Ditopic hydridoborates and hydridoboranes: Bridging ligands in coordination polymers and versatile hydroboration reagents

Article abstract of DOI:10.1039/c0dt01271h

Single crystals of the meta- and para-phenylene-bridged ditopic trihydridoborates (Li(THF)₂)₂[m-C₆H ₄(BH₃)₂] and (Li(THF)₂) ₂[p-C₆H₄(BH₃

Full text of DOI:10.1039/c0dt01271h

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PAPER
Ditopic hydridoborates and hydridoboranes: bridging ligands in coordination
polymers and versatile hydroboration reagents†
Daniel Franz, Michael Bolte, Hans-Wolfram Lerner and Matthias Wagner*
Received 21st September 2010, Accepted 4th November 2010
DOI: 10.1039/c0dt01271h
Single crystals of the meta- and para-phenylene-bridged ditopic trihydridoborates
(Li(THF)2)2[m-C6H4(BH3)2] and (Li(THF)2)2[p-C6H4(BH3)2] have been prepared and investigated by
X-ray crystallography. The compounds turned out to be coordination polymers in which each
trihydridoborate substituent is connected with one trihydridoborate substituent of a neighbouring
+
monomer via two bridging Li(THF)2 ions. (Li(THF)2)2[m-C6H4(BH3)2] and (Li(THF)2)2[p-
C6H4(BH3)2] suffer from poor solubility in all common non-protic solvents. Thus, a more soluble
derivative of (Li(THF)2)2[p-C6H4(BH3)2], equipped with n-hexyl groups at the positions 2 and 5 of the
phenylene ring, has been used for all further investigations (i.e., compound Li2[6]). Treatment of Li2[6]
with Me3SiCl in the presence of excess N(Me)2Et leads to the abstraction of one hydride ion per boron
atom under formation of the ditopic amine-borane adduct p-C6H2(n-hexyl)2(BH2–N(Me)2Et)2 (7). The
compound turned out to be an efficient hydroboration reagent both for internal olefins (i.e.,
1,5-cyclooctadiene) and terminal alkynes (i.e., tert-butyl acetylene) to give p-C6H2(n-hexyl)2(9-BBN)2
(8; 9-BBN = 9-borabicyclo[3.3.1]nonyl) and p-C6H2(n-hexyl)2(B(C(H) C(H)tBu)2)2 (9), respectively.
tion of a number of polystyrene-supported borane complexes
Introduction
that were reported by Ja¨kle et al.14 This lack of oligotopic
For 50 years organoboranes have been used as versatile reagents in
organyl(hydrido)boranes is surprising, because they could be
organic synthesis.1 More recently, they have also found widespread
relevant in various fields of chemistry as illustrated by the following
applications as homogeneous (co)catalysts2–5 and anion sensors.6,7
examples: (i) A borane reagent or catalyst that possesses two
One of the latest advances is the incorporation of boron into
boryl groups for cooperative substrate activation/orientation
can give enhanced reactivity/chemoselectivity.20–24 (ii) When a
monoorganylborane like MesBH2 (Mes = mesityl) is used in
the hydroboration polymerisation of dialkynes,10,11,25 the first
hydroboration event (MesBH2 + HC CR → MesB(H)R¢; R¢:
C(H) C(H)R) occurs under significantly different electronic and
steric conditions than the second addition reaction (MesB(H)R¢ +
HC CR → MesBR¢2). This factor, which has a negative impact
on the polydispersity of the material obtained, can be largely
eliminated by employing an appropriate ditopic hydridoborane
with two spatially separated, but chemically related, B–H func-
tionalities.
Given this background, we have recently begun to systematically
investigate the chemistry of oligotopic hydridoborates and
-boranes and to explore promising applications of this class of
compounds. For example, starting from the ferrocenylene-bridged
hydridoborate Li2[I] (Fig. 1), the corresponding hydridoborane
Fe(C5H4BH2)2 has been liberated by hydride abstraction with
Me3SiCl and trapped as N(Me)2Et- or SMe2-adduct II (Fig. 1).15
In the absence of Lewis basic donors, Fe(C5H4BH2)2 undergoes a
condensation reaction with liberation of B2H6 and formation of
the polymeric borane III (Fig. 1).15
extended p-electron systems, which leads to materials with highly
useful optoelectronic properties.8,9
In many of these cases the hydroboration reaction plays a crucial
role: not only in the course of multistep organic transformations
when the organoborane is only a reaction intermediate,1 but
also as a powerful tool for the preparation of sophisticated
boron-containing frameworks when the organoborane is the
actual target molecule.10–17 Considerable effort has therefore
been devoted to the development of organyl(hydrido)boranes
with optimised reactivity, e.g., improved regioselectivity (cf. 9-
borabicyclo[3.3.1]nonane18) or applicability in asymmetric syn-
thesis (cf. diisopinocampheylborane19). However, compared to
the wealth of information that has been gathered on monotopic
organyl(hydrido)boranes, their oligotopic congeners, i.e., electron-
precise compounds containing more than one R2B–H group,
have so far largely been neglected – with the remarkable excep-
Institut fu¨r Anorganische Chemie, J.W. Goethe-Universita¨t Frankfurt,
Max-von-Laue-Strasse 7, D-60438, Frankfurt (Main), Germany. E-mail:
Matthias.Wagner@chemie.uni-frankfurt.de
† Electronic supplementary information (ESI) available: X-ray crystal
structure analyses of 3, 4, 5, and (Li(THF)2)2[6]. CCDC reference numbers
793257–793260, 793266–793269. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c0dt01271h
Contrary to Fe(C5H4BH2)2, the ditopic borane 9,10-dihydro-
9,10-diboraanthracene is stable not only as the base adduct,
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Fig. 1 The ditopic trihydridoborate Li2[I], the ditopic dihydridoborane
II, and the oligotopic boranes III and IV.
