Highly efficient alkane oxidation catalyzed by [MnV(N)(CN) 4]2-. Evidence for [MnVII(N)(O)(CN) 4]2- as an active intermediate

Article abstract of DOI:10.1021/ja5019546

The oxidation of various alkanes catalyzed by [Mn^V(N)(CN) ₄]^2- using various terminal oxidants at room temperature has been investigated. Excellent yields of alcohols and ketones (>95%) are obtained using H₂O₂ as oxidant and CF₃CH ₂OH as solvent. Good yields (>80%) are also obtained using (NH₄)₂[Ce(NO₃)₆] in CF ₃CH₂OH/H₂O. Kinetic isotope effects (KIEs) are determined by using an equimolar mixture of cyclohexane (c-C₆H ₁₂) and cyclohexane-d₁₂ (c-C₆D₁₂) as substrate. The KIEs are 3.1 ± 0.3 and 3.6 ± 0.2 for oxidation by H₂O₂ and Ce(IV), respectively. On the other hand, the rate constants for the formation of products using c-C₆H₁₂ or c-C₆D₁₂ as single substrate are the same. These results are consistent with initial rate-limiting formation of an active intermediate between [Mn(N)(CN)₄]^2- and H₂O₂ or Ce^IV, followed by H-atom abstraction from cyclohexane by the active intermediate. When PhCH₂C(CH₃)₂OOH (MPPH) is used as oxidant for the oxidation of c-C₆H₁₂, the major products are c-C₆H₁₁OH, c-C₆H₁₀O, and PhCH₂C(CH₃)₂OH (MPPOH), suggesting heterolytic cleavage of MPPH to generate a Mn=O intermediate. In the reaction of H₂O₂ with [Mn(N)(CN)₄]^2- in CF ₃CH₂OH, a peak at m/z 628.1 was observed in the electrospray ionization mass spectrometry, which is assigned to the solvated manganese nitrido oxo species, (PPh₄)[Mn(N)(O)(CN)₄] ^-·CF₃CH₂OH. On the basis of the experimental results the proposed mechanism for catalytic alkane oxidation by [Mn^V(N)(CN)₄]^2-/ROOH involves initial rate-limiting O-atom transfer from ROOH to [Mn(N)(CN)₄]^2- to generate a manganese(VII) nitrido oxo active species, [Mn^VII(N)(O) (CN)₄]^2-, which then oxidizes alkanes (R'H) via a H-atom abstraction/O-rebound mechanism. The proposed mechanism is also supported by density functional theory calculations.

Full text of DOI:10.1021/ja5019546

Article
pubs.acs.org/JACS
V
2−
Highly Efficient Alkane Oxidation Catalyzed by [Mn (N)(CN) ] .
4
VII
2−
Evidence for [Mn (N)(O)(CN) ] as an Active Intermediate
4
Li Ma, Yi Pan, Wai-Lun Man, Hoi-Ki Kwong, William W. Y. Lam, Gui Chen, Kai-Chung Lau,*
and Tai-Chu Lau*
Department of Biology and Chemistry and Institute of Molecular Functional Materials, City University of Hong Kong, Tat Chee
Avenue, Hong Kong, China
*
S Supporting Information
V
ABSTRACT: The oxidation of various alkanes catalyzed by [Mn (N)-
CN)4]2 using various terminal oxidants at room temperature has
−
(
been investigated. Excellent yields of alcohols and ketones (>95%) are
obtained using H O as oxidant and CF CH OH as solvent. Good
2
2
3
2
yields (>80%) are also obtained using (NH ) [Ce(NO ) ] in
4
2
3 6
CF CH OH/H O. Kinetic isotope effects (KIEs) are determined by
3
2
2
using an equimolar mixture of cyclohexane (c-C H ) and cyclohexane-
6
12
d12 (c-C D ) as substrate. The KIEs are 3.1 ± 0.3 and 3.6 ± 0.2 for
6
12
oxidation by H O and Ce(IV), respectively. On the other hand, the rate constants for the formation of products using c-C H
2
2
6
12
or c-C D as single substrate are the same. These results are consistent with initial rate-limiting formation of an active
6
12
2−
IV
intermediate between [Mn(N)(CN)4] and H O or Ce , followed by H-atom abstraction from cyclohexane by the active
2
2
intermediate. When PhCH C(CH ) OOH (MPPH) is used as oxidant for the oxidation of c-C H , the major products are c-
2
3 2
6
12
C H OH, c-C H O, and PhCH C(CH ) OH (MPPOH), suggesting heterolytic cleavage of MPPH to generate a MnO
6
11
6
10
2
3 2
2−
intermediate. In the reaction of H O with [Mn(N)(CN) ] in CF CH OH, a peak at m/z 628.1 was observed in the
2
2
4
3
2
electrospray ionization mass spectrometry, which is assigned to the solvated manganese nitrido oxo species, (PPh )[Mn(N)-
4
−
(
[
O)(CN) ] ·CF CH OH. On the basis of the experimental results the proposed mechanism for catalytic alkane oxidation by
Mn (N)(CN) ] /ROOH involves initial rate-limiting O-atom transfer from ROOH to [Mn(N)(CN) ] to generate a
4
3
2
V
2−
2−
4
4
VII
2−
manganese(VII) nitrido oxo active species, [Mn (N)(O)(CN) ] , which then oxidizes alkanes (R′H) via a H-atom
4
abstraction/O-rebound mechanism. The proposed mechanism is also supported by density functional theory calculations.
INTRODUCTION
oxidation of alkanes using H O and various other oxidants, with
2 2
■
unprecedented yields of >95%. Through a combination of
experimental and computational studies, we provide evidence
that the active intermediate is a manganese(VII) nitrido oxo
The search for efficient catalysts for selective oxidation of alkanes
under mild conditions continues to be a challenge for chemists.
