Nickel(0) complexes with fluorinated alkyne ligands and their reactivity towards semihydrogenation and hydrodefluorination with water

Article abstract of DOI:10.1002/asia.201000601

The current work describes the synthesis and full characterization of zerovalent nickel complexes of the type [(dippe)Ni(I·²- C,C-F_n-alkyne)] (dippe=1,2-bis(di-isopropylphosphino-ethane), F _n-alkyne=fluorinated aromatic al

Full text of DOI:10.1002/asia.201000601

FULL PAPERS
DOI: 10.1002/asia.201000601
Nickel(0) Complexes with Fluorinated Alkyne Ligands and their Reactivity
towards Semihydrogenation and Hydrodefluorination with Water
Rigoberto Barrios-Francisco, Tania Benꢀtez-Pꢁez, Marcos Flores-Alamo,
Alma Arꢂvalo, and Juventino J. Garcꢀa*[a]
Abstract: The current work describes
the synthesis and full characterization
of zerovalent nickel complexes of the
bound alkyne to the corresponding
alkene, accompanied by partial hydro-
defluorination of the aromatic ring.
Different alkynes were tested; on using
the alkyne with five fluorine atoms
over the aromatic ring, partial defluori-
nation was achieved under the mildest
reaction conditions, followed in reactiv-
ity by the alkyne with three fluorine
atoms. The alkyne with only one fluo-
rine atom was barely defluorinated.
The use of triethylsilane as a sacrificial
hydride source resulted in an overall
increase in reactivity towards defluori-
nation.
type
[(dippe)Ni(h2-C,C-Fn-alkyne)]
(dippe=1,2-bis(di-isopropylphosphino-
ethane), Fn-alkyne=fluorinated aro-
matic alkyne, n=1, 3, 5; 3a–c) and
[{(dippe)Ni}2(m2-C,C-Fn-alkyne)]
(4).
Keywords: fluorinated ligands
·
Reactions with complexes 3a–c, and
water as the hydrogen source, yield se-
lective semihydrogenation of the
hydrodefluorination · nickel · semi-
hydrogenation · water chemistry
Introduction
generation of large quantities of highly inert, thermally
stable waste products that pose grave environmental haz-
ards. The presence of fluorinated aerosols in the atmos-
phere, for example, is a well-established cause for the deple-
tion of the ozone layer.[3] Consequently, the selective and ef-
À
The stoichiometric and catalytic activations of C X bonds
(X=halogen) by using transition-metal complexes to form
new C C or C H bonds are topical fields in current organic
and organometallic chemistry.[1a–e] The most reactive sub-
strates for such transformations are typically iodinated or
brominated derivatives, followed by chlorinated derivatives
À
À
À
ficient activation of C F bonds is indeed important and has
been the subject of several keynote reviews.[4]
Hydrodefluorination (HDF) is the process by which a flu-
orine atom is replaced by hydrogen in an organic molecule,
thereby releasing a fluorine free or partially defluorinated
derivative. In the case of aliphatic compounds, the use of
strong Lewis acids to abstract the fluorine atom has been es-
tablished. Typically, catalysts based on main-group elements
are employed.[5] On using transition-metal complexes, the
HDF of fluoro-aromatic compounds can be achieved upon
careful tuning of the reaction conditions.[6] In addition, the
reaction can be made thermodynamically favorable by com-
bination of an additional Lewis acid and a transition-metal
complex. Several groups have reported the use of silanes,
alanes, and boranes as convenient thermodynamic sinks for
this reaction, by irreversibly “trapping” the abstracted fluo-
rine atoms. The advantage in their use is two-fold, thus pre-
venting the formation of extremely toxic hydrogen fluoride
and allowing for the regeneration of the active transition-
metal catalyst.[7]
and then, scarcely, fluorinated substrates, which are general-
[1a,b]
À
ly inactive owing to the high energy of the C F bond.
Fluorinated organic compounds, on the other hand, exhibit
a vast number of applications in medicine, material sciences,
agrochemicals, and in the pharmaceutical industries, as inter-
mediates in fine-chemicals; the large-scale preparations of
which represent major challenges, both synthetically and
from the health and safety aspects.[2] To the latter adds the
[a] Dr. R. Barrios-Francisco, T. Benꢀtez-Pꢁez, Dr. M. Flores-Alamo,
Dr. A. Arꢂvalo, Prof. Dr. J. J. Garcꢀa
Facultad de Quꢀmica
Universidad Nacional Autꢃnoma de Mꢂxico
Mꢂxico D. F. 04510 (Mꢂxico)
Fax : (+52)55-56162010
E-mail: juvent@servidor.unam.mx
Supporting information for this article includes the complete experi-
mental and crystallographic details, ORTEP representations of com-
plexes 4b and 4c, and is available on the WWW under http://
dx.doi.org/10.1002/asia.201000601.
