Lanthanide complexes with zwitterionic amidoximes stabilized by noncoordinating water molecules*

Article abstract of DOI:10.1080/10610278.2017.1405002

We present the first structural report of lanthanides complexed with free amidoxime ligands as part of our ongoing research effort to understand the interactions of amidoximes with metal ions in seawater. Three isomorphous lanthanide complexes with acetamidoxime (AcAO) having the formula Ln₂(NO₃)₆(AcAO)₃(OH₂)₃·3H₂O (Ln?=?Pr³⁺, Nd³⁺, Gd³⁺) have been crystallized and structurally characterised. Notably, the AcAO ligands coordinate in a bridging mode as zwitterions, which creates pre-organised hydrogen bonding cavities into which water molecules are incorporated. Charge-assisted hydrogen bonds to these water molecules appear to stabilize the zwitterionic form and thus the overall complex, underscoring the importance of supramolecular phenomena and overall complex geometry on the relative stability of amidoxime complexes. This has implications for controlling their selectivity towards metal ions.

Full text of DOI:10.1080/10610278.2017.1405002

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20
Lanthanide complexes with zwitterionic
amidoximes stabilized by noncoordinating water
molecules
Steven P. Kelley & Robin D. Rogers
To cite this article: Steven P. Kelley & Robin D. Rogers (2017): Lanthanide complexes with
zwitterionic amidoximes stabilized by noncoordinating water molecules, Supramolecular Chemistry,
DOI: 10.1080/10610278.2017.1405002
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Date: 28 November 2017, At: 02:42

Supramolecular chemiStry, 2017
https://doi.org/10.1080/10610278.2017.1405002
Lanthanide complexes with zwitterionic amidoximes stabilized by
noncoordinating water molecules*
Steven P. Kelleya,b and Robin D. Rogersa,b,c
aDepartment of chemistry, the university of alabama, tuscaloosa, al, uSa; bDepartment of chemistry, mcGill university, montreal, canada; c525
Solutions, inc., tuscaloosa, al, uSa
ABSTRACT
ARTICLE HISTORY
received 31 august 2017
accepted 6 November 2017
We present the first structural report of lanthanides complexed with free amidoxime ligands as
part of our ongoing research effort to understand the interactions of amidoximes with metal ions
in seawater. Three isomorphous lanthanide complexes with acetamidoxime (AcAO) having the
formula Ln2(NO3)6(AcAO)3(OH2)3·3H2O (Ln = Pr3+, Nd3+, Gd3+) have been crystallized and structurally
characterised. Notably, the AcAO ligands coordinate in a bridging mode as zwitterions, which
creates pre-organised hydrogen bonding cavities into which water molecules are incorporated.
Charge-assisted hydrogen bonds to these water molecules appear to stabilize the zwitterionic form
and thus the overall complex, underscoring the importance of supramolecular phenomena and
overall complex geometry on the relative stability of amidoxime complexes. This has implications
for controlling their selectivity towards metal ions.
KEYWORDS
coordination chemistry;
f-elements; amidoxime;
hydrogen bonding
Introduction
their selectivity, as seawater contains many other metal
ions at much higher concentrations than uranium. Some
alternatives to amidoximes have recently been demon-
strated at the laboratory scale, including anionic MOFs
for capturing uranyl ([UO2]2+) ions (8) and cationic amine
resins for capturing anionic complexes (9, 10). However,
the leading strategy has been to improve the selectivity of
amidoximes starting by understanding their interactions
with metal ions, particularly [UO2]2+.
There has been a recent rise in interest in the coordi-
nation chemistry of the amidoxime functional group
(R–C(NH2)=N–OH), notably due to its use in selective
sorbents for the extraction of uranium from seawater (1).
Amidoxime-functionalized resins have been demonstrated
to extract uranium directly from the marine environment
(2), but despite a number of improvements in capacity
(3), recyclability (4, 5), and potential elimination of nonre-
newable chemicals from the process (6), they are not yet
cost-competitive with terrestrial mining (7). The capacity
of these sorbents could be greatly improved by increasing
It has been established through a number of structural
studies that amidoximes can exchange protons for [UO2]2+
to form η2 complexes, as shown in Figure 1(a) and (b) (11,
CONTACT robin D. rogers
rdrogers@ua.edu
*Dedicated to prof. Jerry l. atwood for his 75th birthday.
Supplemental data for this article can be accessed at https://doi.org/10.1080/10610278.2017.1405002.
© 2017 informa uK limited, trading as taylor & Francis Group

