INVESTIGATION OF DIMERIZATION OF PHOSPHAZO COMPOUNDS

BY THE CNDO/2 METHOD WITH OPTIMIZATION OF GEOMETRY

A. S. Tarasevich, A. M. Nesterenko, and I. E. Boldeskul UDC 541.5

In a previous communication [i] we examined the possibility of synchronous dimerization of monomeric phosphazo compounds (PAC), depending on the type of substituent on the P and N atoms. In that work, it was shown that with angles PNR ~ 120 ~ , the most probable is cyclo- addition of two antiparallel-positioned monomers as a result of paired a-dative interaction of the unshared electron pairs (UEP) of the nitrogen atom with the electrophilic phosphorus atoms. The highest occupied molecular orbital (HOMO) is the ~-MO~ contributions to which are made by the px-AOs of the nitrogen and phosphorus. The highest of the occupied a~'-MOs, which are characterized by contributions of s-, p~-, andpz-orbitals of the P and N atoms, is

located below the T-HOMO on the energy scale, and corresponds mainly to the UEP of the nitrogen atom. The lowest unoccupied molecular orbital (LUMO) is also of the o~' type, and it is localized to a considerable degree of the P atom. The tendency of such monomers to dimerize increases with increasing contributions of the AOs of phosphorus to the o~'-LUMO and the contribution of the AOs of nitrogen to the a~'-HOMO, and also with decreasing energy gap between these MOs. The studies that we have cited fell short in that they used exactly the same model geometry of the phosphazo group regardless of the electronic properties of the substituent, even though the interrelations between the electronic and spatial structure of PACs could refine quite substantially our concepts of the mechanism of their dimerization.

To this end, using the MO-SCF method in the CNDO/2 approximation [2] with optimization of the geometry [3], we have carried out a calculation of the electronic structure of the following phosphorus-containing compounds: F3PNH (i), F3PNMe (II), Me3PNH (III), (F3PNMe)z

(IV), F3PNMe (V), F3PO (VI), Me3PO (VII), F3PS (VIII), and Me3PS (IX). The PNR fragment (R

is H or C) was positioned in the YZ plane, and the PM bond (M is N, O, or S) was positioned along the Z axis. The selection of the initial structural parameters for compounds II, III, and IV is described in [i]. Analogous values were used for the other compounds, except for V. The PO and PS bond lengths were assumed to be 1.54 and 2.00 ~, respectively. The struc- ture of compound V was obtained from an optimization of the geometry of compound IV by step- wise plane-parallel separation of its monomeric components.

A characteristic feature that was manifested in the CNDO/2 approximation in optimizing the geometry of the monomeric PACs is a decrease (in comparison with the initial value) of the bond angle at the nitrogen atom, this decrease being the most pronounced for the com- pounds with electronegative substituents on the phosphorus atom (Fig. i). The increase in the excess positive charge on the four-coordinated phosphorus atom, expressed in an increase in the contribution of its AOs to the unoccupied MOs, increases the coordination unsaturation of the phosphorus. The probability of its forming new intramolecular and intermolecular bonds is increased. For an isolated molecule, this leads to a strengthening of the nonvalence bonds of phosphorus to the atoms of the substituent R on the nitrogen (Fig. 2). For the two monomers with antiparallel position of the PN bonds, the increased electrophilicity of the phosphorus atom facilitates its dimerization.

The imperfection of the standard CNDO/2 parametrization for Period III elements (4) apparently leads to overestimation of this effect. Because of the too-strong P...P non- valence interaction in the dimer of IV, for example, the values of the intracyclic angles that are obtained are considerably different from the experimental values [5]. On the other hand, the decrease in the PNC bond angle from 124.2 ~ to 119.1 ~ when the change is made from

Institute of Organic Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev. lated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 18, No. 5, pp. 525-530, September-October, 1982. Original article submitted July 13, 1981.

Trans-

0040-5760/82/1805-0475507.50 �9 !983 Plenum Publishing Corporation 475

, o:~ :,67 ,'/~,~,, \'t 1:.~6 ~

:

II ,.'.>~.~_._ 1173 o

- . "

"i . _ " dT.-: _

:236,',k. . . . : ~ F H l O ~ iF6,T . .

