Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270104030410/gg1227sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270104030410/gg1227IIIsup2.hkl |
CCDC reference: 263063
A solution of Li2[PhB(NtBu)2] (1.00 g, 4.10 mmol) in Et2O (50 ml) was added to a stirred solution of BF3·OEt2 (0.25 ml, 2.05 mmol) in Et2O (50 ml) at 195 K, producing a bright-pink solution. The reaction mixture was allowed to warm to room temperature, whereupon it became a clear colorless solution. After 18 h, the reaction mixture was filtered (Acrodisc syringe filter, diameter 25 mm, pore size 0.45 µm), followed by concentration of the solvent in vacuo. Subsequent cooling (273 K, 18 h) of the resulting violet solution yielded colorless crystals of the title compound, [PhF2B3N3tBu3], (III) (0.30 g). Analysis calculated for C18H32B3F2N3: C 59.90, H 8.94, N 11.64%; found: C 59.48, H 8.45, N 11.76%. Spectroscopic analysis: 1H NMR (D8-thf, 296 K, δ, p.p.m): 7.73–7.22 (m, 5 H, –C6H5), 1.40 (s, 9 H, –C4H9), 1.19 (s, 18 H, –C4H9); 11B NMR (D8-thf, 296 K, δ, p.p.m.): 37 [d, 1J(BF) = 278 Hz], 25 (br s); 19F NMR (D8-thf, 296 K, δ, p.p.m.): −88.9 (s). The Et2O solvent was dried with appropriate drying agents and distilled onto molecular sieves before use. The reaction and manipulation of the moisture- and air-sensitive product were carried out under an atmosphere of argon or under vacuum. All glassware was carefully dried prior to use. The compound Li2[PhB(NtBu)2] was prepared by the literature procedure of Chivers et al. (2002), while BF3·OEt2 was a commercial sample from Aldrich and used as received.
The molecule of (III) lies on a twofold axis, with one of tBu groups disordered over two sites with 0.50 site-occupancy factors. The H atoms were included in the refinement at geometrically idealized positions, with C—H = 0.95 and 0.98 Å and Uiso = 1.5 (methyl) and 1.2 (phenyl) times Ueq of the atoms to which they were bonded. The final difference map was free of any chemically significant features, with the top seven peaks located in bonds.
Data collection: COLLECT (Hooft, 1998); cell refinement: HKL DENZO (Otwinowski & Minor, 1997); data reduction: SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SAPI91 (Fan, 1991); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: SHELXL97 (Sheldrick, 1997).
C18H32B3F2N3 | F(000) = 776 |
Mr = 360.90 | Dx = 1.169 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71069 Å |
Hall symbol: -C 2yc | Cell parameters from 4451 reflections |
a = 13.653 (4) Å | θ = 2.5–25.5° |
