Synthesis, lanthanide coordination chemistry, and liquid-liquid extraction performance of CMPO-decorated pyridine and pyridine N-oxide platforms

Article

pubs.acs.org/IC

Synthesis, Lanthanide Coordination Chemistry, and Liquid−Liquid

Extraction Performance of CMPO-Decorated Pyridine and Pyridine

N‑Oxide Platforms

Daniel Rosario-Amorin,^†Sabrina Ouizem,^†Diane A. Dickie,^†Yufeng Wen,^†Robert T. Paine,*^,†Jian Gao,^†

John K. Grey,^†Ana de Bettencourt-Dias,^‡Benjamin P. Hay,^§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 Nevada, Reno, Nevada 89557, 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 a set of new ligands containing one or two carbamoylmethylphosphine oxide (CMPO) fragments

appended to pyridine and pyridine N-oxide platforms are described. Molecular mechanics analyses for gas phase lanthanide−

ligand interactions for the pyridine N-oxides indicate that the trifunctional NOPOCO molecules, 2-{[Ph₂P(O)][C(O)NEt₂]-

C(H)}C₅H₄NO (7) and 2-{[Ph₂P(O)][C(O)NEt₂]CHCH₂}C₅H₄NO (8), and pentafunctional NOPOP′O′COC′O′ molecules,

2,6-{[Ph₂P(O)][C(O)NEt₂]C(H)}₂C₅H₃NO (9) and 2,6-{[Ph₂P(O)][C(O)NEt₂]CHCH₂}₂C₅H₃NO (10), should be able to

adopt, with minimal strain, tridentate and pentadentate chelate structures, respectively. As a test of these predictions, selected

lanthanide coordination chemistry of the N-oxide derivatives was explored. Crystal structure analyses reveal the formation of a

tridentate NOPOCO chelate structure for a 1:1 Pr(III) complex containing 7 while 8 adopts a mixed bidentate/bridging

monodentate POCO/NO binding mode with Pr(III). Tridentate and tetradentate chelate structures are obtained for several 1:1

complexes of 9 while a pentadentate chelate structure is observed with 10. Emission spectroscopy for one complex,

[Eu(9)(NO₃)₃], in methanol, shows that the Eu(III) ion resides in a low-symmetry site. Lifetime measurements for methanol

and deuterated methanol solutions indicate the presence of four methanol molecules in the inner coordination sphere of the

metal ion, in addition to the ligand, with the nitrate anions most likely dissociated. The solvent extraction performance of 7−10

in 1,2-dichloroethane for Eu(III) and Am(III) in nitric acid solutions was analyzed and compared with the performance of 2,6-

bis(di-n-octylphosphinoylmethyl)pyridine N-oxide (TONOPOP′O′) and n-octyl(phenyl)-N,N-diisobutylcarbamoylmethylphos-

phine oxide (OPhDiBCMPO) measured under identical conditions.

examples of these compounds serve as critical components in

large-scale TALSPEAK,^1,2,11PUREX,^1,2,12and TRUEX^1,2,13

solvent extraction processes. Nonetheless, there remain

limitations with each process that stimulate continued eﬀorts

to derive improved extractant systems. This includes develop-

ment of additional neutral carbamoylmethylphosphine oxide

(CMPO) ligands, RR′P(O)CH₂C(O)NR″₂, 1, for general f-

element ion partitioning from high-level nuclear waste

INTRODUCTION

■

The development of multifunctional ligands as eﬃcient and

selective solvent extraction reagents for inherently diﬃcult f-

element ion separations continues to be an important research

objective, and many of the fundamental challenges central to

extractant design and assembly have been thoroughly

summarized.¹⁻⁷Due to the unique coordination properties of

the relatively hard f-element ions,⁸only a small number of

donor group types have been successfully employed in

extractant designs for these ions. Foremost among these are

neutral and acidic organophosphorus compounds,^1,2,9,10and

Received: November 19, 2012

Published: March 5, 2013

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solutions, as well as phosphorus-free malonamide-based

extractants that are used in the DIAMEX process^{2d,3,5,6b,14,15}

and bis-triazinyl pyridine-based extractants employed in

SANEX and GAMEX processes^2e,3,5,7for diﬃcult An(III)/

Ln(III) group separations.

extraction complexes remain somewhat unclear,^27d,e,29the

preorganized ligands with two, three, and four CMPO

fragments display much improved extractant eﬃciencies and,

in some cases, enhanced selectivity performance compared with

the parent CMPO. For example, the wide-rim substituted

calix[4]arene, 2, with four CMPO fragments, produces

distribution ratios, D = [M_o]/[M_aq], depending upon speciﬁc

conditions, similar to those recorded with solutions that are

∼100 times more concentrated in the parent CMPO.^27a

The known CMPO ligands, in particular, display fundamen-

tally interesting properties. Under typical ligand-loaded biphasic

solvent extraction process conditions, they are known to form

stable lanthanide (Ln) and actinide (An) ion complexes in the

organic phase even in the presence of strong mineral acids, e.g.,

HNO₃and HCl. Furthermore, extraction eﬃciencies unexpect-

edly increase with increasing acid concentration. Ligand

dependency analyses for Ln(III), An(III), and An(IV) cations

indicate formation of organic phase soluble complexes

containing two to four CMPO ligands depending upon the

speciﬁc metal ion and the organic diluent. In the case of

Am(III) in nitric acid solutions, the extraction complex that is

transferred to the organic phase is proposed to be [Am-

(CMPO)₃(NO₃)₃]·3HNO₃.¹⁶Solution infrared and NMR

spectroscopic data suggest that the CMPO ligands bind to

Am(III) in a monodentate mode through the phosphine oxide,

O_P, atoms.¹⁷The three nitrate anions probably reside in the

inner coordination sphere, bonded in a bidentate fashion, and

the coextracted nitric acid molecules are likely coordinated with

the amide carbonyl, O_C, atoms of the CMPO ligands. In

contrast, crystal structure analyses for several isolated

lanthanide−CMPO complexes, as well as for related

lanthanide−carbamoylmethylphosphonate (CMP), (RO)₂P-

(O)CH₂C(O)NR′₂, complexes, typically reveal solid-state

structures containing two CMPO or CMP ligands and three

nitrate anions in the inner coordination sphere. For early

lanthanide ions, the CMPO or CMP ligands are bonded in a

bidentate, O_PO_Cmanner while for late lanthanides the CMPO

or CMP ligands are bonded in a monodentate mode through

the O_Patoms.¹⁸In the latter complexes, the amide carbonyl O_C

atoms are found to be hydrogen bonded with an inner sphere,

lanthanide bound, water molecule. In these cases, the three

nitrate counterions reside in the inner coordination sphere

bound in a bidentate mode. More recently, Odinets and co-

workers¹⁹have also reported the successful isolation and

structural characterization of a 3:1 CMPO:Pr(III) complex in

which the CMPO ligands are all bonded in a bidentate, O_PO_C

manner. The observed structural diﬀerences between the

solution and solid phase complexes are not unexpected given

the competing coordination features present in these systems.

Indeed, subsequent detailed quantum mechanical studies of

lanthanide−CMPO interactions conﬁrm that, even in the gas

phase, there is a sensitive balance of factors responsible for

selection of the CMPO (CMP) binding condition.²⁰In

condensed solution and solid-state phases, the picture is

expected to be even more complicated.

During the maturation of the CMPO family of extractants,

our group also explored additional families of multifunctional

ligands derived by decoration of pyridine platforms. These

ligands are illustrated by the general structures 3−6 that carry

pendent phosphine oxide (X = P(O)R₂),³⁰phosphonate (X =

P(O)(OR)₂),³⁰methyl phosphine oxide (X = CH₂P(O)R₂),³¹

and methyl phosphonate (X = CH₂P(O)(OR)₂)³¹donor group

substituents. The NPO, 3, and NPOP′O′, 4, ligands are easily

obtained in good yields and high purity, and they form

monodentate or bidentate O_P-bonded coordination complexes

with Ln(III) ions. In no case, under the conditions utilized, is

the pyridine N-atom observed to bond to an f-element cation.

With only two exceptions, the corresponding pyridine N-oxide

NOPO, 5, and NOPOP′O′, 6, ligands also are obtained in good

yields and high purity. The exceptions appear in attempts to

prepare the NOPOP′O′ ligands with X = P(O)R₂and

P(O)(OR)₂. Here, oxidative ring degradation reactions

compete with N-oxidation. The NOPO and NOPOP′O′

ligands form stable coordination complexes with Ln(III),

Th(IV), Pu(IV), and U(VI) ions, and X-ray diﬀraction analyses

indicate that the ligands bond to the f-element ions as bidentate

O_NO_Pand tridentate O_NO_PO′_P′chelates, respectively.^30,31The

extraction performance of one NOPO derivative 5, with X =

P(O)(OHx)₂in CHCl₃, toward Ce(III), Eu(III), and Yb(III) in

aqueous nitric acid solutions, is relatively unremarkable.³²Most

notably, the D values decrease signiﬁcantly with increasing acid

concentration as expected if nitric acid competes eﬀectively for

binding with the ligand. However, despite the need to form

larger seven-membered chelate rings, toluene solutions of a

derivative of 5 with X = CH₂P(O)Ph₂provide eﬃcient

extractions at all acid concentrations ([HNO₃] = 10⁻²M to

6 M). In addition, the D values increase with increasing nitric

acid concentration between [HNO₃] = 10⁻²M and 1 M before

decreasing.³³Although the N-oxides 6, with X = P(O)R₂and

P(O)(OR)₂, are not available for extraction analysis, the

extraction performance of methyl phosphine oxides 6 (X =

−CH₂P(O)R₂) have proven to be especially interesting. Initial

survey extractions of Eu(III) and Am(III) in HNO₃with 6 (X =

CH₂P(O)Ph₂) in CHCl₃showed that, similar to the CMPO

ligands, the D values for this NOPOP′O′ molecule increase by

more than two decades with increasing acid concentration.

Between [HNO₃] = 0.5−1.0 M, the D values are similar to

those for equivalent concentrations of CMPO (R = n-octyl, R′

= phenyl, R″ = i-Bu) while at higher nitric acid concentrations

the D values for NOPOP′O′ are greater. A further contrast

between the two ligand families is found in the ligand

dependency analyses. Recall that the CMPO ligand dependency

for Am(III) extractions from nitric acid solutions is consistent

with the presence of three molecules of CMPO in the

extraction complex. However, for NOPOP′O′, the slope

analysis indicates that two molecules of the ligand are present

in the extraction complex. This observation parallels the

stoichiometry found in solid state structures. These results led

to more detailed comparative studies of the extraction of

These observations, as well as general coordination chemistry

principles⁸and molecular mechanics (MM) computational

analyses with other donor ligand systems,²¹suggest that

improved extraction performance might be realized by

preorganization of two or more CMPO or CMP fragments

on a central lipophilic platform. Indeed, several groups have

explored attachment of CMPO fragments to C₃-symmetric

alkyl,²²⁻²⁶1,3,5-trialkyl benzene,²⁵C₃-symmetric amine,²⁶

upper and lower rim-substituted calixarene,²⁷and resorcinar-

ene²⁸platforms. In each case, the platform attachment linkage

involves the amide N-atom of the CMPO fragment. Although

the precise compositions and structures of the resulting

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hexane, 10.7 mL, 17 mmol) was added dropwise, under nitrogen (23

Eu(III) and Am(III) in HNO₃solutions by toluene and

dodecane solutions of 6 (X = CH₂P(O)(2-ethylhexyl)₂),³⁴and

most recently of Eu(III), Am(III), U(VI), Th(IV), Pu(IV), and

Np(V) extractions with toluene solution of 6 (X = CH₂P(O)-

(n-octyl)₂).³⁵In each case, the NOPOP′O′ derivatives display

improved eﬃciency over CMPO and additional comparative

studies continue.

The interesting coordination chemistry and favorable

extraction performance displayed by the NOPO and

NOPOP′O′ ligands led us to consider potential marriages

between these fragments and the CMPO fragment. Of course,

there are several options available for incorporating both

functionalities into hybrid ligand structures. In this Article, we

describe the syntheses, selected lanthanide coordination

chemistry, and initial extraction analyses for a set of new

hybrid ligands, 7−10, wherein the NOPO/CMPO and

NOPOPO/CMPO linkages are made through the methyl

carbon atom of the CMPO. Ligands involving attachment

through the amide N-atom will be reported separately.

°C, 40 min), to

a vigorously stirred solution of 2-

(diphenylphosphinoylmethyl)pyridine P-oxide^31a,37(5.02 g, 17

mmol) in dry toluene (90 mL, 23 °C). Following addition of the

reagents, a clear, pale red solution was obtained which was heated and

stirred (65−70 °C, 1 h). The reaction mixture was then cooled (−40

°C), and diethylcarbamoyl chloride (2.2 mL, 17 mmol) in toluene (10

mL) was added dropwise (30 min, 23 °C) with stirring. The

temperature of the mixture was slowly increased, held at 80 °C (1 h),

and then cooled and stirred (23 °C, 12 h). The resulting yellow

suspension was poured into an ice bath (100 g), and the phases were

separated. The aqueous layer was extracted with CHCl₃(2 × 50 mL),

and the combined organic phases were washed with water (50 mL).

The organic phase was dried (4 Å molecular sieves) and ﬁltered, and

the volatiles were removed by vacuum evaporation. The weakly brown

residue was triturated with Et₂O (3 × 25 mL) and pentane leaving a

white powder, 7_N: yield 3.42 g, 51%; mp 158−160 °C. The compound

is soluble in CHCl₃, CH₂Cl₂, MeOH, and EtOH, slightly soluble in

benzene and acetone, and insoluble in toluene, pentane, and Et₂O.

