Yttrium-iron(III) citric acid complexes with mole ratio Y : Fe = 1 or 3:5, potential precursors for YFe03 and Y3Fe50i2 thin films preparation, have been synthesized in ethylene glycol medium. The compositions, IR, 1H and 13C NMR spectral characteristics of the isolated solid complexes have been studied. Their Chemical nature and the peculiarities of the complexation process in the system Ym-Feni-citric acid-ethylene glycol = 1 : 1 : 8.5 : 35 have been studied and compared with those of the earlier studied lanthanide(III or IV)-titanium(IV) citric acid complexes, prepared under the same condi-tions. The mixed-metal nature of the obtained complexes has been shown. In contrast to the analogous lanthanide-titanium system, no deprotonation of the alcoholic OH groups of citric ligands takes place in the course of the formation of Y-Fe complexes

Dumanova, Desislava G.

Getsova, Miroslava M.

Todorovska, Rumiana V.

Todorovsky, Dimitr S.

Dimitr S. Todorovsky

Desislava G. Dumanova

Rumiana V. Todorovska

Miroslava M. Getsova

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ISSN-0011-1643CCA-2789 Original Scientific PaperPreparation and Characterization of Yttrium-ironCitric Acid ComplexesDimitr S. Todorovsky,a,* Desislava G. Dumanova,a Rumiana V. Todorovska,band Miroslava M. Getsovaaa Faculty of Chemistry, University of Sofia, 1, J. Bourchier Blvd.,Sofia 1126, Bulgariab Institute of Electronics, Bulgarian Academy of Sciences, 72, Tzarigradskoshosse, Sofia 1784, BulgariaReceived May 7, 2001; revised September 4, 2001; accepted October 26, 2001Yttrium-iron(III) citric acid complexes with mole ratio Y : Fe = 1 or3 : 5, potential precursors for YFeO3 and Y3Fe5O12 thin films prepa-ration, have been synthesized in ethylene glycol medium. The com-positions, IR, 1H and 13C NMR spectral characteristics of the iso-lated solid complexes have been studied. Their chemical nature andthe peculiarities of the complexation process in the system YIII–FeIII–citric acid–ethylene glycol = 1 : 1 : 8.5 : 35 have been studied andcompared with those of the earlier studied lanthanide(III or IV)-ti-tanium(IV) citric acid complexes, prepared under the same condi-tions. The mixed-metal nature of the obtained complexes has beenshown. In contrast to the analogous lanthanide-titanium system,no deprotonation of the alcoholic OH groups of citric ligands takesplace in the course of the formation of Y-Fe complexes.Key words: citric complexes, iron(III), lanthanides, titanium(IV), poly-merized complex method.INTRODUCTIONThe citric precursor technique is widely used (in aqueous or ethylene gly-col solutions) to produce a number of multicomponent, phase-homogeneousmaterials, including yttrium-iron garnet (Y3Fe5O12, YIG), other Ln3Fe5O12* Author to whom correspondence should be addressed. (E-mail: cvdt@chem.uni-sofia.bg)CROATICA CHEMICA ACTA CCACAA 75 (1) 155¿164 (2002)and Ln2LnFe5O12 (Ln, lanthanide),1,2 NiFe2O4,3 CoFe2O4,4 Ln2Ti2O7,5,6 etc.The authors pay attention to the precursors' composition but point to thenecessity of a much better knowledge of their chemical nature.5,6 Significantefforts have been made in the last years in this direction by studying thechemistry of the processes involved in BaTiO3,8,9 and Ln2Ti2O7,10–12 produc-tion in ethylene glycol (EG) medium by the polymerized complex method(PCM). However, the reasons for the citric ligands’ alcoholic OH group de-protonation, taking place in the course of the complexation