Organometallic cation-exchanged phyllosilicates: variable-temperature 57Fe Mossbauer spectroscopic and related studies of the adsorption of dimethylaminomethylferroceneon clays and pillared clays Christopher Breen," John S. Brooks, Susan Forder and Julian C. E. Hamer Materials Research Institute, Shefield Hallam University, Shefield, UK S1 1 WB Variable-temperature "Fe Mossbauer spectroscopy, thermogravimetry (TG), powder X-ray diffraction (PXRD) and temperature- programmed solid insertion probe mass spectrometry (TP-SIP-MS) have been used to study the interaction of dimethylaminomethylferrocene (DMAMF) with Westone-L (WL), a low iron montmorillonite. The hydrochloride salt of DMAMF, (ferrocenylmethyl)dimethylammoniumchloride (FMDMACl), was prepared and studied both prior and subsequent to exchange on the interlamellar sites of WL.X-Ray diffraction confirmed that the FMDMA cations were incorporated between the + clay lamellae and the observed spacing of 15.1 A was thermally stable up to 200 "C in air. TP-SIP-MS indicated that a small proportion of the incorporated metallocene was volatilised at temperatures below 400 "C, but that the majority decomposed via loss of cyclopentadienyl ligands leaving the metal centre between the sheets. A similar thermal degradation path was observed for DMAMF on aluminium pillared clay (A1-PILC). 57Fe Mossbauer spectroscopy revealed that the FMDMA cation occupied a + similar environment in the chloride salt, FMDMA+-WL and DMAMF-A1-PILC insofar as the isomer shift, 6, and quadrupole splitting, A, of the incorporated metallocene were essentially the same in all complexes and virtually identical to that observed for FMDMACl(6 =0.34 mm s-l, d =2.32 mm s-l at 300 K).The values for the Debye temperature 8, and recoil-free fractionf determined from variable-temperature 57Fe Mossbauer spectroscopy, were typically 140 K and 0.12, respectively, for FMDMACl and FMDMA+-WL, thus confirming the similar environment occupied by the cation in the chloride salt and in WL. In contrast, the corresponding values for DMAMF-A1-PILC were 118 K and 0.06, respectively, indicating that the the metallocene enjoyed much greater freedom in the galleries of the Al-PILC which exceed the dimensions of the metallocene compared to FMDMA+-WL where the organoiron cation itself determines the layer separation.The incorporation of metallocenes into zeolites and zeotypes continues to attract interest. One particularly attractive goal is the production of catalytically active molecular fragments or small metallic clusters within a host matrix which, in addition to its thermal stability, can impart size and shape selectivity on the product distribution. In pursuit of this goal, considerable emphasis is placed on proving that the molecule is in the channel network and not bound to the surface, the characterisation of the incorporated species, and an expla-nation of how it interacts with its new environment before undertaking a detailed investigation of how it degrades upon thermal treatment.Moller et al.' have presented a detailed investigation of the pyrolysis of ferrocene in zeolites presenting EXAFS and sup-porting mass spectral data which indicate the presence of half-sandwich fragments bound to the oxygens of the zeolite lattice. Indeed, attempts to incorporate neutral metallocenes within the confines of the zeolite framework can prove problematic since protons arising from residual water molecules readily oxidise ferrocene to ferrocenium.',2 Recent studies,., have resulted in the successful inclusion of ferrocene into the channel network of A1P04-5 and AlPO,-8 from which it cannot be sublimed. s7Fe Mossbauer spectroscopy3 indicates that the metallocene is (i) rapidly reorientating within the channel network and (ii) largely unchanged following incorporation.EXAFS analysis,, which provided independent evidence for the presence of unaltered ferrocene at temperatures up to 200 "C, indicated that the thermally degraded composite did not show any evidence of the clustering of iron atoms through either Fe-Fe or Fe-0-Fe interactions. Cobaltocene, however, was oxidised to cobalticenium upon incorporation within the channels of VPI-5.5 Once produced the cobalticenium remained thermally stable up to 130"C, even though VPI-5 converted to AlPO,-8 over this temperature interval. An increase in layer separation, which exhibits enhanced thermal stability, usually confirms incorporation of metallo- cenes into layered componds but the nature of the included species, its interaction with the host and its thermal degradation path are still important.Ferrocenylalkylammonium cations have been adsorbed into a number of layered hosts including a-Sn( HPO,),-H,O (a-SnP), a-Zr( HPO,),.H,O (a-ZrP), MOO, and VOPO,. Dimethylaminomethylferrocene (DMAMF) was readily intercalated into a-SnP from aqueous solution forming a bilayer of protonated amines.6 In MOO,, 15N CP MAS NMR provides evidence for two distinct environments for 15N between the 1aye1-s.~ A minor resonance was tentatively assigned to a small amount of oxidised guest species whilst the major resonance was intermediate between that for (FCCH,CH,~'NH~)+C~-and FcCH,CH,~'NH,, where Fc =Fe(q-C,H,)(q-C,H,). NMR evidence was also utilised to show that the amino group was interacting with the host layer via P-O..