Iminoborane Explained

Iminoboranes comprise a group of organoboron compounds with the formula RB=NR'. They are electronically related to acetylenes but are usually more reactive due to the polarity.[1] [2]

Structure and bonding

The parent iminoborane, HB=NH, is produced by the photolysis of H3BNH3.[3] [4] [5] Bonding in iminoboranes can be described by two resonance structures:[6]

R-\overset\overset-R <-> R-B=\ddot-R

The stability is dramatically affected by bulky substituents. One isolable iminoborane is .[7]

Comparison of bond lengths in simple boron-nitrogen hydrides
MoleculeAmmonia borane[8] Aminoborane[9] Iminoborane[10]
FormulaBNH6BNH4BNH2
Classamine-boraneaminoboraneiminoborane
Analogous hydrocarbonethaneethyleneacetylene
Analogous hydrocarbon classalkanealkenealkyne
Structure
Ball-and-stick model
Hybridisation of boron and nitrogensp3sp2sp
B-N bond length1.658 Å1.391 Å1.238 Å
Proportion of B-N single bond100%84%75%
B-H bond length1.216 Å1.195 Å
N-H bond length1.014 Å1.004 Å
Structure determination methodmicrowave spectroscopymicrowave spectroscopyinfrared spectroscopy

Synthesis

Elimination of fluoro- or chlorosilanes provides a well-tested route. Bulky substituents such as (Me3Si)3Si stabilize the iminoborane with respect to oligomerization:[11]

(Me3Si)3SiB(F)-N(SiMe3)2 → (Me3Si)3Si-B=N-SiMe3 + F-SiMe3

Thermal decomposition of azidoboranes induces migration of R from boron to the nascent nitrene gives iminoboranes:[12]

R2B-N3 → RB=NR + N2

Reactivity

Oligomerization

Iminoboranes tend to oligomerize, often forming cyclic derivatives. Preventing this reaction is the purpose of bulky substituents. Five types of oligomerization product are produced: cyclodimers (s,[7] Di[13]), cyclotrimers (borazines, Tr), bicyclotrimers (Dewar borazines, [14]), cyclotetramers (s, Te[15]), and polymers (polyiminoboranes, Po); which are shown below.[16] Which product is dominant depends on the structures of reactants and the reaction conditions. Some of the products can be interconverted.[17]

Addition reactions

The addition of protic agents is fast and quantitive.[18] Boration reaction of iminoboranes is the addition of B-X single bond to B≡N, where -X can be -Cl (chloro-boration), -N3 (azido-boration), -SR (thio-boration), -NR2 (amino-boration) and R (alkyl-boration). One of these reactions are illustrated here.

Some electron-rich iminoboranes form adducts with Lewis acids.[19]

Cycloaddition

The typical [2+3]-cycloaddition is the addition of B≡N and RN3 to give a BN4 ring.[7] One of the widely investigated [2+2]-cycloadditions is the reaction of aldehydes and ketones.

Coordination to transition metals

Like alkynes, iminoboranes bind transition metals.

