Re-evaluating selectivity as a determining factor in peroxidative methane oxidation by multimetallic copper complexes

Article abstract of DOI:10.1039/c5cy00462d

A series of multimetallic copper(ii) complexes have been re-investigated for methane oxidation with H₂O₂. The preparation and properties of trinuclear copper(ii) complexes of the form [Cu₃(triazole)_n(OH₂)_12-n] (n = 8, 10) are reported. While these complexes are trimeric in the solid-state, ¹H NMR studies suggest that facile ligand dissociation occurs in solution. The oxidation of cyclohexane with H₂O₂ catalyzed by [Cu₃(triazole)_n(OH₂)_12-n] (n = 8, 10) is compared against a literature known oxo-centered tetrameric cluster (Angew. Chem., Int. Ed., 2005, 44, 4345) and these catalysts display moderate activities. The series have also been investigated in methane oxidation at 30 bar and 40 °C. Analytical techniques including a solvent suppression ¹H NMR method have been applied to quantify the liquid- and gas-phase products. The multi-metallic copper(ii) complexes and copper(ii) nitrate control samples produce only methanol and CO₂. While TONs for methanol production range from 1.4-4.6 in all cases approximately 50 times the amount of CO₂ is produced relative to methanol. We conclude that selectivity is a determining factor in methane oxidation under these conditions and should be considered in future studies.

Full text of DOI:10.1039/c5cy00462d

Catalysis
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PAPER
Re-evaluating selectivity as a determining factor in
peroxidative methane oxidation by multimetallic
copper complexes†
Cite this: Catal. Sci. Technol., 2015,
5, 4108
David Palomas,a Christos Kalamaras,b Peter Haycock,a Andrew J. P. White,a
b
c
a
*
Klaus Hellgardt, Andrew Horton and Mark R. Crimmin
A series of multimetallic copperIJII) complexes have been re-investigated for methane oxidation with H2O2.
The preparation and properties of trinuclear copperIJII) complexes of the form ijCu3IJtriazole)nIJOH2)12−n] (n =
8, 10) are reported. While these complexes are trimeric in the solid-state, 1H NMR studies suggest that
facile ligand dissociation occurs in solution. The oxidation of cyclohexane with H2O2 catalyzed by
ijCu3IJtriazole)nIJOH2)12−n] (n = 8, 10) is compared against a literature known oxo-centered tetrameric cluster
(Angew. Chem., Int. Ed., 2005, 44, 4345) and these catalysts display moderate activities. The series have
also been investigated in methane oxidation at 30 bar and 40 °C. Analytical techniques including a solvent
suppression 1H NMR method have been applied to quantify the liquid- and gas-phase products. The multi-
metallic copperIJII) complexes and copperIJII) nitrate control samples produce only methanol and CO2. While
TONs for methanol production range from 1.4–4.6 in all cases approximately 50 times the amount of CO2
is produced relative to methanol. We conclude that selectivity is a determining factor in methane oxidation
under these conditions and should be considered in future studies.
Received 27th March 2015,
Accepted 17th June 2015
DOI: 10.1039/c5cy00462d
www.rsc.org/catalysis
While these studies have prompted renewed investigation
of dinucleating ligand systems capable of the stabilisation of
1. Introduction
The structural correlation between the postulated active site
of particulate methane mono-oxygenase (pMMO) and meth-
ane oxidizing zeolite Cu-ZSM-5 has provoked interest in the
use of multi-metallic copper complexes for methane oxida-
tion.1 In 2010, Rosenzweig and co-workers cloned and
expressed in E. coli. a series of water-soluble proteins that cor-
respond to small fragments of the enzyme pMMO. One con-
clusion of this study was that the most likely active site of the
enzyme is comprised of a dicopper cofactor ligated by a series
of histidine residues.2 Schoonheydt and co-workers have con-
cluded that methane oxidation can occur at a similarly dinu-
clear, bent bisIJμ-oxo)dicopper core stabilized on ZSM-5 or
mordenite zeolites.3 Furthermore, the direct oxidation of
methane with O2 can be catalyzed by copper ions on meso-
porous silica SBA-15, with the selective formation of
formaldehyde.4
