Synthesis and structure of (Ph4P)2MCl6 (M = Ti, Zr, Hf, Th, U, Np, Pu)

Article abstract of DOI:10.1021/ic300179d

High-purity syntheses are reported for a series of first, second, and third row transition metal and actinide hexahalide compounds with equivalent, noncoordinating countercations: (Ph₄P)₂TiF₆ (1) and (Ph₄P)₂MCl₆ (M = Ti, Zr, Hf, Th, U, Np, Pu; 2-8). While a reaction between MCl₄ (M = Zr, Hf, U) and 2 equiv of Ph₄PCl provided 3, 4, and 6, syntheses for 1, 2, 5, 7, and 8 required multistep procedures. For example, a cation exchange reaction with Ph₄PCl and (NH₄)₂TiF₆ produced 1, which was used in a subsequent anion exchange reaction with Me₃SiCl to synthesize 2. For 5, 7, and 8, synthetic routes starting with aqueous actinide precursors were developed that circumvented any need for anhydrous Th, Np, or Pu starting materials. The solid-state geometries, bond distances and angles for isolated ThCl₆^2-, NpCl₆ ^2-, and PuCl₆^2- anions with noncoordinating counter cations were determined for the first time in the X-ray crystal structures of 5, 7, and 8. Solution phase and solid-state diffuse reflectance spectra were also used to characterize 7 and 8. Transition metal MCl ₆^2- anions showed the anticipated increase in M-Cl bond distances when changing from M = Ti to Zr, and then a decrease from Zr to Hf. The M-Cl bond distances also decreased from M = Th to U, Np, and Pu. Ionic radii can be used to predict average M-Cl bond distances with reasonable accuracy, which supports a principally ionic model of bonding for each of the (Ph ₄P)₂MCl₆ complexes.

Full text of DOI:10.1021/ic300179d

Article
pubs.acs.org/IC
Synthesis and Structure of (Ph4P)2MCl6 (M = Ti, Zr, Hf, Th, U, Np, Pu)
Stefan G. Minasian,†,‡ Kevin S. Boland,† Russell K. Feller,§ Andrew J. Gaunt,† Stosh A. Kozimor,*,†
Iain May,† Sean D. Reilly,† Brian L. Scott,§ and David K. Shuh‡
§
†Inorganic, Isotope and Actinide Chemistry Group and Materials Chemistry Group, Los Alamos National Laboratory, Los Alamos,
New Mexico 87545, United States
‡Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: High-purity syntheses are reported for a series
of first, second, and third row transition metal and actinide
hexahalide compounds with equivalent, noncoordinating
countercations: (Ph4P)2TiF6 (1) and (Ph4P)2MCl6 (M = Ti,
Zr, Hf, Th, U, Np, Pu; 2−8). While a reaction between MCl4
(M = Zr, Hf, U) and 2 equiv of Ph4PCl provided 3, 4, and 6,
syntheses for 1, 2, 5, 7, and 8 required multistep procedures.
For example, a cation exchange reaction with Ph4PCl and
(NH4)2TiF6 produced 1, which was used in a subsequent
anion exchange reaction with Me3SiCl to synthesize 2. For 5,
7, and 8, synthetic routes starting with aqueous actinide
precursors were developed that circumvented any need for anhydrous Th, Np, or Pu starting materials. The solid-state
geometries, bond distances and angles for isolated ThCl62−, NpCl62−, and PuCl62− anions with noncoordinating counter cations
were determined for the first time in the X-ray crystal structures of 5, 7, and 8. Solution phase and solid-state diffuse reflectance
spectra were also used to characterize 7 and 8. Transition metal MCl62− anions showed the anticipated increase in M−Cl bond
distances when changing from M = Ti to Zr, and then a decrease from Zr to Hf. The M−Cl bond distances also decreased from
M = Th to U, Np, and Pu. Ionic radii can be used to predict average M−Cl bond distances with reasonable accuracy, which
supports a principally ionic model of bonding for each of the (Ph4P)2MCl6 complexes.
environment provides a well-defined platform from which
actinide electronic structure and bonding can be compared with
that of the main group, transition metal, and lanthanide
elements. As part of a recent effort to better understand trends
in M−Cl orbital mixing as a function of 3d, 4d, 5d, 5f, and 6d
orbital energies and occupancies, transition metal and actinide
INTRODUCTION
■
Recent advances in actinide synthesis1−16 are correlated with
the availability of convenient preparations for actinide starting
materials. For molecular systems, actinide halides have emerged
as the most attractive reagents for anhydrous synthesis17−31 and
have also served as analytes for both spectroscopic and
theoretical studies.32−35 The AnCl4 (An = Th, U, Np)
compounds are well established starting materials for exploring
the anhydrous chemistry of the early actinides. While synthesis
and use of UCl4 is still widespread, the lack of commercial
sources for ThCl4 has prompted the search for alternative
entries into nonaqueous thorium chemistry.31 Using NpCl4 has
become less attractive because it requires significant laboratory
infrastructure to support the high temperature (ca. 200 °C) air
and moisture sensitive synthesis with hexachloropropene, and
associated production of large quantities of flammable organic
neptunium waste. Tetravalent PuCl4 has been observed in the
gas phase, but has not been established as a viable starting
2−
hexachlorides MCl6 (M = Ti, Zr, Hf, Th, U, Np, Pu) were
targeted for Cl K-edge X-ray absorption spectroscopy (XAS)
and time-dependent density functional theory (DFT) studies.40
Syntheses have been reported previously for numerous
2−
octahedral MCl6 salts with alkali metal or noncoordinating
tetraalkylammonium, phosphonium, or arsonium counter-
2−
cations (Table 1). However, high-purity syntheses for MCl6
complexes with equivalent noncoordinating countercations that
met all the spectroscopic requirements of Cl K-edge XAS had
not been developed. Of the known salts that had been
structurally characterized, the M−Cl bond distances varied
widely with different countercations (see Table 1 for crystallo-
graphically characterized examples). Reported here is the
synthesis and structural characterization of a comprehensive
material and is reported to decompose readily in the solid-
state.36 In this sense, actinide hexachloride dianions (MCl6
)
2−
2−
series of d and f block transition metal MCl6 dianions with
are promising alternatives to the tetrachlorides because they are
stable for all of the light actinides (M = Th to Pu).37−39
Octahedral binary chlorides are also attractive for systematic
spectroscopic and theoretical studies. They are known for most
metals in the periodic table, and their octahedral ligand
two common Ph4P+ countercations. Each compound was
Received: January 24, 2012
Published: May 3, 2012
© 2012 American Chemical Society
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2−
Table 1. Previously Published MCl6 Salts (M = Ti, Zr, Hf, Th, U, Np, Pu) with Alkali Metal or Tetraalkylpnictogenium
a
Cations and Average M−X Bond Distances Where Available
2−
2−
2−
2−
2−
2−
2−
cation
TiCl6
ZrCl6
HfCl6
ThCl6
UCl6
NpCl6
PuCl6
K+
2.35(3)75
2.33(5)76
2.35(5)76
2.3344(7)79
2.336(7)48
2.34(1)81
2.570(2)67
2.6799(7)67
2.7(2)68
Rb+
2.4(1)76
Cs+
2.5(1)76
2.81(6)64
2.621(4)77,78
R4N+
Ph4P+
2.452(9)80
2.46(1)44
2.452(9)80
2.44(2)49
2.60(2)47
2.611(3)82
b
Ph4As+
a
2−
Published examples of MX6 salts where M is not six coordinate are not included. Errors indicated in parentheses are equal to the standard
deviation in the average M−X bond distances. Where only one M−X bond distance is observed, the error is taken from the published estimated
b
standard deviation. These structures were reported with cocrystallized CH2Cl2 solvent molecules.
