On the Complex Formation of Boric Acid Esters with Various Organic Compounds

B. V. Tronov and N. D. Strelnikova

Boron has valence 3 and coordination number 4. The atoms and atomic groups surrounding the boron atom are arranged at three corners of a tetrahedron, the fourth corner of which remains free, and at this corner a positive electric field of considerable intensity is produced. The radius of the “ion” \(B^{+3}\) is calculated to be \(0.2 \text{ Å}\); hence, for the intensity of the ionic field one obtains [2] \( \frac{3}{r^2} = 75\), i.e., much greater than, for example, for aluminum \(Al^{+3}\) (9.2).

This circumstance explains the strongly pronounced ability of boron compounds to give stable complexes \(H[BF_4]\), \(F_3B \cdot NH_3\), etc.

In organic chemistry, complex compounds of boric acid esters with alcohols of the type \(H[B(OR)_4]\) have long been known. These compounds possess a clearly expressed acidic character, i.e., the acidic properties of alcohols are greatly enhanced upon addition of boric esters.

In the present work we studied a number of systems which included, on the one hand, boric acid esters and, on the other, alcohols and certain other organic compounds which, according to our assumptions, could form complexes with boric acid esters having noticeable acidic properties. For the compounds studied and their mixtures, activity with respect to metals was determined, using the electrochemical method developed in our laboratory [2].

By this method, the current strength is determined directly in a closed circuit comprising a galvanic cell made up of a pair of metals and the substance under investigation, and a galvanometer with a large internal resistance. The current strength in such a circuit depends on the electromotive force and on the resistance, but we give the measurement results in volts or millivolts, since that is how our galvanometers were calibrated.

In the present work the metals used were sodium and platinum, and galvanometers with internal resistances of 2336 ohms and 2880 ohms.

Of the boric acid derivatives, boromethyl and boroisoamyl esters were taken.

Trimethyl borate was purified by repeated distillation with a large dephlegmator. Its purity was checked by our same electrochemical method. As previous work [2] has shown, this method is distinguished by extraordinary sensitivity. The unpurified preparation showed an e.m.f. equal to 68.2 millivolts; the purified one, an e.m.f. of 0.01 mV, and this value did not change after a new distillation.

The triisoamyl ester of boric acid was synthesized from boric anhydride and isoamyl alcohol. The glassy boric anhydride obtained by heating boric acid was ground and mix-

was washed with twice-distilled isoamyl alcohol from fermentation. A portion of the mixture, consisting of 70 g of anhydride and 528 g of alcohol, was boiled for 4–6 hours with a reflux condenser. After standing, the liquid part of the mixture was filtered off from the unreacted boric anhydride and the boric acid that had partly formed, and was subjected to distillation. Repeated distillations gave a fraction boiling within the range 250–260°. This evidently contained various boric acid esters with two isoamyl alcohols of fusel oil

\[ (\mathrm{CH_3})_2 \cdot \mathrm{CH} \cdot \mathrm{CH_2} \cdot \mathrm{CH_2OH} \quad \text{and} \quad \mathrm{CH_3} \cdot \mathrm{CH_2} \cdot \mathrm{CH} \cdot (\mathrm{CH_3}) \cdot \mathrm{CH_2OH}. \]

Further separation is practically impossible, and for us it was of no interest. The yield of triisoamyl borate averaged 62% of theoretical. In our apparatus, boron isoamyl alcohol produced no electromotive force.

Among other organic compounds we used:

1) isopropyl alcohol \(\mathrm{CH_3 \cdot CHOH \cdot CH_3}\),
2) normal butyl alcohol \(\mathrm{CH_3 \cdot CH_2 \cdot CH_2 \cdot CH_2OH}\),
3) isoamyl alcohol from fermentation,
4) phenol \(\mathrm{C_6H_5OH}\),
5) acetone \(\mathrm{C_2H_5 \cdot CO \cdot CH_3}\),
6) acetylacetone \(\mathrm{CH_3 \cdot CO \cdot CH_2 \cdot CO \cdot CH_3}\),
7) isovaleronitrile \((\mathrm{CH_3})_2\mathrm{CH} \cdot \mathrm{CH_2} \cdot \mathrm{CN}\).

The experiments were carried out at a temperature of 20–22° in toluene solution. The results of the measurements are presented in the form of eight diagrams, without placing in this paper the calculation tables for the diagrams.

In the system triisoamyl borate—isoamyl alcohol (not included in the table), no emf at all was observed, although the formation of a complex compound with certain acidic properties should have been expected. Assuming that the complex formed here, for example, \(\mathrm{H[B(OC_5H_{11})_4]}\), because of the presence of a large hydrocarbon portion, has insufficient acidity for measurement, we decided to try to increase the activity of the complex toward metals by introducing it into a new process of complex formation.

