Covalency in lanthanides. An X-ray absorption spectroscopy and density functional theory study of LnCl6x- (x = 3, 2)

Article abstract of DOI:10.1021/ja510067v

Covalency in Ln-Cl bonds of O_h-LnCl₆^x- (x = 3 for Ln = Ce^III, Nd^III, Sm^III, Eu^III, Gd^III; x = 2 for Ln = Ce^IV) anions has been investigated, primarily using Cl K-edge X-ray absorption spectroscopy (XAS) and time-dependent density functional theory (TDDFT); however, Ce L_3,2-edge and M_5,4-edge XAS were also used to characterize CeCl₆^x- (x = 2, 3). The M_5,4-edge XAS spectra were modeled using configuration interaction calculations. The results were evaluated as a function of (1) the lanthanide (Ln) metal identity, which was varied across the series from Ce to Gd, and (2) the Ln oxidation state (when practical, i.e., formally Ce^III and Ce^IV). Pronounced mixing between the Cl 3p- and Ln 5d-orbitals (t_2g and e_g) was observed. Experimental results indicated that Ln 5d-orbital mixing decreased when moving across the lanthanide series. In contrast, oxidizing Ce^III to Ce^IV had little effect on Cl 3p and Ce 5d-orbital mixing. For LnCl₆^3- (formally Ln^III), the 4f-orbitals participated only marginally in covalent bonding, which was consistent with historical descriptions. Surprisingly, there was a marked increase in Cl 3p- and Ce^IV 4f-orbital mixing (t_1u + t_2u) in CeCl₆^2-. This unexpected 4f- and 5d-orbital participation in covalent bonding is presented in the context of recent studies on both tetravalent transition metal and actinide hexahalides, MCl₆^2- (M = Ti, Zr, Hf, U).

Full text of DOI:10.1021/ja510067v

Article
pubs.acs.org/JACS
Covalency in Lanthanides. An X‑ray Absorption Spectroscopy and
x−
Density Functional Theory Study of LnCl (x = 3, 2)
6
†
†,‡
∥
†
,†
,†
Matthias W. Loble, Jason M. Keith, Alison B. Altman, S. Chantal E. Stieber, Enrique R. Batista,*
̈
†
†
†
⊥
Kevin S. Boland, Steven D. Conradson, David L. Clark, Juan Lezama Pacheco, Stosh A. Kozimor,*
,†
§
†
†
§
Richard L. Martin,* Stefan G. Minasian, Angela C. Olson, Brian L. Scott, David K. Shuh,
Tolek Tyliszczak, Marianne P. Wilkerson, and Ralph A. Zehnder
§
†
#
†
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
‡
Colgate University, Hamilton, New York 13346, United States
§
Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
∥
University of California, Berkeley, California 94720, United States
⊥
Stanford University, Stanford, California 94305, United States
#
Angelo State University, San Angelo, Texas 76909, United States
*
S Supporting Information
x−
ABSTRACT: Covalency in Ln−Cl bonds of O -LnCl (x = 3 for Ln =
h
6
III
III
III
III
III
IV
Ce , Nd , Sm , Eu , Gd ; x = 2 for Ln = Ce ) anions has been
investigated, primarily using Cl K-edge X-ray absorption spectroscopy
(
XAS) and time-dependent density functional theory (TDDFT); however,
x−
Ce L -edge and M -edge XAS were also used to characterize CeCl (x
3,2
5,4
6
=
2, 3). The M -edge XAS spectra were modeled using configuration
5,4
interaction calculations. The results were evaluated as a function of (1) the
lanthanide (Ln) metal identity, which was varied across the series from Ce
to Gd, and (2) the Ln oxidation state (when practical, i.e., formally Ce
and Ce ). Pronounced mixing between the Cl 3p- and Ln 5d-orbitals (t *
and e *) was observed. Experimental results indicated that Ln 5d-orbital
III
IV
2g
g
III
IV
mixing decreased when moving across the lanthanide series. In contrast, oxidizing Ce to Ce had little effect on Cl 3p and Ce
3
−
III
5
d-orbital mixing. For LnCl6 (formally Ln ), the 4f-orbitals participated only marginally in covalent bonding, which was
IV
consistent with historical descriptions. Surprisingly, there was a marked increase in Cl 3p- and Ce 4f-orbital mixing (t * + t *)
1
u
2u
in CeCl62 . This unexpected 4f- and 5d-orbital participation in covalent bonding is presented in the context of recent studies on
−
2−
both tetravalent transition metal and actinide hexahalides, MCl6 (M = Ti, Zr, Hf, U).
INTRODUCTION
use of rare earth elements is increasing in many industrial
■
43
processes e.g. in catalysis, energy production, medicine,
lighting materials, high-performance alloys, magnets, and areas
relevant to national securityit is becoming more important to
understand these anomalies and advance descriptions of
lanthanide electronic structure and covalent bonding.
Despite recent advances, it remains of considerable interest to
better define the roles that f- and d-orbitals play in chemical
bonding in lanthanides and actinides.
and spectroscopic techniques has provided insight into f- and d-
1
−5
Advent of new theory
6
−8
orbital mixing in actinide metal−ligand bonds.
However,
Perhaps the most versatile and direct approach to evaluate
covalency utilizes a combination of ligand K-edge X-ray
bonding for the 4f-lanthanide (Ln) elements is often described
2
−4,6−12
as primarily ionic,
largely because previous calculations
6
−8,44−56
absorption spectroscopy (XAS)
density functional theory (TDDFT).
and time-dependent
Using orbital
indicated that the 4f-orbitals do not extend significantly beyond
6
−8,50,57−60
13−18
the core orbitals.
The viewpoint that lanthanide−ligand
descriptions, the spectroscopic approach involves quantifying
the intensity of bound-state transitions between ligand 1s-
orbitals and virtual (unoccupied) orbitals. Because the hole is
localized on the ligand, the dominant contribution to the dipole
matrix element governing the intensity can be associated with
the component of ligand p-character in the final state.
interactions are best described using ionic bonding models
finds support in the literature from many reactivity studies and
physical measurements. Most notably, optical spectroscopy
suggests that the 4f-orbitals are core-like and only weakly
influenced by ligand fields.
element compounds have been isolated where covalency in
1
9−21
However, a number of 4f-
2
2−38
bonding may be important,
and a growing number of
studies suggested that the 5d-orbitals should be considered
Received: September 30, 2014
1
−5,18,39−42
when evaluating lanthanide−ligand bonding.
As the
©
XXXX American Chemical Society
A
DOI: 10.1021/ja510067v
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
x−
Scheme 1. Cartoon Depicting Electronic Excitations in a
Typical Ligand K-Edge XAS Experiment
analytesreferred to hereafter as LnCl
6
(x = 3, 2)were
carefully selected because they contain highly symmetric O -
LnCl6 anions, which facilitated evaluating Ln−Cl bonding
owing to the separability of the 4f- and 5d-orbitals in an
h
x−
x−
octahedral ligand field. The LnCl6 analytes were attractive
because they can be prepared in large quantities and in high
purity. All of the hexachlorides were made using slight
modifications of previously published synthetic proce-
6
9−73
3−
III
dures.
For instance, the LnCl6
(formally Ln )
compounds were isolated from reactions between anhydrous
LnCl3 and 3 equiv of either NEt Cl or PPh Cl, while
4
4
2−
IV
Ce(SO ) ·xH O was used to make CeCl
(formally Ce ).
4
2
2
6
All of the products were characterized by single-crystal X-ray
diffraction (XRD). A representative thermal ellipsoid plot for
CeCl62 has been provided in Figure 1. Crystallographic
−
Transitions into an unoccupied orbital of principally metal
character (Scheme 1) imply a component of ligand p-character
in that orbital. Measuring the experimental intensities then
provides an estimate of the associated metal−ligand orbital
mixing. Strictly speaking, the observed intensities reflect atomic
orbital mixing in the unoccupied orbitals. However, in the
simple orbital models used to interpret bonding, any atomic
orbital mixing present in an unoccupied orbital reflects mixing
in the occupied orbital, due to the orthogonality constraints on
the two orbitals that contain ligand p-character. Hence, as we
define orbital mixing as a synonym for covalency (in the sense
1
of Heitler and London ), ligand K-edge XAS provides a direct
48
estimate of metal−ligand covalency.
To provide guidance for the interpretation of the
experimental work, the TDDFT computational approach
involves modeling the XAS spectra using a linear response
theory calculation, where the energy and oscillator strength of
excitations from the core levels to virtual orbitals can be
determined. Combined, these experimental (XAS) and
theoretical (TDDFT) methods operate cooperatively and
have been implemented to evaluate orbital mixing in many
Figure 1. Thermal ellipsoid plot of (NEt ) CeCl . Thermal ellipsoids
are drawn with 50% probability. The NEt4 countercations are
omitted.
4
2
6
1+
IV
analysis of (NEt ) CeCl (formally Ce ) was consistent with
4
2
6
69
III
the previous report. Although the Z LnCl (formally Ln )
3
6
structures had not been reported, the Ln−Cl bond distances
and Cl−Ln−Cl angles observed in this study were consistent
3−
6
−8,44−66
transition metal systems.
with other octahedral LnCl63 salts.
−
69−73
In all the LnCl
6
Herein, we have exploited the recent successes of ligand K-
edge XAS and TDDFT in evaluating covalency in
structures, the solid state analyses revealed that the large non-
coordinating cations guarded against bridging Cl interactions
and ensured that the M−Cl bond was probed discretely during
Cl K-edge XAS measurements. Most notably, especially in
regard to the XAS and DFT experiments, the Ln−Cl distances
were equivalent once the metal ionic radii were considered
(Table 1), suggesting that the Ln−Cl bond was best described
using an ionic bonding model. Moreover, there was no
indication from the structural metrics that the amount of
Ln−Cl orbital mixing varied appreciably. As will be discussed,
the XAS and DFT results provided unambiguous evidence for
6
−8,60,66−68
actinides
to provide the first in a series of Ln−Cl
covalency studies. Orbital mixing in the Ln−Cl bond was
evaluated in a series of highly symmetric O -LnCl (Ln = Ce,
x−
h
6
Nd, Sm, Eu, Gd, x = 3; Ln = Ce, x = 2) anions. In contrast to
many traditional assumptions, comparing our spectroscopic and
3−
III
theoretical results from LnCl6 (formally Ln ) revealed that
the 5d-orbitals were involved in covalent Ln−Cl bonding with
2
−
the 4f-orbitals playing a minimal role. For the case of CeCl
,
6
IV
where the unusual Ce oxidation state could be investigated,
we found evidence for both 4f- and 5d-orbital participation in
bonding. Overall, the results are presented in the context of
recent studies on analogous transition metal and actinide
2−
profound changes in orbital mixing between CeCl and the
6
LnCl63 anions.
−
2−
7
compounds, MCl6 (M = Ti, Zr, Hf, U).
