Mechanically Stimulated Solid-State Interaction of Platinum Tetrachloride with Sodium β-Diketonates

A new mechanically stimulated solid-state reaction of PtCl4 with sodium β-diketonates has been discovered. Platinum (II) β-diketonates were obtained by grinding excess sodium trifluoroacetylacetonate Na(tfac) or hexafluoroacetylacetonate Na(hfac) in a vibration ball mill, followed by subsequent heating of the resulting mixture. The reactions occur under much milder conditions (at about 170 °C) compared to similar reactions of PtCl2 or K2PtCl6 (at about 240 °C). Excess diketonate salt plays the role of a reducing agent in the conversion of Pt (IV) salt to Pt (II) compounds. The effect of grinding on properties of the ground mixtures was studied by XRD, IR, and thermal analysis methods. The difference in the course of the interaction of PtCl4 with Na(hfac) or Na(tfac) indicates the dependence of the reaction on the ligand properties. The probable reaction mechanisms were discussed. This method of synthesis of platinum (II) β-diketonates makes it possible to substantially reduce the variety of reagents used, the number of reaction steps, the reaction time, the use of solvents, and waste generation compared to conventional solution-based methods.


Introduction
Due to their unique properties, platinum-group metals β-diketonates and their derivatives are increasingly used in the chemical industry, microelectronics, optics, hydrogen energy systems, medicine, and other fields [1][2][3][4][5][6][7]. Conventional methods for the synthesis of platinum metal β-diketonates are based on the interaction of β-diketone and a metal salt in aqueous or aqueous organic solutions [8][9][10]. As stated earlier by Chimitov et al. [11], there is currently no general method for the synthesis of platinum-group metal β-diketonates, such as M(β-diketonate) 2 (M = Pd, Pt) and M(β-diketonate) 3 (M = Ru, Rh, Ir, Os). In this regard, the possibilities of using alternative technologies for their preparation, e.g., microwave-assisted synthesis [11] or solid-state synthesis [12], are explored by researchers. Note that the use of various mechanochemical techniques (solvent-free/solvent-less processes, liquid-assisted grinding) in many cases makes it possible to accelerate reactions, increase the efficiency of processes, and eliminate or significantly reduce the use of solvents and waste generation. The mechanochemical approach is more environmentally friendly than conventional solution-based reactions [13][14][15][16][17]. In 2019, IUPAC included mechanochemistry in the list of top ten technologies for sustainable development [18,19]. A comprehensive review on the use of alternative technologies for the synthesis of various types of organometallic and coordination compounds has recently been published by Beillard et al. [20].
Only slightly shifted absorption bands of the reactant Na(hfac) were observed in the IR spectra of the mixture after grinding ( Figure 2, Table 1). The IR data corresponded to the results of XRD analysis, and indicated that the reaction passed through a series of intermediate stages and led to the formation of only traces of the final product under the grinding conditions used. The conclusion about the multistage nature of the process under study is consistent with modern ideas about the dynamics of mechanochemical processes, according to which solid-state mechanochemical reactions proceed in several stages, including grinding and physical mixing of reagents, their homogenization at the molecular or cluster level, and the conversion of the activated reactants into reaction products [24]. Earlier, we showed that the transformation of the activated mixture into the final products for a number of mechanically stimulated solid-state reactions occurs when the Figure 1. X-ray diffraction powder patterns of (a) initial PtCl 4 (PDF file 00-019-0914 [21,22]); (b) sodium hexafluoroacetylacetonate Na(hfac); (c,d) the PtCl 4 -4.5Na(hfac) reaction mixture after grinding for 3 and 4 h, respectively; (e) residue after platinum hexafluoroacetylacetonate, Pt(hfac) 2 , vacuum sublimation; and (f) Pt(hfac) 2 sublimed. Symbols denote: , PtCl 4 ; , Na(hfac); , NaCl; •, Pt(hfac) 2 ; Molecules 2023, 28, 3496 2 of 13 milder conditions. To solve this problem, we studied the solid-state interaction of a simple PtCl4 salt with sodium trifluoroacetylacetonate Na(tfac) or hexafluoroacetylacetonate Na(hfac) using mechanical activation (ball milling) in the absence of a solvent, and compared the reactivity of PtCl2, K2PtCl6, and PtCl4 in the reactions.

