Coordination complexes of decamethylytterbocene with 4,4′- disubstituted bipyridines: An experimental study of spin coupling in lanthanide complexes

Article abstract of DOI:10.1021/om051051d

The paramagnetic 1:1 coordination complexes of (C₅Me ₅)₂Yb with a series of 4,4′-disubstituted bipyridines, bipy-X, where X is Me, t-Bu, OMe, Ph, CO₂Me, and CO ₂Et, have been prepared. All of the complexes are paramagnetic, and the values of the magnetic susceptibility as a function of temperature show that these values are less than expected for the cations, [(C₅Me ₅)₂Yb(III)(bipy-X)]⁺, which have been isolated as the cation-anion ion pairs [(C₅Me₅)₂Yb(III) (bipy-X)]⁺[(C₅Me₅)₂YbI ₂]^-, where X is CCV Et, OMe, and Me. The ¹H NMR chemical shifts (293 K) for the methine resonances located at the 6,6′ site in the bipy-X ring show a linear relationship with the values of χT(300 K) for the neutral complexes, which illustrates that the molecular behavior does not depend on the phase, with one exception, viz., (C₅Me ₅)₂Yb(bipy-Me). Single crystals of the 4,4′-dimethylbipyridine complex undergo an irreversible, abrupt first-order phase change at 228 K that shatters the single crystals. The magnetic susceptibility, represented in a χ vs T plot, on this complex, in polycrystalline form undergoes reversible abrupt changes in the temperature regime 205-212 K, which is suggested to be due to the way the individual molecular units pack in the unit cell. A qualitative model is proposed that accounts for the subnormal magnetic moments in these ytterbocene-bipyridine complexes.

Full text of DOI:10.1021/om051051d

3228
Organometallics 2006, 25, 3228-3237
Coordination Complexes of Decamethylytterbocene with
4,4′-Disubstituted Bipyridines: An Experimental Study of Spin
Coupling in Lanthanide Complexes
Marc D. Walter, David J. Berg, and Richard A. Andersen*
Department of Chemistry and Chemical Sciences DiVision of Lawrence Berkeley National Laboratory,
UniVersity of California, Berkeley, California 94720
ReceiVed December 8, 2005
The paramagnetic 1:1 coordination complexes of (C5Me5)2Yb with a series of 4,4′-disubstituted
bipyridines, bipy-X, where X is Me, t-Bu, OMe, Ph, CO2Me, and CO2Et, have been prepared. All of
the complexes are paramagnetic, and the values of the magnetic susceptibility as a function of temperature
show that these values are less than expected for the cations, [(C5Me5)2Yb(III)(bipy-X)]+, which have
been isolated as the cation-anion ion pairs [(C5Me5)2Yb(III)(bipy-X)]+[(C5Me5)2YbI2]-, where X is CO2-
1
Et, OMe, and Me. The H NMR chemical shifts (293 K) for the methine resonances located at the 6,6′
site in the bipy-X ring show a linear relationship with the values of øT (300 K) for the neutral complexes,
which illustrates that the molecular behavior does not depend on the phase, with one exception, viz.,
(C5Me5)2Yb(bipy-Me). Single crystals of the 4,4′-dimethylbipyridine complex undergo an irreversible,
abrupt first-order phase change at 228 K that shatters the single crystals. The magnetic susceptibility,
represented in a ø vs T plot, on this complex, in polycrystalline form undergoes reversible abrupt changes
in the temperature regime 205-212 K, which is suggested to be due to the way the individual molecular
units pack in the unit cell. A qualitative model is proposed that accounts for the subnormal magnetic
moments in these ytterbocene-bipyridine complexes.
Their paramagnetic condition implies that their electronic
structure is based upon Yb(III) with an open shell electron
configuration of 4f13, which requires that the bipyridine ligand
carries a negative charge and is therefore noninnocent. However,
the values of µeff as a function of temperature are much less
than expected for the valence bond structure, [Cp′Yb(III,
4f13)][bipy•-, S ) 1/2], in which the charge carriers are
uncoupled. Two ways that µeff can be lowered are either an
equilibrium between the two valence bond structures [Yb(III),
4f13)][bipy•-, S ) 1/2] and [Yb(II), 4f14)][bipy, S ) 0] or electron
exchange coupling between the spin carriers in the paramagnetic
spin isomer. The equilibrium model is appealing since the
relative population of the two valence bond structures will
determine the value of µeff, which in turn depends on the
temperature, resulting in a temperature dependence of µeff.
However, it has been shown recently for (C5Me5)2Yb(bipy) that
the Yb LIII edge XANES signatures for the Yb(II) and Yb(III)
species do not change over the temperature regime 10-400 K,
which is inconsistent with an equilibrium model.3 The electron
exchange coupling model is difficult to accept since the
conventional view is that electrons in f-orbitals are core electrons
that do not participate to any extent in chemical bonding in
molecular compounds, and therefore the bonds between lan-
thanide metals and their ligands are largely due to electrostatic
forces described by Coulomb’s law. The exchange coupling
model defines the parameter J, which is the coupling constant
Introduction
The preparation and the molecular and electronic structure
of the coordination complexes of cyclopentadienyl-ring-
substituted ytterbocenes with bipyridine of the type Cp′2Yb-
(bipy), where Cp′ is C5H5, 1,3-R2C5H3 (R ) Me3C, Me3Si),
1,2,4-R3C5H2 (R ) Me3C, Me3Si), C5Me4H, or C5Me5 (Cp*),
have been described.1,2 Magnetic susceptibility measurements,
as a function of temperature, of these simple coordination
complexes and therefore their electronic structure are not simple
to understand. The ytterbocene complexes are paramagnetic,
which is unexpected since their stoichiometry implies they are
based upon Yb(II) with a closed shell 4f14 electron configuration.
* To whom correspondence should be addressed. E-mail: raandersen@
lbl.gov.
(1) Schultz, M.; Boncella, J. M.; Berg, D. J.; Tilley, T. D.; Andersen, R.
A. Organometallics 2002, 21, 460-472.
(2) Walter, M. D.; Schultz, M.; Andersen, R. A. New J. Chem. 2006,
30, 238-246.
(3) Booth, C. H.; Walter, M. D.; Daniel, M.; Lukens, W. W.; Andersen,
R. A. Phys. ReV. Lett. 2005, 95, 267202.
(4) van Vleck, J. H. The Theory of Electric and Magnetic Susceptibilities;
Clarendon Press: Oxford, 1932.
(5) Berg, D. J.; Boncella, J. M.; Andersen, R. A. Organometallics 2002,
21, 4622-4631.
(6) Ohsawa, Y.; Whangbo, M.-H.; Hanck, K. W.; DeArmond, M. K.
Inorg. Chem. 1984, 23, 3426-3428.
(7) McInnes, E. J. L.; Farley, R. D.; Rowlands, C. C.; Welch, A. J.;
Rovatti, L.; Yellowlees, L. J. J. Chem. Soc., Dalton Trans. 1999, 4203-
4208.
(12) We mentioned in ref 1 that the magnetic susceptibility of Cp*2Yb-
(bipy-Me) is superimposable on that of Cp*2Yb(bipy-H). We observed the
discontinuity in its magnetic susceptibility vs temperature plot, but at that
time we attributed this to temperature instability in the SQUID magnetometer
that was used. This rationalization is no longer acceptable.
(13) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 6th
ed.; John Wiley & Sons: New York, 1988.
(8) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 185-195.
(9) Finke, R. G.; Keenan, S. R.; Schiraldi, D. A.; Watson, P. L.
Organometallics 1986, 5, 598-601.
(10) Da Re, R. E.; Kuehl, C. J.; Brown, M. G.; Rocha, R. C.; Bauer, E.
D.; John, K. D.; Morris, D. E.; Shreve, A. P.; Sarrao, J. L. Inorg. Chem.
2003, 42, 5551-5559.
(11) Tilley, T. D.; Boncella, J. M.; Berg, D. J.; Burns, C. J.; Andersen,
R. A. Inorg. Synth. 1990, 27, 146-149.
(14) Pauling, L. The Nature of the Chemical Bond; Cornell University
Press: Ithaca, NY, 1960.
