Carbonate-bridged dinuclear lanthanide(III) complexes of chiral macrocycle

Article abstract of DOI:10.1016/j.poly.2019.05.005

Mononuclear Eu(III) and Dy(III) complexes of the chiral hexaaza macrocycle L, 2(R),7(R),18(R),23(R)-1,8,15,17,24,31-hexaazatricyclo[25.3.1.1.0.0]-dotriaconta-10,12,14,26,28,30-hexaene have been obtained as chloride derivatives (1 and 2, respectively) and

Full text of DOI:10.1016/j.poly.2019.05.005

Accepted Manuscript
Carbonate-bridged dinuclear lanthanide(III) complexes of chiral macrocycle
Aleksandra Gerus, Katarzyna Ślepokura, Jerzy Lisowski
PII:
S0277-5387(19)30316-X
https://doi.org/10.1016/j.poly.2019.05.005
POLY 13929
DOI:
Reference:
Polyhedron
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Accepted Date:
6 March 2019
30 April 2019
3 May 2019
Please cite this article as: A. Gerus, K. Ślepokura, J. Lisowski, Carbonate-bridged dinuclear lanthanide(III)
complexes of chiral macrocycle, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.05.005
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Carbonate-bridged dinuclear lanthanide(III) complexes of chiral
macrocycle
Aleksandra Gerus, Katarzyna Ślepokura and Jerzy Lisowski *
Department of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław,
Poland.
E-mail address: jerzy.lisowski@chem.uni.wroc.pl.
Abstract
Mononuclear Eu(III) and Dy(III) complexes of the chiral hexaaza macrocycle L,
2(R),7(R),18(R),23(R)-1,8,15,17,24,31-hexaazatricyclo[25.3.1.1.0.0]-dotriaconta-
10,12,14,26,28,30-hexaene have been obtained as chloride derivatives (1 and 2,
respectively) and characterized by using spectroscopic methods. The X-ray crystal structure
of 2 has been determined. The reaction of two equivalents of mononuclear 1 or 2 complexes
with one equivalent of sodium carbonate results in the formation of dinuclear complexes
[Eu2L2(2-CO3)(H2O)3]Cl4 3 and [Dy2L2(2-CO3)(H2O)3]Cl4 4, respectively. The X-ray crystal
structure of complex 3 reveals two macrocyclic [EuL]3+ units bridged by the carbonate anion.
The carbonate anion is bound to the Eu(III) ions using the 1:2-2 bridging mode. The
binding of carbonate anions to the [EuL]3+ or [DyL]3+ macrocyclic units has also been studied
in solution using spectroscopic methods. The titration of the starting paramagnetic
1
mononuclear chloride derivatives 1 and 2 monitored by H NMR spectroscopy revealed
initial formation of dinuclear species, followed by the formation of complexes possessing
additional carbonate ligands. The dinuclear carbonate complexes 3 and 4 are also formed in
the reactions of the respective mononuclear chloride complexes with gaseous CO2.
Lanthanide(III) complexes of the chiral hexaaza macrocycle L are effective for catalytic
hydrolysis of bis(4-nitrophenyl) phosphate BNPP. Within the series of the studied
mononuclear complexes: nitrate derivatives [LnL(NO3)(H2O)](NO3)2 (Ln = Ho, Er),
[LnL(H2O)2](NO3)3 (Ln = Tm, Yb, Lu) and chloride derivatives [LnL(H2O)2]Cl3 (Ln = Tm, Yb, Lu, Y)
the catalytic activity of the complexes is increased with increasing ionic size. Kinetic studies
revealed that most active is the dinuclear europium(III) complex 3 which catalyzes the

