Assembly of a planar, tricyclic B4N8 framework with s-indacene structure

Article abstract of DOI:10.1039/b709270a

A neutral, formally 16π-electron, tricyclic tetrahydrazidotetraborane was obtained in a two-step procedure involving self-assembly of a dilithiodiborate with B₄N₈ framework and subsequent oxidation of the phenylborate moieties to bor

Full text of DOI:10.1039/b709270a

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Assembly of a planar, tricyclic B4N8 framework with s-indacene
structure{
Hanh V. Ly,a Heikki M. Tuononen,b Masood Parveza and Roland Roesler*a
Received (in Berkeley, CA, USA) 18th June 2007, Accepted 10th August 2007
First published as an Advance Article on the web 6th September 2007
DOI: 10.1039/b709270a
A neutral, formally 16p-electron, tricyclic tetrahydrazidotetra-
borane was obtained in a two-step procedure involving self-
assembly of a dilithiodiborate with B4N8 framework and
subsequent oxidation of the phenylborate moieties to boranes
and biphenyl using Fe(II) as an oxidant.
The chemistry and properties of borazines, the ‘‘inorganic’’
analogues of benzene, were intensely investigated in the mid-
sixties.1 Borazine research has experienced a resurgence in the last
few years, driven by the applications as precursors to boron nitride
fibres and ceramics,2 as well as molecular materials.3 Boron
derivatives of hydrazines have received considerably less attention.
However, a few cyclic tetraazadiborinanes have been synthesized
and structurally characterized.4 A limited number of molecules
with bi- and tricyclic BN frameworks have been reported as well,5
including analogues of phenalene,5b naphthalene5f and Dewar
benzene.5e
Scheme 1 Synthesis of polycycles 3 and 4.
We were interested in studying the ligand properties of the
planar 6p-electron system 1,6a which was obtained through
containing two B2N3 rings connected through a B2N4 ring (Fig. 1).
As indicated by the NMR spectra, the skeleton contains two
quantitative self-assembly of monomethylhydrazine with PhB-
(NMe2)2, according to a reported procedure.6b Deprotonation of 1
borane and two borate moieties. The tetracoordinated boron, B(2)
˚
is situated 0.6 A outside the best plane formed by the other skeletal
with KH followed by methylation with MeI resulted in clean
replacement of the more acidic ring proton in 1 with a methyl
group.{ The disappearance of the signal corresponding to the ring
proton of 1 at 7.48 ppm in the 1H NMR spectrum clearly indicated
the regiochemistry of the reaction. The lithium amide produced by
deprotonation of the exocyclic nitrogen in 2 with LiTMP
dimerized through N–B bond formation, producing the dilithium
1
salt of a tricyclic borate dianion 3 (Scheme 1). The H and 13C
NMR spectra confirmed the presence of two inequivalent phenyl
groups and three inequivalent methyl groups, while the 11B NMR
spectrum featured signals corresponding to the borane and the
borate moieties at 28.5 and 2.7 ppm, respectively. The chemical
7
+
shift of the Li NMR signal (22.18 ppm) is typical for Li(thf)4
,
suggesting the presence of solvent separated ion pairs in THF
solution.7
A crystallographic determination for 3 revealed a centrosym-
metric structure featuring a tricyclic, annelated B4N8 skeleton
aDepartment of Chemistry, University of Calgary, 2500 University Dr.
