Reaction of [P2N2]Ta=CH2(Me) with ethylene: Synthesis of [P2N2]Ta(C2H4)Et, a neutral species with a β-agostic ethyl group in equilibrium with an α-agostic ethyl group ([P2N2] = PhP(CH2SiMe2NSiMe2CH2) 2PPh)

Article abstract of DOI:10.1021/ja0021621

The photolysis of [P₂N₂]TaMe₃ ([P₂N₂] = PhP(CH₂SiMe₂NSiMe₂CH₂) ₂PPh) produces [P₂N₂]Ta=CH₂(Me) as the major product. The thermally unstable methylidene complex decomposes in solution in the absence of trapping agents to unidentified products. However, in the presence of ethylene [P₂N₂]Ta= CH₂(Me) is slowly converted to [P₂N₂]Ta(C₂H₄)Et, with [P₂N₂]Ta(C₂H₄)Me observed as a minor product. A mechanistic study suggests that the formation of [P₂N₂]Ta(C₂H₄)Et results from the trapping of [P₂N₂]TaEt, formed by the migratory insertion of the methylene moiety into the tantalum-methyl bond. The minor product, [P₂N₂]Ta(C₂H₄)Me, forms from the decomposition of a tantalacyclobutane resulting from the addition of ethylene to [P₂N₂]Ta=CH₂(Me) and is accompanied by the production of an equivalent of propylene. Pure [P₂N₂]Ta(C₂H₄)Et can be synthesized by hydrogenation of [P₂N₂]TaMe₃ in the presence of PMe₃, followed by the reaction of ethylene with the resulting trihydride. Crystallographic and NMR data indicate the presence of a β-agostic interaction between the ethyl group and tantalum center in [P₂N₂]Ta(C₂H₄)Et. Partially deuterated analogues of [P₂N₂]Ta(C₂H₄)Et show a large isotopic perturbation of resonance for both the β-protons and the α-protons of the ethyl group, indicative of an equilibrium between β-agostic and an α-agostic interaction for the ethyl group in solution. An EXSY spectrum demonstrates that an additional fluxional process occurs that exchanges all of the ¹H environments of the ethyl and ethylene ligands. The mechanism of this exchange is believed to involve the direct transfer of the β-agostic hydrogen atom from the ethyl group to the ethylene ligand, via the so-called β-hydrogen transfer process.

Full text of DOI:10.1021/ja0021621

1602
J. Am. Chem. Soc. 2001, 123, 1602-1612
Reaction of [P2N2]TadCH2(Me) with Ethylene: Synthesis of
[P2N2]Ta(C2H4)Et, a Neutral Species with a â-Agostic Ethyl Group in
Equilibrium with an R-Agostic Ethyl Group ([P2N2] )
PhP(CH2SiMe2NSiMe2CH2)2PPh)
Michael D. Fryzuk,* Samuel A. Johnson, and Steven J. Rettig†
Contribution from the Department of Chemistry, UniVersity of British Columbia, 2036 Main Mall,
VancouVer, B.C., Canada V6T 1Z1
ReceiVed June 16, 2000
Abstract: The photolysis of [P2N2]TaMe3 ([P2N2] ) PhP(CH2SiMe2NSiMe2CH2)2PPh) produces [P2N2]-
TadCH2(Me) as the major product. The thermally unstable methylidene complex decomposes in solution
in the absence of trapping agents to unidentified products. However, in the presence of ethylene [P2N2]Tad
CH2(Me) is slowly converted to [P2N2]Ta(C2H4)Et, with [P2N2]Ta(C2H4)Me observed as a minor product. A
mechanistic study suggests that the formation of [P2N2]Ta(C2H4)Et results from the trapping of [P2N2]TaEt,
formed by the migratory insertion of the methylene moiety into the tantalum-methyl bond. The minor product,
[P2N2]Ta(C2H4)Me, forms from the decomposition of a tantalacyclobutane resulting from the addition of ethylene
to [P2N2]TadCH2(Me) and is accompanied by the production of an equivalent of propylene. Pure [P2N2]Ta-
(C2H4)Et can be synthesized by hydrogenation of [P2N2]TaMe3 in the presence of PMe3, followed by the
reaction of ethylene with the resulting trihydride. Crystallographic and NMR data indicate the presence of a
â-agostic interaction between the ethyl group and tantalum center in [P2N2]Ta(C2H4)Et. Partially deuterated
analogues of [P2N2]Ta(C2H4)Et show a large isotopic perturbation of resonance for both the â-protons and the
R-protons of the ethyl group, indicative of an equilibrium between a â-agostic and an R-agostic interaction for
the ethyl group in solution. An EXSY spectrum demonstrates that an additional fluxional process occurs that
1
exchanges all of the H environments of the ethyl and ethylene ligands. The mechanism of this exchange is
believed to involve the direct transfer of the â-agostic hydrogen atom from the ethyl group to the ethylene
ligand, via the so-called â-hydrogen transfer process.
Introduction
providing a 3-center, 2-electron bonding interaction.4,15 For the
simplest alkyl group in which this interaction is possible, the
ethyl group, â-agostic interactions are usually associated with
coordinatively unsaturated and cationic complexes.16-19 For
neutral complexes, a few â-agostic ethyl groups have been
postulated on the basis of NMR, EPR, and IR data;20-22
Agostic interactions involving C-H bonds are more than just
a structural curiosity; they have been shown to be important in
C-H activation processes1,2 as well as in olefin polymerization.
With respect to polymerization, â-agostic interactions have been
implicated in the resting state of many early and late metal
polymerization catalysts,3-6 and thus may be relevant to catalyst
activity and polymer molecular weights.7-14 Similarly, R-agostic
interactions have been examined for their ability to direct olefin
insertion.11
(10) Guo, Z.; Swenson, D. C.; Jordan, R. F. Organometallics 1994, 13,
1424.
(11) Grubbs, R. H.; Coates, G. W. Acc. Chem. Res. 1996, 29, 85.
(12) Prosenc, M. H.; Janiak, C.; Brintzinger, H. H. Organometallics 1992,
11, 4036.
There are numerous examples of â-agostic interactions, where
a â-C-H bond of an alkyl ligand interacts with the metal center
† Professional Officer: UBC X-ray Structural Laboratory (Deceased
October 27, 1998).
(1) Ginzburg, A. G. Russ. Chem. ReV. 1988, 57, 1175.
(2) Gleiter, R.; Hyla-Kryspin, I.; Niu, S. Q.; Erker, G. Organometallics
1993, 12, 3828.
(13) Ro¨ll, W.; Brintzinger, H. H.; Rieger, B.; Zolk, R. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 279.
(14) Cavallo, L.; Guerra, G.; Vacatello, M.; Corradini, P. Macromolecules
1991, 24, 1784.
(15) Crabtree, R. H.; Hamilton, D. G. AdV. Organomet. Chem. 1988,
28, 299.
(16) Jordan, R. F.; Bradley, P. K.; Baenziger, N. C.; LaPointe, R. E. J.
Am. Chem. Soc. 1990, 112, 1289.
(3) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250,
395.
(4) Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog. Inorg. Chem.
1988, 36, 1.
(5) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth,
R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143.
(6) Woo, T. K.; Margl, P. M.; Lohrenz, J. C. W.; Blo¨chl, P. E.; Ziegler,
T. J. Am. Chem. Soc. 1996, 118, 13021.
(17) Mole, L.; Spencer, J. L.; Carr, N.; Orpen, A. G. Organometallics
1991, 10, 49.
(18) Conroy-Lewis, F. M.; Mole, L.; Redhouse, A. D.; Lister, S. A.;
Spencer, J. L. J. Chem. Soc., Chem. Commun. 1991, 1601.
(19) Cracknell, R. B.; Orpen, A. G.; Spencer, J. L. J. Chem. Soc., Chem.
Commun. 1984, 326.
(20) Fellmann, J. D.; Schrock, R. R.; Traficante, D. D. Organometallics
1982, 1, 481.
(7) Schmidt, G. F.; Brookhart, M. J. Am. Chem. Soc. 1985, 107, 1443.
(8) Burger, B. J.; Thompson, M. E.; Cotter, W. D.; Bercaw, J. E. J. Am.
Chem. Soc. 1990, 112, 1566.
(9) Resconi, L.; Piemontesi, F.; Franciscono, G.; Abis, L.; Fiorani, T. J.
Am. Chem. Soc. 1992, 114, 1025.
(21) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan,
M. C.; Santasiero, B. D.; Shaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc.
1987, 109, 203.
(22) Lukens, W. W. J.; Smith, M. R. I.; Andersen, R. A. J. Am. Chem.
Soc. 1996, 118, 1719.
10.1021/ja0021621 CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/31/2001

Reaction of [P2N2]TadCH2(Me) with Ethylene
J. Am. Chem. Soc., Vol. 123, No. 8, 2001 1603
however, the only structurally characterized example of a neutral
complex with a â-agostic ethyl group is the 12-electron species
TiCl3(dmpe)Et,23,24 and the bonding in this species continues
to be the subject of theoretical studies.25-29 There are also a
number of reports of R-agostic interactions,4,11 although for the
ethyl group this interaction is believed to be preferred only when
steric bulk disfavors a â-agostic interaction.30
derivative decomposes to NMR inactive unidentified products
over the course of about one month. In an attempt to elucidate
the mechanisms and chemistry involved in this thermal decom-
position, we examined the decomposition of [P2N2]TadCH2-
(Me) in the presence of three potential trapping agents: ethylene,
trimethylphospine, and propylene. Of these, ethylene alone
proved successful in allowing the characterization and isolation
of products of the methylidene complex’s decomposition.
We have recently reported the synthesis of a methylidene
complex of tantalum, [P2N2]TadCH2(Me)31 (where [P2N2] )
PhP(CH2SiMe2NSiMe2CH2)2PPh). While isolable, this com-
pound is not stable when stored in solution at room temperature.
In analogy to the decomposition of the first isolated methylidene
complex, (η5-C5H5)2TadCH2(Me),32 an 18-electron species, we
expected the decomposition of the less electronically saturated
[P2N2]TadCH2(Me) in the presence of ethylene gas would yield
[P2N2]Ta(C2H4)Me. Instead, the major product is the ethylene
ethyl compound [P2N2]Ta(C2H4)Et, which features a â-agostic
interaction in the solid-state structure despite the fact that this
species is neither cationic nor exceptionally coordinatively
unsaturated. Furthermore, the â-agostic interaction appears to
be in rapid equilibrium with an R-agostic structure in solution.
