Organic Magnetic Resonance, 1972, Vol. 4, pp. 831 to 836. Heyden & Son Limited. Printed in Northern Ireland

14N AND 35Cl NQR STUDIES OF ORGANOPHOSPHORUS COMBOUNDS

D. J. OSOKIN, I. A. SAFIN and I. A. NURETDINOV Kazan Physico-technical Institute of the Academy of Sciences of the

USSR, Lobachevskogo 2/31,420084 Kazan 84, USSR

(Received 6 March 1972; accepted 18 May 1972)

Abstract-lPN and 35Cl NQR spectra have been investigated for 24 organophosphorus compounds using a pulse technique. The electron populations of the nitrogen lone pair orbital and the N-P bond are calculated according to the Townes and Dailey method. The experimental data are inter- preted assuming a partial double bond character of the N-P bond due to thep,-d, interaction and p,--a conjugation of the lone pair electrons of the nitrogen atoms. The effect of the different nature of substituents X on the N-P bond populations is observed in X = PR,'(R,N),-, molecules (where X is 0, S, Se, or lone pair electrons and n = 0, 1 , 2). It can be seen from this dependence that the effective electronegativity of the phosphorus atom is largest in selenophosphoramidates and falls in the sequence P=Se > P=S > P=O > P.

I N T R O D U C T I O N

3 5 C ~ NUCLEAR quadrupole resonance (NQR) spectra have been reported for a number of organophosphorus compounds.1*2 In our opinion a simultaneous study of the NQR spectra of both chlorine and nitrogen nuclei seemed to be much more informative and useful for drawing conclusions on the electronic structure and reactivity of certain types of phosphorus compounds. The insufficient sensitivity of the conventional steady-state NQR technique, however, greatly restricts a wider application of the 14N NQR method for this purpose. The pulse technique is much more sensitive and partly removes this restriction3 In this paper we would like to report the results of an attempt to use the pulse technique for a simultaneous study of 14N and 35Cl NQR in molecules of organophosphorus compounds containing N-P bonds.

EXPERIMENTAL Most of the compounds studied in this work were prepared by adaptations of known procedures.'

The element content of each compound was checked by analysis and comparison of the constants (ng', di" and b.p.) with the corresponding literature data. 31P NMR spectra of these compounds provided an additional confirmation of purity. Synthesis of the compounds 22 to 24 (Table 1) will be described elsewhere.

Measurements of 14N NQR frequencies were performed with a coherent pulse ~pectrometer.~ The NQR signals were recorded, following a phase sensitive detector and box car integrator. The sample volume was about 3 cc. A11 measurements were made at liquid nitrogen temperature. As an example, one of the spectra recorded is shown in Fig. 1.

RESULTS A N D DISCUSSION

As can be seen from Table 1, 14N nuclear quadrupole resonance (NQR) spectra of bis- and tris-(N,N-dimethy1)amides of phosphorus acids consist of two or three lines, respectively, (in the spectrum of 13 there are two groups of lines, one of which is twice as intense as the other). 35Cl NQR spectra of N,N-di-alkylamidodichloro- derivatives of phosphorus acids are also split into doublets due to the inequivalent positions of the chlorine atoms. The largest splitting is observed in the spectrum of 2. It is difficult to explain the latter fact only by intermolecular interactions in the solid phase. Probably this splitting is caused (to a certain extent at least) by different distri- bution of electron density around the nitrogen atoms in both amido groups of 2.

