Boron-Nitrogen Chemistry

27 LCAO-MO Calculations on Boron Compounds III. Heteroaromatic Boron Compounds JOYCE

Downloaded by RUTGERS UNIV on January 10, 2018 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0042.ch027

Research

J.

KAUFMAN

Institute

and

JON

for Advanced

R.

HAMANN

Studies, Baltimore

12,

Md.

Molecular orbitals for the π-systems of aromat ic hydrocarbons, where a B - N pair replaces a C - C pair, have been calculated, using an L C A O technique with the Hückel, Pariser-Parr, and Pople approximations. Several theoretical models were used to choose input parameters, and modified values for boron electron-repulsion integrals were calculated, taking into account electron correlation effects. Complete L C A O - M O -SCF calculations were performed for each choice of input parameters. One choice gave good agreement between localization energies calculated by the Pople method and experimental results, as well as correct prediction of the order of stabilization of the borazarenes. The most effective model is discussed with reference to the σ-bonding of heteroaromatic boron compounds and borazines. Ι η the past five years a new class of heteroaromatic boron compounds, in which the boron atoms are incorporated into sixmembered aromatic rings, has been synthesized by Dewar and his colleagues (5). These compounds are very stable and bear a close re semblance to the parent aromatic hydrocarbons. In most of the com pounds reported a C - C pair has been replaced by an isoelectronic B-N pair. It was of interest toperform various types of L C A O - M O calculations, especially L C A O - M O - S C F (linear combination of atomic orbitals molecular orbital - self-consistent field) calculations, on the π-systems of these heteroaromatic boron compounds and to compare their calcu lated properties with those of the corresponding aromatic hydrocarbons. Since it had been shown that even for hydrocarbons a discrepancy exists between predicted chemical reactivities as calculated from charge distributions or localization energies (and also between reactivities pre dicted from localization energies calculated in the three approximations) (2)* a series of localization energy calculations by all three techniques was performed for the compounds of Dewar. The reactivities calcu lated by localization energies, as well as those predicted from charge distributions, may then be compared with the experimental data. 1

273

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

274

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

Details of Calculations The theoretical background of the calculational methods for the quan tum mechanical study of the π-electrons of conjugated molecules has been described (1) as well as the theoretical models utilized for the calculations of B - N pairs and the cnoices of input parameters, and the modified values for boron electron-repulsion integrals which take into account correlation effects. Using the activated complex model of Wheland, the contribution of the ^-electrons to the potential barrier has been calculated by the cal culational methods for three types of chemical reactions: electro philic, nucleophilic,and radical. In the activated complex, the carbon which is attacked in the reaction is surrounded by four single bonds, each containing a pair of electrons, and thus the carbon cannot be part of the delocalized bond. Two electrons are necessary to form the CY bond. For benzene the intermediate complex would have the form


9

Η

Y

A delocalized bond exists that extends over all the carbon skeleton except the carbon which is attacked (3). Localization energy is defined as the energy of the activated complex minus the energy of the original molecule. The numerical calculations reported here were carried out on the IBM 7094 at the Martin Co. Computing Center with the aid of an auto matic program written by G. Bessis of the Centre de Mécanique Ondulatoire Appliquée and with supplementary programs written by Jane Flinn of the Martin Co. Computing Center, using an L C A O technique in the framework of the Hiickel, Pariser-Parr (without iteration), and Pople approximations. The calculations were carried outfor both Model I (1), in which the ^-electron in the aromatic system sees an effective "core" similar to the one which a ^-electron would see in an almost neutral boron or n i trogen atom (this calculational model was effective for describing the properties of borazine itself); and Model Π, corresponding to the B~N+ model for B - N pairs in conjugated systems (this model was un reasonable for borazine, but turns out to be more reasonable for the heteroaromatic compounds). The values of β were estimated by the approximate ratios of the overlap integrals (1). F o r the heteroaromatic B - N compounds Model II may be a promi sing choice for a calculational model, at least for prediction of chem ical reactivities by localization energy criteria. Results and Discussion Coefficients of atomic orbitals in the molecular orbitals, SCF Ham iltonian matrix elements, energy levels, π-charge densities, bond orders, and chemical reactivities were calculated by all three tech niques for the molecules mentioned below. These calculations will be Niedenzu; Boron-Nitrogen Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