Herein we report a mechanistic study of the reaction of [P2N2]-
TadCH2(Me) with ethylene and a description of the â-agostic
interaction and associated fluxional processes observed in [P2N2]-
Ta(C2H4)Et.
Photolysis of [P2N2]TaMe3 in the Presence of C2H4. As
previously reported, a solution of [P2N2]TaMe3 gradually
darkens as it is photolyzed; uncharacterized paramagnetic
impurities are formed, likely from the photolysis of the desired
product, [P2N2]TadCH2(Me).31 The photolysis of [P2N2]TaMe3
in the presence of ethylene is visibly different, as the solution
does not darken even with long photolysis times, and a light
orange/red solution results. The initial product of the photolysis
in the presence of ethylene remains the methylidene [P2N2]Tad
1
CH2(Me), and both the H and 31P{1H} NMR spectra show it
to be the major product; however, performing this reaction in a
NMR tube containing an internal concentration standard dem-
onstrates that a considerable amount of paramagnetic, NMR-
inactive side products must also exist. When the photolysis time
is sufficiently long for all of the [P2N2]TaMe3 starting material
to react, only a ∼50% yield of [P2N2]TadCH2(Me) is obtained
(relative to internal ferrocene by 1H NMR spectroscopy),
although this value varies from 40 to 60% depending on the
exact conditions employed.
Results and Discussion
Reaction of [P2N2]TadCH2(Me) with C2H4. Although
ethylene appears to react immediately with the dark paramag-
netic impurities produced in the photolysis of [P2N2]TaMe3 as
evidenced by the difference in the color of the solution after
photolysis, ethylene also reacts with the methylidene complex
[P2N2]TadCH2(Me). This latter reaction is slow, comparable
in rate to the thermal decomposition of [P2N2]TadCH2(Me).
Monitoring the reaction by 1H and 31P{1H} NMR spectroscopies
over the course of two weeks, two new products appeared, and
no [P2N2]TadCH2(Me) remained. The major product was
identified by a combination of 1H, 13C{1H}, and 31P{1H} NMR
spectroscopies as the ethylene-ethyl complex, [P2N2]Ta(C2H4)-
Et (1), and the minor product as the ethylene-methyl derivative,
[P2N2]Ta(C2H4)Me (2). The total concentration of complexes
1 and 2 was equal to that of the original [P2N2]TadCH2(Me)
by comparison to an internal concentration standard, which
indicates that the source of the NMR-active products was [P2N2]-
TadCH2(Me) and not the NMR-inactive products. The dark red
NMR-inactive products resulting from the reaction with ethylene
have vastly different solubilities than complexes 1 and 2 and
could be removed by washing with hexanes. NMR spectroscopic
analyses (1H and 13C{1H}) and GC-MS showed propylene and
1-butene to be the organic products of this reaction. The tanta-
lum-containing reaction products are depicted in Scheme 1.
We have previously reported the preparation of [P2N2]TaMe3
and its hydrogenation to yield a dinuclear Ta(IV) hydride.31,33
The macrocyclic ancillary ligand [P2N2]34 is a flexible frame-
work of two amide and two phosphine donors that has been
shown to stabilize complexes of the early transition metals,35-37
the main group elements,38 and the lanthanides39 in a variety
of geometries.
Photolysis of [P2N2]TaMe3 generates the isolable methylidene
complex, [P2N2]TadCH2(Me),31 which is reasonably stable in
the solid state; however, in solution at room temperature this
(23) Dawoodi, Z.; Green, M. L. H.; Mtetwa, V. S. B.; Prout, K. J. Chem.
Soc., Chem. Commun. 1982, 802.
(24) Dawoodi, Z.; Green, M. L. H.; Mtetwa, V. S. B.; Prout, K.; Schultz,
A. J.; Williams, J. M.; Koetzle, T. F. J. Chem. Soc., Dalton Trans. 1986,
1629.
(25) Haaland, A.; Scherer, W.; Ruud, K.; McGrady, G. S.; Downs, A.
J.; Swang, O. J. Am. Chem. Soc. 1998, 120, 3762.
(26) Scherer, W.; Priermeier, T.; Haaland, A.; Volden, H. V.; McGrady,
G. S.; Downs, A. J.; Boese, R.; Blaser, D. Organometallics 1998, 17, 4406.
(27) McGrady, G. S.; Downs, A. J. Coord. Chem. ReV. 2000, 197, 95.
(28) Cotton, F. A.; Petrukhina, M. A. Inorg. Chem. Commun. 1998, 1,
195.
(29) Munakata, H.; Ebisawa, Y.; Takashima, Y.; Wrinn, M. C.; Scheiner,
A. C.; Newsam, J. M. Catal. Today 1995, 23, 403.
(30) Etienne, M.; Mathieu, R.; Donnadieu, B. J. Am. Chem. Soc. 1997,
119, 3218.
(31) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. Organometallics 1999,
18, 4059.
Alternate Synthesis and Structure of [P2N2]Ta(C2H4)Et
(1). Complexes 1 and 2 proved difficult to separate by fractional
crystallization. Both are soluble in aromatic solvents and have
poor solubility in hexanes. Attempts to isolate complex 1 by
exposing a benzene solution of these two complexes to air with
the intention that the less sterically crowded methyl complex 2
might be more reactive led to the surprising revelation that both
these complexes are air stable. A benzene solution of 1 and 2
showed no signs of decomposition of either complex after 4 h
of exposure to air. Due to the lack of success in separating
complex 1 from complex 2, an alternate route to 1 was sought,
to determine its structure in the solid state by X-ray crystal-
lography.
(32) Schrock, R. R. J. Am. Chem. Soc. 1975, 97, 6577.
(33) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. Organometallics 2000,
19, 3931.
(34) Fryzuk, M. D.; Love, J. B.; Rettig, S. J. J. Chem. Soc., Chem.
Commun. 1996, 2783.
(35) Basch, H.; Musaev, D. G.; Morokuma, K.; Fryzuk, M. D.; Love, J.
B.; Seidel, W. W.; Albinati, A.; Koetzle, T. F.; Klooster, W. T.; Mason, S.
A.; Eckert, J. J. Am. Chem. Soc. 1999, 121, 523.
(36) Fryzuk, M. D.; Love, J. B.; Rettig, S. J.; Young, V. G. Science
1997, 275, 1445.
(37) Fryzuk, M. D.; Love, J. B.; Rettig, S. J. Organometallics 1998, 17,
846.
(38) Fryzuk, M. D.; Giesbrecht, G. R.; Rettig, S. J. Inorg. Chem. 1998,
37, 6928.
(39) Fryzuk, M. D.; Love, J. B.; Rettig, S. J. J. Am. Chem. Soc. 1997,
119, 9071.

1604 J. Am. Chem. Soc., Vol. 123, No. 8, 2001
Fryzuk et al.
Scheme 1
including those containing Ta, is quite small in 1.31,33 The twist
can be quantified by the difference between the P(1)-Ta(1)-
N(1)-Si(4) and P(2)-Ta(1)-N(1)-Si(1) dihedral angles, which
is 19.2(3)°. Compound 1 is a 16-electron complex if one
includes the donation of electrons from the agostic interaction,
but does not consider possible π-donation from the amide lone
pairs. As has been shown before,31,33 without a twist in the
[P2N2] ligand framework only one linear combination of the
two amido-based lone pair p-orbitals has overlap with an
unoccupied metal orbital of appropriate symmetry, and therefore
compound 1 can be considered an 18-electron species.
NMR Spectral Data for [P2N2]Ta(C2H4)Et. Does the ethyl
group maintain the â-agostic interaction in solution? The
1
resonances for the ethyl group in 1 are prominent in the H-
{31P} NMR spectrum, with a quartet for the R-hydrogens of
the ethyl group at δ -1.27 and a triplet for the â-protons at δ
-0.51. Simulation of the phosphorus-31 coupled 1H NMR
spectrum of the tantalum ethyl group provided the coupling
3
constants of the R and â hydrogens to phosphorus of JPH
)
3.1 Hz and JPH ) 4.4 Hz, respectively. The larger coupling to
the â-hydrogens of the ethyl group (compared to the R) and
the low frequency observed for these protons provide strong
evidence that there is an agostic interaction of a â-C-H bond
with the tantalum metal center in solution. In comparison, for
the related 18-electron species (η5-C5H5)2Nb(C2H4)Et,40 in which
there is no agostic interaction, the â-CH3 resonance appears at
δ 1.83; in a less electronically saturated ethylniobium species
with an R-agostic interaction, the â-CH3 group appears at δ
1.15.30 Both of these groups have chemical shifts in the same
region as ethyl groups in organic compounds. The single
chemical environment for the â-hydrogens of the ethyl group
in 1 indicates that there is rapid rotation of the terminal CH3
group, so that the observed 4.4 Hz coupling is an average of
the P-H couplings arising from the single agostic hydrogen
(quasi two-bond coupling) and the two terminal hydrogens (four-
bond coupling). If it is assumed that the coupling of phosphorus-
31 to the more distant terminal â-hydrogens is much smaller,
the actual coupling of phosphorus-31 to the â-agostic hydrogen
can be estimated as at least three times this 4.4 Hz value, or
13.2 Hz; however, this approximation assumes that the â-agostic
interaction is maintained at all times in solution. Likewise, the
chemical shift for the â-CH3 group (δ -0.51) is an average of
the two terminal and one agostic â-hydrogens, although between
180 and 350 K only one environment is observed for the â-CH3
A viable route to a pure 1 might be through the reaction of
ethylene with a Ta(V) hydride, such as [P2N2]TaH3. Attempts
to prepare this trihydride via the hydrogenation of [P2N2]TaMe3
led only to the dinuclear hydride ([P2N2]Ta)2(µ-H)4,33 which
does not react with ethylene. The hydrogenation of [P2N2]TaMe3
in the presence of ethylene resulted in catalytic hydrogenation
of the ethylene to ethane and the previously characterized
monohydride [P2N2]TaMe2H.33 It proved possible to generate
the mononuclear trihydride [P2N2]TaH3(PMe3), 3, by the
hydrogenation of [P2N2]TaMe3 in the presence of excess PMe3.
