Probing the 5f orbital contribution to the bonding in a U(V) ketimide complex

Article abstract of DOI:10.1021/ja211875s

Reaction of UCl₄ with 5 equiv of Li(N = C^tBuPh) generates the homoleptic U(IV) ketimide complex [Li(THF)₂][U(N = C^tBuPh)₅] (1) in 71% yield. Similarly, reaction of UCl₄ with 5 equiv of Li(N

Full text of DOI:10.1021/ja211875s

Article
pubs.acs.org/JACS
Probing the 5f Orbital Contribution to the Bonding in a U(V)
Ketimide Complex
Lani A. Seaman,† Guang Wu,† Norman Edelstein,*,‡ Wayne W. Lukens,*,‡ Nicola Magnani,‡
and Trevor W. Hayton*,†
†Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States
‡Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: Reaction of UCl4 with 5 equiv of Li(N
CtBuPh) generates the homoleptic U(IV) ketimide complex
[Li(THF)2][U(NCtBuPh)5] (1) in 71% yield. Similarly,
reaction of UCl4 with 5 equiv of Li(NCtBu2) affords
[Li(THF)][U(NCtBu2)5] (2) in 67% yield. Oxidation of 2
with 0.5 equiv of I2 results in the formation of the neutral
U(V) complex U(NCtBu2)5 (3). In contrast, oxidation of 1
with 0.5 equiv of I2, followed by addition of 1 equiv of Li(NCtBuPh), generates the octahedral U(V) ketimide complex
[Li][U(NCtBuPh)6] (4) in 68% yield. Complex 4 can be further oxidized to the U(VI) ketimide complex U(NCtBuPh)6
(5). Complexes 1−5 were characterized by X-ray crystallography, while SQUID magnetometry, EPR spectroscopy, and UV−
vis−NIR spectroscopy measurements were also preformed on complex 4. Using this data, the crystal field splitting parameters of
the f orbitals were determined, allowing us to estimate the amount of f orbital participation in the bonding of 4.
ketimides are noted for being inert coligands that do not
exhibit insertion chemistry39−42 and which are resistant to
electrophilic attack,41 making them good ligands for the
exploration of small molecule chemistry. Ketimides are also
easily synthesized43 and are highly tunable.44−46
INTRODUCTION
■
Interest in understanding the extent of covalency in actinide−
ligand bonding has grown substantially in recent years.1−13 This
interest is motivated, in part, by the need for new extractants in
advanced nuclear fuel cycles.8−10,14 A variety of experimental
techniques have been used to quantify covalency in these
elements,1,15−17 including ligand K-edge X-ray absorption
spectroscopy,4,18 photoelectron spectroscopy,19 EXAFS,11,20
X-ray crystallography,21−28 and thermochemistry.22,29 Optical
spectroscopy can also provide useful information on bonding in
the actinides. For instance, using NIR spectral data, Edelstein
and co-workers were able to extract the θ, Δ, and ξ ligand field
parameters for a series of homoleptic 5f1 actinide halides.30,31
They suggested that the presence of covalency in the An-X σ-
bonding framework best explained the observed trends in these
parameters. Unfortunately, this analysis was limited by the
paucity of complexes with both octahedral symmetry and the
correct electronic configuration. For this technique to be
applied more broadly, the synthesis of new, high symmetry 5f1
complexes is required. In this context, our research group has
demonstrated that high valent, octahedral alkoxide, amide, and
alkyl complexes of uranium are, in fact, readily isolable.32−35
In our continuing efforts to synthesize homoleptic complexes
of uranium in the 5+ and 6+ oxidation states, we have turned
our attention to the ketimide ligand, in particular [N
CtBuPh]− and [NCtBu2]−. Recent studies by our group have
shown that ketimides are capable of supporting high oxidation
states, such as the strongly oxidizing Mn(IV) and Fe(IV)
ions.36,37 This stabilization likely arises from a combination of
the ketimide’s strong σ-donating ability (the pKa of Ph2CNH
is 31)38 and its strong π-donating ability.37,39 Moreover,
Kiplinger and co-workers have synthesized an expansive
series of actinide ketimides with the general formula of
Cp*2An(NCR2)2 (An = Th, U; R = alkyl, aryl).47−57
Theoretical calculations and spectroscopic analysis of this series
of complexes suggests that ketimides exhibit relatively covalent
metal−ligand interactions, due, in part, to their significant π-
donating capability.37,39 Further insight into the metal−
ketimide interaction comes from an electrochemical study of
a series of U(V) imido complexes, Cp*2U(NAr)X (Ar =
2,6-iPr2C6H3; X = OTf, I, Cl, Br, SPh, F, Me, OPh, NCPh2).
For this series, the ketimido derivative exhibited the lowest
U(VI)/U(V) redox potential,49 further demonstrating the
strong electron donating ability of this ligand class. In addition
to the work of Kiplinger, Cummins and Diaconescu reported
the synthesis of a U(III) inverted arene sandwich complex,
K2(μ-η6,η6-C10H8)[U(NC[tBu]Mes)3]2,58 demonstrating that
ketimides can also stabilize the 3+ state of uranium.
Herein we report the synthesis of a series of homoleptic
uranium ketimides in the 4+, 5+, and 6+ oxidation states. In
addition, we present a detailed spectroscopic analysis of an
octahedral U(V) ketimide, allowing us to estimate the amount
of f orbital participation in the uranium−ketimide interaction.
Received: December 20, 2011
Published: February 10, 2012
© 2012 American Chemical Society
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RESULTS AND DISCUSSION
■
Synthesis. Addition of 5 equiv of Li(NCtBuPh) to UCl4
in THF results in the formation of a dark brown solution, from
which [Li(THF)2][U(NCtBuPh)5] (1) can be isolated as a
brown solid in 71% yield by crystallization from concentrated
hexanes (eq 1). Similarly, addition of 5 equiv of Li(NCtBu2)
to UCl4 provides the homoleptic uranium(IV) ketimide,
[Li(THF)][U(NCtBu2)5] (2), which is isolable in 67%
yield after crystallization from hexanes (eq 1).
