Planar, three-coordinate boron monohydrides ligated by Bis(3-methylindolyl) methanes

Article abstract of DOI:10.1021/om2006694

Reactions of tris(3-methylindolyl)methane, bis(3-methylindolyl)-2- methoxyphenylmethane, and bis(3-methylindolyl)-phenylmethane with THFBH ₃ result in hydrogen elimination and yield the corresponding boron monohydride complexes (3-CH₃C₈H₄NH)HC-(3- CH₃C₈H₄N)₂BH (6), (2-CH ₃OC₆H₄)HC(3-CH₃C₈H ₄N)₂BH (7), and PhHC(3-CH₃C₈H ₄N)₂BH (8), respectively. Compositions and structures were confirmed by NMR (¹H, ¹¹B, ¹³C) and IR spectroscopy, elemental analysis, and X-ray crystallography. The molecular structures of 6-8 each possess a trigonal-planar boron atom which resides in the plane of a bidentate diindolylmethane ligand. This planarity, the lack of Lewis acid-base complexation in the presence of THF, and the short B-N distances suggest significant N→B π donation. Thermolysis of 6 did not produce the three-coordinate, pyramidal borane HC(3-CH₃C₈H ₄N)₃B or a tripodal, four-coordinate acid-base complex, presumably due to the significant B-N π bonding in 6. The planarity of 6-8 and the isoelectronic relationship of BH to C: suggest that diindolylmethanes may be useful platforms for the preparation of a new family of N-heterocyclic carbenes.

Full text of DOI:10.1021/om2006694

Article
pubs.acs.org/Organometallics
Planar, Three-Coordinate Boron Monohydrides Ligated by Bis(3-
methylindolyl)methanes
Bingxu Song, Kristin Kirschbaum, and Mark R. Mason*
Department of Chemistry, School for Green Chemistry and Engineering, University of Toledo, Toledo, Ohio 43606-3390s
S
* Supporting Information
ABSTRACT: Reactions of tris(3-methylindolyl)methane, bis(3-methyl-
indolyl)-2-methoxyphenylmethane, and bis(3-methylindolyl)-
phenylmethane with THF·BH3 result in hydrogen elimination and yield
the corresponding boron monohydride complexes (3-CH3C8H4NH)HC-
(3-CH3C8H4N)2BH (6), (2-CH3OC6H4)HC(3-CH3C8H4N)2BH (7),
and PhHC(3-CH3C8H4N)2BH (8), respectively. Compositions and
structures were confirmed by NMR (1H, 11B, 13C) and IR spectroscopy,
elemental analysis, and X-ray crystallography. The molecular structures of
6−8 each possess a trigonal-planar boron atom which resides in the plane
of a bidentate diindolylmethane ligand. This planarity, the lack of Lewis
acid−base complexation in the presence of THF, and the short B−N
distances suggest significant N→B π donation. Thermolysis of 6 did not
produce the three-coordinate, pyramidal borane HC(3-CH3C8H4N)3B or
a tripodal, four-coordinate acid−base complex, presumably due to the significant B−N π bonding in 6. The planarity of 6−8 and
the isoelectronic relationship of BH to C: suggest that diindolylmethanes may be useful platforms for the preparation of a new
family of N-heterocyclic carbenes.
donating than N-pyrrolidinyl groups.6−8 Similar arguments
have been made previously on the basis of N-pyrrolyl and N-
INTRODUCTION
Lewis acidic, three-coordinate boranes continue to be of
■
indolyl compounds of phosphorus9 and transition metals.10
The extent and significance of N→B π bonding varies in
pyrrolyl and indolyl complexes of boron.5,11−17 On the basis of
NMR and crystallographic studies, Wrackmeyer and co-workers
concluded that there is little, if any, significant N→B π bonding
in three-coordinate tris(N-pyrrolyl)borane, tris[N-(2,5-
dimethyl)pyrrolyl]borane, and bicyclic N-pyrrolylboranes.11−15
In contrast, Wang and co-workers interpret B−N bond
distances in (N-R-indolyl)B(mesityl)2 (R = H, 2-Me, 3-Me,
7-Me, 3-Ph) derivatives as support for the presence of BN
interest for anion binding and sensing,1 as well as for catalysts,
cocatalysts, and activators for generation of cationic metal
complexes.2 The recent discovery of phosphine−borane and
amine−borane frustrated Lewis pair systems3 for activation of
small molecules has further increased the interest in Lewis
acidic boranes. Perfluorarylboranes are particularly common,
with B(C6F5)3 serving as the prototypical borane.4 The
enhanced Lewis acidity of perfluoroarylboranes results from
the electron-withdrawing ability of the fluorinated substituents.
