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J . Am. Chem. SOC. 9 94,116, 6445-6446 6445Formation of the Radical Anion of Cubene andDetermination of the Heat of Formation, Heat ofHydrogenation, and Olefin Strain Energy of CubenePaul 0. Staneke? Steen Ingemann? Philip Eaton,$Nico M. M. ibbering,',? and Steven R. Kass'M

Institute of Mass Spectrometry, University of AmsterdamNieuwe Achtergracht 1291018 WS Amsterdam, The NetherlandsDepartment of Chemistry, The University of ChicagoChicago, Illinois 60637Department of Chemistry, U niversity of MinnesotaMinneapolis, Minnesota 55455

Received November 23, 1993Revised Manuscript Received April 5 , 1994Small strained ring compounds have fascinated chemists forgenerations and have been the sub ject of numerou s investigations.'Many of these species are fleetingly stable, and as a result,thermody namic information is usually unavailable. Gas-phaseion chemistry is of consid erablevalue in this regard, and we reportherein on the heat of form ation, heat of hydrogenation , and olefinstrain energy of cubene.The ato mic oxygen radical anion (O+) reacts with many organiccompounds by abstracting both a proton an d a hydrogen atomto afford a new radical ion and water.2 For example, bicyclobutanereacts with 0'- to produce the radical anion of bicyclobutene (eql).3 We have found tha t cubane reacts with the atomic oxygen

ion in a home-built Fourier transform ion cyclotron resonance(FT-IC R) mass spectrometer4 o afford hydroxide and a CsH6.-ion (1,m /z 102)which we believe is the radical anion of cubene(eq

1

2Our structural assignment is based, in part, upon analogy toother recent work, Le., the formation of cubene6 and th e radicalanions of bicyclobu tene3and benzyne.' Ion 1also undergoes fivehydrogen/deuterium exchanges upon reaction with deuteriumt University of Amsterdam.t University of Chicago.)University of Minnesota.1 Alfred P. Sloan Research Fellow.(1) For example, see: (a) Greenberg, A.; Liebm an, J. F. Strained OrganicMolecules; Academ ic Press: New York, 1978. (b) The C hemistry of rheCyclopropyl Group; Rappoport, Z . , Ed.; John Wiley and Sons: New York,1987; Parts 1, 2. (c) W iberg, K. B. Chem. Rev. 1989.89, 973 and the restof this issue.(2) For a recent review on the gas-phase chemistry of the atomic oxygenion, see: Lee, J.; Grabowski, J. J. Chem. Reu. 1992, 92, 1611.(3) Chou, P. K.; Kass, S . R. J . Am. Chem. Soc. 1991, 113, 697.(4) Koning, L. J. de; Nibbering, N. M. M. J. Am . Chem.Soc. 1987,109,1715 and refe renca cited therein.(5) A small amount (S%)f a M - ion is also formed. This spec ies couldbe the conjug ate base of cubene, bu t since it is formed in low yield it was notexplored further. Cuban e and 0- also react in a flowing afterglow device toafford a small signal of the M - 2 ion, but the reaction is inefficient andprecludes a thorough investigation of this ion.(6) Eaton, P.E.;Maggini, M. J . Am. Chem.Soc. 1988, 110, 7230.

OOO2-7863/94/1616-6445$04.50/0

100 p (CeHe) - 3 10 7 mbarp (D20) - 3 10 -7 mbar

0 .0 0 .4 0.8 1. 2Reaction time in s

Figure 1. Hydrogen-deuterium exchange behavior of the radical anionof cubene (1) upon reaction with deute rium oxide.oxide (Figure 1).8 A sixth sequen tial exchange is not observedbecause of the falling intensity of the deuterium containing ionswith time and the presence of a competitive deuteron transferpathway, Le., DO- formation. The only other CgH6* isomerwhich would be expected to undergo at least five H /D exchangesis the radical anion of 1,2-dehydrocyclooctatetraene, but thisspecies can be ruled ou t on thermody namic grounds.9The above result strongly suggests that the acidity of cubylradical (2) is similar to tha t of deuterium oxide ( w , " d = 392.0kcal mol-').lI In accor d with this observatio n, 1does not reactwith weaker acids such as ammonia-d3 or dimethy lamine(wad= 403.6and 396.2 cal mol-', respectively ), but is protona ted byfluorobenzene and methanol ( m a c i d = 387.2 and 381.5 kcalmol-', respectively). Consequently, m0&J(2)392f kcalmol-', which corresponds o a 12kcal mol-' acidity enhancementover the calculated value for cubane (404.3kcal mol-' at theMP2/6-31+G(d)//6-31+G(d)evel). This difference is thesame as between bicyclobutane and the I-bicyclobutyl radical3but somewhat smaller than the benzene/phenyl radical aciditydifferential (22 kcal mol-').7a*bThe cubene radical anion undergoes electron transfer uponreaction with m-CF&H 4CN, CsFa, or CS2 (EA = 0.67, .52,and 0.51 eV, respectively) but not with CH3N02 (E A = 0.48

