Synthesis and characterization of stable N-heterocyclic plumbylenes

Article abstract of DOI:10.1002/ejic.200701260

The reaction of N,N′-dialkyl-1,2-diaminobenzenes with bis[bis(trimethylsilyl)amido]lead(II) leads to the corresponding N-heterocyclic plumbylenes (R = Me: 1; R = Et: 2; R = isobutyl: 3; R = neopentyl: 4). The steric demand of the N,N′-dialkyl substituents

Full text of DOI:10.1002/ejic.200701260

SHORT COMMUNICATION
DOI: 10.1002/ejic.200701260
Synthesis and Characterization of Stable N-Heterocyclic Plumbylenes
F. Ekkehardt Hahn,*[a] Dennis Heitmann,[a] and Tania Pape[a]
Keywords: Lead / N-Heterocyclic plumbylenes / Carbene analogues / X-ray structure determination
The reaction of N,NЈ-dialkyl-1,2-diaminobenzenes with
and 4 to form dimers in the solid state with intermolecular
bis[bis(trimethylsilyl)amido]lead(II) leads to the correspond- [η2-(C6H4)···Pb]2 contacts for 3 and [η6-(C6H4)···Pb]2 contacts
ing N-heterocyclic plumbylenes (R = Me: 1; R = Et: 2; R =
isobutyl: 3; R = neopentyl: 4). The steric demand of the N,NЈ-
dialkyl substituents determines the mode of aggregation of
the plumbylenes. X-ray diffraction analyses show both 3
for 4.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
stituents at the 1,2-diaminobenzene. However, the yields are
generally very good (89–92%) in all cases. Initiation of the
reaction can be monitored visually by a color change of
the reaction mixture from yellow-orange to red or purple,
respectively. The benzimidazolin-derived plumbylenes were
isolated as intensely red (1, 2) or purple (3, 4) crystalline
solids. They are very sensitive towards oxygen and moisture.
Introduction
The heavier analogues of diaminocarbenes E[N(SiR3)2]2
(E = Ge, Sn, Pb) have been described as early as 1974 by
Lappert et al.[1] They are highly reactive, but thermally
stable, coloured and mostly diamagnetic compounds. Later,
the heavier analogues of the Arduengo carbene[2] with di-
valent silicon,[3] germanium[4] and tin[5] have been synthe-
sized. In addition, benzannulated N-heterocyclic carb-
enes,[6] silylenes,[7] germylenes[8] and stannylenes[9] are
known. While some monomeric diaryl- and bis(trimethyl-
silyl)plumbylenes[10] and -diplumbenes[11] have been re-
ported, no N-heterocyclic plumbylenes have been described
in the literature. Lappert et al. mention a benzannulated N-
heterocyclic plumbylene and its carbene adduct[12] but no
spectroscopic or structural data about the plumbylene are
given. We describe here the preparation of four benzannu-
lated N-heterocyclic plumbylenes with different N,NЈ-sub-
stituents and the molecular structures of two of these deriv-
atives.
Scheme 1. Transmetalation of Pb[N(SiMe3)2]2 with N,NЈ-dialkyl-
1,2-diaminobenzene derivatives.
Compound 1 precipitates directly from the reaction
solution and was isolated in pure form by filtration. The
plumbylenes 2–4 were isolated by filtration after concentra-
tion of the reaction solution. The EI mass spectra of all
four plumbylenes show a peak for the molecular ion as the
most abundant signal exhibiting the correct isotope pattern.
