Organophosphorus-Modified Lanthanide Nitrates as Potential Actinide Oxide Aerosol Surrogates

Article abstract of DOI:10.1021/acs.inorgchem.0c02428

In search of suitable simulants for aerosol uranium waste products from Plutonium Uranium Redox Extraction (PUREX) process burns, a series of lanthanide nitrate hydrates ([Ln(κ2-NO3)3·nH2O]) were dissolved in the presence of tributylphosphate (O= P(O(CH2)3CH3)3) referred to as TBP) in kerosene or triphenylphosphate (O= P(O(C6H5) referred to as TPhP) in acetone. The crystal structure of the TPhP derivatives of the lanthanide nitrate series and uranium nitrate were solved as [Ln(κ2-NO3)3(TPhP)3] (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and [U(O)2(κ2-NO3)2(TPhP)2] (U), respectively. The lanthanide-TBP, Ln, and U were further characterized using FTIR spectroscopy, 31P NMR spectroscopy, thermogravimetric analysis, and X-ray fluorescence spectroscopy. Further, thermal treatment of the lanthanide-TBP, Ln, and U using a box furnace to mimic pyrolysis conditions was found by PXRD analyses to generate a phosphate phase [LnP3O9 or UP2O7) for all systems. The resultant nuclear waste fire contaminant particulates will impact both aerosol transport and toxicity assessments.

Full text of DOI:10.1021/acs.inorgchem.0c02428

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Article
Organophosphorus-Modified Lanthanide Nitrates as Potential
Actinide Oxide Aerosol Surrogates
Timothy J. Boyle,* Xavier J. Robinson, Fernando Guerrero, Diana Perales, Joshua A. Hubbard,
and Roger E. Cramer
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ABSTRACT: In search of suitable simulants for aerosol uranium waste products from
Plutonium Uranium Redox Extraction (PUREX) process burns, a series of lanthanide nitrate
hydrates ([Ln(κ2-NO3)3·nH2O]) were dissolved in the presence of tributylphosphate (O
P(O(CH2)3CH3)3) referred to as TBP) in kerosene or triphenylphosphate (OP(O(C6H5)
referred to as TPhP) in acetone. The crystal structure of the TPhP derivatives of the
lanthanide nitrate series and uranium nitrate were solved as [Ln(κ2-NO3)3(TPhP)3] (Ln = La,
Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and [U(O)2(κ2-NO3)2(TPhP)2] (U),
respectively. The lanthanide-TBP, Ln, and U were further characterized using FTIR
spectroscopy, 31P NMR spectroscopy, thermogravimetric analysis, and X-ray fluorescence
spectroscopy. Further, thermal treatment of the lanthanide-TBP, Ln, and U using a box
furnace to mimic pyrolysis conditions was found by PXRD analyses to generate a phosphate
phase [LnP3O9 or UP2O7) for all systems. The resultant nuclear waste fire contaminant
particulates will impact both aerosol transport and toxicity assessments.
both oxide structures, the metals are 8-coordinated, which
consists of eight-coordinate Ce4+ cations and tetrahedrally
bound O2− anions. This approach is valid for a wide number of
research efforts concerning stability to radiation exposure;
however, for aerosol transport during a fire, as would occur in a
Red Oil explosion, the density of the metal oxide is believed to
be a more important parameter. The composition, mobility,
and distribution of aerosolized metal oxide particulates in the
ambient atmosphere are of critical importance for the
modeling aspects of thermal processing of actinide waste
materials. As can be discerned from the density of the oxides
listed in Table 1, CeO2 may not be the best lanthanide
surrogate choice for actinide oxide waste processing.31 The
heavier, late lanthanide cations (Yb and Lu) appear to be the
most similar in terms of oxide density. While UOx is the main
actinide of study, it is of note that plutonium oxide (PuOx) is
also of particular interest for these types of studies, but it is not
feasible to study this material under the same conditions.
Therefore, it is surprising that the solubility and aerosol
transport behavior of these potential simulants in comparison
to UOx have not been previously reported.
I. INTRODUCTION
The Plutonium Uranium Redox Extraction (PUREX) process
relies on solubilizing actinide oxide waste through the
acidification of the waste stream using nitric acid. In order to
separate the actinides from other metals, this mixture is then
transferred to a kerosene system employing organophosphorus
compounds. Historically, this process has been found to
generate a red colored, unspecified organic phase consisting of
a trialkylphosphate and nitric acid, which is referred to as “Red
Oil”.1−11 When this material is heated above 120 °C,
explosions and fires have occurred, resulting in several
incidents in the United States and across the world.1−11 Due
to the potential risks within this system, a great deal of research
into characterizing the properties of Red Oil and identifying
the products formed during this process has been under-
taken.1−14 Studies concerning the scattering of the particulates
from actinide-containing fires rely heavily on particle
dispersion data collected in the early 1970s.15 As part of a
revamping of these outdated data, this effort focused on
identifying the various components of the solution system and
characterizing the particulates formed during a burn event.
Particulate behavior of this system has been reported in
additional reports.16,17
Received: August 13, 2020
While there is no ideal actinide simulant among the
lanthanide cations, the rare earth ceramic oxide materials are
often used due to the similarity in chemistry, phases, oxidation
states, and lack of radioactivity. In particular, ceria (CeO2) is
widely selected as a surrogate for uranium dioxide (UO2) due
to its similar mineral phase and oxidation state (4+).18−30 For
https://dx.doi.org/10.1021/acs.inorgchem.0c02428
© XXXX American Chemical Society
Inorg. Chem. XXXX, XXX, XXX−XXX
A

Inorganic Chemistry
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Article
[Ln(κ2‐NO3)3·6H2O] + (xs)TBP/kerosene → “Ln(κ2‐NO3)3(TBP)x”
Table 1. List of Potential Lanthanide Oxide Simulants,
Phase, Density Data from Ref 35, and General Solubility in
the TBP/Kerosene Mixture
(1)
[Ln(κ2‐NO3)3·6H2O] + 3TPhP/acetone → [Ln(κ2‐NO3)3(TPhP)3]
(2)
oxide
crystal structure (space group)
density (g/cm3)
CeO2
cubic (fluorite)
7.22
6.2
6.9
Ce2O3
Pr2O3
Nd2O3
Sm2O3
Eu2O3
Gd2O3
Tb2O3
Dy2O3
Ho2O3
Er2O3
Tm2O3
Yb2O3
Lu2O3
U3O8
hexagonal (P3
hexagonal (P3
hexagonal (P3
cubic
̅
̅
̅
m1)
m1)
m1)
II. EXPERIMENTAL SECTION
7.24
8.35
7.42
7.407
7.91
7.80
8.41
8.64
8.6
9.17
9.42
8.3
11.0
11.5
The various [Ln(κ2-NO3)3·6H2O] compounds were obtained from
Aldrich Chemical Company and used without further purification.
Depleted uranium metal (d-U(0)) was obtained from Los Alamos
National Laboratories and handled with appropriate personal
protective equipment with all manipulations performed with
double-glove, taped lab-coat in a glovebox, or hood. Additional
chemicals were used as received from Aldrich Chemical Company,
including the following: TBP, TPhP, kerosene, acetone, nitric acid,
and potassium bromide (KBr). All samples were handled under
ambient conditions in a hood. DI-water was freshly collected from a
Millipore Milli-Q Integral water purification system employing a
Progard T3 reverse osmosis filter system. All 31P NMR spectroscopic
data were collected on a Magritek SpinSolve 60 NMR spectrometer
using an aliquot from each reaction mixture. Elemental analyses were
performed on a PerkinElmer 2400 CHN-S/O elemental analyzer. All
samples were externally referenced to an aqueous solution of
tetraphenylphosphonium chloride.
monclinic, cubic
monclinic, cubic
cubic (Ia3
cubic (Ia3
cubic (Ia3
cubic (Ia3
cubic (Ia3
̅
̅
̅
̅
)
)
)
)
)
̅
cubic
cubic
layered
cubic (fluorite)
cubic (fluorite)
UO2
PuO2
30% TBP/Kerosene Stock Solution. In a hood, a 30% v/v stock
solution was prepared under ambient conditions consisting of TBP
(30 mL) and kerosene (70 mL). This mixture was stirred for 0.5 h,
transferred to an amber colored glass bottle, sealed, and stored until
needed.