Water represents an ideal reagent or solvent for sustaina-
ble transformations and, relevant examples of its use for the
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Chem. Asian J. 2011, 6, 842 – 849

reduction or oxygenation of organic compounds, as either
hydrogen and/or oxygen donors, have been reported.[8]
Some reports show that water can carry out defluorination
of the CF3- moiety to yield CO and HF as the final fates of
carbon and fluorine, respectively; the defluorination pro-
cesses have been mediated by either Rh or Ir complexes.[9]
Recently, our research group reported the stereoselective
semihydrogenation of aromatic alkynes by using water as
the hydrogen source. The reactions took place stoichiometri-
cally in the presence of nickel(0) complexes of the type,
Scheme 1. Monometallic complexes 3a–c.
_
_
[(PP)Ni(h2-C, C-alkyne)] (PP=dippe, 1, 2-bis-di-isopropyl-
phosphinoethane, or dtbpe, 1, 2-bis-di-terbutylphosphino-
ethane), and this resulted in the formation of the corre-
sponding trans-alkenes, along with 1, 2-bis-di-isopropylphos-
phino-ethane monoxide (dippeO) and Ni(OH)2. In the pres-
ence of Et3SiH instead of water, a catalytic reduction pro-
cess was applicable with numerous aromatic alkynes. The
reductions were selective towards the formation of cis-al-
kenes only. The results showed that the use of the silane, as
the hydrogen source for these reductions prevented catalyst
of reactions.[12] Illustrated in Figure 1 is a blow-up of the
31P{1H} NMR spectrum of the reaction mixture before heat-
ing, and this shows the coexistence of peaks for species 3a
and 4a. Relevant spectroscopic details for complexes 3a–c
are summarized in Table 1.
decomposition, presumably due to oxidation of the
_
“ACHTUNGTRENNUNG
[(PP)Ni0]” fragment.[10] Herein, we disclose the use of a
similar reactivity of nickel(0) catalysts that yield the selec-
tive semihydrogenation of fluorinated aromatic alkynes to
produce the corresponding alkenes, a process that is, in
tandem, accompanied by the selective hydrodefluorination
of the aromatic rings of the substrates.
Results and Discussion
Preparation and Characterization of Monomeric Nickel(0)
Fluoroalkyne Complexes (3a–3c)
Figure 1. Blow-up of a 31P{1H} NMR spectrum in [D8]THF, at room tem-
By reacting the nickel(I) hydride dimer, [(dippe)Ni
G
perature, showing the coexistence of signals for 3a and 4a.
(1), with two equivalents of the corresponding alkyne (2), a
series of complexes of the type
[(dippe)Ni(h2-C, C-Fn-alkyne)]
(3; Scheme 1) were prepared by
Table 1. Relevant spectroscopic details.
Nuclei[a]
Selected peaks of alkyne-coordinated h2-complexes
following the methodology re-
3a
3b
3c
ported by Jones et al.[11]
31P{1H}
82.3 (m, JPÀP; 40.0 Hz, P1)
81.9 (m, JPÀP; 39.7 Hz, P2)
81.6 (m, JPÀP; 37.9 Hz, P1)
81.1 (m, JPÀP; 40.1 Hz, P2)
82.6 (m, JPÀP; 34.5 Hz, P1)
81.5 (m, JPÀP; 34.3 Hz, P2)
Notably, during the synthesis
of complex 3a, the formation of
an additional species in solu-
tion, subsequently assigned as
the alkyne-bridged complex
19F
À106.9 (m, Fo)
À130.7 (m, Fo)
À139.4 (m, Fp)
À131.0 (m, Fm)
À141.1 (m, Fo)
À165.8 (m, Fp)
À167.1 (m, Fm
h2-C,C-
13C{1H}
142.1 (d, JCÀP; 43.3 Hz, Csp)
132.0 (d, JCÀP; 43.4 Hz, Csp)
140.9 (m, Csp)
124.0 (m, Csp)
129.4 (m, Csp)
121.5 (m, Csp)
[{(dippe)Ni}2{m-C,C-,
(F1-alkyne)}] was detected (4a;
Scheme 2). The latter com-
pound progressed towards the
formation of 3a when further
reacted with the residual free
[a] All NMR experiments were recorded at ambient temperature, in [D8]THF. All chemical shifts (d values)
are reported in parts-per-million (ppm).
