Chiral 4f and 3d-4f Complexes from Enantiopure Salen-Type Schiff Base Ligands

Abstract

This review summarizes the structural characteristics and physicochemical properties of chiral 4f and 3d-4f complexes based on enantiopure salen-type Schiff base ligands. The chirality originates from the enantiopure diamines and is imparted to the Schiff base ligands and complexes and finally to the crystal structures. The reported enantiopure Schiff base ligands derive from the condensation of aromatic aldehydes, such as salicylaldehyde and its various derivatives, and the enantiopure diamines, (1R,2R) or (1S,2S)-1,2-diamino-cyclohexane, (1R,2R) or (1S,2S)-1,2-diamino-1,2-diphenylethane, (R) or (S)-2,2′-diamino-1,1′-binaphthalene, and 1,2-diaminopropane.

1. Introduction

Molecular materials based on metal coordination complexes exhibit versatile electronic and structural features, which arise from the metal ions and the different types and numbers of ligands. These materials are usually prepared by wet chemistry methods, in some cases under environmentally friendly processes, and most importantly, combine different characteristics, such as metal oxidation states and versatile coordination geometries, with redox, catalytic, magnetic, electrical, and optical properties. The physicochemical properties of these materials depend on their exact structure and can be totally altered by small structural changes. Molecular materials with nonlinear optical, molecular magnetic, conducting, and superconducting properties have been proposed for potential applications [1,2]. Over the last decade, the field of molecular materials that combine different physical properties and are subjected to various external stimuli [3] has grown very rapidly. The rational design of these multifunctional molecular materials is of prime importance because of their potential applications in information storage, sensors, spintronics, biology, etc. [4].

Chirality is an intrinsic property of matter and is important in chemistry, biology, and physics, as it can affect the properties and reactivity of molecules. Chiral objects or molecules are not superimposable in their mirror image. The distinct left and right-handed forms are called enantiomers and exhibit different properties and activities. Eminent examples of chiral molecules are amino acids, sugars, and, therefore, proteins and nucleic acids, as well as certain pharmaceutical drugs. Metal coordination complexes can exhibit two types of chirality: (a) chirality at metal, which arises by the certain arrangement of ligands around the metal ion and disappears upon the separation of the metal and the ligands, and (b) chirality, which is imparted by the presence of chiral ligands around the metal ions [5]. Examples of metal complexes that belong in type (a) are the tetrahedral metal complexes with four different ligands, the octahedral tris-bidentate metal complexes, the octahedral bis-bidentate-bis-monodentate metal complexes, the octahedral metal complexes with three different monodentate ligands (MA₂B₂C₂), the octahedral metal complexes with four different ligands in the fac-arrangement (MABCL₃), and the octahedral metal complexes with linear tetradentate ligands in the cis-isomer (MD₄L₂). These chiral complexes are referred to as chiral-at-metal or ‘stereogenic-at-metal’ complexes [6]. From the synthetic point of view, the use of enantiopure ligands for the enantioselective synthesis of chiral coordination complexes is the most efficient method [7].

Chiral metal complexes are important in modern medicine as potential therapeutics (metallodrugs) [8] and can be selectively activated at the target site of a biomolecule by an external stimulus. These compounds can act as ‘prodrugs’ that interact with the biomolecules via various mechanisms such as redox processes, metal coordination, photo- and thermal-activation, and radiation [9]. Besides their importance in the pharmaceutical industry, chiral metal complexes, which combine chirality with magnetic, optical, and electrical properties, have received great attention in recent years and constitute a new and fast-growing class of multifunctional molecular materials. Coordination complexes that combine chirality and single-molecule magnet behavior, and in addition, chirality-induced properties, e.g., ferroelectricity [10,11], second-order nonlinear optical properties [12,13], magnetochiral dichroism [14,15], magneto-optical Faraday effect [16,17], and circularly polarized luminescence [18,19], represent a special class of multifunctional molecular materials.

Lanthanide(III) ions display large spin ground states and magnetic anisotropy, therefore, have been widely used for the synthesis of homo- and hetero-metallic complexes which show single-molecule magnet (SMM) [20,21,22], and single-ion magnet (SIM) [23,24,25,26,27,28] behavior. Schiff bases are formed by the condensation reaction between a primary amine and an aldehyde or ketone. These compounds are widely used in organic synthesis and coordination chemistry as ligands for metal complexes. They are also important in the production of various pharmaceuticals, dyes, and pesticides, in the food and agrochemical industry, in catalysis and energy storage, and for environmental and biomedical applications [29,30,31]. Schiff base ligands can be easily synthesized by various combinations of carbonyl compounds and primary amines and can be easily functionalized by choosing starting materials with the desired side groups. The combination of chiral Schiff base ligands with lanthanide(III) ions has been an effective strategy for the development of multifunctional molecular materials. The present minireview summarizes the chiral 4f and 3d-4f coordination complexes with enantiopure Schiff base ligands derived from the condensation of aromatic aldehydes, such as salicylaldehyde and its various derivatives, and the enantiopure diamines, (1R,2R) or (1S,2S)-1,2-diamino-cyclohexane, (1R,2R) or (1S,2S)-1,2-diamino-1,2-diphenylethane, (R) or (S)-2,2′-diamino-1,1′-binaphthalene, and 1,2-diaminopropane. The complexes reported herein were published up to December 2023. The Schiff base ligands discussed herein are depicted in Scheme 1, Scheme 2 and Scheme 3.

2. Homometallic 4f Complexes

Structural, CD, and magnetic data for the homometallic 4f complexes are listed in

Table 1. Structural, CD, and magnetic data for the homometallic 4f complexes 1–10.

Complex	4f Ion	Space Group	Coordination Number	CD, λmax (nm)	χMT (r.t.) (cm3Kmol−1)	Ueff (K)	t0 (s)	Ref.
1R/1S	Dy	C2	8	-	14.40	39.90 b	3.62 × 10−6	[32]
2R/2S	Dy	P21	7	-	13.84	13.27 c	2.02 × 10−6	[33]
3R/3S	Tb	P21	7	−291, −391 +350 a	11.80	-	-	[33]
4R/4S	Ho	P21	7	-	14.12	-	-	[33]
5R	Ce	P43	8	−343, +320 a	-	-	-	[34]
6R	Dy	P21	8	-	-	-	-	[35]
7S	Er	P21	8	-	-	-	-	[36]
8R/8S	Dy	C2	7	−281, −389, +348 a	13.47	24.61 c	8.49 × 10−8	[33]
9S	Sm	C2	10	-	-	-	-	[37]
10R	Ce	P1	9, 10	-	-	-	-	[38]

^a the values correspond to the R-enantiomer, same values with opposite signs correspond to the S-enantiomer; ^b under 1.5 kOe external dc field; ^c under 2.0 kOe external dc field.

.

