Synthesis of low-valent nickel complexes in aqueous media, mechanistic insights, and selected applications

Article abstract of DOI:10.1021/om500767p

The synthesis of nickel(0) complexes usually requires the employment of strong reducing agents, including powerful hydride donors such as LiHBEt₃, LiAlH₄, and DIBAL-H. Herein, we have reduced the Ni(II) complex [(dippe)NiCl₂] (1a) at room temperature by using KOH in aqueous media to yield the low-valent complex [Ni(dippe)₂] (4a) as the main product, along with the formation of dippeO₂. In order to gain some insight into the reaction mechanism, a series of intermediates were isolated and characterized at different reaction stages. As a result, both Ni(II) hydroxo-bridge (2b) and hydride (3b) complexes were identified as key intermediates. Additionally, the use of water as a hydrogen source in nickel-mediated processes was also investigated and successfully applied to the hydrodefluorination, hydrodesulfurization, and hydrogenation of selected organic substrates.

Full text of DOI:10.1021/om500767p

Article
pubs.acs.org/Organometallics
Synthesis of Low-Valent Nickel Complexes in Aqueous Media,
Mechanistic Insights, and Selected Applications
́
Illan Morales-Becerril, Marcos Flores-Alamo, Adrian Tlahuext-Aca, Alma Arevalo, and Juventino J. García*
́
́
́
Facultad de Química, Universidad Nacional Auton
́ ́ ́
oma de Mexico, Circuito Interior, Ciudad Universitaria, Mexico City, 04510,
Mexico
́
S
* Supporting Information
ABSTRACT: The synthesis of nickel(0) complexes usually
requires the employment of strong reducing agents, including
powerful hydride donors such as LiHBEt3, LiAlH4, and
DIBAL-H. Herein, we have reduced the Ni(II) complex
[(dippe)NiCl2] (1a) at room temperature by using KOH in
aqueous media to yield the low-valent complex [Ni(dippe)2]
(4a) as the main product, along with the formation of
dippeO2. In order to gain some insight into the reaction
mechanism, a series of intermediates were isolated and characterized at different reaction stages. As a result, both Ni(II) hydroxo-
bridge (2b) and hydride (3b) complexes were identified as key intermediates. Additionally, the use of water as a hydrogen source
in nickel-mediated processes was also investigated and successfully applied to the hydrodefluorination, hydrodesulfurization, and
hydrogenation of selected organic substrates.
catalytic processes.16 For this reason, many hydride systems
INTRODUCTION
■
have emerged as model precursors obtained by a series of
pincer-type ligands.17 These complexes have been demon-
strated to be suitable for reaching low-valent products.
However, their synthesis still requires the same powerful
hydride sources as the commonly employed Ni(0) method-
ologies.
Thus, novel synthetic approaches for the synthesis of low-
valent nickel complexes are greatly needed to reach greener
procedures that involve the use of water as reaction media and
avoid strong hydride donors. Herein, we report the use of KOH
in aqueous media as an alternative for the ultimate reduction of
the Ni(II) precursor [(dippe)NiCl2] (dippe = 1,2-bis(diiso-
propylphosphino)ethane). We also provide a few mechanistic
insights into this reaction and demonstrate some selected
applications in the hydrogenation and activation of C−F and
C−S bonds, in nickel-mediated processes using water as the
hydrogen source.
The use of nickel compounds in a variety of applications has
gained increasing attention in the past few years. Particularly
low-valent nickel complexes have been found to be useful in
catalytic applications, including cross-coupling reactions,1
carboxylations,2 hydroaminations,3 hydrodefluorinations,4 hy-
drodesulfurizations,5 cycloadditions,6 and C−H bond activa-
tions.7 Not only the less forcing reaction conditions but also the
capacity of activating inert substrates have positioned nickel
complexes as powerful tools in organic synthesis8 compared to
the other more expensive 10-group metals. However, its use as
a routine option has not been extended due to the lack of mild
and environmentally friendly synthetic methods. Low-valent
nickel complexes are commonly synthesized under an inert
atmosphere by reaction with strong reducing agents, including
the powerful hydride donors LiHBEt3,9 LiAlH4,10 and [(iBu)2-
Al(μ-H)]2.11 These reaction conditions are some of the
limitations faced by new green methodologies.
Since the discovery of the first Ni(0) complex, [Ni(CO)4],
relatively few new synthetic approaches have been developed
for obtaining Ni(0) species under mild reaction conditions.12 In
several cases, auxiliary agents such as phosphines and alcohols
are oxidized, allowing a redox process in the presence of a base
and “wet solvents”.13 These reaction conditions have become
attractive for a variety of catalytic applications and do not
require the use of a starting air-sensitive Ni(0) complex.14
However, to the best of our knowledge, virtually no evidence
nor mechanistic insights have been provided to explain the
overall reduction.