Results
Grinding a PtCl4-4.5 Na(hfac) mixture in a vibration ball mill for 3-4 h produced a yellow powder, which turned brown when heated to ca. 150 °C. Strong reflections of the reactant Na(hfac), weak reflections due to PtCl4, and traces of the products NaCl and Pt(hfac)2, as well as possible intermediates (PtCl2) or by-products, were observed in the Xray powder diffraction patterns of the mixture after grinding. With increased grinding time, the intensity of the NaCl reflections only slightly increased, while that for the reactant PtCl4 decreased (Figure 1). Figure 1. X-ray diffraction powder patterns of (a) initial PtCl4 (PDF file 00-019-0914 [21,22]); (b) sodium hexafluoroacetylacetonate Na(hfac); (c,d) the PtCl4-4.5Na(hfac) reaction mixture after grinding for 3 and 4 h, respectively; (e) residue after platinum hexafluoroacetylacetonate, Pt(hfac)2, vacuum sublimation; and (f) Pt(hfac)2 sublimed. Symbols denote: , PtCl4; □, Na(hfac); ▲, NaCl; •, Only slightly shifted absorption bands of the reactant Na(hfac) were observed in the IR spectra of the mixture after grinding ( Figure 2, Table 1). The IR data corresponded to the results of XRD analysis, and indicated that the reaction passed through a series of intermediate stages and led to the formation of only traces of the final product under the grinding conditions used. The conclusion about the multistage nature of the process under study is consistent with modern ideas about the dynamics of mechanochemical processes, according to which solid-state mechanochemical reactions proceed in several stages, including grinding and physical mixing of reagents, their homogenization at the molecular or cluster level, and the conversion of the activated reactants into reaction products [24]. Earlier, we showed that the transformation of the activated mixture into the final products for a number of mechanically stimulated solid-state reactions occurs when the , PtCl 2 [23].
Only slightly shifted absorption bands of the reactant Na(hfac) were observed in the IR spectra of the mixture after grinding ( Figure 2, Table 1). The IR data corresponded to the results of XRD analysis, and indicated that the reaction passed through a series of intermediate stages and led to the formation of only traces of the final product under the grinding conditions used. The conclusion about the multistage nature of the process under study is consistent with modern ideas about the dynamics of mechanochemical processes, according to which solid-state mechanochemical reactions proceed in several stages, including grinding and physical mixing of reagents, their homogenization at the molecular or cluster level, and the conversion of the activated reactants into reaction products [24]. Earlier, we showed that the transformation of the activated mixture into the final products for a number of mechanically stimulated solid-state reactions occurs when the mixture is heated [25]. In this regard, we carried out a thermoanalytical study of the reaction mixtures. mixture is heated [25]. In this regard, we carried out a thermoanalytical study of the reaction mixtures.  Thermal analysis of the PtCl4-4.5Na(hfac) reaction mixtures after 3-4 h grinding revealed several endothermic DTA peaks corresponding to weight losses in the TG curves. In the temperature range of ca. 50-150 °C, between 5 and 10% of weight was lost (it appears that Na(hfac) decomposition products arise from the Pt (IV)-Pt (II) reduction process). The intensity of the peaks increases markedly with an increase in the grinding time   Thermal analysis of the PtCl 4 -4.5Na(hfac) reaction mixtures after 3-4 h grinding revealed several endothermic DTA peaks corresponding to weight losses in the TG curves. In the temperature range of ca. 50-150 • C, between 5 and 10% of weight was lost (it appears that Na(hfac) decomposition products arise from the Pt (IV)-Pt (II) reduction process). The intensity of the peaks increases markedly with an increase in the grinding time from 3 to h. In our opinion, the data of instrumental investigations may indicate that the reaction proceeds through the stage of reduction of the ground PtCl 4 to finely dispersed PtCl 2 (d 6.48 Molecules 2023, 28, 3496 4 of 13 from 3 to 4 h. In our opinion, the data of instrumental investigations may indicate that the reaction proceeds through the stage of reduction of the ground PtCl4 to finely dispersed PtCl2 (d 6.48 Ǻ, 3.10 Ǻ, 1.90 Ǻ) (Figure 1). It is known that solid PtCl4, when heated to 380 °C, decomposes with the release of chlorine. When grinding PtCl4, especially in the presence of a reducing agent, the decomposition temperature should decrease, which is what we observe in our reaction. To interact with one molecule of chlorine, one molecule of the reducing agent is sufficient. Fine dispersion of PtCl2 formed in this process leads to its high reactivity. To obtain Pt(βdiketonate)2, two moles of the diketone salt are needed. The ratio (ligand):(metal) = 4.5:1 was used. Here, 2 moles of the ligand went to the formation of the complex, 1 mole went to the reduction, and 1.5 moles were taken to ensure the completeness of the reaction. Weak endothermic peaks at about 70 °C and 110 °C, the intensity of which increased with increasing grinding time, indicated the formation of the decomposition products of the hexafluoroacetylacetonate anion. The formation of volatile products can slow down the course of this solid-state reaction due to the dissipation of mechanical energy, which is due to its consumption for the processes of evaporation of volatile products (see below). Therefore, to complete this solid-state reaction, it is necessary to heat the activated mixture.
