Kinetics and mechanistic study on deoxygenation of pyridine oxide catalyzed by {MeReVO(pdt)} 2 dimer

Article abstract of DOI:10.1016/j.jorganchem.2017.04.034

The oxorhenium(V) dimer {MeReO(pdt)}₂ (where pdt?=?1,2-propanedithiolate) catalyze the oxygen atom transfer (OAT) reaction from the pyridine oxide to triphenylarsine (Ph₃As). The rate law is given by ν?=?k[Re-dimer][PyNO] and zero order dependence on Ph₃As. The value of k at 25?°C in CHCl₃ is 139?±?3?L?mol^?1?s^?1. The activation parameters are ΔH^??=?12.2?±?1.0?kcal?mol^?1 and ΔS^??=??7.9?±?3.24?cal?K^?1?mol^?1. According to the proposed mechanism, the rate determining step is the oxidation of Re^VO to Re^VIIO₂ and the pyridine release. The triphenylarsine enters the catalytic cycle after the rate determining step. The reaction constant ρ?=??1.4 obtained from Hammett correlation with σ for different substituted pyridine N-oxide. The computational study indicates that the oxidation of Re^V to Re^VII and release of the pyridine step is insensitive to the nature of the substituent on the pyridine with the average estimated activation barrier ≈11.5?kcal/mol from six different substituted pyridine oxide. It is proposed that electron donor substituent enrich the equilibrium of the first step of the proposed mechanism which is the coordination of the pyridine oxide with one rhenium atom to form I₁ (Scheme 2). The electron donor substituent on the pyridine increase the concentration of I₁ which will increase the rate of the reaction as the ν?=?k₂[I₁].

Full text of DOI:10.1016/j.jorganchem.2017.04.034

Journal of Organometallic Chemistry 842 (2017) 9e20
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Kinetics and mechanistic study on deoxygenation of pyridine oxide
catalyzed by {MeReVO(pdt)} dimer
2
Abdellatif Ibdah*, Salwa Alduwikat
Department of Chemistry, Jordan University of Science and Technology, Irbid, Jordan
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxorhenium(V) dimer {MeReO(pdt)}2 (where pdt ¼ 1,2-propanedithiolate) catalyze the oxygen atom
Received 27 February 2017
Received in revised form
26 April 2017
Accepted 28 April 2017
Available online 3 May 2017
transfer (OAT) reaction from the pyridine oxide to triphenylarsine (Ph3As). The rate law is given by
n
¼ k
[Re-dimer][PyNO] and zero order dependence on Ph3As. The value of
k
at 25 ꢀC in CHCl3 is
1.0 kcal molꢁ1 and
139
3
L
molꢁ1 sꢁ1
.
The activation parameters are
D
Hz
¼
12.2
D
Sz ¼ ꢁ7.9 3.24 cal Kꢁ1 molꢁ1. According to the proposed mechanism, the rate determining step is the
oxidation of ReVO to ReVIIO2 and the pyridine release. The triphenylarsine enters the catalytic cycle after
the rate determining step. The reaction constant
r
¼ ꢁ1.4 obtained from Hammett correlation with
s for
Keywords:
Oxygen atom transfer
DFT
different substituted pyridine N-oxide. The computational study indicates that the oxidation of ReV to
ReVII and release of the pyridine step is insensitive to the nature of the substituent on the pyridine with
the average estimated activation barrier z11.5 kcal/mol from six different substituted pyridine oxide. It is
proposed that electron donor substituent enrich the equilibrium of the first step of the proposed
mechanism which is the coordination of the pyridine oxide with one rhenium atom to form I1 (Scheme
2). The electron donor substituent on the pyridine increase the concentration of I1 which will increase
Kinetics
the rate of the reaction as the
n
¼ k2[I1].
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
The mechanistic studies of oxygen atom transfer catalyzed by
transition metal complexes have been studied as an essential step
in preparing a more efficient catalysis. In addition to experiments,
several computational studies have been employed into the high
valent transition metal oxo complexes [6,8,10,11]. The proposed
catalytic cycle begins with oxidation of Mn¼O by an oxygen donor
to produce Mnþ2O2 followed by reduction of Mnþ2O2 back to Mn¼O
by an oxygen acceptor [1,4,5]. The ReV¼O/ReVIIO2 system can be
constructed to be used as catalysis for oxygen atom transfer. Several
dithiolate ReV¼O complexes has been prepared and their catalytic
activity has been examined [9,12e26]. The high valent rhenium(V)
oxo complexes have shown a great ability to catalyze the oxidation
reaction [16,20e28].