NW, Calgary, AB, T2N 1N4, Canada. E-mail: roesler@ucalgary.ca;
Fax: +1 (403) 289-9488; Tel: +1 (403) 220-5366
Fig. 1 Molecular structure of 3 with 50% probability level thermal
ellipsoids. Hydrogen atoms on the organic substituents have been omitted
for clarity and atoms marked with a prime (9) are at equivalent position
bDepartment of Chemistry, University of Jyva¨skyla¨, P.O. Box 35, FIN-
40014, Jyva¨skyla¨, Finland. E-mail: hetuonon@jyu.fi;
Fax: +358 (14) 260-2501; Tel: +358 (14) 260-2618
˚
(2x, 2y, 1 2 z). Selected bond lengths (A): B(1)–N(2) 1.441(2), B(1)–N(3)
1.408(2), B(2)–N(1) 1.562(2), B(2)–N(3) 1.556(2), B(2)–N(49) 1.566(2),
{ Electronic supplementary information (ESI) available: The cyclovol-
tammogram of 4, NMR spectra, as well as optimized coordinates and
representations of the Kohn–Sham orbitals for 4 and [4]2+. See DOI:
10.1039/b709270a
N(1)–N(2) 1.475(2), N(3)–N(4) 1.445(2), Li(1)–N(1) 2.189(3), Li(1)–N(3)
…
2.326(3), Li(1)–N(49) 2.035(3), Li(1)–O(1) 1.902(3), Li(1) C(109) 2.775(3),
B(1)–C(4) 1.579(2), B(2)–C(10) 1.636(2), N–C 1.446(2)–1.462(2).
4522 | Chem. Commun., 2007, 4522–4524
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observed in borazines (1.42–1.44 A),11 and for a single N–N bond
(vide supra). The phenyl substituents show little deviation from the
ring plane (5.3u), while the C–N bonds form angles of 22.7–31.1u
with this plane, in an all-trans arrangement. The all-carbon analog
of 4, s-indacene, is an unstable molecule with formally antiaro-
matic character according to the Hu¨ckel rule (12p-electrons).12 Its
more stable substituted derivatives13 can be reduced to formally
aromatic, 14p-electron dianions.14 It could therefore be expected
that a two-electron oxidation or reduction of 4 would yield a stable
dication or dianion, respectively, satisfying the Hu¨ckel condition of
aromaticity. In order to explore this hypothesis, theoretical
investigations were performed for 4 as well as for its doubly
reduced and oxidized forms.15
˚
atoms, determining the envelope conformation of the five-
…
membered rings (folding angle along the N(1) N(3) axis ca.
30u) and the chair conformation of the six-membered ring (folding
…
angle along the N(3) N(49) axis ca. 55u). The N–N bond lengths
˚
are 1.443(2) and 1.475(2) A, comparable to the N–N bond length
8
in hydrazine (1.45 A). The B–N bonds involving the borane center
˚
˚
B(1) (1.408(2) and 1.441(2) A) are much shorter than those
˚
involving the borate center B(2) (1.556(2)–1.566(2) A). The lithium
˚
ions are positioned 1.7 A outside the best plane of the ligand, and
are coordinated by the three nitrogen atoms connected to the
borate center. The coordination sphere of the lithium ion is
completed by an intramolecular contact involving the ipso carbon
of the phenylborate moiety, and a THF molecule.
The geometry optimized structure of 4 is in excellent agreement
with the data from the structural determination (Fig. 2). The
analysis of Kohn–Sham orbitals of 4 confirms that it is only
formally a 16p-electron system: though the tricyclic B4N8 frame-
work is essentially planar, the geometry around all methyl
substituted nitrogen atoms is pyramidal, giving rise to MOs with
only partial p-like character (see ESI{). The HOMO is
p-antibonding through all four N–N linkages, which readily
explains the observed long bond distances indicative of single
bonds.
Oxidation of 3 with FeCl2(thf)2 in THF produced the neutral
BN tricycle 4 in low (22%) but reproducible isolated yield. The
other probable product of this reaction, FePh2(thf)2, subsequently
decomposed to metallic iron and biphenyl, which were identified in
the reaction mixture. A similar decomposition pathway was
1
reported for FePh2(PEt3)2 at temperatures above 0 uC.9 The H
and 13C NMR spectra of 4 featured the resonances expected for
one phenyl group and three inequivalent methyl groups, and the
signals corresponding to the two inequivalent boron centers
merged into a broad singlet at 25.1 ppm. The use of a
stoichiometric quantity of I2 as oxidizing agent instead of
FeCl2(thf)2 did not result in the formation of 4. The electro-
chemical oxidation of tetraphenylborate to boric acid and biphenyl
in aqueous conditions at a potential of 0.216 V vs. SCE has been
reported,10 however, the reaction of Li[BPh4] with FeCl2(thf)2
failed to produce BPh3. Thus, the formation of the stable polycycle
appears to be an essential thermodynamic contributor to the
oxidation of the phenylborate 3 to biphenyl and the borane 4.
