Synthesis and structure of pyrrolidinobromodiboranes(4)

Article abstract of DOI:10.1515/znb-2005-0918

The reaction of tetrapyrrolidinodiborane(4) (1) with BBr₃ in a 1:1 molar ratio yields a mixture of 1,2-dibromo-1,2-dipyrrolidinodiborane(4) (2) and bromotripyrrolidino- diborane(4) (3), while a 1:2 molar ratio leads in Et₂O to compound 2 as the main product along with a small amount of [(C₄H₈N)₂B₂Br₃(OEt)] (4). The new compounds have been characterized by NMR and MS data, as well as by X-ray structure analyses of 2 and 4, of which the former exhibits an interesting polymorphism phenomenon.

Full text of DOI:10.1515/znb-2005-0918

1016
Note
that planar B4 rings are unstable. Instead, the de-
halogenation reaction of Me2N(Cl)B-B(Cl)NMe2 pro-
duces orange-red cyclo-(Me2NB)6 [5] with a chair-
like structure, which was first reported in 1980 [9] as
a side product in the dehalogenation of (Me2N)2BCl.
The dechlorination reaction of Et2N(Cl)B-B(Cl)NEt2
leads to green, polyhedral (Et2NB)6 in 40% yield [5],
which was first obtained in low yield by dehalogenat-
ing Et2NBCl2. At elevated temperatures, it rearranged
into its cyclic isomer [10].
Synthesis and Structure of
Pyrrolidinobromodiboranes(4)
Yong Nie, Hans Pritzkow, and Walter Siebert
Anorganisch-Chemisches Institut der Universita¨t
Heidelberg, Im Neuenheimer Feld 270,
D-69120 Heidelberg, Germany
Reprint requests to Prof. W. Siebert.
Fax: + 49-(0)6221/545609.
E-mail: walter.siebert@urz.uni-heidelberg.de
The sterically more hindered iPr2N(Cl)B-B(Cl)
NiPr2 allows the formation of blue folded cyclo-
(iPr2NB)4, and in the case of the corresponding
2,2,6,6-tetramethylpiperidino(tmp) compound the yel-
low tetrahedral (tmpB)4 is produced [11]. Interestingly,
the codehalogenation of a 1:1 mixture of Me2NBCl2
and Me2N(Cl)B-B(Cl)NMe2 affords in low yield col-
orless (Me2N)2B4[B(NMe2)2]2 [12] with a planar B4
rhomb having a unique electronic structure. It is avail-
able in good yield [13, 14] by dehalogenating 1,3-di-
chloro-1,3,5-tris-(dimethylamino)triborane(5) [15].
Considering the similarity between pyrrolidino,
dimethylamino and diethylamino groups, one might
expect that the dehalogenation of 1,2-dichloro-1,2-di-
pyrrolidinodiborane(4) would give cyclic or polyhe-
dral aminoboranes (C4H8NB)n (n = 4, 6). However,
the reaction did not lead to any definite product [5],
and it was anticipated that the more reactive 1,2-
dibromo-1,2-dipyrrolidinodiborane(4) (2) could yield
better results. Herein we report on the reactions of
tetrapyrrolidinodiborane(4) (1) with BBr3, which lead
to the formation of compound 2 and the byproduct
[(C4H8N)2B2Br3(OEt)] (4).
Z. Naturforsch. 60b, 1016 – 1019 (2005);
received April 7, 2005
Dedicated to Professor Heinrich Vahrenkamp on the
occasion of his 65th birthday
The reaction of tetrapyrrolidinodiborane(4) (1) with BBr3
in a 1:1 molar ratio yields a mixture of 1,2-dibromo-1,2-
dipyrrolidinodiborane(4) (2) and bromotripyrrolidino- dibo-
rane(4) (3), while a 1:2 molar ratio leads in Et2O to com-
pound 2 as the main product along with a small amount of
[(C4H8N)2B2Br3(OEt)] (4). The new compounds have been
characterized by NMR and MS data, as well as by X-ray
structure analyses of 2 and 4, of which the former exhibits
an interesting polymorphism phenomenon.