but also in pure form and exists as a unique B–H–B-bridged
coordination polymer in the solid state (cf. IV, Fig. 1).16,17 Polymer
IV has already been successfully employed for the preparation
of boron-doped conjugated p-electron systems via hydroboration
reactions.16,17
Compound IV can be viewed as an ortho-phenylene-bridged
ditopic hydridoborane. The purpose of this paper is to report on
related meta- and para-phenylene-bridged species; as in the case
of the ferrocenylene derivatives Li2[I] and II, the corresponding
hydridoborates will be included into the discussion.
Scheme 1 Synthesis of the meta- and para-phenylene-bridged ditopic
trihydridoborates Li2[m-2] and Li2[p-2]. (i) Toluene, reflux, 1 h. (ii) THF,
rt. (iii) Et2O, 0 ◦C → rt. (iv) THF–Et2O, rt, 12 h.
˚
ions and the boron atoms amount to B(1) ◊ ◊ ◊ Li(1) = 2.469(4) A
28
˚
and B(1) ◊ ◊ ◊ Li(1A) = 2.537(4) A. Using Edelstein’s correlation
of metal-boron distances as a measure of the denticity of a
˚
˚
trihydridoborate group, values of 1.6 0.1 A and 1.36 0.06 A
are estimated for the ionic radii of bidentate and tridentate
Results and discussion
trihydridoborate ligands, respectively. Thus, B ◊ ◊ ◊ Li distances of
about 2.50 A and 2.26 A can be expected for RBH3-h -Li and
2
˚
˚
Meta- and para-phenylene-bridged ditopic hydridoborates
At the first stage, we aimed for the synthesis of the parent systems
Li2[m-2] and Li2[p-2] (Scheme 1).
Table 1 Crystallographic data for (Li(THF)2)2[m-2] and (Li(THF)2)2[p-2]
In order to prepare Li2[m-2], m-bis(dibromoboryl)benzene
(1)26 was first converted via ether cleavage into m-
bis(dialkoxyboryl)benzenes, which could subsequently be trans-
formed into the hydridoborate by means of Li[AlH4]. For the
synthesis of Li2[p-2] we found it convenient to start from
literature-known p-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)benzene27 (3; cf. the ESI† for an X-ray crystal structure
determination of 3); Li[AlH4] was again employed as the hydride
transfer reagent. Even though we were fortunate to obtain a
few single crystals of Li2[m-2] and Li2[p-2] suitable for X-ray
crystallography, it soon became evident that facile access to more
soluble derivatives of Li2[m-2] and Li2[p-2] is mandatory for further
progress (see below).
Li2[m-2] crystallises from THF–Et2O with 4 equiv. of THF
((Li(THF)2)2[m-2]; Fig. 2 top and Table 1). The crystal lattice
of (Li(THF)2)2[m-2] consists of zigzag coordination polymer
strands that are held together by interactions between two Li+
ions and two negatively charged trihydridoborate moieties. A
C2-axis runs through the carbon atoms C(1) and C(4) and an
inversion centre is located at the midpoint of each B2Li2 ring. The
meta-phenylene rings within each polymer chain as well as the
phenylene bridges of different chains adopt crystallographically
imposed coplanar conformations. The distances between the Li+
(Li(THF)2)2[m-2]
(Li(THF)2)2[p-2]
Formula
fw
Colour, shape
T/K
Cryst. syst.
