1481
Organometallics 1986,5, 1481-1490
Mechanisms of Thermal Decomposition of trans-Chloroneopentylbis(tricyclopentylphosphine)platinum( I I)' Robert L. Brainard, Timothy M. Miller, and George M. Whitesides' Depattment of Chemistry, Harvard Universi?v, Cambridge, Massachusetts 02 138 Received January 29, 1986
The most probable mechanism for the thermal decomposition of trans-chloroneopentylbis(tricyc1opentylphosphine)platinum(II) (bPtNpC1,l) to trans-chlorohydridobis(tricyclopentylphoaphhe)phtinum(II) (L2PtHC1,5) and 1,l-dimethylcyclopropane(DMC) in cyclohexane solution involves initial equilibrium dissociation of tricyclopentylphosphine(L) from 1,reversible intramolecular oxidative addition of a 7-C-H bond of the neopentyl moiety to platinum and formation of an intermediate platinacyclobutane, 3, and rate-limiting reductive elimination of 1,l-dimethylcyclopropane(Scheme I). Reassociation of L yields 5. In the absence of added L, the reaction is half-order in 1; with added L (0.068-0.54 M), the reaction is fiist-order in 1 and inverse first-order in L. Arrhenius activation parameters Al and Ea,1for the first-order decom osition of 1 to 5 and dimethylcyclopropane were determined in cyclohexane containing L ([L], = 0.39 h f1: EG1= 49 f 1 kcal/mol; log AI = 21 f 2; AH* (156 OC) = 48 f 1 kcal/mol; AS* (156 "C) = 34 f 1eu. Substitution of deuterium for hydrogen in the neopentyl group of 1results in a deuterium kinetic isotope effect of kH/kD = 2.9. The dependence of the rate of reaction on the concentration of 1 allows estimation of the dissociation constant Kq of L from 1; Kq (156 "C) = 0.017 f 0.002 M. From the Arrhenius parameters in the presence and absence of L, the temperature dependence of Kq can be determined ASq = 30 f 5 eu and A",, = 16 f 3 kcal/mol. A side reaction (3-20%) in the decomposition of 1 produces neopentane and appears to proceed through free neopentyl radicals. trans-Chloro(1-norbornylmethy1)bis(tricyclopentylphosphine)platinum(II)(L2Pt(CH2Nb)C1,6) decomposes thermally at 156 OC and gives 1-methylnorbornane,LzPtHCl (5), and a product, 7, derived from cyclometalation of the phosphine L.
Introduction
This paper describes the mechanism of thermal decomposition of trans-chloroneopentylbis(tricyclopenty1phosphine)platinum(II) (1, L2PtNpC1) to trans-chlorohydridobis(tricyclopentylphosphine)platinum(II) (5, L2PtHC1)and 1,l-dimethylcyclopropane(DMC) (Scheme I). This reaction is interesting for three reasons. First, in general, it provides further information about the details of elementary reactions which break and form C-H and C-C bonds by reaction with a platinum atom. Reforming12,3hydrogenation: intermolecular C-H activation,* intramolecular C-H activation,'-l2 @-hydride eliminaand reductive elimination of C-H and tion-inserti~n~"'~ (1)Supported by the Netional Science Foundation (Grants CHE 8205143 and CHE 85-08702). (2) Davis, S. M.; Somorjai, G. A. J. Phys. Chem. 1983,87, 1545-52. (3) Sinfelt, J. H. Science (Washington, D O 1977, 195, 641-646. Selman,D. M.; Voorhies, A. Ind. Eng. Chem., Prod. Res. Dev. 1975,14, 118-123. Garin, F.; Gault, F. G. In Chemistry and Chemical Engiwering of Catalytic Processes; Prins, R., Shuit, G. C. A., Eds.; Sijthoff and Noordhoff: Germantown, MD, 1980; pp 351-380. (4) James, B. R. Adv. Organomet. Chem. 1979,17,319-405. (5) For example: Janowicz, A. H.; Bergman, R. G. J. Am. Chem. SOC. 1983,106,3929-3939. Hoyano, J. K.; Graham, W. A. G. Ibid 1982,104, 3723-3725. Crabtree, R. H.; Mellea, M. F.; Mihelcic, J. M.; Quirk, J. M. Zbid 1982, 104, 107-113. Baudry, D.; Ephritikhine, M.; Felkin, H.; Holmes-Smith, R. J. Chem. SOC.,Chem. Commun. 