process in (Ba orLn)–Ti–(CA)–EG systems (CA, citric acid) at an uncommonly low pH, havenot been fully elucidated. The role of the overall charge of the complexingagents in this process was also studied by substituting LnIII with CeIV inLn–TiIV–CA–EG system.11,13 It was interesting to compare these resultswith the ones obtained by substituting the other complexing agent (Ti4+), asa partner of LnIII, with another metal ion in oxidation state III, like Fe3+.On the other side, magnetic, magneto-optical and dielectric properties ofrare earth (Ln) ortoferrites and ferrite-garnetes determine their wide rangeof applications, including components of microwave and magneto-optic memorydevices, substrate for microwave compatible YBCO high-temperature super-conducting films, etc. A number of methods had been proposed for the prepa-ration of these materials in polycrystalline form and of YIG thin films. YFeO3films were also produced, but by pulsed laser deposition method only.14 Toour knowledge, only one paper deals with the spray pyrolysis method as atool for YIG film deposition using a solution of Y- and Fe-acetilacetonates.15We explore the possibility preparing of YIG and YFeO3 films by spray pyroly-sis using EG solutions of Y-Fe citric complexes as starting material, whilesubmicron YIG films (on yttrium-aluminum garnet substrate) of a rathergood quality have been already produced.16 Such a relatively new field ofapplication of bimetallic citrates, obtained in (and containing) EG, also re-quires a knowledge of their chemical nature.In the present paper, the results are reported on the synthesis and char-acterization of two types of Y-FeIII citric complexes (potential precursors forYFeO3 and YIG production), prepared in EG medium.EXPERIMENTALMaterialsAnhydrous citric acid (CA, extra pure), Y(NO3)3  6H2O (lab), FeCl3 (for synthe-sis) (Merck) were used. The other reagents used were of p.a. grade. The exact con-tent of Y and Fe in the respective compounds was determined complexometrically.156 D. S. TODOROVSKY ET AL.Synthesis of the ComplexesCA was dissolved in EG at 35 °C. Y(NO3)3  6H2O or/and FeCl3 were added to thesolution so as to enable the mole ratio Y3+(Fe3+) : CA : EG = 1.0 : 4.3 : 17.6 and Y3+ :Fe3+ : CA : EG = 1.0 : 1.0 : 8.5 : 35.0 or 1.00 : 1.67 : 11.30 : 46.50 to be adjusted. Themetal(s) : CA : EG ratio was analogous to the one used in the system (Y3+ or/andTi4+)–CA–EG, applied for Y2Ti2O7 production5 and accounting for the difference inthe oxidation states of TiIV and FeIII. The obtained solutions were agitated in a ther-mostatically heated vessel at (110  3) °C (120 °C for the monometallic Y-compound).After cooling to room temperature, acetone was added as a desalting agent in a vol-ume 20 times as large as the one of the solution. After 24 h, the obtained precipitatewas filtered, washed with acetone, dried at room temperature and stored in a tightlyclosed vessel. No precipitate was obtained by this procedure in the Fe3+–CA–EG systemthough that (according to IR spectrum) a complexation process took place.AnalysisThe content of C and H in the obtained substances was determined by the com-mon organic analysis method. The metals were determined complexometrically mak-ing use of the significant difference in the stability constants of their complexes withEDTA. The relative standard deviations of the latter procedure (derived from 4 par-allel analyses of the model solution, simulating the real one) were 0.9% for Fe and2.4% for Y. IR spectra (4000–400 cm–1) were recorded by a Specord 75 (C. Zeiss, Ger-many) spectrometer in the capillary layer for the solutions and in nujol mulls or KBrpellets for solid substances. 