Synthesis and lanthanide coordination chemistry of phosphine oxide decorated dibenzothiophene and dibenzothiophene sulfone platforms

Article abstract of DOI:10.1021/ic500471w

Syntheses for new ligands based upon dibenzothiophene and dibenzothiophene sulfone platforms, decorated with phosphine oxide and methylphosphine oxide donor groups, are described. Coordination chemistry of 4,6- bis(diphenylphosphinoylmethyl)dibenzothiophene (8), 4,6- bis(diphenylphosphinoylmethyl)dibenzothiophene-5,5-dioxide (9) and 4,6-bis(diphenylphosphinoyl)dibenzothiophene-5,5-dioxide (10) with lanthanide nitrates, Ln(NO₃)₃·(H₂O)_n is outlined, and crystal structure determinations reveal a range of chelation interactions on Ln(III) ions. The nitric acid dependence of the solvent extraction performance of 9 and 10 in 1,2-dichloroethane for Eu(III) and Am(III) is described and compared against the extraction behavior of related dibenzofuran ligands (2, 3; R = Ph) and n-octyl(phenyl)-N,N- diisobutylcarbamoylmethyl phosphine oxide (4) measured under identical conditions.

Full text of DOI:10.1021/ic500471w

Article
pubs.acs.org/IC
Synthesis and Lanthanide Coordination Chemistry of Phosphine
Oxide Decorated Dibenzothiophene and Dibenzothiophene Sulfone
Platforms
Daniel Rosario-Amorin,† Sabrina Ouizem,† Diane A. Dickie,† Robert T. Paine,*,† Roger E. Cramer,‡
Benjamin P. Hay,§ Julien Podair,§ and Lætitia H. Delmau§
†Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States
‡Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822, United States
§Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, United States
S
* Supporting Information
ABSTRACT: Syntheses for new ligands based upon
dibenzothiophene and dibenzothiophene sulfone platforms,
decorated with phosphine oxide and methylphosphine oxide
donor groups, are described. Coordination chemistry of 4,6-
bis(diphenylphosphinoylmethyl)dibenzothiophene (8), 4,6-
bis(diphenylphosphinoylmethyl)dibenzothiophene-5,5-dioxide
(9) and 4,6-bis(diphenylphosphinoyl)dibenzothiophene-5,5-
dioxide (10) with lanthanide nitrates, Ln(NO3)3·(H2O)n is
outlined, and crystal structure determinations reveal a range of
chelation interactions on Ln(III) ions. The nitric acid
dependence of the solvent extraction performance of 9 and 10 in 1,2-dichloroethane for Eu(III) and Am(III) is described
and compared against the extraction behavior of related dibenzofuran ligands (2, 3; R = Ph) and n-octyl(phenyl)-N,N-
diisobutylcarbamoylmethyl phosphine oxide (4) measured under identical conditions.
particular, the rigid 4,6-bis(X)2dibenzofuran, [4,6-(X)2DBF,
1], fragment has been used to form acyclic22−35 and
macrocyclic36−40 ligands decorated primarily with soft donor
groups incorporated in the X substituents. The coordination
chemistry of these ligands has not been extensively explored,
but in some complexes the DBF O atom participates in chelate
formation,23,24,27,33,35 while in other complexes it does
not.25,28−30 Recently, with very different goals in mind, the
decoration of the 4,6-(X)2DBF platform with hard diphenyl-
phosphine oxide fragments was accomplished.41,42 Han and co-
workers41 prepared and studied 4,6-bis(diphenylphosphinoyl)-
dibenzofuran (2, R = Ph) as a host for blue and green
electrophosphorescence materials, while we prepared and
explored the chelation properties of this molecule and the
related 4,6-bis(diphenylphosphinoylmethyl)dibenzofuran (3, R
= Ph) with hard Ln(III) and Pu(IV) ions.42 Bidentate OPO′P′
binding was structurally confirmed in several isolated
complexes of 2 and 3, but no examples of tridentate
OPOfuranO′P′ binding were observed. This preference parallels
the expected weak donor strength of the furan O atom as well
as ligand strain analysis42 that compared both the energetics of
tridentate OPOfuranO′P′ versus bidentate OPO′P′ structures
formed by 2 and 3 and the donor bond vector convergence
in tridentate and bidentate structures. Consistent with the
INTRODUCTION
■
A large number of organic molecules have been used as
platforms to append and organize donor groups such that the
resulting functionalized molecules behave as chelating ligands
toward metal ions. The fundamental coordination properties of
such ligands have been extensively studied, and practical
applications for the ligands and their complexes are numerous.
As examples, we have recently reported on the use of 2-pyridine
N-oxide, [2-(X)C5H4NO], and 2,6-pyridine N-oxide, [2,6-
(X)2C5H3NO], platforms to prepare chelating ligands deco-
rated with functional arms X = phosphine oxide, −[P(O)R2],1,2
phosphonate, −[P(O)(OR)2],3 methylphosphine oxide,
−[CH2P(O)R2],4−13 methylphosphonate, −[CH2P(O)-
(OR)2],14 methylphosphinic acid, −CH2P(O)(OH)2],14 meth-
ylphosphine sulfide, −[CH2P(S)R2],15 and methylamido,
−[CH2C(O)N(H)R]16 and −[CH2CH2C(O)NR2].17 These
ligands generally favor binding with hard lanthanide cations;
the 2-(X)C5H4NO compounds adopt bidentate, ONOP,4,13 or
17
ONOC chelate interactions, while the 2,6-(X)2C5H3NO
11−17
compounds bond as tridentate OPONO′P′
or OCONO′C′
chelates. In turn, favorable solubility and binding/release
thermodynamic and kinetic features for several of the
OPONO′P′-class ligands and complexes support interesting
solvent extraction performance for trivalent f-block element
cations in acidic aqueous solutions.18−21
Furan and dibenzofuran platforms have also been used in the
past to derive multifunctional chelating ligands, and, in
Received: February 27, 2014
Published: May 20, 2014
© 2014 American Chemical Society
5698
dx.doi.org/10.1021/ic500471w | Inorg. Chem. 2014, 53, 5698−5711

Inorganic Chemistry
Article
was added NaBH4 (1.46 g, 38.7 mmol), and the mixture was stirred
(23 °C, 2 h) under nitrogen. The resulting mixture was concentrated
to 10 mL, and water (30 mL) was added. The precipitate formed was
collected by filtration and washed with cold ethanol and CH2Cl2 1to
give 6 as a white powder: yield 2.90 g, 92%; mp 216−218 °C. H
modeling analyses, it was also deduced from solvent extraction
data that 2 (R = Ph) is a better extractant toward Am(III) and
Eu(III) than 3 (R = Ph) and that 2 is comparable in extraction
performance to the commercial extractant n-octyl(phenyl)-
N,N-diisobutylcarbamoylmethylphosphine oxide (OΦDi-
BCMPO, 4).43
In efforts to extend their studies of optoelectronic properties,
Han and co-workers44 recently reported a synthesis for the 4,6-
dibenzothiophene (X2DBT) analogue of 2, 4,6-bis-
(diphenylphosphinoyl)dibenzothiophene, and they examined
the impact of this substitution on electron injection and
transportation. Extending our prior studies of the coordination
behavior of X2DBF platform ligands 2 and 3 on Ln(III) ions,
we report here syntheses for the DBT analogue of 3, 4,6-
bis(diphenylphosphinoylmethyl)dibenzthiophene, its sulfone
derivative and the sulfone derivative of 4,6-bis-
(diphenylphosphinoyl)dibenzothiophene, selected coordina-
tion chemistry with Ln(III) ions, and initial characterization
of the solvent extraction behavior of the sulfone derivatives.
NMR (300 MHz, d6-dimethyl sulfoxide (DMSO)): δ 8.26 (d, JHH
=
5.1 Hz, 2H, H5), 7.52 (m, 4H, H3,4), 5.57 (s, 2H, OH) 4.80 (s, 4H,
H1). 13C{1H} NMR (75.5 MHz, d6-DMSO): δ 136.53 (C7), 136.43
(C6), 135.38 (C2), 124.66 (C4), 124.50 (C3), 120.62 (C5), 62.24 (C1).
FTIR (KBr, cm−1): 1484, 1426, 1349, 1324, 1245, 1173, 1135, 1078,
1060, 1036, 765. HRMS(ESI): m/z (%): 227.0531 (92) [M −
(OH)+]. C14H11OS requires 227.0531; 267.0461 (100) [M + Na+].
