Carbon Dioxide Activation by a Uranium(III) Complex Derived from a Chelating Bis(aryloxide) Ligand

Article abstract of DOI:10.1021/acs.organomet.7b00263

The new dianionic ligand C₆H₄{p-C(CH₃)₂C₆H₂Me₂O^-}₂ (=p-Me₂bp), featuring two aryloxide donors and a central arene ring, has been synthesized and used to prepare the mixed-ligand U(III) compound [U(Cp?)(p-Me₂bp)], which exhibits an η⁶ interaction with the uranium center. Reductive activation of CO₂ was investigated using [U(Cp?)(p-Me₂bp)] in supercritical CO₂, which gave a dinuclear uranium carbonate complex, {U(Cp?)(p-Me₂bp)}₂(μ-η¹:η²-CO₃), cleanly and selectively. Reactivity studies in conventional solvents using lower pressures of CO₂ showed the formation of a rare U(IV) oxalate complex, {U(Cp?)(p-Me₂bp)}₂(μ-η²:η²-C₂O₂), alongside {U(Cp?)(p-Me₂bp)}₂(μ-η¹:η²-CO₃). The relative ratio of the last two products is temperature dependent: at low temperatures (-78 °C) oxalate formation is favored, while at room temperature the carbonate is the dominant product. The U(IV) iodide [U(Cp?)(p-Me₂bp)I] was also synthesized and used as part of an electrochemical study, the results of which showed that [U(Cp?)(p-Me₂bp)] has a U^IV/U^III redox couple of -2.18 V vs FeCp₂^+/0 as well as a possible electrochemically accessible U^III/U^II reduction process at -2.56 V vs FeCp₂^+/0.

Full text of DOI:10.1021/acs.organomet.7b00263

Article
pubs.acs.org/Organometallics
Carbon Dioxide Activation by a Uranium(III) Complex Derived from a
Chelating Bis(aryloxide) Ligand
Christopher J. Inman, Alistair S. P. Frey, Alexander F. R. Kilpatrick, F. Geoffrey N. Cloke,*
and S. Mark Roe
Department of Chemistry, School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QJ, United Kingdom
S
* Supporting Information
ABSTRACT: The new dianionic ligand C6 H4 {p-C-
(CH3)2C6H2Me2O−}2 (=p-Me2bp), featuring two aryloxide donors and
a central arene ring, has been synthesized and used to prepare the
mixed-ligand U(III) compound [U(Cp*)(p-Me2bp)], which exhibits an
η6 interaction with the uranium center. Reductive activation of CO2 was
investigated using [U(Cp*)(p-Me2bp)] in supercritical CO2, which gave
a dinuclear uranium carbonate complex, {U(Cp*)(p-Me2bp)}2(μ-η1:η2-
CO3), cleanly and selectively. Reactivity studies in conventional solvents
using lower pressures of CO2 showed the formation of a rare U(IV)
oxalate complex, {U(Cp*)(p-Me2bp)}2(μ-η2:η2-C2O2), alongside {U-
(Cp*)(p-Me2bp)}2(μ-η1:η2-CO3). The relative ratio of the last two
products is temperature dependent: at low temperatures (−78 °C)
oxalate formation is favored, while at room temperature the carbonate is
the dominant product. The U(IV) iodide [U(Cp*)(p-Me2bp)I] was also synthesized and used as part of an electrochemical
study, the results of which showed that [U(Cp*)(p-Me2bp)] has a UIV/UIII redox couple of −2.18 V vs FeCp2 as well as a
+/0
possible electrochemically accessible UIII/UII reduction process at −2.56 V vs FeCp2
.
+/0
form of a mixed-sandwich framework incorporating a cyclo-
pentadienyl ligand and a dianionic aromatic cycloooctatetraene
or pentalene ligand.9,12 Hence, we were interested in the effect
of changing the dianionic ligand in the latter for an alternative,
“harder” chelating ligand with tunable steric demands. Here we
report a new bidentate ligand featuring two dimethyl-
functionalized aryloxide rings and a central arene ring and the
synthesis of its Cp*UIII derivative and reactivity toward CO2
thereof. Beyond simply providing steric bulk, the arene is
centrally situated to promote stabilization of low-valent U
centers via δ back-bonding (vide supra).
INTRODUCTION
■
Since the development of easily accessible U(III) starting
materials, the field of uranium(III) chemistry has flourished,
utilizing a wide range of ligands to stabilize uranium in this
oxidation stateof particular current interest in the context of
small-molecule activation.1 Aryloxide groups provide a hard
oxygen donor atom, well suited to binding oxophilic uranium in
the +3 and higher oxidation states,2,3 and homoleptic U(III)
derivatives of bulky aryloxide ligands have been shown to
reductively activate N2 and CO and insert CO2.4 Meyer et al.