We have been interested in the design of highly active alkane
oxidation systems that consist of an earth abundant metal catalyst
such as Mn or Fe, and a green oxidant such as O or H O .
1
2
−
2
species, [Mn(N)(O)(CN) ] . Our work suggests that d
4
Mn(V) nitrido complexes are potentially a new class of highly
efficient oxidation catalysts that may be more active and more
robust than conventional Mn(II)/(III)/(IV) oxidation catalysts.
2
2
2
Although a number of Mn(II)/(III)/(IV) alkane oxidation
2
−6
catalysts are known,
the product yields are generally low
(
<50%) using green oxidants such as H O .
We recently reported that the square pyramidal Mn(V) nitrido
2
2
EXPERIMENTAL SECTION
■
compound (PPh ) [Mn(N)(CN) ], prepared by Wieghardt and
co-workers, is a very active and selective catalyst for the
oxidation of alkenes and alcohols in CH CN/CH COOH using
Materials. All chemicals were of reagent grade unless otherwise
specified. (PPh ) [Mn(N)(CN) ] was synthesized according to a
4
2
4
7
4
2
4
7
published procedure. The purity of the complex was determined by
CHN elemental analysis and UV/vis spectrophotometry. All organic
substrates were obtained from Sigma-Aldrich and purified according to
3
3
8
H O as the oxidant, with product yields close to 100%.
2
2
2
Although in a high oxidation state, the d Mn(V) complex is
relatively stable compared with many high-spin Mn(II)/(III)
catalysts. Binding of oxidant to Mn(V) is facilitated by the
presence of a vacant site. According to density functional theory
9
literature methods. Hydrogen peroxide (33%, Sigma-Aldrich) was used
10,11
as received and standardized by iodometry.
2-Methyl-1-phenyl-2-
propyl hydroperoxide (MPPH) was prepared according to a literature
1
2
method, and its purity was determined to be >99% by NMR
spectroscopy. Active MPPH levels were further measured by iodometric
2
−
(
DFT) calculations, [Mn(N)(CN)4] first binds to H O to
2 2
2
−
titrations, which showed >99% peroxide activity. Cyclohexane-d (98+
form the intermediate [Mn(N)(CN) (H O )] ; the coordi-
nated H O is stabilized by hydrogen bonding with the CN
ligands and CH COOH.
12
4
2
2
18
18
−
atom % D) and H2 O (96 atom % O) were purchased from Medical
2
2
8
3
We report here that when CF CH OH is used as the solvent,
Received: February 25, 2014
Published: May 5, 2014
3
2
Mn(N)(CN)4]2 is also a highly efficient catalyst for the
−
[
©
2014 American Chemical Society
7680
dx.doi.org/10.1021/ja5019546 | J. Am. Chem. Soc. 2014, 136, 7680−7687

Journal of the American Chemical Society
Article
a
Table 1. Catalytic Oxidation of Various Alkanes by (PPh ) [Mn(N)(CN) ]/H O
2
4
2
4
2
total
b
entry
substrate
cyclohexane
product (% yield)
% yield
c
1
cyclohexanol (11), cyclohexanone (14)
cyclohexanol (69), cyclohexanone (28)
25
97
97
64
97
97
96
97
96
95
2
cyclohexane
cyclohexane
cyclohexane
cycloheptane
cyclooctane
n-heptane
d
3
e
cyclohexanol (70), cyclohexanone (26), cyclohexyl bromide (1)
cyclohexanol (8), cyclohexanone (56)
4
5
6
7
8
9
cycloheptanol (62), cycloheptanone (35)
cyclooctanol (63), cyclooctanone (34)
2-heptanol (22), 3-heptanol (25), 4-heptanol (13), 2-heptanone (15), 3-heptanone (17), 4-heptanone (4)
2-octanol (17), 3-octanol (19), 4-octanol (18), 2-octanone (15), 3-octanone (14), 4-octanone (14)
2,3-dimethyl-2-butanol (96)
n-octane
2,3-dimethylbutane
10
trans-1,2-dimethyl-
cyclohexane
trans-1,2-dimethylcyclohexanol (36), cis-1,2-dimethylcyclohexanol (1), 2,3-dimethylcyclohexanol (19),
3,4-dimethylcyclohexanol (14), 2,3- and 3,4-dimethylcyclohexanone (25)
11
cis-1,2-dimethyl-
cyclohexane
cis-1,2-dimethylcyclohexanol (84), trans-1,2-dimethylcyclohexanol (1), 2,3-dimethylcyclohexanol (4),
3,4-dimethylcyclohexanol (5), 2,3- and 3,4-dimethylcyclohexanone (2)
96
a
Reaction conditions: (PPh ) [Mn(N)(CN) ], 2.5 mM; alkane, 1.2 M; H O , 0.25 M; CH CO H, 0.25 M; solvent, CF CH OH; T = 23 °C, under
4
2
4
2
2
3
2
3
2
b
c
argon. Time, 5 h. % yield of alcohol = (mol of alcohol/mol of H O ) × 100; % yield of ketone = (mol of ketone/mol of H O ) × 2 × 100. CH CN
was used as solvent. In the presence of 0.5 M BrCCl . Reaction conditions: (PPh ) [Mn(N)(CN) ], 0.5 mM; cyclohexane, 4.0 M; H O , 0.75 M;
2
2
2
2
3
d
e
3
4
2
4
2
2
CH CO H, 0.5 M; solvent, CF CH OH. T = 23 °C. Time, 48 h. TON = 960.
3
2
3
2
Isotopes and used as received. H218O (90 atom % 18O) was obtained
hybrid functional) level using LanL2DZ basis set for Mn and 6-311+
13
14
2
from Icon Services Inc. Bromotrichloromethane was purchased from
Aldrich and was purified by passing through a short column of neutral
alumina prior to use.
+G(d,p) basis set for nonmetal atoms. The solvent effect of CF CH OH
(TFE) was taken into account by the polarizable continuum model.