C14H12O2SNa requires 267.0456; 511.1023 (97) [2 M + Na+].
C28H24O4S2Na requires 511.1014.
4,6-Bis(chloromethyl)dibenzothiophene (7). A suspension of 6
(3.05 g, 12.5 mmol) and triphenylphosphine (9.82 g, 37.5 mmol) in
dry CH2Cl2 (100 mL) was cooled (0 °C), and N-chlorosuccinimide
(5.00 g, 37.5 mmol) was added with stirring. The temperature was
maintained at 0 °C until the mixture became homogeneous, and then
the solution was warmed (23 °C) and stirred (12 h). The resulting
solution was washed with water (3 × 25 mL), dried (anhydrous
MgSO4), and filtered, and the solvent was vacuum evaporated. The
brown residue obtained was purified by column chromatography
(silica gel, eluted with hexane/CH2Cl2 70:30) leaving 7 as a pale
1
yellow powder: yield 2.42 g, 69%; mp 184−186 °C. H NMR (300
MHz, CDCl3): δ 8.15 (dd, JHH = 7.2, 1.6 Hz, 2H, H5), 7.56−7.48 (m,
4H, H3,4), 4.92 (s, 4H, H1). 13C{1H} NMR (75.5 MHz, CDCl3): δ
138.94 (C7), 136.61 (C6), 131.75 (C2), 127.67 (C3), 125.30 (C4),
122.27 (C5), 45.21 (C1). FTIR (KBr, cm−1): 1444, 1425, 1388, 1314,
1264, 1239, 1170, 1147, 1095, 1045, 976, 783, 759, 730, 677.
HRMS(ESI): m/z (%): 302.9771 (86) [M + Na+]. C14H10Cl2SNa
requires 302.9778; 318.9508 (100) [M + K+]. C14H10Cl2SK requires
318.9517.
EXPERIMENTAL SECTION
■
General Information. Organic reagents (Aldrich Chemical) and
metal nitrates (Ventron) were used as received, and organic solvents
(VWR) were dried and distilled by standard methods. Reactions were
performed under dry nitrogen unless specified otherwise. Infrared (IR)
spectra were recorded on a Bruker Tensor 27 benchtop spectrometer.
Solution NMR spectra were recorded with Bruker Avance-300 and
-500 multinuclear spectrometers using Me4Si (1H, 13C) and 85%
H3PO4 (31P) as external standards. Downfield shifts from the reference
resonances were given +δ values.45 Mass spectra for the ligands were
obtained from the University of New Mexico Mass Spectrometry
Center by using electrospray ionization (ESI) with a Waters/
Micromass mass spectrometer operating in the positive ion mode.
Elemental analyses were performed by Galbraith Laboratories.
4,6-Bis(diphenylphosphinoylmethyl)dibenzothiophene (8). A sol-
ution of 7 (1.08 g, 3.84 mmol) in ethyl diphenylphosphinite (4.90 mL,
22.7 mmol) was heated at 140 °C (14 h). After cooling (23 °C), the
resulting white residue was collected and washed with diethyl ether (4
× 30 mL) giving 8 as a white powder: yield 2.27 g, 97%; mp 284−286
°C. 31P{1H} NMR (121.5 MHz, CDCl3): δ 30.1. 31P{1H} NMR
(121.5 MHz, d4-MeOH): δ 34.4. 1H NMR (300 MHz, CDCl3): δ 7.94
(d, JHH = 7.6 Hz, 2H, H5), 7.76 (dd, JHH = 11.2 Hz, 7.3 Hz, 8H, H9),
7.53−7.38 (m, 14H, H3,10,11), 7.29 (t, JHH = 7.5 Hz, 2H, H4), 3.90 (d,
1
JHP = 13.2 Hz, 4H, H1). H NMR (300 MHz, d4-MeOH): δ 8.03 (d,
JHH = 7.8 Hz, 2H, H5), 7.79 (dd, JHH = 11.7 Hz, 7.8 Hz, 8H, H9),
7.60−7.50 (m, 12H, H10,11), 7.32−7.22 (m, 4H, H3,4), 4.07 (d, JHP
=
13.5 Hz, 4H, H1). 13C{1H} NMR (75.5 MHz, CDCl3): δ 140.17 (d,
JCP = 6.6 Hz, C7), 136.41 (C6), 132.42 (d, JCP = 99.8 Hz, C8), 132.01
(C11), 131.19 (d, JCP = 9.4 Hz, C9), 128.63 (d, JCP = 11.7 Hz, C10),
128.26 (d, JCP = 4.3 Hz, C3), 126.30 (d, JCP = 7.5 Hz, C2), 124.97 (C4),
120.47 (C5), 36.98 (d, JCP = 67.3 Hz, C1). FTIR (KBr, cm−1): 2933,
1437, 1388, 1188 (νPO), 1119, 1030, 852, 836, 783, 757, 741, 723,
693, 555, 536, 514. HRMS(ESI): m/z (%): 613.1526 (35) [M + H+].
C38H31O2P2S requires 613.1520; 635.1353 (100) [M + Na+].
C38H30O2P2SNa requires 635.1339; 651.1081 (12) [M + K+].
C38H30O2P2SK requires 651.1071. Anal. Calcd for C38H30O2P2S: C
74.50, H 4.94, P 10.11. Found: C 74.26, H 4.96, P 9.75%.
4,6-Bis(diphenylphosphinoylmethyl)dibenzothiophene-5,5-diox-
ide (9). To a solution of 8 (1.10 g, 1.79 mmol) in dry CH2Cl2 (20 mL)
was slowly added 3-chloroperoxybenzoic acid (77 wt %, 1.60 g, 7.14
mmol). The mixture was stirred (23 °C, 12 h); the resulting mixture
was diluted with additional CH2Cl2 (100 mL) and washed with
aqueous NaOH (2N, 3 × 25 mL) and distilled water (2 × 25 mL).
The organic phase was dried (anhydrous MgSO4) and filtered, and the
volatiles were removed by vacuum evaporation, leaving a white
powder, 9: yield 1.10 g, 95%; mp 278−280 °C. 31P{1H} NMR (121.5
MHz, CDCl3): δ 30.7. 31P{1H} NMR (121.5 MHz, d4-MeOH): δ 34.0.
1H NMR (300 MHz, CDCl3): δ 8.11 (d, JHH = 7.5 Hz, 2H, H5), 7.88−
Ligand Syntheses. 4,6-Diformyldibenzothiophene (5). The
compound was prepared in a fashion similar to that described in the
literature.46 A solution of dibenzothiophene (5.00 g, 27.1 mmol) and
N,N,N′,N′-tetramethylethylene diamine (TMEDA) (10.1 mL, 67.8
mmol) in dry hexane (150 mL) was cooled (0 °C) under nitrogen
atmosphere, and a solution of n-BuLi (1.6 M in hexane, 42.4 mL, 67.8
mmol) was added dropwise over 30 min. The resulting mixture was
refluxed (15 min), then cooled (0 °C), and dimethylformamide
(DMF) (12.6 mL, 163 mmol) was added. The mixture was allowed to
slowly warm (23 °C, 1 h) while being stirred and was then poured
onto an ice bath. A precipitate formed that was collected by filtration,
washed with water (3 × 50 mL), and recrystallized from toluene
leaving 5 as a white powder: yield 3.17 g, 49%; melting point (mp)
238−240 °C. 1H NMR (300 MHz, CDCl3): δ 10.36 (s, 2H, H1), 8.51
(d, JHH = 7.8 Hz, 2H, H5), 8.08 (d, JHH = 7.5 Hz, 2H, H3), 7.75 (t, JHH
= 7.6 Hz, 2H, H4). 13C{1H} NMR (75.5 MHz, CDCl3): δ 191.20 (C1),
140.72 (C7), 135.67 (C6), 133.48 (C5), 131.40 (C2), 127.04 (C3),
125.02 (C4). Fourier transform infrared (FTIR) (KBr, cm−1): 2846,
2827, 1684, 1560, 1461, 1429, 1385, 1328, 1218, 1183, 996, 874, 776.
High-resolution mass spectrometry (HRMS) (ESI): m/z (%):
241.0331 (52) [M + H+]. C14H9O2S requires 241.0323; 263.0153
(100) [M + Na+]. C14H8O2SNa requires 263.0143.
