- Email: [email protected]
New routes to halogenated B8 and B9 boron cages
NOTES
134
DAAG29-80-KOO30, I am also indebted to Dr. Stanislav Heimanek of the Institute of Inorganic Chemistry, Czechoslovak Academy of Sciences, Dr. D. hf. P. Mingos of the Inorganic Chemistry Laboratory, Oxford University (England), and Dr. Russell Grimes of the University of Virginia for useful suggestions and constructive criticisms of earlier versions of this paper. Departmentof Chemistry Universityof Georgia Athens, GA 30602 U.S.A.
R. B. KING
REFERENCES ‘R. B. King, Theor. Chim. Acta., 1981,59,25.
Polyhedron Vol. I. No. 1. pp. 134435, Printed in Great Britain.
‘J. R. Pipal and R. N. Grimes, Inorg. Chem., 1979,18,257, ‘K. Wade, Chem. Comm.;‘1971,792. ‘R. N. Grimes, Ann. N. Y. Acad. Sci., 1974,239, 180. ‘K. Wade, Ado. Inorn. Chem. Radiochem., 1976.18. 1. 6R.B. King and D. H.Rouvray, J. Am. Chem. Six, 1977,99,7834. ‘5. R. Bowser and R. N. Grimes, 1. Am. Chem. Sot., 1978,100, 4623. ‘5. R. Bowser, A. Bonny, J. R. Pipal and R. N. Grimes, J. Am. Chem. SIC.. 1979.101.6229. 9D.N. Cox, fi. M. 6. Mi&os and R. Hoffmann, J.C.S. Dalton, 1981, 1788. “‘E.L. Muetterties and B. F. Beier, Bull. Sot. Chim.Belg., 1978.84, 397.
1982
can-5387/82/010l344$03.@3/0 PergamonPressLtd.
New routes to bslogenated Bs and Be horon cages (Received 17thJu/y 1981) AhstraceBsBrs and B9Br9are formed when B&is and B&l9 are heated with aluminium tribromide. Cage-size reduction occurs on heating B&l,,, and B&l,, with hydrogen to give $CisH and B9CI,Hz,respectively. At least six bromine atoms in B9Br9can be. substituted for methyl groups using SnMe,.
To date complete halogen exchange on boron cage compounds has not been achieved. However, we have found that B&Is can be fully brominated under the relatively mild conditions of 100°C in the presence of aluminium tribromide using boron tribromide as solvent. B&l9 does not react under these conditions but is brominated completely by molten aluminium tribromide (which acts as a solvent) at 260°C. Normally, B&h as isolated from decomposed diboron tetrachloride samples contains substantial amounts of B&l, which are very difficult to remove;’ the differing reactivities of the Bs and B9 systems towards brominechlorine exchange means that separation of the two chlorides is not required prior to reaction. The B&XB&l9 mixture was sealed under vacuum in a Pyrex tube with freshly sublimed aluminium tribromide and vacuum distilled boron tribromide to give a very dark purple solution (the colour being due to BsCls). When the tube was heated to loo” the colour was observed to slowly change to dark brown; after 14 days the tube was opened under vacuum and the boron tribromide removed. The gentle heat of a hot air blower was sufficient to sublime the ahuninium halides and unreacted B&l9 to a remote part of the apparatus leaving behind a dark red-brown solid. This solid sublimed cleanly on heating with a free flame and was identified as BsBrs by mass spectrometry. On resealing the tube containing the yellow mixture of aluminium halides and B&l9 and heating to 260°C for 15hr it was found that the colour slowly changed to deep red; fractional sublimation of the products allowed isolation of pure B9Br9as dark red crystals. To our knowledge BsBrs has not been isolated previously although it has been detected as a minor component among the decomposition products of diboron tetrabromide? It is a very dark reddish-brown, water-
sensitive solid which is soluble in halogenated solvents and which sublimes without melting when heated under vacuum. The parent ion gives rise to the base peak of the mass spectrum, the next most intense peak being due to the loss of BBrs from the parent ion. An as yet unexplained phenomenon is that glass having been in contact with B,Br8 and then sealed with an oxygen-gas flame assumes a light green colour near the point of sealing; in contrast, glass previously used for handling B&l8 takes on a permanganate-purple colour at the seal. The other boron sub-halides do not exhibit this glasscolouring effect. Previously we have described the reaction of B,&l,, B,,Cl,, mixtures with hydrogen? To study the reaction of BloCllo alone with hydrogen the BIOCIIO-BIICIII mixtures obtained from decomposed diboron tetrachloride were heated under vacuum at 350” to pyrolyse the less stable B,,Cl,, leaving the B,&llo intact (small amounts of B&l9 sometimes formed in this procedure do not react with hydrogen below 3OO’C”and so do not affect the