Experimental and theoretical investigations of catalytic alkyne Cross-Metathesis with Imidazolin-2-iminato tungsten alkylidyne complexes

Article abstract of DOI:10.1021/om801119t

The imidazolin-2-iminato tungsten alkylidyne complexes [Me ₃CC=W(NIm^R)(OR )₂] (4a: R = tBu, R = CMe ₃; 4b: R = Dipp, R = CMe₃; 5a: R = tBu, R = CMe(CF ₃)₂; 5b: R = Dipp, R = CMe(

Full text of DOI:10.1021/om801119t

1534
Organometallics 2009, 28, 1534–1545
Experimental and Theoretical Investigations of Catalytic Alkyne
Cross-Metathesis with Imidazolin-2-iminato Tungsten Alkylidyne
Complexes
Stephan Beer,† Kai Brandhorst,‡,† Cristian G. Hrib,† Xian Wu,† Birte Haberlag,†
Jo¨rg Grunenberg,‡ Peter G. Jones,† and Matthias Tamm*,†
Institut fu¨r Anorganische and Analytische Chemie and Institut fu¨r Organische Chemie, Technische
UniVersita¨t Carolo-Wilhelmina, Hagenring 30, 38106 Braunschweig, Germany
ReceiVed NoVember 24, 2008
The imidazolin-2-iminato tungsten alkylidyne complexes [Me3CC≡W(NImR)(OR′)2] (4a: R ) tBu,
R′ ) CMe3; 4b: R ) Dipp, R′ ) CMe3; 5a: R ) tBu, R′ ) CMe(CF3)2; 5b: R ) Dipp, R′ ) CMe(CF3)2
have been prepared from [Me3CC≡W(OCMe3)3] (2) and [Me3CC≡W{OCMe(CF3)2}3(dme)] (3, dme )
1,2-dimethoxyethane) by reaction with the lithium reagents Li(NImtBu) and Li(NImDipp), generated with
MeLi from 1,3-di-tert-butylimidazolin-2-imine (ImtBuNH) or 1,3-bis(2,6-diisopropylphenyl)imidazolin-
2-imine (ImDippNH), respectively. Reaction of 3 with Li[N(tBu)Ar] · OEt2 (Ar ) 3,5-dimethylphenyl)
afforded the amido complex [Me3CC≡W{N(tBu)Ar}{OCMe(CF3)2}2] (6). Addition of Li[OCPh(CF3)2]
to [Me3CC≡WCl3(dme)] (1) produced the dme-free complex [Me3CC≡W{OCPh(CF3)2}3] (7), which,
upon treatment with Li(NImtBu), gave the alkylidene complex 8, presumably formed by activation and
addition of an ortho-C-H bond across the W≡C bond in the intermediate alkylidyne complex. Treatment
of 1 with Li[OC(CF3)3] led to the substitution of only two chloride ligands and formation of
cis-[Me3CC≡WCl{OC(CF3)3}2(dme)] (9), which exhibits long-range through-space 19F-19F coupling
between the fluorine atoms of the two OC(CF3)3 ligands. Reaction of 9 with Li(NImtBu) resulted in partial
cleavage of the ImtBuN ligand and ligand redistribution to afford the dinuclear tungsten alkylidyne complex
10. The propylidyne complex [EtC≡W(NImtBu){OCMe(CF3)2}2] (12) was obtained by treatment of 5a
with 3-hexyne, which proceeded via the metallacyclobutadiene complex 11. Complex 5a is able to rapidly
catalyze alkyne cross-metathesis of 3-heptyne to give a statistical 1:2:1 mixture of 3-hexyne, 3-heptyne
and 4-octyne. The catalytic homodimerization of 1-phenylpropyne under vacuum-driven conditions was
studied for 5a, 5b and 6 at 30 and 80 °C. The molecular structures of complexes 2, 3, 4a, 4b, 5b, 8, 9,
10 and 12 were determined by single crystal X-ray diffraction. High-level DFT calculations employing
the B3LYP functional have been carried out for a series of experimentally studied and other alkylidyne
complexes by choosing alkyne metathesis of 2-butyne as the model reaction.
synthesis”.3 In contrast, the metathesis of stronger carbon-carbon
Introduction
triple bonds4 has been less prominent,5 and only a limited
number of active catalyst systems are known to date that fulfill
Although the principle of carbon-carbon multiple bond
metathesis has been known for more than 40 years,1 there is a
huge divergence between the successes of alkene metathesis
and the related alkyne metathesis. The breaking and making of
carbon-carbon double bonds via highly active alkylidene
catalysts has had a tremendous impact on the development of
new methods for the preparation of complex natural products
and novel materials;2 this is exemplified by the award of the
2005 Nobel prize in chemistry to Chauvin, Grubbs and Schrock
“for the development of the metathesis method in organic
(2) For reviews on olefin metathesis see: (a) Schrock, R. R.; Czekelius,
C. AdV. Synth. Catal. 2007, 349, 55–77. (b) Trnka, T. M.; Grubbs, R. H.
Acc. Chem. Res. 2001, 34, 18–29. (c) Grubbs, R. H. Tetrahedron 2004, 60,
7117–7140. (d) Deiters, A.; Martin, S. F. Chem. ReV. 2004, 104, 2199–
2238. (e) Schrock, R. R.; Hoveyda, A. H. Angew. Chem. 2003, 115, 4740–
4782; Angew. Chem., Int. Ed. 2003, 42, 4592-4633. (f) Connon, S. J.;
Blechert, S. Angew. Chem. 2003, 115, 1944–1968; Angew. Chem., Int. Ed.
2003, 42, 1900-1923. (g) Diver, S. T.; Giessert, A. J. Chem. ReV. 2004,
104, 1317–1382. (h) Fu¨rstner, A. Angew. Chem. 2000, 112, 3140–3172;
Angew. Chem., Int. Ed. 2000, 39, 3012-3043. (i) Buchmeiser, M. R. Chem.
ReV. 2000, 100, 1565–1604. (j) Buchmeiser, M. R. AdV. Polym. Sci. 2005,
176, 89–119. (k) Buchmeiser, M. R. Monatsh. Chem. 2003, 134, 327–342.
(l) Handbook of Metathesis; Grubbs, R. H., Ed.; WILEY-VCH: Weinheim,
2003; Vol. 1-3. (m) Alkene Metathesis in Organic Synthesis; Fu¨rstner, A.,
Ed.; Springer: Berlin, 1998.
(3) Nobel lectures: (a) Chauvin, Y. Angew. Chem. 2006, 118, 3824–
3831; Angew. Chem., Int. Ed. 2006, 45, 3741-3747. (b) Schrock, R. R.
Angew. Chem. 2006, 118, 3832–3844; Angew. Chem., Int. Ed. 2006, 45,
3748-3759. (c) Grubbs, R. H. Angew. Chem. 2006, 118, 3845–3850;
Angew. Chem., Int. Ed. 2006, 45, 3760-3765.
(4) (a) March, J. AdVanced Organic Chemistry; John Wiley & Sons:
New York, 1992, p 24. (b) Grunenberg, J. Angew. Chem. 2001, 113, 4150–
4153; Angew. Chem., Int. Ed. 2001, 40, 4027-4029.
(5) (a) Fu¨rstner, A.; Davies, P. W. Chem. Commun. 2005, 2307–2320.
(b) Fu¨rstner, A. In Handbook of Metathesis; Grubbs, R. H., Ed.; WILEY-
VCH, Weinheim, 2003; Vol. 2, p 432.
* To whom correspondence should be addressed. E-mail: m.tamm@
tu-bs.de.
† Institut fu¨r Anorganische and Analytische Chemie.
‡ Institut fu¨r Organische Chemie.
(1) (a) Calderon, N. Chem. Eng. News 1967, 45 (41), 51. (b) Calderon,
N.; Chen, H. Y.; Scott, K. W. Tetrahedron Lett. 1967, 34, 3327. (c) Bands,
R. L.; Bailey, G. C. Ind. Eng. Chem., Prod. Res. DeV. 1964, 170. (d) Katz,
T. J.; McGinnis, J. J. Am. Chem. Soc. 1975, 97, 1592–1594. (e) Katz, T. J.;
Lee, S. J.; Acton, N. Tetrahedron Lett. 1976, 17, 4247–4250. (f) Katz, T. J.;
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10.1021/om801119t CCC: $40.75
2009 American Chemical Society
Publication on Web 01/26/2009

