Syntheses, X-ray structures, and redox behaviour of the group 14 bis-boraamidinates MPhB(μ-N-t-Bu)22 (M = Ge, Sn) and Li2MPhB(μ-N-t-Bu)22 (M = Sn, Pb)

Article abstract of DOI:10.1139/V08-183

The solid-state structures of the complexes M[PhB(μ-N-t-Bu) ₂]₂ (1a, M= Ge; 1b, M = Sn) were determined to spirocyclic with two orthogonal boraamidinate (bam) ligands N,N'-chelated to the group 14 centre. Oxidation of 1b with₂Cl₂afforded the thermally unstable, blue radical cation {Sn[PhB(m-N-t-Bu)₂]2}-+, identified by electron paramagnetic (EPR) spectroscopy supported by density functional theory (DFT) calculations, whereas the germanium analogue 1a was inert towards SO ₂Cl₂. The reaction between Li2[PhB(μ-N-t-Bu) ₂]₂ and SnCl₂ or PbI₂ in 2:1 molar ratio in diethyl produced the novel heterotrimetallic complexes Li ₂Sn[PhB(μ-N-t-Bu)₂]₂ (2b) and (Et2O-Li)LiPb[PhB(μ-N-t-Bu)₂]₂ (2c-OEt₂), respectively. By contrast, treatment of Li₂[PhB(μ-N-t-Bu) ₂]₂ with C₄H₈O₂-GeCl2 yielded the germanium(IV) 1a via a redox process. The X-ray structures of 2b and 2c-THF revealed polycyclic arrangements in which one bam ligand is N,N'-chelated to the Sn(II) or Pb(II) atom and one of the Li+ cations, while the second bam ligand exhibits a bonding mode, bridging all three metal centres. The fluctional behaviour of 2b was investigated by variable temperature, NMR spectroscopy.

Full text of DOI:10.1139/V08-183

461
Syntheses, X-ray structures, and redox behaviour
of the group 14 bis-boraamidinates M[PhB(m-N-t-
Bu)2]2 (M = Ge, Sn) and Li2M[PhB(m-N-t-Bu)2]2 (M =
Sn, Pb)
Jari Konu, Heikki M. Tuononen, and Tristram Chivers
Abstract: The solid-state structures of the complexes M[PhB(m-N-t-Bu)2]2 (1a, M= Ge; 1b, M = Sn) were determined to
be spirocyclic with two orthogonal boraamidinate (bam) ligands N,N’-chelated to the group 14 centre. Oxidation of 1b with
SO2Cl2 afforded the thermally unstable, blue radical cation {Sn[PhB(m-N-t-Bu)2]2}ꢀ+, identified by electron paramagnetic
resonance (EPR) spectroscopy supported by density functional theory (DFT) calculations, whereas the germanium analogue
1a was inert towards SO2Cl2. The reaction between Li2[PhB(m-N-t-Bu)2]2 and SnCl2 or PbI2 in 2:1 molar ratio in diethyl
ether produced the novel heterotrimetallic complexes Li2Sn[PhB(m-N-t-Bu)2]2 (2b) and (Et2OꢁLi)LiPb[PhB(m-N-t-Bu)2]2
(2cꢁOEt2), respectively. By contrast, treatment of Li2[PhB(m-N-t-Bu)2]2 with C4H8O2ꢁGeCl2 yielded the germanium(IV)
complex 1a via a redox process. The X-ray structures of 2b and 2cꢁTHF revealed polycyclic arrangements in which one
bam ligand is N,N’-chelated to the Sn(II) or Pb(II) atom and one of the Li+ cations, while the second bam ligand exhibits a
unique bonding mode, bridging all three metal centres. The fluctional behaviour of 2b was investigated by variable temper-
ature, multinuclear NMR spectroscopy.
Key words: germanium, tin, lead, boraamidinate, cation radical.
´
´
´
´
`
´
Resume : On a determine que les structures a l’etat solide des complexes M[PhB(m-N-t-Bu)2]2 (1a, M = Ge; 1b, M = Sn)
´
sont spirocycliques avec deux ligands orthogonaux boraamidinate (bam) avec une chelation N,N’ avec le centre du groupe
`
`
14. L’oxydation du produit 1b a l’aide de SO2Cl2 conduit a la formation du cation radical bleu, thermiquement instable
{Sn[PhB(m-N-t-Bu)2]2]ꢀ+ identifie par spectroscopie RPE supportee par des calculs a base de la theorie de la fonctionnelle
´
´
`
´
´
´
`
´
de la densite; l’analogue germanie 1a est inerte vis-a-vis du SO2Cl2. La reaction du Li2[PhB(m-N-t-Bu)2]2 avec le SnCl2
´
´
`
´ ´
ou le PbI2, dans un rapport molaire de 2:1, dans l’ether ethylique, conduit a la formation de nouveaux complexes hetero-
´
metalliques Li2Sn[PhB(m-N-t-Bu)2]2 (2b) et (Et2OꢁLi)LiPb[PhB(m-N-t-Bu)2]2 (2cꢁOEt2). Toutefois, le traitement du
Li2[PhB(m-N-t-Bu)2]2 par du C4H8O2ꢁGeCl2 fournit le complexe de germanium(IV) (1a) par le biais d’un processus redox.
´
´
´ `
´
Les structures des produits 2b et 2cꢁTHF, determinees par diffraction des rayons X, revelent la presence d’arrangements
`
´
`
polycycliques dans lesquels un ligand bam est soumis a une chelation en N,N’ par rapport a l’atome de Sn(II) ou Pb(II) et
un des cations Li alors que le second ligand bam donne lieu a un mode unique de liaison et qu’il cree un pont pour les
+
`
´
´
´
´
`
trois centres metalliques. On a etudie le comportement fluxionnel du produit en faisant appel a la spectroscopie RMN
´
`
´
multinucleaire a temperature variable.
´
´
Mots-cles : germanium, etain, plomb, boraamidinate, cation radical.
´
[Traduit par la Redaction]
Introduction
The dianionic boraamidinate (bam) ligand, [RB(NR’)2]2–
(B), is formally isoelectronic with the extensively studied
monoanionic amidinate (am) ligand, [RC(NR’)2]– (A). A
wide range of main-group element and transition-metal com-
plexes have been structurally characterized and a variety of
bonding modes have been established for the bam ligand.1–4
The most intriguing consequence of the 2– charge is the
facile tendency for redox transformations to occur in which
the dianion B is oxidized to the corresponding monoanion
radical [bam]ꢀ– (C). This paramagnetic ligand can be stabi-
lized through chelation, e.g., to early main-group metals.5,6
In the case of group 13 metals it has been possible to isolate
stable neutral radicals. For example, the intensely coloured
paramagnetic species {M[PhB(m-N-t-Bu)2]2}ꢀ (M = Al, dark
red; M = Ga, dark green) are produced by one-electron oxi-
dation of the corresponding anions with iodine, and the X-
Received 17 October 2008. Accepted 1 December 2008.
Published on the NRC Research Press Web site at
canjchem.nrc.ca on 27 January 2009.
J. Konu and T. Chivers.1 Department of Chemistry, University
of Calgary, Calgary, AB T2N 1N4, Canada.
H.M. Tuononen. Department of Chemistry, University of
¨
¨
¨
¨
Jyvaskyla, P.O. Box 35, FI-40014 Jyvaskyla, Finland.
1Corresponding author (e-mail: chivers@ucalgary.ca).
Can. J. Chem. 87: 461–471 (2009)
doi:10.1139/V08-183
Published by NRC Research Press

462
Can. J. Chem. Vol. 87, 2009
ray structures of these spirocyclic complexes have been de-
termined (Scheme 1).5 The singly occupied molecular orbi-
tal (SOMO) of these neutral radicals is located primarily
and equally in p-orbitals on the four nitrogen atoms of the
two bam ligands; there is very little electron density on the
group 13 centres. The corresponding boron and indium-con-
taining radicals {M[PhB(m-N-t-Bu)2]2}ꢀ (M = B, In) were
characterized only in solution by electron paramagnetic res-
onance (EPR) spectroscopy.6
Scheme 1. Synthesis of spirocyclic {M[PhB(m-N-t-Bu)2]2}ꢀ radicals.
