Unusual ligand-induced reductive elimination in Cp*W(NO)(H)[η2-PPh2C6H4]: A route to the extremely strong π-donor fragment Cp*W(NO)(PPh3)

Full text of DOI:10.1021/ja9635037

J. Am. Chem. Soc. 1997, 119, 1139-1140
1139
Unusual Ligand-Induced Reductive Elimination in
Cp*W(NO)(H)[η2-PPh2C6H4]: A Route to the
Extremely Strong π-Donor Fragment
Cp*W(NO)(PPh3)
David J. Burkey, Jeff D. Debad, and Peter Legzdins*
Department of Chemistry
The UniVersity of British Columbia
VancouVer, British Columbia, Canada V6T 1Z1
ReceiVed October 7, 1996
The fundamental importance of the formation of a C-H bond
by reductive elimination from a metal center in organometallic
chemistry has led to extensive research on this subject.1 One
mechanistic pathway for C-H reductive elimination involves
its inducement by the presence of a potential incoming ligand;
however, well-characterized examples of this process are still
relatively uncommon.2 We report here the facile ligand-induced
reductive elimination of arene in the ortho-metallated phosphine
complex Cp*W(NO)(H)[η2-PPh2C6H4] (1), which provides
access to complexes containing the exceptionally strong π-donor
fragment Cp*W(NO)(PPh3).
We recently reported the synthesis of 1 by the reaction of
Cp*W(NO)(CH2SiMe3)2 with H2 in the presence of PPh3.3 We
have since determined the solid state molecular structure of 1
(Figure 1) which reveals a cis arrangement of the aryl and
hydride ligands; the C(2)-W(1)-H(1) angle is 72(1)°.4 This
geometry contrasts with the trans configuration of R and H
ligands found for other Cp′W(NO)(R)(H)(L) complexes (Cp′
) Cp (C5H5) or Cp* (C5Me5), R ) alkyl or aryl ligand, L )
neutral two-electron donor ligand), even when they are generated
by oxidative addition of R-H.5 Presumably, isomerization of
1 to a trans arrangement of the aryl and hydride ligands is
inhibited by the chelating nature of the ortho-metallated
phosphine ligand.
Figure 1. Solid state molecular structure of 1; 50% probability thermal
ellipsoids are shown.
go cleanly to completion over a period of 48 h at room
1
temperature and 3-6 h at 45 °C, as monitored by H and 31P
NMR spectroscopies.6 Clearly, the presence of a potential
ligand, L, is required to induce reductive elimination of arene
in 1. Previous examples of ligand-induced reductive elimination
for either C-H or C-C bonds have been limited primarily to
phosphines, CO, and alkynes.2,7 Jones and Hessell recently
reported that the reductive elimination of benzene from Tp′Rh-
(H)(Ph)(CNCH2CMe3) (Tp′ ) hydridotris(3,5-dimethylpyra-
zoyl)borate) is induced by neopentyl isocyanide;2a however, to
the best of our knowledge, the use of a ketone or ester to effect
reductive elimination at a metal center is without precedent.
Spectroscopic data for 2-6 reveal that the 16-electron Cp*W-
(NO)(PPh3) fragment is an exceptional π-donor. For example,
the isocyanide complexes 2 and 3 exhibit ν(CN) bands in their
IR spectra at 1844 and 1796 cm-1, decreases of ca. 300 cm-1
when compared to the free isocyanides. The markedly low-
field 13C NMR resonances for the isocyanide carbons of 2 and
3 (226.3 and 234.6 ppm) are also indicative of substantial
π-bonding between the tungsten and the isocyanide ligands.8
Strong tungsten-isocyanide π-bonding is evident in the solid
state molecular structure of 2 (Figure 2),9 as the tert-butyl
isocyanide ligand is bent with a C(11)-N(2)-C(12) angle of
137(1)°.8a,10 Additionally, the W-C(11) bond length of 2.00(1)
Å is significantly shorter than analogous M-C bond lengths in
tungsten and molybdenum complexes with linear isocyanide
ligands, which are typically ca. 2.1 Å or greater.8b,11,12
Despite the cis arrangement of aryl and hydride ligands in 1,
the compound displays reasonable thermal stability, remaining
unchanged in THF-d8 or 1,4-dioxane-d8 solution indefinitely at
room temperature and for several days at 50 °C. However, in
the presence of isocyanides or organic carbonyls, 1 undergoes
facile reductive elimination of arene to afford Cp*W(NO)(PPh3)-
(L) complexes (2-6) in excellent yields (eq 1). The reactions
(6) The isomer of 1 with a trans configuration of aryl and hydride ligands,
which is produced in low yield upon thermolysis of dioxane solutions of 1
at 80 °C for 24 h, does not react with tert-butyl isocyanide or acetone,
even at elevated temperatures (80 °C).
