/: (j Mechanismof ThermalDecomposition of Di-n-butylbis( triphenylphosphine ) platinum ( II )' GeorgeM. Whitesides,*John F. Gaasch,2and Erwin R. StedronskyJ Contributionfrom the Department of Chemistry, MassachusettsInstitute of Technology, Cambridge, Massachusetts 02139. ReceioedAugust 4, I97l Abstract: The thermaldecomposition of di-n-butylbis(triphenylphosphine)platinum(Il)(l) in methylenechloride to n-butane,l-butene, and a complexof platinum(0)is proposedto takeplace by an intramolecularprocess involv- ing an initial dissociationof I equivof triphenylphosphinefrom 1 to yielda three-coordinateintermediate 5, elimi- nationof platinumhydride from one butyl groupof 5 with concomitanttransfer of the resultingl-butene to the vacantcoordination site, several cycles composed of rapidaddition of platinumhydride to thecoordinated butene and subsequentrapid reeliminationof platinumhydride from the resultingbutylplatinum complexes, and a final reductiveelimination of n-butanefrom an intermediatehaving both hydrideand butyl moietiesbonded to platinum. Three significantconclusions concerning the mechanismof thermal decompositionof t have emergedlrom this study. First a vacant coordinationsite on platinum is a prerequisitefor thermal decompositionunder the con- ditionsstudied. Second,the identity of therate-limiting step for the overalldecomposition reaction depends upon the concentrationof triphenylphosphineadded to the solution: in the absenceof addedtriphenyiphosphine, the rate-limitingstep is the dissociationof I to 5; in the presenceof ca. I equivof addedtriphenylphoiphine, the rate-limitingstep is the reductiveelimination of butane. Third, the olefinsparticipating in the platinum hydride addition-eliminationsequence are coordinatedin the intermediateplatinum Complexessufficiently firmly that they do not exchangewith 1-butenefree in solution. f nformation concerning the mechanismsof thermal Mechanisms involving both the homolytic scission of r decomposition of alkyl derivatives of transition carbon-metal a bonds and the 6 elimination of metal metals is pertinent both to theoretical discussionsof hydrides have been proposed for these thermal de- the electronicstructure of carbon-metal c bonds and compositions;{'5 however, the latter course has been practical applications of transition metal organo- establishedas the more common for-n-alkyl derivatives metallic compounds in organic synthesisand catalysis. (4) M. S. Kharasch and O. Reinmuth. "Grignard Reactionsof Non- metallicSubstancEs," Prentice Hall, New York, N. Y., 1954,Chapter 5. (l) Supported'in part by The National ScienceFoundation, Grants (5) Reviews: (a) G. E. Coates, M. L. H. Creen, and K. Wadc, GP-7266and GP-142,17. "Organometallic Compounds," Vol. 2, 3rd ed, Methuen and Co., (?) National Institutes of Health Predoctoral Fellow, 1967-196g: London, 1968, Chapter 7; (b) G. W. Parshall and J. J. Mrowca, E. B. HerschbergFellow, 1966-1967. Adcan.Organometal. Chem.,7,157 (1968): (c) F. A. Cotton, Chem.Reo., (3) National Institutes of Health Predoctoral Fellow. 1967-1970, 52,557 (1955); (d) I. I. Kritskaya,Usp. Khim.,35, 167 (1966). Journal ol'the Anterican Chemical Societ;' I 94:15 I Jult' 26, 1972 of platinum(Il),u rhodiur,n(I),7 and copper(I),E by ideniification of metal hydrides as products of the decompositions, and for derivatives of other metals by less direct techniques.s'e The nature of the factors determining both the rate of metal hydride elimination in the thermal decompositions of these classes of compoundsand the reversibilityof this elimination are unknown. The work reported in this paper dealswith an examinationof the mechanismof thermal decompo- sition of crs-di-n-butylbis(triphenylphosphine)plati- num(Il) (1), a representative alkyl derivative of a dE transition metal ion, whose physical properties make it amenable to detailed mechanistic examination. The purpose of this study was to substantiatea pathway for itreimat decomposition of 1 involving platinum hy- dride elimination and to provide detail to the steps surrounding this elimination. Results Preparation and Characterization of Alkylplatinum- (ID Compounds. Di-n-butylbis(triphenylphosphine)- platinum(II) and di'n-octylbis(triphenylphosphine)- platinum(ll) (2) were preparedby reactionof the corre- iponding n-alkyllithium reagentswith crs-dichlorobis- (triphenylphosphine)platinum(II)in ether-hexane at 0o, using proceduresdeveloped by Chatt and Shaw.t0' Deuterated derivativesof 1 were synthesizedby anal' PhrP\ PhrP\ \^ / ,,CHrCH-CH.,CHr -Dt/ -cDrcH,cH]cH, zl L\ -T-,-r-_I-rrl | | | ,rft-. -cH.,cH.,cH.,cHr ^r 'cD.,cH,,cH.,cH, | | Ph,P/ PhlP- I l-1,1-dt l" 5 loo oo5 (PPM) ogous proceduresusing as starting materials l-bromo- Figure l. Observed100-MHz deuterium-decouplednmr spectril cu. 0.25 rVlsolutions