A tungsten silyl alkylidyne complex and its bis(alkylidene) tautomer. Their interconversion and an unusual silyl migration in their reaction with dioxygen

Article abstract of DOI:10.1021/om049031j

Bis(alkylidene) complex W(=CH-t-Bu)₂(CH₂-t-Bu)(Si-t- BuPh₂) (1a) has been found to be in equilibrium with its alkyl alkylidyne tautomer W(=C-t-Bu)(CH₂-t-Bu)₂(Si-t-BuPh ₂) (1b). Bis(alkylidene) complexes are believed to be intermediates in α-H transfer in alkylidyne complexes W(=CSiMe₃)(CH ₂-t-Bu)₃ and W(=¹³C-t-Bu)(CH ₂-t-Bu)₃. The current study represents a rare observation of an exchange between a bis(alkylidene) and an alkyl alkylidyne complex. Thermodynamics and kinetics of the exchange 1a ? 1b have been studied. The equilibrium constant K_eq, for the exchange 1a ? 1b is 3.34(0.05) at 287(1) K, and the thermodynamic parameters of the equilibrium measured are ?H° = -3.8(0.8) kJ/mol, ?S° = -3(3) J/mol-K, and ?G°_287K = -3(2) kJ/mol. The rate constants k₁ and k_-1 were obtained from 2D exchange spectroscopy between 267 and 297 K and are 0.34(0.03) and 0.10(0.01) s_-1, respectively, at 287 K For the conversion 1a - 1b, activation enthalpy and entropy are ?H? ₁ = 75(5) kJ/mol, ?S?₁ = 8(25) J/mol-K; for the conversion 1b - 1a, ?H?_-1 = 78(5) kJ/ mol, ?S? _-1 = 8(25) J/mol-K. In the reaction of 1 with O ₂, the silyl ligand in 1b was found to undergo an unprecedented migration to the alkylidyne ligand to give the alkylidene oxo complex W(=O)[=C(t-Bu)(Si-t-BuPh₂)](CH₂-t-Bu)₂ (2). The structure of 2 has been determined. Density functional theory calculations have been conducted for a series of model 1,2-migration reactions. The results suggest that the formation of 2 might be initiated by a 1,2-silyl migration to generate a triplet W-alkylidene intermediate, which is then trapped by O ₂.

Full text of DOI:10.1021/om049031j

1214
Organometallics 2005, 24, 1214-1224
A Tungsten Silyl Alkylidyne Complex and Its
Bis(alkylidene) Tautomer. Their Interconversion and an
Unusual Silyl Migration in Their Reaction with
Dioxygen
Tianniu Chen,† Xin-Hao Zhang,‡ Changsheng Wang,‡,# Shujian Chen,†
Zhongzhi Wu,† Liting Li,† Karn R. Sorasaenee,† Jonathan B. Diminnie,†
Hongjun Pan,† Ilia A. Guzei,§, Arnold L. Rheingold,§,| Yun-Dong Wu,*,‡ and
Zi-Ling Xue*,†
Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996-1600,
Department of Chemistry, The Hong Kong University of Science and Technology,
Clear Water Bay, Hong Kong, China, and Department of Chemistry & Biochemistry,
The University of Delaware, Newark, Delaware 19716-2522
Received December 8, 2004
Bis(alkylidene) complex W(dCH-t-Bu)2(CH2-t-Bu)(Si-t-BuPh2) (1a) has been found to be
in equilibrium with its alkyl alkylidyne tautomer W(tC-t-Bu)(CH2-t-Bu)2(Si-t-BuPh2) (1b).
Bis(alkylidene) complexes are believed to be intermediates in R-H transfer in alkylidyne
complexes W(tCSiMe3)(CH2-t-Bu)3 and W(t13C-t-Bu)(CH2-t-Bu)3. The current study repre-
sents a rare observation of an exchange between a bis(alkylidene) and an alkyl alkylidyne
complex. Thermodynamics and kinetics of the exchange 1a h 1b have been studied. The
equilibrium constant Keq for the exchange 1a h 1b is 3.34(0.05) at 287(1) K, and the
thermodynamic parameters of the equilibrium measured are ∆H° ) -3.8(0.8) kJ/mol, ∆S°
) -3(3) J/mol‚K, and ∆G°287K ) -3(2) kJ/mol. The rate constants k1 and k-1 were obtained
from 2D exchange spectroscopy between 267 and 297 K and are 0.34(0.03) and 0.10(0.01)
s-1, respectively, at 287 K. For the conversion 1a f 1b, activation enthalpy and entropy are
∆Hq ) 75(5) kJ/mol, ∆Sq ) 8(25) J/mol‚K; for the conversion 1b f 1a, ∆Hq ) 78(5) kJ/
1
1
-1
mol, ∆Sq-1 ) 8(25) J/mol‚K. In the reaction of 1 with O2, the silyl ligand in 1b was found to
undergo an unprecedented migration to the alkylidyne ligand to give the alkylidene oxo
complex W(dO)[dC(t-Bu)(Si-t-BuPh2)](CH2-t-Bu)2 (2). The structure of 2 has been deter-
mined. Density functional theory calculations have been conducted for a series of model
1,2-migration reactions. The results suggest that the formation of 2 might be initiated by a
1,2-silyl migration to generate a triplet W-alkylidene intermediate, which is then trapped
by O2.
Introduction
and their chemistry is largely unknown. The reactivity
of R-H atoms in such alkyl alkylidyne complexes has
been the subject of many studies, as these hydrogen
atoms often play a pivotal role in the formation of the
high-oxidation-state complexes.2,3 The R-H atoms in d0
Transition metal silyl complexes have shown unique
chemistry different from their alkyl analogues.1 We have
been interested in d0 silyl alkylidyne complexes
M(tCR)(CH2R)2SiR′3 [R′3 ) Ph2-t-Bu, 1b; (SiMe3)3, 3;
R ) t-Bu] in part because there are few such complexes
(2) (a) Schrock, R. R. Chem. Rev. 2002, 102, 145. (b) Schrock, R. R.
Dalton Trans. 2001, 2541. (c) Feldman, J.; Schrock, R. R. Prog. Inorg.
Chem. 1991, 39, 1. (d) Aguero, A.; Osborn, J. A. New J. Chem. 1988,
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p 39. (g) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds;
Wiley: New York, 1988. (h) Kim, H. P.; Angelici, R. J. Adv. Organomet.
Chem. 1987, 27, 51. (i) Schrock, R. R.; Fellmann, J. D. J. Am. Chem.
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Soc. 1978, 100, 6774. (k) Mowat, W.; Wilkinson, G. J. Chem. Soc.,
Dalton Trans. 1973, 1120. (l) Andersen, R. A.; Chisholm, M. H.; Gibson,
J. F.; Reichert, W. W.; Rothwell, I. P.; Wilkinson, G. Inorg. Chem. 1981,
20, 3934. (m) Li, L.; Hung, M.; Xue, Z. J. Am. Chem. Soc. 1995, 117,
12746. (n) Xue, Z.; Li, L.; Hoyt, L. K.; Diminnie, J. B.; Pollitte, J. L. J.
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(3) For dn type carbenes and carbynes, see, e.g., refs 2e-h and: (a)
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R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42, 4592. (c) Pollagi,
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Series C 1993, 392, 71. (d) Yong, B. S.; Nolan, S. P. Chemtracts 2003,
16, 205. (e) Doyle, M. P. Pure Appl. Chem. 1998, 70, 1123.
* To whom correspondence should be addressed. E-mail: xue@
utk.edu; chydwu@ust.hk.
† The University of Tennessee.
‡ The Hong Kong University of Science and Technology.
# Current address: Department of Chemistry, Liaoning Normal
University, Dalian 116029, China.
§ The University of Delaware.
Current address: Department of Chemistry, The University of
Wisconsin-Madison, Madison, WI 53706-1396.
| Current address: Department of Chemistry and Biochemistry,
University of California-San Diego, La Jolla, CA 92093-0332.
(1) (a) Tilley, T. D. In The Silicon-Heteroatom Bond; Patai, S.,
Rappoport, Z., Eds.; Wiley: New York, 1991; p 245 and p 309. (b) Eisen,
M. S. In The Chemistry of Organic Silicon Compounds, Vol. 2,
Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998, Part 3, p 2037.
(c) Corey, J. Y.; Braddock-Wilking, J. Chem. Rev. 1999, 99, 175. (d)
Sharma, H. K.; Pannell, K. H. Chem. Rev. 1995, 95, 1351. (e) Harrod,
J. F.; Mu, Y.; Samuel, E. Polyhedron 1991, 10, 1239. (f) Reichl, J. A.;
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10.1021/om049031j CCC: $30.25 © 2005 American Chemical Society
Publication on Web 02/17/2005

Tungsten Silyl Alkylidyne Complex
Organometallics, Vol. 24, No. 6, 2005 1215
alkyl alkylidyne complexes W(tCR′)(CH2R)3 are also
known to undergo exchanges among the R-C atoms,
leading to alkyl-alkylidyne scrambling in W(t13C-t-
Bu)(CH2-t-Bu)3 and W(tCSiMe3)(CH2-t-Bu)3 (eqs 1, 2).4
Deuterium-labeling and kinetic studies of the R-H
migration in W(tCSiMe3)(CH2-t-Bu)3 showed stepwise
transfer of two H atoms in one alkyl ligand to the
alkylidyne ligand with the proposed bis(alkylidene)
intermediate “W(dCHSiMe3)(dCH-t-Bu)(CH2-t-Bu)2” (eq
1).4b H/D scrambling was also observed in the d2 Os bis-
(alkylidene) complex Os(dCH-t-Bu)2(CD2-t-Bu)2, and
this exchange is believed to involve the alkylidyne
intermediate “Os(tC-t-Bu)(CD2-t-Bu)2(CH2-t-Bu)” (eq
3).5
undergoes an unprecedented migration to the alkylidyne
ligand in 1b to give the alkylidene ligand in 2. In
comparison, we found that W(tC-t-Bu)(CH2-t-Bu)2[Si-
(SiMe3)3] (3),2n which differs from 1b only in the silyl
group, reacts with O2 to give HSi(SiMe3)3 and other
unidentified decomposition products.
