Catalytic oxidation of alkanes by a (salen)osmium(VI) nitrido complex using H2O2 as the terminal oxidant

Article abstract of DOI:10.1039/c5cc03636d

The osmium(vi) nitrido complex, [Os^VI(N)(L)(CH₃OH)]⁺ (1, L = N,N′-bis(salicylidene)-o-cyclohexyldiamine dianion) is an efficient catalyst for the oxidation of alkanes under ambient conditions using H₂O₂ as the oxidant. Alkanes are oxidized to the corresponding alcohols and ketones, with yields up to 75% and turnover numbers up to 2230. Experimental and computational studies are consistent with a mechanism that involves O-atom transfer from H₂O₂ to [Os^VI(N)(L)]⁺ to generate an [Os^VIII(N)(O)(L)]⁺ active intermediate.

Full text of DOI:10.1039/c5cc03636d

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Catalytic oxidation of alkanes by a (salen)osmium(VI) nitrido
complex using H2O2 as the terminal oxidant
Man Chen,a,b,c Pan Yi,b Hoi-Ki Kwong,b Raymond J. Zeng,a,c Kai-Chung Lau*b and Tai-Chu Lau*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The osmium(VI) nitrido complex, [OsVI(N)(L)(CH3OH)]+ (1, L = N,N’-
In an attempt to design more robust catalysts, we plan to
bis(salicylidene)-o-cyclohexyldiamine dianion) is an efficient replace the four monodentate ligands in these metal nitrido
catalyst for the oxidation of alkanes at ambient conditions using catalysts with a quadridentate ligand such as salen or its
H2O2 as the oxidant. Alkanes are oxidized to the corresponding derivatives. Salen ligands are known to stabilize high oxidation
alcohols and ketones, with yields up to 75% and turnover number states, moreover steric and electronic effects can be readily
up to 2230. Experimental and computational studies are tuned by varying substituents on the ligand.6-9 Herein we
consistent with a mechanism that involves O-atom transfer from report the catalytic oxidation of alkanes by
H2O2 to [OsVI(N)(L)]+ to generate an [OsVIII(N)(O)(L)]+ active (salen)osmium(VI) nitrido complex [OsVI(N)(L)(CH3OH)]+ (
a
1
, L =
intermediate.
N,N’-bis(salicylidene)-o-cyclohexyldiamine dianion; Fig. 1),10
using the environmentally benign hydrogen peroxide as the
terminal oxidant.
The selective oxidation of alkanes under mild conditions
continues to be an important research area.1 A variety of
enzymes, such as cytochrome P-450 and methane
monooxygenase, are able to catalyze selective alkane
oxidation.2 On the other hand, relatively few chemical systems
are able to oxidize alkanes efficiently under mild conditions,
especially using green terminal oxidants such as H2O2 or O2.3
Transition metal complexes in oxidation states II or III are
commonly used as catalysts for alkane oxidation. On the other
hand, we recently demonstrated that high-valent d2 metal
nitrido species can function as highly efficient alkane oxidation
catalysts; they can accept an O-atom from an oxidant such as
H2O2 to generate a d0 metal nitrido oxo species as the active
oxidant. For example, we have reported that the d2
[OsVI(N)Cl4]- and [MnV(N)(CN)4]2- complexes are highly efficient
catalysts for alkane oxidation by H2O2.4,5 These square
pyramidal nitrido species possess a vacant site that can readily
bind and activate H2O2. In the case of [OsVI(N)Cl4]-, its catalytic
activity can be greatly accelerated by Lewis acids such as FeCl3
or Sc(CF3SO3)3, which readily bind to the nitrido ligand and
exert an electrophilic effect on the catalyst.4
N
OsVI
N
O
N
O
CH3OH
Fig. 1 Structure of 1.
1
was found to readily catalyze the oxidation of
cyclohexane in CH2Cl2/CH3CO2H (5:2, v/v) with H2O2 at room
temperature under argon (Table 1). Using excess cyclohexane,
a mixture of cyclohexanol (61%) and cyclohexanone (12%) was
obtained (Entry 1). There is also a small amount of
chlorocyclohexane (2%), which probably came from the
reaction of cyclohexyl radicals with CH2Cl2. The total yield is
75%, which corresponds to a turnover number (TON) of 60.