b = 13.637 (5) Å | µ = 0.08 mm−1 |
c = 11.893 (4) Å | T = 173 K |
β = 112.19 (3)° | Block, colorless |
V = 2050.3 (13) Å3 | 0.30 × 0.25 × 0.20 mm |
Z = 4 |
Nonius KappaCCD area-detector diffractometer | 2327 independent reflections |
Radiation source: fine-focus sealed tube | 2052 reflections with I > 2.0 σ(I) |
Graphite monochromator | Rint = 0.019 |
ω and ϕ scans | θmax = 27.5°, θmin = 2.5° |
Absorption correction: multi-scan (SORTAV; Blessing, 1997) | h = −17→17 |
Tmin = 0.977, Tmax = 0.984 | k = −17→17 |
4451 measured reflections | l = −15→15 |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.043 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.124 | H-atom parameters constrained |
S = 1.05 | w = 1/[σ2(Fo2) + (0.0664P)2 + 0.77P] where P = (Fo2 + 2Fc2)/3 |
2327 reflections | (Δ/σ)max < 0.001 |
140 parameters | Δρmax = 0.33 e Å−3 |
0 restraints | Δρmin = −0.18 e Å−3 |
C18H32B3F2N3 | V = 2050.3 (13) Å3 |
Mr = 360.90 | Z = 4 |
Monoclinic, C2/c | Mo Kα radiation |
a = 13.653 (4) Å | µ = 0.08 mm−1 |
b = 13.637 (5) Å | T = 173 K |
c = 11.893 (4) Å | 0.30 × 0.25 × 0.20 mm |
β = 112.19 (3)° |
Nonius KappaCCD area-detector diffractometer | 2327 independent reflections |
Absorption correction: multi-scan (SORTAV; Blessing, 1997) | 2052 reflections with I > 2.0 σ(I) |
Tmin = 0.977, Tmax = 0.984 | Rint = 0.019 |
4451 measured reflections |
R[F2 > 2σ(F2)] = 0.043 | 0 restraints |
wR(F2) = 0.124 | H-atom parameters constrained |
S = 1.05 | Δρmax = 0.33 e Å−3 |
2327 reflections | Δρmin = −0.18 e Å−3 |
140 parameters |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
F1 | 0.31190 (5) | 0.36127 (5) | 0.64973 (8) | 0.0511 (2) | |
N1 | 0.41503 (6) | 0.50626 (6) | 0.66174 (8) | 0.0264 (2) | |
N2 | 0.5000 | 0.34621 (8) | 0.7500 | 0.0291 (3) | |
C1 | 0.5000 | 0.67805 (10) | 0.7500 | 0.0289 (3) | |
C2 | 0.42842 (9) | 0.73129 (8) | 0.78457 (10) | 0.0347 (3) | |
H2 | 0.3783 | 0.6972 | 0.8077 | 0.042* | |
C3 | 0.42918 (10) | 0.83327 (9) | 0.78575 (11) | 0.0449 (3) | |
H3 | 0.3809 | 0.8681 | 0.8112 | 0.054* | |
C4 | 0.5000 | 0.88388 (12) | 0.7500 | 0.0505 (5) | |
H4 | 0.5000 | 0.9535 | 0.7500 | 0.061* | |
C5 | 0.32942 (8) | 0.54496 (7) | 0.54643 (9) | 0.0302 (2) | |
C6 | 0.36083 (10) | 0.63963 (9) | 0.49972 (10) | 0.0410 (3) | |
H6A | 0.3104 | 0.6530 | 0.4172 | 0.062* | |
H6B | 0.4320 | 0.6324 | 0.4990 | 0.062* | |
H6C | 0.3602 | 0.6942 | 0.5531 | 0.062* | |
C7 | 0.22715 (9) | 0.56292 (10) | 0.56699 (12) | 0.0438 (3) | |
H7A | 0.2404 | 0.6097 | 0.6337 | 0.066* | |
H7B | 0.2018 | 0.5009 | 0.5877 | 0.066* | |
