Alcoholic solutions of chloroplatinic acid hexahydrate (H2PtCl6 ⋅ 6H2O) are among the widely used catalysts in the production of organosilicon monomers and polymers in the addition reactions of olefins and oligomers with multiple bonds, respectively, to –SiH groups. As a result of studying the hydrosilylation reaction at sufficiently low temperatures, it has been found [1] that chloroplatinic acid in isopropyl alcohol (Speier’s catalyst, SpC) is extremely active in this reaction. Also, platinum black, platinized carbon, and potassium chloroplatinite K2PtCl4 proved to be highly efficient [1].

Summarizing numerous publications, it can be stated that compounds of platinum in different oxidation states exhibit catalytic properties in SpC. At the same time, it seems that there is no clear understanding of the mechanism of formation of Pt(II) complexes, in particular, from H2PtCl6 ⋅ 6H2O and alcohols, since the works often contain conflicting and even mutually exclusive data. For example, it has been hypothesized in [2–4] that the Pt(II) propylene complex is not formed in the composition of SpC, and Pt(IV) is successively reduced to Pt(0) by reactions (1) and (2):

$$\begin{gathered} {{{\text{H}}}_{2}}{\text{PtC}}{{{\text{l}}}_{6}} + {{({\text{C}}{{{\text{H}}}_{3}})}_{2}}{\text{CHOH}} \\ \to {{{\text{H}}}_{2}}{\text{PtC}}{{{\text{l}}}_{4}} + {{({\text{C}}{{{\text{H}}}_{{\text{3}}}})}_{2}}{\text{C}}{\kern 1pt} = {\kern 1pt} {\text{O}} + 2{\text{HCl}}{\text{,}} \\ \end{gathered} $$
(1)
$$\begin{gathered} {{{\text{H}}}_{2}}{\text{Pt}}{{{\text{C}}}_{4}} + {{({\text{C}}{{{\text{H}}}_{3}})}_{2}}{\text{CHOH}} \\ \to {\text{Pt}} + {{({\text{C}}{{{\text{H}}}_{3}})}_{2}}{\text{C}}{\kern 1pt} = {\kern 1pt} {\text{O}} + 4{\text{HCl}}{\text{.}} \\ \end{gathered} $$
(2)

On the other hand, it has been stated in [5–11] that Pt(II) is coordinated to olefins formed by dehydration of alcohols (3):

$$\begin{gathered} {{{\text{H}}}_{2}}{\text{PtC}}{{{\text{l}}}_{6}} \cdot 6{{{\text{H}}}_{2}}{\text{O}} + 2{{({\text{C}}{{{\text{H}}}_{3}})}_{2}}{\text{CHOH}} \\ \to 0.5[({{{\text{C}}}_{3}}{{{\text{H}}}_{6}}){\text{PtC}}{{{\text{l}}}_{2}}] + {{({\text{C}}{{{\text{H}}}_{3}})}_{2}}{\text{C}}{\kern 1pt} = {\kern 1pt} {\text{O}} + 4{\text{HCl}} + 7{{{\text{H}}}_{2}}{\text{O}}{\text{.}} \\ \end{gathered} $$
(3)

Early studies used anhydrous PtCl4 [12], platinum chloride salts with dry alcohols [13, 14], and anhydrous Na2PtCl6, which was then dissolved in alcohol [15]. In our opinion, dehydration led to the creation of favorable conditions for the intermediate formation of olefins by alcohol dehydration. However, in the SpC medium, chloroplatinic acid already contains 6 moles of water, which can hinder the reaction of alcohol dehydration, since it is known [16] that it proceeds under rather severe conditions in the presence of strong water-removing agents, such as concentrated sulfuric acid.