27

KAUFMAN

AND H AM ANN

MO of Heteroaromafic

Borons

275


published infull elsewhere- In the present article only results of per tinent chemical interest are reported. Localization Energies and Reactivities. I, Π-Borazarene. From general considerations of the effect on chemical reactivities of sub stitution of heteroatoms into an aromatic ring, Dewar has pointed out that for electrophilic attack, nitrogen, being more electronegative than carbon, should deactivate the positions of opposite parity to itself, and boron, being less electronegative than carbon, should activate the posi tions of opposite parity to itself. For I, II-borazarene,

the simplest of t h e B - N compounds, this would imply that positions ΙΠ and V would be deactivated and positions IV and VI would be activated. Therefore, it is to be expected that positions IV and VI will be most susceptible to electrophilic attack. Table I presents the electrophilic localization energies calculated by the three methods for borazarene, utilizing Models land Π. Htickel calculations and both Pariser-Parr (P+Po) and Pople (P+PF) calculations for Model I indicate that posi tion IV is the most reactive, while P+P and P+Pp calculations for 0

Table I.

Electrophilic Localization Energies

β Compound

Position

Hûckel

D

I, IV-borazarene

»

Most stable

I, ΙΠ-borazarene Unstable

This stability order is derived from Dewar s statement, "No deriva tives of I, ΠΙ-borazarene have yet been reported, and the few known compounds containing the ring system I, IV-borazarene are less stable than those containing I, Π-borazarene" (4). f

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

27

KAUFMAN

AND

H AM AN Ν

Table IV.

Μ Ο of Heteroaromatic

_D J

p

Model I A.


Hiickel

D

Atom 1 vm

2 3 4 5 6 7 8 9 10 11 12 13 14

IX X I π ΠΙ IV ν VI VII

4.5 5.6 5, 10 6, 7 7, 8 8, 9 9,10 10, 11 11, 12 12, 13 13, 14 14, 1

+

p

p Model II

Charges 1.0374 0.9443 1. 6339 0.4712 1.0848 0.9182 1.0099 0. 9396 1.0073 0.9252 1.0325 0.9864 1. 0249 0.9844

1.037 0.951 1. 888 0.112 1.049 0.963 1.008 0.975 1.006 0.967 1.033 0.994 1.025 0.992

1. 0066 0.8791 1.4505 0. 6248 1.0757 0.9729 1.0195 0. 9603 1. 0149 0.9217 1. 0598 0. 9959 1.0309 0.9874

1. 0198 0. 7347 1.9558 0. 0902 1.1050 1.0116 1. 0256 0.9593 1.0127 0.9089 1. 1218 0. 9989 1. 0669 0. 9887

Bond Orders

Bond

B-N 3,4

279

ττ-Charge Densities

Β. 1.2 2.3 2, 11

Borons

X , EX-Borazarophenanthrene

Bond

Orders

vm IX

0.648 0.271 0.595

0. 6133 0.3954 0.5692

0.5795 0.4206 0.5822

0. 6183 0.1632 0. 6058

IX, X X I

0.280 0.271 0.648 0.595 0.670 0.653 0.680 0.615 0.388 0.615 0.680 0.653 0.670

0.5480 0. 5426 0.5571 0.5241 0.7045 0.6268 0.6905 0.6043 0.4407 0.5950 0.6953 0. 6350 0.6900

0. 6840 0.5316 0.5393 0.5510 0.7315 0. 6013 0.7205 0.5747 0.4337 0.5740 0.7188 0.6101 0.7190

