Synthesis and X-ray crystal structure of Bis{2,4,6-tris[bis(trimethylsilyl)methyl]phenyl}dibromoplumbane: The first monomeric diorganodihaloplumbane in the crystalline state

Article abstract of DOI:10.1021/om970151j

Reaction of a plumbylene Tbt₂Pb: (1) with carbon tetrabromide in THF gave the first crystallographically characterized dibromoplumbane Tbt₂PbBr₂ (2) bearing two 2,4,6-tris[bis(trimethylsilyl)methyl]-phenyl (Tbt) groups. The X-ray crystallographic analysis revealed that 2 has a monomeric structure with a distorted tetrahedral lead center.

Full text of DOI:10.1021/om970151j

2748
Organometallics 1997, 16, 2748-2750
Syn th esis a n d X-r a y Cr ysta l Str u ctu r e of Bis{2,4,6-
tr is[bis(tr im eth ylsilyl)m eth yl]p h en yl}d ibr om op lu m ba n e:
Th e F ir st Mon om er ic Dior ga n od ih a lop lu m ba n e in th e
Cr ysta llin e Sta te
Naokazu Kano, Norihiro Tokitoh,* and Renji Okazaki*
Department of Chemistry, Graduate School of Science, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113, J apan
Received February 26, 1997X
Sch em e 1
Summary: Reaction of a plumbylene Tbt2Pb: (1) with
carbon tetrabromide in THF gave the first crystallo-
graphically characterized dibromoplumbane Tbt2PbBr2
(2) bearing two 2,4,6-tris[bis(trimethylsilyl)methyl]-
phenyl (Tbt) groups. The X-ray crystallographic analy-
sis revealed that 2 has a monomeric structure with a
distorted tetrahedral lead center.
plumbylenes, although they are expected to be poten-
tially useful for the synthesis of various organolead
compounds.6 In the course of our study on the reactions
of plumbylenes, we have found that a plumbylene Tbt2-
Pb: (1) reacts with an excess amount of carbon tetra-
bromide to give an extremely hindered dibromoplum-
bane, Tbt2PbBr2 (2). Here, we present the synthesis and
X-ray crystallographic analysis of the dibromoplumbane
2, which is the first example of a crystallographically
characterized monomeric diorganodihaloplumbane.
Diorganodihaloplumbanes are one of the most impor-
tant and fundamental compounds in the organic chem-
istry of lead.1 Although they have been known for many
years, very little has been studied about their struc-
tures. Diphenyldihaloplumbanes Ph2PbX2 (X ) Cl, Br,
and I) have been reported to have a polymeric structure
from vibrational spectoscopic analysis,2 but the X-ray
crystallographic structure of diorganodihaloplumbanes
is known only for Ph2PbCl2,3 which indicates the pres-
ence of symmetrical chlorine bridgings giving rise to
polymeric chains of octahedrally coordinated lead atoms.
There have been no reports of the X-ray crystallographic
analysis for the monomeric diorganodihaloplumbanes
with a tetrahedral lead center, in contrast to the well-
known tetrahedral counterparts of silicon, germanium,
and tin.1c
We have been studying the synthesis and isolation
of various kinds of highly reactive chemical species,
which tend to dimerize or polymerize, as stable mono-
meric compounds by taking advantage of steric protec-
tion due to the 2,4,6-tris[bis(trimethylsilyl)methyl]-
phenyl group (denoted as Tbt hereafter).4 We recently
succeeded in the kinetic stabilization of divalent orga-
nolead species, i.e. plumbylenes, by introduction of the
Tbt group and another bulky group on the lead atom.5
There have been only few reports on the reactivities of
Resu lts a n d Discu ssion
We recently reported the synthesis of plumbylenes
Tbt(Ar)Pb: [Ar ) 2,4,6-triisopropylphenyl (Tip) or 2,4,6-
tris(trimethylsilylmethyl)phenyl (Ttm)] by sequential
substitution of Pb[N(SiMe3)2]2 with TbtLi and ArLi.5
The plumbylene Tbt2Pb: (1) bearing two Tbt groups on
a lead atom was synthesized by almost the same
procedure, though the substitution reaction required a
higher temperature (-10 °C) than for those plumbylenes
(-40 °C) (Scheme 1). Plumbylene 1 is stable at room
temperature under an argon atmosphere, though sensi-
tive to the air and moisture. The reaction of 1 with an
excess amount of carbon tetrabromide in THF gave
dibromoplumbane 2 in a moderate yield (59%). This is
in a remarkable contrast with the reaction of methyl
iodide with plumbylenes Tip2Pb:, Tbt(Tip)Pb:, and Tbt-
(Ttm)Pb:, which gives the carbon-iodine insertion
products,5, 6 or that with a plumbylene [(Me3Si)2CH]2Pb:
, which gives lead diiodide.7 Dibromoplumbane 2 can
be purified even by silica gel chromatography and is
stable in the open air. In the solid state, 2 is stable to
the light, though slightly sensitive in solution, especially
in THF, giving TbtH. It is readily soluble in common
organic solvents such as THF, ether, chloroform, dichlo-
romethane, and toluene, but hardly soluble in ethanol,
hexane, and ethyl acetate.
