Uranium(III) complexes with bulky aryloxide ligands featuring metal-arene interactions and their reactivity toward nitrous oxide

Article abstract of DOI:10.1021/ic401532j

We report the synthesis and use of an easy-to-prepare, bulky, and robust aryloxide ligand starting from inexpensive precursor materials. Based on this aryloxide ligand, two reactive, coordinatively unsaturated U(III) complexes were prepared that are maske

Full text of DOI:10.1021/ic401532j

Article
pubs.acs.org/IC
Uranium(III) Complexes with Bulky Aryloxide Ligands Featuring
Metal−Arene Interactions and Their Reactivity Toward Nitrous Oxide
Sebastian M. Franke,† Ba L. Tran,§ Frank W. Heinemann,† Wolfgang Hieringer,‡ Daniel J. Mindiola,*,§,⊥
and Karsten Meyer*,†
†Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander University of Erlangen-Nuremberg,
Egerlandstrasse 1, 91058 Erlangen, Bavaria, Germany
‡Department of Chemistry and Pharmacy, Theoretical Chemistry, Friedrich-Alexander University of Erlangen-Nuremberg,
Egerlandstrasse 3, 91058 Erlangen, Bavaria, Germany
§Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, Indiana 47405, United States
S
* Supporting Information
ABSTRACT: We report the synthesis and use of an easy-to-prepare,
bulky, and robust aryloxide ligand starting from inexpensive precursor
materials. Based on this aryloxide ligand, two reactive, coordinatively
unsaturated U(III) complexes were prepared that are masked by a
metal−arene interaction via δ-backbonding. Depending on solvent
and uranium starting material, both a tetrahydrofuran (THF)-bound
and Lewis-base-free U(III) precursor can easily be prepared on the
multigram scale. The reaction of these trivalent uranium species with
nitrous oxide, N2O, was studied and an X-ray diffraction (XRD) study
on single crystals of the product revealed the formation of a five-
coordinate U(V) oxo complex with two different molecular
geometries, namely, square pyramidal and trigonal bipyramidal.
1), that provides steric protection, by forming the correspond-
ing [(Ar*O)3U(THF)] (1) and Lewis-base-free [(Ar*O)3U]
INTRODUCTION
■
The use of bulky aryloxide and alkoxide ligands for the
stabilization of unsaturated homoleptic metal complexes are
widespread in coordination chemistry.2−7 Rothwell et al., for
example, harnessed the bulky nature of aryloxide ligands
coupled to intramolecular metal−arene interactions to
synthesize reactive species, such as the mononuclear W(II)
species, [W(OC6HPh3-η6-C6H5)(OAr)(PMe2Ph)] (OAr =
2,3,5,6-tetraphenylphenoxide), which can undergo up to four-
electron redox chemistry at a single metal center.8,9 Although
aryloxide ligands, such as HO-2,6-iPr2-C6H3 have been widely
used for uranium chemistry, likely because of their commercial
availability, the small size of these ligands lead to dimerization
via U−arene interactions.10−15 Similarly, sterically demanding
siloxide ligands such as HOSitBu3 have been employed for small
molecule activation.16 Research in our laboratory has focused
on sterically encumbering tacn-,17 N-,18 and mesityl-anchored19
aryloxide chelating ligands; these ligands provide suitable
protection around the U(III) centers; however, steric flexibility
at the primary coordination sphere is restricted because of the
tethered nature of tripodal ligands. Therefore, we sought a
supporting ligand with additional flexibility but without
compromising the steric hindrance, a requirement needed to
protect a large ion such as U(III).
Scheme 1. Synthesis of the Ligands HOAr* and NaOAr*
(2) complexes via convenient salt metathesis or alcoholysis
reactions, respectively (Scheme 2). Both complexes 1 and 2
(Figure 1) feature a metal−(η6-arene) interaction, which is
fundamentally interesting since d and f valence orbitals are
capable of engaging in covalent interactions, and have been
demonstrated to influence the electronic structure of the overall
system.19−24 We also demonstrate the reactive nature of these
masked U(III) systems by the activation of nitrous oxide at
ambient temperature and pressure to produce a terminal U(V)-
oxo.
