New Fe–Ta and Co–Ta oxalate complexes: structural characterization and thermal behaviour – formation of mixed-metal oxides

Abstract

Two novel heterometallic oxalate-based compounds, [M(phen)₃][Ta(OC₂H₅)(C₂O₄)₃]·H₂O [M = Fe (1), Co (2); phen = 1,10-phenanthroline] were synthesized by using an (oxalato)tantalate(V) aqueous solution as a source of complex anions. As a result of the presence of ethanol in the reaction mixture, the ethoxy group coordinated to tantalum(V) and the [Ta(OC₂H₅)(C₂O₄)₃]²⁻ anion was isolated in the crystalline products. The obtained compounds were characterized by means of IR spectroscopy, X-ray single-crystal diffraction and thermal analysis. Both compounds were investigated as possible molecular precursors for the mixed-metal oxides. Thermal decomposition residues of the compounds at different heating temperatures (800, 1000, 1200 °C) were analyzed by powder X-ray diffraction. When 1 was heated up to 800–1200 °C the mixed-metal oxide FeTaO₄, of different crystallite sizes, formed. The thermal decomposition residue of 2, heat-treated up to 1000 °C, contained an up to now non-investigated mixed-metal oxide, Co₄Ta₂O₉, along with the better known mixed-metal oxide CoTa₂O₆. Co₄Ta₂O₉ was for the first time structurally characterized: the structure is composed of (i) layers containing hexagonally arranged CoO₆ octahedra and (ii) layers where both CoO₆ and TaO₆ octahedra form a hexagonal assembly.

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Introduction

The aqueous chemistry of tantalum has been restricted merely to peroxide compounds¹ and certain forms of carboxylates, such as oxalate,^2,3 tartarate,^2,4 citrate,² glycolate,⁵ lactate⁶ and polyaminocarboxylate species.⁷ Most of these tantalum compounds have been utilized as starting materials for the sol–gel or polymeric precursor methods for the preparation of oxide materials, more specifically, mixed-metal oxides.^2,4,5 Amongst a limited number of structurally defined tantalum complexes that were prepared from aqueous solutions, there are only a few heterometallic compounds.^3b,c,4a These well defined heterometallic species are of great interest because their thermal decomposition can lead to the mixed-metal oxides of tantalum. Metal tantalates represent very common functional inorganic materials with various properties and applications, in particular photocatalytic and dielectric.⁸ The molecular precursor-to-material conversion for the preparation of mixed-metal oxides has several advantages: (i) crystalline oxides are produced under significantly milder conditions than are those employed in traditional solid-state syntheses, thus resulting in relatively high specific surface-area materials; (ii) there is a better control of the metal stoichiometry in the final products; (iii) the materials are more homogeneous due to the mixing of metal ions at the molecular level.

The remarkable coordination ability of the oxalate anion, C₂O₄²⁻, has already been proved in many homo- and heteropolynuclear species with various nuclearities or dimensionalities.⁹ Relatively low thermal stability of the heterometallic oxalate compounds (oxalate ligand decomposes at ≈ 400 °C to gaseous CO₂ and CO) makes them suitable for the use as molecular precursors for the mixed-metal oxides.¹⁰ Another advantage of the oxalate ligand is its low cost.

As a continuation of our previous studies on the oxalate chemistry of tantalum,³ in this study we investigated the reactivity of the (oxalato)tantalate solution with the transition-metal complex cations [M(phen)₃]²⁺ (M = Fe, Co). Two new heterometallic complexes [M(phen)₃][Ta(OC₂H₅)(C₂O₄)₃]·H₂O [M = Fe (1), Co (2); phen = 1,10-phenanthroline] were synthesized; their spectroscopic, structural and thermal properties were examined. The mixed-metal oxides FeTaO₄ and a mixture of CoTa₂O₆ and Co₄Ta₂O₉ were obtained upon thermal treatment of 1 and 2 at different temperatures. While the mixed-metal oxides of tantalum have been so far poorly studied, the analogous derivatives of niobium are well understood. For example, FeNbO₄ is found to have gas sensing properties¹¹ and visible-light-induced photocatalytic activity.¹² Magneto-structural studies of the mixed Co–Nb oxides (i.e. CoNb₂O₆ and Co₄Nb₂O₉) have drawn considerable attention due to a rich variety of their magnetic behaviour.¹³ Up to now, such mixed-metal oxides have been synthesized by the solid-state reactions that required long-term heating at relatively high temperatures and repeating grinding procedures. The preparation of FeTaO₄, CoTa₂O₆ and Co₄Ta₂O₉ through the prospective oxalate-based Fe–Ta and Co–Ta precursors (compounds 1 and 2, respectively) stands for a much more convenient way for obtaining mixed-metal oxides in general. The method, therefore, represents a significant contribution to the functional oxide materials chemistry, particularly in the context of a growing need for the multicomponent oxide materials of tantalum(V).