Alkyne Cross-Metathesis
Organometallics, Vol. 28, No. 5, 2009 1535
counterintuitively, was suggested to be the active species.10
Similar Mo alkylidyne complexes such as [EtC≡Mo{N(tBu)-
Ar}3] (III) have been isolated in pure form by Moore and can
be activated for efficient alkyne metathesis by treatment with
phenol derivatives or by capture on silica, which probably leads
to partial or total cleavage of the Mo-N bonds.11 This
assumption is compatible with previous work by Cummins and
co-workers, who demonstrated that well-defined trialkoxy- and
triaryloxymolybdenum alkylidynes such as IV can be conve-
niently synthesized from Mo-triamides by reaction with the
corresponding ROH derivatives. IV is among the very few
catalyst systems that are able to act as efficient alkyne metathesis
initiators even at room temperature.12
Scheme 1. Selected Catalysts for Alkyne Metathesis
Apart from these molybdenum-based systems, Schrock’s
neopentylidyne complex [Me3CC≡W(OCMe3)3] (2)13 clearly
represents the most widely used catalyst for applications such
as alkyne cross-metathesis (ACM) and ring-closing alkyne
metathesis (RCAM),14 and until recently, this classical alkyli-
dyne complex could be regarded as the only catalytically active,
well-defined tungsten-based species among all the alkyne
metathesis catalysts that have been utilized.6a Lately, we were
able to successfully introduce a new design strategy for the
development of alkyne metathesis catalysts that draws on the
structure of the most active alkene metathesis catalysts, stable
molybdenum and tungsten alkylidene complexes,2a,e,i,15 and that
involves the combination of electron-withdrawing alkoxides
such as hexafluoro-tert-butoxide with strongly electron-donating
imidazolin-2-iminato ligands16 as a subtle balance between the
stability and activity of tungsten alkylidyne complexes.17,18
Indeed, the well-defined imidazolin-2-iminato tungsten neopen-
tylidyne complex 5a exhibits high catalytic activity in the ACM
of 1-phenylpropyne at ambient temperature.17 Efficient RCAM
of several R,ω-diynes was also achieved to selectively afford
the expectations for an alkyne metathesis catalyst with regard
to its activity, substrate compatibility, and required reaction
temperature.2a,5a,6 A selection of homogeneous catalysts is
shown in Scheme 1. Mixtures of [Mo(CO)6] (I) and phenol
additives had already been introduced in the mid-1970s by
Mortreux and can be classified as ill-defined,7 since the nature
of the active catalyst generated in situ is unknown. Despite the
high reaction temperatures required for achieving catalytic
activity, these systems have been widely used in the past, in
particular because of the facile application of entirely stable,
inexpensive, and commercially available reagents.8 More re-
cently, several catalytically active systems have been established
that rely on the use and further activation of Cummins′
molybdenum(III) triamido complex [Mo{N(tBu)Ar}3] (II,Ar )
3,5-dimethylphenyl).9 As demonstrated by Fu¨rstner, this amido
complex can be activated by treatment with CH2Cl2 affording
the methylidyne complex [HC≡Mo{N(tBu)Ar}3] and the chloro
complex [ClMo{N(tBu)Ar}3], whereby the latter, somewhat
(10) Fu¨rstner, A.; Mathes, C.; Lehmann, C. W. J. Am. Chem. Soc. 1999,
121, 9453–9454.
(11) (a) Zhang, W.; Kraft, S.; Moore, J. S. Chem. Commun. 2003, 832–
833. (b) Zhang, W.; Kraft, S.; Moore, J. S. J. Am. Chem. Soc. 2004, 126,
329–335. (c) Zhang, W.; Moore, J. S. J. Am. Chem. Soc. 2004, 126, 12796.
(d) Zhang, W.; Moore, J. S. J. Am. Chem. Soc. 2005, 127, 11863–11870.
(e) Zhang, W.; Brombosz, M.; Mendoza, J. L.; Moore, J. S. J. Org. Chem.
2005, 70, 10198–10201. (f) Weissmann, H.; Plunkett, K. N.; Moore, J. S.
Angew. Chem. 2006, 118, 599–602; Angew. Chem., Int. Ed. 2006, 45, 585-
588. (g) Cho, H. M.; Weissmann, H.; Moore, J. S. J. Org. Chem. 2008, 73,
4256–4258.
(6) For reviews, see: (a) Zhang, W.; Moore, J. S. AdV. Synth. Catal.
2007, 349, 93–120. (b) Van de Weghe, P.; Bisseret, P.; Blanchard, N.;
Eustache, J. J. Organomet. Chem. 2006, 691, 5078–5108. (c) Mortreux,
A.; Coutelier, O. J. Mol. Catal. A: Chem. 2006, 254, 96–104. (d) Schrock,
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2005, 308, 216–217.
(7) (a) Mortreux, A.; Blanchard, M. J. Chem. Soc., Chem. Commun.
1974, 786–787. (b) Mortreux, A.; Dy, N.; Blanchard, M. J. Mol. Catal.
1975/76, 1, 101–109. (c) Mortreux, A.; Petit, F.; Blanchard, M. Tetrahedron
Lett. 1978, 4967–4968. (d) Bencheick, A.; Petit, M.; Mortreux, A.; Petit,
F. J. Mol. Catal. 1982, 15, 93–101. (e) Mortreux, A.; Delgrange, J. C.;
Blanchard, M.; Lubochinsky, B. J. Mol. Catal. 1977, 2, 73–82. (f) Mortreux,
A.; Petit, F.; Blanchard, M. J. Mol. Catal. 1980, 8, 97–106.
(8) See for example: (a) Kaneta, N.; Hikichi, K.; Mori, M. Chem. Lett.
1995, 1055–1066. (b) Zhang, W.; Moore, J. S. Angew. Chem., Int. Ed. 2006,
45, 4416–4439. (c) Zhao, D.; Moore, J. S. Chem. Commun. 2003, 807–
818. (d) Brizius, G.; Pschirer, N. G.; Steffen, W.; Stitzer, K.; zur Loye,
H.-C.; Bunz, U. H. F. J. Am. Chem. Soc. 2000, 122, 12435–12440. (e) Ge,
P.-H.; Fu, W.; Herrmann, W. A.; Herdtweck, E.; Campana, C.; Adams,
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3875; Angew. Chem., Int. Ed. 2005, 44, 3806-3808. (g) Miljanic, O. S.;
Vollhardt, K. P. C.; Whitener, G. D. Synlett 2003, 29. (h) Johnson, C. A.;
Lu, Y.; Haley, M. M. Org. Lett. 2007, 9, 3725–3728. (i) Fu¨rstner, A.; Guth,
O.; Rumbo, A.; Seidel, G. J. Am. Chem. Soc. 1999, 121, 11108–11113.
(9) (a) Laplaza, C. E.; Odom, A. L.; Davis, W. M.; Cummins, C. C.;
Protasiewicz, J. D. J. Am. Chem. Soc. 1995, 117, 4999–5000. (b) Laplaza,
C. E.; Cummins, C. C. Science 1995, 268, 861. (c) Laplaza, C. E.; Johnson,
A. R.; Cummins, C. C. J. Am. Chem. Soc. 1996, 118, 709–710.
(12) (a) Tsai, Y.-C.; Diaconescu, P. L.; Cummins, C. C. Organometallics
2000, 19, 5260–5262. (b) Blackwell, J. M.; Figueroa, J. S.; Stephens, F. H.;
Cummins, C. C. Organometallics 2003, 22, 3351–3353.
(13) (a) Schrock, R. R.; Clark, D. N.; Sancho, J.; Wengrovius, J. H.;
Rocklage, S. M.; Pederson, S. F. Organometallics 1982, 1, 1645–1651. (b)
Sancho, J.; Schrock, R. R. J. Mol. Catal. 1982, 15, 75–79. (c) Wengrovius,
J. H.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc. 1981, 103, 3932–3934.
(14) (a) Fu¨rstner, A.; Seidel, G. Angew. Chem. 1998, 110, 1758–1760;
Angew. Chem. Int. Ed. 1998, 37, 1734-1736. (b) Grela, K.; Ignatowska, J.
Org. Lett. 2002, 4, 3747–3749. (c) Song, D.; Blond, G.; Fu¨rstner, A.
Tetrahedron 2003, 59, 6899–6904. (d) Fu¨rstner, A.; Mu¨ller, G. J. Orga-
nomet. Chem. 2000, 606, 75–78.
(15) Schrock, R. R. Chem. ReV. 2002, 102, 145–179.
(16) (a) Tamm, M.; Randoll, S.; Bannenberg, T.; Herdtweck, E. Chem.
Commun. 2004, 876–877. (b) Tamm, M.; Beer, S.; Herdtweck, E. Z.
Naturforsch. 2004, 59b, 1497–1504. (c) Tamm, M.; Randoll, S.; Herdtweck,
E.; Kleigrewe, N.; Kehr, G.; Erker, G.; Rieger, B. Dalton Trans. 2006,
459–467. (d) Tamm, M.; Petrovic, D.; Randoll, S.; Beer, S.; Bannenberg,
T.; Jones, P. G.; Grunenberg, J. Org. Biomol. Chem. 2007, 5, 523–530. (e)
Panda, T. K.; Randoll, S.; Hrib, C. G.; Jones, P. G.; Bannenberg, T.; Tamm,
M. Chem. Commun. 2007, 5007–5009. (f) Stelzig, S. H.; Tamm, M.;
Waymouth, R. M. J. Polym. Sci. Part A., Polym. Chem. 2008, 46, 6064–
6070.
(17) Beer, S.; Hrib, C. G.; Jones, P. G.; Brandhorst, K.; Grunenberg, J.;
Tamm, M. Angew. Chem. 2007, 119, 9047–9051; Angew. Chem. Int. Ed.
2007, 46, 8890-8894.
(18) Beer, S.; Brandhorst, K.; Grunenberg, J.; Hrib, C. G.; Jones, P. G.;
Tamm, M. Org. Lett. 2008, 10, 981–984.