83 1

832 D. J. OSOKIN, I. A. SAFIN and I. A. NURETDINOV

TABLE 1. NQR DATA FOR AMIDES OF PHOSPHORUS ACID

Parameters of the 14N NQR spectra 35c1 NQR V- v+ V q Q ) frequencies,

N O Compound MHz MHz MHz % MHz

1 2 3 4 5 6 7 -

3.423 3.387 3.368 3.361 3.336 3.307 3.277 2.9116 3.5498 3.3386 3.4230 3.3412 3.3684 3.3113 3.3696 3.3955 3.4607 3.1903 3.4210 3.0230

3.0792

3.0749 3.0900

2.9834 3.1981 3.6452 3.6457 3.6061 3441 1 3.3117 3.7550 3.5056

3.5595

2.261 1 2.2873

3.3392 3.6464 3.7897 3.414 3.536 3.4848

4.0145 4.003 3.9765 3.965 3.943 3.816 3.8075 3.4193 3.8810 3.8970 3.9628 3.8559 3.8766 3.8803 3.9478 3.9708 4.0653 3.7667 3.9635 3.5941

3.3784

3.3744 3.3925

3.2643 3.7951 4.0866 4.0868 4.0453 3.8682 3.6252 3.9443 3.5954

3.9356

3.3591 3.3778

3.7735 3.9286 4.0194 3.536 3.645 3.5264

4.856' 4.2206 4.9538 4.8238 4.9239

4.8140'

4.90008 4.5780 4.9230 4.41 14

4.3051

4.31068

4.1653 4'6622 5.1546 5.1550 5.1003 4.8727 4.6246 5.1329 4.7340

4.9967

3.7616a

4.7418

5.12g8

4.6741

23.9" 24.06 13.4 23.15 21.19

21.25'

23-7' 25.22 22.04 25.9

13.9

13.97"

13.54 25.7 17.26 17.10 17.2 17.5 13.56 7.38 3.8

15.05

58.28

18.3

9.98

1.8

18.508

23.135 24,450 23.692 23.892 24.098 24.314

22.224

27.073e 25.785

2444

28.095= 26.990

14N and 35C1 NQR studies of organophosphorus compounds 833

TABLE 1 (contd.)

Parameters of the 14N NQR spectra 35c1 NQR V - y + (e'qQ) 9 frequencies,

NO Compound MHz MHz MHz % MHz

1 2 3 4 5 6 7

H,C \ /

for 1 N-groups

3.6181 3.8171 4.9538 8.05

2.0595 3.2290 2.1026 3.2764 3.5548' 66.10

3.3728 3.7849 4.7718 17.2 3.6358 3.8634 4.9994 9.1 3,3169 3.7453 4.7080 18.2 3.6448 3.9915 3.7940 3.8914 5 ~ 1 0 7 2 ~ 8.69' 3.3397 3.6435 4.6621 13.1 3.6556 3.8483 5.0026 7.7 3.8485 4.0175 5.2440 6.4

a Average values. These data were reported at the Annual Scientific Conference of the A. E. Arbuzov Institute of

In 1 some of the 14N NQR lines were detected by L. Krauze and M. A. Whitehead [J . Chem.

In 18 the 14N NQR spectrum consists of three broad (about 60 kHz) lines, which have the in-

G. K. Semin and T. A. Babushkina, Teor. Eksp. Khim. 4, 835 (1968).

Organic and Physical Chemistry of the Academy of Sciences of the USSR, Kazan, 1970.

Phys. 52, 2787 (1970)l.

tensity ratio of 1 :2: 1 .

4.1 4.0 3.9 3.0 3.7 3.6 3.5 3.4 3.3 FIG. 1. 14N NQR in [N(CH,),],PC,H, at 77°K. The rate of frequency sweep was about

5 kHz min-'. Frequencies are given in MHz.

The conformational inequality of the nitrogen and chlorine atoms is probably re- sponsible for the inequivalent positions of these atoms in the compounds considered, as soon as the electron-donating and electron-accepting properties of the substituents at phosphorus depend substantially on the orientation of the electron lone pair orbitals of a donor and the vacant orbitals of an acceptor.