Ce L -Edge XAS. In light of recent electronic structure
3
,2
studies that have addressed the possibility of intermediate
valency for Ce compounds with formal oxidation states of
RESULTS AND DISCUSSION
■
LnCl6x− Synthesis. The Cl K-edge, Ce L -edge, and Ce
IV,
characterized at the Ce L -edges. The Ce L -edge extended X-
5,24,42,74−93
the CeCl6 and CeCl6 compounds were
3−
2−
3,2
M -edge XAS techniques were used to analyze a series of
5,4
3,2
3
lanthanide hexachloride salts containing organic cations of the
ray absorption fine structure (EXAFS) data were initially used
to confirm that synchrotron experiments were conducted on
pure CeCl6 and CeCl6 analytes. Measurements were made
III
III
III
III
general formula Z LnCl (Ln = Nd , Sm , Eu , Gd and Z =
3
6
III
3−
2−
NEt ; Ln = Ce and Z = PPh ) and (NEt ) CeCl . These
4
4
4 2
6
B
DOI: 10.1021/ja510067v
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Journal of the American Chemical Society
Article
Table 1. Average Bond Distances (Å) for the Z LnCl (Ln =
3
6
Nd, Sm, Eu, Gd and Z = NEt ; Ln = Ce and Z = PPh ) and
4
4
(
NEt ) CeCl Compounds, Compared with the DFT-
4
2
6
a
Calculated Values
expt
DFT
Ln−Cl minus Ln
Ln−Cl
a
a
a
compound
Ln−Cl
SD
ionic radii
mean
SD
CeCl63
−
2.77
−
2.74
0.02
−
0.02
1.76
−
1.75
2.6715
2.860
0.0001
0.008
−
PrCl63
−
NdCl63
PmCl63
SmCl63
EuCl63
GdCl63
CeCl62
−
2.8451
2.8286
2.820
−
−
−
−
_
−
2.71
2.70
2.69
0.01
0.01
0.02
−
1.75
1.75
1.75
0.004
0.021
_
−
2.817
−
2.8273
2.8843
−
2.599(1)
1.73
a
0.00013
a
Standard deviation from the mean. Ionic radii-corrected Ln−Cl
165
distances, using Shannon ionic radii.
Figure 3. Fourier transforms of the Ce L -edge EXAFS function
3
3
3−
2−
k χ(k) for CeCl6 (○, top) and CeCl6 (○, bottom) collected at 100
K. Fits to the data are represented by the green and dashed blue traces.
In Figure 4, the background-subtracted and normalized Ce
3−
2−
L -edge XAS spectra from CeCl and CeCl6 are provided.
3,2
6
3
3−
Figure 2. Ce L -edge EXAFS function k χ(k) for CeCl (aqua trace)
3
6
and CeCl62 (pink trace) collected at 100 K. See Supporting
−
Information for the details of the EXAFS fits.
3
at both 100 and 6 K. In Figure 2, the k χ(k) data (100 K) for
both CeCl63 and CeCl6 are shown. This comparison
highlighted the phase shift and change in frequency for the
EXAFS oscillations between the two analytes, which was
attributed to differences in Ce−Cl bond distances. The data
were modeled by allowing the bond length distribution
−
2−
2
variance, σ (Debye−Waller factor), and the bond length (R)
3
−
to converge to reasonable values, while the number of
neighboring atoms in the inner scattering shell was fixed at
six Cl atoms based on the crystallographic data (Figure 3). The
Figure 4. Ce L -edge XAS spectra of CeCl
(pink trace) and
3,2
6
2
2
−
−
3−
CeCl
(aqua trace) at 6 K. The dashed traces show the CeCl
and
6
6
CeCl6 spectra obtained at 100 K.
proximity of the Ce L -edge (∼6164 eV) constrained
2
meaningful EXAFS data to less than ∼11k. As a result,
From the perspective of the free ion, the edge features in these
spectra originated from electric-dipole allowed transitions from
Ce 2p-orbitals to unoccupied states that contain Ce 5d-
scattering pathways beyond the first coordination shell
(
including multiple scattering pathways, e.g., Cl−Ce−Cl)
character, e.g. in CeCl63 there were 2p 4f 5d →2p 4f 5d
−
6
1
0
5
1
1
were not included. However, satisfactory models of the
EXAFS data (100 K) were obtained and the Ce−Cl bond
transitions. These final states were split into two primary L -
3
2
5
distances were determined to be 2.62(2) Å (σ = 0.0034 ±
and L -edges, owing to spin−orbit coupling of the 2p core−
2
2
6
−
2
3−
0
.0008) for CeCl and 2.79(2) Å (σ = 0.0027 ± 0.0005) for
hole. For CeCl6 , varying the temperature from 100 to 6 K
3
−
CeCl6 . These distances were consistent with the single-crystal
had no detectable effect on the pre-edge peak intensities, as the
XRD measurements, which showed a crystallographically
high-temperature spectra were superimposable on the low-
IV
2−
unique Ce −Cl bond distance of 2.599(1) Å in CeCl and
temperature measurements. Superficially, the L -edge peak
6
3,2
3
−
an average 2.77 Å (standard deviation of 0.02 Å at 1σ) distance
positions from CeCl6 were similar to those of previously
analyzed Ce compounds and were characterized by
in CeCl63 . Although not shown in Figure 3, analyses of EXAFS
−
III
3
−
2−
data from CeCl6 and CeCl6 collected at 6 K were in good
agreement with the 100 K measurements discussed above.
pronounced absorption edges with inflection points of 5723.1
75−93
(L -edge) and 6164.1 eV (L -edge).
The L -edge peak
3
2
3,2
C
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Journal of the American Chemical Society
Article
8
5,90,91
intensity ratio was determined to be 0.47(2) using a graphical
approach based on the integration of the second-derivative
spectrum, and defined as A /(A + A ). Here, A and A
2
forbidden 2p→4f transitions.
The white line doublet was
defined by a set of intermediate- and high-energy peaks. The
intermediate features were modeled with three Lorentzian
functions, (5725.7, 5728.8, and 5730.8 eV; aqua, pink, purple
traces), while the high-energy peaks were modeled using a pair
of Lorentzian functions (5736.2 and 5739.0 eV; orange and
3
3
2
3
represent the total areas under the second derivative of the L -
3
3−
and L -edge peaks, respectively. The CeCl
spectrum was
2
6
III
interesting in comparison to the spectra of other Ce
compounds in that the L -edge peaks clearly consisted of
2−
brown traces). In this sense, the CeCl
spectra were unique in comparison to data previously reported
Ce L3,2-edge XAS
3
,2
6
two dominant features. Consistently, curve-fitting analysis
suggested that the spectrum (100 K; between 5710 and 5730
eV) was best modeled by two Lorentzian functions separated
by 2.6 eV, which was in agreement with the second derivative of
the data.
IV
24,74−93
from Ce compounds,
whose white line doublets often
had less fine structure and were typically defined by only three
functions.
For comparison with previously reported spectra, Ce L -edge
peak intensity ratios for the intermediate- (P ) and high-energy
3
As observed for CeCl63−, the CeCl62− L3,2-edge XAS data also
showed no dependence on temperature. Both the 6 and 100 K
I
(
P ) features were determined using P /(P + P ), where P
H H H I I
2−
and PH represented the total areas under the intermediate-
aqua, pink, and purple traces, 5720−5734 eV) and high-energy
(orange and brown traces, 5734−5742 eV) functions. We note
that curve-fitting metrics from models with the step function at
low energy (as shown in Figure 5) gave similar intensity ratios
to models where the step function was pushed above 5745 eV
L -edge XAS data from CeCl were best described by double
3,2
6
(
white line features and were consistent with previous analyses
IV
83,86
24,74−76,80,90,91,85
of Ce compounds, i.e., Ce(C H ) ,
CeO2,
The inflection points for
8
8 2
78,80
7
5,76
Ce(SO ) ·4H O,
and CeF4.
4
2
2
the rising edges (L = 5724.7 eV and L = 6165.7 eV) were
3
2
approximately 1.5 eV higher in energy than those observed for
7
5,76
CeCl63 . The 0.52(2) L -edge peak intensity ratio was larger
−
(similar to that reported by Bianconni and co-workers
The 0.51(5) intensity ratio determined for CeCl6
).
3,2
2
−
75,76
3−
was
and
than the CeCl6 ratio of 0.47(2). Curve-fitting analyses
indicated that the CeCl6
2−
reminiscent of the ratio determined for CeO (0.54)
2
86
L3-edge XAS spectrum (100 K;
smaller than that reported for Ce(C H ) (0.89). Based on
8
8 2
Figure 5) was modeled best using six Lorentzian functions. This
model provided the best fit with the fewest functions and was in
good agreement with analysis of the second derivative of the
data. There was a low-energy pre-edge feature (red trace,
recent studies, this peak intensity ratio could be interpreted in
1
different ways, either as a 4f contribution to a multiconfigura-
tional ground-state or as resulting from final state effects. These
concepts have ignited new theoretical discussions and it is
surprising that with only simple Cl ligands, CeCl
unusual electronic properties, like CeO2 and Ce(C H ) .
5
720.1 eV) at the onset of a white line doublet, which has been
2
6
−
IV
may exhibit
attributed previously in other Ce spectra to quadropole-
8
8 2
Clearly, understanding electronic structure and bonding in
CeCl62 will benefit from additional X-ray absorption measure-
−
2−
ments. Hence, we compare subsequently the CeCl6 Ce L -
3,2
edge XAS data with Ce M -edge and Cl K-edge XAS results,
5
,4
2
−
while future work is focused on fully characterizing CeCl6
bonding using other approaches.
Ce M -Edge XAS. Because exciting electrons from 3d-core
5
,4
orbitals at the M -edges directly probes the valence 4f-orbitals,
5,4
Ce M -edge XAS provides an opportunity to uniquely evaluate
5
,4
x−
f-electron occupancy and electronic structure in the CeCl
compounds.
6
94−103
The intense multiplet structure associated
with these M -edge XAS spectraas well as the M -edge
5,4
5,4
intensity ratioshas been previously used to characterize Ce
1
04
105
106
orbital occupancies in many lanthanide oxides, metals,
107−110
and other lanthanide materials.
From the perspective of
the free ion, features in Ce M -edge XAS spectra originate
5,4
from electric-dipole-allowed transitions from Ce 3d-orbitals to
unoccupied states that contain some degree of 4f-character, e.g.,
10
n
0
9
n+1
0
94−103
the 3d 4f 5d →3d 4f 5d transition.
For Ce, these final
states are further split into two primary M - and M -edges
5
4
9
owing to spin−orbit coupling of the 3d core−hole.
The background-subtracted M -edge XAS spectra obtained
5,4
from samples of CeCl63 and CeCl6 in transmission mode
using a scanning transmission X-ray microscope (STXM) are
provided in Figure 6 from the particles shown in Figure 7. For
CeCl63 , the Ce M -edge (near 880 eV) and the Ce M -edge
−
2−
−
5
4
(
∼910 eV) portions of the spectra were best described as
having a large peak that was further split into a series of less
intense features. This “sawtooth” splitting pattern was
Figure 5. Ce L -edge XAS experimental data (●) obtained from
3
CeCl63 (top) and CeCl6 (bottom) at 100 K. The Lorentzian
functions (red, aqua, pink, purple, orange, brown traces) and
arctangent (gray) functions used to generate the deconvoluted spectra
−
2−
consistent with previous M -edge spectra obtained from
5,4
III
106
other Ce compounds.
The fine structure observed for
CeCl63 also closely resembles the multiplet structure
−
9
2
106
(
green trace) are included.
calculated for 3d 4f final states by Thole and co-workers
D
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Journal of the American Chemical Society
Article
values was consistent with the anticipated decrease in the
number for 4f-electrons for CeCl62 relative to CeCl6
−
3−
.
However, we note that branching ratio values were equivalent
once the error associated with the measurement was
considered.
The presence of well-separated peaks in the M -edge
5
,4
spectra of CeCl62 (formally Ce ) provided an opportunity to
−
IV
evaluate the Ce 4f-orbital contribution to Ce−Cl bonding. In
2
6
−
many ways, the spectrum from CeCl
was reminiscent of
IV
many other formally Ce compounds. Most notable was the
similarity between two parameters used previously by Kaindl
and co-workers to evaluate f-orbital occupancy, namely (1) the
satellite-to-main peak intensity ratio and (2) the energy
splitting between the main peaks and the satellite features.
For instance, the satellite-to-main peak intensity ratio was
measured to be 0.39(2) for CeCl62 . Likewise, energy splittings
between the main peaks and satellite features were 4.9 eV for
3
6
−
Figure 6. Ce M -edge XAS spectra of CeCl (top, blue trace) and
CeCl6 (bottom, pink trace).
