Derivative of cyclen with three methylene(phenyl)phosphinic acid pendant arms. Synthesis and crystal structures of its lanthanide complexes

Article abstract of DOI:10.1039/a907018d

A new macrocyclic ligand 1,4,7,10-tetraazacyclododecane-1,4,7,10- tetraylmethyltrimethylenetris(phenylphosphinic acid) H₃L¹ was synthesised and its complexes with lanthanides were prepared. X-Ray analysis of seven Ln complexes (Ln = La, Ce, Nd, Eu, Tb, Er, Yb) shows a formation of dimers [LnL¹]₂ with phosphinic acid bridges in the solid state. All complexes are isostructural and the coordination polyhedra are formed by four N atoms and four O atoms of phosphinic acid groups. A water molecule is placed near the O₄ base. The Ln-O_w distance is strongly dependent on Ln(III), it varies from 2.815(6) A in the La complex to 4.448(10) A in the Yb complex. The ³¹P NMR studies show formation of a rather complex mixture of isomers in solution. The dimer stability in solution is verified by reaction with triphenylphosphine oxide (tppo) and with pyridine N-oxide and by cyclic voltammetry. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/a907018d

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Derivative of cyclen with three methylene(phenyl)phosphinic acid
pendant arms. Synthesis and crystal structures of its lanthanide
complexes
a
a
a
b
a
ˇ
Jan Rohovec, Pavel Vojtísˇek, Petr Hermann, Jirí Ludvík and Ivan Lukesˇ *
a Department of Inorganic Chemistry, Universita Karlova (Charles University), Albertov 2030,
128 40 Prague 2, Czech Republic
b J. Heyrovský Institute, Dolejsˇkova 3, 182 23 Prague 8, Czech Republic
Received 31st August 1999, Accepted 19th November 1999
A new macrocyclic ligand 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraylmethyltrimethylenetris(phenylphosphinic
acid) H3L1 was synthesised and its complexes with lanthanides were prepared. X-Ray analysis of seven Ln complexes
(Ln = La, Ce, Nd, Eu, Tb, Er, Yb) shows a formation of dimers [LnL1]2 with phosphinic acid bridges in the solid
state. All complexes are isostructural and the coordination polyhedra are formed by four N atoms and four O atoms
of phosphinic acid groups. A water molecule is placed near the O4 base. The Ln–Ow distance is strongly dependent
on Ln(), it varies from 2.815(6) Å in the La complex to 4.448(10) Å in the Yb complex. The 31P NMR studies show
formation of a rather complex mixture of isomers in solution. The dimer stability in solution is verified by reaction
with triphenylphosphine oxide (tppo) and with pyridine N-oxide and by cyclic voltammetry.
Lanthanide complexes with azacycles usually substituted on
nitrogen atoms by four equal pendant groups are widely
investigated because of their use, e.g., as contrast agents in
MRI,1,2 for labelling antigenic antibodies using radioisotopes2,3
for both diagnostic and therapeutic purposes, and also as
catalysts in the splitting of nucleic acids.4 Cyclen derivatives
with acetate side chains H4DOTA (H4DOTA = 1,4,7,10-tetra-
azacyclododecane-1,4,7,10-tetraacetic acid) and its complexes
are the most thoroughly studied substances of this kind.5 The
ligands with acetate side chains form thermodynamically stable
metal complexes, the most stable being generally formed from
those with di- and tri-valent metals.5
In addition, a study of H4DOTA6,7 analogues with side
chains containing a methylenephosphonic acid8–10 (–CH2–
PO3H2) or methylenephosphinic acid8,11–17 (–CH2–P(R)O2H)
pendant arm was initiated, in a search for ligands with prop-
erties different from compounds containing acetic acid arms.
We focused on a study of the influence of the methylene-
phosphinic acid substituents on complexing properties of the
azacycles. We published results dealing with the synthesis and
complexing properties of 1,4,7-triazacyclononane and 1,4,7,10-
tetraazacyclododecane substituted with methylenephosphinic
acid pendant groups (R = –CH2–P(H)O2H).18 In addition, we
Chart 1
reported the synthesis,19 structure and solution properties of
1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayltetrakis[methyl-
prepare its complexes with yttrium and lanthanides, and to
ene(phenyl)phoshinic acid] H4L2 and 1,4,8,11-tetraazacyclo-
determine their structure by X-ray analysis.
tetradecane-1,4,8,11-tetrayltetrakis[methylene(phenyl)phos-
hinic acid] H4L3. Formulae of the ligands mentioned are shown
in Chart 1 together with their abbreviations.