Although reactions of O2 with metal complexes are
of fundamental importance to many catalytic and
biological processes, studies of such reactions have been
mostly concentrated to dn complexes.9,10 Oxidation of the
metals is often involved in these reactions with O2.
Although many d0 early transition metal complexes are
O2-sentitive, the nature of the reactions of these com-
plexes with O2 is largely unknown. Few studies have
been conducted of the reactions of O2 with d0 high-
oxidation-state transition metal complexes. Schwartz,10a
Gibson,10b and co-workers reported oxygen insertion into
the Zr-R bond in the reaction of ZrCp2RCl with O2.
Similar reactions between ZrCp2R2 and O2 were shown
by Brindley and Scotton to give ZrCp2(OR)2.10c Bercaw
and co-workers studied the conversion of HfCp*2(R)(OO-
t-Bu) to HfCp*2(OR)(O-t-Bu).10d In Cp-free complexes,
Wolczanski, Rothwell, Gibson, and their co-workers
reported reactions of O2 with Ti(OR)2Me2,10e Ta(OAr)2-
Me3,10f and Mo(dNAr)2Me2
to yield Ti(OR)2(OMe)2,
10g
(9) (a) Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidations of
Organic Compounds; Academic Press: New York, 1981. (b) The
Activation of Dioxygen and Homogeneous Catalytic Oxidation; Barton,
D. H., Martell, A. E., Sawyer, D. T., Eds.; Plenum: Press: New York,
1993. Oxygen Complexes and Activation by Transition Metals, Martell,
A. E., Sawyer, D. T., Eds.; Plenum Press: New York, 1988. (c) Chem.
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Espenson, J. H. Adv. Inorg. Chem. 2003, 54, 157. (i) Roesky, H. W.;
Haiduc, I.; Hosmane, N. S. Chem. Rev. 2003, 103, 2579. (j) Zhang, C.
X.; Liang, H.-C.; Humphreys, K. J.; Karlin, K. D. Catal. Met. Complexes
2003, 26, 79. (k) Theopold, K. H.; Reinaud, O. M.; Blanchard, S.;
Leelasubcharoen, S.; Hess, A.; Thyagarajan, S. ACS Symp. Ser. 2002,
823, 75. (l) Tolman, W. B.; Que, L., Jr. J. Chem. Soc., Dalton Trans.
2002, 5, 653. (m) Blackman, A. G.; Tolman, W. B. Struct. Bonding
(Berlin) 2000, 97, 179. (n) Zuberbuhler, A. Chimia 1999, 53, 239. (o)
Suzuki, M. Pure Appl. Chem. 1998, 70, 955. (p) Bianchini, C.; Zoellner,
R. W. Adv. Inorg. Chem. 1997, 44, 263. (q) Bakac, A. Prog. Inorg. Chem.
1995, 43, 267.
(10) (a) Blackburn, T. F.; Labinger, J. A.; Schwartz, J. Tetrahedron
Lett. 1975, 16, 3041. Labinger, J. A.; Hart, D. W.; Seibert, W. E.;
Schwartz, J. J. Am. Chem. Soc. 1975, 97, 3851. (b) Gibson, T.
Organometallics 1987, 6, 918. (c) Brindley, P. B.; Scotton, M. J. J.
Chem. Soc., Perkin Trans. 2 1981, 419. (d) Van Asselt, A.; Santarsiero,
B. D.; Bercaw, J. E. J. Am. Chem. Soc. 1986, 108, 8291. Coughlin, E.
B.; Bercaw, J. E. Organometallics 1992, 11, 465. (e) Lubben, T. V.;
Wolczanski, P. T. J. Am. Chem. Soc. 1987, 109, 424. Lubben, T. V.;
Wolczanski, P. T. J. Am. Chem. Soc. 1985, 107, 701. (f) Wang, R.-J.;
Folting, K.; Huffman, J. C.; Chamberlain, L. R.; Rothwell, I. P. Inorg.
Chim. Acta 1986, 120, 81. (g) Gibson, V. C.; Redshaw, C.; Walker, G.
L. P.; Howard, J. A. K.; Hoy, V. J.; Cole, J. M.; Kuzmina, L. G.; De
Silva, D. S. J. Chem. Soc., Dalton Trans. 1999, 161. (h) Brindley, P.
B.; Hodgson, J. C. J. Organomet. Chem. 1974, 65, 57. (i) Kim, S.-J.;
Jung, I. N.; Yoo, B. R.; Cho, S.; Ko, J.; Kim, S. H.; Kang, S. O.
Organometallics 2001, 20, 1501. (j) Schaverien, C. J.; Orpen, A. G.
Inorg. Chem. 1991, 30, 4968. Schaverien, C. J. J. Chem. Soc., Chem.
Commun. 1991, 458. (k) Adam and co-workers used O2 to oxidize d0
Ti enolates to give R-hydroxy amides. Adam, W.; Metz, M.; Prechtl,
F.; Renz, M. Synthesis 1994, 563. (l) Adam and co-workers used
oxidation of alkenyl complexes Tp*W(dO)2R by singlet 1O2 to yield
allylic hydroperoxides. Adam, W.; Putterlik, J.; Schuhmann, R. M.;
Sundermeyer, J. Organometallics 1996, 15, 4586. (m) In the reaction
of L2Mo(dO)(CH2R)2 (L ) 4,4′-dimethyl-2,2′-dipyridyl) with 18O2, Sen
and Vetter observed the formation of RCHd18O and decomposition of
the complex. Vetter, W. M.; Sen, A. Organometallics 1991, 10, 244.
(n) Bercaw and co-workers found that the formation of Cp*2Ta(CH2R)-
(η2-O2) [and later Cp*2Ta(O-CH2R)(dO)] in the reaction of d0 Cp*2-
Ta(dCHR)H with O2 is consistent with a mechanism involving a d2
intermediate “Cp*2Ta(CH2R)”. van Asselt, A.; Trimmer, M. S.; Henling,
L. M.; Bercaw, J. E. J. Am. Chem. Soc. 1988, 110, 8254.
High-oxidation-state bis(alkylidene) complexes were
first reported by Schrock, Churchill, and co-workers.6
Direct observations of exchanges between alkyl alkyli-
dyne and bis(alkylidene) tautomers are rare.7 The
phosphine-promoted exchange W(tCSiMe3)(CH2SiMe3)3-
(PMe3) h W(dCHSiMe3)2(CH2SiMe3)2(PMe3) was re-
cently reported.7 In the absence of phosphine, bis-
(alkylidene) “W(dCHSiMe3)2(CH2SiMe3)2” was not ob-
served. d0-Silyl alkylidyne complex W(tC-t-Bu)(CH2-t-
Bu)2(Si-t-BuPh2) (1b) has been found to exchange with
its bis(alkylidene) tautomer W(dCH-t-Bu)2(CH2-t-Bu)-
(Si-t-BuPh2) (1a).8 The silyl ligand in 1a/1b clearly plays
an important role in the current alkyl alkylidyne-bis-
(alkylidene) exchange, as no such exchange has been
directly observed in W(t13C-t-Bu)(CH2-t-Bu)3, the alkyl
analogue of 1b (eq 2).4a
We were also surprised to find that the equilibrium
mixture of d0 1a h 1b reacts with O2 to give a silyl-
substituted alkylidene oxo complex, W(dO)[dC(t-Bu)-
(Si-t-BuPh2)](CH2-t-Bu)2 (2). In this reaction, the silyl
ligand in d0 W(tC-t-Bu)(CH2-t-Bu)2(Si-t-BuPh2) (1b)
(4) (a) Xue, Z.; Chuang, S.-H.; Caulton, K. G.; Chisholm, M. H. Chem.
Mater. 1998, 10, 2365. (b) Caulton, K. G.; Chisholm, M. H.; Streib, W.
E.; Xue, Z. J. Am. Chem. Soc. 1991, 113, 6082. (c) Xue, Z.; Caulton, K.
G.; Chisholm, M. H. Chem. Mater. 1991, 3, 384.
(5) (a) LaPointe, A. M.; Schrock, R. R.; Davis, W. M. J. Am. Chem.
Soc. 1995, 117, 4802. (b) LaPointe, A. M.; Schrock, R. R. Organome-
tallics 1995, 14, 1875.
(6) (a) Fellmann, J. D.; Rupprecht, G. A.; Wood, C. D.; Schrock, R.
