A study of new manganese complexes as potential driers … · A study of new manganese complexes as potential driers for alkyd paints E. Bouwman, R. van Gorkum FSCT and OCCA 2007

A study of new manganese complexes as potential driersfor alkyd paints

E. Bouwman, R. van Gorkum

� FSCT and OCCA 2007

Abstract In the search for new autoxidation catalysts,the oxidative crosslinking of ethyl linoleate (EL) hasbeen monitored using time-resolved FT-IR (Fouriertransform IR spectroscopy), size-exclusion chromatog-raphy and GC/MS (gas chromotography/mass spec-troscopy). These methods seem to be quite sensitive tochanges in the structure of the catalyst. It has beenfound that the compound [Mn(acac)3] (Hacac = 2,4-pentanedione) is an efficient catalyst for the oxidationand the oligomerization of EL, which is proposed toproceed not only via hydroperoxide decomposition,but also through substrate activation. The system[Mn(acac)3] with added bpy (=2,2¢-bipyridine) has avery high activity for the oxidation of EL. In situformation of the species [MnII(acac)2(bpy)] and[MnIII(acac)2(bpy)]+, and the high reactivities of thesetwo species with hydroperoxides and with EL, respec-tively, are proposed as an explanation for the observedhigh oxidation rate.

Keywords Alkyd paint, Cobalt-based catalysts,Manganese derivatives, Cobalt derivatives, Driercatalyst, Ethyl linoleate, Bipyridine

Introduction1

Alkyd paints are based on alkyd resin precursors,which are polyesters incorporating unsaturated fatty

acid chains such as oleic acid, linoleic acid, linolenicacid, and ricinoleic acid. In the drying process of apaint film, the solvent evaporates (physical drying),and then chemical reactions take place, resulting in thecuring of the binder.

The chemical ‘drying’ of alkyd paints is a metal-catalyzed, oxidative crosslink-forming process, mainlybased on the autoxidation of the unsaturated fatty acidside chains. The autoxidation of the diene moiety,leading to a dry paint film is illustrated, in a greatlysimplified form, in Fig. 1. The reaction is initiated byhydrogen abstraction of the doubly activated methy-lene group. The resulting radical, R•, reacts withdioxygen, according to the well-known autoxidationmechanism, leading to hydroperoxide species(ROOH). These hydroperoxides decompose in ametal-catalyzed reaction to alkoxyl (RO•) and peroxylradicals (ROO•). Through the recombination of theradicals, a three-dimensional polymer is formed that isresponsible for the hardening of the paint.

Among the environmental challenges arising in thelast decade, that concerning the substitution of sus-pected toxic components in common organic coatingmaterials is of great importance. The search foracceptable alternatives to toxic chemical additives inalkyd paints, especially the antiskinning agents and thecobalt drier catalysts, has recently become a subject ofintense industrial and academic interest.1–8

The most important and recognized role of dryingagents in the paint formulations is to enhance thedecomposition of the relatively stable hydroperoxides.The role of the metal ion in the first initiation step isstill an issue of debate. The most widely used driers forsolventborne paints contain cobalt 2-ethylhexanoate asthe primary drier. To screen and to compare a largenumber of metal complexes for their potential use as adrying catalyst, a fast, simple, and reliable test systemhas been developed. First of all, the performance of thecommercial cobalt driers and manganese driers were

Presented at XXVIIIth FATIPEC Congress, organized jointlyby the Hungarian Chemical Society (MKE) and the PolishAssociation of Chemical Engineers (SITPCHEM), in Budapest,Hungary, June 12–14, 2006.

E. Bouwman (&), R. van GorkumLeiden Institute of Chemistry, Leiden University, P.O. Box9502, Leiden 2300 RA, The Netherlandse-mail: [email protected]

J. Coat. Technol. Res., 4 (4) 491–503, 2007

DOI 10.1007/s11998-007-9041-0

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analyzed; the effect of bipyridine inclusion on thereactivity of these driers has been investigated and theresults compared with the actual effectiveness of thesesystems in real alkyd paints.3 In later stages, new driercatalysts based on manganese were developed.2,7 Also,the effect of the antiskinning agent, methyl ethylketoxime (MEKO), on the activity of a drier has beenmonitored using this test system.6

Development of the test system

Modeling alkyd paint

In order to avoid using complex paint mixtures,compounds can be used that model certain aspects ofthe mixture. Since, in alkyd paint drying, most of thechemistry takes place at the unsaturated fatty acid tailsof the binder compound, the entire binder resin can bemodeled by using compounds that structurally orchemically resemble this fatty acid chain. Dienes suchas 4,7-heptadiene,9 3,6-nonadiene,10 and 2,5-undecadi-ene,11 as well as methyl and ethyl esters of theunsaturated fatty acids linoleic, linolenic, and ricinoleicacid,12–15 have been used as model compounds foralkyd resin precursors. An advantage of this approach,besides having a much simpler system to study, is thatwhen model compound mixtures become ‘dried’, theyremain liquid and consequently are more easily studiedusing standard analytical techniques. In the presentstudy, ethyl linoleate (EL, see Fig. 2), the ethyl ester ofthe fatty acid linoleic acid, was used. A wide range ofanalytical techniques has been used to study theautoxidation of alkyd paint model compounds. Thesetechniques include 13C-NMR,10–13 GC,13 HPLC,9–11

FT-IR,14,16–18 SEC,12,13 and SIMS.15

To screen different metal complexes quickly fortheir ability to enhance the ‘drying’ of EL, the twomore useful parameters to monitor are the rate of theautoxidation reaction and the extent of the crosslinkingthat takes place. Time-resolved Fourier transforminfrared spectroscopy (FT-IR) has been used to studythe kinetics of EL oxidation. Size exclusion chroma-tography (SEC) has been used to measure the extent ofcrosslinking.