next stage of reaction). The red-orange BloCllo was sublimed into a clean piece of apparatus, sealed up with lO-20cm pressure of hydrogen and heated to 150”overnight. On cooling a yellow, crystalline solid condensed out on the cooler parts of the glass tubing; mass spectral analysis showed this to be B&&H contaminated with unchanged B&19. The main fragmentation process in the mass spectrometer is loss of either BC13or BCl*H from the parent ion to give B&b+ and B&H’; the next two most prominent ions were B&13+ and B,Cl,‘. If, as seems likely, the BloCllo molecule possesses a bicapped square antiprismatic boron cage a possible mechanism for the exclusive formation of B&&H may be as follows. Loss of one of the eight equatorial boron atoms from the cage (boron 2) would leave the pre-
NOTES
apical boron 1 relatively coordinatively unsaturated in the open-cage intermediate; it is suggested that the chlorine on this unique boron atom is the one which is substituted by hydrogen. Cage closure to give B&&H then occurs after substitution. The formation’ of B&&H in low yield from the pyrolysis of (H~O)&&, may occur via the same route; hydrogen, formed by partial hydrolysis of the boron cages could attack small amounts of BloCllo produced by oxidation of some of the B&l,~- ions.
viously
I
-
I
BlOCllO’-
@D
open-cage
tntcrmediata
BoCllH Dilute solutions of diboron tetrachloride in boron trichloride decompose slowly at room temperature to give BllClll together with a few per cent of B12C112?A sample of B,,Cl,, prepared in this way was heated to 150°Cwith an excess of hydrogen; small yellow crystals, melting at 93-9X under vacuum, appeared on cooling. In a larger mass the compound, shown by mass spectrometry to be mainly B&l,H2, was red-orange in colour. The two largest peaks in the mass spectrum were due to the ions B,Cl,H,’ and B&H+ (base peak) representing loss of BClp and BC12H,respectively, from the parent ion. The compound B&&H3 was present as an impurity and possibly arises from the attack of hydrogen on B12C112.Hydrogen chloride and boron trichloride were identified by IR spectroscopy as the gaseous products. Morrison has shown that BsBrsMe may be prepared in about 15% yield by the pyrolysis of (Et3NH)2B10Brlo;5 we have studied the thermal decomposition of the corresponding chloride (Et~NH)2B&110 in an effort to make B&&Me. Although the major sublimate product was *Authorto whom correspondenceshouldbe addressed.
Polykifmn, Vol. I. No. 1, pp. 135437, Printed in Great Britain.
135
found to be B&&Me the yield was less than 1% and impurities, thought to be B&l9 and B&l,Me2, were also present. The poor yield from the reaction caused us to abandon the study. The thermal decomposition of (EtaNH)&Cl,o in an atmosphere of chlorine, in an attempt to form B&l,, again gave B&&Me as the main boron-containing sublimate. It appears to be widely assumed that boron cages present in the sub-halides are stabilized by back-donation from halogen to the cage orbitals. Therefore an important question to ask is how many of the halogen atoms may be changed for groups which can’t stabilize the cages by such back-donation. From the experiments described here it is evident that up to three hydrogen atoms or one methyl group may be substituted onto the Bg cage. By heating B,Br, with a large excess of tetramethyltin up to six bromine atoms could be replaced by methyl groups. The whole range of mixed methylbromides B,Br,_.Me. (n = O-6) was formed and unfortunately proved impossible to separate into its components. EXPERIMENTAL Reactionswere carriedout undervacuumin Pyrex glass tubes whichhad previouslybeen baked out in an atmosphereof either boron trichlorideor boron tribromide.In the brominationsthe apparatuswas designed so that the aluminiumtribromidecould be sublimed directly into the reaction vessel from a side-arm, which was then removedby sealingoff a constriction. We had occasion to heat a small sample of B&b in an open test-tube. In the dry, anaerobicconditions at the bottom of the tube the B&l8 sublimed to give a purple vapour which, on “pouring” into the air by inverting the tube, ignited spontaneouslywith a series of mild explosions. A. J. MARKWELL A. G. MASSEY* P. J. PORTAL Departmenl of Chemistry, Universityof Technology, Loughborough, Leicestershire, LEll 3 TU, England REFERENCES ‘A. G. Massey, Chemistryin Britain 1980.588. *J.Kane and k. G. Massky, J. Inorg.Nuci. Chem. 1971,33,1195; N. A. Kutz and J. A. Morrison.Inorp. Chem. 1980.19.329s.