H7C | 0.1735 | 0.5897 | 0.4928 | 0.066* | |
C8 | 0.31327 (11) | 0.47009 (10) | 0.44429 (11) | 0.0484 (3) | |
H8A | 0.2590 | 0.4942 | 0.3688 | 0.073* | |
H8B | 0.2906 | 0.4073 | 0.4664 | 0.073* | |
H8C | 0.3799 | 0.4610 | 0.4325 | 0.073* | |
C9 | 0.5000 | 0.23477 (11) | 0.7500 | 0.0367 (4) | |
C10 | 0.4554 (3) | 0.20132 (19) | 0.6069 (2) | 0.0499 (6) | 0.50 |
H10A | 0.4558 | 0.1296 | 0.6017 | 0.075* | 0.50 |
H10B | 0.5009 | 0.2288 | 0.5677 | 0.075* | 0.50 |
H10C | 0.3831 | 0.2256 | 0.5657 | 0.075* | 0.50 |
C11 | 0.4268 (2) | 0.19741 (17) | 0.8029 (3) | 0.0454 (6) | 0.50 |
H11A | 0.4238 | 0.1257 | 0.7969 | 0.068* | 0.50 |
H11B | 0.3562 | 0.2246 | 0.7589 | 0.068* | 0.50 |
H11C | 0.4513 | 0.2168 | 0.8885 | 0.068* | 0.50 |
C12 | 0.6063 (2) | 0.18882 (19) | 0.8039 (4) | 0.0608 (8) | 0.50 |
H12A | 0.6402 | 0.2086 | 0.8891 | 0.091* | 0.50 |
H12B | 0.6498 | 0.2102 | 0.7591 | 0.091* | 0.50 |
H12C | 0.5990 | 0.1173 | 0.7990 | 0.091* | 0.50 |
B1 | 0.5000 | 0.56175 (11) | 0.7500 | 0.0252 (3) | |
B2 | 0.40799 (9) | 0.40355 (8) | 0.68697 (11) | 0.0301 (3) |
U11 | U22 | U33 | U12 | U13 | U23 | |
F1 | 0.0286 (4) | 0.0348 (4) | 0.0757 (6) | −0.0075 (3) | 0.0038 (3) | 0.0124 (3) |
N1 | 0.0256 (4) | 0.0236 (4) | 0.0288 (4) | 0.0014 (3) | 0.0089 (3) | 0.0023 (3) |
N2 | 0.0297 (6) | 0.0194 (5) | 0.0367 (6) | 0.000 | 0.0111 (5) | 0.000 |
C1 | 0.0326 (7) | 0.0233 (6) | 0.0281 (7) | 0.000 | 0.0083 (5) | 0.000 |
C2 | 0.0372 (6) | 0.0302 (5) | 0.0343 (5) | 0.0044 (4) | 0.0108 (4) | −0.0012 (4) |
C3 | 0.0500 (7) | 0.0323 (6) | 0.0424 (6) | 0.0128 (5) | 0.0062 (5) | −0.0052 (5) |
C4 | 0.0608 (11) | 0.0222 (7) | 0.0504 (10) | 0.000 | 0.0005 (8) | 0.000 |
C5 | 0.0291 (5) | 0.0294 (5) | 0.0286 (5) | 0.0020 (4) | 0.0070 (4) | 0.0018 (4) |
C6 | 0.0460 (6) | 0.0392 (6) | 0.0317 (6) | −0.0020 (5) | 0.0076 (5) | 0.0096 (4) |
C7 | 0.0302 (6) | 0.0532 (7) | 0.0459 (7) | 0.0101 (5) | 0.0121 (5) | 0.0113 (5) |
C8 | 0.0549 (8) | 0.0444 (7) | 0.0352 (6) | 0.0028 (6) | 0.0048 (5) | −0.0080 (5) |
C9 | 0.0383 (8) | 0.0194 (7) | 0.0552 (10) | 0.000 | 0.0207 (7) | 0.000 |
C10 | 0.0781 (19) | 0.0321 (12) | 0.0464 (14) | −0.0162 (12) | 0.0315 (13) | −0.0127 (10) |
C11 | 0.0683 (17) | 0.0241 (10) | 0.0586 (16) | −0.0009 (10) | 0.0406 (14) | 0.0034 (10) |
C12 | 0.0478 (15) | 0.0261 (12) | 0.098 (3) | 0.0047 (10) | 0.0155 (15) | −0.0002 (14) |
B1 | 0.0274 (7) | 0.0232 (7) | 0.0279 (7) | 0.000 | 0.0137 (6) | 0.000 |
B2 | 0.0274 (6) | 0.0262 (6) | 0.0348 (6) | −0.0026 (4) | 0.0096 (5) | −0.0002 (4) |
F1—B2 | 1.3457 (13) | C7—H7A | 0.9800 |
