Preparation of lead oxoalkoxides from lead oxide

Polyhedron 18 (1999) 1089–1093

Preparation of lead oxoalkoxides from lead oxide R. Merkle*, H. Bertagnolli ¨ Physikalische Chemie, Universitat ¨ Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany Institut f ur Received 4 August 1998; accepted 26 October 1998

Abstract Lead alkoxides of the types Pb 4 O(OR) 6 and Pb 6 O 4 (OR) 4 were prepared from PbO and the corresponding alcohols in good yields. Alkoxide exchange reactions extend the number of compounds accessible by this synthesis route. This synthesis avoids the preparation of educts like Pb[N(SiMe 3 )] which were used in earlier work. Due to their high solubility in alcohols, lead oxoalkoxides can provide valuable educts for the sol-gel preparation of lead containing ceramics.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Lead oxoalkoxides; Pb 6 O 4 (OR) 4 clusters; Lead oxide; IR and Raman spectroscopy; Sol-gel process

1. Introduction The synthesis of lead compounds, which are soluble in common organic solvents, is important for the preparation of lead containing ceramics by sol-gel processing, e.g. lead zirconate titanate Pb(Zr / Ti)O 3 or lead niobate Pb(Mg / Ni / Zn) 1 / 3 Nb 2 / 3 O 3 . Easily accessible lead(II) carboxylates, e.g. lead acetate and lead 2-ethylhexanoate, are widely used as lead sources. Lead alkoxides and lead oxoalkoxides which are well soluble in alcohols are another interesting class of precursors. They have the advantage that the alkoxide groups can be removed completely by hydrolysis, yielding materials with an extremely low content of organics. This can help to avoid problems caused by incomplete decomposition of the organic residues in the calcination steps. Some volatile oxoalkoxides could be used for chemical vapour deposition (CVD). In previous papers the synthesis of lead alkoxides from lead acetate and sodium alkoxides in alcoholic solution [1,2] is reported, but the neat alkoxide was not isolated. Lead isopropoxide Pb(O i Pr) 2 was prepared from PbF 2 and potassium isopropoxide in benzene, and transformed to Pb(OEt) 2 and Pb(O t Bu) 2 by alcoholysis and transesterification [3]. Pb[N(SiMe 3 ) 2 ] 2 [4] has proved to be a useful educt for the preparation of lead alkoxides and oxoalkoxides. It reacts with t BuOH to Pb(O t Bu) 2 and Pb 4 O(O t Bu) 6 [5] and with EtOH or i PrOH to the corresponding oxoalkoxides *Corresponding author. Tel.: 149-711-685-4450; fax: 149-711-6854443; e-mail: [email protected] 0277-5387 / 99 / $ – see front matter PII: S0277-5387( 98 )00399-4



Pb 4 O(OR) 6 and Pb 6 O 4 (OR) 4 [6]. The tetranuclear oxoalkoxides Pb 4 O(OR) 6 have a structure consisting of an oxygen centered Pb 4 -tetrahedron with OR groups over each edge, the hexanuclear oxoalkoxides Pb 6 O 4 (OR) 4 consist of a Pb 6 octahedron with four oxygens and four OR groups alternant on the faces [6]. Similar structures were found for Pb 4 O(OSiPh 3 ) 6 [8], Be 4 O(OAc) 6 [9] and for Sn 6 O 4 (OMe) 4 and Sn 6 O 4 (OH) 4 [10]. The reaction of Pb[N(SiMe 3 ) 2 ] 2 with i PrOH, t BuOH, HOCMe 2 Et and MeOEtOH under different conditions leads to the alkoxides Pb(OR) 2 [7], which form oligomeric or polymeric units. In this work we examine an alternative preparation route for lead oxoalkoxides from the commercially available cheap educts PbO and ROH instead of Pb[N(SiMe 3 ) 2 ] 2 .

