Synthesis and properties of new (Phosphinoylmethyl)pyridine N-oxides

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DOI: 10.1002/zaac.201300099

Synthesis and Properties of New (Phosphinoylmethyl)Pyridine N-Oxides

Sylvie L. Pailloux,^[a][‡]Daniel Rosario-Amorin,^[a]Manab Chakravarty,^[a][‡‡]Jean-

Michel Camus,^[a]Karen Ann Smith,^[a]Eileen N. Duesler,^[a]Diane A. Dickie,^[a]

Robert T. Paine,*^[a]Kevin K. Klausmeyer,^[b]Daniel A. Padron,^[b]Benjamin P. Hay,^[c]and

Laetitia H. Delmau^[c]

Dedicated to Professor Heinrich Nöth on the Occasion of His 85th Birthday

Keywords: Phosphine oxides; Pyridine N-oxides; Coordination chemistry; X-ray diffraction; Lanthanides; Actinides / Solvent

extraction

Abstract. Syntheses for 2-[1-(diarylphosphinoyl)-1-(pyridin-2-yl)- nation mode with minimal steric strain. In contrast, 7a cannot form

tetradentate NO(meNO)₂PO chelates; however, tridentate binding

should be accessible with minimal ligand strain. Coordination com-

plexes of 8a,b, 5a,b, 6a,b and 11a,b with Ln(NO₃)₃salts were isolated

and a X-ray crystal structure for [Er(8a)(NO₃)₃(MeOH)₂]·CH₂Cl₂, re-

vealed a monodentate Er–O=P interaction. On the other hand,

complexes formed by a more symmetrical trifunctional phenylphos-

phino-bis-2-methylpyridine N,N,P-trioxide ligand, (meNO)₂PO*,

methyl]pyridines, (8a,b), and 2-[1-(diarylphosphinoyl)-1,1-bis(methyl-

pyridin-2yl)methyl]pyridines, (11a,b), (Ar = C₆H₅and 2-CF₃C₆H₄),

based on substitution of 2-methylpyridine fragments onto the exo

methylene carbon atom of 2-[(diaryl)phosphinoylmethyl]pyridine plat-

forms, are described. N-oxidations of 8a,b and 11a,b produced the 2-

[1-(diarylphosphinoyl)-1-(1-oxy-pyridin-2yl)methyl]pyridine N-oxides

(5a,b) and the 2-[1-(diarylphosphinoyl)-1,1-bis(1-oxy-methylpyridin-

2-yl)methyl]pyridines (6a,b), respectively. The “short-arm“ pyridine

fragment of 11a,b resists N-oxidation, and the fully oxidized mole-

{La[(meNO)₂PO*)](OTf)₂(MeOH)₃(H₂O)⁺}(OTf^–)

and

{Pr[(meNO)₂PO*)](OTf)(MeOH)₄⁺}(OTf^–)₂, realized a tridentate co-

ordination mode. Solvent extraction behaviors for Eu^IIIand Am^IIIin

nitric acid solutions using 5a,b, 6a,b, Ph₃PO and the parent bifunc-

tional ligand 2-[(diphenylphosphanyl)methyl]pyridine N,P-dioxide

(3a) in 1,2-dichloroethane were assessed, and 5a,b and 6a,b were

found to behave more like Ph₃PO than 3a.

cules,

2-[1-(diarylphosphinoyl)-1,1-bis(1-oxy-methylpyridin-2-yl)-

methyl]pyridine N-oxides (7a,b) were not isolated. Molecular mechan-

ics calculations for gas phase 1:1 ligand/lanthanide complexes indi-

cated that 5a should accommodate a tridentate NO(meNO)PO coordi-

strategically and environmentally important metal ions.^[1–7]In

particular, monofunctional organophosphine oxides and

organoamides have found practical applications in LLE separa-

tions for f-element ions.^[2]Furthermore, these two fragments

have been combined to produce a unique, widely adopted fam-

ily of carbamoylmethylphosphine oxide (CMPO) reagents (1)

that display remarkable extraction performance for trivalent f-

element cations in strongly acidic aqueous solutions.^[1,2,8]In-

deed, one example, [(N,N-diisobutylcarbamoyl)methyl](octyl)-

(phenyl)phosphine oxide (1*, R = n-octyl, R’ = phenyl, R’’ =

iBu) is the critical reagent in the large-scale TRUEX process

for trivalent actinide partitioning.^[9]

1 Introduction

There continues to be considerable fundamental and applied

interest in the development of relatively simple organic rea-

gents that can function under harsh conditions as selective

complexants and liquid-liquid extraction (LLE) reagents for

* Prof. Dr. R. T: Paine

E-Mail: rtpaine@unm.edu

[a] Department of Chemistry and Chemical Biology

University of New Mexico

Albuquerque, NM 87131, USA

[b] Department of Chemistry and Biochemistry

Baylor University

Waco, TX 76798, USA

[c] Chemical Sciences Division

Oak Ridge National Laboratory

P.O. Box 2008

Monofunctional pyridine N-oxides also have occasionally

been employed as extractants for f-element cations,^[10]and we

have previously described efforts to combine this platform

with phosphine oxides to obtain the bifunctional and trifunc-

tional chelating reagents 2–4.^[11,12]These compounds act as

bidentate or tridentate complexants toward f-element

cations, and several examples of 3 and 4 display good

extraction performance^[13,14]that, in the cases of

Oak Ridge, TN 37831 USA

[‡] Current address: Department of Chemistry

University of California-Berkeley

Berkeley, CA USA

[‡‡] Current address: Chemistry Department

Birla Institute of Technology and Sciences

Pilani-Hyderabad Campus

Hyderabad, 500078, A.P., India

Supporting information for this article is available on the WWW

under http://dx.doi.org/10.1002/zaac.201300099 or from the au-

thor.

[(2-EtHx)₂P(O)CH₂]₂C₅H₄NO^[14a]

and

[(n-octyl)₂P(O)-

CH₂]₂C₅H₄NO,^[14b]exceed the performance of 1*. As part of

Z. Anorg. Allg. Chem. 2013, 639, (7), 1101–1116

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R. T. Paine et al.

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a continuing interest in extractant development for f-element ments are generated. The structure for a 4:1 complex,

separations, we have been examining the impacts that substitu- [Tb(3a)₄](NO₃)₃·(H₂O)·(EtOH), showed an eight coordinate

ent group modifications have on the coordination chemistry inner sphere geometry produced by the oxygen atoms from

and extraction performance of the platforms represented by 3 four neutral, bidentate 3a ligands.^[12a]All three nitrate counter-

and 4. In the present study, we specifically sought to determine ions are displaced to the outer sphere along with lattice solvent

the effect on chelate formation by addition of 2-(methyl)pyr- molecules. For 3b, (R = 2-CF₃C₆H₄), so far only 1:1 com-

idine N-oxide fragments to the exo-sp³carbon center of 3. The plexes have been isolated and structurally characterized.^[12n]

synthesis, selected coordination chemistry and extraction A crystal structure determination for one of these complexes,

analyses for derivatives of 5 and 6 are described along with [Yb(3b)(NO₃)₃(DMF)]·(DMF)·0.5(H₂O), showed 3b bonded

attempts to prepare 7.

in the inner sphere as a neutral, bidentate NOPO chelate

along with three bidentate nitrate ions and a DMF molecule.

In nearly all of these complexes, the Ln–O(P) bond lengths are

significantly shorter than the Ln–O(N) bond lengths, and the

infrared spectroscopic coordination shifts for the phosphine

oxide group, Δν_PO, are significantly larger than pyridine

N-oxide shifts, Δν_NO, consistent with geometrically and elec-

tronically asymmetric NOPO/Ln ion interactions.

Extensive studies of Ln^IIIcoordination interactions with the

trifunctional ligands 4 show that these ligands adopt tridentate

NOPOPO chelate structures in the solid state,^[12]and IR coor-

dination shifts, recorded from solid samples, are consistent

with tridentate bonding. However, in some complexes, small

IR coordination shifts for ν_NOsuggest that N-oxide coordina-

tion may be labile in solution.^[14a]Lastly, Klausmeyer^[16]has

reported selected late transition metal coordination chemistry

for a related trifunctional ligand, PhP(O)[CH₂C₅H₄NO]₂

(meNO)₂PO*, in which the ligand adopts multiple coordina-

tion conditions, and the (N)O–M bond lengths, in some cases,

are quite asymmetric.

2 Results and Discussion

2.1 Computational Assessment of the Chelation Properties

of 5 and 7

Before describing efforts to prepare and study the lanthanide

coordination chemistry of the new multifunctional ligands 5–

7, some pertinent electronic and steric features related to these

molecules are summarized. Unfortunately, few relevant quanti-

tative thermodynamic data exist for the binding of neutral

monofunctional pyridine N-oxide or phosphine oxide ligands

with Ln^IIIcations in aqueous solution.^[15]However, the isola-

tion of numerous stable complexes from water or methanol

solutions and a host of spectroscopic data qualitatively indicate

that both neutral monofunctional donor fragments readily bond

to these hard, f-element acceptors. Further, it appears that, of

the two fragments, organyl phosphine oxides produce the

stronger donor-acceptor interactions.^[11,12]In contrast, pyridine

is a comparatively weak N-donor ligand toward Ln^IIIcations

in these solutions compared to phosphine oxides and pyridine

N-oxides. Not surprisingly, the marriage of the phosphine ox-

ide and pyridine N-oxide fragments into bifunctional NOPO

ligands of types 2 and 3 results, without exception, in bidentate

NOPO binding with Ln^IIIions in the solid state as opposed to

monodentate NO or PO coordination. For example, with 2c

(R = iPrO), a 2:1 complex, [La(2c)₂(NO₃)₃], has been structur-

In addition to donor group electronics, ligand steric features

ally characterized and found to display bidentate NOPO coor- and accessible chelate ring sizes also influence coordination

dination.^[11b]Similarly, with 3a (R = Ph), 2:1 and 4:1 L:Ln complex stability. For example, it is generally recognized that

complexes have been isolated and characterized. Crystal struc- complexes containing five-membered chelate rings are gen-

ture determinations for the 2:1 complexes revealed that 3a erally more stable than complexes containing donor-related

bonds in a bidentate NOPO mode,^[12a]and together with three six-membered chelate rings, and both are normally more stable

bidentate nitrate anions, ten coordinate inner sphere environ- than complexes formed with higher-membered chelate

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Synthesis and Properties of New (Phosphinoylmethyl)Pyridine N-Oxides

rings.^[15]However, in prior comparative studies,^[11,12]forma-

tion of complexes with examples of 3 and 4, that employ

seven-membered chelate rings, was favored over formation of

complexes with 2 that utilize six-membered chelate rings. In

addition, there appears to be little steric penalty for formation

of congested 4:1 complexes with 3 or 2:1 complexes of 4 with

Ln^IIIions. This is indicated, as mentioned above, by generation

of the complexes [Ln(3)₄](NO₃)₃and [Ln(4)₂(NO₃)](NO₃)₂

where four neutral bidentate NOPO ligands or two neutral tri-

dentate NOPOPO ligands bind by expelling all or two of the

three nitrate counterions, respectively, to the outer sphere.

Lastly, computational modeling for 2:1 and 4:1 complexes of

3b^[12n]suggest that the ortho-CF₃substituents do not sterically

impede formation of these complexes.