N1—B2 | 1.4433 (15) | C7—H7B | 0.9800 |
N1—B1 | 1.4493 (12) | C7—H7C | 0.9800 |
N1—C5 | 1.5214 (14) | C8—H8A | 0.9800 |
N2—B2i | 1.4283 (14) | C8—H8B | 0.9800 |
N2—B2 | 1.4283 (13) | C8—H8C | 0.9800 |
N2—C9 | 1.5197 (18) | C9—C11 | 1.459 (2) |
C1—C2i | 1.3981 (13) | C9—C11i | 1.459 (2) |
C1—C2 | 1.3981 (13) | C9—C12i | 1.486 (3) |
C1—B1 | 1.586 (2) | C9—C12 | 1.486 (3) |
C2—C3 | 1.3907 (17) | C9—C10 | 1.641 (3) |
C2—H2 | 0.9500 | C9—C10i | 1.641 (3) |
C3—C4 | 1.3794 (17) | C10—H10A | 0.9800 |
C3—H3 | 0.9500 | C10—H10B | 0.9800 |
C4—C3i | 1.3794 (17) | C10—H10C | 0.9800 |
C4—H4 | 0.9500 | C11—H11A | 0.9800 |
C5—C7 | 1.5256 (16) | C11—H11B | 0.9800 |
C5—C6 | 1.5295 (16) | C11—H11C | 0.9800 |
C5—C8 | 1.5377 (16) | C12—H12A | 0.9800 |
C6—H6A | 0.9800 | C12—H12B | 0.9800 |
C6—H6B | 0.9800 | C12—H12C | 0.9800 |
C6—H6C | 0.9800 | B1—N1i | 1.4493 (12) |
N1—B1—N1i | 117.05 (12) | H8A—C8—H8B | 109.5 |
N1—B1—C1 | 121.48 (6) | C5—C8—H8C | 109.5 |
N1i—B1—C1 | 121.48 (6) | H8A—C8—H8C | 109.5 |
F1—B2—N2 | 119.54 (10) | H8B—C8—H8C | 109.5 |
F1—B2—N1 | 118.72 (9) | C11—C9—C11i | 139.1 (2) |
N2—B2—N1 | 121.74 (9) | C11—C9—C12i | 47.34 (19) |
B2—N1—B1 | 117.01 (9) | C11i—C9—C12i | 112.53 (18) |
B2—N1—C5 | 115.80 (8) | C11—C9—C12 | 112.53 (18) |
B1—N1—C5 | 127.16 (9) | C11i—C9—C12 | 47.34 (19) |
B2i—N2—B2 | 113.62 (12) | C12i—C9—C12 | 130.1 (2) |
B2i—N2—C9 | 123.19 (6) | C11—C9—N2 | 110.43 (11) |
B2—N2—C9 | 123.19 (6) | C11i—C9—N2 | 110.43 (11) |
C2i—C1—C2 | 117.43 (14) | C12i—C9—N2 | 114.95 (12) |
C2i—C1—B1 | 121.28 (7) | C12—C9—N2 | 114.95 (12) |
C2—C1—B1 | 121.28 (7) | C11—C9—C10 | 107.72 (16) |
C3—C2—C1 | 121.25 (11) | C11i—C9—C10 | 60.10 (17) |
C3—C2—H2 | 119.4 | C12i—C9—C10 | 61.07 (19) |
C1—C2—H2 | 119.4 | C12—C9—C10 | 104.43 (19) |
C4—C3—C2 | 120.05 (12) | N2—C9—C10 | 106.14 (10) |
C4—C3—H3 | 120.0 | C11—C9—C10i | 60.10 (17) |
C2—C3—H3 | 120.0 | C11i—C9—C10i | 107.72 (16) |
C3—C4—C3i | 119.95 (16) | C12i—C9—C10i | 104.43 (19) |
C3—C4—H4 | 120.0 | C12—C9—C10i | 61.07 (19) |
C3i—C4—H4 | 120.0 | N2—C9—C10i | 106.14 (10) |
N1—C5—C7 | 110.44 (9) | C10—C9—C10i | 147.7 (2) |
N1—C5—C6 | 113.26 (9) | C9—C10—H10A | 109.5 |
C7—C5—C6 | 108.78 (10) | C9—C10—H10B | 109.5 |
N1—C5—C8 | 108.30 (9) | H10A—C10—H10B | 109.5 |
C7—C5—C8 | 111.49 (10) | C9—C10—H10C | 109.5 |
C6—C5—C8 | 104.46 (10) | H10A—C10—H10C | 109.5 |
C5—C6—H6A | 109.5 | H10B—C10—H10C | 109.5 |
C5—C6—H6B | 109.5 | C9—C11—H11A | 109.5 |
H6A—C6—H6B | 109.5 | C9—C11—H11B | 109.5 |
C5—C6—H6C | 109.5 | H11A—C11—H11B | 109.5 |
H6A—C6—H6C | 109.5 | C9—C11—H11C | 109.5 |
