Controlling the lability of uranyl(vi) through intramolecular π-π Stacking

Article abstract of DOI:10.1039/c8dt02600a

A reaction of UO₂²⁺ with cyclohexyldiphenylphosphine oxide (OPCyPh₂) in ethanol resulted in a perchlorate salt of the 4-fold homoleptic complex, [UO₂(OPCyPh₂)₄](ClO₄)₂·EtOH. X-ray structure determination revealed that [UO₂(OPCyPh₂)₄]²⁺ shows a highly symmetric molecular structure supported by the intramolecular π-π stacking interactions between the phenyl groups of the neighbouring OPCyPh₂ in the equatorial plane of UO₂²⁺. To clarify whether the reactivity of UO₂²⁺ is affected by such a unique coordination structure, the ligand exchange kinetics of [UO₂(OPCyPh₂)₄]²⁺ in non-coordinating solvents has been studied using an NMR line-broadening method. In CD₂Cl₂ and CD₃NO₂ solutions, both signals of coordinated and free OPCyPh₂ appeared distinctively even at 298 K, while 2D EXSY experiment provided evidence that a chemical exchange between [UO₂(OPCyPh₂)₄]²⁺ and free OPCyPh₂ occurs in these systems. Thus, this ligand exchange reaction is quite slow on the NMR time-scale. The dependency of the apparent first-order rate constant (k_obs) on temperature and the free OPCyPh₂ concentration suggested that this ligand exchange reaction involves associative and dissociative mechanisms both governed by large negative ΔS^? terms predominantly. Compared with the kinetic data of ligand exchange reactions of other UO₂²⁺ complexes reported so far, the lability of UO₂²⁺ in [UO₂(OPCyPh₂)₄]²⁺ is found to be significantly suppressed due to the intramolecular π-π stacking interactions as observed in the crystal structure. The DFT calculations successfully reproduced the intramolecular π-π stacking in [UO₂(OPCyPh₂)₄]²⁺ by considering the empirical dispersion corrections, and provided theoretical rationales to the A and D mechanisms of the ligand exchange reaction of [UO₂(OPCyPh₂)₄]²⁺.

Full text of DOI:10.1039/c8dt02600a

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Controlling the lability of uranyl(VI) through
intramolecular π–π stacking†
Takanori Mashita,a Satoru Tsushima b,c and Koichiro Takao
Cite this: DOI: 10.1039/c8dt02600a
a
*
A reaction of UO22+ with cyclohexyldiphenylphosphine oxide (OPCyPh2) in ethanol resulted in a perchlor-
ate salt of the 4-fold homoleptic complex, [UO2(OPCyPh2)4](ClO4)2·EtOH. X-ray structure determination
revealed that [UO2(OPCyPh2)4]2+ shows a highly symmetric molecular structure supported by the intra-
molecular π–π stacking interactions between the phenyl groups of the neighbouring OPCyPh2 in the
equatorial plane of UO22+. To clarify whether the reactivity of UO22+ is affected by such a unique coordi-
nation structure, the ligand exchange kinetics of [UO2(OPCyPh2)4]2+ in non-coordinating solvents has
been studied using an NMR line-broadening method. In CD2Cl2 and CD3NO2 solutions, both signals of
coordinated and free OPCyPh2 appeared distinctively even at 298 K, while 2D EXSY experiment provided
evidence that a chemical exchange between [UO2(OPCyPh2)4]2+ and free OPCyPh2 occurs in these
systems. Thus, this ligand exchange reaction is quite slow on the NMR time-scale. The dependency of the
apparent first-order rate constant (kobs) on temperature and the free OPCyPh2 concentration suggested
that this ligand exchange reaction involves associative and dissociative mechanisms both governed by
large negative ΔS‡ terms predominantly. Compared with the kinetic data of ligand exchange reactions of
other UO2 complexes reported so far, the lability of UO2 in [UO2(OPCyPh2)4]2+ is found to be signifi-
cantly suppressed due to the intramolecular π–π stacking interactions as observed in the crystal structure.
The DFT calculations successfully reproduced the intramolecular π–π stacking in [UO2(OPCyPh2)4]2+ by
considering the empirical dispersion corrections, and provided theoretical rationales to the A and D
2+
2+
Received 26th June 2018,
Accepted 16th August 2018
DOI: 10.1039/c8dt02600a
rsc.li/dalton
mechanisms of the ligand exchange reaction of [UO2(OPCyPh2)4]2+
.
method. This is one of the main steps of the PUREX reproces-
sing process for spent nuclear fuels.
Introduction
Coordination chemistry of uranium with phosphorus-contain-
As another important class of P-containing ligands, tertiary
ing ligands is highly relevant to nuclear fuel reprocessing.1 phosphine oxides are frequently employed to explore the
Most typically, tri-n-butyl phosphate (TBP) is employed as a fundamental coordination chemistry of metal cations (Mn+
)
2+
2+ 2–12
unique extractant to selectively separate UO2 and Pu4+ from including UO2
.