The synthesis and characterization of the group 13 com-
plexes {M[PhB(m-N-t-Bu)2]2}ꢀ (M = Al, Ga, In) raises the
question of whether the isoelectronic group 14 cation radi-
cals {M[PhB(m-N-t-Bu)2]2}ꢀ+ (M = Si, Ge, Sn) will be ac-
cessible by mild oxidation of the corresponding neutral
precursors. Several bam complexes of group 14 elements
(Si, Ge, Sn, Pb) have been reported.1 Although the metathe-
sis reactions of two equivalents of Li2bam reagents with
MCl4 (M = Ge, Sn) afford the corresponding M(bam)2 com-
plexes,7,8 the synthesis of a silicon analogue (M = Si) by this
method has not been reported. The known silicon complexes
have only one bam ligand chelated to silicon in complexes
of the type RR’Si(bam); they were prepared by indirect ap-
proaches rather than metathesis.9,10 The solid-state structures
of M(bam)2 complexes (M = Ge, Sn) have not been deter-
mined. Interestingly, the reaction of Li2[PhB(m-N-t-Bu)2]
with PbCl4 produces the dimeric lead(II) complex
{Pb[PhB(m3-N-t-Bu)2]}2, which is preferably prepared by
using PbCl2 as a source of lead(II).11 There are no reports
of complexes in which two bam ligands are chelated to a
group 14 metal in the formal +2 oxidation state, i.e.,
[M(bam)2]2– (M = Ge, Sn, Pb). These dianions are isoelec-
tronic with the corresponding group 15 monoanions of the
type [M(bam)2]– (M = As, Sb, Bi), which exhibit interesting
polycyclic structures in the solid state and fluctional behav-
iour in solution.12 The synthesis of dilithio derivatives of the
group 14 dianions [M(bam)2]2– (M = Ge, Sn, Pb) is, therefore,
of interest for comparison with their group 15 analogues.
anhydrous t-BuNH2 in n-hexane at –10 8C and its purity was
checked by 1H NMR spectroscopy. The compounds
PhB[N(H)-t-Bu]2 and Li2[PhB(m-N-t-Bu)2] were prepared as
described earlier.13 The solvents n-hexane, toluene, Et2O,
and THF were dried by distillation over Na/benzophenone
under an argon atmosphere prior to use. Elemental analyses
were performed by Analytical Services, Department of
Chemistry, University of Calgary (Calgary, Alberta).
Spectroscopic methods
1
7
The H, Li, 11B, 13C, and 119Sn NMR spectra were ob-
tained in d8-toluene at 23 8C on a Bruker DRX 400 spec-
trometer operating at 399.59, 155.30, 128.20, 100.49, and
1
149.00 MHz, respectively. H and 13C spectra are referenced
to the solvent signal and the chemical shifts are reported rel-
7
ative to (CH3)4Si. Li and 11B NMR spectra are referenced
externally and the chemical shifts are reported relative to a
1.0 mol/L solution of LiCl in D2O and to a solution of
BF3ꢁEt2O in C6D6, respectively. Similarly, the 119Sn NMR
spectra are referenced externally and the chemical shifts are
reported relative to (CH3)4Sn.
The X-band EPR spectra were recorded on a Bruker EMX
113 spectrometer equipped with a variable-temperature ac-
cessory. EPR spectral simulations were carried out by using
WINEPR SimFonia (version 1.25, 1996) and PEST WinSim
(version 0.98, 2002) softwares.14
X-ray crystallography
In this contribution we report (a) the X-ray structures of the
complexes M[PhB(m-N-t-Bu)2]2 (1a, M = Ge; 1b, M = Sn),
(b) oxidation of 1b with SO2Cl2 to give the radical cation
{Sn[PhB(m-N-t-Bu)2]2}ꢀ+, which was identified by EPR
spectroscopy supported by density functional theory (DFT)
calculations, (c) the synthesis and X-ray structures of the
new heterotrimetallic complexes Li2Sn[PhB(m-N-t-Bu)2]2 (2b)
and (THFꢁLi)LiPb[PhB(m-N-t-Bu)2]2 (2cꢁTHF), and (d) vari-
Crystallographic data for 1a, 1b, 2b, and 2cꢁTHF are sum-
marized in Table 1 (also see the Supplementary data). Crys-
tals of Ge[PhB(m-N-t-Bu)2]2 (1a), Sn[PhB(m-N-t-Bu)2]2 (1b),
Li2Sn[PhB(m-N-t-Bu)2]2 (2b), and (THFꢁLi)LiPb[PhB(m-N-t-
Bu)2]2 (2cꢁTHF) were coated with Paratone 8277 oil and
mounted on a glass fibre. Diffraction data were collected on
a Nonius Kappa CCD diffractometer using monochromated
7
able temperature, multinuclear (1H, 13C, Li, 11B, and 119Sn)
Mo Ka radiation (l = 0.71073 A) at –100 8C. The data sets
˚
NMR investigations of the fluctional behaviour of 2b.
were corrected for Lorentz and polarization effects, and em-
pirical absorption correction was applied to the net inten-
sities. The structures were solved by direct methods using
SHELXS-9715 and refined using SHELXL-97.16 After full-
matrix least-squares refinement of the nonhydrogen atoms
with anisotropic thermal parameters, the hydrogen atoms
Experimental section
Reagents and general procedures
All reactions and the manipulations of products were per-
formed under an argon atmosphere by using standard
Schlenk techniques or an inert atmosphere glove box. The
compounds PhBCl2 (Sigma-Aldrich, 97%), GeCl4 (Strem,
99.99%), C4H8O2ꢁGeCl2 (Sigma-Aldrich, 99%), SnCl4
(Sigma-Aldrich, 1.0 mol/L soln. in heptane), SnCl2 (Sigma-
Aldrich, 98%), PbI2 (Strem, 99.999%), and t-BuNH2
(Sigma-Aldrich, 98%) were used as received. SO2Cl2
(Sigma-Aldrich, 97%) was distilled prior to use. LiN(H)-t-
Bu was prepared by the addition of n-BuLi to a solution of
˚
were placed in calculated positions [C-H = 0.98 A for
˚
C(CH3)3 and 0.95 A for phenyl hydrogens]. The isotropic
thermal parameters of the hydrogen atoms were fixed at
1.2 times to that of the corresponding carbon for phenyl hy-
drogens and 1.5 times for C(CH3)3. In the final refinement
the hydrogen atoms were riding on their respective carbon
atoms.
In the structure of 2b, the Li2 and Sn1 atoms were disor-
dered with the atoms statistically distributed over the two
Published by NRC Research Press

Konu et al.
463
Table 1. Crystallographic data for 1a, 1b, 2b, and 2cꢁTHF.
Compound
1a
1b
2b
2cꢁTHF
C32H54B2Li2N4OPb
753.48
Triclinic
P-1
9.188(2)
10.201(2)
20.649(4)
79.05(3)
84.95(4)
71.49(3)
1801.1(7)
2
Empirical formula
Formula weight
Crystal system
Space group
C28H46B2N4Ge
532.90
Orthorhombic
Pbcn
17.259(4)
8.917(2)
20.256(4)
90.00
90.00
90.00
3117(1)
4
C28H46B2N4Sn
579.00
Monoclinic
C2/c
24.846(5)
8.738(2)
18.065(4)
90.00
128.29(3)
90.00
3078(2)
4
C28H46B2Li2N4Sn
592.88
Monoclinic
P21/c
13.544(3)
11.828(2)
20.059(4)
90.00
104.72(3)
90.00
3108(1)
4
˚
a (A)
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
3
˚
V (A )
Z
T (8C)
–100
1.135
1.004
–100
1.249
0.851
–100
1.267
0.844
–100
1.389
4.712
rcalcd. (g/cm3)
m(Mo Ka) (mm–1)
Crystal size (mm3)
F(000)
0.20 ꢂ 0.16 ꢂ 0.12 0.44 ꢂ 0.36 ꢂ 0.06 0.40 ꢂ 0.06 ꢂ 0.02 0.24 ꢂ 0.20 ꢂ 0.08
1136
1208
1232
760
q range (8)
2.57–25.02
5083
2732
0.0213
2066
0.0360
0.0901
1.059
3.23–25.02
5103
2709
0.0236
2449
0.0311
0.0808
1.033
3.38–25.03
9846
5430
0.0457
4020
0.0468
0.0993
1.103
3.57–25.03
12174
6335
0.0282
5945
0.0234
0.0555
1.032
Reflns. collected
Unique reflns.
Rint
Reflns. [I > 2s (I)]
R1 [I > 2s(I)]a
wR2 (all data)b
GoF on F2
Completeness
0.990
0.996
0.991
0.996
˚
Note: l (MoKa) = 0.71073 A.
aR1 = SjjFoj – jFcjj / SjFoj.
bwR2 = [Sw(Fo – Fc2)2 / SwFo ]1/2
.
2
4
1
atomic sites. The two atoms were constrained to be located
in the same position and the anisotropic thermal parameters
were restricted to be equal. In the final refinement, the site
occupation factors were ca. 93% and 7%, respectively. The
scattering factors for the neutral atoms were those incorpo-
rated with the programmes.
