The ordered lamellar structure thus obtained has been investigated by X-ray diffraction. A relation between the lamellar spacing and the disorder in the film has ...
ELSEVIER
Thin Solid Films 274 ( 1996) 143-146
Preparation and structure of ordered films of iron carboxylate complexes M. Popescu a, C. Turta b, V. Meriacre b, V. Zubareva b, T. Gutberlet ‘, H. Bradaczek ’ aInstituteof Physics
and Technology
of Materials, Bucharest-Magurele. P.O. of Chemistry, Chishinau, Moldova
Box Mg.7, Bucharest.
Romania
h Institute ’ Institut,fiir Kristallagraphie.
Freie Univer.sitiit Berlin, TakustmPe
6, D-141 95 Berlin, Germany
Received 23 August 1994; accepted 10 August 1995
Abstract Several iron complexes containing a tri-iron-oxo-cation linked to short chain fatty acids have been used in the preparation of films by selfassembling from solution on glass slides via two procedures: direct evaporation and dragging out. The ordered lamellar structure thus obtained has been investigated by X-ray diffraction. A relation between the lamellar spacing and the disorder in the film has been obtained from the X-ray diffraction data. Keywwfs: Langmuir-Blodgett films; Organic substances; Structural properties; X-ray diffraction
1. Introduction Organic amphiphilic molecules containing iron or other magnetic ions can be packed as thin films to provide ideal media for the study of the magnetic properties in two-dimensional structures. As an example of such two-dimensional systems. Langmuir-Blodgett films of long chain fatty acids with Fe’+ ions situated in the polar head groups have recently been investigated [ I]. More complex coordinative polynuclear compounds of the transition metals with anions of carboxylic acids are intensively studied by various physical and chemical methods due to their remarkable magnetic, optical and structural properties [ 2,3]. Many compounds from this class have marked catalytic properties [ $1. Among the carboxylates of such compounds, those containing iron are of high interest as models for ferro-proteins [ 51 (ferritin, trans-ferritin, etc.). Complexes with di-, tri-, tetra-, hexa-, octa-, nona-, deca-, undecaand dodecanuclear metal complexes with iron or manganese are known and some have been synthesized [ 6,7]. Recently, even a heptadecanuclear mixed iron/manganese carboxylate has been prepared and investigated [7]. Of the trinuclear oxo-centered iron carboxylates, up to now complexes with acetate anions are known or anions of carboxylic acids with ethyl, propyl or butyl substituents. In a recent investigation long chain fatty acids of myristic, palmitic and stearic acid have also been used as carboxylic ligands [ 81. We report in this paper the preparation of trinuclear iron0x0 carboxylates with short-chain fatty acids hexanoic, 0040~6090/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIOO40-6090(95)07080-X
octanoic, nonanoic and decanoic acid and the structure of ordered films of these compounds obtained on deposition on glass slide and investigated by X-ray diffraction.
2. Experimental 2.1. Materials The
compounds with the chemical formula COO)6 (H,O),Cl) + where x= 5, 7, 8, 9 (Fe,O(C,H,+ l have been synthesized by heating triaqua hexa-p-acetato(0,O’) -CL,-oxo-&i-iron (III) dihydrate chloride with the corresponding fatty acids C,H& + ,COOH in the molar ratio 1:6 followed by dissolution in diethylether, filtering and drying at room temperature. The resulted precipitates were dissolved in acetone, filtered and dried. The powders thus obtained were washed for a few minutes in heptane. Finally, brown-reddish powders with high solubility in benzene, acetone, diethylether, chloroform but not soluble in water or heptane were obtained. X-ray diffraction patterns of the powders show the formation of amorphous phases as proved by broad halos, the first one being centred at about 40” (20) for Cu Ka radiation. 2.2. Chemical, infrared and MGssbauer analyses The chemical analyses show a good agreement between the calculated compositions and those experimentally obtained. For example, for the compound with x = 9 the cal-
