Structural isomers of iron(III) N-methyl diethanolaminate as sol–gel precursors for iron-based oxide nanomaterials

Abstract

A new homoleptic iron(III) complex, [Fe₄(mdea)₆]·6CHCl₃ (mdeaH₂ = N-methyl diethanolamine), isolated in star- or chair-shaped isomeric forms, was evaluated and compared with other homoleptic iron(III) aminoalkoxides for different sol–gel parameters to stabilize nano-sized colloidal suspensions, and to elaborate magnetic Fe₂O₃ and BiFeO₃ nanoparticles as well as thin films by spin coating.

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There is a great interest in nanosized iron oxides for both fundamental and application reasons.¹ The unique and tuneable magnetic properties of iron-based oxides find widespread use in applications as diverse as environmental remediation, magnetic recording and magnetic resonance imaging.^2,3 Besides, the iron oxide nanomaterials are also frequently employed as sensors⁴ or active supports for gold catalysts.⁵ Given the various iron oxide phases (hematite, magnetite and maghemite) present in the Fe–O system⁶ and their different optical and magnetic properties, methods for phase selective preparation of iron oxide nanoparticles/thin films are of significant interest.⁷ The advantages of soft chemical routes such as Sol–Gel (SG) processing, Metal Organic Decomposition (MOD) and Chemical Vapour Deposition (CVD) over traditional methods for the elaboration of nanomaterials are well-documented.⁸ Thermal decomposition of organometallic/metal–organic complexes such as [Fe(CO)₅], [Fe(acac)₃], [Fe(oleate)₃] and [Fe(glycolate)₃] in the presence of various surfactants, typically oleic acid, has been shown to produce iron oxide nanoparticles (NPs) of well-defined shape and size.⁹ However, the relatively high temperature required to decompose these precursors makes them less attractive for the materials processing. The sol–gel (SG) process, which is based on the hydrolysis and condensation of molecular precursors, offers many advantages including processing at moderate temperature. Unfortunately, the SG route to iron-based oxides remains underexplored.¹⁰ Metal alkoxides are the preferred sol–gel precursors for metal oxide ceramics,¹¹ and classical iron alkoxides have been used as precursors for iron-based oxides.⁷ However, the use of classical metal alkoxides in SG process is unattractive due to their extremely high susceptibility towards hydrolysis and poor stability of the resulting colloidal solutions. Functional alcohols bearing additional ether or amino functionalities not only alter their physical and chemical properties such as hydrolysis susceptibility, solubility, viscosity, etc. but also, due to their action as surfactants, stabilize the colloidal solutions obtained during hydrolysis. The iron(III) 2-methoxyethoxide has been utilized as a sol–gel¹² and MOD¹³ precursor for the iron oxide or FeBiO₃ nanomaterials. However, 2-methoxyethanol is toxic, which must be replaced with a safer reagent. Modification of classical metal alkoxides by aminoalcohols has been shown to make them attractive sol–gel precursors for metal oxide nanomaterials because of the favorable hydrolysis and condensation characteristics and good properties in terms of thin film deposition and their transformation into oxide at relatively low temperature.¹⁴ Besides, some of these metal aminoalkoxides are photoreactive which can be exploited for the low-temperature synthesis of (mixed) metal oxide nanomaterials by exciting the corresponding solutions with UV light.¹⁵ These characteristics as well as interesting structural features that play a role in determining the properties of the final ceramic material produced have prompted us to investigate the fundamental properties of the designed metal aminoalkoxides with desired structure, stability and lability.¹⁶ Although a few structurally characterized homo- and heteroleptic aminoalkoxides of iron have been reported,^17–20 almost all of them are concerned with their magnetic properties including their use as single molecule magnets (SMMs). Only recently, a couple of reports have described synthesis of magnetic Fe₃O₄ NPs by aqueous co-precipitation of in situ prepared iron aminoalkoxides, though no structures of the precursors were described.^21,22 In this paper, we wish to report two interesting structural isomers of the iron(III) N-methyl diethanolaminate derivative [Fe₄(mdea)₆]·6CHCl₃, obtained under different synthetic conditions (see ESI for the details†). These were investigated for different sol–gel parameters and employed for the elaboration of magnetic Fe₂O₃ and FeBiO₃ nanoparticles as well as films by spin coating. To study and compare various sol–gel processing parameters to stabilize nano-sized colloidal suspensions, other homoleptic iron(III) aminoalkoxide precursors based on N-methyl substituted aminoethanols, namely [Fe₂(dmea)₆] (A) and [Fe₆(tea)₆] (B) (where dmeaH = N,N-dimethylethanolamine; teaH₃ = triethanolamine) were also prepared by alkoxo–aminoalkoxo exchange reactions in toluene (see ESI for details†).