As a point of reference, the 1:1 binding of the parent NOPO

fragment 3a on Gd^IIIwas evaluated, and a view of the lowest

energy complexed form is shown in Figure 1a. The computed

ΔG_relfor this complex is 3.14 kcal·mol^–1. With two donors

involved in a bidentate interaction, the ΔG_rel/donor is

1.57 kcal·mol^–1. This low value is consistent with the observed

ease that ligands of type 3 display in the formation of bidentate

NOPO structures. A view of the computed 1:1 complex formed

by the NO(meNO)PO ligand 5a (R = Ph) with Gd^IIIis shown

in Figure 1b. It is noted that the conformation of 5a in its crys-

tal structure lies 1.4 kcal·mol^–1above the computed global

minimum (low) structure. This is not entirely unexpected since

the crystal structure of 5a contains 1.5 MeCN lattice solvent

molecules per ligand molecule, vide infra, and solvent disorder

and hydrogen bonding probably perturb the crystal structure

relative to the computed gas phase global minimum (low)

Given these observations, it was anticipated that new as-

semblies of N–O and P=O donor groups onto low-strain or-

ganic frameworks should generate electronically favorable do-

nor fields suitable for binding hard Ln^IIIacceptors. The in-

clusion of a pyridine fragment in the donor set, by comparison,

would be expected to be electronically less favorable. For syn-

thetic convenience, we chose to conceptually start with the

known bifunctional NOPO fragment 3 and determine the im-

pacts of addition of 2-(methyl)pyridine N-oxide fragments to

the exo-sp³carbon atom that would produce 5–7. Since the

energetic impacts of such architectural modifications, going

from 3 to 5–7, are difficult to assess a priori, a force field-

based structure scoring approach, developed by Hay and co-

workers,^[17]was employed. This approach has been used pre-

viously with a wide cross-section of multifunctional ligand

types to anticipate ligand steric preferences on metal cation

chelation energetics. Briefly, the process involves discovery of

ligand:metal structures that display low conformational energy,

small induced strain energy and limited restricted bond rota-

tions upon formation of complexes in the gas phase from free

ligands. For each structure evaluated, the steric energies

(kcal·mol^–1) of the ligand global minimum structure (low), the

binding conformer (binding) and the bound form (bound) are

computed. These energies are then used to calculate

ΔE_comp(bound – binding), ΔE_conf(binding – low) and ΔE_tot

(bound – low) = ΔE_comp+ ΔE_conf.Then, by defining ΔS_rotas

(the number of rotatable bonds frozen on chelation) x 0.31

kcal·mol^–1·bond^–1, an estimate of the relative binding free en-

ergy, ΔG_rel, is given by ΔE_tot+ ΔS_rot. The ΔG_relvalue provides

a quantitative ranking of organization for series of ligands with

constant donor sets. Normalized values of ΔG_rel/donor provide

a measure of the degree of organization per donor group in

ligands with differing numbers of donors. In the present case,

this analysis was applied on 3a, 5a, 6a and 7a to evaluate the

degree of donor group organization provided by each architec-

ture and to assess whether 5 and 6 could readily adopt triden-

tate NONOPO chelate structures, and if 7 could form a tetra-

dentate NONONOPO chelate structure on a Ln^IIIion without

encountering serious ligand strain. A similar approach was re-

cently employed in the assessment of chelate preferences for

ligands derived from pyridine N-oxide platforms decorated

with CMPO pendent arms.^[18]

structure. The tridentate binding mode for 5a exhibits ΔG_rel

=

10.75 kcal·mol^–1and ΔG_rel/donor = 3.58 kcal·mol^–1. This

value is noticeably higher than for bidentate 3a. The additional

(meNO) donor group interaction with a lanthanide ion might

offset the added strain incurred upon tridentate complexation

Figure 1. Geometry optimized structures for 1:1 complexes on Gd^III

with: a) bidentate coordinated ligand 3a and b) tridentate coordinated

ligand 5a.

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although it is known from our prior work that pyridine N-oxide ΔG_rel(kcal·mol^–1) and ΔG_rel/donor (kcal·mol^–1) values are:

donors are more weakly binding than phosphine oxides.

The evaluation of the chelation modes for the

NO(meNO)₂PO ligand 7a (R = Ph) leads to the conclusion

that this ligand simply cannot adopt a tetradentate chelate

structure with reasonable ligand O–Gd bond lengths. As a re-

sult, three more realistic tridentate chelate structures were ana-

lyzed that are labeled as NO(meNO)PO (motif 1), (meNO)₂PO

(motif 2) and NO(meNO)₂(motif 3) where the designation NO

refers to the pyridine N-oxide ring bonded to the Ph₂P(O)

group through the tertiary sp³carbon atom (“short-arm”) and

meNO corresponds to the pyridine N-oxide rings bonded to

the Ph₂P(O) group through CCH₂- groups (“long-arm^”).

Views of these structures are shown in Figure 2. The

motif 1, 8.76 and 2.92; motif 2, 16.24 and 5.41 and motif

3, 13.44 and 4.48. Motifs 1 and 2 have (NO)₂PO donor set

combinations, and it is noteworthy that the mixed “short-arm”–

“long-arm” combination gives the smaller computed ΔG_rel/do-

nor value. Motif 3, on the other hand, would be least favored

of the three due to moderate bond strain and the absence of

the strong PO donor center from the coordination field.

Lastly, for comparison, ΔG_relwas evaluated for the trifunc-

tional ligand reported previously by Klausmeyer.^[16]Since this

is a variant of the (meNO)₂PO form (motif 2) of 7a described

above, it is labeled as (meNO)₂PO*, and a view of the com-

puted, Ln-bonded structure is shown in Figure 3. With ΔG_rel

=

11.48 kcal·mol^–1and ΔG_rel/donor = 3.83 kcal·mol^–1, the analy-

sis suggests that tridentate chelation with this ligand, would be

sterically favored over the tridentate motif 2 of 7a and slightly

disfavored compared to tridentate 5a.

Figure 3. Geometry optimized structures for 1:1 complex on Gd^IIIwith

tridentate coordinated ligand (meNO)₂PO*.

2.2 Synthesis of (Phosphinoylmethyl)pyridine N-Oxide

Ligands

Based upon the computational analyses, examples of 5, that

potentially might produce a tridentate NO(meNO)PO coordi-

nation field, were targeted first for synthetic development. The

approaches explored for 5a,b [Ar = a) C₆H₅; b) 2-CF₃C₆H₄]

are summarized in Scheme 1. Preparations for the intermedi-

ate, 2-(diphenylphosphinoylmethylpyridine, 3-Oa (Ar = Ph),

[12a]

that employ NaPh₂P(O)^[19], Ph₂P(OEt)^[20]or KPPh₂

as

phosphination reagents have been previously reported; how-

ever, we have found a Grignard-based route to be more conve-

nient, efficient and dependable. We have previously

described this approach for the synthesis of 3b^[12n]and 4c

(R = 2-EtHx)^[14a]and full details for 3-Oa are provided here.

Three new routes for the synthesis of 8a, briefly mentioned as

a product of the reaction of a potassium salt of 3-Oa with 2-

(chloromethyl)pyridine,^[19]were explored. Each involves the

deprotonation of the methylene group spanning the pyridine

ring and the phosphine oxide substituent of 3-Oa with a strong

base: LDA (Method 1),^[21]nBuLi (Method 2) or PhMgCl

(Method 3). In the cases of Methods 1 and 2, 8a was isolated

as a spectroscopically pure white solid, following chromato-

graphic purification, with yields of 81% and 69%, respec-

Figure 2. Geometry optimized structures for 1:1 complexes on Gd^III

with tridentate bonded ligand 7a: a) motif 1; b) motif 2; c) motif 3.

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Synthesis and Properties of New (Phosphinoylmethyl)Pyridine N-Oxides

Scheme 1. Synthesis outlines for 8a,b and 5a,b.

tively. While attempting to optimize conditions for the depro- gave a better yield for 5b (99%). Attempts to study the sequen-

tonation reaction of 3-Oa with various Grignard reagents, it

was discovered that a one-pot synthesis (Method 3), employing

diethyl phosphate, 2-(chloromethyl)pyridine and phenylmag-

nesium bromide, in a 1:2:4 combining ratio, gave 8a directly

in 70 Ϯ 5% yield following chromatographic purification. The

crude product mixture also contained small amounts of 3-Oa

and 11a (see Scheme 2). Similar results were obtained with

use of 2-(bromomethyl)pyridine. Only Method 3 was used to

prepare samples of 8b, and it was isolated with an unoptimized

yield of 45%.

tial oxidation process for the two pyridine rings in 8a,b by

using limited amounts of oxidant were inconclusive, and

neither Ar₂P(O)C(H)(C₅H₄N)(CH₂C₅H₄NO), 9a,b nor

Ar₂P(O)C(H)(C₅H₄NO))(CH₂C₅H₄N) 10a,b were obtained ex-

cept in very small amounts in complex reaction mixtures.

However, pure 10a was obtained exclusively via deprotonation

of 3a with nBuLi followed by addition of 2-(bromomethyl)-

pyridine. The compounds were isolated as spectroscopically

pure solids, and high resolution mass spectra were consistent

with the proposed formulae. Infrared spectra for 3a, 8a,b, 5a,b,

and 10a display a strong absorption in the range 1177–

1188 cm^–1that is diagnostic of the P=O stretch in the

Ph₂P(O)CH- substituent, and 5a,b and 10a also display an ab-

sorption in the range 1208–1231 cm^–1that is attributed to the

N–O stretch in pyridine N-oxide fragments. As expected, the

³¹P NMR spectra for the compounds are similar, each showing

1

a single resonance in the range 30–37 ppm. The H and ¹³C

NMR spectra are fully consistent with the proposed structures,

and typical spectra are provided in supporting information.

In order to unambiguously confirm the molecular structures

of the key compounds, single-crystal X-ray diffraction analy-

ses were completed for 8a,b, 5a·1.5MeCN and 5b. Views of

the molecules are shown in Figures 4–7. In each case, the pyr-

idine ring fragments are planar, and the P=O bond vector is

directed away from the space occupied by the pyridine N-atom

lone pair and/or the N–O bond vector, thereby minimizing

electron repulsions. It is interesting to note that in both 8b and

Scheme 2. Synthesis outline for 6a,b.

The N-oxidations of 8a,b were examined with excess per-

oxyacetic acid and m-chloroperoxybenzoic acid. The former

provided slightly better results for 5a (97%), while the latter 5b, a fluorine atom on one of the CF₃groups is located close

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to the backside of the phosphorus atom in the P=O group: 8b

(two independent molecules), P1···F4 296.3(3) pm, P2···F10

293.5(4) pm; 5b, P1···F4 291.8(1) pm. This type of non-

bonded interaction has been observed previously in related

compounds,^[12n,22]and it results from the electrostatic interac-

tion between the electron poor P^Vcenter and an electronega-

tive fluorine atom in one CF₃group. The interaction assists in

the ordering of the aryl groups in the solid state.

Figure 6. Molecular structure and atom labeling scheme for

5a·1.5MeCN (thermal ellipsoids, 30%). Selected bond lengths /pm:

P1–O1 148.8(1), P1–C1 182.3(1), N1–O2 130.9(2), N2–O3 130.8(2).

Figure 4. Molecular structure and atom labeling scheme for 8a (ther-

mal ellipsoids 30%). Selected bond lengths /pm: P1–O1 148.8(1), P–

C7 183.1(2).

Figure 7. Molecular structure and atom labeling scheme for 5b (ther-

mal ellipsoids, 30%). Selected bond lengths /pm: P1–O1 148.6(9), P1–

C1 183.6(1), N1–O2 130.3(1), N2–O3 130.3(1).

white solids with reduced isolated yields: 11a (41%) and 11b

(33%). The N-oxidation reaction of 11a was examined with

excess H₂O₂/acetic acid mixtures at 70 °C (12 h), m-CPBA/

CHCl₃mixtures at 70 °C (12 h) and oxone^®/aq. NaHCO₃,

50 °C (12 h). In all variations examined, only 6a was isolated

with yields of 90%, 92% and 75%, respectively. The N-oxi-

dation of 11b with m-CPBA gave 6b in 85% yield. In no case

was complete N-oxidation, resulting in the formation of 7a,b,

Figure 5. Molecular structure and atom labeling scheme for 8b (ther-

mal ellipsoids, 30%). Selected bond lengths /pm: P1–O1 148.6(3),

184.6(4).

