Direct observation of an equilibrium between (Bu(t)CH2)2W(≡CBu(t))(SiBu(t)Ph2) and (Bu(t)CH2)W(=CHBu(t))2(SiBu(t)PH2) and an unusual silyl migration [4]

Full text of DOI:10.1021/ja982571l

J. Am. Chem. Soc. 1998, 120, 13519-13520
13519
between alkylidene and alkylidyne complexes. Herein, we
describe, for the first time, the direct observation of such an
exchange (ButCH2)W(dCHBut)2(SiButPh2) (2b) a (ButCH2)2W-
(tCBut)(SiButPh2) (2a) and our studies of the process. 2b is also
one of the rare known d0 bis(neopentylidene) complexes; only
tantalum and niobium d0 bis(neopentylidene) complexes have been
reported.2e,5a,6 In addition, we were surprised to find that, in the
reaction of 2 with O2, the silyl ligand in 2a formally underwent
an unprecedented migration to the alkylidyne ligand to give a
silyl-substituted alkylidene complex (ButCH2)2W(dO)[dC-
(But)(SiButPh2)] (3).
Direct Observation of an Equilibrium between
(ButCH2)2W(tCBut)(SiButPh2) and
(ButCH2)W(dCHBut)2(SiButPh2) and an Unusual
Silyl Migration
Tianniu Chen,† Zhongzhi Wu,† Liting Li,†
Karn R. Sorasaenee,† Jonathan B. Diminnie,† Hongjun Pan,†
Ilia A. Guzei,‡ Arnold L. Rheingold,‡ and Ziling Xue*,†
Department of Chemistry, The UniVersity of Tennessee
KnoxVille, Tennessee 37996
Department of Chemistry & Biochemistry
The UniVersity of Delaware, Newark, Delaware 19716
Complex 2 was synthesized as part of our studies of cyclo-
pentadienyl (Cp)-free silyl complexes of the early transition met-
als.6c,d,7,8 2 was made by the reaction of Li(THF)3SiButPh2 in
9
ReceiVed July 20, 1998
Et2O with (ButCH2)2(Cl)WtCBut (1)8a at -40 °C (Scheme 1).10
Warming the solution to -10 °C, followed by workup at this
temperature and crystallization at -30 °C yielded crystalline 2
in 58% yield.10 Spectroscopic properties [1H, 13C{1H},1H-gated-
decoupled 13C, 1H-13C heteronuclear correlation (HETCOR), and
29Si{1H} NMR] of 2a and 2b are consistent with the structure
assignments and the existence of the two isomers in solution.10
The characteristic 13C NMR alkylidene resonance of 2b at 272.30
ppm and alkylidyne resonance of 2a at 318.38 ppm appear,
respectively, as a doublet and a singlet in the 1H-gated-decoupled
13C spectra. There is one 1H NMR resonance at 6.03 ppm (dCH-
But) for the two alkylidene ligands in 2b between 253 and 293
K. It is thus unlikely that these two ligands are involved in a fast
rotation about the WdC bonds. If so, one would expect to observe
three alkylidene R-hydrogen resonances for the anti,anti- and syn,
anti-configurations10 in the 1H NMR spectrum at low temperature.
The presence of a single dCHBut 1H NMR resonance thus
suggests that the two alkylidene ligands adopt an anti,anti-
configuration. Such configuration has been observed in anti,anti-
Os(dCHBut)2(CH2But)2.5a The prochiral tungsten atom in 2a gives
rise to diastereotopic methylene (CHaHbBut) protons with chemical
shifts of 2.08 and -0.77 ppm (2JHa-Hb ) 11.9 Hz). In the 2D-
NOESY spectra of 2a and 2b at 296 K (tmix ) 3 s),10 strong
positiVe cross-peaks were observed between the methylene
(CHaHbBut) protons in 2a and the alkylidene (dCHBut) and
methylene (CH2But) protons in 2b, consistent with a chemical
exchange process between 2a and 2b at this temperature. The
mixture of 2a and 2b is stable as solid, but slowly decomposes
in solution at room temperature, forming HSiButPh2 and unknown
species.