1

³¹P{¹H} NMR (121.5 MHz, CDCl₃): δ 29.5. H NMR (300 MHz,

CDCl₃): δ 8.41 (d, J_HH= 4.2 Hz, 1H, H₁), 7.95−7.70 (m, 5H, H_4,11,11′),

7.57−7.35 (m, 7H, H_{3,12,12′,13,13′}), 7.02−6.95 (t, J_HH= 5.1 Hz, 1H, H₂),

5.31 (d, J_HP= 13.2 Hz, 1H, H₆), 3.41−3.01 (m, 4H, H_8,8′), 0.98 (t, J_HH

= 7.1 Hz, 3H, H₉), 0.90 (t, J_HH= 7.1 Hz, 3H, H_9′). ¹³C{¹H} NMR

(75.5 MHz, CDCl₃): δ 166.11 (C₇), 152.60 (d, J_CP= 5.7 Hz, C₅),

149.07 (C₁), 136.52 (C₃), 132.21 (d, J_CP= 9.4 Hz, C₁₁), 131.93 (d, J_CP

= 101.7 Hz, C_10,10′), 131.89 (d, J_CP= 9.3 Hz, C_11′), 131.83 (C_13,13′),

128.21 (d, J_CP= 12.1 Hz, C_12,12′), 125.33 (d, J_CP= 3.0 Hz, C₄), 122.60

(C₂), 56.28 (d, J_CP= 64.5 Hz, C₆), 43.04 (C₈), 40.90 (C_8′), 14.53 (C₉),

12.70 (C_9′). FTIR (KBr, cm⁻¹): 3054, 2978, 1634 (ν_CO), 1435, 1190

(ν_PO), 1117. HRMS (ESI) m/z (%): 393.1737 (100) [M + H⁺].

C₂₃H₂₆N₂O₂P requires 393.1732; 415.1560 (23) [M + Na⁺].

C₂₃H₂₅N₂O₂PNa requires 415.1551.

2-(Diphenyl-N,N-diethylcarbamoylmethylphosphine oxide)-

pyridine N-Oxide (7). Method A is described here. A sample of 7_N

(2.3 g, 5.87 mmol) was dissolved in glacial acetic acid (10 mL) and

combined with a solution of H₂O₂(30%, 3.3 mL, 29 mmol). The

mixture was stirred (23 °C, 4 d), and then the volatiles were removed

by vacuum evaporation. The remaining white residue was dissolved in

CHCl₃(50 mL) and washed with water (3 × 25 mL). The combined

aqueous wash solutions were extracted with CHCl₃(2 × 25 mL) and

the combined organic phases dried (4 Å molecular sieves). The

organic phase was vacuum evaporated leaving a white solid (7): yield

1.83 g, 76%. Method B follows here. To a solution of 7_N(2.0 g, 5.1

mmol) in dry CH₂Cl₂(10 mL) was slowly added m-chloroperox-

EXPERIMENTAL SECTION

■

General Information. Organic reagents (Aldrich Chemical Co)

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 spectra were recorded on a Bruker Tensor 27

benchtop spectrometer. Solution NMR spectra were measured with

JEOL-GSX400, Bruker FX-250, and 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.³⁶Mass spectra for the ligands were obtained from the UNM

Mass Spectrometry Center by using electrospray ionization (ESI) with

a Waters/Micromass mass spectrometer operating in the positive ion

mode. Mass spectra (ESI) for one lanthanide complex, [Eu(9)-

(NO₃)₃], were recorded at UNR with an Agilent Technologies 6230

TOF mass spectrometer in positive ion mode. In the latter case, the

sample was previously dissolved in deuterated methanol for photo-

physical characterization. Consequently, deuterium partially substitutes

for hydrogen atoms in some of the observed peaks. Elemental analyses

were performed by Galbraith Laboratories.

Ligand Syntheses. 2-(Diphenyl-N,N-diethylcarbamoylmethyl-

phosphine oxide)pyridine (7_N). A solution of n-BuLi (1.6 M in

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ybenzoic acid (77 wt %, 3.77 g, 15.3 mmol). The mixture was stirred

(23 °C, 12 h) and the resulting mixture diluted with additional CH₂Cl₂

(90 mL) and washed with aqueous NaOH (2 N, 3 × 25 mL) and

distilled water (2 × 25 mL). The organic phase was dried (anhydr

MgSO₄) and ﬁltered, and the volatiles were removed by vacuum

evaporation, leaving a white powder, 7: yield 1.5 g, 72%; mp 134−138

°C. The compound is soluble in CHCl₃, CH₂Cl₂, MeOH, and EtOH,

slightly soluble in xylenes, benzene, and acetone, and insoluble in Et₂O

and heptane. ³¹P{¹H} NMR (121.5 MHz, CDCl₃): δ 30.4. In d₄-

MeOH: δ 31.6. ¹H NMR (300 MHz, CDCl₃): δ 8.34 (d, J_HH= 7.8 Hz,

1H, H₁), 8.12 (d, J_HH= 6.3 Hz, 1H, H₄), 7.92−7.78 (m, 4H, H_11,11′),

7.54−7.41 (m, 6H, H_{12,12′,13,13′}), 7.21−7.07 (m, 2H, H_2,3), 6.67 (d, J_HP

= 8.6 Hz, 1H, H₆), 3.48−3.37 (dq, J_HH= 7.0 Hz, 1H, H₈), 3.22 (q, J_HH

= 7.1 Hz, 2H, H_8′), 3.04−2.93 (dq, J_HH= 7.0 Hz, 1H, H₈), 0.97 (t, J_HH

= 7.1 Hz, 3H, H₉), 0.87 (t, J_HH= 7.1 Hz, 3H, H_9′). ¹³C{¹H} NMR

(75.5 MHz, CDCl₃): δ 164.27 (d, J_CP= 3.4 Hz, C₇), 143.80 (C₅),

138.49 (C₁), 132.35 (C₃), 131.68 (d, J_CP= 9.8 Hz, C₁₁), 131.34 (d, J_CP

= 9.8 Hz, C_11′), 130.86 (d, J_CP= 104.1 Hz, C_10,10′), 129.85 (C₁₃),

129.80 (C_13′), 128.66 (d, J_CP= 12.0 Hz, C₁₂), 128.63 (d, J_CP= 12.0 Hz,

aqueous phases were extracted with CHCl₃(2 × 25 mL) and the

combined organic phases dried (4 Å molecular sieves). The solution

was ﬁltered and the ﬁltrate vacuum evaporated leaving a white solid

(8): yield 12.4 g, 61%; mp 146−147 °C. The compound is soluble in

CHCl₃, CH₂Cl₂, MeOH, and EtOH, slightly soluble in xylenes and

benzene, and insoluble in Et₂O and pentane. ³¹P{¹H} NMR (121.5

1

MHz, CDCl₃): δ 32.0. In d₄-MeOH: δ 35.4. H NMR (300 MHz,

CDCl₃): δ 7.99 (d, J_HH= 5.7 Hz, 1H, H₁), 7.86−7.73 (m, 4H, H_12,12′),

7.41−7.32 (m, 6H, H_{13,13′,14,14′}), 7.12 (dd, J_HH= 7.5 Hz, 1H, H₃),

6.99−6.95 (m, 2H, H_2,4), 4.83−4.76 (m, 1H, H₇), 3.41−3.05 (m, 4H,

H_6,9), 2.91−2.69 (m, 2H, H_9′), 0.70 (t, J_HH= 7.1 Hz, 3H, H₁₀), 0.63 (t,

J_HH= 7.1 Hz, 3H, H_10′). ¹³C{¹H} NMR (75.5 MHz, CDCl₃): δ 166.49

(C₈), 148.27 (d, J_CP= 12.7 Hz, C₅), 138.82 (C₁), 131.82 (C_14,14′),

131.78 (d, J_CP= 100 Hz, C₁₁), 131.65 (d, J_CP= 9.5 Hz, C₁₂), 131.40 (d,

J_CP= 9.5 Hz, C_12′), 130.56 (d, J_CP= 99.0 Hz, C_11′), 128.37 (C₃), 128.29

(d, J_CP= 11.2 Hz, C₁₃), 128.15 (d, J_CP= 11.2 Hz, C_13′), 125.60 (C₄),

124.41 (C₂), 42.33 (C₉), 40.71 (C_9′), 38.31 (d, J_CP= 62.6 Hz, C₇),

31.19 (C₆), 13.70 (C₁₀), 12.50 (C_10′). FTIR (KBr, cm⁻¹): 3064, 2980,

2931, 2874, 1634 (ν_CO), 1485, 1437, 1380, 1360, 1323, 1282, 1269,

1242 (ν_N−O), 1227, 1200, 1180 (ν_PO), 1151, 1136, 1119, 1099, 1068,

997, 970, 893, 877, 862, 835, 783, 766, 746, 714, 700, 617, 553, 532,

517, 490. HRMS (ESI) m/z (%): 423.1828 (100) [M + H⁺].

C₂₄H₂₈N₂O₃P requires 423.1838; 445.1637 (32) [M + Na⁺].

C₂₄H₂₇N₂O₃PNa requires 445.1657. Anal. Calcd for C₂₄H₂₇N₂O₃P:

C 68.23, H 6.44, N 6.63. Found: C 67.79, H 6.65, N 6.59.

C_12′), 125.83 (C₄), 124.73 (C₂), 42.98 (C₈), 41.38 (d, J_CP= 63.8 Hz,

C₆), 41.09 (C_8′), 14.09 (C₉), 12.65 (C_9′). FTIR (KBr, cm⁻¹): 3055,

2970, 2933, 2874, 1638 (ν_CO), 1589, 1485, 1435, 1283, 1265 (ν_N−O),

1205, 1192 (ν_PO), 1117, 1099, 1072, 995, 951, 916, 841, 773, 752,

729, 702, 561, 550, 513. HRMS (ESI) m/z (%): 409.1687 (100) [M +

H⁺]. C₂₃H₂₆N₂O₃P requires 409.1681; 431.1500 (18) [M + Na⁺].

C₂₃H₂₅N₂O₃PNa requires 431.1501. Anal. Calcd for C₂₃H₂₅N₂O₃P: C

67.64, H 6.17, N 6.86. Found C 67.37, H 6.30, N 6.65.

2,6-Bis(diphenyl-N,N-diethylcarbamoylmethylphosphine oxide)-

pyridine (9_N). A solution of n-BuLi (1.6 M in hexane, 56.3 mL, 90.1

mmol) was added dropwise (23 °C, 30 min) under nitrogen to a

vigorously stirred slurry of 2,6-bis(diphenylphosphinoylmethyl)-

pyridine^31b(20.28 g, 40 mmol) in dry toluene (250 mL). The solid

dissolves, and the solution turns red during the addition. Following

addition, the solution was heated (80−90 °C, 2 h), and then cooled

(23 °C), and diethylcarbamyl chloride (10.1 mL, 80 mmol), in dry

toluene (50 mL), was added dropwise (30 min) with vigorous stirring.

The mixture was heated and stirred (90 °C, 12 h), and a yellow

suspension was formed. The mixture was poured into an ice bath (100

g), and the phases separated. The aqueous layer was extracted with

CHCl₃(3 × 50 mL), and the combined organic phases were dried (4

Å molecular sieves) and ﬁltered and the volatiles removed by vacuum

evaporation. A light brown residue was recovered, dispersed with

acetone (10 mL), ﬁltered, washed with diethyl ether (3 × 25 mL), and

vacuum-dried leaving a white solid (9_N): yield 8.7 g, 31% (rac/meso

80/20); mp 179−184 °C. The isomeric mixture is soluble in CHCl₃,

CH₂Cl₂, MeOH, and EtOH, partially soluble in acetone, and insoluble

in toluene, ethyl acetate, pentane, and Et₂O. ³¹P{¹H} NMR (121.5

2-[(Diphenyl-N,N-diethylcarbamoylmethylphosphine oxide)-

methyl]pyridine (8_N). A solution of n-BuLi (1.6 M in hexane, 62.8

mL, 0.10 mol) was added dropwise (40 min), under nitrogen, to a

vigorously stirred slurry of diphenyl-N,N-diethylcarbamoylmethyl-

phosphine oxide³⁸(31.63 g, 0.10 mol) in toluene (300 mL, 23 °C).

Following addition of the reagents, a clear, pale red solution was

obtained which was heated (80−90 °C, 2 h). The resulting dark red

solution was cooled (23 °C) and transferred dropwise (1 h) into a

vigorously stirred solution of 2-(chloromethyl)pyridine^31a(12.8 g, 0.10

mol) in toluene (100 mL, 23 °C). The resulting mixture was stirred

and heated (90 °C, 12 h), and then cooled (23 °C). A yellow solid

formed that contained the product and LiCl. The solid was collected

by ﬁltration under nitrogen and washed with water (100 mL) and the

remaining solid dissolved in CHCl₃(100 mL). The aqueous wash

solution was extracted with CHCl₃(2 × 50 mL), and the combined

organic phases were washed with water (3 × 25 mL) and then dried (4

Å molecular sieves). The organic phase was recovered by ﬁltration and

vacuum evaporated leaving a sticky, pale yellow residue that was

washed with diethyl ether and vacuum-dried leaving a bone white solid

(8_N): yield: 20.2 g, 50%; mp 116−118 °C. The product is soluble in

CHCl₃, CH₂Cl₂, EtOH, MeOH and insoluble in toluene, hexane and

MHz, CDCl₃): δ 30.2. ¹H NMR (300 MHz, CDCl₃): δ 7.86 (dd, J_HH

=

7.1 Hz, 4H, H₉), 7.72 (dd, J_HH= 7.1 Hz, 4H, H_9′), 7.51−7.32 (m, 15H,

H

_{1,2,10,10′,11,11′}), 5.24 (d, J_HP= 13.8 Hz, 2H, H₄), 3.31−3.01 (m, 8H,

_6,6′), 0.92 (t, J_HH= 7.1 Hz, 6H, H₇), 0.86 (t, J_HH= 7.0 Hz, 6H, H_7′).

1

Et₂O. ³¹P{¹H} NMR (121.5 MHz, CDCl₃): δ 31.0. H NMR (300

MHz, CDCl₃): δ 8.38 (d, J_HH= 4.1 Hz, 1H, H₁), 8.13 (dd, J_HH= 8.1

Hz, 2H, H₁₂), 7.83 (dd, J_HH= 8.1 Hz, 2H, H_12′), 7.45−7.37 (m, 7H,

¹³C{¹H} NMR (75.5 MHz, CDCl₃): δ 166.14 (C₅), 151.94 (d, J_CP

5.6 Hz, C₃), 137.01 (C₁), 132.29 (d, J_CP= 9.5 Hz, C₉), 132.12 (d, J_CP

=

H

_{3,13,13′,14,14′}), 7.02−6.95 (m, 2H, H_2,4), 4.35 (t, J_HP= 12.6 Hz, 1H, H₇),

101 Hz, C₈), 132.01 (d, J_CP= 9.0 Hz, C_9′), 131.76 (C₁₁), 131.70 (C_11′),

131.47 (d, J_CP= 102 Hz, C_8′), 128.14 (d, J_CP= 11.6 Hz, C_10,10′), 123.78

(C₂), 55.93 (d, J_CP= 64.7 Hz, C₄), 42.98 (C₆), 40.90 (C_6′), 14.68 (C₇),

12.71 (C_7′). FTIR (KBr, cm⁻¹): 3057, 2978, 2936, 1643 (ν_CO), 1439,

1200 (ν_PO). HRMS(ESI): m/z (%): 706.2983 (100) [M + H⁺].