As far as we are aware, syntheses for the tetrafunctional accomplished. In an attempt to address this roadblock, the re-

ligands 11a,b, 6a,b and 7a,b have not been previously re- actions of 3a with nBuLi followed by addition of 2-(halome-

ported. Not surprisingly, attempts to prepare 11a,b via depro- thyl)pyridine were explored, but without success. The steric

tonation of 8a,b with LDA or nBuLi followed by addition of congestion that is indicated in the ligand modeling analysis for

2-(halomethyl)pyridine gave mixtures from which 11a,b were 7a may be responsible for hindering the complete N-oxidation.

not obtained in pure form. However, an adequate approach was

The HRMS spectra for the new compounds are consistent

developed that used the one-pot Grignard synthesis described with the proposed compositions. Infrared spectra for 11a,b and

above for 8a,b, modified to employ a 1:5:4 (EtO)₂P(O)H: 6a,b show a strong absorption in the range 1169–1180 cm^–1

ArMgBr: 2-(chloromethyl)pyridine combining ratio, as sum- that is assigned to the P=O stretching vibration, and the spectra

marized in Scheme 2. The crude yields of 11a,b were typically for 6a,b also contain a strong band at 1243 cm^–1assigned to

80% as estimated by ³¹P NMR spectroscopy. Subsequent chro- the N–O stretch. The ³¹P NMR spectra display a single reso-

matographic purifications provided spectroscopically pure nance in the range 37.6–41.2 ppm. It is noted that in the pyr-

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Synthesis and Properties of New (Phosphinoylmethyl)Pyridine N-Oxides

idine ring compound series, 3-Oa, 8a, 11a, and the pyridine and 5a,b. Once more, there is one short, non-bonded, backside

N-oxide compound series, 5a, 6a, 10a, the ³¹P resonance is P···F interaction: P1···F 293(4) pm.

regularly shifted downfield with each addition of a methyl pyr-

1

idine or methyl pyridine N-oxide fragment. The H and ¹³C

NMR spectra are in good agreement with the assigned struc-

tures of these compounds. There are possible ambiguities over

the assignment of the structure for the incomplete oxidation

products, 6a,b. Namely, the double N-oxidation of 11a,b may

take place, as proposed, on the two “long-arm” methyl pyr-

idine fragments or on one “long-arm” fragment and one “short

arm“ fragment as illustrated by 12. The ¹H and ¹³C NMR spec-

tra clearly show only two types of pyridine ring environments

as expected for the assigned structure instead of three environ-

ments as would be required for 12. This conclusion, based

upon NMR spectroscopic data, is confirmed by a X-ray crystal

structure analysis, vide infra.

Figure 9. Molecular structure and atom labeling scheme for 6b·MeOH

(thermal ellipsoids 30%) with lattice MeOH molecules omitted. Se-

lected bond lengths /pm: P1–O1 148.4(4), P1–C6 190.9(5).

2.3 Lanthanide(III) Coordination Chemistry

In the present study, 1:1 ligand:lanthanide complexation re-

actions of 8a,b, 5a,b, 11a,b and 6a,b were examined in MeOH

solution (23 °C, 2 h) with one or more of Ln(NO₃)₃·xH₂O

(Ln = La, Pr, Eu, Nd, Tb, Er, Lu), and the resulting complexes

were obtained as fine, colorless or faintly colored powders. In

general, compositional (CHN) analyses for these crude com-

plexes or recrystallized samples were most consistent with 1:1

formulae [Ln(L)(NO₃)₃·(solvent)] where the solvent was one

or more of H₂O, MeOH, CH₂Cl₂and/or MeCN). Unfortu-

nately, individual CHN percent compositions varied by Ϯ 1–

5% from expected theoretical values; therefore, the precise

solvent shell compositions remain unclear. Similar variable an-

alytical results have been observed previously with related

complexes where it is known from TGA analyses that inner

and/or outer sphere solvent molecules are loosely held and are

often partially lost during sample handling.^[11,12]Infrared spec-

tra for each complex showed a strong absorption in the region

1160–1140 cm^–1that can be definitively assigned to a Ln^III

coordinated P=O stretching vibration with coordination shifts

of 25–40 cm^–1relative to the respective free ligands. For the

complexes of 5a,b and 6a,b, the spectra also contain a medium

to strong intensity absorption in the range 1235–1220 cm^–1that

is tentatively assigned to the N–O stretching vibration. The

coordination shifts for the complexes containing ligands 5a,b

are small, 1–6 cm^–1, consistent with the absence of a pyridine

N-oxide/Ln^IIIinteraction. Coordination shifts for the com-

plexes containing 6a,b are slightly greater, 10–15 cm^–1, and

these may indicate the presence of modest pyridine N-oxide/

Ln interaction. Alternatively, the small shifts could result from

hydrogen bonding interactions between lattice solvent mole-

cules, H₂O or MeOH, and one or both pyridine N-oxide sites.

In summary, the IR data alone confirm, at the least, mono-

dentate Ln–O(P) binding by these ligands, but the presence

and nature of chelating interactions remains unresolved.

A crystal structure determination for 11b was completed,

and a view of the molecule is shown in Figure 8. The three

pyridine rings are planar and the N lone pair centroids are

twisted away from each other and from the P=O bond vector.

The P=O bond length is slightly longer than that in 8a, 8b,

5a and 5b and there is one short non-bonded P···F interaction

involving one of the CF₃groups: P1···F1, 297.0(1) pm. A mo-

lecular structure determination for 6b·MeOH removes any po-

tential ambiguity over the assignment of the sites of pyridine

N-oxidation deduced from the NMR analysis. The structure

(Figure 9) confirms that the double oxidation of 11b takes

place on both of the “long-arm” methyl pyridine fragments.

The N-oxide vectors are rotated away from the P=O bond vec-

tor and the P=O bond length is comparable to those in 8a,b

Figure 8. Molecular structure and atom labeling scheme for 11b (ther-

mal ellipsoids 30%). Selected bond lengths /pm: P1–O1 152.2(3), P1–

C15 189.5(1).

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Since the elemental analysis and infrared spectroscopic data greater than the value for 5a, but both were expected to be

give only limited information about the compositional and able to adopt tridentate chelate interactions on Ln^IIIions with

structural nature of these lanthanide complexes, considerable minimal steric stress. Indeed, 1:1 combinations of

effort was devoted to obtaining X-ray diffraction quality single (meNO)₂PO* with La(OTf)₃·xH₂O and Pr(OTf)₃·H₂O in

crystals for selected complexes that would provide unambigu- MeOH led to isolation of solid complexes that gave

ous identification of the compositions and ligand binding CHN analyses in agreement with the compositions

modes. Unfortunately, to date, from the complexes formed by La[(meNO)₂PO*](OTf)₃(MeOH)₃(H₂O) and Pr[(meNO)₂-

8a,b, 5a,b, 11a,b and 6a,b, only one complex, [Er(8a)(NO₃) PO*](OTf)₃(MeOH)₄. Infrared spectra for the complexes, un-

₃(MeOH)₂]·(CH₂Cl₂)₂has provided suitable crystals for single- fortunately, do not provide for assignment of reliable coordina-

crystal X-ray diffraction analysis. A view of the structure is tion shifts for the P=O and N–O modes since the region 1100–

shown in Figure 10. The Er^IIIion inner coordination sphere is 1300 cm^–1also contains very strong absorptions due to the

composed of three bidentate nitrate anions, two MeOH mole- triflate anion. However, single crystals for the complexes were

cules and one monodentate ligand 8a bonded through the phos- obtained, and the compositions and structures were unambigu-

phoryl oxygen atom. As expected, the two pyridine N-atom ously determined by X-ray diffraction analyses. Views of the

donor centers are well removed from the Er^IIIatom, and the inner sphere structures for {La[(meNO)₂PO*)](OTf)₂-

coordinated P=O bond length is elongated compared to the (MeOH)₃(H₂O)⁺}(OTf^–) and {Pr[(meNO)₂PO*)](OTf)-

+

distance in the free ligand. The acceptance by Er^IIIof two inner (MeOH)₄}(OTf^–)₂are shown in Figures 11 and 12. The inner

sphere bonded MeOH molecules as opposed to the nitrogen sphere arrangement around the La³⁺atom is a nine-coordinate

atoms of the pyridine or methyl pyridine fragments of 8a is monocapped square antiprism with the vertices of one square

consistent with the expected electronically weak affinity of face occupied by the three oxygen atoms of the tridentate li-

Er^IIIfor pyridine N-atom donors as well as with expected unfa- gand, (meNO)₂PO*, and one oxygen of a methanol molecule.

vorable ligand strains for either bidentate NPO or tridentate Two oxygen atoms of two OTf^–anions and two oxygen atoms

NNPO interactions.

of two methanol molecules occupy the vertices of the other

square face with one molecule of water capping the face. As

often found, the La–O_P1=O3distance, 245.0(2) pm is slightly

shorter than the La–O_N1–O1and La–O_N2–O2distances, 248.2(2)

pm and 251.7(2) pm, respectively. The latter two are more

similar than found in a number of transition metal complexes

of the ligand. The P=O bond length, 149.3(2) pm, and N–O

bond lengths, 133.1(2) and 132.4(3) pm are within the range

of values reported in other work.^{[11,12,16,23]}The average La–O

distance involving the inner sphere triflates, and methanol and

Figure 10. Molecular structure and atom labeling scheme for

[Er(8a)(NO₃)₃(MeOH)₂]·CH₂Cl₂(thermal ellipsoids 30%). Selected

bond lengths /pm: Er1–O2 226.4(3), Er–O_nitrate(range): 239.0(4)–

245.7(4); Er–O_nitrate(avg): 243 Ϯ 2, P1–O1 150.4(4), P1–C15

183.6(5).

Although somewhat of a structural departure from the

NO(meNO)PO ligands 5a,b and the as yet inaccessible

NO(meNO)₂PO ligands 7a,b, the results of a study of the lan-

thanide coordination chemistry for the more symmetrical

(NO)₂PO donor group ligand (meNO)₂PO* is presented here

for comparison. Recall in the ligand modeling analysis, the

Figure 11. Inner sphere molecular structure and atomic numbering

scheme for {La[(meNO)₂PO*)](OTf)₂(MeOH)₃(H₂O)⁺}(OTf^–) (ther-

mal ellipsoids shown at the 50% level) with hydrogen atoms removed

for clarity. Only the major position for the phenyl ring is shown. Se-

lected bond lengths /pm: La1–O1 248.2(2), La1–O2 2.51.7(2), La1–

O3 2.450(2), La1–O8 2.555(2), La1–O11 2.547(2), N1–O1 133.1(2),

calculated ΔG_rel/donor value for (meNO)₂PO* was slightly N2–O2 132.4(3), P1–O3 149.3(2).

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Synthesis and Properties of New (Phosphinoylmethyl)Pyridine N-Oxides

water molecules is 257 Ϯ 3 pm, with the shortest involving also that interest in the new ligands described here, particu-

one triflate anion (La1–O11 = 254.7(2) pm) and the longest larly, 5a,b and 7a,b, was focused on determining if placement

for the interaction with the molecule of water (La1–O4 = of one or two additional 2-(methyl)pyridine N-oxide fragments

260.7(3) pm).

on the 3a framework would favorably or unfavorably impact

coordination and extraction performance. Since 7a,b, at this

point, are synthetically inaccessible, we chose to characterize

the extractions of Eu^IIIand Am^IIIfrom nitric acid solutions

with 0.01 m solutions of 5a,b and 6a,b in 1,2-dichloroethane

(DCE) and to compare these results with extractions using 3a

recorded under identical conditions (25 °C, 0.01 m in DCE). In

addition, for comparison, the extraction performance for

Ph₃P(O) (13) (25 °C, 0.1 m in DCE) was examined. The varia-

tions in distribution ratios, D = [M_org]/[M_aq], with increasing

HNO₃concentration (0.01–4 M) for 3a, 5a,b and 6b are shown

in Figure 13. Data for 6a are not included due to its low solu-

bility in DCE. The shape of the extraction curve for 3a in

DCE obtained here is similar to that for the ligand in toluene

solution.^[14a]In the present case, 3a displays increasing D val-

ues with increasing acid strength from 0.01 m to 0.5 m before

starting to decrease. In addition, the D values, at all acid con-

centrations for both Eu^IIIand Am^III, are significantly greater

for 3a in DCE solutions than in toluene solutions. On the other

hand, the behaviors of 5a,b and 6b are noticeably different

from 3a. Most importantly, in the context of this report, the D

values for 5a,b and 6b, at the same [HNO₃], are significantly

reduced compared to 3a, and the D values tend to decrease

with increasing nitric acid concentration above the initial acid

concentration of 0.01 m. These results clearly indicate that the

addition of one methylpyridine N-oxide arm (in 5a,b) and the

removal of the “short-arm“ N-oxide oxygen atom, as well as

addition of a second “long-arm” methylpyridine N-oxide donor

Figure 12. Inner sphere molecular structure and atomic numbering

scheme for {Pr[(meNO)₂PO*)](OTf)(MeOH)₄⁺}(OTf^–)₂(thermal el-

lipsoids shown at the 50% level) with hydrogen atoms and outer

sphere triflate ions omitted for clarity. Only the major position for the

disordered triflate ion is shown. Selected bond lengths /pm: Pr1–O1

241.6(2), Pr1–O2 240.9 (2), Pr1–O3 242.3(2), N1–O1 132.9(3), N2–

O3 133.7(3), P1–O2 1.500(2).

