NOTES
3299
staggered would make plausible the exchange of hydrogen atoms between upper and lower ethylene molecules. Most commonly, the rate of hydrogenation satisfies'* dPczHe -dt
(1)
JCPHa
The zeroth-order dependence on ethylene pressure follows if sites suitable for trans-diadsorbed ethylene or acetylene are almost completely filled with the latter (the former reacting as fast as formed). The firstorder dependence on hydrogen pressure could result from a concerted addition Of hydrogen to acetylene (either by impact or from a weak complex, e*g*,a hydrogen phYsical1y adsorbed On top Of the chemisorbed acetylene) or by addition Of hydrogen atoms chemisorbed on the metal provided the additional sites for hydrogen adsorption are sparsely occupied. At least according to the simplest view, the concerted thermal addition from impact or weak complex is not in accord with the Woodward-Hoffmann rules so the chemisorbed hydrogen intermediate appears preferable. H
\.
I
'\c .......... . . . .. . . . . . . . . . . . . c' H
/
a small fraction of the ethylene-14C was ever removed from the surface. The authors concluded that only a small fraction of the sites were operative in the reaction, whereas we believe the ethylene-14C molecules served repeatedly as hydrogen donors without ever leaving the surface. We do not propose that this is the only mechanism operative in catalytic hydrogenation. In fact, unless an olefin has a hydrogen atom on each side of the double bond it appears sterically difficult or impossible to diadsorb it in the full trans configuration. Further, it appears that only for ethylene will trans diadsorption place two hydrogen atoms exposed for ready abstraction. This suggests the possibility of using an ethylene predose as catalyst promotor in the hydrogenation of substituted olefins; this possibility is being explored. Acknowledgment. We are indebted to Professor 0. L. Chapman for extremely helpful and stimulating discussion. (12) G. C. Bond, "Catalysis by Metals," Academic Press, New York, N. Y., 1962, pp 239-242. (13) N. C. Gardner and W. Powell, unpublished work. (14) R. Gomer, R. Wortman, and R. Lundy, J . Chem. Phys., 26, 1147 (1957). (15) C. Kemball, J . Chem. Hoc., 735 (1956). 58, (16) S. J. Thomson and J. L. Wishlode, Trans. Faraday SOC., 1170 (1962).
.
H
H
Investigations on Single Crystals of Alkali+ Biphenyl- Radical Salts Figure 2. Transition state for transfer of hydrogen atoms from trans-diadsorbed ethylene to ethylene. M denotes a surface metal atom, dotted lines are bonds in the process of forming or breaking, and the atoms connected b y dotted lines are considered coplanar. A plausible transition state for exchange of hydrogen atoms between the two ethylene molecules results (approximately) from rotating the top ethylene molecule 60' about an axis connecting the two central hydrogen atoms.
The following observations are simply interpreted in terms of this mechanism and are otherwise rather puzzling. (1) Ethylene readily self-hydrogenates on tungsten a t temperatures as low as 160"K,13 yet hydrogen adatoms are negligibly mobile on tungsten below 200°K.14 (2) When ethylene reacts with deuterium on tungsten, nickel, rhodium, or iron the first ethane formed is Ci". Subsequently all possible ethanes are formed, CzDs being the last to appear; this can be accounted for by exchange of hydrogens between chemisorbed and complexed ethylene as previously mentioned. (3) Thomson and Wishlode'6 performed an experiment in which hydrogen and ethylene were admitted to a nickel film containing preadsorbed ethylene-14C. Only
by G. W. Canters, A. A. K. Klaassen, and E. de Boer* Department of Physical Chemistry, University of iVijmegen, Nijmegen, T h e Netherlands (Received February 12, 1970)
It is known that organoalkali salts, such as Li fluorenyl, can be obtained in a crystalline form.' While these crystals are probably ionic, they are not expected to show electronic paramagnetism as the constituent compounds are diamagnetic themselves. On the other hand, it is known that neutral free radicals such as diphenylpicrylhydraxil (DPPH) can form single crystals which are paramagnetic but not ionic.2 Therefore, it was of interest to investigate the feasibility of preparing single crystals of paramagnetic alkali radical salts. Moreover, an investigation
* To whom correspondence should be addressed. (1) J. A. Dixon, P. A. Gwinner, and D. C. Lini, J. A m e r . Chem. Soc., 87, 1379 (1965); A. K. Banerjee, A. J. Layton, R. S. Nyholm, and M. R. Truter, Nature, 217, 1147 (1968).