C₄₁H₄₆N₃O₄P₂requires 706.2964; 728.2787 (11) [M + Na⁺].

C₄₁H₄₅N₃O₄P₂Na requires 728.2783.

3.49−3.39 (m, 1H, H₉), 3.25−3.00 (m, 4H, H_6,9′), 2.78−2.66 (m, 1H,

H₉), 0.71 (t, J_HH= 7.1 Hz, 3H, H₁₀), 0.66 (t, J_HH= 7.1 Hz, 3H, H_10′).

¹³C{¹H} NMR (75.5 MHz, CDCl₃): δ 167.60 (C₈), 158.50 (d, J_CP

=

14.2 Hz, C₅), 148.93 (C₁), 136.06 (C₃), 132.08 (d, J_CP= 9.0 Hz, C₁₂),

131.85 (C_14,14′), 131.79 (d, J_CP= 9.0 Hz, C_12′), 131.29 (d, J_CP= 84.3

Hz, C_11,11′), 128.41 (d, J_CP= 12.1 Hz, C₁₃), 128.21 (d, J_CP= 12.1 Hz,

C

_13′), 123.73 (C₄), 121.47 (C₂), 45.58 (d, J_CP= 61.7 Hz, C₇), 42.42

2,6-Bis(diphenyl-N,N-diethylcarbamoylmethylphosphine oxide)-

pyridine N-Oxide (9). Method A is described here. A sample of 9_N

(7.87 g, 11.2 mmol, rac/meso 80/20) was dissolved in a mixture of

glacial acetic acid (30 mL) and aqueous H₂O₂solution (30%, 6.3 mL,

56 mmol) and the mixture stirred (23 °C, 4 d). The resulting solution

was vacuum evaporated leaving a white solid that was dissolved in

CHCl₃(50 mL) and washed with distilled water (3 × 25 mL). The

organic phase was dried (4 Å molecular sieves) and ﬁltered, and the

volatiles were removed by vacuum evaporation. The residue was

dissolved in acetone (15 mL) and the solution treated with diethyl

ether (100 mL). A precipitate formed that was collected by ﬁltration

and washed with diethyl ether (3 × 10 mL) leaving a colorless

(C₉), 40.67 (C_9′), 36.43 (C₆), 13.47 (C₁₀), 12.38 (C_10′). FTIR (KBr,

cm⁻¹): 3059, 2974, 2931, 1634 (ν_CO), 1589, 1435, 1317, 1192

(ν_PO), 1121, 748, 700, 521. HRMS (ESI) m/z (%): 407.1888 (100)

[M + H⁺]. C₂₄H₂₈N₂O₂P requires 407.1888; 429.1710 (48) [M +

Na⁺]. C₂₄H₂₇N₂O₂PNa requires 429.1708.

2-[(Diphenyl-N,N-diethylcarbamoylmethylphosphine oxide)-

methyl]pyridine N-Oxide (8). A sample of 8_N(19.5 g, 48 mmol)

was dissolved in glacial acetic acid (60 mL), H₂O₂(30%, 27.2 mL, 270

mmol) was added, and the mixture was stirred (23 °C, 4 d). The

resulting solution was vacuum evaporated and the residue dissolved in

CHCl₃(100 mL) and washed with water (3 × 25 mL). The combined

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microcrystalline powder (9): yield 3.96 g, 47% (rac/meso > 95/5). The

ﬁltrate was vacuum evaporated leaving additional 9 as a white powder:

yield 2.28 g, 27% (rac/meso 54/46); global yield 6.24 g, 74% (rac/meso

80/20). Method B follows. To a solution of 9_N(1.0 g, 1.41 mmol, rac/

meso 80/20) in anhydrous CH₂Cl₂(10 mL) was slowly added m-

chloroperoxybenzoic acid (77 wt %, 635 mg, 2.83 mmol), and the

mixture was stirred (23 °C, 12 h). The resulting reaction mixture was

diluted in CH₂Cl₂(50 mL) and washed with aqueous NaOH (2 N, 4

× 10 mL) and then distilled water (2 × 10 mL). The organic phase

was dried (anhydr MgSO₄) and ﬁltered, and the solvents were

removed by vacuum evaporation, leaving 9 as a white powder: yield

1.02 g, 100% (rac/meso 80/20); mp rac-9: 172−174 °C. ³¹P{¹H}

NMR (121.5 MHz, CDCl₃) rac-9: δ 29.6. meso-9: δ 31.3. In d₄-MeOH:

rac-9: δ 35.0. meso-9:δ 36.4. ¹H NMR (300 MHz, CDCl₃) rac-9: δ 8.03

(d, J_HH= 8.1 Hz, 2H, H₂), 7.82−7.74 (m, 8H, H_9,9′), 7.49−7.28 (m,

12H, H_{10,10′,11,11′}), 7.10 (t, J_HH= 8.0 Hz, 1H, H₁), 6.62 (d, J_HH= 9.5

Hz, 2H, H₄), 3.34−2.95 (m, 8H, H_6,6′), 0.85 (t, J_HH= 7.2 Hz, 6H, H₇),

0.82 (t, J_HH= 7.2 Hz, 6H, H_7′). meso-9: 7.92−7.75 (m, 10H, H_2,9,9′),

7.49−7.27 (m, 12H, H_{10,10′,11,11′}), 6.84 (t, J_HH= 8.1 Hz, 1H, H₁), 6.82

(d, J_HH= 12.2 Hz, 2H, H₄), 3.38−2.93 (m, 8H, H_6,6′), 0.91 (t, J_HH= 7.1

Hz, 6H, H₇), 0.84 (t, J_HH= 7.1 Hz, 6H, H_7′).¹³C{¹H} NMR (75.5

MHz, CDCl₃) rac-9: δ 164.64 (d, J_CP= 2.6 Hz, C₅), 142.26 (C₃),

132.07 (C_11,11′), 131.91 (d, J_CP= 102 Hz, C₈), 131.60 (d, J_CP= 9.5 Hz,

C₉), 131.54 (d, J_CP= 9.5 Hz, C_9′), 131.02 (d, J_CP= 102 Hz, C_8′),

128.52−128.15 (C_1,10,10′), 124.91 (C₂), 42.79 (C₆), 42.59 (d, J_CP= 64.4

Hz, C₄), 41.04 (C_6′), 14.26 (C₇), 12.61 (C_7′). meso-9: δ 164.75 (d, J_CP

= 2.5 Hz, C₅), 143.17 (C₃), 132.71−130.49 (C_{8,8′,9,9′,11,11′}), 128.96−

128.21 (C_10,10′), 127.87 (C₁), 124.58 (C₂), 43.30 (d, J_CP= 63.6 Hz, C₄),

42.89 (C₆), 41.12 (C_6′), 14.15 (C₇), 12.69 (C_7′). FTIR (KBr, cm⁻¹):

3057, 2976, 2932, 1643 (ν_CO), 1556, 1483, 1429, 1400, 1313, 1275

(ν_N−O), 1203 (ν_PO), 1126, 1103, 1072, 997, 902, 841, 727, 706, 696,

534, 507. HRMS (ESI) m/z (%): 722.2904 (100) [M + H⁺].

C₄₁H₄₆N₃O₅P₂requires 722.2913. Anal. Calcd for C₄₁H₄₅N₃O₅P₂: C

68.23, H 6.28, N 5.82. Found: C 67.74, H 6.39, N 5.60.

2,6-Bis[(diphenyl-N,N-diethylcarbamoylmethylphosphine oxide)-

methyl]pyridine (10_N). A solution of n-BuLi (1.6 M in hexane, 31.3

mL, 50 mmol) was added dropwise (23 °C, 40 min), under nitrogen,

to a vigorously stirred solution of diphenyl-N,N-diethylcarbamoylme-

thylphosphine oxide³⁸(15.75 g, 50 mmol) in toluene (300 mL).

Following combination of the reagents, a clear, pale yellow solution

was obtained that was heated (75−80 °C, 2 h) and stirred. The

resulting dark red solution was cooled (23 °C) and transferred

dropwise over 1 h, under nitrogen, into a vigorously stirred solution of

2,6-(dibromomethyl)pyridine³⁹(6.63 g, 25 mmol) in toluene (100

mL, −40 °C). The resulting mixture was slowly warmed (23 °C),

stirred, and then heated (80 °C). After about 2 h a yellow precipitate

appeared, and heating and stirring were continued (12 h). The

resulting mixture was cooled, the solid was collected by ﬁltration and

washed with water (100 mL), and the remaining solid dissolved in

CHCl₃(50 mL). The aqueous layer was extracted with CHCl₃(2 × 25

mL), the combined organic phases were dried (4 Å molecular sieves)

and ﬁltered and the volatiles removed by vacuum evaporation leaving a

pale yellow residue. The residue was washed with Et₂O (3 × 25 mL)

leaving a white powder (10_N) consisting of a diasteroisomeric mixture

of racemic isomers (R,R/S,S) and mesomeric isomer (R,S): yield 9.0 g,

49% (rac/meso 60/40). The combined ﬁltrate was vacuum evaporated,

and the residue was dissolved in a minimum of EtOAc (1 mL) and

diethyl ether (20 mL). The resulting precipitate was collected by

ﬁltration and washed with diethyl ether (10 mL): yield 2.2 g, 12%

(rac/meso 13/87); global yield 11.2 g, 61% (rac/meso 51/49). The

compound is soluble in CHCl₃, CH₂Cl₂, MeOH, and EtOH, slightly

soluble in benzene, toluene, acetone, and EtOAc, and insoluble in

pentane and Et₂O. Details for diastereoisomeric resolution follow: In a

100 mL round-bottom ﬂask, 350 mg of 10_N(rac/meso 60/40) was

dissolved in CH₂Cl₂/acetone (1/1, 2 mL), and diethyl ether (60 mL)

was added. The solvent was allowed to slowly evaporate (23 °C). After

one day, block-like crystals of racemic (R,R/S,S) isomers were

separated from the solution containing the meso isomer (R,S) and

washed several time with diethyl ether. The same procedure was

repeated a second time for each isomer: yield rac-10_N160 mg, 46%,

meso-10_N80 mg, 23%; mp rac-10_N186−188 °C; meso-10_N140−142

°C. ³¹P{¹H} NMR (121.5 MHz, CDCl₃) rac-10_N: δ 30.4. meso-10_N: δ

1

30.4. H NMR (300 MHz, CDCl₃) rac-10_N: δ 8.15 (t, J_HH= 8.7 Hz,

4H, H₁₀), 7.91 (t, J_HH= 8.6 Hz, 4H, H_10′), 7.45 (m, 12H, H_{11,11′,12,12′}),

7.26 (t, J_HH= 7.4 Hz, 1H, H₁), 6.84 (d, J_HH= 7.4 Hz, 2H, H₂) 4.30 (t,

J_HP= 13.1 Hz, 2H, H₅), 3.42−3.32 (m, 2H, H₇), 3.14−2.89 (m, 8H,

H

_4,7′), 2.66−2.59 (m, 2H, H₇), 0.61 (t, J_HH= 6.6 Hz, 6H, H₈), 0.41 (t,

J_HH= 6.7 Hz, 6H, H_8′). meso-10_N: δ 8.27 (t, J_HH= 8.9 Hz, 4H, H₁₀),

7.85 (t, J_HH= 9.1 Hz, 4H, H_10′), 7.53−7.40 (m, 12H, H_{11,11′,12,12′}), 7.30

(t, J_HH= 9.4 Hz, 1H, H₁), 6.87 (d, J_HH= 7.6 Hz, 2H, H₂) 4.14 (t, J_HP

=

13.1 Hz, 2H, H₅), 3.50−3.45 (m, 2H, H₇), 3.30−3.04 (m, 8H, H_4,7′),

2.91−2.82 (m, 2H, H₇), 0.80 (t, J_HH= 6.9 Hz, 6H, H₈), 0.69 (t, J_HH

=

6.9 Hz, 6H, H_8′).¹³C{¹H} NMR (75.5 MHz, CDCl₃) rac-10_N: δ 167.43

(C₆), 158.05 (d, J_CP= 15.1 Hz, C₃), 136.68 (C₁), 132.07 (d, J_CP= 9.5

Hz, C₁₀), 131.86 (C_12,12′), 131.79 (d, J_CP= 9.5 Hz, C_10′), 130.76 (C₉),

130.69 (C_9′), 128.40 (d, J_CP= 11.9 Hz, C₁₁), 128.34 (d, J_CP= 11.4 Hz,

C

_11′), 121.89 (C₂), 46.03 (d, J_CP= 62.0 Hz, C₅), 42.30 (C₇), 40.57

(C_7′), 36.39 (C₄), 13.51 (C₈), 12.30 (C_8′). meso-10_N: δ 168.27 (C₆),

158.32 (d, J_CP= 13.8 Hz, C₃), 136.64 (C₁), 132.47 (d, J_CP= 9.3 Hz,

C₁₀), 131.96 (d, J_CP= 9.3 Hz, C_10′), 131.90 (C₁₂), 131.72 (d, J_CP= 75.5

Hz, C₉), 131.67 (C_12′), 131.19 (d, J_CP= 75.5 Hz, C_9′), 128.54 (d, J_CP

=

12.5 Hz, C₁₁), 128.38 (d, J_CP= 12.5 Hz, C_11′), 121.64 (C₂), 46.51 (d,

J_CP= 61.2 Hz, C₅), 42.68 (C₇), 40.92 (C_7′), 36.50 (C₄), 13.88 (C₈),

12.69 (C_8′). FTIR (KBr, cm⁻¹): 3059, 2976, 2934,1634 (ν_CO), 1437,

1379, 1319, 1209, 1190 (ν_PO), 1113. HRMS (ESI) m/z (%): rac-10_N

734.3268 (94) [M + H⁺]. C₄₃H₅₀N₃O₄P₂requires 734.3277; 756.3094

(100) [M + Na⁺]. C₄₃H₄₉N₃O₄P₂Na requires 756.3096; meso-10_N

734.3264 (53) [M + H⁺]. C₄₃H₅₀N₃O₄P₂requires 734.3277; 756.3091

(100) [M + Na⁺]. C₄₃H₄₉N₃O₄P₂Na requires 756.3096.

