252
ROBERTL. COOK AND
ALFRED
P. MILLS
TTol. 65
THE DIPOLE MOMEKTS OF SOME TRII14ETHYL-N-SLL(YLSILAZAi\’ES1 BY ROBERT L. COOKAND ALFREDP. MILLS Department of Chemistry, University of Miami, Coral Gables, Florida Recezved J u l y 8 , 1960
The dipole moments of MeaSiNHEt, MeaSjNHPr, MelSiNHiPr and Me3SiN€IiBu were obtained bp solution measurements in benzene and cyclohexane, and by using Onsager’s equation and pure liquid measurements. All values fell within the range 0.72 to 0.79 D. Yo significant difference between the benzene and cyclohexane measurements indicates no complex formation between benzene and the silazanes, while no significant difference between the moments obtained from pure liquid and solution measurements indicates the absence of hydrogen bonding. The lorn dipole moment could not be accounted for on the basis of single bond contributions. It can, however, be accounted for on the basis of p,-d, bonding between the silicon and the nitrogen.
Stone and Seyferth2summarize and discuss considerable evidence for the utilization of silicon’s 3dorbitals in bonds with Puch electronegative, electron-pair-containing, elements as F, 0, C1 and N. A number of trimethylamine complexes with Auoro- and chlorosilanes have been prepared.3-6 Many of these exhibit a coordination number of five. Burg and Iiuljian’ demonstrated that trisilylamine is a far weaker electron-donor than trimethylamine and suggested that this arises through wandering of the lone-pair on nitrogen into the d-orbitals of silicon. Electron diffraction studies by Hedberg and Stosick*indicate that trisilylamine is planar, a configuration which could be interpreted as indicating n-bonding between the nitrogen and silicon. Dipole moments have been reported9 for (Me8Si)JVH (0.67 0)and (MeaSi)&Me (0.440). I n this work the dipole moments of trimethylX-ethylsilazane, trimethyl-N-propylsilazane, trimethyl - K - isopropylsilazane and trimethyl - K isobutylsilazane were determined in order to study the possible n-bonding between the silicon and the nitrogen in the silazanes. Measurements were made using both benzene and cyclohexane in order to determine whether or not an intermolecular complex is formed between benzene and the silazanes. Such a complex is possible since the n-electrons of the benzene ring are easily polarizable and could possibly fill the silicon d-orbitals forming a d,p, bond. Experimental Dielectric Constant.-All measurements were made a t 30” using a heterodyne beat apparatus which mas constructed,lO with slight modifications, using the circuitry described in \nieissberger.*I The precision capacitor was (1) (a) This research was supported by a Frederick Gardner Cottrell grant from Research Corporation. (b) Presented at the bleeting-inMiniature of the Florida Section of the American Chemical Society, Orlando, Florida, May, 1960. Abstracted from the M.S. thesis of Robert L. Cook. (2) F. G. .4.Stone and n. Seyferth, J . Inorg. & iVuclear Chenk., 1, 112 (1955). (3) H. J. Emel6us and N. llliller, J . Chsm. Soc., 819 (1939). (4) J. 11. Simons (Editor), “Fiuorine Chemistry,” Vol. 1 , Academic Press, Inc., New York, N . Y.. 1950, p. 108. ( 5 ) C. J. Wilkins and D . K. Grant, J. Chem. Soc., 827 (19.53). ( 6 ) A. B. Burg, J . Am. Chem. Soc., 7 6 , 2674 (1954). ( 7 ) A. B. Burg and E. S. Kuljian. ibid., 72, 3103 (1950). ( 8 ) K . IIedberq and A. J. Stosick, Abstracts of the X I 1 International Congress of Pure and Bpplied Chemistry, New York, Sept. 10-13. 1951, p. 543. (9) T . Moeller (Editor), “Inorganic Syntheses,” Vol. V, McGrawHill Book Co., Inc.. New York, N. Y., 1957, p. 58-59. (10) Built by Kimball Electronic Laboratory, 2323 N . W. 14th Ave., Miami, Fla. (11) Weissberger (Editor), “Physical Methods of Organic Cheniistry,” 2nd Ed., Vol. I, Part 11, Interscience Publishers, Inc., New York, N . Y . , 1949, p. 1638.
obtained from General Radio Company (type 722-XD) and the dielectric cell from J. C. Balsbaugh, Marshfield Hills, Mass. (type 350). This cell was a three-terminal monel-nickel cell having an a i r capacitance of 35 ppf. I t was provided with a Pyrex glass jacket, liquid volume 40-50 ml., dimensions 1.5” diameter by 5.5” length. The cell was calibrated with benzene and cycloheuane. Molar Refractions.-The molar refractions were calculated from densities and refractive indices measured in this Laboratory,12 and which will be published separately. These molar refractions check quite closely with those predicted using the method of Vogel.I3 Materials.-Fisher C.P. benzene was puiified by obtaining a constant boiling center fraction, refluxing for about 20 hours over PlOj, and then distilling from P205 just before using. Fisher spectrograde cyclohexane was puiified by distillation prior to uring. The silazanes were prepared in this Laboratory’* and purified by reduced pressure fractionation using a colnmn packed with glass helices and having about fifteen theoretical places The boiling point ranges of the barnplc‘s measured are listed in Table I .
TABLE I Compound
Me3SiNHEt MetSiNHPr Me&?iHiPr MeaSiWHiBu
B p. range, OC.
40 6-41 43.2-43 40 0-40 55 3-55
0 4 3 9
RD 30’
P , mm.
Euptl.
Pred.1’
121 70 58 49
37 74 42.43 42 40 4 i 01
37 71 42.38 42 39 47.03
The solutions for the measurements were made up within the weight fraction range of 0.01-0.05. In malting up the solutions, filling the cell, etc., alk transfers were made in a dry box under a nitrogen atmosphere in order to minimize hydrolysis of the silnznnes.
Experimental Results The molar polarizations at infinite dilution were calculated from a modified Halverstadt and I
