the values of s for 2 are similar to previously observed values
(Table 1), the angle for 3 reveals a structure that is, uniquely,
much closer to being eclipsed (as opposed to staggered).
Treatment of 2 or 3 in toluene with four equivalents of n-
BuLi revealed a broadening of the 11B NMR resonance and a
slight up-field chemical shift change to d 31.2 and 28.8 ppm
respectively as well as the appearance of some minor peaks in
the region associated with four-coordinate boron centres.
After work up and recrystallisation from thf (tetrahydrofu-
ran), crystalline samples of salts containing the imidodiborate(4)
anions, namely Li4(thf)6B2(NPh)4 (6) and Li4(thf)4B2(N-2,6-
Me2C6H3)4 (7)¶(Scheme 2) were isolated both of which were
characterised by X-ray crystallography;* the structure of 7 is
shown in Fig. 2. Both structures are very similar consisting
of a tetra-imidodiborate tetra-anion with the four Li centres
complexing pairs of imido arms and being additionally solvated
by thf molecules. To our knowledge 6 and 7 are the first
structurally characterised examples not only of imidodiborate(4)
anions but also from the family of imido analogues of p-block
oxoanions in which an element–element bond is present in the
central unit.9
in the presence of the ion-contacted lithium atoms which bridge
the imido nitrogen centres in a 1,2-fashion above and below the
B2N4 plane in both imidodiborate structures thereby increasing
the N–B–N angles with a consequent lengthening of the B–B
bond. In 6 a total of six thf molecules are coordinated to the
four lithium centres whereas for 7 it is only four presumably as
a result of the larger steric bulk of the aryl group in the latter.
We note that there is apparently very little previous work on
the deprotonation of primary amido diborane(4) compounds
although a recent study by Patton and co-workers10 has explored
the use of the dianions [B2(NMe2)2(NR)2]2−, derived from
B2(NMe2)2(NHR)2 (R = 2,6-disubstituted aryl), as ligands
towards early transition metal centres. We note also the related
work of Power11 and No¨th7 on the reduction of B2Ph2(NMe2)2
and B2(NMe2)2(NR2)2 respectively affording the corresponding
di-anions, an example being [Li(Et2O)]2[B2(NMe2)2(pyrrolyl)2],7
=
featuring B B double bonding. It is noteworthy in the examples
described by No¨th that on reduction, and compared to their
neutral precursors, B–N bond distances get longer and the
B2N4 units become planar, as seen for 6 and 7, whereas the
B–B bonds get shorter and N–B–N angles decrease slightly.
Finally we note the work on the formation of the related
imidoborate anions [B(NR)3]3− reported by Russell12 and No¨th13
which represent imido analogues of the borate BO33−, and note
also that the oxo-anion related to 6/7, i.e. B2O44− is not currently
known.
Acknowledgements
We thank the EPSRC and the Royal Society (CAR) for financial
support.
Notes and references
† In a typical reaction, a solution of B2(NMe2)4 (0.396 g, 2.0 mmol)
and aniline (0.920 g, 9.9 mmol) in toluene (20 cm3) were refluxed
under dinitrogen for 24 h. White crystals formed on cooling which were
recrystallised from hexane/thf mixtures affording colourless crystals of
1
B2(NHPh)4 (2) (93%). NMR (toluene, ppm): 11B d 33.7; H d 7.02 (m,
20H, Ph), 0.28 (s, 4H, NH). IR (solid) m(N–H) 3358 cm−1. MS(EI) m/z
390. C24H24B2N4 requires C, 73.90; H, 6.20; N, 14.35; found C, 73.90,
H, 6.40; N, 14.30%. Data for 3: 80%. 11B NMR (CDCl3, ppm) d 29.7.
IR (solid) m(N–H) 3395, 3330 cm−1. HRMS(EI) m/z 502.34391 (calc.
502.32440). C32H40B2N4 requires C, 76.50; H, 8.05; N, 11.15; found C,
72.90, H, 8.10; N, 10.60%. Data for 4: 46%. 11B NMR (toluene, ppm)
d 32.4. IR (solid) m(N–H) 3343 cm−1. MS(EI) m/z 894. C24H20B2N4I4
requires C, 32.25; H, 2.25; N, 6.25; found C, 33.00, H, 3.60; N, 7.05%.
