Contributions to the Chemistry of Boron

1,1 -Dimethylhydrazine, 1,2-dimethylhydrazine, and phenylhydrazine were commercial products. They were dried over. BaO and fractionally distilled...
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17 Contributions to the Chemistry of Boron XXIV. Preparation and Properties of Some Hydrazinoboranes HEINRICH NOTH and WILHELM REGNET

Downloaded by UNIV OF GUELPH LIBRARY on July 16, 2012 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0042.ch017

Institut

für Anorganische

Chemie,

Universität

München,

Munich,

Germany

Several types of hydrazinoboranes have been prepared by hydrazinolysis of various chloroboranes and aminoboranes. Chemically, the hydrazinoboranes resemble the aminoboranes, as has been shown by their behavior toward nucleophilic and electrophilic reagents. I nterest in boron-nitrogen chemistry has been focused principally on aminoborane and borazine derivatives. The hydrazinoboranes have attracted less attention, although they offer interesting aspects from both the preparative and theoretical points of view. Dative π-bonding between the nitrogen's lone electron pair into the empty pz-orbital of the boron atom in hydrazinoboranes of the type R B - N H - N H - B R may result in a conjugation effect, well established in the isoelectronic butadiene system. Because of electronegativity considerations, this effect will almost certainly not be as pronounced as in the carbon ana­ log. A study of the B - N linkage in hydrazinoboranes will also show the influence of an electronegative group, Χ (X = NR , NHPh, NHBR ), on the B - N bond in an aminoborane, R B-NHX. In addition to these con­ siderations, the hydrazinoboranes also offer access to new hetero­ cyclic and polymeric boron compounds with a B - N - N - B unit, and a great number of hydrazinoboranes of low molecular weight are to be expected. Early attempts to produce hydrazinoborane, H B - N H - N H - B H , through pyrolysis of hydrazine-bis(borane), H Β· NH - N H · B H , were unsuccessful (10, 30) until Goubeau and Ricker prepared it by pyrolysis of N H 4 - B H (13). Hydrazino-l,2-bis(dialkylboranes), R B-NH-NHBR , were first reported in 1959 (23). They were obtained by hydrazinolysis of trialkylboranes (18), tetraalkyldiborane (25) (chloro)alkylboranes (25), (mercapto)alkylboranes (17), and aminobor­ anes (21, 23, 25). In addition, some (hydrazino) phenylboranes (21) , a B, B', B''-tris(hydrazino)-borazine (22) , and tne cyclic 1,4-diphenyl2, 3, 5, 6-tetraza-l,4-diborine (21) were syntnesized. 2

2

2

2

2

2

3

2

3

2

2

2

3

2

2

Prebaration The methods for preparing hydrazinoboranes are not necessarily the same as those for aminoboranes, altnough tftese methods suggest them­ selves. The normal procedure for aminolysis of a boron nalide can be disturbed in the hydrazinolysis process, because the free amino group 166

In Boron-Nitrogen Chemistry; Niedenzu, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

17

NOTH ET AL

Hydrazinoboranes

167

in the expected hydrazinoborane, Rs B-NH-NH , may act as the hydrogen halide acceptor. Indeed, the yields of hydrazinoboranes obtained by the action of hydrazine on chlorodialkyl- and diphenylboranes are negligible, if the reaction is carried out in ether solution at room temperature. This is due to the formation of an ether-insoluble salt of the composition Rg B C l - 2 N : 2

2

R BC1

+

2

2N H 2

-

4

[R B(N H ) ]C1 2

2

4

(1)

2

Downloaded by UNIV OF GUELPH LIBRARY on July 16, 2012 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0042.ch017

R = Pr, Ph

The composition of these compounds suggests their formulation as cationic complex salts of boron, since similar compounds containing amines instead of hydrazine as a ligand are readily formed (16, 20, 28, 29). Replacement of hydrazine by M e N - N H in the reaction according to Equation 1 leads to reasonable yields of (dimethylhydrazino.)dialkyl- or diphenylborane: 2

2

R BC1

+

2

2 Me N-NH 2

-

2

R B-NH-NMe 2

+ Me N-NH -HCl

2

(2)

2

The difficulties encountered in solvolyzing the B - C l bond by hydrazine demanded a more convenient and versatile preparative method— the hydrazinolysis of aminoboranes. Since these compounds are easily accessible, hydrazinoboranes are also readily prepared. The hydrazinoboranes synthesized by this route as well as by the method of Equation 2 are summarized in Table I. Interaction of M e N - N H with tris(dimethylamino)borane leads to solid, sublimable tris (dimethylhydrazino)borane: 2

