Synthesis, molecular and electronic structure of UV(O) [N(SiMe3)2]3

Article abstract of DOI:10.1021/ic201936j

Addition of 1 equiv of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) to U(NR₂)₃ in hexanes affords U(O)(NR₂) ₃ (2), which can be isolated in 73% yield. Complex 2 is a rare example of a terminal U(V) oxo complex. In contrast, addition of 1 equiv of Me₃NO to U(NR₂)₃ (R = SiMe₃) in pentane generates the U(IV) bridging oxo [(NR₂)₃U] ₂(μ-O) (3) in moderate yields. Also formed in this reaction, in low yield, is the U(IV) iodide complex U(I)(NR₂)₃ (4). The iodide ligand in 4 likely originates from residual NaI, present in the U(NR₂)₃ starting material. Complex 4 can be generated rationally by addition of 0.5 equiv of I₂ to a hexane solution of U(NR₂)₃, where it can be isolated in moderate yield as a tan crystalline solid. The solid-state molecular structures and magnetic susceptibilities of 2, 3, and 4 have been measured. In addition, the electronic structures of 2 and 3 have been investigated by density functional theory (DFT) methods.

Full text of DOI:10.1021/ic201936j

Article
pubs.acs.org/IC
Synthesis, Molecular and Electronic Structure of UV(O)[N(SiMe3)2]3
Skye Fortier,† Jessie L. Brown,† Nikolas Kaltsoyannis,*,‡ Guang Wu,† and Trevor W. Hayton*,†
†Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States
‡Department of Chemistry, Christopher Ingold Laboratories, University College London, 20 Gordon Street, London WC1H 0AJ,
United Kingdom
S
* Supporting Information
ABSTRACT: Addition of 1 equiv of 2,2,6,6-tetramethylpiperidine-
1-oxyl (TEMPO) to U(NR2)3 in hexanes affords U(O)(NR2)3 (2),
which can be isolated in 73% yield. Complex 2 is a rare example of
a terminal U(V) oxo complex. In contrast, addition of 1 equiv of
Me3NO to U(NR2)3 (R = SiMe3) in pentane generates the U(IV)
bridging oxo [(NR2)3U]2(μ-O) (3) in moderate yields. Also formed
in this reaction, in low yield, is the U(IV) iodide complex U(I)-
(NR2)3 (4). The iodide ligand in 4 likely originates from residual NaI, present in the U(NR2)3 starting material. Complex 4 can
be generated rationally by addition of 0.5 equiv of I2 to a hexane solution of U(NR2)3, where it can be isolated in moderate yield as
a tan crystalline solid. The solid-state molecular structures and magnetic susceptibilities of 2, 3, and 4 have been measured. In
addition, the electronic structures of 2 and 3 have been investigated by density functional theory (DFT) methods.
Chart 1
INTRODUCTION
■
While the trans-di(oxo) framework of the uranyl ion is ubiqui-
tous in uranium chemistry,1−3 terminal mono-oxo complexes of
uranium are surprisingly rare.4 This is probably due, in part, to
the potent nucleophilicity of the oxo ligands in these species,5−7
which necessitates the use of bulky ancillary ligands to prevent
the formation of bridging oxo groups.5,6,8−13 For example,
Cp′2U(bipy), where Cp′ is the extremely bulky 1,2,4-tBu3C5H2
cyclopentadienyl ligand, reacts with pyridine-N-oxide in Et2O
to provide the terminal oxo Cp′2U(O)(py) in good yield.5,14 Inter-
estingly, the oxo ligand in the base-free analogue, Cp′2U(O),
rapidly reacts with Me3SiCl to give Cp′2U(OSiMe3)(Cl),
confirming the nucleophilicity of this functional group.5,14 The
identity of the O-atom source is also important in determining
whether a terminal oxo ligand is formed upon atom transfer.
For example, the synthesis of a terminal oxo can be achieved by
addition of CO2 to a U(V) imido, ((tBuArO)3tacn)U(NMes).15
This results in the formation of mesityl isocyanate and a U(V)
terminal oxo complex via a [2 + 2] cycloaddition. In contrast, addi-
tion of an O-atom transfer reagent to the UIII parent complex
generates a U(IV) bridging oxo species, [((tBuArO)3tacn)U]2-
(μ-O)), and not the U(V) terminal oxo.16,17 Addition of N2O
to ((tBuArO)3mes)U also results in the formation of a U(IV)
bridging oxo, [((tBuArO)3mes)U]2(μ-O).7 Similarly, addition of
H2O to [U(tpa)2]I3, (tpa = tris[(2-pyridyl)methyl]amine),
results in the formation of a U(IV) bridged oxo cluster.18,19
Recently, we described the synthesis of a terminal U(V) oxo
complex [Ph3PCH3][U(O)(CH2SiMe2NSiMe3)(N{SiMe3}2)2]
(1) (Chart 1)20 by O-atom transfer from TEMPO (TEMPO =
2,2,6,6-tetramethylpiperidine-1-oxyl). While the O-atom transfer
ability of TEMPO has only been documented in a few in-
stances,21,22 this preliminary result suggests that the reaction of
TEMPO with other actinide complexes may be a fruitful arena
for the generation of new terminal oxos. Previously, Evans and
co-workers demonstrated that reaction of TEMPO with Cp*3Sm
results in ligand oxidation and formation of [Sm(TEMPO)3]2.23
However, no evidence for N−O bond cleavage was observed in
this example.
The isolation of complex 1,20 prompted us to investigate the
chemistry of the closely related U(V) oxo U(O)(NR2)3 (R =
SiMe3) (2). Despite being reported by Andersen in 1979,24 this
complex has received little attention, and both its chemistry and
solid-state structure have yet to be elaborated. This is surprising
given the paucity of uranium complexes with mono-oxo function-
alities and the current interest in molecular U(V) systems.4,25−31
Herein, we revisit the synthesis of U(O)(NR2)3 (R = SiMe3)
(2) and explore its electronic structure with Density Functional
Theory (DFT).
RESULTS AND DISCUSSION
■
Following the literature procedure for the synthesis of 2,24
1 equiv of Me3NO was added to U(NR2)3 (R = SiMe3) in
pentane, generating a dark brown-red solution. Consistent with
the previous report, light yellow crystals were isolated from the
reaction mixture. Surprisingly, however, an X-ray crystallographic
Received: September 2, 2011
Published: January 13, 2012
© 2012 American Chemical Society
1625
dx.doi.org/10.1021/ic201936j | Inorg. Chem. 2012, 51, 1625−1633

Inorganic Chemistry
Article
1
analysis of the material indicated that a U(IV) bridging oxide
[(NR2)3U]2(μ-O) (3) was generated, instead of the anticipated
U(V) terminal oxo complex. Complex 3 can also be synthe-
sized, in slightly better yield, via the treatment of U(NR2)3 with
0.5 equiv of Me3NO (Scheme 1). Interestingly, the oxidation of
The room temperature H NMR spectrum of 3 in C7D8
displays four broad resonances at −28.64, −16.79, −6.50, and
15.23 ppm, occurring in a 1:1:1:3 ratio, respectively. These
relative ratios can be explained by assuming that rotation along
the U−N bonds is restricted, affording two sets of SiMe3
groups in a 3:3 ratio: one group that points toward (endo) the
oxo bridge and one group that points away (exo) from the oxo
bridge. Additionally, slow rotation about the N−Siendo bonds,
because of the interdigitation of the methyl substituents, further
splits the endo SiMe3 group into three inequivalent methyl envi-
ronments, thereby accounting for the overall 1:1:1:3 ratio.