Experimental

Materials and methods

The (oxalato)tantalate(V) solution was prepared by dissolving freshly precipitated Ta₂O₅·nH₂O in H₂C₂O₄·2H₂O, following the procedure described previously.¹⁴ All other chemical reagents used in the synthesis were purchased from commercial sources, and applied without further purification. Elemental analyses for C, H and N in 1 and 2 were carried out using a Perkin Elmer Model 2400 microanalytical analyzer. Infrared spectra were recorded as KBr pellets using a Bruker Alpha-T spectrometer in the 4000–350 cm⁻¹ range. The thermal analysis was performed on a Shimadzu DTG-60H analyzer, from the room temperature to 1200 °C, in the stream of synthetic air at a heating and cooling rate of 10 °C min⁻¹.

Synthesis of [M(phen)₃][Ta(OC₂H₅)(C₂O₄)₃]·H₂O [M = Fe (1), Co (2)]

Ethanol solution (8 mL) of 1,10-phenanthroline (phen·H₂O; 79.3 mg; 0.4 mmol) was added dropwise to an aqueous solution (2 mL) of MCl₂·nH₂O [0.2 mmol; n = 4 (Fe), 6 (Co)]. The reaction mixture was stirred for 15 minutes, and then an (oxalato)tantalate(V) solution (1 mL) [m(Ta) = 18.1 mg; n(Ta) = 0.1 mmol] was added. A small amount of a precipitate (red in the Fe–Ta system, and pink in the Co–Ta system) formed immediately, which was removed by filtration after 30 minutes of stirring. From the clear solution the prism-like single crystals (of both compounds) started to form soon, which were left to grow in the mother liquid. After two days the crystals were filtered off and dried in air (the yield was ≈ 70% for both 1 and 2). Found for compound 1, %: C, 47.55; H, 3.19; N, 7.03. Calcd for C₄₄H₃₁FeTaN₆O₁₄, %: C, 47.85; H, 2.83; N, 7.61. IR, (KBr): [small nu, Greek, tilde] = 3558 (m), 3494 (m), 3439 (m), 3056 (w), 1756 (m), 1723 (vs), 1684 (s), 1578 (w), 1515 (w), 1493 (w), 1428 (m), 1414 (w), 1360 (s), 1333 (s), 1253 (w), 1208 (w), 1195 (m), 1185 (m), 1148 (m), 1108 (m), 1072 (m), 1003 (w), 949 (w), 897 (m), 848 (s), 799 (s), 768 (w), 723 (s), 645 (w), 616 (w), 560 (m), 545 (m), 459 (m), 446 (m), 373 (m) cm⁻¹. Found for compound 2, %: C, 47.44; H, 3.02; N, 7.14. Calcd for C₄₄H₃₁CoTaN₆O₁₄, %: C, 47.71; H, 2.82; N, 7.59. IR, (KBr): [small nu, Greek, tilde] = 3561 (m), 3495 (m), 3440 (m), 3058 (w), 1755 (m), 1723 (vs), 1685 (s), 1625 (m), 1583 (w), 1518 (s), 1496 (w), 1426 (w), 1426 (s), 1358 (s), 1333 (s), 1257 (w), 1225 (w), 1210 (w), 1193 (m), 1147 (m), 1107 (m), 1072 (m), 1010 (w), 959 (w), 897 (m), 884 (w), 868 (w), 851 (s), 800 (s), 770 (w), 726 (s), 643 (w), 560 (m), 545 (m), 462 (w), 424 (w), 374 (w) cm⁻¹.

Single-crystal X-ray study

The X-ray data for the single crystals of 1 and 2 were collected at 293(2) K by ω-scans on an Oxford Diffraction Xcalibur Nova R diffractometer with the graphite monochromated Cu-Kα radiation (λ = 1.54179 Å, microfocus tube, CCD detector). Data reduction was performed with the CrysAlis PRO software package.¹⁵ Solution, refinement and analysis of the structures were performed using the programs integrated in the WinGX system.¹⁶ The structures were solved by direct methods (SIR92)¹⁷ and refined with SHELXL97.¹⁸ The model was refined using the full-matrix least squares refinement; all non-hydrogen atoms were refined anisotropically. Hydrogen atoms attached to the C atoms of the phenanthroline ligands were treated as riding in idealized positions, with the C–H distances of 0.93 Å and displacement parameters assigned as U_iso(H) = 1.2U_eq(C). Hydrogen atoms from the CH₃ fragment of the ethoxy group were treated as riding and were positioned according to the idealized tetrahedral geometry around the C atoms, with the C–H distances of 0.96 Å and displacement parameters assigned as U_iso(H) = 1.5U_eq(C). Hydrogen atoms from the CH₂ fragment of the ethoxy group were refined freely. The geometry of the water molecule was restrained to the target values: O–H distance of 0.90 Å, using the DIFX command, and H–O–H angles of 104°, as U_iso(H) = 1.5U_eq(O). Molecular geometry calculations were performed by PLATON,¹⁹ and molecular graphics were prepared using the ORTEP-III (ref. 20) and CCDC-Mercury (ref. 21) programs. The crystal data, experimental conditions and final refinement parameters are summarized in Table 1.