Complex 1 was then readily synthesized in an overall yield of
95% by the reaction of [P2N2]TaH3(PMe3) with ethylene, as
shown in Scheme 2.
The solid-state molecular structure of 1 is shown in Figure
1, and crystallographic data are given in Table 1. The nine
hydrogen atoms associated with the ethylene and ethyl ligands
were identified in an electron density difference map, and their
locations were refined. Immediately evident from this struc-
ture is the â-agostic interaction. The agostic Ta(1)-C(26)
distance of 2.498(4) Å is only ∼0.25 Å longer than the σ-bound
Ta(1)-C(25) distance of 2.251(3) Å. The ethylene moiety also
has two significantly different Ta-C bond lengths, where the
longer Ta-C bond is with the carbon diagonally opposing the
agostically bound â-carbon of the ethyl group; the Ta(1)-C(27)
distance is 2.251(3) Å, whereas the Ta(1)-C(28) is 2.362(4)
Å. A single hydrogen atom attached to the â-carbon of the ethyl
group is directed toward the metal with a Ta(1)-H(45) distance
of 2.07(4) Å. The back-bonding to the ethylene moiety is
significant enough that it is probably best described as a
metallacyclopropane. The C(25)-C(26) bond length in the ethyl
ligand and the C(27)-C(28) bond length in the ethylene ligand
are identical at 1.449(5) Å. These distances are both slightly
shorter than typical carbon-carbon single bonds, but much
longer than a carbon-carbon double bond. If one considers the
η2-ethylene and η2-ethyl groups to each occupy a single
coordination site, the overall geometry of complex 1 is best
described as distorted octahedral. As is common for complexes
of the [P2N2] ligand, the phosphorus donors are approximately
trans with a P(1)-Ta(1)-P(2) angle of 160.18(3)°, and the nitro-
gen donors are closer to a cis orientation with a N(1)-Ta(1)-
N(2) angle of 97.02(9)°. The “twist” of the amide donors that
has been described for other complexes of the [P2N2] ligand,
1
group in the 500 MHz H NMR spectrum.
On the NMR time-scale complex 1 appears to have a mirror
plane of symmetry such that the two phosphorus donors are
equivalent but the two amides are not. Thus, two signals are
evident in the 1H NMR spectrum for the tantalum-bound
ethylene group. Only one signal was observed for the ethylene
group in the 13C{1H} NMR spectrum, indicating that the C-C
vector of the ethylene group must be arranged perpendicular to
the mirror plane of symmetry. The [P2N2] ligand resonances in
1
the H NMR spectrum are as expected for a complex with
apparent Cs symmetry, with four silyl methyl resonances and
four ligand methylene proton environments. The resonances
associated with the chemically equivalent phenyl substituents
of the phosphine ligands are also consistent with this. Variable-
temperature 1H NMR spectroscopy demonstrates that the
chemical shift of the â-hydrogens of the ethyl group is
significantly temperature dependent, as is one of the resonances
of the tantalum bound ethylene group, presumably due to the
(40) Guggenberger, L. G.; Meakin, P.; Tebbe, F. N. J. Am. Chem. Soc.
1974, 96, 5420.

Reaction of [P2N2]TadCH2(Me) with Ethylene
J. Am. Chem. Soc., Vol. 123, No. 8, 2001 1605
Scheme 2
Table 1. Crystal Data and Structure Refinement for 1
compound
formula
fw
color, habit
cryst size, mm
cryst syst
space group
a, Å
1a
C28H51N2P2Si4Ta
770.96
orange, irregular
0.40 × 0.35 × 0.20
monoclinic
P21/n (No. 14)
12.0328(8)
19.7483(14)
14.5850(2)
90.
92.1381(5)
90.
3463.4(3)
4
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
T, °C
-93
F
calc, g/cm3
1.478
F(000)
1568.00
Mo
34.20
0.5060-1.000
60.1
negligible
30445
8914
radiation
µ, cm-1
transmission factors
2θmax, deg
Figure 1. ORTEP diagram of the solid-state molecular structure of
[P2N2]Ta(C2H4)Et, 1, as determined by X-ray crystallography. Silyl
methyls have been omitted for clarity and only the ipso carbons of the
phenyl rings attached to phosphorus are shown. Selected bond distances
(Å), bond angles (deg), and dihedral angels: C(25)-C(26), 1.449(5);
C(27)-C(28), 1.449(5); Ta(1)-C(25), 2.251(3); Ta(1)-C(26), 2.498-
(4); Ta(1)-C(27), 2.251(3); Ta(1)-C(28), 2.362(4); Ta(1)-H(45), 2.07-
(4); Ta(1)-P(1), 2.5701(8); Ta(1)-P(2), 2.5525(7); Ta(1)-N(1),
2.153(2); Ta(1)-N(2), 2.182(2); C(25)‚‚‚C(28), 3.038(5); C(26)‚‚‚C(27),
3.121(5); P(1)-Ta(1)-P(2), 160.18(3); N(1)-Ta(1)-N(2), 97.02(9);
P(1)-Ta(1)-N(1)-Si(4), -179.87(14); P(2)-Ta(1)-N(1)-Si(1), 160.93-
(13); P(1)-Ta(1)-N(2)-Si(3), 163.12(13); P(2)-Ta(1)-N(2)-Si(2),
-178.12(14).
crystal decay, %
total no. of reflns
no. of unique reflcns
Rmerge
0.029
no. with I g nσ(I)
no. of variables
R
Rw
gof
6684
370
0.044a
0.043a
1.50
0.002
max ∆/σ
residual density e/Å3
2.09, -1.34 (both near Ta)
a Rigaku/ADSC CCD diffractometer,R ) ∑||Fo | - |Fc2||/∑|Fo |; Rw
2
2
) (∑w(|Fo | - |Fc2|)2/∑w|Fo| )1/2
.
2
4
hydrogens directed toward the ethyl ligand. The 1H NMR signal
of the â-hydrogens of the ethyl group shifts to lower frequency
on cooling. These data indicate that the proposed â-agostic
interaction in 1 is fluxional, with the entropically disfavored
â-agostic η2-ethyl group being increasingly favored at lower
temperature. This fluxional process, shown in Figure 2, accounts
for the observed Cs symmetry of 1 on the NMR time scale. An
in-place rotation of the ethyl group would not account for the
apparent Cs symmetry of species 1 in solution.41
The intermediate species in Figure 2 is depicted as having
an R-agostic interaction, despite the fact that so far no data have
been provided as yet to support such an R-agostic-â-agostic
equilibrium. Evidence for this process in the solution structure
of 1 was provided by studying the effect of partial deuteration
of the ethyl group.
low-frequency chemical shift of the â-protons and a large
coupling of these protons to the phosphorus nuclei of the [P2N2]
ligand. Evidence for an R-agostic interaction in equilibrium with
the â-agostic interaction is more difficult to obtain; the chemical
shift of the R-protons is anticipated to be affected by the
proximity of these protons to the metal center regardless of
whether an R-agostic interaction is present, and likewise a three-
bond coupling to phosphorus-31 is not unexpected. A valuable
technique for examining agostic interactions is isotopic pertur-
bation of resonance (IPR).42,43 For species with â-agostic
interactions, this technique involves partially deuterating the
â-CH3 group, so that the â-CH2D and â-CHD2 isomers are also
present in solution. The difference in the zero-point energies
for terminal C-D bonds versus agostic C-D bonds compared
to terminal C-H bonds versus agostic C-H bonds causes the
protons to accumulate in the â-agostic position, whereas the
deuterons will accumulate in the terminal sites. Therefore, if
Isotopic Perturbation of Resonance. It has been noted that
between 180 and 350 K the â-CH3 group of the agostic ethyl
1
moiety in 1 shows only a single resonance in the H NMR
spectrum. Thus, as already mentioned the only evidence for a
(42) Calvert, R. B.; Shapley, J. R. J. Am. Chem. Soc. 1978, 100, 7726.
(43) Green, M. L. H.; Hughes, A. K.; Popham, N. A.; Stephens, A. H.
H.; Wong, L.-L. J. Chem. Soc., Dalton Trans. 1992, 3077.
â-agostic interaction in the solution structure of 1 is an unusually
(41) Tempel, D. J.; Brookhart, M. Organometallics 1998, 17, 2290.

1606 J. Am. Chem. Soc., Vol. 123, No. 8, 2001
Fryzuk et al.
Figure 2. Depiction of the fluxional nature of the â-agostic interaction that results in the Cs symmetry of complex 1 on the NMR time scale. The
intermediate species shown here is drawn with an R-agostic interaction.
Figure 4. The ethyl region of the 1H NMR (500 MHz, 255 K) spectrum
of [P2N2]Ta(C2H4)Et in which the ethyl group is ∼75% deuterated.
1
isotopomers, for example, have slightly different H chemical
shifts for their R-CHD resonances. The effect of deuteration of
the bound ethylene moiety has a negligible affect on its apparent
1
chemical shift in the H NMR spectrum, as was expected.
To understand the effect of deuteration of the R-CH2 group
1
on the chemical shift of the â-CH3 resonance in the H NMR
spectrum of 1, a more highly deuterated sample was prepared
which consisted of ∼75% deuterium in the ethyl group. At room
temperature, the many signals for the â-C-H group overlap;
however, upon cooling the sample to 255 K these signals
separate sufficiently to identify at least six distinct resonances.
By taking into account the degree of deuteration, it was possible
to determine the statistical likelihood of all the possible
isotopomers of the ethyl group, and this assisted with assigning
the resonances. The 1H spectrum of the ethyl group at 255 K is
shown in Figure 4. For the â-CH group, the lowest frequency
resonance at δ -1.16 is assigned to the CD2CHD2 isotopomer,
the resonance at δ -1.10 to the CHDCHD2 isotopomer, and
the higher frequency resonances to CD2CHD2, CHDCHD2,
CD2CH3, and CHDCH3, respectively. The resonances of the
â-C-H protons in isotopomers containing two protons in the
R-C position were too low in intensity to assign. The difference
in the chemical shifts of the â-C-H protons compared to the
spectrum shown in Figure 3 is due to the temperature depen-
dence of the chemical shifts of these protons, as noted
previously. That the lowest frequency corresponds to the
most deuterated sites at the â-C is as expected, because this
causes the protons to preferentially accumulate in the agostic
position.
