The autoxidation activity of new mixed-ligand manganese and iron complexes with tripodal ligands

Article abstract of DOI:10.1016/j.jcat.2007.09.008

The activity of new manganese and iron complexes of dianionic tripodal ligands in the autoxidation of ethyl linoleate (EL) is reported. EL consumption rates were monitored using time-resolved FTIR and the degree of oligomerisation was determined by SEC. Almost all complexes showed the same trend in the autoxidation of EL. After a short induction time, the reaction started at a relatively high constant rate; later, this rate changes to a lower rate, which was again constant and on average was the same for all complexes studied. The observation that the autoxidation rate eventually became the same for all complexes can be explained with the assumption that a catalytic species was formed that was structurally similar for each complex. The crystal structure of a representative new complex, [Mn(papy)(acac)] (H₂papy=N-(2-hydroxybenzyl)-N-(2-picolyl)-glycine; Hacac = acetylacetone), was elucidated, demonstrating a Jahn-Teller distorted octahedral geometry of the Mn(III) ion. To gain insight into the reaction mechanisms by which these manganese complexes react with peroxides, the reaction of [Mn(pppy)(dpm)] with alkyl hydroperoxides was studied using UV-vis and EPR spectroscopy (H₂pppy = 2-[bis(2-hydroxybenzyl)aminomethyl]-pyridine; Hdpm = dipivaloylmethane). It is proposed that in a reaction of [Mn(pppy)(dpm)] with t-butyl hydroperoxide (^tBuOOH), the species [Mn(III)(pppy)(OH)(^tBuOO^s)] is formed. In analogy with the formation of that species, which is postulated to occur through a reduction to Mn(II), it is proposed that in the direct initiation reaction of EL with a manganese(III) complex, a 1 electron-1 proton transfer reaction leads to the formation of Mn(II) and an EL radical. Considering parameters such as autoxidation activity, complex solubility and ease of complex preparation, the compound [Mn(III)(tbpppy)(dpm)] (H₂tbpppy = 2-[bis(2-hydroxy-3,5-di-tert-butylbenzyl) aminomethyl]pyridine) is expected to be the best potential alkyd paint drier.

Full text of DOI:10.1016/j.jcat.2007.09.008

Journal of Catalysis 252 (2007) 110–118
www.elsevier.com/locate/jcat
The autoxidation activity of new mixed-ligand manganese and
iron complexes with tripodal ligands
Remy van Gorkum a, Joris Berding a, Duncan M. Tooke b, Anthony L. Spek b, Jan Reedijk a,
Elisabeth Bouwman a,∗
a
Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands
b
Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University, 3584 CH Utrecht, The Netherlands
Received 13 July 2007; revised 13 September 2007; accepted 13 September 2007
Abstract
The activity of new manganese and iron complexes of dianionic tripodal ligands in the autoxidation of ethyl linoleate (EL) is reported. EL con-
sumption rates were monitored using time-resolved FTIR and the degree of oligomerisation was determined by SEC. Almost all complexes showed
the same trend in the autoxidation of EL. After a short induction time, the reaction started at a relatively high constant rate; later, this rate changes
to a lower rate, which was again constant and on average was the same for all complexes studied. The observation that the autoxidation rate even-
tually became the same for all complexes can be explained with the assumption that a catalytic species was formed that was structurally similar for
each complex. The crystal structure of a representative new complex, [Mn(papy)(acac)] (H papy = N-(2-hydroxybenzyl)-N-(2-picolyl)-glycine;
2
Hacac = acetylacetone), was elucidated, demonstrating a Jahn–Teller distorted octahedral geometry of the Mn(III) ion. To gain insight into the
reaction mechanisms by which these manganese complexes react with peroxides, the reaction of [Mn(pppy)(dpm)] with alkyl hydroperoxides
was studied using UV–vis and EPR spectroscopy (H pppy = 2-[bis(2-hydroxybenzyl)aminomethyl]-pyridine; Hdpm = dipivaloylmethane). It is
2
t
t
·
proposed that in a reaction of [Mn(pppy)(dpm)] with t-butyl hydroperoxide ( BuOOH), the species [Mn(III)(pppy)(OH)( BuOO )] is formed. In
analogy with the formation of that species, which is postulated to occur through a reduction to Mn(II), it is proposed that in the direct initiation
reaction of EL with a manganese(III) complex, a 1 electron–1 proton transfer reaction leads to the formation of Mn(II) and an EL radical. Con-
sidering parameters such as autoxidation activity, complex solubility and ease of complex preparation, the compound [Mn(III)(tbpppy)(dpm)]
(H tbpppy = 2-[bis(2-hydroxy-3,5-di-tert-butylbenzyl) aminomethyl]pyridine) is expected to be the best potential alkyd paint drier.
2
© 2007 Elsevier Inc. All rights reserved.
Keywords: Ethyl linoleate; Manganese complexes; Tripodal ligands; Autoxidation; Catalysts
1. Introduction
the manganese-based catalyst system [Mn(acac)3]/bpy (bpy =
2,2ꢀ-bipyridine), which exhibited very high activity in the
oxidation of unsaturated fatty acids [8,11]. Unfortunately,
[Mn(acac)3] is dark-coloured, precluding its application in
many commercial paint products. Nevertheless, this research
result stimulated further research to find new manganese-based
drying catalyst systems.
