Investigations of the electronic structure of arene-bridged diuranium complexes

Article abstract of DOI:10.1021/om3010367

The electronic structure of the arene-bridged complex (μ-toluene)U ₂(N[^tBu]Ar)₄ (1a₂-μ-toluene, Ar = 3,5-C₆H₃Me₂) has been studied in relation to a variety of mononuclear uranium amide complexes, and their properties have been discussed comparatively. The syntheses, molecular structures (X-ray crystal structures and solution behavior based on variable-temperature NMR spectroscopic data), and corresponding spectroscopic (X-ray absorption near-edge structure and UV-vis-near-IR absorption) and magnetic properties are presented and interpreted with reference to results of density functional theory (DFT) and complete active space self-consistent field with corrections from second-order perturbation theory (CASSCF/CASPT2) calculations performed on model compounds. While the mononuclear compounds display expected electronic and magnetic properties for uranium complexes, 1a₂-μ-toluene shows complicated properties in contrast. XANES spectroscopy, X-ray crystallography, and both density functional and CASSCF/CASPT2 results are consistent with the following electronic structure interpretation: f orbitals host the unpaired electrons, followed energetically by two δ bonds formed by filled uranium f orbitals and LUMOs of toluene.

Full text of DOI:10.1021/om3010367

Article
pubs.acs.org/Organometallics
Investigations of the Electronic Structure of Arene-Bridged
Diuranium Complexes
Bess Vlaisavljevich,† Paula L. Diaconescu,*,‡ Wayne L. Lukens, Jr.,*,§ Laura Gagliardi,*,†
and Christopher C. Cummins*,∥
†Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota, Minneapolis,
Minnesota 55455, United States
‡Department of Chemistry & Biochemistry, University of California, Los Angeles, California 90095, United States
§Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
∥Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S
* Supporting Information
ABSTRACT: The electronic structure of the arene-bridged
complex (μ-toluene)U2(N[tBu]Ar)4 (1a2-μ-toluene, Ar = 3,5-
C6H3Me2) has been studied in relation to a variety of
mononuclear uranium amide complexes, and their properties
have been discussed comparatively. The syntheses, molecular
structures (X-ray crystal structures and solution behavior based
on variable-temperature NMR spectroscopic data), and
corresponding spectroscopic (X-ray absorption near-edge
structure and UV−vis−near-IR absorption) and magnetic
properties are presented and interpreted with reference to results of density functional theory (DFT) and complete active space
self-consistent field with corrections from second-order perturbation theory (CASSCF/CASPT2) calculations performed on
model compounds. While the mononuclear compounds display expected electronic and magnetic properties for uranium
complexes, 1a2-μ-toluene shows complicated properties in contrast. XANES spectroscopy, X-ray crystallography, and both
density functional and CASSCF/CASPT2 results are consistent with the following electronic structure interpretation: f orbitals
host the unpaired electrons, followed energetically by two δ bonds formed by filled uranium f orbitals and LUMOs of toluene.
the mononuclear complexes (THF)U(N[Ad]Ar)3 (2b-THF,
Ad = 1-adamantyl, THF = tetrahydrofuran), IU(N[tBu]Ar)3
(2a-I), IU(N[Ad]Ar)3 (2b-I), and (Me3SiN)U(N[Ad]Ar)3
(2b-NSiMe3), such that a range of uranium formal oxidation
states is surveyed. In order to understand the properties of a
unique compound such as 1a2-μ-toluene, the rest of the series is
based on classical uranium amide compounds, for which there
is no ambiguity about the oxidation state. The syntheses,
molecular structures (X-ray crystal structures and solution
behavior based on variable-temperature NMR spectroscopic
data), and corresponding spectroscopic (X-ray absorption near-
edge structure and UV−vis−near-IR absorption) and magnetic
properties are discussed and interpreted with reference to
results of computational studies performed on model
compounds; X-ray absorption near-edge structure (XANES)
spectroscopic characterization of arene-bridged diuranium
complexes has not been reported previously.
INTRODUCTION
■
Arene-bridged complexes constitute a general bonding motif
for organouranium compounds featuring benzene/toluene,1−9
naphthalene,10 biphenyl,11 cycloheptatrienyl,12,13 or cyclo-
octatetraene10 as the bridging arene ligand. In most cases, as
well as in actinocene complexes, δ bonding14 between f orbitals
of uranium and ligand LUMOs (lowest unoccupied molecular
orbitals) of the appropriate symmetry is considered to play a
major role.1,2,5 Like π bonding in transition-metal chemistry, δ
bonding may be key to understanding uranium complexes;
however, the electronic structures of arene-bridged diuranium
complexes have rarely been investigated beyond the usual
techniques.2,5
Amide ligands have become ubiquitous in transition-metal
chemistry,15−17 and they also proved successful in supporting
interesting actinide complexes.6,18−30 Their versatility is largely
based on the tunability of electronic and steric properties that
takes advantage of the ability to modify the two substituents of
the nitrogen donor.15,31 These properties allowed the isolation
and characterization of arene-bridged diuranium complexes in
which the arene is either toluene or benzene.1
Special Issue: Recent Advances in Organo-f-Element Chemistry
The focus of this report is to investigate the electronic
structure of the arene-bridged complex (μ-toluene)U2(N[tBu]-
Ar)4 (1a2-μ-toluene, Ar = 3,5-C6H3Me2) by comparison with
Received: November 1, 2012
Published: February 20, 2013
© 2013 American Chemical Society
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RESULTS AND DISCUSSION
■
Syntheses and X-ray Crystal Structures. The synthesis
of compound 1a2-μ-toluene and the X-ray crystal structure of
the related (μ-toluene)U2(N[Ad]Ar)4 (1b2-μ-toluene) as well
as reactivity studies have been reported previously.1 Scheme 1
Scheme 1. Syntheses of Uranium Complexes Discussed in
the Text
Figure 1. Structural drawing of 2b-THF with thermal ellipsoids at the
35% probability level and hydrogen atoms omitted for clarity. Selected
distances (Å): U−N(av), 2.346(9); U−O, 2.489(5).
describes the syntheses of all the complexes discussed here.
