Revealing Disparate Chemistries of Protactinium and Uranium. Synthesis of the Molecular Uranium Tetroxide Anion, UO4-

Article abstract of DOI:10.1021/acs.inorgchem.7b00144

The synthesis, reactivity, structures, and bonding in gas-phase binary and complex oxide anion molecules of protactinium and uranium have been studied by experiment and theory. The oxalate ions, An^VO₂(C₂O₄)^?, where An = Pa or U, are essentially actinyl ions, An^VO₂⁺, coordinated by an oxalate dianion. Both react with water to yield the pentavalent hydroxides, An^VO(OH)₂(C₂O₄)^?. The chemistry of Pa and U becomes divergent for reactions that result in oxidation: whereas Pa^VI is inaccessible, U^VI is very stable. The U^VO₂(C₂O₄)^? complex exhibits a remarkable spontaneous exothermic replacement of the oxalate ligand by O₂ to yield UO₄^- and two CO₂ molecules. The structure of the uranium tetroxide anion is computed to correspond to distorted uranyl, U^VIO₂²⁺, coordinated in the equatorial plane by two equivalent O atoms each having formal charges of ?1.5 and U-O bond orders intermediate between single and double. The unreactive nature of Pa^VO₂(C₂O₄)^? toward O₂ is a manifestation of the resistance toward oxidation of Pa^V, and clearly reveals the disparate chemistries of Pa and U. The uranium tetroxide anion, UO₄^-, reacts with water to yield UO₅H₂^-. Infrared spectra obtained for UO₅H₂^- confirm the computed lowest-energy structure, UO₃(OH)₂^-

Full text of DOI:10.1021/acs.inorgchem.7b00144

Article
pubs.acs.org/IC
Revealing Disparate Chemistries of Protactinium and Uranium.
−
Synthesis of the Molecular Uranium Tetroxide Anion, UO4
Wibe A. de Jong,*,† Phuong D. Dau,‡ Richard E. Wilson,§ Joaquim Marcalo,∥
̧
Michael J. Van Stipdonk,⊥ Theodore A. Corcovilos,# Giel Berden,7 Jonathan Martens,7 Jos Oomens,7,8
and John K. Gibson*,‡
†Computational Research Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
‡Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
§Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
∥Centro de Cien
Portugal
̂ ́
cias e Tecnologias Nucleares, Instituto Superior Tecnico, Universidade de Lisboa, 2695-066 Bobadela Loures,
⊥Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282, United States
#Department of Physics, Duquesne University, Pittsburgh, Pennsylvania 15282, United States
7Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525ED Nijmegen, The
Netherlands
8van ‘t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands
S
* Supporting Information
ABSTRACT: The synthesis, reactivity, structures, and bond-
ing in gas-phase binary and complex oxide anion molecules of
protactinium and uranium have been studied by experiment
and theory. The oxalate ions, AnVO2(C2O4)−, where An = Pa
or U, are essentially actinyl ions, AnVO2 , coordinated by an
+
oxalate dianion. Both react with water to yield the pentavalent
hydroxides, AnVO(OH)2(C2O4)−. The chemistry of Pa and U
becomes divergent for reactions that result in oxidation:
whereas PaVI is inaccessible, UVI is very stable. The
UVO2(C2O4)− complex exhibits a remarkable spontaneous
exothermic replacement of the oxalate ligand by O2 to yield
−
UO4 and two CO2 molecules. The structure of the uranium
tetroxide anion is computed to correspond to distorted uranyl, UVIO22+, coordinated in the equatorial plane by two equivalent O
atoms each having formal charges of −1.5 and U−O bond orders intermediate between single and double. The unreactive nature
of PaVO2(C2O4)− toward O2 is a manifestation of the resistance toward oxidation of PaV, and clearly reveals the disparate
−
−
chemistries of Pa and U. The uranium tetroxide anion, UO4 , reacts with water to yield UO5H2 . Infrared spectra obtained for
−
−
UO5H2 confirm the computed lowest-energy structure, UO3(OH)2 .
O−O bonding. In most reported uranium oxide molecules the
INTRODUCTION
■
(nearly) linear uranyl moiety with two strongly bound O atoms
is retained, and the excess O atoms adopt alternative bonding
configurations.17,26,27
Gas-phase chemistry has been demonstrated as an effective
approach for revealing new and fundamental aspects of metal
ion chemistry.1−10 An attribute of these systems is that they are
generally sufficiently simple that accurate computational
modeling is feasible, even for systems that incorporate many-
electron heavy elements such as actinides.11 Bonding and
reactivity of gas-phase actinide oxides, particularly uranium
oxides, has received appreciable attention, both experimentally
and computationally.12−25 Of particular interest are molecules
Whereas uranium oxides have received considerable
attention, oxides of the much less prevalent and more
radioactive actinide protactinium have been relatively neglected.
