STRUCTURE AND ULTRAVIOLET SPECTRA OF

ARYLPHOSPHINES AND ARSINES

R. R. Shagidullin and A. V. Chernova

SOME

UDC 541,6 + 543.422.6 + 547,558,1/2

In our previous papers {I, 2] where the effects of conjugation in organophosphorus and organoarsenic compounds were studied employing optical spectroscopy methods we discussed the UV spectra of some fl- styryl- and phenylphosphines, and also some phenylarsines [I, 2]. In the present paper we wish to discuss some of the properties of the electronic spectra of trivalent phosphorus and arsenic derivatives. In the UV spectra of aromatic phosphines, in most cases two bands are observed in the region >200 nm: an in- tense short-wave band with a maximum around 220 nm, and a long-wave band of medium intensity in the 250-270 nm region [1-7]. In its nature, the short-wave band is analogous to the K-band of benzene, which exhibits bathoehromic shift due to the insertion of the phosphorus atom into the molecule [3-7]. Various opinions were expressed regarding the interpretation of the long-wave band. Some authors [3, 6] compare it to the B-benzene band, while other authors [4, 5] assign the long-wave band to the n --~ ~*-transition (R-type). If the last viewpoint is valid, then in the UV spectra of the aryl-substituted phosphines should be observed three bands, namely K, B, and R, in the mentioned region. The existence in publications, de- voted to the UV spectra of arylphosphines, of information on only two bands, is explained by the superim- position of the stronger R-band on the B-band [7]. In order to corroborate this viewpoint the spectra, in which all three bands are observed separately, have decisive importance.

In the series of compounds studied by us: C6HbCH = CHP(OC2Hb) 2 (D, CsHbCH = CHPCI2 (If), C6H5 ,P(OC2Hb) 2 (Ill), C6HbPCI 2 (IV), (CGHb)2PCI (V), p-CH3CGH4As(C3H7)2 (VI), and p-CH3OC~HtAs(C2Hb) 2 were obtained data that are interesting from this viewpoint. In the UV spectrum of p-tolyldipropylarsine (VI) can be distinctly seen three bands (kK m ~225 nm; log e 4 48; k R .. ~245 nm; log c 3 96; XBmax 275 nm ax " maz~ log e 3.04, $ Fig. I~ curve 3). The validity of assigning the band with a maximnm at 245 nm to the n -- ~-*-

transition is supported by the fact that the tying up of the unshared pair log

�9 " L \ 1

,W t , z }

r , { ~A r

zoo ko ' 2 'o

Fig. 1. Ultraviolet spectra: 1) p-(CH30)C4H4As(C2Hb) z; 2) p-(CH30)C GH4As(O) (C~H5)2; 3) p-CH3CGH4As(C3HT)2; 4) p-CH3C GH4As(O) (C 3H7) 2-

during oxidation of the arsine leads to disappearance of this band (curve 4). The insertion of a stronger electron-donor substituent, for example, the methoxy group in the p-position [compound (VII)] should lead to shifts of the K- and R-bands in opposite directions [8]. In view of the increase in the conjugation, the first bands exhibit bathochromie shift, while the second exhibit hypsoehromic shift since in the latter case an electron-donor grouping hinders the n --~ ~r*-transition. Actually, in the spectrum of (VII) (curve I) the K- and R-bands prove to be combined and the B-band can be seen distinctly, with a fine structure of k Bma x 283 nm (log e 3.49). Oxidation of the product should lead to a disappearance of the R-brand, which is in agreement with a contraction of the peak at 240 nm in the spectrum of the anisyl diethylarsine oxide (curve 2).

The presence of a B-band in the spectrum of compound (IK) (Fig. 2b, curve 1) can be judged by the weak indication of a vibrational struc- ture on the long-wave shoulder" of the R-band, with X max 253 nm. The

there and subsequently the maximum of the extreme long-wave peak with a fine structure is indicated for the B-band.

A, E. Arbuzov Institute of Organic Chemistry, Academy of Sciences of the USSR. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimieheskaya, No. l , pp, 183-185, January, 1971. Original article submitted April 24, 1970.

�9 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, ! N_ Y. 10011. All rights reserved. This article cannot be reproduce~ for any purpose whatsoever without 1 permission of the publisher. A copy of this article is available from the publisher for $15.00.

165

5,D

4~

zog

-log 8

J \ /'",,

f f O 2~g 780 Bg 300 fgg ZfO f~g fGg fSg .J,)l'lm .g ~ Fl m

Fig. 2

log 6 ~

'~iq z r I i i i A~.nm

Fig. 3

Fig. 2. Ul t rav io le t spec t ra , a: 1) C6HsPC12; 2) (C6Hs)21DC1; 3) C6HsP(O)C12. b: 1) C6HsP(OC2Hs)2; 2) C6HsP(O)(OC2Hs)2.

F ig .3 . Ul t ravio le t spec t ra : 1) C6HiCH = CHP(OC2Hs)2; 2) C6HsCH = CHP(O) �9 ( O C 2 H 5 ) 2 ; 3) C6HsCH = CHPC12; 4) C6HsCH = C H 2.

l a t t e r d i sappea r s when going to the phosphoryl der ivat ive , and on this bas i s is ass igned to the n ~ 7r*- t ransi- tion. The addition of e lec t ronega t ive chlorine to phosphorus [compounds (IV) and (V}] should faci l i ta te conjugation of the benzene ring with the d -o rb i t a l s of phosphorus [9], and cause a ba thochromie shift of the K-band when compared with compound (TIT). Here the n ~ 7r*- t rans i t ion is hindered, i . e . , the R-band un- de rgoes hypsochromic shift . In the s pec t r a of compounds (IV) and (V) (Fig. 2a, cu rves 1 and 2, r e s p e c t i v e - ly) the long-wave bands can be in te rp re ted as being f reed of super impos i t ion by the B-band. The m a x i m u m in the 230 nm region in the s p e c t r a of these compounds can be intel l igently a s sumed to be fo rmed as the r e su l t of super impos i t ion by the shifted toward each other K- and R-bands .