Results and discussion
Lanthanide complexes of both tetraacetic and tetrakis-
(methylenephosphinic) acid derivatives have a salt like char-
acter resulting in relatively high osmolality under physiological
conditions. Therefore, new ligands containing acetic acid
pendant arms with charge 3Ϫ have been introduced, such as
H3pydo3a (3,6,9,15-tetraazabicyclo[9,3,1]pentadeca-1(15),11,
13-triene-3,6,9-triacetic acid)20 and H3hp-do3a (1,4,7,10-tetra-
azacyclododecane-1-(2-hydroxypropyl)-4,7,10-triacetic acid).21
The aim of this paper is to synthesise a new cyclen derivative
containing three methylenephosphinic acid arms H3L1, to
Synthesis of H3L1
Synthesis of 1-methyl-1,4,7,10-tetraazacyclododecane followed
Scheme 1. The syntheses of 1 and 2 are known from the
literature.22–25 A cyclisation procedure for formation of 3, is also
known; it was used, for example, for the synthesis of cyclene
and 1,7-dimethylcyclene. However, we prefer the reaction of 1
with tosylated diethanolamine (2) which was found to be more
convenient than using bis(2-chloroethyl)methylamine.22–25 An
alternative cyclisation procedure of N-methyldiethanolamine
J. Chem. Soc., Dalton Trans., 2000, 141–148
This journal is © The Royal Society of Chemistry 2000
141

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Scheme 1
Scheme 2
ditosylate and diethylenetriamine tritosylate was not used due
to difficulties in obtaining the former compound in a pure
form.
The deprotection26 of 3 gave 1-methyl-1,4,7,10-tetraazacyclo-
dodecane tetrahydrochloride 4 in high yield. Subsequently,
H3L1 was synthesised by the Mannich reaction of 4 with para-
formaldehyde and phenylphosphinic acid in aqueous HCl
(Scheme 2). The optimised reaction conditions are described in
the Experimental section. The ligand H3L1 was identified by
ESI/MS spectrum, by both 1H NMR and 31P NMR spectra and
correct organic analysis.
Preparation of the complexes
Direct reaction of an appropriate lanthanide oxide with H3L1
required heating under reflux overnight. Therefore, the reaction
of H3L1 with lanthanide chlorides was found to be more con-
venient. An aqueous solution of LiOH was used for neutralis-
ation of the reaction mixture because of solubility of LiCl in
methanol. The complexes easily crystallise from the reaction
mixture in pure form as hydrates, however they rapidly lose
a part of the hydrated water. The stoichiometry of the
compounds prepared was confirmed by elemental analysis
and by electrospray ionisation mass spectroscopy ESI/MS. Pre-
dominant peaks in the spectra correspond to molecular ions
associated with Hϩ, Liϩ, Naϩ or Kϩ ions.
Fig. 1 View of [YbL1]2ؒ6H2O with the atom numbering scheme.
Structures of the complexes
All compounds were found to crystallise in the space group P21/
c (no. 14) with similar values of lattice parameters (Table 1).
The structures consist of centrosymmetrical electroneutral
dimers. The structure of the dimer is shown in Fig. 1 and crystal
packing is depicted in Fig. 2 and 3 for Ln = Yb. Table 2
lists selected bond distances and angles.
The (L1)3Ϫ is co-ordinated to a lanthanide ion by four nitro-
gen atoms (N1–N4), two oxygen atoms from two monodentate
phosphinate groups (O11, O21), and two oxygen atoms from
two bidentate phosphinate groups (O31, O32i). The nitrogen
and oxygen atoms form bases N4 and O4 which are planar and
parallel within experimental error. Twist angles of the bases
around the local four-fold axis are in the range 28.0Њ– 33.1Њ the
values being independent of ionic radii. Thus, the co-ordination
polyhedra of Ln() seem to be “twisted square prismatic”,
therefore they are nearer to a square antiprism (ideal twist angle
Fig. 2 Crystal packing of [YbL1]2ؒ6H2O (view along the x axis).
142
J. Chem. Soc., Dalton Trans., 2000, 141–148

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Fig. 3 Crystal packing of [YbL1]2ؒ6H2O (view along the y axis).
45Њ) than to a square prism (ideal twist angle 0Њ). The Ln() ion
lies between coplanar O4 and N4 bases (Fig. 4) the distances
between the bases being independent of Ln() radii (2.67(1) Å
for La and 2.65(1) Å for Yb). However, with decreasing ionic
radii, the lanthanide ion Ln() shifts from the O4 base towards
the N4 base, as is shown in Table 2. Simultaneously with these
shifts, distances of O atoms in the O4 square base become
smaller (O11–O31 = 4.631(6) Å and O21–O32i = 4.480(6) Å for
La; O11–O31 = 4.109(4) Å and O21–O32i = 3.876(4) Å for Yb).
The N4 square base seems to be more rigid and differences
in the N ؒ ؒ ؒ N distances vary less (N1–N3 = 4.292(7) Å and
N2–N4 = 4.223(7) Å for La, N1–N3 = 4.192(5) Å and N2–
N4 = 4.126(5) Å for Yb).