Table 2. Summary of Crystallographic Data Collection and Refinement for 1−4
(Ph4P)2TiF6·CH3CN
(Ph4P)2TiCl6·4CH3CN
(Ph4P)2ZrCl6·4CH3CN
(Ph4P)2HfCl6·4CH3CN
1
2
3
4
empirical formula
formula weight (g/mol)
T (K)
C50H43F6NP2Ti
881.69
C56H52Cl6N4P2Ti
1103.56
C56H52Cl6N4P2Zr
1146.88
C56H52Cl6N4P2Hf
1234.15
140(2)
140(2)
140(2)
140(2)
radiation
0.71073 Å
triclinic
0.71073 Å
monoclinic
P21/c
0.71073 Å
monoclinic
P21/c
0.71073 Å
monoclinic
P21/c
crystal system
space group
a (Å)
P1
̅
9.9307(16)
10.5367(13)
11.0318(16)
74.432(6)
72.6920(10)
79.405(3)
1054.8(3)
1
9.4866(16)
19.326(3)
14.839(3)
90
9.5282(12)
19.366(2)
14.9522(18)
90
9.5378(13)
19.356(3)
14.942(2)
90
b (Å)
c (Å)
alpha (deg)
beta (deg)
98.234(2)
90
98.159(2)
90
98.112(2)
90
gamma (deg)
volume (Å3)
Z
2692.6(8)
2
2731.2(6)
2
2730.9(6)
2
density (calcd) (g/cm3)
crystal color
absorption coefficient (mm−1
crystal size (mm)
1.388
1.361
1.395
1.501
colorless
yellow
colorless
0.594
colorless
2.303
)
0.342
0.557
0.20 × 0.18 × 0.16
1.99 to 25.31
456
0.18 × 0.10 × 0.06
2.11 to 26.73
1140
0.36 × 0.10 × 0.06
2.10 to 27.48
1176
0.30 × 0.04 × 0.04
2.10 to 25.30
1240
θ range for data collection (deg)
F(0 0 0)
no. reflns collected
10335
28139
28487
25846
no. of data/restrnts/params
goodness-of-fit on F2
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
3830/48/325
1.038
5694/0/315
1.036
6174/0/315
1.002
4963/0/315
1.003
0.0412, 0.0991
0.0514, 0.1058
0.0360, 0.0744
0.0549, 0.0823
0.0479, 0.0931
0.0919, 0.1085
0.0332, 0.0735
0.0509, 0.0804
150 °C for at least 12 h. Toluene (anhydrous, Fisher) and
tetrahydrofuran (THF, anhydrous, Fisher) were vacuum distilled
from potassium metal and benzophenone, and degassed by three
freeze−pump−thaw cycles. Acetonitrile (anhydrous, Fisher) was dried
over CaH2, degassed by three freeze−pump−thaw cycles, and vacuum
distilled before use. Ph4PCl (Aldrich) was exposed to vacuum (10−3
Torr) for 24 h and stored under He in a glovebox prior to use.
Me3SiCl (Aldrich) was transferred to a Teflon sealable Schlenk flask
for storage and used as received. (NH4)2TiF6 (American Elements),
ZrCl4 (anhydrous, Strem), and HfCl4 (anhydrous, Strem) were used as
received. UCl4 was prepared as previously described from UO3 and
hexachloropropene.17 Aqueous manipulations were performed with
>18 Ω water from a Millipore purification system. Electronic
absorption spectra were collected at room temperature using a Varian
Cary 6000i UV−vis−NIR spectrophotometer with a 0.2 nm spectral
bandwidth. Solid diffuse reflectance spectra were collected using a
Varian Cary 500 with an Internal Diffuse Reflectance Accessory.
Elemental analyses were carried out by Midwest MicroLab, LLC
(Indianapolis, IN), and infrared spectra were recorded on a Nicolet
Magna-IR System 750 spectrometer as KBr pellets.
obtained in high-purity as crystalline solids. In addition, the first
2−
X-ray crystal structures containing discrete ThCl62−, NpCl6
,
2−
and PuCl6 anions supported by noncoordinating cations are
presented. The metrical parameters obtained from single crystal
X-ray diffraction are evaluated as a function of the increasing
ionic radii of the metals, and the resulting implications for ionic
and covalent models of M−Cl bonding are discussed.