10.1021/om051051d CCC: $33.50 © 2006 American Chemical Society
Publication on Web 05/19/2006

Coordination Complexes of Decamethylytterbocene
Organometallics, Vol. 25, No. 13, 2006 3229
Table 1. Redox Potentials of 4,4′-X2-bipy (bipy-X) in 0.1 M
[nBu4N][BF4]-DMF at 293 K6,7
from OEt (and by assumption OMe, used in this work), an
electron-donating group, to CO2Me, an electron-withdrawing
group. This substituent effect is consistent with the Hammett
σ-constants.8 The uncoordinated bipyridine ligands used in this
work cannot be reduced by (C5Me5)2Yb, since its reduction
potential in acetonitrile is +1.78 V relative to Cp2Fe/Cp2Fe+.9
Presumably the reduction potential of the free ligand is lowered
when it binds to the Lewis acid.10
The substituted bipyridine adducts of decamethylytterbocene
are prepared by adding the bipyridine ligand (bipy-X) to a
solution of (C5Me5)2Yb(OEt2), Cp*2Yb(OEt2), in toluene and
crystallizing the red complexes from toluene or pentane, eq 1.
Some physical properties of the complexes are listed Table 2.
a
X
E1 /V
E2/V
OEt
Me
H
Ph
Cl
CO2Et
CO2Me
-2.88b
-2.68c (0.140)d
-2.60 (0.100)
-2.34 (0.060)
-2.24b
-2.05e
-2.03 (0.070)
-2.40e
-2.52 (0.130)
a Potentials quoted relative to [Cp2Fe]+/Cp2Fe, measured relative to Ag-
AgCl. b Irreversible, cathodic peak quoted. c (Ef - Er)/2. d Ef - Er. e 0.1
M [Et4N][PF6]-DMF at -54 °C.6
or exchange integral in the Schroedinger Heisenberg Van Vleck
Hamiltonian, Hˆ ) JBS1‚BS2,3,4 and the value of J must be large
(>500 cm-1) and negative (antiferromagnetic coupling) in order
to account for the low values of µeff observed.1,2 Qualitatively,
the large value of the exchange interaction means that the
unpaired electron in the f-orbital of the Cp*2Yb(III) fragment
and the electron in the ligand LUMO of the bipy•- are strongly
coupled; that is, they are participating in a bonding interaction.
In the electrostatic bonding model, exchange interaction is
introduced by polarization, but these values are small, on the
order of 10-25 cm-1.5 Thus the ytterbocene-bipy compounds
provide fertile ground for exploring bond models that provide
a deeper understanding of the electronic structure of f-block
metal compounds in general and the metallocenes in particular.
The original papers focused on a series of ytterbocene-bipy
coordination compounds in which the substituents on the
cyclopentadienyl ring changed the magnetic properties of the
resulting complexes.1,2 In this paper the focus is on how the
magnetic susceptibility changes when the cyclopentadienyl ring
is kept constant, in this case C5Me5, and the substituents on the
4,4′-position of bipyridine are changed. Therefore, the role of
substituents in tuning the magnetic properties of the coordination
compounds can be experimentally defined, a prerequisite for
developing a bond model.
Cp*2Yb(OEt2) + 4,4′-X2-bipy f
Cp*2Yb(4,4′-X2-bipy) + OEt2
X ) CO2Me, CO2Et, Ph, H, Me, t-Bu, OMe
(1)
These complexes are high-melting solids, when they melt, but
do not sublime when heated in diffusion pump vacuum; they
decompose at temperatures of about 250 °C. Table 2 also lists
three new examples of the cation-anion complexes, [Cp*2Yb-
(bipy-X)][Cp*2YbI2] (X ) CO2Et, OMe, Me), which were
isolated from the reaction of the neutral complexes with AgI in
toluene, eq 2.
The synthetic reaction yields the desired cation [Cp*2Yb-
(bipy-H)][I],1 when the unsubstituted bipyridine is used, which
is the reason it is suggested to be an intermediate in eq 2, but
the ytterbocene cation-anion pair is isolated when the 4,4′-
substituted bipy ligands are used. The reason for the difference
is not obvious, but the ion pairs are valuable for their magnetic
properties, which is the reason for preparing them.
Results and Discussion
Synthesis. The most readily available substituted bipyridine
derivatives are those substituted in the 4,4′-position (the position
para to the nitrogen atom in the ring). The steric bulk of the
substituents at these remote sites should have an insignificant
influence on intramolecular steric effects, but the electronic
effects will change significantly. This is supported by the
reduction potentials of the free ligand, shown in Table 1.
These values show that the substituents used in this study
alter the redox potentials by 0.85 V as the substituent changes
Infrared spectra in the 300 cm-1 region of decamethylytter-
bocene complexes are a qualitative indicator of the ytterbium
oxidation state.5 An absorption in the range 315-290 cm-1 is
indicative of Yb(III), whereas an absorption in the range 275-
265 cm-1 is indicative of Yb(II). This general pattern is apparent
for most of the complexes listed in Table 2; two exceptions are
the bipy-X derivatives, where X ) Me and OMe. These two
complexes have low magnetic moments, and the oxidation state
Table 2. Solid State Properties of Ytterbocene Bipyridyl Complexes
color mp/°C
IR/cm-1
red-brown
compound
µ
eff(300 K)/µB
(Me5C5)2Yb(bipy-H)a
328-330
290, 942, 1553
271, 944, 1595
260, 1602
2.4
0.9
1.1
3.3
2.7
3.2
3.2
4.2
6.4
6.2
6.3
6.3
0
(Me5C5)2Yb(bipy-Me)a,b
deep red
deep red
deep red
red-brown
red
red
red-brown
red
dark red
dark red
dark red
dark green
dark green
dark blue
>300
(Me5C5)2Yb(bipy-OMe)b
206-208
245-248
247-250
>330
(Me5C5)2Yb(bipy-Ph)b
310, 960, 1560
310, 1590
311, 963, 1534
315, 960, 1540
320, 1598
(Me5C5)2Yb(bipy-t-Bu)b
(Me5C5)2Yb(bipy-CO2Et)b
(Me5C5)2Yb(bipy-CO2Me)b
180-181
125-130
264-266
230
234-235
>320
208-210
228-238
209-211
[(Me5C5)2Yb(bipy-H)]+[I]- a
[(Me5C5)2Yb(bipy-H)]+ [(Me5C5)2YbCl2]- a
[(Me5C5)2Yb(bipy-Me)]+[(Me5C5)2YbI2]- b
[(Me5C5)2Yb(bipy-OMe)]+[(Me5C5)2YbI2]- b
[(Me5C5)2Yb(bipy-CO2Et)]+[(Me5C5)2YbI2]- b
1598
300, 322, 1615
300, 325, 1610
300, 330, 1550
265
260
253
a,c
(Me5C5)2Yb(py)2
b
(Me5C5)2Yb(4-picoline)2
(Me5C5)2Yb(py-OMe)2
0
0
b
a From ref 1. b This work. c From ref 11.

3230 Organometallics, Vol. 25, No. 13, 2006
Walter et al.
potential of the bipyridines listed in Table 1. This implies that
the exchange coupling increases as the energy of the LUMO
increases and, conversely, decreases as the LUMO energy
decreases. This in turn implies that as the frontier orbital energy
of the substituted bipyridine radical anion is closer in energy
to that of the frontier orbital energy of the ytterbocene fragment,
the exchange integral increases and the extent of the paramag-
netic condition decreases.
The low values for Cp*2Yb(bipy-OMe) are not due to random
paramagnetic impurities in an otherwise diamagnetic compound.
This supposition is shown by the data in Figure 3, which shows
the experimentally determined values of ø, corrected for a small
amount of a J ) 7/2 impurity, Yb(III), as outlined in ref 2.
Subtraction yields a øcorrected curve that is essentially independent
of temperature, no longer has the “Curie tail”, and yields ø
values > 0, i.e., a paramagnetic molecule.
The magnetization curves for the bipy-Me complex shown
in Figures 4 and 5 are similar at low temperature (T < 205 K)
and high temperature (T > 212 K) to those presented in Figures
1 and 3, but they show a dramatic deviation in the temperature
range 205-212 K.12
Figure 1. Solid state magnetic susceptibility (ø) vs T plot for (C5-
Me5)2Yb(bipy-Ph), (C5Me5)2Yb(bipy-CO2Et), (C5Me5)2Yb(bipy-
CO2Me), (C5Me5)2Yb(bipy-H), and (C5Me5)2Yb(bipy-OMe). The
arrows indicate the maximum value of ø, ømax. The Curie-tails at
low temperature indicate small contributions of Yb(III) impurities,
except for (C5Me5)2Yb(bipy-CO2Et), in which ømax is washed out
by the impurity, yielding a plateau instead of a maximum. This
complex is the most difficult one to crystallize, since it is very
soluble in pentane, and therefore the most difficult to purify,
although it is obtained in analytically pure form.
