Xpril, 19.17
PROTON-DEUTERON EXCHANGE
B E T W E E N h M O N I A .4ND .-hMONIATED
The deposition of manganese on the surface is corroborated visually by the fact that no colloidal manganese dioxide formed in the solutions during the tests, whereas similar tests in which no manganese dioxide surfaces were introduced showed increasing quantities of colloid in the solutions as the peroxide decomposed. These results are interpreted as confirming the conclusion drawn from the electrode potential measurements that the solution adjacent to the manganese dioxide surface is saturated with manganous hydroxide. Acknowledgment.-The authors are indebted to Dr. T. K. Sherwood and Dr. E. R. Gilliland for many helpful suggestions, and t o the Naval Bureau of Ordnance for support and release of this work.
Summary
40v
20
0.
The electrode potential of a manganese dioxide electrode was measured in hydrogen peroxide solutions of various peroxide and ion concentrations. Of a number of possible electrode reactions, the results were in most consistent agreement with the assumption that the potential was controlled by the reaction BhOz
4-4HC + 2e- +M n f f
DIIIORANF,'TAi
I
I
1
+ 2Hz0
and that the solution immediately adjacent to the manganese dioxide surface was saturated with manganous hydroxide, even though the bulk of the solution was not. Measuremlmts of the change of manganese concentration in hydrogen peroxide solutions decomposing in contact with macroscopic manganese dioxide surfaces gave results supporting the
[CONTRIBUTIOS FROM THE
DEPARTMEXT O F CHEMISTRY, THEUNIVERSITY
O F SOUTHERN CALIFORNIA]
Proton-Deuteron Exchange between Ammonia and Ammoniated Diborane BY ANTONB. BURG Earlier publications'~2~s~4 brought forth several The latter reacted further with liquid ammonia, kinds of experimental evidence pointing uniquely evidently drifting toward the equilibrium NH4to the idea that diborane (B2H6) behaves toward (BH3)zNHz -I- 2xNH3 @ 2BHsNH3.xNH3, probbases (electron donors) as if i t consisted of separ- ably not appreciably complicated by (NH4)2able pairs of borine (BH3) radicals, able to share (BH&NH or (NHd)3(BH3)4N. There was no electrons furnished by other molecules. Thus the evidence which could reasonably be interpreted to volatile solid material of the composition BzH6.21L'- favor the formula ( N H ~ ) ~ B z H ~ . ~ Although the borine theory of the base-reactions (CH3)3proved to be the borine complex (CH&N: BH31#5and IhH632NH3 (if prepared carefully by of diborane appears to be fairly generally acan addition :reaction a t -120"), behaved as one cepted, there has recently been a tendency to rewould expect. of the structure N H ~ + ( B H ~ ) z N H ~ -vive . ? the di-protic acid hypothesis, because of its simple relation to the protonated double bond idea (1) A. B. Burl: and H. I. Schlesinger, THISJOURNAL, 69, 780 of the structure of diborane.' It therefore be(1937). (2) H. I. Schlesinger and A. B. Burg, ibid., 60, 290 (1938). came necessary to seek new evidence bearing on (3) H. C. Brown. H. I. Schlesinger and A. B. Burg, ibid., 61, 673 the subject--evidence preferably simple and yet (1939). decisive enough to eliminate all doubt. (4) H. I. Schlesinger, N. W. Flodin and A. B. Burg, ibid., 61, 1078 (1939). (5) S. .H. Bauer, i b i d . . 69, 1804 (1937).
(6) E. Wiberg, 2. anorg. allgem. Chcm., 173, 210 (1928). (7) K. S. Pitzer, THISJOURNAL, 67, 1126 (1945).
748
ANTONB. BURG
Such evidence was found in the study of the proton-deuteron exchange reaction between free ammonia and ammoniated diborane. The experiments were planned in two ways: (1) treatment of B ~ H K ~ Nwith H ~ known proportions of nearly pure ND3, and observing the rate and final extent of substitution of H for D in the ND3; ( 2 ) dissolving B?H6.2ND3 in liquid ND3 and retesting the NlIa for purity.