1536 Organometallics, Vol. 28, No. 5, 2009
Beer et al.
Scheme 2. Preparation of Imidazolin-2-Iminato Tungsten
Alkylidyne Complexes
monomeric or dimeric cycloalkynes via reversible alkyne
metathesis.18 It is the scope of this contribution to give a full
account of the preparation and structural characterization of
various imidazolin-2-iminato tungsten alkylidyne complexes of
this novel type and to present a comparative experimental and
theoretical study of their catalytic activity.
Results and Discussion
Preparation of Imidazolin-2-iminato Tungsten Alkyli-
dyne Complexes. Our initial efforts to obtain dialkoxytung-
Figure 1. ORTEP diagrams of 4a (top) and 4b (bottom, only one
of the two independent molecules shown) with thermal displacement
parameters drawn at 50% probability. For selected bond lengths
and angles, see Table 1.
sten(VI)
neopentylidyne
complexes
of
the
type
[Me3CC≡W(ImN)(OR)2] proceeded from the trichloro complex
[Me3CC≡WCl3(dme)] (1, dme ) 1,2-dimethoxyethane),13a
which was synthesized following the classical route from WCl6
via [(MeO)3WCl3] and [Me3CC≡W(CH2CMe3)3].13 Introduction
of just one imidazolin-2-iminato ligand at this stage by reaction
of 1 with the lithium reagent (ImtBuN)Li, obtained by deproto-
nation of 1,3-di-tert-butylimidazolin-2-imine (ImtBuNH) with
methyl lithium,16d proved unsuccessful, and the formation of
di- and triimido complexes and also reduction to afford dimeric
tungsten(V) complexes were observed instead. An alternative
approach required the preparation of the trialkoxides [Me3CC≡
W(OCCMe3)3] (2) and [Me3CC≡W{OCMe(CF3)2}3(dme)] (3),
which can be obtained from 1 by treatment with the respective
lithium alkoxides (Scheme 2).19 Surprisingly, the solid state
structures of these classical alkylidyne complexes have never
been reported, and therefore, colorless (2) and yellow-orange
(3) single crystals of these complexes were subjected to X-ray
diffraction analyses (see Supporting Information for a presenta-
tion of their molecular structures).
of introducing one imidazolin-2-iminato ligand, by substitution
of a tert-butoxide ligand in 2 or by substitution of a hexafluoro-
tert-butoxide ligand together with the dme solvate molecule in
3 (Scheme 2). The complexes could be isolated in high yield
as light yellow (4a, 4b) or yellow-orange (5a, 5b) crystalline
solids, which gave rise to characteristic low-field 13C NMR
resonances at 273.3 (4a) 276.4 (4b) 285.6 (5a) and 290.5 ppm
(5b) attributable to the alkylidyne carbon atoms. The 1H NMR
spectra of complexes 4b and 5b containing the diisopropylphe-
nyl-substituted ImDippN ligand show two doublets for the
diastereotopic isopropyl CH3 groups, which results from hin-
dered rotation around the N-C(phenyl) bond, indicating at the
same time that rotation around the W-N-C axis is fast at room
temperature on the NMR time-scale. The fluorinated complexes
5a and 5b exhibit two quartets each in the 13C and 19F NMR
spectra, revealing the presence of diastereotopic CF3 groups and
accordingly of conformationally stable Cs-symmetric complexes.
To confirm these spectroscopic results, the crystal structures of
the complexes 4a, 4b and 5b were determined by X-ray
diffraction analyses; Figures 1 and 2 show ORTEP diagrams
of the molecular structures together with that of the previously
published complex 5a.17 In all cases, the formation of mono-
meric tungsten alkylidyne complexes with slightly distorted
tetrahedral geometries is observed. The W-C1 distances are
very similar and range from 1.765(4) to 1.771(3) Å; with
The reaction of 2 and 3 with the lithium imidazolin-2-imides
(ImtBuN)Li and (ImDippN)Li in toluene is a convenient method
(19) Freudenberger, J. H.; Schrock, R. R.; Churchill, M. R.; Rheingold,
A. L.; Ziller, J. W. Organometallics 1984, 3, 1563–1573.
(20) (a) Cotton, F. A.; Schwotzer, W.; Shamshoum, S. Organometallics
1984, 3, 1770–1771. (b) Cotton, F. A.; Schwotzer, W.; Shamshoum, S. J.
Organomet. Chem. 1985, 296, 55–68.

Alkyne Cross-Metathesis
Organometallics, Vol. 28, No. 5, 2009 1537
[EtC≡Mo{N(tBu)Ar}3] by treatment with phenol derivatives.11
Hence, complex 6 was synthesized from 3 by reaction with the
lithium reagent Li[N(tBu)Ar] · OEt223 (Scheme 3) and could be
isolated as a light yellow solid by recrystallization from cold
hexane solution. In agreement with the formation of a dme-
free, Cs-symmetric complex, the 19F NMR spectrum of the light
yellow complex features two quartets at -78.8 and -77.9 ppm
(4JFF ) 18 Hz) for the diastereotopic CF3 groups. The 1H NMR
spectrum exhibits six resonances at 0.66, 1.16, 1.73, 2.13, 6.62
and 6.70 ppm in a 9:9:6:6:1:2 ratio for the two different types
of tBu and Me groups, respectively, and for the ortho- and para-
hydrogen atoms of the aromatic xylyl substituent. In comparison
with complexes 4 and 5, the 13C NMR alkylidyne carbon
resonance for 6 is observed at considerably lower field (298.2
ppm).
Variation of the Alkoxide Ligands. The most active
Schrock-type olefin metathesis catalysts normally contain the
hexafluoro-tert-butoxide ligand OCMe(CF3)2,2a,e,i,15 and that ligand
was also successfully used herein (Vide infra), and previously, to
establish 5a as a highly active, well-defined system for catalytic
alkyne metathesis.17,18 Substitution of the OCMe(CF3)2 ligand in
5a by the related bis(trifluoromethyl)benzyloxy ligand OCPh(CF3)2
required the synthesis of the corresponding tris(alkoxide) tungsten
alkylidyne complex. The reaction of 1 with LiOCPh(CF3)2 afforded
the previously unknown complex [Me3CC≡W{OCPh(CF3)2}3] (7)
as a yellow oil in 70% yield (Scheme 4). Surprisingly, the
characterization of 7 by NMR spectroscopy and elemental analysis
revealed the absence and thus the complete substitution of the
coordinated dme ligand present in the starting material 1. Accord-
ingly, the 1H, 13C, and 19F NMR spectra show resonances that are
in agreement with the formation of a C3v-symmetric complex
containing three equivalent alkoxide ligands; for instance, one
singlet 19F NMR resonance is observed at -73.0 ppm in C6D6.
The attempt to prepare complex [Me3CC≡W(NImtBu){OCPh-
(CF3)2}2] as a benzyl derivative of 5a failed, however, and the
reaction of 7 with the lithium imide LiNImtBu exclusively yielded
the alkylidene complex 8 shown in Scheme 4, which must have
formed by adding an ortho-C-H bond of the phenyl group across
the W≡C bond. Interestingly, a closely related addition across a
WdC bond was observed through the attempted isolation of the
tungsten alkylidene complex [Me3CCHdW(NDipp){OC(C6H3Me)-
(CF3)2}2] (C6H3Me ) para-tolyl), resulting in the formation of the
corresponding cyclometalated neopentyl complex with a W-C
single bond.21 Related C-H activation and cycloaddition reactions
have also been observed for tungsten neopentylidyne complexes,
and two other examples comprise activation of a 2,6-di-tert-
butylphenoxide or a 2,6-diphenylphenoxide ligand, respectively,
to give neopentylidene complexes containing six-membered rather
than five-membered metalacycles.24,25 In this context, it should also
be noted that related intermolecular additions of C-H bonds across
a Ti-C triple bond in a transient titanium alkylidyne complex have
been reported only recently by Mindiola.26,27
Figure 2. ORTEP diagrams of 5a (top, only one of the two
independent molecules shown) and 5b (bottom, only one of the
two independent molecules shown) with thermal displacement
parameters drawn at 50% probability. For selected bond lengths
and angles, see Table 1.
W-C1-C2 angles between 168.8(2)° and 175.4(4)°, these axes
can be regarded as being close to linearity. The same holds true
for the W-N1-C6 axes (Table 1); however, the W-N1
distances, ranging from 1.844(2) to 1.875(5) Å, are significantly
longer than observed for imido tungsten alkylidene complexes,
for example, W-N ) 1.731(6)/1.748(6) Å in [Me3CCHd
W(NDipp)(OCCMe3)2]21 and W-N ) 1.708(17) Å in [PhCHd
W(NDipp){OCMe(CF3)2}2].22 Similar to the orientation of the
Dipp substituent in these complexes, the imidazole planes in
all complexes 4 and 5 adopt horizontal conformations with
regard to the plane containing the atom C1, W, and N1 with
the interplanar angles ranging from 87.2 to 91.7°.
Variation of the Nitrogen Ligand. To investigate the impact
of the nitrogen ligand on the catalytic activity, we wished to
utilize alternatives to the imidazolin-2-iminato ligand in the
bis(hexafluoroalkoxide) complexes 5. The sterically demanding
amide N(tBu)Ar (Ar ) 3,5-dimethylphenyl) was chosen as an
alternative ancillary ligand, since it is also likely to be present
in related catalytically active species generated from
The 19F NMR spectrum of 8 exhibits four quartets at -74.4,
-73.9, -71.9, and -70.5 ppm with 4JFF coupling constants of
20 Hz, which is in agreement with the presence of two different
benzyloxy ligands both featuring diastereotopic CF3 groups. In
(23) Laplaza, C. E.; Davis, W. M.; Cummins, C. C. Organometallics
1995, 14, 577–580.
(24) Churchill, M. R.; Ziller, J. W.; Freudenberger, J. H. M.; Schrock,
R. R. Organometallics 1984, 3, 1554–1562.
(25) Couturier, J.-L.; Paillet, C.; Leconte, M.; Basset, J.-M.; Weiss, K.
Angew. Chem. 1992, 104, 622–624; Angew. Chem. Int. Ed. Engl. 1992, 31,
628-631.
(26) Bailey, B. C.; Fan, H.; Huffman, J. C.; Baik, M.-H.; Mindiola, D. J.
J. Am. Chem. Soc. 2007, 129, 8781–8793.
(21) Schrock, R. R.; DePue, R. T.; Feldmann, 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,
2262–2275.
(22) Schrock, R. R.; DePue, R. T.; Feldmann, J.; Schaverien, C. J.;
Dewan, J. C.; Liu, A. H. J. Am. Chem. Soc. 1988, 110, 1423–1435.

