Buy article online - an online subscription or single-article purchase is required to access this article.
Download citation
Download citation
link to html
The B atoms in the title compound, C8H16B2Br4N2, bridge between the two monomeric moieties, forming a (BN)2 four-membered ring with partial bond orders of the B—N bonds.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270101005388/ln1117sup1.cif
Contains datablocks I, global

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270101005388/ln1117Isup2.hkl
Contains datablock I

CCDC reference: 167003

Comment top

The B2(NR2)2 compounds are important intermediates in the synthesis of B2(OR)4 compounds, which are very useful in the preparation of boronic acids from alkynes (Ishiyama, Matsuda et al., 1996), 1,3-butadienes (Ishiyama, Yamamoto & Miyaura, 1996), α,β-enones (Lawson et al., 1997), methylenecyclopropanes (Ishiyama et al., 1999) and aromatic compounds (Ishiyama et al., 1995). The preparation of B2(NR2)2 involves Wurtz coupling of the dialkylhaloboranes (Brotherton et al., 1960; Nöth & Meister, 1961). Moreover, the structural aspects of diborane chemistry are not well explored and conformational problems are not yet really understood (Moezzi et al., 1992). It has been shown that the bromides (R2N)2BBr provide better yields than the chlorides (Brotherton et al., 1960). In addition, the size of the R groups on the nitrogen influences the yields of B2(NR2)2. In an effort to synthesize B2(NR2)2 compounds other than the methyl derivative and to explore their structural chemistry, we studied the dehalogenation of bis(pyrrolidino)bromoborane. During the course of this investigation, we isolated an unexpected by-product, the title compound, [(Br)2B(pyrrolidine)2B(Br)2], (I), in low yield. Subsequently, the same product was obtained in 90% yield as described below. \sch

The molecular structure of (I) is illustrated in Fig. 1. The molecule has three possible directions for a C2 axis, and it reveals an approximate C2 symmetry about an axis passing perpendicular to the plane, and through the centre, of the four-membered ring. Its structural features, with relatively long B—N bonds [mean 1.603 (7) Å], are similar to those observed in closely related aminoboron dichloride and difluoride dimers (Hazell, 1966; Edwards & Stadler, 1970; Jones, 1984; Clegg et al., 1998; Jansen & Jäschke, 1999), as well as in other aminoboron dimers (Metzler & Nöth, 1995). This confirms that the observed elongation of the B—N bond is a genuine feature of the system. The four-membered (BN)2 ring in this structure has a distorted tetrahedral geometry.

The N5—B7—N6 angle of 92.4 (2)° is significantly less than the ideal tetrahedral value. This results in a corresponding widening of the angles N5—B7—Br2 [113.5 (2)°], N6—B7—Br2 [113.7 (2)°], N5—B7—Br1 [113.9 (2)°] and N6—B7—Br1 [116.0 (2)°], while the Br2—B7—Br1 angle of 107.0 (2)° is relatively close to tetrahedral. The coordination geometry around the N atoms is also distorted from tetrahedral. Although the B7—N5—B8 angle of 86.8 (2)° is significantly less than the ideal tetrahedral value, widening of the C9—N5—C12 angle [100.3 (2)°] is not observed. On the other hand, the angles C9—N5—B7 [117.5 (2)°], C12—N5—B7 [118.0 (3)°], C9—N5—B8 [119.8 (3)°] and C12—N5—B8 [115.7 (2)°] are all expanded. Due to the presence of pseudo C2 symmetry, both B8 and N6 are similar to B7 and N5 in terms of geometry. The relatively long B—N bonds may be attributed to the coordination number of four around the B and N atoms, which, due to the bridging manner of the pyrrolidine groups, does not permit B—N π-bonding interactions, giving a bond order of about one. This is in contrast with an average B—N bond length of 1.48 Å observed in compounds having a B—N π interaction and a bond order of about two (Nöth et al., 1999). In addition, the electronegativity of the heterocyclic pyrrolidino groups also tends to inhibit π-bonding through their N atoms.

The four-membered heterocyclic ring has a bent nature. The dihedral angles between its N—B—N planes and between its B—N—B planes are 16.2 (3) and 15.4 (3)°, respectively. The pyrrolidine groups are puckered and adopt an envelope conformation with the N atoms at the flaps. The corresponding puckering parameters (Cremer & Pople, 1975) for the two rings are q2 = 0.438 (4) and 0.435 (3) Å, and ϕ2 = -178.9 (5) and -174.4 (5)°. The carbon plane of one ring (C13/C14/C15/C16) points up, while the plane of the other ring (C9/C10/C11/C12) points down. The deviations of the N atoms from the respective carbon planes are 0.665 (3) Å for N5 and 0.655 (3) Å for N6. The H atoms of the pyrrolidine rings are eclipsed. The substituents of the four-membered ring are slightly staggered, with torsion angles of 12.7 (3)° for C16—N6—B8—Br4 and 16.0 (3)° for C13—N6—B8—Br3, to decrease eclipsing interactions between H and Br atoms.