VOl. 69
structure seems to be eliminated by a formidable array of facts,2 and, in any case, the strong proton-donor character of the quaternary nitrogen atoms should render the proton-sensitive BH4- ion incapable of existence.
Experimental Apparatus.-A
completely greaseless high-vacuum system was constructed in the form silhouetted in Fig. 1. It was attached a t V to the main high-vacuum manifold, from which purified diborane or ammonia could be delivered. The 200-cc. bulbs S1 and S2 were used for storage of gaseous samples or for measurement of gas volumes of ND, (employing the mercury float-valves as null manomeV ters). The ammoniation of diborane and the subsequent exchange reaction were carried out in the small reaction tube R . The deuterium content of the recovered ammonia then could be estimated by comparison of its vapor tension with that of a known mixture, in the differential tensiometer D . Comparison samples of mixed ammonias could be stored temporarily in tube M. The tensiometer was as shbwn more clearly in Fig. 2. It was essentially a three-way mercury float-valve, with two Fig. 1.-Silhouette of apparatus. arms ending in bulbs A and B. The standard ND3-XHa mixr m s c;ndensed into bulb A Experiments of type (1) showed that the ex- ture (at -183 ; B a t room temchange reaction reaches equilibrium within a perature) and a small part subfew seconds after the solid B ~ H v ~ Nand H ~ND3 limed away, in order to clear The composi- out any possible trace of nonform a liquid solution a t -78'. Xext the tion of the free ammonia then corresponds t o a conrlensable gases. mercury level was raised to cut mixture of all the ammonia (bound and free). off access to A, anti the test This agrees with the borine theory, according t o sample was condenqed into B, with return sublimation. which only the N-H links in N H ~ ( B H s ) z N H again ~ plugs should be able to exchange with ND3. The Then with all three glass floated into place, -4 and B transfer of protons from diborane t o ammonia t o were immersed in ether, conform (NH4)2B2H4would imply one-third more pro- tained in a test-tube dipping ton-contamination of the ND3 than is observable. into propane near its boiling An experiment of type (2) showed no exchange. point. Finally, the mercury lowered half-way, and the The recovered ND3 had received no appreciable was differences of pressure could be contamination by protons from diborane, even read. The tip of the ammonia though the solution had been allowed to warm to vapor tension thermometer" served as a stirrer for the final 18' during many hours. The formula (NDIH)z- approach to thermal equality. B2H4 would have implied an easily detectable 5y0 Sources of Materials.--A m substitution of H for D in the ND3, occurring rnonia-d3 was prepared by the rapidly a t -78". Again there was no H-D ex- reaction of D 2 0 (99.9Tc acchange when B2Hs and ND3 were suddenly cording to label by Stuart Oxygen Co.) with magnesium brought to reaction a t room temperature, with nitride. The procedure was extensive ammonolysis. the same in almost every detail It thus is no longer within reason to assume as that described by Kirschent h a t protons can be transferred directly from di- baum and UreyXz;the reaction place in tube I, from borane to a base. The only possible alternative took which the product was distilled Fig. 2.-The differenthl is that the acidic character of diborane is due to directly into the vacuum svstensiometer. the electron-receptor character of the boron atom. tern and resublimed. ComSince the discovery of the borohydride^,^^^^^^ i t parison with pure ammonia in the differential manometer a vapor tension difference of 62 * 0.3 mm. a t has become necessary to admit a theoretical alter- showedo -42.3 . Calculation from the equationsI2 gave 62.1 mm. native to the idea that the base reactions of di- for S D aassaying 9870 deuterium. It thus seemed reasonborane are all due t o the borine radical. One able to base assay calculations upon 98% original purity may consider also the reaction form BH2+BH4-. of the NDs, and to estimate the probable error as 0.5Y0. In later experiments, the error of reading was lowered to The diammoriiate thus would be (NH3)2BH2f about 0.1 mm., or 0.2Yc. BHI-, in logical symmetry to NH4+(BH3)2NH2-. DiSorane was freed of hydrogen by trapping from the However, such a boraminonium borohydride pump-stream a t - 1 8 3 O , and then distilled in vacuo from a tube a t -15.5". Its vapor tension was 225 mm. at
Th
(8) H. I Schlesinger, R. T. Sanderson and A. B. Burg, THIS
JOURNAL,
61, 536 (1939); 62, 3421 (1940).
