Boron-nitrogen analogues of the fluorenyl anion

Article abstract of DOI:10.1039/c0dt00028k

The synthesis and structural characterisation of [BPh(bipy)] and [BCl(bipy)] are described both of which contain a direduced bipy ligand (bipy = 2,2′-bipyridyl) and are analogues of the fluorenyl anion; DFT calculations highlight differences in the electronic structure of both species compared to the fluorenyl anion. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0dt00028k

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www.rsc.org/dalton | Dalton Transactions
Boron–nitrogen analogues of the fluorenyl anion†‡
Stephen M. Mansell, Nicholas C. Norman* and Christopher A. Russell*
Received 25th February 2010, Accepted 16th April 2010
First published as an Advance Article on the web 29th April 2010
DOI: 10.1039/c0dt00028k
The synthesis and structural characterisation of [BPh(bipy)]
and [BCl(bipy)] are described both of which contain a dire-
duced bipy ligand (bipy = 2,2¢-bipyridyl) and are analogues
of the fluorenyl anion; DFT calculations highlight differences
in the electronic structure of both species compared to the
fluorenyl anion.
Thus, we have recently discovered the synthesis of the persistent
radical species [BCl2(bipy)]∑ (3) from the reaction of B2Cl2(NMe2)2
with bipy,11 but during one particular experiment, we isolated
a small crop of bright orange crystals rather than the intense
purple/black coloured solid characteristic of 3. Analysis of these
orange crystals by X-ray crystallography showed them to be
[BCl(bipy)] (1) (Fig. 1).§ Compound 1 is planar and formally
contains the bipy ligand in its direduced state (vide infra). Inspired
by this observation, we sought a more rational synthesis of 1
from Li2[bipy] and BCl3. However, whilst this reaction did indeed
afford 1, significant quantities of free bipy as well as a number
of other unidentified products were also obtained. In turning our
attention to related species, we found that the phenyl substituted
analogue [BPh(bipy)] (2) could be readily prepared from the
analogous reaction between Li2[bipy] and BPhCl2 in toluene,
followed by filtration and recrystallisation giving a moderate yield
of red crystals. 11B NMR spectroscopy of a C6D6 solution of 2
showed a signal at 20.7 ppm whilst 1H NMR spectroscopy revealed
resonances at significantly lower frequency for the bipy unit (7.64,
7.19, 6.17, 5.92 ppm) compared to those observed in free bipy
(8.75, 8.54, 7.20, 6.68 ppm).
The substitution of pairs of carbon atoms with isoelectronic
B–N units in polycyclic aromatic hydrocarbons leads to a series
of compounds with similar structural features but subtly different
electronic properties.1,2 For example, analogues of pyrene with
internal B–N substitution have recently been synthesised by Piers
and coworkers,3 and the effects of B–N substitution in benzene
analogues have also been investigated both structurally with in-
creasing unsaturation4 and as a result of coordination to Cr(CO)3.5
These advances complement the recent characterisation of boron-
substituted aromatic analogues, such as 9-boraanthracene6 and
also in a dithiophene species containing a B–B unit.7 In com-
parison, BN analogues of aromatic hydrocarbons containing
an excess of nitrogen atoms are comparatively underdeveloped,
but highly fluorescent molecules have been synthesised which
contain N–B–N moieties attached to perylene scaffolds.8,9 In this
context, we have recently described the 1,1- and 1,2-isomers of
bis(diaminobenzene)diborane(4) compounds, which contain 20p
electrons, and our investigations revealed electron partitioning
which avoided the anti-aromatic state.10 We sought to extend the
scope of this family of compounds by probing the interactions
between low-oxidation state boron units and bipyridyl (bipy)
which resulted in the discovery of two novel N–B–N compounds,
namely [BX(bipy)] (1, X = Cl; 2, X = Ph), which are formally
aromatic and analogues of the fluorenyl anion.
Fig. 1 Thermal ellipsoid plot (at 50% probability) of 1. Selected bond
distances and angles are presented in Table 1.
The crystal structure of 2§ (Fig. 2) is similar to that of 1 in most
respects (Table 1) with the boron atom in each case chelated by a
direduced bipy ligand, with the phenyl group tilted with respect to
the B(bipy) plane (dihedral angles of -38.3 and -39.4◦ for each of
the two independent molecules in the asymmetric unit).
University of Bristol, School of Chemistry, Bristol, UK BS8 1TS. E-mail:
Chris.Russell@bris.ac.uk, n.c.norman@bris.ac.uk; Fax: +44 (0)117 929
0509; Tel: +44 (0)117 928 7599
† Dedicated to Professor David W. H. Rankin on the occasion of his
retirement.
