Monomeric Iron Heteroarylalkenolates: Structural Design Concepts

Article pubs.acs.org/crystal

Monomeric Iron Heteroarylalkenolates: Structural Design Concepts and Investigations on Their Application in Chemical Vapor Deposition Gregor Fornalczyk,† Martin Valldor,§ and Sanjay Mathur*,† †

Institute of Inorganic Chemistry, University of Cologne, Greinstraße 6, 50939 Cologne, Germany Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden, Germany

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S Supporting Information *

ABSTRACT: Aryl substituted β-alkenol 1-(dimethyl-1,3-thiazol-2-yl)3,3,3-trifluoropropenol (DMTTFP) was employed as an efficient metal chelator to obtain volatile monomeric precursors containing FeII and FeIII centers. [Fe(DMTTFP)2] (1) and [Fe(DMTTFP)2(OBut)] (2) were synthesized by reacting suitable starting materials with DMTTFP. The molecular structures were elucidated by single-crystal X-ray diffraction analyses, which revealed a distorted tetrahedral and a trigonal-bipyramidal arrangement of ligands around iron atoms in 1 and 2, respectively. Magnetic investigations confirmed [Fe(DMTTFP)2] to exhibit a thermally populated spin-state transition that becomes apparent below 10 K. The high-spin state was gradually transferred to a low-spin state on cooling, suggesting a nonmagnetic ground state. [Fe(DMTTFP)2(OBut)] exhibited enhanced stability, sufficient volatility, and decomposition behavior serving as an efficient FeIII precursor for the growth of iron oxide layers on an Al2O3 substrate via chemical vapor deposition.

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INTRODUCTION The production of phase selective and homogeneous metal oxide materials requires precursors that ensure specific and well-defined properties, such as oxidation state, volatility, and stability against air and moisture, essential for routine handling as starting materials.1−3 A judicious balance between chemical reactivity and kinetic stability is often achieved by the interplay of steric and electronic factors.4 Nevertheless, there exist only few examples of successful precursor design as the incorporation of larger ligands adversely influences the vapor pressure due to the increase of the molecular weight.5 In addition, high thermal stability leads to residual organic contamination in final materials. These properties are tunable by using chelating heteroarylalkenols (Ar-CHCOCF3) as ligands, which enhance volatility and stability due to steric shielding by the aryl moiety and a decreased tendency to form oligomers due to the steric profile. Further, the electronic influence of the trifluoromethyl group shifts the keto−enol equilibrium to the enol side that enables a facile deprotonation as well as direct oxidation of the metals.6 Iron oxide is an important material finding technological applications in a variety of fields, such as solar absorber films,7 pigments,8 photoelectrochemical production of hydrogen,9 catalysis,10 and contrast agents.11 Despite the ever-increasing interest and application potential of iron oxide films, the available single-source precursors delivering both Fe and O in a single molecular unit are rather scant. Some representative examples include [Fe(OBut)3]212 and [Fe(acac)3].13 In this context, the development of new precursors suitable for © XXXX American Chemical Society

application in chemical vapor deposition methods represents a thrust research area integrating molecular chemistry and materials science. This work reports on the application of a new ligand system (Scheme 1) that produced highly manageable Fe(II) and Fe(III) precursors and presents preliminary data on their application to grow iron oxide films. For the deposition of iron oxide films, several precursors are already reported in literature. For various iron oxide modifications, [Fe(OBut)3]2 was frequently used because of its viable volatility and decomposition properties.12,14 Although these compounds are significantly sensitive to air and moisture, it was possible to create different iron oxides either directly by varying the deposition parameters or via post-treatment methods, such as laser interference pattering.15,16 The here presented Fe(II) and Fe(III) complexes that offer both volatility and stability promise to serve as suitable precursors for deposition techniques and the formation of iron oxide films.

