,7482
Vol. 77
has been reported by Burg and his co-workers,4for the trimeric P-dimethylphosphinoborane and the trimeric As-dimethylarsinoborane it is very stable for a boron-nitrogen compound containing two active hydrogens per boron. It is unaffected after weeks in contact with nioist air and is hydrolyzed only very slowly by cold water. The pyrolysis of methylamine-borane has been studied by a number of previous investigators5 but this initial pyrolysis product never has been characterized. Experimental A . Preparation of Trimeric N-Methylaminoborane .In a typical preparation, 3 mmoles of diborane \ ~ a $condensed with liquid nitrogen, together witli 3 small excess of :~nliydrousmethylamine (7 millimoles). The mixture was then allowed to warm u p to room temperature over 3 period of about three hours. A small amount of hydrogel1 wa5 Iiroduced iii this preparation of methylamine boririe. Air was then admitted t o the system and the tube was removed from the vacuuni line, stoppered with a calcium sulfate drying tube, and heated on a steam-bath for two hours. The liquitl in the tube changed into a cr tlie first hour of heating. Tliis whitc solid was then placed in a clean tube, which was replaced on the vacuum liiie. .4 beaker of boiling water was placed around the bottom of t!ie tube and the trimeric ;\'-methylamino-boranf sublimed uiider high vacuum. T h e product collected as a ring of fine needles on the walls of the tube just above the surface of the bath. The tube T T ~ Sremoved from the line a r i d broken on c-ach side of the ring. T h e long white needles n-ere tlieii scrxped from the walls; yield about 0.22 g. or 85%. 'The analysis of the product is sumlnarizcxi in Table 1. T.4131.~
Ohsd.
CUlCtl.
Carbori Hydrogen "Active" hydrogeii Boron S i t roge i i
38,OO 14.03 1.85 2 j .0 32.78
28.011 14. 10 4 , 7('1 26. 23 32 66
B. Molecular Weight Determinations.-The
molecular weight of tlie solid was estimated from tlie freezing point depressions of benzene and nitrobenzene solutions using a qtandard Beckmann apparatus. The results indicated molecular weights of 121 and 111 for two trials in benzene and 134 for nitrobenzene. The molecular weight mas also determined by thc vapor pressure lowering of liquid ammonia solutions. The apparatus designed by Il.R . SchultzG was modified by using a large slusli bath of ethylene chloride as the constant temperature medium. A molecular !wight of 127 was obtained. ,4trimer of CHaSHBH2has a calculated molecular weight of 128.7. C , Decomposition into 1,3,5-Trimethylborazene.-A quantity, 0.2364 g., of ( C H I N H B H ~was ) ~ placed in an evacuated sealed tube and heated a t 200" for four hours. Cpon opening the tube, 5.90 mmoles of hydrogen, identified bv 3 molecular weight of 2.1, was found. This gives a ratio of flytlrogen per mole of starting material of 3.24. The 1,3,3trimethylhornzeric was purified by vacuum condensation, t h e portion collecting in a trap a t -4-5' being retained. A 74% yicld w:i$ obtained based on the equation 200 (C€liSIIBH2)3 +3Ha f ( C H S N B H ) ~ ( 2 ) The 1,3,5-trimethylborazene produced was characterized by vapor pressure measurements over the temperature range 50 to i s " , and by vapor phase molecular weight measure~~
TABLE rr ii
r
.Ai)nl f u r :CITISNBHI),, (,'
~~~.
ments based on vapor density (obsd. = 120; theor. = 122.5). The melting point of the solid was 0 to -1" as compared to -9" reported b y Wiberg, Bolz and Buckheit2 and -7 t o -8" reported by Schaeffer and Anderson.6 I t is believed t h a t the higher value of 0 to - 1O indicates greater purity of the sample studied and does not negate the characterization of the borazene. D . Properties of Trimeric N-Methylaminoborane. The compound may be recrystallized from methyl or ethyl alcohol as long white fibrous needles. An analysis for borori arid "active" hydrogen on this recrystallized material demonstrated t h a t the composition was unchanged. Iti addition to the above solvents, the compound is very soluble in acetone and liquid ammonia. It is moderately soluble in benzene, ether and chloroform and is insoluble in carboti tetrachloride, petroleum ether and water. T h e insolubility in water may be primarily a wetting problem. The density of the compound is 0.90 g./ml. as determined by centrifuging a few crystals in a series of mixtures of carbori tetrachloride and kerosene. T h e crystals would reniaiii suspended in the mixture with a density of 0.90 g./ml. A sample of the solid was allowed to stand exposed to the moist air of the laboratory for three weeks, during which time no noticeable hydrolysis occurred. A portion placed in cold water will float for hours b u t will disappear after about a day. A 20% solution of hydrochloric acid must be heated to boiling to bring about rapid hydrolysis. E. X-Ray Power Diffraction Data for Trimeric N-Methylaminoborane .-In Table I1 are listed the interplanar spacings ( d values) and the relative line intensities found to be characteristic of the compound. The powder patterns were taken with copper KU radiation in a cylindrical camera of 57.3 mm. diameter. The low absorbing glass capillaries u i e d to hold the sample had a diameter of 0.2 m m . and a \v:ill thiclcne.;s of 0.01 m m . J
7 . 1 \'s 1 . 2 6 VS 4.00 M S 3 ,45. M 3 . 0 6 \TV