Depending upon the synthetic procedures employed, the N-methyldiethanolaminate derivative [Fe₄{(OC₂H₄)₂NMe}₆]·6CHCl₃ was crystallized in two isomeric forms: a chair-shaped (1a) and a star-shaped (1b).‡ The structure of the chair-shaped isomer 1a can be described as a central [Fe₂{(μ-OC₂H₄)(OC₂H₄)NMe}₂]²⁺ cation being coordinated with two peripheral metallate [Fe{(μ-OC₂H₄)(OC₂H₄)NMe}₂]⁻ units (Fig. 1a). All the four iron atoms are six-coordinate with NO₅ environment for the iron atoms in the central [Fe₂{(μ-OC₂H₄)(OC₂H₄)NMe}₂]²⁺ moiety and a N₂O₄ environment for the two peripheral [Fe{(μ-OC₂H₄)(OC₂H₄)NMe}₂]⁻ units. The distance between two central Fe1⋯Fe1ⁱ (3.173 Å) is slightly shorter than 3.233 Å found between central-peripheral metals i.e. Fe1⋯Fe2. In contrast, the structure of the star-shaped isomer 1b can be assumed as a central Fe³⁺ cation being coordinated with three peripheral bis-(N-methyldiethanolamino)ferrate [Fe{(μ-OC₂H₄)(OC₂H₄)NMe}₂]⁻ units, which act as a bidentate chelating ligand through two oxygen atoms (Fig. 1b). All the four iron atoms are six-coordinate with an all-oxygen environment O₆ for the central iron atom and a mixed oxygen–nitrogen N₂O₄ environment for the three peripheral iron atoms, the former having more regular octahedral geometry than the latter. All four iron ions are located in a plane, with Fe–Fe–Fe angles of about 120°. The central iron is equally distant with three peripheral iron atoms, the distance 3.207 Å being slightly shorter than those found in other star-shaped tetranuclear metal derivatives [e.g., 3.44 Å for indium derivative].^16a As expected, the average terminal Fe–O bond distance, 1.913 and 1.887 Å for 1a and 1b, is slightly shorter than the average bridging Fe–O one, 2.001 Å and 1.990 Å for 1a and 1b, respectively. The Fe–N distances, which are found in the rage 2.248(3)–2.322(6) Å, compare well with those reported for some other iron aminoalkoxides having terminal Fe–N bonds.^17–20 The planar Fe₂O₂ rings have inner ring angles of 72.4(1)–75.1(1)° and 104.8(1)–107.9(1)° for iron and bridging oxygen atoms, respectively. It should be noted that the star-shaped isomer [Fe₄(mdea)₆]·(solvent)_x, with different number of solvation molecules [solvent = CHCl₃ (x = 4), CH₂Cl₂ (x = 3)],¹⁷ has previously been reported by Saalfrank et al. who synthesized it from the reaction of FeCl₃ and sodium salt of N-methyldiethanolamine. Even though all these isomers crystallize in triclinic crystal system (space group: P1̄), their molecular packings are all very different. These isomers form molecular layers parallel to the different axis of the unit cell where the interlayer distance is controlled by the hydrogen-bonded solvate molecules. Whereas in [Fe₄(mdea)₆]·4CHCl₃, hydrogen bonded chloroform molecules separated the molecular layers of Fe₄-star (shortest interlayer distances of Fe(III) centres = 10.78 Å), in case of [Fe₄(mdea)₆]·3CH₂Cl₂, bridging of these layers by dichloromethane was observed (interlayer distances of Fe(III) centres = 8.57 Å), leading to molecular strands along the a-axis. This reduced distance between the molecular layers in [Fe₄(mdea)₆]·3CH₂Cl₂ compared to Fe₄(mdea)₆·4CHCl₃, strongly affected the molecular magnetism.¹⁷ In case of [Fe₄(mdea)₆]·6CHCl₃ isomers reported here, the increased number of chloroform molecules increased the distance between molecular layers of Fe₄-star further (interlayer distances of Fe(III) centers = 13.003–13.764 Å) (Fig. S2†). The chair-shaped isomer 1a, on the other hand, has more compact packing with inter- and intra-columnar Fe⋯Fe distances being in the range 7.792–11.477 Å (Fig. S3†).