The experimental procedures that were used inthe studies can be found in the original researchpapers.3,7,19

Time-resolved FT-IR experiments

The kinetics of fatty acid oxidation can be studiedby time-resolved FT-IR spectroscopy.2,3 The cisH–C=CH stretching vibration at 3010 cm–1 is espe-cially suitable for following the disappearance of thesubstrate in time.20 In the first step of the reaction, ahydrogen atom is abstracted from the reactive meth-ylene group of the cis,cis-1,4-diene moiety. After therearrangement of the resulting pentadienyl radical andreaction with dioxygen, a hydroperoxide is formed.One of the cis H–C=CH hydrogens has become ahydrogen atom on a secondary carbon atom (seeFig. 3). The decrease of the IR vibration due to thedisappearance of this cis H–C=CH hydrogen atom canthus be associated directly with the first step of theautoxidation reaction.

To monitor the autoxidation of EL in time, anFT-IR spectrum is recorded (automatically) every5 min. Figure 4 shows (part of) several representativespectra of an oxidation experiment. For each spectrum,integration is carried out for the area between 2992 and3025 cm–1. As a result, a table is obtained with the areaof the 3010 cm–1 peak at different times. The logarith-mic plot of these data gives a straight line and thus, as afirst approximation, the oxidation of EL followspseudo first-order kinetics. From the slope of thisgraph, the first-order reaction rate of H• radicalabstraction can be determined. In some cases, aninduction period occurs. This can vary from severalminutes to many hours. The occurrence of an inductiontime can have several explanations. However, the moreimportant factors are the concentration of hydroper-oxides in EL at the start of an autoxidation reaction(see the later section headed ‘Determination of thehydroperoxide concentration in ethyl linoleate’) rela-tive to the catalyst concentration and the metalcomplex that is used as a drying catalyst.

O

O

12

3

4

5

6

7

8

910

11

1213

14

15

16

17

18

Fig. 2: Ethyl ester of (Z,Z)-9,12-octadecadienoic acid; ethyllinoleate

OO

HOO

RR, RORand ROOR

(RH)

(R )

(ROO )

(ROOH)

..

.

.

(ROO ).

(RO ).

OO.

O.

Cross-linked network

Initiator .- H

Catalyst

+ O2

+ RH - R.

Fig. 1: Schematic impression of paint drying: the catalystdecomposes hydroperoxides to form alkoxyl and peroxylradicals. Termination reactions create the crosslinkednetwork


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Since the metal-catalyzed autoxidation of EL is aradical reaction, an optimal catalyst concentration isexpected because with higher concentrations satura-tion would occur. To determine the optimal concen-tration for the screening of different metal catalysts, aseries of measurements with different concentrationsof cobalt 2-ethylhexanoate (Co-EH) has been per-formed. The curve, as obtained from a plot of thereaction rate constants for different molar ratiosCo/EL, indeed shows saturation behavior. Using cobaltconcentrations in a molar ratio of Co/EL higher than4 x 10–3 does not result in a significant increase in thereaction rate, as shown in Fig. 5. Therefore, for all ofthe FT-IR experiments, a molar catalyst/EL ratio of2.5 x 10–3 (EL/catalyst = 400/1) has been used. At thisratio, the reaction rate is still dependent on the metalconcentration, but it is not too sensitive to smallexperimental errors compared with the sensitivity thatassesses the very low metal concentrations.

Size exclusion chromatography

In the metal-catalyzed autoxidation of EL, crosslinksare formed, predominantly because of radical termi-nation reactions. These crosslinks consist of etherbonds, peroxy bonds, and carbon–carbon bonds.13–15

Due to crosslink formation, dimers, trimers and evenoligomers of EL are formed while the autoxidation

reaction proceeds. Taking samples of an autoxidationreaction mixture at increasing reaction times, andmeasuring them using SEC gives some insight in to therate of formation of the different crosslinked species.In Fig. 6 several chromatograms are shown that havebeen obtained at different times during an autoxida-tion reaction. The EL peak decreases in time as EL-hydroperoxides are formed. The hydroperoxide peakwill first increase, and then decrease, as hydroperoxides

Fig. 5: The effect of the molar Co/EL ratio on the reactionrate

Fig. 6: Several size exclusion chromatograms; samplestaken after 2, 4, 8, and 24 h reaction time of an autoxidationreaction mixture of EL with a metal complex drying catalyst.Peak assignments: 1: EL 2: EL-hydroperoxide 3: dimers 4:trimers 5: oligomers

C C

C(RH) (R ).

Initiator.- Hb

+ O2+ RH

- R.

Ha Ha

Hb Hb

HOO

Hb

Ha Ha

(ROOH)

Fig. 3: The cis Ha–C=CH hydrogen of a 1,4-pentadiene unit in a fatty acid disappears in the first step of the autoxidation dueto abstraction of a Hb atom and rearrangement of the resulting pentadienyl radical

Fig. 4: 2700–3200 cm–1 region of the FT-IR spectra of EL atincreasing reaction times (curves at 0, 50, 100, and 750 minare shown) during an autoxidation reaction. The peak at3010 cm–1 decreases as the autoxidation reaction proceeds


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are decomposed to form crosslinked species. Theintensity of the peaks, related to these crosslinkedspecies, increases over time. First, the dimers arevisible, then trimers, and, given enough reaction time,higher oligomers are detected.

Determination of the hydroperoxide concentrationin ethyl linoleate

During storage, the hydroperoxide concentration inEL will slowly increase because of the spontaneousreaction with dioxygen from the air. Since hydroper-oxides have a direct influence on the metal-catalyzedautoxidation, in most cases it is desirable to remove allof the hydroperoxides from EL prior to use in anoxidation experiment. In the remainder of this paper,whenever ‘purified EL’ is mentioned, it implies that ELis used, from which all peroxides have been removed.