2
S. P. KELLEY AND R. D. ROGERS
Figure 1. coordination modes of amidoximate and imidedixoime.
Figure 2. General reactions of amidoximes or amidoximates with metal ions.
12). This coordination mode has only been observed for
complexes of amidoximate with [UO2]2+ or Mo4+/6+ (13).
Adjacent amidoxime groups on a polymer can also react
with each other to form a chelating imide dioxime which is
usually believed to be a more active binding site for [UO2]2+
than the amidoxime itself (14). These chelate [UO2]2+
through the O and central N atoms (15), forming a cycle
(Figure 1(c)). Again, this chelating mode is very reasonable
for [UO2]2+, which can readily accommodate six equatorial
coordinating atoms, but would be unusual for a smaller or
more spherical metal ion.
Improving selectivity for [UO2]2+ requires not only
increasing the strength of its interactions with amidox-
ime but also inhibiting the interactions with other metal
ions. Experimentally determined amidoxime coordination
modes have been recently reviewed by Bolotin et al. (13).
Whereas the η2 coordination of [UO2]2+ occurs through
cation exchange (Figure 2(a)), as this coordination mode
greatly increases the acidity of the oxime group (16), other
metal ions can coordinate to amidoxime without cation
exchange by metallating the oxime N atom (Figure 2(b)) or
the O atom of the zwitterionic tautomer (Figure 2(c)). Even
the amidoximate anion is capable of tautomerization, and
the chelating amidinate tautomer (Figure 2(d)) interacts
more strongly with [VO]2+ than amidoximate does with
[UO2]2+ (17). These observations suggest that controlling
protonation of amidoximes, particularly at the N position,
could be a mechanism for eliminating competition from a
broad range of metal ions.
In practice, major improvements in sorbent perfor-
mance are often accomplished without modifying the
moiety of the extractant that actually coordinates to the
metal ion. These approaches include grafting amidox-
ime groups onto ultra-high surface area porous aromatic
frameworks to increase capacity (18) or crosslinking known
extractant moieties with hydrophilic acrylamide linkers to
improve uptake kinetics (19). Such systems involve nonco-
valent or cooperative effects dictated by the relative posi-
tions of functional groups which are difficult to understand
by techniques that look at molecular structure, but our
group has found that crystal structure determination by
single crystal X-ray diffraction (SCXRD) on carefully chosen
models of such systems can capture some of the phenom-
ena which are occurring. For instance, we have previously
shown through crystal structures of metal complexes
that [UO2]2+-amidoxime interactions could be stabilized
through noncovalent, cooperative interactions with a
pendant cation (11) and that [UO2]2+ could be chelated
by adjacent amidoximes in a 1,2-position (20).
In our present study, we investigate the interactions of
trivalent lanthanides (Ln3+) with acetamidoxime (AcAO),
a simple monoamidoxime ligand with no other coordi-
nating functional groups. Only two structures have ever
been reported which contain Ln3+ ions interacting with