These types of ligands commonly exhibit
fission products and minor actinides in a solvent extraction strong affinity to Mn+ thanks to their highly polarized PvO
bonds. The tertiary phosphine oxides in general are remark-
ably stable due to the protection of the P(V) centre by alkyl or
aryl substituents, so a wide range of applications as robust
ligand systems can be expected.13
aLaboratory for Advanced Nuclear Energy, Institute of Innovative Research,
2+
The coordination chemistry of UO2 with the phosphine
oxides is attracting special attention with particular interest
in coordination structures and homogeneous catalysis.
Triphenylphosphine oxide (OPPh3) has been exclusively
employed in the studies reported so far.2–7 Due to the bulki-
ness of OPPh3, the coordination structure prefers to have
Tokyo Institute of Technology, 2-12-1 N1-32, O-okayama, Meguro-ku,
152-8550 Tokyo, Japan. E-mail: ktakao@lane.iir.titech.ac.jp
bTokyo Tech World Research Hub Initiative (WRHI), Institute of Innovative Research,
Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, 152-8550 Tokyo,
Japan. E-mail: tsushima.s.ab@m.titech.ac.jp
cInstitute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf (HZDR),
Bautzner Landstrasse 400, 01328 Dresden, Germany. E-mail: s.tsushima@hzdr.de
†Electronic supplementary information (ESI) available: Kinetic data of ligand
exchange reactions of different uranyl complexes, IR spectra of the ligand and
complex, drawing of intramolecular π–π stacking, 31P{1H} NMR spectra of the
ligand and uranyl complex in non-coordinating solvents, 2D EXSY, Eyring plots,
and optimized structures of [UO2(OPCyPh2)n]2+. CCDC 1846710. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8dt02600a
2+
lower coordination number (CN) relative to UO2 complexes
with other types of ligands. For instance, [UO2(OPPh3)4]2+ is
formed as a 4-fold homoleptic complex despite the most
common CN = 5 in the UO2 coordination chemistry.4,5 In
2+
such a system, some space still remains in the coordination
sphere of UO22+, so the reactivity of the formed complex can
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be expected. As a matter of fact, the catalytic activity of α emitters and also contain their radioactive daughters.
[UO2(OPPh3)4]2+ as a Lewis acid in acyl substitution reactions Therefore, standard precautions for handling radioactive
has been demonstrated in our recent study.14
materials should be followed.
Besides chelating PvO ligands or their hybrid ones with
Uranyl perchlorate hydrate (UO2(ClO4)2·nH2O, n ∼ 5) was
another functional group specially designed for some specific prepared by dissolving UO3 (lab-made) in equimolar amounts
purposes,6,15–17 any phosphine oxides with various substitu- of HClO4 (aq.). This solution was concentrated near to
ents other than OPPh3 have not been well-tested. However, dryness. To remove slightly excess acid in UO2(ClO4)2·nH2O,
some steric and/or inductive effects on the reactivity of Mn+ the addition of a small amount of ultrapure water and evapor-
can be expected upon substitution of the phenyl group of ation by heating were repeated until no white fume was
OPPh3.
observed. The final residue of UO2(ClO4)2·nH2O was dissolved
2+
In this context, we are currently motivated to clarify the in ultrapure water. The UO2 concentration in this stock solu-
reactivity of the UO2 complexes upon substitution with tion was determined by recording a UV-vis absorption spec-
2+
various monodentate phosphine oxide ligands, OPR3. The trum in 1 M Na2CO3 (aq.). Cyclohexyldiphenylphosphine oxide
reactivity of Mn+ is always of great interest not only in the fun- (OPCyPh2, 121 mg, 0.43 mmol) was dissolved in ethanol
damental coordination and solution chemistry, but also in the (3 mL, Kanto), followed by the addition of the stock solution
wide application fields like homogeneous catalysis in organic of UO2(ClO4)2·nH2O (1.089 M, 97.6 μL). This solution was
syntheses, redox reactions in batteries, and reprocessing of stored upon contact with diisopropyl ether (iPr2O) vapor in a
spent nuclear fuels. A ligand exchange reaction in Mn+ with no closed vessel in the dark. Single crystals of [UO2(OPCyPh2)4]
net reaction and ΔG° = 0 is the simplest. Therefore, its kinetic (ClO4)2·EtOH suitable for X-ray crystallography were deposited
information is frequently employed as a measure to assess the within 3 days. Yield: 72%.
lability of Mn+ under certain conditions.18 In this work, we
report the synthesis and characterization of a UO2 complex
2+
Characterization
with cyclohexyldiphenylphosphine oxide (OPCyPh2) and its [UO2(OPCyPh2)4](ClO4)2·EtOH prepared above was character-
ligand exchange kinetics together with theoretical rationales ized by NMR (JEOL JNM-ECX400), IR (JASCO FT/IR-4700),
supported by DFT calculations. The perturbation of the ligand elemental analysis (Yanaco MT-5), and single crystal X-ray
structure to replace one of the phenyl groups on P with the diffraction (Rigaku RAXIS RAPID). To the sample of elemental
cyclohexyl group (Cy) provides unique intramolecular π–π analysis, WO3 was added for fully converting carbon into CO2
stacking interactions to form a highly symmetric coordination even in the presence of phosphorus. For the X-ray structure
structure around UO22+, which strongly affects the kinetic and determination, a single crystal of [UO2(OPCyPh2)4](ClO4)2·EtOH
mechanistic aspects of the ligand exchange reaction.
