Chapter 43 Boron compounds

Chapter 43

Boron compounds

s. H E ~ M A N E K CONTENTS .............................................. 945 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 946 .................... 947 Mobile phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Detection.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 941 .................................... . 941 ................ Carboranes . . . . . . . . . . . . . . . . . . . . . . Ligand derivatives of boranes and carboranes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , 9 5 0 950 Metallocarboranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....... . . . . . . . . . . . . . . . . . . . . . . . 951 General techniques.

GENERAL TECHNIQUES The separation of rigid boron compounds is based on differences in the sizes and shapes of the molecules, in dipole moments, Brgnsted or Lewis acidities and in the characters of the substituents. In some instances, the use of liquid-solid chromatography is limited by the reactivity of the compounds t o be separated towards water, air or some solvents. However, a careful choice of chromatographic conditions can decrease this sensitivity to an acceptable level. Up to now, classical column chromatography on silica gel or aluminium oxide has been the most widely used technique, but recently dry column chromatography has been introduced into borane chemistry. The main advantages of the latter technique are good separations, rapidity, negligible destruction of the solid-phase column by hydrogen (due to undesirable hydrolysis) and, in particular, the easy selection of conditions by means of thin-layer chromatography. Very good results were obtained not only in separations of coloured compounds (sandwich-type metallocarboranes), but also in separations of Wdetectable compounds (see below), chromatographed in a quartz column or in a column with polyethylene or polypropylene walls. Attempts to use gel permeation chromatography (Coupek and H e h i n e k ) and especially high-efficiency liquid chromatography (KrejEi and H e h i n e k ) are promising, but more detailed studies are necessary. It would appear that other kinds of column chromatography have not been used.

References p . 951

945

946

BORON COMPOUNDS

Stationary phase Water- and air-stable borane compounds (most ligand derivatives, closocarboranes and metallocarboranes) are chromatographed on silica gel or aluminium oxide. With hydride compounds, which are hydrolyzed by water, it is recommended that an adsorbent of activity I should be used so as to prevent evolution of hydrogen and destruction of the adsorbent column. Acidic nido-boranes and nido-carboranes can be separated on silica gel activated in vacuo (100-1 50°C) and deactivated after cooling by the injection of an eluent containing 3-7% of acetic or trifluoroacetic acid into the evacuated flask (Stuchlik et d ) . Some of the air-sensitive boron compounds can be separated in an inert atmosphere on an activated and gas-free stationary phase. An adsorbent is activated in uucuo, the flask filled with nitrogen, the eluent is added, a small part of it is evaporated in uacuo at ambient temperature (so as to remove the last traces of oxygen from the active surface) and the flask is filled with nitrogen. The suspension is then transferred through a connecting tube to the chromatographic column, which has previously been flushed with nitrogen. All of the succeeding operations, i.e., the addition of the mixture to be chromatographed, the stepwise addition of the gas-free eluent and the collection of fractions, are performed in an inert atmosphere in a simple apparatus (Fig. 43.1).

Fig. 43.1. Chromatographic apparatus for the separation of air-sensitive compounds. A, E = football bladders containing nitrogen; B = dropping funnel containing eluent; C = column containing stationary phase; D = collecting flask.

947

BORANES AND SUBSTITUTED BORANES

Mobile phase The best information on eluents is obtained by thin-layer chromatography on appropriate stationary phases. In classical elution chromatography, solvents in which a distinct separation proceeds at an RF value of 0.1-0.2 are t o be preferred. In dry column chromatography, solvents are used that give the best separations within the full length of the plate. Caution: tetrachloromethane is not recommended for the separation of larger amounts of those boron hydride compounds which reduce silver nitrate, because of the danger of an explosion. Detection Some groups of borane compounds are deeply coloured (metallocarboranes), while others are visible in ultraviolet light (higher boranes, nidocarboranes, ligand derivatives). Most nido-compounds absorb in the region of 200-350 nm and reduce aqueous silver nitrate solution, but the detection of closocarboranes is difficult owing to the transmittance of ultraviolet light and low reactivity towards chemical reagents. According to PleSek et al. (1972), fractions that contain closocarboranes in amounts above 1 fig can be monitored by thin-layer chromatography on starch-bound silica gel, followed by detection with iodine vapour. In high-efficiency liquid chromatography, virtually all types of boron compounds were found to be detectable by refractometric monitoring.