RESULTS AND DISCUSSION
■
Synthesis of [Ni(dippe)2]. Inspired by a seminal report by
Pearson et al.,18 we proposed the feasible reduction of the
complex [(dippe)NiCl2] (1a) in the presence of a base and
aqueous media. Recently, Beletskaya and co-workers demon-
strated that Ni(0) species can be generated in situ from Ni(II)
precursors along with the phosphine oxidation in the presence
of water.13b Therefore, we decided to generate low-valent nickel
complexes on the basis of the dippeO2 formation in aqueous
media to avoid a large excess of KOH. After a set of different
Since theoretical studies have invoked hydride intermediates
as Ni(0) precursors,15 nickel−hydride complexes have received
increasing attention as key intermediates for a wide series of
Received: July 25, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om500767p | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 1. Formation of [Ni(dippe)2]
Figure 1. 31P{1H} NMR monitoring for the [(dippe)NiCl2]/KOH reaction mixture in D2O.
reaction conditions, we found that 2 equiv of carefully purified
KOH19 is necessary for the complete consumption in 15 min of
the otherwise insoluble dichloride complex (1a). An orange
homogeneous aqueous solution was obtained at room temper-
ature after stirring for a couple of minutes. After evaporation
under vacuum, a reddish-brown oily product was obtained. This
residue was readily soluble in organic solvents, but insoluble in
water, and showed air-sensitive properties (Scheme 1). After
the workup, the nickel(0) complex [Ni(dippe)2] (4a) was
isolated and characterized by single-crystal X-ray diffraction
analysis. It was then compared with the previous report by Vicic
and Jones.20 Also the corresponding byproducts dippeO2,
Ni(OH)2, and KCl were identified (see the Supporting
Information). In addition, if the reaction was carried out in
the absence of KOH, none of the products can be obtained.
Instead, the starting dichloride complex (1a) was recovered,
which makes evident the use of a base as the key reagent for the
nickel reduction.
in D2O are shown in Figure 1. At the beginning of the reaction,
a narrow singlet labeled (2a) located at 85.0 ppm was slowly
transformed into a complex pattern consisting of four
phosphorus signals labeled as (3a) (vide infra); the relative
ratios of (2a) vs (3a) determined by 31P{1H} NMR are (3.4:1);
(1.5:3.6), and (0.7:6.0) at 1, 3, and 7 days, respectively. At this
point, it is important to underline that, in Figure 1, only the
D2O soluble compounds are observed; consequently, dippeO2
and (4a) cannot be observed due to their null solubility in
water. Instead, it yields a final insoluble brown product clearly
observed after 7 days identified as (4a). Therefore, if vacuum
was not applied, the reaction time to produce (4a) was
substantially increased.
The intermediate (2a) was assigned to the complex
[(dippe)Ni(μ-OH)]2(OH)2 on the basis of the isolation of a
yellow precipitate (2b) produced by ionic exchange process
adding stoichiometric amounts of [(nBu4N)(PF6)], as shown in
Scheme 2. Compound (2b) readily crystallized by evaporation
Considering the reaction depicted in Scheme 1, a
stoichiometric amount of KOH is required to reduce the
Ni(II) complex through the phosphine oxidation. If off-the-
shelf KOH was used, the yield was dramatically reduced to 40%
for the nickel product (4a) after workup, due to the formation
of Ni−carbonate complexes.21 Pure KOH allowed complex
(4a) to be prepared as the only Ni(0) product. This represents
an alternative methodology for the synthesis of low-valent
nickel complexes in water. However, at this point, not only the
KOH participation but also the mechanistic pathway for the
reaction remained unknown. In order to investigate further, we
decided to monitor the reaction to identify the involved
intermediates by 31P{1H} NMR before the water was removed.
A monitoring of the reaction was placed at room temperature
in a sealed NMR tube under argon for several days to allow
detection of some of its key intermediates. Relevant 31P{1H}
NMR spectra of the reaction mixture at different reaction times
Scheme 2. Ionic Exchange Process from (2a) to (2b)
of an acetone solution, and allowed full characterization from a
single-crystal XRD analysis. The corresponding ORTEP
representation of complex [(dippe)Ni(μ-OH)]2(PF6)2 (2b) is
depicted in Figure 2.
The resulting Ni(II) complex (2b) displays a square-planar
structure in a cis-P2-O2 arrangement with averaged (Ni−O),
(Ni−P), and (Ni−Ni) bond distances of 1.893, 2.153, and
2.939 Å, respectively. All of these parameters are similar to
other [L2Ni(μ-OH)]2 motifs,22 including the closely related
[(dippe)Ni(μ-OH)]2(OTf)2 (2c) that has been synthesized,
isolated, and crystallized in the presence of AgOTf from a KOH
free methodology.23
B
dx.doi.org/10.1021/om500767p | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
The signals labeled as (3a) in Figure 1 account for the
following intermediate being formed in the reaction mixture
from (2a). This complex displays four sets of signals in the
31P{1H} NMR spectrum in D2O, located at 90.1, 75.9, 65.4, and
45.9 ppm (see the Experimental Section for δ and couplings in
acetone-d6). Note that characteristic high-order splitting
patterns have been correlated by selective irradiation experi-
ments (see the Supporting Information). The 1H NMR (H2O;
see the Experimental Section) allowed us to observe a key
signal located at −11.1 ppm (dt, 2JH‑P = 105 and 60 Hz, for the
trans and cis couplings, respectively) that is characteristic of a
nickel−hydride complex. Once again, suitable crystals for
single-crystal X-ray studies could be obtained only by addition
of [(nBu4N)(PF6)] to yield compound [(dippe)(dippeO)Ni-
(H)](PF6) (3b).
