Structure, properties and comparison of c,n-chelated and amido-stabilized plumbylenes

Article abstract of DOI:10.1135/cccc2009093

The molecular structure of {2-[(CH3)2NCH2]C6H4}2Pb (1) in the solid state was determined by X-ray diffraction techniques on a single crystalline material. Bulky amido ligand stabilized complexes [(i-C3H7)2C6H3]NHLi (2) and [(i-C3H7)2C6H3N(Me3Si)]2Pb (3) w

Full text of DOI:10.1135/cccc2009093

C,N-Chelated and Amido-Stabilized Plumbylenes
121
STRUCTURE, PROPERTIES AND COMPARISON OF
C,N-CHELATED AND AMIDO-STABILIZED PLUMBYLENES
a
a
a
b
Josef BAREŠ , Vítězslav ŠOUREK , Zdeňka PADĚLKOVÁ , Philippe MEUNIER ,
Nadine PIRIO , Ivana CÍSAŘOVÁ , Aleš RŮŽIČKAa1,* and Jaroslav HOLEČEK
b
c
a
a
Department of General and Inorganic Chemistry, Faculty of Chemical Technology,
University of Pardubice, nám. Čs. legií 565, 532 10, Pardubice, Czech Republic;
1
e-mail: ales.ruzicka@upce.cz
Institute de Chimie Moleculaire de l’Université de Bourgogne,
b
9 avenue Alain Savary, BP 47870, F-21078 Dijon Cedex, France
Department of Inorganic Chemistry, Faculty of Science, Charles University,
c
Hlavova 2030, 128 40 Prague 2, Czech Republic
Received August 3, 2009
Accepted October 26, 2009
Published online February 10, 2010
Dedicated to Professor Pavel Kratochvíl on the occasion of his 80th birthday in recognition of his
outstanding contributions to the area of macromolecular chemistry.
The molecular structure of {2-[(CH3)2NCH2]C6H4}2Pb (1) in the solid state was determined
by X-ray diffraction techniques on a single crystalline material. Bulky amido ligand stabi-
lized complexes [(i-C3H7)2C6H3]NHLi (2) and [(i-C3H7)2C6H3N(Me3Si)]2Pb (3) were synthe-
sized and characterized both in the solid state and solution by 1H, 13C, 15N and 207Pb NMR
and XRD techniques. The structure of both C,N-chelated and bulky amido plumbylenes is
compared.
Keywords: Lead; Bulky amides; C,N-chelates; Plumbylenes; Organoelement chemistry; X-ray
diffraction.
Organolead compounds have been known for almost 150 years. Tetraethyl-
lead became an industrial commodity in the last century but is now being
banned because of the negative environmental impact related to its toxic-
ity1,2
.
Plumbylenes usually occur as reactive intermediates in the preparation of
plumbanes R4Pb and undergo polymerization and/or disproportionation in
the absence of stabilizing groups at the lead(II) center3. Thus, only a few ex-
amples of low-valent plumbylenes have been reported so far4, although the
first diamino- and dialkylplumbylenes were already described by 1H and
13C NMR spectroscopy5,6. Another example of a lead(II) complex, stabilized
by the intramolecular coordination, was described recently by Stalke7.
Collect. Czech. Chem. Commun. 2010, Vol. 75, No. 1, pp. 121–131
© 2010 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc2009093