The reactivity of R-hydrogen atoms in â-hydrogen free alkyl
ligands (e.g., ButCH2 and Me3SiCH2) has been of great interest
primarily for the pivotal role of these atoms in the formation of
high-oxidation-state alkylidene and alkylidyne complexes.1-3 The
R-hydrogen atoms in d0 (ButCH2)3TadCDBut and (ButCH2)3Wt
CSiMe3 are also known to undergo exchange among the R-carbon
atoms.2b,4 In the latter case, deuterium labeling and kinetic studies
are consistent with unimolecular and stepwise transfer of two
hydrogen atoms in one alkyl ligand to the alkylidyne ligand in
(ButCH2)3WtCSiMe3. A bis(alkylidene) reactive intermediate
“(ButCH2)2W(dCHSiMe3)(dCHBut)” was proposed in the trans-
fer [(ButCH2)3WtCSiMe3 a “(ButCH2)2W(dCHSiMe3)(dCH-
But)” a (ButCH2)2W(CH2SiMe3)(tCBut)].4 In a d2 bis(alkyl-
idene) complex Os(dCHBut)2(CD2But)2, hydrogen/deuterium
atoms were found to scramble among the R-carbon atoms at 0
°C.5 This exchange is believed to occur through an alkylidyne
reactive intermediate “(ButCH2)3OstCBut ”. Although the ex-
change of R-hydrogen atoms is a fundamental dynamic process
in these archetypical alkylidene and alkylidyne complexes, there
has been no report of a direct observation of such an exchange
† The University of Tennessee.
‡ The University of Delaware.
(1) (a) Clark, D. N.; Schrock, R. R. J. Am. Chem. Soc. 1978, 100, 6774.
(b) Schrock, R. R.; Clark, D. N.; Sancho, J.; Wengrovius, J. H.; Rocklage, S.
M.; Pedersen, S. F. Organometallics 1982, 1, 1645. (c) Hua, F.; Mowat, W.;
Skapski, A. C.; Wilkinson, G. J. Chem. Soc. Chem. Commun. 1971, 1477.
(d) Mowat, W.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1973, 1120. (e)
Andersen, R. A.; Chisholm, M. H.; Gibson, J. F.; Reichert, W. W.; Rothwell,
I. P.; Wilkinson, G. Inorg. Chem. 1981, 20, 3934. (f) Schrock, R. R. Acc.
Chem. Res. 1986, 19, 342. (g) Murdzek, J. S.; Schrock, R. R. In Carbyne
Complexes; Fischer, H., Hofmann, P., Kreissl, F. R., Schrock, R. R., Schubert,
U., Weiss, K., Eds.; VCH: New York, 1988. (h) Mayr, A.; Hoffmeister, H.
AdV. Organomet. Chem. 1991, 32, 227. (i) Kim, H. P.; Angelici, R. J. AdV.
Organomet. Chem. 1987, 27, 51.
Variable-temperature NMR spectra of the isomerization 2b a
2a were studied, and the equilibrium constants, Keq ) [2a]/[2b],
measured between 237 and 287 K are listed in Table 1. A plot of
ln Keq vs 1/T12 gave a linear fit and yielded ∆H° ) -0.9(0.2)
(2) (a) Schrock, R. R. J. Am. Chem. Soc. 1974, 96, 6796. (b) Schrock, R.
R.; Fellmann, J. D. J. Am. Chem. Soc. 1978, 100, 3359. (c) Li, L.; Hung, M.;
Xue, Z. J. Am. Chem. Soc. 1995, 117, 12746. (d) Schrock, R. R. Acc. Chem.
Res. 1979, 12, 98. (e) Schrock, R. R. In Reactions of Coordinated Ligands;
Braterman, P. S., Ed.; Plenum: New York, 1986. (f) Schrock, R. R. J.
