NOTES
3887
subtracting the conductance of potassium gives Xo (Br-) = 78.23. Finally, from the corrected conductance of cesium bromide, we find ho(Cs+) = 76.77. A repeat rum was also made on the cesium chloride used by Justice,j this time after fusing the salt in a platinum boa1 in a muffle furnace. The salt was kept molten for several minutes and then allowed to cool. Evidently the earlier samples still contained a little water after vacuum drying, because we now obtain do = 153.31 by extrapolation of the data of Table 11, Table I1 : Conductance of Fused Cesium Chloride in Water at 25" 104~
A
8A
118.044 87,936 67.898 47.206 23,859
143.164 144.496 145,480 146.724 148.587
-0,017 0.028 -0.001 -0.006 -0,004
where the last column gives the difference between thle observed conductances and those calculated by th,e equation A = 153.310
95.65~"'
+ 26.2% In c
+ 138.8~ (3)
Applying the same correction (-0.258) as before for impurities gives A,(CsCl) = 153.05; subtracting the chloride ion conductance'5 of 76.35 gives h,,(Cs+) -76.70, in excellent agreement with the value from the bromide. Averaging 77.20 from the i ~ d i d e 76.92 ,~ from the chloride,6 and our present values of 76.70 and 76.77 from the chloride and bromide, respectively, we obtain h o ( C ~ + )= 76.!30 st 0.16. (13) G. C. Benson and A. 12. Gordon, J . Chem. Phys., 13, 473 (1945). (14) G. Jones and G. F. Bickford, J . Am. Chem. Soc., 5 6 , 602 (1934). (15) Correction to line 5 b'elow Table I1 of ref. 5: redace 73.52 by 76.35 (or Cl by E+).
The Use of Sodium Borohydride in Catalytic Deuterium Exchange Reactions'
by G. E. Calf and J. IL. Garnett School of Chemistrzi, T h e University of h'ew South Wales, Kensington, N . S. W . , Australia (Received December $8,1968)
The problems associated with hydrogen reduction and activation of platinum oxide2 and other transition
metal oxides3azbhave already been discussed in the development of the associative and dissociative r-complex substitution mechanisms4 for the metal catalyzed exchange of organic coiiipounds with heavy ~ a t e r . ~A) ~ different type of procedure for the reduction and activation of platinum oxide using organic compounds has also been reported.' This "self-activated" catahyst possesses certain advantages in isotopic hydrogen exchange experiments especially when combined with either ultraviolet or y-radiation.* It is the purpose of this present publication to report the use of a new method for the preparation of active transition metal catalysts for exchange reactions, namely, reduction in aqueous media with sodium borohydride. Brown and B r o ~ nhave ~ ~ already '~ mentioned that the treatment of platinum metal salts with aqueous sodium borohydride yields finely divided black preci pitates which are active catalysts for the hydrogenation of olefins. However, the exchange properties of these catalysts have no1 been investigated but should be important since the activity of the catalyst, particularly the site effect, depends markedly on the method of reduction and actival ion.* Two types of metal catalyst systems have been studied, the water-soluble transition metal oxides and the soluble chloride salts. Benzene and ethylbenzene have been used as representative aromatic systeiiis whde hexane and cyclohexane were examined as examples of aliphatic reactivity.
Experimental Apparatus, general procedures, and low voltage mass spectrometric analytical techniques have been outlined in an earlier publication.2 For the hydrogen runs, reactions were performed with metallic oxide (100 mg.) prereduced with hydrogen or deuterium, benzene (4.0 X mole), and heavy water (12.0 X mole) at a temperature of 130'' for 48 hr. as previously described. 4 5 I n the hydride reductions, sodiuiii borohydride (400 mg.) was added slowly to a suspension of the oxide (100 (1) Part XI of a serieis entitled "Catalytic Deuterium Exchange with Organics." (2) J. L. Garnett and W. A. Sollich, J . Catalysis, 2 , 339 (1963). (3) (a) J. L. Garnett, and W. A. Sollich, Nature, 201, 902 (1964); (b) J. L. Garnett and W. A. Sollich, to be published. (4) J. L. Garnett and W . A. Sollich, J . Catalysis, 2, 350 (1963). (5) J. L. Garnett and W. A. Sollich, Australian J . Chem., 14, 441 (196l\. (6) J. L. Garnett and W . 9. Sollich, ibid.. 15, 56 (1962). (7) J. L. Garnett and W. A. Sollich, J . P h y s . Chem., 6 8 , 436 (1964). (8) W. G. Brown and J L. Garnett, submitted for publication, (9) H. C. Brown and (2. A. Brown, J . Am. Chem. Soc., 8 4 , 1403 (1962). (10) H. C. Brown and C. A. Brown, ibid., 84, 1494 (1962).
