808
Communications to the Editor
(11) D. Turnbull and S.H. Maron, J. Am. Chem. Soc., 65, 212 (1943). (12) E. M. Kosower and H. Dcdiuk, J. Am. Chem. Soc., 98,924 (1976). (13) R. P. Bell, "The Proton in Chemistry", 2nd ed, Chapman and Hall, London, 1973, pp 233-234. Department of Chemistry Tel-Aviv University Ramat Aviv, Tel Aviv, Israel and Department of Chemistry State University of New York Stony Brook, New York 1 1794
Edward M. Kosower
Received October 18, 1976
Oriented Adsorption of HD on ZnO and Catalytic Addltion of HD to Butadiene on I t
Sir: Kokes, Dent, Chang, and Dixonl found a remarkable orientation in the adsorption of the HD molecule on ZnO; the orientation in H
D / 0
I
Zn
I
prevails at room temperature but in D I Zn
H 1 0
I1
at the lower temperatures. In the previous paper,2 it was found that the reaction of butadiene with HD on a MoSz catalyst brings about an oriented addition giving 1.5 times more 1-butene-3-dl than 1-butene-4-dl. This oriented addition of the HD molecule was well explained by the isotope effect for half-hydrogenated intermediates formation. I t should be quite interesting whether such prominent orientation in the HD adsorption on ZnO, being dependent on adsorption temperature, will reflect on the orientation of the HD addition reaction. The reaction of butadiene with HD and/or with an equilibrated mixture of H2,HD, and D2was performed on ZnO catalyst (Kadox-25 from New Jersey Zinc Co.) at the two temperatures: room temperature and -40 "C. The catalyst was evacuated at 450 "C for about 4 h and cooled in vacuo immediately before the experiments, The results are summarized in Table I. The hydrogen molecular identity is highly maintained in the addition reaction as has been reported by several investigators. The apparent isotope effect for the reaction with Hz and D2 was undoubtedly very small, however, the addition of HD molecule represents a remarkable orientation giving 75% 1-butene-3-dland 25 % 1-butene-4-dlat room temperature. The most surprising result is that the orientation of the HD addition to butadiene is not influenced by reaction temperatures, although the adsorption of HD on ZnO prefers configuration I at room temperature by 75% and configuration I1 at -40 "C by 75-80%.l
One unsettled problem in the hydrogenation of unsaturated hydrocarbons on ZnO catalyst is the addition process of the hydrogen to the double bond of hydrocarbons. So far two mechanisms have been proposed (1) H2 is dissociatively adsorbed on ZnO and followed by an addition reaction, and (2) Hz reacts directly with the double bond adsorbed on ZnO, where no dissociation of hydrogen is required. The former mechanism was proposed by Kokes and co-worker~,~~~ and they concluded that only reversible adsorption of hydrogen named type 1 adsorption would participate in the hydrogenation reaction. The type 1adsorption saturates at pressures higher than about 30-40 Torr at room temperature, whereas the kinetics in hydrogen pressures are half-order for the hydrogenation of ethylene and first-order for the hydrogenation of butadiene in wider range of hydrogen pressure, Such a discrepancy has been pointed out by adsorption measurements of hydrogen during the hydrogenation of ethylene6 and of b ~ t a d i e n ethat , ~ is, that the adsorption of hydrogen may be a type 1 adsorption during the hydrogenation reactions, does not correlate with the reaction rates. Tamaru and co-workers3 proposed a second mechanism, direct attack of the hydrogen molecule on the double bond adsorbed on ZnO to account for the discrepancy between kinetics and adsorption of hydrogen and for the maintenance of hydrogen molecular identity in the hydrogenated product. The direct addition of the hydrogen molecule to the double bond, however, might