EXPERIMENTAL SECTION
■
General Considerations. Caution! The 232Th and 238U isotopes are
low specific-activity α-particle emitting radionuclides, and their use presents
hazards to human health. 237Np and 239Pu isotopes are high specific
activity α-particle emitting radionuclides, and their use presents extreme
hazards to human health. This research was conducted in a radiological
facility with appropriate analyses of these hazards and implementation of
controls for the safe handling and manipulation of these toxic and
radioactive materials.
Where noted, all manipulations were performed with rigorous
exclusion of air and moisture using Schlenk and glovebox techniques
under an atmosphere of argon or helium. All glassware was dried at
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Article
Table 3. Summary of Crystallographic Data Collection and Refinement for 5−8
(Ph4P)2ThCl6·4CH3CN
(Ph4P)2UCl6·4CH3CN
(Ph4P)2NpCl6
(Ph4P)2PuCl6·4CH3CN
5
6
7
8
empirical formula
formula weight (g/mol)
T (K)
C56H52Cl6N4P2Th
1287.7
C56H52Cl6N4P2U
1293.69
C48H40Cl6P2Np
1128.49
C56H52Cl6N4P2Pu
1294.72
140(1)
120(1)
141(2)
140(1)
radiation
0.71073 Å
monoclinic
P21/c
0.71073 Å
monoclinic
P21/c
0.71073 Å
triclinic
0.71073 Å
monoclinic
P21/c
crystal system
space group
a (Å)
P1
̅
9.6163(8)
19.4637(16)
15.1524(12)
90
9.5987(11)
19.413(2)
15.0841(17)
90
10.178(6)
10.316(6)
12.108(7)
93.067(7)
99.243(7)
116.144(6)
1115.3(11)
1
9.6574(5)
19.7997(10)
15.2266(8)
90
b (Å)
c (Å)
alpha (deg)
beta (deg)
98.0870(10)
90
98.085(1)
90
98.8540(10)
90
gamma (deg)
volume (Å3)
Z
2807.9(4)
2
2782.5(5)
2
2876.8(3)
2
density (calcd) (g/cm3)
crystal color
absorption coefficient (mm−1
crystal size (mm)
1.523
1.544
1.634
1.498
Colorless
3.037
Blue-Green
3.302
Colorless
2.758
Yellow
)
1.519
0.14 × 0.16 × 0.18
2.09 to 25.34
1276
0.22 × 0.10 × 0.08
2.10 to 28.37
1280
0.26 × 0.26 × 0.32
2.10 to 25.05
538
0.10 × 0.12 × 0.14
2.06 to 28.41
1284
θ range for data collection (deg)
F(0 0 0)
no. reflns collected
26950
31206
10535
32300
no. of data/restrnts/params
goodness-of-fit on F2
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
5136/0/315
1.24
6561/0/315
0.933
3924/0/259
1.096
6820/0/315
0.974
0.0287, 0.0417
0.0466, 0.0443
0.0202, 0.0526
0.0275, 0.0564
0.0288, 0.0911
0.0299, 0.0920
0.0263, 0.0542
0.0475, 0.0614
Crystallographic Details. Crystals of the Np and Pu-containing
compounds 7 and 8 were prepared with three appropriate layers of
containment prior to single crystal X-ray diffraction studies by
following modifications of the published procedures.15,41,42 Single
crystals were coated with epoxy and then mounted inside 0.5 mm
capillaries. The capillaries were sealed with additional epoxy, and their
external surfaces were coated with a thin film of acrylic dissolved in
ethyl acetate (Hard as Nails nail polish). Otherwise, single crystals of
1−6 were mounted on nylon loops with Paratone-N oil (Hampton
Research) and immediately cooled to 140 K in a cold nitrogen gas
stream on a Bruker CCD platform diffractometer. Standard peak
search and indexing procedures, followed by least-squares refinement
yielded the cell dimensions given in Tables 2 and 3. The measured
intensities were reduced to structure factor amplitude, and their
estimated standard deviations by correction for background and
Lorentz and polarization effects. No corrections for crystal decay were
necessary. Systematically absent reflections were deleted and symmetry
equivalent reflections were averaged to yield the set of unique data. All
unique data were used in the least-squares refinements. The analytical
approximations to the scattering factors were used, and all structure
factors were corrected for both real and imaginary components of
anomalous dispersion. Correct atomic position(s) were deduced from
an E-map (SHELX); least-squares refinement and difference Fourier
calculations were used to locate atoms not found in the initial solution.
Where noted, all manipulations were performed with rigorous
exclusion of air and moisture using Schlenk and glovebox techniques
under an atmosphere of argon or helium. All glassware was dried at
150 °C for at least 12 h. Toluene (anhydrous, Fisher) and
tetrahydrofuran (THF, anhydrous, Fisher) were vacuum distilled
from potassium metal and benzophenone, and degassed by three
freeze−pump−thaw cycles. Acetonitrile (anhydrous, Fisher) was dried
over CaH2, degassed by three freeze−pump−thaw cycles, and vacuum
distilled before use. Ph4PCl (Aldrich) was exposed to vacuum (10−3
Torr) for 24 h and stored under He in a glovebox prior to use.
Me3SiCl (Aldrich) was transferred to a Teflon sealable Schlenk flask
for storage and used as received. (NH4)2TiF6 (American Elements),
ZrCl4 (anhydrous, Strem), and HfCl4 (anhydrous, Strem) were used as
received. UCl4 was prepared as previously described from UO3 and
hexachloropropene.17 Aqueous manipulations were performed with
>18 Ω water from a Millipore purification system. Electronic
absorption spectra were collected at room temperature using a Varian
Cary 6000i UV−vis−NIR spectrophotometer with a 0.2 nm spectral
bandwidth. Solid diffuse reflectance spectra were collected using a
Varian Cary 500 with an Internal Diffuse Reflectance Accessory.
Elemental analyses were carried out by Midwest MicroLab, LLC
(Indianapolis, IN), and infrared spectra were recorded on a Nicolet
Magna-IR System 750 spectrometer as KBr pellets.