Lipoxygenases are manganese or iron-containing dioxy-
genases that act on lipid substrates containing one or more
(Z,Z)-1,4-pentadiene moieties [12]; they are widely distrib-
uted among plants and animals. Common polyunsaturated fatty
acids, such as linoleic, linolenic, and arachidonic acids, are the
natural substrates for these enzymes; the primary reaction prod-
ucts are hydroperoxides of conjugated (E,Z)-dienes. In fact,
Cobalt complexes are the most widely used drying cata-
lysts for oxidative air-drying of alkyd paints. However, sev-
eral reports have been published describing the possible car-
cinogenicity of cobalt salts and cobalt complexes [1,2]. Some
commercial paint formulations contain manganese carboxylate
as a primary drier, even though these do not have the same
performance as cobalt-based driers [3,4]. This has initiated a
search to find alternatives for cobalt complexes as drying cat-
alysts in alkyd paint [5–10]. Recently, our group reported on
*
Corresponding author.
E-mail address: bouwman@chem.leidenuniv.nl (E. Bouwman).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.09.008

R. van Gorkum et al. / Journal of Catalysis 252 (2007) 110–118
111
Scheme 2. Overview of the complexes.
2. Experimental
Scheme 1. Ligands used in this study.
model complexes for the lipoxygenase active site could very
well be good catalytic driers for alkyd paints, especially if they
also would be able to catalytically decompose hydroperoxides.
Tripodal ligands with phenol groups have been used extensively
in the synthesis of iron and manganese complexes to model the
active sites of various dioxygenase and phosphatase enzymes
[13–17].
2.1. Materials
Technical ethyl linoleate (EL, 70%; remainder ethyl oleate,
ethyl palmitate and ethyl stearate [20]) was obtained from
Fluka. The small amounts of hydroperoxides that are present in
the EL were removed by passing the oil through a basic alu-
mina column immediately before use. Cumyl hydroperoxide
(80% in cumene) and t-butyl hydroperoxide (5.5 M solution
in decane) purchased from Fisher Scientific were used as re-
ceived. Other chemicals were all of reagent grade and used as
received. The complex [Mn(III)(acac)3] was prepared as de-
scribed previously [21]. The synthesis and characterisation of
the complexes with the tripodal ligands H2pppy, H2tbpppy, and
H2dnpppy have been reported elsewhere [18].
The impetus for the use of tripodal ligands in the search
for paint-drying catalysts is that such tetradentate ligands usu-
ally form relatively stable complexes with manganese and iron.
The formed complexes are expected to be robust and thus are
not readily decomposed under oxidising conditions. This last
point is especially important, because these complexes must
act as radical autoxidation catalysts and are subject to oxidis-
ing conditions. An additional benefit of this ligand type is that
the synthesis of the ligands is straightforward and variations on
the ligand structure are readily possible. β-Diketonates were
chosen as the “secondary” ligand because good autoxidation
results have been obtained with simple, homoleptic manganese
β-diketonate complexes [7,8]. We have synthesised and char-
acterised a series of manganese(III) and iron(III) coordination
compounds with one dianionic tripodal ligand and one auxil-
iary ligand (Schemes 1 and 2) [18]. The resulting complexes
are noncharged, which is expected to result in good solubility
in apolar solvents. A good solubility in the apolar paint medium
is considered to be one of the most crucial properties of poten-
tial paint driers [19].
Herein, we report the autoxidation activity of the complexes
in a model reaction with ethyl linoleate; the different activi-
ties are compared using time-resolved FT-IR and size-exclusion
chromatography (SEC) [9,20]. The new manganese(III) com-
plex [Mn(papy)(acac)] has been synthesised and characterised;
its activity is compared with a series of tripodal complexes. To
gain insight into the reaction mechanisms by which complexes
of the type [Mn(tripod)(β-diketonate)] react with peroxides, we
studied the reactions of cumyl hydroperoxide and tert-butyl
hydroperoxide with [Mn(pppy)(dpm)] using UV–vis and EPR
spectroscopy.
2.2. Synthesis
2.2.1. N-(2-Hydroxybenzyl)glycine
N-(2-Hydroxybenzyl)glycine was synthesised using an ad-
aptation of a literature procedure [22]. Glycine (6.0 g, 80 mmol)
and potassium hydroxide (4.5 g, 80 mmol) were dissolved in
40 mL of H2O, and this mixture was added to one equivalent of
salicylaldehyde (9.6 g, 80 mmol) in EtOH. After 30 min of stir-
ring, a solution of one equivalent of NaBH4 (3.0 g, 80 mmol)
and a few drops of an aqueous sodium hydroxide solution
was slowly added to the yellow mixture. After addition of the
NaBH4, the reaction mixture was white and turbid. After stir-
ring overnight, the solution was filtered and acidified to pH 4
with concentrated hydrochloric acid. On cooling of the reac-
tion mixture a white solid precipitated, which was collected
by filtration and washed with a small amount of water. The
product was obtained in 60% (4.31 g) yield as a white pow-
der. 1H NMR (D2O with some DCl): 7.32 (t, 2H, ar), 6.93 (m,
2H, ar), 4.27 (s, 2H, N–CH2–Ar), 3.90 (s, 2H, N–CH2–CO2).