pioneered the use of the tripodal ligands derived from phenols
of the type (R,R′ArOH)3mes, which incorporates three aryloxide
arms and a central arene ring which provides additional
electronic stabilization via a δ interaction with the uranium
center.5 This approach has led to the successful synthesis of a
U(II) complex6 and the uranium(III) complex
[((Ad,MeArO)3mes)U], which electrocatalytically generates
dihydrogen from water.7 Arnold et al. have also reported a
tetraaryloxide variant which facilitates the complexation of two
uranium centers.8
RESULTS AND DISCUSSION
■
Ligand Synthesis. The neutral bisphenol (p-Me2bp)H2 (1)
was prepared via a modification of a procedure reported in the
patent literature.13 α,α,α′,α′-Tetramethyl-1,4-benzenedimetha-
nol undergoes an acid-catalyzed condensation reaction with
2,4-dimethylphenol (excess, as solvent) to give crude 1
(Scheme 1). Recrystallization from Et2O gave 1 as white
crystals in 40−50% overall yield, with analytical purity
confirmed by elemental analysis.
Uranium(III) complexes featuring a variety of ancillary ligand
2−
sets have been shown to disproportionate CO2 to form CO3
The solid-state molecular structure of 1 was determined by
X-ray diffraction and is shown in Figure 1, together with
complexes and CO.9 Furthermore, we and others have also
reported the reductive coupling of CO2 to yield bridging
oxalate U(IV) complexes, which is a rare transformation in
uranium chemistry (and indeed in the d-block elements).10,11
Our work to date on small-molecule activation chemistry has
focused on the use of a “soft” ligand set around U(III), in the
Special Issue: Organometallic Actinide and Lanthanide Chemistry
Received: April 7, 2017
© XXXX American Chemical Society
A
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spectroscopy; however, these solutions were too dilute to allow
acquisition of 13C NMR spectroscopic data.
Scheme 1. Synthesis of (p-Me2bp)H2 (1)
Mixed-Ligand Uranium(III) Complex. Dipotassium salt 2
reacts with 1 equiv of [UCp*I2(THF)n] (prepared in situ in
THF) to give, after workup, the mononuclear U(III) complex
[U(Cp*)(p-Me2bp)] (3) as dark green-black plates in yields of
60−75% (Scheme 2).
Scheme 2. Synthesis of [U(Cp*)(p-Me2bp)] (3)
Complex 3 is soluble in hydrocarbons and ethers, and single
crystals suitable for X-ray crystallography were obtained by
recrystallization from pentane at −50 °C. The asymmetric unit
contains two crystallographically independent molecules of 3
(Figure 2) featuring the bisaryloxide ligand in a syn
Figure 1. ORTEP diagram for molecular structure of compound 1.
The disordered C6D6 solvent is omitted for clarity; thermal ellipsoids
are given at 50% probability. The numbering scheme is used for NMR
spectroscopy assignments. Selected structural parameters (Å, deg):
C12−C13 = 1.390(17), C12−C14 = 1.3933(18), C6−C9−C12 =
109.05(10), C1−O1 = 1.3843(15).
Figure 2. ORTEP diagram for the molecular structure of compound 3.
One crystallographically independent molecule is shown with 50%
probability thermal ellipsoids, and H atoms are omitted for clarity.
Selected structural parameters (Å, deg): average U−O = 2.179, average
U−C(arene) = 2.774, average Ct(arene)−U = 2.388, average
Ct(Cp*)−U = 2.487, average O1−U−O2 = 117.9.
selected bond lengths and angles. The phenol rings adopt an
anti conformation with respect to the central arene, and the
quaternary sp3 carbon (C9) linking the phenol and arene rings
is tetrahedral (C6−C9−C12 angle of 109.05(10)°), consistent
with a lack of strain. The central arene C−C distances are
essentially the same as those observed in the solid-state
structure of p-xylene (1, average 1.3930 Å; p-xylene, average
1.392 Å), and the central arene displays almost no ring torsion
(1, 0.15°; p-xylene, 0.10°).14 The two phenol rings are
related via a crystallograpic inversion center, and the angle
between the planes of the central arene and each phenol ring is
81.51°.
conformation, allowing both aryloxide oxygens to chelate to
the U center with an average U−O distance of 2.179 Å, which is
within the range observed for other U(III) aryloxides (2.155−
2.338 Å).15 The Cp* ring assumes the usual η5-binding mode
(average Ct(Cp*)−U = 2.487 Å), while the central arene ring is
planar (maximum deviation from C6 plane 0.038 Å) and is
bound to the U center via an η6-arene interaction with an
average U−C(arene) distance of 2.774 Å. This is similar to the
U−C(arene) distance in the mesityl-anchored chelating U(III)
1H and 13C NMR spectroscopic data show 2-fold symmetry,
indicating that in solution the phenol rings freely rotate around
the C(CH3)2 units linking them and the central arene ring.