X Abstract published in Advance ACS Abstracts, May 15, 1997.
(1) For reviews; (a) Rivie`re, P.; Rivie`re-Baudet, M.; Satge´, J .
Comprehensive Organometallic Chemistry; Pergamon: New York,
1982; Vol. 2, p 670. (b) Harrison, P. G. Comprehensive Organometallic
Chemistry II; Pergamon: New York, 1995; Vol. 2, p 305. (c) Harrison,
P. G. Comprehensive Coordination Chemistry; Pergamon: Oxford,
England, 1987; Vol. 3, p 185. (d) Abel, E. W. Comprehensive Inorganic
Chemistry; Pergamon: Oxford, England, 1973; Vol. 2, p 105.
(2) Clark, R. J . H.; Davies, A. G.; Puddephatt, R. J . Inorg. Chem.
1969, 8, 457. Wharf, I.; Cuenca, R.; Besso, E.; Onyszchuk, M. J .
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419.
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115, 2065. (b) Tokitoh, N.; Matsumoto, T.; Manmaru, K.; Okazaki, R.
J . Am. Chem. Soc. 1993, 115, 8855. (c) Tokitoh, N.; Suzuki, H.;
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H.; Tokitoh, N.; Nagase, S.; Okazaki, R. J . Am. Chem. Soc. 1994, 116,
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Ed. Engl. 1994, 33, 2316. (f) Saito, M.; Tokitoh, N.; Okazaki, R. J .
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Okazaki, R. Chem. Lett. 1995, 1087. (h) Tokitoh, N.; Kishikawa, K.;
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Tokitoh, N.; Okazaki, R. Chem. Lett. 1997, 277.
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Sulfur, Silicon Relat. Elem. 1994, 93-94, 189. (b) Tokitoh, N.; Kano,
N.; Shibata, K.; Okazaki, R. Organometallics 1995, 14, 3121.
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34, 1495.
(7) Cotton, J . D.; Davidson, P. J .; Lappert, M. F. J . Chem. Soc.,
Dalton Trans. 1977, 2275.
S0276-7333(97)00151-9 CCC: $14.00 © 1997 American Chemical Society

Notes
Organometallics, Vol. 16, No. 12, 1997 2749
of Pb and C, 2.23 Å,10 suggesting steric repulsion
between two Tbt groups. The Pb-Br bond length
[2.623(1) Å] is shorter than those in Ph3PbBr [2.852(1)
Å]11 and tris{2-[3,5-bis(1,1-dimethylethyl)phenyl]-2-
methylpropyl}bromoplumbane [2.656(7) Å]12 and in the
range of those [2.60-2.82 Å] in (Ph2PbBr)4C.13 This
value slightly exceeds the sum of the covalent radii of
the two atoms, 2.60 Å,10 suggesting that the Pb-Br bond
in 2 is not ionic but covalent, as has been seen in the
other group 14 element analogues. This is the first
successful isolation of a compound carrying two of the
very bulky Tbt groups attached geminally on one
element. Since we have been unable to synthesize such
a type of compound in the case of the other group 14
elements, it is conceivable that the long Pb-C bonds
enable the two Tbt groups to bind to one element for a
steric reason.
The nearest distance between intermolecular lead and
bromine atoms is 9.664(1) Å, showing the absence of
intermolecular contacts which have sometimes been
3
seen in triorganolead halides11,13 and Ph2PbCl2.
The angles of C(1)-Pb-C(1*) [135.8(3)°] and C(1)-
Pb-Br(1*) [114.4(2)°] are larger, and those of Br(1)-
Pb-Br(1*) [95.29(7)°] and C(1)-Pb-Br(1) [95.4(2)°] are
smaller than the usual tetrahedral angle, 109.5°. The
lead center is best regarded as distorted tetrahedral
rather than octahedral as has been seen in Ph2PbCl2.3
The enlarged C-Pb-C bond angle and Pb-C bond
length are most likely interpreted in terms of steric
repulsion between two bulky Tbt groups. Thus, we have
shown that a diorganodihaloplumbane, which tends to
form an octahedral polymer, can be synthesized as a
tetrahedral monomer by taking advantage of steric
protection due to bulky substituents.
