摘要:
Gas-phase, liquid and solid complexes in the POC13-FeC13 system 7Soghomon Boghosian,* George A. Voyiatzis and George N. PapatheodorouInstitute of Chemical Engineering and High Temperature Chemical Processes (ICEIHT-FORTH) andDepartment of Chemical Engineering, University of Patras, PO Box 1414, GR-26500 Patras, GreeceRaman and UV/VIS spectra were obtained at temperatures up to 625 K for the gas-phase complex formed overPOC1,-FeCl, molten mixtures under static equilibrium conditions. Raman spectra were also measured formolten POC1,-FeCl, salt mixtures. A comparison of the spectral features of the POC1,-FeCl, vapours withthose of the POC1,-FeCl, molten mixtures at 525 K indicates that the gas-phase complex has a 1 : 1stoichiometry (POCl,.FeCl,) with characteristic vibrational bands at 95, 362, 530, 121 8 and 1268 cm-l.Thedata indicate a C,, symmetry for the POCl,=FeCl, complex. The energies of the M+L charge-transfertransitions in the electronic absorption spectra of the POCl,*FeCl, gas-phase complex suggest, in agreementwith the Raman data, that complexing occurs through oxygen bridging. The 1 : 1 POCl,.FeCl, molecular liquidcomplex is the predominant species in equilibrium with POCl, and iron chloride at temperatures around 500 K.At temperatures below 450 K and in POC1,-rich mixtures the '3 : 2' ionic liquid compound [Fe(POCl,),]-[FeCl,], was formed at the expense of POCl,-FeCl,(l). Two solids were identified at room temperature,yellow POCI,~FeCl, and red [Fe(POCl,),][FeCl,],, and their Raman spectra have been recorded.The action of phosphoryl chloride as donor molecule towardsinorganic metal halides leading to low-melting complexcompounds is well known and a detailed summary of thechemistry of these complexes is available.' Formation of gas-phase complexes between POCl, and metal halides hasattracted significant interest for removal of aluminium chloridefrom Friedel-Crafts mixtures, while such complexes with ZrC1,and HfC1, have been investigated due to their potential use forseparation of zirconium from hafnium in nuclear reactormaterials by distillation.' However, not much work has beendirected towards the determination of the structural characteris-tics of these complexes in the gas phase.On the contrary, thevibrational and structural properties of a large number ofliquid- and gas-phase metal halide complexes where AlCl,,GaCl,, InCl, and FeCl, act as complexing agents have beene~tablished.~,~ Owing to the high reactivity of these complexesand complexing agents with moisture their handling forspectroscopic and thermodynamic measurements is ratherdifficult.However, with the use of sealed fused-silica containersand glove-box techniques, Raman spectroscopy has been usedfor determining the thermodynamic and/or structural propertiesof the gas-phase complexes POCl,=ZrC1,,4 POCl,~HfC1,,4~5POCl,*GaC1,6 and POC1,.AlC1,.6,7Complex formation of phosphoryl chloride with the 'acidic'iron(m) chloride has been the subject of a few earlyinvestigations. Treatment of FeC1, with an excess of POCl, at95 "C and subsequent cooling gave water-soluble red crystals ofthe 2 : 3 complex (2FeC1,.3POCl3, m.p.98 "C) which lost POCl,at 50°C in uacuo giving yellow-brown crystals of the 1: 1adduct.' A high percentage of the 1 : 1 adduct in the vapourphase from sublimation of the POCl,*FeCl, compound wasreported. Ultraviolet absorption spectra of a dilute solution ofFeCl, in POCl, suggested the presence of FeC1,- and providedevidence for self-ionization of the solvent. lo However,electrolysis of a radioactive "FeCl, complex with POCl,indicated that iron was present in both cationic and anionicspecies and that P0C12+ ions were not present."7 Supplementary data available (No. SUP 57153, 2 pp.): molarabsorption coefficients of the gas-phase complex POCI,*FeCI,. SeeInstructions for Authors, J.Chem. SOC., Dalton Trans., 1996, Issue 