Reactivity of U3+ metallocene allyl complexes leads to a nanometer-sized uranium carbonate, [(C5Me5) 2U]6(μ-κ1:κ2-CO 3)

Article abstract of DOI:10.1021/om400526h

The U³⁺ allyl complexes (C₅Me₅) ₂U[CH₂C(R)CH₂] (R = H, Me) display three different types of reactivity, as exemplified by reactions with PhNi - NPh, cyclooctatetraene, and CO₂. Two equivalents of (C ₅Me₅)₂U[CH₂C(R)CH₂] effect a four-electron reduction of PhNi - NPh to form the bis(imido) complex (C₅Me₅)₂U(i - NPh)₂ and the bis(allyl) species (C₅Me ₅)₂U[CH₂C(R)CH₂]₂. Two-electron reduction of C₈H₈ occurs to form (C ₅Me₅)(C₈H₈)U[CH₂C(R) CH₂] products that contain only one cyclopentadienyl ring per metal. With CO₂ at 80 psi, both reduction and insertion occur. A hexametallic uranium carbonate, [(C₅Me₅) ₂U]₆(μ-κ¹:κ²-CO ₃)₆, is isolated as well as the bis(carboxylate) complexes (C₅Me₅)₂U[κ²-O,O′- O₂CCH₂C(R)i - CH₂]₂. The polymetallic carbonate complex can also be synthesized from [(C ₅Me₅)₂U]₂(μ-η⁶: η⁶-C₆H₆) and [(C₅Me ₅)₂U][(μ-Ph)₂BPh₂] and CO ₂.

Full text of DOI:10.1021/om400526h

Article
pubs.acs.org/Organometallics
Reactivity of U3+ Metallocene Allyl Complexes Leads to a Nanometer-
Sized Uranium Carbonate, [(C5Me5)2U]6(μ‑κ1:κ2‑CO3)6
Christopher L. Webster, Joseph W. Ziller, and William J. Evans*
Department of Chemistry, University of California, Irvine, California 92697-2025, United States
S
* Supporting Information
ABSTRACT: The U3+ allyl complexes (C5Me5)2U[CH2C-
(R)CH2] (R = H, Me) display three different types of
reactivity, as exemplified by reactions with PhNNPh,
cyclooctatetraene, and CO2. Two equivalents of (C5Me5)2U-
[CH2C(R)CH2] effect a four-electron reduction of PhN
NPh to form the bis(imido) complex (C5Me5)2U(NPh)2
and the bis(allyl) species (C5Me5)2U[CH2C(R)CH2]2. Two-
electron reduction of C8H8 occurs to form (C5Me5)(C8H8)-
U[CH2C(R)CH2] products that contain only one cyclo-
pentadienyl ring per metal. With CO2 at 80 psi, both reduction and insertion occur. A hexametallic uranium carbonate,
[(C5Me5)2U]6(μ-κ1:κ2-CO3)6, is isolated as well as the bis(carboxylate) complexes (C5Me5)2U[κ2-O,O′-O2CCH2C(R)CH2]2.
The polymetallic carbonate complex can also be synthesized from [(C5Me5)2U]2(μ-η6:η6-C6H6) and [(C5Me5)2U][(μ-
Ph)2BPh2] and CO2.
The U4+ allyl metallocenes (C5Me5)2U[CH2C(R)CH2]2 dis-
INTRODUCTION
Organometallic complexes of U3+ have proven to be excellent
■
play η1-alkyl reactivity with formal insertion of CO2, as shown
for R = H in eq 1.30 It was of interest to determine how this
sources of new uranium chemistry and have led to an extensive
collection of uranium-based transformations.1−30 The reducing
allyl reactivity would couple with the reduction chemistry of
U3+.