F igu r e 1. ORTEP drawing of Tbt2PbBr2 (2) with thermal
ellipsoid plots (30% probability). Selected bond lengths (Å)
and angles (deg): Pb-C(1) 2.274(6), Pb-Br(1) 2.623(1);
C(1)-Pb-C(1*) 135.8(3), C(1)-Pb-Br(1) 95.4(2), C(1)-Pb-
Br(1*) 114.4(2), Br(1)-Pb-Br(1*) 95.29(7).
Exp er im en ta l Section
Diorganodibromoplumbanes are generally synthe-
sized by bromination of tetravalent organolead com-
pounds with brominating reagents, such as bromine and
hydrogen bromide.1 The synthesis of 2 is the first
example where a dibromoplumbane was synthesized by
oxidative bromination of a plumbylene. The reaction
mechanism is most likely explained by a two times
repetition of the bromine radical abstraction process,
as has been shown in the reaction of dimethylgermylene
with halocarbons.8
Gen er a l Meth od s. All reactions were carried out under
an atmosphere of dry argon. Solvents were dried by standard
methods and freshly distilled prior to use. 1H (500 MHz), 13C
(125 MHz), and 207Pb (105 MHz) NMR spectra were recorded
on a J EOL R-500 MHz spectrometer at 27 °C. Chemical shifts
were measured with tetramethylsilane (δ ) 0) for 1H and 13C
NMR and tetramethylplumbane (δ ) 0) for 207Pb NMR as
external standards, respectively. Preparative HPLC was
performed on an LC-908 (J apan Analytical Industry Co., Ltd.)
equipped with J AI-gel 1H and 2H columns (eluent: toluene).
High-resolution mass spectral data were obtained on a J EOL
SX-102 mass spectrometer. The melting point was determined
by differential scanning calorimetry using a SHIMADZU DSC-
50 differential scanning calorimeter. Elemental analyses were
carried out at the Microanalytical Laboratory of Department
of Chemistry, Faculty of Science, The University of Tokyo.
1-Bromo-2,4,6-tris[bis(trimethylsilyl)methyl]benzene (TbtBr)14
and bis[N,N-bis(trimethylsilyl)amino]lead(II)15 were prepared
according to procedures reported in the literature. Carbon
Dibromoplumbane 2 showed satisfactory spectral and
analytical data. The 1H and 13C NMR spectra indicated
that the two bulky Tbt groups in 2 are equivalent in
solution in spite of their steric hindrance. The molec-
ular structure of 2 was finally determined by X-ray
crystallographic analysis. This is the first example of
a crystallographically characterized diorganodibro-
moplumbane. The ORTEP drawing of 2 is shown in
Figure 1. The lead atom is bound to two Tbt groups
and two bromine atoms with a geometry best described
as distorted tetrahedral. The two Tbt groups and two
bromine atoms are both equivalent in the crystal.
The Pb-C bond, 2.274(6) Å, is notably longer than
that [2.19(3) Å] in tetrahedrally coordinated Ph4Pb9 and
(10) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements;
Pergamon: Oxford, England, 1984; p 431.
(11) Preut, H.; Huber, F. Z. Anorg. Allg. Chem. 1977, 435, 234.
(12) Okazaki, R.; Shibata, K.; Tokitoh, N. Tetraherdron. Lett. 1991,
32, 6601.
(13) Kroon, J .; Hulscher, J . B.; Peerdeman, A. F. J . Organomet.
Chem. 1970, 23, 477.
(14) (a) Okazaki, R.; Unno, M.; Inamoto, N. Chem. Lett. 1987, 2293.
(b) Okazaki, R.; Unno, M.; Inamoto, N.; Yamamoto, G. Chem. Lett.
1989, 493. (c) Okazaki, R.; Unno, M.; Inamoto, N. Chem. Lett. 1989,
791.
3
that [2.12(2) Å] in octahedrally coordinated Ph2PbCl2
and slightly longer than the sum of the covalent radii
(8) Ko¨chter, J .; Lehnig, M.; Neumann, W. P. Organometallics 1988,
7, 1201.
(15) Gynane, M. J . S.; Harris, D. H.; Lappert, M. F.; Power, P. P.;
Rivie`re, P.; Rivie`re-Baudet, M. J . Chem. Soc., Dalton Trans. 1977,
2004.
(9) Busetti, V.; Mammi, M.; Signor, A.; Del Pra, A. Inorg. Chim. Acta.
1967, 1, 424.

2750 Organometallics, Vol. 16, No. 12, 1997
Notes
tetrabromide was purchased from Wako Pure Chemical Ind.,
Ltd., and used as such.