The weak endothermic peak in the range of 150-220 °C with an extremum at ca. 180 °C corresponds to the main stage of the weight loss changing from 28% after 3 h to 44% after 4 h grinding. Visual observation shows that sublimation of the target product Pt(hfac)2 occurs in this temperature range. A weak sharp peak at ca. 230 °C corresponding to a weight loss of ~10% is presumably due to thermal degradation of Pt(hfac)2 and/or Na(hfac) (Figure 3a,b).
To avoid thermal decomposition and contamination with by-products formed upon the decomposition of Na(hfac) during Pt (IV)-Pt (II) reduction, the product Pt(hfac)2 was isolated by vacuum sublimation of the ground mixtures at 140-180 °C. The PtCl4-Pt(hfac)2 conversion after grinding for 4 h reached 71% (vide infra, Experimental). The IR and XRD data for the product are consistent with published data for Pt(hfac)2 [8,12]. The residue after the isolation of Pt(hfac)2 showed intense NaCl reflections and weak Na(hfac) reflections ( Figure 1e).
Comparison of the PtCl4-4.5Na(hfac) reaction to previously studied Na(hfac) reactions with PtCl2 or K2PtCl6 (Figure 3c,d) [12] demonstrated that the reaction with PtCl4 proceeded under much milder conditions (at a temperature ~50-60 °C or more lower than with PtCl2 or K2PtCl6) and results in the formation of Pt(hfac)2 with a higher conversion degree.

, 3.10
Molecules 2023, 28, 3496 4 of 13 from 3 to 4 h. In our opinion, the data of instrumental investigations may indicate that the reaction proceeds through the stage of reduction of the ground PtCl4 to finely dispersed PtCl2 (d 6.48 Ǻ, 3.10 Ǻ, 1.90 Ǻ) ( Figure 1). It is known that solid PtCl4, when heated to 380 °C, decomposes with the release of chlorine. When grinding PtCl4, especially in the presence of a reducing agent, the decomposition temperature should decrease, which is what we observe in our reaction. To interact with one molecule of chlorine, one molecule of the reducing agent is sufficient. Fine dispersion of PtCl2 formed in this process leads to its high reactivity. To obtain Pt(βdiketonate)2, two moles of the diketone salt are needed. The ratio (ligand):(metal) = 4.5:1 was used. Here, 2 moles of the ligand went to the formation of the complex, 1 mole went to the reduction, and 1.5 moles were taken to ensure the completeness of the reaction. Weak endothermic peaks at about 70 °C and 110 °C, the intensity of which increased with increasing grinding time, indicated the formation of the decomposition products of the hexafluoroacetylacetonate anion. The formation of volatile products can slow down the course of this solid-state reaction due to the dissipation of mechanical energy, which is due to its consumption for the processes of evaporation of volatile products (see below). Therefore, to complete this solid-state reaction, it is necessary to heat the activated mixture.
The weak endothermic peak in the range of 150-220 °C with an extremum at ca. 180 °C corresponds to the main stage of the weight loss changing from 28% after 3 h to 44% after 4 h grinding. Visual observation shows that sublimation of the target product Pt(hfac)2 occurs in this temperature range. A weak sharp peak at ca. 230 °C corresponding to a weight loss of ~10% is presumably due to thermal degradation of Pt(hfac)2 and/or Na(hfac) (Figure 3a,b).