nature of U3+, coupled with its Lewis acidity and the
opportunity to access products with U4+, U5+, and U6+
oxidation states, has led to unique types of small-molecule
activation2,3 with substrates such as N2,6−10 CO,9,11−15
CO2,9,11,16−27 and NO.28,29 Carbon dioxide, one of the
substrates in the present study, provides a good example of
U3+ reactivity. CO2 is readily converted into CO and uranium
oxide products by many U3+ complexes,9,11,16−27 including early
studies with (C5H4SiMe3)3U.26 Tris(aryloxide)-substituted
triazacyclononane U3+ complexes formed oxide, carbonyl, and
carbonate products,11,16,20−22,25,27 as well as the first crystal
structure of a (CO2)− radical anion.19 The mixed sandwich U3+
complex (C5Me5)U[C8H6(SiiPr3-1,4)2] generates carbonate as
well as a [(CO)4]2− squarate product from U3+ reduction of the
CO byproduct.18 Recently, it has been shown that carbonate
can also be generated from U3+ siloxide complexes.17 In each of
these cases, the carbonate products are typically bimetallic
uranium complexes with (μ-κ1:κ2-CO3)2− bridges, as exempli-
fied in Figure 1A. One structure that is not bimetallic is the
tetrauranium compound obtained by potassium graphite
reduction of a bimetallic trisaryloxide-substituted triazacyclo-
nonane U4+ carbonate complex that contains four [U-
(ArO)3tacn]+ units located around a {[K(CO3)]4}4− network,
Figure 1B.16
We report here that 1 and 2 can display several types of
reactivity depending on the substrate. This is illustrated in
reactions with azobenzene, cyclooctatetraene, and carbon
dioxide. The azobenzene reactivity is presented first since it
demonstrates that 1 and 2 can engage in a previously observed
pattern of U3+ metallocene reaction chemistry. The other two
substrates involve more complicated reaction sequences
including loss of a (C5Me5)− ring and the unexpected
formation of a hexametallic uranium complex formed by
bridging carbonate ligands, [(C5Me5)2U]6(μ-κ1:κ2-CO3)6.
Interest in large polymetallic uranium compounds has been
stimulated by the results of Burns and co-workers, who have
reported an extensive series of 1.6−1.9 nm diameter polyactinyl
compounds that are linked with peroxide.33−36 An extensive
range of compounds containing up to 60 uranyl units with
formulas such as (UO2)36(O2)41(OH)2636−, [(UO2)-
The recent synthesis of the U3+ allyl metallocene complexes
(C5Me5)2U[CH2C(R)CH2] (R = H, 1; Me, 2)30 provided the
opportunity to study U3+ reactivity with a ligand that can adopt
π and σ metal−carbon bonding modes, namely, the allyl group.
Received: June 10, 2013
© XXXX American Chemical Society
A
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Figure 1. (A) [U(C8H6{SiiPr3-1,4}2)(C5Me5)]2(μ-κ1:κ2-CO3).18 (B) The core structure of {[((Neop,MeArO)3-triazacyclononane)U(CO3)]K}4 with
the triazacyclononane ligand system abbreviated as L.16 (C) The anion in the polyuranyl guanidinium salt [C(NH2)3]6[(UO2)(CO3)2]3.31,32
.
60−
(O2)1.5]4444−, and [UO2(O2)(OH)]60
are readily prepared
solution of 1 (100 mg, 0.182 mmol) in hexane (10 mL). The green
solution quickly turned dark brown and was stirred for 1 h. The
solvent was removed via reduced pressure, and the resulting solid was
washed with cold hexane to leave 5 as a brown, microcrystalline solid
(74 mg, 79%). Removal of solvent from the unfiltered washings left a
solid that contained (C5Me5)2 by H NMR spectroscopy in C6D6. H
NMR (C6D6, 298 K): δ 2.8 (s, C5Me5, 15 H), −32.2, (s, C8H8, 8H),
−33.9 (s, CH2CHCH2, 1H). IR: 2964m, 2901s, 2856s, 2725w, 1630m,
1436m, 1377s, 1253w, 1107w, 1020w, 900w, 722s cm−1. Anal. Calcd
for C21H28U: C, 48.65; H, 5.44. Found: C, 48.20; H, 5.78.
(C5Me5)(C8H8)U[CH2C(Me)CH2], 6. As described for 5, C8H8 (21
μL, 0.19 mmol) reacts with 2 (100 mg, 0.177 mmol) to form 6 as a
brown, microcrystalline solid. Concentrated samples in hexane at −35
°C yielded single crystals suitable for X-ray diffraction (81 mg, 85%).
1H NMR (C6D6, 298 K): δ 4.5 (s, C5Me5, 15 H), −4.1 [s,
CH2C(Me)CH2, 3H], −30.4, (s, C8H8, 8H). IR: 2964m, 2900s, 2856s,
2724w, 1633m, 1435m, 1377s, 1253w, 1106w, 1020w, 900w, 721s
cm−1. Anal. Calcd for C22H30U: C, 49.62; H, 5.68. Found: C, 49.90; H,
5.80.
from uranyl nitrate and hydrogen peroxide in alkaline solution
in the presence of various alkali metal cations. In some cases,
these can self-assemble within 15 min to micrometer-sized
crystals.35 The reports on these peroxides have pointed out the
value of such polyuranium species for nuclear waste stream
remediation using size-selective processes.36 A recent review36
on large polymetallic uranium molecules details how nano-
meter-sized uranium species readily form with oxide or
peroxide linkages, but can also include oxalate, sulfate,
pyrophosphate, and selenate bridges.34,35,37−39 However, the
largest crystallographically characterized complex in which
multiple uranium atoms are connected to each other by
carbonate ligands is the trimetallic uranyl species in Figure
1C.31,32 This is surprising given the numerous examples of
carbonate bridges in bimetallic uranium com-
plexes.16−18,20,21,24,25
1
1
[(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, from (C5Me5)2U(CH2CHCH2), 1.