not be feasible in accordance with the orbital symmetry rule.5 The results obtained in this paper, the temperature independent remarkable orientation in the HD addition with little isotope effect for Hz and D2addition, may rule out the direct addition of the hydrogen molecule, and oblige us to rectify the stepwise addition infered by Kokes et al. In conformity with rather facile ~J-T interconversion during the isomerization of 1-butene on Zn0,3J the isoelectronic intermediates for 1-butene formation and cis2-butene formation from the hydrogenation of butadiene should be quite interesting The reaction of butadiene with Dz and ZnO gives 1-butene-d2in more than 90% yield accompanying several percent of cis-2-butene-d2. The NMR spectrum of the cis-2-butene-d2formed as a minor product showed that the intensity ratio of methyl hydrogens to olefinic hydrogens was 2.04, which indicated the strict 1,4 addition of a deuterium molecule to butadiene forming cis-Zbutene. These findings clearly indicate that the 1,2 addition giving 1-butene and the 1,4 addition giving 2-butene take place independently on the ZnO catalyst via different intermediates. In conformity with the kinetic facility of HD adsorption on ZnO to take the orientation of configuration 11, independent 1,2 and 1,4 addition of hydrogen to butadiene may be described as shown in Scheme I. Recently, Hattori et found 1,4 addition of hydrogen to butadiene on a MgO catalyst evacuated at temperatures as high as 1100 O C . The active sites for cis-Zbutene formation on ZnO may have similar characteristics as that on MgO, on which a n-allyl anion intermediate formation is prominent. In
TABLE I : Hvdronenation of Butadiene on ZnO Products Temp, Conv, Run 101 102 112
Hydrogen used
%
H,
5.7 13.0 5.6
25.0 1.7 1.7
D,
22.7 0.9 0.6
The Journal of Physical Chemlstry, Vol. 81, No. 8 , 1977
do 23.2 6.6 3.5
d, 55.4b 91.0c
cis-2-Butene" d, 21.4 2.3 1.0
do 30.8 13.1
d,
50.6 83.3
d, 18.5 3.6
Hydrogen
H, HD D, 24.7 51.7 23.6 4.2 92.9 2.9 95.4d 2.1 97.1 0.8 72.8% l-butene-3-d,,27.2% a The amount of cis-2-butene in the products was 10% in run 1 0 1 and 9.1%in run 102. 1-butene-4-d,. 75.4% l-butene-3-dl,24.6% 1-butene-4-dl. 75.2% l-butene-3-d,, 24.8% 1-butene-4-dl. "C
RT RT -40
HD 52.3 97.4 97.7
1-Butene
Communications to the Editor
Y B
Scheme I CHD-CH=CH2 (I
HfC ?
1 I 2-addition ; ~
CHz=CH-CH=CHz
+
P
IH+
I
--+3-df-l-butene (75 %)
-Zn-o-
HD
Y B
CHz-CH=CHz
9
-2n-O-
-+
4-dl-l-butene (25 %)
d
d
X - configuration
Y- configuration
the 1,2 addition in Scheme I, the orientation forming l-butene-3-dl might be kinetically facile as has been observed on ZnO adsorption. The apparent small isotope effect for H2and D2addition reaction may be attributable to lack of orientation of H2 and D2 molecules and to the compensation between the bonds forming and breaking a t the transition state. If this is the mechanism, the pressure dependence will be first order in hydrogen pressure, and hydrogen molecular identity will be maintained. References and Notes R. J. Kokes, A. L. Dent, C. C. Chang, and L. T. Dlxon, J. Am. Chem. Soc., 94, 4429 (1972). T. Okuhara and K. Tanaka, J. Chem. SOC.,Chem. Common., 199 (1976); 1.Okhara, T. KO&, and K. Tanaka, Chem. Lett., 717 (1976). S. Nako, Y. Sakurai, H. Shlmizu, T. Ohlshl, and K. Tamaru, BUN. Chem. SOC.Jpn., 43, 2274 (1970); Trans. Faraday Soc., 67, 1529 (1971). A. L. Dent and R. J. Kokes, J. Phys. Chem., 73, 3772 (1909); 74, 3653 (1970). R. 0. Pearson, J. Am. Chem. Soc., 94, 8267 (1972). K. Tanaka and G. Blyholder, J. Phys. Chem., 76, 1394 (1972). C. C. Chang, W. C.Conner, and R. J. Kokes, J. Phys. Chem., 77, 1957 (1973). H. Hattori, Y. Tanaka, and K. Tanabe, J. Am. Chem. Soc., 98,4652 (1976). Sagami Chemical Research Center, NishWnuma, Kanagawa, Japan.
