Ligand-induced β-H transfer in W(VI) dialkyl complexes

Full text of DOI:10.1021/ja971218x

11990
J. Am. Chem. Soc. 1997, 119, 11990-11991
Ligand-Induced â-H Transfer in W(VI) Dialkyl
Complexes
Shu-Yu S. Wang, Khalil A. Abboud, and
James M. Boncella*
Department of Chemistry and Center for Catalysis
UniVersity of Florida, GainesVille, Florida 32611-7200
ReceiVed April 17, 1997
â-Elimination and its microscopic reverse, olefin insertion
into the metal-hydride bond, are among the most fundamental
transformations that occur in organometallic chemistry.1,2 There
is much evidence3-6 that provides compelling support for olefin
insertion into the metal-hydride bond as an initiation step in
Ziegler-Natta polymerization of ethylene and R-olefins, while
â-H elimination serves as a chain-transfer or termination step
in olefin polymerization. In addition, â-H elimination is also
the most common decomposition route for â-hydrogen contain-
ing metal alkyls. Indeed, the facility of this pathway often
precludes isolation of alkyl complexes with â-hydrogen atoms,
especially when the complexes are coordinatively unsaturated.7
We now report the preparation8 of a series of coordinatively
unsaturated tungsten(VI) dialkyl complexes containing â-hy-
drogens (eq 1), from the dichloride precursor, W(NPh)[o-(Me3-
SiN)2C6H4]Cl2.9
Figure 1. Thermal ellipsoid plot of W(NPh)[(o-Me3SiN)2C6H4](CH2-
CH2Ph)2 (4). Selected bond distances (Å): W-N(3), 1.728(4); W-N(2),
2.003(4); W-N(1), 2.000(4); W-C(1), 2.189(5); W-C(9), 2.175(5).
Selected bond angles (deg): C(29)-N(3)-W, 171.8(3); N(1)-W-
N(2), 82.9(2); C(1)-W-C(9), 85.1(2); C(2)-C(1)-W, 117.6(3);
C(10)-C(9)-W, 117.7(4).
vector which causes the SiMe3 groups to be displaced above
the basal plane. The folding of the ligand also tilts the N
coordination planes by 60-65° relative to the basal plane of
the molecule.
The formal electron count at the metal center could be as
high as 18 e- if both sets of nitrogen lone pairs of the [o-(Me3-
SiN)2C6H4]2- ligand can be donated to the metal center. This
is however not possible since there are only three d orbitals
that are of the correct symmetry to form π bonds with the
ligands. Given that the dxz and dyz orbitals form the π bonds
with the imido group, only the dxy orbital is available to overlap
with the N π electrons on the diamido ligand. Thus, the
maximum electron count at the metal center is 16 e-. Further-
more, the structure of 4 shows that the folding of the diamide
ligand brings the p orbitals on the N atoms to within ca. 25-
30° of the xy plane, preventing efficient overlap between the
dxy orbital and the N lone pairs. The structural constraints
imposed by the chelating nature of the [o-(Me3SiN)2C6H4]2-
ligand lead us to suggest that these dialkyl complexes are best
described as 14 e- or at most 16 e- compounds.
Surprisingly, despite their coordinative unsaturation, com-
plexes 1-4 are indefinitely stable at room temperature in the
absence of coordinating ligands. Single crystals of 4 that were
suitable for X-ray diffraction studies were obtained by slowly
cooling a pentane solution of 4 to -20 °C. A thermal ellipsoid
plot of 4 is shown in Figure 1.10 The coordination geometry
about W is a square pyramid with the imido N in the axial
position and the [o-(Me3SiN)2C6H4]2- ligand and two alkyl
ligands in the basal plane. The W-imido bond length is
consistent with a W-N triple bond. The geometry about the
N atoms of the [o-(Me3SiN)2C6H4]2- ligand is planar as is
observed in silyl amines.11 The ligand is folded about the N-N
Upon the addition of trimethylphosphine, complexes 1, 2,
and 4 were converted to the cooresponding olefin complexes
5, 6, and 7 (eq 2).8 It has been shown that the dissociation of
(1) Parshall, G. W. Homogeneous Catalysis; John Wiley & Sons: New
York, 1980; Chapter 3.
