405 2
J . Am. Chem. SOC.1990, 112, 4052-4054
following direct adsorption of acetaldehyde on the zeolite catalysts.Il Further exposure of the catalyst to the atmosphere allowed the acetaldehyde to diffuse out and permitted a spectrum of the tars to be obtained (Figure 2c). Alternatively, further heating of adsorbed acetaldehyde in the presence of oxygen resulted in the formation of acetic acid (Scheme I ) . Control experiments in which water was coadsorbed onto the zeolite with acetylene yielded spectra similar to Figure 2b. The highly paraffinic nature of these tars suggests that the required source of hydrogen may be water. Indeed, in samples that were not deliberately exposed to water, the amount of tars formed correlated with the catalyst activation temperature and thus the degree to which water was present in the catalyst. Furthermore, similar tars were observed to form when authentic samples of acetaldehyde on H Y catalyst were subjected to analogous hightemperature studies.', The dynamics of V were probed by dipolar dephasingl) experiments at 298 and 143 K. At 298 K, the 143-ppm peak survived 50 ps of interrupted decoupling. At 143 K, however, the intensity of the 143-ppm peak was greatly attenuated. The 107-ppm resonance was completely suppressed at both temperatures. These results suggest that V undergoes rapid rotation at 298 K about the C-0 axis, with the effect that the dipolar coupling to the directly bound proton is a t t e n ~ a t e d . ' ~At low temperatures, this rotation is slowed down, and the 143-ppm peak is attenuated.
Table I. Electron-Transfer Rate Constants for the Reduction of [Pd(CNMe),]'+ by Substituted Ferrocenes with Determined E I t 2
Acknowledgment. This work was supported by a grant from the National Science Foundation (Grant CHE8700667). E.J.M. is a National Science Foundation Predcctoral Fellow. J.L.W. and M.L. are Department of Education Fellows.
relating to the role of [Pd(CNMe),]'+ radicals as photogenerated oxidants. The [Pd(CNMe),]*+ radical system provides a rare example for direct observation of both reductive and oxidative behavior of an organometallic radical. A key finding is that transfer of an electron to the d9, Pd(1) radical produces a Pd(0) complex which spontaneously deposits palladium as a metal film. Irradiation (A = 3 13 nm) of an acetonitrile solution of 1 (0.013 mmol) and Fe(C,Me& (0.027 mmol) leads to the disappearance of 1 at 307 nm and the appearance of a band at 777 nm characteristic of [Fe(C5Me5)2]'+,as observed by UV-vis spectrophotometry. A new absorbance at 410 nm also appears and corresponds to [Pd(CNMe)2]n.li,12 This band vanishes over a period of several minutes as metallic palladium is deposited within the quartz photolysis cell. We note that palladium(0) isocyanide oligomers [Pd(CNR),], (R = Pr', C 6 H l l , Ph, p-MeC6Ho, p MeOC6H4)are known to decompose in polar Irradiation of an identical sample at 438 nm, within the Fe(C5Me5)z absorbance band, produced no reaction. Similarly, no reaction was observed in refluxed samples of 1 and Fe(C,Me,),. These results indicate that decamethylferrocene is oxidized by photolysis of 1, producing a Pd(0) isocyanide species which is unstable in acetonitrile with respect to the formation of palladium metal. Laser flash photolysis (7 ns, 355 nm) of 1 (1 mM in acetonitrile) produces an intense transient absorbance at 405 nm. The disappearance of transient absorbance corresponds to second-order recombination of [Pd(CNMe),]'+ radicals (eq 2). Recombination
Registry No. H 2 0 , 7732-1 8-5; CH,CHO, 75-07-0; acetylene, 74-86-2; alumina, 1344-28-1.
( I I ) The chemistry is analogous to the hydrolysis of vinyl ethers. See, for example: Salomaa, P. In The Chemisrry ofrhe Carbonyl Group: Patai, S . , Ed.; Interscience: New York, 1966. (12) Chang and Silvestri also observed the conversion of acetaldehyde to alkyl aromatics: Chang, C. D.; Silvestri, A. J. J. Caral. 1977, 47, 249. (13) Opella. S.J.; Frey, M. H. J . Am. Chem. SOC.1979, 101, 5854. (14) Elementary considerations of the bond angles for sp2 carbons suggest that the angle between the C-H bond vector for the carbon attached to oxygen in species V and the axis of rotation (the C-O bond vector) is 60°, which is close to the "magic angle" of 54.7O, which is effective in averaging dipolar couplings.
Photoreduction of Palladium Radical Cations. Transient Absorbance Kinetics of Electron Transfers to Photogenerated [Pd(CNMe)3]+Radicals Frederick R. Lemke,l Robert M. Granger, David A. Morgenstern, and Clifford P. Kubiak**2 Department of Chemistry, Purdue University West Lafayette, Indiana 47907 Received June I . 1989
The photochemical deposition of metal films provides the basis for a wide variety of imaging proce~ses.~Laser direct writing of conducting metal features has received considerable attention as a means of defining and "wiring" a micron-scale circuit in a single step." We report the photoreduction of organometallic ( I ) Present address: Department of Chemistry, Ohio University, Athens,
OH
45701. (2) Research Fellow of the Alfred P. Sloan Foundation, 1987-1991. (3) Proceedings of the Sevenrh Inrernational Symposium on the Phorochemistry of Coordinarion Compounds; Yersin, H., Vogler, A,, Eds.; Springer-Verlag: Berling. Heidelberg, 1987. (4) Ehrlich, D. J.: Tsao, J. Y. J. Vac. Sci. Technol., B 1983, I , 969. (5) Montgomery, K. R.; Mantei, T. D. Appl. Phys. Letr. 1986, 48, 493. (6) (a) Gross, M. E.;Appelbaum, A.; Gallagher, P. K. J . Appl. Phys. 1987, 61, 1628. (b) Harriott, L. R.; Cummings, K. D.; Gross, M. E.; Brown, W. L. Appl. Phys. t e l l . 1986, 49, 1661.