5,4
2
−
−
CeCl62 . Taken together, the Ce M -edge XAS spectra and
−
5,4
measurements made at the Ce L -edge XAS (discussed above)
3,2
provided a unified picture, suggesting that the Ce 4f-orbital
2−
contribution to bonding in CeCl6 closely resembled that of
IV
other Ce compounds.
To unravel the origin of the satellite in the Ce spectra,
IV
configuration interaction (CI) calculations for the M -edge
5
,4
x−
XAS spectra from CeCl6 (x = 2, 3) were conducted using a
semiempirical approach with the program CTM4XAS by de
1
11−113
Groot.
This model allowed for core−hole-induced
charge transfer, exchange, and multipole interactions to be
116
included in the calculation. To prepare the reader for what
follows, we note that the calculations were performed in the
atomic orbital representation. For CeCl63 (formally Ce ),
−
III
1
0
1
calculations employing a transition from single 3d 4f initial
state to the multiplet states associated with a 3d 4f final state
9
2
configuration suitably modeled the experimental spectra
III
(
Figure 8). In agreement with previously reported Ce M -
5
,4
III
edge XAS calculations, Ce bond covalency could be
accounted for by decreasing the 4f−4f Slater−Condon
117−119
Figure 7. X-ray contrast images taken above and bellow the Ce M5-
edges showing the particles of CeCl63− (top) and CeCl6 (bottom)
used to obtain the Ce M -edge XAS spectra shown in Figure 6.
Coulomb repulsion parameter.
In our calculations this
2−
4f−4f repulsion parameter and the 3d spin−orbit coupling
5,4
(
SOC) parameter were reduced to 79% and 98% of the atomic
Hartree−Fock values, respectively. A Gaussian broadening of
0.25 eV was applied to account for instrumental broadening
and Lorentzian broadenings of 0.2 and 0.4 eV were applied to
and by modeling using the CTM4XAS program (vide
1
11−113
infra).
From an atomic perspective, assuming a single final-state
the M
- and M
-edges, respectively. We note that in the
- and M -edge
5
4
9
1
0
configuration (3d 4f ) for formally tetravalent Ce (4f )
absence of a reduction of the 3d SOC the M
5
4
compounds, we would expect one peak for the Ce M - and
separation was overestimated and in the absence of a reduction
5
3
+
106,111−113
M -edges (similar to La ).
However, the Ce M5,4-
of the 4f−4f Slater−Condon Coulomb repulsion the fine
4
2
−
edge XAS spectrum from CeCl6 was composed of intense
main peaks at 883.7 and 901.6, respectively, and additional
weaker satellite features at 888.6 and 906.4 eV. In this regard
was reminiscent of
experimental measurements from other formally Ce com-
pounds, such as CeO . The M - and M -edges were split by
7.8 eV for CeCl6 and 17.2 eV for CeCl6 , owing to
structure of the M -edge was not well modeled. We interpret
5
the good correlation between the CI calculation and the
experimental data as suggesting that the XAS spectra were
dominated by interactions of the core 3d hole with 4f states,
and contributions from the ligand field were of less concern.
2
−
the Ce M -edge XAS of CeCl
5
,4
6
IV
9
3
2−
IV
For CeCl6 (formally Ce ), the experimental spectra could
2
5
4
2
−
3−
10
0
1
not be modeled successfully by considering only a 3d 4f initial
state and the final states (multiplets) associated with a 3d 4f
configuration. In contrast to the Ce data described above, this
result implied that for CeCl6 the atomic (ionic) limit was not
9
1
differences in spin−orbit coupling with the 3d and 3d3/2
5
/2
III
core−holes. Peak areas were determined using a graphical
114
2−
approach based on integration of the second derivatives. The
M -edge total intensity ratiosdefined as A /(A + A ),
appropriate. In an attempt to better understand the origins for
5
,4
5
5
4
the rich CeCl62 M -edge XAS spectral featuresespecially
−
where A and A were the total areas under all M - or M -edge
5
4
5
4
5,4
9
8,115
peaks
increased from 0.48(2) to 0.50(3) on moving
the higher energy satellitescharge-transfer effects were
2−
3−
120−122
from CeCl6 to CeCl . Given that M -edge intensity ratios
considered.
In the charge transfer model, the initial
6
5,4
1
0
0
have been reported to be dependent on the 4f-electron
state was described by two possible configurations (3d 4f and
3d L4f ) and the final state by the two configurations 3d 4f
8
1,105−108
10
1
9
1
occupancy,
the increase in the magnitude of these
E
DOI: 10.1021/ja510067v
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Journal of the American Chemical Society
Article
Figure 9. UV−vis spectrum from (NEt ) CeCl (1.2 × 10− M)
5
4
2
6
dissolved in acetonitrile. Data were acquired using a quartz cell (path
length = 1 cm).
the peak at 3.3 eV to the lowest LMCT band, given its large
extinction coefficients and because the TDDFT calculations of
the optical spectrum yielded states with appreciable intensity at
2
.9 eV and another at >4 eV. This information was useful for
our CTM4XAS calculations, in that it provided upper bounds
on acceptable values for ΔE and T . In particular, the energy
gs
gs
of the LMCT band should be of the order of ΔE + 2T . In
gs
gs
addition, an NBO analysis of our ground state DFT calculations
gave an effective hopping integral of roughly 1.1 eV. Combined,
this information severely restricted our parameter space. Hence,
we found that ΔE = 2.5 eV and T = 0.7 eV resulted in a
Figure 8. Comparison of the experimental (○, black traces) and the
configuration interaction calculation (pink and aqua traces) for the Ce
gs
gs
calculated output LMCT of 3.6 eV, which was in reasonable
agreement with the direct UV−vis measurement. Within these
constraints, our calculated ground state was modeled roughly
x−
M -edge XAS spectra from CeCl (x = 3, top; x = 2, bottom). The
5,4
6
bars represent the energy and oscillator strength for the calculated
transitions.
10
0
10
1
with 75% 3d 4f and 25% 3d L4f . We note once again that
this description is in the atomic orbital representation, and can
be rewritten in terms of a molecular orbital of similar parentage
with Ln 4f- and Cl 3p-orbital mixing in the ground state. It does
not imply a strong ground state correlation effect.
9
2
and 3d L4f , where L designates a hole in the ligand orbital.
Each configuration was defined by 4f−4f Coulomb repulsion,
4
f−3d
Coulomb repulsion, SOC, and Coulomb exchange
core
The remaining parameter to specify for the XAS calculation
parameters. Four additional parameters were defined to
describe the relative energies and interactions of these states.
These adjustable parameters consisted of the charge transfer
was ΔE . This is defined by the model as ΔE = ΔE + U −
fs
fs
gs
ff
U , where U and U represent the self-repulsion and core
fd
ff
fd
Coulombic repulsion integrals, respectively. In L-edge spectra
energy (ΔE ) between the two ground-state configurations and
gs
from transition metals, the self-repulsion is often ∼2 eV smaller
the effective hopping integral (T ) connecting the two,
gs
than core repulsion. In general, the Hubbard U is notoriously
ff
1
0
1
10
0
ΔE = E(3d L4f ) − E(3d 4f )
̲
gs
large in lanthanides because of the small orbital radius.
However, as de Groot has pointed out previously, generally
the core−valence repulsion (Ufd) is somewhat larger than the
1
0
1
10
0
T = (3d L
̲
4f )|H|(3d 4f )
gs
self-repulsion (U ). This indeed occurred in our calculations for
ff
CeCl62 , as a very good fit to the M -edge XAS experimental
−
and two analogous parameters for the core−hole excited state
5,4
(
ΔE and T ),
data was obtained with ΔE = −1.5 eV. Note this value for ΔE
fs
fs
fs
fs
9
2
9
1
suggested that, in the presence of the core−hole, the effective
final state charge transfer energy was significantly reduced, in
this case becoming negative in value. Hence, the ground state
includes a degree of covlanecy between the Ce 4f- and Cl 3p
orbitals, while the structure in the final state is assigned to a
final state configuration interaction. Note that the latter reflects
a change in covalency in the final state induced by the presence
of the 3d core-hole. A more detailed study of our modeling will
be presented in the future, and more work is needed to test the
generality of our conclusion and the implications on the origins
ΔE = E(3d L4f ) − E(3d 4f )
̲
fs
9
2
9
1
T = (3d L
̲
4f )|H|(3d 4f )
fs
To confine this parameter space, we chose to impose the
constraint that the hopping integrals in the initial and final
states should be identical. In addition, because the difference in
energy between the two roots corresponding to the initial state
CI corresponds to the LMCT band in the model, the
experimental UV−vis spectrum was obtained (Figure 9). The
CeCl62 UV−vis spectrum consists of two intense peaks, one at
−
IV
of the Ce M-edge XAS features.
−
1
−1
3
.3 eV (ε = 4 700 L cm mol ) and an additional peak at 4.4
Finally, we remind the reader that the parameters used for
−
1
−1
2−
IV
eV (ε = 12 000 L cm mol ). We felt comfortable assigning
CeCl6 (formally Ce ) were quite consistent with what was
F
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Journal of the American Chemical Society
Article
used in the CeCl63− (formally Ce ) model. The distinguishing
III
III
characteristic in the Ce case was that the initial state charge
IV
transfer energy was significantly increased relative to Ce . This
3
−
III
10
2
was because for CeCl , the ΔE (Ce ) = E(3d L4f ) −
6
gs
10
1
E(3d 4f ) + U was significantly larger than the ΔE for
ff
gs
CeCl62 (Ce ), owing to the large Coulomb repulsion
−
IV
III
III
associated with Ce charge transfer. As a result, the Ce
ground state was firmly in the atomic limit, which was
consistent with our DFT results that suggested no spin
III
delocalization and that the spin density on the Ce (1.0) was
identical to the value expected in the ionic limit.
x−
LnCl6 Cl K-Edge XAS. The background-subtracted and
2
6
−
normalized Cl K-edge XAS spectra obtained from CeCl and
LnCl63 (Ln = Ce, Nd, Sm, Eu, Gd) are compared in Figure 10.
−
Figure 10. Overlay of the background-subtracted and normalized Cl
K-edge XAS spectra for CeCl62− and LnCl6 (Ln = Ce, Nd, Eu, Sm,
and Gd).
3−
In many ways, the Cl K-edge XAS spectra from these
hexachlorides were quite similar. Most obviously, a dominant
edge peak near 2826 eV superimposed on a step-like absorption
threshold primarily characterized the spectra (see Supporting
Information). More subtle attributes common to each spectrum
included pre-edge features that were indicative of covalent Ln−
Cl bonding (<2825 eV). A deconvoluted model was obtained
Figure 11. Cl K-edge XAS experimental data (○) obtained from the
Z LnCl (Ln = Nd, Sm, Eu, Gd and Z = NEt ; Ln = Ce and Z = PPh )
3
6
4
4
and (NEt ) CeCl compounds. The pre-edge pseudo-Voigt functions
4
2
6
(aqua and pink traces) used to generate the deconvoluted spectra
(green trace) are included. The functions used to describe the edge-
region of the Cl K-edge XAS spectra are summed to give the brown
dashed trace. Feature B from Figure 10 is provided in the cut-out.
for the CeCl62 and LnCl6 Cl K-edge XAS spectra as shown
in Figure 11, summarized in Table 2, and described in the
Experimental Section. Overall, this approach provided high-
quality models of the data, as evidenced by good correlation
coefficients and symmetric residual peaks that were similar in
shape to the corresponding pseudo-Voigt functions. Addition-
ally, the residuals in each spectrum were less than 2% from
−
3−
Because the A-features were quite close to the rising edge in
the LnCl63 (formally Ln ) spectra, care was taken to maintain
consistency in modeling the edge region (2818−2830 eV). The
cautious approach to modeling of the rising edge line shape
allowed changes in A-feature peak intensities to be evaluated as
−
III
2
818 to 2830 eV.