dicopper complexes and the study of their oxidation chemis-
try,5 catalytic studies of methane oxidation are limited.6–8 For
example, in 2005 Pombeiro and co-workers reported a series
of tri- and tetrametallic copperIJII) complexes supported by
ethanolamine ligands capable of not only the peroxidative
oxidation of cyclohexane but also methane.6 In late 2010, a
patent was filed describing the use of a series of triazole
ligands to support putative bimetallic copperIJII) complexes
capable of methane oxidation with some exceptional activi-
ties and liquid-phase selectivities.7 More recently, inspired by
the hypothesis that pMMO oxidises methane at a trimetallic
rather than bimetallic site, Chan and co-workers disclosed a
series of trinucleating ligands believed to stabilise a tricopper
complex capable of the peroxidative oxidation of methane to
methanol with turnover numbers (TON) of up to 18 for reac-
tions conducted in H2O at atmospheric pressure and ambient
temperature (Fig. 1).8 While the authors suggest a model
based upon a hydrophobic effect to explain the stability of
methanol to over-oxidation, no analysis is presented on the
potential gaseous products of over-oxidation, CO and CO2.8b
Herein we re-investigate Pombeiro's ethanolamine-based
catalyst6 and describe the preparation and properties of a
series of triazole-based catalysts which, while previously
claimed as potent methane oxidation catalysts, are ill-
described in terms of synthesis, structure and catalytic
a Department of Chemistry, Imperial College London, South Kensington,
London, SW7 2AZ, UK. E-mail: m.crimmin@imperial.ac.uk
b Department of Chemical Engineering, Imperial College London, South
Kensington, London, SW7 2AZ, UK
c PTI/RE Experimentation, Emerging Technologies Shell Global Solutions
International B.V, P.O Box 38000, 1030 BN, Amsterdam, The Netherlands
† Electronic supplementary information (ESI) available. CCDC 1028073–1028074.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5cy00462d
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for 3 days at 30 °C. 3c was as a light blue powder (170 mg,
0.252 mmol, 28%). Mass spec. (ESI, +ve mode) 263.9 [75%,
CuIJ2c)IJNCMe)], 419.0 [40%, CuIJ2c)2IJOH2)2], 497.0 [20%,
CuIJ2c)2IJdmso)IJOH2)2]. UV-vis (H2O, 25 °C) 204 (sh, ε = 27 900
dm3 mol−1 cm−1), 261 (ε = 2010 dm3 mol−1 cm−1). Infrared
(ATR cell, cm−1) 3131, 1561, 1459, 1287, 1018, 707. Elemental
analysis calc. for C18H18Cu2N10O12 C, 31.18; H, 2.62; N, 20.20
found C, 31.32; H, 2.58; N, 20.05.
Fig. 1 Multi-metallic Cu-based methane oxidation catalysts.
3d: a solution of CuIJNO3)2·2.5H2O (105 mg, 0.45 mmol) in
MeCN (3 mL) was added to a solution of 2d (73 mg, 0.45
mmol) in MeCN (3 mL). Precipitation of a blue solid is
observed immediately and the reaction mixture was stirred
for 3 days at 30 °C. The product was as a light blue powder
(85 mg, 0.126 mmol, 27%). UV-vis (H2O, 25 °C) 219 nm (ε =
16 220 dm3 mol−1 cm−1), 266 nm (ε = 7150 dm3 mol−1 cm−1).
Mass spec. (ESI, +ve mode) 381.09 [100%, CuIJ2d)2]. Infrared
(ATR cell, cm−1) 3126, 1598, 1551, 1439, 1354, 1299, 1074,
782. Elemental analysis calc. for C16H16Cu2N12O12 C, 27.63;
H, 2.32; N, 24.17 found C, 27.51; H, 2.41; N, 23.98.
procedure.7 We report analytical methods to elucidate both
the liquid- and gas-phase products formed from peroxidative
oxidation of methane. We conclude that selectivity is a deter-
mining factor in these reactions and that reporting mass bal-
ance for the carbon containing products is an essential
approach to properly evaluating catalyst performance.
2. Experimental section
For the preparation of the triazole ligands including a
bisIJtriazole) and the details of the cyclohexane oxidation
studies see the ESI.†
Single-crystal X-ray diffraction data
Although single crystal X-ray diffraction studies of 3b (ref. 9)
and 4 (ref. 6) have been conducted previously, in both cases
the compounds crystallised as polymorphs of the known
structures. While analysis of single crystals of complex 3a
confirmed the connectivity represented in Scheme 1, these
data were not of publishable quality. Data are provided in
Fig. 3 and 5 and Table 1, these are included to demonstrate
catalyst characterisation.
