Exclusively Ligand-Mediated Catalytic Dehydrogenation of Alcohols

Article abstract of DOI:10.1021/acs.inorgchem.6b01310

Design of an efficient new catalyst that can mimic the enzymatic pathway for catalytic dehydrogenation of liquid fuels like alcohols is described in this report. The catalyst is a nickel(II) complex of 2,6-bis(phenylazo)pyridine ligand (L), which possesses the above requisite with excellent catalytic efficiencies for controlled dehydrogenation of alcohols using ligand-based redox couple. Mechanistic studies supported by density functional theory calculations revealed that the catalytic cycle involves hydrogen atom transfer via quantum mechanical tunneling with significant k_H/k_D isotope effect of 12.2 ± 0.1 at 300 K. A hydrogenated intermediate compound, [Ni^IICl₂(H₂L)], is isolated and characterized. The results are promising in the context of design of cheap and efficient earth-abundant metal catalyst for alcohol oxidation and hydrogen storage.

Full text of DOI:10.1021/acs.inorgchem.6b01310

Article
pubs.acs.org/IC
Exclusively Ligand-Mediated Catalytic Dehydrogenation of Alcohols
Debabrata Sengupta,† Rameswar Bhattacharjee,Δ Rajib Pramanick,† Santi Prasad Rath,†
Nabanita Saha Chowdhury,† Ayan Datta,*,Δ and Sreebrata Goswami*,†
†Department of Inorganic Chemistry and ΔDepartment of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur,
Kolkata 700 032, India
S
* Supporting Information
ABSTRACT: Design of an efficient new catalyst that can mimic the
enzymatic pathway for catalytic dehydrogenation of liquid fuels like
alcohols is described in this report. The catalyst is a nickel(II) complex
of 2,6-bis(phenylazo)pyridine ligand (L), which possesses the above
requisite with excellent catalytic efficiencies for controlled dehydrogen-
ation of alcohols using ligand-based redox couple. Mechanistic studies
supported by density functional theory calculations revealed that the
catalytic cycle involves hydrogen atom transfer via quantum mechanical
tunneling with significant kH/kD isotope effect of 12.2 0.1 at 300 K. A
hydrogenated intermediate compound, [NiIICl2(H2L)], is isolated and
characterized. The results are promising in the context of design of cheap and efficient earth-abundant metal catalyst for alcohol
oxidation and hydrogen storage.
above proposition further. Engineering of metal complex
catalysts of suitable azo aromatic ligands for alcohol
dehydrogenation is evidently a challenge, which is the primary
objective of this work. Some representative examples of
catalytic alcohol dehydrogenation are shown in Scheme 1.
Herein, we wish to disclose a model nickel catalyst of a
pyridyl containing bis-azo aromatic pincer ligand to address the
issue of reversible hydrogen storage via selective partial alcohol
dehydrogenation using azo−hydrazo couples. In the present
context it is worth noting that selective partial dehydrogenation
of alcohols is presently emphasized because of no CO2 or CO
emission. In the overall process, the two azo functions of the
ligand are found to be operative in the dehydrogenation
reaction, while the metal ion remains as a spectator that acts
only as a template to hold the ligand(s) and substrate. The key
point in this metal redox free alcohol dehydrogenation is that
the model catalyst is capable of dehydrogenation of both
primary and secondary alcohols following galactose oxidase
pathway and also generates a genuine prospect24,25 toward the
application of this class of systems in energy generation.
