Jmm-nal of Mokxdar Catal@s, 64 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM (1991) 133-142 133 zyxwvutsr Preparation and characterization of tetra(4-pyridyl)porphyrinatomanganese(III) cation supported covalently on poly(siloxane) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO H. S. Hilal*, C. Kim, M. L. Sito and A. F. Schreiner** Department (Received of Chemistry, North Carolina State University, Raleigh, NC 27495 (U.S.A.) March 30, 1990; revised July 3, 1990) Abstract Tetra(4-pyridyl)porphyrinatomanganese(III), [Mn”‘(TPyP)]+, has been covalently bonded to the surface of a chlorinated crosslinked poly(siloxane) which contains the immobilized chloropropyl group, -CH-CH&H&l. The metalloporphyrin complex was found to react with the immobilized ligand via a quaternization reaction of the 4-pyridlne N-atom. The porphyrin binding involves a chemical process rather than physical adsorption. There is evidence that two porphyrins species coexist, both bonded to the surface, one being [Mn”‘(TPyP) ] + and the other (HsTPyP), both in the quatemary salt forms, when the quatemization reaction is carried out at higher temperature (150 “C). At moderate reaction temperatures (70-80 “C) the quatemization reaction resulted in only one species, supported Wn”‘(TpYP) I+, as evident from electronic spectra of the solid. Also, visible absorption spectra taken of the solution remainin g after the quatemixation reaction showed no demetallated porphyrin. Solid state electronic absorption, dispersive IR and FT-IR spectra have been used for confirmation. Introduction Metalloporphyrins of the type shown below (Fig. 1) are reported to have been immobilized on several insoluble supports. Zeolites have been employed to support, by coulombic attraction, metalloporphyrins such as [ Mn”‘(TMPyP)]Cl,, using ion exchange [ 1,2 1. Metalloporphyrins immobilized on Nafion polymer surfaces have also been reported [3]. Additionally, activated carbon electrodes have been employed to support such porphyrins by physical adsorption [ 4-7). All these above-mentioned supported porphyrins were immobilized by physical adsorption only. Recently, Marrese et al. [8] have anchored cobalt tetrapyridylporphyrin to a polysiloxane coating of a platinum electrode by covalent bonding (Scheme 1). In Scheme 1 the polysiloxane coating results from the reaction of the -Si(OR), groups with the premodifled platinum surface. A fraction of the Si(OR) groups react with surface R-OH groups *On sabbatical Nablus, W est **Author leave from the Department of Chemistry, A n-Nsjah National University, Bank (via Israel). to whom correspondence should be addressed. 0304-5102/91/$3.50 0 Elsevier Sequoia/Printed in The Netherlands 4-SULFOPHENYL; TSPP” PYRIDYL; TFyP ~~HYLPY~DY~ WyF4+ PHENY L; TF’P R Fig. 1. Structures M = Zn,Pd,Ag,Cd,Cu,Sn,Pb,Mn zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of the rne~opo~~~. ptiL4.5 (CH#XOH) Scheme 1. to yield Pt-0-Si bonds. A ~~rnrno~ feature of these supported po~h~s is that they have been used as electrocatalysts. The supporting solid, therefore, is either the electrode itself, as the case with carbon electrode, or a surface film that is a modifkation of the electrode, as the case with platinum electrodes. Enikolopyan and Soloveva recently reported the use of modified organic copolymers as supports for te~a(p-~o)po~h~atorn~~ese, [Mn(T~P)] [9], and hematopo~h~atom~g~ese [ 101. Unlike other systems, these supported complexes have been used as chemically ‘regenerated catalysts’ in oxidation reactions. (The Mn-porphyrin is covalently anchored as ,Mn***and then chemically reduced to its catalytically active form, Mn”.) In our search for new supported porphyrins which are covalently bonded, we are interested in the preparation of chemically regenerated supported catalysts from molecular starting compounds, i.e. to prepare supported catalysts with the porphyrin bound to the solid surface by genuine covalent chemical bonds. In this article we describe our results using a chlorinated crosslinked poly(siloxane) support, prepared as shown in Scheme 2, which is known to have preferred mechanical properties [ 1 l-l 3 1.The [ Mnm(TPyP) It was anchored to the chlo~ated polysiloxane surface via a quate~ation reaction (Scheme 2). Such reactions have previously been used for metallotetra(4-pyridyl)porphyrins [ 14-171 to prepare molecular solution species. 