methyldiborane

Methylboranes are formed by the reaction of diborane and trimethylborane. This reaction produces four different substitution of methyl with hydrogen on diborane. Produced is 1-methyldiborane, 1,1-dimethyldborane, 1,1,2-trimethyldiborane and 1,1,2,2-tetramethyldiborane. The reaction is complex. At 0 °C when diborane is in excess, monomethyldiborane is initially produced, coming to a steady but low level, and 1,1-dimethyldiborane level increases over a long time, until all trimethylborane is consumed. Monomethyldiborane ends up at equilibrium with a mixture of diborane and dimethyldiborane. At 0° the equilibrium constant for 2B₂H₅Me ←→ B₂H₆ + (BH₂Me)₂ is around 0.07, so monomethyldiborane will typically be the majority of the mixture, but there will still be a significant amount of diborane and dimethyldiborane present.{{cite book|last1=van Aalten|first1=Lloyd|last2=Seely|first2=G. R.|last3=Oliver|first3=Juhn|last4=Ritter|first4=D. M.|title=Kinetics and Equilibria in the Alkylation of Diborane Preliminary Report|volume=32|date=1 June 1961|pages=107–114|doi=10.1021/ba-1961-0032.ch012|publisher=American Chemical Society|url=https://www.thevespiary.org/rhodium/Rhodium/Vespiary/talk/files/4264-BORAX-TO-BORANES3db1.pdf?topic=2297.0|series=Advances in Chemistry|isbn=978-0-8412-0033-3}} Monomethyldiborane yield is best with a ratio of 4 of diborane to 1 of trimethylborane.{{cite journal|last1=Carpenter|first1=J.H.|last2=Jones|first2=W.J.|last3=Jotham|first3=R.W.|last4=Long|first4=L.H.|title=The Raman spectra of the methyldiboranes—II Monomethyldiborane and trimethyldiborane, and characteristic frequencies of the methyldiboranes|journal=Spectrochimica Acta Part A: Molecular Spectroscopy|date=September 1971|volume=27|issue=9|pages=1721–1734|doi=10.1016/0584-8539(71)80227-1|bibcode=1971AcSpA..27.1721C|doi-access=free}} The yield of trimethyldiborane is maximised with ratio of 1 of diborane to 3 of trimethylborane.

When methyllithium reacts with diborane, monomethyldiborane is produced in about a 20% yield.{{cite journal|last1=Massey|first1=A. G.|title=Chapter 2. The typical elements. Part II: Group III|journal=Annual Reports on the Progress of Chemistry, Section A|date=1979|volume=76|pages=13–15|doi=10.1039/IC9797600013}} {{subscription required}}

Tetramethyl lead can react with diborane in a 1,2-dimethoxyethane solvent at room temperature to make a range of methyl substituted diboranes, ending up at trimethylborane, but including 1,1-di, tridiborane. The other outputs of the reaction are hydrogen gas and lead metal.{{cite journal|last1=Holliday|first1=A.K.|last2=N. Jessop|first2=G.|title=The reaction of tetramethyllead with diborane|journal=Journal of Organometallic Chemistry|date=November 1967|volume=10|issue=2|pages=291–293|doi=10.1016/s0022-328x(00)93089-4}}

Other methods to form methyldiboranes include reacting hydrogen with trimethylborane between 80 and 200 °C under pressure, or reacting a metal borohydride with trimethylborane in the presence of hydrogen chloride, aluminium chloride or boron trichloride. If the borohydride is sodium borohydride, then methane is a side product. If the metal is lithium then no methane is produced. dimethylchloroborane and methyldichloroborane are also produced as gaseous products.

When Cp₂Zr(CH₃)₂ reacts with borane dissolved in tetrahydrofuran, a borohydro group inserts into the zirconium carbon bond, and methyl diboranes are produced.{{cite journal|last1=Marsella|first1=John A.|last2=Caulton|first2=Kenneth G.|title=Dealkylation of zirconium(IV) by borane: the intimate mechanism of an alkyl transfer reaction|journal=Journal of the American Chemical Society|date=May 1982|volume=104|issue=9|pages=2361–2365|doi=10.1021/ja00373a005|bibcode=1982JAChS.104.2361M }}

When trimethylgallium reacts with diborane at -45°, methyldiborane is produced along with dimethylgallium borohydride.

