New polycyclic borazine species

Article abstract of DOI:10.1039/c1cc10196j

The new polycyclic borazines B₂{1,2-N₂C ₆H₄}₂{B₂(NMe₂) ₂}₂, B₂{1,8-N₂naph} ₂{B₂(NMe₂)₂}₂ and B ₂(NPh)₄{B₂(NMe₂)₂} ₂ have been prepared from diborate(4) anions and two equivalents of B₂Cl₂(NMe₂)₂ and have been structurally characterised. Aspects of their structure and bonding are discussed and comparison made with corresponding polycyclic aromatic hydrocarbons. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1cc10196j

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COMMUNICATION
New polycyclic borazine speciesw
Xiaochen Xie, Mairi F. Haddow, Stephen M. Mansell, Nicholas C. Norman* and
Christopher A. Russell*
Received 11th January 2011, Accepted 10th February 2011
DOI: 10.1039/c1cc10196j
The new polycyclic borazines B2{1,2-N2C6H4}2{B2(NMe2)2}2,
B2{1,8-N2naph}2{B2(NMe2)2}2 and B2(NPh)4{B2(NMe2)2}2
have been prepared from diborate(4) anions and two equivalents
of B2Cl2(NMe2)2 and have been structurally characterised.
Aspects of their structure and bonding are discussed and
comparison made with corresponding polycyclic aromatic
hydrocarbons.
this methodology lithiation of the 1,1-isomer of
B2{1,2-(NH)2C6H4}2 (2)8 followed by reaction with two
9
equivalents of B2Cl2(NMe2)2 afforded the compound
1,1-B2{1,2-N2C6H4}2{B2(NMe2)2}2 (3) according to Scheme 1.w
The structure of 3 (as a toluene solvate) was determined by
X-ray crystallography (Fig. 1)zy which revealed a polycyclic
species in which a B–B unit derived from each of the two
B2Cl2(NMe2)2 reactants has added in a 1,2-fashion to pairs of
nitrogen atoms in the B2{1,2-(NH)2C6H4}2 precursor.
In general, borazines, i.e. cyclic species containing alternating
BR and NR units, adopt six-membered ring structures with
the general formula {BNR2}x; commonly x = 3 and the
species are isoelectronic with arenes,1 although some examples
are known where x = 4.2 Fused or linked systems related to
naphthalene and biphenyl have also been described, the latter
including species in which the two rings are linked by a B–B
bond.3 One example has been reported of a cyclic B4N4R8
species containing two B–B bonds, i.e. B4(NMe2)4N4Et4,
effectively an isomer of {BNR2}x (x = 4), derived from the
reaction between B2Cl2(NMe2)2 and 1,2-diethylhydrazine.4 In
a recent paper, we described an example of a new class of
boron-rich borazine with the general formula {B2NR3}x where
x = 4, namely B8(NH)4(NMe2)8 (1), in which B–B bonds are
present as well as B–N bonds (a diborazine).5 Herein we report
examples of two new borazine types constructed from
reactions involving diborane(4) compounds. The polycyclic
frameworks of these novel borazines can also be seen in
relation to those of isostructural polycyclic aromatic hydro-
carbons (PAHs), which constitute a highly promising class of
photoelectric materials and the substitution of BN units
for isoelectronic CC units in PAH manifolds is a topic of
considerable current interest owing to the subtly different
electronic properties of the resulting materials.6 In the case
of the species reported herein, the B : N ratio is such that the
compounds have two electrons fewer than their parent PAH.
We have previously shown that diborane(4) compounds
containing primary amido substituents, B2(NHR)4, can be
lithiated by addition of a stoichiometric amount of BunLi to
give the tetra-anionic diborate(4) anions [B2(NR)4]4ꢀ.7 Using
Compound 3 comprises a mixture of fused C6, B4N2 and
BC2N2 rings and resembles those borazines which have a
3
naphthalene-type structure, i.e. B5N5R8 but in this case the
general formula is B6N4R8 and it therefore contains two fewer
electrons. Molecules of 3 lie on a crystallographic centre
of inversion but adopt a markedly non-planar, twisted
conformation due to intramolecular steric interactions between
adjacent NMe2 groups as a result of the conformation about
the B–N bonds required to maximise p-bonding. This twisting
is reflected in the angle between the N2B planes, t, of 58.41 for
the peripheral B2 units, in contrast to the eclipsed conformation
(i.e., t = 0) about the central B2 unit. Also notable are some of
the bond distances. Thus the central B–B bond [B(1)-B(1A)
1.611(4) A] is extremely short,8,10 being substantially shorter
than the outer B–B lengths [B(2)–B(3A) 1.757(4) A] and
comparable to lengths which have been formally assigned as
BQB double bonds.11 The B–N bond lengths also span a wide
range with the exocyclic distances shortest [B(2)–N(3)
1.402(3), B(3)–N(4) 1.402(2) A] and the endocyclic longer in
two groups [B(1)–N(1) 1.427(3), B(1)–N(2) 1.431(3) and
B(2)–N(1) 1.500(3), B(3)–N(2) 1.495(3) A]. These bond lengths
are suggestive of exocyclic BQN character, and endocyclic
units comprising delocalized N–B–N bonding and formal B–N
single bonds respectively.