3
2
15
The geometries were fully optimized without any symmetry or geometry
constraint. The nature of all stationary points was confirmed by
vibrational frequencies calculations. All reactants, intermediates, and
products have no imaginary frequency, while all transition states have
only one imaginary frequency. The connectivity of transition state
structures were confirmed by intrinsic reaction coordinates (IRC)
Instrumentation. Gas chromatographic (GC) analyses were
performed on a HP5890 GC/FID gas chromatograph equipped with
a DB-FFAP (30 m × 0.25 mm i.d.) or a HP-5MS (30 m × 0.25 mm i.d.)
capillary column. GC/MS measurements were carried out on a HP6890
gas chromatograph interfaced to a HP5975 mass selective detector.
Elemental analyses were done on an Elementar Vario EL analyzer. Mn
content was determined by a PerkinElmer Optima 2100 DV ICP optical
emission spectrometer. Electrospray ionization mass spectrometry
16
calculations. The natural bond orbital (NBO) analyses were carried
2
−
out on the [MnN(CN)
level.
]
complex and INT5 obtained at the B3LYP
4
(
ESI/MS) was performed on an Applied Biosystems 3200 Q-trap
mass spectrometer.
Catalytic Oxidation of Alkanes. Typically the oxidant was added
to a pink solution of (PPh ) [Mn(N)(CN) ] in CF CH OH containing
RESULTS AND DISCUSSION
■
Catalytic Oxidation of Alkanes. Although [Mn(N)-
2
4
2
4
3
2
−
substrate and acetic acid at 23 °C. The mixture was vigorously stirred
under argon. Chlorobenzene was then added as an internal standard,
and the mixture was analyzed by GC and GC/MS at various time
(CN)4] is a highly efficient catalyst for the oxidation of alkenes
and alcohols in CH CN/CH COOH by H O at room
3
3
2
2
temperature, when excess cyclohexane was used as substrate
under the same conditions, the product yield was only 25%
Table 1, entry 1). However, when CF CH OH was used as the
3 2
t
intervals. BuOOH and MPPH were added using a syringe pump at 10−
2
5 μL/min.
(
For catalytic oxidation with MPPH, product analysis was carried out
solvent instead of CH CN, the yield became 85%, which was
when all MPPH was consumed, since MPPH itself will decompose in
3
further increased to 97% (turnover number, TON = 97) when a
small amount of acetic acid (0.25 M) was added (entry 2). By
adding more substrate and oxidant, a TON of 960 could be
achieved (entry 4). High yields of >95% were also obtained in the
oxidation of various other alkanes. For linear alkanes, oxidation
occurred only at the secondary C−H bonds (entry 7−8); no
products arising from oxidation of primary C−H bonds were
observed for these substrates. In the oxidation of trans- and cis-
1,2-dimethylcyclohexane, only 1% of epimerized products were
observed (entry 10−11), suggesting that there are no freely
diffusing alkyl radicals in the catalytic reaction. This is also
supported by the formation of only 1% bromocyclohexane in the
the GC injection port to give a variety of products. The consumption of
MPPH was monitored by adding PPh to the solution; when an aliquot
3
of the solution did not produce any PPh O (as monitored by GC)
3
from PPh , then it was assumed that all MPPH was consumed.
3
Kinetic isotope effects (KIEs) were investigated by using an
equimolar mixture of cyclohexane and cyclohexane-d12 as substrate.
The organic products were quantified and identified by GC and GC/
MS. The KIE value was obtained by taking the ratio of the
corresponding areas from GC measurements.
For 1 O-labeling experiments, the O/ O compositions of cyclo-
8
16
18
hexanol and cyclohexanone were determined from the relative
1
6
18
abundances of the peaks at m/z = 100( O), 102( O) for cyclohexanol
16
18
and 98( O), 100( O) for cyclohexanone in GC/MS(EI).
Isolation of Solid from Oxidation of Mn(V). H O (12.5 mmol)
oxidation of cyclohexane in the presence of BrCCl , which is an
2
2
3
was added to a pink solution of (PPh ) [Mn(N)(CN) ] (0.125 mmol)
efficient scavenger for alkyl radicals (entry 3).
4
2
4
in 15 mL of CF CH OH containing 1 mL of cyclohexane and 12.5 mmol
3
2
The oxidation of secondary C−H bonds produced a mixture of
the corresponding alcohols and ketones. The ketone was
probably derived from oxidation of the initial alcohol product.
In the competitive oxidation of a mixture of cycloheptanol and
of acetic acid at 23 °C. The mixture was stirred under argon for 6 h. The
resulting pale pink percipitate was collected by filtration, washed with
water, acetone, dichloromethane, and dried in vacuo. Yield: 15 mg
−1
(
56%). IR (KBr cm ): ν(CN) 2151. Elemental analysis calcd (%) for
2−
cyclohexane (in a mole ratio of 1:1) by [Mn(N)(CN) ] /H O ,
Mn(CN) ·3H O: Mn 25.79; C 22.55, H 2.84, N 26.30; found: Mn
4
2
2
4
2
the products consisted of 34% cycloheptanone, 8% cyclohexanol,
2
5.80; C 22.85, H 3.21, N 25.92.
DFT Calculations. All optimizations and frequency calculations
and 7% cyclohexanone, indicating that alcohol is more rapidly
17
were done at the B3LYP (Becke three-parameter Lee−Yang−Parr
oxidized than alkane by this system.