2. Results The reaction of PbO with the alcohols ROH allows the synthesis of tetra- and hexanuclear lead oxoalkoxides from simple educts. The reaction follows the equation → Pb x O y (OR) 2(x 2y) 1 (x 2 y)H 2 O x PbO 1 2(x 2 y)ROH ← where x54 and y51 or x56 and y54. The water formed in this reaction must be removed in order to shift the equilibrium to the right. The primary step of this complex reaction might be the addition of ROH to PbO followed by elimination of water or alcohol which leads to the formation of Pb–O–Pb bridges

1999 Elsevier Science Ltd. All rights reserved.

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→ RO-Pb-OH PbO 1 ROH ← → RO-Pb-O-Pb-OR 1 H 2 O RO-Pb-OH 1 HO-Pb-OR ← → HO-Pb-O-Pb-OR 1 ROH HO-Pb-OR 1 HO-Pb-OR ← For ROH52-methoxyethanol (MeOEtOH, which can act as a bidentate ligand because of the two oxygen atoms), the coordination of the alkoxide to the Pb 21 is strong enough that Pb 4 O(OEtOMe) 6 is formed in high yields by refluxing PbO in MeOEtOH without water removal. Partial hydrolysis (performed by recrystallization from benzene containing 0.06 wt. % of water (saturated solution at 208C)) yields the hexanuclear oxoalkoxide Pb 6 O 4 (OEtOMe) 4 . For ROH5n-PrOH and n-BuOH, the water formed in the course of the reaction was removed by a Soxhlet extractor filled with dried molecular sieves, through which the condensed solvent runs before returning to the boiling PbO suspension. These alcohols form only the hexanuclear oxoalkoxides Pb 6 O 4 (OR) 4 . The yield (cf. Table 1) is good for n-BuOH (bp. 1188C) where the water is more volatile than the alcohol, and thus is removed effectively from the PbO suspension. The yield decreases for n-PrOH (bp. 978C) where the water removal seems to be less effective. The direct reaction of PbO with i-PrOH is unfavourable due to the further reduced boiling point of i-PrOH (828C) and the higher steric demand of the alkoxide. Such oxoalkoxides can be prepared best by substitution of the n-butoxide groups from Pb 6 O 4 (O n Bu) 4 by refluxing in an excess of the other alcohol. This reaction was carried out with i-PrOH, t-BuOH and EtOH. The reaction of Pb 6 O 4 (O n Bu) 4 with MeOEtOH leads to the formation of the tetranuclear compound Pb 4 O(OEtOMe) 6 . The potentially bidentate methoxyethoxide groups from Pb 4 O(OEtOMe) 6 and Pb 6 O 4 (OEtOMe) 4 cannot be exchanged by the unidentate simple alkoxides. The properties of all the oxoalkoxides prepared in this work are collected in Table 1. The oxoalkoxides were characterized by elementary analysis, IR and Raman spectroscopy, cryoscopy and powder diffraction. Our IR and XRD data are in good agreement with the values given in [6] for Pb 6 O 4 (OEt) 4 and Pb 6 O 4 (O i Pr) 4 , which were prepared from

Fig. 1. IR spectra (KBr tablets) of Pb 6 O 4 (OH) 4 , Pb 6 O 4 (OEtOMe) 4 and Pb 4 O(OEtOMe) 6 . The spectra are shifted along the ordinate for clarity.

