On the mechanism of dihydrogen addition to tantalocene complexes [8]

Full text of DOI:10.1021/ja9739188

J. Am. Chem. Soc. 1998, 120, 5329-5330
5329
transition metal complexes is presumed to occur by an analogous
mechanism, although few systems have been investigated19-21 and
none have been studied with PHIP. Detailed knowledge of the
mechanism of this activation reaction should aid in expanding
the catalytic properties of these systems.
On the Mechanism of Dihydrogen Addition to
Tantalocene Complexes
Susan P. Millar,† Deanna L. Zubris,‡ John E. Bercaw,‡ and
Richard Eisenberg*,†
Bercaw and co-workers have reported several interesting
reactions between hydrogen and bis(pentamethylcyclopentadi-
enyl)tantalum complexes that meet the requirements for PHIP.22-25
Specifically, the benzyne hydride complex 1 reacts with H2 at 0
°C to form a new phenyl dihydride complex 2, which in turn
reacts with H2 at room temperature to generate benzene and the
trihydride complex 3. The overall transformations are shown as
eqs 1 and 2.
Department of Chemistry, UniVersity of Rochester
Rochester, New York 14627
Arnold and Mabel Beckman Laboratories of
Chemical Synthesis, California Institute of
Technology, Pasadena, California 91125
ReceiVed NoVember 17, 1997
Parahydrogen induced polarization (PHIP) is a valuable
mechanistic tool that can provide definitive evidence for pairwise
addition of dihydrogen to unsaturated substrates. PHIP occurs
if H2 enriched in the para spin state adds to a metal complex or
an organic substrate in a pairwise manner, such that spin
correlation is maintained between the two transferred protons that
originated from the same H2 molecule. If the transfer of the
protons occurs fast relative to proton relaxation, non-Boltzmann
populations in their spin states can result, leading to enhanced
absorption and emission lines in the product NMR spectrum. For
the observation of PHIP, dihydrogen addition must lead to a
product in which the transferred protons are magnetically distinct
and J coupled to each other.1-8 To date, PHIP in metal complexes
has only been observed in the oxidative addition of dihydrogen
to late transition metal complexes including those of rhodium,2,9-12
iridium,2,3,13-15 ruthenium,16 and platinum.17,18 The reaction
proceeds in a concerted manner via a 3-centered triangular
transition state. Activation of dihydrogen by low valent early
H2
Cp*2Ta
Cp*2Ta Hc
Cp*2Ta
(1)
(2)
Hl
Hl
Hl
1
2
Hl
H2
Cp*2Ta Hc
Hl
+
Cp*2Ta Hc
Hl
RT
2
3
Complexes 2 and 3 have bent metallocene structures with three
σ-bonded ligands in the central or equatorial “wedge”, resulting
in an inequivalence of hydride ligands. For 2 the hydride ligands
and the phenyl group lie in the equatorial plane with one hydride
ligand in the central position (Hc) and the other hydride ligand
(Hl) and the phenyl group in the lateral positions. In the 1H NMR
spectrum, the hydride ligands of 2 are observed at 2.5 and 0.95
ppm (2JHH ) 12 Hz, benzene-d6).24 In the case of 3, the tantalum
metal and the three hydride ligands lie in a common plane with
† University of Rochester.
‡ California Institute of Technology.
(1) Bowers, C. R.; Weitekamp, D. P. Phys. ReV. Lett. 1986, 57, 2645-
2648.
1
the hydride resonances appearing as an AB2 pattern in the H
(2) Bowers, C. R.; Weitekamp, D. P. J. Am. Chem. Soc. 1987, 109, 5541-
5542. In this communication, the acronym PASADENA was used to describe
the parahydrogen generated polarization effect.
(3) Eisenschmid, T. C.; Kirss, R. U.; Deutsch, P. P.; Hommeltoft, S. I.;
Eisenberg, R.; Bargon, J.; Lawler, R. G.; Balch, A. L. J. Am. Chem. Soc.