IR (neat): 3108, 2548, 1564, 1489, 1460, 1435, 1410, 1378,
1335, 1309, 1278, 1266, 1254, 1188, 1185, 1151, 1112, 1054,
1030, 972, 917, 874, 850, 826, 763, 686, 616, 590, 544, 512,
474, 459, 332 cm−1
.

112
R. van Gorkum et al. / Journal of Catalysis 252 (2007) 110–118
2.2.2. N-(2-Hydroxybenzyl)-N-(2-picolyl)glycine
2.4. Reactivity studies
(H2papy)
This new ligand was synthesised through a slightly modified
previously reported procedure [23]. To 11 mmol (2 g) N-(2-
hydroxybenzyl)glycine in dry MeOH was added one equivalent
of potassium hydroxide (11 mmol, 0.62 g), then one equiva-
lent of pyridine-2-carboxaldehyde (11 mmol, 1.18 g), yield-
ing a clear yellow solution. After 2 h of stirring, one equiva-
lent of NaBH4 (11 mmol, 0.41 g) was slowly added, and the
solution was stirred for 18 h. After evaporation of the sol-
vent, the residue was dissolved in water, and the solution was
acidified with concentrated acetic acid to pH 3. The reaction
mixture was extracted three times with 30 mL of CH2Cl2.
A white solid precipitated in the combined CH2Cl2 fractions.
This solid was collected by filtration. This was the desired
2.4.1. FTIR and SEC measurements
The FTIR and SEC measurements were performed as de-
scribed previously [9,10]. Due to small differences in the con-
figuration of the HPLC system (e.g., the number of detectors
used), the x-coordinate (elution volume in mL) of a peak in a
size-exclusion chromatogram can vary slightly between differ-
ent sets of measurements.
2.4.2. EPR measurements
Electron paramagnetic resonance measurements were per-
formed using a JEOL JES-RE2X ESR spectrometer with a
JEOL X-band microwave, a JEOL electromagnet, and a JEOL
ESPRIT 330 ESR Datasystem unit. A special quartz Dewar
flask was used for measurements at liquid nitrogen temperature
(77 K).
1
product in 32% (0.95 g) yield. H NMR (D2O): 8.60 (d, 1H,
py), 7.90 (t, 1H, py), 7.40 (dt, 5H, py,ar), 6.86 (m, 2H, ar),
4.53 (s, 2H, N–CH2-py), 4.48 (s, 2H, N–CH2-ar), 3.84 (s, 2H,
N–CH2–CO2). IR (neat): 3397, 3007, 1858, 1704, 1628, 1594,
1460, 1436, 1392, 1347, 1307, 1274, 1254, 1200, 1185, 1150,
1114, 1031, 992, 914, 896, 836, 783, 757, 728, 702, 614, 548,
2.4.2.1. Reaction of [Mn(pppy)(dpm)] with cumyl hydroperox-
ide at room temperature Before the actual experiment a spec-
trum was recorded of the empty EPR tube with solid DPPH
as an external reference, thereafter keeping the parameters ex-
actly the same for the experiment (freq. = 9.434 GHz, power =
4 mW, receiver gain = 1000, field modulation 100 kHz, field
modulation width = 1 mT, sweep width 220–420 mT, sweep
time 1 min). 2.12 mg (3.8 µmol) [Mn(pppy)(dpm)] was dis-
solved in 300 µL of toluene. 4 µL (∼24 µmol, six equivalents
based on the complex) of cumyl hydroperoxide was transferred
into the EPR tube, followed by the toluene solution; then the
EPR spectra were recorded at 1.5 min intervals.
524, 507, 458, 415 cm−1
.
2.2.3. Synthesis of [Mn(III)(papy)(acac)] (IV)
A solution of 0.735 mmol (200 mg) H2papy with 2 equiv of
triethylamine (209 µL) in 5 mL of CH2Cl2 was added to a solu-
tion of 0.735 mmol (0.25 g) of [Mn(acac)3] in 5 mL of CH2Cl2.
The resulting green reaction mixture was stirred overnight. The
solvent was evaporated, and the residue was taken up in ace-
tone. Petroleum ether (40–60) was slowly added until a precipi-
tate was visible. After another 15 min of stirring, the precipitate
was filtered off and dried in vacuo over P2O5. The product
was a dark-green powder, yield 63% (196 mg). Single crystals
were obtained by keeping the acetone/petroleum ether filtrate
at −20 ◦C. Elem. analysis found (calc) for C20H21MnN2O5:
C: 56.52 (56.61), H: 5.40 (4.99), N: 6.42 (6.60)%. IR (neat):
∼3500, 1646, 1604, 1564, 1517, 1478, 1456, 1442, 1353, 1261,
1100, 1018, 920, 892, 796, 760, 732, 679, 636, 528, 460, 416,
2.4.2.2. Reaction of [Mn(pppy)(dpm)] with t-butyl hydroper-
oxide EPR measured in frozen solution (77 K): 5 µL t-bu-
tyl hydroperoxide solution (27.5 µmol) was brought into
an EPR tube. Then, 800 µL of a 1.3 mmol solution of
[Mn(pppy)(dpm)] in toluene (1 µmol) was added, and the EPR
tube was quickly stoppered and frozen in liquid nitrogen. The
EPR spectrum was measured using the following parameters:
freq. = 9.115 GHz, power = 5 mW, receiver gain = 1000, field
modulation 100 kHz, field modulation width = 0.4 mT, sweep
width 220–420 or 310–330 mT, sweep time 4 min. The mea-
surement was carried out in the absence of DPPH to observe
all radical signals unperturbed. After the desired spectrum was
recorded, the spectrum was again recorded in the presence of
DPPH as an external reference to establish a reference field
value.