2,6-Bis[(diphenyl-N,N-diethylcarbamoylmethylphosphine oxide)-

methyl]pyridine N-Oxide (10). Method A is described here. To a

solution of 10_N(7.42 g, 10 mmol, rac/meso 60/40) in glacial acetic

acid (30 mL) was added a hydrogen peroxide (30%, 6 mL, 53 mmol).

The mixture was stirred (23 °C, 4 d), and the volatiles were removed

by vacuum evaporation. The residue was dissolved in CHCl₃(100

mL) and washed with distilled water (3 × 25 mL). The organic phase

was dried (4 Å molecular sieves) and ﬁltered, and the volatiles were

removed by vacuum evaporation leaving 10 as white powder: yield

5.56 g, 74% (rac/meso 60/40). Method B follows. To a solution of 10_N

(1.0 g, 1.36 mmol, rac/meso 60/40) in anhydrous CH₂Cl₂(10 mL)

was slowly added m-chloroperoxybenzoic acid (77 wt %, 613 mg, 2.72

mmol). The mixture was stirred (23 °C, 12 h), and the resulting

mixture was diluted with CH₂Cl₂(50 mL) and washed with aqueous

NaOH (2 N, 3 × 15 mL) and distilled water (2 × 10 mL). The organic

phase was dried (anhydr MgSO₄) and ﬁltered, and the volatiles were

removed by vacuum evaporation leaving 10 as a white powder: yield

990 mg, 97% (rac/meso 60/40). Diastereomerically pure isomers of 10

were also prepared starting with diastereomerically pure isomers of

10_N; mp rac-10 128−130 °C; meso-10 108−110 °C. The compound is

soluble in CHCl₃, CH₂Cl₂, MeOH, and EtOH, slightly soluble in

xylenes, toluene, benzene, acetone, ethyl acetate, and THF, and

insoluble in Et₂O and heptane. ³¹P{¹H} NMR (121.5 MHz, CDCl₃)

rac-10: δ 32.5. meso-10: δ 30.8. In d₄-MeOH rac-10: δ 35.1. meso-10: δ

33.7. ¹H NMR (300 MHz, CDCl₃) rac-10: δ 8.12−8.06 (m, 4H, H₁₀),

7.95−7.89 (m, 4H, H_10′), 7.52−7.49 (m, 12H, H_{11,11′,12,12′}), 7.07 (d,

J_HH= 7.6 Hz, 2H, H₂), 6.89 (t, J_HH= 7.6 Hz, 1H, H₁), 4.92 (m, 2H,

H₅), 3.52−3.44 (m, 2H, H₇), 3.30−2.92 (m, 8H, H_4,7′), 2.79−2.72 (m,

2H, H₇), 0.71 (t, J_HH= 6.9 Hz, 6H, H₈), 0.67 (t, J_HH= 7.0 Hz, 6H,

H_8′). meso-10: δ 7.99−7.93 (m, 4H, H₁₀), 7.83−7.77 (m, 4H, H_10′),

7.51−7.35 (m, 12H, H_{11,11′,12,12′}), 6.96 (d, J_HH= 7.6 Hz, 2H, H₂), 6.73

(t, J_HH= 7.7 Hz, 1H, H₁), 4.83 (q, J_HP= 8.1 Hz, 2H, H₅), 3.56−3.46

(m, 2H, H₇), 3.32−3.16 (m, 6H, H_4,7′), 2.99−2.89 (m, 4H, H_7,7′), 0.94

(t, J_HH= 7.0 Hz, 6H, H₈), 0.75 (t, J_HH= 7.1 Hz, 6H, H_8′). ¹³C{¹H}

NMR (75.5 MHz, CDCl₃) rac-10: δ 168.89 (C₆), 148.28 (d, J_CP= 13.2

Hz, C₃), 132.10 (d, J_CP= 9.5 Hz, C_10,10′), 132.04 (C_12,12′), 131.84 (d,

J_CP= 100.2 Hz, C₉), 131.13 (d, J_CP= 99.7 Hz, C_9′), 128.52 (d+d, J_CP

=

12.3 Hz, C_11,11′), 126.83 (C₂), 124.77 (C₁), 42.60 (C₇), 41.08 (C_7′),

39.33 (d, J_CP= 62.0 Hz, C₅), 31.72 (C₄), 13.74 (C₈), 12.74 (C_8′). meso-

10: δ 167.28 (C₆), 147.63 (d, J_CP= 9.3 Hz, C₃), 132.03 (d, J_CP= 99.0

Hz, C₉), 131.94 (C_12,12′), 131.70 (d, J_CP= 9.1 Hz, C₁₀), 131.36 (d, J_CP

=

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9.1 Hz, C_10′), 131.29 (d, J_CP= 99.0 Hz, C_9′), 128.31 (d, J_CP= 12.2 Hz,

C₁₁), 128.10 (d, J_CP= 11.9 Hz, C_11′), 126.72 (C₂), 124.16 (C₁), 42.53

(C₇), 40.81 (C_7′), 38.89 (d, J_CP= 63.3 Hz, C₅), 31.44 (C₄), 13.99 (C₈),

12.58 (C_8′). FTIR (KBr, cm⁻¹): 3057, 2974, 2934, 1632 (ν_CO), 1487,

1458, 1435, 1381, 1317, 1253 (ν_N−O), 1186 (ν_PO), 1117, 1097, 1072,

997, 885, 858, 833, 787, 744, 702, 555, 513. HRMS (ESI) m/z (%):

750.3228 (60) [M + H⁺]. C₄₃H₅₀N₃O₅P₂requires 750.3226; 772.3062

(100) [M + Na⁺]. C₄₃H₄₉N₃O₅P₂Na requires 772.3045. Anal. Calcd

for C₄₃H₄₉N₃O₅P₂: C 68.88, H 6.59, N 5.60. Found: C 66.21, H 6.73,

N 5.19.

Lanthanide Complex Syntheses. The lanthanide coordination

complexes were prepared by combination of 1 equiv of ligand with 1

equiv of Ln(NO₃)₃·xH₂O in MeOH. The mixtures were stirred (23

°C, 2 h), volatiles removed by vacuum evaporation, and the resulting

powders vacuum-dried. Elemental analyses (CHN) and infrared

spectra for representative samples were obtained, and selected samples

were crystallized in order to obtain single crystals for X-ray diﬀraction

analyses. Characterization data for the Pr(III) complexes are

summarized here, and additional data are provided in Supporting

Information. [Pr(7)(NO₃)₃]·4H₂O. FTIR (KBr, cm⁻¹): 1601 (ν_CO),

1211 (ν_NO), 1134 (ν_PO). Anal. Calcd for C₂₃H₃₃N₅O₁₆PPr: C, 34.21;

H, 4.12; N, 8.67. Found: C, 34.62; H, 3.85; N, 8.44. [Pr(8)-

(NO₃)₃]·MeOH. FTIR (KBr, cm⁻¹): 1601 (ν_CO), 1227 (ν_NO), 1151

(ν_PO). Anal. Calcd for C₂₅H₃₁N₅O₁₃PPr: C, 38.43; H, 4.00; N, 8.96.

Found: C, 38.07; H 3.97; N, 8.97. [Pr(9)(NO₃)₃]·4H₂O. FTIR (KBr,

cm⁻¹): 1637 (ν_CO), 1612 (ν_CO), 1246 (ν_NO), 1148 (ν_PO). Anal. Calcd

for C₄₁H₅₃N₆O₁₈P₂Pr: C, 43.91; H, 4.77; N, 7.50. Found: C, 43.77; H,

4.38; N, 7.22. {[Pr(10)(NO₃)(H₂O)]₂(μ-10)}(NO₃)₄·12H₂O. FTIR

(KBr, cm⁻¹): 1620 (ν_CO), 1597 (ν_CO), 1213 (ν_NO), 1153 (ν_PO). Anal.

Calcd for C₁₂₉H₁₇₅N₁₅O₄₇P₆Pr₂: C, 49.10; H, 5.59; N, 6.66. Found: C,

49.24; H, 5.62; N, 6.62.

collection parameters for the ligands and the metal complexes are

presented in Tables 1 and 2, respectively. All heavy atoms were reﬁned

Table 1. Crystallographic Data for Ligands (7)₂·CH₂Cl₂, 8_R,

and 9_R,R

(7)₂·CH₂Cl₂

8_R

9_R,R

empirical

formula

C₄₇H₅₂Cl₂N₄O₆P₂

C₂₄H₂₇N₂O₃P

C₄₁H₄₅N₃O₅P₂

crystal size

(mm³)

0.12 × 0.17 × 0.51 0.37 × 0.53 ×

0.19 × 0.23 ×

0.56

0.57

fw

901.77

triclinic

P1

422.45

triclinic

P1

721.74

cryst syst

space group

orthorhombic

Aba2

unit cell

dimens

a (Å)

b (Å)

c (Å)

11.9000(8)

19.5305(13)

20.4598(13)

101.122(4)

90.588(4)

99.922(4)

4591.4(5)

4

10.6641(5)

14.2715(6)

16.8816(9)

106.043(3)

99.900(4)

109.529(4)

2225.9(2)

4

22.2182(16)

15.4755(12)

10.8569(9)

90

α (deg)

β (deg)

γ (deg)

V (Å³)

Z

90

3733.0(5)

4

T (K)

D_calcd

173(2)

1.284

1.305

1.261

(g cm⁻³

)

μ (mm⁻¹

)

0.263

0.151

0.165

min/max

transm

0.8765/0.9691

0.9184/0.9462

0.9131/0.9696

Photophysical Characterization. Solutions for spectroscopic

studies were prepared in a glovebox with controlled nitrogen

atmosphere (O₂< 0.5 ppm, H₂O < 1 ppm). A sample of

[Eu(9)(NO₃)₃] (17 mg) was initially dissolved in methanol (3 mL)

and stirred (15 min). After measurement in methanol, the solvent was

evaporated, and the sample was redissolved in deuterated methanol (5

mL) for the spectroscopic measurements. Solutions were equilibrated

for at least two days before measurements were made. The emission

spectrum and lifetimes were measured on a Jobin-Yvon Fluorolog-3

spectroﬂuorimeter equipped with a red-sensitive PMT R928 detector

and a Xe ﬂash lamp. The excitation wavelength was 289 nm, and

emission and excitation slits were 5 nm for the spectra and 9 nm for

the lifetime measurements. Additionally, for the lifetime measure-

ments, a time-per-ﬂash of 41 ms, a ﬂash count of 100, an initial delay of

0.06 ms, a sample width and maximum delay of 4 ms for methanol and

6 ms for deuterated methanol, and a delay interval of 0.01 ms were

chosen. The number of coordinated methanol molecules q was

determined through comparison of the emission lifetimes of Eu(III) in

methanol and deuterated methanol, using the equation q =

reﬂns

43 055

25 550

21 088

collected

indep reﬂns

[R_int]

18 761 [0.0743]

9792 [0.0656]

0.0538 (0.1119)

4645 [0.0917]

0.0515 (0.0980)

ﬁnal R indices 0.0705 (0.1612)

[I > 2σ(I)]

R1 (wR2)

ﬁnal R indices 0.1419 (0.1935)

(all data)

R1 (wR2)

0.1053 (0.1335)

0.0980 (0.1124)

anisotropically, and hydrogen atoms were included in ideal positions

and reﬁned isotropically (riding model) with U_iso= 1.2U_eqof the

parent atom (U_iso= 1.5U_eqfor methyl groups) unless noted otherwise.

The structure reﬁnements were well behaved except as indicated in the

following notes. (7)₂·CH₂Cl₂: colorless rods were obtained by slow

evaporation of a CH₂Cl₂/hexane (2/1) solution. Lattice CH₂Cl₂

chlorine atoms are disordered over two sites with occupancies set at

65/35. 8: colorless prisms were obtained by slow evaporation of an

acetone/EtOAc (4/1) solution. 9: colorless platelets were obtained by

slow evaporation of an Et₂O solution containing small amounts of

acetone and CH₂Cl₂. [Pr(7_R)(NO₃)₃(Me₂CO)]: green rods grown

from slow evaporation of an acetone/EtOAc (1/1) solution of the

complex formed from a 1:1 combination of 7 (rac) and Pr-

(NO₃)₃·6H₂O in MeOH. [Pr(8_R)(NO₃)₃(MeOH)-Pr(8_S)-

(NO₃)₃(MeOH)]_n: green rods formed from slow evaporation of

MeOH solution of the complex formed from a 1:1 combination of 8

(rac) and Pr(NO₃)₃·6H₂O. The O−H hydrogen atoms on the inner

sphere methanol molecules were located in the diﬀerence map. All

atoms of one nitrate group, N10, O22, O23, O24, and one O-atom,

O19, of a second nitrate are disordered over two sites with

occupancies set at 60/40. Disordered lattice solvent molecules, likely

either H₂O or MeOH or a combination, were not adequately modeled.

Reﬁnement was improved by application of the SQUEEZE

procedure.⁴³PLATON estimates 64 electrons unaccounted for in

the solvent accessible void volume of 135 Å³. {[Eu(9_R,S)-

(NO₃)₃]·(Me₂CO)_0.75·(H₂O)_0.3}₄: colorless blocks were formed by

slow evaporation of a acetone/MeOH (4/1) solution of the complex

obtained from a 1:1 combination of 9 (rac/meso 54/46) and

2.1(τ⁻¹

− τ⁻¹_MeOD) proposed by Horrocks and Sudnick.⁴⁰All

MeOH

reported lifetimes are the average of at least three independent

measurements. Emission spectroscopy for a microcrystalline sample of

the complex was examined by using a home-built scanning conformal

microscope spectrometer. Brieﬂy, laser excitation from either an argon-

ion or a krypton-ion source was focused to a diﬀraction-limited spot,

and the signal was collected in a backscattering geometry. Scattered

excitation was removed by using long-pass edge ﬁlters, and the PL

spectrum was dispersed and read-out using a CCD spectrograph.

Spectra were not corrected for instrument response.