Figure 3. The ethyl region of the 1H NMR (500 MHz, 295 K) spectrum
of [P2N2]Ta(C2H4)Et, 1, before (top) and after (bottom) C2D4 was added.
an agostic interaction is present, this experiment should result
in an IPR that produces a noticeable chemical shift separation
between the isotopomers present.
The preparation of a variety of deuterated isotopomers of 1
proved to be facile. The addition of d4-ethylene to a solution of
1 provided a mixture of isotopomers. Attempts to monitor this
exchange by 1H NMR spectroscopy were unsuccessful, because
considerable exchange had already occurred by the time the
solution could be transferred to the NMR probe. It is not clear
whether the C2D4 exchanges with the ethyl group or the bound
ethylene or both. Regardless, a mechanism that exchanges proton
environments from the ethyl to the ethylene group exists and
will be discussed later.
Figure 3 shows the ethyl region of the 1H NMR spectrum of
complex 1 before and after a small amount of C2D4 has been
added. After adding C2D4, the resonances for the CH2CH3
isotopomer are still the most intense, with the â-CH3 group at
δ -0.52. The â-CH2D resonance occurs at δ -0.64 ppm, a
shift of -0.12 ppm from the undeuterated resonance. The third
isotopomer, â-CHD2, appears at two apparent resonances, with
an average chemical shift of δ -0.75; the appearance of two
resonances for this signal was unexpected, and it appears to
imply that the degree of deuteration of the adjacent methylene
group also affects the 1H chemical shift of the â-CHD2
resonance. The appearance of a large IPR for the R-CH2 group
was also unanticipated. The resonance shifts from δ -1.28 for
the R-CH2 isotopomer to δ -1.52 for the R-CHD isotopomer.
The resonance at -1.52 is also comprised of more than one
actual chemical shift, so that the CHDCH3 and CHDCH2D
As mentioned above, the appearance of extra signals in the
â-C-H region for isotopomers containing different degrees of
deuteration in the R-C position was not expected. The effect of
increased deuteration at the R-C position is a decrease in the
frequency of the â-C-H resonances. If only the â-agostic
interaction was present in the structure of 1 in solution, then
only three â-C-H resonances should be observed, and no
significant effect should be observed when the R-C is deuterated.
However, if an R-agostic interaction is in equilibrium with the

Reaction of [P2N2]TadCH2(Me) with Ethylene
J. Am. Chem. Soc., Vol. 123, No. 8, 2001 1607
â-agostic interaction, as shown in Figure 2, then the observation
of six â-C-H resonances and the large IPR for the R-C-H
protons can be rationalized. The origin of this effect is similar
to that noted previously, where partial deuteration of the â-C-H
group causes the protons to accumulate in the agostic position,
due to the slightly higher energy of the deuteron in the agostic
position. The effect of increased deuteration of the R-C would
provide a slight destabilization of the R-agostic interaction versus
the â-agostic interaction. If the energy difference between the
â-agostic interaction and the R-agostic interaction is very small,
this small destabilization should favor the â-agostic interaction
sufficiently to cause the slight shift of the â-C-H resonance to
lower frequency, as is observed. The rapid rate of the process
shown in Figure 2 is consistent with a small energy difference
between the â-agostic and R-agostic ethyl group for 1 in
solution.
Unlike the â-C-H resonances, the R-C-H resonances are
not greatly affected by the degree of deuteration at the â-C.
This is consistent with the observation that the chemical shift
of the R-C protons is not greatly temperature dependent, unlike
the â-C protons, so that a shift in the equilibrium between the
R-agostic and â-agostic structure does not result in a significant
change in the chemical shift of the R-C-H resonances. The
relatively small temperature dependence of the R-C proton
resonance eliminates the possibility that the large IPR observed
for 1 is due simply to an equilibrium between a â-agostic and
a non-agostic structure. For an IPR to be observed both a
difference in chemical shift and a difference in C-H bond length
is necessary, and so the significant IPR observed necessitates a
structure in which the two R-C-H environments and C-H bond
lengths are vastly different in the species lacking a â-agostic
interaction. The most probable structure that fits these require-
ments is one containing an R-agostic interaction, as shown in
the center of Figure 2.44
common, it is possible that many complexes undergo equilibria
of this type; however, such processes may be too fast to be
observed on the NMR time scale. In such cases, thorough
labeling studies could provide additional information about the
solution structures of these complexes. To the best of our
knowledge complex 1 provides the first example where an
equilibrium between R-agostic and â-agostic interactions is
identified by a study of the isotopic perturbation of resonance
of both the R and â positions. This could be used to study
isolable polymerization catalyst resting states. The absence of
other reports of equilibria of this type may be due to the
exclusive labeling of the â-C of the ethyl group in other
studies,16,23,24 which prevented the detection of such an equi-
librium in solution. On the other hand, for this effect to be
observed the R-agostic structure and â-agostic structures must
be similar in energy, and this may be uncommon. It has been
suggested that the competition between R-agostic and â-agostic
interactions is likely largely governed by steric factors, where
increasing steric bulk disfavors â-agostic interactions.30
NMR Data for Carbon-13 Labeled [P2N2]Ta(13C2H4)13Et.
The synthesis of an isotopomer of 1 with the ethyl group carbon-
13 labeled was performed to determine the effect of the agostic
1
interactions in species 1 on the JCH values. The reaction of
[P2N2]TadCH2(Me) with 13C2H4 yields the species [P2N2]Ta-
(13C2H4)13Et, where the four carbons of the ethyl and ethylene
groups of complex 1 are 13C labeled. The 13C{1H} NMR
1
spectrum of this reaction mixture illustrates a JCC coupling
constant in the ethyl moiety of 30.0 Hz, as is appropriate for a
1
single bond. From the H NMR spectrum of this 13C-labeled
species, it is possible to determine the 1JCH coupling constants
of 126.5 and 123.1 Hz for the R-carbon and â-carbon of the
ethyl group, respectively. Both are within the values expected
for sp3 hybridized carbons. In some cases a large 1JCH of ∼150
Hz to the R-carbon has been used as evidence for an agostic
interaction;16,20,24 however, there are other complexes in which
â-agostic ethyl groups are implicated where a value similar to
The presence of an R-agostic interaction in a tantalum ethyl
species such as 1 has precedence; R-agostic interactions are
implicated in the formation of tantalum alkylidene complexes
from sterically crowded tantalum alkyls,45 and both R and â
hydrogen abstractions have been observed for tantalum alkyls,
which presumably proceed through agostic interactions.46,47
Additionally, examples of niobium ethyl complexes with
R-agostic interactions but no evidence of â-agostic interactions
have been reported,30,48 and the zwitterionic tantalocene deriva-
tive (η5-C5H5)2Ta(dCH2B(C6F5)3)(Me) has a strong R-agostic
interaction. To our knowledge, only one example exists where
both R-agostic and â-agostic interactions were identified to be
in equilibrium in the 1H NMR spectrum;49 two distinct species
corresponding to an R-agostic and a â-agostic niobium bound
1
that found here was observed.21 The JCH coupling constants
1
for the ethylene fragment are somewhat obscured in the H
NMR spectrum, as these signals are in the same region as the
ligand methylene groups. From the 13C NMR data these
coupling constants were ∼145 Hz. These are larger than those
observed for the ethyl group, but smaller than the ∼156 Hz
coupling constant expected for ethylene complexes with minimal
back-bonding.
The variable-temperature 13C NMR spectrum of [P2N2]Ta-
(13C2H4)13Et was also investigated to complement the variable-
temperature 1H NMR data obtained for 1. The resonance of the
R-C shifts from δ 75.9 at 350 K to δ 53.5 at 180 K, and the
corresponding 1JCH value increases from 123.4 Hz at 350 K to
133.6 Hz at 233 K. These data are consistent with an increased
contribution from an R-agostic structure versus a â-agostic
structure at higher temperatures.15,20 The resonance of the â-C
is less strongly affected and shifts from δ 7.3 at 350 K to δ 2.0
at 180 K and the corresponding 1JCH value changes from 123.8
Hz at 350 K to 122.8 Hz at 233 K. In comparison, the resonance
of the tantalum bound ethylene group is δ 45.5 at 350 K and δ
43.3 at 180 K, which indicates that this resonance is not greatly
affected by temperature.
1
isopropyl group were observed by H NMR spectroscopy, a
result that is unique even in that study. For the early transition
metals, where both R-agostic and â-agostic interactions are quite
(44) A fast equilibrium between an ethylene ethyl and an ethyl ethylidene
structure, formed from an R-hydrogen abstraction from the ethyl group and
insertion of this hydride into the ethylene moiety, is not a viable mechanism.
Such a process would rapidly exchange the chemical environment at the
R-position of the ethyl group in the ethylene ethyl structure with the chemical
environments of the ethylene moiety, and this is not observed in the 1H
NMR spectrum. An EXSY spectrum (vide infra) further demonstrates that
this process does not occur even on a time scale too slow to be observed
1
in the H NMR spectrum.
(45) Schrock, R. R. Acc. Chem. Res. 1979, 12, 98.
(46) Freundlich, J. S.; Schrock, R. R.; Davis, W. M. J. Am. Chem. Soc.
1996, 118, 3643.
(47) Freundlich, J. S.; Schrock, R. R.; Davis, W. M. Organometallics
1996, 15, 2777.
(48) Hierso, J. C.; Etienne, M. Eur. J. Inorg. Chem. 2000, 5, 839.
(49) Jaffart, J.; Mathieu, R.; Etienne, M.; McGrady, J. E.; Eisenstein,
O.; Maseras, F. Chem. Commun. 1998, 2011.
NMR Data for [P2N2]Ta(C2H4)Me (2). The ethylene methyl
complex, 2, is produced as a minor product in the reaction of
1
[P2N2]TadCH2(Me) with ethylene. The H NMR spectrum of
complex 2 contains a low-frequency resonance for the tantalum-
bound methyl group at δ -1.49 coupled to phosphorus (3JPH
)
3.1 Hz). In complex 2 four silyl methyl peaks are present in

1608 J. Am. Chem. Soc., Vol. 123, No. 8, 2001
Fryzuk et al.