In contrast, the Pr⁺³ion resides in a slightly distorted eight- (in 6a) significantly degrade the extraction performances com-

coordinate square antiprismatic environment. The tridentate li- pared to 3a. This is consistent with the conclusion that ligand

gand and a MeOH oxygen atom occupy the vertices on one of binding is probably reduced from bidentate NOPO in 3a to

the square faces. The vertices on the other square face are

occupied by three oxygen atoms of three MeOH molecules and

one oxygen atom of one coordinated triflate anion. The Pr–

O_P1=O2distance, 240.9(2) pm is 4.1 pm shorter than the dis-

tance La–O_P1=O3in-line with the difference (6 pm) in effective

ionic radii: La⁺³(CN = 9), 120 pm; Pr⁺³(CN = 8), 114 pm.^[24]

The distances Pr–O_N1–O1,241.6(2) pm and Pr–O_N2–O3

,

242.3(2) pm, are slightly longer than Pr–O_P1=O2, and they are

contracted by 6.6 pm and 9.4 pm compared to the La–O_N–O

distances. The P=O and the two N–O bond lengths are similar

to those in the La^IIIcomplex: P1=O2, 150.0(2) pm; N1–O1,

132.9(3) pm and N2–O3, 133.7(3) pm. Additionally, the Pr^III

complex presents intermolecular hydrogen bonding between

one of the oxygen atoms on the coordinated triflate and the

hydrogen atoms of two adjacent methanol units.

2.4 Extraction Analyses

As noted above, the extraction properties of 3a in toluene

solution toward Eu^IIIand Am^IIIin nitric acid solutions have

been previously characterized.^[14a]The ligand was found to

have modest extraction efficiency with distribution ratios (D’s)

Figure 13. Americium and europium distribution ratios as a function

of the initial nitric acid concentration. Organic phase 3a, 5a, 5b, 6b at

10 mM, and Ph₃PO (13) at 100mM in 1,2-DCE. Aqueous phase: trace

²⁴¹Am and 0.1 mM of europium nitrate in nitric acid. O/A = 1, T =

increasing with increasing [HNO₃] from 0.01 to 1 m. Recall 25 °C.

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R. T. Paine et al.

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monodentate PO in 5a,b and 6b. For comparison, D values for prohibited extraction analysis, but extraction data for 6b indi-

Ph₃P(O), 13, in DCE are included in Figure 13. At first glance, cated that this compound behaves as a weak extractant more

13 appears to be a stronger extractant than 5a,b and 6b, but like 5a,b and 13 than 3a. Hence, it is concluded that addition

the D values were necessarily measured with extractant solu- of one 2-(methyl)pyridine N-oxide fragment to 3a,b or two 2-

tions ten times more concentrated than the solutions of the (methyl)pyridine N-oxide fragments to 3-Oa adversely impact

other extractants due to the very weak extraction at all acid extraction performance compared to 3a. Whether the degraded

strengths by 13. Clearly, this monofunctional phosphine oxide extraction performances result from donor field strength

displays the smallest D values of the five extractants. Since changes, ligand steric affects or a combination of these influ-

the D values and the solubilities of all of the ligands in DCE ences on extraction complex structures, solubilities and sta-

are modest, the ligand dependencies for the extractions were bilities is presently unclear. The weakened extraction perform-

not determined. In order to accomplish those measurements, ance may also reflect reduced extraction complex phase par-

more DCE soluble derivatives should be prepared.

titioning characteristics. Additional studies with these systems

would be required to better understand the results of these de-

sign/composition changes. In this regard, extraction studies

with the more symmetrical (meNO)₂PO* ligand, that readily

forms 1:1 Ln^IIIcomplexes with tridentate chelate structures,

are planned in the future.

3 Conclusion

The original objective of this study was to determine if ad-

dition of 2-(methyl)pyridine N-oxide fragments to the exo-sp³

carbon atom of 3a would favorably or unfavorably impact the

ligand’s coordination ability and extraction performance. Mo-

lecular modeling of one of the target molecules, 5a, suggested

4 Experimental Section

that it should be able to accommodate

a tridentate

4.1 Materials and Measurements

NO(meNO)PO binding motif on a Ln^IIIcation with a relative

binding free energy ΔG_rel/donor greater than that for the par-

ent bifunctional platform NOPO ligand 3a, but smaller than

that for the more geometrically symmetric, trifunctional

(meNO)₂PO* ligand. The ligands 5a,b were successfully pre-

pared, structurally characterized and selected coordination

chemistry explored. Lanthanide complexes (1:1) were isolated,

and the solids found to display IR stretching frequency coordi-

nation shifts, Δν_PO, consistent with monodentate phosphine ox-

ide binding. Unfortunately, single crystals, suitable for X-ray

diffraction analysis, were not obtained, and the ligand binding

mode in the solid state was not unambiguously confirmed.

However, comparison of the extraction performances for 3a

and 5a,b show dramatic differences. Notably, the performance

of 5a,b more closely compares with the performance of the

monofunctional phosphine oxide 13. Clearly the addition of a

weakly donating, “long-arm” 2-(methyl)pyridine N-oxide

group to the 3a platform at the exo-carbon atom degrades the

extraction performance of 3a.

Molecular modeling for the second target ligand class, 7a,

indicated that this ligand could not realistically adopt a tetra-

dentate chelate mode on a Ln^IIIcation. Furthermore, all

attempts to accomplish full, triple N-oxidation of 11a,b that

would result in 7a,b failed. Instead, the double N-oxidation

products 6a,b were obtained in high yields. This outcome sug-

gests that, if these ligands were available, steric features would

not only hinder tetradentate binding by 7a,b, but also might be

responsible for retarding N-oxidation of the “short-arm“ pyr-

idine fragment of 11a,b. Molecular modeling of three possible

tridentate binding motifs for 6a indicated that a mixed “short-

arm”/”long-arm” NO(meNO)PO ligand construct (motif 1)

would provide a reasonably small ΔG_rel. Indeed, 6a,b readily

form 1:1 complexes with Ln(NO₃)₃, but IR spectra do not pro-

vide for clear assignment of the binding mode. Furthermore,

suitable single crystals for X-ray crystallographic analyses

Organic reagents were purchased from Aldrich Chemical Co., and they

were used as received. Organic solvents were obtained from VWR,

and these were dried by standard procedures. The lanthanide nitrates

were purchased from Ventron. Reactions were generally performed un-

der a dry nitrogen atmosphere by using Schlenk techniques. Infrared

spectra were recorded from KBr pellets on a Bruker Tensor 27

benchtop FTIR spectrometer, and NMR spectra were measured with

Bruker FX-250 and Avance 500 and 300 spectrometers (23 °C) and a

Varian VNMR 500 spectrometer (25 °C) using Me₄Si (¹H and ¹³C)

and 85% H₃PO₄(³¹P) as chemical shift standards. Downfield shifts

were assigned + δ values. Mass spectra were obtained from the UNM

Mass Spectrometry Center, and elemental analyses were performed by

Galbraith Laboratories.

4.2 Ligands and Intermediates Syntheses

4.2.1 2-(Halomethyl)pyridine

A

sample of 2-(bromomethyl)pyridine hydrobromide (20.0 g,

79 mmol) was dissolved in a mixture of water (200 mL) and dichloro-

methane (200 mL), and the combination was cooled (0 °C). A satu-

rated aqueous solution of NaHCO₃was added with stirring until the

aqueous phase reached pH = 7. The phases were separated, the organic

phase vacuum evaporated and dried leaving a red oil of 2-(bro-

momethyl)pyridine. Yield: 11.4 g (84%). This compound decomposes

upon standing at 23 °C, and it should be used promptly in reactions.

Samples of 2-(chloromethyl)pyridine were prepared in a similar fash-

ion.

4.2.2 2-[(Diphenyl)phosphinoylmethyl]pyridine, (3-Oa)

A sample of phenylmagnesium bromide (65 mL, 0.1 m solution in

THF, 65 mmol) was added to a nitrogen purged flask outfitted with a

reflux condenser. To this solution was added dropwise (EtO)₂P(O)H

(3.0 g, 22 mmol) in dry THF (25 mL). During the addition (0.5 h), the

temperature rose, and, following addition, the mixture was then heated

to reflux (1 h). The mixture was cooled (23 °C), and 2-bromometh-

ylpyridine (3.7 g, 22 mmol) in THF (20 mL) was added. The resulting

were not obtained. Poor ligand solubility for 6a in DCE also mixture was heated to reflux (12 h) and afterwards quenched with

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Synthesis and Properties of New (Phosphinoylmethyl)Pyridine N-Oxides

saturated aqueous NH₄Cl solution (70 mL). The organic phase was

separated, and the aqueous phase extracted with CH₂Cl₂(4 ϫ 50 mL).

dried with Na₂SO₄. The solution was vacuum evaporated and the crude

mixture flash chromatographed (CH₂Cl₂/MeOH, 98/2 to 95/5) leaving,

The combined organic phases were dried (Na₂SO₄) and vacuum evapo- 8a as a white powder. Yield: 1.98 g (71%), m.p. 151–153 °C. Single

rated leaving 2-[diphenyl)phosphinolymethyl]pyridine (3-Oa) as a

light tan solid (5.0 g, 77%). Minor impurities were removed by column

chromatography (SiO₂gel, 2/98 MeOH/CHCl₂). Yield: 4.2 g (66%),

m.p. 134–136 °C. Alternatively, a diphenylphosphonomanesium brom-

ide/ THF Grignard solution, Ph₂P(O)MgBr/THF, prepared as described

crystals for X-ray analysis were obtained by slow evaporation of a

solution of 8a in CH₂Cl₂/Et₂O. Elemental analysis: C₂₄H₂₁N₂OP:

calcd. C 74.99, H, 5.51, N, 7.29%; found: C, 74.82; H, 5.52; N,

7.23%. HRESI-MS: m/z (%)

=

385.1489 (100) [M

+

H⁺].

C₂₄H₂₂N₂OP requires 385.1470; 407.1321 (10) [M

+

Na⁺],

above (1 equiv.), was combined with 2-(chloromethyl)pyridine C₂₄H₂₁N₂NaOP requires 407.1289. FT-IR (KBr (cm^–1)): ν = 1590 (s),

(1 equiv.) in THF (10 mL), and the mixture was heated to reflux over-

1569 (m), 1474 (s), 1436 (vs), 1346 (w), 1180 (vs, v_PO), 1118 (m),

night. Product workup, as described above, provided 3-Oa. Yield: 1105 (m), 1072 (w), 1053 (w), 1026 (w), 997 (m), 814 (m), 799 (m),

91%, m.p 134–136 °C. ESI-MS: m/z = 295 (100) [M + H⁺]. ³¹P{¹H}

NMR (101.2 MHz, CDCl₃): δ = 30.4 ppm. ¹H NMR (500 Mz,

CDCl₃): δ = 3.94 (d, J_CP= 17.2 Hz, 2 H, CH₂), 7.07 (m, 1 H, H_pyr),

750 (s), 721 (s), 699 (s), 629 (w), 593 (w), 551 (m), 534 (s), 510 (s).

³¹P{¹H} NMR (101.2 MHz, CDCl₃): δ = 33.5 ppm. ¹H NMR

(500 MHz, CDCl₃): δ = 3.49 (d, J_HH= 14.0 Hz, 1 H, CH₂), 3.68 (dd,

7.43–7.47 (m, 6 H, H_Ph), 7.48 (m, 1 H, H_pyr), 7.55 (t, J_HH= 7.5 Hz, 1 J_HH= 14.5, J_HH= 11.0 Hz, 1 H, CH₂), 4.70 (m, 1 H, CH), 6.86 (d,

H, H_pyr), 7.74–7.78 (m, 4 H, H_Ph), 8.38 (d, J_HH= 6.0 Hz, 1 H, H_pyr).