An alternative, but less utilized, approach for generating
acidic boranes is use of electron-withdrawing N-pyrrolyl, N-
indolyl, or N-carbazolyl groups that benefit from delocalization
of the nitrogen lone pair into the aromatic ring system,
resulting in a reduced tendency for N→B π donation. For
example, tris(N-pyrrolyl)borane is a moderate Lewis acid due
to the electron-withdrawing ability of the η1-N-pyrrolyl
substituent,5 and (N-pyrrolyl)B(C6F5)2 effects methide abstrac-
tion from Cp2ZrMe2 to give an active ethylene polymerization
catalyst.6 Lewis acidity is not likely maximized, however, since
N→B π donation is possible when the pyrrolyl substituent lies
in the trigonal plane of boron, as is observed in the crystal
structure of (N-pyrrolyl)B(C6F5)2.6 Comparison of B−N bond
distances in (N-pyrrolyl)B(C6F5)2 (1.4094(9) Å) and (N-
pyrrolidinyl)B(C6F5)2 (1.3746(6) Å) and comparison of 11B
NMR resonances for B(C6F5)3 (60 ppm), (N-pyrrolyl)B-
(C6F5)2 (40.8 ppm), and (N-pyrrolidinyl)B(C6F5)2 (32.5 ppm)
support the conclusion that N-pyrrolyl groups are less π
double bonds.16 Noth and co-workers concluded that pyrrolyl-
̈
and indolyl-substituted tetraaminodiboranes exhibit a moderate
amount of B−N π bonding.17
A plausible approach to increased Lewis acidity in N-
pyrrolylboranes and N-indolylboranes is to preclude N→B π
donation by constraining the heterocyclic ring such that the π
system is oriented perpendicular to the acceptor orbital on
boron. We proposed to achieve this by use of ligand 118 to form
a base-stabilized complex of bicyclic Lewis acid boratris(N-3-
methylindolyl)methane (4). Not only would a perpendicular
orientation of the heterocyclic π system and the boron acceptor
orbital be strictly enforced in free 4, but the boron would also
be pyramidalized and ready to accept a Lewis base. Unlike the
reorganization energy19 required to convert a trigonal-planar
Received: August 5, 2011
Published: December 13, 2011
© 2011 American Chemical Society
191
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Organometallics
Article
borane to a pyramidal borane adduct with concomitant loss of
π stabilization, no additional reorganization energy would be
expended upon base coordination to 4. A similar hypothesis
was put forth and an analogous strategy successfully employed
by Zhu and Chen in the preparation of 5,20 although 5 is
severely distorted from a trigonal-pyramidal geometry.
Considering the strong acidity anticipated for free 4, we set
out to isolate 4 as a base-stabilized complex. An additional
caveat from use of the tris(3-methylindolyl)methane framework
in 4 is the partially protected ″pocket″ in which to bind the
Lewis base.21
substituents at 2.62 and 2.12 ppm, also in a ratio of 1:2. A
singlet methine resonance appears at 6.09 ppm. An N−H
resonance is observed at 7.56 ppm, consistent with the presence
of one uncoordinated indolyl moiety per molecule. Noticeably
1
absent in the H NMR spectrum are resonances for THF,
ruling out a THF adduct. A broad, low-intensity hump, barely
discernible from the baseline, is present at 6.1 ppm. This is
tentatively assigned to a boron coupled (10B, I = 3; 11B, I = 3/2)
resonance for the B−H hydride.29 For comparison, the hydride
resonance in 10, which was recently obtained by Piers and co-
workers30 by hydride transfer from the thermodynamically
unstable BODIPY borane derivative 9 (eq 2), appears at 5.17
As an extension of our research on reactions of aluminum
and gallium alkyls with CO,22 we were interested in exploring
new boranes such as 4 that might be capable of coordinating
and activating CO.23,24 Herein, we report reactions of tris(3-
methylindolyl)methane (1) and bis(3-methylindolyl)-
arylmethanes (2, 3) with THF·BH3 to prepare the planar,
three-coordinate boranes 6−8. Attempts to convert 6 to base-
stabilized adducts of 4 are also presented.