Experimental top

A solution of boron tribromide (9.92 g, 0.04 mol) in pentane (30 ml) was added over a period of 20 min to a vigorously stirred solution of tris(pyrrolidino)borane (4.38 g, 0.20 mol) in pentane (100 ml) at 273 K. After an additional 3 h the solution was allowed to warm to room temperature. The pentane was then removed by vacuum and the residue was distilled at 1 mm H g (1 mm H g = 133.322 Pa) to give the product as a clear liquid which, upon cooling below 273 K, gave colourless crystals of (I) in 90% yield (b.p. 403 K at 0.5 m mH g). Analysis calculated for C8H16N2B4Br4: C 19.96, H 3.35, N 5.82, Br 66.38%; found: C 20.00, H 3.67, N 5.79, Br 66.26%. 1H and 13C NMR spectra were recorded in CDCl3 solution using a Varian Unity (300 or 75 MHz), with Me4Si as an internal standard. 11B NMR spectra were recorded using a Bruker MSL-400 (128 MHz), with BF3·OEt2 as an external standard. Spectroscopic analysis: 1H NMR (CDCl3, δ, p.p.m.): 1.90 (t, 8H, CH2, 3JH—H = 6.9 Hz), 3.45 (t, 8H, CH2N, 3JH—H = 6.8 Hz); 13C{1H} NMR (CDCl3, δ, p.p.m.): 26.37 (CH2), 51.80 (CH2N); 11B NMR (CDCl3, δ, p.p.m.): 23.23.

Refinement top

H atoms were refined riding, with C—H = 0.9900 Å and Uiso = 1.2Ueq of their parent atoms. Please check this added sentence and if necessary please provide brief details of H-atom refinement.

Computing details top

Data collection: COLLECT (Nonius, 1999); cell refinement: DENZO (Otwinowski, 1985); data reduction: DENZO; program(s) used to solve structure: DIRDIF96 (Beurskens et al., 1996); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: SHELXL97.