Pb[N(SiMe 3 ) 2 ] in [6], indicating the identity of the substances. The presence of hexanuclear clusters Pb 6 O 4 (OR) 4 could be confirmed for the species Pb 6 O 4 (O t Bu) 4 by cryoscopy in t BuOH as an example. It yields a molecular weight of Pb 6 O 4 (O t Bu) 4 of 14856126 g / mol (calc. 1599.2 g / mol for the hexanuclear species). In the case of Pb 4 O(OEtOMe) 6 the tetranuclear cluster is the simplest structure which is consistent with the analytical data and avoids fractional stoichiometric coefficients. Due to the low solubility, the molecular weight of this compound could not be determined cryoscopically. In the IR spectra of the oxoalkoxides, the O–H stretching band of the free alcohols is absent. The C–H vibrations of the carbon atom adjacent to the oxygen are shifted to lower wavenumbers (2700 and 2800 cm 21 for the nalcohols, 2600 and 2820 cm 21 for i-PrOH, and 2690 and 2740 cm 21 for MeOEtOH). The other bands are almost identical for the alcohol and the alkoxide groups. Additional Pb–O vibration bands appear at low wavenumbers in the range of 400–600 cm 21 (cf. Fig. 1). Pb 4 O(OEtOMe) 6 21 exhibits bands at 558, 508, 458 and 406 cm , Pb 6 O 4 (OEtOMe) 4 and the other hexanuclear compounds show only a broad band at 480–540 cm 21 , which shows shoulders that vary slightly depending on the alkoxide group. The IR data for Pb 6 O 4 (OEt) 4 and Pb 6 O 4 (O i Pr) 4 are

Table 1 Yields, melting points and solubilities of the prepared oxoalkoxides Compound

Yield [%] rel. to PbO

mp [8C]

Solubility in parent alcohol [g / g of alcohol] at 258C

Pb 4 O(OEtOMe) 6 Pb 6 O 4 (OEtOMe) 4 Pb 6 O 4 (O n Bu) 4 Pb 6 O 4 (O t Bu) 4 Pb 6 O 4 (O n Pr) 4 Pb 6 O 4 (O i Pr) 4 Pb 6 O 4 (OEt) 4

82 46 68 48 59 65 52

170 mp / decomp. 57.6 117.0 150 decomp. 150 decomp. 150 decomp. 150 decomp.

,0.02 0.69 0.42 0.17 (at 308C) 0.40 0.02 0.06

R. Merkle, H. Bertagnolli / Polyhedron 18 (1999) 1089 – 1093

in good agreement with the data reported in [6]. Pb 6 O 4 (OH) 4 , for which the structure of a hexanuclear cluster is confirmed by Rietveld analysis of neutron scattering data [11], also shows a strong IR band at 492 cm 21 . The Raman spectra of Pb 6 O 4 (OH) 4 , Pb 4 O(OEtOMe) 6 , Pb 6 O 4 (OEtOMe) 4 and MeOEtOH are shown in Fig. 2. The Raman spectra also show a large similarity of the bands of the free alcohol and the alkoxide, and exhibit the same shift of the C–H vibrations adjacent to the oxygen as observed in the IR spectra. An additional small band can be observed at 1370–1380 cm 21 for the tetranuclear as well as for the hexanuclear compounds. The distinction between these structure types can be made by the low wavenumber Raman bands at 220, 340, 427 and 566 cm 21 for the tetranuclear structure of Pb 4 O(OEtOMe) 6 which are missing for the hexanuclear structures. Hexanuclear species exhibit strong bands only at 160 and 300 cm 21 . These peaks are almost identical for all hexanuclear clusters, indicating that they are caused by internal vibrations of the Pb 6 O 4 skeleton. A strong band at 162 cm 21 also appears in the Raman spectrum of Pb 6 O 4 (OH) 4 . The peak at about 100 cm 21 might arise due to the cutoff caused by the filter which removes the Rayleigh scattering. In Fig. 3, the Raman spectra of solid Pb 6 O 4 (O n Bu) 4 , Pb 6 O 4 (O n Bu) 4 dissolved in n-butanol and pure n-butanol are depicted. The comparison of these spectra shows that the peak at 160 cm 21 is caused by an internal vibration of the Pb–O skeleton which is still present in the solution. After addition of Ti(O n Bu) 4 (1 mol Ti per mol Pb, which is the appropriate stoichiometry for the preparation of PbTiO 3 ) this peak vanishes (Fig. 4), indicating that the Pb–O–skeleton is broken. Therefore an intimate and homogeneous mixing of Pb- and Ti-components in the solution can be assumed, which is advantageous for sol-gel processing.