¹³C{¹H} NMR (125.7 Mz, CDCl₃): δ = 41.0 (d, J_CP= 64.7, CH₂),

121.2 (C_pyr), 125.0 (C_pyr), 128.5 (d, J_CP= 11.8 Hz, C_Ph), 131.1 (d, J_CP

J_HH= 7.5 Hz, 2 H, CH_pyr), 6.92 (m, 1 H, CH_pyr), 7.24–7.27 (m, 2 H,

CH_Ph), 7.30–7.34 (m, 2 H, CH_pyr), 7.40–7.49 (m, 5 H, CH_Ph/pyr), 7.63

(pseudo t, J_HH= 7.5, J_HP= 8.0 Hz, 2 H, CH_Ph), 7.96 (pseudo, J_HH

=

= 9.3 Hz, C_Ph), 131.8 (C_Ph), 132.4 (d, J_CP= 100.4 Hz, C_qPh), 136.4 7.0, J_HP= 8.5 Hz, 2 H, CH_Ph), 8.26 (d, J_HH= 4.5 Hz, 1 H, CH_pyr), 8.42

2

(C_pyr), 149.2 (C_pyr), 152.5 (d, J_CP= 7.7 Hz, C_qpyr).

(d, J_HH= 4.5 Hz, 1 H, CH_pyr). ¹³C{¹H} NMR (125.7 MHz, CDCl₃): δ

= 37.2 (CH₂), 48.3 (d, J_CP= 64.8 Hz, CH), 121.4 (C_pyr), 121.8 (C_pyr),

123.8 (C_pyr), 124.8 (C_pyr), 128.2 (d, J_CP= 10.3 Hz, C_Ph), 128.8 (d, J_CP

= 10.9 Hz, C_Ph), 131.4 (d, J_CP= 9.4 Hz, C_Ph), 131.5 (C_Ph), 131.6 (d,

J_CP= 9.5 Hz, C_Ph), 131.9 (C_Ph), 132.1 (d, J_CP= 93.3 Hz, C_qPh), 136.0

(C_pyr), 136.3 (C_pyr), 149.0 (C_pyr), 149.6 (C_pyr), 156.4 (C_qpyr), 158.7 (d,

J_CP= 13.2 Hz, C_qpyr).

4.2.3 2-[1-(Diphenylphosphinoyl)-1-(pyridin-2-yl)-

methyl]pyridine, (8a)

Method 1. Under dry nitrogen, diisopropylamine (0.137 g, 0.20 mL,

1.36 mmol) was added (0 °C) dropwise to a nBuLi solution (0.85 mL,

1.6 m in hexane, 1.36 mmol) in hexane (2 mL) and stirred (23 °C,

5 min.).^[21]The hexane was vacuum evaporated, and THF (1 mL) was

added to the residue. A solution of 3-Oa (0.20 g, 0.68 mmol) in THF

(3 mL) was added dropwise with stirring (0 °C), and the resulting mix-

ture was stirred (0 °C, 30 min). Afterwards, a solution of 2-(bro-

momethyl)pyridine (0.130 g, 0.76 mmol) in THF (2 mL) was added

dropwise, and the mixture was heated to reflux (2 h). The resulting

mixture was quenched with saturated aqueous NH₄Cl solution (5 mL),

the phases separated, and the aqueous phase was extracted with

CH₂Cl₂(3 ϫ 5 mL). The combined organic phases were dried with

Na₂SO₄and concentrated. The residue was purified by column

chromatography (silica gel, MeOH/CH₂Cl₂2/98 eluant) leaving 8a as

a white solid. Yield: 0.21 g (81%).

4.2.4 2-[1-(Bis(2-(trifluoromethylphenyl))phosphinoyl)-1-

(pyridin-2-yl)methyl]pyridine (8b)

A solution of 2-bromobenzotrifluoride (4.14 mL, 30.4 mmol) in dry

THF (5 mL) was added dropwise to magnesium turnings (0.74 g,

30.4 mmol) in dry THF (10 mL) and stirred (2 h). A solution of diethyl

phosphite (930 μL, 7.24 mmol) in dry THF (5 mL) was then added

dropwise, the resulting mixture was heated to reflux (1 h) and after-

wards cooled (23 °C). A solution of 2-(chloromethyl)pyridine (2.3 g,

18.0 mmol) in THF (15 mL) was added, and the reaction mixture was

stirred under reflux (24 h). The resulting mixture was quenched with

saturated aqueous NH₄Cl solution (100 mL), and the aqueous phase

was extracted with CH₂Cl₂(3 ϫ 30 mL). The combined organic frac-

tions were dried with MgSO₄and vacuum evaporated. The crude resi-

due obtained was purified by column chromatography (silica gel,

CH₂Cl₂/MeOH 99/1 to 95/5) leaving 8b as a white powder. Yield:

1.69 g (45%), m.p. 216–218 °C. Single crystals, suitable for X-ray dif-

fraction analysis, were obtained by slow evaporation of a solution of

8b (20 mg) in CH₂Cl₂/Et₂O (4/1, 5 mL). HRESI-MS: m/z (%) =

521.1209 (100) [M + H⁺]. C₂₆H₂₀F₆N₂OP requires 521.1217; 543.1035

(96) [M + Na⁺]. C₂₆H₁₉F₆N₂OPNa requires 543.1037. FT-IR (KBr

(cm^–1)): 1590 (m), 1569 (m), 1471 (s), 1434 (s), 1315 (vs), 1264 (w),

1191 (s), 1177 (vs,v_P=O), 1133 (vs), 1113 (vs), 1035 (m), 995 (w), 780

(s), 752 (w), 645 (w), 604 (w), 540 (s), 520 (m), 500 (w). ³¹P{¹H}

NMR (121.5 MHz, CDCl₃): δ = 33.7 ppm. ¹H NMR (300 MHz,

CDCl₃): δ = 3.56–3.65 (m, 1 H, CH₂), 3.74–3.85 (m, 1 H, CH₂), 5.06

(td, J_HH= 2.8, J_HP= 11.2 Hz, 1 H, CH), 6.86 (d, J_HH= 7.5 Hz, 2 H,

CH_pyr), 6.98 (dd, J_HH= 5.4, J_HH= 6.9 Hz, 1 H, CH_pyr), 7.31–7.44 (m,

Method 2. An aliquot of nBuLi (1.05 mL, 2.5 m in hexane, 2.6 mmol)

was added to a solution of 3-Oa (0.7 g, 2.4 mmol) in THF (50 mL)

and stirred (23 °C, 30 min). Separately, Et₃N (0.72 mL, 5 mmol) was

added to a suspension of 2-(bromomethyl)pyridine hydrobromide

(1.21 g, 4.8 mmol) in THF (70 mL), the combination was stirred

(30 min.), and the resulting mixture filtered. The filtrate was then

added dropwise to the putative [Li⁺][3-Oa^–] solution (23 °C), and the

combination was stirred (12 h). A cloudy mixture was obtained that

was quenched with saturated aqueous NaHCO₃solution (50 mL). The

phases were separated, and the aqueous phase was extracted with

CH₂Cl₂(3 ϫ 20 mL), the combined organic phases were washed with

brine solution and dried with Na₂SO₄. The volatiles were vacuum

evaporated leaving a pink residue that was purified by column

chromatography (SiO₂, MeOH/CH₂Cl₂(5/95) providing 8a as a white

solid. Yield: 0.64 g (69%).

Method 3. Freshly distilled diethylphosphite (1.0 g, 7.2 mmol) in dry 5 H, CH_Ar+pyr), 7.55 (pseudo t, J_HH= 5.5 Hz, 1 H, CH_pyr), 7.68 (pseudo

THF (10 mL) was added to phenylmagnesium bromide (30.4 mL, 1 m

in THF, 30.4 mmol). The mixture was heated to reflux (1 h), cooled

(23 °C), and 2-(chloromethyl)pyridine (2.31 g, 18.2 mmol) in THF

t, J_HH= 7.3 Hz, 1 H, CH_pyr), 7.76–7.83 (m, 2 H, CH_Ar), 8.18–8.23 (m,

2 H, CH_Ar), 8.49 (d, J_HH= 4.2 Hz, 1 H, CH_pyr), 8.60 (dd, J_HH= 7.8,

J_HP= 12.6 Hz, 1 H, CH_pyr). ¹³C{¹H} NMR (75.5 MHz, CDCl₃): δ =

(15 mL) was added dropwise. The mixture was heated to reflux (12 38.0 (CH₂), 48.6 (d, J_CP= 70.0 Hz, CH), 121.5 (C_pyr), 121.8 (C_pyr),

h), and afterwards cooled and quenched with saturated aqueous NH₄Cl

(200 mL) solution. The phases were separated, the aqueous phase ex-

tracted with CH₂Cl₂(3 ϫ 50 mL), and the combined organic phases

123.8 (C_pyr), 125.2 (d, J_CP= 3.4 Hz, C_pyr), 126.7 (m, C_Ar), 128.2 (m,

C_Ar), 128.3 (d, J_CF= 266 Hz, C_q(CF3)),130.6–132.1 (C_qAr+C_Ar), 133.0

(d, J_CF= 37.3 Hz, C_qAr) 134.8 (d, J_CP= 8.8 Hz, C_Ar), 135.4 (d, J_CP

=

Z. Anorg. Allg. Chem. 2013, 1101–1116

R. T. Paine et al.

6.3 Hz, C_Ar), 136.1 (C_py), 149.1 (C_pyr), 149.6 (C_pyr), 156.1 (C_qpyr), (C_Ar), 129.8 (d, J_CP= 81.6 Hz, C_qAr), 130.6 (d, J_CP= 12.1 Hz, C_Ar),

ARTICLE

158.5 (d, J_CP= 14.6 Hz, C_qpyr).

131.6 (d, J_CP= 11.8 Hz, C_Ar), 131.9 (C_Ar), 132.4 (C_Ar), 132.8 (C_qAr),

134.2 (d, J_CP= 7.9 Hz, C_Ar), 135.5 (d, J_CP= 6.9 Hz, C_Ar), 138.9 (C_pyr),

139.7 (C_pyr), 147.0 (C_qpyr), 148.0 (d, J_CP= 14.8 Hz, C_qpyr).

4.2.5 2-[1-(Diphenylphosphinoyl)-1-(1-oxy-pyridin-2yl)-

methyl]pyridine N-oxide (5a)

4.2.7 2-[1-(Diphenylphosphinoyl)-1-(methylpyridin-2-yl)-

methyl]pyridine N-oxide (10a)

A sample of 8a (0.5 g, 1.3 mmol) was dissolved in a mixture of acetic

acid (4 mL) and 35% hydrogen peroxide (4 mL), and the mixture was

heated (12 h, 75 °C). The resulting mixture was vacuum evaporated, A sample of 9a (1.2 g, 3.88 mmol) in dry THF (10 mL) was cooled

the residue dissolved in CH₂Cl₂(20 mL), mixed with saturated aque-

ous NaHCO₃solution and adjusted to pH = 8. The organic phase was

collected, washed with brine solution and dried with Na₂SO₄. Evapora-

(0 °C) and nBuLi (2.24 mL, 1.6 m in THF, 3.88 mmol) was added

dropwise with stirring. The mixture turned dark red, and it was slowly

warmed to 23 °C (30 min). A sample of 2-(bromomethyl)pyridine

tion of the volatiles left 5a as a light yellow powder. Yield: 0.49g (0.67 g, 3.88 mmol) in THF (10 mL) was added slowly with stirring,

(92%), m.p. 217–220 °C. Single crystals for X-ray diffraction analysis and the mixture became red-yellow in color. The stirring was contin-

were obtained by slow evaporation of a solution of 5a (50 mg) in ued (12 h) and then the mixture was quenched with saturated NaHCO₃

MeCN/MeOH (9/1, 10 mL). Elemental analysis: C₂₄H₂₁N₂O₃P: calcd. solution (100 mL). The mixture was extracted with CH₂Cl₂(3 ϫ

C 69.22; H, 5.08; N, 6.73; found: C, 68.75; H, 5.39; N 6.44. HRESI- 30 mL), the combined organic phases dried with Na₂SO₄and the vola-

MS: m/z (%) = 417.1358 (33) [M + H⁺]. C₂₄H₂₂N₂O₃P requires

tiles removed by vacuum evaporation. The residue was purified by

column chromatography (silica gel, CH₂Cl₂/MeOH 98/2 to 95/5) pro-

417.1368; 439.1163 (100) [M + Na⁺]. C₂₄H₂₁N₂NaO₃P requires

439.1187. FT-IR (KBr (cm^–1)): ν = 1487 (s), 1451 (m), 1437(s), 1425 viding 10a as a white solid. Yield: 0.71 g (46%), m.p. 196–198 °C.