Consistent with this analysis, heating the solution to 45 °C
results in the coalescence of the peaks at −28.64, −16.79, −6.50
ppm into a very broad resonance at −14.05 ppm, as expected
upon faster rotation of N−Siendo bonds. Moreover, cooling the
solution to −55 °C produces six well-resolved resonances of
equal intensity, ranging from −43.23 to 80.61 ppm, assignable to
the three endo and three exo methyl environments and consistent
with slow rotation of both the N−Siendo and N−Siexo bonds.
The effective magnetic moment (μeff) for 3 is 1.92 μB per U4+
ion at 300 K (Figure 2), as determined by SQUID magneto-
Scheme 1
the related U(III) tris(amide), U(NN′3) (NN′3 = N(CH2CH2NR)3,
t
R = SiMe2 Bu), with 1 equiv of Me3NO also yields a U(IV)
bridging oxo complex, [(NN′3)U]2(μ-O).32
Complex 3 crystallizes in the monoclinic space group C2/c
with two independent molecules in the asymmetric unit. The
solid-state molecular structure of 3 reveals that each U(IV) center
possesses a pseudotetrahedral geometry comprised of a bridging
O2− group and three silylamide ligands (Figure 1). Additionally,
Figure 2. Temperature-dependent SQUID magnetization data for U(O)-
(NR2)3 (R = SiMe3) (2), [(NR2)3U]2(μ-O) (3), and U(I)(NR2)3 (4).
metry, which is comparable to the 1.82 μB reported for Andersen’s
oxo complex.24 Both values are well below the 3.54 μB expected
for a free U4+ ion,34,35 but are within the lower range established
for U(IV) complexes (e.g., μeff = 1.98 μB for [Li(THF)]2[U-
(OtBu)6]).36 Additionally, the IR spectrum of 3 (KBr pellet)
exhibits an absorption at 932 cm−1, matching a band originally
assigned to the terminal UO stretch of 2 at 930 cm−1. How-
ever, the parent U(III) tris(amide), U(NR2)3, also exhibits a
band at 930 cm−1 (see the Supporting Information) suggesting
this stretch is assignable to the absorptions of the silylamide
ligand, rather than a U−O stretch. Finally, the melting point for
3 was determined to be 155−157 °C, nearly identical to that
reported by Andersen for the putative terminal oxo (157−159 °C).
Overall, the similarity of our characterization data with that
reported by Andersen, and the similar appearance of 3 with
Andersen’s material (green-yellow prisms), suggests that the
material originally reported in 1979 may have been complex 3,
and not the terminal U(V) oxo species 2 as originally proposed.
To gain further insight into the electronic and geometric
structure of 3 we have performed gradient-corrected DFT
calculations. Geometry optimization of a quintet state in the
D3 point group yielded excellent agreement with experiment
(U−O = 2.164 Å calc, 2.145 Å exp (av.); U−N = 2.301 Å calc,
Figure 1. ORTEP diagram of [(NR2)3U]2(μ-O)·2.5C6D6 (R = SiMe3)
(3·2.5C6D6) represented with 50% probability ellipsoids. Hydrogen
atoms and solvent molecules omitted for clarity. Selected bond lengths
(Å) and angles (deg): U1−O1 = 2.142(6), U2−O1 = 2.147(6), U1−
N1 = 2.295(8), U1−N2 = 2.27(1), U1−N3 = 2.297(8), U2−N4 =
2.273(8), U2−N5 = 2.290(8), U2−N6 = 2.293(8), O1−U1−N1 =
115.1(3), O1−U1−N2 = 112.2(3), O1−U1−N3 = 112.1(3), O1−U2−
N4 = 113.5(3), O1−U2−N5 = 112.8(3), O1−U2−N6 = 113.8(3),
U1−O1−U2 = 179.2(4).
the complex exhibits a linear U−O−U bond angle (e.g., U1−
O1−U2 = 179.2(4)°) with U−O distances (e.g., U1−O1 =
2.142(6) Å, U1−O2 = 2.147(6) Å) similar to that found for the
related U(IV) bridging oxo [((tBuArO)3tacn)U]2(μ-O) (U−O =
2.1095(4) Å).33
1626
dx.doi.org/10.1021/ic201936j | Inorg. Chem. 2012, 51, 1625−1633

Inorganic Chemistry
Article
2.286 Å exp (av.); O−U−N = 113.0° calc, 113.3° exp (av.)), and
subsequent calculation of the vibrational frequencies revealed no
imaginary modes. The four unpaired electrons are almost entirely
metal-localized (>90% in all cases), with a uranium Mulliken spin
density of 2.21 per metal center. There are no calculated vibra-
tional frequencies between 880 cm−1 and 1214 cm−1; vibrations
at 853 cm−1 and 855 cm−1, predicted to yield very intense
infrared bands, are associated with modes contained within the
silylamide ligands, and are most likely the cause of the band
observed experimentally at 932 cm−1.
To better understand the formation of 3, the reaction of
1
U(NR2)3 with 0.5 equiv of Me3NO was followed by H NMR
spectroscopy in C6D6. Under these conditions several products
are formed during the reaction, and in addition to the
resonances of 3, resonances assignable to the U(IV) metalla-
cycle U(CH2SiMe2NR)(NR2)2 are also observed.37,38 No
evidence for the presence of 2 (vide infra), even in small
amounts, is observed in these spectra. This is true, regardless of
whether 0.5 equiv or 1 equiv of Me3NO is used in the reaction.
Interestingly, we have found that these samples often exhibit a
sharp singlet at −0.98 ppm in their 1H NMR spectra.
Moreover, the intensity of this resonance is highly dependent
on the batch of U(NR2)3 used in the synthesis of 3. We have
identified this resonance as corresponding to the U(IV) iodide
complex U(I)(NR2)3 (4). The iodide ligand in 4 likely
originates from NaI, which is present in the U(NR2)3 starting
material and can be difficult to completely remove by
recrystallization. In support of this hypothesis, treatment of
U(NR2)3 with 1 equiv of NaI, followed by 0.5 equiv of Me3NO,
generates 4 (Scheme 2) as the major product, as indicated by
Figure 3. ORTEP diagram of U(I)(NR2)3 (R = SiMe3) (4) with 50%
probability ellipsoids. Hydrogen atoms omitted for clarity. Selected
bond lengths (Å) and angles (deg): U1−I1 = 2.9512(8), U1−N1 =
2.238(4), I1−U1−N1 = 101.8(1), N1−U1−N1* = 115.96(8).
decreases to 2.16 μB at 4 K (Figure 2); fully consistent with
the U(IV) oxidation state assignment.7,35,40−42
The failure to generate 2 from U(NR2)3 and Me3NO com-
pelled us to investigate the use of other O-atom transfer re-
agents. As demonstrated by the synthesis of 1,20 TEMPO can
be an effective O-atom source for the actinides. Thus, addition
of 1 equiv of TEMPO to U(NR2)3 in hexane initially affords a
pale orange solution. Within seconds, however, this orange
solution converts to a dark red color. Crystallization from hexane
at −25 °C results in the deposition of 2 as red blocks in 73% yield
(Scheme 1). Notably, the appearance of 2 is substantially different
than that originally reported for this material.24 Complex 2 is also
formed if sublimed U(NR2)3 is used in place of recrystallized
U(NR2)3. The 1H NMR spectrum of 2 in C6D6 exhibits a single
resonance at −0.23 ppm. Interestingly, in solution at room
temperature, 2 slowly decomposes over 24 h, affording HNR2 and
the U(IV) metallacycle U(CH2SiMe2NR)(NR2)2 as the major
products (see Supporting Information, Figure S2). Preliminary
reactivity studies show that 2 readily conproportionates with
U(NR2)3 to produce 3 (Scheme 3). Furthermore, treatment of 2
with 1 equiv of Ph3PCH2 in C6D6 rapidly generates 1 via
deprotonation of a silylamide ligand (Scheme 3). Finally, the
melting point for 2 was determined to be 104−105 °C.