was mounted on a Kapton capillary and put into a temperature-
controlled nitrogen gas stream (123 K). Intensity data were col-
lected using an imaging plate area detector on a Rigaku RAXIS
RAPID with graphite monochromated Mo Kα radiation (λ)
0.71075 Å. The structure was solved by direct methods,
SHELXT,19 and expanded using Fourier techniques. All non-
Experimental
Materials
Cyclohexyldiphenylphosphine (1.0 g, Tokyo Chemical Ind.) hydrogen atoms were anisotropically refined by SHELXL.20
was dissolved in acetone (20 mL, Kanto Chemical Co. Ltd), Hydrogen atoms were refined as riding on their parent atoms
and an excess amount of H2O2 (30 wt% 1.0 mL, Kanto) was with Uiso (H) = 1.2Ueq (C). The final cycle of the full-matrix
added. This mixture was stirred at room temperature for 2 h, least-squares refinement on F2 was based on the observed
dried over MgSO4, filtered, and evaporated under reduced reflections and parameters and converged with the
pressure. In order to remove residual H2O2, the crude product unweighted and weighted agreement factors, R and wR. All
was mixed with pre-dried molecular sieves 3A (6.0 g, Kanto) in calculations were performed using the CrystalStructure
toluene (50 mL, Kanto). The mixture was stirred at room temp- crystallographic software package.21
erature for 18 h, followed by filtration through Celite and eva-
[UO2(OPCyPh2)4](ClO4)2·EtOH:
anal.
calcd
for
poration under reduced pressure to give a colourless powder. C74H90Cl2O15P4U: C, 53.74; H, 5.49. Found: C, 53.56; H, 5.45.
Yield: 79%. OPCyPh2: 1H NMR (399.78 MHz, CD2Cl2, 298 K, vs. 1H NMR (399.78 MHz, CD2Cl2, 298 K, vs. TMS.): δ/ppm 0.8–1.9
TMS.): δ/ppm 1.0–1.7 (m, 11H, Cy), 7.4–7.9 (m, 10H, Ph). 31P (m, 44H, Cy), 7.4–7.8 (m, 40H, Ph). 31P{1H} NMR (161.83 MHz,
{1H} NMR (161.83 MHz, CD2Cl2, 298 K, vs. 85% H3PO4.): CD2Cl2, 298 K, vs. 85% H3PO4.): δ/ppm 60.28. IR (cm−1
,
δ/ppm 33.12. IR (cm−1, neat, diamond ATR): 3080(w), 2950(s), diamond ATR): 3542 (m, EtOH), 3080(w), 2950(m), 2870(m),
2870(m), 1620(w), 1510(w), 1470(w), 1455(m), 1195(s), 1140(s), 2200(w), 1610(w), 1510(w), 1470(w), 1455(m), 1145(s), 1120(bs,
1090(w), 1020(m), 940(w), 910(m), 870(m), 845(m), 770(m), ClO4−), 1068(s), 1045(m), 1010(s), 945(s, ν3, asymmetric
760(m), 740(s), 720(s), 575(s), 550(s).
stretching of [OvUvO]2+), 915(w), 870(w), 845(w), 780(w),
Caution! Perchlorate salts are potentially explosive. 760(s), 745(s), 715(s), 705(s), 645(m), 635(m), 570(s), 553(s),
Handling of all compounds containing perchlorate, especially 530(w), 480(w). Crystallographic data: fw = 1652.35, ortho-
with heated UO2(ClO4)2·nH2O, should be done with great care rhombic, Fddd (no. 70), a = 17.9384(8) Å, b = 21.0551(10) Å, c =
and in small amounts. All the isotopes in natural uranium are 38.4913(16) Å, V = 14 538.0(11) Å3, Z = 8, T = 123 K, Dcalcd
=
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1.510 g cm−1, μ = 24.597 mm−1, GOF = 1.090, R (I > 2σ) = iPr2O successfully gave yellow crystals. The elemental analysis
0.0499, wR (all) = 0.1162.
data well agreed with those of [UO2(OPCyPh2)4](ClO4)2·EtOH.
The IR spectrum has also clearly indicated the presence of
UO2 and OPCyPh2, while the detection of peak shifts of the
2+
Kinetic study on the ligand exchange reaction
PvO stretching was difficult because of the superposition of
multiple IR absorptions in the 1100–1150 cm−1 region (Fig. S1,
ESI†). The IR absorption at 3542 cm−1 can be assigned to an
O–H vibration of the ethanol molecule present in the crystal
structure. The molecular and crystal structures of this complex
were determined by single crystal X-ray diffraction. The mole-
cular structure and selected structural parameters of
[UO2(OPCyPh2)4](ClO4)2·EtOH are shown in Fig. 1 and Table 1,
respectively. As seen from Fig. 1, a highly symmetric coordi-
nation structure of [UO2(OPCyPh2)4]2+ has been observed. The
uranium atom is coordinated by two axial oxygen atoms (Oyl)
and surrounded by four structurally-equivalent oxygen atoms
of OPCyPh2 in the equatorial plane (OL). The bond distances
in U–Oyl and U–OL are 1.763(5) Å and 2.309(4) Å, respectively.