The deoxygenation of heteroaromatic N-oxides are important as
intermediate step in the heterocycles synthesis [29,30]. Many of the
known methods for deprotection of pyridines are limited by the
side reactions or by reduction of the ring substituents. Moreover,
pyridine and other heteroarene N-oxides are important class of
heteroaromatic compounds in medicinal [31] and material chem-
istry [32]. The development of an efficient deoxygenation method
is highly desirable that can be used during the functionalization of
N-heterocycles [33,34].
Since decades, the oxygen atom transfer reaction (OAT) cata-
lyzed by high valent oxo transition metal complexes have been
attracted the scientific attention due to its application in biology [1]
and chemistry [2e4]. In biology, molybdenum and tungsten are
exist in the active-site of certain enzymes. These enzymes catalyze
the oxygen atom transfer reaction and often called oxotransferase
enzyme. Their catalytic cycle runs between M(VI)O2 and M(IV) ¼ O
(M ¼ Mo, W) [1] In chemistry, alkene oxidation, sulfide oxidation to
sulfoxide, and deoxygenation of pyridine oxide through OAT re-
actions are extensively employed in a wide range of areas of pre-
parative organic chemistry. Considering the interest in developing
an efficient catalysis for oxygen atom transfer reaction (OAT),
several transition metal complexes have synthesized and their
catalytic activity have been examined. Most of these complexes
have metal oxo (M ¼ O) bond in their structures like molybdenum,
tungsten [4,5], iron [2], manganese [6,7], and rhenium [8,9].
* Corresponding author.
E-mail address: aaibdah@just.edu.jo (A. Ibdah).
http://dx.doi.org/10.1016/j.jorganchem.2017.04.034
0022-328X/© 2017 Elsevier B.V. All rights reserved.

10
A. Ibdah, S. Alduwikat / Journal of Organometallic Chemistry 842 (2017) 9e20
The purpose of this work is to study the pyridine oxide deoxy-
implemented in GAMESS [39,40]. All the structures were fully
optimized with B3LYP using PCM solvation model of chloroform
with the following basis set: the LANL2DZ ECP augmented with 2f
polarization functions for Re [41]and 6-31G(d) basis sets were used
for other atoms. The structures were confirmed as minima or
transition states by calculating vibrational frequencies at same level
genation catalyzed by oxorhenium dimer {MeReO(pdt)}2 (1)
(pdt ¼ 1,3-propanedithiolate) according to the following equa-
tion. The dimeric catalyst was chosen because, in earlier studies,
dimeric systems had proved to be approximately 100 times more
effective than monomeric analogues [35].
(1)
The above reaction is very important reaction to study because
no uncatalyzed reaction has been observed even though it's ther-
modynamically favored. The As-O (102.6 kcal/mol [36]) bond
dissociation energy is stronger than the bond dissociation energy of
of theory and basis sets. The zero point, entropy change (D
Sꢀ), and
the temperature correction to the energy is calculated with same
basis set and level of theory. Energies were refined with single
point calculations on the optimized geometries using Minnesota
exchange-correlation M11-L [42] with PCM solvation model of
chloroform with the following basis sets: G3Large for As, 6-311(3df)
for S and Cl, 6-311G(d) for O, N, C and H, along with the same basis
set for Re.
N-O (61.7 kcal/mol [37]) and the entropy change (
pected to be very small. So, the above reaction has a negative Gibbs
free energy ( G < 0).
DS z 0) is ex-
D
2. Experimental section
3. Result and discussion
2.1. Reagent
The characterization of the product has been done with Ph3P
using the 31P-NMR. The Ph3P is used only for the purpose of the
product characterization because the dimer reacts with phosphine
to form a monomer (Scheme 1) and phosphine monomer is very
slow comparing to dimer [35,43,44]. The catalysis 1 is stable with
respect to monomerization by arsines, even over long reaction time
no MeReO(pdt) (AsPh3) monomer has not been detected. Also, the
catalysis 1 is found to be stable toward monomerization with pyr-
idine (the reaction product). This result is very important because
similar rhenium dimer can be monomerized with pyridine and
other N-heterocycles aromatic compounds (Scheme 2)
[23,28,44e47].
