Chemical routes to oxides: alkoxide vs. alkoxide–acetate routes: synthesis, characterization, reactivity and polycondensation of MNb2(OAc)2(OPri)10 (M=Mg, Cd, Pb) species Souad Boulmaa�z,a Rene�e Papiernik,a Liliane G. Hubert-Pfalzgraf *,a Bernard Septea and Jacqueline Vaissermannb aL aboratoire de ChimieMole�culaire, URA-CNRS, Universite� de Nice-Sophia Antipolis, 06108 Nice Ce�dex 2, France bL aboratoire de Chimie des Me� taux de T ransition, URA-CNRS, 75230 Paris Ce�dex, France The molecular constitution of solutions containing niobium alkoxides and divalent metal acetates M(OAc)2 (M=Mg, Ba, Pb, Zn, Cd) has been examined.The heterometallic aggregates MNb2(m-OAc)2(m-OPri)4(OPri)6 (M=Mg 1, Cd 2, Pb 3a) have been isolated and characterized by elemental analysis, FTIR, multinuclear NMR (1H, 13C, 207Pb, 113Cd) and by single-crystal X-ray diVraction for M=Mg and Cd.The magnesium derivative crystallizes in the monoclinic system, space group P21/c with the unitcell parameters a=21.238(5), b=10.127(10), c=24.861(3) A ° , b=107.77(2)° and Z=4. The thermal stability of the various species has been investigated. Condensation is induced thermally for PbNb2(OAc)2(OPri)10. The reactivity between MNb2(m-OAc)2- (m-OPri)4(OPri)6 (M=Pb, Mg) and other metallic species has been evaluated.A terheterometallic compound, PbMgNb2O(OAc)2(OPri)10 has been isolated. Whereas no reaction is observed between Ba(OAc)2 and Nb(OPri)5, the reaction between metal alkoxides aVords BaNb2(OPri)12(PriOH)2. Its reactivity shows that the absence of an assembling acetate ligand induces facile separation between the metals.Results of the hydrolysis experiments of 1–3a are given. The powders have been analyzed by TG, SEM–EDX, light scattering and XRD. The merit of the assembling acetate ligand for avoiding the segregation of the metals is emphasized. Mixed-metal oxides represent an important class of advanced reactions has been achieved.Results on the Nb–Cd system have been reported in a communication.5 materials due to their palette of technological applications.1 Their uses include catalysts and structural ceramics, but they have mostly been acclaimed for their applications as sensors, actuators and smart materials.2 Techniques for the formation Experimental of such materials, including chemical vapor deposition (CVD), All manipulations were routinely performed under nitrogen sol–gel processes, metal–organic decomposition and molecular atmosphere using Schlenk tubes and vacuum line techniques beam epitaxy, to name a few, require metal–organic molecules with dried and distilled solvents.Niobium6 and lead alkoxides,7 which have specific physical and chemical properties, as BaNb2(OPri)12(PriOH)2,8 lead9 and zinc10 trimethylsilylam- precursors.The preparation of inorganic materials from ides were prepared according to the literature. Anhydrous metal–organic precursors generally has the advantages over metal acetates were obtained by refluxing metal acetate ‘traditional routes’ of low temperatures of formation and/or hydrates with acetic anhydride over 15 h. 1H, 13C, 113Cd and crystallization, better compositional uniformity and conformal 207Pb NMR spectra were recorded on solutions (concen- coverage in the case of films.3 Metal alkoxides M(OR)n or trations: 1 M for 113Cd and 0.3 M for 207Pb) on a Bruker AC- oxoalkoxides MO(OR)n are versatile molecular precursors of 200 spectrometer. The chemical shifts are reported with respect metal oxides, one of their most attractive features being their to M(NO3)2 (M=Cd, Pb) in aqueous solutions as external solubility in a large variety of solvents and their ability to references, they are positive to low fields.IR spectra were form heterometallic species, especially by mixing alkoxides of registered with a FT–IRS 45 Bruker spectrometer as Nujol diVerent metals.4 However, to overcome the diYculty in handmulls between KBr plates for the air-sensitive derivatives and ling alkoxides and/or in their availability, more commonly as KBr pellets for the powders resulting from hydrolysis. accessible derivatives such as carboxylates (often acetates), Analytical data were obtained from the Centre de nitrates, halides, hydroxides or b-diketonates have often been Microanalyses du CNRS.IR and NMR data are listed in used in conjunction with metal alkoxides. The molecular Table 1. constitution of such solutions is almost unknown, although a Hydrolyses were achieved at room temperature in THF or better understanding could allow a better control of the isopropyl alcohol (0.1–0.05 M) without additives, the water hydrolysis–polymerization process and thus of the properties being added via these solvents.The powders were characterized of the resulting material. by TG–DTA with a Setaram system (nitrogen, heating rate of We report here the results of the investigations of the 5 °C min-1). Powder X-ray diVraction patterns were collected M(OAc)2–Nb(OR)5 (M=Mg, Ba, Zn, Cd, Pb) systems. Insight using Cu-Ka radiation after calcination at various into the molecular composition of homogeneous solutions has temperatures.been gained by using a variety of spectroscopic techniques, IR and nuclear magnetic resonance (1H and 13C as well as metal NMR, namely 207Pb or 