122
Bareš et al.:
Besides, O,C,O- and similar chelated plumbylenes were described8. All of
these compounds contain at least one electron-withdrawing substituent,
which makes the lead atom a Lewis acid allowing intramolecular coordina-
tion. The most recent studies on the field of amido-stabilized plumbylenes
with angular geometry and tendency to form dimers are published in pa-
pers of Hahn9.
We have recently reported reactivity and products of C,N-chelated
plumbylene 1 6 with a zirconocene derivative known as the Negishi re-
agent10 (Scheme 1) where the product is zircona complex 5 formed by C–H
activation and C–C coupling, containing two five-membered rings11. A dif-
ferent product of transmetallation, 6, the product of ligand transfer from
lead to zirconium can be isolated if one equivalent of n-butyllithium in-
stead of two is used for in situ generation of zirconocene12.
The use of different stannylene (stabilized with bulky amide group) in
the same reactions yields the same type of complex but one of the amido
ligands on the tin atom is replaced by n-butyl group (Scheme 2)13.
SCHEME 1
Reactions of zirconocene dichloride derivatives with 1 and 4
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C,N-Chelated and Amido-Stabilized Plumbylenes
123
SCHEME 2
Reactions of zirconocene dichloride derivatives with 7 and 8
1
In this paper, the crystal structure and H, 13C, 15N and 207Pb NMR spec-
tral description of the known C,N-chelated plumbylene and the mutual
comparison of this compound to the new amido-stabilized plumbylene are
described.
RESULTS AND DISCUSSION
The intramolecularly coordinated homoleptic organolead(II) compound 1 6
and bisamido lead compound 3 have been obtained in good yields by reac-
tions of the corresponding organolithium compounds with PbCl2 at low
temperature (for 2 and 3, see Scheme 3).
SCHEME 3
Synthesis of 3
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124
Bareš et al.:
1
Compound 1 was identified by H and 13C NMR spectra in the past6. Two
different polymorphs (1 and 1′) were obtained from diethyl ether or THF
solution. Both polymorphs crystallize in the same crystal system (mono-
clinic, P21/c, see Table I), but there are significant differences in the cell pa-
rameters found. The molecular structures of both polymorphs are slightly
different and also the crystal packing differs significantly. The central lead
atoms in both 1 and 1′ are coordinated with four atoms two carbon and
two nitrogen atoms (Fig. 1). The coordination geometry about lead atoms
in 1 and 1′ is ψ-distorted trigonal bipyramidal with two carbon atoms and
the lone electron pair in equatorial and nitrogen atoms in axial positions.
The Pb–N distances are slightly different (2.727(3) and 2.616(3) Å for 1 or
2.712(3) and 2.682(3) Å for 1′). The geometry of the lead central atom is
very similar to the tin coordination vicinity in organotin(II) analogue14 of
1. Slightly longer distances between lead and carbon or nitrogen atoms are
caused by a higher covalent radius of lead compared to tin. From the
C11–Pb1–C21 angles (99.68(13)° for 1 and 92.01(11)° for 1′), which is the
main difference between both polymorphs, we can conclude that the lone
electron pairs in the compound are situated in radial s-orbital.
FIG. 1
Molecular structure of 1 (ORTEP 40% probability level). Hydrogen atoms are omitted for clar-
ity. Selected interatomic distances (in Å) and angles (in °) of 1 (the appropriate parameters for
the second polymorph 1′ are given in parentheses): Pb1–N1 2.727(3) (2.712(3)), Pb1–N2
2.616(3) (2.682(3)), Pb1–C11 2.311(3) (2.319(3)), Pb1–C21 2.294(4) (2.311(3)); N11–Pb1–N21
163.54(9) (152.67(7)), C11–Pb1–C21 99.68(13) (92.01(11))
Collect. Czech. Chem. Commun. 2010, Vol. 75, No. 1, pp. 121–131

