Probing Ni[S2PR2]2 electronic structure to generate insight relevant to minor actinide extraction chemistry

Article abstract of DOI:10.1021/ic3001587

A method to evaluate the electronic structure of minor actinide extractants is described. A series of compounds containing effective and ineffective actinide extractants (dithiophosphinates, S₂PR₂ ^-) bound to a common transition metal ion (Ni²⁺) was analyzed by structural, spectroscopic, and theoretical methods. By using a single transition metal that provides structurally similar compounds, the metal contributions to bonding are essentially held constant so that subtle electronic variations associated with the extracting ligand can be probed using UV-vis spectroscopy. By comparison, it is difficult to obtain similar information using analogous techniques with minor actinide and lanthanide complexes. Here, we demonstrate that this approach, supplemented with ground state and time-dependent density functional theory, provides insight for understanding why high separation factors are reported for the extractant HS₂P(o- CF₃C₆H₄)₂, while lower values are reported and anticipated for other HS₂PR₂ derivatives (R = C₆H₅, p-CF₃C₆H₄, m-CF₃C₆H₄). The implications of these results for correlating electronic structure with the selectivity of HS ₂PR₂ extractants are discussed.

Full text of DOI:10.1021/ic3001587

Article
pubs.acs.org/IC
Probing Ni[S2PR2]2 Electronic Structure to Generate Insight Relevant
to Minor Actinide Extraction Chemistry
Scott R. Daly, Jason M. Keith, Enrique R. Batista,* Kevin S. Boland, Stosh A. Kozimor,* Richard L. Martin,
and Brian L. Scott
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
S
* Supporting Information
ABSTRACT: A method to evaluate the electronic structure of minor actinide extractants is
described. A series of compounds containing effective and ineffective actinide extractants
−
(dithiophosphinates, S2PR2 ) bound to a common transition metal ion (Ni2+) was analyzed
by structural, spectroscopic, and theoretical methods. By using a single transition metal that
provides structurally similar compounds, the metal contributions to bonding are essentially
held constant so that subtle electronic variations associated with the extracting ligand can be
probed using UV-vis spectroscopy. By comparison, it is difficult to obtain similar information
using analogous techniques with minor actinide and lanthanide complexes. Here, we
demonstrate that this approach, supplemented with ground state and time-dependent density
functional theory, provides insight for understanding why high separation factors are
reported for the extractant HS2P(o-CF3C6H4)2, while lower values are reported and
anticipated for other HS2PR2 derivatives (R = C6H5, p-CF3C6H4, m-CF3C6H4). The implications of these results for correlating
electronic structure with the selectivity of HS2PR2 extractants are discussed.
Here, we describe the latest in a series of studies36−38 focused
on understanding how extractant electronic structure affects
INTRODUCTION
Proposed advanced nuclear fuel cycles offer a means to
■
binding selectivity in minor actinide separations. Although new
responsibly manage inventories of spent nuclear fuel and an
coordination compounds are reported, this manuscript focuses
opportunity to meet increasing global energy demands without
relying on carbon-based energy sources.1−5 One of the greatest
on evaluating a series of effective and ineffective extractants
bound to a common metal ion so that the metal contributions
chemical challenges facing the development of this technology
to the bonding are held constant and only electronic variations
involves removing the trivalent minor actinides (americium and
associated with the ligand orbitals are probed. Described are the
curium) from lanthanide contaminants found in spent nuclear
fuel.6−8 Separating these elements is difficult because they have
syntheses, molecular structures, spectroscopy, and density
functional theory (DFT) calculations for a family of complexes
similar chemical and physical properties, including comparable
−
that contain diaryldithiophosphinate ligands. The S2PR2
ionic radii, a common +3 oxidation state, and redox chemistry
that is difficult to access.9 In the course of developing improved
ligands, where R = o-CF3C6H4, m-CF3C6H4, p-CF3C6H4, and
minor actinide/lanthanide separation methods,6−8,10−16 it has
C6H5 (Ph), were chosen because their selectivity for minor
actinides varies dramatically with the presence and position of
been established that minor actinides can be effectively
removed from lanthanides through liquid/liquid extraction
the CF3 substituent. For example, the ortho-substituted
HS2P(o-CF3C6H4)2 provides exceptionally high separation
using certain “soft-donor” extractants, particularly those
factors (SF) for americium over europium (SF = 100 000).39
The selectivity drops to 1000 for the meta-substituted HS2P(m-
CF3C6H4)2,40 and based on previous reports, the SFs for
HS2P(p-CF3C6H4)2 and HS2PPh2 are anticipated to be even
lower.24 Binding these extractants to Ni2+ results in structurally
similar and highly symmetric Ni[S2PR2]2 complexes with
relatively well understood electronic structures. The divalent
nickel ion is additionally attractive because it provides a well-
established platform for identifying electronic nuances
associated with the ligand as they pertain to metal−ligand
bonding via UV−vis spectroscopy.41−48 By comparison, it is
difficult to obtain equivalent information from analogous UV−
containing S or N.17−29 Despite these empirical observations,
it is not clear what factors contribute to soft-donor extraction
selectivity.
It has long been suggested that the separation of minor
actinides from lanthanides can be heavily influenced by special
attributes associated with the electronic structure of the
extracting ligand, which is proposed to lead to more covalent
metal−ligand bonding with actinide vs lanthanide ele-
ments.9,30,31 While only subtle differences in metal−ligand
covalency between trivalent minor actinides and lanthanides are
expected,32,33 such differences could be large enough to account
for the observed selectivity in soft-donor separations.34−36
However, at present, it is difficult to determine what electronic
properties influence the binding selectivity of an extractant in f-
element separations.