In this experiment the sample is cooled rapidly from 300 to
2 K, then slowly heated as data collection commences. The value
of ø smoothly decreases with increasing temperature to 205 K,
when an abrupt decrease occurs in which ø decreases by 25%
from 205 to 212 K. After this abrupt change, ø then decreases
smoothly to 300 K, whereupon it gradually increases as the
temperature increases to 400 K (Figure 4). On cooling slowly
from 400 K, the original curve is retraced until the temperature
reaches 212 K, when ø slowly increases as the temperature
reaches 200 K. At 200 K, ø abruptly increases over the
temperature range 200-192 K, then the original curve is
retraced. This hysteresis behavior is reproducible over several
cycles of temperature, from sample to sample, and is indepen-
dent of the applied magnetic field strength (5, 20, 40, and 70
kG). The hysteresis is a solid state property of the molecule
Cp*2Yb(bipy-Me), which is ascribed to a first-order crystal-
lographic phase transition, as described in the next section.
The magnetization data of the only other alkyl-substituted
bipyridine complex, Cp*2Yb(bipy-t-Bu), is also different from
those shown in Figures 1 and 4 and rather more complex (Figure
6). Initial cooling of the sample to 2 K followed by slow heating
while collecting the ø-values shows that their values reach a
maximum value at T(1) ) 345 K, but between 355 and 365 K
of the ytterbium in these complexes, and in several of the other
ones, is ambiguous (see later).
Solid State Magnetic Measurements: SQUID. Figure 1
shows the ø vs T plot, and Figure 2 shows the plot of µeff vs T
for four of the substituted bipy-ytterbocene complexes, X )
OMe, Ph, CO2Me, and CO2Et, along with those of the
unsubstituted parent, X ) H. The plots for the parent complex
are included for calibration purposes using the values pub-
lished.1,2 The curves in Figures 1 and 2 define a family of curves
in which ø passes through a minimum value then gradually
increases through a maximum value as T increases; the ømax
value is indicated by the arrow in Figure 1. The values of ø
and µeff depend in a regular way on X, viz., at 400 K, OMe <
H < CO2R (R ) Me, Et) e Ph. It is clear that the extent of the
paramagnetic condition increases when X ) H is replaced by
an electron-withdrawing group and decreases when X is an
electron-donating group, i.e., with the difference in reduction
Figure 2. Solid state magnetic moment (µeff) vs T plot for (C5Me5)2Yb(bipy-Ph), (C5Me5)2Yb(bipy-CO2Et), (C5Me5)2Yb(bipy-CO2Me),
(C5Me5)2Yb(bipy-H), and (C5Me5)2Yb(bipy-OMe).

Coordination Complexes of Decamethylytterbocene
Organometallics, Vol. 25, No. 13, 2006 3231
Figure 5. Solid state magnetic susceptibility (ø) vs T plot and
magnetic moment (µeff) vs T plot (180-230 K) for (C5Me5)2Yb-
(bipy-Me). The sample was cooled to 2 K, and the data were
collected at different fields (5, 20, 40, 70 kG) on heating and cooling
cycles.
Figure 3. Solid state magnetic susceptibility (ø) vs T plot for (C5-
Me5)2Yb(bipy-OMe). The experimental values øexp include a small
magnetic impurity (∼0.6% of a J ) 7/2 impurity), which is removed
in øcorrected. The data clearly show TIP behavior with ø0 ) (4.19 (
0.05) × 10-4 cm3 mol-1
.
as is the Yb-N distance of 2.396(2) and 2.319(6) Å, and the
bipy-ring C(2)-C(2′) distance of 1.464(4) and 1.435(9) Å,
respectively. The torsion angles are slightly different, with the
two rings in the bipy-Me complex being closer to coplanarity.
However, these parameters are on the edge of significance and
the two molecular structures have nearly identical intramolecular
bond distances and angles. After data collection was completed
at 228(2) K the temperature of the liquid nitrogen-cooled stream
was slowly decreased, and the unit cell parameters were
monitored. The unit cell volume decreased smoothly until 185-
(2) K (see Supporting Information for the experimental data),
whereupon the diffraction pattern changed and the crystal
disintegrated to an amorphous powder during the acquisition
of the unit cell parameters. Various crystals exhibited identical
behavior, although different crystal sizes and cooling rates were
explored, resulting in the conclusion that the single crystals
undergo a first-order phase transition that irreversibly shatters
the crystals in the low-temperature regime that is close to the
hysteresis temperature for the polycrystalline material in the
magnetic susceptibility study, Figures 4 and 5.
Figure 4. Solid state magnetic susceptibility (ø) vs T plot and
magnetic moment (µeff) vs T plot for (C5Me5)2Yb(bipy-Me). The
sample was cooled to 2 K, and the data were collected at different
fields (5, 20, 40, 70 kG) on heating and cooling cycles.
A possible reason for the destructive behavior is traced to
the crystal structure of Cp*2Yb(bipy-Me), which crystallizes in
the monoclinic crystal system in space group P21/c, whereas
Cp*2Yb(bipy-H) crystallizes in the orthorhombic crystal system
in space group Pbca.1 Figures 8 and 9 show the packing
diagrams for these two molecules, where the C5Me5 rings have
been removed for clarity of viewing the intermolecular orienta-
tion of the Yb(bipy-X) fragments. The contents of the entire
unit cell are available in the Supporting Information. The
orthorhombic unit cell for Cp*2Yb(bipy-H), Figure 8, is more
open, and the open space follows a zigzag channel that is parallel
to the x-axis and a cylindrical channel that is parallel to the
z-axis. Thus, the environment about the individual molecules
is open and flexible, so the unit cell will easily accommodate a
change in an external variable such as temperature. The unit
cell in Figure 8 shows that two Yb(bipy-H) fragments are related
by an inversion center located at (0.5, 0, 0), which orientates
the bipy ligands so that the carbon atoms in the 3-position of
the rings (meta carbon atoms) have a C‚‚‚C distance of only
3.37 Å. This value is close to the interlayer C‚‚‚C distance of
3.40 Å observed in graphite, from which the van der Waals
radius of a carbon atom in an aromatic ring of 1.70 Å is
deduced.13,14 Thus, the C‚‚‚C distance of 3.37 Å lies very close
to the distance at which the repulsive and attractive forces on
an abrupt, 50%, drop occurs. On cooling, the original curve is
not retraced until the temperature reaches 25 K. Heating slowly
does not retrace the initial heating curve, T(2), but path T(3) is
followed. The complicated behavior is reproducible and is
consistent with the notion that the molecules in the ensemble
undergo substantial reorganization and reconstruction, and these
changes effect ø.
X-ray Crystallography. An ORTEP diagram of Cp*2Yb-
(bipy-Me) is shown in Figure 7. Acquisition of the X-ray
diffraction data presented several difficulties, since the crystals
shatter when placed on a goniometer head that is precooled to
163 K in a stream of liquid nitrogen, the usual way data are
collected. In this case, to get diffraction data, the crystal was
mounted on a goniometer in a stream of liquid nitrogen cooled
to about 225 K and data were collected at 228(2) K, i.e., above
the “magnetic hysteresis temperature”. The crystallographic data
collected at 228(2) K are shown in Table 3, along with those
for Cp*2Yb(bipy-H),1 and bond distances and angles are
compared in Table 4 for Cp*2Yb(bipy-Me) and related ytter-
bocene bipy complexes.1
The average Yb-C distance in the bipy-Me complex is
slightly longer than that in the bipy complex, 2.67 vs 2.62 Å,

3232 Organometallics, Vol. 25, No. 13, 2006
Walter et al.
Figure 6. Solid state magnetic susceptibility (ø) vs T plot for (C5Me5)2Yb(bipy-t-Bu). The arrows indicate T(ømax), and T(1)-T(3) indicate
the different cycles.
