Catalytic oxidation of alcohol via nickel phosphine complexes with pendant amines

Article abstract of DOI:10.1021/cs500853f

Nickel complexes were prepared with diphosphine ligands that contain pendant amines, and these complexes catalytically oxidize primary and secondary alcohols to their respective aldehydes and ketones. Kinetic and mechanistic studies of these prospective electrocatalysts were performed to understand what influences the catalytic activity. For the oxidation of diphenylmethanol, the catalytic rates were determined to be dependent on the concentration of both the catalyst and the alcohol and independent of the concentration of base and oxidant. The incorporation of pendant amines to the phosphine ligand results in substantial increases in the rate of alcohol oxidation with more electron-donating substituents on the pendant amine exhibiting the fastest rates. (Chemical Equation Presented).

Full text of DOI:10.1021/cs500853f

Research Article
pubs.acs.org/acscatalysis
Catalytic Oxidation of Alcohol via Nickel Phosphine Complexes with
Pendant Amines
Charles J. Weiss, Parthapratim Das, Deanna L. Miller, Monte L. Helm, and Aaron M. Appel*
Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MS K2-57, Richland, Washington 99352, United
States
S
* Supporting Information
ABSTRACT: Nickel complexes were prepared with diphos-
phine ligands that contain pendant amines, and these complexes
catalytically oxidize primary and secondary alcohols to their
respective aldehydes and ketones. Kinetic and mechanistic
studies of these prospective electrocatalysts were performed to
understand what influences the catalytic activity. For the
oxidation of diphenylmethanol, the catalytic rates were
determined to be dependent on the concentration of both the
catalyst and the alcohol and independent of the concentration of
base and oxidant. The incorporation of pendant amines to the
phosphine ligand results in substantial increases in the rate of alcohol oxidation with more electron-donating substituents on the
pendant amine exhibiting the fastest rates.
KEYWORDS: alcohol oxidation, nickel, electrochemistry, catalysis, proton relay
studied as electrocatalysts for the production7 and oxidation7e,8
INTRODUCTION
The intermittent availability of renewable electrical energy
■
of hydrogen and for the oxidation of formate.9 The design and
function of electrocatalysts for the oxidation of alcohols is
expected to benefit from the incorporation of pendant amines
due to the required movement of protons.
requires an efficient and economical means of energy storage to
enable widespread utilization. One approach to the storage of
renewable energy is the formation of carbon-based liquid fuels,
such as alcohols, from the reduction of CO2. When energy is
needed, the resulting carbon-based fuels, such as methanol and
ethanol, can then be converted back to CO2 (eq 1)
electrochemically using fuel cells.1 The production and
utilization of alcohols for an energy storage cycle will require
the development of effective electrocatalysts for these chemical
transformations.
The focus of the present work is on the design and use of
nickel diphosphine complexes as catalysts for the oxidation of
alcohols, with an emphasis on the effects of incorporation of a
pendant base. Our goal is to understand the factors that
influence the first step (eq 2) of a multistep electrochemical
oxidation of alcohols using these nickel catalysts, as this step
will be critical to developing catalysts for the complete
oxidation shown in eq 1. Due to the observed rates of alcohol
oxidation being too slow to measure by typical diffusive
electrochemical techniques, a chemical oxidant has been
utilized as an electron acceptor in place of an electrode.
Employing this proxy has enabled the use of in operando NMR
spectroscopy to determine catalytic rates and reaction orders
through directly observing reactant consumption and product
formation. The results reported in this work provide the
foundation and understanding needed for subsequent electro-
chemical studies using analogous catalysts.
CH3OH + H2O ⇌ CO2 + 6H+ + 6e−
(1)
Molecular catalysts for the oxidation of alcohols are well-
known;2 however, molecular electrocatalysts for oxidation of
alcohols for fuel storage cycles are much fewer in number.3 To
the best of our knowledge, all of the reported molecular
electrocatalysts for alcohol oxidation require precious metals,3
unlike reported catalysts for the chemical alcohol oxidation
used in organic syntheses2e,i,4 and Meerwein−Ponndorf−Verley
(MPV) catalysts for transfer hydrogenation between alcohols
and ketones or aldehydes.5 For the electrocatalytic production
and utilization of fuels, the movement of both electrons and
protons is critical (eqs 1 and 2). The incorporation of proton
relays in the second coordination sphere of transition metal
complexes facilitates the movement of protons to and from the
metal center, and this approach has been shown to be effective
in the design of electrocatalysts for a variety of trans-
formations.6 First-row transition metal complexes containing
cyclic diphosphine ligands with pendant amines have been
R2CHOH → R2CO + 2H+ + 2e−
(2)
Received: June 18, 2014
Revised: July 21, 2014
Published: July 23, 2014
© 2014 American Chemical Society
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using the oxidation of diphenylmethanol to benzophenone as a
test reaction, with the results shown in Table 1.