General procedure for the synthesis of 3a–d
A solution of ijCuIJNO3)2·2.5H2O] in acetonitrile was added to
a solution of an equimolar amount of triazole ligand in aceto-
nitrile. The reaction mixture was stirred for 3 days at 30 °C, a
blue precipitate was observed. The product was isolated by
filtration, washed with acetonitrile and hexane and dried
under vacuum for 24 h at 25 °C.
3a: a solution of 2a (1 mmol, 145 mg) in MeCN (4 mL)
and was added to CuIJNO3)2·2.5H2O (1 mmol, 232 mg) in
MeCN (4 mL). After 3 days stirring at 30 °C 3a was isolated as
a blue solid (153 mg, 0.074 mmol, 75%). Infrared (ATR cell,
cm−1) 3120, 1598, 1551, 1440, 1286, 1073, 776. UV-vis (H2O,
25 °C) 221 (ε = 74 600 dm3 mol−1 cm−1), 279 (sh, ε = 8790 dm3
mol−1 cm−1). Mass spec. (ESI, +ve mode) 493.9 [30%,
Cu2IJ2a)2IJdmso)], 415.0 [100%, Cu2IJ2a)2] 353.6 [30%, CuIJ2a)2].
Repeated attempts to acquire satisfactory CHN analysis
failed.
3b: a solution of 2b (2.4 mmol, 352 mg) in MeCN (12 mL)
was added to CuIJNO3)2·2.5H2O (2.4 mmol, 557 mg) in MeCN
(8 mL). After 3 days stirring at 30 °C, 3b was isolated as a
blue solid (460 mg, 0.26 mmol, 87% yield). Infrared (ATR cell,
cm−1) 3121, 1557, 1448, 1390, 1289, 1023, 757. UV-vis (H2O,
25 °C) 228 (ε = 22 000 dm3 mol−1 cm−1). 265 nm IJε = 14 500
dm3 mol−1 cm−1). Mass spec. (ESI, +ve mode) 390.9 (30%,
CuIJ2b)2IJOH2)2), 280.9 [100%, CuIJ2b)IJOH2)4], 263.9 [75%,
CuIJ2b)IJOH2)3], 243.9 [40%, CuIJ2b)IJOH2)2], 227.0 [60%,
Methane oxidation
Experiments were carried out in a reactor containing a
Teflon-lined vessel with a total volume of 50 mL. The vessel
was charged with the catalyst (0.017 mmol), 15 mL solvent,
an aqueous solution of H2O2 (30 wt%, 5 mL, 50 mmol) and
HNO3 (70 wt% in H2O, 65 μL, 1 mmol). After sealing, the
reactor was purged with methane (4×, P = 30 bar) and then
pressurised to 30 bar. The reactor was heated to the desired
temperature and vigorously stirred with a Teflon coated mag-
netic bar at 1500–2000 rpm. After the reaction time is fin-
ished the reactor was cooled with an ice-water bath, degassed
CuIJ2a)IJOH2)]. Elemental analysis calc. for C56H57Cu3N38O22.5
:
C, 37.10; H, 3.17; N, 29.36 found C, 37.14; H, 3.25; N, 29.34.
3c: a solution of CuIJNO3)2·2.5H2O (208 mg, 0.9 mmol) in
MeCN (5 mL) was added to a solution of 2c (143 mg, 0.9
mmol) in MeCN (5 mL). Precipitation of a blue solid is
observed immediately and the reaction mixture was stirred
Scheme 1 Preparation of Cu-triazole complexes.
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13C decoupling to remove 13C satellite signals was achieved
using the pulse program wetdc. All relevant parameters were
selected automatically via execution of the Bruker AU pro-
gram au_lc1d after obtaining a 1H NMR spectrum showing
the solvent chemical shifts.
Table
experiments
1 Selected acquisition data from single-crystal diffraction
Data
3b
4
Molecular formula
C56H56Cu3N32O4·6IJNO3)
·0.5IJH2O)
C
ijBF4]2
24H52B4Cu4N4O17
The custom-made apparatus shown in Fig. 2 allowed
quantification of the gas-phase products from the selective
oxidation of methane over homogeneous copper catalysts.