F(000) ) 3048. A yellow prismatic crystal of 2 having
dimensions of 0.50 × 0.50 × 0.40 mm was mounted on a glass
fiber. The data set was collected on a Rigaku AFC5R diffrac-
tometer with graphite-monochromated Mo KR radiation (λ )
0.710 69 Å) at 296 K and a rotating anode generator. The
structure was solved by direct methods with SHELXS-86.16
The intensities were corrected for Lorentz and polarization
effects. The non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were included but not refined. The final cycle
of full-matrix least-squares refinement was based on 3830
observed reflections (I > 3.00σ(I)) and 312 variable parameters
and converged (largest parameter was 0.02 times its esd) at
R (Rw) ) 0.043 (0.037) with a GOF ) 1.28. All calculations
were performed using the TEXSAN crystallographic software
package of the Molecular Structure Corp. Atomic coodinates,
bond lengths and angles, and thermal parameters have been
deposited at Cambridge Crystallographic Data Centre.
Syn th esis of Tbt2P bBr 2, (2). To an ether suspension (25
mL) of TbtLi prepared from TbtBr (1.58 g, 2.50 mmol) and
t-BuLi (1.60 M in pentane; 1.60 mL, 2.05 equiv) at -40 °C
was added slowly an ether solution (50 mL) of bis[N,N-bis-
(trimethylsilyl)amino]lead(II) (1.32 g, 2.50 mmol) at -30 °C.
After further stirring of the red reaction mixture for 2 h at
-30 °C, an ether suspension (25 mL) of TbtLi (2.50 mmol),
prepared in a way similar to the above, was slowly added to
the reaction mixture at -10 °C. After further stirring of the
reaction mixture for 1 h at -10 °C, the reaction mixture turned
green-blue. After further stirring of the reaction mixture for
1 h at 25 °C, the volatile substances were removed under
reduced pressure. After 25 mL of THF was added, the solution
was stirred at 25 °C for 30 min, and then carbon tetrabromide
(2.49 g, 7.50 mmol) was added to the reaction mixture at 25
°C. After the reaction mixture was stirred for 3 min, insoluble
materials were filtered out through Celite. After removal of
the volatile substances under reduced pressure, the residue
was subjected to HPLC to yield bis{2,4,6-tris[bis(trimethylsilyl)-
methyl]phenyl}dibromoplumbane (2) (2.19 g, 59%). 2: mp
139.4 °C dec; 1H NMR (CDCl3, 500 MHz) δ 0.08 (s, 72H), 0.20
(s, 36H), 1.33 (s, 2H), 1.95 (s, 2H), 3.77 (s, 2H), 6.45 (s, 2H),
6.66 (s, 2H); 13C NMR (CDCl3, 125 MHz) δ 1.06 (q), 1.28 (q),
2.07 (q), 3.01 (q), 3.14 (q), 28.40 (d), 30.51 (d), 31.51 (d), 126.30
(d), 130.62 (d), 145.16 (s), 147.66 (s), 150.15 (s), 167.21 (s); 207Pb
NMR (CDCl3, 105 MHz) δ -83. HRMS (FAB): found, m/z
1390.5555; calcd for C54H11979Br208PbSi12, [M - Br + H]+,
1390.5493. Anal. Calcd for C54H118Br2PbSi12: C, 44.08; H,
8.08; Br, 10.86. Found: C, 43.82; H, 8.14; Br, 10.67.
Ack n ow led gm en t. This work was partially sup-
ported by a Grant-in Aid for Scientific Research No.
05236102 from the Ministry of Education, Science,
Sports, and Culture of J apan. We are also grateful to
Shin-etsu Chemical Co., Ltd., and Tosoh Akzo Co., Ltd.,
for the generous gift of chlorosilanes and alkyllithiums,
respectively.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details of the crystallographic study, tables of complete
crystal and refinement data, bond lengths and angles, and
positional and thermal parameters for 2, and the packing view
along the b axis and a drawing showing the atomic numbering
of 2 (26 pages). Ordering information is given on any current
masthead page.
X-r a y Da ta Collection a n d Str u ctu r e Refin em en t of
Tbt2P bBr 2 (2). Single crystals of 2 were grown by the slow
evaporation of its saturated solution in ethanol and dichlo-
romethane at room temperature. Crystal data for 2: C54H118
-
OM970151J
PbBr2Si12, fw ) 1471.56, monoclinic, space group C2/c, a )
25.49(1) Å, b ) 11.23(1) Å, c ) 26.74(2) Å, â ) 91.01(5)°, V )
(16) Sheldrick, G. M. SHELXS-86, Program for Crystal Structure
Determination. University of Go¨ttingen, Go¨ttingen, Germany, 1986.
7656(8) Å3, Z ) 4, Dc ) 1.276 g cm-3, µ(Mo KR) ) 34.71 cm-1
,