Lewis acid chelation of [NR]2- by 2,2′ -diborabiphenyl: 9,11-diboratacarbazole heterocycles

Article abstract of DOI:10.1021/om049800z

A new heterocyclic framework, the dianionic 9,11-diboratacarbazole, has been prepared from a bis-(trimethylphosphine)-stabilized 2,2′ -diborabiphenyl derivative, 1, and H₂NR (R = Ph., CH ₂C₆H₄-p-OMe). The heterocyclic framework is formally assembled via Lewis acid chelation of the [NR]^2- dianion by 2,2′ -diborabi-phenyl. These dianions are isoelectronic with the well-studied fluorenyl monoanion.

Full text of DOI:10.1021/om049800z

Organometallics 2004, 23, 3085-3087
3085
Lew is Acid Ch ela tion of [NR]2- by 2,2′-Dibor a bip h en yl:
9,11-Dibor a ta ca r ba zole Heter ocycles
Ioan Ghesner,† Warren E. Piers,*,† Masood Parvez,† and Robert McDonald‡
Department of Chemistry, University of Calgary, 2500 University Drive NW,
Calgary, Alberta, Canada T2N 1N4, and X-ray Structure Laboratory,
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received March 18, 2004
Sch em e 1
Summary: A new heterocyclic framework, the dianionic
9,11-diboratacarbazole, has been prepared from a bis-
(trimethylphosphine)-stabilized 2,2′-diborabiphenyl de-
rivative, 1, and H2NR (R ) Ph, CH2C6H4-p-OMe). The
heterocyclic framework is formally assembled via Lewis
acid chelation of the [NR]2- dianion by 2,2′-diborabi-
phenyl. These dianions are isoelectronic with the well-
studied fluorenyl monoanion.
Transposition of carbon atoms for boron and/or ni-
trogen in aromatic hydrocarbyl π donors has been a
fruitful strategy for the development of new ligands for
transition metals with comparable steric properties but
altered electronic characteristics.1 In this way, modifica-
tions to the ubiquitous C5H5 (Cp) anion have led to
isoelectronic borollide2 (C4H4BR) dianions and mono-
anionic azaborollide3 (C3H3BRNR′) donors with applica-
tions in olefin polymerization catalysis.4 Indenyl,5
arene,6 and fluorenyl7 donors have also been manipu-
lated in this fashion. The fluorenyl ligand framework
is also an important one in olefin polymerization
catalysis,8 but the extended conjugative network also
allows for its use as a sensitive chromophore in fluo-
rescent materials.9 Thus, in addition to comparative
coordination chemistry, the photophysical properties of
heterocyclic analogues of the fluorenyl group are also
of interest.
Recently, we reported a route to two synthons for the
chelating Lewis acid 2,2′-diborabiphenyl, which was
shown to bind bifunctional pyridazine Lewis bases,
giving B2N2 analogues of polycyclic aromatic hydro-
carbons.10 The [NR]2- fragment constitutes the simplest
bifunctional, nitrogen-based donor, and chelation of
[NR]2- by this Lewis acid would lead directly to a
dianionic B2N analogue of the fluorenyl anion.
The 2,2′-diborabiphenyl synthon 1 was used to pre-
pare precursors to the 9,11-diboratacarbazole frame-
work, as shown in Scheme 1. Base-stabilized bora-
benzene derivatives are known to undergo reaction with
protic acids via addition of E-H across the BdC bond
of the borabenzene framework;11 deprotonation of the
resultant cyclic borane provides a facile route to bo-
ratabenzenes. This methodology was applied using
aniline and p-methoxybenzylamine as the protic acids.
Addition of 2 equiv of H2NAr to 1 occurs smoothly at
room temperature to give the colorless bis(amido)-
boranes 2, but when only 1 equiv of aniline or amine is
employed, ring closure via addition of the second aniline
or amine proton to the other half of the diborabiphenyl
molecule can be achieved at higher temperatures and
longer reaction times to give the tricyclic B2N hetero-
cycles 3 in about 60% isolated yield.12 Observable
amounts of compounds 2 are also formed in these
reactions and do not thermally convert to the ring-closed
* To whom correspondence should be addressed. Phone: 403-220-
5746. E-mail: wpiers@ucalgary.ca. S. Robert Blair Professor of Chem-
istry (2000-2005).
† University of Calgary.
‡ University of Alberta.
