Formation of a layered framework structure based upon 4-methyl-2,6- bis(methylphosphonic acid) phenol

Article abstract of DOI:10.1039/b603987a

The trifunctional ligands, (HO)₂P(O)CH_{2 2}C₆H₂(R)OH, (5-H₄) (R = CH ₃, Br) were prepared in good yield via an Arbusov reaction between P(OEt)₃ and the respective 4-R-2,

Full text of DOI:10.1039/b603987a

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www.rsc.org/dalton | Dalton Transactions
Formation of a layered framework structure based upon
4-methyl-2,6-bis(methylphosphonic acid) phenol
Xin-min Gan, Iris Binyamin, Sylvie Pailloux, Eileen N. Duesler and Robert T. Paine*
Received 17th March 2006, Accepted 23rd May 2006
First published as an Advance Article on the web 26th June 2006
DOI: 10.1039/b603987a
The trifunctional ligands, [(HO)2P(O)CH2]2C6H2(R)OH, (5-H4) (R = CH3, Br) were prepared in good
yield via an Arbusov reaction between P(OEt)3 and the respective 4-R-2,6-bis(chloromethyl)phenols
followed by acidic aqueous hydrolysis and they were spectroscopically characterized by IR and NMR
techniques. The ligand 5-H4–CH3 readily dissolves lanthanide hydroxide residues and it forms a
crystalline complex from aqueous LaCl3 solutions. This complex was characterized by single crystal
X-ray diffraction methods and found to adopt a complex 2-D lamellar network in the bc plane. The
La(III) inner coordination sphere is seven coordinate formed by oxygen atoms from two water
molecules and five phosphonate oxygen atoms from three different ligands. The phenolic oxygen atom
is not involved in the ligand binding to La(III).
to strong acid solutions. Interestingly, 1-H2 in the presence of lan-
Introduction
thanide hydroxide or aged oxide-hydroxide precipitates produces
water soluble complexes.24 The crystal structure determination for
one complex, Er(1-H−)3(1-H2), reveals a symmetrical eight coor-
Neutral organophosphonates, RP(O)(ORꢀ)2, and organophos-
phine oxides, R3PO, typically behave as modest to weak ligands
toward many transition metal cations while they display moderate
to strong attractions toward harder lanthanide (Ln) and actinide
(An) cations.1 This distinction is accentuated when the phospho-
nate or phosphine oxide groups are part of a multifunctional
ligand that can adopt stable chelate structures on the larger, less
orbitally restrictive f-block ions. Such chelating ligands, e.g. car-
bamoylmethylphosphonates, (RO)2P(O)CH2C(O)NR2, find utility
in biphasic extractive separations of Ln and An ions present in
acidic aqueous nuclear process waste solutions that contain a great
variety of other cations.2 Unfortunately, most organophospho-
nates undergo hydrolytic degradation in aqueous acid solutions
with formation of phosphonic acids, RP(O)(OH)2.3 These species,
even in small amounts, dramatically alter the extraction selectivity
and phase separation behavior of the neutral organophospho-
nates. This no doubt results from less discriminate binding
of phosphonate anions, RPO32−, toward all hard cations. It is
known that organophosphonate anions form a wide variety of
stable complexes with transition metal cations and much of that
chemistry has been thoroughly reviewed by Clearfield.4 Many of
these complexes are relatively insoluble and poorly crystalline,
but where crystalline complexes are obtained, single crystal X-
ray diffraction analyses reveal intricate extended 1-D chain, 2-D
layered and 3-D network structures. Far fewer complexes with
lanthanide cations have been structurally characterized, but where
data are available it is clear that extended structures are also
produced.5–19
dinate structure having four Er–O–N
PO seven-membered
chelate ring interactions.24 Extended ligand donor atom–metal
bridge bonding is absent from the structure; however, extensive
hydrogen bonding involving N–O, P–O and P–OH groups is
present. Ligand 2-H2, with its longer exo-ethyl phosphonic acid
arm does not form chelate interactions. Instead, as reported for