A pair of enantiopure complexes (Et₃NH)[Dy^III(L4)₂] (1R/1S) were reported which contain ligand H₂L4_RR,SS, N,N’-(1,2-cyclohexanediylethylene)bis(3-nitrosalicylideneiminato), Scheme 1 [32]. The asymmetric unit of complex 1R contains two independent [Dy^III(L4_RR)₂]⁻ anions and four (Et₃NH)⁺ cations. Each Dy^III ion is eight-coordinated to two O_phenoxo and two N_imino atoms from two (L4_RR)²⁻ ligands with a triangular dodecahedron (TDD-8) geometry (CShM (Continuous Shape Measures) = 0.82632 (Dy1) and 1.14706 (Dy2)), Figure 1. The CD spectra of both 1R/1S in the MeCN solution display mirror image patterns as expected for a pair of enantiomers. The magnetic properties of mononuclear complexes in which the Ln^III ion is encapsulated by two salen-type ligands are reported for the first time. Both complexes exhibit field-induced SIM behavior combined with chirality. Dual magnetic relaxations are observed under a weak DC field as a result of both single-ion anisotropy and a direct process due to intermolecular interactions. Under a higher dc field, a single thermally activated Orbach relaxation is observed due to single-ion anisotropy. Thus, a field-induced switch from dual relaxation to a single relaxation process is achieved.

Pairs of enantiomerically pure chiral complexes with H₂L5_RR,SS were reported with formula [(L_OEt)Ln(L5)] (Ln^III = Dy (2R/2S, Tb (3R/3S), Ho (4R/4S); L_OEt = [(Cp)Co(P(O)(OEt)₂)₃]) which crystallize with two independent molecules in the asymmetric unit [33]. Both Ln^III ions are seven-coordinated to the two O_phenoxo and two N_imino of (L5)²⁻ and to three O-atoms from L_OEt in distorted monocapped triangular prism geometry. The enantiomeric nature of all complexes was confirmed by CD spectra. The temperature dependence of the magnetic susceptibility for 2R-4R shows a gradual decrease of the χ_MT product in the range 300–100 K and a further sharp decrease at 2 K, which can be attributed to the progressive depopulation of the Stark levels and/or possible antiferromagnetic interactions between the metal ions. The magnetization of 2R-4R at 2 K does not reach saturation at 70 kOe due to the ligand-field-induced splitting of the Stark levels and to the magnetic anisotropy. Complex 2R shows field-induced slow relaxation of magnetization, which follows a thermally activated mechanism. The energy barrier and pre-exponential factor τ₀ (Table 1) were calculated by the Arrhenius law, $\tau ={\tau }_0\mathrm{e}\mathrm{x}\mathrm{p}\left(\frac{\Delta }{k_{B}T}\right)$. Complex 2R represents a rare example of a seven-coordinated chiral SIM. Complexes 2R-4R show second-order nonlinear optical (NLO) effects with responses of 0.3 times that of urea. Unfortunately, no ferroelectric properties were observed for 2R-4R at room temperature.

The Ce^IV ion in complex [Ce^IV(L7_RR)₂] (5R) is eight-coordinated to the two O_phenoxo and the two N_imino atoms from two (L7_RR)²⁻ ligands in distorted square antiprism geometry [34]. The dihedral angle between the best mean planes of the two ligands, excluding the C-atoms of the cyclohexane ring, is 47.1°, and this is a unique architecture for Ce^IV complexes with two Schiff base ligands (Figure 2). The Ce-O and Ce-N bond distances are 2.212(5)-2.241(5) Å and 2.550(7)-2.570(6) Å, respectively. The complex was studied by CD, ¹H NMR, ¹³C NMR, and NOE (nuclear Overhauser effect) studies; the latter showed that the azomethine H-atom is in proximity to the phenyl ring H-atom and the cyclohexane CH₂. The spectral analysis suggests Ce^IV ion in a distorted planarity of the N₂O₂ chromophore group.

The same ligand gave complex (cation)[Dy^III(L7_RR)₂] (6R) [35]. The Dy^III ion is eight-coordinated to two O_phenoxo and two N_imino atoms from two (L7_RR)²⁻ ligands and displays square antiprism geometry with Δ absolute configuration. The countercation was formed in situ from the decomposition of H₂L7_RR with Dy(NO₃)₃ acting as a Lewis acid catalyst. Single crystal samples of 6R at r.t. show ferroelectric behavior with remnant polarization P_r~4.51 μCcm⁻² and E_c~28.11 kVcm⁻¹ and characteristic emission ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions of Dy^III at the solid luminescent spectrum (Figure 3). The enantiopure ligand H₂L7_SS (Scheme 1) gave complex (cation)[Er^III(L7_SS)₂] (7S), which is isomorphous to complex 6R, but in enantiomeric configuration [36]. The Er^III ion shows an eight-coordinate square antiprism geometry with Λ absolute configuration due to the S,S-enantiomer of the diamine used. Single crystal samples of 7S at r.t. show ferroelectric behavior with remnant polarization P_r~7.98 μCcm⁻² and E_c~22.12 kVcm⁻¹ at electric field 24 kVcm⁻¹. The value of P_r is close to that of P_s~8.04 μCcm⁻², which is much larger than that of other ferroelectrics, such as KH₂PO₄, triglycine sulfate, and NaKC₄H₄O₆·4H₂O. Both H₂L7_SS and 7S show second harmonic generation (SHG) efficiency, 0.6 and 1.2 times that of urea, respectively, thus confirming their potential as second-order NLO materials.

A pair of enantiopure complexes, namely [(L_OEt)Dy(L9)] (8R/8S) was also reported (L_OEt = [(Cp)Co(P(O)(OEt)₂)₃]) [33]. In both complexes, the Dy^III ions are seven-coordinated to the (O_phenoxoN_imino)₂ donor set of (L9)²⁻ and to three O-atoms from L_OEt ligand, with distorted monocapped triangular prism geometry (Figure 4). The CD spectra of 8R/8S in KBr pellets confirm the enantiomeric nature of both compounds (Figure 5, Table 1). The complex exhibits field-induced SIM behavior. The relaxation follows a thermally activated mechanism (Table 1). Complex 8R is similar to complex 2R-4R and displays similar second-order NLO effects, with no ferroelectric behavior at r.t.

The zwitterion of enantiopure ligand H₂L10_SS (Scheme 1) gave complex [Sm^III(H₂L10_SS)(NO₃)₃] (9S), which crystallizes with two independent molecules in the asymmetric unit [37]. The Sm^III ion is ten-coordinated to three chelate nitrato groups and to the two O_phenoxo and two O_methoxo atoms of the zwitterion H₂L10_SS. Both N_imino atoms are protonated and form intramolecular N-H···O_phenoxo hydrogen bonds. Hirshfeld surface analysis shows that the crystal packing is dominated by H···H, O···H and C···H contacts.