1538 Organometallics, Vol. 28, No. 5, 2009
Beer et al.
Table 1. Selected Bond Lengths (Å) and Angles (deg) in Complexes 4 and 5
4a
4ba
5aa
5ba
W-C1
1.765(4)
1.867(3)
1.893(2)
1.896(2)
1.299(5)
172.9(3)
165.3(3)
1.765(5)/1.766(5)
1.875(5)/1.873(4)
1.881(4)/1.880(4)
1.881(4)/1.884(4)
1.273(6)/1.275(6)
175.4(4)/173.9(4)
178.5(4)/174.1(4)
1.768(3)/1.764(3)
1.852(2)/1.844(2)
1.929(2)/1.936(2)
1.927(2)/1.923(2)
1.315(4)/1.328(4)
171.8(2)/173.2(2)
164.2(2)/162.1(2)
1.758(3)/1.770(3)
1.860(2)/1.855(2)
1.927(2)/1.918(2)
1.917(2)/1.906(2)
1.289(4)/1.290(4)
175.2(3)/168.8(2)
168.5(2)/164.3(2)
W-N1
W-O1
W-O2
N1-C6
W-C1-C2
W-N1-C6
a Two independent molecules in the asymmetric unit.
Scheme 3. Preparation of an Amido Tungsten Alkylidyne
Complex
Scheme 4. Preparation of OCPh(CF3)2 Tungsten Alkylidyne
and Alkylidene Complexes
Figure 3. ORTEP diagram of one of the two independent molecules
8 with thermal displacement parameters drawn at 50% probability.
Selected bond lengths (Å) and angles (deg) in molecule 1/molecule
2: W-C1 1.901(5)/1.895(6), W-N1 1.792(4)/1.800(3), W-O1
1.988(3)/1.980(3), W-O2 1.997(3)/2.004(3), W-C21 2.175(5)/
2.167(4), N1-C6 1.345(6)/1.348(5), W-C1-C2 138.0(4)/143.0(5),
W-N1-C6 170.0(3)/168.0(4).
W2-C41 distances are 1.901(5) and 1.895(6) Å, only slightly
elongated in comparison with the values reported for tetrahedral
alkylidene complexes such as [Me3CCHdW(NDipp)-
(OCCMe3)2] [W-C ) 1.892(7)/1.863(8) Å]21 and [PhCHdW-
(NDipp){OCMe(CF3)2}2] [W-C ) 1.859(22)].22 At this stage,
the OCPh(CF3)2 ligand was not further considered for the
development of alkyne metathesis catalysts, despite the theoreti-
cally expected high catalytic activity (Vide infra, see complex
C in Table 2). Furthermore, the reactivity of 8 and related
systems, for example, with regard to the reversibility of the C-H
cycloaddition reaction or the use of 8 in olefin metathesis, still
needs to be investigated.
1
addition to the H NMR resonance at 1.18 ppm for the CMe3
hydrogen atoms, the neopentylidene ligand in 8 gives rise to a
low-field CH resonance at 10.30 ppm, and the 13C NMR
spectrum shows the signal for the alkylidene carbon atom at
281.8 ppm. To unambiguously confirm the formation of 8, single
crystals obtained from a saturated hexane solution at -35 °C
were subjected to an X-ray diffraction analysis. 8 crystallized
j
in the space group P1 with two independent, structurally closely
related molecules in the asymmetric unit. The molecular
structure of both molecules s Figure 3 shows molecule 1 s is
best described as a distorted square pyramid with the alkylidene
atoms C1 and C41 at the apical positions. The cyclometalated
benzyloxy ligands form nearly planar five-membered chelate
rings with the tungsten atoms, and the corresponding intraring
O-W-C angles of 74.67(15) and 75.45(15)° constitute the
smallest angles in the basal plane, followed by the O-W-O
angles of 81.14(13) and 78.36(12)°. Accordingly, the sterically
demanding ImtBuN ligand subtends larger N-W-C and N-W-O
angles of 91.44(16)°/93.13(16)° and 99.81(14)°/99.18(14)°,
respectively, and the imidazole planes in both molecules adopt
vertical conformations with nearly coplanar orientations toward
the planes containing N1, W1, C1, H1, and C2 (molecule 1,
interplanar angle ) 5.4°) or N4, W2, C41, H41, and C42
(molecule 2, interplanar angle ) 10.6°). The W1-C1 and
The introduction of the nonafluoro-tert-butoxide ligand OC-
(CF3)3 was also considered, since an enhanced electrophilicity
and catalytic activity could be expected from the resulting
alkylidyne complex [Me3CC≡W(NImtBu){OC(CF3)3}2]. Thus,
28
the trichloride 1 was treated with freshly sublimed LiOC(CF3)3
in a 1:3 molar ratio in diethyl ether to yield a purple crystalline
solid. Elemental analysis suggested that the introduction of three
alkoxide ligands had failed and that the bis(alkoxide) complex
[Me3CC≡WCl{OC(CF3)3}2(dme)] (9) was formed instead
(Scheme 5). In our hands, substitution of the third chloride
(27) (a) Mindiola, D. J. Acc. Chem. Res. 2006, 39, 813–821. (b)
Mindiola, D. J.; Bailey, B. C.; Basuli, F. Eur. J. Inorg. Chem. 2006, 3135–
3146.
(28) Dear, E. A.; Fox, W. B.; Frederickse, R. J.; Gilbert, E.; Huggins,
D. K. Inorg. Chem. 1970, 9, 2590–2591.

Alkyne Cross-Metathesis
Organometallics, Vol. 28, No. 5, 2009 1539
Scheme 5. Preparation of OC(CF3)3 Tungsten Alkylidyne
Complexes
Figure 4. Excerpt from the 19F NMR spectrum of 9 (376.5 MHz
MHz) in C6D6 at 25 °C; the asterisk denotes an impurity.
ligand proved unsuccessful, although the preparation of tris-
(nonafluoro-tert-butoxide) complexes of the type [RC≡Mo-
{OC(CF3)3}3(dme)] (R ) CMe3, Et)29,30 has been reported.
However, it was noted that the reactions involving
[Me3CC≡MoCl3(dme)] and fluoroalkoxides are sensitive to
solvent and the counterion of the alkoxide salt employed, and
for instance, the use KOC(CF3)3 in ether afforded [Me3CC≡
MoCl{OC(CF3)3}2(dme)] as the molybdenum analogue of 9.
On the basis of NMR data, a Cs-symmetric octahedral structure
was assigned to the Mo complex that features a trans-orientation
of the two alkoxide ligands, and accordingly, only one 19F NMR
resonance was observed.29b In contrast, two multiplets at -71.1
and -71.5 ppm are observed for 9, which can only be explained
by a through-space coupling mechanism between the 19F atoms
of two cis-oriented, magnetically inequivalent alkoxides.31,32
Figure 4 shows part of the high-resolution 19F NMR spectrum
of 9, revealing the presence of two decets with JFF ) 1.6 Hz.
Line-shape analysis fully confirms the presence of a 1:9:36:84:
126:126:84:36:9:1 line pattern that is expected for mutual
coupling between the F atoms of two adjacent C(CF3)3 moieties.
In agreement with C1-symmetry, the dme ligand gives rise to
two CH3 singlets and to four individual multiplets for the four
diastereotopic CH2CH2 hydrogen atoms. Finally, the alkylidyne
13C NMR resonance is observed at 315.8 ppm with 1JWC ) 252
Hz. In addition, the NMR spectroscopic structure assignment
could be confirmed by X-ray diffraction analysis (Figure 5),
and 9 displays a distorted octahedral geometry similar to that
observed for 3 (Vide supra). The W-Cl and W-C1 distances
of 2.3742(5) and 1.768(2) Å and also the W-O bond lengths
Figure 5. ORTEP diagram of 9 with thermal displacement
parameters drawn at 50% probability. Selected bond lengths (Å)
and angles (deg): W-C1 1.768 (2), W-O1 2.1800(14), W-O2
2.4317(15), W-O3 1.9319(14), W-O4 1.9708(14), W-Cl 2.3742(5),
W-C1-C2 172.64(16).
are unexceptional and fall in the expected ranges. In accordance
with the observed through-space 19F-19F coupling, two short
intramolecular F · · · F contacts of 2.77 and 2.90 Å that fall below
the sum of the van der Waals radii [rvdW(F) ) 1.47 Å]33 are
observed between the fluorine atoms of the neighboring cis-
OC(CF3)3 ligands.
We anticipated that complex 9 might be equally well suited
for the introduction of an imidazolin-2-iminato ligand, and the
reaction of 9 with Li(NImtBu) in diethyl ether afforded a red
solution after removal of LiCl by filtration; addition of hexane
and cooling afforded single crystals of 10 · 0.5 hexane (Scheme
5) that were subjected to an X-ray diffraction analysis. The
resulting molecular structure is shown in Figure 6, revealing
the formation of a dinuclear complex, in which an intraring C-N
bond s more precisely the C31-N4 bond s of one of the two
ImtBuN ligands has been cleaved to afford an ethylene-bridged
amido-cyanamide ligand. This ligand bridges two tungsten-
neopentylidyne moieties, with the neutrally charged N5-C31≡N6
cyanamide end of the ligand coordinating to the tris(alkoxide)
[Me3CC≡W{OC(CF3)3}3] complex fragment, whereas the ami-
do nitrogen atom N4 binds to the monoalkoxide complex
fragment [Me3CC≡W(NImtBu){OC(CF3)3}]. Since the molecular
formula of dinuclear 10 is twice that of the envisaged complex
[Me3CC≡W(NImtBu){OC(CF3)3}2], we tentatively propose that
this complex might have actually formed, followed by ligand
(29) (a) McCullough, L. G.; Schrock, R. R. J. Am. Chem. Soc. 1984,
106, 4067–4068. (b) Laughlin, G.; McCullough, L. G.; Schrock, R. R.;
Dewan, J. C.; Murdzek, J. C. J. Am. Chem. Soc. 1985, 107, 5987–5998.
(30) Gdula, R. L.; Johnson, M. J. A. J. Am. Chem. Soc. 2006, 128, 9614–
9615.
(31) Berger, S.; Braun, S.; Kalinowski, H. O. NMR Spectroscopy of the
Non-Metallic Elements; John-Wiley and Sons: Chichester, 1997.
(32) For selected references, see: (a) Bro¨ring, M.; Kru¨ger, R.; Link, S.;
Kleeberg, C.; Ko¨hler, S.; Xie, X.; Ventura, B.; Flamigni, L. Chem.-Eur.
J. 2008, 14, 2976–2983. (b) Dorozhkin, E. I.; Ignat’eva, D. V.; Tamm,
N. B.; Goryunkov, A. A.; Khavrel, P. A.; Ioffe, I. N.; Popov, A. A.;
Kuvychko, I. V.; Streletskiy, A. V.; Markov, V. Y.; Spandl, J.; Strauss,
S. H.; Boltalina, O. V. Chem.-Eur. J. 2006, 12, 3876–3889. (c) Kareev,
I. E.; Quin˜ones, G. S.; Kuvychko, I. V.; Khavrel, P. A.; Ioffe, I. N.; Goldt,
I. V.; Lebedkin, S. F.; Seppelt, K.; Strauss, S. H.; Boltalina, O. V. J. Am.
Chem. Soc. 2005, 127, 11497–11504. (d) Tuttle, T.; Gra¨fenstein, J.; Cremer,
D. Chem. Phys. Lett. 2004, 394, 5–13. (e) Arnold, W. D.; Mao, J.; Sun, H.;
Oldfield, E. J. Am. Chem. Soc. 2000, 122, 12164–12168. (f) Ernst, L.; Ibrom,
K. Angew. Chem. 1995, 107, 2010–2012; Angew. Chem. Int. Ed. 1995, 34,
1881-1882.
(33) (a) Bondi, A. J. Phys. Chem. 1964, 68, 441–451. (b) Rowland,
R. S.; Taylor, R. J. Phys. Chem. 1996, 100, 7384–7391.