Scheme 2
analysis of the crude reaction mixture by 1H NMR spectroscopy
(see the Supporting Information). However, the formation of 3
is not completely suppressed under these conditions. More
importantly, complex 3 is also still formed if sublimed U(NR2)3
is used in place of material that was only recrystallized from
hexane, demonstrating that the presence of NaI is not required
for the formation of 3 (see the Supporting Information).
Interestingly, under these conditions very small amounts of 2
The synthesis of 2 is also accompanied with the formation of
1
tetramethylpiperidine (TMPH), as revealed by H NMR spec-
troscopy. Its presence can be explained by invoking formation
of the TMP· radical upon O-atom transfer, followed by abstrac-
tion of H· from the solvent.21 Not surprisingly, performing the
reaction in the presence of 9,10-dihydroanthracene affords the
coupled product 9,9′,10,10′-tetrahydro-9,9′-bianthracene, formed as
a result of H· abstraction by TMP· (see Supporting Information,
Figure S3).20,43
To account for the formation of complexes 2 and 3 we sug-
gest that the relative rates of uranium binding and N−O bond
cleavage between TEMPO and Me3NO are responsible for the
different reaction outcomes. Accordingly, during the reaction of
TEMPO with U(NR2)3, TEMPO coordination is rapid, quickly
consuming all the U(NR2)3 in solution. However, subsequent
N−O bond cleavage occurs at a slower rate, selectively gen-
erating U(NR2)3(O) as the only uranium-containing product.
This hypothesis is supported by the observation of a pale orange
1
are observed in the supernatant by H NMR spectroscopy.
Complex 4 can be rationally synthesized by addition of
0.5 equiv of I2 to a hexane solution of U(NR2)3 (Scheme 2).
Recrystallization from CH2Cl2 affords 4 as light tan crystals in
1
62% yield. Its room temperature H NMR spectrum in C6D6
exhibits a singlet at −0.98 ppm, identical to that observed in the
crude samples of 3. Analysis by X-ray crystallography (Figure 3)
reveals that complex 4 adopts a pseudotetrahedral geometry in the
solid-state (I1−U1−N1 = 101.8(1)°, N1−U1−N1* = 115.96(8)°),
similar to other four-coordinate silylamide complexes.39 Additionally,
4 exhibits an effective magnetic moment of 3.35 μB at 300 K, which
1627
dx.doi.org/10.1021/ic201936j | Inorg. Chem. 2012, 51, 1625−1633

Inorganic Chemistry
Article
Scheme 3
solution at very short reaction times, which could correspond
to the putative TEMPO adduct, UIV(NR2)3(TEMPO). In con-
trast, for the reaction of U(NR2)3 with Me3NO, the relative
rates are reversed. That is, Me3NO coordination is slow (in part
because of its poor solubility in pentane), whereas N−O bond
cleavage is fast. As a result, U(NR2)3(O) is generated in the
presence of unconsumed U(NR2)3, resulting in the formation
of complex 3 via conproportionation.
pseudotetrahedral geometries of complexes 3, 4, U(H)(NR2)3,45
and the group 4 tris(silylamide) complexes MCl(NR2)3 (M = Ti,
Zr, Hf).46 Moreover, the structures of the closely related U(V)
47
imido complex U(NR)(NR2)3 and the Nb(V) oxo
complex Nb(O)(NR2)3 are also pseudotetrahedral.48 In the
niobium example, the niobium atom is positioned 0.416 Å
above the plane defined by the nitrogen atoms. Taken together,
this structural data suggests that the tris(silylamide) framework
can easily adopt a pseudotetrahedral coordination environment.
As such, the trigonal pyramidal geometry of 2 is highly unusual,
suggesting that its structure may be imposed by electronic
effects that exceed both the electrostatic and the steric demands
of the coligands. Accordingly, we returned to DFT to probe
this, and other aspects, of 2.
The electronic properties of 2 have been assessed by SQUID
magnetometry. At 300 K, 2 exhibits an effective magnetic
moment of 1.59 μB which gradually decreases upon cooling to
0.94 μB at 4 K, a temperature response characteristic of U(V)
(Figure 2).15 The μeff of 2 at room temperature is significantly
lower than the theoretical U5+ free ion value (μeff = 2.54 μB)
and surprisingly much lower than values found for the closely
related U(V) oxo complexes 1 (μeff = 1.97 μB) and [U(O)-
The optimized geometry of 2 agrees very well with the
experimental structural data. The computed U−O and average
U−N distances are slightly longer than experiment, at 1.838 Å
and 2.267 Å, respectively and, pleasingly, calculation agrees with
experiment in finding a trigonal pyramidal geometry, with O−
U−N angles of 89.4°, 91.7° and 89.1°, and N−U−N angles of
118.3°, 123.7° and 118.3°.
t
(tacn(OArR)3)] (μeff = 1.98 μB, R = Bu; μeff = 1.92 μB, R =
Ad).15 This low μeff value may be attributable to the quenching
of spin−orbit coupling arising from covalent metal−ligand
interactions.15,34 We have also recorded the EPR spectrum
of complex 2 (see Supporting Information, Figure S29). The
spectrum reveals a highly anisotropic signal, in which g∥ = 2.17
and g⊥ < 0.7. Because we were only able to record a partial spec-
trum, we were limited in the amount of information that could
be extracted. However, the observation of a signal does support
the 5+ oxidation state assignment of this complex.
As expected for a U(V) complex, the singly occupied molec-
ular orbital (SOMO) is a U 5f-based electron (Table 1). The
Table 1. Compositions (Mulliken Analysis, Threshold = 1%)
and Energies (eV) of Selected Canonical α Spin Orbitals of
U(O)(NR2)3 (2)
Complex 2 crystallizes in the monoclinic space group P21/c
(Figure 4). In the solid-state, 2 features a U−Ooxo bond length
principal
bonding
orbital
energy
5f
6d
7p total O total N character
unpaired e−
SOMO
−3.891 93.8
HOMO-1 −5.756 4.8
HOMO-2 −5.809 4.9
HOMO-3 −5.853 11.0
HOMO-13 −7.538 6.7
HOMO-14 −7.633 8.9
HOMO-15 −7.659 8.5
2.6
54.6
53.1
46.5
2.1
N lone pair
N lone pair
N lone pair
U−O σ
U−O π
U−O π
1.8
2.3
7.3
2.7
2.8
2.5
36.2
41.0
40.8
2.2
1.1
SOMO is shown in Figure 5, together with three other orbitals
which, although rather delocalized, clearly demonstrate U−O
covalent bonding. The U−O bonding orbitals are polarized
toward the oxygen, but also possess significant metal character.
