Electrophilic Attack on [Cp*Cl(PPh3)Ru(CCHR)]: Carbyne Formation vs Chloride Abstraction

Article abstract of DOI:10.1021/om030559i

The effect of steric properties of the electrophile and ruthenium (Ru) complexes on the electrophilic addition to the [Cp^*Cl(PPh ₃)Ru(CCHR)] was analyzed. The Ru complexes were treated with a diethyl ether solution at -78 °C. The structural information of the complex was derived by using room-temperature nuclear magnetic resonance (NMR) spectroscopy. The results show that the site of electrophilic attack was changed from Ru complex to halide by changing the electrophile properties.

Full text of DOI:10.1021/om030559i

Organometallics 2003, 22, 5179-5181
5179
Communications
Electr op h ilic Atta ck on [Cp *Cl(P P h 3)Ru (CCHR)]:
Ca r byn e F or m a tion vs Ch lor id e Abstr a ction
Nicholas J . Beach,† Hilary A. J enkins,‡ and Gregory J . Spivak*,†
Departments of Chemistry, Lakehead University, Thunder Bay, Ontario P7B 5E1, Canada,
and Saint Mary’s University, Halifax, Nova Scotia B3H 3C3, Canada
Received J uly 29, 2003
Sch em e 1
Summary: The course of electrophilic addition to the
ruthenium(II) chloro vinylidenes [Cp*Cl(PPh3)Ru(CCHR)]
is influenced by the steric properties of the electrophile
and ruthenium complex. Thus, H+ selectively adds to Câ
of the vinylidene ligand to yield the ruthenium(IV)
carbynes [Cp*Cl(PPh3)Ru(CCH2R)][A] (A ) BF4-, BArf4-),
while the comparably larger Me+ (from MeOTf) abstracts
Cl- to yield, after anion coordination, [Cp*(OTf)(PPh3)-
Ru(CCHR)] and MeCl.
Recent reports describing electrophilic addition reac-
tions of E+ (from EA) to the ruthenium vinylidenes [Ru-
(X)(Y)(CCHR)(PR3)2] (X, Y ) hydride, chloride, carbox-
ylate) revealed the outcome was dependent upon several
factors, including the nature of the counteranion A- of
EA and the ancillary ligands on the metal.1 Remarkably,
despite the presence of hydride, halide, and/or pseudo-
halide ligands (i.e., ligands with lone pairs on the
R-ligand atom) and a coordinatively unsaturated ruthe-
nium center, selective electrophilic addition to Câ of the
vinylidene ligand2 occurred, establishing it as the more
Brønsted basic site in the complexes studied.3 Thus,
depending upon the conditions, ruthenium carbenes,1a
carbynes,1b,d or carbene-carbyne equilibria1c,d were
observed. Intrigued by these results, we decided to
explore further electrophilic additions to ruthenium
vinylidenes, and especially those factors which control
the site selectivity of electrophilic attack. We now report
a different consequence of electrophilic addition to
ruthenium chloro vinylidenes, which reveals that steric
factors can influence the selectivity of the reaction.
The neutral ruthenium(II) vinylidenes [Cp*Cl(PPh3)-
Ru(CCHR)],4 when treated with a diethyl ether solution
of HBF4 (2 equiv) at -78 °C, selectively add H+ to Câ of
the vinylidene ligand to afford, after anion metathesis
with NaBArf (Arf ) 3,5-(CF3)2C6H3), the orange ruthe-
4
nium(IV) carbynes5 [Cp*Cl(PPh3)Ru(CCH2R)][BArf ],
4
(R ) tBu, 1; R ) nBu, 2) in good yields (Scheme 1). The
instantaneous formation of the cations of 1 and 2 upon
adding HBF4 to the ruthenium vinylidene precursor,
even at -78 °C (as observed by NMR spectroscopy),
frustrated our attempts to detect any possible inter-
mediates. Hence, it is unclear whether the cations are
formed via direct attack of H+ at Câ of the vinylidene
ligand6 or through some other mechanism involving
either initial attack at the metal7 or other ligand (i.e.,
(4) (a) Bruce, M. I.; Hall, B. C.; Zaitseva, N. N.; Skelton, B. W.;
White, A. H. J . Chem. Soc., Dalton Trans. 1998, 1793. (b) See the
Supporting Information.