Preceding U in the actinide series, Pa has one less valence
electron and thus typically exhibits PaV as its highest oxidation
state. Because of this fundamental difference in the chemistries
of Pa and U, it is expected that there should not necessarily be a
correspondence between these two elements upon addition of
−
with an excess of oxygen atoms, such as UO4 , in which not all
can be accommodated as O2− moieties within the constraint of
the highest accessible hexavalent oxidation state of uranium. In
such cases the excess O atoms are accommodated either as
open-shell oxo-ligands with radical character, or as ligands with
Received: January 17, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b00144
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Article
yield PaO2(C2O4)−, which was instead prepared by fragmentation of
O atoms beyond the pentavalent anion oxides, AnVO3 .
Already for pentavalent ions the chemistry between Pa and U
is different. PaVO2+ rapidly hydrolyzes in aqueous solution, this
−
−
gas-phase Pa(C2O4)3 .
Caution! The employed 231Pa isotope is highly radioactive with an α-
decay half-life of ∼33000 y; the radiological hazards are highly
exacerbated by the several short-lived progeny in the subsequent decay
chain, starting with daughter 227Ac. Accordingly, 231Pa must be handled
with proper radiological safety controls.34
28−30
in contrast to hydration of UVO2 ,
a difference that has
+
also been observed in the gas phase.31 Reactions of neutral or
charged PaV and UV oxide molecules should reveal fundamental
differences in chemistry, particularly regarding oxygen-rich
species where the excess O atoms can be accommodated in
different manners. In addition to binary oxides, more complex
species comprising PaV and UV/VI with actinyl cores
coordinated by anionic ligands can provide insights into
variations in chemistry between these two neighboring
actinides.
The experiments were performed using an Agilent 6340 ESI QIT
mass spectrometer (QIT/MS) with MSn collision induced dissociation
(CID) capability; the CID energy is an instrumental parameter that
provides an indication of relative ion excitation.
Mass-selected ions in the trap can undergo ion−molecule reactions
at ∼300 K35 by applying a reaction time of up to 10 s. Anion mass
spectra were acquired using the following parameters: solution flow
rate, 60 μL/h; nebulizer gas pressure, 12 psi; capillary voltage offset
and current, −4000 V and 210 nA; end plate voltage offset and
current, −500 V and 3000 nA ; dry gas flow rate, 3 l/min; dry gas
temperature, 325 °C; capillary exit, −50.0 V; skimmer, −36 V;
octopole 1 and 2 DC, −10.9 V and −3.0 V; octopole RF amplitude,
190 Vpp; lens 1 and 2, 10.0, and 91.0 V; trap drive, 50.0. Nitrogen gas
for nebulization and drying was supplied from the boil-off of a liquid
nitrogen Dewar. The background water and oxygen pressures in the
ion trap are estimated as ∼10−6 Torr. Reproducibility of hydration
rates of UO2(OH)+ established that the water pressure was constant to
Previous chemistry and spectroscopy studies of gas-phase
uranium oxide anions, including UO4 , have relied on high-
energy synthesis methods such as laser ablation of solids or
energetic fragmentation of gas-phase precursor complexes that
have been produced under low-energy conditions such as by
−
electrospray ionization (ESI) of complexes in solution.17,27,32
A
goal of gas-phase chemistry is to identify spontaneous
exothermic reactions that can alternatively result in seemingly
exotic high-energy species such as UO4 under thermal
conditions. One approach to achieve such reactions is by
selecting precursors that have ligands that decompose absent
substantial kinetic barriers to yield stable neutral molecules,
such as CO2.
−
within ∼10%;36 reproducibility of O2-addition to UO2(CH3COO)2
−
established a similar constancy of the oxygen pressure.37 The helium
buffer gas pressure in the trap is constant at ∼10−4 Torr.
The ion trap has been modified to allow for the introduction of
reagent gases through a gas manifold and a leak valve.34 The 18O2 gas
was directly introduced into the ion trap. Although the isotopic purity
of the 18O2 was unknown, the relative pressure of 18O2 and 16O2 was
determined by monitoring the previously reported reaction of
The overall objective of the present work was to explore the
chemistries of gas-phase binary and complex protactinium and
uranium oxide species. An emphasis was on differences between
these two neighboring actinides, which contrasts with the very
similar chemistries of the homologous trivalent 4f lanthanides,
Pr and Nd. A further aim was to identify a route to oxygen-rich
molecular ions via ion−molecule reactions under thermal
37
−
−
UO2(CH3CO2)2 with O2 to yield UO2(O2)(CH3CO2)2 .
