Reactions of Ru-alkynyl complexes with electrophilic boranes

Article abstract of DOI:10.1021/om200815s

The species CpRu(PPh₃)₂(C≡CPh) (1) reacts with HB(C₆F₅)₂ to effect the hydroboration of the metal-bound alkyne, giving the orange species CpRu(PPh₃) ₂CH=C(Ph)(B(C₆F₅)₂) (2) in 95% isolated yield. Subsequent photolysis of this species results in the rearrangement of the vinyl group to give the species CpRu(PPh ₃)[PhC≡C(B(H)(C₆F₅)₂)] (3) in 87% yield. The species exhibits a side-on alkyne donation to Ru as well as a bridging BH fragment. Intermediates in this conversion are discussed in relation to known species, and support for a transient zwitterionic intermediate is offered by the isolation of CpRu(PPh₃)₂(C=CPh(C ₆F₄)BF(C₆F₅)₂) (4) from the reaction of CpRu(PPh₃)₂(C≡CPh) with B(C ₆F₅)₃. The compounds 2-4 are crystallographically characterized.

Full text of DOI:10.1021/om200815s

ARTICLE
pubs.acs.org/Organometallics
Reactions of Ru-alkynyl Complexes with Electrophilic Boranes
Michael P. Boone and Douglas W. Stephan*
University of Toronto, Toronto, ON, Canada
S Supporting Information
b
ABSTRACT: The species CpRu(PPh3)2(CtCPh) (1) reacts with HB(C6F5)2 to
effect the hydroboration of the metal-bound alkyne, giving the orange species
CpRu(PPh3)2CHdC(Ph)(B(C6F5)2) (2) in 95% isolated yield. Subsequent
photolysis of this species results in the rearrangement of the vinyl group to give
the species CpRu(PPh3)[PhCtC(B(H)(C6F5)2)] (3) in 87% yield. The species
exhibits a side-on alkyne donation to Ru as well as a bridging BH fragment.
Intermediates in this conversion are discussed in relation to known species, and
support for a transient zwitterionic intermediate is offered by the isolation of
CpRu(PPh3)2(CdCPh(C6F4)BF(C6F5)2) (4) from the reaction of CpRu-
(PPh3)2(CtCPh) with B(C6F5)3. The compounds 2ꢀ4 are crystallographically
characterized.
’ INTRODUCTION
species, a transformation that is important in a number of
metal-mediated transformations.22ꢀ31 In such conversions, the
migrations of carbon-,32ꢀ34 silyl-,35ꢀ43 stannyl-,44,45 sulfenyl-,46
and iodo-substituents47 have been reported and vinylidene
complexes, such as [CpRu{CdC(Ph)Ar}-(dppe)][B(C6F5)4],
have been reported to proceed via electrophilic 1,2-migration.48
This observation suggests that boron-based electrophiles might
also display interesting reactivity with metal alkyne complexes.
Indeed, in the case of early transition-metal acetylide and
metallocumulene complexes, the groups of Rosenthal49ꢀ54 and
Erker55ꢀ61 have demonstrated a rich and unique reactivity
(Scheme 2). With these precedents, we were prompted to
explore reactions of the coordinatively saturated metal acetylide
complex CpRu(PPh3)2(CtCPh) (1) with electrophilic boranes.
Herein, we demonstrate the synthesis of a Ru-vinyl-borane and
subsequent conversion to a borohydride-alkyne complex. In
addition, we also isolate an unusual Ru-vinylidene-borate zwitterion.
