Boron-Nitrogen Chemistry

9 The Force Constants of Various Boron-Nitrogen Compounds JOSEF G O U B E A U

Downloaded by CORNELL UNIV on October 8, 2016 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0042.ch009

Laboratoriurn für anorganische Stuttgart, Germany

Chemie

, Technische

Hochschule,

Force constants of various types of boron-nitrogen compounds have been calculated from the vibrational spectra on the basis of a valence force model. The force constant, k , of amine boranes was found to be about 3.5 mdyn. per Α . , whereas for aminoboranes a value of 7.0 mdyn. per A. was obtained. For borazines calculated values for k range from 5.7 to 6.3 mdyn. per A. Some general features of the force con stants — for instance, their utilization to eval uate bond orders — are discussed. BN

BN

F orce constants can be derived from the vibrational spectra. In particular, the valence force constants, like the bond distances, per mit an evaluation of the bond relations between atom pairs. The present results agree with the results of experimental determination of bond distances. Table I recalls the results for the classical element com binations C - C , C - N , and C - Q . With increasing order of the bond, the bond distance decreases and the force constants increase. However, whereas changes in the bond distance have a range of about 30%, the change in the force constants amounts to about 200%. This is a definite advantage of the force constants. Force constants can be calculated from the vibrational spectra on the basis of certain models. The models utilized are extremely important for the results and also for a possible comparison of different force con stants: Results are comparable only if they are obtained with similar models. A further very important basis for obtaining force constants is the assignment of the found frequencies to the vibrations of the molecule. The following discussion is based on the so-called "valence force model. " In using this model it is assumed that forces exist along the valence bonds of the atoms as well as other forces which preserve the bond angles, and that there are reciprocal forces effective between the valence forces and those forces preserving the angles. The values of those force constants which preserve the angles are normally in the neighborhood of 10 to 20% of the valence force con stants; the reciprocal force constants range from negative values to 30%. Therefore the latter are at present mostly responsible for any un certainty. Basically one has to assume an error of about 5 to 10% in the valence force constants, even if similar models are utilized. The 87

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

ADVANCES

88

Table I.

3

C

H C 2

HC

CH

=

3

2 = CH C

H

NH

3 ° H C = NH H

2

2

= Ν

HC


H

3

C -

C H E M I S T R Y SERIES

Distances and Valence Force Constants of Various Atom Groupings of C - C , C - N , and C - O rA-B,

H

IN

OH

A.

ω

Cm *

ab

f

9

Mdyn/A.

1.543

992

4.5

1.339

1621

9.7

1.205

1974

15.7

1.475

1044

5.0

1.24

1660

10.0

1.156

2100

17.5

1.428

1050

5.2

H C

= Ο

1.230

1750

10.3

C

= Ο

1.128

2155

18.8

2

deviations obtained through assumption of harmonic vibrations are of only very limited importance, especially if atoms with an atomic weight over 10 are considered. Calculations were made mainly according to the Wilson F - G matrix method. Mathematical work increases for more complicated mole cules, but any mathematical operation is easily accomplished with modern computers. A particular difficulty of all calculations is caused by the large number of possible mathematical solutions. These can be limited, however, through the use of isotopes. In Stuttgart Fadini (5) has developed a calculating method which enables the calculations of force constants from frequencies without testing. In the following only valence force constants were used. A. Stock and E . Wiberg identified three basic groups of boronnitrogen compounds in analogy to the isoteric carbon compounds: ι

I

1. Amine borane

- B «— N -

2. Aminoborane

>B —

N-

^=

N
- Β ^ =

N-

I

1

>B

\_

3.

Borazine

=

Of the last group, no monomeric compound has yet been isolated; they are known only in polymeric form. In the early fifties we began a spectroscopic investigation of the various classes of compounds in order to clarify the bonding situation for boron-nitrogen compounds, first, with the aid of the Raman effect, and later also with infrared spectra. Amine

Boranes

For the first group of compounds, amine boranes, which are addi tion compounds of nitrogen derivatives and boranes, the first (very sim plified) calculations revealed values for / B N ^ ^ neighborhood of about 3.5 mdyn. per A . Later calculations considered the effect of coupling and gave values which range from 2.5 to 3.1 mdyn. per A. [ Table II and (13)}. e


9

GOUBEAU

Table II.