Received: January 20, 2012
© XXXX American Chemical Society
A
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Table 1. Crystallographic Data for Ni[S2PR2]2 Complexes (R = o-CF3C6H4, 1; m-CF3C6H4, 2; p-CF3C6H4, 3; Ph, 4) Collected at
120(1) K
a
1
2
3
4
4
formula
FW (g mol−1
cryst syst
space group
a (Å)
C28H16F12NiP2S4
829.30
C28H16F12NiP2S4
829.30
C28H16F12NiP2S4
829.30
C24H20NiP2S4
557.29
C24H20NiP2S4
557.29
monoclinic
C2/c
)
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/n
9.179(3)
7.876(3)
20.948(7)
95.952(4)
1506.1(8)
2
12.032(2)
9.978(2)
12.864(2)
92.663(2)
1542.7(5)
2
10.803(6)
11.195(6)
13.291(7)
92.802(6)
1606(2)
2
13.033(2)
11.442(2)
16.641(2)
103.362(1)
2414(5)
4
13.28(1)
11.55(1)
17.00(1)
103.43(5)
2533
b (Å)
c (Å)
β (deg)
V (Å3)
Z
4
ρcalcd (g cm−3
μ (mm−1
)
1.829
1.785
1.715
1.533
1.461
)
1.124
1.098
1.055
1.293
0.0525
864
data/restraints/params
goodness-of-fit on F2
4146/0/215
1.138
2820/0/214
1.059
3065/48/242
1.141
5742/0/283
1.135
b
R1 [I > 2σ(I)]
0.0515
0.0423
0.0416
0.0397
0.114
c
wR2 (all data)
largest diff. peak and hole (e·Å−3
0.1718
0.1042
0.1005
0.1460
)
0.804/−0.487
0.723/−0.562
0.861/−0.678
0.522/−0.619
a
b
2
2
Obtained from ref 42. Data collected at room temperature using Cu Kα radiation. R1 = ∑ ||Fo| − |Fc||/|∑|Fo| for reflections with Fo > 2σ(Fo ).
c
wR2 = [∑w(Fo − Fc2)2/∑(Fo )2]1/2 for all reflections.
2
2
a
Table 2. Selected Bond Distances and Angles for Ni[S2PR2]2 Complexes
Ni[S2PR2]2
Ni−S (Å)
S−P (Å)
P−C (Å)
S−Ni−S (deg)
S−P−S (deg)
C−P−C (deg)
S−P−C (deg)
Ni[S2P(o-CF3C6H4)2]2 (1)
2.215(1)
2.220(1)
[2.218]
2.016(2)
2.014(2)
[2.015]
1.825(5)
1.827(5)
[1.826]
87.86(4)
99.55(7)
107.2(2)
112.1(2)
112.0(2)
112.6(2)
113.5(2)
112.0(1)
112.5(1)
112.7(1)
113.1(1)
110.8(1)
110.1(1)
113.7(1)
113.7(1)
111.7(1)
112.6(1)
112.6(1)
111.8(1)
112.2(4)
112.1(5)
111.2(4)
112.3(5)
Ni[S2P(m-CF3C6H4)2]2 (2)
Ni[S2P(p-CF3C6H4)2]2 (3)
Ni[S2PPh2]2 (4)
2.2343(9)
2.2431(9)
[2.239]
2.005(1)
2.010(1)
[2.008]
1.804(4)
1.805(4)
[1.804]
88.48(3)
88.79(4)
88.47(3)
88.2(1)
102.15(6)
102.13(6)
100.66(4)
101.2(2)
104.6(2)
106.6(2)
107.5(1)
107.1(6)
2.232(1)
2.239(1)
[2.236]
2.007(2)
2.015(2)
[2.011]
1.803(4)
1.807(4)
[1.805]
2.2161(7)
2.2342(7)
[2.225]
2.016(1)
2.017(1)
[2.017]
1.797(3)
1.807(3)
[1.802]
b
Ni[S2PPh2]2 (4)
2.234(5)
2.242(6)
[2.238]
2.006(8)
2.022(8)
[2.014]
1.79(2)
1.78(2)
[1.78]
a
b
Average bond distances are shown in brackets. Obtained from ref 42.
vis experiments on f-element dithiophosphinate complexes due
to lower symmetry and smaller ligand field effects.49 The
experimental and theoretical results are discussed in relation to
recent S K-edge X-ray absorption spectroscopy (XAS) and
time-dependent density functional theory (TDDFT) calcu-
treating NiCl2·6H2O with two equivalents of HS2PR2 in water
and EtOH (eq 1).
NiCl2 + 2HS2PR2 → Ni[S2PR2]2 + 2HCl
(1)
The CF3-free Ni[S2PPh2]2 (4), which has been reported
previously,42,45 was also prepared for spectroscopic and
structural comparison to 1−3. All four compounds precipitate
from the reaction solutions as they are formed, but there are
noticeable differences in how the reactions proceed. For
example, while the addition of HS2P(o-CF3C6H4)2 to a solution
of NiCl2 causes 1 to immediately precipitate, the synthesis of 2
and 3 required heating and longer reaction times. In the course
of exploring these differences, an alternative procedure was
developed that appears to expedite the synthesis of 2 and 3.
lations conducted on the free S2PR2 ligands,38 which suggest
−
−
that the electronic structure of the S2PR2 extractant can be
correlated with its minor actinide selectivity.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Ni[S2PR2] Com-
plexes. The Ni[S2PR2]2 complexes, where R = o-CF3C6H4 (1),
m-CF3C6H4 (2), and p-CF3C6H4 (3) were prepared in air by
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Figure 1. Molecular structure of Ni[S2P(o-CF3C6H4)2]2, 1. Ellipsoids are drawn at the 35% probability level. The hydrogen atoms were included in
the refinement but have been omitted from the figure.
This approach utilizes biphasic reaction conditions achieved by
contacting concentrated aqueous solutions of NiCl2 and
HS2PR2 mixtures with CH2Cl2. While it is not immediately
clear to us why this approach proceeds more rapidly than the
precipitation route, this approach provides a more efficient
method for isolating 2 and 3 in moderate yields.
0.02 Å shorter and the P−C bond distances are only 0.02 Å
longer compared to those in 2 and 3. The S−P−S angles in
ortho-substituted 1 at 99.55(7)° are comparable to those
observed in Ni[S2PPh2]2 (4) at 100.66(7)° but are subtly
different compared to the S−P−S angles in meta- and para-
substituted 2 and 3 at 102.15(6)° and 102.13(6)°, respectively.
