Identification of Fragments in the Mass Spectra of Dialkyldiboranes CHARLES 0. WILSON, Jr.,l and ISADORE SHAPIR02 Research laboratory, Olin Mathieson Chemical Corp., Pasadena, Calif.
b The mass spectra of 1,I - and 1,2dimethyldiboranes and their ethyl analogs are analyzed. Identification of the fragments is aided by the use of isotopically labeled compounds-1,land 1 , ~ - ( C H ~ ) ~ B(CH3)2B2H4, Z ~ ~ H ~ , (CH312B2D4, (CD&hH4, ( C D ~ ) Z B ~ D and ~ , the corresponding diethyl series. Alkyl rearrangement in the mass spectra of the 1,2-dialkyldiboranes is noted.
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course of studying diborane and its alkyl derivatives (9), particularly the dialkyldiboranes, unequivocal mass spectral analyses were complicated by evidence of atomic and group rearrangements. Prominent peaks shoved up in the mass spectra of isotopically normal lJ2-dialkyldiboranes, which could not be accounted for by simple fragmentation of the molecules. The mass spectra of the 1,2-compounds were suspiciously similar to the 1,ldialkyldiboranes (Figures 1 t o 4), especially the dimethyldiboranes, but infrared spectra ( 8 ) revealed their structural differences. Because of the striking similarities in the mass spectral patterns, the possibility of isomeric mixtures was considered. For example, the peak a t m / e 69 in the spectrum of 1,2diethyldiborane (Figure 4) might have arisen from the presence of the 1,l-isomer as an impurity because the peak corresponds to the fragment (C&.&B+. This and other problems were resolved by examination of the mass spectra of the isotopically labeled dialkyldiboranes.
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Figure 1.
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Anticipated fragments and mass spectra of 1,l -dimethyldiboranes
EXPERIMENTAL
Apparatus. Standard high vacuum techniques mere employed in the preparation and handling of all materials. Cooling baths were made with Freon-12. Mass spectra were obtained with a Consolidated (Model 21-103) mass spectrometer operating a t an ionizing potential of 70 volts. All spectra (obtained on different days) resulted from sample pressures of 30 microns. Reagents. D I B O R ~ N E S .Isotopically labeled diboranes such as BioH6, B2H6, 1 Present address, liational Engineering Science Co., Pasadena, Calif. 2 Present address, Aircraft Division, Hughes Tool Co., Culver City, Calif.
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ANALYTICAL CHEMISTRY
and B2D6 were prepared by the reaction of the appropriate boron trifluoride etherate with lithium aluminum hydride or deuteride in the conventional manner ( 7 ) . The composition of the Blo-enriched diborane was about 96’% B’O, 4% B1llk normal diborane, 80% Bll, 20% B deuterodiborane, 96% B2D6, 4% B d 5 H . TRIMETHYLBORANES. The triniethylboranes-BIO( CH3)3, B (CH3)3, and B(CD3)3-~ere prepared by the Grignard reaction of the appropriately labeled methyl magnesium iodide with boron trifluoride etherate ( 2 ) . TRIETHYLBORAKES. The triethylb~ranes--BlO(C~H~)~,B(C2HL)3, and B(C2D5)3-were prepared by the reaction of the appropriately labeled ethyl-
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ene and diborane (6 to 1 molar ratio) over a catalyst (5). Procedure. PREP.4RhTIOX OF 1,1DIALKYLDIBORANES. The equilibration of diboraiie and trialkylborane ( 5 to 1molar ratio) results in a mixture of terminally substituted alkyldiboranes (6). The 1,l-dialkyldiborane (most abundant) and a small amount of the 1,2-compound mere separated from the mixture by low-temperature fractionation. The entire mixture of methyldiboranes JT as passed through a U-tube cooled to -115’ C.; the products retained in a bath a t - 120” C. w r e the dimethyldiboranes. The corresponding distillation temperatures for the isolation of diethyldiboranes n ere - 72’ and -80” c.
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