Data for 5: 80%. 11B NMR (CDCl3, ppm) d 33.1. IR (CH2Cl2) m(N–
H) 3441, 3392 cm−1. HRMS(EI) m/z 446.28131 (calc. 446.21604).
C28H32B2N4 requires C, 75.35; H, 7.20; N, 12.55; found C, 75.20, H,
7.55; N, 12.50%.
Fig. 2 A view of the molecular structure of compound 7. Se-
lected bond distances (A) and angles (◦) include: B(1)–B(1A)
˚
‡ The structure of B2(NMe2)(NH-2,6-Pri2C6H3)3 was confirmed by
X-ray crystallography, details of which will be reported in a future
publication; C. Feng, unpublished results.
1.814(4), B(1)–N(1) 1.442(2), B(1)–N(2) 1.445(2), Li(1)–N(1) 1.963(3),
Li(1)–N(2A) 1.963(3), Li(2)–N(1) 2.054(3), Li(2)–N(2A) 2.057(3);
N(1)–B(1)–N(2) 133.96(16).
¯
§ Crystal data for 2: C24H24B2N4, M = 390.09, triclinic, space group P1,
˚
a = 11.484(11), b = 11.494(9), c = 15.965(12) A, a = 83.96(4), b =
◦
3
−3
˚
75.16(4), c = 89.92(5) , U = 2025(3) A , Z = 4, Dc = 1.279 Mg m
,
k = 0.71073 A, l(Mo-Ka) = 0.582 mm−1, F(000) = 824, T = 100(2)
˚
K, R1 = 0.0421 [for 4711 reflections with I > 2r(I)].14–17 Crystal data
for 3: C32H40B2N4, M = 502.30, orthorhombic, space group Pccn, a =
3
˚
˚
19.740(4), b = 8.8681(18), c = 16.550(3) A, U = 2897.1(10) A , Z = 4,
Dc = 1.152 Mg m−3, k = 0.71073 A, l(Mo-Ka) = 0.067 mm−1, F(000) =
˚
1080, T = 293(2) K, R1 = 0.0545 [for 2499 reflections with I > 2r(I)].14–17
¶ To a cooled (−78 ◦C) solution of 2 (0.262 g, 0.7 mmol) in toluene
(15 cm3), n-BuLi (1.7 cm3 of a 1.6 M solution, 2.7 mmol) was added
and the resulting pale yellow coloured solution allowed to warm to
room temperature with stirring. After 40 min, the reaction mixture
was brought briefly to reflux and then left to stir for 3 h. Removal
of the toluene afforded a yellow powder which was recrystallised from
cooled thf solutions affording 6 as colourless crystals. Compound 7 was
prepared in an analogous fashion from 3. Isolated crystalline yields of
both 6 and 7 were low preventing satisfactory elemental analytical data
from being obtained although monitoring of both reactions by 11B NMR
revealed little in the way of side products.
Scheme 2 6, R = Ph; 7, R = 2,6-Me2C6H3.
In the unit cells of 6 and 7 the imidodiborate(4) tetra-anion lies
on a crystallographic centre of inversion such that the B2N4 units
are strictly planar (i.e. s = 0◦). The B–B distances (6, 1.781(4)
˚
˚
˚
A and 7, 1.814(4) A) are significantly longer (> 0.05 A) than
the corresponding distances in 2 and 3 as are the B–N distances
˚
albeit to a lesser degree (> 0.02 A, see Fig. captions). Further-
more, the N–B–N angles in 6 [133.24(16)◦] and 7 [133.96(16)◦]
are considerably enlarged relative to the corresponding angles
in 2 [113.11(19), 112.87(18), 112.15(17), 113.30(17)◦] and 3
[116.32(15)◦]. The most likely explanation for these features lies
* Crystal data for 6: C48H68B2Li4N4O6, M = 846.44, orthorhombic,
˚
space group Pbca, a = 17.9789(4), b = 13.8833(6), c = 19.6293(9) A,
3 1 3 8
D a l t o n T r a n s . , 2 0 0 5 , 3 1 3 7 – 3 1 3 9