The Rhenium dimer {MeReO(pdt)}2 (1) (pdt
¼
1,2-
propanedithiolate) was synthesized according to the literature
experimental procedure [38]. Other reagents were purchased from
commercial sources and used without extra purification. The sol-
vent used for kinetics study is chloroform. Bruker 400 MHz spec-
trometers used to obtain 1H and 31P NMR spectra. Benzene-d6 was
used for rhenium(V) dimer 1H -NMR characterization to compare
with literature values.
2.2. Kinetic measurements
The reactions were studied at 25 ꢀC. Shimadzu scanning spec-
trophotometers (UV-2550) were used to record UVeVis spectra and
kinetic curves in cells with 1 cm optical path lengths. The reaction
progress was monitored from the intensity of pyridine oxide,
picoline oxide, and nitro pyridine oxide absorbance at, 307 nm,
309 nm, and 375 nm respectively. The time-absorbance data were
fitted to first-order kinetics, Eq (2), in which Y ¼ absorbance of
pyridine oxide substrate.
obst
Yt ¼ Y∞ þ ðY0 ꢁ Y∞Þeꢁk
(2)
2.3. Computational methods
Computations were carried out using the DFT calculation, as
Scheme 1. Monomerization reaction of {MeReO(pdt)}2 dimer occurs with phosphines.

A. Ibdah, S. Alduwikat / Journal of Organometallic Chemistry 842 (2017) 9e20
11
Scheme 2. Monomerization reaction between dimer and N-heterocycles aromatic compounds.
Fig. 2. The pyridine oxide absorbance at 307 nm decrease with 0.5 mM pyridine oxide,
10.0 mM Ph3As, and 0.01 mM catalysis.
Based on the experimental observation the rate law can be
written as: R ¼ k[PyNO][Re][Ph3As]0. k ¼ 139 3 Mꢁ1sꢁ1. The first
order dependence on the pyridine oxide and the rhenium catalyst
imply that the rate determining step involve the pyridine oxide and
the catalyst only. The zero order dependence on the Ph3As indicates
that it inter the catalytic cycle after the rate determining step.
Fig. 1. 31P-NMR spectrum before (below) and after (above) completion (2.5 h) of a
reaction between 25 mM pyridine N-oxide, 140 mM PPh3, and 3.86 mM Re dimer. The
resonance at 31.9 ppm in CDCl3 is for Ph3PO.
3.1. Para-substituted pyridine-oxide
More kinetic measurements have been done with substituted
pyridine oxide to compare with the pyridine oxide. The 4-
nitropyridine oxide and picoline oxide (4-methyl-pyridine oxide)
were the chosen substrates for the kinetic study. The absorbance
decrease of the nitro-pyridine oxide and picoline oxide were
observed under pseudo first order conditions of Ph3As ([Ph3As] »
[nitropyridine oxide] and [Ph3As] » [picoline oxide]) (Fig. 4). Similar
to PyNO reaction, the absorbance decrease confirmed a first order
dependence on the 4-nitropyridine oxide and picoline oxide but
the nitro substituent is slower the pyridine oxide and the methyl
substituent is faster than the pyridine oxide (Fig. 4).
The Ph3PO was the only oxide product formed from the deox-
ygenation reaction of pyridine oxide (Fig. 1). The rest of the kinetics
measurement were made with Ph3As as oxygen acceptor. The
absorbance decrease of pyridine oxide was monitored under
pseudo first order condition of Ph3As ([Ph3As] » [PyNO]) (Fig. 2).
The absorbance decrease conformed first order dependence on the
pyridine oxide concentration. The k(obs) values obtained from the
first order fitting of Eq (2) conformed zero dependent with respect
to the Ph3As concentration (Fig. 3) and has linear dependence on
the rhenium dimer catalyst concentration confirming the first order
dependence on rhenium dimer (Fig. 3).
The calculated k(obs) values from the first order fitting confirmed
the zero order dependence on the Ph3As and first order depen-
dence on the rhenium catalyst for both 4-nitropyridine oxide and

Fig. 3. (Left) Plot of k(obs) against the Re concentration with 0.5 mM Pyridine oxide and 10.0 mM Ph3As. (Right) Plot of kobs against Ph3As concentration with 0.5 mM pyridine oxide
and 0.01 mM of Re catalyst.