BORANES AND SUBSTITUTED BORANES Higher boranes and their substituted derivatives are mildly sensitive to air and show different sensitivities towards water and heat. They have a relatively high dipole moment (ca. 3 Debye) and various Br4nsted acidities (Bl0HI4,pK, ca. 5; n-BleH22, pKa < 1). Some of them (BI0Hl4,XBloHI3)react with water, forming the strongly acidic HzOborane adducts (pK, 1.8). The separation of these compounds was carried out on silica TABLE 43.1 ELUTION CHROMATOGRAPHY OF BORANES AND THEIR DERIVATIVES ~~~

Compounds separated

Sorbent *

Eluent

Reference

Decaborane < alkyldecaboranes Halodecaboranes: 1-Br > 2-Br; 6-Br > 5-Br; 1€1> 2-CI n-B,,H,, > iso-B,,H,,

Silica gel Silica gel (i)

n-Hexane n-Hexane, benzene

Siege1 and Mack Stuchlik e l al.

Silica gel (activity 11) Silica gel ( i)

n-Hexane

He'fminek and PleSek Hefmanek et al., unpublished

n-B,,H,,

< 6-C6H,CH, -B,,H,,

*(i) = Impregnated with CH,COOH or CF,COOH.

References p. 951

n-Hexane

TABLE 43.2 CHROMATOGRAPHY OF CARBORANES AND THEIR DERIVATIVES Compounds separated

Sorbent

Eluent

Reference

Silica gel All 0, (basic)

n-Hexane n-Pentane Tetrahydrofuran n-Hexane n-Hexane

h f b r et al. Sieckhaus et al. Eoupek et al. PleSek et al. (1970) Stanko and Goltjapin; Stank0 and Anorova; Zakharkin and Kalinin; Zakharkin et al. (1966) Stanko and Goltjapin Gregor et al. Zakharkin et 01. (1970a)

~~

1,2- < 1,6- < 1,10-C2B8H,, < m- < p-carborane 0-< m -< p-carborane o-Carborane > 4-brorno-o-carborane Mono-, di- and trihalogeno-o-carboranes 0-

Halogeno-m- and pcarboranes oCarborane > 1ethoxys-carborane 3-Hydroxy-, 3-acetoxy-o-carborane 1-Acyls-carborane, reaction products

Silica gel 3'

'41203

Silica gel A1203 4

0

3

1-Phenyl-o-, 1-phenyl-m-carborane 3-Phenyl-o-carborane, reaction products

(acid-washed) Silica gel (activity I) Silica gel

1- and 3-fluorphenyl-o-carborane; purification

'412 0 3

1-Fluorphenyls- and -m-carboranes, reaction products 1-Ferrocenyl-o-carborane, reaction products 0-, m-andpCPB,,H,,

Silica gel (activity 11)

A 4 0O,(activity All 3 11)

nHexane n-Hexane n-Hexanechloroform (3:2) n-Hexane n-Pentane n-Hexanebenzene (1 :1) n-Hexane n -Hexane, cyclohexane n-Hexane n-Hexane

Sianko et al. Hawthorne et al. (1 968b) Hawthorne and Wegner Zakharkin et al. (1 969) Hawthorne et a!. (1965) Zakharkm et al. (1970b) Zakharkin and Kysin

b

%

3 5

P

Compounds separated

v)

2

!5 P

TABLE43.3 CHROMATOGRAPHY OF LIGAND DERIVATIVES O F BORANES AND CARBORANES

B10H14

B9H13L

> B9H13 - m F > B10H12L2

(L=pX-C,H4NH2;C,H,N) B,H13L, B,H13L' (L = (CH,CH,),S > X-C5H,N) ((CH3)3 N)2 Bi OH 8 (2,7- > 2,3- + 2,4-) ((CH,),S), BloH,

b

Sorbent

Eluent

Reference

Silica gel Silica gel (activity 11)

Benzene Dichloromethane, diethyl ether

PleSeketal. (1967) Heiminek et al. (1 968)

Al,O, (neutral)

Dichloromethane

Graybill et al.

A40

Dichloroethane

Hertler and Raasch

Benzene -dichloroethane Acetoni trile-dichloroethane Benzene ( L = C, H, 0) Acetonitrile-ethyl acetate (L = CH,CN) Benzene-n-pentane (L = (CH,CH,), S) Benzene

Knoth et al.

3

Al, 0, (acidic)

7,8-C, B, HI, L ( s y m > asym.)