* To whom correspondence should be addressed. E-mail:
greg.spivak@lakeheadu.ca.
† Lakehead University.
(5) Other ruthenium carbynes: (a) Bustelo, E.; J ime´nez-Tenorio, M.;
Mereiter, K.; Puerta, M. C.; Valerga, P. Organometallics 2002, 21, 1903.
(b) J ung, S.; Brandt, C. D.; Werner, H. New J . Chem. 2001, 25, 1101.
(c) Coalter, J . N., III; Bollinger, J . C.; Eisenstein, O.; Caulton, K. G.
New J . Chem. 2000, 24, 925. (d) Amoroso, D.; Snelgrove, J . L.; Conrad,
J . C.; Drouin, S. D.; Yap, G. P. A.; Fogg, D. E. Adv. Synth. Catal. 2002,
344, 757. (e) Roper, W. R. J . Organomet. Chem. 1986, 300, 167.
(6) Reactions between the vinylidene complex [RuHCl(CCH2)(PCy3)2]
and D+ revealed only exclusive attack at Câ of the vinylidene, as
determined by 2H NMR spectroscopy.1b
(7) At least one example involving initial protonation at the metal
of a coordinatively saturated neutral d6 metal vinylidene complex
followed by rearrangement to the isomeric metal carbyne has been
reported: Carvalho, M. F. N. N.; Henderson, R. A.; Pombeiro, A. J . L.;
Richards, R. L. J . Chem. Soc., Chem. Commun. 1989, 1796.
‡ Saint Mary’s University.
(1) (a) Wolf, J .; Stu¨er, W.; Gru¨nwald, C.; Werner, H.; Schwab, P.
Schulz, M. Angew. Chem., Int. Ed. 1998, 37, 1124. (b) Stu¨er, W.; Wolf,
J .; Werner, H.; Schwab, P.; Schulz, M. Angew. Chem., Int. Ed. 1998,
37, 3421. (c) Gonza´lez-Herrero, P.; Weberndo¨rfer, B.; Ilg, K.; Wolf, J .;
Werner, H. Angew. Chem., Int. Ed. 2000, 39, 3266. (d) Gonza´lez-
Herrero, P.; Weberndo¨rfer, B.; Ilg, K.; Wolf, J .; Werner, H. Organo-
metallics 2001, 20, 3672.
(2) Molecular orbital calculations predict electrophilic addition
occurs selectively at Câ of a vinylidene ligand rather than CR: Kostic´,
N. M.; Fenske, R. F. Organometallics 1982, 1, 974.
(3) Selective electrophilic attack has been observed on metal hydride
halides: Kuhlman, R. Coord. Chem. Rev. 1997, 167, 205 and references
therein.
10.1021/om030559i CCC: $25.00 © 2003 American Chemical Society
Publication on Web 11/11/2003

5180 Organometallics, Vol. 22, No. 25, 2003
Communications
chloride) on the metal, followed by hydrogen migration
to Câ. Both solids and solutions of 1 show exceptional
stability at room temperature under nitrogen even over
several days, while 2 proved to be somewhat less stable
under similar conditions. The substantial electron-
donating properties of the Cp* ligand8 likely contribute
to the stabilization of the strongly π-acidic carbyne
ligand and formal Ru(IV) center. Decomposition of 1 and
2 in solution occurs very slowly, yielding mainly Ph3-
PH+ as the only phosphorus-containing species, as
determined by 31P{1H} NMR.