Key words: Boron, Borane, Diborane(4), Polymorphism
Introduction
Diborane(4) derivatives are interesting reagents in
organic synthesis. In particular the tetraalkoxydib-
orane(4) compounds are used for various transi-
tion metal-catalyzed addition and insertion reactions
into multiple bonds [1]. The B-halogeno-diborane(4)
derivatives display a rich chemistry and serve as
starting compounds for, among others, oligoboranes
Results and Discussion
To obtain compound 2, we started from tetrapyrro-
which are obtained via dehalogenation reactions. A lidinodiborane(4) (1), which was prepared in high
yield (90%) by a combination of the literature meth-
ods [1 from (C4H8N)2BCl and NaK2.8, 46% [16], and
1 from (C4H8N)2BBr and Na, 50% [17] ] by using the
two more reactive starting compounds, (C4H8N)2BBr
and NaK2.8. The reaction of 1 with 1 equiv. of BBr3
did not afford pure 2, but a mixture of 2 and the mono-
unique structural interplay between the resulting cyclo-
and polyhedral boron compounds has been demon-
strated [2]. While the existence of the postulated cy-
clo-(Me2NB)4 [3] (from B4Cl4 and Me2NH) with a
planar structure has not been confirmed [4], attempts
[2, 5] have been made to obtain its analogs by the de-
halogenation of diborane(4) reagents R(Cl)B-B(Cl)R bromodiborane(4) species 3 (Scheme 1), as charac-
terized by mass spectrometry which gave the molec-
ular ion peaks for 2 and 3, respectively. In addi-
tion, (C4H8N)2BBr was detected (11B NMR and MS).
(R = NMe2 [6], NEt2 [5, 7]) with NaK2.8. However,
the planar cyclo-tetraboranes were not obtained, which
is in accord with the calculations [8] which indicate
c
0932–0776 / 05 / 0900–1016 $ 06.00 ꢀ 2005 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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Note
1017
This indicates that, in contrast to the results in the
literature [6] on the reaction with tetrakis(dimethyl-
amino)diborane(4),1 equiv. of BBr3 in the present case
is not enough to completely replace two pyrrolidino
groups in 1.
Scheme 1.
When 2 equiv. of BBr3 were used (Scheme 2), color-
less 2 was obtained in 69% yield. A very small amount
of 4 was also formed. The identity of both compounds
has been confirmed by X-ray structure analyses (see Fig. 1. Molecular structure of 2 in the orthorhombic
˚
phase 2’. Selected bond lengths [A]: Br1-B1 1.981(2), N1-
below).
B1 1.381(2), B1-B1A 1.696(4). [Data for the triclinic phase
2”: Br-B 1.978(2), N-B 1.370(2), B-B 1.684(3)].
Scheme 2.
The formation of 4 results obviously from the
Et2O solvent used (see Experimental Section), which
can react with (C4H8N)BBr2, produced in the re-
action and identified by 11B NMR and MS, to
give (C4H8N)B(OEt)Br, which further reacts with
(C4H8N)BBr2 to afford 4. It is unlikely that
(C4H8N)BBr2 dimerizes to form [C4H8NBBr2]2 [18]
which then reacts with Et2O to give 4. The same re-
action in toluene (Scheme 2) gives pure 2 in similar
yield (64%). Compound 2 shows polymorphism (or
dimorphism) phenomena: from a hexane/toluene solu-
tion (−28 ◦C) it crystallizes as 2’ in the orthorhombic
space group P21212 (determined at 103(2) K), while
Fig. 2. Molecular structure of 4, hydrogen atoms omit-
◦
˚
ted for clarity. Selected bond lengths [A] and angles [ ]:
Br1-B1 2.012(3), Br2-B1 2.013(3), Br3-B2 2.045(3), N1-B1
1.578(4), N1-B2 1.655(3), N2-B1 1.580(3), N2-B2 1.618(3),
O(1)-B(2) 1.375(3); B1-N1-B2 86.7(4), B1-N2-B2 88.0(2),
N2-B2-N1 89.1(2), N1-B1-N2, 93.3(2).
◦
neat 2 on cooling (−30 C) crystallizes as 2” in the
π interaction in 2 than in 1 because of replacement
of the two pyrrolidino in 1 by two bromine atoms.