in methylenechloride of : (A) di-rr- butane-/,/-d,.rand l-bromobutane'2,2-d2which were takenof butyl-2,2-d,-[l, l'-bi s(d i phenylphosph i no )ferrocene]plati num( I I ) (3 ) ; (f); (C) PhJP. ,CH,,CDrCHjCH, (B) di-r-buryl-2,2-drbis(triphenylphosphine)platinum(lI) \prl calculatedspectrum of di-rr-buty"l-2,2-drbis(triphenylphosphine)- -/^ "\ -CH2CD:CH2CHi platinum(ll)(ref 14). Ph3P' l-2,2-d2 SchemeI. Synthesisof l-Bromobutane-2,2'dzand A\ Ph, (V)-P\ l-Bromobutane-l,l-dz /cr)cu)cHrcHl 'F?'l' T\, \tt( l. NsOEr,Dtorr Fe CHTCHzCCITCN+' CH'CHrCClzCOzEt I t \ 2. HrO. A/\ ,r^ a*F;,/(^ tHrcHrcHrcHt CH'CH:CDrCH'Br 3 CHTCHTCDzCO'Et ffi;;> -507,; 97%&,3% dt preparedfollowing the reactionsequences outlined in l' Li'\lDr SchemeI. rr Di-n-butyl' and di-n-butyl-2,2-d'rU,I'' cHrcHrcHzco:Et ' t cHrcHzcH:cD:Br 2. HBrll:-ior -50%; 98%dz,27i ttt (6) J. Chatt, R. S. Coffey,A. Gough, and D. T. Thompson,J. Chem. Soc.A, 190(1968), and referencescited therein. bis(diphenylphosphino)ferrocenelplatinum(II)(3 and (1968); also P. (7) R. Cramer, AccountsChem. Res., l, 186 see J. were prepared by reaction of thc Collman. ibid., 136(1968). 3-2,2-d2,respectively) (S) G. fvt. Whitcsidcs,E. R. Stredronskv,C. P. Casev,and J' San correspondinglithium reagentswith dichloro[1,1'-bis- Filippo, Jr.,J. Amer. Chem. 9oc.,92, 1426(19701. Seealso K. Wada, (diphenyl phosphino)ferrocenelplatinum(I I). t 3 lvLTamura. and J. I(ochi, ibid.,92,6656(1970). rH 1- (9) R. F. Heck, AccountsChem. Res.,2, l0 (1969); L. Reich and The deuterium-decoupled nmr spectrum of A. Schindler. "Polymerization by Organometatlic Compounds," 2,2-d!was examined to establishthe configuration of the 4; P' Candlin'K' A' Interscience.Nerv York, N' Y'' 1966,Chaptcr J' alkyl and phosphine groups. The group of lines be- Taylor, and D. T. Thompson, "Reactions of Transition-metalCom- plexes,"Elsevier, Amsterdam, 1967. tween -0.4 and 0.9 ppm in the spectrum shown in (10) (a) J. Chatt and B. L. Shaw,J. Chem.Soc., 705, 4020 (1959); Figure t can be assignedto the terminal ethyl groups 507,<(1962): (b) J. D. Ruddick and B. L. Shaw,J. Chem..Soc.,{, 2801 (1969). ( We found reductionof a-halo estersin a medium containing (13) J. J. Bishop,A. Davison,NI. L. Katcher,D. W. Lichtenberg, I I ) have (1971\' D:O to be a much more etficientmethod of introducingdeuterium c to R. E. Merrilt.and J. C. Smart,J. Organomerai.Chern.,27,24l platinum atoms the carbonyl group than the base-catalyzedexchange methods com- Thereis no evidenceof interactionbetwcen the iron and monly used.l2 in thesecompounds. We have used the ferrocene-l,l'-bis(diphenvi- preparcd (12) A. Murray and D. L. Williams, "Organic Synthesiswith Iso' phosphine)(fdpp)ligand in thesestudies because it is an easily topcs," Interscicnce,Nerv York. N. Y., 1958; M, Fctizon and J. Gra- bi.tenrate chelating phosphine whose dichloroplatinum(II) complcx main, Brrl1.Soc. Chint. Fr.,65l (1969). reactscleanly with n-butyllithium. llthitesides, Gausclt, Stedronskl' ,t Di-n-butv'lbis(tiphenv'lplnspline)platinuntt I lt 5260 of the alkyl chains: the four.-linepattern at 1.04ppm DTNBO is entirely compatible with a mechanismfor and the one visible rerPtsatellite at 1.76ppm arise from thermolysis of I not involving free n-butyl radicals. the methylenegroups d to the platinumatom. Detailed However,the pertinenceof thesedata to discussionsof analysisof the a-methylene"quartet" indicatedthat the thermal behavior of I in the absenceof DTBNO two distinct PPTCH coupling constantswere required should be viewedwith some reservations,since DTBNO to simulatethe spectrumr{and confirmedthat I has forms complexeswith a number of metals16and since the cis configuration.expected by analogy with related a complex of 1 and DTBNO cannot be excludedas the organoplatinum compounds.r0 The spectrum of 3- species giving rise to the products obtained from 2.2-d2(Figure l) was not analyzedin detail; however, decompositionof 1 in the presenceof DTBNO on the its qualitativesimilarity to that of l-2,2-dt suggeststhat basisof presentlyavailable evidence. rT the PPIP and PPIC^F/coupling constants,and by in- In order to test explicitly the hypothesisthat the ferencethe correspondingbond angles,are similar for hydrogen atoms incorporated into the n-alkane por- thesetwo compounds. tion of the hydrocarbon productsoriginated exclusively Productsof the Thermal Decompositionof l. Heat- in the o-bonded n-alkyl groups of the starting organo- ing a 0.10 M solutionof I at 60ofor 24 hr in methylene metallic compound, and not in the solvent or the aryl chloride or benzeneresulted in completeconversion of groupsof the coordinatedtriphenylphosphine ligands, tt rr-butyl moieties to an equimolar mixture of n-butane the octane formed as a product of the thermal decom- and l-butene accompanied by transf,ormationof