Fig. 4. (Left) The picoline oxide absorbance at 309 nm decrease with 0.38 mM picoline oxide, 10.0 mM Ph3As, and 0.01 mM catalysis. (Right) The 4-nitropyridine oxide absorbance
at 375 nm decrease with 0.25 mM 4-nitropyridine oxide, 5.0 mM Ph3As, and 0.05 mM catalysis.
picoline oxide (see supporting information). These kinetics results
are equivalent to the pyridine oxide results. Also, the first order
fitting on pyridine oxide substrates retain even at low concentra-
tion of Ph3As (less than the pseudo order conditions) which
confirm the zero dependence on the arsine. The rate constant k is
16.5 0.8 Mꢁ1sꢁ1 and 388 14 Mꢁ1sꢁ1 for 4-nitropyridine oxide
and picoline oxide, respectively.
The reaction rate became slower with electron withdrawing
substituent (NO2 substituent) on the pyridine oxide and faster with
electron donor substituent (methyl substituent). The rate deter-
mining step remains same and the Ph3As inter the catalytic cycle
after the rate determining step.
3.2. Reaction scheme and the mechanism
The kinetics data allow the construction of the reaction mech-
anism as presented in Scheme 3. The first step of the proposed
mechanism is the coordination of the pyridine oxide to one
rhenium atom concomitant with release one Re-S coordinate bond
to form five coordinated intermediate I1 in fast equilibrium. The
following step is the irreversible release of the pyridine to form of
Scheme 3. The Proposed mechanism of the oxygen atom transfer catalyzed by
{MeReVO(pdt)2 dimer.

A. Ibdah, S. Alduwikat / Journal of Organometallic Chemistry 842 (2017) 9e20
13
catalyst will be restored to start another cycle. Analogous system of
rhenium catalysts adopt similar kinetics dependence that the
ReO(2VII) formation is the slow step followed by fast addition of the
oxygen acceptor (e.g Ph3P/Ph3As) [44,48,49].
On the basis of Scheme 3, the rate expression can be derived
considering prior-equilibrium in the first step (I1 formation) fol-
lowed by the R.D.S formation of I2 (Full derivation of the rate law is
in the supporting information).
k1 k2
v ¼
½1ꢂ½PyNOꢂ
(3)
k
ꢁ1
Eq (3) can be simplified to Eq (4).
v ¼ KeqK2½1ꢂ½PyNOꢂ
(4)
(5)
v ¼ K½1ꢂ½PyNOꢂ; k ¼ Keqk2
3.3. Hammett analysis
Fig. 5. The Hammett analysis of the relative rate constant of pyridine oxide
substituent.
The experimental second order rate constants are sensitive to
the nature of the substituent on pyridine oxide Me > H > NO2, with
reaction constant
r
¼ ꢁ1.4 0.1 (Fig. 5). The reaction constant is
Table 1
estimated from the relative rate constant to the hammett substit-
uent constant (Table 1). The second order rate constant (k) depends
on the Keq and k2 values (Eq (5)). The increase in the rate constant
can be traced to increase of either Keq or k2 or both. The electron
donor substituent on pyridine oxide increases the rate of reaction.
The first equilibrium of the proposed mechanism will be enhanced
with more electron donor substituent as more basic ligand will
coordinate faster. The second step (oxidation of rhenium and
reduction of pyridine oxide) is thought to be inhibited with electron
donor substituent on the pyridine oxide because reduction of the
pyridine ring will be less favored by electron donor substituent. The
The ratio of the rate constant (kX/kH) to hammett substituent content.
X-PyN-O
kX L molꢁ1 sꢁ1
kX/kH
s
NO2-PyN-O
PyN-O
Me-PyN-O
15.3
136.3
390.2
0.11
1.00 (rel)
2.87
0.78
0.0
ꢁ0.17
ReO(2VII) intermediate I2 and it is the rate controlling step. The Ph3As
react with the ReO2(VII) intermediate I2 in fast step to reduce ReVII
and form Re(V)-OAsPh3 intermediate. Then the Re-S coordinate
bond will be regenerated concomitant to release Ph3AsO. The
Scheme 4. The simplified reaction model for the DFT calculation.
Fig. 6. The DFT optimized structure: A is the Me(pdt)ReVIIO2, B is the optimized structure of Me(pdt)ReV-OPyN intermediate, C is the transition state structure of the pyridine release
and Me(pdt)ReVIIO2 formation.

Fig. 7. The DFT calculated reaction coordinate of the simplified model reaction.
