Iminoborane

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.[2][3]

Iminoborane

Iminoborane (parent compound)
Names
Other names
Boraneimine
Identifiers
3D model (JSmol)
  • R,R'=H: InChI=1S/BH2N/c1-2/h1-2H
    Key: LNLSXDSWJBUPHM-UHFFFAOYSA-N
  • R,R'=H: B=N
Properties
BH2N
Molar mass 26.83 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references
Structure of tBuN=B-tBu (tBu = tert-butyl). The B-N bond distance is 126 pm.[1]

Structure and bonding

The parent iminoborane, HB=NH, is produced by the photolysis of H3BNH3.[4][5][6] Bonding in iminoboranes can be described by two resonance structures:[7]

The stability is dramatically affected by bulky substituents. One isolable iminoborane is (CH3)3C−B−≡N+−C(CH3)3.[1]

Comparison of bond lengths in simple boron-nitrogen hydrides
Molecule Ammonia borane[8] Aminoborane[9] Iminoborane[10]
Formula BNH6 BNH4 BNH2
Class amine-borane aminoborane iminoborane
Analogous hydrocarbon ethane ethylene acetylene
Analogous hydrocarbon class alkane alkene alkyne
Structure
Ball-and-stick model
Hybridisation of boron and nitrogen sp3 sp2 sp
B-N bond length 1.658 Å 1.391 Å 1.238 Å
Proportion of B-N single bond 100% 84% 75%
B-H bond length 1.216 Å 1.195 Å
N-H bond length 1.014 Å 1.004 Å
Structure determination method microwave spectroscopy microwave spectroscopy infrared 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 (1,3-diaza-2,4-diboretidines,[1] Di[13]), cyclotrimers (borazines, Tr), bicyclotrimers (Dewar borazines, Tr′[14]), cyclotetramers (octahydro-1,3,5,7-tetraza-2,4,6,8-tetraborocines, 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.[1] 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.

References
  1. Paetzold, Peter; Plotho, Christoph Von; Schmid, Günter; Boese, Roland; Schrader, Bernhard; Bougeard, Daniel; Pfeiffer, Uwe; Gleiter, Rolf; Schüfer, Wolfgang (1984). "Darstellung, Reaktionen und Struktur vontert-Butyl(tert-butylimino)boran". Chemische Berichte. 117 (3): 1089–1102. doi:10.1002/cber.19841170324.
  2. Inorganic ring systems : 7th International symposium : Papers. Chivers, Tristram. Gordon and Breach Science Publishers. 1994. ISBN 978-2-88449-168-6. OCLC 81135356.{{cite book}}: CS1 maint: others (link)
  3. Paetzold, Peter (1987). Iminoboranes. Advances in Inorganic Chemistry. Vol. 31. pp. 123–170. doi:10.1016/s0898-8838(08)60223-8. ISBN 978-0-12-023631-2.
  4. 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. doi:10.1021/ja00787a012. ISSN 0002-7863.
  5. 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. 112 (12): 3811–3827. doi:10.1002/cber.19791121207. ISSN 1099-0682.
  6. Paetzold, Peter; von Plotho, Christoph (1982-08-01). "Über weitere monomere Borimide und ihre Reaktionen". Chemische Berichte. 115 (8): 2819–2825. doi:10.1002/cber.19821150813. ISSN 1099-0682.
  7. 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. 9 (30): 3970–3977. Bibcode:2007PCCP....9.3970M. doi:10.1039/b702480k. hdl:10261/72489. ISSN 1463-9084. PMID 17646885.
  8. Thorne, L. R.; Suenram, R. D.; Lovas, F. J. (1983). "Microwave spectrum, torsional barrier, and structure of BH3NH3". J. Chem. Phys. 78 (1): 167–171. Bibcode:1983JChPh..78..167T. doi:10.1063/1.444528.
  9. Sugie, Masaaki; Takeo, Harutoshi; Matsumura, Chi (1987). "Microwave spectrum and molecular structure of aminoborane, BH2NH2". J. Mol. Spectrosc. 123 (2): 286–292. Bibcode:1987JMoSp.123..286S. doi:10.1016/0022-2852(87)90279-7.
  10. Kawashima, Yoshiyuki (1987). "Detection of HBNH by infrared diode laser spectroscopy". J. Chem. Phys. 87 (11): 6331–6333. Bibcode:1987JChPh..87.6331K. doi:10.1063/1.453462.
  11. Haase, Martin; Klingebiel, Uwe (1985-04-01). "Simple Synthesis of Stable Iminoboranes". Angewandte Chemie International Edition in English. 24 (4): 324. doi:10.1002/anie.198503241. ISSN 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. 121 (1): 61–66. doi:10.1002/cber.19881210110. ISSN 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" (PDF). Acta Crystallographica Section B (in German). 25 (11): 2342–2349. doi:10.1107/s056774086900567x. ISSN 0567-7408.
  14. Steuer, Holger-A.; Meller, Anton; Elter, Gernot (1985). "B-t-butyl-borazine und -diazadiboretidine". Journal of Organometallic Chemistry (in German). 295 (1): 1–6. doi:10.1016/0022-328x(85)88065-7.
  15. Turner H. S. and Warne R. J. 1962 Proc. Chem. Soc. 69.
  16. Advances in inorganic chemistry. Volume 31. Emeléus, H. J. (Harry Julius), Sharpe, A. G. New York: Academic Press. 1987. ISBN 978-0-12-023631-2. OCLC 277086713.{{cite book}}: CS1 maint: others (link)
  17. Maier.G. (1978). "Tetra-tert-butyltetrahedrane". Angew. Chem. Int. Ed. Engl. 17 (7): 520–521. doi: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. 118 (5): 2144–2146. doi:10.1002/cber.19851180536. ISSN 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. 118 (6): 2554–2556. doi:10.1002/cber.19851180631. ISSN 1099-0682.
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