(?) (a ) Guo,Y.; Grabowski, J. J. J . Am. Chem. Soc. 1991,113,5923 . (b)Matimba, H. E.K.; Crabbendam, A. M.; Ingemann, S.;Nibbering, N. M.M. J . Chem.Soc., Chem. Commun. 1991 ,644. (c) Guo, Y.; Grabowski, J.J. Int. J . Mass Specrrom. Ion Processes 1992, 117, 299.(8) Th e slight increase in the relative abundance of th e dl ions at reactiontimes exceeding 0.8 s may be a resu lt of the presence of anothe r species withthe sa me mass as th e dl ion. The present experiments do not give insight intothe origin or structure of this species. Initially, it cannot be anything morethan a minor ion, but it can become more pronounced at longer times becausethe sum of the abundanccs of the d cubene radical anions decreases asa result of deuteron transfer (DO- ormation) and possibly some electrondetachment. In any case, the H-D xchange results are inconsistent with th e1,3- and 1 4-dehydrocu bane radical anions since these species should undergotwo and zero exchanges, respectively.(9) If the radical anion of 1,2-dehydrocyclooctatetraenewas formed andone assumes a vinyl C-H BDE f 110kca l mol-' for 1,3,5,7-cyclooctatetraene,then one can derive a heat of hydrogenation for the neutral dehydrocyclo-octatetraene going to cyclooctatetraene using our experimental data and athermodynamic cycle similar to the one shown in eq s 3-7. Th e hea t ofhydrogen ation would be 98 kcal mol-I, which is unreasonably large given thatthe hydrogenation energy of 2-butyne going to cis-2-butene is 37 kcal mol-l,lOand that of 1,5-cyclooctadien-3-ynegoing to 1,3,5-cyclooctatriene s only 48kcal mol-I.l1 Moreover, several stable dehydrocyclooctatetraeneshave beenprepared.'*(IO) Cox, J. D.; ilcher, G. Thermoc hemistry of O rganic and Organo-metallic Compounds;Academic Press: London, 1970.(1 1) All thermodynamic information including gas-phase acidities andelectron affinities, unlessotherw ise noted, comes from Lias et al.: Lias, S. .;Bartmess, J. E.;Liebman, J. F.;Holmes, J. L.; Levin, R. D.; allard,W. .J. Phy s. Chem. ReJ Dura 1988, 17, Suppl. 1.(12) See: Huang, N. 2.;ondheim er, F. Acc. Chem. Res. 1982,15,96 andreferences therein.

0 1994 American Chemical Society

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6446 J. Am. Chem. Soc.. Vol. 116,No. 4, 1994 Communications to the EditoreV).llJ3 This data indicates that the electron affinity of cubene(3) is 0.50 f .1 eV (11.5 i kcal mol-'), which is simi lar tothat of bicyclobutene (8 i kcal mol-')3 and in accord with theexpectation for a highly bent out-of-plane olefin. Most hydro-carbo ns do not have bound rad ical anions; the loss of an electro nis exothermic. However, the LU MO energy drops and the bindingenergy increases as an alkene is distorted from planarity.14Our da ta can be combined with the C-H bond dissociationenergy (BDE) of cubane (4) and some well-known ancillarythermochemical data to derive the heat of hydrogenation ofcubene. Unfortunately, the C-H BDE of cubane has not beenmeasured, and thus we have used ab initio calculations to obtainthis quantity. Three methods were used: direct calculation ofthe bond energy (10 4 kcal mol-I),'s an isodesmic reactio n givingthe relative BDEs of cubane and isobutane (104 kcal mol-I),'6and a therm odynam ic cycle relating the acidity of cubane a ndthe electron affinity of cubyl radical to the bond dissociationenergy of interest (104 kcal m0l-').l7 All three procedures werecarried out using second-order Morller -Plesset perturbation theoryand 6-31+G(d) or 6-31G(d) optimized geometries (e.g., M P2 /6-3l+G(d)//6-31+G(d))** and gave the same result, 104 kcalmol-', to within 1 kcal mol-'. Thi s value is in accord with thelimited availableexperimentaldata and seems quite reasonable.19Combining this quantity with w & ( 2 ) , EA(3), IE(H'), andBDE(H2) leads to a heat of hy drogenation for cubene of 90 f4 kcal mol-' (eqs 3-7), a heat of form ation of 238i kcal mol-',a strain ene rgy of 227 i kcal mol-', and an olefin strain energy(OSE) of 63 i kcal mol-I.20~2l These values are in very good

