Structural studies on manganese(III) and manganese(IV) complexes of tetrachlorocatechol and the catalytic reduction of dioxygen to hydrogen peroxide

Article abstract of DOI:10.1016/j.ica.2003.09.032

The mononuclear complexes (Bu₄N)[Mn(Cl₄Cat) ₂(H₂O)(EtOH)] and (Bu₄N)₂[Mn(Cl ₄Cat)₃] (H₂Cat=1,2-dihydroxybenzene) have been synthesised and characterised by X-ray diffraction. This work provides a direct, independent, synthesis of these complexes and an interesting example of how solvent effects can promote the formation of either a manganese(III) or manganese(IV) complex of the same ligand. The characterisation of (Bu ₄N)[Mn(Cl₄Cat)₂(H₂O)(EtOH)] supports previous work that manganese(III) is extremely reluctant to form tris (catecholato) complexes due to the short 'bite distance' of catecholate oxygen atoms (2.79 ?) which are unable to span the elongated coordination axes of the Jahn-Teller distorted Mn(III) ion and explains the 2:1 and 3:1 tetrachlorocatechol:manganese ratios in the Mn(III) and Mn(IV) complexes, respectively. Hydrogen peroxide production using dioxygen and hydroxylamine as substrates in acetonitrile/water mixtures, under ambient conditions, can be demonstrated with both complexes, suggesting that neither labile coordination sites nor the oxidation state of the manganese are important to the catalytic system. Turn over frequencies (TOF, moles of H₂O₂ per moles of manganese per hour) of ～10000 h^-1 are obtained and this compares very favourably with the commercial production of hydrogen peroxide by the autoxidation of 2-ethylanthrahydroquinone (AO process).

Full text of DOI:10.1016/j.ica.2003.09.032

Inorganica Chimica Acta 357 (2004) 2494–2502
www.elsevier.com/locate/ica
Structural studies on manganese(III) and manganese(IV)
complexes of tetrachlorocatechol and the catalytic reduction of
dioxygen to hydrogen peroxide
a,
b
c
c
Tippu S. Sheriff *, Pamela Carr , Simon J. Coles , Michael B. Hursthouse ,
b
Jocelyne Lesin , Mark E. Light
c
a
Inorganic Chemistry Research Laboratories, Department of Chemistry, Queen Mary University of London, Mile End, London E1 4NS, UK
b
School of Chemistry, University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB, UK
c
Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK
Received 30 August 2003; accepted 25 September 2003
Available online 21 November 2003
Abstract
The mononuclear complexes (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)] and (Bu4N)2[Mn(Cl4Cat)3] (H2Cat ¼ 1,2-dihydroxybenzene)
have been synthesised and characterised by X-ray diffraction. This work provides a direct, independent, synthesis of these complexes
and an interesting example of how solvent effects can promote the formation of either a manganese(III) or manganese(IV) complex of
the same ligand. The characterisation of (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)] supports previous work that manganese(III) is ex-
ꢀ
tremely reluctant to form tris (catecholato) complexes due to the short Ôbite distanceÕ of catecholate oxygen atoms (2.79 A) which are
unable to span the elongated coordination axes of the Jahn-Teller distorted Mn(III) ion and explains the 2:1 and 3:1 tetrachloro-
catechol:manganese ratios in the Mn(III) and Mn(IV) complexes, respectively. Hydrogen peroxide production using dioxygen and
hydroxylamine as substrates in acetonitrile/water mixtures, under ambient conditions, can be demonstrated with both complexes,
suggesting that neither labile coordination sites nor the oxidation state of the manganese are important to the catalytic system. Turn
over frequencies (TOF, moles of H2O2 per moles of manganese per hour) of ꢀ10 000 hꢁ1 are obtained and this compares very fa-
vourably with the commercial production of hydrogen peroxide by the autoxidation of 2-ethylanthrahydroquinone (AO process).
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Manganese; Tetrachlorocatechol; Catecholate complexes; Hydrogen peroxide; Hydroxylamine; Dioxygen activation catalyst
1. Introduction
vide an interesting challenge for both synthetic and
theoretical chemists [7] (Scheme 1).
Transition metal complexes containing catechol and
semiquinone ligands show a unique facility for electron
transfer between the metal and chelated ligand [1] and
these redox characteristics have been recognised as being
important in reactions catalysed by certain enzymes that
include galactose oxidase [2–4], ribonucleotide reductase
[5] and PSII [6]. The non-innocent, redox-active, nature
of catecholate complexes, in which catechols can be
oxidised to the semiquinone and o-quinone forms, pro-
Thus, in the case of manganese, valence tautomerism
has been observed between MnIV-catecholate and MnII-
semiquinone forms and the interconversion of these
forms can occur with small changes of temperature [8].
