Synthesis and Coordination Properties of Trifluoromethyl Decorated Derivatives of 2,6-Bis[(diphenylphosphinoyl)methyl]pyridine N-Oxide Ligands with Lanthanide Ions

Article abstract of DOI:10.1021/ic802390c

Phosphinoyl Grignard-based substitutions on 2,6-bis(chloromethyl)pyridine followed by N-oxidation of the intermediate 2,6-bis(phosphinoyl)methylpyridine compounds with mCPBA give the target trifunctional ligands 2,6-bis[bis(2- trifluoromethylphenyl)phosph

Full text of DOI:10.1021/ic802390c

Inorg. Chem. 2009, 48, 3104-3113
Synthesis and Coordination Properties of Trifluoromethyl Decorated
Derivatives of 2,6-Bis[(diphenylphosphinoyl)methyl]pyridine N-Oxide
Ligands with Lanthanide Ions
Sylvie Pailloux,† Cornel Edicome Shirima,† Alisha D. Ray,† Eileen N. Duesler,† Robert T. Paine,*,†
John R. Klaehn,‡ Michael E. McIlwain,‡ and Benjamin P. Hay§
Department of Chemistry and Chemical Biology, UniVersity of New Mexico, Albuquerque,
New Mexico 87131, Idaho National Laboratory, P.O. Box 1625, Idaho Falls, Idaho 83415, and
Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee, 37831
Received December 15, 2008
Phosphinoyl Grignard-based substitutions on 2,6-bis(chloromethyl)pyridine followed by N-oxidation of the intermediate
2,6-bis(phosphinoyl)methylpyridine compounds with mCPBA give the target trifunctional ligands 2,6-bis[bis(2-
trifluoromethylphenyl)phosphinoylmethyl]pyridine 1-oxide (2a) and 2,6-bis[bis(3,5-bis(trifluoromethyl)phenyl)phos-
phinoylmethyl]pyridine 1-oxide (2b) in high yields. The ligands have been spectroscopically characterized, the molecular
structures confirmed by single crystal X-ray diffraction methods, and the coordination chemistry surveyed with
lanthanide nitrates. Single crystal X-ray diffraction analyses are described for the coordination complexes Nd(2a)(NO3)3,
Nd(2a)(NO3)3 · (CH3CN)0.5, Eu(2a)(NO3)3, and Nd(2b)(NO3)3 ·(H2O)1.25; in each case the ligand binds in a tridentate
mode to the Ln(III) cation. These structures are compared with the structures found for lanthanide coordination
complexes of the parent NOPOPO ligand, [Ph2P(O)CH2]2C5H3NO.
Introduction
In previous reports,1-6 we have documented the synthetic
development of bis(phosphinoylmethyl)pyridine 1-oxides,7
[R2P(O)CH2]2C5H3NO, (NOPOPO), (R ) alkyl or aryl).
These compounds act as neutral, tridentate chelating ligands
that form stable 1:1 and 2:1 (L:M) complexes with lan-
In addition, the ligands display favorable performance as
liquid-liquid extractants of trivalent Ln(III) and actinide,
thanide, Ln(III),1,3-6,8 Th(IV)1 and Pu(IV)9 ions.
An(III), ions present in strongly acidic aqueous solu-
* To whom correspondence should be addressed. E-mail: rtpaine@
tions,6,10-12 an unusual characteristic shared only with
unm.edu.
†
University of New Mexico.
Idaho National Laboratory.
Oak Ridge National Laboratory.
carbamoylmethyl phosphonates (CMP) and carbamoylmethyl
‡
phosphine oxides (CMPO).13-16 The NOPOPO ligand
§
extraction performance was initially assessed with the tetra-
phenyl derivative, R ) Ph, in chlorinated hydrocarbon
(1) Rapko, B. M.; Duesler, E. N.; Smith, P. H.; Paine, R. T.; Ryan, R. R.
Inorg. Chem. 1993, 32, 2164.
(2) Engelhardt, U.; Rapko, B. M.; Duesler, E. N.; Frutos, D.; Paine, R. T.;
Smith, P. H. Polyhedron 1995, 14, 2361.
(3) Gan, X.; Duesler, E. N.; Paine, R. T. Inorg. Chem. 2001, 40, 4420.
(4) Gan, X.; Paine, R. T.; Duesler, E. N.; No¨th, H. Dalton Trans. 2003,
153.
(5) Gan, X.; Rapko, B. M.; Duesler, E. N.; Binyamin, I.; Paine, R. T.;
Hay, B. P. Polyhedron 2005, 24, 469.
(6) Nash, K. L.; Lavalette, C.; Borkowski, M.; Paine, R. T.; Gan, X. Inorg.
Chem. 2002, 41, 5849.
(7) In prior publications these compounds have been named as bis(pho-
sphinomethyl)pyridine N,P,P′-trioxides. The nomenclature has been
revised to meet current rules.
(9) Matonic, J. H.; Neu, M. P.; Enriquez, A. E.; Paine, R. T.; Scott, B. L.
Dalton Trans. 2002, 2328.
(10) Bond, E. M.; Engelhardt, U.; Deere, T. P.; Rapko, B. M.; Paine, R. T.;
FitzPatrick, J. R. SolV. Extr. Ion Exch. 1997, 15, 381.
(11) Bond, E. M.; Engelhardt, U.; Deere, T. P.; Rapko, B. M.; Paine, R. T.;
FitzPatrick, J. R. SolV. Extr. Ion Exch. 1998, 16, 967.
(12) Bond, E. M.; Gan, X.; FitzPatrick, J. R.; Paine, R. T. J. Alloys Compd.
1998, 271-273, 172.
(13) Horwitz, E. P.; Kalina, D. G.; Kaplan, L.; Mason, G. W.; Diamond,
H. Sep. Sci. Technol. 1982, 17, 1261.
(8) Bond, E. M.; Duesler, E. N.; Paine, R. T.; No¨th, H. Polyhedron 2000,
19, 2135.
(14) Kalina, D. G.; Horwitz, E. P.; Kaplan, L.; Muscatello, A. C. Sep. Sci.
Technol. 1981, 16, 1127.
3104 Inorganic Chemistry, Vol. 48, No. 7, 2009
10.1021/ic802390c CCC: $40.75  2009 American Chemical Society
Published on Web 02/26/2009

2,6-Bis[(diphenylphosphinoyl)methyl]pyridine N-Oxide Ligands
solvent.10-12 However, chlorinated solvents are unacceptable
in process scale operations; therefore, a dodecane soluble
derivative (R ) 2-EtHx) was produced and its extraction
properties characterized.6 During the course of these studies
a new multicomponent extraction process (the Universal
extraction, UNEX) was devised for simultaneous extraction
of Cs, Sr, and An ions from acidic solutions. This employs
Ph2P(O)CH2C(O)N(n-Bu)2 as the actinide extractant17-19 and
phenyl trifluoromethyl sulfone (FS-13) as the diluent solvent.