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EXPERIMENTAL SECTION

Materials and Instrumentation. All manipulations of air- and moisture-sensitive materials were carried out under nitrogen using Stock-type glass assemblies. The ligand 1-(dimethyl-1,3-thiazol-2-yl)3,3,3-trifluoropropenol (DMTTFP) was synthesized as described previously.6 Anhydrous iron(II) and iron(III) chloride, hexamethyldisilazane (HMDS), potassium, sodium, and n-butyllithium were used without further purification. The solvents were dried by standard Received: December 28, 2013 Revised: February 25, 2014

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dx.doi.org/10.1021/cg401930n | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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HMDS, and 4 mL of 2.5 M (10 mmol) n-butyllithium solution in 10 mL of diethyl ether. The crude product was distilled at 80 °C under reduced pressure, obtaining a green liquid that was directed immediately into a suspension of DMTTFP in THF at 77 K. During defrosting, a color change to dark brown was observed. The solvents were removed under reduced pressure (10−3 mbar), and sublimation at 130 °C yielded 880 mg (35%, corresponding to the amount of FeCl2) of an orange solid. Via ligand exchange: [Fe(OBut)2]n was synthesized by distilling Fe(HMDS)2 directly into a mixture of toluene and excessive tert-butanol under nitrogen cooling. After defrosting of the reaction mixture and removal of the solvents under reduced pressure (10−3 mbar), a pale green powder was obtained as product. A 348 mg (1.6 mmol) portion of as-synthesized tert-butoxide was added to 30 mL of acetone, leading to a dark green suspension. Afterward, 720 mg (3.2 mmol) of DMTTFP was added, and the reaction mixture was kept stirring at room temperature until a clear orange solution was obtained. The solvent was again removed under reduced pressure, and sublimation at 130 °C (10−3 mbar) yielded 505 mg (63%) of the product. C16H14S2N2O2F6Fe (500.2) calcd C 38.41%, H 2.82%, N 5.60%, S 12.82%; found C 38.73%, H 3.10%, N 5.18%, S 12.41%. [Fe(DMTTFP)2(OBut)]. Iron(III) tert-butoxide was synthesized using FeCl3 and potassium tert-butoxide in a ratio of 1:3 in a mixture of THF and toluene and was sublimed at 95 °C and 10−3 mbar pressure. A 138 mg (0.25 mmol) portion of the as-synthesized precursor were dissolved in 20 mL of toluene before 225 mg (1.00 mmol) of DMTTFP was added to the solution. The reaction mixture was stirred vigorously at room temperature until the ligand was completely dissolved. Afterward, the solvent was removed under reduced pressure, and sublimation at 160 °C (10−3 mbar) gave 150 mg (52%) of a dark brown solid. C20H27S2N2O3F6Fe (577.4) calcd C 41.90%, H 4.04%, N 4.89%, S 11.18%; found C 41.39%, H 4.23%, N 4.65%, S 9.94%. Chemical Vapor Deposition (CVD). Iron oxide films were grown in a horizontal CVD reactor on Al2O3 substrates placed on a graphite susceptor, which was heated inductively up to 750 °C by a highfrequency field. The precursor [FeIII(DMTTFP)2(OBut)] was introduced by maintaining dynamic vacuum (10−3 mbar) and heating of the precursor flask to 140 °C. The temperatures of precursor and substrates as well as the pressure of the system were monitored during the entire process.