1540 Organometallics, Vol. 28, No. 5, 2009
Beer et al.
Figure 6. ORTEP diagram of 10 with thermal displacement
parameters drawn at 50% probability. The CF3 groups have been
omitted for clarity. Selected bond lengths (Å) and angles (deg):
W1-C1 1.762(6), W1-N1 1.861(5), W1-N4 1.968(5), W1-O1
1.971(4), W2-C32 1.745(6), W2-N6 2.085(5), W2-O2 1.963(4),
W2-O3 1.982(4), W2-O4 1.951(4), W1-C1-C2 172.2(5),
W1-N1-C6 158.6(4), W2-C32-C33 178.0(5), W2 N6 C31
169.9(5).
Figure 7. ORTEP diagram of 12 with thermal displacement
parameters drawn at 50% probability. Selected bond lengths (Å)
and angles (deg): W-C1 1.757(5), W-N1 1.839(3), W-O1
1.931(3), W-O2 1.938(3), N1-C4 1.334(5), W-C1-C2 170.4(4),
W-N1-C4 153.4(3).
Scheme 7. Cross-Metathesis of 3-Hexyne and 1-Phenylpropyne
Scheme 6. Cycloaddition and Cycloreversion Reactions of
Imidazolin-2-iminato Tungsten Alkylidyne Complexes
neopentylidyne tungsten moiety (Me3CC≡W) in 5a for a
propylidyne moiety (EtC≡W) with concomitant formation of
the alkyne Me3CC≡CEt, followed by a [2 + 2] cycloaddition
reaction of the intermediate alkylidyne complex 12 with a
second equivalent of 3-hexyne. To confirm that 12 is a likely
intermediate in this reaction, a toluene solution of 11, prepared
from 5a by treatment with excess 3-hexyne, was evaporated
and dried at 40 °C under vacuum to afford a brownish yellow
solid. Recrystallization of this residue from diisopropyl ether
afforded single crystals, suitable for an X-ray diffraction
analysis. The resulting molecular structure is shown in Figure
7, revealing that the propylidyne complex 12 with a slightly
distorted tetrahedral geometry has indeed formed. The W-C
triple bond is short [W1-C1 ) 1.757(5) Å] and falls in the
expected range for tungsten alkylidyne complexes.15 The
W-C1-C2 and W-N1-C4 angles are 170.4(4) and 153.4(3)°
and thus smaller than observed for the neopentylidyne com-
plexes 4 and 5 (Table 1). Again, the imidazole ring adopts a
perpendicular orientation with respect to the plane containing
the alkylidyne carbon atom C1 and the tungsten and the nitrogen
atoms W and N1 [interplanar angle ) 91.0°], the molecule can
thus be regarded as being essentially Cs-symmetric, with the
mirror plane bisecting the imidazolin-2-iminato ligand and the
tungsten-alkylidyne moiety. The presence of a configurationally
stable Cs-symmetric complex is also confirmed in solution by
cleavage and alkoxide redistribution to afford 10. At present,
the reason for the apparent instability of the intermediate
mononuclear imidazolin-2-iminato complex is not known;
however, these findings indicate that the enhanced electrophi-
licity promoted by the presence of two nonafluoro-tert-butoxide
ligands might be responsible for the observed C-N bond
cleavage and is therefore likely to prevent the use of these
imidazolin-2-iminato complexes of this type in alkyne metath-
esis, despite the theoretically expected high catalytic activity
(Vide infra, see complex D in Table 2).
Reactivity of Imidazolin-2-iminato Tungsten Alkylidyne
Complexes toward Alkynes. In our intial report on imidazolin-
2-iminato tungsten alkylidyne complexes, we demonstrated that
treatment of a solution of 5a with a 10-fold excess of 3-hexyne
(EtC≡CEt) cleanly afforded the metallacyclobutadiene complex
11 as a red crystalline solid (Scheme 6).17 The molecular
structure displayed a clear short-long-short-long alternation
of bond lengths within the WC3 ring and was best described as
a square pyramid with the alkylidene WdC carbon atom at the
apex. The formation of 11 was rationalized by exchange of the
observation of two quartets at -78.3 and -76.4 ppm in the 19
F
NMR spectrum, which can be assigned to the diastereotopic
CF3 groups.
Catalytic Alkyne Cross-Metathesis with Tungsten
Alkylidyne Complexes. The formation of complexes 11 and
12 upon treatment of 5a with 3-hexyne (Vide supra) implies
that this imidazolin-2-iminato tungsten alkylidyne complex is

Alkyne Cross-Metathesis
Organometallics, Vol. 28, No. 5, 2009 1541
alkylidyne complexes.13c,19 To test the new alkylidyne com-
plexes for preparative alkyne metathesis and to compare their
catalytic activities, we performed the homodimerization of
1-phenylpropyne at room temperature under vacuum driven
conditions (Scheme 7). Since the combination of an electron-
donating imidazolin-2-iminato ligand with two electron-
withdrawing alkoxides has been identified as a lead structure
for the design of catalytically active tungsten alkylidyne
complexes,17 only the performance of complexes 5a, 5b, and 6
containing the hexafluoro-tert-butoxide ligand was tested.
In a typical experiment, a solution containing 1-phenylpro-
pyne (300.0 mg, 2.58 mmol) and the catalysts (1 mol%, 26
µmol) in hexane (20 mL) was stirred at 30 °C under reduced
pressure (300 mbar) to remove 2-butyne continuously (Scheme
7). Aliquots of 0.5 mL were drawn every ten minutes without
disconnecting from the vacuum line, and the conversion to
diphenylacetylene (tolane) was monitored by GC (Figure 8).
In agreement with our previous results,17 the alkylidyne complex
5a containing the ImtBuN ligand is able to bring this reaction to
completion within 50 min; the catalytic performance of all other
complexes is clearly inferior. Only the amido complex 6 is able
to produce a noticeable amount (∼30%) of tolane within one
hour. In contrast, 5b featuring the sterically more demanding
ImDippN ligand is hardly active. The same catalytic reaction was
also examined at 80 °C by using toluene as a solvent with a
higher boiling point, producing a similar picture with regard to
the relative catalytic performance of the three complexes.
Specifically, the reaction catalyzed by 5a is greatly accelerated,
and 83% conversion is reached after 10 min and full conversion
within 30 to 40 min. A similar increase of the reaction rate is
observed for catalyst 6, which allows almost total completion
of the reaction within 70 min. Again, 5b is not sufficiently active
(Figure 8).
Figure 8. Conversion-time diagrams for the cross-metathesis of
1-phenylpropyne at 30 °C (top) and 80 °C (bottom).
an active alkyne-metathesizing catalyst at room temperature and
that this reaction follows the conventional [2 + 2] cycloaddition/
cycloreversion mechanism.1d Further corroboration stems from
the observation that the unsymmetrical alkyne 3-heptyne is
rapidly converted into a statistical 1:2:1 mixture of 3-hexyne,
3-heptyne and 4-octyne in the presence of a catalytic amount
(1 mol%) of 5a (Scheme 7, see experimental part for details).
The reaction is too fast at room temperature to be followed
kinetically but is fully equilibrated within less than five minutes
as judged by gas chromatography (GC) and NMR analysis.
Similar activity has been observed for Schrock-type tungsten
Theoretical Investigation of Alkyne Metathesis with
Tungsten Alkylidyne Complexes. From our previous combined
experimental and theoretical study we have learned thatskeeping
in mind all the assumptions and approximations one has to
inherently apply in static, approximate density functional theory
(DFT)swe are at least qualitatively able to predict the activity
of alkyne metathesis catalysts.17 Therefore, we initiated an in
silico screening comprising ten selected catalysts to explain the
experimentally observed catalytic activities (Vide supra) and to
uncover suitable structures for future optimization of our catalyst
Table 2. Relative B3LYP Energies in kcal mol-1 for the Reaction of Selected Catalysts with 2-Butynea
TSb
∆Hq
INb
q
catalyst
∆E0
∆Gq
∆E0
6.31
7.92
-2.47
7.38
∆H298
∆G298
298
298
2′
L ) L′ ) OCMe3
17.26
19.09
9.78
19.27
13.21
9.51
16.63
18.55
8.99
18.18
12.19
8.87
32.92
33.84
26.05
37.55
31.22
25.03
24.66
24.00c
19.47
5.51
7.21
-3.54
6.15
21.00
22.35
13.67
24.20
20.19
11.52
13.19
7.25
4a′
5a′
5b′
6′
A
B
L ) OCMe3, L′ ) ImtBuN
L ) OCMe(CF3)2, L′ ) ImtBuN
L ) OCMe(CF3)2, L′ ) ImDipp
N
L ) OCMe(CF3)2, L′ ) N(tBu)Ar
3.54
2.66
L ) OCMe(CF3)2, L′ ) ImMe
N
-4.33
-3.39
-10.45
-9.48
-5.29
-4.36
-11.60
-10.18
L ) OCMe(CF3)2, L′ ) NPMe3
L ) OCPh(CF3)2, L′ ) ImtBuN
L ) OC(CF3)3, L′ ) ImtBuN
8.25
7.58
C
D
6.21c
4.87
4.81c
4.29
4.83
a For details, see experimental part. b ∆E0: relative energies at 0 K, H298: enthalpies at 298 K, ∆G298: Gibbs free energies at 298 K; all values are
given with respect to the isolated catalysts and 2-butyne. c Calculation reveals the presence of a second-order transition state, which is not fully
converged because of an ultraflat hypersurface; the corresponding values represent an upper limit.