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.⁴¹The structures

were solved by direct methods and reﬁned with full-matrix least-

squares methods on F²with use of SHELXTL.⁴²Lattice and data

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Scheme 1

Eu(NO₃)₃·6H₂O in MeOH. The crystals were found to be twinned

(three components). The non-hydrogen atoms were reﬁned

anisotropically. The H-atoms on the water (H16b, H16c) were

located in the diﬀerence map and allowed to reﬁne with constraints on

the O−H bond length and H−O−H bond angle. The C27 atom was

disordered over two positions that were allowed to reﬁne freely to 75/

25% occupancies with the thermal parameters constrained to be

similar. Two partially occupied solvent molecules were found in the

outer sphere. The acetone was set to 75% occupancy, and the water

(O16) was set to 30% occupancy. [Pr(9_R,S)(NO₃)₃]: green platelets

were obtained from slow evaporation of an acetone/MeOH solution

(4/1) of the complex formed by the 1:1 combination of 9 (rac/meso

54/46) and Pr(NO₃)₃·6H₂O in MeOH. The lattice solvent molecules

were disordered, and the SQUEEZE procedure was applied. PLATON

estimates 368 electrons unaccounted for in the solvent accessible void

volume of 1024 Å³. [Er(9_R,S)(NO₃)₃]·Me₂CO: pink diamond shaped

crystals were deposited from the slow evaporation of a acetone/EtOAc

(4/1) solution of the complex obtained from a 1:1 combination 9

(rac/meso 54/46) and Er(NO₃)₃·5H₂O in MeOH. [Er(9_S,S)-

(NO₃)₂(H₂O)](NO₃)·(MeOH)·(H₂O)_0.4: pink blocks formed in the

same synthesis and crystallization described above for [Er(9_R,S)-

(NO₃)₃]·Me₂CO. The lattice contains a methanol molecule with full

occupancy and a partial water molecule that, when reﬁned, converged

with an occupancy of 40%. {[Pr(10_R,S)(NO₃)(H₂O)]₂(μ-10_R,R)}_-

(NO₃)₄: green rods were obtained by slow evaporation of an

acetone/MeOH (4/1) solution of the complex formed from a 1:1

combination of 10 (rac/meso 60/40) and Pr(NO₃)₃·6H₂O in MeOH.

One ethyl carbon atom, C51, is disordered, and it was modeled over

two sites with 80/20 occupancies. In addition, unbound outer sphere

nitrate ions and lattice solvent molecules are disordered. These were

accounted for by using PLATON/SQUEEZE which found void spaces

of 1120 Å³with an electron count of 478.

Methods. Phases at a 1:1 organic to aqueous (O:A) phase ratio

were added to 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. The method is described

here. Geometry optimizations of the free and metal-bound forms of 7,

8, 9, and 10 were carried out with the MM3 force ﬁeld⁴⁴using a

points-on-a-sphere metal ion⁴⁵as implemented in PCModel

software.⁴⁶Conformational searches to locate the most stable form

for each structure were performed using the GMMX algorithm

provided with this software. Input ﬁles required to repeat these

calculations including additional parameters for treating the metal-

dependent interactions are available as Supporting Information.

RESULTS AND DISCUSSION

■

Ligand Syntheses and Characterization. The inter-

mediate pyridine-based platforms 2-(diphenyl-N,N-diethylcar-

bamoylmethylphosphine oxide)pyridine, 7_N, and 2,6-bis-

(diphenyl-N,N-diethylcarbamoylmethylphosphine oxide)-

pyridine, 9_N, were prepared by combination of

diethylcarbamoyl chloride with the putative lithio reagents

formed from the respective 2-(diphenylphosphinoylmethyl)-

pyridine P-oxide, (DPhNPO), 3, or 2, 6-bis-

(diphenylphosphinoylmethyl)pyridine P,P′ dioxide (TPhNPO-

P′O′), 4, and n-BuLi as summarized in Scheme 1. Compound

7_Nwas isolated as a white powder containing a racemic mixture

of enantiomers with an unoptimized yield of 51%, and 9_Nwas

obtained in 31% yield as a white, solid diastereomeric mixture

of isomers R,R/S,S (rac) and R,S (meso). Both compounds are

spectroscopically pure, and the compositions are supported by

HRMS analyses that show high intensity [M + H⁺] and [M +

Na⁺] ions. The infrared spectra display strong absorptions that

Distribution Studies. Materials. 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 Engineering Research Center (REDC) of ORNL. 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.

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are tentatively assigned to ν_COand ν_POstretching modes based

upon prior assignments:^31a,b,h,387_N, 1634 and 1190 cm⁻¹; 9_N,

1643 and 1200 cm⁻¹, respectively. The ³¹P NMR spectra for

the compounds show a single resonance: 7_N, δ 29.5; 9_N, δ 30.2.

These chemical shifts are similar to shifts reported for the

component molecules Ph₂P(O)CH₂C₅H₄N,^31aδ 30.2, and

Ph₂P(O)CH₂C(O)NEt₂,³⁸δ 27.9. The ¹H and ¹³C NMR

spectra have been fully assigned, and the shifts for the methine

proton and carbon centers are noteworthy: 7_N, H₆, δ 5.31, J_HP

=

13.2 Hz, C₆, δ 56.28, J_CP= 64.5 Hz; 9_N, H₄, δ 5.24, J_HP= 13.8

Hz, C₄, δ 55.93, J_CP= 64.7 Hz.³⁶As expected, the coupling

constants are closely comparable, but the chemical shifts are

noticeably downﬁeld from the corresponding methylene proton

and carbon resonances in the component molecules: Ph₂P-

(O)CH₂C₅H₄N,^31aδ_H3.88, J_HP= 14.2, δ_CH240.7, J_CP= 64 Hz;

Ph₂P(O)CH₂C(O)NEt₂,³⁸δ_H3.3, J_HP= 15.7 Hz, δ_CH239.4, J_CP

= 60.9 Hz.

Subsequent N-oxidations of 7_Nand 9_Nwere accomplished

with peracetic acid or m-CPBA, and the target ligands, 2-

(diphenyl-N,N-diethylcarbamoylmethylphosphine oxide)-

pyridine N-oxide, 7, and 2,6-bis[(diphenyl-N,N-diethylcarba-

moylmethylphosphine oxide)]pyridine N-oxide, 9, were iso-

lated as white powders with yields of 72% and 100%,

respectively. Compound 7 was obtained as a racemic mixture

of enantiomers while 9 was isolated as a 80/20 rac/meso

mixture. The major diastereomer was isolated by precipitation

from acetone or by recrystallization from Et₂O containing a

small fraction of acetone or CH₂Cl₂. The minor diastereomer

was not obtained in pure form. The compound compositions

were supported by CHN elemental analyses and HRMS spectra

that display intense [M + H⁺] and [M + Na⁺] ions. The

infrared spectra show strong absorptions that are tentatively

assigned to ν_CO, ν_NO, and ν_POon the basis of previous

assignments:^31a,b,h,387 1638, 1265, 1192 cm⁻¹; 9 1643, 1275,

1203 cm⁻¹. The ³¹P NMR spectrum for 7 is consistent with the

formation of a racemic mixture of enantiomers with a single

resonance: ³¹P δ 30.4. The ³¹P NMR spectrum for 9, on the

other hand, shows two resonances: ³¹P δ 29.6 (rac) and 31.3

(meso) with relative intensities 4:1. The ¹H and ¹³C resonances

assigned to the methine proton and carbon atoms in 7 and 9

appear at H₆δ 6.67, J_HP= 8.6 Hz, C₆δ 41.38, J_CP= 63.8 Hz, and

H₄δ 6.62, J_HP= 9.5 Hz (rac), δ 6.82, J_HP= 12.2 Hz (meso), C₄δ

42.59, J_CP= 64.4 Hz (rac), δ 43.30, J_CP= 63.6 Hz (meso),

respectively. The assignments for the resonances of 9 are made

possible by determination of the absolute conﬁguration of the

R,R and S,S diastereomers by X-ray crystallography, vide infra.

The proposed molecular structures of 7_R, 7_S, and 9_R,Rwere

conﬁrmed by single crystal X-ray diﬀraction analyses.⁴⁷Views

of the molecules are shown in parts a and b in Figure 1 and in

Figure 2, respectively, and selected bond lengths are

summarized in Table 3. The unit cell for 7 contains four

independent molecules, three of which have the R con-

formation while one has the S conformation. There are also two

CH₂Cl₂solvent molecules in the unit cell, one of which has a

disordered Cl atom. The pyridine N-oxide ring in each

molecule is planar, and the CMPO fragment is bonded through

the central methine carbon atom at the 2-position of the

pyridine ring. The PO and CO bond vectors are rotated

out of the pyridine ring plane (PO, 58.5−73.7°; CO,

49.8−56.7°), and they are oriented syn to each other. The P

Figure 1. Molecular structure of {[Ph₂P(O)][C(O)NEt₂]C(H)-

C₅H₄NO}₂·CH₂Cl₂, (7)₂·CH₂Cl₂: (a) 7_Rand (b) 7_S(thermal

ellipsoids, 50%) with carbon atom labels, H-atoms, and lattice

CH₂Cl₂omitted for clarity.

Figure 2. Molecular structure of (R,R)-{[Ph₂P(O)][C(O)NEt₂]C-

(H)}₂C₅H₃NO, 9_R,R(thermal ellipsoids, 50%), with carbon atom

labels and H-atoms omitted for clarity.

CH₂}C₅H₄NO, 1.4822(9) Å,³¹ⁿand in the CMPO molecule

Ph₂P(O)CH₂C(O)NEt₂, 1.490(3) Å.⁴⁸The average N−O

bond length, 1.307 0.005 Å, is identical to the value in {[2-

(CF₃)C₆H₄]₂P(O)CH₂}C₅H₄NO, 1.306(1) Å,³¹ⁿand the

average carbonyl CO bond length, 1.229 0.011 Å, is the

same as reported for the amide carbonyl bond length in

Ph₂P(O)CH₂C(O)NEt₂, 1.226(5) Å.⁴⁸The structure for 9_R,R

contains a planar pyridine N-oxide ring with two CMPO

fragments bonded through the methine carbon atoms at the 2-

and 6- positions of the pyridine N-oxide ring. As in 7, the PO

and CO bonds in each CMPO fragment are mutually syn to

each other. The PO and CO bond lengths, 1.481(2) and

1.218(4) Å, are identical to the average bond lengths in 7, but

the N−O bond length is slightly longer, 1.328(4) Å. The PO

O bond lengths in the four molecules are identical, PO_avg

=

1.485 0.002 Å, and they compare favorably with the PO

bond length in the NOPO derivative {[2-(CF₃)C₆H₄]₂P(O)-

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proposed composition is supported by HRMS analysis that

displays high intensity [M + H⁺] and [M + Na⁺] ions. The

FTIR spectrum shows absorptions at 1634 and 1192 cm⁻¹that

are tentatively assigned to ν_COand ν_POabsorptions,

respectively, and the ³¹P NMR spectrum contains a single

resonance at 31.0 ppm. These data are essentially identical to

the data recorded for 7_N. However, distinguishing features are

Table 3. Selected Bond Lengths for Ligands (7)₂·CH₂Cl₂, 8_R,

and 9_R,R(Å)

(7)₂·CH₂Cl₂

P−O Bond

8_R

9_R,R

P1−O2 1.483(3)

P2−O5 1.486(3)

P3−O8 1.485(3)

P4−O11 1.487(3)

N−O Bond

P1−O3 1.4886(16)

P2−O6 1.4810(17)

P1−O2 1.481(2)

1

provided by the H and ¹³C NMR spectra. In particular, in the

¹H NMR spectrum, the CMPO fragment methine proton in 8_N

(H₇) is shifted upﬁeld, δ 4.35, J_HP=12.6 Hz, relative to the value

in 7_N(H₆). Similarly, the ¹³C NMR resonance for C₇in 8_N

appears at δ 45.58, J_CP= 61.7 Hz, signiﬁcantly upﬁeld of the

resonance for C₆in 7_N.

N1−O1 1.307(4)

N3−O4 1.312(4)

N5−O7 1.306(4)

N7−O10 1.304(4)

C−O Bond

C7−O3 1.224(4)

C30−O6 1.231(5)

C53−O9 1.222(4)

C76−O12 1.240(4)

N1- O1 1.311(3)

N1−O1 1.328(4)

C5−O3 1.218(4)

N3−O4 1.317(2)

Compound 10_Nwas isolated in overall 61% yield as a

diastereomeric mixture of rac (R,R and S,S) and meso (R,S)

isomers with a 60/40 rac/meso ratio. The HRMS shows high

intensity [M + H⁺] and [M + Na⁺] ions, and the FTIR

spectrum displays absorptions at 1634 and 1190 cm⁻¹

tentatively assigned to ν_COand ν_PO, respectively. On a small

scale, separation of the rac and meso forms was accomplished by

recrystallization from Et₂O containing small fractions of

CH₂Cl₂or acetone. One diastereomeric pair, subsequently

identiﬁed as the racemic R,R/S,S form by single crystal X-ray

diﬀraction analysis, was obtained as blocky crystals while the

meso form was isolated as thin needles. Physical separation of

the R,R/S,S diastereomeric pair facilitated the full assignment of

the NMR spectra. The ³¹P NMR spectrum displays only a

single resonance at 30.4 ppm, but the ¹H and ¹³C NMR spectra

C8−O2 1.233(3)

C32−O5 1.231(3)

and N−O bond lengths are comparable with those in 2,6-

[Ph₂P(O)CH₂]₂C₅H₃NO, 1.480(3) and 1.315(6) Å,^31bre-

spectively.

In order to explore potential chelate ring strain features in

lanthanide ion coordination interactions of 7 and 9, syntheses

for 8 and 10 were also developed. That chemistry is

summarized in Scheme 2. The respective pyridine precursors,

2-[(diphenyl-N,N-diethylcarbamoylmethylphosphine oxide)-

methyl]pyridine, 8_N, and 2,6-bis[(diphenyl-N,N-diethylcarba-

moylmethylphosphine oxide)methyl] pyridine, 10_N, were

prepared by reaction of 2-chloromethylpyridine or 2,6-bis-

(bromomethyl)pyridine with Li{[Ph₂P(O)][C(O)NEt₂]CH},

formed by combination of 1 (R = R′ = Ph, R″ = Et) and n-

BuLi. Compound 8_Nwas isolated in 50% yield as a solid, white,

spectroscopically pure, racemic mixture of enantiomers. No

diﬀerences between the R and S enantiomers were revealed by

NMR or FTIR spectroscopy or TLC on silica plates. The

1

distinguish the rac and meso forms. The methine H and ¹³C

resonances for H₅and C₅in 10_Nappear at δ 4.30, J_HP=13.1 Hz

(rac), δ 4.14, J_HP= 13.1 Hz (meso), and δ 46.03, J_CP= 62.0 Hz

(rac), δ 46.51, J_CP= 61.2 Hz (meso). The proton resonances are

signiﬁcantly upﬁeld compared with the values for the methine

proton resonances for 9_Nwhile the carbon resonances are

slightly downﬁeld of the shifts recorded for 9_N.