(7) (a) Hardy, D. T.; Wilkinson, G.; Young, G. B. Polyhedron 1996,
15, 1363. (b) Foo, T.; Bergman, R. G. Organometallics 1992, 11, 1811. (c)
Komlya, S.; Abe, Y.; Yamamoto, A.; Yamamoto, T. Organometallics 1983,
2, 1466. (d) Tatsumi, K.; Nakamura, A.; Komlya, S.; Yamamoto, A.;
Yamamoto, T. J. Am. Chem. Soc. 1984, 106, 8181.
(8) (a) Adachi, T.; Sasaki, N.; Ueda, T.; Kaminaka, M.; Yoshida, T. J.
Chem. Soc., Chem. Commun. 1989, 1320. (b) Rommel, J. S.; Weinrach, J.
B.; Grubisha, D. S.; Bennett, D. W. Inorg. Chem. 1988, 27, 2945.
(9) Crystal data for 2: monoclinic, space group P21/n, a ) 8.975(4) Å,
b ) 22.082(3) Å, c ) 16.088(3) Å, â ) 103.34(2)°, V ) 3102(1) Å3, Z )
4, R ) 0.034, Rw ) 0.029, and GOF ) 1.38 for 2102 reflections with Io g
3σ(Io) and 365 variables.
(10) (a) Aharonian, G.; Hubert-Pfalzgraf, L. G.; Zaki, A.; Borgne, G. L.
Inorg. Chem. 1991, 30, 3105. (b) Bassett, J.-M.; Berry, D. E.; Barker, G.
K.; Green, M.; Howard, J. A. K.; Stone, F. G. A. J. Chem. Soc., Dalton
Trans. 1979, 1003. (c) Chatt, J.; Pombeiro, A. J. L.; Richards, R. L.;
Royston, G. H. D.; Muir, K. W.; Walker, R. J. Chem. Soc., Chem. Commun.
1975, 708.
(11) (a) Carmona, E.; Contreras, L.; Puebla-Gutie´rrez, E.; Monge, A.;
Sa´nchez, L. Inorg. Chem. 1990, 29, 700. (b) Carmona, E.; Galindo, A.;
Marin, J. M.; Gutie´rrez, E.; Monge, A.; Ruiz, C. Polyhedron 1988, 7, 1831.
(c) Lippard, S. J.; Warner, S. Organometallics 1986, 5, 1716. (d) Guy, M.
P.; Guy, J. T., Jr.; Bennett, D. W. Organometallics 1986, 5, 1696.
(12) The recently reported cis-(CNC6H4NC)2W(dppe)2 has dramatically
short W-C bond lengths of 1.84(1) and 1.882(9) Å for the two isocyanide
ligands. See: Hu, C.; Hodgeman, W. C.; Bennett, D. W. Inorg. Chem. 1996,
35, 1621.
(1) For some representative examples, see: (a) Halpern, J. Acc. Chem.
Res. 1982, 15, 332. (b) Buchanan, J. M.; Stryker, J. M.; Bergman, R. G. J.
Am. Chem. Soc. 1986, 108, 1537. (c) Jones, W. D.; Feher, F. J. J. Am.
Chem. Soc. 1984, 106, 1650. (d) McAlister, D. R.; Erwin, D. K.; Bercaw,
J. E. J. Am. Chem. Soc. 1978, 100, 5966. (e) Hackett, M.; Whitesides, G.
M. J. Am. Chem. Soc. 1988, 110, 1449.
(2) (a) Jones, W. D.; Hessell, E. T. J. Am. Chem. Soc. 1992, 114, 6087.