Bis(tetraphenylphosphonium) Titanium Hexafluoride,
(Ph4P)2TiF6 (1). Compound 1 was synthesized using a modification
of a previously published procedure.43 (NH4)2TiF6 (0.21 g, 1.05
mmol) and Ph4PCl (0.78 g, 2.09 mmol) were each dissolved in boiling
water (10 mL), and the solutions were stirred together. The resulting
solution was allowed to cool and stored at room temperature for 24 h,
which resulted in formation of white crystals. Following filtration, the
crude product was isolated from the reaction mixture as small white
crystals. These solids were dried under vacuum (10−3 Torr) at elevated
temperature (60 °C) for 24 h, brought into a helium-filled glovebox,
and dissolved in hot (60 °C) CH3CN (15 mL). The solution was
allowed to cool slowly to room temperature and then placed in a
freezer (−20 °C) overnight. Compound 1 was isolated as single
crystals suitable for X-ray diffraction studies in 22% yield (0.20 g, 0.22
mmol). Higher crystalline yields are achieved by isolating a second
crop of crystals, which can be obtained by concentrating the mother
liquor and repeating the crystallization procedure. IR (cm−1): 695 (s),
724 (s), 762 (s), 770 (m), 997 (m), 1027 (w), 1108 (s), 1166 (w),
1190 (w), 1317 (w), 1385 (w), 1437 (m), 1444 (m), 1483 (w), 1584
(w), 3022 (w), 3073 (w). Anal. Calcd for C48H40P2TiF6: C, 68.58; H,
4.80; F, 13.56. Found: C, 68.48; H, 4.84; 13.66.
Bis(tetraphenylphosphonium) Titanium Hexachloride,
(Ph4P)2TiCl6 (2). In a helium-filled glovebox, an excess of Me3SiCl
(10 mL) was added to a flask containing a solution of 1 (0.51 g, 0.60
mmol) in CH3CN (10 mL). This caused immediate precipitation of a
yellow solid. The volatiles were removed in vacuo (10−3 Torr), and the
resulting solids were redissolved in CH3CN. Slow diffusion of Et2O
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into this solution produced compound 2 as single crystals suitable for
X-ray diffraction in 35% yield following filtration (0.58 g, 0.18 mmol).
Higher crystalline yields are achieved by isolating a second crop of
crystals, which can be obtained by concentrating the mother liquor and
repeating the crystallization procedure. IR (cm−1): 669 (w), 691 (s),
723 (s), 757 (w), 997 (m), 1028 (w), 1108 (s), 1166 (w), 1187 (w),
1318 (w), 1384 (w), 1438 (s), 1483 (w), 1585 (w), 2218 (w), 2249
(w), 2925 (w), 3058 (w). Anal. Calcd for C48H40P2TiCl6: C, 61.37; H,
4.29. Found: C, 61.10; H, 4.65.
Bis(tetraphenylphosphonium) Zirconium Hexachloride,
(Ph4P)2ZrCl6 (3). In a helium-filled glovebox, a modified version of
the literature procedure was followed.44 A solution of Ph4PCl (0.32 g,
0.85 mmol) in acetonitrile (6 mL) was added to a stirred solution of
ZrCl4 (0.10 g, 0.43 mmol) in CH3CN (6 mL), resulting in the
immediate formation of a white crystalline solid. After isolating the
solid by centrifugation it was dissolved in hot CH3CN. This solution
was allowed to cool slowly to room temperature and then placed in a
freezer at −6 °C. The resulting white crystals were isolated by filtration
and desolvated by exposure to vacuum for several hours. This provided
compound 3 in 49% yield (0.21 g, 0.21 mmol). The desolvated
product was used for analytical measurements. Single crystals suitable
for X-ray diffraction were obtained by slow-cooling of a hot saturated
solution of 3 in CH3CN. Higher crystalline yields are achieved by
isolating a second crop of crystals, which can be obtained by
concentrating the mother liquor and repeating the crystallization
procedure. IR (cm−1): 689 (s), 722 (s), 756 (m), 996 (m), 1027 (w),
1107 (s), 1164 (w), 1188 (w), 1315 (w), 1437 (s), 1483 (w), 1585
(w), 3057 (w). Anal. Calcd for C48H40P2ZrCl6: C, 58.66; H, 4.10.
Found: C, 58.04; H, 4.04.
1338 (w), 1384 (w), 1440 (s), 1484 (m), 1586 (m), 3059 (m). Anal.
Calcd for C48H40P2ThCl6: C, 51.3; H, 3.59. Found: C, 51.5; H, 3.69.
Bis(tetraphenylphosphonium) Uranium Hexachloride,
(Ph4P)2UCl6 (6). A modification of the literature procedure was
followed.47 A vial containing UCl4 (0.22 g, 0.58 mmol) and THF (10
mL) was heated (70 °C) while stirring in a helium-filled glovebox until
all of the UCl4 dissolved. In a separate vial, CH3CN (10 mL) was
added dropwise to a white suspension of Ph4PCl (0.48 g, 1.3 mmol) in
THF (10 mL) and heated at 70 °C until all the Ph4PCl dissolved. The
Ph4PCl solution was then added rapidly to the solution of UCl4
causing immediate precipitation of a pale blue-green solid. After
stirring for 1 h, the mixture was centrifuged and the supernatant was
decanted and discarded. The remaining solids were washed with hot
THF (3 × 5 mL) and dissolved in boiling CH3CN (15 mL). After
filtration, the solution was cooled to −25 °C and after 24 h formed
pale blue-green crystals, which were isolated by decanting the
supernatant, washing with cold THF (10 mL), and drying in vacuo
(10−3 Torr). The desolvated product was obtained as a crystalline solid
(0.42 g, 0.37 mmol) in 64% yield. Single crystals suitable for X-ray
crystallographic analysis were obtained by slow cooling of a hot
concentrated solution of 6 in CH3CN. Higher crystalline yields can be
achieved by isolating a second crop of crystals, which can be obtained
by concentrating the mother liquor and repeating the crystallization
procedure. IR (cm−1): 689 (s), 723 (s), 754 (m), 760 (m), 996 (m),
1027 (w), 1108 (s), 1165 (w), 1188 (w), 1315 (w), 1338 (w), 1385
(w), 1438 (s), 1483 (m), 1585 (m), 3058 (m). Anal. Calcd for
C48H40P2UCl6: C, 51.04; H, 3.57. Found: C, 51.15; H, 3.66.