The process provides for excellent separations performance
when small concentrations of Ln and An ions are present;
however, because of modest CMPO solubility and low
metal-ligand complex solubility in FS-13, the system has
limited utility when Ln and An concentrations are high.20
This issue has stimulated interest in the development of
related new compositions that provide comparable or higher
ligand solubility in FS-13, as well as improved aqueous/
organic phase partitioning. Addition of trifluoromethyl groups
to a molecule can promote its solubility in a fluorinated
solvent such as FS-13; therefore, in the present study the
initial objective was to determine if the organic framework
of NOPOPO-type extractants could be decorated with CF3
substituents. The specific compounds of interest are shown
here. Once available for study, the next objective was to
determine the solubility of 2a and 2b in FS-13 and assess
the effects of CF3 substitution on the coordination behavior
of the ligands toward Ln(III) ions.
obtained from VWR. THF was dried and distilled using standard
procedures. The Eu(NO3)3 ·5H2O was obtained from Aldrich
Ventron, and the Nd (NO3)3 ·5H2O from Aldrich. Infrared spectra
were recorded on a Bruker Tensor 27 benchtop spectrometer;
solution NMR spectra were measured (20 °C) on a Bruker FX-250
spectrometer. The NMR standards were TMS (1H, 13C) and 85%
H3PO4 (31P), and downfield shifts were assigned as +δ. Elemental
analyses were obtained from Galbraith Laboratories. Mass spectra
were obtained from the University of New Mexico Mass Spec-
trometry Facility.
Ligand Synthesis. 2,6-Bis(chloromethyl)pyridine was prepared
as described previously.21 Caution! This reagent and its solutions
should be handled in a well-Ventilated fume hood. Skin and eye
contact must be carefully aVoided since the compound is an
aggressiVe skin and bronchial irritant with Variable indiVidual
response symptoms.
2,6-Bis[bis(2-(trifluoromethyl)phenyl)phosphinoylmethyl]pyri-
dine (1a). A solution of 2-bromobenzotrifluoride (5.7 mL, 42.0 mmol)
in dry tetrahydrofuran (THF, 42 mL) was combined dropwise with a
suspension of magnesium (1.0 g, 42.0 mmol) in dry THF (10 mL).
The temperature rose during the addition. Following addition, the
mixture was stirred (1 h), then cooled to 23 °C, and a solution of
diethylphosphite (1.45 mL, 11.0 mmol) in dry THF (20 mL) was added
dropwise. The mixture was refluxed (1 h), then cooled (23 °C). A
solution of 2,6-bis(chloromethyl)-pyridine (1.0 g, 5.7 mol) in dry THF
(20 mL) was added dropwise at 23 °C, and the mixture stirred and
refluxed (12 h). The resulting mixture was quenched with a saturated
aqueous solution of NH4Cl (100 mL), the aqueous phase was separated,
and then extracted with CH2Cl2 (3 × 20 mL). The organic phases
were combined, dried over Na2SO4 and concentrated. A crude solid
was obtained (4.0 g, 90%), that was recrystallized from methanol to
give a white solid, 1a: yield 3.7 g, 84%; mp 180-182 °C. IR (KBr,
cm-1): 3076 (w), 1585 (s), 1448 (s), 1397 (w), 1315 (vs), 1271 (s),
1179 (vs), 1120 (vs), 1035 (s), 959 (w), 894 (w), 839 (vw), 818 (w),
776 (s), 710 (s), 641 (w), 592 (s), 556 (w), 515 (s), 422 (w). NMR
(CDCl3): 1H δ 8.11 (dm, JHP ) 13.7 Hz, 4H), 7.77-7.62 (m, 12H),
7.39 (t, 3JHH ) 7.7 Hz, 1H), 7.15 (dd, 3JHH ) 7.7 Hz, 4JHH ) 1.7 Hz,
3
4
3
1H), 7.14 (dd, JHH ) 7.7 Hz, JHH ) 1.7 Hz, 1H), 3.82 (d, JHP
)
14.5 Hz, 4H). 13C{1H} δ 151.6 (d, 2JCP ) 8.5 Hz, py), 137.3, 134.8
(d, JCP ) 8.2 Hz) (py), 132.4, 132.0 (d, 1JCP ) 93 Hz), 131.5 (JCP
11.5 Hz), 126.8 (py), 126.3 (q, 1JCF ) 208 Hz, CF3), 42.0 (d, 1JCP
)
)
68.5 Hz). 31P{1H}: δ 30.8. Mass spectrum (ESI): [M+H]+ 780.1059,
[M+Na]+ 802.0915. Anal. calcd for C35H23NO2F12P2: C, 53.93; H,
2.97; N, 1.80. Found: C, 53.55; H, 3.27; N, 1.71.
2,6-Bis[bis-3,5-bis((trifluoromethyl)phenyl)phosphinoylmethyl]-
pyridine (1b). A solution of 3,5-bis(trifluromethyl)bromobenzene
(12.3 mL, 42.0 mmol) in dry THF (42 mL) was combined dropwise
with a suspension of magnesium (1.0 g, 42.0 mmol) in dry THF
(10 mL). The temperature rose during the addition, and the mixture
was stirred (1 h). The resulting mixture was cooled (23 °C), and a
solution of diethylphosphite (1.45 mL, 11.0 mmol) in dry THF (20
mL) was added dropwise. The mixture was refluxed (1 h) and then
cooled (23 °C). A solution of 2,6-bis(chloromethyl)-pyridine (1.0
g, 5.7 mmol) in dry THF (20 mL) was added dropwise, and the
mixture refluxed (12 h). The cooled mixture was quenched with a
saturated aqueous solution of NH4Cl (100 mL), and the aqueous
phase separated from the organic phase. The aqueous phase was
extracted with CH2Cl2 (3 × 20 mL), and the combined organic
phases were dried over Na2SO4 and concentrated. The residue was
crystallized from methanol giving a yellow brown powder, 1b:
yield: 1.77 g, 35%. Slow evaporation of the solvent provided a
Experimental Section
General Information. The organic reagents for the syntheses
were purchased from Aldrich Chemical Co. and used without
purification unless noted otherwise, and organic solvents were
(15) Horwitz, E. P.; Muscatello, A. C.; Kalina, D. G.; Kaplan, L. Sep. Sci.
Technol. 1981, 16, 417.
(16) Horwitz, E. P.; Kalina, D. G.; Muscatello, A. C. Sep. Sci. Technol.
1981, 16, 403.
(17) Romanovskiy, V. N.; Smirnov, I. V.; Babain, V. A.; Todd, T. A.;
Herbet, R. S.; Law, J. D.; Brewer, K. N. SolV. Extr. Ion Exch. 2001,
19, 1.
(18) Law, J. D.; Herbst, R. S.; Todd, T. A.; Romanovskiy, V. N.; Babain,
V. A.; Esimantovskiy, V. M.; Smirnov, I. V.; Zaitsev, B. N. SolV.
Extr. Ion Exch. 2001, 19, 23.
(19) Herbst, R. S.; Law, J. D.; Todd, T. A.; Romanovskiy, V. N.; Babain,
V. A.; Esimantovskiy, V. M.; Smirnov, I. V.; Zaitsev, B. N. SolV.
Extr. Ion Exch. 2002, 20, 429.
(20) Romanovskiy, V. N.; Babain, V. A.; Alyapyshev, M. Yu.; Smirnov,
I. V.; Herbst, R. S.; Law, J. D.; Todd, T. A. Sep. Sci. Technol. 2006,
41, 2111.
(21) Rezzonico, B.; Grigon-Dubois, M. J. Chem. Res. 1944, 142.
Inorganic Chemistry, Vol. 48, No. 7, 2009 3105

Pailloux et al.
second crop of crystals of 1b: total yield 3.77 g, 63%; mp 228-230
°C. IR (KBr, cm-1): 3079 (w), 3036 (w), 2967 (w), 2921 (w), 1845
(w), 1619 (s), 1586 (s), 1457 (s), 1369 (vs), 1284 (vs), 1189 (vs),
1134 (vs), 990 (w), 906 (s), 844 (s), 762 (vw), 742 (w), 692 (s),
2048 (w), 1978 (m), 1855 (w), 1735 (s), 1582 (s), 1494 (vs), 1312
(vs), 1260 (s), 1179 (vs), 1119 (vs, br), 1031 (s), 970 (s), 892 (sh,
s), 856 (s), 820 (s), 724 (s), 637 (s), 589 (s), 569 (sh, s), 505 (s).