An analogous reaction starting with 1,1-B2{1,8-N2naph}2
(4), derived from the reaction between B2(NMe2)49b,12 and two
School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK.
E-mail: Chris.Russell@bris.ac.uk, n.c.norman@bris.ac.uk;
Fax: +44 (0)117 925 1295; Tel: +44 (0)117 928 7599
w Electronic supplementary information (ESI) available: Syntheses
and experimental data for 3–6; molecular structure of 4. CCDC
807912–807915. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1cc10196j
Scheme 1
c
3748 Chem. Commun., 2011, 47, 3748–3750
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Fig. 2 A view of the molecular structure of compound 5. Thermal
ellipsoids are set at the 50% probability level. Hydrogen atoms and the
solvent of crystallisation (toluene) have been omitted for clarity.
Selected bond distances (A): B(1)–B(1A) 1.728(4), B(2)–B(3A)
1.710(3), B(1)–N(1) 1.439(2), B(1)–N(2) 1.435(2), B(2)–N(2) 1.504(2),
B(2)–N(3) 1.393(2), B(3)–N(1) 1.499(2), B(3)–N(4) 1.402(2). Symmetry
transformations used to generate equivalent atoms labelled ‘‘A’’ 2 ꢀ x,
2 ꢀ y, 1 ꢀ z.
Fig. 1 A view of the molecular structure of compound 3. Thermal
ellipsoids are set at the 50% probability level. Hydrogen atoms and the
solvent of crystallisation (toluene) have been omitted for clarity.
Selected bond distances (A): B(1)–B(1A) 1.611(4), B(2)–B(3A)
1.757(4), B(1)–N(1) 1.427(3), B(1)–N(2) 1.431(3), B(2)–N(1) 1.500(3),
B(2)–N(3) 1.402(3), B(3)–N(2) 1.495(3), B(3)–N(4) 1.402(2). Symmetry
transformations used to generate equivalent atoms labelled ‘‘A’’ 12 ꢀ x,
1
2
ꢀ y, 2 ꢀ z.
Scheme 2
equivalents of 1,8-diaminonaphthalene, afforded a similar
species 1,1-B2{1,8-N2naph}2{B2(NMe2)2}2 (5) (Scheme 2).w
We note that this diamine has been employed recently as a
substituent in diborane(4) compounds used in diboration
reactions.13 Both 4 and 5 were characterised by X-ray crystallo-
graphy, the latter as a bis-toluene solvate.wzz8 Complex 4
co-crystallises with two molecules of DMF but the structure is
unexceptional and is similar to the structure of 2 reported in
ref. 8; molecules are strictly planar in the solid state and the
B–B distance is typical of a single bond at 1.691(4) A. The
structure of 5 resembles that of 3 and is shown in Fig. 2.
Molecules of 5 also lie on a crystallographic inversion centre
and are twisted about the peripheral B2 vectors [t = 67.71]
comparable to that found in 3. In 5, however, the B–B
distances are much more similar [B(1)–B(1A) 1.728(4) and
B(2)–B(3A) 1.710(3) A] reflecting the reduced ring strain
in a species composed entirely of six-membered rings. The
B–N distances exhibit a similar pattern to those observed in
compound 3.
Fig. 3 A contour plot of the HOMO-2 of compound 3 (contour value
0.04) showing a B–B p-bonding interaction.
atoms, suggesting that the B–B bond order is higher than 1 in
this case.
In looking to explore further the generality of the reactions
which afforded 3 and 5, the reaction between the lithium salt of
the diborate(4) anion [B2(NPh)4]4ꢀ 7 and two equivalents of
B2Cl2(NMe2)2 was carried out which afforded the compound
B2(NPh)4{B2(NMe2)2}2 (6).w
In order to test whether the short nature of the central B–B
bond in 3 is purely due to ring strain in the molecule or
whether this is evidence of some B–B p bonding, DFT
calculations14,15 were performed on 3 and 5** and the
molecular orbitals of p-symmetry examined. Whilst 5 showed
no evidence of p bonding between the central boron atoms, the
HOMO-2 in 3 (see Fig. 3) shows substantial delocalisation of
the nitrogen lone pairs across the p-orbitals of both boron
Interestingly, the addition of the B–B units occurs in a
1,1-fashion in this case to give a compound containing two
B3N2 rings linked by an unsupported B–B bond. The structure
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3748–3750 3749

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8 Crystal data for 5ꢁ2toluene:
C
orthorhombic, space group Pbca, a = 13.4491(4), b = 9.6202(3),
42H52B6N8,
M
=
733.78,
c = 31.1212(10) A, U = 4026.6(2) A3, Z = 4, Dc = 1.210 Mg mꢀ3
,
l = 1.54178 A, m(Cu-Ka) = 0.541 mmꢀ1, F(000) = 1560, T = 100(2)
K, R1 = 0.0453 [for 3170 reflections with I > 2s(I)], wR2 = 0.1161
[for all 3402 reflections]. Rint = 0.0741. CCDC 807914.