alkyne 2a, at 508C over 30 min. Evidence of such conver-
sion was established spectroscopically by using 31P{1H} NMR
spectroscopy with [D8]THF. As additional support, Pçrschke
and co-workers have reported similar observations concern-
ing the syntheses of closely related complexes, wherein the
binuclear compounds are the kinetic products of these kind
As shown in Table 1, the 13C{1H} NMR peaks assigned to
the coordinated alkyne carbons of complexes 3a–c are
strongly shifted downfield (d=140–120 ppm) when com-
pared to the respective peaks of the corresponding free al-
kynes (d=90–73 ppm). The peaks for the coordinated sub-
strates appear in the same range as those reported for close-
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843

J. J. Garcia et al.
FULL PAPERS
ly related bis(phosphine)-nickel(0) alkyne complexes.[11,12]
Compound 3a was also characterized in the solid state by
using single-crystal X-ray crystallography (ORTEP structure
shown in Figure 2). The complex exhibits a distorted trigo-
nal-planar geometry with a coordinated carbon–carbon mul-
tiple bond length (1.283(2) ꢅ), which is similar to the one
reported for a nonfluorinated alkyne complex analog.[11]
Scheme 2. Bimetallic complexes 4a–c.
shifts observed by 13C{1H} NMR spectroscopy for the coordi-
nated alkyne carbons on these compounds (d=100–89 ppm)
versus those of the mononuclear analogues, 3a–c (d=140–
120 ppm), in which the acetylenic peaks for the latter
appear considerably deshielded. A similar trend is reflected
in the 31P{1H} NMR spectra of the two series presented
above, wherein the lesser extent of s donation of the [(dip-
pe)Ni0] moieties to the coordinated alkyne in the bridging
compounds results in virtual shielding of the phosphorus
atoms (d=60–62 ppm for the bridging compounds versus
d=80–82 ppm for the terminal compounds). Complexes 4a–
c were all fully characterized by using single-crystal X-ray
crystallography. Illustrated in Figure 3 is a selected ORTEP
view of complex 4a, which shows the doubly-coordinated
alkyne bridge in the meridional plane with pseudo-C2 sym-
metry. Selected views for compounds 4b–c are included in
the Supporting Information.
Figure 2. ORTEP drawing for complex 3a, thermal ellipsoids at the 50%
À
À
probability level. Selected distances (ꢅ): Ni(1) C(13) 1.878(2); Ni(1)
À
À
À
C(14) 1.880(2); C(13) C(14) 1.283(3); Ni(1) P(1) 2.1500(6); Ni(1) P(2)
2.1538(6). Selected angles (8): C(13)-Ni(1)-P(1) 112.16(7); C(14)-Ni(1)-
P(1) 151.67(7); C(13)-Ni(1)-P(2) 156.38(7); C(14)-Ni(1)-P(2) 116.46(7);
P(1)-Ni(1)-P(2) 91.42(2); C(13)-Ni(1)-C(14) 39.93(10).
The carbon–carbon bond length of the alkyne bridge is
enlarged by 0.092 ꢅ in compound 4a (1.375 ꢅ) with respect
to 3a (1.283 ꢅ), as a result of coordination to the second
[(dippe)Ni0] unit, indicative of bond orders closer to single
Preparation and characterization of bridging nickel(0)
fluoroalkyne complexes (4a–4c)
À
A series of dinuclear alkyne bridged complexes (4a–4c;
Scheme 2) were prepared by reacting the dimeric complex 1
with the corresponding fluorinated alkyne 2a–c in a 1:1
dimer-to-alkyne mole ratio. Key spectroscopic details of
these compounds are listed in Table 2.
A general feature of all the dinuclear alkyne-bridging
compounds is that the NMR peaks appear slightly broad, as
a result of coordination of the triple bond to two [(dippe)-
Ni0] moieties. Worth noting, are the differences in chemical
around the coordinated C C bond in the bridging structure;
similar features are also observed with the other two ana-
logues 4b and 4c. The d10–d10 interaction between the two
nickel(0) centers, observed in complexes 4a–c, has been pre-
viously described in closely related complexes.[13]
Thermolysis of complexes 4a–c in [D6]benzene at 1008C
yield the formation of the monocoordinated alkyne com-
plexes 3a–c, that is accompanied by gradual precipitation of
metallic nickel particles and free dippe, which result from
the thermal decomposition of
free [(dippe)2Ni0] at such tem-
peratures. Noteworthy, products
Table 2. Key spectroscopic details.
Nuclei[a]
Selected peaks of alkyne-coordinated m2-, h2-complexes
À
derived from any C F bond ac-
tivation were not observed
4a
4b
4c
31P{1H}
61.8 (m, JPÀP; 55.4 Hz, P1)
61.0 (m, JPÀP; 54.9 Hz, P2)
62.3 (m, JPÀP; 53.1 Hz, P1)
61.7 (m, JPÀP; 53.8 Hz, P2)
62.2 (m, JPÀP; 57.3 Hz, P1)
61.5 (m, JPÀP; 58.3 Hz, P2)
under these conditions.