7
681
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Journal of the American Chemical Society
Article
Various other oxidants have also been used (Table 2). Good
yields (85%) of cyclohexanol and cyclohexanone were also
oxidation by H O the pseudo-first-order rate constants (k ) for
2
2
obs
−2
c-C H and c-C D are (1.05 ± 0.05) × 10 and (1.06 ± 0.07)
6
12
6
12
−
2
−1
×
10 min , respectively, at [cyclohexane] = 1.2 M and 23 °C
(Figure 1a). The same rate constants observed for the oxidation
Table 2. Effects of Various Oxidants on the Catalytic
a
Oxidation of Cyclohexane by (PPh ) [Mn(N)(CN) ]
4
2
4
product (% yield)
entry
oxidant
time (h)
alcohol
ketone
total yield (%)
1
2
3
4
tBuOOH
8
40
45
22
6
85
35
20
44
87
0
oxone
5
13
CH CO H
5
14
3
3
m-CPBA
0.5
0.5
5
37
7
b
5
Ce(IV)
38
49
0
6
Cl pyO
2
trace
a
Reaction conditions: (PPh ) [Mn(N)(CN) ], 2.5 mM; cyclohexane,
4
2
4
1
2
.2 M; oxidant, 0.25 M; CH CO H, 0.25 M; solvent, CF CH OH; T =
3 2 3 2
b
3 °C, under argon. CF CH OH/H O (7:3 v/v) was used as solvent.
3
2
2
Yield was calculated based on Ce(IV) acting as a one-electron oxidant.
t
obtained using BuOOH. However, oxone (NaHSO ), peracetic
5
acid (CH CO H), and m-chloroperbenzoic acid (m-CPBA) gave
3
3
much lower yields, while 2,6-dichloropyridine N-oxide (Cl pyO)
2
was completely ineffective. Interestingly, the one-electron
oxidant (NH ) [Ce(NO ) ]{Ce(IV)} gave a high yield of 87%
4
2
3 6
in CF CH OH/H O (7:3 v/v) within 30 min. The yield is
3
2
2
similar (85%) when the oxidation was carried out in
CF CH OH/0.1 M CF CO H in H O (7:3 v/v) (Table 3,
Figure 1. Pseudo-first-order fit of % yield vs time for the catalytic
3
2
3
2
2
oxidation of c-C H (1.2 M) and c-C D (1.2 M) by (PPh ) [Mn-
entry 2). When the Ce(IV) oxidation was carried out in
6
12
6
12
4 2
1
8
18
(N)(CN) ] (2.5 mM) in the presence of CH CO H (0.25 M) at 23 °C.
4
3
2
CF CH OH/H O (96% O-labeled), the cyclohexanol and
3
2
2
1
8
(a) Oxidant = H O (0.25 M) in CF CH OH. (b) Oxidant = Ce(IV)
2 2 3 2
cyclohexanone produced were 85% and 87% O-labeled,
respectively, indicating that the oxygen source came from
water. Oxidation of various alkanes using Ce(IV) also gave good
yields (Table 3). As in the case of H O , only oxidation of
(
0.25 M) in CF CH OH/H O (7:3 v/v).
3 2 2
of c-C H and c-C D , despite a KIE of 3.1 obtained from
2
2
6
12
6
12
secondary C−H bonds was observed in the oxidation of linear
alkanes. Also, in the oxidation of trans- and cis-1,2-dimethylcy-
clohexane, <1% of epimerized products were observed.
competition experiments, suggest that the rate-limiting step in
the catalytic reaction is the formation of the active intermediate
between [Mn(N)(CN)4]2 and H O , followed by H-atom
−
2
2
Kinetic Isotope Effects. KIEs were determined by using an
abstraction from cyclohexane by the active intermediate (see
Discussion). When 0.5 M of cyclohexane was used, the yields
became lower, but the rate constants remained the same: (1.04 ±
equimolar mixture of cyclohexane (c-C H ) and cyclohexane-
6
12
d (c-C D ) as a substrate. The KIEs for oxidation by H O and
12
6
12
2
2
0.07) × 10− min and (1.06 ± 0.05) × 10 min for c-C H
2
−1
−2
−1
Ce(IV) were found to be 3.1 ± 0.3 and 3.6 ± 0.2, respectively
6 12
(Table S2, Supporting Information).
and c-C D , respectively (Table S3 and Figure S1a, Supporting
6 12
Kinetic studies for catalytic cyclohexane oxidation by H O2
Information). Similarly, in oxidation by Ce(IV), kobs values for c-
2
−
1
and Ce(IV) were also performed using c-C H or c-C D as a
C H and c-C D are the same: (1.56 ± 0.07) × 10 and (1.55
6 12 6 12
1
6
12
6
12
−
−1
single substrate. For both oxidants, the formation of products
determined by GC) followed pseudo-first-order kinetics. In
± 0.05) × 10 min , respectively at [cyclohexane] = 1.2 M and
23 °C (Figure 1b), suggesting that the formation of the active
(
a
Table 3. Catalytic Oxidation of Various Alkanes by (PPh ) [Mn(N)(CN) ]/Ce(IV)
4
2
4
total
b
entry
1
substrate
cyclohexane
product (% yield)
% yield
cyclohexanol (38), cyclohexanone (49)
87
85
80
72
67
90
80
c
2
cyclohexane
cycloheptane
cyclooctane
cyclohexanol (41%), cyclohexanone (44%)
cycloheptanol (39), cycloheptanone (41)
cyclooctanol (31), cyclooctanone (41)
3
4
5
6
7
n-heptane
2-heptanol (18), 3-heptanol (19), 4-heptanol (10), 2-heptanone (7), 3-heptanone (8), 4-heptanone (5)
2,3-dimethyl-2-butanol (90)
2,3-dimethylbutane
trans-1,2-dimethyl-
cyclohexane
trans-1,2-dimethylcyclohexanol (8), cis-1,2-dimethylcyclohexanol (trace), 2,3-dimethylcyclohexanol (29),
3,4-dimethylcyclohexanol (7), 2,3- and 3,4-dimethylcyclohexanone (36)
8
cis-1,2-dimethyl-
cyclohexane
cis-1,2-dimethylcyclohexanol (75), trans-1,2-dimethylcyclohexanol (1), 2,3-dimethylcyclohexanol (8),
3,4-dimethylcyclohexanol (2), 2,3- and 3,4-dimethylcyclohexanone (3)
89
a
Reaction condition: (PPh ) [Mn(N)(CN) ], 2.5 mM; alkane, 1.2 M; (NH ) [Ce(NO ) ], 0.25 M; CH CO H, 0.25 M; solvent, CF CH OH/H O
4
2
4
4
2
3
6
3
2
3
2
2
b
IV
(
7:3 v/v); T = 23 °C. Time = 0.5 h. % yield of alcohol = (mmol of alcohol/mmol of Ce ) × 2 × 100%. % yield of ketone = (mmol of ketone/
IV
c
mmol of Ce ) × 4 × 100%. Reaction carried out in CF CH OH/0.1 M CF CO H in H O (7:3 v/v).