The most pronounced structural difference associated with 1−3
involves the planarity of the PS2−Ni atoms. For 1, this four-
membered metallocycle is planar, but for 2 and 3 the
phosphorus atom is puckered out of the NiS4 plane by 0.18
Å and 0.39 Å, respectively. Analogous puckering is common in
the solid-state structures of many diaryldithiophosphinates and
has been attributed to intermolecular π−π van der Waals
interactions.50 Consistent with this analysis, inspection of the
extended crystal structures shows no obvious intermolecular
interactions for 1 but does reveal a possibility for intermolecular
aryl π−π stacking interactions for 2 and 3 (Figure S2,
Supporting Information). The observed π−π overlap in the
structures of 2 and 3 appears to be accommodated by free
rotation of the aryl groups around the P−C bond. By
comparison, rotation of the aryl groups in 1 is likely obstructed
by steric repulsion of the o-CF3 substituents, which are locked
between the S atoms and the CF3 substituents on the adjacent
aryl group. These steric interactions have been reported
previously in the calculated and experimental structures of
Single crystals suitable for X-ray diffraction studies were
prepared by cooling concentrated CH2Cl2 solutions of 1−3
(Table 1). Crystals of Ni[S2PPh2]2 (4) were also grown
following the literature procedure (Figure S1, Supporting
Information). However, in our hands, 4 crystallizes in the P21/n
space group, and not C2/c as previously reported.42 Despite the
difference in space groups, only small variations are observed
when the metrical parameters from both data sets are compared
(Tables 1 and 2). Although ortho-substituted 1 forms magenta-
colored crystals, its structure is remarkably similar to 2−4,
which crystallize as dark blue prisms. In each structure, the four
sulfur atoms form a square-planar arrangement around the Ni2+
ion (Figures 1−3). The bond distances and angles for 1 are
slightly different compared to those in 2 and 3, which have
identical metrics when their standard deviations are considered
(Table 2). For example, the Ni−S bond distances for 1 are only
S2P(o-CF3C6H4)2 and HS2P(o-CF3C6H4)2.37,39,51−53
−
In addition to unique reaction chemistry and slight structural
differences, Ni[S2P(o-CF3C6H4)2]2 (1) also exhibits distinct
NMR spectra. Single resonances observed in the decoupled 31
P
and 19F NMR spectra of ortho-substituted 1 are shifted
dramatically to lower field compared to 2 and 3. For instance,
the 31P NMR resonance for 1 is more deshielded at δ 92.3
compared to δ 80.2 and 79.6 for 2 and 3, and the 19F NMR
resonance is similarly shifted at δ −57.6 compared to δ −64.1
1
(2) and −64.6 (3). The H NMR spectra of 1−3 are readily
assignable based on their peak integrations and isotopic
splitting, and the observed trends in NMR chemical shifts are
consistent with the trends previously reported for their
corresponding HS2PR2 and [PPh4][S2PR2] compounds.37
In comparison to the NMR spectral, structural, and reactivity
differences for 1 compared to 2 and 3, the most profound
distinction for 1 is associated with its magenta color. The
unique color indicates that the electronic structure of 1
appreciably differs from the dark blue 2−4. We have evaluated
these differences using UV−vis spectroscopy, and the spectra of
Figure 2. Molecular structure of Ni[S2P(m-CF3C6H4)2]2, 2. Ellipsoids
are drawn at the 35% probability level. The hydrogen atoms were
included in the refinement but have been omitted from the figure.
C
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Figure 3. Molecular structure of Ni[S2P(p-CF3C6H4)2]2, 3. Ellipsoids are drawn at the 35% probability level. The hydrogen atoms were included in
the refinement but have been omitted from the figure.
1−4 are provided in Figure 4. The profile of each spectrum is
similar to those reported previously for square-planar Ni2+
dithiophosphinates.43−48 A large charge transfer band is
observed around 30 000 cm−1 and is adjacent to a lower
energy shoulder at ca. 25 000 cm−1 in the UV region. In the
visible region, there are two weak intensity features located
between 12 500 and 23 000 cm−1 that have been assigned to
dipole-forbidden d−d transitions (see below). The UV−vis
spectra of Ni[S2P(m-CF3C6H4)2]2 (2) and Ni[S2P(p-
CF3C6H4)2]2 (3), which are superimposable, are almost
identical to the spectrum of Ni[S2PPh2]2 (4), which does not
have any electron withdrawing CF3 substituents. In contrast, all
of the UV−vis features for ortho-substituted 1 are shifted
significantly to higher energies despite the structural similarities
of 1−4 (Table 3). For example, the charge transfer band at
Table 3. Experimental UV−Vis Transitions for Ni[S2PR2]2
Complexes 1−4. The Position of the Band Maxima are
Reported in Units of 1000 cm−1
1
energy
ε (M−1 cm−1
energy
14.3
111
13.5
89
18.8
143
17.7
96
25.5
2120
24.6
30.2
26 900
29.8
R = o-CF3C6H4
)
)
)
)
2
R = m-CF3C6H4
3
ε (M−1 cm−1
1590
24.6
23 300
29.8
energy
13.5
140
13.6
96
17.7
149
17.8
101
R = p-CF3C6H4
ε (M−1 cm−1
energy
2650
24.9
34 100
29.8
a
4
R = phenyl
ε (M−1 cm−1
1590
22 800
a
Initially reported in ref 42 and verified here upon comparison.
30 200 cm−1 in 1 is blueshifted by more than 300 cm−1 while
the two d−d transitions at 18 800 and 14 300 cm−1 are
blueshifted by 800−1100 cm−1 compared to the identical
features in 2−4.
Ground-State Electronic Structure of Ni[S2PR2]2. To
facilitate interpretation of the UV−vis spectra shown in Figure
4, ground state density functional theory calculations (DFT)
were made on compounds 1−4. The optimized structures of
1−4 are consistent with the structures obtained from single-
crystal XRD experiments, and the calculated bond distances,
angles, and atomic coordinates can be found in the Supporting
Information. In general, the molecular orbitals obtained from
the DFT calculations are similar to those derived from a group
theory analysis of Ni[S2PR2]2. For example, for two generic
Figure 4. Top: UV−vis spectra for Ni[S2PR2]2 (R = o-CF3C6H4, 1; m-
CF3C6H4, 2; p-CF3C6H4, 3; Ph, 4). The inset shows the magnified d−
d transitions in the 24 000−12 500 cm−1 visible region. Bottom:
Normalized UV−vis spectra to show the shifted features of 1 relative
to 2−4.