Silica gel

3-(BrC5H4N)-7,8-CPB,Hl

Silica gel

Young e t a1

3

I/)

b U

s

I/)

A

4

C

3U W

0 w b

z E4

Beer and Todd

TABLE 43.4 CHROMATOGRAPHY O F M ETALLOCARBORANES Compounds separated

Sorbent

Eluent

Reference

X-C2B,H,-Co-C5H, ( X = H , -COCH3) l,lO-[(CsHs) Fe(C0),]2-C,B, H, purification (R-C, B,H 10)2N1,[(R-C, B9Hlo),Nil -

Silica gel Silica gel Silica gel

n-Hexane-dichloromethane n-Hexane-benzene (1 :1) n-Hexane-benzene (1: 1 )

Isomeric (R,C,B,H,),Ni, isomerlc [(R2C,B,H,),N1] (C,B,H,,)Co-C,H,, reactlon products (R,C,B,H,)-Pd(C,(C,H,),), reaction product5 [ (C, B,H, I ), Fe] -, (C, B,H ,,S(CH,CH 3 ) 2 ), Fe and (C, B,H )Fe(C,B,H ,,S(CH ,CH 3 ) 2 ) reaction products 1-1C, H, )Fe(CO),] -2-CH,-C,B,,Hl (B,,H ,,S)Co-C,H,, reaction products

Silica gel Silica gel Silica gel Silica gel

n-Hexane-dichloromethane (2: 1 ) n-Hexane-benzene (1:l) rz-Hexane-benzene (4:6) ri-Hexane-benzene (7:3)

Graybill and Hawthorne Smart et al. Hawthorne et al. (1968a); Warren and Hawthorne Paxson et al. Hawthorne er al. (1968a) Hawthorne et al. (1968a) Hawthorne et al. (1971)

Silica gel

n-Hexane- benzene (5:1) Benzene-dichloroethane (5 :1)

Hertler er al.

AL203

Smart et al. \o

P

\D

950

BORON COMPOUNDS

gel (HehBnek and Pleiek, Siege1 and Mack) or silica gel deactivated with acetic acid (Stuchlik et al.) or with trifluoroacetic acid (Herminek et al., unpublished). The separation of boranes and their substituted derivatives is summarized in Table 43.1. In accordance with our experience, we propose that the separation of these sensitive compounds should be carried out by elution chromatography on silanized and air-free silica gel in an inert atmosphere. Our latest results also show the advantage of the use of high-efficiency liquid chromatography in the identification and separation of borane compounds (KrejEi and Hefminek).

CARBORANES The chromatographic behaviour of various carboranes is based on their dipole moments (dependent on the character of the skeleton, the location of hetero-atoms or the different locations of substituents) and on the character and number of substituents. The most stable class is the closo-carboranes, Cz B,&+ *, which are generally stable enough towards heat, oxidation and acid hydrolysis and are separated satisfactorily on silica gel or aluminium oxide. It is of interest that a good separation of o-, m- and p-carborane, Le., compounds that differ significantly in their dipole moments but not in the sizes and shapes of the molecules, was achieved by gel permeation chromatography (toupek and Heiminek), which is believed not to be influenced by the dipole moment. Some examples of the column chromatography of closo-carboranes and their derivatives are reported in Table 43.2. A theoretically possible but rarely used technique is the column chromatography of more sensitive intermediate and higher nido-carboranes (Pleiek et al., unpublished). The optimum separation conditions for these compounds are similar to those for boranes.

LIGAND DERIVATIVES OF BORANES AND CARBORANES The chromatographic character of these relatively air- or hydrolysis-stable compounds is mainly influenced by their high dipole moments, owing to the dative bond between the ligand and the borane framework. Generally, the mobility decreases in following order: mono-ligand S di-ligand derivatives and (CH3CH2CH2CH2)2S >(CH3 CH2)2S >(CH3)z S > (C~H~)~P>CHJC = pyridine N % (C6H5)NH2 (approximately). Examples of chromatographed mixtures are listed in Table 43.3.

METALLOCARBORANES Column chromatography on silica gel is the most useful method for the purification of many types of o-bonded or sandwich-type metallocarboranes, most of which are relatively stable and of low volatility. The chromatographic behaviour of these compounds is determined predominantly by their ionic charge or dipole moment. Good results were achieved in separations of neutral metallocenes from ionic metallocenes (re., compounds with differently charged metal atoms), substituted from unsubstituted compounds, metallocenes with different ligands, or in separations of isomers, with

REFERENCES

951

different positions of the carbon atoms. Mixtures of these compounds are often intensively coloured and therefore well separated by the dry column technique. Typical examples of chromatographed mixtures and experimental conditions are collected in Table 43.4.