To avoid thermal decomposition and contamination with by-products formed upon the decomposition of Na(hfac) during Pt (IV)-Pt (II) reduction, the product Pt(hfac)2 was isolated by vacuum sublimation of the ground mixtures at 140-180 °C. The PtCl4-Pt(hfac)2 conversion after grinding for 4 h reached 71% (vide infra, Experimental). The IR and XRD data for the product are consistent with published data for Pt(hfac)2 [8,12]. The residue after the isolation of Pt(hfac)2 showed intense NaCl reflections and weak Na(hfac) reflections ( Figure 1e).
Comparison of the PtCl4-4.5Na(hfac) reaction to previously studied Na(hfac) reactions with PtCl2 or K2PtCl6 (Figure 3c,d) [12] demonstrated that the reaction with PtCl4 proceeded under much milder conditions (at a temperature ~50-60 °C or more lower than with PtCl2 or K2PtCl6) and results in the formation of Pt(hfac)2 with a higher conversion degree.

, 1.90
Molecules 2023, 28, 3496 4 of 1 from 3 to 4 h. In our opinion, the data of instrumental investigations may indicate that th reaction proceeds through the stage of reduction of the ground PtCl4 to finely dispersed PtCl2 (d 6.48 Ǻ, 3.10 Ǻ, 1.90 Ǻ) (Figure 1). It is known that solid PtCl4, when heated to 380 °C, decomposes with the release o chlorine. When grinding PtCl4, especially in the presence of a reducing agent, the decom position temperature should decrease, which is what we observe in our reaction. To inter act with one molecule of chlorine, one molecule of the reducing agent is sufficient. Fine dispersion of PtCl2 formed in this process leads to its high reactivity. To obtain Pt(β diketonate)2, two moles of the diketone salt are needed. The ratio (ligand):(metal) = 4.5:1 was used. Here, 2 moles of the ligand went to the formation of the complex, 1 mole wen to the reduction, and 1.5 moles were taken to ensure the completeness of the reaction Weak endothermic peaks at about 70 °C and 110 °C, the intensity of which increased with increasing grinding time, indicated the formation of the decomposition products of the hexafluoroacetylacetonate anion. The formation of volatile products can slow down th course of this solid-state reaction due to the dissipation of mechanical energy, which i due to its consumption for the processes of evaporation of volatile products (see below) Therefore, to complete this solid-state reaction, it is necessary to heat the activated mix ture.
The weak endothermic peak in the range of 150-220 °C with an extremum at ca. 180 °C corresponds to the main stage of the weight loss changing from 28% after 3 h to 44% after 4 h grinding. Visual observation shows that sublimation of the target produc Pt(hfac)2 occurs in this temperature range. A weak sharp peak at ca. 230 °C corresponding to a weight loss of ~10% is presumably due to thermal degradation of Pt(hfac)2 and/o Na(hfac) (Figure 3a,b).
To avoid thermal decomposition and contamination with by-products formed upon the decomposition of Na(hfac) during Pt (IV)-Pt (II) reduction, the product Pt(hfac)2 wa isolated by vacuum sublimation of the ground mixtures at 140-180 °C. The PtCl4-Pt(hfac) conversion after grinding for 4 h reached 71% (vide infra, Experimental). The IR and XRD data for the product are consistent with published data for Pt(hfac)2 [8,12]. The residue after the isolation of Pt(hfac)2 showed intense NaCl reflections and weak Na(hfac) reflec tions (Figure 1e).
Comparison of the PtCl4-4.5Na(hfac) reaction to previously studied Na(hfac) reac tions with PtCl2 or K2PtCl6 (Figure 3c,d) [12] demonstrated that the reaction with PtCl proceeded under much milder conditions (at a temperature ~50-60 °C or more lower than with PtCl2 or K2PtCl6) and results in the formation of Pt(hfac)2 with a higher conversion degree.
) (Figure 1). It is known that solid PtCl 4 , when heated to 380 • C, decomposes with the release of chlorine. When grinding PtCl 4 , especially in the presence of a reducing agent, the decomposition temperature should decrease, which is what we observe in our reaction. To interact with one molecule of chlorine, one molecule of the reducing agent is sufficient. Fine dispersion of PtCl 2 formed in this process leads to its high reactivity. To obtain Pt(βdiketonate) 2 , two moles of the diketone salt are needed. The ratio (ligand):(metal) = 4.5:1 was used. Here, 2 moles of the ligand went to the formation of the complex, 1 mole went to the reduction, and 1.5 moles were taken to ensure the completeness of the reaction.