Research Institute for Catalysis Hokkaido University Sapporo, Japan
Toshlo Okuhara Toshihiko Kondo' Ken-ichl Tanaka'
Recelved November 15, 1976
On the Temperature Dependence of Multiple Charge-Transfer Bands In 7r-T Electron Donor-Acceptor Complexes Publlcation costs asslsted by the Naflonal Research Councll of Canada
Sir: The double charge-transfer (CT) absorption bands observed for complexes of tetracyanoethylene (TCNE) with substituted benzenes have been attributed to transitions from the highest and second highest occupied donor levels to the lowest empty acceptor level of TCNE1-e as was proposed originally by Orgel,7 but have also been attributed to rotational isomers.B Resonance Raman excitation profiles of charge-transfer transitions of TCNE complexes recently obtained in this laboratory indicated that the two charge-transfer bands involve different excited electronic states and thus different complex geometries.g The present study was undertaken to obtain more direct evidence of the existence or not of distinct complex isomers associated with the double transitions. On the basis of the results obtained from a study of the temperature dependence of the relative intensities of the two bands for various TCNE complexes we conclude that two most stable
Flgure 1. The two predicted conflguratbns for a TCNE-monosubstituted benzene complex.
isomeric structures exist for each appropriate complex corresponding to maximum overlap for the highest and second highest donor orbital, respectively, with the same (lowest empty) acceptor orbital (Figure 1). This result has particular significance in determining the relative contributions for the various interactions involved in donor-acceptor complexation. Theoretical calculations by Lippert, Hanna, and TrotterBon the p-xylene-TCNE complex have predicted two stable rotational isomers corresponding to the X and Y configurations in Figure 1. These calculations, done for the complex in the gas phase, predict that the exchange repulsion interaction (contributing a 4 kcal mol-' barrier between preferred conformations) is of primary importance in determining complex geometry. In contrast, CNDO/2 calculations have predicted only a single preferred conformation for the p-xylene-TCNE complex with the X and Y configurations corresponding to energy maxima.1° The concept of multiple configurations has been used by Holder and Thompson'l to rationalize the CT band intensities of increasingly sterically hindered alkylbenzenes with TCME. They concluded that substitution of bulkier groups favored the X configuration of TCNE in the complex. The EDA complexes were studied in inert polymer films of poly(methy1 methacrylate) which allowed us to determine the temperature dependence of the CT bands over a wide range, 4-300 K. The data and results are summarized in Table I and Figure 2. For each complex, except p-xylene-TCNE, the ratio of the absorbance of the high-energy band maximum to the low-energy band maximum, Ax/Ay, is reduced at the lower temperature. For benzene substitution by electron-donating groups, theory predicts the low-energy band to arise from the Y configuration and the high-energy band from the X configuration of the complex.* Thus, the Y configuration is energetically preferred for all the complexes investigated except p-xylene-TCNE for which the X configuration is preferred. The relative populations, Nx/Ny, of complex in two configurations with an energy difference, AE, can be expressed by the Boltzmann factor
N , IN, = exp(-AE/RT) Assuming Beer's law to hold, the optical absorbance, A, of a particular conformation is proportional to the number of complex molecules in that configuration and to its extinction coefficient, e. Hence, the relation between the ratio of absorbances for two conformations and the temperature would be
In A , / A ,
=
In
E,
/ey
- AE/RT
If the absorbance is measured at the maxima of two resolved bands, then to a good approximation the ratio, The Journal of Physlcal Chemistry, Vol. 81, No. 8, 1977