Bisphenol 1 is readily deprotonated by 2 equiv of KH in 1,2-
dimethoxyethane (DME) to give the dipotassium salt (p-
Me2bp)K2(DME)n (2) as a fractional solvate (n = 1.5−2) in
85−95% yield. The crude material thus obtained returned
elemental analyses which were slightly high in C and H;
however, it was successfully used in subsequent salt metathesis
reactions without further purification. Compound 2 is
sufficiently soluble in d5-pyridine to allow the degree of DME
5
t
complex C6Me3{CH2C6H2 Bu2O}3U (average 2.73 Å) and
somewhat shorter than those in other U(III) complexes
featuring pendant arenes (e.g., (2,6-Ph2-4-Me-C6H3-O)3U,
2.853 Å; (2,6-Ph2-4-Me-C6H3-O)3U(THF), 2.964 Å)16 and
discrete arenes (e.g., (C6Me6)U(AlCl4)3, 2.93 Å;17 (C6Me6)U-
(BH4)3, 2.93 Å).18 The central arene C−C distances in 3
(average 1.411 Å) are slightly longer than those observed in 1
(average 1.393 Å). Interestingly, the metal center bears no
coordinated Lewis base (i.e., THF), unlike, for example, the
1
solvation in individual samples to be determined by H NMR
B
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mixed-sandwich complexes [U(η8-C8H6{SiMe3-1,4}2)(η5-
9
i
t
C5Me4R) (THF)] (R = Me, Et, Pr, Bu).
The 1H NMR spectrum of paramagnetic 3 contains
resonances over the range 18.3 to −13.5 ppm. Both the CH3
groups of the bridging arms and the central arene protons
appear as two distinct sets of resonances due to the reduced
symmetry imposed by a rigid aryloxide and Cp* coordination
environment.
Carbon Dioxide Activation. The reaction of gases with f-
element complexes in the solid state (i.e., solvent free) has
recently been reported as a potentially high yielding and
selective approach to exploring subsequent reduction chem-
istry.19 Recent work from our own laboratory has used
supercritical CO2 to promote the exclusive formation of
U(IV) bridging carbonate complexes,9 and this approach was
used to investigate the reductive activation of CO2 by 3
(Scheme 3).
Figure 3. ORTEP diagram for molecular structure of compound 4,
with 50% probability thermal ellipsoids shown. Solvent masking was
employed to remove a highly disordered C6H6 from the refinement.
Selected structural parameters (Å, deg): average U−O(aryloxide) =
2.175, η1-U2−O5 2.162(11), average η2-U−O = 2.448, average U−
C(arene) = 3.124, Ct(arene) − U = 2.7902(3), Ct(Cp*)−U =
2.4958(3), O1−U1−O2 = 161.6(3).
Scheme 3. Synthesis of {U(Cp*)(p-Me2bp)}2(μ-η1:η2-CO3)
(4)
central U−arene interaction (average U−C(arene) = 3.124 Å)
significantly weaker than that observed in 3, possibly due to the
larger number of O donors and greater steric congestion in 4.
This weak U−arene interaction is consistent with a pair of
U(IV) centers each binding to a neutral, unreduced central
arene. The complex is twisted around the central carbonate unit
to minimize steric interaction between opposite bisaryloxides
(torsion as measured between opposite arene centroids
Ct1(arene) − U1−U2−Ct2(arene) = 67.95°).
The torsion observed in the solid-state structure of 4·C6H6
1
persists in solution. At 0 °C the H NMR spectrum of 4
(C6D5CD3) consists of eight CH resonances, eight CH3
resonances, and one resonance for the two freely rotating
Cp* ligands paramagnetically shifted between 33.0 and −67.1
ppm. Above 10 °C the aryloxide resonances exhibited varying
degrees of coalescence in addition to the changes in chemical
shift expected for a paramagnet, but there was no resolution to
a discrete, higher symmetry species even at 100 °C.
The reaction of solid 3 in supercritical CO2 resulted in an
immediate color change from green-black to orange, and the
resultant orange powder contained only one major product
(>96% by 1H NMR spectroscopy), confirming the highly
selective nature of the reaction. The product was identified as
the bridging carbonate complex {U(Cp*)(p-Me2bp)}2(μ-η1:η2-
CO3) (4) by X-ray diffraction studies of crystals of 4·C6H6
obtained from C6H6 at ambient temperature. The degree of
The reductive activation of 13CO2 by 3 was also investigated
in solution (Scheme 4). An excess (ca. 2 equiv) of 13CO2 was
added to a solution of 3 in C D CD at −78 °C using a Topler
̈
6
5
3
pump. When the sample was warmed to ambient temperature,
1
1
solvation in 4·C6H6 was determined by H NMR spectroscopy
the color changed from black to orange, and the H NMR
in d8-THF, and satisfactory elemental analysis was obtained for
this solvate.