The ORTEP representation of complex (3b) is depicted in
Figure 3. The Ni−H bond length was calculated to be 1.45 Å
and was refined in agreement with previously reported nickel−
hydride complexes. The molecular structure of (3b) revealed a
Ni(II) in a distorted square-planar geometry that is distorted by
the coordinated bulky phosphine mono-oxide. In fact, the
Ni(1)−P(2) and Ni(1)−P(3) bonds are slightly longer than
the Ni(1)−P(1) probably due to steric effects.
As previously described for complex (2b), a solution of
compound (3b) was evaporated to dryness under vacuum
without formation of (4a). However, the unexchanged aqueous
[(dippe)(dippeO)Ni(H)](OH) (3a) was transformed to
[Ni(dippe)2] (4a) either readily under vacuum or slowly by
just stirring at room temperature. In addition, dippeO2 was also
identified as a byproduct after workup by direct comparison
with the reported 31P{1H} NMR and also confirmed by its
single-crystal XRD analysis (see the Supporting Information).
A mechanistic proposal for this redox process is depicted in
Scheme 3 considering the intermediates isolated described
previously during the monitoring of the reaction and as a result
of anion exchange crystallization. The proposal starts with the
Figure 2. ORTEP representation of complex (2b) with ellipsoids at
the 50% probability level. All H atoms (except hydroxide H1) and
−
PF6 anions are omitted for clarity. Key bond lengths (Å) and angles
(deg): Ni(1)−P(1), 2.1534(10); Ni(1)−O(1), 1.900(3); O(1)−H(1),
0.84(2); P(1)−Ni(1)−P(2), 87.55(4); P(1)−Ni(1)−O(1), 97.08(8);
O(1)−Ni(1)−O(1)#1, 78.36(12).
It is noteworthy that, when the aqueous solution of
[(dippe)Ni(μ-OH)]2(OH)2 (2a) was evaporated to dryness,
[Ni(dippe)2] (4a) was obtained as the major product.
However, neither the anion exchanged complex [(dippe)Ni-
(μ-OH)]2(PF6)2 (2b) nor the synthesized [(dippe)Ni(μ-
OH)]2(OTF)2 (2c) could lead to [Ni(dippe)2] (4a) when
vacuum was applied. This provides evidence for the
participation of the hydroxide anions in the outer Ni(II)
coordination sphere in the phosphine oxidation.
−
Figure 3. ORTEP drawing for nickel−hydride complex (3b) with ellipsoids at the 40% probability level. All H atoms (except the hydride H1), PF6
anion, and solvent molecules are omitted for clarity. Key bond lengths (Å) and angles (deg): Ni(1)−H(1), 1.45(4); Ni(1)−P(1), 2.1357(16);
Ni(1)−P(2), 2.2132(16); Ni(1)−P(3), 2.1896(16); P(4)−O(1), 1.487(5); P(1)−Ni(1)−P(2), 90.63(6); P(2)−Ni(1)−P(3), 111.67(6); P(1)−
Ni(1)−P(3), 157.70(7); P(1)−Ni(1)−H(1), 76(3).
C
dx.doi.org/10.1021/om500767p | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 3. Mechanistic Proposal for the Formation of (4a)
anion exchange of (1a) and dimerization to yield (2a).
Formation of (2e) probably involves an outer OH−
nucleophilic attack to a coordinated phosphine in compound
(2a), and then (2e) forms the hydride complex (3a) involving
the PO bond formation and the hydroxo bridge dissociation
and Ni(OH)2 formation.
The evaporation to dryness or longer reaction times might
account for H2 elimination, particularly for conversion of
complex (3a) to yield (4a). Clearly, when a different anion
such as PF6− or OTf− was used, release of H2 by reaction with
an external OH− is not possible, nor is the formation of (4a).
Applications of the Low-Valent Nickel Complex in the
Presence of Water. Considering the formation of the Ni(0)
complex (4a) in aqueous media, and also the involvement of
some hydride intermediates, we decided to apply this
knowledge for the coordination and activation of some selected
substrates. It was anticipated that water would act as a hydrogen
source based on previous reports from our group.24
Thus, as a proof of concept, hexafluorobutyne (HFB) was
the first substrate to be evaluated in the presence of aqueous-
generated [Ni(dippe)2] (4a). The resulting Ni(0) complex
[(dippe)Ni(HFB)] (5a) was characterized by multinuclear
NMR and easily crystallized from hexane. In the solid state,
complex (5a) exhibits a distorted trigonal-planar geometry with
a coordinated carbon−carbon multiple bond length of 1.301 Å,
as depicted in Figure 4.
