Influence of the torsion angle in 3,3′-Dimethyl-2,2′-bipyridine on the intermediate valence of Yb in (C5Me5) 2Yb(3,3′-Me2-bipy)

Article abstract of DOI:10.1021/om400528d

The synthesis and X-ray crystal structures of Cp*₂Yb(3, 3′-Me₂bipy) and [Cp*₂Yb(3,3′-Me ₂bipy)][Cp*₂YbCl_1.6I_0.4] ·CH₂Cl₂ are described. In both complexes, the NCCN torsion angles are approximately 40. The temperature-independent value of n _f of 0.17 shows that the valence of ytterbium in the neutral adduct is multiconfigurational, in reasonable agreement with a CASSCF calculation that yields a n_f value of 0.27; that is, the two configurations in the wave function are f¹³(π*₁)¹ and f ¹⁴(π*₁)⁰ in a ratio of 0.27:0.73, respectively, and the open-shell singlet lies 0.28 eV below the triplet state (n_f accounts for f-hole occupancy; that is, n_f = 1 when the configuration is f¹³ and n_f = 0 when the configuration is f¹⁴). A correlation is outlined between the value of n _f and the individual ytterbocene and bipyridine fragments such that, as the reduction potentials of the ytterbocene cation and the free x,x′-R₂-bipy ligands approach each other, the value of n _f and therefore the f¹³:f¹⁴ ratio reaches a maximum; conversely, the ratio is minimized as the disparity increases.

Full text of DOI:10.1021/om400528d

Article
pubs.acs.org/Organometallics
Influence of the Torsion Angle in 3,3′-Dimethyl-2,2′-bipyridine on
the Intermediate Valence of Yb in (C5Me5)2Yb(3,3′-Me2‑bipy)
Gregory Nocton,*,†,‡ Corwin H. Booth,§ Laurent Maron,∥ and Richard A. Andersen*,‡,∥
́
†Laboratoire Het
́
er
́
oel
́
em
́
ents et Coordination, CNRS, Ecole Polytechnique, Route de Saclay, 91128 Palaiseau, France
‡Department of Chemistry, University of CaliforniaBerkeley, Berkeley, California 94720, United States
§Chemical Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
∥LPCNO, UMR 5215, Universite
́
de Toulouse-CNRS, INSA, UPS, Toulouse, France
S
* Supporting Information
ABSTRACT: The synthesis and X-ray crystal structures of
Cp*2Yb(3,3′-Me2bipy) and [Cp*2Yb(3,3′-Me2bipy)]-
[Cp*2YbCl1.6I0.4]·CH2Cl2 are described. In both complexes,
the NCCN torsion angles are approximately 40°. The
temperature-independent value of nf of 0.17 shows that the
valence of ytterbium in the neutral adduct is multiconfigura-
tional, in reasonable agreement with a CASSCF calculation
that yields a nf value of 0.27; that is, the two configurations in
the wave function are f13(π*1)1 and f14(π*1)0 in a ratio of
0.27:0.73, respectively, and the open-shell singlet lies 0.28 eV
below the triplet state (nf accounts for f-hole occupancy; that
is, nf = 1 when the configuration is f13 and nf = 0 when the
configuration is f14). A correlation is outlined between the value of nf and the individual ytterbocene and bipyridine fragments
such that, as the reduction potentials of the ytterbocene cation and the free x,x′-R2-bipy ligands approach each other, the value of
nf and therefore the f13:f14 ratio reaches a maximum; conversely, the ratio is minimized as the disparity increases.
When the 2,2′-bipyridine ligand in Cp*2Yb(bipy) is replaced
by methyl-substituted 2,2′-bipyridine ligands symbolized as x-
Mebipy or x,x′-Me2bipy, where x indicates the position of the
methyl group(s) in the 2-pyridyl rings, the ytterbium atom in
the resulting adducts is intermediate valent, but the value of nf
is temperature dependent.3 For example, when x is 5, the value
of nf in Cp*2Yb(5-Mebipy) changes from 0.42 to 0.75 at 300
and 30 K, respectively. The temperature dependence is not due
to a closed-shell singlet ⇌ open-shell triplet equilibrium, a
valence tautomerism, but to an equilibrium between two open-
shell singlet states in which the thermodynamic constants, ΔH
and ΔS, are obtained using the Boltzmann equation. The
computational model showed that two open-shell singlet states,
SS1 and SS2, lie below the triplet by 0.58 and 0.11 eV,
respectively.