RESULTS
■
Synthesis and Characterization. Nickel(II) diphosphine
complexes were prepared by the addition of PR NR′ (1,5-R′-
R2CHOH + 2Et3N + 2Cp*2Fe+
2
2
3,7-R-1,5-diaza-3,7-diphosphacyclooctane) or dcpe (dicyclohex-
ylphosphinoethane) to Ni(CH3CN)6(BF4)2, which generated
2+
NiL2
⎯⎯⎯⎯⎯⎯→ R2CO + 2Et3NH+ + 2Cp*2Fe
(3)
complexes of the formula Ni(PtBu2NR )(CH3CN)n(BF4)2 (n =
2
2−3; R = Ph (1),7h Bn (2),7h tBu (3)), Ni(dcpe)-
(CH3CN)2(BF4)2 (4), and Ni(PR NBn2)2(BF4)2 (R = Cy
Table 1. Measured Turnover Frequencies for the Nickel-
Catalyzed Oxidation of Diphenylmethanol to Benzophenone
2
(5)8f and Ph (6)10), as shown in Figure 1. A single ligand
a
entry
compound
TOF, h−1
1
2
3
4
5
6
[Ni(PtBu2NPh2)(CH3CN)3]2+ (1)
[Ni(PtBu2NBn2)(CH3CN)2]2+ (2)
[Ni(PtBu2NtBu2)(CH3CN)2]2+ (3)
[Ni(dcpe)(CH3CN)2]2+ (4)
[Ni(PCy2NBn2)2]2+ (5)
9.4 3.2
26.8 3.4
114 5.2
5.4 3.4
0.4
[Ni(PPh2NBn2)2]2+ (6)
<0.1
a
Turnover frequencies are the initial rates of alcohol oxidation at 25
0.5 °C in CD3CN with 0.10 equiv of catalyst, 1.9 equiv of Et3N, and
1.9 equiv, thereby resulting in a maximum number of turnovers of 9.5.
Cp*2FeBF4 versus diphenylmethanol. See the Supporting Information
for full experimental details.
The complexes with two PR NBn ligands, 5 and 6, were the
2
2
slowest catalysts in this series, in spite of being active
electrocatalysts for the oxidation of formate to CO2 (with
turnover frequencies of 9.6−12.5 s−1).9 Rates for the oxidation
of diphenylmethanol with these two complexes were found to
improve significantly upon heating the reaction to 55 °C, at
which temperature the rates increased to 3.4 h−1 and 60 h−1 for
R = Ph and Cy, respectively.13 The results with complexes 5
and 6 as well as the higher rates observed with complexes 1−4
are consistent with inhibition of catalysis in the presence of a
second diphosphine ligand. In addition to the higher turnover
frequencies observed for the monodiphosphine complexes, 1−4
(5.4−114 h−1, Table 1), a 10-fold increase in rate is also
observed with increasing the basicity of the pendant amine on
the diphosphine ligand (Table 1, 1−3).14 Furthermore, an
appreciably lower rate is observed when no pendant amine is
present, as indicated by the results with 4.
Figure 1. Nickel complexes used in the electrocatalytic oxidation of
alcohols.
coordinated to the nickel in 1−4 due to the steric encumbrance
of the phosphine substituents. The two new complexes 3 and 4
1
have been characterized by H and 31P NMR spectroscopy, X-
ray crystallography (Figure 2), elemental analysis, and cyclic
voltammetry (see the Supporting Information). Complex 3 has
two coordinated solvent molecules, similar to 2 and distinct
from 1, which was reported with three CH3CN ligands.7h
Complexes 3 and 4 exhibit reversible Ni(II/I) couples and
irreversible Ni(I/0) couples similar to previously reported
compounds 1 and 2.7h
Catalytic Studies. Complexes 1−6 were found to catalyze
the oxidation of primary and secondary alcohols to aldehydes
and ketones using triethylamine (Et3N) and decamethylferro-
cenium (Cp*2Fe+; Cp* = Me5C5)11,12 as the stoichiometric
base and oxidant, respectively, in these model reactions (eq 3).