methods using appropriate desiccating reagents and distilled prior to their use. Elemental analysis was performed on a HEKAtech CHNS Euro EA 3000. Mass spectra were obtained on a Finnigan MAT 95 (20 eV) in m/z (rel %). NMR spectra were recorded on a Bruker Avance II 300 spectrometer; chemical shifts are quoted in parts per million relative to TMS (1H: 300.1 MHz; 13C: 75.7 MHz) and CCl3F (19F: 282.4 MHz). Data collection for X-ray structure elucidation was performed on a STOE IPDS I/II diffractometer using graphitemonochromated Mo Kα radiation (0.71073 Å). The programs used in this work are STOE’s X-Area,17 the WINGX suite of programs,18 including SIR-9219 and SHELXL-9720 for structure solution and refinement. Powder of compound 1 was investigated by means of Xray diffraction using a STADI P powder diffractometer from STOE with Mo K radiation (λ = 0.70926 Å) (Figure S1, Supporting Information). The calculated pattern was obtained with the Rietveld software Fullprof2k.21 The crystallographic data were taken directly from the single crystal refinement. The cell parameters, zero-position, background, peak profiles (Pseudo-Voigt), peak asymmetry, and an overall thermal displacement parameter were refined to obtain a reasonable description of the diffraction data. Note that none of the atomic positions were refined. Hence, the single crystal represents about 95% of the bulk powder sample, as about 5% impurity cannot be resolved by this method and only peaks are seen that belong to the main phase. The refined cell parameters are a = 12.338(2) Å, b = 18.425(4) Å, c = 18.077(4) Å, β = 103.94(2)°, of which all represent a better estimate than from the single crystal data. For powder X-ray diffraction of deposited films, a STOE-STADI MP diffractometer was used in a reflection mode using Cu Kα (λ = 1.5406 Å) radiation. Scanning electron microscopy (SEM) images were collected on an FEI Nova Nano SEM 430 equipped with an energy-dispersive X-ray (EDX) spectrometer Apollo X by EDAX. All magnetic investigations were performed in an MPMS-XL from Quantum Design, and the variable parameters were temperature (2−300 K) and field (up to 7 T). The powder samples were placed in hard gelatin capsules that were sealed with airtight lids. Plastic straws were used as sample holders, as recommended by the magnetometer manufacturer. Dimer simulations of [Fe(DMTTFP)2] were executed with the program JulX written by Bill.22 Thermogravimetric experiments were run on a TGA/DSC1 from Mettler Toledo, Germany, with masses of 5−10 mg. The samples were placed in aluminum cartridges and heated under a nitrogen atmosphere (flow rate: 25 mL/min) using a heating rate of 10 °C/min. 1-(Dimethyl-1,3-thiazol-2-yl)-3,3,3-trifluoropropenol (DMTTFP). A 7.3 mL (60 mmol) portion of 2,4,5-trimethylthiazole was added to a mixture of 25 mL (300 mmol) of pyridine and 160 mL of toluene under continuous stirring. Subsequently, 26 mL (0.19 mol) of trifluoroacetic acid anhydride was added dropwise to the reaction after cooling down to 0 °C. Stirring at room temperature was continued for 18 h. Afterward, 600 mL of a 3% sodium carbonate solution was poured into the reaction flask. The final product was extracted with ethyl acetate (3 × 200 mL) and left for precipitation at 8 °C. It was filtrated under vacuum and dried in air, giving 3.64 g (27%) of a light pink powder. 1H NMR (300.1 MHz, DMSO-d6): δ = 12.80 (s, OH, 1H) 6.05 (s, H3, 1H), 2.19 (s, H7, 3H), 2.16 (s, H8, 3H) ppm. 13C NMR (75.7 MHz, DMSO-d6): δ = 167.2 (C2), 164.4 (C4), 132.0 (C6), 119.1 (C1), 116.3 (C5), 82.6 (C3), 11.5 (C8), 10.9 (C7) ppm. 19F NMR (282.4 MHz, DMSO-d6): δ = −74.1 (1JC,F = 288 Hz, 2JC,F = 32 Hz). C8H8SNOF3 (225.2) calcd C 43.05%, H 3.61%, N 6.27%, S 14.37%; found C 43.36%, H 3.71%, N 6.42%, S 14.42%. [Fe(DMTTFP)2]. Via salt elimination: A 1.6 mL portion of 2.5 M (3.9 mmol) n-butyllithium solution was added dropwise to 0.88 g (3.9 mmol) of DMTTFP in 100 mL of toluene under vigorous stirring at room temperature. After 1 h, the color of the reaction mixture had turned from pale red to yellow. This mixture was poured on 247 mg (1.95 mmol) of iron(II) chloride and stirred for another 36 h. A color change to orange and the formation of a white precipitate were observed. The solvent was then removed under reduced pressure (10−3 mbar), and the final product was carried out via sublimation at 130 °C, giving 528 mg (54%) of an orange solid. Via HMDS-route: Fe(HMDS)2 was prepared using a salt elimination reaction starting from 634 mg (5.00 mmol) of iron(II) chloride, 2.1 mL (10 mmol) of