4,6-Bis(hydroxymethyl)dibenzothiophene (6). The procedure for
conversion of 5 to 6 is similar to that provided in the literature.46 To a
suspension of 5 (3.10 g, 12.9 mmol) in MeOH/CHCl3 (1/1, 80 mL)
7.81 (m, 8H, H9), 7.53−7.42 (m, 16H, H3,4,10,11), 4.11 (d, JHP = 13.8
Hz, 4H, H1). 1H NMR (300 MHz, d4-MeOH): δ 7.84−7.77 (m, 10H,
H5,9), 7.65−7.49 (m, 16H, H3,4,10,11), 4.21 (d, JHP = 13.8 Hz, 4H, H1).
5699
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Inorganic Chemistry
Article
Table 1. Crystallographic Data for Ligands 8, 9·2CH2Cl2, and 10·3CHCl3
8
9·2CH2Cl2
10·3CHCl3
empirical formula
crystal size (mm)
formula weight
crystal system
space group
unit cell dimen.
a (Å)
C38H30O2P2S
0.15 × 0.25 × 0.48
612.62
C40H34Cl4O4P2S
0.18 × 0.20 × 0.27
814.47
C39H29Cl9O4P2S
0.16 × 0.22 × 0.53
974.67
orthorhombic
triclinic
monoclinic
P2(1)2(1)2(1) (No. 18)
P1 (No. 1)
P2(1)/n (No. 14)
5.7594(6)
16.772(2)
31.593(4)
90
8.9081(4)
10.2099(5)
11.4562(5)
91.404(3)
97.115(3)
114.544(3)
937.17(7)
1
14.6937(5)
15.6122(5)
20.4382(6)
90
b (Å)
c (Å)
α (deg)
β(deg)
90
94.086(2)
90
γ (deg)
V (Å3)
90
3051.8(6)
4
4676.6(3)
4
Z
T (K)
173(2)
1.333
173(2)
173(2)
Dcald (g cm−3
)
1.443
1.384
μ (mm−1
)
0.245
0.499
0.689
min/max transmission
reflection collected
0.892/0.964
32 159
6722 [0.1349]
0.878/0.914
21 422
0.660/0.900
40 502
independent reflections [Rint]
8828 [0.0392]
9193 [0.0910]
0.0807(0.2450)
0.2450(0.3070)
a
b
c
d
final R indices [I > 2σ(I)] R1 (wR2)
0.0560(0.1046)
0.0395(0.0759)
final R indices (all data) R1 (wR2)
0.1012(0.1229)
0.0505(0.0822)
a
b
c
d
2
2
R1 = (Σ∥F◦| − |Fc∥)/Σ|F◦|. wR2 = [Σw(F◦ − Fc2)2/Σw(F◦ )2]1/2. Flack parameter = 0.04(9). Flack parameter = 0.03(4).
13C{1H} NMR (75.5 MHz, CDCl3): δ 135.34 (d, JCP = 8.4 Hz, C7),
134.03 (C4), 132.34 (d, JCP = 4.4 Hz, C3), 132.16 (C11), 131.80 (d, JCP
= 103.4 Hz, C8), 131.75 (C6), 131.17 (d, JCP = 9.4 Hz, C9), 130.57 (d,
JCP = 6.3 Hz, C2), 128.74 (d, JCP = 12.1 Hz, C10), 120.16 (C5), 31.98
(d, JCP = 66.8 Hz, C1). FTIR (KBr, cm−1): 3072, 2904, 1587, 1480,
1435, 1288 (νSO(as)), 1211, 1194 (νPO), 1157 (νSO(sy)), 1119, 997,
824, 796, 754, 722, 695, 628, 592, 573, 524, 509, 474. HRMS(ESI): m/
z (%): 645.1423 (46) [M + H+]. C38H31O4P2S requires 645.1418;
667.1246 (100) [M + Na+]. C38H30O4P2SNa requires 667.1238.
4,6-Bis(diphenylphosphinoyl)dibenzothiophene-5,5-dioxide (10).
A solution of dibenzothiophene (3.00 g, 16.3 mmol) and TMEDA
(6.75 mL, 45.3 mmol) in dry hexane (100 mL) was cooled (0 °C)
under nitrogen atmosphere, and a solution of n-BuLi (1.6 M in hexane,
28.4 mL, 45.4 mmol) was added dropwise with stirring. The resulting
mixture was refluxed (15 min), then cooled (0 °C), and a solution of
chlorodiphenylphosphine (5.80 mL, 32.5 mmol) in hexane (40 mL)
was added dropwise over 40 min. The mixture was stirred (23 °C, 12
h), then quenched with water (100 mL), and extracted with CH2Cl2 (3
× 100 mL). The organic layer was dried (anhydrous MgSO4) and
filtered, and the solvent was removed under vacuum. The brown
residue obtained was washed with Et2O (3 × 50 mL) leaving a yellow
powder, which was then dissolved in dry CH2Cl2 (100 mL). The
solution was cooled (0 °C), and 3-chloroperoxybenzoic acid (77 wt %,
21.9 g, 97.7 mmol) was added slowly. The mixture was stirred (23 °C,
4 h); the resulting mixture was diluted with additional CH2Cl2 (100
mL) and washed with aqueous NaOH (2N, 3 × 25 mL) and distilled
water (2 × 25 mL). The organic phase was dried (anhydrous MgSO4)
and filtered, and the volatiles were removed by vacuum evaporation.
The pale yellow residue obtained was purified by column
chromatography (silica gel, eluted with CH2Cl2/MeOH 97:3) leaving
10 as a white powder: yield 4.90 g, 49%; mp > 380 °C decomposition.
31P{1H} NMR (121.5 MHz, CDCl3): δ 27.7. 31P{1H} NMR (121.5
MHz, d4-MeOH): δ 32.0. 1H NMR (300 MHz, CDCl3): δ 7.98 (d, JHH
= 6.0 Hz, 2H, H4), 7.85−7.62 (m, 12H, H2,3,8), 7.55−7.50 (m, 4H,
H10), 7.45−7.37 (m, 8H, H9). 1H NMR (300 MHz, d4-MeOH): δ 8.34
(d, JHH = 6.0 Hz, 2H, H4), 7.76 (m, 2H, H3), 7.63−7.43 (m, 22H,
H2,8,9,10). 13C{1H} NMR (75.5 MHz, CDCl3): δ 140.55 (C6), 135.82
(d, JCP = 8.5 Hz, C2), 132.98 (C3), 132.77 (d, JCP = 10.1 Hz, C8),
132.41 (C10), 131.49 (d, JCP = 107.5 Hz, C7), 131.31 (d, JCP = 78.1 Hz,
C1), 129.81 (d, JCP = 41.3 Hz, C5), 128.46 (d, JCP = 12.7 Hz, C9),
124.94 (d, JCP = 10.6 Hz, C4). FTIR (KBr, cm−1): 1573, 1483, 1437,
1414, 1331 (νSO(as)), 1196 (νPO), 1159 (νSO(s)), 1119, 1104,
1070, 855, 797, 752, 736, 723, 692, 591, 580, 549, 493. HRMS(ESI):
m/z (%): 617.1121 (39) [M + H+]. C36H27O4P2S requires 617.1105;
639.0938 (100) [M + Na+]. C36H26O4P2SNa requires 639.0925;
655.0678 (20) [M + K+]. C36H26O4P2SK requires 655.0664.