390 cm−1
.
2.3. Physical measurements
1H NMR measurements were carried out on a 200 MHz
Jeol FX-200 spectrometer equipped with a Tecmag data sta-
tion. Infrared spectroscopy was done on a Perkin-Elmer 1000
FT-IR spectrophotometer with a diamond ATR device. UV–vis
spectra were recorded on a Varian Cary 50 spectrophotome-
ter, and elemental analysis on C, H, and N was performed
with a Perkin-Elmer series II CHNS/O Analyzer 2400. CV
measurements were done in CH2Cl2 solution with an Auto-
lab PGSTAT 10 potentiostat using an Ag/AgCl reference elec-
trode, a Pt working electrode, and a Pt counter electrode with
0.1 M tetrabutylammonium hexafluoridophosphate as an elec-
trolyte.
2.4.3. UV–vis measurements
Solution UV–vis spectra were recorded with a Varian Cary
50 spectrophotometer. First, 3 mL of a 0.25 mmol solution of
[Mn(pppy)(dpm)] in CH3CN was brought into a quartz cuvette;
then 1 µL of cumyl hydroperoxide 80% was dissolved in 100 µL
CH3CN. From this solution, a 18 µL (∼2 eq based on complex)
sample was obtained and added to the cuvette. Then the UV–
vis spectra of the reaction mixture were recorded automatically
each hour for up to 17 h.

R. van Gorkum et al. / Journal of Catalysis 252 (2007) 110–118
113
Table 1
Selected bond lengths (Å) and angles ( ) for complex IV
◦
Mn1–N102
Mn1–N101
Mn1–O101
2.261(2)
2.129(2)
1.863(2)
Mn1–O102
Mn1–O201
Mn1–O202
2.0968(19)
1.931(2)
1.941(2)
N102–Mn1–N101
O101–Mn1–O102
O101–Mn1–O201
O101–Mn1–O202
O101–Mn1–N102
O101–Mn1–N101
O102–Mn1–O201
O102–Mn1–O202
75.38(9)
93.93(9)
175.50(9)
88.50(9)
92.25(10)
92.84(10)
90.44(9)
99.54(8)
O102–Mn1–N102
O102–Mn1–N101
O201–Mn1–O202
O201–Mn1–N102
O201–Mn1–N101
O202–Mn1–N102
O202–Mn1–N101
154.50(8)
79.62(8)
89.69(8)
84.26(10)
89.03(10)
105.33(9)
178.47(10)
2.5. X-ray data collection and structure determination
A black single crystal of the complex [Mn(III)(papy)(acac)]
was selected and transferred into the cold nitrogen stream
(150 K) on a Nonius KappaCCD diffractometer with graphite-
monochromated MoKα radiation (λ = 0.71073 Å) from a rotat-
ing anode source. The data were processed with EVALCCD and
SADABS. The structure was solved by Direct Methods with
SHELXS-86 [24] and refined by least squares with SHELXL97
[25] against F2 with all reflections. A small amount of disor-
dered solvent (in four solvent accessible voids of 112 Å3 each)
was taken into account by back-Fourier transformation in the
structure factor calculations following the PLATON/SQUEEZE
procedure. Hydrogen atoms were introduced at calculated po-
sitions and refined riding on their carrier atoms. One of the two
crystallographically independent molecules was found to be
slightly disordered (atom C501) and refined with a 57:43% dis-
order model. Molecular illustration, checking for higher sym-
Fig. 1. Projection of the structure of [Mn(papy)(acac)] IV. Hydrogen atoms are
omitted for clarity.
lengths and angles for Mn(2) are almost the same as for Mn(1).
A PLATON/PLUTON representation of the molecular struc-
ture of complex IV is shown in Fig. 1. The Mn(III) ion in IV
has a distorted octahedral coordination geometry, just as the
other [Mn(III)(tripod)(β-diketonate)] complexes reported ear-
lier [18]. In contrast with the pppy-complexes Ia, Ic, and Id,
however, complex IV has both oxygen donor atoms of the acac
ligand (O202 and O201) in the equatorial plane, together with
the amine nitrogen atom N101 and the phenolate oxygen atom
O101. The Jahn–Teller axis of the d4 Mn(III) ion is located
along the bonds Mn1–N(pyridine) and Mn1–O(carboxylate).
The N(pyridine)–Mn1 bond length is slightly longer than ob-
served for the other [Mn(III)(tripod)(β-diketonate)] complexes
[18]. The bond lengths of the phenolate oxygen and the amine
nitrogen to the manganese center are both comparable to those
in the Ia, Ic, and Id complexes. The acac ligand is coordinated
symmetrically, with similar bond lengths for both oxygens. The
acac ligand is not folded over the O201–O202 axis, but the
acac-Mn chelate ring is slightly puckered, as indicated by a
small torsion angle of 5.5(3)◦ between the two C–O groups in
the acac ligand.