Bis(tetraphenylphosphonium) Neptunium Hexachloride,
(Ph4P)2NpCl6 (7). In an argon-filled negative pressure purge box, an
aliquot (0.10 mL, 0.14 mmol) of a green stock solution of dilute
Bis(tetraphenylphosphonium) Hafnium Hexachloride,
(Ph4P)2HfCl6 (4). The procedure for 3 was modified, using instead
HfCl4 (0.10 g, 0.31 mmol) and Ph4PCl (0.24 g, 0.63 mmol).
Compound 4 was isolated as white crystals, desolvated by exposure to
vacuum for several hours, and obtained in 49% yield (0.16 g, 0.15
mmol). Single crystals suitable for X-ray diffraction were formed by
slow-cooling of a hot saturated solution of 4 in CH3CN. The crystals
which had been desolvated by exposure to vacuum for several hours
were used for other analytical measurements. Higher crystalline yields
can be achieved by isolating a second crop of crystals, which can be
obtained by concentrating the mother liquor and repeating the
crystallization procedure. IR (cm−1): 669 (w), 691 (s), 723 (s), 761
(m), 996 (m), 1027 (w), 1108 (s), 1165 (w), 1187 (w), 1316 (w),
1338 (w), 1368 (w), 1438 (s), 1483 (w), 1585 (w), 3059 (w). Anal.
Calcd for C48H40P2HfCl6: C, 53.88; H, 3.77. Found: C, 54.01; H, 3.86.
Bis(tetraphenylphosphonium) Thorium Hexachloride,
(Ph4P)2ThCl6 (5). An adaptation of the published synthesis of
(NH4)4Th(SO4)4 was followed.45 In a fume hood, ThO2 (0.26 g, 0.98
mmol) and (NH4)2SO4 (0.79 g, 6.0 mmol) were ground together in a
mortar and pestle, placed in a Pt foil boat, and slowly heated in a
quartz tube to 365 °C for 3 h. The product, (NH4)4Th(SO4)4, was
then dissolved in H2O (100 mL). While monitoring with a pH meter,
concentrated aqueous NH4OH was added dropwise until the pH was
slightly below 9. A white precipitate, which was formulated as a poly
hydrated thorium hydroxide, was isolated on a medium glass frit,
washed liberally with aqueous NH4OH (0.1 M), and redissolved in
aqueous HCl (1 M, 10 mL). The solution was transferred to an
Erlenmeyer flask and the solvent removed using a Meker burner. After
drying in an oven (100 °C),46 the resulting powder was dissolved in
CH3CN and a solution of Ph4PCl (0.71 g, 1.9 mmol) in CH3CN (5
mL) was added, resulting in a rapid formation of a white precipitate.
The product was isolated on a medium glass frit, washed with cold
CH3CN, and dried in vacuo (10−3 Torr) at 60 °C for 24 h. This
desolvated form of compound 5 was isolated with 70% overall yield as
a white powder (0.77 g, 0.69 mmol). Crystals of compound 5 suitable
for X-ray diffraction studies were grown by slow diffusion of Et2O
vapor into saturated solutions of 5 in CH3CN. Higher crystalline yields
can be achieved by isolating a second crop of crystals, which can be
obtained by concentrating the mother liquor and repeating the
crystallization procedure. IR (cm−1): 689 (s), 723 (s), 754 (m), 760
(m), 997 (m), 1027 (w), 1108 (s), 1165 (w), 1189 (w), 1316 (w),
+
aqueous HCl containing NpO2 (1.4 M) was transferred to a flask
containing a Zn/Hg amalgam. Following a brief agitation by shaking,
which resulted in a color change from green to blue, the solution was
decanted from the reducing agent and exposed to an atmosphere of
air. After approximately 2 h, the complete oxidation from blue Np3+ to
yellow Np4+ was confirmed by UV−vis−NIR spectroscopy. The water
was removed via distillation, a solution of Ph4PCl (0.11 g, 0.28 mmol)
in hot CH3CN (5 mL) was added, and the mixture was heated until all
the solids had dissolved. Vapor diffusion of Et2O into this
concentrated solution produced colorless single crystals of 7 suitable
for X-ray diffraction studies, and were isolated in 41% yield (0.063 g,
0.056 mmol) following filtration from the supernatant. Radiological
concerns prohibited evaluation of the elemental composition of 7.
Alternative Synthesis of Bis(tetraphenylphosphonium)
Neptunium Hexachloride, (Ph4P)2NpCl6 (7). In a fume hood, an
+
2+
aliquot of a stock solution containing mixed NpO2 and NpO2
oxidation states in HCl (309 μL, 0.44 M, 0.14 mmol), was diluted with
additional HCl (1 mL, 6.0 M). The Np oxidation state was adjusted to
Np4+ by adding NH2OH·HCl and heating the solution. Oxidation
state purity was verified by UV−vis−NIR spectroscpy. This green
solution was evaporated to dryness by flowing argon gas for 12 h over
the opening of a vial containing the solution. The orange solid residue
was suspended in CH3CN (1 mL), a solution of Ph4PCl (0.095 g, 0.25
mmol) in CH3CN (2 mL) was added, and the mixture was stirred for
10 min producing a cloudy white suspension. Et2O (8 mL) was added,
and the mixture was stirred at room temperature for 20 min. After
settling, the supernatant was pipetted away from the white solid, which
was transferred to a negative-pressure helium atmosphere drybox and
dried in vacuo to yield a crude powdered product (0.10 g, 0.092
mmol) in 66% yield. The product could be purified further by
dissolving in CH3CN (9 mL) at 50 °C, filtering, and crystallizing by
diffusion of Et2O vapor into the concentrated CH3CN solution at −35
°C for 3 d. Following filtration and drying in vacuo, colorless crystals
suitable for X-ray diffraction were obtained in 61% yield (0.096 g,
0.085 mmol).