(1 3) Proton transfer takes place with C H J N O ~nd com petes with electrontransfer with m-C F$ 2a CN (60% proton transfer and 40% electron transfer).(14) (a) Smith, J. M.; Hrovat, D. A.; Borden, W. T.; Allan, M.; Asmis,K. R.; Bulliard, C.; Haselbach , E.; Meier, U. C. J.Am. Chem.Soc. 1993,115,3816. (b) Borden, W. T. Chem. Rev. 1989, 89 , 1 0 9 5 . (c) Hrovat, D. A.;Borden, W. T. J . Am. Chem. Soc. 1988,110,4710. (d) Yin, T. K.; Miyake,F.; Renzoni, G. E.; Borden, W. T.; Radziszewski, J. G.; Michl, J. J. A m .Chem.Soc. 1986,108, 3544. (e) Jordan, K. D.; Burrow, P. D. Chem. Reo.1987,87, 557. (f) Jordan, K. D.; B urrow, P. D. Acc. Chem. Res. 1978, 11,341.(15) For a recent discussion on the calculation of C-H bond dissociationenergies of alkanes, see: Wiberg, K. B.; Hadad, C. M.; Siebcr, S.; chleyer,P. v. R. J.Am. Chem. Soc. 1992,114, 5820.(16) This method has been used previously with a smaller basis set (6-31G(d)/3-21G). The results ar e quite similar. See: Hrovat, D. A.; Borden,W. T. J.Am. Chem. Soc. 1990,112,3221.(17) See he supplementary material for additional details.(18) (a) Maller, C.; Plesset, M. S. hys. Reo. 1934,46,618. (b) Pople, J.A.; Binkley, J. S.; Seeger, R. Inr. J . Quantum Chem., Sy ". 1976, 10, 1.(19) (a)Choi,S.-Y.;Eaton,P.E.;Newcomb,M.;Yip,Y.C.J.Am.Chem.Soc. 1992,114,6326. (b) Della, E. W.; Head, N. J.; Mallon, P.; Walton, J.C. J . Am. Chem. Soc. 1992,114,10730. (c) Luh, T.-Y.; Stock, L. M. J.Org.Chem. 1978,43, 3271.

accord withan OSE of 58.9 kcal mol-' an da heat of hydrogenationof 82.5 kcal mol-' previously calcu lated by Hrova t and Bordenwith a two-configuration wave function (6-31G(d)/3-21GTCSCF) I&

1 3

392 kcal mol''2&I5 m*+' ( 5 )104 kcal mol"4

-IE(H) - BDE(H2)H++ H' + e' - 2 (6)-417.8 kcal mol"Acknowledgment. We wish to thank D r. PhillipK. hou andProfessor Charles DePuy for carrying out the initial reactionbetween0'-nd cubane and thus showing that an M -2 ion can

be formed. P.O.S., S.I., and N.M.M.N. thank the NetherlandsOrganization for Scientific Research (SON /NW O) for financialsupport.S.K. cknowledges the donors of the Petroleum ResearchFund, administered by the American Chemical Society, theNational Science Found ation, and the M innesota Supercom puterInstitute for their support.Supplementary Mat erial Available: Calculated structures,energies, and the three procedures used to calculate the bonddissociation energy of cubane (6 pages). This ma terial is containe din many lib raries on microfiche, immediately follows this articl ein the microfilm version of the journal, and can beordered fromthe AC S; see any current ma sthead page for ordering information.(20) The s train energies of cubane (163.9 kcal mol-') and cubene werecalculated by subtracting their strain-free heats of formation (obtained usingBenson's group equivalents) from their experimental heats of formation. TheOSE was obtained by taking the difference in the strain energies of cubeneand cubane. (a) Benson,S . W. ThermochemicalKineric~,nd ed.; John Wileyand Sons: New York, 1976. (b) Maier,W. .; Schleyer, P. v. R. J. Am. Chem.

Soc. 1981,103, 1891.(21) The specified uncertainties arc independent of the calculated C-HBDE of cubane a s long as it is accu rate to within 7 kcal mol-'. W e anticip atethat it is much more reliable (+2-3 kcal mol-').