113Cd), and in the most favorable cases Synthesis of MgNb2(OAc)2(OPri)10 1 by single-crystal X-ray diVraction. The heterometallic species MNb2(m-OAc)2(m-OR)4(OR)6 (M=Mg, Cd; R=Pri) have thus Anhydrous Mg(OAc)2 (1 g, 7.02 mmol) was added to a solution of Nb(OPri)5 (5.02 g, 12.94 mmol) in 25 ml of hexane (or been unequivocally characterized.Their reactivity as well as that of a Ba–Nb species having a similar stoichiometry, toluene) at room temperature. After stirring for 1 to 2 h, the excess of Mg(OAc)2 was eliminated by filtration. Crystallization BaNb2(OPri)12(PriOH)2 has been examined.The transformation of the acetatoalkoxides by hydrolysis–polycondensation was achieved at 5 °C (5.88 g, 99%). Anal. Calc. for J. Mater. Chem., 1997, 7(10), 2053–2061 2053Table 1 IR and NMR (1H, 13C, 207Pb) spectroscopic data for mixed-metal acetatoalkoxides NMR (25 °C, d CDCl3)a IR/cm-1 207Pb, 1H 13C{1H} nMOZ 113Cd{1H} compound nCO2 (Z=Ac, Et, Pri) (toluene) CH3(Ac) CH2(Et), CH (Pr) CH3 (Et, Pri) CO(Ac) CH2(Et), CH (Pri) CH3( Et, Pri, Ac) MgNb2(OAc)2(OPri)10 1 1590vs, 1429vs 662m, 616w, 581m, — 1.90 (s,3H) 4.75, 4.67, 4.54 1.33, 1.30, 1.20 177.4 75.4, 75.3 25.8, 25.4, 564m, 529m, 478s, (spt, 25251, 5H) (d, 25251, 30H) 71.8, 70.6 24.6, 24.2 391m CdNb2(OAc)2(OPri)10 2 1574vs, 1423s 579s, 557(sh), 518w, 33.8 1.95 (s,3H) 4.70 (spt, 5H) 1.28 (d, 30H) 177.6 74.1 25.4, 24.7 446s PbNb2(OAc)2(OPri)10 3a 1560vs, 1415vs 659m, 644m, 608(sh), 2390 25 °C: 1.97(s,3H) 25 °C: 4.99, 4.69 25 °C: 1.27 (d,30H) 571vs, 512w, 466m (spt, 352, 5H) 177.8 74.7, 73.9 25.8, 25.3, 432m -50 °C: 1.95(s) -50 °C 4.95, 4.83, 4.65 -50 °C: 1.22 (ov.d) 24.6, 23.8 (spt, 25251) PbNb2(OAc)2(OEt)10 3b 1566vs, 1414m 662s, 555s, 403m 2325 1.75(s,3H) 4.50, 4.30 1.21 (ov.t, 15H) 178.6 67.2, 62.9 23.7, 20.0, (q, 253, 10H) 18.2, 18.1 as=Singlet, d=doublet, q=quartet, spt=septet, m=multiplet, ov.d=overlapping of doublets, ov.t=overlapping of triplets.Coupling constants J=7 Hz (Et), 6 Hz (Pri). 2054 J. Mater. Chem., 1997, 7(10), 2053–2061C34H76MgNb2O14: C, 44.43; H, 8.27; Mg, 2.64; Nb, 20.23.CH3, OAc), 1.35, 1.30, 1.20 (d, J=6.1 Hz, 45452, 60H, CH3); 13C{1H} NMR (CDCl3), d 177.5 (CO, Ac), 75.7, 75.5, 72.9, Found: C, 43.53; H, 8.12; Mg, 2.35; Nb, 20.30%. 71.9, 70.7 (CH), 26.7, 25.5, 25.2, 24.7, 24.3 (CH3, Pri). 207Pb NMR (C7D8), d 4323. Synthesis of CdNb2(OAc)2(OPri)10 2 and of PbNb2(OAc)2(OR)10 (R=Pri 3a, Et 3b) Structure determination of MgNb2(OAc)2(OPri )10·0.5C7H8 Colourless needles were obtained by the same procedure (93%) The selected crystal was mounted on a Enraf-Nonius CAD-4 for 2 and for 3a (99%).Anal. Calc. for C34H76CdNb2O14: automatic diVractometer. The unit-cell parameters and basic C, 39.23; H, 7.45; Cd, 10.76; Nb, 18.92. Found: C, 38.99; H, 7.17; information about data collection at -100 &cture Cd, 10.82; Nb, 18.74.Anal. Calc. for C34H76Nb2O14Pb: C, 37.05; refinement are given in Table 2. Lattice parameters and orien- H, 6.90; Pb, 18.81; Nb, 16.87. Found: C, 36.99; H, 6.87; tation matrices were obtained from least-squares refinement of Pb, 18.79; Nb, 16.85%. the setting angles of 25 well centred reflections in the range Unit-cell parameters for 3a (-100 °C): a=10.308(3), b= 6<2h<35°.The intensities of three standard reflections moni- 14.114(3), c=34.121(3) A ° , b=99.08(2)°. tored every hour showed no decay. Corrections for Lorentz PbNb2(OAc)2(OEt)10 3b was obtained accordingly from and polarization eVects were applied. Nb(OEt)5 and Pb(OAc)2 in toluene in a 65% yield. Anal. Calc. Computations were performed using the PC version of for C24H56Nb2O14Pb: C, 29.96; H, 5.82; Nb, 19.33; Pb, 21.56.CRYSTALS.11 Scattering factors and corrections for anomal- Found: C, 29.36; H, 5.48; Nb, 19.19; Pb, 21.19%. ous dispersion were taken from ref. 12. The structure was The various compounds were soluble in common organic solved using direct methods (SHELXS)13 and standard Fourier solvents including hydrocarbons, 3b is poorly soluble in techniques.One of the isopropyl groups [on O(11)] showed ethanol. relatively large thermal parameters compared with the others. A best solution was found by the introduction of two dis- Synthesis of Pb2Nb4O5(OAc)2(OPri)12 4 ordered carbon atoms [C(23) and C(231)] each with half Anhydrous Pb(OAc)2 (0.67 g, 2.06 mmol) was added to a occupancy. All non-hydrogen atoms, except the two disordered solution of Nb(OPri)5 (1.60 g, 4.12 mmol) in 20 ml of toluene carbon atoms, were refined anisotropically.A diVerence Fourier and the reaction medium was refluxed for 40 h. After evapor- map showed the presence of a toluene molecule located on the ation of the solvent, a yellow oil was obtained. Addition of C2 axis, leading to the given complete formula. The very large isopropyl alcohol induced crystallization of thin needles (1.5 g, values of the thermal parameters for this molecule suggests a 86%) which were highly soluble in organic solvents.Anal. disorder around the C2 axis which could not be solved. Atomic coordinates, thermal parameters, and bond lengths Calc. for C40H90O21Nb4Pb2: C, 28.38, H, 5.36; Nb, 21.95; and angles have been deposited at the Cambridge Pb, 24.47.Found: C, 28.10; H, 5.23; Nb, 22.02; Pb, 24.64%. Crystallographic Data Centre (CCDC). See Information for IR(cm-1): 1570s (nasCO2), 1414m (nsCO2); 1160s, 1122s, 1014s, Authors, J. Mater. Chem., 1997, Issue 1. Any request to the 991s, 960s, 850m, 837m, 826w, 800w (nMMOMM), 663s, 618w; CCDC for this material should quote the full literature citation 598s, 569s, 516s, 467m, 430w (nMMOAc, nMOR). 