Hydroperoxides are removed from EL by eluting itover a basic Al2O3 column. To determine the hydro-peroxide content in EL before and after purification,the HPLC method described by Nakamura and Maedawas used.21 This method is based on the stoichiometricoxidation of triphenylphosphine (TP) to triphenyl-phosphine oxide (TPO) by any hydroperoxides that arepresent in a given sample. The concentration of formedTPO can then be determined by reverse phase HPLCusing an UV detector at 220 nm. All hydroperoxidesare quantitatively reduced within 30 min, and thedetection limit for TPO at 220 nm is lower than10 pmol. Hydroperoxides are also important interme-diates in the autoxidation reaction. Therefore, it isvaluable to determine the hydroperoxide level in anautoxidation reaction mixture at different reactiontimes. For this purpose, a spectrophotometric methodwas used.22

Performance of the commercial driers

FT-IR experiments: comparison of differentcommercial catalysts

It appeared to be possible to compare the drying abilityof different metal complexes in the model system byusing time-resolved FT-IR. In Fig. 7, the resulting first-order fits for three different drying agents are com-pared.3

The commercial drier cobalt 2-ethylhexanoate(Co-EH) and the drier based on manganese 2-ethyl-hexanoate and bipyridine (Mn-EH/bpy) both showexcellent catalytic activity in the disappearance of EL(reaction rate 30–200 min: 0.004 min–1). For theMn-EH/bpy system, a short induction time of 20 minis observed. Mn-EH (manganese 2-ethylhexanoate), incontrast, is not active in the test system during the first100 min. After this induction time, the Mn-EH is alsoactive in the reaction, with a slightly lower reaction

rate of 0.0032 min–1. These results are in reasonablecorrelation with the drying times found for actual alkydpaints.3

Influence of bipyridine

The remarkable enhancing effect of bpy on thecatalytic activity of the manganese octoate in the drierMn-EH/bpy has been explored in a systematic study.Known amounts of bpy were added to Mn-EH, andthese in situ mixtures were tested in the modelreaction. For comparison, the effect of bipyridine onthe catalytic activity of Co-EH was also investigated.3

The addition of only 0.25 equiv. of bipyridine toMn-EH already has a tremendous effect on the catalyticactivity. The induction time drops from 100 to 25 min,and the slope of the graph increases from 3.2 x 10–3 to4.4 x 10–3. By further increasing the bpy/manganeseratio, the induction time can be reduced to 10 min, andthe slope of the graph stays the same within theexperimental error. The in situ measurements ofCo-EH with various amounts of bpy show quite adifferent trend. Here, the induction time remains shortin every measurement. The slope, however, decreasessignificantly with increasing amounts of added bipyri-dine. The resulting graphs resemble the measurementstaken with lower Co concentrations as measured for thedetermination of the optimal catalyst ratio. It can beassumed that by adding bpy, a number of the cobalt ionsis co-ordinated, thereby lowering the concentration ofthe catalytically active species.

During the studies, single crystals of the tetranuclearmanganese cluster [Mn4O2(2-ethylhexanoate)6(bpy)2]were obtained.3A projection of the structure isdepicted in Fig. 8. Due to a disorder in the alkyl

3.7

3.9

4.1

4.3

4.5

4.7

0 50 100 150 200

time (min)

)001*0]L

E[ /t ]L

E[(nL

Co-EH

Mn-EH/bpy

Mn-EH

Fig. 7: FT-IR graphs representing the pseudo first-orderrates for the disappearance of EL in time for the catalyticdriers Co Nuodex ( ), Mn-EH/bpy ( ), and Mn-EH ( )


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chains of the 2-ethylhexanoate, overlaid by stronglydisordered free solvent molecules on the same locationand twinning of the crystals, the structure could not befully refined. The overall structure, however, especiallythe geometry of the Mn4O2 cluster core, can beconsidered as quite fitting. A very similar structure ofan acetate-bridged cluster has been published byVincent et al.23 The cluster was found to contain amixed-valence complex with two of the manganeseions in the Mn(III)oxidation state in a penta-co-ordinated pure oxygen environment. The two othermanganese ions are octahedrally co-ordinated in aN2O4 donor set with much longer bonding distances,and these are in the divalent oxidation state. Sincethere is a strong analogy between the geometry in thecentral Mn4O2 core, it is assumed that in the2-ethylhexanoate-bridged complex the electronic dis-tribution is the same. In the FT-IR experiments withEL, this cluster shows only a short induction time and aslope that is even slightly higher than those of thein situ measurements, indicating that this tetranuclearspecies may be the actual catalyst in the EL oxidation.

Development of new driers basedon manganese2,19

FT-IR measurements

The FT-IR plots of selected experiments are depictedin Fig. 9, and the autoxidation rates and inductiontimes of Mn-EH, Co-EH, [Mn(acac)3], [Mn(acac)3]/bpy and [Mn(acac)2(bpy)] are listed in Table 1. Largevariations in induction times are observed whenunpurified EL is used as the substrate. Unpurified ELcontains hydroperoxides in varying amounts because ofslow air oxidation during storage. The presence of

hydroperoxides directly influences the reaction rateand the induction time. When purified EL is used, theinduction time for the commercial manganese catalystMn-EH increases from 140 to over 700 min. Shortinduction times (varying between 0 and 100 min) areobserved, however, when manganese(III) acetylaceto-nate is used as an oxidation catalyst. The higheroxidation activity for [Mn(acac)3] is compared withthat of Mn-EH can be attributed mostly to the largeinduction time for Mn-EH. When the EL oxidationrate for [Mn(acac)3] is compared with the rates for Mn-EH and Co-EH, it is clear that although [Mn(acac)3]shows a slightly lower rate than Co-EH, its overallactivity is far superior to that of Mn-EH. The additionof 1 equiv. of bipyridine to a reaction mixture of[Mn(acac)3] and EL increases the rate of oxidation ofEL significantly, surpassing even the activity of thecobalt drier. Adding more than 1 equiv., however, doesnot seem to enhance the rate any further.