hydrolysis of BNPP with a rate kobs = 6.5 x 10-5 s-1 (at 0.3 mM concentration of the catalyst)
at pH 8.5 and 37°C.
Graphical abstract
Two mononuclear macrocyclic units containing Dy(III) or Eu(III) can be converted to
carbonate-bridged dinuclear species both in the reactions with sodium carbonate or gaseous
carbon dioxide. The catalytic activity of dinuclear Eu(III) carbonate complex is much higher in
comparison with the analogous mononuclear complexes.
Keywords
Macrocycles; lanthanides; carbonate complexes; dinuclear complexes; catalytic hydrolysis;
BNPP.
1. Introduction
Multinuclear lanthanide(III) complexes are attracting increasing attention in light of their
potential applications as efficient catalysts for the hydrolysis of phosphate esters of different
types [1] and single molecule magnets [2, 3]. However, the labile nature of lanthanide(III)
ions combined with their poorly expressed stereochemical preferences render the control
over the formation of such compounds difficult. The formation of multinuclear
lanthanide(III) systems is typically accomplished by the application of bridging ligands. One
of the simple bridging ligands that can be applied for the construction of polynuclear
lanthanide(III) systems is the carbonate anion [2-4]. For instance, carbonate-bridged
dysprosium(III) complexes have been recently intensively studied as molecular magnets [2].
The multitude of coordination modes of carbonate anions in combination with high
coordination numbers of lanthanide(III) makes it relatively difficult to predict and control the
molecular architectures based on lanthanide carbonate derivatives. Thus, the carbonate
anion may bind to a single Ln(III) ion in a simple end-on 2 coordination fashion [4,3b] or

bridge two, three or six Ln(III) ions using 1:1-2, 2:2-2, 2:2:2-3, 1:1:2-3,
1:2:2-3, 1:1:11:1:1-6 or 1:1:11:1:2-6 coordination modes [2,3].
Moreover, the carbonate-bridged multinuclear lanthanide(III) complexes are most often
obtained serendipitously in reactions involving atmospheric CO2 [2,3].
diastreomer I
diastreomer II
N
N
N
N
N
N
H
H
H
H
N
N
H
H
H
H
N
N
N
N
N
N
N
N
R
R
S
S
N
N
N
N
R
R
S
S
R
S
N
N
N
N
S
R
S
R
R
S
R
R
S
S
R
R
R
R
S
S
R
R
S
S
S
S
H
H
H
H
H
H
H
H
(RRRR)(RRRR)
(SSSS)(SSSS)
(RRRR)(RSRS)
(SSSS)(SRSR)
L
L
L
L
Chart 1. Isomeric forms of L in its lanthanide (III) complexes.
Here we present a more controlled approach to lanthanide(III) carbonate complexes
based on surrounding the Ln(III) ions by a hexadendate macrocyclic ligand and thus limiting
the number of binding sites available for the coordination of carbonate anions. We report
the formation of dinuclear carbonate-bridged Ln(III) complexes based on chiral
hexaazamacrocycle L [5,6], which can exist in different diastereomeric forms (Chart 1). A
similar type of dinuclear complex with a different non-chiral macrocycle has been obtained
in the past accidentally in the reaction with atmospheric carbon dioxide [7]. We demonstrate
that formation of carbonate-bridged macrocyclic Ln(III) complexes is a more general process,
and that various carbonate derivatives can be formed in a controlled manner by the gradual
addition of simple carbonate salts to the solutions of mononuclear lanthanide(III) complexes.
We also report higher catalytic activity of the carbonate dinuclear [Ln2L2(2-CO3)(H2O)3]Cl4
complexes toward the hydrolysis of the BNPP in comparison with that of the mononuclear
complexes.
It should be noted that properly designed dinuclear metal-ion complexes are often more
effective than the mononuclear ones on catalyzing the phosphodiester hydrolysis reactions
because of the cooperative effect of the dinuclear systems that mimic those found in nature
[8]. In general, lanthanide(III) complexes are highly active catalyst for hydrolysis of
phosphodiester bonds [9]. However, the major drawback of lanthanide complex systems is
their very low stability in the alkali solution due to hydrolysis of lanthanide cations. This
problem can be circumvented by the application of Ln-binding ligands in order to form
artificial hydrolase [10]. For instance, nitrogen or oxygen based ligands lead to active Ln(III)
catalysts for hydrolysis [11].
2. Experimental
2.1. Synthesis
Macrocycle L was obtained as described previously described [5, 6].

