Full Text
PHYSICAL CHEMISTRY
A. B. FASMAN, G. G. KUTYUKOV, Academician of the Academy of Sciences of the Kazakh SSR D. V. SOKOL’SKII
KINETICS AND MECHANISM OF THE REDUCTION OF K₂PdBr₄ BY CARBON MONOXIDE IN AQUEOUS SOLUTIONS
Coordination compounds of Pd(II) are active homogeneous catalysts involving carbon monoxide and olefins (¹–⁵). The complex anion \([\mathrm{PdBr}_4]^{2-}\) has the greatest catalytic activity (⁶). It is known that CO forms carbonyl compounds with Pd(II) salts, which are very labile in aqueous solutions (⁷–¹⁰). To establish their composition it is necessary to study the interaction of CO with the catalyst in the absence of an oxidant, proceeding according to the overall equation
\[ [\mathrm{PdBr}_4]^{2-} + \mathrm{CO} + \mathrm{H_2O} \rightarrow \mathrm{Pd} + \mathrm{CO_2} + 2\mathrm{H}^{+} + 4\mathrm{Br}^{-}. \tag{1} \]
\(\mathrm{K_2PdBr_4}\) was prepared by the method of (¹¹). A weighed portion of the complex, calculated to absorb 25 ml of CO at NTP (\(3.37 \cdot 10^{-2}\) mol/l), was placed in an intensively shaken reaction vessel connected to a measuring gas burette. The reduction of the complex was studied by recording the rates of CO absorption and CO₂ evolution during the reaction (¹²).
Fig. 1. Reduction of \(\mathrm{K_2PdBr_4}\) by carbon monoxide in a 25% KBr solution at 5°
Figure 1 shows the results of two parallel experiments: curve 1—absorption of CO by a solution of \(\mathrm{K_2PdBr_4}\) and of CO₂ at the moment of liberation by a 25% KOH solution (\(V_{\mathrm{CO}}\)); 2—absorption of CO₂ by a solution of \(\mathrm{K_2PdBr_4}\) without absorption of CO₂ (\(V_{\mathrm{CO}} - V_{\mathrm{CO_2}}\)). Separate measurement of the rates of CO absorption with absorption and without absorption of CO₂ makes it possible to calculate the rate of hydrolysis of the carbonyl complex of Pd(II) being formed (curve 3).
CO₂ appears in the gas phase long before the precipitation of metallic Pd. Consequently, upon hydrolysis of carbonyl bromide Pd(II), a complex is formed in which the charge of the central atom is equal to 0, since oxidation state 1 is unknown for palladium. This complex gradually decomposes with the liberation of metallic Pd. A number of cases of identification of Pd(0) complexes with coordination number 2, stabilized by electrophilic ligands, have been described in the literature (⁹, ¹³–¹⁶).
Table 1
Reduction of \(\mathrm{K_2PdBr_4}\) by carbon monoxide in aqueous solutions
| \(t^\circ\) | Conc. KBr, % | \(V_{\mathrm{CO}}/V_{\mathrm{CO_2}}\) | \(V^{\mathrm{end}}_{\mathrm{CO_2}}\), ml | \(V^{\mathrm{end}}_{\mathrm{CO}}\), ml | \(n\) | \(\tau\), min | \(t^\circ\) | Conc. KBr, % | \(V_{\mathrm{CO}}/V_{\mathrm{CO_2}}\) | \(V^{\mathrm{end}}_{\mathrm{CO_2}}\), ml | \(V^{\mathrm{end}}_{\mathrm{CO}}\), ml | \(n\) | \(\tau\), min |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 5 | 0 | 3.9 | 10.2 | 35.0 | 0.0 | 2.5 | 25 | 0 | 2.7 | 21.3 | 38.5 | 0.5 | |
| 5 | 5 | 4.1 | 8.5 | 37.0 | −1.2 | 3.0 | 25 | 10 | 3.0 | 21.5 | 40.0 | 0.0 | 1.0 |
| 5 | 10 | 3.8 | 9.7 | 38.5 | −1.0 | 4.0 | 25 | 30 | 2.9 | 22.5 | 39.5 | 0.0 | 1.5 |