Current interest in manganese catecholate chemistry has
also been stimulated by their involvement in catechol
oxidation and dioxygen activity [1,7]. The interaction of
manganese complexes with dioxygen can either be in a
reversible manner forming complexes of the type M–O2
(in which the ÔO2Õ moiety is preserved) [9,10] but whose
identity is somewhat controversial [11,12] or through
non-reversible reactions to produce manganese-o-semi-
quinone complexes [13,14], free benzoquinone [15,16],
* Corresponding author. Tel.: +44-20-7882-7746; fax: +44-20-7882-
7794.
E-mail address: t.s.sheriff@qmul.ac.uk (T.S. Sheriff).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.09.032

T.S. Sheriff et al. / Inorganica Chimica Acta 357 (2004) 2494–2502
2495
Scheme 1.
or even oxidative carbon-carbon bond cleavage prod-
ucts of catechol [14,17]. The discovery of manganese
dioxygenase enzymes, a new class of extradiol dioxy-
genases with Mn(II) at the active site [18–21] and the
Ôcatalase-likeÕ properties [22] of manganese-based sys-
tems has added to the biological significance of higher-
valent manganese chemistry and show that these
systems are implicated in both the oxidative and re-
ductive cycles of oxygen in natural systems. There is also
an interest in the use of these systems as synthetic
magnets [23,24].
Most work in this field has involved the preparation
of model manganese complexes of 3,5-di-tert-butyl-
catechol (dtbCatH2) [25], with little work in the area of
the activation of dioxygen by manganese complexes of
other catechols.
We have, for a number of years, been interested in
manganese-catecholate systems that can efficiently re-
duce dioxygen to hydrogen peroxide. Aqueous, oxy-
genated, mixtures of manganese(II) chloride and Tiron
(1,2-dihydroxybenzene-3,5-disulfonate, disodium salt
monohydrate) efficiently catalyse the reduction of di-
oxygen to hydrogen peroxide with hydroxylamine at pH
8.0 and ambient temperatures (Eq. (1)) [26]:
ported by Pierpont et al. [25] who carried out a struc-
tural study on the complex K[Mn(III)(Cl4Cat)2
(OPPh3Þ] ꢂ H2O ꢂ (CH3)2CO with tetrachlorocatechol
being generated in situ from the reduction of
tetrachlorobenzoquinone using triphenylphosphine.
We describe herein the formation of a new tetrabuty-
lammonium salt of the bis Mn(III) complex from the
oxidation of mixtures of the MnCl2 ꢂ 4H2O, tccH2 ꢂ H2O
and tetrabutylammonium hydroxide with air (dioxy-
gen). By a judicious change of solvent, the tris Mn(IV)
complex can be isolated and single crystal X-ray struc-
tural determinations of both are reported. Recently,
Gatteschi and coworkers [30] have reported a structural
study on the ditetraphenylphosphonium salt of Mn(IV)-
tris-tcc from the oxidation of the potassium salt of
Mn(III)-bis-tcc using tetraphenylphosphonium chloride
as an alkali-metal chelator in acetronitrile. GatteschiÕs
alternative synthetic method for Mn(IV)-tris-tcc was
based on the reaction of MnCl2 ꢂ 4H2O with in situ
generated tcc2ꢁ (from the reaction of tetrachloro-1,2-
benzoquinone and triphenylphosphine neutralised with
a methanolic solution of (Bu)4NþOHꢁ), initially under
argon, with a rather long and complicated work up
procedure. Indeed a similar procedure was first used by
Pierpont [25] who obtained the potassium salt of
Mn(III)-bis-tcc with the use of KOH instead of
(Bu)4NþOHꢁ. The direct synthetic method reported
here using tetrachlorocatechol (rather than the quinone)
is very quick and easier to carry out leading to the for-
mation of both complexes independently.
O2ðairÞ þ 2NH2OH ¼ H2O2 þ N2 þ 2H2O
ð1Þ
This system has been described by the Institute of
Applied Catalysis (iAc) as the Ômost successfulÕ method
for the local and in situ generation of hydrogen peroxide
by Ôenzyme-likeÕ processes [27].
A single crystal structural study has recently been
completed on [Na]5[Mn(3,5-(SO3)2Cat)2] ꢂ 10H2O, and
this has been shown to be based on a bis-catecholate
complex exhibiting the Jahn-Teller distortion to give a
pseudo-octahedral complex with the oxygen atoms of
sulfonate groups in neighbouring molecules occupying
the axial positions [28].
Manganese(III), uniquely amongst other first row
transition metals that forms trivalent ions – Ti, V, Cr
and Fe, is extremely reluctant to form tris (catecholato)
complexes which arise from the short bite distance of
catechol which limits the ability of this ligand to span
the elongated coordination axes of the Jahn-Teller dis-
torted Mn(III) ion [29].
2. Experimental
2.1. Preparation of complexes
These syntheses were performed in open reaction
vessels without any special conditions.
