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CHEMISTRY
L. M. VOL’SHTEIN and I. O. VOLODINA
CIS ISOMER OF THE INNER-COMPLEX SALT OF DIVALENT PLATINUM WITH $\alpha$-ALANINE
(Presented by Academician A. A. Grinberg, 21 XI 1959)
Inner-complex salts of the general formula $[\mathrm{PtA}_2]$*, obtained by the reaction:
\[ \mathrm{K}_2\mathrm{PtCl}_4 + 2\mathrm{AH} \to [\mathrm{PtA}_2] + 2\mathrm{KCl} + 2\mathrm{HCl}, \tag{1} \]
must exist in the form of two geometrical (cis and trans) isomers. In 1931 A. A. Grinberg and B. V. Ptitsyn showed ($^1$) that in the case of GlH reaction (1) leads to the isolation of both isomers $[\mathrm{PtGl}_2]$ in the molecular ratio trans : cis $\simeq 3 : 1$. In 1934 it was shown ($^2$) that the reaction of $\mathrm{K}_2\mathrm{PtCl}_4$ with an excess of GlH, neutralized with alkali, proceeds according to the equation:
\[ \mathrm{K}_2\mathrm{PtCl}_4 + 4\mathrm{GlH} + 4\mathrm{KOH} \to \mathrm{K}_2[\mathrm{PtGl}_4] + 4\mathrm{KCl} + 4\mathrm{H}_2\mathrm{O}. \tag{2} \]
From the soluble salt $\mathrm{K}_2[\mathrm{PtGl}_4]$, by adding the calculated amount of acid, a sparingly soluble compound $\mathrm{H}_2\mathrm{PtGl}_4$ was isolated:
\[ \mathrm{K}_2[\mathrm{PtGl}_4] + 2\mathrm{HCl} \to \mathrm{H}_2\mathrm{PtGl}_4 + 2\mathrm{KCl}, \tag{3} \]
It was shown ($^3$) that $\mathrm{H}_2\mathrm{PtGl}_4$, when heated with water, is almost quantitatively converted into the cis isomer of the inner-complex salt**:
\[ \mathrm{H}_2\mathrm{PtGl}_4 \to 2\mathrm{GlH} + \text{cis-}[\mathrm{PtGl}_2]. \tag{4} \]
In 1937 one of us, together with A. A. Grinberg, showed ($^4$) that reaction (1) in the case of AnH (in contrast to GlH) leads to the isolation only of the trans isomer $[\mathrm{PtAn}_2]$. When $\mathrm{K}_2\mathrm{PtCl}_4$ reacts with an excess of AnH neutralized with alkali, the reaction proceeds according to an equation similar to (2); however, the compound $\mathrm{H}_2\mathrm{PtAn}_4$ did not precipitate upon addition of acid to the salt $\mathrm{K}_2[\mathrm{PtAn}_4]$ (the presence in solution of the anions $[\mathrm{PtAn}_4]^{2-}$ was demonstrated ($^4$) by isolation of the compound $[\mathrm{Pt}(\mathrm{AnH})_4\mathrm{Cl}_2]\mathrm{Cl}_2$, formed from $[\mathrm{PtAn}_4]^{2-}$ as a result of oxidation of Pt(II) to Pt(IV) in an acidic medium). Cis-$[\mathrm{PtAn}_2]$ not only was not detected among the products of reaction (1), but it also could not be obtained by a reaction similar to (4), since the compound $\mathrm{H}_2\mathrm{PtAn}_4$ was not isolated.
In 1957 one of us synthesized ($^5$) both isomers of the inner-complex salt $[\mathrm{PtM}_2]$. However, in the case of MH (similarly to AnH), reaction (1) leads to the isolation only of trans-$[\mathrm{PtM}_2]$. Cis-$[\mathrm{PtM}_2]$ was synthesized from the compound $\mathrm{H}_2\mathrm{PtM}_4$, obtained from $\mathrm{K}_2[\mathrm{PtM}_4]$, i.e., reactions similar to (2), (3), and (4) were carried out. We note that a reaction of type (2), as proved ($^6$), is a general method for obtaining compounds $\mathrm{K}_2[\mathrm{PtA}_4]$. Thus, only in the case of AnH (of the three simplest members of the homologous
* AH denotes monobasic monoamino acids in the general form; GlH — $\alpha$-aminoacetic acid (glycine), AnH — $\alpha$-aminopropionic acid (alanine), MH — $\alpha$-aminobutyric acid.