Figure 6. Two possible reaction mechanisms for the formation of
[P2N2]Ta(C2H4)Et, 1, from the reaction of ethylene with [P2N2]Tad
CH2(Me). The starred carbon atoms illustrate where carbon-13 labels
would appear in the products if the reaction was performed with 13C2H4.
Figure 5. Two possible mechanisms for the formation of [P2N2]Ta-
(C2H4)Me, 2. The starred carbon atoms illustrate where carbon-13 labels
would appear in the products if the reaction was performed with 13C2H4.
Inspection of the 13C{1H} NMR spectrum of the products
of the reaction of [P2N2]TadCH2(Me) with 13C2H4 reveals
that indeed two isotopomers of propylene are produced,
13CH2d13CHCH3 and H2Cd13CH13CH3, clearly supportive of
mechanism B. The tantalum-bound ethylene moiety is fully
labeled, and the tantalum methyl group remains unlabeled. The
metallocyclobutane product once formed appears to decompose
rapidly, as it does not accumulate in solution, and no productive
metathesis, to form [P2N2]Tad13CH2(Me) and 13CH2dCH2, is
observed.
The mechanism of formation of the ethyl complex 2 could
involve a similar mechanism to that proposed for complex 1.
Initial metallocyclobutane formation could be followed by
â-hydrogen elimination; however, if instead of reinserting into
the double bond to form a propylene complex ethylene inserts
into the hydride bond, [P2N2]Ta(CH2CHdCH2)MeEt would
form. If insertion of the double bond into the tantalum methyl
bond then occurs a butene complex would be formed, which
could exchange with ethylene to form the ethyl complex 1 and
1 equiv of butene. This mechanism is labeled mechanism C in
Figure 6.
A second, simpler mechanism is also shown in Figure 6,
mechanism D. In this case the methylidene inserts into the
tantalum-methyl bond, forming an ethyl complex, which is then
trapped by ethylene. The migratory insertion of an alkylidene
into a cis tantalum alkyl linkage has been demonstrated to occur
with cationic tungsten complexes,51-53 late transition metal
carbene complexes,54,55 and a niobium alkylidene bearing
electron-withdrawing substituents,56 presumably because these
factors render the alkylidene more electrophilic. However,
evidence suggests that this insertion is not restricted to elec-
trophilic alkylidenes, as the insertion of an alkylidene into a
tantalum alkyl has been proposed as a reaction step in the
1
the H NMR spectrum, indicating that, as for species 1, this
complex appears to have Cs symmetry. Unfortunately, other
peaks in the same region, particularly the [P2N2] ligand
methylene signals, obscure the 1H NMR signals for the ethylene
protons. An R-agostic interaction may be present in this species;
however, we have not probed this possibility by IPR.
For complex 2, a single isotopomer [P2N2]Ta(13C2H4)Me
results from the reaction of [P2N2]TadCH2(Me) with 13C2H4.
The 13C{1H} NMR spectrum contains a signal for the tantalum-
bound ethylene group at δ 49.1, near that observed for the
ethylene group in complex 1. The tantalum methyl group of 2
is not 13C labeled, and appears as a triplet at δ 51.4.
Mechanism of the Reaction of Ethylene with [P2N2]Tad
CH2(Me). Previous studies on the thermal decomposition of
the 18-electron complex (η5-C5H5)2TadCH2(Me) demonstrated
that it decomposed via dimerization and coupling of the two
bridging methylidene moieties.32,50 The species (η5-C5H5)2Ta-
(C2H4)Me resulted, where the ethylene unit is derived from the
coupled methylidene units. The remaining “(η5-C5H5)2TaMe”
moiety could be trapped using ethylene. This same mechanism
could be responsible for the formation of methyl complex 2,
and is illustrated in Figure 5 as mechanism A. The starred carbon
atoms indicate where the 13C labels would be expected if the
reaction was performed with 13C2H4.
An alternative mechanism to the formation of complex 2
would involve initial metallocyclobutane formation upon ad-
dition of ethylene to the tantalum methylidene, and subsequent
decomposition of this intermediate by â-hydrogen elimination.
Reinsertion of the resulting olefin into the hydride bond in the
opposite manner would generate a propylene complex, [P2N2]-
Ta(CH2dCHMe)Me, which could then further react with
ethylene, eliminating propylene and generating complex 2, as
shown in Figure 5 as mechanism B. As for mechanism A, the
starred carbon atoms indicate where the 13C labels would be
expected in the products if the reaction was performed with
13C2H4.
(51) Hayes, J. C.; Pearson, J. D. N.; Cooper, N. J. J. Am. Chem. Soc.
1981, 106, 3026.
(52) Hayes, J. C.; Cooper, N. J. J. Am. Chem. Soc. 1982, 104, 5570.
(53) Jernakoff, P.; Cooper, N. J. J. Am. Chem. Soc. 1984, 106, 3026.
(54) Thorn, D. L.; Tulip, T. H. J. Am. Chem. Soc. 1981, 103, 5984.
(55) Kleitzein, H.; Werner, H.; Serhadli, P.; Ziegler, M. L. Angew. Chem.,
Int. Ed. Engl. 1983, 22, 46.
(50) Schrock, R. R.; Sharp, P. R. J. Am. Chem. Soc. 1978, 100, 2389.
(56) Threlkel, R. S.; Bercaw, J. E. J. Am. Chem. Soc. 1981, 103, 2650.

Reaction of [P2N2]TadCH2(Me) with Ethylene
J. Am. Chem. Soc., Vol. 123, No. 8, 2001 1609
Figure 7. Depiction of the mechanism believed to exchange the proton environments of the ethyl and ethylene ligands of species 1.
decomposition of alkylidenes only slightly more sterically
crowded than (η5-C5H5)2TadCH2(Me), such as the ethylidene
complex (η5-C5H5)2TadCH2Me(Me).57
tantalum-ethyl and tantalum-ethylene groups when 1 is reacted
with C2D4 strongly suggests the existence of a fluxional process
that interconverts the tantalum ethyl and tantalum ethylene
groups. Such a process is known to occur in the cationic late
transition metal complex [(η5-C5H5)Co(η2-C2H4)(η2-Et)]+, where
the intermediate is the bis(ethylene) hydride [(η5-C5Me5)Co-
(η2-C2H4)2H]+.58,59 Despite this precedent, this mechanism
seems unlikely in complex 1. Although the metal center in
species 1 could formally be considered Ta(III), with a d2
configuration, extensive back-bonding to the tantalum ethylene
moiety suggests the complex is more accurately described as a
Ta(V) metallacyclopropane, with no remaining metal-based
electrons, and no available higher oxidation states. Because it
lacks the electrons necessary to allow back-bonding to two
ethylene moieties to form [P2N2]Ta(C2H4)2H, complex 1 would
be more likely to eliminate ethylene to form [P2N2]Ta(C2H4)H.
Although solutions of 1 turn green over the course of several
months in the absence of ethylene due to a trace amount of a
new complex, the species [P2N2]Ta(C2H4)H could not be
detected by 1H NMR, and the slow rate at which this
decomposition reaction occurs prohibits it from being of
importance in the more rapid exchange of proton environments
described here. Interestingly, many ethylene ethyl compounds
have been prepared, and to our knowledge (with the exception
of the previously mentioned cobalt example) none exhibits
â-agostic interactions or fluxional processes that exchange ethyl
and ethylene proton environments.60-63
A labeling study here should reveal if mechanism C or
mechanism D occurs in the formation of ethyl complex 1. In
Figure 6, the starred carbon atoms indicate where the labeled
atoms are anticipated to be if the reaction of [P2N2]TadCH2-
(Me) is performed with 13C2H4. Although 1H, 13C{1H}, and 31P-
{1H} NMR spectroscopies identify the tantalum product as the
completely labeled species [P2N2]Ta(13C2H4)13CH213CH3, as is
predicted for mechanism C, the 1-butene observed in the product
mixture is fully labeled 13CH2d13CH13CH213CH3, rather than
the partially labeled 1-butene that is anticipated from mechanism
C. The 1-butene observed is therefore simply produced from
the dimerization of ethylene. The similar amount of 1-butene
and 1 produced in the reaction of [P2N2]TaMe3 with ethylene
therefore must be coincidental. Examples of tantalum complexes
that catalyze the dimerization ethylene and also produce higher
oligomers are known; however, neither complex 1 nor 2
catalyzes these reactions, so intermediates or trace impurities
must be involved in 1-butene production. Mechanism D is
therefore most consistent with the data in the formation of 1.
The formation of the fully labeled product can be rationalized
by the exchange of the bound ethyl group with the carbon-13
labeled ethylene. This exchange has been noted before in the
reaction of C2D4 with [P2N2]Ta(C2H4)Et to produce partially
deuterated isotopomers. The reaction of [P2N2]TadCH2(Me)
with C2D4 in a sealed NMR tube results in the appearance of a
Examining the crystal structure of 1, it is notable that the
bond lengths in the tantalum ethyl ligand and the tantalum
ethylene ligand are remarkably similar. The C-C bond lengths
are identical, and the Ta-CR distance is identical to one of the
Ta-C bond lengths of the bound ethylene. The Ta-Câ
interaction is only 0.136 Å longer than the longer Ta-C bond
lengths in to the bound ethylene. Considering the structural
similarities between the ethylene and ethyl ligands in species
1, a valid mechanism for the exchange of proton environments
in complex 1 could simply involve direct transfer of a hydrogen
atom from the ethyl group to the ethylene group, as shown in
Figure 7. The â-agostic interaction will weaken the C-H bond
that is interacting with the metal center in the ground state, and
in the postulated intermediate the hydrogen atom that is
1
resonance in the H NMR spectrum in the region anticipated
for ethylene. This resonance is slightly broadened, presumably
because isotopomers such as C2D3H are the end products. This
provides further evidence that despite forming the fully labeled
product, mechanism D is the likely mechanism for the formation
of 1.