Weak endothermic peaks at about 70 • C and 110 • C, the intensity of which increased with increasing grinding time, indicated the formation of the decomposition products of the hexafluoroacetylacetonate anion. The formation of volatile products can slow down the course of this solid-state reaction due to the dissipation of mechanical energy, which is due to its consumption for the processes of evaporation of volatile products (see below). Therefore, to complete this solid-state reaction, it is necessary to heat the activated mixture.
The weak endothermic peak in the range of 150-220 • C with an extremum at ca. 180 • C corresponds to the main stage of the weight loss changing from 28% after 3 h to 44% after 4 h grinding. Visual observation shows that sublimation of the target product Pt(hfac) 2 occurs in this temperature range. A weak sharp peak at ca. 230 • C corresponding to a weight loss of~10% is presumably due to thermal degradation of Pt(hfac) 2 and/or Na(hfac) (Figure 3a,b).  To avoid thermal decomposition and contamination with by-products formed upon the decomposition of Na(hfac) during Pt (IV)-Pt (II) reduction, the product Pt(hfac) 2 was isolated by vacuum sublimation of the ground mixtures at 140-180 • C. The PtCl 4 -Pt(hfac) 2 conversion after grinding for 4 h reached 71% (vide infra, Experimental). The IR and XRD data for the product are consistent with published data for Pt(hfac) 2 [8,12]. The residue after the isolation of Pt(hfac) 2 showed intense NaCl reflections and weak Na(hfac) reflections (Figure 1e).
Comparison of the PtCl 4 -4.5Na(hfac) reaction to previously studied Na(hfac) reactions with PtCl 2 or K 2 PtCl 6 (Figure 3c,d) [12] demonstrated that the reaction with PtCl 4 proceeded under much milder conditions (at a temperature~50-60 • C or more lower than with PtCl 2 or K 2 PtCl 6 ) and results in the formation of Pt(hfac) 2 with a higher conversion degree.
Thus, the processes occurring during this reaction can be represented by Scheme 1: The solid-state reaction of PtCl4 with Na(tfac) differs somewhat from the reaction described above. Grinding PtCl4-4.5Na(tfac) for 3-4 h leads to the formation of a brown PtCl4-4.5 Na(tfac) mixture.
The X-ray diffraction pattern of the ground mixture consists mainly of NaCl reflections, an amorphous phase at 2θ~20°, and traces of the reactants Na(tfac) and PtCl4 ( Figure  4c). The presence of strong NaCl reflections ( Figure 4) may indicate the formation of substitution products in the ground mixtures. The formation of substitution products also manifested in the IR spectra by a slight shift in the absorption bands of the Na(tfac) C=O group (Table 2), and indicated a difference in the mechanisms of PtCl4 interaction with Na(hfac) and Na(tfac). Mainly, strong reflections of the product NaCl and weak reflections of the reactant Na(tfac) were observed in the residue after Pt(tfac)2 sublimation. The solid-state reaction of PtCl 4 with Na(tfac) differs somewhat from the reaction described above. Grinding PtCl 4 -4.5Na(tfac) for 3-4 h leads to the formation of a brown PtCl 4 -4.5 Na(tfac) mixture.
The X-ray diffraction pattern of the ground mixture consists mainly of NaCl reflections, an amorphous phase at 2θ~20 • , and traces of the reactants Na(tfac) and PtCl 4 (Figure 4c). The presence of strong NaCl reflections ( Figure 4) may indicate the formation of substitution products in the ground mixtures. The formation of substitution products also manifested in the IR spectra by a slight shift in the absorption bands of the Na(tfac) C=O group (Table 2), and indicated a difference in the mechanisms of PtCl 4 interaction with Na(hfac) and Na(tfac). Mainly, strong reflections of the product NaCl and weak reflections of the reactant Na(tfac) were observed in the residue after Pt(tfac) 2 sublimation.  The IR spectra mainly display slightly shifted absorption bands of Na(tfac) and som bands close to those of Pt(tfac)2 ( Figure 5, Table 2). Table 2. IR spectra of Na(tfac), ground mixture, and obtained Pt(tfac)2 (ν, cm −1 ).  Table 2. IR spectra of Na(tfac), ground mixture, and obtained Pt(tfac) 2 (ν, cm −1 ). The IR spectra mainly display slightly shifted absorption bands of Na(tfac) and some bands close to those of Pt(tfac) 2 ( Figure 5, Table 2).  Comparison of the results from instrumental research methods suggests that, unlike reaction in the PtCl4-4.5Na(hfac) mixture (Scheme 1, formation of PtCl4 reduction products), this reaction proceeds via a sequence of addition-elimination steps with the possible formation of intermediates such as Na[PtCl4(tfac)] + , Pt(tfac)2Cl2 or others, and the production of the desired Pt(tfac)2. Taking into account the polymeric structure of PtCl4, the formation of various other intermediates can be assumed [26].