32,33
UI3(THF)4
is a versatile starting material and can be
employed to obtain tris(amido)uranium iodide complexes 2a-I
and 2b-I or can be used to generate 1a2-μ-toluene and 2b-THF
directly. In general, compounds based on the N-tert-butylanilide
ligand are more lipophilic and, in practice, are less crystalline
than compounds based on the N-adamantylanilide ligand.
Reduction of the tris(amido)uranium iodide compounds
affords either arene-bridged diuranium complexes, when the
arene is used as a solvent,1 or uranium tris(amide) complexes
with a molecule of THF coordinated to the uranium center,
when THF is used as a solvent.21 Finally, 2b-NSiMe3 is
obtained from the reaction of Me3SiN3 with 2b-THF, which
can be generated in situ or isolated prior to the reaction (eq 1).
Figure 2. Structural drawing of 2b-I with thermal ellipsoids at the 35%
probability level and hydrogen atoms omitted for clarity. Selected
distances (Å): U−N(av), 2.204(9); U−I, 3.0682(4).
ca. 0.1 Å longer than the corresponding distances in 2b-I
(Figure 2) and 2b-NSiMe3 (Figure 3), at 2.204(9) and
2.245(7) Å, respectively. The trend observed here is in good
agreement with the results of XANES experiments (see below),
which show that the effective charges on the uranium center are
similar for 1a2-μ-toluene and uranium(III) compounds, on one
hand, and for 2b-I and 2b-NSiMe3, on the other hand.
The structure of 2b-NSiMe3 (Figure 3) distinguishes itself by
the U−Nimide distance of 1.943(4) Å, which is ca. 0.3 Å shorter
than the average U−Namide distance (2.245(7) Å) in the same
compound. Short U−Nimide distances and angles close to 180°
at Nimide (170.1(13)° in 2b-NSiMe3) have been associated with
The formulation of these compounds was verified by X-ray
crystallography (Figures 1−3). A metrical parameter present
throughout the entire series is the distance U−Namide(av). This
distance of 2.334(13) Å in 1b2-μ-toluene is slightly shorter than
the 2.346(9) Å found for 2b-THF (Figure 1); both values are
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of the arene and empty metal d orbitals dictates such behavior;
hence, a less electron rich arene, such as benzene, will exchange
for a more electron rich substrate, such as toluene or
mesitylene. For the uranium complexes, the exchange studies
reflect the existence of a back-bond from filled uranium f
orbitals to LUMOs of the arene and a less electron rich ligand is
preferred to a more electron rich analogue.
1a2-μ-toluene is dinuclear in the solid state. In order to test
whether in solution there is an equilibrium between the
dinuclear structure and some mononuclear species (either two
uranium bis(amide) fragments or a uranium bis(amide)
fragment and a coordinated benzene uranium bis(amide)
fragment), the variable-temperature (VT) behavior of 1a2-μ-
toluene was examined between −70 and 120 °C (the
compound is stable at these temperatures during the time of
the experiment). Figure 4 presents the plot of δ versus 1/T for
Figure 3. Structural drawing of 2b-NSiMe3 with thermal ellipsoids at
the 35% probability level and hydrogen atoms omitted for clarity.
Selected distances (Å): U−Namide(av), 2.245(7); U−Nimide, 1.943(4).
multiple-bond character between the uranium center and the
imide nitrogen atom,34 in accordance with our findings from
CASSCF calculations on a model compound (see Computa-
tional Results).
The structure of 1b2-μ-toluene features an average distance
of 2.594(30) Å between uranium and the carbon atoms of the
bridging toluene molecule that is similar to those in other
toluene- or benzene-bridged diuranium complexes.2,5 These
values are among the shortest such distances registered for
carbon atoms of arenes coordinated to uranium centers. For
example, the average U−C distance is 2.647(10) Å in
uranocene, U(η8-C8H8)2,35 and 2.807(18) Å in U(η5-
C5H5)4,36 while in benzene complexes such as U(η6-C6Me6)-
Figure 4. Plot of δ versus 1/T for 1a2-μ-toluene.
all of the protons of the molecule. Although the graphs for the
protons belonging to the bridging toluene are slightly curved,
those for the amide protons are linear, indicating Curie−Weiss
behavior.42 This finding suggests that the dinuclear compound
is the only species in solution detectable by NMR spectroscopy.
Electronic Spectra. Electronic spectra of uranium com-
plexes are usually complicated due to the splitting by ligand-
field and spin−orbit coupling of a multitude of states derived
from fn configurations. The electronic spectra are comprised of
f → f, f → d, and charge-transfer bands. Usually, bands are
assigned on the basis of the magnitude of the molar absorption
coefficient and the position of a band in a spectrum.33,43−45
Therefore, for the compounds discussed here, bands present in
the UV region (200−400 nm), due to their high intensity (ε ≈
105 M−1 cm−1), can be assigned to π → π* transitions of the
arene rings (see the Supporting Information, Figures SX5 and
SX6). Additionally, intense absorption bands present in the
visible region (400−800 nm) that have ε ≈ 103 M−1 cm−1 could
be either f → d or charge-transfer transitions (Figures SX5 and
SX6).
The most interesting region of the electronic spectra for
uranium compounds is the near-IR region (spectra reported
here were recorded from 1500 to 800 nm, Figure 5), because
most compounds show “fingerprint” features. These character-
istics are assigned to Laporte-forbidden f → f transitions and
have molar absorption coefficients in the range 10−102 M−1
cm−1. With the exception of the U(III) compound 2b-THF (ε
= 100−260 M−1 cm−1) the other mononuclear compounds
37
(BH4)3 and U(η6-C6H5Me)(AlCl4)3,38 the average U−C
distance is significantly longer: 2.93(2) and 2.94(1) Å,
respectively. The average C−C distance of 1.438(13) Å for
the bridging toluene in 1b2-μ-toluene is ca. 0.04 Å longer than
the corresponding distances in free toluene.39 In complexes of
toluene such as K(18-crown-6)(toluene),40 C−C distances
average 1.398(21) Å. The longer C−C distances for 1b2-μ-
toluene in comparison to those found in free toluene or toluene
radical anion are consistent with a substantial covalent overlap
between filled uranium f orbitals and LUMOs of the bridging
toluene, as found from DFT calculations on model
compounds.1 Furthermore, high-level CASSCF calculations
support this conclusion (see Computational Results).
