 35.3.2 Concave- η
-Bound Chiral s-Butylcyclopentadienyl Iron Cation Complex of Sumanene Chiral
π-bowl complexes are desirable model structures for chiral endohedral metallofullerenes, asymmetric organometallic catalysts, as well as molecular recognition units. Synthesis of an Fe(II) + complex of 1 having a Cp ligand with a chiral s-butyl group attached was performed in a manner similar to the preparation of 15. The complex 16 also exhibited a concave- face-selective coordination at a flank benzene ring in solution (Fig. 35.12), in which the rotation of the Cp ring is restricted. Magnetic and optical desymmetrization in the sumanene ligand could be shown with the complex 16 [49]. Chirality on sumanene bowl is induced through the asymmetric center on s-butyl group in Cp ligand. This is the first optically active complex with a π-bowl ligand. Recently, an enantiometrically pure corannulene–Rh(II) complex with chiral norbornadiene ligand was reported [50].
η
-Bound Cyclopentadienyl Ruthenium Cation Complex of Sumanene The selectivity for endo complexation might reflect kinetic and/or thermodynamic control in the complexation, thermodynamic control in “bowl-to-bowl inversion,” or both. Corannulene and sumanene ligands themselves are known to exhibit the bowl-to-bowl inversion as described above [20, 31]. Concerning the dynamic behavior of π-bowl transition- metal complexes, the ring-to-ring migration of such as η 6
2 + , Cp*Ru + , (COE)
2 Ir + , and (COE) 2 Rh + is found
on the corannulene ligands [40, 41, 44, 50]. However, the bowl-to-bowl inversion has not been elucidated with π-bowl
transition-metal complexes. The CpRu
+ sumanene complex 17 (Fig. 35.13) was synthesized similarly to 15 and 16 [51]. Unlike them, a pair of two species was observed in the 1 H NMR spectra of [CpRu(sumanene)]PF 6 (17). The major and minor species were assigned to 480 ORGANOMETALLIC COMPLEXES OF SUMANENE (a) (b)
Figure 35.11 Crystal cell and packing structure of 15: (a) top and (b) side views. H atom is omitted for clarity. (See insert for color representation of the figure.) Fe PF 6 16 H
Chiral π-bowl complex 16. the concave-bound complex 17 concave and the convex-bound isomer 17 convex , respectively, by NOE experiments. The ratio of the concave and convex complexes depends to some degree on the solvent. In CD 2 Cl 2 , the occupancy in the equilibrium is 75 : 25 for 17
/17 convex . In more polar solvents, the equilibrium tends to the major isomer 17 concave (up to 90% in CD 3
3 NO 2 ). Two-dimensional EXSY NMR experiment of 17 in CD 2 Cl 2 clearly showed negative cross peaks for the chemical exchange between H a–f
and H a –f
in one-to-one correspondence, as exemplified by a–a (Fig. 35.13) [51]. The endo-benzylic protons H d,endo and H
e,endo of 17 concave show correlations with the exo-benzylic peaks H d ,exo and H
e ,exo of 17 convex (cross peaks: d endo –d
and e endo
– e exo
, respectively), which are in a common region for exo-benzylic protons. Conversely, the exo-benzylic protons H d,exo and H
e,exo of 17 concave show correlations with the endo-benzylic peaks H d ,endo
η 6 -COORDINATION COMPLEXES OF SUMANENE 481 Ru Ru 17 concave 17 convex H H H H H H H H a b c d endo
d exo
e endo
e exo
f a ′ b ′ c ′ d ′ endo d ′ exo e ′ endo e ′ exo f ′ 17.3 kcal/mol 16.7 kcal/mol f e
– e ′ exo d endo
d exo
e exo
f ′ e endo e ′ exo d ′ endo d ′ exo e ′ endo e exo – e ′ endo d exo
– d ′ endo f – f' f – f
′ d exo – d ′ endo e exo
– e ′ endo e endo
– e ′ exo d endo
– d ′ exo d endo
– d ′ exo * a a ′ b a – a ′ a – a
′ b – b
′ c – c
′ c b ′ c ′ b – b ′ c – c ′ PF 6 PF 6 CD 2 Cl 2 303 K Figure 35.13 (a) Bowl-to-bowl inversion of [CpRu(sumanene)]PF 6 (17). (b) 2D EXSY NMR spectra of 17 in CD 2 Cl 2 . (b)
(a) Ru
ORTEP drawings of 17
with thermal ellipsoids set at 40% probability: (a) top and (b) side views. PF 6 anion is
omitted for clarity. and H
e ,endo of 17 convex (cross peaks: d exo –d
and e exo
–e endo
, respectively), which are in a typical region for endo-benzylic protons. The bowl-to-bowl inversion can account for the phenomenon without inconsistency. The quantification of the cross peaks gives an inversion barrier of 16.7 and 17.3 kcal/mol at 303 K for 17
to 17 concave and the reverse inversion, respectively (Fig. 35.13). Linear van’t Hoff plots give a positive
From a mixture of the diastereomers, a single crystal of 17 concave , which is the major species, was obtained. In the X-ray crystallographic analysis, ORTEP diagrams clearly showed the concave coordination (Fig. 35.14) [51]. Once the crystal of
was dissolved, both isomers of 17 appeared again. These results indicate that the preference of the concave or convex isomers depends on the solvent and temperature under thermodynamic control.