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RESULTS AND DISCUSSION Ligand Synthesis. The general synthetic approach to heteroarylalkenols such as 1-(dimethyl-1,3-thiazol-2-yl)-3,3,3trifluoropropenol (DMTTFP) as well as their corresponding metal complexes was recently reported by us.23−25 DMTTFP was synthesized starting from 2,4,5-trimethylthiazole and trifluoroacetic acid anhydride (TFAA) following a procedure reported by Kawase et al. (Scheme 1).26 The product was Scheme 1. Synthesis of 1-(Dimethyl-1,3-thiazol-2-yl)-3,3,3trifluoropropenol (DMTTFP)

recrystallized from ethyl acetate and characterized using elemental analysis and NMR spectroscopy. The 1H NMR spectrum showed signals at 6.05 ppm for the vinylic proton and at 12.80 ppm for the acidic proton. As this proton is involved in a tautomeric exchange process, enol and enaminone forms of the ligand are no longer distinguishable via NMR. However, theoretical calculations for the molecule in the gas phase revealed the enaminone as the preferred mode. These results confirm the electronic influence of the trifluoromethyl group in shifting the keto−enol/enaminone equilibrium nearly B



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Scheme 2. Synthetic Pathways to Homoleptic [Fe(DMTTFP)2] via Salt Elimination (a) and Ligand Exchange (b, c) and the According Yields

Scheme 3. Reaction Pathway for the Synthesis of the Heteroleptic Iron(III) Compound [Fe(DMTTFP)2(OBut)]

a monomeric Fe(II) compound with a 4-fold coordinated iron atom. Synthesis of [FeIII(DMTTFP)2(OBut)]. For the synthesis of the iron(III) compound [Fe(DMTTFP)2(OBut)] (2), dimeric iron(III) tert-butoxide was prepared by a salt elimination reaction between FeCl3 and KOBut as described elsewhere.15 As-synthesized [Fe(OBut)3]2 was used in a partial ligand exchange reaction with DMTTFP in a 1:2 molar ratio. The resulting compound [Fe(DMTTFP)2(OBut)] (2) is constituted by bidentate chelation of two DMTTFP units and a monodentate tert-butoxo group that gave a 5-fold coordinated Fe(III) center (Scheme 3). Attempts to obtain [Fe(DMTTFP)(OBut)2] by an equimolar reaction of [Fe(OBut)3]2 and DMTTFP were not successful and produced the bis-substituted compound and excess (unreacted) [Fe(OBut)3]2, indicating 2 to be the kinetically favored product despite an asymmetric coordination sphere. Single-Crystal X-ray Diffraction Analysis. The single crystals suitable for X-ray diffraction analysis were grown from toluene solutions. 1 crystallizes in a monoclinic crystal system with eight molecules per unit cell. The molecular structure displayed a monomeric Fe(II) complex with a 4-fold coordinated iron atom (Figure 1, left). The slightly distorted tetrahedral environment in [Fe(DMTTFP)2] is revealed best by the differences between the bite angles (N1−Fe1−O1, N2− Fe1−O2, ∼92°) of the ligand and the angles O1−Fe1−N2 and N1−Fe1−O2 (∼114°). The Fe−O bonds of 1.921(2) Å are relatively short but still in good agreement with other tetrahedral Fe(II) complexes of this kind.29 The Fe1−N1 and Fe1−N2 bond lengths (2.063(3) Å) are comparable as well to those of known Fe(II) complexes.30 In the packing, an interaction between the iron and the sulfur atoms (d(Fe−S) = 3.5725(15) Å, 3.7826(15) Å) was observed, leading to infinite chains. Considering these secondary contacts to affect the molecular structure, the distorted tetrahedron is expanded to a strongly distorted octahedral environment with a “4 + 2”