One of the important tools in the synthetic toolbox of the
organic chemist is the activation of alkynes.1 For example,
transition-metal species can be employed to metalate terminal
alkynes to give useful synthons, while transition-metal catalysts
provide avenues to effect CꢀC coupling reactions, such as
oxidative dimerizations,2 alkyneꢀarene coupling reactions,3
and alkynylations of aldehydes.4 On the other hand, main group
reagents, such as boranes, can be employed to derivatize alkynes
to yield synthons, such as vinyl boranes, alkynyl boranes, or
alkynyl borates.5ꢀ7 Alternatively, boranes can be used to effect
hydrostannation8 or hydrogermylation9 of alkynes, intramole-
cular additions of silyl enol ethers to alkynes,10 and cyclization
of dialkynylsilanes employing B(C6F5)3.11 Yamaguchi and
co-workers12 have demonstrated that intramolecular addition
of phosphine and boron to alkyne fragments provides materials
of interest for their electronic properties. In a related sense, we
have previously shown that combinations of sterically demanding
donors and electrophilic boranes, so-called “frustrated Lewis
pairs”, react with terminal alkynes via two pathways.13 Where
the donor is Brønsted basic, such as tBu3P, the reaction with
B(C6F5)3 and PhCtCH results in deprotonation of the alkyne,
yielding the salt [tBu3PH][PhCtCB(C6F5)3] (Scheme 1).
Conversely, using more nucleophilic donors, including phos-
phines, sulphides, amines, pyridines, and pyrroles, has been
shown to add to the alkyne in the presence of boranes to give
zwitterionic alkene derivatives of the form (Donor)C(Ph)d
C(H)B(C6F5)3 (Scheme 1).14,15 More recently, the research
groups of Berke16,17 and Erker18ꢀ21 have discovered that both
terminal and internal alkynes react with B(C6F5)3 exclusively via
1,1-carboboration, affording novel, electrophilic vinyl borane
products of the from RR0CdC(C6F5)B(C6F5)2 (Scheme 1).
In the case of the chemistry of alkynes and transition-metal
reagents, metal alkyne complexes are well known to undergo the
reversible interconversion of terminal alkyne and vinylidene
’ EXPERIMENTAL SECTION
General Remarks. All manipulations were carried out under an
atmosphere of dry, O2-free N2 employing an Innovative Technology
glovebox and a Schlenk vacuum line. Solvents were purified with a
Grubbs-type column system manufactured by Innovative Technology
and dispensed into thick-walled Schlenk glass flasks equipped with
Teflon-valve stopcocks (hexane, toluene, and CH2Cl2) or were dried
over the appropriate agents and distilled into the same kind of storage
flasks (C6H6 and C6H12). All solvents were thoroughly degassed after
purification (repeated freezeꢀpumpꢀthaw cycles). Deuterated solvents
were dried over the appropriate agents, vacuum-transferred into storage
flasks with Teflon stopcocks, and degassed accordingly (CD2Cl2).
1H, 13C, and 31P NMR spectra were recorded at 25 °C on Bruker
400 MHz spectrometers, unless otherwise noted. Chemical shifts are
Received: August 30, 2011
Published: September 26, 2011
r
2011 American Chemical Society
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Scheme 1. Reactions of Alkynes with B(C6F5)3
HB(C6F5)2 (110 mg, 0.319 mmol) to yield an intense orange solution
that was stirred for 1 h. The reaction was then irradiated with UV light
while being stirred for 12 h. The reaction was pumped-dried in vacuo
and washed with hexanes (2 ꢁ 5 mL), and the resulting orange solid was
dissolved in dichloromethane (2 mL). To this, cyclohexane (18 mL) was
added, and upon diffusion, X-ray quality orange crystals were obtained
1
(87%, 242 mg, 0.276 mmol). H NMR (400 MHz, CD2Cl2, 298 K):