Lacking in the literature is the structure that the lanthanide
precursor adopts in this process, the degree of solvation, and
the material conversion properties. The most commonly used
organophosphorus reagent in the PUREX system is tri-n-
butylphosphate (TBP; OP(OBun)3); however, there does
not appear to be any published structures of lanthanide or
actinide ions ligated by TBP. In our hands, we were not able to
crystallize any of these cations with TBP either. Other
modifiers such as triphenyl (TPhP; OP(OPh)3), triiso-
butyl (TIP), triethyl (TEP), or trimethylphosphate (TMP)
have also been employed.12−14,32−36 A search of crystallo-
graphically characterized organophosphate-modified lantha-
nide cations37 yields only a handful of compounds that have
been identified34,35,38−45 with only two possessing a nitrate34,35
coligand. It is of note that other lanthanide nitrates modified
by phosphate derivatives have been structurally character-
ized,37 but due to their polydenticity in comparison to the
monodentate organophosphates, different structure types have
been reported.35,46−48 A few monomeric and U species with
nitrate and organophosphate ligands have been reported as
monomeric species [U(O)2(κ2-NO3)2(OP(OR)3)2] (OR =
OMe49 and OCH2(C(CH3)2)50). The dimeric complex
[U(O)2(κ2-NO3)2(H2O)2]·H2O]. In an argon filled glovebox, d-U(0)
(1.0 g, 4.2 mmol) was placed in a beaker, and (conc, aq) HNO3 (∼20
mL) was added. After the bubbling subsided, the clear yellow solution
was removed from the glovebox and placed in a hood. The reaction
mixture was allowed to slowly evaporate the volatile component until
crystals formed. The mother liquor was decanted, and the crystalline
product was further air-dried and used without further purification.
Yields were not determined but assumed to be quantitative, as the
metal was completely consumed, and the solution was clear. The
structure of the final crystalline material was consistent with that
reported by Burns et al.51
“U(O)2(κ2-NO3)2(TBP)2”. [U(O)2(κ2-NO3)2(H2O)2·H2O] (0.500 g,
1.00 mmol) was added to the 30% organophosphorus/kerosene stock
solution (5 mL) and allowed to stir for 24 h. FTIR (KBr, cm−1)
3432(m, br), 2928(w), 2347(w), 1725(m), 1589(m), 1530(m),
1487(m), 1384(m), 1280(w), 1227(m, sh), 1195(m), 946(m),
904(m), 783(m), 753(m), 687(m), 528(s, sh), 505(s). 31P NMR δ
0.11.
[U(O)2(κ2-NO3)2(TPhP)2]. [U(O)2(κ2-NO3)2(H2O)2·H2O] (0.500 g,
1.00 mmol) was dissolved in acetone, and TPhP (0.978g, 3.00 mmol)
was added with stirring. After 12 h, the reaction mixture was set aside
with the cap loose to allow for slow evaporation until crystals formed.
Yield 80.4% (0.742 g). FTIR (KBr, cm−1) 3064(w), 2557(w),
2299(w), 1946(m), 1901(m), 1780(w), 1735(m), 1591(m),
1485(m), 1456(m), 1384(w), 1280(m), 1228(m), 1090(w),
1072(w), 1016(m), 946(m), 902(m), 809(m), 779(s), 752(s),
721(m, sh), 685(s), 663(m, sh), 616(m), 584(s), 561(m), 528(s),
505(s), 491(s, sh), 442(m). 31P NMR δ −18.15.
a. General Molecular Synthesis. a.i. Lanthanides/TBP. In a vial,
the appropriate [Ln(κ2-NO3)3·6H2O] (0.50 g) was added to a stirring
solution (∼5 mL) of the TBP stock solution. After 12 h, the reaction
was set aside with the cap off to allow any volatile components to
evaporate. In our hands, crystals of the TBP derivative could not be
isolated, but powders were obtained for each reaction by slow
evaporation or oven drying at 120 °C. 31P NMR spectra for several of
the “Ln(κ2-NO3)3(TBP)x” (Ln = Gd, Tb, Dy, Ho, and Tm) could not
be obtained. Meaningful yields could not be calculated due to the
presence of excess TBP in each reaction.
14
[U(O)2(κ2-NO3)(OP(OMe)3)(μ-O2P(OMe)2]2 has also
been found when an acid phosphate ligands is employed.
In this study, a series of lanthanide nitrates ([Ln(κ2-NO3)3·
nH2O]) were mixed with organophosphorus ligands (i.e., TBP
(eq 1) or TPhP (eq 2)), and the resulting products generated
were characterized and evaluated as potential actinide aerosol
simulants. The crystal structures of the lanthanide series and
uranium nitrate51 using TPhP (eq 2) were solved as [Ln(κ2-
NO3)3(TPhP)3] (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy,
Ho, Er, Tm, Yb, Lu collectively referred to as Ln) and
[U(O)2(κ2-NO3)2(TPhP)2] (U). The lanthanide-TBP, Ln,
and U were further characterized using FTIR, 31P NMR
spectroscopy, and thermal gravimetric analysis (TGA). The
thermally treated products from the lanthanide-TBP, Ln, and
U precursors were surprisingly found to form a phosphate
phase by powder X-ray diffraction (PXRD) and X-ray
fluorescence (XRF) studies.
B
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“La(κ2-NO3)3(TBP)x”. [La(κ2-NO3)3·6(H2O)] (0.500 g, 1.15 mmol).
FTIR (KBr, cm−1) 2927(w), 2371(w), 2346(w), 1656(m), 1384(w,
sh), 1086(s, br.), 722(w), 493(m). 31P NMR δ 18.88(d).
“Ce(κ2-NO3)3(TBP) x”. [Ce(κ2-NO3)3·6(H2O)] (0.500 g, 1.15
mmol). FTIR (KBr, cm−1) 3414(w), 2960(w), 1654(w), 1384(m,
sh), 1185(m, br), 1072(s, br), 552(m), 473(m). 31P NMR δ 14.17(s).
“Pr(κ2-NO3)3(TBP) x”. [Pr(κ2-NO3)3·6(H2O)] (0.500 g, 1.15
mmol). FTIR (KBr, cm−1) 3414(m, br), 2927(w), 2391(w),
2127(w), 1618(s), 1384(w, sh), 1090(s, br), 487(m). 31P NMR δ
38.86(s).
(KBr, cm−1) 3065(w), 1566(m), 1475(s), 1454(s), 1292(s), 1168(s),
1072(w), 1011(s), 946(s), 904(s), 747(s), 686(s). 31P NMR δ
−15.61(s). Elemental Analysis for C54H45CeN3O21P3 (MW =
1304.99) Calc’d: 49.70, C%; 3.48, H%; 3.22, N%. Found: 49.69, C
%; 3.80, H%; 3.20, N%.
[Pr(κ2-NO3)3(TPhP)3] (Pr). [Pr(κ2-NO3)3·6(H2O)] (0.250 g, 0.575
mmol) and TPhP (0.563 g, 1.72 mmol). Yield 98% (0.741 g). FTIR
(KBr, cm−1) 3337(w, br), 3065(w), 1677(w), 1583(w), 1487(s),
1290(m) 1250(m), 1213(m), 1152(s), 1011(s), 979(s), 934(m),
901(m), 747(s), 683(s). 31P NMR δ −16.73(s). Elemental Analysis
for C54H45N3O21P3Pr (MW = 1303.78) Calc’d: 49.67, C%; 3.47, H%;
3.22, N%. Found: 50.07, C%; 3.87, H%; 3.24, N%.