135 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK Hz0 , ROH (Et O),SI Scheme + (M e O),SI (CH 2 ),CI 2. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA The ensuing paper [ 181 describes our studies using supported [Mnm(TPyP)] + as a chemically regenerated catalyst for oxidation reactions. zyxwvutsrqponmlkjihgf Experimental Manganous sulfate (MnSO, - HzO) was purchased from Fisher Scientific. The silanes, (EtO)$i and (MeOa)Si(CH&,CI, and PhzSnClz were purchased from Aldrich, as was 5,10,15,20-tetra(4-pyridyl)2lH,23H-porphin. Organic solvents were purified prior to use according to standard methods [ 191. Grating dispersive IR spectra were recorded with a Perkin Elmer 521 grating IR spectrophotometer (model 621 up-date) either as KBr pellets or as Nujol mulls. Electronic absorption spectra of the supported porphyrin were recorded on Varian Gary 2300 and/or Hitachi 110 spectrophotometers, using thin solid layers on quartz cell surfaces. Baseline spectra were obtained with thin layers of chlorinated poly(siloxane) solid. FT-IR spectra (KBr disks) were measured on a Mattson Polaris FI’-IR spectrophotometer, with an H&Cd, _,Te(MCT) detector. Scanning (10 shuttles/block, 50 scans/block) with 8 cm-’ resolution was used. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO Preparatiim of [Mnrlr(TFgP)] + (SO, ) 1/ 2 Tetra(4-pyridyl)porphyrinatomanganese(III) was prepared as described in the literature [ 14, 20, 21). 5,10,15,20-Tetra(4-pyridy1)21H,23H-porphin (81.7 mg, 0.132 mmol) was stirred with excess MnS04.Hz0 (1.525 g, 0.908 mmol) in refluxing NJWimethylformamide (60 ml) for 10 h. The solvent was then evaporated under reduced pressure while using an air stream through the solution to allow complete conversion into [Mnm(TPyP)]+. The resulting residue was then chromatographed on a neutral ahnnina (Bio-Rad AGF, 100-200 mesh) column to remove extra MnSO,, using MeOH + CHCla (15:85 votiol) as eluent. The eluate was then taken and dried under reduced pressure at room temperature. The electronic spectra of the product in DMF showed three distinct bands at 463 (Soret), 569 and 620 run, consistent with the literature [ 201. of chlorinated poly(Xoxane) sueace The poly(siloxane) was prepared by a method similar to the literature [ll-13) with some modifications. (EtO)*Si (20 g, 0.096 mol) was stirred Preparation 136 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA with (MeO),Si(CH&Cl (9.53 g, 0.048 mol), Ha0 (4.75 g, 0.26 mol) and PhzSnCla (1.03 g, 3 mmol). The mixture was then refluxed for 2 h and cooled while stirring. MeOH (15 ml) was then added to the stirred mixture, followed by addition of excess Hz0 (100 ml). The mixture was stirred for 3 min and left to stand without stirring. The thick oily liquid layer that formed in the bottom of the beaker (under the aqueous layer) within 10 min was left to stand under water overnight. A white solid formed, which was isolated, crushed and washed several times with water. It was then washed with EtOH several times to remove extra PhaSnCl,, and dried at 80 “C overnight. IR spectra taken (Nyjol mull or KBr pellets) of the resulting solid showed bands similar to the IR spectra of the original liquid (MeO)$i(CH&Zl. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDC of the zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF supported [MnrD(Tpyp)]+ complex [Mn’“(TPyP)] + was covalently bonded to the poly(siloxane) surface by a quaternization reaction similar to that described for the homogeneous solution reactions of CH3-I with [ Mnm(TPyP)] + [ 141. Since alkylchlorides undergo the quaternization reaction more slowly than alkyliodides [ 16, 171, more vigorous conditions have been employed here. Chlorinated poly(siloxane) (4 g) was stirred with [Mnm(TPyP)]+ (0.03 g, 4.17~ 10m2 mmol) in DMF (20 ml) at 70 “C for 72 h. [Mnm(TPyP)]+(SO&n is highly soluble in DMF. The resulting red-brown solid was isolated, washed with DMF ten times and with CH2C12five times. It was then dried at room temperature under reduced pressure overnight. Attempts were also made to prepare supported [Mn’ll(TPyP)] + under more vigorous conditions. The quaternization reaction was conducted under refluxing DMF (150 “C) for 72 h, using otherwise similar reaction conditions. Preparaticm Results and discussion Metalloporphyrins of the type M(TPyP) are known to undergo the quaternization reaction (eqn. 1) [ 14-171 as free solution molecules, resulting in catonic solution species. The rate of the reaction is affected by a number of factors, such as the nature of substituents on the pyridine ring, the nature of the alkyl group (R) in R-X, the nature of X and solvent polarity. In case of [ Mn”*(TPyP)] + , the rate is expected to decrease in the order X = I > Br > Cl. In this work a chlorinated poly(siloxane) surface is used in place of R-X. The reaction, as expected, was found to be slow. A slow reaction is expected also due probably to steric factors at the surface. To speed up the reaction, attempts were made to convert the chlorinated poly(siloxane) into the iodized form, following the Finkelstein reaction [22, 231. The chlorinated poly(siloxane) surface (6.8 g) was stirred with dry KI (15 g) in dry acetone (100 ml) for 30 min. The mixture was then refluxed ( N 56 “C) for 3 h. After cooling the mixture was further stirred for 60 min. The solid was then isolated, filtered and washed several times with acetone to remove 137 excess KI, and with water to remove any possibly formed KCl. After that, the solid was further washed with acetone and dried overnight at 80 “C. However, these attempts to iodize the chlorinated surface were unsuccessful. IR spectra taken for the treated surface did not show the v(C-I) characteristic band in the region 1190-l 150 cm-’ [24]. More vigorous reaction conditions using overnight refluxing also did not work. The IR spectra of the treated solid matched only those of the original chlorinated polysiloxane. The lack of iodization was also confirmed by another method. The KI-treated surface was allowed to react with [Mnm(TPyP)] + via a quaternization reaction as described in the experimental section. Such reactions are accompanied by halide ion (X-) formation, which in this case could be either Cl- (indicative of chloropoly(siloxane)) or I- (indicative of iodopoly(siloxane)). The porphyrinated surface was then treated as an ion-exchange chromatography resin, through which NaNOa(aq) solution was passed. The eluate, expected to contain X- ions, was actually positive for Cl- and negative for I- (addition of AgNO,(aq)). This indicates that the K&treated surface is in fact still in the chloride form. Due to unsuccessful attempts to prepare the iodized polysiloxane surface, the chlorinated poly(siloxane) surfaces were used to support [Mn?J’PyP)] + . DMF was used because polar solvents are known to increase the quaternization reaction rate [ 161. Preliminary experiments showed that refluxing conditions ( N 150 “C) are not advisable, since demetallization of the porphyrin resulted. Therefore, the quaternization reaction was carried out at 70 “C. The dry yellow/brownish solid (red/brown when wet), which resulted from quaternization was analyzed further to verify if the complex is bonded to the support by a covalent chemical bond. The supported catalyst was treated as a resin for ion exchange chromatography, i.e. aqueous NaNOa was passed through the column, and the column was further eluted with water. The eluate was then treated with aqueous AgNO,. A white precipitate formed, indicating the presence of Cl- in the quaternized solid. This indicates that the quaternization reaction took place. Blank runs were conducted for