Table 2
The enthalpy, entropy, and Gibbs free energy of activation of the proposed slow step with different pyridine oxide substituent and the fast step.
z
za
D
Gꢀ (kcal/mol)
Reaction
D
Hꢀz (kcal/mol)
D
Sꢀ cal Kꢁ1 molꢁ1
11.5
ꢁ0.71
11.66
O
O
S
Me
O
S
Me
O
Re
Re
S
S
N
N
NO 2
NO 2
11.3
11.6
0.167
11.28
11.79
O
O
S
Me
S
Me
O
Re
Re
O
S
S
N
N
Cl
Cl
ꢁ0.64
O
O
S
S
Me
O
S
Me
O
Re
Re
S
N
N
11.7
11.2
11.4
0.39
0.11
0.72
11.55
10.84
11.19
O
O
S
Me
O
S
Me
O
Re
Re
S
S
N
N
Me
Me
O
O
S
Me
O
S
Me
O
Re
Re
S
S
N
N
OMe
OMe
O
O
S
Me
O
S
Me
O
Re
Re
S
S
N
N
NMe 2
Me As
NMe 2
6.1
ꢁ44.1
19.2
O
O
S
S
Me
O
Me
Re
Re
O
S
S
AsMe 3
a
z
z
z
At T ¼ 298.15 K;
D
Gꢀ
¼
D
Hꢀ - T
D
Sꢀ

A. Ibdah, S. Alduwikat / Journal of Organometallic Chemistry 842 (2017) 9e20
15
experimental results shows the electron donor substituent in-
creases the reaction rate constant and the electron withdrawing
substituent decreases the reaction rate constant. This suggests that
the first equilibrium is more affecting by nature of the substituent
than the second oxidation step.
rate determining step and the fast step of the reaction as well. Due
to the time limitation to run the calculation on the actual catalysis
and the reactants, certain simplification has been used in which the
Me(pdt)ReVO-OPyN-X and Me(pdt)ReVIIO2 (Scheme 3) are used in
place of I1 and I2 intermediates in Scheme 2 and Me3As is used in
place of Ph3As. This small model of the dimer is valid as the other
half of the Re dimer isn't directly involved in the proposed
mechanism.
3.4. Computational study
In the kinetics experiments only three substrates have been
used because of the starting materials availability. But in the
To gain a better understanding on the reaction mechanism and
the substituent effect, the DFT calculations have been done on the
Table 3
Selected bond lengths (Å) and angles and Mulliken charges on Re, O, and N.
Re-O2
1.68
Re-O1
2.10
O1-N
1.34
Re-O1-N angle
125.4
Charge, Re
0.44
Charge, O1
Charge, N
O2
Re
ꢁ0.53
ꢁ0.05
S
Me
O1
S
N
NO2
O2
Re
1.68
1.68
1.89
2.08
1.62
1.36
126.6
123.9
0.57
0.44
ꢁ0.52
ꢁ0.54
ꢁ0.21
ꢁ0.07
S
Me
O1
S
N
NO2
O2
Re
S
Me
O1
S
N
Cl
1.68
1.68
1.89
2.08
1.64
1.36
126.2
124.6
0.57
0.45
ꢁ0.52
ꢁ0.55
ꢁ0.23
ꢁ0.07
O2
Re
S
Me
O1
S
N
Cl
O2
S
Me
O1
Re
S
N
O2
1.68
1.88
1.66
126.1
0.58
ꢁ0.53
ꢁ0.22
S
Me
Re
O1
N
S
O2
Re
1.68
1.68
2.07
1.88
1.36
1.66
123.5
126.3
0.45
0.58
ꢁ0.55
ꢁ0.53
ꢁ0.07
ꢁ0.23
S
Me
O1
S
N
O2
Re
S
Me
O1
S
N
O2
Re
1.68
1.68
2.06
1.87
1.37
1.67
124.3
126.3
0.46
0.58
ꢁ0.55
ꢁ0.52
ꢁ0.09
ꢁ0.25
S
Me
O1
S
N
O
O2
S
Me
Re
O1
S
N
O
O2
1.68
1.68
2.05
1.87
1.37
1.68
125.0
126.6
0.46
0.59
ꢁ0.56
ꢁ0.53
ꢁ0.10
ꢁ0.26
S
Me
Re
O1
N
S
NMe2
O2
S
Me
O1
Re
S
N
NMe2
O1
Re
1.69
1.69
0.65
ꢁ0.46
S
Me
O2
S

16
A. Ibdah, S. Alduwikat / Journal of Organometallic Chemistry 842 (2017) 9e20
computational study, more substrates were tested to understand
the substituents effect (Scheme 4). Six substituted pyridine oxide
have been used varying from strong electron withdrawing to strong
electron donor substituent.
mulliken charges on Re increases as results of the oxidation and
the negative charges on the N increases as result from the
reduction. The charges on the nitrogen doesn't change with sub-
stituent even though there are a little increase on the charge with
strong electron donor substituent (MeO-, Me2N-). There is small
elongation observed on the N-O with MeO- and Me2N- substitu-
ent compared to others in both transition state and intermediates.