“Nd(κ2-NO3)3(TBP)x”. [Nd(κ2-NO3)3·6(H2O)] (0.500 g, 1.14
mmol). FTIR (KBr, cm−1) 3458(m, br), 2925(w), 2644(w),
2385(w), 2176(w), 1641(m), 1384(w, sh), 1222(s, br), 1093(s, br),
918(m), 704(w), 498(m). 31P NMR δ 53.94(s).
[Nd(κ2-NO3)3(TPhP)3] (Nd). [Nd(κ2-NO3)3·6(H2O)] (0.250 g,
0.570 mmol) and TPhP (0.558 g, 1.71 mmol). Yield 95.3% (0.712
g). FTIR (KBr, cm−1) 3337(w, br), 3061(w), 1588(w), 1482(s).
1290(m), 1250(m), 1215(m), 1157(s), 1016(m), 976(s), 943(m),
899(m), 749(s), 683(s). 31P NMR δ −14.37(s). Elemental Analysis
for C54H45N3NdO21P3 (MW = 1309.21) Calc’d: 49.54, C%; 3.46, H
%; 3.21, N%. Found: 49.97, C%; 3.68, H%; 3.00, N%.
“Sm(κ2-NO3)3(TBP)x”. [Sm(κ2-NO3)3·6(H2O)] (0.500 g, 1.13
mmol). FTIR (KBr, cm−1) 3403(m, br), 2971(w), 2371(w),
2144(w), 1637(m), 1383(w), 1233(s, br), 1101(s, br), 952(s),
737(w), 497(m). 31P NMR δ −7.73(s).
“Eu(κ2-NO3)3(TBP)x”. [Eu(κ2-NO3)3·6(H2O)] (0.500 g, 1.12
mmol). FTIR (KBr, cm−1) 3383(m, br), 2926(w), 2070(w),
1650(m), 1091(s, br), 897(w), 734(w), 608(w), 468(m). 31P NMR
δ −12.48(s).
[Sm(κ2-NO3)3(TPhP)3] (Sm). [Sm(κ2-NO3)3·6(H2O)] (0.250 g,
0.562 mmol) and TPhP (0.550 g, 1.69 mmol). Yield 95.0% (0.703
g). FTIR (KBr, cm−1) 3070(w), 1585(w), 1475(s), 1290(m),
1241(m), 1215(m), 1154(s), 1009(m), 9799s), 936(m), 894(m),
744(s), 678(s). 31P NMR δ −15.54(s). Elemental Analysis for
C54H45N3O21P3Sm (MW = 1315.24) Calc’d: 49.31, C%; 3.45, H%;
3.19, N%. Found: 49.57, C%; 2.60, H%; 3.06, N%.
“Gd(κ2-NO3)3(TBP)x”. [Gd(κ2-NO3)3·6(H2O)] (0.500 g, 1.11
mmol). FTIR (KBr, cm−1) 3425(m, br), 2133(w), 1637(s),
1097(m, br), 670(w), 586(w), 473(w). 31P NMR signal not observed.
‘Tb(κ2-NO3)3(TBP)x”. [Tb(κ2-NO3)3·6(H2O)] (0.500 g, 1.10
mmol). FTIR (KBr, cm−1) 3455(m, br), 2932(w), 2383(w),
2137(w), 1643(m), 1384(m, sh), 1254(s), 1108(s, br), 913(m),
803(w), 718(w), 565(w), 488(m). 31P NMR signal not observed.
“Dy(κ2-NO3)3(TBP)x”. [Dy(κ2-NO3)3·6(H2O)] (0.500 g, 1.10
mmol). FTIR (KBr, cm−1) 3399(m, br), 2955(w), 2916(w),
2838(w), 2393(w), 2143(w), 1666(m), 1384(m, sh), 1257(m),
1098(s, br), 1014(s), 718(w), 572(w), 458(m). 31P NMR signal not
observed.
[Eu(κ2-NO3)3(TPhP)3] (Eu). [Eu(κ2-NO3)3·6(H2O)] (0.250 g, 0.560
mmol) and TPhP (0.549 g, 1.68 mmol). Yield 98.6% (0.729 g). FTIR
(KBr, cm−1) 3403(m, br), 2974(w), 2911(w), 1672(m), 1637(m),
1491(s), 1451(s), 1445(s), 1405(m), 1337(m), 1283(s), 1194(s),
1037(s), 906(m), 810(s), 742(s), 693(m). 31P NMR δ −24.89(s).
Elemental Analysis for C54H45EuN3O21P3 (MW = 1316.84) Calc’d:
49.25, C%; 3.44, H%; 3.19, N%. Found: 49.60, C%; 3.71, H%; 3.20, N
%.
[Gd(κ2-NO3)3(TPhP)3] (Gd). [Gd(κ2-NO3)3·6(H2O)] (0.250 g,
0.554 mmol) and TPhP (0.542 g, 1.66 mmol). Yield 97.2% (0.712
g). FTIR (KBr, cm−1) 3412(w, br), 3063(w), 2969(w), 1679(m),
1634(w), 1578(m), 1470(s), 1281(s), 1253(s), 1215(s)m 1154(s),
1030(s), 979(s), 904(m), 815(m), 754(s), 683(s). 31P NMR signal
not observed. Elemental Analysis for C54H45GdN3O21P3 (MW =
1322.13) Calc’d: 49.06, C%; 3.43, H%; 3.18, N%. Found: 48.81, C%;
3.47, H%; 3.17, N%.
“Ho(κ2-NO3)3(TBP)x”. [Ho(κ2-NO3)3·6(H2O)] (0.500 g, 1.09
mmol). FTIR (KBr, cm−1) 3438(w, br), 2961(w), 2922(w),
2841(w), 2393(w), 2146(w), 1656(w), 1380(m, sh), 1244(s),
1095(s), 1020(s), 952(m), 744(w), 702(w), 481(m). 31P NMR
signal not observed.
“Er(κ2-NO3)3(TBP)x”. [Er(κ2-NO3)3·6(H2O)] (0.500 g, 1.08 mmol).
FTIR (KBr, cm−1) 2875 (w, br), 2034(w, br), 1633(w), 1142(m),
1001(s),913(s), 664(m). 31P NMR δ −34.02(s).
[Tb(κ2-NO3)3(TPhP)3] (Tb). [Tb(κ2-NO3)3·6(H2O)] (0.250 g, 0.552
mmol) and TPhP (0.540 g, 1.65 mmol). Yield 87.2% (0.637 g). FTIR
(KBr, cm−1) 3396(w, br), 2969(w), 1677(m), 1644(w), 1588(w),
1468(s), 1283(s), 1248(s), 1161(s), 1039(s), 981(m), 906(m),
803(m), 751(s), 683(s). 31P NMR signal not observed. Elemental
Analysis for C54H45N3O21P3Tb (MW = 1323.80) Calc’d: 48.99, C%;
3.43, H%; 3.17, N%. Found: 48.87, C%; 3.44, H%; 3.23, N%.
[Dy(κ2-NO3)3(TPhP)3] (Dy). [Dy(κ2-NO3)3·6(H2O)] (0.250 g,
0.548 mmol) and TPhP (0.536 g, 1.64 mmol). Yield 98.1% (0.713
g). FTIR (KBr, cm−1) 3419(w, br), 3061(w), 2976(w), 1677(w),
1644(w), 1588(w), 1473(s), 1288(s), 1255(s), 1215(m), 1166(s),
1028(s), 974(s), 904(m), 808(m), 747(s), 688(s). 31P NMR δ
−37.25(s). Elemental Analysis for C54H45DyN3O21P3 (MW =
1327.38) Calc’d: 48.86, C%; 3.42, H%; 3.17, N%. Found: 48.87, C
%; 3.63, H%; 3.20, N%.