The catalytic performances of complexes 1−6 were compared
To study the effects of varying steric and electronic
properties of the substrate, experiments were performed with
additional alcohols, as shown in Table 2. Larger substituents on
secondary, aliphatic alcohols resulted in decreased rates for the
oxidation (Table 2, entries 3, 5, and 6), which is consistent with
Figure 2. Thermal ellipsoid plots rendered at 50% probability for the structures of the nickel complexes 3 (left) and 4 (right). Hydrogen atoms,
anions, and noncoordinated solvent have been omitted for clarity.
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The analogous oxidation reactions were also performed with
the primary alcohols methanol, ethanol, and benzyl alcohol (eq
4) using 2 as the catalyst. For methanol, the expected product,
formaldehyde was not observed; however, methylformate was
observed in substoichiometric quantities.16,17 For ethanol,
acetaldehyde was formed, based on the observation of a singlet
Table 2. Oxidation of Secondary Alcohols to Ketones
a
Catalysed by [Ni(PtBu2NBn2)(CH3CN)2]2+ (2)
1
at 9.69 ppm in the H NMR spectrum. However, this product
was quickly converted to ethyl acetate, presumably from either
further oxidation to acetate and subsequent esterification with
ethanol or reaction of two equivalents of acetaldehyde to
generate this four electron oxidized product. In contrast to the
results with ethanol, oxidation of benzyl alcohol (81.4 10.0
h−1) was observed to produce benzaldehyde, with little or no
benzyl benzoate observed. When a similar study is performed
using a 1:1 mixture of benzaldehyde and ethanol, a 2:7 mixture
of ethyl acetate and ethyl benzoate forms.
RCH2OH → RC(O)H → RC(O)OCH2R
(4)
Mechanistic Investigations. To understand what factors
limit the catalytic rates, the oxidation of diphenylmethanol
catalyzed by 2 was studied as a function of the reagent
concentrations, using Et3N and Cp*2FeBF4, as the base and
oxidant, respectively. Increasing the concentration of 2 over the
range of 0.9−7.4 mM resulted in a linear increase in the rate of
oxidation of the alcohol (Figure 3, left), consistent with first-
order dependence on the concentration of the catalyst. The
turnover frequency increased linearly with increasing concen-
tration of diphenylmethanol over the range of 10−218 mM
(Figure 3, right), indicating a first-order dependence on the
concentration of alcohol. In the presence of excess
diphenylmethanol, benzophenone was produced at a constant
rate until all base and oxidant were consumed (Figure 4). This
lack of dependence on the diminishing concentrations of Et3N
and Cp*2FeBF4 indicates that the reaction rate is independent
of the concentrations of these two reagents. No significant
change in initial rate was observed when the oxidation of
diphenylmethanol catalyzed by 2 was examined with ∼4×
higher concentration of Et3N (7.6 mol equiv, 340 mM), further
supporting that the catalytic reaction is independent of base
concentration. Combined with the first-order dependence on
the concentrations of diphenylmethanol and the catalyst, these
results are consistent with the rate law in eq 5.
a
Turnover frequencies are the initial rates of alcohol oxidation at 25
0.5 °C in CD3CN with 0.10 equiv of 2, 1.9 equiv of Et3N, and 1.9
equiv of Cp*2Fe+, thereby resulting in a maximum number of
turnovers of 9.5. See the Supporting Information for full experimental
details. Uncertainty values are two standard deviations calculated
from multiple kinetic runs.
b
a rate-limiting step that involves the binding of the alcohol to
the metal.
To examine electronic effects while minimizing steric
changes, the rate of oxidation of 4′-substituted α-methyl benzyl
alcohols was measured with varying electronic donating and
withdrawing substituents in the para position of the benzene
ring (Table 3). For Br-, H-, and MeO-substituted alcohols
Table 3. Ni(PtBu2NBn2)(CH3CN)2(BF4)2 (2)-Mediated
Oxidation of Secondary Alcohols to Ketones
a
b15
c
entry
substituent (X)
σ-value
TOF, h−1
1
2
3
4
5
-Br
0.26
0.15
28.4 14.8
73.2 20.4
24.4 2.4
-F
-H
0.0
-OMe
-NH2
−0.12
−0.30
25.9 6.4
d
Rateinitial = kobs[2][diphenylmethanol]
(5)
a
Turnover frequencies are the initial rates of alcohol oxidation at 25
The observed rate law is consistent with the binding of the
alcohol limiting the rate due to either slow binding or
unfavorable binding that precedes a rate-determining transition
state. The first-order dependence of the rate on alcohol
concentration was observed over the entire course of the
reaction, in the presence of excess diphenylmethanol (5 times
as much diphenylmethanol relative to the base and oxidant).