The Re-O1 are not changed significantly with substituents for both
optimized minima and transition state. The substituents on the
pyridine oxide don't have any significant changes on the N-O and
Re-O1 bond length and the charges on both N and O1 which
confirm that the rate determining step is not sensitive to the na-
ture of the substituent.
The calculated structure of ReVIIO2 key intermediate is a dis-
torted trigonal bipyramid in which the oxo group occupy the
equatorial position (Fig. 6, A). This result is consistent with earlier
computational studies that showed the dioxo d0 octahedral com-
plexes form stable cis-dioxo complexes [50,51]. The optimized
structure of Me(pdt)ReVO-OPyN and its transition state structure of
pyridine release are illustrated in Fig. 7 (B and C). The other opti-
mized structure and transition state structure of the substituted
pyridine oxide are presented in the supporting information along
with the coordinates. All the transition states have been verified by
the intrinsic reaction coordinate (IRC) calculation on the located
transition state.
The DFT estimated the thermodynamic parameters of the slow
oxidation ReV to ReVII and release of the pyridine step is exothermic
and exergonic for all substituted pyridine oxide (Table 4). But more
The activation barrier for ReVIIO2 formation and pyridine
release were calculated for six pyridine oxide substituents
negative D D
Hꢀ and Gꢀ obtained with electron withdrawing group
because the reduction of N of the aromatic ring will be more
favored with electron withdrawing group.
(Table 2). The calculated barrier (
of the substituent on
approximately z 11.5 kcal/mol. The average Gibbs free energy of
activation (
Gꢀz) is 11.4 kcal/mol and also not affected by the
nature of the substituent. The entropy of activations (
Sꢀz) of the
D
Hꢀz) is unaffected by the nature
the
pyridine
oxide
and
The computational results explain the zero order dependence
on the triphenylarsine but doesn't explain the Hammett depen-
dence. The Hammett analysis shows that the electron donating
substituent increase the rate of the reaction. Based on computa-
tional results, it can be assume that the Hammett substituent effect
came from the first equilibrium step. As basicity of pyridine oxide
increases with electron donor substituent, the first equilibrium step
will shift to right to form more I1 (Scheme 2) which will increase
the reaction rate.
The influence of the substituent on the first equilibrium is not
applicable to study computationally because of large time needed
to run the calculation on the dimer without simplification. But it
can be examined indirectly relative to pyridine according to Eq (3).
D
D
slow step are very small and slightly positive except for NO2 and H
substituent are slightly negative. The activation barrier of the fast
step (nucleophilic addition of the Me3As to ReVIIO2 step) is
6.15 kcal/mol (Table 2) and has large negative entropy of activa-
tion which agree with associative nature of the reaction. The
activation barrier is lower than the activation barrier of the rate
determining step which agree with the experimental finding that
the arsine inter the catalytic after the rate determining step. The
reaction coordinate has been calculated by DFT for the proposed
slow and the fast steps of the mechanism using the above reaction
The DH of Eq (3) indicate how the substituent can influence the first
model (Fig. 7).
equilibrium step compare to pyridine oxide. Since the other
rhenium atoms isn't involve in the reaction, it can be studied using
a small model of the rhenium dimer (Eq (4)). Eq (4) is an isodesmic
reaction in which the error of the computation will be the mini-
mum. Table 5 summarized our results.
z
The above results of
D
Hꢀ promoted us to examine the Re¼O,
Re-O, and N-O bond length of the ReV intermediates and the
transition states in addition to mulliken charges on ReV, O, N, and
ReVII. Table 3 summarize our results. In all transition states the Re-
O length decreases and the O-N bond increases as expected. The
O
O
S
Me
S
S
Me
S
Re
O
Re
O
S
S
R
N+ O-
+
Re
(3)
Me
Re
+
H
N+ O-
Me
S
S
O
O
N
N
H
R
R
O
O
S
S
Re
S
Re
S
R
N+ O-
+
H
N+ O-
+
(4)
Me
N
Me
N
O
O
H
R

A. Ibdah, S. Alduwikat / Journal of Organometallic Chemistry 842 (2017) 9e20
17
Table 4
The enthalpy, entropy, and the Gibbs free energy of the proposed slow step with different pyridine oxide substituent and the fast step.
