Lanthanide(III) and yttrium(III) coordination compounds of diastereomeric (2+2) macrocyclic imines derived from 2,6-diformylpyridine and trans-1,2-diaminocyclopentane

Article abstract of DOI:10.1016/j.poly.2018.02.027

The enantiopure mononuclear coordination compounds [Ln(L1)]Cl₃·nH₂O (Ln = Y(III), Nd(III), Sm(III), Eu(III), Tb(III), Yb(III)) of the chiral macrocyclic (2+2) imine L1 (L1_RRRR and L1_SSSS), derived from (1R,2R) and (1S,2S)-1,2-diaminocyclopentane (DACP) and 2,6-diformylpyridine (DFP), have been synthesized in the templated condensation of the precursors in the presence of the appropriate metal salts. From the racemic trans-DACP and DFP in the templated condensation, a series of racemic mononuclear coordination compounds [Ln(L1_rac)]Cl₃·nH₂O (Ln = Y(III), Nd(III), Sm(III), Eu(III), Tb(III), Yb(III)) have been obtained. In the absence of metal salt, the condensation of the achiral precursors leads to a meso type Schiff base L2, which in the presence of lanthanide(III) salts produced the mononuclear coordination compounds [Ln(L2)]Cl₃·nH₂O (Ln = Y(III), Nd(III), Sm(III), Eu(III), Tb(III)). The mononuclear coordination compounds with the L1 and L2 ligands have been characterised by NMR spectroscopy, mass spectrometry, elemental analyses and/or circular dichroism (CD). ¹H NMR signals of CDCl₃/CD₃OD solutions of the Y(III), Nd(III), Sm(III) and Eu(III) coordination compounds have been assigned on the basis of their COSY and HMQC spectra, and for the remaining lanthanide coordination compounds (Tb(III) and Yb(III)) the signals were tentatively assigned on the basis of linewidths analyses. The properties and stability of the lanthanide(III) and yttrium(II) coordination compounds of the racemic macrocycle L1_rac have been compared with those of the isomeric meso type macrocycle L2 in organic solvents and water. Axial ligand exchange for several coordination compounds was investigated by ¹H NMR titration experiments in organic solvents. The X-ray crystal structures of the representative mononuclear coordination compounds [Nd(L1_RRRR)Cl(H₂O)₂]Cl₂·0.5MeOH·H₂O, [Nd(L1_rac)Cl(H₂O)₂]Cl₂·1.6MeOH·0.2H₂O, [Y(L1_rac)Cl(H₂O)₂]Cl₂·3.5H₂O, [Y(L1_rac)Cl(H₂O)₂][Y(L1_rac)(H₂O)₃]Cl₅·6.8H₂O and [Nd(L2)Cl₂(H₂O)]Cl with the diastereomeric ligands L1 and L2 have been determined.