Reactivity of Complexes 3a–c
19F
À103.8 (br m, Fo)
À130.4 (br m, Fo)
À148.5 (br m., Fp)
À165.8 (br. m., Fm)
À132.6 (br m, Fo)
with Water
À166.1 (br m, Fm)
À170.3 (br m, Fp)
Complexes 3a–c all reacted
13C{1H}
100.8 (s, Csp)
89.1 (s, Csp)
100.4 (s, Csp)
85.9 (s, Csp)
100.32 (s, Csp)
95.5 (d, JCÀP; 23.0, Csp)
with water at 1358C and were
monitored by multinuclear
NMR spectroscopy until the
starting complexes had com-
[a] All NMR experiments were recorded at room temperature, in [D8]THF. All chemical shifts (d values) are
reported in parts-per-million (ppm).
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Chem. Asian J. 2011, 6, 842 – 849

Tandem Reactions
Table 3. Reactivity with water/toluene.
Entry[a] Complex Reaction
time [days][b]
Conversion [%][c] Product distribu-
tions (5:6:7) [%][d]
1
2
3
3a
3b
3c
7
5
1
98
100
96
5a:6a:7a; 32:63:3
5b:6b:7b; 12:74:14
5c:6c:7c; 0:46:50
[a] All reactions were performed at 1358C in [D8]toluene. Reactions
were followed by NMR using 0.38 mmJ. Young tubes. An excess of
300 equiv of water was used in every experiment. [b] Established on the
basis of complete conversion of 3, as gauged by 31P{1H} NMR spectrosco-
py. [c] Determined by direct injection of the reaction mixtures into a GC-
MS. [d] Established on the basis of 19F NMR and GC-MS.
Figure 3. ORTEP drawing of 4a, thermal ellipsoids at the 50% probabili-
À
À
ty level. Selected distances (ꢅ): C(13) C(14) 1.368(7); C(13) Ni(2)
À
À
À
1.897(5); C(13) Ni(1) 1.982(5); C(14) Ni(1) 1.892(5); C(14) Ni(2)
À
À
À
1.987(5); P(1) Ni(2) 2.1913(16); P(2) Ni(1) 2.1834(15); P(3) Ni(1)
À
À
2.1683(14); P(4) Ni(2) 2.1774(16); Ni(1) Ni(2) 2.8566(9). Selected
angles (8): C(14)-Ni(1)-P(3) 154.42(16); C(13)-Ni(1)-P(3) 113.60(14);
C(14)-Ni(1)-P(2) 114.88(16); C(13)-Ni(1)-P(2) 148.09(16); P(3)-Ni(1)-
P(2) 90.00(6); C(14)-Ni(1)-C(13) 41.3(2); C(13)-Ni(2)-P(4) 153.72(15);
C(14)-Ni(2)-P(4) 112.61(15); C(13)-Ni(2)-P(1) 113.30(16); C(14)-Ni(2)-
P(1) 148.35(16); P(4)-Ni(2)-P(1) 90.29(7); C(13)-Ni(2)-C(14) 41.2(2);
Ni(2)-C(13)-Ni(1) 94.8(2); Ni(1)-C(14)-Ni(2) 94.8(2).
pletely disappeared. All of the reactions evolve to form the
corresponding free trans-alkene (6) and also the alkene-co-
ordinated complex (5), both of which have been previously
observed in closely related systems,[10] along with the forma-
Figure 4. Stacked 31P{1H} NMR spectra illustrating the follow up of the
reaction between 3a with water at 1358C, spectrum determined in
[D8]toluene at room temperature.
tion of diphosphine monoxide (iPr)2P-CH2-CH2-P(O)ACTHNUTRGNENUG(iPr)2
(dippeO) and Ni(OH)2. Furthermore, in each reaction the
formation of an additional product was observed, identified
by using GC-MS as the monodefluorinated trans-alkene (7)
(see Table 3 and the Experimental Section).
coordinated nickel(0) complex 5c, was detected in the crude
mixture after heating for 1 day by using NMR spectroscopy
(see Figure S4 in the Supporting Information); this is proba-
bly due to the lower stability of this species in solution,
which for this large number of fluorine atoms, seems to in-
crease the rate by which the mono-defluorinated by-product
is instead produced. Therefore, these results indicate that
As shown in Table 3, the reactivity raises from 3a to 3c
with increasing numbers of fluorine atoms on the aromatic
ring. Reactivity of 3a (7days, 98%) is very similar to that
observed in the case of diphenylacetylene (7days, 100%),[10]
to produce 5a (32%) and 6a (63%), along with the forma-
tion of a small amount of the monodefluorinated alkene
(3%) (Table 3, entry 1). Displayed in Figure 4 is the
31P{1H} NMR spectroscopy follow up for that reaction, the
corresponding follow ups for 3b and 3c are included in the
Supporting Information.