spectrum showed the presence of a mixture of products,
Scheme 4. Synthesis of {U(Cp*)(p-Me2bp)}2(μ-η1:η2-CO3)
(4) and {U(Cp*)(p-Me2bp)}2(μ-η2:η2-C2O2) (5)
The solid-state molecular structure of 4 is shown in Figure 3,
together with selected bond distances and angles, and reveals a
dinuclear structure with a η1:η2-bound carbonate bridging two
2−
uranium centers. The central CO3 unit is disordered in the
crystal, resulting in superposition of η1:η2 and η2:η1 binding
modes with 50:50 occupancy. The resulting model required
isotropic refinement for the central carbonate (which has a
regular planar triangular structure), and as a result the bond
lengths and angles within the central carbonate unit cannot be
determined accurately. The η1-U−O distance (2.162(11) Å) is
shorter than the η2-U−O distances (average 2.448 Å), a feature
which was also observed in the mixed-sandwich U(IV)
carbonate complex {U(η8 -C8 H6 {Sii Pr3-1,4}2 )(η5 -
C5Me4H)}2(μ-η1:η2-CO3) (η1, 2.227(12) Å; η2, average
2.422(10) Å).8b Each U center is chelated by a pair of
bisaryloxide oxygens (average U−O = 2.175 Å), with the
C
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including 4 (ca. 40%). In addition to excess 13CO2 and free
13CO (arising from the reductive disproportionation of 13CO2
to form 4), three paramagnetically shifted resonances were
observed in the 13C{1H} NMR spectrum of the reaction
mixture. A singlet at −101 ppm was assigned to the bridging
13CO3 in 4, and two mutually coupled doublets appeared at
−130 and −199 ppm; all three resonances remained unchanged
suggest that the formation of 4 competes with the formation of
5 and the two pathways can be kinetically manipulated.
Uranium(IV) Iodide Complex. An attempt to prepare 3
using DME as solvent instead of THF using equimolar amounts
of 2 and [UCp*I2(DME)n], produced in situ from UI3 and
KCp* in DME, resulted in white solids (presumed to be KI)
and a dark brown solution which contained several new
products, but no 3 by 1H NMR spectroscopy (Scheme 5). The
1
in the H-coupled spectrum. The observed coupling in the
latter is consistent with coupling between two proximal,
inequivalent 13C atoms (JCC = 70 Hz), thus pointing toward
the formation of an oxalate complex resulting from the
reductive coupling of 13CO2.
Scheme 5. Synthesis of [U(Cp*)(p-Me2bp)I] (6)
Fractional crystallization from benzene and then diethyl
ether produced yellow crystals of the bridging oxalate complex
5 suitable for single-crystal X-ray diffraction. The molecular
structure is shown in Figure 4, together with selected bond
uranium(IV) iodo complex 6 was isolated from this solution as
a microcrystalline red powder in 29% yield, and its formation
from a U(III) starting material must presumably involve a
disproportionation reaction, thus explaining the modest yield of
6. Single crystals of 6 suitable for X-ray diffraction were
obtained from THF and furnished 6·THF, which was used for
NMR and MS studies. Satisfactory elemental analysis results
were obtained for 6·THF.
The molecular structure of 6 displays a syn-aryloxide
chelating to a U center which is η6-bound to the central
arene weakly (average U−C(arene) = 2.985 Å), consistent with
a U(IV) center (Figure 5). The U−O distances (average 2.149
Figure 4. ORTEP diagram for molecular structure of compound 5,
with 50% probability thermal ellipsoids shown. Selected structural
parameters (Å, deg): C1−C2 = 1.531(12), O1−C1−C2−O2 torsion
angle = −22.2(12), average U−O(aryloxide) = 2.147, average
Ct(Cp*)−U = 2.5030, average Ct(arene)−U = 2.828, average O−
U−O (aryloxide) = 160.6, Ct(Cp*)−U−Ct(arene) = 120.184(14).
lengths and angles, and shows a η2:η2 bridging oxalate unit in
which the two carbons are inequivalent (the two Cp* ligands
are effectively cis to one another) and hence consistent with
13C NMR data. Of note is the O1−C1−C2−O2 torsion angle
of 22.2(12)°, which is not seen in the molecular structures of
9
i
{U(η8-C8H6{SiMe3-1,4}2)(η5-C5Me4 Pr)}2(μ-η2:η2-C2O4) and
{((nP,MeArO)3tacn)UIV}2(μ-η2:η2-C2O4),10 possibly due to the
steric hindrance around the uranium center. The U−
O(oxalate) and oxalate C−C and C−O bonds are, however,
consistent with those found in the latter U(IV) oxalate
complexes. The arene centroid to uranium distance of 2.828
Å in 5 is again indicative of a weak interaction between the
uranium center and the central arene ring.
To gain further insight into the distribution of products 4
and 5 formed from 3 and 13CO2, the reaction was repeated but
the reaction mixture was allowed to react at −78 °C for 2 days
followed by slow warming to room temperature over 1 day.
This furnished a mixture of 4 and 5 in a ratio of 30:70, as
judged by the relative integration of the Cp* resonances in the
1H NMR spectrum of the crude reaction mixture. When
addition of 13CO2 to 3 at −78 °C was followed by rapid
equilibration to room temperature, the ratio of 4 to 5 was
found to be 83:17, suggesting that 5 is the kinetic product while
4 is the thermodynamic product. The above observations
Figure 5. ORTEP diagram for the molecular structure of solvent-free
6, with 50% probability thermal ellipsoids shown and H atoms
removed for clarity. Selected structural parameters (Å, deg): average
U−O = 2.149, U−I = 3.0573(6), average U−C(arene) = 2.985,
Ct(arene)−U = 2.6381(3), Ct(Cp*)−U = 2.4788(3), O1−U−O2 =
159.18(16), Ct(Cp*)−U−Ct(arene) = 130.181(8).