(2) G. E. Pake, "Paramagnetic Resonance," W. A . Benjamin, New York, N. Y., 1962, Chapter 4. The Journal of Physical Chemistry, Vol. 74, N o . 17, 1970
NOTES
3300 Table I : Results of the Chemical Analysis of the Alkali Bp Crystals"
Li-Bp-THP Na-Bp-Tg K-Bp-Ttg Rb-Bp-T tg
Proposed compoui-
Metal,
Biphenyl,
tion
%
Solvent,
Total,
%
%
%
1:1:5 2:2:5 1:1:3 1:1:3
1 . 3 f 0 . 1 (1.18) 3 . 8 f 0 . 1 (3.7) 5.0i.OO.3(4.6) 9 . 0fl ( 9 . 4 )
27 f l ( 2 6 . 1 ) 23.1 f 0 . 6 (24.8) 16.2 i.0 . 5 (17.9) 1 6 . 3 ='c 0 . 8 (17.0)
72 f 3 (72.8) 64 f 2 (71.5) 70 f 2 (77.4) 66 f 3 (73.5)
100.3 (100) 90.9 (100) 91.2 (100) 91.3 (100)
The numbers in parentheses represent theoretical weight percentages based on the proposed composition in the second column. Experimental weight percentages are mean values of a number of analyses. The quoted experimental uncertainties are equal to three times the root mean square deviation of these mean values. Q
of the structure of these crystals might give more insight into the structure of alkali radical ion pairs in solution. We have prepared single crystals of the alkali radical ion pairs of LiBp in THP, XaBp in Tg, KBp in Ttg, and RbBp in Ttg. The abbreviations Bp, THP, Tg, and Ttg refer to biphenyl, tetrahydropyran
The chemical analysis of the crystals was performed by dissolving a weighed quantity of crystals in cyclohexane and decomposing them with a small amount of water. From the nmr spectrum of the hexane solution the concentrations of the Bp and the ether solvent, apparently always present in the crystals, could be determined by integrating the respective nmr signals r-1 and comparing the integrals with those of a standard. (C2H40C3H6),triglyme (CH30[CH2CH20IaCH3), and After extraction of the hexane solution with water, tetraglyme (CH20[CH2CH20],CH3,respectively. In the hexane layer was diluted and the Bp concentration this note we describe the preparation and the comwas once again determined spectrophotometrically. I n position of the crystals, together with a discussion the aqueous extract the amount of alkali hydroxide of some physical properties. was determined by a titration. The method was tested by analyzing samples consisting of a mixture of weighed Experimental Section amounts of alkali metal, Bp, and ether solvent. The Preparation of the Crystals. The crystals were preresults showed that the random error in the amount pared from 0.5-1.0 M solutions of the alkali Bp salts of each of the three components usually amounted in the respective ether solvents. For the preparation to less than f10% for a single run. of the solutions standard techniques were e m p l ~ y e d . ~ The experimentally determined percentages of the Bp (BDH) was used directly from stock. T H P components of the alkali Bp crystals are presented (Fluka) was stored over CaC12, dried before use over in Table I together with the experimental uncertainties, calcium hydride or an Na dispersion, and distilled on which are indicated directly behind the quoted perthe vacuum line into a storage bottle containing some centages. Theoretical percentages, calculated on the Na-K alloy. Tg and Ttg were dried in vacuo over an basis of an assumed composition as indicated in column Na mirror, an Na dispersion or Na-Pb alloy for 1 or two of the table, are shown in parentheses behind more days and distilled in vucuo into a container bulb the corresponding experimental percentages. provided with a break-seal. Results Reduction of the solutions was accomplished by Physical Properties. The crystals of the alkali Bp shaking the solution on the metal for 2-4 hr. The salts all show a deep blue-black color. LiBpTHP LiBp solutions were reduced at room temperature, crystallizes in diamond- and NaBpTg in rectangularthe other solutions at elevated temperatures (-50"). shaped