INTRODUCTION
■
A continuous increase in demand for clean and renewable
energy has reintroduced an interest in hydrogen as a form of
chemical energy, although storage of hydrogen is a demanding
task.1−5 Consequently as a genuine alternative, liquid organic
hydrogen carriers like alcohols are now under scan because of
their easy storage, transport, and high hydrogen content.2,6 In
this context, development7 of base metal complex as catalysts
for selective dehydrogenation of alcohols is of interest. During
the past few years, there have been attempts of using transition-
metal complexes of redox-active ligands as dehydrogenation
catalysts, where the transition metal hydride is a key
intermediate for dehydrogenation reaction, and the respective
ligand acts as a supporting backbone.8−10 Recent investigations
by Milstein, Beller, and others on the metal complexes bearing
cooperative pincer ligands have led to the generation of
efficient catalysts for dehydrogenation reaction.7,11−16 They
have shown that the PNP/PNN ligand serves as a cooperative
ligand and that hydrogen can be added reversibly across the
M−N bond.10,12 Grutzmacher et al. have introduced another
̈
novel ligand-based protocol,17−19 which resulted in successful
oxidation of methanol into CO2 and H2 using Ru or Ir metal
RESULTS AND DISCUSSION
■
complex catalysts. In the Grutzmacher catalyst, dehydrogen-
̈
Catalyst Design and Isolation. The catalyst here is a
distorted octahedral nickel(II) complex, [NiIICl2L(H2O)] (1),
as shown in Figure 1, which consists of a tridentate N,N,N-
donor ligand (L = 2,6-bis(phenylazo)pyridine), two chlorides
(Cl−), and a H2O molecule in the coordination sphere. The
compound is paramagnetic with two unpaired spins, μeff (300
K) = 2.45 μB. The ligand was chosen because of its known
ation of methanol is brought about via complete reduction (4e−
+ 4H+) of the two imine chromophores of the ligand. Another
interesting protocol of using nitroxyl radical or azodicarboxylate
as cocatalyst has developed high promise for copper-catalyzed
aerobic dehydrogenation.20,21 The above propositions have
generated genuine possibilities22,23 of designing complex
catalysts of suitable ligands that will be capable of shifting the
overall redox exclusively from metal to ligand center. As an
outcome, it is envisaged21 that the use of azo−hydrazo couple
(2e− + 2H+) would be a potential candidate to perform the
Received: May 31, 2016
© XXXX American Chemical Society
A
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a
Scheme 1. Different Classes of Compounds Utilized in Dehydrogenation Reactions of Alcohols
a
Some represented examples are shown, Type I: Baeckvall, J.-E. Modern Oxidation Methods, 2nd ed.; Wiley-VCH, 2010. Type II: Gunanathan, C.;
Milstein, D. Chem. Rev. 2014, 114, 12024−12087.
Figure 1. (a) Line drawing of the ligand 2,6-bis(phenylazo)pyridine (L) and the synthesis of the catalyst, 1. (b) Molecular view of the catalyst, 1.
Selected bond lengths (Å): Ni1−Cl1, 2.335(3), Ni1−O1, 2.101(4), Ni1−N1, 2.303(4), Ni1−N3, 1.998(4), N1−N2, 1.256(4).
ability26 to transfer multiple electrons and protons, and the
catalyst design was for a labile system to enable coordination of
incoming substrates in the intermediate steps. Its synthesis
involved a chemical reaction between equimolar quantities of L
and hydrated nickel chloride salt in tetrahydrofuran (THF). A
dark greenish-brown product of 1 was obtained in a nearly
quantitative yield (isolated yield, 90%). Usual chemical
characterizations reveal that the compound is a mixed ligand
nickel(II) complex of above composition. Synthesis of 1 and its
complete characterization are collected in Experimental
Section.
Single-crystal X-ray analysis of the nickel complex has
revealed that it is an octahedral complex, which is shown in
Figure 1b. The tridentate ligand L binds Ni(II) in a mer fashion
using a pair of azo-N and a pyridine-N; two coordinated
chloride ligands are mutually trans. A water molecule occupies
the sixth coordination site. The dN−N bond length in the
complex is 1.256(4) Å and indicative of neutral azo- (−N
N−) coordination.26 The NN stretching frequency in the
compound 1 appeared at 1398 cm−1, which also supports27 the
neutral form of the azo chromophores of the coordinated
ligand.
Cyclic voltammetric experiment of the complex 1 in
dichloromethane solution using tetrabutylammonium perchlo-
rate (TBAP) as supporting electrolyte exhibits four single
electron transfer at −0.05, −0.28, −1.04, and −1.3 V (Figure
S1) versus the Ag/AgCl reference electrode. The wave at the
least cathodic potential is reversible, whereas other responses
are quasi-reversible. The cyclic voltammogram of the free L
showed26 two reductions due to L → [L●]− → [L●● 2−
]
at
−1.07 and 1.39 V, successively. Two more possible reductions,
[L
●● 2− → [L●]3− → [L]4−, were not however observed under
]
our experimental conditions.
Catalysis and Mechanism. It is well-known28−32 that the
metal complexes of neutral azo ligands are susceptible to
reductions and can be utilized to perform several unusual
chemical reactions. We thus set out to study aerial catalytic
oxidation of alcohols using the above complex. The complex 1
was found to bring about partial dehydrogenation of both
primary and secondary alcohols catalytically into their
corresponding aldehydes or ketones, respectively, in low
catalyst loading (0.1−0.5 mol %). In a typical reaction, a
mixture of 1 mmol of alcohol, 1 × 10−3 mmol of catalyst (1 mL
from 100 mL stock solution containing 43 mg of catalyst), 1 ×
B
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10−3 mmol of KtBuO (1 mL from 100 mL stock solution
containing 11 mg of KtBuO), and 1.5 mmol of zinc was stirred
in 10 mL of dry THF at room temperature under a positive
pressure of oxygen (using oxygen-filled balloon; see Table 1).