** Computational energy minimisations for 3 and 5 were carried out
using B3LYP/6-31G*, with the geometry of the species in the crystal
structure as the starting point for the optimisation. All calculations
were performed with the Gaussian03 package using the B3LYP
functional and with 6-31G* basis sets on all atoms.14,15
ww Crystal data for 6: C32H44B6N8, M = 605.61, triclinic, space group
ꢀ
P1, a = 11.9997(8), b = 12.1731(9), c = 14.0679(13) A, a =
94.107(6)1, b = 104.802(5)1, g = 119.494(3)1, U = 1681.0(2) A3,
Z = 2, Dc = 1.196 Mg mꢀ3, l = 1.54178 A, m(Cu-Ka) =
0.540 mmꢀ1, F(000) = 644, T = 100(2) K, R1 = 0.0676 [for 4832
reflections with I > 2s(I)], wR2 = 0.1850 [for all 5585 reflections]. The
crystal was a partial merohedral twin with two domains. Each domain
was indexed independently using the Apex II software and integrated
simultaneously using SAINT. The structure was solved using
‘detwinned’ data and refined against all reflections. This resulted in a
ratio of 0.66 : 0.34 for the two domains. CCDC 807915.
Fig. 4 A view of the molecular structure of compound 6. Hydrogen
atoms have been omitted for clarity and thermal ellipsoids are set at
the 50% probability level. Selected bond distances (A) and angles (deg)
include: B(1)–B(4) 1.722(5), B(2)–B(3) 1.723(5), B(5)–B(6) 1.718(5),
B(1)–N(1) 1.462(5), B(1)–N(2) 1.440(5), B(2)–N(1) 1.474(5), B(2)–N(3)
1.405(5), B(3)–N(2) 1.475(5), B(2)–N(4) 1.405(5), B(4)–N(5) 1.457(5),
B(4)–N(6) 1.451(4).
1 P. Paetzold, Pure Appl. Chem., 1991, 63, 345.
2 B. Thiele, P. Schreyer, U. Englert, P. Paetzold, R. Boese and
B. Wrackmeyer, Chem. Ber., 1991, 124, 2209.
3 V. Gutmann, A. Meller and R. Schlegel, Monatsh. Chem., 1964, 95,
314; A. Meller and H. Marecek, Monatsh. Chem., 1968, 99, 1666;
J. L. Adcock and J. J. Lagowski, J. Organomet. Chem., 1974, 72,
323; J. L. Adcock, L. A. Melcher and J. J. Lagowski, Inorg. Chem.,
1973, 12, 788.
4 B. Thiele, P. Paetzold and U. Englert, Chem. Ber., 1992, 125, 2681.
5 G. Bramham, J. P. H. Charmant, A. J. R. Cook, N. C. Norman,
C. A. Russell and S. Saithong, Chem. Commun., 2007, 4605.
6 (a) M. J. D. Bosnet and W. E. Piers, Can. J. Chem., 2009, 87, 8;
(b) K. E. Krahulic, G. D. Enright, M. Parvez and R. Roesler,
J. Am. Chem. Soc., 2005, 127, 4142; (c) A. Wakamiya, K. Mori,
T. Araki and S. Yamaguchi, J. Am. Chem. Soc., 2009, 131, 10850;
(d) E. R. Abbey, L. N. Zakharov and S. Y. Liu, J. Am. Chem. Soc.,
2010, 132, 16340.
7 R. A. Baber, J. P. H. Charmant, A. J. R. Cook, N. E. Farthing,
M. F. Haddow, N. C. Norman, A. G. Orpen, C. A. Russell and
J. M. Slattery, Dalton Trans., 2005, 3137.
8 M. A. M. Alibadi, A. S. Batsanov, G. Bramham, J. P. H. Charmant,
M. F. Haddow, L. MacKay, S. M. Mansell, J. E. McGrady,
N. C. Norman, A. Roffey and C. A. Russell, Dalton Trans., 2009,
5348.
of 6 was also determined by X-ray crystallography and is
shown in Fig. 4.zww The two B3N2 rings are non-planar and
are arranged in an approximately staggered conformation
about the central B–B bond, the angle between the two BN2
planes, t, being 84.61. All B–B bonds are similar in length and
characteristic of single bonds [B(1)–B(4) 1.722(5), B(2)–B(3)
1.723(5), B(5)–B(6) 1.718(5)].8,10 The non-planarity of the
five-membered rings arises because the B2(NMe2)2 units are
twisted [t = 30.21], but this is significantly less than the
twisting observed in 3 and 5. Compound 6 also constitutes a
new class of borazine and is related to a fulvalene but with
2 fewer electrons; 8 vs. 10.
