An intramolecular boron nitrogen lewis acid base pair on a rigid naphthyl backbone

Article abstract of DOI:10.5560/ZNB.2012-0091

The reaction of (C6F5)2BCl with 8-lithio-N,N-dimethyl-1-naphthylamine (1) afforded the fivemembered ring system 8-bis(pentafluorophenyl)boryl-N,N- dimethyl-1-naphthylamine (2) with an intramolecular dative B-N bond. The compound was characterised by elemental analysis, NMR spectroscopy and single-crystal X-ray diffraction.

Full text of DOI:10.5560/ZNB.2012-0091

An Intramolecular Boron Nitrogen Lewis Acid Base Pair on a Rigid
Naphthyl Backbone
Daniel Winkelhaus, Beate Neumann and Norbert W. Mitzel
Lehrstuhl fu¨r Anorganische Chemie und Strukturchemie, Universita¨t Bielefeld,
Universita¨tsstraße 25, 33615 Bielefeld, Germany
Reprint requests to Prof. Dr. N. W. Mitzel. Fax: (+49)521 106 6026.
E-mail: mitzel@uni-bielefeld.de
Z. Naturforsch. 2012, 67b, 589 – 593 / DOI: 10.5560/ZNB.2012-0091
Received March 30, 2012
Dedicated to Professor Wolfgang Beck on the occasion of his 80th birthday
The reaction of (C6F5)2BCl with 8-lithio-N,N-dimethyl-1-naphthylamine (1) afforded the five-
membered ring system 8-bis(pentafluorophenyl)boryl-N,N-dimethyl-1-naphthylamine (2) with an in-
tramolecular dative B–N bond. The compound was characterised by elemental analysis, NMR spec-
troscopy and single-crystal X-ray diffraction.
Key words: Lewis Acid, Boron, Nitrogen, Heterocycle, Naphthalene Derivate
Introduction
anthracenes (B) (Scheme 1) [11] with bond lengths of
˚
the dative B–N bonds of 1.89 and 1.67 – 1.81 A, re-
In the field of polyfunctional Lewis acids [1] the pre- spectively.
organisation of the Lewis acidic sites can be achieved In this contribution we present an 8-(dimethyl-
employing rigid backbones. Ortho-substituted ben- amino)-1-naphthyl derivate with strongly electron-
zenes [2], peri-substituted naphthalene [3 – 6] and 1,8- withdrawing pentafluorophenyl substituents at the
substituted anthracene [7, 8] offer promising applica- boron atom.
tions as such backbones in the field of host-guest chem-
istry [9]. Furthermore, these frameworks can induce
unusual interactions between different functionalities.
Results and Discussion
The combination of Lewis acid and base functions
fixed to rigid backbones leads to the formation of da-
The reaction of N,N-dimethyl-1-naphthylamine
tive bonds in constrained systems. Examples for such with n-butyllithium in Et2O leads to the regioselective
systems include boron nitrogen interactions in 1,8- lithiation in the 8-position (Scheme 2) [12]. In our ex-
substituted naphthalenes (A) [10] and 1,8,9-substituted periments this product 1 could be isolated as a yellow
solid in 70% yield.
Subsequent reaction of 1 with (C6F5)2BCl [13]
at −30 ◦C in toluene afforded the pentafluorophenyl-
X
X
N
B
N
O
B
N
O
X = Cl, Me, 1,2-O2C6H4
A
B
Scheme 2. Reaction of N,N-dimethyl-1-naphthylamine with
n-BuLi.
Scheme 1.
c
ꢀ 2012 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com