The autoclave reactor was charged as described above. After
20 h, the loop (0.5 mL) was filled by the product gas from the
exit of the reactor and injected by a 6-way chromatographic
valve with electrical actuator to the mass spectrometer (Euro-
pean Spectrometry System II) for analysis using a constant
flow of carrier gas (Ar, 300 mL min−1). The mass numbers
Formula weight
1813.01
1139.71
(g mol−1
)
Crystal system
Space group
Temperature (K)
a (Å)
b (Å)
c (Å)
Triclinic
Monoclinic
C2/c
173
14.4651(5)
19.3815(6)
14.1054(5)
—
90.991(3)
—
¯
P1
173
14.1016(5)
14.4022(6)
20.4554(9)
94.939IJ4)°
95.386IJ3)°
119.157IJ4)°
3570.2(3)
2
0.991
1.687
0.0401
0.1021
α (deg)
β (deg)
γ (deg)
V (Å3)
3953.9(2)
4
2.237
+
IJm/z), 15 IJCH3 ), 16 (CH4), 18 (H2O), 28 (CO), 31 IJCH3O+), 32
Z
(O2) and 44 (CO2) were continuously monitored using
Quadstar 32bit software. The purity of the gases used (e.g.,
CH4, CO2, Ar) all provided by BOC gases UK was higher
than 99.95%. The amount of CO2 (μmol) is calculated based
on multiplying the area under the CO2 molar fraction peak
(ppm s) with the carrier gas molar flow rate (mol s−1).
μ (mm−1
)
)
ρ (g cm−3
1.915
R1 (obs)a
0.0341
0.0846
4161/3590
a
wR2 (all data)
Unique/observed
reflections
Rint
13 924/10 946
0.0191
0.0261
P
P
P
P
a
2
R1
=
||Fo| − |Fc||/ |Fo|; wR2 = { ijwIJFo − Fc2)2]/ ijwIJFo2)2]}1/2
;
w
−1 = σ2IJFo2) + (aP)2 + bP.
3. Results and discussion
Catalyst preparation
Condensation of amines and N,N-dimethylformamide azine
and opened. Samples were filtered before being analyzed by
GC-FID and 1H NMR spectroscopy. Liquid phase analytes
were quantified by comparison to standard samples of
known concentration.
dihydrochloride (1) allowed preparation of a series of
4-substituted triazoles (R = Ph, 2a; 2-Py, 2b; Bn, 2c; CH22-Py,
2d).10 Triazole-based copper complexes of ligands 2a–d were
generated from a 1 : 1 reaction between the triazole and CuIJII)
salt at 30 °C. Complexes 3a–d were obtained from ijCuIJNO3)2
·2.5H2O] in CH3CN (Scheme 1).9 In all cases, complexes pre-
cipitated from the reaction media and were isolated as blue
solids by filtration. The copperIJII) complexes have been char-
acterized by a combination of 1H NMR, UV-vis and infrared
spectroscopy, CHN analysis, mass spectrometry and single-
crystal X-ray diffraction studies.
Analytical methods
Liquid-phase products of methane oxidation were analyzed
by GC-FID using an Agilent 7820 equipped with a DB-WAX
column and a double solvent suppression 1H NMR method.
1H NMR spectra were measured at a frequency of 500.13
MHz using a Bruker AVANCE III HD 500 spectrometer operat-
ing at 298 K. A small amount of D2O was added to the sam-
ple. Double solvent suppression (HOD/CH3CN), along with
X-ray diffraction experiments on single crystals of 3a and
3b grown from slow evaporation of concentrated aqueous
solutions revealed a structural motif that has been observed
Fig. 2 Apparatus for analysis and quantification of gas-phase analytes.
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assigned to the n → π* transition of the pyridine unit within
these complexes.12 Mass spectrometry on samples 3a–d
revealed a series of peaks with m/z consistent with mono-
and dicopper fragments. Using electrospray ionization, peaks
associated with the intact trimeric structures of 3a or 3b were
not observed. In the infrared spectra, the CN stretches of
the triazole moiety of 3a–d are observed between 1551–1598
cm−1 and differ only slightly from those of the free ligand.
Magnetic measurements have been made on 3b previously.12
1
Despite their paramagnetic nature, H NMR data collected
on 3a and 3b in D2O demonstrate the expected resonances
albeit broadened and within the chemical shift window δ =
5–25 ppm. In solution, only one ligand environment is
1
observed by H NMR spectroscopy. Analysis of the X-ray data
Fig.