(9) A. B. Burg and H. I. Schlesinger, ibid , 62, 3425 (1940). (IO) H. I . Schlesinger and H. C. Brown,ibid., 62, 3429 (1940).
(11) A . Stock, Z . Eleklrochem., 29, 354 (1923). (12) I. Kirschenbaum and H. C. Urey, J . Chem. Phys., 10. 714 (1942).
April, 1!347
PROTON-DEUTERON EXCHANGE BETWEEN AMMONIA AND AMMONIATED DIBORANE 749
-111.9" (melting CS?). The original sample had been made seven years earlier, by the boron trichloride-hydrogen discharge niethod,13 and several times repurified a t wde intervals. First Experiment, Plan ( 11 .-A samde of diborane amountingio 5.43 cc. (gas a t S. C.) was condensed in tube R (Fig. I ) , the lower walls of which were coated with solid ammonia (25.6 cc. gas). The diborane was absorbed a t - 120 to - 115' and the excess ammonia was sublimed off as the temperature rose to -80" (recovered S H 3 , 14.4 cc.; formula of residue B2H6.2.069H3). Ammonia-& (15.9 cc. gas, 98YcD) now was condensed upon the residue and warmed to -80" without forming a solution. The ammonia was removed a t intervals, assayed, and returned for further reaction. After the first approach to equilibrium, 9.1 cc. more h-D3 was brought in, and equilibrium again was approached. Comparisons were made against a mixed ammonia containing 70Y0 of the 98y0 ND,. The data and calculated resultsrare given by Table I.
TABLE I ND3 EXCHANGE WITH SOLID B2Hs.2.06KH3 Total time, hr. 12 23 35 47 57 59 75
Gas cca D Teomp., C. atoms -130 467 -80 46.7 -80 4G.7 4fj 7 -73 -42 4li.7 -42* 73.5 -42 73.5
Gas C C . ~
H atoms 34.G 31.6 34.6 34.6 34.6 34.6 34.6
APmm. at
-42.5' 1 7 6.7 6.5 7 1 0.5
TnH free ammonia 31 42.6 42.1 41.8 42.6 33.0 32.2
Calcd. % H if solid exchanges 4H+ 6H+ 8H+ 33.7 42.6 4 9 . 3 33.7 42.6 49.3 33.7 42.6 4 9 . 3 33.7 42.6 49.3 3 3 . 7 42 6 4 9 . 3 24.5 32.0 38.2 2 4 , 5 3 2 . 0 '38.2
a For convenience in calculation, the quantities of deuterium and of hydrogen are here computed as though each were a monatomic gas a t standard conditions. Hydrogen atoms in diborane are not here included. Liquid state.
Although these first results lacked precision because of some uncertainty of temperature control, there can be no doubt of the main conclusion, that B2H6.2NHs exchanges six and only six protons with NDz. The extent of exchange after only twelve hours corresponds to exchange of four protons, attributable to the ammonium ion of SH4(BH::)?SH2,, but this cannot be taken as evidence of such a structure because the latter stages of a heterogeneous reaction often are much slower than the earlier course of reaction. In any case, the observation times were too far apart for genuine estimation of exchange-rate. Second Experiment, Plan (1) .-In an attempt to discern stages in the exchange reaction in the liquid phase, a sample of the diammoniate containing 7.67 cc. of B2Hs and 15.5 cc. of SH3 was washed down the tube-wall by 98Y0 NDs (23.7 cc. gas a t S. C . ) , deliquescing a t -80 , and the resulting mixed ammonia was removed after seventeen seconds (reaction tube a t - 183 during assay). The vapor tension of this mixture a t -42" was 0.5 mm. less than that of the standard mixture (€18.6'7~ D) ; hence it contained 30.6y0 H (calcd. for four-proton exchange, 31.7). This could mean a primary exchange on one S H 4 + ion only, except that the final equilibrium was a:most attained during the next 70 seconds a t -80 : the new mixed ammonia showed 5.0 mm. more pressure (at -41.5') than the standard-hence contained 39.0y0 H (calcd. for exchange only of hT-H, 40.8Yc H ) . The final equilibrium (ammonia assay 40.9y0 H ) was established wirhin ten minutes a t -42'; a further ten hours a t -42" showed no change. Thus again the exchange could no'! be made to involve any hydrogen except that originally belonging to ammonia. The liquid-phase reaction was much too fast for definite recognition of stages. Third Experiment, Plan (2).-A 9.92-cc. sample of diborane was brought to reaction a t -120" with 64.3 cc. of (13) H . I. Schlesinger and A . B. Burg, THISJOURNAL, 53, 4321 (1931).