Full text of DOI:10.1016/j.poly.2018.02.027

Accepted Manuscript
Lanthanide(III) and yttrium(III) coordination compounds of diastereomeric
(2+2) macrocyclic imines derived from 2,6-diformylpyridine and trans-1,2-di-
aminocyclopentane.
Janusz Gregoliński, Katarzyna Ślepokura
PII:
S0277-5387(18)30111-6
https://doi.org/10.1016/j.poly.2018.02.027
POLY 13062
DOI:
Reference:
Polyhedron
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Accepted Date:
22 September 2017
22 February 2018
Please cite this article as: J. Gregoliński, K. Ślepokura, Lanthanide(III) and yttrium(III) coordination compounds
of diastereomeric (2+2) macrocyclic imines derived from 2,6-diformylpyridine and trans-1,2-
diaminocyclopentane., Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.02.027
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Lanthanide(III) and yttrium(III) coordination compounds of diastereomeric (2+2)
macrocyclic imines derived from 2,6-diformylpyridine and trans-1,2-
diaminocyclopentane.
Janusz Gregoliński*, Katarzyna Ślepokura
Department of Chemistry, University of Wrocław
14 F. Joliot-Curie, 50-383 Wrocław, Poland.
*janusz.gregolinski@chem.uni.wroc.pl
Abstract
The enantiopure mononuclear coordination compounds [Ln(L1)]Cl3∙nH2O (Ln = Y(III),
Nd(III), Sm(III), Eu(III), Tb(III), Yb(III)) of the chiral macrocyclic (2+2) imine L1 (L1RRRR
and L1SSSS), derived from (1R,2R) and (1S,2S)-1,2-diaminocyclopentane (DACP) and 2,6-
diformylpyridine (DFP), have been synthesized in the templated condensation of the
precursors in the presence of the appropriate metal salts. From the racemic trans-DACP and
DFP in the templated condensation, a series of racemic mononuclear coordination compounds
[Ln(L1rac)]Cl3∙nH2O (Ln = Y(III), Nd(III), Sm(III), Eu(III), Tb(III), Yb(III)) have been
obtained. In the absence of metal salt, the condensation of the achiral precursors leads to a
meso type Schiff base L2, which in the presence of lanthanide(III) salts produced the
mononuclear coordination compounds [Ln(L2)]Cl3‧nH2O (Ln = Y(III), Nd(III), Sm(III),
Eu(III), Tb(III)). The mononuclear coordination compounds with the L1 and L2 ligands have
been characterized by NMR spectroscopy, mass spectrometry, elemental analyses and/or
circular dichroism (CD). 1H NMR signals of CDCl3/CD3OD solutions of the Y(III), Nd(III),
Sm(III) and Eu(III) coordination compounds have been assigned on the basis of their COSY
and HMQC spectra, and for the remaining lanthanide coordination compounds (Tb(III) and
Yb(III)) the signals were tentatively assigned on the basis of linewidths analyses. The
properties and stability of the lanthanide(III) and yttrium(II) coordination compounds of the
racemic macrocycle L1rac have been compared with those of the isomeric meso type
macrocycle L2 in organic solvents and water. Axial ligand exchange for several coordination
compounds was investigated by 1H NMR titration experiments in organic solvents. The X-ray
crystal structures of the representative mononuclear coordination compounds
[Nd(L1RRRR)Cl(H2O)2]Cl2·0.5MeOH·H2O, [Nd(L1rac)Cl(H2O)2]Cl2·1.6MeOH·0.2H2O,
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[Y(L1rac)Cl(H2O)2]Cl2·3.5H2O, [Y(L1rac)Cl(H2O)2][Y(L1rac)(H2O)3]Cl5·6.8H2O and
[Nd(L2)Cl2(H2O)]Cl with the diastereomeric ligands L1 and L2 have been determined.
1. Introduction
Coordination compounds of macrocyclic ligands are found in a few significant biological
systems, such as chlorophyll, haemoglobin and vitamin B12, where they are involved in vital
biological processes, including photosynthesis and dioxygen transport [1]. The high
thermodynamic and kinetic stability of macrocyclic coordination compounds, collectively
known as the macrocyclic effect [2], has contributed to extensive research of the properties
which influence their potential applications. Especially, lanthanide(III) coordination
compounds have been drawing increasing attention because of their various applications, such
as contrast enhancing agents for MRI, artificial nucleases, enantioselective catalysts,
luminescent probes and in biomedicine [3a-h]. They can also exhibit interesting circularly
polarized luminescence applications and non-linear optical properties [3i,j]. Among the
diversity of macrocyclic ligands [3k], hexaaza Schiff base macrocycles derived from diamines
and dicarbonyl compounds are particularly well-matched for the coordination of
lanthanide(III) ions [4]. Chirality has been so far mostly been introduced to designed
macrocycles by application of trans-1,2-diaminocyclohexane [5] and sometimes by chiral
binaphthyl [6] or diphenyl amines [7]. Enantiopure and racemic Ln(III) coordination
compounds of the chiral Schiff base hexaazamacrocycle I [8] and the achiral Ln(III)
coordination compounds of its diastereomeric meso counterpart II [9], derived from trans-1,2-
diaminocyclohexane (DACH) and 2,6-diformylpyridine (DFP), belong to this type of hexaaza
macrocyclic derivative (Figure 1).
Figure 1. The structures of the Schiff base macrocycles I, II and III.
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In this contribution we have focused on the replacement of the commonly used DACH [10]
for the formation of small (2+2) macrocyclic imines and their coordination compounds with
lanthanide(III) and yttrium(III) ions by its smaller homologue, trans-1,2-diaminocyclopentane
(DACP) [11]. It is worth mentioning that in the literature there are only a few examples of
metal coordination compounds where the ligand contains the DACP moiety (-ies) [12] and
even fewer examples were found where the ligand is just a macrocycle [13].
Lately we have presented that the chiral (3+3) macrocyclic nonaazaamine, built of DACP
and DFP moieties [13b], easily coordinates lanthanide(III) and lead(II) ions, providing
helically twisted coordination compounds. This macrocylic amine is derived directly from the
not yet isolated nonaaza macrocyclic Schiff base III. This parental chiral macrocyclic imine
forms in a non-templated reaction of 2,6-diformylpyridine (DFP) with enantiomerically pure
DACP in methanol at room temperature.
In this paper we describe the very similar condensation of enatiomerically pure, but also
racemic, DACP with DFP, performed however, in the presence of lanthanide(III) and
yttrium(III) chlorides acting as templates and led in refluxing methanol. This reaction delivers
enantiomerically pure or racemic Schiff base coordination compounds with the respective
metal ions, [Ln(L1)]Cl3‧nH2O (Ln = Y(III), Nd(III), Sm(III), Eu(III), Tb(III) and Yb(III))
[14] (Figure 2). The condensation of racemic DACP with DFP under analogues conditions,
but in the absence of any templating agent, results in the formation of the achiral meso type
Schiff base L2 [15a]. It is worth mentioning that the higher (3+3) and (4+4) macrocyclic
imine homologues of heterogeneous chiralities (RRRRSS/SSSSRR and RRSSRRSS,
respectively) were also formed in the non-templated condensation carried out in benzene [15].
The small (2+2) achiral macrocycle L2 can react with CdCl2 and transform itself into giant
(6+6) and (8+8) Schiff bases linked with several Cd(II) ions [15]. In the case of the reactions
with lanthanide(III) and yttrium(III) chlorides, the meso type ligand L2 produces the
respective achiral mononuclear coordination compounds, [Ln(L2)]Cl3‧nH2O (Ln = Y(III),
Nd(III), Sm(III), Eu(III) and Tb(III)), resembling their aforementioned chiral
[Ln(L1)]Cl3‧nH2O diasteromers.
The enantiomerically pure and racemic (2+2) macrocyclic imine L1, of RRRR, SSSS or
RRRR/SSSS configuration, respectively, differs in its configurations at the DACP carbon
atoms from its achiral meso type counterpart L2 of the RRSS configurations [9] (Figure 2).
3

Figure 2. The general structures and labelling schemes of the lanthanide(III) and yttrium(III)
coordination compounds with macrocyclic ligands L1 and L2.
It should be noted that the significance of the chiral and meso isomers of macrocycles is
well documented, e.g. in the area of transition metal coordination compounds. It has been
shown that the chirality of tetraazamacrocycles has a strong influence on the properties of
their coordination compounds. It has been found that not only the structure of meso-type
tetraazamacrocyclic coordination compounds but also the reactivity and spectroscopic
properties are clearly different from their chiral isomers [16]. On the contrary, very few meso-
and chiral (including rac) isomers of lanthanide(III) coordination compounds coordinated
with the bigger hexaazamacrocycles have been presented and compared to date [9,17].
2. Results and discussion
2.1. Synthesis of the coordination compounds
The enantiomerically pure chloride coordination compounds, [Ln(L1)]Cl3·nH2O (Ln =
Y(III), Nd(III), Sm(III), Eu(III), Tb(III), Yb(III); L1 = L1SSSS or L1RRRR; n = 4–5.5) were
synthesized in the templated condensation of DFP with (SS)-DACP or (RR)-DACP by the
respective yttrium(III) or lanthanide(III) chlorides [18]. Both enantiomers of DACP were used
as their salts with hydrochloric acid, the (1S,2S)- or (1R,2R) cyclopentane-1,2-diaminium
chlorides, in the presence of the base triethylamine, NEt3. The described chloride coordination
compounds are soluble in methanol and do not precipitate from its solutions. Their isolation
required recrystallization from methanol/acetonitrile solutions to remove the still soluble
triethylammonium chloride by-product. The isolated yield was ca. 70% for the Y(III), Nd(III),
Eu(III) and Tb(III) coordination compounds, but for Sm(III) derivative it exceeded 80%. Only
4