Figure 1. ORTEP diagram of [Li(THF)2][U(NCtBuPh)5] (1) with
50% probability ellipsoids. Selected bond lengths (Å) and angles
(deg): U1−N1 = 2.313(4), U1−N2 = 2.335(3), U1−N3 = 2.248(4),
U1−N4 = 2.260(4), U1−N5 = 2.251(5), C1−N1−U1 = 145.7(3),
C2−N2−U1 = 143.6(3), C3−N3−U1 = 170.9(4), C4−N4−U1 =
173.4(5), C5−N5−U1 = 172.2(4).
Complex 1 is soluble in both nonpolar and ethereal solvents.
1
Its H NMR spectrum in C6D6 displays resonances at 12.33,
7.73, and 7.43 ppm in an 2:1:2 ratio. These resonances are
attributable to the ortho, para, and meta protons on the phenyl
rings, respectively. A resonance at −5.01 ppm can be assigned
to the tert-butyl protons, while the resonances of the THF
molecules are observed at 0.28 and 1.35 ppm. Additionally, 1
Interestingly, addition of 6 equiv of either Li(NCtBuPh)
or Li(NCtBu2) to UCl4 does not result in the isolation of a
hexacoordinate complex. Instead, the aforementioned penta-
coordinate complexes are the only species isolated. The
inability of 1 or 2 to coordinate a sixth ketimide ligand is
likely due to the strongly electron donating nature of the
ketimide ligands. This places significant negative charge density
on the metal center and disfavors coordination of a sixth
ketimide ligand. A similar justification was used to explain the
pentacoordinate structures of [Li(DME)3][U(NC5H10)5] and
7
exhibits a single resonance in the Li NMR spectrum at 14.52
ppm. Complex 2 is also highly soluble in hexanes, toluene, and
ethereal solvents. Its 1H NMR spectrum in C6D6 exhibits
resonances at 4.24, 1.56, and −1.47 ppm, corresponding to the
α-THF protons, the β-THF protons, and the tert-butyl protons,
7
respectively. Its Li NMR spectrum exhibits a single resonance
at 89.38 ppm.
Storage of a concentrated hexanes solution of 1 at −25 °C
affords crystals suitable for X-ray analysis. Complex 1
32,34
t
[Li][UR5] (R = CH2 Bu, CH2SiMe3).
crystallizes in the triclinic space group P1, and its solid state
The solution phase redox properties of complex 2 were
investigated by cyclic voltammetry. In THF at room temper-
ature, complex 2 exhibits a reversible redox couple at −2.03 V
(vs Fc/Fc+) and a quasi-reversible feature at −0.39 V (vs Fc/
Fc+) (see the Supporting Information). We have assigned these
features to the U(IV)/(V) couple and the U(V)/(VI) couple,
respectively. For comparison, Cp*2U(NCPh2)2 exhibits a
U(IV)/(V) redox potential at −0.48 V (vs Fc/Fc+),39,54 while
Cp*2U(NC(Me)Ph)2 exhibits a U(IV)/(V) redox potential
at −0.54 V (vs Fc/Fc+).50 Consistent with the cyclic
voltammetry results, addition of 0.5 equiv of I2 to 2 generates
the neutral five-coordinate U(V) complex, U(NCtBu2)5 (3)
(Scheme 1), which can be isolated by crystallization from
concentrated hexanes as a dark brown microcrystalline solid in
63% yield. Complex 3 is exceedingly soluble in hexanes,
̅
molecular structure is shown in Figure 1. In the solid state, 1
adopts a trigonal bipyramidal geometry. The U−N bond
lengths for the terminal ketimide ligands range from 2.248(4)
to 2.260(4) Å, whereas the U−N bond lengths of the bridging
ketimide ligands are 2.313(4) and 2.335(3) Å. The U−N bond
lengths are within the range of U(IV) amide complexes.32,59
They are also in the range of other uranium ketimide U−N
distances (2.176(12)−2.343(7) Å).49,58 The terminal ketimide
U−N−C bond angles are nearly linear, ranging from 170.9(4)°
to 173.4(5)°, consistent with an sp hybridized nitrogen
participating in π-donation to the uranium metal center.
However, given the steric bulk of the ketimide ligand, it is
possible that the large U−N−C angles are a result of steric
crowding. Finally, the lithium cation in 1 is ligated by two THF
molecules and two nitrogen atoms from the ketimide ligands,
giving the lithium a tetrahedral geometry. Complex 2 also
exhibits a trigonal bipyramidal geometry around the uranium
center (see the Supporting Information), with similar metrical
parameters. As with 1, the lithium cation in 2 is contained
within the secondary coordination sphere; however, its
coordination environment consists of two ketimide nitrogen
atoms, one THF molecule, and agostic interactions with the
ketimide methyl groups.36,37
1
toluene, and ethereal solvents. Its H NMR spectrum in C6D6
consists of a single broad resonance at 1.55 ppm.
Crystals of 3 suitable for X-ray crystallographic analysis were
grown from a dilute toluene solution. Complex 3 crystallizes in
the monoclinic space group P21/c, and its solid-state structure
is shown in Figure 2. In the solid state, 3 exhibits a trigonal
bipyramidal geometry. Its U−N bond lengths range from
2.173(5) to 2.208(4) Å, similar to those observed for complex
2 (avg 2.24 Å) and the uranium(V) amides [Li(DME)3][U-
(NC5H10)6] and [PPh4][U(dbabh)6] (Hdbabh = 2,3:5,6-
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Scheme 1
dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) (range: 2.23(1) to
bond angles ranging from 172.6(4)° to 179.5(5)°, again
consistent with sp hybridization at the nitrogen atom. To our
knowledge, complex 3 is the first UX5-type complex to be
structurally characterized.
2.292(6) Å).32,60 In addition, 3 exhibits nearly linear U−N−C
In contrast to the reactivity observed for complex 2,
oxidation of 1 with 0.5 equiv of I2 only results in
decomposition. Gratifyingly, however, reaction of 1 with 0.5
equiv of I2 in diethyl ether, followed rapidly by the addition of 1
equiv of Li(NCtBuPh), results in the formation of the
hexacoordinate U(V) complex, [Li][U(NCtBuPh)6] (4)
(Scheme 1), which can be isolated by crystallization from
1
concentrated toluene as dark red blocks in 68% yield. The H
NMR spectrum of 4 in C6D6 exhibits resonances at 7.98, 7.66,
and 7.57 ppm, attributable to the meta, para, and ortho protons
on the phenyl rings, while a resonance at 0.35 ppm is assignable
to the tert-butyl groups. Additionally, a single resonance at
7
32.46 ppm is observed in its Li NMR spectrum.