Organomet. Chem. 1986, 300, 249. (g) Feldman, J.; Schrock, R. R. Prog.
Inorg. Chem. 1991, 39, 1. (h) AdVances in Metal Carbene Chemistry; Schubert,
U., Ed.; NATO ASI Series, Series C, Vol. 269, 1989. (i) Aguero, A.; Osborn,
J. A. New J. Chem. 1988, 12, 111. (j) Schrock, R. R. Acc. Chem. Res. 1990,
23, 158.
(3) (a) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds;
Wiley: New York, 1988. (b) Rothwell, I. P. Polyhedron 1985, 4, 177. (c)
ComprehensiVe Organometallic Chemistry; Wilkinson, G., Stone, F. G. A.,
Abel, E. W., Eds.; Pergamon: New York, 1982; Vol. 3, Chapter 25 by J. A.
Labinger; Vol. 8, Chapter 54 by R. H. Grubbs. (d) ComprehensiVe Organo-
metallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;
Pergamon: New York, 1982; Vol. 5, Chapter 2 by D. E. Wigley and S. D.
Gray; Chapter 5 by M. J. Winter and S. Woodward.
(4) (a) Caulton, K. G.; Chisholm, M. H.; Streib, W. E.; Xue, Z. J. Am.
Chem. Soc. 1991, 113, 6082. (b) Xue, Z.; Caulton, K. G.; Chisholm, M. H.
Chem. Mater. 1991, 3, 384. (c) Xue Z.; Chuang, S.-H.; Caulton, K. G.;
Chisholm, M. H. Chem. Mater 1998, 10, 2365.
(5) (a) LaPointe, A. M.; Schrock, R. R.; Davis, W. M. J. Am. Chem. Soc.
1995, 117, 4802. (b) There is no H/D scrambling among the neopentyl and
neopentylidene ligands in Re(tCBut)(dCHBut)(CD2But)2 at 80 °C in toluene-
d8. LaPointe, A. M.; Schrock, R. R. Organometallics 1995, 14, 1875.
(6) (a) Fellmann, J. D.; Schrock, R. R.; Rupprecht, G. A. J. Am. Chem.
Soc. 1981, 103, 5752. (b) Fellmann, J. D.; Rupprecht, G. A.; Wood, C. D.;
Schrock, R. R. J. Am. Chem. Soc. 1978, 100, 5964. See also: (c) Diminnie,
J. B.; Hall, H. D.; Xue, Z. J. Chem. Soc. Chem. Commun. 1996, 2383. (d)
Diminnie, J. B.; Xue, Z. J. Am. Chem. Soc. 1997, 119, 12657.
(7) (a) Tilley, T. D. In The Silicon-Heteroatom Bond; Patai, S., Pappoport,
Z., Eds.; Wiley: New York, 1991; Chapters 9 and 10. (b) Sharma, H. K.;
Pannell, K. H. Chem. ReV. 1995, 95, 1351. (c) Xue, Z. Comments Inorg. Chem.
1996, 18, 223. (d) MacKay, K. M.; Nicholson, B. K. In ComprehensiVe
Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;
Pergamon: New York, 1982; Vol. 6, Chapter 43. (e) Aylett, B. J. AdV. Inorg.
Chem. Radiochem. 1982, 25, 1. (f) Colomer, E.; Corriu, R. J. P. Top. Curr.
Chem. 1981, 96, 79. (g) Schubert, U. Transition Met. Chem. 1991, 16, 136.
(8) (a) Xue, Z.; Li, L.; Hoyt, L. K.; Diminnie, J. B.; Pollitte, J. L. J. Am.
Chem. Soc. 1994, 116, 2169. (b) Li, L.; Diminnie, J. B.; Liu, X.; Pollitte, J.
L.; Xue, Z. Organometallics 1996, 15, 3520. (c) McAlexander, L. H.; Hung,
M.; Li, L.; Diminnie, J. B.; Xue, Z.; Yap, G. P. A.; Rheingold, A. L.
