Carbon-hydrogen bond breaking and making in the open-shell singlet molecule Cp2Yb(4,7-Me2phen)

Article abstract of DOI:10.1021/om500843z

The adducts formed between the 4,7-Me₂-, 3,4,7,8-Me₄-, and 3,4,5,6,7,8-Me₆-phenanthroline ligands and Cp₂Yb are shown to have open-shell singlet ground states by magnetic susceptibility and L_III-edge XANES spectroscopy. Variable-temperature XANES data show that two singlet states are occupied in each adduct that are fit to a Boltzmann distribution for which δH = 5.75 kJ mol^-1 for the 4,7-Me₂phen adduct. The results of a CASSCF calculation for the 4,7-Me₂phen adduct indicates that three open-shell singlet states, SS1-SS3, lie 0.44, 0.06. and 0.02 eV, respectively, below the triplet state. These results are in dramatic contrast to those acquired for the phenanthroline and 5,6-Me₂phen adducts, which are ground state triplets (J. Am. Chem. Soc. 2014, 136, 8626). A model that accounts for these differences is traced to the relative energies of the LUMO and LUMO+1 orbitals that depend on the position the methyl group occupies in the phenanthroline ligand. The model also accounts for the difference in reactivities of Cp₂Yb(3,8-Me₂phen) and Cp₂Yb(4,7-Me₂phen); the former forms a σ C-C bond between C(4)C(4′), and the latter undergoes C-H bond cleavage at the methyl group on C(4) and leads to two products that cocrystallize: Cp₂Yb(4-(CH₂),7-Mephen), which has lost a hydrogen atom, and Cp₂Yb(4,7-Me₂-4H-phen), which has gained a hydrogen atom.

Full text of DOI:10.1021/om500843z

Article
pubs.acs.org/Organometallics
Carbon−Hydrogen Bond Breaking and Making in the Open-Shell
Singlet Molecule Cp* Yb(4,7-Me phen)
Gregory Nocton,* Corwin H. Booth, Laurent Maron, Louis Ricard, and Richard A. Andersen*
́
2
2
,†,‡
§
∥
†
,‡,§
†
‡
§
́
Laboratoire de Chimie Moleculaire, CNRS, Ecole Polytechnique, Route de Saclay, 91128 Palaiseau, France
Department of Chemistry, University of California, Berkeley, California 94720, United States
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
∥LPCNO, UMR 5215, Universite
́
de Toulouse-CNRS, INSA, UPS, 31000 Toulouse, France
*
S Supporting Information
ABSTRACT: The adducts formed between the 4,7-Me2-,
,4,7,8-Me -, and 3,4,5,6,7,8-Me -phenanthroline ligands and
3
4
6
Cp* Yb are shown to have open-shell singlet ground states by
magnetic susceptibility and L -edge XANES spectroscopy.
2
III
Variable-temperature XANES data show that two singlet states
are occupied in each adduct that are fit to a Boltzmann
−
1
distribution for which ΔH = 5.75 kJ mol for the 4,7-Me phen adduct. The results of a CASSCF calculation for the 4,7-Me phen
2
2
adduct indicates that three open-shell singlet states, SS1−SS3, lie 0.44, 0.06. and 0.02 eV, respectively, below the triplet state.
These results are in dramatic contrast to those acquired for the phenanthroline and 5,6-Me phen adducts, which are ground state
2
triplets (J. Am. Chem. Soc. 2014, 136, 8626). A model that accounts for these differences is traced to the relative energies of the
LUMO and LUMO+1 orbitals that depend on the position the methyl group occupies in the phenanthroline ligand. The model
also accounts for the difference in reactivities of Cp* Yb(3,8-Me phen) and Cp* Yb(4,7-Me phen); the former forms a σ C−C
2
2
2
2
bond between C(4)C(4′), and the latter undergoes C−H bond cleavage at the methyl group on C(4) and leads to two products
that cocrystallize: Cp* Yb(4-(CH ),7-Mephen), which has lost a hydrogen atom, and Cp* Yb(4,7-Me -4H-phen), which has
2
2
2
2
gained a hydrogen atom.
INTRODUCTION
In this article, it is shown that the ground-state electronic
■
structure of the 4,7-Me phen adduct of Cp* Yb is an open-shell
The ground-state electronic structure of the neutral 2,2′-
bipyridine and some 1,10-phenanthroline adducts of Cp* Yb,
2
2
singlet like those for bipyridine, which is in contrast to the
2
Cp* Yb(diimine), has been studied with the goal of under-
standing the fundamental nature of how the spin on the f
triplet ground state when x,x′-phen is H,H, 3,8-Me , or 5,6-Me .
2
2
2
13
The different behavior of substituted phenantholines has its
origin in the ground state of their solvent-separated radical
anions, as determined by analysis of their EPR spectra. Thus,
fragment couples with the electron in the diimine LUMO of
these paramagnetic compounds. The bipyridine adducts are
open-shell singlet molecules that result when an open-shell
singlet configuration couples with the closed-shell singlet
configuration, driving the energy of the resultant singlet state
below the triplet state, in violation of Hund’s maximum
multiplicity rules. These states are multiconfigurational, the
spins are antiferromagnetically coupled, and the valence of
•
−
•−
•−
7
the radical anions bipy , 4,4′-Me bipy , 5,5′-Me bipy ,
2
2
•
−
7
•−
8
•−
7
phen
,
2,9-Me
phen
,
4,7-Me
phen
,
and 5,6-
2
2
2
•
−
8
•−
Me phen
have B ground states but 3,4,7,8-Me phen
has a 2A2 ground state. The influence of the electronic
2
1
4
9
structure of the diimine radical anion on the physical and
chemical properties is illustrated by the following example:
14
ytterbium is intermediate: i.e., it lies between Yb(II) (f ) and
13
1−5
Yb(III) (f ). In contrast, when the diimine is a phenanthro-
Cp Ti(bipy) is a spin equilibrium adduct in which the ground-
line ligand, x,x′-Me phen, where x,x′ is H,H, 3,8-Me , or 5,6-
2
2
2
Me , the ground state is a spin triplet in which the spins on the
state singlet is in equilibrium with a triplet excited state with
2
−1
10
individual fragments are ferromagnetically coupled and the
−2J ≈ 600 cm . Although the spin state of Cp Ti(phen) is
2
1
3
6
valence is fully trivalent, Yb(III) (f ). In these phenanthroline
unknown, the 3,4,7,8-Me phen derivative disproportionates
4
adducts, when x,x′ is H,H or 3,8-Me , the monomeric units
11
2
according to the reaction illustrated in Scheme 1. These
couple by forming a σ C−C bond between the C(4)C(4′)
III
13
titanocene results set the stage for the decamethylytterbocene
chemistry that follows.
atoms and the Cp* Yb (f ) fragments are isolated para-
2
+
magnetic Cp* Yb groups bridged by the resulting diamagnetic
2
dianion. The C−C bonds in these dimers are weak, and in
solution a dimer−monomer equilibrium exists in which ΔG ≈
Received: August 19, 2014
Published: November 17, 2014
0
when x,x′ is 3,8-Me .
2
©
2014 American Chemical Society
6819
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Article
11
Scheme 1.
Scheme 2
RESULTS
■
Synthesis and Physical Properties. The substituted
phenanthroline adducts were prepared as illustrated in eq 1.
Some physical properties of the adducts prepared in this article
are given in Table 1.
Cp* Yb(OEt ) + x,x′‐phen
2
2
→
Cp* Yb(x,x′‐phen) + Et O
(1)
2
2
All of the adducts are soluble in either toluene or diethyl
ether, from which they are purified, in most cases, by
crystallization at low temperature. The good solubility of the
neutral adducts is in contrast to the solubility of Cp* Yb(x-
2
5
phen), where x is H, 3-Me, 4-Me, and 5-Me, which are only
sparingly soluble in these solvents. All of the adducts are high-
Figure 1. Temperature-dependent magnetic data for 1−6 and
6
6
Cp* Yb(5,6-Me phen).
2
2
melting-point solids that are thermally stable in solution, except
for the 4,7-Me , 3,4,7,8-Me , and 3,4,5,6,7,8-Me phenanthro-
line adducts, which are best prepared and isolated at 0 °C or
lower; the thermal rearrangement of these adducts is described
below (see Scheme 2).