1H NMR and the reference number 1145/47. (CDCl3, -30 °C): 4.85, 4.69, 4.62 (spt, J=6 Hz, 15454, 12H, CH), 2.02 (6 H, O Ac), 1.29, 1.22, 1.16 (d, J=6 Hz, 72H, Me); 207Pb NMR, d 2482. Results and Discussion Synthesis Synthesis of [PbNb2O(OPri)10]m We have investigated the reactions between anhydrous metal Lead iodide (1.12 g, 2.42 mmol) was added to a suspension of acetates M(OAc)2 (M=Mg, Ba, Cd, Pb) and niobium, namely KNb(OPri)6 (2.36 g, 4.85 mmol) in 25 ml of toluene.After stirring at room temperature for ca. 4 h, refluxing was carried Table 2 Crystallographic data for MgNb2(OAc)2(OPri)10·0.5C7H8 at out for 12 h. Potassium iodide was separated by filtration. -100 °C Cooling of the filtrate at -30 °C aVorded large platelets (1.7 g, 70%), soluble in toluene and isopropyl alcohol.Anal. Calc. for Mw 965.1 C30H70Nb2O11Pb: C, 36.03; H, 7.00; Nb, 18.59; Pb, 20.74. a/A° 21.238(5) Found: C, 35.35; H, 6.77; Nb, 17.90; Pb, 21.05%. IR (cm-1): b/A °10.127(10) c/A ° 24.861(3) 1169m, 1135m, 1122s, 1025m, 998s, 984m, 955s; 852m, 829m, a/° 90 800w, 771w, 721w; 577vs, 461m (nMMOR). 1H NMR (CDCl3), b/° 107.77(2) d 5.00, 4.77, 4.65 (spt, J=6 Hz, 75251, 10H, CH), 1.33, 1.31 (d, c/° 90 J=6 Hz, 60H, Me); 13C{1H} NMR (CDCl3), d 75.2 (CH), V /A ° 3 5110(5) 26.1, 25.1, 22.9 (CH3).Z 4 The same product was obtained by reacting [Pb(OPri)2]2 crystal system monoclinic space group P21/c and Nb(OPri)5 in toluene at room temperature. linear absorption coeYcient m/cm-1 4.9 density r/g cm-3 1.26 Synthesis of PbMgNb2O(OAc)2(OPri)10 diVractometer CAD4 Enraf-Nonius radiation Mo-Ka (0.71069) [Pb(OPri)2]2 (0.87 g, 2.67 mmol) was added to a solution of scan type v–2h MgNb2(OAc)2(OPri)10 (1.76 g, 1.91 mmol) in 32 ml of a mixscan range/° 0.80+0.345 tan h ture of hexane–toluene (1551). After stirring for 20 h at room h limits/° 2–20 temperature, the excess of lead isopropoxide was removed by octants collected (hkl) h 0–20, k 0–9, l -23 to 23 filtration; crystallization of 5 as thin needles occurred at no.of data collected 4940 no. of unique data collected 4761 -30 °C (1.25 g, 57.3%). Anal. Calc. for C34H76O15MgNb2Pb: no. of unique data used for refinement 2891 [(Fo)2>3s(Fo)2] C, 35.76; H, 6.71; Mg, 2.13; Nb, 16.27; Pb, 18.14. Found: Rint 0.034 C, 35.53; H, 6.58; Mg, 2.5; Nb, 16.20; Pb, 18.07%.IR(cm-1): R=SdFo|-|Fcd/S|Fo| 0.0544 1605ns, 1582ns, (nasCO2); 1428s (nsCO2); 1329m, 1260w, 1161ns, Rw=[Sw(|Fo|-|Fc|)2/SwFo2]1/2 0.0656(w=1.0) 1125ns, 1017ns, 998ns, 969ns, 953(sh), 899m, 847m, 836m, extinction parameter 0 828(sh), 667m; 604s, 580s, 560ns, 516m, 500m, 467m, 425m, goodness of fit, s 3.9 no. of variables 496 415m, 314m, 291m (nMMOAc, nMOR). 1H NMR (CDCl3), Drmin,max/e A°-3 -0.30, 0.38 d 4.75, 4.67, 4.54 (spt, J=6.1 Hz, 45452; 10H, CH), 1.9 (s, 6H, J. Mater. Chem., 1997, 7(10), 2053–2061 2055ethoxide and isopropoxide. The choice of these systems was is inert toward Nb(OR)5, even by heating in the presence of the parent alcool. The poor reactivity of barium acetate, which motivated by the attractive properties of niobates and tantalates as electrooptical ceramics [LiNbO3, (Sr,Ba)Nb2O6 is actually a polymer based on tetranuclear units, is generally overcome by adding acetic acid,20 but we observed no dissolu- (SBN), (Pb,La)(Ti,Nb)O3 (BLNT), Pb(Sc,Nb)O3 (PSN)],14 as ceramics for microwave resonators [PbMg1/3Nb2/3O3 (PNM), tion even in refluxing conditions by adding variable amounts of AcOH (up to 17 equivalents per Ba, this amount leading to BaZn1/3Ta2/3O3 (BZT), BaMg1/3Ta2/3O3 (BMT)],15,16 or as dielectric ceramics (CdNb2O6).5 Metal acetates are the most gelification).A BaNb2(OPri)12(PriOH)2 species, obtained either by mixing the isopropoxides or by metathesis reaction common precursors associated with metal alkoxides. Among the various coordination modes of the acetate ligand, the between BaI2 and KNb(OPri)6, provides an alternative ‘singlesource’ precursor for Ba–Nb materials.8 Anhydrous zinc acetate assembling ones (bridging or bridging–chelating) are generally favored, this suggests that carboxylates could be a means of is actually a soluble oxide acetate Zn4O(OAc)6, its reaction with Nb(OPri)5 is thus more diYcult to control and less maintaining the stoichiometry along the various steps through the hydrolysis–polycondensation process.These characteristics, selective than for the other divalent metal acetates as shown by the several nasCO2 absorption bands (1590, 1577, which have been exploited for the formation of gels or fibers,17 are however less attractive for MOCVD purposes.18 1515 cm-1). With the exception of the barium acetate, the anhydrous acetates of divalent metals M(OAc)2 (M=Mg, Cd, Reactions between metal alkoxides and carboxylates have generally been considered to proceed by elimination of an Pb) are more reactive than these based on lanthanides since heating was required for the latter.21 ester as a volatile byproduct, thus giving oxo derivatives.However, such reactions can occur in