The stirring was continued for 2−4 h depending upon the
substrate. Table 2 collects the yields of corresponding oxidized
products.
Table 2. [NiIICl2L(H2O)] (1) Catalyzed Oxidation of
Alcohols into Ketones/Aldehydes
ab
,
Table 1. Optimization of [NiIICl2L(H2O)] (1) Catalyzed
a
Oxidation of 1-Phenylethanol into Acetophenone
catalyst
base
time (h)
isolated yield (%)
1
1
1
1
KtBuO
3
3
3
3
3
3
3
81
76
75
73
b
NaOH
b
NaOMe
c
d
KtBuO
NR
c
d
free ligand
NiCl2
KtBuO
NR
c
d
KtBuO
NR
a
Reaction condition: [1] = 0.001 mmol, [substrate] = 1 mmol,
[KtBuO] = [catalyst,1], Zn dust = 1.5 mmol (1.5 equiv with respect to
substrate), 10 mL of solvent (dried). The reactions were studied in
b
positive pressure of O2, DMSO was added for a making clear
c
d
solution. [KtBuO] = 0.001 mmol. NR = No reaction.
To understand the courses of the above oxidation reaction,
kinetics measurements on the dehydrogenation of 1-phenyl-
ethanol were made as a function of the catalyst concentration
using KtBuO as a base in a catalytic quantity. Increase in the
concentration of 1 over the range from 0.8 × 10−4 to 1.5 × 10−4
times with respect to alcohol resulted in a linear rise in the rate
kobs for oxidation of alcohol (Figure S2). Moreover, the kobs
values were found to be dependent linearly on the substrate
concentration as well over a range of 80−150 times of alcohol
with respect to catalyst (see Figure S3). Thus, the rate law for
the above dehydrogenation reaction is as in the following eq 1,
which indicates alcohol (substrate) binding to the catalyst at
the rate-limiting state.21,33
rate = k[1][1‐phenylethanol]
(1)
The reaction does occur in the absence of base. However, it is
found that addition of catalytic quantity of base augments the
yields of the products by ∼5−10% (Table 1). It may only be
due to proton abstraction from alcohol to initiate the first
catalytic cycle. In subsequent cycles the reaction proceeds via
proton abstraction by one of the intermediates (see later). The
catalytic cycle began with single electron reduction of the
catalyst by zinc metal. To quantify the single electron transfer
step, stoichiometric reactions in an inert atmosphere were
monitored by mixing equimolar quantities of 1 and 1,1-
diphenyl carbinol and varied amounts of cobaltocene (CoCp2),
0.25−1.5 equiv with respect to 1. A maximum yield of
benzophenone was obtained when equimolar quantities of 1
and CoCp2 were used.
To trap an intermediate, if any, a stoichiometric reaction
between the compound 1 and 2-propanol in dry THF medium
using 1.5 equiv of zinc metal in an inert condition was
attempted. An intense blue compound 2 with S = 1 (μeff
(300K) = 2.35 μB) was isolated, which upon crystallization
yielded fine crystals. The compound is sensitive to oxidation;
reversal of 2→1 is fast and quantitative in positive pressure of
oxygen (Figure S5). Generation of H2O2 from the above
reaction is identified chemically from a bulk experiment in air
a
Reaction condition: [1] = 0.001−0.005 mmol, [substrate] = 1 mmol,
[KtBuO] = [catalyst, 1], [Zn dust] = 1.5 mmol (1.5 equiv with respect
to substrate), 10 mL of solvent (dried). The reactions were studied in
b
positive pressure of O2. Details of reaction condition of each reaction
c
are collected in Supporting Information, Table S2. Isolated yield.
d
e
GCMS yield. Detected from NMR spectroscopy from the reaction
mixture; due to high volatility absolute yield of the product is not
obtained.
(see Supporting Information for details and Figure S6). X-ray
structure determination of 2 revealed a distorted trigonal
bipyramidal geometry that consisted of two axial chlorine (Cl)
coordination along with an azo−hydrazo tridentate coordina-
tion of the ligand (Figure 2b). Most notable part of this
complex is that one of the two azo functions of the ligand L in
catalyst 1 underwent hydrogenation to produce an unsym-
metrical ligand containing an azo and a hydrazo function.