NMR Spectroscopy Studies on 1a2-μ-toluene: Probing
Stability and Fluxionality. 1H NMR analysis showed that the
bridging toluene in 1a2-μ-toluene exchanges slowly with C6D6
(5% exchange in 24 h) at room temperature. Since 1a2-μ-
toluene is thermally stable,1 arene exchange was also studied at
higher temperatures. Preliminary results show that the bridged
toluene exchanges with C6D6 faster than the bridged benzene in
(μ-benzene)U2(N[tBu]Ar)4 (1a2-μ-benzene)1 exchanges with
toluene-d8. Furthermore, the bridging toluene did not exchange
with p-xylene over a 24 h period. These results are similar to
those reported by Evans et al.2 and show an opposite trend
from that found for transition-metal complexes.41 For
transition-metal complexes, the bond between the HOMOs
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Figure 5. Near-IR spectra at 25 °C of 2b-THF in THF (top left), 2b-I in toluene (top right), 2b-NSiMe3 in toluene (bottom left), and 1a2-μ-toluene
in toluene (bottom right).
Figure 6. Plots of 1/χ (left) and μeff (right) versus T for 1a2-μ-toluene.
show weak bands in the near-IR region (ε < 100 M−1 cm−1). As
observed for other toluene or benzene-bridged diuranium
systems,2,5 1a2-μ-toluene has intense bands (ε = 200−600 M−1
cm−1) in this region. The increased intensity of the f−f
transitions in uranium complexes has previously been attributed
to intensity stealing,46 which is an increase in intensity in
formally forbidden transitions due to the presence of significant
covalent bonding.47 The observation of intense f−f bands for
only 1a2-μ-toluene suggests that the bonding between the
uranium centers and the bridging toluene ligand is significantly
covalent. Previous reports used the similarity between the near-
IR spectra of toluene or benzene-bridged diuranium systems
and uranium(III) complexes as an indication that the electronic
structure of the diuranium compounds is consistent with the
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Figure 7. Plots of 1/χ versus T for 2a-I (left) and 2b-I (right).
Figure 8. Plots of 1/χ versus T for 2b-THF (left) and 2b-NSiMe3 (right).
presence of uranium(III) metal centers, in agreement with our
XANES results (see below).
The magnetic behavior of the mononuclear complexes is as
expected and follows some general trends. For example, at low
temperatures, the graphs for 2a-I and 2b-I (Figure 7) show
temperature-independent paramagnetism (TIP, 5−25 K for 2a-
I and 5−15 K for 2b-I). TIP behavior is specific to even-
electron species, since at low temperatures the ground state can
be an orbital singlet.52 Kramers ions, i.e. odd-electron species,
such as the U(III) (2b-THF) and U(V) (2b-NSiMe3)
compounds (Figure 8), would never present a singlet ground
state; therefore, their low-temperature magnetic behavior is
different from that of U(IV) compounds. The determined μeff
values for 2b-THF (3.20 μB), 2b-I (3.52 μB, 20−300 K), and
2a-I (3.18 μB, 50−300 K) are within the range for uranium(III)
and uranium(IV) complexes. The magnetic moment obtained
for 2b-NSiMe3 is 1.81 μB, smaller than the other magnetic
moments of the mononuclear compounds but consistent with
values for similar uranium(V) compounds.53
X-ray Absorption Near-Edge Structure (XANES) Spec-
troscopy Results. The chemical shift of the absorption edge
reflects the effective charge of the absorbing atom.54,55 For
uranium complexes, the U L3 absorption edge, which
corresponds to a 2p3/2 to 6d5/2 transition, has been shown to
vary systematically with the uranium oxidation state.56,57
Therefore, the chemical shifts of the U L3 absorption edge
are affected by changes in shielding of the 2p3/2 electrons and
Solid-State Magnetic Susceptibility Measurements.
Although magnetic properties of uranium compounds are
usually difficult to interpret,48,49 SQUID measurements were
carried out on the whole series of complexes considered here in
order to compare the behavior of the mononuclear compounds
to that of 1a2-μ-toluene. It is notable that the magnetic moment
for 1a2-μ-toluene (Figure 6) is temperature dependent (from
0.25 μB at 5 K to 1.50 μB at 300 K; values for one uranium
center), while the mononuclear compounds present Curie−
Weiss behavior in the 5−300 K temperature range (except for
the TIP intervals for 2a-I and 2b-I, Figure 7). In addition, for
1a2-μ-toluene (Figure 6), although paramagnetic behavior is
observed over the temperature intervals 5−50 and 170−300 K,
as the temperature is lowered to around 125 K, the magnetic
susceptibility of the sample passes through a maximum and
begins to decrease at lower temperatures. Between 95 and 125
K the minimum values in the 1/χ versus T graph are
characteristic of a transition to antiferromagnetic behavior.5,50,51
This overall behavior is in contrast to that found for the
analogous bridging-toluene complex [(U(BIPMTMSH)(I))2(μ-
η6:η6-C6H5CH3)] (BIPMTMS = C(PPh2NSiMe3)2), which did
not show strong antiferromagnetic coupling.5
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can be used to compare the effective charges on the uranium
centers in different complexes. Although effective charges can
be correlated to the oxidation state of the absorbing atom, other
factors, including coordination geometry and the degree of
covalency in the ligand−metal interactions, may also be
significant. For a series of formally trivalent organometallic
and inorganic uranium compounds, the average chemical shift
of the absorption edge relative to a UO2Cl2 sample is −6.0(5)
eV, and for a series of formally tetravalent uranium complexes,
the average shift is −3.1(6) eV, as shown in Table 1 and Figure
9. Both sets of compounds include different coordination
geometries and ligands with widely varying electronegativities.
a
Table 1. U L3 Chemical Shifts of Uranium Complexes
edge shift versus
0.1 M UO2Cl2 (eV)
compound
1a2-μ-toluene
oxidation state
−5.1
−6.3
−6.4
−5.0
−6.3
−5.7
−5.9
−6.4
−5.6
−3.3
−3.8
−2.0
−2.4
−2.9
−3.5
−3.6
−2.6
−2.4
−3.7
−3.6
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
4
U[N(SiMe3)2]3
Figure 9. U L3 absorption edges of selected organouranium
complexes. The edge height is normalized such that the absorption
at the edge step is equal to 1. The edge energies are referenced to the
half-height of a 0.1 M UO2Cl2 in 1 M HCl solution set at 17163 eV.
