J . Phys. Chem. 1986, 90,6749-6750 Technology was supported by National Science Foundation Grant CHE84-19828. C.-M.C. and W.-M.L. acknowledge support from the Committee on Conference and Research Grants of the University of Hong Kong. The Center for Fast Kinetics Research
6749
is supported jointly by the Biotechnology Branch of the Division of Research Resources of N I H (RR00886) and by the University of Texas at Austin. W.-M.L. acknowledges a Croucher Foundation studentship (1984-1986).
Kinetic and Spectroscopic Characterlzatlon of the Diarylphosphonyl Radical, (2,4,6-But3C6H,)@=0 Nicholas Jeremy Winter: Jacques Fossey,* Bruno Beccard, Yves Berchadsky, Francis Vila, Lawrence Werbelow, and Paul Tordo* Laboratoire de Structure et R&activitZ des EspPces Paramagnztiques, CNRS UA 126, Universite de Provence, Marseille Cedex 13, France (Received: September 15, 1986)
The diarylphosphonyl radical, A r 2 M (Ar = 2,4,6-But3C,H2), was generated (1) by hydrogen abstraction from the corresponding phosphine oxide, Ar2P(0)H, and (2) from the precursor, Ar2P(0)CI, by using methods pioneered by Lappert. Kinetic, spectroscopic, and theoretical methodologies were used to explore the structure and stability of this radical. It was discovered that the apparent lifetime of the A r 2 h 0 species was dependent upon the method of generation. For radicals generated from Ar2P(0)H, the first-order decay constant can be represented by the Arrhenius expression, k = 1.5 X loL4 exp(-93 kJ/RT). The ESR spectrum of the "0-enriched radical was uniquely informative. The experimental determination that the I7O isotropic hyperfine coupling is negative, A. = -9.4 G, provides strong evidence that the unpaired electron is delocalized. Ab initio MO calculations concur with this finding.
Recently, a large number of sterically protected phosphorus compounds have been synthesized by various methods.' The ready preparation and easy handling of these compounds opens up broad new possibilities for the generation of persistent, phosphoruscentered radicals.2 Some of the most interesting and more versatile members of this radical family are the phosphonyl radicals, Y2P=0 (Y = R, OR, Ar, X). Because of the intriguing chemical properties of phosphonyl free radicals, it is believed that studies involving these radicals will emerge as an influential area of chemical research. Not surprisingly, electron spin resonance (ESR) has been the principal spectroscopic technique used to study the physicochemical properties of phosphonyl radical^.^ However, previous investigations have afforded little insight or comment regarding the electronic structure or modes of decay of these species. It is the purpose of this Letter to address these fundamental topics. The specific radical chosen for this study was the sterically protected diarylphosphonyl radical, Ar2%0, where Ar = 2,4,6-tri-tert-butylphenyl.For one set of studies, this radical was generated by photolysis (1000 W, Hg-Xe lamp) of di-tert-butyl peroxide in the presence of the parential phosphine oxide, Ar2P(0)H. The chosen solvent was cyclopropane. Over temperatures ranging from -30 to +30 OC, the ESR spectrum obtained from this photolyzed mixture was dominated by an intense doublet which was assigned to the Ar2*0 radical.2b The observed phosphorus (isotropic) hyperfine coupling, A,, was 371 G. For phosphonyl radicals, it has been suggested38that an approximate linear relationship exists between A , and the one-bond, hydrogen-phosphorus spin-spin coupling constant, J p H , of the protic parent, Y2P(0)H. This relationship, A , (in G) = 1.52(JpH (in Hz) -230), predicts A, = 377 G for the Ar2P=0, radical (using the value JPH = 478 Hz obtained from the 31PNMR spectrum of Ar2P(0)H). This is in good agreement with the experimental value of 371 G. Once successfully generated from Ar,P(O)H, the subsequent decay of A r 2 M was monitored (by ESR spectroscopy!). It was determined that the decomposition of this radical obeyed first-order kinetics, (-d/dt) In [ A r 2 M ] = k. The associated rate constant University College, London. Thiais, France.
4 LECSO-CNRS,
0022-3654/86/2090-6749$01 .50/0
SCHEME I
% SCHEME I1
1 ERO'
is represented faithfully, throughout the temperature range studied, by the ex ression k = 1.5 X 1014exp(-93 kJIRT). It is presumed that Ar2 -0 undergoes intramolecular hydrogen abstraction producing the radical
L
% which then dimerizes (see Scheme I). The preexponential factor for this isomerization is somewhat larger than for most intramolecular hydrogen abstraction^.^ It is postulated that steric hinderance restricts the structure around the radical center in such (1) Cowley, A. H. Polyhedron 1984, 3, 389, and references cited therein. (2) (a) Gynane, M. A. S.;Hudson, A.; Lappert, M. F.; Power, P. P. J . Chem. SOC.,Dalton Trans. 1980, 2428. (b) Cetinkaya, B.; Hudson, A.; Lappert, M.F.; Goldwhite, H. J. Chem. SOC.,Chem. Commun. 1982, 609. (c) Cetinkaya, B.; Hitchcock, P. B.; Lappert, M. F.; Thorne, A. J.; Goldwhite, H. J. J . Chem. SOC.,Chem. Commun. 1982, 691. (3) (a) Davies, A. G.; Dennis, R. W.; Griller, D.; Ingold, K. U.; Roberts, B. P. Mol. Phys. 1973,25,989. (b) Roberts, B. P.; Singh, K. J. Organometal. Chem. 1978, 159, 31. (c) Davies, A. G.; Griller, D.; Roberts, B. P. J. Am. Chem. SOC.1972,94, 1782. (d) Anpo, M.; Ingold, K. U.; Wan, J. K. S. J . Phys. Chem. 1983, 87, 1964. (e) Anpo, M.; Sutcliffe, R.; Ingold, K. U. J. Am. Chem. SOC.1983, 105, 3580. (4) See, E. G.; Watkins, K. W. J. Am. Chem. SOC.1971,93,6355. Can. J. Chem. 1972,50, 3138. J . Phys. Chem. 1973, 77, 2938. Mintz, K.J.; Leroy, D.J. Can. J . Chem. 1973, 51, 3534.