To evaluate the LnCl6 Cl K-edge XAS spectra, we found it
x−
2−
III
III
convenient to begin the discussion with CeCl6 (formally
Ce ). Parameters from the curve-fitting analysis of the CeCl6
a function of metal identity, from Ce to Gd . As shown by
the dashed brown trace in Figure 11, which represented the
summation of functions used to model the spectral edge
regions (>2825 eV; fully deconvoluted spectra provided in the
Supporting Information), the rising edge line shape deviated
IV
2−
spectrum showed a large pre-edge feature near the onset of the
rising edge at 2823.6 eV (aqua trace, A-feature in Figure 10)
and a second low-energy feature at 2820.6 eV (B-feature in
Figure 10; pink trace in Figure 11). Both features were well
resolved from the rising edge, which had an inflection point of
III
III
only slightly from Ce to Gd . For instance, the rising edge
position was constant for all the data, as evidenced from the
first inflection point varying by only 0.47 eV. Additionally, the
slope of the rising edges (dashed brown trace, Figure 11) was
nearly equivalent for all of the trivalent hexachlorides (average
slope = 0.57 with a standard deviation of ±7.0%). These results
suggested that changes in relative intensity for the pre-edge A-
features could be determined with confidence despite their
proximity to the rising edge. Error in the A-feature peak
2
825.7 eV. The large peak area (defined hereafter as peak
intensity) of 0.70(7) for the B-feature was comparable to that
of the A-feature at 0.84(8). The spectra from the LnCl
3−
6
III
2−
(
formally Ln ) were similar to that obtained from CeCl6 in
that they also contained intense A-features near the rising edge
∼2824 eV; aqua trace in Figure 11) with intensities ranging
from 0.93(9) to 0.48(5).
(
G
DOI: 10.1021/ja510067v
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Journal of the American Chemical Society
Article
lanthanide series was traversed from CeIII to Gd , close
III
Table 2. Comparison of the Experimental Cl K-Edge XAS
Pre-edge Peak Energies, Intensities, Amounts of Cl 3p-
Character, and TDDFT Calculated Oscillator Strengths (f)
Associated with Electronic Excitations from Cl 1s-Orbitals to
the t * and t * + t * Orbitals for the Z LnCl (Ln = Nd,
inspection revealed systematic variations between spectra. In
general, moving to the right of Ce on the periodic table
III
caused the energy for the B-feature to broaden and shift to
lower energy until it disappeared into the baseline of the
2
g
2u
1u
3
6
GdCl63 spectrum. In contrast to CeCl6 (formally Ce ),
where both the A- and B-features could be evaluated with
confidence, the low intensity of the B-features associated with
−
2−
IV
Sm, Eu, Gd and Z = NEt ; Ln = Ce and Z = PPh ) and
4
4
(
NEt ) CeCl Compounds
4
2
6
Cl 3p/bond (%)
3−
the LnCl6 challenged the sensitivity of the experiment.
Despite these issues, the data in Figure 11 were best modeled
for Ce , Sm , and Eu with an unconstrained B-feature. To
obtain a model for Nd that could be compared directly with
Ce , Eu , and Sm , the peak height and width for the Nd B-
feature had to be constrained between that observed for Ce
and Eu . The error associated with the Nd B-feature
overlapped with that of Ce and Eu and, in general,
energy
(eV)
intensity
(exp)
TDDFT
f, ×1000
a
assignment
exptl
calcd
III
III
III
2−
O -CeCl
h
III
6
Cl 1s→t *
2823.6
2820.6
0.84(8)
0.70(7)
11(1)
9.9(9)
6.0
4.90
5.50
III
III
III
III
2g
Cl 1s→4f
15.5
III
3
6
−
O -CeCl
h
III
III
Cl 1s→t *
2824.0
2822.8
0.93(9)
0.07(2)
13(1)
1.0(3)
3.9
2.7
3.90
1.30
III
III
2g
Cl 1s→4f
O -NdCl
3−
uncertainties in the intensity for all of the LnCl6 B-features
3
−
h
6
were estimated to be quite high. Hence, although we
acknowledge the presence of B-features and while the data
suggests that B-features were quite small, we refrain from
attempting to quantify the B-feature intensities.
LnCl6 Ground-State Electronic Structure. Density
functional theory (DFT) calculations were conducted on the
Cl 1s→t *
2824.1
2823.8
0.83(8)
0.07(2)
12(1)
1.0(3)
3.9
3.8
3.98
1.33
2g
Cl 1s→4f
D -SmCl
6
3
−
4h
Cl 1s→e *+b *
2824.1
2822.4
0.60(6)
0.03(1)
8.5(9)
0.4(1)
3.7
1.1
3.95
1.30
x−
g
2g
Cl 1s→4f
3−
D -EuCl
6
x−
4h
LnCl6 anions at their optimized geometries. This computa-
Cl 1s→e *+b *
2824.2
2821.2
0.59(6)
0.02(3)
8.4(8)
0.3(3)
4.3
6.4
4.61
1.99
III
III
g
2g
tional effort included Pr and Pm , which were not
experimentally evaluated. Unrestrictive Kohn−Sham calcula-
tions were carried out using relativistic effective core potentials
Cl 1s→4f
3−
O -GdCl
h
6
Cl 1s→t *
Cl 1s→4f
2824.1
0.48(5)
6.8(7)
3.3
2.4
3.75
2g
(
RECPs) on the high-spin ground states (Hund’s first rule).
−
−
−
0.013
The computational results for the α spin−orbitals are
summarized in Table 3, as well as in the quantitative orbital
level diagram shown in Figure 12. We have limited this
discussion to only unoccupied orbitals relevant to the XAS
experiments (Scheme 2) because the electronic structures for
a
The calculated Cl 3p/bond values were determined by multiplying
the total Cl 3p-character from the Mulliken populations by the sum of
the squares of the normalization constants for the corresponding
ligand-orbital wave functions (see Experimental Section).
x−
18,123−126
O -LnCl6 anions have been previously described.
h
intensity was estimated at 10% based on our ability to
reproduce the data on beamlines 6-2 and 4-3 at the Stanford
Synchrotron Radiation Lightsource (SSRL) synchrotron as well
Results from our DFT calculations were consistent with group
theoretical considerations and were summarized as follows. For
x−
O
-LnCl
anions, a symmetry-adapted linear combination
h
6
as at the Institut fu
̈
r Nukleare Entsorgung (INE) beamline at
(SALC) of six Cl 3p-atomic orbitals having σ-symmetry with
+ t1u symmetries.
the Angstromquelle Karlsruhe (ANKA) synchrotron during
̈
respect to the Ln−Cl bond spanned a1g + e
g
multiple experimental periods.
There were also 12 Cl 3p-SALCs with π-symmetry with respect
to the Ln−Cl bond that transformed as t1g + t2g + t1u + t2u. With
this designation, the Ln 5d-orbitals were allowed by symmetry
Our curve-fitting models suggested that the intensity for the
A-features systematically decreased as a function of lanthanide
metal identity (CeIII to Gd ), which corresponded with an
III
to mix and form Ln−Cl σ- and π-interactions of e
and t2g
g
increase in the effective nuclear charge of the metal (Z ). For
symmetries, respectively, which is well established for the
eff
III
x−
7,127−132
example, although the intensities of 0.93(9) for Ce and
MCl
6
transition metal analogues.
Superimposed on
III
0
.83(8) for Nd were equivalent once the error was considered
this orbital picture (and at lower energy) was the Ln 4f-
manifold, which in an octahedral ligand field transformed with
a2u + t2u + t1u symmetries. Hence, the Ln 4f-orbital of a2u
symmetry was rigorously nonbonding, the t2u orbitals formed
Ln−Cl π-interactions, and the t1u orbitals formed Ln−Cl σ- and
π-interactions. This left the Cl t1g orbitals as nonbonding Cl
lone pair orbitals.
(
Table 2 and Figure 11), marked intensity decreases were
observed when non-adjacent lanthanides on the periodic table
were compared. The most extreme example was associated with
III
III
the decrease from 0.93(9) in Ce to 0.48(5) for Gd . Given
that the pre-edge peak intensities are directly related to the
degree of Ln−Cl orbital mixing, these results suggested that
M−Cl covalency decreased systematically with increasing Z
It is in this orbital framework that the experimental XAS
eff
IV
III
across the lanthanide series. As a reminder, the structural
analysis provided in Table 1 showed no evidence of such
changes.
spectra were interpreted. Moving from Ce to Ce then to
III
Gd resulted in successive filling of the 4f-orbitals, such that
there was not double occupancy in any of the calculated ground
3
−
The Cl K-edge XAS spectra from the LnCl
(formally
states. The orbital description in O -symmetry (described
6
h
III
2−
IV
2−
Ln ) differed from that of CeCl (formally Ce ) in that the
Ln B-features were quite small and best described as
above) was in good agreement for the closed-shell CeCl and
GdCl6 systems and for CeCl6 and NdCl6 , where the a
and t orbitals were half-filled. However, for PrCl , PmCl6
SmCl6 , and EuCl6 the calculations converged to D4h
structures with slight changes in the axial and equatorial bond
distances. This structural distortion was small with the standard
6
6
III
3−
3−
3−
2u
3−
3
6
−
shoulders at the base of the A-features. Comparisons within
,
1
u
the LnCl63 (formally Ln ) series revealed subtle differences.
For instance, while the energies of the A-features (2824 eV)
and edge-peaks (2827 eV) remained relatively constant as the
−
III
3−
3−
H
DOI: 10.1021/ja510067v
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Journal of the American Chemical Society
Article
Table 3. Calculated Atomic Compositions of Selected Molecular Orbitals for the Z LnCl (Ln = Nd, Sm, Eu, Gd and Z = NEt ;
3
6
4
a,b,c
Ln = Ce and Z = PPh ) and (NEt ) CeCl Compounds
4
4 2
6
composition (%)
Ln 5d
composition (%)
Ln 5d
c
c
MO
energy (eV)
Cl 3p
Ln 4f
MO
energy (eV)
Cl 3p
Ln 4f
3
6
−
3−
O -CeCl
h
O -PmCl6
h
e *
9.27
7.32
6.44
5.96
0.96
9.9
5.2
0.4
1.8
0.0
92.2
94.0
0.0
0.0
0.0
e *
8.95
7.18
11.6
90.2
93.8
0.0
0.0
0.0
g
g
t *
t *
5.2
1.2
2.4
0.0
2g
2g
t *
99.4
97.5
100.0
t *
3.88
98.8
97.5
100.0
2u
2u
t *
0.0
t *
−2.89
−3.56
0.0
1u
1u
a2u (nb)
0.0
a2u (nb)
0.0
3−
3−
D -PrCl
D -EuCl6
4h
4
h
6
b *
9.19
9.12
7.33
7.24
5.96
5.76
5.14
4.80
−0.61
10.8
10.6
5.0
5.2
1.6
0.0
2.2
0.8
16
91.7
91.7
94.1
93.9
0.0
0.0
0.0
b *
9.65
9.56
8.4
9.2
5.6
5.8
85.7
83.6
94.1
93.8
0.0
0.0
0.0
1g
1g
a *
a *
1g
1g
eg*
0.0
b *
8.44
0.0
2g
b *
0.0
e *
8.27
0.0
2g
g
a *
97.7
100.0
97.2
99.1
84.1
a *
1.75
24.8
1.2
0.4
0.0
0.4
75.0
98.8
99.7
100.0
99.5
2u
2u
b1u (nb)
e *
0.0
e *
−2.88
−3.15
−3.16
−4.10
0.0
u
2
0.0
b *
0.0
u
2u
b *
0.0
b1u (nb)
0.0
2u
1
e *
0.0
e *
u
0.0
u
3−
3−
O -NdCl
h
O -GdCl6
h
6
e *
9.11
7.28
11.0
5.2
0.0
2.0
2.6
91.0
94.0
0.0
0.0
0.0
e *
9.17
7.50
9.8
4.4
0.4
0.0
1.4
92.5
94.8
0.0
0.0
0.0
g
g
t *
t *
2g
2g
a2u (nb)
t *
4.97
100.0
97.4
97.5
t *
−3.10
−3.10
−3.13
99.6
100.0
98.4
2u
4.77
0.0
a2u (nb)
0.0
2u
t *
−1.90
0.0
t *
1u
0.0
1u
3
6
−
2−
D -SmCl
O -CeCl6
h
4
h
b *
9.20
9.15
11.6
1.4
4.8
5.2
4.6
0.0
0.8
0.4
0.1
90.5
90.5
94.0
94.0
0.0
0.0
0.0
e *
9.38
6.82
3.19
3.18
3.11
18.0
8.0
9.9
3.2
0.0
86.9
90.4
0.0
0.0
0.0
1g
g
a *
t *
1g
2g
eg*
7.35
0.0
t *
89.6
96.8
100.0
1u
b *
7.34
0.0
t *
0.0
2g
2u
e *
3.32
95.1
100.0
99.3
99.7
98.8
a2u (nb)
0.0
u
b1u (nb)
−2.55
−2.95
−3.68
−3.78
0.0
e *
0.0
u
b *
0.0
2u
a *
0.0
2u
a
b
Alpha spin−orbital energies are reported. The use of a non-orthogonal basis set can cause Mulliken analysis to have nonphysical results such as
compositions >100% or <0.