In summary, we have shown that BN-rich polycyclic
systems, which may be viewed both as new types of borazines
and as isostructural with polycyclic aromatic hydrocarbons,
may be constructed in comparatively few steps from simple
diborane(4) precursors.
9 (a) H. Noth and W. Meister, Z. Naturforsch., Teil B, 1962, 17, 714;
¨
(b) M. J. G. Lesley, N. C. Norman and C. R. Rice, Inorganic
Syntheses, 2004, 34, 1.
10 W. Clegg, M. R. J. Elsegood, F. J. Lawlor, N. C. Norman,
N. L. Pickett, E. G. Robins, A. J. Scott, P. Nguyen, N. J. Taylor
and T. B. Marder, Inorg. Chem., 1998, 37, 5289.
11 See for example: (a) P. P. Power, Chem. Rev., 1999, 99, 3463;
(b) Y. Wang, B. Quillian, P. Wei, C. S. Wannere, Y. Xie,
R. B. King, H. F. Schaefer, P. v. R. Schleyer and
G. H. Robinson, J. Am. Chem. Soc., 2007, 129, 12412;
We thank the University of Bristol and the EPSRC for
funding, Dr Chris Adams (Bristol) for useful discussions
concerning the electronic structures of these molecules and
Claire McMullin (Bristol) for generating the molecular orbital
pictures associated with this work.
Notes and references
(c) H. Noth, J. Knizek and W. Ponikwar, Eur. J. Inorg. Chem.,
¨
1999, 1931.
z Single crystals of 3–6 were recrystallised from saturated solutions of
an appropriate solvent (see ESIw), mounted in inert oil and transferred
to the cold gas stream of the diffractometer. Structures were solved
using SHELXS and refined using SHELXL.16
y Crystal data for 3ꢁtoluene: C27H40B6N8, M = 541.53, monoclinic,
space group C2/c, a = 27.515(5), b = 12.652(2), c = 8.8516(14) A,
b = 102.852(9)1, U = 3004.2(8) A3, Z = 4, Dc = 1.197 Mg mꢀ3, l =
1.54178 A, m(Cu-Ka) = 0.544 mmꢀ1, F(000) = 1152, T = 100(2) K,
R1 = 0.0502 [for 1616 reflections with I > 2s(I)], wR2 = 0.1351
[for all 2129 reflections], Rint = 0.0672. CCDC 807912.
12 (a) A. L. McCloskey, R. J. Brotherton, J. L. Boone and
H. M. Manasevit, J. Am. Chem. Soc., 1960, 82, 6245;
(b) R. J. Brotherton, in Progress in Boron Chemistry, ed.
H. Steinberg and A. L. McCloskey, Macmillan, New York,
1964ch. 1; (c) H. Noth and W. Meister, Chem. Ber., 1961, 94,
¨
509; (d) T. Ishiyama, M. Murata, T. Akiko and N. Miyaura, Org.
Synth., 2000, 77, 176.
13 N. Iwadate and M. Suginome, J. Am. Chem. Soc., 2010, 132, 2548
and references therein.
z Crystal data for 4ꢁ2DMF: C26H30B2N6O2, M = 480.18, monoclinic,
14 (a) A. D. Becke, J. Chem. Phys., 1993, 98, 5648; (b) P. J. Stevens,
J. F. Devlin, C. F. Chabalowski and M. J. Frisch, J. Phys. Chem.,
1994, 98, 11623; (c) C. Lee, W. Yang and R. G. Parr, Phys. Rev. B,
1988, 37, 785.
space group P21/n, a = 12.8216(7), b = 13.9708(8), c = 14.5010(8) A,
,
b = 104.001(3)1, U = 2520.4(2) A3, Z = 4, Dc = 1.265 Mg mꢀ3
l
= 0.71073 A, m(Mo-Ka) = , F(000) = 1016,
0.082 mmꢀ1
T = 100(2) K, R1 = 0.0569 [for 4239 reflections with I > 2s(I)].
wR2 = 0.2216 [for all 7023 reflections]. Rint = 0.0357. CCDC 807913.
15 Gaussian 03, Revision D.02, Gaussian, Inc., Wallingford CT, 2004.
16 G. M. Sheldrick, Acta Crystallogr., Sect. A, 2008, 64, 112.
c
3750 Chem. Commun., 2011, 47, 3748–3750
This journal is The Royal Society of Chemistry 2011