Reductive coupling of six carbon monoxides by a ditantalum hydride complex

Full text of DOI:10.1021/ja9007276

Published on Web 02/25/2009
Reductive Coupling of Six Carbon Monoxides by a Ditantalum Hydride
Complex
Takahito Watanabe, Yutaka Ishida, Tsukasa Matsuo, and Hiroyuki Kawaguchi*
Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8551, Japan
Received January 29, 2009; E-mail: hkawa@chem.titech.ac.jp
Sequential coupling of carbon monoxide, which leads to forma-
tion of oxocarbons (CO)n, has represented a basic challenge to the
development of CO chemistry. A number of metal complexes are
known to promote incorporation of CO as carbon units into organic
substrates,1 while the chaining of CO beyond two is rare due to
the great tendency of neutral (CO)n toward dissociation into CO.2-4
To overcome this synthetic difficulty, one promising approach
is reductive coupling of CO to produce anionic oxocarbons, which
are stable relative to the corresponding neutral form.5 High reduction
potentials of low-valent f-elements can be applicable to reductive
CO coupling. For instance, lanthanocenes of La(II) and Sm(II) have
been reported to reduce CO at 90 psi, yielding a dianionic ketene
carboxylate unit.6 Recently, U(III) complexes have been demon-
strated to undergo reductive cyclo-trimerization and -tetramerization
of CO under mild conditions.7 Here we describe reduction of CO
with a ditantalum hydride complex, resulting in head-to-head C-C
coupling of six CO molecules.
renders the two halves of the molecule equivalent. The C6O6 unit
is bound to Ta(1) and Ta(1′) in a κ2O,O mode, while it is bound to
Ta(2) and Ta(2′) in a κ2C,O mode. Each tantalum atom has an
octahedral geometry, with the remainder of the coordination sphere
being completed by a tridentate [OOO] ligand and a THF ligand.
As a consequence of H2 release, THF binds to the Ta to offset the
loss of the hydride ligands.
We have previously reported the synthesis of 1 by the reaction
of [(OOO)TaCl2]2 with KBHEt3 (H3[OOO] ) 2,6-bis(3-tert-butyl-
5-methyl-2-hydroxybenzyl)-4-tert-butylphenol, the C-H activated
ligand is denoted as [OOCO]4-), in which a methylene linker of
the [OOO] ligand backbone undergoes cyclometalation.8 This C-H
activation is reversible, and 1 can serve as a low-valent tantalum
precursor. Thus we examined reduction of CO by 1 (Scheme 1).
Exposure of a THF solution of 1 to an atmosphere of CO under
1 atm at room temperature produced a gradual color change from
yellow to deep green over a period of 60 min. After removal of
volatiles in vacuo, slow diffusion of hexane into a DME solution
of the residue afforded 2 as green crystals in 31% isolated yield.
Formally, one can consider the oxidation state of each Ta in 1 and
2 to be Ta(V) and the C6O6 unit as an octaanion. The stoichiometry
assumes that 2 equiv of 1 furnish eight electrons needed to convert
6 equiv of CO to a [C6O6]8- unit. Since 2 contains no C-H
activated ligand, two of the three hydrides in each 1 migrate to the
[OOCO] ligands. The other hydride remains as a terminal ligand
in the product 2. Overall, the eight-electron reduction can be
accounted for by four two-electron C-H reductive eliminations.
A single crystal X-ray diffraction study of 2 confirmed its
formulation.9 The presence of hydride ligands in 2 was further
established by other means.
The reaction of metal hydrides with proton (H+) sources is a
well-known reaction, and formation of H2 and a solvated metal
complex has been used as a diagnostic test for metal hydrides.10
Performing protonation of 2 with [Me3NH][BPh4] in THF resulted
in H2 gas evolution and the generation of 3 with concomitant
precipitation of K[BPh4] as a white solid.
Slow diffusion of hexane into the THF solution of 3 yielded
green crystals that were suitable for X-ray structure analysis (Figure
1). A striking feature of the molecular structure is the encapsulation
of a C6O6 unit within a shell comprised of four Ta fragments. There
is a crystallographic inversion center between C(3) and C(3′) that
Figure 1. Molecular structure of 3. All methyl and tert-butyl groups
attached to the aryloxides have been omitted for clarity.