(vs), 1313 (w), 1257 (s), 1231 (vs,v_NO), 1188 (vs, v_PO), 1143 (m), 1116

Elemental analysis: (10a + MeOH) C₂₅H₂₅N₂O₃P: calcd. C 69.43; H,

(m), 1107 (m), 1069 (w), 1048 (w), 987 (w), 881 (w), 844 (m), 774 5.83; N, 6.48. found: C, 69.82; H, 5.26; N, 6.63. HRESI-MS: m/z =

(vs), 759 (m), 745 (m), 718 (s), 691 (s), 617 (m), 561 (s), 532 (s), 519 401.1403 (100) [M + H⁺]. C₂₄H₂₂N₂O₂P requires 401.1419; 423.1235

(s), 497 (m), 476 (m), 441 (m). ³¹P{¹H} NMR (101.2 MHz, CDCl₃):

(18) [M + Na+]. C₂₄H₂₁N₂NaO₂P requires 423.1238. FT-IR (KBr

1

δ = 34.9 ppm. H NMR (500 MHz, CDCl₃): δ = 3.54 (m, 1 H, CH₂), (cm^–1)): ν = 1582 (s), 1479 (s), 1429 (vs), 1333 (w), 1322 (s), 1292

3.67 (m, 1 H, CH₂), 6.10 (m, 1 H, CH), 6.93–6.98 (m, 2 H, CH_pyr), (s), 1208 (vs, ν_NO), 1182 (vs, ν_PO), 1107 (vs), 991 (s), 931(w), 845

7.01–7.03 (td, J_HH= 1.9, J_HH= 7.5 Hz, 1 H, CH_pyr), 7.12–7.15 (m, 2

(s), 789 (vs), 689 (vs), 602 (vs), 545 (vs), 503 (vs). ³¹P{¹H} NMR

H, CH_Ph), 7.26–7.29 (m, 2 H, CH_Ph), 7.34 (t, J_HH= 6.7 Hz, 1 H, (101.2 MHz, CDCl₃): δ = 34.6 ppm. ¹H NMR (500MHz, CDCl₃): δ =

CH_pyr), 7.53 (m, 3 H, CH_Ph+py), 7.75 (dd, J_HH= 7.6, J_HP= 11.4 Hz, 2 3.52–3.57 (m, 2 H, CH₂), 5.89–5.92 (m, 1 H, CH), 6.86–6.92 (m, 2

H, CH_Ph), 7.90 (d, J_HH= 6.4 Hz, 1 H, CH_pyr), 7.97 (d, J_HH= 7.0 Hz, 1 H, CH_pyr), 7.01–7.07 (m, 2 H, CH_pyr), 7.19–7.36 (m, 4 H, CH_Ph), 7.58

H, CH_pyr), 8.10–8.12 (m, 3 H, CH_Ph+pyr). ¹³C{¹H} NMR (125.7 MHz, (m, 3 H, CH_Ph+pyr), 7.74 (pseudo t, 2 H, J_HP= 9.4 Hz, CH_Ph), 7.85

CDCl₃): δ = 31.6 (CH₂), 33.2 (d, J_CP= 69.5 Hz, CH), 124.2 (C_pyr),

124.5 (C_pyr) 125.2 (C_pyr), 126.1(C_pyr), 126.8 (C_pyr), 128.4 (d, J_CP

11.4 Hz, C_Ph), 129.0 (d, J_CP= 10.4 Hz, C_Ph), 130.7 (m, C_Ph), 131.4

(m, C_Ph), 131.9 (d, J_CP= 76.5 Hz, C_qPh), 132.0 (C_Ph), 132.4 (C_Ph),

139.1 (C_py), 139.7 (C_py), 148.0 (C_qpy), 148.5 (C_qpy).

(pseudo t, 2 H, J_HP= 8.6 Hz, CH_Ph), 7.98 (pseudo t, 2 H, J_HH= 8.1 Hz,

CH_pyr), 8.31 (s, 1 H, CH_pyr).¹³C{¹H} NMR (125.7 MHz, CDCl₃): δ =

36.9 (d, J_CP= 66.2 Hz, CH), 37.0 (CH₂), 121.5 (C_pyr), 122.8 (C_pyr),

123.7 (C_pyr), 125.1 (C_pyr), 126.6 (C_pyr), 128.1 (d, J_CP= 12.2 Hz, C_Ph),

128.7 (d, J_CP= 11.8 Hz, C_Ph), 130.5 (d, J_CP= 9.8 Hz, C_Ph), 131.2 (d,

J_CP= 8.7 Hz, C_Ph), 131.5 (d, J_CP= 95.9 Hz, C_qPh), 131.7 (C_Ph), 131.9

(C_Ph), 136.2 (C_pyr), 138.8 (C_pyr), 148.2 (C_qpyr), 149.3 (C_pyr), 157.6

(C_qpyr).

=

4.2.6 2-[1-(Bis(2-(triluoromethylphenyl))phosphinoyl)-1-(1-

oxy-pyridin-2yl)methyl]pyridine N-oxide (5b)

To a solution of 8b (1.33 g, 2.56 mmol) in dry CH₂Cl₂(10 mL), was

slowly added m-chloroperoxybenzoic acid (77 wt.-%, 1.32 g,

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

ture diluted with additional CH₂Cl₂(90 mL) and washed with aqueous

NaOH (2N, 3 ϫ 30 mL) and distilled water (2 ϫ 10 mL). The organic

4.2.8 2-[1-(Diphenylphosphinoyl)-1,1-bis(methylpyridin-2yl)-

methyl]pyridine (11a)

A sample of phenylmagnesium bromide (36.2 mL, 1 m solution in

THF, 36.2 mmol) was placed in a nitrogen purged Schlenk flask out-

phase was dried with MgSO₄, filtered and the volatiles removed by fitted with

a reflux condenser. A solution of freshly distilled

vacuum evaporation, leaving 5b as a white powder. Yield: 1.4 g

(99%), m.p. 230–232 °C. Suitable single crystals for X-ray analysis

were obtained by slow evaporation of a solution of 5b (20 mg) in

(EtO)₂P(O)H (1.0 g, 7.2 mmol) in dry THF (10 mL) was added drop-

wise with stirring. The temperature of the mixture rose during the

addition. When the addition was complete, the mixture was heated

CH₂Cl₂/Et₂O (4/1, 5 mL). HRESI-MS: m/z (%): 553.1121 (30) [M + (50 °C, 1 h) and afterwards cooled (23 °C). A sample of 2-(chlo-

H⁺]. C₂₆H₂₀F₆N₂O₃P requires 553.1116; 575.0943 (100) [M + Na⁺].

C₂₆H₁₉F₆N₂O₃PNa requires 575.0935. FTIR (KBr (cm^–1)): 1487 (s),

romethyl)pyridine (3.68 g, 28.9 mmol) in THF (10 mL) was added and

the mixture heated (50 °C, 12 h). The mixture was then cooled and

1427 (s), 1314 (vs), 1265 (s), 1231 (m, v_N–O), 1182 (vs, v_P=O), quenched with saturated aqueous NH₄Cl solution (100 mL). The or-

1121(vs), 1034 (s), 891(w), 870 (w), 846 (m), 776 (vs), 714 (m), 646 ganic layer was separated, the aqueous layer treated with CH₂Cl₂(3

(m), 614 (w), 599 (w), 559 (w), 536 (w), 520 (w), 484 (m). ³¹P{¹H}

ϫ 50 mL), the combined organic layers washed with distilled water (2

NMR (121.5 MHz, CDCl₃): δ = 36.4 ppm. ¹H NMR (300 MHz, ϫ 20 mL), dried with Na₂SO₄and then evaporated to dryness. The

CDCl₃): δ = 3.40–3.51 (m, 1 H, CH₂), 3.70–3.78 (m, 1 H, CH₂), 6.28 residue was purified by flash chromatography (SiO₂gel, MeOH/

(pseudo t, J_HP= 9.6 Hz, 1 H, CH), 6.93–7.00 (m, 3 H, CH_pyr), 7.04– CH₂Cl₂, 2/98, 5/95) leaving 11a as a colorless solid. Yield: 1.4 g

7.10 (m, 1 H, CH_pyr), 7.15 (t, J_HH= 7.6 Hz, 1 H, CH_pyr), 7.46 (pseudo (41%), m.p. 62–64 °C. Elemental analysis: C₃₀H₂₆N₃OP; calcd. C

q, J_HH= 7.1 Hz, 2 H, CH_Ar), 7.62–7.73 (m, 2 H, CH_Ar), 7.82–7.86 (m, 60.97; H, 7.37; N, 2.84%; found: C, 60.91; H, 7.38; N, 2.68%.

3 H, CH_Ar+pyr), 8.16–8.23 (m, 3 H, CH_Ar+pyr), 8.81 (dd, J_HH= 8.1, J_HP

HRESI-MS: m/z = 476.1887 (100) [M + H⁺]. C₃₀H₂₇N₃OP requires

= 13.8 Hz, 1 H, CH_pyr). ¹³C{¹H} NMR (75.5 MHz, CDCl₃): δ = 32.4 476.1892; 973.3507 (30) [2M + Na⁺] requires 973.3525. FT-IR (KBr

(CH₂), 33.9 (d, J_CP= 74.0 Hz, CH), 121.5 (C_q(CF3)), 124.4 (C_pyr), 124.6

(C_pyr), 124.9 (C_pyr), 125.3 (C_pyr), 126.1 (C_pyr), 127.4 (C_Ar), 128.0

(cm^–1)): ν = 3425 (w), 3054 (vs), 3005 (s), 2926 (w), 1581 (vs), 1471

(vs), 1429 (vs), 1302 (w), 1236 (w), 1180 (vs, v_PO), 1101 (vs), 993 (s),

1112

www.zaac.wiley-vch.de

Z. Anorg. Allg. Chem. 2013, 1101–1116

Synthesis and Properties of New (Phosphinoylmethyl)Pyridine N-Oxides

898 (w), 790 (vs), 750 (vs), 704 (vs) 631 (s), 523(vs). ³¹P{¹H} NMR C₃₀H₂₆N₃O₃P: calcd. C 71.0; H, 5.16; N, 8.28; found: C, 64.24; H,

1

(101.2 MHz, CDCl₃): δ = 37.6 ppm. H NMR (500 Mz, CDCl₃): δ =

4.06 (m, J_HH= 12.5 Hz, 4 H, CH₂), 6.84 (dd, J_HH= 6.0, J_HH= 4.0 Hz,

2 H, CH_pyr), 7.00 (d, J_HH= 10.0 Hz, 2 H, CH_pyr), 7.08 (dd, J_HH= 6.5,

5.68; N, 7.32. HRESI-MS: m/z = 508.1781 (13) [M + H⁺].

C₃₀H₂₇N₃O₃P requires 508.1790; 530.1594 (100) [M

+

Na⁺].

C₃₀H₂₆N₃O₃PNa requires 530.1609. FT-IR (KBr (cm^–1)): 3419 (s),

1655 (w), 1579 (s), 1482 (s), 1432 (vs), 1243 (vs, v_N–O), 1175 (vs,

v_P=O), 1103 (vs), 999 (w), 849 (s), 762 (s), 703 (vs), 599 (s), 540 (vs).