Nd(2-H−)3,25 the 2-H-anions bridge bond between Nd(III) cations
resulting in a chain structure that does not involve the N-oxide in
the six coordinate Nd(III) inner sphere. However, the N-oxide is
involved in the secondary hydrogen bonding structure. Finally, the
tetra-acid, 3-H4, forms complexes Ln(3-H2−2)(3-H−3) where both
ligand types act as tridentate chelates and Ln(III) ions are bridged
by Ln–O–P connections forming an extended chain structure.26
In support of the characterization of the extraction performance
of a new family of phosphinomethyl pyridine N-oxide ligands,20–23
we have prepared and examined the ligation behavior of the pre-
viously unknown phosphonic acids 1-H2,24 2-H225 and 3-H426 that
form under extended exposure of the parent organophosphonates
Much remains to be revealed about the range of structural
chemistry displayed by the pyridyl N-oxide phosphonic acids
1-H2, 2-H2 and 3-H4 and work continues with these ligands in
our group. We have also prepared related ligands on a phenol
backbone and some aspects of the coordination chemistry of
the neutral phosphine oxide derivatives [Ph2P(O)CH2)]2C6H2-
(R)OH (R = Me, tBu) (4) have been reported.27 These compounds
act as neutral, tridentate chelating ligands forming complexes
Department of Chemistry, University of New Mexico, Albuquerque, NM,
87131, USA. E-mail: rtpaine@unm.edu
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of the general type Ln(L)(NO3)3. Unlike the neutral pyri-
dine N-oxide,28 [Ph2P(O)CH2]2C6H3NO, the bis(diphenylphos-
phinomethyl)phenols do not appear to form 2:1 ligand:Ln com-
plexes, [Ln L2(NO3)](NO3)2. Since solvent extraction studies for
the phenol derivatives are planned, it is appropriate to determine if
these ligands form phosphonic acids 5-H4 and, if formed, evaluate
how they behave as coordinating ligands. We report here the direct
synthesis of two tetra-acids, [(HO)2P(O)CH2]2C6H2(R)(OH) (R =
Me, Br), and the formation and structural characterization of one
complex formed with La(III).
31P NMR analysis for monitoring hydrolysis of the phosphonate
or phosphine oxide ligands when exposed to strong acids and/or
radiation fields. It is noteworthy that short-term (7 day) acid
exposures of the phosphine oxides shows no evidence of hydrolysis.
The behavior of the phosphonic acids, 5-H4, as ligands was
evaluated in two ways. Initially, since we are interested in potential
uses of the acids as decontamination agents, we qualitatively
examined the lanthanide hydroxide dissolution behavior of 5-H4–
CH3. Lanthanum hydroxide, La(OH)3, precipitates were prepared
by addition of aqueous NaOH to La(NO3)3 solution: (0.1 M).
Colloids or precipitates form rapidly, depending upon the specific
conditions used. Addition of sufficient (1 : 1) 5-H4–CH3 rapidly
dissolves these precipitates and suspended colloids resulting in
completely clarified solutions. In addition, when the La(OH)3
solid was heated to 80 ◦C in water for several days then
isolated, dried and stored for one week, the resulting solid easily
redissolves when exposed to an aqueous solution containing 5-
H4–CH3. These initial tests support further evaluation of 5-H4 as
a decontamination agent for surfaces coated with hydroxide waste
residues. Dissolution studies using actinide bearing materials are
planned for the near future.
The solution coordination chemistry of 5-H4–CH3 has also been
examined. Initially, we attempted to isolate the complexes formed
from the dissolution of La(OH)3 precipitates as described above.
Simple evaporation of the aqueous solutions produced solids with
slightly variable elemental compositions that probably result from
variable hydration of the solids. Attempts to obtain X-ray quality
single crystals in this fashion were unsuccessful. However, when
LaCl3 and 5-H4–CH3 were combined in a 1 : 1 ratio in dilute
(0.02 M) HCl and the solution heated in a sealed tube at 125 ◦C for
two days, the hydrothermal conditions provided X-ray diffraction
quality crystals formed as colorless platelets.