2.1.1. (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)]
Mn(ClO4)2 ꢂ 4H2O (0.38 g, 1.1 mmol) was added to a
solution of tetrachlorocatechol monohydrate (0.66 g, 2.5
mmol) in 20% aqueous EtOH (50 ml). (Bu)4NþOHꢁ
(ꢀ40% aqueous, 1.0 ml, 1.5 mmol) was added and the
solution darkened immediately. This dark green solu-
tion was refluxed for 1 h and then left to cool slowly to
The potassium salt of the bis Mn (III) complex of
tetrachlorocatechol (Cl4CatH2, tccH2) has been re-

2496
T.S. Sheriff et al. / Inorganica Chimica Acta 357 (2004) 2494–2502
room temperature. Long dark yellow–green needles
were isolated. Yield: 0.38 g (40%).
spectrophotometrically using a Perkin–Elmer Plasma 40
spectrophotometer. Hydrogen peroxide production runs
were performed in a dreschel bottle immersed in a water-
bath at 20 ꢃ 1 °C. Dioxygen was provided from a
cylinder via a sintered-glass inlet into a well-stirred so-
lution. Plastic spatulae and de-ionised water were used
to avoid contamination by metal ions that might de-
compose H2O2 and to avoid addition of adventitious
sources of manganese(II).
2.1.2. (Bu4N)2[Mn(Cl4Cat)3]
Mn(ClO4)2 ꢂ 4H2O (0.38 g, 1.1 mmol) was added to
solution of tetrachlorocatechol monohydrate (0.88 g, 3.5
mmol) in acetone (50 ml). (Bu)4NþOHꢁ (ꢀ40% aque-
ous, 1.5 ml, 2.3 mmol) was added and the solution
darkened immediately. This dark blue solution was
refluxed for 1 h and then left for several days to con-
centrate where upon small dark green crystals were
isolated. Yield: 0.67 g (48%).
2.3. Crystallographic data and refinement of structure
The crystal data of (Bu4N)2[Mn(Cl4Cat)2(H2O)
(EtOH) and (Bu4N)2[Mn(Cl4Cat)3] and the parameters
of data collection are summarised in Table 1. Data were
collected on a Bruker–Nonius KappaCCD situated at
the window of an FR591 rotating anode generator
2.2. Physical measurements
Infrared spectra were recorded on Perkin–Elmer
Paragon FT-IR 1000 spectrophotometer as KBr discs.
Magnetic measurements were carried out using a Sher-
wood Scientific magnetic susceptability balance. A Phi-
lips PU 8720 spectrophotometer was used for UV–Vis
spectra on aqueous solutions of the complex and for the
determination of hydrogen peroxide colorimetrically. C,
H, N and Cl analysis was carried out by MEDAC Ltd.
(Brunel University) and manganese was determined
ꢀ
[k(Mo Ka) ¼ 0.71073 A]. The structures were solved by
direct methods using the crystallographic package
SHELXS97 [31] and refined using full-matrix least-
squares [32] on F2. Non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were located from the
difference map and isotropically refined except for those
attached to carbon, which were placed in calculated
Table 1
Crystal data and structure refinement for (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)] and (Bu4N)2[Mn(Cl4Cat)3]
Identification code
Empirical formula
Formula weight (g moleꢁ1
Temperature (K)
(Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)]
30H44Cl8MnNO6
853.20
(Bu4N)2[Mn(Cl4Cat)3]
C50H72Cl12MnN2O6
C
)
1277.44
120(2)
150(2)
0.71073
monoclinic
ꢀ
Wavelength Mo Ka (A)
0.71073
monoclinic
Crystal system
Space group
P21=n
10.1751(3)
P21
9.6128(3)
ꢀ
a (A)
ꢀ
b (A)
24.2470(7)
15.2289(6)
18.9994(6)
16.2805(5)
ꢀ
c (A)
b (°)
V (A )
104.9560(10)
3629.9(2)
97.836(2)
2945.66(16)
3
ꢀ
Z
4
1.561
2
1.440
dx (Mg mꢁ3
)
l (mmꢁ1
F ð000Þ
Crystal
)
0.995
1760
0.815
1326
needle; dark yellow
0.46 ꢄ 0.08 ꢄ 0.06
3.24–25.40
block; dark green
0.10 ꢄ 0.10 ꢄ 0.10
Crystal size (mm)
h range (°)
Index ranges
3.03–25.02
ꢁ9 6 h 6 11; ꢁ22 6 k 6 20; ꢁ19 6 l 6 19
ꢁ12 6 h 6 12; ꢁ21k 6 29; ꢁ18 6 l 6 18
21133
Ncollect
19318
9344 [Rint ¼ 0:0619]
0.987
Nindt
6538 [Rint ¼ 0:0913]
0.978
Data completeness
Absorption correction
Maximum and minimum transmission
semi-empirical from equivalents
0.9427 and 0.6575
full-matrix least-squares on F 2
6538/36/451
0.9229 and 0.9229
full-matrix least-squares on F 2
9344/1/640
Refinement method
Data/restraints/parameters
Goodness-of-fit on F 2
R1
1.003
0.0689 [F 2 > 2rF 2ꢅ
0.948
0.0457 [F 2 > 2rF 2ꢅ
0.0766
0.504
wR2
0.1558
0.888
ꢁ3
ꢀ
ꢀ
qmax (e Aꢁ3
)
qmin (e A
)
)0.557
)0.345

T.S. Sheriff et al. / Inorganica Chimica Acta 357 (2004) 2494–2502
2497
positions. Some disorder is observed in the terminal
groups of the butyl chains in the TBA molecule, which
was modelled over two fractionally occupied sites.