The oxidation of 8_Nwas accomplished with peracetic acid,

while the oxidation of 10_Nwas performed with both peracetic

Scheme 2

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acid and m-CPBA. The resulting solid, white pyridine N-oxides

were obtained as a racemic R/S mixture in 61% yield (8), and

as a rac/meso 60/40 mixture (10) in 74% (peracetic acid) and

97% (m-CPBA) yields. The latter contained a low level

impurity, as indicated by CHN analyses, that was not

successfully removed. Small samples of diastereomerically

pure 10 were obtained by oxidation of diastereomerically

pure rac and meso samples of 10_N. The HRMS analyses of both

8 and 10 show high intensity [M + H⁺] and [M + Na⁺] ions.

The FTIR spectrum for 8 displays absorptions centered at

discovery of structures produced in gas phase metal ion

coordination events that have low conformational energy, low

degrees of induced strain, and few restricted bond rotations.

The calculated values for the relative strain free energy (G) per

bonded donor group in 7 (2.86 kcal/mol/donor group) and 8

(2.81 kcal/mol/donor group), wherein the donor groups are

identical within the set, suggest that the NOPOCO ligands

would both be expected to behave as tridentate chelators. Views

of the calculated complexed forms of 7 and 8 are shown in

Figure 4. Although the diﬀerence is small, the values of G per

1634, 1242, and 1180 cm⁻¹tentatively assigned to ν_CO, ν_NO

,

and ν_PO, respectively. The corresponding absorptions for 10

appear centered at 1632, 1253, and 1186 cm⁻¹. The ³¹P NMR

spectrum for 8 displays a single resonance at δ 32.0 (CDCl₃) or

at δ 35.4 (d₄-MeOH) and the methine proton (H₇) and carbon

(C₇) resonances appear at δ 4.79 (m) and 38.31 (J_CP= 62.6

Hz). On the other hand, the ³¹P NMR spectrum for 10

contains two resonances, δ 32.5 (rac) and 30.8 (meso), and the

methine proton (H₅) and carbon (C₅) resonances appear at δ

4.92 (rac), 4.83 (meso), and 39.33, J_CP= 62.0 Hz (rac), 38.89,

J_CP= 63.3 Hz (meso).

The molecular structure of 8_Rwas conﬁrmed by single crystal

X-ray diﬀraction techniques. A view of the molecule is provided

in Figure 3, and selected bond lengths are listed in Table 3. The

Figure 4. Geometry optimized structures for tridentate coordination

in 1:1 ligand/Pr(III) complexes: (a) ligand 7, (b) ligand 8.

Figure 3. Molecular structure of (R)-{[Ph₂P(O)][C(O)NEt₂]C(H)-

CH₂}C₅H₄NO, 8_R, (thermal ellipsoids, 50%) with carbon atom labels

and H-atoms omitted for clarity.

donor group suggest that the “ﬂoppier” ligand, 8, that would

utilize larger eight-membered chelate rings, should produce

slightly more stable complexes compared to 7. In contrast, the

less “ﬂoppy” NOPOP′O′COC′O′ ligand, 9, has smaller values

of strain free energy per donor group than 10 when forming

pentadentate chelate structures: 9_R,R/S,S, 4.33 kcal/mol/donor

group, 9_R,S, 4.44 kcal/mol/donor group; 10_R,R/S,S, 4.54 kcal/

mol/donor group, 10_R,S,5.14 kcal/mol/donor group. Views of

the pentadentate chelate structures are shown in Figure 5.

Although this steric analysis suggests that the maximum

chelating conditions for 7 and 8 (tridentate) and 9 and 10

(pentadentate) should be accessible without serious ligand

strain on a naked Ln(III) cation, the impact of inner sphere

charge compensating anions (nitrate ions in particular) is

ignored. The steric component of anion inclusion in the inner

coordination sphere can be addressed in part by analysis of

existing lanthanide structural data in the Cambridge Structural

Database.⁵⁰From a total of 758 structures of lanthanide

complexes, Ln(L)_n(NO₃)₃, that contain three bidentate nitrate

anions and ligands L that provide additional O-donor atoms in

the inner coordination sphere, a plot of the number of

additional O-donor atoms versus Shannon lanthanide ionic

crystal used was obtained by slow evaporation of an acetone/

EtOAc (80/20) solution of the racemic reaction product. There

are two independent molecules in the unit cell, and both have

the R conﬁguration.⁴⁹The pyridine N-oxide ring is planar, and

the N−O bond vector is rotated away from the PO and C

O bonds in the attached CMPO fragment. The PO, CO,

and N−O bond lengths, 1.485

0.004, 1.232

0.001, and

1.314 0.003 Å (av), respectively, are identical to the bond

lengths in 7. Several crystals of 10_Nwere examined, but all gave

X-ray diﬀraction data of marginal quality. Nonetheless, the

composition was conﬁrmed, and each crystal contained a

racemic mixture of R,R and S,S enantiomers. Data from one

crystal are deposited in the Supporting Information.

Ligand Computational Modeling. The inclinations of

these hybrid ligands to adopt maximal tridentate (7 and 8) and

pentadentate (9 and 10) binding modes on a Ln(III) ion were

evaluated by using a force ﬁeld-based structure scoring

approach described previously.^21hThe objective involves

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would be expected that the relative order of O-donor atom base

strength would be O_P≫O_N> O_C, so tridentate chelate

structures involving 9 and 10 will likely utilize asymmetric

NOPOP′O′ docking arrangements while the tetradentate and

pentadentate structures will add one or two amide carbonyl O-

atoms to the inner sphere environment. In order to test these

predictions, the coordination chemistry of 7−10 was explored.

Lanthanide Ion Coordination Chemistry: One-Armed

Ligands. Equimolar combinations of the one-armed ligands

rac-7 with Ln(NO₃)₃·x(H₂O) (Ln = La, Pr, Tb, Er) and rac-8

with Ln(NO₃)₃·x(H₂O) (Ln = La, Pr, Eu, Dy, Er) in MeOH

led to isolation of 1:1 complexes as solid powders. Elemental

analyses of the powders are consistent with the 1:1 composition

[Ln(L)(NO₃)₃], although best agreement requires addition of

water or MeOH as solvate molecules. For example, the CHN

analyses for the Pr(III) complexes support the compositions

[Pr(7)(NO₃)₃]·4H₂O and [Pr(8)(NO₃)₃]·MeOH, and the

presence of lattice solvent molecules is consistent with FTIR

spectra. Solution ³¹P NMR analyses for most of the complexes

generally produce broad resonances due to lanthanide para-

magnetic eﬀects; however, spectra for diamagnetic La(III)

complexes display sharp resonances. The ³¹P NMR spectrum

for crude [La(7)(NO₃)₃] in d₄-MeOH contains a single

resonance, δ 35.8, shifted downﬁeld from the free ligand in

d₄-MeOH, δ 31.6. Addition of a second equivalent of 7 provides

Figure 5. Geometry optimized structures for pentadentate coordina-

tion in 1:1 ligand/Pr(III) complexes: (a) ligand 9_R,R/S,S, (b) ligand

a

³¹P NMR spectrum with a single resonance, δ 34.6. Whether

10_R,R/S,S

.

this small shift, relative to the 1:1 complex, indicates formation

of a 2:1 complex in solution or simply rapid ligand exchange is

not certain. All attempts to isolate a 2:1 complex were

unsuccessful. The ³¹P NMR spectrum for crude [La(8)(NO₃)₃]

in d₄-MeOH also displays a single resonance, δ 40.0, shifted

downﬁeld from the free ligand in d₄-MeOH, δ 35.4. Once again,

addition of a second equivalent of 8 results in a small upﬁeld

shift relative to the shift for the 1:1 complex, δ 38.4, and eﬀorts

to isolate a 2:1 complex of 8 were unsuccessful. Shifts of the

infrared stretching frequencies ν_CO, ν_NO, and ν_POfor the solid

complexes in KBr relative to the free ligands provide some

additional clues regarding the interactions of 7 and 8 with

Ln(NO₃)₃fragments. However, it must be noted that

assignments for the bands are tentative especially for ν_NO

which is of only modest intensity and falls in a region

containing several other absorptions. The spectrum for the

crude 1:1 complex [Pr(7)(NO₃)₃] shows coordination shifts

Δν_CO= 36 cm⁻¹, Δν_NO= 54 cm⁻¹, and Δν_PO= 58 cm⁻¹, and

these shifts are consistent with all three O-atom donors

participating in the coordinate interaction. Similar shifts appear

for the related complexes formed with the other Ln(III) ions.

For [Pr(8)(NO₃)₃], the coordination shifts are Δν_CO= 33

cm⁻¹, Δν_NO= 15 cm⁻¹, and Δν_PO= 29 cm⁻¹. These data also

are consistent with a tridentate, albeit weaker, chelate

interaction between 8 and Pr(III).

Crystal Structure Analyses: One-Armed Ligand Com-

plexes. Single crystals for one of the complexes containing 7

were obtained by dissolution of crude [Pr(7)(NO₃)₃]·4H₂O in

acetone/ethyl acetate solution followed by slow evaporation of

the homogeneous solution. The crystal structure determination

reveals a composition for the single crystals as [Pr(7_R)-

(NO₃)₃(Me₂CO)]. A view of the molecule is shown in Figure

7, and selected bond lengths are summarized in Table 4. The

Pr(III) ion inner coordination sphere environment (CN = 10)

is generated by three O-atoms from a tridentate ligand 7 in the

R conformation, six O-atoms from three bidentate nitrate ions,

and an O-atom provided by an acetone molecule. Although the

radii (CN = 9) can be derived, and this plot is shown in Figure

6. The mean number of additional inner sphere O-atoms

Figure 6. Plot of mean number of added ligand O-atoms vs Shannon

ionic radii (Å) for Ln(III) ions (CN = 9).

reﬂects the amount of steric volume available on the lanthanide

ion surface. This, in turn, parallels the size of the ion. If the

mean donor atom number is less than 3.5, it is more commonly

observed that three extra O-donor atoms are provided by the

ligand(s) L, while if the mean donor atom number exceeds 3.5,

four or more extra O-donor atoms are provided by the

ligand(s). As expected, the latter condition is typically found for

lanthanide ions larger than Gd(III). This analysis is consistent

with the prediction that ligands 7 and 8, each with three

available O-donor atoms, should be able to employ all three to

form tridentate NOCOPO chelate structures on any Ln(III)

cation. Ligands 9 and 10 will likely form tridentate chelate

structures on smaller, later lanthanide ions, due primarily to the

limited available Ln(III) surface area, but tetradentate and

perhaps pentadentate chelate structures may appear with the

larger, early Ln(III) ions. Further, on the basis of limited data, it

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the respective distances in [Pr(7_R)(NO₃)₃(Me₂CO)]. Each of

the PO, NO, and CO bond lengths in the complex are

elongated compared to the distances in the free ligand as

expected with involvement of each donor site in the binding

mode.

Lanthanide Ion Coordination Chemistry: Two-Armed

Ligand 9. Equimolar combinations of the two-armed ligands 9

(rac/meso 54/46) with Ln(NO₃)₃·x(H₂O) (Ln = La, Pr, Nd,

Eu, Dy, Er, Yb) in MeOH for two hours at 23 °C produced 1:1

complexes that were isolated as powders following evaporation

of the volatiles. Characterization data for the crude complexes

with Pr(III) are described as illustrative of these samples.

Elemental analysis (CHN) data for the crude Pr(III) complex

with 9 are most consistent with the formula [Pr(9)-

(NO₃)₃]·4H₂O. An electrospray ionization mass spectrum for

the related complex, {[Eu(9)(NO₃)₃]·(Me₂CO)_0.75(H₂O)_0.3}₄,

following its dissolution in MeOH and d₄-MeOH and use in

photophysical studies, vide infra, shows a second most intense

peak at 998.1834 amu corresponding to [Eu(9)(NO₃)₂]⁺

consistent with a 1:1 ligand/metal composition. The peak

and matching isotope pattern are shown in Supporting

Information (Figure S.66). The most intense peak at

973.0988 amu corresponds to a species containing 9, one

Eu(III) ion, two methoxy anions, and two water molecules,

[Eu(9)(OCH₃)₂]⁺·H₂O·HDO (Figure S.67, Supporting In-

formation). The FTIR spectrum for the solid [Pr(9)-

(NO₃)₃]·4H₂O shows bands at 1637, 1612, 1246, and 1148

cm⁻¹. These are tentatively assigned to uncoordinated CO,

coordinated CO, coordinated NO, and coordinated PO

stretching modes, respectively. The coordination shifts, Δν_CO

= 6 cm⁻¹, Δν_CO= 31 cm⁻¹, Δν_NO= 29 cm⁻¹, and Δν_PO= 55

cm⁻¹, are consistent with 9 acting as a tetradentate,

NOPOPOCO ligand with one of the two amide carbonyl O-

atoms not involved in the chelate interaction. The spectrum

remains unchanged for samples obtained from reactions

performed in MeOH at 23 °C for 12 h, in reﬂuxing MeOH

or acetone for two hours or for single crystals obtained from

acetone/MeOH (4/1) mixtures. The same band patterns, with

slight frequency diﬀerences, are obtained for La, Nd, and Eu

complexes. The infrared spectrum of the residue obtained from

a 1:1 combination of 9 with Er(NO₃)₃·5H₂O in MeOH (23 °C,

2 h) contains bands at 1244 and 1161 cm⁻¹that are tentatively

assigned to NO and PO stretching vibrations, as well as a

broader, more asymmetric band in the carbonyl region, 1660−

1580 cm⁻¹, with a maximum at 1614 cm⁻¹. The last absorption

is consistent with at least one of the carbonyl O-atoms

participating in ligand binding. Crystallization of the crude

Er(III) complex by slow evaporation of a acetone/ethyl acetate

(4/1) solution produced two crystal morphologies (rectangular

prisms and diamond-like plates) that were separated by hand.