vw
Thus, the processes occurring during this reaction can be represented by Scheme 2: The DTA curve of the reaction mixture after grinding for 4 h shows weak endothermic peaks at 98 and 110 • C corresponding to a weight loss of~4% in the TG curve ( Figure 6). In the range of 110-180 • C, an intense exothermic peak was observed with a maximum at~170 • C, corresponding to the main stage of weight loss. Visual obser- A difference in the course of PtCl4 reactions with Na(hfac) (Scheme 1) and Na(tfac) (Scheme 2) indicates a significant dependence of the reaction course on the structural, electronic, and steric properties of organic ligands. Na(hfac) easily reduces PtCl4 with the formation of highly volatile products and PtCl2, while the reaction with Na(tfac) proceeds via addition-elimination steps. It can be expected that PtCl4 reactions with salts of less acidic β-diketones will also proceed similarly to the reaction with Na(tfac).
Thermal analysis data show that the main stage of weight loss (sublimation of target products) corresponds to the weak endothermic effect, with an extremum at ca. 178 °C for the reaction of PtCl4 with Na(hfac) (Figure 3) and the exotherm with a maximum at ca. 171 °C for the reaction with Na(tfac) ( Figure 6); this is in contrast to the analogous reactions with PtCl2 or K2PtCl6 accompanied by exothermic effects with maxima at ca. 240 °C [12]. These data indicate a decrease in the formation temperature of Pt(II) diketonates by about 60 °C when using PtCl4 instead of PtCl2 or K2PtCl6.
The degree of conversion of the initial platinum salt in the considered solid-state reactions is comparable to that for conventional solution-based methods [8]. However, the solid-state synthesis approach makes it possible to achieve the following: significantly reduce the variety of reagents used, the number of reaction steps, and the reaction time; reduce or eliminate the use of solvents; and significantly reduce waste generation. Further research is necessary to elucidate the mechanism and optimize the conditions of the reactions under consideration.
Previously, in the course of a systematic study, we studied the feasibility and some features of the solid-state mechanochemical synthesis of various classes of coordination and organometallic compounds, such as cyclopentadienyl complexes of d- [27] and f-elements [28], Fe, Co, and Ni metallocenes [29], bis-dicarbollyl complexes of Fe, Co, and Cr [30], β-diketonates [25,31], carboxylates [32], dialkyldithiocarbamates [33], and other complexes of a number of metals.
The data obtained by us and by other researchers make it possible to reveal some typical features that make a difference between solid-state mechanochemical reactions and other mechanochemical technologies and conventional solution-based reactions. These features are associated primarily with the impact of mechanical energy on the crystalline reactants (grinding), which results in their physical mixing, fragmentation, and homogenization, an increase in the contact area between them, the formation of various Comparison of the results from instrumental research methods suggests that, unlike reaction in the PtCl 4 -4.5Na(hfac) mixture (Scheme 1, formation of PtCl 4 reduction products), this reaction proceeds via a sequence of addition-elimination steps with the possible formation of intermediates such as Na[PtCl 4 (tfac)] + , Pt(tfac) 2 Cl 2 or others, and the production of the desired Pt(tfac) 2 . Taking into account the polymeric structure of PtCl 4 , the formation of various other intermediates can be assumed [26].
Thus, the processes occurring during this reaction can be represented by Scheme 2: Comparison of the results from instrumental research methods suggests that, unlike reaction in the PtCl4-4.5Na(hfac) mixture (Scheme 1, formation of PtCl4 reduction products), this reaction proceeds via a sequence of addition-elimination steps with the possible formation of intermediates such as Na[PtCl4(tfac)] + , Pt(tfac)2Cl2 or others, and the production of the desired Pt(tfac)2. Taking into account the polymeric structure of PtCl4, the formation of various other intermediates can be assumed [26].