590
D. Winkelhaus et al. · An Intramolecular Boron Nitrogen Lewis Acid Base Pair on a Rigid Naphthyl Backbone
dimethylamino group to the boron atom which leads to
a five-membered ring structure. This was subsequently
confirmed by X-ray diffraction. Single crystals of 2
were obtained from a concentrated toluene solution at
−30 ◦C. Compound 2 crystallises in the orthorhombic
space group Pbca; its structure is displayed in Fig. 1;
selected structural parameters are listed in Table 1.
The structure of 2 features a five-membered ring
system. Due to the rigid naphthalene backbone
the B/N ring does not exhibit a typical envelope
Scheme 3.
naphthylamine (1) with (C6F5)2BCl.
Reaction
of
8-lithio-N,N-dimethyl-1-
substituted borane 2 as a colourless solid in 87% yield conformation as it is observed in the related five-
(Scheme 3). membered ring system with a dative B–N bond,
Compound 2 was characterised by elemental analy- (C6F5)2B(CH2)3NMe2 [17]. By contrast, compound
sis and by NMR spectroscopy (1H, 11B, 13C, 19F). The 2 shows only a small deviation from planarity.
1H NMR spectrum shows signals for the protons of The torsion angles B(1)–C(13)–C(22)–C(21) and
the naphthalene ring between δ = 7.60 and 6.52 ppm. N(1)–C(21)–C(22)–C(13) have values of 6.4(2)◦ and
The singlet for the methyl group is detected at δ = 5.9(2)◦, respectively. The length of the bond created by
2.17 ppm. The corresponding signals of the naphtha- the strongly Lewis acidic boron atom and the nitrogen
lene ring in the 13C NMR spectrum are observed be- atom is 1.742(2) A (Table 1). This is remarkably longer
˚
tween δ = 152.2 and 111.3 ppm. A broad resonance than the B–N bond length in (C6F5)2B(CH2)3NMe2
˚
for the carbon atom in 8-position at δ = 145.7 ppm in- [1.672(2) A] and is a consequence of the ring strain
dicates the formation of a B–C bond. In the 11B NMR established by the rigid naphthyl backbone which
spectrum a single resonance at δ = 4.8 ppm (ν1/2
=
prevents a shorter B-N contact. According to the
higher electronegativity of the boron substituents, the
B–N bond in 2 is clearly shorter than that in the related
pinacol-substituted species, (Me2N)C10H6B(Pin)
180 Hz) is observed. The 19F NMR signals of the C6F5
groups are detected at δ = −162.7 (m-C6F5), −156.1
(p-C6F5) and −127.7 ppm (o-C6F5). The small sepa-
ration of the p- and m-C6F5 19F resonances indicates
the presence of a tetra-coordinate boron atom [14 – 16].
The NMR spectroscopic data are consistent with
the formation of an intramolecular coordination of the
˚
(A, Scheme 1) (1.89 A) [10].
A
comparison
with the derivatives of 1,8-bis(dimethylamino)-9-
borylanthracene (B, Scheme 1) reveals values for
˚
the B–N bond of 1.809(2), 1.739(2) and 1.664(3) A
Fig. 1. (a) Molecular struc-
ture of 2 in the crystal. Hy-
drogen atoms are omitted
for clarity; (b) illustration
of the B-C3-N ring geome-
try of 2.