(b) Gell, K. I.; Schwartz, J. J. Am. Chem. Soc. 1981, 103, 2687. (c) Pedersen,
A.; Tilset, M. Organometallics 1993, 12, 3064. (d) Jones, W. D.;
Kuykendall, V. L.; Selmeczy, A. D. Organometallics 1991, 10, 1577.
(3) Debad, J. D.; Legzdins, P.; Lumb, S. A. Organometallics 1995, 14,
2543.
(4) Crystal data for 1: triclinic, space group P1h, a ) 10.3786(5) Å, b )
10.8927(6) Å, c ) 12.7051(6) Å, R ) 95.549(1)°, â ) 97.927(1)°, γ )
116.282(1)°, V ) 1255.25(11) Å3, Z ) 2, R1 ) 0.0233, wR2 ) 0.0547, and
GOF(F2) ) 1.073 for 4283 reflections and 397 variables.
(5) (a) Legzdins, P.; Martin, J. T.; Einstein, F. W. B.; Jones, R. H.
Organometallics 1987, 6, 1826. (b) Martin, J. T. Ph.D. Thesis, University
of British Columbia, Vancouver, Canada, 1987.
S0002-7863(96)03503-2 CCC: $14.00 © 1997 American Chemical Society

1140 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
Communications to the Editor
high-energy HOMO. However, the metal center will be more
electron-rich in Cp*W(NO)(PPh3) than in [Cp′Re(NO)(PPh3)]+
due to the absence of the cationic charge. The increased electron
density at the metal will enhance the η2-interaction with organic
carbonyls and will disfavor η1-coordination by reducing the
electron-withdrawing character of the metal.16 The effect of
the increased electron density in Cp*W(NO)(PPh3) is enough
to promote exclusive η2-coordination of even weakly π-acidic
ketone and ester ligands.19 It should be noted that the Cp*W-
(NO)(PPh3) fragment binds the ketone and ester ligands in 4-6
diastereoselectively; only one isomer is observed for all three
complexes.
The reaction of 1 with excess PMe3, in addition to producing
the expected reductive elimination product Cp*W(NO)(PPh3)-
(PMe3) (7), also yields the formal phosphine substitution product
Cp*W(NO)(H)(PMe3)(η1-C6H4PPh2) (8) (eq 2); the ratio of 7/8
Figure 2. Solid state molecular structure of 2; 50% probability thermal
ellipsoids are shown.
Spectroscopic data for 4-6 reveal that the ketone and ester
ligands are coordinated exclusively in an η2-fashion to the
tungsten. Evidence for this is provided by the lack of any ν(CO)
absorbances above 1500 cm-1 typical of η1-coordinated ketone
or ester ligands in the IR spectra of 4-6.13 Also diagnostic for
η2-coordination are the 13C NMR chemical shifts for the
carbonyl carbons in 4-6, which occur at 70.3, 80.9, and 101.8
ppm, dramatically upfield from those of the uncomplexed
species; these peaks exhibit one-bond coupling constants to
tungsten (ca. 50 Hz). Indeed, the extent of the π-bonding is
such that the tungsten-carbonyl interaction in 4-6 is probably
best described as a three-membered metallaoxirane ring.14 It
should be noted that the acetone ligand of 4 does not exchange
with added acetone-d6 (50 equiv), even after heating at 50 °C
for 24 h.
Although there are now several examples of η2-ketone
complexes,14,15 6 represents the first example of a transition
metal complex with an η2-ester ligand. The lack of any previous
examples of η2-ester complexes can been attributed to the poor
π-accepting ability of esters relative to ketones or aldehydes,
since esters possess a higher energy CO π*-orbital due to the
destabilizing presence of the OR group.14,16 That the Cp*W-
(NO)(PPh3) fragment is able to form an η2-interaction with the
ethyl acetate ligand in 6 thus attests to its extremely strong
π-donor ability.
produced in the reaction is 3:1. Spectroscopic data for 8, notably
the upfield resonance for the hydride ligand at -0.37 ppm and
2
the large JPH coupling constant of 94 Hz, are most consistent
with a trans arrangement of aryl and hydride ligands, in contrast
to their cis disposition in 1.5 Monitoring the reaction by 31P
NMR spectroscopy reveals that the two products are formed
by separate reactions and that 8 does not reform 1 through
dissociation of PMe3.
