Stereochemical activity of the metal-centered lone electron pair in group 14 metallocenes. Crystal structure of the linear sandwich complex [C5(iPR)3H2]2Pb

Article abstract of DOI:10.1021/ic9905401

Stereochemically active valence electrons are not present in the structure of [C₅Pr(i))₃H₂]₂Pb, which, like [C₅Me₄(SiMe₂Bu(t))]₂Pb, possesses parallel cyclopentadienyl rings. There are no close intramolecular contacts that force the rings to be parallel; the structure is consistent with the lack of a strong electronic preference for bending in heavy group 14 metallocenes.

Full text of DOI:10.1021/ic9905401

Inorg. Chem. 2000, 39, 153-155
153
Notes
pattern of (C5Ph5)2Ge is similar to that of (C5Ph5)2Sn, and it is
likely that the germanium compound is also linear.15
Stereochemical Activity of the Metal-Centered
Lone Electron Pair in Group 14 Metallocenes.
Crystal Structure of the Linear Sandwich
Complex [C5(iPr)3H2]2Pb
The accumulation of structural data on the linear metal-
locenes, combined with more sophisticated theoretical work,16
has served to refine the stereochemical role for the lone pair of
electrons in group 14 metallocenes.17 We now report the
synthesis and crystal structure of a second linear plumbocene,
which in concert with other related compounds allows us to
evaluate the importance of intramolecular steric crowding as a
contributor to the structure of linear group 14 metallocenes.
David J. Burkey,† Timothy P. Hanusa,*,† and
John C. Huffman‡
Department of Chemistry, Vanderbilt University,
Nashville, Tennessee 37235 and Molecular Structure Center,
Department of Chemistry, Indiana University,
Bloomington, Indiana 47405
Experimental Section
Materials. Anhydrous PbI2 was a commercial sample and was used
as received. KCp3i ([Cp3i]- ) 1,2,4-(C3H7)3C5H2) was prepared as
previously described.18 Its isomeric purity was ensured by reaction of
the originally prepared (C3H7)3C5H3 hydrocarbon with KH, followed
by hydrolysis and a subsequent secondary reaction with KH; 1H NMR
analysis was used to check the composition. Solvents for the reaction
were distilled under nitrogen from sodium or potassium benzophenone
ketyl.19
Physical Measurements. Proton and carbon (13C) NMR spectra were
obtained on a Bruker NR-300 spectrometer at 300 and 75.5 MHz,
respectively, and were referenced to the residual resonances of C6D6
(δ 7.15 and 128.0) or THF-d8 (δ 3.58 and 67.4). Assignments of the
signals in the 13C NMR spectrum were made with the help of DEPT
pulse experiments. Infrared data were obtained on a Perkin-Elmer 1600
Series FT-IR spectrometer as a neat sample between sealed KBr plates.
ReceiVed May 17, 1999
Introduction
Both stannocene (Cp2Sn) and plumbocene (Cp2Pb) possess
structures with nonparallel rings in the solid state,1-3 in
solution,4,5 and in the gas phase.6 The operation of a stereo-
chemically active lone pair of metal valence electrons in the
complexes has traditionally been cited as the source of the bent
ring structures,7 a conclusion reinforced by early MO calcula-
tions.8 Not surprisingly, therefore, the report9 of the structure
of the linear stannocene (C5Ph5)2Sn received close scrutiny, as
it was the first structurally authenticated group 14 metallocene
to possess parallel rings.10 Subsequently, other linear metal-
locenes of the group 14 elements were discovered, including
one conformer of Cp*2Si,11 two more stannocenes ([C5Me4-
(SiMe2But)]2Sn12 and [C5(Pri)5]2Sn),13 and the plumbocene
[C5Me4(SiMe2But)]2Pb.14 Although its structure has not been
determined by single-crystal X-ray diffraction, the X-ray powder
Synthesis of (Cp3i)2Pb. KCp3i (0.67 g, 2.93 mmol) and PbI2 (0.67
g, 1.46 mmol) were added to 45 mL of THF, and the orange suspension
was stirred for 12 h. The THF was removed under vacuum, leaving a
reddish-orange residue, which was extracted with 50 mL of hexanes.