The Gopinathan−Jug U−O bond order is calculated to be
2.34,49 very similar to that found for the analogous bond in 1
(2.29).20 The Mulliken charge of the uranium is +1.93 in 2,
in comparison with +1.85 in 3, consistent with the increase in
formal oxidation state.
Figure 4. ORTEP diagram of U(O)(NR2)3 (R = SiMe3) (2) with 50%
probability ellipsoids. Hydrogen atoms omitted for clarity. Selected
bond lengths (Å) and angles (deg): U1−O1 = 1.817(1), U1−N1 =
2.235(1), U1−N2 = 2.244(2), U1−N3 = 2.242(1), O1−U1−N1 =
92.53(6), O1−U1−N2 = 92.16(6), O1−U1−N3 = 92.48(5), N1−
U1−N2 = 119.30(5), N1−U1−N3 = 118.16(5).
Directly below the SOMO come six orbitals with nitrogen
character. HOMO-4 to HOMO-6 are very delocalized, with
only about 25% nitrogen content, but HOMO-1 to HOMO-3
are much more nitrogen-based. They are shown in Figure 6,
together with the analogous orbitals following Boys−Foster50
localization of the canonical Kohn−Sham levels. The localized
orbitals are about 80% nitrogen in content, and display U−N
bonding character.
of 1.817(1) Å, comparable to the UO bond lengths of 1
(U−O = 1.847(2) Å) and ((RArO)3tacn)U(O) (U−O = 1.848(8) Å,
R = tBu; U−O = 1.848(4) Å, R = Ad).15 Furthermore, 2 adopts a
trigonal pyramidal geometry about the metal center (e.g., O1−
U1−N1 = 92.53(6)°, N1−U1−N2 = 119.30(5)°) with its
uranium atom lying only 0.0933(8) Å above the plane defined
by the amide nitrogen atoms. This lies in stark contrast to the
pyramidal molecular structure of U(NR2)3,44 as well as the
1628
dx.doi.org/10.1021/ic201936j | Inorg. Chem. 2012, 51, 1625−1633

Inorganic Chemistry
Article
in Figure 7 (top). The U−O and average U−N bond lengths
increase by only 0.007 Å over this distortion, clearly demonstrating
Figure 5. Representations of (clockwise from top left) the α spin 93a
(SOMO), 80a (HOMO-13), 79a (HOMO-14), and 78a (HOMO-15)
molecular orbitals of 2. The isosurface level is 0.05 in all cases. H
atoms omitted for clarity.
Figure 7. Changes in the total energy, sum of the prerelaxation
Pauli repulsion and electrostatic energies, and orbital mixing energies
(kJ·mol−1) in 2 as a function of O−U−N angle (shown as values in
excess of 90°), relative to the values at 90°.
that the energy change does not result from significant alterations
in the lengths of the bonds to the metal center.
At each step of the lineartransit we have decomposed the
total molecular energy using the Ziegler−Rauk scheme.51,52
The changes in the pre-SCF relaxation electrostatic energy term
are approximately a factor of 10 smaller than the changes in
the pre-SCF relaxation Pauli (steric) repulsion and post-SCF
relaxation orbital mixing energies. Figure 7 shows the changes in
the orbital mixing term throughout the distortion, together with
the sum of the pre-SCF relaxation electrostatic and Pauli
interactions. As might be expected, increasing the O−U−N
angle from 90° causes the sum of the pre-SCF relaxation terms to
become more favorable as the ligands move apart from each other
and the geometry approaches pseudotetrahedral. Toward the end
of the distortion, however, the pre-SCF relaxation term becomes
more positive once again, presumably as a result of increasing
repulsive interactions between the bulky N(SiMe3)2 ligands. The
orbital mixing term shows the opposite trend to the pre-SCF
relaxation terms, that is, the most negative (stabilizing) value of
this contribution to the total energy comes at 90°. We therefore
conclude that the origin of the trigonal pyramidal geometry of
2 lies in the orbital mixing term, although the requirement to run
Figure 6. Representations of (left column) the α spin 92a (HOMO-
1), 91a (HOMO-2), and 90a (HOMO-3) molecular orbitals of 2
(viewed down the U−O vector), and (right column) the analogous
orbitals following Boys−Foster localization. The isosurface level is 0.05
in all cases. H atoms omitted for clarity.
To probe the origin of the trigonal planar geometry, we per-
formed a series of constrained geometry optimizations
(lineartransit) in which the three O−U−N angles were set
equal to one another and simultaneously altered from 90° to
110° in steps of 2°, allowing all other geometric variables to relax
at each step. The total molecular energy becomes gradually less
negative during the course of this distortion, with the 110° struc-
ture being 35.1 kJ·mol−1 less stable than that at 90°, as shown
1629
dx.doi.org/10.1021/ic201936j | Inorg. Chem. 2012, 51, 1625−1633

Inorganic Chemistry
Article
U[N(SiMe3)2]3 was 56%. All other reagents were obtained from
commercial sources and used as received.
the lineartransit calculations without symmetry constraints
precludes further analysis of the orbital interaction energy.
Eisenstein et al. have invoked a second order Jahn−Teller
mechanism to explain the pseudo C3v geometries of trigonal
complexes of the lanthanides.53,54 In this explanation, C3v-like
structures are favored over planar D3h through enhanced bond-
ing as a result of mixing between metal 5d (for Ln) or 6d (for
An) orbitals and ligand levels, which is symmetry forbidden in
the planar geometry. Evidence for this mechanism comes in the
form of enhanced orbital mixing and metal orbital populations
in the pseudo C3v structures.
NMR spectra were recorded on a Varian UNITY INOVA 500 spec-
1
trometer. H NMR spectra are referenced to SiMe4 using the residual
protio solvent peaks as internal standards. 31P{1H} NMR spectra were
referenced to external 85% H3PO4. Elemental analyses were performed
by the Micro-Mass Facility at the University of California, Berkeley.
UV−vis/NIR spectra were recorded on a UV-3600 Shimadzu spectro-
photometer. IR data were collected using a Nicolet 6700 FT-IR
spectrometer. Electron paramagnetic resonance spectra were obtained
at 8 K using a Varian E-12 spectrometer equipped with an Oxford
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).
Table 2 presents the uranium Mulliken atomic populations
and atomic orbital character of the HOMO-1−HOMO-3 of 2
Magnetism Measurements. Magnetism data were recorded
using a Quantum Design MPMS 5XL SQUID magnetometer. The
experiments were performed between 4−300 K using 50−100 mg of
powdered, crystalline solid. The solids were loaded into an NMR tube,
which was subsequently flame-sealed. The solids were kept in place
with approximately 100 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 experiments were performed using a
0.5 T field. Diamagnetic corrections (χdia = −3.99 × 10−4 cm3·mol−1 for 2;
χdia = −8.04 × 10−4 cm3·mol−1 for 3; χdia = −4.38 × 10−4 cm3·mol−1
for 4) were made using Pascal’s constants.56
Table 2. Mulliken Atomic Populations (e−) of Uranium and
Average U and N Content of the N-based Canonical α Spin
Orbitals of U(O)(NR2)3 (2)
% U in
% N in
O−U−N
angle
(deg)
HOMO-1− HOMO-1−
HOMO-3
(av.)
HOMO-3
(av.)