Figure 4. ORTEP representation of [(dippe)Ni(HFB)] complex (5a)
with ellipsoids at the 60% probability level. Key bond lengths (Å) and
angles (deg): Ni(1)−P(1), 2.1536(8); Ni(1)−C(9), 1.869(3); C(9)−
C(9′), 1.301(7); P(1)−Ni(1)−P(1′), 91.58(5); P(1)−Ni(1)−C(9),
113.88(11); C(8)−C(9)−C(9′), 139.80(19).
The use of complex (5a) in the presence of excess water
promotes the alkyne reduction that generates the correspond-
ing trans alkene isomer in a selective manner (Table 1).
Further, water was demonstrated to be the hydrogen source by
the use of D2O; consequently, deuterated products were
obtained and characterized by a GC-MS analysis. As in our
previous report,24 a time-dependent isomerization process was
observed at short reaction times. The kinetic cis product was
originally obtained, and it was then slowly transformed into the
thermodynamic trans isomer (entries 1−3, Table 1).
We also found that water concentration plays an important
role. The use of stoichiometric amounts of water in THF
allowed the hydrodefluorination of the cis-alkene under
relatively mild reaction conditions. In fact, the isomerization
process was a competing reaction vs the hydrodefluorination
reaction; i.e., the more water was used, the more the
isomerization products were generated (see Table 1, entries 1
and 4). Also, we speculate that the use of lower amounts of
water avoids quenching of intermediates, ultimately leading to
HDF.
D
dx.doi.org/10.1021/om500767p | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
Table 1. Tandem Hydrogenation/HDF Reaction of [(dippe)Ni(HFB)] with Water
entry
time
6 h
H2O (eq)
(a)
(b)
(c)
(d)
conv.
1
2
3
4
5
300
300
300
5
33
97
77
11
8
61
1
6
2
0
0
28
48
97
27
51
2 days
7 days
5 h
13
48
0
9
0
41
59
0
3 days
5
33
a
Reaction mixtures prepared by using 3.5 mL of THF. All conversions were determined by GC-MS.
a
Chart 1. HDS Conversions of DBT
a
Experiments were carried out in 3.5 mL of (1:1) H2O:THF (v:v) mixture. 24 h. Using [(dippe)NiCl2]:KOH:DBT:ADS molar proportions
(1:2:1:2), respectively. All conversions were determined by GC-MS.
Apparently, a sequential hydrodefluorination reaction occurs
at longer reaction times using more stoichiometric amounts of
water (entry 5, Table 1). This result differs from other
methodologies that employ electrophilic main-group Lewis
acids, where the partially defluorinated products (−CHF2,
−CH2F) were not observed (see the Supporting Informa-
tion).25 Regarding the regioselectivity for the HDF reaction, we
speculate that the second −CF3 group acts as an electron-
withdrawing group along with the weakening of C−F bonding
in every successive HDF step (C−F bond energies: −CF3 >
−CHF2 > −CH2F).
On the other hand, another selected substrate was
dibenzothiophene (DBT), whose reactivity in the presence of
low-valent nickel complexes has been previously reported.20,26
However, herein, we first report the use of water as the
hydrogen source in a metal-mediated process. The reaction was
carried out under relatively mild conditions using complex
(4a), yielding biphenyl as the main hydrodesulfurization
(HDS) product and chromatographic conversions close to
the 84% at 50 °C. Again, the use of D2O corroborated the
incorporation of deuterium to yield dideuterobiphenyl.
Efforts to perform this reaction with the use of the air-stable
[(dippe)NiCl2] (1a) and KOH as basic media to generate in
situ the hydride nickel complex (3a) were made. Unfortunately,
low conversions were obtained with this method, even with the
use of an assistant desulfurating agent (ADS) to thermody-
namically enhance the HDS process. Chart 1 depicts the main
results for HDS conversions at 70 °C.
CONCLUSIONS
■
In summary, the synthesis of the Ni(0) complex [Ni(dippe)2]
has been achieved with the use of [(dippe)NiCl2] in aqueous
basic media. Key intermediates for the nickel reduction were
isolated and characterized by anion exchange, which demon-
strated that KOH promotes phosphine oxidation. It was also
observed that water can be used as a hydrogen source in metal-
mediated processes and promotes hydrodesulfurization, hydro-
defluorination, and hydrogenation of unreactive substrates such
as dibenzothiophene and hexafluorobutyne. HFB was hydro-
genated and hydrodefluorinated when stoichiometric amounts
of water were employed. The metal-mediated HDS process has
been achieved using Ni(0) and a Ni(II) system in aqueous
media. Efforts are underway in our group to achieve catalytic
deep desulfurization in aqueous media.
