Mechanism of thermal decomposition of dibutylbis(triphenylphosphine)platinum(II)

TLDR: The thermal decomposition of di-n-butylbis(triphenylphosphine)platinum(Il) (l) in methylene chloride to n-butane, l-butene, and a complex of platinum(0) is proposed to take place by an intramolecular process involving an initial dissociation of I equiv of triphenyl phosphine from 1 to yield a three-coordinate intermediate 5, elimination of platinum hydride from one butyl group of 5 with concomitant ransfer of the resulting l

Abstract: The thermal decomposition of di-n-butylbis(triphenylphosphine)platinum(Il) (l) in methylene chloride to n-butane, l-butene, and a complex of platinum(0) is proposed to take place by an intramolecular process involving an init ial dissociation of I equiv of triphenylphosphine from 1 to yield a three-coordinate intermediate 5, elimination of platinum hydride from one butyl group of 5 with concomitant ransfer of the resulting l-butene to the vacant coordination site, several cycles composed of rapid addition of platinum hydride to the coordinated butene and subsequent rapid reelimination of platinum hydride from the resulting butylplatinum complexes, and a final reductive elimination of n-butane from an intermediate having both hydride and butyl moieties bonded to platinum. Three significant conclusions concerning the mechanism of thermal decomposition of t have emerged lrom this study. First a vacant coordination site on platinum is a prerequisite for thermal decomposition under the conditions studied. Second, the identity of the rate-l imiting step for the overall decomposition reaction depends upon the concentration of triphenylphosphine added to the solution: in the absence of added triphenyiphosphine, the rate-l imiting step is the dissociation of I to 5; in the presence of ca. I equiv of added triphenylphoiphine, the rate-limiting step is the reductive elimination of butane. Third, the olefins participating in the platinum hydride addition-elimination sequence are coordinated in the intermediate platinum Complexes ufficiently firmly that they do not exchange with 1-butene free in solution. f nformat ion concerning the mechanisms of thermal r decomposi t ion of a lkyl der ivat ives of t ransi t ion metals is pert inent both to theoret ical d iscussions of the electronic structure of carbon-metal c bonds and pract ical appl icat ions of t ransi t ion metal organometal l ic compounds in organic synthesis and catalysis. ( l ) Supported' in part by The Nat ional Science Foundat ion, Grants GP-7266 and GP-142,17. (?) Nat ional Inst i tutes of Heal th Predoctoral Fel low, 1967-196g: E. B. Herschberg Fel low, 1966-1967. (3) Nat ional Inst i tutes of Heal th Predoctoral Fel low. 1967-1970, Journal ol'the Anterican Chemical Societ;' I 94:15 I Jult ' 26, 1972 Mechanisms involving both the homolytic scission of carbon-metal a bonds and the 6 elimination of metal hydrides have been proposed for these thermal decomposi t ions;{ '5 however, the lat ter course has been established as the more common for-n-alkyl derivatives (4) M. S. Kharasch and O. Reinmuth. \"Gr ignard React ions of Nonmetal l ic SubstancEs,\" Prent ice Hal l , New York, N. Y. , 1954, Chapter 5. (5) Reviews: (a) G. E. Coates, M. L. H. Creen, and K. Wadc, \"Organometal l ic Compounds,\" Vol . 2, 3rd ed, Methuen and Co., London, 1968, Chapter 7; (b) G. W. Parshal l and J. J. Mrowca, Adcan. Organometal . Chem.,7,157 (1968): (c) F. A. Cotton, Chem. Reo., 52 , 557 (1955) ; (d ) I . I . K r i t skaya , Usp . Kh im. ,35 , 167 (1966) . of platinum(Il),u rhodiur,n(I),7 and copper(I),E by ideniif ication 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 compounds and the reversibi l i ty of th is el iminat ion are unknown. The work reported in th is paper deals wi th an examinat ion of the mechanism of thermal decomposition of crs-di-n-butylbis(triphenylphosphine)platinum(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 substantiate a pathway for itreimat decomposition of 1 involving platinum hydride elimination and to provide detail to the steps surrounding this elimination.