Nd(2a)(NO3)3 ·(CH3CN)0.5. A sample of 2a (0.2 g, 0.25 mmol)
was dissolved in acetone (5 mL) and combined and stirred with a
solution of Nd (NO3)3 ·5H2O (0.055 g, 0.126 mmol) in MeOH (3
mL). A white powder was recovered by filtration after 12 h. Violet
crystals, suitable for X-ray analysis, were obtained by recrystalli-
zation from acetonitrile.
Eu(2a)(NO3)3. A sample of 2a (0.5 g, 0.6 mmol) was dissolved
in MeOH (5 mL) and combined with Eu(NO3)3 ·5H2O (0.269 g,
0.5 mmol) in MeOH (5 mL). A white precipitate formed after 1 h.
The solid was recovered by filtration. It is insoluble in MeOH and
partially soluble in CH2Cl2 and DMF. Recrystallization from DMF
slowly produced small crystals. IR: (KBr, cm-1): 3160 (w), 3080
(m), 3008 (m), 2940 (m), 2519 (w), 2448 (w), 2306 (w), 1730 (w),
1581 (m), 1494 (vs), 1435 (s), 1313 (vs), 1275 (m, sh), 1230 (m,
sh), 1167 (vs), 1120 (vs), 1033 (s), 969 (w), 858 (m), 820 (m), 771
(s), 726 (s), 688 (w), 639 (w), 591 (w), 569 (w), 512 (s).
Nd(2b)(NO3)3 ·(H2O)1.25. A sample of 2b (0.12 g, 0.09 mmol)
was dissolved in acetone (5 mL) and combined with a solution of
Nd (NO3)3 ·5H2O (0.04 g, 0.09 mmol) in acetone (3 mL). The
mixture was stirred at 23 °C (12 h). A white powder was recovered
after slow evaporation (7 d). Single crystals were obtained after
recrystallization from hot dichloromethane and slow evaporation:
mp: 200 °C (decomposed). IR (KBr, cm-1): 3449 (w, br), 3271
(vw), 3254 (vw), 3139 (w), 3093 (sh, w), 2929 (w, br), 2855 (w),
2769 (vw), 2674 (vw), 2524 (vw), 1944 (vw), 1847 (w), 1737 (w),
1620 (w), 1577 (w), 1495 (s), 1368 (s), 1284 (vs), 1184 (s), 1136
(vw), 1031 (w), 970 (w), 909 (w), 860 (w), 838 (w), 750 (w), 709
(sh, s), 690 (s), 641 (w), 613 (w), 569 (w), 514 (w).
X-ray Diffraction Analyses. Crystals of the ligands and complexes
were placed in glass capillaries and mounted on a Bruker X8 Apex2
CCD-based X-ray diffractometer outfitted with an Oxford Cryostream
700 low temperature device and a normal focus Mo-target X-ray tube
(λ ) 0.71073Å) operated at 1500 W (50 kV, 30 mA). The data frames
were integrated with the Bruker SAINT software package and
processed with SADABS. The structures were solved and refined with
the Bruker SHELXTL software package.22 Lattice and data collection
parameters are summarized in Table 1. Individual comments on each
structure follow. 1a and 2a·MeOH: The structure solutions and
refinements were well behaved. Non-hydrogen atoms were refined
anisotropically, and H-atoms were included and refined in ideal
positions with fixed Uiso ) 1.2Ueq of C. 1b·CH3OH: All non-hydrogen
atoms were refined anisotropically and hydrogen atoms were placed
in idealized positions and refined with fixed Uiso ) 1.5Ueq of C and O
of the MeOH-[O3, C40]. There are eight CF3 groups on the molecule
and a MeOH in the lattice and all are disordered. The treatment of the
disorder is fully described in the Supporting Information.
2b·(CH3)2CO: All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were included in idealized portions and refined with
Uiso ) 1.2Ueq of C. The eight CF3 groups and the acetone have large
Uiso values and they are disordered. The disorder treatment is described
in the Supporting Information. Nd(2a)(NO3)3: All non-hydrogen atoms
were refined anisotropically. The hydrogen atoms on C were included
in idealized positions and refined with Uiso ) 1.2Ueq of the parent
atom. There is disorder of the F-atom positions on C14. The treatment
of the disorder is described in the Supporting Information.
1
648 (vw), 611 (s), 520 (s), 486 (s). NMR (CDCl3): H δ 8.30 (d,
3
3JHP ) 11.2 Hz, 8H), 8.03 (s, 4H), 7.52 (t, JHH ) 7.7 Hz, 1H),
7.22 (dd, 3JHH ) 7.7 Hz, 4JHH ) 2.0 Hz, 1H), 7.20 (dd, 3JHH ) 7.7
4
2
Hz, JHH ) 2.0 Hz, 1H), 3.80 (d, JHP ) 18.1 Hz, 4H). 13C{1H} δ
151.0 (d, 2JCP ) 11 Hz, py), 138.9 (py), 134.7 (d, 1JCP ) 99.0 Hz),
2
3
2
133.0 (dq, JCF ) 34 Hz, JCP ) 12 Hz), 131.8 (d, JCP ) 12 Hz),
126.8, 124.7 (py), 123.1 (1JCF ) 270 Hz, CF3), 40.0 (d, JCP
)
1
65.8 Hz). 31P{1H}: δ 26.3. Mass spectrum (ESI): [M+H]+
1052.0435, [M+Na]+ 1074.0393. Anal. Calcd for C39H19NO2F24P2:
C, 44.55; H, 1.82; N, 1.33. Found: C, 43.72; H, 1.77; N, 1.38.
Ligand Oxidations. A sample of 1a or 1b (1 equiv) was
dissolved in CH2Cl2 and combined with m-chloroperbenzoic acid
(77%, 1.3 equiv). The mixture was stirred and refluxed (12 h). The
resulting mixture was quenched with saturated aqueous NaHCO3
(30 mL). The aqueous phase was extracted with CH2Cl2 (3 × 20
mL). Organic phases were combined, dried with Na2SO4 or MgSO4,
and concentrated.
2,6-Bis[bis(2-(trifluoromethyl)phenyl)phosphinoylmethyl]pyri-
dine N-Oxide (2a). A white powder 2a was obtained by crystal-
lization from methanol: yield 0.79 g, 78%; mp: 118-120 °C. IR
(KBr, cm-1): 3074 (w), 1569 (w), 1488 (w), 1428 (s), 1315 (vs),
1260 (s), 1179 (vs), 1118 (vs), 1035 (vs), 968 (s), 895 (w), 860
(w), 822 (w), 774 (vs), 710 (s), 639 (w), 592 (w), 518 (s), 489 (s).
NMR (CDCl3): 1H δ 8.10 (dm, JHP ) 14.3 Hz, 4H), 7.70 (m, 4H),
3
3
7.60 (m, 10H), 7.05 (t, JHH ) 8.7 Hz, 1H), 4.20 (d, JHH ) 13.8
Hz, 4H). 13C{1H} δ 143.0 (d, JCP ) 13 Hz, py), 133.0 (d, JCP
)
2
9.1 Hz), 131.9, 131.4 (d, 1JCP ) 95 Hz), 131.3 (d, JCP ) 11.8 Hz),
127.5 (m), 125.6 (q, 1JCF ) 193 Hz, CF3), 125.5 (py) 33.3 (d, 2JCP
) 72.8 Hz). 31P{1H}: δ 32.1. Mass spectrum (ESI) [M+H]+
796.0980, [M+Na]+ 810.0844. Anal. Calcd for C35H23NO3F12P2:
C, 52.85; H, 2.91; N, 1.76. Found: C, 52.17; H, 3.37; N, 1.60.