The enantiopure ligand H₂L12_RR (Scheme 1) gave the chiral 2D coordination polymer [Ce^III₂(H₂L12_RR)₆(NO₃)₆(H₂O)]_n (10R) [38]. Ce(2) is nine-coordinated to three chelate nitrato groups and three O_phenoxo from three different H₂L12_RR ligands. Ce(1) is ten-coordinated, exhibiting the same coordination environment as Ce(2) and also an additional water molecule. Complex 10R shows 2D square grid network architecture. Both H₂L12_RR and complex 10R were tested on MDA-MB-231 breast cancer cells by MTT assay and showed antiproliferative activity, which is higher for complex 10R compared with H₂L12_RR.

Figure 5. The molecular structure of one of the independent molecules in 17R (CCDC-1046379, left) and 20R (CCDC-1046384, right) [39]. Color code: Dy pink, Yb tan, O red, N blue, C grey.

Figure 5. The molecular structure of one of the independent molecules in 17R (CCDC-1046379, left) and 20R (CCDC-1046384, right) [39]. Color code: Dy pink, Yb tan, O red, N blue, C grey.

3. Heterometallic 3d-4f Complexes

The structural, CD, and magnetic data for the heterometallic 3d-4f complexes are summarized in Table 2, Table 3 and Table 4. Other spectroscopic data are listed in Table 5.

The enantiopure ligand H₂L2_RR (Scheme 1) gave a family of CuLn dinuclear complexes, [CuLn(L2_RR)(NO₃)₃(H₂O)] (Ln^III = Ce (11R), Nd (12R)), and [CuLn(L2_RR)(NO₃)₃] (Ln = Sm (13R), Eu (14R), Gd (15R), Tb (16R), Dy (17R), Ho (18R), Er (19R) and Yb (20R)), which crystallize with two independent dimers in the asymmetric unit (Figure 5) [39]. Temperature-controlled reversible conversion of one chiral crystal (15R-17R) to another chiral crystal (15′R-17′R) was observed, which leads to polymorphism and space group transformation from P1 to P2₁. The metal ions are linked through two O_phenoxo bridges. The Cu^II ion in 11R-17R is coordinated to the two O_phenoxo and the two N_imino atoms of (L2_RR)²⁻ in a square planar geometry, whereas in 15′R-17′R and 18R-20R a weak bond to one O_nitrato leads to square pyramidal geometry. The Ln^III ion in 13R-17R is ten-coordinated to two bridging O_phenoxo, two O_methoxo, and three chelate nitrato groups, whereas, in 11R and 12R, the Ln^III ion is eleven-coordinated due to an additional coordinated water molecule. In 15′R-17′R and 18R-20R, one Ln^III ion is ten-coordinated as in 13R-17R, and the second Ln^III ion is nine-coordinated because one of the nitrato groups is monodentate. The Cu···Ln distances are in the range 3.2287(7)-3.4382(13) Å in going from 20R to 11R. The CD spectra of the aforementioned complexes confirmed their optical activity and enantiomeric nature. Complexes 15R-17R and 15′R-17′R show ferromagnetic coupling between the metal ions. The Tb^III, Dy^III, and Ho^III compounds, 16R/16′R, 17R/17′R, and 18R display field-induced magnetic relaxation phenomena (Table 3). The magnetic difference between these complexes results from the lanthanide contraction effect of the Ln^III ions. Complex 15′R and [CuLu(L2_RR)(NO₃)₃] (21R) were previously reported [40].

Complexes [NiLn(L2_RR)(NO₃)₃(H₂O)] (Ln = Ce (22R), Nd (23R)) and [NiLn(L2_RR)(NO₃)₃] (Ln = Nd (24R), Sm (25R), Eu (26R), Gd (27R), Tb (28R), Dy (29R), Yb (30R), and Lu (31R)) are isomorphous to abovementioned complexes 11R-14R, 15′R-17′R, 20R and 21R [40,41,42]. Complexes 28R and 29R exhibit field-induced SMM behavior due to the strong anisotropy and crystal field effect of the Tb^III or Dy^III ions (Figure 6, Table 2).

Complex [ZnLu(L2_RR)(NO₃)₂(CH₃CO₂)] (32R) consists of a five-coordinate Zn^II ion with square pyramidal geometry defined by the two O_phenoxo and the two N_imino atoms of the ligand and one O-atom from the μ₂-CH₃CO₂⁻, and a nine-coordinate Lu^III ion bound to two bridging O_phenoxo, two O_methoxo, two chelate nitrato groups and one O-atom from the μ₂-CH₃CO₂⁻ [40]. The enantiomeric nature of all the abovementioned complexes was confirmed by solid-state CD spectroscopy.

Figure 6. Temperature dependence of the χ_ac at different frequencies with H_dc = 2 kOe and H_ac = 2.5 kOe and the least-squares fit of the experimental data to the Arrhenius equation for 28R (a) and 29R (b) [41]. Copyright © 2015 Elsevier B.V. All rights reserved.

Figure 6. Temperature dependence of the χ_ac at different frequencies with H_dc = 2 kOe and H_ac = 2.5 kOe and the least-squares fit of the experimental data to the Arrhenius equation for 28R (a) and 29R (b) [41]. Copyright © 2015 Elsevier B.V. All rights reserved.

A pair of enantiomer tetranuclear complexes [Cu₂Tb₂(L2)(N₃)₆(MeOH)₂] (33R/33S) crystallize with one cluster in the asymmetric unit (Figure 7) [43]. The CuTb subunits are related to each other through a pseudo inversion center and are bridged by two end-on azides. The two Cu^II ions are octahedral and coordinated to the two O_phenoxo and the two N_imino atoms of (L2)²⁻, one μ_1,3-N₃⁻, and one MeOH. The two Tb^III ions are eight-coordinated to the two bridging O_phenoxo and the two O_methoxo of (L2)²⁻, one μ_1,3-N₃⁻, one terminal N₃⁻ and two μ_1,1-N₃⁻ in bicapped trigonal prism geometry. The ac susceptibility studies under zero dc field showed slow relaxation of magnetization (Table 3). The SMM behavior was confirmed by the hysteresis loops observed on field-oriented single crystals of 33R below 4 K (Figure 8). This is the first time that angular resolved magnetometry measurements and ab initio calculations were performed on polynuclear SMMs in order to determine the magnetic anisotropy axis of each individual Tb^III ion.