2
4
6
μeff decreasing rapidly below ca. 25 K, indicating a change in
population of the crystal field states, since the crystal field
splitting is small, and/or an intermediate valent nature of the
ground state, as observed in the neutral methyl-substituted
The effective magnetic moments of these adducts in the solid
state at 300 K are substantially lower than those in the x-phen
1
3
14
bipyridine adducts of Cp* Yb, for which the relative f to f
2
6
3
adducts mentioned above and in Cp* Yb(5,6-Me phen) and
configuration fractions depend on temperature.
2
2
are lower than that expected for two uncorrelated spins of
Yb(III) (f ) and a radical anion of F and S1/2, respectively,
The L -edge XANES spectra are able to distinguish between
III
13
2
2
these two possibilities. In contrast to the case for Cp* Yb(5,6-
7
/2
2
1
3
for which a μ value of 4.85 μ is expected. However, the value
Me phen), which shows only one temperature-independent f
eff
B
2
for the cation [Cp* Yb(4,7-Me phen)]I (6) is as expected for
feature at 8946 eV (Figure 2a), Cp* Yb(4,7-Me phen) (2)
2 2
2
2
2
an isolated F7 Yb(III) paramagnetic compound at 4.54 μB.
presents two different features, in agreement with the presence
of both f14 and f contributions, for which the intensity ratio
changes with the temperature (Figure 2b). This situation wa3s
observed in previous work with some substituted bipy adducts
/2
13
The lower effective magnetic moments at 300 K for the adducts
given in Table 1 clearly show that the spins are correlated and
that antiferromagnetic coupling is greater when two methyl
groups occupy the 4- and 7-positions in the phenanthroline
ring, in contrast to the case when they occupy the 3,8- or 5,6-
positions.
when the ligand can accept the electron from Yb in Cp* Yb
2
into several accessible π* orbitals (π* , π* , ...), which results in
1
2
several possible open-shell singlets in which both the relative π*
13 14
The temperature dependence of the effective magnetic
moments in the solid state is shown in Figure 1; the plots of χ,
contribution and the f :f ratio are different, accounting for
the temperature dependence in the L -XANES spectra. For
III
−1
13
χ , and χT vs T are available in the Supporting Information.
[Cp* Yb(4,7-Me phen)] (2) the f configuration fraction (also
2
2
13
These plots clearly illustrate the reduced moments of the
known as the relative f occupancy or the f-hole occupancy, n )
f
neutral adducts in comparison to those of Cp* Yb(5,6-
varies from 0.81 at low temperature to 0.58 at 296 K. This
situation also accounts for the low magnetic moment of
Cp* Yb(4,7-Me phen)] (2). When the number of methyl
2
Me phen) and [Cp* Yb(4,7-Me phen)]I (6). In all cases, the
2
2
2
general shapes of the curves are similar for the neutral adducts,
2
2
a
Table 1. Solid-State Properties of the Compounds 1−6
IR (cm−1)
μeff(300 K)
b
compound
color
mp (°C)
Cp* Yb(2,9-Me phen) (1)
deep purple
deep purple
dark brown
dark brown
dark purple
gold yellow
deep purple
313−317
265−268
264−266
270−272
288−292
214−218
285−287
1622, 1592, 1505, 848
1622, 1577, 1520, 849
1611, 1567, 1518, 810
1.62
1.98
1.79
1.67
2.05
4.44
3.68
2
2
Cp* Yb(4,7-Me phen) (2)
2
2
Cp* Yb(3,4,7,8-Me phen) (3)
2
4
Cp* Yb(3,4,5,6,7,8-Me phen) (4)
2
6
Cp* Yb(2,9-Me -4,7-Ph phen) (5)
1622, 1569,1547, 880
1631, 1534, 1430, 728
1605, 1584, 1480, 804
2
2
2
[
Cp* Yb(4,7-Me phen)]I (6)
2 2
6
Cp* Yb(5,6-Me phen)
2
2
a
b
The atom numbering scheme for the phenanthroline is given in Scheme 2. In μ per Yb(III), calculated from a plot of χT vs T, where μ
=
B
eff
.828(χT)1 at T = 300 K.
/2
2
6
820
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Article
Figure 2. Yb L -edge XANES data for the complexes (a) Cp* Yb(2,9-Me phen) (1) Cp* Yb(2,9-Me -4,7-Ph phen) (5), and Cp* Yb(5,6-
III
2
2
2
2
2
2
6
Me phen) at 300 K (dashed) and 30 K (solid), (b) Cp* Yb(4,7-Me phen) (2) at various temperatures, (c) Cp* Yb(3,4,7,8-Me phen) (3) at
2
2
2
2
4
various temperatures, and (d) Cp* Yb(3,4,5,6,7,8-Me phen) (4) at various temperatures.
2
6
Figure 3. (a) Plot of n vs T (K) for 2−4. The best fits are obtained from a Boltzmann distribution between two singlet states using the parameters
f
reported in Table 2. (b) Plot of χT vs temperature for complexes 2−4. The black lines for each compound show the fit to the data using the
parameters reported in Table 2.
Table 2. Thermodynamic Constants Derived for the SS1 ⇌ SS2 Equilibrium from n vs T and χ vs T plots (Figure 3)
f
a
a
b
b
c
c
compound
nf(gs)
nf(exptl)
χ(gs)
χ(exptl)
ΔH(nf)
ΔH(χ)
ΔS(nf)
ΔS(χ)
% imp
Cp* Yb(4,7-Me phen) (2)
0.81
0.61
0.37
0.58
0.50
0.27
0.00182
0.00125
0.000108
0.00060
0.00060
0.00062
5.75
3.2
5.75
3.6
31
21
25
25
21
21
7.0
6.8
4.7
2
2
Cp* Yb(3,4,7,8-Me phen) (3)
2
4
Cp* Yb(3,4,5,6,7,8-Me phen) (4)
3.0
3.9
2
6
a
−1
b
−1
c
−1
In units of emu mol . In units of kJ mol . In units of J (mol K) .
13 14
groups is increased in the phenanthroline ligand, the f :f ratio
decreases and the spectra continue to show a temperature
dependence.
Me phen) (4), and the fits are presented in Figure 3, while
6
the parameters are reported in Table 2. Only two singlet states,
SS1 and SS2, are used in all three complexes. In the same
manner, the magnetic curves are fit (Figure 3) with a
Boltzmann distribution of two open-shell singlet states below
the triplet and the enthalpy and entropy changes obtained from
both fits (XANES and magnetism) agree well. The enthalpy
In the Cp* Yb adducts with substituted bipyridine ligands,
2
the temperature dependence is due to the presence of several
open-shell singlet states (SS) that develop below the triplet (T)
and it is therefore possible to determine their relative
3
−1
populations using a Boltzmann equation. A similar method-
change for Cp* Yb(4,7-Me phen) (2) is 5.75 kJ mol and lies
2
2
ology is used in this work for Cp* Yb(4,7-Me phen) (2),
in the reported range for the enthalpy changes in substituted
2
2
Cp* Yb(3,4,7,8-Me phen) (3), and Cp* Yb(3,4,5,6,7,8-
bipyridine adducts of Cp* Yb. As an additional note, the
2
4
2
2
6
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percentage of 2F7/2 impurities used for the fit of the magnetic
data is relatively high, in comparison to previous work, but is
rationalized by the thermal instability of 2−4 (see Reactivity
below). The L -edge XANES data give n values of 0.41 and
four molecules in the two crystal structures of Cp* Yb(5,6-
2
Me phen), respectively. The Yb−N distances follow a similar
2
pattern; the distance in Cp* Yb(4,7-Me phen) is 0.035 and
2
2
0.030 Å longer than the equivalent distances in Cp* Yb(phen)
III
f
2
0
.55 at 30 K for Cp* Yb(2,9-Me phen) (1) and Cp* Yb(2,9-
monomer and Cp* Yb(5,6-Me phen), respectively. The Yb−
2
2
2
2
2
Me -4,7-Ph phen) (5), respectively, and show no temperature
C(Cp*) and Yb−N distances in Cp* Yb(2,9-Me -4,7-Ph phen)
2
2
2
2
2
dependence for Cp* Yb(2,9-Me phen) (1), while n decreases
(5) are much longer, 0.098 and 0.16 Å, respectively, than the
equivalent distances in Cp* Yb(4,7-Me phen) (2) and are
2
2
f
to 0.37 at 300 K for Cp* Yb(2,9-Me -4,7-Ph phen) (5). The
2
2
2
2
2
reason for the low n values and variable-temperature behavior
close to the equivalent distances in Cp* Yb(py) of 2.74 and
2 2
12
f
of these two complexes is not addressed in a quantitative
manner in this work, but the general behavior is similar to that
of Cp* Yb(6,6′-Me bipy) and Cp* Yb(6-Mebipy), respec-
2.565 ± 0.005 Å, respectively.