Table 3. Selected Crystal Data and Data Collection
Parameters for (C5Me5)2Yb(bipy-H) and
(C5Me5)2Yb(bipy-Me)
(C5Me5)2Yb(bipy-H)a (C5Me5)2Yb(bipy-Me)b
formula
fw
YbN2C30H38
599.68
YbN2C32H42
627.72
space group
a (Å)
b (Å)
Pbca (#61)
16.976(1)
30.060(5)
21.255(3)
90
10846.4
16
1.469
P21/c
12.347(1)
12.907(1)
18.619(1)
104.465(1)
2873.1(4)
4
c (Å)
â (deg)
V (Å3)
Z
d
calc (g/cm3)
1.451
32.8
µ(Mo KR)calc (cm-1
size (mm)
)
34.56
0.24 × 0.20 × 0.16
0.22 × 0.20 × 0.12
temperature (K)
scan type, θmax
no. of reflns integrated
298(2)
228(2)
θ-2θ, 22.5°
7780
ω, 24.7°
12 508
no. of unique reflns, Rint 7081, 0.000
4715, 0.0398
4088
328
Figure 7. ORTEP diagram of (C5Me5)2Yb(bipy-Me) (50% prob-
ability ellipsoids).
no. of good reflns
4494
variables
595
transmission range
R1
wR2
Rall
GOF
0.66-0.56
0.033
0.034
0.092
1.653
0.69-0.53
the van der Waals potential energy curve are equal.15 The
monoclinic unit cell for Cp*2Yb(bipy-Me), Figure 9, is less open
compared to the orthorhombic unit cell of Cp*2Yb(bipy-H) since
the voids are filled with Yb(bipy-Me) fragments and the
monoclinic unit cell has fewer degrees of freedom than the
orthorhombic one. The individual methyl groups on the 4,4′-
position are aligned one on top of the other such that the C-H
groups in the 5,5′-positions of the bipyridine ring have C‚‚‚C
contact distances that range from 3.74 to 3.87 Å, with an average
distance of 3.82 Å. The C‚‚‚C distance of 3.82 Å is larger than
the closest interplanar distance in Cp*2Yb(bipy-H) and graphite
and is close to the distance at which the intermolecular forces
of attraction on the van der Waals potential energy curve are at
a maximum, 3.8 Å.15 The intermolecular orientation and
interplanar distance of the Yb(bipy-Me) fragments in the unit
cell are likely to play an important role in why the crystal
shatters on cooling since, as the temperature is reduced from
228 K, the unit cell contracts, forcing all of the molecules in
the ensemble closer together. At some temperature the repulsive
forces become dominant and the molecules try to find a more
open crystalline lattice in order to relieve the lattice pressure.
This process is either gradual or sudden; if sudden, the crystal
shatters.
0.0343
0.0917
0.0403
1.073
a Taken from ref 1. b This work.
regime at which the crystals shatter and the magnetic hysteresis
occurs is comparable. On heating, the molecules in the ensemble
expand, but this expansion does not destroy the long-range order.
Cooling reconstructs the original order but at a different
temperature, which is related to the lattice dynamics in a single
crystal relative to that found in a polycrystalline sample.
Experiments designed to test this qualitative model are under-
way.
Solution 1H NMR Spectroscopy. The 1H NMR spectra are
more sensitive than the infrared and perhaps even the UV-vis
spectra as a qualitative determination of whether a molecule is
paramagnetic or diamagnetic, particularly when its magnetic
moment is low, as is the case for the ytterbocene-bipyridine
complexes.1,2 The proton chemical shifts in a paramagnetic
molecule are determined by the Fermi contact, δFc, and
pseudocontact (dipolar), δpc, terms, both of which describe the
The sudden change in the unit cell on cooling is likely to
correlate with the hysteresis behavior since the temperature
(15) Devic, T.; Yuan, M.; Adams, J.; Fredrickson, D. C.; Lee, S.;
Venkataraman, D. J. Am. Chem. Soc. 2005, 127, 14616-14627.

Coordination Complexes of Decamethylytterbocene
Organometallics, Vol. 25, No. 13, 2006 3233
Table 4. Selected Bond Distances (Å) and Angles (deg) for [1,3-(Me3C)2C5H3]2Yb(bipy-H), (C5Me5)2Yb(bipy-Me),
(C5Me5)2Yb(bipy-H), and [(C5Me5)2Yb(bipy-H)]+[(C5Me5)2YbCl2]-
[1,3-(Me3C)2C5H3]2Yb(bipy-H)a (C5Me5)2Yb(bipy-Me)b (C5Me5)2Yb(bipy-H)a [(C5Me5)2Yb(bipy-H)]+[(C5Me5)2YbCl2]-a,c
Yb-Cring (mean)
Yb-Cring (range)
Yb-centroid
centroid-Yb-centroid
Yb-N
2.75
2.67
2.62
2.59
2.674(6)-2.803(6)
2.624(5)-2.695(3)
2.592(9)-2.647(7)
2.582(3)-2.614(3)
2.47
132
2.50
15
2.40
143
2.40
0.7
2.34
139
2.32
3
2.30
142
2.37
7
torsion angle
N-C-C-N
torsion angle
19
0.2
3
8
C-C-C-C
a From ref 1. b This work. c The distances and angles of the Cp*2Yb fragment are average values for the individual ytterbocene fragments.
where D and D′ are the magnetic susceptibility tensors and G
and G′ are the geometric factors, which are related to the
distance from the paramagnetic center by r-3. In these molecules
in which intermolecular exchange between free and bound
bipyridine is slow on the NMR time scale, the resonances whose
chemical shifts are determined by δpc are therefore related to D
and D′ and therefore directly proportional to the magnetic
susceptibility expressed as øT.17 This assumes that the chemical
shifts in solution are averaged over all molecular orientations
and the orientations of the individual molecules are randomly
distributed in the polycrystalline powders in the magnetization
experiments. The chemically inequivalent bipy resonances
should be most sensitive, since they occur over a wide range,
but this assumes that they can be assigned. The 1H NMR
chemical shifts at 20 °C of the Cp*2Yb(bipy-X) compounds
are listed in Table 5. A pattern is reasonably clear since the
ellipsoids). The Cp* rings have been removed for clarity.
Figure 8. Packing diagram for Cp*2Yb(bipy-H) (50% probability
C5Me5 resonance occurs over a relatively narrow range, 3.3 to
5.5 ppm downfield of Me4Si, and the integrated intensity leaves
no doubt about the assignment. The four bipyridine resonances
(three in the 4,4′-disubstituted ligands) span a chemical shift
range from +215 to -20 ppm, and the chemical shifts of the
neutral derivatives follow a pattern. Thus, all seven complexes
show a resonance that is strongly deshielded, which is labeled
A. Two other resonances, labeled B and C, appear upfield of
the A resonances in a rather narrow range of +11 to -20 and
+4 to -20, respectively. These A, B, and C resonances are of
area ratio 2:2:2. The other resonance(s), labeled D, is (are) easily
assigned due its (their) intensity, and their chemical shifts span
a relatively narrow range. An assignment of the A, B, and C
resonances is possible by noting that the most strongly shifted
resonances are likely to be due to those C-H’s in closest
proximity to the paramagnetic center (whose chemical shifts
are determined by the pseudocontact contribution), the A set,
and these are assigned to the C-H in the 6,6′-positions of the
bipyridine ring (the site ortho to the nitrogen atom). The other
two resonances, B and C, are then assigned to the hydrogens in
either 3,3′- or 5,5′-position (the site meta to the nitrogen atom).
A choice may be made in Cp*2Yb(bipy-Ph), where the
resonance at δ 10.7 is a doublet while the resonance at δ 4.2 is
a singlet, and therefore δ 10.7 is located at the 5,5′-site or C
and the δ 4.2 resonance is on the 3,3′-site or B. The line width
of the resonances in the other complexes is larger than the
expected coupling constant and cannot be used to assign them,
but the general chemical shift pattern supports the assignment
suggested in Table 5.
Figure 9. Packing diagram for Cp*2Yb(bipy-Me) (50% probability
ellipsoids). The Cp* rings have been removed for clarity.
mechanism by which unpaired spin effects the chemical shift
of the ligand and both are proportional to the magnetic
susceptibility ø.16,17 The Fermi contact term is described by eq
3,
γe
i γH
gâS(S + 1)
3kT
δFc ) -a
(3)
where ai is the isotropic hyperfine splitting constant. The
pseudocontact term, δpc, for a nonaxially symmetric molecule,
depends on through-space interactions of a given nucleus relative
to the paramagnetic center as defined in eq 4,
Assuming these assignments are correct, eqs 3 and 4 show
that a plot of the solution chemical shift (at 300 K) of the 6,6′
hydrogens vs øT (at 300 K) in the solid state is linear, Figure
10. As the chemical shift range for the A resonances is relatively
large due to the dominance of the pseudocontact term, a better
correlation is expected and observed than when the B and C
resonances are used. The solid state magnetic susceptibility
δpc ) -D G(θ, r)i - D′ G′(θ, Ω, r)i
(4)
(16) Fischer, R. D. Lanthanide and Actinide Complexes. In NMR of
Paramagnetic Molecules; La Mar, G. N., Horrocks, W. D., Holm, R. H.,
Eds.; Academic Press: New York, 1973; pp 521-553.
(17) øT ) C ) µeff2/2.8282.