2,6-Bis[bis(3,5-bis(trifluoromethyl)phenyl)phosphinoylmethyl]-
pyridine N-Oxide (2b). A white powder 2b was obtained by
crystallization from methanol: yield 0.76 g, 75%. A white cotton was
obtained from acetone: yield 0.71 g, 70%. Suitable single crystals were
obtained from a mixture of acetone and methanol: mp 235-240 °C.
IR (KBr, cm-1): 3086 (vw), 2964 (vw), 2921 (s), 1575 (w), 1454 (w),
1415 (w), 1367 (s), 1283 (vs), 1189 (vs), 1136 (vs), 908 (s), 839 (s),
796 (w), 710 (s), 692 (s), 613 (w), 517 (s), 489 (s). NMR (CDCl3): 1H
δ 8.40 (d, 3JHP ) 11.8 Hz, 8H), 8.05 (s, 4H), 7.43 (dd, 3JHH ) 7.7 Hz,
4JHH ) 2 Hz, 1H), 7.41(dd, 3JHH ) 7.7 Hz, 4JHH ) 2 Hz, 1H) 7.15 (t,
3JHH ) 7.5 Hz, 1H), 4.00 (d, 2JCP ) 15.2 Hz, 4H). 13C{1H} δ 142.7
(d, 2JCP ) 10.7 Hz, py), 134.0 (d, 1JCP ) 100 Hz), 132.4 (dq, 2JCF
)
3
2
34 Hz), JCP ) 13 Hz), 131.7 (py), 131.1 (d, JCP ) 10 Hz), 126.6
(m), 126.0 (py), 122.6 (q, 1JCF ) 273 Hz, CF3) 34.7 (d, 1JCP ) 68.1
Hz). 31P{1H}: δ 27.9. Mass spectrum (ESI) [M+H]+ 1068.0607,
[M+Na]+ 1090.0330. Anal. Calcd for C39H19NO3F24P2: C, 43.88; H,
1.79; N, 1.31. Found: C, 43.20; H, 1.95; N, 1.30.
Synthesis of Complexes. Nd(2a)(NO3)3. A sample of 2a (0.10
g, 0.13 mmol) was dissolved in acetone (5 mL) and combined with
a solution of Nd (NO3)3 ·5H2O (0.055 g, 0.13 mmol) in MeOH (3
mL). The solution was stirred (12 h) and a white powder was
recovered by filtration. Violet crystals suitable for X-ray analyses
were obtained by recrystallization from a mixture of methanol,
acetonitrile and acetone with slow evaporation (5 days): yield 0.072
g, 49%; mp: >250 °C. IR (KBr, cm-1): 3448 (w, br), 3163 (w),
3079 (s), 3005 (s), 2932 (s), 2854 (w), 2620 (w), 2572 (w), 2515
(m), 2445 (m), 2357 (m), 2304 (m), 2235 (w), 2158 (w), 2092 (w),
(22) The crystallographic software used in these structure determinations
include: Sheldrick, G.M., SHELXTL, v. 6.14; Bruker Analytical X-ray
Madison, WI, 2001; Sheldrick, G.M., SADABS, v. 2.10. Program for
Empirical Absorption Correction of Area Detector Data, University
of Go¨ttingen:Go¨ttingen, Germany, 2003, SAINTPlus, v. 7.01, Bruker
Analytical X-ray, Madison, WI, 2003.
3106 Inorganic Chemistry, Vol. 48, No. 7, 2009

2,6-Bis[(diphenylphosphinoyl)methyl]pyridine N-Oxide Ligands
Table 1. Crystallographic Data
1a
2a ·CH3OH
1b·CH3OH
C40H23F24NO3P2
2b·(CH3)2CO
C42H25F24NO4P2
empirical formula
formula weight,
color, cryst. size (mm)
crystal system
space group
a (Å)
C35H23F12NO2P2
779.48
C36H27F12NO4P2
827.53
1083.53
1125.57
colorless, 0.52 × 0.52 × 0.27 colorless, 0.44 × 0.41 × 0.28 colorless, 0.41 × 0.08 × 0.08 colorless, 0.39 × 0.32 × 0.28
monoclinic
P2(1)/n
18.0813(19)
9.4524(10)
19.956(2)
90
triclinic
triclinic
triclinic
j
P1
j
P1
j
P1
8.5721(6)
13.1785(10)
16.8087(13)
71.228(4)
83.435(4)
77.558(4)
1753.5(2)
2
12.1654(5)
12.5478(5)
15.3684(7)
83.752(2)
88.970(2)
87.809(2)
2330.11(17)
2
12.1283(8)
12.5924(8)
15.3841(10)
97.640(3)
92.001(4)
90.857(3)
2326.8(2)
2
b (Å)
c (Å)
R (deg)
ꢁ (deg)
103.949(5)
90
3310.1(6)
4
γ (deg)
V (Å3)
Z
T, (K)
228
0.234
1.564
228
0.230
1.567
228
0.227
1.544
225
0.232
1.607
µ (mm-1
)
Dcalcd (gcm-3
)
Fooo
1576
840
1080
1124
no. of reflections
88321
55811
53409
54230
no. independent reflections 11136 (0.0261)
(Rint.
max/min transmission
13145 (0.0202)
14291 (0.0366)
11537 (0.0191)
)
0.93/0.88
0.939/0.906
0.0538, 0.1428
0.0688, 0.1582
0.980/0.910
0.0772, 0.2204
0.1292, 0.2591
0.93/0.91
0.0809, 0.2343
0.0941, 0.2541
final R1a, wR2 [I > 2σ(I)] 0.0389, 0.0981
b
R1, wR2 (all data)
0.0653, 0.1224
Nd(2a)(NO3)3
Nd(2a)(NO3)3 ·(CH3CN)0.5
Eu(2a)(NO3)3
Nd(2b)(NO3)3 ·(H2O)1.25
empirical formula
formula weight,
color, cryst. size (mm)
crystal system
space group
a (Å)
C35H23F12N4NdO12P2
1125.75
C36H24.5F12N4.5NdO12P2
1146.28
C35H23EuF12N4O12P2
1133.47
C39H21.5F24N4NdO13.25P2
1420.28
lilac, 0.28 × 0.25 × 0.10 lilac, 0.25 × 0.20 × 0.13
colorless, 0.18 × 0.16 × 0.07 colorless, 0.35 × 0.23 × 0.17
orthorhombic
P2(1)2(1)2(1)
10.1697(4)
18.9472(7)
21.0152(8)
90
monoclinic
P2(1)/c
11.9160(6)
23.4118(11)
15.6768(7)
90
94.698(4)
90
43587.7(4)
4
orthorhombic
P2(1)2(1)2(1)
10.1129(5)
18.8663(10)
21.1388(10)
90
triclinic
j
P1
11.0642(6)
15.0247(8)
16.2086(9)
87.033(3)
79.624(2)
75.122(3)
2561.5(2)
2
b (Å)
c (Å)
R (deg)
ꢁ (deg)
90
90
4049.4(3)
4
90
90
4033.1(3)
4
γ (deg)
V (Å3)
Z
T, (K)
225
1.484
1.847
225
1.381
1.747
228
1.758
1.867
225
1.228
1.841
µ (mm-1
)
Dcalcd (gcm-3
)
Fooo
2220
2264
2232
1391
no. of reflections
124872
101819
13350 (0.1189)
128628
15577 (0.0860)
81790
19848 (0.0201)
no. independent reflections 15578 (0.0422)
(Rint.
max/min transmission
)
0.86/0.68
final R1a, wR2 [I > 2σ(I)] 0.0369, 0.0805
R1, wR2 (all data) 0.0534, 0.0873
0.83/0.708
0.0586, 0.1218
0.1202, 0.1456
2
0.888/0.738
0.0410, 0.0819
0.0649, 0.0898
0.810/0.699
0.0325, 0.0811
0.0407, 0.0899
b
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) {∑[w(Fo - Fc2)2]/∑w(Fo )2}1/2
.
b
2
Nd(2a)(NO3)3 ·(CH3CN)0.5: All non-hydrogen atoms were refined
anisotropically. The hydrogen atoms of 2a were placed in ideal
positions and refined with Uiso ) 1.2Ueq of the parent atom. There is
a partially occupied CH3CN solvent molecule, and its H-atoms were
placed in ideal positions and refined with Uiso ) 1.5Ueq. There is a
small degree of disorder of the F-atoms on C28 and C35.