Å) are similar to those observed in the U(IV) complex 4, and
the U−I bond length of 3.0573(6) Å is typical for a U(IV)
iodide: e.g., 3.059(2) Å in Cp3UI.20 Complex 6·THF is soluble
1
in ethers and aromatic solvents, and the H NMR spectrum
(C6D6) of 6·THF shows 11 peaks paramagnetically shifted over
the range 37.2 to −64.9 ppm. Two sets of meso-CH3 and
central arene proton resonances indicate a nonfluxional
bisaryloxide coordination environment, as observed for 3.
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Electrochemistry. To investigate the redox behavior of the
bis(aryloxide)/Cp* complexes and enable comparison with
similar U(III) and U(IV) systems, voltammetric data for 3−6
were obtained in [nBu4N][B(C6F5)4]/THF. Tabulated electro-
chemical data can be found in the Supporting Information.
The cyclic voltammogram of 3 is quite complex and displays
several processes, as shown in Figure 6. Four quasi-reversible
processes were observed at −2.56, −2.18, −1.71, and +0.13 V
The electrochemistry of 5 was studied in 0.05 M [nBu4N]-
[B(C6F5)4]/THF and showed two quasi-reversible events at
−2.24 and −2.56 V vs FeCp2+/0 (Figure 7). These processes are
vs FeCp2+/0. In addition, the CV of 3 shows a minor oxidation
+/0
feature at ca. −1 V vs FeCp2
.
Figure 7. Overlaid CV scans (5 cycles) for 5 in THF/0.05 M
[nBu4N][B(C6F5)4] (scan rate 100 mV s−1).
assigned to the [UIV−UIV]/[UIV−UIII]− and [UIV−UIII]−/
[UIII−UIII]2− couples on the basis of reasonable agreement
with corresponding processes observed in 3, 4, and 6 and in
9
i
[U(η8-C8H6{SiMe3-1,4}2)(η5-C5Me4 Pr)]2(μ-η2:η2-C2O4).
Peak current ratios (ipc/ipa) for these processes of 1.32 and 1.72,
respectively, indicate that the species generated on the cathodic
scan are not completely oxidized back on the return anodic
Figure 6. Overlaid CV scans (10 cycles) for 3 in THF/0.05 M
[nBu4N][B(C6F5)4] (scan rate 100 mV s−1).
I−II
scan. The ΔE1/2 value of 320 mV is indicative of a metal−
metal interaction. Two minor anodic processes were observed
at −520 and −1007 mV, which we are unable to assign with any
certainty.
Cyclic voltammetry was carried out on the U(IV) derivative
6 to gain insight into the nature of the processes seen in 3.
Complex 6 shows an irreversible reduction wave at −2.19 V vs
CONCLUSION
■
+/0
FeCp2 (Figure S4 in the Supporting Information), which is
The combination of the soft pentamethylcyclopentadienyl
ligand and a hard, chelating aryloxide ligand affords the U(III)
complex [U(Cp*)(p-Me2bp)], whose UIV/UIII redox couple is
essentially identical with that of the mixed-sandwich complexes
of the type U(η-C8H6{SiR3-1,4}2)(η-C5Me4R′). The work
presented here demonstrates that the new ligand (p-
Me2bp)2− in [U(Cp*)(p-Me2bp)] can significantly alter the
reactivity toward CO2. The new bidentate ligand presented can
be synthesized cleanly in moderate yield and provide a system
analogous to the COT- and pentalene-based mixed sandwiches
already established in actinide chemistry. Using this ligand, a
U(III) complex has been used to reductively disproportionate
and reductively couple CO2 to give uranium carbonate and
oxalate complexes, respectively. Studies have shown that the
reactivity toward CO2 can be controlled via temperature.
Electrochemical studies show the UIV/UIII couple has a
reduction potential of −2.18 V vs FeCp2+/0, indicating that
this system is an ideal candidate to support metal centers for
the activation of small molecules.
in excellent agreement with the quasi-reversible process at
−2.18 V vs FeCp2 seen in 3 and is assigned to the UIV/UIII
+/0
couple in this system. This value is similar to other UIV/UIII
redox couples found in other UIII systems studied by us and
others8,21 and is consistent with a strongly reducing metal
center. Further supporting this assignment is the observation of
a single quasi-reversible reduction process at −2.19 V vs
+/0
FeCp2 in the dinuclear U(IV) complex 4 (Figure S2 in the
Supporting Information).
Upon scanning to more negative potentials, 3 and 6 showed
a quasi-reversible process observed at −2.56 and −2.59 V vs
FeCp2+/0 respectively. This process lies at an extremely negative
potential and may be attributable to a ligand-based reduction;
however, the free ligand 2 did not show a cathodic process
within this potential region (Figure S1 in the Supporting
Information). Another possibility is a UIII/UII reduction
process, and Meyer and co-workers have recently reported a
trivalent uranium monoarene complex derived from the
chelating tris(aryloxide)arene ligand [{(Ad.MeArO)3-mes}UIII],
which shows a nearly reversible and chemically accessible
Future studies will focus on the reactivity of [U(Cp*)(p-
Me2bp)] toward other small molecules, alongside the
modification of the (p-Me2bp)2− ligand, which may lead to
the discovery of new reactivity toward substrates.