‘S. B. Awad, D. W. Prest and A. G. Massey, 1’ In&g. Nucl. Chem.1978,40,395. ‘J. A. Forstner, T. E. Haas and E. L. hfuetterties,Inorg. Chem. 1964,3,155. ‘D. Saalys and J. A. Morrison,Inorg. Chem. 1980,19,3057.
19132
On7-5387/82/0101M03103.~/0 Pergamon Press Ltd.
The thiocyanato adduct of chromiumlff) acetate (Received 30 July 1981) Ah&a&-The first dichromiumcomplex with axially bonded anions, [NE4]2[Cr,(0,CCH,)4(NCSk],was obtained by reactionof [NEt,]NCS with [Cr&CCHs)4(OH2k]in ethanol. INTRODUCTION
The axial water molecules in binuclear chromium(H) acetate [Cr2(02CCH3)4(0H2)2]can be replaced by various neutral bases, e.g. pyridine, pyrazine,’ piperidine,’
substituted pyridines3 and ammonia.’ ligands, and in the absence of axial ligands quadruple bonds are formed? Recently, reported6 that chromium(H) forms many
The Cr-Cr “supershort” it has been thiocyanato-
134
DAAG29-80-KOO30, I am also indebted to Dr. Stanislav Heimanek of the Institute of Inorganic Chemistry, Czechoslovak Academy of Sciences, Dr. D. hf. P. Mingos of the Inorganic Chemistry Laboratory, Oxford University (England), and Dr. Russell Grimes of the University of Virginia for useful suggestions and constructive criticisms of earlier versions of this paper. Departmentof Chemistry Universityof Georgia Athens, GA 30602 U.S.A.
R. B. KING
REFERENCES ‘R. B. King, Theor. Chim. Acta., 1981,59,25.
Polyhedron Vol. I. No. 1. pp. 134435, Printed in Great Britain.
‘J. R. Pipal and R. N. Grimes, Inorg. Chem., 1979,18,257, ‘K. Wade, Chem. Comm.;‘1971,792. ‘R. N. Grimes, Ann. N. Y. Acad. Sci., 1974,239, 180. ‘K. Wade, Ado. Inorn. Chem. Radiochem., 1976.18. 1. 6R.B. King and D. H.Rouvray, J. Am. Chem. Six, 1977,99,7834. ‘5. R. Bowser and R. N. Grimes, 1. Am. Chem. Sot., 1978,100, 4623. ‘5. R. Bowser, A. Bonny, J. R. Pipal and R. N. Grimes, J. Am. Chem. SIC.. 1979.101.6229. 9D.N. Cox, fi. M. 6. Mi&os and R. Hoffmann, J.C.S. Dalton, 1981, 1788. “‘E.L. Muetterties and B. F. Beier, Bull. Sot. Chim.Belg., 1978.84, 397.
1982
can-5387/82/010l344$03.@3/0 PergamonPressLtd.
New routes to bslogenated Bs and Be horon cages (Received 17thJu/y 1981) AhstraceBsBrs and B9Br9are formed when B&is and B&l9 are heated with aluminium tribromide. Cage-size reduction occurs on heating B&l,,, and B&l,, with hydrogen to give $CisH and B9CI,Hz,respectively. At least six bromine atoms in B9Br9can be. substituted for methyl groups using SnMe,.