The IRMPD experiments were performed at the Free Electron
Laser for Infrared Experiments (FELIX) Laboratory.38 The
UO2(C2O4)− complex was produced by ESI of a similar solution
and under similar conditions as employed for the gas-phase reactivity
−
−
studies described above; the UO5H2 complex resulted from the
conditions; a new synthetic route to molecular UO4 was
sequential reaction of UO2(C2O4)− with O2 and H2O as described
below. The IRMPD spectra were acquired using a QIT/MS similar to
that previously employed to study uranyl-crown and organouranyl
complexes.39,40 The QIT/MS has been modified41,42 such that the
high-intensity tunable IR beam from FELIX can be directed onto the
ion packet, resulting in multiphoton dissociation that is appreciable
only when the IR frequency is resonant with a sufficiently intense
absorption vibrational mode of the particular mass-selected complex
being studied. The FEL produces ∼5 μs long IR pulses with an energy
of ∼40 mJ/pulse at a repetition rate of 10 Hz. For the IRMPD
measurements described here, ions were irradiated for 200 ms,
corresponding to two FEL pulses, before being scanned out of the trap
and detected.
identified. The gas-phase pentavalent actinyl complexes
PaO2(C2O4)− and UO2(C2O4)− were produced by very
different approaches, a disparity that reflects very different
chemistries of PaV and UV. Gas-phase reactions of the two
pentavalent actinyl-oxalate complexes with H2O and O2 were
studied in a quadrupole ion trap (QIT), with contrasting results
that reveal different chemistries of Pa and U. Infrared
multiphoton dissociation (IRMPD) spectra were acquired for
selected gas-phase uranium complexes. The structures and
bonding of the oxalate complexes and reaction products were
computed using density functional theory (DFT); agreement
between experiment and theory provides validation of the
computational methodology, as well as of computational results
not directly evaluated by experimental results.
COMPUTATIONAL METHODS
■
All the calculations in the manuscript were performed with the latest
development version of the open-source NWChem software suite.43
Density functional theory calculations were carried out with the
PBE044 density functional, using the Stuttgart small-core effective
core-potential for the actinide atoms45 and all-electron DFT optimized
valence triple-ζ polarized (TZVP) basis sets46 for the light atoms in
the complex. An unrestricted wave function was used for all open-shell
DFT calculations. The geometries of the complexes were optimized,
followed by frequency calculations to ensure the calculated structure
had no imaginary frequencies and was in a minimum energy
configuration. All calculated energetics presented in this paper include
the zero-point energy correction. For each experimentally observed
species all possible bonding and coordination patterns were explored
at various accessible spin states. Oxidation states were assigned based
on Mulliken atomic spin populations. Atomic spin populations have
been shown to provide a reliable picture of the actinide oxidation state
EXPERIMENTAL APPROACH
■
The general experimental approach has been described previously.33
The anionic actinyl carboxylate complex, UO2(C2O4)−, was produced
by ESI of an ethanol solution prepared by diluting a 10 mM UO2Cl2
stock solution (238U isotope; pH = 2) to 100 μM; H2C2O4 was added
to yield a H2C2O4:U ratio of 3:1. A 14 mM solution of Pa in 0.1 M
HCl, containing a 6:1 ratio of NH4F:Pa so as to solubilize the Pa2O5·
xH2O was used as a stock solution. This Pa stock was diluted with 1 M
H2C2O4 corresponding to a H2C2O4:F− ratio of 500:1, in an effort to
displace the strongly coordinating F− from the Pa. Prior to addition of
−
excess oxalate, PaF6 was evident in the ESI mass spectrum. The Pa
oxalate solution was introduced to the mass spectrometer as a 50%
vol/vol ethanol to Pa(aq) mixture, resulting in a final H2C2O4:Pa ratio
of 3500:1. As discussed below, ESI of the Pa solution did not directly
B
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Article
a
Table 1. Computed Reaction Energies
reaction
An = Pa
An = U
−
−
1
An(C2O4)3 → AnO(C2O4)2 + CO2 + CO
206
189
AnO(C2O4)2 → AnO2(C2O4)− + CO2 + CO
−
2
3
AnO2(C2O4)− + H2O → AnO2(C2O4)(H2O)−
AnO2(C2O4)− + H2O → AnO(OH)2(C2O4)−
−73
−121
−79
−132
52
−75
−95
4
AnO(C2O4)2 + H2O → AnO(C2O4)2(H2O)−
−
5
−
−
6
AnO(C2O4)2 + H2O → An(OH)2(C2O4)2
7
AnO2(C2O4)− + O2 → AnO2(C2O4)(O2)−
−218
78
AnO2(C2O4)(O2)− → AnO4 + 2(CO2)
−45
7
−
8
AnO2(C2O4)− + O2 → AnO4 + 2(CO2)
−140
−57
−171
−
9
AnO4 + H2O → AnO4(H2O)−
−9
−187
−
11
12
−
−
AnO4 + H2O → AnO3(OH)2
a
ΔE in kJ/mol, with zero point energy correction.
by Maurice et al.47 A QTAIM analysis of the bond critical points in the
actinyls was performed with the AIMAll code.48
than U−O bonds due to energy-degeneracy driven cova-
lency.31,49,50 A QTAIM analysis of two of the bond critical
point (BCP) metrics along the An-O bond, namely the electron
(Pa = 0.237 au, U = 0.275 au) and energy (Pa = −0.168 au, U =
−0.229 au), show an increase in value that is indicative of an
increased covalency going from Pa to U.50 The greater
deviation from linearity for protactinyl can be attributed to
this lesser covalency, which relaxes the necessity for symmetry
correspondence between the Pa 5f and O 2p orbitals. The
RESULTS AND DISCUSSION
■
Synthesis and Structures of AnO2(C2O4)− (An = Pa, U).