In view of the potential utilisation of these complexes as
contrast agents, an important feature of these structures is the
position of the water molecule labelled O99. The vector Ln–
O99 has the same direction in all the structures studied, and
thus the molecule is situated approximately in the same direc-
tion toward the O4 plane. However, the Ln–O99 distance is
strongly dependent on Ln() and increases in the order: 2.815
Å La, 2.854 Å Ce, 3.171 Å Nd, 3.745 Å Eu, 4.278 Å Tb, 4.050 Å
Er and 4.448 Å in the Yb complex. Thus, this water molecule
(O99) can be considered as co-ordinated in La and Ce com-
plexes and obviously unco-ordinated in the complexes of Eu,
Tb, Er and Yb. The distance in [NdL]2 is a little higher than
that for the co-ordinated O atoms. Nevertheless, very weak
Coulombic interaction is assumed even for Yb ؒ ؒ ؒ O99. The
O99 is also connected with O12 and O94 (3.02 Å and 3.11 Å in
the Yb complex) through weak hydrogen bonds. From this
point of view the position of the water molecule seems to be
given rather by the room between phosphinic pendant arms
above the O4 base than by the interactions mentioned
previously.
As was mentioned above, the size of the N4 square bases in
these structures are virtually the same as the common square
{3333} conformation of the tetrazacyclododecane. This con-
formation corresponds to that of the lowest strain energy.27 All
chains are aligned in a cis-arrangement, i.e., on one side of the
ring. There are three types of torsion angles in the tetraaza-
macrocycles of these compounds, around C(n)–C(n ϩ 1) bonds
(average value Ϫ61Њ) and two conformationally non-equivalent
angles around C–N bonds, N1–C1 (gauche) and around C2–N2
(anti). The angles around the C–N bonds give the average
values of 76Њ (gauche) and 163Њ (anti).
The co-ordination of the phenylphosphinic acid group
causes chirality of the phosphorus atoms. Configurations of
the stereogenic phosphorus centres are the same in the seven
J. Chem. Soc., Dalton Trans., 2000, 141–148
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Table 2 Selected bond lengths (Å) and angles (Њ) in structures [LnL1]2ؒxH2OؒyMeOH
Ln
x ; y
La
6 ; 0
Ce (170 K)
5 ; 1
Nd (170 K)
7 ; 0
Eu (170 K)
5 ; 2
Tb
6 ; 0
Er
5 ; 0
Yb (170 K)
6 ; 0
Ln–O(99)
Ln–O(11)
Ln–O(21)
2.815(6)
2.854(6)
2.392(4)
2.476(4)
2.434(4)
2.380(4)
0.9207(3)
2.851(5)
2.789(5)
2.802(5)
2.769(4)
1.8225(5)
2.7291(6)
167.71(3)
168.5(1)
18.2(1)
0.843(2)
1.822(2)
4.310(7)
4.210(6)
4.567(5)
4.486(5)
Ϫ29.9(2)
Ϫ30.2(1)
Ϫ29.1(2)
Ϫ30.9(2)
Ϫ30(2)
3.171(3)
2.362(7)
2.403(6)
2.392(5)
2.340(6)
0.948(3)
2.776(7)
2.732(8)
2.752(8)
2.720(7)
1.734(3)
2.6770(4)
173.2(2)
165.1(1)
16.5(1)
0.942(3)
1.738(4)
4.261(10)
4.203(10)
4.414(9)
4.290(9)
Ϫ30(3)
3.745(12)
2.333(5)
2.341(4)
2.364(4)
2.285(4)
1.0139(3)
2.716(5)
2.652(5)
2.675(5)
2.664(5)
1.6589(4)
2.6701(3)
173.66(2)
159.1(2)
21.2(2)
1.011(2)
1.658(3)
4.244(7)
4.159(7)
4.315(6)
4.068(6)
Ϫ31(2)
4.278(19)
4.050(30)
4.448(10)
2.243(3)
2.259(3)
2.296(3)
2.205(3)
1.0853(2)
2.631(3)
2.579(3)
2.616(3)
2.606(3)
1.5732(2)
2.6529(3)
172.44(1)
157.0(1)
27.1(1)
1.003(1)
1.573(2)
4.192(5)
4.126(5)
4.109(4)
3.876(4)
Ϫ32.8(1)
Ϫ32.8(1)
Ϫ32.6(1)
Ϫ31.3(1)
Ϫ32.4(1)
5.968(1)
121.47(1)
56.63(1)
81.3(1)
2.420(5)
2.466(4)
2.464(4)
2.382(4)
0.8485(3)
2.858(5)
2.807(5)
2.827(5)
2.803(5)
1.8337(4)
2.6758(3)
2.319(6)
2.292(7)
2.329(7)
2.247(7)
1.0357(4)
2.678(8)
2.629(8)
2.656(8)
2.641(8)
1.6241(5)
2.6575(5)
2.254(16)
2.260(14)
2.281(14)
2.219(15)
1.036(1)
2.652(17)
2.584(19)
2.616(19)
2.634(19)
1.597(2)
2.631(2)
175.12(9)
157.2(5)
22.6(5)
1.033(8)
Ln–O(31)