3234 Organometallics, Vol. 25, No. 13, 2006
Table 5. 1H NMR Data of Bipyridyl Ytterbocene Complexesa
Walter et al.
compound
C5Me5
bipy-H (A)
bipy-H (B)
bipy-H (C)
bipy-X (D)
(C5Me5)2Yb(bipy-H)
(C5Me5)2Yb(bipy-Me)
(C5Me5)2Yb(bipy-OMe)
(C5Me5)2Yb(bipy-Ph)
3.98 (9)
3.80 (8)
4.10 (10)
4.21 (10)
160.1 (52)
144.1 (60)
169.4 (108)
178.3 (74)
6.4 (48)
8.2 (57)
-12.9 (14)
-9.5 (22)
-20.0 (40)
10.7 (15)
H 26.5 (36)
CH3 -9.3 (11)
CH3 1.21 (4)
m,o-CH 6.53 (7)
p-CH -16.8 (10)
t-Bu 1.07 (3)
CH3 4.04 (5)
CH2 4.66 (41)
CH3 0.82
-18.6 (98)
4.21 (10)
(C5Me5)2Yb(bipy-t-Bu)
(C5Me5)2Yb(bipy-CO2Me)
(C5Me5)2Yb(bipy-CO2Et)
3.95 (7)
4.45 (13)
4.43 (18)
141.1 (55)
211.0 (90)
209.9 (110)
11.4 (45)
-10.5 (80)
-9.1 (87)
-8.9 (10)
-19.6 (11)
-19.5 (22)
[(C5Me5)2Yb(bipy-H)]I
[(C5Me5)2Yb(bipy-Me)]+
[(C5Me5)2YbI2]-
3.32 (50)
3.61 (72)
2.76 (32)
5.30 (50)
4.26 (75)
5.51 (60)
4.05 (35)
329.0 (410)
330.0 (390)
57.0 (34)
56.6 (34)
-15.3 (24)
-14.7 (23)
H 9.2 (30)
CH3 8.29 (10)
[(C5Me5)2Yb(bipy-OMe)]+
[(C5Me5)2YbI2]-
334.6 (400)
332.9 (500)
58.4 (34)
58.0 (90)
-15.3 (24)
-13.1 (80)
CH3 10.04 (16)
[(C5Me5)2Yb(bipy-CO2Et)]+
[(C5Me5)2YbI2]-
CH2 6.50 (60)
CH3 3.68 (80)
a Recorded in C6D6 (neutral complexes) or CD2Cl2 (cation-anion complexes) at 20 °C. The line widths at half-peak height (Hz) are given in parentheses.
Figure 10. Solid state magnetic susceptibility (øTsolid) vs para-
magnetic chemical shift (δpara) at 300 K for the “A” set of
resonances in various Cp′2Yb(bipy-X) complexes.
values are used since the solution magnetic moments are likely
to have large errors associated with them since the moments
are low.2
The plot in Figure 10 is anchored by [1,2,4-(Me3Si)3C5H2]2-
Yb(bipy-H), where the 6,6′ resonance is assigned to the most
deshielded resonance in the only diamagnetic compound in this
series,1,2 and [Cp*2Yb(bipy-H)][I], an authentic example of Yb-
(III), in which the magnetic moment is normal, i.e., without
exchange coupling. The assignment of chemical shifts of the
bipyridine resonances in the cation and in the cation-anion ion
pairs in Table 5 is analogous to that used in the neutral
derivatives. Thus the feature identified as A is the most strongly
deshielded, on the order of +330 ppm, while the others fall
into narrow regions around +57, -15, and +10 ppm due to
the B, C, and D resonances. The strong correlation between
the isotropic chemical shift, that is, δpara ) δobs - δdia, where
δdia is the chemical shift in [1,2,4-(Me3Si)3C5H2]2Yb(bipy-H),
of the resonances assigned to the 6,6′-position relative to øT
may be extended to the other ytterbocenes described earlier.1,2
The correlation in Figure 10 supports a thesis developed in this
and earlier papers that the magnetic susceptibilities of molecules
with low magnetic moments are difficult to measure with
confidence and the confidence level increases when two different
physical methods correlate, as in Figure 10.
Figure 11. (a) Chemical shift (δ) vs T-1 plot of 1H NMR
resonances of (C5Me5)2Yb(bipy-Ph) in toluene-d8 at temperatures
1
from -70 to +100 °C. (b) Chemical shift (δ) vs T-1 plot of H
NMR resonances of (C5Me5)2Yb(bipy-Ph) in toluene-d8 at temper-
atures from -70 to +100 °C. The C5Me5 and bipy-H (A) resonances
have been omitted for clarity.
The variable-temperature 1H NMR spectra of the deca-
methylytterbocene complexes with the 4,4′-disubstituted bipy-
ridine are similar to each other and similar to that of Cp*2Yb-
(bipy-H) reported earlier.1,2 A plot of δ vs T-1 for the bipy-Ph
derivative is shown in Figures 11a and 11b, and the others are
available as Supporting Information. The most deshielded
resonance, assigned to A, has a nonlinear temperature depen-
dence, and this pattern is observed in all of the derivatives. The
resonance ascribed to B also is nonlinear in T-1, but that due
to C has a linear dependence. The nonlinear dependence may

Coordination Complexes of Decamethylytterbocene
Organometallics, Vol. 25, No. 13, 2006 3235
The nonlinear behavior of the chemical shifts as a function
of temperature of the bipy resonances is difficult to rationalize,
but it is clear that the shifts are due to intramolecular processes
since no exchange between free and bound bipy resonances is
observed on the NMR time scale. The nonlinear chemical shifts
presumably reflect the change in unpaired spin density at a given
site, which is related to the mechanism of transmission of the
spin density. Experimental and calculational studies on the
mechanism are underway.3
Conclusions
This and earlier papers1,2 show that the 1:1 coordination
complexes between ytterbocenes and bipyridine do not have a
closed shell electronic configuration, as anticipated from their
stoichiometry. The complexes are paramagnetic, and the extent
of their paramagnetism, measured by ø or µeff, depends on the
substituents on cyclopentadienyl and bipyridine ligands. In all
cases studied, the observed values of ø (or µeff) are lower than
expected for the two uncoupled spin carriers in the molecules
represented by the valence bond structure [Yb(III), 4f13)][bipy•-,
S ) 1/2]. The Yb(III) spin carrier is a bent sandwich fragment
in which the unpaired electron is located in a b1-symmetry
orbital of f-parentage20,21 and, therefore, a hole with b1 symmetry
(in molecular C2V symmetry). In addition, an empty b1-symmetry
orbital of d-parentage is available, which is Yb-Cp ring
antibonding, that can hybridize with the f-parentage orbitals of
the same symmetry. Since the unpaired electron in bipy resides
in an orbital with b1 symmetry (in C2V molecular symmetry),22
these electrons in the spin carriers can mix, providing a pathway
for antiferromagnetic exchange coupling. However, the magnetic
susceptibility curves do not fit to a Boltzmann distribution of
the two spin isomers with S ) 0 and S ) 1, as is the case for
Cp2Ti(bipy);23 thus the ytterbocene-bipyridine complexes can-
not be described by a spin equilibrium model. The Yb LIII edge
XANES data of Cp*2Yb(bipy) unequivocally show that the
ground state is not a single configuration ground state (e.g., it
cannot be described by a single wave function), but it is a
multiconfigurational or mixed-valent ground state.3 Thus the
mechanism of electron communication in these 1:1 complexes
is not trivial to understand using the physical methods familiar
to chemists. Further studies using the physical methods available
to the physics community will be published when complete.