Lanthanide Complex Syntheses. The lanthanide coordination
complexes were prepared by combination of 1 equiv of ligand (50−
100 mg) with 1 equiv of Ln(NO3)3·xH2O in MeOH. The mixtures
were stirred (23 °C, 1 h); volatiles were removed by vacuum
evaporation, and the resulting powders were vacuum-dried. Elemental
analyses (CHN) and IR spectra for representative samples were
obtained, and selected samples were crystallized to obtain single
crystals for X-ray diffraction analyses. [La(8)(NO3)3]: 31P{1H} NMR
1
(121.5 MHz, d4-MeOH): δ 36.2. H NMR (300 MHz, d4-MeOH): δ
8.03 (d, JHH = 7.9 Hz, 2H, H5), 7.79 (dd, JHH = 11.4 Hz, 7.8 Hz, 8H,
H9), 7.62−7.51 (m, 12H, H10,11), 7.25 (t, JHH = 7.5 Hz, 2H, H4), 7.08
(d, JHH = 7.2 Hz, 2H, H3), 4.13 (d, JHP = 13.2 Hz, 4H, H1). FTIR (KBr,
cm−1): 1240 (νPO). [Pr(8)(NO3)3(H2O)]: FTIR (KBr, cm−1): 1240
(νPO). Anal. Calcd for C38H32N3O12P2PrS: C 47.66, H 3.37. Found C
48.31, H 3.63%. [La(9)(NO3)3(H2O)]: 31P{1H} NMR (121.5 MHz,
d4-MeOH): δ 37.2. 1H NMR (300 MHz, d4-MeOH): δ 7.87−7.84 (m,
10H, H5,9), 7.67−7.57 (m, 12H, H10,11), 7.42 (t, JHH = 7.8 Hz, 2H, H4),
6.87 (m, 2H, H3), 4.33 (d, JHP = 13.2 Hz, 4H, H1). FTIR (KBr, cm−1):
1286 (br, νSO(as)), 1155 (br, νPO, νSO). Anal. Calcd for
[C38H30LaN3O13P2S]·5H2O: C 43.07, H 3.80, N 3.97. Found C
43.03, H 3.51, N 3.75%. [Pr(9)(NO3)3]: FTIR (KBr, cm−1): 1288 (br,
νSO(as)), 1151 (br, νPO, νSO). Anal. Calcd for [C38H30N3O13P2PrS]·
4H2O: C 43.73, H 3.67, N 4.03. Found C 43.74, H 3.33, N 3.85%.
[Eu(9)(NO3)3]: FTIR (KBr, cm−1): 1281 (br, νSO(as)), 1151 (br, νPO,
νSO). [Dy(9)(NO3)3]: FTIR (KBr, cm−1): 1286 (br, νSO(as)), 1155 (br,
νPO, νSO). [Er(9)(NO3)3]: FTIR (KBr, cm−1): 1288 (br, νSO(as)), 1151
(br, νPO, νSO). [Lu(9)(NO3)3]: FTIR (KBr, cm−1): 1284 (br, νSO(as)),
1151 (br, νPO, νSO). [La(10)(NO3)3]: 31P{1H} NMR (121.5 MHz, d4-
MeOH): δ 34.5. 1H NMR (300 MHz, d4-MeOH): δ 8.41 (d, JHH = 7.8
Hz, 2H, H4), 7.86 (t, 2H, JHH = 7.7 Hz, H3), 7.69−7.62 (m, 12H,
H8,10), 7.54−7.41 (m, 10H, H2,9). FTIR (KBr, cm−1): 1311 (br,
νSO(as)), 1155 (br, νSO). [Pr(10)(NO3)3]: FTIR (KBr, cm−1): 1305 (br,
νSO(as)), 1169 (br, νPO), 1151 (br, νSO). [Eu(10)(NO3)3]: FTIR (KBr,
cm−1): 1305 (br, νSO(as)), 1155 (br, νPO, νSO). [Dy(10)(NO3)3]: FTIR
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Scheme 1
(KBr, cm−1): 1300 (br, νSO(as)), 1155 (br, νPO, νSO). [Er(10)(NO3)3]:
FTIR (KBr, cm−1): 1298 (br, νSO(as)), 1155 (br, νPO, νSO).
{[Eu(9)Cl3]2(μ-9)}·5(MeCN) was prepared by combination of
EuCl3·6H2O (28.4 mg, 1 equiv) with an excess of 9 (200 mg, 3
equiv) in MeOH (4 mL). The mixture was stirred (23 °C, 1 h), and
volatiles were removed by vacuum evaporation leaving a colorless
powder. Anal. Calcd for C114H90Cl6Eu2O12P6S3: C 55.87, H 3.70.
Found C 55.59, H 3.57%. FTIR (KBr, cm−1): 1290 (br, νSO(as)), 1157
(br, νPO, νSO). The colorless powder was recrystallized from MeCN/
MeOH (9/1, 4 mL) by slow evaporation.
parameters of the affected atoms. Colorless prisms of [Pr(10)(NO3)3-
(MeOH)2]·MeOH were obtained by slow evaporation of a MeCN/
MeOH (9:1) solution.
Distribution Studies. All salts and solvents were reagent grade
and were used as received. Extraction experiments were carried out
using 1,2-dichloroethane (OmniSolv, EM Science) as the diluent. The
aqueous phases were prepared using nitric acid (J. T. Baker, Ultrex II)
and europium nitrate (Aldrich, 99.9%). Distilled, deionized water was
obtained from a Barnstead Nanopure filter system (resistivity at least
18.2 MΩ cm) and used to prepare all the aqueous solutions. The
radioisotope 241Am was provided by the Radiochemical and Engineer-
ing Research Center of Oak Ridge National Laboratory. The
radiotracer 152/154Eu was obtained from Isotope Products, Burbank,
CA. Both were added as spikes to the aqueous phases in the sample
equilibration vials in the extraction experiments.
X-ray Diffraction Analyses. Crystals of the ligands and lanthanide
complexes were coated with Paratone oil and mounted on a CryoLoop
attached to a metal pin with epoxy. Diffraction data were collected
with a Bruker X8 Apex II CCD-based X-ray diffractometer equipped
with an Oxford Cryostream 700 low-temperature device and normal
focus Mo-target X-ray tube (λ = 0.710 73 Å) operated at 1500 W
power (50 kV, 30 mA). Data collection and processing were
accomplished with the Bruker APEX2 software suite.47 The structures
were solved by direct methods and refined with full-matrix least-
squares methods on F2 with use of SHELXTL.48 Lattice and data
collection parameters for the ligands and the metal complexes are
presented in Tables 1 and 2, respectively. All heavy atoms were refined
anisotropically, and hydrogen atoms were included in ideal positions
and refined isotropically (riding model) with Uiso = 1.2Ueq of the
parent atom (Uiso = 1.5Ueq for methyl groups) unless noted otherwise.
The structure refinements were well-behaved except as indicated in the
following notes. Colorless rods of 8 were obtained by holding a
MeOH/CH2Cl2 (2 mL, 95:5) solution of 8 (30 mg) at −20 °C (12 h).
Colorless rods of 9·2CH2Cl2 were obtained by holding a MeOH/
CH2Cl2 (5 mL, 95:5) solution of 9 at −20 °C (12 h). Colorless rods of
10·3CHCl3 were obtained by slow evaporation of a CDCl3 solution of
10 (50 mg) at 23 °C. The chlorine atom Cl8 is disordered over two
sites, and the occupancies were allowed to refine freely. One molecule
of CHCl3 could not be acceptably modeled, and it was accounted for
by using SQUEEZE.49 Colorless blocks of [Pr(8)(NO3)3(MeCN)]·
[Pr(8)(NO3)3(H2O)]·3MeCN were formed by slow evaporation of a
saturated MeCN/MeOH (4:1) solution. Colorless blocks of [La(9)-
(NO3)3(H2O)] were obtained by slow evaporation of a MeCN/
MeOH (9:1) solution. All atoms of one nitrate group, N3, O8, O9,
O10, and one inner sphere molecule of water, O14, are disordered
over two positions. Constraints on bond lengths and thermal
displacement parameters were applied to the disordered species.
Further disordered solvent was accounted for by using SQUEEZE.49
Pink platelets of [Er(9)(NO3)3]·2MeCN were formed by slow
evaporation of a MeCN/MeOH (9:1) solution. A colorless rod-like
specimen was cut from a larger crystal of {[Eu(9)Cl3]2(μ-9)}·
5(MeCN) obtained by slow evaporation of a MeCN/MeOH (9:1)
solution. One of the five independent MeCN lattice solvent molecules
was found disordered equally over two positions. The occupancy was
freely refined with constraints on the anisotropic displacement
Phases at a 1:1 organic to aqueous (O/A) phase ratio were
combined in 2 mL polypropylene microtubes, which were then capped
and mounted by clips on a disk that was rotated in a constant-
temperature air box at 25.0
0.5 °C for 1 h. After the contacting
period, the tubes were centrifuged for 5 min at 3000 rpm and 25 °C in
a Beckman Coulter Allegra 6R temperature-controlled centrifuge. A
250 μL aliquot of each phase was subsampled and counted using a
Canberra Analyst pure Ge Gamma counter. Counting times were
sufficient to ensure that counting error was a small fraction of the
precision of the obtained distribution ratios, considered from a
combination of volumetric, replicate, and counting errors to be 5%.
