5425
Communications to the Editor
the observation of two new, corresponding signalsI2 of 4 ( b in Time-Resolved CIDEP and ESR Studies 4a). On the other hand, 3 and 4 behave like enantiomers in of Heavy Metal-Organic Radical Complexes. their chiroptical properties with respect to nickel, i.e. [0]630 ca. The Uranyl-Phenanthroquinone Radical Ions - 1 190 and 1260. Therefore, the diastereomerization 3 Sir: 4 can also be monitored by time-dependent circular dichroism (mutarotation). The interconversion rates are cleanly first The rather turbulent debate as to the origins of the CIDEP order for 3 in tetralin solution; both methods define a common (chemically induced dynamic electron polarization) observed Eyring plot with AH* = 22.5 (40.8) kcal/mol and AS* = in the ESR spectra of photogenerated radicals has abated, and, - 1 5 (f3)cal K-' mol-'. The AG* values of 27.0 (at room as it is now generally accepted that there are two distinctly temperature) or 29.3 kcal/mol (at 453 K for comparison with different mechanisms,' interest has turned to chemical ap1) thus confirm the lower limit2 of 321.8 kcal/mol for 1 ( R ' plications. While almost all of the CIDEP initial polarization = C6H5, R2 = CH3, R3 = C2H5). systems reported to date involve the photochemical triplet of Does the bulky bornane skeleton perturb such barriers? W e organic carbonyl compounds,l our recent efforts in CIDEP measured the inversion 5 + 6 for comparison with the known2 applications have been mainly directed towards metal-quinone activation enthalpy 17.3 kcal/mol (at 358 K in cyclohexane) complexes, particularly the o-phenanthroquinone (PQ).2 We of 1 (R' = R2 = CH3, R 3 = C2H5). Both processes are fast on report here our first successful application of time-resolved the N M R time scale and can be followed by coalescence CI DEP to heavy metal-organic radical complexes: the urastudies on several pairs of diastereotopic protons. Although 5-6 nyl-phenanthroquinone radical ions. Historically the photois too sensitive to be isolated in pure form, its very large and chemistry of uranyl ion has played an important role3 in the characteristic chemical ' H N M R shifts permit the easy evaldevelopment of modern photochemistry. The CIDEP results uation of AG* = 17 ( f l ) kcal/mol (at 345 K in benzenewill shed some light on the primary photochemical processes tetralin). Thus a t least for R' = CH3 the comparison of 5-6 of uranyl ions and the ESR characterization of the uranylwith the above 1 shows no distinct perturbation by the bornane quinone complex ions should be of wider interest to chemistry moiety. in general. Barriers of such heights are very unusual'3 for open-shell The laser flash photolysis (Molectron 1-MW N2 pulsed tetrahedra. When diastereomerization of 3 was performed in laser) and the time-resolved dc detection CIDEP observation the presence of racemic ligand, the third possible, completely system were assembled similarly to those reported by Kim and asymmetric (C, in 7a) diastereomer 7 was also o b ~ e r v e d ' ~ ~ W ' ~e i ~ s r n a n .The ~ total spectrometer d c response time was as R R S (and/or its antipode2 R S S ) . Both 4 and 7 N M R measured and found to be 0.2 ws. A detailed examination of signals appear with comparable rates but now somewhat faster the system performance and the analysis of the relaxation than in the absence of free ligand. Since 7 can be formed from measurements will be described elsewhere. 3 only by ligand exchange, it is clear that substitution can be When a degassed T H F containing M each of PQ and faster than configurational inversion. Some mechanistic feaUOr(N03)2-6H2O was exposed briefly to light in the ESR tures of these competing pathways will be published shortly. cavity with 100-kHz modulation a t -60 O C , a well-resolved spectrum was observed (Figure 1A). Prolonged UV irradiation led to the disappearance of the spectrum and in its place a new Acknowledgment. We are grateful to Professor E. Wunsch, Dr. E. Jaeger, and Dr. S . Knof (Abteilung Peptidchemie spectrum (Figure I B) was developed when irradiation was terminated. Both spectra are characterized by a distinctly low des Max-Planck-Instituts fur Biochemie, Martinsried bei g factor and their ESR parameters are given in Table I . The Munchen) for access to and intense technical assistance with analyses of the hyperfine structures are consistent with the the dichrographe. This research is supported by the Stiftung assignments of [UO2PQ]+. and [U02HPQl2+., respectively, Volkswagenwerk and the Deutsche Forschungsgemeinin which the unpaired spin is associated mainly with the pheschaft. nanthroquinone moiety. The low g factor can be explained by References and Notes the coordination to the uranium nucleus having a large spinorbit coupling. Hyperfine interaction due to 235U ( I = 7/2, Sheldrick. W. S.;Knorr. R.; Polzer, H. Acta CfystaIIogr., Sect. B 1979, 35,