a
Reaction
D
Hꢀ (kcal/mol)
D
Sꢀ cal Kꢁ1 molꢁ1
D
Gꢀ (kcal/mol)
ꢁ21.7
46.6
ꢁ35.6
O
S
O
Me
O
S
Me
O
Re
Re
N
NO2
S
S
N
NO2
ꢁ19.9
45.9
ꢁ33.6
O
S
O
Me
O
S
Me
Re
Re
N
Cl
S
O
S
N
Cl
ꢁ17.7
ꢁ16.8
44.1
46.9
ꢁ30.8
ꢁ30.8
O
S
O
Me
O
S
Me
Re
Re
N
O
S
S
N
O
S
Re
O
Me
O
S
Me
Re
N
Me
O
S
S
N
Me
ꢁ17.7
47.7
ꢁ31.9
O
S
O
Me
O
S
Me
O
Re
Re
N
OMe
S
S
N
OMe
ꢁ14.6
ꢁ17.5
46.8
ꢁ28.6
ꢁ4.1
O
O
S
Me
S
Me
O
Re
Re
N
NMe2
O
S
S
N
NMe2
ꢁ44.8
O
O
S
Me
O
Me
O
S
S
Me3As
Re
Re
S
AsMe3
a
At T ¼ 298.15 K;
D
Gꢀ
¼
D
Hꢀ - T
D
Sꢀ.
Table 5
The enthalpy and Gibbs free energy of reaction 4 with different substituent.
a
ꢀ
S4 cal Kꢁ1 molꢁ1
ꢀ
DG4 kcal/mol
ꢀ
Reaction
D
H4 kcal/mol
D
O
O
3.80
ꢁ2.72
4.61
S
S
Re
S
Re
S
N+ O-
+
Me
N
Me
N
O2
N
N+ O-
+
H
O
O
H
O2
N
O
O
0.16
0.00
ꢁ1.81
0.70
0.00
S
S
Re
S
Re
S
N+ O-
+
Me
Me
Cl
N+ O-
+
H
O
O
N
N
H
Cl
O
O
0.00
S
S
Re
Re
N+ O-
+
Me
N
Me
N
H
N+ O-
+
H
S
S
O
O
H
H
H
H
O
O
ꢁ1.73
ꢁ2.58
ꢁ5.71
ꢁ1.69
ꢁ3.39
ꢁ6.95
ꢁ1.22
ꢁ1.57
ꢁ3.64
S
S
Re
S
Re
S
N+ O-
+
Me
N
Me
N
+
H
N+ O-
O
O
O
O
S
S
Re
Re
N+ O-
+
Me
N
Me
N
MeO
N+ O-
+
H
S
S
O
O
MeO
O
O
S
S
Re
Re
Me
N+ O-
Me
+
Me2
N
N+ O-
+
H
S
S
O
O
N
N
H
Me2
N
a
At T ¼ 298.15 K;
D
Gꢀ
¼
D
Hꢀ - T Sꢀ.
D

18
A. Ibdah, S. Alduwikat / Journal of Organometallic Chemistry 842 (2017) 9e20
ꢀ
Fig. 8. (Left) Plot of enthalpy of Eq (4) (D s) substituent constant, (Right) Plot of DG4 of Eq (4) against Hammett (s) substituent.
H4ꢀ) against Hammett (
The enthalpy (
D
H4ꢀ) and Gibbs free energy (
D
G4ꢀ) of reaction 4
energy of Eq (4) (
constant (Fig. 8).