166
c
2−
The calculated values are normalized to the t -symmetric highest occupied molecular orbital of CeCl
.
6
1
g
deviation from the mean varying between 0.004 and 0.7% from
the calculated values (Table 1). Hence, the calculated distances
were still in agreement with the experimental structures but
consistent with a Jahn−Teller distortion. In the text that
possible to a one-electron orbital picture. The neglect of the
spin−orbit interaction in our calculations was a first concern,
136
but previous experience with actinide compounds suggested
it would not qualitatively change the comparison of theory and
experiment. A greater concern was associated with the fact that
the TDDFT calculation provided a single particle−hole
approximation, and that the full manifold of final states
associated with the core−hole excitation was not addressed. To
best understand what was included versus omitted in the
calculations, consider the ionic limit where all the f-orbitals
3
−
follows, orbitals from the D -LnCl
calculations were
4
h
6
discussed using symmetry labels for the parent O -orbitals.
h
Overall, the calculated systematic variation in orbital
composition and energy was in relatively good agreement
with experimental observations, suggesting that these calcu-
lations provided an appropriate guide for interpreting the Cl K-
edge XAS spectra.
n
were localized on the metal. If the ground state was f , then the
n+1
Spectral Interpretation. To guide our spectral interpreta-
tions, TDDFT calculations were used to simulate the LnCl6x−
Cl K-edge XAS spectra. Analogous computational methods
were previously employed to model spectra for several other
transition metal and actinide systems and had good
excited state would be associated with the configuration f . In
this scenario, there are several ways to couple the orbital and
n+1
spin angular momenta of the states arising from f , each
leading to a different final state multiplet. For instance, in the
2
case of an f ground state (Hund’s rule suggests S = 1, L = 5, or
6
−8,57−60,66−68,133−135
3
3
experimental agreement.
Before discus-
H), excitation of a Cl 1s electron would lead to an f
sing the results in detail, we found it pertinent to discuss several
caveats that were particularly relevant in this series. To provide
guidance for experiment, it was useful to stay as close as
configuration. Hence, there would be three spin states
associated with three unpaired electrons, a quartet and two
doublets. There would also be a number of angular momentum
I
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Journal of the American Chemical Society
Article
of this doublet state is an average of the two actual doublet
states and the oscillator strength a similar average. Nevertheless,
this crude TDDFT particle−hole approximation has been
effective in the past for assigning the spectra.
The computational results are compared with the exper-
imental data in Figure 13 and Table 2. In Figure 13, the total
Figure 12. Orbital diagram of α orbitals from DFT ground-state
x−
calculations for the LnCl6 (Ln = Ce, x = 2; Ln = Ce, Pr, Nd, Pm, Sm,
Eu, Gd, x = 3) anions. Orbitals containing primarily 5d- (blue lines)
and 4f-character (pink lines) are shown. In the calculations, the
III
III
III
symmetry was reduced from O to D for Pr , Sm , and Eu , owing
h
4h
to Jahn−Teller distortions. The calculated values are normalized to the
2
−
highest occupied t -symmetric orbital of CeCl .
6
1
g
Scheme 2. Relevant Unoccupied Kohn−Sham Orbitals That
2
−
Have Primarily 5f- or 6d-Character for CeCl
6
Figure 13. Comparison between experimental Cl K-edge spectra (○)
x−
and results from the TDDFT calculations for the LnCl6 (Ln = Ce, x
=
2; Ln = Ce, Nd, Sm, Eu, Gd, x = 3) anions. The orange bars
represent the energy and oscillator strength for the calculated
transitions. The partial densities of states derived from the TDDFT
for final states associated with 4f- (pink trace) and 5d- orbitals (aqua
and green traces) are also shown.
TDDFT output has been represented by orange bars and the
partial density of 4f-states derived from the TDDFT
calculations were shown using pink traces, while t * and e *
2g
g
states were represented by aqua and green traces. For D -
4
h
LnCl63 anions, states of t * and e * parentage in O -symmetry
−
2g
g
h
couplings available to the three unpaired electrons. To fully
address the states associated with this configuration requires
multielectron excitations relative to the single determinant
were also shown using aqua and green traces, respectively. Each
calculated spectrum has been shifted by a uniform +64 eV to
account for omission of electronic relaxations and other effects
3
137,138
describing the H ground state. However, our TDDFT
(see Experimental Section).
In general, the calculations
calculations generate only single particle−hole excitations and
agreed well with the experimental data and were consistent with
expectations based on group theory analyses. For example,
assuming that only transitions associated with electric dipole-
allowed final states will be observed by Cl K-edge XAS
are therefore not able to describe all these final states. In
2
particular, for the f ground state the TDDFT allowed
excitations yield a quartet and only one doublet. The energy
J
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Journal of the American Chemical Society
spectroscopy, we anticipated only transitions from the A
Article
1
with the DFT and TDDFT calculations, and could not have
been anticipated on the basis of the structural analyses
described above.
1g
1
0
0
x−
ground-state to T final states for a 4f 5d O -LnCl anion
1u
h
6
(
dipole operator t ). As such, the TDDFT calculations showed
1u
electronic excitations from Cl 1s-orbitals to unoccupied orbitals
that resulted from mixing between Cl 3p-orbitals and either the
Ln 4f- (in O symmetry orbitals of t * and t * symmetries) or
It is instructive to begin by considering only the Ln−Cl 5d-
interactions (top panel in Figure 14 and Table 2). For all
h
1u
2u
5
d-orbitals (in O symmetry t * and e *). Thus, for the
h 2g g
III IV
x−
LnCl6 anions (formally containing Ln and Ce ), the
TDDFT results attributed the low-energy B-feature to
overlapping electronic excitations from Cl 1s-orbitals to orbitals
of t * and t * symmetries (pink trace), which were primarily
1
u
2u
of Ln 4f character. Higher energy A-features were primarily
associated with Cl 1s electronic excitations to 5d π-orbitals
(
aqua trace; t * in O -symmetry), while transitions associated
2g h
with the 5d σ-orbitals (e * in O -symmetry) were calculated to
be buried beneath the rising edge (green trace).
g
h
In support of this spectral interpretation, both experiment
and theory agreed in trends observed for 4f- and 5d-orbital
energies. For example, while transitions to the t * (probed by
2g
XAS and DFT) and e * (DFT only) orbital energies remained
g
constant, the energy for both the singly occupied and
unoccupied 4f-orbitals decreased as the central ion identity
IV
III
III
III
was changed from Ce to Nd , Sm , and Eu . These trends
in 4f- and 5d-orbital/transition energies were reasonably
139
rationalized using Slater’s rules. The energy range for the
Figure 14. Comparison of amount of Cl 3p-character per bond
determined using Cl K-edge XAS (pink, left axis), Mulliken orbital
population analyses (blue, left axis), and TDDFT (dashed green, right
axis). The top panel shows values for the 5d-orbital of t * symmetry,
calculated transitions involving 4f-orbitals also increased for
III
III
III
III
Ce to Nd , Sm , and Eu . This broadening was largely
attributed to the multiplets resulting from the increase in 4f-
electron count. Unfortunately, we could not use Cl K-edge XAS
to confirm this observation because of the low-intensity and
poorly resolved pre-edge B-features. We anticipated that larger
energy broadening would be computationally observed once
spin−orbit effects were considered, and future efforts are
focused on incorporating these contributions into our
2
g
and the summation for the 4f-orbitals is provided in the bottom panel.
LnCl6x− (x = 2, 3) analytes probed, significant amounts of Cl
3
p-character were experimentally determined in the 5d π-
orbitals of t symmetry. These experimental values ranged from
2g
7 to 13% per Ln−Cl bond and were in reasonable agreement
with those determined from the Mulliken population analysis of
the ground-state DFT calculations (ranging from ∼3 to 4%).
Across the lanthanide series, both the DFT and TDDFT
(Figure 14) calculations showed virtually no change in the
amount of Cl 3p-character in orbitals of t * symmetry (in the
III
theoretical model. For Gd , a resolved low-energy B-feature
was not observed experimentally. Consistently, the calculations
indicated that transitions involving the 4f-orbitals were pushed
high in energy, as exciting a Cl 1s electron to the half-filled Gd
7
4
f -manifold was accompanied by electronic repulsion resulting
2g
from the double occupancy in the final state. The only
transition calculated at low energy (near 2819 eV) was
associated with an a1g (5s of Rydberg type) orbital, but a
corresponding pre-edge feature was not observed experimen-
tally.
D -point group, orbitals of t * parentage). Meanwhile, the
4h 2g
experimental measurement indicated a decrease in Ln−Cl t
2
g
III
III
orbital mixing from Ce to Gd . When the experimental error
III
was considered, changes between adjacent Ln -containing
compounds were indiscernible while a substantial decrease in
III
Ln−Cl Orbital Mixing. It has been established that the
transition intensities observed for the pre-edge peaks in Cl K-
edge XAS spectra are directly related to the amount of Cl 3p
Ln−Cl t * orbital mixing was apparent when comparing Ce ,
2g
III
13(1)%, and Gd , 6.8(7)% (Table 2, Figure 14). It was unclear
if this difference between theory and experiment was associated
with our computational model or with a systematic
experimental bias associated with resolving the t * pre-edge
4
8
x−
character in M−Cl bonds. Hence, the LnCl6 data described
above offers an opportunity to quantify changes in Ln 4f- and
2
g
5d-covalency as a function of (1) the lanthanide metal identity
feature (A-feature in Figure 4). For the 5d σ-orbitals (e *),
g
and (2) the lanthanide oxidation state. To directly compare the
experimental and DFT results, we used the well-defined
experimental values could not be determined, owing to the
proximity of these transitions to the rising edge. However, we
could evaluate the DFT results. In this case, as observed for the
t * orbitals, the calculations indicated that changing the metal
Cs CuCl4 Cl K-edge XAS intensity standard to convert
2
experimental pre-edge peak intensities to the amount of Cl
2
g
III
III
3
p-character (%) per Ln−Cl bond (an intensity of 0.53 = 7.5%
from Ce to Gd had little effect on the amount of Cl 3p-
48,140
Cl 3p-character per bond).