(1) For recent reviews see: (a) Fu, G. C. Adv. Organomet. Chem.
2001, 47, 101. (b) Herberich, G. E.; Ohst, H. Adv. Organomet. Chem.
1986, 25, 199. (c) Siebert, W. Adv. Organomet. Chem. 1993, 35, 187.
(2) Herberich, G. E.; Hostalek, M.; Laven, R.; Boese, R. Angew.
Chem., Int. Ed. Engl. 1990, 23, 317.
(3) (a) Liu, S.-Y.; Lo, M. M.-C.; Fu, G. C. Angew. Chem., Int. Ed.
2002, 41, 174. (b) Ashe, A. J ., III; Fang, X. Org. Lett. 2000, 2, 2089.
(4) (a) Bazan, G. C.; Donnelly, S. J .; Rodriguez, G. J . Am. Chem.
Soc. 1995, 117, 2671. (b) Pastor, A.; Kiely, A. F.; Henling, L. M.; Day,
M. W.; Bercaw, J . E. J . Organomet. Chem. 1997, 528, 65. (c) Nagy, S.;
Krishnamurti, R.; Etherton, B. P. PCT Int. Appl. W09634,-021; Chem.
Abstr. 1997, 126, 19432j.
(5) Ashe, A. J ., III; Yang, H.; Fang, X.; Kampf, J . W. Organometallics
2002, 21, 4578.
(6) (a) Komon, K. J . A.; Bazan, G. C. In Contemporary Boron
Chemistry; Davidson, M. G., Highes, A. K., Marder, T. B., Wade, K.,
Eds.; The Royal Society of Chemistry: Cambridge, U.K., 2000; p 3. (b)
Ashe, A. J ., III; Al-Ahmed, S.; Fang, X. J . Organomet. Chem. 1999,
581, 92.
(7) Wehmschulte, R. J .; Khan, M. A.; Twamley, B.; Schiemenz, B.
Organometallics 2001, 20, 844.
(8) (a) Alt, H. G.; Ko¨ppl, A. Chem. Rev. 2000, 100, 1205. (b) Ewen,
J . A.; J ones, R. L.; Razavi, A.; Ferrara, J . D. J . Am. Chem. Soc. 1988,
110, 6255. (c) Dietrich, U.; Hackmann, M.; Rieger, B.; Klinga, M.;
Leskela¨, M. J . Am. Chem. Soc. 1999, 121, 4348. (d) Miller, S. A.;
Bercaw, J . E. Organometallics 2002, 21, 934.
(10) Emslie, D. J . H.; Piers, W. E.; Parvez, M. Angew. Chem., Int.
Ed. 2003, 42, 1251.
(9) Tirapattur, S.; Belletete, M.; Drolet, N.; Leclerc, M.; Durocher,
G. Macromolecules 2002, 35, 8889 and references therein.
(11) Rogers, J . S.; Lachicotte, R. J .; Bazan, G. C. J . Am. Chem. Soc.
1999, 121, 1288.
10.1021/om049800z CCC: $27.50 © 2004 American Chemical Society
Publication on Web 05/22/2004

3086 Organometallics, Vol. 23, No. 13, 2004
Communications
Sch em e 2
bonds, while the C-C bonds of the flanking six-
membered rings alternate between short (C(1)-C(2) and
C(3)-C(4)) and long (C(2)-C(3) and C(4)-C(5)), indicat-
ing localization of π bonds in this system.
F igu r e 1. Crystalmaker representation of the structure
of 3a . Isopropyl hydrogen atoms are omitted for clarity.
Selected bond lengths (Å): N(1)-B(1), 1.459(2); C(1)-B(1),
1.545(2); C(1)-C(2), 1.351(2); C(2)-C(3), 1.462(2); C(3)-
C(4), 1.349(2); C(4)-C(5), 1.504(2); C(5)-B(1), 1.569(2),
C(1)-C(1)*, 1.466(3). Selected bond angles (deg): C(9)-
N(1)-B(1), 125.71(8); B(1)-N(1)-B(1)*, 108.57(16); C(1)-
B(1)-C(5), 117.46(12); N(1)-B(1)-C(5), 133.82(13); N(1)-
B(1)-C(1), 108.72(12).