Accordingly the two dN−N bond lengths are entirely different:
C
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Figure 2. (a) Isolation of complex 2 in deaerated condition. (b) Molecular view of the complex 2. Selected bond lengths (Å): Ni1−Cl1, 2.212(2),
Ni1−Cl2, 2.230(2), Ni1−N1, 2.310(6), Ni1−N3, 2.015(5), Ni1−N5, 2.411(5), N1−N2, 1.270(8), N4−N5, 1.402(8). (c) Isotope labeling
experiments: N−H stretching frequencies of complex 2, isolated from cyclohexanol-H (blue line) and cyclohexanol-D (red line).
1.402(8) Å, for hydrazo30 and 1.270(8) Å for azo groups,26
respectively (Figure 2). The hydrazo nitrogens are sp3
hybridized, whereas that of the azo function is sp2 hybridized.
IR spectrum of 2 isolated from cyclohexanol-H substrate
displayed νN−H at 3220 and 3070 cm−1.34 To ascertain the
source of hydrogen in the hydrazo function, an isotope labeling
experiment was performed similarly using cyclohexanol-D in
place of cyclohexanol-H. A similar compound was isolated,
which showed two νN‑D stretches at 2335 and 2205 cm−1
(Figure 2c).34 So we conclude that dehydrogenation of alcohol
in the above catalytic cycle proceeds with subsequent
hydrogenation of one of the azo functions of coordinated L.
The experimental data on reaction mechanism has been
supported well by DFT calculation (see below).
−1.1 kcal/mol, resulting in an activation electronic energy
barrier of 0.4 kcal/mol. Therefore, TS1 is an early transition
state and possesses minimal structural distortion. In the
optimized geometry of the intermediate VI the bond length
dN1−N2 = 1.32 Å is significantly longer than dN4−N5 = 1.25 Å
indicating an azo−anion radical and neutral azo oxidation
states, respectively.26 That the unpaired electron is localized
primarily on azo−anion radical function bearing N1−N2 is also
verified by natural bond orbital analysis (see Experimental
Section for details, Figure S23). Experimentally, the electron
paramagnetic resonance (EPR) spectrum of the chemically
reduced intermediate [1]− showed an isotropic spectrum
(Supporting Information, Figure S4). In the subsequent steps,
hydrogen atom transfer (HAT) from the β-C−H of the
attached 2-propanol33,40 to the adjacent electron-rich nitrogen
atom (N1) of the azo group occurs with the fast release of
acetone, which has a barrier of 25.7 kcal/mol (TS2). This rate-
limiting step is computed to be highly exergonic (ΔG = −15.4
kcal/mol). The structure of TS2 is depicted in Figure 3, and the
distance of the migratory hydrogen atom from both carbon and
nitrogen is found to be similar ∼1.32 Å. This HAT step is
expected to have a significant contribution from quantum
mechanical tunneling (QMT) under ambient conditions, as the
barrier is rather narrow.41 For verification with the experimental
data, we also computed kinetic isotope effect (KIE) for
cyclohexanol. The free energy of activation in the latter case is
25.3 kcal/mol, which clearly indicates that replacement of 2-
propanol by cyclohexanol does not alter the mechanistic
pathway discussed above. Experimentally the observed cyclo-
hexanol-H/cyclohexanol-D KIE is 12.2 0.1 at 300 K, which is
in excellent agreement with a QMT model (TST/Eckart
computed KIE = 12.9 using TheRate program42) and is
significantly larger than the transition-state theory (TST;
computed classical KIE, 4.2). The effect is more prominent at
DFT (SMD35/M0636/TZVP,37 LANLTZ(f)38 (Ni), imple-
mented in Gaussian 09,39see Supporting Information for
details) calculated ground-state structures for the two isolated
complexes, 1 and 2, match well with the experimental values
(see Supporting Information, Figures S18 and S19). The first
step is a single electron reduction (intermediate I →
intermediate II). Notably, the Ni−Cl bond lengths are
elongated in II (Ni1−Cl1 = 2.49 Å, Ni1−Cl2 = 2.36 Å) and
in 1 (Ni1−Cl1 = 2.40 Å, Ni1−Cl2 = 2.32 Å). The next step is
dissociation of one Cl− from the anionic complex, [1]−
resulting in a penta-coordinated complex, III. The formation
of III is though highly endergonic in the gas phase (ΔG = 33.5
kcal/mol); it becomes less endergonic (ΔG = 7.7 kcal/mol) in
THF. Experimentally, the yield of products in hexane and
toluene is significantly lower than that in THF (Table S2). In
the first cycle, the intermediate complex III undergoes
deprotonation by the external base, tert-butoxide, to form of
a relatively stable intermediate VI via an intermediate complex
V. The total distortion energy associated with IV → TS1 is 1.5
kcal/mol, while the interaction energy in TS1 is found to be
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Figure 3. Computed reaction profile with Arrhenius plot. (a) Gibb’s free energy profiles computed for the key steps involved in the catalytic cycle.