Crystals of 4 suitable for X-ray crystallographic analysis were
grown from a dilute toluene solution. Complex 4 crystallizes in
the rhombohedral space group, R3, and its solid-state molecular
̅
structure is shown in Figure 3. The uranium atom in 4 resides
on a site with S6 symmetry, which generates the ketimide
ligands in an idealized octahedral geometry. Additionally, the Li
cation is disordered over two positions. The U−N bond length
is 2.217(2) Å, which is comparable to the bond lengths of the
homoleptic uranium(V) amide complexes; [PPh4][U(dbabh)6]
(2.23(1)−2.27(1) Å) and [Li(DME)3][U(NC5H10)6]
(2.250(6)−2.292(6) Å)32,60 but longer than that of the U(V)
ketimide complex, Cp*2U(N-2,6-iPr2C6H3)(NCPh2)
(2.199(4) Å).49 The lithium cation is coordinated by three
nitrogen atoms of the ketimide ligands and by π interactions
with the three phenyl rings, which are arranged in a propeller-
Figure 2. Solid-state molecular structure of U(NCtBu2)5 (3) with
50% probability ellipsoids. Selected bond lengths (Å) and angles
(deg): U1−N1 = 2.180(5), U1−N2 = 2.193(5), U1−N3 = 2.208(4),
U1−N4 = 2.173(5), U1−N5 = 2.181(5), U1−N1−C1 = 172.6(4),
U1−N2−C2 = 172.7(5), U1−N3−C3 = 175.2(4), U1−N4−C4 =
179.5(5), U1−N5−C5 = 175.3(4).
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Figure 4. ORTEP diagram of U(NCtBuPh)6·C7H8 (5·C7H8) with
50% probability ellipsoids. Asterisks denote symmetry related atoms.
Toluene solvate omitted for clarity. Selected bond lengths (Å) and
angles (deg): U1−N1 = 2.173(3), U1−N2 = 2.175(3), U1−N3 =
2.181(3), U1−N1−C1 = 171.6(3), U1−N2−C2 = 176.3(3), U1−
N3−C3 = 178.0(3).
Figure 3. ORTEP diagram of [Li][U(NCtBuPh)6] (4) with 50%
probability ellipsoids. Selected bond lengths (Å) and angles (deg):
U1−N1 = 2.217(2), U1−N1−C1 = 176.9(2).
like fashion around the lithium atom. Finally, the U1−N1−C1
bond angle is nearly linear (176.9(2)°), suggestive of sp
hybridization at the nitrogen atom.
nearly linear U−N−C bond angles, ranging from U1−N1−C1
= 171.6(3)° to U1−N3−C3 = 178.0(3)°, indicative of π-
donation into the metal center.
The cyclic voltammogram of 4, in THF at room temperature,
displays a reversible oxidation feature at −1.52 V (vs Fc/Fc+)
which we have assigned to the U(V)/U(VI) redox couple. This
value is similar to that of the homoleptic U(V) amide
[Li(DME)3][U(NC5H10)6] (E = −1.51 V, vs Fc/Fc+)32 but
lower than those reported for analogous alkoxide33,61 and alkyl
complexes,35 suggesting that the ketimide has a π-donating
ability comparable to that of a di(alkyl)amide ligand. Consistent
with the electrochemical data, oxidation of 4 with 0.5 equiv of I2
at −25 °C provides the homoleptic uranium(VI) complex
U(NCtBuPh)6 (5) (Scheme 1) in 56% yield. Complex 5
Attempts to generate a six-coordinate analogue of complex 3
have been unsuccessful, as addition of Li(NCtBu2) to 3
results in no reaction. We suggest that the bis(tert-butyl)
ketimide ligand is likely too bulky to permit formation of an
octahedral complex, in contrast to the phenyl-tert-butyl
ketimide ligand. We have also attempted to oxidize complex
3 by one electron, in an attempt to form the U(VI) cation,
[U(NCtBu2)5]+. However, reaction of 3 with a variety of
oxidants leads to either decomposition or simply return of
starting material.
1
exhibits a H NMR spectrum consistent with a diamagnetic
complex. For instance, a resonance at 1.46 ppm is assignable to
the tert-butyl protons, while resonances at 7.11 ppm and 7.35
ppm can be attributed to the aryl protons of the phenyl
substitutent. The cyclic voltammogram of 5, in THF at room
temperature, displays a reversible reduction feature at −1.49 V
(vs Fc/Fc+) (see the Supporting Information), consistent with
the electrochemical parameters observed for complex 4. Also,
present in the cyclic voltammogram of 5 is an irreversible
feature at 0.21 V (vs Fc/Fc+), which we have assigned to a
ketimide ligand oxidation event.
Crystals of 5 suitable for X-ray crystallographic analysis were
grown from a dilute toluene solution. Complex 5 crystallizes in
the monoclinic space group P21/n as a toluene solvate, 5·C7H8.
Its solid-state structure is shown in Figure 4. Like complex 4, 5
adopts an octahedral geometry in the solid-state. Notably, the
phenyl substituents are also no longer arranged in a propeller-
like fashion, as there is no longer a lithium cation enclosed in
the secondary coordination sphere. The U−N bond lengths
range from U1−N1 = 2.173(3) Å to U1−N3 = 2.181(3) Å.
These bond lengths are comparable to the U−N bond lengths
in similar uranium(VI) amides, U(NC5H10)6 and U(dbabh)6
(range: 2.187(6)−2.239(5) Å), but they are shorter than those
observed in complex 4, consistent with the smaller ionic radius
of U6+.32,60 As with complexes 1 and 4, complex 5 exhibits
Electronic Structure and Bonding. To better understand
the electronic structures of complexes 1 and 4, their magnetic
susceptibilities were measured using SQUID magnetometry. A
plot of their effective magnetic moments (μeff) is shown in
Figure 5. Complex 1 exhibits a μeff of 2.45 μB at 300 K, much
smaller than that of the free 5f2 ion, which has a value of 3.58
μB.51,62,63 On cooling to 73 K, the μeff value of 1 drops slightly
to 2.41 μB; however, on further cooling to 4 K, the effective
magnetic moment decreases to 0.94 μB. This behavior contrasts
with that observed for related trigonal bipyramidal U(IV)
complexes, [Li(DME)][U(NC5H10)5] and [Li][UR5] (R =
32,34
t
CH2 Bu, CH2SiMe3),
but is consistent with the behavior
normally observed for U(IV).34,51,62,64−70 Complex 4 possesses
a μeff value of 1.38 μB at 300 K, considerably smaller than the
2.54 μB calculated for the U5+ ion in a 2F5/2 ground state.49 This
value is also lower than those observed for other U(V)
complexes, which typically range from 1.9 to 2.5 μB at room
temperature.62,64,71−74 However, it is comparable to the
effective magnetic moments found for the homoleptic amido
complex [Li(DME)3][U(NC5H10)6],32 and the U(V) alkyl
complexes [Li(DME)3][U(OtBu)2(CH2SiMe3)4] and [Li-
(DME)3][U(CH2SiMe3)6].35 On cooling, the μeff value of 4
decreases to 1.09 μB at 4 K. This temperature response is in
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Figure 5. Temperature dependent magnetic susceptibility at 1 T for 1 (blue squares) and 4 (red diamonds) from 4 to 300 K.