Fig. 1 Perspective view of two isomers of [Fe₄(mdea)₆]·6CHCl₃: (a) the chair-shaped isomer (1a) and, (b) the star-shaped isomer (1b). Ellipsoids are drawn at the 50% probability level and the chloroform molecules and H-atoms are omitted for clarity. Selected bond length (Å) and bond angle (°) (1a): Fe1–O1 1.978(2), Fe2–O3 2.010(3), Fe2–O4 1.907(3), Fe1–O5 2.035(3), Fe1–O2ⁱ 1.928(3), Fe2–N2 2.314(3), Fe1–N1ⁱ 2.248(3), O1–Fe1–O3 101.14(1), O3–Fe2–O6 100.40(1), O1–Fe1–O1ⁱ 75.14(1), O3–Fe2–N3 146.68(1), O1–Fe1–O2ⁱ 97.48(1), O4–Fe2–O5 102.33(1), O1–Fe1–N1ⁱ 151.02(1), O3–Fe1–O5 72.42(1), O4–Fe2–N2 78.56(1), O1ⁱ–Fe1–O3 162.42(1), O2ⁱ–Fe1–O3 91.86(1), O5–Fe2–O6 113.38(1), O3–Fe1–N1ⁱ 107.81(1), O5–Fe2–N3 77.34(1), O2ⁱ–Fe1–O5 157.56(1), O5–Fe1–N1ⁱ 89.21(1), N2–Fe2–N3 136.48(1), O2ⁱ–Fe1–N1ⁱ 80.31(1), O3–Fe2–O4 112.88(1), O3–Fe2–O5 72.53(1), symmetry code: (i) 2 − x, −y, −z. (1b): O9–Fe1 1.997(4), O1–Fe1 1.992(4), O11–Fe1 1.990(5), O3–Fe1 1.977(5), O5–Fe2 1.998(4), O6–Fe2 1.900(5), O5–Fe1 1.992(4), O7–Fe1 1.996(4), O7–Fe2 2.012(5), N5–Fe2 2.322(6), O8–Fe2 1.905(5), Fe2–N6 2.300(6), O7–Fe1–O5 73.61(18), O7–Fe1–O1 162.1(2), O5–Fe1–O1 93.76(19), O7–Fe2–O5 73.15(17), O7–Fe2–O8 114.0(2), O5–Fe2–O8 100.2(2), O7–Fe2–O6 100.3(2), O5–Fe2–O6 113.7(2).

The isomers 1a and 1b present similar decomposition pattern in thermogravimetric analysis. The TG curve shows a three-step decomposition in air that lasts up to 450 °C (Fig. 2a). The compound loses majority of the solvated CHCl₃ molecules in the first step (50–120 °C, endothermic peak at 104 °C). No significant mass loss was subsequently observed until the onset of decomposition at 215 °C, with decomposition occurring in two steps as indicated by two prominent exothermic peaks at 240 and 390 °C. The residual mass (27.3%) at 450 °C was higher than necessary for complete decomposition to Fe₂O₃ (19.5%), suggesting retention of a part of the mdea ligand. Complete hydrolyses of the derivatives 1, A and B were performed in boiling water and the resulting powders obtained were separated by centrifugation and, after washing with water and ethanol, characterised by FT-IR, TG-DTA as well as XRD at variable temperatures. The XRD patterns of the as-prepared powders showed that they were amorphous. Presence of hydroxyl group and some remaining organic residues was indicated by FT-IR spectra of these as-prepared powders (Fig. S5†), the latter could be eliminated on calcinations below 400 °C, as shown by TGA and DTA curves in Fig. S6.† The TG-DTA curves of hydrolyzed powders obtained from the derivative 1 showed a 2-step decomposition process with an endothermic peak at 85 °C and an exothermic peak at about 230 °C due to loss of adsorbed solvent or water (5.9% loss) and pyrolysis of residual organics (8.2% loss), respectively. The hydrolysis of 1 followed by calcination at 500 °C led to crystallized magnetic maghemite-C, syn-Fe₂O₃ nanoparticles (JSPDS 00-039-1346) (Fig. 2b and c). Hydrolyses of 1 was also performed in the presence of tert-butyl ammonium bromide in boiling water but this also led to the formation of amorphous as-prepared powders. Use of tert-butyl ammonium bromide salt as an additive in the sol–gel synthesis has previously led to the formation of highly crystalline TiO₂ nanoparticles at low temperature.²³ The presence of this salt had some influence though on the hydrolytic behaviour of 1 as the calcination of the as-prepared powder at 500 °C resulted in a mixture of two phases of Fe₂O₃ (JSPDS 00-039-1346 and JSPDS 04-006-6579, Fig. S7†). In contrast, the amorphous as-prepared powders obtained from the hydrolysis of A and B were converted majorly into crystallized non-magnetic hematite, syn-Fe₂O₃ nanoparticles (JSPDS 00-033-0664) on calcination at 500 °C, although another phase (JSPDS 04-011-7764) was also present as a minor component in the case of A (Fig. S8†). The size of these magnetic and non-magnetic Fe₂O₃ nanoparticles, as calculated by the Debye–Scherrer formula, was found to be in the range 15–28 nm.