Eu(2a)(NO3)3: All non-hydrogen atoms except disordered F-atoms
on C30 were refined anisotropically. The H-atoms on C were placed
in ideal positions and refined with Uiso ) 1.2Ueq of the parent atom.
The treatment of the F-atom disorder is described in the Supporting
Information. Nd(2b)(NO3)3 ·(H2O)1.25: There are two Nd(2b)(NO3)3
complexes and 2.5 molecules of water per unit cell. All non-hydrogen
atoms were refined anisotropically. The H-atoms on C were placed in
ideal positions and refined with Uiso ) 1.2Ueq. There are three
disordered CF3 groups (C14, C21, C28) and their treatment is described
in Supporting Information. Selected bond distances for the ligands and
the complexes are summarized in Table 2.
carried out with the Gaussian 03 suite of programs.23 Analytic
calculations of the energy Hessian confirmed that minimum energy
structures had no imaginary frequencies. Given that Pople type
triple-ꢀ are adequate for phosphorus atoms,24 all calculations were
(23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, J. A.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, G. A., Gaussian 03, Revision
C.02; Gaussian, Inc.: Wallingford, CT, 2004.
Computational Analyses. Geometry optimization of ground-
state structures and corresponding vibrational frequencies were
Inorganic Chemistry, Vol. 48, No. 7, 2009 3107

Pailloux et al.
performed in the gas-phase with the B3YLP hybrid density
functional25 and 6-311G(d,p) and 6-311++G(d,p) Pople basis sets.
On the basis of evaluation of the resulting vibrational frequency
values produced for both basis sets for 2a and several model
compounds, that is, pyridine-1-oxide and triphenylphosphine oxide,
the 6-311G(d,p) basis set offered the best balance between
computational cost and frequency accuracy. The geometries and
energies were not corrected for basis set superposition error (BSSE)
since the larger basis sets combined with Density Functional Theory
were expected to produce smaller errors.26 The harmonic frequency
algorithm employed by Gaussian 03 generates a set of atom motions
for each frequency. The visualizer option in GaussView 3.027 was
employed to identify the frequency values corresponding to νPO
and νNO. The geometry optimized coordinates and frequencies for
the compounds of interest to this paper can be found in the
Supporting Information.
Results and Discussion
As detailed in a prior report,3 several symmetrical alkyl
and aryl substituted NPOPO ligands can be prepared in good
yields and purities by using traditional Arbusov or Michaelis-
Becker reactions28,29 on 2,6-bis(chloromethyl) pyridine.
Subsequent N-oxidation of the NPOPO pyridine intermedi-
ates produces the desired NOPOPO ligands. In addition, we
found that phosphinoyl Grignard reagents, in combination
with 2,6-bis(chloromethyl) pyridine, provide facile entry to
both symmetrical and asymmetrical alkyl and aryl NPOPO
intermediates. This route is often preferred because of the
relative efficiency of the one-pot formation and transforma-
tion of the Grignard reagents. In the present study, the
intermediate aryl Grignard reagents, 2-CF3C6H4MgBr,30 and
3,5-(CF3)2C6H3MgBr31 were prepared in dry THF and mixed
with (EtO)2P(O)H (∼4:1). The respective phosphinoyl
Grignard reagents [(2-CF3)C6H4]2 P(O)MgBr and [3,5-
(CF3)2C6H3]2P(O)MgBr were obtained in good yields and,
without isolation, these were combined (2:1) with 2,6-
bis(chloromethyl) pyridine. Work-up of the product mixture
provided the NPOPO compounds {[(2-CF3)C6H4]2P(O)CH2}2-
C5H3N (1a) and {[(3,5-(CF3)2C6H3]2P(O)CH2}2C5H3N (1b),
as solids, in good yields: 90% and 63%, respectively. Both
intermediates were dissolved in CH2Cl2 and N-oxidized with
m-chloroperbenzoic acid. Following standard solution work-
up, the target NOPOPO ligands {[(2-CF3)C6H4]2P(O)CH2}2-
C5H3NO (2a) and {[(3,5-CF3)2C6H3]2P(O)CH2}2C5H3NO
(2b) were isolated as white solids in 78% and 75% yields,
respectively. The syntheses are summarized in Scheme 1.
Characterization data substantiate the proposed formula-
tions of 1a, 1b, 2a, and 2b. Each compound displays a
(24) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639.
(25) Sousa, S. P.; Fernandes, S. A.; Ramos, M. J. J. Phys. Chem. A 2007,
111, 10439.
(26) Zhano, Y.; Truhlar, D. G. J. Chem. Theory Comput. 2005, 1, 415.
(27) Dennington, Roy, II; Keith, Todd; Millam, John; Eppinnett, Ken;
Hovell, W. Lee; and Gilliland, Ray; GaussView, Version 3.09;
Semichem, Inc.: Shawnee Mission, KS, 2003.
(28) Emsley, J.; Hall, D. The Chemistry of Phosphorus; Harper and Row:
London, 1976.
(29) Quin, L. D. A Guide to Organophosphorus Chemistry; Wiley-
Interscience: New York, 2000.
(30) McKinstry, L.; Livinghouse, T. Tetrahedron 1994, 50, 6145.
(31) Casalnuovo, A. L.; RajanBabu, T. V.; Ayers, T. A.; Warren, T. H.
J. Am. Chem. Soc. 1994, 116, 9869.
3108 Inorganic Chemistry, Vol. 48, No. 7, 2009

2,6-Bis[(diphenylphosphinoyl)methyl]pyridine N-Oxide Ligands
Scheme 1
Table 3. Gaussian Computation of P)O and N-O Stretching
Frequencies
vibrational
mode
computed
experimental
compound
1a
frequency (cm-1
)
frequency (cm-1
)
νPO
1219
1222
1207
1200
1205
1216
1225
1281
1201
1267
1197
1211
1282
1313
1187
1202
1239
1322
1179
1b
1c
νPO
νPO
1189
1195
2a
νPO
1179
νNO
νPO
νNO
νPO
1260
1189
1283
1188
2b
2c
νNO
1234
satisfactory CHN elemental analysis, and intense [M+H+]
and [M+Na+] ions in the high resolution ESI-MS are
observed. In prior infrared analyses of alkyl and aryl
substituted NPOPO and NOPOPO ligands,1-6 despite a
crowded fingerprint region 1700-700 cm-1, we have gener-
ally succeeded in making tentative frequency assignments
in the regions 1200-1150 and 1290-1230 cm-1 to P)O
and N-O stretching modes, respectively. However, because
of the numerous CF3 stretching and bending modes in 1a,b
and 2a,b, the fingerprint region is even more complex in
comparison to the parent aryl NPOPO, [Ph2P(O)CH2]2C5H3N
(1c) and NOPOPO, [Ph2P(O)CH2]2C5H3NO (2c). This has
made assignments of the νPO and νNO stretching modes in
the CF3 decorated ligands less certain. Nonetheless, the
experimental νPO stretch has been assigned to strong bands
centered at 1179 (1a) and 1189 cm-1 (1b). These values
compare favorably with the νPO band assigned in 1c,2 1195
cm-1. As expected, no strong band is observed in the N-O
stretching region 1290-1230 cm-1. Following N-oxidation,
the phosphoryl stretching frequencies in 2a and 2b remain
unchanged from those observed for their pyridine progenitors.