Preliminary kinetic investigations of the reaction of 1 with
acetone and PMe3 have revealed that the rate increases linearly
with increasing concentration of incoming ligand (up to ca. 50
equiv). Additional mechanistic information is provided by the
observation that clean formation of Cp*W(NO)(PPh3)(L) from
1 appears to require reactants that are relatively good σ-donors.
For example, the reaction of 1 with excess P(OMe)3 in THF
does give the expected reductive elimination product Cp*W-
(NO)(PPh3)(P{OMe}3) (9), but in greatly reduced yield (ca.
25%) compared to the formation of 2-6. In addition, treatment
of 1 with CO (1 atm), instead of generating Cp*W(NO)(PPh3)-
(CO), simply leads to decomposition of 1. Further mechanistic
investigations of the ligand-induced reductive elimination of
arene in 1 as well as reactivity studies on the new low-valent
tungsten complexes 2-7 are currently in progress.
A rationale for the strong π-donor characteristics of Cp*W-
(NO)(PPh3) is provided by a comparison to the isoelectronic
[Cp′Re(NO)(PPh3)]+ fragment extensively studied by Gladysz.17
These investigations have revealed that [Cp′Re(NO)(PPh3)]+ is
a strong π-donor capable of forming η2-aldehyde complexes,
but only η1-ketone complexes. Molecular orbital calculations
on [Cp′Re(NO)(PPh3)]+ have attributed its π-donor nature to a
metal-centered HOMO that is high in energy, since it is not
stabilized by π-bonding with the NO ligand.18 The isoelectronic
Cp*W(NO)(PPh3) fragment will have an analogous π-donor,
Acknowledgment. We are grateful to the Natural Sciences and
Engineering Research Council of Canada (grants to P.L. and a
postgraduate scholarship to J.D.D.) and to the National Science
Foundation (postdoctoral fellowship to D.J.B.) for support of this work.
We also acknowledge Victor G. Young, Jr., of the X-ray Crystal-
lographic Laboratory at the University of Minnesota and Steven J. Rettig
of this department for solving the X-ray crystal structures of 1 and 2,
respectively.
(13) (a) Caldarelli, J. L.; Wagner, L. E.; White, P. S.; Templeton, J. L.
J. Am. Chem. Soc. 1994, 116, 2878. (b) Willard, P. G.; Liu, Q.-Y. J. Am.
Chem. Soc. 1992, 114, 348.
(14) Bryan, J. C.; Mayer, J. M. J. Am. Chem. Soc. 1990, 112, 2298.
(15) (a) Harman, W. D.; Fairlie, D. P.; Taube, H. J. Am. Chem. Soc.
1986, 108, 8223. (b) Williams, D. S.; Schofield, M. H.; Schrock, R. R.
Organometallics 1993, 12, 4560. (c) Hill, J. E.; Fanwick, P. E.; Rothwell,
I. P. Organometallics 1992, 11, 1771. (d) Wood, C. D.; Schrock, R. R. J.
Am. Chem. Soc. 1979, 102, 5421. (e) Mayer, J. M.; Bercaw, J. E. J. Am.
Chem. Soc. 1982, 104, 2157.
(16) Delbecq, F.; Sautet, P. J. Am. Chem. Soc. 1992, 114, 2446.
(17) Boone, B. J.; Klein, D. P.; Seyler, J. W.; Me´ndez, N. Q.; Arif, A.
M.; Gladysz, J. A. J. Am. Chem. Soc. 1996, 118, 2411 and references therein.
(18) (a) Czech, P. T.; Gladysz, J. A.; Fenske, R. F. Organometallics
1989, 8, 1806. (b) Schilling, B. E. R.; Hoffmann, R.; Faller, J. W. J. Am.
Chem. Soc. 1979, 101, 592.
Supporting Information Available: Experimental procedures and
complete characterization data for complexes 2-9 and full details of
the crystal structure analyses including associated tables for 1 and 2
(25 pages). See any current masthead page for ordering and Internet
access instructions.
JA9635037
(19) For a recent discussion of the chemical consequences associated
with a change from cationic rhenium to isoelectronic neutral tungsten
complexes, see: Heinekey, D. M.; Schomber, B. H.; Radzewich, C. E. J.
Am. Chem. Soc. 1994, 116, 4515.