There are also significant disagreements on the stability of π-complexes of platinum chlorides with olefins. For example, it has been concluded in [5] that the chloride ion released in a chemical transformation according to equation (4),

$$\begin{gathered} {{{\text{H}}}_{2}}{\text{PtC}}{{{\text{l}}}_{6}} \cdot 6{{{\text{H}}}_{2}}{\text{O}} + 2{{({\text{C}}{{{\text{H}}}_{3}})}_{2}}{\text{CHOH}} \\ \to {\text{H}}[({{{\text{C}}}_{{\text{3}}}}{{{\text{H}}}_{6}}){\text{PtC}}{{{\text{l}}}_{3}}] + {{({\text{C}}{{{\text{H}}}_{3}})}_{2}}{\text{C}}{\kern 1pt} = {\kern 1pt} {\text{O}} + 3{\text{HCl}} + 7{{{\text{H}}}_{2}}{\text{O}}{\text{,}} \\ \end{gathered} $$
(4)

stabilizes the anionic complex H[(C3H6)PtCl3], while in [8] it has been stated that [(C3H6)PtCl3]– decomposes with the release of propylene under the action of chloride ions at room temperature.

At the same time, there are studies that have shown that Pt(II) forms complexes with alcohols [17, 18], ketones [19, 20], and water [21]. Surprisingly, but none of the researchers paid attention to the fact that water can be formed during the exchange reaction between alcohol and hydrogen chloride, which is widely used in the production of chloroalkyls [22]:

$${\text{ROH}} + {\text{HCl}} \rightleftarrows {\text{RCl}} + {{{\text{H}}}_{2}}{\text{O}}{\text{.}}$$
(5)

In exchange reactions (5), Lewis acids–metal chlorides (for example, AlCl3, ZnCl2, etc.)—are mainly used as a catalyst. It is quite possible that platinum chloride compounds can act as a catalyst in the above system.

Since, as noted above, the relevance of expanding knowledge about the composition and transformations of the SpC over time and under various conditions is obvious, to refine the above characteristics of SpC, in this work we have studied the behavior of H2PtCl6 ⋅ 6H2O in an excess of 2-octanol. The choice of 2-octanol is due to the rather high boiling point of octene-1, which makes it easy to identify it when it is formed by alcohol dehydration both in the composition of the complex and in the free state.

EXPERIMENTAL

The 1Н, 13С, and 29Si NMR spectra were recorded on Bruker AM-360 and AVANCE 600 spectrometers. All chemical shifts were expressed in ppm relative to TMS. The solvent was CDCl3.

GLC analysis was carried out on a Kristall 2000 chromatograph; 1 m × 3 mm column filled with 5% SE-30 on Chromaton N–AW HMDS as a stationary phase; carrier gas, helium; detector, katharometer.

The molecular weight and structure of the compounds were determined on an Agilent 7890A triple quadrupole GC/MS mass spectrometer with an Agilent 240 ion trap unit.

The IR spectra were recorded on a Specord M-80 IR Fourier spectrophotometer.

Reaction of chloroplatinic acid hexahydrate with octanol-2. H2PtCl6 ⋅ 6H2O (1 g, 0.0019 mol) was dissolved in 20 mL (0.1259 mol) of 2-octanol. The solution was stirred for 10 min at room temperature, then stirred at 100°C until the precipitation of platinum began and the solution became completely clear. Samples for analysis were taken after 1, 3, 7, 10, and 13 h. The weight of the isolated product was 16.9 g. The product composition: 0.11 g (0.00056 mol) (0.65%) of colloidal platinum(0), 0.43 g (0.0013 mol) (2.55%) of H2PtCl4, 16.1 g (0.1238 mol) (95.44%) of 2-octanol, 0.23 g (0.0018 mol) (1.36%) of 2-octanone, and trace amounts of 2-chlorooctane.

Reaction of chloroplatinic acid hexahydrate with ethanol and 1-octene. Crystalline H2PtCl6 ⋅ 6H2O (0.5 g, 0.00097 mol) was mixed with 0.045 g (0.00097 mol) of ethanol and 0.11 g (0.00097 mol) of 1-octene and stirred at 75°C for 6 h. Final composition of the reaction mixture: 0.3 g (0.00089 mol) of H2PtCl4, 0.08 g (0.00054 mol) of 2-chlorooctane, and trace amounts of 1-octene.