Cyclic voltammetry was used to compare the oxidising
power of the potential paint drier complexes [18]. The cyclic
voltammogram of [Mn(papy)(acac)] IV is shown in Fig. 2.
The redox couple 1 (1ox = 0.96 V; 1red = 0.78 V) of com-
plex IV with an E1/2 value of 0.87 V versus Ag/AgCl for the
oxidation of Mn(III) to Mn(IV) was quite similar to that of
[Mn(dnpppy)(acac)] III [18]. The electron-withdrawing NO2
groups on both phenolate arms of the tripodal ligand ap-
parently had the same effect on the total electron density
at the metal atom as the substitution of one phenolate arm
of the tripodal ligand for a carboxylate group. The lower
donor capacity of the ligands papy and dnpppy resulted in a
metry and geometry calculations were performed with the PLA-
∗
TON package [26]. Crystallographic data: C20H21MnN2O5
,
Mr = 424.33∗, orthorhombic, space group P na21 (no. 33),
Z = 8, a = 21.0513(15), b = 11.2954(10), c = 17.8209(19) Å,
V = 4237.5(7) Å3, T = 150 K, F(000) = 1128∗, μ (MoKα) =
0.687 mm−1∗, 63109 reflections measured (Rσ = 0.0440),
8119 unique (Rint = 0.0789), R = 0.0377, wR2 = 0.0897, S =
1.044, ꢀρmin = −0.32 eÅ−3, ꢀρmax = 0.30 eÅ−3. Starred
quantities are without the contribution of the disordered sol-
vent.
3. Results
3.1. Synthesis and characterisation of potential drier
complexes
The synthesis, spectroscopic characterisation, and structures
of the complexes I–III have been described elsewhere [18].
For the complex [Mn(papy)(acac)] (IV), single crystals were
obtained, and the crystal structure was determined. Table 1
presents a selection of molecular bond lengths and angles.
The crystal structure of complex IV contains two crystallo-
graphically independent but chemically equivalent molecules
in the asymmetric unit. One of the acac-CH3 groups in one
of these complexes is disordered over two positions. Only the
data for the nondisordered molecule are listed in Table 1; bond

114
R. van Gorkum et al. / Journal of Catalysis 252 (2007) 110–118
Fig. 2. Cyclic voltammogram of the complex [Mn(papy)(acac)] IV. Scan rate: 200 mV/s.
metal centre that is much easier to reduce; the peak potential
2red of complex IV (−0.39 V) was shifted to a considerably
more positive potential relative to the value for complex Ia
(−1.08 V) [18]. However, the reduction of [MnIII(papy)(acac)]
to [MnII(papy)(acac)]− was irreversible; another species with
an oxidation at 3ox (−0.01 V) was formed in solution. Ac-
cordingly, the redox couple 1 for the oxidation to Mn(IV) also
shifted to a significantly more positive E1/2 value (0.61 V for
Ia) and became more reversible, indicating greater stability for
[MnIV(papy)(acac)]+ than for [MnIV(pppy)(acac)]+.
3.2. Autoxidation activity determined with FTIR and SEC
To test the new complexes I–IV for their potential paint-
drying capacities, the rate of EL autoxidation was determined
for each complex, using the FTIR method reported earlier [9].
The Mn(III) complexes themselves are not air-sensitive and are
quite stable over long periods (days) in solution; no changes
to the spectrum of the complexes were observed by UV–vis
measurements in solution in air. Reaction occurred only with
both EL and dioxygen present or with hydroperoxides present,
as demonstrated by the EPR and UV–vis experiments (Sec-
tion 3.3). When hydroperoxides were removed from EL, autox-
idation was initiated only when dioxygen was present as well.
All autoxidation reactions were performed using purified
EL, that is, EL from which all hydroperoxides have been re-
moved [7]. The EL autoxidation rates for each complex are
collected in Table 2. For nearly all complexes, the autoxida-
tion reaction proceeded at two different rates over the course of
the measurement: after the induction time, the reaction started
with a high initial rate (termed rate 1), which changed to a lower
rate (termed rate 2) after a certain time interval. This time inter-
val, the “duration of rate 1,” is also given in Table 2 for those
complexes that show two different rates. The FTIR plot for
the autoxidation of EL catalysed by [Mn(pppy)(dpm)] (Id) is
shown in Fig. 3 as a typical example. For the manganese com-
plexes with a tbpppy ligand (II), no clear discrimination could
be made between different rates, and, consequently, only the
initial rate is listed in Table 2.
−1
Fig. 3. Logarithmic plot of the percentage of decrease of the IR 3010 cm
peak area integrals of EL vs time for [Mn(pppy)(dpm)] (Id).
−1
Fig. 4. Logarithmic plots of the percentage of decrease of the IR 3010 cm
peak area integrals of EL vs time for the complexes IIa,c,d, III, and IV.
(acac)] (Ib) and [Fe(tbpppy)(acac)] (IIb), did not show any
catalytic activity in the autoxidation of EL.