[EuL(H2O)2]Cl3 (1) 68.7 mg (0.187 mmol) of EuCl3·6H2O and 81.5 mg (0.187 mmol) of
macrocycle L were dissolved in 3 mL of distilled water. The mixture was refluxed for 2 h,
cooled down and the volume was reduced to 0.5 mL by evaporation in a stream of N2. The
formed precipitate was filtered off, washed with a small amount of water and dried in
vacuum. The product was purified by recrystallization from methanol. Yield 89.21 mg (66%)
final product 1 after recrystallization. Calcd (anal.) for EuC26H60N6O11Cl3: C 35.04 (34.85); H
1
6.79 (6.21); N 9.43 (9.22). H NMR (300 MHz, D2O, 300K): -9.05, -8.70, -2.40, -1.82, -1.60, -
0.79, 1.95, 4.02, 6.16, 19.63.
[DyL(H2O)2]Cl3 (2) 73.9 mg (0.194 mmol) of DyCl3·6H2O and 84.4 mg (0.194 mmol) of
macrocycle L were dissolved in 3 mL of distilled water. The mixture was refluxed for 2 h,
cooled down and the volume was reduced to 0.5 mL by evaporation in a stream of N2. The
formed precipitate was filtered off, washed with small amount of water and dried in
vacuum. The product was purified by recrystallization from methanol. Yield 122 mg (90%).
1
Calcd (anal.) for DyC26H54N6O8Cl3: C 36.84 (36.78); H 6.42 (5.91); N 9.92 (9.95). ). H NMR
(300 MHz, D2O, 300K): -179.90, -108.04, -38.62, 10.45, 20.18, 43.66, 67.95, 87.50, 136.39,
203.36. 1H NMR (500 MHz, CD3CN, 300K): δ -114.19, -84.54, -63.72, -35.33,-29.02, -26.18, -
13.88, 20.81, 23.34, 31.08, 32.49, 49.27, 51.42, 54.26, 62.46, 116.71, 132.80, 135.96, 158.35.
[Eu2L2(2-CO3)(H2O)3]Cl4 (3) The solution of 1.5 mg (0.011 mmol) of sodium carbonate in
69.3 μl of water was added to a solution of 16.3 mg (0.022 mmol) of [EuL(H2O)2]Cl3 in 2 mL
of water. The clear solution was left to evaporate for 7 days till the volume was reduced to
ca. 0.5 mL. The crystalline product (4) was collected and dried in vacuum. Yield 33 mg (95%).
Calcd (anal.) for Eu2C53H100N12O15Cl4: C 40.00 (40.22); H 6.33 (6.21); N 10.56 (10.57). 1H NMR
(500 MHz, D2O, 300K): -19.12, -17.81, -14.11, -11.27, -6.89, -5.04, -3.31, -2.42, -1.70, -0.86,
1.82, 3.84, 6.54, 21.73, 27.52.
[Dy2L2(2-CO3)(H2O)3]Cl4 (4) was obtained in a similar manner as the above complex 3. Yield
33 mg (92%). Calcd (anal.) for Dy2C53H94N12O12Cl4: C 40.85 (40.72); H 6.08 (6.20); N 10.79
1
(10.56). H NMR (300 MHz, CD3CN, 300K): -115.27, -112.13, -84.69, -80.76, -70.18, -62.73, -
49.40, -36.07, -29.80, -23.52, -13.72, 21.17, 23.13, 29.40, 31.36, 31.11, 49.01, 51.36, 54.10,
62.34, 67.04, 75.67, 80.37, 82.33, 116.05, 123.50, 132.52, 136.83, 158.39.
2.2 Methods
The NMR spectra were taken on Bruker Avance 500 and AMX 300 spectrometers. The CD
spectra were measured on Jasco J-715 Spectropolarimeter. The electrospray mass spectra
were obtained using Bruker microOTOF-Q instrument. The elemental analyses were carried
out on a Perkin-Elmer 2400 CHN elemental analyzer.
The diffraction data for the crystals [DyLCl(H2O)[Cl2·0.5MeOH·2.5H2O (2) and
[Eu2L2(CO3)(H2O)3]Cl4·10H2O (3) were collected at 100(2) K on κ-geometry automated four-
circle diffractometers with graphite-monochromatized MoKα radiation: Kuma KM4-CCD or
Agilent Technologies Xcalibur R (ω scans) (see details in Table S1). Data collection, cell