The complex UO2(C2O4)− was produced directly by ESI of a
solution containing uranyl and oxalic acid (Figure S2).
Formation of the UVO2 oxalate complex indicates reduction
+
of UVIO22+ during ESI. Also apparent in the ESI mass spectrum
2+
is a peak attributed to the UVIO2 oxalate dianion complex,
+
greater propensity for PaO2 , when compared with other
UO2(C2O4)22−, which upon CID yields UO2(C2O4)− and
C2O4− (the latter ion is not detected due to the low-mass cutoff
of the QIT/MS during CID). In contrast to uranyl, protactinyl
is not a stable species in solution but rather undergoes
hydrolysis.28,29 Accordingly, PaO2(C2O4)− did not appear in
the ESI mass spectrum. Instead, the gas-phase protactinyl
complex was produced by CID of the PaV complex,
pentavalent actinyl ions, to hydrolyze has similarly been
attributed to its more ionic bonding character.51 The results
of the bonding analysis for UO2(C2O4)− and PaO2(C2O4)− in
Table 2 indicate spin populations and states, and atomic
charges, in accord with characterization of these as pentavalent
actinyl complexes. The parameters in Table 2 are indicative of
pentavalent actinyl complexes that are formally Pa(5f0)O2+ and
−
U(5f1)O2 coordinated by an oxalate dianion, C2O42−. The
+
Pa(C2O4)3 , which was a prominent species in the ESI mass
−
spectrum (Figure S1). Primary CID (MS2) of Pa(C2O4)3
PaO2(C2O4)− complex synthesized in this work is a rare
−
example of a PaVO2 species, which are virtually unknown due
+
yields predominantly PaO(C2O4)2 . Secondary CID of PaO-
(C2O4)2 (MS3) yields PaO2(C2O4)−, along with substantial
−
to the propensity of protactinyl to hydrolyze.52 This
accomplishment is an illustration of the ability to produce
stable prototypical bonding motifs that may be inaccessible in
the condensed phase due to high reactivity; the proclivity for
PaO2(C2O4)− to hydrolyze during CID is in correspondence
with the intrinsic solution chemistry of PaV.
PaO(OH)2(C2O4)−, which is attributed to a hydrolysis reaction
with background water in the ion trap (see below). The
syntheses of the protactinyl oxalate complex was achieved by
CID reactions 1 and 2 in Table 1, which are facilitated by the
formation of the stable CO2 and CO molecules.
The computed ground state structures of UO2(C2O4)− and
PaO2(C2O4)− are shown in Figure 1. The O−An−O actinyl
Addition of H2O to AnO2(C2O4)− and PaO(C2O4)−.
Experimental results for reactions of PaO2(C2O4)− and
UO2(C2O4)− with background gases in the ion trap are
shown in Figure 2. As has been discussed previously,36,37 the
background pressures of H2O and O2 in the QIT are estimated
to be on the order of ∼10−6 Torr, and are found to remain
constant to within ∼10%. For the Pa complex the only
observed reaction is water-addition, which is also observed for
the U complex albeit with an efficiency more than ten times
lower and with additional reaction pathways as discussed below.
Water-addition can result in a hydrate by simple adduct
formation (reaction 3 in Table 1), or a bis-hydroxide by
hydrolysis (reaction 4 in Table 1). The computed structures for
both the hydroxide and hydrate isomers, which are similar for
An = Pa and U, are shown in Figure 1. The computed
energetics for hydrate and hydroxide formation are given in
Table 1 (reactions 3 and 4, respectively), where it is apparent
that the hydroxide is predicted to be the more stable isomer
upon addition of a water molecule to both the Pa and U
complexes. Whereas the physisorption hydration reactions
exhibit very similar energies for both complexes, the
Figure 1. Computed structures of AnO2(C2O4)−, AnO-
(OH)2(C2O4)−, and AnO2(C2O4)(H2O)− for An = Pa and U.
bond angles are 154.2° and 165.0°, and the An−Oyl bond
distances are 1.87 and 1.81 Å for An = Pa and U, respectively
(all geometric coordinates are in the Supporting Information).
Both actinyls are distorted from linear, with that for protactinyl
substantially more so. Although the net bond dissociation
+
energy, to An+ and two O atoms, of PaO2 (∼1580 kJ/mol) is
+
somewhat greater than that of UO2 (∼1520 kJ/mol), it has
been established that molecular Pa−O bonds are less covalent
C
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Table 2. Spin States, Mulliken Atomic Charges (q), Atomic Spin Populations (μ) of Actinyl Oxygens, and Assigned Oxidation
States for PaO2(C2O4)−, UO2(C2O4)−, PaO2(C2O4)(O2)−, UO2(C2O4)(O2)−, PaO4 , and UO4
−
−
species
An
qAn
μAn
qO‑AnO
μO‑AnO
qO−O
μO−O
2
oxidation/spin state
2
2
2
AnO2(C2O4)−
Pa
U
1.67
1.60
1.78
1.75
1.56
1.70
0.00
1.12
−0.69
0.00
V (5f0)/singlet
V (5f1)/doublet
−0.67
−0.59
−0.57
−0.77/−0.51
−0.66/−0.69
−0.05
0.30
a
AnO2(C2O4)(O2)−
Pa
U
0.24
−0.23
−0.21
0.51
0.52
V (5f0)/triplet
−0.05
0.00
0.00
VI (5f0)/doublet
V (5f0)/singlet
VI (5f0)/doublet
−
AnO4
Pa
U
0.00/0.00
0.19/0.41
−0.20
a
Triplet is energetically degenerate with the open-shell singlet.