The compounds are (a) U[N(SiMe3)2]3, (b) [Cp′′2UCl]2,58 (c)
58
[Cp‡ UOH]2
2
58
[Cp‡ UF]2
2
58
[Cp‡ UCl]2
2
58
[Cp‡ UBr]2
2
58
[Cp‡ UOH]2,58 (d) (μ-C7H8)[U(N[tBu]Ar)2]2 (1a2-μ-toluene), (e)
[Cp′′2UF]2
[Cp′′2UCl]2
[Cp′′2UBr]2
2
IU(N[tBu]Ar)3 (2a-I), (f) IU(DME)(NC[tBu]Mes)3,10 and (g)
(Me3SiN)U(N[Ad]Ar)3 (2b-NSiMe3). The point closest to the half-
height is circled. The average edge shifts of U(III) and U(IV)
complexes are indicated by the vertical lines.
58
58
10
IU(DME)(NC[tBu]Mes)3
2a-I
58
[Cp‡ UO]2
2
58
Cp‡ UF2
2
58
Cp‡ UCl2
2
58
Cp‡ UBr2
2
58
Cp‡ UI2
2
58
Cp′′2UF2
Cp′′2UCl2
Cp′′2UBr2
Cp′′2UI2
58
58
58
a
Abbreviations: Cp‡ = 1,3-(Me3C)2C5H3, Cp′′ = 1,3-(Me3Si)2C5H3,
DME = 1,2-dimethoxyethane.
In the family of complexes (μ-arene)[U(N[R]Ar)2]2, the
bonding lies somewhere between two extremes: (1) a neutral
arene ligand coordinated by two U(II) centers and (2) an arene
tetraanion with two U(IV) centers (Figure 10). A series of the
U L3 absorption edges of selected uranium complexes is shown
in Figure 9. Since the chemical shift of 1a2-μ-toluene is −5.1
eV, the effective charge of the U center in the complex is mostly
similar to that of U(III) complexes. A description of the
bonding more detailed than that presented here requires
information about the overlap between the ligand and metal
orbitals,59 as well as their relative energies. Nonetheless, the
observed chemical shift of the U L3 edge is consistent with a
strong, covalent interaction between the arene πu* orbitals and
the δu orbitals of the two uranium centers. In particular, the
observed chemical shift of 1a2-μ-toluene is consistent with its
formulation as two U(III) centers bridged by a toluene(2−)
ligand, where the f electrons on the U(III) center are stabilized
by back-bonding with the bridging ligand.
Figure 10. Bonding scenarios in arene-bridged diuranium complexes.
The relative energies of the 5f and πu orbitals affect the XANES edge
shift.
frontier orbitals for both the metal and the arene ligand. The
differences consist of the oxidation state of the metal and the
energy of arene orbitals. As pointed out recently,63 tuning of
both contributors influences greatly the extent of covalency in
actinide complexes.
Computational Results. DFT Calculations. Geometries
were optimized for the DFT ground state for model systems
based on the 2b-THF, 2b-I, 2b-NSiMe3, and 1a2-μ-toluene
structures. 2b-THF and 2b-I are U(III) and U(IV) compounds
and have quartet and triplet ground states, respectively. In both
cases, the experimental and calculated structures are in very
It is important to note that the bonding picture described
here shares characteristics with those of actinocene complexes
An(C8H8)2 (An = actinide), which have been intensely
researched and investigated.60−62 The similarities between the
two classes of complexes stem from the analogous symmetry of
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Table 2. Average Distances (Å) and Angles (deg) for Calculated and Experimentally Obtained Structures
av distance (Å)
angle (deg)
U−N
U−X
X−U−N
U−Y−Z
2b-THF (X = O)
exptl
B-97D
PBE
2.35
2.30
2.32
2.20
2.23
2.23
2.25
2.27
2.28
2.33
2.33
2.33
2.49
2.50
2.41
3.07
3.07
3.07
1.94
1.94
1.95
2.59
2.62
2.60
98.89/108.2/116.0
92.3/115.6/116.3
96.0/107.8/121.9
94.3/113.3/126.2
92.2/113.0/123.2
91.7/113.9/125. 8
103.2/103.7
2b-I (X = I)
exptl
B-97D
PBE
2b-NSiMe3 (X, Y = Nimide; Z = Si)
1a2-μ-toluene (X = N; Y = U; Z = N)
exptl
B-97D
PBE
170.3
102.4/103.2
168.8
171.2
106.0/108.6
exptl
B-97D
PBE
103.2/103.7
126.4/127.0/129.8/129.9
124.7/126.2/128.2/130.6
126.4/127.0/128.3/129.5
104.2/105.3
103.8/104.6
Figure 11. δ bonding natural orbitals66 from 1a2-μ-toluene. From left to right, the natural orbital occupation numbers are 1.86 and 1.87. Legend: U,
light blue; N, blue; C, gray; H, white.
good agreement, as shown in Table 2 and Table S1
(Supporting Information). Both doublet and quartet spin
states were explored for 2b-NSiMe3. The doublet is 33.3 kcal/
mol lower in energy at the B-97D level of theory than for the
quartet. Moreover, the doublet geometry is in good agreement
with experimental parameters, whereas the quartet has an
average U−Namide distance of 2.40 Å, as opposed to the 2.24 Å
observed experimentally. Finally, 1a2-μ-toluene was optimized
for the quintet state. While the singlet and triplet spin states
should be considered, these states suffer from spin contam-
ination at the DFT level, and therefore the energetics are
unreliable. For this reason, only the quintet state was
considered with DFT, while all of the spin states were studied
at the CASSCF/CASPT2 level of theory (see below). Average
distances and angles are in good agreement with experimental
parameters (Table 2). The dihedral angles are expected to
deviate more than the other parameters due to the ligand
truncation; however, the calculated values are within 10° of
experiment (Table S1), with the exception of the B-97D
Nimide−U−N−Ctert‑butyl dihedral angle in 2b-NSiMe3, which
deviates by 15°.