3
2
3
2
2
7
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Journal of the American Chemical Society
Article
intermediate between [Mn(N)(CN)4]2− and Ce(IV) is also rate-
limiting. Again the yields are lower but the rate constants are
unchanged when [cyclohexane] was decreased to 0.5 M: (1.57 ±
study the kinetics of the catalytic oxidation by UV/vis
spectrophotometry (Figure S4, Supporting Information).
Catalytic Oxidation with PhCH C(CH ) OOH (MPPH).
The interaction of a metal catalyst with a peroxide (ROOH) may
occur by homolytic (eq 1) or heterolytic (eq 2) pathways to
generate intermediates that can abstract H-atoms from alkanes.
2
3 2
.10) × 10− and (1.55 ± 0.08) × 10 min for c-C H and c-
1
−1
−1
0
6
12
2−
C D , respectively. The oxidation of [Mn(N)(CN) ] by
6
12
4
Ce(IV), however, is about 15 times faster than oxidation by
H O .
2
2
n+
+ ROOH → Mn+1‐OH + RO•
+ ROOH → Mn+2=O + ROH
M
M
(1)
(2)
The kinetics have also been studied at various temperatures.
Activation parameters obtained from Erying plots (Figure S2,
n+
‡
−1
Supporting Information) are ΔH = 16.2 ± 1.5 kcal mol and
‡
−1 −1
‡
In order to distinguish between these possibilities when
[Mn(N)(CN)4]2 is used as catalyst, we carried out catalytic
cyclohexane oxidation using PhCH C(CH ) OOH (MPPH) as
ΔS = −(20 ± 4) cal mol
K
for H O ; ΔH = 17.1 ± 1.0 kcal
2
2
−
−1
‡
−1
−1
mol and ΔS = −(13 ± 3) cal mol
K
for Ce(IV). The
negative ΔS values are consistent with the proposed bimolecular
reaction between Mn(V) and the oxidant in the rate-limiting
step.
2
3 2
the oxidant. MPPH has been used as a mechanistic probe to
distinguish between free radical (RO· from homolytic cleavage)
and metal-based (MO from heterolytic cleavage) hydrocarbon
Reaction of Mn(V) with Oxidant in the Absence of
1
2,18
oxidation.
When MPPH undergoes homolytic O−O bond
Alkane. Upon adding Ce(IV) or H O to Mn(V) in
2
2
cleavage, the resulting tert-alkoxyl radical (PhCH C(CH ) O·) is
known to undergo very rapid β-scission (eq 3); thus H-atom
CF CH OH, a pink solid which is insoluble in common solvents
2
3 2
3
2
−1
was gradually formed. IR shows a ν band at 2151 cm , which
CN
−1
abstraction from an alkane by PhCH C(CH ) O· cannot occur.
is shifted from 2116 cm in the Mn(V) catalyst (Figure S3,
Supporting Information). Mn, C, H, and N analyses approx-
imately correspond to the formula “Mn(CN) ·3H O”. This
2 3 2
Subsequent reactions of PhCH · will result in products such as
2
PhCH OH and PhCHO. On the other hand, heterolytic cleavage
2
4
2
of MPPH will result in the formation of PhCH C(CH ) OH
insoluble solid is probably a cyano-bridged Mn-CN-Mn polymer.
When the oxidant is H O , O could be detected (by GC/TCD)
2
3 2
(
MPPOH) and MO (eq 2); the latter species can oxidize RH
2
2
2
to give ROH.
in 78% yield in the absence of alkane, suggesting that Mn(V) also
catalyzes the disproportionation of H O . The kinetics of this
•
•
2
2
PhCH C(CH ) O → PhCH + (CH ) CO
(3)
2
3 2
2
3 2
reaction has been followed by monitoring the yield of O as a
2
When MPPH was used as oxidant, apparently products arising
from both heterolytic (MPPOH, cyclohexanol, and cyclo-
hexanone) and homolytic cleavage (benzyl alcohol and
benzaldehyde) of MPPH were observed (Table 4). However,
function of time (Figure 2). kobs was found to be (1.08 ± 0.09) ×
Table 4. Catalytic Oxidation of Cyclohexane by
a
(
PPh ) [Mn(N)(CN) ]/MPPH
4 2 4
products (% yield)
heterolytic cleavage homolytic cleavage
substrate
b
c-C H
c-C H OH (9), c-C H O (3), PhCH OH (16), PhCHO (14),
6
12
6
11
6
10
2
MPPOH (65)
(CH ) CO (27)
3
2
c
c-C H
c-C H OH (12),
PhCH OH (11), PhCHO (7),
6
12
6
11
2
c-C H O (48),
(CH ) CO (16)
6
10
3 2
MPPOH (75)
b
c
c-C H
c-C H OH (25),
PhCH OH (9), PhCHO (11),
Figure 2. Pseudo-first-order fit of % yield of O vs time for the reaction
8
16
8
15
2
2
c-C H O (36),
(CH ) CO (20)
8
14
3 2
of (PPh ) [Mn(N)(CN) ] (2.5 mM) and H O (0.25 M) in 1 mL of
4
2
4
2
2
MPPOH (78)
CF CH OH at 23 °C. % Yield = (mmol of O /1/2 mmol of H O ) ×
3
2
2
2
2
c-C H
c-C H OH (18),
PhCH OH (4), PhCHO (5),
8
16
8
15
2
100%.