Room-temperature NMR spectroscopy provides im-
portant structural information which supports the
formation of a carbyne ligand and oxidized ruthenium
in 1 and 2. The 13C{1H} NMR spectra of 1 and 2 reveal
a number of important resonances, most notably the
downfield-shifted carbyne CR carbon resonances (δ 348.3
for 1; δ 347.0 for 2), which appear as doublets due to
F igu r e 1. Molecular structure of the cation of 1‚CH2Cl2
2
coupling with PPh3 (2J PC ) 15.7 Hz for 1; J PC ) 17.1
(the hydrogen atoms, BArf - counterion, and CH2Cl2 solvate
4
Hz for 2). These couplings are lower compared to the
2J PC couplings observed for the ruthenium(II) vinylidene
precursors (2J PC ca. 24 Hz),4 which is consistent with a
lengthening of the Ru-PPh3 bond as a result of dimin-
ished back-bonding with Ru(IV) vs Ru(II).9 The Cp*
signals of 1 and 2 in the 13C{1H} NMR spectra are
shifted downfield vs [Cp*Cl(PPh3)Ru(CCHR)],4 which
also points to an increase in oxidation state of the
have been omitted for clarity). Selected bond lengths (Å)
and angles (deg): Ru(1)-C(11) ) 1.710(3), Ru(1)-P(1) )
2.3641(9), Ru(1)-Cl(1) ) 2.3715(9), Ru(1)-C(1) ) 2.393-
(3), Ru(1)-C(2) ) 2.391(3), Ru(1)-C(3) ) 2.255(3), Ru(1)-
C(4) ) 2.232(3), Ru(1)-C(5) ) 2.240(3); C(11)-Ru(1)-Cl(1)
) 101.4(1), C(11)-Ru(1)-P(1) ) 90.0(1), P(1)-Ru(1)-Cl-
(1) ) 90.22(3), C(11)-C(12)-C(13) ) 114.4(3), C(12)-
C(11)-Ru(1) ) 174.1(3).
1
ruthenium center.9b,10 The room-temperature H NMR
appealing mixtures of products (which typically include
ca. 30-60% Ph3PH+) after warming to room tempera-
ture, as evidenced by 31P{1H} and 1H NMR spectros-
copy. However, monitoring the reaction at low temper-
atures (-75 °C) using NMR spectroscopy (CD2Cl2)
immediately after adding HBF4 at -78 °C reveals rapid
and quantitative formation of the carbyne [Cp*Cl-
(PPh3)Ru(CCH2Ph)][BF4] (3). No other species are de-
tected under these conditions. At -75 °C complex 3
exhibits spectroscopic features similar to those observed
for 1 and 2 (see Supporting Information), including two
inequivalent Câ methylene protons (δ 4.05 and 3.52,
spectra of 1 and 2 are particularly interesting and
clearly show a separate signal for each of the two Câ
methylene protons of the carbyne ligand (for 1, δ 2.51
2
and 1.76, both doublets, J HH ) 20.3 Hz; for 2, δ 2.35
2
and 1.86, both multiplets, J HH ) 20.5 Hz). These
observations are consistent with restricted rotation
about the CR-Câ bond of the carbyne ligands,11 which
causes the two protons on Câ to become inequivalent.
The cumulative steric effects of the Cp*, PPh3, and
carbyne R groups likely contribute to this rotational
barrier. Coalescence of the Câ methylene proton signals
was not observed at higher temperatures (C6D6, 70 °C)
for 1, while complex 2 proved to be unstable at elevated
temperatures and decomposed to a number of unidenti-
fied phosphorus-containing species.
2
both doublets, J HH ) 20.5 Hz), which again can be
ascribed to restricted rotation about the CR-Câ bond of
the carbyne ligand. The NMR spectra of 3 remain
essentially unchanged upon warming the solution to 0
°C, although 1H NMR spectroscopy reveals the Câ
methylene protons show an unusual temperature de-
pendence within this temperature range (the separation
between doublets increases by ca. 0.5 ppm as the
temperature approaches 0 °C). Rapid (minutes) decom-
position to many products occurs above 0 °C, with Ph3-
PH+ representing the main (>50%) decomposition prod-
uct (the remaining products could not be confidently
identified). Coalescence of the Câ methylene proton
signals of 3 was also not observed under these condi-
tions.
The R group of the vinylidene ligand in [Cp*Cl(PPh3)-
Ru(CCHR)] appears to influence the stability of the
resulting carbyne complex, but not the selectivity of H+
addition. For example, extending these reactions to
include [Cp*Cl(PPh3)Ru(CCHPh)]4a (i.e., decreasing the
nucleophilicity of Câ by replacing electron-donating alkyl
with electron-withdrawing phenyl) yields only un-
(8) (a) Tellers, D. M.; Skoog, S. J .; Bergman, R. G.; Gunnoe, T. B.;
Harman, W. D. Organometallics 2000, 19, 2428 and references therein.
