Formation and reaction of O=MnV species in the oxidation of phenolic substrates with H2O2 catalysed by the dinuclear manganese(IV) 1,4,7-trimethyl-1,4,7-triazacyclononane complex [Mn IV2(μ-O)3(TMTACN)2](PF 6)2

Article abstract of DOI:10.1039/b315427k

The oxidation of phenolic substrates with H₂O₂ catalysed by [Mn^IV₂(μ-O)₃(TMTACN) ₂](PF₆)₂1, (TMTACN, 1,4,7-trimethyl-1,4,7- triazacyclononane) has been investigated by use of ESI mass spectrometry. The role of the phenols as one-electron reductants and as co-ligands in the stabilisation and reaction of an intermediate O=Mn^V species has been analysed and the presence of a variety of manganese species in solution has been explained. Our results lead to a proposed mechanism for the catalytic oxidation of phenols in this system.

Full text of DOI:10.1039/b315427k

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Formation and reaction of O᎐MnV species in the oxidation
᎐
of phenolic substrates with H2O2 catalysed by the dinuclear
manganese(IV) 1,4,7-trimethyl-1,4,7-triazacyclononane complex
[MnIV (ꢀ-O)3(TMTACN)2](PF6)2
2
Bruce C. Gilbert,*a John R. Lindsay Smith,*a Antoni Mairata i Payerasa and John Oakesb
a University of York, Department of Chemistry, Heslington, UK YO10 5DD
b Unilever Research, Port Sunlight Laboratories, Quarry Road East, Bebington, Merseyside,
UK L63 3JW
Received 1st December 2003, Accepted 19th February 2004
First published as an Advance Article on the web 18th March 2004
The oxidation of phenolic substrates with H2O2 catalysed by [MnIV2(µ-O)3(TMTACN)2](PF6)2 1, (TMTACN, 1,4,7-
trimethyl-1,4,7-triazacyclononane) has been investigated by use of ESI mass spectrometry. The role of the phenols as
one-electron reductants and as co-ligands in the stabilisation and reaction of an intermediate O᎐MnV species has
᎐
been analysed and the presence of a variety of manganese species in solution has been explained. Our results lead
to a proposed mechanism for the catalytic oxidation of phenols in this system.
reacts with periodic acid to give the corresponding biphenol
in 90% yield (the same methodology was used to convert
Introduction
Manganese complexes derived from TMTACN ligands
(e.g. 1) have been found to be highly active catalysts both for
α-terpinene to p-cymene in >95% yield). Mechanistic studies
point to an active species of the type O᎐MnV–MnIV which is
᎐
low-temperature bleaching1 and, also for the epoxidation of
formed from the oxidation of a MnIII–MnIV species by periodic
alkenes2–5 by H2O2, a reaction which has been improved by the
acid or Oxone®.11
inclusion of a co-ligand, such as oxalate6 or ascorbic acid.7
Our interest in O᎐MnV species as potential oxidants also
᎐
We have previously described8 a mechanistic investigation of
reflects the suggestion that they are the active species in systems
the oxidation of a number of azonaphthol dyes with H2O2 and
involving Mn–salen12,13 and Mn–porphyrins,14 as well as in
1, as well as related mononuclear analogues, and have reported
enzymatic cycles such as that of PSII.15 The aim of the experi-
preliminary findings relating to the oxidation of phenols.9,10 For
ments described here was to establish the formation and role
example, we have identified polyphenols and diphenoquinones
of O᎐MnV species in reactions of a range of phenols and H O
᎐
2
2
as oxidation products in the reaction of 4-methoxyphenol and
2,6-dimethoxyphenol by 1 and H2O2;9 these evidently arise from
typical coupling reactions of phenoxyl radicals and provide
support for the occurrence of one-electron transfer reactions.
From EPR experiments it is also clear that the dinuclear com-
plex 1 is capable of reduction by phenols, via a MnIII/MnIV
species, ultimately to mononuclear MnII, and that addition of
H2O2 regenerates the active oxidant.9 We have also utilised ESI-
catalysed by 1 and to study the influence of the nature of the
phenolic substrate on their stability and reactivity. ESI-MS
has thus been used to study oxidation of the phenolic substrates
3–9 shown below, chosen to provide examples with different
potential for binding to Mn (as monomers and/or dimers) and
for differing ease of oxidation. Electrospray ionisation (ESI) is
now recognised as being capable of producing intact ions, with
multiple charges, from remarkably large, complex and fragile
parent species16 (cf. the Feichtinger and Plattner17,18 study
of the detailed mechanism of oxygen transfer from iodo-
sylbenzene to a Mn()–salen complex, which identified the
oxomanganese species formed upon oxidation).
mass spectrometry to identify an O᎐MnV-containing species in
᎐
the oxidation of 4-methoxyphenol with H2O2 catalysed by 1.