1.Non-SZ unit employed: atm = 101 325 Pa.The aim of the present work is the spectroscopiccharacterization of a yellow-green gas-phase complex formed inthe iron(m) chloride-phosphoryl chloride system (Fe-0-P-Cl)under static equilibrium conditions according to equation (1).Raman and electronic absorption spectroscopy are used attemperatures up to 625 K to determine the vibrationalproperties of the complex formed as well as the type of bondingand Fe-Cl interaction. The vibrational frequencies of thecomplex(es) formed in the vapour as well as in the liquid statewere determined. The electronic absorption spectra and molarabsorption coefficients of the gas-phase complex weremeasured. The data are discussed in terms of possible structuresin the vapour and/or the liquid phase.Finally, liquid and solidcomplexes in both molecular and ionic form were found tooccur in the temperature range 300-450 K and at liquid-nitrogen temperature.ExperimentalHigh-purity anhydrous FeCl, was prepared from thecorresponding Cerac/Pure Inc. reagent by repeated slowsublimations in fused-silica tubes under vacuum. It was handledin a nitrogen-filled glove-box having a water vapour content ofless than 2 ppm. Liquid POCl, was obtained from Fluka.The Raman cells containing the melt mixtures consisted offused-silica tubing (outside diameter 4 k 0.1 mm, insidediameter 2 k 0.1 mm, FZ 3 cm long). Those containing vapourmixtures comprised silica tubing of outside diameter 20 k 0.2mm, and inside diameter 18 k 0.2 mm. Preweighed amounts ofFeCl, were transferred into clean, dry, flamed and degassedoptical cells of known volume.The cells were then attached toan all-glass vacuum line to which a container with liquid POCl,was also connected. Phosphoryl chloride was then allowed tovaporize and occupy a confined bulb of known volume and wastransferred by distillation and condensed in the bottom of theoptical cell which was immersed in liquid nitrogen. The opticalcell was then sealed under vacuum, the chemicals were allowedto react inside a side tube (outside diameter 6 k 0.1 mm, insidediameter 4 k 0.1 mm), vapour transported into the main cellJ.Chem. Soc., Dalton Trans., 1996, Pages 3405-3410 340compartment and finally the side tube was sealed off. In thisway high-purity samples were obtained.Raman spectra were excited with the 514.5 and 488.0 nm linesof a Spectra Physics 164 argon-ion laser and the 647.1 and 676.4nm lines of a Spectra Physics 2020 krypton-ion laser. Thescattered light was collected at an angle of 90" (horizontalscattering plane) and analysed with a Spex 1403,0.85 m doublemonochromator equipped with a -20°C cooled RCAphotomultiplier and EG&G/ORTEC rate meter and photon-counting electronics. The optical furnace used and theprocedures for obtaining Raman spectra at high temperatureshave been described in detail el~ewhere.'~,'~ It should bepointed out that use of the blue to green argon-ion laser lines forexciting gas-phase Raman spectra resulted in the formation of ayellow-brown deposit at the focusing point of the laser beam inthe optical cell.As explained in detail elsewhere l4 this is due toabsorption of the laser by the Fe-containing coloured vapoursand the photodissociation (2), and thus the 'rotating cellFeCl,(g) e FeCl,(s) + Cl,(g) (2)te~hnique"~ was used for obtaining spectra with the argon-ionexcitation lines.Rectangular fused-silica cells of 1 or 0.5 cm path length(Hellma, Mullheim, Baden, Germany) were used for theUV/VIS measurements of the vapours. The cells were degassedand filled with preweighed amounts of FeCl, in the glove-box.Gaseous POCl, was added to the cells using the vacuum-gas-addition line system and trapped with liquid nitrogen asdescribed above.The absorption measurements were performedwith a Hitachi U-3000 spectrophotometer equipped with ahigh-temperature optical furnace. ' During the experiments theportion of the cell interrupting the optical path was at atemperature 2-3 "C higher than the rest of the cell. Thisprevented condensation of solids or liquids in the optical pathand permitted absorption measurements of vapours inequilibrium with condensed phases. Spectroscopic methods forinvestigating gaseous equilibria involving complexes have beendescribed previously. l 6Results and DiscussionRaman spectra of POCI,-FeCl, gas- and liquid-phase mixturesFig. 