accord with the magnetic behavior normally observed for
U(V).49,62,64,72,74
The electronic structure and bonding in 4 were further
investigated using a combination of EPR spectroscopy, NIR
spectroscopy, and crystal-field modeling. The EPR spectrum of
powdered 4 at 2 K is shown in Figure 6 along with the
the bonding in octahedral U(V) halide complexes.30,31
Although this model is based on pure Oh symmetry, we use
it as a starting point for the analysis of the electronic structure
of 4 and expect it to provide at least a qualitative picture of the
bonding.
In the absence of spin−orbit coupling, the Oh crystal field
splits the seven 5f orbitals into a singly degenerate orbital, a2u,
and two triply degenerate sets of orbitals, t2u and t1u. As shown
by Burns and Axe, the bonding in octahedral U(V) complexes
can be described using three parameters, θ, Δ, and ζ, where ζ is
the spin−orbit coupling constant. In the limit of zero spin−
orbit coupling, Δ is the energy of t2u relative to the a2u orbital,
and t1u is destabilized by Δ + θ relative to a2u.75 The a2u orbital
is nonbonding; the t2u orbital is π-antibonding with a group
overlap corresponding to (5/2) 5f-2p π-antibonding orbitals;
and the t1u orbital is both σ- and π-antibonding with a group
overlap that corresponds to two 5f-2p σ-antibonding orbitals
and (3/2) 5f-2p π-antibonding orbitals. Therefore, the strength
2
of a single 5f-2p π-interaction is /5Δ, and the strength of a
1
1
single 5f-2p(s) σ-interaction is /2θ + /5Δ; this information is
summarized in Table 1. Spin−orbit coupling splits the t2u state
Figure 6. EPR spectrum of 4 at 2 K. The complex displays axial
symmetry with g∥ = 2.132 and g⊥ = 0.746.
Table 1. Bonding and Energetics of the MOs Derived from
the 5f-Orbitals in an Octahedral U(V) Complex
simulated spectrum. Complex 4 displays axial symmetry with g∥
= 2.132 and g⊥ = 0.746, which is consistent with the S6 site
symmetry of the uranium center in 4. In addition, μeff for the
ground state of 4 was determined to be 1.19 μB from the g-
values using eq 2, slightly greater than the value of 1.09 μB
determined by the susceptibility. The good agreement between
the values of μeff determined by EPR and magnetic
susceptibility supports the assignment of the EPR spectrum
shown in Figure 6.
orbital
group overlap
bonds
energy relative to a2u
a2u
t2u
t1u
0
(5/2)1/2⟨πp|f⟩
(3/2)1/2⟨πp|f⟩ + (2)1/2⟨σp|f⟩
5/2π
3/2π + 2σ
Δ
Δ + θ
into a Γ8 quartet and a Γ7′ doublet and splits the t1u state into a
Γ8′ quartet and a Γ6 doublet; the ground a2u state becomes a Γ7
doublet when spin−orbit coupling is included. A useful
schematic of this orbital picture can be found in ref 31. For
an octahedral complex, θ, Δ, and ζ may be determined if the
NIR spectrum can be assigned to determine the energies of the
low-lying Γ7′, Γ8′, and Γ6 excited states (the energy of Γ8 is too
low to be observed in the present study). In 4, the symmetry is
lowered from octahedral due to the fact that the ketimide
ligand is not axially symmetric and therefore not a π-donor
along both the x- and y-axes (taking the U−N vector as the z-
2
2
g
+ 2g
⊥
μ
=
eff
(2)
2
The coordination geometry of 4 is very close to octahedral,
especially if the substituents on the ketimide ligands are
ignored. This geometry enables a more extensive exploration of
the bonding in 4 using the approach previously used to describe
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axis). Each of the Γ8 and Γ8′ quartets are split into two doublets
by this asymmetry with respect to π-bonding. Keeping this in
mind, the NIR spectrum of 4 may be assigned as shown in
Figure 7. Note that the parameters Δ and θ are linearly related
The main difference between calculation and experiment is that
the EPR spectrum that is used as the basis for the calculation
corresponds to a moment ∼10% greater than the measured
moment, which leads to the calculated moment being ∼10%
larger than the measured moment.
Using the relationships in Table 2 and the crystal field data in
Table 3, the strength of the interactions between the uranium
5f orbitals and the nitrogen 2p orbitals may be estimated. It
should be noted that one of the parameters, ζ, was not
permitted to freely vary, which could result in a systematic
error. In addition, the large standard deviations in the values of
the crystal field parameters (given in parentheses in Table 3)
result in relatively large errors in the bond strengths, so the
results are largely qualitative. Using the data in Table 2, the
ketimide σ-bond strength is 8 2 kcal·mol−1, and the strength
of the π-interaction is 2 1 kcal·mol−1. However, complex 4
has only half of the 2p-5f π-interactions as the UX6− complexes
upon which the model is based, so the strength of the 2p-5f π-
interaction in 4 is actually 4 3 kcal mol−1, double that given
by the relationships in Table 2. The corrected values of the σ-
and π-interactions are given in Table 3, along with the values
Figure 7. NIR spectrum of 4 showing assignments of the transitions to
low-lying excited states with their labels for octahedral symmetry. The
Γ8′ state is split by the lower symmetry of 4. The peak at 1300 cm−1 is
due to a ligand-based absorption (see Figure S21 of the Supporting
Information).