(1a) as a white powder (0.442 g, 83%). The H, 11B, and
13C NMR spectra were in good agreement with the literature
values.7 Anal. calcd. for C28H46B2N4Ge: C 63.10, H 8.70, N
10.51; found: C 62.85, H 8.94, N 10.71. Crystallization from
n-hexane afforded colourless X-ray quality crystals of
Ge[PhB(m-N-t-Bu)2]2 (1a).
Computational details
Synthesis of Sn[PhB(m-N-t-Bu)2]2 (1b)
All calculations were done with the Gaussian 03 program
using density functional theory.17 Molecular structures were
optimized using the hybrid PBE1PBE exchange-correlation
functional18 together with the Ahlrichs’ TZVP basis sets19;
an effective core potential basis set of similar valence qual-
ity was used for the heavy tin nuclei.20 Hyperfine coupling
constants were then calculated by single-point calculations
employing the optimized geometries and the basis set com-
bination used for optimizations. The orbital plot was ob-
tained by the program gOpenMol.21
A solution of Li2[PhB(m-N-t-Bu)2] (0.244 g, 1.00 mmol)
in 25 mL of diethyl ether was added to a solution of SnCl4
(0.5 mL of a 1.0 mol/L solution in heptane, 0.50 mmol) in
25 mL of diethyl ether at –80 8C. The reaction mixture was
stirred for 30 min at –80 8C and 6 h at 23 8C. The precip-
itate of LiCl was removed by filtration and the solvent was
evaporated under vacuum giving Sn[PhB(m-N-t-Bu)2]2 (1b)
1
as a white powder (0.257 g, 87%). The H, 11B, and 13C
NMR spectra were in excellent agreement with the litera-
ture values.7 119Sn NMR d: –83.0 (cf. –117.8 ppm for
Sn[(2,4,6-iPr3C6H2)B(m-N-t-Bu)2]2 in CDCl3).8 Anal. calcd.
for C28H46B2N4Sn: C 58.08, H 8.01, N 9.68; found: C 58.30,
H 7.93, N 9.03. Crystallization from n-hexane afforded col-
ourless X-ray quality crystals of Sn[PhB(m-N-t-Bu)2]2 (1b).
Synthesis of Ge[PhB(m-N-t-Bu)2]2 (1a)
A solution of Li2[PhB(m-N-t-Bu)2] (0.366 g, 1.50 mmol)
in 30 mL of diethyl ether was added to a solution of GeCl4
(0.161 g, 0.75 mmol) in 30 mL of diethyl ether at –80 8C.
The reaction mixture was allowed to reach room temperature
in 30 min and it was then heated to 35 8C for 15 h. The pre-
cipitate of LiCl was removed by filtration and the solvent
was evaporated under vacuum giving Ge[PhB(m-N-t-Bu)2]2
Oxidation of Sn[PhB(m-N-t-Bu)2]2 (1b) with SO2Cl2
A solution of SO2Cl2 (0.040 mL, 0.067 g, 0.50 mmol)
was dissolved in 10.0 mL of diethyl ether; 1.0 mL
(0.05 mmol) of this solution was added to a solution of 1b
Published by NRC Research Press

464
Can. J. Chem. Vol. 87, 2009
(0.058 g, 0.10 mmol) in 80 mL of diethyl ether at –80 8C.
The reaction mixture was allowed to warm up to ca. –40 8C
at which point the colourless solution became bright blue
and a small amount of white powder (LiCl) appeared. An
aliquot of the reaction solution was used for EPR spectro-
scopic measurements. The remaining reaction solution was
allowed to reach room temperature, which resulted in the
decomposition of the paramagnetic species at ca. 0 8C as in-
dicated by disappearance of the blue colour.
Method B
The Pb(II) complex {Pb[PhB(m3-N-t-Bu)2]}2 was prepared
by a modification of the procedure reported earlier11 from
the reaction of PbI2 (0.461 g, 1.00 mmol) and Li2[PhB(m-N-
t-Bu)2] (0.244 g, 1.00 mmol) in diethyl ether. The purity of
1
the compound was checked by H NMR spectroscopy. A
solution of Li2[PhB(m-N-t-Bu)2] (0.244 g, 1.00 mmol) in
30 mL of diethyl ether was added to a solution of
{Pb[PhB(m3-N-t-Bu)2]}2 (0.437 g, 0.50 mmol) in 30 mL of
diethyl ether at –80 8C. The reaction mixture was allowed
to reach room temperature in 30 min and it was then heated
to 35 8C for 18 h. The precipitate of LiI was removed by
filtration and the solvent was evaporated under vacuum giv-
ing a pale yellow, amorphous powder (0.546 g). The 1H
NMR spectrum of this powder in d8-toluene revealed a com-
plex mixture of products. The reactions described in meth-
ods A and B were also carried out in THF at 55 8C without
significant improvement in the yield of 2c.
Synthesis of Li2Sn[PhB(m-N-t-Bu)2]2 (2b)
A solution of Li2[PhB(m-N-t-Bu)2] (0.366 g, 1.50 mmol)
in 25 mL of diethyl ether was added to a suspension of
SnCl2 (0.142 g, 0.75 mmol) in 25 mL of diethyl ether
at –80 8C. The reaction mixture was stirred for 30 min
at –80 8C and 5 h at 23 8C. The precipitate of LiCl was
removed by filtration and the solvent was evaporated under
vacuum. The product was then washed with cold n-hexane
(ca. 10 mL) to give 2b as a white powder (0.364 g, 75%).
Reaction of Li2[PhB(m-N-t-Bu)2] with C4H8O2ꢁGeCl2
A solution of Li2[PhB(m-N-t-Bu)2] (0.183 g, 0.75 mmol)
in 25 mL of diethyl ether was added to a solution of
C4H8O2ꢁGeCl2 (0.174 g, 0.75 mmol) in 25 mL of diethyl
ether at –80 8C. The reaction mixture was stirred for
30 min at –80 8C and 16 h at 23 8C. The precipitate of
LiCl was removed by filtration and the solvent was evapo-
rated under vacuum giving a pale yellow, amorphous pow-
der (0.185 g, 78% calculated as a 1:2 mixture of 1a and
{Ge[PhB(m-N-t-Bu)2]}2, see discussion). 1H NMR (d8-toluene,
23 8C) d: 7.02–7.96 (m, 30H, C6H5), 1.41 (s, 36H,
C(CH3)3), 1.39 (s, 36H, C(CH3)3), 1.30 (s, 36H, C(CH3)3).
1H NMR (d8-toluene, 23 8C) d: 6.98–7.72 (m, 10H, C6H5),
1.39 (shoulder, br, 9H, C(CH3)3), 1.19 (s, br, 27H,
1
C(CH3)3). H NMR (0 8C) d: 6.96–7.72 (m, 10H, C6H5),
1
1.44 (s, 9H, C(CH3)3), 1.21 (s, 27H, C(CH3)3). H NMR
(–20 8C) d: 6.92–7.70 (m, 10H, C6H5), 1.46 (s, 9H, C(CH3)3),
1.24 (s, 9H, C(CH3)3), 1.21 (s, 18H, C(CH3)3). 13C NMR
(–20 8C) d: 133.9–125.8 (C6H5), 54.4 (s, C(CH3)3), 52.1
(s, C(CH3)3), 51.1 (s, C(CH3)3), 37.0 (s, C(CH3)3), 36.6
(s, C(CH3)3), 36.1 (s, C(CH3)3). 11B NMR (–20 8C) d:
7
36.6. Li NMR (–20 8C) d: 1.10. 119Sn NMR (d8-toluene,
23 8C, ppm) d: –167.9. Anal. calcd. for C32H56B2Li2N4Sn:
C 56.72, H 7.82, N 9.45; found: C 55.45, H 7.99, N 9.18.
X-ray quality crystals of 2b were obtained by dissolving
the white powder in boiling n-hexane and then allowing
the solution to cool down to room temperature in ca. 3 h.
11B NMR (–20 8C) d: ca. 36.5 (br). Crystallization from
n-hexane afforded a few colourless crystals of Ge[PhB(m-
N-t-Bu)2]2 (1a).
Synthesis of (Et2OꢁLi)LiPb[PhB(m-N-t-Bu)2]2 (2cꢁOEt2)
Results and discussion
X-ray structures of M[PhB(m-N-t-Bu)2]2 (1a, M = Ge; 1b,
M = Sn)
Method A
A solution of Li2[PhB(m-N-t-Bu)2] (0.244 g, 1.00 mmol)
in 30 mL of diethyl ether was added to a suspension of PbI2
(0.231 g, 0.50 mmol) in 30 mL of diethyl ether at –80 8C.
The reaction mixture was allowed to reach room tempera-
ture in 30 min and it was then heated to 35 8C for 12 h.