“Tm(κ2-NO3)3(TBP)x”. [Tm(κ2-NO3)3·6(H2O)] (0.500 g, 1.08
mmol). FTIR (KBr, cm−1) 2941(w), 2880(w), 2822(w), 2356(m),
1819(m, br), 1473(m), 1311(s), 1098(m), 1011(m), 943(w),
758(m), 679(s). 31P NMR signal not observed.
“Yb(κ2-NO3)3(TBP)x”. [Yb(κ2-NO3)3·6(H2O)] (0.500 g, 1.07
mmol). FTIR (KBr, cm−1) 3386(m, br), 1634(m), 1533(s),
1471(s), 1325(s), 1273(s), 1241(s), 1023(s), 803(m), 757(s). 31P
NMR δ −26.67(s).
“Lu(κ2-NO3)3(TBP)x”. [Lu(κ2-NO3)3·6(H2O)] (0.500 g, 1.06
mmol). FTIR (KBr, cm−1) 3451(w, br), 2961(w), 2870(w),
1662(w), 1468(m), 1273(s), 1173(m), 1114(m), 1023(s), 989(s),
893(w), 841(w), 744(m), 656(w). 31P NMR δ −3.39(s).
a.ii. Lanthanides/TPhP. In a vial, the appropriate [Ln(κ2-NO3)3·
6H2O] was added to a stirring solution (∼1.5 mL) of acetone and
TPhP (3 equiv). The mixture was sonicated, allowed to stir 15 min,
and then slowly dried in vauco. Samples resulted in off-yellow/brown
oils that ultimately crystallized over time. Meaningful 31P NMR
spectra for some of the [Ln(κ2-NO3)3(TPhP)3] (Ln = Gd, Tb, and
Tm) species could not be obtained.
[Ho(κ2-NO3)3(TPhP)3] (Ho). [Ho(κ2-NO3)3·6(H2O)] (0.250 g,
0.544 mmol) and TPhP (0.533 g, 1.63 mmol). Yield 70.2% (0.508
g). FTIR (KBr, cm−1) 3424(w, br), 3058(w), 2974(w), 2360(w),
1681(w), 1637(w), 1583(w), 1473(s), 1292(s), 1255(s), 1220(m),
1154(s), 1035(s), 967(s), 932(s), 901(m), 812(m), 751(s), 686(s).
31P NMR δ −32.55(s). Elemental Analysis for C54H45HoN3O21P3
(MW = 1329.81) Calc’d: 48.77, C%; 3.41, H%; 3.16, N%. Found:
49.08, C%; 3.99, H%; 2.97, N%.
[La(κ2-NO3)3(TPhP)3] (La). [La(κ2-NO3)3·6(H2O)] (0.500 g, 1.16
mmol) and TPhP (1.13 g, 3.47 mmol). Yield 98.0% (1.49 g). FTIR
(KBr, cm−1) 3386(m, br), 2480(w), 2363(w), 1632(m), 1426(s),
1292(s), 1160(s), 1042(m), 943(m), 817(m), 744(m), 672(m). 31P
NMR δ −14.15(s). Elemental Analysis for C54H45LaN3O21P3 (MW =
1303.78) Calc’d: 49.75, C%; 3.48, H%; 3.22, N%. Found: 50.08, C%;
3.97, H%; 2.84, N%.
[Er(κ2-NO3)3(TPhP)3] (Er). [Er(κ2-NO3)3·6(H2O)] (0.250 g, 0.549
mmol) and TPhP (0.530 g, 1.63 mmol). Yield 99.2% (0.716 g). FTIR
(KBr, cm−1) 3574(w, br), 3080(w), 2529(w), 1869(w), 1580(w),
1506(m), 1480(s), 1311(m), 1239(s), 1210(s), 1149(s), 1014(s),
975(s), 939(s), 894(m), 784(s), 747(s), 683(s). 31P NMR δ
[Ce(κ2-NO3)3(TPhP)3] (Ce). [Ce(κ2-NO3)3·6(H2O)] (0.250 g, 0.576
mmol) and TPhP (0.563 g, 1.73 mmol). Yield 98% (0.736 g). FTIR
C
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Table 2. List of Properties of Ln(NO3)3·6H2O/O = POR3 Powders
a
b
nitrate
POstretch FTIR (cm‑1)
31P NMR (δ, acetone)
TGA/DSC temp (°C)
PXRD phase (1000 °C)
XRF
TBP
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
U
TPhP
La
1275 (s)
1086 (s, br)
1072 (s, br)
1090 (s, br)
1093 (s, br)
1101 (s, br)
1091 (s, br)
1097 (m, br)
1108 (s, br)
1098 (s, br)
1095 (s)
−0.98 (s)
18.88 (d)
14.17 (s)
38.86 (s)
53.94 (s)
7.73 (s)
−12.48 (s)
NSO
NSO
NSO
NSO
−34.02 (s)
NSO
26.67(s)
3.39(s)
0.11 (s)
−17.65
−14.15 (s)
−15.61 (s)
−16.73 (s)
−14.37 (s)
−15.54 (s)
−24.89 (s)
NSO
240
265
230
235
240
240
250
210
245
240
205
190
260
260
175
LaP3O9 (PDF00-033-07176)
CeP3O9 (PDF00-033-0336)
PrP3O9 (PDF00-033-1077)
NdP3O9 (PDF00-027-0322)
Sm(PO4)·xH2O (PDF01-084-0918)
Eu(PO3)3 (PDF00-034-1453)
Gd(PO3)3 (PDF00-052-1761)
TbP3O9 (PDF00-031-1379)
Dy(PO3)3 (PDF00-052-1760)
Ho(PO3)3 (PDF00-052-1763)
Er(PO3)3 (PDF00-055-0209)
Tm(PO3)3 (PDF00-052-1759)
YbP3O9 (PDF00-045-0653)
Lu(PO3)3 (PDF00-035-0399)
U(P2O7) (PDF04-019-3508)
La, P
Ce, P
Pr, P
Nd, P
Sm, P
Eu, P
Gd, P
Tb, P
Dy, P
Ho, P
Er, P
Tm, P
Yb, P
Lu, P
U, P
TPhP
La, P
Ce, P
Pr, P
Nd, P
Sm, P
Eu, P
Gd, P
Tb, P
a
1001 (s)
1011 (m)
1023 (s)
1023 (s)
1227 (m)
1189 (s)
1160 (s)
1011 (s)
1152 (s)
1157 (s)
155(24%), 235(36%)
195(10%), 250(56%)
La(PO4) (PDF 04-002-9468)
Ce(PO4) (PDF00-032-0199)
Pr(PO4) (PDF00-032-0885)
Nd(PO4) (PDF00-025-1066)
Sm(PO4) (PDF00-032-0938)
EuP3O9 (PDF04-001-9480)
Gd(PO4) (PDF04-008-8499)
TbP3O9 (PDF04-006-0692)
Tb(PO4) (04-002-0128)
Ce
Pr
220
230
235
Nd
Sm
Eu
Gd
Tb
1154 (s)
1194 (s)
1154 (s)
1161 (s)
175(15%), 245(40%)
240
215
NSO
Dy
Ho
1166 (s)
1154 (s)
−37.25 (s)
−32.55 (s)
245
220
Dy(PO4) (PDF00-026-0593)
Ho(PO4)·3H2O (PDF00-020-0476)
Ho(PO3)3 (PDF00-052-1763)
Er(PO4) (PDF04-008-3615)
TmP3O9 (PDF04-001-9474)
Yb(PO4)·3H2O (PDF00-020-1398)
Lu(PO4) (PDF00-043-0003)
U(P2O7) (PDF04-019-3508)
Dy, P
Ho, P
Er
1149 (s)
1154 (s)
1011 (s)
1011 (s)
−38.01 (s)
NSO
−18.63 (s)
−14.10 (s)
−18.15 (s)
210
180(5%), 250(34%)
180(5%), 255(30%)
220
Er, P
Tm, P
Yb, P
Lu, P
U, P
Tm
Yb
Lu
U
1228 (m)
180
a
b
NSO = no single observed. Argon and Ca were observed as well from the background gas and holder, respectively.