C11H21BLiO2
203.03
colourless, block
173(2)
Monoclinic
C2/c
14.456(2)
11.0059(11)
16.369(2)
90
106.218(11)
90
2500.7(5)
8
1.079
888
0.068
0.52 ¥ 0.45 ¥ 0.38
5116
2300 (0.0209)
2300/0/158
1.052
0.0642, 0.1711
0.0759, 0.1808
0.539 and -0.303
C11H21BLiO2
203.03
colourless, block
173(2)
Triclinic
¯
Space group
P1
˚
a/A
8.8448(8)
9.0494(8)
9.1349(9)
73.851(7)
68.733(7)
66.814(7)
618.47(10)
1
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
Dc/g cm-3
1.090
222
0.069
0.37 ¥ 0.33 ¥ 0.32
14793
2176 (0.0470)
2176/0/147
1.075
0.0679, 0.1865
0.0755, 0.1927
0.687 and -0.404
F(000)
m/mm-1
Cryst. size/mm
Reflections collected
Indep. reflns (Rint
)
Data/restraints/params
GOOF on F2
R1, wR2 (I > 2s(I))
R1, wR2 (all data)
Largest diff peak and
-3
˚
hole/e A
2434 | Dalton Trans., 2011, 40, 2433–2440
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Fig. 2 Molecular structures of (Li(THF)2)2[m-2] (top) and (Li(THF)2)2[p-2] (bottom) in the solid state; hydrogen atoms attached to carbon have been
˚
˚
omitted for clarity. Selected bond lengths (A), atom ◊ ◊ ◊ atom distances (A), and angles (deg): (Li(THF)2)2[m-2]: B(1)–C(2) 1.616(3), B(1) ◊ ◊ ◊ Li(1) 2.469(4),
B(1) ◊ ◊ ◊ Li(1A) 2.537(4), Li(1) ◊ ◊ ◊ Li(1A) 3.138(7); B(1) ◊ ◊ ◊ Li(1) ◊ ◊ ◊ B(1A) 102.4(1), Li(1) ◊ ◊ ◊ B(1) ◊ ◊ ◊ Li(1A) 77.6(1). Symmetry transformation used to
generate equivalent atoms: A: -x+1, -y+1, -z+1. (Li(THF)2)2[p-2]: B(1)-C(1) 1.622(3), B(1) ◊ ◊ ◊ Li(1) 2.504(4), B(1) ◊ ◊ ◊ Li(1A) 2.522(4), Li(1) ◊ ◊ ◊ Li(1A)
3.182(7); B(1) ◊ ◊ ◊ Li(1) ◊ ◊ ◊ B(1A) 101.5(1), Li(1) ◊ ◊ ◊ B(1) ◊ ◊ ◊ Li(1A) 78.6(1). Symmetry transformation used to generate equivalent atoms: A: -x+1, -y+1,
-z+1.
3
RBH3-h -Li coordination modes, respectively (ionic radii of Li+ =
discrete dimeric entities.15 The anionic constituents of these dimers
adopt syn-conformations as indicated in Fig. 1 and are bridged by
four Li+ counterions via B–H–Li interactions.
29
˚
0.90 A (c. n. 6), 0.73 (c. n. 4) ). This leads to the conclusion
that each Li+ ion in (Li(THF)2)2[m-2] is h -coordinated to one
2
RBH3- ligand, whereas the binding mode to the other RBH3- unit
2
1
is intermediate between h and h . The coordination sphere of
Solubilised para-phenylene-bridged ditopic hydridoborates and
hydridoboranes
each Li+ ion is completed by two THF ligands.
The solid-state structure of (Li(THF)2)2[p-2] (Fig. 2 bottom
and Table 1) closely resembles that of (Li(THF)2)2[m-2], apart
from the fact that, because of the para-phenylene linker, the
polymer backbone is now straighter. Each hydridoborate unit
in (Li(THF)2)2[p-2] acts as an h -ligand towards two Li+ ions
(B(1) ◊ ◊ ◊ Li(1) 2.504(4) A, B(1) ◊ ◊ ◊ Li(1A) 2.522(4) A).
The polymeric nature of (Li(THF)2)2[m-2] and (Li(THF)2)2[p-
2] forms a striking contrast to the solid-state structure of the
ferrocenylene-bridged hydridoborate Li2[I] (Fig. 1). Li2[I] has been
crystallised as Et2O adduct and its crystal lattice consists of
With regard to the preparation of soluble 2-type molecules,
we focused on the para-derivatives, because the doubly n-hexyl-
substituted congener 5 (Scheme 2) of our previously employed
starting material 3 (Scheme 1) is literature-known.30 Compound
5 had been synthesised by esterification of the correspond-
ing boronic acid with pinacol; the boronic acid, in turn, is
accessible via lithium-halogen exchange between 1,4-dibromo-
2,5-di-n-hexylbenzene (4)31,32 and n-BuLi, followed by addition
of trimethyl borate and acidic aqueous workup.33 However,
2
˚
˚
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Table 2 Crystallographic data for 7 and 8
7
8
Formula
fw
Colour, shape
T/K
C26H54B2N2
416.33
colourless, plate
173(2)
C34H56B2
486.41
colourless, plate
173(2)
Cryst. syst.
Space group
Monoclinic
P21/c
10.3561(10)
10.8699(6)
12.2614(10)
90
94.386(7)
90
1376.22(19)
2
1.005
468
0.056
0.32 ¥ 0.28 ¥ 0.13
13090
Monoclinic
P21/c
12.5723(11)
9.5902(6)
13.5233(11)
90
108.187(6)
90
1549.1(2)
2
1.043
540
0.057
0.36 ¥ 0.33 ¥ 0.23
10470
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
Dc/g cm-3
F(000)
m/mm-1
Cryst. size/mm
Reflections collected
Indep. reflns (Rint
)
2800 (0.0479)
2800/6/163
1.029
2727 (0.0647)
2727/8/162
1.070
Data/restraints/params
GOOF on F2
Scheme 2 Synthesis of the solubilised ditopic trihydridoborate Li2[6]
and the corresponding amine-borane adduct 7 (Bpin: 4,4,5,5-tetra-
methyl-1,3,2-dioxaborolyl; [Pd]: Pd2dba3·CHCl3/P(Me)tBu2). (i) Toluene,
90 ◦C, 4 d. (ii) Et2O, 0 ◦C → rt, 2 h. (iii) THF, -196 ◦C → rt, 12 h.