A comparison of 1 and 2 reveals a number of interesting
features (Table 1). Within the pyridyl units, the C–C bond
lengths may be grouped into those with more single-bond
‡ Electronic supplementary information (ESI) available: Syntheses and
experimental data for 1 and 2 and the UV-vis spectrum of 2. Packing
diagrams for both structures in the solid state. Computed dimensions
for A and B. CCDC reference numbers 767737 and 767738. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00028k
˚
character [1.437(2) to 1.414(2) A] and those with greater double-
˚
bond character [1.345(2) to 1.359(2) A]. Most strikingly, the
5084 | Dalton Trans., 2010, 39, 5084–5086
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◦
˚
Table 1 A structural comparison of bond lengths (A) and angles ( ) in [BCl(bipy)] (1), [BPh(bipy)] (2) and their calculated structures
1
2a
Calculated 1
Calculated 2
B(1)–Cl(1)
B(1)–N(1), B(1)–N(2)
1.7650(15)
1.4193(19), 1.4202(19)
1.776
1.430, 1.430
1.443(2), 1.442(2)
1.444(2), 1.438(2)
1.3979(18), 1.3933(18)
1.3941(18), 1.3944(19)
1.345(2), 1.348(2)
1.345(2), 1.348(2)
1.431(2), 1.437(2)
1.434(2), 1.433(2)
1.357(2), 1.358(2)
1.355(2), 1.358(2)
1.416(2), 1.414(2)
1.421(2), 1.417(2)
1.4208(17), 1.4185(17)
1.4192(17), 1.4185(17)
1.380(2), 1.379(2)
103.38(12), 103.55(12)
108.19(12), 108.08(12)
108.15(12), 108.08(12)
-38.3, -39.4
1.445, 1.445
1.387, 1.387
1.358, 1.358
1.437, 1.437
1.368, 1.368
1.418, 1.418
1.424, 1.424
N(1)–C(1), N(2)–C(10)
C(1)–C(2), C(10)–C(9)
C(2)–C(3), C(9)–C(8)
C(3)–C(4), C(8)–C(7)
C(4)–C(5), C(7)–C(6)
C(5)–N(1), C(6)–N(2)
1.3927(18), 1.3930(18)
1.348(2), 1.346(2)
1.388, 1.388
1.357, 1.357
1.438, 1.438
1.368, 1.368
1.417, 1.417
1.427, 1.427
1.437(2), 1.4372(19)
1.358(2), 1.359(2)
1.4161(18), 1.4183(19)
1.4273(16), 1.4252(16)
C(5)–C(6)
1.3785(19)
106.10(11)
108.10(11), 108.29(11)
1.389
105.72
108.18, 108.18
1.389
103.67
108.06, 108.06
N(1)–B(1)–N(2)
N(1)–C(5)–C(6),
N(2)–C(6)–C(5)
N(1)–B(1)–C(11)–C(16)
-45.9
a Two independent molecules in the asymmetric unit.
with 116.26◦ in free bipy.12 The B–N bond lengths [1.419(2) to
10
˚
1.444(2) A] are similar to those seen in other B–N compounds.
We note also that in the structure of 1, the [BCl(bipy)] molecules
˚
stack parallel to each other with an inter-bipy distance of 3.43 A,
˚
whilst slightly shorter distances of 3.27 and 3.30 A between parallel
molecules of 2 are observed (see ESI‡).
Compounds containing the bipy ligand in either its mono- or
di-reduced forms are rare compared to the very many examples
of bipy acting as a neutral bidentate ligand. However, a few
examples have been reported such as [Li(thf)4][Al(bipy)2] (4)13
and [Si(bipy)2] (5)14 which have been characterised by X-ray
crystallography and have been assigned as having the aluminium
and silicon centres in oxidation states +3 and +4 respectively. In
addition, predominantly ionic species have also been structurally
characterised including the disodium salt Na2[bipy] (6)15 and the
dirubidium salt Rb2[bipy] (7).16 The structural features in all of
these species mimic those in 1 and 2; in particular the diagnostic
inter-ring distances [i.e., analogous to C(5)–C(6) in 1 and 2] show
significant multiple bond character [varying between 1.35(1) and
14
13
˚
˚
˚
1.34(1) A in 5, 1.36 and 1.37 A in 4, 1.375 and 1.376 A
in 615 and 1.399(6) A in 7; cf. 1.488 A in free bipy ]. We
also note the syntheses of the related boron-containing species
[B(bipy)2]∑ and [B(NMe2)(bipy)] using di-reduced and/or mono-
reduced bipy with suitable boron substrates. Both species are air-
sensitive and interestingly, [B(bipy)2]∑ was shown to be a persistent
radical, although no solid-state structures were obtained for either
species.17
16
12
˚
˚
Fig. 2 Thermal ellipsoid plot (at 50% probability) of one of the two
crystallographically independent molecules of 2. Selected bond distances
and angles are presented in Table 1.
