1126
Journal of the American Chemical Society
(23) The facile occurrence of the H-abstraction step, i.e., the second stage of reaction 7, which serves to "trap" the initially produced CeH5CHCH3 radical irreversibly and inhibit reversal of the first step, finds precedent in earlier studies on other low-spin cobalt(l1) complexes, e.g., [ C O ( C N ) ~ ] ~tRCHCH3 [ H C O ( C N ) ~ ] ~ - RCH=CHz. The competition between this process and the alternative combination reaction, [CO(CN)S]~- RCHCH3 -,[R(CH&HCo(CN)p13-, is favored for secondary (Le.. (CH&CH) relative to primary (e.g., CH3CHz) radicals.18 Tachkova, E. M.; Rtidakova, I. P.; Yurkevich. A. M. Zh. Obshch. Khim. 1974, 44,2574. Blaser, H. U.; Halpern, J. J. Am. Chem. Soc., in press. One of the few transition metal-alkyl bond dissociation energies to have been directly determined, i.e., that of CH3Mn(CO!6,(-30 kcal/rnol), also lies in this low range.*' The "weakness" of transition metal-alkyl bonds thus appears to be fairly widespread. Brown, D. L. S.; Connor, J. A,; Skinner, H. A. J. Organornet. Chem. 1974, 81, 403. On sabbatical leave from the University of Waterloo, Waterloo, Ontario, Canada.
-
(24)
(25) (26)
(27) (28)
+
+
/
101:23
/
Nocember 7 , I979
Scheme I Me
Me
\\
B
,CI
,'
Si,'
+
Me
\1 .
S,-CH3
/ Me
".: ' ,Si--CH2
Me
+ 8'' +
+
Fragments
BH'
-I
Our experimental procedure is outlined in Scheme 1." Trimethylsilyl chloride, a base (B) of known strength and an inert buffer gas (Ar or Kr) are added to an 1CR spectrometer i n approximate ratio of 2: 1 :50- 100 and total pressure on the order of 1 OT5 Torr. Electron impact results in the initial production of trimethylsilyl cation (via rapid halide loss from the radical cation of trimethylsilyl chloride) and, depending on base B, of B+. and of fragment ions of B. If base B is sufficiently strong, it will be capable of proton abstraction from the trimethylsilyl Heat of Formation of 1,l-Dimethylsilaethyleneby cation, giving rise to an ion of mass corresponding to BH+ and Ion Cyclotron Resonance Spectroscopy concurrently to the neutral molecule 1,l -dimethylsilaethylene. Therefore, by employing a series of abstracting bases of known Sir: and increasing strength, and by monitoring the onset of proInterest has time and again focused upon the generation and duction of BH+ (verified by standard double resonance techcharacterization of molecules containing formal carbon-silicon niquesI0.' to arise from the trimethylsilyl cation), it is possible double For example, the existence of 1,l-dimethto determine an approximate enthalpy for deprotonation of the ylsilaethylene (I), as an intermediate in the pyrolysis reaction silicon-centered cation. Piperidine (enthalpy of proton transfer ( l ) , has been suggested2a and the rate interpreted to yield a 2 1.9 kcal mol-' greater than that of the ammonia standard)I2 was the strongest base considered which was not observed to Me Me abstract a proton from trimethylsilyl cation; the weaker bases, I I Me-Sii ethylmethylamine (19.4 kcal mol-') and trimethylamine (21.2 Me-Sikcal mol-'), did not result in proton abstraction. IsoproI __* pylethylamine (AHo of protonation 23.4 kcal mol-' above I "3) was the weakest base for which proton abstraction was observed; the stronger bases di-n-propylamine (23.9 kcal heat of formation of 12-29 kcal mol-' for the specie^.^^,^ Mass mol-') and diisopropylamine (26.0 kcal mol-') also lead to spectral evidence supporting the existence of I in the gas phase proton abstraction. These results were unaffected by further has only recently been advanced.2w The low-temperature addition of buffer gas. matrix infrared spectra of both 1,l -dimethylsilaethylene (I)5a.b We conclude, therefore, that the proton affinity of I , ] (obtained from the pyrolysis reaction 1) and I , I ,2-trimethdimethylsilaethylene is >21.9 kcal mol-' (relative to ammoylsilaethylene ( II)5c,d (obtained upon photolysis of trimethnia) but