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−
S2PR2 ligands in D2h symmetry there are S 3p symmetry
splitting of the molecular orbitals containing Ni 3d character,
2
2
adapted linear combinations (SALCs) of atomic orbitals of ag
and b1g symmetries that can form Ni−S σ bonds using the Ni
3d orbitals of the same symmetry (dz and dxy). There are also S
particularly for those participating in Ni−S π-bonding (dx −y
,
dxz, and dyz). This interpretation was confirmed by comparing
the calculations on 1−4 with additional calculations conducted
on Ni[S2PMe2]2 (5), where analogous P−C(aryl) π-inter-
actions are not present. Because the orbital splitting observed
for Ni[S2PR2] (R = o-CF3C6H4, m-CF3C6H4, p-CF3C6H4, and
Ph; 1−4) is not present in the calculation for Ni[S2PMe2]2, we
conclude that the unique splitting in 1−4 is indeed associated
with P−C π aryl interactions.
2
3p SALCs of ag, b2g, and b3g symmetry that are available to form
2
2
Ni−S π bonds with the Ni dx −y , dxz, and dyz orbitals (ag, b2g,
and b3g, respectively). A small energy difference is expected
between the b2g (dxz) and the b3g (dyz) orbitals due to slight
deviation of D2h-Ni[S2PR2]2 from D4h symmetry. By compar-
ison, in a perfectly symmetrical d8 square-planar complex with
monatomic ligands (D4h-MX4n−), these two orbitals are
degenerate. It should also be noted that lowering of the point
group symmetry from D4h to D2h causes a change in the
location of the C2(x) and C2(y) rotation axes, which coincide
with the metal−ligand bonds in D4h-MX4n− and lie between the
Ni−S bonds in D2h-Ni[S2PR2]2.
For the simplest complex in the series, Ni[S2PMe2]2 (5), the
2
2
2
calculations reveal that the MOs containing Ni dz and dx −y
character (1ag and 2ag; Figure 6) are essentially nonbonding
The DFT calculations indicate that the lowest unoccupied
molecular orbital (LUMO) has b1g symmetry and is highest in
energy due to the highly destabilized Ni−S σ* bonds that result
from strong directional overlap between Ni dxy atomic orbital
and the associated S b1g SALC. The calculated energies for the
remaining molecular orbitals decrease from dxy (b1g, LUMO) to
2
2
2
dxz (b2g, HOMO), dyz (b3g), dz (ag), and dx −y (ag). The
quantitative molecular orbital diagram generated from the DFT
calculations show that at lower energy are nonbonding and
bonding molecular orbitals, which were not shown in the
qualitative diagram provided in Figure 5. In this lower energy
region, analysis of the Kohn−Sham orbitals shows that π-
contributions from the ligand aryl groups cause additional
Figure 6. DFT calculated MO correlation diagram of Ni[S2PR2]2
complexes 1−5. The energies have been normalized so that the energy
of the HOMO is 0.0 eV. The energies of the HOMO−LUMO gaps
are shown in red.
with respect to sulfur, as evidenced by the lack of splitting into
bonding and antibonding pairs and the low percentage of S 3p
character (3.6%; Table 4). By comparison, the molecular
orbitals containing Ni dxy, dxz, and dyz character (b1g, b2g, and
b3g) all form bonding and antibonding pairs and have
substantially more S 3p character (28.8−57.6%). The non-
2
2
2
bonding nature of the Ni dz and dx −y orbitals is consistent
with calculations previously performed on the structurally
similar D2h Ni(II) dithiolenes.54
Overall, the calculated molecular orbital diagrams of 1−4 are
remarkably similar, aside from one key difference: the
HOMO−LUMO gap is nearly identical for 2−4 (3.460−
3.475 eV) but increases by more than 0.1 eV (800 cm−1) for o-
CF3 substituted 1 (3.583 eV). The magnitude of the increased
HOMO−LUMO gap of 1 is consistent with its uniquely shifted
UV−vis spectra, which is also shifted by 800−1100 cm−1 in
comparison to 2−4. While the origin of this shift is not
immediately clear from the calculations reported here, it is
consistent with our previous spectroscopic and theoretical
analyses on S2PR2 ligands.38 For example, S K-edge X-ray
−
absorption spectroscopy (XAS) measurements and DFT
calculations performed on the free anions show a large increase
−
in S 3p mixing with the aryl π-orbitals for S2P(o-CF3C6H4)2
−
−
relative to S2P(m-CF3C6H4)2 , S2P(p-CF3C6H4)2 , and
−
S2PPh2 . The increased mixing was accompanied by a
concomitant increase of ∼800 cm−1 in the calculated energy
of the HOMO. It is important to point out that the HOMO for
Figure 5. Qualitative ligand-field splitting diagram of Ni[S2PR2]2
complexes in D2h point group symmetry. Only the S 3p atomic
orbitals are included from the S2PR2− ligands in the molecular orbitals
to show the nature of the Ni−S bonds (σ* vs π*).
−
the S2P(o-CF3C6H4)2 ligand is the same orbital involved in
formation of the Ni−S σ bonds and appears to be associated
with the uniquely shifted LUMO in Ni[S2P(o-CF3C6H4)2]2.
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Time-Dependent Density Functional Theory Calcu-
lations. To aid in our interpretation of the UV−vis spectra
shown in Figure 4 and support our tentative conclusions that
the increased HOMO−LUMO for Ni[S2P(o-CF3C6H4)2]2 (1)
vs Ni[S2P(m-CF3C6H4)2]2 (2), Ni[S2P(p-CF3C6H4)2]2 (3), and
Ni[S2PPh2]2 (4) is responsible for the observed differences,
time-dependent density functional theory (TDDFT) calcu-
lations were performed. Overall, the simulated TDDFT spectra
of 1−4 are in excellent agreement with the experimental data.
At high energy (between 31 000 and 32 000 cm−1), the
calculations show ligand-to-metal charge transfer (LMCT)
transitions, the largest of which involves an occupied ligand-
based orbital of b3u symmetry and the LUMO (b1g; Figure 7).