δ 7.49ꢀ7.45 (m, 2H, Ph), 7.37ꢀ7.21 (m, 12H, Ph), 7.00ꢀ6.91 (m,
6H, Ph), 4.63 (s, 5H, Cp), ꢀ8.02 (s (br), 1H, RuHB). 1H{11B} NMR (400
MHz, CD2Cl2, 298 K): δ 7.49ꢀ7.45 (m, 2H, Ph), 7.37ꢀ7.21 (m, 12H,
Ph), 7.00ꢀ6.91(m, 6H, Ph), 4.63(s, 5H, Cp), ꢀ8.02 (d, 1H, RuHB, 2JPH
=
12.8 Hz). 31P{1H} NMR (162 MHz, CD2Cl2, 298 K): δ 40.29 (s, PPh3).
11B NMR (128 MHz, CD2Cl2, 298 K): δ ꢀ18.9 (s (br)). 19F NMR (376
MHz, CD2Cl2, 298 K): δ ꢀ128.55 to ꢀ128.78 (m, 2F, o-C6F5), ꢀ129.47
Scheme 2. Reactions of Early Metal Cumulenes with
Electrophilic Boranes
3
to ꢀ129.92 (m, 2F, o-C6F5), ꢀ159.92 (t, 1F, p-C6F5, JFF = 20.3 Hz),
3
ꢀ160.64 (t, 1F, p-C6F5, JFF = 20.3 Hz), ꢀ164.01 to ꢀ164.42 (m, 2F,
m-C6F5), ꢀ165.21 to ꢀ165.47 (m, 2F, m-C6F5). 13C{1H} NMR (101
MHz, CD2Cl2, 298 K, partial): δ 135.43 (s, Ph), 135.01 (s, Ph), 133.61
(d, ipso-Ph, 1JCP = 11.4 Hz), 132.73 (s, Ph), 129.45 (s, Ph), 128.38 (s, Ph),
133.57 (d, o-Ph, 2JCP = 9.1 Hz), 127.56 (s, Ph), 83.31 (s, alkyne), 83.03
(s, Cp). The BꢀC and CꢀF carbons were not observed. Anal. Calcd for
C43H26BF10PRu C6H12: C, 61.33; H, 3.99%. Found: C, 60.77; H, 4.02%.
3
Synthesis of CpRu(PPh3)2[CdC(Ph)((C6F4)B(F)(C6F5)2)] (4).
Compound (1) (33 mg, 0.042 mmol) in toluene (2 mL) was added to
solid B(C6F5)3 (22 mg, 0.043 mmol). The reaction was heated at 120 °C
for 12 h, resulting in a red oil to crash out. Solution was decanted off and
oil taken up in dichloromethane (1 mL) and subsequently precipitated
out with hexanes (10 mL). The precipitate was then dissolved in
benzene (12 mL) and layered with hexanes (8 mL), resulting in X-ray
quality red crystals upon diffusion (40%, 22 mg, 0.017 mmol). 1H NMR
(400 MHz, CD2Cl2, 298 K): δ 7.45ꢀ7.37 (m, 7H, Ph), 7.25ꢀ7.11
(m, 16H, Ph), 7.00ꢀ6.91 (m, 12H, Ph), 5.27 (s, 5H, Cp). 31P{1H} NMR
(162 MHz, CD2Cl2, 298 K): δ 40.29 (s, PPh3). 11B NMR (128 MHz,
CD2Cl2, 298 K): δ ꢀ0.5 (d, 1JBF = 64 Hz). 19F NMR (376 MHz, CD2Cl2,
213 K): δ ꢀ134.64 to ꢀ134.38 (m, 2F, C6F4), ꢀ135.18 to ꢀ135.43 (m,
4F, o-C6F5), ꢀ142.16 to ꢀ142.41 (m, 2F, C6F4), ꢀ162.67 (t, 2F, p-C6F5,
3JFF = 20.8 Hz), ꢀ166.87 to ꢀ167.08 (m, 4F, m-C6F5), ꢀ192.04 (s (br),
1F, BF). 13C{1H} NMR (101 MHz, CD2Cl2, 298 K, partial): δ
133.42ꢀ133.21 (m, Ph), 131.17 (s, Ph), 129.37 (s, Ph), 129.05 (s, Ph),
128.77ꢀ128.54 (m, Ph), 128.37 (s, Ph), 128.13 (s, Ph), 95.24 (s, Cp). The
RuꢀC, BꢀC, and CꢀF carbons were not observed. Anal. Calcd for
C60H40BF15P2Ru: C, 61.72; H, 3.09%. Found: C, 61.72; H, 3.10%
(repeated analyses gave low carbon values, suggesting formation of metal
or B carbides during combustion).