The free energy changes (in kcal/mol) obtained from both the condensed phase (ΔGsol) and gas phase (ΔGgas). (b) DFT-computed optimized
geometries of all the transition states involved in the reaction. (c) Arrhenius plot for primary KIE for HAT from cyclohexanol to the azo group of
compound 6 without QMT effects (TST calculations, black dash line) and with QMT corrections (Eckart corrections for TST calculations, red solid
line). The experimental KIE data at T = 283, 293, 300 K is shown as a blue filled circle.
lower temperatures as is evident in the curvature of Arrhenius
plot of the KIE (see Figure 3c). To confirm it further, two sets
of experiments were performed at 283 and 293 K; the observed
KIE are 17.76 0.2 and 14.42 0.25, which are in excellent
agreement with TST/Eckart calculated KIE = 18.7 and 14.9,
respectively.43 Hydrogen atom transfer via five-membered
chelate ring in GO model complex was first demonstrated33
by Tolman and others. The rate-determining step for such
transfer is associated with large KIE. The following steps in the
catalytic cycle involve oxidation of the ketyl radical with
subsequent rejection of ketone. We propose that the above
oxidation process is intramolecular and brought about by the
concomitant reduction of the other unreduced azo function of
L (intermediate VII).33,44,45 Intramolecular oxidations of ketyl
radicals in galactose oxidase (GO) model has been previously
established.46 To support our proposition further DFT studies
on competing pathways like intramolecular versus intermo-
lecular oxidation and the reduction of the unreduced azo
function versus that of the nickel(II) center were also
considered. The possibility of intermolecular oxidation is
ruled out, as the ketyl radical is unstable along the potential
energy surface and should, therefore, have at best fleeting
existence (Supporting Information, Movie). A representation of
singly occupied molecular orbital of VII, illustrated in Figure
S21, shows the unpaired electron is delocalized over azo
functions with a minimal contribution of Ni d-orbital.
Intramolecular oxidation of ketyl radical with concomitant
reduction of unreduced azo function is expected to be the
preferred pathway. Alcohol oxidation by the use of the model
complexes primarily of copper(II) have been studied
extensively; however, the primary difference in the present
case is that hydrogen atoms of alcohol are added across the
coordinated azo function resulting in the formation of a
hydrazo complex.33,47,48
Participation of an azo function in catalytic alcohol oxidation
was proposed primarily by Marko et al. using azo-dicarboxylate
́
as a cocatalyst.20,48 This example, as far as we know, is the most
prominent proof (achieved by isolation of a hydrazo
intermediate) of the involvement of azo−hydrazo couple in
alcohol dehydrogenation. Oxidation of the intermediate VII by
aerial oxygen results in the azo−hydrazo intermediate VIII
through an endergonic process (ΔG = 21.9 kcal/mol). The
subsequent cycle begins with the addition of another alcohol to
VIII in a stepwise fashion to produce the dehydrogenated
penta-coordinated complex (XII). The intermediate complex 2
is crystallized from the mixture as NiCl2(LH2) by a simple
substitution of alkoxide in XII by Cl−. In the final step,
E
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oxidation of XII regenerates the catalyst with the formation of
H2O2 (ΔG = 1.0 kcal/mol).
Thus, the complete catalytic cycle, based on the experimental
data and supported by DFT calculation, is as in Figure 4. Here
CONCLUSION
■
The present results on combined experimental and theoretical
studies highlight the use of azo−hydrazo (2e− + 2H+) couple in
pure ligand-mediated catalytic dehydrogenation of alcohols.
Metal ion of the catalyst behaves innocently and acts as a
template. This protocol is a classic shift from the conventional
ones in many respects and has generated scopes of designing
efficient catalysts with earth-abundant 3d metal ions for their
possible application in DAFCs. Mechanistic studies have clearly
established that the dehydrogenation process proceeds via H
atom transfer, which is more similar to what happens in
enzymatic GO pathway. Our work on similar dehydrogenation
reaction using a closely related zinc complex shows very high
prospect, which will be reported in due course.
EXPERIMENTAL SECTION
■
Materials. [Ni(H2O)6]Cl2 was purchased from Merck, India.
Different substituted alcohols were purchased from Sigma-Aldrich and
Arora-Matthey Limited. All other reagents and chemicals were
obtained from commercial sources and used without further
purifications. Solvents were dried and deoxygenated before use.
Tetrabutylammonium perchlorate was prepared and recrystallized as
reported earlier.49 Caution! Perchlorates have to be handled with care
and appropriate safety precautions.