-H-N hydrogen bonds in Z~(HPO,),(FCCH,CH,~~NH~)~.,(H20), (x=0.1-0.5).7 The increase in layer expansion upon incorporation of FCCH,CH,'~NH, in both a-ZrP and MOO, suggests that the guests formed bilayers between the layers of each host.Large increases in d spacing, i.e. layer expansion, were also observed when ferrocenylalkylammonium iodides were incorporated between the layers of VOPO,. The length of the alkyl bridging unit apparently influences both the layer spacing and the extent to which the ferrocene moiety is oxidised upon intercalation.' DMAMF was the molecule of choice in this investigation for several reasons. Firstly, it is easily transformed into the hydrochloride salt thus allowing straightforward replacement of the Na' cations resident on the exchange sites of a low- +iron Texas bentonite.Secondly, incorporation into H -exchanged WL (H+-WL) should be possible via the in situ formation of the conjugate base, ferrocenylmethyldimethylam-monium (FMDMA+ ), in the interlamellar region. This second J. Muter. Chem., 1996, 6(5), 849-859 849 approach also has the added attraction of neutralising the protons which may contribute to the oxidation of the ferrocene unit Thirdly, it was our intention to incorporate DMAMF into the interlamellar gallery of an aluminium pillared inter- layer clay (Al-PILC) by using the protons, formed during calcination of the pillar, to produce FMDMA+ ions All these approaches have proven successful and the products have been characterised using a range of instrumental techniques includ- ing X-ray fluorescence spectrometry (XRF), powder X-ray diffraction (PXRD), thermogravimetry (TG), temperature-pro- grammed solid insertion probe mass spectrometry (TP-SIP- MS), and Fourier transform IR (FTIR) spectroscopy Moreover, variable-temperature Mossbauer spectroscopy has been extensively employed to determine how strongly DMAMF is held within the Al-PILC, where the gallery height exceeds the dimensions of the metallocene, compared to the situation in FMDMA+-WL where the dimensions of the metallocene itself determines the interlayer separation Experimental Materials The clay used in all the experiments was Westone-L (WL) a Texas bentonite, supplied by ECC International, which has a cation exchange capacity (cec) of 81 mequiv (100 g)-' and a low iron content of 05% m/m Fe203 This clay and the procedures used to convert it into the (nominal) < 2 ym particle size, Na-exchanged form, subsequently referred to as Na+-WL, have been described in detail elsewhere The H+-exchanged form of Westone-L, H+-WL, was obtained by treating Na+-WL with aqueous 1 mol dmP3 sulfuric acid for 2 h at 25 "C, and washing until the residual conductivity of the supernatant was <30 pS The product was dried at room temperature Elemental analysis using XRF spectrometry indicated that this treatment, as anticipated based on related results,1° had little effect on the layer composition This was confirmed when 27Al and 29S1 MAS NMR spectra of WL were unchanged following the acid treatment The aluminum pillared clay was prepared using the method described by Schoonheydt et a1 l1 Na+-WL was suspended in 100 cm3 of water and stirred for 6 h An aqueous solution of NaOH (17 cm, 0 4 mol dme3) was added dropwise at 1 cm3 min-l to an aqueous solution of AlCl, 6H,O (17 cm3, 0 2 mol dmP3, 10 cec) with vigorous stirring The resulting solution was heated at reflux for 3 h and then added dropwise at 8 cm3 min-' to the Na+-WL suspension This was stirred for 12 h, and then washed with deionised water until the conductivity of the supernatant fell below 30 pS The clay was air-dried and then calcined at 500 "C for 1h This yielded aluminym pillared WL (Al-PILC) with an interlayer spacing of 18 8 A (Ferrocenylmethyl)dimethylammonium chloride (FMD-MAC1) was prepared using N,N-dimethylaminomethylferro-cene (DMAMF) supplied by Aldrich Chemicals DMAMF (1g, 4 12 mmol) was added dropwise with stirring to 50 cm3 of 1mol dm-3 HCl This was evaporated (in vacuo) to give a green solid Recrystallisation from CHCl,-Et,O gave long golden brown crystals in 87% yield (Analysis Found C, 55 59, H, 640, N, 502 Calc C, 5585, H, 649, N, 501%) The cationic portion of this salt will subsequently be referred to as FMDMA+ FMDMA +-WL was prepared using three different methods In the first method FMDMACl(0 23 g, 1cec, 0 81 mmol) was dissolved in 50 cm3 of deionised water, and 1 g of powdered Na+-WL, dried at 120"C, was added The suspension was stirred for 8 h at 25°C before the clay was isolated and the process repeated twice more The product was then washed (5 x 120 cm3 deionised water) as above The product is sub- sequently referred to as FMDMA+-WL( 1) (Analysis Calc 100% exchange, C, 14 2 Found C, 11 O%, equivalent to 80% 850 J Mater Chem, 1996, 6(5),849-859 exchange) In the second method DMAMF (046 g, 2 cec, 162 mmol) was suspended in 50 cm3 deionised water, and 5 cm3 of 1 mol dmP3 HCl (an excess) added dropwise with stirring to give a solution of FMDMACl Ground Na+-WL powder (1 g), dried at 120"C, was added and the resulting suspension was stirred for 18 h at 25 "C The clay was isolated by centrifugation and the process repeated twice more Finally, the product [referred to as FMDMA f-WL(2)] was washed with deionised water (5x 120 cm3), and air dried at room temperature In the third method, 1 g of H+-WL, dried at at 120"C, was added to DMAMF (046 g, 2 cec, 162 mmol) dissolved in 50 cm3 of methanol The resulting suspension was stirred for 18 h at 25 "C, centrifuged and finally washed with methanol (5 x 120 cm3) The product [referred to as FMDMA+-WL( 3)] was then air dried at room temperature DMAMF (1 29 g, 3 cec, 243 mmol) was suspended in 100 cm3 of deionised water, and 1g of powdered Na+-WL (dned at 120°C) was added The suspension was left to stir for 6 h at 25 "C The clay was then isolated by centrifuga- tion and the process repeated twice more The final prod- uct, DMAMF-WL, was then washed (3 x 120 cm3 H,O, 3 x 120 cm3 MeOH, 1 x 120 cm3 H20) DMAMF (043 g, 2 cec, 1 62 mmol) was dissolved in 50 cm3 of methanol and 1 g of the calcined pillared clay, pretreated at 120°C, added The suspension was stirred for 18 h, washed (5 x 120 cm3 methanol), and collected in the normal manner The product is subsequently referred to as DMAMF-AI-PILC (Analysis Calc 100% exchange, C, 142 Found C, 42% equivalent to 29% exchange) Adsorption isotherms Methanolic solutions (20 cm3) of DMAMF-FMDMACI in the range 0-3 cec were prepared, and the absorption