SUPRAMOLECULAR CHEMISTRY
3
amidoximate ions, both of which are mixed Ln3+/Ni2+ com-
plexes where anionic Ni2+ amidoximate complexes act as
chelators, rather than the amidoxime ligands themselves
(21, 22). We expected that Ln3+ ions would be useful for
understanding structural features which can be attributed
to the unusually strong interaction between [UO2]2+ and
amidoximes, since Ln3+ ions overlap with [UO2]2+ in size,
but lack these interactions. Additionally, [UO2]2+/Ln3+ sep-
arations are relevant in non-seawater applications such as
the use of amidoxime sorbents to recover uranium from
spent nuclear fuel processing streams (23). We sought to
begin our investigation of this under-explored system
by crystallizing complexes formed from the reaction of
Ln(NO3)3·6H2O (Ln = Nd, Gd, or Pr) and AcAO.
discrete, neutral, dinuclear complexes residing on crystal-
lographic 2-fold axes which pass through one of the AcAO
ligands (Figure 3). The 2-fold symmetry causes the two Ln3+
centres to be identical, relates two of the AcAO ligands
to each other, and causes the third to be disordered over
two symmetry related positions. There are three nonco-
ordinating water molecules per Ln2(NO3)6(AcAO)3(OH2)2,
two of which are related by symmetry and one of which
sits on the 2-fold axis.
The metal centres have coordination numbers of 10 and
are coordinated to three bidentate [NO3]− ions, one water
molecule, and three bridging AcAO ligands (Figure 4). The
coordination geometry is highly distorted from ideal poly-
hedral and can be described as a capped trigonal cupola,
as observed for 10-coordinate Pu3+ in polyborate networks
(24). Each [NO3]− ion chelates unsymmetrically, which is
typical for lanthanide nitrate complexes. The AcAO ligands
bridge two metal centres solely through the oxygen atom,
so that the coordination polyhedra of the two metals are
face-sharing.
The AcAO ligands are in a zwitterionic tautomeric form,
also seen in the crystal structure of UO2(NO3)2(AcAO)2 (25).
The manner in which their negatively charged oxygen
atoms bridge the Ln3+ centres is similar to that observed
in crystal structures of binuclear, face-sharing dilantha-
nide complexes bridged by alkoxides (26, 27). In all of
these structures, the μ2-oxygen atoms are pyramidal,
rather than planar. Two of the pyramids are oriented with
their vertices 120° to each other, presumably minimising
repulsion of the nonbonding lone pair located there. The
third oxygen atom can only have this orientation relative
to one of the neighbours but not both, and the result-
ing frustration appears to give rise to the disorder in the
AcAO ligand. The similarity of the bridging oxygen atoms
in Ln2(NO3)6(AcAO)3(OH2)2 to alkoxides indicates a high
degree of charge separation between the N and O. Thus,
Results and discussion
Crystals of Ln2(NO3)6(AcAO)3(OH2)2·3H2O were formed
by reacting aqueous solutions of the corresponding lan-
thanide nitrate hexahydrate and AcAO (Equation (1)).
Combining the solutions did not result in any precipitate,
nor did any solids form upon vapor diffusion of acetone
into the resulting solutions. The compounds were ulti-
mately crystallized by allowing the acetone-water mix-
tures to evaporate to dryness under ambient conditions,
yielding crusts of different coloured solids depending on
the metal (green for Pr3+, pink for Nd3+, and colourless for
Gd3+). Single crystals were isolated directly from these
reaction mixtures and analyzed by SCXRD.
(
)
3(aq)
(
2Ln NO3
+ 3AcAO(aq) + 5H2O(l)
)
(1)
(
)
→ Ln2 NO3 (AcAO)3 OH2 ⋅ 3H2O(s)
6
2
All three compounds were isomorphous and crystallized
in the face-centred orthorhombic space group Fdd2
with Z′ = 0.5. The Ln2(NO3)6(AcAO)3(OH2)2 molecules are
Figure 3. (colour online) Fifty per cent probability ellipsoid plots of pr2(No3)6(acao)3(oh2)2·3h2o (left), Nd2(No3)6(acao)3(oh2)2·3h2o
(centre), and Gd2(No3)6(acao)3(oh2)2·3h2o (right), emphasising the isostructural nature of the complexes. Dashed lines indicate shortest
intermolecular interactions.
Note: Disorder omitted for clarity.