R. J. Am. Chem. Soc. 1978, 100, 5964. (b) Churchill, M. R.; Youngs,
W. J. Inorg. Chem. 1979, 18, 1930. (c) Fellmann, J. D.; Schrock, R. R.;
Rupprecht, G. A. J. Am. Chem. Soc. 1981, 103, 5752. See also: (d)
Diminnie, J. B.; Hall, H. D.; Xue, Z.-L. Chem. Commun. 1996, 2383.
(e) Diminnie, J. B.; Blanton, J. R.; Cai, H.; Quisenberry, K. T.; Xue, Z.
Organometallics 2001, 20, 1504.
(7) Morton, L. A. Zhang, X.-H.; Wang, R.; Lin, Z.; Wu, Y.-D.; Xue,
Z.-L. J. Am. Chem. Soc. 2004, 33, 10208.
(8) Preliminary results were published earlier. Chen, T.-N.; Wu, Z.-
Z.; Li, L.-T.; Sorasaenee, K. R.; Diminnie, J. B.; Pan, H.-J.; Guzei, I.
A.; Rheingold, A. L.; Xue, Z.-L. J. Am. Chem. Soc. 1998, 120, 13519.

1216 Organometallics, Vol. 24, No. 6, 2005
Chen et al.
Scheme 1. Preparation of d0 W Silyl Alkylidyne
Complex 1b and Its Exchange with the
Bis(alkylidene) Tautomer 1a
Table 1. Equilibrium Constants (KEq) for 1a h 1b
a
T (K)
Keq[σKeq(ran)]
287(1)
282(1)
277(1)
272(1)
267(1)
262(1)
257(1)
252(1)
247(1)
242(1)
237(1)
3.34(0.05)
3.45(0.04)
3.520(0.005)
3.620(0.015)
3.760(0.004)
3.860(0.001)
3.990(0.016)
4.130(0.011)
4.260(0.004)
4.450(0.002)
4.590(0.006)
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
.
Ta(OAr)2(OMe)3, and [Mo(dNAr)2Me(µ-OMe)]2, respec-
tively (eq 4). Autoxidation of M(CH2R)4 by O2 was found
to be rapid.10h Yoo and co-workers found that reactions
of O2 with chelating diamides Ti(N∼N)MeX (X ) Me,
Cl; N∼N ) silacyclo-alkane or -alkene bridge) gave [Ti-
(N∼N)(µ-OMe)X]2.10i In contrast, the reaction of alkyl-
free Ti(N∼N)Cl2 with O2 led to diamide ligand oxida-
tion.10i Schaverien and Orpen reported that AlLY(µ-
Me)2Me2 (L ) porphyrin) activates O2 to give AlLY(µ-
OMe)2Me2.10j
and 20 °C. This observation suggests that the two
alkylidene ligands adopt an anti,anti-configuration, and
it is unlikely that these two ligands are involved in a
fast rotation about the WdC bonds. Barriers to the
rotation of alkylidene ligands about the MdC bonds are
usually high.2a The anti,anti-configuration has also been
observed in Os(dCH-t-Bu)2(CH2-t-Bu)2.5a The prochiral
W atom in 1b gives rise to diastereotopic methylene
(CHaHb-t-Bu) protons with chemical shifts of 2.08 and
-0.77 ppm (2JHa-Hb ) 11.9 Hz). The mixture of 1a and
1b is stable as a solid, but slowly decomposes in solution
at room temperature, forming HSi-t-BuPh2 and un-
known species.
In the solid-state CPMAS (cross-polarization magic
angle spinning) 13C{1H} NMR spectrum of crystalline
1 at 23 °C, both 1a and 1b were observed, indicating
that both isomers are present even in the crystalline
solids.13 Crystals of 1 were found to be severely disor-
dered, and attempts to fully refine the structure of 1
were unsuccessful. The presence of both 1a and 1b as
well as their R-H exchange in the solid state, as observed
in the 13C SSNMR of 1 (1a/1b),13 may lead to the
disorder in the structure of 1.
In the 2D NOESY spectra of 1a and 1b at 296 K (tmix
) 3 s), strong positive cross-peaks were observed
between the methylene (CHaHb-t-Bu) protons in 1b and
the alkylidene (dCH-t-Bu) and methylene (CH2-t-Bu)
protons in 1a, consistent with a chemical exchange
process between 1a and 1b at this temperature.
Thermodynamic Studies of the Exchange 1a h
1b. Variable-temperature NMR spectra of the isomer-
ization bis(alkylidene) 1a h alkylidyne 1b were studied,
and the equilibrium constants, Keq ) [1b]/[1a], mea-
sured between 237 and 287 K are listed in Table 1. A
plot of ln Keq vs 1/T (Figure 1) gave a linear fit and
yielded ∆H° ) -3.8(0.8) kJ/mol and ∆S° ) -3(3) J/mol‚
K. 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 1b is favored, and increasing the tem-
perature shifts the equilibrium toward bis(alkylidene)
1a. The process 1a h 1b is slightly exothermic with ∆H°
) -3.8(0.8) kJ/mol. This enthalpy change outweighs the
entropy change [∆S° ) -3(3) J/mol‚K] in the isomer-
ization 1a h 1b to give ∆G° ) -2.9(1.7) kJ/mol at 287-
(1) K in favor of 1b. Increasing the temperature shifts
the equilibrium toward the alkylidyne tautomer. In
comparison, the bis(alkylidene) tautomer is favored in
n/2 O2
MRn
8 M(OR)n
(4)
We have investigated thermodynamics and kinetics
of the unusual bis(alkylidene) h alkyl alkylidyne ex-
change W(dCH-t-Bu)2(CH2-t-Bu)(Si-t-BuPh2) (1a) h
W(tC-t-Bu)(CH2-t-Bu)2(Si-t-BuPh2) (1b). Experimental
and theoretical studies have also been conducted to
explore the mechanistic pathways in the formation of
the silyl-substituted alkylidene oxo complex W(dO)-
[dC(t-Bu)(Si-t-BuPh2)](CH2-t-Bu)2 (2). Theoretical stud-
ies have been performed to model the silyl migration in
1b to give 2 and potential silyl migration in 3. These
studies and the preparation and characterization of 1
and 2 are reported.
Results and Discussion
Preparation and Characterization of d0 W Silyl
Alkylidyne Complex 1b. The addition of 1 equiv of
HCl to W(tC-t-Bu)(CH2-t-Bu)311 led to the formation of
thermally unstable alkylidyne W(tC-t-Bu)(CH2-t-Bu)2-
Cl (4). The reaction of 4 with 1 equiv of Li(THF)3Si-t-
BuPh2 (5)12 at -40 °C (Scheme 1), followed by workup
of the product at -10 °C and crystallization at -30 °C,
yielded crystalline 1 in 58% yield (Scheme 1). Spectro-
scopic properties [1H, 13C{1H}, 1H-gated-decoupled 13C,
1H-13C heteronuclear correlation (HETCOR), and 29Si-
{1H} NMR] of 1a and 1b were consistent with the
structure assignments and the existence of the two
tautomers in solution. The resonances of the alkylidene
ligands in 1a and alkylidyne ligand in 1b appeared as
a doublet at 272.30 ppm and a singlet at 318.38 ppm,
respectively, in the 1H-gated-decoupled 13C spectra.
1
There was one H NMR resonance at 6.03 ppm (dCH-
t-Bu) for the two alkylidene ligands in 1a between -20
(11) Schrock, R. R.; Sancho, J.; Pederson, S. F. Inorg. Synth. 1989,
26, 44.
(12) Campion, B. K.; Heyn, R. H.; Tilley, T. D. Organometallics 1993,
12, 2584.
(13) See Supporting Information for details.

Tungsten Silyl Alkylidyne Complex
Organometallics, Vol. 24, No. 6, 2005 1217
temperature 1D NMR, I1a1a and I1b1b are the diagonal
peak intensities of 1a and 1b, respectively, and I1a1b
and I1b1a are the cross-peak intensities.
k′ ) (1/tm) ln[(r + 1)/(r - 1)]
(5)
r ) {4X1aX1b(I1a1a + I1b1b)/(I1a1b + I1b1a)} -
(X1a - X1b)2 (6)
Considering that the forward and reverse rates (R1,
R-1) at the equilibrium 1a h 1b are equal (eq 7), the
rate constants for the conversions 1a f 1b and 1b f
1a are given in eq 8.
Figure 1. Plot of ln Keq vs 1/T of the equilibrium 1a h
1b.
R1 ) X1ak1 ) X1bk-1 ) R-1
(7)
k1 ) k′/(1 + X1a/X1b) and k-1 ) k′/(1 + X1b/X1a) (8)
the other known, PMe3-promoted alkylidyne h bis-
(alkylidene) exchange, W(tCSiMe3)(CH2SiMe3)3(PMe3)
h W(dCHSiMe3)2(CH2SiMe3)2(PMe3). Keq ranges from
9.37(0.12) at 303(1) K to 12.3(0.2) at 278(1) K, and ∆H°
) -8(2) kJ/mol, ∆S° ) -6(7) J/mol‚K for this equilib-
rium.7
Two separate experiments were performed at each
temperature. Within each experiment, two independent
values of r1 and r2 were obtained from the intensities
of CHd (1a)/CHaHb (1b) and CH2 (1a)/CHaHb (1b)
peaks, respectively. The averages of r1 and r2 from the
two experiments r1-av and r2-av are listed in Table 2.