susceptible to hydrolysis in aqueous solution, whereas PaO3+
exhibits a substantially greater stability.53 The gas-phase
hydrolysis results are found to be parallel to the solution
hydrolysis behavior of PaO2 and PaO3+, where the former is
+
more susceptible toward hydrolysis. In particular, the much
+
more facile hydrolysis of the PaO2 core to yield PaO-
(OH)2(C2O4)− is in accord with the unstable nature of this
dioxo cation. This chemistry is not fully understood but is
generally attributed to highly ionic character of the Pa−O
+
bonds in PaO2 , which results in more facile attack by water to
yield Pa−OH bonds upon hydrolysis.31,49−51
Reactions of AnO2(C2O4)− with O2: The Synthesis of
−
−
UO4 . A particularly notable result is the appearance of UO4
upon reaction of UO2(C2O4)− with background gases in the
ion trap (Figure 2b). The corresponding PaO4 product is not
Figure 2. Mass spectra acquired after isolation of (a) PaO2(C2O4)−
and (b) UO2(C2O4)− and reaction with background gases (H2O and
O2) in the ion trap for 50 and 500 ms, respectively. The black arrow
−
−
apparent in Figure 2a. To confirm that UO4 results from the
reaction of UO2(C2O4)− with O2, an excess of isotopically
labeled 18O2 was introduced into the ion trap. The result is
shown in Figure 3, where the dominant product peak at 306 m/
−
points to the UO4 , whereas the red arrows point to PaO2(C2O4)− in
panel a and UO2(C2O4)− in panel b. Blue arrows signify water addition
products.
chemisorption hydrolysis reaction is calculated to be 26 kJ/mol
more exothermic for PaO2(C2O4)− versus UO2(C2O4)−, which
could account for the faster reaction rate for the Pa complex.
The greater propensity for hydrolysis of the protactinyl
complex is in accord with previous results for water-addition
+
+
to PaO2 versus UO2 , where the former hydrolyzes to a bis-
hydroxide whereas the latter forms a hydrate adduct. This
disparity in chemistry may be ascribed to lesser covalency of
molecular Pa−O bonds, as discussed previously.31,49,50
Addition of water to the AnO2(C2O4)− complexes, whether
the product is a bis-hydroxide or a hydrate, results in no change
in oxidation state with the result that the energetics for the Pa
and U complexes are similar. This is in contrast to reactions for
which there is a change in oxidation state, specifically from UV
to UVI as occurs in the addition of O2 to yield a superoxide; for
this oxidation reaction the energetics are significantly more
favorable for UV versus PaV because of the stability of the UVI
oxidation state.
Figure 3. Mass spectrum acquired after isolation of UO2(C2O4)− for
50 ms in the ion trap containing 18O2. The 16O2/18O2 ratio was
estimated as ∼1:8 using the method described in the Experimental
Section.
z corresponds to reaction of UO2(C2O4)− with 18O2 to yield
UO218O2 (unless specified otherwise, O refers to the 99.8%
natural abundance 16O isotope). The minor peak at 304 m/z is
attributed to oxo-exchange of UO218O2− with background water
in the trap to yield UO318O−. That this exchange process occurs
The computed energetics for the hydrate and hydroxide
−
−
formation for PaO(C2O4)2 (reactions 5 and 6) and
was established by demonstrating exchange of UO4 with
PaO2(C2O4)− (reactions 3 and 4) are found to be very similar.
H218O (Figure S4). The minor peak at 302 m/z corresponds to
−
Because primary and sequential CID of Pa(C2O4)3 yielded
reaction of UO2(C2O4)− with background O2 in the trap to
both PaO(C2O4)2 and PaO2(C2O4)− it was possible to
yield UO4 , as is apparent in Figure 2b. There is also a minor
−
−
directly compare the hydrolysis rates of these two anion oxo
complexes. The results (Figure S3) reveal that PaO2(C2O4)−
hydrolyzes much more rapidly, by a factor of ∼100-fold, as
peak at 376 m/z resulting from addition of H2O to
UO2(C2O4)−; the intensity of this peak is much lower than
in Figure 2b, largely due to the shorter reaction time, 50 ms
there versus 500 ms in Figure 3.
−
compared with PaO(C2O4)2 . Given the reaction energetics
were found to be similar, the difference in reactivity is driven by
Spontaneous exothermic replacement of C2O4 by O2 in
−
−
a higher kinetic barrier for PaO(C2O4)2 . The two species can
UO2(C2O4)− to yield UO4 is a remarkable process. Notably,
be considered as PaO2 and PaO3+ coordinated by one or two
this replacement reaction is not observed for PaO2(C2O4)−.