The evolving model of the electronic structure of the
bipyridine adducts of Cp*2Yb is (i) the ground state is an open-
shell singlet, (ii) the configurations of both fragments, Cp*2Yb
and the bipyridines, are multiconfigurational, (iii) the number
and position of the methyl groups on the bipyridine ligand are
significant, and (iv) the nf values qualitatively track the
reduction potential of the free bipyridine ligand, at least for
those bipyridines whose reduction potentials are available in the
INTRODUCTION
■
Previous studies of the 2,2′-bipyridine adduct of (C5Me5)2Yb,
abbreviated Cp*2Yb(bipy), have shown that the electronic
ground state is multiconfigurational; that is, the f-orbital
portion of the ground state wave function includes Ψ = c1|
Yb(III,f13)(bipy•−)⟩ + c2|Yb(II,f14)(bipy)0⟩, which is an
admixture of two configurations, where c1 and c2 are their
respective coefficients.1 The f13(bipy•−) configuration is an
open-shell singlet, and f14(bipy)0 is a closed-shell singlet; a
configuration interaction is therefore possible, since the two
configurations have the same symmetry. The outcome of the
configuration interaction is that the valence of the ytterbium in
Cp*2Yb(bipy) is neither +II, where c1 = 0, nor +III, where c2 =
0, but is between these two extreme values and therefore
intermediate valent. Experimentally, the valence of the
ytterbium was obtained from Yb LIII-edge XANES spectroscopy
and expressed as nf, the number of f holes: for Cp*2Yb(bipy), nf
2
= c1 = 0.83(2) is temperature independent, and f13(bipy•−) is
the dominant configuration. The concept that the ground state
is multiconfigurational, initially posited by Fulde,2 was
developed from CASSCF calculations, initially using the
model (C5H5)2Yb(bipy)1 and then extended to (C5Me5)2Yb-
(bipy) and including four empty bipy orbitals, π*1, π*2, π*3,
and π*4, in the active space. The calculated value of nf was 0.86,
in agreement with experiment.3
Received: June 8, 2013
© XXXX American Chemical Society
A
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literature. Item iv provides a strategy for manipulating the value
of nf and therefore the magnetic properties of the adducts. The
reduction potential of a ligand is a measure of the HOMO−
LUMO energy gap,1,2 which can be changed by substituents
that are electron donors or electron acceptors and by the
torsion angle between the 2-pyridyl rings, usually reported as
the NCCN angle, obtained from solid-state crystal structures.
Electron-donating groups, such as a methyl group, increase the
reduction potential relative to hydrogen (Table 1), while a
AgI in toluene (Scheme 2). In this case, the identity of the
anion depends upon the crystallization solvent.11,12 If the
cation−anion pair is crystallized from warm toluene, the ion
pair crystallizes as [Cp*2 Yb(3, 3′-Me2 bipy)]-
[Cp*2YbI2]·1/3PhMe. If, on the other hand, the ion pair is
crystallized by layering a CH2Cl2 solution with pentane, the ion
pair has the composition [Cp*2Yb(3,3′-Me2bipy)]-
[Cp*2YbCl1.5I0.5]·CH2Cl2, as determined by combustion
1
analysis and integrated intensities in the H NMR spectrum
(see the Experimental Section for details). Although the
composition of a single crystal is slightly different ([Cp*2Yb-
(3,3′-Me2bipy)][Cp*2YbCl1.6I0.4]·CH2Cl2; see below), it is
clear that chloride for iodide exchange occurs in dichloro-
methane, a result not observed in earlier studies of ytterbocene
bipyridine cations.11,12
Table 1. Redox Potential of Selected N-Aromatic
Heterocycles and Ytterbocenes
ligand
E1/2 (V) vs Fc+/Fc
ref
py
−3.16
−2.88
−2.68
−2.60
−2.03
−1.78
−1.65
5
6
6
7
6
8
9
4,4′-(OEt)2bipy
4,4′-Me2bipy
bipy
X-ray Crystal Structures. ORTEP drawings of the neutral
and cationic adducts are shown in Figures 1 and 2, respectively.
The crystallographic details are available in the Supporting
Information and some bond length and angles are given in
Table 2. In both adducts, the coordination number and
geometry about ytterbium are the same, as are the torsion
angles in the coordinated 3,3′-Me2bipy ligands, the intra-
molecular bond angles, and the C(2)−C(2′) distances; the
average Yb−N and Yb−C distances, however, are rather
different. The average Yb−C distance in the neutral adduct of
2.71 0.01 Å is the same as that found in Cp*2Yb(py)2 of 2.74
0.04 Å13 and in the three independent molecules of
a
4,4′-CO2Me
(C5Me5)2Yb+
(C5H4Me)2Yb+
a
The approximate value of 3,3′-Me2bipy lies below −2.8 V (vs Fc+/Fc,
THF/0.1 M [NBu4][PF6]) but could not be measured precisely under
our conditions. Under similar conditions the value of bipy is 2.63 V (vs
Fc+/Fc).
CO2Me group decreases the potential. Increasing the torsion
angle should raise the reduction potential, since twisting the 2-
pyridyl groups out of coplanarity will move the energy of π*1
closer to the energy of π*2 and π*3 orbitals. Although the value
of the reduction potential for 3,3′-Me2bipy is not available in
the literature, an approximate value is listed in footnote a in
Table 1, the energy of the empty orbital should behave
qualitatively like those in twisted ethylene.4 Several examples of
methyl- and dimethyl-substituted bipyridine adducts of Cp*2Yb
have been reported that are consistent with the correlation
between nf and the reduction potential. For this article, we
prepared the neutral and cationic adducts between 3,3′-
Me2bipy and Cp*2Yb in order to explore the physical
properties of the adducts with a bipyridine ligand with a large
torsion angle. The results of these and earlier studies support
the contention that the redox properties of the individual
molecular fragments that comprise the adduct, Cp*2Yb and
x,x′-Me2bipy, determine the extent of electron correlation
between them.
Cp*2Yb(6,6′-Me2bipy) of 2.74
average Yb−N distance of 2.485
0.002 Å.3 However, the
0.003 Å is significantly
shorter than the equivalent distances in Cp*2Yb(py)2 and
Cp*2Yb(6,6′-Me2bipy) of 2.565 0.005 and 2.506 0.009 Å,
respectively. The average Yb−C distance in the cation of 2.60
0.01 Å is identical with that in [Cp*2Yb(bipy)]+ of 2.59 0.01
Å, as are the Yb−N distances of 2.396
0.02 and 2.372
0.005 Å, respectively. The Yb−C and Yb−N distances in the
neutral adduct are approximately 0.10 Å longer than in the
cation, showing that the valences of Yb are different.