Double deprotonation of the heterocycle 3a with a
lithium amide base leads to the dianionic 9,11-dibo-
ratacarbazole derivative 4a ‚nTHF (Scheme 2).15 Reac-
tion is rapid and quantitative at room temperature and
is signaled in the 11B NMR spectrum by a significant
upfield shift from ∼54 to 31.7 ppm, characteristic of an
amido-substituted boratabenzene moiety. 7Li NMR spec-
troscopy in THF reveals a signal at -6.1 ppm relative
to external LiCl, which is indicative of a contact ion pair
in which the Li is interacting with the π system of an
aromatic ring.16 The resonance remains unchanged at
all accessible temperatures, indicating a symmetrical
structure, but a fast exchange of the lithium ions
between flanking rings cannot be excluded.
products, indicating that the addition of N-H bonds to
the borabenzene unit is irreversible under these condi-
tions. Heterocycles 3 can be separated from compounds
2 via fractional crystallization from isopentane solutions
as light yellow solids. All compounds were characterized
by NMR and electronic spectroscopy and high-resolution
mass spectrometry.13 Since the boron centers are three-
coordinate aminoboranes, they exhibit a diagnostic
resonance at ≈41 ppm (2a ,b) and 54 ppm (3a ,b)
(relative to BF3‚Et2O at 0 ppm). In addition, 3a was
characterized by X-ray crystallography.14
Crystals of 4a ‚nTHF were not of X-ray quality, but
treatment with 2 equiv of TMEDA (N,N,N′,N′-tetram-
ethylethylenediamine) followed by crystallization from
toluene/isopentane afforded good-quality single crystals
of the bis-TMEDA adduct 4a ‚2TMEDA‚PhMe (Figure
A Crystalmaker diagram of 3a is shown in Figure 1,
along with selected metrical data. Despite the presence
of an sp3 carbon center in the six-membered rings, the
atoms of the three cycles are essentially coplanar, with
the N-phenyl group twisted out of this plane by 34.5°.
The B-C bond lengths are in the range found for single
(15) Synthesis of 4a ‚THF: lithium bis(trimethylsilyl)amide (60 mg,
0.36 mmol) in 4 mL of THF was added dropwise to a THF solution (6
mL) of 3a (59 mg, 0.18 mmol) at room temperature. The mixture was
further stirred for 10 min at room temperature, and the volatiles were
removed under vacuum. The product was obtained as a yellow solid
after washing with pentane (60 mg, 98%). Dec pt: 163 °C. For
crystallization purposes N,N,N′,N′-tetramethylethylenediamine was
added to a solution of 4a ‚THF in THF, the mixture was stirred for 30
min at room temperature, and the volatiles were removed under
vacuum. Single crystals were grown by cooling a solution of 4a ‚
2TMEDA in toluene/isopentane at -35 °C. 1H NMR (THF-d8, 400
MHz): δ 1.30 (d, 12H, CH3 iPr, 3J HH ) 6.9 Hz), 1.78 (m, 4H, CH2 THF),
(12) Synthesis of 3a : aniline (49 mg, 0.52 mmol) in 2 mL of toluene
was added at room temperature to a solution of the bis-PMe3 adduct
of 2,2′-diborabiphenyl (205 mg, 0.53 mmol) in 4 mL of toluene. The
solution was stirred for 20 h at 65 °C, and the volatiles were removed
under vacuum. The product was obtained as a light yellow solid after
recrystallization from isopentane at -35 °C (110 mg, 63%). Mp: 155
3
2.89 (sept, 2H, CH iPr, J HH ) 6.9 Hz), 3.62 (m, 4H, CH2 THF), 6.25
3
3
(d, 2H, CH1, J HH ) 10.3 Hz), 6.52 (t, 1H, C6H5 para, J HH ) 7.1 Hz),
7.08 (m, 4H, C6H5 ortho + CH2), 7.63 (s, br, 2H, CH4), 7.76 (d, 2H,
C6H5 meta, 3J HH ) 8.3 Hz). 13C{1H} NMR (THF-d8, 100 MHz): δ 26.54
3
