Reports of the Academy of Sciences of the USSR
1962. Vol. 143, No. 2

CHEMISTRY

Academician A. N. NESMEYANOV, Corresponding Member of the Academy of Sciences of the USSR D. N. KURSANOV, V. N. SETKINA, N. V. KISLYAKOVA, N. S. KOCHETKOVA, and R. B. MATERIKOVA

ISOTOPIC EXCHANGE OF HYDROGEN IN CYCLOPENTADIENYLMANGANESE TRICARBONYL

The recently discovered \((^1)\) reaction of isotopic hydrogen exchange of ferrocene and its derivatives in acidic media apparently opens up the possibility of establishing a quantitative characteristic of the relative electrophilicity of various metallocene and aromatic systems. In this connection, it was of interest to investigate the reaction of isotopic hydrogen exchange of cyclopentadienylmanganese tricarbonyl \((\mathrm{C}_5\mathrm{H}_5\mathrm{Mn}(\mathrm{CO})_3)\) (I) with acids and to compare them with data on the hydrogen exchange of ferrocene and benzene. Until now, only a few reactions of electrophilic substitution of \(\mathrm{C}_5\mathrm{H}_5\mathrm{Mn}(\mathrm{CO})_3\) have been known: alkylation \((^{2a,2})\), acylation \((^{3-6})\), and sulfonation \((^7)\). A reaction of competitive acylation of benzene, cyclopentadienylmanganese tricarbonyl (I), methylcyclopentadienylmanganese tricarbonyl (II), and anisole was also carried out, as a result of which the following series of decreasing reactivity was found: anisole \(>\) II \(>\) I \(>\) benzene.

We investigated the reaction of isotopic hydrogen exchange of \(\mathrm{C}_5\mathrm{H}_5\mathrm{Mn}(\mathrm{CO})_3\) with trifluoroacetic acid in benzene under the same conditions in which we had previously studied the hydrogen exchange of ferrocene \((^1)\). Under these conditions, benzene does not enter into the exchange reaction, whereas ferrocene readily exchanges hydrogen atoms for deuterium \((K_{\mathrm{BO}}^* = 1.6 \cdot 10^{-4}\ \mathrm{sec}^{-1}\) at \(25^\circ)\). It turned out that, under the indicated conditions, \(\mathrm{C}_5\mathrm{H}_5\mathrm{Mn}(\mathrm{CO})_3\), like benzene, practically did not enter into the hydrogen-exchange reaction.

Table 1

Hydrogen exchange of \(\mathrm{C}_5\mathrm{H}_5\mathrm{Mn}(\mathrm{CO})_3\) and benzene in a solution of \(\mathrm{CH}_2\mathrm{Cl}_2\) with \(\mathrm{CF}_3\mathrm{COOD}\), in the presence of \(\mathrm{D}_2\mathrm{SO}_4\)

The hydrogen exchange of \(\mathrm{C}_5\mathrm{H}_5\mathrm{Mn}(\mathrm{CO})_3\) was accomplished by increasing the acidity of the medium through the addition of sulfuric acid to it. Under these conditions, however, hydrogen exchange of benzene is also observed simultaneously. To compare the electrophilic activity of \(\mathrm{C}_5\mathrm{H}_5\mathrm{Mn}(\mathrm{CO})_3\) and benzene, we studied the kinetics of the hydrogen exchange of these compounds under the conditions indicated below, i.e., in a solution of trifluoroacetic acid with additions of sulfuric acid and methylene chloride as the solvent. The results of the investigations are given in Table 1.

* \(K_{\mathrm{BO}}\) here and below denotes the rate constant of hydrogen exchange.

It was found that the rate constants of the hydrogen exchange of \( \mathrm{C_5H_5Mn(CO)_3} \) are 1.3–2 times higher than those of benzene. These results show that the electrophilic activity of \( \mathrm{C_5H_5Mn(CO)_3} \) only slightly exceeds the electrophilic activity of benzene.

In experiments on the competing hydrogen exchange of \( \mathrm{C_5H_5Mn(CO)_3} \) and benzene, simultaneously present in the reaction medium, this difference appeared more clearly (Table 2).

Table 2

Competing hydrogen exchange of \( \mathrm{C_5H_5Mn(CO)_3} \) and benzene, simultaneously present in the reaction medium
(molar ratio of reagents
\( \mathrm{C_5H_5Mn(CO)_3:(C_6H_6):CF_3COOD:D_2SO_4:CH_2Cl_2 = 1:1:3:0.2:3} \))

* Experiments were carried out at 25°.

Experimental Part

1. Starting reagents. Cyclopentadienylmanganesetricarbonyl, m.p. 76–77°, obtained by the method of Piper, Cotton, and Wilkinson \((^8)\), was purified by recrystallization from heptane and by sublimation in vacuum. Deuterated trifluoroacetic acid was obtained by treating trifluoroacetic acid with deuterosulfuric acid and was then distilled on a column with an efficiency of 30 theoretical plates.

2. Hydrogen-exchange experiments. The hydrogen exchange of cyclopentadienylmanganesetricarbonyl was carried out in a medium consisting of methylene chloride, trifluoroacetic acid, and sulfuric acid. In the course of the kinetic studies, the reaction was stopped by pouring separate samples into ice water. The cyclopentadienylmanganesetricarbonyl isolated from the experiment was purified by sublimation. Experiments on the hydrogen exchange of benzene were carried out in an analogous manner. The deuterium content in the substances studied was determined from the excess density of combustion water by the drop method. Since all kinetic experiments were carried out under similar conditions, we shall confine ourselves to describing only a few typical cases.

a) Hydrogen exchange of cyclopentadienylmanganesetricarbonyl in a medium of \( \mathrm{CH_2Cl_2} \), \( \mathrm{CF_3COOD} \), and \( \mathrm{D_2SO_4} \) at molar ratios of 1 : 3 : 3 : 0.2, respectively (experimental temperature 25°).

Table 3

* E.d.c.w. — excess density of combustion water.

b) Hydrogen exchange of benzene under conditions analogous to experiment (a).

Table 4

c) Competitive hydrogen exchange of \( \mathrm{C_5H_5Mn(CO)_3} \) and benzene in a solution of \( \mathrm{CF_3COOD} \), \( \mathrm{D_2SO_4} \), and \( \mathrm{CH_2Cl_2} \) (molar ratios \(= 1 : 1 : 3 : 0.2 : 3\), respectively) at \(25^\circ\)

Table 5

It has been established that cyclopentadienylmanganese tricarbonyl enters into the reaction of hydrogen isotope exchange in an acidic medium. A comparison of the kinetics of hydrogen exchange of cyclopentadienylmanganese tricarbonyl and benzene under identical conditions shows that the rate constant of hydrogen exchange for the former is 1.3–2 times higher than that for benzene.

Institute of Organoelement Compounds
Academy of Sciences of the USSR

Received
12 XII 1961

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