the isolated yields for both enantiomeric Yb(III) coordination compounds were lower (ca.
46%) (see Supporting Information for details).
The racemic chloride coordination compounds [Ln(L1rac)]Cl3·nH2O (Ln = Y(III), Nd(III),
Sm(III), Eu(III), Tb(III), Yb(III); L1rac = L1SSSS/RRRR, n = 3.25-4.5) were synthesized in a
similar fashion as their enantiomerically pure derivatives by the templated condensation of
DFP with racemic DACP (used as its salt, racemic trans–cyclopentane-1,2-diaminium
chloride, in the presence of triethylamine, NEt3) in boiling methanol. The isolated yields
ranged, after purification, from 14% for Nd(III) to 70% for Yb(III) coordination compounds
(see Supporting Information for details, Supporting Figures S1-S6).
The [Ln(L2)]Cl3‧nH2O coordination compounds (Ln = Y(III), Nd(III), Sm(III), Eu(III) and
Tb(III), n= 1.25-2) were synthesized from the pure macrocycle L2 [15a] and the appropriate
lanthanide(III) or yttrium(III) chloride hydrate in methanol by stirring the resulting solutions
for short time at RT. The isolated yields were in the range 40-60% (see Supporting
Information for details). The applied method requires using relatively mild conditions and
short reaction times, otherwise admixtures enriched in lanthanide(III) coordination
compounds of L1rac can be obtained. It is worth mentioning that the reaction of the meso
ligand L2 with YbCl3‧5H2O, even as an “in situ” reaction in the NMR tube, delivers a
complicated mixture of products and it was not possible to isolate the [Yb(L2)]Cl3
coordination compound.
The identity of all the coordination compounds has been confirmed by elemental analyses
and their mass spectra. The purity of each coordination compound was examined by its NMR
spectra. For each enantiomeric pair of [Ln(L1RRRR)]Cl3/[Ln(L1SSSS)]Cl3 coordination
compounds, the CD spectra were recorded.
2.2. Crystal structures of the coordination compounds with the L1 and L2 ligands
Five crystals [19] of the formulae [Nd(L1RRRR)Cl(H2O)2]Cl2·0.5MeOH·H2O,
[Nd(L1rac)Cl(H2O)2]Cl2·1.6MeOH·0.2H2O, [Y(L1rac)Cl(H2O)2]Cl2·3.5H2O,
[Y(L1rac)Cl(H2O)2][Y(L1rac)(H2O)3]Cl5·6.8H2O and [Nd(L2)Cl2(H2O)]Cl have been obtained
(Figure 3, Supporting Figure S7, Supporting Table S1). Each asymmetric unit of the crystal
(3-n)
structures consists of 1 or 2 similar coordination compound cation(s), [Ln(L)Cln(H2O)(3-n)
]
(Ln = Nd(III) or Y(III), L = L1 or L2, n = 0, 1 or 2), labelled A or B respectively, chloride
anions and solvent molecules of crystallization. Each Ln(III) central atom of the coordination
5

compound cations adopts a characteristic nine coordination number and it is coordinated to
six nitrogen atoms of its own macrocyclic unit and to three axial ligands of chloride anion(s)
and water molecule(s) (Figure 3, Supporting Figure S7c). The Ln–N, Ln–Cl and/or Ln–Ow
bond lengths are listed in the Supporting Tables S2 and S3.
The chiral macrocyclic ligand in the two A and B cations, [Nd(L1RRRR)Cl(H2O)2]2+, of the
enantiomerically pure [Nd(L1RRRR)Cl(H2O)2]Cl2·0.5MeOH·H2O possesses the RRRR
configuration of both diaminocyclopentane rings in each unit.
The isomorphous [Nd(L1rac)Cl(H2O)2]Cl2·1.6MOH·0.2H2O and
[Y(L1rac)Cl(H2O)2]Cl2·3.5H2O crystals, and also
[Y(L1rac)Cl(H2O)2][Y(L1rac)(H2O)3]Cl5·6.8H2O, include the racemic ligand L1rac. The first
two centrosymmetric structures contain equal amounts of the chiral cations
[Ln(L1rac)Cl(H2O)2]2+ (Ln = Nd(III) or Y(III)) with opposite chirality of the macrocyclic
units, L1RRRR and L1SSSS, respectively. The third centrosymmetric structure,
[Y(L1rac)Cl(H2O)2][Y(L1rac)(H2O)3]Cl5·6.8H2O, includes two different cations:
[Y(L1rac)Cl(H2O)2]2+ labelled A and [Y(L1rac)(H2O)3]3+ labelled B. These two cations also
exist with equal amounts of two opposite enantiomers of the macrocyclic units.
The asymmetric unit of the achiral [Nd(L2)Cl2(H2O)]Cl crystal comprises one
[Nd(L2)Cl2(H2O)]+ cation, including the achiral meso ligand L2 with the RRSS configuration
of its cyclopentane moieties.
The three axial ligands (chloride anions and water molecules) of each coordination
compound can be positioned in a different fashion (Figure 3, Supporting Figure S7c). In
[Nd(L1RRRR)Cl(H2O)2]Cl2·0.5MeOH·H2O, [Nd(L1rac)Cl(H2O)2]Cl2·1.6MeOH·0.2H2O and
[Y(L1rac)Cl(H2O)2]Cl2·3.5H2O, each [Ln(L1)Cl(H2O)2]2+ (Ln = Nd(III) or Y(III)) cation has
two water molecules lying on the same side of macrocycle and one chloride anion occupying
the opposite side of the macrocyclic ring. In [Y(L1rac)Cl(H2O)2][Y(L1rac)(H2O)3]Cl5‧6.8H2O,
the [Y(L1rac)Cl(H2O)2]2+ cation, labelled A, has one water molecule and one chloride anion
coordinated on the same side of macrocyclic ring and the second water molecule is bound on
the opposite site of macrocycle, whereas the [Y(L1rac)(H2O)3]3+ cation, labelled B, has three
coordinated water molecules, two of which are located on the same side and the third one lies
on the opposite side of the macrocycle.
6

