CONTRIBUTORS TO THIS VOLUME Peter Day A. G. Massey Melvin B. Robin A. G. Sykes Advances in INORGANIC CHEMISTRY AND RADIOCHEMISTRY EDITORS H. J. EMELEUS A. G. SHARPE University Chemical Laboratory Cambridge, England VOLUME 10 I967 ACADEMIC PRESS New York and London 0 COPYRIGHT 1967, BY ACADEMIPCR ESS INC. ALL RIGHTS RESERVED. NO PART OF THIS BOOK MAY BE REPRODUCED IN ANY FORM, BY PHOTOSTAT, MICROFILM, OR ANY OTHER MEANS, WITHOUT WRITTEN PERMISSION FROM THE PUBLISHERS. ACADEMIC PRESS INC. 111 Fifth Avenue, New York, New York 10003 United Kingdom Edition published by ACADEMIC PRESS INC. (LONDON) LTD. Berkeley Square House, London W.l LIBRARYO F CONGRESCSA TALOGC ARDN UMBER59:- 7692 PRINTED IN THE UNITED STATES OF AMERICA LIST OF CONTRIBUTORS Numbers in parentheses indicate the pages on which the authors’ contributions begin. PETERD AY( 247), Inorganic Chemistry Laboratory, University of Oxford, Oxford, England A. G. MASSEY( l),D epartment of Chemistry, Queen Mary College, Univer- sity of London, London, England MELVIN B. ROBIN( 247), Bell Telephone Laboratories, Inc., Murray Hill, New Jersey A. G. SYKES( 153), Department of Chemistry, The University, Leeds, England THE HALIDES OF BORON A. G. Massey Department of Chemistry, Queen Mary College, University of London. London, England I. Introduction . . 1 11. Preparation of the Boron Halides . . 3 A. Boron Trihalides . . 3 B. Diboron Tctrahalides . . 4 C. Tetraboron Tetrahalides . . 7 D. Other Boron Halides . . 7 E. Mixed Boron Halides . . 8 111. Structures of the Boron Halides . . 12 A. Boron Trihalides . . 12 B. Diboron Tetrahalides . . 16 C. Tetraboron Tetrachloride . . 17 D. Octaboron Octachloride . . . 18 E. B,,X& and B,,X& Ions . . 19 IV. Chemistry of the Boron Halides . . 19 A. Boron Trihalides . . 19 R. Diboron Tetrahalides . . 102 C. Other Boron Halides . . 125 V. Conclusion . . . 128 References . . 129 1. Introduction It is now almost 150 years since Gay-Lussac and Thenard first isolated ammonia-boron trifluoride, H3NBF3, and so initiated a study of the boron halides. Since that time, notably during the past 30 or so years, a tremendous volume of research has been carried out on the halides, ranging from their use in syntheses of hydrides and borohydrides required for rocketry and wartime atomic energy projects (279, 534) to the very extensive use of boron trifluoride as a catalyst in the petroleum industry, polymer production, and organic chemistry generally (817 ). This review will be concerned only with the chemistry of the boron halides, and will neglect studies of their catalytic activity and of their more physical properties except where the latter (e.g., structural studies) have a direct bearing on the chemistry. Nomenclature is always a headache and never more so than in boron chemistry; thus B2Cl, might be more correctly called tetrachloro- diborane-4, but few chemists would immediately recognize this as 1 2 A. G. MASSEY diboron tetrachloride. For this reason I have elected to call compounds by what appears to be their most frequently used literature-name and so, while BC13 is boron trichloride, BMe, is written as trimethylborane ; similarly diboron tetrachloride is used for B,CI,, whereas B2(0Me), is called tetramethoxydiboron (97). The review deals first with the preparation and molecular structures of the boron halides, and then an account is given of their main chemical TABLE I THEH ALIDEOSF BOROP x BnXm X=F =C1 X = Br X=I EX BI u 2, y = nrikriowri. Compounds marked with astcrisk are known to be irivolatilc in high vacuum. ‘‘UI&!