1 shows representative Raman spectra of POC1,-Fe2Cl,vapours in equilibrium with a 1: 1 POC1,-FeCl, moltenmixture at 550 and 625 K.The Raman spectra of gaseousphosphoryl chloride and iron chloride vapours l4 are wellknown. At ca. 500 K iron chloride vapours consist of dimericFe2C16, while at ca. 625 K traces of monomeric FeC1, can bedetected in the Raman spectra because of the dimerdissociation. l4 The spectra in Fig. 1 consist of a superpositionof bands due to POCl,(g) and Fe,Cl,(g) plus some extra bandsC attributed to a new gas-phase complex. The data indicate thatthe complex vaporizes dissociatively. Five bands are assigned toit and their wavenumbers and polarization characteristics aresummarized in Table 1. Several other bands due to the complexmight have been obscured by those of the component gases.It is noteworthy that when using the 514.5 and/or 488.0 nmexcitation lines of the argon-ion laser no bands due to thecomplex could be observed.Absorption of the laser line bycoloured vapours, the electronic absorption bands of whichoverlap with the energy range of the excitation lines used, isknown to lead to increased local spectroscopic temperatures orphotodissociation of gas-phase c ~ m p l e x e s . ~ , ~ , ' ~ As will bedemonstrated in the next section, such an overlap occurs for theFe-0-P-C1 gas-phase complex. Absorption of the argon-ionlaser lines by the complex molecules resulted in higher sampletemperatures along the beam than indicated by the furnaceC- i1 I I I 11300 1100 600 400 200 0Raman shift/cm-'Raman spectra of vapours over a 1 : I POC1,-FeCl, mixture at550 and 625 K.Bands due io the gas-phase complex are-marked C.h, = 647.1 nm; laser power, w = 200 mW; spectral slit width, S.S.W. =6 cm-'; time constant, z = 1 s; scan speed, S . S . = 30 cm-' min-'. VVand HV denote the vertical-vertical and horizontal-vertical spectrapolarizations, respectivelycontroller and thermal dissociation of the complex to itscomponents occurs. Furthermore the 647.1 nm krypton-ionlaser line used for exciting the Raman spectra of vapours (Fig.1) overlaps with the tail of the electronic absorption bands ofthe gas-phase complex, leading to a probable preresonanceenhancement of the complex band intensities. Thus, as pointedout in ref. 13, the relative Raman intensities of vapours cannotbe used for determining the stoichiometry and thermodynamicsof reaction (1).The 1 : 1 mixture could be sublimed across a temperaturegradient giving yellow needle-shaped crystals, which weretransformed into a dark brown-red liquid melting at ca.160 "C.The gas-phase complex is presumably POCl,*FeCl,(g) inagreement with what has been established for other POCl,-MCl, (M = A1 or Ga) systems and according to a proposal ofSUVO~OV,~ who reported a high percentage of POCl,~FeCl,(g)in the gas phase over POC1,-FeC1, mixtures. More informationabout the number of bands due to the complex can be obtainedby considering the spectra from liquid-state samples.Several POCl,-FeCl, molten mixtures with P : Fe ratios of5 : 1,2 : 1, 3 : 2, 1 : 1 and 1 : 2 were placed in cells.A careful studyof the concentration dependence of the spectral features at 525K indicates that complexation occurs and that the compoundformed most probably has a 1 : 1 stoichiometry, POCl,*FeCl,.This can be deduced by comparing the relative intensities of theRaman spectra of POC1,-FeCl, molten mixtures with 2 : 1, 1 : 1and 1 : 2 compositions (Fig. 2). The assignment of the observedbands and their polarization characteristics are given in Table 1.In POC1,-rich melts (P: Fe = 2: 1) three bands can be assignedas due to the vl, v2 and v3 POCl,(l) modes,I7 while in FeC1,-rich melts (P : Fe = 1 : 2) the spectra are dominated by the liquidiron chloride modes.'