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise noted, all operations
were carried out in an MBraun glovebox (<1 ppm of H2O and O2) or
by using high-vacuum and standard Schlenk techniques under an
argon atmosphere. THF was dried and distilled from dark purple
E
dx.doi.org/10.1021/om500767p | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3
solutions of sodium/ketyl benzophenone. Deuterated solvents for
NMR experiments were purchased from Aldrich and stored over 3 Å
molecular sieves in the glovebox. Celite and silica gel were dried by
heating at 200 °C under vacuum for 20 h and were also stored in the
glovebox. Complex [(dippe)NiCl2] was prepared from a solution of
NiCl2·6H2O and the corresponding chelating bisphospine dippe (1,2-
bis(diisopropylphosphino)ethane) by adapting reported procedures.27
All other chemicals and filter aids were reagent grade and were used as
received. The purification of the new compounds was carried out by
crystallization. All reagents were loaded in the glovebox using Schlenk
flasks equipped with Rotaflo high-vacuum stopcocks. Crude reaction
mixtures for each catalytic run were immediately analyzed by GC-MS.
GC-MS determinations were performed using an Agilent 5975C
system equipped with a 30 m DB-5MS capillary (0.32 mm i.d.)
1.13 (d, JH‑H = 7.6 Hz, 24 H, CH3), −1.59 (s, 2H, OH); 31P{1H}
NMR (Acetone-d6, 121 MHz): δ 85.0 (s, dippe), −146 (m, 1JP‑F = 708
−
Hz, PF6−); 19F NMR (Acetone-d6, 282 MHz): δ −72.3 (d, 1JF‑P = 708
Hz, PF6 ). Anal. Calcd for C28H66F12Ni2O2P6: C, 34.81; H, 6.89; F,
23.60. Found: C, 34.65; H, 6.77; F 23.50.
Synthesis of [(dippe)(dippeO)Ni(H)](PF6). [(dippe)NiCl2]
(0.098 g, 0.25 mmol) was slowly added to a solution of purified
KOH (0.028 g, 0.50 mmol) in H2O (3.5 mL) under constant stirring
at room temperature. After 7 days, the yellow solution was filtered,
then slowly added with [(Bu4N)(PF6)] (0.010 g, 0.025 mmol), and
stored in the drybox. Brown crystals were obtained from the mother
liquors after a couple of weeks. Yield 74%. 1H NMR (Acetone-d6, 300
MHz): δ 2.25−2.05 (m, 8 H, CH), 1.93−1.77 (m, 8 H, CH2), 1.12−
2
1
0.93 (m, 48 H, CH3), −11.15 (dt, JH‑P = 105.3, 59.4 Hz, 1H, Ni-H);
column; a toluene internal standard was used for quantification. H,
31P{1H} NMR (Acetone-d6, 121 MHz): δ 90.1 (d, JP‑P = 204.8,
2
19F, and 31P{1H} NMR spectra were recorded at room temperature on
2
dippeO), 75.9 (m, dippe), 65.4 (m, dippe), 45.8 (d, JP‑P = 204.8 Hz,
a 300 MHz Varian Unity spectrometer in THF-d8, D2O, or CDCl3,
unless otherwise stated. 1H chemical shifts (δ, ppm) are reported
relative to the residual proton resonance in the corresponding
deuterated solvent. 31P{1H} NMR spectra were referenced to an
external 85% H3PO4 solution. All air-sensitive NMR samples were
handled under an inert atmosphere using thin-wall (0.38 mm) Wilmad
NMR tubes equipped with J. Young valves.
X-ray Structure Determinations. Single-crystal X-ray diffraction
determinations were performed on an Oxford Diffraction Gemini Atlas
diffractometer equipped with radiation sources of λMοKα = 0.71073 Å
and λCuKα = 1.5418 Å, and a Cryojet System for low-temperature
collection data. CrysAlisPro and CrysAlis RED software packages28
were used for data collection and data integration. Structure solution
and refinement were carried out with the program(s): SHELXS97 and
SHELXL97;29 and the software used to prepare material for
publication: WinGX.30 Full-matrix least-squares refinement was carried
out by minimizing (Fo2 − Fc2)2. All non-hydrogen atoms were refined
anisotropically. H atoms attached to O atoms were located in a
difference map and refined isotropically with Uiso(H) = 1.5 Ueq for
(O). H atoms attached to C atoms were placed in geometrically
idealized positions and refined as riding on their parent atoms, with
C−H = 0.95−1.00 Å and Uiso(H) = 1.2 Ueq(C), or 1.5 Ueq(C) for
methylene, methyne, and methyl groups.