Ln–O(32)i
Ln–Q(O)
Ln–N(1)
Ln–N(2)
Ln–N(3)
Ln–N(4)
Ln–Q(N)
Q(O)–Q(N)
Q(O)–Ln–Q(N)
O(99)–Ln–Q(N)
O(99)–Ln–Q(O)
Ln–(plane O4)
Ln–(plane N4)
N1 ؒ ؒ ؒ N3
171.53(3)
168.7(1)
14.4(2)
175.20(4)
156.5(3)
24.6(3)
0.838(2)
1.030(3)
1.855(2)
4.292(7)
4.223(7)
4.631(6)
4.480(6)
1.626(4)
4.217(10)
4.156(11)
4.234(9)
3.983(10)
1.596(10)
4.184(26)
4.135(28)
4.115(20)
3.881(23)
N2 ؒ ؒ ؒ N4
O(11) ؒ ؒ ؒ O(31)
O(21) ؒ ؒ ؒ O(32)i
twist angle of pen.1
pen.2
Ϫ28.4(2)
Ϫ28(2)
Ϫ28(2)
Ϫ32.3(2)
Ϫ29.2(2)
Ϫ30.9(3)
Ϫ32.3(7)
Ϫ29.2(3)
Ϫ29.9(2)
Ϫ33.1(3)
Ϫ30.6(3)
6.132(1)
117.1(1)
62.3(2)
Ϫ29.7(2)
Ϫ30.9(2)
Ϫ32.5(2)
Ϫ31(2)
6.070(1)
119.86(1)
60.34(1)
81.0(2)
Ϫ29.4(3)
Ϫ31.1(3)
Ϫ32.0(3)
Ϫ30.9(3)
Ϫ29.7(7)
Ϫ31.2(7)
Ϫ32.0(6)
Ϫ31.3(7)
6.021(11)
121.25(7)
59.8(1)
pen.3
pen.4
mean twist angle
Lni ؒ ؒ ؒ Ln
6.208(1)
6.202(1)
114.65(2)
61.76(2)
76.7(1)
6.024(1)
Lni–Ln–Q(N)
Lni–Ln–Q(O)
Lni–Ln–O(99)
113.89(1)
64.60(2)
77.2(1)
120.36(2)
59.09(2)
83.1(3)
77.7(1)
81.6(5)
Q(O) is a centroid of the O4 base, Q(A) is a centroid of the N4 base.
Chart 2
Fig. 4 Lanthanide co-ordination sphere, position of Ln between N4
and O4 planes.
structures studied. The usual R,S notation can be used for P1
and P2 atoms of non-bridging monodentate phenylphosphinic
acid groups (Chart 2). To express the configuration of the P3
atom of the didentate bridging phenylphosphinic acid group
using common rules is difficult. On the basis of a comparison
of the torsion angles the configuration of the P3 atom is closer
to the P1 atom and, therefore, it should be considered as R.
The pendant arms bearing the phenylphosphinic acid groups
can be orientated clockwise (enantiomer Λ) or counterclockwise
(enantiomer ∆) as is shown in Chart 3.
Both Λ and ∆ isomers occur in each molecule of the dimeric
complex in the ratio 1:1 because of the presence of the inverse
centre. Thus, a molecule of the dimer can be regarded as the
meso form, i.e., a combination of two “monomeric” units with
opposite configurations (∆ and Λ) of the pendant arms.
As was mentioned above, the coordination polyhedra are
linked through an eight-membered ring, which is typical of
phosphinates.28 The presence of two P atoms and in particular
Chart 3
two Ln() atoms with a flexible geometry leads to an irregular
centrosymmetric conformation. The independent part of this
ring can be described by the pattern Ϫg,ϩg,(Ϫg),ϩg, where
are gauche and (Ϫg) represents an intermediate position
g
between gauche and anti.29
The packing of the dimeric complex molecules leaves infinite
channels in the crystals which are parallel with the monoclinic
axis. In all the seven structures studied, these channels are
occupied by water and water and/or MeOH molecules of crys-
tallisation, hydrogen-bonded to each other (distances Ow–Ow in
the range 2.8–3.2 Å) and, some of them, to outward non-
co-ordinated oxygen atoms from the phenylphosphinic acid
groups (distances Ow–On2 in the range 2.7–3.0 Å). The system
of hydrogen bonds could be studied in detail only in the Yb
complex, where all hydrogen atoms were determined. The non-
co-ordinated oxygen atoms are linked to solvate molecules by a
144
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Table 3 Cyclic voltammetric data for the electrooxidation of [CeL2]Ϫ and [CeL1]2 in non-aqueous media. Concentration of substances
1–4 × 10Ϫ4 mol lϪ1, 0.05 M [Bu4N]PF6 (Epa = anodic peak potential, Epc = cathodic peak potential, all data are in V vs. SCE)
Solvent
[CeL2]Ϫ Epa
Epc
EpaЈ
[CeL1]2 Epa(1)
Epc(1)
Epa(2)
Epc(2)
AN/EtOHa
AN
DMF
0.71
0.64
0.58
0.62
0.55
0.36
0.76–0.80
0.83
0.72
1.04
0.95
a Acetonitrile:EtOH, 9:1.