The rectangular prisms display an IR spectrum similar to the

crude material with a broad ν_COband centered at 1614 cm⁻¹

that is consistent with carbonyl O-atom involvement in ligand

coordination. On the other hand, the spectrum for the

diamond-like crystals shows a single ν_COband centered at

1649 cm⁻¹(Δν_CO= −6 cm⁻¹). This single, up-frequency

shifted CO vibration suggests that neither amide carbonyl O-

atom of 9 is involved in coordination with the Er(III) ion;

therefore, in this case the ligand is likely behaving only as a

tridentate NOPOP′O′ chelate. Similar observations are also

made for samples containing Dy(III) and Yb(III) ions.

Figure 7. Molecular structure and atom labeling scheme for

[Pr(7_R)(NO₃)₃(Me₂CO)] (thermal ellipsoids, 50%) with carbon

atom labels and H-atoms omitted for clarity.

phosphine oxide is generally considered to provide a

signiﬁcantly more basic donor O-atom, the Pr−O bond lengths

involving 7 are remarkably similar: Pr1−O3(P) 2.457(7) Å,

Pr1−O1(N) 2.479(8) Å, and Pr1−O2(C) 2.448(8) Å. In

addition, the PO, NO, and CO bond lengths in the

complex, 1.502(8), 1.320(12), and 1.258(13) Å, respectively,

are all elongated compared to the average bond lengths in the

free ligand. These parameters are consistent with the IR

coordination shifts observed for the crude complex. Similarly, a

complex, [Pr(5)₂(NO₃)₃], containing the related bifunctional

NOPO ligand (5) (X = CH₂P(O)Ph₂), displays average Pr−

O(P) and Pr−O(N) bond lengths of 2.457(6) and 2.446(6) Å,

respectively.^31aA structure for a CMPO/Pr(III) complex is not

available for direct comparison, but the average NdO(P) and

NdO(C) bond lengths in {Nd[Ph₂P(O)CH₂C(O)NEt₂]-

(NO₃)₃} are 2.457(4) and 2.493(4) Å, respectively.^18bIt is also

of interest to compare the tridentate ligand binding mode

displayed by 7 with the unexpected chelation interaction found

between Pr(III) and the NOPOPO-type ligand [Ph₂P(O)]-

[Ph₂P(O)CH₂CH₂]C(H)C₅H₄NO. In this case, the N-oxide

O-atom and the long-arm phosphine oxide substituent’s O-atom

generate a bidentate chelate interaction while the short-arm

phosphine oxide substituent forms a bridging interaction with

another molecular unit.^31j

The structure determination for single crystals recovered by

slow evaporation of a solution containing the 1:1 combination

of rac-8 with Pr(NO₃)₃·6H₂O in MeOH unexpectedly reveals

formation of a 1D polymeric structure. The repeating unit is

shown in Figure 8, and selected bond lengths are listed in Table

4. Each Pr(III) ion is 10 coordinate with the coordination

positions occupied by the O-atoms from a bidentate POCO

chelating fragment of a 8_Rligand, the O-atom of a pyridine N-

oxide fragment of a second ligand molecule of opposite

conﬁguration, 8_S, the O-atoms of three bidentate nitrate ions,

and the O-atom of an inner sphere molecule of MeOH. This

provides an empirical formula, [Pr(8_R)(NO₃)₃(MeOH)−Pr-

(8_S)(NO₃)₃(MeOH)]_n, that is consistent with the CHN

analytical data for the crude complex. The bridge linkage of

the units occurs through the pyridine N-oxide O-atom. It is

noteworthy that the average M-O(P) distance, 2.446 0.001

Å, is the same as in the complex [Pr(7_R)(NO₃)₃(Me₂CO)], but

the average Pr−O(C) distance, 2.471 0.007 Å is longer and

the average Pr−O(N) distance is shorter, 2.435 0.007 Å, than

Photophysical Characterization. Europium(III) is an

emissive ion, due to metal-centered f−f transitions. Therefore,

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Figure 8. Molecular structure and atom labeling scheme for [Pr(8_R)(NO₃)₃(MeOH)−Pr(8_S)(NO₃)₃(MeOH)]_n(thermal ellipsoids, 50%) with

carbon atom labels and H-atoms omitted for clarity.

in order to gain additional structural information, the emission

spectroscopy of the Eu(III) complex of 9 was examined in the

solid state and in methanol solution. The emission spectrum for

the crystallographically characterized solid,{[Eu(9)(NO₃)₃]-

(Me₂CO) _0.75(H₂O)_0.3}₄, vide infra, is shown in Figure 9a,

and the spectrum for [Eu(9)(NO₃)₃] in methanol is presented

in Figure 9 b. The spectra are similar, and contain the

characteristic emission peaks of Eu(III) centered at 579, (591)

594, 614, 617, 649, (685) 689, and (696) 700 nm (solid) and

579, 590, 613, 648, and 691 nm (MeOH solution). These

correspond to the ⁵D₀→ ⁷F_J(J = 0, 1, 2, 3, 4) transitions. The

appearance and intensities of the transition to the J = 0 ground

state and the electric dipole forbidden transition to J = 2 are

consistent with the absence of an inversion center in the

complex. A spectrum obtained from a frozen (77 K) methanol

solution shows splitting of the J = 1 transition into at least three

components and the J = 2 transition into at least three, possibly

up to ﬁve, components. This additionally indicates a low

symmetry environment around the metal ion. Following the

method of descending symmetry, such splitting is consistent

with the point groups C₁, C_s, C₂, and C_2v.⁵¹This spectrum is

distinct from the one obtained from the solid sample (Figure 9

a) indicating that the structure in the frozen methanol solution

is diﬀerent from the solid structure. Attempts to compare the

emission spectrum at 77 K with the one from the solution at

room temperature showed similar broadening of the peaks.

However, due to low resolution, the peak splittings were not

observed. To further clarify the room-temperature solution

structure of the metal center, the emission lifetime of Eu(III) in

methanol, 0.3627

0.0388 ms (Figure S.68, Supporting

Information), and in deuterated methanol, 1.2743 0.1073 ms

(Figure S.69, Supporting Information), were compared by

using the Horrocks’ equation.⁴⁰This indicates that four

methanol solvent molecules replace the three nitrate anions

in solution, maintaining a low symmetry environment around

the metal ion.

Lanthanide Ion Coordination Chemistry: Two-Armed

Ligand 10. Equimolar combinations of the two-armed ligand

10 (rac/meso 60/40) with Ln(NO₃)₃·x(H₂O) (Ln = La, Pr, Eu,

Dy, Er) in MeOH for two hours at 23 °C gave powder samples

following evaporation of the volatiles. Characterization data for

the crude complex with Pr(III) are described as illustrative of

these samples. Interpretation of the analytical data for this

complex was initially unclear. However, following a X-ray

structure determination for crystals obtained by crystallization

of the crude complex from MeOH/acetone, vide infra, a

composition corresponding to [Pr₂(10)₃(NO₃)₆(H₂O)₂]·-

12H₂O was suggested, and found to be in good agreement

with the analytical data. The infrared spectrum obtained from

Figure 9. Emission spectrum of (a) {[Eu(9)(NO₃)₃](MeOH)(H₂O)

(crystalline solid), λ_exc= 488 nm, 1 μW, and (b) [Eu(9)(NO₃)₃] in

methanol (77 K). λ_exc= 289 nm.

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the crude Pr(III) complex with 10 displays bands centered at

1601, 1213, and 1155 cm⁻¹assigned to CO, NO, and PO

stretching modes, respectively. These correspond to coordina-

tion shifts of Δν_CO= 31 cm⁻¹, Δν_NO= 40 cm⁻¹, and Δν_PO= 31

cm⁻¹. It is noted that the band assigned to the carbonyl stretch

in this complex is roughly doubled in width compared to the

width of the band assigned to the CO stretching mode for the

free ligand. This may indicate that there are overlapping bands

for coordinated and uncoordinated amide carbonyl groups.

Supporting this conclusion, an IR spectrum for single crystals of

the complex containing Pr(III) and 10 shows two resolved

carbonyl frequencies centered at 1620 and 1597 cm⁻¹. Similar

band patterns and frequencies are observed for the La, Eu, Dy,

and Er complexes of 10.

Crystal Structure Analyses: Two-Armed Ligand Com-

plexes. The spectroscopic data outlined above support the

conclusion that 9 and 10 act as chelating ligands toward Ln(III)

ions, but the detailed nature of the ligand/lanthanide

interactions in both solution and the solid state remained

unclear. As a result, single crystal X-ray diﬀraction analyses were

undertaken for six complexes of 9 and one of 10. For the

crystalline complexes of 9, the starting materials were the crude

powders obtained from the equimolar combinations of 9 (rac/

meso 54/46) and Ln(NO₃)₃·6H₂O in MeOH (23 °C, 2 h). The

crude complexes were dissolved in a minimum of acetone/

MeOH (4/1) and the solutions allowed to slowly evaporate

whereupon suitable single crystals for the Pr(III) and Eu(III)

complexes were obtained. The structures appear to be

isomorphous although diﬃculty was encountered with the

reﬁnement of the outer sphere solvent atoms in both

complexes. The reﬁnement for the Eu(III) complex leads to

Figure 11. Molecular structure and atom labeling scheme for

[Pr(9_R,S)(NO₃)₃] (thermal ellipsoids, 50%) with carbon atom labels,

H-atoms, and outer sphere solvent molecules omitted for clarity.

conformer arm) rotated away from the Ln(III) center. The

ligand docking asymmetry is also indicated by signiﬁcantly

diﬀerent Pr−O(P) bond lengths: Pr1−O10 2.415(2) Å and

Pr1−O13 2.478(2) Å; a similar asymmetry pattern is found in

the Eu(III) complex. The Pr−O(N) distance, Pr1−O12,

2.513(2) Å, is notably longer as often observed with tridentate

complexes of 6 (X = CH₂P(O)Ph₂).³¹The Pr−O(C) bond

length, Pr1−O14, 2.470(2) Å, is comparable to the distances in

the Pr(III) complexes of 7 and 8 described earlier. It is noted

that the asymmetric tetradentate chelate interactions observed

in the crystal structures are consistent with the IR data recorded

from KBr pellets for these early lanthanide ion complexes, and

with the emission spectroscopic data for the solid and methanol

solutions of the Eu(III) complex, vide supra.

Interestingly, as mentioned above, crystallization by slow

evaporation of a acetone/ethyl acetate (4/1) solution of the

crude complex of 9 with Er(NO₃)₃provided crystals with two

diﬀerent morphologies: diamond-like plates and rectangular

prisms. The molecular structure obtained from the diamond-

like crystals is shown in Figure 12, and selected bond lengths

are presented in Table 4. The complex, [Er(9_R,S)-

(NO₃)₃]·(Me₂CO), contains a nine-coordinate Er(III) with

the inner sphere coordination polyhedron generated by six O-

an empirical formula {[Eu(9_R,S)(NO₃)₃]·(Me₂CO)_0.75

-

·(H₂O)_0.3}₄in which the solvent void volume is not fully

ordered and occupied. A suitable disorder model was not found

for the Pr(III) complex, so in this case the SQUEEZE

procedure was applied. Views for {[Eu(9_{R , S})-

(NO₃)₃]·(Me₂CO)_0.75·(H₂O)_0.3}₄and [Pr(9_R,S)(NO₃)₃] are

shown in Figures 10 and 11, and selected bond lengths are

provided in Table 4. In both cases the Ln(III) ion is bonded to

three bidentate nitrate anions and one neutral, meso,

tetradentate ligand, 9_R,S, in which the NOPOP′O′CO chelate

interaction is asymmetric with one amide (S conformer arm)

coordinated to the Ln(III) and the second amide arm (R

Figure 10. Molecular structure and atom labeling scheme for

{[Eu(9_R,S)(NO₃)₃]·(Me₂CO)_0.75·(H₂O)_0.3

}

(thermal ellipsoids,

Figure 12. Molecular structure and atom labeling scheme for

[Er(9_R,S)(NO₃)₃]·(Me₂CO) (thermal ellipsoids, 50%) with carbon

atom labels, lattice Me₂CO, and H-atoms omitted for clarity.

4

50%) with carbon atom labels, lattice Me₂CO and H₂O, and H-

atoms omitted for clarity.

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atoms from three bidentate nitrate anions and three O-atoms

provided by a neutral, tridentate NOPOP′O′ chelated ligand, 9,

in its meso form . Both amide carbonyl units are rotated away

from the Er(III) ion coordination pocket, and they are not

involved in the inner sphere coordination interaction. This

tridentate chelation mode is consistent with the IR spectrum

that shows a single strong, relatively sharp carbonyl stretch at

1647 cm⁻¹that is essentially unperturbed from the free ligand

vibration, ν_CO=1643 cm⁻¹, vide supra. The ligand is

symmetrically docked on the Er(III) as indicated by nearly

equivalent Er−O(P) and Er−O(N) bond lengths: Er1−O2

2.300(2) Å, Er1−O4 2.293(2) Å, and Er1−O1 2.321(2) Å. In

this sense, the ligand behaves much like the previously studied

NOPOP′O′ ligands, 6, [X = CH₂P(O)PR₂].³¹For example, the

Er−O bond lengths can be compared with the distances in the

related 1:1 complexes, [Er(6)(NO₃)₃] (X = CH₂P(O)PPhBz):

Er−O(P) 2.280(7) and 2.281(7) Å and Er−O(N) 2.278(7)

Å.^31hFor [Er(6)(NO₃)₃] (X = CH₂P(O)Cy₂), the distances

follow: Er−O(P) 2.249(2) and 2.253(2) and Er−O(N)

2.318(2) Å.

structure is consistent with the IR spectrum for the complex

which displays a very broad absorption in the carbonyl stretch

region.

The crude complexes formed by 9 and two other late

lanthanides, Dy(III) and Yb(III), in MeOH solutions, following

crystallization from acetone/ethyl acetate (4/1) solution, also

provide crystals with two diﬀerent morphologies (diamond-like

and needles (Yb) or diamond-like and microcrystalline powder

(Dy)). X-ray crystal structure determinations for both

diamond-like crystals show that the complexes are isomorphous

with the complex [Er(9_R,S)(NO₃)₃]·(Me₂CO). The relevant

structural data are provided in Supporting Information. It is

noted as well that these complexes display IR spectra that are

nearly identical and each shows a relatively narrow,

unperturbed ν_COband, compared to the free ligand, consistent

with tridentate coordination of 9. Unfortunately, the needle

crystalline form of the Yb(III) complex was not of suﬃcient

quality to permit a X-ray structure determination; however, it is

likely that the structure for this complex resembles the structure

of [Er(9_S,S)(NO₃)₂(H₂O)](NO₃)·(MeOH)·(H₂O)_0.4that con-

tains a tetradentate chelated ligand 9.