at 435 nm, characteristic of both DMAMF and FMDMACl, measured Clay (0 1g) dried at 120 "C was then added and the suspensions shaken overnight These were then centrifuged, and the absorb- ance of the supernatant measured using a Hitachi U-2000 double-beam UV-VIS spectrophotometer, with cells of path- length 1cm Thermogravimetry Thermogravimetry was performed using a Mettler TG50 ther- mobalance connected to a Mettler TClOA processor Samples (5-10 mg) were heated from 25 to 800 "C, at a rate of 20 "C min-l, in a dynamic atmosphere of dry N, gas flowing at 20 cm3 min-l X-Ray diffraction X-Ray diffraction traces of pressed powder samples were recorded using a Phihps PW1830 diffractometer using Cu-Ka radiation (i=15418 A) yhereas a Philips PW1050, using Co- Ka radiation (A= 1789 A) was used to study partially oriented samples on glass slides A heating stage manufactured accord- ing to Brown et a1 l2 was used to heat the partially oriented samples in the temperature range 20-400 "C X-Ray fluorescence XRF analyses of samples presented as lithium tetraborate beads were obtained using a Philips PW2400 X-ray spec- trometer calibrated using certified reference materials C,H,N analyses were obtained from Medac Ltd Mossbauer The Mossbauer spectrometer, cryostat, sample presentation and fitting routines have been described in detail elsewhere Absorbers of Mossbauer t values <1, with a maximum iron concentration of 7 mg cmP2 for FMDMACl, were studied The 100 200 300 cec offered (%) Fig.1 Isotherms for the adsorption, from methanol, of DMAMF on Na+-WL (0)and Al-PILC (0)and FMDMA' on Na+-WL (A), H+-WL (W) and on Na+-WL in the presence of [H'] (0) values of the isomer shift, 6, the quadrupole splitting, A, and the linewidths, r,quoted are relative to the source, 57C0 in a rhodium matrix at room temperature.Results Adsorption isotherms The adsorption of DMAMF onto Na+-WL and Al-PILC from methanol resulted in an uptake equivalent to only 10% of the cec (Fig. 1). The loading on the A1-PILC was disap- pointing given that when the pillared clay is calcined at 500 "C, protons are released as the aluminium oxide pillars are formed, which then migrate into the layers. These protons can be enticed into the interlayer using strong bases such as amrnonia.I3 Thus it was anticipated that interaction with these protons might provide the driving force to draw DMAMF into the galleries in the pillared clay.Formation of the hydro- chloride salt, FMDMACl, followed by contact with the clay in methanol proved successful yielding 53YO exchange. Production of the chloride salt in situ, by the addition of acid to the methanolic solutions followed by contact with the clay, was undertaken and this resulted in 48% exchange. In the final experiment of the series, DMAMF was contacted with H +-WL. Exchange was successful although the loading achieved was only 45% of the theoretical value, perhaps implying that the upper limit for exchange using methanol as solvent was near 50% cec. Elemental analyses The loadings achieved following one contact in methanol were disappointing so attempts were made to increase the level of exchange by contacting Na+- or H+-WL three times with the metallocene using water as solvent.In the main this proved more successful as the following results show. The theoretical value for the Na,O content of fully Na+-exchanged WL, given a cec of 81 mequiv (100 g clay)-', is 2.09 mass% whereas the Fe203 content should increase from 0.5 to 5.6% m/m if FMDMA' ions occupy all the exchange sites. Note, however, that the calculations for iron content assume that no oxidation or volatilisation of the metallocene occurred, in line with previous observation^.^,^,^ Table 1 lists the results of the XRF and C,H,N analyses and expresses these values as the percent- age of exchange sites occupied by the metallocene or vacated by the Na' ions.When the metallocene is adsorbed in the cationic form the number of resident Na' ions replaced should correlate with the amount of iron adsorbed given that both species carry a single positive charge. Thus the discrepancy between the number of Na' ions displaced, the amount of iron adsorbed and the C,H,N analysis was a cause of initial concern (Table 1). For example, C,H,N analysis indicated that FMDMA' ions occupied 80% of the exchange sites in FMDMA'-WL( l), a figure which agreed with the number of Na+ ions displaced (76% cec) but not with the amount of iron determined by XRF (59%). The figures for DMAMF-Al-PILC behave in a similar manner. C,H,N analysis indicated that DMAMF occupied 29% of the exchange sites, whereas XRF data suggested a value of 33%.The value of 67% Na+ displacement when DMAMF was contacted directly with Na+-WL (DMAMF-WL) was unexpected. It is proposed that there were sufficient protons present during this process to protonate enough DMAMF to cause this level of exchange. Thermogravimetry The derivative thermograms presented in Fig. 2 were obtained after each sample had been pre-conditioned in the nitrogen purge gas for 15 min. This procedure removes much of the physisorbed water, which contributes little information, and serves to emphasise the maxima associated with desorption of strongly bonded species. The derivative thermogram for Na+-WL [Fig. 2(a)] shows that the desorption of the remain- ing physisorbed water was essentially complete by 100 "C, with dehydroxylation of the structure reaching a maximum at 680"C.14 Liquid DMAMF boils at 200°C and so little infor- mation regarding its decomposition was gained.The corre- sponding chloride salt, FMDMACl, began to decompose at ca. 150°C with an associated mass loss of 27%. Further mass losses of 15, 9.4 and 14.5% occurred with associated maxima at 350, 460 and 520 "C, respectively [Fig. 2( b)]. The derivative thermogram for FMDMACl provides a useful fingerprint for the protonated moiety insofar as a number of the mass losses were