4
S. P. KELLEY AND R. D. ROGERS
Figure 4. (colour online) Left: labelled ball and stick plot of asymmetric unit of pr2(No3)6(acao)3(oh2)2·3h2o with symmetry equivalent
atoms added to show full metal coordination sphere. Dashed lines indicate shortest intermolecular interactions. o10a is the symmetry
equivalent of acao oxygen atom o10. Right: Stick plot emphasising the shared face.
interaction, and the ability of the complex to form six
such interactions while adopting an undistorted geome-
try preferred by lanthanide alkoxide complexes is likely a
source of significant additional stabilization. The hydrogen
bonding to water also has the consequence of reducing
homomolecular hydrogen bonding between AcAO lig-
ands, which would weaken the metal-ligand interaction
of the hydrogen bond acceptor.
There are no other lanthanide amidoxime structures
to determine by comparison if the hydrogen bonding
to water molecules affects the metal-AcAO interaction.
However, the hydrogen bonding patterns in the crystal
structure do suggest that water molecules are delocalizing
some of the positive charge from the AcAO nitrogen cen-
tres. All water molecules act as hydrogen bond donors, as
well as Lewis bases, either by coordinating to the metal ion
or accepting hydrogen bonds. Water molecules ligated to
the metal centre donate the shortest hydrogen bonds out
of any water molecules in the structure, noncoordinating
Figure 5. (colour online) Ball and stick plot showing hydrogen
water molecules interacting with the iminium nitrogen
bonding of lattice water molecules to zwitterionic acao ligands.
donate the next shortest, and those interacting only with
amide groups donate long, bifurcated hydrogen bonds
(Table 1). This suggests that the water molecules which
interact with more acidic sites are themselves more acidic,
whether it be through coordination to the metal ion or
hydrogen bonding to the zwitterionic amidoxime.
Lattice water molecules also often play an important
role in stabilizing the packing of crystals by forming hydro-
gen bonded networks which buffer effects such as size
mismatches between ions and offer additional interactions
with polar functional groups. However, it can be seen in
Figure 6 that the Ln2(NO3)6(AcAO)3(OH2)2 complexes form
a hydrogen bonded network on their own, and the lattice
it is the ability of amidoxime to coordinate as a zwitterion
which stabilizes these particular complexes, and the fac-
tors which stabilize the zwitterion warrant investigation.
While lattice water molecules appear to be common
in, for instance, uranyl amidoximate complexes, the coor-
dination geometry of Ln2(NO3)6(AcAO)3(OH2)2 produces
an exceptional situation. The geometry of the complex
pre-organises the AcAO zwitterions into cavities which
are ideally suited for donating hydrogen bonds to a sin-
gle water molecule. This gives rise to the large hydrogen
bonded cycle shown in Figure 5. Each of these hydro-
gen bonds is a strong and in some cases charge-assisted