:2(CH₃)₃Ga + B₂H₆ → (CH₃)₂GaBH4 + CH₃B₂H₅.{{cite journal|last1=Schlesinger|first1=H. I.|last2=Brown|first2=Herbert C|title=The Borohydrides of Gallium|journal=Journal of the American Chemical Society|date=September 1943|volume=65|issue=9|page=1787|doi=10.1021/ja01249a512|bibcode=1943JAChS..65.1786S |url=http://pubs.acs.org/doi/abs/10.1021/ja01249a035?journalCode=jacsat|access-date=17 August 2015|url-access=subscription}}

At room temperature trimethylgallium reacts with diborane to make a volatile substance that decomposes to gallium metal along with methyldiborane.

:(CH₃)₃Ga + 3B₂H₆ → Ga + 3CH₃B₂H₅ +1.5H₂.

The compound boils at −43 °C.{{cite web|title=Thermal reaction of diborane with trimethylborane|url=https://digital.library.unt.edu/ark:/67531/metadc64257/m1/1/|date=4 September 1958|author1=Lamneck, John H Jr |author2=Kaye, Samuel|publisher=National Advisory Committee for Aeronautics}}

Methyldiborane liquid has a density of 0.546 g/ml at −126° At −78.5 the vapour pressure is 55 torr.

Methyldiborane HCH₃BH₂BH₂ has one methyl group and a hydrogen on a boron atom. The other boron atom is only bound to hydrogen atoms. A bridge of two hydrogen atoms links the boron atoms together. The methyldiborane molecule has the following measurements: B¹ is the boron atom not attached to the methyl group and B² is the boron atom that has methyl group attached, and H^μ is one of the bridge hydrogen atoms between the boron atoms. The distance between boron atoms is 1.82 Å, Distance between boron atoms and bridging hydrogen atoms is 1.34 Å. Distances to non bridging hydrogens from B¹ are 1.195 and 1.187 Å. B² distance to non bridging hydrogen is 1.2 Å. Distance between two bridging hydrogen atoms is 1.96 Å. The carbon to boron bond is 1.49 Å long. The angle subtended from the bridging hydrogens to the boron to boron axis is 47°. The angle of carbon to the boron-boron axis is 120°. The dipole moment of methyldiborane is 0.56 D.{{cite journal|last1=Chiu|first1=C. W.|last2=Burg|first2=A. B.|last3=Beaudet|first3=R. A.|title=Molecular structure determination of methyldiborane|journal=Inorganic Chemistry|date=March 1982|volume=21|issue=3|pages=1204–1208|doi=10.1021/ic00133a064}} It has a vapour pressure of 61 mm Hg at −77.2 °C. The predicted heat of formation for the liquid is ΔH⁰_f=−14 kcal/mol, and for the gas −9 kcal/mol.{{cite web|last1=Altschuller|first1=Aubrey P.|title=Calculated Heats of Formation and Combustion of Boron Compounds (Boron, Hydrogen, Carbon, Silicon)|series=NACA Research Memorandum|url=http://naca.central.cranfield.ac.uk/reports/1955/naca-rm-e55g26.pdf|publisher=National Advisory Committee for Aeronautics|access-date=14 August 2015|location=Cleveland, Ohio|page=22|date=4 October 1955}}

A gas chromatograph can be used to determine the amounts of the methyl boranes in a mixture. The order they pass through are diborane, monomethyldiborane, trimethylborane, 1,1-dimethyldiborane, 1,2-dimethyldiborane, trimethyldiborane, and last tetramethyldiborane.{{cite journal|last1=Seely|first1=G. R.|last2=Oliver|first2=J. P.|last3=Ritter|first3=D. M.|title=Gas-Liquid Chromatographic Analysis of Mixtures Containing Methyldiboranes|journal=Analytical Chemistry|date=December 1959|volume=31|issue=12|pages=1993–1995|doi=10.1021/ac60156a032}}

The nuclear resonance shift for the bridge hydrogen is 10.09 ppm, compared to 10.49 for diborane.{{cite journal|last1=Leach|first1=John B.|last2=Ungermann|first2=Charles B.|last3=Onak|first3=Thomas P.|title=Proton magnetic resonance studies on methyl and chloro substituted diboranes|journal=Journal of Magnetic Resonance|date=January 1972|volume=6|issue=1|pages=74–83|doi=10.1016/0022-2364(72)90088-1|bibcode=1972JMagR...6...74L}}