Other solvents such as CF3CH2OH or CH3CN give lower yields
(Table S1). By lowering the concentration of
1 and adding H2O2
slowly using a syringe pump, a maximum TON of 2230 could be
achieved (Table S2, entry 1). The observed higher TON when
H2O2 is added slowly suggests that the active intermediate can
also oxidize H2O2, hence keeping [H2O2] low would minimize its
decomposition. When the reaction was carried out in air, the
yield increased from 75% to 82% (Table 1, entry 2). On the
other hand, when the alkyl radical scavenger BrCCl3 was added
to the system, 6% of bromocyclohexane could be detected
(Entry 3). These results indicate that some freely diffusing
cyclohexyl radicals are formed in the system, which can
abstract halogen atoms from the solvent CH2Cl2 or from added
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BrCCl3. The cyclohexyl radical can also react with O2 to epimerization. The alcohol/ketone ratio increases when
generate a mixture of cyclohexanol and cyclohexanone.11 By [cyclohexane] increases or when [H2O2] decreases (Table S4),
using an equimolar mixture of C6H12 and C6D12 as substrate, a suggesting that the ketone is derived from further oxidation of
deuterium isotope effect of 4.2 was obtained, suggesting that the initial alcohol product by the active intermediate.
the rate-limiting step in the oxidation of cyclohexane by the
A metal catalyst may react with ROOH via homolytic
active intermediate involves C-H bond cleavage. Although we (equation 1) or heterolytic (equation 2) pathways to generate
previously found that the catalytic oxidation of alkanes by species that can abstract H
[Os(N)Cl4]- is greatly accelerated by Lewis acids,4 no
enhancement of catalytic activity was found in this case when
Lewis acids such as BF3, FeCl3 and Sc(OTf)3 were added to the
system (Table 1, entries 4-6). This result is perhaps not
−atoms from RH.
surprising, since
1 is a cationic species and its affinity for Lewis
acids is expected to be low. The UV/Vis spectrum of the
catalyst is also not affected by the presence of various Lewis
acids.
One way to distinguish between these two possibilities is
to use PhCH2C(CH3)2OOH (MPPH) as the oxidant.12-14 If MPPH
undergoes homolytic O−O bond cleavage on interaction with
the metal catalyst (equation 1), the resulting tert-alkoxyl
Table 1. Catalytic oxidation of cyclohexane by 1
/H2O2 under various conditionsa
radical PhCH2C(CH3)2O• would undergo very rapid β-scission
Product yieldb
Cy=O
12%
Total
yield
75%
82%
73%
77%
74%
73%
Entry
Catalyst
(equation 3); thus H−atom abstraction from the alkane by
Cy-OH
61%
68%
63%
63%
58%
59%
Cy-Cl
2%
2%
2%
2%
4%
3%
PhCH2C(CH3)2O• cannot occur. Subsequent reactions of
PhCH2• would result in products such as PhCH2OH and PhCHO.
On the other hand, heterolytic cleavage of MPPH would
produce PhCH2C(CH3)2OH (MPPOH) and M=O (equation 2), the
latter species may oxidize RH to give ROH. When cyclohexane
1
2
3
4
5
6
1
1c
1d
12%
8%
1/BF3
1/FeCl3
1/Sc(OTf)3
12%
12%
11%
oxidation was carried out using 1/MPPH, the major products
aReaction conditions: 1 (0.625 mM), Lewis acid (1.25 mM), cyclohexane (1.2 M),
CH2Cl2/CH3CO2H (5:2, v/v), H2O2 (50 mM, 80 equiv), T = 23 oC, time = 3h, under
argon. bProducts yields were calculated based on H2O2 acting as a two-electron
oxidant. cReaction in air. dReaction in the presence of BrCCl3 (50 mM), 6%
bromocyclohexane could also be detected.
are those derived from heterolytic cleavage of MPPH and
subsequent alkane oxidation by the M=O species, including
89% MPPOH, 40% cyclohexanol, 3% cyclohexanone and 3%
chlorocyclohexane. There are also minor products derived
from homolytic cleavage of MPPH, including 2% benzaldehyde,
7% benzyl alcohol and 1% benzyl chloride. We conclude that
MPPH undergoes predominantly heterolytic cleavage on
The catalytic oxidation of various other alkanes by 1/H2O2
has also been investigated (Table 2). As in the case of
cyclohexane, good yields were obtained and the major
products are the corresponding alcohols. Oxidation of linear
alkanes occurs only at the secondary C-H bonds (Entries 4-5).