J_HH= 6.0 Hz, 1 H, CH_pyr, 7.19–7.26 (m, 6 H, CH_Ph), 7.35 (dd, J_HH

=

8.5, J_HH= 6.5 Hz, 2 H, CH_pyr), 7.44 (dd, J_HH= 8.5, J_HH= 6.0 Hz, 1

H, CH_pyr), 7.59–7.65 (m, 5 H, CH_Ph+pyr), 8.18 (s, 2 H, CH_pyr), 8.43

(d, J_HH= 4.0 Hz, 1 H,CH_pyr). ¹³C{¹H} NMR (125.7 Mz, CDCl₃): δ =

40.7 (CH₂), 55.5 (d, J_CP= 62.9 Hz, C_q), 121.0 (C_pyr), 121.7 (C_pyr),

³¹P{¹H} NMR (101.2 MHz, CDCl₃):

δ =

38.4 ppm. ¹H{³¹P}

(500 MHz, CDCl₃): δ = 4.29 (m, J_HH= 21, J_HH= 21, J_HH= 16 Hz, 4

H, CH₂), 6.78–6.86 (m, 4 H, CH_pyr), 7.12 (m, J_HH= 6.0, J_HH= 5.0 Hz,

125.9 (C_pyr), 126.3 (C_pyr), 127.5 (d, J_CP= 11.3 Hz, C_Ph), 130.9 (C_Ph), 1 H, CH_pyr), 7.21 (d, J_HH= 8.0 Hz, 2 H, CH_Ph), 7.27 (m, 2 H, CH_pyr),

132.5 (C_qPh), 132.8 (d, J_CP= 7.9 Hz, C_Ph), 134.9 (C_pyr), 135.1 (C_pyr),

147.5 (C_pyr), 148.0 (C_pyr), 158.1 (d, J_CP= 9.2 Hz, C_pyr), 158.5 (C_qpyr).

7.35–7.48 (m, 7 H, CH_Ph+pyr), 7.85 (d, J_HH= 7.5 Hz, 3 H, CH_Ph+pyr),

7.93 (d, J_HH= 6.0 Hz, 2 H, CH_pyr), 8.44 (d, J_HH= 3.0 Hz, 1 H, CH_pyr).

¹³C{¹H} NMR (125.7 MHz, CDCl₃): δ = 32.5 (CH₂), 53.9 (d, J_CP

=

4.2.9 2-[1-(Bis(2-(triluoromethylphenyl))phosphinoyl)-1,1-

bis(methylpyridin2yl)methyl]pyridine (11b)

60.5 Hz, C_q), 122.5 (C_pyr), 123.5 (C_pyr), 124.5 (C_pyr), 125.3 (C_pyr),

128.3 (d, J_CP= 11.6 Hz, C_Ph), 128.4 (d, J_CP= 11.6 Hz, C_Ph), 130.4 (d,

J_CP= 92.8 Hz, C_qPh), 132.1 (C_Ph), 133.1 (d, J_CP= 7.8 Hz, C_Ph), 135.6

(C_pyr), 139.1 (C _pyr), 148.2 (C_pyr), 149.5 (d, J_CP= 12.6 Hz, C_pyr), 156.8

(C_qpyr).

A solution of 2-bromobenzotrifluoride (5.91 mL, 43.3 mmol) in dry

THF (10 mL) was added dropwise to Mg turning (1.06 g, 43.3 mmol)

in dry THF (10 mL) and stirred for 2 h. A solution of diethyl phosphite

(930 μL, 7.24 mmol) in dry THF (5 mL) was added dropwise and the

resulting mixture was heated to reflux (1 h) and then cooled (23 °C).

A solution of 2-(chloromethyl)pyridine (4.59 g, 36.0 mmol) in THF

(20 mL) was added, and the reaction mixture was stirred under reflux

for 24 h. The resulting mixture was quenched with saturated aqueous

NH₄Cl solution (100 mL), and the aqueous phase was extracted with

CH₂Cl₂(3 ϫ 30 mL). The combined organic fractions were dried with

MgSO₄and vacuum evaporated. The crude residue obtained was puri-

fied by chromatography column (silica gel, CH₂Cl₂/MeOH 99/1 to 95/

5) leaving 11b as a weakly brown powder. Yield 1.4 g, 33% m.p. 162–

164 °C. Suitable single crystals for X-ray diffraction analysis were ob-

tained by slow evaporation of a solution of 11b (20 mg) in CH₂Cl₂/

MeOH (4/1, 5 mL). HRESI-MS: m/z (%) = 612.1622 (100) [M +

H⁺].C₃₂H₂₅F₆N₃OP requires 612.1639. FT-IR (KBr (cm^–1)): 3426 (w),

3062 (w), 1589 (s), 1570 (m), 1473 (s), 1434 (vs), 1311 (vs), 1269

(m), 1169 (s, v_P=O), 1147 (s), 1114 (vs), 1098 (s), 1036 (s), 995 (w),

967 (w), 890 (w), 797 (m), 770 (s), 713 (m), 700 (w), 681 (w), 646

(m), 629 (w), 600 (m), 582 (w), 554 (w), 536 (s), 521 (m), 500 (w).

³¹P{¹H} NMR (121.5 MHz, CDCl₃): δ = 38.1 ppm. ¹H NMR

(300 MHz, CDCl₃): δ = 4.07 (t, 2 H, J_HH= 13.5 Hz, CH₂), 4.28 (t, 2

H, J_HH= 12.9 Hz, CH₂), 6.86 (t, J_HH= 6.0 Hz, 2 H, CH_pyr), 6.98 (d,

J_HH= 7.8 Hz, 2 H, CH_pyr), 7.07 (t, J_HH= 6.1 Hz, 1 H, CH_pyr), 7.17

(pseudo t, J_HH= 7.6 Hz, 2 H, CH_Ar), 7.31 (pseudo t, J_HH= 7.5 Hz, 2

4.2.11 2-[1-(Bis(2-(trifluoromethylphenyl))phosphinoyl)-1,1-

bis(1-oxy-methylpyridin-2-yl)methyl] pyridine (6b)

To a solution of 11b (1.0 g, 1.64 mmol) in dry CH₂Cl₂(10 mL), was

slowly added m-chloroperoxybenzoic acid (77 wt.-%, 1.3 g,

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

ture diluted with additional CH₂Cl₂(90 mL) and washed with aqueous

NaOH (2N, 3 ϫ 30 mL) and distilled water (2 ϫ 10 mL). The organic

phase was dried with MgSO₄, filtered and the volatiles removed by

vacuum evaporation, leaving 6b as a weakly brown powder. Yield:

0.89 g (85%), m.p. 202–204 °C. Single crystals suitable for X-ray dif-

fraction analysis were obtained by slow evaporation of a solution of

6a (80 mg) in CH₂Cl₂/MeOH (1:1, 50 mL). HRESI-MS: m/z (%) =

644.1507 (72) [M + H⁺]. C₃₂H₂₅F₆N₃O₃P requires 644.1538; 666.1329

(100) [M + Na⁺]. C₃₂H₂₄F₆N₃O₃PNa requires 666.1357. FT-IR (KBr

(cm^–1)): 3060 (w), 2204 (w), 1611 (m), 1585 (m), 1488 (s), 1468 (w),

1437 (vs), 1313 (vs), 1243 (m, v_N–O), 1172 (s, v_P=O), 1145 (s), 1118

(vs), 1036 (s), 921 (m), 842 (m), 779 (s), 730 (s), 714 (s), 645 (m), 600

(w), 550 (m), 532 (w), 494 (m). ³¹P{¹H} NMR (121.5 MHz, CDCl₃): δ

= 41.2 ppm. ¹H NMR (300 MHz, CDCl₃): δ = 4.44–4.66 (m, 4 H,

CH₂), 6.80–6.89 (m, 4 H, CH_pyr), 7.11–7.19 (m, 2 H, CH_Ar), 7.28–

7.39 (m, 3 H, CH_pyr), 6.63 (m, 4 H, CH_Ar), 7.74–7.79 (m, 2 H, CH_Ar),

7.91–7.94 (m, 2 H, CH_Ar), 8.53–8.58 (m, 3 H, CH_pyr). ¹³C{¹H} NMR

(75.5 MHz, CDCl₃): δ = 1.9 (CH₂), 56.9 (d, J_CP= 62.5 Hz, C_q), 121.3

(C_pyr), 122.7 (C_pyr), 123.2 (d, J_CF= 275 Hz, C_q(CF3)), 123.5 (C_pyr),

124.6 (C_pyr), 128.0 (C_qAr), 128.1 (C_Ar), 130.2 (d, J_CP= 83.4 Hz, C_qAr),

130.7 (d, J_CP= 11.7 Hz, C_Ar), 132.0 (C_Ar), 135.6–135.8 (C_pyr), 139.0

(C_pyr), 148.0 (C_pyr), 149.3 (C_qpyr), 156.5 (C_qpyr).

H, CH_Ar), 7.41–7.51 (m, 3 H, CH_pyr), 7.71 (dd, J_HH= 4.6, J_HH

7.6 Hz, 2 H, CH_Ar), 7.79–7.90 (m, 3 H, CH_Ar+pyr), 8.26 (d, J_HH

=

3.9 Hz, 2 H, CH_pyr), 8.42 (d, J_HH= 3.9 Hz, 1 H, CH_pyr). ¹³C{¹H}

NMR (75.5 MHz, CDCl₃): δ = 41.5 (CH₂), 58.7 (d, J_CP= 65.1 Hz,

C_q), 121.3 (C_pyr), 122.0 (C_pyr), 123.4(d, J_CF= 255 Hz, C_qCF3), 125.9

(C_pyr), 126.7 (C_pyr), 127.6 (C_Ar), 129.7 (d, J_CP= 11.3 Hz, C_Ar), 131.2

(C_Ar), 132.5 (d, J_CF= 55.0 Hz, C_qAr), 132.7 (d, J_CP= 85.0 Hz, C_qAr),

135.4 (C_pyr), 135.5 (C_pyr), 136.1 (d, J_CP= 8.8 Hz, C_Ar), 147.9 (C_pyr), 4.3 Synthesis of the Complexes

148.1 (C_pyr), 157.9 (d, J_CP= 10.4 Hz, C_qpyr), 158.9 (C_qpyr).

[Er(8a)(NO₃)₃(MeOH)₂]·CH₂Cl₂:

A

sample of 8a (0.145 g,

0.377 mmol) in MeOH (5 mL) was combined with Er(NO₃)₃·5H₂O

(0.154 g, 0.347 mmol) in MeOH (5 mL), and the mixture was stirred

(2 h). The resulting solution was filtered, the filtrate evaporated to

dryness and the residue crystallized from CH₂Cl₂/MeOH. Elemental

analysis: C₂₇H₃₁Cl₂ErN₅O₁₂P: calcd. C 36.57; H, 3.52; N, 7.90; found:

C, 35.91; H, 3.45; N, 7.33. FT-IR (KBr (cm^–1)): 2924 (s), 2854 (s),

1602 (s), 1477 (vs), 1439 (vs), 1293 (vs), 1155 (vs, ν_PO), 1105 (s),

1022(vs), 804 (s), 749 (vs), 697 (s), 525 (s), 510 (s).

4.2.10 2-[1-(Diphenylphosphinoyl)-1,1-bis(1-oxy-

methylpyridin-2-yl)methyl]pyridine (6a)

A sample of 11a (0.807 g, 1.7 mmol) in CHCl₃(15 mL) was treated

with m-chloroperoxybenzoic acid (77%, 1.75 g, 10 mmol) and the

mixture stirred (23 °C, 4 h). The resulting mixture was vacuum evapo-

rated, dissolved in CH₂Cl₂(20 mL) and treated with saturated aqueous

NaHCO₃solution until pH = 8. The organic phase was collected,

washed with aqueous brine solution and dried with Na₂SO₄. Evapora-

tion of the volatiles left a solid residue that was purified by column {La[(meNO)₂PO*)](OTf)₂(MeOH)₃(H₂O)⁺}(OTf^–): A stirred solu-

chromatography (SiO₂, MeOH/CH₂Cl₂10/90) leaving 6a as a pale yel- tion of (meNO)₂PO* (0.070 g, 0.205 mmol) in MeOH (5 mL) was

low solid. Yield: 0.79 g (92%), m.p. 112–114 °C. Elemental analysis:

added to a stirred solution of La(OTf)₃(0.120 g, 0.205 mmol) in

Z. Anorg. Allg. Chem. 2013, 1101–1116

R. T. Paine et al.

ARTICLE

MeOH (5 mL), and the resulting mixture was stirred (2 h). No precipi-

ments were well behaved except as indicated in the following notes.

tate was observed. The solvent was vacuum evaporated leaving a white 5a. The structure contains 1.5 acetonitrile molecules per ligand. One

powder. Colorless blocks were obtained by slow diffusion of diethyl

ether into a MeOH solution of the complex (–5 °C). Yield 88%

of these [N3,C25,C26] lies close to a center of symmetry and is at

half-occupancy. The other whole MeCN is disordered over two posi-

(0.189 g, 0.181 mmol). ³¹P NMR (202.31 MHz, [D₈]THF, 298.1 K): δ tions with equal occupancy. The hydrogen atoms of these methyl

= 38.8 ppm. Elemental analysis: C₂₁H₂₁F₉LaN₂O₁₄PS₃·2H₂O (962.47): groups were included in ideal positions and refined with U_iso

=

calcd.: C, 26.20; H, 2.19; N, 2.91; found: C, 26.12; H, 2.17; N, 2.79%.