The precipitate of LiI was removed by filtration and the
solvent was evaporated under vacuum giving a pale yellow,
amorphous powder (0.294 g). 1H NMR spectroscopy re-
vealed a mixture of products among which the lead(II) com-
plex, {Pb[PhB(m-N-t-Bu)2]}2, was identified (d: 1.15 ppm,
d8-toluene, 23 8C; lit. value11 d: 1.19 in C6D6) in addition
to the resonances of 2cꢁOEt2 (see discussion of the VT
The germanium(IV) and tin(IV) complexes 1a and 1b
were obtained by a modification of the literature synthesis7
in which the dilithium reagent Li2[PhB(m-N-t-Bu)2] was iso-
lated and purified prior to reaction with MCl4 (M = Ge, Sn)
in a 2:1 molar ratio in diethyl ether. This new procedure af-
forded analytically pure 1a and 1b in ca. 85% yields, cf. lit.
1a (31%) and 1b (44%).7 Attempts to make the silicon ana-
logue Si[PhB(m-N-t-Bu)2]2 by a similar procedure were un-
successful.
The X-ray structural determinations of 1a and 1b con-
firmed the expected spirocyclic structures with two orthogo-
nal bam ligands N,N’-chelated to the distorted tetrahedral
metal centres (Fig. 1). In both complexes the molecule lies
on a crystallographic twofold axis that imposes equivalence
on the two bam ligands. The diamagnetic complexes 1a and
1b are isostructural with the paramagnetic group 13 ana-
logues {M[PhB(m-N-t-Bu)2]2}ꢀ (M = Al, Ga).5 The centro-
symmetric germanium(IV) complex 1a represents the first
structurally characterized germanium bam complex.1
Selected bond lengths and bond angles for 1a and 1b are
summarized in Table 2. The distorted tetrahedral germanium
centre in 1a exhibits equal Ge–N bond lengths, which are
1
NMR experiments). 2cꢁOEt2: H NMR (d8-toluene, 23 8C)
d: 7.02–7.78 (m, C6H5), 3.40 (q, 6H, (CH3CH2)2O), 1.27
1
(s, br, 36H, C(CH3)3), 1.00 (t, 6H, (CH3CH2)2O). H NMR
(0 8C): d: 7.08–7.82 (m, C6H5), 3.34 (q, 6H, (CH3CH2)2O),
1.35 (shoulder, 9H, C(CH3)3), 1.29 (s, 18H, C(CH3)3), 1.25
(shoulder, 9H, C(CH3)3), 0.97 (t, 6H, (CH3CH2)2O). 1H NMR
(–10 8C) d: 7.10–7.83 (m, C6H5), 3.34 (q, 6H, (CH3CH2)2O),
1.36 (s, 9H, C(CH3)3), 1.30 (s, 18H, C(CH3)3), 1.25 (s, 9H,
C(CH3)3), 0.96 (t, 6H, (CH3CH2)2O). Li NMR (–20 8C) d:
0.87. Crystallization from THF afforded a few colourless
7
crystals of (THFꢁLi)LiPb[PhB(m-N-t-Bu)2]2 (2cꢁTHF).
Published by NRC Research Press

Konu et al.
465
Fig. 1. Molecular structure of Ge[PhB(m-N-t-Bu)2]2 (1a) with the
atomic numbering scheme. Hydrogen atoms have been omitted for
clarity. The tin complex 1b is isostructural with 1a. Symmetry op-
puckered SnBN2 four-membered rings in 1b are 4.6(2)8 and
6.5(3)8.
Although all fourꢀ isostructural compounds (1a, 1b, and
M[PhB(m-N-t-Bu)2]2 (M = Al, Ga)) exhibit a centre of sym-
metry at the group 13 or 14 atom, the tin complex 1b exhib-
its different crystal packing from the other three complexes
as indicated by the crystal system and space group [mono-
clinic C2/c for 1bꢀ vs orthorhombic Pbcn for 1a and
M[PhB(m-N-t-Bu)2]2 (M = Al, Ga)].
a
eration: –x, y, 0.5 – z (E = Ge); 1 – x, y, 1.5 – z (E = Sn).
EPR spectra and DFT calculations of the radical cation
{Sn[PhB(m-N-t-Bu)2]2}ꢀ+
The intensely coloured spirocyclic group 13 radicals
{M[PhB(m-N-t-Bu)2]2}ꢀ (M = Al, Ga) (Scheme 1) are stable
in the solid state,5 while the indium analogue {In[PhB(m-N-
t-Bu)2]2}ꢀ has only been characterized in solution by EPR
spectroscopy.6 In an attempt to generate the isoelectronic
radical cations with a group 14 metal centre, {M[PhB(m-N-
t-Bu)2]2}ꢀ+ (M = Ge, Sn), we have investigated the reactions
of 1a and 1b with various oxidizing agents (Scheme 2).
The attempted one-electron oxidation of 1a or 1b to the
corresponding radical cation by one-half equivalent of I2 in
diethyl ether or THF resulted in no reaction even at elevated
temperatures. The tin(IV) complex 1b was also inert towards
[NO][SbF6] and, surprisingly, the germanium(IV) complex
1a showed no indication of a reaction with SO2Cl2. How-
ever, the treatment of 1b with one-half equivalent of
SO2Cl2 in diethyl ether at low temperatures produced a
bright blue solution of {Sn[PhB(m-N-t-Bu)2]2}ꢀ+ (vide infra),
which was persistent below –40 8C, but decomposed above
0 8C, cf. the thermal instability of the isoelectronic indium-
containing neutral radical {In[PhB(m-N-t-Bu)2]2}ꢀ.6
The EPR spectrum of the blue solution in diethyl ether dis-
plays an eight-line pattern and a highly broadened lineshape
(Fig. 2); the g-value of this radical is 2.0055. Several at-
tempts to resolve additional fine structure in the EPR spec-
trum by variations in the solvent, concentration of the
solution, and temperature, as well as changes in the measure-
ment parameters, were unsuccessful. A reasonable simulation
of the observed spectrum is obtained by using a line width of
2.85 G (95% Gaussian shape) and by including hyperfine
coupling constants to one boron atom (11B, I = 3/2, 9.88 G)
and two equivalent nitrogen centres (14N, I = 1, 9.18 G)
(Fig. 2) thus suggesting localization of the unpaired electron
in only one of the bam units, in contrast to the delocalization
over both bam ligands observed for the neutral group 13 rad-
icals {M[PhB(m-N-t-Bu)2]2}ꢀ (M = Al, Ga, In).5,6 This elec-
tron distribution is, however, reminiscent of the behaviour
noted for the persistent radicals {(Et2O)Li[PhB(m-N-t-
Bu)2]2M}ꢀ (M = Mg, Zn), which exhibit localization of the
unpaired electron on the spin-active nuclei in only one of the
bam ligands as a consequence of the coordination of the ad-
ditional lithium counter-cation to the second bam unit.6
The apparent localization of the unpaired electron in only
one of the bam ligands, and the strong coordinating nature
of the Cl– counter-anion, raised the possibility of the forma-
tion of neutral radicals, {Sn[PhB(m-N-t-Bu)2]2Cl}ꢀ, compris-
ing a Sn-Cl or B-Cl contact instead of the ion-separated
radical cation, {Sn[PhB(m-N-t-Bu)2]2}ꢀ+ (Scheme 3), cf. the
Zwitterionic arsenic(V) complex, {[PhB(m-N-t-Bu)2]As[(m-
N-t-Bu)2B(Cl)Ph]}, with a covalent B–Cl bond.22 Conse-
˚
noticeably shorter (by ca. 0.20 A) than the corresponding
M–N bonds in the heavier tin analogue 1b. Somewhat sur-
prisingly, the Ge–N bonds in 1a are also significantly
shorter than the Ga–N bond lengths of 1.922(2) A observed
in the neutral radical Ga[PhB(m-N-t-Bu)2]2 , despite the dif-
ference of only one electron between the two centres (Ge
and Ga). The Ge–N bond lengths are, however, equal within
estimated standard deviation with the Al–N bond lengths of
˚
ꢀ 5
˚
1.842(2) and 1.852(2) A reported for {Al[PhB(m-N-t-
Bu)2]2}ꢀ,5 for which the difference between the two metal
centres (Ge and Al) is nine electrons. These trends in bond
lengths may be attributed to the weaker Coulomb interaction
in the paramagnetic group 13 complexes where the total charge
on the ligands is formally 3– compared to the diamagnetic
group 14 complexes for which the formal charge is 4–. Inter-
˚
estingly, the Ge–N bonds in 1a are also ca. 0.04 A longer than
the As–N bond lengths of 1.797(4)–1.811(4) A observed in the
˚
isostructural, and isoelectronic, group15 cation As[PhB(m-N-t-
Bu)2]2 (in the GaCl4 salt).22 Since the single bond cova-
+
–
23
˚
lent radius is the same for Ge and As (1.22 A), the
shorter As–N bonds are probably due to the stronger elec-
trostatic interaction between the positively charged arsenic
centre and the two dianionic bam ligands. In the tin(IV)
complex 1b the Sn–N bonds within each bam ligand are
equal in length and are close to the values of 2.043(3)–
˚
A
2.066(3)
observed in the eight-membered rings
{Me2Sn[m-N(Me)B(R)NMe]2SnMe2} (R = 2,4,6-i-Pr3C6H2,
2,4,6-t-Bu3C6H2), which also involve Sn(IV) centres,8,24
but they are somewhat shorter than the value of 2.105(1)
˚
A found for the Sn–N bond involving a three-coordinate ni-
trogen atom in the dimeric Sn(II) complex, {Sn[MeB(m-
NSiMe3)2]}2.25 The B–N bond lengths in both 1a and 1b
are equal and represent typical values for the bam ligand.1
The M-N-B angles in 1a and 1b are close to 908 and the
geometry about boron atom is trigonal planar as indicated
by the sum of bond angles (3608). The N-B-N and all N-
Ge-N angles in 1a are close to the corresponding bond an-
gles observed in {Al[PhB(m-N-t-Bu)2]2}ꢀ5 due to the identi-
cal Ge-N/Al-N and B–N bond lengths, whereas the longer
Sn–N bonds result in a narrower N-Sn-N angle by ca. 78
and 38 within the bam ligands in 1b. The GeBN2 four-mem-
bered rings in 1a are planar with torsion angles of 0.5(2)8
and 0.6(2)8 for ff N2-Ge1-N1-B1 and Ge1-N1-B1-N2, respec-
tively, while the corresponding torsion angles for the slightly
Published by NRC Research Press

466
Can. J. Chem. Vol. 87, 2009
˚
Table 2. Selected bond lengths (A) and angles (8) in 1a and 1b.
