Interconversion of boron hydrides. Part 2. Cothermolysis of tetraborane(10), B4H10, with other boranes at 75 °C

摘要

1398 J.C.S. Daltonlnterconversion of Boron Hydrides.borane(lO), B4H10, with Other BoranesBy Terence C. Gibb, Norman N. Greenwood,. TrevorPart 2.l Cothermolysis of Tetra-at 75 "CR. Spalding, and Derek Tavlorson, Department oftnorganic and Structural Chemistry, The University of Leeds, Leeds-LS2 9JTA mass spectrometric method of analysis has been used to study the cothermolysis of B4H,o with B2H6, B,H,,BaHll, B6Hlo, and B6H12 at 75 "C. The reactions are complicated. The variations with time of the concentrationsof the major products are compared with results obtained from the thermolysis of each borane singly to elicitinformation about the role of B4H10 and possible reactive species formed from B4H1, in the cothermolysis. A seriesof reactions are suggested which invoke (B4H,) as a reactive species of major importance from B4H10.WE have recently developed a mass spectrometricmethod for the continuous quantitative analysis ofborane rnixtures.l As part of a general study of gas-phase interconversions of boranes we report here the useof the method in an initial, non-kinetic, investigation ofcothermolyses of B4H10 with B2H6, B5H9, B,H,,, B6H10,and B12.By comparing the time variation of theconcentrations of species during the cothermolysis withthe results from the thermolysis of individual boranes,it has been possible to form some conclusions about therole played in interconversion reactions by B4H1o andreactive species produced therefrom.Evidence from both mass-spectrornetrically analysedtherrnolysis studies 2-4 and chemical reactions has ledto the proposal of three highly reactive species in B4H10chemistry reactions (1) and (Z).Such species areB4H10 {B4H8) + H2 (1)B4H10 c- {B3H7} + (BH3) (2)involved in most suggested mechanisms of borane inter-conversions, e.g. B,H, thermolysis6 The importance ofreaction (1) is worth noting since similar reactions are nolonger regarded as significant for B5H11 or B6H12,although there is evidence for loss of H2 from BgH15,' andloss of H, from {B3H9) has often been conjectured.sThe removal of a {BH,} group by reactions related to (2)has been reported for B5H118a and B9H15, 86 and suggestedfor {B3H9) and B6H12.' Stafford, has proposed thatreaction (1) proceeds faster than (2) on the basis of hiswork on B4H10 thermolysis.However, only at tem-peratures above 180 "C was evidence of {B4H8} obtainedand small quantities of (BH,} or (B3H7) compared to{B4H8) could not be excluded.2EXPERIMENTALThe preparation of boranes and the method of analysis ofthe mass spectra of mixtures have been discussed in Part 1.1For each cothermolysis reaction the reactants and argonwere metered into a known volume and allowed to mix atroom temperature for several minutes. The mixture wasexpanded into the reaction vessel which was then isolated.The spectrum of the mixture was recorded as previouslydescribed and the initial partial pressures of boranes(usually of the order of 3 mmHg) t and argon were calcu-lated.1 The reaction vessel was heated from room tem-perature to 76 "C in 3 min.The zero reaction time wastaken 2 min after the start of heating. Spectra wererecorded at set time intervals up to 120 min. At the endof each reaction high-resolution measurements were made t ocheck the proportions of B,H, and B,Hll, and B,Hl, andB,H,,, in the pentaborane and hexaborane regions of thespectra.' As with the previous work, no quantitativeanalyses of H, or solid polymeric boranes were attempted.RESULTS AND DISCUSSIONCothermolysis of B2H6 and B4HIo at 75 "C.-The re-activity of both and B,H, was increased in thecothermolysis compared to the thermolysis of eachsingly, the effect being more noticeable for B$6, asshown in the Figure deposited in SupplementaryPublication No.SUP 22542 (4 pp.).