This is consistent with the results in 1,3-dichloro-
1,3,5-tris(dimethylamino)triborane(5) and the corre-
sponding bromine and iodine analogs [15]. The tor-
sion angles Br1-B1-B1A-Br1A and N1-B1-B1A-N2 in
2’ are −122.8(2) and −122.8(3)◦, respectively, while
the corresponding values in 2” are −86.3(15) and
−82.9(19)◦, respectively.
triclinic space group P1 bar (determined at 293(2) K).
Fig. 1 shows only 2’ (orthorhombic), as the bond
lengths and angles are similar in 2’ and 2”, which
differ in their conformation. The B-Br bond lengths
˚
[1.981(2) in 2’ and 1.978(2) A in 2”] are similar to
˚
those in B2(NMe)2Br2 [1.979(13) A] [19] as to the B-
˚
B bond distances [1.696(4) in 2’, 1.684(3) in 2” A]
˚
[1.682(16) A in B2(NMe)2Br2], but are shorter than
˚
those in the starting compound 1 (1.739 A) [17]. The
shorter B-N bond lengths [1.381(2) in 2’, 1.371(2) in
The structure of 4 is shown in Fig. 2. The four-
˚
2”, cf. 1.424/1.427 A in 1] indicate a stronger B-N membered B-N-B-N ring is not planar but folded (tor-
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1018
Note
sion angle B1-N2-B2-N1: 12.5◦). The B2-Br3 bond
length of 2.045 A is longer that those in 2, and also
Table 1. Crystal data and details of data collection and struc-
ture solution of 2’, 2” and 4.
˚
slightly longer than the B1-Br1 and B1-Br2 distances
(2.012/2.013 A), due to the different orientation of the
2’
2”
4
Empirical formula C8H16B2Br2N2
C8H16B2Br2N2
321.67
triclinic
P1 bar
6.8944(5)
8.0273(6)
11.7240(9)
99.210(2)
102.527(2)
91.176(2)
624.21(8)
2
C10H21B2Br3N2O
446.64
monoclinic
P21/c
10.9113(10)
10.3105(10)
13.4627(12)
90
91.344
90
1514.2(2)
4
˚
Formula weight
Crystal system
Space group
321.67
orthorhombic
P21212
11.2890(6)
11.6672(6)
4.6012(3)
90
90
90
606.03(6)
2
pyrrolidino rings and the steric requirements of the OEt
group. The presence of the OEt group also influences
the B-N bond lengths: the distances B2-N1 and B2-
˚
a [A]
˚
b [A]
˚
c [A]
˚
N2 (1.655/1.618 A) are longer than those involving B1
α [◦]
β [◦]
γ [◦]
˚
atom (1.578/1.580 A, respectively). However, the B-Br
and B-N bond lengths in 4 are similar to those in the re-
3
˚
V [A ]
lated structure [C4H8BBr2]2 (av. B-Br distance 1.999,
Z
ρ (calcd.), [g/cm3] 1.763
1.711
6.458
316
1.959
7.982
872
˚
B-N 1.603 A) [20].
µ [mm−1
F(000)
]
6.651
316
The results of the dehalogenation experiments with
2 show that the reactions with NaK2.8 in hexane or
toluene afford mixtures, in which surprisingly small
amounts 1 could be identified by a comparison of its
NMR and HR-MS data with those of an authentic
sample.
Crystal size [mm] 0.45×0.12×0.10 0.60×0.14×0.12 0.52×0.27×0.13
Θmax [◦]
Temp, [K]
Refl. collected
Refl. indep.[Rint
Refl. observed
parameters
32.03
103(2)
9541
32.00
32.00
103(2)
5134
5134 [0.030]
4167
247
293(2)
10729
4211 [0.027]
3351
200
1.037
]
2085 [0.035]
1983
97
GOOF
1.060
1.055
R1[I > 2σ(I)]
0.0194
0.0469
0.0268
0.0654
0.0320
0.0779
Experimental Section
wR2 (all data)
Min.−/m3 ax. resid.
˚
All reactions and manipulations were performed in dry
glassware under nitrogen using standard Schlenk techniques.
Solvents were dried, distilled, and saturated with nitrogen.
NMR specta were recorded on a Bruker DRX 200 spectrom-
eter (1H: 200.13 MHz, 11B: 64.21 MHz, 13C: 50.32 MHz)
in CDCl3. Et2O•BF3 was used as external standard for
11B NMR. As internal references for 1H and 13C NMR,
the signals of CDCl3 were used and shifts were calculated
relative to TMS. MS: ZAB-2F VH Micromass CTD spec-
trometer, and a JEOL MS Station JMS 700 spectrometer.