ppm. A stretch at 2588 cm−1 in the IR spectrum of 6 confirms
the presence of the hydride by comparison with that observed
for a three-coordinate tetraaminoperylene borane (2588
cm−1)31 and the dipyrromethanato borane 10 (2610 cm−1).30
The infrared spectrum of 6 also exhibits an N−H stretch for the
unreacted indole moiety at 3277 cm−1. The 11B NMR spectrum
of 6 in CDCl3 exhibits a broad resonance at 25.7 ppm (W1/2
=
1160 Hz) which sharpens at 60 °C (W1/2 = 614 Hz), but
coupling to the hydride remains unresolved. The 11B NMR
chemical shift is the same as that reported for compound 1030
and is similar to the chemical shift reported for B(N-indolyl)3
(28.0 ppm).12
The molecular structure of 6 in the solid state was confirmed
by X-ray crystallography (Figure 1). The structure consists of
two η1-N-indolyl groups bound to a boron atom to form a six-
membered chelate ring. The third indolyl moiety is protonated
at nitrogen and is not coordinated to boron, as indicated by a
large B(1)−N(3) distance of 4.096(5) Å. Instead, the
coordination sphere of boron is completed by a hydride at a
B−H distance of 1.14(3) Å. The boron is approximately
trigonal planar, with angles totaling 359.9°. The two
coordinated indolyl moieties and the boron are coplanar,
consistent with the presence of N→B π bonding. The B−N
distances of 1.420(5) and 1.424(5) Å also substantiate the
presence of significant N→B π bonding, although less so than
in typical boron amido complexes. For comparison, B−Nindolyl
and B−Npyrrolyl distances in the four-coordinate complexes (N-
indolyl)BH2·(imidazole), (N-pyrrolyl)BH2·(imidazole), and
(N-pyrrolyl)BH2·(pyridine) range from 1.503(6) to 1.522(6)
Å in the absence of π bonding.28 In contrast, the B−N distance
in three-coordinate (N-pyrrolidinyl)B(C6F5)2 is 1.366(3) Å,
and the B−Namido distances in tetraaminodiboranes [(N-
pyrrolyl)(Me2N)B]2 and [(N-indolyl)(Me2N)B]2 are in the
range 1.387(3)−1.394(2) Å.17 These shorter distances and
orientation of the amido groups relative to the trigonal plane of
RESULTS AND DISCUSSION
■
Reaction of 1 equiv of pyrrole with a borane adduct is known to
result in hydrogen elimination and formation of a pyrrole boron
dihydride derivative such as (N-pyrrolyl)BH2·THF,25,26 (N-
pyrrolyl)BH2·NEt3,27 (N-pyrrolyl)BH2·(pyridine),25,28 and (N-
pyrrolyl)BH2·(imidazole).28 Koster27 and Wrackmeyer12 re-
̈
ported that 3 equiv of pyrrole reacts with Et3N·BH3 or
THF·BH3 with hydrogen elimination to yield tris(N-pyrrolyl)-
borane. Reaction of tris(3-methylindolyl)methane (1) with
THF·BH3 in refluxing THF similarly results in hydrogen
elimination and formation of colorless crystals of 6 in 23% yield
(eq 1). The 1H NMR spectrum of 6 in CDCl3 exhibits two sets
of indolyl resonances, inconsistent with the C3v symmetry
anticipated for compound 4 or a base-stabilized adduct of 4.
For example, doublets for H7 are observed at 7.88 and 7.53
ppm in a ratio of 1:2, as are singlets for the indole methyl
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Organometallics
Article
shifted from that observed for free ligand 2 (3.75 ppm); thus,
ligation of the methoxy oxygen to boron is not obvious. There
are no resonances for coordinated THF. Similar to the case for
compound 6, compound 7 exhibits a broad, barely discernible
1
resonance at 6.1 ppm in the H NMR spectrum and a B−H
stretch in the IR spectrum at 2611 cm−1, indicative of a hydride.
The 11B NMR spectrum of 7 exhibits a broad room-
temperature resonance at 25.8 ppm (W1/2 = 778 Hz) which
sharpens at 60 °C (W1/2 = 458 Hz), but as for compound 6,
coupling to the hydride remains unresolved. IR and NMR (1H,
11B, 13C) spectra for compound 8 exhibit features similar to
those observed for 6 and 7 and suggest that 8 is a structurally
analogous three-coordinate boron hydride of approximate C2v
symmetry.