CASSCF/CASPT2 Results. The electronic structure was
further explored by performing complete active space self-
consistent field calculations with corrections from second-order
perturbation theory (CASSCF/CASPT2) on the optimized
geometries. In CASSCF, a set of orbitals in the valence region is
defined, together with the electrons associated with these
orbitals. Within this orbital space, which is referred to as the
active space, the electronic configurations that can be obtained
by distributing the electrons in the active orbitals in all possible
ways are considered.64 The total wave function is constructed
as a linear combination of all these electronic configurations.
The orbitals lower in energy than the active space are doubly
occupied, while those that are higher in energy are unoccupied.
When possible, all valence orbitals should be included in the
active space; however, in practice this is not always required.
For the higher oxidation states of uranium, it is common
practice to include only the seven U 5f orbitals in the active
space, since the 6d and 7s orbitals are higher in energy and
consequently unoccupied.65 For 2b-THF and 2b-I, the ligands
do not engage in strongly covalent interactions with the U
center, and as a result only the 5f orbitals need to be included in
the active space with either three or two electrons, respectively
(see Figures S8 and S9 in the Supporting Information). The
ground state of 2b-THF is a quartet, while the ground state of
2b-I is a triplet, as expected for U(III) and U(IV) compounds,
respectively.
Alternatively for 2b-NSiMe3, including only the seven U 5f
orbitals in the active space would not properly describe the
covalent bonding between the imide nitrogen and uranium. For
this reason, the three N 2p orbitals must be included in the
active space as well, resulting in an active space of 7 electrons in
10 orbitals. However, in practice this space was too small, as
CASSCF and CASPT2 predicted different ground states. We
found that including 6 additional doubly occupied orbitals
containing contributions from the U 6p, U 5d, and N 2s
orbitals in the active space was very important in obtaining
consistency between CASSCF and CASPT2 energies. With this
larger space of 19 electrons in 15 orbitals, the doublet was the
ground state at the CASSCF and CASPT2 levels by 51.5 and
18.7 kcal mol−1, respectively. Furthermore, the bond between
U and Nimide is a double bond, as shown in Figure S10
(Supporting Information).
For 1a2-μ-toluene, the active space consists of 8 electrons in
14 orbitals. One can think of this as including the 7 U 5f
orbitals on each center and the corresponding electrons;
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Figure 12. Singly occupied natural orbitals of 1a2-μ-toluene. Legend: U, light blue; N, blue; C, gray; H, white.
however, the resulting molecular orbitals corresponding to U−
arene−U bonding contain contributions from both 5f and 6d
orbitals. The singlet, triplet, quintet, and septet spin states were
explored. The ground state at the CASPT2 level is the singlet;
however, the triplet and quintet are only 0.7 and 2.5 kcal mol−1
higher in energy, respectively. Spin−orbit effects were not
included in these calculations. Additionally, the orbital pictures
are the same for all three states. Two sets of δ bonds composed
of occupied uranium 5f orbitals donating into π antibonding
orbitals on the toluene group are present along with four singly
occupied 5f orbitals (Figures 11 and 12). The quintet is high
spin and consists of one dominating configuration contributing
87% to the total wave function. Alternatively, the singlet and
triplet are much more multireference in nature and the total
wave function contains contributions from several electronic
configurations (Table 3). Finally, the septet state was explored
and is 34.5 kcal mol−1 higher in energy than the singlet.
Table 4. LoProp Charges for the Ground State CASSCF
Wave Function
2b-THF
2b-I
2b-NSiMe3
1a2-μ-toluene
U
N
2.20
2.76
3.04
2.33/2.34
−0.54
−0.52
−0.84
−0.77
−0.80
−0.84
−0.86
−0.85
−0.79
−0.85
−0.83
−0.54
−0.54
other
−0.55 (O) −0.75 (I) −1.29 (Nimide
)
−1.95 (toluene (sum))
a doublet, corresponding to a U(V) center. The partial charge
on uranium in 2b-NSiMe3 is consistent with this assignment, as
it is higher than in 2b-I. Finally, the partial charges on the U
centers in 1a2-μ-toluene are consistent with a +3 oxidation
state. Additionally, the sum of the charges on the bridging
toluene group is −1.95, indicating that charge transfer occurs
from the uranium centers to the bridging toluene ligand, which
is consistent with both the observed reactivity (1a2 coordinated
preferentially the least electron rich bridging arene) and the
chemical shift observed by XANES spectroscopy.
Table 3. Electronic Configurations Contributing to the Total
Wave Function in 1a2-μ-toluene
Consistent with a +5 oxidation state assignment, the reaction
of 2b-NSiMe3 with lithium under argon or with KC8 in THF
resulted in the formation of [Li(OEt2)][(Me3SiN)U(N[Ad]-
Ar)3] (Li[2b-NSiMe3]; Figure 13) or K[(Me3SiN)U(N[Ad]-
spin state
configuration
% of the total wave function
quintet
triplet
δ2δ25f15f15f15f1
δ2δ25f15f25f15f0
δ2δ25f25f15f05f1
δ2δ25f05f15f25f1
δ2δ25f15f05f15f2
δ2δ25f25f25f05f0
δ2δ25f25f05f25f0
δ2δ25f15f15f15f1
δ2δ25f05f25f05f2
δ2δ25f05f05f25f2
87.1
17.1
21.2
20.6
17.4
14.2
13.9
14.3
13.7
13.5
Ar)3] (K[2b-NSiMe3]; eq 2). The longer distances U−Namide
=
singlet
LoProp Charges. Atomic charges were computed from the
CASSCF results using the LoProp approach (Table 4). LoProp
charges are reported, since this procedure is stable with respect
to the basis set and provides physically meaningful localized
properties.67 First, by comparison of the uranium partial
charges it is observed that, as expected, the partial charge of the
U(III) compound 2b-THF is less than that of the U(IV)
compound 2b-I. Additionally, the ground state of 2b-NSiMe3 is
2.357(5) Å (av) and U−N
= 2.050(3) Å as compared to
SiMe3
the corresponding values in 2b-NSiMe3 are indicative of a more
electron rich uranium center.