The values of k′1-av and k′2-av were calculated from r1-av
and r2-av, and their averages were used to yield k1 and
k-1, which are the first-order rate constants for the
conversions 1a f 1b and 1b f 1a, respectively. In the
current systems, the exchange rates k1 of 0.12(0.03) s-1
and k-1 of 0.035(0.008) s-1 at 297(1) K are much higher
It is interesting to note that the d0 alkylidyne complex
1b is thermodynamically close in energy to its bis-
(alkylidene) isomer 1a, although 1b is slightly more
stable. In the R-H exchange in W(tCSiMe3)(CH2-t-Bu)3
h W(CH2SiMe3)(tC-t-Bu)(CH2-t-Bu)2, the proposed bis-
(alkylidene) intermediate “W(dCHSiMe3)(dCH-t-Bu)-
(CH2-t-Bu)2” is so much higher in energy than the
ground-state alkylidyne structure that this intermediate
is not observed.4
than those [kfor ) 10.71(0.10) × 10-6 s-1 and krev
)
10.55(0.10) × 10-5 s-1, respectively, at 298(1) K] in the
only other known, phosphine-promoted bis(alkylidene)
h alkyl alkylidyne exchange W(dCHSiMe3)2(CH2SiMe3)2-
(PMe3) h W(tCSiMe3)(CH2SiMe3)3(PMe3).7 k1 and k-1
were then used in least-squares Eyring plots (Figure 2)
to give the activation parameters.
Kinetic Studies of the Exchange 1a h 1b. 2D
exchange spectroscopy (2D EXSY) based on the 2D
NOESY pulse sequence has been established as a
powerful structure determination method.14-17 Quan-
titative exchange rates could also be derived from this
2D EXSY experiment with a careful choice of the mixing
time lengths.16 The kinetics of the exchange among R-H
atoms in 1a and 1b was investigated between 267 and
297 K by 2D exchange spectroscopy. Representative
The theoretical studies conducted by Prof. Zhen-Yang
Lin and co-workers reveal that thermodynamically the
d0 alkylidyne complex W(tC-t-Bu)(CH2-t-Bu)2(Si-t-
BuPh2) (1b) and its bis(alkylidene) tautomer W(dCH-
t-Bu)2(CH2-t-Bu)(Si-t-BuPh2) (1a) were close in energy,
and the alkylidyne tautomer 1b is slightly more stable.18
These studies show that the relative stabilities of the
tautomeric pairs of W and Mo alkylidyne complexes
M(tCH)(CH3)2(X) and bis(alkylidene) complexes M(d
CH2)2(CH3)(X) (X ) Cl, CH3, CF3, SiH3, and SiF3)
increase with the increasing π-accepting ability of X.
When X is a silyl ligand, the tautomeric pair have
similar stabilities. These results have been explained
in terms of π interaction between ligand X and the
electron density in the metal-alkylidyne/alkylidene
bonds.
1
phase-sensitive H EXSY spectra of (1a/1b) are shown
in the Supporting Information.13 At temperatures e267
K, cross-peaks between CHd (1a)/CHaHb (1b) and
between CH2 (1a)/CHaHb (1b) were too weak to be
detected,13 indicating that the exchange was nearly
frozen at these temperatures. As the temperature was
increased, the intensities of the exchange cross-peaks
increased. Between 277 and 297 K, the spectra13 dis-
played high-intensity cross-peaks for CHd (1a)/CHaHb
(1b) and for CH2 (1a)/CHaHb (1b), respectively, as
expected for the exchange process. The 2D EXSY data
were treated quantitatively using the methods sum-
marized by Perrin and Dwyer.16 Each of the sets of
diagonal and cross-peak intensities was processed,
assuming a two-site exchange system according to eqs
5 and 6, in which k′ is the sum of the forward and
reverse rate constants, tm is the mixing time, X denotes
the mole fractions of the two sites, which is a variable
and calculated from Keq in the aforementioned variable-
Reaction of a d0 Silyl Alkylidyne Complex with
O2. Preparation and Characterization of a W
Alkylidene Oxo Complex, W(dO)[dC(t-Bu)(Si-t-
BuPh2)](CH2-t-Bu)2 (2). When a yellow-orange solution
of 1 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 alkylidene oxo complex W(dO)-
[dC(t-Bu)(Si-t-BuPh2)](CH2-t-Bu)2 (2) in this reaction in
32% yield by NMR (Scheme 2). Formally the silyl ligand
(14) Beck, S.; Lieber, S.; Schaper, F.; Geyer, A.; Brintzinger, H. J.
Am. Chem. Soc. 2001, 123, 1483.
(15) Carpentier, J.; Maryin, V. P.; Luci, J.; Jordan, R. F. J. Am.
Chem. Soc. 2001, 123, 898.
(16) Perrin, C. L.; Dwyer, T. J. Chem. Rev. 1990, 90, 935.
(17) Orrell, K. G.; Sˇik, V. Annu. Rep. NMR Spectrosc. 1993, 27, 103.
(18) Choi, S.-H.; Lin, Z.; Xue, Z. Organometallics 1999, 18, 5488.

1218 Organometallics, Vol. 24, No. 6, 2005
Table 2. Exchange Rate Constants Derived from the 2D H EXSY Spectra of 1a h 1bb
Chen et al.
1
b
b
Ta (K)
X1a/X1b
tm (s)
r1-av
k′1-avc (s-1
)
r2-av
k′2-avc (s-1
)
k1d (s-1
)
k-1d (s-1
)
297
292
287
282
277
272
267
0.341(5)
0.307(6)
0.299(5)
0.289(4)
0.284(5)
0.276(2)
0.266(4)
0.3
0.4
0.5
0.6
0.7
0.8
1.0
4.832(3)
3.141(2)
1.402(4)
0.832(2)
0.411(4)
0.213(2)
0.131(2)
0.071(4)
0.036(6)
4.578(4)
5.619(3)
1.478(3)
0.901(4)
0.472(2)
0.269(3)
0.179(3)
0.079(2)
0.047(5)
1.10(4)
0.66(4)
0.34(3)
0.19(3)
0.12(3)
0.059(6)
0.033(6)
0.35(1)
0.20(1)
0.10(1)
0.054(8)
0.035(8)
0.016(1)
0.009(1)
9.772(3)
8.547(4)
15.925(5)
22.053(4)
35.483(4)
55.054(6)
12.364(2)
15.925(5)
31.303(3)
42.667(8)
a The estimated uncertainty in temperature measurement was 1 K. b r1 and r2 were calculated from the intensities of the CHd (1a)/
CHaHb (1b) and CH2 (1a)/CHaHb (1b) peaks, respectively, according to eq 6. The averages r1-av and r2-av from two experiments are given
here. c k′1-av and k′2-av were calculated from r1-av and r2-av according to eq 5. The total uncertainties σk/k in k′1 and k′2 were calculated
from σk(ran)/k and the estimated systematic uncertainty σk(sys)/k ) 5% by σk/k ) [(σk(ran)/k)2 + (σk(sys)/k)2]1/2 d k1 and k-1 were calculated
.
according to eq 8 from the averages of k′1-av and k′2-av
.
Figure 2. Eyring plots for the 1a h 1b exchange.
Scheme 2. Reactions of an Equilibrium Mixture 1a
h 1b with O2 and H2O
Figure 3. ORTEP of 2 showing 50% probability thermal
ellipsoids.
Complex 2 exhibits distorted tetrahedral geometry
around the W center with interligand angles in the
range 104.1(3)-114.8(3)°. The WdC bond distance of
1.920(7) Å is similar to those observed in other d0
tungsten alkylidene complexes.20 The WdO bond dis-
tance of 1.686(5) Å is also similar to those observed in
tungsten oxo complexes.20a 2, which is thermally stable
at room temperature, reacts further with excess O2 to
give unknown species.
Preparation and Characterization of W(tC-t-
Bu)(CH2-t-Bu)2(OSi-t-BuPh2) (6) and Studies of the
Mechanistic Pathway in the Formation of W(dO)-
[dC(t-Bu)(Si-t-BuPh2)](CH2-t-Bu)2 (2). The interest-
ing silyl migration reaction to give 2 likely proceeds
through one of two possible pathways (Scheme 3). In
pathway A, one O atom inserts into the W-Si bond in
W(tC-t-Bu)(CH2-t-Bu)2(Si-t-BuPh2) (1a) to give the
intermediate W(tC-t-Bu)(CH2-t-Bu)2(OSi-t-BuPh2) (6).
This insertion is followed by -Si-t-BuPh2 migration to
the alkylidyne ligand to give 2. In pathway B, -Si-t-
BuPh2 migration yields a d2 W(IV) alkylidene complex
7 prior to silyl migration. The reaction of 7 with O2 gives
8, and then 2.
in W(tC-t-Bu)(CH2-t-Bu)2(Si-t-BuPh2) (1b) migrates to
the alkylidyne ligand in this reaction to give the
alkylidene ligand dC(t-Bu)(Si-t-BuPh2) in 2. To our
knowledge, this observation is the first such silyl ligand
migration. 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 W(dCHPh)-
(dX)TpBr [Tp ) tris(pyrazolyl)borate; X ) NR, O] with
Br2.19 The formation of 2 through the reaction of 1 with
H2O was excluded (Scheme 2).