Values for the population of the lone pair orbitals, the N--P bond of the nitrogen atoms and the CI-P bonds are presented in Table 2. These values were calculated from our experimental data in terms of the Townes and Dailey theory6 with the help of the formulae.' The population of the N-C bond was assumed to be equal to

834 D. J. OSOKIN, I. A. SAFIN and I. A. NURETDINOV

TABLE 2. ELECTRON POPULATIONS OF NITROGEN AND CHLORINE ORBITALS

Populations of 14N orbitals Populations of the Compound Lone pair N-P a-bond chlorine a-bond

3

1.874 1 .so 1.865 1.869 1.875 1.863 1.879 1.841 1.879 1.817 1.79 1 .I7 1.85 1.90 1.898 1.896 1.865 1.91 1.824 1.834

1.874

1.88 1.85

1.81 -

1.852

1.863 1.851 1.876 1.844 1.861 1.829 1.884

4

1.393 1.36 1.33 1.388 1.382 1.375 1.394 1.386 1.394 1.399 1.32 1.28 1.397 1.355 1.355 1.35 1.35 1.255 1.35 1.256

1.256

1.32 1.36

1.256 -

1.393

1.304 1.347 1.303 1.357 1.296 1.322 1.295

5

1.5978

a s-Character of the CI-P hybrid a-bond has been taken equal to 15 %.

c Average values. For (CH,),N-groups.

Calculated by using the data of Livingstone [R. J. Livingstone, J. Php . Chem. 57, 496 (1953)l.

1.25 in all compounds with sp2-hybridisation for the nitrogen atom.* The quadrupole coupling constant (QCC) produced by a 2p electron at the nitrogen atom is supposed to be equal to 8.4 MHz.

Changes in the lone pair orbital population of the nitrogen atom and the N-P o-bond are rather small among bis-(N,N-dialky1)-amide derivatives. Of the substit. uents listed in Table 2, the chlorine atom has the largest influence upon the nitrogen

I4N and 35CI NQR studies of organophosphorus compounds 835

orbital population. Decrease in the orbital population of the a-bond can probably be explained by the inductive effect of the chlorine atom in this particular case. On the other hand, the fact that the inductive effect of other substituents is negligible requires an additional explanation. I t is quite possible that in the case of compounds containing substituents other than chlorine, the inductive effect can be masked by a stronger one (the mesomeric effect, for example). On the other hand, the phosphorus atom can display properties of an ‘electronic capacity’, due to which transmission of the inductive effect becomes difficult. The electron-donating properties of the sub- stituents fall in the order F > C1 > Br, which results in a decrease in the orbital population of the N-P bond in N,N-dialkylamidohalogenophoramidites (7 to 9, Table 2). Substitution of the halogen atoms by alkyl or arylgroups, however, increases the population of the N-P a-bond. I t is apparent that the N-P a-bond polarity depends on both electron-donating properties and electronegativity of substituents at the phosphorus atom.

The concept of the px-a conjugation appeared to be useful for explanation of the electron density distribution in the molecules of amides of phosphorus acids. The same concept has been used by Lucken for the interpretation of the anomalously low 35Cl NQR frequencies in the spectra of chlor~fluoromethanes.~ In terms of the valence bond theory a contribution of the resonance structure RRC=F+Cl- with the charge transfer from electron-donating groups to the chlorine atom could be responsible for a substantial decrease of the 35Cl NQR frequencies. This effect should result in a simultaneous increase of electron density at the Cl-P a-bond and a complementary decrease of electron density at the nitrogen lone pair orbital. As compared to N,N-dimethylamide of diethylphosphinous acid ( I l ) , where the electron affinity of ethyl groups and thus the contribution of the structure with the charge transfer are extremely low, there is a deficit of electron density equal to 0.11 units at the lone pair orbital of the nitrogen atom in compound 8. The population of both CI-P a-bonds in 8 exceeds by 0.10 units that in PCl,, where the pn-a conjugation can be neglected. In 2 these values are equal to 0.14 (for both amido groups) and 0.15, respectively. The results obtained prove the existence of a correlation between the dx-a conjugation and the electron density distribution in the compounds considered. The dependence of the electron density deficit at the lone pair orbital of nitrogen on the electron-accepting properties of the substituents should be a logical consequence of the abovementioned pn-a conjugation. One can notice this relationship when looking through Table 2.

