1,2,4,3-Triazaborole-based neutral oxoborane stabilized by a Lewis acid

Article abstract of DOI:10.1039/c4cc03379e

The first example of 1,2,4,3-triazaborole-based oxoborane has been synthesized via hydrogen migration upon the coordination of AlCl₃ to the corresponding borinic acid. X-ray diffraction analysis and computational study disclosed the partial BO double-bond property. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03379e

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1,2,4,3-Triazaborole-based neutral oxoborane
stabilized by a Lewis acid†
Cite this: Chem. Commun., 2014,
50, 8561
Ying Kai Loh,‡ Che Chang Chong,‡ Rakesh Ganguly, Yongxin Li, Dragoslav Vidovic
Received 6th May 2014,
Accepted 9th June 2014
and Rei Kinjo*
DOI: 10.1039/c4cc03379e
www.rsc.org/chemcomm
The first example of 1,2,4,3-triazaborole-based oxoborane has been breakthrough by Braunschweig and co-workers demonstrated
synthesized via hydrogen migration upon the coordination of AlCl3 to that a late-transition metal effectively stabilizes dicoordinate boron
the corresponding borinic acid. X-ray diffraction analysis and compu- V where the BO fragment contains boron–oxygen triple bonds.13
Q
tational study disclosed the partial B O double-bond property.
More recently, Yamashita et al. reported a ruthenium–boronate
complex VI displaying a B–O–Ru bond angle close to 901, which
Oxoboranes [RBOs], compounds containing boron–oxygen induced a short B–O bond with double-bond character and a long
bonds with multiple-bonding character, are important transient Ru–O bond.14 These pioneering results demonstrated that (i) kinetic
intermediates in organoboron chemistry,1,2 and could only be stabilization by introducing bulky ligands at the BO unit and (ii) the
detected directly in the high-temperature gas phase or in low- coordination of Lewis or Brønsted acid to the BO unit are effective
temperature matrices;3 otherwise, they readily trimerize under for isolation of oxoboranes as the monomeric form, which
ambient conditions to form boroxine [B3O3R3] due to the high prompted us to investigate the preparation of the novel type of
oxophilic nature of the unsaturated boron atoms.4 Recent oxoborane derivative to understand deeply the BO multiple-bonding
advances in synthetic main group chemistry, however, allowed nature. Here, we propose to extend the neutral oxoborane family
for the stabilization of such persistent species,5,6 and indeed with a five-membered ring system, and report the preparation of
several bottleable oxoboranes have been reported (Fig. 1). In the 1,2,4,3-triazaborole-based oxoborane stabilized by aluminum
2005, Cowley et al. reported the first isolation of AlCl3-stabilized trichloride. In addition, we perform single crystal X-ray diffrac-
neutral oxoborane Ia supported by a bulky b-diketiminate, and tion and computational studies, revealing the bonding character
revealed the existence of the short B–O bond with considerable of boron–oxygen units in the oxoborane molecule.
double-bond character (the plausible resonance forms Ia1–4 are
To construct the oxoborane based on the five-membered
shown in Fig. 1).7,8 Similarly, neutral oxoboranes Ib coordinating ring framework, we chose amidrazone ligands because simple
to Lewis acidic boranes [B(C6F5)3 and BCl3] were synthesized by cyclocondensation of boron trihalide with amidrazones readily
Cui et al.9,10 They also reported the first metal-free anionic affords a 3,4-dihydro-2H-1,2,4,3-triazaborole skeleton, as described
derivatives II by deprotonation from the corresponding borinic previously by Weber and co-workers.15 With slight modification
acid with N-heterocyclic carbenes (NHCs), and a hydrogen bond in their method, we first attempted the reaction of BCl3 with
was confirmed between the BO fragment in anions and the H amidrazone 1 generated in situ from the reaction of an imidoyl
atom at C2 of an imidazolium cation in II.9 Interestingly, Curran chloride and phenyl hydrazine (Scheme 1). A toluene solution
et al. observed the relatively short B–O distance in NHC- of 1 and BCl3 was refluxed for 3 hours, and after work-up,
coordinated dihydroxyborenium cation III.11 Similarly, Ingleson 3,4-dihydro-2H-1,2,4,3-triazaborole 2 was obtained as a pale
et al. proposed that cationic boroxine derivatives IVa–b feature yellow solid in 66% yield (Scheme 1). Next, treatment of com-
the B–O units with partial double-bond character.12 A recent pound 2 with one equivalent of H2O and NEt3 in dichloro-
methane yielded borinic acid 3 quantitatively (73% yield), and
the 11B-NMR spectra of 3 show a broad signal at 23.4 ppm. The
Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, Nanyang Link 21,
Singapore 637371, Singapore. E-mail: rkinjo@ntu.edu.sg; Tel: +65-6592-2625
† Electronic supplementary information (ESI) available: Experimental and calculation
details, and crystallographic information for 2, 3 and 5. CCDC 1000877–1000879.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4cc03379e
solid state structure of compounds 2–3 were characterized by
X-ray diffraction analysis.16 The presence of a proton on the
oxygen atom was supported by a B1–O1 single-bond distance
(1.351(7) Å) (Fig. 2, top). This indicates that spontaneous
hydrogen migration from the OH group does not occur, pre-
sumably due to the strong basicity of O atoms of the OH group.