H6B—C6—H6C | 109.5 | H11A—C11—H11C | 109.5 |
C5—C7—H7A | 109.5 | H11B—C11—H11C | 109.5 |
C5—C7—H7B | 109.5 | C9—C12—H12A | 109.5 |
H7A—C7—H7B | 109.5 | C9—C12—H12B | 109.5 |
C5—C7—H7C | 109.5 | H12A—C12—H12B | 109.5 |
H7A—C7—H7C | 109.5 | C9—C12—H12C | 109.5 |
H7B—C7—H7C | 109.5 | H12A—C12—H12C | 109.5 |
C5—C8—H8A | 109.5 | H12B—C12—H12C | 109.5 |
C5—C8—H8B | 109.5 | ||
C2i—C1—C2—C3 | −0.67 (8) | B2—N2—C9—C10 | 55.75 (13) |
B1—C1—C2—C3 | 179.33 (8) | B2i—N2—C9—C10i | 55.75 (13) |
C1—C2—C3—C4 | 1.34 (16) | B2—N2—C9—C10i | −124.25 (13) |
C2—C3—C4—C3i | −0.66 (8) | B2—N1—B1—N1i | 15.16 (6) |
B2—N1—C5—C7 | 78.63 (11) | C5—N1—B1—N1i | −167.00 (10) |
B1—N1—C5—C7 | −99.24 (11) | B2—N1—B1—C1 | −164.84 (6) |
B2—N1—C5—C6 | −159.09 (9) | C5—N1—B1—C1 | 13.00 (10) |
B1—N1—C5—C6 | 23.05 (13) | C2i—C1—B1—N1 | −109.58 (7) |
B2—N1—C5—C8 | −43.73 (12) | C2—C1—B1—N1 | 70.42 (7) |
B1—N1—C5—C8 | 138.41 (9) | C2i—C1—B1—N1i | 70.42 (7) |
B2i—N2—C9—C11 | 119.27 (15) | C2—C1—B1—N1i | −109.58 (7) |
B2—N2—C9—C11 | −60.73 (15) | B2i—N2—B2—F1 | −163.58 (13) |
B2i—N2—C9—C11i | −60.73 (15) | C9—N2—B2—F1 | 16.42 (13) |
B2—N2—C9—C11i | 119.27 (15) | B2i—N2—B2—N1 | 16.46 (7) |
B2i—N2—C9—C12i | 170.64 (19) | C9—N2—B2—N1 | −163.54 (7) |
B2—N2—C9—C12i | −9.36 (19) | B1—N1—B2—F1 | 147.38 (9) |
B2i—N2—C9—C12 | −9.36 (19) | C5—N1—B2—F1 | −30.71 (14) |
B2—N2—C9—C12 | 170.64 (19) | B1—N1—B2—N2 | −32.66 (13) |
B2i—N2—C9—C10 | −124.25 (13) | C5—N1—B2—N2 | 149.25 (9) |
Symmetry code: (i) −x+1, y, −z+3/2. |
Experimental details
Crystal data | |
Chemical formula | C18H32B3F2N3 |
Mr | 360.90 |
Crystal system, space group | Monoclinic, C2/c |
Temperature (K) | 173 |
a, b, c (Å) | 13.653 (4), 13.637 (5), 11.893 (4) |
β (°) | 112.19 (3) |
V (Å3) | 2050.3 (13) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.08 |
Crystal size (mm) | 0.30 × 0.25 × 0.20 |
Data collection | |
Diffractometer | Nonius KappaCCD area-detector diffractometer |
Absorption correction | Multi-scan (SORTAV; Blessing, 1997) |
Tmin, Tmax | 0.977, 0.984 |
No. of measured, independent and observed [I > 2.0 σ(I)] reflections | 4451, 2327, 2052 |
Rint | 0.019 |
(sin θ/λ)max (Å−1) | 0.650 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.043, 0.124, 1.05 |
No. of reflections | 2327 |
No. of parameters | 140 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.33, −0.18 |
Computer programs: COLLECT (Hooft, 1998), HKL DENZO (Otwinowski & Minor, 1997), SCALEPACK (Otwinowski & Minor, 1997), SAPI91 (Fan, 1991), SHELXL97 (Sheldrick, 1997), ORTEPII (Johnson, 1976).