−38.01(s). Elemental Analysis for C54H45ErN3O21P3 (MW =
1332.13) Calc’d: 48.69, C%; 3.41, H%; 3.15, N%. Found: 48.97, C
%; 3.78, H%; 3.43, N%.
X-ray Crystal Structure Information. For each sample, the
individual single crystals were mounted onto a loop from a pool of
Fluorolube or Parabar 10312 and immediately placed in a 100 K N2
vapor stream. X-ray intensities were measured using a Bruker APEX-II
CCD diffractometer with MoKα radiation (λ = 0.71070 Å) for all
compounds. Indexing, frame integration, and structure solutions were
performed using the BrukerSHELXTL52−54 software package within
Apex354 and/or OLEX255 suite of software. All final CIF files were
checked using the CheckCIF program (http://www.iucr.org/).
Additional information concerning the data collection and final
structural solutions can be found in the Supporting Information or by
accessing CIF files through the Cambridge Crystallographic Data
Base.37 The unit cell parameters for all compounds are available in
Table S1 (see the Supporting Information). Due to the isostructural
nature of 1−14, crystal structures were solved using the coordinates of
the Sm adduct (5) as a basis for refinement.
For each analysis, the samples generated from the above mixtures
were dried (in vacuo), the resulting powder was ground and further
dried, and a free-flowing powder remained. These powders were used
without further manipulation.
Fourier Transformed Infrared Spectroscopy (FTIR). The dried
powders were pressed into KBr pellets on the benchtop. Spectra were
collected on a Bruker Vector 22 MIR spectrometer under an
atmosphere of flowing nitrogen.
[Tm(κ2-NO3)3(TPhP)3] (Tm). [Tm(κ2-NO3)3·6(H2O)] (0.250 g,
0.540 mmol) and TPhP (0.528 g, 1.62 mmol). Yield 99.8% (0.719 g).
FTIR (KBr, cm−1) 3354(w, br), 2981(w), 1684(w), 1641(w),
1470(s), 13679w), 1278(s), 1255(s), 1154(s), 1025(s), 972(m),
904(m), 798(m), 749(s), 681(m). 31P NMR signal not observed.
Elemental Analysis for C54H45N3O21P3Tm (MW = 1333.81) Calc’d:
48.63, C%; 3.40, H%; 3.15, N%. Found: 49.18, C%; 4.05, H%; 2.84, N
%.
[Yb(κ2-NO3)3(TPhP)3] (Yb). [Yb(κ2-NO3)3·6(H2O)] (0.250 g, 0.535
mmol) and TPhP (0.524 g, 1.61 mmol). Yield 98.3% (0.706 g). FTIR
(KBr, cm−1) 3391(w, br), 3072(w), 3012(w), 1951(w), 1583(m),
1480(s), 1318(m), 1295(s), 1269(m), 1245(s), 1152(s), 1014(s),
976(s), 899(s), 808(m), 777(s), 747(s), 681(s). 31P NMR δ
−18.63(s). Elemental Analysis for C54H45N3O21P3Yb (MW =
1338.94) Calc’d: 48.44, C%; 3.46, H%; 3.14, N%. Found: 48.87, C
%; 3.44, H%; 3.23, N%.
[Lu(κ2-NO3)3(TPhP)3] (Lu). [Lu(κ2-NO3)3·6(H2O)] (0.250 g, 0.533
mmol) and TPhP (0.525 g, 1.60 mmol). Yield 93.3% (0.670 g). FTIR
(KBr, cm−1) 3335(w, br), 1677(w), 1578(w), 1484(s), 1229(m),
1248(m), 1213(m), 1145(s), 1030(s), 1011(s), 979(s), 939(m),
904(m), 808(w), 749(s), 679(s). 31P NMR δ −14.10(s). Elemental
Analysis for C54H45LuN3O21P3 (MW = 1340.85) Calc’d: 48.37, C%;
3.46, H%; 3.13, N%. Found: 47.93, C%; 4.05, H%; 2.92, N%.
Thermogravimetric Analysis/Differential Scanning Calorim-
etry (TGA/DSC). All powder samples were carefully loaded under
ambient conditions into a prefired (1000 °C) ceramic boat using a
D
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Figure 1. Structure plot for [Yb(κ2-NO3)3(TPhP)3] (Yb). Thermal ellipsoids are drawn at 30% for the heavy atoms. Hydrogen atoms are omitted
for clarity. The dotted lines represent the bonds for the two (50%) occupancy disordered O53 O atoms.
Mettler Toledo TGA/DSC 1 Stare System. All analyses were
conducted under a flowing argon atmosphere from room temperature
to 1000 °C at a ramp rate of 10 °C/min.
Thermal Processing. For all thermal processing, the sample
Table 3. List of Select, Average Metrical Data for (Ln = La,
Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and
b
[U(O)2(κ2-NO3)2(TPhP)2] (U)
solutions were carefully loaded into a prefired (1000 °C) ceramic boat
and heated in a ThermoScientific Thermolyne (FB1315m) Box
furnace under ambient atmosphere in a ventilated hood. Each sample
was heated to preselected temperatures (450, 650, and 1000 °C), and
after each temperature, the room temperature powder was analyzed
by PXRD.
Powder X-ray Diffraction (PXRD). Powder X-ray diffraction
(PXRD) was performed by a PANalytical X’Pert Pro diffractometer
employing Cu Kα radiation (1.5406 Å) and a RTMS X’Celerator
detector. Data were collected over a 2θ range of 10−100° at a scan
rate of 0.15°/s, and a zero-background holder was employed. The
XRD patterns were analyzed using JADE 9 software (Materials Data,
Inc., Livermore CA) and indexed using The Powder Diffraction File
PDF-4+ 2013.
X-ray Fluorescence (XRF) Spectroscopy. Elemental concen-
trations were determined using a ThermoFisher ARL (West Palm
Beach, FL, US) Quant’X EDXRF spectrometer utilizing UniQuant
software that was used for all analyses. The system uses a
Fundamental Parameters approach based on the Sherman equation
for direct measurement of elemental concentrations based on
integrated fluorescent peak intensities. In air, using a medium count
rate, a 1 repetition, multiscan excitation [C Thin (5 kV, 60 s); Al (12
kV, 100 s), Pd Thick (28 kV, 100 s); Cu Thick (50 kV, 100 s)] was
used to evaluate each sample.
distances (Å)
angles (deg)
Ln−
Ln−
Ln−
O−
Ln−
Ln-TPhP
OP
O2NO
O−N
N−O
OP
ref
La
Ce
Pr
Pr-dmpp
Nd
Sm
Eu
Gd
Tb
Ho
Dy
Dy-dipp
Ho
Ho-dipp
Er
Er-dipp
Tm
Yb
2.50
2.46
2.44
2.45
2.43
2.39
2.38
2.36
2.34
2.32
2.34
2.24
2.32
2.22
2.31
2.22
2.29
2.29
2.28
2.42
2.59
2.53
2.53
2.60
2.51
2.49
2.48
2.47
2.45
2.43
2.44
2.47
2.43
2.47
2.42
2.44
2.42
2.41
2.40
2.50
2.51
96.5
96.4
96.5
102.1
96.6
95.9
96.2
95.9
95.7
94.4
95.5
95.3
94.4
95.1
95.8
95.5
95.3
95.6
95.6
97.4
97.6
116.8
116.9
116.2
117.3
115.7
116.6
116.0
116.2
116.4
118.23
116.5
116.7
118.23
116.4
115.5
115.9
116.4
115.6
115.4
114.4
114.3
154.3
153.8
153.6
153.7
153.6
152.6
152.5
152.1
151.7
150.9
151.2
141.6
150.9
142.9
150.7
140.0
149.9
149.9
149.5
139.0
164.3
a
a
a
35
a
a
a
a
a
a
a
34
a
34
a
34
a
a
a
a
Lu
U
U-OBui
III. RESULTS AND DISCUSSION
2.37
33
a
b
This work. The model compounds have been added near the
The PUREX system has established the utility of organo-
phosphorus compounds in terms of extraction of actinide
waste. Due to the use of nitric acid to solubilize the various
metals found in the process, nitrate ligated species are expected
to be found in the aqueous solution.1−13,56 Some of the
organophosphate uranium nitrate products that have been
crystallographically identified include the monomeric [UO2(κ2-
NO3)2(L)2] (where L = TMP, TEP, TIP) or dimeric [UO2(κ2-
NO3)(μ-L)(L)]2 (where L = TMP, TEP) com-
pounds.12,13,32,33,57 As mentioned, lanthanide cations are
often used as simulants to mitigate the complications
particular cation for completeness. TPhP = triphenylphosphate; dmpp
= (dimethylphosphonato)phenylmethanol; dipp = 2,6-diisopropyl-
phenylphosphate.