The ability of complex 1 to react with labeled ethylene
such as 13C2H4 to exchange both the ethylene and ethyl groups
can be explained by two possible mechanisms. The most ob-
vious mechanism would involve â-elimination of ethylene to
form an ethylene hydride, [P2N2]Ta(C2H4)H, which could then
react with 13C2H4 to generate the labeled product. The stability
of 1 in air and the observation that ethylene is not lost from
solutions of 1 over several days indicates that the mechanism
is probably not this straightforward. The bound ethylene moiety
could also be involved in an exchange mechanism by which
the insertion of ethylene into the bound ethylene forms a
metallocyclopentane, which can then eliminate ethylene and
result in exchange. This olefin exchange mechanism is believed
to occur in the mechanism of the formation of 2, where
propylene is displaced by ethylene, and therefore seems a likely
mechanism for the exchange of free ethylene with bound
ethylene in 1.
(57) Sharp, P. R.; Schrock, R. R. J. Organomet. Chem. 1979, 171, 43.
(58) Brookhart, M.; Lincoln, D. M.; Bennett, M. A.; Pelling, S. J. Am.
Chem. Soc. 1990, 112, 2691.
(59) Brookhart, M.; Green, M. L. H.; Pardy, R. B. A. J. Chem. Soc.,
Chem. Commun. 1983, 691.
(60) Basickes, N.; Hutson, A. C.; Sen, A.; Yap, G. P. A.; Rheingold, A.
L. Organometallics 1996, 15, 4116.
(61) Spencer, M. D.; Morse, P. M.; Wilson, S. R.; Girolami, G. S. J.
Am. Chem. Soc. 1993, 115, 2057.
(62) Fe´rnandez, F. J.; Go´mez-Sal, P.; Manzanero, A.; Royo, P.; Jacobsen,
H.; Berke, H. Organometallics 1997, 16, 1553.
(63) Barry, J. T.; Chacon, S. T.; Chisholm, M. H.; Huffman, J. C.; Streib,
W. E. J. Am. Chem. Soc. 1995, 117, 1974.
Exchange of Proton Environments in Ethyl Complex 1.
The rapid rate at which deuterium is scrambled into both the

1610 J. Am. Chem. Soc., Vol. 123, No. 8, 2001
Fryzuk et al.
ligand. However, no polymerization activity is noted in this
complex due to this considerable back-bonding to the ethylene
ligand. The elongation of the C-C bond of the ethylene unit
from this back-bonding interaction renders the geometry of the
bound ethylene quite similar to that of the â-agostic ethyl ligand.
This similarity in structure undoubtedly assists in an exchange
process whereby the â-agostic hydrogen atom from the ethyl
group is transferred to the ethylene moiety, exchanging these
groups without the intermediacy of an ethylene hydride species,
a process that is believed to be important in olefin polymeri-
zation chain termination.66
Experimental Section
Unless otherwise stated all manipulations were performed under an
atmosphere of dry oxygen-free dinitrogen by means of standard Schlenk
or glovebox techniques (Vacuum Atmospheres HE-553-2 glovebox
equipped with a MO-40-2H purification system and a -40 °C freezer).
Hexanes were predried by refluxing over CaH2, and then distilled under
argon from sodium benzophenone ketyl with tetraglyme added to
solubilize the ketyl. Anhydrous diethyl ether was stored over sieves
and distilled from sodium benzophenone ketyl under argon. Toluene
was predried by refluxing over CaH2 and then distilled from sodium
under argon. Nitrogen was dried and deoxygenated by passing the gases
through a column containing molecular sieves and MnO. Deuterated
benzene and toluene were dried by refluxing with molten potassium
metal, and molten sodium metal, respectively, in a sealed vessel under
reduced pressure, then trap-to-trap distilled, and freeze-pump-thaw
Figure 8. A portion of the EXSY spectrum (τ ) 0.4 s) of [P2N2]Ta-
(C2H4)Et, illustrating four cross-signals between the ethyl and ethylene
ligands, and cross-signals between pairs of silyl-methyl environments.
transferred has bonding interactions with two carbon atoms and
the tantalum center.
Unfortunately, the rate at which this exchange reaction occurs
is too slow to be monitored by variable-temperature 1H NMR.
In an attempt to verify that this process does occur even in the
absence of a labeled ethylene such as C2D4, a phase-sensitive
EXSY (exchange spectroscopy)64,65 spectrum of complex 1 was
obtained at 300 K, and a portion of this spectrum is shown in
Figure 8. This 2-D spectrum displays four cross-signals of the
correct sign and intensity to be due to chemical exchange
between both resonances of the ethyl unit and both resonances
of the tantalum ethylene, thus indicating that as implied from
the mechanism shown in Figure 7, exchange between both ethyl
group environments and both ethylene ligand environments is
possible. Another possible mechanism,20 involving transfer of
an R-agostic hydrogen atom from the ethyl group to generate
the intermediate [P2N2]TadCHMe(Et), would not allow for
exchange of all the observed chemical environments. Further-
more, cross-signals occur between pairs of the ligand silyl
methyl environments, as would be expected if a fluxional
process occurred which exchanged the location of the ethyl and
ethylene ligands in complex 1. Cross-signals between pairs of
ligand methylene environments should also be observed;
however, it appears that the pairs of signals are too close together
(δ 1.34, 1.41; 1.70, 1.70) for these cross-signals to be distin-
guishable from the large diagonal signals.
1
1
degassed three times. Unless otherwise stated, H, 31P, H{31P}, 13C-
{1H}, 13C, and variable-temperature NMR spectra were recorded on a
1
Bruker AMX-500 Instrument operating at 500.1 MHz for H spectra.
1H NMR spectra were referenced to internal C6D5H (7.15 ppm),
CDHCl2 (5.32 ppm), and C7D7H (2.09 ppm), 31P{1H} NMR spectra to
external P(OMe)3 (141.0 ppm with respect to 85% H3PO4 at 0.0 ppm),
and 13C spectra to 13CC5D6 (128.4 ppm) and 13CD2Cl2 (54.0 ppm).
Elemental analyses were performed by Mr. P. Borda of this department.
Photolysis of [P2N2]TaMe3 under C2H4. A sample of [P2N2]TaMe3
(1.05 g, 1.38 mmol) in 100 mL of hexanes was sealed in a glass vessel
equipped with a Teflon valve under an atmosphere of ethylene. The
vessel was exposed to a Vitalux sunlamp for 30 min and then stirred
for two weeks at room temperature. The ethylene was then removed
under vacuum, and an orange solid (0.55 g) crystallized that had poor
solubility in hexanes but was soluble in aromatic solvents. The solid
was identified as a mixture of [P2N2]Ta(C2H4)Et (1) and [P2N2]Ta-
(C2H4)Me (2). Exact ratios and yields of these products depended
considerably on the conditions used, such as photolysis duration and
intensity, but [P2N2]Ta(C2H4)Et was always the major product. A
product ratio of compounds 1 to 2 as high as 10:1 and as low as 2:1
was observed. Both compounds 1 and 2 are air stable but moisture
sensitive. 1H NMR of selected peaks (C6D6, 295 K, 500 MHz): δ -1.49
3
3
(t, JPH ) 3.1 Hz, [P2N2]Ta(C2H4)CH3), -1.28 (A2M3X2, JHH ) 7.8
Hz, 3JHP ) 3.1 Hz, [P2N2]Ta(C2H4)CH2CH3), -0.53 (A3M2X2, 3JHH
)
7.8 Hz, JHP ) 4.4 Hz, [P2N2]Ta(C2H4)CH2CH3), 0.13, 0.23, 0.37, and
0.51 (s, [P2N2]Ta(C2H4)Et ligand SiCH3), -0.01, 0.30, 0.46, and 0.58
(s, [P2N2]Ta(C2H4)Me ligand SiCH3).
Conclusions
The reaction of [P2N2]TadCH2(Me) with ethylene produces
the complex [P2N2]Ta(C2H4)Et (1) as the main product along
with [P2N2]Ta(C2H4)Me (2). Extensive evidence exists for a
â-agostic interaction in 1, and this same interaction was also
present in the solid-state X-ray structure. By careful examination
Photolysis of [P2N2]TaMe3 under 13C2H4. A sample of [P2N2]TaMe3
(20 mg, 0.026 mmol) in 1 mL of C6D6 was sealed in a NMR tube
under 1 atm of 13C2H4. The tube was exposed to a Vitalux sunlamp for
30 min and then stirred for two weeks at room temperature. The two
NMR-active tantalum-containing products were identified as [P2N2]-
Ta(13C2H4)13Et and [P2N2]Ta(13C2H4)Me and were present in ap-
proximately a 2:1 ratio. The 13C NMR spectrum identified the 1-butene
isotopomer as the fully 13C-labeled 13CH2d13CH213CH213CH3 and the
two propylene isotopomers as CH2d13CH213CH3 and 13CH2d13CH2-
1
of the H NMR spectrum of a mixture of partially deuterated
isotopomers of 1, evidence was collected that supports an
equilibrium between the â-agostic structure and an R-agostic
interaction of the ethyl ligand in solution. Such interactions are
of great importance in early transition metal olefin polymeri-
zation catalysts, and this complex is particularly interesting
because it contains an ethyl ligand arranged cis to an ethylene
1
CH3. H NMR, selected peaks (C6D6, 299 K, 500 MHz): δ -1.51 (t,
1
[P2N2]Ta(13C2H4)CH3), -1.29 (m, JCH ) 126.8 Hz, [P2N2]Ta13CH2-
13CH3), -0.52 (m, 1JCH ) 123.2 Hz, [P2N2]TaCH2CH3). 31P(C6D6, 299
K): δ 21.3 (vt, 2JCP ) 4.2 Hz, [P2N2]Ta(13C2H4)Me), 24.0 (m, [P2N2]-
Ta(13C2H4)13Et). 13C{1H} NMR (C6D6, 299 K, 129.76 MHz): δ 6.3
(64) Jeener, J.; Meier, B. H.; Bachmann, P.; Ernst, R. R. J. Chem. Phys.
1979, 71, 4546.
(65) Perrin, C. L.; Dwyer, T. J. Chem. ReV. 1990, 90, 935.
(66) Margl, P.; Deng, L.; Ziegler, T. J. Am. Chem. Soc. 1999, 121, 154.