Results and discussion
The phosphonic acids 5-H4–CH3 and 5-H4–Br were prepared in
high yield by the two step reaction sequence outlined in Scheme 1.
The Arbusov reactions are rapid and proceed with complete
conversion to the phosphonate esters 6-CH3 and 6-Br that were
characterized by 1H, 13C and 31P NMR spectroscopy. The 31P
chemical shifts d 26.8 and 25.8 for R = CH3 and Br, respectively,
are comparable to the shift observed for the pyridine N-oxide
analog [(EtO)2P(O)CH2]2C5H3NO, d 23.7.29 Since our only interest
in examples of 6 is as intermediates, no effort was made to
isolate these compounds in analytically pure form. Instead, they
were briefly heated in concentrated HCl (37%, 1–2 h), which
induced hydrolysis of the ester groups and complete conversion
to the phosphonic acids 5-H4–CH3 and 5-H4–Br. The rates of
hydrolyses for 6 are notably faster than that observed in the
hydrolysis of [(EtO)2P(O)CH2]2C5H3NO.26 The new phosphonic
acids were isolated as analytically pure, white crystalline solids
that are soluble in water, slightly soluble in DMSO and insoluble
in common organic solvents. Mass spectra obtained for both
compounds show intense parent, (M+) and (M + H+), ions and
fragment ions that would be expected for substituted phenols.
The compound crystallizes in the centric monoclinic space
group P21/c. There is one molecule, La (5-H1–CH3)(H2O)2 in the
asymmetric unit and four molecules per unit cell. A view of the
asymmetric unit is shown in Fig. 1. The two water molecules are
1
The 31P{ H} NMR spectra show a sharp singlet at d 25.4 (R =
CH3) and 24.6 (R = Br). These shifts are downfield of the shift
observed for the pyridine N-oxide analog 3-H4, d 17.9, when
recorded in D2O solutions or d 19.3 when recorded in DMSO.27
The shifts are slightly upfield of the shifts of the respective
phosphonate esters, 6-CH3 and 6-Br. The shifts for the phosphonic
acids are also upfield of the resonances for the phosphine oxide
derivatives [R2P(O)CH2]2C6H2(R)OH.26 This allows for the use of
˚
directly coordinated to the La(III) cation with La–O(8) 2.646(2) A
˚
and La–O(9) 2.608(2) A. The 5-H4–CH3 ligand has lost three of
four available protons and the resulting trianionic ligand, 5-H1–
CH33−, is bonded in a bidentate fashion to the La(III) through
Scheme 1
Fig. 1 Asymmetric unit for La(5-H1–CH3)(H2O)2.
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the phosphonate oxygen atoms O(4) and O(7): La–O(4)P(1),
The resulting La–O–P interactions, along with extensive hydrogen
bonding, produce a lamellar network in the bc plane, and a view
of the network is shown in Fig. 3. The gap between the sheets of
˚
˚
2.393(2) A and La–O(7)P(2), 2.448(2) A. The shorter distance
results from bonding of the fully ionized phosphonate arm,
2−
˚
CH2PO3 while the longer distance appears with the partially
carbon atoms formed by the pyridine rings is 3.6(5) A.
ionized phosphonate arm, CH2PO2(OH)−1. The phenol oxygen
atom remains protonated under the synthetic conditions utilized.
The phenol oxygen points in the general direction of the La(III)
The P–O bond lengths involving the –CH2PO3 arm are P(1)–
2−
˚
˚
O(2) 1.535(2) A, P(1)–O(3) 1.512(2) A and P(1)–O(4) 1.515(2)
˚
A. The elongation in P(1)–O(2) is a consequence of a strong
˚
cation, but the separation distance, 3.165 A, indicates that a
hydrogen bond interaction between O(2) and the P(2)O(6)–H
La · · · O(1) interaction is, at the most, very weak.