C,N-Chelated and Amido-Stabilized Plumbylenes
125
In 207Pb NMR spectrum, a signal at 2624 ppm was observed. An impor-
tant tool in evaluation of coordination between donor nitrogen and metal
atoms is the 15N NMR spectroscopy. The value of 15N NMR chemical shift
(–351.11 ppm) indicates a weak interaction comparable to that found in
tetraorganotin(IV) compounds bearing the same ligand15.
The synthesis of compound 3 and many other compounds of current in-
terest includes the lithiation, silylation and further lithiation of bulky
anilido ligands. Compound 2 was isolated after the first step of reaction
procedure. The nitrogen atoms (N1, N1a) and lithium atoms (Li1, Li1a) in 2
(Fig. 2) are in the same plane, while the oxygen atoms of diethyl ether coor-
dinated to lithium atoms are below or above this plane. The planes formed
by phenyl rings are perpendicular to this plane and parallel to each other.
The center of symmetry of this compound is in the cross point of Li1–Li1a
and N1–N1a bisectors.
Compound 3 was prepared from 2 by silylation and further reaction with
0.5 molar equivalent of PbCl2. Both isopropylphenyl groups are oriented
perpendicularly to the plane described by N1, Pb1 and N2 atoms (Fig. 3) be-
cause of high steric repulsion. In the comparison with the tin analogue, the
geometry of this compound is almost the same16. Small differences in dis-
FIG. 2
The molecular structure of 2·Et2O (ORTEP 40% probability level). Hydrogen atoms and ethyl
groups of diethyl ether are omitted for clarity. Selected interatomic distances (in Å) and angles
(in °): Li1–O1 1.918(2), Li1–N1 2.025(2), Li1–N1a 1.989(2), Li1–Li1a 2.473(3), N1–C1
1.3843(14), N1–N1a 3.1616(13); C1–N1–Li1 118.03(9), N1–Li1–N1a 103.93(10), Li1–N1–Li1a
76.07(9), O1–Li1–N1 129.47(11)
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Bareš et al.:
tances between lead or tin and nitrogen atoms are caused by a higher cova-
lent radius of the lead atom. Judging from the value of the N1–Pb1–N2
angle (111.34(10)°), the structure can be described as singlet 1σ2. Both
nonbonding and bonding electron pairs occupy sp2-hybrid orbitals and the
p orbital is unoccupied. An alternative description of the bonding situation
is that the 6s2 electrons, stabilized by relativistic effects, play no part, the
bonding electrons are in 6p orbitals, and the large size of Pb atom leaves
room for several weaker interactions that give apparent coordination num-
bers greater than 2. The difference from the theoretical model (120°) is
caused by steric requirements of the ligands (mainly SiMe3 groups). There is
a weak interaction between lead and carbon atoms (or the aromatic system
of the phenyl ring) in the molecule due to suitable orientation of lead and
the ipso-carbon (dashed lines in Fig. 3). These interactions (2.877(3) and
2.940(3) Å) are weaker than those found in 1-plumba-closo-heptaboranes17,
but much stronger than those found for dimeric plumbylenes of Hahn
(3.367 Å). On the other hand, normal Pb–C bond lengths range from 2.3 to
2.4 Å 18
.
FIG. 3
The molecular structure of 3 (ORTEP 40% probability level). Hydrogen atoms are omitted for
clarity. Selected interatomic distances (in Å) and angles (in °): Pb1–N1 2.204(3), Pb1–N2
2.198(3), Pb1–C1 2.877(3), Pb1–C21 2.940(3), C1–N1 1.432 (4), C21–N2 1.430(4), N1–N2
3.635(4), N1–Si1 1.735(3), N2–Si2 1.732(3); N1–Pb1–N2 111.34(10), C1–Pb1–C21 164.39(9),
Pb1–N1–Si1 137.73(16), Pb1–N2–Si2 132.05(16), Pb1–N1–C1 102.6(2), Pb1–N2–C21 106.3(2)
Collect. Czech. Chem. Commun. 2010, Vol. 75, No. 1, pp. 121–131

C,N-Chelated and Amido-Stabilized Plumbylenes
127
The structure of 3 in solution seems to be very close to that in the solid
state judging from the 1H NMR spectra pattern with non-equivalent iso-
propyl groups. From the 207Pb NMR chemical shift value (4070 ppm) can be
concluded that the 207Pb NMR shift is strongly dependent on the coordina-
tion geometry of lead atom compared with the chemical shift 2624 ppm
found for 1, which has a distorted trigonal bipyramidal geometry (ψ-TBP)
of the lead central atom, where the organic substituents together with lone
electron pair occupy the equatorial positions and intramolecularly coordi-
nated amino groups axial ones. The chemical shifts of other kinetically
stabilized monomeric dialkyl- and diarylplumbylenes (δ(207Pb) = 10050–
8884 ppm)19 are much greater than those for plumbylenes in which the lead
atom has a coordination number greater than 2 (1981–4878 ppm)19c,20 and
diaminoplumbylene (4916 ppm)19b. Diarylplumbylene (δ(207Pb) = 3870 ppm)
bearing bulky terphenyl substituents resonates at a much higher fields than
other kinetically stabilized plumbylenes21. Similar discrepancy has been
found for four-coordinated plumbylene with one O,C,O-chelating and one
bulky alkyl ligands of Jurkschat8a (δ(207Pb) = 6874 ppm).
EXPERIMENTAL
All procedures were carried out using the modified Schlenk techniques under an argon at-
mosphere. The resulting plumbylenes need to be protected from light.
Synthesis of 1
2-[(CH3)2NCH2]C6H4Li (1.5 g, 0.011 mol) in diethyl ether (100 ml) was added slowly to a
mixture of PbCl2 (1.67 g, 0.006 mol) with diethyl ether (50 ml) and stirred at room tempera-
ture for 24 h. The slightly green solution was filtered off, concentrated to about half of the
initial volume and placed in a freezer at –20 °C for 1 day. Compound 1 was obtained as col-
orless crystals suitable for X-ray crystallography, which decompose in solution on daylight
to elemental lead and an unidentified green solution. Yield 2.06 g (0.0043 mol, 78%). For
C
18H24N2Pb (475.58) calculated: 45.46% C, 5.09% H, 5.89% N; found: 45.38% C, 5.06% H,
5.93% N. The compound was identified by NMR spectroscopy as a single product: 1H NMR
(360 MHz, toluene-d8, 300 K): 2.41 (s, 12 H, CH3); 3.83 (s, 4 H, CH2); 7.37 (t, 2 H, CH(6),
3J(1H,1H) = 5.6); 7.51 (d, 2 H, CH(4), 3J(1H,1H) = 5.7); 7.62 (t, 2 H, CH(5), 3J(1H,1H) = 6.0);
8.37 (d, 2 H, CH(3), 3J(1H,1H) = 7.0). 207Pb NMR (75 MHz, toluene-d8, 300 K): 2624. 15N NMR
(36.5 MHz, toluene-d8, 300 K): –351.11.
Synthesis of (i-C3H7)2C6H3NHSiMe3
To (i-C3H7)2C6H3NH2 (9.5 g, 0.052 mol) in diethyl ether (30 ml) was added CH3SiCl (5.91 g,
0.055 mol) and the reaction mixture stirred at –30 °C for 1 h. The orange solution was fil-
tered and the solvents were evaporated. The product was obtained as an orange oil. Yield
11.68 g (0.0466 mol, 89.7%).
Collect. Czech. Chem. Commun. 2010, Vol. 75, No. 1, pp. 121–131

