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C O M M U N I C A T I O N S T O THE EDITOR
by a recently developed epr m e t h ~ d . Nmr ~ is not sufficiently sensitive at low deuteration incorporation for this purpose and mass spectrometry does not distinguish positional substitution (e.g. , naphthalene-a-dl us. naphthalene-/I-&). For the first time, we now wish to report that using the above experimental technique^,^ we have been able to observe in the deuteration of a representative number of polycyclics (i) initial heterogeneous exchange as distinct from competing randomization and (ii) identical deuteration patterns between heterogeneous platinum data and the corresponding homogeneous platinum(I1) system. The results (Tables I and 11) show that in
Homogeneous and Heterogeneous Platinum-Catalyzed Isotopic Hydrogen Exchange in pOIYcYclic
Aromatic Hydrocarbons
Publication costs borne completely by The Journal of Physical Chemistr~
Sir: Exchange between aromatic compounds and heavy water is catalyzed by both homogeneous and heterogeneous platinum. 1 , 2 Using data essentially from the exchange of the monosubstituted benzenes, common n-complex mechanisms have been proposed for the two This type of reactivity is fundamen-
Table I : Homogeneous and Heterogeneous Platinum-Catalyzed Deuteration of Polycycliosa Solvent Run
Compound
Phase
wt, g x 10
1 2 3 4 5 6 7 8 9 10 11 12 13
Naphthalene Naphthalene Naphthalene Naphthalene Naphthalene Anthracene Pyrene Pyrene Diphenyl p-Terphenyl p-Terphenyl m-Terphenyl m-Terphenyl
Homogeneous Heterogeneous Homogeneous Heterogeneous Heterogeneous Homogeneous Homogeneous Heterogeneous Homogeneous Homogeneous Heterogeneous Homogeneous Heterogeneous
2.5 4.0 4.0 4.0 4.0 1.0 0.5 4.0 4.0 1.0 1.0 1.0 5.0
a
vol,
Cat. wt,
OC
Time, hr
ml
g
75 150 75 150 150 75 75 150 75 116 150 116 150
0.5 168 168 2 X 168b 4 X 168" 1.0 0.7 4 39 3 X 2.5b 3 X 336b 3 X 2.5b 336
10.0 5.0 16.0 2 X 5.0b 4 X 5.0b 13.0 20.0 5.0 8.0 3 X 15.0b 3 x 10.0b 3 X 15.0b 5.0
Temp,
Experimental exchange procedures as in relevant references.1.2
tally significant since such relationships indicate that the chemistry of adsorbed molecules and the chemistry of inorganic coordination complexes are intimately related. An important class of compounds in mechanistic studies of these catalytic systems are the polycyclic aromatic hydrocarbons. In particular, to develop the above correlations further, studies in isotope orientation with these hydrocarbons are essential and, a t present, only homogeneous catalysis data are available.' No equivalent systematic heterogeneous study has been performed with the polycyclics because of the difficulty of separating the exchange reaction from the competing randomization process, which only occurs heterogeneously. For a direct comparison between homogeneous and heterogeneous catalytic exchange it is necessary to be able to observe initial deuteration rates and orientation in both systems. This is now possible
b
x
108
8.0 2 X 2.5b 4 X 2.5b 5.0 20.0 10.0
%D
% D (theoret)
(exptl)
56.6 95.7 56.6 99.8 100.0 98.8 96.2 96.8 38.6 100.0 100.0 100* 0 94.4
1.8 17.9 44.2 42.3 98.5 1.50 5.2 5.6 37.6 41.2 93.9 48.4 42.8
Number of equilibrations.
exchange with both homogeneous and heterogeneous platinum catalysts, the polycyclic aromatic hydrocarbons fall into two groups, e.g., condensed polycyclics (naphthalene) and the polyphenyls (biphenyl). With the condensed polycyclics, initial deuteration is exclusively stepwise and to the /I position or its equivalent, whereas the polyphenyls exhibit multiple deuteration and exchange predominantly, one ring at a time, in the meta and para positions. Thus naphthalene gives naphthalene-/I-& after initial exchange with either (1) R.J. Hodges and J. L.Garnett, J. Phys. Chem., 73, 1525 (1969). (2) J. L.Garnett and W. A. Sollich-Baumgartner, Advan. Catal., 16, 95 (1966). (3) R. R.Fraser and R. N. Renaud, J . Arne?'. Chem. Soc., 88, 4365 (1966). (4)K.P.Davis, J. L. Garnett, and J. H. O'Keefe, Chem. Commun., in press. The Journal of Phy8ku.l Chemistry, Vol. 76, No. 8, 1971
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homogeneous or heterogeneous platinum.