| 5 | 15 | 4.4 | 20.0 | 38.0 | −1.0 | 3.5 | 30 | 0 | 2.8 | 18.3 | 28.0 | 1.0 | |
| 5 | 20 | 4.3 | 18.7 | 37.5 | −1.0 | 4.5 | 30 | 10 | 2.8 | 21.6 | 38.0 | 0.0 | 1.5 |
| 5 | 25 | 4.5 | 14.5 | 38.0 | −1.0 | 5.0 | 30 | 30 | 2.9 | 19.2 | 39.0 | 0.0 | 1.5 |
| 5 | 30 | 4.2 | 17.6 | 38.5 | −1.0 | 10.0 | 40 | 0 | 2.9 | 21.8 | 25.0 | 0.5 | |
| 15 | 0 | 4.2 | 10.4 | 35.5 | 0.0 | 1.0 | 40 | 10 | 2.6 | 22.3 | 36.0 | 0.0 | 1.0 |
| 15 | 10 | 4.9 | 18.5 | 36.0 | −1.0 | 1.0 | 40 | 30 | 2.9 | 23.4 | 36.0 | 0.0 | 1.0 |
| 15 | 30 | 4.1 | 19.0 | 37.0 | 3.0 | 50 | 0 | 2.8 | 20.0 | 20.0 | 0.5 | ||
| 20 | 0 | 3.5 | 19.8 | 37.0 | 0.0 | 0.5 | 50 | 10 | 3.1 | 22.7 | 31.0 | 0.0 | 0.5 |
| 20 | 10 | 3.4 | 21.2 | 37.5 | 0.0 | 0.5 | 50 | 30 | 2.9 | 24.6 | 35.0 | 0.0 | 0.5 |
| 20 | 30 | 3.1 | 22.4 | 39.0 | 2.0 |
Note. \(V_{\mathrm{CO}}/V_{\mathrm{CO_2}}\) is the ratio of the volume of CO absorbed to the volume of \(\mathrm{CO_2}\) evolved in 0.25–0.5 min before the appearance of metallic Pd; \(V^{\mathrm{end}}_{\mathrm{CO_2}}\) is the volume of \(\mathrm{CO_2}\) evolved by the end of the reaction; \(V^{\mathrm{end}}_{\mathrm{CO}}\) is the volume of CO absorbed by the end of the reaction; \(n\) is the reaction order with respect to \(\mathrm{Br^-}\); \(\tau\) is the time of appearance of metallic Pd.
Experimental data on the reduction of \(\mathrm{K_2PdBr_4}\) by carbon monoxide are given in Table 1.
It follows from the data in Table 1 that at temperatures above \(20^\circ\), \(V_{\mathrm{CO}}/V_{\mathrm{CO_2}} \approx 3\). This means that one volume of CO is used for the reduction of Pd(II) to Pd(0), while the other two coordinate with Pd(0), forming the complexes \(\mathrm{Pd(CO)_2}\) or \(\mathrm{K_2[Pd(CO)_2Br_2]}\). The reduction process, in the general case, may be considered as consecutive reactions:
Fig. 2. Effect of temperature on the value of \(K_{\mathrm{B}}\) (\(1\)—without KBr, \(2\)—10% KBr, \(3\)—30% KBr)
Fig. 3. Effect of temperature on the value of \(K_{\mathrm{g}}\) (\(1\)—without KBr, \(2\)—10% KBr, \(3\)—30% KBr)
may be considered as consecutive reactions:
\[ [\mathrm{PdBr_4}]^{-2}+\mathrm{CO} \underset{K_{-\mathrm{B}}}{\stackrel{K'_{\mathrm{B}}}{\rightleftarrows}} [\mathrm{PdBr_3CO}]^{-1}+\mathrm{Br^-}, \tag{2} \]
\[ [\mathrm{PdBr_3CO}]^{-1}+\mathrm{H_2O} \xrightarrow{K_{\mathrm{g}}} \mathrm{Pd^0}+\mathrm{CO_2}+3\mathrm{Br^-}+2\mathrm{H^+}, \tag{3} \]
where: \(K'_{\mathrm{B}}\) is the rate constant for the introduction of CO into the inner coordination sphere of the bromide complex of Pd(II), \(K_{-\mathrm{B}}\) is the rate constant of the reverse reaction, and \(K_{\mathrm{g}}\) is the rate constant of hydrolysis. It should be borne in mind that carbonyl bromide of Pd(II) is hydrolyzed in solutions with a small excess concentration of ligand. This is indicated by the zero order of the reaction for exchange of coordinated \(\mathrm{Br^-}\) with \(\mathrm{Br^-}\) ions in solution, found by Grinberg-
gen ($^{17}$). Therefore oxidative hydrolysis may also proceed at the expense of inner-sphere water.