F1—B2 | 1.3457 (13) | N2—B2i | 1.4283 (14) |
N1—B2 | 1.4433 (15) | N2—C9 | 1.5197 (18) |
N1—B1 | 1.4493 (12) | C1—B1 | 1.586 (2) |
N1—C5 | 1.5214 (14) | ||
N1—B1—N1i | 117.05 (12) | B2—N1—B1 | 117.01 (9) |
N1—B1—C1 | 121.48 (6) | B2—N1—C5 | 115.80 (8) |
N1i—B1—C1 | 121.48 (6) | B1—N1—C5 | 127.16 (9) |
F1—B2—N2 | 119.54 (10) | B2i—N2—B2 | 113.62 (12) |
F1—B2—N1 | 118.72 (9) | B2i—N2—C9 | 123.19 (6) |
N2—B2—N1 | 121.74 (9) | B2—N2—C9 | 123.19 (6) |
B2—N1—B1—N1i | 15.16 (6) | B2i—N2—B2—N1 | 16.46 (7) |
C5—N1—B1—N1i | −167.00 (10) | C9—N2—B2—N1 | −163.54 (7) |
B2—N1—B1—C1 | −164.84 (6) | B1—N1—B2—F1 | 147.38 (9) |
C5—N1—B1—C1 | 13.00 (10) | C5—N1—B2—F1 | −30.71 (14) |
B2i—N2—B2—F1 | −163.58 (13) | B1—N1—B2—N2 | −32.66 (13) |
C9—N2—B2—F1 | 16.42 (13) | C5—N1—B2—N2 | 149.25 (9) |
Symmetry code: (i) −x+1, y, −z+3/2. |
Recently, we reported the first boraamidinate [RB(NR')2]2− complexes of group 13 elements. We described the syntheses, spectroscopic characterization and X-ray structures of [µ-Li(OEt2){PhB(µ-NtBu)2}2E], (I) (E = Ga, In), in which the spirocyclic anion [{PhB(µ-NtBu)2}2E]− is N,N'-chelated to a monosolvated lithium cation (Chivers et al., 2003), and the solvent-separated ion-pair complexes [Li(OEt2)4][{PhB(µ-NDipp)2}2E], (II) (E = Ga, In; Dipp = 2,6-diisopropylphenyl), where the spirocyclic anion [{PhB(µ-NDipp)2}2E]− is stabilized by the steric bulk provided by the four Dipp substituents attached to the N atoms (Chivers et al., 2004). In an effort to synthesize the boron analogue of (I), E = B, the dilithiated boraamidinate {Li2[PhB(NtBu)2]}2 (Brask et al., 2002) was treated with BF3·OEt2 in a 2:1 molar ratio. A single-crystal X-ray diffraction experiment revealed that, rather than the expected complex, (I) (E = B), the asymmetrically substituted borazine derivative [PhF2B3N3tBu3], (III), was isolated. Borazines serve as precursor molecules in boron nitride (BN) materials science, since they are relatively robust when compared with other boron-nitrogen compounds (Haberecht et al., 2004). We present here the crystal structure of (III).