F ~ 5 , 5~ ://.9~...~l ) oe"

F o

If ,",//

H ~

/V .~'" F ~

f ~e6 :05~ :7 :s,:~/ ' s H- ---~ /'7 "~:' '~

:II/ ::8: ~ -v-::: :/ ] ! ,, , t F H H

~S

Fig. i. Equilibrium geometry of compounds I-IX. Bond lengths in A.

Ph3P = N -- C6H4p-Br [6] to Ph2FP = N -- CH 3 [7] can be fully explained, apart from steric

factors, by the change in the inductive effect when Ph is replaced by F on the P atom. The increase in reactivity of the four-coordinated phosphorus atom with increasing positive charge on the atom correlates well with the capability of the PACs for dimerization [8].

Thus, even without complete quantitative coincidence of the optimized geometric param- eters with the experimental parameters, the trends in the changes of parameters in a series of corresponding monotypical compounds is entirely reasonable. In Table 1 we have listed the orbital energies e of the frontier MOs of different types of symmetry, and the coeffi- cients C (M) and C*(P) for the Px-AOs of the M and P atoms in the ~-HOMO and v-LUMO,

respectively. The values in parentheses for compounds VI-IX refer to the o-MOs, for which the Z axis is a third-order symmetry axis. The quantities pertaining to unoccupied MOs are

denoted by asterisks.

On the basis of the relationships we have obtained for the changes in forms of the frontier molecular orbitals and their energy levels, it is possible to judge the change in reactivity of the corresponding molecules. When the geometry is optimized, the electronic structure of the monomeric PACs changes significantly. The decrease in the bond angle at the nitrogen atom leads to a reinforcement of P...P nonvalence interaction (Fig. 2), a lengthening of the PN bond (Fig. i), and a change in character of the ov'-HOMO. The contri- bution of the pz-AO to this MO is increased, and that of the py-AO is decreased; i.e., the

direction of the hybrid AO of nitrogen in this MO approaches the axis of the PN bond. Here,

476

-0,25F

�9 *O,d! P

_424r/'"

1,85 N -0,26

"~H +0,,t6

H ~l H,O,06

H c-0,1j

~P ~ ~-~,~

.Y f ) --- ..\o H - - f , i ~ . + o o q

H - - - I T C -015

§ 4' ~ H +O05

+ 74 ~ ~62 O, '4 p " . N - d 2 2

4 ,,-, /+O,,lt t / .ool - o , 2 l f s

-424F \ ___---- C ~------~+4ot ,0,01 +O06 H 080 ' ' e ~ . . . H - - - - c

< H +0,0! ~ 1-~24 r-o,2s \ -o.z~ . /

-0,25F \ V_ O, lg N

1,48 N 021 f " ~ 2 4

t~ ~ : - , " - t

/ ~\','~ \ / " ~ ' < , o . , j ", ~ ,<- l.----.t~._ I ~2e !

, ~ ~ss -o23E ! o o) ~ c +o go. p " o -o23 " \~ r o # ~ /

r ~ _v; o67~ 2.~8 s o , z " / p

+o, o5// . . . . - o 2 ; r / / ~-" ~r r " I +0o~" o 9 s l /

oo~ ~/~2" F-OZ3 , . ~ c -o,u § "'- C-o/" 4 \ ~ .

~_ , 2,01 * ' 05 \P ~ C , *0,26P .v' w0-o,55 u, ~ , - i ~ / ,

k,~/ # e

, ,

H H H H

Fig. 2. Charges on atoms and Wiberg indexes of bonds in compounds I-IX.