According to Moiseev, Vargaftik, and Syrkin ($^{18}$), the equilibrium constant for the reaction of incorporation of $\mathrm{C_2H_4}$ into the inner sphere of $[\mathrm{PdCl_4}]^{-2}$ is $\simeq 20$, which indicates a shift of the equilibrium toward formation of the $\pi$-complex. It has been shown that, upon evacuation of the system, the process is completely reversible. In the case of the carbonyl bromide of Pd(II) this is unlikely, since CO is much more reactive than ethylene: if reverse displacement of CO from the inner sphere of the complex does occur, it does so at very low rates. Therefore it is assumed below that $K_{\mathrm{v}} \gg K_{-\mathrm{v}}$.
At lower temperatures ($\leq 15^\circ$) $V_{\mathrm{CO}}/V_{\mathrm{CO_2}} \simeq 4$. Under these conditions the anion $[\mathrm{PdBr_3CO}]^{-1}$ accumulates in solution. Addition of a strong oxidizing agent ($n$-benzoquinone) causes a sharp increase in the rate of hydrolysis, and $\mathrm{CO_2}$ is additionally evolved into the gas phase.
Table 2
Effect of conditions on the order of the reaction of reduction of $\mathrm{K_2PdBr_4}$ by CO
| $P_{\mathrm{CO}}$, atm | without KBr, $n$ 15° | without KBr, $n$ 30° | 10% KBr, $n$ 15° | 10% KBr, $n$ 30° | 30% KBr, $n$ 15° | 30% KBr, $n$ 30° |
|---|---|---|---|---|---|---|
| 0.2—0.4 | 1.1 | 0.9 | 1.2 | 0.8 | 0.5 | 0.5 |
| 0.4—0.6 | 1.1 | 1.0 | 0.9 | 0.8 | 0.4 | 0.7 |
| 0.6—0.8 | 1.0 | 1.1 | 1.1 | 1.1 | 0.4 | 0.8 |
| 0.8—1.0 | 1.1 | 1.0 | 1.1 | 1.0 | 0.5 | 0.9 |
After completion of the reduction process, a certain portion of CO remains in the bound state. For the reduction of $\mathrm{K_2PdBr_4}$, instead of the calculated 25 ml of CO, from 8.5 to 24.5 ml is consumed, depending on the experimental conditions. In all probability, at low temperatures a fairly stable dimeric complex containing Pd(II) and Pd(0) is formed in solution, as in ($^{19}$). With increasing temperature, $V_{\mathrm{CO_2}}^{\mathrm{end}}$ approaches the calculated value (25 ml), and the reduction of $\mathrm{K_2PdBr_4}$ proceeds according to equation (1).
The influence of the partial pressure of CO in mixtures with Ar on the reaction order is shown in Table 2.
It follows from the data in Table 2 that $n$ is close to 1, with the exception of 30% KBr solution.
On the basis of a system of equations for two consecutive reactions (2), (3), $K_{\mathrm{v}}$ and $K_{\mathrm{g}}$ were calculated. The solubility values of CO at different temperatures were taken from a handbook ($^{20}$).
The possibility of formation of the carbonyl bromide of Pd(II) in a $\mathrm{K_2PdBr_4}$ solution without excess $\mathrm{Br^-}$ decreases with increasing temperature, since the rate of hydrolysis of $[\mathrm{PdBr_3CO}]^{-1}$ considerably exceeds the rate of its formation (Figs. 2, 3). In 10 and 30% KBr solutions, at first there is an increase and then a decrease in the rate of formation of the carbonyl bromide of Pd(II). Under the same conditions, $K_{\mathrm{g}}$ increases, especially sharply in 10% KBr.