Americium and europium distribution ratios were calculated as the
ratio of the volumetric count rates of the 241Am and 152/154Eu isotopes
in each phase at equilibrium.
Molecular Mechanics Calculations. Geometry optimizations of
the free and metal-bound forms of 8, 9, and 10 were carried out with
the MM3 force field50 using a points-on-a-sphere metal ion51 as
implemented in PCModel software.52 Conformational searches to
locate the most stable form for each structure were performed using
the GMMX algorithm provided with this software.
RESULTS AND DISCUSSION
■
Ligand Syntheses. The initial target DBT ligand, 8, was
prepared in four steps from dibenzothiophene following the
pathway summarized in Scheme 1. The precursors 4,6-
diformyldibenzothiophene, 5, and 4,6-bis(hydroxymethyl)-
dibenzothiophene, 6, were prepared essentially as described
in the literature46 by a double formylation of dibenzothiophene
followed by reduction with NaBH4. The conversion of 6 to 4,6-
bis(chloromethyl)dibenzothiophene, 7, was accomplished in a
manner similar to the procedure described previously for the
preparation of 4,6-bis(chloromethyl)dibenzofuran.42 Lastly, the
initial target, 4,6-bis(diphenylphosphinoylmethyl)-
dibenzothiophene, 8, was obtained with 97% yield from an
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Arbuzov reaction of 7 with Ph2POEt. The composition of 8 is
supported by elemental analysis (CHP), and HRMS analysis
shows high-intensity [M + H+], [M + Na+], and [M + K+] ions.
The FTIR spectrum of 8 contains a strong absorption at 1188
cm−1 that is assigned to a νPO stretching mode, and the 31P
NMR spectrum displays a single resonance that is solvent
dependent: δ 30.1 (CDCl3); 34.4 (d4-MeOH). These shifts are
c o m p a r a b l e t o t h o s e r e p o r t e d f o r 4 , 6 - b i s -
(diphenylphosphinoylmethyl)dibenzofuran, 3 (R = Ph): δ
HRMS that shows high-intensity [M + H+] and [M + Na+]
ions. The FTIR spectrum displays strong absorptions
tentatively assigned to νSO(as), νPO, and νSO(s) stretching
vibrations at 1288, 1194, and 1157 cm−1, respectively. The 31P
NMR spectrum contains a single, solvent-dependent resonance:
δ 30.7 (CDCl3); 34.0 (d4-MeOH). These resonances are
slightly shifted from the resonances for the precursor 8. The 1H
and 13C NMR spectra (CDCl3) are also consistent with the
proposed structure. Notably, the proton and carbon resonances
assigned to the methylene groups spanning the DBT(O)2 ring
and the phosphine oxide group, H1, δ 4.11 (JHP = 13.8 Hz) and
C1, δ 31.98 (JHP = 66.8 Hz), are significantly shifted from the
respective resonances for 8. The molecular structure of 9,
cocrystallized with two molecules of CH2Cl2, was determined
by single-crystal X-ray diffraction analysis. A view of the
molecule is shown in Figure 2, and selected bond lengths are
29.8 (CDCl3); 34.5 (d4-MeOH).42 The H and 13C NMR
1
spectra (CDCl3) are consistent with the proposed structure,
and the shifts for the methylene group are noteworthy: H1, δ
3.90 (JHP = 13.2 Hz), C1, δ 36.98 (JHP = 67.3 Hz). The
molecular structure of 8 was confirmed by single-crystal X-ray
diffraction analysis. A view of the molecule is shown in Figure 1,
Figure 1. Molecular structure of 4,6-[Ph2P(O)CH2]2DBT, 8, (thermal
ellipsoids, 50%) with carbon atom labels and H atoms omitted for
clarity.
Figure 2. Molecular structure of 4,6-[Ph2P(O)CH2]2DBT(O)2·
2CH2Cl2, 9·2CH2Cl2 (thermal ellipsoids, 50%) with carbon atom
labels, H atoms and lattice CH2Cl2 omitted for clarity.
Table 3. Selected Bond Lengths for Ligands 8, 9·2CH2Cl2,
and 10·3CHCl3
listed in Table 3. The dibenzothiophene backbone is slightly
distorted in response to the oxidation and hybridization change
of the sulfur atom. This is indicated by the torsion angle
between the two aromatic rings: C4−C5−C6−C7, 7.2(4)°.
The PO bond vectors are rotated out of the dibenzothio-
phene backbone in opposite directions, and the POav bond
bond type
P−O
8
9·2CH2Cl2
10·3CHCl3
P1−O2
P1−O1
P1−O1
1.493(3)
1.494(2)
1.485(4)
P2−O1
P2−O2
P2−O2
length, 1.489
0.005 Å, is indistinguishable from that in 8,
1.490(3)
1.484(2)
1.491(4)
1.491 0.002 Å.
The C−Sav bond length, 1.769
elongated compared to that for 8, 1.755
indistinguishable from the average distance, 1.764 0.001 Å, in
dibenzothiophene-5,5-dioxide.53 The SOav bond length,
1.438 0.002 Å, and the O3−S−O4 angle, 116.80(12)°, are
indistinguishable from the corresponding bond length and
bond angle in dibenzothiophene-5,5-dioxide, 1.439(3) Å and
117.3(3)°.53
S−O
S1−O3 1.439(2) S1−O3 1.439(4)
S1−O4 1.437(2) S1−O4 1.439(4)
0.002 Å, is slightly
0.002 Å, and is
P−C(CH2 or DBT)
P1−C5 1.817(3)
P1−C13
P1−C10
1.838(2)
1.821(6)
P2−C10
P2−C26
P2−C25
1.829(4)
1.823(3)
1.822(6)
S−C
S1−C7 1.758(4)
S1−C8 1.753(4)
S1−C11
S1−C11
1.767(2)
1.785(5)
S1−C12
S1−C12
1.771(3)
1.790(5)
The related sulfone, 4,6-bis(diphenylphosphinoyl)-
dibenzothiophene-5,5-dioxide, 10, was prepared in a one-pot
synthesis, as summarized in Scheme 1, by reaction of
dibenzothiophene with an n-BuLi/TMEDA solution followed
by treatment with Ph2PCl. The putative intermediate, 4,6-
[Ph2P]2DBT, was treated, without isolation, with an excess of
m-CBPA, resulting in the oxidation of both the phosphorus and
the sulfur atoms.54 Compound 10 was isolated as a white solid
in 49% yield. The formation of the sulfone is supported by
HRMS analysis that shows high-intensity [M + H+], [M +
Na+], and [M + K+] ions. The IR spectrum displays strong
and selected bond lengths are given in Table 3. The
dibenzothiophene backbone is planar, and the PO bond
vectors are rotated out of the backbone plane in opposite
directions. The average (av) PO bond length, 1.491 0.002
Å, is indistinguishable from that reported for the furan analogue
3 (R = Ph), 1.491 0.003(av) Å.42
S-Oxidation of 8 with an excess of meta-chloroperoxybenzoic
acid (m-CPBA) at room temperature gave 4,6-bis-
(diphenylphosphinoylmethyl)dibenzothiophene sulfone, 9,
with 95% yield. The composition of 9 is supported by a
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absorptions at 1331, 1196, and 1159 cm−1 that are tentatively
assigned to νSO(as), νPO, and νSO(s) stretching modes,
respectively. The 31P NMR spectrum contains a single
resonance, δ 27.7 (CDCl3), δ 32.0 (d4-MeOH) similar to the
shift reported for the DBF analogue, 4,6-bis-
(diphenylphosphinoyl)dibenzofuran, 2: δ 25.2 (CDCl3).42
1
The H and 13C{1H} NMR spectra are consistent with the
proposed structure, and the shifts are similar to those reported
for 2.42 Single-crystal X-ray diffraction analysis of 10,
cocrystallized with CHCl3, confirmed the composition, and a
view of the molecule and selected bond lengths are presented in
Figure 3 and Table 3, respectively. Unfortunately, the
Figure 3. Molecular structure of 4,6-[Ph2P(O)CH2]2DBT(O)2·
3CHCl3, 10·3CHCl3, (thermal ellipsoids, 50%) with carbon atom
labels, H atoms, and lattice CHCl3 omitted for clarity.
refinement of the structure is marginal due to instability of
the crystals from solvent loss. X-ray diffraction analysis with
crystals obtained by slow evaporation of a CH2Cl2/MeOH
(9:1) solution did not provide improved results. Nonetheless,
the POav bond length, 1.488
0.003 Å, and SO bond
length, 1.439(4) Å, are similar to the bond lengths in 9.