1987, 109, 8089-8091.
NMR spectrum. The two lateral hydride ligands are chemically
equivalent and appear as a doublet at -0.91 ppm, while the Hc is
a triplet and resonates at 1.11 ppm (2JHH ) 13 Hz, benzene-d6).26
The formation of 2 and 3 is thought to proceed via Ta(III) species,
which are generated respectively by benzyne insertion into the
metal-hydride bond and reductive elimination of benzene.
Dynamic NMR experiments provide evidence for the existence
of a Ta(III) intermediate in eq 1.23,24 An alternative mechanism
for the H2 addition reactions is via a four-centered transition state
corresponding to σ-bond metathesis. The present study was
undertaken since PHIP can provide unambiguous evidence for
exclusion of one or the other of these mechanisms.
(4) Pravica, M. G.; Weitekamp, D. P. Chem. Phys. Lett. 1988, 145, 255-
258.
(5) Kirss, R. U.; Eisenschmid, T. C.; Eisenberg, R. J. Am. Chem. Soc. 1988,
110, 8564-8566.
(6) Bargon, J.; Kandels, J.; Kating, P.; Thomas, A.; Woelk, K. Tetrahedron
Lett. 1990, 31, 5721-5724.
(7) Eisenberg, R. Acc. Chem. Res. 1991, 24, 110-116.
(8) Harthun, A.; Selke, R.; Bargon, J. Angew. Chem., Int. Ed. Engl. 1996,
35, 2505-2507.
(9) Duckett, S. B.; Eisenberg, R. J. Am. Chem. Soc. 1993, 115, 5292-
5293.
Reaction of parahydrogen and submilligram samples of 1 leads
to PHIP in both of the hydride resonances of 2 in the H NMR
(10) Duckett, S. B.; Eisenberg, R.; Goldman, A. S. J. Chem. Soc., Chem.
Commun. 1993, 1185-1187.
1
(11) Duckett, S. B.; Newell, C. L.; Eisenberg, R. J. Am. Chem. Soc. 1994,
116, 10548-10556.
spectrum. Equally important, no discernible polarization is
observed in any of the phenyl resonances of 2. This result
indicates pairwise addition of H2 to the Ta center, thus strongly
supporting the proposal of a 16-electron Ta(III) intermediate,
rather than direct reaction of H2 with the formally Ta(V) benzyne
hydride complex. Optimization of polarized resonances for 2
occurs at 308 K and the spectrum is shown in Figure 1a.
Polarization at 308 K decays over a period of 10 min. As the
initial polarized peaks of 2 decay, more 2 forms and its polarized
(12) Duckett, S. B.; Barlow, G. K.; Partridge, M. G.; Messerle, B. A. J.
Chem. Soc., Dalton Trans. 1995, 3427-3429.
(13) Eisenschmid, T. C.; McDonald, J.; Eisenberg, R.; Lawler, R. G. J.
Am. Chem. Soc. 1989, 111, 7267-7269.
(14) Duckett, S. B.; Newell, C. L.; Eisenberg, R. J. Am. Chem. Soc. 1993,
115, 1156-1157.
(15) Sleigh, C. J.; Duckett, S. B.; Messerle, B. A. J. Chem. Soc., Chem.
Commun. 1996, 2395-2396.
(16) Duckett, S. B.; Mawby, R. J.; Partridge, M. G. J. Chem. Soc., Chem.
Commun. 1996, 383-384.
(17) Jang, M.; Duckett, S. B.; Eisenberg, R. Organometallics 1996, 15,
2863-2865.
(22) van Asselt, A.; Burger, B. J.; Gibson, V. C.; Bercaw, J. E. J. Am.
Chem. Soc. 1986, 108, 5347-5349.
(18) Millar, S. P.; Jang, M.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chim.
Acta 1998, 270, 363-375.
(23) Parkin, G.; Bunel, E.; Burger, B. J.; Trimmer, M. S.; van Asselt, A.;
Bercaw, J. E. J. Mol. Catal. 1987, 41, 21-39.