Calculated amounts of [UO2(OPCyPh2)4](ClO4)2·EtOH and
OPCyPh2 were dissolved in CD3NO2. For this sample, the 31P
{1H} NMR spectra were recorded at various temperatures with
1
the H decoupling mode using a JEOL ECX-400 NMR spectro-
meter (1H, 399.78 MHz; 31P, 161.83 MHz). Kinetic analysis for
ligand exchange reactions in [UO2(OPCyPh2)4]2+ was performed
using the NMR line-broadening method in the two-site model.
The full line width at the half-maximum (Δνm) of the 31P
signal was obtained by Lorentz deconvolution using Igor Pro
6.02AJ. A line width of [UO2(OPCyPh2)4]2+ in the absence of
free OPCyPh2 at each temperature, Δνm0, was subtracted from
each spectrum recorded in the ligand exchange experiments.
The apparent first-order rate constant (kobs) under each con-
dition was calculated from the following relationship under
the slow exchange limit.22
The bond angles around U in all directions (Oyl–U–OL
=
93.70(9)° and OL–U–OL = 91.32(13)°) are close to 90°. Thus,
kobs ¼ πðΔνm ꢀ Δνm0Þ=4
ð1Þ
Quantum chemical calculations
Quantum chemical calculations were performed using the
Gaussian 16 program (Gaussian Inc.)23 employing the density
functional theory (DFT) using a conductor like polarizable con-
tinuum model.24 Structure optimizations were performed in
ethanol at the B3LYP-D3, CAM-B3LYP-D3, and B97D3 levels, all
of which include empirical dispersion corrections (D3 version
of Grimme’s dispersion with the original D3 damping func-
tion).25 Structure optimization was followed by vibrational fre-
quency analysis to confirm the absence of any imaginary fre-
quencies. Two sets of effective core potentials (ECPs) and basis
sets were used. For B3LYP-D3 calculations, the energy-consist-
ent small-core ECP and the corresponding basis set suggested
by Küchle et al. were used for U,26 whereas the 6-31G* basis
sets were used for all other elements.25,27 For CAM-B3LYP-D3
and B97D3 calculations, small-core ECP was used on U
whereas large-core ECP was used on other elements28 except
H. For H, the 6-31G basis set was used. The most diffuse basis
functions on uranium with the exponents 0.005 (all s, p, d,
and f type functions) were omitted. These basis functions have
a very small effect on the reaction energies (less than 1 kJ
mol−1).29 The spin–orbit effects and basis set superposition
error corrections were neglected. All the calculations were per-
formed on a TSUBAME 3.0 supercomputing system at the
Tokyo Institute of Technology.
Fig. 1 ORTEP drawing of [UO2(OPCyPh2)4]2+ in the ClO4 salt de-
−
posited from EtOH/iPr2O at the 50% probability level. Hydrogen atoms,
ClO4− and EtOH are omitted for clarity. Symmetry operation: 2; −X + 3/4
+ 1, Y, −Z + 3/4, 3; −X + 3/4 + 1, −Y + 3/4 + 1, Z.
Table 1 Selected
structural
parameters
of
[UO2(OPCyPh2)4]
(ClO4)2·EtOH
Bond distances (Å)
Bond angles (°)
Results and discussion
Synthesis and characterization of [UO2(OPCyPh2)4]
U(1)–O(1)
U(1)–O(2)
P(1)–O(1)
2.309(4)
1.763(5)
1.536(4)
O(1)–U(1)–O(2)
O(1)–U(1)–O(1)3
O(2)–U(1)–O(2)2
U(1)–O(1)–P(1)
93.70(9)
91.32(13)
180.00
(ClO4)2·EtOH
157.5(2)
[UO2(OPCyPh2)4](ClO4)2·EtOH was prepared from an EtOH
solution of UO2(ClO4)2(aq.) and OPCyPh2 in a 1 : 4 mole ratio.
Recrystallization of [UO2(OPCyPh2)4](ClO4)2·EtOH from EtOH/
Symmetry operation: 2; −X + 3/4 + 1, Y, −Z + 3/4, 3; −X + 3/4 + 1, −Y + 3/
4 + 1, Z.
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the coordination geometry around U is tetragonal bipyramidal in this substituent is tilted towards the direction of Oyl by
in
a
D4h symmetry. These structural properties of 16.6°. As a result, the hydrogen atoms in the cyclohexyl groups
[UO2(OPCyPh2)4]2+ described here are quite common to those can escape from steric hindrance with other phenyl groups of
of other uranyl and actinyl complexes with phosphine oxides the neighbouring ligands. In contrast, if [UO2(OPPh3)4]2+ tries
reported so far.4–6,8 For example, [UO2(OPPh3)4]2+ shows to show a similar ligand arrangement, its hydrogen atoms
similar structural features like U–Oyl = 1.758(4) Å, U–OL
=
should always suffer from steric hindrance with other phenyl
2.30 Å (mean), Oyl–U–OL = 90.4° (mean), and OL–U–OL = 90.0° groups and Oyl because of the strong planarity. Finally, this
(mean).5 The ethanol solvent molecule present in this crystal phenyl group of [UO2(OPPh3)4]2+ cannot find any stable
structure is disordered, and not directly bound to U.