In the cases of compounds 3b and 3c, complete conver-
sion of the starting nickel(0) alkyne compounds was ach-
ieved in shorter reaction times (5 days and 1 day, respective-
ly). The formation of both the reduced free alkene and its
defluorination by-product were largely favored over the
pentafluorinated analogue 3c (6c:7c estimated yield 46 and
50% by GC-MS, respectively). No evidence of the alkene-
À
the defluorination step originates from C F metal activa-
tion, in which a decrease in activation energy for defluorina-
tion, as a result of polyfluorination of the aryl ring, and this
leads to a faster cleavage rate by nickel. Semihydrogenation
of the coordinated alkyne to produce the corresponding
alkene species was observed in all three cases, which indi-
À
cates a stepwise process of which the C F activation is prob-
ably the last step and could involve a reaction between free
[(dippe)Ni0] fragments in solution and free alkene mole-
cules. Recoordination of the alkene molecules to nickel(0)
would thus be consistent with a reversible process, the rate
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J. J. Garcia et al.
FULL PAPERS
À
of which competes with that of the C F cleavage. If such
conjecture were true, it is clear that polysubstitution of the
phenyl ring by five fluorine atoms overcomes the rate at
which recoordination takes place, therefore precluding any
further formation of the C=C-coordinated alkene compound
(5c).
described reactions performed in [D8]THF, it was difficult to
unambiguously establish the configuration, owing to the
presence of several inseparable isomeric products. At lower
temperatures in THF the hydrodefluorination is slightly in-
hibited (Table 4, entries 1 and 2).
With the aim of confirming the role of water as the hydro-
gen source for alkyne reduction, deuterium labeling experi-
ments were performed. A comparison between the use of
water versus deuterium oxide (D2O) for such process at
1008C is included in Table 5.
The addition of deuterium to the triple bond of the sub-
strates, along with the substitution of fluorine atoms for deu-
terium over the aromatic ring (entry 1), to virtually the
same extent as water (entry 2) confirms the role of water as
the source of hydrogen in the reduction reaction (Table 5,
entry 2). Consistently, the molecular weights of all deuteri-
um/protio-incorporated products are in agreement with the
expected weights. The differences noted in product distribu-
tions are adduced to a kinetic isotope effect (KIE) operating
in this transformation.
In all the above reactions, the final fate of oxygen was
dippeO, which formed at the expense of the ancillary ligand.
In the case of the defluorination reaction, the formation of
HF was expected and, consequently, its reaction with the
glassware was anticipated, provided that the formation of
NiF2 was not observed. Both types of reactions resulted in
the quenching of any further reactivity beyond the stoichio-
metric cases. The use of triethylsilane (Et3SiH) as a sacrifi-
cial agent was explored, aiming to avoid oxidation of the an-
cillary ligand, additionally it could also act as a fluorine
We extended this study to a series of reactions involving
complex 3c and water. The reactions were performed in
[D8]THF over different periods of time, aiming to establish
the differences between semihydrogenation versus hydrode-
fluorination in a more polar media. Our findings are sum-
marized in Table 4.
Table 4. Reactivity with water/THF.
Entry[a] T [8C] t [h][b] Conversion [%][c]
Product distribution
(isomeric products)[d]
8:9:10
1
2
3
70
80
100
18
14
10
96
98
98
36.5 (2):50.6 (4):8.67 (4)
19.2 (2):58.7 (4):20.2 (4)
15.2 (2):67.8 (5):15.1 (3)
[a] All reactions were performed in 0.38 mm J. Young tubes. An excess
of 300 equiv of water was used in every experiment. [b] Heating was
stopped after complete consumption of 3c as gauged by 31P{1H} NMR
spectroscopy. [c] Determined by direct injection of the reaction mixtures
into a GC-MS. [d] Determined also by their relative chromatographic
areas following injection into a GC-MS. The numbers in parentheses
refer to the number of isomeric products as gauged by the mass spectra
associated to the chromatographic peaks in the GC plot.
À
trap, as a result of the strength of the Si F bonds. Regard-
less, the Et3SiH/water system was found to be more reactive.
Regeneration of the active species, [(dippe)Ni0], was
deemed feasible and capable of three defluorination turn-
overs. Analysis of the produced silicon-derived species by
using GC-MS revealed the presence of Et3SiOH and
(Et3Si)2O, as oxygen sinks, along with Et3SiF, as the one for
fluorine (Supporting Information and Table 6).
On using [D8]THF a faster reaction was found compared
with the same reaction in [D8]toluene; even as this behavior
is consistent with a better miscibility of water in THF, the
oxidative addition step potentially involved in both semihy-
drogenation and hydrodefluorination processes would be
also increased in polar solvents. Indeed, a different product
distribution was found, for instance at 1008C for 10 h yield-
ed 15.2% of two nondefluorinated products (Table 4,
entry 3), thereby suggesting the presence of both cis- and
trans-alkenes; 67.8% of five
Although a complete study on the mechanism for de-
fluorination is currently underway, we speculate on the par-
ticipation of nickel(0) h2-fluoroarene fluxional species as
active intermediates in the process. The latter conjecture is
À
invoked on the basis of a previous study on the C CN bond
monodefluorinated
products
the cis- and trans-alkenes de-
fluorinated in o-, m-, and p-po-
sitions; additionally, 15.1% of
three di-defluorinated products
in the same reaction. In the
case of the reactions performed
in [D8]toluene, the trans- con-
figuration assigned to the re-
sulting olefin was made after
analysis of the 1H NMR spec-
trum for the isolated free
Table 5. Labeling studies.