refinement, data reduction and analysis were carried out with CrysAlisPRO [13]. The
structures were solved by direct methods using the Shelxt-2014 [14] program and refined on
F2 by a full-matrix least-squares procedure using Shelxl-2014 [15]. Figures presenting the
molecular structures were made using the Diamond program [16] and Mercury program
[17]. Details of structure solution and refinement are presented as Supplementary Material.
Kinetic experiments for the hydrolysis of the model substrate bis(4-nitrophenyl) phosphate
(BNPP) [18] were performed by following the absorbance of 4-nitrophenol (NP) using
AquaMate UV-vis spectrophotometer (see Supplementary Material for details). The
instrumental pH values measured in D2O calibrated with standard buffers in H2O were
corrected to pH values according to Krężel and Bal formula: pH=pH*×0.929+0.42 [12].
3. Results and Discussion
3.1. Crystal structures
As it has been observed previously, the macrocycle L can exist in two diastereomeric forms
(form I and form II) in its lanthanide complexes [5]. In the mononuclear
[DyLRICl(H2O)]Cl2·MeOH·2H2O complex 2, isomorphic to the Yb(III) complex [5], the eight-
coordinate Dy(III) ion is bound to six nitrogen atoms of the macrocycle L and to an axial
water molecule and chloride anion (Figure 1, Supporting Figure S12). The two axial ligands
are positioned on the same side of the macrocycle. This arrangement is accompanied by the
presence of the diastereomeric form I of the ligand L. In this form the nitrogen atoms adopt
the RRRR configuration (Chart 1) and the macrocycle is highly twisted in such a way that only
one face of the macrocycle is available for axial ligands.
Fig. 1. Molecular structure of the complex cation [DyLRICl(H2O)]2+ of complex 2.
Complex 3 is composed of two macrocyclic Eu(III) units bridged by carbonate anion (Figure 2,
Figures S13, S14). This anion is disordered in such a way that with 50% occupancy one of its
oxygen atom is bound to one of the two Eu(III) while two of its oxygen atoms are bound to
the other Eu(III) ion and with 50% occupancy the mode of binding to the two Eu(III) ions is
reversed. Thus the carbonate anion is bound in a 1:1-2 fashion and the two Eu(III) ions in

a given molecule are not equivalent. Both ions are eight-coordinate, but their sets of axial
ligands are different. For one of the Eu(III) ions the set of axial ligands correspond to a
monodendate carbonate and water molecule at one side of the macrocycle as well as
additional axial water molecule at the opposite side; for the other Eu(III) ion in a dimer the
set of axial ligands corresponds to a bidendate carbonate anion and a water molecule at the
opposite side of the macrocycle. The bridging carbonate anion resides in a kind of cleft
formed by the pyridine fragments of the macrocyclic ligands. Unlike in the case of 2, the axial
ligands are coordinated at both faces of the macrocycle and the diastereomeric form II of
the macrocycle L is observed in the dimeric Eu(III) complex 3. This form of the ligand is
related to the RSRS configuration at amine nitrogen atoms and a saddle conformation of the
macrocycle.
The mutual orientation of pyridine rings in the diastereomeric form II observed in 3 can be
characterized by the value of the pseudo-torsion angle C2-C4-C15-C17 equal to 82°; in
comparison this value is equal to -5° for the diastereomeric form I observed in the
mononuclear Dy(III) complex 2.
Although the quality of the crystals of the dinuclear Dy(III) carbonate derivative 4 were not
sufficient to satisfactory solve the crystal structure [20], crude structural model indicated
complex similar to that of the above complex 3. Here too, the disordered carbonate anion
bridges two lanthanide(III) ions in a 1:1-2 fashion.
Fig. 2. Molecular structure of the complex cation [Eu2L2(CO3)(H2O)3]4+ of complex 3.
3.2. Synthesis and spectra