1.5U_eqiv.8a and 8b. The structures were refined as racemic twins. All

reflections were averaged including Friedel pairs and the absolute con-

figuration was not determined. 11b. One [C₆H₄CF₃] group is disor-

dered over two positions with occupancies set at 78% and 22%. Some

bond lengths and angles in the minor component deviate from the

corresponding data for the major component, and all are listed in the

deposited data. 6b. The structure contains one molecule of MeOH per

ligand, and its carbon (C33) and oxygen (O4) atoms have large aniso-

tropic thermal parameters. The MeOH hydrogen atoms were included

in idealized positions and refined with U_iso= 1.5U_equivof the parent

atom. [Er(8a)(NO₃)₃(MeOH)₂]·CH₂Cl₂. The crystal was of marginal

quality. The CH₂Cl₂outer-sphere solvent molecule was disordered

over two positions sharing a central carbon atom (C54) with occupanc-

ies 69.7% and 30.3%. The hydrogen atoms were refined in ideal posi-

tions with U_iso= 1.5U_eqivfor the terminal methyl hydrogen atoms.

{La[(meNO)₂PO*)](OTf)₂(MeOH)₃(H₂O)⁺}(OTf^–). The complex ex-

hibited a disorder of the phenyl ring over two positions with occupanc-

ies in a 59/41 ratio. The carbon atoms of the two rings were restrained

to have identical thermal parameters for equivalent atoms.

{Pr[(meNO)₂PO*)](OTf)(MeOH)₄⁺}(OTf^–)₂. The bound triflate was

disordered over two positions with a 72/28 ratio of occupancies. Iden-

tical atoms of the anion were restrained to have identical thermal pa-

rameters.

{Pr[(meNO)₂PO*)](OTf)(MeOH)₄⁺}(OTf^–)₂: A stirred solution of

(meNO)₂PO* (0.073 g, 0.217 mmol) in MeOH (5 mL) was added to

a stirred solution of Pr(OTf)₃(0.127 g, 0.217 mmol) in MeOH (5 mL),

and the resulting mixture stirred (2 h). No precipitate was observed.

The solvent was vacuum evaporated leaving a light-green powder. Col-

orless blocks were obtained by slow diffusion of diethyl ether into a

MeOH solution of the complex (–5 °C). Yield 85% (0.203 g,

0.186 mmol). Elemental analysis: C₂₁H₁₉F₉N₂O₁₃PPrS₃·H₂O (946.45):

calcd.: C, 26.65; H, 2.02; N, 2.96%; found: C, 26.65; H, 2.17; N,

2.87%.

4.4 X-ray Crystallography

Crystals were coated with Paratone oil and mounted on a CryoLoop

attached to a metal pin with epoxy. Diffraction data were collected

with Bruker X8 Apex II CCD-based X-ray diffractometers equipped

with an Oxford Cryostream 700 low temperature device and normal

focus Mo-target X-ray tube (λ = 0.71073 Å) operated at 1500 W power

(50 kV, 30 mA). Data collection and processing were accomplished

with the Bruker APEX2 software suite.^[25]The structures were solved

by direct methods after corrections of the data using SADABS.^[26]

There were no signs of crystal decomposition during data collection.

The data were refined with full-matrix least-squares techniques on F²Crystallographic data (excluding structure factors) for the structures

with use of SHELXTL.^[27]Lattice and data collection parameters are have been deposited with the Cambridge Crystallographic Data Centre,

presented in Table 1 (ligands) and Table 2 (complexes). All heavy CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data

atoms were refined anisotropically and hydrogen atoms were included can be obtained free of charge by quoting the depository numbers

in ideal positions and refined isotropically (riding model) with U_iso

1.2 U_eqof the parent atom unless noted otherwise. The structure refine-

=

CCDC-838611 (8a), CCDC-838612 (5a), CCDC-838613 (8a), CCDC-

838614 (5b), CCDC-838615 (11b), CCDC-923838 (6b), CCDC-83861

Table 1. Crystallographic data for ligands.

5a·1.5MeCN

5b

8a

8b

11b

6b·MeOH

Empirical formula

Crystal size /mm

C₂₇H_25.5N_3.5O₃P C₂₆H₁₉F₆N₂O₃P C₂₄H₂₁N₂OP

C₂₆H₁₉F₆N₂OP

0.37 ϫ 0.16 ϫ

0.09

C₃₂H₂₄F₆N₃OP

0.28 ϫ 0.23 ϫ

0.09

C₃₃H₂₈F₆N₃O₄P

0.19 ϫ 0.24 ϫ

0.36

0.30 ϫ 0.23 ϫ

0.12

0.31 ϫ 0.17 ϫ

0.05

0.50 ϫ 0.21 ϫ

0.18

Formula weight

477.98

552.40

384.40

520.40

611.51

675.56

Crystal system

Space group

a /Å

b /Å

c /Å

α /°

β /°

triclinic

P1

Triclinic

P1

orthorhombic

P2₁2₁2₁

5.8983(3)

16.3942(8)

20.1274(10)

90

orthorhombic

Pca2(1)

15.8836(9)

8.2841(5)

34.706(2)

90

4566.6(5)

8

monoclinic

P2₁/c

12.8408(3)

13.9554(3)

15.7076(3)

90

99.0240(10)

90

2779.94(10)

4

triclinic

P1

¯

9.0627(4)

10.3215(4)

13.6874(6)

88.348(2)

86.221(2)

75.377(2)

1236.06(9)

2

9.4656(5)

11.2077(6)

12.2022(6)

71.395(3)

85.708(3)

79.039(3)

1204.33(11)

2

8.6661(11)

10.2685(13)

17.5874(19)

76.146(8)

85.444(7)

81.417(8)

1501.0(3)

2

90

γ /°

V /Å³

1946.28(17)

4

Z

T /K

150(2)

1.284

0.146

150(2)

1.523

0.194

225(2)

1.312

0.158

150(2)

1.514

0.193

150(2)

1.461

0.172

173(2)

1.495

0.174

D_calcd./mg·m^–3

μ /mm^–1

F(000)

502

564

808

2128

1256

696

reflection collected

independent reflections

32000

8105[0.0257]

31999

9198[0.0292]

42109

3408[0.0542]

90343

7085[0.0698]

67894

10152[0.0349]

11291

5477[0.0777]

[R_int

]

R₁^a), wR₂^b)[I Ͼ 2σ I]

R₁(wR₂(all data))

0.0583, 0.1473

0.0725, 0.1593

0.0413(0.0927)

0.0603(0.1028)

0.0380(0.0881)

0.0532(0.0943)

0.0689(0.1756)

0.0794(0.1843)

0.0444(0.1114)

0.0670(0.1228)

0.0714(0.1726)

0.1684(0.2132)

2

2 2

2 2 ½

a) R₁= (Σ||F_o|–|F_c||)/Σ|F_o|, b) wR₂= [Σw(F_o–F_c) /Σw(F_o) ] .

1114 www.zaac.wiley-vch.de

Z. Anorg. Allg. Chem. 2013, 1101–1116

Synthesis and Properties of New (Phosphinoylmethyl)Pyridine N-Oxides

Table 2. Crystallographic data for coordination complexes.

[Er(8a)(NO₃)₃(MeOH)₂]·CH₂Cl₂{La[(meNO)₂PO*)](OTf)₂(MeOH)₃(H₂O)⁺}(OTf^–) {Pr[(meNO)₂PO*)](OTf)(MeOH)₄⁺}(OTf^–)₂

Empirical formula C₂₇H₃₁Cl₂ErN₅O₁₂

Crystal size /mm 0.21 ϫ 0.14 ϫ 0.07

P

C₂₄H₃₁F₉LaN₂O₁₆PS₃

0.115 ϫ 0.221 ϫ 0.280

1040.57

C₂₆H₃₇F₉N₂O₁₇PPrS₃

0.286 ϫ 0.297 ϫ 0.315

1088.64

Formula weight

Crystal system

Space group

a /Å

b /Å

c /Å

886.70

Triclinic

triclinic

¯

P1

14.2425(6)

15.2756(12)

18.0856(7)

106.085(3)

112.706(2)

95.995(3)

3387.1(3)

4

10.0544(13)

12.0918(16)

16.899(2)

74.031(7)

86.484(7)

86.566(7)

1969.5(4)

2

10.5210(4)

11.8820(4)

17.7754(6)

71.083(2)

83.079(2)

80.860(2)

2069.73(13)

2

α /°

β /°

γ /°

V /Å³

Z

T /K

225(2)

110(2)

D_calcd./mg·m^–3

μ /mm^–1

reflection

collected

independent

reflections

1.739

2.750

152084

1.755

1.394

21388

1.747

1.477

43423

26352

8053

10170

b)

R₁^a), wR₂

0.0510(0.1203)

0.0281(0.0667)

0.0357(0.0849)

[I Ͼ 2σ I]

R₁(wR₂(all data)) 0.0968(0.1544)

GOF on F²

1.041

0.0334(0.0692)

1.035

0.0375(0.0861)

1.091

2

2 2

2 2 ½

a) R₁= (Σ||F_o|–|F_c||)/Σ|F_o|, b) wR₂= [Σw(F_o–F_c) /Σw(F_o) ] .

(Er complex), CCDC-923807 (La complex), CCDC-923806 (Pr com-

plex).

4.6 Molecular Mechanics Calculations

Method: Geometry optimizations of the free and metal-bound forms

of 7, 8, 9 and 10 were carried out with the MM3 force field^[28]using

a points-on-a-sphere metal ion^[29]as implemented in PCModel soft-

ware.^[30]Conformational searches to locate the most stable form for

each structure were performed using the GMMX algorithm provided

with this software.

(Fax: +44-1223-336-033; E-mail: deposit@ccdc.cam.ac.uk, http://

www.ccdc.cam.ac.uk).

4.5 Distribution Studies

Supporting Information (see footnote on the first page of this article):

Representative copies of the HRESI-MS plots and ¹H, ¹³C and ³¹P

NMR spectra for the new ligands and intermediates are provided in

Supporting Information.

Materials: All salts and solvents were reagent grade and were used as

received. Extraction experiments were carried out using 1,2-dichloroe-

thane (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 Iso-

tope Products, Burbank, CA. Both were added as spikes to the aqueous

phases in the sample equilibration vials in the extraction experiments.

Acknowledgments

Financial support for the studies 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

Methods: Phases at an 1:1 organic to aqueous (O:A) phase ratio were X-ray diffractometer (CHE-0443580) and NMR spectrometers (CHE-

added to 2 mL polypropylene micro-tubes, which were then capped

and mounted by clips on a disk that was rotated in a constant-tempera-

ture air box at 25.0 Ϯ 0.5 °C for 1 hour. After the contacting period,

0840523 and CHE-0946690) at UNM. B.P.H. and L.H.D. acknowledge

support from the Division of Chemical Sciences, Geosciences and Bio-

sciences, Office of Basic Energy Sciences, U. S. Department of En-

the tubes were centrifuged for 5 min at 3000 RPM and 25 °C in a ergy. K.K.K acknowledges support provided by the Robert A. Welch

Beckman Coulter^TMAllegra 6R temperature-controlled centrifuge. A Foundation (AA-1508) and in part by funds from the University Re-

250 μL aliquot of each phase was sub-sampled and counted using a

Canberra Analyst pure Ge Gamma counter. Counting times were suf-

ficient to ensure that counting error was a small fraction of the pre-

cision of the obtained distribution ratios, considered from a combina-

tion of volumetric, replicate and counting errors to be Ϯ5%. Americ-

ium 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.

search Committee and the Vice Provost for Research at Baylor Univer-

sity.

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Received: February 19, 2013

Published Online: April 11, 2013

1116

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Synthesis and properties of new (Phosphinoylmethyl)pyridine N-oxides

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