SUPRAMOLECULAR CHEMISTRY
5
Table 1. hydrogen bond donor-to-acceptor (o⋯o) distances in Å for hydrogen bonds donated by the water molecules in
ln2(No3)6(acao)3(oh2)2·3h2o.
Bond
Pr2(NO3)6(AcAO)3(OH2)2·3H2O
Nd2(NO3)6(AcAO)3(OH2)2·3H2O
Gd2(NO3)6(AcAO)3(OH2)2·3H2O
coordinated water molecules
o12⋯o6
o12⋯o9
2.705(4)
2.751(3)
2.711(6)
2.758(5)
2.732(4)
2.762(4)
Water molecules accepting iminium hydrogen bonds
o1S⋯o1
o1S⋯o3
2.836(4)
2.937(4)
2.828(5)
2.934(6)
2.820(4)
2.932(4)
Water molecules accepting only amide hydrogen bonds
o2S⋯o7
o2S⋯o9
3.302(4)
3.325(5)
3.305(6)
3.321(8)
3.323(4)
3.293(6)
Figure 6. (colour online) hydrogen bond motifs between [No3]−/acao groups (top) and [No3]−/oh2 ligands (bottom), both reinforced by
interactions with lattice water molecules, in ln2(No3)6(acao)3(oh2)2·3h2o.
water molecules simply support that network without
creating any new hydrogen bonded bridges. The struc-
ture packs by forming a dense hydrogen bonded network.
Infinite layers form perpendicular to the a axis through
hydrogen bonds between the water molecule occupy-
ing the 2-fold axis and the nitrate groups of adjacent
molecules (Figure 6, top). These layers are reinforced by
hydrogen bonds from the amide groups and coordinated
water molecules. The layers interact along a through more
hydrogen bonds involving the lattice water molecules and
nitrate groups (Figure 6, bottom). As the water molecules
have only an ancillary role in the packing, this further sup-
ports that they are incorporated in the structure due to the
strength of their interactions with the complex rather than
as just waters of crystallization.
Conclusions
The first crystal structures of lanthanide ions with free ami-
doxime ligands, Ln2(NO3)6(AcAO)3(OH2)2·3H2O have been
described. As hard acids, lanthanides prefer to coordinate
with the anionic oxygen atom of the zwitterionic form of
AcAO. However, the preferred geometry for such a com-
plex also creates three pre-organised hydrogen-bonding
cavities per complex, resulting in the crystallization of a tri-
hydrate. These hydrogen bonds further strengthen the met-
al-ligand interaction through stabilization of the zwitterion
and donation of additional electron density to the ligand.
The protonation state of amidoxime, particularly at
the imine nitrogen atom, has been shown to be a key fac-
tor in how it coordinates. These new structures present