Figure 12. Chemical shift (δ) vs T-1 plot of 1H NMR resonances
B and D of (C5Me5)2Yb(bipy-H) and (C5Me5)2Yb(bipy-Me) in
toluene-d8 at temperatures from -70 to +100 °C. The resonances
B and D are assigned as 3,3′-H2-bipy and 4,4′-X2-bipy (X ) H
and Me), respectively.
be due to the mechanism by which the unpaired spin density is
distributed to the various sites in the molecule, but the separation
of the contributions of δpc and δFc is not an easy task except in
cubic symmetry, when δpc vanishes.18 Experimentally the
dominance of the Fermi contact term can be demostrated, since
aH and aCH3 have different signs and consequently different
slopes as a function of temperature.19 Thus, when a methyl group
replaces a hydrogen atom at a given site in a ligand that is
remote from the paramagnetic center, their temperature depend-
ences will have opposite slopes when the Fermi contact shift
dominates the pseudocontact shift. The two molecules (C5Me5)2-
Yb(bipy-H) and (C5Me5)2Yb(bipy-Me) fit these criteria, and the
δ vs T-1 plots for the B and D resonances in the two complexes
are shown in Figure 12. The B resonances in the two complexes,
which are assigned to the C-H’s at the 3,3′-site, are superim-
posable. In contrast the resonances due to the C-H and C-Me
groups at the 4,4′-site have opposite slopes. This striking result
suggests that the contributions of δpc and δFc to the C-H
chemical shifts located in the 3,3′-site are similar but that the
δ
Fc term dominates the δpc term for the 4,4′-site. Two additional
observations are consistent with this deduction. The temperature
dependence of the tert-butyl chemical shift in (C5Me5)2Yb(bipy-
t-Bu) (see Supporting Information for details) is essentially
independent of temperature, which is consistent with the view
that the CMe3 group is less capable of hyperconjugation than a
CH3 group. In addition, the phenyl ring hydrogens in (C5Me5)2-
Yb(bipy-Ph) in the ortho and para C-H sites are shielded
relative to the chemical shifts in the free ligand. The temperature
dependence of the ortho and para chemical shifts is nonlinear
with negative slopes, implying a dominance of the contact term,
Figure 11b. The slope of the meta C-H is positive and the
shift is linear in T-1. Resonance structures are useful to illustrate
how negative spin density localizes on the ortho and para sites.
Experimental Section
General Comments. All reactions and product manipulations
have been carried out under a dry nitrogen atmosphere using
standard Schlenk and glovebox techniques. Dry, oxygen-free
solvents were employed throughout. NMR spectra were taken on
Bruker AVQ-400 and AV-300 spectrometers. All chemical shifts
are reported in δ units with reference to the residual protons of the
deuterated solvents, which are internal standards, for proton
chemical shifts. Melting points were measured on a Thomas-Hoover
melting point apparatus in sealed capillaries and are uncorrected.
The elemental analyses were performed by the analytical facilities
at the University of California at Berkeley. Magnetic susceptibility
data were obtained on a Quantum Design MPMS XL7 SQUID
magnetometer as described previously.2 (C5Me5)2Yb(bipy-Me)1 and
1
The nonzero contribution of the contact term to the H NMR
chemical shifts means that unpaired spin density is located in
the bipy ring in the ytterbocene complexes.
(18) Bleaney, B. J. Magn. Reson. 1972, 8, 91-100.
(20) Green, J. C.; Hohl, D.; Ro¨sch, N. Organometallics 1987, 6, 712-
720.
(21) Ortiz, J. V.; Hoffmann, R. Inorg. Chem. 1985, 24, 2095-2104.
(22) Kaim, W. J. Am. Chem. Soc. 1982, 104, 3833-3837
(23) McPherson, A. M.; Fieselmann, B. F.; Lichtenberger, D. L.;
McPherson, G. L.; Stucky, G. D. J. Am. Chem. Soc. 1979, 101, (13), 3425-
3430.
(19) (a) La Mar, G. N.; Horrocks, W. D.; Holm, R. H. NMR of
Paramagnetic Molecules; Academic Press: New York, 1973; pp 85-178.
(b) Wicholas, M.; Drago, R. S. J. Am. Chem. Soc. 1968, 90, 6946-6950.
(c) Horrocks, W. D.; Taylor, R. C.; LaMar, G. N. J. Am. Chem. Soc. 1964,
86, 3031-3038. LaLancette, E. A.; Eaton, D. R.; Benson, R. E.; Phillips,
W. D. J. Am. Chem. Soc. 1962, 84, 3968-3970.

3236 Organometallics, Vol. 25, No. 13, 2006
Walter et al.
(C5Me5)2Yb(OEt2)11 were prepared as previously reported. The 4,4′-
disubstituted 2,2′-bipyridine ligands were purified by crystallization
and/or sublimation; 4-picoline and 4-methoxypyridine were distilled
from sodium prior to use.
dicarboxylate-2,2′-bipyridine diethyl diester (0.36 g, 1.2 mmol) were
weighed into a Schlenk flask and dissolved in 60 mL of pentane
with stirring. The orange-red solution was stirred 1 h at room
temperature and filtered, and the filtrate was concentrated to 5 mL.
Cooling at -80 °C for several days produced deep red crystals
(0.79 g, 0.95 mmol, 79%), which collapsed to a brick red powder
on exposure to vacuum. Mp: >330 °C (dec). Anal. Calcd for
C36H46N2O4Yb: C, 58.13; H, 6.23; N, 3.77. Found: C, 57.77; H,
(C5Me5)2Yb(bipy-OMe). (C5Me5)2Yb(OEt2) (0.57 g, 1.1 mmol)
and the 4,4′-dimethoxy-2,2′-bipyridine (0.22 g, 1.0 mmol) were
weighed into a Schlenk flask and dissolved in 30 mL of toluene
with stirring. The dark red solution was stirred for 1 h at room
temperature and filtered, and the filtrate was concentrated to ca.
20 mL. Cooling at -20 °C for several days produced large deep
red crystals (0.44 g, 0.67 mmol, 67%). Mp: 206-208 °C (rev).
Anal. Calcd for C32H42N2O2Yb: C, 58.26; H, 6.42; N, 4.25.
1
6.19; N, 3.75. H NMR (C6D6, 20 °C): δ 209.9 (2H, ν1/2 ) 110
Hz, bipy-H), 4.66 (4H, ν1/2 ) 41 Hz, CH2), 4.43 (30H, ν1/2 ) 18
Hz, C5Me5), 0.82 (6H, t, 3JCH ) 5.2 Hz, CH3), -9.1 (2H, ν1/2 ) 87
Hz, bipy-H), -19.5 (2H, ν1/2 ) 22 Hz, bipy-H). IR (Nujol mull;
CsI windows; cm-1): 2730 (w), 2626 (w), 2540 (w), 1732 (m),
1708 (vs), 1611 (m), 1534 (vs), 1502 (vs), 1372 (s), 1365 (m),
1284 (s), 1267 (vs), 1202 (vs), 1161 (w), 1134 (sh m), 1116 (s),
1037 (s), 963 (vs), 920 (vw), 862 (br m-s), 800 (br vs), 748 (m-
s), 724 (w), 686 (br m), 663 (w), 536 (w), 443 (s), 390 (br s), 311
(s), 294 (w).
1
Found: C, 58.24; H, 6.38; N, 4.20. H NMR (C6D6, 20 °C): δ
169.4 (2H, ν1/2 ) 108 Hz, bipy-H), 4.1 (30H, ν1/2 ) 9.8 Hz, C5-
Me5), 1.21 (6H, ν1/2 ) 3.6 Hz, bipy-OMe), -18.6 (2H, ν1/2 ) 98
Hz, bipy-H), -20.0 (2H, ν1/2 ) 40 Hz, bipy-H). IR (Nujol mull;
CsI windows; cm-1): 2720 (w), 1602 (vs), 1560 (m), 1492 (s),
1422 (s), 1368 (m), 1330 (w), 1310 (m), 1282 (w), 1265 (s), 1248
(m), 1210 (s), 1190 (w), 1102 (w), 1060 (m), 1038 (s), 872 (w),
862 (w), 858 (m), 842 (w), 832 (w), 800 (w), 745 (m), 722 (m),
590 (m), 260 (br s).
(C5Me5)2Yb(bipy-CO2Me). (C5Me5)2Yb(OEt2) (0.42 g, 0.81
mmol) and the 4,4′-dicarboxylate-2,2′-bipyridine dimethyl diester25
(0.22 g, 0.81 mmol) were weighed into a Schlenk flask and
dissolved in 40 mL of toluene with stirring. The dark red solution
was stirred 1 h at room temperature, and the solvent was removed
under dynamic vacuum, leaving a red powder. The red residue was
dissolved in 100 mL of pentane and filtered, and the filtrate was
concentrated to ca. 20 mL. Cooling at -20 °C overnight produced
deep red blocklike crystals (0.50 g, 0.70 mmol, 86%). Mp: 180-
181 °C (dec). Anal. Calcd for C36H46N2O4Yb: C, 57.05; H, 5.91;
(C5Me5)2Yb(bipy-Ph). (C5Me5)2Yb(OEt2) (0.19 g, 0.36 mmol)
and the 4,4′-diphenyl-2,2′-bipyridine (0.11 g, 0.36 mmol) were
weighed into a Schlenk flask and dissolved in 20 mL of toluene
with stirring. The red-purple solution was stirred for 1 h at room
temperature, then the solvent was removed under dynamic vacuum,
leaving a red-purple powder. The red-purple residue was dissolved
in 50 mL of pentane and filtered, and the filtrate was concentrated
to ca. 20 mL. Cooling at -20 °C overnight produced deep red
crystals (0.1 g, 0.13 mmol, 37%). Mp: 245-248 °C (dec). Anal.