A family of Zn₂Dy chiral SIMs were reported by using enantiopure H₂L2_RR and H₂L2_SS ligands, complexes [Zn₂Dy(L2_RR)₂(X)₂(H₂O)](anion) (X = Cl⁻, anion = ClO₄⁻ (34R); X = Cl⁻, anion = CF₃SO₃⁻ (35R); X = Br⁻, anion = CF₃SO₃⁻ (36R)) and [Zn₂Dy(L2_SS)₂(X)₂(H₂O)][Zn₂Dy(L2_SS)₂(X)₂(MeOH)](anion)₂ (X = Cl⁻, anion = ClO₄⁻ (37S); X = Cl⁻, anion = CF₃SO₃⁻ (38S); X = Br⁻, anion = CF₃SO₃⁻ (39S)) [44]. All complexes are isomorphous. The asymmetric unit of 34R-36R contains two independent molecules, whereas, in 37S-39S, the two molecules differ in the coordinated solvent molecule. Each Zn^II ion is five-coordinated to the two O_phenoxo and the two N_imino atoms of (L2)²⁻ and to the X⁻ ligand, which occupies the apex of the square pyramid. Each Dy^III ion is nine-coordinated to the two bridging O_phenoxo and the two O_methoxo atoms of each (L2)²⁻ and one solvent molecule in muffin (MFF-9) geometry (Dy1, Dy2 (34R, 37S), Dy1 (35R, 36R, 38S, 39S)) and spherical tricapped trigonal prism, TTP-9 (Dy2 (35R, 36R, 38S, 39S)). The molecular structure of complex 34R is given in Figure 9. Solid-state CD spectra of all complexes confirmed their chiral nature and optical activity (Table 4). Both the coordinated anions and the counterions, as well as the coordinated solvent molecules, have important effects on the SIM behavior and on the second-harmonic generation and third-harmonic generation NLO properties of the chiral complexes (Table 5). All complexes display field-induced magnetic relaxation as indicated by the peaks in the χ″ vs. T curves. The data in the lnτ vs. 1/T plots are not linear, which suggests that the relaxation has an Orbach process in addition to the Raman process. The curves were fitted with the equation ${\mathsf{\tau }}^{-1}=\mathrm{C}{\mathrm{T}}^{\mathrm{n}}+{\mathsf{\tau }}_0^{-1}\mathrm{e}\mathrm{x}\mathrm{p}(-\frac{{\mathrm{U}}_{\mathrm{e}\mathrm{f}\mathrm{f}}}{\mathrm{k}\mathrm{T}})$ to consider both processes and give the values listed in Table 4. Theoretical calculation further confirmed the experimental data. The study of the NLO properties showed that the SHG signal intensity of the 37S-39S complexes is much stronger than that of the 34R-36R because of the MeOH coordination in the former (Figure 10). Moreover, the bromide coordination and the perchlorate counteranion favor the SHG properties between the 37S-39S complexes. The third harmonic generation (THG) signal intensity of the 34R-36R complexes is much stronger than that of the 37S-39S, which suggests that the coordination of the H₂O molecule favors the THG properties. Moreover, the triflate counteranion favors the THG properties between the 34R-36R complexes.

Enantiopure ligands H₂L2_RR and H₂L2_SS gave a family of 3d-3d′-4f chiral complexes, [(Tp*)Fe(CN)₃Cu(L2)Ln(NO₃)₃(H₂O)₃][(Tp*)Fe(CN)₃] (Ln^III = Gd (40R/40S), Tb (41R/41S), Dy (42R/42S); Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate) which are isomorphous (Figure 11, Table 3) [45]. The Fe^III ions are six-coordinated to the three C-atoms from CN⁻ and the three N-atoms from pyrazolyl groups in a distorted octahedral geometry. The Cu^II ions are five-coordinated to the two O_phenoxo and the two N_imino atoms of (L2)²⁻ and the bridging cyanide group in the apex of a square pyramid. The Ln^III ions are nine-coordinated to the two bridging O_phenoxo and the two O_methoxo of (L2)²⁻, one chelate nitrate, and three aqua ligands in a geometry between spherical capped square antiprism (CSAPR-9) and capped square antiprism J10 (JCSAPR-9). CD spectra confirm the enantiomeric nature of the complexes. Magnetic studies revealed ferromagnetic interactions between the metal ions without signs of SMM behavior under zero or 1 kOe dc field. This indicates fast QTM relaxation as a result of the observed distorted coordination symmetry, which disfavors the axial anisotropy of the Tb^III and Dy^III ions.

The enantiopure ligand H₂L3_RR (Scheme 1) gave the chiral complex [CuEu(L3_RR)(NO₃)₃] (43R), which crystallizes with two independent molecules in the asymmetric unit (Table 3). The Cu^II ion is square planar, bound to the (O_phenoxoN_imino)₂ donor set of (L3_RR)²⁻, and the Eu^III is ten-coordinated bound to the (O_phenoxoO_ethoxo)₂ donor set of (L3_RR)²⁻ and to three chelate nitrato groups [46]. The chiral 1D zigzag chains of [CuGd(L3)(NO₃)₃]_n (44R/44S) were also reported (Table 3). The polymer structure is promoted by the semicoordination of nitrate to Cu^II. Magnetic studies showed strong ferromagnetic coupling between the metal ions [47].

The enantiopure ligand H₂L6_RR (Scheme 1) was synthesized by the condensation reaction of 4-formyl-3-hydroxybenzoic acid and (1R,2R)-(-)-1,2-diaminocyclohexane. The ligand gave four chiral 1D coordination polymers, [Mn₂Ln₂(L6_RR)₂Cl₂(NO₃)₂(dmf)₆(H₂O)₂]_n (Ln^III = Pr (45R), Nd (46R), Sm (47R), Gd (48R)), (Table 2) [48]. The Mn^III ion is coordinated to the two O_phenoxo and the two N_imino of (L6_RR)²⁻, one chlorido, and one monodentate nitrato group in a distorted octahedral geometry. Each carboxylato group of (L6_RR)²⁻ links two Ln^III ions in μ₂-fashion, thus forming a distorted square paddle-wheel unit Ln^III₂. Each Ln^III ion is eight-coordinated to four O_carboxylato, three dmf, and one H₂O molecules (Figure 12). The activity of 45R-48R as heterogeneous enantioselective catalysts for the sulfoxidation reaction of various alkyl and aryl sulfides revealed that 45R-47R show comparable activity for the oxidation of methyl p-tolylsulfide, whereas 48R displays maximum conversion of 100% methyl phenylsulfoxide with 88% selectivity after 16 h and could be reused three times without loss of activity.