Fig. 2. Raman spectra of Pb 6 O 4 (OH) 4 , Pb 6 O 4 (OEtOMe) 4 , Pb 4 O(OEtOMe) 6 and MeOEtOH. The spectra are shifted along the ordinate for clarity.

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Fig. 3. Raman spectra of solid Pb 6 O 4 (O n Bu) 4 , Pb 6 O 4 (O n Bu) 4 dissolved in n-butanol and pure n-butanol. The spectra are shifted along the ordinate for clarity.

Some of the oxoalkoxides prepared in this work have an extremely high solubility in their parent alcohol. The solubilities are also collected in Table 1. They decrease from n- to iso- or tert-alkoxides and from n-butoxide to ethoxide. The solubility of the pure alkoxides in non-polar solvents such as hexane is usually low, but small amounts of the parent alcohol (present e.g. in the crude alkoxides) strongly increases the solubility. Most of the oxoalkoxides decompose at about 1508C without melting. The decomposition and melting points are also listed in Table 1.

3. Summary Lead oxoalkoxides with a composition of Pb 6 O 4 (OR) 4 can be prepared from PbO and primary, secondary and

Fig. 4. Raman spectra of Ti(O n Bu) 4 dissolved in n-butanol, Pb 6 O 4 (O n Bu) 4 dissolved in n-butanol, and Pb 6 O 4 (O n Bu) 4 1Ti(O n Bu) 4 dissolved in n-butanol. The spectra are shifted along the ordinate for clarity.

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tertiary alcohols ROH by direct reaction or with subsequent alkoxide exchange. Pb 4 O(OEtOMe) 6 is formed in the direct reaction of PbO with MeOEtOH. The Pb 4 O and Pb 6 O 4 skeleton remains intact when the oxoalkoxides are dissolved in alcohols, but it is destructed by addition of titanium alkoxides to the solution.

h in 50 ml of hexane to remove residual amounts of MeOEtOH. Removal of the supernatant solvent and excessive drying yields 6.0 g (yield 82%) of white needles of Pb 4 O(OEtOMe) 6 . Pb calc. 64.0 found 63.7, C calc. 16.7 found 16.8, H calc. 3.3 found 3.4, N found 0; IR (Pb–OR, KBr) 558(vs), 511(vs), 457(w), 410(m) cm 21 , Raman (Pb–O, Pb–OR) 220(m), 340(w), 427(w), 566(m) cm 21 .

4. Experimental

4.3. Pb6 O4 ( OEtOMe)4 All reactions were performed under dry nitrogen in Schlenk tubes. The solvents were dried by standard methods. The products were dried at room temperature under vacuum to remove remaining solvents. FT-IR spectra (400–4000 cm 21 ) of solid samples were recorded in transmission mode from KBr tablets and Nujol mulls (prepared in a glove box) with a BRUKER IFS 66 spectrometer. Fourier transform Raman spectra (60–3200 21 cm ) were recorded in 1808 reflection geometry with a BRUKER RFS 100 spectrometer with an excitation wavelength of 1064 nm. Both the solid and the liquid samples were measured in sealed glass capillaries. The Pb content was determined complexometrically with EDTA and xylenolorange as indicator [12] after hydrolyzing the samples which were previously weighed under dry N 2 . CHN analysis could be carried out successfully only for Pb 4 O(OEtOMe) 6 (data given below) which is most stable against hydrolysis, because exposure to air could not be avoided during insertion of the samples into our CHN analyzer. Thus, the other oxoalkoxides underwent partial hydrolysis before CHN analysis, which yielded results consistent with a composition of Pb 6 O 4 (OR) 42x (OH) x with 0.3#x#0.6. Diffractograms were recorded at room temperature on a STOE STADI P powder diffractometer equipped with a position sensitive detector. The samples were contained in sealed glass capillaries and measured with Cu Ka radiation in Debye–Scherrer geometry. The molecular weight of Pb 6 O 4 ( t Bu) 4 was determined cryoscopically in t BuOH.