Plumbylene 1 is insoluble in polar and nonpolar organic
solvents, while 2–4 are sparingly soluble in toluene and ben-
zene. The solubility increases with the steric bulk of the
Results and Discussion
The preparation of the benzimidazolin-derived plumbyl-
enes 1–4 (Scheme 1) was achieved by the transamination
reaction of bis[bis(trimethylsilyl)amido]lead(II)[1] with a
suitable 1,2-diaminobenzene derivative in toluene. The
preparation of various differently N,NЈ-alkylated 1,2-diami-
nobenzene precursors has been described.[8d,8e,9c] The driv-
ing forces for the transamination reaction are believed to be
a combination of chelate and entropy effects. The reaction
rate decreases with increasing steric bulk of the N,NЈ-disub-
1
N,NЈ-disubstituents. The H NMR spectroscopic data for
compounds 2–4 show a downfield shift of about 1.8 ppm
for the signal of the methylene protons next to the nitrogen
atoms relative to the free 1,2-diaminobenzenes. Plumbylene
formation was also detected in the 13C NMR spectra by the
downfield shift of the resonance signal of the ipso-carbon
atoms from δ ≈ 137 ppm for the 1,2-diaminobenzenes to δ
≈ 156 ppm for the plumbylenes.
[a] Institut für Anorganische und Analytische Chemie, Westfälische
Wilhelms-Universität Münster,
Corrensstraße 36, 48149 Münster, Germany
Fax: +49-251-8333108
E-mail: fehahn@uni-muenster.de
Suitable single crystals for X-ray diffraction analyses
could be obtained for 3 and 4 by cooling of a saturated
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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F. E. Hahn, D. Heitmann, T. Pape
SHORT COMMUNICATION
toluene solution. Both compounds are found to be dimers fashion (range of the sum of angles at N1 and N2 359.6–
in the solid state (Figures 1 and 2). However, the dimeric 359.8°). The Cring–N–Pb angles deviate most significantly
structures do not arise from intermolecular Pb–Pb or Pb– from 120° [range 115.5–116.5(6)°]. The Pb–N distances in 3
N contacts but from interactions of the lead atom of one [range 2.150(7)–2.164(8) Å] are, as expected, elongated
molecule with the aromatic six-membered ring of a second when compared to the equivalent distances in heterocyclic
molecule.
benzannulated stannylenes (Sn–N ca. 2.08 Å)[9] and germyl-
enes (Ge–N ca. 1.87 Å).[8] The N–E–N angle, however, be-
comes more acute for E = Pb (75.7°) when compared to E
= Sn and E = Ge.
The dimer of 3 is formed by intermolecular interactions
of ring atoms C4 (Pb*–C4 3.367 Å) and C5 (Pb*–C5
3.390 Å) with the symmetry-related atom Pb*. The other
intermolecular Pb*–C distances are about 0.4 Å longer
(Pb*–C3/C6 ca. 2.70 Å, Pb*–C1/C2 ca. 4.10 Å).
A slightly different mode of dimerization was found for
4. Here, one molecule of 4 (the asymmetric unit contains
only 1/2 molcule of 4, related to the other half by a mirror
plane containing the lead atom and bisecting the C3–C3*
bond) is placed on top of a second molecule with the lead
atoms residing on top of the midpoint of the six-membered
benzene ring of the adjacent molecule. This leads to six Pb–
C separations which fall in the range from 3.348 Å for Pb–
C3/C3* to 3.601 Å for Pb–C1/C1*. The distance between
the lead atom and the midpoint of the six-membered ring
of the adjacent molecule measures 3.17 Å. A similar type
of [η6-(C6H4)···Sn]2 dimerization has been described for a
benzannulated N-heterocyclic stannylene.[9a]
Figure 1. Molecular structure of dimeric 3 in the solid state. Se-
lected bond lengths [Å] and angles [°] for molecule 1 [molecule
2]: Pb–N1 2.150(7) [2.164(8)], Pb–N2 2.160(7) [2.154(8)], N1–C1
1.379(11) [1.379(11)], N1–C7 1.448(13) [1.448(3)], N2–C2 1.359(11)
[1.373(13)], N2–C11 1.453(13) [1.446(14)], C1–C2 1.462(14)
[1.438(14)]; N1–Pb–N2 75.6(3) [75.8(3)], Pb–N1–C1 116.5(6)
[115.5(6)], Pb–N1–C7 122.7(6) [123.0(7)], C1–N1–C7 120.6(8)
[121.2(8)], Pb–N2–C2 116.5(6) [115.8(7)], Pb–N2–C11 122.8(6)
[123.5(7)], C2–N2–C11 120.4(8) [120.3(9)].