The enantiopure ligands H₂L8_RR and H₂L8_SS (Scheme 1) gave pairs of chiral complexes [ZnLn(L8)(N(SiHMe₂)₂)(AABP)] (Ln^III = Y (49R/49S), Lu (50R/50S), Dy (51R/51S), Sm (52R/52S), La (53R/53S); AABP = alkoxy-amino-bis(phenolato)), (Table 4) [49]. The Zn^II ion is five-coordinate to the two O_phenoxo and the two N_imino atoms of (L8)²⁻ and to the nitrogen of N(SiHMe₂)₂ in distorted square pyramidal geometry. The Ln^III ion is six-coordinated to the two bridging O_phenoxo atoms of (L8)²⁻ and to the three O- and the N-atoms of AABP in distorted octahedral geometry (Figure 13). Complexes 51R/51S and 52R/52S are highly efficient catalysts for copolymerization of CO₂ with cyclohexene oxide. The transformation of CO₂ to other chemicals is very important, both economically and environmentally, and can be achieved via the ring-opening copolymerization of CO₂ with epoxide to produce polycarbonates. The reaction requires selectivity in order to avoid other byproducts. It was shown that the ionic radii of the lanthanide ions influence the carbonate linkage, thus leading to excellent selectivity for the isolation of polymers only.

Table 2. Structural, CD, and magnetic data for the heterometallic Mn/4f and Ni/4f complexes.

Complex	3d/4f Ions	Space Group	Coordination Number	CD, λmax (nm)	χMT (r.t.) (cm3Kmol−1)	Ueff (K)	t0 (s)	Ref.
22R	Ni/Ce	P1	11	−280, −400 +558 a	0.86	-	-	[41]
23R	Ni/Nd	P1	11	−280, −400 +558 a	2.36	-	-	[41]
24R	Ni/Nd	P1	10	-	-	-	-	[42]
25R	Ni/Sm	P1	10	−280, −400 +558 a	0.18	-	-	[41]
26R	Ni/Eu	P1	10	−280, −400 +558 a	1.05	-	-	[41]
27R	Ni/Gd	P1	10	−280, −400 +558 a	8.12	-	-	[41]
28R	Ni/Tb	P1	10	−280, −400 +558 a	12.26	29.12 b	3.21 × 10−9	[41]
29R	Ni/Dy	P1	10	−280, −400 +558 a	14.87	18.40 b	7.39 × 10−6	[41]
30R	Ni/Yb	P1	10	−280, −400 +558 a	1.46	-	-	[41]
31R	Ni/Lu	P1	10	-	-	-	-	[40]
54R	Ni/Ce	C2	10	-	-	8.5 c	7.7 × 10−5	[50]
55R	Ni/Nd	C2	10	-	-	9.2 b	1.9 × 10−5	[50]
56R/56S	Ni/Eu	C2	10	−325, +290, +370, +410 a	-	-	-	[50]
57R	Ni/Dy	C2	10	-	-	9.3 b	2.1 × 10−5	[50]
58R	Ni/Er	C2	10	-	-	18.4 b	1.7 × 10−6	[50]
59S	Ni/Yb	C2	10	-	-	18.1 b	2.1 × 10−6	[50]
45R	Mn/Pr	C2	8	-	-	-	-	[48]
46R	Mn/Nd	C2	8	-	-	-	-	[48]
47R	Mn/Sm	C2	8	-	-	-	-	[48]
48R	Mn/Gd	C2	8	-	-	-	-	[48]

^a the values correspond to the R-enantiomer, same values with opposite signs correspond to the S-enantiomer; ^b under 1.0 kOe external dc field; ^c under 0.5 kOe external dc field.

Table 2. Structural, CD, and magnetic data for the heterometallic Mn/4f and Ni/4f complexes.

Complex	3d/4f Ions	Space Group	Coordination Number	CD, λmax (nm)	χMT (r.t.) (cm3Kmol−1)	Ueff (K)	t0 (s)	Ref.
22R	Ni/Ce	P1	11	−280, −400 +558 a	0.86	-	-	[41]
23R	Ni/Nd	P1	11	−280, −400 +558 a	2.36	-	-	[41]
24R	Ni/Nd	P1	10	-	-	-	-	[42]
25R	Ni/Sm	P1	10	−280, −400 +558 a	0.18	-	-	[41]
26R	Ni/Eu	P1	10	−280, −400 +558 a	1.05	-	-	[41]
27R	Ni/Gd	P1	10	−280, −400 +558 a	8.12	-	-	[41]
28R	Ni/Tb	P1	10	−280, −400 +558 a	12.26	29.12 b	3.21 × 10−9	[41]
29R	Ni/Dy	P1	10	−280, −400 +558 a	14.87	18.40 b	7.39 × 10−6	[41]
30R	Ni/Yb	P1	10	−280, −400 +558 a	1.46	-	-	[41]
31R	Ni/Lu	P1	10	-	-	-	-	[40]
54R	Ni/Ce	C2	10	-	-	8.5 c	7.7 × 10−5	[50]
55R	Ni/Nd	C2	10	-	-	9.2 b	1.9 × 10−5	[50]
56R/56S	Ni/Eu	C2	10	−325, +290, +370, +410 a	-	-	-	[50]
57R	Ni/Dy	C2	10	-	-	9.3 b	2.1 × 10−5	[50]
58R	Ni/Er	C2	10	-	-	18.4 b	1.7 × 10−6	[50]
59S	Ni/Yb	C2	10	-	-	18.1 b	2.1 × 10−6	[50]
45R	Mn/Pr	C2	8	-	-	-	-	[48]
46R	Mn/Nd	C2	8	-	-	-	-	[48]
47R	Mn/Sm	C2	8	-	-	-	-	[48]
48R	Mn/Gd	C2	8	-	-	-	-	[48]

^a the values correspond to the R-enantiomer, same values with opposite signs correspond to the S-enantiomer; ^b under 1.0 kOe external dc field; ^c under 0.5 kOe external dc field.