Table 1 Crystallographic data and the structure refinement details for 1 and 2

Compound	1	2
a b c
Empirical formula	C44H31FeTaN6O14	C44H31CoTaN6O14
Formula weight/g mol^−1	1104.55	1107.63
Crystal colour, habit	Red, prism	Orange, prism
T/K	293(2)	293(2)
Crystal system	Triclinic	Triclinic
Space group	P1̄	P1̄
a/Å	11.424(5)	11.538(5)
b/Å	12.763(5)	12.683(5)
c/Å	15.546(5)	15.657(5)
α/°	108.799(5)	108.147(5)
β/°	92.429(5)	92.156(5)
γ/°	108.397(5)	109.194(5)
V/Å^3	2009.3(13)	2031.0(14)
Z	2	2
ρcalcd/g cm^−3	1.826	1.811
μ/mm^−1	8.537	8.781
F(000)	1096	1098
θ range/°	3.04–75.59	3.01–76.1
Measured reflections	19 287	20 185
Independent reflections	8245	8389
Observed reflections	7928	7551
Parameters, restraints	611, 3	611, 3
Rint	0.0504	0.0355
R^a, wR^b [I > 2σ(I)]	0.0370, 0.0981	0.0350, 0.0927
R, wR [all data]	0.0387, 0.0994	0.0391, 0.0947
Goodness of fit, S^c	1.044	1.046
Δρmax, Δρmin/e Å^−3	1.289; −1.747	0.948; −1.699

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Powder X-ray structural and microstructural study

X-ray powder diffraction (XRPD) patterns were measured in the reflection mode with the monochromated Cu-Kα radiation on a PW1830 diffractometer in the 2theta range 10–70° (with the step size of 0.02° and at 10 s per step). The Rietveld structure refinement was performed by Panalytical HighScore X'pert Plus ver. 2.1. The polynomial model was used to describe the background. Diffraction profiles were described by the Pseudo-Voigt function. During the refinement a zero shift, scale factor, half-width parameters (U, V, W), asymmetry parameters and peak shape parameters were refined simultaneously. Microstructural characterization was carried out by the XBroad program,²² which performs Stokes deconvolution²³ in order to separate pure diffraction and instrumental broadening, followed by the Warren-Averbach analysis.²⁴ Namely, for each diffraction line XBroad calculates the first 20 Fourier coefficients and generates the F(L) vs. L plot, where t is the order of the Fourier coefficient and L is as follows:

The initial slope of the F(L) plot when L approaches zero,

gives the negative reciprocal value of the average column length, i.e. the average crystallite size in the direction perpendicular to the lattice planes. The Si powder was used as an instrumental broadening standard. Molecular graphics were prepared using the VESTA program.²⁵

Results and discussion

Synthesis

Compounds 1 and 2 were synthesized in a one-step procedure, with high yields and an easy product isolation, as previously described for the compound [Ni(phen)][Ta(OC₂O₅)(C₂O₄)₃]·H₂O (3).^3c The reaction of an aqueous solution of the (oxalato)tantalate anions with a predominantly ethanolic solution of the complex [M(phen)₃]²⁺ cations (M = Fe, Co) yielded single-phase crystalline products, 1 and 2, respectively. The crystals were separated from the solution by decantation and dried in air. Crystallization of 1 and 2 is a very fast process that occurred in a one hour period after the reactants were mixed. The crystals of both compounds are soluble in water under heating, and can be recrystallized.