* Absence of bands due to ionic speciessuch as FeC1,- from the spectra of Fig.2 points to a molecularnature for the POCl,-FeCl, liquid compound at 525 K. Therelative band positions and the polarization properties forPOC1,-FeCl, are similar to those of the previously studiedPOCl,-GaC1,6 and POC1,*A1C1,6*7 gas-phase and molecularliquid complexes.Bonding in the gas-phase and liquid POCl,-FeCl, adductpresumably occurs via a P-0-Fe bridge, as in a series of known3406 J. Chem. SOC., Dalton Trans., 1996, Pages 3405-341Table 1 Observed vibrational Raman wavenumbers (cm-') for POCl,-FeCI, molten mixtures (Figs. 2,4 and 5) and gaseous POC13-FeC13 (Fig. 1)"POC1,-FeCI, liquid mixturesP:Fe = 5:l300 K114s (dp)134w (dp)191s (dp)x 210 (sh) dp268m (PI297m (P)33WP)356m (PI385w486vs (p)521m (P)P:Fe = 2:l360 K' 525 K 525 KP:Fe = I : ]97s (dp) 97s (dp) 97m (dp)127w (sh) dp 127w (dp)1 14m (dp)132 (sh)179w (p) 179vw (br)202m (dp) 202m (dp) 201w (dp)268vw (p) 268vw (p) 268vw (p)305m (p) 305m (p) 305m (p)332m (PI362s (p) 362s (p) 362s (p)x404w (p) 403m (p)x 420 (br) p 420w (dp)486w (p) 486m (p) 486w (p)521m (P)537m (p) 537m (p) 537m (p)583w (br) dp 583vw (p)P:Fe = 1:2 POC13-FeC13(g)525 K Assignments 625 Kx 70w (dp) 'FeCl,' 95m (dP)97vw (br) dp POC13*FeC13 (E)126w (br) dp POCl,-FeCl, (E)'2 (FeC14 - ) , CFe (poc13) 61 [ FeC14] 3POC13~FeCl, + [Fe(POC13)6][FeC1,]3v4(FeC14 -1, cFe(POCl,)6ICFeCl,I,POCI3.FeCl3 (A,)POC13~FeC13 (E)v6(poc13),v5(Fe06)2 CFe(POC13)61[FeC14]3V3(POCI3), A,[Fe(P0C13)61 CFeC141 3304w (p) POC13~FeC13 (A,)-1, CFe(POC13)6][FeC14]31(Fe06), [Fe(POC13)61[FeC141 3362m (p) v(Fe-Cl), POC13.FeC13 (A,)405s (PI 'FeCl,'[Fe(P0C13) 6 1 CFeC141 3362s (p) [v(Fe-Cl)].I _ x625vw (br) x625vw (br) 618vw 6;) x615vw (br) POCl,-Fkl, (E)1199w (p) 1199w (p) 1194w (br) (p?) 1192vw (br) (p?) POC13-FeC13 (A,) 1218w (p) [v(P-O)]1268w (dp) 1268w (dp) 1267vw (dp) 1269vw (dp) 1269vw (dp) POC13.FeC13 (E) + [Fe(POC13)6][FeCI,]3d 1268vw1298m (p) 1298w (p) 1298 w V1(POCl3), A," Abbreviations: s = strong; m = medium; w = weak; br = broad; sh = shoulder; p = polarized; dp = depolarized. Assignments ofPOC13*FeC13 are based on assumed C3v symmetry Group-theory classification of vibrational modes: linear P-0-Fe bridge (C," symmetry), rvib = 6A,(Raman, IR) + A, + 7E(Raman, IR); bent P-0-Fe bridge (C, symmetry); rvib = 13 A'(Raman, IR) + 8A"(Raman, IR).'Supercooled.Overlapping bands.F F IFig. 2 Raman spectra of molten POC1,-FeCI, mixtures at 525 K.Bands due to iron chloride, phosphoryl chloride and to the complex aremarked by F, P and C respectively. h, = 647.1 nm, w = 35 mW,SAW. = 4cm-', z = 0.3 s, S.S. = 60cm-' min-'POCl, addition corn pound^.^^^^'^ From a structural point ofview a C,, symmetry involving a linear P-0-Fe bridge or analternative C, configuration with a bent P-0-Fe bridge can beconsidered. Assignment of five polarized and five depolarizedbands as due to the complex (see Table 1) points to C,, ratherthan to C, as the most plausible structure.Furthermore, it isnoteworthy that the v(P-Cl) stretching frequency of thecomplex is blue-shifted relative to the corresponding band ofPOCl,(g), while v(P-0) and v(Fe-C1) are red-shifted relative tothe respective modes of POCl,(g) and FeCl,(g). These shifts arecompatible with the proposed type of bonding, indicating that,as expected, the formation of the P-0-Fe bridge weakens theP-0 and Fe-CI and strengthens the P-Cl bond.Electronic absorption spectra of POCl,FeCl, vapoursMolar absorption coefficient of the Fe-0-P-Cl gas-phasecomplex. The absorption spectra of vapours obtained from cellscontaining