A solution of (C5Me5)2U(CH2CHCH2) (100 mg, 0.182 mmol) in
benzene (8 mL) in a Fischer−Porter high-pressure apparatus in an
argon-containing glovebox was sealed and attached to a high-pressure
line. The vessel was evacuated to the solvent vapor pressure and
charged with CO2 (80 psi). The solution turned from dark green to
orange over 1 h, and orange crystals grew over 3 days. The CO2
pressure was reduced to 15 psi on the vacuum line, and the residual
CO2 was removed under vacuum in the glovebox. The mother liquor
was decanted, and a small sample of single crystals was taken for
analysis by X-ray diffraction. The rest were washed three times with
benzene and dried under reduced pressure (51 mg, 96% yield based on
ligand redistribution). IR: 2979w, 2902m, 2857w, 1534w, 1475s,
1433s, 1400s, 1080m, 1021w, 831m, 766w, 675w, 652w cm−1. Anal.
Calcd for C126H180O18U6·C6H6: C, 45.44; H, 5.37. Found: C, 45.01; H,
5.80. The benzene washings were combined with the mother liquor,
EXPERIMENTAL INFORMATION
■
All syntheses and manipulations described below were conducted
under argon with rigorous exclusion of air and water using glovebox,
Schlenk, and vacuum line techniques. Solvents were sparged with UHP
argon and dried over columns containing Q-5 alumina. NMR solvents
were dried over sodium−potassium alloy, degassed using three freeze−
pump−thaw cycles, and vacuum transferred before use. PhNNPh
(99%) was purchased from Sigma-Aldrich and placed under vacuum
(10−3 Torr) for 12 h prior to use. 1,3,5,7-Cyclooctatetraene (98%) was
purchased from SigmaAldrich and distilled under nitrogen before use.
13CO2 gas (99%) was purchased from SigmaAldrich and used as
received. (C5Me5)2U[CH2C(R)CH2] (R = H, 1; Me, 2),30
(C5Me5)2U[κ2-O,O′-O2CCH2C(R)CH2]Cl (R = H, Me),30
[(C5Me5)2U][(μ-Ph)2BPh2],40 and [(C5Me5)2U]2(μ-η6:η6-C6H6)41
were synthesized as previously described. 1H NMR spectra were
recorded with a Bruker DRX500 MHz spectrometer. Infrared spectra
were recorded as KBr pellets on a Varian 1000 FT-IR spectrometer.
Elemental analysis was performed on a Perkin-Elmer 2400 Series II
CHNS analyzer.
1
and the solvent was evaporated. The H NMR spectrum of the solids
in C6D6 indicated (C5Me5)2U(κ2-O,O′-O2CCH2CHCH2)2,30 8, was
the exclusive byproduct.
[(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, from (C5Me5)2U[CH2C(Me)CH2],
2. Following the procedure above, 2 was reacted with CO2 (80 psi).
Complex 7 was identified by elemental analysis, IR spectroscopic
analysis, and X-ray unit cell analysis of a single crystal matching that for
7·C6H6. No significant difference in yield or rate of crystal formation
was observed. (C5Me5)2U(κ2-O,O′-O2CCH2C(Me)CH2)2,30 9, was
(C5Me5)2U(NPh)2 from (C5Me5)2U(CH2CHCH2), 1. In a
glovebox, PhNNPh (4 mg, 0.02 mmol) was added quickly to a
vigorously stirred solution of (C5Me5)2U(CH2CHCH2), 1 (12 mg,
0.022 mmol), in hexane (5 mL). The green solution quickly turned
black and was stirred for 1 h. The solvent was removed via reduced
1
identified via H NMR spectroscopy as the only other product.
42
pressure to yield a tacky black solid. (C5Me5)2U(NPh)2 and
[(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, from [(C5Me5)2U]2(μ-η6:η6-C6H6).
A solution of [(C5Me5)2U]2(μ-η6:η6-C6H6) (100 mg, 0.091 mmol) in
benzene (10 mL) was reacted with CO2 as described for 1, but with a
reaction time of 12 h to produce orange single crystals of 7 (83 mg,
78%). Elemental analysis, infrared spectroscopic analysis, and X-ray
unit cell analysis of the single crystals formed matched that for 7·C6H6.
Anal. Calcd for C126H180O18U6·C6H6: C, 45.44; H, 5.37. Found: C,
45.08; H, 5.15.