On the other hand, oxidation of 1,2-dimethylcyclohexanes
occurs predominantly at the tertiary C-H bonds (Entries 6-7),
hence for this catalytic system the reactivity order is 30>20>>10.
Around 5% of epimerized products were observed in the
oxidation of 1,2-dimethylcyclohexanes, implying that the
tertiary alkyl radical is sufficiently long-lived to undergo some
interaction with 1.
Catalytic cyclohexane oxidation under argon using H218O2
in H216O was also carried out. The cyclohexanol product was
found to be 100% 18O-labeled, while the cyclohexanone was
12% 18O-labeled (Table S5). O-exchange between alcohol and
water is known to be very slow while O-exchange between
ketone and water occurs readily at room temperature.4 This
result indicates that the oxygen source in the organic products
came from H2O2.
All the experimental results are consistent with
a
a
Table 2. Catalytic oxidation of alkanes by 1/H2O2
Entry
Substrate
cyclohexane
cycloheptane
cyclooctane
n-hexane
Products (% yield) b
Total yield
75%
1
2
3
4
cyclohexanol (61%), cyclohexanone (12%), chlorocyclohexane (2%)
cycloheptanol (60%), cycloheptanone (10%)
70%
cyclooctanol (65%), cyclooctanone (6%)
71%
2-hexanol (24%), 3-hexanol (28%), 2-hexanone (7%), 3-hexanone (8%)
2-heptanol (23%), 3-heptanol (23%), 4-heptanol (12%),
67%
5
6
n-heptane
69%
75%
2-heptanone (5%), 3-heptanone (4%), 4-heptanone (2%)
cis-1,2-dimethylcyclohexanol (68%), trans-1,2-dimethylcyclohexanol (5%),
cis-2,3-dimethylcyclohexanol (1%), cis-3,4-dimethylcyclohexanone (1%)
trans-1,2-dimethylcyclohexanol (48%), cis-1,2-dimethylcyclohexanol (4%)
trans-2,3-dimethylcyclohexanol (8%), trans-2,3-dimethylcyclohexanone (3%),
trans-3,4-dimethylcyclohexanol (5%), trans-3,4-dimethylcyclohexanone (2%)
cis-1,2- dimethylcyclohexane
trans-1,2- dimethylcyclohexane
7
70%
aReaction conditions: 1 (0.625 mM), alkane (1.2 M), CH2Cl2/CH3CO2H (5:2, v/v), H2O2 (50 mM, 80 equiv.), under argon, T = 23 oC, t = 3h. bProducts yields were calculated
based on H2O2 acting as a two-electron oxidant.
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Scheme 1. Reaction mechanism (H2O2 is cis to the nitrido ligand) obtained at B3LYP level. ΔG (298K) free energies are in kcal/mol. Basis sets: LanL2DZ for Os and Fe, 6-31G(d) for
non-metal atoms. Solvent: CH3CO2H.
heterolytic mechanism that involves O-atom transfer from CH3CO2H, and an Os(VIII) nitrido oxo complex, cis-
ǂ
1
H2O2 to
1
, [(L)OsVI(N)]+, to generate an osmium(VIII) nitride oxo [OsVIII(N)(O)(L)]+
(
INT3) via TS2 (ΔG298 = 29.6 kcal mol− ). The
species, [(L)OsVIII(N)(O)]+, as the active intermediate which oxo ligand in Os(VIII) then abstracts
a
H-atom from
oxidizes RH via a H-atom abstraction/O-rebound mechanism in cyclohexane
followed
by
O-rebound to give
ǂ
1
the rate-limiting step to generate ROH, i.e. OsVIII=O + RH → [OsVI(N)(L)(C6H11OH)]+ (INT4) via TS3 (ΔG298 = 23.3 kcal mol− ).