1542 Organometallics, Vol. 28, No. 5, 2009
Beer et al.
Figure 9. PLUTO drawings of the calculated metallacyclobutadiene
intermediates IN(5a′) (left) and IN(5b′) (right).
Figure 10. PLUTO drawings of the calculated structures 6′ (left)
and IN(6′) (right).
system. Since the design strategy is based on subtly balanced
electron-donating and electron-withdrawing effects, the impact
of this push-pull situation was reviewed by systematic variation
of both the imido and the alkoxide ligands. The symmetric
metathesis of 2-butyne (MeC≡CMe) was chosen as the model
reaction, and based on the conventional [2 + 2] cycloaddition-
cycloreversion mechanism,34,35 the first three relevant stationary
pointsscatalyst plus 2-butyne, transition state TS and metalla-
cyclobutadiene intermediate INsof the rate-determining alkyne
metathesis step were characterized for each MeC≡W model
system (Table 2). It should be noted that a full catalytic cycle
additionally involves the interconversion between two identical
metallacyclobutadiene intermediates. However, since this rear-
rangement has a significantly lower barrier, no attempts have
been made to locate this second transition state. Table 2
summarizes the B3LYP energies, which are subject of discussion
hereafter. In view of a comparison between our gas phase
computations with experimental results, we note that, for the
associative mechanism at work, entropic effects are expected
to be smaller in the condensed phase, which should afford
considerably lower free activation barriers.36
orientation of the diisopropylphenyl and the alkoxide substit-
uents. As a consequence, IN(5b′) exhibits a significantly greater
deviation from an ideal coplanar orientation of the five-
membered imidazole ring with respect to the metallacyclo-
butadiene moiety than observed for IN(5a′), which results in a
less favorable electronic metal nitrogen interaction and might
account for the destabilization of both the intermediate IN(5b′)
and of the corresponding transition state TS(5b′).
In contrast to 5b, the amido complex 6 showed moderate
activity in the metathesis of 1-phenylpropyne (Vide supra), and
this experimental result is in agreement with a lowering of the
calculated barrier [∆Gq298(5b′) ) 37.55 kcal mol-1 versus
∆Gq298(6′) ) 31.22 kcal mol-1]. For this system, the computed
structures of the catalyst and the metallacyclobutadiene inter-
mediate IN(6′) are assembled in Figure 10. The structure of the
catalyst is almost perfectly Cs-symmetric with the N(tBu)Ar
ligand (Ar ) 3,5-dimethylphenyl) adopting a coplanar orienta-
tion of the C(Ar)-N-C(tBu) plane toward the linear W≡C-C
axis. The cis-arrangement of the aryl group with the tungsten
alkylidyne moiety represents the global minimum, and this
conformation was calculated to be stabilized by 1.22 kcal mol-1
over the trans-conformer. Visual inspection of IN(6′) reveals
that steric congestion does not account for the lower activity
with regard to 5a′ and therefore suggests that in this case the
different electronic ligand properties, that is, the stronger
electron-donating ability of the ImtBuN ligand, play the dominant
role in effecting the catalytic performance.
As previously shown, the use of the catalyst 5a′37 leads to a
significantly lower activation barrier in comparison with the
standard Schrock catalyst 2′ [∆Gq298(5a′) ) 26.05 kcal mol-1
versus ∆Gq298(2′) ) 32.92 kcal mol-1].17 The importance of
the electron-withdrawing hexafluoro-tert-butoxide ligand in 5a′
becomes evident by comparison with the corresponding non-
fluorinated complex 4a′, which exhibits a barrier of ∆Gq
)
298
33.84 kcal mol-1, suggesting that this complex should be
inactive at room temperature. Substitution of the ImtBuN ligand
in 5a for the ImDippN ligand in 5b resulted in a dramatic decrease
of the catalytic activity in 1-phenylpropyne metathesis (Vide
supra). This observation is in full agreement with the calculated
barrier of ∆Gq298(5b′) ) 37.55 kcal mol-1, which even exceeds
that of 2′. Since the electronic properties of the two imidazolin-
2-iminato ligands in 5a′ and 5b′ should be very similar, these
differences are probably based on steric grounds. To illustrate
In addition to the systems described above, we have
performed similar calculations for the catalysts A-D, which
do not relate to experimentally studied systems or have defied
synthesis, but could help to uncover suitable structures for future
catalyst development and optimization (Table 2). Complex A
contains the 1,3-dimethylimidazolin-2-iminato ligand and two
hexafluoro-tert-butoxy ligands, and comparison of the calculated
B3LYP energies with those of 5a′ indicates that decreasing the
size of the N-substituents by going from tBu in 5a′ to Me in A
does not have a pronounced effect on the activation barrier,
which is only lowered by about one kcal mol-1. Accordingly,
no significant improvement of the catalytic performance should
be expected, albeit this prediction might only be valid for the
metathesis of 2-butyne and not for the metathesis of sterically
more demanding alkynes. Furthermore, it should be noted that
the catalyst stability in the condensed phase can also be strongly
affected by the ligand size and that less sterically protected
systems might suffer from faster catalyst degradation and
deactivation. In view of the striking similarity between nucleo-
philic carbenes of the imidazolin-2-ylidene type and electron-
the different spatial dimensions of the ImtBuN and ImDipp
N
ligands, the calculated structures of the corresponding metal-
lacyclobutadiene intermediates IN(5a′) and IN(5b′) are shown
in Figure 9. In fact, the ImDippN ligand appears to be consider-
ably twisted in order to minimize steric congestion by optimal
(34) Zhu, J.; Jia, G.; Lin, Z. Organometallics 2006, 25, 1812–1819.
(35) Poater, A.; Solans-Monfort, X.; Clot, E.; Copeˆret, C.; Eisenstein,
O. J. Am. Chem. Soc. 2007, 129, 8207–8216.
(36) Vyboishchikov, S. F.; Bu¨hl, M.; Thiel, W. Chem.-Eur. J. 2002,
8, 3962–3975.
(37) The prime indicates the substitution of the neopentylidyne moiety
W≡CCMe3 in the experimentally studied complexes for the ethylidyne
moiety W≡CMe in our theoretical model alkylidyne complexes.