~II’’a nd “B12CI11” appear, from their physical properties, to be thc same compound, but it is not pctrfcctly clear which formula is the correct one (see text). properties; by far the most research has been carried out on the tri- halides and, while attempting to cover the maximum number of references in the minimum of space, some have obviously had to be left out or given under a collective review reference. Table I lists by formulas the boron halides known to date (1966). As these compounds are all water-sensitive and one or two ignite spontaneously in air, they are best handled in a glass high-vacuum apparatus, preferably out of contact with hydrocarbon- or silicone-based stopcock lubricants. Most constructional metals appear to be unaffected by the halides at reasonable tempera- tures; although data are mainly lacking for the action of the halides on polymeric materials, a careful study on boron trifluoride (421)s hows it THE HALIDES OF BORON 3 to be without action on Teflon, polystyrene, Araldite, and pure poly- ainylchloride up to at least 80°, whereas rubber tubing, phenol-form- vldehyde resins, Nylon, cellulose, and commercial polyvinylchloride are readily attacked. The literature has been covered up to June 1966. [The reader is referred to Martin et al. (82, 330, 528, 817) for earlier accounts of the chemistry of boron trifluoride, Lappert et al. (288,476,528)f or chemistry involving the other boron trihalides, and Holliday and Massey (410)f or a discussion of the diboron tetrahalides. General articles in which the boron halides are frequently mentioned are to be found in references (187,280, 378, 622, 793).] II. Preparation of the Boron Halides A. BORONTR IHALIDES The trifluoride (430, 868), trichloride (250, 265), tribromide (849), and triiodide (549)c an be prepared by treating elemental boron with the appropriate halogen. A better method for the trifluoride is to heat a mixture of fluospar, concentrated sulfuric acid, and boric oxide (86), Bz03 + 3CaFz + 3HzS04 3 2BF3 + 3CaS04 + 3Hz0 while the triiodide is formed in good yield by refluxing sodium boro- hydride and iodine in either hexane or benzene (452, 757). Having obtained one boron trihalide it is possible to make the others from it by various halogen-exchange processes, of which the following are typical : AlrCIs (267) BC13 While these reactions are normally of little value to the preparative chemist (all four halides are now available commercially), they can be employed in making an isotopically labeled trihalide, e.g., l0BCI3f rom 4 A. G. MASSEY an available sample of 'OBF, (439,499,506)A. lso, since some of the halo- gen-interchange reactions appear to be quite general in scope they can be of help in synthesizing new boron compounds, as in the fluorinations carried out with antimony trifluoride : - +Slil's h F4 (246) CFz=CFBClz +C F2=CFUFz (78% (CH2=CH)zBCI (CHz=CH)zBF (92) ClzBCzHdBClz FzBCzH4UFz (149) __f CsHsBCIz CsHsBFz (557) Sometimes it is necessary to prepare a boron trihalide labeled with radioactive halogen atoms for use in tracer experiments. Special synthetic methods can be devised, such as heating silver chloride with boron (504) A&I* + B +BOO" B C13* (7096 yield) VBClIIlII1 but exchange of halogen atoms between a suitable reagent and a boron trihalide is more convenient. Tetramethylammoniuin chloride, contain- ing radioactive chlorine, exchanges rapidly with boron trichloride and this has been used to prepare specimens of the labeled trichloride (392- 394); the rricthod should be applicable to other trihalides : X*- i BX3 + BX:- + X- + BX3* eq. X=F (3&t) The rapid exchange between free bromine and boron tribromide (238) could also make a useful route to the labeled tribromide. B. DIBOROTNE TRAHALIDES Stock made the first conipound of this series, diboroii tetrachloride, in low yield (ca. lo/o)b y striking an arc between zinc electrodes im- mersed in liquid boron trichloride ; the product contained about 120,: of silicon tetrachloride impurity (800).S ubsequently it was foiwd better to carry out the reduction of the trichloride in the gaseous phase using an electrical discharge between either mercury (406,545,7 18, 751,826,843) or copper (845)e lectrodes, and about 1 mmole/hr can now be obtained relatively easily. Exact details of the synthesis can be found in references (545,845);c arc has to be taken in the construction of the cell so that the hot discharge does not touch the glass walls at any point, otherwise silicon is etched away and contaminates the diboron tetrachloride with SiC14, SipCl,, Si3Cl,, and C13SiOSiCl, (535, 542). 