Reactivity of Bi- and Monometallic Trifluoroacetates Towards Amorphous SiO₂

Abstract

The reactivity of alkali–manganese(II) and alkali trifluoroacetates towards amorphous SiO₂ (a-SiO₂) was studied in the solid-state. K₄Mn₂(tfa)₈, Cs₃Mn₂(tfa)₇(tfaH), KH(tfa)₂, and CsH(tfa)₂ (tfa = CF₃COO⁻) were thermally decomposed under vacuum in fused quartz tubes. Three new bimetallic fluorotrifluoroacetates of formulas K₄Mn₃(tfa)₉F, Cs₄Mn₃(tfa)₉F, and K₂Mn(tfa)₃F were discovered upon thermolysis at 175 °C. K₄Mn₃(tfa)₉F and Cs₄Mn₃(tfa)₉F feature a triangular-bridged metal cluster of formula [Mn₃(μ₃-F)(μ₂-tfa)₆(tfa)₃]⁴⁻. In the case of K₂Mn(tfa)₃F, fluoride serves as an inverse coordination center for the tetrahedral metal cluster K₂Mn₂(μ₄-F). Fluorotrifluoroacetates may be regarded as intermediates in the transformation of bimetallic trifluoroacetates to fluoroperovskites KMnF₃, CsMnF₃, and Cs₂MnF₄, which crystallized between 250 and 600 °C. Decomposition of these trifluoroacetates also yielded alkali hexafluorosilicates K₂SiF₆ and Cs₂SiF₆ as a result of the fluorination of fused quartz. The ability to fluorinate fused quartz was observed for monometallic alkali trifluoroacetates as well. Hexafluorosilicates and heptafluorosilicates K₃SiF₇ and Cs₃SiF₇ were obtained upon thermolysis of KH(tfa)₂ and CsH(tfa)₂ between 200 and 400 °C. This ability was exploited to synthesize fluorosilicates under air by simply reacting alkali trifluoroacetates with a-SiO₂ powder.

Graphical Abstract

The reactivity of alkali–manganese(II) and alkali trifluoroacetates towards amorphous SiO₂ was studied in the solid-state with an eye towards the synthesis of alkali fluorosilicates.

Introduction

For a number of years our group has been working on the synthesis, crystal-chemistry, and reactivity of bimetallic trifluoroacetates.^1–5 We have established that the trifluoroacetato ligand (tfa = CF₃COO⁻) can bridge atoms with dissimilar electronic and geometric requirements and that this ability can be exploited to synthesize bimetallic trifluoroacetates featuring alkali–manganese(II),^{1, 5} alkaline-earth–manganese(II),² and alkali–alkaline-earth pairs.⁴ K₂Mn₂(tfa)₆(tfaH)₂(H₂O), CsMn(tfa)₃, K₂Mn(tfa)₄, Cs₃Mn₂(tfa)₇(tfaH), Ca_3–xMn_x(tfa)₆(H₂O)₄, and RbCa(tfa)₃ are some examples of this family of solid-state materials. These solids can be prepared as single-phase polycrystalline materials, which is advantageous to establish reactivity patterns. On this basis, we have extensively studied the thermal decomposition of bimetallic trifluoroacetates under inert atmosphere. Taking K₂Mn₂(tfa)₆(tfaH)₂(H₂O) and CsMn(tfa)₃ as examples, we have demonstrated that these solids serve as self-fluorinating single-source precursors to the corresponding fluoroperovskites KMnF₃ and CsMnF₃.¹ Likewise, thermolysis of K₂Mn(tfa)₄ and Cs₃Mn₂(tfa)₇(tfaH) provides synthetic access to layered fluoroperovskites K₂MnF₄ and Cs₂MnF₄, respectively.⁵