very mild conditions The importance of the solvent on the formation of mixedmetal acetatoalkoxides is noteworthy.Whereas the reaction (room temperature and non-polar solvents), giving compounds whose formulation results from a simple addition. The reac- between Cd(OAc)2 and Nb(OPri)5 proceeds at room temperature, no reaction is observed with [Nb(OEt)5]2, even in tions between divalent metal acetates of magnesium, cadmium and lead, and niobium isopropoxide, illustrate these features.refluxing toluene. Addition of small amounts of ethanol allows the reaction to proceed, probably as a result of the formation These reactions proceed smoothly and quantitatively in hydrocarbons over 1 or 2 h, with progressive dissolution of the of the Nb(OEt)5(EtOH) monomer; CdNb2(OAc)2(OEt)10 is thus obtained.5 The Pb(OAc)2–Nb2(OEt)10 system is acetate according to eqn. (1).The excess of metal acetates is easily removed by filtration, while the novel compounds are more reactive than the Cd(OAc)2–Nb2(OEt)10 system; PbNb2(OAc)2(OEt)10 3b is obtained in toluene, under con- crystallized out almost quantitatively from the filtrate. ditions similar to these of the isopropoxide analog, 3a.The choice of a polar solvent which might act as a ligand toward M(OAc)2+2Nb(OPri)5 CCCCA hexane, 25 °C M=Mg 1, Cd 2, Pb 3a MNb2(OAc)2(OPri)10 M(OAc)2 can be an unfavorable feature for complexation by a metal alkoxide. By contrast with hydrocarbons, no reaction The nasCO2 stretching frequencies of the carboxylate ligands occurs between Pb(OAc)2 and Nb(OEt)5 (152 stoichiometry) in the resulting complexes MNb2(OAc)2(OPri)10 are generally in THF, although solubilization of the acetate is achieved.shifted to higher frequencies with respect to those of the 207Pb NMR, which is a quite convenient tool [l=1/2, sensihomometallic acetates (Table 1). This shift excludes the forma- tivity=20.6%, wide range (#10 000 ppm of chemical shift)]22 tion of mixed crystals M(OAc)2,2Nb(OPri)5. The diVerence for the analysis of the solutions of lead derivatives, only shows D=nasCO2- nsCO2 suggests a chelating or bridging–chelating the presence of complexed lead acetate (d=2230).One can behavior for the OAc ligands.19 notice that whereas Pb6O4(OEt)4 reacts with Nb(OEt)5 to While the reactions between niobium isopropoxide and form Pb6(m4-O)4(m3-OEt)4Nb4(OEt)20,23 the use of Pb(OAc)2 cadmium, magnesium or lead acetates occur easily, diVerent as the source of lead oxide allows to acceed to precursors behaviors are observed in the case of barium and of zinc.having a diVerent Pb–Nb stoichiometry and thus to expand Scheme 1 summarizes the various routes investigated for access the range of ‘single-source’ precursors available for Nb–Pb to Nb–MII species (M=Mg, Ba, Zn, Cd, Pb).Barium acetate oxide materials. The new species are soluble in common organic solvents, thus allowing their characterization by NMR. All compounds 1, 2, 3a, 3b are fluxional, the exchange rate being dependent upon the size of the central nucleus. The exchanges between the diVerent types of alkoxide ligands are frozen out already at room temperature for 1 while lower temperatures (-50 °C) are necessary for 2, 3a and 3b.The acetate ligands are observed as a unique peak in the 1H NMR spectra whereas the alkoxide signals appear as three resonances in a 25251 ratio. The acetate ligands act as clips between the diVerent metals and the solidstate structure is retained upon dissolution in non-polar as well as polar solvents.This is also confirmed by the NMR of the other nuclei (13C, 207Pb, 113Cd). Molecular structures of Nb2M(m-OAc)2(m-OPri)4(OPri)6 The connectivity between the diVerent metals has been established for the magnesium and cadmium derivatives by a singlecrystal X-ray structure determination. Selected bond lengths and angles are collected in Table 3 for 1 and in Table 4 for 2; the molecular structure of MgNb2(OAc)2(OPri)10 is displayed in Fig. 1. The structure is related to that of CdNb2(OAc)2(OPri)10. These trinuclear species display a bent, open-shell structure (NbMMgMNb 139.45° for 1), with alternating Nb and M atoms, all metal atoms being 2 KNb(OPri)6 BaI2 BaNb2(OPri)12(PriOH)2 toluene–PriOH, heat –2 KI MNb2(OAc)2(OPri)10 2 Nb(OR)5 Ba(OPri)2 toluene–PriOH, heat Ba(OAc)2 toluene–ROH, heat M(OAc)2 toluene, RT Zn4O(OAc)6 non-selective Pb(OAc)2 THF Cd(OAc)2 toluene, heat toluene PbNb2(OAc)2(OEt)10 CdNb2(OAc)2(OEt)10 toluene–EtOH M = Mg 1 Cd 2 Pb 3a 3b R = Et and/or Pri Scheme 1 Various routes to Nb–MII species (M=Mg, Ba, Cd, Pb) six-coordinate. The NbMO bond distances span the range 2056 J.Mater.Chem., 1997, 7(10), 2053–2061Table 3 Selected bond distances (A° ) and angles (degrees) for MgNb2(m-OAc)2(m-OPri)4(OPri)6 0.5C6H5CH3 1 Mg(1)MO(1) 2.042(9) Mg(1)MO(3) 2.044(5) Nb(1)MO(10) 1.872(8) Nb(1)MO(11) 1.879(8) Mg(1)MO(5) 2.096(4) Mg(1)MO(6) 2.093(8) Nb(2)MO(2) 2.177(8) Nb(2)MO(5) 2.012(7) Mg(1)MO(7) 2.081(8) Mg(1)MO(8) 2.080(9) Nb(2)MO(7) 2.027(5) Nb(2)MO(12) 1.878(8) Nb(1)MO(4) 2.175(6) Nb(1)MO(6) 2.025(7) Nb(2)MO(13) 1.882(8) Nb(2)MO(14) 1.877(7) Nb(1)MO(8) 2.012(7) Nb(1)MO(9) 1.878(6) Mg(1)···Nb(1) 3.203(4) Mg(1)···Nb(2) 3.194(3) O(3)MMg(1)MO(1) 90.9(3) O(5)MMg(1)MO(1) 89.0(3) O(10)MNb(1)MO(9) 96.0(3) O(11)MNb(1)MO(4) 