Phosphine oxide functionalised imidazolium ionic liquids as tuneable ligands for lanthanide complexation

Article abstract of DOI:10.1039/c2cc31544k

A series of novel, phosphine oxide functionalised ionic liquids have been synthesised and their application as tuneable lanthanide complexing agents is demonstrated.

Full text of DOI:10.1039/c2cc31544k

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COMMUNICATION
Phosphine oxide functionalised imidazolium ionic liquids as tuneable
ligands for lanthanide complexationwz
Jorge Alvarez Vicente, Agata Mlonka, H. Q. Nimal Gunaratne, Ma$gorzata Swadz
´
´
ba-Kwasny
and Peter Nockemann*
Received 29th February 2012, Accepted 30th April 2012
DOI: 10.1039/c2cc31544k
A series of novel, phosphine oxide functionalised ionic liquids
have been synthesised and their application as tuneable lanthanide
complexing agents is demonstrated.
In this communication, we report on the synthesis of the
first diphenyl phosphine oxide-functionalised ionic liquids
(POFILs) and their tuneable ability to complex lanthanides.
These POFILs were synthesised in two steps by reacting alkyl
bromide-functionalised 1,2-dimethyl-imidazolium ionic liquids
with [Tf2N]ꢀ or [PF6]ꢀ anions and potassium diphenylphosphine
(Scheme 1). The resulting phosphine functionalised ionic
liquid was then oxidised using hydrogen peroxide. The synthesis
delivered clean products in fairly good yields (around 70%);
all ionic liquids have been characterised using 1H, 13C,
31P NMR and IR spectroscopy, and ESI mass spectrometry
(see ESIz).
Selective extraction of metal ions from aqueous solutions is a
major challenge in mining, nuclear fuel recycling and in
sustainable metal waste recovery.1 Ionic liquids have been
described for a number of applications in the separation and
extraction of specific metal ions from aqueous solution.2–5
Davis and Rogers et al. introduced the concept of functiona-
lised (‘‘task-specific’’) ionic liquids designed for the extraction
of heavy metal ions.6,7 Functional groups selective towards
metal complexation can be tethered covalently either to the
cation (such as thioether, thiourea, nitriles,8 carboxylic acids9) or
the anion (b-diketonates2,10). Davis reported on phosphoramide-
functionalised imidazolium ionic liquids for the extraction of
actinides.11 Odinets et al. reported on bidentate carbamoylmethyl-
phosphine oxide (CMPO) functionalised ionic liquids for the
actinide and rare earth metal recovery.12 Ionic liquids based on
quaternary ammonium cations bearing phosphoryl moieties and
their application in the liquid–liquid extraction of U(VI) were
reported by Ouadi et al.13 Synthesis of a PQO functionalised
imidazolium salt as an intermediate towards carbene–phosphine
complexes has been reported by Tsoureas et al.14
The PQO stretching vibration in IR was not greatly affected
by the length of the ꢀ(CH2)nꢀ (n = 2, 3, 4, 6, 8) spacer unit,
whereas the 31P NMR chemical shift increased with the
increasing spacer length. The phase transitions for the ionic
liquids were determined by Differential Scanning Calorimetry
(DSC) and are shown in Table 1. All of the bistriflimide ionic
liquids were liquids at RT, except for 1C, which exhibits a
melting point.
All compounds had well defined glass transitions, ranging from
ꢀ24.9 1C to ꢀ5.8 1C (see Table 1). [DMImC3P(O)Ph2][PF6] (2)
was expected to have a higher melting point, allowing for
crystallisation and the determination of the crystal structure.
Fig. 1 shows the cation of the crystal structure of 2 in
Besides metal extraction, ionic liquids have been used as
solvents for lanthanide spectroscopy, giving high luminescence
quantum yields and an extraordinary photostability compared
to organic solvents. These can be incorporated as components
of lanthanide containing hybrid materials, like luminescent
polymer films or ionogels.15–17 Phosphine oxides are generally
well-known complexing ligands for lanthanides18–20 due to
their ability to remove coordinating water molecules from the
first coordination sphere and thereby avoiding vibronic O–H
luminescence quenching in optical applications.
The QUILL Research Centre, School of Chemistry and Chemical
Engineering, The Queen’s University of Belfast, Belfast BT9 5AG,
UK. E-mail: p.nockemann@qub.ac.uk
w This article is part of the ChemComm ‘Ionic liquids’ web themed
issue.
z Electronic supplementary information (ESI) available: Synthesis and
characterisation of the ionic liquids; IR spectra; ES mass spectra;
crystallographic data. CCDC 869702 and 869703. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2cc31544k
Scheme 1 Reaction scheme and structures of the ionic liquids
1-alkyl-diphenylphosphine-oxide-2,3-dimethylimidazolium-cations
[DMImCnP(O)Ph2]+ and the anions [A]ꢀ bis(trifluoromethylsulfonyl)-
imide, [Tf2N]ꢀ (chain length n = 2, 3, 4, 6, 8; 1A–1E) and [PF6]ꢀ
(chain length n = 3, 2).
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6115–6117 6115