Boron-Nitrogen

mdyn/A

B N j

-NH

H B-NH

3

3

3.08

r

(H C)^o-NH

3

3

2.91

E^-NE^U

3

2

2.48

1.60

BN


89

Force Constants of Some Addition Compounds of Boron and Nitrogen Compounds FjB

&

Force Constants

2.46

1.63

These values differ about 20%. Since they are calculated with com parable models, these differences are beyond the normal limit of error. However, at present these differences will not be discussed. Neverthe less, it is interesting to compare the experimentally obtained values with those which are expected. For a calculation of these expected values, an empirical formula as given by Siebert (12) has been found very helpful: *AB

-

7

' ° 2

Η

Α

Χ

Ω

Β

where Ζ = number of nuclear charges and η - main quant number of the bonding electrons. This equation is valid to the first approximation for the sp hybridi zation (6). For demonstration purposes, the relationships between the various force constants of some nitrogen single bonds of a few elements of the first group of the periodic system are combined in Table ΠΙ. 3

Table III.

Force Constants of Various Nitrogen Single Bonds BeN

BN

CN

NN

According to Siebert

3.1

3. 9

4. 7

5. 5

Exptl.

1.5

2.9

4.8

5.4

Difference, %

-52

-26

+2

-2

The experimental values of Table ΠΙ are derived from C ^ B e * 2HN(CH ) , Η Β · Ν Η , H CNHg , and [ H N N H ]** . The values for CN and NN are in good agreement with the values calculated according to Siebert s method. The values for BN and especially for BeN are con siderably below the values obtained by Siebert s method. These devia tions are definitely beyond the normal limit of errors. The increasing deviation in the direction of beryllium can be considered confirmation that a systematic deviation of force constants towards increasing polarity of the bond occurs for these elements. In general, high polarity results in a pronounced weakening of the force constant. The polarity of the boron-nitrogen bond in these addition compounds is evidenced also by their high dipole moments. 3

2

3

3

3

3

3

1

1

Aminoboranes The spectroscopic investigation of the second class of boron-nitrogen compounds — namely, the aminoboranes — byBecher (2) showed that an almost characteristic frequency in the neighborhood of 1400 to 1500 Niedenzu; Boron-Nitrogen Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

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

90

c m . " canbeassigned to the BN vibration. From these values a force constant, &BN, of about 7.0 mdyn. per A . has been calculated. This value is far above those for addition compounds of the first type. There fore the existence of a coordinative ττ-bonding was assumed. One ob tains a certain similarity with ethylene derivatives, indicated unequi vocally in these compounds by a tendency to polymerize. Because of the polarity of the bond, polymerization occurs easily, frequently leading to dimers, and less frequently to trimeric compounds. As indi cated by the crystal structure investigation of [ Clz BN(CH ) ] (9) » a four-membered ring system, BgNjj, exists in these dimers. The BN bond is influenced by the substituents which have an effect on the force constant and on the tendency to polymerize. 1


3

Trimeric

2

2

Borazines

The next group of boron-nitrogen compounds to be considered is the trimeric borazines. Edsall and Crawford (4) calculated for this ben zenelike substance a force constant & = 6.3 mdyn. per A . This value is probably the upper limit, whereas the lower limit will be about 5.7. A wide variety of material was obtained from several investigators for this type of compound (3, 10). The most important result is that ring vibrations, in contrast to the isosteric benzene derivatives, do not show the constance observed in the aromatic system. This indicates (in analogy to both other classes of boron-nitrogen compounds) that the in fluence of the mass of substituents, the coupling of vibrations of sub stituents with the ring vibrations, and the occurrence of mesomerism are of utmost importance. If one considers the strong infrared ring vibration near 1350 to 1500 c m . (compare Table IV), it is clear that for the interpretation of the relationships a change in the force con stants in the B N ring has to be assumed. B N

- 1

3

3

Table IV.