rings with the remaining two oxygen atoms from either
two water or two ethanol molecules (Fig. 1). The two
Mn–O(catechol) bond lengths on either side of the
ꢀ
molecule are almost identical at 1.896(4) and 1.916(3) A
(Table 2). The bond distance between the manganese
and the water or ethanol oxygen atoms are much longer
2.4. Hydrogen peroxide production
ꢀ
Samples of the complexes (1.25, 2.50, 5.00 or
10:0lmol) were separately added to a dreschel bottle
containing ACS grade NH2OH (3.30 g, 50%, w/w,
aqueous, 0.0500 M) in a 1:1 mixture of acetonitrile and
de-ionised water (total volume 100.0 cm3). Dioxygen gas
at a rate of 1.5 dm3 minꢁ1 was bubbled through the
rapidly stirred solution which was equilibrated 20 ꢃ 1
°C. A blank was also run with no complex added.
Aliquots of the solution were removed at defined periods
and analysed for hydrogen peroxide concentration using
the colorimetric method based on titanium(IV) sulphate
[33,34].
(Mn–OH2 ¼ 2.294(4) A and Mn–OHCH2CH3 ¼ 2.280
ꢀ
A) due to the effects of Jahn-Teller distortion. Each
[Mn(Cl4Cat)2(H2O)(EtOH)]ꢁ complex ion is associated
with a tetrabutylammonium cation (the asymmetric unit
consists of two half molecules of [Mn(Cl4Cat)2(H2O)
(EtOH)]ꢁ whose charges are balanced by a single TBA
and hence Fig. 1 shows the connectivity and numbering
scheme but does not exhibit the correct stoichiometry).
The disorder in the terminal parts of the butyl arms of
the TBA molecule is not shown in Fig. 1 for clarity
purposes.
The isolation of this complex provides further evi-
dence that manganese(III) is extremely reluctant to form
tris (catecholato) complexes due to the short Ôbite dis-
tanceÕ of catecholate oxygen atoms, calculated to be 2.79
A, which are unable to span the elongated coordination
axes of the Jahn-Teller distorted Mn(III) ion.
3. Results and discussion
ꢀ
3.1. Preparation of complexes
The preparative methodology employed is interesting
in that the use of the bulky (Bu)4NþOHꢁ base would
appear to be important in keeping anionic forms of
tetrachlorocatechol apart thus mitigating against poly-
merisation reactions [28,35] and possibly stabilising re-
duced, anionic, forms of tetrachlorocatechol for
subsequent coordination to the manganese(II) ion.
Thereafter, rapid oxidation to either Mn(III)-bis-tcc or
Mn(IV)-tris-tcc occurs, with the use of the more polar
aqueous-ethanol solvent mixture stabilising the forma-
tion of Mn(III)-bis-tcc by occupying coordination sites
around the Mn(III) ion while the use of the relatively
non-polar, poorly coordinating, acetone as solvent leads
to the formation of Mn(IV)-tris-tcc. The use of alkali
metal containing bases are reported to lead to the for-
mation of particularly stable alkali metal-catecholate
complexes which would prevent the formation of the tris
Mn(IV)-tcc complex [30]; this was avoided in this syn-
thetic method. The formation of these complexes indi-
cate that the catecholate oxygens still possess strong
electron-donating properties, despite the electronegative
effect of the four chlorine atoms on the benzene ring, to
stabilise tetravalent manganese [8,13,29,36–39].
3.2.2. (Bu4N)2[Mn(Cl4Cat)3]
This structure consists of discrete mononuclear pro-
peller-like anions with a 2-fold symmetry and two tet-
rabutylammonium cations (Fig. 2). The manganese
atom is coordinated to six oxygen atoms from the bid-
entate catecholate ligands in a trigonally distorted oc-
tahedral geometry.