Figures top
[Figure 1] Fig. 1. The molecular structure of (I), with displacement ellipsoids at the 50% probability level. H atoms are shown as small spheres of arbitrary radii.
Bis(µ-pyrrolidine-N:N)bis(dibromoboron) top
Crystal data top
C8H16B2Br4N2F(000) = 912
Mr = 481.49Dx = 2.316 Mg m3
Monoclinic, P21/cMo Kα radiation, λ = 0.71070 Å
a = 12.8750 (2) ÅCell parameters from 3271 reflections
b = 6.9960 (1) Åθ = 2.6–27.9°
c = 16.4480 (3) ŵ = 11.63 mm1
β = 111.2230 (8)°T = 110 K
V = 1381.05 (4) Å3Prism, colourless
Z = 40.20 × 0.15 × 0.10 mm
Data collection top
Nonius KappaCCD
diffractometer
3271 independent reflections
Radiation source: fine-focus sealed tube2850 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.041
Detector resolution: 56 µm pixels mm-1θmax = 27.9°, θmin = 2.6°
ϕ scansh = 016
Absorption correction: multi-scan
(SORTAV; Blessing, 1995)
k = 09
Tmin = 0.155, Tmax = 0.341l = 2120
5628 measured reflections
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.029H-atom parameters constrained
wR(F2) = 0.069 w = 1/[σ2(Fo2) + (0.0308P)2 + 3.5877P]
where P = (Fo2 + 2Fc2)/3
S = 1.01(Δ/σ)max = 0.019
3271 reflectionsΔρmax = 0.94 e Å3
145 parametersΔρmin = 0.81 e Å3
0 restraintsExtinction correction: SHELXL97 (Sheldrick, 1997), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
Primary atom site location: structure-invariant direct methodsExtinction coefficient: 0.00076 (17)
Crystal data top
C8H16B2Br4N2V = 1381.05 (4) Å3
Mr = 481.49Z = 4
Monoclinic, P21/cMo Kα radiation
a = 12.8750 (2) ŵ = 11.63 mm1
b = 6.9960 (1) ÅT = 110 K
c = 16.4480 (3) Å0.20 × 0.15 × 0.10 mm
β = 111.2230 (8)°
Data collection top
Nonius KappaCCD
diffractometer
3271 independent reflections
Absorption correction: multi-scan
(SORTAV; Blessing, 1995)
2850 reflections with I > 2σ(I)
Tmin = 0.155, Tmax = 0.341Rint = 0.041
5628 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0290 restraints
wR(F2) = 0.069H-atom parameters constrained
S = 1.01Δρmax = 0.94 e Å3
3271 reflectionsΔρmin = 0.81 e Å3
145 parameters
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Br10.57268 (3)0.10962 (5)0.40957 (2)0.01405 (9)
Br20.80023 (3)0.03289 (5)0.37569 (2)0.01522 (10)
Br30.94374 (3)0.34596 (5)0.56567 (2)0.01505 (9)
Br40.75048 (3)0.37905 (5)0.64711 (2)0.01568 (9)
N50.7858 (2)0.0199 (4)0.55275 (17)0.0076 (5)
N60.7032 (2)0.2675 (4)0.45883 (17)0.0090 (5)
B70.7156 (3)0.0412 (5)0.4509 (2)0.0099 (7)
B80.7944 (3)0.2495 (5)0.5556 (2)0.0099 (7)
C90.8905 (3)0.0988 (5)0.5792 (2)0.0126 (7)
H9A0.87490.22720.55210.015*
H9B0.94800.03590.56160.015*
C100.9284 (3)0.1128 (5)0.6782 (2)0.0213 (8)
H10A0.96540.23680.69910.026*
H10B0.98100.00830.70650.026*
C110.8213 (3)0.0950 (5)0.6983 (2)0.0192 (8)
H11A0.82540.01650.73650.023*
H11B0.80880.21170.72760.023*
C120.7286 (3)0.0689 (5)0.6099 (2)0.0147 (7)
H12A0.69460.19330.58590.018*
H12B0.66980.01680.61450.018*
C130.7291 (3)0.3975 (5)0.3953 (2)0.0151 (7)
H13A0.74840.52750.41990.018*
H13B0.79180.34650.38060.018*
C140.6225 (3)0.4004 (5)0.3155 (2)0.0202 (8)
H14A0.61410.52390.28440.024*
H14B0.62240.29590.27490.024*
C150.5282 (3)0.3724 (5)0.3511 (2)0.0190 (8)
H15A0.48150.26090.32330.023*
H15B0.48030.48740.34040.023*
C160.5881 (3)0.3387 (5)0.4495 (2)0.0131 (7)
H16A0.54820.24210.47110.016*
H16B0.59320.45890.48250.016*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Br10.01065 (17)0.01145 (17)0.01671 (17)0.00229 (12)0.00095 (13)0.00177 (13)
Br20.01612 (18)0.01887 (18)0.01300 (16)0.00500 (13)0.00805 (14)0.00079 (13)
Br30.00910 (17)0.01413 (17)0.02053 (18)0.00211 (12)0.00371 (14)0.00210 (14)
Br40.0219 (2)0.01401 (17)0.01275 (17)0.00307 (13)0.00820 (14)0.00306 (13)
N50.0074 (13)0.0055 (12)0.0088 (12)0.0011 (10)0.0015 (10)0.0011 (10)
N60.0087 (13)0.0101 (13)0.0089 (12)0.0020 (11)0.0041 (11)0.0010 (11)