Fig. 2 (a) TG-DTA curves of 1, (b) XRD pattern of the Fe₂O₃ NPs obtained after the hydrolysis of 1 and calcined at 500 °C, (c) digital image of these magnetic NPs in the presence of a magnet, and (d) SEM image of Fe₂O₃ film deposited by spin-coating of the sol obtained from 1 on SiO₂ substrate and calcination at 400 °C.

Colloidal suspensions or gelous media are usually required for the elaboration of thin films via solution routes. The stability of the colloidal media is also of importance for industrial applications, hence, partial hydrolysis and aging phenomena were investigated. Partial hydrolyses of 1 and A were performed by mixing 0.05 M–0.3 M solutions in isopropanol with a 0.2–0.8 M solutions of water in the same solvent, to achieve h = 2, 4, 5, 6 or 8. The poor solubility of triethanolamine derivative B in common solvents precluded evaluation of its hydrolytic behaviour. Hydrolyzed solutions were stored under argon for few hours at room temperature and evolution of the particle size in solution was recorded by light scattering measurements. The behaviour of precursors 1 and B were different for different hydrolysis ratio (h). For 0.057 M solutions of A, immediate precipitation was observed with amount of precipitate getting increased gradually from hydrolysis ratio (h) = 2 to 8. However, in the presence of parent alcohol N, N-dimethylethanolamine (dmeaH), clear sols were obtained for h = 2–6, which were stable for several weeks. An average particle size of 540, 461, 421 and 352 nm were obtained for h = 2, 4, 5 and 6, respectively. The 0.072 M solutions of N-methyldiethanolamine derivative 1 in isopropanol behaved better than that of N,N-dimethylethanolamine derivative A and formed clear sols based on particles of about 230–330 nm for h = 2, 4, 5 and 6, which were stable for 2–3 days. On increasing the concentration up to 0.3 M, the average particle size of the obtained sol also increased to 330 nm.

Following above observations, 1 was used to elaborate Fe₂O₃ films by spin coating. In the first experiment, 0.1 ml of the sol obtained by hydrolysis of the solution of 1 (conc. = 0.09 M, h = 2) was dropped rapidly on the pre-cleaned SiO₂ substrate of 500 nm thickness, spinning at a speed of 1000 rpm for 30 seconds. The substrate was then dried at 130 °C in the air and the whole procedure was repeated 5 times to increase the thickness of the coating. The film was finally annealed at 400 °C (10 °C min⁻¹) for 4 h in air. The scanning electron microscopic (SEM) images of this Fe₂O₃ film showed a continuous but slightly porous film of about 20 nm thickness (Fig. 2d). By changing deposition conditions (spinning rate 500 rpm, annealing at 80 °C under inert atmosphere and number of coating increased to 10), we obtained films of about 50–60 nm thickness, as determined by cross-sectional SEM measurements (Fig. S9†). These films adhered very well to the substrate and passed the Scotch tape test. We are currently studying other parameters such as increase in concentration of iron precursor and viscosity of the sol to enhance further the thickness up to 100 nm and reduce the porosity, which would allow us to have additional characterization of the films (XRD, RBS, SIMS …). The derivative 1 was further investigated as iron source for the fabrication of Fe-based perovskite FeBiO₃ nanomaterials, which has attracted tremendous attention as a unique material that is both magnetic and a strong ferroelectric at room temperature.³ The iron-based metal–organic precursors used so far for the above materials by Chemical Solution Deposition (CSD) methods are limited to [Fe(acac)₃], [Fe(OR)₃] (R = Et, Bu^t) and [Fe(propionate)₃].^10a The dark brown coloured sol obtained after the controlled hydrolysis of the toluene solution containing equivalent amounts of 1 and [Bi₂(mdea)₂(mdeaH)₂]²⁴ gave monolithic crack-free FeBiO₃ xerogel on keeping at 60 °C for few days (Fig. 3a). A calcination temperature of 500 °C was chosen for this xerogel following its TG-DTA curves, which showed elimination of most of the organic parts before this temperature. The XRD of the obtained magnetic nanoparticles showed them to be single phase FeBiO₃ with all the peaks indexing with the JCPDS file 04-009-2327 (Fig. 3b and c). The problems associated with obtaining phase-pure FeBiO₃ are well-described in the literature.³ It is important to prepare FeBiO₃ at low temperature to avoid bismuth loss, which generates other phases like Bi₂Fe₄O₉ (as confirmed by further calcination of above NPs at 600 °C, Fig. S10†). Sol–gel preparation of FeBiO₃ have also been reported using metallic salt as reagents but these risk retention of anions as impurities in the final materials. Further studies are currently underway to optimize the conditions for the elaboration of non-porous and sufficiently thick films of FeBiO₃ by spin coating.