U s
U p
U d
U f
90
2.187
2.161
5.685
5.715
1.582
1.573
2.616
2.569
6.9
5.3
51.4
51.9
102
Synthesis of U(O)[N(SiMe3)2]3 (2). To a cold (−25 °C) stirring
solution of U[N(SiMe3)2]3 (0.518 g, 0.720 mmol) in hexane (5 mL)
was added a cold (−25 °C) solution of TEMPO (0.113 g, 0.723 mmol)
in hexane (2 mL) dropwise. Upon addition, the solution imme-
diately turned pale orange in color. Within seconds, however, this
orange solution converted to a dark red color. After stirring for 5 min
at room temperature, the volume of the solution was reduced by half
in vacuo. Storage of the solution at −25 °C for 24 h resulted in the
at the start of the lineartransit and at the point at which the orbital
mixing term is least negative (i.e., at an O−U−N angle of 102°).
With the exception of the p population, all the other uranium
populations are smaller at the larger angle, as is the uranium con-
tribution to the three metal−nitrogen orbitals. These observations
are consistent with the reduced orbital mixing energy found in the
Ziegler−Rauk breakdown, and suggest that there is no second
order Jahn−Teller driver toward increased O−U−N angles in 2.
1
deposition of red crystalline material, 0.384 g, 73% yield. H NMR
(500 MHz, 25 °C, C6D6): δ −0.23 (s, 54H, NSiMe3). Anal. Calcd for
C18H54N3OSi6U: C, 29.40; H, 7.42; N, 5.72. Found: C, 29.57; H, 7.39;
N, 5.47. UV−vis/NIR (C7H8, 14.3 mM, 25 °C, L·mol−1·cm−1): 1016
(ε = 54.4), 1116 (ε = 32.6), 1254 (ε = 80.1), 1662 (overlap with
solvent absorption). IR (KBr pellet, cm−1): 2960 (m), 2950 (m), 2897
(w), 2362 (w), 2337(w), 1923 (w), 1845 (w), 1431 (w), 1416 (w),
1401 (w), 1250 (s), 1182 (w), 930 (s), 840 (s), 816 (sh m), 767 (s),
729 (w), 682 (w), 672 (w), 650 (m), 609 (s). Melting point: 104−
105 °C (dec.).
SUMMARY
■
We have demonstrated that addition of Me3NO to U(NR2)3 does
not produce the terminal oxo complex U(O)(NR2)3 as originally
reported,24 but instead generates the U(IV) bridging oxo
[(NR2)3U]2(μ-O). In contrast, treatment of U(NR2)3 with
TEMPO readily affords U(O)(NR2)3 in good yield. This com-
plex is marked by a trigonal pyramidal geometry in the solid-state.
The short UO bond found for U(O)(NR2)3 suggests a
covalent UO interaction leading to electronic control of the
geometry. DFT calculations support this assertion not only by
identifying covalent bonding within the U−O σ and π molecular
orbitals but also, via energy decomposition analysis, by demon-
strating that the adoption of the trigonal pyramidal geometry is
orbitally driven. For future work we intend to further examine the
chemistry of U(O)(NR2)3 and the reactivity of its oxo group.
Reaction of U[N(SiMe3)2]3 with TEMPO in the Presence of
9,10-Dihydroanthracene. To a deep purple solution of U[N-
(SiMe3)2]3 (0.020 g, 0.028 mmol) in C6D6 (0.7 mL) was added
9,10-dihydroanthracene (0.007 g, 0.039 mmol). TEMPO (0.004 g,
0.026 mmol) was subsequently added to the deep purple reaction mixture
1
resulting in formation of a red solution. A H NMR spectrum of the
solution revealed the presence of 2, 2,2,6,6-tetramethylpiperidine, 9,9′,
10,10′-tetrahydro-9,9′-bianthracene, and unreacted 9,10-dihydroanthra-
cene. The presence of 2,2,6,6-tetramethylpiperidine in the product mix-
ture was confirmed by comparison of its spectral properties to a solution
of commercially obtained 2,2,6,6-tetramethylpiperidine in C6D6. The pres-
ence of 9,9′,10,10′-tetrahydro-9,9′-bianthracene in the product mixture was
confirmed by comparison of the resonances to reported values.57
Synthesis of [(N(SiMe3)2)3U]2(μ-O) (3) from U(NR2)3 and 0.5
equiv of Me3NO. To a solution of U[N(SiMe3)2]3 (0.103 g, 0.143 mmol)
in pentane (2 mL) was added Me3NO (0.006 g, 0.080 mmol).
The reaction mixture gradually turned dark brown-red. After 1 h the
volatiles were removed in vacuo and the solid was dissolved in toluene
(8 mL). The resulting solution was filtered through a Celite column
(2 cm ×0.5 cm) supported on glass wool. Storage of the filtrate at
−25 °C for 24 h resulted in the deposition of light yellow crystals.
0.055 g, 53% yield. The crystals of 3 used for X-ray crystallography
were grown from a dilute C6D6 solution over 4 d at room temperature,
affording the solvate 3·2.5C6D6. 1H NMR (500 MHz, 25 °C, C7D8): δ
−28.64 (br s, 18H, fwhm = 1300 Hz, NSiMe3), −16.79 (br s, 18H,
fwhm = 1300 Hz, NSiMe3), and −6.50 (br s, 18H, fwhm = 1300 Hz,
EXPERIMENTAL SECTION
■
General Information. All reactions and subsequent manipulations
were performed under anaerobic and anhydrous conditions under an
atmosphere of argon or nitrogen. Diethyl ether, tetrahydrofuran (THF),
and hexane were dried using a Vacuum Atmospheres DRI-SOLV
Solvent Purification system. Pentane and CH2Cl2 were dried over acti-
vated 4 Å and 3 Å molecular sieves, respectively, for 24 h prior to use.
All deuterated solvents were purchased from Cambridge Isotope
Laboratories Inc. and were dried over activated 4 Å molecular sieves
for 24 h prior to use. U[N(SiMe3)2]3 and Ph3PCH2 were synthe-
sized according to published procedures. U[N(SiMe3)2]3 was purified
by recrystallization from hexane or sublimation at 110 °C under
reduced pressure. When isolated by sublimation the yield of
24
55
1
NSiMe3), 15.23 (br s, 54H, fwhm = 2300 Hz, NSiMe3). H NMR
1630
dx.doi.org/10.1021/ic201936j | Inorg. Chem. 2012, 51, 1625−1633

Inorganic Chemistry
Article
(500 MHz, −3 °C, C7D8): δ −32.38 (s, 18H, NSiMe3), −19.52 (s, 18H,
NSiMe3), −7.57 (s, 18H, NSiMe3), 8.76 (br s, 54 H, fwhm = 19,000
Hz, NSiMe3). 1H NMR (500 MHz, −13 °C, C7D8): δ −34.07 (s, 18H,
NSiMe3), −20.36 (s, 18H, NSiMe3), −8.11 (s, 18H, NSiMe3),
resonances assignable to 54 silylamide methyl protons not observed.