RESULTS AND DISCUSSION
The synthesis of the free phenol HOAr* (with Ar* = 2,6-Ph2-
C6H4-Me, 2,6-bis(diphenylmethyl)-4-methylphenyl) (Scheme
■
Herein, we describe the facile multigram synthesis of a novel
monoanionic aryloxide ligand, HOAr* (with Ar* = 2,6-Ph2-
C6H4-Me, 2,6-bis(diphenylmethyl)-4-methylphenyl) (Scheme
Received: June 18, 2013
Published: August 30, 2013
© 2013 American Chemical Society
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Scheme 2. Synthesis of U(III) Complexes 1 and 2
Table 1. Selected Bond Distances for 1 and 2 in Å
structural parameter
1
2
U−Oavg.
C−Oavg.
U−Carene
U−Arcentroid
U−OTHF
2.221(2)
1.342(3)
2.964(3)
2.614
2.158(2)
1.357(3)
2.853(3)
2.484
2.554 (2)
0.339(1)
Uout‑of‑plane shift
0.987(1)
C−C bond distances of 1.438(13) Å observed in [{Ar[R]-
N)2)U}2(μ-η6:η6-C7H8)] (Ar = 3,5-C6H3-Me2).26 The equato-
rial sites are occupied by three aryloxide ligands with an average
U1−aryloxide bond distance of 2.221(2) Å. The U1−O−
C(ipso) angles are different for all three aryloxide ligands.
While U1−O2−C(ipso) and U1−O3−C(ipso) are slightly
comparable at 171.4(2)° and 163.0(2)°, respectively, U1−O1−
C(ipso) is considerably more bent at 142.2(2)° with respect to
the latter angles. This significant bending of the ligand is
required for the phenyl group to situate itself in proper
proximity and orientation to participate in U−arene δ-
backbonding. Aside from the coordinated THF, complex 1
closely resembles the mononuclear, mesityl-anchored tripodal
U(III) complex [((t‑BuArO)3mes)U];1 therefore, prompting us
to pursue the isolation of a potentially more reactive precursor,
namely, the Lewis-base free analogue of 1. Accordingly,
coordinating solvents were strictly avoided throughout the
reaction, and treatment of [(Me3Si)2N)3U] with 3 equiv of
HOAr* in benzene produced analytically pure, dark brown
solids in 65% isolated yield (Scheme 2).27 An XRD study on
single crystals obtained from a saturated benzene solution at
room temperature clearly shows the absence of a coordinated
THF ligand, while the U−(η6-arene) feature observed in 1 is
retained. To our surprise, other phenyl groups did not
participate in metal−arene interactions. The structure of
[(Ar*O)3U] (2) can be described as adopting a distorted
tetrahedral geometry with U1−Oavg = 2.158(2) Å and an
average U1−(η6-arene) bond distance of 2.853(3) Å, which is
slightly longer compared to 1, although the U−Arcentroid
distance is shorter in 2 (2.484 Å) compared to 1 (2.614 Å).
The solid state structures of both 1 and 2 exhibit flanking aryl
groups above and below the O1−O2−O3 plane; and thus, in 1
and 2 one side of the U1 center is blocked via the U1−(η6-
arene) interaction, while 1 additionally shows a bound THF
trans to the U−arene interaction. Complex 2, however,
1) involves an adopted protocol from the aniline analogue
recently reported by Marko
́
et al.25 This protocol allows for
rapid and solvent-free conditions that afford the aryl alcohol in
multigram quantities with high yield (85%) and of high purity.
