1,6-Bis(2,4,6-tri-tert-butylphenyl)-1,6-dibora-2,5-diaza-hexa-1,5-diine, the first
compound containing two B·N triple bonds
Thomas Albrecht, Gernot Elter and Anton Meller*
Institut für Anorganische Chemie der Universität, Tammannstr. 4, D-37077 Göttingen, Germany.
E-mail: punger@gwdg.de
Received (in Cambridge, UK) 21st September 1998, Accepted 21st October 1998
The elimination of hydrogen fluoride from N,NA-bis[(2,4,6-
Notes and references
tri-tert-butylphenyl)fluoroboryl]ethylenediamine 1 gives the
† Preparative details and selected spectroscopic data for 1 and 2: All
title compound 2 as a thermally very stable moiety; water is
easily added across the B·N triple bonds.
reactions under dry nitrogen. Mes*BF2 was obtained as described.4
1: to 1.2 g (0.02 mol) ethylenediamine dissolved in 100 ml hexane, 17 ml
of a solution (23% in hexane) of BuLi (0.04 mol) were added with stirring.
The mixture was refluxed for 2 h and then cooled to 215 °C. To the stirred
suspension was added 11.8 g (0.04 mol) Mes*BF2 dissolved in 80 ml
hexane and the reaction mixture warmed to ambient temperature and then
refluxed (2 h). The solvent was removed under reduced pressure and volatile
byproducts removed at 160 °C (0.01 Torr). Sublimation in a three-bulb tube
yielded 6.9 g (57%) of yellowish 1, subl.p 240 °C (0.01 Torr) (air bath
temp.). After short path resublimation the mp was 208 °C. C38H64B2F2N2
(608.61): satisfactory analytical results. EIMS: m/z (%) = 608 (4) [M]+, 551
(3) [M 2 CMe3]+, 57 (100).
1
Iminoboranes R–B·N–RA and aminoiminoboranes R2N–B·N–
2
RA are now quite well known species,3 particularly due to the
work of Paetzold1 and Nöth.2 Iminoboranes are kinetically
stabilized species and their thermal and hydrolytic stability is
dependent upon the steric requirement of the substituents and
their resistance against intramolecular chemical attack from the
thermodynamically unstable (BN) triple bond. In the past we
have studied systematically the stabilization of these moieties
by the 2,4,6-tri-tert-butylphenyl (supermesityl, Mes*) group.4,5
If iminoboranes carrying the supermesityl group have the
methyl group as the other substituent, their stability against
dimerization is retained up to 100 °C. While if the other
substituent is ethyl, benzyl, phenyl, tert-butyl or trimethylsilyl
these iminoboranes will not dimerize, but upon heating will
rearrange to give the corresponding benzo[1]borolanes at
temperatures between 170 and 350 °C.
NMR data: 1H (CDCl3, SiMe4, 250 MHz) d 1.34 (s, 18H, CMe3 :4), 1.43
(d, 6JHF 1.1 Hz, 36H, CMe3 :2/6), 3.22 (m, 4H, CH2), 3.34 (br, d, 3JHF 17.6
Hz, 2H, NH), 7.40 (s, 4H : 3/5); 13C (CDCl3, TMS, 100.6 MHz) d 31.3
5
(CMe3 :4), 33.0 (d, JCF 2.5 Hz, CMe3 :2/6), 34.9 (CMe3 :4), 37.5
(CMe3 :2/6), 41.6 (d, 3JCF 2.1 Hz, CH2), 121.0 (3/5), 127.5 (br, 1), 149.8 (d,
3
5JCF 0.6 Hz:4), 153.6 (d, JCF 2.1 Hz:2/6); 11B (CDCl3, BF3·OEt2 ext.,
80.25 MHz) d 33.0; 19F (CDCl3, C6F6 int., 188.3 MHz) d 70.3.
2: to a suspension of 4.0 g (0.0066 mol) of 1 in 70 ml hexane, 9 ml of a
15% solution of ButLi in pentane (0.0132 mol) was added dropwise with
stirring at 0 °C. The pentane was distilled off and the reaction mixture
refluxed for 2 h. The solution was filtered through a pressure funnel, the
residue washed with three portions of hexane (20 ml each) and the solvent
distilled off under reduced pressure. By short path sublimation in a high
vacuum the remainder yields 1.5 g (41%) of colourless 2; mp 311 °C, subl.p
180 °C (0.0001 Torr) (bath temp.). 2 is only sparingly soluble in organic
solvents. C38H62B2N2 (568.55). Satisfactory elemental analyses were
obtained. EIMS: m/z (%) = 568 (2) [M]+, 511 (5) [M 2 CMe3]+, 284 (100)
[M/2]+; FDMS: m/z (%) = 568 (100). NMR data: 1H (CDCl3, SiMe4, 250
MHz) d 1.33 (s, 18H, CMe3 :4), 1.57 (s, 36H, CMe3 :2/6), 3.73 (s, 4H,
CH2), 7.33 (s, 4H : 3/5); 13C (CDCl3, SiMe4, 100.6 MHz) d 31.3 (CMe3 : 4),
31.9 (CMe3 :2/6), 35.3 (CMe3 : 4), 36.7 (CMe3 :2/6), 44.9 (CH2), 114.0 (br,
1), 119.6 (3/5), 152.4 (4), 159.1 (2/6); 11B (CDCl3, BF3·OEt2 ext., 80.25
MHz) d 5.9. IR: n (11BN) = 2020 cm21, n (10BN) = 2066 cm21 (in
KBr).