A close look at the bond distances within the Ta4C6O6 core gives
insight into electron localization within the complex. The average
Ta-O and Ta-C distances 1.964(5) and 2.198(8) Å are in the
ranges for the corresponding single bonds.11,12 The C-O distances
[average 1.379(9) Å] are comparable to enolate C-O bond
distances.12,13 The C(1)-C(2)-C(3)-C(3′)-C(2′)-C(1′) linkage
exhibits short-long bond alternation as follows: 1.367(11), 1.455(10),
and 1.357(16) Å, respectively. These metric parameters suggest an
important contribution from a hexatriene-hexaolate form as shown
in Scheme 1.
Complexes 2 and 3 are further characterized by NMR spectros-
copy. The 1H NMR spectrum of 2 at 253 K in THF-d8 exhibits the
number of the peaks expected for a molecule with Ci point group
symmetry, as observed in the solid state. A singlet resonance typical
of a terminal tantalum hydride is observed at 17.59 ppm integrating
to 2H.11,14 As the temperature is raised, these signals are severely
broadened. For 3, the low-temperature 1H NMR spectrum indicated
the presence of isomers15 in equilibrium. Warming the sample
results in observation of a time-average C2h symmetric molecule.
These behaviors are caused by a fluxional process involving
dynamic dissociation and recoordination of the THF ligands.
To confirm the origin of the C6O6 unit, the isotopically enriched
product 2-13C was analogously prepared by treatment of 1 with
13CO. Subsequent protonation of 2-13C yielded 3-13C. The 13C NMR
spectrum of 2-13C exhibits three resonances for the C6O6 unit at
143.1, 160.2, and 219.3 ppm, but C-C coupling is not adequately
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3474 J. AM. CHEM. SOC. 2009, 131, 3474–3475
10.1021/ja9007276 CCC: $40.75
2009 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
resolved even at low temperatures owing to a broadening of
resonances associated with a reversible THF-dissociation process.
In the 13C NMR spectrum of 3-13C at 323 K, the corresponding
resonances appear as multiplets and are downfield shifted at 145.1,
165.5, and 221.3 ppm. These data unambiguously confirm that all
carbon atoms in the C6O6 unit arise from external CO.
Acknowledgment. The authors thank the Ministry of Education,
Culture, Sports, Science and Technology, Japan for funding (Nos.
18064016 and 8GS0207A).
Supporting Information Available: Experimental procedures in
PDF format. X-ray structural data of 2, 3, and 4 in CIF format. This
material is available free of charge via the Internet at http://pubs.acs.org.
Additional evidence of the lability of the THF ligands is provided
by isolation of a desolvated product from 3. One observation is
that loss of THF undergoes a striking color change from green to
purple upon exposure of 3 to vacuum in solid or dissolution of 3
in toluene. Addition of THF to the purple product results in
regeneration of the diagnostic green color of 3. Standing a saturated
pentane solution of 3 afforded purple crystals identified as 4 by
X-ray structure analysis.9 Dissociation of the two THF ligands
creates two trigonal-bipyramidal Ta centers, while the other Ta
metals remain octahedral. The Ta4C6O6 core is reserved, and its
internal C-C bond distances exhibit a pattern similar to that found
in 3.
References
(1) 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.
(2) des Abbayes, H; Salau¨n, J.-Y. Dalton Trans. 2003, 1041–1052.
(3) Fagan, P. J.; Manriquez, J. M.; Marks, T. J.; Day, V. W.; Vollmer, S. H.;
Day, C. S. J. Am. Chem. Soc. 1980, 102, 5393–5396.
(4) (a) Jiao, H.; Wu, H.-S. J. Org. Chem. 2003, 68, 1475–1479. (b) Frapper,
G.; Cu, C.-X.; Halet, J.-F.; Saillard, J.-Y.; Kertesz, M. Chem. Commun.
1997, 2011–2012. (c) Sabzyan, H.; Noorbala, M. R. THEOCHEM 2003,
626, 143–158.
(5) (a) Morris, R. M.; Klabunde, K. J. J. Am. Chem. Soc. 1983, 105, 2633–
2639. (b) Carnahan, E. M.; Protasiewicz, J. D.; Lippard, S. J. Acc. Chem.