The pair of enantiomers, ligands H₂L10_RR and H₂L10_SS (Scheme 1), gave the chiral complexes [NiLn(L10)(NO₃)₃] (Ln^III = Ce (54R), Nd (55R), Eu (56R/56S), Dy (57R), Er (58R), Yb (59S)) and [ZnLn(L10)(NO₃)₃(MeOH)] (Ln^III = Ce (60R), Nd (61R), Eu (62R/62S), Dy (63S), Er (64S), Yb (65S)) [50]. Both complexes 56R and 56S crystallize with two independent molecules in the asymmetric unit (Table 2). The Ni^II ion is bound to the two O_phenoxo and the two N_imino atoms of (L10)²⁻ in a square planar geometry, and the Eu^III ion is ten-coordinated to the two bridging O_phenoxo and the two O_methoxo of (L10)²⁻ and to three bidentate nitrato groups with sphenocorona geometry (CShM = 3.24). Complexes 62R and 62S crystallize with two independent molecules in the asymmetric unit (Table 4). Complexes 62R/62S are similar to 56R/56S, with the only difference being that the Zn^II ion in the former is bound to a MeOH molecule in the apex of a square pyramid. The solution CD spectra of the enantiomer pairs display mirror image patterns as expected (Figure 14). Complexes 54R, 55R, 59S, 60R, and 61R, i.e., complexes with Ln^III = Ce, Nd, and Yb, show slow relaxation of magnetization under an externally applied dc field, which is applied in order to avoid quantum tunneling of magnetization (QTM), and represent scarce examples of molecules with SIM behavior from ‘uncommon magnetic lanthanides’. The fitting of the data by using the generalized Debye model according to the equation $\ln \left(\frac{{\mathsf{\chi }}_{\mathrm{M}}^{″}}{{\mathsf{\chi }}_{\mathrm{M}}^{\prime }}\right)=\ln \left(\mathsf{\omega }{\mathsf{\tau }}_0\right)-{\mathrm{U}}_{\mathrm{e}\mathrm{f}\mathrm{f}}/{(\mathrm{k}}_{\mathrm{B}}\mathrm{T})$ gave the values listed in Table 2 and Table 4. Additionally, complexes with Ln^III = Dy and Er, i.e., 57R, 58R and 63S show field-induced SIM behavior, whereas ZnEr complex 64S does not show signs of slow relaxation of the magnetization. The U_eff (τ₀) values for 57R were calculated according to the generalized Debye model (Table 2). For 58R and 63S, the data were fitted according to the Arrhenius equation $\ln \left(\frac{1}{2\mathsf{\pi }\mathsf{\omega }}\right)=\ln \left(\frac{1}{{\mathsf{\tau }}_0}\right)-{\mathrm{U}}_{\mathrm{e}\mathrm{f}\mathrm{f}}/\left({\mathrm{k}}_{\mathrm{B}}\mathrm{T}\right),$ which gave the values listed in Table 2 and Table 4, respectively.

The pair of enantiomers [ZnDy(L10)(NO₃)₃(H₂O)] 66R/66S are isomorphous and crystallize with two independent molecules in the asymmetric unit (Table 4) [51]. The molecular structure of both complexes is analogous to that of the 63S above. The molecules of 66R are linked through hydrogen bonds between the coordinated H₂O molecule and one of the nitrato O-atoms. Ac susceptibility measurements for 66R under 4 kOe dc field showed two maxima at the χ″ vs. T plots, which are consistent with two relaxation processes following the Orbach pathway. The data were fitted to the Arrhenius equation for the low- and high-temperature relaxations (Table 4). The complexes show chiroptical activity and circularly polarized luminescence (CPL) in dmf solution, and also magnetic circular dichroism (MCD) signals at r.t. in the range 280–600 nm with g_max(MCD)~6.7 × 10⁻² T⁻¹ attributed to the large orbital angular momentum of aromatic π-conjugated systems (Figure 15). These results suggest that the complexes may have potential applications in magnetochiral devices. The CPL spectra of both complexes show mirror images in the range 430–600 nm, with the g_PL for the two enantiomers being ±2.5 × 10⁻³ (Figure 16).

Complexes [ZnLn(L10)(NO₃)₂(O₂CMe)] (Ln^III = Nd (67S), Dy (68R/68S), Yb (69R/69S)) were also reported (Table 4) [52,53,54]. In all complexes, the Zn^II ion is located in the (O_phenoxoN_imino)₂ pocket of (L10)²⁻ and the Ln^III ion is coordinated to the (O_phenoxoO_methoxo)₂ donor set and to two chelate nitrato groups. In addition, a bridging μ₂-O₂CMe links the two metal ions. The Zn^II ion displays square pyramidal geometry, and the Ln^III ion is nine-coordinated with intermediate geometry between the spherical capped square antiprism and the spherical tricapped trigonal prism. Complex 67S displays near-infrared luminescence in MeOH solution [52]. The pair of enantiomers 68R/68S were justified by solid-state CD spectroscopy. Both enantiomers display field-induced SIM behavior. Under 1.5 kOe external dc field, two relaxation processes are observed in the frequency and temperature dependences of the ac susceptibilities (Figure 17a). The fit of the data to the Arrhenius law gave the values given in Table 4 for the low- and the high-temperature relaxation, respectively. Polarization measurements as a function of the applied electric field on single crystals showed a hysteresis loop in the P vs. E curve at 400 K with a large coercitive field of ~17 kVcm⁻¹ with polarization P_s~9.1 μCcm⁻², which is comparable to other molecular ferroelectrics (Figure 17b, Table 5) [53]. The molecular crystals of 69R/69S display electrical bistability up to 573 K, which is the highest temperature reported for molecular ferroelectric materials. Moreover, this complex also exhibits optical activity, high- and low-temperature luminescence, para- and super-paramagnetic behavior at high- and low-temperature, respectively. The chiral complexes 69R/69S also display ferroelectric behavior and room temperature magnetoelectric coupling, therefore combining magnetic and electric polarizabilities in the same phase [54].

The enantiopure ligand H₂L11_RR (Scheme 1) gave the chiral complex [CuGd(L11_RR)(NO₃)₃] (70R), Table 3, [55]. The molecular structure consists of a square planar Cu^II ion located in the (O_phenoxoN_imino)₂ compartment of (L11_RR)²⁻ and a ten-coordinated Gd^III ion bound to the (O_phenoxoO_methoxo)₂ donor set of (L11_RR)²⁻ and to three chelate nitrato groups. The chiral complex exhibits efficiency in second harmonic generation 0.3 times that of urea. The magnetic susceptibility studies revealed ferromagnetic coupling between the metal ions with exchange parameter J = 1.3 cm⁻¹ (g_Cu = 2.10, g_Gd = 2.01).

The enantiopure ligand H₂L13_SS (Scheme 2) gave the chiral complex [ZnNd(L13_SS)(NO₃)₃(dmf)₂] (71S), Table 4, [52]. The Zn^II ion is five-coordinated to the two O_phenoxo and two N_imino atoms of (L13_SS)²⁻ and one dmf molecule, whereas the Nd^III is nine-coordinated to three O-atoms of the ligand, two chelate nitrates, one monodentate nitrate, and one dmf molecule. Complex 71S displays near-infrared luminescence in MeOH solution.

A chiral 1D coordination polymer [CuGd(L14_SS)(thd)₂]_n (72S) was reported (Hthd = 2,2,6,6-tetramethyl-3,5-heptanedione), Table 3, [56]. The ligand H₂L14_SS in shown in Scheme 2. The Cu^II ion is square planar and is coordinated to the (O_phenoxoN_imino)₂ donor atoms of the ligand. This subunit acts as a metalloligand to the two Gd^III ions, Gd(1) and Gd(2). The two O_phenoxo and the one O_methoxo atoms coordinate at one side of the Gd^III ion as a tridentate ligand. The O_amido coordinates with another Gd^III ion as a monodentate ligand. The eight-coordination of each Gd^III ion is completed by two bidentate thd⁻ ligands (Figure 18). The chiral 1D chains extend along the b axis. The magnetic studies revealed ferromagnetic coupling between the metal ions, compatible with spin S = 4 in the ground state.