The extract was then filtered through a glass frit, and removal of the
hexanes from the resulting red filtrate gave 0.58 g (68% yield) of (Cp3i)2-
1
2
Pb as an orange-red oil. H NMR (C6D6), δ: 5.64 (s, J(Pb-H) )
34.4 Hz, 4 H, ring CH); 3.06 (septet, J ) 6.8 Hz, 2 H, CHMe2); 2.88
(septet, J ) 6.8 Hz, 4 H, CHMe2); 1.20-1.24 (two overlapping
doublets, 24 H, CH3); 1.18 (d, J ) 6.8 Hz, 12 H, CH3). 13C NMR
(C6D6), δ: 134.5 (ring CCHMe2); 134.2 (ring CCHMe2); 102.6 (1J(Pb-
C) ) 61.3 Hz, ring CH); 27.9 (CH3); 27.6 (CHMe2); 26.5 (CH3); 26.2
(CH3); 25.7 (CHMe2). Principal IR bands (neat) cm-1: 2960 (s), 2870
(sh), 1462 (m), 1452 (m, sh), 1408 (w), 1381 (m), 1361 (m), 1325
(w), 1278 (w), 1262 (w), 1176 (w), 1150 (w), 1084 (m), 1021 (s), 980
(w), 864 (w), 800 (m), 668 (m), 502 (w), 465(w). Although initially
isolated as an oil, samples of (Cp3i)2Pb left standing (7-10 days) at
ambient temperature partially solidified to form deep red, needle-shaped
crystals (mp 34-35 °C).
* Corresponding author. Fax: 615-343-1234. E-mail: t.hanusa@
vanderbilt.edu.
† Vanderbilt University.
‡ Indiana University.
(1) Atwood, J. L.; Hunter, W. E.; Cowley, A. H.; Jones, R. A.; Stewart,
C. A. J. Chem. Soc., Chem. Commun. 1981, 925-927.
(2) Overby, J. S.; Hanusa, T. P.; Young, V. G., Jr. Inorg. Chem. 1998,
37, 163-165.
(3) Beswick, M. A.; Lopez-Casideo, C.; Paver, M. A.; Raithby, P. R.;
Russell, C. A.; Steiner, A.; Wright, D. S. Chem. Commun. 1997, 109-
110.
(4) Fischer, E. O.; Grubert, H. Z. Anorg. Allg. Chem. 1956, 286, 237-
242.
(5) Fischer, E. O.; Grubert, H. Z. Naturforsch., B 1956, 11, 423-424.
(6) Almenningen, A.; Haaland, A.; Motzfeldt, T. J. Organomet. Chem.
1967, 7, 97-104.
(7) Dave, L. D.; Evans, D. F.; Wilkinson, G. J. Chem. Soc. 1959, 3684-
3688.
(8) Jutzi, P.; Kohl, F.; Hofmann, P.; Krueger, C.; Tsay, Y. H. Chem. Ber.
1980, 113, 757-769.
(9) Heeg, M. J.; Janiak, C.; Zuckerman, J. J. J. Am. Chem. Soc. 1984,
106, 4259-4261.
(15) Heeg, M. J.; Herber, R. H.; Janiak, C.; Zuckerman, J. J.; Schumann,
H.; Manders, W. F. J. Organomet. Chem. 1988, 346, 321-332.
(16) Recent ab initio calculations place the energy difference between the
bent (preferred) and linear forms of Cp2Sn and Cp2Pb at 17 and 2.8
kJ/mol, respectively (Armstrong, D. R.; Duer, M. J.; Davidson, M.
G.; Moncrieff, D.; Russell, C. A.; Stourton, C.; Steiner, A.; Stalke,
D.; Wright, D. S. Organometallics 1997, 16, 3340-3351). Preliminary
density functional calculations on stannocene and plumbocene suggest
that the bent and linear forms are nearly isoenergetic (Hanusa, T. P.,
Smith, J. D. Unpublished results).
(10) Williamson, R. L.; Hall, M. B. Organometallics 1986, 5, 2142-2143.
(11) Jutzi, P.; Holtmann, U.; Kanne, D.; Krueger, C.; Blom, R.; Gleiter,
R.; Hyla, K. I. Chem. Ber. 1989, 122, 1629-1639.
(12) Constantine, S. P.; Hitchcock, P. B.; Lawless, G. A.; DeLima, G. M.
J. Chem. Soc., Chem. Commun. 1996, 1101-1102.
(13) Sitzmann, H.; Boese, R.; Stellberg, P. Z. Anorg. Allg. Chem. 1996,
622, 751-755.
(17) Burkey, D. J.; Hanusa, T. P. Comments Inorg. Chem. 1995, 17, 41-
77.
(18) Williams, R. A.; Tesh, K. F.; Hanusa, T. P. J. Am. Chem. Soc. 1991,
113, 4843-4851.
(19) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon: Oxford, U.K., 1988.
(14) Constantine, S. P.; Hitchcock, P. B.; Lawless, G. A. Organometallics
1996, 15, 3905-3906.