The observed reaction presumably occurs by addition of O2 to
+
+
oxalate dianions, respectively. It is known that PaO2 is highly
D
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Article
−
Figure 4. Computed structures of AnO2(C2O4)(O2)− and AnO4 for An = Pa, U.
the oxalate complex, reaction 7 in Table 1, followed by
elimination of two CO2 molecules, reaction 8. The weakly
bound C2O4 gas-phase CO2 dimer complex is calculated to be a
few kJ/mol lower in energy than two separated CO2 molecules
and should not remain bound under the ∼300 K experimental
conditions; the product of reaction 8 is accordingly given as
two CO2 molecules rather than the dimer. The computed
energies for the O2-addition reaction 7, CO2-elimination
reaction 8, and the net O2/C2O4 replacement reaction 9 are
given in Table 1. For protactinium the O2 association reaction
is unfavorable, and as a result the replacement reaction is not
observed in experiment. In contrast, the O2 association with the
uranium complex is highly exothermic. In the formation of
UO2(C2O4)(O2)− the uranium gets oxidized to UVI. Protactium
cannot be oxidized further, and forms an unstable biradical
complex where the triplet and open-shell singlet are nearly
degenerate, with one unpaired electron on the O2 and one
electron distributed over the PaO2 unit. The experimental
result that the association complex UO2(C2O4)(O2)− is not
observed suggests that it efficiently eliminates two CO2
energy below the 78 kJ/mol needed for this process. The
proposed reaction pathways and energetics are shown in Figure
5.
Figure 5. Computed reaction energetics for conversion of
−
AnO2(C2O4)− to AnO4 by reaction with O2. Although the overall
reaction is predicted to be exothermic for An = Pa and U, the initial
O2-assciation step is endothermic for Pa but exothermic for U.
−
molecules to yield UO4 according to reaction 9 in Table 1.
Reaction 10, in which UVO2 anion complexes are oxidized
+
2+
to UVIO2 by addition of O2 to form superoxides, was
As is apparent in Figure 2, the only reaction of PaO2(C2O4)−
with the H2O/O2 background gases in the ion trap is water
addition, which results in no change in oxidation state.
Oxidation of closed-shell [Rn]5f0 PaV is chemically implausible
with the result that addition of O2 to yield the superoxide
PaO2(C2O4)(O2)− is not favorable. The calculated energetics
show that reaction 7 is endothermic by 52 kJ/mol for Pa, and is
270 kJ/mol less favorable than for U. This is a manifestation of
relatively facile oxidation of UV to UVI, this in contrast to the
inaccessibility of the PaVI oxidation state. As indicated in Table
2, the oxidation state of the hypothetical PaO2(C2O4)(O2)− is
predicted to stay PaV. This complex is also computationally
shown to be unstable relative to PaO4− and two CO2 molecules
(reaction 9 in Table 1).
previously reported to occur spontaneously for X = F, Cl, Br, I,
OH, NO3, ClO4, HCO2, CH3CO2, CF3CO2, CH3COS, NCS,
and N3.37
UVO2X2− + O2 → UVIO2X2(O2)−
(10)
The energy for reaction 10 was computed for X = F, Cl, Br, I,
OH, and NCS; the results indicated the reaction to be
exothermic by 143 kJ/mol (for X = I) to 195 kJ/mol (for X =
OH). The computed energy of reaction 7 for An = U is only 23
kJ/mol more exothermic than is reaction 10 for X = OH. Some
care should be taken comparing the energetics as the density
functional used here differs from the previous work. Even so, it
is apparent that reaction 7 for An = U is directly analogous to
2−
reaction 10, with two X− ligands replaced by one C2O4
−
The computed structures of hypothetical PaO4 and
observed UO4− are shown in Figure 4, with all of the structural
parameters included in the Supporting Information. The
ligand.
The computed structures and properties of UO2(C2O4)-
(O2)− are consistent with the characterization as a UVI
−
structure of PaO4 incorporates a distorted protactinyl moiety
−
superoxide complex (Figure 4 and SI). The UVO2X2 and
in which the Oax−Pa−Oax angle is 144° and the Pa−Oax bond
UVO2(C2O4)− complexes are exothermically oxidized to
+
distances are both 1.91 Å. The formally PaO2 unit is
UVIO2 by addition of O2 to yield superoxides. For the
2+
coordinated by a peroxide ligand to yield the PaVO4 anion
−
UVIO2X2(O2)− products there is no low-energy dissociation
pathway and a substantial fraction of the adduct cools by
collisions with the helium bath gas in the ion trap and thus
remains intact prior to adiabatic dissociation to the reactants.
For the UO2(C2O4)(O2)− association product, the energy for
the pathway corresponding to elimination of two CO2
molecules is much lower than the association energy and is
thus enabled after association. Notably, UO2(C2O4)(O2)− was
not detectedthere is no peak at 390 m/z in Figure 2b
suggesting that carbon dioxide elimination predominates during
the period before the association complex has cooled to an
complex with Pa−Oper distances of 2.19 Å and an Oper−Oper
−
distance of 1.46 Å. The distinctive UO4 complex, where the
O2 is clearly dissociated (bond distance of over 3 Å), is
computed to have a 5f0 configuration corresponding to UVI.