Physical Measurements. A PerkinElmer Lambda 950 spectro-
photometer with Unisoku CoolSpeK UV USP-203-A attachments was
used to record variable-temperature UV−vis spectra. Infrared spectra
were obtained using a PerkinElmer 783 spectro-photometer. 1H NMR
spectra were recorded on a Bruker Avance 400 MHz or 500 MHz
spectrometer, and SiMe4 was used as the internal standard. A
PerkinElmer 240C elemental analyzer was used to collect micro-
analytical data (C, H, N). All electrochemical measurements were
performed using a PC-controlled PAR model 273A electrochemistry
system. Cyclic voltammetric experiments were performed in dichloro-
methane solution containing supporting electrolyte, 0.1 M Bu4NClO4
under the nitrogen atmosphere using a Ag/AgCl reference electrode, a
Pt disk working electrode, and a Pt wire auxiliary electrode. E1/2 for the
ferrocenium−ferrocene couple under our experimental conditions was
0.39 V. X-band EPR spectra were recorded with a JEOL JES-FA200
spectrometer. Room-temperature magnetic moment measurements for
complexes 1 and 2 were performed with Gouy balance (Sherwood
Scientific, Cambridge, U.K).
X-ray Crystallography. Crystallographic data for compounds 1
and 2 were collected in Table S1. Suitable X-ray quality crystals of
complexes 1 and 2 were obtained by the slow diffusion of a
dichloromethane solution of the complex into hexane. All data were
collected on a Bruker SMART APEX-II diffractometer, equipped with
graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å), and were
corrected for Lorentz polarization effects. Data for 1: a total of 10 954
reflections were collected, of which 1744 were unique (Rint = 0.050),
satisfying the I > 2σ(I) criterion, and were used in subsequent analysis.
Data for 2: A total of 20 125 reflections were collected, of which 3031
were unique (Rint = 0.164). The structures were solved by employing
the SHELXS-97 program package50 and were refined by full-matrix
least-squares based on F2 (SHELXL-97).51 All hydrogen atoms were
added in calculated positions.
Figure 4. Proposed mechanism for the alcohol dehydrogenation
reaction.
the cycle is entirely controlled by the ligand redox, without any
mass loss and no involvement of metal redox. The individual
steps of the catalytic cycle along with their optimized structures
are collected in Supporting Information (Figure S22).
Furthermore, the above catalytic cycle also is operative with
equal ease in the presence of 1,4-benzoquinone as an
oxidant.16,18,24 Thus, catalytic dehydrogenation of 1,1-diphenyl
carbinol was successfully achieved (in 80% yield) in deaerated
conditions. The two products benzophenone and 1,4-hydro-
quinone both were detected by gas chromatography mass
spectrometry (GCMS; Figure S7). To quantify the oxidative
equivalent of the oxidant we used 2 mol equivalent of
benzoquinone to perform a catalytic reaction in deaerated
conditions. GCMS analysis of the resultant products indicated
that 0.5 mol equivalent of benzoquinone remained unused and
1.5 mol equivalent of 1,4-hydroquinone was formed.
In an attempt of decoupling of electron(s) and proton(s)
transfer events (direct alcohol fuel cell (DAFC) require-
ment)8,24 we successfully performed the dehydrogenation
reactions of the compound 2 using ferrocenium hexafluor-
ophosphate (FcPF6) as the oxidant in an alkaline medium (see
Supporting Information for details). The product benzophe-
none was isolated in 72% yield after 4 h of stirring with 5 mol %
catalyst loading under deaerated conditions.
Synthesis. The ligand L was synthesized52−54 and purified by
following a procedure slightly modified from that for 2-(phenylazo)-
pyridine; 2,6-diaminopyridine was used in place of 2-aminopyridine in
a 1:2 molar proportion. 2,6-Diaminopyridine (1 g) was dissolved in 20
mL of pyridine and mixed with 2 g of nitrosobenzene and 10 mL of
60% aqueous sodium hydroxide. The reaction mixture was refluxed for
12 h. The dark red solution was cooled, diluted with water, and
extracted with toluene. The crude mass, obtained by evaporation of
the solvent in vacuum, was purified by preparative thin-layer
chromatography (TLC) using toluene as eluent. Yield: 30%.
Synthesis of [NiIICl2L(OH2)], 1. 287 mg of L, dissolved in moist
THF, was added dropwise to a wet THF solution containing 237 mg
F
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of [Ni(H2O)6]Cl2 with constant stirring for 4 h. During this period,
the color of the solution changed from orange to dark green. The
crude mass, obtained by evaporation of the solvent in vacuum, was
purified by fractional crystallization from dichloromethane and ether
solvent mixture. The precipitate was recrystallized by slow diffusion of
a dichloromethane solution of the compound into hexane. Its yield and
characterization data are as follows: Green colored solid. Yield: 90%.