entirely to the left side (Scheme 1), where, here, the molecule is depicted as an enol. Synthesis of [FeII(DMTTFP)2]. Homoleptic iron(II) compound [Fe(DMTTFP)2] (1) was synthetically accessible via three different reaction pathways (a, b, and c in Scheme 2) involving air- and moisture-sensitive constituents. For salt elimination reactions, anhydrous iron(II) chloride was used in a reaction with the lithium salt of DMTTFP. In line with the previous reports on reactive behavior of this ligand toward a wide range of transition-metal chlorides, a facile reaction between FeCl2 and Li-DMTTFP led to the aspired product (1) in satisfactory yield (54%). 24 When compared to salt elimination reactions, the ligand exchange reactions based on higher basicity of the incoming ligands are more promising for higher yields and pure products. Reactions of [Fe(N{Si(CH3)3}2)2] (N{Si(CH3)3}2 = HMDS) and polymeric tertbutoxide [Fe(OBut)2]n with heteroarylalkenols appeared to be adequate since both hexamethyl disilazane and tert-butyl alcohol are good leaving groups and easy to remove out of the reaction mixture.27,28 FeII(HMDS)2 was synthesized from iron(II) chloride and Li-HMDS, giving a green liquid that was used directly after distillation. Reaction between Fe(HMDS)2 and DMTTFP occurred instantaneously, leading to a ligand exchange reaction at room temperature. The yield was relatively low (35%), which was possibly due to the very high sensitivity of the iron(II) amide. To circumvent this, Fe(HMDS)2 was reacted with tert-butanol to obtain the corresponding tertbutoxide [Fe(OBut)2]n as an air-sensitive solid, which was less sensitive then the amide, and the ligand exchange reaction was complete after a few minutes of stirring at ambient conditions with 63% yield. In all the three cases, a sublimable orange solid (130 °C, 10−2 mbar) was obtained (Scheme 2). The products were characterized by elemental analysis and single-crystal Xray diffraction studies. The single crystals suitable for diffraction experiments were grown from the gas phase during sublimation under reduced pressure. The molecular structure of 1 displayed C



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was calculated to be 0.75, indicating the molecule to exist merely as a slightly distorted trigonal bipyramid (τmax = 1) rather than a square pyramid (τmin = 0) (Figure 2, right).31 The axial N1−Fe1−N2 angle is 167.8°, revealing the weak distortion of the bipyramidal structure. The bite angles of around 87° are approximately 5° smaller than the corresponding angles in 1, which may be attributed to the additional alkoxo ligand and its sterical demand in the ligand sphere. The Fe−O distances of 1.938(2) (Fe1−O1) and 1.921(2) Å (Fe1−O2) are found in the typical range determined for iron chelate complexes known in the literature and are only slightly longer compared to the homoleptic derivative, [Fe(DMTTFP)2].32 The view along the a axis shows the space demanding trifluoromethyl groups pointing at each other building fluorine containing channels in the packing of the unit cell, while a significant π−π stacking of the aromatic units could not be observed from any direction (Figure 3). The Fe1−O3 distance (alkoxo group) of 1.808(2) Å matches closely that of the terminal alkoxo groups found in homoleptic iron(III) tertbutoxide (d(Fe−O) = 1.798, 1.784 Å).33 The Fe−N distances (2.186(2) Å) are significantly longer than those of 1 but still in good agreement with known Fe(III) thiazole complexes.34 Magnetic Properties of [FeII(DMTTFP)2]. For the magnetic investigations, a powder sample of [Fe(DMTTFP)2] (1) was prepared. This sample was in addition characterized via powder X-ray diffraction experiments, showing a very good accordance between the experimental pattern and the one that was calculated from the structural data of the single crystal of 1 (Figure S1, Supporting Information). [Fe(DMTTFP)2] exhibits a paramagnetic moment of 4.93 μB/Fe, suggesting a high-spin d6 state without orbital contributions to the magnetic moment (left inset in Figure 4). The plotting of χT vs T also reveals the paramagnetic behavior of the compound. The relatively large negative Weiss constant (θCW) cannot be easily understood, because the Fe ions appear as monomers in the crystal structure. This structural fact prevents even dimer (singlet) formation, also because the Fe ions cannot connect through reasonable superexchange paths. Normally, a negative θCW suggests that the spins are fluctuating with a preference for antiparallel spin orientations in the paramagnetic range and that an antiferromagnetic ground state should result. However, the magnetic anomaly close to 10 K (lower right inset in Figure 4) does not agree with a transition into a Néel state, due to the following reasons. First, the transition is relatively broad in temperature, although a relatively small magnetic field is used. Second and most important, it is commonly known that χ of an antiferromagnetic ground state is about 2/3 of the observed maximum in χ, i.e., at TN.35 At 2 K, χ of [Fe(DMTTFP)2] is clearly below this value. By simple extrapolation of the curve down to the ground state (0 K), it is obvious that χ is close to zero, i.e., a nonmagnetic state. Hence, another explanation has to be found for this decrease in χ. Close at hand is a spin-state transition of the d6 ion from a high-spin (S = 2) to a low-spin state (S = 0), and the gradual change might coincide with a thermal population of both spin states (Figure S2, Supporting Information). A similar behavior has been theoretically suggested for quasi octahedrally coordinated d6 ions,36 and such a transition to a nonmagnetic ground state is perhaps realized in the here presented [Fe(DMTTFP)2]. It is likely that a minor reconstruction within the crystal structure changes the crystal field of Fe2+, thus enabling a more square-planar crystal