hydrogen bond network (O12–O99, O12–O96, O22–O88, O22–
O96). Further hydrogen bonds connecting the molecules of
solvent were also found. The diameters of channels (4.5–6.5 Å)
are close to those observed in structures Li[Ln(H2O)L2]ؒ10H2O
(ref. 30) and smaller than in similar structures of benzylphos-
phinates.31 Due to the size of the channels, it is not surprising
that water (or MeOH) molecules have room enough to move
along the channels and such a structure gives ample pos-
sibilities for disorder (both static and dynamic) of the channel
contents. This feature really made the structure-solution dif-
ficult. In addition, the crystals outside in the mother liquid
(even under an inert atmosphere) are highly unstable and thus,
it is understandable, that solvation of single crystals varied,
and precise determination was not possible by X-ray experi-
ments. The cumulative effects of these factors, together with an
extremely high mechanical brittleness of crystals, reduce sub-
stantially the accuracy and precision of the structure determin-
ations of, e.g., Er and Tb complexes. Similar problems are
common with this type of compound.
form even in solution. If we assume the centrosymmetric struc-
ture of the bridging phosphinate groups, then 16 isomers from
(∆RR–ΛSS) to (∆SS–ΛRR) should occur in solution and in the
31P NMR spectrum, 32 signals corresponding to the isomers
and additional signals from bridging phosphorus atoms could
be observed. If the isomerisation occurred on the bridging
phosphorus atoms, the number of isomers and signals would be
higher. Therefore, the 31P NMR spectra of [LnL1]2 , where Ln is
La, Lu or Y, are complex and about 20 peaks are observed in a
narrow region of 29–34 ppm. The number of peaks did not
change even at 50 ЊC. The 13C NMR spectra were also found to
be complex, except for a sharp signal at 46 ppm corresponding
to the methyl group.
Stability of the dimer and its ability to dissociate was tested
by reactions with triphenylphosphine oxide (tppo) and with
pyridine N-oxide, which are known to be good donors for
lanthanide ions,37 in a molar ratio of complex:oxide = 1:10.
The 31P NMR spectra showed no changes in the complex signal
pattern in the range of 29–34 ppm after addition of tppo, only
the intensity of the tppo signal increased with the increasing
ratio of tppo/complex. The same results were obtained from the
experiments with pyridine N-oxide. No changes of the spectra
were observed after heating. Thus, the experiments confirmed
the high stability of the dimer in methanolic solution.
A carboxylic analogue of H3L1, H3DO3A forms a slightly dif-
ferent co-ordination sphere with a lanthanide under the same
conditions, i.e., in slightly basic solution.32 The X-ray analysis
showed that Gd(DO3A) crystallized as [(GdDO3A)3ؒ
Na2CO3]ؒ17H2O in which each Gd atom is coordinated to four
nitrogens of the macrocycle and the oxygen from each carb-
oxylic arm. The coordination sphere is completed by co-
ordination of two oxygens of the carbonate ion. Neither water
molecule was co-ordinated to the Gd atom nor formation of
the carboxylate bridges was observed. However, single carb-
oxylic bridge formation was observed in the [La(HDOTA)-
La(DOTA)] complex.33 Nevertheless, a comparison of H3L1
and H3DO3A complexes confirms the well-known better ability
of the phosphinic acid group for didentate coordination and
formation of the bridges connecting co-ordination polyhedra.
Thus, the N4O4(H2O) co-ordination sphere observed for [LnL1]2
complexes is closer to that found for complexes with ligands
containing three or four pendant arms such as H4DOTA33,34 or
H4L4. In addition to the same conformation of the tetraaza-
dodecane ring, a similar range of the twist angles between N4
and O4 bases around the four-fold axes is present. The distances
between the N4 and O4 bases are close to those previously
observed33,34 and the trend of the lanthanide moving towards
the N4 base dependent on its size is also analogous to that in the
structures with four pendant ligands.
The dimeric form, [CeL1]2, was also investigated by cyclic
voltammetry. Electrochemical properties of the dimer were
also compared with the monomeric complex [CeL2]Ϫ. Electro-
chemical oxidation of the [CeL1]2 in acetonitrile proceeds in two
one-electron diffusion-controlled reversible steps at potentials
Epa(1) 0.830 V and Epa(2) 1.040 V. This potential range is typical
of a Ce()/Ce() redox pair at a platinum electrode.38 Thus,
the [CeL1]2 is considered a molecule with two equivalent redox
active centers (two Ce()-ions).39 The fact that the oxidation
proceeds in two steps proves that both Ce atoms are oxidised
independently. There is no consecutive reaction, i.e., mixed
valence complexes Ce()–Ce() are sufficiently stable due to
electronic delocalisation over the whole [CeL1]2 molecule, where
the oxidised part of the molecule acts as a substituent decreas-
ing the electron density on the rest of the molecule. Therefore,
the oxidation of the second Ce() is more difficult. The
experiment verified an existence of the dimeric form in solution.