Calcd for C42H46N2Yb: C, 67.09; H, 6.17; N, 3.73. Found: C,
66.73; H, 6.36; N, 3.60. 1H NMR (C6D6, 20 °C): δ 178.3 (2H, ν1/2
) 74 Hz, bipy-H), 10.7 (2H, ν1/2 ) 15 Hz, bipy-H), 6.53 (8H, ν1/2
≈ 7 Hz, br signal with some coupling structure (overlapping triplet/
doublet), m,o-C6H5), 4.21 (32H, ν1/2 ) 10 Hz, C5Me5 and bipy-H),
1
N, 3.91. Found: C, 57.35; H, 5.93; N, 3.75. H NMR (C6D6, 20
°C): δ 211.0 (2H, ν1/2 ) 90 Hz, bipy-H), 4.45 (30H, ν1/2 ) 13 Hz,
C5Me5), 4.04 (6H, ν1/2 ) 5 Hz, bipy-CO2Me), -10.5 (2H, ν1/2
)
80 Hz, bipy-H), -19.6 (2H, ν1/2 ) 11 Hz, bipy-H). IR (Nujol mull;
CsI windows; cm-1): 3100 (vw), 2720 (vw), 2630 (vw), 2540 (vw),
2400 (vw), 2380 (vw), 1720 (s), 1708 (vs), 1695 (vs), 1540 (vs),
1505 (vs), 1450 (s), 1432 (vs), 1338 (m), 1295 (w), 1288 (m), 1258
(m-s), 1240 (m), 1220 (br vs), 1190 (m), 1115 (s), 1095 (m), 1042
(s), 980 (m), 960 (vs), 905 (w), 880 (vw), 830 (w), 805 (m), 775
(vw), 750 (vs), 735 (vw), 690 (m), 410 (vw), 360 (vw), 350 (vw),
315 (vs).
1
-16.8 (2H, ν1/2 ) 6 Hz, p-C6H5). H NMR (C7D8, 21 °C): 178.0
3
(2H, ν1/2 ) 97 Hz, bipy-H), 10.9 (2H, d, JCH ) 6.4 Hz, bipy-
H5,5′), 6.67 (4H, d, 3JCH ) 7.2 Hz, o-C6H5), 6.47 (4H, “t”, 3JCH
)
6.6 Hz, m-C6H5), 5.17 (2H, ν1/2 ) 50 Hz, bipy-H), 4.22 (30H, ν1/2
) 10 Hz, C5Me5), -16.8 (2H, ν1/2 ) 10 Hz, p-C6H5). IR (Nujol
mull; CsI windows; cm-1): 1598 (m), 1580 (m), 1560 (br s), 1495
(w), 1488 (w), 1300 (vw), 1290 (m), 1262 (m), 1242 (vw), 1100
(br m), 1022 (m), 1000 (vw), 960 (br m), 880 (w), 800 (br m), 750
(s), 720 (m), 690 (br m), 630 (m), 530 (w), 310 (br s).
[(C5Me5)2Yb(bipy-Me)]+[Cp*2YbI2]-. Method 1. Bis(penta-
methylcyclopentadienyl)ytterbium 4,4′-dimethyl-2,2′-bipyridine (0.3
g, 0.48 mmol) in toluene (15 mL) was contacted with AgI (0.11 g,
0.48 mmol) for 24 h. The solvent was removed under dynamic
vacuum, and the residue was washed with pentane (2 × 10 mL).
The precipitate was then dissolved in CH2Cl2 (10 mL), and the red
CH2Cl2 solution was filtered from the silver metal formed in the
reaction. Pentane (30 mL) was carefully layered onto the CH2Cl2
solution, and diffusive mixing at room temperature overnight
resulted in dark red crystals (0.2 g, 0.15 mmol, 63%). Mp: 230 °C
(dec). Anal. Calcd for C52H72N2I2Yb2: C, 47.14; H, 5.48; N, 2.11.
(C5Me5)2Yb(bipy-t-Bu). (C5Me5)2Yb(OEt2) (0.80 g, 1.6 mmol)
and the 4,4′-di(tert-butyl)-2,2′-bipyridine (0.43 g, 1.6 mmol) were
weighed into a Schlenk flask and dissolved in 20 mL of toluene
with stirring. The color changed immediately to dark red-brown.
The solvent was removed under dynamic vacuum. The residue was
extracted into pentane (30 mL), filtered, concentrated, and cooled
to -25 °C overnight, yielding red-brown crystals (0.76 g, 1.07
mmol, 67%). Mp: 247-250 °C (dec). Anal. Calcd for C38H54N2-
Yb: C, 64.11; H, 7.65; N, 3.94. Found: C, 64.48; H, 7.96; N, 3.75.
1H NMR (C6D6, 20 °C): δ 141.1 (2H, ν1/2 ) 55 Hz, bipy-H), 11.4
(2H, ν1/2 ) 45 Hz, bipy-H), 3.95 (30H, ν1/2 ) 7 Hz, C5Me5), 1.07
(18H, ν1/2 ) 3 Hz, bipy-CMe3), -8.9 (2H, ν1/2 ) 10 Hz, bipy-H).
IR (Nujol mull; CsI windows; cm-1): 1610 (m), 1592 (sh m), 1588
(m), 1550 (m), 1405 (m), 1380 (s), 1310 (br w), 1260 (s), 1200
(w), 1198 (vbr vs), 1020 (vbr vs.), 902 (vw), 845 (m), 800 (br vs),
730 (br w), 670 (w), 602 (m), 400 (vbr s), 310 (vbr s), 280 (vbr s).
(C5Me5)2Yb(bipy-CO2Et). 4,4′-Dicarboxylate-2,2′-bipyridine di-
ethyl diester was prepared by esterification of the diacid according
to a literature procedure.24 The crude diester was recrystallized twice
from boiling ethanol and then sublimed in an oil pump vacuum at
200 °C. (C5Me5)2Yb(OEt2) (0.63 g, 1.2 mmol) and the 4,4′-
1
Found: C, 47.62; H, 5.16; N, 2.24. H NMR (CD2Cl2, 20 °C): δ
330.0 (2H, ν1/2 ≈ 390 Hz, bipy-H), 56.6 (2H, ν1/2 ) 34 Hz, bipy-
H), 8.29 (6H, ν1/2 ) 10 Hz, bipy-Me), 3.61 (30H, ν1/2 ) 72 Hz,
C5Me5), 2.76 (30H, ν1/2 ) 32 Hz, C5Me5), -14.7 (2H, ν1/2 ) 23
Hz, bipy-H). IR (Nujol mull; CsI windows; cm-1): 2720 (w), 1615
(s), 1310 (m), 1268 (w), 1245 (w), 1228 (vw), 1022 (s), 920 (br
vw), 895 (vw), 850 (m), 830 (m), 805 (br vw), 732 (br s), 698 (w),
560 (m), 530 (m), 480 (w), 438 (w), 390 (vbr w), 322 (vs), 300 (br
s).
Method 2. (C5Me5)2YbI‚thf1 (0.12 g, 0.17 mmol) and 4,4′-
dimethyl-2,2′-bipyridine (0.03 g, 0.17 mmol) were weighed into a
Schlenk tube, and toluene (20 mL) was addded. The slurry was
stirred at room temperature overnight, the supernatant was dis-
carded, and the residue was washed with pentane (2 × 10 mL)
and extracted into dichloromethane (5 mL). Pentane (30 mL) was
carefully layered onto the CH2Cl2 solution, and diffusive mixing
(24) Elliot, C. M.; Hershenhart, E. J. J. Am. Chem. Soc. 1982, 104, 7519-
7526.
(25) Garelli, N.; Vierling, P. J. Org. Chem. 1992, 57, 3046-3051.

Coordination Complexes of Decamethylytterbocene
Organometallics, Vol. 25, No. 13, 2006 3237
at room temperature overnight resulted in dark red crystals (0.08
g, 0.06 mmol, 71%). The physical properties were identical to those
obtained by method 1.
became deep blue-green. The solvent was removed under dynamic
vacuum, and the residue was washed with pentane and dissolved
in hot toluene (20 mL). After filtration, the solution was slowly
cooled to -25 °C to give dark green crystals (0.58 g, mmol, 83%),
which were found by microanalysis and hydrolysis to contain one
toluene of crystallization. The compound was insoluble in pentane
and only sparely soluble in toluene. Mp: 228-238 °C (dec). Anal.