13C and 1H NMR spectra were taken using a Bruker 250MHz spectrometer. The amount of water was determined thermogravimetrically witha Paulik-Paulik-Erdey (MOM, Hungary) derivatograph. X-ray powder diffractometrywas performed using a TUR-M62 (Germany) diffractometer at Cu-K, 34 kV, 15 mA,0.05 ° / 2 s.RESULT AND DISCUSSIONComposition and Some Spectral Characteristics of Isolated ProductsThe elemental composition, water content, mass loss during the heatingto 1000 °C of the isolated products as well as data on the ligands' esterifica-tion and the relative content of EG, derived from NMR spectra, are shownin Table I. These data along with the ones from IR spectral analysis lead tothe following formulae of the complexes:YHCitROH2– (HCitROH)2R0.22– HCitROHRHCit0.23– (H2O)  0.1HOROH  3H2O (for the Y complex, labeled later in the text as Y120),11 where R =(CH2)2, HCit = OOCCH2C(COO)(OH)CH2COO;YFe(HCitHROH)2– (HCitH)1.52– (HCitROH)0.52–  0.4HOROH  8H2O (for the1 : 1 Y-Fe complex, labeled as YFe110) andYTTRIUM-IRON CITRIC COMPLEXES 157Y0.6Fe(HCitHROH)1.8– (HCitH)1.42– (HCitROH)0.12– (H2O)3  0.5HOROH  6H2O(for the 3 : 5 Y-Fe complex, labeled as 3Y5Fe110).The coincidence between the experimentally found values for the com-plex composition and the calculated ones according to the above formulae iswithin the limits of 5%.A broad band with a maximum at 3350 cm–1 in the IR spectra (Figure 1)confirms the presence of hydrate H2O in all of the separated complexes. Thenon-dissociated OH groups of citric ligands and those from their adducts withEG also contribute to the 3200–3400 cm–1 band intensity.The complexation is proven by the bands of as(COO–) at 1595 cm–1 andof s(COO–) at 1440 and 1385 cm–1 (Figure 1). The band positions are thesame as in the analogous Y-Ti citrate complex spectrum11 and suggest thatCOO– groups are monodentate coordinated. The slight asymmetry of the ab-sorption maximum in Y-Fe citrate spectra, assigned to the asymmetric stretch-ing vibration, to the lower wavenumbers shows that the bridging type of co-ordination may also take place. The 13C NMR peak at 79.7 ppm (Figure 2) isassigned5 to the quaternary alcoholic carbon in citrate ligands. The shift tohigh frequency in comparison with pure CA ( 75 ppm)6 is due to the coordi-158 D. S. TODOROVSKY ET AL.Figure 1. IR spectrum of Y-citrate (1, nujol mull) and of Y-Fe citrate with the moleratio Y : Fe = 1 (2, KBr pellet) and Y : Fe = 3 : 5 (3, nujol mull).nation of OH to metal ions. The signal at 183.0 ppm can be attributed to thecentral COO– group.13 Formation of a five-membered ring when the OH andCOO– groups attached to the central carbon of a citrate ligand coordinate tometal ion is characteristic of Fe- and Ni citric complexes.17,18 Consideringthat terminal groups of CA resonate about 176 ppm13 and that the corre-sponding carboxylate anion has a shift of  5 ppm to the high frequencies,19we assigned the peak at 179.8 ppm to terminal COO– groups coordinated tometal ions. The shift at 174.8 ppm can be caused by protonated and esterifi-cated terminal carbonyl groups, whose signals are overlapped. The presenceof EG, bonded as adduct, is confirmed by the IR band at 1040 cm–1 (Ref. 20)(Figure 1) and by the chemical shifts at 65 ppm and 3.65 ppm in the 13C and1H NMR spectra,10 respectively (Figures 2, 3).The resonance signals at 62 and 69 ppm in 13C- and 3.8 and 4.2 ppm in1H NMR spectra are associated with the CH2 groups of esterificatied EG.10The peak at 4.35 ppm appears only in the Y-complex 1H NMR spectrum andYTTRIUM-IRON CITRIC COMPLEXES 159Figure 2. 