R1, wR2 (I > 2s(I))
R1, wR2 (all data)
Largest diff peak and
0.0495, 0.1231
0.0653, 0.1302
0.264 and -0.234
0.0579, 0.1540
0.0795, 0.1646
0.259 and -0.271
-3
˚
hole/e A
1
we find it more convenient to synthesise 5 in one step via
a transition metal-mediated borylation reaction starting from
4 (Scheme 2), 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin),
NEt3 and Pd2dba3·CHCl3/P(Me)tBu2 as the catalyst (dba: diben-
zylideneacetone; yield of 5: 67%; cf. the ESI† for X-ray crystal
structure determinations of 4 and 5).34
Compound 5 reacts cleanly with 2.5 equiv. of Li[AlH4] to give
the lithium trihydridoborate Li2[6] (Scheme 2) as evidenced by a
quartet at -26.2 ppm in the 11B NMR spectrum of the reaction
product (1JBH = 75 Hz). The corresponding hydrogen substituents
give rise to a 1 : 1 : 1 : 1 quartet at 1.02 ppm in the 1H NMR
such that JBH coupling could not be resolved. The result of an
X-ray crystal structure analysis of 7 is shown in Fig. 3 (cf. Table 2
for key crystallographic data).
In the solid state, the molecule possesses a centre of inversion
in the middle of the benzene ring. Both the N(1)–B(1) vector
and the C(5)–C(4) vector are almost orthogonal to the phenylene
plane (torsion angles: C(2)–C(1)–B(1)–N(1) = 84.4(1)◦, C(2)–
C(3)–C(4)–C(5) = -91.5(1)◦). The B(1)–N(1) bond length of
˚
1.651(2) A possesses a similar value to that of the B–N bond in the
related monotopic borane adduct (C5H5)Fe(C5H4BH2–N(Me)2Et)
12
˚
(1.655(7) A).
spectrum. In the IR spectrum, characteristic B–H stretches are
-1
observed at n= 2252, 2190 cm (cf. Li[PhBH3]: d(11B) = -26.4
˜
Hydroboration of 1,5-cyclooctadiene and tert-butyl acetylene
(1JBH = 76 Hz); n(B–H) = 2241, 2189 cm ).35 X-ray quality crystals
-1
˜
with 7
of (Li(THF)2)2[6] were grown from Et2O–THF. Apart from the n-
hexyl tethers, the polymeric solid-state structure of (Li(THF)2)2[6]
is virtually identical to that of (Li(THF)2)2[p-2] (cf. the ESI†
for detailed information). The only major difference lies in the
fact that not all repeat units are symmetry related, but that two
crystallographically independent molecules, (Li(THF)2)2[6]A and
(Li(THF)2)2[6]B, alternate along the polymer strand. As a result,
adjacent para-phenylene rings include dihedral angles of 7.0◦,
whereas they are coplanar in (Li(THF)2)2[p-2]. Contrary to 4, the
n-hexyl sidechains do not adopt an all-s-trans conformation but
are coiled in (Li(THF)2)2[6].
Treatment of Li2[6] with excess Me3SiCl/N(Me)2Et in THF
resulted in the replacement of one hydride ion per trihydridoborate
moiety by a neutral N(Me)2Et donor. The resulting ditopic amine-
borane adduct 7 (Scheme 2) is characterised by an 11B NMR signal
at -3.2 ppm, which, compared to Li2[6], is deshielded by more
than 20 ppm (cf. (C5H5)Fe(C5H4BH2–N(Me)2Et): d(11B) = -3.4).
In line with the lower local symmetry about the boron atom in 7
as opposed to Li2[6], the 11B resonance is significantly broadened
The reactivity of 7 was tested both with an internal alkene (1,5-
cyclooctadiene) and with a terminal alkyne (tert-butyl acetylene).
The 9-borabicyclo[3.3.1]nonyl (9-BBN) derivative 8 (Scheme 3)
formed in almost quantitative yield after a solution of 1,5-
cyclooctadiene and 7 (2 : 1) in benzene had been heated to 60 ◦C
for 18 h (NMR spectroscopic control). Repeated recrystallisation
of the crude product from hexane gave an analytically pure sample.
The 11B NMR shift of 8 (86 ppm) is characteristic of three-
coordinate triorganylboranes.36 A comparison of the alkyl region
of the 1H and 13C NMR spectra of 8 with the corresponding data
of other 9-BBN derivatives36 indicates the successful generation
of the 9-BBN cage. Moreover, the proton integral ratios of 8
unequivocally prove the presence of two 9-borabicyclo[3.3.1]nonyl
substituents per para-phenylene bridge. The following results of
this hydroboration experiment are noteworthy: (i) compound 7
can be used as hydroboration reagent even though the boron
atom bears a rather strongly Lewis basic N(Me)2Et donor, (ii)
the reaction product 8 is a free borane (already prior to workup)
2436 | Dalton Trans., 2011, 40, 2433–2440
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Scheme 3 Synthesis of the hydroboration products 8 and 9. (i) Benzene,
Fig. 3 Molecular structure of 7 in the solid state; hydrogen atoms attached
to carbons have been omitted for clarity. Displacement ellipsoids are
60 ◦C, 18 h.