OsVII-OH + R• → OsVI + ROH. The alcohol is then further The weakly bound cyclohexanol is then released from INT4 to
ǂ
oxidized by the active intermediate to generate the yield [OsVI(N)(L)]+ and C6H11OH. The ΔG298 of TS3 for the
corresponding ketone. The alkyl radicals generated from the
oxidation of C6D12 has also been computed and is found to be
initial H-atom abstraction process are sufficiently long-lived to 24.1 kcal/mol, this yields a theoretical value of 3.9 for
cause epimerization of 1,2-dimethylcyclohexane or to diffuse deuterium isotope effect, which is in good agreement with the
from the solvent cage to react with solvent, O2 or BrCCl3.
The proposed mechanism is also supported by DFT
experimental value of 4.2.
The possible effects of a Lewis acid such as FeCl3 on the
calculations. The potential energy surface (PES) for catalytic catalytic reaction were also investigated by DFT calculations
oxidation of cyclohexane by /H2O2 in CH3CO2H is shown in (Scheme S1). The mechanism is similar to that without FeCl3.
1
Scheme 1. [OsVI(N)(L)]+ first binds with one H2O2 and one The barrier heights of the O-O cleavage are almost the same
ǂ
ǂ
CH3CO2H molecule to form [OsVI(N)(L)(H2O2)]+·CH3CO2H (INT1). (via TS2a, ΔG298 = 29.6 kcal mol−1 without FeCl3 vs. ΔG298 = 29.7
H2O2 is a very poor ligand,15 however, in the presence of kcal mol− with FeCl3); this means that FeCl3 has no effect on
1
CH3CO2H the Os-(HOOH) intermediate is stabilized by the catalytic oxidation, which is consistent with our
hydrogen bonding. This explains the activating effects of experimental observations. We have also studied the reaction
CH3CO2H and similar molecules such as CF3CH2OH in catalytic pathways with the H2O2 initially bound to osmium trans to the
oxidation by H2O2.5,16 In INT1, H2O2 is cis to the nitrido ligand. nitrido ligand, which forms trans-[OsVIII(N)(O)(L)]+ as the active
INT1 then undergoes proton transfer from H2O2 to CH3CO2H to intermediate (Scheme S2). The reaction mechanism is also
generate an Os-OOH species (INT2) via TS1 with a barrier similar, however in this case the barrier for the O-O cleavage
ǂ
1
height (ΔG298 ) of 28.4 kcal mol−1. O-O bond elongation with process (39.1 kcal mol− ) is much higher than that of the
1
concomitant proton rebound then occurs to produce H2O, pathway with initial cis binding of H2O2 (29.6 kcal mol−
,
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13 P. A. MacFaul, K. U. Ingold, D. D. M. Wayner and L. Que, Jr., J.
Am. Chem. Soc., 1997, 119, 10594.
Scheme 1). The barrier for subsequent H-atom abstraction
from cyclohexane by trans-[OsVIII(N)(O)(L)]+ (31.5 kcal mol−1) is
also much higher than that by cis-[OsVIII(N)(O)(L)]+ (23.3 kcal
mol−1).
14 K. U. Ingold and P. A. MacFaul, in Biomimetic Oxidations
Catalyzed by Transition Metal Complexes, ed. B. Meunier,
Imperial College Press, London, 2000, pp. 45-89.
15 A. G. DiPasquale and J. M. Mayer, J. Am. Chem. Soc., 2008,
130, 1812.
In conclusion, we have demonstrated that the d2
(salen)osmium(VI) nitrido complex
1 is a robust and efficient
16 H. K. Kwong, P. K. Lo, K. C. Lau and T. C. Lau, Chem. Commun.,
2011, 47, 4273.
catalyst for alkane oxidation by H2O2. Our work suggests that a
new class of oxidation catalysts may be designed based on the
d2 metal nitrido platform with a variety of multidentate
ancilliary ligands.
The work described in this paper was supported by Hong
Kong University Grants Committee Area of Excellence Scheme
(AoE/P-03-08), the Research Grants Council of Hong Kong
(CityU 101713 and CityU 101512), the Shenzhen Science and
Technology Research Grant (JCYJ20120613115247045) and the
Shenzhen Research Institute, City University of Hong Kong.
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