Thus, the processes occurring during this reaction can be represented by Scheme 2: A difference in the course of PtCl 4 reactions with Na(hfac) (Scheme 1) and Na(tfac) (Scheme 2) indicates a significant dependence of the reaction course on the structural, electronic, and steric properties of organic ligands. Na(hfac) easily reduces PtCl 4 with the formation of highly volatile products and PtCl 2 , while the reaction with Na(tfac) proceeds via addition-elimination steps. It can be expected that PtCl 4 reactions with salts of less acidic β-diketones will also proceed similarly to the reaction with Na(tfac).
Thermal analysis data show that the main stage of weight loss (sublimation of target products) corresponds to the weak endothermic effect, with an extremum at ca. 178 • C for the reaction of PtCl 4 with Na(hfac) (Figure 3) and the exotherm with a maximum at ca. 171 • C for the reaction with Na(tfac) ( Figure 6); this is in contrast to the analogous reactions with PtCl 2 or K 2 PtCl 6 accompanied by exothermic effects with maxima at ca. 240 • C [12]. These data indicate a decrease in the formation temperature of Pt(II) diketonates by about 60 • C when using PtCl 4 instead of PtCl 2 or K 2 PtCl 6 .
The degree of conversion of the initial platinum salt in the considered solid-state reactions is comparable to that for conventional solution-based methods [8]. However, the solid-state synthesis approach makes it possible to achieve the following: significantly reduce the variety of reagents used, the number of reaction steps, and the reaction time; reduce or eliminate the use of solvents; and significantly reduce waste generation. Further research is necessary to elucidate the mechanism and optimize the conditions of the reactions under consideration.
Previously, in the course of a systematic study, we studied the feasibility and some features of the solid-state mechanochemical synthesis of various classes of coordination and organometallic compounds, such as cyclopentadienyl complexes of d- [27] and f -elements [28], Fe, Co, and Ni metallocenes [29], bis-dicarbollyl complexes of Fe, Co, and Cr [30], β-diketonates [25,31], carboxylates [32], dialkyldithiocarbamates [33], and other complexes of a number of metals.
The data obtained by us and by other researchers make it possible to reveal some typical features that make a difference between solid-state mechanochemical reactions and other mechanochemical technologies and conventional solution-based reactions. These features are associated primarily with the impact of mechanical energy on the crystalline reactants (grinding), which results in their physical mixing, fragmentation, and homogenization, an increase in the contact area between them, the formation of various defects in the crystal structure, and an increase in temperature of the reaction mixture. All these factors increase the reactivity of the reactants.
The accumulation of energy by mixtures during their grinding can be most clearly detected using thermal analysis. During grinding, a new exothermic effect appears in the DTA curves of the reaction mixtures, which is absent in the DTA curves of the reactants and products. With an increase in grinding time, its temperature decreases due to the formation of smaller particles of reactants in the mixture. This exotherm corresponds to the interaction of the reactants with the formation of final products. When the reaction temperature approaches the ambient temperature, the reactants are instantly converted into products in the thermal explosion mode [34].
Milled mixtures with too high temperatures of the exothermic effect (above 200 • C) do not reach the stage of transformation into final products even with prolonged grinding under experimental conditions. Instrumental research methods indicate the presence of only ground reactants in such mixtures (broadened absorption bands in the IR spectra and broadened reflections in powder X-ray diffraction patterns). Evidently, the power of the used vibration mill is not enough to heat the reactor to the temperature of the reaction initiation. These mixtures can be converted into products by further heating after grinding [12].
The yield of products is very sensitive to the quality of the reactants and decreases drastically in the presence of trace water or other volatile compounds because the presence or formation of liquid or viscous compounds prevents the transfer of mechanical energy to crystalline reactants due to energy dissipation. In this case, other regularities are observed [35].
In some cases, mixtures that have accumulated mechanical energy may remain reactive for some time, and they can be called activated mixtures. This allows them to be used to conduct or study reactions of the synthesis in the mode of self-propagation. The advantage of such systems is a low temperature of the process (less than 300 • C), and the possibility of visual control of the process due to a change in the color of the reaction mixture during the reaction. The reaction in the Na(acetylacetonate)-CrCl 3 system is one of the first examples of the synthesis of complex compounds in the self-propagation mode. Reactions of this type demonstrate the relationship between the phenomena of mechanical activation, thermal explosion, and reactions in the self-propagation mode [25,34,36].