°C. 1H NMR (C6D6, 400 MHz): δ 1.28 (d, 12H, CH3 iPr, J HH ) 6.9
i
3
Hz), 2.12 (d, 4H, BCH1 3J HH ) 4.2 Hz), 2.65 (sept, 2H, CH Pr, J HH
,
2
) 6.7 Hz), 6.02 (t, 2H, CH2, 3J HH ) 4.4 Hz), 7.03 (m, 2H, C6H5 ortho or
meta), 7.09 (m, 1H, C6H5 para), 7.24 (m, 2H, C6H5 ortho or meta), 7.50
(s, CH2 THF), 26.61 (s, CH3 iPr), 36.41 (s, CH Pr), 68.38 (s, CH2 THF),
i
107.79 (s, CH1), 116.38 (s, C6H5 para), 117.78 (s, CH4), 122.50 (s, CH
ortho), 123.29 (s, C ipso), 128.51 (s, C6H5 meta), 128.86 (s, CH2), 138.90
(s, br, C ipso), 155.57 (s, C ipso).11B NMR (THF-d8, 128 MHz): δ 31.71
(br). 7Li NMR (THF-d8, 155 MHz): δ -6.08 (s). ESI-MS: m/z 338 [4a Li2
- H+]-. UV/vis (THF): λmax, nm (ꢀ) 509 br (3259), 343 sh (8763), 278
(d, 2H, CH4, J HH ) 0.9 Hz). 13C{1H} NMR (C6D6, 100 MHz): δ 20.10
4
(s, BC1H2), 22.72 (s, CH3 iPr), 35.41 (s, CH Pr), 124.02 (s, C6H5 ortho
i
or meta), 124.62 (s, C6H5 para), 124.91 (s, CH2), 129.22 (s, C6H5 ortho
or meta), 134.48 (s, CH4), 141.24 (s, br, C ipso), 144.18 (s, C ipso), 145.94
(s, C ipso). 11B NMR (C6D6, 128 MHz): δ 54.15 (br). EI-MS: m/z 327
[M+], 284 [M+ - CH(CH3)2], 242 [M+ - 2 CH(CH3)2], 43 [CH(CH3)2+].
EI-HRMS: m/z 327.232 13 (calcd 327.232 96 amu, C22H27N11B2). UV/
vis (THF): λmax, nm (ꢀ) 364 br (15 414), 324 sh (11 986), 276 sh (13 736),
244 sh (22 224), 221 (25 987). Fluorescence emission (THF): λmax, nm
(λexcitation 242 nm) 333, 485, 664.
sh (11 580), 249 (19 735). Fluorescence emission (THF):
(λexcitation 404 nm) 496.
λmax, nm
(16) (a) Elschenbroich, C.; Salzer, A. Organometallics: A Concise
Approach, 2nd ed.; VCH: Weinheim, Germany, 1992; pp 24-27. (b)
Dixon, J . A.; Gwinner, P. A.; Lini, D. C. J . Am. Chem. Soc. 1965, 87,
1379.
(13) See the Supporting Information.
(17) Crystallographic data for C34H57B2Li2N5‚C7H8: monoclinic,
(14) Crystallographic data for C22H27B2N: monoclinic, space group
C2/c, radiation Mo KR (λ ) 0.710 73 Å), a ) 20.924(7) Å, b ) 11.808(5)
Å, c ) 7.700(3) Å, â ) 99.47(3)°, V ) 1876.5(12) Å3, Z ) 4, absorption
coefficient 0.065 mm-1, F(000) ) 704, crystal size 0.16 × 0.10 × 0.4
mm3, T ) 173(2) K, θ range for data collection 3.4-27.5°, index range
-27 e h e 27, -15 e k e 13, -9 e l e 9, 3725 reflections collected,
2136 independent reflections (R(int) ) 0.031), completeness to θ ) 27.5°
99.3%, absorption correction multiscan method, refinement method
full-matrix least squares on F2, number of data/restraints/parameters
2136/0/117, goodness of fit on F2 1.01, final R indices (I > 2σ(I) R1 )
0.046 and wR2 ) 0.106, final R indices (all data) R1 ) 0.076 and wR2
space group P21/n (an alternate setting of P21/c [No. 14]), radiation
Mo KR (λ ) 0.710 73 Å), a ) 18.9342(14) Å, b ) 11.7733(9) Å, c )
19.3032(15) Å, â ) 101.2639(18)°, V ) 4220.1(6) Å3, Z ) 4, absorption
coefficient 0.060 mm-1, crystal size 0.27 × 0.21 × 0.20 mm3, T ) 193
K, 2θ range for data collection 4.42-51.62°, index range -23 e h e
22, -14 e k e 14, -24 e l e 23, 28 615 data collected, 8627
independent reflections (R(int) ) 0.1179), absorption correction mul-
tiscan (SADABS), refinement method full-matrix least squares on F2
(SHELXL-93), number of data/restraints/parameters 8627 (Fo2 g -3σ-
2
(Fo2))/0/452, goodness of fit on S 0.976 (Fo g -3σ(Fo2)), final R indices
R1 ) 0.0727 (Fo2 g 2σ(Fo2)) and wR2 ) 0.2342 (Fo2 g -3σ(Fo2)), largest
) 0.121, largest difference peak and hole 0.19/-0.17 e Å-3
.
difference peak and hole 0.422/-0.241 e Å-3
.