Scheme 1. Formation of complexes 1 -4.
The mononuclear chloride derivatives 1 and 2 form easily from the free macrocycle L and
appropriate lanthanide chloride in water solution (Scheme 1). The solid samples of these
complexes can be obtained by slow evaporation of water solutions in an inert atmosphere.
On the other hand, slow evaporation of water solutions of the 1 and 2 complexes results in
the formation of carbonate derivatives. In particular, an attempt to obtain single crystal of 1
by slow evaporation of water solution resulted in the formation of single crystals of 3. In the
latter complex the carbonate anion must have originated from the fixation of atmospheric
carbon dioxide. In order to verify the formation of carbonate complexes, the starting
mononuclear chloride complex 1 and 2 were dissolved in D2O. CO2 was passed through these
1
solutions and changes over time were monitored by H NMR spectroscopy (Figure 3). The
starting 1H NMR spectrum of the D2O solution of the mononuclear Eu(III) chloride derivative
1 changes in time under CO2 exposition. The reaction with the gaseous carbon dioxide for 1h
results in a final spectrum practically identical to that of the synthesized dinuclear complex 3
(Supporting Fig. S3). This experiment clearly indicates fixation of carbon dioxide and
formation of dinuclear complex 3 from mononuclear complex 1.
Fig. 3. 1H NMR spectras of the D2O solution of the 1 complex with CO2 passed through these
solution.

The accidental formation of lanthanide(III) carbonate complexes from atmospheric carbon
dioxide has been observed previously, in fact the majority of the reported polynuclear Ln(III)
complexes with bridging carbonate anions have been obtained in this way [2,3]. The
formation of carbonate derivative is often facilitated by the presence of a base during
exposition to air, in the present case, however, the fixation of CO2 must correspond to the
evolution of HCl as required by the reaction balance. This effect must arise from the stronger
binding of the carbonate anion to the Eu(III) ion in comparison with the binding of the
chloride anion. The conversion of the chloride derivative to the carbonate derivative upon
exposition to atmospheric carbon dioxide was also observed for the Dy(III) complex 2,
although it was much slower. This qualitative difference may result from the various
preferences of the diastereomeric forms I and II of the macrocycle L; as it was previously
observed [5,6] the larger Ln(III) ions tend to prefer form II, while the smaller Ln(III) ions
prefer the form I. Since the conversion of Ln(III) chloride derivative to the Ln(III) carbonate
derivative requires conversion of the ligand form I to the ligand form II, this process is more
difficult for the Dy(III) complexes of L in comparison with the Eu(III) complexes.
In order to verify the formation of carbonate complexes, the starting mononuclear chloride
derivatives were reacted with 1/2 equivalent of sodium carbonate. The isolated product 3
was identical to that originating from atmospheric carbon dioxide. The binding of carbonate
anions in solution was also studied with the use of NMR titration experiments (Figure 4).
Fig. 4. NMR titration of the 1 complex with potassium carbonate solution (D2O, 300 MHz,
300 K).