arrangements in a coordination structure involving intra-
The most striking structural character of [UO2(OPCyPh2)4]2+ molecular π–π stacking like [UO2(OPCyPh2)4]2+.
is the highly ordered ligand arrangement in the equatorial
plane. Fig. 2 shows the side views of this complex drawn in the
Kinetic study on the ligand exchange reaction
thermal ellipsoids (left) and CPK model (right). Indeed, the A ligand exchange phenomenon is the simplest reaction with
centroids of the phenyl groups are rigorously located on the ΔG° = 0, and the most suitable to assess the reactivity of Mn+
.
mean plane of U(1) and four O(1) atoms within 0.2 Å deviation. If the intramolecular π–π stacking observed in Fig. 2 is also
Therefore, all the phenyl groups of the ligands are strictly maintained even in a solution, the reaction rate of the ligand
located in the UO2 equatorial plane, while the cyclohexyl exchange in [UO2(OPCyPh2)4]2+ will be affected regardless of
2+
groups are oriented in both axial directions above and below any of the possible mechanisms like dissociation of the
the plane. Such a characteristic ligand arrangement seems to leaving group and association of the extra ligand. In this
be ascribed to intramolecular π–π stacking interactions context, we were strongly motivated to study the ligand
between the phenyl groups of the neighboring OPCyPh2. exchange reaction kinetics of [UO2(OPCyPh2)4]2+.
According to Janiak,30 π–π stacking is commonly defined as
First, we selected CD2Cl2 as a solvent in this study because
stacks of aromatic groups with approximately parallel mole- of its non-coordinating nature. Actually, the 31P{1H} NMR spec-
cular planes separated by centroid–centroid distances ranging trum of the CD2Cl2 solution dissolving [UO2(OPCyPh2)4]
from 3.4 Å to 4.6 Å. Two aromatic rings are parallel, but often (ClO4)2·EtOH shows a singlet signal at 59.69 ppm, which is in
show displacement defined by the angles between the ring- a much lower field than that of free OPCyPh2 (33.58 ppm) as
centroid vector and the ring normal to one of the ring planes. shown in Fig. S3 (ESI†). Such a downfield shift indicates
The displacement angle averages 27° and most examples lie coordination of OPCyPh2 to form [UO2(OPCyPh2)4]2+, being
between 16° and 40°. In Fig. S2 (ESI†), typical structural para- supported by a similar behaviour found in [UO2(OPPh3)4]2+.5
meters related to the π–π stacking interactions in When the free OPCyPh2 was added to [UO2(OPCyPh2)4]2+ in
[UO2(OPCyPh2)4]2+ were extracted from the original molecular CD2Cl2, the 31P signals of both free and coordinated ligands
structure. The centroid–centroid distances of the stacked were observed distinctively at 298 K (Fig. S3, ESI†). This
phenyl groups are 3.90 Å and 4.19 Å. The displacement angles implies that the OPCyPh2 ligands are stable enough to be
are 28.0° and 35.4°. All these structural data well agree with silent in their exchange reaction, or at least such a reaction is
the above criteria of the offset π–π stacking, indicating that quite slow under these conditions. If the ligand exchange reac-
these intramolecular interactions result in the highly ordered tion actually occurs in [UO2(OPCyPh2)4]2+, we can observe sig-
ligand arrangement of [UO2(OPCyPh2)4]2+ shown in Fig. 1 and nificant temperature dependency of the NMR signals like line-
2. It is worth noting that any π–π stacking interactions have broadening and coalescence. However, there is an experi-
not been found in [UO2(OPPh3)4]2+ despite the similarity in mental difficulty in further exploring this chemistry because of
the ligand structure.4,5 Looking carefully at the structure of the low boiling point of CD2Cl2 (bp 313 K). Hence, we had to
[UO2(OPCyPh2)4]2+, the cyclohexyl groups are in the chair- find another non-coordinating solvent with a higher and wider
shape conformation, and therefore the mean plane of C atoms temperature range.
Nitromethane-d3 shows a higher boiling point (bp 374 K)
and low donor number,31 which indicates that this solvent is
nearly non-coordinating to Mn+. Therefore it would be suitable
for the purpose here. The 31P{1H} NMR spectrum of the
CD3NO2 solution dissolving [UO2(OPCyPh2)4](ClO4)2·EtOH
actually shows a 31P signal at 60.28 ppm, which is in agree-
ment with that in CD2Cl2, and is also in a much lower field
than that of the free ligand (33.76 ppm) in CD3NO2 (Fig. S4,
ESI†). The quite similar 31P chemical shifts of the coordinated
OPCyPh2 in the different solvents indicate that
[UO2(OPCyPh2)4]2+ is also present in CD3NO2. Although several
exceptional cases of CH3NO2 solvates of other Mn+ have been
Fig. 2 Side views of the molecular structure of [UO2(OPCyPh2)4]2+
drawn in the thermal ellipsoids (left) and CPK model (right). Counter
anions ClO4−, EtOH, and hydrogen atoms are omitted for clarity.
known,32,33 both additional coordination of CD3NO2 and
ligand substitution between OPCyPh2 and CD3NO2 will result
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in fully different 31P NMR signals as reported so far5 or
vide infra. Therefore, this is not the case in the current system.