Entry[a]
Hydrogen source
t [h][b]
Conversion
[%][c]
Olefin
[%][d]
Monodefluorinated
[%][d]
Didefluorinated
[%][d]
1
2
H2O
D2O
10
14
98
15.2 (2)
67.8 (5)
15.1 (4)
G
N
ACHTUNGTRENNUNG
97
(m/z=272)
G
ACHTUNGTRENNUNG
[a] All reactions were performed in 0.38 mm J. Young tubes at 1008C. An excess of 300 equiv of water was
used in every experiment. [b] Heating was stopped after complete consumption of 3c as monitored by
31P{1H} NMR spectroscopy. [c] Determined by direct injection of the reaction mixtures into a GC-MS. [d] De-
termined also by their relative chromatographic areas following injection into a GC-MS. The numbers in pa-
rentheses refer to the number of isomeric products as gauged by the mass spectra associated to the chromato-
alkene. However, for the above graphic peaks in the GC plot.
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Chem. Asian J. 2011, 6, 842 – 849

Tandem Reactions
Table 6. Use of a sacrificial agent.
nistic insights in addition to extending this chemistry to a
catalytic system. The use of nickel, a fairly inexpensive
metal, is highlighted in view of its remarkable versatility to
perform these types of chemical transformations.
Experimental Section
General Procedures and Materials
Unless otherwise noted, all manipulations were carried out by using
Schlenk and glovebox techniques under high-purity argon (Praxair:
99.998%). An Mbraun glovebox operating at <1 ppm H2O and O2 was
used. Protio solvents (THF and diethyl ether; J.T. Baker) were dried over
sodium/benzophenone ketyl, distilled under an inert atmosphere and
stored in the glovebox over 3 ꢅ molecular sieves and sodium. Water was
distilled and degassed (3ꢆ) by the freeze-pump-thaw method prior to
use. All fluorinated alkynes were prepared by a Sonogashira cross-cou-
pling reaction, according to a well established protocol.[16] Deuterated
solvents ([D8]THF, [D8]toluene, [D6]benzene and CDCl3; Cambridge Iso-
tope Laboratories) were stored over 3 ꢅ molecular sieves in the glovebox
for at least 24 h, prior to their use. The bisphosphine ligand, dippe, was
synthesized from 1,2-bis(dichlorophosphino)ethane (Aldrich) and isopro-
pylmagnesium chloride solution in THF (2.0m, Aldrich).[17] The nickel(I)
dimer, [(dippe)NiH]2 (1), was prepared from [(dippe)NiCl2][18] by drop-
wise addition of LiHBEt3 (“super-hydride”, Aldrich) to an hexanes solu-
tion of the former, similarly to the informed procedure.[19] All other sub-
stances, filters, and chromatographic materials were reagent grade and
were used as received. The organometallic complexes and organic deriva-
tives prepared in this work were purified either by crystallization or by
column chromatography. 1H, 19F, 13C{1H} and 31P{1H} NMR spectra of
nickel(0) complexes and organics in this work were recorded at ambient
temperature by using either a 300 MHz Varian Unity or a 400 MHz
Varian Inova spectrometer. All air and moisture sensitive samples in this
work were handled under inert atmosphere by using thin wall (0.38 mm)
WILMAD NMR tubes equipped with J. Young valves. Heating of
charged NMR tubes was done by using stirred thermostated silicon oil
baths. 1H and 13C{1H} NMR chemical shifts (d, ppm) are reported relative
to residual proton or deuterium resonances of the corresponding deuter-
ated solvent. 31P{1H} NMR spectra of nickel compounds and free phos-
phines are reported relative to external 85% H3PO4. 19F NMR spectra
are reported relative to external trifluoroacetic acid. Coupling constants
(J values) are given in Hz. GC-MS determinations were performed by
using a Varian Saturn 3 equipped with a 30 m DB-5MS capillary column.
Mass spectrometric (MS-EI+) of pure compounds were performed by
USAI-UNAM by using a Thermo-Electron DFS. Elemental analyses
(EAs) were also performed by USAI-UNAM with a Perkin–Elmer mi-
croanalyzer 2400.