Table 4. Calculated Molecular Orbitals for Ni[S2PR2]2
Containing Ni 3d Character
a
Ni[S2PR2]2
MO d orbital % Ni 3d % S 3p energy (eV)
R = o-CF3C6H4 (1)
2b1g
4b2g
3b3g
3ag
dxy
dxz
dyz
47.0
40.5
46.2
81.8
13.6
27.3
19.5
30.7
37.1
38.6
5.5
39.2
54.8
41.6
10.0
6.8
−2.265
−5.848
−5.948
−6.154
−7.443
−7.494
−7.610
−7.655
−7.824
−7.857
−7.867
−7.950
−2.702
−6.171
−6.246
−6.502
−7.765
−7.781
−7.897
−7.989
−8.082
−8.159
−8.195
−8.272
−2.822
−6.282
−6.359
−6.617
−7.855
−7.870
−7.979
−8.082
−8.193
−8.267
−8.280
−8.370
−2.205
−5.680
−5.755
−6.019
−7.161
−7.177
−7.347
−7.387
−7.575
−7.585
−7.667
−7.729
−2.354
−5.887
−5.905
−6.195
−7.537
−7.680
−7.828
−7.837
2
dz
2b3g
3b2g
2b2g
2ag
dyz
dxz
dxz
19.2
22.8
4.8
2
2
dx −y
1b1g
1b3g
1b2g
1ag
dxy
dyz
dxz
54.1
28.6
15.4
2.0
2
2
dx −y
53.4
48.4
38.9
42.8
85.4
38.2
5.5
R = m-CF3C6H4 (2)
R = p-CF3C6H4 (3)
R = C6H5 (4)
2b1g
4b2g
3b3g
3ag
dxy
dxz
dyz
38.8
56.4
44.8
6.4
2
dz
3b2g
2b3g
2b2g
2ag
dxz
dyz
dxz
18.4
3.0
16.4
32.2
44.3
36.6
5.2
18.0
4.8
2
2
dx −y
1b3g
1b1g
1b2g
1ag
dyz
dxy
dxz
27.6
56.4
14.4
2.0
2
2
dx −y
52.8
47.5
38.9
42.9
88.3
28.0
10.5
24.9
30.7
44.3
36.6
6.4
2b1g
4b2g
3b3g
3ag
dxy
dxz
dyz
38.0
56.8
44.8
3.6
2
dz
Figure 7. Natural transition orbitals of the ligand and metal-based
molecular orbitals participating in the most intense LMCT transition
at 31 314 cm−1 in the TDDFT spectrum of Ni[S2PPh2]2 (4).
3b2g
2b3g
2b2g
2ag
dxz
dyz
dxz
12.3
7.2
26.4
5.6
2
2
dx −y
At lower energy, the calculations show a single weak LMCT
transition near 25 000 cm−1. The calculations show the first d−
d transition, which corresponds to 2ag → LUMO (2b1g),
around 18 000 cm−1 and three closely matched d−d transitions
that involve excitations from the 3ag, 4b2g, and 3b3g orbitals to
the LUMO (2b1g) near 11 000 cm−1. The relatively good
agreement between the calculated transition energies with the
peak positions observed experimentally provide confidence in
our interpretation of the UV−vis data, which can be
summarized for 1−4 as having a strong LMCT band at
30 000 cm−1 with a weak LMCT shoulder at 25 000 cm−1 and
lower energy peaks associated with d−d transitions between the
1b3g
1b1g
1b2g
1ag
dyz
dxy
dxz
27.6
56.6
15.4
2.0
2
2
dx −y
56.1
48.5
38.7
42.6
88.2
6.3
2b1g
4b2g
2b3g
3ag
dxy
dxz
dyz
39.2
56.8
45.2
3.6
2
dz
2b3g
3b2g
2b2g
2ag
dyz
dxz
dxz
5.6
16.3
29.6
16.3
48.5
13.3
36.4
70.3
51.4
40.4
41.5
88.5
59.7
58.1
87.3
35.9
6.0
31.6
6.0
2
2
dx −y
1
1
1Ag ground state and the B1g, B3g, and 1B2g excited states. We
refrain from comparing the relative intensities of the
experimental peaks with the calculated oscillator strengths for
the d−d transitions because spin−orbit coupling, which gives
these features intensity in the UV−vis spectra, was not included
in the DFT calculations. As a result, there is no appreciable
oscillator strength associated with the calculated d−d
transitions (Table 5).
1b3g
1b2g
1b1g
1ag
dyz
dxz
dxy
28.6
22.0
56.8
2.0
2
2
dx −y
R = CH3 (5)
2b1g
2b2g
2b3g
2ag
dxy
dxz
dyz
42.0
57.6
47.2
3.6
2
dz
To graphically compare the position and splitting of the
calculated low energy features in the spectra for 1−4, the
calculated oscillator strengths in Figure 8 have been artificially
increased for the d−d transitions. For 2−4, both experiment
and theory show that the energies for the d−d transitions are
nearly identical (Table 5), which is exemplified using the dotted
lines in Figure 8. However, for Ni[S2P(o-CF3C6H4)2]2 (1), the
TDDFT and experimental UV−vis data show a high energy
1b2g
1b3g
1ag
dxz
dyz
39.2
28.8
3.6
2
2
dx −y
1b1g
dxy
57.6
a
The MO symmetry labels correspond to those used in Figure 6.
F
dx.doi.org/10.1021/ic3001587 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 5. TD-DFT Calculated Transitions and Transition Assignments for Ni[S2PR2]2 Complexes
R = o-CF3C6H4
energy
R = m-CF3C6H4
energy
R = p-CF3C6H4
energy
R = C6H5
transition
(primary)
oscillator
oscillator
oscillator
energy
(cm−1
oscillator
strength
(cm−1
)
strength
(cm−1
)
strength
(cm−1
)
strength
)
3ag → 2b1g
4b2g → 2b1g
3b3g → 2b1g
2ag → 2b1g
LMCT
11179
11302
11685
18396
24344
31754
0.000
0.000
0.000
0.000
0.005
0.666
10192
10388
10767
17447
23068
31463
0.000
0.000
0.000
0.000
0.005
0.734
10244
10417
10795
17475
23051
31313
0.000
0.000
0.000
0.000
0.006
0.763
10329
10476
10845
17513
23121
31314
0.000
0.000
0.000
0.000
0.005
0.977
LMCT (major)
shift of approximately 850−1000 cm−1 for all four d−d
transitions, as well as the low energy LMCT peak near 25 000
cm−1. Overall, this is consistent with the ground state DFT
calculations and can be attributed to the increased HOMO−
LUMO gap of 1 relative to 2−4.