Bis(tetraphenylphosphonium) Plutonium Hexachloride,
(Ph4P)2PuCl6 (8). In a fume hood, an aliquot of a Pu4+ stock solution
(0.15 mL, 0.71 M, 0.10 mmol) in HCl was diluted with HCl (0.5 mL,
6 M) and stirred briefly until homogeneous. This red solution was
evaporated to dryness by flowing argon gas for 12 h over the opening
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more favorable Th starting materials such as ThCl4.31 In
contrast to redox active actinide oxides (MO2, M = U, Np, and
Pu), ThO2 is not easily dissolved. As a result, ThO2 was first
converted to water-soluble (NH4)4Th(SO4)4 by reaction with
(NH4)2SO4 at 365 °C, using a procedure similar to that
described by Mudher and co-workers (Scheme 3).45 The
of a vial containing the solution. A solution of Ph4PCl (0.079 g, 0.21
mmol) dissolved in CH3CN (3 mL) was added to this solid residue,
which quickly produced a yellow precipitate. The reaction was allowed
to go to completion while stirring at room temperature for 5 min, then
40 °C for 10 min, and finally allowed to cool back to room
temperature. The resulting mixture contained both a yellow solution
and a yellow precipitate. To force precipitation of the partially soluble
yellow product, Et2O (12 mL) was added while stirring over 10 min.
The supernatant was then pipetted away from the solids, which were
subsequently transferred to a negative-pressure He atmosphere drybox
and dried in vacuo to yield a crude powdered product (0.10 g, 0.092
mmol) in 88% yield. For further purification, this yellow solid was
dissolved in CH3CN (6 mL), heated to 50 °C, and filtered. Storing the
filtrate at −35 °C for 5 h followed by filtration gave 8 as X-ray quality
single crystals in 82% yield (0.097 g, 0.086 mmol). Radiological
concerns prohibited evaluation of the elemental composition of 8.
Scheme 3
ammonium thorium sulfate was dissolved in H2O, precipitated
with NH4OH, and redissolved in dilute HCl. After removing
the water at elevated temperature using a Meker burner, the
resulting solid was treated with Ph4PCl in CH3CN. Anhydrous
5 was obtained by gently heating the reaction product under
vacuum, and isolated as single crystals in 49% yield from a
Et2O/CH3CN solution.
RESULTS AND DISCUSSION
■
Synthesis. The general reaction involving combination of
the anhydrous MCl4 and Ph4PCl in organic solvent consistently
provided large quantities (crystalline yields ranging 49 to 69%)
of pure zirconium, hafnium, and uranium hexachlorides (3, 4,
and 6) following crystallization from CH3CN (Scheme 1).
Aqueous synthetic routes were also investigated to obtain
sufficient quantities of (Ph4P)2NpCl6 (7) and (Ph4P)2PuCl6
(8), and are related to those used to generate Cs2NpCl6 and
Cs2PuCl6.51,52 Pure stock solutions of Np4+ were prepared by
Scheme 1
+
reducing a mixed stock solution containing NpO22+ and NpO2
2−
with either Zn/Hg amalgam or NH2OH·HCl (Scheme 4).
Some syntheses have been reported for MCl6 salts (M = Ti,
Zr, Hf, U) with noncoordinating Ph4P+ countercations,44,47−49
However, the (Ph4P)2MCl6 salts typically cocrystallized with
chlorinated CH2Cl2 solvent molecules, which are not
compatible with our interest in conducting Cl K-edge XAS
studies focused on the MCl62− anions.50 The elemental analysis
and single crystal X-ray structures (see below) confirm that the
syntheses reported herein provided (Ph4P)2MCl6 salts with
rigorous exclusion of a secondary chlorine species.
Scheme 4
UV−visible spectroscopy indicated that if a solution containing
NpO22+/NpO2 was reduced with Zn/Hg amalgam, a solution
+
Upon initial inspection the synthetic approach used for 3, 4,
and 6 seemed appropriate for all the (Ph4P)2MCl6 complexes.
containing almost exclusively Np3+ was generated which
subsequently oxidized to Np4+ upon exposure to air (for ca. 2
h).53−55 Alternatively, if the milder NH2OH·HCl reducing
agent was used, Np3+ was not observed and a clean tetravalent
Np solution was obtained directly. After obtaining a pure Np4+
solution, removal of the solvent followed by reactions with
Ph4PCl in CH3CN provided colorless 7 in moderate crystalline
yields, which ranged from 41% when using Zn/Hg, to 61% if
NH2OH·HCl was used. A similar approach was developed for
the Pu salt (8); however, no valence adjustment step was
required because solutions containing exclusively Pu4+ were
readily available from anion exchange purification procedures,
which are routinely employed in aqueous Pu chemistry.56 After
removing the water, the Pu4+ solid was partially dissolved in
CH3CN and treated with Ph4PCl. From this saturated solution,
yellow X-ray quality single crystals of 8 were obtained in 82%
yield (Scheme 5).
2−
However, this was not feasible for the Ph4P+ salts of TiCl6
2−
2−
2−
(2), ThCl6 (5), NpCl6 (7), or PuCl6 (8), and alternative
syntheses of these anhydrous materials were designed. For
example, because TiCl4 reacts with many solvents that
solubilize Ph4PCl (such as CH3CN), 2 was prepared in two
steps from commercially available (NH4)2TiF6 (Scheme 2).
Scheme 2
First, compound 1 was prepared in low crystalline yield (22%)
using a modification of the published procedure for
(Ph4P)2ZrF6,43 which involved a cation exchange reaction
between (NH4)2TiF6 and Ph4PCl. The product 1 was
precipitated from H2O, dried at elevated temperature under
vacuum, and crystallized from CH3CN. A subsequent anion
exchange reaction between 1 and Me3SiCl conducted with
rigorous exclusion of air and moisture provided 2 in 35% yield
following crystallization from Et2O/CH3CN.