also observed in FMDMA'-WL( 1) [Fig. 2(c)]. For example, a maximum in the derivative thermogram for FMDMA+-WL( 1) at 200 "C was clearly visible and there was evidence for the presence of a maximum at 350°C.The maximum at 625°C may reflect some combination of the FMDMACl maximum at 620 "C and the structural dehydrox- ylation of WL which maximised at 680°C [Fig. 2(a)]. In addition, a new maximum at 740°C which corresponded to twice the mass loss associated with the maximum at 200°C was observed, perhaps indicating that the FMDMA 'cation follows a different decomposition pathway when exchanged onto WL. The derivative thermogram for the desorption of DMAMF from A1-PILC exhibited a small maximum at 200 "C but was dominated by a peak at 550"C, which accounted for 8% of the initial mass or 43% of the total mass loss. TP-SIP-MS TP-SIP-MS was used to explore the way in which the various complexes were thermally degraded.The maxima in the total ion current (TIC) for the desorption of metallocene from WL and Al-PILC correlate quite well with those observed in the derivative thermograms, which is reassuring given the different conditions under which they are obtained." The TIC for the thermal decomposition of the incorporated metallocene, which reached maxima for FMDMA+-WL and DMAMF-A1-PILC at 225 and 250 "C respectively, represented the combination of a large number of mass fractions. In particular, peaks at m/z 214 (NCH,Fc), 200 (CH,Fc), 186 (Cp,Fe), 134 [CH,(q5-C5H4)Fe] and 121 (CpFe) [Fc= Fe(q-CSH,)(q-CSH4); Cp =$-C5H5] proved that iron was volatilised from the sample, although this process was essentially complete by 400 "C. Above 400 "C the decomposition products contained only ligand, with characteristic peaks at m/z 79 (CH,Cp) and 66 (Cp).No iron was desorbed. This behaviour is summarised in Fig. 3 where the intensity of peaks, selected to distinguish between metallocene and ligand desorption, are plotted as a function of sample temperature. It is important not to equate TIC with the amount of metallocene desorbed. Reference to the derivative thermograms in Fig. 2, where the area under the peaks is directly related to the mass loss, indicates that the amount of material desorbed below 400°C was not as signifi- cant as the TIC suggests, yet the mass spectra quite clearly J. Muter. Chem., 1996, 6(5),849-859 851 Table 1 Summary of elemental analysis data for the samples described in the text Fe contenta Na content" (+O 1)YO (fO 1)Yo (m/m) (m/m) Na+-WL 04 19 H+-WL 04 01 AI-PILC 04 00 FMDMA+-WL( 1) 34 05 FMDMA' WL(2) 41 01 FMDMA+-WL( 3) 34 01 DMAMF-WL 33 07 DMAMF-Al-PILC 20 01 " Based on XRF analysis figures Based on C,H,N analysis figures I I I I t I 1 400 Mm 800200 TI"C Fig.2 Derivative thermograms for (a) Na+-WL, (b) FMDMACl, (c) FMDMA+-WL( l),(d) A1-PILC and (e) DMAMF-Al-PILC corroborate the loss of some iron which explains the discrep- ancies noted in the elemental analyses above Mass spectral analysis of the peaks contributing to the large maximum at 680°C in the TIC for the desorption of DMAMF from Al- PILC proved that this maximum was due to dehydroxylation of the structure and the pillar Powder X-ray diffraction The quality of the X-ray diffraction traces collected using pressed powder samples is shown in Fig 4 and the index for each pattern is ogiven in Table 2 The basal spacing for Na'-WL was 12 5 A, which is commensurate with one water layer between adjacent clay layers, whlst the spacing for FMDMA+-WL( 1)[Fig 4(b)] was 15 1A The diffraction trace for the Al-PILC, after firing for 1 h at 5OO0C, exhibited a 852 J Muter Chem , 1996,6(5), 849-859 Fe adsorbed" Na desorbed" metallocene (% cec) (YOcec) adsorbedb - - - 94 - 100 59 76 72 95 59 95 57 67 33 - spacing of 18 8 A,thus confirming that the pillaring process had been successful Variable-temperature X-ray diffraction VTXRD provides the first real indication that the metallocene cation was present within the interlayer of WL At room tempFrature and humidity Na'-WL exhibits a d spaFing of 12 5 A which upon heating to 50 "C decreases to 9 6 A This latter value is diagnostic of an Na+-exchanged clay from which all the interlamellar water has been expelled Incorporation of a large species such as the FMDMA' cation between the aluminosilicate layers increases the d spacing and makes it more thermally stable than the corresponding water-expanded material The 15 1 A spacing, as evidenced by the 001 and 003 reflections, remained essentially constant until the tomposite was heated to 200 "C, whereupon it collapsed to 13 0 A (Fig 5) This reduction in the d spacing coincides with the onset of the first major mass loss in the derivative thermogram for FMDMAfWL( 1) [Fig 2(c)] and there was no evidence of a 9 6 A spacing, characteristic of a completely collapsed clay, which indicates that the decomposition products of the FMDMA' cations remained in the interlayer region The PXRD trace for the pillared clay provided little information regarding the location of tbe metallocene because the spacing remained constant at 18 8 A and no extra peaks were observed, suggesting that the metallocene was not mixed with Al-PILC in a powder form The variable-temperature Mossbauer spec- troscopic data (vide znfra) support this observation Mossbauer spectroscopy 57Fe Mossbauer data were obtained for FMDMA', as the chloride salt, and after incorporation into Westone-L, FMDMA+-WL( 1)-( 3) and A1-PILC, respectively, over the temperature range 15-300 K Selected spectra are shown in Fig 6 and 7 and the parameters derived from the fitting process are listed in Tables 3-5 FMDMACl was fitted as a resolved quadrupole doublet whereas the fitting strategies for the clay- supported complexes had to allow for the small amount of iron in the clay structure, the absorption of which became increasingly more significant as the temperature increased It is common for the recoil-free fraction of inorganic species to decrease more slowly with temperature than that for an organometallic species This was particularly evident in DMAMF-A1-PILC (Fig 7) where the combination of a low loading, equivalent to 29% cec, and the much lower recoil- free fraction meant that the contribution from the structural iron in WL dominated the spectrum above 200 K The similarity of the Mossbauer parameters, 6 and A, for FMDMA' as the chloride salt and when incorporated in the host aluminosilicate indicates that there was no substantial change in the organometallic cations upon exchange In par- ticular, the absence of a component with a reduced quadrupole splitting suggested that the ferrocene unit had not been oxidised 100 (a) -TIC 0 .c 0 200 400 600 200 400 600 E,O0 .