22
+/0
reduction at −2.495 V vs FeCp2
.
When 4 was studied in 0.1 M [nBu4N][PF6]/THF, the first
UIV to UIII reduction event occurred at −2.25 V vs FeCp2
+/0
(Figure S3 in the Supporting Information). For comparison
reduction processes in the carbonate-bridged complexes based
EXPERIMENTAL SECTION
■
General Information. All manipulations involving air- or
moisture-sensitive materials were performed under an inert atmos-
phere of argon using standard Schlenk techniques or in an MBraun N2
or Ar-filled glovebox. Solvents were dried over appropriate drying
on the [U(η8-C8H6{SiMe3-1,4}2)(η5-C5Me4R)] (R = Et, Pr,
i
tBu) mixed-sandwich ligand system were observed between
−2.11 and −2.17 V vs FeCp2+/0 in 0.1 M [nBu4N][PF6]/THF.9
E
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agents (NaK2.8 for pentane, Et2O, and tBuOMe; K for THF, benzene,
and toluene) before distilling under N2 and degassing before use.
Solvents were stored over K mirrors, with the exception of THF and
Et2O, which were stored over activated 4 Å molecular sieves.
Deuterated solvents were dried over K, vacuum-distilled and freeze−
pump−thawed before storage under N2. NMR spectra were recorded
{U(Cp*)(p-Me2bp)}2CO3 (4). Complex 3 (75 mg, 0.097 mmol)
was placed in the sapphire NMR tube, which was then pressurized
with CO2 to 55 bar, causing an immediate change in color from dark
olive green/black to orange. The tube was sealed and warmed to 40
°C, causing the CO2 to form a supercritical phase, observed as a loss of
meniscus and visibly increased density throughout the tube. The tube
was maintained at this temperature for 20 min before cooling (liquid
CO2 state reestablished) and venting of excess gas. Solid, orange 4 was
retrieved inside the glovebox (71 mg, 0.044 mmol, 91%). Solvent-free
4 was used for NMR and MS studies. Crystalline samples of 4·C6H6
spontaneously crystallized out from solutions of 4 in C6H6 at room
termperature and were used for elemental analysis. 1H NMR
(C6D5CD3, 273 K): δ 32.89 (s, 6 H, CH3), 29.69 (s, 6H, CH3),
28.81 (s, 6H, CH3), 27.22 (s, 2H, CH), 19.01 (s, 2H, CH), 17.57 (s,
2H, CH), 14.81 (s, 6H, CH3), 13.76 (s, 2H, CH), 10.64 (s, 6H, CH3),
5.22 (s, 6H, CH3), 0.15 (s, 6H, CH3), −3.94 (s, 6H, CH3), −10.65 (s,
30H, Cp*), −51.26 (s, 2H, CH), −61.22 (s, 2H, CH), −63.30 (s, 2H,
CH), −67.11 (s, 2H, CH). Anal. Calcd for C77H94O7U2·C6H6: C,
59.14; H, 5.98. Found: C, 59.50; H, 5.61. MS (ESI)+: 1608 m/z (M+).
{U(Cp*)(p-Me2bp)}2C2O4 (5). A 50 mL J. Young ampule was
charged with 3 (300 mg, 0.386 mmol) and toluene (3 mL). The
solution was cooled to −78 °C and degassed, and 4 equiv of CO2 gas
was added. A color change from black to orange was observed shortly
after the addition of CO2. The reaction mixture was stirred at −78 °C
for 2 days and then at room temperature for 1 day. Volatiles were
removed in vacuo, benzene (5 mL) was added, and the suspension was
heated at almost reflux and slowly cooled to ambient temperature
overnight to afford 4·C6H6 as a microcrystalline red solid. The latter
was collected by filtration, and washed with benzene (3 × 3 mL), and
the washings were combined with the original filtrate. This solution
was then concentrated to ca. 3 mL and allowed to stand at room
temperature overnight, depositing a further, small amount of 4·C6H6,
which was filtered off. Finally, the filtrate from this second
crystallization was pumped to dryness, dissolved in Et2O (3 mL),
and stored at −35 °C to give small yellow crystals of 5 (32 mg, 10%).
1H NMR (C6D6, 273 K): δ 39.34 (s, 12 H, CH3), 31.67 (s, 4 H, Ar-H),
25.45 (s, 4 H, Ar-H), 22.68 (s, 12 H, CH3), 14.82 (s, 12 H, CH3),
−3.77 (s, 12 H, CH3), −7.62 (s, 30 H, Cp*), −53.91 (s, 4 H, Ar-H),
−62.33 (s, 4 H, Ar-H). Anal. Calcd for C78H94O8U2: C, 57.28; H, 5.79.
Found: C, 57.00; H, 6.02.