However, when equally limiting concentrations of alcohol, base,
and oxidant are used, the apparent rate law changed after ∼50%
conversion (Figure S9). No significant change in rate was
observed in the presence of added benzophenone, suggesting
that the change in rate is not due to product inhibition. Due to
the constant rate of diphenylmethanol oxidation with excess
alcohol, catalyst deactivation is also unlikely. The observed
slowing of the reaction may be due to a more complex rate law
than shown in eq 5, perhaps resulting from competition
between multiple catalytic pathways. One possible source of
additional catalytic pathways is the variable number of solvent
0.5 °C in CD3CN with 0.10 mol equiv of nickel, 1.9 mol equiv of Et3N,
and 1.9 mol equiv of Cp*2FeBF4 versus diphenylmethanol, thereby
b
resulting in a maximum number of turnovers of 9.5. σ-Values quantify
the electronic effects of the para substituent with positive values having
a net electron withdrawing effect and negative values having a net
c
electron donating effect. Uncertainty values are two standard
d
deviations calculated from multiple kinetic runs. Little or no alcohol
oxidation observed.
(Table 3, entries 1, 3, and 4), an average TOF of ∼26 h−1 was
observed with minimal variation, suggesting no significant
electronic effect. Based on the lack of significant variation in the
rates using the Br-, H-, and MeO-substituted alcohols, the
deviation of catalytic rates for F- and NH2-substituted alcohols
is not expected to result from differences in simple electronics.
In the case of the amine-substituted alcohol, the reaction may
be impeded by possible binding of the amine functionality to
the metal center.
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Figure 3. Plot of catalytic rate versus [catalyst] (mM) (left) and versus [diphenylmethanol] (mM) (right) for the Ni(PtBu2NBn2)(CH3CN)2(BF4)2
(2)-catalyzed oxidation of diphenylmethanol to benzophenone at 25 °C with Et3N and Cp*2FeBF4.
molecules coordinated to the metal center7h,18 and therefore
possible variation in the number of alcohols or bases that bind
to the nickel at some point in the catalytic cycle.
In contrast with the first-order dependence on alcohol
concentration for diphenylmethanol, the analogous oxidation of
2-propanol under the same conditions was observed to have a
half-order dependence on the concentration of alcohol over the
entire course of the reaction (see Figure S11). The dependence
of the catalytic rate on the concentration of alcohol varied over
the range of 0.5 to 1 for the studied alcohols.
When iPr2EtN was used as the base for the oxidation of
diphenylmethanol, the catalytic rate with 2 increased to 78.4
7.2 h−1. This rate is approximately three times faster than the
rate with Et3N, despite the observed independence from the
1
concentration of Et3N. The H NMR spectra of 2 in the
Figure 4. Plot of time (s) versus [diphenylmethanol] (M) for the
presence of 3.7 equiv of Et3N (Figure 5) contains resonances
Ni(PtBu2NBn2)(CH3CN)2(BF4)2 (2)-catalyzed (4 mM) oxidation of
for the base that are shifted downfield by 0.07−0.18 ppm,
diphenylmethanol to benzophenone at 25 °C with Et3N, Cp*2FeBF4,
suggesting base binding to the nickel center. The same
and a 5-fold excess of diphenylmethanol versus base and oxidant. The
i
experiment with Pr2EtN results in no shifting of the base
resonances. The 31P NMR spectra of 2 in the presence of Et3N
for the above experiments contain two new species in addition
reaction is complete after 300 s due to complete consumption of base
and oxidant. The linear trend indicates that the excess alcohol has
resulted in a pseudo-zero-order reaction.
1
Figure 5. H NMR (500 MHz) spectra in CD3CN (1.94 ppm) of Et3N (blue) without a nickel complex (bottom) and Et3N (2.0 μL, 1.4 × 10−5
mol) with Ni(PtBu2NBn2)(CH3CN)2(BF4)2 (2) (2.9 mg, 3.8 × 10−6 mol) (top). The downfield shift of the Et3N resonances suggests an interaction
between the amine and the nickel complex.