The molecular structures of 7·THF and 8 were confirmed by
X-ray crystallography (Figures 2 and 3). Each structure consists
Figure 1. ORTEP diagram for 6. Anisotropic displacement ellipsoids
are drawn at the 50% probability level. Hydrogen atoms, except for
those bound to boron and nitrogen, are omitted for clarity. Selected
distances (Å) and angles (deg): N(1)−B(1) = 1.420(5), N(2)−B(1) =
1.424(5), B(1)−H(1B) = 1.14(3); N(1)−B(1)−N(2) = 115.9(3),
N(1)−B(1)−H(1B) = 122.3(13), N(2)−B(1)−H(1B) = 121.7(14).
boron are consistent with the presence of strong π bonding. B−
N distances for three-coordinate tris(N-pyrrolyl)borane,12
tris[N-(2,5-dimethyl)pyrrolyl]borane,12 and (N-R-indolyl)B-
(mesityl)2 (R = H, 2-Me, 3-Me, 7-Me, 3-Ph)16 are slightly
longer and are in the range 1.438(3)−1.457(3) Å. Despite
Wang’s assertions that the distances in these latter complexes
represent BN double bonds, these distances clearly suggest
weaker π bonding than for the B−Namido complexes, partially
due to the nonzero torsion angles (24−51°) between the
heterocycle and the trigonal plane of boron, and partially due to
the reduced π donation from the pyrrolyl and indolyl groups.
The B−N distances in 6 are similar to that observed for (N-
pyrrolyl)B(C6F5)2 (1.401(3) Å).6 The retention of the planar
structure for 6, even in the presence of the Lewis base THF,
also argues for significant N→B π donation.
Figure 2. ORTEP diagram for 7·THF. Anisotropic displacement
ellipsoids are drawn at the 50% probability level. Hydrogen atoms,
except for that bound to boron, are omitted for clarity. Selected
distances (Å) and angles (deg): N(1)−B(1) = 1.419(2), N(2)−B(1) =
1.419(2), B(1)−H(1B) = 1.05(2); N(1)−B(1)−N(2) = 116.4(2),
N(1)−B(1)−H(1B) = 121.5(9), N(2)−B(1)−H(1B) = 122.1(9).
of two η1-N-indolyl groups bound to a trigonal-planar boron
atom to form a six-membered chelate ring. Each boron atom is
additionally ligated by a hydride. The methoxy group in 7 is
oriented above the molecular plane and toward boron, but it is
not coordinated. The B(1)−O(1) distance of 3.252(2) Å is
more than twice the distance observed for B−O coordinate-
covalent bonds in borane adducts such as Ph2BCl·THF
(1.569(2) Å)32 and tris(3,3-dimethyl-1-butynyl)borane·THF
(1.605(1) Å).33 The disordered THF molecule in 7·THF
occupies a void in the crystalline lattice and is also not
coordinated to boron. The B−N distances of 1.419(2) Å (7)
and 1.426(2) Å (8), as well as the planarity of the
diindolylmethane moiety and boron, indicate significant N→
B π bonding.
The N−B π bonding and resulting stability of the planar
structure observed for 6, as well as for 7 and 8, at least partially
account for the unobserved formation of base-stabilized target
compound 4·THF upon reaction of 1 with THF·BH3 in
refluxing THF. Although it was clear that more forceful
conditions would be required to promote dihydrogen
elimination from 6, refluxing 6 in higher boiling solvents such
as toluene and xylenes led only to complicated mixtures of
Reactions of 2 and 3 with THF·BH3 yielded 7 and 8 as
colorless crystals in yields of 40 and 44%, respectively (eq 3).