Figure 1. Left: Molecular structure of the homoleptic tetrahedral [Fe(DMTTFP)2] (1). The atomic displacement ellipsoids are drawn at the 50% probability level; hydrogen atoms are omitted for clarity. Right: Packing of the molecular units in the unit cell, where the Fe−S distances (d(Fe−S) = 3.5725(15) Å, 3.7826(15) Å) are highlighted with dashed lines; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1−O1 1.919(2), Fe1−O2 1.922(2), Fe1−N1 2.059(3), Fe1−N2 2.067(3), Fe1−S1 3.5725(15), Fe1−S2 3.7826(15); O1−Fe1−O2 134.57(11), O2−Fe1−N1 114.73(11), O1−Fe1−N2 114.90(11), N1−Fe1−N2 107.94(10), N1−Fe1−O1 91.65(10), N2−Fe1−O2 92.30(11).

arrangement of ligands with two weak interactions (Figure 1, right). Since the distances between the closest iron centers are 5.6289(15) and 5.450(15) Å in each direction, a pairwise arrangement of molecules could not be observed. Calculations with the software JulX confirmed our assumptions regarding the monomeric nature of [Fe(DMTTFP)2] (1).22 Further characterization and magnetic properties of 1 in particular are given later on. The sublimation (160 °C, 10−3 mbar) of [Fe(DMTTFP)2(OBut)] (2) produced crystals suitable for X-ray diffraction analysis. The compound crystallizes in a triclinic crystal system with two molecules per unit cell (see Table 1). The molecular structure (Figure 2, left) showed a heteroleptic, monomeric complex containing two chelating ligands and one remaining tert-butoxo group with oxygen atoms residing in equatorial positions and nitrogen atoms occupying apical sites. The τ value for the angles α (N1−Fe1−N2) and β (O1−Fe1−O3) Table 1. Crystallographic Data for the Compounds [Fe(DMTTFP)2] and [Fe(DMTTFP)2(OBut)] Collected at 293(2) K compound

[Fe(DMTTFP)2]

[Fe(DMTTFP)2(OBut)]

chemical formula molecular weight crystal system space group a (Ǻ ) b (Ǻ ) c (Ǻ ) α (deg) β (deg) γ (deg) V (Ǻ 3) Z R1, wR2 (2σ) R1, wR2 (all data) goodness-of-fit on F2