Ligand Computational Modeling. As thoroughly de-
scribed in a prior report on phosphine oxide decorated DBF
ligands,42 ligand strain free energies, derived from molecular
mechanics (MM) computations on 1:1 Ln(III)/ligand
complexes, absent of competing inner sphere anions and
solvent molecules, provide useful assessments of preferred
ligand chelate docking structures.55−57 Extending this treatment
to the current set of phosphine oxide decorated DBT-based
ligands, the strain energetics for 8, 9, and 10 on the medium-
sized lanthanide Eu(III) were evaluated. The inherently weak
donor ability of the S atom in the dibenzothiophene platform
and hard−soft acid−base (HSAB) principles58 portend that 8
would bind to Ln(III) ions with only its two pendent
phosphine oxide O atoms. Evaluation of the relative strain
free energy (G) per coordinated O atom for the geometry-
optimized bidentate OPO′P′ structure in [Eu(8)3+], shown in
Figure 4a, provides a value of 1.47 kcal mol−1/donor group, and
the nonbonded S atom in this structure resides 3.74 Å from the
Eu(III) center. Therefore, the low strain energy, which is
slightly smaller than that calculated for the DBF analogue 3 (R
= Ph) in a bidentate chelate condition, 1.60 kcal mol−1/
donor,59 is also consistent with 8 adopting a bidentate OPO′P′
chelate structure. The sulfone derivative 9 offers possibilities of
forming bidentate OPO′P′ or OPOS as well as tridentate
OPOSO′P′ chelate structures, and the energy-minimized
tridentate structure is shown in Figure 4b. The computed
relative strain free energy, 0.91 kcal mol−1/donor group is
small, and this augurs well for tridentate docking. For
Figure 4. Geometry-optimized structures for selected maximally O-
donor atom bonded ligands in gas phase, [Eu(L)3+] complexes: (a)
ligand 8, (b) ligand 9, and (c) ligand 10.
comparison, the computed value for tridentate OPONO′P′
docking by 2,6-bis(diphenylphosphinoylmethyl)pyridine N-
oxide (TPhNOPOPO, 11) is 1.27 kcal mol−1/donor, and this
ligand routinely adopts tridentate chelate structures with
Ln(III) ions.4−13 The latter also displays efficient solvent
extraction properties. Lastly, the docking preference for 10 was
evaluated. Here, bidentate OPO′P′ and OPOS, as well as
tridentate OPOSO′P′ coordination conditions are available, but
the lowest strain energy condition is found for a novel
OPOSO′SO′P′ tetradentate structure shown in Figure 4c. This
structure provides a strain energy value of 1.93 kcal mol−1/
donor group, which is comparable to the strain energy (1.85
kcal mol−1/donor group) for 2 acting as a bidentate OPO′P′
chelating ligand. Interestingly, the energy-minimized tridentate
structure is actually found to be 7.4 kcal mol−1 higher in strain
energy than the tetradentate structure. In summary, it can be
expected that ligand strain should not preclude the formation of
chelate structures with maximal denticity interactions by 9 and
10.
Lanthanide Coordination with Ligand 4,6-[Ph2P(O)-
CH2]2DBT (8). On the basis of our previous study with DBF
analogues,42 HSAB principles, and the molecular modeling
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Figure 5. Molecular structure of {[Pr(8)(NO3)3(MeCN)]·[Pr(8)(NO3)3(H2O)]}·3MeCN: (a) [Pr(8)(NO3)3(MeCN)] and (b) [Pr(8)(NO3)3-
(H2O)]·(thermal ellipsoids, 50%) with carbon atom labels, H atoms, and lattice MeCN omitted for clarity.
results described above, it was expected that 8 would most
likely form 1:1 and perhaps 2:1 ligand/metal complexes with
Ln(III) ions utilizing only bidentate OPO′P′ chelate interactions
with no involvement of the DBT S atom in forming the
coordination field. Complexes, prepared from equimolar
combinations of 8 with Ln(NO3)3·xH2O (Ln = La, Pr) in
MeOH (23 °C, 1 h) followed by evaporation of the volatiles,
were isolated as colorless and green powders, respectively.
Elemental analysis (CH) for the Pr complex shows good
agreement with a 1:1 composition, [Pr(8)(NO3)3(H2O)]. The
FTIR spectra for several complexes are essentially identical;
they show a strong absorption at 1140 cm−1, tentatively
assigned to νPO, that corresponds to a down-frequency
coordination shift, Δν = 48 cm−1, compared to the free ligand.
Further, the complexation of La(III) with 8 (1−4 equiv) was
followed by 31P and 1H NMR in d4-MeOH. The spectra show a
single resonance with very small shift displacement as the ligand
concentration increases, consistent with the formation of a
single complex undergoing rapid ligand exchange. Since these
data do not unambiguously identify the ligand denticity, single
crystals of the 1:1 Pr/complex were grown by dissolution of the
crude powder [Pr(8)(NO3)3(H2O)] in a mixture of MeCN/
MeOH (4:1) followed by slow evaporation of the solution.
Single-crystal X-ray diffraction analysis revealed the formation
of two chemically distinguishable complexes cocrystallized with
three molecules of MeCN, resulting in an overall composition
of {[Pr(8)(NO3)3(MeCN)]·[Pr(8)(NO3)3(H2O)]}·3MeCN.
Views of the independent complex units are shown in Figure
5a,b, and selected bond lengths are listed in Table 4. As
anticipated, each Pr(III) ion coordinates with one bidentate
OPO′P′ chelating ligand, three bidentate nitrate anions, and
either an O-bonded water or N-bonded acetonitrile molecule
forming nine-member inner coordination spheres. As expected,
the S atoms are nonbonded with separations of Pr1···S1
3.351(1) Å and Pr2···S2 3.300(1) Å.60 As observed in the
related complex containing the DBF ligand, [Pr(3)(NO3)3-
(MeCN)], the bidentate chelate interaction in [Pr(8)(NO3)3-
(MeCN)] is slightly asymmetric as indicated by the bond
lengths Pr1−O1 2.365(3) Å and Pr1−O2 2.339(3) Å. The
chelate interaction is more symmetrical in the second unit,
[Pr(8)(NO3)3(H2O)]: Pr2−O3 2.367(3) Å and Pr2−O4
2.364(3) Å. In this complex the coordinated water molecule
is hydrogen bonded with two of the lattice acetonitrile
molecules: (O23−H23B 0.83(2) Å, N9···H23B 2.20(3) Å,
O23···N9 2.981(7) Å, O23−H23B···N9 156.0°; O23−H23C
0.83(2) Å, N7#1···H23C 2.08 (3) Å, O23···N7#1 2.868(6) Å,
O23−H23C···N7#1 157.0°. The four PO bond lengths (P
Oav = 1.506 0.004 Å) are identical within 3σ of the estimated
standard deviations, and they are elongated relative to the free
ligand bond length. It is also noted that attempts to isolate and
structurally characterize complexes with 2:1 ligand/metal
compositions were unsuccessful.
Lanthanide Coordination with 4,6-[Ph2P(O)CH2]2DBT-
(O)2 (9). The coordination chemistry of 9 with Ln(NO3)3·
xH2O (Ln = La, Pr, Eu, Dy, Er, Lu) was studied using 1:1 and
2:1 ligand/metal combining ratios in MeOH and/or in MeOH/
EtOAc (4:1), and the resulting complexes were isolated as
powders after evaporation of the volatiles. Elemental analyses
(CHN) for the 1:1 La(9) and Pr(9) complexes are consistent
with compositions La(9)(NO3)3(H2O)5 and Pr(9)(NO3)3-
(H2O)4, respectively. Once more, the FTIR spectra for the
isolated powders suggest phosphine oxide binding to the
Ln(III) ions, but they do not provide unambiguous assignment
for the ligand denticity due in part to overlapping adsorptions.