To investigate the influence of bipyridine in moredetail, the complex [MnII(acac)2(bpy)] was synthesized

Fig. 9: Time-dependent integral plots of the 3010 cm–1 IRpeak of purified EL reaction with different catalysts

Fig. 8: Molecular structure of [Mn4O2(2-ethylhexanoate)6

(bpy)2]

Table 1: Oxidation rates and induction times for theoxidation of ELa

Inductiontime (min)

Rate constant k(10–3 min–1)

Co-EH 0 3Mn-EH >700 2[MnIII(acac)3] <100 1[MnIII(acac)3] + 1 equiv. bpy 30 4[MnIII(acac)3] + 2 equiv. bpy 50 4[MnIII(acac)3] + 3 equiv. bpy 45 5[MnII(acac)2(bpy)] 430 2

a EL was purified by eluting over Al2O3 before use. Molarratio catalyst/EL = 1/400, reaction in neat EL at room tem-perature. Due to the radical nature of the reactions theaverage experimental error is about 20%


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separately,24 and was tested for its autoxidation activ-ity. A rather large induction time is observed when thismanganese(II) compound is used in combination withpurified EL (see Table 1). Initiation does not occurwith manganese(II) complexes in the absence ofperoxides because manganese(II) is not able to oxidizethe substrate under these reaction conditions. Thus, aninduction time of over 400 min is observed in whicheither the substrate or the complex is slowly air-oxidized, after which the catalytic radical autoxidationcan start. To check whether or not the induction timeresults from the absence of peroxides, the oxidationexperiment was also performed with unpurified EL.Then no induction time is observed and the oxidationreaction starts immediately with a rate of 4 x 10–3 min–1,comparable to the rates observed for [Mn(acac)3] withbpy added in situ.

Structure of [Mn(acac)2(bpy)]24

While searching for novel manganese-based catalystsfor the oxidative drying of alkyd paints, the X-raystructure of the well-known compound [Mn(acac)2

(bpy)] was determined. Although it has been claimedthat this complex has an octahedral co-ordinationgeometry,25 it appears to be the first example of amixed–ligand complex with innocent didentate ligandsthat possesses the trigonal prismatic co-ordinationgeometry.

A projection of the complex is shown in Fig. 10. Themanganese(II) ion has a almost perfect N2O4 trigonal-prismatic co-ordination environment, with two acetyl-acetonate ligands and one bipyridine ligand. Themanganese to nitrogen distances are 2.288(3) and2.283(2) A. The manganese to oxygen distance for theacetylacetonate ligands is, for each Mn–O bond, almostthe same, and lie in the range 2.1480(16) to2.1580(18) A. The bite angles for each of the acetyl-acetonate ligands are also nearly identical, being81.69(6)� (O1–Mn1–O2) and 81.92(6)� (O3–Mn1–O4).Since the bite angles are rather small and the Mn–O

distance rather large, the Mn–O–C angles are alsolarge, in the range of 130.39(17) to 131.78(16)�. Thebipyridine ligand has a bite angle (N1–Mn1–N2) of70.41�. The two acetylacetonate ligands are nearlyplanar, but the angle between the least-square planesthrough the rings of the bipyridine ligand amounts to3.79(12)�. This ligand is therefore not planar. For allthree ligands, the atoms comprising the chelate ringsdeviate little from their least-square mean plane. Theobtuse angles between the least-square mean planes ofthe chelate rings lie in the range of 117.67(7) to121.30(6)�, in accordance with the trigonal prismaticco-ordination geometry.

The two trigonal faces of the prism constitute oneoxygen atom of each of the acetylacetonate ligands andone nitrogen atom, thus forming O1–O3–N1 andO2–O4–N2. In Fig. 11 the trigonal prismatic co-ordination geometry around manganese is shown inmore detail. The lengths of the triangular sides are inthe range of 2.901(2) to 3.030(3) A for the triangleO1–O3–N1, and 2.846(2) to 3.007(3) A for the triangleO2–O4–N2. All of the angles are in the range of56.88(7) to 62.25(7)�. The four acetylacetonate oxygenatoms make up an almost exact square, the sides ofwhich are in the range of 2.816(2) to 2.901(2) A. Theremaining two faces of the prism are trapezoids consist-ing of two oxygen atoms of one acetylacetonate ligand,and are joined by the two nitrogen atoms of thebipyridine. Density functional theory (DFT) calcula-tions have been performed to address the question of thepreference for trigonal prismatic vs octahedral geome-try, comparing this complex with the related octahedralphenanthroline complex, [Mn(acac)2(phen)].