X-ray Data Collection and Reduction. Crystals were coated in
Paratone-N oil in the glovebox, mounted on a MiTegen Micromount,
and placed under a N2 stream, thus maintaining a dry, O2-free environ-
ment for each crystal. The data were collected on a Bruker Apex II
diffractometer using a graphite monochromator with Mo Kα radiation
(λ = 0.71073 Å). The data were collected at 150((2) K for all crystals.
The frames were integrated with the Bruker SAINT software package
using a narrow-frame algorithm. Data were corrected for absorption
effects using the empirical multiscan method (SADABS).
given relative to SiMe4 and referenced to the residue solvent signal
(1H, 13C) or relative to an external standard (31P: 85% H3PO4). Chemi-
cal shifts are reported in parts per million and coupling constants as scalar
values in Hz. Combustion analyses were performed in house employing a
PerkinElmer CHN Analyzer. B(C6F5)3 was purchased from Boulder
Scientific Company and used as received. CpRu(PPh3)2(CtCPh) (1)
and HB(C6F5)2 were prepared by literature methods.62,63
Synthesis of CpRu(PPh3)2[CHdC(Ph)(B(C6F5)2)] (2). Com-
pound (1) (33 mg, 0.042 mmol) in CH2Cl2 (2 mL) was added to solid
HB(C6F5)2 (16 mg, 0.045 mmol) to yield an intense orange solution
that was stirred for 4 h. The reaction was then pumped-dried in vacuo
and the resulting residue dissolved in toluene (2 mL). To this was added
hexanes (4 mL), and upon cooling to ꢀ35 °C, X-ray quality orange crystals
were obtained (95%, 46 mg, 0.040 mmol). 1H NMR (400 MHz, CD2Cl2,
298 K): δ 12.55 (t, 1H, vinyl H, 3JPH = 11.7 Hz), 7.39ꢀ7.27 (m, 10H, Ph),
7.26ꢀ7.12 (m, 15H, Ph), 7.04ꢀ6.96 (m, 10H, Ph), 4.05 (s, 5H, Cp).
31P{1H} NMR (162 MHz, CD2Cl2, 298 K): δ 51.62 (s, PPh3). 11B NMR
(128MHz, CD2Cl2, 298 K): δ46.1 (s (br)). 19F NMR (376 MHz, CD2Cl2,
298 K): δ ꢀ129.88 (s (br), 2F, o-C6F5), ꢀ131.89 (s (br), 2F, o-C6F5),
ꢀ157.65 (s (br), 1F, p-C6F5), ꢀ158.17 (s (br), 1F, p-C6F5), ꢀ162.84
(s (br), 2F, m-C6F5), ꢀ164.54 (s (br), 2F, m-C6F5). 19F NMR (376 MHz,
CD2Cl2, 213 K): δ ꢀ129.96 to ꢀ130.21 (m, 2F, o-C6F5), ꢀ131.68 to
3
Structure Solution and Refinement. The structures were
solved by direct methods using XS and refined by full-matrix least-
squares on F2 using XL as implemented in the SHELXTL suite of
programs. All non-hydrogen atoms were refined anisotropically. Carbon-
bound hydrogen atoms were placed in calculated positions using an
appropriate riding model and coupled isotropic temperature factors.
(see Table 1 and the Supporting Information).