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cuIated composition is (wt. %) : C (55.41%), H (9.30%), Cl (2.73%) and Fe (12.88%) while the composition found from analysis is: C (54,94%), H (9.25%), Cl (2.40%) and Fe (13.05%). Infrared spectra, recorded in the range 4000-250 cm- ’ on a Specord MS-80 spectrometer using fine powders embedded in Vaseline and in fluorate oil show absorption bands characteristic to carboxylic anions [ 91 C,H,+zCOO(x= 7, 8, 9): u,,(COO) = 1580 cm-‘/1585 cm-‘; v,(COO) = 1450cm-‘/1450cm-‘; 6(OCO) =670cm-‘/ 670 cm-‘; rr(CO0) =615 cm-‘/615 cm-‘; v,,(CH/CH~) =2960 cm-‘/2960 cm-‘; v,(CH/ CH2) =2930 cm-‘/2925 cm-‘; v,(CHICH3) =2880 cm-‘/2880 cm-‘; v,(CHICH,) = 2860 cm-‘/2858 cm-‘; S,(CH,), = 1465 cm-‘/1470 cm-‘; &(CH*),= 1320 cm-‘/1320 cm-‘; v(C-C) +6(OH) =970 cm-‘; p(CH,).=725 cm-‘/720cm-‘. The water molecules contribute by a broad absorption band situated at 3500-3300 cm- ’ ascribed to V(H20) and a band ascribed to 6(H20) vibration at 1530 cm- ’ in all the compounds. The vibration frequency V, of the Fe,0 cluster is observed at 605 cm- ’ and ud vibration of the FeO, configuration at 380 cm-‘. These data allow one to conclude that during the powder processing the compound does not change its initial structure as a trinuclear compound with the oxygen atoms in the centre of the triangle having the iron cations at its peaks. The Mossbauer spectra have been recorded in an electrodynamics apparatus with uniformly accelerated movement of the Co-57 source using a multichannel analyser ICA-70. From the double line spectra the isomer shift (IS) has been calculated: IS = 0.68 mm s - ’ at 300 K and IS = 0.77479 mm s-l at 80 K, these values corresponding to Fe3+ cations in the high spin state (S = 5/2). The results obtained from the above analyses allowed to suggest a structural model for the iron carboxylate molecule as shown in Fig. 1. 2.3. Film deposition
The deposition of films on solid supports by the LangmuirBlodgett technique was unsuccessful. Ordered films were successfully prepared by deposition from saturated solutions of the compounds dissolved in benzene on glass substrates by two different procedures. In the first procedure a thin film was dragged out on the surface of the slide starting from the saturated solution dropped in front of another gliding slide. We call this procedure the dragging procedure [ 9, lo]. In the second procedure a pre-definite volume of saturated solution was spread on a clean glass substrate and evaporated in three variants corresponding to different evaporation rates:
2
C-R
C-R
lR : CXH~,Y+~ ; o Fe
3+
,
l
0
Fig. 1. Structure of the iron carboxylate
molecule.
low rate, i.e. evaporation in a limited space in a covered crucible in about one day; intermediate rate (evaporation in about 3 h) ; and normal rate (evaporation in the open atmosphere in about 10 min). We call this procedure the evaporation procedure.
2.4. X-ray difsraction measurements The films deposited on glass slides were studied by X-ray powder diffraction by the conventional &20 method. A highly collimated Ni-filtered X-ray beam of Cu Kar radiation in a conventional diffractometer (Philips PW lOSO>, monochromatized by a graphite crystal monochromator mounted in the diffracted beam in front of the detector was used in the measurements [ 10,111.
3. Results The recorded diffractograms of thin ordered films of trinuclear, iron oxo-carboxylate with octa-, nona- and decanoic acid with the thickness of some hundreds of nanometers, obtained in the dragging procedure show several Bragg peaks ascribed to a lamellar ordering in the films. The calculated spacings suggest the disposal of the molecules in a monolayer arrangement with the six hydrophobic tails distributed symmetrically on opposed sides of the Fe,0 group. Fig. 2 shows the X-ray diffraction patterns of the thin films of the dragging procedure. In the evaporation procedure thick ordered films with the thickness of some tens of micrometers were formed. Here the quality of the packing of the ordered layers strongly depends on the evaporation rate. In the X-ray diffraction diagrams of the hereby obtained thick films of composition corresponding to x = 9 the relation between the degree of ordering in the films and the evaporation rate during deposition is most distinct by the change in half width of the measured lamellar Bragg lines. The films become more disordered and show
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M. Popescu et al. /Thin Solid Films 274 (1996) 143-146
XIOJ 1
0
!A
JL
120
NORMAL
EVAPORATION RATE
20(” ) Fig. 3. X-ray diffraction patterns of thick films of iron carboxylate compound COO),( HZO)a] (Cl) with x = 9 prepared by the evapora(Fe30(C,H,+, tion procedure with different evaporation rates
28(“) Fig. 2. X-ray diffraction patterns of thin films of iron carboxylate compounds COO),(H,O),J (Cl) with x=7, 8, 9 dragged out on glass (Fe,O(C,H1,+, slides.