Optical Spectroscopy of 7 and 8. Diffuse solid-state
reflectance spectra and the UV/visible/NIR absorbance spectra
of solutions of 7 and 8 in CH3CN show the anticipated f → f
and f → d transitions associated with the Np4+ (f3) and Pu4+
Scheme 5
The synthesis of the Th analogue (5) was achieved beginning
with ThO2 because of the limited commercial availability of
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(f4) ions (Figures 1 and 2). For both compounds, the number
and intensity of bands observed in the visible region are
space group without any CH3CN. Unit cells for crystalline 1−8
contain discrete metal halide molecules that are well-separated
from the Ph4P+ countercations and for 1−6 and 8 there are no
2−
obvious interactions between MCl6 anions and the CH3CN
2−
solvent molecules. A thermal ellipsoid plot for the PuCl6
molecule in 8 is shown in Figure 3, and thermal ellipsoid plots
Figure 3. Thermal ellipsoid (50%) plot of 8. Noncoordinating Ph4P+
countercations and acetonitrile molecules have been omitted.
Figure 1. UV spectra of (Ph4P)2NpCl6 (panel A) and (Ph4P)2PuCl6
(panel B) in CH3CN.
for 1−7 are provided in the Supporting Information, Figures
2−
S1−S7. In each MCl6 molecule the metal geometry is best
described as octahedral; however, all six M−Cl bond distances
are not crystallographically equivalent, and the Cl−M−Cl
angles are not rigorously 90 degrees. Each of the three
crystallographically inequivalent M−Cl bond distances and
angles are given in Table 4, along with the mean M−Cl bond
distances and standard deviations from the means (σ).
For 1, the F−Ti−F angles only slightly deviate from 90
degrees (90.49(6), 89.96(6), and 89.92(6) degrees) and the
three unique Ti−F bond distances (1.8671(13), 1.8644(14),
and 1.8567(14) Å) average 1.863 Å with σ = 0.005 Å. This
compares well with the 1.86 Å (σ = 0.01 Å) average distance
reported in the crystal structure of TiF62− with noncoordinating
aminoguanidinium cations.62,63 In 2, the average Ti−Cl bond
distance is appreciably longer at 2.34 Å (σ = 0.01 Å). Changing
the metal from Ti to Zr and Hf increases the M−Cl bond
distance, and average values of 2.47 Å (σ = 0.01 Å) and 2.45 Å
(σ = 0.01 Å) were observed for 3 and 4, respectively.
Structures of the thorium and uranium analogues (5 and 6)
are similar to the transition metal compounds, with an average
Th−Cl distance of 2.68 Å (σ = 0.01 Å) and an average U−Cl
distance of 2.62 Å (σ = 0.01 Å). The average Th−Cl distance is
shorter than what was observed in the X-ray crystal structure of
Cs2ThCl6 (Th−Cl = 2.81(6) Å; each Cs atom is also
coordinated to 12 Cl atoms at an average distance of 3.74
Å).64 However, because of the large error bars associated with
the Cs2ThCl6 metric parameters, the Th−Cl bond distances in
both crystal structures are equivalent within three σ. The
average U−Cl bond distance compares well with that observed
in (Ph4P)2UCl6·2CH2Cl2 (average U−Cl = 2.60 Å, σ = 0.02
Å).47 To the best of our knowledge, the structure of 5 is the
Figure 2. Visible-NIR spectra of (Ph4P)2NpCl6 (panel A) and
(Ph4P)2PuCl6 (panel B), obtained by reflectance of solid samples
(black traces) and absorbance of dilute solutions in CH3CN (red
traces).
consistent with the usually pale colors observed for single
crystals of 7 (colorless) and 8 (yellow). The solid and solution-
state optical spectra are very similar, and in agreement with
2−
2−
previous reports on NpCl6 and PuCl6 with different
cations.57−61 They exhibit a series of well-resolved absorptions
in the near-IR region, which is a characteristic of Np4+ and Pu4+
coordination compounds and suggests that the speciation for
the neptunium and plutonium ions in solution is equivalent to
that in the solid state.
2−
only structural report for a ThCl6 salt with noncoordinating
countercations. Overall, the similarities between observed bond
2−
distances and geometries in these MX6 molecules (M = Ti,
X-ray Crystal Structure Descriptions. The complexes 2−
6 and 8 are isomorphous and crystallize in the P21/c space
group with two molecules of CH3CN (Tables 2 and 3). For
Zr, Hf, Th, U; X = F, Cl) confirm that alternative
noncoordinating countercations and cocrystallized solvent
molecules have an insignificant effect on the solid-state
structure.
reasons not well understood, compound 7 crystallizes in the P1
̅
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Table 4. Selected Bond Lengths (Å) and Angles (deg) for Complexes 1−8
2−
2−
2−
2−
2−
2−
2−
2−
1, TiF6
2, TiCl6
3, ZrCl6
4, HfCl6
5, ThCl6
6, UCl6
7, NpCl6
8, PuCl6
M(1)−X(1)
M(1)−X(2)
M(1)−X(3)
av. M(1)−X
X(1)−M(1)−X(2)
X(1)−M(1)−X(3)
X(2)−M(1)−X(3)
1.8671(13)
1.8644(14)
1.8567(14)
1.863(5)
89.96(6)
90.49(6)
89.92(6)
90.1(3)
2.3543(6)
2.3252(6)
2.3458(6)
2.34(1)
2.4771(8)
2.4530(8)
2.4699(8)
2.47(1)
2.4605(10)
2.4414(10)
2.4526(10)
2.45(1)
2.6901(8)
2.6699(9)
2.6824(8)
2.68(1)
2.6326(6)
2.6087(5)
2.6224(6)
2.62(1)
2.6281(16)
2.5826(14)
2.6198(16)
2.61(2)
2.5944(8)
2.5757(7)
2.5936(8)
2.59(1)
a
90.27(2)
90.280(19)
89.89(2)
90.1(2)
90.50(3)
90.47(3)
89.79(3)
90.3(4)
90.36(3)
90.38(3)
89.84(3)
90.2(3)
91.31(3)
90.90(3)
89.57(3)
90.6(9)
91.048(18)
89.600(16)
90.768(16)
90.5(8)
89.59(6)
89.93(4)
88.31(4)
89.3(9)
90.97(3)
90.84(3)
89.61(3)
90.5(8)
a
av. X−M(1)−X
a
The error in the average bond lengths and angles is equal to the standard deviation in the three experimental values.
