Hexaarylcyclotriplumbane: A molecule with a homonuclear ring system of lead

Article abstract of DOI:10.1021/ja036299d

The reaction of lead(II) bromide with RMgBr, R = 2,4,6-Et₃C₆H₂, furnishes black crystals ofhexakis(2,4,6-triethylphenyl)cyclotriplumbane (1) as the first molecule with a homonuclear lead ring. 1 adopts a skewed structure with highly stretched Pb-Pb bond lengths of 3.185 A (average), which indicate that the three-membered ring is formed by weak interactions of 3 singlet plumbylene molecules. Copyright

Full text of DOI:10.1021/ja036299d

Published on Web 07/31/2003
Hexaarylcyclotriplumbane: A Molecule with a Homonuclear Ring System of
Lead
Frank Stabenow,† Wolfgang Saak,† Heinrich Marsmann,‡ and Manfred Weidenbruch*,†
Department of Chemistry, UniVersity of Oldenburg, D-26111 Oldenburg, Germany, and Department of Chemistry,
UniVersity of Paderborn, D-33095 Paderborn, Germany
Received May 23, 2003; E-mail: manfred.weidenbruch@uni-oldenburg.de
The reactions of lead(II) halides with aryl Grignard reagents,
RMgBr, depend not only on the spatial demands of the respective
group R but also on how the reaction is performed. While the
reaction with R ) 2-tBu-4,5,6-Me3C6H furnishes a monomeric
plumbylene,1 the use of the only slightly smaller 2,4,6-triisoprop-
ylphenyl group affords the diplumbene 1 as the first example of a
molecule with a lead/lead double bond.2 A further reduction in the
spatial requirements of the group to R ) 2,4,6-triethylphenyl leads
to the formation of the molecule 2 in which two plumbylene units
are linked through MgBr2.3 Finally, when R ) mesityl, a plumb-
ylene dimer in which the two lead atoms are each stabilized by a
magnesium bromide molecule is obtained.4
To separate the MgBr2(THF)4 formed in the reaction between
2,4,6-triethylphenylmagnesium bromide and lead(II) bromide in
THF and thus suppress the formation of 2, we performed the
Figure 1. Molecule of 3 in the crystal (hydrogen atoms omitted). Ellipsoids
are drawn at 50% probability. Selected bond lengths (Å) and angles (deg):
Pb(1)-Pb(2) ) 3.1812(3), Pb(2)-Pb(3) ) 3.2014(2), Pb(1)-Pb(3) )
reaction in the presence of dioxane. We then obtained in 34%
isolated yield black crystals of the cyclotriplumbane 3 which can
be heated to 80 °C without decomposition (eq 1).5
3.1727(3), Pb-C ) 2.30 (average); Pb(3)-Pb(1)-Pb(2) ) 60.510(6),
Pb(2)-Pb(3)-Pb(1) ) 59.876(6), Pb(1)-Pb(2)-Pb(3) ) 59.614(6), C-Pb-C
) 102.9 (average).
The thermal stability of 3 as well as the 207Pb NMR signal at
1160 ppm clearly demonstrate that the structural integrity of 3 in
the solid state is also preserved in solution. This NMR shift differs
substantially from those observed with typical plumbylenes that
range from 3870 to 8888 ppm.1,6 The structure of compound 3 was
confirmed by an X-ray crystallographic analysis7 that revealed some
unusual structural parameters (Figure 1). With an average value of
3.184 Å, the Pb/Pb bond lengths are markedly longer than those
of a typical diplumbane such as Ph3Pb-PbPh3 with 2.844(4) Å.8
They rather correspond to the Pb/Pb separation in the diplumbylene
RPb-PbR where R ) 2,6-Tip2C6H3 (Tip ) 2,4,6-triisopropyl-
phenyl), determined by Power et al. as 3.1881(1) Å.9
Figure 2. Details of the structure of 3.
c-Pb3H6, for example, Pb/Pb bond lengths of 3.231 Å and
displacement angles of 50° for the hydrogen atoms.10,11
The calculated and observed structures clearly demonstrate that
the Pb/Pb bonds in 3 are not classic single bonds but rather that
the structure is held together by weak interactions between three
singlet plumbylene molecules (Figure 3).10 This is in contrast to
all other homonuclear ring systems of group 14 elements, which
can be described as normal single bonds. The weakness of these
interactions and the thus resulting widened bonds is probable due
to the relativistic contraction12 of the 6s orbital of one lead atom,
which then only incompletely overlaps with the unoccupied 6p
orbitals of the respective other lead atom. The Pb-C lengths and
C-Pb-C angles, which do not differ substantially, are in the region
typical for diarylplumbylenes.
Equally unusual is the orientation of the substituents at the lead
atoms which are twisted by about 37° out of their ideal positions
(Figure 2). The structural parameters observed here are in accord
with those obtained from calculations on the parent compound,