Similarly, the amount of Cl 3p-
character in the e * orbitals, which fluctuated between 2.9 and
g
character determined from the Mulliken population analysis
was converted to a per-bond basis as described previously and
3.8%.
The influence of lanthanide oxidation state on orbital mixing
in the 5d π-orbitals (t *) was also investigated by comparing
7
outlined in the Experimental Section. As described sub-
2
g
CeCl63 with CeCl6 (Figure 14 and Table 2). Oxidizing
−
2−
sequently, there were appreciable differences in the exper-
−
2−
imental pre-edge peak intensities, which in turn indicated that
CeCl63 to CeCl6 only slightly decreased the amount of Cl
3p-character from 13(1) to 11(1)% observed experimentally;
however, these values were equivalent within error. These data
IV
III
the amount of Ce −Cl and Ln −Cl orbital mixing varied
substantially. These experimental observations were consistent
K
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Journal of the American Chemical Society
Article
compared well with the analogous calculated values of 3.9 and
calculated values (3.3−3.8%) also suggested that Ln 5d and Cl
x−
6
.0%. Overall, the results indicated that the change from
3p σ-orbital mixing occurred in the LnCl6 anions. In
III
IV
formally Ce to Ce had little effect on the amount of Cl 3p-
and Ce 5d-orbital mixing.
comparison to the trends described above for the Ln 4f-
orbitals, our results indicated that systematically changing the
III
III
In regard to 4f-orbital mixing (bottom panel, Figure 14),
both theory and experiment were in reasonable agreement. For
metal identity from Ce to Gd had very little effect on the
t * and e * (primarily 5d) orbital energies. In terms of t *
2g
g
2g
the LnCl63 (formally Ln ), both experimental and DFT
analyses provided unambiguous evidence that the 4f-orbitals
participated in weak covalent bonding and were significantly
less engaged than the 5d-orbitals of t * symmetry (in the D -
−
III
orbital compositions, the experimental values showed a
III
III
decrease in Cl 3p-character from Ce to Gd , while the
DFT and TDDFT calculations showed no appreciable change.
One explanation that accounted for this discrepancy came from
2g
4h
1
III
III
point group, orbitals of t * parentage). Our results for the 4f
the TDDFT analyses, which suggested that for Sm , Eu , and
2g
CeCl63 , which showed only 1(1)% Cl 3p-character per Ln−Cl
bond, were in reasonable agreement with the analysis by
Edelstein and Lukens using UV−vis spectroscopy that also
showed only marginal involvement of the t and t orbitals in
−
III
Gd the t * peaks broadened, likely resulting from increased
2g
multiplet effects. We additionally anticipate further broadening
from SOC, which was not included in the TDDFT calculations.
If this calculated range of transitions is correct, it is possible that
additional pre-edge peak intensities are masked within the
rising absorption edge and that the experimental measurements
did not provide the entire picture. Hence, future efforts are
focused on determining if experimental conditions can be
identified that provide better resolution for these t * pre-edge
1u
2u
III
141
Ce −Cl bonding. Moving across the lanthanide series from
left to right, both experiment and theory indicated that the
energy of the orbitals decreased while the amount of Cl 3p-
character determined in the combined 4f-orbitals remained
relatively constant (Figure 14 and Table 2). The DFT-
determined amount of Cl 3p-character per bond in the 4f-
orbitals fluctuated between 1.1 and 3.8% for Ce , Nd , Sm ,
and Gd , while the calculated value for Eu 4f was increased
to 6.4%.
2g
peaks and confirm that Ln 5d- and Cl 3p-orbital mixing
III
III
III
III
III
decreased from Ce to Gd .
III
III
6
Changing from CeCl6 to CeCl62− had a surprising
influence on Ce−Cl covalent mixing. Increasing the Ce
oxidation statein essence“turned on” Ce 4f- and Cl 3p-
orbital mixing, such that both experiment and theory showed
that the amount of Cl 3p-character in the t * + t * (4f)
3−
III
While the amount of Ln −Cl mixing was experimentally and
computationally found to be small, oxidizing Ce to Ce
III
IV
showed unambiguous evidence for pronounced Ce 4f- and Cl
p-orbital mixing. For instance, the amount of Cl 3p-character
2
u
1u
3
orbitals was comparable to that in the t * or e * (5d) orbitals.
2g g
per Ce−Cl bond in the t and t2u orbitals was experimentally
determined to be 9.9(9)%, which was similar to the 15% value
Although we have not characterized the generality of this
observation, it seems likely that this is not an isolated case, and
we assume that 4f-orbital participation in covalent bonding
occurs in other systems. In transition metal chemistry, a similar
observation has been made for high-temperature super-
1
u
determined by DFT. The pronounced Ce−Cl orbital mixing
2
−
IV
observed in CeCl
(formally Ce ) is reasonable given the
6
IV
uniqueness of Ce in comparison to the rest of the trivalent
IV
142
II
9
lanthanides; e.g., the atomic configuration for Ce is
Xe]4f 5d 6s . These values were comparable to previous
conducting cuprate materials.
In this case the Cu
d
1
1
2
[
ground state is reasonably ionic in character; however, upon
III
measurements made on transition metal and actinide
hole doping, the effective “Cu ” site has significantly increased
IV
II
hexahalides and the Ce −Cl uniqueness has been addressed
hybridization with O 2p orbitals. Once again, Cu is more ionic
III
in more detail in the following section. Despite the relatively
good agreement between experiment and theory for all of the
Ln−Cl interactions probed here, in general, the DFT
calculations slightly overestimated the amount of Ln−Cl orbital
mixing for 4f-orbitals and underestimated the participation of
the 5d-orbitals. Previous studies on actinide 5f- and Cl 3p-
than Cu due to the rather large U parameter associated with
dd
10
II
the dd repulsion in the LMCT final state (Ld ) in Cu . The
IV
III
striking difference in covalent Ce −Cl vs Ce −Cl bonding
could not be predicted on the basis of the structural metrics
obtained from the single-crystal XRD measurements. Hence,
these results clearly caution against using Shannon ionic radii
and structural metrics to infer conclusions regarding electronic
structure and covalent bonding.
6
,7
orbital mixing provided similar results.
OUTLOOK AND CONCLUDING REMARKS
2−, the Cl K-edge XAS results and the X-ray
■
For CeCl
6
These results have shown that Cl K-edge XAS can be used to
evaluate both Ln 4f- and 5d-orbital mixing with Cl 3p-orbitals
in Ln−Cl bonds. Weak pre-edge features associated with
transitions to π and σ + π orbitals of primarily 4f-character (t *
absorption spectra collected at the Ce L3,2- and M5,4-edges were
all internally consistent. Overall these data showed explicit
evidence for substantial Ce−Cl orbital mixing. The analyses of
2−
CeCl
at the Ce L3,2-edge exhibited the double white line
1
u
6
and t * in O -symmetry) were observed by Cl K-edge XAS and
absorptions, and the Ce M5,4-edge XAS data contained satellite
features similar to those observed from CeO . While still
2
u
h
TDDFT that suggested only marginal involvement of the 4f-
2
III
III
III
III
III
orbitals in Ln −Cl bonding (Ln = Ce , Nd , Sm , Eu ,
heavily debated, these types of spectra have been interpreted as
III
Gd ). Both theory and experiment indicated that moving
being diagnostic of a multiconfigurational ground
III
III
5,24,42,74−93
across the lanthanide series (from Ce to Gd ) dropped the
overall energy of the 4f-manifold, while the amount of Ln 4f-
and Cl 3p-orbital mixing remained small and relatively
constant. In contrast, we observed appreciable mixing between
the Ln 5d- and Cl 3p-orbitals, as was evident from the
measured and calculated amounts of Cl 3p-character in the π-
bonding t * orbitals, which ranged from 6.8(7) to 13(1)% by
state.
It is interesting that these types of spectr−a
can be observed for Ce complexes containing simple Cl
ligands. However, our CI calculations conducted using
CTM4XAS provided a good description of the CeCl6 M -
2−
5,4
edge data, associating it primarily with a multiconfigurational
final state. Additional work on this interesting problem is
certainly needed here.
For f-elements, understanding the roles of the frontier f- and
d-orbitals has represented a longstanding challenge in both
2
g
Cl K-edge XAS and from 3.3 to 4.3% by DFT. Although the σ-
bonding e * orbitals could not be probed experimentally, the
g
L
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Journal of the American Chemical Society
Article
rare-earth and actinide chemistry. For lanthanides, however, the
option for covalent bonding is often dismissed, and the
lanthanide−ligand interaction is regarded as being largely ionic.
Here, we have provided experimental and theoretical results
that indicate both the 4f- and 5d-orbitals should be considered
when evaluating Ln−Cl bonding. Moreover, these results
provided an opportunity to compare orbital mixing with
transition metal and actinide halides that have previously been
analyzed using Cl K-edge XAS. Previous Cl K-edge XAS results
4f/5f- and 5d/6d-orbital mixing vary as both the lanthanide and
actinide series are traversed.
EXPERIMENTAL SECTION
■
General Considerations. Except where noted, the syntheses and
manipulation of all compounds reported herein were conducted under
helium or argon atmospheres with rigorous exclusion of air and water
by standard Schlenk, glovebox, and glovebag techniques. The
(
NEt ) CeCl trivalent lanthanide hexahalide salts, Z LnCl (Ln =
4
2
6
3
6
2
−
Nd, Sm, Eu, Gd and Z = NEt
; Ln = Ce and Z = PPh ), were prepared
4 4
from octahedral MCl6 hexahalides (M = Ti, Zr, Hf, U) are
using synthetic procedures that deviated slightly from those previously
2−
compared with the CeCl6 measurements described here in
Figure 15. For the tetravalent metals, these comparisons
69−73
reported.
Prior to XAS analyses, the hexahalide salts were
crystallized and characterized by single-crystal XRD. Acetonitrile and
diethyl ether (anhydrous, Aldrich) were distilled from calcium hydride
or sodium/benzophenone, respectively, and degassed by three freeze−
pump−thaw cycles. The D -Cs CuCl intensity and energy calibration
2
d
2
4
4
6,48,144−146
147
standard
was prepared as previously described.
Single
crystals were mounted in a nylon cryoloop with Paratone-N oil.
Crystallographic 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
148
and cell refinement were handled using APEX II software. Frame
integration, including Lorentz-polarization corrections, and final cell
149
parameter calculations were carried out using SAINT+ software.
Figure 15. Comparison of the amount of Cl 3p-character per M−Cl
The data were corrected for absorption using redundant reflections
1
50
bond (%) experimentally determined using Cl K-edge XAS for O -
and the SADABS program. 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.
The final refinement included anisotropic temperature factors on all
non-hydrogen atoms. Structure solution and refinement were
h
CeCl63 and O -M Cl
−
IV
2−
(M = U,7 Ce, Hf,7 Zr, Ti ). For the
7
7
h
6
octahedral compounds, the amount of Cl 3p-character (%) in the t2g
orbitals (aqua bars) and in the f-orbitals (pink bars) is shown.
1
51
performed using SHELXTL.
showed an increase in M−Cl π-orbital mixing (t *) as the
2
g
IV
IV
Bis(tetraethylammonium) Cerium(IV) Hexachloride,
periodic table was descended from Ti (3d) to Zr (4d), then
(
NEt ) CeCl . In a fume hood under an atmosphere of air, an
IV
IV
IV
4 2
6
to Hf (5d) and Ce (5d), and ending with U (6d). To
intimate mixture of NEt Cl (1.996 g, 12 mmol) and Ce(SO ) ·xH O
4
4
2
2
better understand these trends, first-order perturbation theory
(
2.000 g, 6 mmol) was dissolved in cold HClaq (12 M; 10 mL).
7
,118,143
has been considered, as described previously.