The molecular structure determination for the complex

isolated from the equimolar combination of 10 and Pr-

(NO₃)₃·6H₂O in MeOH and crystallized from acetone/MeOH

(4/1) solution provides an additional interesting result. A view

of the structure is shown in Figure 14, and selected bond

lengths are summarized in Table 4. The complex, {[Pr(10_R,S)-

(NO₃)(H₂O)]₂(μ-10_R,R)}(NO₃)₄, is composed of two [Pr-

(10_R,S)(NO₃)(H₂O)] units, each containing a central Pr(III)

ion bonded to one bidentate nitrate anion, a molecule of water,

and an asymmetrically docked, pentadentate NOPOP′O′CO-

C′O′ ligand, 10_R,S. The two equivalent fragments are bridged

through PrOP bonds by a third ligand molecule, 10_R,R. As

a result, each Pr(III) is nine-coordinate. Four disordered nitrate

counterions appear in the outer sphere, and these were treated

through SQUEEZE. Four of the ﬁve Pr−O bond lengths

involving the pentadentate ligand are relatively similar, Pr1−

O3(P) 2.426(4) Å, Pr1−O5(P) 2.492(4) Å, Pr1−O1(N)

2.452(4) Å, Pr1−O4(C) 2.482(4) Å. The coordinate bond

formed by the second amide carbonyl group is longer, Pr1−O2

2.589(4) Å, and more comparable to the bond lengths

involving the inner sphere nitrate ion, Pr1−O9 2.589(4) and

Pr1−O10 2.5888(4) Å. The bridging Pr1−O8(P) bond length,

2.478(4) Å, and the coordinated water distance, Pr1−O15,

2.475(5) Å, are similar to the shorter Pr−O bond lengths

involving the pentadentate ligand. The functional group bond

lengths are consistent with the coordination modes displayed

by the ligand: N1−O1 1.327(6) Å, P1−O3 1.503(4) Å, P2−O5

1.506(4) Å, C8−O2 1.236(8) Å, C27−O4 1.242(7) Å, P3−O8

1.498(4) Å, C49−O7 1.220(7) Å. The unbound pyridine N-

oxide bond length, N4−O6 1.324(9) Å, is somewhat longer

than expected.

The molecular structure determined from the rectangular

prisms, on the other hand, revealed a diﬀerent composition,

[Er(9_S,S)(NO₃)₂(H₂O)](NO₃) ·(MeOH)·(H₂O)_0.4. A view of

the complex is shown in Figure 13, and selected bond lengths

Figure 13. Molecular structure and atom labeling scheme for

[Er(9_S,S)(NO₃)₂(H₂O)](NO₃)·(MeOH)·(H₂O)_0.4(50% thermal el-

lipsoids) with carbon atom labels, outer-sphere nitrate ion, lattice

solvents, and H-atoms except for inner sphere water molecule omitted

for clarity.

are presented in Table 4. The central Er(III) is eight-coordinate

with the coordination positions occupied by four O-atoms

provided by two bidentate nitrate anions and four O-atoms

from a neutral, tetradentate, NOPOP′O′CO-bonded ligand 9 in

its S,S enantiomeric form. In contrast to the interactions of 9

with the early lanthanide ions Pr(III) and Eu(III), the free

amide arm is not rotated away from the Er(III) ion. Instead, it

points directly at and is hydrogen bonded with a H-atom of an

Er(III)-bound water molecule with O15−H15B 0.77(3) Å,

O3···H15B 1.92(3) Å, O15···O3 2.690(3) Å, and O3−H15B−

O15 176.0°. The relevant bond lengths involving 9 and the

Er(III) ion are Er−O(P), Er1−O2 2.283 (2) Å, and Er−O4

2.310(2) Å, Er−O(N) Er1−O1 2.466(2) Å, Er−O(C) Er1−O5

2.310(2) Å, and Er−O(H) Er−O15 2.288(2) Å. The donor

group bond lengths in the ligand, P−O 1.497(2) and 1.496(2)

Å, N−O 1.313(4) Å, and C−O 1.252(3) and 1.236(3) Å, are all

elongated relative to the bond lengths in the free ligand. For the

amide fragments, the greater C−O bond elongation occurs, as

expected, for the carbonyl bonded to the Er(III) ion. This

Solvent Extraction Analyses. The interesting lanthanide

coordination chemistry displayed by 7−10 encouraged studies

of the solvent extraction performance of the new hybrid ligands.

The aryl derivatives described here are not appreciably soluble

in the preferred organic diluent dodecane; however, each is

readily soluble in 1,2-dichloroethane (DCE), and this diluent

was used for initial screening extraction measurements.

Distribution ratios, D = [M_org]/[M_aq], were measured for

Eu(III) and Am(III) in nitric acid solutions under identical

conditions at 25 °C by using 0.01 M solutions of 7−10 in DCE.

The variations of D values on nitric acid concentration for the

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Figure 14. Molecular structure and atom labeling scheme for{[Pr(10_R,S)(NO₃)(H₂O)]₂(μ-10_R,R)}(NO₃)₄(50% thermal ellipsoids) with carbon

atom labels, outer-sphere nitrate ions, and H-atoms omitted for clarity.

“one-armed” ligands 7 and 8, as well as for OPhDiBCMPO, are

shown in Figure 15. It is apparent that, at all nitric acid

The dependencies of D values on nitric acid concentration

for extractions with DCE solutions of the “two-armed” ligands

9 and 10 are displayed in Figure 16 along with data for

Figure 15. Americium and europium distribution ratios as a function

of the initial nitric acid concentration. Organic phase: 7, 8, or

OPhDiBCMPO at 10 mM in 1,2-DCE. Aqueous phase: trace ²⁴¹Am

and 0.1 mM of europium nitrate in nitric acid. O/A = 1, T = 25 °C.

Figure 16. Americium and europium distribution ratios as a function

of the initial nitric acid concentration. Organic phase: 9, 10,

(TO)NOPOPO, or OPhDiBCMPO at 10 mM in 1,2-DCE. Aqueous

phase: trace ²⁴¹Am and 0.1 mM of europium nitrate in nitric acid. O/A

= 1, T = 25 °C.

concentrations, 7 is a superior extractant compared to 8, and

OPhDiBCMPO is intermediate in performance between 7 and

8. This result contrasts with the steric modeling prediction that

the “ﬂoppier” ligand 8 would provide slightly smaller steric

strain per donor group than 7. Of course, additional factors

beyond steric strain contribute to the magnitudes of D values.

All three compounds show increasing D values for Eu(III) and

Am(III) with increasing nitric acid concentration in the range

0.01−1 M. At higher acid concentrations, the D values decrease

likely as a result of competing ligand protonation reactions.

This behavior is also observed with CMPO, NOPO, and

NOPOPO extractions.^{13,16,32−35}In addition, each compound,

at the same [HNO₃], displays better extractions for Am(III)

than for Eu(III) with the greatest diﬀerentiation occurring with

7.

OPhDiBCMPO and TONOPOPO measured under identical

conditions. First, it is pointed out that although previous studies

have indicated that the NOPOPO-class of extractants provides

slightly improved performance relative to the CMPO-class

extractants,³²⁻³⁵accurate comparisons have not been possible

since the extraction data have not been obtained under identical

experimental conditions. The data in Figure 16 provide for a

direct evaluation albeit in a chlorocarbon solvent, DCE. At all

initial nitric acid concentrations, the D values for TONOPOPO

are signiﬁcantly greater than the D values for OPhDiBCMPO.

Further, it is observed that both compounds exhibit increasing

D values with increasing initial nitric acid concentration from

0.01 to 0.3 M.⁵²From 0.3 to 1 M [HNO₃], the D values for

TONOPOPO begin to decrease while the D values for

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OPhDiBCMPO continue to increase up to 1 M [HNO₃] and

then decrease. In both cases, at all [HNO₃], the D values for

Am(III) are slightly greater than for Eu(III). Extractions with

10 generally parallel the performance of OPhDiBCMPO in the

initial acid range 0.01−0.3 M, but at higher acid concentrations

the D values for 10 continue to increase up to 3 M [HNO₃]

before declining at the highest acid concentrations. At all

[HNO₃] up to 3 M the D values for 10 are signiﬁcantly smaller

than those observed with TONOPOPO. Furthermore, the D

values for Am(III) are slightly greater than those for Eu(III) at

all [HNO₃].

The particularly interesting observations appear in the

extractions performed with 9. Here, the D values are much

greater than those for 10 or OPhDiBCMPO at all nitric acid

concentrations. In the acid concentration range 0.01−0.3 M the

D values for 9 and TONOPOPO extractions of Am(III) are

essentially identical while the D values for 9 are smaller than for

TONOPOPO extractions of Eu(III). However, above 1 M

[HNO₃], 9 is a dramatically better extractant than TONOPO-

PO for both Am(III) and Eu(III), and the D values for both

continue to increase up to at least 5 M [HNO₃]. For this case,

the f-element cation very favorably competes against proton for

the ligand donor sites. At all acid concentrations D_Amis

signiﬁcantly greater than D_Eu, and at 1 M [HNO₃] the

separation ratio D_Am/D_Eu∼ 10. This separation factor is

noticeably larger than typically encountered with many all-O-

atom donor chelating extractants such as CMPO and

NOPOPO, comparable with a few “calix-tethered”-CMPO

extractants,^27b,jbut smaller than reported for several soft donor

extractants.⁵⁻⁷

and perhaps pentadentate ligand/metal ion interactions persist

under extraction conditions with 9 and likely with 10.

CONCLUSION

■

Synthetic procedures have been successfully designed and

implemented for the marriage of carbamolymethylphosphine

oxide fragments to pyridine N-oxide and methylpyridine N-

oxide platforms. Computational analyses of steric strain

incurred by the ligands when adopting energy minimized,

maximal multidentate docking interactions on Ln(III) ions

indicate that the “one-armed” ligands, 7 and 8, and the “two-

armed” ligands, 9 and 10, should be able to accommodate

tridentate and pentadentate chelate binding modes, respec-

tively. Selected coordination chemistry with lanthanide nitrates

was surveyed, and several 1:1 complexes were isolated. Under

the conditions explored, 2:1 complexes were not isolated

although the formation of such species, with reduced ligand

denticities, may well occur especially in noncoordinating

organic solutions. Spectroscopic analyses suggest that the

ligand docking interactions in organic solutions are asymmetric

and hemilabile-like. Crystal structure analyses for complexes

containing the “one-armed” ligands indicate that 7 adopts the

tridentate NOPOCO chelate condition while 8 prefers to

utilize a mixed bidentate POCO/bridging NO binding mode.

Structures for complexes containing the “two-armed” ligands, 9

and 10, not surprisingly, show greater diversity with examples

of tridentate, tetradentate, and pentadentate binding. Initial

solvent extraction screening using solutions of the ligands

dissolved in DCE in contact with Eu(III) and Am(III) in

aqueous nitric acid solutions reveal impressive performance for

7 and especially 9. Stimulated by these results, more detailed

spectroscopic studies of the ligand/Ln(III) coordination

interactions in solution, additional crystallographic analyses of

isolated coordination complexes, and expanded extraction

analyses are in progress. The results from these studies, as

well as parallel discoveries involving the development of related

NOPOCO and NOPOPOCOCO ligands with attachments

through the amide N-atom, will be reported separately.

Lastly, ligand dependency analyses for extractions of Am(III)

and Eu(III) by 9, TONOPOPO, and OPhDiBCMPO in DCE,

measured at 1 M [HNO₃] without correction for ligand

protonation, are summarized in Figure 17. As expected, the

ASSOCIATED CONTENT

* Supporting Information

■

S

Selected IR, HRMS, emission and NMR spectra, summary of

separation factors, and CIF data for the crystal structures. This

material is available free of charge via the Internet at http://

pubs.acs.org. X-ray data for 8_R.8_Shave also been deposited with

the Cambridge Crystallographic Data Centre with the

deposition number 831193. These ﬁles may be accessed free

of charge at http://www.ccdc.cam.ac.uk/conts/retrieving.html.

AUTHOR INFORMATION

Corresponding Author

*E-mail: rtpaine@unm.edu.

■

Figure 17. Dependence of distribution ratios D for Eu(III) and

Am(III) on concentration of 9, (TO)NOPOPO, and OPhDiBCMPO

in DCE from 1 M HNO₃at 25 °C.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

ligand dependencies for Am(III) and Eu(III), with each ligand,

are similar, and the slopes are ∼1.4, 1.8, and 2.1, respectively.

These data suggest that the ligands probably form a

combination of 1:1 and 2:1 ligand/metal complexes in DCE

with the 1:1 stoichiometry most prevalent with 9. This is

consistent with the conclusion that asymmetric, tetradentate

Financial support for this study at the University of New

Mexico was provided by the Division of Chemical Sciences,

Geosciences and Biosciences, Oﬃce of Basic Energy Sciences,

U.S. Department of Energy (Grant DE-FG02-03ER15419

(R.T.P)). In addition, funds from the National Science

Foundation assisted with the purchases of the X-ray

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B.; Glatz, J. P.; Malmbeck, R.; Modolo, G. Solvent Extr. Ion Exch. 2009,

27, 26−35.

diﬀractometer (CHE-0443580) and NMR spectrometers

(CHE-0840523 and -0946690). R.T.P. also wishes to thank

Dr. Brian M. Rapko for his contributions to the initial stages of

this study. B.P.H. and L.H.D. acknowledge support from the

Division of Chemical Sciences, Geosciences and Biosciences,

Oﬃce of Basic Energy Sciences, U.S. Department of Energy.

A.d.B.-D. acknowledges ﬁnancial support from the NSF (CHE-

1058805) and Dr. Sebastian Bauer’s help with acquisition of the

HRMS spectra.

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dx.doi.org/10.1021/ic3025342 | Inorg. Chem. 2013, 52, 3063−3083

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Synthesis, lanthanide coordination chemistry, and liquid-liquid extraction performance of CMPO-decorated pyridine and pyridine N-oxide platforms

DOI: 10.1021/ic3025342

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