This is similar to the observations found for the conversion
of 1c to 2c. In that case, the νPO frequency in 2c is slightly
lower, 1188 cm-1, than in 1c. In addition, 2a and 2b display
additional strong bands at 1260 and 1283 cm-1, respectively,
and these are tentatively assigned to the νNO mode. These
appear at higher frequency than the νNO band for 2c,2 1235
cm-1. The up-frequency shifts may result from increased
group electronegativities for the CF3 decorated
[(CF3)C6H4]2P(O)CH2 and [(CF3)2C6H3]2P(O)CH2 groups in
2a and 2b.
In an effort to provide additional support for the tentative
νPO and νNO vibrational mode assignments made here and
in our past studies,1-6 Gaussian calculations were performed
on several simple phosphine oxide fragments, as well as on
1a, 1b, 2a, and 2b. The computational and experimental data
are summarized in Table 3. The computed values of νPO for
Ph3PO, Ph2(Me)PO, and Ph2(CF3)PO fall within the region
1240-1150 cm-1 typically ascribed to phosphine oxides with
aryl substituent groups.28,32,33 It might be expected that the
calculated νPO values would increase with increasing electron
withdrawal on the P-atom resulting in an order Ph2(Me) PO
< Ph3PO < Ph2(CF3)PO. The fact that the calculated νPO
for Ph2(Me)PO falls in between the νPO for the other two
Ph3PO
νPO
νPO
νPO
νNO
1205-118534,35
1202-117235,36
122637
Ph2(Me)PO
Ph2(CF3)PO
pyrN-O
1340-126538,39
molecules may result from a secondary influence on the
electron population in the P)O bond, or this may simply
reflect the uncertainty in the computed values which is
estimated to be ( 30 cm-1. The values computed for 1a,
1b, 2a, and 2b are all very similar to each other and to the
model fragments. These observations support the νPO band
assignments given above. The computed νNO frequencies for
pyridine N-oxide, 1322 cm-1, 2a, 1281 cm-1, and 2b, 1267
cm-1 are similar to each other. The differences may reflect
electronic differences or they may once more mirror the
larger computational error found with pyridine N-oxide
derivatives ((50 cm-1). It is well-known that experimental
νNO frequencies are sensitive to electronic effects exerted
by ring substituents, as well as the phase of the sample used
to record spectra.39,40
The 31P NMR spectra for 1a, 1b, 2a, and 2b each display
a single resonance in the narrow chemical shift range δ
33-26 and, as observed previously,2,3 the 31P resonance for
each N-oxide is slightly (1-2 ppm) downfield of its pyridine
progenitor. It is also noteworthy that the 31P chemical shifts
in 1a and 2a are identical to the shifts recorded previously3
for the tolyl analogues, and the shifts for 1b and 2b are ∼4
ppm upfield of the respective shifts in 1a and 2a. Hence,
the 2-CF3 substituents in 1a and 2a and the 3,5-(CF3)2
substituents in 1b and 2b apparently do not dramatically
impact the electronic character of the P)O groups as assessed
1
by 31P NMR. The H and 13C{1H} NMR spectra are also
comparable to the spectra observed for the phenyl and tolyl
respective NOPO and NOPOPO analogues.2,3
To confirm the molecular structure of this class of ligands,
the single crystal X-ray diffraction molecular structure
determination for 1a was undertaken. A view of the molecule
is shown in Figure 1, and selected bond lengths are
(32) NIST Standard Reference Data Program Collection, Origin: Sadtler
Research Laboraories under US-EPA contract.
(33) DeBolster, M. W. G.; Groeneveld, W. L. Top. Phosphorus Chem. 1976,
8, 273.
(34) Gramstad, T. Acta Chem. Scand. 1992, 46, 1087.
(35) An unattributed compendium found on the website at www.bu.edu/
gale/analyses/nuPO_Data_File/nuPO%20data%20.
(36) Shvets, A. A.; Safaryan, G. P.; Shvets, S. A. Russ. J. Gen. Chem.
2000, 70, 1037.
Inorganic Chemistry, Vol. 48, No. 7, 2009 3109

Pailloux et al.
Figure 1. Molecular structure and atom labeling scheme for 1a (20%
thermal ellipsoids; all hydrogen atoms omitted for clarity).
summarized in Table 2. As expected, the molecule consists
of a planar pyridine ring with pendent -CH2P(O)(2-CF3C6H4)2
groups on the 2- and 6- positions. The P)O bond vectors
are rotated away from the space occupied by the in-plane
pyridine N-atom lone electron pair. This is illustrated by
calculation of mean planes containing P1O1C6 and NC1-
C2C3C4C5C6; the angle between these planes is 86.7°. This
arrangement is anticipated based upon the structure reported
for [Ph2P(O)CH2]2C5H3N, 1c.41 The average P)O bond
length, 1.478(1) Å, is comparable to the average value
observed in 1c, 1.480(7) Å. However, an interesting unique
feature appears in 1a compared to 1c: the non-bonded
distances F(4) · · · P(1) 2.883Å and F(10) · · · P(2) 2.932Å are
unusually short compared to the sum of van der Waal radii,
3.27 Å.42 In addition, the distances F(1) · · · P(1), 3.205 Å,
and F(7) · · ·P(2), 3.133 Å, are slightly shorter than expected.
This feature, involving P(V) centers and aryl CF3 groups,
has also recently been observed in (2-CF3C6H5)2P(S)(SH)
where the related non-bonded distances are F· · · P 2.968 Å
and 3.165 Å.43 These suggest that weak electronic interac-
tions exist between the electronegative F atoms and the
backside of the P(V) center in both families of compounds.
The geometry and bond angles about P(1) and P(2) further
support this conclusion. Both P(1) and P(2) are tetrahedral
with average angles 109.5° at P(1) and 109.4° at P(2).
However, the tetrahedra are distorted, with larger X-P-Y
angles facing the positions occupied by F(1), F(4), F(7), and
F(10).44
Figure 2. Molecular structure and atom labeling scheme for 1b·CH3OH
(20% thermal ellipsoids; all hydrogen atoms and disordered CH3OH omitted
for clarity).
Figure 3. Molecular structure and atom labeling scheme for 2a·CH3OH
(20% thermal ellipsoids; all hydrogen atoms and CH3OH omitted for clarity).
solvent molecule CH3OH. These features obscure the
analysis of the non-bonded F · · · P distances, but it appears
that none of the F-atoms makes close approach to the
central P-atoms. The majority of F · · ·P distances exceed 4.0
Å with the shortest F(16) · · ·P(1), 3.55 Å, being significantly
greater than the sum of van der Waals radii. This is expected
as the 3- and 5- positions are well removed from the P-atom
environment. A view of 1b is shown in Figure 2 and selected
bond lengths are presented in Table 2. The average P)O
bond length in 1b, 1.483(1) Å, is comparable to that in 1a
and 1c. The phosphorus atom geometries are both tetrahedral,
and the bond angles show greater min/max variation than in
1a: 103.07°-114.55°.
