NOTES
July, 1958
87 1
CIHsO:BF3, (CH&0:BF3, (CzH&0:BF3, (iso-CsHT)z:BF3
100
O
I
I
10
10
1 ;
I
30
40
50
I
I
I
I
*O
IO
W
80
100
Mole % Diborane.
Fig. 3.-The
ethylene oxide-diborane system.
peratures slightly above the melting points of the mixtures. Dimethyl ether, tetrahydrofuran and tetrahydropyran (Figs. 4A, 4B and 4C, respectively) all form 1:1 borine complexes. The whole range of compositions could not be investigated, as the diborane pressure exceeded one atmosphere when the mole fractions of diborane were above 0.3-0.4.
This order cannot be explained on the basis of the inductive effect of the alkyl groups, but is accounted for by the effect of steric strains. The same order of relative stability was found, as expected, for the first three corresponding borine complexes by Rice and Uchida.6 It would be expected however that a borine complex of methyl ethyl ether would be intermediate in stability between those of methyl and ethyl ether, and that ethylene oxide would form a strong complex. That they do not show up on the phase diagrams may indicate that a t low temperatures in these solvents the concentration of borine from the dissociation of diborane is too low to permit the formation of 1:1 complexes. The inductive effect of the highly fluorinated substituents in perfluoroether and cyclo-C4F80reduces the basic strength of these ethers so they cannot form complexes even with boron trifluoride. The unexpected compounds found with methyl ethyl ether and diethyl ether could be regarded as molecular addition compounds of diborane t o which the formulas CH30C2H5.B2H, and (CzHb)20: BH3. BzHe could be applied: By analogy to the corresponding boron trifluoride complexes, lo the formulations CH30C2H6.2BH3and (C2H5)20.3BH3 are preferred. (IO) H. E. Wirth, M. J. Jackson and H. W. Gritlitha, THISJOURNAL, 62, 871 (1958).
COMPLEXES OF ETHERS WITH BORON TRIFLUORIDE 2
OL
I 2 00
I
I
I
10
20
30
BY HENRYE. WIRTH, MIRIAMJ. JACKSON A N D HOWARD W. GRIFFITHS 40
0
30
20
IO
40
Department of Chemistry, Syracuse University, Syracuse, AT. Y. Received January 1 1 1968 ~
I00
140
- 1I4 0 0
10
20
30
40
0
20
Mole % D i b o r a n e .
40
eo
no.
__
ieo
Fig. 4.-A, dimethyl ether-diborane; B, tetrahydrofurandiborane; C ,tetrahydropyran-diborane; D, perfluoroetherdiborane.
Perfluoroether and diborane (Fig. 4D) formed a partially miscible liquid system, with no indication of compound formation. Cyclo-C4F80 formed a similar system. Two liquid phases also were observed when perfluoroether and boron trifluoride were mixed a t 155°K. Brown and Adamss have shown that the relative stability of the etherates of boron trifluoride decreases in the order: (9) H. C. Brown and M. Adams, J . A m . Chem. SOC.,64, 2557 (1942).
Although 1:1 ether-boron trifluoride complexes are well known there are no reports in the literature of compounds containing more than one mole of boron trifluoride per mole of ether except for a single reference’ to a second complex of diethyl ether containing 60-90 mole % BF3. I n their monograph Booth and Martin2refer to the compounds 2(CH3)2O.BF3 and 2(CzH&0.BF3,but the reference cited3 discusses compounds of the alcohols. Since recent work4 in this Laboratory has demonstrated the existence of the borine complexes (CzHs)20.3BH3and CH30C2H6.2BH3, a survey of several ether-boron trifluoride systems was made to see whether similar compounds are formed with boron trifluoride. Experimental The apparatus described in the previous article4was used. Methyl n-propyl ether and methyl isopropyl ether were prepared by the reaction of sodium methylate with 1bromopropane and 2-bromopropane, respectively, in methyl alcohol solution. The mixtures were refluxed for 3 hours, and the material distilling below 60” collected. Thme products were washed with water, dried successively over (1) A, F. 0. Gerrnann and M. Cleaveland, Science, 63, 582 (1921). (2) H. S. Booth and D. R. Martin, “Boron Trifluoride and Its
Derivatives,” John Wiley and Sons, Inc., New York, N. Y., 1949. (3) H. Meerwein and W. Pannwitz, J . prakt. Chem., 141, 123 (1934). (4) H. E. Wirth, F. E. Massoth and D. X . Gilbert, TIUS JOURNAL^ 62, 870 (1958).