ω Ring

(HNBF)

3

(HNBH)

3

(HNBC1)

1497 1465 3

(H CNBH) 3

3

(H C NBCH ) 3

2

1517

3

1442

1452

1412

1411

1402

3

On the basis of the highest value observed for the fluor ο compounds, one has to assume that the highest force constant of a BN borazine ring system is obtained in these compounds. Very often a split of this in frared absorption is observed which has to be related to the presence of both boron isotopes (n B N 51%, B g B N 38%, B B N 10%, B N 1%). Deviation of fe in the borazines is probably in the neighborhood of 10%, as observed for the addition compounds. If one assumes in these three types of compounds (as in the isoteric carbon derivatives) in first approximation the bond orders of 1, 1.5, and 2.0, one obtains an increase in the force constants with the bond order, just as in the corresponding carbon compounds (Figure 1). Thus 3

1 0

3

3

1 1

3

1 0

3

1 1

1 0

2

BN


3


1

2 BOND

3

ORDER

Figure 1. Comparison of the force constants CC-(O) and boron-nitrogen compounds (Θ) force constants can be utilized for obtaining the bond order. Some ex amples are summarized in Table V. It is difficult to fix force constants for the normal single and the normal double bond. As a standard one canuse either the experimental force constants of the addition compound, and then the value BO I is obtained, or a second value (Siebert's value) can be used, equal to a steady increase with the bond order, and then the for the addition compounds (as indicated earlier) are below the value of the normal single bonds. Independent of any standard is the fact that the values f o r / g ^ range between a single and a double bond. A comparison of these results with the corresponding ones of other element-boron bonds shows an interesting picture, illustrated in Table VI. It is obvious that the experimental value for a boron-nitrogen single bond is too low, especially if compared with the experimental values for other element combinations. However, these combinations were takenfrom the high symmetrical compounds B X " , a type of compound not known in the boron-nitrogen combination. The most important re sult, however, is that the boron-nitrogen combination at room tempera ture is capable of forming the highest bond orders. Of all element com binations of the boron, this is the only one which at room temperature obtains a value of nearly 2 for bond orders. The combination boronoxygen and boron-fluorine have a value of approximately 1.4. During 4


92

A D V A N C E S IN CHEMISTRY

SERIES

Table V . Force Constants and Bond Orders Substance X

B - N Y

f

0

2.8

1.0

V s ' ^ S

4.0

1.3

R B-NCO

5.8

1.7

3

6.0

1.7

3

6.3

1.8

2

7.0

2.0

"

7.0

2.0

8. 3

2.3

3

3

2

B-(NR ) 2

H B -(NH) 3

X


B

BN

3

B - N Y

2

B - N Β

3 2

- Ν (band spectra)

1

the last few years we often tried to obtain boron-oxygen compounds of a higher bond order. Thus we tried to obtain salts of the dimethyl boric acids (7). In the course of these investigations, we observed that the ion disproportionates into trimethylborane and methylborates, which polymerize via addition of dimethylborates. However, other investi gators have demonstrated within the last few years that in monomeric Bg O , B 0 , and H B O a very high force constants occur at high tem peratures. Thus at high temperatures the maximum of the bond order is shifted from the boron-nitrogen bond to the boron-oxygen linkage. The boron-fluorine combination is never capable of forming a high bond order, because of the high polarity of the bonds. If one compares the relationships between the boron-nitrogen bond with the combinations of nitrogen with other elements of the first row of the periodic system, as well as with elements of the second period (as indicated in Table VII), one obtains completely normal behavior. If one excludes the combination N F , which is not capable of forming higher bond orders (since too many electrons are present in this particular combination), the result is that combinations with strongly electronegas