1aa
1bb
1aa
1bb
˚
Bond lengths (A)
M1–N1
M1–N2
1.846(2)
1.842(2)
2.047(2)
2.047(2)
B1–N1
B1–N2
1.438(3) 1.436(4)
1.439(4) 1.438(4)
Bond angles (8)
N1-M1-N2
N1-M1-N1’
N1-M1-N2’
75.9(1)
69.2(1)
M1-N2-B1 90.2(2)
91.1(2)
128.6(1)c
127.8(1)c
129.2(2)d
132.8(1)d
135.5(1)d
91.2(2)
N1-B1-N2
N1-B1-C9
N2-B1-C9
104.0(2) 107.9(2)
127.3(2) 126.5(3)
128.7(2) 125.6(3)
N2’-M1-N2’ 129.8(1)c
M1-N1-B1
90.0(2)
aM = Ge.
bM = Sn. Symmetry operation for the atoms marked with a single quote (’).
c–x, y, 0.5 – z.
d1 – x, y, 1.5 – z.
Scheme 2. One-electron oxidation of {M[PhB(m-N-t-Bu)2]2} (M =
Ge, Sn).
Fig. 2. (a) Simulated and (b) experimental EPR spectra of
{Sn[PhB(m-N-t-Bu)2]2}ꢀ+Cl– measured in diethyl ether at –20 8C.
Scheme 3. Possible structures of the product of one-electron oxida-
tion of 1b with SO2Cl2.
quently, the replacement of the Cl– anion by the weakly co-
ordinating SbF6 was performed by an in situ reaction with
only one bam ligand (Fig. 3). A D2d-symmetric structure,
analogous to that observed for the neutral group 13 radicals
{M[PhB(m-N-t-Bu)2]2}ꢀ (M = Al, Ga) (Scheme 1), is a
transition state with respect to Sn-N stretching vibrations
and only 5 kJ mol–1 higher in energy. Hence, it appears
that the spiroconjugative interactions in 1b are not suffi-
ciently strong to give rise to a fully delocalized electron
distribution in the corresponding cation,26 and the system
adopts a structure with the two bam ligands bearing differ-
ent formal charges, i.e., Sn(IV)(bam)2–(bam)–ꢀ. This is con-
sistent with the finding that optimizations carried out for
the lighter group 14 congeners of {M[PhB(m-N-t-Bu)2]2}ꢀ+
(M = Si, Ge) give minima with significantly shorter NꢁꢁꢁN
–
AgSbF6. The thermal stability of the radical cation was not
improved, but a colour change from blue to purple was ob-
served along with the formation of AgCl. The EPR spectrum
of the purple solution, however, was unaffected by the
change of the counter-anion, suggesting that the one-electron
oxidation product of 1b is present in very dilute diethyl
ether solutions as the ion-separated radical cation I rather
than the ‘‘contact ion-pairs’’ II or III.
Density functional theory calculations conducted for
the cation radical {Sn[PhB(m-N-t-Bu)2]2}ꢀ+ predict a C2v-
symmetric minimum structure and localized spin density on
Published by NRC Research Press

Konu et al.
467
Fig. 3. SOMO of the radical cation {Sn[PhB(m-N-t-Bu)2]2}ꢀ+
( 0.05 isodensity surfaces).
½1Š
2Li2½PhBðmÀNÀtÀBuÞ2Š þ SnCl2
! Li2Sn½PhBðmÀNÀtÀBuÞ2Š2 þ 2LiCl
ð2bÞ
The reaction of Li2[PhB(m-N-t-Bu)2] with PbI2 in 2:1 mo-
lar ratio was conducted under various conditions in an effort
to produce the analogous lead(II) complex Li2Pb[PhB(m-N-t-
1
Bu)2]2 (2c). The H NMR spectrum of the reaction mixture,
however, revealed several products, one of which was iden-
tified as the previously reported mononuclear lead(II) com-
plex, {Pb[PhB(m3-N-t-Bu)2]}2.11 In an alternative synthetic
approach, this dimeric Pb(II) complex was prepared and iso-
lated prior to treatment with two equivalents of Li2[PhB(m-
N-t-Bu)2]. However, this two-step method also afforded a
mixture of products from which only a few crystals of 2c
were isolated as the tetrahydrofuran solvate, (THFꢁLi)-
LiPb[PhB(m-N-t-Bu)2]2 (2cꢁTHF).
The reaction of Li2[PhB(m-N-t-Bu)2] with C4H8O2ꢁGeCl2
in 2:1 molar ratio in diethyl ether produced several products
(1H NMR). Consequently, the reaction was repeated in 1:1
molar ratio in an attempt to form the unknown germaniu-
m(II) complex, Ge[PhB(m-N-t-Bu)2], for subsequent reaction
with Li2[PhB(m-N-t-Bu)2]; this two-step approach was previ-
ously found to be necessary for the successful synthesis of
the aluminum complex (Et2OꢁLi)Al[PhB(m-N-t-Bu)2]2.5 The
separations and, consequently, only D2d-symmetric struc-
tures are found.
The hyperfine coupling constants calculated for
Sn[PhB(m-N-t-Bu)2]2}ꢀ+ (2 ꢂ 14N, 7.35 G; 2 ꢂ 14N, 0.05 G;
11B, –10.96 G; 11B, –0.32 G) are in good agreement with the
values determined from the experimental spectrum. Similar
to the previous observations for the persistent radicals
{(Et2O)Li[PhB(m-N-t-Bu)2]2M}ꢀ (M = Mg, Zn),6 the calcu-
lated nitrogen couplings are too small, whereas the coupling
to the boron nuclei is slightly larger than the value inferred
from the simulation; there is an apparent tendency of this
functional basis set combination to underestimate couplings
that result from the spin density of the unpaired electron
while overestimating those that arise from spin-polarization
effects.5,6 The undulating baseline of the EPR spectrum and
the low intensity humps visible at both ends of the spectrum
most likely arise from the presence of small amounts of iso-
topomers, which contain the spin-active tin nuclei (117Sn, I =
1/2, 7.6%; 119Sn, I = 1/2, 8.6%). The magnitudes of the two
Sn couplings cannot, however, be obtained with the current
computational method due to the indirect treatment of rela-
tivistic effects via effective core potential basis sets. The ob-
served broad lineshape can, at least in part, be attributed to
the numerous smaller couplings to the hydrogen atoms of
the t-Bu groups.27
1H NMR spectrum of the product(s) showed three equally
intense resonances for N-t-Bu groups at 1.41, 1.39, and
1.30 ppm (d8-toluene, 23 8C). The chemical shift of
1.30 ppm is identical with the value observed for the t-Bu
groups in the germanium(IV) complex Ge[PhB(m-N-t-
Bu)2]2 (1a),8 suggesting that a redox process has occurred.