Spectroscopic properties of 2 are consistent with the
structure assignment. The alkylidene resonance of 2 at
269.65 ppm appears as a singlet in the 1H-gated-
decoupled 13C NMR spectrum. The molecular structure
of 2 has been determined by X-ray crystallography and
is shown in Figure 3. Crystallographic data and selective
bond lengths and angles are given in Tables 3 and 4.
To our knowledge, the silyl migration observed here
and a silyl-substituted alkylidene oxo complex have not
(20) (a) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds;
Wiley: New York, 1988. (b) Churchill, M. R.; Youngs, W. J. Inorg.
Chem. 1979, 18, 2454. (c) VanderLende, D. D.; Abboud, K. A.; Boncella,
J. M. Organometallics 1994, 13, 3378. (d) 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.
(19) (a) Ahn, S.; Mayr, A. J. Am. Chem. Soc. 1996, 118, 7408. (b)
Mayr, A.; Ahn, S. Inorg. Chim. Acta 2000, 300-302, 406.

Tungsten Silyl Alkylidyne Complex
Organometallics, Vol. 24, No. 6, 2005 1219
Table 3. Crystal Data and Structure Refinement
for 2
Scheme 3. Proposed Pathways in the Formation
of 2
empirical formula (fw)
temperature
C31H50OSiW (650.65)
-50(2) °C
wavelength
0.71073 Å
cryst syst
orthorhombic
Pna21
space group
unit cell dimens
a ) 21.8650(5) Å, R ) 90°
b ) 17.2786(2) Å, â ) 90°
c ) 8.31220(10) Å, γ ) 90°
3140.32(9) Å3
volume
Z
density(calcd)
abs coeff
F(000)
cryst size
θ range for data collection
index ranges
4
1.376 Mg/m2
3.736 mm-1
1328
0.20 × 0.20 × 0.16 mm3
1.50-28.26°
-28 e h e 28,
-19 e k e 13,
-11 e l e 4
no. of reflns collected
no. of indep reflns
completeness to θ ) 23.32°
abs corr
9837
Scheme 4. Preparation of Silonate 9 and 6
3931 [R(int) ) 0.0334]
99.8%
semiempirical from
equivalents
refinement method
no. of data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]a
R indices (all data)a
largest difference peak
and hole
full-matrix least-squares on F2
3930/1/307
1.035
R1 ) 0.0291, wR2 ) 0.0544
R1 ) 0.0529, wR2 ) 0.0638
0.638 and -1.313 e‚Å-3
absolute struct param
0.015(13)
a wR2 ) [∑w(Fo2 - Fc )2/∑w(Fo2)2]1/2; R ) ∑||Fo| - |Fc||/∑|Fo|; w
2
) 1/[σ2(Fo2) + (aP)2 + bP]; P ) [2Fc + max(Fo2,0)]/3.
2
(CH2-t-Bu)2 (2) was however observed over a period of
a week at room temperature. Pathway A to 2 in Scheme
3 is thus unlikely.
Table 4. Selected Bond Lengths (Å) and Angles
(deg) for 2
DFT Studies of the Mechanistic Pathway B.
Theoretical studies have been conducted to explore the
possibility of pathway B in the formation of 2. All
calculations were performed with the Gaussian 03
package23 using the density functional theory method
of B3LYP.24 Full geometric optimizations were carried
out with basis set I (BSI): LanL2DZ25 for W with f
polarization functions, for Si, Cl with d polarization
functions,26 and 6-311+G* for the rest of the elements.
Single-point energies were calculated with basis set II
(BSII): SDDAll for Si, Cl, and W, and 6-311++G** for
other elements. Vibration frequency calculations were
performed for all the structures with the BSI. The
calculated relative energies shown in the text have been
W-O
1.686(5)
2.112(9)
W-C(11)
W-C(6)
1.920(7)
2.118(9)
W-C(1)
O-W-C(11)
104.1(3)
O-W-C(1)
O-W-C(6)
111.8(3)
C(11)-W-C(1)
C(11)-W-C(6)
C(11)-Si-C(26)
C(26)-Si-C(20)
C(2)-C(1)-W
109.7(3)
109.0(3)
105.8(3)
103.4(3)
125.6(5)
107.6(8)
107.4(7)
124.8(5)
120.8(4)
110.4(6)
110.3(5)
109.5(3)
112.4(4)
114.8(3)
114.8(3)
120.6(5)
109.3(8)
112.3(7)
114.4(5)
114.0(6)
120.5(6)
123.6(6)
C(1)-W-C(6)
C(11)-Si-C(20)
C(11)-Si-C(16)
C(7)-C(6)-W
C(9)-C(7)-C(10)
C(9)-C(7)-C(6)
C(12)-C(11)-W
C(18)-C(16)-Si
C(21)-C(20)-Si
C(25)-C(20)-Si
C(9)-C(7)-C(8)
C(8)-C(7)-C(10)
C(12)-C(11)-Si
Si-C(11)-W
C(17)-C(16)-Si
C(19)-C(16)-Si
been observed.21 We have conducted studies to elucidate
the mechanistic pathway in the formation of 2 in the
current work. First we prepared W(tC-t-Bu)(CH2-t-
Bu)2(OSi-t-BuPh2) (6), a proposed intermediate in path-
way A, from the reaction of W(tC-t-Bu)(CH2-t-Bu)2Cl
(4) with LiOSi-t-BuPh2 (9) to see if it converted to 2
(Scheme 4). 9 was prepared directly from Li-t-Bu and
(Ph2SiO)3 in 73% yield.22 NMR studies of 6 showed that
it slowly decomposed in solution during crystallization
at -30 °C. No isomerization from W(tC-t-Bu)(CH2-t-
Bu)2(OSi-t-BuPh2) (6) to W(dO)[dC(t-Bu)(Si-t-BuPh2)]-
(23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, Jr., T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma,
K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui,
Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.;
Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challa-
combe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; Pople, J. A. Gaussian 03, Revision B.03; Gaussian, Inc.:
Pittsburgh, PA, 2003.
(21) (a) Chisholm, M. H.; Hoffman, D. M.; Huffman, J. C. Inorg.
Chem. 1983, 22, 4291. (b) Cotton, F. A.; Schwotzer, W.; Shamshoum,
E. S. Organometallics 1984, 3, 1770. (c) Listemann, M. L.; Schrock, R.
R. Organometallics 1985, 4, 74.
(22) MOSi-t-BuPh2 (M ) Li, 9; Na, K) were initially prepared from
silanol HOSi-t-BuPh2 (Sieburth, S. M.; Mu, W. J. Org. Chem. 1993,
58, 7584) and alkali metals. The NMR spectra of these complexes
showed that they were relatively pure compounds. However their
reactions with 4 did not yield 6. We suspect that the reactions between
alkali metals and the silanol HOSiButPh2 perhaps were not complete.
(24) (a) Becke, A. D. Phys. Rev. 1988, A38, 3098. (b) Becke, A. D. J.
Chem. Phys. 1993, 98, 1372, 5648. (c) Lee, C.; Yang, W.; Parr, R. G.
Phys. Rev. 1988, B37, 785.
(25) (a) Huzinaga, S. Andzelm, J.; Klobukowski, M.; Radzio-And-
zelm, E.; Sakai, Y.; Tatewaki, H. Gaussian Basis Sets for Molecular
Calculations; Elsevier: Amsterdam, 1984. (b) Wadt, W. R.; Hay, P. J.
J. Chem. Phys. 1985, 82, 284.

1220 Organometallics, Vol. 24, No. 6, 2005
Chen et al.
Scheme 5. Different Migration Groups R in the
Equilibrium of 11a-f h 12a-f
corrected with zero-point energy (ZPE). Atomic charges
were analyzed by natural bond analysis.27
Theoretical Studies of the Silyl Migration. As
depicted in Scheme 3, the key step in pathway B is the
1,2-silyl migration from W to the alkylidyne C atom.
Therefore, we first studied a series of 1,2-migration
reactions28 with different migrating groups including H,
CH3, Cl, and three silyl groups (Scheme 5) to have a
good understanding of the general features of the
reaction. Among them, the -SiMe3 ligand in 11e is to
model the -Si-t-BuPh2 ligand in W(tC-t-Bu)(CH2-t-
Bu)2(Si-t-BuPh2) (1b), while -Si(SiH3)3 in 11f acts as a
model for the -Si(SiMe3)3 ligand in W(tC-t-Bu)(CH2-
t-Bu)2[Si(SiMe3)3] (3). The migration reaction can be
formally regarded as a reductive elimination that
reduces W(VI) in the silyl alkylidyne complexes to
W(IV). The W(IV)-alkylidene intermediates are a d2
system. Therefore, they can be in either low-spin state
(S ) 0, singlet) or high-spin state (S ) 1, triplet).
Calculated structures of W-alkylidyne reactant 11e and
W-alkylidene intermediate in singlet state 12e(S) and
triplet state 12e(T), derived from the silyl migration of
11e, are shown in Figure 4. 11e adopts a distorted
tetrahedral geometry with a small Si-WtC angle of
93°. Although in different spin states, 12e(S) and 12e-
(T) have quite similar geometries; both are in a planar
geometry like the organic double bond. The high-spin
species has a slightly longer WdC bond. All intermedi-
ates derived from other compounds have planar geom-
etries.13
Figure 4. Calculated geometries and relative energies of
11e, the singlet transition state 11e-TS(S), and the triplet
and singlet products of silyl migration in 11e (bond
length: Å; bond angle: deg; energy: kJ/mol).