-E VIc s .-0 200 400 600 200 400 600 TI"C Fig.3 TP-SIP-MS data for (a) (b) FMDMA+-WL( 1) and (c) (d)DMAMF-Al-PILC Table 2 PXRD data for Na+-WL and FMDMA+-WL( 1) 281degrees I h k 1 dobslA dca,clA Na+-WL" 14.17 8 0 0 2 6.24 6.25 19.87 3 1 1 0 4.46 4.45 020 28.43 27004 3.14 3.13 35.96 2 0 0 5 2.49 2.50 43.48 3 0 0 6 2.08 2.08I I FMDMA +-WL( 1)b110,020 5.86 1 0 0 0 0 1 15.10 15.10 11.65 3 0 0 2 7.59 7.55 17.65 1 7 0 0 3 5.02 5.03 19.84 8 1 1 0 4.47 4.45 0 2 0 23.62 4 0 0 4 3.76 3.78 e2 .-u) 0.0 40 I do I do ' lo I do I do 29.63 34.7 1 41.91 5 2 1 0 1 0 0 3 0 5 0 7 3.01 2.58 2.15 3.02 2.60 2.16 61.94 2 0 6 0 1.49 1.50 upon intercalation.The variation of the absorption area data with temperature, for the organometallic cation, was analysed and the resulting plots of log (area) us.temperature for the 250al! 110.020 I I samples under investigation are presented in Fig. 8. The values of OD andf, obtained using software which uses the full Debye 900. integral, are presented in Table 6. The illustrative data pre- sented concentrates on FMDMA+-WL and DMAMF-AI- PILC, but these are representative of all the samples studied as the data in Table 6 and the plots in Fig. 8 confirm. 4 I Discussion The adsorption isotherm data (Fig. 1) and the results of the elemental analyses confirm that DMAMF, and the correspond- Fig. 4 PXRD profiles, obtained using Cu-Kcr radiation, for ing FMDMA' cation, was adsorbed onto WL from both (a) Na+-WL and (b) FMDMA+-WL(1) (0s small quantity of fine methanol and water with varying degrees of !uccess to produce grained opaline silica impurity, K =kaolin) a composite with a basal spacing of 15.1 A.The method of J. Muter. Chem., 1996, 6(5), 849-859 853 2 3 4 5 6 7 8 0 10 11 12 13 14 15 16 17 18 192021 2223 242526 2eldegrees Fig. 5 PXRD profiles, obtained using Co-Ka radiation, for FMDMA+-WL(1) at (i) 20, (ii) 50, (iii) 100, (iv) 150, (v) 200 and (vi) 250°C introduction of the FMDMA' cation into the interlayer region influences the amount of cation adsorbed but, in general, acceptable levels of incorporation are achieved from multiple contacts in water. The poor uptake of DMAMF from methanol was attributed to a combination of two distinct factors. Firstly, incomplete separation of the layers due to the solvent (meth- anol), and secondly, there was little to favour incorporation of a neutral species between the layers of Na+-WL.Indeed, similar loadings of cationic half-sandwich compounds of iron were achieved using methanol as solvent.' The low loading of the DMAMF on A1-PILC, from both methanol and water, was a combination of two factors. Firstly, the protons generated during the firing process may not have been available for complexation with the dimethylamino group on the metallo- cene. Secondly, it is probable that sites at the periphery of the interlayer region were filled first thus preventing the diffusion of further molecules into the structure. The observed basal spacing of 15.1A indicates that, in contrast to the bilayer formation in a-SnP,6 MOO, and a-ZrP,7 and VOP04,* only a single layer of metallocene resides in the interlamellar region ofWL. The reduction in the basal spacing from 15.1 to 13.0A when FMDMA+-WL(1) was heated to 200°C must be attributed to the decomposition of the FMDMA cation witbin the interlayer region.Moreover, + the final value of 13.0A indicates quite clearly that the decomposition was not complete. These observations are in accord with both the derivative thermograms (Fig. 2) and the TP-SIP-MS results (Fig. 3) and suggest that some residue containing iron was left between the aluminosilicate lamellae. The volatilisation of some of the metallocene, which contrasts with recent studies of organoiron species on WL9 and AlP04- 5,394 at temperatures around 200 "C may contribute, at least in part, to the reduction in basal spacing.The data presented in Table 1 indicate that the FMDMA' cation displaced Na' ions from the exchange sites on WL; thus it is unlikely that the volatilised metallocene arose solely from surface sites, 854 J. Muter. Chem., 1996,6(5), 849-859 although the possibility is not rejected. The similarity of the desorption profiles, derived from mass spectrometry (Fig. 3), for FMDMA+-WL( 1) and DMAMF-Al-PILC suggest that the metallocene molecules are desorbed from similar sites with the proportion of strongly bonded molecules, which only lose ligand upon thermal treatment, outnumbering those which desorb near 200°C.The mass spectral data give no indication that the aminomethylmetallocene is changed upon incorpor- ation into the host structures and this is supported by the parameters for the incorporated metallocenes derived from the 57Fe Mossbauer data, Fig. 5 shows that the 15.1 A basal spacing waso stable at 200°C. The thickness of the metallocenc is 6.65 which, when added to th,e layer thickness of 9.6 A, should result in a d spacing of 16.3 A. However, the incorporation of FMDMoA+ cations in VOP04* only rFsulted in an expansion of 5.8 A, a value close to that of 5.5 A observed here. It is common for metalloc5ne expanded layered materials to display d spacings up to 1A less than the value anticipated from