The resonances at 1.41 and 1.39 are tentatively assigned to
the t-Bu groups attached to the three- and four-coordinate
nitrogen atoms of the dimeric complex germanium(II) com-
plex {Ge[PhB(m-N-t-Bu)2]}2; however, attempts to isolate
this second product were unsuccessful.
X-ray structures of Li2Sn[PhB(m-N-t-Bu)2]2 (2b) and
(THFꢁLi)LiPb[PhB(m-N-t-Bu)2]2 (2cꢁTHF)
Crystals of 2b and 2cꢁTHF were obtained by recrystalliza-
tion from boiling n-hexane and THF, respectively.28 The
crystal structures of 2b and 2cꢁTHF with the atomic number-
ing scheme are depicted in Fig. 4 and the pertinent bond pa-
rameters are summarized in Table 3. The structures reveal
novel polycyclic arrangements, which bear a close similarity
with each other despite the introduction of a solvent mole-
cule in 2cꢁTHF. In both frameworks one of the bam ligands
is N,N’-chelated to both the M(II) atom (M = Sn, Pb) and
one of the Li+ ions while the second bam ligand exhibits a
unique bonding motif by bridging all three metal centres.1
While one of the Li+ ions (Li1) in both structures is coordi-
nated to three nitrogens, the second lithium in 2cꢁTHF is co-
ordinated to two nitrogen atoms and the solvent molecule.
The absence of the solvent molecule in 2b affords a third,
Synthesis of Li2Sn[PhB(m-N-t-Bu)2]2 (2b) and
(THFꢁLi)LiPb[PhB(m-N-t-Bu)2]2 (2cꢁTHF)
The reactions of Li2[PhB(m-N-t-Bu)2] with group 14 diha-
lides in a 2:1 molar ratio were investigated with a view to
isolating and structurally characterizing complexes of the
type Li2M[PhB(m-N-t-Bu)2]2 in which the dianions
2–
M[PhB(m-N-t-Bu)2]2 (M = Ge, Sn, Pb) are isoelectronic
with the previously reported group 15 anions M[PhB(m-N-t-
–
Bu)2]2 (M = As, Sb, Bi), which were obtained as mono-
lithium derivatives.12 The outcome of these reactions was
markedly dependent on the nature of the group 14 metal.
˚
albeit relatively weak, LiꢁꢁꢁN close contact [2.557(6) A] for
The reaction between Li2[PhB(m-N-t-Bu)2] and SnCl2 in a
2:1 molar ratio in diethyl ether proceeded cleanly to produce
Li2Sn[PhB(m-N-t-Bu)2]2 (2b) in 75% yield (eq. [1]). The un-
solvated heterotrinuclear complex 2b was characterized in
solution by multinuclear NMR spectroscopy and in the solid
state by single crystal X-ray crystallography.
the second lithium cation that results in a formal coordina-
tion number of five for the N4 atom (Fig. 4a).
Lithium salts of the group 15 monoanions M[PhB(m-N-t-
Bu)2]2 (3, M = Sb; 4, M = Bi),12 which are isoelectronic
–
2–
with the group 14 dianions M[PhB(m-N-t-Bu)2]2 (M = Sn,
Published by NRC Research Press

468
Can. J. Chem. Vol. 87, 2009
˚
Fig. 4. Crystal structures of (a) Li2Sn[PhB(m-N-t-Bu)2]2 (2b) and
(b) (THFꢁLi)LiPb[PhB(m-N-t-Bu)2]2 (2cꢁTHF) with the atomic
numbering scheme. Hydrogen atoms have been omitted for clarity.
The average Sn–N bond length of 2.187(4) A in 2b
(Fig. 4a, Table 3) is significantly longer than the values of
2.043(3)–2.066(3) A observed in 1b and in the Sn(IV) com-
˚
plexes {Me2Sn[m-N(Me)B(R)NMe]2SnMe2} (R = 2,4,6-i-
Pr3C6H2, 2,4,6-t-Bu3C6H2),8,24 but slightly shorter than the
˚
distance of 2.254(13) A found for the Sn–N bonds involving
four-coordinate nitrogen atoms in the dimeric Sn(II) complex
{Sn[MeB(m3-NSiMe3)2]}2.25 Expectedly, the Sn–N bond
lengths are also close to the values of 2.145(5) and 2.198(5)
˚
A observed for the Sb–N bonds to the four-coordinate ni-
trogens in 3 (the single bond covalent radii for Sn and Sb
23
˚
are 1.40 and 1.43 A, respectively). The Li1–N distance to
˚
the bridging bam ligand is ca. 0.18 A shorter than the two
Li1–N bonds to the chelating bam ligand. To compensate
for the long Li2ꢁꢁꢁN4 contact, the two Li2–N bonds of
˚
1.869(5) and 1.884(5) A are among the shortest lithium–
nitrogen bonds observed for this ligand system.1
The introduction of a second Li+ ion in 2b results in
opening the angle between BN2Sn and LiN2Sn four-mem-
bered rings as illustrated by the N1-Sn1-N3 and Li2-N4-B2
angles, which are ca. 48 and 98 wider, respectively, than the
corresponding angles in the related antimony complex, 3.12
Widening of these angles and elongation of the Sn1-N1 and
Li2-N4 contacts in 2b compared to the analogous Sb–N and
Li–N bonds, however, allows B1 and N2 atoms to incline
towards the central SnLiN2 four-membered ring and, conse-
quently, the Sn1-N1-B1 and N4-Li2-N2 angles are ca. 48 and
58 narrower than the comparable angles in 3. The N-Sn-N
angle within the chelating bam ligand in 2b is ca. 5.78
smaller than the corresponding angle in 1b owing to the
slightly longer Sn–N bonds in 2b.
The average Pb–N bond length in 2cꢁTHF (Fig. 4b, Ta-
˚
˚
ble 3) of 2.330(3) A is identical to the value of 2.329(6) A
observed for the Pb–N bond involving a four-coordinate
nitrogen atom in the dimeric lead(II) complex {Pb[PhB(m3-
Pb), provide an interesting benchmark for comparison with
the structures of 2b and 2cꢁTHF (Scheme 4). Curiously, the
lighter complexes in both group 14 (M = Sn) and group 15
(M = Sb) seem to favour unsolvated structures, whereas sol-
vation of the Li+ cation is observed for the heavier conge-
ners (M = Pb, Bi). The molecular structure of the antimony
complex 3 is comprised of two four-membered rings, BN2Li
and BN2Sb, connected by Li–N and Sb–N bonds to form a
tricyclic compound with both three- and four-coordinate ni-
trogens.12 The introduction of a second Li+ ion in 2b has a
surprisingly small effect on the overall structure; the mole-
cule retains the ladder-like backbone and cisoid arrangement
of the PhBN-t-Bu units. Most significantly, however, the
Li2ꢁꢁꢁN4 contact in 2b (Fig. 4a, dashed line in Scheme 4) is
N-t-Bu)2]}2.10 The Pb–N bonds are ca. 0.06 A longer than
˚
the three Bi–N bonds in 4ꢁOEt2 (cf. the single bond covalent
23
˚
radii of 1.54 and 1.52 A for Pb and Bi, respectively), but
˚
they are ca. 0.11 A shorter than the fourth, elongated, Bi–N
bond in the bismuth complex.12 The Li-N distances to the
N2 atom in the bridging ligand in 2cꢁTHF are somewhat
shorter than the remaining three Li-N interactions (ca. 0.12
˚
and 0.07 A for Li1-N2 and Li2-N2, respectively). All these
distances are consistent with, for example, those in the
group 13 and 15 complexes (Et2OꢁLi)M[PhB(m-N-t-Bu)2]2,
which show two (M = Ga, In, As) or three (M = Bi) Li-N
interactions.4,12 The N-Pb-N angle in the chelating ligand is
ca. 108 narrower than the N-Li-N angle at the unsolvated Li+
ion due to the longer Pb–N bonds.