1
on a Varian VNMR 400 MHz spectrometer at 303 K, with H NMR
spectra run at 399.5 MHz and 13C NMR spectra run at 100.5 MHz.
Chemical shifts are quoted in parts per million and are referenced
internally to residual protic solvent shifts (1H) or deuterated solvent
shifts (13C). EI-MS was performed by Dr. A. K. Abdul-Sada at the
University of Sussex using a VG Autospec Fisions instrument (EI at 70
eV). Elemental analyses were performed by Mikroanalytisches Labor
Pascher or Bristol University Microanalytical Laboratory. 13CO2
(99.7% enrichment) gas was purchased from EuroIsotop and was
transferred via an accurately calibrated Topler pump. The reaction
̈
with supercritical CO2 (99.999% BOC) was performed in a 10 mm
sapphire NMR tube equipped with a titanium valve, connected to a
high-pressure stainless steel line equipped with a high-pressure piston
pump (High Pressure Equipment Company).
Caution! Depleted uranium is a weak α emitter (4.197 MeV) with a
half-life of 4.47 × 109 years. All manipulations should be undertaken in
a well-ventilated and -monitored fumehood or under an inert
atmosphere in a glovebox, and uranium-containing residues should
be disposed of according to local regulations.
(p-Me2bp)H2 (1). α,α,α′,α′-Tetramethyl-1,4-benzenedimethanol
(10 g, 50 mmol) dissolved in 2,4-dimethylphenol (20 g) was added
with stirring over 4 h to 2,4-dimethylphenol (45 g) containing p-
tolylsulfonic acid (0.30g, 1.5 mmol, 3 mol %) at 150 °C. Na2CO3 (0.60
g, 5 mmol) was added to neutralize the catalyst, and the mixture was
stirred another 40 min. Excess 2,4-dimethylphenol was distilled off
under vacuum, the solid, beige residue was taken up in Et2O, and this
solution was rinsed with NaOH (0.6 M, 1 × 100 mL), water (3 × 150
mL), and brine (2 × 50 mL), dried over MgSO4, and stripped to
provide a crude white solid. Recrystallization from Et2O at −50 °C and
drying for 2 h (80 °C, 0.01 mbar) provided 1 as a white powder (9.5 g,
23.6 mmol, 47%). 1H NMR (assigned according to Figure 1) (CDCl3,
303 K): δ 7.36 (s, 4H, H13,14), 7.18 (s, 2H, H5), 6.95 (s, 2H, H3),
4.28 (s, 2H, OH), 2.39 (s, 6H, H8), 2.18 (s, 6H, H7), 1.73 ppm (s,
12H, H10,11). 13C{1H} NMR (assigned as per Figure 1) (CDCl3, 303
K): δ 149.9 (C2), 147.3 (C12), 134.6 (C6), 130.4 (C3), 129.1 (C4),
127.1 (C13,14), 126.2 (C1), 124.9 (C5), 41.7 (C9), 30.0 (C10,11),
21.2 (C8), 16.4 ppm (C7). Anal. Calcd for C28H34O2: C, 83.54; H,
8.51. Found: C, 83.26; H, 8.49. MS (EI)+: m/z 402 (M+).
[U(Cp*)(p-Me2bp)I] (6). A deep blue-green solution of [UI2Cp*-
(DME)n] was prepared by adding DME (70 mL) to KCp* (0.26 g, 1.5
mmol) and UI3 (0.929 g, 1.5 mmol) and stirring for 5 days followed by
cannula filtration. To this was added (p-Me2bp)K2(DME)1.66 (0.60 g,
0.95 mmol, 0.63 equiv) as a suspension in DME (15 mL). The dark
brown reaction mixture was stirred overnight before filtration through
Celite, reduction to 20 mL, and layering with pentane (20 mL). The
mother liquors were stripped, taken up in toluene (10 mL) and
pentane (10 mL), and stirred overnight. The brown mixture was
(p-Me2bp)K2(DME)1.65 (2). A solution of 1 (2.66 g, 6.62 mmol) in
DME (40 mL) was added dropwise over 1 h to a stirred suspension of
KH (0.533 g, 13.3 mmol) in DME (60 mL). After 18 h the reaction
mixture was cooled to −30 °C for 1.5 h and filtered through a frit, and
the solids thus obtained were dried in vacuo for 2.5 h to give the
product as a white powder of sufficient purity for further reactions
t
filtered via cannula and stripped, and BuOMe (5 mL) was added,
1
(3.95 g, 95%). H NMR (C5D5N, 303 K): δ 7.43 (s, 4H, Ar-H), 7.42
giving a dark brown solution which spontaneously deposited red
crystals over 3 days at room temperature. These crystals were rinsed
with pentane at −78 °C and dried in vacuo to give 6 (388 mg, 0.431
mmol, 29% based on UI3). Recrystallization of 6 from THF at −50 °C
gave 6·THF as red crystals which were used for NMR, MS, and
(s, 2H, Ar-H), 7.11 (s, 2H, Ar-H), 3.51 (s, 6.7H, DME CHs, 6.6H,
DME CH2), 3.29 (s, 9.9H, DME CH3), 2.54 (s, 6H, Ar-CH3), 2.33 (s,
6H, Ar-CH3) 1.90 (s, 12H, C(CH3)2). Anal. Calcd for
C34.6H48.5O5.3K2: C, 66.2; H, 7.79. Found: C, 69.2; H, 8.19.