** It has recently been established that this reaction proceeds in two stages; in the first of them the compound $\mathrm{HPtGl}_3$ is formed.
of the series of α-amino acids) the cis-isomer of the inner-complex salt remained unknown. The assumption that there were any obstacles (for example, steric ones) preventing the formation of cis-\([{\rm PtAn}_2]\) became unlikely after the synthesis of the cis-isomers \([{\rm PtGl}_2]\) and \([{\rm PtM}_2]\). Therefore we considered it fundamentally important to attempt again to synthesize this isomer.
Experimental Part
When acid was added to a solution of \({\rm K}_2[{\rm PtAn}_4]\), as noted above, the compound \({\rm H}_2{\rm PtAn}_4\) did not precipitate. However, the compound \({\rm H}_2{\rm PtAn}_4\) should be present in solution, and we assumed that, on heating the solution (by analogy with the behavior of the isolated solid \({\rm H}_2{\rm PtGl}_4\) and \({\rm H}_2{\rm PtM}_4\)), \({\rm H}_2{\rm PtAn}_4\) would be converted into cis-\([{\rm PtAn}_2]\). The experiment confirmed our assumption.
To 5 mmoles of \({\rm K}_2{\rm PtCl}_4\) were added 20 mmoles each of AnH and KOH; the total volume of the solution was \(\sim 20\) ml. The mixture was heated for about 1 hour on a water bath. To the almost colorless solution, \(1\,M\) HCl was added (calculated as 2 mmoles of HCl per 1 mmole of the initial \({\rm K}_2{\rm PtCl}_4\)) and the mixture was heated for 2–3 hours on a water bath, maintaining the solution volume at 15–12 ml. On cooling the solution, a white precipitate separated in abundance; it was filtered off, washed with water, alcohol, and ether, and dried at \(105^\circ\). The precipitate proved to be pure \([{\rm PtAn}_2]\) (yield about 30%).
\[ \begin{aligned} &\text{Found \%: } {\rm Pt}\ 52.55;\ 52.59;\ 52.63;\quad {\rm N}\ 7.62;\ 7.72\\ &[{\rm PtAn}_2].\ \text{Calculated \%: } {\rm Pt}\ 52.57;\quad {\rm N}\ 7.54 \end{aligned} \]
The properties of the \([{\rm PtAn}_2]\) preparations isolated by the method described differ substantially from the properties of the trans-\([{\rm PtAn}_2]\) preparations (identical in composition) obtained\({}^{(4)}\) by reaction (1). The solubility of cis-\([{\rm PtAn}_2]\) in water (1.1% at \(25^\circ\)) is \(\sim 14\) times greater than the solubility of the trans-isomer. The trans-isomer forms plate-like crystals; the cis-isomer forms crystals of bar-like shape. The isomers differ especially sharply in their behavior toward hydrochloric acid. When concentrated HCl is added to the cis-isomer, even in the cold, the initial precipitate disappears instantaneously and completely, giving a bright-yellow solution; the trans-isomer under these conditions turns slightly yellow but does not go into solution. On gentle heating, the cis-isomer dissolves even in the theoretical amount of \(1\,M\) HCl (2 moles of HCl per 1 mole of cis-\([{\rm PtAn}_2]\)).