It is noteworthy that the torsion angles of approximately 40°,
defined by either the four atoms NCCN and CCCC or the
dihedral angle between the planar 3-Me-2-pyridyl rings, are the
same in both adducts. The large torsion angle in [Cp*2Yb(3,3′-
Me2bipy)]+ of 40° and the small angle in [Cp*2Yb(bipy)]+ of
7° results in only a slight elongation of the Yb−N distance
(0.012 Å) in the former. Hence, the large torsion angle and the
resulting “misdirection” of the lone pair of electrons on the
bidentate chelate in these cations only slightly lengthens the
average Yb−N distances. This, presumably, reflects the small
effect on the bond enthalpy difference resulting from the
“misdirection”. Several crystal structures of 3,3′-Me2bipy with d
transition metals are known, and the twist angle, generally
defined as the intersection of the dihedral angle between the
planar 3-Me-2-pyridyl rings, ranges from 29 to 37°.14−20 One
exception is [Ru(Me3tacn)(3,3′-Me2bipy)(OH2)]2+, in which
the reported twist angle is 54°. However, the structure in the
CCDC indicates that the NCCN and CCCC angles are 29 and
43°, respectively.17
RESULTS
■
Synthesis and Physical Properties. The preparation of
Cp*2Yb(3,3′-Me2bipy) follows the general synthesis procedure
used in earlier articles: that is, addition of 3,3′-Me2bipy to
Cp*2Yb(OEt2)10 in toluene followed by crystallization at −20
°C. The green crystals melt at 310−312 °C (Scheme 1).
The cation in [Cp*2Yb(3,3′- Me2bipy)][Cp*2YbI2] is
obtained by addition of the neutral adduct to a suspension of
Scheme 1
1H NMR Studies. The 1H NMR chemical shifts at 20 °C of
the previously prepared bipyridine and substituted bipyridine
adducts of Cp*2Yb fall into a pattern in which δ(6-H) (∼160
ppm) > δ(4-H) (∼30 ppm) > δ(5-H) (∼10 ppm) > δ(3-H)
(∼−15 ppm). The assignments are determined by H for Me
group replacement; the approximate values of the chemical
shifts are in parentheses.21 The 1H NMR spectra show that the
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Scheme 2
adducts have a similar degree of paramagnetism, an inference
that is developed below.
A plot of δ vs 1/T (Figure 3) of Cp*2Yb(3,3′-Me2bipy)
shows that the most downfield resonance has a strong
dependence on temperature and the shift increases with
increasing temperature, i.e., the resonance shows anti-Curie
behavior. A resonance that follows a Curie Law, δ = C/T, has a
positive slope in a δ vs 1/T plot, but a slope that is negative
displays anti-Curie behavior. This anti-Curie dependence is not
due to the exchange between free and coordinated 3,3′-
Me2bipy, since addition of 3,3′-Me2bipy results in a spectrum at
20 °C due to free and coordinated 3,3′-Me2bipy resonances.
Unfortunately, the inverse temperature dependence of the
downfield resonance cannot be interpreted, since the resonance
cannot be assigned definitively, although it is most reasonably
ascribed to the 6-H position and therefore the value of its
chemical shift is dominated by the dipolar contribution to the
chemical shift.12 It is noteworthy that anti-Curie behavior has
been shown in low-spin Fe(III) porphyrins to result from a
difference in sign between the dipolar and contact shift terms in
the chemical shift tensor.23
Figure 1. ORTEP drawing of Cp*2Yb(3,3′-Me2bipy) (thermal
ellipsoids at the 50% level). All non-hydrogen atoms have been
refined anisotropically, and the hydrogen atoms have been placed in
calculated positions but not refined.
The neutral and cationic adducts of 3,3′-Me2bipy have C2
1
symmetry in the solid state (Figures 1 and 2). The H NMR
spectrum of the cation in CD2Cl2 at 300 K contains four
bipyridine resonances at 313, 52, 11, and −18 ppm; the
intensity of the last resonance shows that it is due to the methyl
groups. Unfortunately, the resonance at δH 313 ppm disappears
into the baseline as the temperature is lowered and its
temperature dependence cannot be determined. In solution,
the adducts could have either C2 or C2v symmetry depending
on the value of the torsion angle. In C2v symmetry the average
torsion angle is 0° and therefore the methyl groups on each 2-
pyridyl ring are eclipsed. Thus, the dynamic process C2 ⇌ C2v
⇌ C2 will have a high activation energy.24−27 This process
cannot be observed in [Cp*2Yb(3,3′-Me2bipy)]0,+, since the
two methyl groups are chemically equivalent in either C2 or C2v
symmetry. A way to answer this question would be to prepare
adducts in which the C2 axis is removed: for example, by
preparing CpCp′Yb(3,3′-Me2bipy). Unfortunately, a synthetic
methodology for preparing mixed-ring ytterbocences is
unknown at the present time.
Figure 2. ORTEP drawing of the cation [Cp*2Yb(3,3′-Me2bipy)]+
(thermal ellipsoids at the 50% level). The anionic partner
[Cp*2YbCl1.6I0.4]− and the cocrystallized molecule of CH2Cl2 have
been removed for clarity. All non-hydrogen atoms have been refined
anisotropically, and the hydrogen atoms have been placed in calculated
positions but not refined.