the molecular dimensions of the guest, particularly in swelling layer lattices such as a-Zr( HP0J2,17 VOP04,18 and V205 Given the .7319 uncertainty regarding the increase in d spacing of metallocene expanded layered hosts, conclusions regarding the orientation of the FMDMA' cation are difficult to reach.However, the observed 15.1 A d spacing of FMDMA+-WL( 1) is consistent with the cation adopting an orientation where the plane of the cyclopentadienyl ring is perpendicular to the basal surface with the side chain accommodated in the interlamellar space, thus making no contribution to the layer expansion [Fig. 9(a)]. It is more difficult to ascertain the orientation of the metallocene in DMAMF-Al-PILC because the height of the pillars, which exceed the dimensions of DMAMF, determine the interlayer spacing [Fig. 9(b)] and this spacing does not alter after the PILC has been fired.The 57Fe Mossbauer spectrum for FMDMACl consisted of a single symmetric doublet, with a quadrupole splitting, A, of 2.34 mm s-', which remained constant between 15 and 300 K, whereas the isomer shift, 6, exhibited a typical second-order Doppler shift effect, falling steadily from an initial value of 0.41 mm s-l at 15 K to a final value of 0.34 mm s-' at 300 K (Table 3). Analysis of the normalised area us temperature data (Fig. 7)yielded a Debye temperature, OD, of 144 K and a recoil free fraction,f,,, K, of 0.14 when an effective recoiling mass of 57 u was assumed. The low OD, which is typical of organometal- lic compounds, may be further reduced in this instance owing to the difference in size between the large FMDMA+ cation and the smaller chloride anion.The halfwidth at half height, r/2,of the absorption peaks varied from 0.13 mm s-' at 15 K to 0.16mm s-' at 300 K. This broadening arises owing to increased vibration within the lattice as the temperature of the solid was increased. Fig. 6 illustrates how the 57Fe Mossbauer spectrum for FMDMA+-WL varied with temperature. The sharp, outer doublet, which dominates the spectra at low temperatures, was assigned to the FMDMA' cation (uide infra). The broad, ill defined absorption seen between the wings of this sharp, outer doublet has been attributed to two components. The first is a weak, broad doublet arising from the small amount of Fe"', present owing to isomorphous substitution in the octahedral sheet of WL [Q(l) in Table 41, whilst the second is a broad singlet, characteristic of Feo [S(l) in Table41.This singlet arises from the small quantity of iron which was added to the graphite rod to aid machining when making the sample holders. This contribution is not normally observed, but owing to the low iron content of the materials under study, the absorption becomes significant. The broadness of the Fe"' doublet indicates that the iron present in the clay occupies a range of closely related sites. When exchanged into WL the FMDMA' cations exhibited isomer shifts and quadrupole splittings which were very similar to those determined for FMDMACl (Table 3) and varied little 100 f Y 97.5 !! I + 99.3 100 240 K 1 2- Ti 3 100 A8 Y c .- v) .-E chc.96.8 I 99.0 15 K 2 1 T 100 t- 3 100 96.7 98.4 -4 0 2 4-4 -2 0 2 4 vlmm s-l Fig. 6 Variable-temperature 57FeMossbauer spectra collected for FMDMA+-WL( 1) at the temperatures indicated despite the different routes by which the clay/metallocene were prepared. The similarity of these values indicates that the FMDMA cations occupied similar environments in both the+ chloride salt and in WL. This is firm evidence that WL simply expanded to accommodate the FMDMA' cation, with no oxidation of the iron centre nor distortion in the orientation of the cyclopentadienyl rings. Given that the increase in the d spacing upon incorporation of the FMDMA' cation into WL was l.OA less than anticipated, and that previous studies2' have shown that a 9" tilt in the cyclopentadienyl rings reduces the isomer shift by 0.02 mm s-' and the quadrupole splitting by 0.11 mm s-', the similarity of the observed parameters was surprising.Nonetheless, it is consistent with an earlier study where a similar low d spacing did not alter appreciably the Mossbauer parameters of half sandwich organoiron com-pounds when they were incorporated into WL.' J. Muter. Chem., 1996, 6(5),849-859 855 80K 300 K ln n2 i -3 I 00 # 99 1 995 50K 240 K 1-2 100 -3 n 100 h s v c 0 u)fn 6 *c 98 5 99 6 15 K 100 100 99 5 -4 -2 0 2 4-4 -2 0 2 4 vlmm s-l Fig.7 Vanable-temperature 57Fe Mossbauer spectra collected for DMAMF-AI-PILC at the temperatures indicated The Mossbauer spectra for the complex formed when Al- PILC was treated with DMAMF (Fig 7) were of lower quality than those recorded for the FMDMA+-WL( 1)-( 3) samples because the amount of iron present was only equivalent to 29% cec, and the recoil-free fraction fell off much more rapidly Nonetheless, the values of 6 and d determined from these spectra (Table S), together with the resistance of the incorpor- ated metallocene to the washing procedures, suggests that the 856 J Muter Chem , 1996, 6(5),849-859 incorporated species was the FMDMA’ cation The narrow doublet [Q( 1) in Table 51 became evident in the Mossbauer spectrum of the fired Al-PILC pnor to contact with DMAMF The origin of this doublet has not been studied extensively The consistency of the values of d and 6 determined for the FMDMA+ cations in WL together with their similanty to the values for the chlonde salt suggested that the organoiron species was the same in all the samples Yet the TP-SIP-MS Table 3 Isomer shifts, quadrupole splittings and linewidths derived from a variable-temperature 57Fe Mossbauer study of FMDMACl 15 0.4 1 1.18 0.13 0.13 2.04 0.504 33 0.41 1.18 0.13 0.13 1.89 0.772 50 0.41 1.18 0.14 0.14 1.82 1.074 80 0.40 1.17 0.16 0.15 1.56 0.663 100 0.40 1.17 0.14 0.14 1.35 1.065 120 0.39 1.17 0.16 0.15 1.21 0.872 140 0.39 1.17 0.15 0.14 1.05 0.810 160 0.39 1.17 0.17 0.16 0.93 0.897 