The N-oxidized derivative 2a, contains a CH3OH molecule
in the unit cell, and all heavy-atom positions refined properly.
Like in 2c,2 the P)O bond vectors are rotated away from
the N-O bond vector. In this case the mean P1O2C6 plane
makes an angle of 63.7° with the mean N1C1C2C3C4C5C6
plane. A view of the molecule without the CH3OH solvent
molecule appears in Figure 3, and selected bond distances
are given in Table 2. The average P)O bond length,
1.4824(13) Å, is essentially identical to the average values
in 1a and 2c. Hence, as noted before, N-oxidation and CF3
substituted do not appear to significantly impact P)O bond
lengths or P)O stretching frequencies (vide supra). The
N-O bond length in 2a is 1.3003(18) Å, and this is slightly
shorter than the value in 2c, 1.315(6) Å.2 The question then
becomes: does N-oxidation alter the F · · · P non-bonded
interactions between the 2-CF3 substituents and the P-atoms?
Examination of the environments around P(1) and P(2) in
The observation of short non-bonded F · · · P interactions
in 1a led us to also determine the molecular structures of
1b, 2a, and 2b. For molecule 1b with CF3 groups at the 3-
and 5- positions of the aryl rings (total of eight CF3 groups),
all CF3 groups show large anisotropic thermal parameters,
and four (on C(14), C(15), C(22), and C(31)) are
disordered. In addition, the unit cell contains a disordered
(37) Lindner, E.; Kranz, H. Z. Naturforsch. B. 1967, 22B, 675.
(38) NIST Standard Reference Data Program Collection, Origin: Coblentz
(No. 10231).
(39) Mielke, Z. Spectrochim. Acta 1983, 34A, 141.
(40) Herlocker, D. W.; Drago, R. S.; Meek, V. I. Inorg. Chem. 1966, 5,
2012.
(41) Szbyk, E.; Zhang, Z. Y.; Palenik, G. J.; Palenik, R. C.; Colgate, S. O.
Acta Crystallogr. 1989, C45, 1234.
(42) Boudi, J. Phys. Chem. 1964, 68, 441.
(43) Klaehn, J. R.; Peterman, D. R.; Harrup, M. K.; Tillotson, R. D.; Luther,
T. A.; Law, J. D.; Daniels, L. M. Inorg. Chim. Acta 2008, 361, 2522.
(44) For example F(1)/C(7)-P(1)-C(14), 112.65(6)°; F(4)/O(1)-P(1)-C(14),
111.82(6)°; F(7)/C(22)-P(2)-C(29), 113.61(7)°, F(10)/O(2)-P(2)-
C(21), 111.83(7)°.
3110 Inorganic Chemistry, Vol. 48, No. 7, 2009

2,6-Bis[(diphenylphosphinoyl)methyl]pyridine N-Oxide Ligands
Figure 5. Molecular structure and atom labeling scheme for Nd(2a)(NO3)3
(20% thermal ellipsoids; all hydrogen atoms omitted for clarity).
Figure 4. Molecular structure and atom labeling scheme for 2b·(CH3)2CO
(20% thermal ellipsoids; all hydrogen atoms and (CH3)2CO omitted for
clarity).
Nd(2a)(NO3)3. An infrared spectrum for the complex in KBr
shows a strong band at 1119 cm-1 that is tentatively assigned
to a metal coordinated P)O stretch. This represents a 60
cm-1 coordination shift from the free ligand consistent with
astrong2a/Nd(III)interaction.Althoughtheregion1340-1240
cm-1 is rich in adsorptions, a band at 1260 cm-1 is tentatively
assigned to coordinated pyridine N-O. A single crystal X-ray
analysis for the complex, confirms that 2a forms a 1:1
tridentate chelate complex with Nd(NO3)3, and a view of
the molecule is shown in Figure 5. Selected bond lengths
are presented in Table 2. The Nd(III) is coordinated to one
tridentate ligand 2a and three bidentate NO-3 resulting in a
distorted nine-coordinate sphenoid polyhedron. One CF3
(C(14)) is disordered over two positions, and the C(15)-C(20)
ring is slightly irregular. The structural parameters can be
compared against data for the complex Nd(2c)(NO3)33. The
Nd-O(P) distances in the new complex, 2.362(2) and
2.372(2) Å, are slightly shorter than observed in Nd(2c)-
(NO3)3, 2.390(3) and 2.378(3) Å, and the Nd-O(N) distance,
2.459(2) Å, is significantly longer than in Nd(2c)(NO3)3,
2.382(3) Å. The longer Nd-O(N) distance is consistent with
a weaker N-oxide/Nd interaction that may be in response to
electron withdrawal by the CF3 groups or increased steric
factors because of the presence of the large CF3 groups. The
P)O and N-O distances, 1.489(2), 1.493(2), and 1.329(3)
Å are comparable with the related bond lengths in
Nd(2c)(NO3)3: 1.500(3), 1.504(3), and 1.337(4) Å. These
observations are consistent with the observed P)O coordina-
tion shifts in the IR spectra for Nd(2a)(NO3)3 and the
difficulty in locating a shifted N-O stretch for the new
complex. Lastly, the Nd-O distances involving the nitrate
groups suggest that one nitrate is symmetrically coordinated
in a bidentate fashion while two are asymmetrically bonded.
This may indicate that one or both of these anions might be
replaced in the inner coordination sphere by another neutral
molecule of 2a. This is observed with 2c where Nd(2c)(NO3)
can be converted to [Nd(2c)2(NO3)](NO3)2.3
2a show, once more, four interactions shorter than the sum
of P and F van der Waals radii: F(1) · · · P(1), 3.117 Å,
F(4)· · ·P(1), 2.989 Å, F(7)· · ·P(2), 2.945 Å, and F(10) · · ·P(2),
2.980 Å. The solvent molecule CH3OH weakly hydrogen
bonds with a neighboring phosphoryl O-atom.45
The molecular structure of 2b is shown in Figure 4. The
unit cell includes a molecule of acetone that is disordered
over two positions, and this molecule is not shown in Figure
4. In addition, the CF3 groups are disordered in a fashion
similar to that found with its pyridine progenitor 1b. None
of the non-bonded F· · · P distances are shorter than 3.27 Å,
and the two shortest distances, F(7) · · ·P(2) and F(10) · · ·P(2),
are greater than 3.6 Å. The molecule shows its two P)O
bond vectors rotated away from the N-O bond vector with
the average P)O bond length 1.480(2) Å and N-O bond
length 1.288(3) Å.
The observation that 2a and 2b are obtained in atom-
efficient syntheses as easily purified solids that are soluble
in the process solvent FS-13 (∼0.01 M) makes them
potentially attractive as liquid-liquid extraction reagents
since the alternative hydrocarbon soluble extractant [(2-
EtHx)2P(O)CH2]2C5H3NO is a heavy oil that must be purified
with care. However, before embarking on time-consuming
extraction analyses with actinide ions and fission product
species, it is important to explore the coordination behavior
of the new ligands and compare this against the coordination
behavior of the related ligand, [Ph2P(O)CH2]2C5H3NO, 2c,
using Ln(NO3)3 compounds as surrogates for trivalent
actinides. Combining ratios 2a or 2b with the early lan-
thanides Nd(III) and Eu(III) L/Ln ) 1:1, 2:1 and excess:1
were explored with several solvent combinations. On the
basis of earlier studies with 2c and related aryl and alkyl
analogues,1,3 it was expected that 2:1 complexes would form.