(2) Cross, R. J. The Chemistry of the Metal-Carbon Bond; Hartley, F.
R., Patai, S., Eds.; John Wiley & Sons: New York, 1985; Vol. 2, Chapter
8.
(3) (a) Watson, P. L. J. Am. Chem. Soc. 1982, 104, 337-339. (b) Watson,
P. L.; Roe, D. C. J. Am. Chem. Soc. 1982, 104, 6471-6473.
(4) (a) Jordan, R. F.; Dasher, W. E.; Echols, S. F. J. Am. Chem. Soc.
1986, 108, 1718-1719. (b) Jordan, R. F.; Bajgur, C. S.; Willett, R.; Scott,
B. J. Am. Chem. Soc. 1986, 108, 7410- 7411.
labile ligands (e.g., PMe3) to generate a coordinatively unsatur-
ated species is often the first step in the â-H elimination
process.12 However, the â-H transfer chemistry of 1, 2, and 4
is promoted by adding a ligand to the metal center. This
behavior is clearly unexpected since it implies that â-H transfer
is induced by increasing the coordination number of the
complex. To understand the role of PMe3 and clarify the
mechanism, we have undertaken a kinetic study of the reaction
of 1 with PMe3.
(5) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann,
H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091.
(6) (a) Soto, J.; Steigerwald, M. L.; Grubbs, R. H. J. Am. Chem. Soc.
1982, 104, 4479. (b) Clawson, L.; Soto, J.; Buchwald, S. L.; Steigerwald,
M. L.; Grubbs, R. H. J. Am. Chem. Soc. 1985, 107, 3377-3378.
(7) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987.
(8) Full details of the synthetic procedures are found in the Supporting
Information.
(9) Boncella, J. M.; Wang, S.-Y. S.; VanderLende, D. D.; Huff, R. L.;
Abboud, K. A., Vaughan, W. M. J. Organomet. Chem. 1997, 530, 59.
(10) 4: orange crystals, triclinic, P1h, a ) 10.0571(1) Å, b ) 10.1133(2)
Å, c ) 19.1479(4) Å, R ) 93.043(1)°, â ) 99.667(1)°, γ ) 112.048(1)°,
V ) 1765.06(5) Å3, Z ) 2, R1 ) 3.4%, GOF on F2 ) 0.921, T ) 173 K.
(11) Lappert, M. F.; Power, P. P.; Sanger, A. R.; Srivastava, R. C. Metal
and Metalloid Amides; John Wiley: New York, 1980; pp 244-249.
(12) (a) Whitesides, G. M.; Gaasch, J. F.; Stedronsky, E. R. J. Am. Chem.
Soc. 1972, 94, 5258. (b) Reger, D. L.; Culberston, E. C. J. Am. Chem. Soc.
1976, 98, 2789.
S0002-7863(97)01218-3 CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 49, 1997 11991
Scheme 1
The kinetics of the conversion of 1 to 5 in toluene-d8 was
examined by following the disappearance and appearance of
the trimethylsilyl peaks of 1 and 5 using 1H NMR spectroscopy
in the presence of a pseudo-first-order excess of PMe3. The
reaction is first order with respect to 1 and PMe3, with a pseudo-
first-order rate constant at 293 K of k ) 9.61(82) × 10-4 s-1
(t1/2 ) 12 min). Activation parameters calculated from an
Eyring plot over the temperature range 263-293 K are ∆Hq )
5.1(2.6) kcal mol-1 and ∆Sq ) -54.9(9.7) cal mol-1 K-1 for
the formation of 5. The observation that the addition of ethylene
has no effect on the rate of thermolysis as well as the large,
negative entropy of activation suggests that liberation of ethylene
does not occur prior to the rate-determining step in this reaction.