6
S. P. KELLEY AND R. D. ROGERS
evidence for the extent to which supramolecular interac-
tions can stabilize the zwitterionic form of an amidoxime,
which would in turn prevent the proton exchange reaction
needed for irreversible adsorption. Such phenomena are
related to structural elements, such as the overall geom-
etry of a metal-amidoxime complex, which are under-in-
vestigated compared to the metal-amidoxime interaction
itself. A better understanding of how factors such as
hydration of the metal complexes affect their stability
could lead to ligand design strategies which augment the
selectivity of amidoxime-based sorbents for uranium ions
without changing the fundamental nature of the amidox-
ime-metal interaction. For instance, the hydrogen bonds
in Ln2(NO3)6(AcAO3)3(OH2)2·3H2O could be inhibited by
changing the functional groups at noncoordinating posi-
tions of the ligand (such as the R groups of the noncoor-
dinating amide), allowing the selectivity of amidoxime to
be tuned without changing the fundamental nature of its
interaction with metal ions.
The ability to stabilize or destabilise protonation of the
N and O positions of the oxime group may prove to be a
tool for improving selectivity for [UO2]2+ since it affects the
availability of these sites for coordination. Since [UO2]2+
coordinates to both atoms of an oximate anion, while the
ions that outcompete it such as Na+ and V4+/5+ coordinate
only to one of the atoms, the protonation states of ami-
doxime groups likely have a role in selectivity that is not
yet fully understood.
removed by rotary evaporation to give a white powder,
which was re-dissolved in hot methanol and recrystallized
on standing overnight to give long, colourless prisms of
AcAO. 1H NMR (300 MHz, dmso-d6) δ: 8.65 (s,=N–OH), 5.34
(s, –NH2), 1.61 (s, –CH3). IR (cm−1, neat sample in ATR mode):
3490 (m), 3366 (m), 3153 (m, br), 2769 (m), 1653 (s), 1586
(s), 1433 (w), 1393 (s), 1364 (m), 1104 (w, br), 1047 (w), 1021
(w), 891 (s), 816 (m, br).
Crystallization of lanthanide-AcAO complexes
Ln2(NO3)6(AcAO)3(OH2)2·3H2O
Approximately 50 mg each of Ln(NO3)3·6H2O salt was
dissolved in 200–300 μL of deionized water. Three molar
equivalents of AcAO were weighed out, dissolved com-
pletely in deionized water, and added to the Ln(NO3)3 solu-
tions. The mixtures were sealed in vials containing ca. 1 mL
of acetone, which transferred completely into the aqueous
solution through vapor diffusion without any precipitation
occurring. The water/acetone solutions were then allowed
to evaporate under ambient conditions. Crystals for struc-
ture determination were obtained after ca. 1 month.
Crystal structure determination
Crystals were selected under an optical polarising micro-
scope, mounted on a glass fibre with silicone grease, and
cooled to the collection temperature under a stream of
cold nitrogen using an Oxford N-Helix cryostat (Oxford
Cryosystems, Oxford, UK). A strategy of scans about the
omega and phi axes were used to collect a hemisphere
of unique data for each crystal under Mo-Kα radiation
(λ = 0.71073 Å). Data collection, unit cell determination,
integration, scaling, and absorption correction were all
performed using the Bruker Apex2 software suite (29).
All three structures were solved by direct methods. The
metal atom site was initially located and refined, and all
remaining non-hydrogen atoms were located from the
difference map. All non-hydrogen atoms were refined ani-
sotropically by full matrix least squares refinement against
F2. All structures contained an AcAO ligand which was dis-
ordered by symmetry over two positions and whose site
occupancy factors were therefore fixed at 50%. Hydrogen
atoms bonded to strong hydrogen bond donors were
located from the difference map. The positions of these
hydrogen atoms were freely refined, while their thermal
parameters were constrained to ride on the carrier atom. It
was found that these hydrogen atoms converged to posi-
tions that were realistic based on molecular geometry and
the positions of nearby hydrogen bond acceptors, although
they often refined to unrealistic N–H or O–H bond distances.
This is a typical error for hydrogen atom positions that are
refined against hard X-ray data collected on a heavy-atom
Experimental
Materials and methods
Acetonitrile (EMD Chemicals, Billerica, MA), hydroxylamine
(50 wt.% solution in water, Alfa-Aesar, Ward Hill, MA), and
Ln(NO3)3·6H2O (Strem Chemical, Newburyport, MA) were
used as received from their commercial sources. Deionized
water was obtained from an in-house deionizer system
(Culligan International Co., Rosemont, IL; typical resistivity
17.8 MΩ·cm). 1H NMR was measured using a Bruker Avance
300 MHz NMR spectrometer (Bruker Corporation, Billerica,
MA). Infrared spectroscopy was measured using a Bruker
ALPHA FT-IR with an ATR accessory (Bruker Optics, Billerica,
MA). SCXRD was measured using a Bruker diffractometer
equipped with a 3-circle PLATFORM goniometer and an
Apex II CCD area detector (Bruker-AXS, Madison, WI).
Synthesis of AcAO
AcAO was synthesised according to a published procedure
(28). Acetonitrile and 50 wt.% hydroxylamine in water were
separately mixed with methanol and combined at a 1:1 M
ratio of acetonitrile to hydroxylamine. The reaction mix-
ture was stirred for 48 h at room temperature. Solvent was

SUPRAMOLECULAR CHEMISTRY
7
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further restraints or constraints to these hydrogen atoms
since they converged to positions that effectively illustrated
the hydrogen bonding and protonation states of the struc-
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lattice water molecules in Pr2(NO3)6(AcAO)3(OH2)2·3H2O
lacking a hydrogen bond acceptor. This molecule donates
a bifurcated hydrogen bond which is missed by the test due
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No potential conflict of interest was reported by the authors.
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