Res. 1993, 26, 90–97. (c) Chatani, N.; Shinohara, M.; Ikeda, S.; Murai, S.
J. Am. Chem. Soc. 1997, 119, 4303–4304. (d) Zhang, X.-X.; Parks, G. F.;
Wayland, B. B. J. Am. Chem. Soc. 1997, 119, 7938–7944. (e) Wolczanski,
P. T.; Bercaw, J. E. Acc. Chem. Res. 1980, 13, 121–127.
(6) (a) Evans, W. J.; Grate, J. W.; Hughes, L. A.; Zhang, H.; Atwood, J. L.
J. Am. Chem. Soc. 1985, 107, 3728–3730. (b) Evans, W. J.; Lee, D. S.;
Ziller, J. W.; Kaltsoyannis, N. J. Am. Chem. Soc. 2006, 128, 14176–14184.
(7) (a) Summerscales, O. T.; Cloke, F. G. N.; Hitchcock, P. B.; Green, J. C.;
Hazari, N. Science 2006, 311, 829–831. (b) Summerscales, O. T.; Cloke,
F. G. N.; Hitchcock, P. B.; Green, J. C.; Hazari, N. J. Am. Chem. Soc.
2006, 128, 9602–9603. (c) Frey, A. S.; Cloke, F. G. N.; Hitchcock, P. B.;
Day, I. J.; Green, J. C.; Aitken, G. J. Am. Chem. Soc. 2008, 130, 13816–
13817.
The UV-visible spectra of the C6O6 complexes deserve some
comments. High oxidation aryloxide complexes are usually light-
colored. For example, 1 is yellow. In contrast, intense colors are
noted for the C6O6 complexes. The UV-visible spectra of 2, 3,
and 4 contain broad absorptions in the region between 500 and
700 nm with extinction coefficients from 8300 to 12 000 M-1 cm-1
,
which are assigned to the HOMOfLUMO transitions.9 Since the
HOMO has mainly hexatriene π orbital character and the LUMO
gains contribution from tantalum d orbitals in addition to the π*
orbital of the hexatriene unit, the observed absorptions are attribut-
able to ligand-to-metal charge transfer transition.16
(8) Kawaguchi, H.; Matsuo, T. J. Am. Chem. Soc. 2003, 125, 14254–14255.
(9) See Supporting Information (SI) for details of the X-ray crystal structures
and UV-visible spectra.
(10) Kubas, G. J. Chem. ReV. 2007, 107, 4152–4205.
(11) Mulford, D. R.; Clark, J. R.; Schweiger, S. W.; Fanwick, P. E.; Rothwell,
I. P. Organometallics 1999, 18, 4448–4458.
(12) Proulx, G.; Bergman, R. G. J. Am. Chem. Soc. 1996, 118, 1981–1996.
(13) (a) Hofmann, P.; Frede, M.; Stauffert, P.; Lasser, W.; Thewalt, U. Angew.
Chem., Int. Ed. Engl. 1985, 24, 712–713. (b) Barger, P. T.; Santarsiero,
B. D.; Armantrout, J.; Bercaw, J. E. J. Am. Chem. Soc. 1984, 106, 5178–
5186.
(14) (a) Fryzuk, M. D.; MacKay, B. A.; Patrick, B. O. J. Am. Chem. Soc. 2003,
125, 3234–3235. (b) Gavenonis, J.; Tilley, T. D. Organometallics 2004,
23, 31–43.
We have shown that multielectron reductive chemistry of
transition metals can be applicable to the chaining of CO. The
sequence of reductive coupling beginning with the hydride complex
1 ceases with the C6O6 complex, which was not found to react
with CO. Prevention of further CO homologation is possibly due
to the lack of d-electrons available for reducing CO. The C6O6
complexes are remarkably stable as long as they are not exposed
to O2 and water. Coordination to tantalum is ascribed to stabilizing
an acyclic CO linkage.
(15) Each THF ligand can be situated above and below the Ta4C6O6 plane.
(16) To gain more insight into the electronic structures of the complexes, DFT
calculations were performed. See SI for details of these results.
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