Entry[a]
1
Conversion [%][b]
99
Product distribution [%][c]
8
9
10
11
3.4 (1)
15.1 (5)
54.3 (7)
25.0 (3)
[a] All reactions were performed in 0.38 mm J. Young tubes at 1008C for
10 h. Excess of 300 equiv of water and 6 equiv of Et3SiH were used in
every experiment. Heating was stopped after complete consumption of
3c as gauged by 31P{1H} NMR spectroscopy. [b] Determined by direct in-
jection of the reaction mixtures into a GC-MS. [c] Determined also by
their relative chromatographic areas following injection into a GC-MS.
The numbers in parentheses refer to the number of isomeric products as
gauged by the mass spectra associated to the chromatographic peaks in
the GC plot.
activation of benzonitrile[14] and the closely related aromatic
nitriles by [(dippe)Ni0],[15] wherein the existence of similar
fluxional arene-intermediates was established by both exper-
imental and computational methods. A mechanistic proposal
considering some of the evidence presented in the current
report is presented in Scheme 3.
Conclusions
Novel mono- and bi-metallic nickel(0)complexes of the
type, [(dippe)Ni(h2-C,C-Fn-alkyne)] and [{(dippe)Ni}2(m2-
C,C-Fn-alkyne)], were prepared in high yield from the
nickel hydride dimer, [{(dippe)NiACHTNUTRGNEUNG(m-H)}2], and aryl-fluori-
nated alkynes. The monometallic complexes engage into a
stoichiometric reaction that involves both semihydrogena-
tion of the triple bond by water and hydrodefluorination of
the fluorinated aryl substituent in tandem. Reactivity to-
wards the semihydrogenation–hydrodefluorination was
found to increase in direct relationship with the number of
fluorine atoms over the aryl ring, thereby making perfluori-
nated alkynes more susceptible to these transformations.
Substitution of two fluorine atoms of the aryl ring was possi-
ble in the presence of water, slightly increased for up to
three if Et3SiH was used as a substitute hydrogen source.
Use of Et3SiH also prevents decomposition of reactive [(dip-
pe)Ni0] species, by acting as an oxygen and fluoride scaveng-
er in the mixtures, ultimately making it a sacrificial hydro-
gen donor. Overall, use of Et3SiH yielded a more efficient
metal-mediated process, whereas the presence of water re-
sulted in a less reactive yet more selective process. In either
case, the tandem processes occurred under relatively mild
reaction conditions. Studies are underway to provide mecha-
X-ray diffraction measurements were performed on an Oxford Diffrac-
tion Gemini-Atlas diffractometer. Data collection routine and data re-
duction were carried out with CrysAlisPro, Oxford Diffraction Ltd.,
Oxford Diffraction.[20a] The structures of all molecules were solved by
using SHELXS-97[20b] and refined by using SHELXL-97. All non-hydro-
gen atoms were refined anisotropically and the hydrogen atoms were
found in difference Fourier maps, placed at geometrically calculated posi-
tions and refined by using the riding model. The obtained bond lengths
and angles of each compound are normal and are available from the Sup-
porting Information.
Reactivity Assessment in Toluene
Reactivity of complex 3a with water: A solution of complex 3a (0.037 g,
0.92 mmol, 1 equiv) in [D8]toluene (0.7 mL) was added 300 equiv of
water (0.45 mL, 0.45 mg, 27 mmol). The reaction mixture was heated to
1358C. Over 7days the reaction mixture was monitored by using
31P{1H} NMR spectroscopy until complex 3a disappeared. After this time,
the reaction was quenched with air and the contents were analyzed by
using GC-MS. Conversion 98% with a product distribution; IE+-MS:
m/z: 198.08 (95%, one product, [F-C6H4-CH=CH-C6H5]+), 180.1 (3%,
Chem. Asian J. 2011, 6, 842 – 849
ꢄ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
847

J. J. Garcia et al.
FULL PAPERS
31P{1H} NMR spectroscopy until com-
plex 3c disappeared. After this time,
the reaction was quenched by expo-
sure to air and the contents were ana-
lyzed by using GC-MS. Conversion
100%. Product distribution, IE+-MS:
m/z: 270.05 (46%, one product, [C6F5-
CH=CH-C6H5]+), 252.06 (50%, three
products, [C6F4H-CH=CH-C6H5]+).
Reactivity assessment of 3c in THF
General Procedure: A series of NMR
tubes were charged with 3c (0.055 g,
0.92 mmol,
1 equiv)
and
water
(0.45 mL, 0.45 mg, 27 mmol) in
[D8]THF (0.7 mL). Reactions were
heated at the appropriate tempera-
tures. The reaction mixtures were
monitored by using 31P{1H} NMR spec-
troscopy until complex 3c disap-
peared, at which time the reactions
were quenched by exposure to air and
the contents were analyzed by using
GC-MS.
Reaction at 708C: Reaction was
stopped after 18 h, conversion 96%.
Product distribution; IE+-MS: m/z:
270.05 (36.5%, two products, [C6F5-
CH=CH-C6H5]+), 252.06 (50.6%, four
products,
[C6F4H-CH=CH-C6H5]+),
234.1 (8.6%, four products, [C6F3H2-
CH=CH-C6H5]+).