POC1,-Fe,Cl, mixtures show UV bands withmaxima near 350 and 260 nm. Owing to overlapping bands ofpure POCl,(g), Fe,Cl,(g) and the Fe-0-P-Cl gas-phasecomplex, careful measurements are needed in order todetermine and assign the molar absorption coefficients of thespecies.The number of iron(m) participating in the gas-phasecomplex [ie.the value of n in equation (l)] is not known andonly apparent values of the molar absorption coefficient, E, permole of Fe"' in the gas phase can be calculated. However, asdiscussed in the previous section, the gas-phase complex mostprobably has a 1 : 1 stoichiometry, and is formed according tothe equilibrium (3). For determining the apparent molarabsorption coefficient of the complex a sufficiently large excessof phosphoryl chloride (Ppocl, = 2.5-6.75 atm) had to beJ. Chem. SOC., Dalton Trans., 1996, Pages 3405-3410 340present in the cell, in order to assure that the contribution ofthe iron chloride sample (which was small enough to vaporizecompletely) to the absorbance in the homogeneous gas-phaseregion was negligible.In such a case it could be assumed that allthe iron in the cell was in the form of POCI,*FeCl,(g) and themolar absorption coefficient of this species could be determined.Three different spectrophotometric cells were used to determinethe apparent molar absorption coefficients of the gas-phasecomplex at 346 and 257 nm (&346 and E ~ ~ ~ ) (see SUP 57153).Fig. 3 shows the overall spectrum from cell E-2 at 550 K [spec-trum (a)] and Fe,Cl,(g) at 500 K [spectrum (b)]. The molarabsorption coefficients of the Fe2C16(g) UV bands at 360 and245 nm were found to be in agreement with the data reported inref.20. The temperature dependence (550-635 K) of &346 andE~~~ of the complex was found from the measurements in thethree cells and can be represented by the relations (4) and (5)..3 52B9 *s.3d 2E346 = 11 067.4 - 11.7T dm3molp' cm-', s.d. = 29.9 (4)625 K(Hquld)426 K(liquid)300 K(iuparcooledliquid)300 K(iolld)E~~~ = 7605 - 9.7T dm3 mol-' cm-', s.d. = 26 (5)The data (see SUP 57 153) indicate that within experimentalerror the apparent molar absorption coefficients are independ-ent of POCl, pressure (varied in the range 2.5-6.75 atm). As forother gas-phase complexes,16 this indicates that either onegaseous species is present or that two or more species with equal'atomic' (in terms of Fe"') absorptivities are formed.Attempts to use spectrophotometry in order to determine thethermodynamics of the reaction FeCl,(s) + POCl,(g) +POC1,-FeCl,(g) in cells containing an excess of iron chlorideand small amounts of POCl, (i.e.corresponding to pressuresof ~ 0 . 2 atm) were unsuccessful. This was due to the form-ation of a stable liquid even at temperatures around 400 K(the melting point of FeCl, is 581 K).Charge-transfer spectra. The type of bonding and M-CIinteraction in the POCl,.FeCl, gas-phase complex can bestudied by measuring the energies of the ligand-to-metal( M t L ) charge-transfer transitions. A comparison of spectra(a) and (b) in Fig. 3 shows the differences between the charge-transfer transition energies of POCl,*FeCl,(g) (at 346 and 257nm) and Fe,Cl,(g) (at 360, ~ 2 8 2 and 245 nm).However, athird charge-transfer band of the POCl,-FeCl, gas-phasecomplex at ~ 2 1 0 nm is obscured by the strong POCl,(g) bandin the far UV [see band tail in Fig. 3(a)]. The POCl,(g) band isdue to a forbidden (n --- n*) transition from a non-bondingoxygen orbital to the antibonding molecular orbital of the P-0bond.21 The occurrence of the band at 210 nm is illustrated inspectrum (c), Fig. 3, which was obtained from the vapours inequilibrium with a condensed phase [POCl,-FeCl,(l) +'FeCl,(l)'?] in a cell with no excess of POCl, at 475 K. Thecontribution of gaseous Fe,Cl, was subtracted from spectrum(c), which can thus be assigned to POC1,-FeCl,(g). Table 2summarizes the M t L charge-transfer