1
The H NMR spectrum of 7 is consistent with a structure of
C2v symmetry. Indolyl moieties are chemically equivalent and
exhibit doublets for H7 (7.87 ppm) and H4 (7.44 ppm), as well
as triplets for H6 (7.31 ppm) and H5 (7.24 ppm). Triplets at
7.09 and 6.71 ppm, as well as doublets at 6.95 and 6.86 ppm,
are well-resolved for aryl resonances of the 2-methoxyphenyl
substituent. The methoxy resonance at 3.89 ppm is only slightly
193
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Organometallics
Article
Table 1. Summary of X-ray Crystallographic Data for 6,
7·THF, and 8
6
7·THF
8
formula
fw
C28H24N3B
413.31
monoclinic
P21/c
C30H31N2O2B
462.38
C25H21N2B
360.25
monoclinic
P21/n
cryst syst
space group
a, Å
triclinic
P1
̅
11.709(4)
22.392(9)
8.758(3)
90
8.917(2)
12.367(3)
13.132(3)
117.198(3)
100.154(4)
100.595(4)
1208.7(4)
2
9.6244(13)
9.4559(13)
20.650(3)
90
b, Å
c, Å
α, deg
β, deg
γ, deg
V, Å3
Z
110.970(7)
90
98.746(3)
90
2144.2(3)
4
1857.5(4)
4
D
calcd, g cm−3
1.280
1.270
1.288
T, K
μ(Mo Kα), mm−1
120
120
120
0.075
0.079
0.075
Figure 3. ORTEP diagram for 8. Anisotropic displacement ellipsoids
are drawn at the 50% probability level. Hydrogen atoms, except for
that bound to boron, are omitted for clarity. Selected distances (Å)
and angles (deg): N(1)−B(1) = 1.426(2), N(2)−B(1) = 1.426(2),
B(1)−H(1B) = 1.089(16); N(1)−B(1)−N(2) = 115.64(13), N(1)−
B(1)−H(1B) = 122.8(8), N(2)−B(1)−H(1B) = 121.6 (8).
λ, Å
0.710 73
1.000−0.643
48.0
0.710 73
1.000−0.896
52.0
0.710 73
1.000−0.807
57.5
transmissn coeff
2θ limit, deg
total no. of data
no. of unique data
10 325
3360
11 073
11 882
4374
4701
a
no. of obsd data
2014
3433
3425
unidentified products. Preliminary attempts at alternate routes
to the formation of base-stabilized adducts of 4, such as
deprotonation of 1 with nBuLi followed by treatment with BCl3
or Et2O·BF3 and transamination of B(NEt2)3 with 1, were
also unsuccessful.
no. of params
385
363
337
ab
,
R1
0.0680
0.1502
0.0496
0.0489
0.1124
c
wR2
0.1258
34
max, min peaks, e/Å3 0.216, −0.274
0.190, −0.287
c
0.249, −0.268
2
a
b
I > 2σ(I). R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑[w(Fo − Fc2)2]/
2
∑[w(Fo )2]]1/2
.
CONCLUSIONS
■
Chemical shifts for 1H and 13C are reported relative to
tetramethylsilane. 11B NMR chemical shifts are reported relative to
Et2O·BF3. Tentative 13C NMR assignments are based on previously
reported NMR assignments for bis- and tris(3-methylindolyl)-
methanes18 as well as on NMR assignments of indole derivatives
reported by Park and co-workers.40 In the reporting of NMR data,
indole hydrogen and carbon atoms are referred to by the standard
numbering scheme for indoles. Infrared spectra were obtained on a
Nicolet 5DX FT-IR infrared spectrometer. Elemental analyses were
performed by Schwarzkopf Microanalytical Laboratories, Woodside,
NY.
Reactions of bis- and tris(3-methylindolyl)methanes with
THF·BH3 readily yield planar, three-coordinate boranes with
significant N→B π bonding, as elucidated by X-ray
crystallography. Although conversion of 6 to base-stabilized
derivatives of 4 was not successful, the planarity of 6−8 and the
isoelectronic relationship of BH to C:35d suggest that
diindolylmethanes may be useful platforms for the preparation
of a new family of fused N-heterocyclic carbenes with less
common six-membered rings,35,36 such as 11, for use as ligands
for inorganic and organometallic chemistry. Hydride abstrac-
tion from 6−8 may also provide access to borenium ions, as
demonstrated by Piers for the dipyrromethanato borane 10.30,37
Preparation of (3-CH3C8H4NH)HC(3-CH3C8H4N)2BH (6). A
solution of THF·BH3 (2.4 mL, 2.4 mmol, 1.0 M in THF) was
added via syringe to a stirred solution of 1 (1.00 g, 2.4 mmol) in THF
(20 mL). The resulting colorless solution was stirred at room
temperature for 1 h and then refluxed overnight. The solution was
cooled, concentrated under vacuum, and stored in a freezer at −20 °C
to yield colorless crystals which can be recrystallized from toluene or
THF. The product was isolated by filtration and dried in vacuo. Yield:
0.23 g, 23%. 1H NMR (CDCl3, 400 MHz): δ 7.88 (d, 3JHH = 8 Hz, 2H,
H7), 7.56 (s, 1H, NH), 7.53 (d, 3JHH = 8 Hz, 1H, H7), 7.45 (d, 3JHH
=
3
3
8 Hz, 2H, H4), 7.33 (t, JHH = 8 Hz, 2H, H6), 7.29 (t, JHH = 8 Hz,
2H, H5), 7.07−7.02 (m, 3H, H4, H5, H6), 6.1 (br, BH), 6.09 (s, 1H,
CH), 2.62 (s, 3H, CH3), 2.12 (s, 6H, CH3). 11B NMR (CDCl3, 128.3
MHz): δ 25.7 (br, W1/2 = 1160 Hz at 20 °C; W1/2 = 614 Hz at 60 °C).