1
The starting H NMR spectrum of the D2O solution of the mononuclear Eu(III) chloride
derivative 1 does not show all the 17 non-exchangeable signals expected for the C2
symmetry; moreover the signals are much broader than those observed in the NMR
spectrum of CDCl3/CD3OD solution of Eu(III) mononuclear nitrate derivative [5]. In contrast,
19 signals were observed in the case of NMR spectrum of [EuL(H2O)2]Cl3 in deuterated
organic solvents. This fact indicates additional broadening of the signals due to a dynamic
chemical exchange process, most likely an exchange of axial ligands. The addition of 1/2
equivalent of sodium carbonate to the D2O solution of the mononuclear Eu(III) complex 1
results in a spectrum practically identical to that of the synthesized dinuclear complex 3
(Supporting Fig. S3). This indicates clean formation of a complex where one carbonate anion
bridges two macrocyclic Eu(III) units in agreement with the X-ray and elemental analyses
data. Further addition up to 2 equivalents of carbonate anions results in a gradual shift of
1
the broad H NMR signals (e.g. down-field shift of the signals with the most negative
chemical shifts). These spectral changes point to a chemical exchange between 3 and
another macrocyclic Eu(III) carbonate derivative. The process that corresponds to a chemical
exchange is fast on the NMR time-scale for most of the signals while for signals with the
largest differences in  values the intermediate exchange regime is reached. The addition of
3 equivalents of sodium carbonate results in turn in an up-field shift of the signals with the
most negative chemical shifts. This spectral change indicates the formation of yet another
carbonate derivative. The addition of further portions of carbonate results in the sharpening
of signals with little effect on chemical shifts.
A similar situation was observed in the case of NMR titration of Dy(III) derivative 2 (Figure 5).
In this case the signals of the D2O solution of the starting mononuclear complex 2 were even
more severely broadened with most of the signal broadened beyond detection due to
chemical exchange and paramagnetic broadening – only three signals are clearly observed at
300 K and four signals at 350 K. The addition of 1/2 equivalent of sodium carbonate to the
D2O solution of the complex 2 results in a spectrum practically identical to that of the
synthesized dinuclear [Dy2L2(2-CO3)(H2O)3]Cl4 complex 4 (Supporting Fig. S4). The addition
of 1.25 equivalents leads to a new spectrum corresponding to a mixture of two new
carbonate complexes. A further addition of sodium carbonate up to 3 equivalents results in
an increase of the signals of one of these species at the expense of the other. For instance
the two up-field signals at ca -80 and -130 ppm practically vanish, while the signals at ca -78
and -118 ppm grow in intensity. The final spectrum is dominated by the set of 15 signals
corresponding to a macrocyclic complex of C2 symmetry.
The above NMR titration experiments reveal that apart from the dinuclear complex with one
carbonate anion bridging two macrocyclic units, at least two other carbonate derivatives can
be formed when more sodium carbonate is added. These new species may correspond to
dinuclear carbonate-bridged forms with additional carbonate anions bound on the outside
of the macrocyclic system, such as [(LnL)2(2-CO3)(CO3)2] or mononuclear derivatives with
two carbonate anions bound at the two faces of the macrocycle L, such as [LnL(CO3)2]-.

The two diastereomeric forms 2 and 4 give rise to different CD spectra (Fig. 5). The
difference is, however, much less pronounced in comparison with the difference in CD
spectra of diasteromers I and II in their nitrate derivatives [5, 6].
Fig. 5. The comparison of the CD spectra of water solutions of 2 and 4.
3.3. Catalytic activity towards hydrolysis of phosphodiester bonds of BNPP
The Ln(III) complexes of chiral macrocycle L’, which is the Schiff base analogue of amine
macrocycle L, were shown to act as enantioselective catalysts towards hydrolytic cleavage of
plasmid DNA [21]. Since the analogous complexes of L should be more resistant to hydrolytic
ligand decomposition, we have tested them as catalyst for this reaction. While initial results
regarding cleavage of plasmid DNA indicated substantial DNA aggregation, we turned our
attention to a model substrate – bis(p-nitrophenyl)phosphate (BNPP). The hydrolysis of
BNPP generates 4-nitrophenol (NP) and 4-nitrophenyl phosphate (NPP); the latter in turn is
hydrolysed into the mixture of inorganic phosphate and 4-nitrophenol (Scheme 2). In most
cases the rate of the second step is much faster than that of the first one. Therefore, for
these particular cases production of 2 equiv. of 4-nitrophenol at a rate constant k1 can be
assumed.
O2N
O
O-
O2N
O
-O
O-
O-
O-
O-
P
k2
k1
P
P
O
O
NO2
O
O
NPP
4-nitrophenyl phosphate
inorganic
phosphate
BNPP
bis(p-nitrophenyl)phosphate
-O
NO2
-O
NO2
NP
4-nitrophenolate
NP
4-nitrophenolate