Figure 3. Top and side views of the cationic coordination compounds: [Nd(L1RRRR)Cl(H2O)2]2+ cation
A (1) from [Nd(L1RRRR)Cl(H2O)2]Cl2·0.5MeOH·H2O; [Y(L1RRRR)Cl(H2O)2]2+ cation A (2) and
[Y(L1RRRR)(H2O)3]3+ cation B (3) from [Y(L1rac)Cl(H2O)2][YL1rac(H2O)3]Cl5·6.8H2O and
[Nd(L2)Cl2(H2O)]+ cation (4) from [Nd(L2)Cl2(H2O)]Cl. Hydrogen atoms in the third line are omitted
for clarity. Grey – carbon, white – hydrogen, blue – nitrogen, green – chlorine, red – oxygen atoms.
Contrary to the aforementioned chiral coordination compounds, in the achiral
[Nd(L2)Cl2(H2O)]Cl, the three axial ligands are coordinated in a different mode. The
[Nd(L2)Cl2(H2O)]+ cation has one chloride anion and one water molecule coordinated on the
same side of the macrocycle and one chloride anion on the other side of the macrocyclic ring
(Figure 3, Supporting Figure S7c).
The macrocyclic ligand in each chiral coordination compound cation found in the crystals of
[Nd(L1RRRR)Cl(H2O)2]Cl2·0.5MeOH·H2O, [Nd(L1rac)Cl(H2O)2]Cl2·1.6MeOH·0.2H2O,
[Y(L1rac)Cl(H2O)2]Cl2·3.5H2O and [Y(L1rac)Cl(H2O)2][Y(L1rac)(H2O)3]Cl5·6.8H2O is
slightly twisted [5b,5c,5f,17e]. The twist can be described by the angle between two flat
pyridine rings, defined by the pseudo-torsion angle C1–C5–C13–C17 [17e], the values of
which are listed in Supporting Tables S2 and S3. This angle varies from -23.5(11)° to -
29.5(3)° for the chiral coordination compounds [Ln(L1)Cln(H2O)(3-n)]
(3-n) (Ln = Nd(III) or
Y(III), n = 0, 1 or 2) and it is much smaller (-2.5(8)°) for the achiral cation
[Nd(L2)Cl2(H2O)]+ found in [Nd(L2)Cl2(H2O)]Cl.
The macrocycle in [Ln(L)Cln(H2O)(3-n)
or 2) is also moderately bent. This bending can be measured by the angle determined by the
]
(3-n) (Ln = Nd(III) or Y(III), L = L1 or L2, n = 0, 1
7

pyridine nitrogen atoms and the metal ion (N1–Ln–N4). The values of this angle, ranging
from 157.25(17) to 169.9(3)°, are listed in the Supporting Tables S2 and S3. The bent
conformation of the macrocycle in the [Ln(L)Cln(H2O)(3-n)]
(3-n) coordination compounds can
be explained by the different steric requirements of the two axial ligands positioned below
and the one axial ligand positioned above the macrocycle plane.
The twist-bent conformation of the ligand L1 in the discussed chiral coordination
compounds is similar to that of the chloride coordination compounds with the
diaminocyclohexene anologue macrocycle I, [Gd(I)(H2O)3]Cl3∙3H2O [5f] or [La(I)(Cl)3] [5b].
This type of conformation is characteristic for chiral coordination compounds of hexaaza
tetraimine macrocycles with the ligand I.
The macrocycle L2 in its Nd(III) coordination compound has a flatter conformation when
compared with the free form of L2 [9,15a] It maintains, however, some features of the
stepped conformation (Figure 3, Supporting Figures S7c). In the free form, the two pyridine
rings lie on two parallel planes separated by 3.55 Å, while in the Nd(III) coordination
compound these two rings have a slope with a bending angle (N1–Ln–N4) equal to ca. 169.9°,
which is near to being positioned in one plane. A similar observation was also made for the
[Sm(II)(NO3)2(CH3OH)]+ cation of the meso macrocycle II derived from 2,6-
diformylpyridine and racemic trans-1,2-diaminocyclohexeane [9].
In the case of the macrocycle L2 in the [Nd(L2)Cl2(H2O)]+ coordination compound of C1
symmetry, the RR and SS configurations of the two diaminocyclopentane rings impose a Cs
symmetry of the macrocycle. For this reason the [Nd(L2)Cl2(H2O)]+ coordination compound
does not exhibit a helical twist. The non-zero -2.5(8)° angle is probably a consequence of the
stepped conformation and not of a helical twist of the ligand L2.
The stereochemistry of the macrocyclic cations [Ln(L)Cln(H2O)(3-n)]
(3-n) (Ln = Nd(III) or
Y(III), n = 0, 1 or 2) with the L1 and L2 ligands might also be described using the Klyne-
Prelog system in the extended Dale nomenclature [3k]. According to this system the
conformations of the two described macrocycles in their coordination compounds are [1818]2 and
[1818]m for the chiral and meso macrocycles, respectively (Supporting Figure S8). A comparison
between the chiral macrocyclic Nd(L1RRRR)Cl(H2O)2]+ cations A and B and the achiral
[Nd(L2)Cl2(H2O)]+ cation was drawn by a superposition of these cations (Supporting Figure
S9).
8

As among the crystal structures presented above there are only four types of different
coordination cations (Figure 3) of the general formula [Ln(L)Cln(H2O)(3-n)
(3-n) (Ln = Nd(III)
]
or Y(III), n = 0, 1 or 2, L = L1 or L2), only these examples were taken into account for
consideration of their coordination spheres. All of them have been defined as hula-hoop
coordination polyhedra (HH-9), as calculated by the SHAPE program [19e-g] (Supporting
Figure S10, Supporting Table S4).
2.3. Spectroscopic characterization
(a) Mass spectra of the coordination compounds
The mass spectra (Supporting Figures S11-S33) of almost all the synthesized metal
coordination compounds of L1 and L2 were measured in acetonitrile/water solutions. Only
the spectra of the Tb(III) and Y(III) coordination compounds with the L2 ligand were
recorded in methanol solutions since they were too unstable in water/acetonitrile solutions and
decomposed during measurements (see further text for details). All the spectra show the
presence of two main peaks at m/e values corresponding to the [Ln(L)(Cl)2]+ and
[Ln(L)(Cl)]+2 (L = L1 or L2) ions. Additionally, spectra measured in the presence of water
show peaks of the coordination compounds with coordinated OH- anions, [Ln(L)(Cl)(OH)]+,
[Ln(L)(OH)]+2). The presence of these species indicates the axial coordination of chloride or
hydroxide anions rather than clusterization of the ions under the conditions of the ESI-MS
measurements. However, for the spectra recorded in methanol solutions a peak (of the type
[Ln(L)(Cl)2(CH3OH)]+) of the adduct with one methanol molecule was detected (Supporting
Figure S14 and S30). All the yttrium(III) and lanthanide(III) coordination compounds showed
the correct isotopic pattern for each metal.
(b) CD spectra of the enantiomerically pure coordination compounds with the ligand L1.
Figure 4 shows the CD spectra of the enantiomeric pair of coordination compounds
[Yb(L1RRRR)]Cl3‧5.5H2O / [Yb(L1SSSS)]Cl3‧5.5H2O. These spectra of the two enantiomers
are mirror images of each other. Other pairs of enantiomers, [Y(L1RRRR)]Cl3‧5.5H2O /
[Y(L1SSSS)]Cl3‧5.5H2O, [Nd(L1RRRR)]Cl3‧4H2O / [Nd(L1SSSS)]Cl3‧4H2O,
[Sm(L1RRRR)]Cl3‧4H2O / [Sm(L1SSSS)]Cl3‧4H2O, [Eu(L1RRRR)]Cl3‧5H2O /
[Eu(L1SSSS)]Cl3‧5H2O and [Tb(L1RRRR)]Cl3‧5H2O / [Tb(L1SSSS)]Cl3‧5H2O, give very similar
spectra (Supporting Figures S32–S38) to the corresponding Yb(III) coordination compounds.
9