84.4(3) O(5)MMg(1)MO(3) 168.3(3) O(6)MMg(1)MO(1) 93.7(3) O(11)MNb(1)MO(6) 165.3(4) O(11)MNb(1)MO(8) 92.5(4) O(6)MMg(1)MO(3) 90.5(3) O(6)MMg(1)MO(5) 101.2(3) O(11)MNb(1)MO(9) 95.9(4) O(11)MNb(1)MO(10) 95.0(4) O(7)MMg(1)MO(1) 89.7(3) O(7)MMg(1)MO(3) 93.6(3) O(5)MNb(2)MO(2) 85.5(3) O(7)MNb(2)MO(2) 83.3(3) O(7)MMg(1)MO(5) 74.7(3) O(7)MMg(1)MO(6) 174.7(3) O(7)MNb(2)MO(5) 77.7(3) O(12)MNb(2)MO(2) 85.8(3) O(8)MMg(1)MO(1) 167.1(4) O(8)MMg(1)MO(3) 90.5(3) O(12)MNb(2)MO(5) 168.5(3) O(12)MNb(2)MO(7) 93.7(3) O(8)MMg(1)MO(5) 92.1(3) O(8)MMg(1)MO(6) 73.5(3) O(13)MNb(2)MO(2) 178.4(2) O(13)MNb(2)MO(5) 93.8(3) O(8)MMg(1)MO(7) 103.0(3) O(6)MNb(1)MO(4) 85.2(3) O(13)MNb(2)MO(7) 95.2(3) O(13)MNb(2)MO(12) 94.8(4) O(8)MNb(1)MO(4) 85.7(3) O(8)MNb(1)MO(6) 76.4(3) O(14)MNb(2)MO(2) 84.7(3) O(14)MNb(2)MO(5) 93.3(3) O(9)MNb(1)MO(4) 179.6(3) O(9)MNb(1)MO(6) 94.5(3) O(14)MNb(2)MO(7) 165.5(3) O(14)MNb(2)MO(12) 93.4(3) O(9)MNb(1)MO(8) 94.1(3) O(10)MNb(1)MO(4) 84.2(3) O(14)MNb(2)MO(13) 96.8(3) O(10)MNb(1)MO(6) 94.2(3) O(10)MNb(1)MO(8) 166.8(3) Nb(1)MO(6)MMg(1) 102.2(3) Nb(2)MO(5)MMg(1) 102.0(3) Nb(1)MO(8)MMg(1) 103.1(3) Nb(2)MO(7)MMg(1) 102.0(3) C(1)MO(2)MNb(2) 130.4(7) C(14)MO(8)MNb(1) 136.5(8) C(26)MO(12)MNb(2) 163.7(13) C(5)MO(5)MMg(1) 120.50 C(3)MO(4)MNb(1) 130.9(5) C(20)MO(10)MNb(1) 163.2(12) C(32)MO(14)MNb(2) 163.6(11) C(8)MO(6)MMg(1) 121.4(7) C(8)MO(6)MNb(1) 134.8(6) C(23)MO(11)MNb(1) 168.8(11) C(29)MO(13)MNb(2) 145.4(11) C(11)MO(7)MMg(1) 121.8(5) C(17)MO(9)MNb(1) 147.2(9) C(5)MO(5)MNb(2) 136.8(4) C(1)MO(1)MMg(1) 128.2(7) C(14)MO(8)MMg(1) 120.4(8) C(23)MO(11)MNb(1) 142.7(10) C(11)MO(7)MNb(2) 135.0(6) C(3)MO(3)MMg(1) 128.1(7) 1.872(8)–2.175(6) A ° for 1 and vary according to work is related to that of the structurally characterized Ta–Zn oxoisopropoxide, Ta4Zn2(m3-O)2(m-O)2I2(m-OPri)6(OPri)8.25 NbMOR(t)<NbMm-OR<NbMOAc.The magnesium– oxygen distances spread over the range 2.042(9)–2.096(4) A ° ; Similar observations, formation of diVerent species if the reaction is performed at room temperature or at reflux were the MgMOAc distances are slightly shorter than the MgMOR ones.These data are in accord with the observations made on reported by us for the Pb(OAc)2–Ti(OPri)4 system although the species obtained diVered more in their solubility prop- CdNb2(m-OAc)2(OPri)10.5 The non-bonding Mg,Nb distances are 3.369(8) A ° (av.) and thus reflect the smaller size of erties.26 In contrast to 3a, the oxoacetatoalkoxide Pb2Ti2O(OAc)2(OPri)8, obtained at room temperature, is not the central metal as compared to the Cd–Nb species.The p character of the terminal NbMOR bonds, a feature commonly modified by further heating, probably because the metals are already assembled by an oxo ligand. observed for early transition metals,24 is evidenced by the short NbMO bond distances [1.88 A ° (av.)] as well as by the large NbMOMC angles, 164.8(12)° (av.) and 144.1(10)° (av.) for the equatorial and for the apical NbMOR linkages, respectively.The various metals are six-coordinate but display a distorted surrounding, the distortion is the most severe for the central metal, magnesium, with OMMgMO angles ranging from 73.5(3) to 168.3(3)°. The distortion of the central atom toward a trigonal prismatic surrounding has also been observed for the related heterometallic species CdNb2(m- OAc)2(m-OPri)4(OPri)6 (Table 4) and BaNb2(m-OPri)4- (OPri)8(PriOH)2.8 Reactivity The reactivity of the mixed-metal acetatoalkoxides has been investigated (Scheme 2).Compounds 1 and 2 are quite stable with respect to further condensation by elimination of ester and no evolution is detected after ca. 20 h in refluxing toluene. The behavior of the lead system is more complex. In the case of 3a, FTIR monitoring has shown the formation of isopropylacetate (nasCO2=1745 cm-1) during heating along with Significant diVerences were found if the reaction between that of a new metallic derivative 4, characterized by nasCO2= Nb(OPri)5 and Pb(OAc)2 (151 stoichiometry) was performed 1586 cm-1 (the absorption bands corresponding to 3a pro- in refluxing toluene instead of at room temperature: the gressively decrease); 4, analyzing as Pb2Nb4O5(OAc)2(OPri)12, formation of another oxoacetatoalkoxide 5 is observed.In is more soluble than 3a probably as a result of a close structure contrast to 4, 5 is poorly soluble, the presence of broad with peripheral organic groups.Its crystallization could be absorptions bands in the range 800–650 cm-1 in its FTIR achieved in the presence of isopropyl alcohol. Unfortunately, spectrum suggests extended NbMOMNb bonds and thus a the thin needles obtained were unsuitable for single-crystal X- polymeric nature.27 Compound 5 also displays absorption ray diVraction studies. Low-temperature 1H NMR data indi- bands characteristic of nasCO2 and nsCO2 vibrations at 1547 cate three types of OR groups in a 45454 integration ratio and 1401 cm-1 respectively.These results may explain the and one type of acetate ligand. A structure of type A is in evolution of the solubility over time reported in the literature by refluxing Pb(OAc)2 and Nb(OEt)5 in the course of the accord with the overall spectroscopic data.The overall frame- J. Mater. Chem., 1997, 7(10), 2053–2061 2057Table 4 Selected bond lengths (A ° ) and angles (degrees) for Nb2Cd(m-OPri)4(m-OAc)2(OPri)6 2 Cd(1)MNb(1) 3.3820(8) Cd(1)MNb(2) 3.3962(8) Nb(1)MO(8) 1.991(4) Nb(1)MO(9) 1.878(4) Cd(1)MO(1) 2.260(4) Cd(1)MO(3) (2) 2.270(4) Nb(1)MO(10) 1.877(4) Nb(1)MO(11) 1.882(4) Cd(1)MO(5) 2.291(4) Cd(1)MO(6) 2.312(4) Nb(2)MO(2) 2.169(4) Nb(2)MO(5) 2.008(4) Cd(1)MO(7) 2.332(4) Cd(1)MO(8) 2.313(4) Nb(2)MO(7) 2.003(4) Nb(2)MO(12) 1.876(4) Nb(1)MO(4) 2.167(4) Nb(1)MO(6) 2.025(4) Nb(2)MO(13) 1.887(4) Nb(2)MO(14) 1.876(4) O(5)MCd(1)MO(1) 88.2(2) O(5)MCd(1)MO(3) 159.7(2) O(7)MCd(1)MO(6) 116.2(1) O(8)MCd(1)MO(6) 68.4(1) O(6)MCd(1)MO(1) 159.1(1) O(6)MCd(1)MO(3) 86.4(2) O(7)MCd(1)MO(5) 68.3(1) O(8)MCd(1)MO(5) 110.4(1) O(6)MCd(1)MO(5) 97.2(1) O(8)MCd(1)MO(1) 90.8(1) O(7)MCd(1)MO(6) 116.2(1) O(8)MCd(1)MO(7) 175.2(1) O(7)MCd(1)MO(3) 92.1(1) O(8)MCd(1)MO(3) 89.6(2) O(6)MNb(1)MO(4) 82.6(2) O(7)MNb(2)MO(5) 80.6(2) O(10)MNb(1)MO(8) 170.0(2) O(14)MNb(2)MO(2) 84.1(2) O(8)MNb(1)MO(4) 87.9(2) O(12)MNb(2)MO(2) 86.0(2) O(11)MNb(1)MO(6) 166.1(2) O(14)MNb(2)MO(7) 164.9(2) O(9)MNb(1)MO(6) 95.2(2) O(12)MNb(2)MO(7) 93.7(2) O(11)MNb(1)MO(9) 97.8(2) O(14)MNb(2)MO(13) 96.3(2) O(10)MNb(1)MO(6) 90.8(2) O(13)MNb(2)MO(2) 179.4(2) Nb(1)MO(6)MCD(1) 102.3(2) Nb(2)MO(5)MCD(1) 104.2(2) O(10)MNb(1)MO(9) 93.2(2) O(13)MNb(2)MO(7) 96.1(2) Nb(1)MO(8)MCD(1) 103.3(2) Nb(2)MO(7)MCD(1) 102.9(2) O(11)MNb(1)MO(4) 84.4(2) O(14)MNb(2)MO(5) 90.2(2) Nb(1)MO(4)MC(3) 138.8(4) Nb(2)MO(2)MC(1) 137.2(4) O(11)MNb(1)MO(8) 94.0(2) O(14)MNb(2)MO(12) 93.8(2) Nb(1)MO(6)MC(8) 127.2(4) Nb(2)MO(5)MC(5) 127.8(4) O(11)MNb(1)MO(10) 93.1(2) O(5)MNb(2)MO(2) 86.1(2) Nb(1)MO(8)MC(14) 137.0(4) Nb(2)MO(7)MC(11) 137.2(4) O(8)MNb(1)MO(6) 80.7(2) O(7)MNb(2)MO(2) 83.4(2) Nb(1)MO(9)MC(17) 137.0(5) Nb(2)MO(12)MC(26) 167.5(5) O(9)MNb(1)MO(4) 177.6(2) O(12)MNb(2)MO(5) 170.7(2) Nb(1)MO(10)MC(20) 150.9(5) Nb(2)MO(13)MC(29) 141.7(5) O(9)MNb(1)MO(8) 92.7(2) O(13)MNb(2)MO(5) 93.5(2) Nb(1)MO(11)MC(23) 155.6(4) Nb(2)MO(14)MC(32) 153.6(5) O(10)MNb(1)MO(4) 85.8(2) O(13)MNb(2)MO(12) 94.4(2) 2058 J.Mater. Chem., 1997, 7(10), 2053–2061The reactivity of BaNb2(OPri)12(PriOH)2 7 was also con- Fig. 1 Molecular structure of MgNb2(m-OAc)2(m-OPri)4(OPri)6 show- sidered (Scheme 3).Attempts to control its hydrolysis by ing the atom numbering scheme (ellipsoids at 20% probability) adding acetic acid in toluene at room temperature oVered poorly soluble species, one of them having being identified as Nb4O4(m-OAc)4(OPri)8 (nsCO2 1591, nMMOR 489, 410 cm-1) by comparison with an authentic sample.30 The reaction between 7 and lead acetate is governed by redistribution phenomena with extrusion of barium as barium acetate and formation of a Nb–Pb isopropoxide derivative, [PbNb2O(OPri)10]m 8, which was identified by comparison with an authentic sample obtained more directly by reacting lead iodide and KNb(OPri)6.BaNb2(OPri)12(PriOH)2 displays two solvating alcohol molecules, these may act as functional ligands since their acidity is enhanced by coordination thus allowing their reactivity toward labile metallic species.4,31 Trimethylsilylamides are attractive candidates for the introduction of another metal, the by-product being the volatile amine HN(SiMe3)2.Zinc trimethylsilylamide however was found to be inert toward BaNb2(OPri)12(PriOH)2 at room temperature in hexane. Heating promotes a reaction but zinc separates out from the MNb2(OAc)2(OPri)10 toluene, heat PbMgNb2O(OAc)2(OPri)10 6 Pb4O(OPri)6 toluene–hexane RT M = Mg ePriOH [Pb(OPri)2] M = Mg, Cd toluene–hexane RT [Ba(OPri)2] hexane RT no modification Pb2Nb4O5(OAc)2(OPri)12 4 1– 2 M = Pb toluene, heat –AcOPri reaction medium as insoluble zinc isopropoxide and the initial Scheme 2 Reactivity of the MNb2(OAc)2(OPri)10 (M=Mg, Cd, Pb) Ba–Nb species is recovered by adding isopropyl alcohol.Lead species trimethylsilylamide Pb[N(SiMe3)2]2 is more reactive. Its reaction with 7 at room temperature is evidenced by the discoloration of the reaction medium as well as by the presence in the 207Pb NMR spectrum of two species having chemical shifts preparation of a PNM ceramic.28 Allowing the formation of diVerent from lead isopropoxide derivatives known so far.The the mixed-metal species to proceed, prior to heating, can be major species, which could be isolated, corresponds to a crucial for the homogeneity of the system. derivative 9 containing three metals, Ba, Nb and Pb, and Terheterometallic alkoxides have been reported recently.29 characterized by a single peak at d 4059 in its 207Pb NMR The reactivity of some of the heterometallic species toward spectrum.Attempts to obtain crystals suitable for X-ray other metal complexes has been estimated. In view of the diVraction studies were unsuccessful as a result of its poor formulation of ternary oxides, MgNb2(OAc)2(OPri)10 1 seemed stability. In the presence of small amounts of isopropyl alcohol, to be a good candidate for such investigations.No reaction lead oxoisopropoxide, Pb4O(OPri)6, is obtained together with was observed between 1 and [Ba(OPri)2]2 or Pb4O(OPri)6 at the initial Ba–Nb species 7. The overall results indicate that room temperature, even after two days. The polymeric lead terheterometallic species based on a combination of metals of isopropoxide was more reactive and its dissolution was interest for materials are quite diYcult to stabilize and an observed at room temperature in toluene, 207Pb NMR spectroscopy was used to get some insight into the molecular composition of the reaction medium.Several signals are detected (d 4323, 3838, 3803 and 2591). The predominant species, characterized by the chemical shift at d 4323, could be isolated by crystallization (57% yield) and contains niobium, magnesium and lead. Analytical data imply the empirical formula PbMgNb2O(OAc)2(OPri)10 6.The proton NMR spectrum at room temperature indicates the presence of three types of methine groups in an integration ratio 45452. Lowtemperature spectra show a broadening but give no additional information. 