13C NMR spectrum of Y-Fe citrate (mole ratio Y : Fe = 1).160 D. S. TODOROVSKY ET AL.TABLEICompositionofisolatedproductsaProductsymbolbHCYFeH2OResidueafterburningto1000°CcMoleratio,accordingtoNMRdatadHOROH/CitROH/CitR/CitY1204.37(4.43)29.96(31.45)15.30(15.52)––8.7(9.4)19.3(19.7)0.07(0.06)0.87(0.88)0.11(0.12)YFe1104.00(4.20)29.30(30.20)7.25(7.50)4.16(4.71)11.0(12.1)15.9(16.3)0.10(0.10)0.62(0.63)0(0)3Y5Fe1104.91(4.98)28.60(29.11)5.35(5.25)5.34(5.49)16.0(15.9)15.2(14.3)0.15(0.15)0.58(0.58)0(0)aFound(calc.)/%.Calculationsbasedontheproposedformulae.bSeethetext.cY2O3,YFeO3andYIGaretakenasfinalproducts.dCit=CH2COOC(O)COOCH2COO,R=(CH2) 2.TABLEIISomemaincharacteristicsofisolatedcomplexesProductsymbolaLbAveragechargeoftheligandsDestc,dL–/LeY1201.801.671.03(1.12)0YFe1102.001.501.24(1.25)03Y5Fe1102.061.450.93(0.93)0YTi120f1.502.332.56(2.52)0.6Ce4Ti100g1.502.671.96(2.00)1.0aSeethetext.bL=numberofligands,relatedtoonemetalion.cDest=(ROH+2R)/L,R=(CH2) 2.dFound(calc.);calculatedaccordingtotheformulaepresentedinthetextorintherespectivepapers.eL–=numberofligandswithdeprotonatedalcoholicOHgroup,relatedtoonemetalion.fYIII -TiIVcitrate,preparedat120°C;11YTiHCitROHRHCit0.63–CitROH1.63–(CitROH) 2R0.14–(H2O)0.7HOROH5H2O.gCeIV-TiIVcitrate,preparedat100°C;13CeTi(HCitROH) 0.12–(CitROH) 2.63–(H2O)0.1HOROH4.7H2O.is caused by methylene protons of EG, bonded as ester with the central COOHgroup of citric ligands.11 The intensity ratios of the cited proton signals andthat of the CH2 in CA at 2.7–3.1 ppm allow the degree and the mode ofesterification (with one or with both of the EG OH groups) to be determined(Table I). The formed esters exhibit an IR absorption maximum at 1735 cm–1(	(C=O)), strongly overlapped by the ones of the adduct (1725 cm–1) and ofthe free COOH (1710 cm–1). The band near 1200 cm–1 (Figure 1) is due tothe C–O stretching vibration modes in the ester (1195 cm–1) and in theadduct (1225 cm–1).20The isolated products are X-ray amorphous. The final products of theirheating to 1000 °C are Y2O3, YFeO3 and YIG, as confirmed by the mass loss(Table I) and X-ray powder diffractometry.YTTRIUM-IRON CITRIC COMPLEXES 161Figure 3. 1H NMR spectrum of Y-Fe citrate (mole ratio Y : Fe = 1).Chemical Nature of the Yttrium-iron CitratesOne of the main questions arising is whether a mixed-metal complex ora mixture of unimetallic complexes is formed. The problem has been alreadydiscussed in the literature considering the analogous systems precursors ofa number of high-quality materials produced by the PCM. The comparativestudy carried out for the respective monometallic and bimetallic Y-Ti citratecompounds11 shows, fairly definitely, the mixed-metal nature of the latter.Due to our failure to obtain a solid Fe citrate, comparison could be madeonly between Y- and Y-Fe complexes. At the same time, this failure is the evi-dence, although not fully undoubtedly, that the bimetallic precipitate couldnot be a mixture of complexes. A few other facts support this supposition:(i) the shift at 4.35 ppm observed in the Y-complex 1H NMR spectrum doesnot exist in the bimetallic complex spectrum; (ii) the ratio of the 3.8 and 