The interaction between the lone pair electron orbital of nitrogen and the vacant d-orbitals of phosphorus is the other effect which results in a decrease of populations of the former orbitals. In this case the electron-donating substituents should give rise to a competition for the vacant d-orbitals of phosphorus and thus lead to an enlargement of the electron lone pair orbital populations and the QCC of the nitrogen atom. This dependence manifests itself in N,N-dialkyldichlorophosphoramidites 7 to 9. The electron affinities of fluorine, chlorine and bromine atoms are approxi- mately the same, and consequently one should expect almost the same population of the electron lone pair orbital of nitrogen in these compounds. The electron-donating properties of the substituents under discussion vary in the following order Br < C1 < F. The QCC and the population of the lone pair orbital of nitrogen vary in the same way (Table 2). Alkyl groups in the R,NP( :) and R,NP(X) (X = 0, S, Se) are unable

836 D. J. OSOKIN, I. A. SAFIN and 1. A. NURETDINOV

to act as electron donors. Therefore one should expect a more pronounced interaction between the amido groups R,N- and the phosphorus atom in such compounds, and thus also small values of the QCC and of the nitrogen lone pair orbital population as compared to the halogen-containing compounds. The QCC and the nitrogen lone pair population, however, appeared to be increased, as can be seen in Table 2. This fact points to comparable contributions of the p,,--o and p,-d,, interactions to the electron density distribution in the molecules of the compounds investigated.

Values of the asymmetry parameter of the electric field gradient at the nitrogen 14N nucleus and the N-P bond population in phosphoramidates are less than those in phosphoramidites. This is in accordance with the higher electronegativity of the four-coordinated phosphorus atom. The increase in the effective electronegativity of the phosphorus atom can be explained by a rise in the positive extra charge on the latter due to the donor-acceptor nature of the semipolar P -+ X bond. The reverse charge transfer from X due to p,-d, interaction of the lone electron pair of X (X = 0, S, Se) and the vacant d-orbital of the phosphorus atom is unable to balance the positive charge on P. The 14N NQR data show that the effective electronegativity of the phosphorus atom is largest in selenophosphoramidates and falls in the sequence P + Se > P -+ S > P -+ 0 > P. Thus, there is no doubt that the degree of multiple bonding of P -+ 0 is larger than that of P --f S and P + Se.

The 14N QCC and the population of the electron lone pair orbital of nitrogen in N,N-dimethylamides of the phosphorus acids 5 and 3 practically do not depend on the co-ordination of the phosphorus atom. This may be caused by two different mechan- isms. Firstly, ability of the vacant d-orbitals of phosphorus to accept electrons is very little affected by the positive extra charge in the P - X bonding. Secondly, an enhancement of the electron-accepting properties of phosphorus in its four-co- ordinated compounds is compensated by an increase in the number of substituents capable of acting as electron donors.

R E F E R E N C E S 1. E. A. Lucken and M. A. Whitehead, J . Chem. SOC. 2459 (1961). 2. G. K. Semin and T . A. Babushkina, Teor. Eksp. Khim. 4,835 (1968). 3. I. A. Safin, B. N. Pavlov and D. J. Shtern, Zavodsk. Lab. No 6,40 (1964). 4. I. A. Nuretdinov and N. P. Grechkin, Izv. Akad. Nauk SSSR, Ser. Khim. Nauk 1883 (1964); V.

Sasse, Merhoden der organischen Ckemie, XII/I , XII/2, Georg Thienie Verlag, Stuttgart, 1964. 5. I. A. Safin and D. J. Osokin, Pribory Tekhn. Eksperim. No 1, 154 (1971). 6. C. H. Townes and B. P. Dailey, J. Chem. Phys. 17,782 (1949); 23, 118 (1955). 7. D. J. Osokin, I. A. Safin and I. A. Nuretdinov, Dokl. Akad. Nauk SSSR 190, 357 (1970). 8. L. V. Vilkov and L. S . Haykin, Dokl. Akad. Nauk SSSR 168, 810 (1968). 9. E. A. C. Lucken, J. Chem. SOC. 2954 (1959).