purely inorganic framework.16 The isolation of these paramagnetic
complexes suggests that redox behaviour may be a more important
feature of boraamidinate complexes than heretofore recognized.
Notes and references
{ Crystal data for 2a: C28H46AlB2N4, Mr 5 487.29, orthorhombic, Pbcn,
a 5 17.252(4), b 5 8.911(2), c 5 20.212(4) s, V 5 3107.1(1) s3, Z 5 4,
T 5 173(2) K, m(Mo-Ka) 5 0.086 mm21, 15287 reflections collected, 2712
unique (Rint 5 0.068), R1 5 0.0577 [I . 2s(I)], wR2 5 0.1468 (all data). 2b:
C28H46GaB2N4, Mr 5 530.03, orthorhombic, Pbcn, a 5 17.173(3),
b 5 8.945(2), c 5 20.337(4) s, V 5 3123.8(11) s3, Z 5 4, T 5 173(2) K,
m(Mo-Ka) 5 0.902 mm21, 49170 reflections collected, 2756 unique
(Rint 5 0.056), R1 5 0.0395 [I . 2s(I)], wR2 5 0.0926 (all data). CCDC
lographic data in CIF or other electronic format.
1 For a recent review, see: P. P. Power, Chem. Rev., 2003, 103, 789–809.
2 As defined in ref. 1, the term ‘‘stable’’, as applied to radicals, is taken to
indicate ‘‘a species that can be isolated and shows no sign of
decomposition under an inert atmosphere at room temperature’’,
whereas a ‘‘persistent’’ radical has ‘‘a relatively long lifetime under the
conditions it is generated’’.
3 (a) W. Kaim and W. Matheis, Chem. Commun., 1991, 597–598 and
references therein; (b) F. G. N. Cloke, C. I. Dalby, P. J. Daff and
J. C. Green, J. Chem. Soc., Dalton Trans., 1991, 181–184.
4 For a short review, see: P. Blais, J. K. Brask, T. Chivers, C. Fedorchuk
and G. Schatte, ACS Symp. Ser., ed. P. J. Shapiro and D. A. Atwood,
2002, vol. 822, pp. 195–207.
5 For reviews, see: (a) F. T. Edelmann, Coord. Chem. Rev., 1994, 137,
403–481; (b) J. Barker and M. Kilner, Coord. Chem. Rev., 1994, 133,
219–300.
6 (a) D. R. Manke, Z.-H. Loh and D. G. Nocera, Inorg. Chem., 2004, 43,
3618–3624; (b) D. R. Manke and D. G. Nocera, Inorg. Chem., 2003, 42,
4431–4436.
7 T. Chivers, C. Fedorchuk, G. Schatte and M. Parvez, Inorg. Chem.,
2003, 42, 2084–2093.
8 J. K. Brask, T. Chivers and G. Schatte, Chem. Commun., 2000,
1805–1806.
9 T. Chivers, C. Fedorchuk, G. Schatte and J. K. Brask, Can. J. Chem.,
2002, 80, 821–831.
10 The monocyclic structure of 3 was confirmed by X-ray crystallography.
Details will be given in a full account of this work.
11 R. D. Shannon, Acta Crystallogr., Sect. A, 1976, A32, 751–767.
12 W. W. Schoeller and S. Grigoleit, J. Chem. Soc., Dalton Trans., 2002,
405–409.
Fig. 4 Experimental (a) and simulated (b) X-band EPR spectra of a
diethyl ether solution of 2b at 295 K.
Table 1 Experimentala and calculated hyperfine coupling constants
[G]
Exptl.
(R 5 Bu)
Calc.c
(R 5 Me)
t
{[PhB(m-NR)2]2M}
M 5 Ald
Isotope
nb
27Al
14N
10B
1
4
2
11.47
4.66
1.72
5.15
28.00
35.40
4.69
1.73
5.20
16.86
4.17
2.01
6.03
45.61
57.67
4.27
2.03
6.10
11B
M 5 Gae
69Ga
71Ga
14N
10B
1
4
2
11B
a
13 H. E. Simmons and T. Fukunaga, J. Am. Chem. Soc., 1967, 89,
5208–5215.
Parameters were determined from simulations optimized to match
the experimental spectra; very close agreement between experiment
and simulation was obtained (Figs. 3 and 4). Number of nuclei.
b
14 The largest deviations occur for the metal centres and arise from two
primary sources. Firstly, the hfc values of metal centres arise purely from
spin polarization effects, which are known to be extremely difficult to
model accurately.15 Secondly, the basis set used for gallium is energy-
optimized, not property-optimized, and does not include any enhanced
s-part, a requirement for obtaining more than semi-quantitative
accuracy15.
15 M. L. Munzarova´, in Calculation of NMR and EPR parameters: Theory
and Applications, ed. M. Kaupp, M. Bu¨hl and V G. Malkin, VCH,
Weinheim, 2004, pp. 463–482.
c
Details of computational methods used are given in the
supplementary information. 100% Lorentzian line shapes with a
d
e
LW of 1.05 G. 100% Lorentzian line shapes with a LW of 1.30 G.
and DFT calculations indicate uniform spin delocalization
throughout both boraamidinate ligands in 2a and 2b and confirm
the retention of spirocyclic structures in solution.
16 A spirophenalenyl-based radical with a central tetrahedral BO2N2 unit
has been reported, but the spin density is highly delocalized onto the
phenalene rings. X. Chi, M. E. Itkis, B. O. Patrick, T. M. Barclay,
R. W. Reed, R. T. Oakley, A. W. Cordes and R. C. Haddon, J. Am.
Chem. Soc., 1999, 121, 10395–10402.
In summary, the neutral radicals 2a and 2b are the first
examples of complexes in which a boraamidinate anion radical
[PhB(NtBu)2]2 is stabilized by coordination to a metal centre.
?
They also provide an unique example of spiroconjugation in a
3932 | Chem. Commun., 2005, 3930–3932
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