ꢀ131.94 (m, 2F, o-C6F5), ꢀ156.83 (t, 1F, p-C6F5, JFF = 21.2 Hz),
3
ꢀ157.27 (t, 1F, p-C6F5, JFF = 21.2 Hz), ꢀ161.97 to ꢀ162.21 (m, 2F,
m-C6F5), ꢀ163.17 to ꢀ163.41 (m, 2F, m-C6F5).13C{1H} NMR (101
MHz, CD2Cl2, 298 K, partial): δ 247.18 (t, vinyl C, 2JCP = 13.7 Hz), 151.74
1
(s, Ph), 138.68ꢀ137.59 (m, Ph), 133.57 (t, ipso-C, Ph, JCP = 5.0 Hz),
130.35 (s, Ph), 129.19 (s, Ph), 129.06 (s, Ph), 128.26 (s, Ph), 127.69
(t, ipso-C, Ph, 1JCP = 5.0 Hz), 127.40 (s, Ph), 125.31 (s, Ph), 125.31 (s, Ph),
86.72 (s, Cp). The BꢀC and CꢀF carbons were not observed. Anal. Calcd
for C61H41BF10P2Ru: C, 64.39; H, 3.63%. Found: C, 64.72; H, 4.10%.
Synthesis of CpRu(PPh3)[PhCC(B(H)(C6F5)2)] (3). Compound
(1) (240 mg, 0.303 mmol) in CH2Cl2 (5 mL) was added to solid
’ RESULTS AND DISCUSSSION
The species CpRu(PPh3)2(CtCPh)62 (1) was reacted with
HB(C6F5)2 to give an intense orange solution after 4 h.
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Table 1. Crystallographic Data
2 C7H8 0.5C6H14
3 C6H12
4 C6H6
3
3
3
3
formula
wt
C71H56BF10P2Ru C73H46BF15P2Ruu C49H38BF10PRu
1272.98
monoclinic
P21/c
1381.92
triclinic
P1
959.64
triclinic
P1
cryst syst
space grp
a (Å)
22.0269(9)
12.9666(5)
21.5355(9)
90.00
11.7355(17)
16.195(2)
17.148(3)
94.811(9)
99.771(9)
106.499(9)
3049.4(8)
2
9.7474(10)
18.4021(16)
24.467(3)
87.381(7)
81.646(7)
74.766(6)
4189.5(8)
4
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
107.048(2)
90.00
5880.6(4)
4
Figure 1. POV-ray depiction of 2: C, black; H, gray; P, orange; Ru,
deep-green; B, yellow-green; F, pink.
d (cal), g cmꢀ1 1.386
1.505
1.521
R(int)
0.0384
0.394
0.1599
0.401
0.1549
0.493
μ, cmꢀ1
total data
129 313
17 960
766
46 224
67 942
2
>2σ(Fo
)
13 226
19 144
variables
R (>2σ)
Rw
829
1118
0.0406
0.1213
0.973
0.0934
0.2611
0.978
0.0827
0.1686
0.966
GOF
Scheme 3. Synthesis of 2ꢀ4
Figure 2. POV-ray depiction of 3: C, black; H, gray; P, orange; Ru,
deep-green; B, yellow-green; F, pink.
planar B results in restricted rotation about the BꢀC(vinyl) bond
accounting for the inequivalence of the C6F5 rings. The RuꢀP,
RuꢀC, and CdC bond lengths in 2 were found to be 2.3059(5),
2.3116(5), 2.0272(19), and 1.403(3) Å, respectively. The re-
maining metric parameters were unexceptional.
Subsequent isolation afforded an orange crystalline product 2 in
95% yield (Scheme 3). The 1H NMR spectrum of 2 revealed a
triplet at 12.55 ppm that showed a PꢀH coupling of 11.7 Hz,
suggesting a vinylic proton coupling to two equivalent P atoms.
In addition to the aromatic signals, a singlet resonance was
observed at 4.05 ppm corresponding to the Cp group. The
31P{1H} NMR spectrum of 2 showed a single peak at 51.62 ppm,
whereas the 11B NMR spectrum gave rise to a resonance at
46.1 ppm. The corresponding 19F NMR spectrum showed two
sets of broad resonances attributable to inequivalent C6F5 rings.