Alkyne Cross-Metathesis
Organometallics, Vol. 28, No. 5, 2009 1543
rich organophosphanes in terms of their ligand properties,38
phosphoraneiminato ligands R3PN can be regarded as close
analogues of imidazolin-2-iminato ligands ImN.39 Indeed,
introduction of the Me3PN ligand affords complex B, and the
calculated barrier of ∆Gq298(B) ) 24.66 kcal mol-1 indicates
that phosphoraneiminato ligands are also promising ancillary
ligands for the design of highly active alkyne-metathesizing
catalysts.
To study the impact of other fluorinated alkoxide ligands, the
ImtBuΝ complexes C and D containing bis(trifluoromethyl)benzyl-
oxide OCPh(CF3)2 or nonafluoro-tert-butoxide OC(CF3)3 ligands
were also considered. Unfortunately, we have not been able to
locate a first-order transition state TS(C), which can be attributed
to a very flat hypersurface. Nevertheless, based on the low energies
calculated for the second-order transition state and also for the
intermediate IN(C), it could be assumed that this catalyst might
exhibit high catalytic activity. However, our experiments showed
that this system is not isolable and undergoes C-H activation to
form the alkylidene complex 8 instead (cf. Scheme 4). Finally,
maximizing the electron-withdrawing ability of the alkoxide ligands
by nonafluorination leads to a considerable decrease of the
activation barrier [∆Gq298(D) ) 19.47 kcal mol-1]. In fact, this is
the lowest calculated free energy barrier in our computational series.
However, the resulting high electrophilicity of the catalyst D does
not only increase the catalytic activity, but is also likely to open
up other concurrent reaction pathways such as its own decomposi-
tion by ligand cleavage and redistribution (cf. Scheme 5).
(4 Å) prior to use. The 1H, 13C, and 19F NMR spectra were recorded
on Bruker DPX 200 (200 MHz), Bruker DRX 400 (400 MHz),
and Bruker DPX 600 (600 MHz), devices. The chemical shifts are
expressed in parts per million (ppm) using tetramethylsilane (TMS)
as internal standard (1H, 13C). The 19F NMR spectra have reference
to CFCl3. Coupling constants (J) are reported in Hertz (Hz), and
splitting patterns are indicated as s (singlet), d (doublet), t (triplet),
q (quartet), m (multiplet), sept (septet), dec (decet), and br (broad).
Elemental analysis (C, H, N) succeeded by combustion and gas
chromatographical analysis with an Elementar varioMICRO. GC
analysis was performed on a SHIMADZU GC-2010, GC-MS were
accomplished on a JEOL AccuTOF, MS on a Finnigan MAT 95
(EI), Finnigan MAT 95 XL (ESI), respectively and HR-MS from
Bruker-Demo QTOF micro. Flash chromatography was performed
using Merck silica gel (230-400 mesh). Unless otherwise stated,
all starting materials were obtained from Aldrich and used without
further purification. Compounds 1,13a 2,13a 3,19 5a,17 11,17 ImRNH
23
(R ) tBu, Dipp)16d and Li[N(tBu)Ar] · OEt2 were prepared
following literature procedures. Complex 10 was only characterized
by X-ray diffraction analysis.
Synthesis and Characterization of [Me3CC≡W(NImtBu)-
(OCMe3)2] (4a). A toluene solution of Li(NImtBu) (173 mg, 0.860
mmol) was added to the tungsten alkylidyne complex
[Me3CC≡W(OCMe)3] 2 (400 mg, 0.847 mmol), which was
dissolved in 33 mL toluene. After stirring for two hours at ambient
temperature, the orange suspension was filtered to remove any
insoluble byproduct. Evaporation of the solvent afforded the product
as a light-yellow crystalline powder, which can be recrystallized
1
from a cold iPr2O solution. Yield: 97%; H NMR (400.1 MHz,
C6D6, 25 °C): δ 1.43 (18 H, s, NC(CH3)3), 1.44 (9 H, s,
W≡CC(CH3)3), 1.65 (18 H, s, OC(CH3)3), 6.03 (2 H, s, CH); 13C
NMR (150.9 MHz, C6D6, 25 °C): δ 29.1 (s, NCCH3), 33.5 (s,
OCCH3), 34.6 (s, CCCH3), 50.5 (s, CCMe3), 55.9 (s, NCMe3), 77.5
Conclusions
This combined experimental and theoretical study of imida-
zolin-2-iminato tungsten alkylidyne complexes confirms the high
potential of this class of compounds for catalytic alkyne
metathesis. It can be clearly deduced that a combination of
electron-donating ligands such as imidazolin-2-imides with
electron-withdrawing fluorinated alkoxides is beneficial for
achieving high catalytic activity as exemplified by our still most
active catalyst [Me3CC≡W(NImtBu){OCMe(CF3)2}2] (5a). Theo-
retically, increasing the electron-withdrawing ability of the
alkoxides, e. g. by going from hexafluoro- to nonafluoro-tert-
butoxide ligands, should improve the catalytic performance;
however, this contribution also showed that the experimental
realization of promising computed structures might be impos-
sible because of consecutive reactions such as C-H activation
and C-N bond cleavage. In addition, steric factors are also
important, and catalyst activity and stability must be balanced.
Although no catalyst system could be identified at present that
is superior to 5a, this study also indicates that amides and also
phosphoraneimides are promising ancillary ligands and might
be useful to discover even more active catalyst species in the
future.
(s, OCMe3), 108.5 (s, NCdCN), 159.1 (s, NCN), 273.3 (1JCW
)
278 Hz, s, W≡C); Anal. calcd for C24H47N3O2W: C 48.57%, H
7.98%, N 7.08%; Found: C 47.96%, H 7.88%, N 6.52%; MS (EI):
593 (M).
Synthesis and Characterization of [Me3CC≡W(NImDipp)-
(OCMe3)2] (4b). 1,3-Bis(2,6-diisopropylphenyl)imidazolin-2-imine
(350 mg, 0.87 mmol) was dissolved in a mixture of Et2O and
pentane and treated with one equivalent MeLi at -30 °C. After
2 h stirring at low temperature the solution was allowed to reach
ambient temperature. Evaporation of the solvent afforded the
product in 89% yield as a white powder. The lithium salt (43 mg,
0.11 mmol) was dissolved in toluene and added slowly to a solution
of 2 (50 mg, 0.11 mmol) at ambient temperature. After stirring for
4 h at r.t. evaporation of the solvent afforded a light brownish-
orange powder. The product was extracted with a minimum amount
of hexane in order to remove any byproduct. Recrystallization from
a cold hexane solution afforded the product in 91% yield. 1H NMR
(200.1 MHz, C6D6, 25 °C): δ 1.17 (12 H, d,3JHH ) 6.9 Hz,
CH(CH3)2), 1.22 (18 H, s, OC(CH3)3), 1.34 (9 H, s, W≡CC(CH3)3),
1.49 (12 H, d, 3JHH ) 6.9 Hz, CH(CH3)2), 3.19 (4 H, sept, CHMe2),
5.91 (2 H, s, CH), 7.07-7.24 (6 H, m, ArH); 13C NMR (75.5 MHz,
C6D6, 25 °C): δ 24.0 (s, CHCH3), 24.2 (s, CHCH3), 28.8 (s,
CHMe2), 32.4 (s, OCCH3), 34.3 (s, CCCH3), 49.9 (s, CCMe3), 77.2
(s, OCMe3), 114.6 (s, NCdCN), 123.6 (s, Ar), 129.2 (s, Ar), 133.9
(s, Ar), 147.8 (s, Ar), 156.7 (s, NCN), 276.4 (s, W≡C); Anal. calcd
for C40H65N3O2W: C 59.77%, H 8.15%, N 5.23%; Found: C
59.98%, H 7.78%, N 5.51%.
Experimental Section
General Procedures. All operations with air and moisture-
sensitive compounds were performed in a glovebox under a dry
argon atmosphere (MBraun 200B) or on a high vacuum line using
Schlenk techniques. All solvents were purified by a solvent
purification system from MBraun and stored over molecular sieve
Synthesis and Characterization of [Me3CC≡W(NImDipp)-
{OCMe(CF3)2}2] (5b). The DIPP-ImN-lithium salt (110 mg, 0.27
mmol) was dissolved in toluene and added slowly to a solution of
3 (238 mg, 0.27 mmol) at ambient temperature. After stirring for
24 h at r.t. evaporation of the solvent afforded a light brownish-
orange powder. The product was extracted with a minimum amount
of hexane to remove any byproduct. Recrystallization from a cold
(38) Herrmann, W. A. Angew. Chem. 2002, 114, 1342–1363. Herrmann,
W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309. (b) Hahn, F. E.; Jahnke,
M. Angew. Chem. 2008, 120, 3166–3216; Angew. Int. Ed. 2008, 47, 3122-
3172.
(39) Dehnicke, K.; Krieger, M.; Massa, W. Coord. Chem. ReV. 1999,
182, 19–65.