4.1. Pb6 O4 ( OH)4 Preparation according to [11] from an aqueous solution of Pb(OAc) 2 ?3H 2 O with NH 3 . IR (Pb–OH, KBr) 495(vs) cm 21 , Raman (Pb–O) 131(w), 162(vs), 206(w), 368(w) ˚ c59.314 A ˚ (a58.023 cm 21 , XRD tetragonal, a58.038 A, ˚ c59.318 A ˚ from [11]). A,

4.2. Pb4 O( OEtOMe)6 A 5 g sample of yellow PbO was refluxed in 100 ml MeOEtOH for 1–2 h. After filtering the hot suspension, the MeOEtOH was evaporated at room temperature under vacuum to yield 6.5 g of crude product. After recrystallization from hot MeOEtOH, the product was suspended for 2

A 0.5 g sample of Pb 4 O(OEtOMe) 6 was refluxed for 30 min in 20 ml of benzene saturated with water. The solvent was removed under vacuum and the residue was dissolved in 10 ml of hexane. The filtered solution was evaporated to a volume of 2–3 ml and cooled in ice / rock salt. Then 0.23 g of white needles precipitated (yield 55% related to Pb 4 O(OEtOMe) 6 , 46% related to PbO). Raman spectra prove the complete transformation of Pb 4 O(OEtOMe) 6 to Pb 6 O 4 (OEtOMe) 4 . Pb calc. 77.3 found 77.6; IR (Pb–OR, KBr) 512(sh), 492(vs) cm 21 , Raman (Pb–O) 130 (w), 160(vs), 297(m) cm 21 .

4.4. Pb6 O4 ( O n Bu)4 A 5 g sample of yellow PbO was refluxed in 150 ml of n-BuOH for 7 h. The condensed n-BuOH was ran through ˚ to a Soxhlet extractor filled with dry molecular sieve (3A) remove the water formed during the reaction. After filtering the hot suspension, the n-BuOH was removed at room temperature under vacuum which yields 6.0 g of waxy crude product. 40 ml of hexane was added to the crude product, the suspension was filtered after 5 min of stirring, and about 20 ml of hexane was removed under vacuum. Cooling in ice / rock salt leads to the precipitation of white, plate like crystals of Pb 6 O 4 (O n Bu) 4 . Yield 4.1 g (68%). Pb calc. 77.7 found 76.8; IR (Pb–OR, KBr) 544(sh), 508(vs), 481(sh) cm 21 , Raman (Pb–O) 130(w), 161(vs), 297(m) cm 21 .

4.5. Pb6 O4 ( O t Bu)4 Preparation via alkoxide exchange from Pb 6 O 4 (O n Bu) 4 : 3.0 g of the pure Pb 6 O 4 (O n Bu) 4 was refluxed in 50 ml of t-BuOH for 4 h and the solvent evaporated at room temperature under vacuum. After repetition of this procedure, 50 ml of hexane was added to the waxy crude product. The suspension was filtered and about 30 ml of hexane evaporated under vacuum. Cooling in ice / rock salt leads to the precipitation of white crystals of Pb 6 O 4 (O t Bu) 4 . Yield 2.1 g (70% related to Pb 6 O 4 (O n Bu) 4 , 48% related to PbO). Raman spectra prove the complete substitution of O n Bu by O t Bu.