0 1986 American Chemical Society
Letters
6750 The Journal of Physical Chemistry, Vol. 90, No. 26, 1986
Figure 2. Optimized planar geometry (SCF-UHF, 6-31G*//6-31G*) of
li
the radical H 2 h 0 .
A”O
Figure 1. ESR spectrum of Ar2(20% I7O, Ar = 2,4,6-But3C6H2), recorded at 11 O C in cyclopropane-benzene (50/50) solution.
a way that the ideal conformation for the intramolecular hydrogen abstraction is reached with a minimum change of geometry. Seeking a complementary method of radical generation, we photolyzed ArzP(0)Cl, dissolved in cyclopropane. Over the temperature range, -100 to -50 OC, only the Ar’ radical was generated. This presumably occurred via the photolytic cleavage of a P-C bond. In contrast, photolysis of Ar2P(0)Cl in the presence of an electron-rich olefin (ERO) roduced the A r 2 M radical (along with a slight amount of Arz )Zb The specific ERO used in this study was 1,1’,3,3’-tetraethyl-2,2’-biimidazolidinylidene. The decomposition rate of AT2generated by this technique was again first order. However, the. specific rate constant was significantly larger than when A r 2 M was generated from Ar2P(0)H by the method detailed earlier. Also, this decay rate was dependent upon the concentration of the ERO. Since addition of A r 2 M to the ERO is highly unlikely, it is probable that the increased decay rate of A r 2 M in the presence of the ERO results from an electron-transfer process (see Scheme 11): In order to probe the electronic properties of the A r 2 P e 0 radical, we determined the magnitude and sign of the I7Oisotropic hyperfine coupling. This was effected by procuring a 20% I7Oenriched sample of C13P0 by reacting H 2 ” 0 with PC15.5 The labeled precursor, Ar2P(0)Cl, was then prepared and purified according to the procedure described by Yoshifuji et a1.6 An ESR spectrum obtained subsequent to the photolysis of a 5050 cyclopropane-benzene solution containing Ar2P(0)Cl (20% I7O enriched) in the presence of the ERO is shown in Figure 1. ESR signals from both the I6O (a simple doublet) and I7O (a doublet of sextets) isotopomers are clearly discernable. From the spectrum depicted in Figure 1, it can be determined that the absolute value of the I7Oisotropic hyperfine coupling Ao, is 9.4 G. Although not so obvious, this spectrum also reveals that the sign of A. is negative. The temporal correlation between the
;
(5) Laulicht, D.; Pinchas, M. J.; Sadeh, D. S.;Samuel, M. J . Chem. Phys. 1964, 41, 789. (6) Yoshifuji, M . ; Shima, I.; Inamoto, N. Tetrahedron Lett. 1979, 3963.
31Pand I7O anisotropic hyperfine interactions is responsible for the unique line width variation apparent in Figure 1. This peculiar spectral feature enables the relative signs of A , and A. to be determined.7 From the experimental spectrum reproduced in Figure 1, it was deduced that A , and A. are oppositely signed. Given the large magnitude of A,, one is quite certain that the sign of this coupling is positive, and, therefore, A. is negative. Complete details of this line width analysis will be published elsewhere.* Owing to the dramatic steric bulk of the chosen Ar grouping, the structure of the AT2radical is expected to be quasi-plana:. Hence, the valence-bond structures, Ar2&0 (A) and Ar2P-0 (B), can be considered “main contributors” to the spatial distribution of the unpaired electron. For valence-bond structure A, the (2s)spin density at oxygen would be negative and, therefore, A. would be positive (the magnetic moment of ”0is negative). Conversely, structure B would be associated with a negative Ao. The experimental fact that A,, is negative provides decisive evidence that the va!ence-bond structure B is an important canonic form of the Ar2P=0 radical. To examine the electronic properties of phosphonyl radicals in greater detail, SCF-UHF ab-initio calculations were performed on the simplified H2P=0 radical. To mimic more closely the Ar2species, a planar geometry was imposed. Optimization of bond lengths and angles gave the results shown in Figure 2. From these ab-initio MO calculations, it was determined, for this hypothetical radical, that both phosphorus and oxygen exhibit positive s orbital spin densities (0.106 and 0.0089 respectively). From these spin densities, A , (+385 G) and A. (-14.7 G) were calculated. These theoretical values are in good agreement with the experimental values, +371 and -9.4 G. It is worth mentioning that the planar geometry is not the most stable geometry for the radical, H I M . The preferred geometry is pyramidal with the unpaired electron primarily localized on the oxygen atom (A, = +130 G, A. = -27 G ) . A detailed ab initio study of the structure of phosphonyl radicals, Y2-, bearing different Y substituents, will be published in a forthcoming paper. Acknowledgment. We have benefited from numerous constructive criticisms raised by a most helpful referee. (7) Freed, J. H.; Fraenkcl, G. K. J . Chem. Phys. 1964,40, 1815. Werbelow, L. G.; Marshall, A. G. Chem. Phys. Lett. 1973, 22, 586. (8) Ayant, Y.; Thevand, A. Werbelow, L.; Tordo, P. J . Magn. Reson., submitted for publication.