In contrast to the [CeL1]2, electrochemical oxidation of the
anionic compound [CeL2]Ϫ proceeds generally in a single one-
electron diffusion controlled reversible process at potentials
(Epa) between 0.550 V–0.700 V (see Table 3) depending on the
solvent used. With increasing basicity of the solvent, the oxid-
ation potential is shifted to less positive potentials and the
cathodic counter-peak becomes broader. In acetonitrile, the
anodic process involves two steps indicating a consecutive reac-
tion after the primary electron transfer. The first step corre-
sponds to the one-electron oxidation Ce()/Ce() of the
[CeL2]Ϫ anion. The oxidized molecule undergoes most probably
an intramolecular electron transfer resulting in oxidative
splitting of one PPh(O)OϪ group under regeneration of the
oxidation state ϩ at the central Ce ion. The second, more
positive oxidation step (EpaЈ) then corresponds to the Ce()/
Ce() oxidation of the uncharged molecule bearing only three
PPh(O)OϪ groups. This reaction pathway is supported by the
results of the above reported oxidation experiments of [CeL1]2,
In contrast to carboxylates, coordination of the phosphinic
acid group leads to the formation of a centre of chirality. It
seems to be understandable that the RS orientation of the
phenylphosphinic acid group observed in [LnL1]2 complexes is
analogous to the RSRS orientation in Li[LnL2] (ref. 30) and
different from the RRRR orientation observed in [LnL4]Ϫ
complexes containing benzylphosphinic groups.31
Solution properties
The 31P NMR studies of lanthanide complexes with phos-
phonic10,35,36 and phosphinic acid derivatives14,31 of cyclen
showed the formation of isomeric mixtures in solution. This
effect is also assumed to occur in [LnL1]2 solutions. Cyclic
voltammetry (see below) confirmed the presence of the dimeric
J. Chem. Soc., Dalton Trans., 2000, 141–148
145

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where the potential of the first oxidation step of [CeL1]2 (with
three PPh(O)OϪ groups around each Ce() ion) is very close to
the EpaЈ (Table 3). As for the reduction of [CeL2]Ϫ or [CeL1]2, no
cathodic process has been observed in the studied solvents at
the platinum electrode up to Ϫ1.6 V.
DSS or TMS (internal) and 85% H3PO4 (external) standards.
The eluent for TLC of complexes on silica plates is (CH3)3-
COH, H2O and CH3CN in a 2:9:2 volume ratio.
Synthesis of precursors
Luminescence spectra of [EuL1]2 and [TbL1]2 in methanolic
solution were also studied. The phenyl groups on the phos-
phorus atoms show a characteristic absorption at 254 nm and
hence, the absorption band could be used as an antenna for
excitation of a proximate lanthanide ion.11,40 The bands
observed consist of 5D0–7FJ transitions and 5D4–7FJ transitions.
Compounds (TsNHCH2CH2)2NMe, (TsOCH2CH)2NTs, were
synthesised by the previously described procedures.21–25 The
purity of products was checked by conventional methods (1H
NMR and 31P NMR, if appropriate, spectra and elemental
analysis). Solvents were purified and dried using established
procedures.45
5
The electric–dipole-allowed D0–7F2 transition is known to be
hypersensitive its intensity being strongly affected by polaris-
ibility of the ligand.31b,41 It is absent if the Eu3ϩ ion is in an
Synthesis of 1-methyl-4,7,10-tritosyl-1,4,7,10-tetraazacyclo-
dodecane (3). The disodium salt of 1 was prepared by addition
of a solution of 2.65 g Na in 50 ml absolute ethanol to 24 g NЉ-
methyl-NЈ,Nٞ-ditosyldiethylenetriamine in 42 mL of absolute
ethanol at 40 ЊC. The reaction was completed by refluxing the
solution for 20 min. The solvent was stripped off on a rotary
evaporator at 80 ЊC and the white disodium salt was dissolved
in 565 mL of dry DMF. To the solution formed, a solution of
92 g N-tosyldiethanolamine ditosylate in 300 mL of dry DMF
was added during 3 h at 100–105 ЊC while stirring. This tem-
perature was maintained for 1 h. To the cooled solution, 4 L of
H2O was added. The precipitated product was filtered off after
standing overnight, washed with water and dried at 80 ЊC. The
product was then purified by refluxing in ethanol, leaving the
suspension to cool to room temperature and filtering off the
white 1-methyl-4,7,10-tritosyl-1,4,7,10-tetraazacyclododecane
from the slightly brownish solution. Yield 48 g, 65%, mp
201 ЊC. 1H NMR: s 2.17 CH3N (3H), s 2.43 CH3Ar (6H), s 2.46
CH3Ar (3H), bt 2.61 CH2N (4H), t 3.06 CH2N (4H), t 3.27
CH2N (4H), t 3.51 CH2N (4H), m AAЈBBЈ 7.29–7.64 CH
aromatic (8H), m AAЈBBЈ 7.34–7.83 CH aromatic (4H). 1-
5
inversion centre. In the spectrum of [EuL1]2 the D0–7F2 signal
5
is about twice as intensive as the D0–7F1, which indicates the
strong polarisibility of (L1)3Ϫ. For phosphinate complexes of
the [Eu(DOTA)(H2O)]Ϫ type, this transition produces one of
the most intensive bands in the spectrum.42 In the 5D0–7F2
region of the luminescent spectrum of [EuL1]2, two well separ-
ated singlets are found whose intensities are comparable with
that for the 5D0–7F1 transition. This is consistent with the
absence of an inversion centre in the emitting species and polar-
isibility of the ligand, enhanced by the presence of phenyl rings
on phosphorus atoms. The 5D0–7F3 transition is of low intensity
in the spectrum, similarly to other [Eu(DOTA)(H2O)]Ϫ type
complexes.43 The intensity of the 5D0–7F4 multiplet in [EuL1]2 is
smaller in comparison with that of 5D0–7F1,2, in contrast to the
complexes Li[Eu(L2)].30
In the spectrum of [TbL1]2, all the 5D4–7FJ transitions, where
J = 3,4,5,6, were observed. The spectrum is the same as spectra
of analogous Li[TbL2] (ref. 30) or [TbL4] (ref. 31, 44), which
corresponds to the fact that luminescent spectra of Tb3ϩ are not
as sensitive to the changes outside the first co-ordination
sphere.