Visible Spectroscopy. Room-temperature vis−near-IR
spectra for Cp*2Yb(3,3′-Me2bipy) and [Cp*2Yb(3,3′-
Me2bipy)][Cp*2YbCl1.5I0.5], recorded in toluene and CH2Cl2,
respectively, are shown in Figure 4. Two main bands at around
500 nm (20000 cm−1) and 900 nm (11110 cm−1) are present
in Cp*2Yb(3,3′-Me2bipy). These two transitions, attributed to
ligand-based π → π* and π* → π* transitions, are present as
principal features in the spectrum of Li(bipy)28 and are
reasonably ascribed to radical anion signatures in this
ytterbocene adduct. If correct, the absorption coefficients are
low, implying a low concentration of 3,3′-Me2bipy radical
anion. The intensity of the ligand-based π* → π* transitions
adducts have C2v symmetry in solution. The chemical shifts of
the 3,3′-Me2bipy adduct also show four bipy resonances at 20
°C with δH values of 32.7, 12.1, 8.5, and 0.58 ppm. The only
resonance that can be assigned with certainty is δH 0.58, due to
the methyl groups in the 3,3′-positions; the δH 12.1 resonance
is a doublet and therefore is likely due to δ(4-H). The relatively
small spread in chemical shift values relative to the large spread
in the previously reported adducts of Cp*2Yb(x,x′-bipy) where
x = H and x′ = Me or x = x′ = Me, implies that the extent of
paramagnetism is less in the 3,3′-Me2bipy adduct. The small
range in chemical shift values of the bipy resonances in
Cp2Yb(bipy) of δH 53, 18, 11. and 0.622 implies that the two
1
and the relatively small spread of the H NMR chemical shifts
imply that the extent of the paramagnetism of Cp*2Yb(3,3′-
C
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Table 2. Comparison of Bond Lengths (Å) and Angles (deg) for Cp*2Yb(3,3′-Me2bipy) and [Cp*2Yb(3,3′-Me2bipy)]+
a
Cp*2Yb(3,3′-Me2bipy)
[Cp*2Yb(3,3′-Me2bipy)]+
Δ
Yb−C(ring) (av)
Yb−C(ring) (range)
Yb−Cp(cent)
2.71 0.01
2.60 0.01
0.11
2.693(3)−2.732(3)
2.43
2.585(4)−2.629(4)
2.31
0.12
0.09
0.003
6
Yb−N (av)
2.485 0.003
1.494(5)
147
2.393 0.002
1.491(6)
141
C(25)−C(26)
Cp(cent)−Yb−Cp(cent)
Cp(cent)−Yb−N
N−Yb−N
103, 107
66.6(1)
38
105, 108
69.6(1)
37
2, 1
−3
1
N−C−C−N
C−C−C−C
44
43
1
3-Me-2-pyridyl
41
41
0
a
Δ is the value in the neutral adduct minus that in the cation in Å or deg.
coordination environments of the anionic partner in the anion
pair.
Magnetism and XANES Studies. The effective magnetic
moment at 300 K for Cp*2Yb(3,3′-Me2bipy) is 0.8 μB, low
relative to the value expected for two uncorrelated spin carriers,
Yb(III) f13 and bipy•−, for which a value of 4.85 μB is expected.
The low value of μeff is an indication that the Yb in the neutral
adduct may be intermediate valent and therefore a member of
the family of the bipyridine adducts of Cp*2Yb reported earlier
in which effective moments range from 2.4 to 0.8 μB at 300 K.
The temperature dependence of μeff from 2 to 300 K is shown
in Figure 5 (plots of χ, 1/χ, and χT vs T are available in the
Supporting Information).
The value of μeff decreases slowly in the neutral adduct with
decreasing temperature to a value of 0.6 μB at 30 K and then
rapidly decreases to 0.3 μB at 2 K. The latter decrease is likely
due to intermolecular antiferromagnetic coupling at low
temperature, as previously observed.1 In contrast, and as
expected, the value of μeff for [Cp*2Yb(3,3′-Me2bipy)]-
[Cp*2YbI2] of 6.1 μB is in accord with the value for two
Figure 3. Chemical shift (δ) vs 1/T plot of the proton resonances of
Cp*2Yb(3,3′-Me2bipy) in toluene-d8 over a temperature range of 180
to 360 K.
2
Yb(III) J7/2 isolated paramagnets of 6.4 μB. Although the low
effective moment for the neutral adduct is a signature of
intermediate-valent ytterbium, a spectroscopic measurement is
necessary to confirm this hypothesis. The Yb LIII-edge
XANES1,3 spectra in Figure 6 show that the measurements at
30 and 300 K are nearly superimposable and therefore the ratio
of Yb(II) f14 to Yb(III) f13 is independent of temperature; this
rules out a valence tautomeric equilibrium as a possible
explanation for the temperature dependence of μeff. The
XANES spectra can be fit into f14 and f13 components in a ratio
of 83:17 at both temperatures, and the resulting nf value is
therefore 0.17. These results place Cp*2Yb(3,3′-Me2bipy) into
a small subset of bipyridine adducts with low values of μeff and
nf at 300 K reported previously (Table 3).