˚
elongated by ca. 0.28 A compared to the analogous bond in
the antimony complex due to the pseudo-five-coordination
of the nitrogen atom. The lithium cation in the bismuth
complex 4ꢁOEt2 (Scheme 4) is four-coordinate, being N,N’-
chelated by one bam ligand and bonded to one nitrogen of
the second bam ligand; the fourth coordination site is occu-
pied by an Et2O molecule.12 The introduction of a second
Li+ ion in 2cꢁTHF ruptures the bond between the solvated
Li+ ion and the second bam ligand, and cleaves the fourth
M–N bond (M = Bi) that exists in the structure of 4ꢁOEt2,
The bridging bam ligand in both 2b and 2cꢁTHF exhibits
a significant disparity in the B–N bond lengths (ca. 0.12 and
˚
0.08 A, respectively), presumably as a result of the different
environments of N1 and N2, whereas the equality of the B–
N bonds in the chelating bam ligand is not significantly per-
turbed by the weak Li2ꢁꢁꢁN4 contact. The N-B-N angle in the
chelating ligand in both 2b and 2cꢁTHF is somewhat smaller
(by 7.18 and 6.48, respectively) than the analogous bond
angle in the bridging bam ligand. Nevertheless, the boron
atom assumes trigonal planar geometry in the bam ligands
in both complexes. The bond angles at the nitrogen atoms
in the chelating ligand are close to 908, whereas the loss of
although it is elongated compared to the other three Bi–N
12
bonds (by ca. 0.18 A). As a result, both Li+ ions are
˚
three-coordinate and all four nitrogen atoms are four-coordi-
nate in 2cꢁTHF.
Published by NRC Research Press

Konu et al.
469
˚
Table 3. Selected bond lengths (A) and angles (8) in 2b and 2cꢁTHF.
2ba
2cꢁTHFb
2ba
2cꢁTHFb
˚
Bond lengths (A)
M1–N1
M1–N3
M1–N4
Li1–N2
Li1–N3
Li1–N4
Li2–N1
2.194(4)
2.346(3)
2.335(3)
2.309(2)
1.921(6)
2.039(6)
2.030(6)
2.055(6)
Li2–N2
Li2–N4
B1–N1
B1–N2
B2–N3
B2–N4
1.884(5) 1.974(6)
2.557(6)
1.520(6) 1.490(4)
1.408(7) 1.412(4)
1.434(7) 1.447(5)
1.466(6) 1.444(5)
2.189(3)
2.179(4)
1.915(9)
2.075(9)
2.115(9)
1.869(5)
—
Bond angles (8)
N1-M1-N3
N1-M1-N4
N3-M1-N4
N1-B1-N2
N3-B2-N4
M1-N1-B1
Li2-N1-B1
M1-N1-Li2
Li1-N2-B1
Li2-N2-B1
Li1-N2-Li2
M1-N3-B2
108.3(1)
99.5(1)
105.2(1)
59.8(1)
Li1-N3-B2
M1-N3-Li1
M1-N4-B2
M1-N4-Li1
Li1-N4-B2
Li2-N4-B2
N2-Li1-N3
N2-Li1-N4
N3-Li1-N4
N1-Li2-N2
N4-Li2-N2
76.4(4)
84.7(3)
89.7(3)
84.0(3)
74.5(3)
131.3(3)
132.7(5) 131.1(3)
121.9(4) 132.3(3)
66.5(3)
80.6(2)
104.0(2)
77.7(2)
81.5(2)
89.9(2)
82.3(2)
78.1(2)
—
99.1(1)
63.5(1)
112.0(4)
104.9(4)
122.3(3)
80.5(3)
89.5(2)
108.2(4)
82.8(3)
75.2(3)
90.1(3)
112.9(3)
106.5(3)
120.1(2)
77.5(2)
103.1(2)
109.5(3)
102.6(2)
88.7(3)
69.4(2)
73.8(2)
—
88.8(2)
aM = Sn.
bM = Pb.
bon atoms of the t-Bu groups; three singlets are also
observed for the methyl carbons at 37.0, 36.6, and
36.1 ppm, in addition to the phenyl resonances. Throughout
the temperature range (from –80 to +23 8C) only one broad
Scheme 4. Comparison of the structures of Li2M[PhB(m-N-t-Bu)2]2
(M = Sn, Pb) and LiM[PhB(m-N-t-Bu)2]2 (M = Sb, Bi).
7
singlet is observed in both the 11B and Li NMR spectrum
(at 36.6 and 1.10 ppm, respectively, at –20 8C in d8-tol-
uene). The 119Sn NMR spectrum of 2b shows a single reso-
nance at –167.9 ppm, which exhibits a significant shift to
higher field from the value of 120 ppm reported for the di-
meric tin(II) complex, {Sn[MeB(m3-NSiMe3)2]}2.25 How-
ever, chemical shift values of –151.8 to –158.1 ppm have
been reported for the related tin(II) complexes {Sn[RSi(m3-
N-t-Bu)2]}2 (R = C3H6, C4H6, C4H8 and C5H10),30 which ex-
hibit a coordination environment around the tin(II) centres
similar to that in 2b. The three resonances with a 1:1:2 in-
1
tensity ratio observed at –20 8C in the H NMR spectrum
of 2b are assigned to the tert-butyl groups attached to N1,
N2, and N3/N4, respectively (Fig. 4a). The observation of
only one broad resonance for the inequivalent boron and
lithium atoms in the structure of 2b even at low tempera-
tures is attributed to overlap of these resonances.
the fourth M–N bond results in a wider M1-N1-B1 angle
by ca. 328 (M = Sn) and 308 (M = Pb).
NMR spectra and fluctional behaviour of Li2Sn[PhB(m-
N-t-Bu)2]2 (2b)
The fluctional behaviour observed for 2b is reminiscent of
the exchange processes exhibited by the related group 15
complexes, 3 and 4ꢁOEt2, which both showed only a single
t-Bu resonance at room temperature.12 These types of fluc-
tional processes in lithium derivatives of polyimido anions
of p-block elements are known to have low activation ener-
gies.31 In the case of 3 and 4ꢁOEt2, the fluctionality was de-
termined to originate from a Berry pseudorotation.12 In a
similar manner, the single resonance observed at room tem-
1
The H NMR spectrum of 2b in d8-toluene at 23 8C ex-
hibits a broad, poorly resolved signal for t-Bu groups, in ad-
dition to typical resonances for the phenyl groups. When the
temperature is lowered to 0 8C, this broad resonance re-
solves into two well-separated signals at 1.44 and 1.21 ppm
in a 1:3 intensity ratio. Cooling the solution to –20 8C re-
sults in resolution of the latter resonance to give three sin-
glets at 1.46, 1.24, and 1.21 ppm in a 1:1:2 intensity ratio.29
No further changes are apparent at lower temperatures.
Consistently, the 13C NMR spectrum of 2b at –20 8C exhib-
its three singlets at 54.4, 52.1, and 51.1 ppm for the a-car-
1
perature in the H NMR spectra of 2b can be attributed to a
rapid exchange in which each of the four nitrogens, in turn,
occupy the site that is not coordinated to the tin centre.
Published by NRC Research Press

470
Can. J. Chem. Vol. 87, 2009
References
Conclusions
(1) For a recent review see: Fedorchuk, C.; Copsey, M. C.; Chivers,
T. Coord. Chem. Rev. 2007, 251, 897. doi:10.1016/j.ccr.
2006.06.010.
(2) Manke, D. R.; Nocera, D. G. Inorg. Chem. 2003, 42, 4431.
doi:10.1021/ic0340478. PMID:12844317.
(3) Manke, D. R.; Loh, Z.-H.; Nocera, D. G. Inorg. Chem. 2004,
43, 3618. doi:10.1021/ic049795r. PMID:15180415.
(4) Chivers, T.; Fedorchuk, C.; Schatte, G.; Parvez, M.
Inorg. Chem. 2003, 42, 2084. doi:10.1021/ic0206286.
PMID:12639145.
Comparison of the properties of the cation radicals
{M[PhB(m-N-t-Bu)2]2}ꢀ+ (M = Si, Ge, Sn) with those of the
ꢀ
isoelectronic neutral radicals M[PhB(m-N-t-Bu)2]2 (M = Al,
Ga, In) was limited to the tin derivative as a result of the
inertness of Ge[PhB(m-N-t-Bu)2]2 towards oxidizing agents
and the inaccessibility of Si[PhB(m-N-t-Bu)2]2. The blue rad-
ical cation {Sn[PhB(m-N-t-Bu)2]2}ꢀ+, which exhibits ther-
mal instability similar to that of the neutral group 13
ꢀ
analogue In[PhB(m-N-t-Bu)2]2 , was characterized at low
temperatures by EPR spectroscopy. In contrast to the isoe-
lectronic, D2d-symmetric indium bis-bam radical, DFT cal-
culations predict a C2v symmetry for the radical cation
{Sn[PhB(m-N-t-Bu)2]2}ꢀ+ with spin density localized on
only one of the bam ligands owing to the weak spiroconju-
gative NꢁꢁꢁN interactions that, presumably, prevent the de-
localization of the unpaired electron over both ligands.
(5) Chivers, T.; Eisler, D. J.; Fedorchuk, C.; Schatte, G.; Tuono-
´
nen, H. M.; Boere, R. T. Chem. Commun. (Camb.) 2005,
3930. doi:10.1039/b506253e. PMID:16075075.