(C5Me5)2U(CH2CHCH2)2, 3,30 were identified by 1H NMR spec-
troscopy in a 1:1 ratio and were the only products observed.
(C5Me5)2U(NPh)2 from (C5Me5)2U[CH2C(Me)CH2], 2. In an
analogous reaction, PhNNPh reacted with (C5Me5)2U[CH2C(Me)-
42
CH2], 2, to form (C5Me5)2U(NPh)2 and (C5Me5)2U[CH2C-
(Me)CH2]2, 4,30 which were identified by 1H NMR spectroscopy in a
1:1 ratio.
(C5Me5)(C8H8)U(CH2CHCH2), 5. C8H8 (21 μL, 0.19 mmol) was
added dropwise over 15 s using a 25 μL syringe to a vigorously stirred
[(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, from [(C5Me5)2U][(μ-Ph)2BPh2].
A solution of [(C5Me5)2U][(μ-Ph)2BPh2] (60 mg, 0.072 mmol) in
B
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benzene (10 mL) was reacted with CO2 as described for 1. Crystals of
7 were identified by X-ray crystallography, and although crystals were
seen to form after 24 h, only 3 mg (7%) was isolated even after 7 days.
No starting material was observed by H NMR spectroscopy. The
NMR spectrum contained resonances for BPh3.
Table 1. X-ray Data and Collection Parameters on
(C5Me5)(C8H8)U[CH2C(Me)CH2], 6, {[(C5Me5)2U](μ-
κ1:κ2-CO3)}6, 7, and {[(C5Me5)2U](μ-κ1:κ1-O,O′-
O2CCH2CHCH2)}2, 10
1
{[(C5Me5)2U](μ-κ1:κ1-O,O′-O2CCH2CHCH2)}2, 10. K(Hg) (1 wt
%) (44 mg K, 1.1 mmol) was poured into a vigorously stirred solution
of (C5Me5)2U(O2CCH2CHCH2)Cl30 (100 mg, 0.159 mmol) in
hexane (10 mL). Over 10 min, the orange solution turned green and
was stirred for 1 h. After removing gray insoluble solids via
centrifugation and filtration, the solvent was removed via reduced
pressure to yield a dark green, microcrystalline solid. Concentrated
samples in hexane at −35 °C yielded single crystals suitable for X-ray
diffraction (88 mg, 93%). 1H NMR (C6D6, 298 K): δ −5.6 (s, C5Me5,
30 H). No resonance for the allylcarboxylate moiety was observed
between 200 and −200 ppm. IR: 2977m, 2916s, 2723m, 1556s, 1434s,
1397m, 1277m, 1242m, 895w, 840m, 746w, 618w cm−1. Anal. Calcd
for C48H70O4U2: C, 48.56; H, 5.94. Found: C, 48.70; H, 5.50.
{[(C5Me5)2U][μ-κ1:κ1-O,O′-O2CCH2C(Me)CH2]}2, 11. As de-
scribed for 10, (C5Me5)2U(O2CCH2C(Me)CH2)Cl30 (100 mg,
0.156 mmol) was converted to 11, which was isolated as a dark green,
microcrystalline solid (92 mg, 97%). 1H NMR (C6D6, 298 K): δ −6.1
(s, C5Me5, 30 H), −10.1 [s, CH2C(CH3)CH2, 1 H], −15.0 [s,
CH2C(CH3)CH2, 1 H], −19.2 [s, CH2C(CH3)CH2, 3 H], −40.1
[s, CH2C(CH3)CH2, 2 H]. IR: 2977m, 2914s, 2856m, 2723m,
1559s, 1438w, 1397m, 1277m, 1242m, 986w, 896w, 840m, 746w,
686w cm−1. Anal. Calcd for C48H70O4U2: C, 49.42; H, 6.14. Found: C,
49.80; H, 5.91.
6
7
10
empirical formula
temperature (K)
cryst syst
space group
a (Å)
C22H30U
83(2)
C126H180O18U6·C6H6
143(2)
C48H70O4U2
83(2)
triclinic
trigonal
monoclinic
P21/n
P1
̅
R3
̅
8.8131(4)
14.2385(6)
16.0147(6)
106.6569(5)
98.8404(5)
90.4651(5)
1899.52(14)
4
31.339(2)
31.339(2)
10.9920(8)
90
9.7800(5)
15.6266(9)
15.0373(8)
90
b (Å)
c (Å)
α (deg)
β (deg)
90
102.8544(6)
90
γ (deg)
volume (Å3)
120
9349.4(15)
3
2240.5(2)
2
Z
ρcalcd (Mg/m3)
1.862
1.859
1.760
μ (mm−1
)
8.544
7.830
7.260
R1 (I > 2.0σ(I))
0.0178
0.0292
0.0818
0.0145
wR2 (all data)
0.0396
0.0332
a
2
Definitions: R1 = ∑∥Fo| − |Fc∥/∑|Fo|, wR2 = [∑w(Fo − Fc2)2/
2
∑w(Fo )2]1/2
.
