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1,2-Dimethyldiborane

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| Name = 1,2-Dimethyldiborane

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| ImageCaption1 = trans-1,2-Dimethyldiborane

| PIN = 1,2-Dimethyldiborane(6)

| OtherNames = Symmetrical dimethyldiborane

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| CASNo = 17156-88-6

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| SMILES = C[BH]1[H][BH](C)[H]1

| StdInChI = 1S/C2H10B2/c1-3-5-4(2)6-3/h3-4H,1-2H3

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| StdInChIKey = CMZSWMSJMMTTMF-UHFFFAOYSA-N

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|Section2={{Chembox Properties

| Formula = (CH3BH2)2

| MolarMass = 55.72 g mol−1

| Appearance = Colorless gas

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| MeltingPtC = -124.9

| BoilingPtC = 4

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1,2-Dimethyldiborane is an organoboron compound with the formula [(CH3)BH2]2. Structurally, it is related to diborane, but with methyl groups replacing terminal hydrides on each boron. It is the dimer of methylborane, CH3BH2, the simplest alkylborane.{{cite journal|last1=Srebnik|first1=Morris|last2=Cole|first2=Thomas E.|last3=Brown|first3=Herbert C.|title=Methylborane - a remarkable unhindered monoalkylborane which achieves the controlled sequential hydroboration of representative alkenes|journal=Tetrahedron Letters|date=January 1987|volume=28|issue=33|pages=3771–3774|doi=10.1016/s0040-4039(00)96380-9}} 1,2-Dimethyldiborane can exist in a cis- and a trans arrangement.{{cite journal|last1=Low|first1=M. J. D.|title=Characteristic Infrared Frequencies of Methyldiboranes|journal=The Journal of Chemical Physics|date=1968|volume=48|issue=5|pages=2386–2387|doi=10.1063/1.1669454|bibcode=1968JChPh..48.2386L}} 1,2-Dimethyldiborane is an easily condensed, colorless gas that ignites spontaneously in air.

An isomer of 1,2-dimethyldiborane is 1,1-dimethyldiborane, known as unsymmetrical dimethyldiborane, which has two methyl groups on one boron atom. Other methylated versions of diborane including methyldiborane, trimethyldiborane, tetramethyldiborane. Trimethylborane exists as a monomer.

Preparation

Methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.{{cite journal|last1=Long|first1=L. H.|last2=Wallbridge|first2=M. G. H.|title=646. The Chemistry of Boron. Part VI. New Preparative Methods and Decomposition Studies Relating to Methyldiboranes|journal=Journal of the Chemical Society (Resumed)|date=1965|pages=3513–3520|doi=10.1039/JR9650003513|url=http://pubs.rsc.org/en/content/articlepdf/1965/jr/jr9650003513|url-access=subscription}} {{subscription required}}{{cite journal|last1=Schlesinger|first1=H. I.|last2=Walker|first2=A. O.|title=Hydrides of Boron. IV. The Methyl Derivatives of Diborane|journal=Journal of the American Chemical Society|date=April 1935|volume=57|issue=4|pages=621–625|doi=10.1021/ja01307a009|bibcode=1935JAChS..57..621S }}

In a more modern synthesis, 1,2-dimethyldiborane is produced by treating lithium methylborohydride with hydrogen chloride:

:2 LiCH3BH3 + 2 HCl → (CH3BH2)2 + 2 H2 + 2 LiCl

Instead of hydrogen chloride, methyl iodide or trimethylsilyl chloride can be used.

Lithium methylborohydride can be made by treating methylboronic esters with lithium aluminium hydride.{{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 Sgents. 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}}

=Miscellaneous routes=

Methylboranes arise the reaction of diborane and trimethylborane. This reaction produces 1-methyldiborane, 1,1-dimethyldiborane, 1,1,2-trimethyldiborane, and 1,1,2,2-tetramethyldiborane. By treating monomethyldiborane with ether, dimethyl ether borane (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|pages=132|doi=10.1039/QR9480200132}} The reaction is complex.