The time-evolution of peroxides in EL oxidation

The concentration of peroxides was monitored overtime for the autoxidation of EL in the presence of thevarious catalysts. The results are shown in Fig. 12. Theperoxide value for the unreacted EL, as received from

Fig. 10: Projection of [Mn(acac)2(bpy)]. Hydrogen atomswere omitted for clarity

Fig. 11: Trigonal prismatic geometry of [Mn(acac)2(bpy)] indetail


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the supplier, was determined to be 4 mmol/mol EL. Inthe presence of Co-EH, a peroxide concentration wasreached of about 50 mmol/mol in 5 h. As the oxidationof EL proceeds, the amount of peroxides increasescontinuously and a maximum value of 170 mmol/mol isreached in about 30 h. Then, the peroxide valuedecreases rapidly to a level of approximately40 mmol/mol. These observations are in accordancewith EL-autoxidation studies reported by Mallegolet al.26,27

In the presence of [Mn(acac)3], a much higherconcentration of peroxides (240 mmol/mol) is reachedafter a much longer reaction time of 100 h, and therate-of-decrease of the peroxide value after attainingthe maximum value is much less than for the oxidationwith Co-EH. Even after 300 h, a significant amount ofperoxide of 130 mmol/mol still remains.

When bpy is added to [Mn(acac)3], a totally differ-ent observation can be made. The concentration ofperoxides is, at all times, much lower than is observedfor the other two cases: a maximum value is reached ofabout 45 mmol/mol in just 10 h of reaction time. Theamount of peroxide then decreases to a very low levelof 10 mmol/mol over the course of the experiment(up to 300 h).

Oligomerization during EL oxidation

The oxidation reactions of EL generally lead to theformation of oligomers, which is pivotal for the dryingand hardening of real alkyd coatings. In Fig. 13, theSEC chromatograms are shown of EL after various

periods of oxidation catalyzed by Co-EH, [Mn(acac)3],and [Mn(acac)3]/bpy (see Fig. 6 for assignments). Afteroxidation of EL for 8 h catalyzed by Co-EH, dimers,trimers, and higher oligomers could be readily ob-served. As the oxidation proceeds for 24 h, moretrimers and higher oligomers are formed as the dimerpeak slightly decreases.13 After 48 h, both the dimerand trimer peaks have decreased relative to their levelsafter 24 h because of the formation of higher oligomers,and possibly because of the oxidative degradation ofthe fatty acid chains via b-scission reactions. The ELpeak does not disappear completely even after 48 h,which is because of the presence of about 8% ofnonreactive saturated esters and the presence of 19% ofthe less reactive ethyl oleate13,28 in the technical-grade

Fig. 12: The evolution of peroxide amounts during theoxidation of EL in the presence of various catalysts. Linesare added to aid the eye

Fig. 13: SEC of EL after different periods of oxidation in thepresence of various catalysts: (a) Co-EH, (b) [Mn(acac)3],and (c) [Mn(acac)3]/bpy


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EL. The peak appearing at 17 min is due to ELhydroperoxides3,4,12 and to other oxygen-containingEL derivatives (for example, epoxide species).12 Theamount of hydroperoxides first increases and thendecreases significantly over the course of the reaction,in agreement with the results obtained for the peroxidevalue determination.

In the oxidation of EL with [Mn(acac)3], the sameobservations can be made after 8 h as for the oxidationwith Co-EH. Dimers and trimers are formed, althoughto a lesser extent than for the Co-EH catalyzedreaction. In contrast to the oxidation with Co-EH,however, the amount of (higher) oligomers onlyincreases with prolonged reaction times, indicatinglittle degradation of the oligomerized polymer net-work, in agreement with the head-space GC-MS resultsdiscussed below, which showed less hexanal andpentanal formation for [Mn(acac)3] in comparison withthe other catalysts. The peak due to hydroperoxidesremains significant for the entire duration of theexperiment (48 h), again in agreement with the resultsobtained for the peroxide value determination for theoxidation with [Mn(acac)3]. The peak due to theEL-monomer also remains considerable, which was inperfect agreement with the difference in the ELoxidation rates previously found for Co-EH and[Mn(acac)3] using FT-IR.

In the oligomerization of EL with [Mn(acac)3]/bpy,the same trends in the formation of the dimer, trimer,and higher oligomers are seen as for the oxidation withCo-EH. Also, for this catalyst, a decrease in the dimerand trimer peaks is observed over time, which can, inpart, be attributed to the degradation of the polymernetwork, as was clear from the GC-MS results dis-cussed below. The peroxide peak is rather low inintensity at all times, in accordance with the observa-tions made in the peroxide value determination (seeFig. 12).

Formation of volatile byproducts during ELoxidation

The characteristic odor of air-drying alkyd paints is dueto the formation of volatile aldehydes, typically

hexanal and pentanal. These aldehydes are formedthrough b-scission reactions of alkoxy radicals.10 Theamount of hexanal and pentanal formed during theoxidation of EL in the presence of different catalystshas been quantified by head-space GC-MS, and theresults are shown in Fig. 14. In the reaction that wascatalyzed by Co-EH, the amount of hexanal andpentanal increases steadily over the course of thereaction (105 h), reaching levels of 16.6 and 0.4 mmol/mol EL, respectively. During the first 50 h of theoxidation with the [Mn(acac)3]/bpy system, much morehexanal is generated than when Co-EH is used (seeFig. 11a). However, after 100 h, the hexanal levels forboth catalysts are at a similar level. The amount ofpentanal that is generated with the [Mn(acac)3]/bpycatalyst is significantly higher. In the oxidation withonly [Mn(acac)3] as a catalyst, lower amounts of bothhexanal and pentanal are formed compared with thereactions with Co-EH or [Mn(acac)3]/bpy.