In addition, such protocol is highly modular and therefore
permits various functional groups to be installed in the p-aryl
position of the phenol. Formation of the corresponding sodium
salt, NaOAr*, was readily accomplished by treatment of the
phenol with NaN(SiMe3)2 in diethyl ether, leading to a yellow
suspension, which can be filtered to afford pure product in
∼75% isolated yield (Scheme 1). Straightforward salt meta-
thesis reaction of NaOAr* (3 equiv) and UI3(THF)4 in
tetrahydrofuran (THF) provided dark-brown solids of
[(Ar*O)3U(THF)] (1) in ∼95% isolated yield (Scheme 2).
Single-crystals for an X-ray diffraction (XRD) study were
obtained from a saturated benzene solution at room temper-
ature. The solid state structure revealed a U(III) center
confined in a trigonal bipyramidal geometry, in which the axial
positions feature a U1−(η6-arene) interaction with a distance of
2.614 Å between the U(III) center and the centroid and an
average U−Carene distance of 2.964(3) Å, as well as one
molecule THF with U1−O4 = 2.554(2) Å that is weakly
coordinated to the U(III) center (Table 1). The coordinated
arene functionality in 1 is virtually unactivated based on the
average C−C bond distance of 1.396(3) Å compared to longer
Figure 1. Molecular structures of 1 (left) and 2 (right) are shown with thermal ellipsoids at 50% probability level. Hydrogen atoms, some phenyl
groups not bound to uranium, and co-crystallized solvent molecules have been omitted for clarity.
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possesses an open coordination site trans to the U−arene
moiety that is readily available for substrate binding and
activation chemistry, while this coordination site is taken by
THF in 1.
Despite the coordinated THF ligand in 1, characterization of
1
1 and 2 by H NMR and UV/vis spectroscopy indicate that
these complexes have similar dynamic behavior in solution. For
instance, 1H NMR (25 °C, C6D6) spectroscopy for both 1 and
2 display five resonances between 4−12 ppm. 1H NMR spectra
of 1 show no indication of free THF, which would result in
sharp resonances, and resonances for the coordinated THF
ligand in 1 cannot be observed; it is likely that the THF signals
are paramagnetically shifted and broadened beyond detection.
In this context, it is noteworthy to mention that there is no sign
of THF loss in 1 even under dynamic vacuum (as confirmed by
elemental analysis). Therefore rapid exchange of the bound
THF in benzene solution is quite unlikely. UV/vis spectroscopy
Figure 3. Graphical representation of singly occupied orbitals HOMO-
1 (left) and HOMO-2 (right) in 1 according to the density functional
calculations.
been able to find a situation with two η6 phenyl-U interactions
(i.e., a sandwich-type coordination of U) in the calculations.
It is interesting to further quantify the interactions of the
coordinated phenyl and the THF with U(III) in compounds
like 1 and 2. To this end, we have calculated the corresponding
binding energies in the model system [(MeO)3U(η6-C6H6)
(THF)], where the coordinating phenyl ring has been replaced
by benzene and the aryloxide ligands (Ar*O−) in 1 and 2 have
been truncated to methoxide (MeO−). In this simplified
system, the benzene and THF dissociation energies are
calculated to be 109 and 41 kJ/mol, respectively. Hence, the
benzene ligand is bound more strongly to the U center than the
THF ligand. The trans effect brings about a reduction in
binding energies by approximately 30 kJ/mol for both ligands
and as expected, the binding energies of the ligands are higher
in the absence of another ligand in the trans position. This can
be gathered from the calculated binding energies of benzene or
THF to bare U(OMe)3, which amounts to 139 kJ/mol
(benzene) and 70 kJ/mol (THF), respectively. Further details
on the calculations on 1 and 2 and the model systems can be
found in the Supporting Information.
further supports the closely related nature of trivalent 1 and 2
3
and shows several Laporte-allowed metal-centered 5f
to
5f 26d 1 transitions for both complexes in the same visible light
region (492 nm, ε = 1040 (1)/ 1400 (2); 671 nm, ε = 600 (1)/
780(2); 798 nm, ε = 670 (1)/ 900 (2)) (Figure 2). While the
The reactivity of complexes 1 and 2 was probed in an
oxygen-atom transfer reaction. Treating either 1 or 2 with an
atmosphere of N2O in THF solution yields, after removal of
volatiles, red-orange solids that could be crystallized by vapor
diffusion of hexane into a concentrated benzene solution and
identified as U(V) terminal oxo complex [(Ar*O)3(THF)U-
(O)] (3) (Scheme 3). It should be noted that terminal mono-
Figure 2. UV/vis spectrum of complexes 1, 2, and, for comparison,
[((t‑BuArO)3mes)U]1 recorded in toluene.