Reaction of chloroplatinic acid with trimethylchlorosilane and 2-octanol. H2PtCl6 ⋅ 6H2O (0.62 g, 0.0012 mol) was mixed with 1.56 g (0.0144 mol) of (CH3)3SiCl and stirred for 1 h at room temperature. Then, 0.31 g (0.0024 mol) of 2-octanol was added, and the mixture was stirred at 100°C for 8 h. The following products were identified in the final reaction mixture: (CH3)3SiOSi(CH3)3, (CH3)3SiCl, (CH3)3SiO[(CH3)2SiO]nSi(CH3)3, platinum(II) and platinum(0) compounds, 2-octanol, 2-octanone, and 2-chlorooctane.

Reaction of chloroplatinic acid with trimethylchlorosilane, 1-octene, and 2-octanol. H2PtCl6 ⋅ 6H2O (2 g, 0.0039 mol) was mixed with 5.03 g (0.0463 mol) of (CH3)3SiCl and stirred for 1 h at room temperature. Then, 8.5 mL (0.0542 mol) of 1-octene was added, and the mixture was stirred at 75°С for 6.5 h. Next, 0.61 mL (0.0039 mol) of 2-octanol was added, and stirring was continued at 75°C for 9 h until colloidal platinum(0) appeared, which was decanted, washed 3 times with acetone, and calcined at 250°C for 1 h. Dry colloidal platinum(0) (0.22 g, 0.0011 mol) was isolated in 28.9% yield.

RESULTS AND DISCUSSION

To accelerate chemical processes in the selected H2PtCl6 ⋅ 6H2O–2-octanol system, the mixture was subjected to thermal treatment. The initial solution was analyzed by 1H NMR spectroscopy and the spectrum was compared with that of reference 2-octanol [23]. Along with the signals of the 2-octanol protons, a new signal was observed at δ = 2.72 ppm, corresponding to the protons of hydrate water of chloroplatinic acid.

After stirring at 100°C for 1 h, no changes were observed in the 1H NMR spectrum of the reaction mixture, and an absorption band at 1712 cm–1 appeared in the IR spectrum, which corresponded to the stretching vibrations of the C=O bond (νC=O) in 2-octanone. An increase in the intensity of the νC=O band in the IR spectra of the samples taken after 3 and 7 h indicated that the content of 2-octanone was growing. The results of the analysis of the solution by GLC after 13 h, when some of the platinum precipitated in the form of fine platinum black, showed that the content of 2-octanone increased to 2 wt %, while the content of 2-octanol decreased by 6%. The formation of 2-octanone was also confirmed by NMR: the 13C NMR spectrum showed a signal at δ = 208.9 ppm, characteristic of the C=O bond; the 1H NMR spectrum showed the resonances of CH3 and CH2 protons at the carbonyl group (δ = 2.37 and 2.08 ppm, respectively). The 1H NMR spectrum of the product also showed a signal from the CHCl group of the alkyl chloride in the form of a sextet at 3.97 ppm, which confirmed the formation of 2-chlorooctane. The presence of 2-octanone and 2-chlorooctane in the reaction products was demonstrated by GC/MS.

It follows from the research results that no formation of 1-octene and 2-octene by the dehydration reaction of 2-octanol under the action of H2PtCl6 ⋅ 6H2O is observed; the products are 2-octane (reactions (6), (7)) and 2-chlorooctane yielded by exchange reaction (5) of 2-octanol with hydrogen chloride.