encountered using radioactive materials; however, the
structures adopted by organophosphorus modified [Ln(κ2-
NO3)3·6H2O] salts and the subsequent materials they are
converted to are less explored. In fact, only two nitrate
organophoste derivatives have been reported: [Pr(κ2-
NO3)3(κ2-L)] where L = (dimethylphosphonato)-
E
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cooling, antisolvents) from this mixture or stoichiometric
reactions also failed to produce acceptable crystalline materials.
The oils formed with the TBP derivatives may be a reflection
of the alkyl chain present, polymer formation, or potential
hydrolysis products that alkyl phosphates reportedly under-
go.59,60 In order to distinguish between the crystallographically
characterized species, the TBP compounds have been
identified using quotes, inferring only potential empirical
formulations. For the TPhP samples, other minor byproducts
were observed that are currently being fully investigated. In
order to produce “pure” materials, the reaction mixture was
generated, sonicated, and then slowly dried in vacuo to
eliminate any “aged” products. For the samples that crystal-
lized, the structures obtained were identical to those previously
observed, and the additional analytical data of either the 12 h
sample or the sonicated TPhP reactions were self-consistent.
Analytical data (FTIR, 31P NMR, TGA/DSC, PXRD, XRF,
and elemental analyses) were independently collected on the
product material to characterize the products.
Sample Characterization. The FTIR spectrum of the
organophosphorus precursors TBP and TPhP readily displayed
the PO stretches at 1275 and 1189 cm−1, respectively. A
fresh solution of the various [Ln(κ2-NO3)3·6H2O]/organo-
phosphorus reactions was dried to a powder, whereupon each
sample clearly displayed both NO3 and PO bends and
stretches. While the NO3 bends and stretches were consistent
with the parent precursors, the PO of the organophosphate
stretches were shifted. Table 2 lists the PO stretches
observed for the lanthanide TBP and Ln powders. These shifts
are similar to other lanthanide organophosphate species
([Ln5(μ3-OH)(dipp)6(κ2-NO3)x(CH3OH)y(H2O)z]2+ dipp =
2,6-diisopropylphenylphosphate])34 which had strong absorp-
tions for PO noted at 1100 cm−1. Based on these data, it is
clear that the organophosphates have bound to the various
metals without displacing the nitrates.
Table 4. H-Bonding Interactions between P−OPh and NO3
Moieties for Gd
H-bond interaction
(atoms)
C−O
H−O
C−H−O
(Å)
(Å)
(deg)
symmetry
C(523)−H(523)−O(22)
C(533)−H(533)−O(22)
C(623)−H(623)−O(21)
3.47
2.55
2.69
2.61
163.8
158.3
158.2
x−1/2,11/2−
y,1−z
3.592
3.515
x−1/2,11/2−
y,1−z
2−x,y−1/2,
11/2−z
C(535)−H(535)−O(33)
C(424)−H(424)−O(33)
C(543)−H(543)−O(33)
3.314
3.418
3.374
2.44
2.52
2.63
157.8
157.3
135.9
x−1,y,z
x−1,y,z
x−1/2,1/2−y,
1−z
C(533)−H(533)−O(32)
3.408
2.67
135.5
x−1/2,11/2−
y,1−z
C(442)−H(442)−O(23)
C(626)−H(626)−O(13)
3.354
3.084
2.72
2.94
124.9
89.6
x−1,y,z
x−1,y,z
phenylmethanol35 and a series of polymeric lanthanide
dimethylphosphate compounds.36 It is of note that a wide
range of complex organophosphate ligands are available for the
lanthanide cations, including the following: tris(bis(2,6-
dimethylphenyl)hydrogen phosphate)-tris(bis(2,6-
dimethylphenyl)phosphato)-neodymium(iii) benzene sol-
vate,42 tetrakis(m-bis(2,6-dimethylphenyl)phosphato)-(bis-
(2,6-dimethylphenyl)hydrogen phosphate)-bis(bis(2,6-
dimethylphenyl)phosphato)-dineodymium(III) benzene sol-
vate,42 catena(tris(μ-dimethylphosphato-O,O″)-(trimethyl-
phosphate-O)-lanthanum),38 catena-(dodecakis(μ-diethyl-
phosphato)-bis(triethylphosphate)tetracerium(III)),45 bis(tri-
isobutylphosphonato-O)-tris(4-benzoyl-3-methyl-1-phenyl-5-
pyrazolonato-O,O′)-europium,40,58 hexakis(μ3-η5-cyclopenta-
dienylcarboxylato)-bis(μ−η5-cyclopentadienylcarboxylato)-
tetrakis(trimethyl phosphate)-tetracosacarbonyl-dierbium-non-
amanganese toluene solvate,44 and tris(diethyl(1-methylvinyl)-
phosphate)-tris(picrato)-europium(III).43
As can be determined from this family of compounds, the
molecules formed from the standard TBP or TPhP ligands
with the lanthanide cations have not been previously
crystallographically characterized. In order to fill this void,
the synthesis of the [Ln(κ2-NO3)3·nH2O] series in the
presence of TBP and TPhP was undertaken, and the
organophosphorus lanthanide products formed were evaluated
by FTIR spectroscopy, 31P NMR spectroscopy, TGA, and
single crystal X-ray studies when possible. The compounds
were also thermally processed to mimic pyrolysis, and the final
materials were characterized by PXRD and XRF.
Further characterization of these powders redissolved in the
parent solution was undertaken using NMR spectroscopy. The
paramagnetic nature of the various Ln cations prevents
obtaining sharp peaks in the spectral data. Further, all attempts
for the TBP compounds were obfuscated by the excess TBP or
kerosene solvent necessary for dissolution. For the TPhP
1
samples, the H NMR spectra revealed road peaks for the
phenyl ring and did not significantly add to an understanding
of the purity of the final compounds. Therefore, efforts focused
on using 31P NMR, with the resultant data being tabulated in
Table 2. For the “free” organophosphates, a single 31P
resonance was observed: TBP (δ = −0.98 ppm, kerosene)
and TPhP (δ = −17.65 ppm, acetone). When a 31P signal
could be obtained on the lanthanide-TBP or TPhP (Ln)
samples, only a single resonance was noted. Addition of the
free ligand to these mixtures only resulted in a shift of the
signal and not an ingrowth of a new resonance. This is
attributed to the rapid exchange the Ln cations are known to
undergo, especially for monodentate ligands. In some instances
when the Ln samples did not display a 31P signal, but this was
overcome by running samples at higher concentrations.