10.1021/ic9905401 CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/18/1999

154 Inorganic Chemistry, Vol. 39, No. 1, 2000
Table 1. Crystallographic Data for (Cp3i)2Pb
Notes
formula: C28H46Pb
fw ) 589.87
Z ) 1
space group: P1h (No. 2)
T ) -171 °C
λ ) 0.710 69 Å
a ) 9.318(1) Å
b ) 9.677(1) Å
c ) 8.475(1) Å
R ) 95.011(8)°
â ) 116.249(6)°
γ ) 75.709(8)°
V ) 664.0(4) Å3
F
calcd ) 1.475 g cm-3
µ ) 64.13 cm-1
Ra (I > 2.33σ(I)) ) 0.0224
Rwb (I > 2.33σ(I)) ) 0.0206
GOF ) 0.911
a R ) ∑||Fo| - |Fc||/Σ|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑w(Fo )]1/2
Table 2. Selected Bond Lengths (Å) for (Cp3i)2Pb
.
2
Pb(1)-C(2)
Pb(1)-C(3)
Pb(1)-C(4)
Pb(1)-C(5)
Pb(1)-C(6)
Pb(1)-ring centroid
C(2)-C(3)
2.744(4)
2.748(5)
2.767(4)
2.743(3)
2.738(4)
2.470
1.420(6)
1.414(6)
1.416(7)
1.403(7)
1.418(7)
C(2)-C(7)
1.513(7)
C(4)-C(10)
C(6)-C(13)
C(7)-C(8)
1.513(6)
1.512(6)
1.518(9)
1.528(9)
1.517(8)
1.518(9)
1.535(8)
1.512(8)
C(7)-C(9)
C(10)-C(11)
C(10)-C(12)
C(13)-C(14)
C(13)-C(15)
C(2)-C(6)
Figure 1. ORTEP plot of the non-hydrogen atoms of (Cp3i)2Pb.
Thermal ellipsoids are shown at the 30% level.
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
Structure of (Cp3i)2Pb. A low-temperature X-ray structure
analysis of (Cp3i)2Pb found that the molecule lies on a
crystallographic inversion center with the [C5(iPr)3H2]- rings
rigorously parallel (Figure 1). The complex is isomorphous with
the analogous transition metal metallocenes (Cp3i)2M (M )
V-Co;22-25 Ru26). It is only the second crystallographically
characterized plumbocene that has parallel cyclopentadienyl
rings, but it is the sixth linear group 14 metallocene. The
cyclopentadienyl ligands of (Cp3i)2Pb exhibit nearly symmetric
pentahapto coordination to the lead atom, as indicated by the
narrow range of Pb-C distances (2.738(4)-2.767(4) Å);
similarly narrow ranges are found for (C5Ph5)2Sn,9 the linear
conformer of Cp*2Si,11 and [C5Me4(SiMe2But)]2Pb.14 The
average Pb-C distance in (Cp3i)2Pb (2.748(8) Å) is somewhat
shorter than the corresponding distances in other structurally
characterized bent plumbocenes (e.g., 2.79(2) Å in Cp*2Pb1 and
[C5(CH2Ph)5]2Pb27) but is indistinguishable from that in the
linear plumbocene [C5Me4(SiMe2But)]2Pb (2.743(11) Å).14
The isopropyl groups in (Cp3i)2Pb display orientations of 73.6,
66.9, and 16.6° relative to the ring plane. The methyl groups
on the last (C(11), C(12)) are actually bent toward the metal,
with displacements of 0.72 and 0.38 Å below the ring plane
for C(11) and C(12), respectively.
CH3-CH-CH3 (av)
110.2(9)°
within 0.004 Å
180°
planarity of rings
ring centroid-Pb(1)-ring centroid
av displacement of methine carbon
from ring plane
0.079 Å
X-ray Crystallography. Crystals of (Cp3i)2Pb were isolated directly
from the solidification of the initially formed oil. Data were collected
on a suitable orange crystal, measuring 0.10 × 0.10 × 0.12 mm, in a
low-temperature study performed at Indiana University. Relevant
crystallographic data for the present study are given in Table 1.
A systematic search of a limited hemisphere of reciprocal space
revealed a set of diffraction maxima having symmetry consistent with
the triclinic space group P1h. Subsequent solution and refinement of
the structure confirmed the choice; no additional symmetry was detected
with the ADDSYM algorithm of PLATON. Data were collected using
a continuous θ-2θ scan with fixed backgrounds at each extreme of
the scan. Data were corrected for Lorentz and polarization effects and
equivalent data averaged. The structure was readily solved by direct
methods (MULTAN-78) and standard Fourier techniques. Non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
located in a difference Fourier synthesis and were refined isotropically
in the final least-squares cycles. A final difference Fourier map revealed
a large peak (2.03 e Å-3) at the site of the Pb atom; the map was
otherwise featureless, with the largest peak being 0.25 e Å-3. Selected
bond distances listed in Table 2.
