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]

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]

HB-≡N+H ↔ HB=NH

The stability is dramatically affected by bulky substituents. One isolable iminoborane is (tert-Bu)B=N(tert-Bu).[1]

Synthesis

Elimination of fluoro- or chlorosilanes provides a well tested route. Bulky substituents such as (Me3Si)3Si stabilize the iminoborane with respect to oligomerization:[8]

(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:[9]

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 (1,3,2,4-diazadiboretidines,[1] Di[10]), cyclotrimers (borazines, Tr), bicyclotrimers (Dewar borazines, Tr’[11]), cyclotetramers (octahydro-1,3,5,7-tetraza-2,4,6,8-tetraborocines, Te[12]), and polymers (polyiminoboranes, Po); which are shown below.[13] Which product is dominant depends on the structures of reactants and the reaction conditions. Some of the products can be interconverted.[14]

Addition reactions

The addition of protic agents is fast and quantitive.[15] 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.[16]

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: 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.CS1 maint: others (link)
  3. Paetzold, Peter (1987). Iminoboranes. Advances in Inorganic Chemistry. 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. 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.
  9. 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.
  10. 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.
  11. Steuer, Holger-A.; Meller, Anton; Elter, Gernot (1985). "B-t-butyl-borazine und -diazadiboretidine". Journal of Organometallic Chemistry. 295 (1): 1–6. doi:10.1016/0022-328x(85)88065-7.
  12. Turner H. S. and Warne R. J. 1962 Proc. Chem. Soc. 69.
  13. 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.CS1 maint: others (link)
  14. Maier.G. (1978). "Tetra-tert-butyltetrahedrane". Angew. Chem. Int. Ed. Engl. 17: 520. doi:10.1002/anie.197805201.
  15. 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.
  16. 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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