In the first two adducts in Table 3, the metallocenes are bipy
adducts of (C5Me5)2Yb in which the substituents on the
bipyridines are methyl groups in the 3,3′-positions and
methoxy groups in the 4,4′-positions. These two groups are
electron donating, since the reduction potentials of the
corresponding free ligands are more negative than that in
unsubstituted bipy and therefore more difficult to reduce
(Table 1). The third and fourth adducts given in Table 3 show
the influence of the substituents on the metallocene: C5Me5 is
more strongly reducing than C5H4Me, as shown in Table 1, and
presumably C5H5, consistent with the relative value of nf;
therefore, the intermediate valence of ytterbium can be
influenced by the redox potentials of the individual fragments
Figure 4. Room-temperature visible spectra of Cp*2Yb(3,3′-Me2bipy)
(blue line) in toluene and [Cp*2Yb(3,3′- Me2bipy)][Cp*2YbCl1.5I0.5]
(red line) in CH2Cl2.
Me2bipy) is small, which in turns implies that the f14
configuration dominates the f13(bipy•−) configuration in the
ground state electronic structure.
The cation in [Cp*2Yb(3,3′-Me2bipy)][Cp*2YbCl1.5I0.5]
possesses a simpler visible spectrum at room temperature and
is similar to the reported spectra of cationic Cp*2Yb adducts
with N-aromatic heterocycles.11,28 The presence of several f−f
transitions in the 950−1020 nm range accounts for the multiple
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Figure 5. Temperature-dependent magnetic susceptibility data for the neutral (left) and cationic ytterbocenes (right) between 2 and 300 K.
The calculational results are consistent with those obtained
on related bipy adducts, in that the electronic ground states are
open-shell singlet states that are multiconfigurational. Only one
singlet is found below the triplet in each adduct in Table 4. The
singlet state is composed of the two configurations f13(π1*)1
and f14(π1*)0; the f12(π1*)2 configuration is less than 2%. The
calculated values of nf agree with the experimental values very
well for (C5H5)2Yb(bipy) and (C5Me5)2Yb(4,4′-(OMe)2bipy),
but the agreement is less good for the 3,3′-Me2bipy adduct. In
each case, the dominant configuration is f14, in contrast to the
results for (C5Me5)2Yb(bipy).
DISCUSSION
■
The original reason for preparing the 3,3′-Me2bipy adduct of
Cp*2Yb was to determine the role of the torsion angle of the
bipyridine ligand on the intermediate valence of ytterbium in
the resulting adduct. The NCCN torsion angle in the x,x′-
Me2bipy adducts of Cp*2Yb are small; the torsion angles are 1
and 4° for 4,4′-Me2bipy and 5,5′-Me2bipy adducts, respectively,
and the 2-pyridyl rings are nearly coplanar.3,12 When the
methyl groups are in the 3,3′-positions of the 2-pyridyl rings,
the rings are forced out of planarity. The twisted geometry
raises the energy of the LUMO, π1*, and the reduction
potential of the free ligand (see Table 1), which in turn affects
the configuration of the open-shell singlet ground state and
ultimately the magnetism of the adduct. This expectation is met
in Cp*2Yb(3,3′-Me2bipy) since the torsion angle is 38°; the
magnetic moment is one of the lowest observed at 300 K, as is
the nf value of 0.17. These experimental results are in
reasonable agreement with the calculated value of nf in Table
4, and the dominant configuration of the open-shell singlet
ground state is f14(π1*).
The connection between nf and the redox properties of the
ytterbocene and the bipyridine ligand can be extended to
Cp*2Yb(4,4′-(MeO)2bipy) and Cp2Yb(bipy), two adducts with
low values of nf are 0.13 and 0.30, respectively, which are
independent of temperature. A methoxy group is a better
electron donor than a methyl group; the Hammett σp values are
−0.27 and −0.17, respectively,29 and the reduction potential of
4,4′-(EtO)2-bipy is 0.20 V more negative than that of 4,4′-
Me2bipy (Table 1). The nf value for Cp*2Yb(4,4′-(MeO)2bipy)
is 0.13, in accord with the larger reduction potential of the free
ligand. The calculated value of nf of 0.11 agrees with the
experimental value. The reduction potential of (C5Me5)2Yb+ is
more negative than that of (C5H4Me)2Yb+ by 0.13 V (Table 1);
the value for (C5H5)2Yb+ is not available but is likely to be
slightly lower. The nf values of the adducts of (C5Me5)2Yb and
Figure 6. Yb LIII-edge XANES spectra for Cp*2Yb(3,3′-Me2bipy) at
30, 150, and 300 K. The main peak at 8939 eV is indicative of the
Yb(II) contribution, and the peak at 8946 eV is indicative of the
Yb(III) contribution.
Table 3. μeff and nf Values for Some Selected Cp2′Yb(x,x′-
bipy) Adducts
Cp′
x,x′
μeff (μB)
nf(300 K)
ref
C5Me5
C5Me5
C5H5
3,3′-Me2
4,4′-(OMe)2
H,H
0.8
1.1
1.2
2.4
3.2
0.17(2)
0.13(9)
0.30(1)
0.83(2)
0.84(2)
this work
1, 12
1, 22
1
C5Me5
C5Me5
H,H
4,4′-CO2Me
1
in the adducts. This element of control was advanced in earlier
work,1,3 and the new adduct described here provides additional
support for and extends this postulate to a geometrical
dependence of the bipyridine ligands.
Calculations. The CASSCF methodology, similar to
methods used in previous studies, gives the calculated
singlet−triplet energy separations and nf values shown in
Table 4.