Therefore, all samples were ultimately analyzed at as high
concentration as possible. A 31P signal was not observed for the
following cations: TBP derivatives: Gd, Tb, Tm, Dy, and Ho
versus TPhP derivatives: Gd, Tb, Tm. This may be a reflection
of low solubility or the signal being outside of the 31P spectral
window of the instrument used. Further, while lanthanides
have unpaired electrons and f-orbitals, electron spin can be
Sample Preparation. Full dissolution of the [Ln(κ2-
NO3)3·6H2O] in the 30% TBP/kerosene stock solution was
facile for the majority of starting materials, but low heating was
required for [Ln(κ2-NO3)3·6H2O] (TBP: Ln = Yb, Lu). There
was no visual evidence of any changes upon dissolution for
these samples with the solution adopting the color of the
powders used. Due to solubility of the TPhP, the appropriate
[Ln(κ2-NO3)3·6H2O] was dissolved in acetone, and then a
stoichiometric amount (3 equiv) of TPhP was added. No
samples required heating to dissolve. The mixtures were stirred
for 12 h with a pale-yellow color forming over time for the
noncolored samples. Slow evaporation of the volatile
component of the TBP/kerosene led to oils, while the
TPhP/acetone mixtures led to a mixture of crystalline materials
in an organic matrix. Other attempts to generate crystals (i.e.,
F
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Figure 2. PXRD pattern of processed (a) YbP3O9 (PDF 00-045-0653) from “Yb(κ2-NO3)3(TBP)x” and (b) YbPO4·3H2O (PDF 00-020-1398)
from [Yb(κ2-NO3)3(TPhP)3] (Yb).
delocalized into σ and π orbitals of the ligand. For the n-Bu
and Ph moieties of the TBP and TPhP ligands, respectively,
different σ and π orbitals will be available and may also explain
the variation noted above for active 31P signals. While the
collective observed lanthanide-TBP 31P resonances were
shifted downfield of the TBP free ligand, the observed
TPhP-lanthanide shifts appear to be more aligned with what
nuclei they are bound to. The early (La-Sm) and late (Yb-Lu)
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hydrogen bonds, the enantiomers must differ by at least an av
of 0.58 kcal/mol (5.2 kcal/mol/9 bonds). Differences in H-
bonding energies of this magnitude are reasonable and could
account for the failure to observe the missing enantiomer. It
would be useful to compare these intermolecular interactions
with the enantiomeric partner, but again, surprisingly, none
were isolated.
Further evaluation of the model Gd system, reveals nine C−
H···O hydrogen bonding interactions between molecules (see
Table 4). The bonds represent donations from a C−H group
of an OC6H5 ring to an oxygen on a NO3 ligand: five to
terminal and four to Gd-bonded NO3 oxygens, with no clear
difference between the two types. A structure literature search
for a C−H group from a P−OPh ring that H-bonds to a NO3
ion yielded 26 structures.37 Of these, the H-bonding was found
to primarily occur through the meta phenyl ring H, and some
para interactions but no ortho protons were involved in H-
bonding. The majority of structures (17) has one such
interaction, six have two, two have three, and one has four
OPh−H−NO3 bonds.37 In contrast, the Ln structures reported
here have eight (the last entry in Table 4 should not be
considered a H-bond) and are the principle force guiding the
packing of the final molecules. The metrical parameters found
for the C−H···O hydrogen bonds of these Ln species are
consistent with those observed in the literature.37
Elemental analyses of the resulting products were under-
taken for the TPhP samples. Again, the TBP products were not
evaluated by this means as they could not be meaningfully
analyzed due to the excess TBP present. The data proved to be
consistent with three TPhP species bound to a hydrate free
“Ln(NO3)3”. Further, if this analysis was a result of the simple
mixture of the two components in the correct stoichiometry,
significantly less %C would be available. For example, the Lu
sample for mixed (C, 44.79; H, 3.97; N, 2.90%) versus reacted
(C, 48.37; H, 3.46; N, 2.90%) illustrate the difference.
Therefore, the bulk materials were consistent with the
observed crystal structures.
TGA/DSC. The thermal behavior of the organophosphate
modified [Ln(κ2-NO3)3·6H2O] species was investigated
through TGA/DSC experiments. While the overall weight
losses could not be calculated accurately for either the TBP or
TPhP species due to the inclusion of byproducts or unreacted
precursor, the main decomposition temperatures were of
interest to understand the various ligand losses and conversion
during heating. The TBP derivatives surprisingly had a single
weight loss step at ∼250 °C except for the Er, Tm, and Tb
bearing samples which occurred at lower temperatures (∼200
°C). This indicates that the nitrate and TBP ligands are lost at
the same temperature, which may have important implications
for fire experiments, where the nitrates may act as a fire
retardant. In comparison, the majority of TPhP species was
also thermally decomposed in a single step around 250 °C, but
several species (La, Ce, Eu, Tm, and Yb) had two steps with
the first weight loss around ∼180 °C. This multistep
decomposition may represent the nitrate-TPhP species or
other compounds known to be present in the complex mixture.
This decomposition and the lack of higher temperature weight
loss was found to be consistent with the thermal decom-
p o s i t i o n o f t h e [ L n 5 ( μ 3 - O H ) ( d i p p ) 6 ( κ 2 -
NO3)x(CH3OH)y(H2O)z]2+ system.34
Figure 3. Structure plot of [U(O)2(NO3)2(TPhP)2] (U). Thermal
ellipsoids are drawn at 30% for the heavy atoms. Hydrogen atoms are
omitted for clarity.
were shifted downfield, and the middle cations either were not
observed or shifted significantly upfield.
The structure literature indicates two organophosphorus
ligands should be present on the uranium12,13,32,33,57 and
possibly four for Ln35 (two bidentate phosphates) nitrate
species. All attempts to crystallize the final products to aid in
identifying the TBP binding were not successful in our hands;
however, the Ln species all successfully crystallized. The
isomorphous family of compounds were identified as
monomeric compounds, where the lanthanide cation adopts
a 9-coordinated, tricapped trigonal prismatic (ttp) geometry.
Figure 1 shows the general structure type, using Yb as the
representative compound (note: there was some disorder in the
oxygen of one of the ligands indicated by dashed lines to the
50% occupancy). For each of these structures, the three κ2-
NO3 ligands were located in the capping position, and the
TPhP ligands were coplanar in the equatorial (trigonal)
positions. For the structure solutions, all of the heavier Ln
cations were found to have slightly disordered OPh groups,
which did not occur for the lighter metals. This is produced
from an offset of the phenyl ring, in one of two positions, as
noted in the Ln−O−C angle. The metrical data of the first
lanthanide nitro-triphenyl phosphate derivatives reported were
self-consistent (see Table 3). For comparison, literature data
for related compounds35,36 are included in the table.
All of the Ln structures isolated in this work are
isomorphous and isostructural, crystallizing in the P212121
space group, which has only translational symmetry.
Approximately 90% of the crystals isolated in this space
group are chiral, and enantiomeric pairs should therefore be
available. For each structure, the coordinates of Sm were used
as a starting point for refinement; thus, only one of the two
possible packing patterns occurs, and the material sponta-
neously resolves upon crystallization as the same enantiomer.
Remarkably the structures all refined to Flack parameters of 0
( 0.05), indicating that the 14 structures all adopt the same
enantiomeric structure. If the odds of 14 examples of a binary
choice are the same, this sets a lower limit for ΔG < 5.2 kcal/
mol (i.e., K = 214 or 16384); as there are nine C−H···O
The residual powder by XRF analyses indicated the
appropriate lanthanide cations were present along with P.
The presence of the P was surprising, as literature refers only
H
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Figure 4. PXRD patterns of U(P2O7) (PDF 04-019-3508) from (a) “U(O)2(NO3)2(TBP)2” and [U(O)2(NO3)2(TPhP)2] (U).
to oxides being formed.16 Therefore, additional studies were
undertaken where a TPhP stock solution of the [Sm(κ2-NO3)3·
6H2O] sample was placed in a box furnace and heated to 450
°C in air. The material proved to be amorphous by PXRD. The
sample was heated (650, 750, and 850 °C) but remained
amorphous until it was heated to 1000 °C, whereupon it was
identified by PXRD as samarium phosphate (PDF 01-084-
0918). Due to the unexpected phase formed, additional
[Ln(κ2-NO3)3·6H2O] mixed in TBP and TPhP stock solutions
was produced and processed at 1000 °C in air. PXRD patterns
and XRF spectra were collected, and Figure 2a and 2b show
the representative PXRD patterns for the thermal byproduct
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for the Yb-TBP and Yb-TPhP (Yb) powders, respectively.