Anal. Calcd for C17H15Cl2N5NiO: C, 46.95; H, 3.48; N, 16.10, Found
C, 46.76; H, 3.50; N, 16.05%. UV−vis (CH2Cl2): λ [nm] (ε, M−1
cm−1) = 255(17 320), 325(15 100), 395(13 180). IR (KBr disk, cm−1):
ν (NN): 1398 cm−1.
the reaction mixture. The separated aqueous layer was then acidified
with H2SO4 to pH = 2 to stop further oxidation. To it, 1 mL of a 10%
solution of KI and three drops of a 3% solution of ammonium
molybdate were added. Hydrogen peroxide oxidizes I− to I2, which
−
reacts with excess I− to form I3 according to the following chemical
reactions: (i) H2O2 + 2I− + 2H+ → 2H2O + I2, (ii) I2(aq) + I− → I3−
(Figure S6).55−57 The reaction rate was slow but increases with
increasing concentrations of acid; ammonium molybdate solution
catalyzes the oxidation reaction.
Catalytic Reactions Using Benzoquinone as the Oxidant. In a
round-bottom flask containing 1 mmol of 1,1-diphenyl carbinol in dry
THF medium was mixed with 0.001 mmol of catalyst, 0.001 mmol of
KtBuO, and 2 mmol of benzoquinone. The mixture was stirred for 4 h
at 300 K in an inert atmosphere glovebox and then was extracted with
ethyl acetate. After evaporation of the solvent the residue was
dissolved in acetonitrile and subjected to GCMS analysis. Figure S7
shows the chromatogram, which identifies both benzophenone and
hydroquinone (generated from benzoquinone) conclusively. GCMS
analysis indicated that 0.5 mol equiv of benzoquinone remained
unused and that 1.5 mol equiv of 1,4-hydroquinone was detected.
Catalytic Reactions using Ferrocenium Hexafluorophos-
phate as Oxidant. In a round-bottom flask containing 1 mmol of
1,1-diphenyl carbinol in dry THF medium was mixed with 0.05 mmol
of catalyst, 0.05 mmol of KtBuO, and 3.5 mmol of ferrocenium
hexafluorophosphate. The mixture was stirred for 4 h at 300 K in an
inert atmosphere glovebox and then was extracted with ethyl acetate.
After evaporation of the ethyl acetate, the residue was dissolved in
acetonitrile, filtered through silica gel, and subjected to GCMS
analysis. Up to 72% of benzophenone was detected by GCMS.
Computational. All geometry optimization and vibrational
frequency calculations were performed in the gas phase at M0636/6-
31+G(d),58 LANL2DZ (Ni)59 level of theory using Gaussian 09.39 For
simplicity this method is represented as BS1. For complexes having
unpaired electrons, the unrestricted version of DFT was employed.
The stationary point is confirmed by the absence any imaginary
frequency, and the transition states were confirmed by one unique
imaginary frequency. For further verification of the transition state,
intrinsic reaction coordinate (IRC) calculations were performed.
Additional single-point calculations (at the BS1 structures) for the full
catalytic cycle were performed using the M06/TZVP,37 LANLTZ(f)
(Ni)38 for better estimation of electronic energy, which we represent
as BS2. The role of solvation by THF was explored by an implicit
PCM solvation model (SMD).35 All free energy calculations were
performed at SMD/BS1//BS2 incorporating zero point energy (ZPE)
correction at 298 K.
General Procedure for Catalysis. The catalytic reactions were
performed following a general procedure. In a round-bottom flask
containing 1 mmol substrate in dry THF medium was mixed with
0.001 mmol of catalyst (1 mL from 100 mL of stock solution
containing 40 mg of catalyst), 0.001 mmol of KtBuO (1 mL from 100
mL stock solution containing 11 mg of KtBuO) and 1.5 mmol of zinc
was stirred at 300−323 K in the presence of continuous bubbling of
oxygen (using oxygen-filled balloon). The stirring was continued for
2−4 h depending upon substituent. The crude product, thus obtained,
was purified on preparative alumina TLC plate using hexane as eluent.