further confirmation. When the supported complex was replaced by other materials such as silica surface or a chlorinated poly(siloxane) (unreacted), no AgCl precipitate formed on treatment with aqueous AgNO,. Addition of aqueous AgNOB to aqueous properly diluted solutions of [Mnm(lPyP)]+(SO&n used in the quaternization also did not yield a white precipitate. Blank experiments of aqueous NaNOs with aqueous AgNOa also showed no white precipitate, indicating that the original NaNOS solution is free of Clion contamination. Thus the Cl- ion did not originate from any possible traces of PhzSnClz which may be remaining on the solid surface. This was I ’ / / cr O\ o- SIw*h-+N w OH zyxwvuts 138 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA also shown by experiment. From these observations, it can be concluded that the quaternization reaction took place, and that the reaction product is the only source of Cl- ions. Solid state electronic absorption spectra of the Soret region were recorded for the supported [Mn*(TPyP)] + (Table 1). The spectra were taken of the solid which was first powdered and suspended in acetone. A clean quartz cell surface was painted with the solid-acetone suspension. The acetone was then evaporated, leaving a thin uniform layer of the supported complex on the cuvette surface. The spectra were recorded against a reference of chlorinated poly(siloxane). In the case of supported complex prepared in refluxing DMF (150 “C), two Soret bands appeared in the spectrum at 420 and 460 nm. This indicates that two supported species coexist at the surface. The 460 run band is attributed to the presence of supported [Mn?J’PyP)] + in its quaternary ionic form (Scheme 2), because the 460 nm band is close to the 463 nm Soret band associated with [Mnm(TMPyP)]C1, in water [14, 251. The other band at 420 nm is attributed to the presence of supported (HaTPyP) porphin in its quaternary ionic form, free from a central Mn atom, because this band is close to the 422 nm Soret band reported for aqueous [HBTMPyP]Cl~ [ 261. This is indicative of the demetallation process taking place under refluxing conditions. Electronic absorption spectra of an aliquot of the remixing DMF solution also showed the existence of demetallated [H,TPyP] (417 run, l=4.25 X lo5 M-’ cm-‘) as a major component, whereas the [M#*(TPyP)]+ (460 run, ~=8.5X lo4 M-’ cm-‘) is a minor component. However, in the case of the supported complex prepared in DMF at 70 “C, the solid state spectra showed only the one 460 nm Soret band, indicative of the presence of only the supported [Mnm(TPyP)] + species. The absence of bands in the 417420 nm region indicates that there is no demetallated porphin anchored. Furthermore, electronic absorption spectra taken of the DMF solution following the quaternization reaction showed no bands characteristic of demetallated porphin, [H,TPyP]. It is this batch of supported [Mn’“(TPyP)] + that is of interest as a catalyst, work to be reported in an ensuing paper [ 181. Analysis of the extent of anchoring of [Mnm(TPyP)] + was undertaken spectrophotometrically. The amount of supported [Mn”‘(TPyP)] + was calculated by subtracting the amount of [Mnm(TPyP)] + remaining in solution (after quaternization) from the original amount used at the start of the quaternization. The weight percent of [Mnm(TPyP)](S04)In of the total weight of the final solid was 0.65%. The amount of unreacted [~n”*(TPyP)] * remaining in solution after quatemization was measured spectrophotochemically, using ~=8.5X lo4 M-’ cm-’ (464 run) for [Mn”‘(TPyP)] + [ 141. Similar methods of calculating the uptake of binding species by solid surfaces are known [ 11 I. Although the percent [Mn(TPyP)](SO,),, seems to be relatively low, high catalyst turnovers (up to 14 x 103) have been observed for this supported complex in olefin oxidations ]181. The electronic absorption spectra of the Soret region of the anchored porphyrin showed no indication of the presence of any unreacted 139 140 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA [Mn”‘(TPyP)]+ (464 run) or (HaTPyP)) (420 run) adsorbed on the surface. Blank experiments were carried out as follows to confirm this. [Mnm(TPyP)] + was stirred with a nonhalogenated silica surface, under conditions identical to those used for supporting the complex on the chlorinated surfaces. After washing several times, the surfaces were found to be porphyrin-free when tested by the sensitive electronic absorption spectra. Other blank experiments were done using chlorinated poly(siloxane) surfaces. The surface was stirred with [Mn”(TPyP)] + for 60 min at room temperature, then isolated, washed with DMF, then with water, DMF and CHaCla. Though the resulting solid showed some very faint yellow color, the electronic absorption spectra showed no bands characteristic of porphyrins. Solid state IR spectra were also obtained of the supported [Mnm(TPyP)] + complex. Nujol mull and KBr pellet techniques were employed. The grating dispersive IR(D-IR) spectra, however, were inconclusive, presumably due to the low porphyrin complex concentration in the solid. Solid state FT-IR spectra were then recorded for KBr pellets of the supported [M@(TPyP)] + (4 mg supported complex and 300 mg KBr). The spectra were recorded against a blank pellet of KBr (300 mg) with poly(siloxane) (4 mg). The FI’-IR spectrum (500 scan average) was compared to other D-IR spectra of molecular [ Mn”‘(TPyP)] zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCB +, [H,TPyP] and poly(siloxane), all taken in KBr pellets. Not only did the supported [Mnm(TPyP)]+ FI’-IR spectrum resemble the D-IR spectrum of [Mnm(TPyP)]+ more than the spectrum of [HaTPyP], but some bands that appeared exclusively in the [HaTPyP] D-IR spectrum (e.g. 1470 cm-‘), and not in the spectrum of [Mn(TPyP)]+, did not appear in the supported complex Fl’-IR spectrum. Furthermore, the band (1380 cm-‘) that appeared exclusively in the [Mn(TPyP)]+ IR spectrum, but not in [HaTPyP], appeared in the supported complex FI’-IR spectrum. Therefore, it can be concluded again that the supported complex is in the [Mn’“(TPyP)] + form and not in the demetallated form, [H,TPyP]. For further confirmation, the supported complex FI’-IR spectrum was compared to a FYI’-IR spectrum taken for a standard mixture of molecular [Mnm(TPyP)]+ (0.25 mg) and chloropoly(siloxane) (4 mg) in a KBr (300 mg) pellet. The two Mn”‘(porphyrin) spectra matched each other well, indicating that the supported complex is indeed [ Mnm(TPyP) ] + . By comparing the two FI’-IR spectra, it was also possible to determine the concentration of [Mn”‘(TPyP)] + within the total solid. For quantitative purposes, the band at 1598 cm-‘, which appeared in both of these FI’-IR spectra, was used. This band resembles the IR- 1600 cm-’ bands of I@=@) or v(C-N) for other porphyrin pyrrole ring modes [29]. The 1598 cm-’ band was chosen 141 for quantitative analysis, because it appeared in the region with virtually no interference from the poly(siloxane). This was evident from KE3rpellet D-IR spectra of molecular [Mn’“(TPyP)] +, [H,TPyP ] and poly(siloxane) species. The amount of [Mn”‘(TPyP)](S04)In was 0.50 wt.%. The concentration values measured by ET-IR spectra are thus close to those measured from electronic absorption spectra as shown above. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON Acknowledgements The authors would like to thank Dr. L. Jones for access to his GC instrument, Mrs. D. Knight for the D-IR spectra and Mr. W. Kalsbeck for the Fl’-IR spectra. The Nbright scholarship and the NCSU visiting associate professorship provided to H.S.H. are also gratefully acknowledged. References 1 B. de Viimes, F. Bedioui, J. Devynek and C. Bied-Charreton, J. Electroan&. Chem., 187 (1985) 197. 2 B. de Vismes, F. Bedioui, J. Devynek, C. Bied-Charreton and M. Perree-Fauvet, Nouv. J. Chim., 10 (1986) 81. 3 H. Segawa, T. Shim&u and K. Honda, Polym. J., 20 (1988) 441. 4 K. Maxuyama, H. Tamiaki and S. Kawabata, J. Chem. 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