13C NMR spectra however show five types of methine groups.The overall NMR data (1H, 13C and 207Pb NMR) are in accord with a structure of type B. This structure, in which all metals display their usual coordination numbers, namely six for magnesium and niobium, and five for lead, can be viewed as the association between 1 and a PbO moiety. It BaNb2(OPri)12(PriOH)2 7 [PbNb2O(OPri)10]m + Pri 2O 8 1–m –[Ba(OAc)2] –PriOH Pb(OAc)2 toluene, heat toluene AcOH Nb4O4(OAc)4(OPri)8 + poorly soluble species –Pb4O(OPri)6 ePriOH ePriOH 'PbBaNb...' 9 Zn[N(SiMe3)2]2 hexane [BaNb2(OPri)12]m 1–m [Zn(OPri)2] + + 2HN(SiMe3)2 heat RT Pb[N(SiMe3)2]2 is also supported by the facile dissociation of 6 in the presence Scheme 3 Reactivity of BaNb2(OPri)12(PriOH)2 (ePriOH stands for small amount) of trace amounts of isopropyl alcohol to give 1.J. Mater. Chem., 1997, 7(10), 2053–2061 2059appropriate set of ligands is required as in the case of the smooth hydrolysis of PbNb2(OAc)2(OPri)10, that of [PbNb2O(OiPr)10]m 8 in isopropyl alcohol proceeds with compound 6. immediate precipitation. The X-ray diVraction patterns after thermal treatment of the powder (obtained for h=30) show Hydrolysis–condensation reactions no PbNb2O6 but crystalline Pb3Nb4O13 at 600 °C together Hydrolysis–condensation reactions have been achieved for with PbNb4O11 at 750 °C.These observations suggest that various hydrolysis ratios h [h=[H2O]/[MM¾(OR)n+n¾] in THF segregation between the metals occurs at the early stages of or in the parent alcohol (0.1–0.01 M). Powders obtained for the hydrolysis–polycondensation process.This is also conlarge excess of water (h=30) have been analyzed by FTIR, firmed in the Mg–Nb–OR system. thermogravimetry (TG), diVerential thermal analysis (DTA) MgNb2(OAc)2(OPri)10 1, MgNb2(OEt)12(EtOH)2 10 and and X-ray diVraction (XRD) at various temperatures. MgNb2(OPri)12 are all potential precursors of MgNb2O6. Ligands such as carboxylates or b-diketonates are expected These molecules are fragments of a chain made up of to reduce the hydrolysis rates and to modify morphology three octahedra and are thus related to a multicompoand/ or size of the particles.14 The influence of the ancillary nent oxide which displays a columbite-type structure ligands on the properties of the final material can be estimated [(NbO6) (MO6) (NbO6)]2.Hydrolysis proceeds with the forwhen mixed-metal species having the same MM¾ stoichiometry mation of amorphous powders in all cases.As for the powder but a diVerent set of ligands are available. The Mg–Nb, Pb–Nb derived from the hydrolysis of 3a, the thermal elimination of systems, discussed here, and the Pb–Ti system reported prethe acetate ligand is complete below 500 °C for the powder viously26 oVer such opportunities. derived from acetatoalkoxide 1 as precursor [Fig. 3(a)]. The influence of the ancillary ligand, OR vs. OAc, can be Crystallisation starts around 500 °C and the MgNb2O6 phase illustrated with the Pb–Nb system. As expected, diVerential is obtained pure and well crystallized at 600 °C [Fig. 3(b)]. By hydrolysis is observed for the acetatoalkoxide derivatives and contrast, the crystallization only starts at ca. 700 °C for the the powders resulting from the hydrolyses indicate residual powder resulting from hydrolysis of 10 and quite high temperacarboxylate ligands (nasCO2 at 1572 and 1545 cm-1 for 1 and tures are required for the formation of the multicomponent 3a respectively). The hydrolysis of PbNb2(OAc)2(OPri)10 in oxide [Fig. 4(b)]. Partial retention of carboxylate ligands after isopropyl alcohol (0.1 M) gives a homogeneous clear solution the hydrolysis of 1, as shown by FTIR, assists the build-up of up to h=5. PbNb2(OAc)2(OPri)10 remains actually inert an ordered, crystalline material. The formation of sols of toward the addition of water up to h=2 in isopropyl alcohol nanosize particles over a large range of hydrolyses ratios (up (FTIR and 207Pb NMR).Fig. 2(a) shows representative TG to h=24) in isopropyl alcohol, which makes MgNb2(OPri)12 and DTA curves for the powder derived from the hydrolysis an interesting precursor for thin film applications, is however (large hydrolysis ratios) of 3a. A nearly continuous mass loss an unfavorable feature on account of crystallization and homois observed between 80 and 420 °C.This indicates that the geneity. Light scattering measurements have shown that the residual carboxylate ligands are burnt out at relative low sols are polydisperse [20 (88%) and 223 (12%) nm]. Electron temperatures. The DTA curve shows