In our previous papers related to the Ba,Ta–oxalate (ref. 3b) and Ni,Ta–oxalate (ref. 3c) species we have shown that the (oxalato)tantalate solution can contain at least three different (oxalato)tantalate anions: [TaO(C₂O₄)₃]³⁻, [Ta(C₂O₄)₄]³⁻ and [Ta(OH)(C₂O₄)₃]²⁻. The first listed, tris(oxalato)oxometalate(V) type of anion is the most frequent oxalate form of the closely related metal, niobium.^26–28 In this anionic complex form, metal atom is doubly bonded with one oxygen atom (Ta=O; Nb=O). Gray and coworkers have provided information on the oxo ligand in the mono–oxo complexes with d⁰–d² electron configurations in a tetragonal environment.²⁹ Terminal oxo ligands in the high-valent metal complexes are found to be electrophilic because of the π bonding between the oxygen lone pairs and the empty metal d orbitals.²⁹ Such oxo complexes tend to react with nucleophiles. The formation of [Ta(OC₂H₅)(C₂O₄)₃]²⁻ that occurs during the mixing of the (oxalato)tantalate(V) solution with ethanol could be explained by an electrophilic nature of the Ta=O oxygen in the [TaO(C₂O₄)₃]³⁻ anion existing in the parent (oxalato)tantalate(V) solution. A similar reaction has been observed when the mixed-metal polyoxoanion [(NbW₅O₁₈)₂O]⁴⁻ was added to alcohol (e.g. methanol, ethanol, 2-propanol).³⁰ Functionalization occurred at the terminal NbO oxygen, and appropriate alkoxide-derivated polyoxoanions were obtained in very good yields.

IR spectroscopy

The IR spectra of 1 and 2 are nearly equal, indicating the presence of similar constituent units (chelating bidentate oxalate and 1,10-phenanthroline ligands, ethoxy groups and water molecules). All absorption bands observed in the IR spectra of 1 and 2 are given in Experimental section. The absorption bands of medium intensity in the 3600–3200 cm⁻¹ region, originate from the O–H stretching vibration [ν(OH)] of crystallization water molecules. The absorption bands characteristic for the bidentate oxalate ligands together with the v[Ta–O(oxalate)] absorption bands for both compounds are summarized in Table 2.³¹

Table 2 Selected bands in the IR spectra of 1 and 2

Comp.	Bidentate oxalate group			v[Ta–O(ox)]/cm^−1
	νas(CO)/cm^−1	νs(CO)/cm^−1	δ(OCO)/cm^−1
1	1756, 1723, 1684	1360, 1333	799	560, 545
2	1755, 1723, 1685	1358, 1333	800	560, 545

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The C–O stretching vibration [ν_s(CO)] of the ethoxy group is recognized in the absorption band present at 1072 cm⁻¹ in the spectra of both compounds. The same band (1074 cm⁻¹) was also observed in the spectrum of 3, as well as the other bands mentioned above.^3c,31 Other absorption bands mainly correspond to different vibrations of coordinated 1,10-phenanthroline. In the spectra of the high-spin complexes of iron(II) with 1,10-phenanthroline there is usually a band of high intensity around 1515 and 1495 cm⁻¹, which is associated with a strong coupling of the C–C and C–N stretching vibrations, whereas in the spectra of the low-spin iron(II) complexes these bands are extremely weak.³² In the spectrum of 1, weak absorption bands are observed at 1518 and 1493 cm⁻¹ indicating the low-spin [Fe(phen)₃]²⁺ complex. The corresponding bands in the spectra of 2 and 3 (ref. 3c) are more pronounced.

Crystal structure

Compounds 1 and 2 are mutually isostructural and also isostructural to previously crystallographically characterized compound 3.^3c The asymmetric units of the three compounds showing a very good overlapping of the unit cells are given in Fig. S1.† The hydrophilic part in the asymmetric unit of 1 and 2 is composed of the complex anion [Ta(OC₂H₅)(C₂O₄)₃]²⁻ and one water molecule. The metal centres in the complex cations, [Fe(phen)₃]²⁺ in 1 and [Co(phen)₃]²⁺ in 2, have trigonally distorted octahedral environment, also observed in [Ni(phen)₃]²⁺ in 3 (Fig. S2†). Comparing the values of the N–M–N bite angle in these three compounds, where nitrogen atoms belong to the same phen ligand, metal centre in 1 deviates less from the ideal octahedral geometry then the centres in 2 and 3 (on average: 82.58° for 1; 78.49° for 2; 79.71° for 3). The values of trans N–M–N angles also indicate somewhat less deformation in the [Fe(phen)₃]²⁺ polyhedron (on average: 174.88° for 1; 171.21° for 2; 172.08° for 3). The average Fe–N bond length in 1 [1.980 Å; range 1.971(3)–1.987(3) Å] is 0.15 Å shorter than the average Co–N bond length in 2 [2.134 Å; range 2.110(4)–2.154(4) Å]. The Fe–N bond lengths shorter than 2 Å are typical for the low-spin iron(II) complexes; in the high-spin iron(II) complexes these bonds are elongated for ≈ 0.2 Å.³² The Co–N bond lengths are in good accord with similar Co–N(phen) bonds in the high-spin cobalt(II) complexes.³² Also, the Co–N bond lengths are more similar to the Ni–N bond lengths in the [Ni(phen)₃]²⁺ unit from 3 [2.093 Å; range 2.077(2)–2.114(2) Å].^3c Selected bond lengths and angles in the cations are given in Table S1.† The ORTEP-III drawing of the complex cation in 1 is shown in Fig. 1.