In the spectral region of ca 200-350 nm, all the spectra show positive and negative Cotton
effects with a quite strong intensity ( ~ 50 M-1cm-1) of the absorption band.
Figure 4. CD spectra of [Yb(L1RRRR)]Cl3∙5.5H2O (red) and [Yb(L1SSSS)]Cl3∙5.5H2O (blue) in
methanol (C = 1 x 10-4 M, cuvette 1 mm).
(c) NMR spectra of the [Ln(L)]Cl3 coordination compounds
The investigated chiral mononuclear yttrium(III) and lanthanide(III) compounds,
[Ln(L1)]Cl3‧nH2O, (Ln = Y(III), Nd(III), Sm(III), Eu(III), Tb(III) and Yb(III)) give rise to
seven signals of 1:2:2:2:2:2:2 intensity in their 1H NMR spectra and seven signals in their 13C
NMR spectra (measured in organic solvents such as CD3OD or mixture of CDCl3/CD3OD
(v/v 2/1), which is consistent with the presence of diastereotopic cyclopentane protons. This
indicates the D2 symmetry of the [Ln(L1)]Cl3‧nH2O coordination compounds in the
investigated solutions. On the other hand, the solid-state X-ray crystal structures of the related
enantiomerically pure [Nd(L1RRRR)Cl(H2O)2]Cl2·0.5MeOH·H2O and racemic
[Nd(L1rac)Cl(H2O)2]Cl2·1.6MeOH·0.2H2O, [Y(L1rac)Cl(H2O)2]Cl2·3.5H2O and
[Y(L1rac)Cl(H2O)2][Y(L1rac)(H2O)3]Cl5·6.8H2O coordination compounds (Figure 3,
Supporting Figures S7–S10) show a lower symmetry of the coordination compounds with a
twist-bent conformation of the ligand and an unsymmetrical coordination of the axial chloride
anions and/or water molecules. Some of these structures, of approximately C2 symmetry,
should give rise to 13 1H NMR signals of the same intensity and 12 13C NMR signals,
whereas the structures of C1 symmetry would show the presence of 26 1H NMR signals and
24 13C NMR signals.
10

The discrepancy in the number of expected and observed resonances can be clarified by
dynamic exchange of the chloride anions, e.g. in a dissociative fashion. When the process is
fast on the NMR timescale, averaged signals are observed for respective pairs of protons
which would give different signals for the static structure. In other words, the top and the
bottom of the macrocycle become equivalent, giving rise to an effective D2 symmetry and 7
signals in the 1H NMR spectra and 7 signals in 13C NMR spectra are observed.
The meso coordination compounds [Ln(L2)]Cl3‧nH2O (Ln = Y(III), Nd(III), Sm(III),
Eu(III) and Tb(III)) dissolved in organic solvents such as CD3OD or a mixture of
CDCl3/CD3OD (v/v 2/1), also give rise to seven signals of intensities 1:2:2:2:2:2:2:2 in their
1H NMR spectra, and seven signals in their 13C NMR spectra. The number of resonances
corresponds to an effective C2h symmetry of the [Ln(L2)]Cl3‧nH2O coordination compounds.
This symmetry results, as mentioned previously, from the fast dynamic exchange of axial
chloride ligands on the two sides of the macrocycle on the NMR timescale, accompanied by
conformational averaging of the Cs-symmetric structure of the macrocycle L2, observed in the
solid state for [NdL2Cl2(H2O)]Cl with C1 symmetry. For this latter symmetry, 26 1H NMR
signals of the same intensity and 24 13C NMR signals should be observed.
Figure 5. Comparison of the 1H NMR spectra (500 MHz, CDCl3/CD3OD v/v 2/1)
[Sm(L1RRRR)]Cl3‧4H2O (bottom), [Sm(L1rac)]Cl3‧3.25H2O (middle) and [Sm(L2)]Cl3‧2H2O (top).
* Denotes residual solvent signals.
11

The NMR spectra of diamagnetic ([Y(L1)]Cl3‧nH2O / [Y(L2)]Cl3‧1.25H2O) and slightly
paramagnetic ([Sm(L1)]Cl3‧nH2O / [Sm(L2)]Cl3‧2H2O) diastereomeric pairs of coordination
compounds, recorded in the same solvent e.g. CDCl3/CD3OD (v/v 2/1) look very similar (see
Figure 5 and Supporting Figure S39 and compare Supporting Figures S43, S44, with S53, S54
as well as S48, S49 with S58 and S59). This is the result of the fact that the chiral ligand L1
and achiral ligand L2 are diastereomers and their chemical nature is similar, in spite of
possessing different configurations at the chiral carbon atoms.
Figure 6. Comparison of cosy (A, C) and HMQC (B, D) NMR spectra (500 MHz, CDCl3/CD3OD v/v
2/1) of [Eu(L1RRRR)]Cl3·5H2O (A, B) and [Eu(L2)]Cl3·2H2O (C, D). * Denotes residual solvent
signals.
12