1544 Organometallics, Vol. 28, No. 5, 2009
Beer et al.
hexane solution afforded the product in 65% yield. 1H NMR (400.1
NCCH3), 33.6 (s, CC(CH3)3), 43.2 (s, CHCMe3), 59.8 (2 C, s,
NCCH3), 87.0 (2 C, m, OCCF3), 111.7 (2 C, s, NC)CN), 123.7
(CF3), 125.6 (s, C-Ar), 126.1 (s, C-Ar), 126.7 (s, C-Ar), 128.8 (s,
C-Ar), 129.2 (s, C-Ar), 129.6 (s, C-Ar), 136.5 (s, C-Ar), 139.7 (s,
C-Ar), 145.6 (s, C-Ar), 151.1 (s, NCN), 192.4 (s, W-CAr), 281.8
3
MHz, C6D6, 25 °C): δ 1.09 (12 H, d, JHH ) 6.8, Hz, CHMe2),
3
1.16 (9 H, s, W≡CC(CH3)3), 1.36 (12 H, d, JHH ) 6.8, Hz,
CHMe2), 1.54 (6 H, s, OCCH3), 3.05 (4 H, m, CHMe2), 5.93 (2 H,
3
s, CH), 7.08 (4 H, d, JHH ) 3.2 Hz, ArH), 7.21 (2 H, m, ArH);
13C NMR (100.6 MHz, C6D6, 25 °C): δ 23.6 (s, CHCH3), 24.7 (s,
CHCH3), 29.1 (s, CHMe2), 31.9 (s, OCCH3), 33.9 (s, CCCH3), 50.3
(s, CCMe3), 82.7 (m, OCCF3), 116.8 (s, NC)CN), 124.5 (s, Ar),
124.8 (q, CF3), 130.3 (s, Ar), 132.7 (s, Ar), 146.8 (s, Ar), 159.8 (s,
NCN), 290.5 (s, W≡C);); 19F NMR (188.3 MHz, C6D6, 25 °C): δ
-77.9 (4JFF ) 19 Hz, 6 F, q, CF3), -77.3 (4JFF ) 19 Hz, 6 F, q,
CF3); Anal. calcd for C40H51F12N3O2W: C 37.21%, H 5.05%, N
4.13%; Found: C 37.79%, H 7.88%, N 6.52%.
(s, WdC); 19F NMR (376.5 MHz, C6D6, 25 °C): δ -74.4 (4JFF
)
20 Hz, 3 F, q, CF3), -73.9 (4JFF ) 20 Hz, 3 F, q, CF3), -71.9
(4JFF ) 20 Hz, 3 F, q, CF3), -70.5 (4JFF ) 20 Hz, 3 F, q, CF3);
Anal. calcd for C34H39F12N3O2W: C 43.74%, H 4.21%, N 4.50%;
Found: C 43.88%, H 4.32%, N 4.47%.
Synthesis and Characterization of [Me3CC≡WCl{OC(CF3)3}2-
(dme)] (9). For the synthesis of 9, it was necessary to prepare the
lithium salt Li[OC(CF3)3] in advance. Nonafluoro-tert-butanol (1.80
g, 7.62 mmol) was dissolved in diethyl ether and treated with LiH
(7.50 mmol). After stirring for 2 h, the solvent was evaporated,
and the remaining white solid was purified by sublimation. 19F NMR
(188.3 MHz, CDCl3, 25 °C): δ -77.9 (s, CF3). 1 (300 mg, 0.66
mmol) was added to a solution of Li[OC(CF3)3] (350 mg; 1.45
mmol) in diethyl ether at ambient temperature. After stirring for
15 min, the resulting purple solution was filtered and evaporated.
Purple crystals were isolated in 74% yield from a cold (-35 °C)
hexane solution. 1H NMR (400.1 MHz, C6D6, 25 °C): δ 1.08 (9 H,
s, W≡CC(CH3)3), 2.19 (1 H, ddd, 2JHH ) 10 Hz, O(CH2)2O), 2.52
(1 H, ddd, 2JHH ) 10 Hz, O(CH2)2O), 2.90 (3 H, s, OCH3), 3.05 (1
H, m, O(CH2)2O), 3.25 (1 H, m, O(CH2)2O), 3.69 (3 H, s, OCH3);
13C NMR (100.6 MHz, C6D6, 25 °C): δ 33.2 (s, CC(CH3)3), 49.4
(s, CCMe3), 58.7 (s, dme), 68.2 (s, dme), 75.5 (s, dme), 78.2 (s,
Synthesis and Characterization of [Me3CC≡W{OCMe-
(CF3)2}2{N(3,5-C6H3Me2)tBu)] (6).
A toluene solution of
Li[NtBuAr] · OEt2 (51 mg, 0.200 mmol) was added to the tungsten
alkylidyne complex 3 (153 mg, 0.198 mmol), which was dissolved
in 12 mL toluene and stirred for 30 min at room temperature.
Evaporation of the solvent and any volatile byproduct gave a light
yellow residue. The powder was redissolved in 8 mL hexane and
cooled to -35 °C for 6 h. Filtration and evaporation of the solvent
gave the product as a light-yellow powder, which can be recrystal-
lized from a cold hexane solution. 1H NMR (400.1 MHz, C6D6, 25
°C): δ 0.66 (9 H, s, C(CH3)3), 1.16 (9 H, s, C(CH3)3), 1.73 (6 H,
m, C(CH3)(CF3)2), 2.13 (6 H, s, ArCH3), 6.62 (1 H, m, ArH), 6.70
(2 H, s, ArH); 13C NMR (150.9 MHz, C6D6, 25 °C): δ 20.2 (s,
OCCH3), 21.3 (s, ArCH3), 28.7 (s, CC(CH3)3), 31.4 (s, C(CH3)3),
51.5 (2JCW ) 39 Hz, CCCH3), 60.5 (s, NCMe3), 81.9 (m, OCCF3),
123.9 (1JCF ) 287 Hz, q, CF3), 124.2 (1JCF ) 287 Hz, q, CF3),
126.2 (s, C aryl) 127.4 (s, C aryl meta), 128.3 (s, C aryl), 137.0 (s,
C aryl meta), 159.2 (s, C aryl ipso), 298.2 (1JCW ) 286 Hz, s,
W≡C); 19F NMR (376.5 MHz, C6D6, 25 °C): δ -78.8 (4JFF ) 18
Hz, 6 F, q, CF3), -77.9 (4JFF ) 18 Hz, 6 F, q, CF3); Anal. calcd
for C25H33NF12O2W: C 37.94%, H 4.20%, N 1.77%; Found: C
36.97%, H 4.66%, N 2.11%.
1
dme), 84.8 (m, OC(CF3)3), 121.2 (q, CF3, JCF ) 294 Hz,), 121.8
1
1
(q, CF3, JCF ) 294 Hz), 315.8 (s, JCW ) 252 Hz, W≡C); 19F
NMR (376.5 MHz, C6D6, 25 °C): δ -71.1 (dec, JFF ) 1.6 Hz,
CF3), -71.5 (dec, JFF ) 1.6 Hz, CF3); Anal. calcd for
C17H19ClF18O4W: C 24.06%, H 2.26%; Found: C 25.56%, H 3.06%.
Synthesis and Characterization of [EtC≡W(NImtBu){OCMe-
(CF3)2}2] (12). Thirteen equivalents of 3-hexyne (80 mg, 0.989
mmol) were added to a toluene solution of the tungsten alkylidyne
complex 5a (60 mg, 0.074 mmol). Immediately after the addition
of the alkyne the solution turned deep red. The solution was stirred
for 10 min at ambient temperature. Applying high vacuum at 40
°C for five hours afforded a brownish-yellow residue, which gave
yellow crystals upon dissolving in cold iPr2O for two days. 1H NMR
Synthesis and Characterization of [Me3CC≡W{OCPh-
(CF3)2}3] (7). For the synthesis of 7, it was necessary to prepare
the lithium salt Li[OCPh(CF3)2] in advance. nBuLi (1.1 equiv; 1,6
M in Et2O; 7.0 mL; 11.3 mmol) was slowly added to a cold (-20
°C) Et2O solution of the corresponding alcohol (2.50 g; 10.24 mmol)
and stirred for one hour, allowing the solution to warm to -10 °C.
Evaporation of the solvent afforded the salt as a white powder. 1
(200 mg, 0.45 mmol) was added to an Et2O solution of the lithium
salt (334 mg, 1.34 mmol) at ambient temperature and was stirred
for 15 min. The brown suspension was filtered to remove any
insoluble side products. Evaporation of the solvent and any volatile
3
(200.1 MHz, C6D6, 24 °C): δ 0.94 (3 H, t, JHH ) 7.5 Hz,
CCH2CH3), 1.29 (18 H, s, NCCH3), 1.88 (6 H, br, CCH3), 3.86 (2
H, q, 3JHH ) 7.5 Hz, CCH2CH3), 5.96 (2 H, s, CH); 19F NMR (188.3
MHz, C6D6, 24 °C): δ -78.3 (4JFF ) 9.8 Hz, 6 F, q, CF3), -76.4
(4JFF ) 9.8 Hz, 6 F, q, CF3). Anal. calcd C22H31F12N3O2W: C
33.82%, H 4.00%, N 5.38%; Found: 34.18%, H 4.61%, N 4.46%.
Reactivity toward 3-Heptyne. A hexane solution (3.5 mL) of
3-heptyne (202.2 mg, 2.00 mmol) was added to 5a (17.0 mg; 2.1
10-5 mol; 1 mol%), which was dissolved in 5 mL of hexane. The
initially yellow solution immediately darkened upon addition of
the alkyne. The GC-analysis of this solution revealed that 3-hexyne,
3-heptyne and 4-octyne were formed in a statistical 1:2:1 ratio. In
addition, this reaction was studied as an NMR experiment, wherein
3-heptyne (30.00 mg; 0.31 mmol) was added to a C6D6 solution of
5b (2.4 mg; 3.1 10-6 mol; 1 mol%). The 1H NMR spectrum showed
1
byproduct afforded the product as a yellow oil in 70% yield. H
NMR (200.1 MHz, C6D6, 25 °C): δ 0.27 (9 H, s, W≡CC(CH3)3),
6.93-7.08 (9 H, m, Ar-H), 7.66-7.70 (6 H, m, Ar-H); 13C NMR
(100.6 MHz, C6D6, 25 °C): δ 31.5 (s, CC(CH3)3), 52.3 (s, CCMe3),
88.0 (m, OCPh(CF3)2), 123.5 (q, CF3, JCF ) 289 Hz), 127.5 (s,
C-Ar), 129.1 (s, C-Ar), 130.7 (s, C-Ar), 132.0 (s, C-Ar), 307.2 (s,
W≡C); 19F NMR (188.3 MHz, C6D6, 25 °C): δ -73.0 (s, CF3);
Anal. calcd for C32H24F18O3W: C 39.13%, H 2.46%; Found: C
38.41%, H 3.00%.
1
a statistical ratio of 3-hexyne, 3-heptyne and 4-octyne. H NMR
Synthesis and Characterization of the Alkylidene Complex
8. A toluene solution of (ImN)Li (56 mg, 0.254 mmol) was added
to the tungsten alkylidyne complex 7 (250 mg, 0.254 mmol), which
was dissolved in 18 mL toluene and stirred for 24 h at room
temperature. The yellow-orange suspension was filtered through a
Schlenk frit to remove any byproduct. Evaporation of the solvent
afforded a yellow crystalline powder. Single crystals could be
obtained by cooling a saturated hexane solution to -35 °C. Yield:
202 mg of yellow crystals (85%). 1H NMR (400.1 MHz, C6D6, 25
°C): δ 1.18 (9 H, s, W)CHCH3), 1.28 (18 H, s, NCCH3), 5.79 (2
H, s, CH), 7.01-7.84 (7 H, m, ArH), 8.58 (2 H, m, ArH), 10.30 (1
H, s, WdCH); 13C NMR (100.6 MHz, C6D6, 25 °C): δ 28.6 (br,
(200.1 MHz, C6D6, 25 °C): δ 0.86-1.04 (24 H, m, CH2CH3 of
3-hexyne, 3-heptyne and 4-octyne), 1.43 (8 H, sext, CH2CH2Me
of 3-heptyne and 4-octyne), 1.96-2.10 (16 H, m, CH2Me of
3-hexyne and 3-heptyne, CH2CH2Me of 3-heptyne and 4-octyne).
Comparative Alkyne Cross-Metathesis with 1-Phenylpro-
pyne. A hexane (30 °C) or toluene (80 °C) solution (5 mL) of
1-phenylpropyne (300 mg, 2.583 mmol) was added to the respective
catalyst (1 mol%, 2.583 10-5 mol), which was dissolved in 15 mL
of hexane or toluene, respectively. After the addition, vacuum (300
mbar) was applied and every 10 min aliquots of 0.5 mL were taken
and filtered over alumina and analyzed via GC.

Alkyne Cross-Metathesis
Organometallics, Vol. 28, No. 5, 2009 1545
X-Ray Crystal Structure Determinations. Data were recorded
on various area detectors (Bruker, Oxford Diffraction) at low
temperature. Structures were refined anisotropically using the
program SHELXL-97 (G.M. Sheldrick, University of Go¨ttingen,
Germany). Hydrogen atoms were included using rigid methyl
groups or a riding model. Details of all X-ray crystal structure
determinations and pertinent data in tabulated form can be found
in the Supporting Information.
an imaginary frequency of about -20 cm-1 (IV). Enthalpic and
entropic contributions were estimated from the partition functions
calculated at room temperature (298 K) under a pressure of one
atm using Boltzmann thermostatistics and the rigid rotor harmonic
oscillator approximation as implemented in the Gaussian03 set of
programs.44
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft (DFG) through grant Ta
189/6-2. We wish to thank Dr. Kerstin Ibrom (Braunschweig)
for the high-resolution NMR measurements.
ElectronicStructureCalculations.TheB3LYPhybridfunctional40,41
was employed for all calculations. All atoms except tungsten were
described by a standard triple-ꢀ all electron basis set augmented
with one set of polarization functions (6-311G(d,p)). For tungsten,
a double-ꢀ basis set optimized for use with effective core potentials
(ECP) in combination with the corresponding Stuttgart ECP was
employed,42 and an additional polarization function was added (ꢁ(f)
) 0.823).43 After the relevant stationary points were localized on
the energy surface, they were further characterized as minima or
transition states by normal-mode analysis based on the analytical
energy second derivatives. The self-consistent field iteration was
terminated after reaching the convergence criterion of 10-7 hartree.
The root-mean-square of the internal forces is always below 0.0003
hartree bohr-1. Some normal-mode analysis revealed imaginary
frequencies between -10 and 0 cm-1 for local minima, which can
be attributed to rounding errors, since the potential energy surface
around these conformations is almost planar. However, all relevant
imaginary eigenvalues for the transition states have their frequency
in the range from -100 to -40 cm-1 with one exception having
Supporting Information Available: Details of the X-ray crystal
structure determinations; presentations of the molecular structures
of 2 and 3; crystal data in CIF format and all calculated structures
in mol2, pdb, and xyz format. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM801119T
(44) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
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