3 The crystal structure of 3b. H-atoms omitted for clarity.
Selected bond lengths (Å), CuIJ1)–NIJ21) 1.996(2), CuIJ1)–NIJ81) 2.023(2),
CuIJ1)–NIJ101) 2.170(2), CuIJ1)–NIJ1) 2.205(2), CuIJ1)–NIJ41) 2.218(2), CuIJ2)–
NIJ42) 2.002(2), CuIJ2)–OIJ160) 2.039(2), CuIJ2)–OIJ170) 2.321(2).
suggests that at least three environments should be present
in the static structure (one terminal and two bridging, trans-
and cis- to the unique terminal ligand). Upon mixing of sam-
ples of 3a and 3b in D2O only two ligand environments are
observed by 1H NMR spectroscopy one for triazole 2a and a
second for triazole 2b. Furthermore, addition of samples of
2a to trimeric copper complex 3b in either D2O or dmso-d6
gives data consistent with a mixture of 3a and 3b in solution.
In order to obtain an idea of the aggregation state of the spe-
cies observed in solution by 1H NMR spectroscopy we
performed a series of DOSY experiments. Diffusion coeffi-
cients were measured at 25 °C in d6-dmso for 2a–d (0.1–0.2
M, D = 3.9–4.2 × 10−10 m2 s−1) and 3a–b (0.01 M, D = 2.6–3.1 ×
10−10 m2 s−1).13
previously.9 Hence, in the solid state both 3a and 3b exist as
a tricopper array in which an octahedral central copper atom
is complexed by the nitrogen of six triazole ligands (Fig. 3).
These ligands bridge to two terminal copper atoms through
coordination of the α-heteroatom and the coordination
sphere at each terminus is completed by either an additional
triazole ligand or H2O. While both complexes can be related
by an S2-symmetry operation in the solid-state, 3a takes the
form ijCu3IJ2a)10IJOH2)2] and 3b ijCu3IJ2b)8IJOH2)4] and these
two species differ due to the number of water/triazole ligands
coordinated to the terminal copper atoms (Scheme 1).
Attempts to grow single crystals of 3c and 3d failed.11
UV-vis measurements on aqueous solutions of 3a–d dis-
play a maxima at very short-wavelengths with an associated
shoulder shifted to slightly longer wavelengths and tailing
into the visible region (Fig. 4). These absorptions are tenta-
tively assigned to a π → π* of the triazole ligand and MLCT
respectively. For example, 3a demonstrates transitions at 221
and 279 nm. Complexes 3b and 3d also show a second max-
ima centred close to 270 nm overlapping the shoulder and
The DOSY data suggest that the triazole ligand environ-
ments observed by 1H NMR spectroscopy are not associated
with a large trimetallic cluster. In combination with the
cross-over experiments these data can be accounted for by
two processes: (i) the reversible dissociation of the terminal
ligand of the trimeric complexes 3a–b in solution and the
1
observation of the time-averaged H NMR environment of the
triazole contact shifted by the paramagnetic copper complex
or (ii) disintegration of the cluster to monomeric species
including ijCuIJtriazole)nIJOH2)6−n] (n = 3, 4) in solution and
facile inter- and intra-molecular ligand exchange within the
isomers of this series. A monomeric octahedral Mn complex
of ligand 2b has been reported previously.14
The oxo-centered tetrameric copper cluster 4 was obtained
as green crystals according to the procedure reported by
Pombeiro and co-workers.6 Analysis of 4 from a series of
preparations provided suitable mass spectrometry and single
crystal data on 4 that is consistent with the literature (Fig. 5).
Cyclohexane oxidation
As a means to benchmark the catalysts we investigated the
oxidation of cyclohexane. Reactions were conducted in MeCN
using aq. H2O2 as an oxidant and HNO3 as an additive. Com-
plex 4 was originally reported by Pombeiro and co-workers
for cyclohexane oxidation at 25 °C and data were reproduced
with independently synthesized samples with minor modifica-
tion of the catalyst/peroxide ratio (Table 2, entries 1 and 2).6
Fig. 4 UV-vis data on 3a–d.
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is not entirely clear, one possibility is that the acid helps to
slow the decomposition of the peroxide.17,19
Methane oxidation
The oxidation of methane was performed with catalysts 3a–b
and 4 in liquid–gas phase experiments using 1000–3000
equiv. of H2O2 per equiv. of catalyst. Reactions were
conducted at 3 bar MeH at 40 °C using a Teflon lined batch
reactor. While previous studies have reported 4 and triazole-
based catalysts for methane oxidation, full analysis of the
both the liquid and gas-phase analytes has not been forth-
coming.6,7 As such, the selectivities of these reactions are
unknown and the usefulness of the catalyst cannot be accu-
rately evaluated.