UV/vis spectra of our tacn-anchored and N-anchored U(III)
complexes show few but very intense absorptions at ∼500
nm,28,29 the UV/vis spectra of 1 and 2 show a striking similarity
to the spectrum of [((t‑BuArO)3mes)U], for which the metal−
arene interaction was established by DFT calculations.1 All
three compounds show medium intensity absorption bands in
the range from 450 to 900 nm that seem to be characteristic for
U(III) complexes experiencing a δ-backbonding interaction.
The binding energy of the THF molecule in 1 is estimated to
be 63 kJ/mol with the dispersion-corrected density-functional
method used. A major part of this binding energy indeed
appears to derive from dispersion interactions because the
interaction energy is diminished to only 6 kJ/mol without
dispersion correction. Hence, the calculations suggest only a
very weak electronic interaction with the uranium center.
In both 1 and 2, one of the peripheral phenyl rings is bound
in a η6 fashion to the U(III) center. A brief analysis of the
electronic structure reveals a δ-type interaction that is mediated
by two singly occupied molecular orbitals shown in Figure 3 for
complex 1. The interaction can be described in terms of the U f
atomic orbitals with antibonding π orbitals of the phenyl ring
(two nodal planes). A similar bonding interaction has been
described in [((t‑BuArO)3mes)U].1 We note that we have not
Scheme 3. Synthesis of U(V)-oxo 3 from Precursor 1 or 2
oxo complexes of uranium(V) are quite a rare class of
compounds that can generally be obtained via two-electron
oxidation of uranium(III) with an oxygen atom transfer reagent,
such as Me3NO,30,31 TEMPO,31 py-NO,32 O2,33,34 or via
multiple bond metathesis of uranium(V) imido complexes with
CO2 with concomitant formation of isocyanate.35 Nitrous
oxide, N2O, has been used as an oxidant for U(III) before;
however, with one exception,36 only bridging μ-oxo complexes
could be obtained.28,37,38 In fact, because of its stability and
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poor ligand properties,39−41 reactions with N2O rarely lead to
terminal metal oxo species; and thus, there are only very few
examples reported in the literature.42−50
An XRD study on red prism-shaped single crystals revealed
two crystallographically independent but chemically equivalent
molecules of the complex in the asymmetric unit (Z = 2).
Interestingly, the solid-state structures of compounds 3A and
3B show remarkable differences, with regards not only to their
bond lengths and angles but also to their general coordination
geometries as well (Figure 4). Compound 3A adopts a
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Complexes 3A and 3B
structural parameter
3A
3B
U1,2O4,9
1.842(8)
2.103(9)
2.127(8)
2.129(8)
2.434(8)
1.818(12)
2.141(9)
2.072(8)
2.130(8)
2.398(10)
U1,2−O1,8
U1,2−O3,6
U1,2−O2,7
U1,2−OTHF
O4,9−U1,2−O1,8
O4,9−U1,2−O3,6
O1,8−U1,2−O3,6
O−U1−Oavg.,square‑plane
160.6(4)
119.6(5)
98.5(4)
99.7(3)
89.8
143.0(5)
95.5(3)
THF as well as another aryloxide ligand in the axial positions.