Fig. 1 ORTEP-III drawing of the [Fe(phen)₃]²⁺ cation in 1, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are depicted as spheres of arbitrary radii.

The tantalum(V) atom in the complex anions of 1 and 2 is coordinated with one ethoxy group and three oxalate ligands in a distorted pentagonal bipyramid geometry (Fig. 2). The geometrical arrangement is typical for seven-coordinate complexes with one monodentate and three bidentate ligands.³³ Bond distances and angles of the [Ta(OC₂H₅)(C₂O₄)₃]²⁻ anions in 1 and 2 are given in Table S2.† The Ta–OEt bond lengths [1.852(3) Å in 1 and 1.861(3) Å in 2] are consistent with those observed in the isostructural complex 3 [1.847(2) Å].^3c

Fig. 2 ORTEP-III drawing of the [Ta(OC₂H₅)(C₂O₄)₃]²⁻ anion in 1, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are depicted as spheres of arbitrary radii.

Due to the rigidity of the oxalate ligand the Ta–O(ox) bond lengths are longer (average value 2.091 Å), with similar values being found in other Ta–oxalate complexes.³ Deformation of the tantalum(V) coordination polyhedron is reflected in the O1–Ta–O2 angles [161.73(12)° in 1 and 162.4(1)° in 2] that are significantly deviating from the ideal value (180°).

The complex cations in compounds 1–3 are involved in two types of interactions: aromatic stacking interaction through the (C4 → C12)⋯(C4 → C12) pyridyl rings and parallel fourfold aryl embrace [(OFF)(EF)₂] (ref. 34) that includes two aryl (pyridyl) rings of two different phen molecules oriented in a way that the edge of one ring is directed towards the face of another ring (edge-to-face, EF), and two aryl (pyridyl) rings of two different phen molecules, which are parallel and offsetting (offset-face-to-face, OFF). In the [101] direction stacking interactions and (OFF)(EF)₂ contacts are appearing alternately, forming a one-dimensional motif of the [M(phen)₃]²⁺ cations (Fig. 3 and S3†). Geometric parameters of the aromatic stacking interactions and (OFF)(EF)₂ contacts are given in Table 3 and 4.

Fig. 3 Arrangement of the [Fe(phen)₃]²⁺ cations in 1 driven by aromatic stacking interactions (green) and (OFF)(EF)₂ contacts (yellow). Analogous arrangement of [M(phen)₃]²⁺ is observed in 2 and also in 3 (Fig. S3†).

Table 3 Geometric parameters of the aromatic stacking interactions for 1 and 2

Compound	Cg(i)⋯Cg(j)^a	Cg(i)⋯Cg(j)/Å	α^b/°	β^c/°	Cg(i)⋯plane[Cg(j)]/Å	Symm. op.
a Cg = centre of gravity of the aromatic ring.b α = angle between the planes of two aromatic rings.c β = angle between a Cg⋯Cg line and the normal to the plane of the first aromatic ring.
1	(C4 → C12)⋯(C4 → C12)	3.914(3)	0.00	28.69	3.4331(17)	−x, 1 − y, 1 − z
2	(C16 → C24)⋯(C16 → C24)	3.840(3)	0.00	26.28	3.4426(18)	−x, 1 − y, 1 − z

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Table 4 Geometric parameters of the (OFF)(EF)₂ contacts for 1 and 2

Compound	Angle between the two relevant intramolecular N–M–N planes for the pair of embracing cations/°	Angle between the planes of aryl rings in contact/°	∠(Cdistal–M⋯M)^a/°	M⋯M/Å
a Cdistal = carbon atom in phen molecule which gives the largest Cdistal–M⋯M angle.
1	0.00	87.95, 0.00	173.16	9.086
2	0.00	86.02, 0.00	172.94	9.273

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In the crystal packing of 1 and 2 hydrogen bonds are generated between crystallization water molecules and the complex anions [Ta(OC₂H₅)(C₂O₄)₃]²⁻, forming one-dimensional chains presented in Fig. 4. Hydrogen-bonding geometric parameters for 1 and 2 are listed in Table 5. Hydrogen bonding chains along the b axis and the alternating aromatic stacking interactions and (OFF)(EF)₂ contacts in the [101] direction form an overall 2D structural arrangement in compounds 1 and 2. It is interesting to note that the change of the metal centre (Fe, Co or Ni) in the complex cation does not affect the crystal arrangement of compounds 1–3 (Fig. S3 and S4†).