3.2.3. General comments
The average Mn–O (catecholate) bond lengths in
both (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)] (range 1.896–
ꢀ
1.916 A) and (Bu4N)2[Mn(Cl4Cat)3] (range 1.889–1.911
ꢀ
ꢀ
A) are about 1.90 A and are comparable to that found
for other structurally characterised manganese catech-
olate complexes [28,29,37] (note that the Mn–O bond
ꢀ
lengths in (Bu4N)2[Mn(Cl4Cat)3] (range 1.877–1.892 A,
ꢀ
average 1.88 A) isolated by Gatteschi [30] are somewhat
lower and may reflect the lower temperature of 20 K at
which the structural study was carried out compared to
120 K in our study). This similarity in the Mn–O bond
lengths despite the difference in charge on the metal ion
has been commented on before [25] and indeed quite
similar bond lengths have been found in previously
structurally
(range 1.906–1.907 A, average 1.91 A) [28] and
determined
ꢀ
[MnIII(3,5-(SO3)2Cat)2]5ꢁ
ꢀ
3.2. Descriptions of the structures
IV
ꢀ
ꢀ
[Mn (dtbCat)2] (range 1.874–1.922 A, average 1.90 A)
[29,37], showing that the Mn–O bond length is also quite
independent of the nature of the substituent on the ca-
techolate ring. The average O–Mn–O bite angle is about
94° in (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)] and 86° in
(Bu4N)2[Mn(Cl4Cat)3]. In the latter, this deviation from
an ideal octahedron is due to the constraints by three
3.2.1. (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)]
This structure consists of manganese (III) ions occu-
pying a crystallographic centre of symmetry within a
distorted octahedral environment being coordinated by
six oxygen atoms. Four of these oxygen atoms arise
from two centrosymmetrically related coplanar catechol

2498
T.S. Sheriff et al. / Inorganica Chimica Acta 357 (2004) 2494–2502
Fig. 1. Diagram of (Bu4N)[Mn(Cl4Cat)2(H2O) (EtOH)] showing the atomic numbering scheme.
5-membered chelate rings, resulting in the observed tri-
gonal distortion of the manganese coordination sphere
(average twist angle of about 54°). These values are
again typical for catechol complexes. Metal-catecholates
measurement produced a value for l of 3.97 BM. This
fits for Mn4þ (d3) high spin (l ¼ 3:87 BM).
3.4. UV–Vis and IR spectroscopy
ꢀ
generally have C–O bond lengths of 1.34–1.36 A, e.g.
3ꢁ
[Cr(Cat)3]3ꢁ 1.349 (5) A; [Fe(Cat)3] 1.349 (3) A [40].
The C–O bond lengths in (Bu4N)[Mn(Cl4Cat)2(H2O)
(EtOH)] and (Bu4N)2[Mn(Cl4Cat)3] are in the range
3.4.1. (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)]
k
ꢀ
ꢀ
max/nm (CH3CN) ꢀ700 (sh, e/dm3 molꢁ1 cmꢁ1 270)
and 613.3 (323.5) can be assigned to d–d transitions to
produce a pale green solution; kmax/nm 436.0 (1320)
probably a LMCT (ligand-to-metal charge transfer) on
the basis of its high molar absorptivity which is re-
sponsible for the dark yellow colour of the complex in
the solid state and in more dilute solution; kmax/nm 297
(sh, 10 784), 287 (sh, 11 275) and 214.4 (279 804) due to
tetrachlorocatechol. The LMCT at 436.0 nm can be
assigned to a LMCT transition of catecholato ! MnIII,
this being at exactly the same wavelength as that pre-
viously found for [Na]5[Mn(3,5-(SO3)2Cat)2] ꢂ 10H2O
[26]. The d–d transitions at ꢀ700 and 613.3 nm are
ꢀ
1.33–1.35 A and longer than that expected for metal-
ꢀ
semiquinone complexes (1.28 A); the C1–C2 distances in
ꢀ
the two complexes are in the range 1.37–1.40 A) which is
ꢀ
also consistent with coordinated catechols (ꢀ1.40 A)
ꢀ
rather than coordinated semiquinones (1.44 A).
3.3. Analysis and magnetic study of complexes
3.3.1. (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)]
Found: C, 42.78; H, 5.35; N, 1.81; Cl, 33.23; Mn,
6.03. C30H44Cl8MnNO6 requires C, 42.23; H, 5.20; N,
1.64; Cl, 33.24; Mn, 6.44. A magnetic susceptability
measurement produced a value for l of 4.87 BM. This
fits for Mn3þ (d4) high spin (l ¼ 4:90 BM).
equivalent to energy gaps of ꢀ171 and 195.2 kJ molꢁ1
,
respectively, which are again quite similar to that found
for [Na]5[Mn(3,5-(SO3)2Cat)2] ꢂ 10H2O. Gatteschi [30]
reported only a single d–d transition at 635 nm (150 dm3
molꢁ1 cmꢁ1) for K[Mn(Cl4cat)2(OPPh3)] ꢂ H2O ꢂ Me2
CO. However, by using a 4 cm path length (cell) we were
able to obtain much greater spectral detail and a much
more accurate value for the higher energy transition.