The activity for crosslink-formation and oligomerisation of
EL was examined for the selected complexes Ia,d, IIa,d, III,
The FTIR plots obtained for the complexes IIa,c,d, III, and
IV are shown in Fig. 4. The two iron complexes, [Fe(pppy)-

R. van Gorkum et al. / Journal of Catalysis 252 (2007) 110–118
115
Table 2
The induction times and autoxidation rates for the autoxidation of purified EL, catalysed by the complexes I–IV as determined with FTIR
a
−3
−1
a
Compound
Induction
time (min)
Rate 1 (×10 min )/
Rate 2
−3
−1
duration (min)
(×10 min
)
Ia
Ib
Ic
Id
[Mn(III)(pppy)(acac)]
[Fe(III)(pppy)(acac)]
[Mn(III)(pppy)(dbm)]
[Mn(III)(pppy)(dpm)]
[Mn(III)(pppy)(quin)]
[Mn(III)(tbpppy)(acac)]
[Fe(III)(tbpppy)(acac)]
[Mn(III)(tbpppy)(dpm)]
[Mn(III)(tbpppy)(OAc)]
[Mn(III)(dnpppy)(acac)]
[Mn(III)(papy)(acac)]
<25
–
4/155
Not active
6/85
4/120
3/225
1
2
<10
40
75
45
–
40
30
40
80
0
3
2
2
Ie
IIa
IIb
IIc
IId
III
IV
Not active
3
2
3/270
4/220
3
2
2
b
Co-EH
a
The average error in the rates is about 20%.
Co-EH is cobalt(II) 2-ethylhexanoate, the commercial paint drier, given here as a reference.
b
3.3. The reaction of [Mn(pppy)(dpm)] Id with hydroperoxides
studied with UV–vis and EPR spectroscopy
To gain more insight in the reaction mechanisms by which
the manganese complexes might function as autoxidation cata-
lysts, complex Id was reacted with alkyl hydroperoxides. Com-
plex Id was selected as a general example for all complexes
of the type [Mn(III)(tripod)(β-diketonate)]. The EPR and UV–
vis experiments with cumyl hydroperoxide and tert-butyl hy-
droperoxide were performed under argon, to exclude reactions
with dioxygen and enhance resolution and signal-to-noise ratio.
Cumyl hydroperoxide (2-phenyl-prop-2-yl hydroperoxide)
or t-butyl hydroperoxide were added to the complex in solu-
tion, and the changes in the UV–vis or the EPR spectra over
time were monitored. The changes in the UV–vis spectrum of
complex Id in CH3CN solution after addition of two equiva-
lents of cumyl hydroperoxide was monitored for 500 min, and
a spectrum was recorded each hour (supplementary material,
Fig. S2). The two most obvious changes in the spectrum were
an increased intensity of the peak at 430 nm and a decreased ab-
sorption at 321 nm, giving rise to an isosbestic point at 334 nm.
When the reaction time was extended beyond 500 min (up to
1000 min), all absorptions in the spectrum decreased slowly at
a constant rate.
The reaction was also monitored using X-band EPR spec-
troscopy at room temperature. Representative EPR spectra are
shown in Fig. 6. To a dark red/brown solution of [Mn(pppy)-
(dpm)] in toluene was added six equivalents of cumyl hy-
droperoxide, and the EPR spectra of the resulting solution were
recorded at 1.5-min intervals, for up to 15 min. After 15 min,
a brown precipitate was visible in the EPR tube, and the colour
of the remaining solution turned from dark red/brown to brown.
Immediately after addition of the alkyl hydroperoxide, a large
radical signal was visible at 335 mT (g = 2.0104). Over time,
this radical signal decreased, and a broad, six-line signal typical
of Mn(II) appeared at 334 mT (g = 2.0181), which is normal
for Mn(II). The manganese(II) signal was split into six lines by
the 55Mn nucleus, which had a nuclear spin of 5/2. The aver-
Fig. 5. Size exclusion chromatograms for the autoxidation of unpurified EL,
catalysed by the complex [Mn(papy)(acac)] IV, measured after different EL
autoxidation times.
and IV using SEC as described previously [10]. All SEC exper-
iments were performed using unpurified EL, to minimize the in-
duction times. The initiation characteristics are better evaluated
through the FTIR measurements, and are not important for the
oligomerisation experiments. A representative chromatogram
for the catalyst [Mn(papy)(acac)] IV is shown in Fig. 5; the
chromatograms of the other complexes are very similar, with
only some differences in the relative amount of peroxides (sup-
plementary material, Fig. S1). Both compounds with a pppy
ligand, complexes Ia and Id, showed a steady increase in the
ROOH concentration over a period of 8 h. The complexes IIa
and IV also demonstrated an increased ROOH concentration in
8 h, but the total concentration reached in 8 h was lower than
that for the complexes with a pppy ligand. For the complexes
IId and III, a maximum ROOH concentration was attained be-
tween 6 and 8 h of reaction time; the ROOH concentration after
8 h was lower than after 6 h. Although different complexes
showed slightly different trends in the time evolution of the
ROOH peak, the chromatogram after 24 h was very similar for
each complex.