The longer Yb−C(Cp*) and Yb−N distances at 173 K in 2
2
2
2
and 5, relative to these distances in Cp* Yb(phen) and
2
3
6
tively; the presence of a methyl group(s) in the position α
to nitrogen destabilizes the f configuration, presumably
Cp* Yb(5,6-Me phen), indicate that the population of the
Yb(II) (f ) configuration is greater in the former adducts,
consistent with the lower values of nf.
2
2
1
3
14
because of the steric hindrance. The low values of n for 1
f
and 5 also account for the low values of effective moments (see
Although the Yb−C(Cp*) and Yb−N distances in Cp* Yb-
2
1
4
Table 1; n = 0 indicates diamagnetic f ) and for the chemical
(4,7-Me phen) (2) are longer than those in Cp* Yb(phen), the
f
2
2
1
shift in their solution H NMR spectra.
C−C and C−N distances in the phenanthroline ligands are
X-ray Crystallography. The ORTEP drawings of Cp* Yb-
consistent with occupation of the b and a symmetry orbitals.
2
1
2
6
(
4,7-Me phen) (2) and Cp* Yb(2,9-Me -4,7-Ph phen) (5) are
Using the method outlined in an earlier article, the distances
labeled A, I, F and L, B and P, H and J (Scheme 2) change in
opposite directions depending on whether the b or a orbitals
2
2
2
2
shown in Figures 4 and 5, respectively. Some bond lengths and
1
2
are populated (see Table 4 and Scheme 1 and Table 4 of ref 6).
The bond length alternation for Cp* Yb(4,7-Me phen) (2) and
2
2
Cp* Yb(phen) monomer indicate that both of these symmetry
2
orbitals are populated but that the relative values of Δ are
different, implying that the relative energies of these orbitals
depend on the position of the methyl groups in the ring.
Although these bond length changes are only qualitative, they
point toward population of the a2 orbital in which the
coefficients for the carbon pπ orbitals in the 4,7-positions in
the ring are greater than those in b ; this difference has
1
ramifications for the reactivity at these positions (see below).
Solution Properties. The UV−vis spectra at 20 °C in
toluene for the 4,7-Me phen and related adducts are shown in
2
Figure 6. The absorption around 550 nm is due to the π−π*
15
transition in the radical anion.
Figure 4. ORTEP drawing for the complex Cp* Yb(4,7-Me phen)
2
2
These adducts also have an intense absorption around 900
(2). Thermal ellipsoids are at the 50% probability level.
nm, which was assigned to an f−-f transition in Cp* Yb-
2
15
(
phen). Although this transition is forbidden, its intensity is
attributed to the formation of a ligand-to-metal charge transfer
1
15
state, LMCT, in which the intensity is stolen.
1
The solution H NMR spectra in C D at 20 °C for the
6
6
dimethylphenanthroline adducts are given and assigned in
Table 5. The assignments are made by the changes that result
when a pair of hydrogens is replaced by a pair of methyl groups
at a given position in the phenanthroline ring. Three patterns
emerge from the changes in the chemical shift values. (i) The
values of δ2,9 are strongly shifted downfield, consistent with the
dipolar contribution dominating the chemical shift tensor. This
is expected, since the dipolar contribution is related to the
geometry of the hydrogen atoms and H2,9 are proximal to the
paramagnetic center. The chemical shifts of δ2,9 also align the
magnetic z axis collinear with the N−N vector and
perpendicular to C −Yb−C . (ii) The values of δ and δ
Figure 5. ORTEP drawing for the complex Cp* Yb(2,9-Me -4,7-
Ph phen) (5). Thermal ellipsoids are at the 50% probability level.
2
2
2
t
t
4,7
3,8
are less strongly shifted downfield, and when H4 or H3,8 are
,7
angles are given in Table 3, and crystal data are given in the
Experimental Section and the Supporting Information. It is
clear that these two adducts are monomers in the solid state, in
replaced by methyl groups, the sign of the chemical shift
changes, showing that the contact contribution to the chemical
shift tensor dominates the dipolar contribution. (iii) The δ5,6
values are the farthest upfield and the sign does not change
when H5,6 are replaced by methyl groups.
16
contrast to the case for the phenanthroline and 3,8-Me phen
2
6
adducts. The average Yb−C(Cp*) distance in Cp* Yb(4,7-
2
Me phen) of 2.642 ± 0.007 Å is 0.032 and 0.020 Å longer than
In the neutral adducts, the order of chemical shift is δ2,9 > δ4,7
2
the equivalent distances in Cp* Yb(phen) monomer and in the
> δ3,8 > δ , but in the cation, δ and δ3,8 exchange places in
2
5,6
4,7
6
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Table 3. Selected Bond Distances (Å) and angles (deg) at 173 K for Complexes 2 and 5
Cp* Yb(4,7-Me phen) (2)
Cp* Yb(2,9-Me -4,7-Ph phen) (5)
2 2 2
2
2
Yb−C (distance range)
2.630(4)−2.653(4)
2.699(10)−2.781(7)
Yb−C(ring) (mean)
2.642 ± 0.007
2.35
2.74 ± 0.02
2.46
a
Yb−C (mean)
t
a
C −Yb−Ct
139.5
140.0
t
Yb−N(mean)
2.35 ± 0.01
1.486(6)
1.507(5)
1
2.51 ± 0.002
1.496(9)
1.506(9)
3
b
C(23)−Cexo
C(28)−Cexo
c
torsion angle N−C−C−N
torsion angle C−C−C−C
1
1
a
b
c
C is the centroid of the C Me ring. C in 2 is C(33) and C(35) in 5. C is C(34) in 2 and C(41) in 5.
t
5
5
exo
exo
,
a b
Table 4. Bond Length (Å) Changes Δ in 2, 5, and
It is informative to compare the chemical shifts of the neutral
Cp* Yb(phen)
bipyridine and the neutral phenanthroline adducts of Cp* Yb
2
2
using the graphic in Scheme 3.
Cp* Yb(4,7-
2
Cp* Yb(2,9-Me -4,7-
2 2
2
c
6
bond
Me phen) (2)
Cp* Yb(phen)
Ph phen) (5)
2
2
Scheme 3
A
−0.018
+0.035
+0.006
+0.019
+0.015
−0.020
+0.014
+0.041
+0.006
−0.026
−0.018
−0.011
−0.006
−0.008
−0.001
I
F, L
B, P
H, J
a
Δ is the bond length in the adduct minus that in the free ligand in Å.
b
For free 4,7-Me phen, see ref 13, and for free 2,9-Me -4,7-Ph phen,
2
2
2
c
see ref 14. The letter refers to the C−C or C−N bond label in
Scheme 2.
The labels α, β, and γ are the hydrogens on the α, β, and γ
carbon atoms relative to N, b denotes bipyridine, and p denotes
phenanthroline. The values of the chemical shift at 20 °C in
4
,5
6
C D for Cp* Yb(bipy) and Cp* Yb(phen) are, to the
6
6
2
2
nearest whole number, as follows: α 160, β 6, and γ 26 in the
b
b
b
bipyridine complexes; α 140, β 14, γ 48 in the phenanthro-
p
p
p
line complexes. In the bipyridine and phenanthroline adducts
δ ≫ δ < δ , with δ values in each adduct approximately equal
α
β
γ
α
but δ and δ in the phenantholine complexes being about twice
β
γ
the values of δ and δ in the bipyridine complexes. Since the
β
γ
chemical shifts of the hydrogens on the β and γ sites are largely
determined by the contact contribution, this implies that the
unpaired spin density at the γ site is greater than that at the β
site and is greater in the phenantholine complexes than in the
bipyridine complexes; as mentioned above, this difference plays
a role in the reactivity at these positions (see below).
Calculations. As in earlier articles on the bipyridine
adducts, the CASSCF methodology has been used to determine
the ground-state electronic structure of the phenanthroline
Figure 6. UV−vis spectra of Cp* Yb(4,7-Me phen) (2; green),
2
2
Cp* Yb(3,4,7,8-Me phen) (3; red), and Cp* Yb(3,4,5,6,7,8-
2
4
2
1
−4,6
Me phen) (4; blue).
6
adducts of Cp* Yb.
The phenanthroline adduct of Cp* Yb
2
2
is calculated to consist of a pair of nearly degenerate triplets, T1
and T2, which are 2.1 eV below the open-shell singlet exited
6
state. When two methyl groups are attached to the 3,8- and
the order, consistent with the dipolar contribution dominating
the chemical shift tensor in the cation. The value of δCp* in the
neutral and cation adducts is around 4 ppm in all cases.