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to the starting complex, whereas the analogous 31P NMR
spectra in the presence of Pr2EtN contain only the starting
acidity resulting from coordination of the alcohol to the metal
center.
i
complex. These results are consistent with hindered binding of
iPr2EtN due to increased sterics. To test if Et3N displaces one
of the CH3CN ligands in complexes 1−3, Et3N and 1 were
dissolved in CD2Cl2, leading to the liberation of acetonitrile
concurrent with the binding of Et3N (Figure S19). For
comparison to the alkylamine bases, KOH and tBu-P4
phosphazene were also tested and found to be effective for
the oxidation of diphenylmethanol; however, problems with
solubility prevented quantification of catalytic rates using these
bases.
The initial binding of the alcohol appears to be an
unfavorable equilibrium that precedes deprotonation. No
alcohol binding is observed by 31P NMR spectroscopy when
2 and diphenylmethanol are mixed in the absence of base (see
the Supporting Information). The observed reaction order with
respect to alcohol was found to be first-order for
diphenylmethanol, whereas it was approximately half-order
for 2-propanol. The variation in apparent reaction order with
respect to alcohol concentration may be the result of multiple
competing reaction pathways with different orders with respect
to alcohol concentration.
The lack of variation in catalytic rates with different electron
withdrawing and donating substituents on the substrates (Table
3) suggests, if the mechanism proposed in Scheme 1 is correct,
that there is a balance between the alcohol binding strength and
the acidity of the resulting metal-bound alcohol. The more
donating substituents may result in improved binding of the
alcohol to the metal center but would also be expected to yield
a less acidic coordinated alcohol. Based on the lack of variation
for the methoxy- and bromo-substituted alcohols relative to the
unsubstitued equivalent (see Table 3), these two effects appear
to be equally opposed.
Low Temperature Stoichiometric Studies. The addition
of excess benzyl alcohol and Et3N to 1 and 2 at −40 °C
resulted in the formation of nickel hydrides, with resonances
consistent with previous Ni(II)-hydride complexes with two
P2N2 ligands.19 For 2, the resulting hydride was not observed to
be stable when warmed to 25 °C; however, for the hydride
from the reaction of 1 was stable at 25 °C for over an hour. The
analogous reaction with 3 did not result in observable hydride
resonances. An X-ray crystal structure of a tetrahedral complex
of nickel with two PtBu2NPh ligands was obtained (Figure 6),
2
While the rate of oxidation of diphenylmethanol by 2 is
independent of base concentration, the analogous reaction
using the complex without a pendant amine, 4, is approximately
first-order with respect to the concentration of Et3N (see the
Supporting Information). These two results suggest that
deprotonation influences the rate-determining step, but that
the complex without a pendant amine is dependent upon
exogenous base for this step. The presence of the pendant base
appears to be responsible for the observed base concentration
independence for 2.
The subsequent steps are proposed to involve the net
transfer of a hydride from the bound alkoxide to the metal
complex, followed by oxidation and deprotonation to return to
the starting Ni(II) species. The catalytic studies provide very
little information about these steps, given that they are after the
rate-determining step. However, the formation of nickel-
hydride species is supported by the observation of hydrides
in stoichiometric reactions with alcohol and base at low
temperature. This observation is consistent with the mechanism
proposed in Scheme 1.
The pendant base within the ligand appears to play an
important role in the catalytic reaction, as the rates are higher
with the pendant amine and increase with increasing basicity of
the ligand. The pendant amine may be involved in the transfer
of the hydrogen from carbon to nickel through a proton-
coupled electron transfer reaction,9b or the pendant amine may
be involved in the oxidation and deprotonation steps that
convert the proposed hydride back to the Ni(II) complex.21
Regardless of the precise mechanism, the catalytic rate of the
pendant amine containing complexes is independent of the
concentration of either oxidant or base, and so these proton
and electron transfer steps that are later in the cycle do not
appear to be limiting.
Figure 6. Thermal ellipsoid plots of Ni(PtBu2NPh2)2(BF4) rendered at
50% probability. Hydrogen atoms, anions, and noncoordinated solvent
have been omitted for clarity.
but there was no indication of the presence of a hydride in the
crystal structure. Similarly, no hydride resonance was observed
1
in the H NMR spectrum collected by dissolving additional
crystals from the same batch used to obtain the X-ray structure,
suggesting the identity of the complex was the Ni(I) complex,
Ni(PtBu2NPh2)2(BF4), rather than the analogous Ni(II)-hydride.