2

3 Me N-NH 2

+

2

B(NMe ) 2

-

3

(Me N-NH)^B

+

2

3 HNMe

(3)

2

The evolution of dimethylamine begins at room temperature, but heating to 60° to 80°C. is required to drive the reaction to completion. Similarly, (Me N - N H ) BH was prepared from (Me N ) BH;the B - H bond does not react with primary or secondary amines at temperatures below 120° (26). The B - B bond in tetrakis(dimethylamino)diboron is also not affected by treatment with Me N - N H J J : 2

2

2

2

2

B (NMe ) 2

2

+

4

4 H N-NMe 2

2

-

B (NH-NMe ) 2

2

4

+

4HNMe

(4)

2

This is somewhat surprising, because the B - B bond is readily broken down by the action of primary amines on R(Me N)B-B(NMe )R(27) and less easily on Bg (NMe ) (7). Transamination of B(NMe ) with phenylhydrazine in various molar quantities resulted in the formation of B(NH-NHPh) , but not of Me NB(NH-NHPh) or (Me N ) B-NH-HNPh (reactions to be reported elsewhere), while with PhB(NMe ) the reactions 2

2

2

4

2

3

3

2

2

2

2

2

PhB(NMe )

2

+

H N-NHPh

PhB(NMe )

2

+

2 HgN-NHPh

2

2

2

-

2

PhB (NMe ) NH -NHPh 2

-

PhB (NH-NHPh)

2

+ HNMe

+ 2 HNMe

In Boron-Nitrogen Chemistry; Niedenzu, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

2

2

(5)

A D V A N C E S I N C H E M I S T R Y SERIES

168

Table I. B.

Downloaded by UNIV OF GUELPH LIBRARY on July 16, 2012 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0042.ch017

Yield, Method

No.

Compound

1.

Et B-NH-NH-BEt 2

2

Β

85

65-8 (9 mm.)

Β

92

122 (12 mm.)

Β

90

118 (1 mm.) 147

2.

Pr B-NH-NH-BPr

3.

Bu B-NH-NH-BBu

4.

Ph B-NH-NH

2

Β

95

5.

Bu B-NH-NH

2

Β

99

38-39

2

2

2

2

77,85

2

Α,Β

41,82

135 (5 mm.)

Α,Β

8.

PhB(NH-NMe ) 2

HB(NH-NMe ) 2

c

Α,Β

Ph B-NH-NMe

9.

C

2

Bu B-NH-NMe

7.

2

2

48

6.

2

2

P.,

°C.

%

0,

Synthesis

2

2

(mm.)

52,77

76-78 (1 mm.)

Β

91

33-35

Β

95

103

Β

97

72-75

C

C

C

10.

B(NH-NMe )

11.

B (NH-NMe )

12.

PhB (NMe )NH-NHPh

Β

36

57-58

13.

PhB(NH-NHPh)

Β

21

122-125

14.

Bu B-NH-NHPh

Β

85

144 (1 mm. )

15.

Ph B-NH-NHPh

Β

92

94

16.

[PhB-NH-NH]

Β

80-97

257-58

a

k

2

2

3

2

4

2

2

2

2

2

ρ

C

C

C

C

Method Β corresponds to transamination of an aminoborane with a hydrazine, method A to hydrazinolysis of the ap­ propriate chloroborane. Determined cryoscopically in benzene solution.

occurred simultaneously at room temperature. In refluxing toluene solution, 2 moles of M e N H are evolved per mole of phenylhydrazine and the aminoborane and a tetraphenylcyclo- 2,3,5,6-tetraza-1,4diborine are formed. 2

Η Ph I

I

N-N /

2 PhB(NMe ) 0

0

+ 2 H N-NHPh 0

-

\

Ph-B

B - P h + 4 HNMe

0

(7)

N-N I

I

Ph H

However, it has not yet been determined whether this or the other possible Ν isomer has been obtained. This reaction shows that re­ placement of the hydrogen atom of the PhHN group proceeds only at elevated temperatures and therefore the reaction sequence Bu B-NH 2

2

+

H N-NHPh 2

^°

Bu B-NH-NHPh 2

+

NHg

(8)

ο

Bu B-NH-NHPh 2

+ Bu B-NH 2

2

^ *

Bu B-NH-N(Ph)BBu 2

2

In Boron-Nitrogen Chemistry; Niedenzu, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

+ NH^

(9)

17

NOTH ET AL

Hydrazinoboranes

of Hydrazinoboranes