Comparison of Reported UV−Vis Assignments for
Ni[S2PR2]2. The electronic structure and UV−vis spectra for
Ni(II) dithiophosphinates have been previously evaluated in
several studies, and there exist conflicting reports regarding the
relative ordering of molecular orbitals for Ni[S2PR2]2 and the
corresponding UV−vis assignments.43,44 While it is generally
acknowledged that the LUMO contains Ni dxy character, the
ordering of the subsequent Ni-based molecular orbitals has
been disputed, particularly with regards to the identity of the
HOMO (ag vs b2g or b3g). Our ground state DFT data are most
consistent with previously reported conclusions based on
magnetic circular dichroism44 and suggest that the HOMO is
best described as an orbital of b2g symmetry in the D2h point
group. However, the TDDFT calculations reveal that the low-
energy d−d feature in the UV−vis spectra for the Ni[S2PR2]2
complexes consists of nearly overlapping transitions from
orbitals of ag, b2g (HOMO), and b3g symmetry. The close
energy of these transitions, which manifest as only one feature
in the UV−vis spectra, may account for some of the
inconsistencies in spectral interpretation and molecular orbital
assignments.
CONCLUDING REMARKS
■
In summary, we report here that HS2P(o-CF3C6H4)2 reacts
differently than HS2PR2 (R = m-CF3C6H4, p-CF3C6H4, and Ph)
with NiCl2. Although the molecular geometry for the resulting
Ni[S2P(o-CF3C6H4)2]2 (1) varies only slightly in comparison to
2−4, the electronic structure of 1 is unusually distinct and
shows a substantially increased gap between the LUMO and
the occupied molecular orbitals. The origin of the increased
destabilization of the LUMO in 1 compared to 2−4 can be
reasonably interpreted as arising from increased covalent
mixing in the Ni−S σ bonds formed between the Ni 3dxy
orbitals and the S 3p lone pairs.55 The unique electronic
structure of Ni[S2P(o-CF3C6H4)2]2 is noteworthy because
HS2P(o-CF3C6H4)2 exhibits exceptionally high selectivity for
minor actinides compared to other diaryldithiophosphinates.39
While it is obvious that the chemistries of nickel and
actinides are vastly different and that transition metals do not
represent surrogates to understand electronic structure and
bonding in f elements, the square planar Ni[S2PR2]2 complexes
do provide a highly symmetric and well-established coordina-
tion environment that is sensitive to subtle differences in the
ligand electronic structure. By holding the metal identity
constant, metal contributions to bonding remain equivalent,
and electronic variations associated with the extracting ligand
can be directly probed. Here, the UV−vis and TDDFT analysis
identified that the HOMO−LUMO energy gap was directly
related to the extractant identity and correlated with the ligand
performance in minor actinide separations chemistry. Given
that d8 square-planar complexes represent a class of compounds
that have been heavily studied, it seems likely that other
spectroscopic techniques previously employed to characterize
Ni2+ ligand field effects could also be used to identify special
Figure 8. TD-DFT simulated spectra (solid lines) and calculated
transitions (bars) for Ni[S2PR2]2 complexes: (a) Ni[S2P(o-
CF3C6H4)2]2 (1, red), (b) Ni[S2P(m-CF3C6H4)2]2 (2, blue), (c)
Ni[S2P(p-CF3C6H4)2]2 (3, gray), and (d) Ni[S2PPh2]2 (4, black). The
insets show the magnified TD-DFT transitions between 8000 and 26
000 cm−1. The oscillator strengths of these transitions, which are
provided in Table 5, have been artificially increased to compare their
relative positions.
−
electronic properties associated with S2P(o-CF3C6H4)2 vs
G
dx.doi.org/10.1021/ic3001587 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
UV−vis (CH2Cl2): 29 800 cm− (ε = 23300 M−1 cm−1), 24 600 cm− (ε
= 1590 M−1 cm−1), 17 700 cm− (ε = 96 M−1 cm−1), 13 500 cm− (ε =
89 M−1 cm−1).
−
−
−
S2P(m-CF3C6H4)2 , S2P(p-CF3C6H4)2 , and S2PPh2 anions.
Hence, we are currently in the process of investigating the
origin of the unique electronic structure of Ni[S2P(o-
CF3C6H4)2]2 using S K-edge X-ray absorption spectroscopy
and TDDFT calculations in support of efforts to advance
extractant design relevant to minor actinide separations.
Bis[bis(p-trifluoromethylphenyl)dithiophosphinato]Ni(II),
Ni[S2P(p-CF3C6H4)2]2 (3). H2O (3 mL) was added to a mixture of
NiCl2·6H2O (0.035 g, 0.15 mmol) and HS2P(p-CF3C6H4)2 (0.114 g,
0.295 mmol) followed by CH2Cl2 (15 mL). The CH2Cl2 layer slowly
turned dark blue as the newly formed 3 dissolved in the organic layer.
The mixture was stirred for 30 min at room temperature. The organic
layer was separated and evaporated to dryness under a vacuum. The
dark blue solid was dissolved in 12 mL of CH2Cl2, filtered, and stored
at −20 °C to yield 57 mg of dark blue prisms. The mother liquor was
concentrated to 4 mL and stored at −20 °C to yield an additional 21
mg of dark blue prisms. Yield: 0.078 g (64%). Anal. Calcd. for
C28H16F12NiP2S2: C, 40.6; H, 1.94; F, 27.5. Found: C, 40.6; H, 1.98; F,
27.5. 1H NMR (CDCl3, 20 °C): δ 7.78 (d, 3JHH = 7 Hz, meta-H, 8H),
EXPERIMENTAL SECTION
■
The bis(trifluoromethylphenyl)dithiophosphinic acids HS2P(o-
CF3C6H4)2, HS2P(m-CF3C6H4)2, and HS2P(p-CF3C6H4)2 were
prepared as previously described.37,39 Ni[S2PPh2] (4) was prepared
as previously described using HS2PPh2 (Alfa Aesar).45 NiCl2 and
NiCl2·6H2O (Strem) were used as received. H2O was purified to 18.2
MW-cm resistivity using a Thermo-Scientific Barnstead Nanopure
water purification system. All reactions were carried out in the air with
solvents that were used as received from commercial sources. CDCl3
(Aldrich) was used as received. Elemental analyses were carried out by
Midwest MicroLab, LLC (Indianapolis, IN). The infrared spectra were
recorded on a Nicolet Magna-IR System 750 spectrometer as KBr
pellets. The 1H, 31P, and 19F NMR data were obtained using a Bruker
Avance NMR spectrometer at 300.1 MHz, 121.5 MHz, and 282.4
MHz, respectively. Chemical shifts are reported in δ units (positive
shifts to high frequency) relative to TMS (1H), H3PO4 (31P), or CFCl3
(19F). The UV−vis spectra were collected on a Varian Cary 6000i
UV−vis−NIR Spectrophotometer. Molar absorptivity (ε) values were
determined by plotting measured absorbance values at three
Ni[S2PR2]2 concentrations ranging between 10−4 and 10−5 M.