A rapid pre-equilibrium coordination of PMe3 is proposed
as the initial step in the reaction because of the first-order
dependence on PMe3 and the fact that the PMe3 adduct,
W(NPh)[o-(Me3SiN)2C6H4](PMe3)Et2 (1-P), has been observed
at -50 °C. This adduct exists as a mixture of two isomers as
determined by 13C and 31P NMR spectroscopy.13 The major
isomer has trans ethyl groups with PMe3 coordinated cis to the
imido ligand. The minor isomer has the PMe3 coordinated trans
to the imido ligand, leaving the ethyl groups mutually cis.
Though saturation of the rate at high concentrations of PMe3 is
predicted, we have not been able to observe such behavior even
at PMe3 concentrations of 1.6 M. We suggest that this is due
in part because the reaction proceeds through the minor isomer
of 1-P in which the cis disposition of the ethyl groups is retained.
The â-deuterated analogue, W(NPh)[o-(Me3SiN)2C6H4](CH2-
CD3)2 (1-d6), was prepared to provide additional information
regarding the mechanism. The conversion of 1-d6 to 5-d2 was
hybridized carbon in the formation of alkyne/aryne complexes,
while Buchwald22 and Schrock23 have proposed direct â-hy-
drogen abstraction processes in the reactions of Cp2Zr(SCH2-
Ph)(Me) and [Me3SiNCH2CH2]3NTa(Et)2, respectively.
The mechanism that is outlined in Scheme 1 is consistent
with all of the above experimental data.24 The initial step in
the reaction is coordination of PMe3, which is supported by the
observation of the PMe3 adduct, 5-d2, and the first-order
dependence of the reaction on [1] and [PMe3]. The observation
of a large kinetic isotope effect and no deuterium scrambling
are consistent with the proposed direct â-H abstraction pathway.
The combination of the bimolecular nature of this reaction and
the highly ordered transition state, imposed by the geometric
requirement for the â-H to transfer to the R-C of the other alkyl
group, accounts for the large negative entropy value.22,25
Acknowledgment. We acknowledge the National Science Founda-
tion (CHE-9523279) for the support of this work. K.A.A. wishes to
acknowledge the National Science Foundation for funding of the
purchase of the X-ray equipment.
Supporting Information Available: Full details of the experimental
procedures for the syntheses of 1-7, a table of NMR data, representa-
tive kinetics plots, a table of rate constants, and a full description of
the X-ray structure determination of 4, including tables of bond lengths
and angles and positional and thermal parameters (20 pages). See any
current masthead page for ordering and Internet access instructions.
JA971218X
(17) (a) Whitesides, G. M. Pure Appl. Chem. 1981, 53, 287. (b)
McCarthy, T. J.; Nuzzo, R. G.; Whitesides, G. M. J. Am. Chem. Soc. 1981,
103, 1676, 3396, 3404.
(18) Burger, B. J.; Thompson, M. E.; Cotter, W. D.; Bercaw, J. E. J.
Am. Chem. Soc. 1990, 112, 1566-1577.