(b) Gassman, P. G.; Winter, C. H. Organometallics 1991, 10, 1592. (c)
Ryan, M. F.; Siedle, A. R.; Burk, M. J .; Richardson, D. E. Organome-
tallics 1992, 11, 4231.
(9) (a) Becker, E.; Slugovc, C.; Ru¨ba, E.; Standfest-Hauser, C.;
Mereiter, K.; Schmid, R.; Kirchner, K. J . Organomet. Chem. 2002, 649,
55. (b) Mauthner, K.; Slugovc, C.; Mereiter, K.; Schmid, R.; Kirchner,
K. Organometallics 1997, 16, 1956.
(10) (a) Kondo, H.; Yamaguchi, Y.; Nagashima, H. Chem. Commun.
2000, 1075. (b) Yamaguchi, Y.; Nagashima, H. Organometallics 2000,
19, 725. (c) Kondo, H.; Kageyama, A.; Yamaguchi, Y.; Haga, M.;
Kirchner, K.; Nagashima, H. Bull. Chem. Soc. J pn. 2001, 74, 1927.
(d) Gemel, C.; Kalt, D.; Mereiter, K.; Sapunov, V. N.; Schmid, R.;
Kirchner, K. Organometallics 1997, 16, 427.
The solution structures of the cations of 1-3 are
supported by an X-ray crystallographic study on com-
plex 112 (Figure 1). The ruthenium center in the
structure of complex 1 possesses a distorted three-legged
piano-stool coordination geometry. A notable feature of
1 is the relatively short ruthenium-CR carbyne bond
(11) Rotation about the metal-CR bond is also possible, since MO
calculations reveal the rotational barrier around the metal-carbon
triple bond in metal carbynes is quite low: Kostic´, N. M.; Fenske, R.
F. J . Am. Chem. Soc. 1982, 104, 3879.
(12) Crystallographic data for C67H55BCl3F24PRu (1‚CH2Cl2): a )
17.730(1) Å, b ) 19.077(1) Å, c ) 20.365(1) Å, R ) 90°, â ) 94.349(1)°,
γ ) 90°, Z ) 4 in space group P21/n, R1 ) 0.0871 (all data), wR2 )
0.1690 (all data), 15 419 reflections, 1098 refined parameters.

Communications
Organometallics, Vol. 22, No. 25, 2003 5181
distance of 1.710(3) Å, which is shorter when compared
to the structurally characterized vinylidenes [Cp*Cl-
(PPh3)Ru(CCHR)] (range ca. 1.80-1.85 Å).4a Equally
notable are the slightly longer Ru-P bond (2.3641(9)
Å) and slightly shorter Ru-Cl bond (2.3715(9) Å) in the
cation of 1 (vs Ru-P ) ca. 2.305-2.315 Å and Ru-Cl
) ca. 2.395-2.408 Å in [Cp*Cl(PPh3)Ru(CCHR)]4a), both
of which are consistent with an increase in oxidation
state of the ruthenium (vide supra). The Ru-CR-Câ
linkage is bent only slightly (174.1(3)°), possibly because
extent, PPh314b) likely inhibit access of the comparably
larger CH3 (vs H+) to either the vinylidene or the
+
coordinatively saturated metal. Alternatively, complex
4 may be formed via a mechanism involving direct
+
electrophilic attack of CH3 on the chloride ligand of
[Cp*Cl(PPh3)Ru(CCHPh)].15 Halide abstraction involv-
ing nonmetal electrophiles are thought to proceed
through direct attack at the halide.16 This is certainly
reasonable, considering the greater accessibility of the
chloride lone pairs. Furthermore, filled-filled ruthe-
nium d orbital-chloride lone pair repulsions likely
enhance the basicity of the chloride ligand.17
t
of steric crowding among Cp*, PPh3, and the Bu group
on Câ of the carbyne ligand.