Also the general patterns of the NMR spectra of the paramagnetic ([Nd(L1)]Cl3‧nH2O /
[Nd(L2)]Cl3‧2H2O and [Eu(L1)]Cl3‧nH2O / [Eu(L2)]Cl3‧1.5H2O) pairs of coordination
compounds bear some resemblance, however the chemical shifts for both diastereomers are
evidently different (see Figure 6, Supporting Figures S40 and S41 but also compare
Supporting Figures S45–S47 with S55–S57 and S50–S52 with S60–S62). In the case of the
strongly paramagnetic pair of diastereomeric coordination compounds with the Tb(III) ion
([Tb(L1)]Cl3‧nH2O / [Tb(L2)](Cl3)‧1.5H2O), their NMR spectra are very different (Figure
7).
Figure 7. Comparison of the 1H NMR spectra (500 MHz, CDCl3/CD3OD v/v 2/1) and labelling of
signals of [Tb(L1RRRR)]Cl3·5H2O (bottom), [Tb(L1rac)]Cl3·2.5H2O (middle) and [Tb(L2)]Cl3·1.5H2O
(top). * Denotes residual solvent signals.
2.4. Stability of the [Ln(L2)]Cl3 coordination compounds - conversion of [Ln(L2)]Cl3
coordination compounds into [Ln(L1rac)]Cl3 diastereomers.
The [Ln(L2)]Cl3‧nH2O coordination compound solutions in methanol, heated at 328 K,
show a slow conversion into the diastereomeric [Ln(L1rac)]Cl3 coordination compounds. This
process is faster in the case of [Y(L2)]Cl3·1.25H2O and [Tb(L2)]Cl3·1.5H2O, which after
seven days of heating convert cleanly and completely into their chiral racemic diastereomers
[Y(L1rac)]Cl3 and [Tb(L1rac)]Cl3, respectively (Figure 8, Supporting Figure S63). The
progression, however, is much slower for [Eu(L2)]Cl3‧1.5H2O. The sample of that derivative,
on being heated for 4 weeks at 328 K in methanol, apart from the signals of the newly created
13

[Eu(L1rac)]Cl3 coordination compound and traces of unidentified species, still displays the
presence of the initial [Eu(L2)]Cl3 coordination compound (in a molar ratio of 0.18 / 1 for
[Eu(L2)]Cl3 / [Eu(L1rac)]Cl3) (Supporting Figure S64). For [Sm(L2)]Cl3‧2H2O and
[Nd(L2)]Cl3‧2H2O, even a 4-week conversion time at 328 K is too short to complete the
reaction. After that time, the conversion progress is in the middle for the Sm(III) coordination
compound with the meso ligand L2 and still at the beginning for the respective Nd(III)
compound (Supporting Figure S65, S66).
Figure 8. Fragment of the 1H NMR spectra (500 MHz, CDCl3/CD3OD v/v 2/1) for
[Y(L2)]Cl3·1.25H2O (a), after heating a methanol solution of [Y(L2)]Cl3·1.25H2O at 328 K for 7 days
(b), [Y(L1rac)]Cl3·3.5H2O (c). * Denotes residual solvent signals.
It is worth adding that the racemic coordination compounds [Ln(L1rac)]Cl3‧nH2O heated
under the same conditions for 4 weeks do not show any signs of change. This observation
indicates that the coordination compounds of L1rac are thermodynamically more stable than
their diastereomers with the meso ligand L2. Presumably in the conversion process,
[Ln(L2)]Cl3 → [Ln(L1rac)]Cl3, they are formed as a result of hydrolytic cleavage of the meso
Schiff base macrocycle L2 of RRSS configuration, and subsequent rearrangement of the
14

diamine fragments into the racemic RRRR/SSSS form of the more stable coordination
compounds of macrocycle L1rac.
2.5. NMR spectra of the [Ln(L)]Cl3 coordination compounds in D2O (L = b and L2)
(a) D2O solutions of the [Ln(L1)]Cl3 coordination compounds - formation of -hydroxo
dimers [22]
Although the 1H NMR spectra of the pure chiral monomeric lanthanide coordination
compounds [Ln(L1)]Cl3 measured immediately after dissolution of each sample in
CDCl3/CD3OD (v/v 2/1) solution confirm the presence of only one single product (seven
signals), the 1H NMR spectra of the same coordination compounds recorded in D2O solution
may show the presence of additional species immediately after dissolution [22]. Apart from
seven signals coming from parental monomeric [Ln(L1)(D2O)n]3+ coordination compounds (n
= 2), where axial chloride ligands were replaced by water molecules, after time a new set of
13 signals of the same intensity can clearly be noticed. With time or upon heating the D2O
solutions of the [Ln(L1)]Cl3 coordination compounds, the new signals gradually grow,
whereas the parental signals disappear. After some time however, also signs of decomposition
of the coordination compounds are seen in the diamagnetic region as the signals of the
partially released L1 ligand or its decomposition products.
The formation of new coordination compounds, which are -hydroxo dimers, is faster in the
case of enantiomerically pure as well as racemic [Yb(L1)]Cl3 coordination compounds. In this
case, the formation of the new coordination compound occurs even at room temperature
(Figure 9). The conversion is practically finished 99 h after dissolution of the chiral
[Yb(L1)]Cl3 coordination compounds in D2O (L1 = L1RRRR, L1rac). The same result may be
achieved instantly after addition of one equivalent of NaOH to the parental coordination
compound solution, confirming that the new set of signals comes from -hydroxo dimers
[Yb2(L1)2(-OH)2(D2O)2]+4. The presence of additional signals for -hydroxo dimers was
also noticed and investigated in D2O solutions of lanthanide(III) coordination compounds
-
[Ln(I)]X3 (X = Cl-, NO3 ) with the related chiral ligand I, consisting of DACH and DFP
moieties [22a,b].
For the chiral coordination compounds with other metal ions (Y(III), Nd(III), Sm(III),
Eu(III), and Tb(III)) and the ligand L1, the formation of their -hydroxo dimers in water
15

solutions is much slower and involves heating of the respective D2O sample solution for a
prolonged time (Supporting Figures S67-S71).
Figure 9. 1H NMR spectra of [Yb(L1RRRR)]Cl3·5.5H2O measured in D2O immediately after dissolution
of the coordination compound (a), 3.5 (b), 18.5 (c), 40.5 (d) and 99 h (e), respectively. Blue trace (f)
after addition of 1 eq. NaOH to the [Yb(L1RRRR)]Cl3·5.5H2O solution. Black circles denote parental
coordination compound signals, blue squares signals of -hydroxo dimers [Yb2(L1RRRR)2(-
OH)2(D2O)2]+4, asterisk residual solvent signal.
The formation of -hydroxo dimers slows down in the case of lighter lanthanide(III)
coordination compounds, which are more resistant to hydrolysis. For example, a D2O solution
of the Nd(III) coordination compound, even after being heated for 140 h at 323 K, still shows
the majority presence of the parental monomeric coordination compound (Supporting Figure
S67), whereas for the Sm(III) derivative, which was heated for the same period, the newly
created -hydroxo dimer dominates the NMR spectrum (Supporting Figure S68), however,
signals of the parental monomeric coordination compound as well as signals of the free ligand
L1 are still noticed. The Eu(II) coordination compound converts almost completely into its
dimer after 120 h of heating at 323 K (Supporting Figure S69), but in the case of the Tb(III)
16