group in a neighboring molecular unit: P(2)O(6)H(6) · · · O(2) [x,
˚
˚
Inspection of the unit cell (Fig. 2) reveals that the remainder
of the La seven vertex inner coordination sphere is completed
with phosphonate oxygen atoms from three neighboring asymmet-
ric units containing La(1), La(2) and La(3). Two of the interactions
result from bridge bonding with –CH2PO32− fragments, La–O(31)
1.5 − y, z − 0.5] 1.677 A, O(6) · · · O(2) 2.575 A O(6)–H(6) · · · O(2)
◦
˚
171.3 . Correspondingly, the P(2)–O(6) bond length 1.557(2) A is
˚
significantly longer than the bond lengths P(2)–O(5), 1.517(2) A
−1
˚
and the P(2)–O(7) 1.509(2) A in the –CH2PO2(OH) arm. There
is also a short intramolecular hydrogen bond involving a coor-
˚
˚
2.421(2) A and La–O(22) 2.490(2) A, while the third occurs with
dinated water molecule and the phenol group: O(1)H(1) · · · O(9)
◦
˚
˚
the partially ionized –CH2PO2(OH) fragment, La–O(53) 2.552(2)
A. These seven nearest neighbor oxygen atoms provide a mono-
capped trigonal prismatic coordination polyhedron. An additional
1.94 A, O(1) · · · O(9) 2.650 A, O(1)–H(1) · · · O(9) 138 . There is
also a short inter-asymmetric unit hydrogen bond involving the
same water molecule and O(5) [x, 1 + y, z] in another unit:
˚
˚
˚
oxygen atom, O(73), on the –CH2PO2(OH) arm, with La · · · O(73)
O(9)H(9) · · · O(5) 1.88 A, O(9) · · · O(5) [x, 1 + y, z] 2.712 A, O(9)–
2.971(2) A, is not considered to be part of the inner sphere coor-
H(9) · · · O(5) [x, 1 + y, z] 155.8◦.
˚
dination environment. From the perspective of the ligand 5-H1–
CH−33, the exo –C(7)H2P(1)O(2)O(3)O(4)−2 arm forms three P–
O–La bridge bonds to three different La(III) ions while the second
exo –C(8)P(2)O(5)O(7)O(6)H−1 arm forms P–O–La bridge bonds
with two La(III) ions. The third oxygen atom remains protonated.
In summary, it is observed that the 4-methyl-2,6-bis(methyl-
phosphonic acid) phenol, 5-H4–CH3 readily dissolves lanthanide
hydroxide precipitates and it forms a crystalline lamellar solid
under hydrothermal synthesis conditions with a 1 : 1 ligand/
metal stoichiometry. The solid state structure is distinctly
Fig. 2 Extended bc-plane structure for La(5-H1–CH3)(H2O)2.
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Fig. 3 Lamellar network in the bc plane: La polyhedra (green), phosphate tetrahedral (magenta), phenol O atom (red) and carbon atoms (grey).
different from that produced by the related phosphonic acid 2,6-
bis(methylphosphonic acid)pyridine N-oxide. That ligand, even
with 1 : 1 reactant mixtures, forms 2 : 1 complexes, Ln(3-H2−2)(3-
H1−3), in which the lanthanide ion is seven coordinate and the
ligands act as tridentate chelates. The seventh coordination site
is occupied by a bridging phosphate interaction that produces a
unique chain structure. Further studies with these and related
phosphonic acid ligands are in progress that may lead to a
systematic understanding of the structural and electronic factors
operating in the assembly of these complex structures with
lanthanide ions.
from Ventron. All reactions were performed under dry nitrogen
atmosphere unless noted otherwise. Infrared spectra were recorded
on a Mattson 2020 FTIR and NMR spectra were obtained
from Bruker FX-250 and JEOL-GSX-400 spectrometers using
Me4Si(1H, 13C) and H3PO4(31P) as shift standards. Mass spectra
were obtained from the Midwest Center for Mass Spectrometry
at the University of Nebraska. Elemental analyses were obtained
from Galbraith Laboratories.
Liquid syntheses
4-Methyl-2,6-bis(methylphosphonic acid)phenol (5-H4–CH3).