Bis[bis(o-trifluoromethylphenyl)dithiophosphinato]Ni(II),
Ni[S2P(o-CF3C6H4)2]2 (1). A solution of HS2P(o-CF3C6H4)2 (0.401 g,
1.03 mmol) in EtOH (15 mL) was added to a solution of NiCl2
(0.0673 g, 0.519 mmol) in H2O (15 mL). A magenta precipitate
immediately formed. The mixture was stirred for 3 h at room
temperature. The mixture was filtered, and the solid was washed with
H2O (3 × 10 mL) and EtOH (3 × 5 mL) and allowed to evaporate to
dryness in air. The solid was dissolved in a 1:8 hexane/CH2Cl2
solution, filtered, and crystallized by slow evaporation to yield a
crystalline magenta solid. Yield: 0.290 g (70%). The solid could also be
crystallized by cooling concentrated CH2Cl2 or acetone solutions.
Anal. Calcd. for C28H16F12NiP2S2: C, 40.6; H, 1.94; P, 7.47; F, 27.5.
Found: C, 40.6; H, 1.99; P, 7.69; F, 27.2. 1H NMR (CDCl3, 20 °C): δ
3
3
7.89 (dd, JPH = 13 Hz, JHH = 8 Hz, ortho-H, 8H). 31P{1H} NMR
(CDCl3, 20 °C): δ 79.6. 19F{1H} NMR (CDCl3, 20 °C): δ −64.6 (s,
CF3). IR (cm−1): 498 w, 525 w, 593 sh, 602 s, 620 m, 633 w, 643 m,
708 s, 735 w, 783 w, 835 s, 960 w, 974 w, 1016 s, 1062 vs, 1107 s, 1127
s, 1142 s, 1176 s, 1287 w, 1323 vs, 1396 s, 1500 w, 1571 w, 1609 m,
1811 w, 1931 m, 3047 w, 3092 w. UV−vis (CH2Cl2): 29 800 cm− (ε =
34 100 M−1 cm−1), 24 600 cm− (ε = 2650 M−1 cm−1), 17 700 cm− (ε
= 149 M−1 cm−1), 13 500 cm− (ε = 140 M−1 cm−1).
Crystallographic Details. Single crystals of 1−3 obtained from
CH2Cl2 were mounted in a nylon cryoloop with Paratone-N oil under
an argon gas flow. Single crystals of 4 were prepared as previously
described.45 The data were collected on a Bruker D8 diffractometer,
with an APEX II charge-coupled-device (CCD) detector, and cooled
to 120(1) K using an American Cryoindustries low temperature
device. The instrument was equipped with graphite monochromatized
Mo Kα X-ray source (λ = 0.71073 Å) and a 0.5 mm monocapillary. A
hemisphere of data was collected using ω scans, with 10−30 s frame
exposures and 0.5° frame widths. Data collection and initial indexing
and cell refinement were handled using APEX II software.56 Frame
integration, including Lorentz-polarization corrections, and final cell
parameter calculations were carried out using SAINT+ software.57 The
data were corrected for absorption using redundant reflections and the
SADABS program.58 Decay of reflection intensity was not observed as
monitored via analysis of redundant frames. The structure was solved
using direct methods and difference Fourier techniques. All hydrogen
atom positions were idealized. The final refinement included
anisotropic temperature factors on all non-hydrogen atoms. Structure
solution, refinement, graphics, and creation of publication materials
were performed using SHELXTL.59 Data collection and refinement
details are listed in Table 1.
Theoretical Calculations. UV−vis spectra were calculated at
geometries optimized with density functional theory (DFT) in the gas
phase with the hybrid functional B3LYP60,61 as implemented in
Gaussian 09.25 Ni was modeled with the effective core potential and
associated basis sets of Hay and Wadt (LANL2DZ)62 augmented with
a set of f polarization functions (exponent = 3.130).63 All other atoms
were modeled using a Pople style double-ζ 6-31G(d′,p′) all-electron
basis set with polarization functions optimized for heavy atoms.64,65
From these geometries, the spectra were simulated using time
dependent density functional theory (TDDFT). Specifically, this
analysis involves a linear response calculation for extracting the
probability amplitudes from the transition densities and dipole
moments between the calculated excited states and the ground
state.66 Natural transition orbital analysis was performed to further
validate the individual TDDFT excitiations.67
3
7.64−7.74 (m, meta,para-H, 8H), 7.89 (t, JHH = 7 Hz, meta-H, 4H),
8.95 (dd, JPH = 19 Hz, JHH = 8 Hz, ortho-H, 4H). 31P{1H} NMR
(CDCl3, 20 °C): δ 92.3. 19F{1H} NMR (CDCl3, 20 °C): δ −57.6 (s,
CF3). IR (cm−1): 481 w, 504 w, 574 s, 589 m, 594 w, 622 m, 646 m,
669 w, 709 m, 737 w, 772 s, 783 m, 887 w, 967 w, 996 w, 1036 s, 1097
s, 1115 s, 1121 s, 1139 s, 1151 sh, 1172 sh, 1182 s, 1263 s, 1291 m,
1309 vs, 1437 m, 1473 w, 1571 w, 1591 w, 3027 w, 3070 m. UV−vis
(CH2Cl2): 30 200 cm− (ε = 26 900 M−1 cm−1), 25 500 cm− (ε = 2120
M−1 cm−1), 18 800 cm− (ε = 143 M−1 cm−1), 14 300 cm− (ε = 111
M−1 cm−1).