200 0.36 1.17 0.17 0.16 0.70 0.799 250 0.34 1.16 0.18 0.16 0.46 0.975 300 0.34 1.16 0.20 0.16 0.3 1 1.726 Table 4 Isomer shifts, quadrupole splittings, linewidths and areas derived from a variable-temperature 57Fe Mossbauer study of FMDMA+-WL T/K phase" 6 (&0.02)/mm s-' 42 (f0.02)/mm s-' wY1) (+0.02)/mm s-' r/2 (r) (&0.02)/mm s-' normalised area relative area (&2.5%) x2 15 0.24 0.48 0.18 15 0.604 15 0.33 0.4 1 0.24 0.24 0.05 5 0.604 15 0.43 1.21 0.14 0.15 0.93 80 0.604 50 0.24 0.48 0.17 16 0.559 50 0.33 0.43 0.24 0.24 0.54 5 0.559 50 0.42 1.22 0.15 0.15 0.82 79 0.559 80 0.24 0.48 0.17 18 0.582 80 0.33 0.4 1 0.24 0.24 0.07 7 0.582 80 0.42 1.21 0.14 0.15 0.69 75 0.582 160 0.24 0.49 0.17 28 0.572 160 0.29 0.42 0.24 0.24 0.07 11 0.572 160 0.39 1.21 0.13 0.13 0.39 61 0.572 240 0.24 0.50 0.20 44 0.540 240 0.29 0.45 0.23 0.23 0.03 7 0.540 240 0.35 1.20 0.12 0.14 0.23 49 0.540 300 0.23 0.48 0.18 54 0.607 300 0.30 0.49 0.20 0.20 0.02 6 0.607 300 0.32 1.20 0.13 0.14 0.13 40 0.607 " S( 1)=Fe"' present in sample holder; Q(1)=Fe"' present due to isomorphous substitution in WL; Q(2) =Fe" present in ferrocene unit of incorporated metallocene.results suggested quite strongly that there were two adsorption (area) us. temperature data reported by Simopoulos et uLZ1for sites, one where the entire metallocene was desorbed at tem- dimethyltin species adsorbed on montmorillonite exhibited a peratures below 400°C,and a second environment where the discontinuity near 210K at which the gradient of the line metal centre was retained and only ligand was desorbed. In increased considerably, although linearity was retained.This an effort to determine whether the fitting of the 57Fe Mossbauer feature was attributed to the melting of the interlayer water absorption data would support a two site model the spectra which resulted in a lower recoil-free fraction for the Sn atoms. obtained at 15, 25, 50, 80, 100, 140, 180, 220 and 300 K for Dehydration of the samples removed the discontinuity, hence both FMDMA+-WL( 3) and DMAMF-Al-PILC were sub- corroborating the interpretation.In contrast, the Debye tem- jected to a P(Q) analysis. The P(Q) fitting program assumes perature and recoil-free fraction of the N-methyl-3-( triphenyl- a distribution of sites and an effective distribution of electric stanny1)pyridinium cation changed little upon adsorption onto field gradients and corresponding quadrupole splittings. Some the cation-exchange sites in montmorillonite,22 whereas the line broadening away from the theoretical natural linewidth is half-sandwich iron cations, tricarbonyl(~5-2,4-dimethylcyclo-expected owing to saturation effects and sample inhomogen- hexadienyl )iron ( 1+), and tricarbonyl (q5-2-methoxycyclo hex- eity. This fitting procedure indicated that there was no evidence adienyl)iron( 1 +), typically gave Debye temperatures 30 K for more than one unique site which would result in changes lower when occupying the exchange sites in WL than when in the quadrupole splitting larger than the normal line broaden- incorporated in a PF6-lattice.g This was attributed to the ing effects.This indicated that the Mossbauer data would only cations being less tightly bound when incorporated between support one type of site. Consequently, it must be considered the layers of the clay than when locked within the anionic that at temperatures below 300K the adsorption sites for (PF6-)lattice. Clearly, the more hydrophobic the incorporated +FMDMA cations were indistinguishable. organometallic species, the less influence the melting of incor- The Debye temperature, OD, and corresponding recoil-free porated solvent has on the recoil free fraction, hence no fraction,f, provide information regarding how tightly the iron- discontinuities are observed.containing species is bound within a structure. Previous varia- The values for eDandf,,, for the samples studied here ble-temperature Mossbauer studies of organometallic species are collected in Table 6. The similar Debye temperatures adsorbed in clays have shown that incorporation may result obtained for the FMDMA+ cation in WL, irrespective of in a lower Debye temperature and recoil-free fraction. The In preparation method, suggests that the recoiling mass in WL is J. Muter. Chem., 1996,6(5),849-859 857 Table 5 Isomer shifts, quadrupole splittings, linewidths and areas derived from a variable-temperature 57Fe Mossbauer study of DMAMF-AI- PILC T/K phasea 6 (f0.02)/mm s-' A/2 (+O.O2)/mm s-' r/2(1) (&0.02)/mm s-' r/2 (r) (_+0.02)/mms-l normalised area relative area (_+2.5%) ~~~ x2 14 -0.03 0.19 0.20 0.20 0.10 13 0.674 14 0.66 0.19 0.30 0.30 0.07 9 0.674 14 0.43 1.22 0.18 0.18 0.61 78 0.674 25 -0.03 0.19 0.20 0.20 0.11 14 0.586 25 0.66 0.19 0.30 0.30 0.07 10 0.586 25 0.43 1.22 0.18 0.18 0.57 76 0.586 50 -0.03 0.19 0.20 0.20 0.12 18 0.591 50 0.66 0.19 0.30 0.30 0.09 14 0.591 50 0.43 1.22 0.14 0.15 0.42 68 0.591 80 -0.08 0.17 0.20 0.20 0.09 17 0.720 80 0.63 0.19 0.30 0.30 0.09 17 0.720 80 0.42 1.20 0.21 0.2 1 0.34 66 0.720 100 -0.03 0.19 0.20 0.20 0.1 1 22 0.599 100 0.66 0.19 0.30 0.30 0.08 16 0.599 100 0.41 1.21 0.20 0.20 0.3 1 62 0.599 140 -0.05 0.19 0.20 0.20 0.10 25 0.573 1 40 0.66 0.19 0.30 0.30 0.08 19 0.573 140 0.41 1.20 0.25 0.25 0.23 56 0.573 180 -0.05 0.19 0.20 0.20 0.10 32 0.575 180 0.68 0.19 0.30 0.30 0.08 24 0.575 180 0.40 1.20 0.17 0.17 0.14 44 0.575 220 -0.08 0.18 0.20 0.20 0.09 35 0.571 220 0.66 0.19 0.30 0.30 0.07 28 0.571 220 0.39 1.20 0.18 0.18 0.09 36 0.571 300 -0.08 0.19 0.20 0.20 0.09 45 0.594 300 0.68 0.19 0.30 0.30 0.07 35 0.594 300 0.37 1.20 0.16 0.17 0.04 20 0.594 -Q(1)=Fe"' present in fired Al-PILC; Q( 2) =Fe"' present due to isomorphous substitution in WL; Q(3)=Fe" present in ferrocene unit of incorporated metallocene.0.4 Table 6 Values for the Debye temperature and recoil free fraction for samples described in the text 0.0 sample FMDMACl 144 0.14 FMDMA+-WL( 1) 139 0.13 h a FMDMA+-WL( 