98% KD3. The temperature was allowed to rise during sixty hours to 18'. The glassy solution now was cooled to -183" and 0.10 cc. of uncondensed gas (probably H D ) mas pumped off. Comparison of the recovered ND3 with fresh 98% NDI showed no difference of vapor tension a t -46" (readings *0.2 mm.). An exchange reaction involving even one proton from diborane should have increased the proton content of the ND3 from 2 to 4.4YC,a change which would have been ea$ly detectable as a pressure difference of 1.3 mm. a t -46 . This experiment disposed of any doubt that it was only nitrogen-hydrogen bonds which were involved in the previously observed exchange. Any exchange involving protons from diborane must be indescribably slow even a t room temperature. This result cannot be reconciled with any theory which requires transfer of protons from diborane to a base. Fourth Experiment.-As a final check, it was considered essential to carry out a more violent reaction between diborane and ND3, such as woiild occur if the two were suddenly mixed a t room temperature. Accordingly a 28.2-cc. sample of pure diborane and 74.i cc. of 98Y0 XDs mere condensed together in the reaction tube (R, Fig. 1) a t -183" and suddenly warmed by a beaker of water a t 40". The tube was a t once filled with a dense fog and a viscous liquid appeared. After five minutes a t room temperature, the non-condensable gas was pumped off through a trap a t -183', and measured as 19.5 cc. Taking this to be HD, it represented one-stage ammonolysis of nearly half of the available BHd groups. The recovered ammonia amounted to 19.7 cc.-hence the residue contained one T\J per B. This ammonia proved to have the same vapor tension (within 0.1 mm.) a t -40" as the original 98% SD3-hence no exchange with the diborane occurred even during the extensive ammonolysis reaction. The horine theory thus applies as well a t room temperature as a t -120'.
Discussion The results of these experiments evidently remove the last doubt of the validity of the borine theory of the base-reactions of diborane, but they do not directly support any specific theory of its structure. If one assumes the two-moton bridee H\~:''H'':~/H _H/ _
' ~ H s ' \H
originally proposed by Dilthey14
and st&gly-favored by Longuet-Higgins and Bell,15s'6 i t must now be recognized that the bridge protons are not directly removable, as would be implied by the designation "protonated double bond.'17 It thus would become a difficult necessity to explain why the BzH2 bridge, held together by only four electrons, breaks two resonating H-B bonds instead of losing a t least one proton. I t is especially difficult to account for the ease with which a base molecule adds (fast a t - 120') to a boron atom as nearly tetrahedrally-surrounded by hydrogen as the twoproton bridge structure implies. This bridge structure might be accepted if all of the conceivable alternative models were proved to be wrong, but several possibilities seem to have been neglected. Proponents of an ethane-like structure usually have assumed a symmetry strictly like that of ethane, whereas a pseudo(14) W (15) H. (1943). (16) R. (London),
Dilthey, Z. angew, Chem., 34, 596 (1921). C. Longuet-Higgins and R. P. Bell, J . Chcm. Soc