While H+ addition selectively yields the carbynes
[Cp*Cl(PPh3)Ru(CCH2R)]+ from the vinylidenes [Cp*Cl-
(PPh3)Ru(CCHR)], the electrophile Me+ follows a much
different reaction course. For example, adding excess
(3 equiv) MeOTf to a room-temperature solution of
[Cp*Cl(PPh3)Ru(CCHPh)] results in the complete con-
sumption of the vinylidene complex and slowly but
cleanly yields [Cp*(OTf)(PPh3)Ru(CCHPh)], 4 (Scheme
In summary, we have described an interesting site
selectivity in electrophilic addition reactions to ruthe-
nium chloro vinylidenes. The observed selectivity em-
phasizes how easily the site of electrophilic attack can
be diverted from vinylidene to halide just by changing
the properties of the electrophile (H+ vs CH3+). Thus,
steric factors should be considered when there is direct
competition between different sites for an electrophile,
since selectivity can be influenced by the relative
accessibility of these sites. Current efforts in our labora-
tory are directed toward expanding upon the work
presented herein.
1
1), along with 1 equiv (based on H NMR integrations)
of MeCl within 30 min of mixing, as determined by NMR
spectroscopy. A stoichiometric amount of MeOTf yields
similar results under the same conditions but requires
several hours for completion. Mechanistically, this reac-
tion might proceed initially via direct attack at the
metal, or through electrophilic attack at Câ,13 followed
by methyl migration to the metal. In either case (i.e.,
ion oxidative addition), reductive elimination would
subsequently yield MeCl and 4. Despite the relatively
slow production of 4 under the conditions studied, we
see no direct spectroscopic evidence supporting either
initial electrophilic attack on the vinylidene or the
formation of ruthenium methyl species (the carbene
[Cp*Cl(PPh3)Ru(C(OTf)(CH(Me)Ph))] is not observed to
form1a). Unfortunately, extending these reactions to
Ack n ow led gm en t. We gratefully acknowledge fi-
nancial support from the Lakehead University Senate
Research Committee and Dr. Andreas Decken (Depart-
ment of Chemistry, University of New Brunswick) for
collecting the X-ray diffraction data for complex 1.
Su p p or tin g In for m a tion Ava ila ble: Text describing full
synthetic procedures and spectroscopic data, along with text
and tables giving crystallographic details for complex 1. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM030559I
t
include [Cp*Cl(PPh3)Ru(CCHR)] (R ) Bu, nBu) only
(15) The tendency for the chloride of [Cp*Cl(PPh3)Ru(CCHPh)] to
ionize in methylene chloride (the solvent used in the reactions studied)
most likely would be marginal.
resulted in much poorer selectivity and the rapid
production of many unidentified products (as observed
by NMR), although MeCl was detected by 1H NMR
spectroscopy. These results should not be surprising,
since steric factors (especially Cp*14a and, to a lesser
(16) For example, see: (a) Eaborn, C.; Farrel, N.; Murphy, J . L.;
Pidock, A. J . Chem. Soc., Dalton Trans. 1976, 58. (b) Eaborn, C.; Farrel,
N.; Pidock, A. J . Chem. Soc., Dalton Trans. 1976, 289. (c) Connor, J .
A.; Hudson, G. A. J . Organomet. Chem. 1974, 73, 351. (d) Druce, P.
M.; Lappert, M. F.; Riley, P. N. K. J . Chem. Soc. D 1967, 486. (e)
Aizenberg, M.; Milstein, D. J . Chem. Soc., Chem. Commun. 1994, 411.
(f) Huang, D.; Streib, W. E.; Eisenstein, O.; Caulton, K. G. Angew.
Chem., Int. Ed. 1997, 36, 2004. (g) Kuhlman, R.; Streib, W. E.;
Huffman, J . C.; Caulton, K. G. J . Am. Chem. Soc. 1996, 118, 6934.
(17) Caulton, K. G. New J . Chem. 1994, 18, 25.
(13) Birdwhistell, K. R.; Tonker, T. L.; Templeton, J . L. J . Am. Chem.
Soc. 1985, 107, 4474.
(14) (a) Kitajima, N.; Tolman, B. W. Prog. Inorg. Chem. 1995, 43,
419. (b) Tolman, C. A. Chem. Rev. 1977, 77, 313.