3.3.2. (Bu4N)2[Mn(Cl4Cat)3]
Found: C, 46.03; H, 5.97; N, 2.08; Cl, 33.44; Mn,
4.50. C50H72Cl12MnN2O6 requires C, 47.01; H, 5.68; N,
2.19; Cl, 33.30; Mn, 4.30. A magnetic susceptability

T.S. Sheriff et al. / Inorganica Chimica Acta 357 (2004) 2494–2502
2499
IR data 1=k/cmꢁ1: 3418 (broad, O–H stretch, water
and ethanol); 1433 (C–O stretch, catecholate); 810 (C–
Cl) stretch.
Table 2
ꢀ
(a) Selected bond lengths (A) and angles (°) for (Bu4N)[Mn(Cl4
Cat)2(H2O)(EtOH)]
Bond lengths
Mn(1) – O(1)
Mn(1)–O(2)
Mn(1)–O(3)
Mn(2)–O(4)
Mn(2)–O(5)
Mn(2)–O(6)
O(1)–C(1)
1.913(3)
1.896(4)
2.294(4)
1.899(4)
1.903(4)
2.280(5)
1.334(7)
1.369(8)
1.723(6)
1.707(7)
3.4.2. (Bu4N)2[Mn(Cl4Cat)3]
k
max/nm (CH3CN) ꢀ700 (sh, ꢀ6000), 615.2 (ꢀ/dm3
molꢁ1 cmꢁ1 6290) and ꢀ425 (sh, ꢀ1300) can be assigned
to LMCTs on the basis of their high molar absorptivity
which are responsible for the dark green colour of the
complex in the solid state and in solution; kmax/nm 312.6
(27 161), ꢀ250 (sh, 36 500) and 222.0 (91 035) due to tet-
rachlorocatechol. The absorption band at 615.2 nm with
two distinct shoulders at ꢀ700 and ꢀ425 nm is typical of
high-valent MnIV catecholate species [8,13,29,30,36–39].
The 615.2 nm band can be assigned to a LMCT transition
of catecholato ! MnIV on the basis of comparison to
previously reported tris (catecholate) MnIV complexes
[29,30,36,37] while the remaining two bands may, on the
basis of the spectrum for (Bu4N)[Mn(Cl4Cat)2(H2O)-
(EtOH)] (above), be assigned to LMCT transitions of
catecholato ! MnIII.
C(1)–C(2)
C(2)–Cl(1)
C(8)–Cl(5)
Bond angles
O(1)–Mn(1)–O(2)
O(1)–Mn(1)–O(2)i
O(1)–C(1)–C(2)
C(1)–O(1)–Mn(1)
O(4)–Mn(2)–O(5)
O(4)–Mn(2)–O(5)ii
O(4)–C(7)–C(8)
C(7)–O(4)–Mn(2)
94.23(16)
85.77(16)
124.4(5)
110.8(3)
93.82(18)
86.18(18)
122.3(6)
110.5(4)
ꢀ
(b) Selected bond lengths (A) and angles (°) for (Bu4N)2[Mn(Cl4
Cat)3]
Bond lengths
IR data 1=k/cmꢁ1: 1422 (C–O stretch, catecholate);
793 (C–Cl stretch).
Mn(1)–O(1)
Mn(1)–O(2)
O(1)–C(1)
C(1)–C(2)
C(2)–Cl(1)
1.902(3)
1.911(3)
1.333(5)
1.384(6)
1.737(5)
3.5. Hydrogen peroxide production
The curves for the production of H2O2 using hy-
droxylamine (NH2OH, 0.500 M) and dioxygen (1.5 dm3
minꢁ1) as substrates in 1:1 aqueous acetontirile with
different weights of (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)]
and (Bu4N)2[Mn(Cl4Cat)3] are shown in Figs. 3 and 4,
respectively. With no complex added there is virtually
Bond angles
O(1)–Mn(1)–O(2)
O(1)–Mn(1)–O(6)
O(1)–C(1)–C(2)
C(1)–O(1)–Mn(1)
84.95(14)
94.41(13)
124.4(5)
111.9(3)
Fig. 2. Diagram of (Bu4N)2[Mn(Cl4Cat)3] showing the atomic numbering scheme.

2500
T.S. Sheriff et al. / Inorganica Chimica Acta 357 (2004) 2494–2502
no H2O2 production. The rates of H2O2 production
then increase steadily with increasing amounts of com-
plex between 1.25 and 5.00 lmol. With 10.0 lmol of
each complex (not shown), the rate of H2O2 production
is faster than that using 5.00 lmol, but there is then
quite rapid decomposition of the H2O2, suggesting a
slower manganese catalysed decomposition of H2O2.