‡ These authors contributed equally to this work.
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Fig. 1 Examples of structurally characterized boron compounds featuring BO fragments(s) with multiple bonding property.
Fig. 2 ORTEP drawings of 3 (top) and 5 (bottom), all hydrogen atoms
except for OH in 3 and NH in 5 are omitted for clarity. Thermal ellipsoids
are set at the 30% probability level. Selected bond lengths [Å] and angles [1]
for 3: B1–O1 1.351(7), B1–N1 1.437(8), B1–N2 1.439(7), C1–N1 1.401(6),
C1–N3 1.291(7), N2–N3 1.412(6), O1–B1–N1 130.5(5), O1–B1–N2 126.2(5),
N1–B1–N2 103.3(5). For 5: B1–O1 1.304(3), O1–Al1 1.7073(16), B1–N1 1.480(3),
B1–N2 1.437(3), C1–N1 1.345(3), C1–N3 1.319(3), N2–N3 1.385(2), Al1–O1–B1
169.44(18), O1–B1–N1 127.81(19), O1–B1–N2 129.5(2), N1–B1–N2 102.68(17).
Scheme 1 Synthesis of oxoborane 5 and its resonance forms (5a–d)
(Dipp = 2,6-diisopropylphenyl).
We reasoned that the coordination of Lewis acid to the oxygen
atom of the OH group in 3 could decrease the basicity of the oxygen
atom, and consequently the acidity of the hydrogen atom would be
increased, allowing for the oxydrilic proton migration from the OH
group as reported by Cui et al.9 Then, we added a stoichiometric Ia (87.3 ppm).7 By layering hexane on a dichloromethane solution
amount of AlCl3 to a dichloromethane solution of 3 at room of the product at room temperature, single crystals of compound 5
temperature. After removing the solvent in vacuo, the solid residue were obtained (95% yield).17 Although we could not detect inter-
was dissolved in deuterated dichloromethane, and the 11B NMR mediate 4, it is reasonable that compound 5 was formed via fast
spectrum showed a singlet at 18.9 ppm. A broad singlet at 10.2 ppm proton migration from 4.
was detected in the 1H NMR spectrum indicating the presence of a
X-ray diffraction unambiguously revealed the structure of 5 in
significantly acidic H atom. In the 27Al{1H} NMR spectra, a broad which the coordination of AlCl3 to the oxygen atom was confirmed
resonance at 87.3 ppm was observed, which is identical to that of (Fig. 2, bottom). Note that compound 5 is the first example of
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oxoborane featuring a five-membered ring skeleton. The sum of the
bond angles around the boron atom is 359.991 indicating the
trigonal-planar geometry. The B1–O1–Al1 angle (169.44(18)1) in 5
is similar to the value of 169.2(1)1 reported in Ia.7 The B1–O1 bond
length of 1.304(3) Å is ca. 3.5% shorter than the corresponding
B1–O1 single bond in 3, and comparable to that observed in Ia
(1.304(2) Å) but slightly shorter than that in IbꢀB(C6F5)3 (1.311(3) Å).9
The O1–Al1 bond length is 1.7073(16) Å which is slightly shorter
than that in Ia (1.720(1) Å). The B1–N1 bond length (1.480(3) Å) is
longer than B1–N2 (1.437(3) Å), presumably owing to the steric
repulsion between the bulky Dipp group on the N1 atom and the
OAlCl3 moiety on the B atom. The C1–N3 bond (1.319(3) Å) in 5 is
longer than the corresponding C–N bond (1.291(7) Å) in 3. Mean-
while, the C1–N1 bond (1.345(3) Å) in 5 is 0.05 Å shorter than the
corresponding C–N bond (1.401(6) Å) in 3, which indicates that the
cationic charge induced by the coordination of hydrogen on the N3
atom delocalizes over the N1–C1–N3 fragment.