2

2

Table VI. Force Constants of Various Element Combinations with Boron BO

BF

BC

BN

Siijgle bond experimental

3.5

2.8

4.6

A c c o r d i n g to Siebert

3.5

3.9

4.5

5.1

Highest value at r o o m temperature

3.6

7.0

6.3

7.2

B o n d o r d e r at room temperature

1.1

1.8

1.4

1.4

Band spectra

8.3

13.5

7.7

Bond order

2.1

3.0

1.5


5.1

9

GOUBEAU

Boron-Nitrogen

Force Constants

93

of Various Boron-Nitrogen Compounds BO II 0.72


1.0

:°o

2

1.5

:J - * 2 R B - Ν =C =Ο

R B

1.5

(R N) B - Ν R

(R N) B-NR

1.6 1.8

-BH X B-N

1.8

N = Β = Ν

b

r

2

2

2

2

2

B

Y

N R

N-C=0 2

2

" Ν - Η - BH X B NY

1

2

B

2

H

2

2

2

3-

2.1

tive elements like oxygen, nitrogen, and carbon permit the formation of a triple bond. The CN compounds already show a strong tendency to polymerize, indicating the decrease in the stability of the multiple bonding. This effect is more pronounced in the boron-nitrogen com pounds, which are capable only of forming double bonds. In addition they show a strong tendency for polymerization — i . e . , instability. This tendency to polymerize is so strong that in compounds where one could expect a triple bond, only polymers are obtained. Finally, for beryllium only force constants which are far below the expected value of a single bond have been observed. A l l reactions which in boron chemistry lead to the formation of a double bond — for instance, the elimination of HC1 of amine addition compounds according to the equation Cla Be· 2HN(CH ) 3

2

~

H C 1

> ClBeN(CH ) · NH(CH ) s

2

3

2

do not show the expected course of the reaction even at very high tem peratures (400· to 500·) (11). With decreasing electronegativity of a bond partner of the nitrogen, the capability to form multiple bondings

Table VII. Force Constants and Bond Orders of Various Nitrogen Bonds With Various Elements CN

NN

ON

BeN

BN

Siebert

3.1

3.9

4.7

5.5

6.3

I max

1.5

7.0

17.9

22.6

23.9

3.0

3.0

Bond order I

0.5

1.8

MgN

A1N

SiN

PN

3

SN

3.1

3.3

3.6

3.7

I max

1.9

4.1

6.1

12.4

Bond order I

0.6

1.2

1.7

3

Siebert

2.8


94

ADVANCES

C H E M I S T R Y SERIES

towards nitrogen decreases. The boron-nitrogen combination is exactly on the border, with an electronegativity value of 2.0. The second period of the periodic system presents a similar picture. Again chlorine is not considered, and sulfur with approximately the same electronegativity as carbon is able to form a triple bond. In the case of phosphorus, if one neglects the PN molecule, which is stable only at very high temperatures (although it has a high force constant and a high bond order), the highest force constant for the PN combina tion is obtained for the ( C H ) Ρ - N H with the value 6.1 (1). This force constant corresponds to a bond order of 1.7. In analogy to the boron (which has approximately the same electronegativity as phos phorus), phosphorus compounds polymerize very easily, as illustrated by the examples F P N C H and C1 PNCH (8). In the case of silicon the tendency towards the formation of higher bond orders decreases rapidly, and aluminum corresponds in its behavior completely with that of beryllium. This short review indicates clearly that the boronnitrogen bond readily falls in line with all the other nitrogen-bonded elements. In analogy to the C-Cbond, the boron-nitrogen bond is the border of those element combinations which are capable of multiple bonding, but in addition show a strong tendency to polymerize. This fact is respon sible to a large degree for the particular properties of boron-nitrogen compounds. 3

3


IN

Literature

3

3

2

3

+

3

Cited

(1) Appel, R., private communication. (2) Becher, H. J., Advan. Chem. Ser., No. 42, 71 (1963). (3) Beyer, H., Hynes, J. B., Jenne, H., Niedenzu, K., Ibid., No. 42, 266 (1963). (4) Edsall, J. T., Crawford, B. L., Jr., J. Chem. Phys. 7, 223 (1939). (5) Fadini, A., in press. (6) Goubeau, J., Angew. Chem. 69, 77 (1957). (7) Goubeau, J., Ewers, J. W., Z. Anorg. Allgem. Chem. 304, 230 (1960). (8) Hassemann, P., Dissertation, Techn. Hochschule, Stuttgart, 1963. (9) Hess, H., Acta Cryst., in press. (10) Laubengayer, A. W., Watterson, K., Bidinosti, D. R., Porter, R. F., Inorg. Chem. 2, 519 (1963). (11) Rakintzis, N., Dissertation, Techn. Hochschule, Stuttgart, 1957. (12) Siebert, H., Z. Anorg. Allgem. Chem. 273, 170 (1953). (13) Taylor, R. C., Advan. Chem. Ser., No. 42, 59 (1963). Received July 24, 1963.


Boron-Nitrogen Chemistry

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