Values [ Pd(CN Me),]" k.. M-' s-I
E1121 V vs S C E
ferrocene
radical cations by electron transfers to photogenerated [Pd(CNMe),]'+ radicals. It is generally recognized that photogenerated 'ML, radicals are potentially stronger both as oxidants and as reductants than their parent ground-state, metal-metal-bonded L,M-ML, c~mplexes.'~The complex [Pd,(CNMe),] [PF,], (1) was found previously to exhibit photochemical u p * Pd-Pd bond h o m o l y ~ i s . ~Photogenerated *~~ [Pd(CNMe),]'+ radicals derived from photolysis of 1 are rapid and potent reductants of a variety of electron acceptor^,^ A, eq I . We now report our findings [Pd(CNMe),]'+
+A
k,
solvent
[Pd(CNMe),(solvent)12+
+ A'(1)
2[Pd(CNMe),]'+
k,
[Pd2(CNMe)6]2+ 1
of [Pd(CNMe),]'+ radicals occurs with a rate constant, k , = 1 X IO9 M-' s-I , n ear the diffusion-controlled limit in a~etonitrile.~ The addition of decamethylferrocene (10 pM) to a solution of 1 (1 mM in acetonitrile) dramatically accelerates the observed rate of disappearance of [Pd(CNMe),]'+ radicals. The synchronous appearance of the decamethylferricinium ion indicates that the disappearance of [Pd(CNMe),]'+ is due to electron transfer. The kinetics of electron transfer to photogenerated [Pd(CNMe),]'+ radicals was examined for each of the ferrocenes, Fe(C5MeS)z, Fe(C5HS)(C5Me5),13 Fe(C5H4Me)2,Fe(CSH5)(C,H4CH20H), (7) Meyer, T. J.; Caspar, J. V. Chem. Rev. 1985, 85, 187. (8) Hepp, A. F.; Wrighton, M. S. J. Am. Chem. Sor. 1981, 103, 1258. (9) Metcalf, P. A.; Kubiak, C. P. J. Am. Chem. SOC.1986, 108, 4682. (IO) Reinking, M. K.; Kullberg, M. L.; Cutler, A. R.; Kubiak, C. P. J . Am. Chem. SOC.1985, 107, 3517. ( 1 I ) Malatesta, L. J . Chem. SOC.1955, 3924. ( 1 2) Fischer, E. 0.;Werner, H. Chem. Ber. 1962, 95, 703.
0002-7863/90/ 15 12-4052$02.50/0 0 1990 American Chemical Society
Communications to the Editor
J . Am. Chem. SOC.,Vol. I 1 2, No. 10, I990 4053
two possible sources of the observed rate saturation: ( I ) nonadiabaticity resulting from poor electronic coupling between the photogenerated [Pd(CNMe)3]*+radical and the electron donor ferrocene;I6 (2) preequilibrium changes in one of the reactants leading to observed rate constants, keobsd= PI*ke,modulated by the preequilibrium constant, We have examined both of these possibilites. Effects of nonadiabaticity may be described by the kinetic equation of Balzani et a1.I6 or the equivalent expression of Meyer et a1.22 A fit of our data to this model produced values of the intrinsic barrier, AG*(O) = 1.3 kcal/mol and Eo(Pd(CNMe)3+/o)= +0.35 V vs SCE. Inasmuch as the intrinsic barrier is unrealistically lower than the value determined for ferrocene self-exchange alone,23we reject the nonadiabaticity hypothesis. We have linearized our observed electron-transfer data, keobsd,within a preequilibrium model by constraining the diffusion-corrected keobsdto the Marcus condition: RT In ( k J (k22(ferrocene)1/2))vs Eo(ferricinium/ferrocene) is linear with slope = Details may be found in supplementary material. Our studies suggest the preequilibrium dissociation of methyl isocyanide is required for palladium radical reduction, eqs 3-4. Ka. Pd(CNMe)3'+ ePd(CNMe)2*+ CNMe (3)
\ \
\
2
+
i
-0 30
4 io
0 10
0 50
0.30
0 70
Eo vs SCE lor lerrwnes
Figure I . Log plot of k,"bd, the rate constant for electron transfer from differently substituted ferrocenes to photogenerated palladium(I) radicals vs E"(ferricinium/ferroene). The curve represents a fit of the complete Marcus Agmon-Levine expression for the electron-transfer cross react ion. I6q1
/
Fe(C5H5)2,Fe(C,H,I)(C,H,), and Fe(CsH4C1)2,14J5 by laser flash photolysis. The rate constants for electron transfer from the differently substituted ferrocenes for photogenerated [Pd(eNMe),]*+ radicals are summarized in Table I. The electron-transfer rate constants clearly depend on potentials for ferricinium ion formation. Rate constants, k,"b"d,for electron transfer increase as the ferrocenes become progressively easier to oxidize and converge on a value of -IO8 M-' s-I for decamethylferrocene, Eo( Fe(C5MeS)2+/0)= -0.09 V vs SCE. A log plot of the observed electron-transfer rate constants vs Eo (ferricinium/ferrocene) is presented in Figure I . The rate constant for dichloroferrocene, the ferrocene most stable to oxidation, Eo(Fe(C,H,CI)2+~o)= +0.77 V vs SCE, was too slow to measure meaningfully (