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Varian XL-300 and Varian INOVA-501 spectrometers at room
temperature unless specified otherwise. Chemical shifts are reported
with respect to internal or external solvent: 7.16 ppm (C6D6). UV−vis
spectra were recorded on a HP spectrophotometer from 200 to 1100
nm using matched 1 cm quartz cells, and near-IR spectra were
recorded on a PS spectrophotometer from 800 to 1500 nm using
matched 1 cm quartz cells; all spectra were obtained using a solvent
reference blank. Numerical modeling of all data was done using the
program Origin 6.0. CHN analyses were performed by H. Kolbe
Mikroanalytisches Laboratorium (Mulheim an der Ruhr, Germany).
̈
Synthesis of KN[Ad]Ar. A 500 mL round-bottom flask was
charged with HN[Ad]Ar (8.52 g, 33 mmol) and diethyl ether (300
mL), and the solution was frozen. Solid KCH2C6H5 (4.35 g, 33 mmol)
was added to the thawing solution, and the reaction mixture was
warmed to room temperature and stirred for 2−3 h. Filtration of the
resulting solution afforded a white solid that was washed with pentane
and dried under reduced pressure. The KN[Ad]Ar obtained amounted
to 7.93 g (27 mmol, 82% yield). 1H NMR (300 MHz, C6D6, 22 °C): δ
6.28 (s, 1H, p-Ar); 6.22 (s, 2H, o-Ar); 2.23 (s, 6H, Ar-Me); 1.95 (s, 3H,
Ad-CH); 1.83 (s, 6H, Ad-CH2); 1.52 (s, 6H, Ad-CH2).
Synthesis of (THF)U(N[Ad]Ar)3 (2b-THF). A 100 mL round-
bottom flask was charged with UI3(THF)4 (0.526 g, 0.56 mmol),
KN[Ad]Ar (0.510 g, 1.74 mmol, 3 equiv), and a stirring bar and then
placed in the cold well. Thawing THF (50 mL) was added to the solid
mixture as quickly as possible and the reaction mixture stirred for 45
min. Filtration of the reaction mixture through Celite afforded a
solution from which the solvent was removed. The solid obtained was
collected on a frit and washed with small portions of diethyl ether (2 ×
15 mL). The solid obtained on the frit was dried and redissolved in
diethyl ether, and the solution was concentrated and placed in a freezer
at −35 °C. After several days, the solution was decanted and the black
microcrystalline solid (2b-THF; 0.304 g, 0.29 mmol, 51% yield) dried
Figure 13. Structural drawing of [Li(OEt2)][(Me3SiN)U(N[Ad]Ar)3]
(Li[2b-NSiMe3]) with thermal ellipsoids at the 35% probability level
and hydrogen atoms omitted for clarity. Selected distances (Å): U−
Nligand(av), 2.357(5); U−Nimide, 2.050(3).
CONCLUSIONS
■
The compound 1a2-μ-toluene was studied in relation to a
variety of mononuclear uranium amide complexes, and its
properties were discussed with reference to their properties.
While the mononuclear compounds display the expected
electronic and magnetic properties, 1a2-μ-toluene showed
complicated characteristics in contrast. The optical and
magnetic properties of 1a2-μ-toluene are difficult to relate to
reported examples of mononuclear uranium organometallic
complexes. XANES spectroscopy, X-ray crystallography, and
computational studies corroborate the following electronic
structure interpretation: the f orbitals of the two uranium
centers host the four unpaired electrons, followed energetically
by two covalent δ bonds formed by filled uranium f orbitals
overlapping with the LUMOs of toluene, in accord with our
original analysis.1 An effective electronic charge of the metal
centers was determined by XANES and is comparable to values
encountered for classical uranium(III) compounds; these
results are consistent with the presence of a covalent bond
between uranium and toluene that is reflected in the metrical
parameters of 1b2-μ-toluene, as determined by X-ray
crystallography.
1
under reduced pressure. H NMR (300 MHz, C6D6, 22 °C): δ 37.55
(s, 4H, THF-CH2); 0.65 (bs, 9H, o- and p-Ar); 0.23 (s, 9H, Ad-CH);
0.13 (d, 18H, Ad-CH2); −0.13 (d, 18H, Ad-CH2); −6.64 (s, 18H, Ar-
Me); −15.80 (s, 4H, THF-CH2). Anal. Calcd for C58H83N3OU: C,
64.72; H, 7.77; N, 3.90. Found: C, 64.79; H, 7.93; N, 3.72.
Synthesis of (Me3SiN)U(N[Ad]Ar)3 (2b-NSiMe3). (a). From 2b-
THF. Solutions in THF of 2b-THF (1.169 g, 1.14 mmol, 80 mL) and
Me3SiN3 (0.144 g, 1.25 mmol, 1.1 equiv, 20 mL) were frozen. To the
thawing solution of 2b-THF was added dropwise a solution of
Me3SiN3, and the reaction mixture was warmed to room temperature
and stirred for 1 h. After the reaction was finished, the solvent was
removed under reduced pressure, the obtained solid was dissolved in
pentane, and the new solution was concentrated and stored at −35 °C
for several days. 2b-NSiMe3 was obtained as a black, crystalline solid in
two crops amounting to 0.586 g (0.56 mmol, 49% yield).
(b). From UI3(THF)4 Directly. UI3(THF)4 (1.193 g, 1.31 mmol) and
KN[Ad]Ar (1.156 g, 3.94 mmol, 3 equiv) were mixed as solids in a
250 mL round-bottom flask and placed in the cold well. To the stirred
mixture was added thawing THF (100 mL). The reaction mixture was
warmed to room temperature and stirred for a total of 35 min, after
which it was filtered through Celite and and the resulting solution
frozen again. To this thawing solution was added dropwise a thawing
solution of Me3SiN3 (0.136 g, 1.18 mmol, 0.9 equiv) in THF (20 mL).