Tetramethyl lead reacts with diborane to give a range of methyl-substituted diboranes, ending up at trimethylborane, but including 1,1-dimethyldiborane, and trimethyldiborane. Other products 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 treating hydrogen with trimethylborane between 80 and 200 °C under pressure, or treating 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 diborane, 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 }}

In ether dimethylcalcium reacts with diborane to produce dimethyldiborane and calcium borohydride:{{cite book|last1=James|first1=B. D.|last2=Wallbridge|first2=M. G. H.|editor1-last=Lippard|editor1-first=Stephen J.|title=Progress in Inorganic Chemistry, Volume 11|date=1970|publisher=Wiley|isbn=0471-54081-1|page=185|chapter-url=https://books.google.com/books?id=8N6nFvNzfv0C&pg=PA185|chapter=Metal Tetrahydroborates}}

:Ca(CH3)2 + 2 B2H6 → Ca(BH4)2 + B2H4(CH3)2

1,2-Dimethyldiborane is produced by the room temperature disproportionation of trimethyldiborane.{{cite journal|last1=Lehmann|first1=Walter J.|last2=Wilson|first2=Charles O.|last3=Shapiro|first3=I.|title=Infrared Spectra of Alkyldiboranes. V. Tri- and Tetramethyl- and Ethyldiboranes|journal=The Journal of Chemical Physics|date=1961|volume=34|issue=3|pages=783|doi=10.1063/1.1731675|bibcode=1961JChPh..34..783L}}

Physical and spectroscopic properties

cis-1,2-Dimethyldiborane melts at −132.5 °C; trans-1,2-dimethyldiborane melts at −102 °C.{{cite journal|last1=Hedberg|first1=Lise|last2=Hedberg|first2=Kenneth|last3=Kohler|first3=David A.|last4=Ritter|first4=David M.|last5=Schomaker|first5=Verner|title=Electron-diffraction investigations of the molecular structures of cis- and trans-1,2-dimethyldiborane|journal=Journal of the American Chemical Society|date=May 1980|volume=102|issue=10|pages=3430–3434|doi=10.1021/ja00530a021|bibcode=1980JAChS.102.3430H }} The cis-1,2-dimethyldiborane molecule has point group Cs. A trans-1,2-dimethyldiborane molecule has point group C2. Unsymmetrical dimethyldiborane melts at −150.2 °C.{{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|url=https://books.google.com/books?id=AHEPoIx22iwC&pg=PA284|accessdate=14 August 2015|isbn=9780080580043}} Vapour pressure is approximated by Log P = 7.363−(1212/T). The vapour pressure for the symmetrical isomer is given by Log P = 7.523−(1290/T).

Gas chromatography can be used to determine the amounts of the methyl boranes in a mixture. The order of elution 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 9.55 ppm for the unsymmetrical isomer and 9.73 ppm for the symmetrical isomers, 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}}

Reactions

Methylborane shows little tendency to disproportionate (redistribute) at room temperature. It reacts stepwise with alkenes to produce mono and dialkylmethylboranes. More methylated boranes are less stable.

1,2-Dimethyldiborane slowly converts to 1,1-dimethyldiborane.{{cite journal|last1=Lehmann|first1=Walter J.|last2=Wilson|first2=Charles O.|last3=Shapiro|first3=I.|title=Infrared Spectra of Alkyldiboranes. III. 1,2-Dimethyl- and 1,2-Diethyldiboranes|journal=The Journal of Chemical Physics|date=1960|volume=33|issue=2|pages=590|doi=10.1063/1.1731190|bibcode=1960JChPh..33..590L}}

Methylborane hydrolyzes to methylboronic acid:

:(MeBH2)2 + 4 H2O → CH3B(OH)2 + 4 H2

Symmetrical dimethyldiborane reacts with trimethylamine to yield a solid adduct trimethylamine-methylborane (CH3)3N·BH2CH3.

When dimethyldiborane is combined with ammonia and heated, B-methyl borazoles are produced. These borazoles can have one, two or three methyl groups substituted on the boron atoms.{{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 }}

Under normal conditions dimethyldiborane does not react with hydrogen.{{cite book|last1=Adams|first1=Roy M. |title=Metal-Organic Compounds |date=September 1959|volume=23 |page=92|doi=10.1021/ba-1959-0023.ch010|accessdate=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=0-8412-0024-6|chapter=Organoboron Compounds }}