A series of analytical methods were developed to address
this problem. Liquid-phase oxidation products were analyzed
by a combination of GC-FID analysis and a double-solvent
suppression 1H NMR method.20 Based on the work of
Hutchings and co-workers,21 an 1H NMR method was devel-
oped that allows quantitative analysis of C1- and C2-oxidation
products MeOH, EtOH, HCO2H, MeC(O)H in mixtures of
proteo-water and acetonitrile (Fig. 6). Both formaldehyde and
acetic acid are not observed in this experiment and while we
attribute this to reversible exchange of H2CIJOH2)2 with H2O
and overlap of the diagnostic methyl resonance of MeCO2H
with that of the suppressed MeCN signal, acetic acid is read-
ily quantified by GC-FID. Quantitative gas-phase analysis of
CO2 was performed by sampling of an aliquot directly from
the high-pressure reactor, diluting with a known volume of a
carrier gas and analyzing by mass spectrometry. While this
method also has the capacity to quantify CO and O2/MeH,
the former was not observed during methane oxidation and
the latter do not provide direct information about the selec-
tivity of methane oxidation.
Complexes 3a–b and 4 proved active for the oxidation of
methane to methanol (Table 3, entries 1–3). In all cases
both the liquid- and gas-phase analytes were quantified.
Comparison of the data in Table 3 reveals that the double
solvent-suppression NMR method reproduces the GC-FID
data for the quantification of methanol within acceptable
error. While in all cases, triazole based catalysts 3a–b dem-
onstrated a higher activity for methanol production than 4,
TONs for MeOH for the series range from 1.4–4.6. These
data are consistent with a recent report of Chan and co-
workers that demonstrated a trimeric copper catalyst capa-
ble of methane oxidation to methanol with a TON of up to
18 using portionwise addition of 100 equiv. of H2O2.8
These studies have demonstrated that the productive cata-
lytic cycle is competitive with an abortive cycle that con-
sumes further equivalents of H2O2 and that optimal TONs
can be achieved by controlled addition of the H2O2 to the
reaction mixture. Based on data from lower TON experi-
ments (TON = 6), using 20 equiv. of H2O2 Chan and
coworkers suggest an efficient and selective reaction which
produces approximately 1 equiv. of methanol per 3 equiv.
Fig. 5 The crystal structure of 4. H-atoms omitted for clarity, selected
bond lengths (Å), CuIJ1)–OIJ23) 1.9253IJ15), CuIJ1)–NIJ1) 2.000(2), CuIJ1)–
OIJ4) 2.029(4) CuIJ1)–OIJ10) 2.031(4), BIJ1)–OIJ4) 1.510(5), BIJ1)–OIJ4′)
1.437(6).
Under similar conditions 3a–d give similar turnover numbers
(10.3–17.5) and selectivity with a bias towards forming the
alcohol (cyclohexanol, 68–77%; cyclohexanone, 23–32%).
Reducing the catalyst concentration by a factor of 10 resulted
in a 5000 : 1 ratio of peroxide to catalyst and improved cata-
lyst activity. Pombeiro and co-workers first suggested using
low catalyst loadings and high peroxide to catalyst ratios
to improve catalyst performance.17 Under these conditions
3a–d oxidized cyclohexane with higher turnover numbers
(80.6–90.6) and lower selectivities (cyclohexanol, 60–62%;
cyclohexanone, 40–38%). Control experiments with ijCuIJNO3)2
·2.5H2O] under the optimized conditions gave only trace
amounts of oxidation products. Furthermore attempts to gen-
erate active catalysts in situ by mixing ijCuIJNO3)2·2.5H2O] with
2a or 2b did not reproduce the activity of as isolated samples
of 3a or 3b respectively.