482 ORGANOMETALLIC COMPLEXES OF SUMANENE 35.4 CONCLUDING REMARKS AND FUTURE PROSPECTS The coordination chemistry of sumanene (1) reported to date was reviewed here. Stepwise selective benzylic lithiation of 1 was presented. The benzylic anion species exhibits the bowl-to-bowl inversion. Recent study on 1 revealed that the single crystal of 1 shows high electron transport ability with anisotropy [52]. In the prospective view, trapping of such anion is considered to enable various substitutions at the benzylic positions stereoselectively, which is one of the promising approaches to create the functional materials based on 1. Complexation with CpFe + demonstrated selective formation of the first concave-bound complex, which is expected to lead to the inclusion complexes of π bowls. The inversion behavior observed in the CpRu + complex may provide the idea of a dynamic catalytic system. Thus, some characteristic features of sumanene complexes are becoming apparent. In the future, π bowls such as 1 are expected to provide novel electrical materials, organometallic catalysts, etc.
The authors wish to thank all their talented coworkers for their great contributions reported here. Financial support by Grants-in-Aid for Scientific Research (A), Young Scientists (A), and Challenging Exploratory Research from Japan Society for the Promotion of Science is gratefully appreciated. Financial support from JST, ACT-C is also acknowledged. REFERENCES 1. Fagan, P. J.; Calabrese, J. C.; Malone, B. Science 1991, 252 , 1160. 2. Balch, A. L.; Olmstead, M. M. Chem. Rev. 1998, 98 , 2123. 3. Nakamura, E. J. Organomet. Chem. 2004, 689 , 4630. 4. Sawamura, M.; Kuninobu, Y.; Toganoh, M.; Matsuo, Y.; Yamanaka, M.; Nakamura, E. J. Am. Chem. Soc. 2002, 124 , 9354. 5. Chai, Y.; Guo, T.; Jin, C.; Haufler, R. E.; Chibante, L. P. F.; Fure, J.; Wang, L.; Alford, J. M.; Smalley, R. E. J. Phys. Chem. 1991, 95 , 7564. 6. Shinohara, H. Rep. Prog. Phys. 2000, 63 , 843. 7. Scott, L. T.; Bronstein, H. E.; Preda, D. V.; Ansems, R. B. M.; Bratcher, M. S.; Hagen, S. Pure Appl. Chem. 1999, 71 , 209. 8. Rabideau, P. W.; Abdourazak, A. H.; Folsom, H. E.; Marcinow, Z.; Sygula, A.; Sygula, R. J. Am. Chem. Soc. 1994, 116 , 7891. 9. Rabideau, P. W.; Sygula, A. Acc. Chem. Res. 1996, 29 , 235. 10. Wu, Y.-T.; Siegel, J. S. Chem. Rev. 2006, 106 , 4843. 11. Tsefrikas, V. M.; Scott, L. T. Chem. Rev. 2006, 106 , 4868. 12. Scott, L. T.; Petrukhina M. Fragments of Fullerenes and Carbon Nanotubes: Designed Synthesis, Unusual Reactions, and Coordination Chemistry; John Wiley & Sons, Inc.: Hoboken, NJ, 2011. 13. Higashibayashi, S.; Sakurai, H. Chem. Lett. 2011, 40 , 122. 14. Amaya, T.; Hirao, T. Chem. Commun. 2011, 47 , 10524. 15. Faust, R.; Vollhardt, K. P. C. J. Chem. Soc., Chem. Commun. 1993, 1471. 16. Barth, W. E.; Lawton, R. G. J. Am. Chem. Soc. 1966, 88 , 380. 17. Scott, L. T.; Hashemi, M. M.; Meyer, D. T.; Warren, H. B. J. Am. Chem. Soc. 1991, 113 , 7082. 