5
,6-positions in the phenanthroline ligand, the ground state of
the adducts is still a triplet but the open-shell singlet excited
state is now only 0.08 and 0.09 eV above the triplet. In these
1
a
Table 5. H NMR Chemical Shifts (δ) of the Phenanthroline Adducts at 300 K
compound
2,9
4,7
47.9
3,8
14.0
5,6
0.47
Cp*
ref
b
Cp* Yb(phen)-monomer
139.9
109.1
95.5
4.11
3.63
3.36
3.95
3.73
3.82
6
2
b
Cp* Yb(4,7-Me phen)
−21.4 (Me)
51.1
16.7
−10.0 (Me)
14.7
3.15
this work
2
2
b
b
Cp* Yb(3,8-Me phen)
3.83
6
2
2
Cp* Yb(5,6-Me phen)
137.4
301
44.1
0.03 (Me)
6
2
2
c
[
[
Cp* Yb(4,7-Me phen)]I
8.4 (Me)
9.5
53.4
−3.5
−2.5
this work
6
2
2
c
Cp* Yb(phen)]I
281
52.4
2
a
b
c
δ values in ppm relative to Me Si. In C D or C D . In CD Cl .
4
6
6
7
8
2
2
6
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Organometallics
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13
compounds, the f configuration is the only active one and the
ytterbium atoms are fully trivalent; the computational results
are in agreement with the magnetic and XANES experiments
6
for these adducts. In contrast, the calculated ground state of
the 4,7-Me phen adduct is not a triplet but consists of three
2
open-shell singlets, SSI−SS3, that are lower in energy relative to
the triplet by 0.44, 0.06, and 0.02 eV, respectively. The three
open-shell singlets have different contributions of f13 and f14
from Yb and π* and π* contributions from the radical anionic
1
2
ligands and are therefore multiconfigurational singlets. The
1
3
14
13
SS1−SS3 states are given by 0.92 f + 0.08 f , 0.34 f + 0.66
1
4
13
14
f , and 0.63 f + 0.37 f , respectively. The π* and π*
1
2
contribution to SS1 is 0.89 π* + 0.11 π* , and for SS2 and SS3,
1
2
the contributions are 0.35 π* + 0.65 π* and 0.68 π* + 0.32
1
2
1
Figure 7. ORTEP drawing for the brown crystal obtained from
toluene in the thermal reaction of Cp* Yb(4,7-Me phen) (2) (thermal
ellipsoids are given at the 50% probability level) and referred to as
“compound” 7. The carbon atoms represented in orange are due to a
positional disorder of a methyl group connected to the phenanthroline
ring, and the carbon atom represented in pink is due to the methylene
group.
13
π* , respectively. The most stable state, SS1, is dominated by f
2
2
2
and π* configurations, and in the nearly degenerate SS2 and
1
SS3 states, these configurations are less dominant. For SS1 the
2
III 13
•−
n value is 0.92, the value of c in Ψ = c |Yb (f )(phen )⟩ +
f
1
1
II 14
0
c |Yb (f )(phen) ⟩, such that n = 1 when c = 1 and c = 0 and
2
f
1
2
n = 0 when c = 0 and c = 1. The calculated value of n for SS1
f
1
2
f
of 0.92 is in reasonable agreement with the experimental value
of 0.81 at low temperature, assuming that only SS1 is
Scheme 4
populated. As the temperature is increased, the n value of
f
the excited state decreases to 0.58, in reasonable agreement
with the calculated values of SS2 and SS3, where the average
values in these two states are 0.48.
Reactivity. As mentioned in Synthesis and Physical
Properties, the yield of Cp* Yb(4,7-Me phen) (2) is higher
2
2
and purification is easier when the temperature is kept at 0 °C.
In the synthesis of the other two adducts of Me phen and
4
Me phen the temperature must be maintained at −70 °C
6
during the synthesis stage. It was not clear why the temperature
is important until the thermal stability of isolated Cp* Yb(4,7-
2
1
Me phen) (2) was monitored by H NMR spectroscopy as a
2
function of time. At 60 °C, the resonances of Cp* Yb(4,7-
2
graphic solution are given in the Experimental Section and in
the Supporting Information.
Me phen) (2) in C D disappear while a new set appears, a
2
7
8
1
transformation that is complete over 15 days. The H NMR
spectrum is complex, but two different sets of resonances can
be identified, labeled A and A , since the ligand resonances are
In both cases, a pyridyl ring has lost its aromaticity, which
results in a shorter Yb−N(2) distance of 2.286(3) Å in
comparison to the Yb−N(1) distance of 2.363(3) Å, a
consequence of the localization of negative charge on the
nitrogen N(2) (Figure 7). Washing the brown crystals with
pentane results in a purple solution and a green residue.
Cooling the pentane solution (−20 °C) afforded very small
crystals (too small for conventional X-ray crystallographic use),
1
2
asymmetric, and the Cp* resonances appear as a broad feature
at ∼3.7 ppm. The spectrum is available in the Supporting
Information. Although the resonances cannot be conclusively
assigned, they may be tentatively assigned as due to a hydrogen
atom transfer reaction analogous to that illustrated in Scheme 1,
as shown in Scheme 7 in the Experimental Section. This
attribution is supported by the following experiments.
1
whose H NMR spectrum in C D is identical with those
7
8
labeled as A , with the exception that the Cp* resonances are
1
(
1) The thermal reaction conducted on a synthetic scale
resolved as two singlets at δ 3.94 and 3.58 ppm due to 15
yields brown crystals isolated by crystallization from toluene
protons each.
1
solution (referred to as 7). The H NMR spectrum contains the
(
2) Hydrolysis (H O) of the products of thermal trans-
2
resonances referred to as A and A , above. A brown crystal was
1
2
formation, on an NMR tube scale, results in an asymmetric set
selected, and an X-ray data set was collected (see Figure 7).
Analysis of the X-ray data indicate the presence of two different
molecules that cocrystallize in a 72:28 ratio. The major
molecule consists of a dimethylphenanthroline ligand that has
lost a H atom from a methyl group, leading to the terminal
methylene represented in pink in Figure 7 (C28 and C34)
1
of H resonances due to H A , that is, 4,7-Me phen + 2H,
2
1
2
which evolve into resonances due to 4,7-Me phen over 1 day.
2
No resonances attributable to A were identified. Examination
2
of the solution by GCMS showed Cp*H and 4,7-Me phen.
2
1
When the thermal transformation was monitored by H NMR
spectroscopy in C D in the presence of excess dihydroan-
7
8
(
Scheme 4). The minor molecule consists of a dimethylphe-
thracene or 1,4-cyclohexadiene, neither anthracene nor benzene
was detected in the spectrum and the t1/2 value of the reaction
was not altered by these two free radical traps. Monitoring the
nanthroline to which a H atom is added to the phenanthroline
ring at carbon C28. This leads to a C(Me)(H) group in two
possible positions at C28 (up and down). This position
disorder is represented in orange in Figure 7 (Scheme 4)
transformation under an atmosphere of D also did not alter
2
the rate but HD was indeed observed. These experiments do
(
C28A−-C34A and C28B−C34B). Details of the crystallo-
not identify A and A with confidence but allow us to ascribe
1
2
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them as resulting from a disproportionation reaction analogous
to that shown in Scheme 1.
Discussion. The key point that emerges when the
A model to account for the energy separation between the b1
and a orbitals, on the basis of the CI calculations, is presented
2
as a guide to understand how the methyl groups can control the
bipyridine and phenanthroline adducts of Cp* Yb are
spin state in Cp* Yb(x,x′-phen). Figure 9 shows a cartoon
2
2
compared is that the bipyridine adducts are all open-shell
singlets but the phenanthroline adducts are either open-shell
multiconfigurational singlets in Cp* Yb(4,7-Me phen) (2) or
2
2
triplets in the phenanthroline, 3,8-Me phen, and 5,6-Me phen
2
2
adducts. Thus, two methyl groups in different positions on the
phenantholine ligand change the magnetic properties of the
resulting adduct. A qualitative MO model was presented to
account for the difference between Cp* Yb(bipy) and
2
Cp* Yb(phen), which are ground-state singlets and triplets,
2
6
respectively. The model is straightforward for bipyridine, since
only the b orbital is low enough in energy to accept an electron
1
in the charge transfer ground state, resulting in a singlet state.