−
for UX6 determined using the relationships in Table 2.
Regardless of the large errors in the strengths of the 5f-2p
interactions for the ketimide ligand, it is clear that they are
relatively weak and, surprisingly, are comparable to those
observed for [UX6]− (X = F, Cl, Br). While these values seem
to imply that the interactions between U(V) and the ketimide
ligands are weak, it should be remembered that the uranium 6d
orbitals may interact more strongly with the ligands than do the
5f orbitals,8,9,77−79 and the values given here reflect only the
contribution of the 5f orbitals to metal−ligand bonding.80
to the more general crystal field parameters B4 and B6
0
0
(defined using the Wybourne convention).76
If the energies of the transitions assigned to the split Γ8′ state
are averaged, the values for θ and Δ are determined to be 5100
990 cm−1 and 1920 1456 cm−1, respectively, with ζ set
equal to 1800 cm−1. The value of ζ was chosen such that both
the NIR and EPR data were well modeled by the crystal field
parameters. It should be noted that the values of θ and Δ
depend on the value of ζ. The assignment of the NIR spectrum
was checked by transforming the octahedral coordinate system
to a C3v system (the z-axis is the U−Li vector). The distortion
CONCLUDING REMARKS
■
In this contribution we report the synthesis and character-
ization of a series of homoleptic ketimide complexes, including
the high valent species U(NCtBu2)5, [Li][U(NCtBuPh)6],
and U(NCtBuPh)6. Their isolation further demonstrates that
the ketimide ligand platform is strongly electron donating and
capable of stabilizing high oxidation states. A cyclic
voltammetry study of [Li][U(NCtBuPh)6] suggests that
the ketimide ligand is similar to a di(alkyl)amide ligand in terms
of electron donating ability. As with the amide ligand, the
ketimide ligand is likely both a strong σ and π donor. However,
analysis of the electronic structure of [Li][U(NCtBuPh)6]
reveals that the strength of the σ- and π-interactions between
the ligand 2p and the uranium 5f orbitals is similar to those
exhibited by [UX6]− (X = F, Cl, Br). This similarity suggests
that uranium 5f participation in the bonding of [UX6]−-type
complexes does not vary widely among the complexes
examined here and that the seemingly strong uranium−
ketimide interaction (as evidenced by the low U(V)/U(VI)
redox potential, for example) may be due to the participation of
the uranium 6d orbitals. For future work we will attempt to
quantify the level of 6d participation in the bonding of this class
of materials.
from Oh symmetry was modeled by changing the B2
0
parameter, which is zero in Oh symmetry, and fitting the
optical spectrum by allowing the B4 and B6 parameters to
3
3
vary, which splits the Γ8′ state. The best agreement with the
EPR data was found for B2 set equal to 2400 cm−1; the
0
experimental and calculated observables are given in Table 2,
Table 2. Experimental and Calculated Observables for 4
Determined with B2 = 2400 cm−1, B4 = 11230 cm−1, and
0
3
B6 = 4460 cm−1
3
observable
exptl
2.132
calcd
2.123
g∥
g⊥
0.746
0.796
Γ7′
Γ8′
Γ8′
Γ6
6080 cm−1
9843 cm−1
10850 cm−1
12195 cm−1
6290 cm−1
9740 cm−1
10560 cm−1
12520 cm−1
and the combined crystal field parameters are given in Table S5
of the Supporting Information. The experimental and
calculated values agree to within 350 cm−1 (1 kcal·mol−1),
which permits a reasonable estimate of the strengths of the σ
and π interactions between the uranium 5f-orbitals and the
ketimide ligands. The magnetic moment as a function of
temperature may be calculated from the crystal field parameters
(Figure S34) and is in reasonable agreement with experiment.
EXPERIMENTAL SECTION
■
General. All reactions and subsequent manipulations were
performed under anaerobic and anhydrous conditions either under a
high vacuum or an atmosphere of argon or nitrogen. Diethyl ether,
hexanes, toluene, and THF were dried using a Vacuum Atmospheres
DRI-SOLV Solvent Purification system. DME was distilled from
sodium benzophenone ketyl. All deuterated solvents were purchased
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Table 3. Crystal Field Splitting Data for a Series of U(V) Complexes
complex
θ (cm−1
)
Δ (cm−1
4479
)
ζ (cm−1
)
2p-5f σ (kcal mol−1
)
2p-5f π (kcal mol−1
)
a
[UF6]−
6882
1885
1931
1925
1800
12
7
5
[UCl6]−
3371
2375
2963
2935
3
[UBr6]−
5
3
b
b
,
c
d
[U(NCtBuPh)6]−
5100(990)
1920(1456)
8(2)
4(3)
a
b
c
Data for [UX6]− (X = F, Cl, Br) from ref 31. The value in parentheses is the standard deviation from the crystal field model. The value appears
d
−
−
low because 4 has half the π-interactions of UX6 . The value is doubled due to the fact that 4 has half the π-interactions of UX6 .
from Cambridge Isotope Laboratories Inc. and were dried over
activated 4 Å molecular sieves for 24 h prior to use. UCl4,81 Li(N
CtBu2), and Li(NCtBuPh) were synthesized according to published
procedures.44,82,83 All other reagents were obtained from commercial
sources and used as received.
eigenvectors obtained with the parameters given in Table 2 for the
best fit to the optical spectrum and the EPR g values.87
[Li(THF)2][U(NCtBuPh)5] (1). To a stirring solution of UCl4
(0.2709 g, 0.71 mmol) in THF (3 mL) was added dropwise a solution
of Li(NCtBuPh) (0.602 g, 3.61 mmol) in THF (3 mL). The
reaction mixture immediately turned dark brown. After stirring for 15
min, the solvent was removed in vacuo to afford a dark brown powder.
The solid was dissolved in hexanes, and a white precipitate was
removed by filtration of the solution through a Celite column (2 cm ×
0.5 cm) supported on glass wool. The filtrate was concentrated in
vacuo and stored at −25 °C for 24 h to afford dark brown crystals.
NMR spectra were recorded on a Varian UNITY INOVA 500
spectrometer or a Bruker Avance III Ultashield Plus 800 spectrometer.