+
+
739-741. Knorr, R.; Weiss, A.; Polzer, H.; Rapple, E. J. Am. Chem. SOC.1977, 99, 650-65 1. Knorr, R.; Weiss. A. J. Chem. Soc., Chem. Commun. 1977, 173-174, 392. Knorr, R.; Weiss. A.; Polzer, H. Tefrahedron Left. 1977, 459-462. Whitesides. T. H. J. Am. Chem. SOC. 1969, 91, 2395-2396. Elian, M.; Hoffmann, R. Inorg. Chem. 1975, 74, 1058-1076. Lohr. L. L. J. Am. Chem. SOC. 1978, 100, 1093-1099. R is used as a simplified notation for the 1R4R configuration of &(+)camphor. McGeachin, S.G. Can. J. Chem. 1968,46, 1903-1912. Structural assignments of these compounds were based on full spectroscopic characterization and correct elemental analyses. Configurational assignments at nickel are arbitrary but in no way relevant for the conclusions drawn here. Optically active 3 crystallized in pure form from ethyl acetate-ethanol: mp 187-189 OC;molecular mass (benzene) calcd 718, found 690; magnetic dipole moment, found 3.14 in (CIpCD)*at 26 O C . Knorr. R.; Polzer, H.;Bischler. E. J. Am. Chem. SOC. 1975, 97, 643644. The reduced NMR shifts" of these 4-H's are iY +25.7 and +29.3 ppm for 3, 4-27.0 and +28.3 ppm for 4, +26.4 and +30.5 ppm for 7. See ref 2 and 5 for some relevant references. The resultant mixture showed the same NMR shifts as a solution of the nickel complex8 (mp 237.5-239 "C)prepared from the racemic ligand.
Rudolf Knorr,* Friedrich Ruf Institute of Organic Chemistry. University of Munich Karlstr. 23, 8000 Munich 2, West Germany Received April 9, I979 0002-78631791 I501-5425$01 .OO/O
Figure 1. ESR spectra of ( A ) [UOzPQ]+ in T H F at -60 OC, (B) IUO?HPQ]?+i n T t I F a t -60 OC.
0 I979 American Chemical Society
Journal of the American Chemical Society
5426
/
101:18
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August 29, 1979
’Fable 1. ESR Parameters“ of Various Phenanthraquinone Complex Radicals in THF complex
T , OC
g
[ UO?PQ]+* [ UOlH PQ] 2+*
-60 -60 20 20 20
I ,9940 f 0.000 I 1.9945 2.0045 2,0046 2.0005
PO-. K+PQ-. C H 3 Hg- PQ
G I .43 0.16 (methyl
UH(1.I’)
0 H (2.2‘)
UH(3.3‘)
aH(4.4’)
I .86 I .86 I .54 I .43 1.69
0.43 0.43 0.32 0.26 0.4 I
1.86 1.86 I .70 I .68 1.85
0.43 0.43 0.4 I 0.44 0.4 I
The numbering of protons follows the same convention as in ref 5. The extra proton in [UO>HPQ]*+.is assumed to be placed between thc two carbonyl oxygens natural abundance 0.71%) was not resolved. The assignment of the primary [UO2PQ]+. radical ion was further supported by a separate ion-exchange experiment5 in which the K+PQ-. ion pair was first prepared in T H F by metal reduction. Upon addition of uranyl ions via a side-arm arrangement exchange took place and the resulting [UO2PQ]+. (or the ion pair U022+PQ-.) gave a spectrum identical with that in Figure I A. Time-resolved CIDEP studies in the laser flash photolysis reveal a large initial polarization in [UO2PQ]+. but not in the secondary [U02HPQ12+. ion. A typical transient response a t -60 OC from the laser generated [U02PQ]+. ion is shown in Figure 2. In the absence of uranyl ions, neither thermalized nor polarized PQ. was observed. However, laser flash photolysis of a T H F solution containing PQ and triethylamine yields a strongly polarized PQ-. exhibiting both initial and radical-pair polarizations. With the uranyl-PQ system both the uranyl ion and P Q absorb a t the laser wavelength of 337 nm. However, in the experiment the absorption by PQ at 337 nm is approximately two orders of magnitude larger than the uranyl ion. The initial polarization can thus be accounted for by the mechanism shown in eq 1-4 where * denotes electron spin polarization. Reactions 3 and 4 can be regarded as a single step involving the spin-polarized triplet PQ reacting with U0.2+.nHz0 to form [UOzPQ]+-. Subsequent experiments using traces of added acid or base indicate, however, that the stepwise mechanism is more appropriate. PQ
+ 337 nm
3PQ* t UO2’+nH?O
ISC w+
+
3PQ*
‘(U02’+-nHrO)*
+
~ ( u o ~ ~ + . ~ H -* ~ ouoz+.* )* H+ UOr’.*
+ PQ
(1)
-
+ PQ
+ OH.
[UOlPQ]+**
(2)‘ (3) (4)
From the analysis of the transient response of the polarized [UOlPQ]+. radical ion, an estimated T Iof