Origin of the Parallel Ring Geometry. Steadily accumulat-
ing experimental and theoretical evidence has made it apparent
that the link between a metal-centered lone pair of electrons
and bent group 14 metallocenes is more subtle than originally
supposed.17 To a substantial degree, especially with the heavier
elements, the valence electrons are held in nondirectional, metal-
based ns orbitals, a fact that precludes stereochemical activity.
This conclusion is directly supported by photoelectron spec-
Results and Discussion
Synthesis and Chemical Properties. (Cp3i)2Pb was prepared
from the metathetical reaction of KCp3i and PbI2 in THF; it
was separated from the KI byproduct with hexanes extraction.
Spectroscopic data (1H and 13C NMR) for the metallocene
confirm the proposed formulation.
(Cp3i)2Pb melts near room temperature (34-35 °C) and begins
to decompose within minutes in air, although complete decom-
position requires several hours (cf. (Cp3i)2Sn: mp -18 °C and
decomposition within minutes in air).20 (Cp3i)2Pb is thermo-
chromic; the metallocene gradually changes from bright yellow
at -196 °C to dark red at its melting point. Analogous thermo-
chromic behavior has been reported for Cp2Pb,4 (C5Ph4H)2Pb,
[C5(ButPh)Ph4]2Pb, and (C5Ph5)2Pb.15,21
(22) Overby, J. S.; Jayaratne, K. C.; Schoell, N. J.; Hanusa, T. P.
Organometallics 1999, 18, 1663-1668.
(23) Overby, J. S.; Schoell, N. J.; Hanusa, T. P. J. Organomet. Chem. 1998,
560, 15-19.
(24) Hays, M. L.; Burkey, D. J.; Overby, J. S.; Hanusa, T. P.; Yee, G. T.;
Sellers, S. P.; Young, V. G., Jr. Organometallics 1998, 17, 5521-
5527.
(25) Burkey, D. J.; Hays, M. L.; Duderstadt, R. E.; Hanusa, T. P.
Organometallics 1997, 16, 1465-1475.
(26) Overby, J. S.; Farrar, J. M.; Hanusa, T. P. Acta Crystallogr., Sect. C
1997, C53, 1605-1606.
(20) Burkey, D. J.; Hanusa, T. P. Organometallics 1995, 14, 11-13.
(21) Schumann, H.; Janiak, C.; Zuckerman, J. J. Chem. Ber. 1988, 121,
207-218.
(27) Schumann, H.; Janiak, C.; Hahn, E.; Kolax, C.; Loebel, J.; Rausch,
M. D.; Zuckerman, J. J.; Heeg, M. J. Chem. Ber. 1986, 119, 2656-
2667.

Notes
Inorganic Chemistry, Vol. 39, No. 1, 2000 155
troscopic studies of germanocenes, stannocenes, and plumbo-
cenes,28-30 which indicate that the metal valence electrons are
tightly bound relative to the ring electrons. In addition, 119Sn
Mo¨ssbauer spectra have confirmed that the valence electrons
in stannocenes are largely confined to the 5s orbital.31
Indirect support for the lack of direct correlation between the
metal valence electrons and structures of group 14 metallocenes
is provided by the bent geometries of most group 2 metallocenes
of calcium, strontium, and barium,18,32,33 whose metal centers
possess no valence electrons. Bosnich34 and Allinger35 have
suggested that attractive van der Waals forces between the
cyclopentadienyl ligands contribute to the bent structures of
group 2, group 14, and lanthanide metallocenes. In any case,
the accumulated evidence supports the characterization of these
molecules as conformationally “floppy” systems, with low
barriers to rearrangement and bending.
Figure 2. Space-filling drawings of (Cp3i)2Pb (left) and [C5Me4(SiMe2-
But)]2Pb (right).19 The closest Me‚‚‚Me′ contacts in each are 4.17 and
4.30 Å, respectively.
its cyclopentadienyl rings, there is enough room around the
metal center for a bent geometry in the less heavily substituted
(Cp3i)2Pb. In addition, it should be noted that (Cp3i)2Ca is bent
(centroid-Ca-centroid ) 169.7°), even though the Ca-C
distance is substantially shorter (2.62(2) Å) than the analogous
distance in (Cp3i)2Pb.32 It appears that, given the combination
of metal-ligand distance and the trisubstitution of the cyclo-
pentadienyl rings, intramolecular steric crowding cannot be
realistically maintained as the source of the linear geometry of
(Cp3i)2Pb.