The X-ray diffraction study of 4 reveals two very similar,
centrosymmetric, independent molecules containing planar B4N8
frameworks with s-indacene structure (Fig. 2). The tricyclic
skeleton is regular, with all B–N and N–N bonds having the
The B4N8 framework is significantly nonplanar in the anion
[4]22 due to occupation of a MO with B–N antibonding character.
Hence, a two-electron reduction of 4 leads to a structure which
does not fulfill the general conditions of aromaticity. On the other
hand, the two-electron oxidation of 4 yields a dication with a
nearly planar structure because two electrons are removed from
the N–N antibonding HOMO (see ESI{). The minor geometric
distortions present in the system arise from steric interactions, as
evidenced by the geometry optimized structure of a hydrogen
substituted tricycle [B4N8H8]2+, displaying a perfectly planar
Hu¨ckel aromatic geometry. Interestingly, the closed shell
singlet state SCF solution of [4]2+ has an internal instability
indicating that the singlet ground state of this dication has
nitrogen-centered diradical character. Hence, the open-shell
singlet diradical state of [4]2+ was modeled using broken symmetry
formalism and the calculated structure was compared to the
lowest energy triplet state. The calculations showed that the two
spin states are very close in energy, with the formally aromatic
open-shell singlet state being ca. 5 kJ mol21 lower than the triplet
state.
˚
same length, between 1.423(3) and 1.441(2) A. This length is
characteristic of a partial multiple bond character for N–B, as
Unfortunately the experimental search for a stable oxidation
product of 4 remained fruitless. Cyclovoltammetric measurements
were performed in THF in the range of 22.5 to 1.5 V, and they
showed two irreversible oxidation steps at 0.35 and 0.90 V vs. SCE
(20.21 and 0.34 V vs. Fc). Chemical oxidation with [Cp2Fe]PF6
produced Cp2Fe and a solid that was insoluble in organic solvents
and was not further characterized. Radical species were not
detected by EPR upon in-situ electrochemical oxidation. We note
that stable radicals with s-indacene-like frameworks are well-
known in chalcogen–nitrogen chemistry.19
Fig. 2 One of the two independent molecules in the structure of 4 with
50% probability level thermal ellipsoids. Atoms marked with a prime (9)
˚
are at equivalent position (2x, 2 2 y, 2z). Selected bond lengths (A) and
angles (u); calculated values are reported in square brackets: B–N 1.423(3)–
1.440(3) [1.429–1.440], N–N 1.436(2)–1.441(2) [1.411–1.416], N–C
1.446(3)–1.463(3) [1.440–1.447], C–B 1.556(3) [1.565], 1.570(3) [1.565],
B(39)–N(5)–N(6) 113.72(15) [113.5], N(5)–N(6)–B(3) 122.09(15) [122.2],
N(6)–B(3)–N(59) 124.18(18) [124.2], B(3)–N(6)–B(4) 108.26(17) [108.0],
N(6)–B(4)–N(8) 106.85(17) [106.7], B(4)–N(8)–N(7) 109.49(15) [109.8],
N(8)–N(7)–B(3) 106.70(15) [107.1], N(7)–B(3)–N(6) 108.35(16) [108.1].
This work was supported by the Natural Sciences and
Engineering Research Council of Canada, the Canada
Foundation for Innovation, the Academy of Finland and the
Alberta Science and Research Investments Program. The authors
wish to thank Ms Tracey L. Roemmele and Dr Rene´ T. Boere´ for
assistance with the EPR and cyclovoltammetric measurements.
This journal is ß The Royal Society of Chemistry 2007
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2s(I). CCDC 649698. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b709270a
Notes and references
{ All operations were conducted under strict exclusion of air and moisture.
Solvents were dried prior to use and methylhydrazine was dried over CaH2.