Reaction of [P2N2]TadCH2(Me) with Ethylene
J. Am. Chem. Soc., Vol. 123, No. 8, 2001 1611
(dt, 1JCC ) 30.0 Hz, 2JPC ) 4.5 Hz, [P2N2]Ta(13C2H4)13CH213CH3), 13.6
(ddd, 1JCC ) 34.5 Hz, 2JCC ) 2.2, 3JCC ) 4.2, 13CH2d13CH213CH213CH3),
19.7 (d, 2JCC ) 42.2 Hz, CH2d13CH213CH3), 27.3 (dd, 1JCC ) 34.3 Hz,
ligand PC). Anal. Calcd for C27H54N2P3Si4Ta: C, 40.90; H, 6.86; N,
3.54. Found: C, 41.03; H, 6.80; N, 3.39.
Synthesis of [P2N2]Ta(C2H4)Et (1). A stirred red solution of [P2N2]-
TaH3(PMe3) (1.3 g, 1.64 mmol) in 20 mL of hexanes was sealed in a
glass vessel equipped with a Teflon valve under an atmosphere of
ethylene. After 5 min the gases were evacuated and the vessel was
again charged with ethylene. This was repeated and the light orange
solution was allowed to stir for 6 h. The gases were evacuated again,
and the remaining liquid was transferred to an Erlenmeyer flask in a
glovebox. Over 30 min a microcrystalline pale orange solid precipitated
from solution. The solid was collected by filtration and dried in vacuo,
producing [P2N2]Ta(C2H4)Et in 95% yield. X-ray quality single crystals
were obtained by slow evaporation of a benzene and hexamethyldisi-
loxane solution. The species [P2N2]Ta(C2H4)Et is soluble in aromatic
2
1JCC ) 41.5 Hz, 13CH2d13CH213CH213CH3), 45.1 (vt, JPC ) 4.8 Hz,
[P2N2]Ta(13C2H4)13Et), 49.1 (vt, 2JPC ) 4.2 Hz, [P2N2]Ta(13C2H4)Me),
70.6 (dt, 1JCC ) 30.0 Hz, 2JPC ) 9.0 Hz, [P2N2]Ta(13C2H4)13CH213CH3),
113.8 (dd, 1JCC ) 69.9 Hz, 3JCC ) 4.2 Hz, 13CH2d13CH213CH213CH3),
1
1
116.2 (d, JCC ) 69.5 Hz, 13CH2d13CH2CH3), 134.0 (d, JCC ) 69.9
Hz, 13CH2d13CH2CH3), 134.0 (d, 1JCC ) 42.2 Hz, CH2d13CH213CH3),
140.8 (ddd, JCC ) 69.9 Hz, JCC ) 41.5 Hz, JCC ) 2.2 Hz, 13CH2d
13CH213CH213CH3). 13C NMR (C6D6, 299 K, 129.76 MHz): δ 6.3 (dt,
1JCC ) 30.0 Hz, 1JCH ) 123.2 Hz, [P2N2]Ta(13C2H4)13CH213CH3), 45.1
1
1
2
(m, JCH ∼ 140 Hz, [P2N2]Ta(13C2H4)13Et), 49.1 (m, JCH ∼ 140 Hz,
[P2N2]Ta(13C2H4)Me), 70.6 (dt, 1JCC ) 30.0 Hz, 1JCH ) 126.8 Hz, [P2N2]-
Ta(13C2H4)13CH213CH3).
1
1
1
solvents but insoluble in hexanes. H NMR (C6D6, 25 °C, 500 MHz):
3
Variable-Temperature 13C NMR spectra of [P2N2]Ta(13C2H4)13Et.
13C NMR, selected peaks (C7D8, 350 K, 129.76 MHz): δ 7.3 (m, 1JCH
) 123.8 Hz, [P2N2]Ta(13C2H4)13CH213CH3), 45.5 (m, [P2N2]Ta(13C2H4)13-
Et), 75.9 (m, 1JCH ) 123.4 Hz, [P2N2]Ta(13C2H4)13CH213CH3). 13C NMR,
δ -1.27 (A2M3X2, 3JHH ) 7.8 Hz, JHP ) 3.1 Hz, 2H, TaCH2CH3),
3
-0.51(A3M2X2, JHH ) 7.8 Hz, JHP ) 4.4 Hz, 3H, TaCH2CH3), 0.13,
0.23, 0.37, and 0.51 (s, 24H total, SiCH3), 0.55 and 1.47 (m, 4H total,
Ta(C2H4), 1.34, 1.41, 1.70, and 1.70 (AMX, 8H total, CH2 ring), 7.07
(m, 2H, p-H), 7.19 (m, 4H, m-H), 7.66 (m, 4H, o-H). 31P NMR (C6D6,
25 °C): δ 24.0 (s). 13C NMR (C6D6, 25 °C, 125.76 MHz): δ 5.4 (vt,
JPC ) 3.2 Hz, SiCH3), 5.8 (vt, JPC ) 1.6 Hz, SiCH3), 6.0 (vt, JPC ) 1.9
Hz, SiCH3), 6.2 (t, JPC ) 4.5 Hz, TaCH2CH3), 7.0 (vt, JPC ) 3.2 Hz,
SiCH3), 20.4 and 21.2 (s, CH2 ring), 45.0 (vt, JPC ) 4.8 Hz, Ta(C2H4)),
69.9 (t, JPC ) 9.0 Hz, TaCH2CH3), 128.6 (vt, JPC) 4.3 Hz, m-C), 129.2
(s, p-C), 131.0 (vt, JPC ) 4.8 Hz, o-C), 140.9 (dd, JPC )15.3, 16.2 Hz,
ipso-C). Anal. Calcd for C28H51N2P2Si4Ta: C, 43.62; H, 6.67; N, 3.63.
Found: C, 43.93; H, 6.79; N, 3.54.
1
selected peaks (C7D8, 330 K, 129.76 MHz): δ 6.7 (m, JCH ) 123.6
Hz, [P2N2]Ta(13C2H4)13CH213CH3), 45.2 (m, [P2N2]Ta(13C2H4)13Et), 73.4
(m, 1JCH ) 125.3 Hz, [P2N2]Ta(13C2H4)13CH213CH3). 13C NMR, selected
peaks (C7D8, 300 K, 129.76 MHz): δ 5.7 (m, 1JCH ) 123.7 Hz, [P2N2]-
Ta(13C2H4)13CH213CH3), 44.7 (m, [P2N2]Ta(13C2H4)13Et), 69.3 (m, 1JCH
) 127.5 Hz, [P2N2]Ta(13C2H4)13CH213CH3). 13C NMR, selected peaks
1
(C7D8, 273 K, 129.76 MHz): δ 4.8 (m, JCH ) 123.1 Hz, [P2N2]Ta-
(13C2H4)13CH213CH3), 44.2 (m, [P2N2]Ta(13C2H4)13Et), 65.4 (m, 1JCH
)
128.5 Hz, [P2N2]Ta(13C2H4)13CH213CH3). 13C NMR, selected peaks
1
(C7D8, 253 K, 129.76 MHz): δ 4.1 (m, JCH ) 123.1 Hz, [P2N2]Ta-
X-ray Crystallographic Analysis of 1. Crystallographic data for 1
appear in Table 1. The final unit-cell parameters were obtained by least-
squares methods on the setting angles for 21854 reflections with 2θ )
4.0-60.1°. The data were processed and corrected for Lorentz and
polarization effects. The structure of 1 was solved by heavy-atom
Patterson methods and expanded using Fourier techniques. The ethylene
and ethyl hydrogen atoms were refined isotropically, and the remaining
hydrogen atoms were fixed in calculated positions with C-H ) 0.98
Å. Atomic coordinates, anisotropic thermal parameters, complete bond
lengths and bond angles, torsion angles, intermolecular contacts, and
least-squares planes are included as Supporting Information.
Reaction of [P2N2]Ta(C2H4)Et (1) with C2D4. A solution of [P2N2]-
Ta(C2H4)Et (40 mg, 0.052 mmol) in 1 mL of d8-toluene in a NMR
tube was placed under 1 atm of C2D4 then frozen in liquid N2 and
sealed. A 1H NMR spectrum was obtained immediately after thawing.
1H NMR spectrum of selected peaks (C6D6, 295 K, 500 MHz): δ -1.52
(br m, [P2N2]Ta(C2(H/D)4)(CH(D)Me)), -1.28 (m, [P2N2]Ta(C2(H/D)4)-
(CH2Me)), -0.75 (br m, [P2N2]Ta(C2(H/D)4)(CH2CH(D)2)), -0.64 (br
m, [P2N2]Ta(C2(H/D)4)(CH2CH2(D)), -0.52 (br m, [P2N2]Ta(C2(H/D)4)-
(CH2CH3).
(13C2H4)13CH213CH3), 44.0 (m, [P2N2]Ta(13C2H4)13Et, 62.6 (m, JCH
)
1
131.1 Hz, [P2N2]Ta(13C2H4)13CH213CH3). 13C NMR, selected peaks
1
(C7D8, 233 K, 129.76 MHz): δ 3.5 (m, JCH ) 122.8 Hz, [P2N2]Ta-
(13C2H4)13CH213CH3), 43.7 (m, [P2N2]Ta(13C2H4)13Et), 59.9 (m, 1JCH
)
133.6 Hz, [P2N2]Ta(13C2H4)13CH213CH3). 13C NMR, selected peaks
(C7D8, 213 K, 129.76 MHz): δ 2.8 (m, [P2N2]Ta(13C2H4)13CH213CH3),
43.5 (m, [P2N2]Ta(13C2H4)13Et), 57.2 (m, [P2N2]Ta(13C2H4)13CH213CH3).
13C NMR, selected peaks (C7D8, 193 K, 129.76 MHz): δ 2.3 (m, [P2N2]-
Ta(13C2H4)13CH213CH3), 43.3 (m, [P2N2]Ta(13C2H4)13Et), 55.0 (m, [P2N2]-
Ta(13C2H4)13CH213CH3). 13C NMR, selected peaks (C7D8, 180 K, 129.76
MHz): δ 2.0 (m, [P2N2]Ta(13C2H4)13CH213CH3), 43.2 (m, [P2N2]Ta-
(13C2H4)13Et), 53.5 (m, [P2N2]Ta(13C2H4)13CH213CH3).
Photolysis of [P2N2]TaMe3 under C2D4. A sample of [P2N2]TaMe3
(20 mg, 0.026 mmol) in 1 mL of C6D6 was sealed in a NMR tube
under 1 atm of C2D4. The tube was exposed to a Vitalux sunlamp for
30 min and then stirred for two weeks at room temperature.
