J. MATER. CHEM., 1994, 4(8), 1325-1329 1325 Intercalation of Polymerized 3-Methyl- and 3,4=Dimethyl=pyrrole in the VOP04 lnterlayer Space Hiroshi Nakajima and Gen-etsu Matsubayashi* Institute of Chemistry, College of General Education, Osaka University, Toyonaka, Osaka 560, Japan 3-Methylpyrrole (3-Mepyrr) and 3,4-dimethylpyrrole (3,4-Me2pyrr) react with powdered VOP0,.2H20 suspended in ethanol to yield intercalation compounds consisting of the reduced VOPO, lattice and poly-3-Mepyrr or poly-3,4-Me2pyrr chains; VOP04~(H20),.4-(EtOH)o~2~(MeC4HNH)o.65 Poly-3-Mepyrr and poly-3,4-Me2pyrr and VOP0,~(H20),~8~(EtOH)0.2~(Me2C4NH)0.6. moieties in the VOPO, interlayer space afford single interlayer spacings of 8.2 and 9.7 A, respectively. Pyrrole (Pyrr) also reacts with VOP04.2H20 solids suspended in ethanol to yield an oxidatively polymerized compound formed on the surface of the VOPO, grains.Factors causing the polymerization of 3-Mepyrr and 3,4-Me2pyrr in the VOP04 interlayer space and electronic interactions of the reduced VOPO, host lattice with the intercalated poly-3-Mepyrr and poly-3,4-Me2pyrr moieties are described based on powder X-ray diffraction patterns, infrared and X-ray photoelectron spectra together with theoretical calculations of molecular geometries and electron spin densities. Various intercalation compounds having alternating organic and inorganic layers have been extensively studied.'9, In particular, the intercalation of organic polymers into inorganic layered lattices has recently attracted much attention and some examples have been rep~rted.~-~ Electrically conducting poly-pyrrole, -thiophene, and -aniline have been thoroughly studied for application,' but their microstructural properties, such as overall chain conformation, packing and degrees of crosslinking, have not been adequately clarified.Laminated composites of alternating conducting polymer/inorganic layers seem to be useful for elucidating the low-dimensional struc- tures of the polymers. Only a few examples of intercalative polymerizations of pyrrole, aniline, and bithiophene using layered Fe0Cl3q4 and V,05*nH,05'6 have been reported, where factors necessary for the polymerizations in the layered space remain unclarified. Vanadyl phosphates, VOP0410-12 and a-VOP04.2H20,11,13-15 known as layered compounds behave as oxidants and undergo intercalation reactions with ~rganic'~-~' compounds.The VOP04 and organ~metallic~~-~~ moieties can be reduced by organoammonium iodides25 and ferrocenyl corn pound^^^-^^ to include the cation molecules in the layered lattices. Thus, oxidative polymerization of pyrrole and its methyl-substituted derivatives is expected to occur in the VOP04 interlayer space. However, polymerization in the layered lattices is difficult, since polymers are often formed oxidatively on the surface of the layered solids to cover them. We have tried to polymerize them in an ethanol solution suspended with powdered VOP04-2H20 and found successful intercalation/polymerization of 3-methylpyrrole (3-Mepyrr) and 3,4-dimethylpyrrole (3,4-Me2pyrr) in the VOP04 lattice. This paper demonstrates an important role of 3-and 3,4-methyl substituents on the pyrrole ring for the intercalative polymerization process and describes characteristic inter-actions between the reduced VOP04 lattice and the interca- lated poly-3-Mepyrr and poly-3,4-Me2pyrr moieties.A preliminary report of the present work has already appeared.26 Experimental Materials VOP0,.2H20 was prepared according to the literature." 3-Mepyrr and 3,4-Me2pyrr were also prepared according to the literat~re.,~ Pyrrole (Pyrr) was commercially available. They were purified by distillation under reduced pressure prior to use. Intercalation/Polymerizationof Pyrr, 3-MePyrr and 3,4-Me2Pyrr in the VOP04 Lattice Finely powdered VOP04-2H20 (33 mg, 1.5 mmol) was suspended in ethanol (10 ml) solution containing Pyrr (300 mg, 4.5 mmol) and the solution stirred for 48 h at 30°C in darkness.The resulting solid was collected by centrifugation, washed with ethanol and acetone several times and dried in vucuo to afford VOPO,(H,O),.,( EtOH)o~2(C4H2NH)o~6 (1). Found: (2, 13.72; H, 3.00; N, 3.44%. CdC. for C,.,H~.