D. Winkelhaus et al. · An Intramolecular Boron Nitrogen Lewis Acid Base Pair on a Rigid Naphthyl Backbone
591
Table 1. Selected bond
lengths (A) and angles
(deg) for 2 with estimated
standard deviations in
parentheses.
N(1)–C(21)
N(1)–C(23)
N(1)–C(24)
N(1)–B(1)
B(1)–C(13)
B(1)–C(7)
B(1)–C(1)
C(21)–N(1)–C(23)
C(21)–N(1)–C(24)
C(23)–N(1)–C(24)
C(21)–N(1)–B(1)
C(23)–N(1)–B(1)
C(24)–N(1)–B(1)
1.488(2)
1.506(2)
1.508(2)
1.742(2)
1.609(2)
1.633(2)
1.650(2)
112.1(1)
105.3(1)
106.9(1)
104.4(1)
117.4(1)
110.3(1)
C(13)–B(1)–C(7)
C(13)–B(1)–C(1)
C(7)–B(1)–C(1)
C(13)–B(1)–N(1)
C(7)–B(1)–N(1)
119.3(1)
106.6(1)
112.1(1)
97.7(1)
107.2(1)
113.4(1)
109.6(1)
133.1(1)
110.6(1)
127.5(1)
114.9(1)
116.7(1)
121.9(1)
˚
C(1)–B(1)–N(1)
C(22)–C(13)–B(1)
C(14)–C(13)–B(1)
C(22)–C(21)–N(1)
C(20)–C(21)–N(1)
C(21)–C(22)–C(13)
C(14)–C(13)–C(22)
C(20)–C(21)–C(22)
˚
for 1,2-O2C6H4, Me and Cl substitutents at boron, in (C6F5)2B(CH2)3NMe2 [1.637(2) A]. The two N–C
respectively.
bonds of the NMe2 fragment have values of 1.506(2)
˚
As a consequence of the formation of the dative and 1.508(2) A for N(1)–C(23) and N(1)–C(24), re-
B–N bond the bond angles about C(13) and C(21) spectively. This is slightly longer than the endocyclic
feature distorted geometries. The bond angles C(22)– N(1)–C(21) bond at 1.488(2) A.
˚
C(13)–B(1) and C(22)–C(21)–N(1) within the five-
membered ring system have small values of 109.6(1)
and 110.6.(1)◦, respectively. Consequently, the an-
gles C(14)–C(13)–B(1) and C(20)–C(21)–N(1) adopt
larger values with 133.1(1) and 127.5(1)◦. Further-
more, the formation of the five-membered ring leads
to a distortion of the naphthalene backbone. The in-
tramolecular C···C distance between C(13) and C(21)
Conclusion
The reaction of the chloroborane (C6F5)2BCl with
8-lithio-N,N-dimethyl-1-naphthylamine (1) affords
a five-membered B/N ring system with a rigid naphtha-
lene backbone (2). Single-crystal X-ray diffraction has
revealed a strained ring system with an intramolecular
˚
has a value of 2.37 A and is thus distinctly shorter
˚
dative B–N bond length of 1.742(2) A. A comparison
than in unsubstituted naphthalene with a value of
with data for other substitution patterns at boron
suggests that this B–N distance is largely determined
by the substituent electronegativity.
◦
˚
2.48 A [18]. Both, the C(14)–C(13)–C(22) [116.7(1) ]
and the C(20)–C(21)–C(22) angle [121.9(1)◦] show
a distortion in comparison to unsubstituted naphtha-
lene, in which this angle is 119.8◦ [18]. Deforma-
tions of the molecular geometry are also observed in
many substituted aryl derivates [19] and can be ex-
plained by the VSEPR model. The electronegative ni-
trogen atom requires less space, the electropositive
boron atom more space than a hydrogen atom at the C
atoms which leads to an increase and decrease of these
bond angles, respectively.
Experimental Section
All manipulations were performed under a rigorously
dried inert atmosphere of argon using standard Schlenk
and glove-box techniques. Toluene was dried with potas-
sium before being employed in reactions. C6D6 was dried
with Na/K alloy and degassed. (C6F5)2BCl was synthesised
according to a literature procedure [13]. N,N-Dimethyl-1-
naphthylamine was purchased from ABCR and degassed
prior to use. NMR measurements were undertaken with
Bruker DRX 500 and Avance 500 instruments. NMR chemi-
cal shifts were referenced to the residual peaks of the protons
of the used solvents (1H, 13C) or externally (11B, BF3·OEt2;
19F, CFCl3). Elemental analysis was performed by using
a CHNS elemental analyser HEKAtech EURO EA.
The geometries about the boron and the nitrogen
atom in 2 are distorted tetrahedral. The smallest bond
angle at the boron and the nitrogen atom is observed
within the five-membered ring system with values
of 97.7(1) [C(13)–B(1)–N(1)] and 104.4(1)◦ [C(21)–
N(1)–B(1)], respectively. All other bond angles about
the boron and nitrogen atom exhibit larger values
which range up to 119.3(1) [C(13)–B(1)–C(7)] and
117.4(1)◦ [C(23)–N(1)–B(1)], respectively.
8-Lithio-N,N-dimethyl-1-naphthylamine (1)
According to a literature procedure [12], n-BuLi (34 mL,
54 mmol, 4.5 eq, 1.6 M) was added to a solution of N,N-
The endocyclic B–C bond [C(13)–B(1)] has a length
˚
of 1.609(2) A and is thus shorter than the B–C bond dimethyl-1-naphthylamine (2.0 g, 12 mmol) in 25 mL Et2O