c-C H O (62),
(CH ) CO (8)
8
14
3 2
MPPOH (90)
c
Nil
MPPOH (54)
PhCH OH (15), PhCHO (30),
2
(
CH ) CO (44)
3 2
0−2 min−1 at 23 °C, which is the same as the rate constant for
1
a
Reaction conditions: substrate, 1.2 M; solvent, CF CH OH, T = 23
C. (PPh ) [Mn(N)(CN) ], 2.5 mM; MPPH, 25 mM. (PPh ) [Mn-
N)(CN) ], 1 mM; MPPH, 10 mM. Theoretically yield of MPPOH +
3
2
cyclohexane oxidation by Mn(V)/H O , again in accordance
b
c
2
2
°
(
4 2 4 4 2
with the formation of the active species between Mn(V) and
H O being the rate-limiting step. In the presence of excess
4
2
2
(CH ) CO (= PhCH OH + PhCHO) = 100%.
3 2 2
C H (1.2 M), <1% O could be detected, indicating that
6
12
2
oxidation of C H by the active species competes effectively with
6
12
the disproportionation of H O . However, when excess C D
it is possible that apart from reacting with cyclohexane, the Mn
O species generated from heterolytic cleavage of MPPH may also
abstract an H-atom from the relatively weak benzylic C−H or
O−H bonds of MPPH (eqs 4 and 5). The resulting radicals
would decompose to benzyl alcohol, benzaldehyde, and other
organic products. The overall reaction is disproportionation of
MPPH catalyzed by Mn(V), which is not unexpected since we
have demonstrated in the above section that Mn(V) also
catalyzes disproportionation of H O . To confirm this
2
2
6
12
(
1.2 M) was used as substrate, 22% O could be detected; in this
2
case the competing H O disproportionation reaction becomes
2
2
significant, as a result of the slower rate of oxidation of C D by
6
12
the active species. This is the main reason for the lower yield of
organic products for C D (71%) than for C H (97%, Figure
6
12
6
12
1
a and Table S3, Supporting Information). We were unable to
obtain higher TON due to the gradual decomposition of the
catalyst to an insoluble solid. Formation of this solid also caused
the solution to become turbid, and hence we were unable to
2
2
disproportionation, Mn(V) was treated with excess MPPH in
7
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Journal of the American Chemical Society
Article
the absence of alkane. It was found that the MPPH was readily
decomposed, and the yield of MPPOH is approximately equal to
that of acetone (which should be equal to yield of benzyl alcohol
+
benzaldehyde), consistent with catalytic disproportionation of
MPPH (Table 4, entry 5). This disproportionation reaction may
account for the low yields of cyclohexanol and cyclohexanone
(
Table 4, entry 1, [Mn]/[MPPH]/[c-C H ] = 1:10:48). On the
6 12
other hand, when lower concentrations of catalyst and MPPH
were used, the yields of MPPOH, cyclohexanol, and cyclo-
hexanone increased (Table 4, entry 2, [Mn]/[MPPH]/[c-
C H ] = 1:10:120), in accordance with a faster rate of reaction of
6
12
MnO with cyclohexane than with MPPH. Moreover, when
cyclooctane, which has a lower C−H bond dissociation energy
Figure 3. Mass spectrum of the reaction mixture of (PPh ) [Mn(N)-
4
2
−1
−1
(
92.6 kcal mol ) than cyclohexane (95.4 kcal mol ) was used,
(
CN) ] (4.5 mM) and H O (5.3 mM) in CF CH OH/H O (9:1 v/v)
4 2 2 3 2 2
the yields of products arising from heterolytic cleavage of MPPH
are much higher than those arising from hemolytic cleavage
at 2 min. Insets show the expanded spectra of the peak at m/z 628 when
18
18
(a) H O was used; (b) H O (90% O-labled) was used.
2
2
2
2
(
Table 4, entry 3−4). On the basis of these results, we conclude
that catalytic alkane oxidation by MPPH occurs predominantly
VII
observed at m/z = 630, and the relatively intensity of this peak
increased with time.
via a heterolytic cleavage mechanism to generate Mn O and
t
MPPOH. When BuOOH was used as oxidant, no mixed
On the basis of the ESI/MS data, the rate constant for oxygen
peroxides such as c-C H OOCMe could be detected,
6
11
3
VII
t
exchange between Mn O and H
2
O is estimated to be (9 ± 1)
suggesting that homolytic cleavage of BuOOH to generate
−3 −1
t
12,18
×
10
When [Mn(N)(CN)4] was treated with higher concentrations
of H , a minor peak at m/z 644.1 was also observed, which is
s at 23 °C (Figures S7 and S8, Supporting Information).
BuO· also does not occur.
2−
MnVII(N)(O)(CN)4] + PhCH C(CH ) OOH
2−
O
2 2
[
2
3 2
tentatively assigned to the nitrido peroxo species, (PPh )[Mn-
4
VI
2−
•
−
→
[Mn (N)(OH)(CN) ] + PhC HC(CH ) OOH
(N)(O
tion).
)(CN)
2
4
] ·CF
CH
3
2
OH (Figure S9, Supporting Informa-
4
3 2
(4)
Mechanism of Catalytic Alkane Oxidation by [Mn(N)-
2
−
MnVII(N)(O)(CN)4] + PhCH C(CH ) OOH
2−
(CN) ] . On the basis of our experimental results, a mechanism
4
[
2
3 2
V
2−
for catalytic alkane oxidation by [Mn (N)(CN) ] /ROOH is
4
VI
2−
•
→
[Mn (N)(OH)(CN) ] + PhCH C(CH ) OO
proposed (eqs 6−8). The first step is rate-limiting O-atom
4
2
3 2
2
−
transfer from ROOH to [Mn(N)(CN) ] to generate a
(
5)
4
VII
manganese(VII) nitrido oxo active species, [Mn (N)(O)-
Detection of Active Intermediate by ESI/MS. We then
attempted to detect the active intermediate in catalytic alkane
oxidation by electrospray ionization mass spectrometry (ESI/
2−
(
CN) ] , which then oxidizes alkanes (R′H) via a H-atom
4
abstraction/O-rebound mechanism that is commonly accepted
for C−H activation by cytochrome P450 and various metal oxo
20−22
MS). The mass spectrum of (PPh ) [Mn(N)(CN) ] in
4
2
4
species.