In the phenanthroline radical anion, the LUMO and LUMO+1
orbitals of b and a symmetry (in C symmetry) are low
enough in energy to be populated in the charge-transfer ground
state of the adducts, resulting in a triplet state, as shown in the
left-hand side of the diagram in Figure 8. The spin state of
Figure 9. Cartoon energy-level diagram demonstrating the effect of
singlet−triplet configurations closed-shell singlet (CSS), open-shell
singlet (OSS), and triplet (T) on a singly occupied f orbital coupled
with one unpaired electron in a ligand π* orbital. The orbital energies
are only qualitative, but they reflect the CASSCF energies.
1
2
2v
energy-level diagram that is modified from Figure 8 in ref 4.
The diagram illustrates how the open-shell singlet (OSS) and
the closed-shell singlet (CSS) energy levels mix under
configuration interaction. The diagram is idealized, since it
only shows a singly occupied f electron interacting with a single
electron in a π* orbital and how these two orbitals change
when CI is turned on as the triplet state energy is held constant.
The energy separation, Δ , on the left-hand side of Figure 9
SS
represents the large separation in OSS and CSS in Cp* Yb-
2
(
phen). Replacing two hydrogens by methyl groups on the
phenanthroline ligand reduces Δ , since the methyl group is a
σ donor relative to hydrogen. This raises the energy of the
SS
HOMO of the ligand, moving it closer to the energy of the
II 14
empty d-parentage orbital of Cp* Yb (f ) as the bond
2
develops and stabilizing the configuration. In addition, the
methyl substituents raise the LUMO of the ligand, driving up
the π* orbital energy, as shown by the increased value of E1/2,
relative to Fc0 . The values in thf for phen and 3,4,7,8-
/+
Figure 8. Qualitative MO diagram comparing bonding in Cp* Yb(3,8-
9
2
Me phen are −2.55 and −2.68 V, respectively.
4
Me phen) (a) and Cp* Yb(4,7-Me phen) (b). The 4f parentage
2
2
2
The reduction potentials for the other dimethylphenanthro-
orbitals in the center of the MO diagram are associated with the
lines are not available in the literature. The σ-donor substituent
Cp* Yb fragment in C symmetry; the two σ bonding and
2
2v
antibonding orbitals used in the bonding of Cp* Yb to the diimine
destabilizes OSS with the net effect that Δ gets smaller, when
SS
2
ligands are not shown, but the three empty d parentage orbitals are
shown. The orbitals on the extreme left-and right-hand sides are the
LUMO and LUMO+1 of the diimine ligands. The orbitals shown in
CI is turned on, driving the energy separation between the
singlet- and triplet-state energies, Δ , closer to each other, as
ST
in the middle illustration in Figure 9. The qualitative picture
agrees with the calculated separation between the singlet and
triplet states in the phen, 3,8-Me phen, and 5,6-Me phen
(
a) and (b) are the two possible electron configurations in the charge-
transfer state of the adducts.
2
2
species, with the triplet state being lower in energy. As Δ
SS
continues to contract, the multiconfigurational state, composed
of the OSS and CSS configurations, falls below the triplet state,
as found in the calculation for Cp* Yb(4,7-Me phen) (2). A
Cp* Yb(x,x′-phen) is qualitatively determined by the energy
2
that separates the b and a symmetry orbitals. When the
1
2
2
2
separation is on the same order as or smaller than that in
phenanthroline, a triplet state results when x,x′ is 3,8 or 5,6, as
shown in the left-hand side of the diagram in Figure 8, since the
symmetries of the SOMOs (singly occupied molecular orbitals)
are different and the spins of the electrons in these orbitals can
have the same sign. When the separation is larger, the spin
qualitative picture for these energy changes is provided by
inspection of the relative coefficients on the carbon p orbitals
π
on the b and a symmetry orbitals. The coefficients on the
1
2
carbon p orbitals in the 5,6-positions of the b orbital are small,
π
1
the a orbital has a node in the 3,8-positions, and the coefficient
2
in the 4,7-positions in both orbitals is large (Scheme 5). The
net effect is that methyl groups occupying the 3,8- and 5,6-
positions destabilize the OSS state. The destabilization is
substantially larger in both orbitals when methyl groups occupy
the 4,7-positions, lowering the ΔSS value and driving the
arrangement approaches that of Cp* Yb(bipy), resulting in a
2
singlet ground state, as shown in the right-hand side of Figure
8
, since the symmetries of the SOMOs are the same and the
electrons in them cannot have the same sign.
6
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Organometallics
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6
Scheme 5.
(iii) an intermolecular proton transfer process. A free radical
mechanism is ruled out by the results of the radical trapping
experiments, which leaves the intermolecular pathways in which
a hydrogen atom (ii) or a proton (iii) is transferred. These two
processes are illustrated in Scheme 6. The pathways differ by
Scheme 6
multiconfigurational singlet state below the triplet when CI is
turned on. As more methyl groups replace hydrogens in the
phenanthroline ligand, the stabilization continues, consistent
with the experimental results in Table 2.
the nature of the σ-C−C bond between the two radical anions;
in (ii) the spins are weakly correlated as in a pair of radicals,
and in (iii) the spins are more strongly correlated as in a bond.
The experimental information does not allow a distinction to
The change in the electronic structure of the adducts plays a
key role in determining the site of reactivity in the
phenanthroline adducts. Thus, Cp* Yb(phen) and Cp* Yb-
be made, but the formation of HD when D is present seems to
2
favor the proton transfer mechanism.
2
2
Conclusion. The ground-state electronic structure of the
(
3,8-Me phen) are dimers in the solid state due to formation of
2
paramagnetic charge-transfer diimine adducts of Cp* Yb range
2
a σ-C−C bond between the carbon atoms in the 4-position of
6
from open-shell singlets to triplets, depending on the identity of
the diimine and the number and position of methyl substituents
in the diimine. The different electronic structures are perhaps
surprising, since the neutral ligands have similar donor and
the phenanthroline ligand. The 3,8-Me phen adduct exists in a
2
dimer ⇌ 2 monomer equilibrium in solution, but the
insolubility of the phen adduct prevents the acquisition of an
equilibrium constant. In the solid-state structures of these
dimers, the C−C bond length is longer and presumably weaker
1
7
acceptor properties and the magnetic properties are
determined by the symmetry and energy of the SOMO
orbital(s) in the radical anion. All of the 2,2′-bipyridine adducts
studied have open-shell singlet ground states whose wave
in the 3,8-Me phen dimer than in the unsubstituted phen
2
dimer, likely due to steric repulsion between the two
monomeric units in the dimer. Steric repulsion is presumably
a factor contributing to the monomeric solid-state structure of
Cp* Yb(5,6-Me phen) as well as the distribution of the
III 13
•−
function is described by Ψ = c |Yb (f )(bipy )⟩ + c |
1
2
II 14
0
4
Yb (f )(bipy) ⟩. All of the methyl and dimethyl bipy adducts
are open-shell singlets in which the coefficients c and c
depend on their position in the ring. In contrast, the various
2
2
6
1
2
unpaired spin density at the 4-carbon pπ orbital.
3
When two methyl groups are located on the 4,7-positions,
the repulsion between the methyl groups in a hypothetical
dimer is larger than the stabilization resulting from σ-C−C
phenanthroline adducts range from a triplet, c = 0, when 1,10-
2
6
phenanthroline is the diimine, to open-shell singlets in the 4,7-
Me phen adduct. The origin of this difference is traced to the
2
bond formation. The increase in spin density on the carbon p
π
energy separation between the open-shell singlet (Yb↑L↓) and
orbitals at the 4,7-positions is relieved by delocalization of the
0
the closed-shell singlet (Yb↑↓L ) and therefore their extent of
spin density in the p orbital into the σ*-C−H bonds of a
π
configuration-interaction mixing, which depends on the
number and position of the methyl groups in the ring. The
electronic structure of these phenanthroline adducts alters their
methyl group. This delocalization results in weakening the C−
H bond of the methyl group and strengthening the CC bond
as the negative charge in the pyridyl ring rearranges, forming A
1
1
1
reactivity patterns. For example, the 3,8-Me phen adduct exists
2
and A in the Cp* Yb adducts, as well in the Cp Ti adducts.