Organometallics 1996, 15, 5231. (d) Wu, Z.; Diminnie, J. B.; Xue, Z.
Organometallics 1998, 17, 2917.
(9) Campion, B. K.; Heyn, R. H.; Tilley, T. D. Organometallics 1993, 12,
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(10) See Supporting Information for details.
10.1021/ja982571l CCC: $15.00 © 1998 American Chemical Society
Published on Web 12/10/1998

13520 J. Am. Chem. Soc., Vol. 120, No. 51, 1998
Communications to the Editor
Scheme 1
Table 1. Equilibrium Constants (Keq) for 2b a 2aa
T (K) Keq ( σKeq(ran) T (K) Keq ( σKeq(ran) T (K) Keq ( σKeq(ran)
287 ( 1 3.34 ( 0.05
282 ( 1 3.45 ( 0.04
267 ( 1 3.760 ( 0.004 247 ( 1 4.260 ( 0.004
262 ( 1 3.860 ( 0.001 242 ( 1 4.450 ( 0.002
277 ( 1 3.520 ( 0.005 257 ( 1 3.990 ( 0.016 237 ( 1 4.590 ( 0.006
272 ( 1 3.620 ( 0.015 252 ( 1 4.130 ( 0.011
a The largest random uncertainty is σKeq(ran)/Keq ) 0.05/3/34 ) 1.5%.
The total uncertainty σKeq/Keq of 5.2% was calculated from σKeq(ran)
/
)
Keq ) 1.5% and the estimated systematic uncertainty σKeq(sys)/Keq
5% by σKeq/Keq ) [(σKeq(ran)/Keq)2 + (σKeq(sys)/Keq)2]1/2 11
.
Figure 1. ORTEP of 3 showing 30% thermal ellipsoids. Selected bond
distances (Å) and angles (deg): W-O 1.686(5), W-C(1) 2.112(9),
W-C(6) 2.118(9), W-C(11) 1.920(7), O-W-C(1) 111.8(3), O-W-
C(6) 109.5(3), O-W-C(11) 104.1(3).
kcal/mol and ∆S° ) -0.6(0.8) eu.10,13 The equilibrium constants
Keq range from 4.590(0.006) at 237 K to 3.34(0.05) at 287 K,
indicating that the alkylidyne isomer 2a is favored, and increasing
the temperature shifts the equilibrium toward 2b. The process
2b f 2a is slightly exothermic with ∆H° ) -0.9(0.2) kcal/mol.
This enthalpy change outweighs the entropy change [∆S° ) -0.6-
(0.8) eu] in the isomerization 2b f 2a to give ∆G° ) -0.7(0.4)
kcal/mol at 287(1) K in favor of 2a. It is interesting to note that
the d0 alkylidyne complex 2a is thermodynamically close in
energy to its bis(alkylidene) isomer 2b, although 2a is slightly
more stable. In the R-hydrogen exchange in (ButCH2)3WtCSiMe3
a (ButCH2)2W(CH2SiMe3)(tCBut), the proposed bis(alkylidene)
intermediate “(ButCH2)2W(dCHSiMe3)(dCHBut)” is so much
higher in energy than the ground-state alkylidyne structures that
this intermediate is not observed.4 It is not clear why the energy
difference between 2a and 2b is small, and why 2b can be directly
observed in the current studies.
In the solid-state CPMAS (cross-polarization magic angle
spinning) 13C{1H} NMR of crystalline 2, both 2a and 2b were
observed, indicating that both isomers are present in the crystalline
solids. Crystals of 2 were found to be severely disordered, and
attempts to refine the structure of 2a(2b) were unsuccessful.