[(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, from {[(C5Me5)2U](μ-κ1:κ1-O,O′-
O2CCH2CHCH2)}2, 10. A solution of 10 (100 mg, 0.084 mmol) in
benzene (10 mL) was reacted with CO2 as described for 1. Elemental
analysis, infrared spectroscopic analysis, and X-ray unit cell analysis of
the single crystals formed (48 mg, 98% based on ligand redistribution)
matched those for 7·C6H6. Anal. Calcd for C126H180O18U6·C6H6: C,
45.44; H, 5.37. Found: C, 45.30; H, 5.20.
These U3+ reactions are analogous to the reaction of the
“(C5Me5)2UCl” units in [(C5Me5)2UCl]3 that engage in
reductions to form (C5Me5)2UCl2 as a byproduct.46 The half-
reaction for the U3+ metallocene chloride is shown in eq 4,
Detection of the CO Byproduct. A solution of 1 (15 mg, 0.027
mmol) in C6D6 (ca. 0.3 mL) was placed into a J-Young NMR tube.
This solution was degassed using three freeze−pump−thaw cycles on
a high-vacuum line (10−5 Torr). 13CO2 at 5 psi was added to the J-
Young tube, and it was sealed. After 1 h, the solution turned orange,
and after 5 h the 13C NMR spectrum contained a resonance at 181.4
1
ppm consistent with 13CO. The H NMR spectrum contained the
resonances for (C5Me5)2U(κ2-O,O′-O2CCH2CHCH2)2,30 8.
X-ray Data Collection, Structure Determination, and Refine-
ment. Crystallographic information for complexes 6, 7, and 10
(CCDC 928812−928814) is summarized in Table 1. Full details are
given in the Supporting Information.
where n = 2 for U4+ products and n = 4 for a mixture of U6+ and
U4+ products. This method has been used to reduce PhN
NPh to two (NPh)2− units with (C5Me5)2UCl2Na,44 and we
have found as part of this study that the neutral chloride
(C5Me5)2UCl(Et2O)46 reacts similarly with azobenzene to
form (C5Me5)2U(NPh)2.
RESULTS
■
C8H8. In contrast to the four-electron reactivity with
azobenzene, 1 and 2 react with 1,3,5,7-cyclooctatetraene as
two-electron reductants. In this case, a (C5Me5)− ligand is lost
and the mixed ligand U4+ complexes (C5Me5)(C8H8)U[CH2C-
(R)CH2] (R = H, 5; Me, 6) are formed, eq 5. No bis(allyl)
PhNNPh. The allyl complexes (C5Me5)2U[CH2C(R)-
CH2] (R = H, 1; Me, 2) react rapidly with PhNNPh in a
transformation that demonstrates their capacity to effect four-
electron reduction. As shown in eq 2, both 1 and 2 reduce
azobenzene by four electrons to form the bis(imido) U6+
complex (C5Me5)2U(NPh)2, which was fully characterized
in the past by Burns and co-workers.42−45 Both reactions also
form the bis(allyl) U4+ metallocenes (C5Me5)2U[CH2C(R)-
CH2]230 as byproducts. The formal half-reaction is shown in eq
3, where n = 4 and Cp* = C5Me5.
ligand redistribution products were observed as found in eq 2.
Hence, the two electrons added to C8H8 formally arise from a
single equivalent of the U3+ allyl complex according to a new
half-reaction for these complexes, eq 6. One electron formally
comes from a U3+/U4+ redox couple. The observation of
(C5Me5)2 as a byproduct by 1H NMR spectroscopy is
C
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consistent with a second electron coming from a (C5Me5)−/
(C5Me5) redox couple, which is common in sterically crowded
rare earth and actinide organometallic complexes.47 The
combination of the cyclopentadienyl, allyl, and cyclooctate-
traenyl ligands in eq 5 could form a sterically crowded
intermediate with sufficient steric bulk for sterically induced
reduction, since the composition “(C5Me5)2U(μ-C8H8)”
appears to be too sterically crowded to isolate.48
those in the closely related amidinate (C5Me5)(C8H8)U-
[iPrNC(Me)NiPr],48 which has 2.516 Å and 136.4° values.