Reaction of [(dippe)NiCl2] with KOH. Synthesis of [Ni-
(dippe)2]. The compound [(dippe)NiCl2] (0.098 g, 0.25 mmol)
was slowly added into a solution of purified KOH (0.028 g, 0.50
mmol) in H2O (3.5 mL) under constant stirring at room temperature.
After 15 min of reaction, an orange solution was then observed. At this
point, the solvent was evaporated to dryness in vacuum, and the
obtained red-brown residue was redissolved in THF (5.0 mL) and
filtrated via cannula using a Schlenk flask. [Ni(dippe)2] was obtained
as a brown solid in 60% yield (0.034 g, 0.06 mmol). The solid was
redissolved in THF-d8 and stored in the drybox fridge at −30 °C. After
a couple of days, light-yellow crystals suitable for X-ray diffraction
studies were obtained. 1H NMR (THF-d8, 300 MHz): δ 1.93 (m, 3JH‑H
= 7.3 Hz, 4 H, CH), 1.33 (s, 8 H, CH2), 1.20−0.99 (d, 3JH‑H = 7.3 Hz,
24 H, CH3); 31P{1H} NMR (121 MHz, THF-d8): δ 53.9 (s, dippe).
Anal. Calcd for C28H64NiP4: C, 57.65; H, 11.06. Found: C, 57.6; H,
11.10.
1
−
dippeO), −144.2 (m, JP‑F = 708 Hz, PF6 ); 19F NMR (Acetone-d6,
1
−
282 MHz): δ −72.3 (d, JF‑P = 708 Hz, PF6 ). Anal. Calcd for
C28H65F6NiOP5: C, 45.12; H, 8.79; F, 15.29. Found: C, 45.03; H, 8.80;
F 15.18.
Reaction of [Ni(dippe)2] with HFB. A stream of 1,1,1,4,4,4-
hexafluorobutyne (0.100 g, 0.617 mmol) was bubbled into a solution
of [Ni(dippe)2] (0.058 g, 0.10 mmol) in hexanes (3.5 mL) over an
iced-bath under an argon atmosphere. Immediately, a dark green
precipitate was obtained. The precipitate was then filtered, dried, and
redissolved in THF. After a couple of days of cooling in the drybox
fridge at −30 °C, yellow crystals suitable for X-ray diffraction studies
1
were obtained. Yield 92%. H NMR (THF-d8, 300 MHz): δ 2.15 (m,
3JH‑H = 7.1 Hz, 4 H, CH), 1.81 (s, 2 H, axialCH2) 1.78 (s, 2 H,
equatorialCH2), 1.20−0.07 (d, JH‑H = 7.1 Hz, 24 H, CH3); 31P{1H}
3
NMR (THF-d8, 121 MHz): δ 87.5 (m, 4JP‑F = 8.6, 3.0 Hz, dippe); 19
F
4
NMR (THF-d8, 282 MHz): δ −54.5 (dd, JF‑P = 8.6, 3.0 Hz, HFB).
Anal. Calcd for C18H32F6NiP2: C, 44.75; H, 6.68; F, 23.60. Found: C,
44.94; H, 6.53; F 23.53.
Reaction of [(dippe)Ni(HFB)] with H2O. Drops of H2O (0.540 g,
0.030 mol; 0.009 g, 0.50 mmol) were added into a solution of
[(dippe)Ni(HFB)] (0.048 g, 0.10 mmol) in THF (3.5 mL) under an
argon atmosphere. Then, the Shlenk flask was closed and heated in an
oil bath at 100 °C and monitored at different reaction times. Aliquots
were taken and immediately analyzed by GC-MS. The 1,1,1,4,4,4-
hexafluorobut-(E)-ene was identified as the major product when an
excess of water (0.540 g, 0.030 mol) was employed. Yield 97%; GC-
MS m/z 163.9; H NMR (300 MHz, CDCl3): δ 6.07 (s, CH); 19F
1
NMR (282 MHz, CDCl3): δ −66.4 (s, CF3). When a stoichiometric
amount of water was used (0.009 g, 0.50 mmol), the following product
distribution was observed: 1,1,1,4,4,4-hexafluorobutene; yield 8%; GC-
MS m/z 163.9. 1,1,1,4,4-pentafluorobutene; yield 59%; GC-MS m/z
145.0. 1,1,1,4-tetrafluorobutene; yield 33%; GC-MS m/z 127.9.
Reaction of [Ni(dippe)2] with DBT. [Ni(dippe)2] (0.058 g, 0.10
mmol) dissolved in 3.5 mL of THF was added with DBT (0.018 g,
0.10 mmol), and then with H2O (0.540 g, 0.030 mol) under argon.
The mixture was constantly stirred at 50 °C for 3 days. The final HDS
products were determined and quantified by GC-MS.
Reaction Monitoring. [(dippe)NiCl2] (0.039 g, 0.10 mmol) was
slowly added into a purified KOH (0.011 g, 0.20 mmol) solution in
D2O (1.5 mL) with constant stirring at room temperature. The
obtained orange solution was immediately transferred to an NMR tube
and monitored for several days.