More recently, we began studying the thermal decomposition of bimetallic trifluoroacetates in fused quartz sealed tubes. Our main goal was to probe the ability of these solids to fluorinate amorphous SiO₂ (a-SiO₂), which could eventually open up a new solid-state route to ternary and quaternary fluorosilicates. Additionally, we sought to capture decomposition transients to shed light on the transformation of an organic–inorganic hybrid into a fully inorganic solid. Results from these studies are presented herein in two distinct sections. The first section is devoted to the thermolysis of bimetallic trifluoroacetates K₂Mn(tfa)₄ and Cs₃Mn₂(tfa)₇(tfaH) in the 175–600 °C temperature range. Single-crystal and powder X-ray diffraction were used to identify thermal decomposition products; hexafluorosilicates K₂SiF₆ and Cs₂SiF₆ were among these products. This observation prompted us to investigate the reactivity of alkali trifluoroacetates KH(tfa)₂ and CsH(tfa)₂ towards a-SiO₂. We were motivated by the fact that, although alkali trifluoroacetates have been used as trifluoromethylating agents,^6–10 they have not been considered as reagents for the solid-state synthesis of fluorosilicates. Fluorosilicates M₂SiF₆ (M = Li–Cs) and M′₃SiF₇ (M′ = K–Cs) are extensively used as hosts for Mn⁴⁺ downconverting red phosphors.^11–19 Typically, these materials are synthesized through solution-phase routes that use aqueous HF as the fluorine source;²⁰ alternatively, HF is generated in situ by dissolving MHF₂ in H₃PO₄²¹ or NH₄F in HCl.^{15, 22} In either approach, the presence of HF imposes stringent requirements to synthetic procedures and equipment. Thus, the second section of this article focuses on probing the reactivity of KH(tfa)₂ and CsH(tfa)₂ towards a-SiO₂; specifically, on their ability to act as mild fluorinating agents. KH(tfa)₂ and CsH(tfa)₂ were decomposed in fused quartz tubes at temperatures ranging between 200 and 400 °C. Thermolysis experiments were also carried out under air in the presence of commercially available a-SiO₂ powder. Powder X-ray diffraction was used to identify decomposition products. Results presented in this article are discussed from the standpoint of streamlining the solid-state synthesis of fluorosilicates.

Experimental

Synthesis of Bi- and Monometallic Trifluoroacetates.

All experiments were carried out under nitrogen atmosphere using standard Schlenk techniques. K₂CO₃ (99%), Cs₂CO₃ (99.9%), MnCO₃ (99.9%), amorphous SiO₂ (99.8%, surface area 175–225 m² g⁻¹), and anhydrous CF₃COOH (99%) were purchased from Sigma-Aldrich and used as received. Double-distilled water was used throughout. Polycrystalline bi- and monometallic trifluoroacetates were synthesized via solvent evaporation.^{5, 23} The procedure for the preparation of phase-pure K₄Mn₂(tfa)₈ and Cs₃Mn₂(tfa)₇(tfaH) is described in detail elsewhere.⁵ Monometallic trifluoroacetates KH(tfa)₂ and CsH(tfa)₂ were synthesized by dissolving the corresponding metal carbonate (1 mmol) in a mixture of 3 mL of tfaH and 3 mL of double-distilled water in a 50 mL two-neck round-bottom flask. A colorless transparent solution was thus obtained. The flask containing the reaction mixture was immersed in a sand bath and solvent evaporation took place at 65 °C for 48 h under a constant flow of dry nitrogen (140 mL min⁻¹). The resulting white solids were stored in a nitrogen-filled glove box.

Thermal Decomposition of Trifluoroacetates.

Thermolysis of trifluoroacetates was carried out in three different experimental configurations; these are depicted in Figure 1.

Figure 1.

Experimental configurations used for thermolysis of bi- and monometallic trifluoroacetates. The tube furnace is 22 × 1′′ (length × diameter). The chamber of the box furnace is 4 × 5 × 7′′ (width × length × depth).

Setup I.

Bimetallic (≈70–80 mg) and monometallic trifluoroacetates (≈120–150 mg) were first decomposed in setup I (Figure 1a). Polycrystalline samples were loaded into a fused quartz tube (length ≈120 mm, outer diameter ≈10 mm, wall thickness ≈1.0 mm). The tube was sealed under vacuum (≈45–60 mTorr), placed in a tube furnace at 100 °C, heated to a predefined target temperature (175–600 °C), and allowed to dwell at that temperature for a given time (2–12 h). A heating rate of 150 °C h⁻¹ was employed in all cases except in experiments conducted at 175 °C, which were aimed at isolating single crystals of decomposition intermediates; in those experiments the heating rate was set to 6 °C h⁻¹. Once the dwelling time was completed, the furnace was allowed to cool to 100 °C, the quartz tube was opened under air, and products were stored in a nitrogen-filled glove box. Off-white to brown powders were obtained.

Setup II.