The U2−O9 bond distance of 1.818(12) Å is short compared
to previous reports but is still well within the range of a U(V)
oxo bond distance (vide supra). Interestingly, the bond distance
in 3B is even shorter than the respective bond in 3A, which has
an aryloxide ligand trans to the mono-oxo group. In addition to
these geometric features, 3B has three different U−OArO bond
lengths (U2−O6, 2.072(8); U2−O7, 2.130(8), and U2−O8,
2.141(9) Å). Both observations may be attributed to the steric
pressure imposed by the flanking diphenyl groups of the
aryloxide ligand, which should be higher in a trigonal-
bipyramidal coordination geometry. The Addison parameter
(τ) is an indicator of the degree of distortion from trigonal-
bipyramidal and square-pyramidal geometry53 and is 0.21 for
3A and 0.84 for 3B, which is in good agreement with the
description of 3A as square-pyramidal and 3B as trigonal-
1
bipyramidal. H NMR spectroscopic data shows five strongly
broadened signals between 0−9 ppm, indicating that both
complexes 3A and 3B can interchange in solution.
Variable temperature (2−300 K) direct current (dc)
magnetic susceptibility measurements were performed for 1,
2, and 3 to probe the uranium ion’s formal oxidation state and
electronic structures (Figure 5). Superconducting Quantum
Interference Device (SQUID) magnetization data for 1 and 2
are similar to each other, and both complexes exhibit highly
temperature dependent magnetic moments, μeff, of 2.37 (1) and
2.52 μB (2) at 300 K, which gradually decrease with decreasing
temperature to μeff = 1.21 (1) and 1.27 μB (2) at 2 K. Complex
3 has a magnetic moment of 1.96 μB at 300 K that is reduced to
Figure 4. Molecular structures of 3A (top) and 3B (bottom) as balls
and sticks representation (see Supporting Information for different
views and ORTEP representations). Hydrogen atoms and flanking
phenyl groups were omitted for clarity.
distorted square-pyramidal geometry with two aryloxide
ligands, the coordinated THF, and the terminal oxo ligand in
the square plane and another axial aryloxide ligand. The U1−
O4 bond distance of 1.842(8) Å (Table 2) compares well with
those found for U(V) oxo complexes reported in the literature
(1.842−1.859 Å).31,32,35 Noteworthy, 3A has two almost
identical U−OArO bond lengths (U1−O2, 2.129(8) Å and
U1−O3, 2.127(8) Å); however, the bond distance for U1−O1
of 2.103(9) Å, the one bound trans to the mono-oxo ligand, is
slightly shorter compared to the other two bond lengths. This
observation clearly can be attributed to the inverse trans
influence (ITI).51,52 The average bond angle for the O−U−O
angles of the square plane is 89.8°, which is well in agreement
with a uranium center coordinated in a square-pyramidal
fashion. In contrast, 3B features a distorted trigonal-bipyramidal
coordination geometry with two aryloxide ligands and the
mono-oxo moiety in the trigonal plane and the coordinated
Figure 5. Temperature-dependent SQUID magnetization data of
compounds 1, 2 (top), and 3 (bottom). Data were corrected for
underlying diamagnetism. Reproducibility was checked by three
independently synthesized and measured samples for each compound
(see Supporting Information).
