Structure and reactivity of the β-agostic [ansa-Cp-arene]Ta( nPr) cation: Ambivalent behavior induced by intramolecular arene coordination

Article abstract of DOI:10.1021/ja073956j

Cationic tantalum complexes of the type {η⁶-Ar-CMe₂-η⁵-C₅H₄]TaR}⁺ (Ar = 3,5-dimethylphenyl, R = nPr, H) were prepared. In these complexes, the pendant arene group on the cyclopentadienyl ancillary ligand is coordinated intramolecularly to the metal center. Structural and spectroscopic studies revealed that the arene moiety is partly reduced by the low-valent metal center, leading to ambivalent Ta(III)-Ta(V) behavior. The compounds show significant differences in structure and reactivity to the isoelectronic neutral tantalocene derivatives; for example, the nPr complex has a β-agostic alkyl ground state structure, and the system is able to catalyze the hydrogenation of 1-alkenes; the metal center is also able undergo oxidative addition reactions. The use of an intramolecularly coordinating arene group presents a strategy to add stability of low and intermediate valencies of early transition metals without the use of classic strong π-acceptor ligands. Copyright

Full text of DOI:10.1021/ja073956j

Published on Web 07/27/2007
Structure and Reactivity of the â-Agostic [ansa-Cp-arene]Ta(nPr) Cation:
Ambivalent Behavior Induced by Intramolecular Arene Coordination
Edwin Otten, Auke Meetsma, and Bart Hessen*
Centre for Catalytic Olefin Polymerization, Stratingh Institute for Chemistry, UniVersity of Groningen,
Nijenborgh 4, 9747 AG, Groningen, The Netherlands
Received June 1, 2007; E-mail: b.hessen@rug.nl
Scheme 1
Early transition metal catalysis largely involves complexes with
the metal center in its highest (d0) oxidation state. Oxidative
addition-reductive elimination sequences, which are of prime
importance to many processes catalyzed by noble metal complexes,1
are therefore very rare for these metals. To extend the possibilities
for useful catalytic conversions with non-noble metals, it would
be desirable to facilitate two-electron redox reaction steps for these
metals. Recently, we reported cationic monocyclopentadienyl
titanium catalysts that selectively trimerize ethylene to 1-hexene,
by virtue of a pendant arene moiety on the Cp ligand.2 Computa-
tional studies on the catalytic cycle have shown the process to
involve the Ti(II)/Ti(IV) couple, in which the low-valent state in
particular is stabilized by interaction with the pendant arene.3 Thus,
a pendant arene group may be more generally useful to stabilize
low-valent states for non-noble metals.
For the group 5 metal Ta, examples of “low-valent” complexes
considerable π-localization, with contracted C(110)-C(111) and
C(113)-C(114) bond lengths and elongation of the remaining C-C
bonds (Table S1). The ring is folded by 8.6(5)° along the
C(19)-C(112) axis. It should be noted that the distortions of the
arene moiety are much smaller than in Ta(III) arene complexes
where the arene is not linked to a Cp ligand and the overlap between
metal d- and arene π*-orbitals is optimal (ring folding up to 34.4°
has been observed for (C6Me6)Ta(OAr)2Cl).4 Nevertheless, the
structural parameters of 3 clearly indicate significant resonance
contribution from the Ta(V)/dienediyl structure (B, Scheme 1). The
1H NMR resonances of the pendant arene in 3 are shifted upfield
with η6-arene ligands have been reported, but these are more
appropriately described as 7-metallanorbornadienes, through reso-
nance contribution of a Ta(V) species, with a doubly reduced arene
(cyclohexa-2,5-diene-1,4-diyl).4 Here we report the cationic tanta-
n
lum complexes [η6-Ar-CMe2-η5-C5H4]Ta(R)+ (R ) Pr, H; Ar )
3,5-dimethylphenyl), bearing an arene-substituted cyclopentadienyl
ligand involving ansa-(η5-C5H4,η6-arene) coordination. These spe-
cies show migratory insertion reactions as well as ambivalent
[Ta(III)TTa(V)] character.
As starting material, the complex [Ar-CMe2-η5-C5H4]TaMe3Cl
(1) was prepared by treatment of TaMe3Cl2 with [Ar-CMe2-C5H4]-
Li in toluene and obtained as yellow-orange crystals in 67% isolated
n
yield. Addition of 2 equiv of PrMgCl to 1 at -50 °C in Et2O
afforded, after gradual warming to room temperature and extraction
with and crystallization from pentane, dark red crystals of the
cyclometalated species [η1-3,5-Me2C6H2-CMe2-η5-C5H4]Ta(nPr)-
(C3H6) (2) in moderate yield (33%) (Scheme 1).5 1H NMR
spectroscopy shows that 2 is present in C6D6 solution as a 80:20
mixture of two isomers, presumably corresponding to two orienta-
tions of the methyl substituent on the coordinated propene moiety.