1H NMR (500 MHz, −26 °C, C7D8): δ −36.30 (s, 18H, NSiMe3),
−21.96 (s, 18H, NSiMe3), −8.56 (s, 18H, NSiMe3), resonances
NaI (0.007 g, 0.047 mmol). The solution was stirred for 30 min at
room temperature, whereupon Me3NO (0.002 g, 0.027 mmol) was
added. After 1 h of stirring, the solution became brown-orange in
color. The volatiles were removed in vacuo, and the crude solid was
1
dissolved in C6D6 and analyzed by H NMR spectroscopy (see the
Supporting Information). The presence of U(I)[N(SiMe3)2]3 in the
product mixture was confirmed by comparison with a spectrum of an
independently prepared sample of 4. NOTE: NaI and Me3NO are not
observed to react with each other under these reaction conditions.
Synthesis of [Ph3PCH3][U(O)(CH2SiMe2NSiMe3)(NR2)2] (R =
SiMe3) (1) from the reaction of 2 with Ph3PCH2. An NMR tube
equipped with a J-Young valve was charged with a solution of 2 (0.012 g,
0.016 mmol) in C6D6 (0.7 mL) to which Ph3PCH2 (0.005 g, 0.018
mmol) was added. Upon addition, the deep red solution immediately
turned red-brown in color. Analysis of the solution by 1H and 31P{1H}
NMR revealed the formation of [Ph3PCH3][U(O)(CH2SiMe2NSiMe3)-
(NR2)2] as indicated by comparison of the spectra to independently
prepared material.20 1H NMR (500 MHz, 25 °C, C6D6): δ −6.00 (s,
36H, NSiMe3), −4.09 (s, 9H, CH2SiMe2NSiMe3), 8.38 (s, 3H, p-aryl
CH), 8.97 (s, 6H, aryl CH), 9.20 (s, 6H, aryl CH), 14.13 (s, 6H,
CH2SiMe2NSiMe3), 16.01 (s, 3H, Ph3PCH3), 36.71 (br s, 2H, CH2Si-
Me2NSiMe3). 31P{1H} NMR (202 MHz, 25 °C, C6D6): δ 30.28 (s).
X-ray Crystallography. Data for 2 and 3·2.5C6D6 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), while the data was for 4 was 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). The crystals of 2 and 3·2.5C6D6 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.
The crystal of 4 was mounted on a glass fiber under Paratone-N oil,
and all data were collected at 150(2) K using an Oxford nitrogen gas
cryostream system. A hemisphere of data was collected using ω scans
with 0.5° frame widths. Frame exposures of 5, 10, and 15 s were used
for 2, 3·2.5C6D6, and 4, respectively. Data collection and cell param-
eter determination were conducted using the SMART program.58
Integration of the data frames and final cell parameter refine-
ment were performed using SAINT software.59 Absorption correction
of the data for 2 and 3·2.5C6D6 was carried out using SADABS,60
while the absorption correction of the data for 4 was carried out
empirically based on reflection ψ-scans. Subsequent calculations were
carried out using SHELXTL.61 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. Structure solution, refinement, graphics, and creation of
publication materials were performed using SHELXTL.61
Complex 3·2.5C6D6 contains two disordered C6D6 molecules. Each
disordered C6D6 molecule was modeled over two positions with
50:50 occupancies. The carbon atoms of the C6D6 molecules were not
refined anisotropically and hydrogen atoms were not assigned to these
carbons. A summary of relevant crystallographic data for complexes 2,
3·2.5C6D6, and 4 is presented in Table 3.
Computational Details. Spin-unrestricted, gradient corrected
DFT calculations were carried out using the PBE functional,62,63 as
implemented in the Amsterdam Density Functional 2010.0264,65 code.
The Zeroth Order Regular Approximation (ZORA) Hamiltonian was
employed in all calculations. Slater Type Orbital ZORA basis sets of
DZP quality were used for all atoms except U, for which a TZP ZORA
basis set was employed. The frozen core approximation was employed
for all atoms except H; C(1s), N(1s), O(1s), Si(2p), U(5d). The geom-
etry of 2 was optimized without symmetry constraints, and that of
3 within the D3 point group, using the default self consistent field
(SCF) and geometry convergence criteria, together with an integration
grid of 4.5. Ziegler−Rauk bond energy decomposition analysis was
performed.51,52 Gopinathan−Jug bond orders were also computed.49
1
assignable to 54 silylamide methyl protons not observed. H NMR
(500 MHz, −41 °C, C7D8): δ −39.76 (s, 18H, NSiMe3), −24.30 (s,
18H, NSiMe3), −19.42 (br s, 18H, fwhm = 2500 Hz, NSiMe3), −9.56
(s, 18H, NSiMe3), 14.32 (br s, 18H, fwhm = 2500 Hz, NSiMe3), 71.99
(br s, 18H, fwhm = 3000 Hz, NSiMe3). 1H NMR (500 MHz, −55 °C,
C7D8): δ −43.23 (s, 18H, NSiMe3), −26.79 (s, 18H, NSiMe3), −22.98
(br s, 18H, fwhm = 900 Hz, NSiMe3), −10.53 (s, 18H, NSiMe3), 14.21
(br s, 18H, fwhm = 900 Hz, NSiMe3), 80.61 (br s, 18H, fwhm = 900
1
Hz, NSiMe3). H NMR (500 MHz, 45 °C, C7D8): δ −14.05 (br s,
54H, fwhm = 5000 Hz, NSiMe3), 13.49 (br s, 54H, fwhm = 2000 Hz,
NSiMe3). Anal. Calcd for C36H108N6OSi12U: C, 29.73; H, 7.50; N,
5.78. Found: C, 29.87; H, 7.46; N, 5.59. UV−vis/NIR (C7H8, 4.7 mM,
25 °C, L·mol−1·cm−1): 516 (ε = 34.2), 574 (ε = 15.5), 646 (ε = 11.3),
686 (ε = 89.5), 848 (ε = 12.6), 880 (ε = 10.0), 1076 (ε = 25.5), 1168
(ε = 34.1), 1420 (ε = 14.3), 1578 (ε = 21.7), 1772 (ε = 23.4). IR (KBr
pellet, cm−1): 2960 (sh m), 2955 (m), 2901 (m), 1629 (w), 1432 (w),
1405 (w), 1249 (s), 1182 (w), 932 (m), 882 (s), 848 (s), 836 (sh s),
774 (m), 760 (m), 707 (w), 682 (m), 659 (m), 632 (w), 613 (m), 463
(m). Complex 3 exhibits an effective magnetic moment of 3.84 μB at
300 K, which decreases to 1.33 μB at 4 K. Melting point: 155−157 °C.
Synthesis of [(N(SiMe3)2)3U]2(μ-O) (3) from U(NR2)3 and
1 equiv of Me3NO. To a solution of U[N(SiMe3)2]3 (0.099 g, 0.138
mmol) in pentane (2 mL) was added Me3NO (0.011 g, 0.146 mmol).
The reaction mixture gradually turned dark brown-red. After
2 h the volatiles were removed in vacuo affording a dark brown solid.
The solid was washed with hexanes (3 × 2 mL) providing a light yellow
powder. The powder was dissolved in THF (6 mL) and the pale yellow
solution was filtered through a Celite column (2 cm ×0.5 cm) sup-
ported on glass wool. Storage of the filtrate at −25 °C for 24 h resulted
in the deposition of light yellow crystals. 0.033 g, 33% yield.