Most of the systems studied are characterized by the presence of both reaction products and a new exothermic effect in the DTA curves of milled mixtures. In these systems, both final or intermediate products and activated mixtures of reactants are formed as a result of grinding. On the thermograms of these mixtures, an exotherm is observed, the temperature of which decreases with an increase in the grinding time, but does not reach the inner temperature (temperature of the reactor). The superposition of the exothermic effects of the reaction and the endothermic effects of melting or sublimation of the products can lead to a Molecules 2023, 28, 3496 9 of 12 complex shape of the DTA curves. In the X-ray powder diffraction patterns of the milled mixtures, the intensity of the reflections of the reactants decreases and that of the reaction products increases. The IR spectra indicate a decrease in the intensity of the absorption bands of reactants and an increase in the absorption bands of products. There is a gradual decrease in the amount of reactants, and an increase in the amount of reaction products in the ground mixtures. The reactions slow down due to dilution of the reaction mixture by the reaction products. In this case, heating of ground mixtures can significantly reduce time of synthesis, increase the product yield, and prevent mechanochemical decomposition of the target products [33].
The data obtained suggest that activating devices of a vibration-mill type are efficient enough for creating optimal conditions for mechanochemical synthesis of coordination and organometallic compounds. More powerful devices, such as planetary mills, are hardly suitable for the purpose since they can cause decomposition of the product and a decrease in the yield. Similar patterns in relation to the use of apparatuses of various types were observed in the synthesis of boron hydride derivatives [37].
A comparison of the three considered types of reactions indicates that the difference between them is of no fundamental importance and is caused only by the influence of external factors, namely, the temperature of the process. It can be assumed that when milling at low temperatures, the transformation of the activated mixture into products will be hindered, while at elevated temperatures most of the reactions will proceed in a purely mechanochemical mode.
The advantages of solid-state methods also lie in the fact that they make it possible to solve a number of problems that are difficult or impossible to solve by conventional methods, such as the preparation of non-solvated or hydrolysis-sensitive products [38], as well as products of complete replacement of chloride ligands in platinum compounds under mild conditions (this work) compared to reactions in solution or liquid-assisted grinding reactions [39].
Thus, the study of the effect of grinding on mixtures of solid metal chlorides with anionic derivatives of organic compounds made it possible to expand the existing ideas on the reactivity of solids, develop solid-state methods for the synthesis of certain classes of compounds, and discover a number of phenomena previously unknown in the chemistry of coordination and organometallic compounds.

Materials and Methods
Sodium trifluoroacetylacetonate Na(tfac) and hexafluoroacetylacetonate Na(hfac) were obtained as described previously [31]. Freshly prepared β-diketonates were used in the reactions. Platinum (IV) chloride (Sigma-Aldrich; 96% purity) was used without further purification. Preparation of mixtures of starting materials, samples for instrumental studies, loading and unloading of a ball mill grinding jar, as well as other operations with airsensitive substances were carried out in a nitrogen atmosphere in a drybox.
Grinding the reaction mixtures was carried out in a stainless-steel grinding jar with a volume of 85 cm 3 using a custom-made vibration ball mill [12]. Weighed amounts of the reactants (~1 g) and 20 steel balls 12.3 mm in diameter (~150 g) as milling bodies were loaded into the jar. The jar was sealed, mounted on the vibration ball mill, and subjected to vibration (amplitude 11 mm, frequency 12 Hz; energy intensity of~0.5 W/g) for a preset time. Then, the jar was opened and the reaction mixture was separated and used for instrumental studies and isolation of target products. The properties of the reactants, reaction mixtures, and products were studied by X-ray phase analysis, IR spectroscopy, and thermal analysis. The products were identified according to the data of chemical analysis and instrumental research methods. X-ray diffraction powder patterns of the samples were recorded on an ADN-2-01 diffractometer (CuKα radiation, Ni filter). IR spectra of the reactants, reaction mixtures, and products were recorded on a Bruker Vertex 70 v Fourier-transform infrared spectrometer. Thermal studies were performed on an