and Y(III) coordination compounds the process is almost finished after 71 h of heating the
D2O sample solutions at the same temperature (Supporting Figure S70, S71). A longer heating
time than was mentioned for each coordination compound causes precipitation of the
hydrolysis products.
The phenomenon of formation of -hydroxo dimers can be explained by the fact that both
enantiomericaly pure and racemic monomeric yttrium(III) and lanthanide(III) coordination
compounds of L1 ligand easily react with OH- anions coming from water and form -hydroxo
derivatives. As it was said in the experimental part (see Supporting Information for details),
the formation of -hydroxo dimers was also observed in the case of the synthesis of some
monomeric coordination compounds. During the templated condensation of dialdehyde (DFP)
with diamine (DACP) water is produced as one of the products, but also lanthanide (or
yttrium) salts were used as hydrates. This presence of water in the reaction mixture is the
reason why sometimes the synthesized monomeric coordination compounds can co-exist with
their dimers.
(b) Stability of [Ln(L2)]Cl3 coordination compounds in D2O solutions
The 1H NMR spectra of [Ln(L2)]Cl3‧nH2O (Ln = Eu(III), Sm(III) and Nd(III)) recorded in
D2O also displays the seven signals of the [Ln(L2)(D2O)2]+3 coordination compounds, where
axial chloride ligands were replaced by water molecules. Although the spectral patterns of the
[Ln(L2)(H2O)2]+3 and [Ln(L1)(H2O)2]+3 diastereomers are similar, their chemical shifts are
clearly different, confirming the not identical chirality of each ligand (L1 vs. L2) in the
respective coordination compounds (Supporting Figures S67–S69). With time, however, the
coordination compounds gradually decay in D2O, even at RT, [Eu(L2)]Cl3·1.5H2O after 2.5
hours, [Sm(L2)]Cl3‧2H2O after 18 hours and [Nd(L2)]Cl3·2H2O, which is the most stable,
survives about 2 days. [Tb(L2)]Cl3‧1.5H2O and [Y(L2)]Cl3·1.25H2O are completely instable
in water solutions. Their 1H NMR spectra measured in D2O show immediately profound signs
of decomposition of parental [Ln(L2)(D2O)2]+3 (Ln = Tb(III) and Y(III)) coordination
compounds (Supporting figures S70, S71). This instability in water was also the reason why
the mass spectra of [Tb(L2)]Cl3·1.5H2O and also [Y(L2)]Cl3·1.25H2O could not be recorded
in water/acetonitrile solutions (see experimental part for details).
17

2.6. Axial ligand exchange in [Ln(L)]Cl3∙nH2O coordination compounds (L = L1 and L2) –
-
NMR titration with NO3 , CH3COO- and OH- anions.
Several [Ln(L)]Cl3∙nH2O coordination compounds (Ln = Yb(III), Eu(III), Sm(III) and
Y(III)) have been subjected to NMR titration in CDCl3/CD3OD solutions by application of
Bu4NNO3, CH3COONa and NaOH solutions. It was found that all the titrated
[Ln(L)]Cl3∙nH2O coordination compounds easily exchange their axially coordinated chloride
ligands into new ones, producing different species, the type of which depends not only on the
kind of the anion used, but also on the nature of the macrocyclic ligand (chiral L1 vs. mezo
L2).
a) Titration with the nitrate anion
The simplest situation occurred when the [Ln(L)]Cl3∙nH2O coordination compounds were
titrated with Bu4NNO3 solution. In each case the parental signals of each [Ln(L)Cl2]+
substrate gradually shift into the final positions of respective [Ln(L)(NO3)2]+ nitrate product
when the amount of nitrate anion increases. The mechanism is the same for all the
investigated coordination compounds with such metals as Yb(III), Eu(III), Sm(III) or Y(III)
and the chiral ligand L1 as well as the meso ligand L2 (Supporting Figures S72-S76).
b) Titration with the acetate anion
The [Ln(L)]Cl3∙nH2O coordination compounds titrated with the acetate anion [9] behave in
the same way both for the chiral ligand L1 and the meso ligand L2 (Figure 10, Supporting
Figures S77-S80). However, on the contrary to the titration with the nitrate anion, the titration
with acetate shows different results. This time, after addition from 0.5 up to 2.0 equivalents of
the CH3COO- anion a new set of 13 signals appears. It comes most probably from
[Ln(L)(Cl)(CH3OO)]+ cations. It is worth noting that this type of cation should exhibit C2
symmetry due to the unsymmetrical axial ligation of the chloride and acetate anions placed on
the opposite side of the L1 or L2 macrocycle. For this symmetry of a coordination compound,
a set of 14 signals should be observed (13 from the macrocycle and 1 from the CH3COO-
group) The parental signals of the [Ln(L)(Cl)2]+ cations gradually vanish. After addition of
more than 2 equivalents, only one set of 8 signals dominates the spectrum. It suggests the
formation of the [Ln(L)(CH3OO)2]+ coordination product, with the symmetrical coordination
of two acetate groups lying on the both sides of macrocyclic ligand L1 or L2. The symmetry
of the final products is D2 in the case of the derivatives with chiral ligand L1 or C2h in the
18