Previously\({}^{(4)}\), trans-dichloride \([{\rm Pt}({\rm AnH})_2{\rm Cl}_2]\) was obtained from trans-\([{\rm PtAn}_2]\) by boiling with an excess of concentrated HCl (10 ml per 1 g of the inner-complex salt). To obtain the cis-dichloride, we added only 2 ml of concentrated HCl to 1 g of cis-\([{\rm PtAn}_2]\) and heated for less than one minute on a water bath. After cooling, a yellow precipitate of cis-dichloride separated; the precipitate was transferred to a glass filter with concentrated HCl, washed with ether, and dried at \(100^\circ\).
\[ \begin{aligned} &\text{Found \%: } {\rm Pt}\ 43.83;\ 43.76;\quad {\rm N}\ 6.22;\ 6.17\\ &[{\rm Pt}({\rm AnH})_2{\rm Cl}_2].\ \text{Calculated \%: } {\rm Pt}\ 43.94;\quad {\rm N}\ 6.30 \end{aligned} \]
The cis-dichloride was obtained in yields up to 80%. Under the action of HCl on isomeric inner-complex salts, the alanine rings are opened:
\[ [{\rm PtAn}_2] + 2{\rm HCl}\to[{\rm Pt}({\rm AnH})_2{\rm Cl}_2]. \]
Both dichlorides are dibasic acids, quantitatively titrated with alkali in the presence of phenolphthalein; the products of the reaction
\[ [{\rm Pt}({\rm AnH})_2{\rm Cl}_2] + 2{\rm KOH}\to[{\rm PtAn}_2] + 2{\rm KCl} + 2{\rm H}_2{\rm O} \]
are isomeric inner-complex salts.
The isomeric dichlorides show a considerable difference in properties. The cis-dichloride is considerably more soluble in water than the trans-isomer (both are poorly soluble in concentrated HCl); conversely, in ether the trans-dichloride dissolves considerably better than the cis-dichloride.
The isomers differ in their behavior toward thiourea (Thio) and other reagents. Under the action of thiourea (under completely identical conditions), the cis-dichloride gives, in a yield of about 80%, a yellow precipitate \([\mathrm{Pt}(\mathrm{Thio})_4]\mathrm{Cl}_2\) (found: Pt—34.29; 34.02%; calculated: 34.21%), whereas from the trans-dichloride there is obtained\(^{4}\) a white precipitate \([\mathrm{Pt}(\mathrm{Thio})_2(\mathrm{AnH})_2]\mathrm{Cl}_2\).
The absence of the cis isomer \([\mathrm{PtAn}_2]\) in the products of reaction (1) appears quite natural, since free HCl is present in the solution; in its presence cis-\([\mathrm{PtAn}_2]\) very readily passes into the readily soluble cis-dichloride. The same was observed in the interaction of \(\mathrm{K}_2\mathrm{PtCl}_4\) with MH; the solubilities in water of cis-\([\mathrm{PtAn}_2]\) and cis-\([\mathrm{PtM}_2]\)\(^{5}\) are almost identical. Cis-\([\mathrm{PtGl}_2]\) is appreciably less soluble in water (0.18% at \(25^\circ\)) than its analogs; therefore it is obtained, although in low yield, in the interaction of \(\mathrm{K}_2\mathrm{PtCl}_4\) with GlH.
Thus, the cis- and trans-isomers \([\mathrm{PtA}_2]\) have now been obtained for all three of the simplest \(\alpha\)-amino acids.
In conclusion, we note that the compound \(\mathrm{H}_2\mathrm{PtAn}_4\), from which, as we suppose, cis-\([\mathrm{PtAn}_2]\) was obtained, has also very recently been isolated by us (as yet in low yield) and will be described later.
Dnepropetrovsk Chemical-Technological Institute
named after F. E. Dzerzhinsky
Received
19 XI 1959
REFERENCES
- A. A. Grinberg, B. V. Ptitsyn, Izv. Platin. inst., 9, 55 (1932).
- A. A. Grinberg, L. M. Volshtein, DAN, 7, 485 (1935).
- A. A. Grinberg, L. M. Volshtein, Izv. AN SSSR, ser. khim., 1, 3 (1937).
- A. A. Grinberg, L. M. Volshtein, Izv. AN SSSR, ser. khim., 4, 885 (1937).
- L. M. Volshtein, N. S. Velikanova, ZhNKh, 2, 2383 (1957).
- L. M. Volshtein, M. F. Mogilevkina, DAN, 110, 83 (1956); 114, 99 (1957).