2
2
2
in a dimer−monomer equilibrium in solution in which the σ-
The qualitative picture that is outlined above also fits the
C−C bond that connects the monomeric halves in the dimer is
experimental observation that Cp* Yb(4,4′-bipy) is a thermally
2
−1
−1
6
weak; the BDE is ca. 8 kcal mol . The 4,7-Me phen adduct
2
stable monomer but Cp* Yb(4,7-Me phen) is not, even though
2
2
undergoes an irreversible disproportionation in solution in
both have open-shell singlet multiconfigurational ground states
and similar values of n . Inspection of the b symmetry orbitals
which one C−H is broken while another one is formed
f
1
(
Scheme 6). A tentative suggestion is offered for the different
in both ligands provides a possible answer: the coefficients of
reactivity patterns that involves the extent of unpaired spin
density in the carbon p orbital on the 4- and 7-positions. This
the p carbon orbitals on the p C-4 position in each radical
π
π
•
−
•−
π
anion is smaller in bipy than in phen . This difference results
postulate, if true, provides a way to control the reactivity
patterns in these organometallic compounds.
in localization of less spin density on a given carbon atom in
•
−
bipy and a smaller driving force for C−C bond formation or
C−H bond breaking. This conjecture, viz., the spin density
distribution in the radical anions, means that determining the
experimental spin density map in these radical anions is the
next step in unraveling the physics and chemistry of these
molecules.
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,
pentane, and diethyl ether were dried over sodium and distilled, while
Three possible mechanisms may be considered for the
reaction illustrated in Scheme 1: (i) an intramolecular free
radical process, (ii) an intermolecular radical-pair process, and
CH Cl2 was purified by passage through a column of activated
2
alumina. Toluene-d8 and CH Cl -d were dried over sodium. All
2
2
2
1
solvents were degassed prior to use. H NMR spectra were recorded
6
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on Bruker AVB 400 MHz, DRX 500 MHz, AVB 600 MHz, and
Advance 300 MHz spectrometers. H chemical shifts are in δ units
(4,7-Me phen; 0.150 g, 0.721 mmol), and toluene (50 mL) was added
2
1
at 0 °C. The deep purple solution was stirred for 8 h at 0 °C,
concentrated to 15 mL, warmed to room temperature to dissolve the
dark residue, and slowly cooled to −20 °C. Dark purple-red crystals
relative to TMS, and coupling constants (J) are given in Hz. Infrared
spectra were recorded as Nujol mulls between KBr plates on a Thermo
Scientific Nicolet IS10 spectrometer. Samples for UV−vis−NIR
spectroscopy were obtained in a Schlenk-adapted quartz cuvette and
obtained on a Varian Cary 50 scanning spectrometer. Melting points
were determined in sealed capillaries prepared under nitrogen and
were uncorrected. Elemental analyses were determined at the
Microanalytical Laboratory of the College of Chemistry, University
of California, Berkeley. X-ray structural determinations were
performed at CHEXRAY, University of California, Berkeley. Magnetic
susceptibility measurements were made for all samples at 1, 5, and 40
kOe in a 7 T Quantum Design Magnetic Properties Measurement
System that utilizes a superconducting quantum interference device
1
suitable for X-ray diffraction formed overnight (295 mg, 63%). H
NMR in C D , elemental analysis, and X-ray diffraction indicated the
6
6
1
presence of half a molecule of toluene. H NMR (toluene-d , 300 K; δ
8
(ppm)): 109.13 (2H, 2,9-phenH), 16.69 (2H, 3,8-phenH), 3.63 (30H,
C Me ), 3.15 (2H, 5,6-phenH), −21.35 (6H, 4,7Me-phen). Mp: 265−
5
5
268 °C. Anal. Calcd for C3 H N Yb: C, 64.54; H, 6.64; N, 4.01.
7,5
46
2
−1
Found: C, 64.13; H, 6.11; N, 3,98. IR (cm ): 1622 (m), 1577 (w),
1520 (m), 1436 (m), 1374 (s), 1352 (s), 1256 (m), 1221 (m), 1183
(m), 1145 (s), 1077 (m), 981 (s), 849 (s), 790 (m), 728 (m), 694 (w).
Cp* Yb(3,4,7,8-Me phen) (3). The complex Cp* Yb(OEt )
2
4
2
2
(0.114 g, 0.221 mmol) was dissolved in Et O (5 mL) at −77 °C
2
(
SQUID). Sample containment and other experimental details have
and added dropwise over 30 min to a cold ether suspension (10 mL,
18
been described previously. The sample integrity was verified by
observing the absorption spectra of an oxygen “canary” that was always
loaded into one slot of each multislot sample holder, typically a
divalent ytterbocene such as Cp* Yb(OEt ). Diamagnetic corrections
−77 °C) of 3,4,7,8-tetramethyl-1,10-phenanthroline (3,4,7,8-Me phen;
4
0.056 g, 0.221 mmol). The suspension turned brown. After the
addition was complete, the dark brown reaction mixture was stirred at
−77 °C for 2 h. No color change was observed during this time. The
dark brown suspension was warmed to −40 °C while the solvent was
removed under reduced pressure to ca. 3 mL. The suspension was
stored overnight (16 h) at −40 °C, and a brown powder formed that
was collected by filtration at −77 °C. The dark brown powder (141
2
2
were made using Pascal’s constants. 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
4
oxygen and water. X-ray absorption measurements were made at the
Stanford Synchrotron Radiation Lightsource on beamline 11-2. The
mg, 85%) was washed with cold pentane (3 × 5 mL) at −77 °C and
1
samples were prepared and loaded into a liquid helium-flow cryostat at
dried under reduced pressure. H NMR (toluene-d , 300 K; δ (ppm)):
8
4
the beamline as described previously. Data were collected at
50.40 (2H, phen), 6.54 (2H, phen), 2.64 (30H, C Me ), −6.54 (6H,
5
5
temperatures ranging from 30 to 300 K, using a Si(220) double-
crystal monochromator. Fit methods were the same as described
Me-phen), −16.46 (6H, Me-phen). Mp: 264−266 °C. Anal. Calcd for
C H N Yb: C, 63.63; H, 6.77; N, 4.13. Found: C, 63.90; H, 6.81; N,
36
46
2
4
−1
previously. Low-temperature (ca. 2 K) EPR spectra were obtained
4.39. IR (cm ): 1611 (w), 1567 (w), 1418 (s), 1465 (m), 1438 (w),
1276 (m), 1232 (m), 1176 (m), 1088 (m), 1009 (m), 970 (m), 950
(vw), 907 (wv), 810 (s), 733 (w), 721 (s), 669 (m), 633 (m).
Cp* Yb(3,4,5,6,7,8-Me phen) (4). The complex Cp* Yb(OEt )
with a Varian E-12 spectrometer equipped with an EIP-547 microwave
frequency counter and a Varian E-500 gaussmeter, which was
calibrated using 2,2-diphenyl-1-picrylhydrazyl (DPPH; g = 2.0036).
Calculations. The ytterbium center was treated with a small-core
2
6
2
2
(0.086 g, 0.166 mmol) was dissolved in Et O (2 mL) at −77 °C and
2
19
relativistic pseudopotential (RECP) ([Ar] + 3d) in combination with
its adapted basis set (segmented basis set that includes up to g
functions). The carbon, nitrogen, and hydrogen atoms were treated
added dropwise over 30 min to a cold ether suspension (10 mL, −77
°C) of 3,4,5,6,7,8-hexamethyl-1,10-phenanthroline (3,4,5,6,7,8-
Me phen; 0.0323 g, 0.166 mmol). The suspension turned deep
4
20
with an all-electron double-ζ 6-31G(d,p) basis set. All calculations
green and then brown. After the addition was complete, the dark
brown reaction mixture was stirred at −77 °C for 2 h. No color change
was observed during this time. The suspension was filtered at −77 °C,
and the green-brown residue was washed with cold ether (−77 °C, 3 ×
2
1
were carried out with the Gaussian 03 suite of programs and the
22
ORCA suite of programs either at the density functional theory
23
(
DFT) level using the B3PW91 hybrid functional or at the CASSCF
1
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
of phenanthroline.
3 mL) and dried under reduced pressure (74 mg, 70%). H NMR
(toluene-d , 300 K; δ (ppm)): 42.54 (2H, phen), 2.46 (30H, C Me ),
8
5
5
1.50 (6H, Me-phen), −5.71 (6H, Me-phen), −12.44 (6H, Me-phen).
Mp: 270−272 °C. Anal. Calcd for C H N Yb: C, 64.48; H, 7.12; N,
38
50
2
3.96. Found: C, 64.80; H, 6.85; N, 4.10.
Syntheses. The ligands 3,4,7,8-tetramethylphenanthroline (3,4,7,8-
Cp* Yb(2,9-Me -4,7-Ph phen)·xPhMe (5; x = 0.25−0.5). The
2 2 2
complex Cp* Yb(OEt ) (0.240 g, 0.464 mmol) was combined with
Me phen), 2,9-dimethylphenanthroline (2,9-Me phen), 4,7-dimethyl-
4
2
2
2
phenanthroline (4,7-Me phen), and 2,9-dimethyl-4,7-diphenylphenan-
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (2,9-Me -4,7-Ph phen;
2
2 2
throline (2,9-Me -4,7-Ph phen) were purchassed from Aldrich.