Our attempts to prepare a bis(imino)borane by HF-elimina-
tion from Mes*BF–NHNH–FBMes*6 upon treatment with
ButLi, MeLi or MN(SiMe3)2 (M = Li, Na) failed. Apparently
the two supermesityl groups are too large to permit the
necessary approach considering the short bond lengths in the
corresponding bis(imino)borane.
By using an ethylene bridge as a spacer to avoid excessive
steric interference we have now prepared the first bis(imino)-
borane, containing two isolated B·N triple bonds (Scheme
1).†
Mes*B(F) NH (CH2)2 NH (F)BMes*
1
+ 2 ButLi –2 LiF, –2 BuH
‡ Preparative details and selected spectroscopic data for 3: to a stirred
suspension of 2.0 g (0.0035 mol) of 2, suspended in 30 ml Et2O was added
1 ml H2O (0.055 mol, excess). The mixture was refluxed for 2 h and the
liquids removed under reduced pressure leaving colourless 3. Yield: 1.9 g
(91%), mp 221 °C; 3 is fairly soluble in THF. C38H66B2N2O2 (604.58).
Satisfactory elemental analyses were obtained. EIMS: m/z (%) = 604 (1)
[M]+, 547 (3) [M 2 CMe3]+, 302 (15) [M/2]+, 257 (100) [Mes*BO 2 Me]+.
NMR data: 1H (THF-d8, SiMe4, 250 MHz) d 1.28 (s, 18H, CMe3 : 4), 1.44
(s, 36H, CMe3 : 2/6), 3.22 (m, 4H, CH2), 3.47 (m, br, 2H, NH), 6.46 (d, 4JHH
1.5 Hz, 2H, OH), 7.31 (s, 4H : 3/5); 13C (THF-d8, SiMe4, 100.6 Hz) d 31.8
(CMe3 :4), 34.0 (CMe3 :2/6), 35.4 (CMe3 :4), 38.5 (CMe3 : 2/6), 43.2
(CH2), 121.4 (3/5), 134.8 (br, 1), 148.7 (4), 153.5 (2/6); 11B (THF-d8,
BF3·OEt2 ext., 80.25 MHz) d 32.7. IR: n (OH) = 3605 cm21 (in KBr). No
dehydration was observed up to the mp.
Mes*B
N
(CH2)2
N
BMes*
2
Scheme 1
The bis(imino)borane 2 is thermally stable up to its melting
point of 311 °C. It neither dimerizes or oligomerizes nor reacts
in an intramolecular fashion to give a benzo[1]borolane
derivative. Heating above its melting point, however, delivers
several decomposition products, which we could not identify
until now. The 11B NMR signal for 2 is at d 5.9 (for
Mes*B·NMe, d 5.35) and the IR spectrum shows n (11BN) =
2020 and n (10BN) = 2066 cm21, typical for iminoboranes.5
Compound 2 reacts readily with water to yield the corre-
sponding N,NA-bis[(2,4,6-tri-tert-butylphenyl)hydroxyboryl]e-
thylenediamine derivative 3 (Scheme 2).‡
1 P. Paetzold, Adv. Inorg. Chem, 1987, 31, 123.
2 H. Nöth, Angew. Chem. Int. Ed. Engl., 1988, 27, 1603.
3 Gmelin Handbook of Inorganic and Organometallic Chemistry, 8th edn.,
1991, Boron Compounds 4th Suppl. Vol. 3a, pp. 160 and 210.
4 G. Elter, M. Neuhaus, A. Meller and D. Schmidt-Bäse, J. Organomet.
Chem., 1990, 381, 299.
5 G. Elter, M. Geschwentner and A. Meller, Z. Anorg. Allg. Chem., 1993,
619, 1474.
6 M. Geschwentner, G. Elter and A. Meller, Z. Naturforsch., Teil B, 1994,
49, 459.
+ 16 H2O
2
Mes*B(OH) NH (CH2)2 NH B(OH)Mes*
Et2O
3
Scheme 2
We are grateful to the Fonds der Chemischen Industrie for
financial support of this work.
Communication 8/07320A
Chem. Commun., 1998, 2583
2583
Albrecht, Thomas