The
Subsequently, a second aliquot of NEt Cl (0.020 g, 0.12 mmol) was
4
theory states that orbital mixing is directly proportional to
spatial overlap of the atomic orbitals and is inversely
proportional to the energy difference between the two atomic
orbitals. In this context, it is likely that as the quantum number
increased for the metal valence orbitals (from 3d to 4d, 5d, and
added, and the solution was cooled to 5 °C. The yellow precipitate
that formed was isolated by vacuum filtration, dried, and washed with
acetone (20 mL). The solid was dissolved in hot acetonitrile. After
slow cooling, single-crystals formed, and (NEt ) CeCl was isolated in
4
2
6
2
1% crystalline yield. Prior to XAS analysis, the crystals were
characterized by single-crystal XRD. Diffraction. Anal. Calcd. For
6
d), more d-orbital covalency resulted from improved spatial
C H Cl CeN ; C, 31.47; H, 6.61; N, 4.59; Cl, 34.39. Found, C,
16
40
6
2
overlap of the increasingly diffuse d-orbitals.
3
1.52; H, 6.60; N, 4.51; Cl, 34.48.
Tris(tetraphenylphosphonium) Cerium(III) Hexachloride,
2−
Surprisingly, in the MCl6 framework, we observed the 4f-
2−
orbital participation in CeCl6 to be more than twice that
(PPh ) CeCl . In an argon-filled glovebox, PPh Cl (0.454 g, 0.12
4
3
6
4
observed for the more diffuse 5f-orbitals in UCl62 , as was
−
mmol) dissolved in acetonitrile (5 mL) was added to a vial charged
with CeCl (0.100 g, 0.4 mmol) suspended in acetonitrile (10 mL).
2−
evident from the amount of Cl 3p-character for CeCl
vs
3
6
UCl62 , 9.9(9) vs 5.7(6)%, respectively. In this case, we
−
7
The solution was heated to 50 °C for several hours. After cooling to
room temperature the solution was filtered and the volume was
reduced to 10 mL in vacuo. Approximately 5 mL of diethyl ether was
allowed to diffuse at room temperature and after several days single-
crystals of (PPh ) CeCl were obtained in 81% crystalline yield. Prior
2−
attributed the increase in CeCl6 4f-orbital mixing to the lower
energy of the Ce 4f- vs U 5f-orbitals. Another example where
near-degeneracy seems to influence covalency in bonding to a
larger extent than orbital spatial overlap is borne out by
comparing the appreciable Ce 4f- and Cl 3p-orbital mixing in
4
3
6
to XAS analysis, the crystals were characterized by single-crystal XRD.
Diffraction. Anal. Calcd. For C H Cl CeP ; C, 63.20; H, 4.42; P,
72
60
6
3
CeCl62 (formally Ce ) to CeCl (formally Ce ), where 4f-
−
IV
3−
III
6
6.80; Cl, 15.35. Found, C, 61.73; H, 4.57; P, 6.55; Cl, 15.11.
orbital participation in covalent bonding was negligible. Overall,
these comparisons highlight the complex interplay between
orbital spatial overlap and energy near-degeneracy-driven
covalency in transition metal, lanthanide, and actinide elements.
Current efforts are focused on better understanding how these
two variables influence orbital mixing across the periodic table
and as a function of formal oxidation state. Our efforts in doing
so will be heavily supported by attempts to extend this work to
the transuranic actinides and comparatively evaluate how both
Tris(tetraethylammonium) Neodymium Hexachloride,
4 3 6 4
mmol) dissolved in acetonitrile (5 mL) was added to a vial charged
(
NEt ) NdCl . In an argon-filled glovebox, NEt Cl (0.660 g, 4
with NdCl (0.333 g, 1.3 mmol) suspended in acetonitrile (10 mL).
3
The solution was heated to 50 °C for several hours. After cooling to
room temperature a microcrystalline solid precipitated. The supernate
was removed and the solid washed with small fractions of acetonitrile.
The solid was dissolved in approximately 3 mL of boiling acetonitrile
and slowly cooled to room temperature. After several days single-
crystals of (NEt ) NdCl were obtained in 76% crystalline yield. Prior
4
3
6
M
DOI: 10.1021/ja510067v
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
to XAS analysis, the crystals were characterized by single-crystal XRD.
Anal. Calcd. For C H Cl NdN ; C, 38.80; H, 8.15; N, 5.66. Found,
C, 39.10; H, 8.46; N, 5.41.
circularly polarized radiation. For these measurements, the X-ray beam
was focused with a zone plate onto the sample, and the transmitted X-
rays were detected. Images at a single energy were obtained by raster-
scanning the sample and collecting transmitted monochromatic X-rays
as a function of sample position. Spectra at each image pixel or
particular regions of interest in the sample image were extracted from
the “stack,” which is a collection of images recorded at multiple, closely
spaced photon energies across the absorption edge. Dwell times used
to acquire an image at a single photon energy were ∼1 ms per pixel.
To quantify the absorbance signal, the measured transmitted intensity
(I) was converted to optical density using the Beer−Lambert law: OD
= ln(I/I ) = μρd, where I is the incident photon flux intensity, d is the
24
60
6
3
Tris(tetraethylammonium) Samarium Hexachloride,
(
NEt ) SmCl . Single crystals of (NEt ) SmCl were obtained from
4
3
6
4
3
6
NEt Cl (0.600 g, 3.6 mmol) and SmCl (0.313 g, 1.2 mmol) as
4
3
described above for (NEt ) NdCl in 61% crystalline yield. Prior to
4
3
6
XAS analysis, the crystals were characterized by single-crystal XRD.
Anal. Calcd. For C H Cl SmN ; C, 38.29; H, 8.04; N, 5.58. Found,
24
60
6
3
C, 38.53; H, 8.27; N, 5.42.
Tris(tetraethylammonium) Europium Hexachloride, (NE-
t ) EuCl . The (NEt ) EuCl complex was synthesized from NEt Cl
4
3
6
4
3
6
4
0
0
(
(
0.600 g, 3.6 mmol) and EuCl (0.321 g, 1.2 mmol) in analogy to the
3
sample thickness, and μ and ρ are the mass absorption coefficients and
density of the sample material, respectively. Incident beam intensity
was measured through the sample-free region of the Si N windows.
NEt ) NdCl synthesis described above. Single crystals were grown
4
3
6
by allowing diethyl ether to slowly diffuse into a saturated acetonitrile
3
4
solution, such that (NEt ) EuCl was obtained as single-crystals in
Regions of particles with absorption of >1.5 OD were omitted to
ensure the spectra were in the linear regime of the Beer−Lambert law.
During the STXM experiment, particles of CeCl63 showed no sign of
radiation damage and each spectrum was reproduced several times on
independent particles and unique samples. Accurate and reproducible
4
3
6
82% crystalline yield. Prior to XAS analysis, the crystals were
−
characterized by single-crystal XRD. Anal. Calcd. For C H Cl EuN ;
24
60
6
3
C, 38.24; H, 8.03; N, 5.58. Found, C, 38.76; H, 8.31; N, 6.08.
Tris(tetraethylammonium) Gadolinium Hexachloride,
(
NEt ) GdCl . Single crystals of (NEt ) GdCl were obtained from
2−
4
3
6
4
3
6
spectra of CeCl6 were obtained by averaging data obtained from a
NEt Cl (0.600 g, 3.6 mmol) and GdCl (0.316 g, 1.2 mmol) as
4
3
large sample area comprised of more than 50 particles, owing to
radiation damage during long acquisition times on small particles. The
data were fit with a first-order polynomial in the pre-edge region below
described above for (NEt ) NdCl . The (NEt ) GdCl salt was
4
3
6
4
3
6
obtained in 93% crystalline yield. Prior to XAS analysis, the crystals
were characterized by single-crystal XRD. Anal. Calcd. For
the M -edge, then subtracted from the experimental data to eliminate
5
C H Cl GdN ; C, 37.98; H, 7.98; N, 5.54. Found, C, 38.33; H,
24
60
6
3
the background of the spectrum. Second-derivative spectra were used
as guides to determine the number and position of peaks, and the area
under the M - and M -edges was determined by integration. Both of
8
.38; N, 5.65.
Ce L3,2-Edge XAFS. Samples of (PPh ) CeCl and (NEt ) CeCl
6
4
3
6
4
2
5
4
for the Ce L -edge X-ray measurements were prepared under an
3,2
these operations were conducted using the program IGOR 6.0.
Cl K-Edge XAS Sample Preparation. Solid-state XAS samples
were prepared in an argon-filled glovebox and mounted in an
aluminum sample holder. The holder was an aluminum plate with 5 ×
argon atmosphere. The samples were diluted with boron nitride, which
−3
was dried at elevated temperature (200 °C) under vacuum (10 Torr)
for 48 h prior to use. A mixture of the analyte and BN were weighed
out, such that the edge jump for the absorbing atom was calculated to
be at 1 absorption length in transmission. Samples were loaded into a
sample well equipped with Kapton windows. The samples were
removed from the glovebox and immediately submerged into liquid
N . These samples were analyzed in a He cryostat at variable
2
0 × 1 mm slots. Single-sided tape that had been confirmed to contain
low concentrations of Cl by Cl K-edge XAS spectroscopy was fixed to
the back side of the holder. The analyte was prepared by finely
grinding the sample (ca. 10−20 mg) for 2 min in polystyrene canisters
with Plexiglas pestles using a Wig-L-Bug grinder to obtain a
homogeneous fine powder. Samples were painted onto the single-
sided tape using paintbrushes. The sample holder was sealed by
adhering a 4 μm polypropylene window to the front side of the sample
holder with double-sided tape. All samples were sealed in Mylar bags,
removed from the glovebox, and shipped. Once unpackaged, the
samples were immediately attached to the sample translator and
inserted into a sample chamber flushed with He gas.
Cl K-Edge XAS Measurements. All Cl K-edge XAS data were
either collected at the beamlines 6-2 (SSRL), 4-3 (SSRL), or the INE
beamline for Actinide Research at ANKA. The Cl K-edge XAS
measurements made at SSRL were conducted under dedicated
operating conditions (3.0 GeV, 500 mA) at beamline 4-3. This
beamline was equipped with a 20-pole, 2.0 T wiggler, utilized a liquid-
2
liq
temperatures (100 to 6 K). The cryostat was attached to the beamline
1-2 XAFS rail (SSRL), which was equipped with three ionization
1
chambers through which nitrogen gas was continually flowed. One
chamber was positioned before the cryostat to monitor the incident
radiation (I ). The second chamber was positioned after the cryostat
0
so that sample transmission (I ) could be evaluated against I and so
1
0
that the absorption coefficient (μ) could be calculated as ln(I /I ). The
0
1
third chamber (I ) was positioned downstream from I so that the
2
1
XANES of a calibration foil could be measured against I . Samples
1
were calibrated to the energy of the first inflection point of the K-edge
of a chromium foil (5989 eV), whose spectrum was measured before
and after each analyte spectrum.
The X-ray absorption spectra (XAS) and extended X-ray near edge
fine structure (EXAFS) were measured at the Stanford Synchrotron
Radiation Lightsource (SSRL) under dedicated operating conditions
N -cooled double-crystal Si[111] monochromator and employed flat,
2
bent vertically collimating and focusing mirrors (fwhm beam spot size
=3 × 16 mm). At ANKA, operating conditions of 2.5 GeV and 120
mA were used. The beamline was equipped with a 1.5 T bending
magnet and a water-cooled Si[111] double-crystal monochromator.
The crystals were detuned by 30% from the rocking curve
(3.0 GeV, 5%, 500 mA using continuous top-off injections) on end
station 11-2. This beamline, which was equipped with a 26-pole, 2.0 T
wiggler, utilized a liquid nitrogen-cooled double-crystal Si[220]
monochromator and employed collimating and focusing mirrors. A
single energy was selected from the white beam with a liquid-N2-
cooled double-crystal monochromator utilizing Si[220] (φ = 0)
crystals. Harmonic rejection was achieved by detuning the second
crystal of the monochromator by 70% at 7000 eV. The horizontal and
vertical slit sizes were 2.5 and 1 mm, respectively.