The increase in the activity of acids with respect to metals is achieved in complex formation in two ways:

1) Complex addition of the acid anion to the positive ion or to the positive part of the molecule of another component of the complex. In the chemistry of inorganic complex compounds, very many examples of such enhancement of acidity are known. The acidic properties of hydrogen fluoride are strengthened upon complex formation with \(\mathrm{BF_3}\) or \(\mathrm{SiF_4}\), i.e., in the complexes \(\mathrm{H[BF_4]}\) and \(\mathrm{H_2[SiF_6]}\).

The very weak acid \(\mathrm{HCN}\) gives the strong acids \(\mathrm{H_3[Fe(CN)_6]}\) and \(\mathrm{H_4[Fe(CN)_6]}\), etc.

In organic chemistry we have an analogous example in the strengthening of the acidity of alcohols in complexes with boric esters and even alcoholates.

2) Our work on the electrochemical investigation of complex-forming systems of organic chemistry indicates that there is sometimes a great increase in activity with respect to metals (and this, like an increase in electrical conductivity, also denotes a strengthening of acidic properties) in another type of complex formation, when the hydrogen of a weakly acidic compound attaches to a negatively charged atom of another complex-former. Thus, the magnitude of the electromotive force arising in the reaction of carboxylic acids with metals increases sharply in the presence of amines or nitro compounds. Especially great an increase

an electromotive force hundreds of times greater was observed with isovaleric acid. With an amine, the acid most probably gives an ammonium salt:

\[ (\mathrm{C_2H_5})_3\mathrm{N}+\mathrm{HOCO}\cdot\mathrm{C_4H_9} \rightarrow (\mathrm{C_2H_5})_3\mathrm{N}\ldots\mathrm{HO}\cdot\mathrm{CO}\cdot\mathrm{C_4H_9}. \]

\[ (\mathrm{C_2H_5})_3\mathrm{N}\ldots\mathrm{HO}\cdot\mathrm{CO}\cdot\mathrm{C_4H_9} \rightleftarrows [(\mathrm{C_2H_5})_3\mathrm{NH}]^{+}+[\mathrm{OCO}\cdot\mathrm{C_4H_9}]^{-} \]

With nitro compounds one should expect, first of all, the formation of a complex with a hydrogen bond

\[ \begin{array}{c} \phantom{\mathrm{C_6H_5}-}\overset{+}{\mathrm{N}}-\overset{-}{\mathrm{O}}\ldots\mathrm{HOCOC_4H_9}.\\[-2mm] \mathrm{C_6H_5}-\ \ \Vert\\[-1mm] \phantom{\mathrm{C_6H_5}-}\mathrm{O} \end{array} \]

It is possible that this complex also dissociates electrolytically into the acid anion and an ammonium-type cation

\[ \left[ \begin{array}{c} \phantom{\mathrm{C_6H_5}-}\overset{+}{\mathrm{N}}-\mathrm{OH}\\[-1mm] \mathrm{C_6H_5}-\ \ \Vert\\[-1mm] \phantom{\mathrm{C_6H_5}-}\mathrm{O} \end{array} \right]^{+}, \]

the hydrogen of which is readily split off under the action of the metal.

A large increase in activity with respect to metals was observed for phenols [1] in mixtures with nitro compounds, amines, and carbonyl compounds. Comparing a number of facts, M. M. Bortovoi came to the conclusion that phenol with an amine also combines by a hydrogen bond:

\[ \mathrm{C_6H_5OH}\ldots\mathrm{NR_3}. \]

Sometimes the complexes are more complicated in their properties and structure [3], but in any case, upon their formation a hydrogen bond arises, and electrolytic dissociation must give a cation with an active proton.

Analogous complex formation, with an increase in the reactivity of hydrogen, may be expected in the interaction with amines, nitro compounds, and other electron-donor reagents of complex acids, for example, acids composed of boric esters with alcohols. The resulting ternary complexes should be more active with respect to metals than the complex acids themselves.

Below are given the results of experiments with systems that included:

1) Boronoisoamyl acid \(\mathrm{H[B(OC_5H_{11})_4]}\) and pyridine (Fig. 8).

2) Boronoisoamyl acid \(\mathrm{H[B(OC_5H_{11})_4]}\) and the nitro compounds nitromethane (curve 1), nitrobenzene (curve 2), o-nitrotoluene (curve 3, Fig. 7).

Discussion of the Results

For alcohols, the data of the present work do not make it possible to detect those definite compounds with boric acid esters that are known from the literature, i.e.,

\[ \mathrm{B(OR)_3\cdot ROH} \quad\text{or}\quad \mathrm{H[B(OR)_4]}. \]

The greatest increase in electromotive force was observed in mixtures containing a larger number of alcohol molecules:

\[ (\mathrm{CH_3O})_3\mathrm{B}\cdot 3\mathrm{C_5H_{11}OH}; \quad (\mathrm{CH_3O})_3\mathrm{B}\cdot 4\mathrm{C_6H_5OH}; \]

\[ (\mathrm{C_5H_{11}O})_3\mathrm{B}\cdot 9\mathrm{C_3H_7OH}; \quad (\mathrm{C_5H_{11}O})_3\mathrm{B}\cdot 9\mathrm{C_4H_9OH}; \]

\[ (\mathrm{C_5H_{11}O})_3\mathrm{B}\cdot 9\mathrm{C_6H_5OH}. \]

Fig. 1

Inscriptions in the figure:

• \(B(OC_5H_{11})_3\), iso
• \(N1\ (CH_3)_2CHOH\)
• \(N2\ nC_4H_9OH\)
• Alcohols — \(9.720\,m\gamma\)
• \(0.940\,m\gamma\)
• \(N1\)
• \(N2\)
• “Low-boiling boric acid ester —
  a) isopropyl alcohol
  b) normal butyl alcohol.”