Synthesis of [(N(SiMe3)2)3U]2(μ-O) (3) from U[N(SiMe3)2]3
and 2. To a stirring solution of U[N(SiMe3)2]3 (0.065 g, 0.090 mmol)
in toluene (5 mL) was added dropwise a solution of 2 (0.066 g, 0.090 mmol)
in toluene (5 mL). Upon addition, the solution immediately turned
light yellow in color. Storage of the solution at −25 °C for 24 h
resulted in the deposition of light yellow crystalline material, which was
collected by decanting the supernatant (0.053 g). Concentration and
storage of the supernatant for 24 h at −25 °C resulted in the further
deposition of yellow crystals (0.040 g). Total: 0.093 g, 71% yield.
Synthesis of U(I)[N(SiMe3)2]3 (4). To a stirring solution of
U[N(SiMe3)2]3 (0.270 g, 0.375 mmol) in hexanes (3 mL) was added
dropwise a solution of I2 (0.045 g, 0.176 mmol) in Et2O (2 mL). Upon
addition, a nearly colorless microcrystalline precipitate formed. This
solid was isolated by decanting off the supernatant. The material was
subsequently dissolved in CH2Cl2 (16 mL) and filtered through a
Celite column (2 cm ×0.5 cm) supported on glass wool. Storage of the
solution at −25 °C for 24 h resulted in the deposition of light tan
crystalline blocks; 0.199 g, 62% yield. The crystals of 4 used for X-ray
crystallography were grown by storage of dilute hexane/Et2O solution at
−25 °C for 24 h. C18H54N3ISi6U: C, 25.55; H, 6.43; N, 4.97. Found: C,
25.31; H, 6.58; N, 4.86. 1H NMR (500 MHz, 25 °C, C6D6): δ −0.98 (s,
54H, NSiMe3). UV−vis/NIR (THF, 9.8 mM, 25 °C, L·mol−1·cm−1): 452
(sh, ε = 14.0), 495 (ε = 13.0), 520 (ε = 18.7), 524 (ε = 15.6), 554 (sh,
ε = 7.6), 608 (ε = 6.6), 640 (sh, ε = 6.5), 660 (sh, ε = 10.9), 688 (ε =
21.7), 796 (ε = 5.6), 882 (ε = 3.7), 940 (ε = 4.9), 1034 (ε = 20.9), 1050
(ε = 20.8), 1340 (ε = 14.5), 1532 (ε = 10.1). IR (KBr pellet, cm−1):
2956 (m), 2898 (m), 1592 (w), 1408 (w), 1251 (s), 1182 (w), 1072
(w), 983 (sh w), 921 (sh m), 891 (s), 847 (s), 770 (m), 755 (m), 678
(w), 656 (w), 613 (m). Complex 4 exhibits an effective magnetic
moment of 3.35 μB at 300 K which decreases to 2.16 μB at 4 K.
Reaction of U[N(SiMe3)2]3 with Me3NO in the Presence of
NaI. To a deep purple solution of U[N(SiMe3)2]3 (0.035 g, 0.049
mmol) in hexane (2 mL) was added THF (50 μL) and finely ground
1631
dx.doi.org/10.1021/ic201936j | Inorg. Chem. 2012, 51, 1625−1633

Inorganic Chemistry
Article
(4) Hayton, T. W. Dalton Trans. 2010, 39, 1145−1158.
Table 3. X-ray Crystallographic Data for 2, 3·2.5C6D6, and 4
(5) Zi, G.; Jia, L.; Werkema, E. L.; Walter, M. D.; Gottfriedsen, J. P.;
Andersen, R. A. Organometallics 2005, 24, 4251−4264.
(6) Lam, O. P.; Heinemann, F. W.; Meyer, K. Chem. Sci. 2011, 2,
1538−1547.
2
3·2.5C6D6
4
empirical
formula
C18H54N3OSi6U C51H123N6OSi12U2 C18H54IN3Si6U
crystal habit,
color
block, red
block, yellow
block, tan
(7) Lam, O. P.; Bart, S. C.; Kameo, H.; Heinemann, F. W.; Meyer, K.
Chem. Commun. 2010, 46, 3137−3139.
crystal size
(mm)
0.20 × 0.20 × 0.20 0.21 × 0.16 × 0.15 0.30 × 0.30 × 0.10
(8) Avens, L. R.; Barnhart, D. M.; Burns, C. J.; McKee, S. D.; Smith,
W. H. Inorg. Chem. 1994, 33, 4245−4254.
crystal system monoclinic
monoclinic
C2/c
trigonal
R3c
(9) Kraft, S. J.; Walensky, J.; Fanwick, P. E.; Hall, M. B.; Bart, S. C.
Inorg. Chem. 2010, 49, 7620−7622.
space group
volume (Å3)
a (Å)
P21/c
3263.4(1)
12.4187(3)
18.1898(4)
14.8632(4)
90
30929(1)
30.8900(7)
19.9441(4)
50.226(1)
90
5008.9(3)
18.2954(5)
18.2954(5)
17.280(1)
90
(10) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1995, 117, 9448−
9460.
b (Å)
(11) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1993, 115, 9840−
9841.
(12) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Polyhedron 2004, 23,
2689−2694.
(13) Gardner, B. M.; Lewis, W.; Blake, A. J.; Liddle, S. T. Inorg. Chem.
2011, 50, 9631−9641.
c (Å)
α (deg)
β (deg)
γ (deg)
Z
103.600(1)
90
91.699(1)
90
90
120
4
16
6
(14) Barros, N.; Maynau, D.; Maron, L.; Eisenstein, O.; Zi, G.;
Andersen, R. A. Organometallics 2007, 26, 5059−5065.
(15) Bart, S. C.; Anthon, C.; Heinemann, F. W.; Bill, E.; Edelstein,
N. M.; Meyer, K. J. Am. Chem. Soc. 2008, 130, 12536−12546.
(16) Castro-Rodriguez, I.; Meyer, K. J. Am. Chem. Soc. 2005, 127,
11242−11243.
formula weight 735.21
(g/mol)
1649.69
846.11
density
1.496
1.417
4.404
1.683
6.012
(calculated)
(Mg/m3)
absorption
coeff−ic1ient
(mm )
5.208
(17) Lam, O. P.; Anthon, C.; Meyer, K. Dalton Trans. 2009, 9677−
9691.
F000
1468
13296
2472
7595
(18) Karmazin, L.; Mazzanti, M.; Pecaut, J. Inorg. Chem. 2003, 42,
total no.
reflections
34039
104426
5900−5908.
(19) Lukens, W. W.; Beshouri, S. M.; Blosch, L. L.; Andersen, R. A.
J. Am. Chem. Soc. 1996, 118, 901−902.
unique
reflections
12424
29399
2291
(20) Fortier, S.; Kaltsoyannis, N.; Wu, G.; Hayton, T. W. J. Am.
Chem. Soc. 2011, 133, 14224−14227.
final R indices R1 = 0.0220
(I > 2σ(I)]
R1 = 0.0614
wR2 = 0.1149
R1 = 0.0281
wR2 = 0.0803
(21) Lippert, C. A.; Soper, J. D. Inorg. Chem. 2010, 49, 3682−3684.
(22) Lucarini, M.; Marchesi, E.; Pedulli, G. F.; Chatgilialoglu, C.
J. Org. Chem. 1998, 63, 1687−1693.
wR2 = 0.0433
largest diff.
1.355 and −0.931 2.534 and −5.483 1.567 and −1.072
peak −a3nd hole
−
(e Å )
(23) Evans, W. J.; Perotti, J. M.; Doedens, R. J.; Ziller, J. W. Chem.