In the solid state, 2 is found to be present as a single isomer (Figure
1), although redissolution of these crystals again produced the above
mixture of isomers, suggesting an equilibrium. The coordinated
propene shows predominant tantalacyclopropane character, as seen
from its Ta-C bond lengths (that with 2.174(4) and 2.150(4) Å lie
within the range observed for tantalum alkyls) and the elongated
C-C bond of 1.472(6) Å.6
Treatment of 2 with 1 equiv of the Brønsted acid [PhNMe2H]-
[B(C6F5)4] in C6D5Br results in liberation of propene and quantita-
tive formation of {[η6-Ar-CMe2-η5-C5H4]Ta(nPr)}[B(C6F5)4] (3,
Scheme 1). On a preparative scale, 3 was isolated in 88% yield as
dark greenish-brown crystals from bromobenzene/hexane. An X-ray
structure determination (Figure 1) shows the presence of a â-agostic
nPr group. The C-C distances within the C6 fragment manifest
Figure 1. Molecular structures of (from left to right and top to bottom) 2,
3, 4-THF, and 5 with 50% probability ellipsoids. The anion (for 3, 4-THF,
and 5) and hydrogens (except those on C(118) in 3 and Ta-H in 4-THF)
are omitted for clarity.
9
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J. AM. CHEM. SOC. 2007, 129, 10100-10101
10.1021/ja073956j CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
and often stabilized by electron donation of a M-L σ-bond into
the CO π* orbital,11 5 is a stable compound in spite of the apparent
high-valent character of the metal center and absence of Ta-alkyl
σfπ* backdonation (Calkyl-Ta-Ccarbonyl ) 84.9(2)°).11c Reaction
of 3 with PhSSPh at room temperature immediately gives the
cationic dithiolate alkyl complex 6. Liberation of the pendant arene
1
moiety from the metal center in 6 is seen from the shift of its H
NMR signals back into the aromatic region (δ 7.3-6.9 ppm). This
oxidative addition reaction shows that, despite the considerable
Ta(V)/dienediyl (B) character of 3, it can readily display reactivity
corresponding to a Ta(III) compound.
Thus, in cationic ansa-Cp-arene complexes of tantalum, the
coordination of the pendant arene allows resonance stabilization
of low-valent species, resulting in ambivalent character. It causes
considerable differences in chemistry relative to the isoelectronic
neutral tantalocene analogues. The cationic ansa-Cp-arene tantalum
complexes can display both migratory insertion and oxidative
addition reactions. Use of these intramolecular interactions thus
could provide a fruitful strategy to give early transition metals a
more “noble” character.
(o-ArH: δ 4.55/4.23; p-ArH: 2.22 ppm), consistent with formation
1
of an η6-adduct and partial disruption of the aromaticity. The H
NMR spectrum of 3 at -30 °C also shows features characteristic
for a â-agostic alkyl group. The R-CH2 protons are diastereotopic
(δ 0.61 and -0.54 ppm), and the â-CH2 resonances are broad and
shifted significantly upfield (δ -1.9 and -5.9 ppm). The â-agostic
Ta-nPr structure of 3 stands in marked contrast to the isoelectronic
neutral group 5 metal (ansa-)metallocene analogues, which have
olefin-hydride ground states.6a,7 Two-Dimensional EXSY NMR
spectroscopy at 70 °C (at which temperature the diastereotopic R-
and â-CH2 groups are coalesced) shows exchange between all
Acknowledgment. Financial support was provided by NRSC-
Catalysis.
Supporting Information Available: Text giving full experimental
and characterization data, and crystallographic data for compounds 2,
3, 4-THF, and 5. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
propyl H resonances, suggesting the dynamics occur through a
thermally accessible olefin-hydride intermediate.7
References
The reaction of 3 with H2 in bromobenzene results in immediate
formation of the corresponding cationic hydride 4 (Scheme 2). As
is the case for 3, the NMR spectra of 4 indicate a C1 symmetric
structure (possibly indicating solvation by bromobenzene)8 and an
(1) (a) Van Leeuwen, P. W. N. M. Homogeneous Catalysis: Understanding
the Art; Kluwer Academic Publishers: Dordrecht, The Netherland, 2004;
pp 36-41. (b) De Meijere, A., Diederich, F., Eds. Metal-Catalyzed Cross-
Coupling Reactions; Wiley-VCH: Weinheim, Germany, 2004; pp 1-40.
(2) (a) Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Angew. Chem., Int. Ed.
2001, 40, 2516. (b) Deckers, P. J. W.; Hessen, B.; Teuben, J. H.
Organometallics 2002, 21, 5122.