B70.0074 (17)0.0110 (17)0.0107 (16)0.0018 (14)0.0026 (14)0.0004 (14)
B80.0127 (18)0.0048 (16)0.0119 (16)0.0001 (14)0.0041 (14)0.0027 (14)
C90.0110 (16)0.0105 (16)0.0137 (16)0.0052 (13)0.0015 (13)0.0003 (13)
C100.026 (2)0.0181 (19)0.0141 (17)0.0046 (16)0.0001 (15)0.0028 (15)
C110.029 (2)0.0163 (17)0.0108 (16)0.0002 (15)0.0049 (15)0.0036 (14)
C120.0194 (18)0.0124 (16)0.0154 (16)0.0051 (14)0.0101 (14)0.0005 (14)
C130.0177 (18)0.0147 (17)0.0118 (16)0.0019 (14)0.0039 (14)0.0046 (14)
C140.024 (2)0.0196 (19)0.0135 (17)0.0071 (15)0.0031 (15)0.0070 (15)
C150.0118 (18)0.0179 (18)0.0189 (18)0.0045 (14)0.0044 (14)0.0007 (15)
C160.0095 (16)0.0125 (16)0.0169 (16)0.0028 (13)0.0043 (13)0.0033 (14)
Geometric parameters (Å, º) top
Br1—B72.014 (4)C10—H10B0.9900
Br2—B71.990 (4)C11—C121.523 (5)
Br3—B81.988 (4)C11—H11A0.9900
Br4—B82.006 (4)C11—H11B0.9900
N5—C91.507 (4)C12—H12A0.9900
N5—C121.521 (4)C12—H12B0.9900
N5—B71.597 (4)C13—C141.517 (5)
N5—B81.610 (4)C13—H13A0.9900
N6—C131.511 (4)C13—H13B0.9900
N6—C161.517 (4)C14—C151.540 (6)
N6—B71.601 (4)C14—H14A0.9900
N6—B81.604 (4)C14—H14B0.9900
C9—C101.525 (5)C15—C161.537 (5)
C9—H9A0.9900C15—H15A0.9900
C9—H9B0.9900C15—H15B0.9900
C10—C111.535 (6)C16—H16A0.9900
C10—H10A0.9900C16—H16B0.9900
C9—N5—C12100.3 (2)C12—C11—C10105.0 (3)
C9—N5—B7117.5 (2)C12—C11—H11A110.8
C12—N5—B7118.0 (3)C10—C11—H11A110.8
C9—N5—B8119.8 (3)C12—C11—H11B110.8
C12—N5—B8115.7 (2)C10—C11—H11B110.8
B7—N5—B886.8 (2)H11A—C11—H11B108.8
C13—N6—C16100.6 (2)C11—C12—N5104.6 (3)
C13—N6—B7119.0 (2)C11—C12—H12A110.8
C16—N6—B7116.1 (3)N5—C12—H12A110.8
C13—N6—B8117.9 (3)C11—C12—H12B110.8
C16—N6—B8117.6 (2)N5—C12—H12B110.8
B7—N6—B886.8 (2)H12A—C12—H12B108.9
N5—B7—N692.4 (2)N6—C13—C14104.4 (3)
N5—B7—Br2113.5 (2)N6—C13—H13A110.9
N6—B7—Br2113.7 (2)C14—C13—H13A110.9
N5—B7—Br1113.9 (2)N6—C13—H13B110.9
N6—B7—Br1116.0 (2)C14—C13—H13B110.9
Br2—B7—Br1107.04 (17)H13A—C13—H13B108.9
N6—B8—N591.8 (2)C13—C14—C15105.1 (3)
N6—B8—Br3113.3 (2)C13—C14—H14A110.7
N5—B8—Br3113.3 (2)C15—C14—H14A110.7
N6—B8—Br4113.9 (2)C13—C14—H14B110.7
N5—B8—Br4115.9 (2)C15—C14—H14B110.7
Br3—B8—Br4108.11 (17)H14A—C14—H14B108.8
N5—C9—C10104.5 (3)C16—C15—C14104.8 (3)
N5—C9—H9A110.9C16—C15—H15A110.8
C10—C9—H9A110.9C14—C15—H15A110.8
N5—C9—H9B110.9C16—C15—H15B110.8
C10—C9—H9B110.9C14—C15—H15B110.8
H9A—C9—H9B108.9H15A—C15—H15B108.9
C9—C10—C11105.1 (3)N6—C16—C15104.9 (3)
C9—C10—H10A110.7N6—C16—H16A110.8
C11—C10—H10A110.7C15—C16—H16A110.8
C9—C10—H10B110.7N6—C16—H16B110.8
C11—C10—H10B110.7C15—C16—H16B110.8
H10A—C10—H10B108.8H16A—C16—H16B108.8
C9—N5—B7—N6133.3 (3)C9—N5—B8—N6131.1 (3)
C12—N5—B7—N6106.3 (3)C12—N5—B8—N6108.6 (3)
B8—N5—B7—N611.2 (2)B7—N5—B8—N611.1 (2)
C9—N5—B7—Br216.1 (4)C9—N5—B8—Br314.8 (3)
C12—N5—B7—Br2136.5 (2)C12—N5—B8—Br3135.1 (2)
B8—N5—B7—Br2106.0 (2)B7—N5—B8—Br3105.2 (2)
C9—N5—B7—Br1106.8 (3)C9—N5—B8—Br4111.1 (3)
C12—N5—B7—Br113.6 (4)C12—N5—B8—Br49.2 (4)
B8—N5—B7—Br1131.1 (2)B7—N5—B8—Br4128.9 (2)
C13—N6—B7—N5131.5 (3)C12—N5—C9—C1043.4 (3)
C16—N6—B7—N5108.1 (3)B7—N5—C9—C10172.7 (3)
B8—N6—B7—N511.2 (2)B8—N5—C9—C1084.3 (3)
C13—N6—B7—Br214.5 (4)N5—C9—C10—C1127.8 (3)
C16—N6—B7—Br2134.8 (2)C9—C10—C11—C120.8 (4)
B8—N6—B7—Br2105.8 (2)C10—C11—C12—N526.2 (3)
C13—N6—B7—Br1110.3 (3)C9—N5—C12—C1142.9 (3)
C16—N6—B7—Br110.0 (3)B7—N5—C12—C11171.9 (3)
B8—N6—B7—Br1129.4 (2)B8—N5—C12—C1187.4 (3)
C13—N6—B8—N5132.4 (3)C16—N6—C13—C1444.2 (3)
C16—N6—B8—N5106.8 (3)B7—N6—C13—C1483.8 (3)
B7—N6—B8—N511.1 (2)B8—N6—C13—C14173.5 (3)
C13—N6—B8—Br316.0 (3)N6—C13—C14—C1530.4 (4)
C16—N6—B8—Br3136.8 (2)C13—C14—C15—C164.5 (4)
B7—N6—B8—Br3105.3 (2)C13—N6—C16—C1541.2 (3)
C13—N6—B8—Br4108.1 (3)B7—N6—C16—C1588.7 (3)
C16—N6—B8—Br412.7 (3)B8—N6—C16—C15170.6 (3)
B7—N6—B8—Br4130.6 (2)C14—C15—C16—N622.9 (3)