case of the derivatives with the meso ligand L2, and is the same as for the parental chloride
coordination compounds.
Figure 10. 1H NMR titration (in CDCl3/CD3OD v/v 2/1) of [Yb(L1SSSS)]Cl3·5.5H2O with
CH3COONa. Asterisks denote residual solvent signals, black squares signals of initial
[Yb(L1SSSS)(Cl)2]+ cation, red squares signals of [Yb(L1SSSS)(Cl)(CH3OO)]+ intermediate and green
squares signals of final [Yb(L1SSSS)(CH3OO)2]+ product.
c) Titration with the hydroxide anion
The titration of chiral [Ln(L1)]Cl3∙nH2O coordination compounds with OH- anions has
given analogous results as for the chiral coordination compounds with the macrocyclic Schiff
base I derived from enantiomerically pure or racemic DACH, described earlier by Lisowski
[22b,d].
Initially after addition of up to 1 equivalent of OH- anions to the parental [Ln(L1)]Cl3∙nH2O
substrates, -hydroxo dimers of the type [Ln2(L1RRRR)2(-OH)2Cl2]+2 or [Ln2(L1SSSS)2(-
OH)2Cl2]+2 (or both when L1 is racemic L1rac), are formed. They exhibit C2 symmetry,
resulting in 13 signals in their 1H NMR spectra. After addition of the next 1 equivalent of OH-
19

anions, a new set of more than 10 NMR signals (should be 13) dominate the spectrum. This
means that all remaining chloride anions have been replaced by hydroxide ligands, giving the
other -hydroxo dimers of the type [Ln2(L1RRRR)2(-OH)2(OH)2]+2 or [Ln2(L1SSSS)2(-
OH)2(OH)2]+2 (or both when L1 is racemic L1rac), also showing C2 symmetry (Figure 11,
Supporting Figures S81, S83).
Figure 11. 1H NMR titration (in CDCl3/CD3OD v/v 2/1) of [Yb(L1SSSS)]Cl3·5.5H2O with NaOH
solution. Asterisks denote residual solvent signals, green squares the signals of [Yb2(L1SSSS)2(-
OH)2Cl2]+2 -hydroxo dimer and red squares the signals of [Yb2(L1SSSS)2(-OH)2(OH)2]+2 -hydroxo
dimer.
The titration of the diastereomeric [Ln(L2)]Cl3∙nH2O coordination compounds runs in a
different way to that observed for the chiral [Ln(L1)]Cl3∙nH2O derivatives. The formation of
the highly symmetrical [Ln(L2)(OH)2]+ species after addition of 2 equivalents of OH- anions
to the C2h-symmetrical achiral [Ln(L2)]Cl3∙nH2O coordination compounds (Supporting
Figures S82, S84) suggests rather that the mechanism should be analogous to the mechanism
observed earlier during the titration of these complexes with acetate anions. First, after
addition of 1 equivalent of OH-, the C2 symmetrical [Ln(L2)Cl(OH)]+ intermediates are
formed, giving rise to 13 1H NMR signals. Next, after addition of the subsequent 1 equivalent
20

of OH-, these intermediates can convert into C2h-symmetrical [Ln(L2)(OH)2]+ species, which
exhibit only 7 1H NMR resonances. However, it is possible that the mechanism is different
with formation of -hydroxo dimers [Ln2(L2)2(-OH)2Cl2]+2 first. These derivatives, after
addition of 1 additional equivalent of OH-, would dissociate into [Ln(L2)(OH)2]+ monomers.
3. Conclusions
The crystal structures of chiral and achiral Y(III) and Nd(III) coordination compounds with
the ligands L1 and L2 show 9-coordinated metal cations, linked to 6 nitrogen donor atoms of
the macrocycle and 3 axial ligands (water and/or chloride anion(s)) positioned below and
above the macrocyclic ring in a different fashion. The structures of the cationic coordination
compounds [Ln(L)Cln(H2O)(3-n)]
(3-n) (Ln = Y(III) or Nd(III), L = L1 or L2, n = 0, 1, 2) in the
solid state exhibit lower symmetry, C2 (or C1) for the chiral coordination compounds with the
ligand L1 and C1 for the achiral coordination compounds with the ligand L2, due to the
different number and coordination mode of the axial ligands lying above and below the
macrocycle. Thanks to fast (on the NMR timescale) exchange of axial ligands occurring in
solutions, the symmetry of the resulting coordination compound cations, of the type
[Ln(L)Cl2]+ (in CD3OD / CDCl3 1/2 v/v. mixture) or [Ln(L)(D2O)2]+ (in D2O), is higher. It is
D2 for the chiral coordination compounds with the ligand L1 and C2h for the achiral
coordination compounds with the ligand L2. This is manifested by the observation of 7
resonances, both in the 1H and 13C NMR spectra recorded in CD3OD / CDCl3 mixture or in
D2O for the [Ln(L1)]Cl3 and [Ln(L2)]Cl3 coordination compounds.
The chiral coordination compounds [Ln(L1)]Cl3 (Ln = Y(III), Nd(III), Sm(III), Eu(III),
Tb(III) and Yb(III)) are stable in organic solvents, whereas the achiral [Ln(L2)]Cl3
coordination compounds (Ln = Y(III), Nd(III), Sm(III), Eu(III), Tb(III)) convert with time to
their racemic diastereomeric counterparts [Ln(L1rac)]Cl3. This process is faster in the case of
the heavier lanthanide(III) coordination compounds.
In D2O solutions the chiral coordination compounds [Ln(L1)]Cl3 link with OH- anions to
produce the respective -hydroxo dimers, [Ln2(L1)2(-OH)2(D2O)2]+4, the reaction of which
is fastest for the Yb(III) coordination compound and slowest for the Nd(III) coordination
compound. The achiral coordination compounds [Ln(L2)]Cl3 are much less stable in D2O
solutions. The coordination compounds with higher lanthanide(III) ions are completely
21

instable in water, whereas the coordination compounds of lighter lanthanide(III) ions survive
in D2O for circa 1 day.
The titration experiments performed in organic solvent have proved that the [Ln(L)]Cl3
coordination compounds (L = L1 and L2) easily exchange their axially coordinated chloride
ligands into nitrate, acetate or hydroxide anions, giving different types of monomeric or
dimeric products.
Appendix A. Supplementary data
CCDC 1568640-1568644 contain the supplementary crystallographic data for the Y(III) and
Nd(III) coordination compounds with the ligands L1 and/or L2. 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.
Acknowledgement
This research was supported by the NCN grant (Narodowe Centrum Nauki, Poland)
2011/01/D/ST5/02816.
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24

Graphical abstract
Condensation of 2,6-diformylpyridine with trans-1,2-diaminocyclopentane in the presence of
Ln(III) ions leads to chiral macrocyclic (2+2) imine coordination compounds. Without metal
ions, the reaction produces the meso (2+2) imine macrocycle. It binds Ln(III) ions, delivering
achiral coordination compounds. These Ln(III) derivatives convert in methanol into their
thermodynamically more stable chiral diastereomers.
25