The alcohol can be further oxidized to give the
CF CH OH exhibits a peak at m/z 512.2, which is assigned to
3
2
corresponding ketone.
−
the ion {(PPh )[Mn(N)(CN) ]} (Figure S5, Supporting
4
4
2
−
V
2−
Information). At lower m/z, [Mn(N)(CN) ] ·CF CH OH
[Mn (N)(CN) ] + ROOH
4
3
2
4
2−
and [Mn(N)(CN) ] ·2CF CH OH can also be identified at m/
k
4
3
2
6
VII
2−
→
[Mn (N)(O)(CN) ] + ROH
(6)
(7)
(8)
z 136.8 and 186.9, respectively. Intense solvent peaks such as
4
−
[
(
(CF CH OH)(CF CH O)] and [(CF CH OH) -
CF CH O)] are also found at m/z 199.3 and 299.3,
3 2 3 2 3 2 2
VII
2−
−
[Mn (N)(O)(CN) ] + R′H
4
3
2
respectively. When 1.2 equiv of H O was added to [Mn(N)-
k7
2
2
→ [Mn (N)(OH)(CN) ] + R′•
VI
2−
2
−
4
(
CN) ] , a new peak at m/z 628.1 was found, which is assigned
to the solvated nitrido oxo species, (PPh )[Mn(N)(O)(CN) ] ·
CF CH OH (Figure 3). MS/MS of this ion (m/z 628.1) shows
4
−
4
4
Mn (N)(OH)(CN) ] + R′•
VI
2−
[
4
3
2
k
two peaks at m/z 512.1 and 147.0, which are assigned to
8
V
2−
→
[Mn (N)(CN) ] + R′OH
−
−
4
{
(PPh )[Mn(N)(CN) ]} and [Mn(N)(CN) ] , respectively
4 4 3
(
Figure S6, Supporting Information). Metal oxo species are
The rates of oxidation of the various alkanes shown in Table 1
were found to be similar. Also, the rate of oxidation of C H is
known to readily undergo loss of O atoms by collision-induced
dissociation in the MS. When H2 O (90% O-labeled) was
6
12
19
18
18
the same as that of C D and is independent of [cyclohexane].
2
6 12
used, a new peak at m/z 630 appeared, consistent with the
However, when an equimolar mixture of C H and C D was
6 12 6 12
1
8
−
species {(PPh )[Mn(N)( O)(CN) ]} ·CF CH OH (Figure 3,
used as substrate, there were less deuterated products formed;
KIE = [C H OH + C H O]/[C D OH + C D O] = 3.1.
4
4
3
2
inset b). Analysis of the relative intensities of the peaks at m/z
6
11
6
10
6
11
6
10
1
8
6
28−630 indicates that the manganese oxo species is 45% O-
These results suggest that k ≪ k ≪ k ; i.e. the rate limiting step
6
7
8
VII
labeled, which could be due to some oxygen exchange between
is the formation of the active intermediate, [Mn (N)(O)-
(CN) ] . The observed similar KIEs for cyclohexane oxidation
2−
the manganese oxo species and H O. This is supported by
2
4
independent ESI/MS experiments (Figure S7, Supporting
Information) on the reaction of (PPh ) [Mn(N)(CN) ] with
by H O and Ce(IV) is in accordance with a common active
2
2
intermediate. The use of Ce(IV) to generate a metal oxo species
from a low-valent aqua complex by proton-coupled electron
4
2
4
16
18
18
H2 O in CF CH OH containing H O (96% O-labeled),
2
3
2
2
1
8
−
23
where (PPh )[Mn(N)( O)(CN) ] ·CF CH OH was also
transfer is well documented (eq 9). Since catalytic oxidation by
4
4
3
2
7
684
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Journal of the American Chemical Society
Article
2−
Scheme 1. Potential Energy Surfaces for the Oxidation of Cyclohexane by [Mn(N)(CN) ] /H O at the B3LYP Level in
4
2
2
CF CH OH Solvent
3
2
a
Scheme 2. Structures of Intermediates and Transition States at the B3LYP Level in CF CH OH Solvent
3
2
a
The nitrido ligand is in axial position and cis to the oxo ligand.
V
V
2−
IV
Ce(IV) was carried out in CF CH OH/H O (7:3 v/v), the Mn
3
2
2
[Mn (N)(CN) (OH )] + 2Ce
4
2
probably exists as an aqua species, which can be oxidized by
Ce(IV) to generate the Mn O active intermediate according
→ [MnVII(N)(O)(CN)4]2− + 2H + 2Ce
+
III
VII
(9)
to eq 9.
7
685
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Journal of the American Chemical Society
Article
DFT Calculations. In order to gain more insight into the
mechanism of alkane oxidation, DFT calculations have been
MnN bond length of 1.527 Å is slightly longer than that of
[Mn (N)(CN) ] (1.507(2) Å).
4
V
2−
7
performed. The potential energy surface for the [Mn(N)-
The possibility of alternative reaction mechanisms has also
been explored. For example, attempts were made but failed to (i)
locate TS structure for direct O atom abstraction by cyclohexane
from INT4 to form INT6; (ii) locate TS structure for 1,2-proton
CN)4]2 catalyzed oxidation of cyclohexane with H O in
−
(
2
2
CF CH OH (TFE) is shown in Scheme 1, and the structures of
3
2
the intermediates and transition states are shown in Scheme 2.