To date complete halogen exchange on boron cage compounds has not been achieved. However, we have found that B&Is can be fully brominated under the relatively mild conditions of 100°C in the presence of aluminium tribromide using boron tribromide as solvent. B&l9 does not react under these conditions but is brominated completely by molten aluminium tribromide (which acts as a solvent) at 260°C. Normally, B&h as isolated from decomposed diboron tetrachloride samples contains substantial amounts of B&l, which are very difficult to remove;’ the differing reactivities of the Bs and B9 systems towards brominechlorine exchange means that separation of the two chlorides is not required prior to reaction. The B&XB&l9 mixture was sealed under vacuum in a Pyrex tube with freshly sublimed aluminium tribromide and vacuum distilled boron tribromide to give a very dark purple solution (the colour being due to BsCls). When the tube was heated to loo” the colour was observed to slowly change to dark brown; after 14 days the tube was opened under vacuum and the boron tribromide removed. The gentle heat of a hot air blower was sufficient to sublime the ahuninium halides and unreacted B&l9 to a remote part of the apparatus leaving behind a dark red-brown solid. This solid sublimed cleanly on heating with a free flame and was identified as BsBrs by mass spectrometry. On resealing the tube containing the yellow mixture of aluminium halides and B&l9 and heating to 260°C for 15hr it was found that the colour slowly changed to deep red; fractional sublimation of the products allowed isolation of pure B9Br9as dark red crystals. To our knowledge BsBrs has not been isolated previously although it has been detected as a minor component among the decomposition products of diboron tetrabromide? It is a very dark reddish-brown, water-
sensitive solid which is soluble in halogenated solvents and which sublimes without melting when heated under vacuum. The parent ion gives rise to the base peak of the mass spectrum, the next most intense peak being due to the loss of BBrs from the parent ion. An as yet unexplained phenomenon is that glass having been in contact with B,Br8 and then sealed with an oxygen-gas flame assumes a light green colour near the point of sealing; in contrast, glass previously used for handling B&l8 takes on a permanganate-purple colour at the seal. The other boron sub-halides do not exhibit this glasscolouring effect. Previously we have described the reaction of B,&l,, B,,Cl,, mixtures with hydrogen? To study the reaction of BloCllo alone with hydrogen the BIOCIIO-BIICIII mixtures obtained from decomposed diboron tetrachloride were heated under vacuum at 350” to pyrolyse the less stable B,,Cl,, leaving the B,&llo intact (small amounts of B&l9 sometimes formed in this procedure do not react with hydrogen below 3OO’C”and so do not affect the next stage of reaction). The red-orange BloCllo was sublimed into a clean piece of apparatus, sealed up with lO-20cm pressure of hydrogen and heated to 150”overnight. On cooling a yellow, crystalline solid condensed out on the cooler parts of the glass tubing; mass spectral analysis showed this to be B&&H contaminated with unchanged B&19. The main fragmentation process in the mass spectrometer is loss of either BC13or BCl*H from the parent ion to give B&b+ and B&H’; the next two most prominent ions were B&13+ and B,Cl,‘. If, as seems likely, the BloCllo molecule possesses a bicapped square antiprismatic boron cage a possible mechanism for the exclusive formation of B&&H may be as follows. Loss of one of the eight equatorial boron atoms from the cage (boron 2) would leave the pre-
NOTES
apical boron 1 relatively coordinatively unsaturated in the open-cage intermediate; it is suggested that the chlorine on this unique boron atom is the one which is substituted by hydrogen. Cage closure to give B&&H then occurs after substitution. The formation’ of B&&H in low yield from the pyrolysis of (H~O)&&, may occur via the same route; hydrogen, formed by partial hydrolysis of the boron cages could attack small amounts of BloCllo produced by oxidation of some of the B&l,~- ions.