Methyl-1,4,7,10-tetraazacyclododecane tetrahydrochloride
4
(N-methylcyclene) was released from 3 by modification of the
Lázár procedure,26 starting from 0.5 g of 3 and 4 mL of conc.
1
H2SO4 and giving 85% yield, mp 237–239 ЊC (dec), H NMR:
Conclusion
s 2.54 CH3N (3H), br m 2.8–3.4 NCH2 ring (16H).
A new macrocyclic ligand derived from cyclene substituted with
three methylene(phenyl)phosphinic acid pendant arms was
synthesised and its complexes with lanthanides were prepared.
X-Ray analysis of seven Ln() complexes (Ln = La, Ce, Nd,
Eu, Tb, Er, Yb) showed the formation of dimers [LnL1]2 in the
solid state. All complexes are isostructural and a coordination
polyhedron is formed by four N atoms and four O atoms of the
phosphinic acid groups. A water molecule is placed near the O4
base; however, only in La and Ce structures can it be considered
a part of the co-ordination sphere. The compounds are insol-
uble in H2O, but soluble in methanol, forming complex
mixtures of isomers in this solvent.
Synthesis of H3L1. Phenylphosphinic acid (0.55 g, 3.8 mmol)
and N-methylcyclene tetrahydrochloride (0.2 g, 0.6 mmol) were
dissolved in a mixture of 36% HCl (2 mL) and H2O (2 mL). The
mixture was heated in an oil bath at 95–100 ЊC and paraform-
aldehyde (0.17 g, 5.6 mmol) was added while stirring in 30
min intervals during 7 h. After the addition was completed, the
mixture was heated for 2 h. The solution obtained was evap-
orated to dryness using a rotary evaporator and co-distilled
with H2O three times to remove excess of formaldehyde. The
colourless oil was dissolved in H2O, loaded on Dowex 50W-X8
in Hϩ cycle and eluted with H2O to remove excess of phenyl-
phosphinic acid. Ligand H3L1 was eluted using dilute HCl.
Ligand H3L1 was obtained in 57% yield. Analysis found (calcu-
lated): C30H43N4O6P3ؒ5H2OؒHCl: C 46.81 (46.49), H 7.41
(7.02), N 7.55 (7.23)%. 1H NMR: s 1.68 CH3 (3H), br m 2.3–2.7
The 31P NMR studies showed isomerisation of chiral phos-
phorus atoms and formation of rather complex isomeric
mixtures in solution. The dimer stability was verified by reac-
tions with triphenylphosphine oxide (tppo) and with pyridine
N-oxide, which are good donors for lanthanide ions. The
experiments confirmed high stability of the dimers in meth-
anolic solution.
The cyclic voltammetry confirmed that the oxidation pro-
ceeds in two steps, proving that both Ce atoms are oxidised
independently. There is no consecutive reaction, i.e., the mixed-
valence complexes Ce()–Ce() are sufficiently stable.
2
NCH2 ring (16H), d 2.95 NCH2P (2H) JPH = 8.9 Hz, d 2.98
2
NCH2P (4H) JPH= 9.1 Hz, m 6.9–7.2 aromatic CH (15 H).
31P{1H} NMR: s 29.79, (2P), s 29.60 (1P).
Preparation of the complexes [LnL1]2ؒ6H2O. These complexes
were prepared starting from an appropriate lanthanide oxide,
which was dissolved in conc. hydrochloric acid upon warming
in an oil bath. The solution formed was evaporated to dryness
using a rotary evaporator. The residue obtained was dissolved
in a methanol–water (80/20 v/v) mixture. A stoichiometric
amount of H3L1 in a 40% solution of the same solvent was
added and the mixture was heated to 50 ЊC. Then, 1% aqueous
LiOH was added very slowly to this solution while stirring
vigorously (during ca. 5 min) until the pH reached 8–9. During
this addition a white precipitate was formed at about pH 7,
Experimental
Instrumentation
The melting points were determined with a Boethius apparatus
and are uncorrected. 1H and 31P NMR spectra were taken on a
Varian Inova 400 MHz spectrometer, using ca. 0.1 M solutions
in D2O or CDCl3, with chemical shifts reported in ppm from
146
J. Chem. Soc., Dalton Trans., 2000, 141–148

View Article Online
which redissolved at a higher pH. The slightly turbid solution
obtained was heated for 30 min and then filtered through filter
paper (blue strip) and allowed to crystallise in a desiccator over
P2O5. The crystals of [LnL1]2ؒ6H2O formed were filtered off
and air-dried. Analytical data of these compounds are in
agreement with the formula given. Single crystals for X-ray dif-
fraction studies were prepared from methanol–water mixtures
in the manner given above, but their isolation was done by the
technique described in the Crystallography section. The solv-
ation of the latter complexes is higher than that of air-dried
complexes (see Table 1). The purity of the prepared complexes
(especially with respect to uncomplexed Ln3ϩ) was proved by
TLC in the mixture acetonitrile–water–tBuOH 2:9:2 (v/v) on
silica plates, using detection with a Xylenol Orange solution.