The slower rate of H2O2 production with (Bu4N)2
[Mn(Cl4Cat)3] compared to (Bu4N)[Mn(Cl4Cat)2(H2O)
(EtOH)] at 1.25 lmol may be due to the difficulty in
weighing out very small amounts of solid and the sen-
sitivity of the catalytic process to these small variations
in the amount of complex. Nevertheless, the strong
similarity in the rates of production of H2O2 production
with the two complexes suggest that neither the oxida-
tion state of the manganese nor labile coordination sites
around the metal are important in catalysis. Fig. 5
shows a log plot of weight of manganese complex
against initial rate of reaction (taken by the gradient
through t ¼ 0, 2, 5 and 10 min). The slopes for both
Fig. 4. The catalytic production of H2O2 with O2 using NH2OH as
substrate at 20 ꢃ 1 °C with different weights of (Bu4N)2[Mn(Cl4Cat)3].
In all runs the initial [NH2OH] was 0.500 M (3.30 g, 50% aqueous). O2
(1.5 dm3 minꢁ1) was introduced into a rapidly stirred 1:1 aqueous-
acetonitrile solution (total volume 100 cm3). Weights of complex: (s)
none; (N) 1.6 mg (1.25 lmol); (ꢀ) 3.2 mg (2.50 lmol) (r) 6.4 mg (5.00
lmol).
complexes are quite similar and average to about 0.74;
this fractional order in manganese complex may suggest
the involvement of radical species in the mechanism for
H2O2 production. Both these complexes formed a dark
blue solution in acetonitrile but this colour was rapidly
discharged on the addition of aqueous hydroxylamine
suggesting the formation of manganese(II) species. The
use of 1:1 aqueous-acetonitrile mixtures is interesting in
that there appears to be no dependence of the catalytic
system on the concentration of tetrachlorocatechol,
unlike that observed previously with [Na]5[Mn(3,5-
(SO3)2Cat)2] ꢂ 10H2O in aqueous solution [28] where an
ꢀ30-fold excess of Tiron over MnII was required to
ensure a good rate of H2O2 production. Unpublished
work suggests a dependence of the rate of H2O2 pro-
duction on the ratio of water and acetonitrile in the
reaction mixture, with lower and higher amounts of
acetonitrile than that used here producing lower rates.
These results suggest that a 1:1 aqueous acetonitrile
mixture may provide the ideal conditions for stabilising
radical species formed during this catalytic process.
Fig. 3. The catalytic production of H2O2 with O2 using NH2OH as
substrate at 20 + 1 °C with different weights of (Bu4N)[Mn
(Cl4Cat)2(H2O)(EtOH)]. In all runs the initial [NH2OH] was 0.500 M
(3.30 g, 50% aqueous). O2 (1.5 dm3 minꢁ1) was introduced into a
rapidly stirred 1:1 aqueous-acetonitrile solution (total volume 100
cm3). Weights of complex: (s) none; (N) 1.1 mg (1.25 lmol); (ꢀ) 2.2
mg (2.50 lmol); (r) 4.4 mg (5.00 lmol).

T.S. Sheriff et al. / Inorganica Chimica Acta 357 (2004) 2494–2502
2501
catechol in which hydroxylamine acts as a sacrificial
reductant to regenerate the catalyst from the oxidised
quinone form of the catechol. While the results here
would lend support to such a mechanism, this does not
explain why the substrate requirements in this system
are so specific to hydroxylamine (or hydrazine) with a
requirement for at least one hydrogen atom at each end
of the substrate molecule (e.g. no H2O2 production with
1,1-dimethylhydrazine) [16]. It could be that the reaction
proceeds via a hydrogen-bonded stabilised intermediate
that is crucial to the catalytic reduction of dioxygen by
inorganic amine.
The coordination chemistry of manganese higher
valent complexes with O and N donor ligands is an area
of tremendous variety in which patterns are developing
based on both the electronic and structural motif char-
acteristics provided by the ligand environment. The
isolation and characterisation of Na5[Mn(3,5-(SO3)2
Cat)2] ꢂ 10H2O [28] and (Bu4N)[Mn(Cl4Cat)2 (H2O)
(EtOH)] provides further clear evidence of the inability
of manganese(III) to form tris (catecholato) complexes.
It was thought this provided a firm basis for postulating
a mechanism by which this system can activate small
inorganic molecules by the utilisation of vacant sites on
the manganese atom. However the almost exactly simi-
lar catalytic characteristics exhibited by using (Bu4N)2
[Mn(Cl4Cat)3] indicate that neither vacant sites (nor the
oxidation state of the manganese atom) are important in
the catalytic mechanism. This result would lend support
to an outer sphere mechanism for hydrogen peroxide
production involving the reduction of bound O2 by
coordinated catechol in which hydroxylamine acts as a
sacrificial reductant to regenerate the catalyst from the
oxidised quinone form of the catechol, as proposed by
Pierpont and Lange [1]. The electronic, non-innocent,
characteristics provided by catecholate ligands, in par-
ticular their ability to undergo two electron, single step,
oxidation to semi-quinone and quinone species together
with the electronic fine-tuning provided by the substit-
uents on the catecholate ring and the complimentary
nature of the manganese atom forming stable oxidation
states of +II, +III and +IV, provide the enzyme-like
characteristics of this systemÕs unique ability to activate
O2 for reduction to hydrogen peroxide using small in-
organic amines. A similar electronic-structural motif
involving the o,o0-dihydroxy azo structural unit has been
found to be important for the efficient MnII oxidation of
organic dyes in alkaline media [41]. With the system
described here, clean technology processes can be en-
visaged which can, not only generate hydrogen peroxide
in situ in aqueous solution, but also selectively catalyse
important organic oxidation transformations under
ambient conditions.