592
D. Winkelhaus et al. · An Intramolecular Boron Nitrogen Lewis Acid Base Pair on a Rigid Naphthyl Backbone
at ambient temperature. The reaction mixture was stirred for Table 2. Crystal structure data for 2.
48 h, and the resulting solid was filtered off, washed with
Formula
Mr
C24H12BF10
515.16
0.72×0.72×0.60
orthorhombic
Pbca
7.6624(2)
21.7295(5)
24.5402(6)
4085.95(17)
8
N
3 mL Et2O and dried in a vacuum. The product was obtained
as a yellow solid and used without further characterisation.
Yield: 1.5 g (70%).
Crystal size, mm3
Crystal system
Space group
˚
8-Bis(pentafluorophenyl)boryl-N,N-dimethyl-1-naphthyl-
amine (2)
a, A
˚
b, A
˚
c, A
3
˚
To a solution of 8-lithio-N,N-dimethyl-1-naphthylamine
(1) (74 mg, 0.4 mmol) in 4 mL toluene was added a solu-
tion of (C6F5)2BCl (150 mg, 0.4 mmol) in 4 mL toluene at
−30 ◦C. After warming to ambient temperature the reac-
tion mixture was stirred over night and filtered. The sol-
vent was removed in a vacuum to leave a colourless solid.
Crystals suitable for X-ray diffraction were obtained from
a concentrated toluene solution at −30 ◦C. Yield: 173 mg
(87%). – 1H NMR (500.1 MHz, C6D6, 298 K): δ = 7.60
(d, J = 6.8 Hz, 1 H, H-7), 7.52 (d, J = 8.3 Hz, 1 H, H-
4), 7.48 (d, J = 8.0 Hz, 1 H, H-5), 7.40 (dd, J = 6.8 Hz,
J = 8.0 Hz, 1 H, H-6), 7.14 (m, 1 H, H-3), 6.52 (d, J =
7.4 Hz, 1 H, H-2), 2.17 ppm (s, 6 H, CH3). – 11B{1H} NMR
(160.4 MHz, C6D6, 298 K): δ = 4.8 ppm (ν1/2 = 180 Hz). –
13C{1H} NMR (125.7 MHz, C6D6, 298 K): δ = 152.2 (C-1),
V, A
Z
Dcalcd, g cm−3
µ(MoKα ), cm−1
F(000), e
1.68
0.2
2064
hkl range
((sinθ)/λ)max, A
Refl. measured / unique / Rint 66732 / 5950 / 0.063
±10, ±30, ±34
−1
˚
0.703
Param. refined
327
R(F) / wR(F2) (all refs.)
GoF (F2)
0.0666 / 0.0968
1.063
0.27 / −0.24
−3
˚
∆ρfin (max / min), e A
diffractometer. The measurement was carried out with MoKα
radiation (λ = 0.71073 A). The structure was solved by Di-
˚
rect Methods and refined by full-matrix least-squares cycles
(program SHELXS/L-97 [20]). Further details of the crystal
structure data are given in Table 2.
CCDC 873670 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data request/cif.
1
148.5 (dm, JCF = 241.1 Hz, C6F5), 145.7 (br, C-8), 140.3
(dm, 1JCF = 251.3 Hz, C6F5), 137.7 (dm, 1JCF = 249.9 Hz,
C6F5), 132.9 (C-4a), 132.3 (C-8a), 129.9 (C-6), 128.5 (C-7),
126.5 (C-4), 125.6 (C-3), 123.8 (C-5), 118.0 (br, i-C6F5),
111.3 (C-2), 51.2 ppm (CH3). – 19F NMR (470.5 MHz,
C6D6, 298 K): δ = −162.7 (m, 4F, m-C6F5), –156.1 (t,
3JFF = 20.8 Hz, 2F, p-C6F5), −127.7 ppm (m, 2F, o-C6F5).
– C24H12BF10N (515.15): calcd. C 55.96, H 2.35, N 2.72;
found C 53.03, H 2.36, N 3.04.
Acknowledgement
X-Ray crystallographic structure determination
We thank K.-P. Mester and G. Lipinski for recording the
NMR spectra and B. Michel for performing the elemental
analysis. We gratefully acknowledge the financial support of
the Fonds der Chemischen Industrie (stipend for D. W.) and
the DFG within the priority program SPP1178.
A single crystal suitable for X-ray diffraction measure-
ment was picked inside a glove-box, suspended in a paratone-
N/paraffin oil mixture, mounted on a glass fibre and trans-
ferred onto the goniometer of the Bruker Nonius Kappa CCD
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