Although powder X-ray diffraction has been used to
distances and metal ionic radii. A linear fit to these data has a
near unit slope of 0.98 0.02 and a y-intercept of 1.75 0.02,
which is close to the chlorine ionic radius of 1.81 Å.70 On the
basis of previously reported structural criteria for establishing
bonding modes,71 these data indicate that the M−Cl
interactions in each of the (Ph4P)2MCl6 salts are best described
as predominantly ionic in character.
characterize Cs2NpCl6,18 to the best of our knowledge, the
structure of 7 represents the first single crystal diffraction data
2−
containing a NpCl6 anion. Three unique Np−Cl bond
distances were observed (Table 4), which provide an average
2−
Np−Cl bond distance of 2.61 Å (σ = 0.02 Å). For PuCl6
,
powder X-ray diffraction has also been used to characterize
(Me4N)2PuCl6,65 and (Et4N)2PuCl6,65,66 but the only single
crystal X-ray diffraction studies of the PuCl62− anion have been
performed on salts with alkali metal countercations. In these
studies, Pu−Cl bond distances were found to vary significantly
if the countercation was K+ (2.570(2) Å),67 Rb+ (2.6799(8)
Å),67 or Cs+ (2.7(2) Å).68 Each of the alkali metals in these
crystal structures was coordinated by 12 Cl atoms with K−Cl,
Rb−Cl, and Cs−Cl distances that averaged 3.680, 3.643, and
3.71 Å, respectively. The structure of 8 is the first with a
CONCLUDING REMARKS
■
Described here are high-purity syntheses conducted with
rigorous exclusion of chlorinated solvents for octahedral
2−
2−
TiF6 (1) and MCl6 (M = Ti, Zr, Hf, Th, U, Np, Pu; 2−
8) molecules as salts with Ph4P+ countercations. By avoiding
the introduction of additional Cl atoms into the lattice and
using noncoordinating cations to prevent M−Cl−M bridging,
the synthetic procedures provide metal hexahalide salts that are
compatible with Cl K-edge XAS experiments and guarantee
that the M−Cl interactions are probed discretely. Compounds
5, 7, and 8 have the potential to serve as well-defined starting
materials for nonaqueous Th, Np, and Pu coordination
chemistry, and have already been employed as convenient
precursors for anhydrous low-valent Pu coordination chem-
istry.72,73 Achieving efficient high purity syntheses that do not
require extensive manipulations is especially important for the
2−
PuCl6 anion and noncoordinating cations. The mean Pu−Cl
distance in 8 is 2.59 Å (σ = 0.01 Å), which more closely
resembles the Pu−Cl distance in K2PuCl6, than those in the
67
2−
PuCl6 salts with Rb+ or Cs+ countercations.
In summary, crystallographically equivalent M−Cl bond
distances increase when moving from the first row (Ti) to the
second (Zr) or third row (Hf), and decrease across the actinide
series (Th to Pu).69 However, average U−Cl, Np−Cl, and Pu−
Cl bond distances are equivalent within the standard deviation
of the mean. All the transition metal and actinide M−Cl bond
distances are essentially invariant when compared to the six
coordinate ionic radii. Figure 4 depicts this graphically by
showing the linear relationship between mean M−Cl bond
2−
2−
NpCl6 and PuCl6 salts, because radiological concerns limit
the amount of material that can be handled safely.
The M−Cl bond distances follow well-established periodic
trends for transition metals and actinides.69 These trends in
bond distance also parallel the ionic radius of the metal, which
suggests that the M−Cl bonding in each (Ph4P)2MCl6 salt can
be described appropriately using a primarily ionic bonding
model. On the other hand, a variety of theoretical,
spectroscopic and thermodynamic studies indicate that
particularly for actinidesmore direct techniques and precise
terminology are needed to identify subtle differences in
covalent bonding.32−35,74 Along these lines, we are currently
using F and Cl K-edge X-ray absorption spectroscopy as direct
probes of electronic structure and metal−ligand orbital mixing
for the transition metal and actinide compounds described
here.40
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic information files (CIF) and thermal ellipsoid
plots for 1−8. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 4. Plot of the mean M−Cl bond distances (Å) in (Ph4P)2MCl6
(M = Ti, Zr, Hf, Th, U, Np, Pu) relative to the ionic radius of M.70
A
AUTHOR INFORMATION
■
linear fit to the data (dashed line) gives the equation y = (1.75 0.02
Å) + (0.98 0.02)x. Error bars are taken from the standard deviation
in the mean distance of crystallographically inequivalent M−Cl bonds.
Corresponding Author
*E-mail: stosh@lanl.gov; LA-UR 11-06883.
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Inorganic Chemistry
Article
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The authors declare no competing financial interest.
47, 8577.
ACKNOWLEDGMENTS
■
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Morss, L., Edelstein, N. M., Fuger, J., Eds.; Springer: Berlin, Germany,
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(36) Clark, D. L.; Hecker, S. S.; Jarvinen, G. D.; Neu, M. P. In The
Chemistry of the Actinide and Transactinide Elements; 3rd ed.; Morss, L.,
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This work was supported by the Division of Chemical Sciences,
Geosciences, and Biosciences, Office of Basic Energy Sciences,
U.S. Department of Energy at LBNL (contract DE-AC02-
05CH11231) and under the Heavy Element Chemistry
Program at LANL (operated by Los Alamos National Security,
LLC, for the National Nuclear Security Administration;
contract DE-AC52-06NA25396). Additional parts of this
work (A.J.G., S.D.R.) were supported by the Office of Science,
U.S. Department of Energy Early Career Research Program
(contract DE-AC52-06NA25396), and Fuel Cycle Research
and Development Program of the Office of Nuclear Energy,
U.S. Department of Energy. Parts of this work were also
supported at LANL by Glenn T. Seaborg Institute Postdoctoral
Fellowships (S.G.M.).
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