Fig. 3 (a) digital image of BiFeO₃ xerogel obtained after the controlled hydrolysis of 1 and [Bi₂(mdea)₂(mdeaH)₂], (b) the magnetic BiFeO₃ NPs obtained after the calcination of xerogel at 500 °C, and (c) XRD powder pattern of these BiFeO₃ NPs.

Conclusions

We reported new homoleptic iron(III) aminoalkoxides based on N-methyl substituted aminoethanols {Me_3−xN(CH₂CH₂O)_x} (x = 1–3) showing interesting structural diversity, as revealed by isolation of a chair-shaped or a ‘Mitsubishi’ type star-shaped isomers for the N-methyldiethanolaminate derivative 1. The complete hydrolysis of these iron alkoxides yielded magnetic or non-magnetic Fe₂O₃ nanoparticles, depending on the aminoalkoxide ligand employed. From the various sol–gel processing parameters (concentration, hydrolysis ratio (h), solvent…) and evolution of the particle size in solution studied by light scattering measurements, the derivative 1 was found to be the most suitable candidate to stabilize nano-sized colloidal suspensions and to elaborate thin films of Fe₂O₃ by spin coating. The efficiency of 1 as a sol–gel precursor was further demonstrated by preparing monolithic xerogel and its subsequent conversion to phase-pure BiFeO₃ nanoparticles at low temperature.

Acknowledgements

We thank Dr L. Vandroux and M. Bernard (CEA, Grenoble) for spin-coating experiments as well as Y. Aizac and F. Bosselet (IRCELYON) for XRD measurements and Dr. A. Demessence (IRCELYON) for helping in digital photographs of NPs and xerogel.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental section, perspective views of molecular packing of 1a and 1b, FT-IR and TG-DTA curves of 1a and 1b and as-prepared Fe₂O₃ NPs obtained from the hydrolysis of the precursors, SEM image of Fe₂O₃ films, XRD of Fe₂O₃ and BiFeO₃ NPs, and 2 X-ray crystallographic files in CIF format. CCDC 1437704 and 1437705. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra24627j

‡ Crystal structure data for 1a (CCDC no. 1437704): C₃₀H₆₆Fe₄N₆O₁₂·6CHCl₃, M_r = 1642.5, triclinic, P1̄, a = 11.477(1) Å, b = 11.487(1) Å, c = 14.298(2) Å, α = 89.101(8)°, β = 74.396(10)°, γ = 70.145(9)°, V = 1701.6(3) ų, Z = 1, μ = 1.59 mm⁻¹, T = 100 K, 24 358 measured reflections, 7809 independent reflections, R_int = 0.041, R[F² > 2σ(F²)] = 0.041, wR(F²) = 0.105, S = 0.83, reflections/restrains/parameters 7809/514/436, Δρ_max = 0.77 e Å⁻³, Δρ_min = −0.57 e Å⁻³. 1b (CCDC no. 1437705): C₃₀H₆₆Fe₄N₆O₁₂·6CHCl₃, M_r = 1642.5, triclinic, P1̄, a = 11.3859(2) Å, b = 13.7637(3) Å, c = 22.7538(4) Å, α = 87.818(1)°, β = 80.207(1)°, γ = 76.405(1)°, V = 3415.4(1) ų, Z = 2, μ = 1.59 mm⁻¹, T = 150 K, 30 710 measured reflections, 16 296 independent reflections, R_int = 0.046, R[F² > 2σ(F²)] = 0.104, wR(F²) = 0.084, S = 1.15, reflections/restrains/parameters 9657/24/686, Δρ_max = 2.09 e Å⁻³, Δρ_min = −1.11 e Å⁻³.

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