Prolonged deuteration with both catalysts yields predominantly naphthalene-p-d4, although some scrambling to the a positions is observed heterogeneously since temperatures of 150' are required under these conditions to obtain appreciable amounts of the d4 species within reasonable periods of exchange. Again, in both catalytic systems, analogous stepwise, p selectivity is observed with anthracene, phenanthrene, chrysene, and tetracene. Pyrene deuterates successively and preferentially in the 2- then the 2,7followed by the 1, 2, 3, 6, 7, 8 positions. With homogeneous platinum(I1) , the stepwise p selectivity for the condensed polycyclics has been attributed to bond localization.1 In naphthalene, it has been proposed1 that the intermediate T complex is formed with the 1-2 bond, i.e., the bond of highest bond order. Because of steric hindrance with the a position, exchange is confined to the p position where multiple T-u conversions are possible, although the system exhibits an M value of unity. Because the present heterogeneous results are remarkably similar to the previous homogeneous data,l it is suggested that the original T complex exchange mechanisms2 postulated for molecules such as naphthalene under heterogeneous conditions can now be modified. Thus it is plausible to propose that condensed polycyclic~exhibit n-bond localization on the catalyst surface and are adsorbed as ?r-olefin type complexes. A .rr-olefin type complex is consistent with the observed strong toxicity of naphthalene in heterogeneous exchange with heavy water.2 Further, the p orientation, together with stepwise exchange in naphthalene and similar compounds, supports accumulating which indicates that 7r-dissociative, and not rassociative, processes predominate in heterogeneous deuteration. If a a-associative mechanism were significant, steric hindrance from the adjacent a! position in naphthalene would be minimal and appreciable deuteration in both Q and p positions would be expected during initial rates of exchange. Again, with the polyphenyls (Tables I and 11) initial isotope orientation under both homogeneous and heterogeneous conditions is identical. By contrast with the condensed polycyclics, the polyphenyls deuterate via a multiple process and exhibit strong ortho deactivation. Because of the importance of the present results to catalytic theory and also to general deuteration and tritium labeling work, a final check on the isotope orientation data has been made by subjecting all of the above polycyclics to the back exchange techniques where gradual protonation of the fully deuterated molecule is effected. Identical exchange patterns have been observed under homogeneous and heterogeneous
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1 The J O U T Mof~Physical Chemistry, Vol. 76, No. 8,1071
(5) R. J. Harper, S. Siegel, and C. Kemball, J . Catal., 6,72 (1966). (6) G.E.Calf and J. L. Garnett, Chem. Commun., 373 (1969).