$$\begin{gathered} {{{\text{H}}}_{2}}{\text{PtC}}{{{\text{l}}}_{6}} \cdot 6{{{\text{H}}}_{2}}{\text{O}} + {\text{C}}{{{\text{H}}}_{3}}{{({\text{C}}{{{\text{H}}}_{2}})}_{5}}{\text{CHOHC}}{{{\text{H}}}_{3}} \\ \rightleftarrows {{{\text{H}}}_{2}}{\text{PtC}}{{{\text{l}}}_{4}} + {\text{C}}{{{\text{H}}}_{3}}{{({\text{C}}{{{\text{H}}}_{2}})}_{5}}{\text{C(O}}){\text{C}}{{{\text{H}}}_{3}} + 2{\text{HCl}} + 6{{{\text{H}}}_{2}}{\text{O}}{\text{,}} \\ \end{gathered} $$
(6)
$$\begin{gathered} {{{\text{H}}}_{2}}{\text{PtC}}{{{\text{l}}}_{4}} + {\text{C}}{{{\text{H}}}_{3}}{{({\text{C}}{{{\text{H}}}_{2}})}_{5}}{\text{CHOHC}}{{{\text{H}}}_{3}} \\ \to {\text{Pt}} + {\text{C}}{{{\text{H}}}_{3}}{{({\text{C}}{{{\text{H}}}_{2}})}_{5}}{\text{C}}({\text{O}}){\text{C}}{{{\text{H}}}_{3}} + 4{\text{HCl}}{\text{.}} \\ \end{gathered} $$
(7)

Using GLC data, the amount of the resulting 2-octanone was determined and then it was calculated that Pt(IV) was 100% reduced to Pt(II) and about 30% was further reduced to Pt(0). The same result was obtained in the gravimetric study of precipitated colloidal Pt(0).

In the next experiment, H2PtCl6 ⋅ 6H2O was treated with (CH3)3SiCl for its dehydration according to reaction (8). To accurately exclude the presence of water in the system, a small excess of (CH3)3SiCl was taken, which does not affect the processes under study.

$$\begin{gathered} {{{\text{H}}}_{2}}{\text{PtC}}{{{\text{l}}}_{6}} \cdot 6{{{\text{H}}}_{2}}{\text{O}} + 12{{({\text{C}}{{{\text{H}}}_{3}})}_{3}}{\text{SiCl}} \\ \to {{{\text{H}}}_{2}}{\text{PtC}}{{{\text{l}}}_{6}} + 6{{({\text{C}}{{{\text{H}}}_{3}})}_{3}}{\text{SiOSi}}{{({\text{C}}{{{\text{H}}}_{3}})}_{3}} + 12{\text{HCl}}{\text{.}} \\ \end{gathered} $$
(8)

The resulting mixture of H2PtCl6 and hexamethyldisiloxane was stirred in 2-octanol at 100°C for 8 h. 2-Octanone was obtained similarly to reaction (6), which has been confirmed by the 13C NMR spectrum, which shows a signal from the carbonyl group at 211.89 ppm, while no resonances of C=C groups are observed in the range 114–139 ppm. Therefore, even in a partially dehydrated H2PtCl6–2-octanol system, the reaction of alcohol dehydration to form octenes does not occur.

When H2PtCl6 treated with trimethylchlorosilane was mixed with 1-octene at 75°С for 6.5 h, the 1H NMR spectrum of the reaction mixture showed a signal at δ = 4.03 ppm, arising from the corresponding –CHCl group, and signals at δ = 4.99 and 5.86 ppm from 1-octene traces. After stirring this mixture with the addition of 2-octanol at 75°С for 9 h, the mixture contained trace amounts of 1-octene and 2-chlorooctane (the –CСlН signal at δ = 58.0 ppm in the 13С NMR spectrum). The 29Si NMR spectrum showed signals at –19.5, –21.2, –21.8, and –22.5 ppm arising from –Si(CH3)2O– dimethylsiloxy units. These units could only be a result of the elimination of one of the methyl substituents in trimethylchlorosilane, which was taken in excess, or hexamethyldisiloxane formed during reaction (8). Hence, the following transformations (9) took place in this system:

$$\begin{gathered} \left. \begin{gathered} {{({\text{C}}{{{\text{H}}}_{3}})}_{3}}{\text{SiCl}} + {\text{HCl}}\xrightarrow{{{{{\text{H}}}_{2}}{\text{PtC}}{{{\text{l}}}_{6}} + {{{\text{H}}}_{2}}{\text{PtC}}{{{\text{l}}}_{{\text{4}}}}}}{\text{C}}{{{\text{H}}}_{4}} + {{({\text{C}}{{{\text{H}}}_{3}})}_{2}}{\text{SiC}}{{{\text{l}}}_{2}} \hfill \\ {{({{({\text{C}}{{{\text{H}}}_{3}})}_{3}}{\text{Si}})}_{2}}{\text{O}} + 2{\text{HCl}}\xrightarrow{{{{{\text{H}}}_{2}}{\text{PtC}}{{{\text{l}}}_{6}} + {{{\text{H}}}_{2}}{\text{PtC}}{{{\text{l}}}_{{\text{4}}}}}}{\text{2C}}{{{\text{H}}}_{4}} + {{({\text{C}}{{{\text{H}}}_{3}})}_{2}}{\text{ClSiOSiCl}}{{({\text{C}}{{{\text{H}}}_{3}})}_{2}} \hfill \\ \end{gathered} \right\} \\ \xrightarrow{{ + {{{\text{H}}}_{2}}{\text{O}}}}\;{{({{({\text{C}}{{{\text{H}}}_{3}})}_{2}}{\text{SiO}})}_{4}} + {{({\text{C}}{{{\text{H}}}_{3}})}_{3}}{\text{SiOSi}}{{({\text{C}}{{{\text{H}}}_{3}})}_{2}} + {{({\text{C}}{{{\text{H}}}_{3}})}_{3}}{\text{SiO}}{{[{{({\text{C}}{{{\text{H}}}_{3}})}_{2}}{\text{SiO}}]}_{n}}{\text{Si}}{{({\text{S}}{{{\text{H}}}_{3}})}_{3}}. \\ |\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\ {\text{OH}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\ \end{gathered} $$
(9)

In the experiment on the preparation of the π-complex of Pt(II) with 1-octene, H2PtCl6 ⋅ 6H2O was mixed with ethanol and 1-octene combined in the 1 : 1 : 1 molar ratio. The ratio of the components was taken in such a way that the amount of ethanol did not exceed that required for Pt(IV) reduction to Pt(II) by analogy with reaction (1), and Pt(II), in turn, formed a π-complex with 1-octene. No precipitation of Pt(0) was observed during the reaction. The 13С NMR study of the reaction products has shown that the С=С bond is almost completely hydrochlorinated to give the С–Сl bond, which is confirmed by the presence of a new signal at δ = 63.35 ppm in the spectrum. There is no resonance signal of the –CH2–OH group at 3.7 ppm in the 1Н NMR spectrum, which indicates that ethanol is completely consumed for the reduction of Pt(IV) to Pt(II) by analogy with reaction (1) with the only difference that acetaldehyde is formed instead of acetone.

CONCLUSIONS

(1) Based on the analysis of literature data describing the behavior of platinum chlorides in alcohol media, it can be stated that platinum compounds in different oxidation states have catalytic activity in them.

(2) It has been determined that Pt(IV) in H2PtCl6 ⋅ 6H2O under the action of an excess of 2-octanol at 100°C is completely reduced to Pt(II), which is then reduced by 30% to Pt(0) within 13 h due to the oxidation of the alcohol to 2-octanone.

(3) It has been shown for the first time that in the H2PtCl6 ⋅ 6H2O–2-octanol system, 2-chlorooctane is formed through the exchange reaction between 2-octanol and hydrogen chloride.

(4) It has been proven that both hydrated and dehydrated H2PtCl6 and H2PtCl4 do not cause the formation of 1-octene and 2-octene by dehydration of 2-octanol.

(5) It has been demonstrated that even H2PtCl6 dehydrated with trimethylchlorosilane catalyzes the formation of 2-chlorooctane by hydrochlorination of 1-octene.

(6) It has been found that a dehydrated mixture of H2PtCl6 and H2PtCl4 catalyzes the elimination of methyl groups from trimethylchlorosilane and hexamethyldisiloxane to form polydimethylsiloxanes.

(7) It has been assumed that the resulting π-complex of H2PtCl4 with 1-octene is not stable in hydrochloric acid medium and rapidly decomposes with the release of chloroalkyl.