˚
drawn at the 50% probability level. Selected bond lengths (A), angles
(deg), and torsion angles (deg): B(1)–N(1) 1.651(2), B(1)–C(1) 1.624(2);
N(1)–B(1)–C(1) 112.0(1), C(3)–C(4)–C(5) 114.2(1); C(2)–C(1)–B(1)–N(1)
84.4(1), C(1)–B(1)–N(1)–C(12) –57.6(1), C(2)–C(3)–C(4)–C(5) –91.5(1).
and not an amine adduct, and (iii) the assembly of the bulky
9-BBN framework is not sterically prevented by the ortho-n-
hexyl group. To gain further insight into the degree of steric
crowding in 8, we have characterised the compound by X-ray
crystallography (Fig. 4; Table 2). In the centrosymmetric molecule,
the 9-BBN cage is rotated away from the n-hexyl substituent such
as to minimise unfavourable intramolecular interactions (C(2)–
C(1)–B(1)–C(11) = -128.2(2)◦; the shortest contact is established
between one of the 9-BBN bridgehead hydrogen atoms and one
Fig. 4 Molecular structure of 8 in the solid state; hydrogen atoms have
been omitted for clarity. Displacement ellipsoids are drawn at the 50%
˚
a-CH2 hydrogen atom and amounts to 2.156 A). The sum of the
bond angles about the boron atoms of 8 is 359.5◦, thereby giving
no indication for a sterically induced deviation from the ideal
trigonal-planar configuration. All in all, we come to the conclusion
that the solubilising n-hexyl sidechains are compatible even with
large boryl substituents and therefore do not affect the suitability
of 7 for extensive further derivatisation by hydroboration.
˚
probability level. Selected bond lengths (A), angles (deg), and torsion
angles (deg): B(1)–C(1) 1.569(3), B(1)–C(11) 1.573(3), B(1)–C(15) 1.568(3);
C(1)–B(1)–C(11) 126.6(2), C(1)–B(1)–C(15) 122.2(2), C(11)–B(1)–C(15)
110.7(2), C(3)–C(4)–C(5) 112.4(2); C(2)–C(1)–B(1)–C(11) –128.2(2),
C(2)–C(3)–C(4)–C(5) 108.4(2).
3
The reaction between 7 and tert-butyl acetylene also proceeded
cleanly and gave 9 under similar reaction conditions as had been
applied for the synthesis of 8 (Scheme 3). This time, an excess
of the C–C-unsaturated component was employed (6.5 equiv.),
because tert-butyl acetylene is highly volatile and unreacted
starting material can easily be removed in vacuo. The successful
hydroboration reaction is indicated in the proton NMR spectrum
by two doublets at d(1H) = 6.96 and 6.76 with a JHH coupling
constant of 17.7 Hz, which fits to an E-olefin; the proton integral
ratios point towards the introduction of four tert-butylvinyl
substituents. The chemical shift value of the only 11B resonance
(65 ppm) testifies to the presence of the free borane rather than
the N(Me)2Et adduct (cf. (CH2 CH)3B: d(11B) = 56.4;37 Ph3B:
d(11B) = 60.238).
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warm to rt and the solid was isolated by filtration and washed
with Et2O (20 mL). Li2[m-2] was extracted into warm THF
(17 mL). X-ray quality crystals of (Li(THF)2)2[m-2] were grown
by gas-phase diffusion of Et2O into the extract over a period
of 14 d.
Conclusions
This paper reports on rare examples of ditopic trihydrido-
borates and dihydridoboranes, i.e., (Li(THF)2)2[m-C6H4(BH3)2],
(Li(THF)2)2[p-C6H4(BH3)2], (Li(THF)2)2[p-C6H2(n-hexyl)2(BH3)2]
and p-C6H2(n-hexyl)2(BH2–N(Me)2Et)2. The hydridoborate salts
not only serve as precursors for the corresponding boranes,
but are also potentially useful ligands for the generation of
organic/inorganic hybrid compounds. Especially the solubilised
dianion [p-C6H2(n-hexyl)2(BH3)2]2- holds great promise in this
respect, because it can act as a bridging ligand in coordination
polymers (cf. the polymeric solid-state structure of (Li(THF)2)2[p-
C6H2(n-hexyl)2(BH3)2]).
As a proof of principle, we have shown that the amine-borane
adduct p-C6H2(n-hexyl)2(BH2–N(Me)2Et)2 is an efficient hydro-
boration reagent both for internal olefins (i.e., 1,5-cyclooctadiene)
and terminal alkynes (i.e., tert-butyl acetylene). The n-hexyl
sidechains not only have a solubilising effect, but at the same
time serve the purpose of increasing the regioselectivity of the
hydroboration reaction.
Synthesis of Li2[p-2]
In a Schlenk tube, a solution of p-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)benzene (0.079 g, 0.24 mmol) in THF (3 mL)
was carefully layered first with THF (8 mL) and then with a
solution (0.3 M) of Li[AlH4] in Et2O (4.2 mL, 1.3 mmol). After
1 h, a colourless solid began to precipitate; after the vessel had
been left standing at rt for 12 h, the supernatant was separated
from the solid phase via syringe. The solid was washed with Et2O
(10 mL) and dried in vacuo. After the addition of neat N,N,N¢,N¢-
tetramethylethylenediamine (6 mL; dried over solid n-BuLi)
crystalline material was identified among the microcrystalline
precipitate and single crystals for X-ray diffraction analysis could
be selected manually ((Li(THF)2)2[p-2]).