DISCUSSION
■
The proposed catalytic cycle for the oxidation of alcohols by
complexes 1−3 is shown in Scheme 1. Starting from the upper
left, the first step is proposed as the coordination of the alcohol
to the metal complex, followed by deprotonation to form the
metal-bound alkoxide. Given that trialkylamines are not
typically strong enough bases to deprotonate alcohols,20 the
deprotonation step is proposed to be enabled by the increase in
Summary and Conclusions. Homogeneous nickel phos-
phine complexes were prepared and found to catalyze the
oxidation of primary and secondary alcohols to their respective
aldehydes and ketones. The results of mechanistic studies are
consistent with rate-limiting binding of the alcohol to the metal
center, followed by deprotonation of the metal-bound alcohol.
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Scheme 1. Proposed Catalytic Cycle for the Oxidation of Alcohols Using [Ni(PtBu2NR )(CH3CN)2]2+ (2)
2
1
The addition of pendant amines to the phosphine ligand results
in substantial increases in the rate of alcohol oxidation. While
the precise nature of the rate enhancement by the pendant
amine is not yet clear, variation of substituents at the pendant
amine has a significant effect on upon catalytic rates. These
studies demonstrate that alcohol oxidation is possible using
these complexes, which is an essential first step in developing
electrocatalysts based on this platform. Additional studies are
underway, including the examination of this reaction by
electrochemical methods.
external methanol temperature calibration standard. The H
and 13C NMR spectra are referenced versus tetramethylsilane
(0.00 ppm) using internal CD3CN (1H: 1.94 ppm; 13C:
118.69/1.39 ppm) and CD2Cl2 (1H: 5.32 ppm; 13C: 54.00)
solvent resonances, while 31P NMR spectra are referenced
against a H3PO4 external standard (0.0 ppm).
Electrochemical Procedure. Cyclic voltammetry was
performed with 4−5 mM nickel complex dissolved in 1.0 mL
solution of 0.1 M nBu4NPF6 in CH3CN as the supporting
electrolyte. All potentials were measured with a CH Instru-
ments model 620D or 660C potentiostat and reported versus
Cp2Fe+/0 (0.0 V). Measurements were performed using
standard three-electrode cell containing a 1 mm PEEK-encased
glassy carbon working electrode (Cypress Systems EE040), a 3
mm glassy carbon rod (Alfa) as the counter electrode, and a
silver wire suspended in electrolyte solution and separated from
the analyte solution by a Vycor frit as the pseudoreference
electrode. Prior to the acquisition of each voltammogram, the
working electrode was polished using 0.1 μm γ-alumina (BAS
CF-1050) and rinsed with CH3CN.
EXPERIMENTAL SECTION
■
General Procedures. Syntheses were performed under dry
N2 using a glovebox and standard Schlenk techniques. Protio
solvents were purchased as anhydrous from Alfa-Aesar, VWR,
and Fisher further dried using activated alumina columns and
stored under N2. Acetonitrile-d3 and methylene chloride-d2
(Cambridge Isotope Laboratories, 99+ atom % D) were dried
over CaH2 and degassed under high-vacuum (10−6 Torr)
before being distilled and stored under dry N2 until use.
Paraformaldehyde, 1,2-bis(dicyclohexylphosphino)ethane
(dcpe), tBu-P4 phosphazene, bis(pentamethylcyclopenta-
dienyl)ferrocene, p-benzoquinone, and 1,3-dichloropropane
were purchased from Aldrich and Alfa Aesar and used as
received. All other phosphines were purchased from Strem.
Triethylamine, ethydiisopropylamine, 1,3,5-trimethoxybenzne,
alcohols, ketones, and benzaldehyde were purchased from
Aldrich, degassed under vacuum, and dried over multiple beds
of activated 3 Å molecular sieves (liquids) or by pulling under
vacuum (10−3 Torr) overnight (solids). Compounds Ni-
Synthesis of Ni(PtBu2NtBu2)(CH3CN)2(BF4)2 (3). Ni-
(CH3CN)6(BF4)2 (110 mg, 0.230 mmol) and PtBu2NtBu (183
2
mg, 0.489 mmol) were dissolved in 5 mL of acetonitrile and
stirred for 2 h resulting in a deep red solution. The solution was
filtered and concentrated to approximately 1 mL. The product
was crystallized by ether diffusion at 21 °C to yield deep red
1
crystalline material (24.4 mg, 0.0334 mmol, 14.5% yield). H
NMR (CD3CN, 25 °C): δ 4.07−3.64 (bs, 4H, CH2), 3.51−3.37
(m, 4H, CH2), 1.96 (s, 6H, CH3CN) 1.37 (s, 18H, C(CH3)3),
1.20 (s, 18H, C(CH3)3). 1H NMR (CD3CN, −40 °C): δ 3.92−
3.70 (bs, 4H, NCH2P), 3.56−3.44 (m, 4H, NCH2P), 1.96 (s,
acetonitrile) 1.42 (s, 18H, C(CH3)3), 1.25 (s, 18H, C(CH3)3).