After the addition was finished, the removal of solvent was started
immediately. The obtained solid was extracted with pentane, and the
solution was filtered through Celite. The solvent was removed, and the
last two operations were repeated. The new solution was concentrated
to ca. 20 mL and placed in a −35 °C freezer. 2b-NSiMe3 was obtained
as a black, crystalline solid in two crops amounting to 0.521 g (0.50
EXPERIMENTAL SECTION
■
General Considerations. Unless stated otherwise, all operations
were performed in a Vacuum Atmospheres drybox under purified
nitrogen or using Schlenk techniques under an argon atmosphere.
Anhydrous diethyl ether was purchased from Mallinckrodt; n-pentane,
n-hexane, and tetrahydrofuran (THF) were purchased from EM
Science. Diethyl ether, toluene, benzene, n-pentane, and n-hexane were
dried and deoxygenated by the method of Grubbs.68 THF was distilled
under nitrogen from purple sodium benzophenone ketyl and was
transferred under nitrogen into glass vessels before being pumped into
the drybox. C6D6 was purchased from Cambridge Isotopes and was
degassed and dried over 4 Å sieves. The 4 Å sieves, alumina, and Celite
were dried under reduced pressure overnight at a temperature just
above 200 °C. UI3(THF)4,33 KC8,69 KCH2C6H5,70 HN[Ad]Ar,71 and
compounds 1a2-μ-toluene,1 1b2-μ-toluene,1 2a-I,1 2b-I21 were
prepared according to literature methods. Me3SiN3 was passed
through alumina and stored in a refrigerator at −35 °C. Other
1
mmol, 38% yield). H NMR (300 MHz, C6D6, 22 °C): δ 11.95 (bs,
2H, o-Ar); 8.28 (s, 3H, Si-CH3); 6.24 (s, 1H, p-Ar); 2.03 (s, 6H, Ar-
Me); 1.09 (s, 3H, Ad-CH); −0.06 (d of d, 6H, Ad-CH2); −5.59 (bs,
6H, Ad-CH2). Anal. Calcd for C57H81N4SiU: C, 62.90; H, 7.50; N,
5.15. Found: C, 62.80; H, 7.52; N, 5.11.
Synthesis of [Li(OEt2)][(Me3SiN)U(N[Ad]Ar)3]. Small cubes of
lithium (two to three) were washed with hexanes and transferred
under argon to a round-bottom flask charged with a magnetic stirring
bar. To this flask was transferred via cannula a THF solution (25 mL)
1
chemicals were used as received. H NMR spectra were recorded on
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of 2b-NSiMe3 (0.699 g, 0.67 mmol) prepared in the glovebox. The
reaction mixture was stirred for 2 h at room temperature, after which
the solvent was removed under reduced pressure. The flask was taken
into the box, the solid obtained was extracted with pentane, and the
new solution was filtered through Celite. After the solvent was
removed from the filtrate, extraction with pentane and filtration were
repeated and the new solution was concentrated and placed in a −35
°C freezer. [Li(OEt2)][(Me3SiN)U(N[Ad]Ar)3] was obtained as
orange crystals in two crops amounting to 0.502 g (0.43 mmol, 64%
Radiation Lightsource (SSRL) at beamline 11-2 or 4-1 using a
Si(220) double-crystal monochromator detuned 50% to reduce the
higher order harmonic content of the beam. X-ray absorption spectra
were obtained in the transmission mode at room temperature using
argon-filled ionization chambers. The data analysis was performed by
standard procedures using the EXAFSPAK suite of programs
developed by G. George of the SSRL. The background was removed
by fitting a polynomial to the pre-edge data. Edge shifts are determined
from the half-height of the U L3 absorption edge at 17166 eV and are
referenced to the half-height of a 0.1 M solution of UO2Cl2 in
hydrochloric acid.
Magnetic Susceptibility Measurements. Magnetic susceptibil-
ity measurements were recorded using a SQUID magnetometer at
5000 G. The samples were prepared in the glovebox (50−100 mg),
loaded in a gelatin capsule that was positioned inside a plastic straw,
and carried to the magnetometer in a tube under N2. The sample was
quickly inserted into the instrument and centered, and data were
obtained from 5 to 300 K. The contribution from the sample holders
was not accounted for. The diamagnetic contributions were calculated
and subtracted from χmol. Effective magnetic moments were calculated
1
yield). H NMR (300 MHz, C6D6, 22 °C): δ 11.49 (bs, 6H, Et2O-
CH3); 10.61 (bs, 6H, o-Ar); 9.12 (s, 9H, Si-CH3); 8.10 (s, 3H, o-Ar);
7.13 (d, 4H, Et2O-CH2); 2.06 (s, 18H, Ar-Me); −1.99 (d of d, 18H,
Ad-CH2); −3.34 (s, 9H, Ad-CH); −15.16 (bs, 18H, Ad-CH2). Anal.
Calcd for C61H91N4SiOLiU: C, 62.59; H, 7.78; N, 4.79. Found: C,
62.60; H, 8.35; N, 4.71.
Synthesis of K[(Me3SiN)U(N[Ad]Ar)3]. A thawing slurry of KC8
(0.188 g, 1.39 mmol, 2.8 equiv) in THF (15 mL) was added dropwise
to a thawing THF solution (20 mL) of 2b-NSiMe3 (0.518 g, 0.50
mmol). The reaction mixture was warmed to room temperature and
stirred for 1.5 h, after which the solvent was removed under reduced
pressure. The solid obtained was extracted with diethyl ether and the
new solution filtered through Celite. After the solvent was removed
from the filtrate, the extraction with diethyl ether and the filtration
were repeated and the new solution was concentrated and placed in a
−35 °C freezer. K[2b-NSiMe3] was obtained as dark orange crystals in
either by linear regression from plots of 1/χmol versus T (K) for
1/2
Curie−Weiss behavior or by using the formula 2.828(Tχmol
)
for
non-Curie−Weiss behavior. Samples used were recrystallized multiple
times. Measurements for the same compound were carried out on
differently recrystallized samples.