Bimetallic trifluoroacetates (≈70–80 mg) were decomposed in setup II (Figure 1b). This experimental configuration differs from setup I in that the trifluoroacetate sample is not in direct contact with fused quartz. Polycrystalline samples were loaded into a closed-one-end alumina tube (length ≈70 mm, outer diameter ≈6.4 mm, wall thickness ≈1.0 mm). This tube was placed within a second alumina tube (length ≈70 mm, outer diameter ≈8.5 mm, wall thickness ≈1.0 mm). The open end of the tube containing the sample faced the closed end of the wider tube. Ceramic wool was used to fill the gap between the closed end of the tube containing the sample and the open end of the wider tube (≈2 mm). The whole assembly was placed in a fused quartz tube (length ≈120 mm, outer diameter ≈10 mm, wall thickness ≈1.0 mm) and sealed under vacuum (≈45–60 mTorr). Thermolysis was then carried out as in setup I. Off-white to brown powders were obtained.

Setup III.

Monometallic trifluoroacetates were decomposed in setup III (Figure 1c). This experimental configuration differs from setups I and II in that (i) trifluoroacetates are decomposed in the presence of amorphous SiO₂ (alkali:Si molar ratio = 2:1), and (ii) thermolysis is carried out under air. KH(tfa)₂:a-SiO₂ (≈100 mg) and CsH(tfa)₂:a-SiO₂ (≈160 mg) mixtures were prepared in a nitrogen-filled glove box. These were transferred to 5 mL alumina crucibles, which were subsequently covered with alumina disks. Crucibles were placed in a box furnace at 100 °C, heated to a predefined target temperature (300–400 °C), and allowed to dwell at that temperature for a given time (6–12 h). A heating rate of 10 °C min⁻¹ was employed in all experiments. Once the dwelling time was completed, the furnace was allowed to cool to 100 °C and crucibles were removed from the furnace. Ash grey powders were obtained. Dwelling temperatures were selected based on thermal analyses conducted under inert atmosphere which showed that K₄Mn₂(tfa)₈ and Cs₃Mn₂(tfa)₇(tfaH) decompose between 150 and 275 °C (see Figure S1 in the ESI and Figure 4 in reference 5). In the case of KH(tfa)₂ and CsH(tfa)₂, decomposition has been shown to take place between 150 and 250 °C.^{24, 25}

Single-Crystal X-ray Diffraction (SCXRD).

SCXRD analysis was carried out to establish the crystal structures of intermediates formed upon thermolysis of K₄Mn₂(tfa)₈ and Cs₃Mn₂(tfa)₇(tfaH) at 175 °C in setups I and II. Colorless crystals of K₄Mn₃(tfa)₉F (0.48 × 0.32 × 0.25 mm), Cs₄Mn₃(tfa)₉F (0.05 × 0.05 × 0.02 mm), and K₂Mn(tfa)₃F (0.21 × 0.11 × 0.11 mm) were selected for structure determination and mounted in Paratone N oil. Diffraction data were collected using a Bruker X8 Apex diffractometer. X-ray intensities were measured at 100 K using Mo Kα radiation (λ = 0.71073 Å). Frames were integrated using Bruker SAINT. Experimental data were corrected for Lorentz, polarization, and absorption effects; for the latter, the multiscan method was employed using Bruker SADABS.²⁶ Structure solution was accomplished using a dual-space approach as implemented in SHELXT²⁷ and difference Fourier maps as embedded in SHELXL-2014/7²⁸ running under ShelXle.²⁹ VESTA was used to visualize crystal structures.³⁰ Table 1 summarizes crystal data for K₄Mn₃(tfa)₉F and K₂Mn(tfa)₃F. Cs₄Mn₃(tfa)₉F was found to be isostructural to its potassium counterpart, except for positional disorder of some cesium atoms and trifluoroacetate ligands. Full details on data collection and structure refinement are given in the ESI (Tables S1–S10 and Figures S2–S4). Crystal data were deposited in the Cambridge Crystallographic Data Centre with numbers 2184179 (K₄Mn₃(tfa)₉F), 2201197 (Cs₄Mn₃(tfa)₉F), and 2184129 (K₂Mn(tfa)₃F).

Table 1.

Crystal and Structural Determination Data of K₄Mn₃(tfa)₉F and K₂Mn(tfa)₃F

Chemical formula	K4Mn3(tfa)9F	K2Mn(tfa)3F
Formula weight (g)	1357.47	491.20
Crystal system	Monoclinic	Orthorhombic
Space group	P21/n	Pbcn
a, b, c (Å)	17.5481 (9), 13.7303 (7), 19.5024 (9)	12.4013 (10), 14.4480 (12), 7.3981 (6)
α, β, γ (°)	90, 109.993 (2), 90	90, 90, 90
Volume (Å3)	4415.7 (4)	1325.55 (19)
Z	4	4
R[F2 > 2σ(F2)]	3.5%	10.3%
wR(F2)	7.7%	25.2%
S	1.01	1.27

Powder X-ray Diffraction (PXRD).