4.2 ppmsignal intensities clearly shows that the bimetallic product EG does not formesters through both of its OH groups simultaneously. This type of esterifica-tion is found11 in Y complexes (Table I), even in that prepared at 40 °C; (iii)the thermal decomposition of the Y- and Y-Fe citrate complexes proceeds indifferent ways.21Along with the obvious analogy between Y-Ti and 1:1 Y-Fe citrates, thereare two significant differences: in the number of ligands and in the lack ofligands with deprotonated alcoholic OH groups in Y-Fe complexes. In con-trast to analogous Y-Ti,11 CeIV-Ti,13 La-Ti10 citric complexes, no 13C resonancesignal of quaternary carbon in CA with dissociated alcoholic OH group existin the NMR spectrum (Figure 2). (In the earlier studied complexes the citedsignal appeared at 91 ppm). Therefore, the statement5 that the deprotona-tion of the alcoholic OH group proceeds only in the formation of a mixed-metal citric complex cannot be related to Ti-free systems. Furthermore, itwas shown11 that the alcoholic OH deprotonation could take place in the courseof the monometallic complex formation, as well. Thus, it seems that the abovementioned statement5 has to be ignored. The present results fully correspondto the concept11 that the OH deprotonation is one of the few competitive pro-cesses that take place in the course of complexation, including also adductformation, esterification and complexation through deprotonated COOHgroups. Balance between these processes depends on the nature of the com-plexing agent(s) (oxidation state, i.e. overall charge, catalytic activity on theesterification), temperature of the synthesis, nature of the ligands and thelimitations introduced by the steric factor. All of the results obtained todate10,11,13 support the supposition that the OH deprotonation may becomean energetically advantageous process when a high degree of esterificationcreates an insufficiency of COOH groups available for coordination and thisinsufficiency cannot be avoided by the increase of the ligand number. The162 D. S. TODOROVSKY ET AL.increase of the overall charge of the complexing agents in CeIV-TiIV complex,compared to the LnIII-TiIV ones, causes a sharp increase of the relative con-tent of ligands with a deprotonated OH group (Table II). In contrast, the ratherlow degree of esterification in Y-Fe citrates, along with the higher ligand num-ber than in the analogous Y-Ti citrates, makes a deprotonation of OH groupsunnecessary – more than enough COOH groups are available for coordina-tion. The higher ligand number in Y-Fe citrates could be due to their smallereffective size, as a result of the lower esterification degree.The fact that the alcoholic OH group deprotonation is observed (so far,at least) in TiIV-containing systems only, suggests a possible catalytic func-tion, especially of Ti4+ ions in this process. However, the deprotonation wasnot observed in the YIII–TiIV–CA–EG system heated at 40 °C.11 Even if thedeprotonated OH groups also exist as traces in the thus obtained product, itis clear that such a catalytic effect could not be the sole reason for the dis-cussed deprotonation.CONCLUSIONThe reported data suggest that mixed YIII-FeIII citrates are formed at100 °C in the Y3+–Fe3+–CA–EG systems but their composition and the wayof bonding are different from those of the analogous YIII-TiIV bimetallic citriccomplexes, prepared under similar conditions. The reported results contrib-ute