2
followed by H NMR in toluene. In both the absence and
presence of PMe3, no scrambling of the deuterium labels among
the R- and â-carbons was observed. The products of the
reaction are exclusively W(NPh)[o-(Me3SiN)2C6H4](η2-CH2-
CD2)(PMe3)2 (5-d2), and CH2DCD3.14 The above observation
suggests an irreversible â-H abstraction process as proposed
by Yamamoto15 and Buchwald.22
At 293 K, the observed kH/kD for the thermolysis of 1/1-d6 is
6.1(5), which strongly suggests a near linear transition state for
the â-H transfer process and is consistent with the direct transfer
of one â-H to the other R-carbon atom as is proposed in Scheme
1. The large KIE in our system is significantly different from
those compounds which decompose by â-elimination followed
by reductive elimination, e.g. PdEt2(PMePh2)2 (KIE ) 1.4),15
CoEt2(acac)(PMe2Ph)2 (KIE ) 2.3),16 PtEt2(PEt3)2 (KIE ) 1.4-
1.7),17 Cp*2ScCH2CH2Ph (KIE ) 2.0),18 and (CO)(PPh3)2IrCH2-
CH2C6H13 (KIE ) 2.3).19 Buchwald20 and Erker21 have
proposed the direct â-hydrogen abstraction process from an sp2-
(19) Evans, J.; Schwarts, J.; Urguhart, P. W. J. Organomet. Chem. 1974,
81, C37-C39.
(20) Buchwald, S. L.; Lum, R. T.; Dewan, J. C. J. Am. Chem. Soc. 1986,
108, 7441-7442.
(21) Erker, G. J. Organomet. Chem. 1977, 134, 189-202.
(22) Buchwald, S. L. and Nielsen, R. B. J. Am. Chem. Soc. 1988, 110,
3171-3175.
(23) Freundlich, J. S., Schrock, R. R., and Davis, W. M. J. Am. Chem.
Soc. 1996, 118, 3643-3655.
(24) A reviewer has suggested an alternative mechanism which involves
transfer of the â-H atom to the imido nitrogen followed by R-abstraction
of the N-H of the resultant amido group by the remaining ethyl group.
While we have no direct evidence that excludes this suggestion, we believe
that such a mechanism is unlikely because of observations that we have
made on the conversion of 4 to the styrene complex 7. This reaction proceeds
very much more slowly (t1/2 ) 26 min at 95 °C) than 2 is converted to 5,
and the data indicate that the conversion of 4 to 7 proceeds via a different
mechanism (KIE is 2.3 and ∆Sq ca. -28 cal mol-1 K-1) than the conversion
of 2 to 5. When complex 4 binds PMe3, only the isomer with PMe3 cis to
the imido group and the Et groups trans to one another is observed via
low-temperature NMR. Apparently, this is responsible for the change in
mechanism because the alkyl groups are no longer cis to one another. If
transfer of the â-H to the imido is the mechanism that is followed, we
would expect that changing the geometry of the PMe3 adduct in this fashion
would not cause such a dramatic change in the reaction rate because the
alkyl groups remain cis to the imido group.
(13) Compound 1-P in the presence of 15 equiv of PMe3 has two isomers
at low temperature as observed by 31P NMR in a ratio of ca. 20:1. 31P
NMR (-80 °C, C7D8): δ -21.9 (1JP-W ) 240 Hz, major isomer, PMe3 cis
to imido and Et groups mutually trans), -28.1 (broad, minor isomer with
PMe3 trans to imido and Et groups mutually cis).
(25) (a) The observed value of ∆Sq is a combination of PMe3 binding
and subsequent steps to reach the transition state. Values of ∆Sq of between
-30 and -40 cal mol-1 K-1 are often observed in purely associative
substitution reactions. Thus the majority of the observed value of ∆Sq is
accounted for in the binding of PMe3.25b Loss of the remaining 10-20 cal
mol-1 K-1 is accounted for by formation of the highly constrained transition
state necessary for the proposed â-H transfer.22 (b) Wilkins, R. G. Kinetics
and Mechanism of Reactions of Transition Metal Complexes, 2nd ed.;
VCH: New York, 1991.
(14) The formation of CH2DCD3 is supported by EI-MS, displaying a
peak at 34. The same peak is also observed for the product of the reaction
between CD3CH2MgCl and D2O.
(15) Ozawa, F.; Ito, T.; Yamamoto, A. J. Am. Chem. Soc. 1980, 102,
6457.
(16) Ikariya, T.; Yamamoto, A. J. Organomet. Chem. 1976, 120, 257-
284.