~NO~~O~PV: c', 13.86; H, 2.72; N, 3.46%. By a similar procedure, reactions of 3-MePyrr or 3,4-Me2Pyrr with powdered VOP04-2H20 suspended in etha- nol gave VOPO4( H20)1,4(EtOH)o~,(MeC4HNH)o,65(2) and VOPO,(H,O),.,( EtOH)0.2(MezC4NH)0.6 (3), respectively. For 2 found: C, 17.96; H, 3.03; N, 3.61%.Calc. for C3,7H7~lNo~6506~,PV:C, 17.70; H, 2.84; N, 3.68%. For 3 found: C, 19.06; H, 3.82; N, 3.45%. Calc. for C4,0H9~oN,~,07~oPV: C, 18.52; H, 3.50; N, 3.24%. Oxidative Polymerization of 3,4-Me2Pyrr Electrochemical oxidation of 3,4-Me2pyrr was performed in an ethanol solution containing [NBu",][BF,] as an electro- lyte, using a glass cell constructed with a glassy carbon working electrode, platinum file counter-electrode and a satu- rated calomel reference electrode (SCE) according to the literature.28 Poly-3,4-Me2pyrr was grown on the working electrode by oxidation of the monomer under a constant potential (800 mV us. SCE). The content of the BF4 ion in the obtained polymer (0.2-0.3 per 3,4-Me2pyrr unit) was determined by X-ray photoelectron spectra.Poly-3,4-Me2pyrr was also prepared by a reaction of a 10% 3,4-Me2pyrr toluene solution with a 30% FeCl, aqueous solution by using the previously described pr~cedure.~' Physical Measurements Infrared (IR) spectra were recorded on a Hitachi 215 spectro-photometer for the region 4000-650 cm-' in KBr pellets. Powder X-ray diffraction patterns were measured for com- pressed pellet samples with a Shimadzu XD-3A diffractometer employing Cu-Ka irradiation at 30 kV and 20 mA. X-Ray photoelectron spectra (XPS) were measured for compressed pellet samples by Mg-Ka irradiation at 30 kV and 8 mA using a Shimadzu ESCA 750 spectrometer equipped with a Shimadzu ESCA PAC 760 computer analyser. All the spectra were referenced to the gold 4f,,, signal (83.80 eV) for correc- tion of the charge effect.The error of the binding energy determination was estimated to be 0.1eV. Differential thermal analysis (DTA) was carried out on a Seiko I & E TG/DTA instrument under nitrogen atmosphere. Theoretical Calculations Theoretical studies of molecular geometries, electron spin densities and energies for pyrrole and its derivatives were performed using the PM3 molecular-orbital meth~d.~',~~ The MOPAC program, which was revised as SXOS version-6.02 for the use on an NEC SX-2N supercomp~ter,~~ was used for the calculation. Final molecular geometries and energies were obtained by optimizing total molecular energies with respect to all structural variables.Electron spin densities of the radical cations and their heats of formation were optimized using a half-electron (HE) method with restricted Hartree-Fock (RHF) formalism. Results and Discussion Intercalation and Polymerization of Pyrr, 3-Mepyrr and 3,CMe,pyrr in the VOP04 Lattice Compounds 1, 2 and 3, obtained by reaction of Pyrr, 3-Me- pyrr or 3,4-Me2pyrr with powdered VOPO4.2H,O suspended in ethanol, include 0.6-0.65 monomer units per VOPO, moiety. IR spectra of these compounds clearly show the presence of poly-Pyrr, poly-3-Mepyrr or poly-3,4-Me2pyrr without any evidence of free monomers, as illustrated in Fig. 1 which also shows the spectrum of poly-3,4-Me2pyrr prepared chemically. Intercalated poly-3-Mepyrr and poly-3,4-Me,pyrr have been isolated from 2 and 3 by dissolving the VOPO, framework in an NaOH aqueous solution, followed by the J.MATER. CHEM., 1994, VOL. 4 treatment of the solids by an excess HC1 aqueous solution. XPS of these compounds have exhibited bands of vanadium 2pSi2 electrons at 516.8 eV, which indicates the reduction of the Vv sites of the VOPO, host lattice to the VIv state. In accordance with these findings, these compounds show intense, broad electron paramagnetic resonance (EPR) signals due to the VIv state, as described below. X-Ray powder diffraction patterns of these compounds exhibit clear (001) and (200) reflections, revealing that the layered structures of the VOPO, lattice are preserved for them (Fig. 2), as reported for other VOPO, intercalation corn pound^.^^ The interlayer djstances for 2 and 3 can be determined to be 12.8 and 13.8 A, respectively.The net expansions of the interlayero space for these compounds are estimated to be 8.7 and 9.7 A based on the interlayer distance of 4.1 A for anhydrous VOP0,.12 These findings suggest that poly-3- Mepyrr and poly-3,4-Me2pyrr moieties are located in the interlayer space, their molecular planes being ordered approxi- mately perpendicular to the VOPO, sheet. These arrange- ments are compatible with that of the poly-Pyrr moiety in the FeOC1-poly-Pyrr intercalation c~mpound.