The macrocycle enantiopure ligand H₆L16_RR (Scheme 3) gave the chiral complex [Zn₃Er(L16_RR)(O₂CMe)(NO₃)₂(H₂O)_1.5(MeOH)_0.5] (73R), Table 4, [57]. Each Zn^II ion is five-coordinated to two O_phenoxo and two N_imino atoms from (L16_RR)⁶⁻ and one monodentate acetate, nitrate, or methanol ligands at the apical position of a square pyramid. The Er^III ion is nine-coordinated to the six O_phenoxo atoms of the ligand at equatorial positions, one bidentate nitrate, and a H₂O molecule at apical sites. Complex 73R displays field-induced SIM behavior under 1.0 kOe external dc field (Figure 19).

Table 3. Structural, CD, and magnetic data for the heterometallic Cu/4f complexes.

Complex	3d/4f Ions	Space Group	Coordination Number	CD, λmax (nm)	χMT (r.t.) (cm3Kmol−1)	Ueff (K)	t0 (s)	Ref.
11R	Cu/Ce	P1	11	−290, −380, +600 a	1.05	-	-	[39]
12R	Cu/Nd	P1	11	−290, −380, +600 a	1.52	-	-	[39]
13R	Cu/Sm	P21	10	−290, −380, +600 a	0.64	-	-	[39]
14R	Cu/Eu	P1	10	−290, −380, +600 a	1.54	-	-	[39]
15R/15R′	Cu/Gd	P1, P21	10	−290, −380, +600 a,b	8.67, 8.22	-	-	[39]
16R/16R′	Cu/Tb	P1, P21	10	−290, −380, +600 a,b	12.35, 13.10	30.02, 26.16 c	9.81 × 10−9, 2.81 × 10−9	[39]
17R/17R′	Cu/Dy	P1, P21	10	−290, −380, +600 a,b	14.76, 14.45	5.96, 15.72 c	3.66 × 10−6, 1.70 × 10−7	[39]
18R	Cu/Ho	P21	9, 10	−300, −375, +600	14.05	8.46 d	3.03 × 10−7	[39]
19R	Cu/Er	P21	9, 10	300, −375, +600	10.36	-	-	[39]
20R	Cu/Yb	P21	9, 10	300, −375, +600	2.59	-	-	[39]
33R/33S	Cu/Tb	P1	8	-	25.15	27.60 c	2.03 × 10−5	[43]
40R/40S	Cu, Fe/Gd	P21	9	−380, −530, +470, +610 a	9.11	-	-	[45]
41R/41S	Cu, Fe/Tb	P21	9	−380, −530, +470, +610 a	14.13	-	-	[45]
42R/42S	Cu, Fe/Dy	P21	9	−380, −530, +470, +610 a	17.69	-	-	[45]
43R	Cu/Eu	P21	10	-	-	-	-	[46]
44R/44S	Cu/Gd	P212121	10	−237, −285, −390, +219, +261, +334 a	8.38	-	-	[47]
70R	Cu/Gd	P212121	10	-	8.20	-	-	[55]
72S	Cu/Gd	P3221	8	-	8.14	-	-	[56]
79	Cu, Mo/Gd	-	-	-	9.11	-	-	[58]
80	Cu, Mo/Tb	-	-	-	13.50	-	-	[58]
81	Cu, Mo/Dy	P21	9	-	-	-	-	[58]
82	Cu, W/La	P21	9	-	0.70	-	-	[58]
83	Cu, W/Gd	P21	9	-	8.90	-	-	[58]
84	Cu, W/Tb	P21	9	-	13.20	-	-	[58]
85	Cu, W/Dy	P21	9	-	-	-	-	[58]
86	Cu, Mo/La	C22221	10	-	0.76	-	-	[58]
87	Cu, Mo/Pr	-	-	-	-	-	-	[58]

^a the values correspond to the R-enantiomer, same values with opposite sings correspond to the S-enantiomer; ^b for 15R′-17R′ the λ_max values are −300, −375, +600 nm; ^c under 2.0 kOe external dc field; ^d under zero dc field.

Table 3. Structural, CD, and magnetic data for the heterometallic Cu/4f complexes.

Complex	3d/4f Ions	Space Group	Coordination Number	CD, λmax (nm)	χMT (r.t.) (cm3Kmol−1)	Ueff (K)	t0 (s)	Ref.
11R	Cu/Ce	P1	11	−290, −380, +600 a	1.05	-	-	[39]
12R	Cu/Nd	P1	11	−290, −380, +600 a	1.52	-	-	[39]
13R	Cu/Sm	P21	10	−290, −380, +600 a	0.64	-	-	[39]
14R	Cu/Eu	P1	10	−290, −380, +600 a	1.54	-	-	[39]
15R/15R′	Cu/Gd	P1, P21	10	−290, −380, +600 a,b	8.67, 8.22	-	-	[39]
16R/16R′	Cu/Tb	P1, P21	10	−290, −380, +600 a,b	12.35, 13.10	30.02, 26.16 c	9.81 × 10−9, 2.81 × 10−9	[39]
17R/17R′	Cu/Dy	P1, P21	10	−290, −380, +600 a,b	14.76, 14.45	5.96, 15.72 c	3.66 × 10−6, 1.70 × 10−7	[39]
18R	Cu/Ho	P21	9, 10	−300, −375, +600	14.05	8.46 d	3.03 × 10−7	[39]
19R	Cu/Er	P21	9, 10	300, −375, +600	10.36	-	-	[39]
20R	Cu/Yb	P21	9, 10	300, −375, +600	2.59	-	-	[39]
33R/33S	Cu/Tb	P1	8	-	25.15	27.60 c	2.03 × 10−5	[43]
40R/40S	Cu, Fe/Gd	P21	9	−380, −530, +470, +610 a	9.11	-	-	[45]
41R/41S	Cu, Fe/Tb	P21	9	−380, −530, +470, +610 a	14.13	-	-	[45]
42R/42S	Cu, Fe/Dy	P21	9	−380, −530, +470, +610 a	17.69	-	-	[45]
43R	Cu/Eu	P21	10	-	-	-	-	[46]
44R/44S	Cu/Gd	P212121	10	−237, −285, −390, +219, +261, +334 a	8.38	-	-	[47]
70R	Cu/Gd	P212121	10	-	8.20	-	-	[55]
72S	Cu/Gd	P3221	8	-	8.14	-	-	[56]
79	Cu, Mo/Gd	-	-	-	9.11	-	-	[58]
80	Cu, Mo/Tb	-	-	-	13.50	-	-	[58]
81	Cu, Mo/Dy	P21	9	-	-	-	-	[58]
82	Cu, W/La	P21	9	-	0.70	-	-	[58]
83	Cu, W/Gd	P21	9	-	8.90	-	-	[58]
84	Cu, W/Tb	P21	9	-	13.20	-	-	[58]
85	Cu, W/Dy	P21	9	-	-	-	-	[58]
86	Cu, Mo/La	C22221	10	-	0.76	-	-	[58]
87	Cu, Mo/Pr	-	-	-	-	-	-	[58]

^a the values correspond to the R-enantiomer, same values with opposite sings correspond to the S-enantiomer; ^b for 15R′-17R′ the λ_max values are −300, −375, +600 nm; ^c under 2.0 kOe external dc field; ^d under zero dc field.