13C{1H} NMR (CDCl3, 100.6 MHz): δ 139.30 (s, C7a), 136.09 (s,
C7a free), 133.44 (s, C2 free), 133.24 (s, C2), 132.70 (s, C3a), 128.67
(s, C3a free), 123.49 (C5), 122.92 (s, C6), 122.15 (s, C5 free), 119.36
(s, C6 free), 118.92 (s, C4 free), 118.70 (s, C4), 115.94 (s, C3),
112.05 (s, C7), 110.78 (s, C7 free), 106.70 (s, C3 free), 31.88 (s, CH),
9.54 (s, CH3), 8.57 (s, CH3). IR (KBr, cm−1): 2588 (νB−H), 3277
(νN−H). Anal. Calcd for C28H24N3B: C, 81.36; H, 5.85; N, 10.17.
Found: C, 80.35; H, 6.23; N, 10.02.
EXPERIMENTAL SECTION
■
General Considerations. All reactions were performed under an
inert atmosphere of purified nitrogen using standard inert-atmosphere
techniques. Tris(3-methylindolyl)methane (1),38 bis(3-methylindol-
yl)-2-(methoxyphenyl)methane (2),18 and bis(3-methylindolyl)-
phenylmethane (3)18,39 were prepared as previously described. All
other chemical reagents were purchased from Aldrich Chemical Co.
and used without further purification. Tetrahydrofuran was distilled
from sodium benzophenone ketyl, and toluene was distilled from
sodium prior to use. Hexane was distilled from calcium hydride.
Chloroform-d was dried by storage over activated molecular sieves and
degassed with purified nitrogen. Solution NMR spectra were recorded
on a Varian Unity 400 MHz spectrometer using a deuterated solvent.
Preparation of (2-CH3OC6H4)HC(3-CH3C8H4N)2BH (7). A sol-
ution of THF·BH3 (1.3 mL, 1.3 mmol, 1.0 M in THF) was added via
194
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Organometallics
Article
syringe to a stirred solution of 2 (0.50 g, 1.3 mmol) in THF (20 mL).
located at the University of Toledo was established with grants
from the Ohio Board of Regents and ONR. We thank Chris G.
Gianopoulos and Dr. Yong Wah Kim for acquisition of 11B
NMR data.
Subsequent reaction conditions and workup were as described for the
1
preparation of 6. Yield: 0.20 g, 40%. H NMR (CDCl3, 400 MHz): δ
7.87 (d, 3JHH = 8 Hz, 2H, H7), 7.44 (d, 3JHH = 8 Hz, 2H, H4), 7.31 (t,
3JHH = 8 Hz, 2H, H6), 7.24 (t, 3JHH = 8 Hz, 2H, H5), 7.09 (t, 3JHH = 8
3
3
Hz, 1H, aryl), 6.95 (d, JHH = 8 Hz, 1H, aryl), 6.86 (d, JHH = 8 Hz,
3
REFERENCES
1H, aryl), 6.71 (t, JHH = 8 Hz, 1H, aryl), 6.33 (s, 1H, CH), 6.1 (br,
■
BH), 3.89 (s, 3H, OCH3), 2.10 (s, 6H, CH3). 11B NMR (CDCl3, 128.3
MHz): δ 25.8 (br, W1/2 = 778 Hz at 20 °C; W1/2 = 458 Hz at 60 °C).