$ In the cothermo-lysis after 120 min at 75 "C the concentration of B,H,was reduced by almost 30 from its original value (3.3mmHg) compared to a reduction of ~ 8 after the sametime in the thermolysis of at 100 "C.l Further, itshould be noted that B2H6 is itself produced on thermo-lysing B4HIo at 75 "C to an extent of ca. 10 of theinitial concentration of B4H1, in 120 min.l In thecothermolysis the initial B4H1, concentration (3.3 mmHg)decreased by ca. 93 compared to a decrease of 86 intherrnolysis of B4H10 alone.Be-tween 60 and 120 min after the reaction had started theconcentration of B5H11 was about three times that foundin the thermolysis of B4H1, alone, where again B11was the most abundant product. Other, comparativelyminor, products were hexaboranes (B,Hlo and B6H12 ina ratio of 2 : 3 from high-resolution measurements) andBioH14.These appeared a t lower rates and in lessabundance than from the thermolysis of B4H10 singly.The reaction (3) between B2H6 and B4H10 to produceBy far the most abundant product was B,H,,.2B4H10 + B2H6 * 2B,H11 + 2H2 (3)B5H11 and H2 has long been suggested to be an equili-brium.9 A study of the kinetics of the forward reactionbetween 72.5 and 92.9 "C was interpreted as showingB2H6 + (B4H8) - B5H11 + (BH3} (*)that the loss of H2 from B4H10 was the rate-determiningstep (1) followed by reaction (4) to give B5Hll and+ Throughout this paper: 1 mmHg = 13.6 x 9.8 Pa.For details see Notices to Authors No. 7, J.C.S.Dalton, 1978,Index issue1979 1399{ BH,) .lo Earlier workers had suggested that B4H10decomposed by two simultaneous first-order paths to(B4H8) and {B3H7} species, the route to the latter beingthe rate-controlling step.ll The arguments in favour of(1) and (4) as the major reactions giving B5H11 comparedto a route via {B3H7) formed directly from B4H10 includeevidence that {B3H7} and B2H6 react rapidly to giveB,H1o,lO* l2 and mass spectrometric confirmation of(B4Hs> but not (B3H7> or {BH,) as species in the thermo-lysis of B4H10.2'4'8a Convincing as this may seem, wecannot completely rule out other, probably less im-portant, routes to B5H11 by reactions of R,Hlo with{BH,}, {B3H7}, or even {B4H8), reaction (5). Under our{B4H8> + B4H10 - B5H11 $- {B3H7) (5)conditions it seems unlikely that reactions of B2H6 alonewould be important sources of {BH,} and {B,H7).lIt is possible that the BeH10 observed originates byreaction (6), and B,H12 by reaction (7).If reactions2{B4H8) - B2H6 + B6H10 (6){B4H8) + B2H6--tB6H12 + H2 (7)such as (4), and less importantly (7), were significantthen it would be expected that the concentration ofBaH10 would be lower than from a thermolysis of B4H10alone since B2H6 is competing for (B,H,). If this wereso then the concentration of B1oH14 would also be lower(as observed) since B6H10 is generally thought to be anintermediate along with BOH15 and BsH12 in the form-ation of B10H14.6 A lack of (B3H7) would also act toinhibit production of B1oH14.Cothermolysis of B5H, and B4H10 at 75 "C.-As far asthe formation of products is concerned this reaction isvery similar to the thermolysis of B,Hl0 alone, B5H,being unreactive when thermolysed at 75 "C.l A Figuredeposited in SUP 22542 shows the variation of productconcentration with time.The major product is B5H11with B2H6, BloH14, and hexaboranes (B6HlO andin the ratio 2 : 3) also present. A slight reduction inB5H, concentration was observed with small increasesin the concentrations of and BloHl, after 120 rnincompared to the thermolysis of B4H10 alone. Reactionssuch as (8) and (9), followed by conversion of B6H10 intoB10H14, may account for these findings.{B4H8> + Bi5H9-(B3H7) + B6H10 (8){B3H7} + B5H9 - B2H6 $- B6H10 (9)The reaction of {BH,) and B5H, at ca.450 "C has beenreported to produce a hexaborane but the hydrogencontent was not asce13ained.l~Attempted Cothermolysis of B5H11 and B4H10 at 75 "C.-The method used for the preparation of B6H12 resultedin the isolation of a small amount of B5H11 contaminatedwith B6H12 (ca. 20 of the B5H11 concentration). TheB5H11 so obtained could not be further purified and wasused in the cothermolysis with B4H10; the results of thispreliminary study (Figure 1) may therefore be lessreliable than those of the other cothermolyses reportedhere, although it was established that the concentrationof B6H10 was zero at the start of the reaction.With the thermolysis of B4H10 itself producing B5H11,the concentration of B5H11 increased during the first 45min and thereafter decreased.This is in accord withthe previous observations that B4H10 is more reactivethan B5H11 and that B5H11 is the major product from thethermolysis of B4Hio.l Of the other products, bothB2H6 and B1oH14 were formed in larger amounts after120 min than would have been expected from equivalentamounts of B4H10, B5H11, and B6H12 thermolysedseparately. This can be attributed to the increasedimportance of reaction (6) with an increased concen-tration of {B4H8) being, supplied via reaction (10).B5H11 (B4H8} + {BH3) (10)The BeHl,) would be subsequently converted intoB1oH14. The concentration of hexaboranes increasesOIIEEQ1.0050 100t / minFIGURE 1 Cothermolysis of B,H,, and B 5 H 1 1 at 75 "C.Bor-B4Hlo B5H11 (0)J B6H10-B6H18 (a), B,H6B10H14 (A)? B8H12 (a), BSH15 (0)during the first 45 rnin of reaction and then decreases.After 120 min the ratio ofCothermolysis of B6H10 and B4H10 at 75 "C.