A set of VT NMR results shown in Fig. 3 implies that the
ligand exchange reaction of [UO2(OPCyPh2)4]2+ is slow enough
When [UO2(OPCyPh2)4]2+ and free OPCyPh2 were mixed in on the NMR timescale to enable the detection of both signals
CD3NO2, the 31P signals of these species appear at the same of the free and coordinated OPCyPh2 separately in the studied
2+
chemical shifts of those dissolved individually. Fig. 3 shows temperature range. To understand the lability of UO2 in this
the 31P{1H} NMR spectra of [UO2(OPCyPh2)4]2+ and OPCyPh2 system quantitatively, we performed further VT NMR experi-
in CD3NO2 at different temperatures. The line widths of both ments at various free OPCyPh2 concentrations ([OPCyPh2]free
)
signals are actually broadened with elevating temperature. from 22.66 mM to 130.20 mM. The apparent first-order rate
However, these signals never coalesced even at 363 K. The line- constant (kobs) under each condition was evaluated by using
broadening may suggest the chemical exchange between the the NMR line-broadening method in the two-site model at the
free and coordinated OPCyPh2, whereas another possible slow exchange limit.22 The obtained kobs values were plotted as
explanation is the temperature dependency of the natural a function of the reciprocal T in Fig. 4. As a result, kobs clearly
widths of these signals. To further examine this system, we increases with an increase in [OPCyPh2]free at any of the temp-
performed 2D 31P{1H} EXSY (or NOESY) NMR homonuclear eratures studied here. In Fig. 5, kobs values at each T were
correlation experiment, which is sensitive enough to detect plotted as a function of [OPCyPh2]free. Consequently, kobs is
short contacts between non-bonded pairs of nuclei and chemi- proportional to the first-order of [OPCyPh2]free with a signifi-
cal exchange phenomena.34 Consequently, the correlation cant intercept in the range of [OPCyPh2]free examined here. In
between the 31P signals of the free and coordinated OPCyPh2 summary, kobs can be expressed as,
has been clearly observed as shown in Fig. S5 (ESI†). No short
contacts are predictable between the 31P nuclei of the free and
coordinated OPCyPh2 in the sample solution. The 2D EXSY
result clearly indicates the occurrence of the following ligand
exchange reaction of [UO2(OPCyPh2)4]2+ in CD3NO2.
kobs ¼ k1 þ k2½OPCyPh2ꢁfree
ð3Þ
where k1 and k2 are the first-order and second-order rate con-
stants, respectively. This relationship suggests that the ligand
exchange reaction of [UO2(OPCyPh2)4]2+ proceeds through two
pathways independent of and dependent on [OPCyPh2]free. The
k1 path can be categorized as a dissociative (D) mechanism,
where the rate-determining step is the dissociation of one of
the OPCyPh2 ligands from [UO2(OPCyPh2)4]2+ to form a 3-fold
complex like [UO2(OPCyPh2)3]2+ as an intermediate. The other
pathway of k2 should be assigned to an associative (A) mecha-
nism, where an extra OPCyPh2 is inserted into the inner
½UO2ðOPCyPh Þ ꢁ2þ þ OPCyPh2*
2
4
ð2Þ
! ½UO2ðOPCyPh Þ ðOPCyPh *Þꢁ2þ þ OPCyPh2
2
3
2
where the asterisk on OPCyPh2 is a typographical distinction
only. We also considered the use of other non-coordinating
solvents with higher boiling points like o-dichlorobenzene
(bp 453 K). To the best of our knowledge, no appropriate
solvents are, however, available because of the low solubility of
[UO2(OPCyPh2)4](ClO4)2·EtOH in such solvents.
2+
sphere of UO2 to give a 5-fold complex intermediate like
[UO2(OPCyPh2)5]2+. Another possibility for the k2 path is an
interchange (I) mechanism through an outer-sphere complex
formation, because kobs also exhibits dependency on
[OPCyPh2]free in this mechanism. In this case, kobs linearly
increases with an increase in [OPCyPh2]free, while it tends to
draw a curve to gradually approach a constant value at the
Fig. 3 31P{1H} NMR spectra of CD3NO2 solution dissolving
[UO2(OPCyPh2)4](ClO4)2·EtOH (10.0 mM) and OPCyPh2 (44.1 mM at
initial) recorded at various temperatures.
Fig. 4 Temperature dependence of kobs for the ligand exchange reac-
tion of [UO2(OPCyPh2)4]2+ in CD3NO2. The smooth line for each data set
is the best fit of the Eyring relationship.
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Table 2 Activation parameters and kinetic data of the ligand exchange
reactions of 4-fold UO22+ complexes, [UO2(L)4]2+
ΔH‡ a
‡
c
L (solvent)
Mech.
D
ΔS‡ b
ΔG298
k298 d
OPCyPh2 e (CD3NO2)
6.6 1.0
−202
6.9 1.0
−174
7.1 0.3
66.9
58.8
43.5
65.3
58.0
12f
3
OPCyPh2 e (CD3NO2)
OPPh3 h (CD2Cl2)
HMPAi (CD2Cl2)
HMPAi (CD2Cl2)
A
A
D
A
3.1 × 102 g
1.4 × 105 g
23f
3
−122
1
14
3
−172 11
22.2
3
4.3 × 102 g
−120
9
a Activation enthalpy in kJ mol−1 b Activation entropy in J mol−1 K−1
. .