viously
I
-
I
BlOCllO’-
@D
open-cage
tntcrmediata
BoCllH Dilute solutions of diboron tetrachloride in boron trichloride decompose slowly at room temperature to give BllClll together with a few per cent of B12C112?A sample of B,,Cl,, prepared in this way was heated to 150°Cwith an excess of hydrogen; small yellow crystals, melting at 93-9X under vacuum, appeared on cooling. In a larger mass the compound, shown by mass spectrometry to be mainly B&l,H2, was red-orange in colour. The two largest peaks in the mass spectrum were due to the ions B,Cl,H,’ and B&H+ (base peak) representing loss of BClp and BC12H,respectively, from the parent ion. The compound B&&H3 was present as an impurity and possibly arises from the attack of hydrogen on B12C112.Hydrogen chloride and boron trichloride were identified by IR spectroscopy as the gaseous products. Morrison has shown that BsBrsMe may be prepared in about 15% yield by the pyrolysis of (Et3NH)2B10Brlo;5 we have studied the thermal decomposition of the corresponding chloride (Et~NH)2B&110 in an effort to make B&&Me. Although the major sublimate product was *Authorto whom correspondenceshouldbe addressed.
Polykifmn, Vol. I. No. 1, pp. 135437, Printed in Great Britain.
135
found to be B&&Me the yield was less than 1% and impurities, thought to be B&l9 and B&l,Me2, were also present. The poor yield from the reaction caused us to abandon the study. The thermal decomposition of (EtaNH)&Cl,o in an atmosphere of chlorine, in an attempt to form B&l,, again gave B&&Me as the main boron-containing sublimate. It appears to be widely assumed that boron cages present in the sub-halides are stabilized by back-donation from halogen to the cage orbitals. Therefore an important question to ask is how many of the halogen atoms may be changed for groups which can’t stabilize the cages by such back-donation. From the experiments described here it is evident that up to three hydrogen atoms or one methyl group may be substituted onto the Bg cage. By heating B,Br, with a large excess of tetramethyltin up to six bromine atoms could be replaced by methyl groups. The whole range of mixed methylbromides B,Br,_.Me. (n = O-6) was formed and unfortunately proved impossible to separate into its components. EXPERIMENTAL Reactionswere carriedout undervacuumin Pyrex glass tubes whichhad previouslybeen baked out in an atmosphereof either boron trichlorideor boron tribromide.In the brominationsthe apparatuswas designed so that the aluminiumtribromidecould be sublimed directly into the reaction vessel from a side-arm, which was then removedby sealingoff a constriction. We had occasion to heat a small sample of B&b in an open test-tube. In the dry, anaerobicconditions at the bottom of the tube the B&l8 sublimed to give a purple vapour which, on “pouring” into the air by inverting the tube, ignited spontaneouslywith a series of mild explosions. A. J. MARKWELL A. G. MASSEY* P. J. PORTAL Departmenl of Chemistry, Universityof Technology, Loughborough, Leicestershire, LEll 3 TU, England REFERENCES ‘A. G. Massey, Chemistryin Britain 1980.588. *J.Kane and k. G. Massky, J. Inorg.Nuci. Chem. 1971,33,1195; N. A. Kutz and J. A. Morrison.Inorp. Chem. 1980.19.329s.
‘S. B. Awad, D. W. Prest and A. G. Massey, 1’ In&g. Nucl. Chem.1978,40,395. ‘J. A. Forstner, T. E. Haas and E. L. hfuetterties,Inorg. Chem. 1964,3,155. ‘D. Saalys and J. A. Morrison,Inorg. Chem. 1980,19,3057.
19132
On7-5387/82/0101M03103.~/0 Pergamon Press Ltd.
The thiocyanato adduct of chromiumlff) acetate (Received 30 July 1981) Ah&a&-The first dichromiumcomplex with axially bonded anions, [NE4]2[Cr,(0,CCH,)4(NCSk],was obtained by reactionof [NEt,]NCS with [Cr&CCHs)4(OH2k]in ethanol. INTRODUCTION
The axial water molecules in binuclear chromium(H) acetate [Cr2(02CCH3)4(0H2)2]can be replaced by various neutral bases, e.g. pyridine, pyrazine,’ piperidine,’
substituted pyridines3 and ammonia.’ ligands, and in the absence of axial ligands quadruple bonds are formed? Recently, reported6 that chromium(H) forms many
The Cr-Cr “supershort” it has been thiocyanato-