C16H14S2N2O2F6Fe 500.26 monoclinic C2/c 12.323(5) 18.408(5) 18.015(5) 90.000(5) 104.012(5) 90.000(5) 3965.0(2) 4 0.0489, 0.1347 0.0721, 0.1536 1.031

C20H23S2N2O3F6Fe 573.37 triclinic P1̅ 8.1848(13) 10.8195(17) 14.7741(19) 76.019(17) 86.053(18) 70.005(17) 1192.9(3) 2 0.0416, 0.0858 0.0875, 0.0961 0.847 D



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Figure 2. Left: Molecular structure of the heteroleptic trigonal bipyramid of monomeric [Fe(DMTTFP)2(OBut)]. Right: Simplified view of the geometry to reveal the angles α (N1−Fe1−N2) and β (O1−Fe1−O3). The atomic displacement ellipsoids are drawn at the 50% probability level; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1−O1 1.938(2), Fe1−O2 1.921(2), Fe1−O3 1.808(2), Fe1− N1 2.183(2), Fe1−N2 2.189(2); O1−Fe1−O2 118.97(10), O2−Fe1−O3 117.79(9), O1−Fe1−O3 123.16(10), N1−Fe1−N2 167.78(9), N1− Fe1−O1 86.73(9), N2−Fe1−O2 87.43.

domain effect. Low-temperature Mössbauer spectroscopy data would settle this matter, but no such data are available yet. Chemical Vapor Deposition (CVD) Experiments. The decomposition behaviors of the prepared compounds [Fe(DMTTFP)2] and [Fe(DMTTFP)2(OBut)] were investigated via thermogravimetric measurements (TG). The TG curves show rapid mass losses at around 235 °C (1) and (2), indicating single-step decompositions (Figure 5). The relative mass loss of 88% for 1 is higher than the theoretical mass losses for the formation of Fe2O3 (84%) and Fe3O4 (77%), indicating a partial sublimation of the precursor during the decomposition process. [Fe(DMTTFP)2(OBut)] loses 66% of its mass, representing a higher remaining weight than the theoretical values for the mass losses leading to Fe2O3 (86%) and Fe3O4 (87%). In this case, we assume unvolatile decomposition products to cause the higher weight of the residue. In addition, the volatility of [Fe(DMTTFP)2(OBut)] (2) under atmospheric pressure is lower than the one of [Fe(DMTTFP)2] (1), leading to the conclusion that 1 decomposes at lower temperatures, faster and more complete. For the deposition process, the substrate temperatures were held close to the sublimation points of the compounds obtained under vacuum conditions. Substrate temperatures were set up between 750 and 800 °C. Although the TG results showed decompositions in similar temperature regions, compound 1 did not show sufficient deposition of layers on the given substrates which were suitable for further analysis. A possible reason might be the homoleptic nature of the complex where the iron(II) center is strongly shielded by two sterically demanding ligands. The chelating effect is stabilizing the compound while contact or reactive groups like alkoxides are missing in the ligand field. Although the decomposition should run in a single-step mechanism after a crucial temperature, as it was revealed by TG measurements, it was not able to reach sufficient conditions for this process in our CVD system. Concerning the stability of homoleptic compound 1, the usage of reactive gases could lead to a higher decomposition rate.

Figure 3. View along the a axis onto the unit cell of [Fe(DMTTFP)2(OBut)], where neighboring trifluoromethyl groups are highlighted.

field than the distorted tetrahedral one observed at higher temperatures. Hence, it is expected that [Fe(DMTTFP)2] contains a spin-state instability with a “gap” of a few Kelvin. This would also fit the magnetization data (lower right inset in Figure 4); at low temperatures, it is possible to enforce a population of the higher spin state through a meta magnetic transition, keeping in mind that 1 T has about the energy of 0.7 K. As field-cooled (FC) and zero-field-cooled (ZFC) data superimpose below 10 K, this effect is not due to a magnetic E



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Figure 4. χT vs T of [Fe(DMTTFP)2] with both field-cooled (FC) and zero-field-cooled (ZFC) data. The insets contain magnetization data at several different temperatures (upper right), Curie−Weiss fit (lower left), and a magnification of the low-temperature region (lower right), in which χ of a normal 3D antiferromagnetic (AF) ground state is indicated.