We proposed that oxidation of the phenol to the appropriate
bi- and poly-phenols, as described in Scheme 1, leads to the
formation of a mixed complex, MnIII(TMTACN)(biphenol)
that is oxidised by H O to the analogous O᎐MnV complex 2,10
᎐
2
2
Results and discussion
the potential active species.
1
Speciesformedduringthereactionofphenolswith[Mn2(ꢀ-O)3-
(TMTACN)2](PF6)2 and H2O2
a
Initial ESI-MS studies; reactions of 4-methoxyphenol 3.
In a typical experiment, 3 was reacted with catalyst 1 in the
presence and absence of H2O2, and the species detected by
ESI-MS analysed as a function of time. Experiments were
carried out in aqueous solution at pH ca. 10.5 (borate buffer)
and ca. 60 ЊC, with, in some cases, added acetonitrile or
methanol. Typical concentrations employed were [1] 10Ϫ4 mol
dmϪ3, [substrate (phenol)] 10Ϫ2 mol dmϪ3 and [H2O2] 10Ϫ1 mol
dmϪ3, and reactions were followed over a period of ca. 45 min.
In the absence of H2O2, an aqueous solution of 1 gave a
mass spectrum with two major peaks, m/z 250 and 645, which
are assigned to 10 and its complex with PF6Ϫ, respectively (see
Fig. 1). When an excess of 3 was present in the ratio of 1 : 100
(1 : 3), a new peak at m/z 500 was immediately detected, suggest-
ing the occurrence of a one-electron transfer reaction from the
Scheme 1
A
possibly related oxidation of phenols by 1 and
either periodic acid or Oxone® in pyridine has been reported by
Barton and co-workers.11 For example, 2,4-di-t-butylphenol
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 1 1 7 6 – 1 1 8 0
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
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phenol to 1 to give the MnIII–MnIV species 11 (see Fig. 2), which
has been identified both by EPR spectroscopy at 77 K and
ESI-MS (see e.g. refs 9 and 10). When the mixture of 1 and 3,
at pH 10.5, was left to stand at 60 ЊC for 5 min, a new peak at
m/z 470 (assigned to 12) was also observed. When H2O2 was
added to a mixture of 1 and 3 (1 : 3 : H2O2, 1 : 100 : 500) this
became a major peak after a short induction period (ca. 5 min)
and a new peak at m/z 486 assigned to 2 also appeared (see
Fig. 3a). Further support for this assignment derives from
corresponding observations of reactions of the ethoxy counter-
part 4 (see below) and from experiments with H218O and H218O2
described previously.10
In experiments with 3, the ratio between the relative inten-
sities of 12 and 2 was found to be dependent on the concen-
tration of H2O2 used with the latter favoured by higher [H2O2],
suggesting that 2 is formed by oxidation of 12. Furthermore, in
an experiment with increased hydrogen peroxide concentration
(ratio of 1000 to 1 for H2O2 to 1), the build-up of 12 after 5 min
at 60 ЊC was clearly followed by that of 2 after 10–15 min.
For the following 30 min the species 2 with m/z 486 was effec-
tively the only species detected (Fig. 3b). After approximately
45 min, new peaks at m/z 170, 172, 188, associated with the
ligand (m/z 172) and its decomposition products (m/z 170, 188)
were detected (Fig. 3c; see also ref. 8). The proposed cleavage
of the dinuclear complex into 12 and 2 is discussed later in
the text.
b
Reaction of other phenols. Using the substrates 4–9 we
explored the possibility of forming MnIII and MnV species,
equivalent to 12 and 2, with potential chelate ring-sizes of 5, 6
and 7. In addition, the substituents on the phenoxy ligands
should provide information on the importance of electronic
effects on the stabilisation and reactivity of any MnV species
generated. Co-ligands which do not have two hydroxy groups
will, of course, require oxidation and phenolic coupling to
generate the appropriate chelating species. The standard con-
ditions described above were used in comparative experiments
to identify the species formed when the potential co-ligands
were mixed with 1, and to determine which co-ligands are able
to stabilise O᎐MnV species after addition of H2O2. In general,
᎐
as different conditions were explored, we noted that the form-
ation of MnV over MnIII (for those substrates that give both)
was favoured at higher values of pH, temperature and H2O2
concentration. We also noted that only for the ligands 4 and 5,
was an induction period observed as described for 3 (above).