We are currently exploring the potential of ditopic hydro-
boration reagents like p-C6H2(n-hexyl)2(BH2–N(Me)2Et)2 for the
preparation of novel Lewis acidic organocatalysts, and for the
synthesis of boron-doped extended p-systems via hydroboration
polymerisation protocols.
Synthesis of 5
A tube-shaped glass vessel equipped with a Teflon Young’s tap
and a magnetic stirrer bar was charged in a glove box with
Pd2dba3·CHCl3 (0.304 g, 0.29 mmol). The solid was dissolved in
toluene (20 mL) and a solution of P(Me)tBu2 (0.195 g, 1.22 mmol)
in toluene (20 mL) was added with stirring via syringe at 0 ◦C. The
mixture was allowed to warm to rt and stirring was continued for
2 h. The resulting pale orange solution was frozen at -196 ◦C. NEt3
(18.5 mL, 13.4 g, 132 mmol) was vacuum-transferred from Na/K
alloy into the reaction flask. The mixture was again warmed to rt
and freshly sublimed 1,4-dibromo-2,5-di-n-hexylbenzene (12.25 g,
30.30 mmol), toluene (12 mL), and HBpin (12.0 mL, 10.6 g,
82.7 mmol) were added in this order. The mixture was cooled
to -78 ◦C, the glass vessel was depressurised to approximately
250 torr, sealed by closing the Young’s tap, and heated in an
oil bath at 90 ◦C for 4 d. The glass tube was cooled to rt
and a saturated aqueous NH4Cl solution (100 mL) was added
carefully with stirring (Caution: H2 gas is evolved). The organic
phase was separated and the aqueous phase extracted with Et2O
(2 ¥ 100 mL). The combined organic phases were dried over
Na2SO4. After filtration, the filtrate was evaporated to dryness
under reduced pressure and the pale brown residue recrystallised
from hot MeOH. The obtained off-white solid was dissolved in
hexane and separated from small amounts of an insoluble black
material by filtration. Removal of all volatiles from the filtrate
in vacuo gave analytically pure 5. Yield: 10.10 g, 67% (Found:
C, 72.52; H, 10.63. Calc. for C30H52B2O4 [498.34]: C, 72.31; H,
10.52). Single crystals suitable for X-ray diffraction analysis were
obtained by slow evaporation of an Et2O solution of 5.
Experimental
General Considerations
All reactions were carried out under an N2 atmosphere in carefully
dried solvents using Schlenk tube techniques or a glove box.
NMR: Bruker Avance 300 and Avance 400. Chemical shift values
1
(1H, 13C{ H}) are reported in parts per million relative to SiMe4
1
and were referenced to residual solvent signals. 11B{ H} NMR
spectra are referenced to external BF3·Et2O. J values are given
in Hz. Abbreviations: s = singlet, d = doublet, t = triplet, q =
quartet, m = multiplet, br = broad, n.r. = not resolved, n.o. =
signal not observed. Elemental analyses were performed by the
Microanalytical Laboratory of the University of Frankfurt or the
Mikroanalytisches Labor Pascher, Remagen, Germany. Reagents
purchased from commercial sources were used as received unless
otherwise noted. m-Bis(dibromoboryl)benzene,26 p-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)benzene,27 and 1,4-dibromo-
2,5-di-n-hexylbenzene31,32 were prepared according to previously
reported procedures.
Synthesis of Li2[m-2]
m-Bis(dibromoboryl)benzene 1 (6.47 g, 15.50 mmol) was dissolved
in toluene (30 mL) and Et2O (10 ml), and the mixture was heated
at reflux temperature for 1 h. All volatiles were driven off in vacuo,
the residue was redissolved in THF (30 mL)/tBuOMe (30 mL)
1H NMR (300.0 MHz, CDCl3): d = 7.52 (s, 2 H, ArH), 2.81
(m, 4 H, a-CH2), 1.56–1.46 (m, 4 H, b-CH2), 1.34 (s, 24 H, Bpin-
CH3), 1.34–1.25 (m, 12 H, g-, d-, e-CH2), 0.88 (m, 6 H, CH3);
1
and the solution was stirred at rt until the 11B{ H} NMR spec-
trum indicated full conversion to m-bis(dialkoxyboryl)benzene.