Fig. 4 The infinite one-dimensional hydrogen-bonding pattern along the b axis in the crystal packing of the [Ta(OC₂H₅)(C₂O₄)₃]²⁻ anions and water molecules of 1. An analogous hydrogen-bonding pattern is observed in 2 and also in 3 (Fig. S4†).

Table 5 Hydrogen-bonding geometry in 1 and 2

Compound	D–H⋯A	D–H/Å	H⋯A/Å	D⋯A/Å	D–H⋯A/°	Symm. op. on A
1	O14–H14A⋯O7	0.91(6)	2.25(5)	2.954(8)	134(5)	x, 1 + y, z
	O14–H14B⋯O12	0.90(6)	2.07(6)	2.821(8)	140(6)
2	O14–H14A⋯O7	0.91(8)	2.10(9)	2.878(7)	143(10)	x, 1 + y, z
	O14–H14B⋯O12	0.90(5)	2.14(5)	3.014(8)	164(4)

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Thermal analysis

A simultaneous TG and DTA analysis of compounds 1 and 2 was carried out in the stream of synthetic air. The TG and DTA curves of 1 and 2 are presented in Fig. 5 and 6, respectively. Thermoanalytical data are summarized in Table 6. The thermal decomposition is very similar for the two compounds, and comparable to that for 3 (Fig. S5 and S6†). The water molecule was being released slowly, up to 130–140 °C in 1 and 2, the release starting immediately after the beginning of heating. Compound 3 is somewhat more stable, water loss occurs in the temperature range of 80–140 °C. The second decomposition step is associated with ethoxy group decomposition, accompanied by an endothermic maximum around 230 °C for all compounds. With further heating decomposition of phen molecules and then oxalate ligand takes place. The final thermal decomposition step in 1 ends at 430 °C, in 2 at 485 °C, and in 3 around 500 °C.

Fig. 5 The TG and DTA curves (heating and cooling) of 1 measured in the synthetic air.

Fig. 6 The TG and DTA curves (heating and cooling) of 2 measured in the synthetic air.

Table 6 Thermoanalytical data for 1 and 2

Comp.	ΔT/°C	Δm/%		Loss	DTApeak/°C
		Exp.	Calcd
1	50–130	1.75	1.63	H2O
	218–240	4.65	4.08	Ethoxy group	237 endo
	240–365	47.45	48.95	3phen	250, 359 exo
	365–450	19.02	18.11	4.5CO2 + 1.5CO	373 exo
2	40–130	1.79	1.63	H2O
	200–230	4.60	4.07	Ethoxy group	226 endo
	230–430	49.46	48.80	3phen	320, 425 exo
	430–430	17.24	18.79	4.5CO2 + 1.5CO	450 exo

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Thermal decomposition residue of 1 heated at 800 °C and above (1200 °C) is a mixed-metal oxide FeTaO₄ (exp. 27.23%; calcd 27.13%). Thermal decomposition products of 2 change with increasing heating temperature. An endothermic peak accompanied by a slight decrease in mass was observed at 908 °C in the TG–DTA curve of 2 indicating the reduction of Co^III to Co^II (XRPD confirmed the presence of Co₃O₄ at 800 °C). Thermal decomposition residue of 2 heated up to 1200 °C is a mixture of oxides: 40 wt% of Co₄Ta₂O₉ and 60 wt% of CoTa₂O₆ (exp. 26.91%; calcd 26.71%). During the cooling of the thermal decomposition residues no mass change or DTA peaks were observed. Heat treatment of 3 indicated the formation of mixture of oxides NiTa₂O₆ and NiO.^3c

Mixed-metal oxides structural characterization

Different nanocrystalline products were obtained by thermal decomposition of metal-complex precursors 1 and 2 heated up to the selected temperatures (800, 1000 and 1200 °C); these products were characterized by powder X-ray diffraction at RT.

It was found that thermal treatment of precursor 1 in air up to 1200 °C leads to the formation of the nanocrystalline single-phase mixed-metal oxide, FeTaO₄, without any additional impurity phases. The FeTaO₄ crystallizes in the P4₂/mnm tetragonal unit-cell with the lattice parameters a = 4.6771(4) Å and c = 3.0471(3) Å. The graphical result of the Rietveld structure refinement of precursor 1 thermally treated at 1200 °C is given in Fig. 7. The rutile-derived FeTaO₄ lattice consists of infinite chains of the edge-shared Fe_0.5Ta_0.5O₆ octahedra running parallelly to the c axis, chains being mutually interconnected via corners (inset in Fig. 7).

Fig. 7 The graphical result of the Rietveld structure refinement of the product obtained by heat treatment of precursor 1 at 1200 °C. Experimental intensity is shown in red, the calculated pattern is blue and the difference curve is given below. The 2theta positions of the FeTaO₄ reflections are given as the grey vertical lines. The structure of FeTaO₄ in the ab plane is drawn in the inset.