, 250
P. Bell and H . C. Longuet-Higgins, Proc. R o y . SOC. 8183, 357 (1945).
750
J. GORDON ERDMAN AND XLSOPH H. CORWIN
Vol. 69
duced if one permits the boron atom to depart from the sp2 plane when cross-linkage first is established, or during the plane-sliding. Quite as dificult to overthrow as the pseudoethane possibilities would be many others involving what might be described as a four-proton bridge. The example shown in Fig. 3 would be consistent with the theory of partial bond orderslg (roughly extrapolated to orders less than unity), for the total bond order of each proton would be 1.0 and of each boron atom, 3.6. Each boron atom would have a total bonding of 0.G outside the Fig. 3:-A possible four-proton-bridge model for dibororiginal BH3 plane. Such structures, as well as ane (H4 and H5 lie above and below the plane, and cormost of the pseudo-ethane types, offer adequate respond t o H2 and H3). electronic "bareness" on boron to account for the ethane structure in which two protons interact unhindered addition of base molecules. In view of so many possible and inadequately a little more or less than the other four would be easier to reconcile with two strong Raman lines considered alternatives, it would seem quite inas far apart as 21,01and 2523 One thus visual- appropriate to accept a t this time any specific izes a trigonal antiprism consisting of two sp2 model as equivalent to the real structure of diborane. planar €{€I3 groups, held together by B-B and B-H partial cross-linkages, using electrons originally Summary assigned only to the sp2orbitals. Such resonance The H-D exchange reaction between ND3 and cross-linkage would account for the high barrier against internal rotation'8 quite as would the B2Hs.2T\'H3reaches equilibrium very rapidly in The extent of extwo-proton bridge, and a slight tipping of one liquid ND3 solution a t -7s'. RH:$ prism-face would provide increased or di- change corresponds only to the N-H links in B2&.2NH3, even a t temperatures up to 18'. lninished cross-linkage of two protons. Another method of varying this pseudo-ethane The reaction between BzHo and ND3 is not such structure would be to slide one triangular face in as to contaminate the excess ND3 with protons; the direction of the median line from a vertex this is equally true, whether the reaction is carried through the opposite base. Two widely sepa- out a t low temperatures (and followed by soaking rated protons thus approach each other and four in liquid ND3) or a t relatively high temperatures, others adjust out of their planes, until the sym- with extensive ammonolysis. Diborane thus metried two-proton bridge model results. Along fails to yield protons to bases and the borinethe C O ~ I ' S C of this change of models, one finds an complex theory of its base reactions is justified. infinite number of different possible structures for Possible structural models for diborane are diborane. Airlothercontinuous variable is intro- briefly discussed. H2
117) T. F, bnderson and A , B . Burg, J . (1'938).
Chcm. Pkys., 6, 586
LOS ASGELES,CALIF.
(IS)I . Stitt, J . C'/wm. P k y s . , 8, 981 (1940).
[(:ONTRIBUTION
FROM THE CHEMICAL
(19) J. E. Lennard-Jones and C. A. Coulson, Trans. Fnroday Sac., 35, 817 11439).
RECEIVED SOVEMBER 9. 1946
LABORATORIES O F THEJOHNS HOPKINS UNIVERSITY ]
The Nature of the Non-Porphyrin Bonding in Derivatives of Ferrihemel BY J. GORDON E R D M A AKD N ~ XLSOPHH. CORWIN The nature of the bonding in the ferric hemin enzymes such as catalase and the peroxidases requires further study. These enzymes are believed to form, in addition t o their protein links, complexes with hydrogen peroxide which are intermediates in the enzymatic processes. In our consideration of the peculiarities of the chemistry of hemoglobin,' we have postulated that some, a t least, of these phenomena might find an explana(1) Studies in t h e Pyrrole Series, X I X . Paper X V I I I , Corwin and Erdman, THISJOURNAL, 68, 2473 (1946). ( 2 ) Present address: Mellon Institute, University of Pittsburgh, Pittsburgh, Pennsylvania.
tion if we assume that globin is bound to ferroheme by two iron to protein links, whereas a single linkage would not permit advancing a geometric argument of the type proposed. If we extend this speculation to the enzymes containing ferriheme, assuming two bonds to the protein, we are faced with the objection that we must assume seven bonds to iron, one more than the usual coordination number of iron. We may refer to the fact that the assumption of the heptavalence of iron in isolated cases is not a new and in the (3) Pauling and Coryell, Proc. Na1. Acad. Sci., 28, 210 (1936).