0.2118 0.2710 0.6313 0. 6039 0.6844 0.6410 0.6908 0.6079 0.3605 0. 6034 0. 6962 0. 6312 0.6971

I, Π π, m ΙΠ, IV IV V V, VI VI, V n VII, V m

It is possible to calculate the derealization energies of the various borazarenes by an extension of the methods customarily utilized for aromatic hydrocarbons. For normal aromatic compounds, from the total energy of the molecule the energies of the separated pairs are sub tracted to obtain the derealization energies, which are in turn directly related to stabilities. The results of these calculations for the bora zarenes indicate that while Model I does not reproduce the order of sta bilities, Model Π correctly predicts the order ΐ,π > Ι,ΐν » I, ΠΙ Using Model II, the signs of the derealization energies (in electron volts) are negative for I, Π - and I, IV- (as one would expect for stable molecules) but positive for I, ΙΠ-. This implies that Ι, ΙΠ- may actually be unstable with respect to the separated pairs. Conclusions From the results of localization energy calculations for Π, I-boraz aronaphthalene in which Model Π predicts the correct position for Niedenzu; Boron-Nitrogen Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

280

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

electrophilic attack and Model I does not, one might be tempted to i n fer that Model II is preferable for calculations of B-N pairs in these heteroaromatic boron systems. While it would not be judicious to base a choice of input models solely on the agreement with one ex perimental observation, the additional corroboration furnished by the de localization energy calculations (in which the stability order i s c o r rectly predicted by Model Π but not by Model I) indicates that Model Π must not be too unreasonable a choice for B - N in the heteroaromat ic compounds. For these compounds Model Π (or a model close to it) seems preferable to Model I. The changes in the behavior of the σ -frameworks as a function of the neighboring atoms lends credence to the conclusion that, while for bora zine Model I (B-N) was the more effective, for the heteroaromatic boron compounds Model Π ( Β " - N ) would be the more effective, since here initially the boron will have a higher σ-electron density and the nitrogen a lower σ-electron density than B - N pairs in borazine. This analysis in terms of the changes in σ-f rame work to be expected for B - N pair s in different molecular environments appears to be a fairly sound criterion on which to base an initial choice of input model for calculations of the π-electron systems of these molecules. Obviously, research remains to be done, in order to obtain the opti mum sets of input parameters for L C A O - M O calculations on B - N com pounds. The studies reported here were undertaken as preliminary calculations from which to project the direction of subsequent research in this field. From these studies theoretical models have been found to enable one to calculate desired properties for bothborazines and hetero aromatic boron compounds with at least a moderate degree of accuracy.


+

Acknowledgment We thank Raymond Daudel, Odilon Chalvet, and G. Bessis, Centre de Mécanique Ondulatoire Appliquée, Paris, France, for the PariserP a r r - S C F program used for these calculations. We also thank Sol James, Chief of Automatic Computations, Martin Co. Computing Center, for arranging to have the calculations run at the center and Jane Flinn for her help in writing the supplementary programs and performing the computations. Literature

Cited

(1) Chalvet, Ο., Daudel, R., Kaufman, J. J., Advan. Chem. Ser., No. 42, 251 (1963). (2) Chalvet, Ο., Daudel, R., Kaufman, J. J., Division of Physical Chemistry, 144th Meeting, ACS, Los Angeles, Calif., April 1963. (3) Daudel, R., Lefebvre, R., Moser, C . , "Quantum Chemistry. Methods and Applications," Interscience, New York, 1959. (4) Dewar, M. J. S., Advan. Chem. Ser., No. 42, 227 (1963). (5) Dewar, M. J. S., Kubba, V. P., Pettit, R., J. Chem. Soc. 1958, 3073, and subsequent articles in series. Received May 27, 1963. Research supported by the Air Force Office of Scientific Research, Office of Aerospace Research, under Contract No. AF49(638)-1220. Niedenzu; Boron-Nitrogen Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

Boron-Nitrogen Chemistry

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