1H and 13C{1H} NMR spectra are referenced to external SiMe4 using
the residual protio solvent peaks as internal standards (1H NMR
experiments) or the characteristic resonances of the solvent nuclei
7
(13C NMR experiments). Li{1H} NMR spectra are referenced to an
1
0.6009 g, 71% yield. H NMR (500 MHz, 25 °C, C6D6): δ −5.01 (s,
external saturated solution of LiCl in deuterium oxide. Elemental
analyses were performed at the Micro-Mass Facility at the University
of California, Berkeley. UV−vis/NIR spectra were recorded on a UV-
3600 Shimadzu spectrophotometer. IR spectra were recorded on a
Mattson Genesis FTIR or a Thermo Scientific Nicolet 6700 FT-IR
spectrometer.
t
45H, Bu), 0.28 (s, 8H, β-THF), 1.35 (s, 8H, α-THF), 7.43 (s, 10H,
7
m-CH), 7.73 (s, 5H, p-CH), 12.33 (s, 10H, o-CH). Li{1H} NMR
(194 MHz, 25 °C, C6D6): δ 14.52 (s). Anal. Calcd for
C63H86LiN5O2U: C, 63.57, H, 7.28, N, 5.88. Found: C, 63.19, H,
7.06, N, 5.91. IR (KBr pellet, cm−1): 1632(s, br), 1583(sh), 1487(sh),
1477(m), 1471(m), 1443(m), 1392(m), 1358(m), 1311(w),
1254(sh), 1217(m), 1192(s), 1161(sh), 1072(m), 1057(m),
1054(m), 1018(w), 991(w), 945(s), 903(s), 872(sh), 858(w),
822(w), 775(s), 733(sh), 704(s), 688(sh), 623(m), 609(sh),
561(m), 515(w), 484(sh), 444(m). UV−vis/NIR (DME, 4.9 mM,
25 °C): 736 (sh, ε = 311.0 L·mol−1·cm−1), 850 (sh, ε = 121.1
L·mol−1·cm−1), 986 (ε = 121.6 L·mol−1·cm−1), 1156 (ε = 221.5
L·mol−1·cm−1), 1284 (ε = 153.2 L·mol−1·cm−1), 1416 (ε = 109.5
L·mol−1·cm−1), 1562 (ε = 75.5 L·mol−1·cm−1).
Cyclic Voltammetry Measurements. CV experiments were
performed using a CH Instruments 600c Potentiostat, and the data
were processed using CHI software (version 6.29). All experiments
were performed in a glovebox using a 20 mL glass vial as the cell. The
working electrode consisted of a platinum disk embedded in glass (2
mm diameter), and both the working and reference electrodes
consisted of a platinum wire. Solutions employed during CV studies
were typically 1 mM in the uranium complex and 0.1 M in
[Bu4N][PF6]. All potentials are reported versus the [Cp2Fe]0/+ couple.
Magnetism Measurements. Magnetism data were recorded
using a Quantum Design MPMS 5XL SQUID magnetometer. All
experiments were performed between 4 and 300 K using 50−100 mg
of powdered, crystalline solid. Samples were loaded into NMR tubes,
which were subsequently flame-sealed. The solid was kept in place
with approximately 80 mg of quartz wool packed on either side of the
sample. The data was corrected for the contribution of the NMR tube
holder and the quartz wool. The experiment for 1 was performed using
a 1 T field. Diamagnetic corrections (χdia = −7.29 × 10−4 cm3·mol−1
for 1, χdia = −7.33 × 10−4 cm3·mol−1 for 4) were made using Pascal’s
constants.84 The magnetization of 4 was corrected for the presence of
a ferromagnetic impurity using data taken at 0.5, 1, and 3 T in
combination with eq 3, where Msample(T) and Mmeasured(T) are the
magnetization of the sample and measured magnetization, respectively,
and Mferro is the magnetization of the ferromagnetic impurity, which is
field and temperature independent.
[Li(THF)][U(NCtBu2)5] (2). To a stirring solution of UCl4 (0.221
g, 0.58 mmol) in THF (3 mL) was added dropwise a suspension of
Li(NCtBu2) (0.433 g, 2.94 mmol) in THF (3 mL). The reaction
mixture immediately turned dark brown. The reaction mixture was
allowed to stir at room temperature for 1 h, after which the solvent was
removed in vacuo, affording a dark brown solid. The solid was
dissolved in hexanes, and the solution was filtered through a Celite
column (2 cm × 0.5 cm) supported on glass wool. The filtrate was
concentrated in vacuo and stored at −25 °C for 24 h, resulting in the
deposition of dark-brown crystals. 0.3965 g, 67% yield. 1H NMR (500
MHz, 25 °C, C6D6): δ −1.47 (s, 90H, tBu), 1.56 (s, 4H, β-THF), 4.24
(s, 4H, α-THF). 7Li{1H} NMR (194 MHz, 25 °C, C6D6): δ 89.38 (s).
Anal. Calcd for C49H98LiN5OU: C, 57.80; H, 9.70; N, 6.88. Found: C,
57.11; H, 9.61; N, 7.28. IR (KBr pellet, cm−1): 1616(s, br), 1505(sh),
1481(s), 1465(sh), 1448(sh), 1384(s), 1360(s), 1335(w), 1304(w),
1214(s), 1037(s), 955(s), 928(sh), 882(w), 839(w), 808(w), 806(w),
724(w), 682(s), 674(sh), 555(s), 491(w), 484(w), 435(m), 428(sh).
UV−vis/NIR (DME, 4.9 mM, 25 °C): 890 (sh, ε = 116.7
L·mol−1·cm−1), 1012 (ε = 93.1 L·mol−1·cm−1), 1158 (ε = 111.1
L·mol−1·cm−1), 1248 (ε = 107.3 L·mol−1·cm−1), 1378 (ε = 63.6
L·mol−1·cm−1), 1592 (sh, ε = 104.3 L·mol−1·cm−1).
M
(T) = M
sample
(T) − M
measured ferro
(3)
EPR Spectroscopy. EPR spectra were obtained at 2 K with a
Varian E-12 spectrometer equipped with a flowing liquid He cryostat,
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). The spectrum was fit using a version of the code
ABVG modified to use a pseudo-Voigt line shape and modified to fit
spectra using the downhill simplex method.