The 1.975 and 1.973 Å U−(C8H8 ring centroid) distances are
not as close to the 2.008 Å distance in the amidinate complex,
but they fall in the 1.894−2.008 Å range typical for
[(C5Me5)(C8H8)U]+ complexes.49 The 2.627(3)−2.697(3) Å
U−C distances to the end carbon atoms of the allyl group are
longer than the 2.469(3) Å U−C bond in the sigma-bound
alkyl complex (C5Me5)(C8H8)U[CH(SiMe3)2],48 as is typical
for allyl complexes.30
The solid-state structure of 6 was determined by X-ray
crystallography, Figure 2, and reveals that the allyl ligand binds
The σ-bonded (C5Me5)(C8H8)UR′ complexes, where R′ =
Me, Et, Np, and Ph, readily engage in C−H bond activation to
form the tuck-in complex (η5:η1-C5Me4CH2)(C8H8)U.50 This
has not been observed with 5 and 6, which are stable at 100 °C
for 14 h in toluene.
CO2. The reaction of 1 and 2 with CO2 (80 psi) showed yet
another mode of reactivity for these U3+ allyl species. Two
products are isolated from this reaction: an unprecedented
hexametallic U4+ carbonate, [(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7,
and a U4+ bis(allylcarboxylate) complex, (C5Me5)2U[κ2-O,O′-
O2CCH2C(R)CH2]2 (R = H, 8; Me, 9), eq 7. Complex 7
was separated by crystallization and identified by X-ray
crystallography, Figure 3. The IR spectrum of 7 shows a
characteristic strong carbonate absorption at 1400 cm−1.51 The
Figure 2. Thermal ellipsoid plot drawn at the 50% probability level of
one of the two crystallographically independent molecules of
(C5Me5)(C8H8)U[CH2C(Me)CH2], 6, in the unit cell. Hydrogen
atoms are omitted for clarity, and only one of the two molecules in the
asymmetric unit is shown.
in a trihapto manner. The 2.520 and 2.521 Å U−(C5Me5 ring
centroid) distances and the 136.3° and 136.5° (C5Me5 ring
centroid)−U−(C8H8 ring centroid) angles are very close to
Figure 3. (A) Thermal ellipsoid plot at the 50% probability level of [(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, with hydrogen atoms omitted for clarity. (B)
Side view of the uranium carbonate core of 7 with (C5Me5)− ligands omitted.
D
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previously reported 8 and 930 were identified by H NMR
spectroscopy.
carboxyl binding mode, oxidation state, and coordination
number.
Compounds 10 and 11 also produce 7 under CO2 (80 psi)
with a half-reaction analogous to eq 3 with n = 2 and
carboxylate instead of allyl. Scheme 1 shows both possible
pathways for formation of 7.
1
The formation of uranium carbonates from CO2 and U3+
complexes has been well documented in recent
years16−18,20,21,23−25 and is thought to occur through reductive
2−
disproportionation of two CO2 to CO3 and CO using two
equivalents of U3+, eq 8.
Complex 7 crystallizes as the hexameric cyclic structure
[(C5Me5)2U]6(μ-κ1:κ2-CO3)6, Figure 3, with one crystallo-
graphically unique repeat unit. The crystallographic parameters
of the [(C5Me5)2U]2+ metallocene moiety are conventional.
For example, the 2.445 and 2.472 Å U−(C5Me5 ring centroid)
distances and the 137.3° (C5Me5 ring centroid)−U−(C5Me5
ring centroid) are similar to the 2.478 Å average distance and
137.4° angle in (C5Me5)2U(κ2-O,O′-O2CCH2CHCH2)2, 8,
which has two carboxylates coordinated to the metallocene
unit.30 The latter complex, which has a higher formal
coordination number, has longer U−O bonds, 2.42(2)−
2.465(2) Å, compared to 2.360(4) and 2.372(3) Å U−O
distances for the κ2 part of the carbonate ligand. The U−O
distance for the κ1 part of the carbonate, 2.229(3) Å, is much
shorter, although the C−O distances within the carbonate fall
in the narrow range 1.280(6)−1.290(6) Å. In comparison, the
U−O distance in [(C5Me5)2FU]2(μ-O) is 2.118(3) Å.54
The two most distant (C5Me5)− methyl carbon atoms in 7
span a length of 1.97 nm. The complex adopts a “chair”
conformation generated by the bridging (μ-κ1:κ2-CO3)2−
carbonates, which are oriented with 60° angles between one
carbonate plane and the next. This twists each metallocene by
60° around the ring.
2−
2U3 + 2CO2 → 2U4+ + CO3 + CO
(8)
The generation of CO in eq 7 was confirmed using 13CO2 at 5
psi followed by 13C NMR spectroscopy. NMR data were not
obtainable for 7 because of its insolubility in common solvents.