Conclusions
Four benzannulated N-heterocyclic plumbylenes have
been prepared and characterized. The solubility and dimer-
ization behaviour of these plumbylenes is determined by the
steric bulk of the N,NЈ-disubstituents. The plumbylenes 3
and 4 are dimeric in the solid state exhibiting inter-
molecular [η2-(C6H4)···Pb]2 or [η6-(C6H4)···Pb]2 contacts
which are shorter than the sum of the van der Waals radii
for carbon and lead (3.72 Å).
Experimental Section
Starting Materials, Reaction Conditions and Instrumentation: All
manipulation were carried out under argon using Schlenk or glove-
box techniques. Toluene was dried with sodium/benzophenone and
was freshly distilled prior to use. C6D6 was dried with Na/K alloy.
The preparation of differently substituted N,NЈ-dialkyl-1,2-diami-
nobenzenes has been described.[8d,8e,9c] Pb[N(SiMe3)2]2 was pre-
pared as described in the literature.[1]
Figure 2. Molecular structure of dimeric 4. Selected bond lengths
[Å] and angles [°]: Pb–N1 2.180(4), N1–C1 1.373(7), N1–C4
1.457(6) C1–C1* 1.440(10); N1–Pb–N1* 75.4(2), Pb–N1–C1
115.3(3), Pb–N1–C4 121.8(4), C1–N1–C4 122.2(5).
General Procedure for the Synthesis of Plumbylenes 1–4: A sample
of bis[bis(trimethylsilyl)amido]lead(II) (0.268 g, 0.5 mmol) was
added to
a solution of an N,NЈ-dialkyl-1,2-diaminobenzene
(0.5 mmol) in toluene (8 mL). The reaction mixture was then
stirred under argon at ambient temperature for 36 h. Subsequently,
all volatiles were removed in vacuo, and the residue was recrys-
tallized from toluene with the exception of 1 which is insoluble in
toluene. Plumbylene 1 precipitated in analytically pure form from
Plumbylene 3 crystallizes with two essentially identical
molecules in the asymmetric unit. Two identical molecules
from neighbouring asymmetric units form one dimer (Fig-
ure 1). The five- and six-membered rings in 3 are planar,
and the nitrogen atoms are surrounded in a trigonal-planar the toluene reaction solution.
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Eur. J. Inorg. Chem. 2008, 1039–1041

Stable N-Heterocyclic Plumbylenes
[N,NЈ-Dimethylbenzene-1,2-diamido]lead(II) (1): Yield 0.151 g
(89%) of a red crystalline solid. MS (EI, 70 eV): m/z (%) = 342
(100) [M]+, 208 (4.3) [Pb]+, 133 (65.7) [M – Pb]+, 119 (16.5) [M –
Pb – CH3]+.
reflections [IՆ2σ(I)], refinement against |F2|with anisotropic ther-
mal parameters for all non-hydrogen atoms and hydrogen atoms
in calculated positions. The molecule resides on a crystallographic
mirror plane orientated perpendicularly to the ring plane. The mir-
ror plane contains the Pb atom and passes through the C3–C3*
bond.
[N,NЈ-Diethylbenzene-1,2-diamido]lead(II) (2): Yield 0.150 g (80%)
of a dark red crystalline solid. 1H NMR (200 MHz, C6D6): δ =
3
7.00 (m, 2 H, Ar-H), 6.68 (m, 2 H, Ar-H), 4.80 (q, J = 6.7 Hz, 4
H, NCH2CH3), 0.92 (t, 3J = 6.7 Hz, 6 H, NCH2CH3) ppm. 13C
NMR (50.3 MHz, C6D6): δ = 155.5 (Ar-Cipso), 117.4 (Ar-Cmeta),
112.5 (Ar-Cortho), 44.9 (NCH2CH3), 19.0 (CH2CH3) ppm. MS (EI,
70 eV): m/z (%) = 370 (100) [M]+, 342 (44) [M – Et]+, 208 (4.3)
[Pb]+, 164 (43) [M – Pb]+.