3
3
Bis[bis(m-trifluoromethylphenyl)dithiophosphinato]Ni(II),
Ni[S2P(m-CF3C6H4)2]2 (2). H2O (3 mL) was added to a mixture of
NiCl2·6H2O (0.075 g, 0.32 mmol) and HS2P(m-CF3C6H4)2 (0.242 g,
0.63 mmol) followed by CH2Cl2 (20 mL). The CH2Cl2 fraction slowly
turned dark blue as the newly formed 2 dissolved in the organic layer.
The mixture was stirred for 30 min at room temperature. The organic
layer was separated and evaporated to dryness under a vacuum. The
dark blue solid was dissolved in 10 mL of CH2Cl2, filtered, and stored
at −20 °C to yield dark blue prisms. Yield: 0.093 g (35%). Anal. Calcd.
for C28H16F12NiP2S2: C, 40.6; H, 1.94; P, 7.47; F, 27.5. Found: C,
1
40.5; H, 1.95; P, 7.09; F, 27.2. H NMR (CDCl3, 20 °C): δ 7.66 (t,
3JHH = 8 Hz, meta-H, 4H), 7.86 (d, JHH = 7 Hz, para-H, 4H), 7.90
3
3
3
3
ASSOCIATED CONTENT
(dd, JPH = 14 Hz, JHH = 8 Hz, ortho-H, 4H), 8.05 (d, JPH = 15 Hz,
■
ortho-H, 4H). 31P NMR (CDCl3, 20 °C): δ 80.2 (p, JPH = 15 Hz).
3
S
* Supporting Information
19F{1H} NMR (CDCl3, 20 °C): δ −64.1 (s, CF3). IR (cm−1): 467 w,
507 w, 535 w, 577 s, 583 sh, 614 w, 629 m, 639 s, 653 m, 656 m, 677
w, 695 s, 718 s, 742 w, 800 s, 813 s, 899 m, 915 m, 934 w, 943 w, 996
m, 1075 s, 1092 sh, 1112 vs, 1123 vs, 1145 s, 1165 s, 1174 sh, 1275 s,
1312 s, 1328 vs, 1418 s, 1429 m, 1478 w, 1602 m, 3045 sh, 3068 m.
X-ray crystallographic data (CIF format). Atomic coordinates,
Mullikan charges, and bond distances and angles for calculated
structures of 1−5. This material is available free of charge via
the Internet at http://pubs.acs.org.
H
dx.doi.org/10.1021/ic3001587 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(26) Modolo, G.; Kluxen, P.; Geist, A. Radiochim. Acta 2010, 98,
193−201.
AUTHOR INFORMATION
Corresponding Author
*E-mail: stosh@lanl.gov (S.A.K.); LAUR 12-00235.
Notes
■
(27) Lewis, F. W.; Harwood, L. M.; Hudson, M. J.; Drew, M. G. B.;
Desreux, J. F.; Vidick, G.; Bouslimani, N.; Modolo, G.; Wilden, A.;
Sypula, M.; Vu, T.-H.; Simonin, J.-P. J. Am. Chem. Soc. 2011, 133,
13093−13102.
The authors declare no competing financial interest.
(28) Ozcubukcu, S.; Mandal, K.; Wegner, S.; Jensen, M. P.; He, C.
Inorg. Chem. 2011, 50, 7937−7939.
ACKNOWLEDGMENTS
■
(29) Bremer, A.; Ruff, C. M.; Girnt, D.; Muellich, U.; Rothe, J.;
Roesky, P. W.; Panak, P. J.; Karpov, A.; Mueller, T. J. J.; Denecke, M.
A.; Geist, A. Inorg. Chem. 2012, 51, 5199−5207.
(30) Diamond, R. M.; Street, K., Jr.; Seaborg, G. T. J. Am. Chem. Soc.
1954, 76, 1461−1469.
We would like to thank Drs. Dean R. Peterman and John L.
Klaehn for initially providing a sample of HS2P(o-CF3C6H4)2.
This work was supported by U.S. Department of Energy, Office
of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences (Heavy Element Chemistry
Program); the U.S. Department of Energy, Office of Nuclear
Energy (Fuel Cycle R&D Program); the Glenn T. Seaborg
Institute Postdoctoral Fellowship (S.R.D.); and a LANL
Director’s Postdoctoral Fellowship (J.M.K.). Los Alamos
National Laboratory is operated by Los Alamos National
Security, LLC, for the National Nuclear Security Admin-
istration of U.S. Department of Energy under Contract
DEAC52-06NA25396.
(31) Jensen, M. P.; Bond, A. H. J. Am. Chem. Soc. 2002, 124, 9870−
9877.
(32) Kozimor, S. A.; Yang, P.; Batista, E. R.; Boland, K. S.; Burns, C.
J.; Clark, D. L.; Conradson, S. D.; Martin, R. L.; Wilkerson, M. P.;
Wolfsberg, L. E. J. Am. Chem. Soc. 2009, 131, 12125−12136.
(33) Minasian, S. G.; Keith, J. M.; Batista, E. R.; Boland, K. S.; Clark,
D. L.; Conradson, S. D.; Kozimor, S. A.; Martin, R. L.; Schwarz, D. E.;
Shuh, D. K.; Wagner, G. L.; Wilkerson, M. P.; Wolfsberg, L. E.; Yang,
P. J. Am. Chem. Soc. 2012, 134, 5586−5597.
(34) Minasian, S. G.; Krinsky, J. L.; Rinehart, J. D.; Copping, R.;
Tyliszczak, T.; Janousch, M.; Shuh, D. K.; Arnold, J. J. Am. Chem. Soc.
2009, 131, 13767−13783.
(35) Krinsky, J. L.; Minasian, S. G.; Arnold, J. Inorg. Chem. 2011, 50,
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(36) Keith, J. M.; Batista, E. R. Inorg. Chem. 2012, 51, 13−15.
(37) Daly, S. R.; Klaehn, J. R.; Boland, K. S.; Kozimor, S. A.;
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(38) Daly, S. R.; Keith, J. M.; Batista, E. R.; Boland, K. S.; Clark, D.
L.; Kozimor, S. A.; Scott, B. L. J. Am. Chem. Soc. 2012, accepted.
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