The catalytic activities are approximately an order of mag-
nitude lower than those reported in the patent literature,7
but are comparable to a series of recently reported multi-
metallic copper complexes. For example, Roy and co-workers
have reported a series of tetranuclear CuIJII) Schiff-base com-
plexes for the oxidation of cyclohexane with TONs of approxi-
mately 10–20 and a slight selectivity for cyclohexanol.15 While
Chan, Yu and coworkers have reported a series of trinuclear
copper complexes with activities commensurate with those of
3a–b,8b,16 Pombeiro, Shul'pin and co-workers have reported a
significant number copperIJII) multi-metallic clusters based
on chelating N,O-based ligands with a range of activities
some of which exceed the optimized activity of 3a–b.18
A
number of studies have demonstrated the positive effect of
small amounts of acid in peroxidative alkane oxidation and
control experiments conducted without HNO3 resulted in
lower catalyst activities. While the beneficial effect of the acid
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Table 2 Cyclohexane oxidation with isolated multi-metallic copperIJII) complexes
TONd
Ratio of H2O2 :
Entry
Catalyst
catalyst
t (h)
Cyclohexanol
Cyclohexanone
Totale
1a
2b
4/HNO3
4/HNO3
400
500
72
72
10.5
11.4
4.9
3.8
15.3
15.2
3b
4b
5b
6b
3a/HNO3
3b/HNO3
3c/HNO3
3d/HNO3
500
500
500
500
20
20
20
20
12.7
13.5
7.1
4.2
4.0
3.2
3.5
16.9
17.5
10.3
11.0
7.5
7c
8c
9c
3a/HNO3
3b/HNO3
CuIJNO3)2/HNO3
5000
5000
5000
20
20
20
49.8
54.1
0
30.8
36.5
0
80.6
90.6
0
a
b
Data from ref. 6. Catalyst (0.0125 mmol), H2O2 (5 mmol), cyclohexane (0.63 mmol), HNO3 (0.12 mmol), 25 °C. Catalyst (0.01 mmol), H2O2
c
d
(5 mmol), cyclohexane (5 mmol), HNO3 (0.2 mmol). Catalyst (0.001 mmol), HNO3 (0.02 mmol). Turnover number (moles of product per mol
of catalyst) analysed by GC-FID using chlorobenzene as internal standard. e Cyclohexanol + cyclohexanone.
Fig. 6 (a) Liquid phase analytes from the oxidation of methane catalysed by 4. (b) Double solvent-suppression NMR method for the quantification
of liquid-phase analytes of methane oxidation, inset is the downfield region showing the deshielded resonances of MeCHO and HCO2H.
Table 3 Methane oxidation with isolated multi-metallic copperIJII) complexes 3a–b, 4 and ijCuIJNO3)2·2.5H2O]
MeOH
Ratio of H2O2 :
catalyst
HCO2H
(μmol)
EtOH
(μmol)
MeCO2H
(μmol)
MeCOH
(μmol)
CO
(μmol)
CO2
(μmol)
GCc
(μmol)
NMRd
(μmol)
Entry
Catalyst
TONe
1a
2a
3b
3a/HNO3
3b/HNO3
4/HNO3
3000
3000
1000
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
3700
3900
3650
64.0
73.5
69.4
57.1
77.9
71.6
3.4–3.8
4.3–4.6
1.4
4a
CuIJNO3)2/HNO3
3000
0
0
0
0
0
3000
43.8
34.9
2.1–2.6
a
b
Reaction conditions: catalyst (0.017 mmol), H2O2 (50 mmol), HNO3 (1 mmol), methane (30 bar), V = 20 mL. The literature conditions ref. 6
c
d
were scaled to V = 20 mL, catalyst (0.05 mmol), H2O2 (50 mmol), HNO3 (1 mmol), methane (30 bar). Analysed by GC-FID. Analysed by
1H-NMR following a solvent suppression protocol, DMSO as internal standard. e Turnover number (moles of product per mol of catalyst).
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of H2O2. No analysis of the gas phase products has been
forthcoming.
3 (a) M. H. Groothaert, P. J. Smeets, B. F. Sels, P. A. Jacobs
and R. A. Schoonheydt, J. Am. Chem. Soc., 2005, 127, 1394;
(b) P. J. Smeets, R. G. Hadt, J. S. Woertink, P. Vanelderen,
R. A. Schoonheydt, B. F. Sels and E. I. Solomon, J. Am. Chem.
Soc., 2010, 132, 14736.
In the current case, analysis of the gas-phase analytes
reveals production of significant quantities of CO2. For exam-
ple, the reaction of methane with 4, 1000 equiv. of H2O2 and
a HNO3 : catalyst ratio of 20 produces 70 2 μmol of methanol
and 3650 μmol of CO2. A control experiment using CuIJNO3)2
·2.5H2O in place of 3a–b or 4 revealed only minor changes on
the product distribution suggesting that, in this instance and
in the presence of such a large excess of H2O2, the ligand
sphere has very limited control on the catalytic pathway
(Table 3, entry 4). Any comment on the reaction mechanism,
or speculation that these catalysts are playing a role other
than generating peroxide radicals in situ, is unwarranted
based on the current data.