an exotherm around dispersive X-ray analysis (EDAX) by transmission electron 600 °C suggesting the formation of a crystalline material.This microscopy (TEM) indicate that the small particles are mixedis confirmed by XRD. The powders appear amorphous at metal species (Mg5Nb 152) whereas larger particles result room temperature but crystallize at 600 °C as the pure from metal segregation and more reorganization is thus neces- PbNb2O6 phase [Fig. 2(b)], traces of PbNb4O11 are observed sary.Ceramics, such as for instance CdNb2O6 and MgNb2O6 at 800 °C as a result of some loss of lead oxide. By contrast to Fig. 3 TG profile (a) and XRD patterns at various temperatures (b) Fig. 2 TG profile (a) and XRD patterns at various temperatures (b) for the powder resulting from the hydrolysis of PbNb2(OAc)2(OPri)10 for the powder resulting from the hydrolysis of MgNb2(OAc)2(OPri)10 2060 J.Mater. Chem., 1997, 7(10), 2053–20612 L. G. Hubert-Pfalzgraf, Polyhedron, 1994, 13, 1181; Mater. Res. Soc. Symp. Proc., 1992, 271, 15. 3 C. D. Chandler, C. Roger and M. J. Hampden-Smith, Chem. Rev., 1993, 93, 1205. 4 K. G. Caulton and L. G. Hubert-Pfalzgraf, Chem. Rev., 1990, 90, 969. 5 S. Boulmaa� z, R. Papiernik, L. G. 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Sheldrick, SHEL XS in Crystallographic Computing 3, ed. G. M. Sheldrick, C. Kruger and R. Goddard, Oxford University Press, Oxford, 1985, p. 175. 14 L. G. Hubert-Pfalzgraf, Chemical Processing of Ceramics, ed. B. I. L. Lee and E. J. A. Pope, M.Dekker, New York, 1994, ch. 2, p. 23; M. I. Yanovskaya, E. P. Turevskaya, V. G. Kessler, I. E. Obvintseva and N. Ya. Turova, Integrated Ferroelectrics, 1992, 1, 343. Fig. 4 TG profile (a) and XRD patterns at various temperatures (b) 15 F. Chaput, J. P. Boilot, M. Lejeune, R. Papiernik and for the powder resulting from the hydrolysis of MgNb2(OPri)12 L. G. Hubert-Pfalzgraf, J. Am. Ceram. Soc., 1989, 72, 1355. 16 O. Renoult, J. P. Boilot, F. Chaput, R. Papiernik, L. G. Hubert- Pfalzgraf and M. Lejeune, Ceramics T oday—T omorrow’s are already well crystallized at 600 °C, while temperatures Ceramics, Elsevier, London, 1991, p. 1991. >1000 °C are required for conventional routes.32 17 J. Livage, M. Henry and C. Sanchez, Prog. Sol. State Chem., 1988, 18, 259. 18 L. G.Hubert-Pfalzgraf, Appl. Organomet. Chem., 1992, 6, 627. Conclusion 19 G. B. Deacon and R. J. Phillips, Coord. Chem. Rev., 1980, 33, 227. 20 A. Mosset, I. Gautier-Luncau, J. Galy, P. Strehlow and Mixed-metal acetatoalkoxides can be obtained readily by H. Schmidt, J. Non-Cryst. Solids, 1988, 100, 339. ting niobium isopropoxide and anhydrous acetates 21 S. Daniele, L. G. Hubert-Pfalzgraf, J.C. Daran and R. Toscano, M(OAc)2 (M=Mg, Cd, Pb) at room temperature in non-polar Polyhedron, 1993, 12, 2091. solvents. The reactions are selective and proceed to the forma- 22 J. J. Dechter, Prog. Inorg. Chem., 1982, 29; R. K. Harris, tion of the smallest aggregates which allow the metals to J. J. Kennedy and W. McFarlane, NMR and the Periodic Table, ed. R. K. Harris and B.E. Mann, Academic Press, London, 1978. achieve their common coordination numbers regarding the 23 R. Papiernik, L. G. Hubert-Pfalzgraf, J. C. Daran and Y. Jeannin, steric demand of the alkoxide ligand. Niobium tends to be six- J. Chem. Soc., Chem. Commun., 1990, 695. coordinate and this is possible via simple addition products. 24 M. H. Chisholm, Chemtracts: Inorg. Chem., 1992, 4, 273. The experimental conditions, choice of solvent and temperature 25 S. Boulmaaz, L. G. Hubert-Pfalzgraf, S. Halut and J. C. Daran, of the reaction are important in determining the reaction J. Chem. Soc., Chem. Commun., 1994, 601. products. The system based on lead is the most reactive and 26 S. Daniele, R. Papiernik, L. G. Hubert-Pfalzgraf, S. Jagner and M. Hakansson, Inorg. Chem., 1995, 34, 628; L. G. Hubert-Pfalzgraf, condensation can be induced thermally. The acetate groups S. Daniele, R. Papiernik, M. C. Massiani, B. Septe, J. Vaissermann act always as assembling ligands as evidenced by the X-ray and J. C. Daran, J.Mater. Chem., 1997, 7, 753. structural data and by the diVerence in the stretching frequen- 27 L. G. Hubert-Pfalzgraf, M. Postel and J. G. Riess, Comprehensive cies nasCO2-nsCO2<200 cm-1. It assists the building up of Coordination Chemistry, Pergamon Press, London, 1987, ch. 34. an ordered, crystalline material in the course of the hydrolysis– 28 T. Fukui, C. Sakurai and M. Okuyama, J. Non Cryst. Solids, 1991, polycondensation process, thus favoring the formation of 134, 293. 29 M. Veith, S. Mathur and V. Huch, J. Am. Chem. Soc., 1996, 118, single-phase materials at quite low temperatures. 903; M. Veith, S. Mathur and V. Huch, J. Chem. Soc., Dalton T rans., 1996, 2485. The authors are grateful to the CNRS (GRECO) for financial 30 R. Papiernik, L. G. Hubert-Pfalzgraf and J. Vaissermann, to be support and Dr F. Chaput (Ecole Polytechnique, Palaiseau) published. for powder X-ray diVraction experiments. 31 B. A. Vaarstra, J. C. HuVman, W. E. Streib and K. G. Caulton, Inorg. Chem., 1991, 30, 3068; V. G. Kessler, L. G. Hubert-Pfalzgraf, S. Halut and J. C. Daran, J. Chem. Soc., Chem. Commun., 1994, 705. 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