Synthesis of [P2N2]TaH3(PMe3) (3). A yellow solution of [P2N2]-
TaMe3 (1.00 g, 1.32 mmol) in 120 mL of ether was transferred to a
500 mL thick wall glass vessel equipped with a Teflon valve and stir
bar. The mixture was degassed, and a 5-fold excess of PMe3 was
vacuum transferred into the reaction vessel, which was then sealed under
4 atm of hydrogen gas. The mixture was stirred for 3 days in the absence
of light. The solution was then evaporated to dryness and the resulting
solid was rinsed with minimal pentanes, filtered, and dried, yielding
red [P2N2]TaH3(PMe3) in 95% yield. Some 1H NMR coupling constants
were determined with the assistance of Lorentz-Gaussian resolution
Variable-Temperature 1H NMR Study of [P2N2]Ta(C2H4)Et (1).
A solution of [P2N2]Ta(C2H4)Et (20 mg, 0.026 mmol) in 1 mL of d8-
1
toluene was sealed under vacuum in a NMR tube. H NMR (C6D6,
299.9 K, 500 MHz): δ -1.33 (m, 2H, TaCH2CH3), -0.58 (m,
TaCH2CH3), 0.11, 0.22, 0.34, and 0.49 (s, 24H total, SiCH3), 0.44 and
1.38 (m, 4H total, Ta(C2H4), 1.32, 1.38, 1.68, 1.68 (AMX, 8H total,
1
CH2 ring), 6.98-7.63 ppm (aromatic region). H NMR (C6D6, 279.9
1
enhancement. H NMR (500 MHz, C6D6, 30 °C): δ 0.34, 0.38, 0.38,
K, 500 MHz): δ -1.31 (m, 2H, TaCH2CH3), -0.68 (m, TaCH2CH3),
0.14, 0.24, 0.36, and 0.50 (s, 24H total, SiCH3), 0.33 and 1.36 (m, 4H
total, Ta(C2H4), 1.30, 1.36, 1.66, and 1.67 (AMX, 8H total, CH2 ring),
6.98-7.63 ppm (aromatic region). 1H NMR (C6D6, 259.9 K, 500
MHz): δ -1.28 (m, 2H, TaCH2CH3), -0.79 (m, TaCH2CH3), 0.17,
0.26, 0.36, and 0.50 (s, 24H total, SiCH3), 0.27 and 1.34 (m, 4H total,
Ta(C2H4), 1.27, 1.34, 1.64, and 1.65 (AMX, 8H total, CH2 ring), 6.98-
2
0.39, 0.40, 0.40, 0.53, and 0.65 (s, 24H total, SiCH3), 0.67 (d, JHP
)
7.2 Hz, 9H, P(CH3)3), 0.87 and 1.27 (AMX, 2JHH ) 13.9 Hz, 2H total,
CH2 ring), 1.40 (ABX, 2JHH ) 14.3 Hz, 1H, CH2 ring), 1.44 (ABMX,
2JHH ) 14.3 Hz, JHH ) 1.4 Hz, 1H, CH2 ring), 1.54 and 2.10 (AMX,
4
2
2JHH ) 14.4 Hz, 2H total, CH2 ring), 1.56 and 1.73 (AMX, JHH
)
13.4 Hz, 2H total, CH2 ring), 7.08 and 7.10 (m, 2H total, PPh p-H),
7.17 and 7.22 (m, 4H total, PPh m-H), 7.80 (AMNXYZ, JHP ) 110
1
7.63 ppm (aromatic region). H NMR (C6D6, 239.9 K, 500 MHz): δ
2
2
Hz, JHP ) 104 Hz, JHP(B) ) 18.6 Hz, JHH(B) ) 7.1 Hz, JHH(C) ) 7.1
Hz, 1H, TaHA), 8.17 (AMNXYZ, JHP(C) ) 61.9 Hz, JHP(A) ) 24.1 Hz,
JHP(B) ) 16.4 Hz, 2JHH(C) ) 9.3 Hz, 2JHH(A) ) 7.1 Hz, 1H, TaHB), 7.98
and 8.03 (m, 4H total, PPh o-H), 9.87 (AMNOXYZ, JHP(B) ) 60.2 Hz,
-1.26 (m, 2H, TaCH2CH3), -0.87 (m, TaCH2CH3), 0.20, 0.28, 0.38,
and 0.53 (s, 24H total, SiCH3), 0.21 and 1.34 (m, 4H total, Ta(C2H4),
1.24, 1.34, 1.64, and 1.64 (AMX, 8H total, CH2 ring), 6.98-7.63 ppm
(aromatic region). 1H NMR (C6D6, 200.0 K, 500 MHz): δ -1.21 (m,
2H, TaCH2CH3), -1.03 (m, TaCH2CH3), 0.28, 0.33, 0.41, and 0.59 (s,
24H total, SiCH3), 0.11 and 1.34 (m, 4H total, Ta(C2H4), 1.19, 1.34,
1.61, and 1.61 (AMX, 8H total, CH2 ring), 6.98-7.63 ppm (aromatic
region). 1H NMR (C6D6, 183.0 K, 500 MHz): δ -1.17 (m, 2H, TaCH2-
2
2
JHP(A) ) 37.0 Hz, JHP(C) ) 9 Hz, JHH(B) ) 9.3 Hz, JHH(A) ) 7.1 Hz,
4JHH ) 1.4 Hz, 1H, TaHC). 31P{1H}NMR (C6D6, 30 °C): δ -21.5
(dd,2JPP ) 31.7, 2JPP ) 78.2, TaPMe3, PA), 20.2 (dd, 2JPP ) 31.7, 2JPP
2
2
) 86.6, [P2N2] ligand PB), 37.2 (dd, JPP ) 86.6, JPP ) 78.2, [P2N2]

1612 J. Am. Chem. Soc., Vol. 123, No. 8, 2001
Fryzuk et al.
CH3), -1.10 (m, TaCH2CH3), 0.32, 0.37, 0.43, and 0.62 (s, 24H total,
SiCH3), 0.08 and 1.35 (m, 4H total, Ta(C2H4), 1.16, 1.29, 1.62, and
1.62 (AMX, 8H total, CH2 ring), 6.98-7.63 ppm (aromatic region).
Variable-Temperature Isotopic Perturbation of Resonance Ex-
periment. To a sample of [P2N2]Ta(C2H4)Et (30 mg, 0.0389 mmol) in
benzene was condensed C2D4 gas (6.4 mL, 0.26 mmol) to produce a
mixture of isotopomers containing ∼75% deuterium labels in the ethyl
group. After reacting for several months, this solution was evaporated
to dryness, dissolved in d8-toluene, and sealed in a NMR tube. The
calculated relative intensities of the NMR signals for the various
possible isotopomers with 75% deuterium labels are as follows:
R-position CH2CH3, 0.2; CHDCH3, 0.6; CD2CH3, 0.0; CH2CH2D, 1.8;
CHDCH2D, 5.3; CD2CH2D, 0.0; CH2CHD2, 5.3; CHDCHD2, 15.8; CD2-
CHD2, 0.0; CH2CD3, 5.3; CHDCD3, 15.8; CD2CD3, 0.0. â-position
CH2CH3, 0.3; CHDCH3, 1.8; CD2CH3, 2.6; CH2CH2D, 1.8; CHDCH2D,
10.5; CD2CH2D, 15.8; CH2CHD2, 2.6; CHDCHD2, 15.8; CD2CHD2,
23.7; CH2CD3, 0.0; CHDCD3, 0.0; CD2CD3, 0.0. 1H NMR, ethyl region
(C7H8, 295 K) -0.65 (br overlapping, CHDCH3 and CD2CH3), -0.75
(br overlapping, CHDHCH2D and CD2CH2D), -0.81 (br, CHDCHD2),
-0.85 (br, CD2CHD2), -1.37 (br overlapping, CH2CHD2 and
CH2CD3), -1.57 (overlapping, CHDCHD2 and CHDCD3).1H NMR,
ethyl region (C7H8, 274.6 K) -0.81 (br overlapping, CHDCH3 and
CD2CH3), -0.85 (br, CHDCH2D), -0.90 (br, CD2CH2D), -0.96 (br,
CHDCHD2), -1.00 (br, CD2CHD2), -1.34 (br overlapping, CH2CHD2
and CH2CD3), -1.53 (overlapping, CHDCHD2 and CHDCD3).
1H NMR, ethyl region (C7H8, 254.6 K) -0.90 (br, CHDCH3), -0.95
(br, CD2CH3), -1.00 (br, CHDCH2D), -1.05 (br, CD2CH2D), -1.10
(br, CHDCHD2), -1.16 (br, CD2CHD2), -1.31 (overlapping multiplets,
CH2CHD2 and CH2CD3), -1.49 (overlapping, CHDCHD2 and
CHDCD3). 1H NMR, ethyl region (C7H8, 234.6 K) -0.99 (br,
CHDCH3), -1.05 (br, CD2CH3), -1.10 (br, CHDCH2D), -1.17 (br,
CD2CH2D), -1.19 (br, CHDCHD2), -1.28 (br, CD2CHD2), -1.28
(overlapping, CH2CHD2 and CH2CD3), -1.46 (overlapping, CHDCHD2
and CHDCD3).
1
EXSY Spectrum of [P2N2]Ta(C2H4)Et (1). H EXSY (C6D6, τ )
0.4 s, 300 K) cross-signals δ -0.51, 0.55 and -0.51, 1.47 (TaCH2-
CH3, Ta(C2H4)); -1.28, 0.55 and -1.28, 1.47 (TaCH2CH3, Ta(C2H4));
0.13, 0.37 and 0.23, 0.51 (ligand SiCH3, ligand SiCH3).
Acknowledgment. We gratefully acknowledge both the
NSERC of Canada and the Petroleum Research Fund, admin-
istered by the American Chemical Society, for funding in the
form of research grants. Financial support was also provided
by the NSERC of Canada in the form of a postgraduate
scholarship for S. A. Johnson, as well as the University of British
Columbia in the form of a University Graduate Fellowship.
Supporting Information Available: Full experimental
details, tables of X-ray parameters, fractional coordinates and
thermal parameters, bond lengths, bond angles, and torsion
angles, and an ORTEP diagram for compound 1 (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA0021621