REFERENCES Beer, D. C. and Todd, L. J., J. Organometal. Chem., 36 (1972) 77. coupek, I. and H e h i n e k , S., unpublished results. t o u p e k , J., Hefminek, S., Plebek, J. and PokornQ, S., unpublished results. Graybill, B. M. and Hawthorne, M. F., Inorg. Chem., 8 (1969) 1799. Graybill, B. M., Pitochelli, A. R. and Hawthorne, M. F., Inorg. Chem., 1 (1962) 626. Cregor, V., Hehninek, S. and Plesek, J., Collect. Czech. Chem. Commun., 33 (1968) 980. Hawthorne, M. F., Berry, T. E. and Wegner, P. A., J. Arner. Chem. Soc., 87 (1965) 4746. Hawthorne, M. F., Warren, L. F., Callahan, K. P. and Travers, N. F., J. Amer. Chem. Soc., 93 (1971) 2407. Hawthorne, M. F. and Wegner, P. A., J. Amer. Chem. Soc., 90 (1968) 896. Hawthorne, M. F., Young, D. C., Andrews, T. D., Howe, D. V., Pilling, R. L., Pitts, A. D., Reintjes, M., Warren, L. F., Jr. and Wegner, P. A., J. Amer. Chem. SOC, 9 0 (1968a) 879. Hawthorne, M. F., Young, D. C., Garret, P. M., Owen, D. A,, Schwerin, S. G., Tebbe, F. N. and Wegner, P. A.,J. Amer. Chem. Soc., 90 (1968b) 862. Heiminek, S. and PleSek, J., Collecr. Czech. Chem. Commun.,35 (1970) 2488. Hefmanek, S., PleSek, I. and Fetter, K., unpublished results. Heiminek, S., P l e k k , J., h b r , B. and Hanousek, F., Collect. Czech. Chem. Commun., 33 (1968) 2177. Hertler, W. R., Klanberg, F. and Muetterties, E. L., Inorg. Chem., 6 (1967) 1696. Hertler, W. R. and Raasch, M. S., J. Amer. C7zem. Soc., 86 (1964) 3661. Knoth, W. H., Hertler, W. R. and Muetterties, E. L., Inorg. Chem., 4 (1965) 280. Krej&,' M. and Heirninek, S., unpublished results. Paxson, T. E., Kaloustian, M. K., Torn, C . M., Wierserna, R. J . and Hawthorne, M. F., J. Amer. Chem. Soc., 94 (1972) 4882. Plekk, J., Gregor, V. and Hefrninek, S., Collect. Czech. Chem. Commun.,35 (1970) 346. Plekk, J., Cregor, V. and Heirninek, S., J. Chrornatogr., 74 (1972) 149. Pleiek, J., H e h a n e k , S. and Stibr, B., Collect. Czech. Chem. Commun.,33 (1967) 691. Plekk, J., h i b r , B. and H e h i n e k , S., unpublished results. Sieckhaus, J. F., Sernenuk, N. S., Knowles, T. A. and Schroeder, H., Inorg. Chem., 8 (1969) 2452; C.A., 69 (1968) 87047. Siegel, H. and Mack, J. L.,Phys. Chem., 63 (1959) 1212. Smart, J. C., Garrett, P. M.and Hawthorne, M. F.,J. Amer. Chem. Soc., 91 (1969) 1031. Stanko, V. 1. and Anorova, G. A., Zh. Obshch. Khim., 41 (1971) 1521. Stanko, V. 1. and Goltjapin, Ju. V., Zh. Obshch. Khim., 4 0 (1970) 127. Stanko, V. I., Klirnova, A. I., Tschapovskij, Ju. A. and Klirnova, T. P., Zh. Obshch. Khim., 36 (1966) 1779. Stibr, B., PleBek, J. and Hefrninek, S., unpublished results. Stuchlik, J., Hefrninek, S., Pleiek, J. and ktibr, B., Collect. Czech. Chem. Commun.,35 (1970) 339. Warren, L. F., Jr. and Hawthorne, M. F.,J. Amer. Chem. Soc., 92 (1970) 1157. Young, D. C., Howe, D. V. and Hawthorne, M. F.,J. Amer. Chem. Soc., 91 (1969) 859. Zakharkin, L. I. and Kalinin, V. N., Zh. Obshch. Khim., 37 (1967) 939. Zakharkin, L. I., Kalinin, V. N. and Gedyrnin, V. V., Zh. Obshch. Khim., 4 0 (1970a) 2653. Zakharkin, L. I., Kalinin, V. N. and Snyakin, A. P., Zh. Obshch. Khim., 4 0 (1970b) 2246. Zakharkin, L. l., Kalinin, V. N., Snyakin, A. P. and Kvasov, B. A., J. Organometal. Chem., 1 8 (1 969) 19. Zakharkin, L. 1. and Kysin, V. l., Zh. Obshch. Khim., 4 0 (1970) 2234. Zakharkin, L. I., Stanko, V. 1. and Klimova, A. I., Izv. Akad. Nauk SSSR, Ser. Khim., (1966) 1946.