The UV spec t rum of diethyl f i - s ty ry l phosphonate is analogous to the spec t rum of s ty rene (Fig. 3, cu rves 2 and 3, respec t ive ly) , all th ree bands (A, K, and B) of which exhibit ba thochromic shift due to in- t e rac t ion with the phosphorus a tom. Here the K-band, which is e x t r e m e l y sens i t ive to resonance effects [10], undergoes the g r ea t e s t shift and m a s k s the B-band. A marked broadening of the K-band toward longer wavelengths is obse rved when going to the s pec t r a of compounds (I) and (II) (curves 1 and 3). Fo r compound (1) it can be seen that this broadening occurs due to the appearance of a shoulder . On this bas i s it is pos - s ible to a s sume that the indicated broadening tes t i f ies to the appearance of the additional band of the n ~ 7r*- t rans i t ion in the s p e c t r a of the s ty ry l de r iva t ives of t r ivalent phosphorus .

As a resu l t , the obtained exper imen ta l data sugges ts the exis tence of n ~ v * - t r ans i t ions in the m o l e - cules of the phenyl compounds of t r iva len t phosphorus and a r sen ic . This conclusion is in ag reemen t with the fact that in the ground s ta te the i r unshared p a i r s of e lec t rons are local ized on the h e t e r o a t o m s and do not take p a r t in the conjugation with the ~r-electron s y s t e m [7].

It had been prev ious ly es tab l i shed [7, 11] that t r iva lent phosphorus in the ground s ta te exhibits a - M - effect, i . e . , i t s d -o rb i t a l s take p a r t in the conjugation with the ~r-electron sy s t em. The optical s p e c t r o . scopy data [1, 2], re la t ing to the exci ted state, a re in a g r e e m e n t with this m e c h a n i s m of e lec t ronic i n t e r - action in both phosphorus and a r sen ic de r iva t ives . We had mentioned [1] that when going f r o m the f i - s ty ry l and phenyl de r iva t ives of t e t racoord ina ted phosphorus to the cor responding t r ivalent phosphorus de r iva t ives the optical indications of conjugation a re mani fes ted m o r e c l ea r ly in a number of cases [compounds (II) and (IV) ]. An examinat ion of the UV s p e c t r a of the d i scussed compounds r evea l s st i l l another in te res t ing c h a r - ac te r i s t i c . In those cases where the B-band is c l ea r ly seen, for example , in the s p e c t r a of compounds (VI) and (VII), it is located for the t r ivalent de r iva t ives at somewhat longer wavelengths than for the compounds with t e t racoord ina ted he t e roa toms .

E X P E R I M E N T A L

The UV spec t r a were taken on an SF-8 spec t ropho tomete r . The solutions were p r e p a r e d in n-hexane, except for the a r sen ic de r iva t ives . The l a t t e r were d issolved in dioxane. The concentra t ion was approx- imate ly 5- 1 0 - 2 M / l i t e r . The p r e p a r a t i o n of the solutions and the filling of the cel ls was done in a c h a m b e r filled with argon.

166

CONCLUSIONS

I. The appearance of R-bands in the UV spectra of arylphosphines and arsir~es was confirmed.

2. The bathochromic shift of the B-bands for the trivalent arsenic derivatives is greater than for the tetraeoordinated analogs.

LITERATURE CITED

i. R.R. Shagidullin, A. V. Chernova, V. S. Galeev, and Ya. A. Levin, Data of Papers Delivered at Scientific Conference, A. E. Arbuzov Institute of Organic and Physical Chemistry, Academy of Sciences of the USSR [in Russian], Kazan' (1969), p. 138.

2. R.R. Shagidullin, B. D. Chernokal'skii, I. A. Lamanova, A. S. Gel'fond, and G. Kho Kamai, Izv. Akad. Nauk SSSR, Ser. Khim., 1490 (1969).

3. H.H. Jaffe and L. D. Freedman, J. Am. Chem. Soc., 74, 1069 (1952). 4. C.N.R. Rao, J. Ramaehandran, iVi. S. C. lab, S. Somasekhara, and T. V. Rajakumar, Nature,

183, 1475 (1959). 5. H. Goetz, F. Nerdel, and K. H. Wieehel, Ann. Chem., 665, ! (1963). 6. G.P. Shiemenz, Tetrahedron Letters, 2729 (1964). 7. B.I. Stepanov, A. I. Bokanov, and B. A. Korolev, Teor. Eksp. Khim., 4, 354 (1968)o 8. K. Higasi, H. Baba, and A. Rembaum, Quantum Organic Chemistry [Russian translation], Mir (1967),

p. 214. 9. R, Hudson, Structure and Mechanism in Organophosphorus Chemistry [Russian translation], Mir

(1967), po72. I0. L. Doub and J. M. Vandenbelt, J. Am. Chem. Soc., 7__1, 2414 (1949). II. E.N. Tsvetkov, M. M. Makhamatkhanov, al~d M. I. Kabaehnik, Teor. Eksp. Khim., 3, 824 (1967).

167