These signals sharpened at ꢀ60 °C. These data appeared to
suggest the formulation of 2 as a Ru-vinyl species; the precise
geometry of product 2 was confirmed crystallographically
(Figure 1). Species 2 was confirmed as CpRu(PPh3)2CHd
C(Ph)(B(C6F5)2), in which the borane has undergone hydro-
boration of the alkynyl fragment on Ru to give the cis-addition
product. This reaction is regioselective with the B adding
exclusively to the beta-carbon, presumably a result of steric
demands and the electron-rich character of the beta-carbon of
metal acetylides. The steric congestion about the pseudotrigonal
Subsequent irradiation of 2 with ultraviolet light for 12 h
resulted in the isolation of a new orange product 3, which was
isolated as orange crystals in 87% yield (Scheme 3). Most
notably, the resonance attributable to the vinyl proton in 2 was
not observed, while the signal attributable to the Cp protons was
shifted to 4.63 ppm. In addition, and interestingly, a new broad
signal was observed at ꢀ8.02 ppm. This latter signal became a
simple doublet upon 11B decoupling and exhibited a PꢀH
coupling of 12.8 Hz. The 31P{1H} NMR spectrum of 3 showed
a singlet at 40.29 ppm, whereas the 11B NMR spectrum gave a
broad signal at ꢀ18.9 ppm. The 19F NMR spectrum of 3 showed
inequivalent C6F5 rings. As these data were not conclusive, the
nature of 3 was determined unambiguously to be CpRu(PPh3)-
[PhCtC(B(H)(C6F5)2)] (Figure 2). The photoinduced rear-
rangement results in a CpRu fragment that is coordinated to a
single PPh3 as well as the alkynyl moiety and BꢀH fragment
of the alkynyl-borate ligand. The RuꢀP distances in the two
molecules in the asymmetric unit were found to average 2.307(2) Å,
whereas the RuꢀC distances to the alkynyl unit were 2.207(7)
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Scheme 4. Proposed Pathways from 2 to 3
and 2.256(6) Å, for the α and β carbons, respectively. The
corresponding CꢀC bond length was 1.236(10) Å. The RuꢀH
approaches on average 1.51 Å, with a BꢀH bond length of 1.22 Å
and a RuꢀB bond length of 2.470(11) Å. This is significantly
shorter than typical RuꢀB bond distances of η1 BꢀH complexes
that range from 2.586 to 2.797 Å.63ꢀ65 It is also noteworthy that
the Ph and borate groups on the alkynyl-borate ligand adopt a
pseudo-transoid disposition as the angle between the CꢀCꢀC
and CꢀCꢀB planes is 16.6° with the BH unit disposed toward
the Ru center. This is in contrast to traditional binding modes of
alkynes to metal centers and may result from the BH binding to
Ru. Similar transoid binding geometries have been previously
reported for alkynyl-borane complexes of Ni.64
The mechanism of this rearrangement from 2 to 3 is a subject
of some speculation; related literature precedent suggests two
possibilities. One possibility involves photoinduced rearrange-
ment of the Ru-vinyl species 2 to a vinylidene hydride species
with loss of phosphine generating species A. Subsequent re-
arrangement of this vinylidene species to the corresponding
alkyne complex with formation of the bridging BꢀHꢀRu
fragment yields the observed product 3. A second possibility
would involve hydride abstraction by the Lewis acidic B center,
generating a transient zwitterionic Ru-cation/hydroborate spe-
cies B, which could also undergo vinylidene to alkyne rearrange-
ment, affording the product 3 (Scheme 4). These two pathways
differ only in the initial step. It is noteworthy that, in the first
pathway, the intermediate A is analogous to the Os hydride-
vinylidene species Cp(iPr3P)Os(H)(CdCHPh) isolated and
characterized by Baya and Esteruelas.66 Related ruthenium
vinylidene hydride species have also been investigated by Caulton
and co-workers67,68 and more recently by Goosen’s group.69 On
the other hand, the intermediate B in the second pathway is
analogous to the cationic Ru-vinylidenes [CpRu{CdC(Ph)Ar}-
(dppe)][BArf4] described by Mutoh et al. In either pathway, the
latter steps are reasonable to propose given the established and
recent work of Mutoh et al.,48 where the reversible interconver-
sion of vinylidene and alkyne complexes was demonstrated.