Powder XRD patterns were collected using a Bruker D2Phaser diffractometer operated at 30 kV and 10 mA. Cu Kα radiation (λ = 1.5418 Å) was employed. A nickel filter was used to remove Cu Kβ. Diffractograms were collected in the 10−60° 2θ range using a step size of 0.012° and a step time of 0.4 s, unless otherwise noted.

Results and Discussion

Section I. Reactivity of Bimetallic Trifluoroacetates K₄Mn₂(tfa)₈ and Cs₃Mn₂(tfa)₇(tfaH).

Bimetallic trifluoroacetates K₄Mn₂(tfa)₈ and Cs₃Mn₂(tfa)₇(tfaH) were thermally decomposed at 175, 250, 450, and 600 °C for 2 h in setups I and II. Crystalline phases obtained after each thermolysis experiment are summarized in Table 2. We begin our discussion of these results with the crystal structures of K₄Mn₃(tfa)₉F and K₂Mn(tfa)₃F, which were obtained at 175 °C in setups I and II, respectively. Crystal structures are shown in Figure 2. K₄Mn₃(tfa)₉F crystallizes in the monoclinic P2₁/n space group and its building block consists of chains featuring corner-, edge-, and face-sharing MnO₅F and K(O,F)_10,11 polyhedra (Figure 2a). These chains run along the c axis and form layers that extend in the bc plane (Figure 2b). Stacking these layers along the a axis results in the observed three-dimensional structure in which each chain is connected to three adjacent chains (Figure 2c). An interesting structural feature of K₄Mn₃(tfa)₉F is the presence of a triangular-bridged cluster of Mn²⁺ cations that may be described as [Mn₃(μ₃-F)(μ₂-tfa)₆(tfa)₃]⁴⁻ (Figure 2d). A fluoride ion connects three MnO₅F octahedra by bridging Mn²⁺ in a trigonal planar geometry (∠Mn1–μ₃-F–Mn2 = 119.2°, ∠Mn2–μ₃-F–Mn3 = 122.1°, ∠Mn3–μ₃-F–Mn1 = 118.7°, Mn–F = 2.11–2.13 Å). Six trifluoroacetato ligands sitting above and below the plane of the Mn₃(μ₃-F) core bridge metal ions in pairs. The remaining three trifluoroacetato ligands complete the coordination sphere of manganese. The [Mn₃(μ₃-F)(μ₂-tfa)₆(tfa)₃]⁴⁻ cluster was also encountered in fluorotrifluoroacetates Cs₄Mn₃(tfa)₉F and Na₄Mn₃(tfa)₉F, which are isostructural to K₄Mn₃(tfa)₉F. Cs₄Mn₃(tfa)₉F was obtained upon thermal decomposition of Cs₃Mn₂(tfa)₇(tfaH) at 175 °C. Na₄Mn₃(tfa)₉F was synthesized in the course of exploratory thermolysis experiments conducted using a bimetallic sodium manganese trifluoroacetate previously reported by our group (see ESI, Tables S11–S14 and Figure S5).¹ It is worth mentioning that triangular-bridged clusters of formula M₃(μ₃-F)(tfa)₆L₃ (M = Mg, Fe, Mn, Co, Ni, Zn; L = tfa, tfaH, OCH₃, py, H₂O) have been observed in fluorotrifluoroacetate crystals grown from solution at room temperature.^31–35 Altogether, these observations point to the relevance of these clusters as building blocks. Further, the fact that they are encountered both at room temperature and at 175 °C suggests their potential as synthons for the preparation of organic–inorganic hybrid materials. Another structurally interesting hybrid of formula K₂Mn(tfa)₃F was discovered upon thermolysis of K₄Mn₂(tfa)₈ at 175 °C in setup II. K₂Mn(tfa)₃F crystallizes in the orthorhombic Pbcn space group and can be visualized as chains built of corner-sharing MnO₄F₂ octahedra and edge-sharing K(O,F)₁₂ polyhedra (Figure 2e). These chains run along the c axis and are connected to each other through face-sharing polyhedra (Figure 2f). This connectivity results in layers that extend in the (110) and ($\overline{1}$10) planes. The observed three-dimensional structure of the hybrid results from the assembly of these two sets of layers (Figure 2g). Void channels that run parallel to the c axis are observed in K₂Mn(tfa)₃F; the presence of micropores in mono- and bimetallic haloacetates is not uncommon.^{1, 23, 36, 37} An unusual structural motif we observe in K₂Mn(tfa)₃F is a K₂Mn₂(μ₄-F) cluster in which fluoride bridges two K⁺ and two Mn²⁺ cations (Figure 2h). The bridging fluoride sits on a C₂ axis and the four metal ions are arranged in a highly distorted tetrahedral geometry (∠K–μ₄-F–K = 116.9°, ∠Mn–μ₄-F–Mn = 121.0°, ∠K–μ₄-F–Mn = 94.1°, K–F = 2.72 Å, Mn–F = 2.13 Å). Although μ₄-F is known to serve as an inverse coordination center in tetrahedral metal–organic complexes,³⁸ a comprehensive search shows that fluorotrifluoroacetates featuring μ₄-F metal clusters have not been reported neither in the literature nor in the Cambridge Structural Database. Attempts to isolate Cs₂Mn(tfa)₃F by decomposing Cs₃Mn₂(tfa)₇(tfaH) in setup II were unsuccessful; Cs₄Mn₃(tfa)₉F was invariably obtained.