Compound 1 was prepared according to literature procedures
[16, 17].
[eA
]
−0.308/1.138
−0.543/0.749
−0.615/1.701
b) Analogous procedure as described above. (C4H8N)4B2
(3.3 g, 10.9 mmol), BBr3 (5.5 g, 21.9 mmol). The colorless
residue wa◦s distilled to give a colorless liquid as the first frac-
tion at 30 C/2 × 10−1 torr, which contained (C4H8N)BBr2
(1.62 g, 31%) and ◦a very small amount of 4. The sec-
ond fraction at 90 C/7 × 10−2 torr (1.08 g) was a mix-
ture of (C4H8N)BBr2 and 2, the third fraction, obtained at
110 ◦C/7×10−2 torr as a colorless oil, was identified to be 2
(2.44 g, 69%). 1H NMR (CDCl3): 1.82 (t, 3J = 6.8 Hz, 4 H,
3
NCH2CH2), 1.85 (t, J = 6.8 Hz, 4 H, NCH2CH2), 3.33 (t,
3J = 6.8 Hz, 4 H, NCH2), 3.46 (t, 3J = 6.8 Hz, 4 H, NCH2). –
11B NMR (CDCl3): δ = 35.8. – 13C NMR (CDCl3): δ =
25.6, 27.1 (NCH2CH2), 50.6, 51.4 (NCH2). – EI-MS: m/z
(%) = 322 (43) [M+], 241 (100) [M+ −Br]. – HR-MS (EI):
1,2-Dibromo-1,2-dipyrrolidino-diborane(4) (2)
a) To a solution of (C4H8N)4B2 (3.2 g, 10.6 mmol) in
Et2O (20 ml) at −40 ◦C was added a solution of BBr3 (2.7 g,
10.7 mmol) in toluene (15 ml) within 20 min. The mixture
was stirred for 4 h and then overnight at r.t. After the removal
of the solvents under reduced pressure, the yellow residue
was distilled◦to give a colorless liquid [(C4H8N)2BBr, 2.1 g,
84%] at 70 C/5 × 10−2 torr, and a colorless oil (2.0 g) at
m/z = 319.9819 [M+]; calcd. for 12C8 H1611B279Br214N2
1
319.9866 (∆m = −4.7 mmu). On cooling the first fraction
◦
(ca. −40 C) colorless crystals of 4 were formed. Attempts
to measure the melting point was not successful because of
the presence of liquid (C4H8N)BBr2 at ambient tempera-
ture. Different types of crystals of 2 (m.p. 78 – 80 ◦C) were
obtained by cooling either the neat compound or its hex-
ane/toluene (1:1, v/v) solution.
◦
100 C/6 × 10−2 torr, identified to be a mixture of 2 and 3.
1H NMR (CDCl3): δ = 1.6 (NCH2CH2), 1.8 (NCH2CH2).
3.3 (NCH2CH2). 3.5 (NCH2CH2); All signals are over-
lapping and broad. 11B NMR (CDCl3): δ = 36.2 (br.); –
13C NMR (CDCl3): δ = 25.6, 25.8, 26.7, 27.1 (NCH2CH2),
49.5, 49.8, 50.6, 51.4 (NCH2). – EI-MS: m/z (%) = 322 (22)
X-ray structure determinations
The crystal data and details of data collection and struc-
[2+], 311 (32) [3+], 241 (53) [2+ − Br], 232 (80) [3+ − Br], ture solution are listed in Table 1. The intensity data were col-
163 (32) [2+ −2Br+1], 151 (100) [3+ −2Br]. – HR-MS (EI): lected with a Bruker AXS Smart 1000 CCD diffractometer
m/z = 311.1325 [3+]; calcd. for
311.1340 (∆m = −1.5 mmu).