DFT calculations were employed in order to further investigate
the electronic structures of 1 and 2.¶ The calculated HOMO for 2 is
shown in Fig. 3 and the calculated bond lengths and angles for both
1 and 2 (Table 1) coincide well with what is observed in the solid-
state structures. Nucleus-independent chemical shifts (NICS-1)18
which measure the absolute chemical shielding of ghost atoms
C–C bond linking the two pyridyl groups are relatively short
12
˚
˚
[1.379(2) to 1.380(2) A, cf. 1.488 A in free bipy ], clearly
demonstrating multiple-bond character. Whilst it is tempting to
describe 1 and 2 as bipy stabilised B(I) species, this bond length
analysis is indicative of bipy in its direduced form which implies
a boron atom in its usual +3 oxidation state. Another striking
feature is the bending that the bipy group has undergone in order
to chelate to the boron atom as evidenced by the angles N(1)–C(5)–
C(6) and N(2)–C(6)–C(5): 108.08(12) to 108.29(11)◦, compared
˚
placed 1 A above the centres of rings has emerged as a useful means
of estimating aromaticity in molecules.10 Calculated NICS-1 values
for 1, 2, the fluorenyl anion, 2,2¢-bipy and model compounds based
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Notes and references
§ Single crystals of 1 and 2 were obtained from saturated solutions of n-
hexane and toluene respectively stored at -20 ◦C. Data were collected for
crystals mounted in inert oil and transferred to the cold gas stream of the
diffractometer. Structures were solved using SHELXS and refined using
SHELXL.21 Crystal Data for 1: C10H8BClN2, M = 202.44, monoclinic,
˚
space group P21/c, a = 3.84030(10), b = 14.0971(4), c = 16.9200(5) A, b =
◦
3
-3
˚
˚
90.269(2) , U = 915.99(4) A , Z = 4, Dc = 1.468 Mg m , l = 0.71073 A,
m(Mo-Ka) = 0.368 mm-1, F(000) = 416, T = 100(2) K, R1 = 0.0298 [for
1799 reflections with I > 2s(I)], wR2 = 0.0761 [for all 2072 reflections],
CCDC = 767737. Crystal Data for 2: C16H13BN2, M = 244.09, monoclinic,
˚
space group P21/c, a = 20.144(2), b = 17.3230(19), c = 7.1036(7) A, U =
3
-3
˚
˚
2474.0(5) A , Z = 8, Dc = 1.311 Mg m , l = 0.71073 A, m(Mo-Ka) =
0.077 mm-1, F(000) = 1024, T = 100(2) K, R1 = 0.0414 [for 4049 reflections
with I > 2s(I)], wR2 = 0.1055 [for all 5645 reflections], CCDC = 767738.
¶ Calculations were carried out using the Gaussian03 package22 with the
structures of 1 and 2 optimised at the B3LYP/6-31g* level. Single point
calculations using B3LYP/6-311g**+ and the GIAO methodology23 were
Fig. 3 A view of the HOMO calculated for 2.
on the related boracycles, 2,3-dihydro-1H-1,3,2-diazaboroles19
(B), and 1,2-dihydro[1,3,2]diazaborolopyridines20 (A) are shown
in Fig. 4. These results suggest that the boron-containing five-
membered rings in 1 and 2 display a similar and substantial
degree of aromaticity. Interestingly, removing the fused pyridyl
ring(s) to form A and B results in a decrease in aromaticity in the
diazaboracycle. The NICS-1 values of the central ring in 1 and
2 are in fact higher than the corresponding ring of the fluorenyl
anion calculated using the same methodology. However, for 1, 2
and the model 1,2-dihydro[1,3,2]diazaborolopyridine, the values
for the adjacent 6-membered pyridyl ring(s) are much reduced
compared to those in free bipy and the equivalent rings in the
fluorenyl anion, suggesting that the aromaticity in 1 and 2 is largely
localised on the central ring at the expense of the pyridyl rings,
whereas it is delocalised over the entire framework in the fluorenyl
anion.
˚
used to predict the NICS values of ghost atoms 1 A above the centroid of
each ring being studied.
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Fig. 4 NICS-1 values for 1, 2, the fluorenyl anion, 2,2¢-bipy and model
compounds based on related diazaboracycles.
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In summary, two BN-containing analogues of the fluorenyl
anion have been prepared. The pattern of bond lengths in
both species point to bipy in its direduced form and DFT
calculations suggest a shift in electronic structure compared to the
fluorenyl anion with the central BN2-containing five-membered
ring showing appreciable aromatic character and the two pyridyl
rings conversely showing less aromaticity.
5086 | Dalton Trans., 2010, 39, 5084–5086
This journal is
The Royal Society of Chemistry 2010
©