Scheme 6
Since no perpendicular isomer was located, the planar
geometry of 12 reveals that the WdC π bond is formed
by the dyz of W and the pz of the C atoms. The π
antibonding character makes dyz the highest d orbital
Table 5. Calculated Reaction Energies (kJ/mol)
with ZPE Corrections of Migration Reactions of
11a-f
2
2
(Scheme 6). In the meanwhile, the recombined dx -y and
dxy orbitals are destabilized by the ligands in the xy
2
reactant ∆E (high spin) ∆E (low spin)
plane, while dz and dxz remain basically unchanged.
Although not totally degenerated, the energy levels of
R ) H (11a)
0.0
0.0
0.0
0.0
0.0
0.0
65.1
96.4
258.7
78.1
65.7
112.2
102.1
133.0
298.8
115.1
102.6
151.9
2
d
xz and dz are very close. Therefore, the two d electrons
R ) CH3 (11b)
R ) Cl (11c)
2
in W(IV) prefer to occupy both dxz and dz , giving a high-
spin state (Scheme 6). As shown in Table 5, for each
migrating ligand, the reaction is endothermic. For each
system, the high-spin state is ca. 36-40 kJ/mol more
stable than the low-spin state in the migration product
12. Therefore, further reactions were studied only with
the high-spin state products.
R ) SiH3 (11d)
R ) SiMe3 (11e)
R ) Si(SiH3)3 (11f)
ligands. Migrations of the H atom in 11a and a methyl
group in 11b cost ca. 65.1 and 96.4 kJ/mol, respectively.
However, the Cl migration in 11e is very difficult,
having a reaction energy of ca. 258.7 kJ/mol. Among the
four silyl groups, -SiMe3 has the lowest reaction energy
(65.7 kJ/mol), while -Si(SiH3)3 has the highest reaction
energy (112.2 kJ/mol). The 46.5 kJ/mol difference in the
reaction energy between 11e and 11f is consistent with
the observation that only W(tC-t-Bu)(CH2-t-Bu)2(Si-t-
BuPh2) (1b), with a -Si-t-BuPh2 ligand, undergoes the
migration/oxidation reaction to give the W oxo complex
2, while W(tC-t-Bu)(CH2-t-Bu)2[Si(SiMe3)3] (3), with a
-Si(SiMe3)3 ligand, does not.
It is interesting that the reaction energy for the
migration reaction varies significantly with different
(26) (a) Ehlers, A. W.; Bo¨hme, M.; Dapprich, S.; Gobbi, A.; Ho¨llwarth,
A.; Jonas, V.; Ko¨hler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking,
G. Chem. Phys. Lett. 1993, 208, 111. (b) Ho¨llwarth, A.; Bo¨hme, M.;
Dapprich, S.; Ehlers, A. W.; Gobbi, A.; Jonas, V.; Ko¨hler, K. F.;
Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993,
208, 237.
(27) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88,
899.
(28) Suresh, C. H.; Koga, N. Organometallics 2001, 20, 4333.

Tungsten Silyl Alkylidyne Complex
Organometallics, Vol. 24, No. 6, 2005 1221
Table 6. Calculated Structural Parameters, Natural Charges, and Wiberg Bond Indices of W Alkylidyne
Complexes (bond lengths: Å; bond angles: deg)
natural charge
Wiberg bond index
WtC
W-R
W-C
R-WtC
qW
qX a
qR
WtC
W-X
R ) H (11a)
1.745
1.748
1.744
1.748
1.750
1.750
1.726
2.112
2.303
2.569
2.577
2.565
2.100
2.112
2.101
2.098
2.103
2.099
97.3
103.0
108.2
92.5
0.94
1.06
0.97
0.76
0.74
0.81
-0.20
-0.95
-0.32
0.38
-0.20
-0.31
-0.32
-0.12
0.01
2.635
2.610
2.541
2.596
2.596
2.581
0.886
0.880
0.907
0.894
0.858
0.865
R ) CH3 (11b)
R ) Cl (11c)
R ) SiH3 (11d)
R ) SiMe3 (11e)
R ) Si(SiH3)3 (11f)
93.8
1.28
95.5
-0.38
-0.19
a X is the atom of group R that directly connects to W.
Figure 5. Schematic representation of the singlet and
triplet PES along the Si-C distance in the silyl migration
process.
The reaction energies are roughly correlated with the
calculated R-WtC angles and the bond strength in the
reactant, as shown in Table 6. Chlorine is a good π-donor
to the empty d orbitals of the metal center, and the
donation strengthens the W-Cl bond in 11c. The lone
pair on the chlorine atom are also repulsive with the π
orbitals of the WtC bond in 11c. As a result, the Cl-
WtC angle (108.2°) in 11c is the largest. On the other
hand, the silyl ligands in 11d-f are π-acceptors. How-
ever, they cannot receive back-donation from the d0
W(VI) centers to strengthen the W-Si bonds, but can
have stabilizing interaction with the in-plane π orbital
of the WtC bonds. Therefore, the Si-WtC angles are
small, close to 90°. The three methyl groups in -SiMe3
in 11e behave as electron-withdrawing groups,29 while
the three -SiH3 groups in Si(SiH3)3 in 11f are electron-
donating groups. As a result, the former has a smaller
Si-WtC angle than the latter. This is also indicated
by the calculated NBO charges, as shown in Table 6.
Among the three compounds 11d-f with a silyl group,
the electron density of the Si-W bond gradually shifts
from the Si to W in the order Si(SiH3)3, SiH3, and SiMe3.
11e has a less positive W and a slightly positive silyl
group. On the other hand, 11f has a more positive W
and a negative silyl group. This order parallels with the
calculated relative reaction energies. It seems that the
electron-poor silyl ligand facilitates such a migration
reaction, because low electron density on the Si will
increase its π-accepting ability, which promotes the
tautomerization of a W alkylidyne to a W bis(alkylidene)
complex.18
Figure 6. Calculated geometries and relative energies of
O2 adduct (13e) of 12e for the reaction of 11e with O2 and
the final oxidation product of 14e (bond length: Å; bond
angle: deg; energy: kJ/mol).
could not be located in high-spin configuration. Since
the silyl migration reaction is product determined
(endothermic) and the high-spin state is ca. 36-40 kJ/
mol more stable than the low-spin state, according to
the Hammond postulate the real transition state is
expected to be more stable than the low-spin transition
state 11e-TS(S). While the location of a true transition
state for the silyl migration is difficult because it
involves a spin-crossing, we approximated the transition
state by locating the crossing point of the potential
surfaces of the two states. The partial optimization
approach was adopted to locate the two-state crossing
point.30 The Si-C bond was chosen as the reaction
coordinate. A set of Si-C bond-fixed structures were
separately optimized at two different potential energy
surfaces (S ) 0 and S ) 1). The result is shown in Figure
5. The crossing point of the two states occurs earlier
than the low-spin transition state 11e-TS(S). That
means the system switches from a low-spin state to the
high-spin state without passing the low-spin transition
state. A barrier of about 102.1 kJ/mol was found with
the crossing point.
We have located the transition state [11e-TS(S)] for
the formation of silyl migration product from 11e in low-
spin configuration (Figure 5). The calculated activation
energy is ca. 121.6 kJ/mol, and ca. 19.0 kJ/mol higher
than the reaction energy. As depicted in Figures 4 and
5, 11e-TS(S) closely resembles the product 11e(S),
indicating a late transition state. The transition state
(30) (a) Harvey, J. N. In Computational Organometallic Chemistry,
Cundari, T. R., Ed.; Marcel Dekker: New York, 2001. (b) Poli, R.;
Harvey, J. N. Chem. Soc. Rev. 2003, 32, 1.
(29) Carbon is more electronegative than silicon (the Pauling
electronegativities of carbon and silicon are 2.55 and 1.90, respectively).

1222 Organometallics, Vol. 24, No. 6, 2005
Chen et al.
trap and was transferred quantitatively through a gas mani-
fold. Li-t-Bu in pentane (Strem, 2.0 M) was purified by
removing pentane in vacuo and subliming the residue at 50-
60 °C to give Li-t-Bu as a solid on a coldfinger. Elemental
analyses were performed by Complete Analysis Laboratories
Inc., E&R Microanalytical Division, Parsippany, NJ 07054-
4909.
Theoretical Studies of the O2 Attack. The reaction
between 11e and dioxygen was then studied. First of
all, we found that 11e cannot form a stable complex with
O2. That is in line with the assumption that the
oxidation takes place after silyl migration. Since the
silyl migration step is endothermic, it is expected that
the O2 attack assists the shifting of the equilibrium 11e
h 12e toward 12e. The intermediate 12(T) reacts with
O2, leading to three possible spin states, S ) 0, S ) 1,
or S ) 2, depending upon the ways of coupling the spins
on the W atom with the spin on O2. However, all efforts
to locate the S ) 1 and S ) 2 intermediates failed. Only
the singlet intermediate 13e with an energy of -247.9
kJ/mol with respect to separated 11e and O2 was found.