A similar procedure was made using D2O instead of H2O for
labeling purposes.
Reaction of [(dippe)NiCl2] with DBT. [(dippe)NiCl2] (0.039 g,
0.10 mmol) was slowly added into a solution of purified KOH (0.011
g, 0.20 mmol) in a mixture of (1:1) H2O:THF (v:v) (3.5 mL) with
constant stirring at room temperature. After 15 min of reaction, a
reddish solution was observed; then the organic substrate DBT (0.018
g, 0.10 mmol) along with 2 equiv of an assistant desulfurating agent
(ADS) was added [BaCl2, CaCl2, MgCl2, Ba(OH)2, Ca(OH)2,
Mg(OH)2, MgH2, Mg, Zn]. Then, the flask was closed and heated
in an oil bath at 70 °C for 1 day. After this time, the heating was
stopped, and an aliquot of the reaction mixture was immediately
analyzed by GC-MS.
Synthesis of [(dippe)Ni(μ-OH)]2(PF6)2. [(dippe)NiCl2] (0.098 g,
0.25 mmol) was slowly added into a solution of purified KOH (0.028
g, 0.50 mmol) in H2O (3.5 mL) with constant stirring at room
temperature. The orange solution was stirred for 30 min and then
slowly added with [(Bu4N)(PF6)] (0.020 g, 0.050 mmol) to yield a
yellow precipitate, which filtered and further redissolved in deuterated
acetone (0.7 mL). After a couple of days, orange crystals were
obtained by slow evaporation from the mother liquor in the drybox.
1
3
Yield 80%. H NMR (Acetone-d6, 300 MHz): δ 2.40 (m, JH‑H = 7.6
Hz, 4 H, CH), 1.66 (s, 2 H, axialCH2) 1.62 (s, 2 H, equatorialCH2), 1.41−
F
dx.doi.org/10.1021/om500767p | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(b) Breitenfeld, J.; Scopelliti, X.; Hu, R. Organometallics 2012, 31,
2128−2136. (c) Rossin, A.; Peruzzini, M.; Zanobini, F. Dalton Trans.
2011, 40, 4447−4452. (d) Lai, K.; Ho, W.; Chiou, T.; Liaw, W. Inorg.
Chem. 2013, 52, 4151−4153.
ASSOCIATED CONTENT
* Supporting Information
Figures, selected multinuclear NMR spectra, GC-MS, crystallo-
graphic data, and complementary synthetic details for
complexes. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
(18) Pearson, R.; Louw, W.; Rajaram, J. Inorg. Chim. Acta 1974, 9,
251−255.
(19) Purified according to standard methodologies: (a) Sipos, P.;
May, P.; Hefter, G. Analyst 2000, 125, 955−958. (b) Wei, Y.; Oshima,
M.; Motomizu, S. Analyst 2002, 127, 424−427.
(20) Vicic, D.; Jones, W. D. J. Am. Chem. Soc. 1999, 121, 7606−7617.
(21) See the Supporting Information, sections 1−4.
(22) (a) Cornella, J.; Gomez, R.; Martin, E. J. Am. Chem. Soc. 2013,
135, 1997−2009. (b) Dible, B.; Sigman, M.; Arif, A. Inorg. Chem. 2005,
44, 3774−3776. (c) Wikstrom, J.; Filatov, A.; Mikhalyova, E.; Shatruk,
M.; Foxman, B.; Ryvak, E. Dalton Trans. 2010, 39, 2504−2514.
(d) Carmona, E.; Marin, J.; Palma, P.; Paneque, M.; Poveda, M. Inorg.
Chem. 1989, 28, 1895−1900.
(23) (a) Li, J.; Li, W.; Sharp, P. Inorg. Chem. 1996, 35, 604−613. (b)
We crystallized and resolved the corresponding X-ray structure for 2c.
For further details, see the Supporting Information.
AUTHOR INFORMATION
Corresponding Author
*E-mail: juvent@unam.mx (J.J.G.).
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank PAPIIT-DGAPA-UNAM (IN-210613) and CON-
ACYT (0178265 and Grant 271105) for their financial support
of this work.
́
(24) Barrios, R.; Benítez, T.; Flores-Alamo, M.; Arevalo, A.; García, J.
J. Chem.Asian J. 2011, 6, 842−849.
REFERENCES
■
(25) (a) Caputo, C.; Hounjer, L.; Dobrovetsky, R.; Stephan, D.
Science 2013, 341, 1374−1377. (b) Doubris, C.; Ozerov, O. Science
2008, 321, 1188−1190. (c) Stahl, T.; Klare, F.; Oestreich, M. ACS
Catal. 2013, 3, 1578−1587.
(1) (a) Zultansky, S.; Fu, G. J. Am. Chem. Soc. 2013, 135, 624−627.