to further elucidation of the chemistry of the processes taking place inthe application of the polymerized complex method and especially of the me-chanism of deprotonation of the citric ligands alcoholic OH group.The Y : Fe mole ratios in the prepared complexes are rather close to thedesired ones and, from this point of view, they are suitable starting materi-als for YFeO3 and YIG film depositions.Acknowledgment. – The study was financially supported by the Sofia UniversityScience Foundation.REFERENCES1. V. K. Sankaranarayanan and N. S. Gajbhiye, Thermochim. Acta 135 (1989) 337–341.2. V. K. Sankaranarayanan and N. S. Gajbhiye, J. Am. Ceram. Soc. 73 (1990) 1301–1307.3. S. Prasad and N. S. Gajbhiye, J. Alloys Compd. 265 (1998) 87–92.4. N. S. Gajbhiye, S. Prasad, and G. Balaji, IEEE Trans. Magn. 35 (1999) 2155–2161.5. M. Kakihana, M. Milanova, M. Arima, M. Yoshima, and M. Yoshimura, J. Am.Ceram. Soc. 79 (1996) 1673–166.YTTRIUM-IRON CITRIC COMPLEXES 1636. M. Milanova, M. Kakihama, M. Arima, M. Yashima, and M. Yoshimura, J. AlloysCompd. 242 (1996) 6–10.7. N. S. Gajbhiye and S. Prasad, Thermochim. Acta 285 (1996) 325–336.8. M. Kakihana, M. Arima, Y. Nakamura, M. Yoshima, and M. Yoshimura, Chem.Mater. 11 (1999) 438–450.9. M Kakihana and M. Yoshimura, Bull. Chem. Soc. Jpn. 72 (1999) 1422–1443.10. M. Milanova, M. Arnaudov, M. Getsova and D. Todorovsky, J. Alloys Compd. 264(1997) 95–103.11. M. M. Getsova, D. S. Todorovsky, and M. G. Arnaudov, Z. Anorg. Allg. Chem. 626(2000) 1488–1492.12. M. M. Getsova and D. S. Todorovsky, J. Mater. Sci., in the press.13. M. M. Getsova, D. S. Todorovsky, and I. Wawer, unpublished work.14. R. Krishnan, A. Lisfi, M. Guyot, and V. Cagan, J. Magn. Mater. 147 (1995) L221–L224.15. J. L. Deschanvres, M. Langlet, M. Labean, and J. L. Jombert, IEEE Trans. Magn.26 (1990) 187–189.16. R. V. Todorovska, St. Groudeva-Zotova, D. S. Todorovsky, G. Tzvetkov, and P. Ste-fanov, unpublished work.17. T. Fujita, Chem. Pharm. Bull. 30 (1982) 3461–3465.18. J. Strouse, S. W. Layten, and C. E. Strause, J. Am. Chem. Soc. 99 (1977) 562–571.19. G. S. Levy, R. L. Lichter, and G. L. Nelson, Carbon-13 Nuclear Magnetic Reso-nance Spectroscopy, Wiley, New York, 1980, p. 59.20. M. Arnaudov, M. Getsova, D. Todorovsky, and M. Milanova, Anal. Lab. 7 (1998)70–74.21. D. S. Todorovsky, R. V. Todorovska, and St. Groudeva-Zotova, Mat. Lett., in thepress.SA@ETAKPriprava i karakterizacija kompleksa itrija i `eljezas limunskom kiselinomDimitr S. Todorovsky, Desislava G. Dumanova, Rumiana V. Todorovskai Miroslava M. GetsovaPripravljeni su kompleksi itrija i `eljeza(III) s limunskom kiselinom u etilen-gli-kolnom mediju, kao mogu}ih prete~a za pripravu YFeO3 i Y3Fe5O12 tankih filmova.Kompleksi su ispitani IR te 1H i 13C NMR spektroskopijom. Rezultati dobiveni zasustav YIII–FeIII–citronska kiselina–etilen glikol = 1 : 1 : 8.5 : 35 uspore|eni su s ra-nije dobivenim rezultatima za mje{ovite komplekse lantanoida(III i IV) i titanija(IV) slimunskom kiselinom, koji su pripravljeni pod istim uvjetima. Za razliku od kom-pleksa lantanoida i titanija, pri stvaranju kompleksa itrija i `eljeza ne dolazi do de-protoniranja alkoholne OH skupine u ligandu.164 D. S. TODOROVSKY ET AL.

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Preparation and Characterization of Yttrium-iron Citric Acid Complexes

Preparation and Characterization of Yttrium-iron Citric Acid Complexes

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