Commun. 2001, 2326−2327.
GOF
1.009
1.079
1.079
(24) Andersen, R. A. Inorg. Chem. 1979, 18, 1507−1509.
(25) Graves, C. R.; Kiplinger, J. L. Chem. Commun. 2009, 3831−
3853.
ASSOCIATED CONTENT
* Supporting Information
■
S
(26) Graves, C. R.; Vaughn, A. E.; Schelter, E. J.; Scott, B. L.;
Thompson, J. D.; Morris, D. E.; Kiplinger, J. L. Inorg. Chem. 2008, 47,
11879−11891.
(27) Graves, C. R.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L.
Organometallics 2008, 27, 3335−3337.
Experimental procedures, crystallographic details (as CIF files),
and spectral data for 2−4; optimized Cartesian coordinates for
2 and 3. This material is available free of charge via the Internet
at http://pubs.acs.org.
(28) Graves, C. R.; Yang, P.; Kozimor, S. A.; Vaughn, A. E.; Clark,
D. L.; Conradson, S. D.; Schelter, E. J.; Scott, B. L.; Thompson, J. D.;
Hay, P. J.; Morris, D. E.; Kiplinger, J. L. J. Am. Chem. Soc. 2008, 130,
5272−5285.
AUTHOR INFORMATION
Corresponding Author
*E-mail: hayton@chem.ucsb.edu (T.W.H.), n.kaltsoyannis@
ucl.ac.uk (N.K.).
■
(29) Graves, C. R.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. J. Am.
Chem. Soc. 2007, 129, 11914−11915.
ACKNOWLEDGMENTS
(30) Benaud, O.; Berthet, J.-C.; Thuery, P.; Ephritikhine, M. Chem.
Commun. 2011, 47, 9057−9059.
■
We thank the University of California, Santa Barbara, and the
Department of Energy (BES Heavy Element Program) for
financial support of this work. We are grateful to UCL for com-
puting resources via the Research Computing “Legion” cluster
and associated services, and the U.K. EPSRC for computing
resources under grant GR/S06233 and via its National Service
for Computational Chemistry Software (http://www.nsccs.ac.
uk). We also thank Wayne W. Lukens, Jr. (LBNL) for re-
cording the EPR spectrum of 2.
(31) Cooper, O. J.; Mills, D. P.; McMaster, J.; Moro, F.; Davies, E. S.;
Lewis, W.; Blake, A. J.; Liddle, S. T. Angew. Chem., Int. Ed. 2011, 50,
2383−2386.
(32) Roussel, P.; Boaretto, R.; Kingsley, A. J.; Alcock, N. W.; Scott, P.
Dalton. Trans. 2002, 1423−1428.
(33) Castro-Rodriguez, I.; Olsen, K.; Gantzel, P.; Meyer, K. Chem.
Commun. 2002, 2764−2765.
(34) Castro-Rodriguez, I.; Olsen, K.; Gantzel, P.; Meyer, K. J. Am.
Chem. Soc. 2003, 125, 4565−4571.
(35) Schelter, E. J.; Yang, P.; Scott, B. L.; Thompson, J. D.; Martin,
R. L.; Hay, P. J.; Morris, D. E.; Kiplinger, J. L. Inorg. Chem. 2007, 46,
7477−7488.
REFERENCES
■
(1) Fortier, S.; Hayton, T. W. Coord. Chem. Rev. 2010, 254, 197−214.
(2) Denning, R. G. Struct. Bonding (Berlin) 1992, 79, 215−276.
(3) Denning, R. G. J. Phys. Chem. A 2007, 111, 4125−4143.
(36) Fortier, S.; Melot, B. C.; Wu, G.; Hayton, T. W. J. Am. Chem.
Soc. 2009, 131, 15512−15521.
1632
dx.doi.org/10.1021/ic201936j | Inorg. Chem. 2012, 51, 1625−1633

Inorganic Chemistry
Article
(37) Dormond, A.; El Bouadili, A.; Aaliti, A.; Moise, C. J. Organomet.
Chem. 1985, 288, C1−C5.
(38) Simpson, S. J.; Turner, H. W.; Andersen, R. A. Inorg. Chem.
1981, 20, 2991−2995.
(39) Andersen, R. A.; Zalkin, A.; Templeton, D. H. Inorg. Chem.
1981, 20, 622−623.
(40) Castro-Rodríguez, I.; Meyer, K. Chem. Commun. 2006, 1353−
1368.
(41) Lam, O. P.; Feng, P. L.; Heinemann, F. W.; O’Connor, J. M.;
Meyer, K. J. Am. Chem. Soc. 2008, 130, 2806−2816.
(42) Kozimor, S. A.; Bartlett, B. M.; Rinehart, J. D.; Long, J. R. J. Am.
Chem. Soc. 2007, 129, 10672−10674.
(43) Arnold, P. L.; Pecharman, A.-F.; Hollis, E.; Yahia, A.; Maron, L.;
Parsons, S.; Love, J. B. Nat. Chem. 2010, 2, 1056−1061.
(44) Stewart, J. L.; Andersen, R. A. Polyhedron 1998, 17, 953−958.
(45) Edwards, P. G.; Andersen, R. A.; Zalkin, A. J. Am. Chem. Soc.
1981, 103, 7792−7794.
(46) Airoldi, C.; Bradley, D. C.; Chudzynska, H.; Hursthouse, M. B.;
Malik, K. M. A.; Raithby, P. R. Dalton Trans. 1980, 2010−2015.
(47) Zalkin, A.; Brennan, J. G.; Andersen, R. A. Acta Crystallogr., Sect.
C 1988, 44, 1553−1554.
(48) Hubert-Pfalzgraf, L. G.; Tsunoda, M.; Le Borgne, G. J. Chem.
Soc., Dalton Trans. 1988, 533−535.
(49) Gopinathan, M. S.; Jug, K. Theo. Chem. Acc. 1983, 63, 497−509.
(50) von Neissen, W. J. Chem. Phys. 1972, 56, 4290.
(51) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1558.
(52) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1755.
(53) Perrin, L.; Maron, L.; Eisenstein, O. Faraday Discuss. 2003,
124, 25.
(54) Perrin, L.; Maron, L.; Eisenstein, O.; Lappert, M. F. New J.
Chem. 2003, 27, 121.
(55) Bestmann, H. J.; Stransky, W.; Vostrowsky, O. Chem. Ber. 1976,
109, 1694−1700.
(56) Bain, G. A.; Berry, J. F. J. Chem. Educ. 2008, 85, 532.
(57) Berke, C. M.; Streitwieser, A. J. Organomet. Chem. 1980, 197,
123−134.
(58) SMART Apex II, Version 2.1; Bruker AXS Inc.: Madison, WI,
2005.
(59) SAINT Software User’s Guide, Version 7.34a; Bruker AXS Inc.:
Madison, WI, 2005.
(60) Sheldrick, G. M. SADABS; University of Gottingen: Gottingen,
̈
̈
Germany, 2005.
(61) SHELXTL PC, Version 6.12; Bruker AXS Inc.: Madison, WI,
2005.
(62) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77,
3865−3868.
(63) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78,
1396.
(64) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra,
C.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem.
2001, 22, 931−967.
(65) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J.
Theor. Chem. Acc. 1998, 99, 391−403.
1633
dx.doi.org/10.1021/ic201936j | Inorg. Chem. 2012, 51, 1625−1633