Iron(III) tantalate prepared by heat treatment at 1200 °C is characterized by an average crystallite size of approximately 35(2) nm, as obtained by the size-strain analysis during the Rietveld refinement. Crystallite sizes in particular directions were calculated by the Warren-Averbach method²⁴ implemented in the XBroad program.²² The area-weighted sizes of the coherent diffraction domain in the directions perpendicular to the lattice planes 020 and 002 are shown in Fig. 8. As can be seen, the size in the direction of the a axis amounts to 349(9) Å and in the direction of the c axis amounts to 363(8) Å, indicating the isometric crystallite shape.

Fig. 8 The result of the line broadening analysis of the sample obtained by heat treatment of 1 at 1200 °C. The coherent diffraction domain is calculated in directions perpendicular to the lattice planes 020 and 002, respectively.

Whilst it was anticipated that the phase composition will be indifferent to the lowering of temperature of thermal treatment, thermally induced changes in the microstructure could be expected. Therefore, heat treatment of precursor 1 was also conducted up to 1000 and 800 °C and a pronounced difference in the XRD line broadening of the thermal decomposition residue was noticed. Crystallite sizes decreased significantly, from 35(2) nm at 1200 °C to only 10(1) nm at 800 °C. Fig. 9 shows area-weighted sizes of the coherent diffraction domain in the directions perpendicular to the lattice planes 020 and 002, once more indicating the symmetrical shape of the prepared oxide nanoparticles, these being of the size 111(8) and 100(9) Å in the direction of the a and c axis, respectively. Surface related properties of FeTaO₄ prepared at lower temperature (with smaller crystallite sizes) are expected to be enhanced.

Fig. 9 The result of the line broadening analysis of the sample obtained by heat treatment of 1 at 800 °C. Crystallite sizes are calculated in the directions perpendicular to the lattice planes 020 and 002, respectively.

Thermal treatment of 2 up to 800 °C in the stream of air led to the appearance of the tetragonal CoTa₂O₆ phase. The sample obtained at this temperature contains the additional phase Co₃O₄, as well as a small amount of an amorphous phase. A quantitative Rietveld analysis revealed that the product contained 77.1(1) wt% of CoTa₂O₆ and 22.9(4) wt% of Co₃O₄. Cobalt tantalate, CoTa₂O₆, crystallizes in the well-known tri-rutile tetragonal lattice with the lattice parameters a = 4.735(1) and c = 9.170(3) Å. When the thermal treatment of 2 was increased to 1000 °C a different thermal decomposition residue was obtained. The XRD pattern of this sample contains additional diffraction lines beside the CoTa₂O₆ phase, whereas diffraction lines attributed to the Co₃O₄ phase did not appear (Fig. 10).

Fig. 10 The graphical result of the Rietveld structure refinement of the product obtained by heat treatment of precursor 2 up to 1000 °C. Experimental intensity is shown in red, the calculated pattern is blue and the difference curve is given below. The magenta vertical lines represent reflections of Co₄Ta₂O₉, while the reflections of CoTa₂O₆ are given by the green vertical lines.

A new mixed-metal oxide phase, Co₄Ta₂O₉, appeared in the amount of 40.1(2) wt%. All the reflections of this additional phase were indexed by the NTREOR (ref. 35) routine implemented in the EXPO2012 (ref. 36) package; the trigonal P3̄c1 unit-cell with the lattice parameters a = 5.173(1), c = 14.148(4) Å was characterized by the largest figure of merit; the compound is found to be isostructural with Co₄Nb₂O₉.³⁷ Although the unit cell and space group of Co₄Ta₂O₉ has long been known,^37b to date this oxide has not been structurally characterized. In this paper we report the fractional atomic coordinates and thermal parameters of Co₄Ta₂O₉ (Table 7), and also the crystal data and a summary of structure refinement of Co₄Ta₂O₉ (Table 8).

Table 7 Fractional atomic coordinates and thermal parameters for Co₄Ta₂O₉

Atom	Wyck.	x	y	z	Biso/Å^3
Ta	4c	0.0000	0.0000	0.1445(2)	0.63(2)
Co(1)	4d	0.3333	0.6667	0.3316(7)	0.81(4)
Co(2)	4d	0.3333	0.6667	0.0081(1)	0.79(1)
O(1)	6f	0.3041(5)	0.0000	0.2500	0.71(4)
O(2)	12g	0.3517(5)	0.5496(7)	0.1923(4)	0.91(2)

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Table 8 Crystal data and summary of Co₄Ta₂O₉ structure refinement