When a yellow-orange solution of 2 in benzene-d6 was exposed
to 1 equiv of gaseous O2 at room temperature, a rapid reaction
occurred and the color of the solution turned red. We were
surprised to find the formation of an oxo-alkylidene complex
(ButCH2)2W(dO)[dC(But)(SiButPh2)] (3) in this reaction in 32%
yield by NMR (Scheme 2).10 Formally the silyl ligand in (But-
CH2)2W(tCBut)(SiButPh2) (2a) migrates to the alkylidyne ligand
in this reaction to give an alkylidene ligand [dC(But)(SiButPh2)]
in 3. To our knowledge, this is the first observation of such a
migration of a silyl ligand. It is likely that this is an oxygen-
induced silyl migration, although we cannot at present rule out
other possible pathways. Ahn and Mayr have reported a formal
insertion of an alkylidyne group into a W-N bond and the
elimination of HBr in the reaction of TpW(dCHPh)(dX)Br [Tp
) tris(pyrazolyl)borate; X ) NR, O] with Br2.14
Scheme 2
Spectroscopic properties of 3 are consistent with the structure
assignment.10 The 13C NMR alkylidene resonance of 3 at 269.65
ppm appears as a singlet in the 1H-gated-decoupled 13C spectrum.
The molecular structure of 3 has been determined by X-ray
crystallography, and is shown in Figure 1.15 Complex 3 exhibits
distorted tetrahedral geometry around the tungsten center. The
WdC bond distance of 1.920(7) Å is similar to those observed
for other d0 alkylidene complexes of tungsten.3a,16 The WdO bond
distance of 1.686(5) Å is also similar to those observed for
tungsten-oxo complexes.3a Complex 3, which is thermally stable
at room temperature, reacts further with excess O2 to give
unknown species. Studies are currently underway to probe the
mechanism of the reaction of 2 with O2.
Acknowledgment. We are grateful to the National Science Foundation
Young Investigator Award (CHE-9457368), DuPont Young Professor
Award, Camille Dreyfus Teacher-Scholar Award, and the University of
Tennessee (Faculty Research Award) for support of this research. We
also thank Dr. Jeffrey C. Bryan for his assistance.
Supporting Information Available: Experimental details, the com-
putation of errors in ∆H° and ∆S°, a chart of anti,anti- and syn,anti-2b,
a plot of ln Keq vs 1/T, a partial phase-sensitive 2D-NOESY spectrum of
2 and a complete list of the crystallographic data for 3 (13 pages, print/
PDF). See any current masthead page for ordering information and Web
access instructions.
JA982571L
(15) Crystal data for 3 at 223 K:10 orthorhombic, Pna21, a ) 21.8650(5)
Å, b ) 17.2786(2) Å, c ) 8.31220(10) Å, V ) 3140.32(9) Å3, Z ) 4, R(Rw)
) 2.91(5.29)% for 3931 unique reflections with F > 4σ(F), GOF ) 1.035.
(16) See, e.g.: (a) Churchill, M. R.; Youngs, W. J. Inorg. Chem. 1979,
18, 2454. (b) VanderLende, D. D.; Abboud, K. A.; Boncella, J. M.
Organometallics 1994, 13, 3378. (c) Schrock, R. R.; DePue, R. T.; Feldman,
J.; Yap, K. B.; Yang, D. C.; Davis, W. M.; Park, L.; DiMare, M.; Schofield,
M.; Anhaus, J.; Walborsky, E.; Evitt, E. Kru¨ger, C.; Betz, P. Organometallics
1990, 9, 2262.
(11) Taylor, J. R. An Introduction to Error Analysis: The Study of
Uncertainties in Physical Measurements; University Science Books: Mill
Valley, CA 1982; Chapter 4.
(12) Levine, I. N. Physical Chemistry, 3rd ed.; McGraw-Hill: New York,
1988; pp 307-309.
(13) The uncertainties in ∆H° and ∆S° were computed from the error
propagation formulas derived from the equation -RT ln Keq ) ∆H° - T∆S°.10
(14) Ahn, S.; Mayr, A. J. Am. Chem. Soc. 1996, 118, 7408.