Table 2.

Crystalline Products of the Solid-State Thermolysis of Bimetallic Trifluoroacetates

Precursor	Setup	Dwelling Temperature and Time
		175 °C, 2 h	250 °C, 2 h	450 °C, 2 h	600 °C, 2 h
K4Mn2(tfa)8	I	K4Mn3(tfa)9F	KMnF3	KMnF3 K2SiF6 Unindexed phase(s)	KMnF3 K2SiF6
	II	K2Mn(tfa)3F	KMnF3	KMnF3 K2SiF6	KMnF3 K2SiF6
Cs3Mn2(tfa)7(tfaH)	I	Cs4Mn3(tfa)9F	CsMnF3 Cs2MnF4 Cs2SiF6	Cs2SiF6 MnF2	Cs2SiF6 MnF2
	II	Cs4Mn3(tfa)9F	CsMnF3 Cs2MnF4 Cs2SiF6	CsMnF3 Cs2SiF6 MnF2	CsMnF3 Cs2SiF6

Figure 2.

Crystal structures of K₄Mn₃(tfa)₉F (a‒d) and K₂Mn(tfa)₃F (e‒h). Building blocks (a, e), layers (b, f), and extended perspectives (c, g) are shown for each structure. (d) and (h) depict the coordination of bridging atoms μ₃-F and μ₄-F, respectively. Atom splitting in disordered positions is omitted for clarity; only major occupancy sites are shown. Unit cells in (c) and (g) are depicted with solid black lines. Only the polyhedral framework is shown in (c).

As shown in Table 2 and as it will be discussed below, thermolysis of K₄Mn₂(tfa)₈ and Cs₃Mn₂(tfa)₇(tfaH) at or above 250 °C led to the formation of fluoroperovskite phases KMnF₃, CsMnF₃, and Cs₂MnF₄. Fluorotrifluoroacetates K₄Mn₃(tfa)₉F, Cs₄Mn₃(tfa)₉F, and K₂Mn(tfa)₃F isolated at 175 °C could therefore be regarded as intermediates in the transformation of bimetallic trifluoroacetates to fluoroperovskites. This conjecture results from compositional and structural considerations. From a compositional standpoint, organooxygen (from carboxylate groups) and organofluorine (from trifluoromethyl groups) are partially displaced from the coordination sphere of metal atoms upon going from trifluoroacetates to fluorotrifluoroacetates. As an example, manganese atoms in K₄Mn₂(tfa)₈ are solely coordinated by organooxygen.⁵ By contrast, MnO₅F and MnO₄F₂ octahedra present in K₄Mn₃(tfa)₉F and K₂Mn(tfa)₃F, respectively, feature organooxygen and fluoride as ligands. The stepwise replacement of trifluoroacetato ligands by fluoride anions has been observed in the thermolysis of Fe(tfa)₃.³⁹ From a structural standpoint, the connectivity of metal atoms in K₄Mn₃(tfa)₉F and K₂Mn(tfa)₃F may be regarded as intermediate between that observed in bimetallic trifluoroacetates and fluoroperovskites. For clarity, the crystal structure of cubic KMnF₃ is shown in Figure 3. Continuing with the example of manganese atoms, no Mn–organofluorine–Mn bridges are present in K₄Mn₂(tfa)₈.⁵ However, Mn–fluoride–Mn bridges are encountered in K₄Mn₃(tfa)₉F and K₂Mn(tfa)₃F; these bridges build the framework of MnF₆ octahedra in KMnF₃ (Figure 3a). Additionally, K–fluoride–K and K–fluoride–Mn bridges are observed in the case of K₂Mn(tfa)₃F; such bridges are present in KMnF₃. From this perspective, the presence of a K₂Mn₂(μ₄-F) cluster in K₂Mn(tfa)₃F may be visualized as an intermediate towards the formation of the K₄Mn₂(μ₆-F) core in KMnF₃ (Figure 3b). Similar compositional and structural relationships can be established between Cs₃Mn₂(tfa)₇(tfaH), Cs₄Mn₃(tfa)₉F, hexagonal CsMnF₃, and tetragonal Cs₂MnF₄.