transition energies foriron(m) chloride compounds.The positions of the charge-transfer band maxima move to higher energies on going fromFe,Cl,(g) to POC1,-FeCl,(g) (see Fig. 3 and Table 2). This isexpected from the alteration of the chloride ligand environmentcaused by the formation of a P-0-Fe bridge.2Molecular and ionic complexes in POCI,-rich mixtures. Fromthe above discussion of the Raman spectra of liquids obtainedat 525 K it follows that the molecular nature of the POC1,-FeC1, molten mixtures can be considered established at T > 500K. However, a study of the temperature dependence of theRaman spectra of POC1,-rich mixtures indicates that below 450K bands due to a new species appear. This is illustrated in Fig.4,where the temperature dependence of the spectra is shown forthe 2 : 1 POC1,-FeCI, mixture. The measured vibrational bandwavenumbers are listed in Table 1, where a tentative assignment~avenumber/l o3 cm-'30 20 50 40I' I I I I I200 400 600 aimFig. 3 Molar absorption coefficients of (a) POC1,-FeCl,(g) inequilibrium with POCl,(g) (Po,,,,3, 600 = 4.5 atm) at 550 K and (6)Fe,C16(g) at 500 K. The spectrum of POCI,*FeCl,(g) at 475 K is shownin arbitrary intensity units, (c)l ah a I1 I I I I I I 1 1800 800 400 200 0Raman shiftkm-'Fig. 4 Raman spectra of POC1,-FeCI, molten mixtures (P : Fe = 2 : 1)at (a) 525, (b) 425 and (c) 360 K and of the solids at 300 K ( d ) . Bands dueto POCl,(l) are marked by P, while those due to POCl,~FeCl,(l) and[Fe(POCl,),][FeCl,],(1) are marked by a and p, respectively.Spectrum( d ) was obtained from a mixture of the '1 : 1 yellow' and '3 : 2 red' solidcompounds (see text). Parameters as in Fig. 2is also given. By lowering the temperature from 525 to 425 Knew bands appear at 114, 332 and 521 cm-' and becomeprogressively stronger by further lowering the temperature to360 K, attaining their maximum intensities in the solid state at25 "C and/or at liquid-nitrogen temperature. The bands at 332and 114 cm-' are assigned as due to the v1 and v2 modes ofFeC1,- ' * and point to an ionic nature for the 'low'-temperatureliquid species, which, judged from the spectra in Fig. 4, isformed at the expense of POCl,(l) and POCl,-FeCl,(l), mostprobably according to the equilibrium (6).POCl,(l) + 2(POCl,~FeCl,)(l) (3POC1,*2FeCl3)(l) (6)Equilibrium (6) is shifted to the right by increasing the POCl,content.Indeed the spectrum of the 5 : 1 liquid mixture at roomtemperature (Fig. 5) exhibits bands due to POCl,(l) and to the3POC1,.2FeCl3 liquid complex and the observed bandwavenumbers are listed and assigned in Table 1 (first column).The spectrum of the 2 : 1 liquid mixture at 525 K which consistsof POCl,~FeCl,(l) and POCl,(l) is included in Fig. 5 forcomparison. The '3:2' ionic liquid could be formulated as[Fe(POC1,)6][FeC14], and contain the [FeCl,] - anion and the[Fe(POC1,)6]3+ cation as a result of interactions between thelone electron pair of the oxygen of the POCl, solvent molecules3408 J, Chem.SOC., Dalton Trans., 1996, Pages 3405-341Table 2BEtJ[FeCl,]" 2.9 x lo-, Fe,Cl,(g) (500 K) FeCl,(g) (875 K) FeAlCl,(g) (500 K) POCl,.FeCl,(g) (550 K)mol dm-, in MeOH27.2 27.8' (5545)b9d 27.8 ' (4250) 27.8 (5500) 28.9 (4650)35.5' (1800)d.f 39.2' (3230)' 38.9 (2300)40.3 40.8' (5400)'~~ 46.5" (1600)' 40.8 (7200) 47.69" Ref. 22. * Ref. 20. ' Also observed in ref. 20.Gaussians. Band observed at 475 K from gaseous sample in equilibrium with condensed phase; obscured at higher temperatures (see text).Ligand-to-metal ( M t L ) charge-transfer transitions in iron(rr1) chloride compounds [ 10-3P/cm-' (&/dm3 mol-' cm-')IE,,, " Observed in this work by resolving into Gaussians. Value determined by resolving into800 600 400 200 0Raman shift/cm-'Fig.5 Raman spectra of POC1,-FeCl, molten mixtures; (a) P: Fe =2 : 1 at 525 K consisting of POCl,~FeCl,(l) and POCl,(l); (b) P:Fe =5 : 1 at 300 K consisting of [Fe(POCl,),][FeC14],(1) and