1
η6-coordinated arene. The Ta-H H NMR resonance at δ -4.65
ppm corresponds to 1H intensity, revealing a contrast to neutral
metallocene analogues, which form trihydrides (Cp2MH3).9 Upon
addition of 1 bar of propene to a solution of 4, quantitative
regeneration of 3 is observed within 1 h. In agreement with this, 4
can catalyze the hydrogenation of 1-hexene at ambient temperature
(15 t.o. in 16 h). Although 4 is stable for at least 24 h in cold (-25
°C) bromobenzene solution, attempted isolation by removal of the
volatiles led to decomposition. Addition of THF to solutions of 4
gave the THF adduct 4-THF, which was isolated in crystalline form.
Its 1H NMR spectrum shows a singlet at -3.98 ppm, and in the IR
spectrum, a broad band at 1760 cm-1 is observed, both consistent
with the presence of a Ta-H moiety. An X-ray structure (Figure
1) confirmed the overall geometry of the complex.
Treatment of 3 with 1 bar of CO at room temperature results
in the formation of the CO adduct 5 (Scheme 2). The ν(CO) IR
band at 2033 cm-1 lies at considerably higher frequency than in
Cp2TaR(CO) (R ) H, alkyl; 1880-1895 cm-1).10 The reduced
ability of the metal center in 5 for π-backdonation to CO reflects
the acceptor properties of the η6-arene and is in agreement with
substantial Ta(V) character. The X-ray structure of 5 (Figure 1)
shows again that the arene moiety is slightly folded by 11.4(4)°.
The CO bond is only modestly elongated at 1.107(8) versus
1.148(7) Å in Cp2TaR(CO) (R ) CH2SiH(tBu)2).10b Whereas CO
adducts of d0 early transition metal centers are in general weak
(3) (a) Blok, A. N. J.; Budzelaar, P. H. M.; Gal, A. W. Organometallics 2003,
22, 2564. (b) De Bruin, T. J. M.; Magna, L.; Raybaud, P.; Toulhoat, H.
Organometallics 2003, 22, 3404. (c) Tobisch, S.; Ziegler, T. Organome-
tallics 2003, 22, 5392.
(4) (a) Bruck, M. A.; Copenhaver, A. S.; Wigley, D. E. J. Am. Chem. Soc.
1987, 109, 6525. (b) Arney, D. J.; Wexler, P. A.; Wigley, D. E.
Organometallics 1990, 9, 1282.
(5) The presence of MeMgCl as the only NMR active component in the white
residue suggests that trans-alkylation is a key step in this transformation.
(6) (a) Chirik, P. J.; Zubris, D. L.; Ackerman, L. J.; Henling, L. M.; Day, M.
W.; Bercaw, J. E. Organometallics 2003, 22, 172. (b) Fryzuk, M. D.;
Johnson, S. A.; Rettig, S. J. J. Am. Chem. Soc. 2001, 123, 1602. (c)
Abbenhuis, H. C. L.; Feiken, N.; Haarman, H. F.; Grove, D. M.; Horn,
E.; Kooijman, H.; Spek, A. L.; Van Koten, G. Angew. Chem., Int. Ed.
1991, 30, 996.
(7) (a) Burger, B. J.; Santarsiero, B. D.; Trimmer, M. S.; Bercaw, J. E. J.
Am. Chem. Soc. 1988, 110, 3134. (b) Green, M. L. H.; Sella, A.; Wong,
L. L. Organometallics 1992, 11, 2650. (c) Shin, J. H.; Parkin, G. Chem.
Commun. 1999, 887. (d) Ackerman, L. J.; Green, M. L. H.; Green, J. C.;
Bercaw, J. E. Organometallics 2003, 22, 188.
(8) (a) Wu, F.; Dash, A. K.; Jordan, R. F. J. Am. Chem. Soc. 2004, 126,
15360. (b) Bouwkamp, M. W.; Budzelaar, P. H. M.; Gercama, J.; Del
Hierro Morales, I.; De Wolf, J.; Meetsma, A.; Troyanov, S. I.; Teuben, J.
H.; Hessen, B. J. Am. Chem. Soc. 2005, 127, 14310.
(9) Antinolo, A.; Carrillo-Hermosilla, F.; Fajardo, M.; Fernandez-Baeza, J.;
Garcia-Yuste, S.; Otero, A. Coord. Chem. ReV. 1999, 193-195, 43.
(10) (a) Klazinga, A. H.; Teuben, J. H. J. Organomet. Chem. 1979, 165, 31.
(b) Berry, D.; Koloski, T.; Carroll, P. Organometallics 1990, 9, 2952.
(11) (a) Marsella, J.; Curtis, C.; Bercaw, J.; Caulton, K. J. Am. Chem. Soc.
1980, 102, 7244. (b) Guo, Z.; Swenson, D.; Guram, A.; Jordan, R.
Organometallics 1994, 13, 766. (c) Burckhardt, U.; Tilley, T. D. J. Am.
Chem. Soc. 1999, 121, 6328.
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