Under dry nitrogen, 4-methyl-2,6-bis(chloromethyl)phenol (6.0 g,
0.03 mol),27 was added with stirring at 23 ◦C to triethylphosphite
(20.0 g, 0.12 mol). The temperature of the mixture rose during
the addition and when addition was complete the mixture was
briefly heated (10 min) at 150 ◦C. The excess (EtO)3P was removed
Experimental
The organic reagents were purchased from Aldrich Chemical
Co. Organic solvents were purchased from VWR and dried
by standard methods. The lanthanum nitrate was purchased
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˚
by vacuum evaporation leaving a colorless liquid, 4-methyl-2,6-
Table 1 Selected bond lengths (A) for La(5-H1–CH3)(H2O)2
bis(diethylphosphonomethyl)phenol 6-CH3 (11.6 g, 95%). NMR
La–O(23)#2a
La–O(31)#1
La–O(53)#3
La · · · O(73)#4
2.489(2)
2.420(2)
2.552(2)
2.971(2)
(23 ◦C, CDCl3): 31P{ H} d 26.8; 1H d 1.16 (t, J = 7.0 Hz,
1
La–O(4)
La–O(7)
La–O(8)
La–O(9)
2.393(2)
2.448(2)
2.646(2)
2.608(2)
OCH2CH3) 2.14 (s, 4-CH3); 3.11 (d, J = 21.4 Hz, –CH2P); 3.95
1
(m, OCH2), 6.82 (s, C3–H); 8.40 (s, OH); 13C{ H} d 16.1 (d, J =
3.0 Hz, CH3), 20.2 (s, 4-CH3), 28.4 (d, J = 137.6 Hz, –CH2P), 62.3
(d, J = 7.0 Hz, OCH2), 120.2 (t, J = 6.0 Hz), 129.8 (s), 130.8 (t,
J = 5.0 Hz), 151.1 (t, J = 5.9 Hz).
a The numbers, #n, refer to the three other molecules in the unit cell.
and stirred briefly. The solution was then transferred to a Pyrex
tube and sealed. The tube was heated to 125 C (2 d). Colorless
platelets deposited that were suitable for X-ray diffraction analysis.
IR (KBr, cm−1): 3541 (m), 3495 (m), 3427 (m), 3115 (m), 2920 (m),
1650 (w), 1602 (m), 1483 (m), 1313 (w), 1203 (m), 1161 (s), 1111
(s), 1078 (vs), 1028 (s), 966 (s), 864 (w), 738 (w), 538 (m), 470 (m).
A sample of 6-CH3 (10.0 g, 0.025 mol) was added to con-
centrated hydrochloric acid (37%, 60 mL) and the mixture
refluxed (1 h). The resulting solution was evaporated to dryness
under vacuum and the residue treated with acetone (100 mL).
The resulting suspension was stirred at 23 ◦C (1 h), the solid
collected by filtration, washed with fresh acetone (2 × 40 mL)
and dried in vacuum (7.0 g, 96%). The product, 4-methyl-2,6-
bis(methylphosphonic acid)phenol, 5-H4–CH3, is a white solid that
is recrystallized from H2O/acetone mixtures: mp 205–206◦. Solu-
ble in H2O, sl. Sol in DMSO, insol in common organic solvents.
Found: C, 36.27; H, 4.45. C9H14O7P2 requires C, 36.50; H, 4.77.
Mass spectrum (FAB m/z): 297 (100%, M + H+), 296 (90%, M+),
279 (50%, M–OH+). IR (KBr, cm−1): 3502 (m), 2922 (m), 1487 (m),
1215 (m), 1120 (s), 1022 (sh), 960 (s), 935 (s), 879 (sh), 841 (m),
◦
X-Ray diffraction analysis
A single crystal (0.7 × 0.37 × 0.07 mm) was mounted on a glass
fiber and data were collected on a Siemans R3m/V diffractometer
equipped with a graphite monochromator and using Mo Ka
˚
radiation (k = 0.71073 A). Crystal data: C9H15LaO9P2, M =
468.06, monoclinic space group P21/c, a = 12.012(1), b = 8.605(1),
◦
3
˚
˚
c = 13.538(1) A, b = 98.29(1) ; V = 1384.7(2) A , Z = 4,
l = 3.358 mm−1, T = 20 ◦C, 5108 reflections collected, 2440
independent reflections, (Rint = 0.0189). The final refinement
indices were R1 = 0.0194, wR2 = 0.0534 [I ≥ 2r (I)], R1 (all
data) = 0.0206, wR2 = 0.0538. All calculations were performed
with SHELX-9730 and a face indexed absorption correction was
applied. The structure was solved by Patterson map interpretation.