1
[U(Cp*)(p-Me2bp)] (3). A deep blue-green solution of [UC-
p*I2(THF)n] was prepared by adding THF (70 mL) to KCp* (0.35 g,
2.0 mmol) and UI3 (1.24 g, 2.0 mmol), with overnight stirring
followed by filtration. To this was added (p-Me2bp)K2(DME)2.0 (1.12
g, 1.7 mmol, 0.85 equiv) as a suspension in THF (15 mL). The dark
brown reaction mixture was stirred overnight before being stripped to
dryness, taken up in pentane (60 mL), filtered through Celite, and
reduced to ca. 15 mL. Slow cooling to −50 °C gave 3 as dark olive
green platelets, which were rinsed with pentane and dried in vacuo
(0.94 g, 72%). Analytically pure 3 was obtained by recrystallization
from pentane at −50 °C. 1H NMR (C6D5CD3, 303 K): δ 18.28 (s, 2H,
Ar-H), 10.05 (s, 2H, Ar-H), 9.05 (s, 2H, Ar-H), 4.54 (s, 6H, CH3),
4.14 (s, 6H, CH3), −2.24 (s, 6H, CH3), −3.89 (s, 15H, Cp*), −13.56
(s, 2H, Ar-H). Anal. Calcd for C38H47O2U: C, 58.98; H, 6.12. Found:
C, 58.96; H, 6.35.
elemental analysis. H NMR (C6D6): δ 37.23 (s, 6H, CH3), 36.39 (s,
6H, CH3), 31.32 (s, 2H, Ar-H), 26.38 (s, 2H, Ar-H), 15.25 (s, 6H,
CH3), 3.58 (m, 4H, THF), 1.42 (m, 4H, THF), 1.31 (s, 6H, CH3),
−5.59 (s, 15H, Cp*), −53.92 (s, 2H, Ar-H), −64.90 (s, 2H, Ar-H).
Anal. Calcd for C38H47IO2U(C4H8O): C, 51.86; H, 5.70. Found: C,
51.85; H, 5.71. MS (EI)+: m/z 765 (M+ − THF − Cp*).
X-ray Crystallographic Studies. Data sets for 1 and 3−6 were
collected on an Agilent Gemini Ultra diffractometer with an Enhance
source (Mo Kα or Cu Kα) equipped with an Eos CCD area detector
and an Oxford Cryosystems low-temperature device (173 K),
operating in ω scanning mode with ψ and ω scans to fill the Ewald
sphere. The program used for control, integration, and absorption
correction was CrysalisPro.23 The crystals were mounted on glass
fibers or MiTiGen loops with vacuum oil. All solutions and
refinements were performed using the OLEX2 package.24 All non-
F
DOI: 10.1021/acs.organomet.7b00263
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
hydrogen atoms were refined using anisotropic thermal parameters,
and hydrogens were added using a riding model. In the case of 4 a
highly disordered molecule of benzene was found in the asymmetric
unit that could not be successfully modeled, and as such it was treated
using solvent masking. Single-crystal XRD data for 5 were collected by
the UK National Crystallographic Service (NCS) at the University of
Southampton25 on a Rigaku FR-E+ Ultra High Flux diffractometer
(Mo Kα) equipped with VariMax VHF optics and a Saturn 724+ CCD
area detector. The data were collected at 100 or 150 K using an
Oxford Cryostreams Cobra low-temperature device. An empirical
absorption correction was carried out using the MULTI-SCAN
program. Data collected by the NCS were processed using
CrysAlisPro, and unit cell data parameters were refined against all data.
J.; Avens, L. R. Inorg. Chem. 1998, 37, 4040−4045. (c) Wilkerson, M.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00263.
■
S
́
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Crystal structure and refinement data for 1, 3, 4·C6H6, 5,
and 6, electrochemical data, and solution 1H NMR
spectra for 1−6 (PDF)
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Roesky, P. W.; Meyer, K. Chem. - Eur. J. 2014, 20, 13501−13506.
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Accession Codes
CCDC 1550780−1550784 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail for F.G.N.C.: f.g.cloke@sussex.ac.uk.
ORCID
F. Geoffrey N. Cloke: 0000-0001-7822-6593
Notes
The authors declare no competing financial interest.
■
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ACKNOWLEDGMENTS
■
We thank the ERC (Project 247390) and EPSRC (EP/
M023885/1) for financial support. Thanks go to Dr. Alaa
Abdul-Sada (University of Sussex) for mass spectrometry and
Dr. Nikolaos Tsoureas (University of Sussex) for invaluable
advice and guidance. We also thank Dr. Graham J. Tizzard and
EPSRC UK National Crystallography Service at the University
of Southampton for the collection of the crystallographic data
for 5.
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DOI: 10.1021/acs.organomet.7b00263
Organometallics XXXX, XXX, XXX−XXX