1
Computational Details. Geometry optimizations were performed
with DFT using the Perdew−Burke−Ernzerhof (PBE)73 exchange-
correlation functional and the dispersion corrected B-97D functional74
with def-TZVP basis sets75 for all atoms as implemented in the
TURBOMOLE 5.10.2 package.76 The corresponding def-ECP77 was
used for U, and the resolution of the identity (RI) approximation was
used for the Coulomb integrals.64,78 All stationary points were
confirmed as minima by vibrational analysis. Due to the large size of
the ligands, the 1-adamantyl groups were replaced with tert-butyl
groups and the 3,5-C6H3Me2 groups were replaced with phenyl
groups.
one crop amounting to 0.415 g (0.38 mmol, 77% yield). H NMR
(300 MHz, C6D6, 22 °C): δ 20.67 (s, 3H, Si-CH3); 4.75 (s, 1H, p-Ar);
1.99 (s, 3H, Ad-CH); 0.82 (d of d, 6H, Ad-CH2); 0.09 (s, 6H, Ar-Me);
−1.05 (bs, 2H, o-Ar); −2.58 (bs, 6H, Ad-CH2).
Thermal Stability of 1a2-μ-toluene. In one experiment, variable-
temperature 1H NMR studies in octane-d18 showed that 1a2-μ-toluene
is stable up to 110 °C. The spectra were acquired from 20 to 110 °C at
10 °C intervals, and after reaching 110 °C, another spectrum was
obtained on the same sample back to 20 °C. In another experiment, 40
mg of 1a2-μ-toluene in 20 mL of heptane was heated at 80 °C for 24 h.
1
A H NMR spectrum of a sample taken from that solution indicated
Subsequently, the electronic structure was further investigated using
complete active space self-consistent field theory (CASSCF)79 with
second-order perturbation theory (CASPT2)80,81 on top of the B97-D
geometry using the Molcas 7.7 package.82 Relativistic effects were
included through the use of the scalar Douglas−Kroll−Hess (DKH)
Hamiltonian.83,84 ANO-RCC basis sets of triple-ζ quality were used
for U, O, and N, while a minimal basis set was used for peripheral C
and H atoms. In 1a2-μ-toluene, the C and H atoms of toluene were
treated with the ANO-RCC basis set of double-ζ quality.85,86
Additionally, the Cholesky decomposition technique was used
combined with local exchange screening to reduce the computational
costs involved in generating the two-electron integrals signifi-
cantly.87−90 Atomic charges were computed at the CASSCF level for
the ground state using the LoProp procedure.67
that the compound did not decompose.
Arene Exchange Experiments. 1a2-μ-toluene was dissolved in
C6D6, the solution was transferred to an NMR tube, and the NMR
tube was sealed and then placed in a heated oil bath. After 24 h at 90
°C, analysis of the 1H NMR spectrum indicated 3% exchange based on
the integration of the peaks at ca. −7 ppm (Me-Ar). Integration of the
same peak indicated 14% exchange after an additional 24 h at 100 °C
and 18% exchange after another 48 h. A similar experiment conducted
with 1a2-μ-benzene dissolved in toluene-d8 showed 6% exchange after
48 h at 100 °C and 11% exchange on the basis of the integration of the
t-Bu peaks (ca. 7 ppm) after a total of 96 h. For the exchange
experiment with p-xylene, 1a2-μ-toluene was dissolved in p-xylene, the
solution was transferred to a tube, and the tube was sealed and heated
in an oil bath at 90 °C. After 24 h, the tube was taken into the box and
broken and its contents were transferred to a vial. Volatiles were
1
removed. Analysis of the compound’s H NMR spectrum (C6D6, 300
ASSOCIATED CONTENT
■
MHz, 22 °C) indicated no transformation of 1a2-μ-toluene.
S
* Supporting Information
X-ray Crystal Structures. X-ray data collections were carried out
on a Siemens Platform three-circle diffractometer with a CCD detector
using Mo Kα radiation (λ = 0.71073 Å). The data were processed
utilizing the program SAINT supplied by Siemens Industrial
Automation, Inc. The structures were solved by direct methods
(SHELXTL v5.03 by G. M. Sheldrick and Siemens Industrial
Automation, Inc., 1995) in conjunction with standard difference
Fourier techniques.72
Text, tables, figures, and files giving details of the X-ray
crystallographic study, NMR and absorption spectra, coor-
dinates of DFT optimized geometries, and CASSCF orbitals.
This material is available free of charge via the Internet at
http://pubs.acs.org.
XANES Measurements. In an Ar-filled glovebox, approximately
10 mg of uranium complex was powdered and mixed with dry boron
nitride. The samples were packaged in aluminum holders with Kapton
tape. The samples were sealed in glass jars with Teflon tape inside the
drybox. Everything except for the Teflon tape was baked out at 110 °C
for several days. The samples showed no signs of decomposition either
before or immediately after the XANES experiment. After several
hours in the air, the samples began to discolor around the edges. X-ray
absorption spectra were acquired at the Stanford Synchrotron
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: pld@chem.ucla.edu (P.L.D.); wwlukens@lbl.gov,
(W.L.L.); gagliard@umn.edu (L.G.); ccummins@mit.edu
(C.C.C.).
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
For support of this work, we are grateful to the National
Science Foundation (CHE-9988806), the Packard Foundation
(Fellowship to C.C.C., 1995-2000), and the National Science
Board (Alan T. Waterman award to C.C.C., 1998). P.L.D.
thanks Daniel J. Mindiola for help with the structures of 2b-I
and 2b-THF, Dr. Daniel Grohol and Professor Daniel G.
Nocera for suggestions and discussions of the magnetic
properties, and Aetna Wun for help with recording the near-
IR spectra. Portions of this work were supported by the 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 and at the Stanford
Synchrotron Radiation Lightsource, a Directorate of the
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. Computa-
tional studies were funded by the DOE through Grant No. DE-
SC002183.
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