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Bareš et al.:
Synthesis of 2
To a mixture of (i-C3H7)2C6H3NHSiMe3 (11.68 g, 0.047 mol) and hexane (100 ml) was added
n-butyllithium (30 ml) in hexane (c = 1.6 mol/l, 0.048 mol). The mixture was stirred for at
room temperature for 3 h. The solid state product was filtered off, washed with hexane and
dried in vacuo. Yield 10.48 g (0.041 mol, 87%).
Synthesis of 3
To PbCl2 (5.06 g, 0.018 mol) in THF (50 ml) was added 2 (9.33 g, 0.036 mol) in THF (100 ml)
at room temperature. The mixture was stirred for 4 h, its color changed to red. The solvents
were removed under reduced pressure, and the residue was extracted with hexane. The ex-
tract was filtered, concentrated to ca. 60% of the initial volume and placed in a freezer at
–20 °C for 2 days. Compound 3 was obtained as red crystals. Yield 10.29 g (0.0146 mol,
81%), m.p. 220.1 °C (decomp.). The compound was identified by NMR as a single product.
1H NMR (360 MHz, toluene-d8, 300 K): 0.32 (s, 18 H, Si-CH3); 1.05, 1.23 (dd, 24 H, CH-CH3,
J(A,B) = 68.9, 3J(1H,1H) = 6.9); 3.68 (heptet, 4 H, CH, 3J(1H,1H) = 6.9); 6.85 (t, 2 H, CH(4),
3J(1H,1H) = 7.8); 7.09 (d, 4 H, CH(3), 3J(1H,1H) = 5.7. 207Pb NMR (75 MHz, toluene-d8,
300 K): 4070.
X-ray Diffraction
Single crystals of the studied compounds suitable for X-ray diffraction studies were obtained
at –20 °C from solutions in various solvents (mainly diethyl ether). Crystallographic data
were obtained on a Nonius KappaCDD diffractometer with an area detector, source of MoKα
and a graphite monochromator. Suitable single crystals were mounted on glass fibre in an
inert oil and measured at the wavelength 0.71073 Å.
The structures were solved by direct methods (SIR92)22. All reflections were used in the
structure refinement based on F2 by full-matrix least-squares technique (SHELXL97)23. Heavy
atoms were refined anisotropically. Hydrogen atoms were mostly localized on a difference
Fourier map; however, to ensure uniformity of crystal treatment, all hydrogen atoms were
recalculated in idealized positions (riding model) and assigned temperature factors Hiso(H) =
1.2Ueq (pivot atom) or of 1.5Ueq for the methyl group. Absorption corrections were made
using the Gaussian integration from crystal shape24
.
CCDC 740899 (for 1), 740900 (for 1′), 740901 (for 2) and 740902 (for 3) contain the sup-
plementary crystallographic data for this paper. These data can be obtained 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).
NMR Spectroscopy
NMR spectra of the studied compounds were measured in deuterated solvents (toluene-d8,
benzene-d6) at 220–360 K on Bruker AMX 360 and Bruker Avance 500 spectrometers in
pulsed mode with Fourier transformation. 1H, 15N and 207Pb NMR spectra were measured at
frequencies 360.13 (500.14) MHz, 36.5 MHz and 75.34 MHz, respectively, in a 5 mm wide-
zone tuneable sampler. The values of 1H NMR chemical shifts were calibrated to internal
inert standard tetramethylsilane (δ(1H) = 0.00 ppm) or to residual signals of benzene
Collect. Czech. Chem. Commun. 2010, Vol. 75, No. 1, pp. 121–131