Calcd for C39H52N2Yb: C, 64.89; H, 7.26; N, 3.88. Found: C,
65.25; H, 6.82; N, 4.03. The low solubility of the product prevented
the acquisition of an NMR spectrum in C6D6. IR (Nujol mull; CsI
windows; cm-1): 2720 (w), 1612 (s), 1505 (m), 1495 (m), 1265
(m), 1230 (w), 1215 (m), 1080 (vbr m), 1020 (vbr m), 1010 (m),
812 (vs), 737 (vs), 730 (sh), 700 (s), 670 (w), 540 (w), 505 (s),
475 (m), 260 (br s).
(C5Me5)2Yb(py-OMe)2. 4-Methoxypyridine (1 mL, 0.88 mmol)
was added with stirring to a green toluene solution of (C5Me5)2-
Yb(OEt2) (0.40 g, 0.77 mmol). The solution immediately became
deep green. The solvent was removed under dynamic vacuum, and
the residue was washed with pentane and dissolved in toluene (20
mL). After filtration, the solution was concentrated and cooled to
-25 °C to give dark blue crystals (0.33 g, 0.50 mmol, 65%). Mp:
209-211 °C (dec). Anal. Calcd for C32H44N2O2Yb: C, 58.08; H,
6.70; N, 4.23. Found: C, 57.92; H, 6.79; N, 4.15. 1H NMR (C6D6,
20 °C): δ 8.32 (4H, br s, py-H), 6.36 (4H, br s, py-H), 3.01 (6H,
s, p-OMe), 2.12 (30H, s, C5Me5). A spin-spin coupling for the
pyridine-CH resonances was not observed, as the resonances were
significantly broadend, suggesting exchange on the NMR time scale.
IR (Nujol mull; CsI windows; cm-1): 3050 (m), 3030 (m), 2720
(w), 1608 (s), 1568 (m), 1550 (s), 1340 (w), 1302 (vs), 1210 (s),
1040 (vs), 1002 (s), 860 (w), 820 (s), 696 (w), 660 (vw), 572 (w),
550 (w), 470 (w), 375 (br w), 253 (br s).
[Cp*2Yb(bipy-OMe)]+[Cp*2YbI2]-. Method 1. Bis(penta-
methylcyclopentadienyl)ytterbium 4,4′-dimethoxy-2,2′-bipyridine
(0.32 g, 0.48 mmol) in toluene (15 mL) was contacted with AgI
(0.11 g, 0.48 mmol) for 24 h. The solvent was removed under
dynamic vacuum, and the residue was washed with pentane (2 ×
10 mL). The precipitate was then dissolved in CH2Cl2 (10 mL),
and the red CH2Cl2 solution was filtered from the silver metal
formed in the reaction. Pentane (30 mL) was carefully layered onto
the CH2Cl2 solution, and diffusive mixing at room temperature
overnight resulted in dark red blocks (0.23 g, 0.17 mmol, 71%).
Mp: 234-235 °C (dec). Anal. Calcd for C52H72N2O2I2Yb2: C,
1
46.02; H, 5.35; N, 2.06. Found: C, 47.20; H, 5.10; N, 2.10. H
NMR (CD2Cl2, 20 °C): δ 334.6 (2H, ν1/2 ≈ 400 Hz, bipy-H), 58.4
(2H, ν1/2 ) 34 Hz, bipy-H), 10.04 (6H, ν1/2 ) 16 Hz, bipy-OMe),
5.30 (30H, ν1/2 ) 50 Hz, C5Me5), 4.26 (30H, ν1/2 ) 75 Hz, C5-
Me5), -15.3 (2H, ν1/2 ) 24 Hz, bipy-H). IR (Nujol mull; CsI
windows; cm-1): 3060 (w), 2720 (w), 1610 (br s), 1562 (m), 1555
(m), 1502 (m), 1495 (m), 1420 (m), 1350 (s), 1328 (m), 1295 (w),
1287 (w), 1272 (m), 1262 (s), 1232 (br m), 1190 (vw), 1042 (s),
1030 (s), 1008 (w), 920 (vw), 895 (vw), 892 (vw), 875 (vw), 860
(vw), 850 (vw), 840 (vw), 730 (br w), 620 (vw), 600 (w), 400 (br
m), 325 (vs), 300 (vs).
Method 2. (C5Me5)2YbI‚thf1 (0.12 g, 0.17 mmol) and 4,4′-
dimethoxy-2,2′-bipyridine (0.036 g, 0.17 mmol) were weighed into
a Schlenk tube, and toluene (20 mL) was addded. The slurry was
stirred at room temperature overnight, the supernatant was dis-
carded, and the residue was washed with pentane (2 × 10 mL)
and extracted into dichloromethane (5 mL). Pentane (30 mL) was
carefully layered onto the CH2Cl2 solution, and diffusive mixing
at room temperature overnight resulted in dark red blocks (0.078
g, 0.057 mmol, 68%). The physical properties were identical to
those obtained by method 1.
[(C5Me5)2Yb(bipy-CO2Et)]+[Cp*2YbI2]-. Bis(pentamethylcy-
clopentadienyl)ytterbium 4,4′-dicarboxylate-2,2′-bipyridine diethyl
diester (0.73 g, 0.98 mmol) in toluene (15 mL) was contacted with
AgI (0.23 g, 0.98 mmol) for 24 h. The solvent was removed under
dynamic vacuum, and the residue was washed with pentane (2 ×
10 mL). The precipitate was then dissolved in CH2Cl2 (10 mL),
and the red CH2Cl2 solution was filtered from the silver metal
formed in the reaction. Pentane (30 mL) was carefully layered onto
the CH2Cl2 solution, and diffusive mixing at room temperature
overnight resulted in dark red blocks (0.4 g, 0.32 mmol, 65%). Mp
> 320 °C (dec). Anal. Calcd for C56H76N2O4I2Yb2: C, 46.67; H,
5.32; N, 1.94. Found: C, 45.54; H, 5.18; N, 2.02. 1H NMR (CD2-
Cl2, 20 °C): δ 332.9 (2H, ν1/2 ≈ 500 Hz, bipy-H), 58.0 (2H, ν1/2
) 90 Hz, bipy-H), 6.50 (4H, ν1/2 ) 60 Hz, CH2), 5.51 (30H, ν1/2
) 60 Hz, C5Me5), 4.05 (30H, ν1/2 ) 35 Hz, C5Me5), 3.68 (6H, ν1/2
) 80 Hz, CH3), -13.1 (2H, ν1/2 ) 80 Hz, bipy-H). IR (Nujol mull;
CsI windows; cm-1): 3080 (vw), 2720 (vw), 1738 (vs), 1730 (sh),
1615 (m), 1550 (m), 1400 (vs), 1300 (s), 1270 (s), 1250 (s), 1235
(m), 1170 (vbr w), 1140 (s), 1120 (vbr w), 1065 (vw), 1020 (vs),
912 (w), 880 (w), 860 (w), 800 (vbr w), 778 (s), 722 (s), 690 (m-
s), 672 (vw), 390 (vbr m), 330 (vs), 300 (br s).
Crystallographic data are deposited with the Cambridge Crystal-
lographic Data Centre. Copies of the data (CCDC 292029) can be
obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/
cif, by e-mailing data_request@ccdc.cam.ac.uk, or by contacting
The Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB 1EZ, UK; fax +44 1223 336033.
Acknowledgment. This work was supported by the Director,
Office of Energy Research, Office of Basic Energy Sciences,
Chemical Sciences Division of the U.S. Department of Energy
under Contract No. DE-AC03-76SF00098. We thank Dr. Fred
Hollander and Dr. Allen Oliver for assistance with the crystal-
lography and the German Academic Exchange Service (DAAD)
(M.D.W.) and the NSERC (Canada) (D.J.B.) for fellowships.
We thank Wayne W. Lukens and Corwin H. Booth for endless
hours of discussion about what these molecules are trying to
tell us.
Supporting Information Available: Crystallographic data,
labeling diagrams, tables giving atomic positions, anisotropic
thermal parameters, bond distances, bond angles, torsion angles,
least-squares planes, packing diagrams, temperature dependence of
the unit cell parameters of (C5Me5)2Yb(bipy-Me), and variable-
1
temperature H studies. This material is available free of charge
via the Internet at http://pubs.acs.org. Structure factor tables are
available from the authors.
(C5Me5)2Yb(4-picoline)2‚C7H8. 4-Methylypyridine (1 mL, 1.02
mmol) was added with stirring to a green toluene solution of (C5-
Me5)2Yb(OEt2) (0.50 g, 0.97 mmol). The solution immediately
OM051051D