Table 4. CASSCF Calculational Results
S−T gap (eV) nf(calcd)
nf(exptl)
(C5H5)2Yb(bipy)
−0.6
0.32
0.863
0.27
0.11
0.30
0.831
0.17
0.13
(C5Me5)2Yb(bipy)
−0.283
−0.28
−0.007
(C5Me5)2Yb(3,3′-Me2bipy)
(C5Me5)2Yb(4,4′-(MeO)2bipy)
E
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Organometallics
Article
(C5H5)2Yb with bipy are 0.83 and 0.30, respectively, consistent
with the proposition advanced above. The calculated values of
nf of 0.86 and 0.32 agree with experiment, and these values
show that the dominant configuration in the multiconfigura-
tional ground state is inverted by changing the number of
methyl substituents on the cyclopentadienyl rings.
Table 5. Selected Crystal Data for Cp*2Yb(3,3′-Me2bipy)
and [(Cp*2Yb(3,3′-Me2bipy)][Cp*2YbCl1.6I0.4]·CH2Cl2
Cp*2Yb(3,3′-
[(Cp*2Yb(3,3′-Me2bipy)]
[Cp*2YbCl1.6 0.4]·CH2Cl2
Me2bipy)
I
formula
C32H42N2Yb
0.3 × 0.25 × 0.10
orthorhombic
Pbca
C53H74N2Cl3.6I0.4Yb2
0.45 × 0.37 × 0.15
triclinic
cryst size (mm)
cryst syst
space group
V (Å3)
The relation between the redox potential and nf provides a
guide to the extent of unpaired spin density located in the bipy
and related heterocyclic amine adducts of ytterbocenes. The
redox properties of the uncoordinated fragments in the
molecular adducts, however, are only a guide, since these
values change on coordination; the redox potentials in Table 1
show that Cp*2Yb is thermodynamically incapable of reducing
bipy, but electron transfer does occur. The distribution of
unpaired spin density in the ligand orbitals, after the initial
electron transfer event, alters the configurations involved in the
multiconfigurational ground state and therefore the magnetic
properties of the adducts. The amount of unpaired spin density
at a given site in the ligand orbitals is crucial for understanding
chemical reactivity, which is our ultimate goal. One such
example, cleavage of a specific C−H bond in the 4,5-
diazafluorene adduct of Cp*2Yb, has been reported30 and the
other will be reported in due course.
P1
̅
5827(3)
17.550(5)
18.129(5)
18.314(5)
90
2582.8(6)
9.4957(14)
17.024(2)
17.198(3)
72.940(2)
76.451(2)
87.439(2)
2
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
90
γ (deg)
90
Z
8
formula wt
calcd density (g cm−3
627.72
1263.65
1.749
)
1.431
abs coeff (mm−1
)
3.247
4.060
F(000)
3024
1322
temp (K)
153(2)
110(2)
diffractometer
θ range for data
collection (deg)
transmission range
abs cor
SMART APEX
1.96−25.34
SMART 1000
2.44−25.48
CONCLUSIONS
0.393−0.723
multiscan
0.132−0.481
multiscan
■
The experimental and computational studies outlined in this
and previous articles1,3 on the adducts formed between various
ytterbocenes and substituted bipyridine ligands develops a
model that accounts for their unusual magnetic properties. The
experimental facts are that the ytterbium adducts are
intermediate-valent molecular compounds; that is, the valence
of ytterbium is neither Yb(II), f14, nor Yb(III), f13, but lies
between these two extreme values. The electronic ground state
is comprised of two configurations, f14(π1*)0 and f13(π1*)1,
resulting in an open-shell singlet ground state that lies below
the triplet state. The relative population of these two
configurations, expressed as nf, depends on the redox properties
of the individual fragments of the adducts. Accordingly, the
substituents on each fragment play a role in determining the
composition of the multiconfigurational ground state and
ultimately the magnetic properties of the resulting adducts.31
The multiconfigurational ground state is, in this model, the way
ytterbium avoids the extreme values of its possible oxidation
numbers, which allows the adducts to obey Pauling’s
electroneutrality principle and behave as if they are covalent
molecules.31−33
total no. of rflns
49048
41199
no. of unique rflns
(Rint)
5319 (0.0455)
9410 (0.0264)
final R indices (I >
2σ(I))
R = 0.0247, Rw
=
=
R = 0.0280, Rw = 0.0684
R = 0.0328, Rw = 0.0706
1.521 and −0.901
1.047
0.0522
R indices (all data)
R = 0.0354, Rw
0.0574
largest diff peak and
1.518 and
−0.732
1.078
hole (e Å−3
)
GOF
at CHEXRAY, University of California, Berkeley. Magnetic suscept-
ibility measurements were made for all samples at 5 and 40 kOe in a 7
T Quantum Design Magnetic Properties Measurement System, which
utilizes a superconducting quantum interference device (SQUID).