Crystal Field Calculations. The Hamiltonian and matrix elements
for an f1 configuration in Oh symmetry have been given many times
(see, for example, ref 85). The conversion of the Oh crystal field
U(NCtBu2)5 (3). To a cold (−25 °C), stirring solution of 2
(0.297 g, 0.29 mmol) in Et2O (4 mL) was added dropwise a solution
of I2 (0.037 g, 0.15 mmol) in Et2O (2 mL). The reaction mixture was
stirred for 5 min, after which the solvent was removed in vacuo to
afford a dark brown solid. Dissolution of the solid in hexanes was
followed by removal of a white precipitate by filtration of the solution
through a Celite column (2 cm × 0.5 cm) supported on glass wool.
The solution was stored at −25 °C for 24 h, resulting in the formation
of dark red brown crystals. 0.1726 g, 63% yield. X-ray quality crystals
1
parameters to C3v symmetry is described by Gorller-Walrand and
̈
were grown from a dilute solution of 3 in toluene. H NMR (500
Binnemans.86 The calculated magnetic susceptibility was determined
from the Van Vleck susceptibility equation using the energies and
MHz, 25 °C, C6D6): δ 1.55 (br s, 90H, Bu). Anal. Calcd for
t
C45H90N5U: C, 57.54; H, 9.66; N, 7.46. Found: C, 57.12; H, 9.47; N,
4937
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Table 4. X-ray Crystallographic Data for Complexes 1−4 and 5·C7H8
1
2
3
4
5·C7H8
C73H92N6U
empirical formula
crystal habit, color
crystal size (mm)
crystal system
space group
volume (Å3)
a (Å)
C63H86LiN5O2U
block, brown
0.30 × 0.20 × 0.15
triclinic
C49H98LiN5OU
block, brown
0.1 × 0.05 × 0.05
monoclinic
P21/c
C45H90N5U
C66H84LiN6U
brick, dark red
0.40 × 0.25 × 0.25
hexagonal
plate, brown
0.20 × 0.10 × 0.02
monoclinic
P21/c
brick, dark green
0.45 × 0.35 × 0.2
monoclinic
P21/n
P1
R3
̅
̅
2961.8(5)
13.2886(13)
13.9258(14)
18.2372(17)
76.405(5)
77.740(5)
65.596(4)
2
5242.0(3)
13.8905(4)
14.6036(4)
25.8603(7)
90
9702(2)
20.967 (3)
19.715(3)
23.470(3)
90
4635.7(4)
12.1214(8)
12.1214(8)
36.432(3)
90
3248.6(6)
14.2791(16)
11.9428(14)
19.064(2)
90
b (Å)
c (Å)
α (deg)
β (deg)
92.1840(10)
90
90.215 (8)
90
90
92.216(2)
90
γ (deg)
120
Z
4
8
3
2
formula weight (g/mol)
density (calcd) (mg/m3)
1190.34
1.335
1018.29
1.290
939.25
1206.36
1.296
1291.56
1.320
1.286
absorption coeff (mm−1
)
2.785
3.133
3.378
2.668
2.544
F000
1220
2120
3896
1851
1328
total no. reflections
unique reflections
28902
47640
65831
9511
27029
14614
15852
19821
1779
6565
final R indices [I > 2σ(I)]
R1 = 0.0452, wR2 =
R1 = 0.0293, wR2 =
R1 = 0.0410, wR2 =
R1 = 0.0270, wR2 =
R1 = 0.0272, wR2 =
0.1081
0.0771
0.0922
0.0524
0.0626
largest diff. peak and hole (e− 5.918 and −3.774
1.473 and −1.369
1.862 and −1.149
0.670 and −0.486
0.872 and −0.753
Å−3
)
GOF
1.116
0.979
1.058
1.071
0.980
1
7.27. IR (KBr pellet, cm−1): 1763(w), 1689(w), 1610(s, br), 1481(s),
1470(m), 1446(sh), 1385(m), 1358(s), 1335(w), 1267(w), 1213(s),
1038(m), 957(s), 939(m), 904(sh), 891(w), 841(w), 796(w), 791(w),
739(w), 685(s), 550(s), 434(s). UV−vis/NIR (DME, 4.3 mM, 25
°C): 904 (sh, ε = 231.8 L·mol−1·cm−1), 1022 (ε = 146.3
L·mol−1·cm−1), 1272 (ε = 135.1 L·mol−1·cm−1), 1604 (ε = 204.0
L·mol−1·cm−1).
crystals of 5 were grown from a dilute solution in toluene. H NMR
(500 MHz, 25 °C, C6D6): δ 1.46 (s, 54H, CH3), 7.11 (m, 6H, p-CH),
7.35 (m, 24H, o-CH and m-CH). 13C{1H} NMR (201 MHz, 25 °C,
C6D6): δ 180.40 (s, CN), 164.93 (s, ipso-C), 128.38 (s, o-CH),
126.55 (s, m-CH), 125.76 (s, p-CH), 72.81 (s, C(CH3)3), 27.90 (s,
C(CH3)3). Anal. Calcd for C66H84N6U: C, 66.09, H, 7.06, N, 7.01.
Found: C, 65.34, H, 7.06, N 6.87. IR (KBr pellet, cm−1): 1622(s, br),
1579(sh), 1564(sh), 1556(m), 1532(sh), 1496(sh), 1487(m), 1474(s),
1468(s), 1440(s), 1389(m), 1359(m), 1257(m), 1215(s), 1197(s),
1182(sh), 1164(w), 1072(m), 1027(m), 1001(w), 942(s), 904(s),
847(w), 837(w), 774(s), 737(m), 704(s), 671(sh), 619(s), 560(s),
467(w), 444(s).
[Li][U(NCtBuPh)6] (4). To a cold (−25 °C), stirring solution of
1 (0.1345 g, 0.11 mmol) in Et2O (8 mL) was added dropwise a
solution of I2 (0.015 g, 0.06 mmol) in Et2O (2 mL). The reaction
mixture was stirred for 5 min, whereupon a suspension of Li(N
CtBuPh) (0.019 mg, 0.11 mmol) in Et2O (8 mL) was added dropwise.
The reaction mixture was then stirred for an additional 5 min, after
which the solvent was removed in vacuo, affording a dark red solid.