After drying in vacuo, a fraction of the product (0.67 g) was
treated with Et2O (4 mL) and the resulting mixture was cooled
to 0 ◦C. A solution (1 M) of Li[AlH4] in Et2O (10.0 mL,
10.0 mmol) was added slowly via syringe under vigorous stirring,
whereupon a colourless solid formed. The mixture was allowed to
1
13C{ H} NMR (75.5 MHz, CDCl3): d = 146.1 (ArC-2,5), 136.5
(ArC-3,6), 83.3 (Bpin-CCH3), 35.5 (a-CH2), 33.8 (CH2), 31.8
(CH2), 29.6 (CH2), 24.8 (Bpin-CH3), 22.7 (e-CH2), 14.2 (CH3),
n.o. (ArC-1,4); 11B{ H} NMR (96.3 MHz, CDCl3): d = 31.9 (h1/2
=
1
550 Hz).
2438 | Dalton Trans., 2011, 40, 2433–2440
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Synthesis of Li2[6]
cyclooctadiene (0.24 g, 2.22 mmol). Benzene (10 mL) was added,
the mixture was frozen at -196 ◦C, the vessel was evacuated, the tap
was closed, and the mixture was heated with stirring in an oil bath
at 60 ◦C for 18 h. After cooling to rt, the vessel was flushed with
N2 and stored for 2 d. The benzene was removed by freeze-drying
under dynamic vacuum. The solid residue was recrystallised twice
from hexane (5 mL/4 mL) at -30 ◦C/0 ◦C in order to obtain an
analytically pure sample. Yield: 0.23 g, 45% (Found: C, 83.89; H,
11.63. Calc. for C34H56B2 [486.41]: C, 83.96; H, 11.60).
A solution of 5 (2.78 g, 5.58 mmol) in Et2O (40 mL) was cooled
to 0 ◦C. A solution (1 M) of Li[AlH4] (14.0 mL, 14.0 mmol) in
Et2O was added dropwise under vigorous stirring over a period
of 40 min, whereupon a colou◦rless precipitate formed. Stirring
was continued for 45 min at 0 C and for another 45 min at rt.
The insolubles were collected on a frit and washed with Et2O
(20 mL). The solid was then transferred to a flask, treated with
THF (35 mL), and the resulting suspension was stirred for 12 h
at rt. After filtration, Li2[6] was precipitated from the filtrate by
dropwise addition of Et2O (40 mL) over a period of 1 h. The
precipitate was allowed to settle, the supernatant was removed
via cannula, and Li2[6] was dried in vacuo. Yield: 1.57 g, 65%
(Found: C, 72.67; H, 11.59. Calc. for C18H34B2Li2 [285.93] ¥ 2
C4H8O [72.11]: C, 72.60; H, 11.72). Concentration of the mother
liquor under reduced pressure gave a second crop (0.38 g) of Li2[6].
X-ray quality crystals of (Li(THF)2)2[6] were obtained by gas-
phase diffusion of Et2O into a THF solution of Li2[6].
1H NMR (300.0 MHz, C6D6): d = 7.62 (s, 2 H, ArH), 2.89 (m,
4 H, a-CH2), 2.22 (n.r., 4 H, BBNH-a), 2.07 (n.r., 20 H, BBNH-
b,c), 1.66 (m, 4 H, b-CH2), 1.49 (n.r., 4 H, BBNH-c), 1.37 (m, 4
1
H, g-CH2), 1.24 (m, 8 H, d-, e-CH2), 0.85 (m, 6 H, CH3); 13C{ H}
NMR (75.5 MHz, C6D6): d = 143.5 (ArC-2,5), 132.4 (ArC-3,6),
36.6 (a-CH2), 35.0 (b-CH2), 34.7 (BBNC-b), 33.0 (br, BBNC-a),
32.1 (CH2), 30.0 (g-CH2), 23.7 (BBNC-c), 23.0 (CH2), 14.2 (CH3),
1
n.o. (ArC-1,4); 11B{ H} NMR (96.3 MHz, C6D6): d = 86 (h1/2
=
1000 Hz).
-1
1
˜
IR: n/cm = 2252, 2190 (BH). H NMR (400.1 MHz, THF-
d8): d = 6.80 (s, 2 H, ArH), 2.53 (m, 4 H, a-CH2), 1.55 (m, 4 H,
b-CH2), 1.30 (m, 12 H, g-, d-, e-CH2), 0.88 (m, 6 H, CH3), 1.02
Synthesis of 9
A glass tube equipped with a Teflon Young’s tap and a magnetic
stirrer bar was charged with 7 (0.41 g, 0.98 mmol) and tert-butyl
acetylene (0.81 mL, 0.54 g, 6.57 mmol). Benzene (15 mL) was
added, the mixture was frozen at -196 ◦C, the vessel was evacuated,
the tap was closed, and the mixture was heated with stirring in an
oil bath at 60 ◦C for 18 h. After cooling to rt, the vessel was
flushed with N2. Subsequently, all volatiles were removed in vacuo
to obtain 9 as a colourless turbid oil. Purification of the crude
product by recrystallisation failed due to its high solubility in all
common non-coordinating solvents (including hexane at low tem-
perature). Attempts at a purification by vacuum sublimation led to
thermolysis with formation of tris(tert-butylvinyl)borane (NMR
spectroscopic control). Chromatographic workup is precluded by
the pronounced air and moisture sensitivity of the compound.
1H NMR (300.0 MHz, C6D6): d = 7.42 (s, 2 H, ArH), 6.96 (d,
4 H, J = 17.7, C2H2), 6.76 (d, 4 H, J = 17.7, C2H2), 2.87 (m, 4
H, a-CH2), 1.71 (m, 4 H, b-CH2), 1.37–1.20 (m, 12 H, g-, d-, e-
(q, J = 75, BH); 13C{ H} NMR (100.6 MHz, THF-d8): d = 141.5
1
(ArC-2,5), 136.8 (ArC-3,6), 37.8 (a-CH2), 33.3 (CH2), 32.8 (CH2),
31.2 (CH2), 23.7 (e-CH2), 14.5 (CH3), n.o. (ArC-1,4); 11B{ H}
1
NMR (128.4 MHz, THF-d8): d = -26.2 (h1/2 = 30 Hz); 11B NMR
(128.4 MHz, THF-d8): d = -26.2 (q, J = 75).