D s) substituent
G4ꢀ) values with the Hammett (
provides a qualitative analysis how the coordination to rhenium
center is influenced by the nature of the substituent on the pyridine
oxide. The negative enthalpy and Gibbs free energy of reaction with
electron donor substituent (entries 1 and 2) indicates that the
rhenium coordinate better with electron donor substituted pyri-
dine oxide and prefer the electron donor substituent compare to
pyridine oxide. This will shift the equilibrium (Keq) to right to form
more I1 intermediate (Scheme 2). The positive enthalpy and Gibbs
free energy of reaction with electron withdrawing substituent
(entries 4, 5 and, 6) indicate that rhenium center coordinate weaker
with electron withdrawing substitute. This will shift the equilib-
rium (Keq) of the proposed mechanism to the left to form less I1
intermediate. These results agree with experimental finding that
NO2 substituent is slower than pyridine oxide and the methyl
substituent is faster than pyridine oxide. Also, there is a good cor-
3.5. Activation parameters
The second-order rate constant for the pyridine oxide reaction,
as defined in Eq (1), was determined at the different temperatures
of 15.0, 20.0, 25.0, and 35.0 ꢀC. The data were fit by the transition-
state theory equation
ꢀ
ꢀ
lnðk=TÞ ¼ lnðkB=hÞ þ
D
Sz R ꢁ
D
Hz RT
(6)
Fig. 9 displays the plot of ln(k/T) vs 1/T for pyridine oxide. The
plots of nitro and methyl substituent are available in supporting
information. From slope and the intercept the
and
Sz cal Kꢁ1 molꢁ1 were obtained. Table 6 summarize our
results.
The experimental entropy of activation
D
Hz in kcal molꢁ1
D
relation between the enthalpy of Eq (4) (D
H4ꢀ) and Gibbs free
ꢀ
D
Sz ¼
DS1
þ
S2 z. The
ꢀ
D
D
Sꢀ1 is substantially negative because two molecules associate to
one molecules in the first equilibrium. The
Table 2) are slightly positive for NO2 and H substituent and slightly
negative methyl substituent. The experimental
Sz for the three
S+1 is the most
D
Sz2 (estimated in
D
pyridine oxide substrates are negative means that
anticipating in the total entropy of activation.
D
The experimental
stituent attached to pyridine oxide (
D
Hz increases as electron withdrawing sub-
D
HNz O2 HzH HMz e). These
> D > D
results agree with experimental reaction rate that the picoline N-
oxide is faster than pyridine N-oxide and pyridine N-oxide is faster
than
D
4ꢀ-nitropyꢀridine
N-oxide.
The
experimental
Hz ¼
D
H1
þ
D
H2 z. The DFT estimated values
D
Hz2 are 11.7, 11.6,
and 11.5 kcal/mol for 4-picoline N-oxide, pyridine N-oxide, and 4-
nitropyridine N-oxide, respectively (Table 2). The projected values
of
D
H1+
(
D
H1+
¼
D
Hz
-
D
Hz2(DFT)) are 2.7, 0.5, and ꢁ1.2 kcal/mol,
(exp)
respectively. The
D
H+1 shows how the thermodynamic of the first
equilibrium changed by the nature of the substituent. It is less
Table 6
The estimated activation parameters of reaction 1
substrate
D
Hz/(kcal/mol)
D
Sz (cal/mol.k)
4-nitropyridine N-oxide
pyridine N-oxide
4-picoline N-oxide
14.3 1.0
12.2 1.0
10.5 0.2
ꢁ10
ꢁ8
3
1
4
Fig. 9. Plot of ln(k/T) versus 1/T of the pyridine oxide (0.5 mM) reaction with triphenyl
arsine (1 mM), and rhenium dimer (0.014 mM).
ꢁ11

A. Ibdah, S. Alduwikat / Journal of Organometallic Chemistry 842 (2017) 9e20
19
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substituent). With methyl substituent, the
H+1 is negative indi-
D
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termediate. The above results agree with experimental the rate
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Pyridine oxide with electron donor substituent coordinate better
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In conclusion, the kinetics and the DFT calculation results shows
that the reaction is first order with respect to the pyridine oxide and
the catalysis and zero order with respect to the Ph3As. The pyridine
oxide coordinate to one rhenium atom of the catalyst followed by
oxidation of ReV to ReVII and release of pyridine (RDS). The Ph3As
inter the catalytic cycle after the rate determining step to reduce
ReVII to ReV. The reaction rate is
r
¼ ꢁ1.4 means that the reaction
rate is sensitive to the nature of the substituent on the pyridine
oxide and increases with electron donor substituents. The DFT
calculation on the proposed RDS shows that it is unaffected by the
nature of the substituent on the pyridine oxide but the first equi-
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Acknowledgments
This work was supported by Jordan University of Science and
Technology, Irbid, Jordan (20140257).
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Appendix A. Supplementary data
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