Fig. 4 Selected molecular orbitals of 50 and 6.
Cowley et al. also theoretically supported that the boron–oxygen
functionality of Ia retains considerable double bond character. Thus,
because their calculation results revealed that the B–O distance
(1.316 Å) of the optimized Ia was only increased by 1.9% with
respect to that (1.292 Å) of the Lewis acid-free model compound
LBQO (L = b-diketiminate) which displayed BQO p-bonding in the
HOMO ꢁ 6, they concluded that the B–O bond in Ia has double
bond character. We also performed natural bond order (NBO)
analysis for 50 at the M052X level of theory using the 6-311G(d,p)
basis set, which showed that the B–O s bond is formed by the sp1.71
hybrid orbital of the boron and the sp0.44 hybrid orbital of the oxygen
atom, respectively. The characteristic donor–acceptor interaction
was found between the lone pair orbital on the O atom and the
p-orbital on the B atom, which gives rise to a stabilization energy
of 80.15 kcal molꢁ1 (see Table S2 in the ESI†). Concomitant with
a high occupancy (0.43 e) in the p-orbital on the B atom, the
Wiberg bond index (WBI) value of 1.07 for the B–O moiety was
confirmed. These computational studies supported the partial
BQO double-bond property.
In summary, we synthesized a novel type of oxoborane derivative
5 with a 1,2,4,3-triazaborole skeleton, via the oxydrilic hydrogen
transfer promoted by the coordination of AlCl3 to the corresponding
borinic acid 3. X-ray diffraction analysis of 5 revealed the
presence of the B–O unit coordinating to AlCl3, and computa-
tional study supported the partial BQO double-bond character.
Investigation of the reactivity and synthesis of other analogues
featuring BQX bonds is underway in our laboratory.
In the solid-state IR spectrum of 5, a characteristic peak was
observed at 1636 cmꢁ1 which is assigned to a B–O stretch based
on theoretical calculation (see the ESI,† Fig. S4),18 and the value
is larger than the corresponding stretch (1467 cmꢁ1) of VI
which Yamashita et al. reported,14 indicating the somehow
stronger B–O bond of 5 than that of VI.
The bonding properties and the electronic structure of 5 were
further explored theoretically using ab initio calculations at the
B3LYP level of theory with the 6-31G(d) basis set. The structural
optimization on 5 well reproduced the experimental geometry and
the structural parameters. We also performed the calculations on
the model compound 6 which features the B–O moiety without the
coordination of AlCl3. The resulting optimized structures (50 and 6)
are summarized in Fig. 3. The N–B–N angle of 101.511 in 50 is wider
than the corresponding angle in 6 (97.951). Shortening of both the
B–N bonds when AlCl3 is coordinated to the oxoborane 6 indicates
the enhanced B–N pbonding character, probably attributed to the
decrease of electron-donation of the lone pair on the O atom to the
p-orbital of the B atom. The calculated B–O distance (1.297 Å) in 50 is
only 0.033 Å longer than that in the free oxoborane 6. The p-bonding
interaction on the B–O unit in 50 was found in HOMO ꢁ 12 where
the nitrogen 2p orbitals, the CQN p orbital, and chlorine lone pairs
were involved (Fig. 4, left). In contrast, HOMO ꢁ 2 of 6 exhibits the
BQO p-bonding orbital in which participation of other p-orbitals on
the five-membered ring was not observed (Fig. 4, right). Note that
We are grateful to Nanyang Technological University and
Agency for Science, Technology, and Research, Singapore (A*STAR)
(PSF-SERC: 1321202066), for financial support.
Notes and references
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structure of 2, see the ESI,† Fig. S1.
17 Although the coordination of other Lewis acids (BF3, BCl3, and B(C6F5)3)
was also examined, all of them provided a complex mixture, presumable
due to the competitive coordination of the Lewis acids to the N atom in
the five-membered ring which induced the destruction of the ring.
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probably due to overlap with a broad peak at 3418 cmꢁ1, see the ESI†.
8564 | Chem. Commun., 2014, 50, 8561--8564
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