(19) Barefield, E. K.; Parshall, G. W.; Tebbe, F. N. J. Am. Chem. Soc.
1970, 92, 5234-5235.
(24) Trimmer, M. S. Ph.D. Thesis, California Institute of Technology, 1989.
(25) Antonelli, D. M.; Schaefer, W. P.; Parkin, G.; Bercaw, J. E. J.
Organomet. Chem. 1993, 462, 213-220.
(20) Tebbe, F. N.; Parshall, G. W. J. Am. Chem. Soc. 1971, 93, 3793-
3795.
(21) Bell, R. A.; Cohen, S. A.; Doherty, N. M.; Threlkel, R. S.; Bercaw,
J. E. Organometallics 1986, 5, 972-975.
(26) Gibson, V. C.; Bercaw, J. E.; Bruton, J. W., Jr.; Sanner, R. D.
Organometallics 1986, 5, 976-979.
S0002-7863(97)03918-8 CCC: $15.00 © 1998 American Chemical Society
Published on Web 05/14/1998

5330 J. Am. Chem. Soc., Vol. 120, No. 21, 1998
Communications to the Editor
positions, with the original hydride ligand remaining in the central
position, then PHIP could not be observed since the added hydride
ligands would be magnetically equivalent; in effect, the polariza-
tion for the added hydride ligands would exactly cancel.
The stereochemistry of H2 addition was established indepen-
dently by using complementary labeling experiments. Reaction
of 1 with H2 (or D2) at 258 K to form 2 (or 2-d2) followed by
removal of the initial gas and reaction with D2 (or H2) affords
3-d2 (or 3-d1), as shown in eqs 3 and 4.
H
H2
–C6H6
D2
1
[Cp*2Ta H]
(3)
(4)
Cp*2Ta
D
Cp*2Ta
H
H
D
2
3-d2
D
H2
D2
–C6H5D
[Cp*2Ta D]
Cp*2Ta
H
1
Cp*2Ta
D
D
H
2
3-d1
1
The hydride resonance for 3-d2 is observed in the H NMR
spectrum (toluene-d8) as a broad peak at -1.05 ppm, with a peak
width at half-height of about 7 Hz. The broadness is the result
of unresolved JHD couplings to the deuterides in both central and
lateral positions. The hydride resonance of the 3-d2 isotopomer
is shifted upfield 50 ppb from the Hl resonance of the trihydride
complex, which occurs at -1.00 ppm in toluene-d8. The lateral
hydride resonance for 3-d1 is seen in the 1H NMR spectrum as a
doublet at -1.016 ppm (2JHH ) 13 Hz, toluene-d8), and is shifted
upfield 16 ppb.27 The chemical shift and appearance of the Cp*
resonance is not affected by deuterium incorporation in either
3-d2 or 3-d1. Under the conditions of eqs 3 and 4, no other
isotopomers for 3-d2 and 3-d1 are detected. No isotopic scram-
bling is observed in the Cp*2TaH3 system, in contrast to that seen
for the d1 isotopomers of the isoelectronic cation [Cp*2WH3]+.28,29
The formation of 3-d2 in eq 3 and 3-d1 in eq 4, together with the
parahydrogen studies, proves unambiguously that the addition of
H2 occurs in a pairwise fashion to one side of the initial hydride
ligand in the Ta(III) intermediate. Additionally, photolysis of
the trihydride complex under D2 yields 3-d2 after 10 min of
irradiation, indicating that photochemically driven reductive
elimination of H2 from 3 takes place. After 80 min of irradiation,
more isotopomers are present, as the label is scrambled with the
reductive elimination of HD as well as of H2 and D2.