The molecule of (III) resides on a twofold axis which trisects atoms C9, N2, B1 and the phenyl group. A view of the structure of (III) is shown in Fig. 1, with selected bond distances and angles listed in Table 1. The B3N3 ring in (III) is distorted from an ideal planar structure, with torsion angles B2—N1—B1—N1i = 15.16 (6)°, B2i—N2—B2—N1 = 16.46 (7)° and B1—N1—B2—N2 = −32.66 (13)° [symmetry code (i) as in Table 1]. Atoms N1 and N1i lie 0.3477 (14) Å above and below the least-squares plane formed by B1/B2/B2i, while atom N2 does not deviate from this plane. In (III), steric congestion forces one tert-butyl substituent [α-carbon is atom C5] to reside on one side of the ring, while the symmetry-related tert-butyl group [α-carbon is atom C5i] sits below the plane. As a result, distortion of the six-membered B3N3 ring occurs, in which atoms N1 and N1i are on opposite sides of the B1/B2/B2i/N2 plane.
Most structurally characterized borazine compounds are symmetrically substituted (i.e. the three B atoms have the same substituent and identical atoms or groups are attached to the three N atoms) and tend to have planar B3N3 rings, as in the compound [HB-NPh]3 (Jaska et al., 2003). Very few asymmetrically substituted borazines have been characterized by single-crystal X-ray analyses (Jaschke et al., 2002; Nöth & Habereder, 2001; Srivastova et al., 1998; Welker et al., 1989). One such example is compound (IV) (Jaschke et al., 2002), in which five of the B3N3 ring substituents are identical to those in (III). The B3N3 rings in (IV) adopt boat conformations. For example, atoms B1 and N5 (in one ring) are 0.598 and 0.355 Å, respectively, out of the plane formed by the other four ring atoms, N4/B2/B3/N6. Perusal of borazine compounds, whether symmetrically or asymmetrically substituted, characterized in the solid state (Cambridge Structural Database, Version 5.25, 2003 release; Allen, 2002), reveals that the conformation of the central B3N3 ring in (III) has not previously been observed. To date, planar (or very slightly deviating from planar), chair (ideal and puckered), and boat (ideal and twisted) conformations of the B3N3 ring have been reported.
Although the B3N3 ring in [Ph3GeMe2B3N3iPr3] adopts a twisted-boat conformation, resulting from steric repulsion between the Ph3Ge group and the isopropyl groups attached to the N atoms, the B—N bond lengths [1.439 (4)–1.451 (4) Å] remain almost unaffected by this twisting (Nöth & Habereder, 2001). In contrast, the B—N distances in the title compound range from 1.428 (1) to 1.449 (1) Å. The longest B—N bond lengths are B1—N1 and B2—N1, which are similar within experimental error [1.4433 (15) and 1.4493 (12) Å], despite the up-down distortion at N1 and N1i. The shortest B—N bond length is B2—N2 [1.4283 (14) Å], due to the F substituents that reside on these B atoms and the resulting π-interaction. This variability in bond lengths within the B3N3 ring is also observed in (IV) [1.408 (3)–1.491 (3) Å], on account of the F substituents on two of the three B atoms and the boat conformation, which lengthens the B1—N4 and B1—N6 [average 1.488 (3) Å], and B3—N5 and B2—N5 [average 1.441 (3) Å] distances. In the symmetrically substituted borazine [HB-NPh]3, the B—N—B angles [120.6 (2)–121.1 (2)°] and N—B—N angles [118.7 (2)–119.4 (2)°] are all close to 120° (Jaska et al., 2003), while those in (III) deviate from 120°. The B2i—N1i—B1, B2—N1—B1 and N1—B1—N1i angles are 117.1°, while the largest deviation occurs at N2, with a B2i—N2—B2 angle of 113.6 (1)°. This deviation is also observed in (IV), with a range of 110.2 (2)–120.1 (2)°. The phenyl group in (III) is twisted with respect to the B3N3 ring, with an angle of 62.87 (5)° between the two planes. This angle is larger than those observed in the borazine [HB-NPh]3 (48.8, 43.7 and 42.3°; Jaska et al., 2003), but is similar to those in the structure of hexaphenylborazine [PhB-NPh]3, which has angles between the ring planes in the range 62.5–71.4° (Lux et al., 1979). The structure is devoid of any C—H···F type intermolecular interactions, due to the disposition of the tBu groups preventing such interactions.