There are no especially close intermolecular contacts in either
(Cp3i)2Pb or [C5Me4(SiMe2But)]2Pb; the minimum Me‚‚‚Me′
distances are 3.74 and 3.75 Å, respectively, which are longer
than the 3.3-3.5 Å contacts typically found in bent (C5Me5)2M
complexes.33 Nevertheless, it is possible that crystal packing
helps enforce the linear geometries observed in the plumbocenes.
With the exception of (Cp3i)2Ca,32 all structurally authenticated
metallocenes containing [Cp3i]- or [C5Me4(SiMe2But)]- rings
adopt fully staggered configurations, with the metals located
on crystallographic inversion centers. A staggered, parallel
packing arrangement may be especially favorable for these two
cyclopentadienyl ligands.
The structural characterization of (Cp3i)2Pb provides an
opportunity to analyze the factors that could contribute to its
linear geometry. It is clear that the structures of (Cp3i)2Pb and
[C5Me4(SiMe2But)]2Pb cannot be attributed to strong inter-ring
steric repulsions between the two Cp′ ligands, as was claimed
for (C5Ph5)2Sn.9 The closest intramolecular Me‚‚‚Me′ contacts
in (Cp3i)2Pb and [C5Me4(SiMe2But)]2Pb are 4.17 and 4.30 Å,
respectively, well outside the sum of the van der Waals radii
for two methyl groups (4.0 Å)36 (cf. inter-C5 ring C(phenyl)‚‚‚
C(phenyl)′ contacts in (C5Ph5)2Sn as close as 3.61 Å). The
average displacement of the methine carbon atoms of the
isopropyl groups in (Cp3i)2Pb from the C5 ring plane is only
0.079 Å (0.072 Å for the methyl carbons/silicon atom of
[C5Me4(SiMe2But)]2Pb), further underscoring the lack of sig-
nificant interaction between the rings. This can be appreciated
in space-filling drawings of the two plumbocenes (Figure 2),
in which the relatively open coordination spheres of the metal
centers are apparent. Further evidence for the lack of a sterically
imposed geometry in the case of (Cp3i)2Pb is provided by the
bent geometry found for decaisopropylplumbocene, [(C5(Pri)5]2-
Pb, with an angle between the ring normals of 170°.37 Obvi-
ously, if a plumbocene can bend with five isopropyl groups on
In summary, we have found a second linear plumbocene
whose metal-centered lone pair of electrons is stereochemically
inactive. Its geometry is almost certainly not a consequence of
intramolecular steric forces. Of the ∼20 group 14 metallocenes
that have been crystallographically characterized, roughly a
quarter are now known to possess linear geometries; such
structures should not continue to be regarded as anomalous.
(28) Baxter, S. G.; Cowley, A. H.; Lasch, J. G.; Lattman, M.; Sharum, W.
P.; Stewart, C. A. J. Am. Chem. Soc. 1982, 104, 4064-4069.
(29) Bruno, G.; Ciliberto, E.; Fragala, I. L.; Jutzi, P. J. Organomet. Chem.
1985, 289, 263-270.
(30) Cradock, S.; Duncan, W. J. Chem. Soc., Faraday Trans. 2 1978, 74,
194-202.
(31) Dory, T. S.; Zuckerman, J. J. J. Organomet. Chem. 1984, 264, 295-
303.
(32) Burkey, D. J.; Hanusa, T. P.; Huffman, J. C. AdV. Mater. Opt. Electron.
Acknowledgment is made to the National Science Founda-
tion for support of this research. D.J.B. was the recipient of an
NSF Predoctoral Fellowship.
1994, 4, 1-8.
(33) Williams, R. A.; Hanusa, T. P.; Huffman, J. C. Organometallics 1990,
9, 1128-1134.
(34) Bosnich, B. Chem. Soc. ReV. 1994, 23, 387-395.
(35) Timofeeva, T. V.; Lii, J.-H.; Allinger, N. L. J. Am. Chem. Soc. 1995,
117, 7452-7459.
Supporting Information Available: An X-ray crystallographic file,
in CIF format, for (Cp3i)2Pb. This material is available free of charge
via the Internet at http://pubs.acs.org.
(36) Pauling, L. The Nature of the Chemical Bond, 3rd. ed.; Cornell
University Press: Ithaca, NY, 1960.
(37) Sitzmann, H. Z. Anorg. Allg. Chem. 1995, 621, 553-556.
IC9905401