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mmol of ZnCl2 (5.20 g)) dropwise via a glass pipet. After stirring for
0.5 h, the reaction mixture solidified, and the reaction was allowed to
proceed for an additional 2 h to ensure completion. The reaction flask
was cooled to room temperature, and the crude solids extracted into
CH2Cl2 and washed once with water and twice with brine. All volatiles
were removed by rotary evaporator, and cold MeOH (50 mL) was
added to precipitate clean white solids of the product. Yield = 84% (27
μeff = 0.86 at 2 K, which is typically observed for uranium(V)
complexes.35,54,55
CONCLUSION
■
In summary, we report the convenient and multigram synthesis
of a new, sterically encumbering aryloxide ligand and present its
ligand properties in coordination complexes of uranium. Single-
crystal XRD studies on the homoleptic U(III) tris(aryloxide)
and its THF-coordinated derivative revealed U−arene inter-
action via δ-backbonding in both complexes. In addition, the
coordinated ligand features diphenyl groups that act as a “picket
fence” above and below the plane of the U(III) center. In a
preliminary reactivity study it is shown that both U(III)
complexes, the THF-coordinated and the coordinatively
unsaturated species, react with N2O to form a U(V) complex
with a terminal oxo ligand. The XRD study on single crystals of
this five-coordinate oxo complex revealed two independent
molecules per unit cell, which exhibit distinct molecular
geometries, namely, a square pyramidal and trigonal bipyr-
amidal, but that are interchanging in solution at room
1
g, 61.2 mmol). H NMR (25 °C, 270 MHz, CDCl3): δ = 7.32−7.26
(m, 12H, Ar-H), 7.26−7.09 (m, 8H, Ar-H), 6.50 (s, 2H, Ar-H), 5.67
(s, 2H, CH(Ph)2), 4.43 (s, 1H, OH), 2.07 (s, 3H, Me). 13C NMR (25
°C, 67.8 MHz, CDCl3): δ 149.05 (Ar), 142.79 (Ar), 130.77 (Ar),
129.33 (Ar), 128.46 (Ar), 126.57 (Ar), 51.05 (Ph2CH), 20.97 (Me).
Anal. Calcd for C33H28O: C, 89.96; H, 6.41. Found: C, 89.83; H, 6.15.
Synthesis of NaOAr*. To a white suspension of HOAr* (5.00 g,
11.3 mmol) in diethyl ether (200 mL) at room temperature was added
a solid portion of NaN(SiMe3)2 (2.18 g, 11.9 mmol) over 15 min to
produce a homogeneous yellow solution. The mixture was allowed to
proceed for 12 h to give a faint yellow suspension. Subsequently, the
faint yellow solids were collected by vacuum filtration, and any residual
1
solvent was removed in vacuo. Yield = 73%. H NMR (25 °C, 270
MHz, C6D6): δ = 7.17−6.80 (m, 20H, Ar-H), 6.67 (s, 2H, Ar-H), 5.47
(s, 2H, CH(Ph)2), 2.06 (s, 3H, Me). 13C NMR (25 °C, 67.8 MHz,
C6D6): δ 146.77 (Ar), 131.35 (Ar), 129.57 (Ar), 129.38 (Ar), 125.94
(Ar), 54.07 (Ph2CH), 20.98 (Me).
1
temperature, which is indicated by H NMR spectroscopy.
This observation emphasizes the ligand’s flexibility to
accommodate different geometries while, at the same time,
the substantial steric bulk protects the reactive species from
bimolecular decomposition reactions and can mask a low-valent
U(III) ion. Accordingly, we are actively exploring the potential
of this system for further small molecule activation chemistry.
Synthesis of [(Ar*O)3U(THF)] (1). A 20 mL scintillation vial was
charged with a magnetic stirring bar and UI3(THF)4 (1.00 g, 1.10
mmol) in 8 mL of THF. To this dark blue homogeneous solution was
slowly added solid NaOAr* (1.53 g, 3.3 mmol) with a spatula. The
reaction mixture quickly became dark red, and the reaction was
allowed to proceed overnight. The suspension was filtered through a
medium porosity frit containing Celite, and all volatiles were removed
from the filtrate. The brown crude product was dissolved in benzene
and filtered again over Celite and washed with benzene until washing