higher lamellar spacings when the evaporation rate is increased (Fig. 3). The films with nonanoic acid show a very intense lamellar peak for low evaporation rates but a new ordered phase with higher spacing does also appear. The sample of nonanoic acid prepared from acetone solution seems to be monophasic but it exhibits a large degree of disorder as shown by the low height, broad peaks (diffraction data not shown). No ordered films were obtained by the evaporation procedure with octanoic acid. Also, no kind of organisation of the films prepared in both procedures was found for the short chain complex with hexanoic acid (C,H, ,COOH) . The lamellar spacings calculated of the obtained diffractograms of the different iron carboxylate compounds are shown in Table 1. While the thin films prepared in the dragging procedure, show a single phase ordering, the thick ones Table 1 LamelIar spacing in iron carboxylate Compound
tFe,O(C,H,,CO,),(H,O),I(C1) [Fe,O(C,H,,CO,)n(H,O),I(CI)
[Fe~OIC,H,,CO,),(H,O)II(C1)
prepared in the evaporation procedure, show a more complicated picture. The thick films with decanoic acid as the carboxylic ligand are nearly monophasic. A faint additional first-order diffraction peak in the neighbourhood of the first order of the lamellar peaks speaks in favour of a minor phase of spacing 27.5 A. The intensity of the secondary maximum is about 1% from the main peak. Similar minor phases have been observed also by Nakamoto et al. [ 81 in the diffraction patterns of samples of the oxo-centered trinuclear iron carboxylate complexes Fe,O( C,H, + ,C02) 6( py ) 3 with longchain fatty acids as carboxylic ligand (x= 13, 15, 17). Different spacings are characteristic of different evaporation rates during the preparation of thick films. This fact is being undoubtedly related to the details of the molecular organisation, particularly the spatial orientation of the carboxylic tails with the formation of oblong molecules and to the tilting of the molecules in the layers.
4. Discussion Firstly, it is obvious that the lamellar spacing of the obtained thin films by the controlled dragging-out procedure is related to the length of the carboxylic chains. An average
films prepared in two procedures Dragging
procedure
Evaporation
procedure
(thick films) (A)
Low rate
Intermediate
20.3 23.7 (29.1) 26.6
_
_ _
_
23.1 22.8
24.6
27.3
+ deposited from acetone solution. ( ) minor phase (20%).
rate
Normal rate
(thin films) (8)
25.4’
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M. Popescu et al. /Thin Solid Films 274 (1996) 143-146
difference of _ 3 A can be reasonably ascribed to the difference of one CH, group from one complex to another and, consequently, to a shift in the interlayer distance corresponding to two CH2 groups. Therefore, the increase in lamellar spacing found in these samples can be, simply, explained by the increase in the chain length of the complexated fatty acid. The behaviour of the thick films obtained by evaporation is completely different. Apparently, no variation of the interlayer distance due to the length of the complexating fatty acid occurs. The minimum spacing seems to be around 23 A. Nevertheless, during preparation other phases with higher spacing appear. The difference in spacing between phases seems to be N 2 A. This value can probably be ascribed to definite tilting steps of the fatty chains in the bilayer plane. Our findings on additional phases here agree with the observations of Nakamoto et al. [ 81 on such compounds with long-chain fatty acids as carboxylic ligands. There the X-ray diffraction measurements showed a single-layered arrangement of the molecules. In such an orientation the Fe,O’+ cation on top of the fatty acid head groups must be opposed to the end of the hydrophobic alkyl chains of the next layer, probably sheltered by the pyridinium ligands present in the complexes, thus reducing the repulsion between the hydrophilic and hydrophobic moieties in the system. The structural configuration of the molecules, which build our films, can only be supposed, nevertheless the layered structure with alkyl chains opposed to each other is rather obvious in contrast to Nakamoto’s observations. A more detailed coordination scheme and organisation of the tri-iron0x0 cation cannot be predicted by the current X-ray powder diffraction measurements due to the low resolution of the patterns, but should be deducible by spectroscopic measurements of the samples prepared as thin ordered films. 5. Conclusion The tri-iron carboxylate compounds with short-chain fatty acids of aliphatic chain length C,-C, as anions show inter-
esting ordering properties at the molecular scale by evaporation from solution on glass substrates. The evaporation rate of the solvent controls not only the disorder in the films but also the type of the lamellar phase characterised from X-ray data by the distinct steps of spacing between the molecular layers due to specific molecular tilting. By dragging out the solution on glass substrates, monophasic, well ordered and reproducible thin films of definite interlayer spacing as a function of the length of the aliphatic chains are produced. The self-organization capability and the large interlayer distances in the ordered films make these iron carboxylates compounds suitable for applications in long-wavelength monochromators and in devices for energy-dispersive analyses for light elements.
References
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