Scheme 2. Sequential hydrolysis of bis(4-nitrophenyl)phosphate.
We have tested the catalytic activity of carbonate, nitrate and chloride derivatives of Ln(III)
complexes with macrocycle L. For the nitrate derivative [YbL(H2O)2](NO3)3] the rates were
measured at pHs varying from 7.0 to 9.0 (Figure 6).
Table 1. pH dependence of the rate constants (kobs) for the hydrolysis of BNPP at 37 °C.
([[YbL(H2O)2](NO3)3] = 3 x 10-4 M, [BNPP] = 2 x 10-3 M, [TRIS-HCl] = 5 x 10-2 M).
pH
7,0
V0
kobs
1,133E-07
2,199E-07
9,785E-07
1,329E-06
1,767E-06
2,213E-06
2,114E-06
1,945E-06
1,520E-06
1,716E-06
2,266E-10
4,398E-10
1,957E-09
2,658E-09
3,534E-09
4,425E-09
4,227E-09
3,890E-09
3,039E-09
3,431E-09
7,5
8,0
8,23
8,34
8,5
8,61
8,73
8,88
9,0
Fig. 6. Plot of the initial rate vs. pH for the hydrolysis of BNPP at 37 °C. For each case, the
concentration of [YbL(H2O)2](NO3)3 complex was 3 x 10-4 M and the concentration of BNPP
was 2 x 10-3 M.
The rate constants were found to increase with increasing pH until pH 8.5 at which value the
highest activity toward BNPP hydrolysis is observed. The pH dependence of the rate
constants can be best rationalized by considering the Brønsted acidity of water molecules
bound to the metal centres (vide infra). In other word, increasing the pH of the solution will
produce deprotonated species (LnM-OH), and activate OH- nucleophile, in addition to
possible formation of dimeric hydroxo-bridged species. The cationic lanthanide complexes

can form hydrate-, hydroxo-, or oxo-bridged species at high pH due to the presence of inner-
sphere coordinated water molecules/hydroxide ions, thereby altering their reactivity [22].
1
Indeed, the H NMR spectra (Figure S6) measured for the [YbL(H2O)2](NO3)3 complex in D2O
buffers with different pH values corresponding to catalytic studies indicate formation of new
complex species at higher pH.
1
For comparison, H NMR titration of [YbL(H2O)2](NO3)3 complex with NaOH solution in D2O
was performed (Figure S7). The spectral changes are more complicated in comparison with
analogous titrations of Ln(III) complexes of macrocycle L’ [21,23] and more difficult to
interpret because of combination of fast and slow exchange regimes on the NMR time-scale
as well as accompanying line broadening effects. Nevertheless, these spectra indicate
formation of at least four different hydroxo-type complexes (see Supporting Scheme S1 for
the possible forms of hydroxo complexes).
1
The above H NMR spectra of the derivatives [YbL(H2O)2](NO3)3 complex indicate that the
catalytically active form pH = 8.5, which produces the most prominent catalyst in the
hydrolysis of BNPP, is a hydroxo derivative, e.g. mononuclear complex cation
[YbL(H2O)(OH)]2+. At higher pH values likely the dimeric di-2-hydroxo-bridged species of
complex are formed (Supporting Scheme S1).
The influence of the size of central Ln(III) ion on the catalytic activity was investigated for a
series of nitrate derivatives [LnL(NO3)(H2O)](NO3)2 (Ln = Ho, Er) and [LnL(H2O)2](NO3)3 (Ln =
Tm, Yb, Lu) as well as chloride derivatives [LnL(H2O)2]Cl3 (Ln = Tm, Yb, Lu, Y) [5]. These kinetic
studies were limited to the complexes soluble in water and were performed at the optimum
pH value of 8.5 established for Yb(III) complex. For the series of monomeric nitrate species
the catalytic activity was increasing with the radius of metal ion and [HoL(NO3)(H2O)](NO3)2
complex was shown to have the highest reactivity (Supporting Figure S10, S11, Table 2).
The complexes in series of chloride derivatives [LnL(H2O)2]Cl3 (Ln = Tm, Yb, Lu, Y) are much
less reactive for promoting BNPP hydrolysis in comparison with nitrate derivatives (Figure 5,
Table 2) which may suggest inhibitory effect of Cl- anions, what is observed by simple anions
indicating high affinity of neutral hydroxo complexes toward anionic species of BNPP [24].
On the other hand, it was found that dimeric carbonate species 3 and 4 were more reactive
than all measured monomeric species (Supporting Figures S10, S11, Table 2). Complex 4 is
about 4.1 times more active catalyst than HoL(NO3)(H2O)](NO3)2 complex and complex 3 is
about 5.4 times more active than HoL(NO3)(H2O)](NO3)2 complex. The higher activity of
carbonate complexes likely result from cooperative action of the two Ln(III) ions within the
dinuclear carbonate-bridges species. The catalytic activity of dinuclear 3 complex in
hydrolysis of BNPP is comparable with other active lanthanide(III) complexes with
macrocyclic or polychelate ligands [25].