POCl,(l).Parameters as in Fig. 2and the Fe3 + cation. The band at 521 cm-' is then assigned asdue to the v(P-Cl) stretching mode. Within the [Fe(POC1,),]3+complex ion the iron atom is thus surrounded by six oxygenatoms in a near-to-octahedral co-ordination resulting in a(FeO,)(PCl,), configuration containing six Fe-0-P bridges. Itshould then be possible to identify the Raman-active modes ofthe FeO, octahedron which span the representation vl(Al,) +v2(Eg) + v5(TZg). The Fe-0 'stretching' frequency of differentcompounds containing six-co-ordinated iron bound to ligandsthrough oxygen is not greatly affected by the type of ligand andfor certain complexes occurs below 400 cm-'.,, Thus thepolarized band at 356 cm-' observed in our spectra (Fig. 5 andTable 1, first column) could be assigned as due to the v1 mode of'FeO,' in the cationic complex; v2 is usually of very weakintensity and is rarely seen in the Raman spectra.Thedepolarized shoulder band at ~ 2 1 0 cm-' (see Fig. 5 and Table1, first column) is assigned to the v5 mode. This assignment issupported by the fact that the v1 : v5 frequency ratio is close tothe value for a series of 'octahedral' xY6 type molecules andions for which these two frequencies have been measured.', Itshould be pointed out that a [Fe(POC13),][FeC1,], configura-tion is in conformity with earlier indications for self-ionizationof the POCl,-FeCl, liquids, suggesting that iron is present inboth cationic and anionic species in solution and that thecationic species is not POCl, +."," Previ~usly,~~ the formationof [M(POCl,),] + complex ions has been deduced from Ramanstudies of POCl,-MAlCl, (M = Na or Li) liquids.As will be discussed below, spectrum ( d ) in Fig. 4 is due to anorange-yellow solid mixture of the compounds POCl,~FeCl,(s)and 3POC1,*2FeCl3(s). The existence of the '1 : 1' and '3:2'solid compounds in the POC1,-FeCl, system is well known.'The behaviour is somewhat analogous to that of the POCl,-AlCl, system, where cooling of the molecular liquid complexesleads to ionic solid^,^,^,^^ which depending on the P : A1 ratio of1300 1100 600 400 200 0Raman shift/crn-'Fig.6 Raman spectra of polycrystalline '1 : 1 yellow' POC1,-FeCl, (a)and '3 :2 red' ~Fe(POCl,),][FeCl,], (b) at 300 K. h, = 647.1 nm; w =30 mW; S.S.W. = 4 cm-'; '5 = 0.3 s; S.S. = 60 cm-' min-'the liquid mixture can be formulated as [Al(POCl,),] [AlCl,],and/or [Al(POCl,),] [AlCl,] ,.Heat treatment of cells containing the 3 : 2 and 5 : 1 mixturesat 110-120 "C resulted in mixtures of dark orange-red andyellow solids which were indistinguishable by visual inspection.A temperature gradient (1 10-30 "C) had to be applied along thecell containing the 5 : 1 mixture in order to condense the largeexcess of gaseous POCl, in the cold part of the tube away fromthe solids.Removal of this cell from the furnace resulted indissolution of the solid phase (which is presumably a mixture ofthe '1 : 1' and '3: 2' compounds) in POCl,. By heating theorange-yellow solid mixture at 50 "C under vacuum the POCl,could be condensed at a cold part of the tube and a pale yellow(with some orange shades) solid was obtained. The yellow solidis POCl,*FeCl,, while the orange-red unstable compound is[Fe(POCI,),] [FeCI,],.l The occurrence of the two solids isdemonstrated in Fig. 6. Spectrum (a) was obtained from theyellow solid phase containing mainly the molecular POCl,-FeC1, compound as indicated by the close resemblance with thespectrum of the 1 : 1 liquid at 525 K [see for example Fig. 4(a)].Spectrum (b) was obtained from the red [Fe(POC13),][F&14],solid.This last compound was prepared by treating overnight at95 "C iron(1rr) chloride with an eight-fold excess of POCl, in avacuum-sealed U-shaped quartz cell, resulting in a bright redsolid-liquid mixture. In order to separate the red crystals fromthe excess of POCl, the side of the tube containing