Results are summarised in Table 1.
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Table 1 MnIII and O᎐MnV species detected by ESI-MS during reaction of H2O2, 1, and phenols 3–9a
᎐
Substrate
Mn() species cf. 12
m/z (ϩ)ESI-MS
Mn() species cf. 2
m/z (ϩ)ESI-MS
Other species
m/z (Ϫ)ESI-MS
3
4
5
6
7
8
9
Ί
Ί
Ί
Ί
Ί
Ί
Ί
470
498
466
334
410
384
470
Ί
486
514
482
–
426
—
×
×
×
Ί
×
×
×
—
—
—
271
—
Ί
Ί
×
(Ί)
×
Ί
486
—
a For conditions, see text.
Fig. 1 Species detected by (ϩ) ESI-MS from 1 in aqueous solution (see text).
Fig. 2 Species detected by (ϩ)ESI-MS after addition of 4-methoxyphenol to an aqueous solution of 1 (see text).
i
4-Ethoxyphenol, 4. This behaved in a manner closely
O᎐MnV(TMTACN)(9, R = Et), equivalent to 12 and 2 respec-
᎐
similar to 3. With concentration ratio 1 : 4 : H2O2 of 1 : 10 :
1000, ESI-MS showed a peak at m/z 498 after 5 min, and then
another at m/z 514 started to grow after ca. 10 min. These
peaks are assigned to MnIII(TMTACN)(9, R = Et) and
tively (see Table 1). The relative proportion of the O᎐MnV and
᎐
the MnIII species and its variation with time was identical to
that observed from 3. In the absence of H2O2, ESI-MS showed
immediate formation of 11 (cf. also similar behaviour of 5).
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oxidised and the complex formed with MnIII is readily
destroyed upon addition of oxidant.
iv 2,2Ј-Biphenol, 7. A mixture of 7 and the complex 1 led
to immediate detection of 11 (m/z 500) and the complex MnIII-
(TMTACN)(7) (m/z 410). No induction period was observed
under the standard conditions with hydrogen peroxide, the
ESI-mass spectrum of the complex MnIII(TMTACN)(7) also
being detected. By using higher concentrations of H2O2 it was
possible to detect a small peak from the appropriate O᎐MnV
᎐
species (with m/z 426). The results suggest that, under standard
conditions with H2O2, 7 is capable of stabilising the MnIII but
not the O᎐MnV species, and that electron-donating groups,
᎐
as in 9, are necessary to stabilise the higher oxidation state
sufficiently for it to be readily detectable by ESI-MS.
v
1,8-Dihydroxynaphthalene, 8. The formation of the
species MnIII(TMTACN)(8) (m/z 384) was observed immedi-
ately after mixing 8 with 1 in the absence of H2O2. However,
after 2–3 min a brown solid (possibly MnO2) precipitated out
of solution; the substrate was also degraded more rapidly in the
presence of H2O2.
vi 5,5Ј-Dimethoxy-2,2Ј-biphenol, 9, R = Me. The reaction
of 9, R = Me with 1 and H2O2 was very similar to that of 3
with the exception that it showed no induction period, with
immediate observation of ions with m/z 470 and 486, assigned
to 12 and 2. This suggests that the induction period observed
with the phenolic substrates 3–5 is due to the necessity of
prior formation of the appropriate biphenol, and that once this
is formed, it reacts rapidly with 1 to form the complex
MnIII(TMTACN)(biphenol) 12. In the absence of hydrogen
peroxide, strong peaks from 11 and 12 were detected
instantaneously.
The trends in the results of these substrates can be broadly
described as follows. First, we note the apparent strong ability
for dihydroxy compounds to bind to manganese, and this
increases in the cases where its binding leads to the formation
of a ring size of 5 or 6. We suggest that, in the case of 6 and 8,
this makes the formation of a MnIII(co-ligand)2 complex more
favourable at the expense of the appropriate mixed MnIII-
(TMTACN)(co-ligand) complex, (and its O᎐MnV counterpart).
᎐
Secondly, the use of biphenols 6–9 removes the induction
period, suggesting that the induction period for the formation
of O᎐MnV species observed with monophenols may be due to
᎐
the formation of sufficient biphenol to bind to manganese,
rather than to the breaking up of 1, which appears to be rapid
once the biphenol is formed. The rapidity of initial electron
transfer is also confirmed by the ESI-MS experiments with 6–9.
Thirdly, there appears to be an influence of the electronic
effect of substituents on the ligand on the stabilisation of the
higher oxidation state of the manganese, in the order OMe ≈
OEt > Me > H). As would be anticipated, electron-donating
groups stabilise the O᎐MnV complex.