Experimental details

Crystal data
Chemical formulaC8H16B2Br4N2
Mr481.49
Crystal system, space groupMonoclinic, P21/c
Temperature (K)110
a, b, c (Å)12.8750 (2), 6.9960 (1), 16.4480 (3)
β (°) 111.2230 (8)
V3)1381.05 (4)
Z4
Radiation typeMo Kα
µ (mm1)11.63
Crystal size (mm)0.20 × 0.15 × 0.10
Data collection
DiffractometerNonius KappaCCD
diffractometer
Absorption correctionMulti-scan
(SORTAV; Blessing, 1995)
Tmin, Tmax0.155, 0.341
No. of measured, independent and
observed [I > 2σ(I)] reflections
5628, 3271, 2850
Rint0.041
(sin θ/λ)max1)0.658
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.029, 0.069, 1.01
No. of reflections3271
No. of parameters145
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.94, 0.81

Computer programs: COLLECT (Nonius, 1999), DENZO (Otwinowski, 1985), DENZO, DIRDIF96 (Beurskens et al., 1996), SHELXL97 (Sheldrick, 1997), ORTEPIII (Burnett & Johnson, 1996), SHELXL97.

Selected geometric parameters (Å, º) top
Br1—B72.014 (4)N5—B71.597 (4)
Br2—B71.990 (4)N5—B81.610 (4)
Br3—B81.988 (4)N6—B71.601 (4)
Br4—B82.006 (4)N6—B81.604 (4)
B7—N5—B886.8 (2)N5—B7—N692.4 (2)
B7—N6—B886.8 (2)N6—B8—N591.8 (2)
B8—N5—B7—N611.2 (2)B8—N6—B7—N511.2 (2)
B8—N5—B7—Br2106.0 (2)B8—N6—B7—Br2105.8 (2)
B8—N5—B7—Br1131.1 (2)B8—N6—B7—Br1129.4 (2)
 

Subscribe to Acta Crystallographica Section C: Structural Chemistry

The full text of this article is available to subscribers to the journal.

If you have already registered and are using a computer listed in your registration details, please email support@iucr.org for assistance.

Buy online

You may purchase this article in PDF and/or HTML formats. For purchasers in the European Community who do not have a VAT number, VAT will be added at the local rate. Payments to the IUCr are handled by WorldPay, who will accept payment by credit card in several currencies. To purchase the article, please complete the form below (fields marked * are required), and then click on `Continue'.
E-mail address* 
Repeat e-mail address* 
(for error checking) 

Format*   PDF (US $40)
   HTML (US $40)
   PDF+HTML (US $50)
In order for VAT to be shown for your country javascript needs to be enabled.

VAT number 
(non-UK EC countries only) 
Country* 
 

Terms and conditions of use
Contact us

Follow Acta Cryst. C
Sign up for e-alerts
Follow Acta Cryst. on Twitter
Follow us on facebook
Sign up for RSS feeds