4 Y. L. An, D. An, Q. Zhang and Y. Wang, J. Phys. Chem. C,
2008, 112, 13700.
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Braun, F. F. Pfaff, S. Mebs, K. Ray and C. Limberg, J. Am.
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Inorg. Chem., 2011, 50, 2133.
6 A. M. Kirillov, M. N. Kopylovich, M. V. Kirillova, M. Haukka,
M. F. C. Guedes da Silva and A. J. L. Pombeiro, Angew.
Chem., Int. Ed., 2005, 44, 4345.
The propensity, and precedent,22 for MeCN to act as a C1-
source prompted us to further investigate the reactivity of the
solvent under the reported reaction conditions. The decom-
position of MeCN to CO2 was investigated by a further con-
trol reaction conducted without MeH, although small
amounts of CO2 where observed (48 μmol) this experiment
does not account for the large amounts of CO2 produced
under catalytic conditions. Consistent with the more facile
oxidation of methanol than methane under the reaction con-
ditions, approximately 50 times more CO2 is produced than
MeOH regardless of the nature of the catalyst.
7 R. Elgammal and S. Foister, International Patent, WO 2011/
035064 A2, 2011.
8 (a) S. I. Chan, Y.-J. Lu, P. Nagababu, S. Maji, M.-C. Hung,
M. M. Lee, I.-J. Hsu, P. D. Minh, J. C.-H. Lai, K. Y. Ng, S.
Ramalingam, S. S.-F. Yu and M. K. Chan, Angew. Chem., Int.
Ed., 2013, 52, 3731; (b) P. Nagababu, S. S.-F. Yu, S. Maji, R.
Ramu and S. I. Chan, Catal. Sci. Technol., 2014, 4, 930.
9 (a) O. G. Shakirova, A. V. Virovets, D. Y. Naumov, Y. G.
Shvedenkov, V. N. Elokhina and L. G. Lavrenova, Inorg.
Chem. Commun., 2002, 5, 690; (b) J. Liu, Y. Song, Z. Yu, J.
Zhuang, X. Huang and X. You, Polyhedron, 1999, 18, 1491.
10 (a) A. D. Naik, J. Marchand-Brynaert and Y. Garcia, Synthesis,
2008, 149; (b) S. B. Muñoz III, W. K. Foster, H.-J. Lin, C. G.
Margarit, D. A. Dickie and J. M. Smith, Inorg. Chem.,
2012, 51, 12660.
11 In subsequent experiments a 1 : 1 ligand : metal ratio implied
by the CHN analysis is assumed. Possible structural motifs
include the coordination of nitrate to Cu. See for example,
(a) A. F. Stassen, W. L. Driessen, J. G. Haassnoot and J.
Reedijk, Inorg. Chim. Acta, 2003, 350, 57; (b) S. Amaral, W. E.
Jensen, C. P. Landee, M. M. Turnbull and F. M. Woodward,
Polyhedron, 2001, 20, 1317.
4. Conclusions
We have re-investigated
a
series of triazole- and
aminoethanol-based multimetallic copperIJII) catalysts for the
oxidation of cyclohexane and methane using H2O2. We con-
clude that, regardless of the minor differences in the TON
observed for methanol production, selectivity is an important
factor in these reactions. If useful homogeneous catalysts are
to be developed that reproduce the activity of enzymatic
(pMMO) or heterogeneous (M-ZSM-5) systems then selectivity
must be considered as
reactions.
a determining factor in these
12 J. A. Joule and K. Mills, Heterocyclic Chemistry, 2010, 5th
edn., Chichester, Blackwell Publishing.
13 DOSY data were processed within Topspin using the SimFit
Alogorithm.
Acknowledgements
We are grateful to the Royal Society for the provision of a Uni-
versity Research Fellowship (MRC). Royal Dutch Shell for pro-
ject funds (DP, CK).
14 B. Ding, L. Yi, P. Cheng, H.-B. Song and H.-G. Wang,
J. Coord. Chem., 2004, 57, 771.
15 P. Roy and M. Manassero, Dalton Trans., 2010, 39, 1539.
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Yu and S. I. Chan, Adv. Synth. Catal., 2012, 354, 3275; (b)
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