Independent of the cation or the organophosphorus modifier
used, each sample was found to possess both Ln and P by XRF
and to form a phosphate phase (see Table 2). From these data,
it was found that the majority of the TBP samples formed the
single-phase anhydrous lanthanide metaphosphate phase. The
one exception was the Sm derivative that was identified as the
phosphate hydrate material. In contrast, the TPhP species
mainly generated the anhydrous phosphate powders with both
Eu and Tm forming the metaphase. Two samples (Tb and
Ho) had both the easily interconverted metaphosphate and
phosphate phases present, which was surprising based on the
fact that the literature on potential PUREX aerosol efforts has
focused on metal oxide behavior. In particular, cerium oxide
(CeO2) is often sprinkled over paper “trash” to mimic
contaminated waste.61 Traditional efforts have merely focused
characterizing the percent of transferred metal;61,62 however,
this may be an erroneous approach due to the actual formation
of phosphates from the PUREX route.
Uranium. In order to verify the lanthanide simulants
possessed the same chemistry as uranium, a U-nitrate,
organophosphorus sample was prepared. Crystalline material
(U) was isolated by slow evaporation of the reaction mixture
and is shown in Figure 3 (see Table 3 for metrical data). The
monomeric U metal center has a central core that consists of
two κ2-NO3 and two trans-TPhP ligands forming an 8-
coordinated geometry best described as a slightly distorted
hexagonal bipyramid (D6h)), with an ∼10° torsional twist in
the plane. The remainder of the coordination is filled by the
two oxo moieties. The U−OP and U−O2NO distances are
slightly longer after correction for cation size.63 Interestingly,
the U−OP angle is about 10° smaller than those noted for
the Ln compounds.
the XRF revealed the presence of both U and P and the PXRD
pattern (see Figure 4a and 4b for the TBP and TPhP,
respectively, powder patterns) showed the formation of the
pyrophosphate (U(P2O7): PDF 04-019-3508) phase for both
of the U-organophosphorus derivatives. While the phase
observed for the final material from the U or the Ln systems
was not identical, both were phosphates and not the often used
oxide61,62 phase for aerosol studies. Stable phosphate phases
noted for uranium favor the countercation groups, such as the
Autunite group of minerals (M3n(n)+[(UO2)(PO4)]2·xH2O)
(M = Ca, Na)64 or the Cs[UO2)x(HPO3)y(H2O)z]65 family of
materials. It is of note that metaphosphate (U(PO3)4) has been
shown to decompose into the pyrophosphate (U(P2O7),66
which may explain the variation in the phases noted, as the
LnPO4 materials67 are reportedly stable to >900 °C.
IV. SUMMARY AND CONCLUSION
The investigation of the pyrolysis behavior of lanthanide
simulants for actinide aerosol PUREX species was undertaken.
The synthesis of extraction solutions (kerosene and organo-
phosphorus extractants) with [Ln(κ2-NO3)3·6H2O)] led to the
isolation of [Ln(κ2-NO3)3(TPhP)3] for the TPhP solutions,
which were crystallographically characterized as 9-coordinated
monomers. [U(O)2(κ2-NO3)2(TPhP)2] (U) was isolated from
a similar reaction mixture, as a 8-coordinated species. Thermal
processing of the Ln precursors and U led to the phosphate
material, not the oxide, that is often assumed to form upon
pyrolysis, which may impact aerosol transport, toxicity, and
other safety assessment determinations. In general, the Ln
family of organophosphorus nitrates and the U derivative have
enough similarities to suggest they would act as reasonable
surrogates for aerosol investigations. Studies on the pyrolysis
behavior of the resultant materials have been reported
elsewhere and indicated substantial differences exist between
the lanthanide and uranium organophosphorus species during
aerosolization.16 Efforts to understand the subtle differences
that influenced this behavior of these to precursors are
underway.
For comparison, only three uranyl nitrate phosphate
derivatives (OP(OR)3 R = Me,13 Et,32,12 and Bui33) are
available. These derivatives are similar to U, adopting a
hexagonal bipyramid (hbp) geometry. The best model is the
33
OBui derivative (see Table 3). Not surprisingly, the metrical
data are in agreement except the U−OP angle. For the other
uranium species, this angle ranges from 153.4−166.52°,
whereas U has a much smaller angle of 139.0°. This angle
reflects the Ph rings bending away from each other, causing the
molecule to flatten. In evaluating the Ln species versus the U
product, it is obvious that different coordination spheres
(coordination and ligands) and oxidation states exist for these
compounds: Ln3+ (ttp with three TPhP) versus U6+ (hbp with
two TPhP and two oxides). While this would be problematic in
a chemistry sense in terms of surrogates, for materials
applications, the similarity of the various ligands present and
the monomeric nature bodes well for similar thermal
conversion products and their ultimate utility of the Ln as
surrogates.
The dried crystalline U was analyzed under identical
conditions noted for the Ln species (see Table 2) to determine
the utility of these compounds as surrogates. As can be
observed within the FTIR data, there is a shift in the OP
stretch upon complexation of the TBP and TPhP ligands
versus the free ligands stretch. This overall shift is much
smaller for the TPhP ligand than the TBP ligand. Further, the
shift for the ligands is much smaller when binding to U in
comparison to the lanthanide ions. Thermal analyses revealed a
much lower decomposition temperature and a much smaller
31P chemical shift for U vs the Ln complexes. After processing,
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02428.
Crystallographic data for La, Ce, Pr, Nd, Sm, Eu, Gd,
Tb, Dy, Ho, Er, Tm, Yb, Lu, and U, respectively (PDF)
Accession Codes
CCDC 2017493−2017507 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Timothy J. Boyle − Advanced Materials Laboratory, Sandia
National Laboratories, Albuquerque, New Mexico 87106,
United States; orcid.org/0000-0002-1251-5592;
Phone: (505)272-7625; Email: tjboyle@sandia.gov;
Fax: (505)272-73363939
J
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J. Radioanal. Nucl. Chem. 2014, 300, 93−98.
Authors
Xavier J. Robinson − Advanced Materials Laboratory, Sandia
National Laboratories, Albuquerque, New Mexico 87106,
United States
Fernando Guerrero − Advanced Materials Laboratory,
Sandia National Laboratories, Albuquerque, New Mexico
87106, United States
Diana Perales − Advanced Materials Laboratory, Sandia
National Laboratories, Albuquerque, New Mexico 87106,
United States; orcid.org/0000-0001-6885-9856
Joshua A. Hubbard − Sandia National Laboratories,
Albuquerque, New Mexico 87110-0828, United States
Roger E. Cramer − Department of Chemistry, University of
Hawaii - Manoa, Honolulu, Hawaii 96822-2275, United
States; orcid.org/0000-0002-3934-3401
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Complete contact information is available at:
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Notes
Any subjective views or opinions that might be expressed in
the paper do not necessarily represent the views of the U.S.
Department of Energy of the United States Government.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was funded by the Nuclear Safety Research and
Development Program, which is managed by the Office of
Nuclear Safety, within the Office of Environment, Health,
Safety and Security to provide corporate-level leadership
supporting nuclear safety research and development through-
out the DOE. Sandia National Laboratories is a multimission
laboratory managed and operated by National Technology &
Engineering Solutions of Sandia, LLC, a wholly owned
subsidiary of Honeywell International Inc., for the U.S.
Department of Energy’s National Nuclear Security Admin-
istration under contract DE-NA0003525. This paper describes
objective technical results and analysis.
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