NOTES
872
Vol. 62
CaCL and LiAlHd, and fractionally distilled under reduced pressure before use. The other ethers were treated as described previously.(
Methyl Isopropyl Ether.-Methyl isopropyl ether (m.p. 119OK.) also formed two compounds: CH30CH(CH3)2.BF3, m.p. 239"K., and CHIOCHResults (CH&2BF3, m.p. 206°K.; eutectic (63 mole % Diethyl Ether.-The phase diagram for the sys- BF3), m.p. 201". tem diethyl ether-boron trifluoride is given in Fig. Ethylene Oxide.-Excess ethylene oxide reacts 1. In addition to the 1:1 complex melting at with BF3 to give dioxane and a polymer.* With an excess of BF3,a 1: 1 complex is formed below On warming to room temperature, the white complex decomposes. These results were qualitatively confirmed. In a quantitative experiment, a ten-fold excess of BF3 was condensed on ethylene oxide at liquid nitrogen temperature. On warming the mixture, just at the melting point of BF3a vigorous reaction took place, and a white solid (presumably a complex) formed. This solid was insoluble in liquid BF3. At a temperature just above the boiling point of BF3, 1.1 moles of BF3 per mole ethylene oxide were retained in the solid. Other Ethers.-Tetrahydropyran and tetrahydrofuran gave 1:l complexes melting at 283°K. (lit.9 255°K.) and 284°K. (lit.' 281-283"K.), respectively, but the phase equilibria could not be investigated beyond 52-55 mole % BF3, as the pressure exceeded one atmosphere. When a ten-fold excess of BF3 was condensed on these ethers, as in Mole BF3, the experiment with ethylene oxide, 1.5 and 2.0 Fig. 1.-Diethyl ether-boron trifluoride system. moles of BF3 per mole of ether were retained by tetrahydropyran and tetrahydrofuran, respectively. 214°K. (literature values 215.5",5 212.8",8 222OK.7) Diisopropyl ether absorbed two moles of BF3 per a second complex (CzH&0.3BF3 was found, mole of ether when treated with an excess of BF3. m.p. 202°K. The eutectic between the two comDimethyl ether appears to form only the 1 : l plexes melted at 186°K. (69 mole % BF3). Within the accuracy of the temperature measurement complex. (i0.5") the complexes formed do not lower the Discussion freezing point of either ether or boron trifluoride (monotectic behavior). The compounds ether.2BFa formed with methyl The system is complicated by the fact that in ethyl, methyl n-propyl and methyl isopropyl ether, addition to the two known solid phases of diethyl and the compound (CzH6)20.3BF3are less stable ether (m.p. 156.9 and 149.9"K.) the 1:l complex than the 1:l complexes, but do give congruent also appears t o have two solid modifications. This melting points at temperatures above the boiling probably accounts for the spread in freezing points point of BF3 (172°K.). On being heated a few dereported. Between 0 and 50 mole % BF3the solu- grees above their melting points they dissociate tions supercooled so that freezing points were not into BFI and the 1:1 complex. It is also possible well defined. Above 76 mole % BF3 the pressure that tetrahydropyran, tetrahydrofuran and isoproexceeded one atmosphere before the complex dis- pyl ether give very unstable higher complexes. solved completely in the liquid. The higher complexes of methyl ethyl and diMethyl Ethyl Ether.-Two compounds were formed, CH30C2Hb.BF3, m.p. 210°K. (lit.6 175°K.) ethyl ether with boron trifluoride are analogous to and CH30CzH6.2BF3,m.p. 198°K. The eutectic the corresponding borine c~mplexes.~The ether.2(62 mole % BF3) melted at 189°K. It is probable BF3 complexes found with methyl ethyl and the propyl ethers may also be analogous to the that there are two solid modifications of the 1 : l methyl compounds RzO.2BCI20R (R = methyl or ethyl) complex. The pressure exceeded one atmosphere reported by Wiberg and Sutterlin'O for which strucbefore the 1:2 complex dissolved for compositions tures involving coordination of both boron atoms containing more than 70 mole % BF3. Monotectic with the ether oxygen are assumed.11 This attracbehavior was exhibited at the ether and BFs extive explanation does not account for the complex tremes of the system. . dimethyl ether offers the Methyln-Propyl Ether.-This ether (map.139°K.) ( C Z H & O ~ B F ~Since formed the two compounds CH30(CHz)2CH3. least steric hindrance of the series of aliphatic BF3 (m.p. 222°K.) and CH30(CHz)zCH3.2BF3(m.p. ethers to the formation of the 1:1 complex, it would 201'). The eutectic (60 mole % BF3) melted at be expected to form a fairly stable higher complex if the boron fluorides are both coordinated with 198°K. oxygen. Such a complex was not found. 220
(5) N. N. Greenwood, R. L. Martin and H. J. EmelBue, J . Chem. Soc., 3030 (1950). (6) A. Laubengayer and G. R. Finlay, J . Am. Chem. Soc., 66,
W.
884 (1943). (7) H.C. Brown and R. M. Adams, ibid., 64,2557 (1942).
(8) F. G. A. Stone and H. J. EmelBus, J . Chem. SOC.,2755 (1950). (9) J. Grimley and A. K. Holliday, ibid., 1215 (1954). (10) E. Wiberg and W. Siitter'in, I.anow. Chem., 202, 1 (1931). (11) E. Wiberg and W. Sutterlin, ibid., 202, 22 (1931).
LI
c
*