Sample containment and other experimental details have been
described previously.22 The samples were prepared for X-ray
absorption experiments as described previously, and the same methods
were used to protect the air-sensitive compounds from oxygen and
water.1 X-ray absorption measurements were made at the Stanford
Synchrotron Radiation Lightsource on beamline 11-2. The samples
were prepared and loaded into a liquid helium-flow cryostat at the
beamline, as described previously.1 Data were collected at temper-
atures ranging from 30 to 300 K, using a Si(220) double-crystal
monochromator. Fit methods were the same as those described
previously.1
Calculations. The ytterbium center was treated with a small-core
relativistic pseudopotential (RECP) ([Ar] + 3d)34 in combination with
its adapted basis set (a segmented basis set that includes up to g
functions). The carbon, nitrogen, oxygen, and hydrogen atoms were
treated with an all-electron double-ζ, 6-31G(d,p),35 basis set. All the
calculations were carried out with the Gaussian 03 suite of programs36
and ORCA suite of program37 either at the density functional theory
(DFT) level using the B3PW9138 hybrid functional or at the CASSCF
level; only one active space and inactive orbitals were used in the
calculation. The geometry optimizations were performed without any
symmetry constraints at either the DFT or the CASSCF level. The
electrons were distributed over four 4f orbitals and the two π* orbitals
in the bipyridines. The CASSCF calculations were done using the SCF
orbitals. The subsequent analysis of the mixture of configuration was
achieved by projecting the wave function on the initial orbitals, namely
EXPERIMENTAL SECTION
■
General Considerations. All reactions were performed using
standard Schlenk-line techniques or in a drybox (MBraun). All
glassware was dried at 150 °C for at least 12 h prior to use. Toluene
and pentane were dried over sodium and distilled, while CH2Cl2 was
purified by passage through a column of activated alumina. Toluene-d8
and C6D6 were dried over sodium. All the solvents were degassed prior
to use. Infrared samples were prepared as Nujol mulls and taken
between KBr plates and recorded on a Thermo Scientific Nicolet IS10
spectrometer. Samples for ultraviolet, visible, and near-infrared
spectrometry were prepared in a Schlenk-adapted quartz cuvette,
and spectra were obtained using a Varian Cary 50 scanning
spectrophotometer. Melting points were determined in sealed
capillaries prepared under nitrogen and are uncorrected. Elemental
analyses and mass spectra (EI) were determined by the Micro-
analytical Laboratory of the College of Chemistry, University of
California, Berkeley. X-ray structural determinations were performed
F
dx.doi.org/10.1021/om400528d | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
the SCF orbitals. This ensures that no mixing between f and π* is
present in the starting set of orbitals.
[Cp*2YbCl1.6I0.4]·CH2Cl2 were coated in Paratone-N oil and mounted
on a Kaptan loop. The loop was transferred to Bruker SMART 100043
and SMART APEX diffractometers equipped with a CCD area
detector for Cp*2Yb(3,3′-Me2bipy) and [(Cp*2Yb(3,3′-Me2bipy)]-
Ligand. The original synthesis of 3,3′-dimethyl-2,2′-bipyridine
(3,3′-Me2bipy) described in the literature used the Ullmann
methodology to couple two 2-bromo-3-methylpyridine molecules.39
The bipyridine was reported as a viscous liquid with bp 293−298 °C
(1 atm). A slight modification was later reported,40 and the ligand was
described as a colorless liquid whose UV spectrum agreed with the
[Cp*2YbCl1.6I0.4
]·CH2Cl2, respectively,44 Preliminary orientation
matrices and cell constants were determined by collection of 10 s
frames, followed by spot integration and least-squares refinement. Data
were intergrated by the program SAINT45 to a maximum 2θ value of
50.68° for Cp*2Yb(3,3′-Me2bipy) and 50.96° for [(Cp*2Yb(3,3′-
Me2bipy)][Cp*2YbCl1.6I0.4]·CH2Cl2. The data were corrected for
Lorentz and polarization effects. Data were analyzed for agreement
and possible absorption using XPREP. A semiempirical multiscan
absorption correction was applied using SADABS.46 This models the
absorption surface using a spherical harmonic series based on
differences between equivalent reflections. The structures were solved
by direct methods using SHELX47 and the WinGX program.48 Non-
hydrogen atoms were refined anisotropically, and hydrogen atoms
were placed in calculated positions and not refined. Selected crystal
data are given in Table 5.
literature values.41 A H NMR spectrum in Me2CO-d6 was reported
1
somewhat later.42 The ligand used in the work reported here was
purchased from SYNTHON Chemical (ChemiePark Bitterfeld Wolfen
Area A, Werkstattstrasse 10, 06 766 Wolfen, Germany) as a white
crystalline solid that sublimed at 90−100 °C onto a water-cooled cold
finger under dynamic vacuum (∼10−2 mm). 1H NMR (C6D6, 293 K, δ
(ppm)): 8.45 (d, J5,6 = 4 Hz, 2H, 6-H), 7.08 (d, J4,5 = 8 Hz, 2H, 4-H),
6.71 (dd, J4,5 = 8.5 Hz, J5,6 = 5 Hz, 2H, 5-H), 2.12 (s, 6H, Me). The
ultimate proof that pure 3,3′-Me2bipy is a solid was obtained from the
X-ray crystal structures of the adducts described below.
Cp*2Yb(3,3′-Me2bipy). The complex Cp*2Yb(OEt2) (0.325 g,
0.629 mmol) was combined with 3,3′-dimethyl-2,2′-bipyridine (3,3′-
Me2bipy, 0.116 g, 0.629 mmol), and toluene (50 mL) was added at
room temperature. The brown suspension was stirred at room
temperature for 2 h, concentrated to ca. 25 mL, and cooled to −20 °C.
A dark green microcrystalline powder formed overnight (323 mg,
82%), which was recrystallized at −20 °C from warm toluene (40 °C).