Cyclic voltammetry

In order to compare the oxidizing power of thecomplex [Mn(acac)3] with and without bipyridine,cyclic voltammetry has been used.2 The cyclic voltam-mograms of [Mn(acac)3] and [Mn(acac)2(bpy)] areshown in Fig. 15. The Mn(II)/Mn(III) couple (peaks1/1¢ and a/a¢) shifts towards a more positive potentialfor [Mn(acac)2(bpy)] compared with [Mn(acac)3]. Thevoltammograms of [Mn(acac)2(bpy)], obtained at dif-ferent scan rates, show that the relative intensity ofpeak a¢ increases with the increasing scan rate. Thisindicates that the species [MnIII(acac)2(bpy)]+ is unsta-ble under these conditions and therefore it is likely thatthe more stable [Mn(acac)3] complex is formed. Peak bresults from the oxidation of [Mn(acac)3] and peak b¢from the reduction of [MnIV(acac)3]+. Indeed, the factthat the Mn(III)/(IV) transitions in the voltammo-grams (peaks 2,2¢ and b/b¢) occur nearly at the samepotentials suggests also that they must both arise fromthe same species (i.e. [Mn(acac)3]). Peak c mostprobably results from the reduction of [MnIII(acac)3]to [MnII(acac)3]–, which can be concluded by compar-ison with peak 1¢. The reduced complex [MnII(acac)3]–

Fig. 14: The formation of volatile byproducts (a) hexanal and (b) pentanal during the oxidation of EL in the presence ofdifferent catalysts


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reacts with bpy in solution to form the more stablecomplex [MnII(acac)2(bpy)], which is supported by theabsence of a (re)oxidation peak for [MnII(acac)3]–.The cyclic voltammogram of an in situ mixture of[Mn(acac)3] with bpy is nearly identical to that of[Mn(acac)2(bpy)], confirming the instability of the com-plexes and the occurrence of rapid equilibrium reactions.

Experiments in real alkyd paint

The complex [Mn(acac)3] was also tested as a drier in areal alkyd paint formulation, both in the absence and inthe presence of bpy. The drying time of the paint wasdetermined using two methods that are common in thepaint industry: using the ‘thumb-test’ and the use ofa Braive recorder.29,30 The drying times for[Mn(acac)3] and [Mn(acac)3]/bpy and the resultsobtained for the commercial combination of cobalt

driers and manganese driers are listed in Table 2. Fromthe results presented in Table 2, it is clear that[Mn(acac)3] and [Mn(acac)3]/bpy have paint-dryingactivities which are considerably better than the activ-ities of the standard manganese paint driers because thetotal drying times were halved for the new driers. Thedrying results, compared with the drying times obtainedwith the cobalt driers, are comparable or even slightlybetter. It is also interesting to note that the drying timesfor [Mn(acac)3] appear to be even shorter than that forthe [Mn(acac)3]/bpy system.

Discussion

Radical initiation by [Mn(acac)3]

Mn(III) is well known as a radical initiator, mostly inthe form of [Mn(OAc)3] in acetic acid, at elevatedtemperatures and often in the presence of radical-chainstarters such as NHPI (N-hydroxyphthalimide).31,32

[Mn(acac)3], in combination with co-catalysts such asacetic acid33,34 or benzyl bromide35 is also known to actas a radical initiator in the radical polymerization ofvarious substrates.

The induction time in the oxidation of EL is ameasure of the ability of the catalyst to initiate theautoxidation radical-chain reaction. Consequently,from the short induction times observed in the FT-IRmeasurements, it seems that [Mn(acac)3] is able toinitiate the radical autoxidation of EL directly and atroom temperature. Initiation therefore occurs even inthe absence of any acids, halides, peroxides or othereasily oxidizable radical-chain starters. A tentativemechanism for the initiation reaction is depicted inFig. 16. Radical initiation is proposed to occur via ahydrogen atom abstraction pathway, in analogy with amechanism for the oxidation of 1,4-cyclohexadieneby tris(hexafluoroacetylacetonato)-manganese(III) asdescribed by Bryant et al.36 H-atom transfer from ELto [MnIII(acac)3] results in the formation of [MnII

(acac)2], a free acetylacetone ligand and an ethyl

Table 2: Drying times of real alkyd paint, using different catalystsa

Drier Drying time by hand Braive recorder drying time

Surface dry Through dry Stage a Stage b Stage c

Co–Ca–Srb 1 h 45 min 2 h 15 min 1 h 15 min 2 h 15 min 6 h 45 minMn–Ca–Zrc >6 h – 1 h 30 min 12 h 30 min 16 hMn-bpyd 3 h >6 h 1 h 30 min 6 h 30 min 10 h[Mn(acac)3] 2 h 2 h 30 min 1 h 15 min 2 h 15 min 4 h[Mn(acac)3] + bpy 2 h 30 min 3 h 1 h 15 min 3 h 5 h

a The drying time was measured in a clear varnish based on Uralac AD 152 WS-40 from DSM Resins (a medium oil alkydresin based on soy-bean oil, 47% oil)b Commercial cobalt combination drierc Commercial manganese combination drierd Commercial manganese drier based on manganese 2-ethylhexanoate and bipyridine

Fig. 15: Cyclic voltammograms of [Mn(acac)3] and [Mn(acac)2(bpy)] in CH3CN, [nBu4N](PF6) as electrolyte, scanrate 200 mV/s selected potentials: 1/1¢ 0.222/–0.373 V; a/a¢0.50/0.203 V; 2/2¢ 1.111/0.862 V; b/b¢ 1.165/0.891 V; c –0.28 V


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linoleate radical (EL•). This ethyl linoleate radicalreacts immediately with dioxygen (O2) to form aperoxy radical, which can form a hydroperoxide byfurther abstraction of an H• from another EL mole-cule.

Oxidation of EL

The FT-IR results show that [Mn(acac)3] catalyzes theoxidation of EL at a reasonable rate, although this rateis lower than that of the oxidation with Co-EH. Fromthe data relating to the amounts of peroxide, itbecomes clear that the high oxidation rate for the[Mn(acac)3]/bpy system might be attributed to anextremely efficient capability of hydroperoxide decom-position.