R. Merkle, H. Bertagnolli / Polyhedron 18 (1999) 1089 – 1093

Pb calc. 77.7 found 77.6; IR (Pb–OR, KBr) 520(vs), 488(sh), 442(m) cm 21 , Raman (Pb–O, Pb–OR) 130(w), 148(m), 165(vs), 226(w), 297(m), 363(w), 449(w), 493(w) cm 21 , molecular weight cryoscopically 14856126 g / mol, calc. 1599.2 g / mol

4.6. Pb6 O4 ( O n Pr)4 The reaction of PbO with n-PrOH analogously to the preparation of Pb 6 O 4 (O n Bu) 4 yields 4.5 g of crude product and 2.9 g (50%) of pure Pb 6 O 4 (O n Pr) 4 after recrystallization from hexane. Preparation via alkoxide exchange from Pb 6 O 4 (O n Bu) 4 : 3.0 g of the crude Pb 6 O 4 (O n Bu) 4 was refluxed in 50 ml of n-PrOH for 4 h and the solvent evaporated at room temperature under vacuum. After repetition of this procedure, 25 ml of hexane was added to the waxy crude product. The suspension was filtered and about 15 ml of hexane was evaporated under vacuum. Cooling in ice / rock salt leads to the precipitation of white, needle shaped crystals of Pb 6 O 4 (O n Pr) 4 . Yield 1.7 g (59% related to PbO). Raman spectra prove the complete substitution of O n Bu by O n Pr. Pb calc. 80.5 found 80.6; IR (Pb–OR, KBr) 522(vs), 489(vs) cm 21 , Raman (Pb–O) 130(w), 162(vs), 297(m) cm 21 .

4.7. Pb6 O4 ( O i Pr)4 Preparation via alkoxide exchange from Pb 6 O 4 (O n Bu) 4 : 3.0 g of the crude Pb 6 O 4 (O n Bu) 4 was refluxed in 50 ml of i-PrOH for 4 h and the solvent evaporated at room temperature under vacuum. After refluxing for 4 h in another 50 ml of i-PrOH, a fine white precipitate of Pb 6 O 4 (O i Pr) 4 was formed after evaporation of about 25 ml of the solvent and cooling in ice / rock salt. Yield 1.9 g (95% related to Pb 6 O 4 (O n Bu) 4 , 65% related to PbO). Raman spectra prove the complete substitution of O n Bu by O i Pr. Pb calc. 80.5 found 80.4; IR (Pb–OR, KBr) overlapping 527(vs), 508(vs), 492(vs) cm 21 , IR (Pb–OR, Nujol) overlapping 527(vs), 505(vs), 292(vs) cm 21 , Raman (Pb–O, Pb–OR) 130(w), 161(vs), 233(w), 295(m), 396(w), 467(w), 515(w) cm 21 .

4.8. Pb6 O4 ( OEt)4 Preparation via alkoxide exchange from Pb 6 O 4 (O n Bu) 4 :

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3.0 g of the pure Pb 6 O 4 (O n Bu) 4 was refluxed in 50 ml of EtOH for 4 h, the solvent evaporated at room temperature under vacuum, and the solid residue refluxed again for 4 h in 50 ml of EtOH. Cooling in ice / rock salt and partially evaporating the solvent leads to the precipitation of white crystals of Pb 6 O 4 (OEt) 4 . Yield 2.2 g (77% related to Pb 6 O 4 (O n Bu) 4 , 52% related to PbO). Raman spectra prove the complete substitution of O n Bu by OEt. Pb calc. 83.6 found 84.6; IR (Pb–OR, KBr) 498(vs), 465(sh) cm 21 , IR (Pb–OR, Nujol) 498(vs), 460(sh) cm 21 , Raman (Pb–O) 130(w), 160(vs), 296(m) cm 21 , XRD ˚ b512.456 A ˚ c517.451 A ˚ b5 monoclinic, a520.050 A ˚ ˚ ˚ 95.888 (a520.024 A, b512.473 A, c517.409 A, b 5 95.868 at 2808C from [6]).

Acknowledgements We thank Mrs. C. Ansorge for assistance in the syntheses, and Prof. Dr. A. Schmidt for advice concerning analytical questions.

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