The symmetry of the species is C2v: there are two uranyl double
bonds with U−O distances of 1.84 Å and an Oax−U−Oax angle
of 157°, and two longer and weaker bonds having U−O
distances of 1.92 Å and an Oax−U-Oax angle of 114°. The
atomic properties are in Table 2. Alternative structures were
found to be higher in energy, for example the UVO2 -peroxo
+
was calculated to be 130 kJ/mol higher in energy. The
E
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
computed structure of UO4− is very similar to that reported and
analyzed previously.17,54 The doublet spin state indicates one
unpaired electron distributed between the two more weakly
bonded oxygen atoms. This interpretation is very similar to that
of Michelini and co-workers, who find the radical character to
be delocalized between the same “equatorial” oxygen atoms.17
calculated to be substantially more energetically favorable than
formation of the hydrate; the energy disparities between
physisorption and chemisorption are greater than for water
addition to the oxalates as given by reactions 3 and 4 (Table 1).
The formation of hydroxide rather than hydrate motif upon
reaction of gaseous metal oxo complexes with water is not
unprecedented.55 The computed structures of all of the water-
addition species are included in the Supporting Information.
More recently Su et al.54 studied the unusual structure of UO4
−
and came to a consistent conclusion that the complex is not
2−
−
fully represented with fractional oxidation states 2Oax and
The structure of ground-state doublet UO3(OH)2 is nearly
2O−1.5, as suggested here, but is rather intermediate between
this and four O−1.75 ligands as would be the case if all U−O
bonds would be equal. Although the bonding representation
presented here is a somewhat simplified model, it conveys the
essential nature of the species.
linear uranyl (∠Oax−U-Oax = 177°) with three ligands, two OH
and one O, in the equatorial plane, separated by angles close to
120°. The U−Oax distances of 1.79 Å are typical of uranyl
bonds. The U−OH distances of 2.20 Å are characteristic of U−
O single bonds. The U−Oeq distance, 2.18 Å, is similarly
indicative of a single bond such that the complex can be
−
Reaction of UO4 with H2O. The addition of water to
−
2+
considered as UVIO2 coordinated by two hydroxide ligands
UO4 is apparent in Figure 2b. It is conceivable from these
−
results that UO5H2 is produced from a secondary reaction of
and a radical oxygen atom with an unpaired electron. The
computed structure of ground-state singlet PaO3(OH)2− is very
different: Pa is coordinated by a double-bond oxygen, two
hydroxides, and a peroxide moiety to yield the stable
pentavalent oxidation state.
UO2(OH)2(C2−O4)−. To confirm direct water addition to the
tetroxide, UO4 was isolated and reacted with the background
gases. The result is shown in Figure 6, where it is apparent that
−
IRMPD Spectrum of UO5H2 . IRMPD spectra were
acquired in an ion trap at FELIX that has a similar design to
that employed for the CID and reactivity studies at LBNL. The
time scale of the IRMPD experimentsion accumulation and
IRMPD dissociationis ∼200 ms, which is shorter but roughly
comparable to the 500 ms time scale of the reactivity results
shown in Figure 2b. As is apparent in Figures 2 and 6, the
−
product resulting from addition of H2O to UO4 is unreactive
−
Figure 6. Mass spectrum acquired after isolation of UO4 for 300 ms
on this same time scale under the experimental conditions. It
was thus possible to acquire an IRMPD spectrum for this
species. The spectrum, shown in Figure 7, was obtained by
in the ion trap containing background gases. The result demonstrates
−
the spontaneous addition of H2O to UO4 .
−
−
isolating UO4 (302 m/z), after which the UO5H2 water-
addition product was also present in the ion trap; both of these
species were present during the IRMPD experiment. Upon
varying the FEL wavelength it was observed that depletion of
water addition to UO4− occurs spontaneously. As is the case for
water addition to the oxalates, the two plausible structures for
the water-addition product are a hydrate, UO4(H2O)−, and a
hydroxide, UO3(OH)2 . The calculated energies for the two
water-addition processes, reactions 11 and 12, are given in
−
−
UO5H2 resulted in enhancement of the IRMPD product ion
UO4 . The IRMPD spectrum was thus acquired by measuring
the increase in UO4 above the baseline intensity as the
−
−
−
−
Table 1 for both hypothetical PaO4 and observed UO4 . In
both cases the formation of the hydroxide, reaction 12, is
wavelength was varied. The resulting spectrum acquired in the
Figure 7. Two IRMPD spectra (red and blue traces) of UO5H2− obtained under the same experimental conditions using the approach described in
the text. The observed peak at 860 cm−1 is in accord with the single peak computed for the lower-energy UO3(OH)2− chemisorption (hydrolysis)
structure, and not with the UO4(H2O)− physisorption (hydrate) structure.