For example, the strong band at 1194 cm−1 assigned to the
νPO stretching mode for the free ligand disappears in the
spectra for the complexes, but the expected down-frequency
shifted νPO absorption is not clearly resolved from a broad
adsorption centered at 1152 cm−1. In addition, the spectra
display two very broad bands centered at 1285 4 cm−1 and
1152
3 cm−1 that are tentatively assigned to νSO(as) and
νSO(sy), and these are relatively unperturbed compared to the
free ligand frequencies. However, it is noted that the νSO(as)
band falls in a region that also contains adsorptions due to
nitrate modes; therefore, coordination shifts in the SO
stretching modes cannot be determined with certainty. The 31
P
NMR spectra for d4-MeOH solutions with 4:1, 2:1, and 1:1 9/
La combining ratios each display a single resonance at 35.2,
36.1, and 37.2 respectively, shifted slightly from the free ligand
resonance (Δδ 1.2−3.2) consistent with rapid ligand exchange.
Crystal structure determinations were undertaken for two 1:1
complexes (La and Er) obtained by dissolution of the initially
formed powders in a mixture of MeCN/MeOH 9:1 followed by
slow evaporation of the homogeneous solutions. The crystal
structure determination for the lanthanum complex reveals a
composition [La(9)(NO3)3(H2O)] in which one nitrate anion
and the inner sphere water molecule are disordered over two
positions.61 Additional outer-sphere solvent molecules (likely
water and/or methanol) are disordered and could not be
adequately modeled. Hence, they were treated using the
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SQUEEZE procedure.49 A view of the 10-coordinate inner
sphere complex is shown in Figure 6, and selected bond lengths
similar efforts with LnCl3 salts produced complexes that
contain weakly bound solvent molecules that complicate
analysis and crystal growth. However, in one case, the
combination of excess 9 with EuCl3·6H2O produced a powder
that readily lost lattice solvent in vacuo, and the resulting solid
gave satisfactory elemental analysis consistent with a
composition of {[Eu(9)Cl3]2(9)}. The IR spectrum for the
solid showed strong, broad absorptions centered at 1290 cm−1
(νSO(as)) and 1157 cm−1 (νSO(sy) + νPO). Slow evaporation
of a MeOH/MeCN solution of the powder provided single
crystals, and the crystal structure determination revealed a
solvated composition of {[Eu(9)(Cl)3]2(μ-9)}·5MeCN with
the two [Eu(9)(Cl)3] units bridged by another molecule of 9. A
view of the structure is shown in Figure 8. The bridging
interaction employs the phosphine oxide atoms: Eu1−O11
2.363(3), P5−O11 1.501(3), and Eu2−O12 2.338(3), P6−O12
1.501(3) Å. The two [Eu(9)(Cl)3] units are compositionally
equivalent and oriented trans to each other across the
DBT(O)2 plane. Each Eu(III) inner-sphere coordination
polyhedron is seven-coordinate formed by the O atoms from
a tridentate OPOSO′P′ chelating ligand 9, a bridging OP atom,
and three chlorides. However, the metrical parameters in the
two units differ significantly. The geometry about Eu1 is more
symmetric with the Eu−OP bond lengths, Eu1−O4 2.358(3)
and Eu−O3 2.360(3) Å, indistinguishable from the bridging
Eu−OP interaction, Eu1−O11, while the Eu−OS bond length,
Eu1−O1 2.572(3) Å, is longer. The second unit is more
asymmetric with Eu2−O8 2.302(3) and Eu2−O7 2.342(3) Å,
the latter indistinguishable from the bridging distance, Eu2−
O12. The distance involving the sulfone O atom, 2.674(3) Å, is
significantly longer than it is in the first unit. Various attempts
to drive these complexes to a mononuclear 2:1 composition
have so far failed.
Figure 6. Molecular structure of [La(9)(NO3)3(H2O)] (thermal
ellipsoids, 50%), with carbon atom labels and H atoms omitted for
clarity.
are summarized in Table 4. Clearly, as predicted by the steric
modeling analysis, the ligand 9 is bonded to La(III) in a
tridentate OPOSO′P′ fashion with ligand/La bond lengths La−
OP 2.469(3), La−O′P′ 2.503(3), and La−OS 2.615(3) Å. The
remaining positions in the La(III) inner sphere are occupied by
three bidentate nitrate anions and a water molecule.
Single crystals for the erbium complex, [Er(9)(NO3)3]·
2MeCN, were also obtained, and, in this case, recrystallization
of the crude complex from MeCN/MeOH resulted in complete
displacement of water from both the inner and outer
coordination spheres. A view of the structure is provided in
Figure 7. The [Er(9)(NO3)3] unit is cocrystallized with two
Lanthanide Coordination with 4,6-[Ph2P(O)]2DBT(O)2
(10). The lanthanide coordination chemistry with the more
rigid ligand 10 was examined using 1:1 and 2:1 ligand/Ln
combining ratios (Ln = La, Pr, Eu, Dy, Er) in MeOH or
CHCl3/MeOH (9:1). The complexes were isolated as powders
after evaporation of the volatiles. The FTIR spectra recorded
from these samples typically display a broad band assigned to
νSO(as) centered in the region of 1304 6 cm−1 (Δνav = 29
cm−1). The νPO stretch of the free ligand is absent in the
spectra of the complexes, and a second broad band appears
centered in the region of 1154
3 cm−1. This likely
corresponds to overlapped νPO and νSO(s) modes. In one
exception, the spectrum for the 1:1 10/Pr complex contains
two resolved bands in this region: 1169 and 1151 cm−1 for the
crude complex and 1174 and 1153 cm−1 for isolated crystals.
The 31P NMR spectra in d4-MeOH display single resonances at
33.6, 34.2, and 34.5 for solutions containing ligand/metal ratios
4:1, 2:1, and 1:1, respectively, shifted downfield 1.6 to 2.5 ppm,
Figure 7. Molecular structure of [Er(9)(NO3)3]·2MeCN (thermal
ellipsoids, 50%) with carbon atom labels, H atoms, and outer-sphere
MeCN omitted for clarity.
1
relative to the free ligand. The H NMR spectrum for the 1:1
complex is relatively unperturbed compared to the free ligand.
Single crystals for the praseodymium complex were obtained
by dissolution of a powder sample described above in MeCN/
MeOH (9:1) followed by slow evaporation of the solution. The
X-ray structure determination reveals a composition of
[Pr(10)(NO3)3(MeOH)2]·MeOH. A view of the structure is
shown in Figure 9, and selected bond lengths are listed in Table
4. The Pr(III) ion is ten-coordinate with the coordination
positions occupied by the O atoms from a bidentate ligand 10
bonded in an unexpected bidentate OPOS chelating mode, the
O atoms of three bidentate nitrate anions, and the O atoms
outer-sphere MeCN molecules, and the nine-vertex Er(III)
inner-sphere environment is composed of O atoms from a
tridentate OPOSO′P′ bonded ligand 9 with [Er−OP 2.2436(15)
and 2.2557(15) Å and Er−OS 2.3290(14) Å] and three
bidentate nitrate anions. The absence of an inner-sphere
solvent molecule and reduced Er(III) coordination number are
consistent with the smaller size of the Er(III) cation.
Attempts to isolate and characterize 2:1 complexes of 9 with
Ln(NO3)3 salts from various solvents were unsuccessful, and
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Figure 8. Molecular structure of {[Eu(9)(Cl)3]2(μ-9)}·5MeCN (thermal ellipsoids, 50%) with carbon atom labels, H atoms, and outer-sphere
MeCN omitted for clarity.
is necessary to employ extractants that are soluble in a
hydrocarbon diluent, for example, dodecane. This typically
requires the placement of lipophilic substituents, for example,
n-octyl, 2-ethylhexyl, on the ligand to achieve good solubility
and phase separation performance. Unfortunately, this often
complicates the extractant synthesis and purification. Therefore,
survey extraction studies aimed at identifying potentially useful
extractant types are initially accomplished with more easily
prepared and purified phenyl substituted ligands that are
soluble in less process-desirable chlorocarbon solvents.
Consistent with this, the acid dependencies of distribution
ratios, D = [Morg]/[Maq], for europium(III) and americium(III)
in nitric acid solutions in contact with 10 mM solutions of
phenyl-substituted 9 and 10 in 1,2-dichloroethane (DCE) were
measured. The resulting data are displayed in Figure 10 along
with data for two phosphine oxide decorated dibenzofurans, 2
and 3 (R = Ph)42 and data for the classic CMPO extractant,
OPhDiBCMPO, 4, all recorded under identical conditions.