In Fig. 17, a set of radical reactions is given that areoften proposed for hydroperoxide (ROOH) build-upin metal-catalyzed autoxidation reactions.37 Thehydroperoxide concentration, at a given time, dependson the ratio of the rates for hydroperoxide decompo-sition vs hydroperoxide formation.38 According to thereactions depicted in Fig. 17, the higher the rate forreactions 4 and 5, the lower the hydroperoxideconcentration will be. It appears that the [Mn(acac)3]/bpy catalyst is able to decompose ROOHalmost as soon as it is formed, thus attaining a very lowROOH concentration. In Fig. 18, a tentative overviewis given for the metal-catalyzed peroxide decomposi-tion reactions for each catalyst used in the presentstudy.

The cobalt catalyst is mainly a robust peroxidedecomposition catalyst, where the rate for reaction 7will probably be comparable with the rate for reaction8. It is clear from Fig. 12, in which it is shown that theROOH concentration decreases steadily after themaximum has been reached, that the cobalt catalystretains its decomposition activity for at least 150 h. Theoxygen uptake data for the Co-EH-catalyzed EL

oxidation show that significant over-oxidation of ELtakes place (oxygen uptake is higher than 1 molO2/mol EL). The most common pathways for over-oxidation are: (1) through radical addition toconjugated double bond systems, generating a newcarbon-centered radical which can again react withdioxygen,15 or (2) via oxidation of the products that aregenerated by a b-scission reaction of an alkoxyl radical.

Reactions 9 and 10 given in Fig. 18 are the proposedROOH decomposition reactions for the catalyst[Mn(acac)3]. Additionally, the direct activation of ELvia reaction 11 also plays a role for [Mn(acac)3], asshown in Fig. 16. The most important difference forthe activity of [Mn(acac)3] compared with Co-EH isthe rate with which reactions 9 and especially 10proceed (apparent from the Raman data and the ratefor oxygen uptake). A lower rate for these reactionscan explain the significantly higher hydroperoxideconcentration that is attained through reactions 2 and3 in Fig. 17. Additional hydroperoxides can be formedthrough reaction 11 in Fig. 18.

As previously mentioned, the [Mn(acac)3]/bpy cat-alyst shows a very high rate for the oxidation of EL.The high oxidation activity is ascribed to the in situformation of the compound [MnII(acac)2(bpy)]through the sequence of reactions 9 and 12 (or 11and 12). The species [MnII(acac)2(bpy)] has mostprobably a very high rate for reaction 13, formingspecies [MnIII(acac)2(bpy)]+. The cyclic voltammetrymeasurements have shown that this complex has areduction wave at 0.203 V (vs Ag/AgCl in CH3CN).This makes the species [MnIII(acac)2(bpy)]+ a muchmore potent oxidizing agent than, for example,[MnIII(acac)3], which has the corresponding reductionwave at –0.373 V (vs Ag/AgCl in CH3CN). Thereduction of Mn(III) is always the slower reaction inHaber–Weiss reactions.31 The addition of bpy thuseffectively ‘de-bottlenecks’ the autoxidation reactionby speeding up the slower step. Reactions 13, 14, and15 shown in Fig. 18 will thus proceed at a much higherrate than reactions 9, 10, and 11. The extremely highrate of dioxygen uptake, coupled to the very low levelsof detected hydroperoxides, may be explained by theassumption that the oxidation will most likely bedominated by reactions 13 and 15, once the species[MnII(acac)2(bpy)] is formed. The alkoxy radicalsformed (see reaction 13) have a high tendency toundergo b-scission, thus forming hexanal and pentanal.The formation of alkoxy radicals disrupts the radical

Fig. 16: Radical initiation by [Mn(acac)3] through hydrogenatom abstraction of the activated methylene group in ethyllinoleate

Fig. 17: General metal-catalyzed (M) autoxidation reactions, which are responsible for hydroperoxide (ROOH) build-up anddecomposition


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chain of reactions 2 and 3 (see Fig. 17), and conse-quently the build-up of a high ROOH concentration.An explanation for the complete cessation of theuptake of oxygen, once all hydroperoxides are decom-posed, could be that the [MnIII(acac)2(bpy)]+ complexcan only rapidly abstract a doubly allylic hydrogenatom. Once each molecule of EL has reacted with atleast one molecule of dioxygen, no doubly allylichydrogen atoms and hydroperoxides are available(because the hydroperoxides are rapidly decomposed)and hence reactions 13, 14, and 15 will stop.

Oligomerization of EL

A major difference between the SEC results forCo-EH and that for [Mn(acac)3] is the amount ofoligomers that are formed over a period of 48 h. Thelevels of dimer, trimer, and higher oligomers areclearly greater for the [Mn(acac)3] catalyst. Thisobservation can be explained by the high ROOH levelthat is generated through the cycle of reactions 2, 3,and 11. Each hydroperoxide group that is formedresults in the formation of a set of conjugated doublebonds. A high ROOH concentration therefore impliesa high concentration of conjugated double bonds thatare especially prone to radical addition reactions,because of the possibility of forming a relatively stableallylic radical.15 Consequently, each time an alkoxy, orespecially a peroxo radical species, is formed throughdecomposition of a hydroperoxide, a probable reactionpathway for the radical is to react with a conjugateddouble bond to form a higher oligomer. Anotherreason for the lower level of oligomerization, asobserved for Co-EH, is degradation of the polymernetwork that is formed. The head-space GC-MS datafor the Co-EH-catalyzed EL oxidation reaction showthat constant hexanal and pentanal formation takes

place, a sure sign of b-scission reactions. For [Mn(acac)3], the amounts of hexanal and pentanal that areformed are lower, but the same trend is followed, aspreviously observed for the autoxidation of EL cata-lyzed by Co-EH. This observation supports theassumption that a lower rate for the ROOH decom-position reactions is probably the most prominentdifference between the oxidation mechanisms by which[Mn(acac)3] and Co-EH function.