31P{1H} NMR (CD3CN, −40 °C): δ 26.7 (bs). Anal. Calcd For
C26H53B2F8N5NiP2: C, 42.78; H, 7.32; N, 9.59. Found: C,
42.77; H, 7.28; N, 10.58. CV (0.1 M nBu4NPF6 in CH3CN,
scan rate 100 mV/s): E1/2, V vs Fc+/0 (ΔEp, mV) −0.75 (103),
−1.79 (irrev).
22
7h
7h
23
(CH3CN)6(BF4)2, PtBu2NBn
,
2
PtBu2NPh
,
2
PtBu2NtBu
,
2
Ni-
(PtBu2NBn2)(CH3CN)2(BF4)2 (2), and Ni(PtBu2NPh2)-
(CH3CN)3(BF4)2 (1) were synthesized by literature-reported
procedures.7h The identity of ketones, benzaldehyde, and esters
produced in catalytic oxidation reactions were confirmed by
comparing the H and 13C NMR spectra against those of
1
commercially obtained samples or literature values.24 The H,
1
13C, and 31P NMR spectra were collected on a Varian Inova 500
MHz spectrometer at 25 °C unless otherwise indicated. The
temperature for kinetic experiments was determined by an
Synthesis of Ni(dcpe)(CH3CN)2(BF4)2 (4). Ni-
(CH3CN)6(BF4)2 (133 mg, 0.278 mmol) and dcpe (128 mg,
0.303 mmol) were dissolved in 5 mL of acetonitrile and stirred
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ACS Catalysis
Research Article
for 90 min resulting in a yellow solution. The solution was
concentrated to approximately 2 mL, and ether was diffused
overnight at 21 °C to yield yellow crystals (152 mg, 0.197
No alcohol oxidation is observed in the absence of nickel
catalysts. The Ni(CH3CN)6(BF4)2 complex does not catalyze
2-propanol oxidation, indicating that the phosphine ligand is
required for the reaction. No H2 gas has been observed in the
1H NMR spectra of alcohol oxidation reactions in the present
study. Noncatalytic oxidation of 2-propanol to acetone is
observed without chemical oxidant in the presence of 2 and
Et3N. The product of this reaction appears to be paramagnetic
based on the NMR spectra, and the stoichiometry is consistent
with Ni(I), in that only one-half equivalent of alcohol is
oxidized.
Hydride Formation. A 1.0 mL acetonitrile solution of
complexes 1−3 (1.6 × 10−5 mol) and benzyl alcohol (6.0 μL,
5.8 × 10−5 mol) was loaded into an NMR tube sealed with a
rubber septum and placed in a −40 °C NMR spectrometer.
The cold sample was briefly removed from the spectrometer to
inject Et3N (15.0 μL, 1.08 × 10−4 mol), mix by inversion, and
promptly return to the spectrometer at −40 °C. The reactions
were monitored NMR spectroscopy over the course of multiple
hours.
X-ray Structural Analyses. A single crystal of Ni-
(PtBu2NtBu2)(CH3CN)2(BF4)2 (3), Ni(dcpe)(CH3CN)2(BF4)2
(4), or Ni(PtBu2NPh2)2(BF4) was placed on a nylon loop with
Paratone-N oil and then mounted on a Bruker APEX-II CCD
diffractometer for data collection at 140 K for 3 and 100 K for
4. Using Olex2,26 the structure for 3 and Ni(PtBu2NPh2)2(BF4)
was solved with the olex2.solve27 structure solution program
using Charge Flipping. The structure of 4 was solved using
direct methods within the ShelXS program package. Both
crystals were refined with the ShelXL28 refinement package
using Least Squares minimization. All hydrogen atoms were
placed at idealized positions.
1
mmol, 70.9% yield). H NMR (CD3CN, 25 °C): δ 2.31−2.23
(m, 4H, Cy), 2.22−2.12 (m, 4H, Cy), 2.12−2.06 (m, 4H,
CH2P), 2.01−1.89 (m, 8H, Cy), 1.98−1.95 (m, CH3CN),
1.89−1.81 (m, 4H, Cy), 1.79−1.72 (m, 4H, Cy), 1.63−1.27 (m,
20H, Cy). 31P{1H} NMR (CD3CN, 121 MHz 25 °C): δ 98.8
(s). Anal. Calcd For C30H54B2F8N2NiP2(CH3CN): C, 49.40; H,
7.38; N, 5.40. Found: C, 49.05; H, 7.40; N, 5.16. CV (0.1 M
nBu4NPF6 in CH3CN, scan rate 100 mV/s): E1/2, V vs Fc+/0
(ΔEp, mV) −0.95 (76), −1.65 (irrev).