plates, Crystals with dimensions of about 4 X Crystals were grown by storing the reduced solutions 15 X 15 and 2 X 7 X 25 mm, respectively, could be for a few days at room temperature. The crystals easily grown. KBpTtg crystallizes in closely packed were dried by decanting the solution into a side arm, small plates and the crystals of RbBpTtg are needles distilling the remaining solvent by cooling this comabout 1 mm thick and 5-7 mm long (see ref 4, where a partment with liquid nitrogen, and sealing it off. When picture of the crystals is given). The NaBp crystals T H P is used, the crystals can be easily dried, but in melt at 55-58', while the crystals of the other three the case of glymes complete removal of solvent is systems decompose before melting, losing solvent not always possible because of their high boiling points. up on heating. Composition of the Crystals. As the alkali salts of Bp decompose in the presence of oxygen and moisture, (3) D. E. Paul, D. Lipkin, and S. I. Weissman, J . Amer. Chem. Soc., the crystals were handled in a drybox filled with ni78, 116 (1956). trogen previously dried over molecular sieves. Under (4) G. W. Canters, Thesis, University of Nijmegen, Nijmegen, The these conditions the crystals decomposed only slowly. Ketherlands, 1969. The JOUTnal of Physical Chemistry, Vol. 74, No. 17,1970
NOTES Preliminary magnetic ,resonance experiments were performed on crystals of NaBpTtg. Esr experiments were performed on a Varian V 4502 X-band spectrometer. The samples consisted of a crystallite approximately 1 X 1 X 3 mm in size, sealed in a Pyrex tube. Z3Na nmr measurements were performed on a Varian DP 60 spectrometer equipped with a V 4210 variable frequency transmitter. The resonance frequency, 15.1 MHz, was stabilized by a crystal stabilizer. The nmr sample consisted of a sealed thin-walled 10-mm 0.d. Pyrex tube filled with polycrystalline material. All esr and nmr experiments were performed at room temperature. The esr spectrum of the crystals consisted of a single strong signal a t g = 2 with a derivative peak width of 0.3-0.4 G. No low-field signals were observed. The Na nmr spectrum of the crystals consisted of a single signal with a derivative peak to peak width of 4-6 G and shifted upfield from the Na signal of a 1 M solution of NaCl in HzO by an amount of 5-6 G (-400 ppm). Discussion The data shown in Table I indicate that for LiBpTHP the experimental and the theoretical percentages of the different component.s are the same within the experimental error. For the other systems this appears to be true only for the metal percentages; the experimental percentages of Bp and glyme appear to be too low by, respectively, 0.7-1.7% and 7-7.5010 while also the total amount of material found at the end of the analysis is 8-9% smaller than the amount weighed in. These discrepancies may be caused by side reactions at the start of the analysis, when the crystals are decomposed by the addition of a small amount of water.6s6 This means that the observed discrepancies are inherent to the applied method of analysis. The constancy of the errors throughout the analysis of samples of different composition seems to confirm this interpretation. Therefore we feel that the composition of the crystals as presented in column 2 of the table is probably correct. This means that the crystals have the stoichiometric formulas LiBpTHP,, NazBpzTg5, KBpTtga, and RbBpTtg,. The composition of the crystals makes it clear that the solvent plays an important role in the formation of the crystals. Probably the solvation of the alkali ions is the main cause for the thermodynamic stability of the crystals. The importance of the solvation energy for the formation Of stable ion pairs and stable Crystals may become apparent also from the following observations. (1) It is impossible to reduce B~ completely h poorly solvating agents such as T H P and MTHF.6,’ (2) The alkali Bp crystals decompose as soon as the solvent is extracted from the crystals. Removal of solvent fmm the LiBpTHP, crystals could be achieved