TABLE i. Energy Levels s (in eV) of Frontier MOs of Different Types, and Maximal Coefficients C in H-HOMO and ~-LUMO for Px-orbitals of Atoms Connected by a Formal Double Bond

COLT- * * * * p o u n d - - P a ~ ' (8o) - - e~ ~ (~ , (8o.) ~;~ C~ (M) C~ (P)

I lI

III V

VI VII

Vlll IX

16,70 13,09 14,98 14,34

(17,67) (13,90) (17,58) (14,11)

16,75 12,76 14,28 14,47 18,81 14,01 t6,52 12,68

--2,04 +2,29 --2,01 --1,89

(--2,75) (+2,o5) (--2,Ol) (+2 ,o3)

+0 ,92 +3,77 +1,12 +0,97 +0,30 +3,46 --0,87 +2,14

0,86 0,83 0,75 0,76 O, 87 0,77 0,77 0,75

0,81 0,57 0,8l 0,80 0,83 O, 59 0,53 0,45

477

H 113"

6065 F ~ P- ~b2 =N ~ "

%

Fig . 3. T o t a l ene rgy of dimer o f IV as a f u n c t i o n o f d i s - t a n c e be tween i t s "monomeric" PN bonds , and geomet ry of t r a n s i t i o n s t a t e in r ~ a c t i o n of m o n o m e r i z a t i o n . Bond l e n g t h s a re g i v e n in A.

f o r the Me3PNH , where the PNH a n g l e i s c l o s e s t to 120 ~ , t he o r d e r of p o s i t i o n s of the ~- HOMO and the o~'-HOMO i s no t changed by o p t i m i z a t i o n of the geome t ry ; in c o n t r a s t , f o r F3PNH , the o r d e r i s r e v e r s e d . The p o p u l a t i o n of the py-AO of n i t r o g e n i s d e c r e a s e d by the o p t i m i z a t i o n , and t h a t of the py-AO of the phosphorus i s i n c r e a s e d . A l l t h e s e f a c t o r s

shou ld a f f e c t t he mechanism of i n t e r m o l e c u l a r i n t e r a c t i o n o f monomerie PACs. For example , f o r the h y p o t h e t i c a l t r i f l u o r o p h o s p h a z o h y d r i d e I , t h e p o s s i b i l i t y of synch ronous c y c l o - d i m e r i z a t i o n u s i n g the UEF of the n i t r o g e n must be r e d u c e d . There i s a c o r r e s p o n d i n g i n - c r e a s e i n the p r o b a b i l i t y o f t h i s compound fo rming p o l y p h o s p h a z e n e c h a i n s (wi th s p l i t t i n g out of HF).

When the fluorine atoms in trifluorophosphazo hydride are replaced by methyl groups, i.e., when the change is made from compound I to compound II, the ionization potential of the PAC, estimated on the basis of the energy of the HOMO, decreases, but the energy level of the LUMO increases still more. As a result, the energy gap AE becomes larger. This factor, together with the significant delocalization of positive charge from the phosphorus to the methyl groups, is responsible for the existence of compound II in the form of the monomer. At the same time, the replacement~of H by Me in trifluorophosphazo hydride I, with relatively little delocalization of the negative charge from the atom to the methyl group through a hyperconjugation mechanism, leads to a substantial decrease in the ionization potential and in the energy gap AE in the trifluorophosphazomethyl III. For this compound, optimization of the geometry gives the smallest value of the bond angle at the nitrogen atom that has been observed in this series of monomeric PACs. The above-noted corresponding change in direction of the hybrid AO of nitrogen in the o~-HOMO, the ~-character of the HOMO, and steric factors tend to increase the probability of dimerization of the monomers of III in the plane of the PN~-bonds. Owing to the high polarity of these bonds, the symmetry prohibition for such interaction [9] is not strict.

From the data listed in Table i, we can explain the monomeric nature of the phosphoryl and thiophosphoryl compounds that have been examined for comparison. The excessively large energy gap AE = 19.11 eV between the frontier MOs of the ~-type for the trifluorophosphine

oxide VI hinders its cyclomerization, despite the considerable polarity of the phosphoryl bond. In compounds VII-IX, the energy factors are more favorable for dimerization. The contribution of the Px-AO of the M atom to the v-type HOMO is quite large, even in the tri- fluorophosphine sulfide VIII with a positively charged sulfur atom. However, delocalization of the positive charge from the phosphorus atom to methyl groups or to a sulfur atom leads to a considerable decrease in the contribution of the Px-AO of phosphorus to the v-LUMO, and this prevents cyclodimerization of these compounds.