X-ray -suitable dark green crystals formed (160 mg, 41%). The filtrate
was concentrated to 15 mL and cooled to −20 °C. Second and third
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, figures, and CIF files giving information
concerning magnetic susceptibility, vis−near-IR spectroscopy,
1H variable-temperature NMR, X-ray crystal data for Cp*2Yb-
(3,3′-Me2bipy) (CCDC 943122) and [Cp*2Yb(3,3′-
Me2bipy)][Cp*2YbCl1.6I0.4]·CH2Cl2 (CCDC 943123), and
calculated Cartesian coordinates for Cp2Yb(bipy), Cp*2Yb-
(3,3′-Me2bipy), and Cp*2Yb(4,4′-(OMe)2bipy). This material
is available free of charge via the Internet at http://pubs.acs.org.
1
crops of crystals were obtained (77 mg, combined yield 70%). H
NMR (toluene-d8, 300 K, δ (ppm)): 32.71 (2H), 12.12 (2H, d, J = 6
Hz), 8.49 (2H), 2.23 (30H, Me5C5), 0.58 (6H, Me). Mp: 310−313
°C. Anal. Calcd for C32H42N2Yb: C, 61.23; H, 6.74; N, 4.46. Found: C,
61,45; H, 6.47; N, 4.32. IR (cm−1): 1567 (m), 1436 (s), 1411 (m),
1380 (w), 1090 (w), 1064 (w), 1040 (w), 790 (m), 741 (m), 698 (w).
[Cp*2Yb(3,3′-Me2bipy)]+[Cp*2YbI2]−. The complex Cp*2Yb-
(OEt2) (0.204 g, 0.394 mmol) was combined in the drybox with
3,3′-Me2bipy (0.073g, 0.394 mmol) and AgI (0.093g, 0.394 mmol).
Toluene (20 mL) was added at room temperature, and the brown
suspension was stirred at room temperature for 16 h (overnight). The
resulting brown suspension was filtered, and the solvent was removed
under reduced pressure. The solid was triturated in pentane, the
pentane was removed under reduced pressure, and the residue was
crystallized from warm toluene. The crystals were recrystallized twice
from warm toluene (185 mg, 36%). Toluene in the ratio 1:0.33 was
AUTHOR INFORMATION
Corresponding Authors
*E-mail for G.N.: greg.nocton@polytechnique.edu.
*E-mail for R.A.A.: raandersen@lbl.gov.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Work at the University of California, Berkeley, and at Lawrence
Berkeley National Laboratory 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-AC02-05CH11231. X-ray absorption
data were collected at the Stanford Synchrotron Radiation
Lightsource, a Directorate of SLAC National Accelerator
Laboratory and an Office of Science User Facility operated
for the U.S. Department of Energy Office of Science by
Stanford University. We thank Wayne Lukens for several
discussions on the Hubbard molecular model and Antonio
DiPasquale at CHEXRAY Berkeley for his help with crystal
structures.
1
1
found in the H NMR spectrum and by elemental analysis. H NMR
(CD2Cl2, 300 K, δ (ppm)): 331.3 (2H), 58.51 (2H), 9.84 (2H), 7.28−
7.17 (m, 1.7 H, toluene), 4.17 (30H, Me5C5), 3.78 (30H, C5Me5), 2.10
1
(s, 1H, toluene), −17.81 (6H, Me). H NMR (toluene-d8, 300 K, δ
(ppm)): 58.46 (2H), 10.38 (2H), 6.98 (30H, Me5C5), 4.31 (30H,
C5Me5), −17.59 (6H, Me). The downfield resonance due to 2H was
not found in toluene. Mp: >330 °C. Anal. Calcd for
C54.33H74.67N2I2Yb2 (2·0.33Tol): C, 48.13; H, 5.55; N, 2.07. Found:
C, 48.17; H, 5.36; N, 2.14. IR (cm−1): 1604 (m), 1580 (m), 1437 (s),
1378 (m), 1188 (w), 1130 (w), 1113 (w), 1027 (w), 794 (w), 720
(m), 695 (w).
[Cp*2Yb(3,3′-Me2bipy)][Cp*2YbCl1.6I0.4]·CH2Cl2. When the
product of the reaction was recrystallized by layering pentane on
top of CH2Cl2, instead of toluene, crystals suitable for an X-ray
determination were obtained in better yield (75%); however, some of
the iodide atoms were replaced by chlorine atoms. In the X-ray
experiment, the compound was found to cocrystallize with CH2Cl2,
giving the empirical formula [(Cp*2Yb(3,3′-Me2bipy)]-
[Cp*2YbCl1.6I0.4]·CH2Cl2. The combustion analysis fits a slightly
different formula: [(Cp*2Yb(3,3′-Me2bipy)][Cp*2YbCl1.5I0.5]·CH2Cl2.
Anal. Calcd for C53H74N2I0.5Cl3.5Yb2: C, 50.01; H, 5.86; N, 2.20.
Found: C, 49.72; H, 5.52; N, 2.26. IR (cm−1): 2850 (s), 2713 (w),
1582 (m), 1573 (m), 1439 (s), 1406 (m), 1378 (s), 1280 (m), 1237
(w), 1191(w), 1182 (w), 1167 (w), 1019 (s), 992 (w), 851 (m), 796
(s), 731 (s), 708 (m), 700 (m), 616 (w), 593 (w).
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dx.doi.org/10.1021/om400528d | Organometallics XXXX, XXX, XXX−XXX