C,N-Chelated and Amido-Stabilized Plumbylenes
129
TABLE I
Crystallographic parameters for compounds 1, 1′, 2 and 3
Parameter
1
1
2
3
Empirical
formula
C
18H24N2Pb
C18H24N2Pb
C12H18LiN
C30H52N2PbSi2
Crystal system
Space group
a, Å
monoclinic
P21/c
monoclinic
P21/c
monoclinic
C2/c
monoclinic
P21/c
10.1091(3)
10.0179(4)
17.3922(6)
90.00
12.1570(2)
14.8668(3)
10.6320(3)
90.00
13.4343(3)
11.6970(2)
21.9955(5)
90.00
15.0339(2)
9.6941(2)
24.0312(4)
90.00
b, Å
c, Å
α, °
β, °
96.934(2)
90.00
113.702(1)
90.00
104.843(1)
90.00
103.954(1)
90.00
γ, °
Z
4
4
8
4
V, Å3
1748.44(11)
1.807
1759.52(7)
1.795
3341.32(12)
1.023
3398.91(10)
1.376
Dc, g/cm
Crystal size, mm
Crystal shape
µ, mm–1
F(000)
0.25×0.2×0.15
colorless block
9.645
0.2×0.18×0.15
colorless plate
9.585
0.25×0.25×0.40
colorless block
0.061
0.4×0.3×0.25
orange plate
5.053
912
912
1136
1424
h, k, l range
–13/13, –12/12,
–22/22
–15/15, –19/19,
–13/13
–17/17, –15/15,
–18/28
–19/19, –12/12,
–31/23
θ range, °
3.56–27.47
1.83–27.52
1.92–27.46
1.75–27.45
Reflections mea-
sured
26322
30036
26526
41321
Reflections inde-
3988 (0.0499)
3590
4034 (0.0401)
3144
3819 (0.0202)
2945
7763 (0.0495)
6177
a
pendent (Rint
)
Reflections ob-
served [I > 2σ(I)]
Parameters
refined
195
195
172
331
Max/min τ, e Å–3
GOFb
1.518/–1.311
1.039
0.806/–0.574
1.123
0.254/–0.210
1.028
1.001/–1.265
1.027
Rc/wRc
0.0243/0.0561
0.0208/0.0454
0.0465/0.1278
0.0296/0.0594
2
2
b
2
c
a Rint = ∑|Fo – Fo,mean2|/∑Fo
;
S = [∑(w(Fo – Fc2)2)/(Ndiffrs – Nparams)]1/2
;
weighting scheme:
w = [σ2(Fo2) + (w1P)2 + w2P]–1, where P = [max(Fo2) + 2Fc2], R(F) = ∑||Fo| – |Fc||/∑|Fo|, wR(F2) =
[∑(w(Fo – Fc2)2)/(∑w(Fo2)2)]1/2
.
2
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Bareš et al.:
(δ(1H) = 7.16 ppm) or toluene (δ(1H) = 2.09 ppm). The values of 15N chemical shifts were
calibrated to a signal of external nitromethane standard in a coaxial capillary (δ(15N) =
0.0 ppm). The values of 207Pb chemical shifts were calibrated to Pb(NO3)2 in water and recal-
culated to PbEt4 (δ(207Pb) = 0.00 ppm). Positive values of chemical shifts indicate a shift to
lower field in compared with the standard. The coupling constants (J, Hz) were found in the
spectra measured with digital resolution better than 0.5 Hz/point.
The Pardubice group would like to thank the Grant Agency of the Academy of Sciences of the Czech
Republic (Grant KJB401550802) and the Ministry of Education, Youth and Sports of the Czech
Republic (Grant VZ 0021627501) for financial support.
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