However, in no case were the 2:1 complexes isolated and
structurally characterized. Crystalline 1:1 complexes with
Nd(III) and Eu(III) were obtained and selected features of
four complexes are summarized.
Nd(2a)(NO3)3 · (CH3CN)0.5. Combination of 2a and Nd-
(NO3)3 in a 2:1 ratio in the mixed solvent acetone/MeOH
gave a white powder; however, attempts to recrystallize this
solid from acetone or MeOH failed. On the other hand, when
the initial white powder was recrystallized from CH3CN,
violet crystals were obtained, and the molecular structure
Nd(2a)(NO3)3. The 1:1 combination of 2a and Nd(NO3)3
in a mixed acetone/MeOH solution led to the isolation of
(45) The H-bonding parameters for D-H· · ·A O(4)-H(4A) · · ·O(3)#1 are:
D-H, 0.77(4)Å; H · · · A, 1.95(4)Å; D · · · A, 2.713(3)Å; D-H-A,
171(4)°.
Inorganic Chemistry, Vol. 48, No. 7, 2009 3111

Pailloux et al.
Figure 8. Molecular structure and atom labeling scheme for Nd(2b)(NO3)3
(H2O)1.25 (20% thermal ellipsoids; all hydrogen atoms and water omitted
for clarity).
Figure 6. Molecular structure and atom labeling scheme for
Nd(2a)(NO3)3 ·(CH3CN)0.5 (20% thermal ellipsoids; all hydrogen atoms and
CH3CN omitted for clarity).
Figure 7. Molecular structure and atom labeling scheme for Eu(2a)(NO3)3
(20% thermal ellipsoids; all hydrogen atoms omitted for clarity).
Figure 9. Computed structure for [Nd(2a)2(NO3)+2].
determined by X-ray diffraction methods. A view of the
molecule, with the outer sphere CH3CN omitted, is shown
in Figure 6, and selected bond lengths are given in Table 2.
Although the ligand/metal combining ratio is 2:1, the
complex formed is 1:1 and coordination number remains 9.
In this complex the coordination polyhedron more closely
approximates a monocapped square antiprism. This results
from more symmetrical binding of the three nitrate groups
on Nd(III). This is not the only difference. Here the ligand
2a has a shorter Nd-O(N) distance 2.385(3) Å and longer
Nd-O(P) distances, 2.397(3) and 2.425(3) Å compared to
the values in Nd(2a)(NO3)3.
Eu(2a)(NO3)3. The 1:1 combination of 2a and Eu(NO3)3
in MeOH gave a white solid that slowly recrystallized from
DMF providing single crystals of this complex. A view of
the molecule is shown in Figure 7, and selected bond lengths
are listed in Table 2. The structure is very similar to that
found for Nd(2a)(NO3)3 with a distorted nine coordinate
sphenoid polyhedron. The Eu-O(N) distance, 2.424(3) Å,
is longer than the Eu-O(P) distances, 2.334(2) and 2.320(2)
Å, and the small differences in bond lengths between the
Eu and Nd complexes appear to be in line with the
differences in ionic radii: Eu (C.N. ) 8) 1.21Å, Nd (C.N. )
8) 1.26Å.
formation of a 1:1 complex, and a view of the molecule with
the outer sphere water omitted for clarity is shown in Figure
8. Selected bond lengths appear in Table 2. The Nd
polyhedron approximates a nine coordinate distorted mono-
capped square antiprism. The water molecule (2.5 H2O/2Nd
in unit cell) is disordered along with three of the CF3 groups
but this likely has no effect on the ligand binding to Nd.
This structure is very similar to that described for
Nd(2a)(NO3)3 · (CH3CN)0.5 with more symmetrical binding
of the nitrate anions and tighter (shorter) binding of the N-O
donor group to Nd(III), 2.374(1) Å, and longer M-O(P)
bond lengths, 2.457(1) and 2.396(1) Å.
Conclusions
The trifluoromethyl phenyl decorated NOPOPO ligands
have been prepared in an effort to obtain a NOPOPO-like
extractant that can be used with the sulfone process solvent
FS-13. Although somewhat less soluble than the CMPO
extractant Ph2P(O)CH2C(O)NBu2 in FS-13, the derivative
2a has sufficient solubility (∼0.01 M) to encourage further
study. Before undertaking that work it was important to
characterize the new ligands and determine if the CF3
substituents impact the coordination of the ligands with
lanthanide ions. Besides being soluble in FS-13, which is
not the case with EtHx-NOPOPO, the two ligands 2a and
Nd(2b)(NO3)3 ·(H2O)1.25. The 2:1 combination of 2b with
Nd(NO3)3 in acetone gave a white solid that was recrystal-
lized from CH2Cl2. The structure determination revealed the
3112 Inorganic Chemistry, Vol. 48, No. 7, 2009

2,6-Bis[(diphenylphosphinoyl)methyl]pyridine N-Oxide Ligands
2b are solids. This makes the purification of 2a and 2b much
easier than for the heavy oil EtHxNOPOPO. Structural
analyses indicate that the free ligands do not adopt a preferred
preorganized tridentate metal binding geometry but as found
with CMPO other NOPOPO ligands, the molecules are able
to reorganize under mild conditions to the chelating geom-
etry. This is confirmed by the isolation and structural
characterization of four different Ln(III) 1:1 complexes. It
is noted that all attempts to form 2:1 ligand/Ln complexes
gave no crystalline complexes. Whether this is an outcome
of reduced ligand base strength because of the CF3 substit-
uents or steric effects is not clear at this time. However,
recall, vide supra, that the parent NOPOPO 1c does form
2:1 complexes with Ln(III) ions, [Ln(1c)2(NO3)](NO3)2, in
which two nitrate ions are displaced from the inner coordina-
tion sphere to the outer sphere, and both ligands 1c are
bonded in a tridenate fashion.1,3 This results in coordination
numbers of eight in these complexes. To potentially shed
further light on the steric issue, the hypothetical molecule
Nd(2a)2(NO3)3 was modeled with an inner sphere composi-
tion, [Nd(2a)2(NO3)2+]; the complex is found to be sterically
realistic as shown in Figure 9. Efforts will continue to isolate
such 2:1 complexes under modified conditions. In addition,
extraction analyses will be undertaken.
1a and 2a. It is not clear that this geometry exists in solution,
but this interaction may play a role in solution thermody-
namics for extraction. It is also possible that this interaction
partially offsets the expected “first order” electron withdrawal
of electrons by the CF3 groups.46
Acknowledgment. Acknowledgment is made to the U.S.
Department of Energy (DOE), Chemical Sciences, Geo-
sciences and Biosciences Office, Office of Basic Energy
Sciences (Grant DE-FG02-03ER15419) for financial support
at UNM (R.T.P.).
Supporting Information Available: CIF files for the crystal
structures, all-atom ORTEP plots, IR spectra and computational
material. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC802390C
(46) If σ orbital effects dominate the electron distribution in these molecules,
it would be expected that addition of CF3 substituents to the phenyl
groups would result in withdrawal of electron density from the P)O
groups causing, for example, decreased basicity, downfield 31P NMR
shifts and increased VPO. However, if one or more fluorine atoms
associate with the back side of the P(V) center in solid and solution
conditions, then this may offset some of this first order electron
withdrawal. This would, in turn, offset shifts in 31P and VPO. Pi-electron
shifts also certainly contribute in these molecules and further study
will be required to provide a more detailed picture of the impacts of
CF3 substituents on the bonding and spectra for these and related
compounds.
Another feature of the free ligands proved interesting.
Namely, in the solid state the 2-CF3 substituents on the
phenyl groups interact with the backside of the P-atom in
Inorganic Chemistry, Vol. 48, No. 7, 2009 3113