F
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
range of 650−1000 cm−1 (Figure 7) shows a single intense
peak at 860 cm−1. The single observed peak can be assigned to
the uranyl asymmetric stretch frequency predicted from
calculations to be at 938 cm−1 with PBE0 and 896 cm−1 with
the B3LYP functional (both unscaled) for the hydroxide
−
results for addition of water to PaO(C2O4)2 and
PaO2(C2O4)−, gas-phase reaction of U16O4 with
−
H218O, and complete computational results for all
protactinium and uranium species considered (PDF)
−
structure, UO3(OH)2 . While the calculated frequency for the
AUTHOR INFORMATION
■
hydroxide structure is too high, it is well-known that with the
chosen PBE0 or B3LYP functional frequencies are over-
estimated and need to be scaled to lower values.56 Energetically,
the hydroxide structure is predicted to be 114 kJ/mol lower in
energy than the hydrate (Table 1). In addition, the computed
spectrum for the alternative hydrate structure has two intense
absorption features split by about 50 cm−1, instead of the single
peak observed in the experiment (see Figure S5). Both the
difference in spectral features and relative energetics lead to the
Corresponding Authors
*E-mail: wadejong@lbl.gov.
*E-mail: jkgibson@lbl.gov.
ORCID
Wibe A. de Jong: 0000-0002-7114-8315
Joaquim Marca̧ lo: 0000-0001-7580-057X
Theodore A. Corcovilos: 0000-0001-5716-1188
Jonathan Martens: 0000-0001-9537-4117
John K. Gibson: 0000-0003-2107-5418
assignment of the 860 cm−1 peak to UO3(OH)2 rather than
−
UO4(H2O)−.
Notes
The authors declare no competing financial interest.
CONCLUSIONS
■
The two gas-phase pentavalent actinyl-oxalate anion complexes,
UO2(C2O4)− and PaO2(C2O4)−, were prepared by different
routes. The uranium complex resulted from ESI and/or
fragmentation during transfer to the ion trap. In contrast, the
protactinium complex was not apparent upon ESI/transfer but
was instead produced by CID of Pa(C2O4)3− in the ion trap. It
is feasible that UO2(C2O4)− was not generated by ESI, but
ACKNOWLEDGMENTS
■
This work was supported by the U.S. Department of Energy,
Office of Basic Energy Sciences, Heavy Element Chemistry, at
LBNL under Contract No. DE-AC02-05CH11231 [P.D.D. and
J.K.G], by the U.S. Department of Energy Basic Energy
Sciences, Early Career Research Award Program, under
−
rather in the gas phase by fragmentation of UO(C2O4)2 or
Contract DE-AC02-06CH11357 [R.E.W.], by the Fundaca̧ o
para a Ciencia e a Tecnologia through RNEM-Portuguese Mass
̂
̃
−
U(C2O4)3 , which may be more susceptible to fragmentation
−
than Pa(C2O3)3 .
Spectrometry Network and project UID/Multi/04349/2013
[J.M.], by start-up funds from the Bayer School of Natural and
Environmental Sciences and Duquesne University [M.V.S. and
T.A.C.], and by The Netherlands Organisation for Scientific
Research (NWO) under vici-grant no. 724.011.002 and the
Stichting Physica [J.O.]. This research used resources of the
National Energy Research Scientific Computing Center
(NERSC), which is supported by the Office of Science of the
U.S. Department of Energy under Contract No. DE-AC02-
05CH11231. Some of the calculations were performed using
the Oak Ridge Leadership Computing Facility, which is a DOE
Office of Science User Facility supported under Contract DE-
AC05-00OR22725. An award of computer time was provided
by the Innovative and Novel Computational Impact on Theory
and Experiment (INCITE) program. The authors are grateful
to Professor Lester Andrews of the University of Virginia for
providing the 18O2 used in this work.
The UO2(C2O4)− complex spontaneously reacts with O2 to
−
yield UO4 , a process enabled by the creation of two CO2
−
molecules. The computed structure of UO4 is doublet C2v
with a U 5f0 configuration; the structure is essentially a
distorted UVIOc2+ complex with two U−Oyl oxygen bonds and
two equivalent U−Oeq bonds with a bond order intermediated
between one and two. Formation UO4 from UO2(C2O4)−
−
proceeds via addition of O2 to yield the superoxide complex
UVIO2(C2O4)(O2)−. The PaO2(C2O4)− complex does not
exhibit similar reactivity, which can be traced to the resistance
for oxidation from PaV to PaVI, resulting in endothermic
addition of O2 to yield PaO2(C2O4)(O2)−. The disparate
reactivities of UO2(C2O4)− and PaO2(C2O4)− with O2 are well-
predicted by the computed reaction energies.
The IRMPD spectrum of the isolated UO5H2− species in the
uranyl asymmetric stretch region was measured. Calculated
structures, energetics and IR spectra show that the species is
−
that of a hydroxide, UO3(OH)2 , rather than the hydrate,
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DOI: 10.1021/acs.inorgchem.7b00144
Inorg. Chem. XXXX, XXX, XXX−XXX