116
R. van Gorkum et al. / Journal of Catalysis 252 (2007) 110–118
age coupling constant due to the manganese hyperfine coupling
was 95 G.
To gain further insight into the reaction of Id with hydroper-
oxides, the reaction mixture of Id in toluene was immediately
frozen in liquid nitrogen directly after the addition of 27.5 eq
of t-butyl hydroperoxide. The frozen solution EPR spectrum of
this reaction mixture is depicted in Fig. 7. A large, anisotropic,
radical signal is visible, with components at 320 mT (g1 =
2.0353), 324 mT (g2 = 2.0095), and 325 mT (g3 = 2.0039)
(Fig. 7, inset). The broad Mn(II) signal, which at room tempera-
ture was not visible until after several minutes, was immediately
noticeable in the frozen solution spectrum. The EPR spectrum
measured directly after thawing the frozen solution showed a
single radical signal, just as was observed for the experiment
with cumyl hydroperoxide (Fig. 5a, spectrum after 1 min). Sub-
sequent EPR spectra at room temperature showed the same
trend as for the reaction with cumyl hydroperoxide, a decreased
radical signal over time and an increased six-line signal charac-
teristic for manganese(II).
4. Discussion
4.1. The reaction of [Mn(pppy)(dpm)] with hydroperoxides
The high radical signal observed in the room temperature
EPR spectra (Fig. 6) of the reaction of complex Id with cumyl
hydroperoxide quickly decreased to form a growing signal aris-
ing from Mn(II). The UV–vis spectra showed a decrease in the
peak at 321 nm. This peak was assigned as the MLCT transi-
tion from the manganese ion to the dpm ligand [18]. On reac-
tion with cumyl hydroperoxide, the coordination mode of the
dpm ligand apparently changed, or dpm dissociated from the
complex; thus, the specific Mn → dpm MLCT transition disap-
peared. The EPR measurement at 77 K supports the idea that
the secondary ligand was replaced; the anisotropic radical sig-
nal in Fig. 7 has been linked to a coordinated peroxyl radical
[27,28]. Thus, the species responsible for the radical signal is
proposed to be [Mn(III)(pppy)(OH)(t BuOO·)]. Scheme 3 pro-
poses a set of reactions for the reaction of complex Id with an
alkyl hydroperoxide, based on the observations in the UV–vis
and EPR and in analogy with a similar mechanism reported for
Fig. 6. Room-temperature X-band EPR spectra of a toluene solution of
[Mn(pppy)(dpm)] Id (13 mM) with 6 eq of cumyl hydroperoxide added,
recorded after (a) 1, (b) 4, and (c) 10 min. Conditions are given in Section 2.
The signal indicated with * is due to an impurity in the cavity.
t
the reaction of [Co(acac)2] with BuOOH [29]. Although the
experiments with alkyl hydroperoxides were performed with
complex Id, the mechanism is expected to be valid for all of
the Mn(III) tripodal ligand complexes. At room temperature,
the formed species [Mn(III)(pppy)(OH)(t BuOO·)] likely react
rapidly with other t-butylhydroperoxide molecules, resulting in
a decrease of the radical signal and formation of Mn(II).
Fig. 7. Frozen solution (77 K) X-band EPR spectrum of [Mn(pppy)(dpm)] Id
(1.3 mM) in toluene with 27.5 eq of tert-butyl hydroperoxide added. The inset
shows a magnification of the 314–330 mT range. Signals with * are due to
Mn(II). Conditions are given in Section 2.
t
t
·
[Mn(III)(pppy)(dpm)] + BuOOH → [Mn(II)(pppy)(Hdpm)] + BuOO
1
2
3
t
t
·
[Mn(II)(pppy)(Hdpm)] + BuOOH → [Mn(III)(pppy)(OH)] + Hdpm + BuO
[Mn(III)(pppy)(OH)] + BuOO → [Mn(III)(pppy)(OH)( BuOO )]
t
·
t
·
Scheme 3. Proposed reaction mechanism for the reaction of [Mn(III)(pppy)(dpm)] Id with t-butyl hydroperoxide, forming a coordinated t-butyl peroxyl radical
species.

R. van Gorkum et al. / Journal of Catalysis 252 (2007) 110–118
117
for the other tripodal ligand complexes. This observation can be
explained by the assumption that the fragment [Mn(II)(tbpppy)]
is more stable than the fragment [Mn(II)(tripod)] for complexes
with other tripodal ligands, due to the electron-donating t-butyl
groups. Consequently, the secondary ligand can more easily
dissociate from the complex, and the catalytic structure respon-
sible for rate 2 in the other complexes can form much more
quickly.
The autoxidation rate for the compound [Mn(III)(tbpppy)-
(acac)] (IIa) is quite low compared with the rates for other
tripodal ligand complexes, only 1 × 10−3 min−1 (see Table 2
and Fig. 4). Complex IIa is also the only complex to show
reversible ligand oxidation [18], which might explain the ob-
served lower reactivity. The complex will be less reactive if
coordinated radicals are stabilised by delocalisation of the rad-
ical over the phenolate arms.