In intermediate 13e, the O2 occupies one of the tetra-
hedral positions. It is nearly coplanar with one of the
W-CH3 bonds. The two W-O bonds are 1.925 and 1.904
Å, respectively. The O-O distance is about 1.473 Å,
indicating that it is a typical peroxo O-O bond. The O2
binding process is considered to undergo a three-center
oxidative addition. The reaction is exothermic by about
-313.6 kJ/mol. No transition state could be located for
this coupling reaction between the two diradical species.
The conversion of 13e to the final product 14e (a
model of 2) might occur with different pathways because
13e should be a very reactive species. One possibility
is that 13e reacts with 12e(T) to give two molecules of
14e. We have not calculated a transition state for the
reaction. However, the calculated large exothermicity
of ca. -380.8 kJ/mol for the reaction suggests that the
oxygen transfer from peroxo complex 13e to 12e(T)
should be a very fast process. We suggest that it is this
remarkably high exothermic driving force that pushes
the whole reaction. In other words, this is an oxygen-
induced silyl migration.
Preparation of 1 [W(dCH-t-Bu)2(CH2-t-Bu)(Si-t-BuPh2)
(1a) h W(tC-t-Bu)(CH2-t-Bu)2(Si-t-BuPh2) (1b)]. HCl (0.54
mmol, 1.0 M in Et2O) was added dropwise to a vigorously
stirred solution of W(tC-t-Bu)(CH2-t-Bu)3 (0.25 g, 0.54 mmol)
in Et2O (30 mL) at -78 °C. The color of the solution changed
slowly from dark yellow to green-yellow when the solution was
allowed to gradually warm to -40 °C in 3 h. All the volatiles
were removed under vacuum at -40 °C. Li(THF)3Si-t-BuPh2
(5, 0.25 g, 0.54 mmol) in Et2O was added dropwise to the solid
at -40 °C, and the solution was warmed slowly to -10 °C.
Removal of volatiles in vacuo, followed by extraction with
pentane, filtration, and crystallization at -30 °C gave 0.20 g
of 1 (0.32 mmol, 58% yield). Data for 1: Anal. Calcd for C31H50-
SiW: C, 58.67; H, 7.94. Found: C, 58.30; H, 8.31. Data for
1a: 1H NMR (benzene-d6, 250.1 MHz) δ 7.79-7.75, 7.24-7.15
2
(m, 10H, C6H5), 2.08 (d, 2H, CHaHbCMe3, JH-H ) 11.9 Hz),
1.53 (s, 9H, tCCMe3), 1.28 (s, 9H, SiCMe3Ph2), 1.06 (s, 18H,
CH2CMe3), -0.77 (d, 2H, CHaHbCMe3); 13C{1H} (benzene-d6,
62.9 MHz) δ 318.38 (tCCMe3), 143.81, 137.90, 128.59, 127.91
(C6H5), 134.52 (CH2CMe3), 53.87 (tCCMe3), 37.66 (CH2CMe3),
34.14 (CH2CMe3), 32.29 (tCCMe3), 30.60 (SiCMe3), 21.63
(SiCMe3); 29Si{1H} NMR (benzene-d6, 79.5 MHz) δ 73.74 (Si-
t-BuPh2). Data for 1b: 1H NMR (benzene-d6, 250.1 MHz) δ
7.86-7.84, 7.24-7.15 (m, 10H, C6H5), 6.03 (s, 2H, dCHCMe3),
1.29 (s, 9H, SiCMe3Ph2), 1.19 (s, 18H, dCHCMe3), 0.96 (s, 9H,
CH2CMe3), 0.35 (s, 2H, CH2CMe3); 13C{1H} (benzene-d6, 62.9
MHz) δ 272.32 (dCHCMe3, 1JC-H ) 104.1 Hz), 144.31, 137.69,
128.18, 127.61 (C6H5), 131.21 (CH2CMe3), 45.49 (dCHCMe3),
37.31 (CH2CMe3), 35.01 (CH2CMe3), 32.04 (dCHCMe3), 30.30
(SiCMe3), 20.69 (SiCMe3); 29Si{1H} NMR (benzene-d6, 79.5
MHz) δ 62.11 (Si-t-BuPh2).
Preparation of W(dO)[dC(t-Bu)(Si-t-BuPh2)](CH2-t-
Bu)2 (2). 1 (0.50 g, 0.79 mmol) was dissolved in 2.0 mL of
benzene-d6 in a J. Young NMR tube, and the solution was
frozen at -60 °C. The tube was then evacuated, and O2 (0.79
mmol) was introduced into the NMR tube. The solution was
then warmed to room temperature. In 20 min, the color of the
solution changed from brown to red-orange. All the volatiles
were then removed in vacuo. NMR spectra of the solid showed
that ca. 32% (0.25 mmol) of 2 had reacted with O2; 68% of 1
still remained in the reaction mixture. The solid was redis-
solved in pentane, and the solution was filtered. Slow cooling
of the solution to -30 °C gave 2 as a crystalline solid (isolated
weight: 8.8 mg, 0.014 mmol, 5.5% yield based on the amount
of 1 that had reacted with O2). Data For 2: 1H NMR (benzene-
d6, 250.1 MHz) δ 7.79, 7.22-7.15 (m, 10H, Si-t-BuPh2), 3.27
The theoretical calculations here on model 1,2-migra-
tions from W(Me)2(R)(tCH) to W(Me)2(dCHR) indicate
that the reactivity is sensitive to the nature of the -R
ligand. -SiMe3 migration is much more favorable than
-Si(SiMe3)3 migration. The results suggest that the
formation of 2 from the reaction of 1b with O2 might be
initiated by a 1,2-silyl migration in 1b to generate a
triplet W(IV)-alkyldene intermediate, which is trapped
by O2.
Experimental Section
General Procedures. All manipulations, unless otherwise
noted, were performed under a dry nitrogen atmosphere with
the use of either a drybox or standard Schlenk techniques. All
solvents were purified by distillation from potassium/ben-
zophenone ketyl. Benzene-d6 and toluene-d8 were dried over
activated molecular sieves and stored under N2. One-dimen-
2
(d, 2H, CHaHbCMe3, JH-H ) 14.1 Hz), 1.31 (s, 9H, SiCMe3-
Ph2), 1.52 [s, 9H, dC(SiCMe3Ph2)CMe3], 1.19 (s, 18H, CHaH-
bCMe3), 0.34 (d, 2H, CHaHbCMe3); 13C{1H} (benzene-d6, 62.9
MHz) δ 269.65 (WdC), 138.56, 136.52, 130.68, 127.81 (Si-t-
BuPh2), 130.06 (CH2CMe3), 48.24 [dC(SiCMe3Ph2)CMe3], 36.08
(CH2CMe3), 35.27 (CH2CMe3), 34.53 [dC(SiCMe3Ph2)CMe3],
31.48 (SiCMe3), 23.56 (SiCMe3). Anal. Calcd for C31H50OSiW:
C, 57.22; H, 7.75. Found: C, 56.93; H, 7.88.
Preparation of LiOSi-t-BuPh2 (9). To a solution of
hexaphenylcyclotrisiloxane (1.428 g, 2.40 mmol, Gelest) in
Et2O (30 mL) at -56 °C with stirring was added dropwise Li-
t-Bu (0.554 g, 8.64 mmol) in pentane (40 mL) at -56 °C. The
mixture was stirred and gradually warmed to 23 °C overnight.
The solution was then refluxed at 40 °C overnight. All the
volatiles were then removed in vacuo to give 9 as a crude white
solid (1.38 g, 5.26 mmol, 73% yield). The solid was then washed
with cold hexanes and subsequently dried before being sub-
mitted for elemental analysis. Data For 9: 1H NMR (benzene-
1
sional H and 13C NMR spectra, unless noted, were recorded
at 23 °C on a Bruker AC-250 or AMX-400 Fourier transform
(FT) spectrometer and referenced to solvents (residual protons
in the 1H spectra). 1H-13C heteronuclear correlation (HET-
COR) and 29Si{1H} (DEPT) NMR experiments were conducted
at 23 °C on a Bruker AMX-400 FT spectrometer. The 29Si-
{1H} (DEPT) NMR spectra were referenced to SiMe4. 2D
NOESY spectra were recorded at 23 °C on a Varian Mercury
300 FT spectrometer (ns ) 32, tmix ) 3 s, increments ) 128).
2D EXSY spectra (297-267 K) were recorded on an updated
Bruker AMX-400 FT spectrometer. Li(THF)3Si-t-BuPh2 (5)12
and W(tC-t-Bu)(CH2-t-Bu)312 were prepared by the literature
procedures. HCl in Et2O (1.0 M, Aldrich) was used as received.
O2 gas (99.6% purity) was dried by passing through a -78 °C

Tungsten Silyl Alkylidyne Complex
Organometallics, Vol. 24, No. 6, 2005 1223
Table 7. T1 (s) Values for the r-H Atoms of 1a and
d6, 250.1 MHz) δ 7.71-7.55, 7.22-7.15 (m, 10H, C6H5), 0.97
(s, 9H, CMe3); 13C{1H} (benzene-d6, 62.9 MHz) δ 134.80, 129.08,
127.06, 126.67 (C6H5), 28.43 (CMe3), 27.21 (CMe3). Anal. Calcd
for C16H19OSiLi: C, 73.25; H, 7.30. Found: C, 73.18; H, 7.29.