Synthesis of 7
A solution of N(Me)2Et (3.2 mL, 2.2 g, 30.0 mmol) in THF (10 mL)
was condensed from Na/K alloy onto solid (Li(THF))2[6] (1.40 g,
3.25 mmol). After warming to rt, more THF (20 mL) was added
via syringe, and the mixture was again frozen at -196 ◦C. Me3SiCl
(4.2 mL, 3.6 g, 33.1 mmol) was added by vacuum transfer from
CaH2, the reaction mixture was allowed to warm to rt and
stirred overnight, whereupon a colourless suspension formed. All
volatiles were removed in vacuo and the residue was stirred with
Et2O (25 mL)/pentane (5 mL) for 1.5 h at rt. After filtration, the
filtrate was freed of solvents under reduced pressure to obtain
analytically pure 7. Yield: 1.21 g, 90% (Found: C, 74.53; H, 12.78;
N, 6.44. Calc. for C26H54B2N2 [416.33]: C, 75.01; H, 13.07; N,
6.73). Single crystals suitable for X-ray diffraction analysis were
obtained by slow evaporation of an Et2O solution of 7 under
reduced pressure without stirring.
1
CH2), 0.99 (s, 36 H, C(CH3)3), 0.86 (m, 6 H, CH3); 13C{ H} NMR
(75.5 MHz, C6D6): d = 169.4 (C2H2), 141.3 (ArC-2,5), 132.6 (ArC-
3,6), 130.6 (br, C2H2), 37.0 (a-CH2), 35.5 (C(CH3)3), 33.3 (CH2),
32.2 (CH2), 29.7 (CH2), 28.9 (C(CH3)3), 23.0 (e-CH2), 14.3 (CH3),
1
n.o. (ArC-1,4); 11B{ H} NMR (96.3 MHz, C6D6): d = 65 (h1/2
=
1500 Hz).
1H NMR (300.0 MHz, CDCl3): d = 7.16 (s, 2 H, ArH), 2.81
(q, J = 7.3, 4 H, NCH2CH3), 2.66 (m, 4 H, a-CH2), 2.36 (s, 12
H, NCH3), 1.46 (m, 4 H, b-CH2), 1.29–1.12 (m, 12 H, g-, d-, e-
CH2), 1.08 (t, J = 7.3, 6 H, NCH2CH3), 0.76 (m, 6 H, CH3), n.o.
Crystal structure determinations of ((Li(THF)2)2[m-2]),
((Li(THF)2)2[p-2]), 7, and 8
Data were collected on a STOE IPDS II two-circle diffractometer
with graphite-monochromated Mo-Ka radiation (l = 0.71073 A).
1
(BH); 13C{ H} NMR (100.6 MHz, CDCl3): d = 143.9 (ArC-2,5),
˚
140.1 (ArC-3,6), 56.2 (NCH2CH3), 47.5 (NCH3), 35.6 (a-CH2),
33.1 (CH2), 32.0 (CH2), 29.9 (CH2), 22.7 (e-CH2), 14.1 (CH3), 8.3
The structures were solved by direct methods using the program
SHELXS39 and refined against F2 with full-matrix least-squares
techniques using the program SHELXL-97.40
In (Li(THF)2)2[m-2], two atoms of a THF ligand are disordered
over two positions with a site occupation factor of 0.64(1) for
the major occupied site. In (Li(THF)2)2[p-2], one atom of a THF
ligand is disordered over two positions with a site occupation
factor of 0.51(2) for the major occupied site. In 7, three atoms of an
n-hexyl chain are disordered over two equally occupied positions.
1
(NCH2CH3), n.o. (ArC-1,4); 11B{ H} NMR (96.3 MHz, CDCl3):
d = -3.2 (h1/2 = 370 Hz). 11B NMR (96.3 MHz, CDCl3): d = -3.2
(h1/2 = 460 Hz).
Synthesis of 8
A glass tube equipped with a Teflon Young’s tap and a magnetic
stirrer bar was charged with 7 (0.44 g, 1.06 mmol) and 1,5-
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In 8, three atoms of an n-hexyl chain are disordered over two
positions with a site occupation factor of 0.508(7) for the major
occupied site. The disordered atoms in 8 were refined isotropically.
CCDC reference numbers: 793257 ((Li(THF)2)2[m-2]), 793258
((Li(THF)2)2[p-2]), 793259 (7), and 793260 (8).
17 A. Lorbach, M. Bolte, H.-W. Lerner and M. Wagner, Chem. Commun.,
2010, 46, 3592–3594.
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Acknowledgements
M.W. acknowledges financial support by the Beilstein Institute,
Frankfurt/Main, Germany, within the research collaboration
NanoBiC. The authors are also grateful to Merck KGaA,
Darmstadt for further financial funding.
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©