Figure 3.

Crystal structure of cubic KMnF₃. Unit cell (a) and local coordination of the μ₆-F atom (b) are shown.

K₄Mn₂(tfa)₈ and Cs₃Mn₂(tfa)₇(tfaH) were also decomposed at 250, 450, and 600 °C for 2 h in setups I and II. PXRD patterns of the decomposition products are given in Figures 4 and 5. Thermolysis of K₄Mn₂(tfa)₈ at 250 °C in setup I led to cubic KMnF₃ (PDF No. 01-073-9430) as the sole crystalline product (Figure 4a). The formation of K₂SiF₆ (PDF No. 01-075-0694) was observed upon increasing the decomposition temperature to 450 °C while keeping the dwelling time constant. Under these conditions, KMnF₃ and K₂SiF₆ coexisted with one or multiple crystalline phases (X) whose diffraction maxima could not be indexed (Figure 4b). Finally, only KMnF₃ and K₂SiF₆ were identified as crystalline products upon thermolysis at 600 °C (Figure 4c). Similar results were obtained in setup II except that (i) no crystalline phases other than KMnF₃ and K₂SiF₆ were observed at 450 °C, and (ii) the fraction of K₂SiF₆ relative to KMnF₃ at 600 °C was significantly lower than that observed in setup I (Figure 4d–f). In the case of Cs₃Mn₂(tfa)₇(tfaH), CsMnF₃ (PDF No. 01-075-2034), Cs₂MnF₄, and Cs₂SiF₆ (PDF No. 01-073-6564) were observed as crystalline products upon decomposition at 250 °C in setup I (Figure 5a). A tentative crystal structure of layered perovskite Cs₂MnF₄ was proposed by us in a recent article;⁵ the corresponding reflection list is given in the ESI (Table S15). The hygroscopic nature of this phase complicated collection and indexing of diffraction data;⁵ as a result, the presence of trace amounts of additional phases cannot be discarded. Increasing the decomposition temperature from 250 to 450 °C while keeping the dwelling time constant yielded Cs₂SiF₆ and MnF₂ (PDF No. 01-070-2499) as products (Figure 5b); CsMnF₃ was not observed under these conditions. Further increasing the temperature to 600 °C did not result in significant changes (Figure 5c). Similar results were obtained at 250 and 450 °C in setup II, except that for the latter temperature a very small fraction of hexagonal CsMnF₃ coexisted with Cs₂SiF₆ and MnF₂ (Figure 5d and 5e). At 600 °C, by contrast, hexagonal CsMnF₃ was the major phase and coexisted with a minor fraction of Cs₂SiF₆; no MnF₂ was observed under these conditions (Figure 5f). As mentioned in the Introduction, our goal was to establish reactivity patterns towards a-SiO₂. Results presented in this paragraph, however, provide a starting point to formulate some hypotheses for future mechanistic studies aimed at elucidating fluorinating species, reaction pathways, and transients. The formation of alkali hexafluorosilicates was observed for both compounds and in both setups, implying that byproducts from trifluoroacetate thermolysis reacted with the quartz tube (or with the ceramic wool, which contains a-SiO₂) to form a silicon-containing gas such as SiF₄. Two pathways may be envisioned for the formation of this species. The first pathway involves fluorination of a-SiO₂ by difluorocarbene (equation (1));^{40, 41} CF₂ has been proposed as a byproduct of trifluoroacetate thermolysis.^{4, 42, 43} The second pathway involves etching of a-SiO₂ by gaseous hydrogen fluoride (equation (2)).^{44, 45} HF may be formed upon hydrolysis of trifluoroacetic anhydride.