The SEC chromatogram for the oxidation of EL,catalyzed by [Mn(acac)3]/bpy, is similar to that for theCo-EH catalyzed EL oxidation, the exception beingthe low intensity of the hydroperoxide-peak (in agree-ment with the results for the hydroperoxide determi-nation). As discussed in the previous section, the[Mn(acac)3]/bpy catalyst will probably predominantlyoperate through reactions 13 and 15 and thus willgenerate a higher level of alkoxy radicals relative tothe other catalysts. The high amounts of hexanal andpentanal that are formed compared with those withCo-EH and [Mn(acac)3] are in agreement with thisassumption. A system that predominantly generatesalkoxy radicals will have a lower extent of oligomer-ization because these radicals generally have morepathways to form other products of low molecularweights.10

The best paint drier

The cobalt(II) 2-ethylhexanoate catalyst is the mostwidely used oxidative drier for alkyd paints. Judgingonly by the oxidation rate of the alkyd modelcompound EL, it is tempting to say that the manganesecatalyst system [Mn(acac)3]/bpy would be an idealreplacement for the cobalt catalyst as a drier. Theresults pertaining to the formation of higher oligomersshow, however, that the slower catalyst [Mn(acac)3]

Fig. 18: Proposed sets of reactions for each of the catalyst systems


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yields a much greater extent of oligomerization.This observation is supported by the drying results inactual alkyd paint formulations, where the catalyst[Mn(acac)3] shows slightly better drying performancethan does the system [Mn(acac)3]/bpy. In real alkydsystems, cobalt complexes are never used as the onlydrier: so-called ‘secondary’ driers are added, some-times used to retard the activity of the cobalt drier.39

The lesser performance of [Mn(acac)3]/bpy in realalkyd paints (see Table 2) might indeed be attributedto the fact that it is too active: the autoxidationreactions yield predominantly alkoxy radicals, whichresult in less oligomerization. Thus, although the[Mn(acac)3] catalyst shows a lower rate of oxidationin our EL test system, as a paint drier it is the bettercandidate to replace the cobalt catalyst systems.

Conclusions

It has become clear that the oxidation reaction of EL,as monitored with time-resolved FT-IR spectroscopy,may well be suitable as a high-throughput screeningmodel reaction for new catalysts. The rates of disap-pearance of EL in the reaction with dioxygen, ascatalyzed by the commercial cobalt driers and themanganese driers, correlate reasonably well with theactual drying times as found in the real alkyd paints.The method seems to be quite sensitive to changes inthe catalyst’s structure, which is shown by the variousresults obtained with different amounts of bipyridineadded to Mn-EH or Co-EH.

Some early insight into the active catalytic species inthe drying of alkyd paints has been gained. Thepresence of the tetranuclear cluster [Mn4O2(2-ethyl-hexanoate)6(bpy)2] in the commercial Mn-EH/bpydrier and its catalytic activity in the oxidation of EL,gives a strong indication that this type of cluster mayplay an important role in catalytic oxidative drying.The compound [Mn(acac)3] is an efficient catalyst forthe oxidation and the oligomerization of EL, which isproposed to proceed not only via hydroperoxidedecomposition, but also through substrate activation.[Mn(acac)3] also functions as a very good dryingcatalyst in a real alkyd system, better than conven-tional manganese driers and comparable with, or evenbetter than a cobalt combination drier. The system[Mn(acac)3] with added bpy has a very high activity forthe oxidation of EL. The in situ formation of thespecies [MnII(acac)2(bpy)] and [MnIII(acac)2(bpy)]+

and the high reactivities of these two species withROOH and EL, respectively, are proposed as anexplanation for the observed high oxidation rate. In theoxidation of EL by the [Mn(acac)3]/bpy catalyst, lessoligomerization and a greater amount of volatileproducts were observed, probably due to the genera-tion of predominantly alkoxy radicals in hydroperoxidedecomposition. In real alkyd systems, adding bpy to[Mn(acac)3] is therefore not advantageous. This is also

evident from the longer drying times that have beenobserved for [Mn(acac)3]/bpy in a real alkyd systemwhen compared with the drying times for [Mn(acac)3].

In subsequent studies, the presented new modelreaction can be applied to screen large numbers ofmanganese complexes and iron complexes for theirpotential as drying catalysts. Since of the continuingshift of solventborne alkyds to water-based systems,attention has also been directed to the role of water inour model reaction.7

Acknowledgments These investigations have beensupported by the Netherlands Research Council forChemical Sciences (CW) with financial aid from theNetherlands Technology Foundation (STW) in thePriority Programme Materials and by the InnovationOriented Research Programme on Heavy Metals (IOPZware Metalen, no IZW99241c), sponsored by theNetherlands Ministry of Economic Affairs.

The authors are grateful to Dr ST Warzeska, DrJ-C Hierso, Dr S Grecea, Dr J-Z Wu, Dr ZO Oyman(Technical University Eindhoven), and Dr W Ming(Technical University Eindhoven) for their work andcollaboration on this topic. The authors thank Dr JBieleman (Elementis, Delden), Dr WJ Muizebelt, andDr W Buijs (DSM, Geleen) for fruitful discussions.We are indebted to Mr J ter Borg (Elementis,Delden) for the determination of the drying time inalkyd paints. The authors are grateful to Dr HKooijman (Utrecht University) for the X-ray diffrac-tion studies.

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A study of new manganese complexes as potential driers … · A study of new manganese complexes as potential driers for alkyd paints E. Bouwman, R. van Gorkum FSCT and OCCA 2007