Table 1 Properties of the phosphine oxide functionalised ionic
liquids
Phase
transition/1C cmꢀ1
n(PQO)/ 31P NMR
shift/ppm
Ionic liquid
[DMImC2P(O)Ph2][Tf2N] (1A) ꢀ5.79 (Tg)
[DMImC3P(O)Ph2][Tf2N] (1B) ꢀ8.25 (Tg)
[DMImC4P(O)Ph2][Tf2N] (1C) ꢀ8.97 (Tg)/
96.4 (m.p.)
1179
1181
1177
32.67
37.39
37.81
[DMImC6P(O)Ph2][Tf2N] (1D) ꢀ20.24 (Tg) 1177
38.22
38.28
[DMImC8P(O)Ph2][Tf2N] (1E) ꢀ24.87 (Tg) 1179
Fig. 2 31P NMR complexometric titration. The 31P chemical shifts of
the respective [DMImCnP(O)Ph2][Tf2N] ionic liquids (n = 2, 3, 4, 6, 8)
with increasing amounts of La(OTf)3ꢁ6H2O were monitored.
Fig. 1 Cation in the crystal structure of 1-propyl-diphenylphosphine-
oxide–2,3-dimethylimidazolium [PF6]ꢀ showing an OꢁꢁꢁH contact to an
adjacent cation. O10 is at (x ꢀ 1, y, z) and H8A00 is at (1 + x, y, z). The
ellipsoids are shown at 50% probability. Anions are not shown for clarity.
the presence of a PQOꢁ ꢁ ꢁH contact between the phosphine
oxide functionality and protons on the a-carbon to phos-
phorus of an adjacent cation, with a C8–H8Aꢁ ꢁ ꢁO10 separa-
tion of 2.58 A [with O10 at (x ꢀ 1, y, z)] and a C8–H8Aꢁ ꢁ ꢁO10
angle of 1581.
It has been demonstrated for nitrile functionalised ionic
liquids that the length of the spacer chain between the CN
group and the positively charged methyl-pyrrolidinium cation
has a distinct influence on the electron density of the nitrile
functionality and therefore on the donor properties towards
metal centres.8 These electron-withdrawing effects were found
to have some degree of influence over six CH2 groups between
the positive charge and the CN group. As a measure for the
coordinating ability of the phosphine oxide functionalised
ionic liquids, a complexometric titration with an increasing
amount of La(OTf)3ꢁ6H2O has been carried out by 31P NMR
spectroscopy. The 31P NMR chemical shifts of the respective
[DMImCnP(O)Ph2][Tf2N] ionic liquids (n = 2, 3, 4, 6, 8) with
increasing amounts of La(OTf)3ꢁ6H2O were monitored.
Lanthanum has been used, since it is diamagnetic, as opposed
to complexes of paramagnetic lanthanides (such as Eu, Tb, Nd).
The triflate, [OTf]ꢀ, has been used, since it is a weakly
coordinating anion towards the lanthanides in solution. The
results of the NMR titration are shown in Fig. 2.
Fig. 3 Determined binding constants (triangles) and 31P NMR shifts
(rhombuses) of the [DMImCnP(O)Ph2][Tf2N] ionic liquids (n = 2, 3, 4,
6, 8).
higher (40.8) than for n = 3 (see Fig. 3). ES mass spectra of
La(OTf)3ꢁ6H2O solutions in methanol were obtained in the
presence of a 10-fold excess of [DMImCnP(O)Ph2][Tf2N] ionic
liquids (n = 2, 3, 4, 6, 8). The expected masses corresponding
to lanthanide–ionic liquid complexes such as [La(OTf)x-
(DMImCnP(O)-Ph2)y(Tf2N)z]+ (x = 1–4; y = 1–6; z = 1–3)
were identified to be present in these solutions and a full table
with the assigned fragments is shown in the ESI.z With
increasing spacer length, species with more phosphine oxide
ligands were detected, i.e. larger values for y (e.g. species y = 6
only for n = 6, 8; see ESIz).
We further examined the possibility of using phosphine
functionalised ionic liquids as additional ligands for lanthanide
There is a distinct difference in the 31P shifts for the spacer
lengths with 2 and 3 CH2 groups between the cation and the
Ph2PQO group. The effect significantly levels off as the chain
length increases beyond 4 or more CH2 groups. The binding
constants were calculated from these 31P NMR shifts and
range from 23.2 to 140.8, reflecting the ability of increased
binding of the lanthanide to the phosphine oxide ligand with
increasing lengths of the spacer unit; however, the binding
constant of the compound with the shortest spacer is slightly
Fig. 4 Structure of the europium complex Eu(hfa)4(DMImC3P(O)Ph2).
c
6116 Chem. Commun., 2012, 48, 6115–6117
This journal is The Royal Society of Chemistry 2012

great potential for applications in lanthanide, actinide and
transition metal separation and extraction, for light emitting
devices and for composite materials with diverse polymers.
Further investigations, involving simulations, are underway to
examine the formation of the europium complex with the
unusual coordination geometry, and whether the spacer unit
has an influence on a potential ion-pairing effect. We are
currently exploring other hydrophobic functionalised cations,
like tri-alkyl phosphonium, as well as their luminescence
characteristics of lanthanide complexes (Vis and NIR emitters).
A.M. thanks IAESTE for an internship at the QUILL
research centre. P.N. thanks the EPSRC for a RCUK fellowship.
The EPSRC UK National Crystallography Service (NCS) is
acknowledged for crystal data collection. The QUILL IAB is
acknowledged for their financial support.
Fig. 5 Crystal structure of the complex Eu(hfa)4(DMImC3P(O)Ph2).
Notes and references
The ellipsoids are shown at 40% probability.
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(1)
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In summary, we have synthesised and characterised a new
class of phosphine oxide bearing ionic liquids and demon-
strated their tuneable metal ligating abilities. Increasing
lengths of the spacer unit between cations and the phosphine
oxide group appear to increase the binding ability of the ligand
towards lanthanide metals. These novel ionic liquids have a
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c
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Chem. Commun., 2012, 48, 6115–6117 6117