is clear. The residual solvent was removed in vacuo to give a brown
EXPERIMENTAL SECTION
■
General Considerations. All air- and moisture-sensitive experi-
ments were performed under dry nitrogen atmosphere using standard
Schlenk techniques or in MBraun inert-gas glovebox containing an
atmosphere of purified dinitrogen. The glovebox is equipped with a
−35 °C freezer. Solvents were purified using a two-column solid-state
purification system (Glass Contour System, Irvine, CA), transferred to
the glovebox without exposure to air, and stored over molecular sieves
and sodium (where appropriate). NMR solvents were obtained
packaged under argon and stored over activated molecular sieves and
sodium (where appropriate) prior to use. Celite, alumina, and 4 Å
molecular sieves were activated under vacuum overnight at 200 °C. All
other chemicals were used as received from Sigma-Aldrich unless
otherwise stated. Precursor complexes [(UI3(dioxane)1,5] and [U(N-
(SiMe3)2)3] were prepared as described by Kiplinger et al.56
1
solid. Yield: 1.72 g (1.06 mmol, 96%). H NMR (25 °C, 270 MHz,
C6D6): δ = 11.49, 6.53, 5.83, 4.67, 4.17. IR (KBr pellet, cm−1): 3057
(m), 3024 (m), 2918 (w), 2860 (w), 1598 (m), 1492 (s), 1442 (s),
1288 (w), 1265 (m), 1249 (m), 1209 (m), 1132 (m), 1076 (w), 1029
(m), 914 (w), 862 (m), 842 (m), 759 (w), 750 (w), 702 (s), 678 (m),
621 (w), 603 (m), 563 (w), 532 (w). Anal. Calcd. for C103H89O4U: C,
75.95; H, 5.51. Found: C, 75.91; H, 5.49.
Synthesis of [(Ar*O)3U] (2). A 20 mL scintillation vial equipped
with a magnetic stirring bar was charged with [U(N[SiMe3])3] (525.9
mg, 0.73 mmol) in 5 mL of benzene. To this solution was added
dropwise a benzene (3 mL) solution of HOAr* (966.5 mg, 2.19
mmol), and the reaction was allowed to proceed overnight. The
resulting brown solution was filtered, washed with 2 mL of benzene,
and filtered again through a medium porosity frit. The residual solvent
was removed in vacuo to give pure 2. Yield: 736 mg (0.47 mmol,
65%). 1H NMR (25 °C, 270 MHz, C6D6): δ = 10.91, 8.46, 6.09, 5.77,
4.78, 4.12. IR (KBr pellet, cm−1): 3057 (m), 3024 (m), 2918 (w),
2860 (w), 1598 (m), 1492 (s), 1442 (s), 1288 (w), 1265 (m), 1249
(m), 1209 (m), 1132 (m), 1076 (w), 1029 (m), 914 (w), 862 (m),
842 (m), 759 (w), 750 (w), 702 (s), 678 (m), 621 (w), 603 (m), 563
(w), 532 (w). Anal. Calcd for C99H81O3U: C, 76.38; H, 5.24. Found:
C, 76.20; H, 5.09.
1H NMR spectra were recorded on a JEOL ECX 400 or 270
instrument at a probe temperature of 23 °C. 1H and 13C NMR spectra
are reported with reference to solvent resonances of C6D6 at 7.16 ppm
and 128.0 ppm, respectively. Electronic absorption spectra were
recorded from 400 to 1800 nm (Shimadzu (UV-3101PC)) in the
indicated solvent. Results from elemental analysis were obtained from
the Analytical Laboratories at the Friedrich-Alexander-University
Erlangen-Nurnberg (Erlangen, Germany) on Euro EA 3000. XRD
̈
data were collected on a Bruker Smart APEX 2 diffractometer under a
stream of N2 (g) at 100 K for 1 and 3 and on a Bruker-Nonius Kappa
CCD system under a stream of N2 (g) at 150 K for 2. Magnetism data
of crystalline powdered samples (20−30 mg) were recorded with a
SQUID magnetometer (Quantum Design) at 10 kOe (2−300 K for 1,
2, and 3). Values of the magnetic susceptibility were corrected for the
underlying diamagnetic increment (χdia = −988.77 × 106 cm3 mol−1
(1), −928.33 × 106 cm3 mol−1 (2), −964.97 × 106 cm3 mol−1 (3)) by
using tabulated Pascal constants and the effect of the blank sample
holders (gelatin capsule/straw).57
Synthesis of [(Ar*O)3U(O)(THF)] (3). A 20 mL scintillation vial was
charged with a magnetic stirring bar and 1 (200 mg, 0.13 mmol) in 3
mL of THF. This solution was fitted with a balloon containing N2O.