Figure 1. 1H NMR spectra obtained by using a submilligram sample of
Cp*2Ta(benzyne)H (1) and para-enriched hydrogen in toluene-d8. (a) PHIP
spectrum of Cp*2Ta(phenyl)H2 (2) at 308 K. (b) PHIP spectrum of 2
and Cp*2TaH3 (3) at 313 K. (c) PHIP spectrum of 3 at 323 K. Resonances
for solvent (s) and Cp* protons for 1, 2, and 3 are labeled.
hydride resonances become unsymmetrical, as a result of overlap
of the resonances of newly polarized and relaxed phenyl dihydride
product. Significantly, up to 333 K hydrogen addition to 2 is
not reversible, as shown by the fact that 2 in the presence of
parahydrogen does not give polarization of the dihydride reso-
nances.
Further reaction of 2 in the presence of parahydrogen yields
the trihydride complex with polarization observed in both hydride
resonances of 3, consistent with pairwise addition of H2 to the
putative Cp*2Ta-H fragment. Reaction of parahydrogen with a
mixture of 1 and 2 produces polarized hydride resonances in both
2 and 3 (Figure 1b). Once all of 1 has been consumed, the
polarization of 2 decays, but as 2 reacts with parahydrogen,
polarization for 3 is observed. The largest polarization obtained
for 3 occurs at 323 K and is shown in Figure 1c. The hydride
resonance at -0.91 ppm is observed as an antiphase doublet with
2
emission and enhanced absorption lines separated by JHH of 13
This is the first time that PHIP has been observed with an early
transition metal hydride complex. Addition of parahydrogen to
Cp*2Ta(benzyne)H gives strong polarization in the hydride
resonances of Cp*2Ta(phenyl)H2, providing evidence for a 16-
electron intermediate. Further reaction yields Cp*2TaH3, and in
the presence of parahydrogen, polarization of the hydride
resonances is observed. Deuterium labeling experiments comple-
ment the PHIP studies and establish unambiguously the stereo-
chemistry of H2 addition to the monohydride Cp*2TaH interme-
diate. The labeling results also rule against a PHIP-silent σ-bond
metathesis mechanism in the formation of Cp*2TaH3. The
experiments reported thus provide definitive evidence of pairwise
addition of dihydrogen to two low-valent tantalum complexes and
strongly support the notion of concerted H2 oxidative addition
via a triangular transition state.
Hz, while the hydride resonance at 1.11 ppm appears as an
antiphase triplet with the central line missing and the outside lines
in enhanced absorption and emission separated by 2J (26 Hz).
The antiphase triplet can be rationalized by using the energy level
diagram for an AB2 spin system, and the notion that parahydrogen
adds pairwise to A (central) and B (lateral) positions, giving a
non-Boltzmann distribution of spins.2,13 Once the trihydride
complex is formed, it is stable and is not fluxional on the NMR
time scale, even upon warming to 333 K. Placement of 3 under
D2 does not lead to incorporation of deuterium in the hydride
positions, and exposure of 3 to parahydrogen up to 333 K does
not give enhanced polarized resonances, indicating the absence
of hydrogen cycling in this system under these conditions. The
inertness of the Cp*2TaH3 system to hydrogen exchange contrasts
with the behavior shown by Cp2TaH3, Cp2NbH3, and Cp*2-
NbH3.19-21 The observed polarization enhancements for both 2
and 3 are greater than 100-fold. PHIP is observed for both
complexes in toluene-d8, benzene-d6, and THF-d8.
Acknowledgment. Financial support of this work from the National
Science Foundation (Grant No. CHE 94-09441) is gratefully recognized.
JA9739188
The results from our parahydrogen studies for the formation
of 3 provide unambiguous evidence for hydrogen addition to a
tantalum hydride intermediate such that the new hydride ligands
add to both the central and lateral positions of the trihydride
product. In contrast, if H2 addition proceeded solely to the lateral
(27) The central hydride resonance for 3-d1 is not observed. This resonance
is expected to be broad due to deuterium coupling and is obscured by residual
pentane in the sample.
(28) Parkin, G.; Bercaw, J. E. Polyhedron 1988, 7, 2053-2082.
(29) Parkin, G.; Bercaw, J. E. J. Chem. Soc., Chem. Commun. 1989, 255-
257.