1
55
maximum.
Two mirrors were utilized, the first mirror vertically
collimating and the second mirror vertically and horizontally focusing.
At both facilities, samples were analyzed within a chamber that was
separated from the beam pipe by a 4 μm polypropylene window and
continually flushed with He gas. An additional window of the same
material separated the sample from a Vortex Si fluorescence detector
(4-element, SSRL; 1-element, ANKA) that was positioned at 90° from
the incident beam and sufficiently far from the sample to ensure dead
times that were less than 3%. Sample fluorescence was measured using
the Vortex detector against the incident radiation (I0), which was
monitored using an ionization chamber through which He was
continually flowed. The ion chamber was isolated from the sample
chamber with a 4 μm polypropylene window. Spectra were collected
with dwell times of 1−3 s. Dead time corrections to all experimental
Ce M5,4-Edge XAS Measurements. Samples for STXM measure-
ments were encapsulated between two 100 nm Si N membranes
3
4
6
0,100,152,153
(
Silson), as described previously.
Single-energy images and
Ln M -edge XAS spectra were acquired using the STXM instrument
5,4
at the Molecular Environmental Science Beamline 11.0.2 of the
Advanced Light Source, which is operated in topoff mode at 500 mA,
154
in a ∼0.5 atm He-filled chamber.
performed at the Ne K-edge for Ne (867.3 eV). The energy resolution
fwhm) was estimated at 0.2 eV, and spectra were collected using
Energy calibrations were
(
N
DOI: 10.1021/ja510067v
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
scans collected at SSRL were achieved by measuring fluorescence
intensities of the Vortex elements vs the total counts and opening the
horizontal slit (starting at 0.1 mm slit width) in steps of 0.1 mm until
full saturation was achieved. Detector dead times were evaluated by
obtaining a response curve for the single channel analyzer (SCA) vs
the incoming count rate (ICR) of the detector. The SCA-to-ICR
CTM4XAS Multiplet Calculations. Multiplet calculations were
implemented using CTM4XAS, which is a program based on the
1
11
112,113
original code by Cowan and further developed by de Groot.
Effects of the crystal field are typically minimal in f-sytems, so they
were not included.
1
11
III
For Ce , the graphical user interface was
implemented with Fff at 79%, SOC at 98%, and Fdf and Gdf at 100% of
atomic values. A Gaussian broadening of 0.25 eV was applied to
account for instrumental broadening and Lorentzian broadenings of
−rt
relationship was modeled assuming that r = kr e , where t is the dead
1
time, r is the SCA count rate, r is the ICR count rate, and k is a
1
0
.2 and 0.4 eV were applied to the M - and M -edges, respectively.
proportionality constant. These analyses showed dead time contribu-
tions of less than 3%.
5 4
Charge transfer for f-block systems is not implemented in the GUI
IV
interface of the program, so all calculations for Ce were run from the
Cl K-Edge XAS Data Analysis. Data manipulation and analysis
4
8
command line (sample input files may be found in the Supporting
was conducted as previously described by Solomon and co-workers.
Energy calibrations were conducted externally using the maxima of the
IV
Information). For Ce , F was left at atomic values, F reduced to
ff
fd
48
4
0%, SOC reduced to 99%, and G reduced to 80% of atomic values.
first pre-edge features in Cs CuCl4 (2820.02 eV). Data were
fd
2
9
1
For the 3d 4f configuration, these reductions result in values of F =
analyzed by fitting a line to the pre-edge region, 2702.1−2815.0 eV,
which was subsequently subtracted from the experimental data to
eliminate the background of the spectrum. The data were normalized
by fitting a first-order polynomial to the post-edge region of the
spectrum (2836−3030 eV) and setting the edge jump at 2836 eV to an
intensity of 1.0. This normalization procedure gave spectra normalized
to a single Cl atom or M−Cl bond. A deconvoluted model for the Cl
K-edge XAS data was obtained using a modified version of
fd
2
Δ
.622 eV, SOC = 7.293 eV, and G = 3.621 eV. Δ was set to 2.5 eV,
fd
IS
9
2
= −1.5 eV, and T = T = 0.7 eV. For the 3d L4f configuration,
these reductions result in values of F = 10.102 eV, F = 2.396 eV,
FS
IS
FS
ff
fd
SOC = 7.297 eV, and G = 3.247 eV. Δ was set to 2.5 eV, Δ = −1.5
fd
IS
FS
eV, and T = T = 0.7 eV. A Gaussian broadening of 0.25 eV was
IS
FS
applied to account for instrumental broadening and Lorentzian
broadenings of 0.4 and 0.6 eV were applied to the M - and M -edges,
5
4
156
respectively.
EDG_FIT
in IGOR 6.0. The pre-edge regions (<2824.8 eV)
TDDFT Calculations. For the LnCl 3 and CeCl62− anions, the Cl
−
6
were best modeled by two symmetry-constrained pseudo-Voigt line
shapes. Approximate peak positions were determined using first and
second derivatives of each spectrum. The edge regions were modeled
with two pseudo-Voigt functions (for the edge peak) and an additional
function consisting of a 1:1 ratio of arctangent and error function
contributions, which was used to model the absorption threshold.
Deconvoluted spectral models were performed over several energy
ranges. To facilitate comparison with previously reported Cl K-edge
K-edge XAS spectra were simulated using TDDFT calculations, which
6−8,57−60,66−68,133−135
were conducted as previously described.
Specif-
ically, this analysis involved a linear response calculation for extracting
the transition amplitudes from the transition densities and dipole
moments between the calculated excited states and the ground-states.
The excitations originating from all of the intermediate states between
the Cl 1s-orbitals and the highest occupied molecular orbital were
excluded so that only excitations from the core levels to virtual orbitals
could be analyzed. This allowed the virtual orbitals to relax under the
influence of the Cl core−hole, but not the occupied orbitals. Although
excluding relaxations in the occupied space resulted in large errors
associated with absolute calculated transition energies, the relative
energies addressed the first-order changes in virtual orbitals
7
XAS spectra for (PPh ) MCl (M = Ti, Zr, Hf, U), the energy
4
2
6
position for the step function was optimized near the value used
previously. The areas under the pre-edge peaks (hereafter defined as
the intensity) are equal to the fwhm × peak height. Spectra were
obtained in triplicate, with unique samples of the lanthanide
hexahalides being analyzed during four different synchrotron
scheduling periods to establish reproducibility and evaluate error
associated with the measurements.
137,138
accompanying the core−hole excitation. As discussed previously,
an energy shift was established to account for the omission of the
atomic and extra-atomic relaxation associated with the core excitation,
relativistic stabilization, and errors associated with the functional. This
was achieved by setting the energy of transitions simulated for the 6d-
orbitals to be equal to those in the experimental spectra, which
resulted in an energy shift of +64 eV for all simulated spectra of all
treated compounds, respectively.
Ground-State DFT Calculations. Ground-state electronic
3
−
structure calculations were performed on the LnCl (Ln = Ce, Pr,
6
2
−
Nd, Pm, Sm, Eu, Gd) and CeCl complexes using B3LYP hybrid
6
1
57,158
159
3−
DFT)
in the Gaussian 09 code. For all LnCl6 complexes, an
unrestricted calculation was employed. In this case the two density
matrices can differ because the SCF is carried out separately for the α
and β spins, which can result in different orbitals for α and β (referred
to as α and β spin−orbitals). The Stuttgart 97 RECP and associated
basis sets (without the most diffuse function) were used for all
ASSOCIATED CONTENT
Supporting Information
Background-subtracted and normalized X-ray absorption data
and analyses, complete ref 159, XRD data for the LnCl6
■
*
S
160−162
metals.
Chlorine was modeled using a Pople style double-ζ 6-
3
−
3
1G(d′) basis set with polarization functions optimized for heavy
1
63,164
atoms.
Spin−orbit interactions were not included in the
compounds, and CTM4XAS sample input files. This material is
available free of charge via the Internet at http://pubs.acs.org.
The structures for the LnCl63 compounds were additionally
submitted to the Cambridge Structural Database (CCDC Nos.
calculations. These functionals and basis sets have been extensively
tested for organometallic and inorganic systems and shown to give
good agreement with experimental data.
−
6
−8,57−60,66−68,133−135
The
populations for the Cl 3p-orbitals were then obtained by Mulliken
population analyses in each orbital. For comparison with experimental
values, calculated values of the % Cl 3p-character in each orbital were
normalized to per-bond values by multiplying the total Cl 3p-character
by the sum of the squares of the normalization constants for the
corresponding ligand−orbital wave functions. For example, the 3.9%
Cl 3p-character found in the triply degenerate set of t orbitals for
1
021660−1021664).
AUTHOR INFORMATION
■
Corresponding Authors
erb@lanl.gov
*
*stosh@lanl.gov
2
g
CeCl62 was corrected by (1/2)2 + (1/2)2 + (1/2) = 3/4 in
−
2
*rlmartin@lanl.gov
accordance with the mathematical representation for the d , d , and
xy
xz
Notes
1
1
dyz metal orbitals, which are / (x + y + y + x ), / (y + x + x +
2
1
2
3
4
2
1
5
3
The authors declare no competing financial interest.
1
7,66
y ), and / (x + y + y + x ), respectively.
Where a given orbital
6
2
2
5
4
6
was found to have both σ- and π-bonding character (e.g., the t1u 5f-
orbitals), the σ- and π-bonding components were normalized
independently first and then summed together to provide the
calculated per-bond values in Tables 1 and 2.
ACKNOWLEDGMENTS
We are grateful for the comments provided by the reviewers of
this paper, especially the anonymous reviewer that encouraged
■
O
DOI: 10.1021/ja510067v
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
us to conduct the CTM4XAS calculations. We also thank Prof.
Frank M. F. de Groot for his help with the CTM4XAS
computational effort, and Prof. Eric Schelter for providing
insightful discussions in regard to this research. The work was
supported under the Heavy Element Chemistry Program at
LANL by the Division of Chemical Sciences, Geosciences, and
Biosciences, Office of Basic Energy Sciences, U.S. Department
of Energy (Batista, Boland, Conradson, Clark, Kozimor, Martin,
Scott, Wilkerson, Zehnder), and at LBNL (Minasian, Shuh) by
the Director, Office of Science, Office of Basic Energy Sciences,
Division of Chemical Sciences, Geosciences, and Biosciences
Heavy Element Chemistry Program of the U.S. Department of
Energy (contract DE-AC02-05CH11231). Portions of this
research were carried out at the Stanford Synchrotron
Radiation Lightsource, a Directorate of SLAC National
Accelerator Laboratory and an Office of Science User Facility
operated for the U.S. Department of Energy Office of Science
by Stanford University. We acknowledge the Synchrotron Light
Source Angstroemquelle Karlsruhe (ANKA) for provision of
instruments at their INE-Beamline. We are also grateful for
experimental time at the Advanced Light Source which is
supported by the Director, Office of Science, Office of Basic
Energy Sciences (as is Tyliszczak); the Molecular Environ-
mental Science Beamline 11.02 that is supported by the
Director, Office of Science, Office of Basic Energy Sciences,
Division of Chemical Sciences, Geosciences, and Biosciences
Heavy Element Chemistry and Condensed Phase and
Interfacial Molecular Sciences programs; all of the U.S.
Department of Energy at Lawrence Berkeley National
Laboratory under Contract No. DE-AC02-05CH11231.
Portions of this work were supported at LBNL by a Berkeley
Actinide Postdoctoral Fellowship, at LANL by a Glenn T.
Seaborg Institute Postdoctoral Fellowship (Minasian, Olson,
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