this mixturewas heated gently at 4 W 5 "C in a tube furnace in such a waythat POC1, distilled into the empty cell side located outside thefurnace. Further heating at M 75 "C resulted after several hoursin the yellow solid POCl,-FeCl,. The wavenumbers ofvibrational bands of the 1 : 1 and 3 : 2 solids are listed in Table 3.It can now be visualized that a superposition of bands due toPOCl,.FeCl,(s) and to [Fe(POCl,),][FeCl,],(S) would resultin a spectrum similar to the one shown in Fig.4(d) previouslyassigned to a mixture of the two solids (see above).J. Chem. SOC., Dalton Trans., 1996, Pages 3405-3410 340Table 3 Observed vibrational Raman wavenumbers (cm-’) forPOCl ,.FeCI ,(s) and [Fe(POCI 3 ) 6 ] [FeCl,] 3(s) (Fig. 5 )POC13.FeCl 3( s)103vs131s203m1 14vs130 (sh)200s291w303w33%339w (br)z 360w362vs394m423wz 480w (br)518s533m545m631w651w1202m1272wz635w (br)1199w1269wConclusionRaman and UVjVIS spectra obtained from POC1,-FeCl,vapour mixtures at temperatures up to 650 K indicated theexistence of the POCl,*FeCl, gas-phase complex in equilibriumwith POCl,(g) and Fe,Cl,-FeCl,(g). Above 500 K the POCI,-FeCl, molten system can be considered as a molecular liquidmixture consisting of POCl,(l), FeC1,(1) and POCl,~FeCl,(l),while below 450 K the 3:2 ionic liquid formulated as[Fe(POCI,),] [FeCl,], was formed in mixtures withxpoc,, > 0.5 at the expense of POCl,(l) and POCl,~FeCl,(l).Two solids were identified at room temperature: ( i ) ‘1 : 1 yellow’POCl,+FeCl, and (ii) ‘3 : 2 red’ [Fe(POCl,),][FeCl,],.References1 R.H. Tomlinson, in Mellor’s Comprehensive Treatise on Inorganicand Theoretical Chemistry, Longmans, London, 1971, vol. 8, Suppl.3, pp. 466517.2 G. N. Papatheodorou, in Current Topics in Material Science, ed.E. Kaldis, North-Holland, New York, 1982, vol. 10, pp. 249-352.3 M. Brooker and G. N. Papatheodorou, Adv. Molten Salt Chem.,1983,5, 27.4 S. Boghosian, G.N. Papatheodorou, R. W. Berg and N. J. Bjerrum,Polyhedron, 1986,5, 1393.5 G. N. Papatheodorou, ACS Symp. Ser., 1982,179,309.6 S. Boghosian, D. A. Karydis and G. A. Voyiatzis, Polyhedron, 1993,7 F. Birkeneder, R. W. Berg and N. J. Bjerrum, Acta Chem. Scand.,8 V. V. Dadape and M. R. A. Rao, J. Am. Chem. Soc., 1955,77,6192.9 A, V. Suvorov, Zh. Koord. Khim., 1977,3,1141.12, 771.1993, 47, 344.10 V. Gutmann and M. Baaz, Monatsh. Chem., 1960,91,537.11 J. A. Cade, M. Kasrai and I. R. Ashton, J. Inorg. Nucl. Chem., 1965,27, 2375.12 G. N. Papatheodorou, in Proceedings of the 10th Materials ResearchSymposium on Characterization of High Temperature Vapors andGases, ed. J. W. Hastie, NBS Publication 561, National Bureau ofStandards, Washington, DC, 1979, pp. 647-678.13 S. Boghosian and G. N. Papatheodorou, J. Phys. Chem., 1989,93,415.14 L. Nalbandian and G. N. Papatheodorou, High Temp. Sci., 1990,28,49.15 L. Nalbandian, Ph.D. Thesis, University of Patras, 1990.16 G. N. Papatheodorou, J. Phys. Chem., 1973,77,472; G. H. Kucera17 K. Olie and D. J. Stufkens, Spectrochim. Acta, Part A , 1976, 32,18 G. A. Voyiatzis and G. N. Papatheodorou, unpublished work.19 S. Boghosian, G. D. Zissi and G. A. Voyiatzis, in Molten Salts,eds. C. L. Hussey, D. S. Newman, G. Mamantov and Y. Ito, TheElectrochemical Society, Pennington, NJ, 1994, vol. 94-99, p. 276.and G. N. Papatheodorou, J. Phys. Chem., 1979,83,3213.469.20 C.-F. Shieh and N. W. Gregory, J. Phys. Chem., 1975,79,828.21 M. Halmann, J. Chem. Soc., 1963,2853.22 R. S. Drago, R. L. Carlson and K. F. Purcell, Inorg. Chem., 1965,4,15.23 K. Nakamoto, Infrared and Ruman Spectra of Inorganic andCoordination Compounds, 4th edn., Wiley-Interscience, New York,1986, pp. 150,229,245.24 F. Birkeneder, R. W. Berg, H. A. Hjuler and N. J. Bjerrum,Z. Anorg. Allg. Chem., 1989, 573, 170.Received 3 1st January 1996; Paper 6/00744I3410 J. Chem. SOC., Dalton Trans., 1996, Pages 3405-341
ISSN:1477-9226
DOI:10.1039/DT9960003405
出版商:RSC
年代:1996
数据来源: RSC