Fig. 5 [OPCyPh2]free dependence of kobs for the ligand exchange reac- c Gibbs free energy for activation at 298 K in kJ mol−1
.
d Rate constant
.
g Second-order
at 298 K. e This work. f First-order rate constant in s−1
tion of [UO2(OPCyPh2)4]2+ in CD3NO2.
rate constant in M−1 s−1
.
Ref. 5. i Ref. 35.
h
higher [OPCyPh2]free. However, such a tendency was not
observed at least in the studied [OPCyPh2]free range at any of
the temperatures tested here. In summary, the rate-determin-
ing steps of the ligand exchange reaction of [UO2(OPCyPh2)4]2+
are shown in Scheme 1. In the later discussion, k1 and k2 are
denoted as kD and kA for clarity.
The Eyring plots for kD and kA have been obtained as func-
tions of the reciprocal temperature as shown in Fig. S6 (ESI†).
These tendencies allow us to estimate the activation enthalpy
(ΔH‡) and entropy (ΔS‡) in each of the kD and kA pathways.
Table 2 summarizes the activation parameters and related
kinetic information of the ligand exchange reaction of
[UO2(OPCyPh2)4]2+ together with those of [UO2(OPPh3)4]2+ in
various solvents. The activation parameters of the kD path have
been evaluated as ΔH‡ = 6.6 1.0 kJ mol−1 and ΔS‡ = −202
3 J mol−1 K−1, while those in the kA path are 6.9 1.0 kJ mol−1
and −174 3 J mol−1 K−1, respectively. The Gibbs free energies
for activation at 298 K (ΔG‡298) in the kD and kA paths were cal-
culated as 66.9 kJ mol−1 and 58.8 kJ mol−1, respectively. With
these activation parameters, the rate constants at 298 K in
both pathways were estimated to be kD = 12 s−1 and kA = 3.1 ×
102 M−1 s−1
.
For the kD path, the large negative ΔS‡ significantly contrib-
utes to ΔG‡298 in the D process of [UO2(OPCyPh2)4]2+, indicating
that this pathway is entropy-controlled. The negative ΔS‡ for kD
despite its D mechanism could be related to the rearrange-
ment of solvation through the activation process. The ΔG2‡98
and kD at 298 K obtained here are similar to those of
[UO2(HMPA)4]2+ (HMPA = hexamethylphosphoryl triamide,
ΔG2‡98 = 65.3 kJ mol−1, kD = 23 s−1 at 298 K), while the acti-
vation parameters are rather different (ΔH‡ = 14 3 kJ mol−1
,
ΔS‡ = −172 11 J mol−1 −1).35 To the best of our knowledge,
K
there are, however, no other kinetic data of the D ligand
2+
exchange reaction of 4-fold UO2 complexes. In contrast, the
2+
kinetic studies on the 5-fold complexes of UO2 have been
quite commonly reported as shown in Table S1 (ESI†), and
most of them are known to proceed in the D mechanism.36–39
Although the k values at 298 K in the different 5-fold com-
plexes are rather scattered, the ligand exchange reactions in
these systems seem to be enthalpy-controlled. Usually, the
2+
coordination number (CN) in the equatorial plane of UO2
ranges from 3 to 6, and CN = 5 is the most common. Thus,
some space for additional coordination is still left in CN = 4
like [UO2(OPCyPh2)4]2+. Therefore, further dissociation of the
ligand to give a 3-fold intermediate would not be preferred.
For the kA path in the A mechanism of [UO2(OPCyPh2)4]2+,
the large negative ΔS‡ was also obtained as a main contributor
to ΔG‡298, implying that this reaction is also entropy-controlled.
The kinetic data of the kA path can be compared with those of
the A ligand exchange reaction of [UO2(OPPh3)4]2+ in CD2Cl2
(Table 2), because the molecular structures of these 4-fold
uranyl complexes are quite similar to each other in terms of
UvOax (1.75–1.76 Å) and U–OL distances (2.29–2.32 Å).5 While
ΔH‡ is quite similar to each other, ΔS‡ of [UO2(OPCyPh2)4]2+ is
much lower than that of [UO2(OPPh3)4]2+.5 As a result, the A
ligand exchange reaction of [UO2(OPCyPh2)4]2+ exhibits higher
ΔG‡298 by 15 kJ mol−1 and ca. 480 times smaller kA at 298 K
compared with those of [UO2(OPPh3)4]2+. These differences are
Scheme 1 Rate-determining steps of the ligand exchange reaction of
[UO2(OPCyPh2)4]2+
.
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clearly found despite the similarity in the molecular structures
2+
of 4-fold UO2 complexes with phosphine oxides, and could
be ascribed to the additional effect of the intramolecular π–π
stacking interactions in [UO2(OPCyPh2)4]2+ observed in Fig. 2.