After 3−4 min, the reaction mixture turned deep red and was stirred
for another 30 min. The volatiles were removed, and the red
1
precipitate was dried in vacuo. Yield: 201 mg (0.13 mmol, 100%). H
NMR (25 °C, 270 MHz, C6D6): δ = 6.72, 3.44, 2.76, 1.27, 0.97. IR
(KBr pellet, cm−1): 3057 (m), 3024 (m), 2958 (m), 2902 (w), 2864
(w), 1599 (m), 1493 (s), 1443 (s), 1288 (w), 1261 (m), 1211 (m),
1184 (m), 1134 (m), 1116(w), 1076 (w), 1029 (m), 916 (w), 862
(m), 844 (m), 760 (w), 743 (w), 702 (s), 682 (m), 621 (w), 603 (m),
563 (w), 534 (w). Anal. Calcd for C99H81O3U: C, 75.21; H, 5.45.
Found: C, 75.24; H, 5.56.
Synthesis of HOAr* [Ar* = 2,6-Ph2-C6H4-4-Me]. In a 250 mL
round-bottom flask was charged p-cresol (7.92 g, 73.2 mmol),
diphenylmethanol (27.0 g, 146.5 mmol), and a large stirring bar.
The reaction flask was heated to 140 °C to produce a melt followed by
the addition of a solution of HCl/ZnCl2 (2.22 mL, 73.2 mmol HCl; 37
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Article
X-ray Crystal Structure Determinations. CCDC-919929 (for
1), CCDC-919930 (for 2), and CCDC-919931 (for 3A and 3B)
contain the supplementary crystallographic data for this paper. This
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/
products/csd/request/ (or from Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge, CB2 1EZ, U.K. Fax: ++44−
1223−336−033; e-mail: deposit@ccdc.cam.ac.uk).
AUTHOR INFORMATION
Corresponding Authors
*E-mail: karsten.meyer@fau.de.
*E-mail: mindiola@sas.upenn.edu.
■
Present Address
⊥Department of Chemistry, University of Pennsylvania, 231 S.
34 Street, Philadelphia, PA 19104-6323, United States.
Crystallographic Details. Black blocks of 1 were grown by slow
diffusion of n-hexane into a concentrated benzene solution containing
a drop of THF, brown prisms of 2 were grown by further
concentrating a benzene solution through slow diffusion into 1,2,3,4-
tetrahydronaphthalene, and orange plates of 3 were grown from slow
diffusion of n-hexane into a concentrated benzene solution at room
temperature. Crystals were coated with isobutylene oil on a
microscope slide. Intensity data were collected using MoKα radiation
(λ = 0.71073 Å, graphite monochromator) either at 100 K on a Bruker
Smart APEX 2 diffractometer for compounds 1 and 3 or at 150 K on a
Bruker-Nonius KappaCCD for 2. Data were corrected for Lorentz and
polarization effects; semiempirical absorption corrections were
performed on the basis of multiple scans using SADABS.58 All
structures were solved by direct methods and refined by full-matrix
least-squares procedures on F2 using SHELXTL NT 6.12.59 All
hydrogen atoms were placed in positions of optimized geometry; their
isotropic displacement parameters were tied to those of the
corresponding carrier atoms by a factor of 1.2 or 1.5.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
D.J.M. thanks the Chemical Sciences, Geosciences and
Biosciences Division, Office of Basic Energy Science, Office
of Science, U.S. Department of Energy (DE-FG02-07ER15893)
for financial support of this research. K.M. and W.H. would like
to acknowledge the funding of the Deutsche Forschungsge-
meinschaft (DFG) through the Cluster of Excellence Engineer-
ing of Advanced Materials.
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ASSOCIATED CONTENT
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* Supporting Information
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