The Rf of the complexes is 0.4, while Rf of the uncomplexed
Ln3ϩ is 0.
an argon atmosphere using non-aqueous solvents and their
mixtures: acetonitrile (Merck, for UV spectroscopy), dimethyl-
formamide (purified45 by azeotropic distillation followed by
vacuum fractionation), methanol (Fluka, for HPLC) and
ethanol (Lachema, freshly destilled). The supporting electrolyte
[Bu4N]PF6 (Fluka, p.a. for polarographic use) was used in
concentration 0.05 mol dmϪ3. The standard three-electrode
system consisted of a stationary platinum disk electrode 1 mm
in diameter (working electrode), a platinum-wire auxiliary
electrode and a saturated calomel electrode (to which all
potentials are referenced) separated from the solution by a
salt bridge. The concentration of the studied substances varied
between (1–5) × 10Ϫ4 mol lϪ1, the scan rates were 10–1000 mV
s
Ϫ1. The stock solution of Li[CeL2] was 0.025 M in MeOH, the
stock solution of [CeL1]2 was 0.025 M in acetonitrile–MeOH
1:1. computer-controlled electrochemical system ETP
A
The compounds [LnL1]2ؒ6H2O are soluble in methanol, spar-
ingly soluble in water and almost insoluble in higher alcohols
and less polar organic solvents. They have the same colours as
hydrated Ln3ϩ cations. The compounds lose water extremely
easily in the laboratory atmosphere forming lower hydrates. The
decomposition of a single crystal of the compound could be
seen after ca. 2–3 min.
Polarosensors (Prague) was used with the POLARO 2.0
software.
Luminescence spectra
Luminescence experiments were carried out on a Perkin-Elmer
LS 50B luminescence spectrometer in 1 cm cell. The excitation
wavelength was 250 nm for Eu complexes and 265 nm for Tb
complexes. Europium() emission spectra were recorded in the
range 550–900 nm, Tb spectra in the range 450–760 nm, using
the excitation slit 10 nm, emission slit 2.5 nm, delay time 0.1 ms
and filter 430 nm in both cases. The luminescence decay was
observed up to 7.7 ms for Eu and up to 19 ms for Tb.
X-Ray analysis
The well-formed and highly unstable (due to loss of solvent
molecules) crystals of the compounds studied were obtained by
slow evaporation of the solvent mixture (MeOH–H2O 80:20) at
room temperature and under normal pressure for a couple of
weeks. The suitable crystals were selected in the mother liquor,
picked up in a cool room (at 4 ЊC) and mounted at random
orientations on glass fibres using a hydrophilic acrylate glue.
The crystals were coated with a thin paraffin film and scanned
at room temperature or at 170 K (Table 1). All attempts to
protect the crystals in Lindenmann capillares (with and without
mother liquor) failed due to their brittleness. Similar attempts
using low-temperature methods and protecting the crystals with
different silicone oils or other hydrophobic materials were also
unsuccessful probably due to cracking of single crystals caused
by inner temperature and pressure gradients. The stability of
the crystals outside the mother liquor, without protecting
paraffin films, is about 1 min at room temperature, about 10 min
at 4 ЊC, and from 6–11 h with the films (depending on the
crystal size and quality of the paraffin coating) at room
temperature.
An Enraf-Nonius CAD4 diffractometer was used for meas-
urements at 293(2) or 170(1) K in a dry nitrogen stream with
Mo-Kα radiation (λ = 0.71073 Å), with the exception of Ce().
Selected crystallographic and other relevant data for all
compounds are listed in Table 1. The Ce() complex was
measured at 170 K using a KUMA-diffractometer with
imaging plate. The data were corrected for Lorenz polarisation,
but not for absorption.
The extinction correction was applied using the standard
procedure included in SHELXL97.46 The structures were
solved and refined by standard procedures (SHELXS86,
SHELXL97).47,46 Hydrogen atoms in CH3, CH2 and CH frag-
ments were included in the calculated positions (SHELXL97),
but hydrogen atoms from water molecules could not be found
due to the bad quality of the data, with the exception of the Yb
structure, where H atoms were found in the Fourier map and
refined isotropically. We tried to refine the ocupancy of all
uncoordinated water molecules.
Acknowledgements
This work was supported by the Grant Agency of the Czech
Republic (Project 203/97/0252), by grant No. VS96140 of the
Ministry of Education of the Czech Republic and by the EU
COST D8 programme.
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