Higher concentration of catalyst and relatively higher temperatures
and reaction time were needed for primary alcohols. The yields are
collected in Table 2, and spectral characterizations of the products are
as follows:
Isolation of [NiCl2(LH2)], 2. In a glovebox, 1 mmol of the complex 1
was mixed with the equimolar quantity of 2-propanol and 1.5 mmol
zinc in dry and deoxygenated THF solvent. The solution was stirred at
300 K for 3 h. The color of the solution became dark blue. The
solution was evaporated under vacuum, and the crude product was
purified by rapid precipitation from dichloromethane solution of the
complex by hexane. Finally, the compound was crystallized by slow
diffusion of the dichloromethane solution of complex 2 into hexane.
The yield and spectral characterization of the product are as follows:
Blue colored solid. Yield: 85%. UV−vis (CH2Cl2): λ [nm] (ε, M−1
cm−1) = 250(19 460), 315 (10 660), 402sh(3390), 605 (1165). IR
(KBr disk, cm−1): ν (NN): 1399, ν (N−N): 1167, ν (N−H): 3220,
3070. Because of its high air sensitivity, reproducible C, H, N data for
complex 2 were not obtained.
Kinetic and Isotope Labeling Experiments. Isolation of
[NiCl2(LD2)]. In an argon atmosphere Schlenk line, 1 mmol of the
complex 1 was mixed with the equimolar quantity of cyclohexanol-D
and 1.5 mmol zinc in dry and deoxygenated THF solvent. The
solution was stirred at 300 K for 6 h. The color of the solution changed
from green to dark blue. The solution was filtered; solvent was
evaporated under vacuum. The IR spectrum of the resultant
compound showed charecteristic streching due to N−D bonds. IR
(KBr disk, cm−1): ν (NN): 1405, ν (N−N): 1172, ν (N−D): 2335,
2205.
The role of QMT on the ZPE-corrected potential energy surface
was incorporated within the Eckart tunneling model using TheRate42
program. To this regard, the one-dimensional Eckart analytical
potential energy function was fitted to the ZPE-corrected energies of
the reactant, product, and concerned transition-state along the IRC.
Replacement of the corresponding H atom by D atom and scaling the
harmonic frequencies by mass of D provides the primary KIE.
A similar experiment was also performed with cyclohexanone-H for
comparison. The isolated compound was similar to the previously
noted complex 2 (isolated from 2-propanol).
Complex 2 to 1 Conversion. The complex 1 is regenerated from
compound 2 upon exposure to air. The regeneration was fast at room
temperature. However, we were able to monitor the transformation,
spectroscopically at a low temperature (263 K) in acetonitrile solvent.
A 1.1 × 10−4 molar acetonitrile solution of the complex 2 was
continuously stirred in the air, and the spectral changes were
monitored by UV−vis spectroscopy (Figure S5). The transformation
was found to be quantitative.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01310.
Detection of Hydrogen Peroxide during the Catalytic
Reactions. The catalytic reactions produced H2O2, which was
detected by following the gradual development of the charecteristic
Kinetics details with plots, EPR spectrum of intermedi-
ate, NMR, and GCMS spectrum of isolated ketones/
aldehydes and theoretical details. (PDF)
Ketyl radical is unstable along the potential energy
surface. (AVI)
−
band for I3 spectro-photometrically (λmax= 350 nm; ε = 26 000 M−1
cm−1), upon reaction with I−. In a round-bottom flask containing 1
mmol of 1,1-diphenyl carbinol in 10 mL of dry THF was mixed with
0.005 mmol of catalyst, 0.005 mmol of KtBuO, and 1.5 mmol of zinc,
and the mixture was stirred at 300 K for 1 h. An equal volume of water
was added subsequently to the reaction mixture and was extracted with
dichloromethane to remove the leftover reactants and products from
CCDC Nos. 1469110 and 1469111 contain the
supplementary crystallographic data. (CIF)
G
DOI: 10.1021/acs.inorgchem.6b01310
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: icsg@iacs.res.in. (S.G.)
*E-mail: spad@iacs.res.in. (A.D.)
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The research was supported by the Department of Science and
Technology (DST), India, and Council of Scientific and
Industrial Research (CSIR) funded projects, SR/S2/JCB-09/
2011, EMR/2014/000520 and 01(274)/13/EMR-II, respec-
tively. S.G. and D.S. sincerely thank DST-SERB for a J. C. Bose
fellowship and a research fellowship, respectively. A.D. thanks
DST-SERB and BRNS for partial support. R.B., R.P., and S.P.R.
are thankful to the CSIR for their fellowship support.
Crystallography was performed at the DST-funded National
Single Crystal Diffractometer facility at the Department of
Inorganic Chemistry, IACS. We thank Prof. S. Bhattacharya of
Jadavpur Univ. for providing GC-MS data. Computational
facility of the IACS-CRAT initiative is duly acknowledged. We
are also thankful to Dr. S. K. Roy for valuable discussion on
mechanism.
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