13C{1H} NMR (CDCl3, 100.6 MHz): δ 155.29 (s, COCH3), 139.33 (s,
C7a), 135.91 (s, C2), 133.40 (s, C3a), 131.40 (s, Ph), 129.79 (s, Ph),
127.75 (s, Ph), 122.88 (s, C5), 122.43 (s, C6), 121.34 (s, Ph), 118.40
(s, C4), 114.51 (s, C3), 111.84 (s, C7), 110.93 (s, Ph), 55.75 (s,
OCH3), 33.02 (s, CH), 8.40 (s, CH3). IR (KBr, cm−1): 2611 (νB−H).
Anal. Calcd for C26H23N2BO: C, 79.83; H, 6.01; N, 7.08. Found: C,
79.27; H, 6.34; N, 7.08.
(1) See, for example: (a) Wade, C. R.; Broomsgrove, A. E. J.;
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Preparation of PhHC(3-CH3C8H4N)2BH (8). A solution of
THF·BH3 (2.4 mL, 2.4 mmol, 1.0 M in THF) was added via syringe
to a stirred solution of 3 (0.87 g, 2.4 mmol) in THF (20 mL).
Subsequent reaction conditions and workup were as described for the
1
2009, 48, 9839. (e) Momming, C. M.; Otten, E.; Kehr, G.; Frohlich,
̈
̈
preparation of 6. Yield: 0.38 g, 44%. H NMR (CDCl3, 400 MHz): δ
7.88 (d, 3JHH = 8 Hz, 2H, H7), 7.47 (d, 3JHH = 8 Hz, 2H, H4), 7.31 (t,
3JHH = 8 Hz, 2H, H6), 7.26 (t, 3JHH = 8 Hz, 2H, H5), 7.2−7.1 (m, 5H,
aryl), 6.1 (br, BH), 5.75 (s, 1H, CH), 2.20 (s, 6H, CH3). 11B NMR
(CDCl3, 128.3 MHz): δ 25.7 (br, W1/2 = 791 Hz at 20 °C; W1/2 = 476
Hz at 60 °C). 13C{1H} NMR (CDCl3, 100.6 MHz): δ 142.83 (s, Ph-
ipso), 139.45 (s, C7a), 135.24 (s, C2), 133.24 (s, C3a), 128.85 (s, Ph-
ortho), 127.53 (s, Ph-meta), 126.67 (s, Ph-para), 123.26 (s, C5),
122.62 (s, C6), 118.60 (s, C4), 114.89 (s, C7), 112.01 (s, C3), 40.40
(s, CH), 9.04 (s, CH3). IR (KBr, cm−1): 2598 (νB−H). Anal. Calcd for
C25H21N2B: C, 83.34; H, 5.88; N, 7.78. Found: C, 82.80; H, 5.94; N,
7.71.
X-ray Crystallography. Crystals of 6, 7·THF, and 8 were grown
from THF solutions at −20 °C. X-ray diffraction data were collected
on a Siemens three-circle platform diffractometer equipped with a 1K
CCD detector. Data were acquired with SMART 5.05441 using Mo Kα
radiation. Cell constants were determined from the complete data set,
and frames were integrated using SAINT.42 Absorption and decay
correction were performed using SADABS.43 Structures were solved
by direct methods and refined by least-squares methods against F2
using SHELX.43 With the exception of the severely disordered THF
molecules in 7·THF, all non-hydrogen atoms were refined with
anisotropic displacement parameters and all hydrogen atoms were
located and refined with isotropic displacement parameters. For
7·THF, one THF molecule with an occupancy of 0.5 could be located
and refined, but no satisfactory model for the residual electron density
could be determined. Therefore, Platon/SQUEEZE44 was used to
create an hkl file containing solvent-free data, which was used in all
further refinements. The electron density in the solvent void was
approximated as one THF per molecule of 7. The formula unit,
density, etc. reflect this approximation. Details of data collection,
solution, and refinement are given in Table 1.
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ASSOCIATED CONTENT
■
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S
* Supporting Information
CIF files giving crystal data for 6, 7·THF, and 8. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
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ACKNOWLEDGMENTS
■
Acknowledgment is made to the donors of the Petroleum
Research Fund, administered by the American Chemical
Society (Grant 37172-AC3), for partial support of this research.
The CCD facility of the Ohio Crystallography Consortium
donors such as CO. For details, see: Jacobsen, H.; Berke, H.; Doring,
̈
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