Complexes 1 and 2 could affect analogous U3+ reduction of
CO2 with the formal half-reaction in eq 3. This would form the
bis(allyl) complexes, (C5Me5)2U[CH2C(R)CH2]2, 3 and 4, as
byproducts. If these are formed, they would not be observable
since they react quickly with CO2 to form the U4+
bis(carboxylate) byproducts 8 and 9,30 which are observed.
However, as described in the next paragraph, another pathway
to 7 is possible in which CO2 insertion occurs to form
allylcarboxylate products before the formation of 7.
1H NMR studies of 1 and 2 under only 1 atm of CO2 suggest
that there is rapid formation of the U3+ carboxylates
{[(C5Me5)2U][μ-κ1:κ1-O,O′-O2CCH2C(R)CH2]}2 (R = H,
10; Me, 11). Complexes 10 and 11 were identified after
independent synthesis by K(Hg) reduction of (C5Me5)2U[κ2-
O,O′-O2CCH2C(R)CH2]Cl,30 eq 9. The structure of 10 was
determined by X-ray crystallography, Figure 4. It is
Since the yield of 7 from 1 and 2 or from 10 and 11 can be
no higher than 50%, other syntheses of 7 from U3+ metallocene
precursors were examined, Scheme 2. Two U3+ metallocenes
with an extensive reduction chemistry were examined:
[(C5Me5)2U][(μ-Ph)2BPh2]40 and [(C5Me5)2U]2(μ-η6:η6-
C6H6).41 Both compounds were found to react with CO2 to
generate polymetallic 7, with the latter compound giving the
best yield: 78%. The formation of the carbonate from U3+
sources thus appears to be general.
DISCUSSION
■
Figure 4. Thermal ellipsoid plot drawn at the 50% probability level of
{[(C5Me5)2U](μ-κ1:κ1-O,O′-O2CCH2CHCH2)}2, 10. Hydrogen
atoms are omitted for clarity.
The reactions of (C5Me5)2U[CH2C(R)CH2] (R = H, 1; Me, 2)
with azobenzene, eq 2, demonstrate that these U3+ metal-
locenes can participate in the traditional U3+ metallocene
(C5Me5)2UX reduction chemistry shown in eq 4, in which two
equivalents of (C5Me5)2UX deliver two electrons and form
(C5Me5)2UX2 as a byproduct. The X ligand historically has
been limited to chloride,45,46 but eq 2 shows that X can be
extended to allyl. The CO2 reactions with 1 and 2 may also
have a component of this reaction pattern, and the reduction of
CO2 by {[(C5Me5)2U](μ-κ1:κ1-O,O′-O2CCH2CHCH2)}2,
10, Scheme 1, shows that X in eq 4 can also be carboxylate.
isomorphous with [(C5Me5)2Sm(μ-O2CCH2CHCH2)]2.52
The metrical parameters for 10 given in Table 2 are roughly
similar to those in the samarium complex considering that U3+
has a 0.067 Å larger radial size.53 Comparison of the 2.356(1)
and 2.375(1) Å U−O(μ-O2CR) distances with the 2.42(2)−
2.449(2) Å U−O(κ2-O2CR) distances in (C5Me5)2U(κ2-
O2CCH2CHCH2)2 is not direct due to the difference in
E
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Organometallics
Article
Table 2. Selected Bond Distances (Å) and Angles (deg) for (C5Me5)(C8H8)U[CH2C(Me)CH2], 6, [(C5Me5)2U]6(μ-κ1:κ2-
CO3)6, 7, and {[(C5Me5)2U](μ-κ1:κ1-O,O′-O2CCH2CHCH2)}2, 10
6
7
10
(C5Me5 centroid)−U
2.520/2.521
2.445/2.472
137.3
2.509/2.493
133.8
(C5Me5 centroid)−U−(C5Me5 centroid)
(C5Me5 centroid)−U−(C8H8 centroid)
(C8H8 centroid)−U
136.3/136.5
1.975/1.973
C19/20/21−U1
C41/42/43−U2
C19−C20−C21
2.627(3)/2.797(3)/2.697(3)
2.670(3)/2.790(3)/2.645(3)
122.1(3)
C41−C42−C43
122.0(3)
U−O(κ1-CO3 and RCO2)
2.229(3)
2.356(1)
2.375(1)
U−O(κ2-CO3)
2.360(4)/2.372(3)
C21−O1/O2/O3
1.280(6)/1.286(6)/1.290(6)
1.259(2)/ 1.257(2)/−
Scheme 1. Synthesis of the Hexauranium Carbonate 7; (a) −6 (C5Me5)2U[κ2-O,O′-O2CCH2C(R)CH2]2, −6 CO
hapticity over sigma-bonded ligands in U3+ metallocene
Scheme 2. Additional Syntheses of 7; (a) −6 CO, −3 Ph2, −6
BPh3; (b) −6 CO, −3 C6H6
complexes such as (C5Me5)2U(η1-alkyl). However, only one
such example exists for comparison, (C5Me5)2U[CH-
(SiMe3)].46 The formation of the monometallic monocyclo-
pentadienyl complex, 6, rather than a bimetallic bis-
(cyclopentadienyl) C8H8 reduction product such as
(C5Me5)2U[CH2C(R)CH2](μ-C8H8)U(C5Me5)2[CH2C(R)-
CH2] can similarly be rationalized by the likely steric strain in
the latter species.