Preparation of W(tC-t-Bu)(CH2-t-Bu)2(OSi-t-BuPh2)
(6). Anhydrous HCl (0.32 mmol, 1.0 M in Et2O) was added
dropwise with vigorous stirring over a 30 min period to W(t
C-t-Bu)(CH2-t-Bu)3 (0.146 g, 0.313 mmol) in Et2O (10 mL) at
-78 °C. The solution was warmed to 0 °C over a period of 2 h
and then stirred for 20 min. A solution of LiOSi-t-BuPh2 (9)
(0.082 g, 0.313 mmol) in Et2O (10 mL) was added at -30 °C.
The mixture was warmed to room temperature in 40 min and
then stirred for another 20 min. The volatiles were removed
in vacuo, and the residue was dissolved in a small amount of
hexanes and filtrated. Volatiles in the filtrate were removed
to give a dark red oil. 4,4′-Dimethylbiphenyl (5.0 mg) as an
1b at 287 and 267 K
T (K)
CHd (1a)
CH2 (1a)
CHaHb (1b)
CHaHb (1b)
287
267
1.35
1.19
0.86
0.55
0.67
0.33
0.63
0.33
3 s (>4T1, vide infra), the initial t1 was 3 µs, and the mixing
time tm was varied according to the experimental temperature
(vide infra). The pulse sequence was repeated for 128 values
of t1; that is, the F1 dimension contained 128 points. The
number of scans per experiment was 32, giving a total
experiment time of ∼270 min. The optimal mixing time tm,opt
was chosen on the basis of eq 11 to minimize the relative error
in the rate constant.15 The activation parameters determined
from the Eyring analysis (Figure 2) were obtained by using
the average rate constants k1 and k-1 for the temperature
range from 267 to 297 K. The activation enthalpies (∆Hq) and
entropies (∆Sq) were calculated from an unweighted nonlinear
least-squares procedure contained in the SigmaPlot Scientific
Graph System. Spin-lattice relaxation times T1 were mea-
sured by using the standard inversion-recovery method. T1
values for the hydrogen atoms of 1a and 1b measured at 287
and 267 K are listed in Table 7.
1
NMR internal standard was added, and the H NMR of the
mixture showed the yield of 6 at 32%. 6 was found unstable,
and repeated attempts to purify it by crystallization led to the
formation of dark red crystals of 6 mixed with unknown solid
impurities.
In another preparation of 6 involving 0.227 g (0.487 mmol)
of W(tC-t-Bu)(CH2-t-Bu)3, 0.487 mL (0.487 mmol) of 1.0 M
HCl in Et2O, and 0.128 g (0.487 mmol) of LiOSi-t-BuPh2 (9)
by a similar procedure, 4.0 mg of dark red crystals for
elemental analysis reported below were isolated from the solid
precipitation from a pentane solution at -30 °C.
tm,opt ≈ 1/(T1-1 + k1 + k-1
)
(11)
Data For 6: 1H NMR (benzene-d6, 250.1 MHz) δ 7.79-7.19
2
Control experiments established that rate constants deter-
mined by EXSY are not significantly affected by a (50%
variation in tm,opt. Uncertainties in rate constant values were
estimated assuming (1% uncertainty in the finite s/N ratios
of the computed spectra and (2% uncertainty in 2D signal
integrations. At least two separate experiments performed for
all temperatures gave the uncertainty. The maximum random
uncertainty in the equilibrium constants was combined with
the estimated systematic uncertainty, ca. 5%. The total
uncertainties in the rate constants were used in the Eyring
plots in Figure 2 and error propagation calculations. The
estimated uncertainty in the temperature measurements for
an NMR probe was 1 K. The uncertainties in ∆Hq and ∆Sq
were computed from the following error propagation formulas
(eqs 12 and 13), which were derived from R ln(kh/kbT) )
-∆Hq/T + ∆Sq.31
(m, 10H, C6H5), 1.88 (d, 2H, CHaHbCMe3, JH-H ) 14.4 Hz),
1.36 (d, 2H, CHaHbCMe3), 1.22 (s, 18H, CHaHbCMe3), 1.20 (s,
9H, tCCMe3), 1.18 (s, 9H, SiCMe3); 13C{1H} (benzene-d6, 62.9
MHz) δ 314.13 (WtCCMe3), 135.44-127.84 (C6H5), 97.06 (CH2-
CMe3), 52.01 (tCCMe3), 34.37 (CH2CMe3), 33.94 (CH2CMe3),
32.71 (tCCMe3), 27.12 (SiCMe3), 20.18 (SiCMe3). Anal. Calcd
for C31H50OSiW: C, 57.22; H, 7.75. Found: C, 57.22; H, 8.06.
Reaction of W(tC-t-Bu)(CH2-t-Bu)2[Si(SiMe3)3] (3) with
O2. 3 (43.9 mg, 0.0683 mmol) in benzene-d6 was added to 1.5
equiv of O2 in a J. Young NMR tube. The mixture was kept at
23 °C for 6 h. A 1H NMR spectrum of the solution then revealed
that HSi(SiMe3)3 was the major product along with other
unidentified compounds.
Thermodynamic Studies of the 1a h 1b Exchange. For
the thermodynamic studies, the equilibrium constants Keq were
obtained from at least two separate experiments at a given
temperature, and their averages are listed (Table 1). The
maximum random uncertainty in the equilibrium constants
was combined with the estimated systematic uncertainty, ca.
5%. The total uncertainties in the equilibrium constants were
used in the ln Keq vs 1/T plot in Figure 1 and error propagation
calculations. The estimated uncertainty in the temperature
measurements for an NMR probe was 1 K. The enthalpy (∆H°)
and entropy (∆S°) changes were calculated from an un-
weighted nonlinear least-squares procedure contained in the
SigmaPlot Scientific Graph System. The uncertainties in ∆H°
and ∆S° were computed from the following error propagation
formulas (eqs 9 and 10), which were derived from -RT ln Keq
) ∆H° - T∆S°.
2
(σ∆Hq)2 ) R2Tmax Tmin2/∆T2{(σT/T)2[(1 + Tmin∆L/∆T)2 +
(1 + Tmax∆L/∆T)2] + 2(σk/k)2} (12)
(σ∆Sq)2 ) R2/∆T2{(σT/T)2[Tmax2(1 + Tmin∆L/∆T)2 +
Tmin2(1 + Tmax∆L/∆T)2] + (σk/k)2(Tmax2 + Tmin2)} (13)
where ∆L ) [ln(kmax/Tmax) - ln(kmin/Tmin)] and ∆T ) (Tmax
Tmin).
-
X-ray Crystal Structure Determination for 2. The
structure of 2 was determined at the University of Delaware.
A suitable crystal was selected and mounted in a thin-walled
glass capillary under an inert atmosphere. The data were
collected on a Siemens P4 diffractometer equipped with a
SMART/CCD detector. The systematic absences in the diffrac-
tion data are consistent for space groups Pna21 and Pnma.
Even though the E-statistics suggested the centrosymmetric
space group, the value of Z and the absence of a molecular
mirror plane indicated the non-centrosymmetric space group.
Both possibilities were explored, but the only solution in the
non-centrosymmetric option yielded chemically reasonable and
computationally stable results of refinement. The structure of
2 was solved using direct methods, completed by subsequent
2
2
(σ∆H°)2 ) (σT/T)2R2(Tmax Tmin4 + Tmin Tmax4) ×
[ln(Keq(max)/Keq(min))]2/∆T4 + 2R2Tmax Tmin2(σKeq/Keq)2/∆T2
2
(9)
2
(σ∆S°)2 ) 2R2Tmin Tmax2[ln(Keq(max)/Keq(min))]2 ×
(σT/T)2/∆T4 + R2(Tmax2 + Tmin2)(σKeq/Keq)2/∆T2 (10)
where ∆T ) (Tmax - Tmin).8
Kinetic Studies of the 1a h 1b Exchange. In the kinetic
studies by 2D EXSY, the pulse sequence was t0-90°-t1-90°-
tm-90°-FID. The initial relaxation delay time t0 was typically
(31) More, P. M.; Spencer, M. D.; Wilson, S. R.; Girolami, G. S.
Organometallics 1994, 13, 1646.

1224 Organometallics, Vol. 24, No. 6, 2005
Chen et al.
difference Fourier syntheses, and refined by full-matrix least-
squares procedures. Absorption corrections were not required
because there was less than 10% variation in the integrated
ψ-scan intensity data. All non-hydrogen atoms were refined
with anisotropic displacement coefficients, and hydrogen atoms
were treated as idealized contributions. All software and
sources of the scattering factors are contained in the SHELXTL
(5.03) program library (G. Sheldrick, Siemens XRD, Madison,
WI).
0212137) and the Research Grant Council of Hong Kong
(HKUST6193/00P) for financial support.
Supporting Information Available: Partial solid-state
CPMAS 13C{1H} and 2D EXSY NMR spectra of the 1a h 1b
exchange, a complete list of crystallographic data for 2,
calculated total energies and calculated geometries with
coordinates of 11a-14e. These materials are available free of
charge via the Internet at http://pubs.acs.org.
Acknowledgment is made to the National Science
Foundation (CHE-9457368, CHE-9904338, and CHE-
OM049031J