0.167 g, 0.464 mmol), and toluene (20 mL) was added at room
temperature. The dark purple solution was stirred for 16 h at room
temperature, concentrated to ca. 7 mL, and cooled to −20 °C. Large
dark purple crystals suitable for X-ray determination formed overnight.
Two crops were obtained, which were combined and recrystallized in
2
2
3
,4,5,6,7,8-Hexamethylphenanthroline (3,4,5,6,7,8-Me phen) was a
6
gift from Prof. S. J. Buchwald at MIT. All ligands were purified by
sublimation between 80 and 200 °C prior to use.
Cp* Yb(2,9-Me phen) (1). The complex Cp* Yb(OEt ) (0.222 g,
2
2
2
2
1
0
.430 mmol) was combined with 2,9-dimethyl-1,10-phenanthroline
toluene at −20 °C (256 mg, 69%). H NMR spectra and combustion
analysis on the bulk indicated the presence of 0.25 molecule of toluene
(
2,9-Me -phen, 0.089 g, 0.429 mmol), and toluene (30 mL) was added
2
at room temperature. The pink/purple solution was stirred for 2 h at
room temperature. A dark precipitate formed. The suspension was
cooled to −20 °C, and the dark powder was collected by filtration
per complex; however, in the X-ray experiment 0.5 molecule of
1
toluene was found in the unit cell. H NMR (toluene-d , 300 K; δ
8
(ppm)): 16.94 (6H, Me), 10.70 (2H, phen-H ), 8.11 (t, J = 7.2 Hz,
3,8
(
198 mg, 71%). The dark powder was purified by crystallization in hot
2H, Hpara), 7.88 (d, J = 7.6 Hz, 4H, Hortho), 6.83 (t, J = 7.2 Hz, 4H,
toluene (10 mL) (145 mg, 52%). X-ray-quality crystals were obtained
Hmeta), 6,83 (2H, phen-H ), 1.72 (30H, Cp*). Mp: 288−290 °C.
5,6
1
by this method. H NMR (toluene-d , 300 K; δ (ppm)): 12.94 (2H, d,
Anal. Calcd for C H N Yb: C, 69.35; H, 6.34; N, 3.39. Found: C,
8
47.75 52
2
−1
J = 6.8, phen), 8.57 (2H, d, J = 6.8, phen), 7.29 (6H, Me-phen), 6.88
69.12; H, 6.10; N, 3.37. IR (cm ): 1622 (m), 1569 (w), 1547 (m),
1487 (m), 1439 (m), 1411 (w), 1379 (w), 1355 (w), 1261 (vw), 1077
(w), 1027 (m), 100 (m), 880 (s), 842 (vw), 831 (s), 772 (s), 729 (s),
703 (s).
(
2H, s, phen), 1.84 (30H, C Me ). Mp: 313−317 °C. Anal. Calcd for
5
5
C H N Yb: C, 62.66; H, 6.50; N, 4.30. Found: C, 62.74; H, 6.31; N,
34
42
2
−1
4
1
8
.12. IR (cm ): 1622 (w), 1592 (w), 1505 (m), 1435 (s), 1374 (m),
354 (w), 1316 (vw), 1300 (vw), 1206 (vw), 1148 (m), 1027 (m),
48 (s), 814 (vw), 784 (vw), 730 (s), 694 (w), 637 (w).
+
−
[Cp* Yb(4,7-Me phen)] I (6). The complex Cp* Yb(OEt )
2
2
2
2
(0.174 g, 0.337 mmol) was combined with 4,7-dimethylphenanthro-
line (0.070 g, 0.337 mmol) and AgI (0.079 g, 0.337 mmol). Toluene
(30 mL) was added at room temperature, and the purple solution was
Cp* Yb(4,7-Me phen) (2). The complex Cp* Yb(OEt ) (0.373 g,
2
2
2
2
0
.721 mmol) was combined with 4,7-dimethyl-1,10-phenanthroline
6
827
dx.doi.org/10.1021/om500843z | Organometallics 2014, 33, 6819−6829

Organometallics
Article
stirred for 16 h at room temperature (overnight). The volatile material
was removed under reduced pressure, and the brown residue was
crystals suitable for X-ray structure determination formed after 3 days
(350 mg, 41%). X-ray diffraction data were collected on one of these
1
triturated in pentane and extracted in CH Cl . The orange solution
2
2
crystals, resulting in the ORTEP drawing in Figure 7, The H NMR
was concentrated to ca. 5 mL and cooled to −20 °C. Large gold-yellow
spectrum of these brown crystals indicated the presence of A and A
1
2
1
crystals formed (134 mg, 52%). Mp: 214−218 °C. H NMR (CD Cl ,
2
2
in approximately equal amounts. The brown crystals were extracted
into pentane, and the resulting purple solution was filtered to afford a
green-brown residue. The pentane solution was concentrated to ca. 10
mL and cooled to −20 °C. Purple needles appeared after 2 days that
3
00 K, δ (ppm)): 295.6 (2H, phen), 52.45 (2H, phen), 8.41 (6H,
phen), 3.69 (30H, Me C ), −2.89 (2H, phen). Anal. Calcd for
5
5
C H N YbI·CH Cl : C, 48.68; H, 5.14; N, 3.24. Found: C, 49.47; H,
34
42
2
2
2
−1
5
1
6
.56; N, 3.06. IR (cm ): 1631 (w), 1534 (w), 1430 (s), 1417 (m),
377 (w), 1332 (m), 1301 (w), 1273 (w), 855 (m), 728 (s), 682 (w),
12 (w). EPR (solid 2 K): g = 7.136, g = 1.829, g = 1.214. Using
were found to be too small for an X-ray diffraction experiment (78 mg,
1
2
0%). H NMR (toluene-d , 300 K; δ (ppm)): 192.7 (1H), 87.81
8
1
2
3
(1H), 24.11 (3H), 14.66 (1H), 3.94 (Cp*, 15H), 3.58 (Cp*, 15H),
these g values gives μ (2 K) = 3.7 μ .The value of μ (0 K) from 12 to
eff
B
eff
4
5 K and extrapolation to 0 K gives μ (0 K) = 3.6 μ .
0.92 (1H), −2.71 (1H), −12.86 (1H), −22.87 (3H); one 1-H
resonance is missing. Mp: 257−260 °C. EIMS: {Cp* Yb(4,7-Me -4H-
eff
B
1
Reactivity of the Complex Cp* Yb(4,7-Me phen). H NMR
2
2
2
2
Studies. An NMR tube containing a C D solution of the complex
Cp* Yb(4,7-Me phen) was heated to 60 °C over a period of 15 days
and followed by H NMR spectroscopy. The initial set of five
resonances in a 2:2:2:6:30 ratio decreased in intensity, while two new
sets of peaks (A and A ) appeared and increased with time. After 15
6
6
phen) − H}, m/z 651. Anal. Calcd for C H N Yb: C, 62.56; H, 6.64;
34
13
2
−1
2
2
N, 4.29. Found: C, 62.37; H, 6.44; N, 4.02. IR (cm ): 2963 (m), 2905
m), 2851 (m), 2723 (w), 1583 (s), 1516 (s), 1485 (m), 1467 (m),
394 (m), 1374 (m), 1259 (s), 1186 (m), 1152 (m), 1076 (s), 1012
1
(
1
1
2
(s), 967 (m), 865 (w), 792 (s), 729 (w), 694 (w).
days the resonances due to Cp* Yb(4,7-Me phen) had disappeared
and only those due to A and A remained in the spectrum. The two
2
2
Reactivity of the Complex Cp* Yb(3,4,7,8-Me phen). Heating
2
4
1
2
a solution of Cp* Yb(3,4,7,8-Me phen) was more complicated than
2
4
sets were associated with the formation of the complexes Cp* Yb(4,7-
2
the reaction of Cp* Yb(4,7-Me phen), since several species appear to
Me -4H-phen) (set A , Scheme 7) and Cp* Yb(4-Me-7-(CH )-phen)
2
2
2
1
2
2
form, none of which could be identified with confidence. In contrast,
(
set A , Scheme 7) in a 1:1 ratio.
2
heating a solution of Cp* Yb(3,4,5,6,7,8-Me phen) formed only two
2
6
sets of resonances, much like the case for Cp* Yb(4,7-Me phen).