2) 144 0.152 -0.5 FMDMA+-WL( 3) 139 0.12 Y DMAMF-WL 138 0.12 0,-0 DM AMF-Al-PILC 118 0.06 -1 .o -1 .E 0 100 200 300 TIK Fig. 8 Variation of Mossbauer absorption line area with temperature for 0,FMDMACl; A, FMDMA+-WL(1); 0, FMDMA+-WL(2);V,FMDMA+-WL(3); H,DMAMF-WL and @, DMAMF-A1-PILC the same as that in chloride salt. In contrast, the Debye temperature, OD, and corresponding recoil-free fraction, fZg1 K, for the DMAMF-A1-PILC complex were much lower at 118 K and 0.06, respectively, revealing that the metallocene was much less tightly bound.This is commensurate with a model in which the metallocene, which is probably the FMDMAf Cation, resides in a much freer environment in the PILC where Fig. 9 Schematic illustrations of the probable orientation of the the gallery height is larger than the FMDMA' cation. This FMDMA+ cation in (a)WL and (b)Al-PILC 858 J. Muter. Chem., 1996,6(5), 849-859 contrasts with the situation in FMDMA+-WL where the organoiron cation itself determines the layer expansion. Consequently, it is envisaged that in FMDMA+-WL the rings of the metallocene unit are keyed into the aluminosilicate layer and are tightly held [Fig.9(a)]. This would account for the lower than expected layer expansion and would mean that the Fe would be the only recoiling mass. In the DMAMF-A1- PILC it is reasonable to assume that the metallocene is anchored via the dimethylamino group and that the cyclopen- tadienyl rings are not tightly held [Fig. 9(b)]. Hence the metallocene unit would enjoy considerably more freedom in the gallery space of the PILC than in the cramped, interlayer environs of FMDMA+-WL( 1)-(3). Other workers3 have found that unsubstituted ferrocen?, which is essentially spherical with an effective diameter of 7 A, appears to have almost complete three-dimensional freedom at room temperature in A1PO4-5 and AlP04-8, Poth of which have channels of diameter greater than 7.8 A.This rapid rotation of the ferrocene molecule changes the average electric- field gradient for 57Fe to zero and consequently a singlet is observed in the Mossbauer spectrum. In FMDMA+-WL and DMAMF-Al-PILC, a doublet is observed at all temperatures indicating that the aminomethylmetallocene does not rotate rapidly within these layered hosts, Fig. 9, which depicts the schematic orientation of FMDMA' ion in both WL and Al- PILC, shows that there is little room for the FMDMA' cation to rotate in WL and the bulky side chain must prevent this cation rotating within the larger gallery space in Al-PILC. The influence of bulky side chains on the freedom of organoiron species in clays has been noted previously.' Conclusions FMDMACl has been prepared and characterised using a +number of techniques.FMDMA cations have been success-fully used to displace the resident Na+ cations from the interlamellar exchange sites in WL and loadings up to 80% cec have been achieved. PXRD indicates that a single layer of metallocene is incorporated bttween the sheets and the resulting layer spacing of 15.1 A is lower than anticipated, suggesting some keying of the molecule into the aluminosilicate layer. The single-lay5r complex is stable to 200°C whereupon it collapses to 13.0A. TP-SIP-MS data clearly show that a small proportion of the incorporated metallocene is volatilised at temperatures below 40O0C, but that the majority of the metallocene degrades via loss of the cyclopentadienyl ligands.A similar thermal degradation path was observed for DMAMF-A1-PILC. 57Fe Mossbauer spectroscopy revealed that the FMDMA cation occupied a similar environment in + the chloride salt, FMDMA+-WL and DMAMF-A1-PILC, insofar as the isomer shift and quadrupole splitting of the incorporated metallocene were essentially the same in all complexes. However, variable-temperature 57Fe Mossbauer spectroscopy confirmed that the metallocene enjoyed much greater freedom in the galleries of the A1-PILC. We are indebted to Dr. Rob Brown and Gareth Parkes of the Catalysis Research Unit at Leeds Metropolitan University for the TP-SIP-MS results. References 1 K. Moller, A. Borvornwattananont and T. Bein, J. Phys. Chem., 1989,93,4562.2 G. A. Ozin and J. Godber, J.Phys. Chem., 1989,93,878. 3 A. Lund, D. G. Nicholson, R. V. Parish and J. P. Wright, Acta Chem. Scand., 1994,48,738. 4 A. Lund, D. G. Nicholson, G. Lamble and B. Beagley, J. Mater. Chem., 1994,4,1723. 5 M. Endregard, D. G. Nicholson, M. Stocker and B. Beagley, J.Mater. Chem., 1995,5,485. 6 E. Rodriguez-Castellon, A. Jiminez-Lopez, M. Martinez-Lara and L. Moreno-Real, J.Znclusion Phenom., 1987,6, 335. 7 S. J. Mason, L. M. Bull, C. P. Grey, S. J. Heyes and D. O'Hare, J.Mater. Chem., 1992,2, 1189. 8 S. Okuno and G. Matsubayashi, J. Chem. SOC., Dalton Trans., 1992,2441. 9 C. Breen, J. S. Brooks, S. Forder, A. A. Maggs, G. Marshall and G. R. Stephenson, J. Mater. Chem., 1995,5,97. 10 C. Breen, J. Madejova and P. Komadel, J. Mater. Chem., 1995, 5,469. 11 R. Schoonheydt, J. van den Eynde and W. Stone, Clays Clay Miner., 1994,41, 598. 12 G. Brown, B. Edwards, E. G. Ormerod and A. H. Weir, Clay Miner., 1972,9,407. 13 A. Molinard, PhD Thesis, The University of Antwerp, 1994. 14 C. Breen, A. T. Deane and J. J. Flynn, Clay Miner., 1987,22,169. 15 P. A. Barnes and G. M. B. Parkes, J. Thermal Anal., 1993,39,607. 16 N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, Pergamon Press, Oxford, 1984,p. 1236. 17 J. W. Johnson, J. Chem. SOC.,Chem. Commun., 1980,263. 18 P. Aldebert and V. Paul-Boncour, Mater. Res. Bull., 1983,18, 1263. 19 C. F. Lee, L. K. Myers, K. G. Valentine and M. E. Thompson, J. Chem. SOC., Chem. Commun., 1992,201. 20 M. Hillman and A. G.Nagy, J. Organomet. Chem., 1980,184,433. 21 A. Simopoulos, D. Petridis, A. Kostikas and N. Gangas, HyperJine Interact., 1988,41, 843. 22 K. C. Molloy, C. Breen and K. Quill, Appl. Orgunomet. Chem., 1987,1,21. Paper 5/05572E; Received 22nd August, 1995 J. Mater. Chem., 1996, 6(5), 849-859 859