Methylated hydrazines are highly toxic and probable carcinogens, and their
handling requires special precautions; all residues were neutralized using
commercial bleach solution.
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Synthesis of 2: A solution of 16 (0.700 g, 2.65 mmol) in THF (20 mL) was
slowly added to a suspension of KH (0.106 g, 2.65 mmol) in THF (15 mL).
KH dissolved with evolution of hydrogen producing a yellow solution that
was stirred at ambient temperature for another 2 h. MeI (0.165 mL,
2.65 mmol) was added to the mixture and the colourless suspension was
stirred for an additional hour and then concentrated under vacuum to ca.
3 mL. Hexane (30 mL) was added and the KI by-product was filtered off.
The solvent was subsequently removed in vacuo, leaving behind the product
as a colourless powder (638 mg, 88%). dH (400 MHz, C6D6, 25 uC): 2.32 (d,
3H, 3JHH = 6.4 Hz, HNCH3), 2.88 (s, 6H, (NCH3)2), 3.76 (q, 1H, 3JHH
=
=
3
6.4 Hz, HNCH3), 7.25–7.34 (m, 6H, m- + p-C6H5), 7.74 (d, 4H, JHH
6.6 Hz, o-C6H5); dC (100 MHz, C6D6, 25 uC): 31.7 (s, (NCH3)2), 43.5 (s,
HNCH3), 128.8 (s, m-C6H5), 129.1 (s, p-C6H5), 134.4 (s, o-C6H5), 135.0 (s,
br, i-C6H5); dB (128 MHz, THF-d8, 25 uC): 28.6 (s, br).
Synthesis of 3: A solution of lithium 2,2,6,6-tetramethylpiperidide,
LiTMP was prepared from 1.6 M n-butyllithium in hexane (1.12 ml,
1.80 mmol) and tetramethylpiperidine, TMP, (0.254 g, 1.80 mmol) in THF
(3 mL). The solutions of 2 (0.500 g, 1.80 mmol) in THF (3 mL) and LiTMP
were pre-cooled to 235 uC and mixed, yielding an orange solution that was
stored at 235 uC for a day. Subsequently it was allowed to warm to room
temperature and volatiles were removed in vacuo, leaving behind a yellow
residue that was washed twice with hexane (30 mL) and dried under
vacuum. The product was obtained as a colourless powder (388 mg,
60.5%). dH (400 MHz, THF-d8, 25 uC): 2.01 (s, 6H, NCH3), 2.28 (s, 6H,
NCH3), 2.71 (s, 6H, NCH3), 7.12–7.31 (m, 12H, m- and p-C6H5), 7.53 (d,
3
3
4H, JHH = 6.8 Hz, o-C6H5), 7.77 (d, 4H, JHH = 6.8 Hz, o-C6H5); dC
(100 MHz, THF-d8, 25 uC): 34.2 (s, NCH3), 39.1 (s, NCH3), 41.3 (s,
NCH3), 126.3 (s, p-C6H5), 127.2 (s, p-C6H5), 127.5 (s, m-C6H5), 128.0 (s,
m-C6H5), 134.0 (s, o-C6H5), 135.9 (s, o-C6H5); dB (128 MHz, THF-d8,
25 uC): 2.7 (s) and 28.5 (s, br); dLi (155 MHz, THF-d8, 25 uC): 22.18 (s).
Needle-like X-ray quality crystals were obtained by recrystallization of 3
from THF.
7 H. V. Ly, T. D. Forster, D. Maley, M. Parvez and R. Roesler, Chem.
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602.
9 K. Maruyama, T. Ito and A. Yamamoto, Transition Met. Chem., 1980,
5, 14.