The two iron complexes are not active in the autoxidation
of EL, probably because it is too difficult to reduce Fe(III)
to Fe(II). Often in oxidation catalysis with iron complexes,
polypyridyl ligands are used [31–34]. These polypyridyl lig-
ands are generally good σ-donors, and moderate-to-weak π-
donors [35]. The dianionic tripodal ligands used in this study
are good σ-donors and good π-donors as is clear from the
UV–vis studies [18]. Apparently, the dianionic tripodal ligands
donate too much electron density to the metal, thereby stabilis-
ing the Fe(III) ion and preventing reduction to Fe(II).
The time-resolved SEC studies of EL catalysed by the se-
lected complexes Ia,d, IIa,d, III, and IV show little difference
for each complex, with some differences observed only in the
time evolution of the hydroperoxide peak. For all complexes,
the chromatograms show the formation of dimers, trimers, and
oligomers in the same time period and in roughly the same con-
centrations relative to the monomer peak. The chromatograms
measured after 24 h of reaction are all very similar. This obser-
vation compares well with the postulation that the catalytically
active complexes formed over the course of the autoxidation are
analogous for each complex.
Scheme 4. Proposed reaction mechanism for the initiation of EL-autoxidation
by tripodal ligand complexes, in this example the reaction is shown for com-
plex IV.
4.2. Autoxidation and oligomerisation activities
Except for compounds IIa,c and IId, which contain the
tbpppy ligand, all manganese tripodal ligand complexes showed
the same trend in the autoxidation of (purified) EL. After a short
induction time, the reaction began at a relatively high rate for a
varying period (95–270 min). The high initial rate then changed
to a lower rate, which again was constant and on average was
the same for all complexes. Because the final rate was con-
stant, the change in rate most likely was not related simply to
a decreased concentration of the active catalyst, but rather to a
change in the structure of the catalytically active complex. An-
other feature that the complexes have in common is the short
induction time. A short induction time indicates that the com-
plexes are able to initiate the autoxidation reaction, because
initially no peroxides are present in the purified EL. Initiation
requires reduction of Mn(III) to Mn(II) [8]. As discussed in the
previous section, the complexes are readily reduced by alkyl
hydroperoxides (reaction 1 in Scheme 3). Because the bond dis-
sociation energy of the ROO–H bond (90 kcal mol−1) is higher
than that of a double-allylic R–H bond (80 kcal mol−1) [30],
the tripodal ligand complexes also should be capable of react-
ing directly with EL, thus initiating the reaction.
Scheme 4 illustrates this reaction pathway. The uncharged
secondary ligand that is formed will not coordinate very
strongly to the metal center and can dissociate from the com-
plex. The structural fragment that remains, [Mn(II)(tripod)], is
nearly the same for all of the synthesised compounds. Con-
sequently, a catalytically active complex that is structurally
very similar for each of the complexes may be formed over the
course of the reaction, regardless of the structure of the start-
ing complex. This assumption is supported by the observation
of similar values for “rate 2” (see Table 2) for almost all com-
plexes. It seems that the catalyst that is formed is quite stable,
because the reaction proceeds at the constant “rate 2” for a sig-
nificant time interval. More studies are needed to elucidate the
exact structure of the species responsible for rate 2.
4.3. Tripodal ligand complexes as potential paint driers
Although the only way to really determine whether a com-
plex is a good paint drier is to test it as a drier in alkyd paint,
some predictions can be made based on the results discussed
in this paper. Basically, all manganese complexes except com-
plex IIa showed autoxidation activity in the autoxidation of EL
greater than that of the complex [Mn(acac)3], which has been
established to be a good paint drier [11]. The autoxidation ac-
tivity is also roughly the same for each complex. Thus, based
on these facts, it can be concluded that all of the tripodal–ligand
manganese complexes are potentially good paint driers. How-
ever, although the secondary ligands apparently do not have a
major influence on the autoxidation and oligomerisation char-
acteristics of the different complexes, they do have an influence
on the solubility of a complex. Complexes with an acac or
dpm ligand are much more soluble in apolar solvents, which
is of considerable significance for potential alkyd paint driers.
Thus, complexes Ia,d, IIa,c, III, and IV are expected to be the
The high initial rate 1 might simply be a consequence of the
initiation of radical chain reactions by the direct reaction of the
starting complex with EL. Once the entire complex has been
transformed (as shown in Scheme 4) and the secondary ligand
has dissociated, direct initiation may no longer be possible, and
the autoxidation reaction proceeds predominantly through hy-
droperoxide decomposition.
The FTIR plots of the complexes with a tbpppy ligand do
not show the pronounced change in autoxidation rate observed

118
R. van Gorkum et al. / Journal of Catalysis 252 (2007) 110–118
better candidates for testing as driers in alkyd paint. The com-
plex [Mn(III)(tbpppy)(dpm)] (IIc) is probably the best potential
paint drier, due to its excellent solubility, good autoxidation ac-
tivity, and ease of preparation of the complex.
UK (fax: +44 1223 336 033; email: deposit@ccdc.cam.ac.uk
or www: http://www.ccdc.cam.ac.uk).
Please visit DOI: 10.1016/j.jcat.2007.09.008.
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of an uncharged secondary ligand. This mechanism is proposed
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Acknowledgments
This work was carried out as part of the SENTER/Innovation
Oriented Research Program on Heavy Metals (IOP), IZW-
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Size-exclusion chromatograms for the autoxidation of un-
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