-WithB4H10 present the initial concentration of BaHlo wasreduced by 86 in 120 min. Thermolysis of B6H10alone showed a reduction of ca. 9 in 120 min.l Thecothermolysis (Figure 2) produced B5H11, B2H6, andB1oH14 as major products and a small amount of B,H,after 120 min. Substantial concentrations of B,H12 andBgH15 built up over the first 30-40 min and then de-creased. At the end of the reaction time the ratio ofB6H;O to B6H12 present was 3 : 1. The increased pro-portions of B,H6, B8H12, BgH15, and B10H14 observed canbe explained simply by reactions (11)-(13), or by moreto B6H12 is ca.1 : 1.(B4H8) + B6H10 - B9H15 + (BH3)B9H15 * B8H12 + (BH3)B8H12 + {B4H8> - B10H14 + B2H6(11)(12)(13)complicated routes, e.g. those involving { B,H,,) and{B3H7} as proposed by Long in the thermolysis ofB2H,. There is good evidence for reactions (11) l4and (12),8b but (13) has not been studied separately1400 J.C.S. DaltonAs well as in (13), B2H6 would be produced via reaction(6). The observation of relatively small amounts ofB6H12 and B,H, (ca. 10 of pentaboranes at t = 120min) can be explained in terms of reaction (7) givingB6H12 which then either decomposes by (14) or reactswith (B4H8), reaction (15).B6H12 --b BSHB + iBH3){B4H8> + B6H12 - B6HB + B5H11(14)(15)Cothermol'ysas of B,H,, and B4H10 at 75 "C (Figure 3).-The rate of decomposition of B6H12 in cothermolysiswith B4HIo appeared to be slower than B6H12 alone.This was due to the formation of B6H10 from BgH10,lproducing a mixture of B6H12 and BBH10.After 120min the ratio of BO. to B6H12 was 1 : 3.Greater concentrations of B2H6 and Bl,Hl, wereobserved after 120 min than would have been expectedfrom separate thermolyses of B4H1, and BaH12. Amixture of pentaboranes B5H, and BSH1, in the ratio2 : 3 was found after 120 min. To understand theseobservations one can postulate that the important re-actions in the cothermolysis are (14) and (15) givingB5H, and B5H11, (10) and (1) giving {B,H,), (6) givingB6H10, and (11)-( 13) producing B1oH14. Reaction (8)could play a part in B6H1, formation and other reactionsinvolving (B3H,) may occur to some extent.In conclusion, we have shown that the recentlydeveloped method of analysis of mixtures of boranesbased on mass spectrometry provides consistent resultson the cothermolysis of boranes, which can be inter-preted in a self-consistent manner.Comparison of these0550 1000t / m i nFIGURE 2 Cothermolysis of B,Hlo. and B,Hlo ( x ) at 75 "C.Boranes as in Figure 1Q 2.01.0050 100t / minCothcrinolysis of B,€Ilo. and B,Hl, ( x ) a t 75 "C. FIGURE 3Roranes as in Figure 1results with data from the thermolysis of individualboranes is a very useful approach to the complexproblem of understanding the thermolytic intercon-version reactions of boranes.In particular, it nowappears that (B4H,) is a reactive intermediate ofmajor importance in reactions of B4Hlo with B2H6,B5H,, B5H11, B,Hlo, and B,H12, and that B6Hlo is themost important stable borane precursor for the efficientproduction of BloHl, under the experimental conditionsused.We thank the S.R.C. for support, the University of Leedsfor the award of a Lowsoii Scholarship (to D. T.), and Mr.D. Singh for his help in obtaining the mass spectra.8/1838 Received, 19th October, 19781REFERENCESTaylorson, preceding paper.Stafford, J . Amer. Chem. Soc., 1966, 88, 929.Phys., 1970, 53, 782.Academic Press, New York, 1975.l Part 1, T. C. Gibb, N. N. Greenwood, T. R. Spalding, and D.A. €3. Baylis, G. A. Pressley, M. E. Gordon, and F. E.F. E. Stafford, Bull. SOC. chim. belges, 1972, 81, 81.P. S. Ganguli, L. P. Gordon, and H. A. McGee, J . Chem.E. L. Muetterties (ed.), ' Boron Hydride Chemistry,'L. H. Long, Pvogr. Inovg. Chem., 1972, 15, 1.L. C. Ardini and T. P. Fehlner, Inovg. Chem., 1973, 12, 798,and refs. therein.( a ) R. E. Hollins and F. E. Stafford, Inovg. Chem., 1970, 0,577; (b) J. F. Ditter, J. R. Spielmann, and R. E. Williams, Inorg.Chem., 1966, 5, 118.A. B. Burg and H. I. Schlesinger, J . Amev. Chem. SOC., 1933,55, 4009.lo J. A. Dupont and K. Schaeffer, J . Inovg. Nuclear Chem.,1960, 15, 310.l1 R. K. Pearson and L. J . Edwards, Abs. 132nd NationalMeeting of the Anaerican Chemical Society, New York, September1957, p. 15N.l2 R. Schaeffer, J . Inorg. Nuclear Chem., 1960, 15, 190.l3 T. P. Fehlner and S. -4. Fridmann, Inovg. Chem., 1972, 11,J. Rathke and R. Schaeffer, Inovg. Chem., 1974, 18, 3008.936