Fig. 4. Effect of KBr concentration on the values of the constants $K_{\mathrm{v}}$ (1) and $K_{\mathrm{g}}$ (2). Temperature — 5°
On the basis of the data obtained, the activation energy of the process of CO incorporation in the absence of, and in 10% KBr solution was calculated to be $10 \pm 0.5$ kcal/mol. When the KBr concentration in solution is increased from 0 to 10—15%, the reaction order with respect to $\mathrm{Br^-}$ at low temperatures changes from zero to inverse first order, while at higher temperatures it remains zero. It is known that at low KBr concentrations in solution the complex $[\mathrm{PdBr_4}]^{-2}$ is hydrolyzed to a significant extent ($^{17}$); therefore, upon incorporation of CO, ...
there is displacement from the inner sphere of the complex of $\mathrm{H_2O}$ or $\mathrm{OH^-}$. This is also indicated by the fact that the ligands participating in the process are arranged in the following order according to the strength of the trans effect, and consequently also according to the stability of the complexes formed ($^{21}$):
\[ \mathrm{CO} \gg \mathrm{Br^-} > \mathrm{OH^-} > \mathrm{H_2O}. \]
The same effect explains the constancy of $K_{\mathrm{v}}$ and the decrease of $K_{\mathrm{g}}$ in the KBr concentration range 0–15% (Fig. 4). At KBr concentrations $>15\%$, the introduction of CO is accompanied by displacement of $\mathrm{Br^-}$ and a decrease in the value of $K_{\mathrm{v}}$. Under these conditions, $K_{\mathrm{g}}$ does not depend on the KBr concentration.
The nature of the limiting stage of the overall process depends on the experimental conditions and may change even within a single experiment. In most cases, reaction (1) proceeds in the transition region, i.e., the rates of stages (2) and (3) are comparable in magnitude.
Kazakh State University
named after S. M. Kirov
Alma-Ata
Received
17 VI 1964
LITERATURE CITED
$^{1}$ V. A. Golodov, A. B. Fasman, D. V. Sokol’skii, DAN, 151, 98 (1963).
$^{2}$ A. B. Fasman, V. A. Golodov, D. V. Sokol’skii, DAN, 155, 434 (1964).
$^{3}$ J. Smith, W. Haffner, Angew. Chem., 71, 176 (1959).
$^{4}$ I. I. Moiseev, M. N. Vargaftik, USSR Author’s Certificate No. 137511, 25 IV 1961.
$^{5}$ A. B. Fasman, D. V. Sokol’skii et al., Ninth All-Union Conference on the Chemistry of Complex Compounds, Abstracts, Tashkent, 1963, p. 79.
$^{6}$ I. I. Moiseev, M. N. Vargaftik, Ya. K. Syrkin, DAN, 130, 820 (1960).
$^{7}$ W. Manschot, J. König, Ber., 59, 883 (1926).
$^{8}$ L. Mond, H. Hirtz, M. Dalton-Cowap, ZS. anorg. Chem., 68, 207 (1910).
$^{9}$ R. J. Irving, E. A. Magnusson, J. Chem. Soc., 1958, No. 6, 2283.
$^{10}$ J. Burianova, Z. Burianec, Coll. Czechoslov. Chem. Commun., 28, 2138 (1963).
$^{11}$ Synthesis of Complex Compounds of the Platinum-Group Metals, “Nauka,” 1964, p. 172.
$^{12}$ A. B. Fasman, V. A. Golodov, D. V. Sokol’skii, Tr. Inst. Khim. Nauk AN KazSSR, 8, 137 (1962).
$^{13}$ E. O. Fischer, H. Werner, Chem. Ber., 93, 2075 (1960).
$^{14}$ E. Colton, Österreich. Chem.-Ztg., 58, 227 (1957).
$^{15}$ J. Chatt, F. A. Hart, H. R. Watson, J. Chem. Soc., 1962, No. 7, 2337.
$^{16}$ L. Orgel, Introduction to the Chemistry of Transition Metals, Moscow, 1964.
$^{17}$ A. A. Grinberg, A. E. Nikol’skaya, G. A. Shagisultanova, DAN, 101, 1059 (1955).
$^{18}$ I. I. Moiseev, M. N. Vargaftik, Ya. K. Syrkin, DAN, 152, 147 (1963).
$^{19}$ A. D. Gel’man, V. Meilakh, DAN, 36, 188 (1942).
$^{20}$ Chemist’s Handbook, 3, L.—M., 1962, p. 210.
$^{21}$ J. Chatt, L. A. Dunkanson, L. M. Venanzi, J. Chem. Soc., 1955, No. 12, 4456.