As described above, [UO2(HMPA)4]2+ is also another available
2+
4-fold UO2 complex,35 and exhibits both D and A mecha-
nisms in its ligand exchange reaction in a similar manner to
the current [UO2(OPCyPh2)4]2+ system. While the kA of
[UO2(OPCyPh2)4]2+ is still smaller than that of [UO2(HMPA)4]2+, it
could be difficult to compare these data directly as large differ-
ences in both ΔH‡ and ΔS‡ are present. As mentioned, the 4-fold
Fig. 6 Optimized structures of [UO2(OPCyPh2)4]2+ (panel A) and
[UO2(OPCyPh2)5]2+ (panel B) at the B97D3 level in the top views along
2+
coordination structure of UO2 may occur due to the steric hin-
2+
the axial UO2 moieties. Distances between the centroids (green dots)
drance between bulky ligand systems like phosphine oxides and
HMPA. Therefore, there would be still some space to allow the
additional coordination of an extra ligand to form a 5-fold inter-
mediate in the activation process of the A mechanism.
of the phenyl groups involved in the π–π stacking interactions are also
displayed.
Structure optimization and relative Gibbs energy among
[UO2(OPCyPh2)n]2+ with different numbers of ligands
(n = 3, 4, and 5)
second shell), and also CN 5 and CN “4 + 1”. The results are
summarized in Fig. 7. Although there were some difficulties in
obtaining convergence of some of the structures, the general
trend appears to be clear. The 4-fold coordination is always
preferred above CN 3 and 5 irrespective of the methods. The
complex with CN 3 is unstable and lies 89 to 122 kJ mol−1
above CN 4. There is hardly any π–π stacking interaction in the
case of CN 3, and also the equatorial shell remains loosely
occupied. There is no distinct transition state upon transition
from CN 4 to CN 3 + 1 and the energy increases monotonously
as one of the ligands dissociates. This fits more to the dis-
sociative interchange (ID) ligand-exchange pathway rather than
to the pure D one. The complex with CN 5 lies only 21 to 36
kJ mol−1 above CN 4, and therefore CN 5 is readily accessible.
Since there is a large structural transition in going from CN 4
+ 1 to CN 5, we were not able to identify the transition state
between the two states. However, the activation Gibbs energy
estimated for the A pathway from the Eyring plot (58.8
kJ mol−1) does not contradict with the Gibbs energy differ-
ences of the complexes obtained by the DFT calculations. In
Fig. 6 and S7 (ESI†), the optimized structure of CN 5 obtained
at the B97D3 level is also given. It can be seen that in the
complex with CN 5, in order to fulfil the larger coordination
number, the equatorial shell is somewhat distorted. Despite
The quantum chemical calculations may provide helpful
insights to theoretically explain the structural chemistry and
reaction mechanisms observed experimentally. Especially in
the current system, the intramolecular π–π stacking interaction
in [UO2(OPCyPh2)4]2+ is of great interest. First, we tried to opti-
mize the structures of [UO2(OPCyPh2)n]2+ using hybrid DFT
methods without including dispersion corrections. However,
for n = 4 we obtained the structure without a clear π–π stacking
interaction, and for n = 5 it was not possible to keep 5
OPCyPh2 ligands in the equatorial shell if dispersion inter-
actions are not taken into account. Clearly, inclusion of dis-
persion interaction is mandatory for the system we are study-
ing here. Since MP2 calculation is too costly for the systems
like [UO2(OPCyPh2)n]2+, we applied DFT including empirical
dispersion corrections, namely B3LYP-D3, CAM-B3LYP-D3, and
B97D3. Two types of ECP/basis sets were used and details are
described in Experimental. The optimized structures of
[UO2(OPCyPh2)4]2+ at the B97D3 and CAM-B3LYP-D3 levels
have overall good agreement with its crystal structure (Fig. 6
and S7,† respectively). Finally, the intramolecular offset π–π
stacking interactions were successfully reproduced, while the
centroid–centroid distances are somewhat shorter than the
experimental results (3.91 and 4.18 Å). Furthermore, the calcu-
lated U–Oyl distances are also somewhat longer (1.80 and 1.83 Å
for CAM-B3LYP-D3 and B97D3, respectively) when compared to
the experimental U–Oyl distance of 1.762 Å. These results are
presumably (partially) due to the overestimation of the dis-
persion interactions including the π–π stacking and the four
C–H⋯O hydrogen bonds between the cyclohexyl group and Oyl
in the scheme of DFT. At the B3LYP-D3 level, we obtained a
somewhat distorted structure for [UO2(OPCyPh2)4]2+ without
having clear parallel displaced π–π stacking.
The relative stabilities of the 3-, 4-, and 5-fold
[UO2(OPCyPh2)n]2+ were studied at different levels of theory.
We compared Gibbs energy of the complexes with CN 4 and
Fig. 7 Schematic diagram of the relative Gibbs energies of
CN “3 + 1” (3 ligands in the first shell and 1 ligand in the UO2(OPCyPh2)42+ and intermediates in D and A mechanisms.
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the larger CN, the distances between ring-centroids are par-
tially elongated in CN 5 compared to CN 4 and there are looser
Acknowledgements
stacking interactions. This suggests that the activation barrier We thank Prof. Emer. Yasuhisa Ikeda for stimulating discus-
having the fifth ligand in the equatorial shell is not very high.
sion and his helpful advice.
Notes and references
Conclusion
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