Fig. 2

Inscriptions in the figure:

• \(B(OC_5H_{11})_3 - C_6H_5OH\), iso
• “Boric isoamyl ester—phenol”
• \(40.304\,m\gamma\)
• \(24.016\,m\gamma\)

Fig. 3

Inscriptions in the figure:

• \(B(OC_5H_{11})_3\) · iso-ketones
• a) \(N1\) acetone
• b) \(N2\) acetylacetone
• \(5.050\,m\gamma\)
• \(2.736\,m\gamma\)
• \(N2\)
• \(N1\)

Fig. 4

Fig. 5

Fig. 6

The ratio 1 : 9 (moreover an approximate one) does not resemble ordinary stoichiometric ratios. It is possible that in these systems not definite chemical compounds are formed, but complex aggregates, in which one molecule of boric ester is surrounded by a large number of alcohol molecules located, perhaps, at various distances from the boron atom. An analogous phenomenon was observed by us [3] in a colorimetric study of the interaction of nitro compounds with amines. For example, in the system metadinitrobenzene–pyridine the maximum coloration corresponded to the composition \(C_5H_5N \cdot 9C_6H_4(NO_2)_2\) (also approximately, since few points were taken for the investigation).

Fig. 7

Fig. 8

The greatest electromotive-force values were given by systems that included phenol: with trimethyl borate, \(220\ \text{mV}\) (curve 1, Fig. 6), and with triisoamyl borate, \(40.3\ \text{mV}\) (curve 2, Fig. 2).

The greatest increase in electromotive force was observed in mixtures of boric esters with acetone (Fig. 3, curve 1), acetylacetone (Fig. 3, curve 2), nitromethane (Fig. 5), and isovaleronitrile (Fig. 4).

The composition of the most active mixtures was:
$\mathrm{B(OC_5H_{11})_3 \cdot 3CH_3COCH_3}$; $\mathrm{B(OC_5H_{11})_3 \cdot 4CH_3COCH_2COCH_3}$; $\mathrm{B(OC_5H_{11})_3 \cdot 3CH_3NO_2}$; $\mathrm{B(OC_5H_{11})_3 \cdot 4C_4H_9CN}$.

Ternary systems, in the study of which triisoamyl borate and isoamyl alcohol were taken in an equimolecular ratio, showed a considerable increase in electromotive force. The highest values of the latter corresponded to the composition:

\[ \mathrm{H[B(OC_5H_{11})_4] \cdot 4C_5H_5N} \;—\; 22.80\ \mathrm{mV}. \]

\[ \mathrm{H[B(OC_5H_{11})_4] \cdot 3CH_3NO_2} \;—\; 10.64\ \mathrm{mV}. \]

\[ \mathrm{H[B(OC_5H_{11})_4] \cdot 4C_6H_5NO_2} \;—\; 19.15\ \mathrm{mV}. \]

\[ \mathrm{H[B(OC_5H_{11})_4] \cdot 4CH_3C_6H_4NO_2} \;—\; 15.50\ \mathrm{mV}. \]

Conclusions

1. Fourteen systems were studied, composed of trimethyl and triisoamyl esters of boric acid with various representatives of other classes of organic compounds: alcohols, phenols, nitro compounds, nitriles, and amines. The investigation was carried out by the electrochemical method of B. V. Tronov and L. P. Kulev.

2. It was shown that boric acid esters enter into complex-forming interactions with all the substances taken, but the maximum values of the electromotive force arising in the reaction with sodium do not always correspond to a definite stoichiometric ratio of the components of the system.

3. It was shown that the activity toward metals of acids composed of trialkyl borate and the corresponding alcohol can be increased by adding a third complex-forming agent possessing electron-donor reactivity. In this case, ternary compounds with a hydrogen bond arise, which may dissociate according to the type of ammonium salts.

Literature

1. Grinberg A. V. Introduction to the Chemistry of Complex Compounds. 2nd ed., 1951.

2. Tronov B. V. and Kulev L. P. A new electrochemical method for investigating organic compounds and their mixtures. Proceedings of the Tomsk Polytechnic Institute, vol. 64, pp. 3–37, 1948.

3. Tronov B. V. and Strel’nikova N. D. Complexes of nitro compounds with other organic compounds. Proceedings of the Tomsk Polytechnic Institute, vol. 71, pp. 63–99, 1952.