Volume 68, Number 12 December, 1.9f;4
NOTES
3888
mg.) in water (50 ml,), hydrolysis of the excess sodium borohydride was completed by warming the reaction to 70 O, the prereduced catalysts were washed free from salts with water, the water was decanted, and the remaining reagents were added. Pumping off excess water prior to the last step resulted in appreciable loss in catalyst activity, presumably due to thermal sintering. For the soluble metal salts, the following modified hydride activation procedure was used. Sodium borohydride (400 mg.) was added don-ly to a solution of the metallic chloride (containing the equivalent of 100 nig. of metal) in water (50 ml.). A finely divided black precipitate immediately formed with all salts except iridium, which necessitated warming to 70" for reaction completion. The precipitate was washed free from salts with water, the remaining reagents were added, and exchange was performed as previously, without shaking to accentuate differences in reactivity.2
Results and Discussion The significant feature of the results (Tables I and 11) is the general improvement in isotope incorporation of the aromatics froin the hydride technique for almost all catalysts studied, the effect being particularly marked for ruthenium, rhodium, and iridium in Table I and for all metals in Table 11. With the soluble metal chlorides, the only salts readily reduced by hydrogen were those of rhodium, palladium, and platinum, whereas all chlorides yielded active catalysts in borohy-
Table I: Exchange Reactions with Metal Oxides Act,ivated by Reduction with Hydrogen or Sodium Borohydride
Table I1 : Exchange Reactions with Metal Chlorides Activated by Reduction with Hydrogen or Sodium Borohydride
Metal
Catalyst
atom
Fe" CO" Nic
D
Benzene,b atom % D
0 04 0 10 8 0
Ru" Rh Pd
0 7
Irc Pt
13 9
4 5
3 12 38 38 6 46
0 9 0 4 1 1
atom 70 D
0 0 18 1
01
06 1 0
25 0 25 1
25 9
a Method of catalyst preparation-hydrogen a t 30". D, = 50.0%. Catalyst preparation-NaBHr. D, = 50.0y0 for benzene, 37.570 for ethylbenzene. Since little or no reduction of these aqueous chlorides was observed with hydrogen, there was correspondingly insignificant exchange in benzene.
dride reduction. For platinum oxide, accurate kinetic studies would be necessary before a decision as to whether sodium borohydride reduction was better than hydrogen could be substantiated since the benzene samples from hydrogen and hydride reduction procedures had both virtually reached equilibrium. With nickel, rhodium, and palladium, reduction of the soluble salts yields a catalyst which is more active for deuterium exchange in both benzene and ethylbenzene than the corresponding oxide. The same result is obtained with ruthenium catalysts for the deuteration of benzene only. From Table I, sodium borohydride reduction of all catalysts is no better than hydrogen reduction for the exchange of aliphatic hydrocarbons as predicted by rr-complex chemisorption theory. A consistent predominance of ring deuteration in the ethylbenzenes is observed for all catalysts except nickel where the orientation effect favors the aliphatic side chain. Distribution studies with the labeled benzenes show that both stepwise and multiple exchange processes2 occur with the two types of systems, i.e., both hydrogen and borohydride activated catalysts. As a synthetic tool for general deuterium and tritium labeling,14the sodium borohydride technique possesses several most important advantages. Reductions may now be performed very quickly and efficiently in minutes at 30' without the necessity for using vacuum line 5511-13
Metal
Fe co Ni Ru Rh Pd
os Ir Pt
Catalyst
Benaene,a atom % D
0.05 0.03 0.04 0.04 0.13 2.13 11.8 0.04 0.09 0.01 46.4
Ethylbenzene,b atom % atom % Hexane,b D D atom % D
0.08 0.08 0.05 0.20 8.1 8 4 2.0 0 06 9.4 0.1 45.7
0.02 0.02
0.01