Reaction at 808C: Reaction was
stopped after 14 h, conversion 98%.
Product distribution; IE+-MS: m/z:
270.05( 19.2%, two products, [C6F5-
CH=CH-C6H5]+), 252.06 (58.7%, four
products,
[C6F4H-CH=CH-C6H5]+),
234.1 (20.2%, four products, [C6F3H2-
CH=CH-C6H5]+).
Reaction at 1008C: Reaction was
stopped after 10 h. Conversion 98%.
Product distribution; IE+-MS: m/z:
270.05 (15.2%, two products, [C6F5-
CH=CH-C6H5]+), 252.06 (67.8%, four
Scheme 3. Mechanistic proposal.
products,
[C6F4H-CH=CH-C6H5]+),
234.1 (15.1%, four products, [C6F3H2-
CH=CH-C6H5]+).
one product, [C6H5-CH=CH-C6H5]+). Isolated product (E)-2’-fluorostil-
bene (90%). 1H NMR (400 MHz,CDCl3): d=7.06–7.12 (m, 1H), 7.13–
7.15 (m, 1H), 7.19–7.30 (m, 4H), 7.35–7.38 (m, 2H), 7.53–7.54 (m, 2H),
7.59–7.63 ppm (m, 1H); 13C{1H} NMR (100 MHz, CDCl3): d=116.1 (d,
J=22.1 Hz), 121.2 (d, J=3.8 Hz), 124.5 (d, J=3.3 Hz), 125.5 (d, J=
12.6 Hz), 127.0, 127.3 (d, J=4.2 Hz), 128.3, 129.0 (m), 131.2 (d, J=
5.1 Hz), 137.5, 159.5, 162.0 ppm.
Reaction of 3c with deuterium oxide: An NMR tube was charged with
3c (0.055 g, 0.92 mmol; 1 equiv) and D2O (0.3 mL, 27 mmol, 29 equiv) in
[D8]THF (0.7 mL). The tube was heated at 1008C over 14 h until com-
plete conversion of complex 3c, conversion 97%. Product distribution;
IE+-MS: m/z: 272.1 (26.33%, two products, [C6F5-CD=CD-C6H5]+),
255.1 (61.0%, five products, [C6F4D-CD=CD-C6H5]+), 238.0 (10.1%, four
products, [C6F3D2-CD=CD-C6H5]+).
Reactivity of complex 3b with water: The experiment was performed by
following the above general description, adding water (0.45 mL,
27 mmol, 300 equiv) to a [D8]toluene (0.7 mL) solution of 3b (0.05 g,
0.92 mmol, 1 equiv). The reaction was heated to 1358C for 5 days until
complete conversion of the complex (as monitored by using
31P{1H} NMR spectroscopy). Conversion based on GC-MS=100 %. Prod-
uct distribution; IE+-MS: m/z:(No. products); 86% (234.1, M+)(one
product, [F3-C6H2-CH=CH-C6H5]+), 14% (216.08, M+)(one product, [F2-
C6H3-CH=CH-C6H5]+).
Reactivity study of 3a with Et3SiH/H2O system.
General procedure: An NMR tube was charged with 3c (0.055 g,
0.92 mmol, 1 equiv), Et3SiH (0.64 g, 5.55 mmol, 6 equiv) and water
(0.45 mL, 27 mmol) in 0.7 mL of [D8]THF. Reactions were heated at
1008C over 14 h until complex 3c disappeared in 31P{1H} NMR. After
this time, the reaction was quenched by exposure to air and the content
was analyzed by GC-MS, conversion 100%. Product distribution; IE+-
MS: m/z: 270.05 (3.4%, one product, [C6F5-CH=CH-C6H5]+), 252.06
(15.1%, five products, [C6F4H-CH=CH-C6H5]+), 54.3% (234.1, seven
products, [C6F3H2-CH=CH-C6H5]+), 25.0% (216.1, three products,
[C6F2H3-CH=CH-C6H5]+).
Reactivity of complex 3c with water: Addition of water (0.45 mL,
0.45 mg, 27 mmol, 300 equiv) to
a solution of complex 3c (0.055 g,
0.92 mmol, 1 equiv) in [D8]toluene (0.7 mL). The reaction was heated at
1358C for 1day. The reaction mixture was monitored by using
848
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ꢄ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 842 – 849

Tandem Reactions
CCDC 790028 (3a), 790029 (4a), 790030 (4b), 790031 (4c) contain the
supplementary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif
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We thank PAPIIT-DGAPA-UNAM (IN-201010) and CONACYT
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for a Ph.D. grant.
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Received: August 24, 2010
Published online: November 24, 2010
Chem. Asian J. 2011, 6, 842 – 849
ꢄ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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