Acknowledgments
The authors thank the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie for financial support.
[N,NЈ-Diisobutylbenzene-1,2-diamido]lead(II) (3): Yield 190 mg
1
(88%) of a purple crystalline solid. H NMR (200 MHz, C6D6): δ
= 6.87 (m, 2 H, Ar-H), 6.68 (m, 2 H, Ar-H), 4.81 (d, J = 6.5 Hz,
3
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6.5 Hz, 12 H, CH(CH3)2] ppm. 13C NMR (50.3 MHz, C6D6): δ =
155.0 (Ar-Cipso), 117.4 (Ar-Cmeta), 113.3 (Ar-Cortho), 58.2
(NCH2CH), 29.8 [NCH2CH(CH3)2], 21.2 [NCH2CH(CH3)2] ppm.
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[N,NЈ-Dineopentylbenzene-1,2-diamido]lead(II) (4): Yield 209 mg
1
(92%) of a purple crystalline solid. H NMR (200 MHz, C6D6): δ
= 7.16 (m, 2 H, Ar-H), 6.83 (m, 2 H, Ar-H), 4.90 [s, 4 H,
NCH2C(CH3)3], 0.88 [s, 18 H, C(CH3)3] ppm. 13C NMR
(50.3 MHz, C6D6): δ = 155.6 (Ar-Cipso), 116.8 (Ar-Cmeta), 113.5
(Ar-Cortho), 60.2 [NCH2C(CH3)3], 34.6 [CH2C(CH3)3], 28.3
[CH2C(CH3)3] ppm. MS (EI, 70 eV): m/z (%) = 454 (100) [M]+, 397
(11.0) [M – C(CH3)3]+, 248 (6.4) [M – Pb]+.
X-ray Diffraction Studies: Diffraction data for 3 and 4 were col-
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with a rotation anode at 153(2) K using graphite-monochromated
Mo-Kα radiation (λ = 0.71073 Å). Diffraction data were collected
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using the heavy-atom method and were refined with SHELXL-
97[15] against |F2| using first isotropic and later anisotropic thermal
parameters for all non-hydrogen atoms. Hydrogen atoms were
added to the structure models in calculated positions. CCDC-
673663 (3) and CCDC-673664 (4) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
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Crystal Data for Plumbylene 3: C14H22N2Pb, M = 425.53, triclinic,
P1, Z = 4, a = 10.529(3), b = 11.398(3), c = 13.280(3) Å, α =
¯
73.398(4), β = 89.522(4), γ = 76.447(5)°, V = 1481.8(6) Å3, 12061
measured reflections, 5250 unique reflections (Rint = 0.0332), R =
0.0424, wR = 0.1082 for 3833 contributing reflections [IՆ2σ(I)],
refinement against |F2|with anisotropic thermal parameters for all
non-hydrogen atoms and hydrogen atoms in calculated positions.
The asymmetric unit contains two almost identical molecules of 3.
[13] SMART, Bruker AXS, 2000.
[14] SHELXS-97: G. M. Sheldrick, Acta Crystallogr., Sect. A 1990,
Crystal Data for Plumbylene 4: C16H26N2Pb, M = 453.58, tetrago-
nal, P42mbc, Z = 8, a = 12.4313(11), c = 21.323(4) Å, V =
3295.2(7) Å3, 21823 measured reflections, 1341 unique reflections
(Rint = 0.0944), R = 0.0294, wR = 0.0491 for 1146 contributing
46, 467.
[15] G. M. Sheldrick, SHELXL-97, University of Göttingen, 1997.
Received: November 23, 2007
Published Online: January 29, 2008
Eur. J. Inorg. Chem. 2008, 1039–1041
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