3301 by heating or by pumping the solvent off on a vacuum line. From the KBpTtg, and the RbBpTtg, crystals the solvent could be removed by raising the temperature. From the NazBpzTg5crystals solvent could be extracted by shaking with a nonpolar solvent like n-hexane. (3) NaBp and KBp form crystals in those solvents which are known to specifically solvate alkali ions.4 The presence of solvent molecules in the crystals makes it improbable that the crystals can be considered as purely ionic crystals. Their soft consistency, the low melting point of NazBpzTg6,and the ease with which the solvent can be removed from the crystals characterize them as van der Waals type or molecular crystals. The esr experiments on the NazBpzTg6 crystals reveal a strong exchange interaction between the unpaired electrons in the crystal: the width of 0.3 G observed for the esr signal of the crystals is typical for an exchange narrowed esr line. Similar phenomena have been observed for crystals which are built up of neutral radicalsaZ For instance, the exchange narrowed esr line of a DPPHsCaHa crystal has a width of 3 G, yielding an estimated value of lo--” sec for the “electron exchange time.”2 For the NazBpzTgS crystals with a signal width of 0.3-0.4 G, one expects an “electron exchange time” of 10-l1 to 10-l2 sec. The relative sharpness of the Na nmr signal of the NazBpzTg5 crystals shows that the anisotropic magnetic dipolar interaction of the metal nucleus with the unpaired electron is largely “averaged out” by the electronic exchange i n t e r a c t i ~ n . ~Furthermore, the absence of a quadrupole fine structure may indicate that the solvent molecules are chelated in a symmetric configuration around the metal ions. Conclusion The results presented in the preceding sections have shown that single crystals of alkali+ Bp- salts can be prepared from concentrated solutions of these salts. Solvent molecules appear to be present in the crystals in stoichiometric quantities, probably as chelating agents of the alkali ions. The physical properties of the crystals resemble those of van der Waals type or molecular crystals. Esr experiments on NazBpzTg6 have shown a strong paramagnetism and electron ex-
(5) N. D. Scott, J. F, Walker, and V. L. Hansley, J . Amer. Chem.
sot., 58,2442 (1936)).
( 6 ) J. L. Down, J. Lewis, 3767 (1959)*
B. Moore, and G. Wilkinson, J . Chem.
(7) R. V. Slates and M. Sewarc, J . Amer. Chem. Sot., 89, 6043 (1967); A . I. Shatenstein and E . S. Petrov, Russ. Chem. Rev.,36, 100 (1967); A. Rembaum, A. Eisenberg, R. Haack, and R. F. Landel, J . sOc.v 8991062 (1967). (8) L. L. Chan and J. Smid, ibid., 89, 4547 (1967); M. Sninohara, J. Smid, and M , sswarc, ibid., 2175 (1968). (9) J. H. van Vleck, Phys. Rev.,74, 1168 (1948). The Journal of Physical Chemistry, Vol. 74, N o . 17, 1970
3302 change interaction, the electron correlation time being of the order of 10-l1 to 10-l2 sec. A more elaborate study of the properties and structure of alkali Bp crystals, especially X-ray studies, may give further information on the structure of the alkali radical ion pairs in the solid state and in solution.
The Journal o j Physical Chemistry, Val. 7 4 , N o . 17, 1970
NOTES Acknowledgments. The authors wish to express their gratitude to Dr. B. M. P. Hendriks for experimental assistance. The present investigations have been carried out under the auspices of the Netherlands Foundation for Chemical Research (S.O.N.) and with the aid of the Netherlands Organisation for the Advancement of Pure Research (Z.W.O.).