Unfortunately, the stability of 8 toward aqueous nitric acid
Figure 9. Molecular structure of [Pr(10)(NO3)3(MeOH)2]·MeOH
(thermal ellipsoids, 50%) with carbon atom labels, H atoms except
solutions is limited; therefore, distribution data for it are not
included. Each ligand, except 10, shows a general trend of
increasing D values with increasing nitric acid concentration.
The D values for 3 and 9 are similar in the acid range of 0.01−1
M and smaller, at all acid concentrations, than the D values for
the CMPO ligand 4. As noted in our prior study,42 the D values
for 2 are more comparable with those for 4, and the improved
performance for 2 relative to 3 was attributed to better OP-
donor group lone pair vector convergence upon formation of
bidentate OPO′P′ complexes by 2.42 Since the computed ligand
steric strain for 9 in a 1:1 tridentate OPOSO′P′ complex is low,
the donor strength of the sulfone O atoms is expected to be
greater than that for the furan O atom in 2 and 3, and the
donor group vector convergence is favorable, it was expected
that extraction performance for 9 would be good. However, the
rather modest performance of 9 relative to that of 2, 3, and 4
suggests that other features may be operative. These may
hydrogen bond donor (H14A), and outer-sphere MeOH omitted for
clarity.
from two inner-sphere molecules of MeOH. The second
phosphine oxide group, P2−O2, is hydrogen bonded with one
of the Pr-coordinated MeOH molecules (O14···O2 2.684(2) Å,
O2−H14A−O14 142.0°). The unit cell also contains an outer-
sphere MeOH that is hydrogen bonded with one nitrate anion
and one coordinated MeOH. The bidentate chelate condition
adopted by 10 may reflect the higher strain energy computed
for this ligand and the hydrogen bonding between O2 and the
bound methanol molecule.
Solvent Extraction Analyses. In addition to exploring the
coordination chemistry of 8, 9, and 10 it was of interest to
survey the solvent extraction performance of these new ligand
classes to identify structure types that might be of use in f-
element separations. In practical solvent extraction processes it
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Inorganic Chemistry
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structure determination for one complex, {[Pr(8)-
(NO3)3(MeCN)]·[Pr(8)(NO3)3(H2O)]}·3MeCN, confirmed
this expectation. Similar behavior was previously observed for
the related DBF ligand, 3. On the other hand, computations for
the sulfone ligand 9 indicated a preference for tridentate
OPOSO′P′ chelate binding, and subsequent studies of the
coordination chemistry of 9 with Ln(NO3)3 salts showed
formation of 1:1 complexes with the anticipated tridentate
OPOSO′P′ chelate interaction. Attempts to isolate 2:1
complexes that might be expected to form in a ligand-rich
extraction environment were not successful. However, combi-
nation of excess 9 with EuCl3 produced a complex, {[Eu(9)-
(Cl)3]2(μ-9)}, in which two [Eu(9)Cl3] units, with 9 bonded in
a tridentate chelate fashion, are bridged by another molecule of
9 bonded through Eu−OP interactions.
Molecular modeling of the potential interactions available
with the more sterically constrained ligand 10 revealed that a
unique tetradentate OPOSO′SO′P′ chelate structure was steri-
cally accessible and strain energy favored. However, a
lanthanide complex utilizing this bonding mode has not yet
been isolated. In one structurally characterized complex,
[Pr(10)(NO3)3(MeOH)2]·MeOH, the ligand was observed to
be coordinated via an OPOS bidentate interaction. These initial
observations indicate that the coordination chemistry of 8, 9,
and 10 is rich in variety, and additional aspects remain under
study.
Survey solvent extraction analyses for Eu(III) and Am(III)
were also undertaken to determine if the new ligand structure
types displayed potentially useful separation behavior. Un-
fortunately, 8 displayed poor long-term stability in aqueous
nitric acid solutions, and a complete survey of extraction
performance was not completed. The performance of 9
indicated that this ligand is a weaker extractant than the
classical OPhDiBCMPO extractant for both Eu(III) and
Am(III), while the separation factors, over the acid range of
0.01−3.0 M, are comparable. Ligand 10, on the other hand, is a
much stronger extractant than 9 or the CMPO ligand in the
acid range of 0.01−0.3 M. This performance is interesting, and
it encourages continuing efforts to prepare a dodecane-soluble
derivative and to undertake more extensive extraction analyses.
Figure 10. Americium and europium distribution ratios as a function
of the initial nitric concentration. Organic phase: 2, 3, 4, 9, or 10 at 10
mM in 1,2-DCE. Aqueous phase: trace Am and 0.1 mM of europium
nitrate in nitric acid. O/A = 1, T = 25 °C.
include incomplete phase transfer of the extraction complexes
due to limited solubility under extraction conditions and/or
formation of only a 1:1 ligand/metal complex under extraction
conditions. Detailed, comparative ligand dependency studies
for extractions by each ligand are required to address the latter
point, and these studies will be undertaken with substituent-
modified ligands soluble in dodecane solutions.
The acid dependency behavior for extractions performed
with 10 is interesting in that, in the nitric acid concentration
range of 0.01−0.7 M, this ligand acts as the strongest extractant
of the set. The large D values, at low nitric acid concentration,
are consistent with strong ligand binding resulting from
tridentate OPOSO′P′ chelation or perhaps from the tetradentate
interaction predicted in the modeling analysis. The decrease in
D values at higher acid concentrations (>0.1 M) likely results
from competitive extraction of nitric acid by the ligand. This
point and ligand dependency analyses will be explored further
with a hydrocarbon-soluble derivative of 10. Lastly, typical of
most O-donor extractants, both 9 and 10, like OPhDiBCMPO,
extract Am with a very small preference over Eu at all acid
concentrations (separation factor SFAm/Eu < 2 at 3 M HNO3).
Interestingly, the DBF analogues, 2 and 3, show the opposite
preference,42 and this feature will continue to be examined as
additional studies are undertaken.
ASSOCIATED CONTENT
* Supporting Information
■
S
Selected IR, HRMS, vis−UV, and NMR spectra and CIF files
for the crystal structures. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: rtpaine@unm.edu.
■
CONCLUSION
■
Efficient syntheses for the multifunctional phosphine oxide
decorated ligands 8, 9, and 10, based upon dibenzothiophene
and dibenzothiophene sulfone platforms, were developed; the
new ligands were spectroscopically characterized, and their
molecular structures were verified by single-crystal X-ray
diffraction techniques. Molecular modeling of potential strain-
free chelate binding modes of the ligands on a medium-sized
lanthanide ion, Eu(III), were completed, and the coordination
chemistry with selected lanthanide nitrates was explored. In the
case of 8, the computations suggested that, based upon steric
strain, this ligand should coordinate utilizing a bidentate OPO′P′
binding mode with the dibenzothiophene S atom remaining
nonbonded toward the lanthanide ion. An X-ray crystal
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support for this study at the University of New
Mexico was provided by the U.S. Department of Energy
(DOE), Office of Basic Energy Sciences (BES), Division of
Chemical Sciences, Geosciences, and Biosciences (Grant DE-
FG02-03ER15419 (R.T.P)). In addition, funds from the
National Science Foundation assisted with the purchases of
the X-ray diffractometer (CHE-0443580) and NMR spectrom-
eters (CHE-0840523 and −0946690). Support for liquid−
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Article
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DOE, Office of Science, BES, Chemical Sciences, Geosciences,
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This estimate is much shorter than the computed separation in the gas
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{[Pr(8)(NO3)3(MeCN)]·[Pr(8)(NO3)3(H2O)]}·3MeCN, and it
would be beyond the expected upper limit of a S-donor/Pr(III)
acceptor interaction distance. By comparison, the sum [2.69 Å] of the
Pr and O [0.66(2) Å] covalent radii can be compared to the average
Pr−OP coordinate bond length [2.36 0.02 Å] in {[Pr(8)(NO3)3-
(MeCN)]·[Pr(8)(NO3)3(H2O)]}·3MeCN. Covalent radii estimates
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Echeverría, J.; Cremades, E.; Barragan
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́
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(61) Single crystals obtained by recrystallization of the crude powder
[Pr(9)(NO3)3]·4(H2O) were also examined and found to suffer from
crystal quality shortcomings as described for the [La(9)(NO3)3-
(H2O)] complex. The complexes are isomorphous with disorder in
one nitrate anion and the bound water molecule.
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