Synthesis of Cp*2FeBF4. Using a similar procedure as
described in the literature,25 bis(pentamethylcyclopenda-
dienyl)ferrocene (0.979 g, 3.00 mmol) and p-benzoquinone
(0.649 g, 6.00 mmol) were loaded into a 250 mL Erlenmeyer
flask in open air and dissolved in 100 mL of diethyl ether. To
the solution was added dropwise 1.7 mL of HBF4(Et2O) to
immediately form a green precipitate, and the solution was
stirred for an additional 10 min before filtering and washing
with diethyl ether (400 mL). The solid was dried under vacuum
to yield a green powder (1.18 g, 2.86 mmol, 95% yield). Anal.
Calcd For C20H30BF4Fe: C, 58.15; H, 7.32. Found: C, 58.32; H,
7.11.
t
Synthesis of Bu2P(CH2)3PtBu2. In a 50 mL Schlenk flask,
t
a solution of Bu2PH (1.6 g, 11 mmol) in 10 mL hexanes was
treated dropwise with nBuLi solution in hexanes (2.5 M, 4.4
mL, 11 mmol) and stirred for 1 h to form a white, insoluble
powder. The solvent was removed under vacuum, and the
white product was dissolved in 10 mL of THF. The yellow
solution was cooled to −78 °C, and 1,3-dichloropropane (0.61
g, 5.4 mmol) was added dropwise. After 15 min, the solution
was allowed to warm to room temperature and stir for an
additional 17 h. The solvent was removed under vacuum, and
the product was extracted with 30 mL hexanes and filtered.
Removal of solvent yielded a pale yellow, viscous oil (1.41 g,
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, control experiments, kinetic data,
NMR spectra, and crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
4.24 mmol, 79% yield). H NMR (C6D6, 23 °C, 300 MHz): δ
2.13−1.95 (m, 2H, CH2CH2CH2), 1.68−1.58 (m, 4H, PCH2),
1.25−1.17 (m, 36H, (CH3)3CP). 31P{1H} NMR (C6D6, 23 °C,
121 MHz): δ 26.3 (s).
AUTHOR INFORMATION
Corresponding Author
*E-mail: aaron.appel@pnnl.gov.
■
Typical Kinetic Procedure. A 1.0 mL CD3CN solution of
catalyst (4.2 × 10−6 mol), alcohol (4.2 × 10−5 mol),
Cp*2FeBF4 (32 mg, 7.7 × 10−5 mol), and 1,3,5-trimethox-
ybenzene internal standard (10−20 mg) was loaded into an
NMR tube and sealed with a rubber septum. Triethylamine (11
μL, 7.8 × 10−5 mol) was injected into the NMR tube, and the
contents were mixed by inversion before the tube was promptly
inserting into a 25.0 0.5 °C NMR spectrometer. Single-scan
1H NMR spectra were collected at regular intervals with no
spinning to minimize crystallization of generated Cp*2Fe on
the NMR tube wall. Kinetic measurements were obtained by
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. John C. Linehan, Prof. Elliott B. Hulley, Dr.
Jonathan M. Darmon, and Dr. Elizabeth L. Tyson for helpful
discussions. Research by C.J.W., P.D., D.L.M., and A.M.A. was
supported by the US Department of Energy, Office of Basic
Energy Sciences, Division of Chemical Sciences, Geosciences &
Biosciences. Research by M.L.H. was supported as part of the
Center for Molecular Electrocatalysis, an Energy Frontier
Research Center funded by the U.S. Department of Energy,
Office of Science, Basic Energy Sciences. Pacific Northwest
National Laboratory is operated by Battelle for the US
Department of Energy.
1
integrating product or alcohol H NMR resonances relative to
the 1,3,5-trimethoxybenzene internal integration standard.
Initial rates were measured by linear fit of alcohol consumption
at the start of the reaction. Turnover-frequencies were
calculated using the following eq 6 and reported as the average
of multiple experiments. Uncertainties are two standard
deviations calculated from three or more experiments.
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■
Alcohol Oxidation (mol/s)
TOF (h−1) =
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