At -78.5 °C methyldiborane disproportionates slowly first to diborane and 1,1-dimethyldiborane.{{cite book|editor=Duward F. Shriver|last1=Bunting|first1=Roger K.|title=Inorganic Syntheses, Volume 19|date=22 Sep 2009|publisher=John Wiley and Sons|pages=237–238|chapter-url=https://books.google.com/books?id=Tk3HUFj9XnkC&pg=PA237|chapter=55 1-Methyldiborane|isbn=978-0-471-04542-7}} In solution methylborane is more stable against disproportionation than dimethylborane.{{cite journal|last1=Brown|first1=Herbert C.|last2=Cole|first2=Thomas E.|last3=Srebnik|first3=Morris|last4=Kim|first4=Kee Won|title=Hydroboration. 79. Preparation and properties of methylborane and dimethylborane and their characteristics as hydroborating agents. Synthesis of tertiary alcohols containing methyl groups via hydroboration|journal=The Journal of Organic Chemistry|date=December 1986|volume=51|issue=25|pages=4925–4930|doi=10.1021/jo00375a031}}

:2MeB₂H₅ {{eqm}} 1,1-Me₂B₂H₄ + B₂H₆ K=2.8 Me=CH₃.{{cite book|last1=Onak|first1=Thomas|editor1-last=Stone|editor1-first=F. G. A.|editor2-last=West|editor2-first=Robert|title=Advances in Organometallic Chemistry|date=1 January 1966|publisher=Academic Press|location=New York, London|page=284|chapter-url=https://books.google.com/books?id=AHEPoIx22iwC&pg=PA266|access-date=19 August 2015|chapter=Carboranes and Organo-Substituted Boron Hydrides|isbn=978-0-08-058004-3}}

By reacting methyldiborane with ether, dimethylether borine is formed (CH₃)₂O.BH₃ leaving methylborane which rapidly dimerises to 1,2-dimethyldiborane.{{cite journal|last1=Bell|first1=R. P.|last2=Emeléus|first2=H. J.|title=The boron hydrides and related compounds|journal=Quarterly Reviews, Chemical Society|date=1948|volume=2|issue=2|page=132|doi=10.1039/QR9480200132}} {{subscription required}}{{cite book|last1=Adams|first1=Roy M.|title=Metal-Organic Compounds|chapter=Organoboron Compounds|date=September 1959|volume=23|issue=Metal-Organic Compounds|page=92|doi=10.1021/ba-1959-0023.ch010|access-date=17 August 2015|chapter-url=https://www.thevespiary.org/rhodium/Rhodium/Vespiary/talk/files/4262-AIC-023-METAL-ORGANIC-COMPOUNDS-13db1.pdf?topic=2297.0|series=Advances in Chemistry|isbn=978-0-8412-0024-1}}

Methyldiborane is hydrolyzed in water to methylboronic acid CH₃B(OH)₂.

Methyldiborane reacts with trimethylamine to yield solid derivatives trimethylamine-methylborane (CH₃)₃N—BHCH₃ and trimethylamine-borane (CH₃)₃N—BH₃.

Methyldiborane is pyrophoric, spontaneously inflaming when exposed to air.

When methyldiborane is oxidised around 150 °C a substance 2-methyl-1,3,4-trioxadiboralane is produced. This is a ring of three oxygen atoms and two boron atoms, with methyl attached to one boron atom. At the same time dimethyltrioxadiboralane and trimethylboroxine are also formed, and also hydrocarbons, diborane, hydrogen, and dimethoxyborane (dimethyl methylboronic ester).{{cite journal|last1=Barton|first1=Lawrence|last2=Crump|first2=John M.|last3=Wheatley|first3=Jeffrey B.|title=Trioxadiborolanes from the oxidation of methyldiborane|journal=Journal of Organometallic Chemistry|date=June 1974|volume=72|issue=1|pages=C1–C3|doi=10.1016/s0022-328x(00)82027-6}}

When methyldiborane, or dimethyldiborane is combined with ammonia, aminodimethylborine (NH₂BMe₂) is formed and on heating around 180 °C B-methyl borazoles are produced. These borazoles can have zero, one, two or three methyl groups substituted on the boron atoms (B₃N₃H₆, MeB₃N₃H₅, Me₂B₃N₃H₄ or Me₃B₃N₃H₃).{{cite journal|last1=Sheldon|first1=J. C.|last2=Smith|first2=B. C.|title=The borazoles|journal=Quarterly Reviews, Chemical Society|date=1960|volume=14|issue=2|page=202|doi=10.1039/QR9601400200}}{{cite journal|last1=Schlesinger|first1=H. I.|last2=Horvitz|first2=Leo|last3=Burg|first3=A. B.|title=Hydrides of Boron. VI. The Action of Ammonia on the Methyl Diboranes|journal=Journal of the American Chemical Society|date=March 1936|volume=58|issue=3|pages=409–414|doi=10.1021/ja01294a008|bibcode=1936JAChS..58..409S }}

A specific way to make 1,2-dimethyldiborane is to react methyldiborane with a sufficient amount of a Lewis base. This will strip off borane to combine with the Lewis base, and let two methyl borane molecules dimerise.{{cite book|last1=Wade|first1=Kenneth|title=Electron Deficient Compounds|date=1971|publisher=Thomas Nelson|isbn=978-1-4684-6056-8|pages=91–92|chapter-url=https://books.google.com/books?id=7oXgBwAAQBAJ&pg=PA92|access-date=17 August 2015|chapter=The General Chemistry of the Boron Hydrides}}

Methyldiborane can methylate tetraborane.