The refinement was well behaved and the non-hydrogen atoms
were refined anisotropically. The H atoms on the oxygen atoms
and C(9) were located in difference maps and the H-atoms on
the remaining C-atoms were placed in idealized positions (Riding
model) (Table 1).
◦
1
727 (m), 497 (w). NMR (23 C, D2O): 31P{ H} d 25.4; 1H d 2.15 (s,
4-CH3), 2.95 (d, J = 21.1 Hz, –CH2P), 6.87 (s, C3–H), 10.43 (br,
OH); 13C{ H} d 20.2 (s, 4-CH3), 30.3 (d, J = 133.9 Hz, CH2P);
1
121.7 (t, J = 5.1 Hz), 127.8 (s), 130.0 (s), 151.5 (t, J = 6.1 Hz).
4-Bromo-2,6-bis(methylphosphonic acid)phenol (5-H4Br). Un-
der dry nitrogen, 4-bromo-2,6-bis(chloromethyl)phenol27 (5.0 g,
0.18 mol) in benzene (30 mL) was added dropwise to (EtO)3P
(13.0 g, 0.078 mol) held at 114 ◦C. Following addition, the
mixture was stirred and held at 114 ◦C (20 min) and then
excess (EtO)3P removed in vacuo leaving a colorless oil, 4-bromo-
2,6-bis(diethylphosphonomethyl)phenol 6-Br (8.5 g, 97%). NMR
(23 ◦C, CDCl3): 31P{ H} d 25.8; 1H d 1.20 (t, J = 7.0 Hz,
1
CCDC reference number 601851.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b603987a
OCH2CH3), 3.13 (d, J = 21.5 Hz, –CH2P), 3.99 (m, OCH2), 7.16
1
(s, C3–H) 8.83 (s, OH); 13C{ H} d 16.2 (d, J = 3.1 Hz, CH3), 28.4
(d, 136.2 Hz, CH2P), 62.6 (d, J = 7.0 Hz, –OCH2), 112.4 (s), 122.9
(t, J = 6.0 Hz); 132.7 (t, J = 5.0 Hz), 152.8 (t, J = 5.8 Hz).
A sample of 6-Br (7.7 g, 0.016 mol) was added to concentrated
HCl (37%, 40 mL), the mixture stirred (60 ◦C, 1.5 h) and
then evaporated to dryness at 23 ◦C. The residue was washed
with Et2O (30 mL) and then CHCl3 (30 mL), collected by
filtration and then washed again with acetone (2 × 30 mL). The
remaining solid 4-bromo-2,6-bis(methylphosphonic acid)phenol,
5-H4–Br, was recrystallized from H2O/acetone mixtures: mp >
250 ◦C. Soluble in H2O, sl. sol in DMSO. Found: C, 26.32; H, 2.96
C8H11O7P2Br requires C, 26.62; H, 3.07. Mass spectrum (FAB,
m/z): 362 (90%, M + H+), 361 (100%, M+), 344 (15%, M–OH+).
IR (KBr, cm−1): 3408 (m), 2920 (m), 1467 (m), 1367 (w), 1258 (m),
1207 (s), 1141 (s), 1016 (s), 927 (s), 875 (w), 727 (m), 509 (m). NMR
Acknowledgements
Financial support for this study came from the US Department
of Energy, Office of Basic Energy Sciences, Grant No. DE-FG02-
03ER15419.
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