Figure 5. TG curves of [Fe(DMTTFP)2] (left) and [Fe(DMTTFP)2(OBut)] (right).

In 2, we have, in addition to bidentate ligands, a tert-butoxo group saturating the iron(III) center, which acts as a contact surface, as it could decompose to isobutene and hydroxo groups, which can, thus, interact with the substrate surface. Here, we obtained a dark film on the substrate that was characterized by X-ray powder diffraction. By comparing the measurement data with reference patterns, matching signals of hematite and magnetite modifications were found (Figure 6). Reflexes that did not match to iron oxide phases could be assigned to substrate material aluminum oxide. However, few reflexes are present in the measured diffraction pattern, which showed no matching with known phases. Here, the increase of phase purity can be a goal of future experiments dealing with these precursor systems. Preparation of phase-pure iron oxide using this process could be achieved by post-treatment methods, such as annealing under oxygen or vacuum.

In Figure 7, the scanning electron microscope (SEM) images are shown for the layer deposited starting from the [Fe(DMTTFP)2(OBut)] precursor on still visible aluminum oxide crystallites. The SEM images show layers of iron oxide, which is geared to the grain boundaries of the aluminum oxide substrate. Energy-dispersive X-ray spectroscopy (EDX) showed the presence of iron and oxygen at the surface, while there is no evidence of significant impurities, such as carbon or nitrogen, caused by the ligand decomposition (Figure 7, right).

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CONCLUSION Molecular compounds [Fe(DMTTFP)2 ] (1) and [Fe(DMTTFP)2(OBut)] (2) were synthesized for the first time and structurally characterized by XRD methods. The introduction of fluorinated bidentate chelating ligands imparted an enhanced stability against moisture and air as demonstrated by our group before.23−25 Synthesis of 1 was demonstrated via F



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ASSOCIATED CONTENT

S Supporting Information *

X-ray powder diffraction pattern of compound 1. X-ray crystallographic data files in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to the University of Cologne for infrastructural support and financial assistance. We gratefully acknowledge the University of Cologne and SOLAROGENIX Project (EC-FP7- Grant Agreement No. 310333) for the financial support. M.V. would like to acknowledge the German Science Foundation (DFG) for its support through SFB608.

Figure 6. X-ray diffraction pattern of the obtained layer using [Fe(DMTTFP)2(OBut)] in a CVD process compared to reference patterns of hematite, magnetite, and corundum.

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REFERENCES

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three different pathways including salt elimination and ligand exchange. Comparably shorter Fe−O bonds in 1 led to the presumption of uncommon magnetic properties, which were investigated by SQUID measurements, to observe a spin-state transition that occurred below 10 K, whereby the high-spin state was transferred into a low-spin state on cooling. This could be explained by a reconstruction within the crystal structure enabling a more square-planar crystal field than a distorted tetrahedral one. 2 was prepared in a partial ligand exchange starting from iron(III) tert-butoxide forming a heteroleptic, monomeric complex that existed merely as a trigonal bipyramid (τ = 0.75). Both compounds show enhanced stability compared to precursors known from the literature. The sublimation points of 130 and 160 °C, respectively, are suggestive of sufficient volatility for the synthesis of iron oxide materials by vapor phase techniques, such as chemical vapor deposition (CVD) or atomic layer deposition. As a proof of concept, 2 was used to grow thin iron oxide layers on aluminum oxide substrates via CVD. The CVD deposit obtained was identified as a mixed-phase containing both hematite and magnetite, which indicated the partial reduction of Fe(III) centers during thermolysis. Further studies on the comparison of materials obtained from 1 and 2 are currently underway.

Figure 7. SEM images of iron oxide films deposited on Al2O3 after decomposition of [Fe(DMTTFP)2(OBut)] (left, middle) and the corresponding EDX spectrum (right). G



Article

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Monomeric Iron Heteroarylalkenolates: Structural Design Concepts

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