Fig. 5. Log weight of complex against log of initial rate of reaction. (r)
(BU4N) [Mn(Cl4Cat)2(H2O)(EtOH)]; (ꢀ) (Bu4N)2[Mn(Cl4Cat)3].
4. Conclusions and perspectives
The similarity of the hydrogen peroxide curves
for (Bu4N)[Mn(Cl4Cat)2(H2O)(EtOH)] and (Bu4N)2
[Mn(Cl4Cat)3] suggests that the mechanism of hydrogen
peroxide production is the same in both cases. It is
worthy of note that although (Bu4N)[Mn(Cl4Cat)2
(H2O)(EtOH)] contains the MnIII ion and (Bu4N)2[Mn
(Cl4Cat)3] the MnIV ion, it is probable that these pre-
catalysts are reduced to MnII in the presence of NH2OH
and that it is the in situ generated [Mn(Cl4Cat)2(H2O)
(EtOH)]2ꢁ and [Mn(Cl4Cat)3]4ꢁ that are the active form
of the catalyst [28]. Pecoraro [22] has shown that on
model manganese catalase enzymes, hydroxylamine can
be used as a good reducing agent to prepare the fully
reduced (MnI2I) active form from MnIII=IV forms. This
may explain why the different starting oxidation states
of the manganese atom in the two complexes are un-
important to the catalytic reduction of dioxygen to hy-
drogen peroxide. The fact that the presence of labile
coordination sites around the metal are also unimpor-
tant would suggest that the reduction of dioxygen occurs
via an outer-sphere mechanism without the coordina-
tion dioxygen and hydroxylamine to the manganese
atom. Pierpont and Lange [1] have proposed a mecha-
nism for hydrogen peroxide production in this system
based on the reduction of bound O2 by coordinated
A remarkable feature of this enzyme-like system are
the high TOF of 10 000 hꢁ1 which are due to the rapid
production of hydrogen peroxide and the tiny (lM)

2502
T.S. Sheriff et al. / Inorganica Chimica Acta 357 (2004) 2494–2502
[3] B.G. Malmstrom, L.E. Andreasson, B. Reinhammer, in: P.D.
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1975, p. 507.
quantities of manganese complex which are used. These
results are obtained under near ambient conditions
(room temperature, pH 8.0, only slight positive pres-
sures of O2) and compare very favourably with the
commercial production of hydrogen peroxide by the
auto-oxidation of 2-ethylanthrahydroquinone in an or-
ganic solvent. The characteristics of this system are even
more remarkable when one considers the ability of these
relatively high concentrations of H2O2 and NH2OH to
coexist. This is probably due to the absence of ÔfreeÕ
metal ions to catalyse their mutual redox decomposi-
tion. In PecoraroÕs manganese-catalase systems [22], the
addition of hydrogen peroxide to the enzyme in the
presence of hydroxylamine results in a rapid conversion
to a catalytically inactive, superoxidised form that con-
tains a dioxo-bridged MnIII=IV dinuclear metal centre.
Clearly the nature of the ligand environment around a
manganese atom can radically change the behaviour and
properties of the resulting complex.
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Work is continuing to search for alternative sub-
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economically viable as well as using this and similar
systems to activate other small molecules under ambient
conditions and in the modelling of natural systems.
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5. Supplementary material
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Complete crystallographic data (excluding structure
factors) for the structures reported in this paper have
been deposited at the Cambridge Crystallographic Data
Centre with CCDC No. 218044 for (Bu4N)[Mn(Cl4
Cat)2(H2O)(EtOH)] and CCDC No. 218045 for (Bu4N)2
[Mn(Cl4Cat)3]. Copies of the data can be obtained free
of charge on application to: The Director, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-
336-033, email: deposit@ccdc.cam.ac.uk or www: http://
<www.ccdc.cam.ac.uk>).
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€
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Acknowledgements
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J.L. would like to thank The Nuffield Foundation for
the provision of an Undergraduate Research Bursary
(URG02). We are grateful Dr. Ann L. Bingham at the
EPSRC X-ray crystallography service in the Depart-
ment of Chemistry at the University of Southampton for
coordinating the data collection of our samples.
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