478

In Fig. 3 we show the relationship we have obtained between the total energy of the dimer IV and theodistance d between its "monomeric" PN bonds. The values of d were incre@sed

O

in steps of 0.2 A from the equilibrium value to 3 A, and then (with a larger step) to 25 A. At each fixed value of d, we optimized all of the other geometric parameters. A transition state corresponding to the maximum value of the total energy of the system is realized at a distance approximately equal to the sum of the van der Waals radii of the P and N. Weakening of the "dimeric" PN bonds is accompanied by a strengthening of all of the other valence bonds of the phosphorus. The PNC and FgPN angles in the monomers are increased. With further di- lution of the monomers, the total energy of the system drops off slowly, mainly because of a strengthening of the nonvalence bond of the phosphorus with the carbon atom of the methyl group. Here, the PNC angle decreases, and the "monomeric" PN bond is lengthened. The optimized final structure of the monomer of V is shown in Fig. i. In contrast to II, the monomer of V is characterized by the energetically less favorable masked conformation.

Thus, optimization of the geometry of phosphoryl, thiophosphoryl, and phosphazo compounds, using the standard CNDO/2 method, does not lead to structures that are identical with the experimentally determined structures. Nonetheless, this approach may be useful in qualitative studies of the influence of substituents on reactivity in a series of monotypical molecules. One particular fact that becomes evident is the increase in coordination un- saturation of compounds of four-coordinated phosphorus with increasing electronegativity of its substituents. W~en the bond angle in PACs at the nitrogen atom is reduced to values below 120 ~ , the probability of synchronous cyclodimerization using the UEP of the nitrogen should be lowered. The replacement of a hydrogen atom at the nitrogen by a methyl group facilitates dimerization, since the corresponding increase in the energy level of HOMO leads to a reduction of the gap between the HOMO and LUMO. The analysis we have performed on the changes in the energy gap and in the distribution of electron density on the reaction centers has made it possible to explain the reasons for the lack of any dimerization of phosphoryl and thiophosphoryl compounds.

The authors wish to express their appreciation to V. V. Penkovskii for assistance in this work, and to Yu. P. Egorov for his steady interest in the work.

LITERATURE CITED

i. A. S. Tarasevich and Yu. P. Egorov, "Quantum-chemical interpretation of dimerization of phosphazo compounds," Teor. Eksp. Khim., 13, No. 6, 809-811 (1977).

2. V.G. Maslov, "Program for CNDO calculation of molecular systems including up to 232 orbitals," Zh. Strukt. Khim., 18, No. 2, 414-415 (1977).

3. A.M. Nesterenko and V. G. Maslov, "Programs for optimization of geometry of molecules in basis of up to 232 orbitals in the CNDO/2 and INDO approximations," in: Summaries of Papers, 5th All-Union Conference "Use of Computers in Spectroscopy of Molecules and in Chemical Research," Novosibirsk, September 9-15, 1980 [in Russian], Novosibirsk (1980), p. 154.

4. P. Scharfenberg, "Eine Variante der CNDO-Verfarens unter Einbeziehung von d-Funktionen," Theor. Chim. Acta, 49, No. 2, 115-122 (1978).

5. A. Almenningen, B. Andersen, and E. E. Astrup, "An electron diffraction investigation of the molecular structure of fluoro-N,N-dimethyl-l,2,3,4-diazadiphosphetidine, (F3PNCH3)2, in vapor phase," Acta. Chem. Scand., 23, No. 6, 2179-2180 (1969).

6. M.E. Hewlins, "Crystal structure of p-bromophenylimino(triphenyl)-phosphorane," J. Chem. Soc. B, No. 5, 942-945 (1975).

7. J.C. Adamson and J. C. Bart, "Crystal and molecular structure of diphenylfluoro-N- methyl-phosphine imide," J. Chem. Soc. A, No. 9, 1452-1456 (1970).

8. I.N. Zhmurova and A. V. Kirsanov, "Trichlorophosphazoaryls," Zha Obshch. Khim., 30, No. 9, 3044-3054 (1960).

9. R.B. Woodward and R. W. Hoffman, The Conservation of Orbital Symmetry, Verlag Chemie, Weinheim/Bergstr. (1970).

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