(6) Chivers, T.; Eisler, D. J.; Fedorchuk, C.; Schatte, G.; Tuono-
nen, H. M. Inorg. Chem. 2006, 45, 2119. doi:10.1021/
ic0520014.
(7) Fest, D.; Habben, C. D.; Meller, A.; Sheldrick, G. M.;
Stalke, D.; Pauer, F. Chem. Ber. 1990, 123, 703. doi:10.
1002/cber.19901230409.
(8) Albrecht, T.; Elter, G.; Noltemeyer, M.; Meller, A. Z. Anorg.
Allg. Chem. 1998, 624, 1514. doi:10.1002/(SICI)1521-
3749(199809)624:9<1514::AID-ZAAC1514>3.0.CO;2-5.
(9) Paetzold, P.; Hahnfeld, D.; Englert, U.; Wojnowski, W.;
Dreczewski, B.; Pawelec, Z.; Walz, L. Chem. Ber. 1992,
125, 1073. doi:10.1002/cber.19921250514.
(10) Luthin, W.; Stratmann, J.-G.; Elter, G.; Meller, A.; Heine,
A.; Gornitzka, H. Z. Anorg. Allg. Chem. 1995, 621, 1995.
doi:10.1002/zaac.19956211204.
(11) Heine, A.; Fest, D. J. Chem. Soc. Chem. Commun. 1990,
742. doi:10.1039/c39900000742.
(12) Konu, J.; Balakrishna, M. S.; Chivers, T.; Swaddle, T. W.
Inorg. Chem. 2007, 46, 2627. doi:10.1021/ic0622757.
PMID:17326629.
(13) Chivers, T.; Fedorchuk, C.; Schatte, G.; Brask, J. K. Can. J.
Chem. 2002, 80, 821. doi:10.1139/v02-111.
(14) (a) WINEPR SimFonia, version 1.25; Bruker Analytische:
Messtechnik, Germany, 1996; (b) Duling, D. R. J. Mag.
Res. Ser. B 1994, 104, 105. doi:10.1006/jmrb.1994.1062.
(15) Sheldrick, G. M. SHELXS-97: Program for Crystal Structure
In the heterotrimetallic complexes Li2M[PhB(m-N-t-Bu)2]2
[M = Sn, Pb (THF omitted)], one of the bam ligands exhibits
a novel bonding mode in which it bridges all three metal
centres.1 The solid-state structures of these group 14 com-
plexes are comparable with those of the previously reported
group 15 complexes LiM[PhB(m-N-t-Bu)2]2 [M = Sb, Bi
(OEt2 omitted)].12 Despite the introduction of a second Li+
ion, the tin complex maintains the ladder-like backbone and
cisoid arrangement of the bam ligands found in the antimony
analogue. The intrusion of a second Li+ ion in the lead com-
plex results in rupture of the bond between the solvated Li+
ion and one of the bam ligands, as well as cleavage of the
fourth M–N bond (M = Bi, Pb) that exists in the structure of
(Et2OꢁLi)Bi[PhB(m-N-t-Bu)2]2. In view of these structural
similarities it is, perhaps, not surprising that the variable
temperature NMR spectra of Li2Sn[PhB(m-N-t-Bu)2]2 show
trends comparable to those previously observed for the group
15 complex LiSb[PhB(m-N-t-Bu)2]2,12 implying that the fluc-
tional processes are also similar.
¨
¨
Supplementary data
Determination; University of Gottingen: Gottingen, Ger-
many, 1997.
Supplementary data for this article are available on the
journal Web site (canjchem.nrc.ca) or may be purchased
from the Depository of Unpublished Data, Document
Delivery, CISTI, National Research Council Canada, Ottawa,
ON K1A 0R6, Canada. DUD 3875. For more information
on obtaining material refer to cisti-icist.nrc-cnrc.gc.ca/cms/
unpub_e.shtml. CCDC 705641–705644 contain the crystal-
lographic data for this manuscript. These data can be ob-
tained, free of charge, via www.ccdc.cam.ac.uk/conts/
retrieving.html (Or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
(16) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure
¨
¨
Refinement; University of Gottingen: Gottingen, Germany,
1997.
(17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G.
E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.;
Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.;
Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.;
Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.;
Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Acknowledgements
The authors gratefully acknowledge financial support
from the Natural Sciences and Engineering Research Coun-
cil of Canada and the Academy of Finland. Professor M. Ba-
lakrishna carried out a preliminary reaction of PbI2 with
Li2[PhB(m-N-t-Bu)2], which produced the crystals that were
used in the X-ray structural determination of 2cꢁTHF.
Published by NRC Research Press

Konu et al.
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.;
471
neutral group 13 radicals {M[PhB(m-N-t-Bu)2]2}ꢀ (M = Al,
Ga) (see ref. 5 and refs. cited therein).
Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.
(27) DFT calculations were also performed for the neutral (C1
symmetric) systems II and III (Scheme 3), which comprise
a Sn–Cl or a B–Cl bond, respectively. The paramagnetic spe-
cies II is ca. 80 kJ mol–1 lower in energy, but the calculated
hyperfine coupling constants for II and III are in poor agree-
ment with those obtained from the experimental spectrum.
The Sn–Cl bonded radical II exhibits a large (–11.56 G) cou-
pling to the 11B centre but, due to the lack of symmetry, the
hfc couplings to the two nitrogen atoms are not equal (9.15 G
and 5.56 G). The B–Cl bonded system III displays a cou-
pling of –7.37 G to the 11B isotope, and two inequivalent
couplings of 11.24 and 2.26 G to the 14N nuclei.
(18) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett.
1996, 77, 3865 doi:10.1103/PhysRevLett.77.3865.; (b) Per-
dew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78,
1396 doi:10.1103/PhysRevLett.78.1396.; (c) Perdew, J. P.;
Burke, K.; Ernzerhof, M. J. Chem. Phys. 1996, 105, 9982;
(d) Ernzerhof, M.; Scuseria, G. E. J. Chem. Phys. 1999,
110, 5029.
¨
(19) (a) Schafer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994,
100, 5829; (b) Eichkorn, K.; Weigend, F.; Treutler, O.;
Ahlrichs, R. Theor. Chem. Acc. 1997, 97, 119.
(20) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chim. Acta 1990, 77, 123. doi:10.1007/BF01114537.
(21) (a) Laaksonen, L. J. Mol. Graph. 1992, 33, 10; (b) Bergman,
D. L.; Laaksonen, L.; Laaksonen, A. J. Mol. Graph. Model.
1997, 15, 301. doi:10.1016/S1093-3263(98)00003-5.
(28) Crystals of 2b could not be obtained from diethyl ether. The
attempted recrystallization of 2b from THF resulted in a yel-
low solution, which contained the tin(IV) complex 1b [1H
NMR spectrum (in d8-THF)] as one component.
1
(22) Konu, J.; Tuononen, H. M.; Chivers, T.; Corrente, A. M.;
(29) The variable temperature H NMR spectra of the lead com-
´
Boere, R. T.; Roemmele, T. Inorg. Chem. 2008, 47, 3823.
plex 2c exhibit similar trends; the broad singlet observed for
t-Bu groups at 1.27 ppm at 23 8C resolves into three reso-
nances at 1.36, 1.30, and 1.25 ppm (with a 1:2:1 intensity
ratio) at –10 8C. However, these and other NMR data for 2c
were obtained on a reaction mixture and, consequently, any
conclusions are necessarily tentative.
doi:10.1021/ic702435e.
(23) (a) Pauling, L. The Nature of the Chemical Bond, 3rd ed.;
Cornell University Press: Ithaca, NY, 1960; (b) Housecroft,
C. E.; Sharpe, A. G. Inorganic Chemistry, 2nd ed.; Pearson
Education Ltd.: Harlow, UK. 2005.
(24) Geschwentner, M.; Noltemeyer, M.; Elter, G.; Meller, A. Z.
Anorg. Allg. Chem. 1994, 620, 1403. doi:10.1002/zaac.
19946200813.
(30) Kim, S.-J.; Lee, Y.-J.; Kim, S. H.; Ko, J.; Cho, S.; Kang, S.
O. Organometallics 2002, 21, 5358. doi:10.1021/om020587l.
(31) Brask, J. K.; Chivers, T. Angew. Chem. Int. Ed. Engl. 2001, 40,
3960. doi:10.1002/1521-3773(20011105)40:21<3960::AID-
ANIE3960>3.0.CO;2-U.
¨
(25) Fusstetter, H.; Noth, H. Chem. Ber. 1979, 112, 3672. doi:10.
1002/cber.19791121115.
(26) Spiroconjugation is thought to contribute to the stability of the
Published by NRC Research Press