† University of Oldenburg.
‡ University of Paderborn.
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J. AM. CHEM. SOC. 2003, 125, 10172-10173
10.1021/ja036299d CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
compound RMgBr in THF (35 mL), prepared in turn from 1-bromo-2,4,6-
triethylbenzene (2.41 g, 10 mmol) and magnesium turnings (0.9 g, 37
mmol), was added dropwise to a suspension of lead(II) bromide (1.84 g,
5 mmol) in THF (10 mL) and dioxane (5 mL). The reaction mixture,
which initially turned yellow, was allowed to warm to 0 °C under constant
stirring, and finally adopted a reddish-brown color. The solvents were
removed under reduced pressure, the brownish residue was extracted with
30 mL of n-hexane, and the precipitate of magnesium salt was filtered
off. Storing at -30 °C furnished 0.91 g (34% yield) of black crystals,
dark red in thin layers. 1H NMR (toluene-d8): δ 1.55 (t, 18H, 3J ) 7.4
Hz,), 1.17, 1.19 (m, two overlapping signals, 36H,), 2.50 (q, 24H, 3J )
7.7 Hz, o-CH2), 2.70 (q, 12H, 3J ) 7.7 Hz, p-CH2), 7.22 (s, 12H, aromat.
H). 13C NMR (toluene-d8): 16.04 (Cp), 29.28 (Cs), 29.47 (Cs), 32.06 (Cp),
128.25 (Caromat), 143.48 (Caromat), 144.18(Caromat), 150.85 (Caromat). 207Pb
NMR (toluene-d8): 1152. UV-vis (n-hexane): λ (ꢀ) 502 nm (1665).
Figure 3. Interaction of three singlet plumbylenes R2Pb.
The cyclotriplumbane 3 is the first compound of lead with a
homocyclic ring system.13 This is worthy of note because the lighter
homologues of group 14 form numerous ring systems. Although
in the past few years a number of homo- and heteronuclear cluster
compounds of lead have also been prepared,14 they contain bonding
situations different from that in compound 3.
(6) Brooker, S.; Buijink, J.-K.; Edelmann, F. T. Organometallics 1991, 10,
25. Simons, R. S.; Pu, L.; Olmstead, M. M.; Power, P. P. Organometallics
1997, 16, 1920. Tokotoh, N.; Kano, N.; Shibata, K.; Okazaki, R.
Organometallics 1995, 14, 3121.
(7) Crystallographic data for 3 at 213 K with Mo KR (µ ) 0.71073 Å)
radiation, a ) 13.1218(6) Å, b ) 13.1291(5) Å, c ) 20.8767(11) Å, R )
94.223(5)°, â ) 98.564(6)°, γ ) 105.832(5)°; V ) 3396.8(3) Å3; space
group P1h, Z ) 2, M ) 1589.11 g mol-1; F ) 1.554 g cm-3. Data
collection: Stoe IPDS. The structure was solved by direct phase
determination and refined by full-matrix least-squares techniques against
F2 with the SHELXL-97 program system. R1/wR2 for 14 264 data )
0.0242/0.0534. The data have been deposited with the Cambridge
Crystallographic Data Centre: CCDC 211 375.
Acknowledgment. Financial support for our work by the
Deutsche Forschungsgemeinschaft and the Fonds der Chemischen
Industrie is gratefully acknowledged.
(8) Preut, H.; Huber, F. Z. Anorg. Allg. Chem. 1976, 419, 92.
(9) Pu, L.; Twamley, B.; Power, P. P. J. Am. Chem. Soc. 2000, 122, 3524.
(10) Nagase, S. Polyhedron 1991, 10, 1299.
(11) Srinivas, G. N.; Kiran, B.; Jennis, E. D. J. Mol. Struct. (THEOCHEM)
1996, 361, 205.
Supporting Information Available: X-ray crystallographic file for
compound 3. This material is available free of charge via the Internet
at http://pubs.acs.org.
References
(12) Pyykko¨, P. Chem. ReV. 1988, 88, 563.
(13) Cambridge Crystallographic Data Centre, release April, 2003.
(1) Stu¨rmann, M.; Weidenbruch, M.; Klinkhammer, K. W.; Lissner, F.;
Marsmann, H. Organometallics 1998, 17, 4425.
(2) Stu¨rmann, M.; Saak, W.; Marsmann, H.; Weidenbruch, M. Angew. Chem.
1999, 111, 145; Angew. Chem., Int. Ed. 1999, 38, 187.
(3) Stu¨rmann, M.; Saak, W.; Weidenbruch, M. Z. Anorg. Allg. Chem. 1999,
625, 705.
(4) Stu¨rmann, M.; Saak, W.; Weidenbruch, M.; Klinkhammer, K. W. Eur. J.
Inorg. Chem. 1999, 579.
(5) All manipulations were carried out in oven-dried glassware under an
atmosphere of dry argon. At -110 °C, a solution of the Grignard
(14) Edwards, P. A.; Corbett, J. D. Inorg. Chem. 1977, 16, 903. Eichhorn, B.
W.; Haushalter, R. C. J. Chem. Soc., Chem. Commun. 1990, 937. Fa¨ssler,
T. F.; Hunziker, M. Inorg. Chem. 1994, 33, 5380. Hunziker, M.; Fa¨ssler,
T. F. Z. Anorg. Allg. Chem. 1996, 622, 837. Campbell, J.; Dixon, D. A.;
Mercier, H. P. A.; Schrobilgen, G. J. Inorg. Chem. 1995, 34, 5798.
Campbell, J.; Mercier, H. P. A.; Franke, H.; Santry, D. P.; Dixon, D. A.;
Schrobilgen, G. J. Inorg. Chem. 2002, 41, 86.
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