~ On the other ha@, the interlayer distance of 1 has been determined to ke 6.5 A; the net expansion of the interlayer space is only 2.4 A. This suggests that no poly-Pyrr moieties are intercalated into the VOPO, interlayer space, the Pyrr molecule being poly- merized on the surface of the VOPO, grains.This seems to be consistent with the presence of only water molecules in the VOPO, interlayer space of 1, as deduced from the thermal analysis.26 The DTA curve for VOPO4.2H,O shows two endothermal bands corresponding to the release of two kinds of water molecule with the rise in temperature; the endother- mal band observed around 120°C corresponds to the release of the water molecule coordinated to the vanadium site and the band around 60°C is ascribed to the release of the water molecule held in the VOPO, interlayer space through the hydrogen bonding to the coordinated water molecule (Scheme l).16934 Compound 3 has exhibited a broad DTA band around 60°C due to the release of water and ethanol molecules.This comes from some destruction of the layered VOPO, structure assisted by the coordinated and hydrogen- bonded water and ethanol molecules16 caused by the intercal- ation of poly-3,4-Me2pyrr moieties. A similar broad DTA band has also been observed for 2. On the other hand, 1 exhibits two bands around 70 and 130°C, which indicate clearly the release of two kinds of water molecule, as observed for VOP04-2H20, although the endothermal band at lower temperature is somewhat broad.26 This supports the fact that no Pyrr moiety is intercalated in the VOPO, lattice but I I 1 A 10 I$OO 1400 1200 1000 800 650 I I I I I I wavenurnber/crn-' 5 10 15 20 25 30 20/degrees Fig.1 IR spectra of: (a) 1; (b) 2; (c) 3; and (d) poly-3,4-Me2pyrr prepared chemically according to the literature2' Fig. 2 X-Ray powder diffraction patterns of (a) 1; (h)2; and (c) 3 J. MATER. CHEM., 1994, VOL. 4 0 II 0 0 II II -V--V-I I H2O H20 Scheme 1 pyrrole is polymerized only on the surface of the VOP04 grains. Structures of the Polymers formed in the VOPO, Interlayer Space 3,4-Me2pyrr can be polymerized oxidatively by coupling only through 2,Scarbons to yield a polymer with a straight chain. Although Pyrr is also polymerized through the oxidative coupling at 2,5-carbons predominantly, couplings at 2,3-, 2,4- and 3,4-carbons can partially occur to form a polymer having a crosslinked or shortly terminated chain str~cture.~,~~ Fig.3 shows XPS bands of carbon lsl,z electrons for 1-3. Compound 1 exhibits an asymmetric band with shoulders at higher energies. These broad structural bands are ascribed to carbons of crosslinked, shortly terminated chain structures or non- coupled 2,Scarbons as well as carbons of partially saturated pyrrole rings.34 On the other hand, 3 exhibits a rather symmetric band with narrower halfwidth (1.8 eV) than that of 1 (2.2eV). The overall linewidth and asymmetry of the band having a peak at 285 eV ascribed to methyl carbons for 3 are appreciably decreased. Thus, the poly-3,4-Me2pyrr moi- eties seem to have less varied kinds of carbon, which is ascribed to the 2,5-coupling owing to 3,4-substituted methyl groups.Compound 2 shows a band-shape intermediate between those of 1 and 3. Considering the occurrence of polymerization of 3-Mepyrr in the VOP04 interlayer space, the poly-3-Mepyrr moiety is also likely to have an essentially 1 1 I I 290 285 280 binding energylev Fig. 3 XPS bands of carbon electrons of the polymers included in the VOPO, solids: 1 (-); 2 (-.-); and 3 (---) straight-chain structure, although the polymer skeletons may have somewhat disordered carbons compared with the poly-3,4-Me2pyrr moieties. Since poly-3-Mepyrr is insoluble in organic solvents, it is difficult to anticipate the chain structure chemically or spectro-scopically. In order to estimate chain structures of‘ poly-3-Mepyrr, semi-empirical PM3 calculations have been per- formed on electron spin densities and heats of formation (AfH)of radical cations of the monomer, dimers and trimers as intermediate compounds to poly-3-Mepyrr, as wits calcu- lated for p01y-Pyrr.