Notes and References

  1. Book: Inorganic ring systems : 7th International symposium : Papers.. 1994. Gordon and Breach Science Publishers. Chivers, Tristram. 978-2-88449-168-6. 81135356.
  2. Book: Paetzold, Peter. Iminoboranes. 31. 123–170. en. 10.1016/s0898-8838(08)60223-8. Advances in Inorganic Chemistry. 1987. 978-0-12-023631-2.
  3. Lory. Earl R.. Porter. Richard F.. 1973-03-01. Infrared studies of matrix isolated species in the hydrogen-boron-nitrogen system. Journal of the American Chemical Society. 95. 6. 1766–1770. 10.1021/ja00787a012. 0002-7863.
  4. Paetzold. Peter. Richter. Anette. Thijssen. Theo. Würtenberg. Stefan. 1979-12-01. Bildung, Struktur und Reaktivität von (Pentafluorphenyl)bor-tert-butylimid und seinem Cyclodimeren. Chemische Berichte. en. 112. 12. 3811–3827. 10.1002/cber.19791121207. 1099-0682.
  5. Paetzold. Peter. von Plotho. Christoph. 1982-08-01. Über weitere monomere Borimide und ihre Reaktionen. Chemische Berichte. en. 115. 8. 2819–2825. 10.1002/cber.19821150813. 1099-0682.
  6. Mó. Otilia. Yáñez. Manuel. Pendás. Angel Martín. Bene. Janet E. Del. Alkorta. Ibon. Elguero. José. 2007-07-23. Unusual substituent effects on the bonding of iminoboranes. Phys. Chem. Chem. Phys.. en. 9. 30. 3970–3977. 10.1039/b702480k. 17646885. 2007PCCP....9.3970M. 1463-9084. 10261/72489.
  7. 10.1002/cber.19841170324. Darstellung, Reaktionen und Struktur vontert-Butyl(tert-butylimino)boran. 1984. Paetzold. Peter. Plotho. Christoph Von. Schmid. Günter. Boese. Roland. Schrader. Bernhard. Bougeard. Daniel. Pfeiffer. Uwe. Gleiter. Rolf. Schüfer. Wolfgang. Chemische Berichte. 117. 3. 1089–1102.
  8. L. R. . Thorne . R. D. . Suenram . F. J. . Lovas . Microwave spectrum, torsional barrier, and structure of BH3NH3 . . 78 . 1 . 1983 . 167–171 . 10.1063/1.444528 . 1983JChPh..78..167T .
  9. Masaaki . Sugie . Harutoshi . Takeo . Chi . Matsumura . Microwave spectrum and molecular structure of aminoborane, BH2NH2 . . 123 . 2 . 1987 . 286–292 . 10.1016/0022-2852(87)90279-7 . 1987JMoSp.123..286S .
  10. Yoshiyuki . Kawashima . Detection of HBNH by infrared diode laser spectroscopy . . 1987 . 87 . 11 . 6331–6333 . 10.1063/1.453462 . 1987JChPh..87.6331K .
  11. Haase. Martin. Klingebiel. Uwe. 1985-04-01. Simple Synthesis of Stable Iminoboranes. Angewandte Chemie International Edition in English. en. 24. 4. 324. 10.1002/anie.198503241. 1521-3773.
  12. Paetzold. Peter. Eleftheriadis. Eleftherios. Minkwitz. Rolf. Wölfel. Volker. Gleiter. Rolf. Bischof. Peter. Friedrich. Gert. 1988-01-01. Bildung, Struktur und Reaktionen von Methyl(methylimino)boran. Chemische Berichte. en. 121. 1. 61–66. 10.1002/cber.19881210110. 1099-0682.
  13. Hess. H.. 1969-11-15. Strukturbestimmungen an Bor–Stickstoff-Verbindungen. IV. Die Kristall- und Molekularstruktur von Hexakis(trimethylsilyl)-2,4-diamino-1,3,2,4-diazadiboretidin. Acta Crystallographica Section B. de. 25. 11. 2342–2349. 10.1107/s056774086900567x. 0567-7408.
  14. Steuer. Holger-A.. Meller. Anton. Elter. Gernot. B-t-butyl-borazine und -diazadiboretidine. Journal of Organometallic Chemistry. de. 295. 1. 1–6. 10.1016/0022-328x(85)88065-7. 1985.
  15. Book: Turner H. S. and Warne R. J. 1962 Proc. Chem. Soc. 69.
  16. Book: Advances in inorganic chemistry. Volume 31. 1987. Academic Press. Emeléus, H. J. (Harry Julius), Sharpe, A. G.. 978-0-12-023631-2. New York. 277086713. registration.
  17. Maier.G.. 1978. Tetra-tert-butyltetrahedrane. Angew. Chem. Int. Ed. Engl.. 17. 7. 520–521. 10.1002/anie.197805201.
  18. Nöth. Heinrich. Weber. Siegfried. 1985-05-01. Beiträge zur Chemie des Bors, 154. Addition von Trimethylsily-Verbindungen und von anderen Elektrophilen an (tert-Butylimino) (tetramethylpiperidino)boran. Chemische Berichte. en. 118. 5. 2144–2146. 10.1002/cber.19851180536. 1099-0682.
  19. Nöth. Heinrich. Weber. Siegfried. 1985-06-01. Beiträge zur Chemie des Bors, 158. Addukte von Aluminium- und Galliumhalogeniden an ein Aminoiminoboran. Chemische Berichte. en. 118. 6. 2554–2556. 10.1002/cber.19851180631. 1099-0682.

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