However, a conclusive assignment of the two sets was not possible. H
2
2
Scheme 7
1
NMR after 15 days (C D ; δ (ppm)): 232.3 (1H,), 210.0 (1H), 62.85
6
6
(3H), 60.01 (3H), 27.20 (3H), 25.32 (3H), 23.45 (3H), 23.31 (3H),
7
.66 (3H), 3.92 (3H), 3.48 (br, ν1/2 = 380 Hz, Cp*, 60H), −3.83
(
(
1H), −13.48 (d, J=13 Hz, 1H), −14.96 (3H), −17.04 (3H), −23.83
d, J = 12.6 Hz, 1H), −27.30 (3H). One 1-H resonance was not found
for one of each species.
In all three thermal reactions, the half-times of the reactions were
similar when similar concentrations were used; the t1/2 value was about
1
H NMR after 15 days (C D ; δ (ppm)): 192.7 (1H, A ), 186.8
6
6
1
5
days at a 5 mM concentration at 60 °C.
(
1H, A ), 87.81 (1H, A ), 83.47 (1H, A ), 24.11 (3H, A ), 21.07 (3H,
2 1 2 1
X-ray Crystallography. Single crystals of the compounds
A ), 14.66 (1H, A ), 4.88 (1H, A ), 3.73 (br, ν = 380 Hz, Cp*,
2
2
1
1/2
Cp* Yb(4,7-Me phen) (2), Cp* Yb(2,9-Me -4,7-Ph phen) (5), and
2
2
2
2
2
6
0H), 0.92 (1H, A ), −2.54 (1H, A ), −2.71 (1H, A ), −10.89 (1H,
1
2
1
7
were coated in Paratone-N oil and mounted on a Kaptan loop. The
A ), −12.86 (1H, A ), −12.95 (1H, A ), −22.34 (1H, A ), −22.87
2
1
2
2
24
1
loop was transferred to a Bruker SMART 1000 diffractometer for 5
and a SMART APEX diffractometer for 2 and 7. Both are equipped
with a CCD area detector. 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
(
3H, A ). One H resonance was missing in each compound; changing
1
the temperature did not help locate them.
The rearrangement was performed in the presence of 1,10-
dihydroanthracene, 1,4-cyclohexadiene, or D . Neither anthracene
nor benzene was formed in the two first reactions, but the presence of
2
5
2
D led to the formation of HD. The half-time of the reaction was
26
2
the program SAINT to a maximum 2θ value of 50.8° for 2, 51.0° for
concentration dependent, but no kinetic rate law could be found to fit
the data. The presence of D did not change the half-time at the same
concentration.
When only A and A were observed in the H NMR spectrum, the
mixture was hydrolyzed (H O) and the hydrosylate was examined by
H NMR in C D . H NMR (C D ; δ (ppm)): 9.15 (s, 2H, phen),
.42 (s, 1H, A ), 8.03 (s, 2H, phen), 7.55 (s, 2H, phen), 7.06 (d, J =
.6 Hz, 1H, A ), 7.00 (d, J = 7.8 Hz, 1H, A ), 6.67 (1H, A ), 5.94 (d, J
7.2 Hz, 1H, A ), 4.54 (m, 1H, A ), 3.88 (m, 1H, A ), 2.84 (s, 6H,
phen), 2.22 (s, 3H, A ), 1.34 (3H, d, J = 6.8 Hz, A ). After 1 day, the
5
, and 50.8° for 7. The data were corrected for Lorentz and
2
polarization effects. Data were analyzed for agreement and possible
1
absorption using XPREP. A semiempirical multiscan absorption
1
2
27
correction was applied using SADABS. This models the absorption
2
1
1
surface using a spherical harmonic series on the basis of differences
between equivalent reflections. The structures were solved by direct
6
6
6
6
8
7
=
1
2
8
29
methods using SHELX 2013 and the WinGX program. Non-
hydrogen atoms were refined anisotropically, and hydrogen atoms
were placed in calculated positions and not refined.
1
1
1
1
1
1
1
1
A set of resonances disappeared and resonances for free 4,7-Me phen
For 2 and 5, disordered toluene molecules were found in the lattice
and were solved. For 7, a number of restraints were used in order to
refine this structure. (i) A highly disordered solvent molecule was
determined with the SQUEEZE routine. (ii) Two different chemical
entities cocrystallized on the same site, in a 72/28 ratio with formulas
C H N Yb (A ) and C H N Yb (A ), respectively. The second
1
2
ligand, due to the rearomatization of the negatively charged A and A2
1
ligands, appeared. The resonances associated with complex A2
disappeared too rapidly and were not detected in the 1H NMR
spectrum. A GCMS identified free 4,7-Me phen as the only product
2
formed.
34
41
2
2
34 43
2
1
Synthesis of the Complex Cp* Yb(4,7-Me -4H-phen) (8) (A ).
2
2
1
entity displayed a disorder of the C34 methyl group. In order to
anisotropically refine these entities, EADPs were used and DFIX
The complex Cp* Yb(OEt ) (0.670 g, 1.30 mmol) was combined with
2
2
4
,7-dimethyl-1,10-phenanthroline (4,7-Me phen; 0.70 g, 1.30 mmol),
2
instructions were applied to force the CH−CH groups at C28 in a
3
and toluene (30 mL) was added at room temperature. The deep
purple solution was stirred for 14 days at 60 °C as the solution turned
brown. The solution was concentrated to ca. 10 mL and warmed to
dissolve the brown residue, the resulting solution was filtered while
warm, and the filtrate was slowly cooled to −20 °C. Dark brown
slightly out of plane conformation. (iii) The Cp* rings also were
disordered and were refined as idealized entities using AFIX 9 and
SIMU instructions. The resolution of the data did not allow releasing
these restraints.
6
828
dx.doi.org/10.1021/om500843z | Organometallics 2014, 33, 6819−6829

Organometallics
Article
(
15) Da Re, R. E.; Kuehl, C. J.; Brown, M. G.; Rocha, R. C.; Bauer, E.
ASSOCIATED CONTENT
■
D.; John, K. D.; Morris, D. E.; Shreve, A. P.; Sarrao, J. L. Inorg. Chem.
003, 42, 5551.
16) La Mar, G. N.; Horrocks, W. D.; Holm, R. H. NMR of
*
S
Supporting Information
2
(
Text, tables, figures, and CIF files giving information
concerning magnetic susceptibility, vis−NIR spectroscopy,
Paramagnetic Molecules: Principles and Applications; Elsevier: Am-
sterdam, 1973.
1
and H variable-temperature NMR for new compounds and
crystal data for 2, 5, and 7 (CCDC 1019971, 1019972, and
019973. This material is available free of charge via the
Internet at http://pubs.acs.org.
(17) McWhinnie, W. R.; Miller, J. D.; Emeleu
Adv. Inorg. Chem. Radiochem. 1970, 12, 135.
18) Walter, M. D.; Schultz, M.; Andersen, R. A. New. J. Chem. 2006,
0, 238.
19) Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1989, 90, 1730.
(20) Harihara, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
21) Frisch, M. J., et al. Gaussian 03, Revision E-01; Gaussian Inc.:
Pittsburgh, PA, 2001.
22) Neese, F. ORCA, Version 2.4; Chemie, M.-P.-I. f. B., Ed.
Mulheim and der Ruhr, 2004.
(23) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
́
s, H. J.; Sharpe, A. G.
1
(
3
(
AUTHOR INFORMATION
■
(
Notes
The authors declare no competing financial interest.
(
̈
ACKNOWLEDGMENTS
■
(
(
(
24) Bruker Analytical X-Ray Systems: Madison, WI, USA, 2007.
25) Bruker Analytical X-Ray Systems: Madison, WI, USA, 2007.
26) SAINT; Bruker Analytical X-Ray Systems: Madison, WI, USA,
G.N. thanks the CNRS and Ecole Polytechnique for funding.
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, Geosciences and Biosciences Division,
Heavy Element Chemistry Program, 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 Antonio DiPasquale and Fred
Hollander at CHEXRAY Berkeley for their help with crystal
structures and Wayne W. Lukens for the EPR spectrum and
discussions. L.M. is a member of the Institut Universitaire de
France. Cines and CALMIP are acknowledged for a generous
grant of computing time. L.M. also thanks the Humboldt
Foundation for a fellowship.
2
007.
(
(
(
27) Blessing, R. Acta Crystallogr., Sect. A 1995, 51, 33.
28) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.
29) Farrugia, L. J. Appl. Crystallogr. 1999, 32, 837.
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