$${\text{a-SiO}}_2+2{\text{CF}}_2\to {\text{SiF}}_4+2\text{CO}$$ (1)

$${\text{a-SiO}}_2+4\text{HF}\to {\text{SiF}}_4+2{\text{H}}_2\mathrm{O}$$ (2)

(CF₃CO)₂O has been identified as a byproduct of trifluoroacetate thermolysis^{4, 43, 46–48} and the presence of residual water cannot be ruled out under our experimental conditions. Both fluorination pathways may be operating if water is present since hydrolysis of (CF₃CO)₂O leads to the formation of trifluoroacetic acid which in turns produces difluorocarbene.^{44, 45} Another result that deserves further investigation is whether alkali hexafluorosilicates form through reaction of fluoroperovskites with SiF₄. The presence of MnF₂ following the decomposition of Cs₃Mn₂(tfa)₇(tfaH) suggests this reaction may be

$$2{\text{CsMnF}}_3+{\text{SiF}}_4\leftrightharpoons {\text{Cs}}_2{\text{SiF}}_6+2{\text{MnF}}_2$$ (3)

occurring (equation (3)). At the same time, the fact that MnF₂ was not detected in the decomposition of K₄Mn₂(tfa)₈ raises the question of the dependence of the mechanism by which alkali hexafluorosilicates form on the alkali metal. Hypotheses regarding reaction pathways and transients should be considered with the caveat that decomposition reactions were not sequential because reaction mixtures were allowed to dwell for 2 h at each temperature. Future mechanistic studies should obviously employ a different experimental design.

Figure 4.

PXRD patterns of the products resulting from thermolysis of bimetallic trifluoroacetate K₄Mn₂(tfa)₈ in setups I (a‒c) and II (d‒f) at three different temperatures.

Figure 5.

PXRD patterns of the products resulting from thermolysis of bimetallic trifluoroacetate Cs₃Mn₂(tfa)₇(tfaH) in setups I (a‒c) and II (d‒f) at three different temperatures. Patterns shown in (a) and (d) were collected with a step time of 1.4 s.

Section II. Reactivity of Monometallic Trifluoroacetates KH(tfa)₂ and CsH(tfa)₂.

The observation that bimetallic trifluoroacetates reacted with the quartz tube prompted us to investigate the reactivity of monometallic alkali trifluoroacetates KH(tfa)₂ and CsH(tfa)₂ towards a-SiO₂. Specifically, we were interested in establishing whether these solids could be used as reagents for the solid-state synthesis of fluorosilicates. KH(tfa)₂ and CsH(tfa)₂ crystallize in the monoclinic space group C2/c and are isostructural (see ESI, Tables S16–S19 and Figures S6 and S7).⁴⁹ These solids were decomposed in setups I and III to probe their reactivity towards a-SiO₂ first in vacuum and then under air. Thermal decomposition was carried out at temperatures between 200 and 400 °C for 6 to 12 h. Crystalline phases obtained after each thermolysis experiment are summarized in Table 3. PXRD patterns of the decomposition products are given in Figure 6. Owing to the hygroscopic nature of some of the products, diffraction patterns were collected immediately after opening the quartz tube (setup I) or after removing the alumina crucible from the box furnace (setup III), unless noted otherwise. Thermolysis of KH(tfa)₂ at 200 °C for 12 h in a sealed quartz tube resulted in the formation of K₂SiF₆ (PDF No. 01-075-0694) as the sole crystalline product (Figure 6a). Increasing the decomposition temperature to 300 °C led to the appearance of K₃SiF₇ (PDF No. 01-073-1396); at 400 °C, this was the only crystalline phase observed. As expected, increasing temperature stabilized K₃SiF₇ relative to K₂SiF₆.^50–52 Analysis of the decomposition products of CsH(tfa)₂ was significantly more challenging due to the extremely hygroscopic nature of Cs₃SiF₇.⁵³ Unlike K₃SiF₇, which decomposed after several hours under our experimental conditions (≈20–25% relative humidity), Cs₃SiF₇ decomposed within minutes. X-ray analysis of the products resulting from thermolysis of CsH(tfa)₂ at 200 °C for 6 h was not possible because the powder turned into a liquid right after opening the quartz tube. Increasing the decomposition temperature to 300 °C allowed us to observe the coexistence of Cs₂SiF₆ (PDF No. 01-073-6564) and Cs₃SiF₇ (PDF No. 01-071-0997, Figure 6b). Collection of a diffraction pattern of the same sample after 30 min of air exposure showed that Cs₃SiF₇ had already decomposed, leaving Cs₂SiF₆ as the only crystalline phase. Only maxima arising from Cs₂SiF₆ were observed in the products obtained upon thermolysis at 400 °C. Altogether, results from thermolysis experiments conducted in setup I established the ability of KH(tfa)₂ and CsH(tfa)₂ to fluorinate quartz under vacuum and yield ternary fluorosilicates. We then decided to probe whether this reactivity pattern was maintained under air. To this end, mixtures of KH(tfa)₂:a-SiO₂ and CsH(tfa)₂:a-SiO₂ (2:1 molar ratio) were decomposed in setup III. Thermolysis of KH(tfa)₂ was carried out at 300 and 400 °C for 12 h. Both K₂SiF₆ and K₃SiF₇ could be accessed in this temperature range (Figure 6c). CsH(tfa)₂ was decomposed at 400 °C for 6 h in an attempt to obtain phase pure Cs₂SiF₆. However, a mixture of Cs₂SiF₆ and Cs₃SiF₇ was obtained (Figure 6d); as expected, the latter phase decomposed within minutes. The most significant outcome of these experiments was that alkali trifluoroacetates were able to fluorinate a-SiO₂ in an experimental setup similar to that used for routine solid-state reactions. Further, control experiments performed in setup III using KF as a reagent showed that no reaction occurred with a-SiO₂ (see ESI, Figure S8a). Likewise, CsF was much less reactive towards a-SiO₂ than its trifluoroacetate counterpart (see ESI, Figure S8b), demonstrating that the strong fluorinating ability of KH(tfa)₂ and CsH(tfa)₂ stems from the trifluoroacetato ligands. This distinct reactivity of alkali trifluoroacetates could be exploited to design all-solid-state routes to alkali fluorosilicates as an alternative to currently used solution-based syntheses, which typically entail using aqueous HF.^{20, 54} Tuning the stoichiometry of the reaction mixture and reaction conditions (mass, heating rate, and dwelling temperature and time) should enable the preparation of single-phase fluorosilicates using trifluoroacetates as a metal and fluorine source.