(b) Tasker, S.; Guitierrez, A.; Jamison, T. Angew. Chem., Int. Ed. 2014,
53, 1858−1861. (c) Han, F. Chem. Soc. Rev. 2013, 42, 5270−5298.
(2) Leon, T.; Correa, A.; Martin, R. J. Am. Chem. Soc. 2013, 135,
1221−1224.
(26) Eisch, J.; Hallenbeck, L.; Han, K. J. Org. Chem. 1983, 48, 2963−
2968.
(27) Scott, F.; Kruger, C.; Betz, P. J. Organomet. Chem. 1990, 387,
113−121.
(28) CrysAlis PRO, CrysAlis CCD, and CrysAlis RED; Agilent
Technologies: Yarnton, U.K., 2012.
(3) Ge, S.; Green, R.; Hartwig, J. J. Am. Chem. Soc. 2014, 136, 1617−
1627.
(4) (a) Fischer, P.; Gotz, K.; Eichhorn, A.; Radius, U. Organometallics
2012, 31, 1374−1383. (b) Zhao, W.; Wu, J.; Cao, S. Adv. Synth. Catal.
2012, 354, 574−578.
(29) Sheldrick, G. M. SHELXS97 and SHELXL97; University of
(5) Torres, J.; Arev
2228−2233.
́
alo, A.; García, J. J. Organometallics 2007, 26,
Gottingen: Gottingen, Germany, 2008.
̈
̈
(30) Farrugia, L. J. J. Appl. Crystallogr. 2012, 45, 849−854.
(6) (a) Jenkins, A.; Herath, A.; Song, M.; Montgomery, J. J. Am.
Chem. Soc. 2011, 133, 14460−14466. (b) Song, W.; Ackermann, L.
Chem. Commun. 2013, 49, 6638−6640. (c) Thakur, A.; Facer, M.;
Louie, J. J. Angew. Chem., Int. Ed. 2013, 52, 12161−12165.
(7) (a) Liu, D.; Liu, C.; Li, H.; Lei, A. Angew. Chem., Int. Ed. 2013, 52,
4453−4456. (b) Wu, X.; Zhao, Y.; Ge, H. J. Am. Chem. Soc. 2014, 136,
1789−1792.
(8) Zhang, Z. Synlett 2005, 5, 877−878.
(9) (a) Cariou, R.; Graham, T.; Stephan, D. Dalton Trans. 2013, 42,
́
4237−4239. (b) Crestani, M.; Munoz, M.; Arevalo, A.; Acosta, A.;
̃
García, J. J. J. Am. Chem. Soc. 2005, 127, 18066−18073.
(10) Belykh, L.; Titova, Y.; Umanets, V.; Rokhin, A.; Schmidt, F.
Appl. Catal., A 2011, 401, 65−72.
(11) Sato, Y.; Takimoto, M.; Hayashi, K.; Katsuhara, T.; Takagi, K.;
Mori, M. J. Am. Chem. Soc. 1994, 116, 9771−9772.
(12) Wilke, G. Angew. Chem., Int. Ed. 1988, 27, 185−206.
(13) (a) Sacco, A.; Mastrorilli, P. J. Chem. Soc., Dalton Trans. 1994,
2761−2764. (b) Ananikov, V.; Gayduk, K.; Starikova, Z.; Beletskaya, I.
Organometallics 2010, 29, 5098−5102. (c) Dutta, A.; Lense, S.; Hou,
J.; Engelhard, M.; Roberts, H.; Shaw, W. J. Am. Chem. Soc. 2013, 135,
18490−18496.
(14) (a) Aihara, Y.; Chatani, N. J. Am. Chem. Soc. 2014, 136, 898−
901. (b) Song, W.; Lackner, S.; Ackermann, L. Angew. Chem., Int. Ed.
2014, 53, 2477−2480. (c) Chen, X.; Ke, H.; Zou, G. ACS Catal. 2014,
4, 379−385.
(15) Chen, S.; Rousseau, R.; Raugei, S.; Dupuis, M.; DuBois, D.;
Bullock, R. Organometallics 2011, 30, 6108−6118.
(16) (a) Chakraborty, S.; Zhang, J.; Krause, J.; Guan, H. J. Am. Chem.
Soc. 2010, 132, 8872−8873. (b) He, T.; Tsvetkov, N.; Andino, J.; Gao,
X.; Fullmer, B.; Caulton, K. J. Am. Chem. Soc. 2010, 132, 910−911.
(c) Berkefeld, A.; Mecking, S. J. Am. Chem. Soc. 2009, 131, 1565−
1574. (d) Tran, B.; Pink, M.; Mindiola, D. Organometallics 2009, 28,
2234−2243.
(17) (a) Matsumoto, T.; Nagahama, T.; Cho, J.; Hizume, T.; Suzuki,
M.; Ogo, S. Angew. Chem., Int. Ed. 2011, 50, 10578−10580.
G
dx.doi.org/10.1021/om500767p | Organometallics XXXX, XXX, XXX−XXX