:CH₃B₂H₅ + B₄H₁₀ → 2-CH₃B₄H₉ + B₂H₆{{cite book|last1=Onak|first1=Thomas|title=Organoborane chemistry|date=1975|publisher=Academic Press|location=New York|isbn=978-0-12-526550-8|pages=193–194|url=https://books.google.com/books?id=AKl_M5FrL_sC&pg=PA193|access-date=17 August 2015}}

Bis(trimethylphosphine) methyldiborane is an adduct of methyldiborane formed when methylpentaborane (1-CH₃B₅H₈ or 2-CH₃B₅H₈) is reacted with trimethylphosphine.{{cite journal|last1=Kameda|first1=Mitsuaki|last2=Driscoll|first2=Jerry A.|last3=Kodama|first3=Goji|title=Formation and reactions of bis(trimethylphosphine)-methyldiborane(4)|journal=Inorganic Chemistry|date=September 1990|volume=29|issue=19|pages=3791–3795|doi=10.1021/ic00344a028}}

{{Reflist|30em}}

• {{cite journal|last1=Carpenter|first1=J. H.|last2=Jones|first2=W. J.|last3=Jotham|first3=R. W.|last4=Long|first4=L. H.|title=Laser-source Raman spectroscopy and the Raman spectra of the methyldiboranes|journal=Chemical Communications|date=1968|issue=15|page=881|doi=10.1039/C19680000881}}
• {{cite journal|last1=Lehmann|first1=Walter J.|last2=Wilson|first2=Charles O.|last3=Shapiro|first3=I.|title=Infrared Spectra of Alkyldiboranes. I. Monomethyldiboranes|journal=The Journal of Chemical Physics|date=1960|volume=32|issue=4|page=1088|doi=10.1063/1.1730853|bibcode=1960JChPh..32.1088L}}
• {{cite book|last1=Crompton|first1=T. R.|title=Gas Chromatography of Organometallic Compounds|date=6 December 2012|publisher=Springer|pages=80–83|url=https://books.google.com/books?id=cW_SBwAAQBAJ&pg=80|access-date=17 August 2015|isbn=978-1-4684-4226-7}} methyldiborane in a gas chromatograph
• {{cite journal|last1=Penn|first1=R. E.|last2=Buxton|first2=L. W.|title=Microwave spectrum of methyldiborane|journal=The Journal of Chemical Physics|date=1977|volume=67|issue=2|page=831|doi=10.1063/1.434845|bibcode=1977JChPh..67..831P}}
• {{cite book|title=Inorganic Chemistry of the Main-Group Elements|editor1=C. C. Addison|page=57|chapter=Elements of Group III|last1=Davidson|first1=G.|chapter-url=https://books.google.com/books?id=5FDADhBgE54C&pg=PA57|isbn=978-0-85186-752-6|year=1973|publisher=Royal Society of Chemistry }} Infrared lines
• {{cite book|title=Borax to Boranes|chapter-url=https://www.thevespiary.org/rhodium/Rhodium/Vespiary/talk/files/4264-BORAX-TO-BORANES3db1.pdf?topic=2297.0|pages=135–136|date=1961|volume=32|series=Advances in Chemistry Series|publisher=American Chemical Society|author=Isadore Shapiro |author2=C. O. Wilson |author3=J. F. Ditter |author4=W. J. Lehmann |doi=10.1021/ba-1961-0032.ch014|chapter=Mass Spectrometry in Boron Chemistry|isbn=978-0-8412-0033-3}} mass spectroscopy
• {{cite journal|last1=Kapshtal|first1=V. N.|last2=Sverdlov|first2=L. M.|title=Sum rules for the frequencies and squares of the frequencies of the vibrations in methyl-substituted diborane derivatives|journal=Soviet Physics Journal|date=November 1968|volume=11|issue=11|pages=120–122|doi=10.1007/BF00816081|bibcode=1968SvPhJ..11k.120K|s2cid=121517718}}

Category:Alkylboranes