Table 3.

Crystalline Products of the Solid-State Thermolysis of Monometallic Trifluoroacetates^a

Precursor	Setup	Dwelling Temperature and Time
		200 °C, 12 h	300 °C, 12 h	400 °C, 12 h
KH(tfa)2	I	K2SiF6	K2SiF6 K3SiF7	K3SiF7
	III		K2SiF6 K3SiF7	K2SiF6 K3SiF7
CsH(tfa)2	I		300 °C, 6 h	400 °C, 6 h
			Cs2SiF6 Cs3SiF7	Cs2SiF6
	III			Cs2SiF6 Cs3SiF7

^aUnder a relative humidity of 20–25%.

Figure 6.

PXRD patterns of the products resulting from thermolysis of monometallic trifluoroacetates KH(tfa)₂ and CsH(tfa)₂ in setups I (a, b) and III (c, d).

Conclusions

The reactivity of alkali–manganese(II) and alkali trifluoroacetates towards a-SiO₂ was probed in a number of experimental configurations. Three new bimetallic fluorotrifluoroacetates were discovered upon thermolysis of K₄Mn₂(tfa)₈ and Cs₃Mn₂(tfa)₇(tfaH) under vacuum. K₄Mn₃(tfa)₉F, Cs₄Mn₃(tfa)₉F, and K₂Mn(tfa)₃F may be regarded as intermediates in the transformation of the bimetallic trifluoroacetates to ternary fluoroperovskites. Decomposition of bimetallic trifluoroacetates also yielded alkali hexafluorosilicates K₂SiF₆ and Cs₂SiF₆ as a result of the fluorination of a-SiO₂. This reactivity pattern was exploited to create a straightforward all-solid-state route to hexa- and heptafluorosilicates via thermal decomposition of monometallic trifluoroacetates KH(tfa)₂ and CsH(tfa)₂ under air. These findings enlarge the library of fluorinated organic–inorganic hybrid materials and the toolbox of synthetic routes to fluorosilicates.

Future research avenues include (1) expanding the proposed solid-state route to more compositionally complex targets such as quaternary fluorosilicates,⁵⁵ (2) developing the chemistry to incorporate optically relevant dopants such as Mn⁴⁺ during the thermal decomposition stage, and (3) understanding the mechanistic aspects of the formation of fluorosilicates from metal trifluoroacetates (i.e., fluorinating species, phase equilibria, and kinetics).