C
H2411B279Br14N3 (Mo-Kα radiation, λ = 0.71073 A, graphite monochroma-
12
1
˚
12
tor, ω-scans). Data were corrected for Lorenz-polarization
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Note
1019
and absorption effects (semiempirical, SADABS [21]). The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ,
structures were solved by direct methods (SHELXS-86) [22] UK (Fax: int. code + 44 (1223)336-033; e-mail for inquiry:
and refined by least-squares methods based on F2 with all fileserv@ccde.cam.ac.uk).
measured reflections (SHELXL-97) [22]. All non-hydrogen
atoms were refined anisotropically. Crystallographic data
Acknowledgements
for the structures have been deposited with the Cambridge
Crystallographic Data Center: CCDC-267548 (2’), −267549
(2”), −267550 (4), −267551 [(CH3)2NB]6. Copies of the
Support of this work by the Deutsche Forschungsgemein-
schaft (Schwerpunktprogramm Polyeder) and the Fonds der
Chemischen Industrie is gratefully acknowledged. We thank
data can be obtained free of charge on application to The Lorenz Lo¨rsch for preparative assistance.
[11] C.-J. Maier, H. Pritzkow, W. Siebert, Angew. Chem.
[1] For a recent review on diboron compounds, V. M. Dem-
bitsky, H. Abu Ali, M. Srebnik, Adv. Organomet. Chem.
51, 193 (2004).
111, 1772 (1999); Angew. Chem. Int. Ed. 38, 1666
(1999).
[12] A. Maier, M. Hofmann, H. Pritzkow, W. Siebert,
Angew. Chem. 114, 1600 (2002); Angew. Chem. Int.
Ed. 41, 1529 (2002).
[2] C.-J Maier, A. Maier, P. Greiwe, M. J. Bayer, M. Hof-
mann, H. Pritzkow, W. Siebert, Pure Appl. Chem. 75,
1277 (2003).
[13] C. Pra¨sang, M. Hofmann, G. Geiseler, W. Massa,
A. Berndt, Angew. Chem. 114, 1597 (2002); Angew.
Chem. Int. Ed. 41, 1526 (2002).
[14] W. Mesbah, Ph. D. Thesis, University of Marburg
(2003).
[3] G. Urry, A. G. Garrett, H. I. Schlesinger, Inorg. Chem.
2, 396 (1963).
[4] J. A. Morrison, Chem. Rev. 91, 35 (1991).
[5] A. Maier, Ph. D. Thesis, University of Heidelberg
(2002).
[15] G. Linti, D. Loderer, H. No¨th, K. Polborn, W. Rattay,
Chem. Ber. 127, 1909 (1994).
[6] H. No¨th, H. Schick, W. Meister, J. Organomet. Chem.
1, 401 (1964).
[16] D. Loderer, H. No¨th, H. Pommerening, W. Rattay,
H. Schick, Chem. Ber. 127, 1605 (1994).
[7] M. Baudler, K. Rockstein, Z. Anorg. Allg. Chem. 628,
1741 (2002).
[17] H. Abu Ali, I. Goldberg, M. Srebnik, Eur. J. Inorg.
Chem. 73 (2002).
[18] O. C. Musgrave, J. Chem. Soc. 4305 (1956).
[19] A. Moezzi, M. M. Olmstead, P. P. Power, J. Chem.
Soc., Dalton Trans. 2429 (1999).
[8] M. L. McKee, Inorg. Chem. 38, 321 (1999).
[9] H. No¨th, H. Pommerening, Angew. Chem. 92, 481
(1980); Angew. Chem. Int. Ed. Engl. 19, 482
(1980). Our redetermination of the structure of cyclo-
(Me2NB)6 showed a molecule in the chair config-
uration, crystallographically imposed symmetry 2/m,
[20] H. Abu Ali, I. Goldberg, M. Srebnik, Acta Crystallogr.
Sect. C 57, 770 (2001).
˚
with almost equal B-B distances (1.714, 1,720(2) A).
[21] G. M. Sheldrick, SADABS, Univ. Go¨ttingen (1999).
[22] G. M. Sheldrick, SHELXS86, Univ. Go¨ttingen (1986);
G. M. Sheldrick, SHELXL97, Univ. Go¨ttingen (1997).
Space group C2/m, T = 190 K, 1898 independent re-
flections, 94 parameters, R1 = 0.059, wR2 = 0.185
[CCDC −267551].
[10] M. Baudler, K. Rockstein, W. Oehlert, Chem. Ber. 124,
1149 (1991).
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