Synthesis of 4: A solution of 3 (80.0 mg, 0.112 mmol) in THF (3 mL) was
added to solid FeCl2(thf)220 (38.0 mg, 0.141 mmol). The dark mixture was
allow to stand at room temperature for a day, and was then concentrated
to 1 mL and cooled to 235 uC for several days until colourless crystals of 4
formed and were separated by decantation (10 mg, 22%). dH (400 MHz,
THF-d8, 25 uC): 2.66 (s, 6H, NCH3), 2.83 (s, 6H, NCH3), 2.87 (s, 6H,
10 (a) W. R. Turner and P. J. Elving, J. Phys. Chem., 1965, 69, 1067; (b)
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3
NCH3), 7.31–7.33 (m, 6H, m- and p-C6H5), 7.55 (d, 4H, JHH = 5.6 Hz,
o-C6H5); dC (100 MHz, THF-d8, 25 uC): 32.9 (s, NCH3), 33.7 (s, NCH3),
42.4 (s, NCH3), 128.4 (s, m-C6H5), 128.9 (s, p-C6H5), 134.2 (s, o-C6H5); dB
(128 MHz, THF-d8, 25 uC): 25.1 (s, br); MS (EI+, 70 eV): m/z (%): 400 (51)
[M]+, 385 (13) [M 2 Me]+, 278 (40) [M 2 Ph 2 3Me]+, 154 (41) [B4N8]+;
HRMS for H28C18N811B4: calc. 400.2809, found 400.2838.
Cyclovoltammetry of 4: The measurements were performed in THF at an
analyte concentration of 1 mM, and using 0.1 M [nBu4N]PF6 as a
supporting electrolyte. A PARstat 2273 potentiostat was used at scan rates
between 50 and 1000 mV s21. The cell had a platinum disk working
electrode, a platinum wire auxiliary electrode, and a silver wire pseudo-
reference electrode. [Cp2Co]0/+1 with E0 = 21.36 V vs. ferrocene and
20.80 V vs. the SCE was used as an internal standard.
15 The structures of 4, [4]2+ and [4]22 were optimized with the PBE0 hybrid
functional16 using the Ahlrichs’ TZVP basis sets.17 All calculations were
done with the Turbomole 5.9.1 and Gaussian 03 program packages18.
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H. Horn and C. Ko¨lmel, Chem. Phys. Lett., 1989, 162, 165; (b) Gaussian
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2004.
Crystal data for 3: C38H54B4Li2N8O2, M = 712.02, monoclinic, space
˚
group P21/n, a = 13.149(5), b = 9.299(4), c = 16.145(6) A, b = 100.17(3)u,
3
V = 1943.1(13) A , Z = 2, Dc = 1.217 g cm23, F(000) = 760, m(Mo-Ka) =
˚
0.074 mm21, l = 0.71073 A, T = 123(2) K. 8425 reflections measured, 4398
˚
unique data (2hmax = 55.0u, Rint = 0.027). GOF on F2 = 1.02, wR2(F2) =
0.126 for all data, R1(F) = 0.047 on reflections having I . 2s(I). CCDC
649697.
19 (a) J. M. Rawson, A. Alberola and A. Whalley, J. Mater. Chem., 2006,
16, 2560; (b) J. L. Brusso, S. Derakhshan, M. E. Itkis, H. Kleinke,
R. C. Haddon, R. T. Oakley, R. W. Reed, J. F. Richardson,
C. M. Robertson and L. K. Thompson, Inorg. Chem., 2006, 45,
10958, and references therein.
¯
Crystal data for 4: C18H28B4N8, M = 399.72, triclinic, space group P1,
˚
a = 17.240(3), b = 9.157(6), c = 16.983(12) A, a = 75.88(3), b = 83.19(4), c =
3
76.55(4)u, V = 1059.7(11) A , Z = 2, Dc = 1.253 g cm23, F(000) = 424,
˚
m(Mo-Ka) = 0.077 mm21, l = 0.71073 A, T = 173(2) K. 6791 reflections
˚
measured, 3693 unique data (2hmax = 50.0u, Rint = 0.031). GOF on F2 =
20 S. D. Ittel, A. D. English, C. A. Tolman and J. P. Jesson, Inorg. Chim.
Acta, 1979, 33, 101.
1.01, wR2(F2) = 0.146 for all data, R1(F) = 0.046 on reflections having I .
4524 | Chem. Commun., 2007, 4522–4524
This journal is ß The Royal Society of Chemistry 2007