Related species

  • Lithium trihydromethylborate [CH3BH3]−.
  • Isomers of diethyldiborane can be produced by analogous methods.{{cite journal|last1=Mikhailov|first1=B. M.|title=The Chemistry Of Diborane|journal=Russian Chemical Reviews|date=April 1962|volume=31|issue=4|page=209|bibcode=1962RuCRv..31..207M|doi=10.1070/RC1962v031n04ABEH001281|s2cid=250909492 }}
  • 1,2- 2,2- and 2,4-dimethyltetraborane,{{cite journal|last1=Deever|first1=William R.|last2=Ritter|first2=David M.|title=Methyltetraboranes. I. 2-Methyl and 1,2-, 2,2-, and 2,4-dimethyl derivatives|journal=Inorganic Chemistry|date=November 1969|volume=8|issue=11|pages=2461–2467|doi=10.1021/ic50081a043}} 1,2-dimethylpentaborane{{cite book|last1=Addison|first1=C. C.|title=Inorganic Chemistry of the Main-Group Elements|last2=Davidson|first2=G. |date=1973|volume=1|page=68|doi=10.1039/9781847556370-00053|chapter-url=http://pubs.rsc.org/en/content/chapter/bk9780851867526-00053/978-0-85186-752-6|isbn=978-0-85186-752-6|chapter=Elements of Group III}} {{subscription required}} 2,3-dimethylpentaborane,{{cite journal|last1=Onak|first1=Thomas|last2=Friedman|first2=Lawrence B.|last3=Hartsuck|first3=Jean A.|last4=Lipscomb|first4=William N.|title=Rearrangement of 1,2- to 2,3-Dimethylpentaborane(9)|journal=Journal of the American Chemical Society|date=July 1966|volume=88|issue=14|pages=3439–3440|doi=10.1021/ja00966a051|bibcode=1966JAChS..88.3439O }} 4,5-dimethylhexaborane,{{cite book|last1=Shore|first1=S. G.|editor1-last=Muetterties|editor1-first=Earl L|title=Boron Hydride Chemistry|date=1975|publisher=Academic Press|isbn=0-12-509650-X|page=150|chapter-url=https://books.google.com/books?id=hLUy6or0In8C&pg=PA150|chapter=Nido and Arachno Boron Hydrides}} and 5,6- 6,8- 6,9-dimethyldecaborane.{{cite journal|last1=Dunstan|first1=I.|last2=Williams|first2=R. L.|last3=Blay|first3=N. J.|title=970. Boron hydride derivatives. Part V. Nucleophilic substitution in decaborane|journal=Journal of the Chemical Society|date=1960|issue=IV|pages=5012–5015|doi=10.1039/JR9600005012|url=http://pubs.rsc.org/en/content/articlepdf/1960/jr/jr9600005012|accessdate=19 August 2015|url-access=subscription}}

References

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Extra reading

  • {{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|pages=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|pages=1088|doi=10.1063/1.1730853|bibcode=1960JChPh..32.1088L}}
  • {{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—I 1, 1-dimethyldiborane and tetramethyldiborane|journal=Spectrochimica Acta Part A: Molecular Spectroscopy|date=June 1970|volume=26|issue=6|pages=1199–1214|doi=10.1016/0584-8539(70)80027-7|bibcode=1970AcSpA..26.1199C}}
  • {{cite thesis|title=Reactions of Methyl Substituted Diboranes and 2,2-Dimethyltetraborane with Amine Bases|first1=Francis M.|last1=Jungfleisch|publisher=Ohio State University|date=1973|url=https://etd.ohiolink.edu/!etd.send_file?accession=osu1298565601&disposition=inline|accessdate=30 July 2015|archive-date=4 March 2016|archive-url=https://web.archive.org/web/20160304192445/https://etd.ohiolink.edu/!etd.send_file?accession=osu1298565601&disposition=inline|url-status=dead}}
  • {{cite book|title=Borax to Boranes|url=https://www.thevespiary.org/rhodium/Rhodium/Vespiary/talk/files/4264-BORAX-TO-BORANES3db1.pdf?topic=2297.0|pages=134–136|date=1961|volume=32|series=Advances in Chemistry Series|publisher=American Chemical Society|author1=Isadore Shapiro |author2=C. O. Wilson |author3=J. F. Ditter |author4=W. J. Lehmann|doi=10.1021/ba-1961-0032.ch014}} mass spectroscopy
  • {{cite journal|last1=Levison|first1=K. A.|last2=Perkins|first2=P. G.|title=Methylaluminium compounds I. The Electronic Structure of Some Methylaluminium and Methylboron Hydrides|journal=Theoretica Chimica Acta|date=1970|volume=17|issue=1|pages=1–14|doi=10.1007/BF00526759|s2cid=198179226}} charge distribution and atom location calculations

{{Boron compounds}}

Category:Alkylboranes