1983, 788-89. (6) Jones, W. D.; Feher, F. J. J. Am. Chem. SOC. 1984,106,1650-63. 1984, 106, (7) Fendrick, C. M.; Marks, T. J. J. Am. Chem. SOC. 2214-2216. (8)Watson, P. L. J. Am. Chem. SOC. 1983,105,6491-93. (9) 'Nip, T. H.; Thorn, D. L. J. Am. Chem. Soc. 1981,103,2448-2450. (10) Empsall, H.D.; Hyde, E. M.; Markham,R.; McDonald, W. S.;
Norton, M. C.; Shaw, B. L.; Weeks, B. J. Chem. SOC.,Chem. Commun. 1977,589-590. AI-Salem, N. A.; McDonald, W. S.;Markham,R.; Norton, Dalton Tram. 1980, 59-63. M. C.; Shaw, B. L. J. Chem. SOC., (11) Foley, P.; DiCoeimo, R.; Whitesides, G. M. J. Am. Chem. SOC. 1980,102,6713-6725. (12) DiCoeimo, W,Moore, S.S.;Sowinski, A. F.; Whitesides, G. M. J. Am. Chem. SOC. 1982,104, 124-133. (13) Brainard, R. L.; Whitesides, G. M. Organometallics 1985, 4, 1550-1567. ~ . . ~~. . (14) McCarthy, T. J.; Nuzzo, R. G.; Whitesides, G. M. J. Am. Chem. SOC. 1981,103, 3396-3403.
Scheme I. Postulated Mechanism for Conversion of 1 to 5 and 1,l-Dimethylcyclopropane (L = Tricyclopentylphosphine) L\ /
Y
Pt
CI
\
L
I
4
k-l
L
+
L\
Y
Pt
C;
k2
-
k-2
2
5
.
H L\ I P t 2 CI' 3
+
L
4
C-C b ~ n d s " J ~ Jall ~ 'involve ~ these types of elementary reactions. Second, comparison of this reaction with analogous studies of the mechanism of thermal decomposition of (Et3P),PtNp2 to (Et3P)2PtCH2C(CH3)2CH," 1 l and of ( C Y ~ P ) ~ P ~ C H ~ C ( C H to ~(Cy3P),Pt0 ) ~ C H ~ and 1,ldimethylcycl~propane~~ permits us to draw mechanistically useful inferences concerning the structural factors determining the rates of these reactions. Third, the kinetic order of the reaction in 1 changes from half-order to first-order on addition of tricyclopentylphosphine (L). This kinetic feature permits us, for the first time in our studies of organoplatinum compounds, to analyze separately the thermodynamics of the preequilibrium dissociation of L from the platinum center. We chose to use tricyclopentylphosphine(L) rather than, e.g., triethylphosphine or tricyclohexylphosphine in this (15) Thorn, D. L.; Hoffmann, R. J. Am. Chem. SOC.1978, 100, 2915-2916. (16) Abis, L.; Sen, A.; Halpern, J. J. Am. Chem. SOC. 1978, 100, 2915-2916. (17) Michelin, R. A.; Faglia, S.; Uguagliati, P. Inorg. Chem. 1983,22, 1831-1834.
(18)Eisch, J. J.; Piotrowski, A. M.; Han, K. I.; Kruger, C.; Tsay, Y. H. Organometallics 1985,4, 224-231. (19) DiCosimo, R.; Whitesides, G. M. J . Am. Chem. SOC. 1982, 104, 3601-3607.
0276-733318612305-1481$01.50/00 1986 American Chemical Society
1482 Organometallics, Vol. 5 , No. 7, 1986
Brainard et al.
Scheme 11. Products of Thermal Decomposition of la F(Cti3),
Hydrocorbon Products (%)
C L L , Time of ( M 1 Deconposition D(
NPH
0.00
Ih
94
3.5
0. 39
12 h
81
16
Np2
0
0.5
0.9
0.6
5.2
1.0