Presumably, in the present case, the interaction with the hydride
stabilizes product 3 and precludes the reverse reaction.
Figure 3. POV-ray depiction of 4: C, black; H, gray; P, orange; Ru,
deep-green; B, yellow-green; F, pink. Two of the Ph's from PPh3 groups
are omitted for clarity.
BꢀF coupling of 64 Hz consistent with the presence of a direct
BꢀF bond. The corresponding 19F NMR resonance was ob-
served at 192.04 ppm, whereas the resonances arising from
the fluoroaryl groups showed multiplets at ꢀ134.5 and
ꢀ142.3 ppm attributable to a C6F4 fragment, in addition to the
signals arising from the C6F5 rings. Efforts to observe the
vinylidene carbons by 13C NMR or HMBC spectroscopy were
unsuccessful despite prolonged data acquisition. Nonetheless,
these spectral data suggest the substitution of one of the para-
fluorine atoms of B(C6F5) with concurrent transfer of the
fluorine to B. The precise nature of 4 was unambiguously
determined by X-ray crystallography (Figure 3). This confirmed
the formulation of 4 as the zwitterionic species CpRu(PPh3)2-
(CdC(Ph)((C6F4)BF(C6F5)2) in which the formerly β-alkyne
carbon of 1 is now linked to the para position of one of the
fluoroarene rings of B(C6F5)3 with concurrent fluoride transfer
to B. This generates a zwitterionic species with an anionic borate
center and a cationic Ru-vinylidene unit. The RudCdC frag-
ment gives rise to RuꢀC and CdC bond lengths of 1.835(6) and
1.346(9) Å, with a RuꢀCꢀC angle of 167.8(6)°. This differs
slightly from those seen in [Cp(Ph3P)2RudCdC(Me)Ph][I],
where the RuꢀC distance is 1.863(10) Å and the CdC dis-
tance is 1.293(15) Å.70 The RuꢀC distance in the cation
[CpRu(PPh3)2(CdC(Me)Ph]+ is similar to that in 4, while
the shorter CdC bond likely results from the electron-with-
drawing fluorinated aryl ring. It is also noteworthy that the
RuꢀC distance is longer than that reported in Cl2(iPr3P)2-
RudCdC(Me)SiMe3 (1.787(5) Å).71 This may reflect dimin-
ished π-back-bonding to C as a result of the formal cationic
charge at Ru in 4.
The formation of 4 is an interesting observation and provides a
structural analog of the intermediate B in the latter proposed
mechanism for the formation of 3. Although this cannot be
regarded as conclusive, the characterization of 4 lends support to
this postulate. It is also important to note that the formation of 4
indicated the nucleophilic nature of the beta-carbon of the
acetylide species (1), consistent with alkylations and protona-
tions of metal acetylides to give vinylidene species. In this regard,
one can also image species B as the intermediate en route to the
hydroboration of the alkyne, giving 2. It is presumably the
photolabilization of phosphine as well as the stabilization of
To probe a related reaction further, we reacted (1) with
B(C6F5)3 at 120 °C for 12 h. This resulted in the separation of
a red oil. Subsequent isolation and crystallization afforded a red
1
crystalline product 4 in 40% yield (Scheme 3). The H NMR
signal attributable to the Cp protons was observed at 5.27 ppm,
whereas the 31P{1H} and 11B NMR spectra gave resonances at
40.29 and ꢀ0.5 ppm, respectively. The latter signal exhibited a
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’ ASSOCIATED CONTENT
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S
Supporting Information. Crystallographic details are avail-
b
able. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: dstephan@chem.utoronto.ca.
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’ ACKNOWLEDGMENT
The financial support of LANXESS Inc., NSERC of Canada,
and the Ontario Centres of Excellence is gratefully acknowl-
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