Communications
Organometallics, Vol. 23, No. 13, 2004 3087
‘02 indicates that the HOMO is largely associated with
the flanking six-membered rings rather than the B2C2N
core, whereas the HOMO-1 has substantial contribu-
tions from all of the atoms in the C10B2N framework.13
The energetic ordering of these two orbitals is reversed
in the isoelectronic fluorenyl monoanion; this is reflected
in the electrostatic potential maps for the two com-
pounds,13 as the two anionic charges in the 9,11-
diboratacarbazole framework avoid each other on the
peripheral rings of the C10B2N tricycle. These observa-
tions suggest that η6 bonding can be expected to
dominate in the coordination chemistry of 4a with
transition metals, as the η5 mode is emphasized in the
coordination chemistry of the fluorenyl anion.9 On the
other hand, the energy surface between η5 and η6
bonding in the fluorenyl anion is somewhat soft,18 and
given the similarly close energies of the highest energy
orbitals in the diboratacarbazole dianion, analogous
flexibility in bonding modes might be expected here. We
are therefore currently exploring the coordination chem-
istry of these novel heterocycles, as well as their
electronic structure through spectroscopic studies.
F igu r e 2. Crystalmaker representation of the structure
of 4a ‚2TMEDA‚PhMe. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å): N(31)-Li(1), 2.105(6);
N(32)-Li(1), 2.144(6); C(11)-Li(1), 2.368(6); C(12)-Li(1),
2.365(6); C(13)-Li(1), 2.353(6); C(14)-Li(1), 2.358(6);
C(15)-Li(1), 2.417(6); N(1)-B(1), 1.474(4); N(1)-B(2),
1.485(4); C(15)-B(1), 1.516(5); C(25)-B(2), 1.514(5); C(11)-
B(1), 1.521(5); C(21)-B(2), 1.518(4); C(11)-C(21), 1.483(4);
N(1)-C(1), 1.402(4). Selected bond angles (deg): C(1)-
N(1)-B(1), 126.4(3); C(1)-N(1)-B(2) 125.9(3); B(1)-N(1)-
B(2), 107.1(2); N(1)-B(1)-C(15), 136.6(3); N(1)-B(1)-
C(11), 109.6(3).
Ack n ow led gm en t. Funding for this work was pro-
vided by the NSERC of Canada through a Discovery
grant to W.E.P. and the Alberta Ingenuity Fund for an
Associateship to I.G.
2).17 The lithiums are associated with the flanking six-
membered rings above and below the 9,11-boratacar-
bazole plane and are asymmetrically bonded to the
various atoms of the ring. In particular, the Li-B
distances of 2.456(7) and 2.497(6) Å are somewhat
longer than the Li-C distances, which range from
2.316(6) to 2.417(6) Å. Distances and angles within the
C10B2N ring are similar to those in 3a , with the
exception of the C-C distances, which are now more
equalized due to the rearomatization of these rings.
Computational exploration of the diboratacarbazole’s
electronic structure at the 3-21G level using Spartan
Su p p or tin g In for m a tion Ava ila ble: Text, tables, and
figures giving experimental details, crystal data, atomic
coordinates, bond lengths and angles, ORTEP diagrams, and
anisotropic displacement parameters for 3a and 4a ‚2TMEDA‚
PhMe and details on the computations; X-ray data for 3a and
4a ‚2TMEDA‚PhMe are also available as CIF files. This mate-
rial is available free of charge via the Internet at http://
pubs.acs.org.
OM049800Z
(18) (a) J ohnson, J . W.; Treichel, P. M. J . Am. Chem. Soc. 1977, 99,
1427. (b) U¨ ffing, C.; Ko¨ppe, R.; Schno¨ckel, H. Organometallics 1998,
17, 3512.