Dissolution of the solid in toluene (5 mL) was followed by removal of
a gray precipitate by filtration through a Celite column (2 cm × 0.5
cm) supported on glass wool. The solution was concentrated in vacuo
and stored at −25 °C for 24 h, resulting in the formation of dark red
crystals. 0.0921 g, 68% yield. Crystals of 4 suitable for X-ray analysis
X-ray Crystallography. Data for 4 and 5·C7H8 were collected on
a Bruker 3-axis platform diffractometer equipped with a SMART-1000
CCD detector using a graphite monochromator with a Mo Kα X-ray
source (α = 0.71073 Å). A hemisphere of data was collected using ω
scans with 0.3° frame widths. Frame exposures of 20 s were used for
both complexes 4 and 5·C7H8. Data for 1, 2, and 3 were collected on a
Bruker KAPPA APEX II diffractometer equipped with an APEX II
CCD detector using a TRIUMPH monochromator with a Mo Kα X-
ray source (α = 0.71073 Å). The crystals of 1, 2, and 3 were mounted
on a cryoloop under Paratone-N oil, and all data were collected at
100(2) K using an Oxford nitrogen gas cryostream system. Frame
exposures of 10 and 20 s were used for 2 and 3, respectively, while
frame exposures of 10 and 15 s were used for 1. Data collection and
cell parameter determination were conducted using the SMART
program.88 Integration of the data frames and final cell parameter
refinement were performed using SAINT software.89 Absorption
correction of the data for 1, 2, and 3 was carried out using the
multiscan method SADABS,90 while the absorption correction of the
data for 4 and 5 was carried out empirically on the basis of reflection
ψ-scans. Subsequent calculations were carried out using SHELXTL.91
Structure determination was done using direct or Patterson methods
and difference Fourier techniques. All hydrogen atom positions were
idealized, and rode on the atom of attachment with exceptions noted
in the subsequent paragraph. Structure solution, refinement, graphics,
and creation of publication materials were performed using
SHELXTL.91 A summary of relevant crystallographic data is presented
in Table 4.
1
were grown from a dilute hexanes solution. H NMR (500 MHz, 25
°C, C6D6): δ 0.35 (s, 54H, tBu), 7.57 (s, 12H, m-CH), 7.66 (s, 6H, p-
CH), 7.98 (s, 12H, o-CH). 7Li{1H} NMR (194 MHz, 25 °C, C6D6): δ
32.46 (s). Anal. Calcd C66H84LiN6U: C, 65.71, H, 7.02, N, 6.97.
Found: C, 65.34, H, 6.96, N 6.85. IR (KBr pellet, cm−1): 1664(sh),
1642(sh), 1627(s), 1600(sh), 1581(sh), 1475(m), 1464(m), 1455(m),
1391(m), 1361(m), 1263(w), 1217(w), 1195(s), 1111(w, br),
1073(w), 1030(m), 944(s), 907(s), 772(s), 706(s), 623(w), 564(m),
442(m). UV−vis/NIR (DME, 3.8 mM, 25 °C): 814 (sh, ε = 264
L·mol−1·cm−1), 930 (sh, ε = 135 L·mol−1·cm−1), 1018 (ε = 116
L·mol−1·cm−1), 1300 (ε = 13 L·mol−1·cm−1), 1644 (ε = 32
L·mol−1·cm−1).
U(NCtBuPh)6 (5). To a cold (−25 °C), stirring solution of 4
(0.098 g, 0.08 mmol) in Et2O (3 mL) was added dropwise a solution
of I2 (0.011 g, 0.04 mmol) in Et2O (1 mL). The solution was stirred
for 5 min, and the solvent was removed in vacuo, affording a dark
green solid. The solid was dissolved in toluene (3 mL), and a white
precipitate was removed by filtration of the solution through a Celite
column (2 cm × 0.5 cm) supported on glass wool. The solution was
concentrated in vacuo and cooled to −25 °C for 24 h, resulting in the
deposition of dark green crystals. 0.0585 g, 56% yield. X-ray quality
4938
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For complex 1, a tert-butyl group was found to be disordered
between two positions about the tertiary carbon, in a 50:50 ratio. In
complex 2, two tert-butyl groups were disordered about their tertiary
carbons, each in a 44:56 ratio. Hydrogen atom positions were not
assigned to disordered carbon atoms. For complex 4, the single Li
cation was disordered over two sites in a 50:50 ratio, while for complex
5, a toluene molecule was disordered over two sites in a 50:50 ratio.
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I.; Reilly, S. D.; Scott, B. L.; Wu, G. Chem. Commun. 2011, 47, 7647−
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Haverkort, M. W.; Sawatzky, G. A.; Conradson, S. D.; Clark, D. L.;
Kozimor, S. A.; Boland, K. S. Phys. Rev. B 2010, 81, 193104.
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Darovsky, A.; Reynolds, P. A. Inorg. Chem. 1998, 37, 4559−4566.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, crystallographic details (as CIF files),
magnetic susceptibility plots, tabulated cyclic voltammetry data,
and spectral data for 1−5. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
(20) Denecke, M. A.; Rossberg, A.; Panak, P. J.; Weigl, M.;
Schimmelpfennig, B.; Geist, A. Inorg. Chem. 2005, 44, 8418−8425.
(21) Zhurov, V. V.; Zhurova, E. A.; Pinkerton, A. A. Inorg. Chem.
2011, 50, 6330−6333.
Corresponding Author
hayton@chem.ucsb.edu; wwlukens@lbl.gov; nmedelstein@lbl.
gov
(22) Minasian, S. G.; Krinsky, J. L.; Arnold, J. Chem.Eur. J. 2011,
Notes
17, 12234−12245.
The authors declare no competing financial interest.
(23) Liddle, S. T.; McMaster, J.; Mills, D. P.; Blake, A. J.; Jones, C.;
Woodul, W. D. Angew. Chem., Int. Ed. 2009, 48, 1077−1080.
(24) Brennan, J. G.; Stults, S. D.; Andersen, R. A.; Zalkin, A.
Organometallics 1988, 7, 1329−1334.
ACKNOWLEDGMENTS
■
Research at UCSB was supported by the U.S. Department of
Energy, Office of Basic Energy Sciences, Chemical Sciences,
Biosciences, and Geosciences Division under Contract No. DE-
FG02-09ER16067, and by the National Science Foundation
under Award No. 1059097. Portions of this work were
supported by U.S. Department of Energy, Basic Energy
Sciences, Chemical Sciences, Biosciences, and Geosciences
Division and were performed at Lawrence Berkeley National
Laboratory under Contract No. DE-AC02-05CH11231. We
also thank Skye Fortier and Richard Lewis for assistance with
the SQUID magnetization measurements.
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