Phase	Co4Ta2O9
Formula	Ta4Co8O18
Formula weight/g mol^−1	1483.24
ρcalcd/g cm^−3	7.4957
Weight fraction/%	50.1(2)
Space group (no.)	P3̄c1(165)
a/Å	5.1746(3)
c/Å	14.1679(9)
V/10^6 pm^3	328.53(8)
Fitting mode	Structure Fit
Profile function	Pseudo Voigt
U	0.6(4)
V	−0.021(2)
W	0.187(7)
Asymmetry parameter 1	−0.03(1)
Peak shape parameter 1	0.3(1)
Peak shape parameter 2	0.027(3)
R (weighted profile)/%	4.287
R (profile)/%	3.001

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There are two crystallographically independent Co atoms [Co(1) and Co(2)] in the crystal structure of Co₄Ta₂O₉. All metal atoms in Co₄Ta₂O₉ are octahedrally coordinated with oxygen atoms. The structure of Co₄Ta₂O₉ (shown in Fig. 11) can be described by several structural motives: (i) the isolated dimer Ta₂O₁₂ consisting of the face-shared TaO₆ octahedra (pale pink), (ii) chains of the corner-connected Co(1)O₆ octahedra (light blue) and (iii) sheets of the edge-shared Co(2)O₆ octahedra (dark blue). In Fig. 12a is shown a hexagonal arrangement of the edge-shared Co(2)O₆ octahedra centred on the three-fold axis. The next layer, shown in Fig. 12b, contains similar hexagonal rings with the alternating edge-sharing Co(1)O₆ and TaO₆ octahedra.

Fig. 11 Crystal structure of Co₄Ta₂O₉; Ta is given in pale pink, Co1 in light blue, and Co2 in dark blue colour. Polyhedra around Co atoms are shown as light and dark blue octahedra, respectively, while polyhedra around Ta are omitted for clarity.

Fig. 12 Two types of layers present in the structure of Co₄Ta₂O₉ viewed along the c axis: (a) hexagonal arrangement of edge-shared Co(2)O₆ octhaedra; (b) hexagonal assembly containing edge-shared Co(1)O₆ (light blue polyhedra) and TaO₆ octahedra (pink polyhedra) in alternating manner.

Compound 2 was also heat treated at 1200 °C, but beside better crystallinity of the sample no changes in the sample composition was noticed. The X-ray diffraction patterns of the samples obtained by heating compound 2 up to 800 and 1200 °C and cooling to room temperature are shown in Fig. S7.†

Conclusions

Oxalate species of tantalum, up to now poorly studied, have proved to be very interesting for the design of new heterometallic compounds that are stable under atmospheric conditions and are soluble in water. Our previous study on the nickel–tantalum oxalate system (3) has enabled us to optimize reaction conditions in the reactions of (oxalato)tantalate anions with other transition-metal cations. Here we reported on the synthesis and thermal and structural properties of two new heterometallic [M(phen)₃][Ta(OC₂H₅)(C₂O₄)₃]·H₂O [M = Fe (1), Co (2)] complexes, obtained in very good yields. The two compounds have the same structural arrangement composed of one-dimensional hydrogen-bonding chains and alternating aromatic stacking interactions and parallel fourfold aryl embraces.

From the TG/DTA analysis of 1 and 2, as well as of previously reported compound 3 it follows that there are some similarities in the early stage of decomposition, while final decomposition temperatures and products depend on the nature of the metal in the cation. Thermal treatment of compound 1 up to 800 °C yielded the mixed-metal oxide FeTaO₄. As determined by microstructural characterization, the FeTaO₄ samples prepared at this temperature have significantly smaller crystallite sizes than those obtained by thermal treatment at higher temperatures (10 and 35 nm at 800 and 1200 °C, respectively). Depending on the temperature of heating, thermal treatment residues of compound 2 had different compositions. The mixed-metal oxides Co₄Ta₂O₉ and CoTa₂O₆ were obtained when 2 was heated up to 1000 °C. Co₄Ta₂O₉, so far hardly known, was fully structurally characterized by powder X-ray diffraction. The investigation of properties of the nanophase oxide powders FeTaO₄, and CoTa₂O₆ and Co₄Ta₂O₉, prepared from molecular precursors 1 and 2, respectively, are underway. Due to the solubility in water, 1 and 2 could also be used for the preparation of oxides in the form of thin films by the spin coating technique.

Acknowledgements

This research was supported by the Ministry of Science, Education and Sports of the Republic of Croatia (grant no. 098-0982904-2946).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Cif files, tables with bond distances and angles in cation and anion of 1 and 2, structural overlay of 1–3, TG and DTA curves of 1–3, XRD pattern of thermal decomposition residues of 2. CCDC 991418 and 991419. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra05855k

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