1 Thc stcpwisc fluorination of nlkoxyboron compounds by sulfur tetrafluoride should also prove to be an extremely useful synthctic t,echiiiqtic, e.g. (210): SV B(OR)3 --!+ successive replacement of OR by F --f UF3 THE HALIDES OF BORON 5 When boron trichloride vapor at 1-2 mm pressure is passed through a resonance cavity operating in the microwave region of the spectrum, a glow discharge is set up in the vapor and diboron tetrachloride is formed (256): 2BC13 + BzC14 + Clz Since chlorine reacts with diboron tetrachloride (18),i t must be removed rapidly by fractionation ; the method is useful when small intermittent supplies of diboron tetrachloride are required, and can be made more efficient by passing the effluent gases from the cavity through a plug of copper wool that reacts with and removes the chlorine (88).T he spectra of discharges through boron trichloride vapor have been shown to contain lines due only to BC1 (419, 691),a nd it is thought that BCl, not BCl,, may actually be an intermediate in the preparation of diboron tetrachloride (419,5 46). It is possible to make diboron tetrachloride without resorting to the use of electrical discharges. The simplest method in theory is to pass boron trichloride over boron at 1000°C but, in practice, attack of the reaction vessel at these temperatures makes the method very difficult to operate (633). When boron trichloride is passed over heated boron monoxide a 13% yield ofdiboron tetrachloride is obtained (552,553): The boron monoxide (which has a structure involving boron-to-boron bonds, eq. 626) can be made either by heating boric oxide with boron (425, 436, 744) or by first hydrolyzing tetrakis(dimethy1amino)diboron carefully at low temperature (647)a nd then dehydrating the sub-boric acid so formed (842, 842): Bz(NMe2)4 + 4Hz0 +4 HNMe2 + Bz(0H)d +heat B zOz + 2Hz0 Tetrakis(d imethy1amino)diboron is the only diboron compound commercially available, being prepared on a large scale by treating bis(dimethy1amino)boron chloride or bromide with highly dispersed sodium (96, 647): 2(MezN)zBCl + 2Na --f 2NaCl + BZ(NMe2)d Much effort has been expended in trying to convert the rather un- reactive tetrakis(dimethy1amino)diboron to more useful diboron re- agents. Transamination, which may involve some rupture of the boron- 6 9. G. MASSEY boron bond (650),t akes place with primary and secondary amines (15, 96, 121, 638, 650), Hz(SMc2)4 + 4RR'NH + Bz(NRR')b + 4MezNH (R = alkyl; R' = alkyl or H) while alcohols, at low temperatures and in the presence of hydrogen chloride, produce the tetraalkoxydiborons in good yield (97, 615): RZ(SMe2)4 + 4ROH + 4HC1 +~ 80" B z(OR)+~ 4M02NH&1 Although one, two, or four dimethylamino groups in tetrakis(dimethy1- amino)diboron can be replaced with halogen atoms on treatment with HC1, -H Br, or HF (120, 524, 632, 648),B X, (X = F, C1, or Br) (6'54, or BCI, NMe, (ISO),n o free diboron tetrahalides can be isolated from the reaction mixtures; e.g. : + + Bz(NMrz)b 4HC1 + (Me2N)zBzCIz 2MIozNHzCl B2(XMe2)4+ BC13 + NMez(Cl)&(CI)NMe2 + (Me2N)zBCl i UdNMed4 (NM~z)zBz(C~)NM~~ + + Bz(NMc~)~6 HX + BzX4.2HNMea 2Me2NH2X Addition of the B-NMe, groups in tetrakis(dimethy1amino)diboron across the N=C bond of phenyl isocyanate occurs when the two are mixed in hcxane solution at room temperature (2'4) : The corresponding symmetrical bis(dimethy1ainino)dialkyldiborons can be made by a Wiirtz-type coupling reaction, using sodium-potassium alloy (95, 1'338): MezN(R)BCI + Na/K MezN(H)BZ(R)SMez --f Diboroii tetrafluoride is formed during the fluorination of boron monoxide, sub-boric acid, or tetraalkoxydiborons using sulfur tetra- fluoride (98, 266),s o that the production of the tetrafluoride is not now dependent on diboron tetrachloride from which it had been previously synthesized using antimony trifluoride (246) or ally1 fluoride (149).