[
Mn(N)(CN)4]2 first binds with one H O and two TFE
−
shift when there is only one TFE bonded with [MnN(CN)4]
2−
2
2
2−
molecules to form [Mn(N)(CN) ·(H O )·2TFE] (INT1),
which is stabilized by intermolecular hydrogen bonding. In
and H O in INT2. Also H-atom abstraction from cyclohexane
4
2
2
2
2
by the nitrido ligand in INT5 is not feasible as the barrier height
‡
−1
INT1, H O is cis to the nitrido ligand. INT1 then undergoes a
(ΔG
= 31.2 kcal mol ) is significantly higher.
2
2
298
2
−
1
(
,2-proton shift to form [Mn(N)(CN) ·(O-OH )·2TFE]
INT4) with assistance from TFE molecules in three sequential
steps: (i) INT2, [Mn(N)(CN) (CNH)(OOH)·2TFE] , is
formed by a series of proton transfer relays from H O to
Concluding Remarks. In conclusion, we have demonstrated
that [Mn (N)(CN) ] is a highly efficient catalyst for the
4
2
V
2−
4
2
−
oxidation of alkanes by various oxidants. Unprecedented yields of
>95% are obtained for H O at room temperature. As far as we
3
2
2
2
2
adjacent TFE(1) to TFE(2) to cyanide ligand via TS1 with a
are aware, this is also the first example of manganese catalyzed
‡
−1
barrier height (ΔG ) of 14.7 kcal mol . A short O···H
alkane oxidation using Ce(IV) as the oxidant and H O as the
2
98
2
hydrogen bonding (distance ≈ 1.547 Å) is found between O(1)
of the hydroperoxo ligand (MnOOH) and TFE(1) in INT2; (ii)
rearrangement occurs so that the proton in TFE(1) is now
oxygen source, with yields >80%. We provide experimental
evidence that the active intermediate in oxidation by ROOH and
VII
Ce(IV) is a Mn(VII) nitrido oxo species, [Mn (N)(O)-
‡
2−
coordinated to the O(2) atom in MnOOH via TS2 (ΔG = 6.5
(CN)
4
] , which is also supported by DFT calculations. The
298
kcal mol− relative to INT2) and form [Mn(N)(CN) (H)·(O−
1
V
2−
catalytic alkane oxidation activity of [Mn (N)(CN)
higher in CF CH OH solvent than in CH CN. CF CH
help to stabilize the [Mn (N)(CN)
]
4
is much
OH may
4
OH)·2TFE]2 (INT3), and the MnOOH···CN hydrogen bond
is broken; (iii) a series of proton relays occurs from CN to
TFE(2) to TFE(1) to O(2) atom in MnOOH and forms INT4
via TS3 (ΔG = 8.6 kcal mol relative to INT3). The O−O in
the oxywater ligand in INT4 elongates and eventually a Mn(VII)
−
3
2
3
3
2
V
2−
(ROOH)] intermediate
4
by H-bonding, as supported by DFT calculations. This H-
‡
−1
bonding could also facilitate heterolytic O−O cleavage to
298
VII
2−
generate the [Mn (N)(O)(CN)
]
active species, as well as
4
2
−
nitrido oxo complex, [Mn(N)(O)(CN) ·(H O)·2TFE]
subsequent H-atom abstraction/O-rebound with alkane. H-
bonding between CF CH OH and cyanide was also observed
experimentally by solution IR spectroscopy. When CF CH OH
(4 vol %) was added to the Mn(V) catalyst in CH Cl , the peak at
2 2
4
2
‡
−1
(
INT5) is formed via TS4 (ΔG
= 26.1 kcal mol ). The
3
2
2
98
formation of INT5 is accompanied by a large release of Gibbs
3
2
−1
free energy (≈25 kcal mol relative to INT4). The Mn(VII) oxo
ligand in INT5 then abstracts a H-atom from cyclohexane
followed by O-rebound to give [Mn(N)(CN) ·(C H OH)·
H O)·2TFE] (INT6) via TS5 (ΔG
Since there is only one TS in this process, the O-rebound step is
2116 cm− shifted to 2127 cm (Figure S11, Supporting
1
−1
V
Information). Our results suggest that Mn N is potentially a
4
6
11
2−
‡
−1
(
= 17.2 kcal mol ).
new and useful platform for the construction of highly efficient
2
298
oxidation catalysts.
probably barrierless. The weakly bound cyclohexanol is then
2−
ASSOCIATED CONTENT
Supporting Information
Experimental details, ESI/MS, and DFT data. This materials is
available free of charge via the Internet at http://pubs.acs.org.
released from INT6 to yield [Mn(N)(CN) ] , C H OH, H O,
4
6
11
2
■
and 2TFE.
*
S
16
Natural bond orbital (NBO) charge analysis has been carried
out on INT3, TS4, and INT4 (Table S4 and Scheme S1,
Supporting Information), which shows that the O(12) bears
more negative charge than the O(11). The polarization of the
O(12)−O(11) bond in INT4 favors heterolytic bond cleavage
and forms INT5 via TS4. NBO has also been carried out on
AUTHOR INFORMATION
Corresponding Authors
K.C.L.) E-mail: kaichung@cityu.edu.hk.
T.C.L.) E-mail: bhtclau@cityu.edu.hk.
■
(
(
VII
2−
[
Mn (N)(O)(CN) ] (INT5) (Figure 4 and Table S6). In the
4
optimized structure of INT5, the MnO distance of 1.621 Å is
VII
24
Notes
slightly shorter than those of KMn O (av. 1.659 Å), while the
4
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work described in this paper was supported by the Research
Grants Council of Hong Kong (CityU 101612 and 101512) and
the Hong Kong University Grants Committee (AoE/P-03-08).
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3
2
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CH CN and CF CH OH, but the oxidation of cyclohexane is much
3
3
2
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3
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