Table 2. The complex dependence of the rate constants (kobs) for the hydrolysis of BNPP at
37°C. ([complex] = 3 x 10-4 M, [BNPP] = 2 x 10-3 M, [TRIS-HCl] = 5 x 10-2 M).
Complex
V0
kobs
[LuL(H2O)2](NO3)3
[YbL(H2O)2](NO3)3
[TmL(H2O)2](NO3)3
[ErL(NO3)(H2O)](NO3)2
[HoL(NO3)(H2O)](NO3)2
[TmL(H2O)2]Cl3
5,43E-09
6,59E-09
7,94E-09
9,99E-09
2,42E-08
4,22E-09
4,21E-09
4,32E-09
3,72E-09
1,30E-07
1,00E-07
2,72E-06
3,30E-06
3,97E-06
5,00E-06
1,21E-05
2,11E-06
2,11E-06
2,16E-06
1,86E-06
6,50E-05
5,00E-05
[YbL(H2O)2]Cl3
[LuL(H2O)2]Cl3
[YL(H2O)2]Cl3
[Eu2L2(CO3)(H2O)3]Cl4
[Dy2L2(CO3)(H2O)3]Cl4
4. Conclusions
The chloride derivatives of mononuclear lanthanide(III) complexes of macrocyclic ligand L
can be converted into carbonate derivatives not only by serendipitous sequestering of
atmospheric carbon dioxide, but also by controlled addition of CO2 or reaction with
carbonate salts. The crystal structure of the Eu(III) complex indicates the presence of
dinuclear complex, where two macrocyclic units are linked by the bridging carbonate anion
bound in the 1:1-2 fashion. This dinuclear macrocyclic complex turned out to be more
active catalyst for the hydrolysis of BNPP in comparison with the corresponding
mononuclear species.
Appendix A. Supplementary data
CCDC Nos. 1891898–1891899 contains the supplementary crystallographic data for
complexes
2
and 3. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgements
This research was supported by NCN (Narodowe Centrum Nauki, Poland) grant
2013/11/N/ST5/01373.
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Synopsis
Two mononuclear macrocyclic units containing Dy(III) or Eu(III) can be converted to
carbonate-bridged dinuclear species both in the reactions with sodium carbonate or gaseous
carbon dioxide. The catalytic activity of dinuclear Eu(III) carbonate complex is much higher in
comparison with the analogous mononuclear complexes.