JOSEPH
L. WEININGER
silyl ethers and amines should be stronger Lewis bases than their methyl analogs and that the base strength should decrease as silyl groups are replaced by methyl groups. However, in the amine series, the reverse has been found to be the case, (siH3)~N being the weakest base and (CH3)3N being the strongest in the series.20i26This has been attributed to d,-p, bonding involving the lone pair of electrons on the nitrogen and the 3d orbitals of the silicon20v26-28It also has been observed by using B2H6 as a reference acid that (SiH3)s0 is a much weaker Lewis base than (CH3)20,2gthe latter, but not the former compound forming an adduct with a t --78°.19,29 The results of the present investigation show that SiH30CH3 is also a much weaker Lewis base than (CH3)20since it forms no addition compound with a t -78'. However, diborane is not a sufficiently strong acid to differentiate between the relative base strengths of SiH30CH3 and (SiH3)zO. Boron trifluoride was of no use in measuring the base strength of SiH30CH3 since SiH3F was liberated a t - 127.8' and on warming to room temperature the reaction represented by equation 8 occurred. This reaction is completely analogous
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The relative basicities of the ethers given above may be explained in two ways: (1) One SiH3 group in SiH30CH3 abstracts electrons from the oxygen atom by means of d,-p, back-coordination approximately as strongly as do two SiH3 groups in (SiH3)20. Until the Si-0-C bond angle is ascertained, the validity of this somewhat unlikely assumption cannot be determined. ( 2 ) The enthalpy for the process BzHs
+2BH3
(9)
to that occurring between (SiH&O and BF3 under similar experimental ~ o n d i t i o n s . * ~ ~ ~ ~
is 28.4 k ~ a l . / m o l e . ~If~ the interaction energy of a BH3 group with both SiH30CH3 and (SiH3)20 were less than that required to dissociate B2H6, then neither ether would form an isolable addition compound with B2H6; hence, even if SiH3OCHa were in reality a considerably stronger base than (SiH3)20, it would appear to have the same base strength. That this is indeed quite likely is apparent from the fact that (CH3)20.BH3is an unstable c o m p o ~ n d and ' ~ ~ therefore ~~ any slight decrease in electron-donor ability of the oxygen could readily decrease the ether-BH3 interaction energy to a point where insufficient energy for the dissociation step would be available. Acknowledgment.-The authors wish to thank Dr. E. R. Nixon for helpful discussions on the interpretation of infrared spectral data.
( 2 6 ) A. B. Burg and E. S . Kuljian, J . 4 n i . Chein. Soc., 72, 3103 (1950). (27) S. Sujishi and S. Witz, ibid.,1 6 , 4631 (1954). ( 2 8 ) A. G . MacDiarmid, Quarl. Rev., 10, 208 (195G). (29) (a) 5 . Sujishi, E. L. Gasner and A . D. Payton, Jr., Abstracts of papers presented a t 133rd National Meeting of t h e American Chemical Society, San Francisco, .19