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Benzene in MTHF glass produces a very broad absorption through the visible apparently benzene does not produce a well defined anion under these conditions. The spectrum is quite different from that of trapped electrons in MTHF but does not show the narrower band at 435 nm produced by potassium reduction of benzene in dimethoxyethane at - 80°K.8 (In the latter case, the ion may be stabilized by the Acknowledgment. We thank the Australian Research solvent or the counterion.) Stabilization of benzene Grants Committee and the Australian Institute of anions in a nonpolar solvent therefore appears unNuclear Science and Engineering for the support of likely, but Ekstrom assigns bands a t 525 and 930 nm in this research. 3-methylpentane to CeH6- and (CBH6)2- on the basis of DEPARTMENT OF PHYSICAL CHEMISTRY K. P. DAVIS their removal by carbon tetrachloride. * However, OF NEWSOUTH WALES J. L. GARNETT these wavelengths agree well with ours (550 and 930 THEUNIVERSITY KENSINQTON, N.S.W. 2033, AUSTRALIA nm) obtained in the halide glass and assigned to monoRECEIVED JANUARY18, 1971 mer and dimer cations while Gallivan and Hamill state that the 930-nm band in 3-methylpentane is enhanced by the electron scavengers, carbon tetrachloride and isopropyl chloride, but suppressed by the positive Comment on 6‘Ionic Species Formed from charge scavengers, 2-methylpentene-1, triethylamine, and MTHF.g Benzene during Radiolysis of Its Solutions I n solutions containing carbon tetrachloride, Ekstrom in 3-Methylpentane at 77°K” by A. Ekstrom observed bands at 320 and 1030 nm which he ascribed Publication costs borne completely by The Journal of to monomer and dimer cati0ns.l The 320-nm band Physical Chemistry cannot be the same transition as our band at 550 nm; (which is consistent with photoelectron spectral data;1° Sir: The object of this note is to question Ekstrom’s this band is masked in CC14 solutions). It could be a assignment of an optical absorption band to the species, higher transition8 but it may be due to the radical, (CeHa)z -. CaH,..” The identity of the species absorbing at 1030 The term “dimer cation” is used for species, M2+, nm is not clear. where M is a stable neutral molecule such as an aromatic I conclude that there is as yet no definite evidence hydrocarbon: as in the similar case of excimers, M2*, for a dimer anion. Their apparent instability remains the interaction between the two partners is believed to puzzling. An explanation given recently for the case be weak, i.e., no rearrangement of bonds occurs. of the naphthalene dimer anion4 is not likely to be Dimer cations have been clearly identified by their generally applicable. esr spectra; their optical spectra have been studied and they have been detected in mass spectrometers (see ref 1-3 and references therein). Simple explanations (1) A. Ekstrom, J . Phys. Chem., 74,1705 (1970). (2) B . Badger, B. Brocklehurst, and R . D. Russell, Chem. Phys. of the stability of dimer cation^^-^ suggest that dimer Lett., 1,122 (1967). anions, Mz-, should be equally stable. It is surprising, (3) B . Badger and B. Brocklehurst, Trans. Faraday SOC.,65, 2576, therefore, that the numerous esr studies of aromatic 2582,2588 (1969). hydrocarbon anions in solution have not detected such (4) B. Badger and B. Brocklehurst, ibid.,66,2939 (1970). species. (Anion dimers, M22-, do exist, e.g., where h!I is (5) K. Kimura, H. Yamada, and H. Tsubomura, J . Chem. Phys., 48, 440 (1968). benzoquinone;6 these species have two bonding elec(6) W. H. Hamill in “Radical Ions,” E. T . Kaiser and L. Kevan, Ed., trons instead of one). Interscience, New York, N. Y., 1968, p 321. The radiolysis of solutions in organic glasses forms a (7) T . Shida and W. H. Hamill, J . Chem. Phys., 44,4372 (1966). convenient method of preparing ions of the solutes. (8) C. L. Gardner, ibid., 45,572 (1966). Anions are produced in ethers (such as methyltetrar (9) J. B . Gallivan and W. H. Hamill, ibid., 44,2378 (1966). (10) D. W. Turner, Advan. Phys. Org. Chem., 4 , 3 1 (1966). hydrofuran, MTHF), cations in halides, and both in (11) T . Shida and W. H . Hamill, J . Amer. Chem. Soc., 88, 3689 hydrocarbons.6 Having studied the formation of (1966); T . Shida and I. Hanazaki, Bull. Chem. SOC.Jap., 43, 646 dimer cations in butyl chloride-isopentane mixtures,a (1970). we made a search for dimer anions in MTHF. Only monomer anions could be detected in naphthalene B. BROCKLEHURST solutions up to 1 M and pyrene to 0.5 M concentration CHEMISTRY DEPARTMENT THEUNIVERSITY even on annealing after radiolysis. Comparison with 53 7HF, ENQLAND SHEFFIELD, the cation results suggests that the dimer anions are not RECEIVED SEPTEMBER 8, 1970 stable.
conditions using both forward and back exchange methods, This remarkable correlation in isotope orientation and multiplicity of exchange during initial isotope incorporation suggests that analogous n-complex mechanisms operate under homogeneous and heterogeneous conditions. The data thus unequivocally support the general n-complex theory of metal catalysis. lv2
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The Journal of Physical Chemistry, Vol. 76, No. 8, 1971