Fig. 3 a Species detected by (ϩ)ESI-MS from an aqueous solution of
1, 4-methoxyphenol and hydrogen peroxide (1 : 100 : 500) after 5 min at
60 ЊC b as Fig. 3a, after 15 minutes c as Fig. 3a, after 45 minutes.
᎐
2
Proposed catalytic cycle for the oxidation of phenols by
[Mn2(ꢀ-O)3(TMTACN)2](PF6)2
ii 2,4-Dimethylphenol, 5. As with 3 and 4 there was an
induction period of ca. 5 minutes before the peak at m/z 466
[assigned to the manganese() complex of the appropriate
dimer, MnIII(TMTACN)(13)] was observed. In due course a
The outline mechanism in Scheme 2 is proposed to account
both for the species observed by ESI-MS and for the catalytic
oxidation of phenols, for which evidence has previously been
obtained. This involves two initial one-electron oxidation
steps in which, for example, the anion of 3 is oxidised to the
appropriate phenoxyl radical and 1 is reduced to 11 (detected
by ESR and ESI-MS) and 14, respectively. It is known19 that
the MnIV–MnIV species 1 is capable of oxidising phenols, and
pre-mixing 1 with 3 prior to the addition of H2O2 shortens
the lag phase from ca. 2–3 min to around 30 s, indicating that
the initial oxidation sequence is responsible for the lag phase.
We suggest that Mn2III(µ-O)3(TMTACN)2, 14, is very unstable
and splits into mononuclear species, which together with the
biphenol form the monomeric complex 12 (detected by ESI-
MS). It is proposed that the latter is then oxidised by hydrogen
peak at m/z 482 [assigned to O᎐MnV (TMTACN)(13)] was
᎐
also observed, but its size was relatively small compared to the
corresponding O᎐MnV species for 3 and 4.
᎐
iii Catechol, 6. The good binding and reducing properties
of catechol for Mn are reflected in the immediate observation
of 11 (m/z 500) and the rapid formation of a species with
m/z 334, assigned to MnIII(TMTACN)(6), and also a small
proportion of MnIII(6)2 (m/z 271, negative ion ESI-MS), when
1 and catechol were mixed under the standard conditions
but with no hydrogen peroxide. No oxidation by H2O2 of the
MnIII(TMTACN)(6) complex to the corresponding O᎐MnV
᎐
species was observed. We believe that catechol is very easily
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The sample injection was carried out over a period of 45 min,
and the build up and decay of the intermediates was followed
with time. Positive ESI-MS allowed the detection of the
intermediates in the catalytic cycle, whereas negative ESI-MS
allowed the detection of the phenolic ligands and their oxid-
ation products.
Acknowledgements
A. M. P. gratefully acknowledges financial support from
Unilever Research (UK) and the University of York. We
also thank Dr. Trevor A. Dransfield (Univ. of York, Dept. of
Chemistry) for assistance with ESI-MS experiments, and the
EPSRC for part funding the mass spectrometer.
Scheme 2
References
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᎐
is stabilised by the biphenol: this is then reduced by 3؊ via one-
electron transfer to give an (undetected) species 15. A further
one-electron reduction gives 12 to close the catalytic cycle.
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Experimental
Chemical reagents
Commercially available chemicals were purchased from
Aldrich, Sigma, and Lancaster and were used without further
purification. Deionised water was used in all experiments.
All other solvents used were analytical grade. Hydrogen per-
oxide solution (31% by weight) was purchased from Fisons
and the peroxide content was regularly checked during the
course of this research. [Mn2(µ-O)3(TMTACN)2](PF6)2 was
provided by Unilever. 1,8-Naphthalenediol (8) was provided by
Dr. J. Ragot. Compound 9 was synthesised by the procedure
in ref. 20.
Electrospray mass spectrometry
Electrospray mass spectra were recorded on a Finnigan LCQ
MAT Standard Edition Spectrometer. The intermediates were
detected using the settings obtained after optimisation of the
system for the detection of species 2 (m/z 486). The following
settings were employed: sheath gas flow rate (arb) = 40, octapole
1 offset = Ϫ2.25 V, auxiliary gas flow rate (arb) = 0, lens voltage
= Ϫ30 V, spray voltage = 4.2 kV, octapole 2 offset = Ϫ5 V,
capillary temp = 250 ЊC, octapole RF amplitude = 400 V p-p,
capillary voltage =15 V, flow rate = 5–50 µl minϪ1, tube lens
offset = 0 V.
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