18. Seiders, T. J.; Elliott, E. L.; Grube, G. H.; Siegel, J. S. J. Am. Chem. Soc. 1999, 121 , 7804. 19. Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc. 2000, 122 , 6323. 20. Sakurai, H.; Daiko, T.; Hirao, T. Science 2003, 301 , 1878. 21. Mehta, G.; Shah, S. R.; Ravikumar, K. J. Chem. Soc., Chem. Commun. 1993, 1006. 22. Priyakumar, U. D.; Sastry, G. N. J. Phys. Chem. A 2001, 105 , 4488. 23. Sakurai, H.; Daiko, T.; Sakane, H.; Amaya, T.; Hirao, T. J. Am. Chem. Soc. 2005, 127 , 11580. 24. Hanson, J. C.; Nordman, C. E. Acta. Crystallogr. B 1976, 32 , 1147. 25. Ayalon, A.; Rabinovitz, M.; Cheng, P.-C.; Scott, L. T. Angew. Chem. Int. Ed. Engl. 1992, 31 , 1636. 26. Ayalon, A.; Sygula, A.; Cheng, P.-C.; Rabinovitz, M.; Rabideau, P. W.; Scott, L. T. Science 1994, 265 , 1065. 27. Zabula, A. V.; Filatov, A. S.; Spisak, S. N.; Rogachev, A. Y.; Petrukhina, M. A. Science 2011, 333 , 1008. 28. Amaya, T.; Mori, K.; Wu, H.-L.; Ishida, S.; Nakamura, J.; Murata, K.; Hirao T. Chem. Commun. 2007, 1902.
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The design and synthesis of luminescent tri- and tetracoordinate organoboron compounds have received considerable attention in the past two decades [1]. The four-coordinate organoboron compounds are in general more stable compared to three- coordinate ones, which require bulky substituents to stabilize them. Consequently, the four-coordinate boron compounds have emerged as very attractive materials for various optoelectronic applications, sensory and biological imaging, and as photoresponsive materials [2]. Several luminescent tetracoordinate organoboron compounds of the type (L ∩ L )BR
2 (R = alkyl, aryl; L ∩ L = chelating ligands) containing a variety of conjugated anionic chelating ligands have been synthesized and reported [2a, 3]. The Lewis-acidic boron atom stabilizes negative anionic chelating ligands by forming covalent bonds and dissipating the negative charges of the ligand, thus reinforcing π-conjugation in the chelate. The lowest unoccupied molecular orbital (LUMO) of boron compounds is usually localized on the π-conjugated chelating ligand, while the highest occupied molecular orbital (HOMO) is localized either at the chelating ligand or the R group, depending on the nature of the latter. The π→π* electronic transitions of the chelate or charge-transfer transition from the R group to the chelate are usually responsible for their luminescent properties. Hence, variations on the chelating ligand or R group of the molecule would influence the HOMO–LUMO levels and thereby the color of emission. The boron dipyrromethene (BODIPY) family of tetracoordinate organic boron– fluoride complexes [4] and the four-coordinate boron-functionalized polymers [5] have been reviewed recently and will not be covered here. Thus, this chapter focuses on the recent advances in the synthesis of new tetracoordinate organoboron compounds and their applications. Download 11.05 Mb. Do'stlaringiz bilan baham: 
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