~~ In the monomer and dimer radical cations [Fig.4(u)-(d)], the spin density on 2 (or 5)-carbon is significantly larger than that on unsubstituted 3 (or 4 )-carbon atoms. These findings may predict essentially selective coup- lings of the radical cations with the 3-Mepyrr monomer through 2,5-carbons. And species (b)or (c) seems to be formed preferentially as a dimer based on calculated AfH values. Trimers (e)-(h)can be formed by reactions of the dimer radical cation (b) or (c) with the monomer.The difference of spin densities on 2,5-carbons and on unsubstituted 3 (or 4)-carbons of the trimer radical cations is rather smaller than those of the monomer and dimer radical cations. However, the coup- ling at the unsubstituted 3 (or 4)-carbons seems to be difficult because of some steric hindrance of neighbouring methyl groups and/or pyrrole rings. Thus, trimer radical cations are also likely to couple with the monomer at terminal 5-carbons predominantly, which leads to an enlargement of the straight chain. These calculations indicate that methyl groups substi- tuted on 3 (or 4)-carbons cause significant effects both steri- cally and electronically on the 2,5-coupling, as supported by the calculation on pyrrole 01igomers.~~ As discussed above, poly-Pyrr generated on the surface of the VOP04 grain may have a more or less branched-chain structure caused by the crosslinking and 2,3-, 2.4-and 3,4-couplings.Thus, a poly-Pyrr moiety formed at the interface 0.200iY0.20N H 0.03+ 0.19 0.17 \I H 4H= 213.1 kcal mor’ AfH= 212.7 kcal md-I (e1 (f1 H AfH= 212.3 k-1 md-l 4H= 212.5 kcal moT’ (91 (h1 Fig. 4 Spin densities and heats of formation (A,H) of radical cations calculated by the PM3 method J. MATER. CHEM., 1994, VOL. 4 VOPO, VOPO4 IJ;*13.8A 9.7 A I I I I 405 400 395 1 I VOPO, I Fig.5 Schematic diagram of the intercalation of (a) poly-Pyrr and (b)poly-Me,pyrr moieties into the VOP04 interlayer space of the VOPO, lattice is prevented from intercalation into the VOP04 interlayer space, as illustrated in Fig.5(u). On the other hand, the poly-3,4-Me2pyrr moiety having a straight- chain structure generated through coupling at 2,5-carbons is suitable for insertion into the restricted VOPO, interlayer space [Fig. 5(b)].The poly-3-Mepyrr moiety having an essen- tially straight-chain structure, as described above, can also intercalate into the VOPO, lattice. Fig. 6 shows XPS bands of nitrogen lsl,? electrons of 2 and 3 as well as that of poly-3,4-Me2pyrr obtained by the electro- chemical polymerization. Poly-3,4-Me2pyrr from electro-chemical polymerization exhibits an asymmetric band having shoulders at higher binding energies. The resolution fitting indicates the dominant peak at 399.4 eV and shoulders at 400.7 and 402.5 eV suggesting varying electronic states of the nitrogen atoms.In this polymer the BF4- anion is located in the proximity of the positively charged nitrogens of the polymer chain, giving effective electric fields at the proximal nitrogen atoms. The main band is due to the nitrogen atoms which are less affected by the BF4-ion and positively charged nitrogens countered by the BF4-ion (0.2-0.3 per monomer unit) give shoulder bands at higher energies.38 On the other hand, 2 and 3 show rather symmetric bands at 399.7 and 400.0 eV with halfwidths of 1.9 and 2.4 eV, respectively. For both the compounds the reduced VOPO, host lattice works as the counter-anion, in which negative charges are delocalized to form an average field of negative charges against the nitrogen atoms of the intercalated polymers.Thus, the positive charges are distributed homogeneously through the nitrogen atoms. Conclusions Methyl-substituted pyrrole derivatives (3-Mepyrr and 3,4-Me2pyrr) can be oxidatively polymerized in the VOPO, interlayer space, whereas pyrrole is polymerized only on the surface of the VOP04-2H20 grains without intercalation. Intercalation of these polymer fragments into the VOPO, interlayer space depends on the regularity of their chain structures: poly-3-Mepyrr and po1y-3,4-Me2pyrr form a binding energyIeV Fig. 6 XPS bands of nitrogen lsl,2 electrons of 2 (---); 3 (-.-); and electrochemically prepared poly-Me,pyrr (-) containing the BF4- ion straight-chain structure suitable for the intercalation and partially branched chains of poly-Pyrr cannot be inserted.Poly-3-Mepyrr and poly-3,4-Me2pyrr fragments in the inter- layer space are affected characteristic average electronic inter- actions by the host lattice. We are greatly indebted to Professor H. Yoneyama, Faculty of Engineering, Osaka University, for the use of the X-ray powder diffractometer and Mr I. 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