The synthesis of [(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, from 1, 2,
10, [(C5Me5)2U][(μ-Ph)2BPh2], and [(C5Me5)2U]2(μ-η6:η6-
C6H6) reveals yet another aspect of U3+ metallocene reactivity.
This hexametallic carbonate ring system is unique to carbonate
bridging, although there are many polymetallic molecules
bridged with carbonate in the literature of transition metals,58
alkaline earths,59 and lanthanides.60 The isolation of 7 suggests
that new options in shape and size are available not only to
polyuranium species but also to polymetallic carbonate
compounds in general. Complex 7 also indicates carbonate
can be added to the list of bridging ligands that make large
polymetallic species, i.e., oxide, peroxide, oxalate, sulfate,
pyrophosphate, and selenate bridges.33−39
The shape and nuclearity of 7 can be traced to the angular
requirements of the (μ-κ1:κ2-CO3)2− binding mode coupled
with the presence of the metallocene moieties. Burns has
pointed out the importance of the bent U−(O2)−U angles in
peroxide-bridged compounds in forming large polymetallic
species.36 In the carbonate {[(UO2)(CO3)2]3}6−, Figure 1,31,32
the combination of a linear uranyl unit and three (μ-κ1:κ2-
CO3)2− ligands leads to a planar array with 120° angles
between the monomeric units that closes with only three
Given the diversity of X = Cl, allyl, and carboxylate, it is likely
that other (C5Me5)2UX complexes will behave similarly.
The reactions of (C5Me5)2U[CH2C(R)CH2] (R = H, 1; Me,
2) with cyclooctatetraene, eq 5, show that allyl U3+ metal-
locenes can engage in other types of reduction in which the
(C5Me5)− ligands become active reductants. This reactivity is
similar to (C5Me5)2UX reactivity where X = C5Me5.47,55−57 In
the (C5Me5)3U case, this “sterically induced reduction” arises
because the three (C5Me5)− groups cannot get close enough to
the metal center to achieve their normal stabilization. Although
1 and 2 do not have the steric crowding found in (C5Me5)3U,
the presence of the X = allyl ligand evidently leads to sterically
crowded intermediates in which (C5Me5)− can function as a
reductant. This may be possible with allyl due to its enhanced
F
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Article
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■
́ ́
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The (C5Me5)2UX U3+ metallocenes with X = allyl, i.e.,
(C5Me5)2U[CH2C(R)CH2] (R = H, 1; Me, 2), have the
capacity for several types of reductive reactivity. They can
engage in four-electron reduction as found for X = Cl with a
(C5Me5)2UX2 byproduct, eq 4, but they can also effect two-
electron reductions involving loss of a cyclopentadienyl ring
and formation of products containing {(C5Me5)U[CH2C(R)-
CH2]}2+ moieties. The U3+ allyl metallocenes can also lead to
unusual types of polymetallic uranium complexes, as demon-
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[(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7. The reaction with CO2 of 1
and 2 also led to another variant of (C5Me5)2UX complexes,
namely, those with X = carboxylate, {[(C5Me5)2U][μ-κ1:κ1-
O,O′-O2CCH2C(R)CH2]}2 (R = H, 10; Me, 11), which can
also engage in U3+ metallocene reaction chemistry including the
reduction of CO2 to the hexauranium carbonate compound.
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ASSOCIATED CONTENT
■
(28) Frey, A. S. P.; Cloke, F. G. N.; Coles, M. P.; Hitchcock, P. B.
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S
* Supporting Information
X-ray data collection, structure solution and refinement (PDF),
and X-ray diffraction details of compounds 6, 7, and 10 (CIF,
CCDC 928812−928814) are available free of charge via the
Internet at http://pubs.acs.org
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AUTHOR INFORMATION
Corresponding Author
*E-mail: wevans@uci.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Chemical Sciences, Geosciences, and Biosciences
Division of the Office of Basic Energy Sciences of the
Department of Energy (DE-SC0004739) for support and
Jordan F. Corbey for crystallographic assistance.
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