Table 4. Structural, CD, and magnetic data for the heterometallic Zn/4f complexes.

Complex	3d/4f Ions	Space Group	Coordination Number	CD, λmax (nm)	χMT (r.t.) (cm3Kmol−1)	Ueff (K)	t0 (s)	Ref.
32R	Zn/Lu	P1	9	-	-	-	-	[40]
34R	Zn/Dy	P1	9	−305, −372	14.19	212.1 b	7.0 × 10−10	[44]
35R	Zn/Dy	P1	9	−310, −386	14.21	203.5 c	1.0 × 10−10	[44]
36R	Zn/Dy	P1	9	−290, −409	14.18	207.3 c	1.5 × 10−5	[44]
37S	Zn/Dy	P1	9	+305, +372	28.36	194.5 b	3.1 × 10−9	[44]
38S	Zn/Dy	P1	9	+310, +386	28.33	70.1/231.6 c	8.0 × 10−7/5.4 × 10−10	[44]
39S	Zn/Dy	P1	9	+290, +409	28.40	218.1 c	3.5 × 10−11	[44]
49R/49S	Zn/Y	P21	6	-	-	-	-	[49]
50R/50S	Zn/Lu	P21	6	-	-	-	-	[49]
51R/51S	Zn/Dy	P21	6	-	-	-	-	[49]
52R/52S	Zn/Sm	P21	6	-	-	-	-	[49]
53R/53S	Zn/La	P21	6	-	-	-	-	[49]
60R	Zn/Ce	P21	10	-	-	4.7 d	2.5 × 10−5	[50]
61R	Zn/Nd	P21	10	-	-	15.9 e	3.7 × 10−6	[50]
62R/62S	Zn/Eu	P21	10	−235, −300, +218, +265, +342, +395 a	-	-	-	[50]
63S	Zn/Dy	P21	10	-	-	17.7 e	8.3 × 10−7	[50]
64S	Zn/Er	P21	10	-	-	-	-	[50]
65S	Zn/Yb	P21	10	-	-	-	-	[50]
66R/66S	Zn/Dy	P21	10	−301, −360, +406 a	14.49	11.9/46.1 f	4.23 × 10−5/8.85 × 10−8	[51]
67S	Zn/Nd	P21	9	-	-	-		[52]
68R/68S	Zn/Dy	P21	9	−262, −303, +382 a	13.44	19.40/51.82 g 20.48/51.72 h	1.23 × 10−8/3.75 × 10−9 8.97 × 10−9/3.55 × 10−9	[53]
69R/69S	Zn/Tb	P21	9	−261, −303, +386 a	-	-	-	[54]
71S	Zn/Nd	C2	9	-	-	-	-	[52]
73R	Zn/Er	P212121	9	-	-	8.1 i	5.3 × 10−7	[57]

^a the values correspond to the R-enantiomer, same values with opposite sings correspond to the S-enantiomer; ^b under 1.8 kOe external dc field, n = 5.49, C = 1.6 × 10⁻³ s⁻¹K^−5.49 (34R), n = 5.72, C = 7.6 × 10⁻⁴ s⁻¹K^−5.72 (37S); ^c under 2.0 kOe external dc field, n = 5.06, C = 1.2 × 10⁻² s⁻¹K^−5.06 (35R), n = 5.29, C = 1.9 × 10⁻² s⁻¹K^−5.29 (36R), n = 4.54, C = 3.9 × 10⁻² s⁻¹K^−4.54 for the slow relaxation and n = 5.98, C = 1.8 × 10⁻⁴ s⁻¹K^−5.98 for the fast relaxation (38S), n = 4.78, C = 5.4 × 10⁻² s⁻¹K^−4.78 (39S); ^d under 0.5 kOe external dc field; ^e under 1.0 kOe external dc field; ^f under 4.0 kOe external dc field, for the low- and high-temperature relaxation; ^g under 1.5 kOe external dc field for the R-enantiomer, for the low- and high-temperature relaxation; ^h under 1.5 kOe external dc field for the S-enantiomer, for the low- and high-temperature relaxation; ⁱ under 1.0 kOe external dc field.

Table 4. Structural, CD, and magnetic data for the heterometallic Zn/4f complexes.

Complex	3d/4f Ions	Space Group	Coordination Number	CD, λmax (nm)	χMT (r.t.) (cm3Kmol−1)	Ueff (K)	t0 (s)	Ref.
32R	Zn/Lu	P1	9	-	-	-	-	[40]
34R	Zn/Dy	P1	9	−305, −372	14.19	212.1 b	7.0 × 10−10	[44]
35R	Zn/Dy	P1	9	−310, −386	14.21	203.5 c	1.0 × 10−10	[44]
36R	Zn/Dy	P1	9	−290, −409	14.18	207.3 c	1.5 × 10−5	[44]
37S	Zn/Dy	P1	9	+305, +372	28.36	194.5 b	3.1 × 10−9	[44]
38S	Zn/Dy	P1	9	+310, +386	28.33	70.1/231.6 c	8.0 × 10−7/5.4 × 10−10	[44]
39S	Zn/Dy	P1	9	+290, +409	28.40	218.1 c	3.5 × 10−11	[44]
49R/49S	Zn/Y	P21	6	-	-	-	-	[49]
50R/50S	Zn/Lu	P21	6	-	-	-	-	[49]
51R/51S	Zn/Dy	P21	6	-	-	-	-	[49]
52R/52S	Zn/Sm	P21	6	-	-	-	-	[49]
53R/53S	Zn/La	P21	6	-	-	-	-	[49]
60R	Zn/Ce	P21	10	-	-	4.7 d	2.5 × 10−5	[50]
61R	Zn/Nd	P21	10	-	-	15.9 e	3.7 × 10−6	[50]
62R/62S	Zn/Eu	P21	10	−235, −300, +218, +265, +342, +395 a	-	-	-	[50]
63S	Zn/Dy	P21	10	-	-	17.7 e	8.3 × 10−7	[50]
64S	Zn/Er	P21	10	-	-	-	-	[50]
65S	Zn/Yb	P21	10	-	-	-	-	[50]
66R/66S	Zn/Dy	P21	10	−301, −360, +406 a	14.49	11.9/46.1 f	4.23 × 10−5/8.85 × 10−8	[51]
67S	Zn/Nd	P21	9	-	-	-		[52]
68R/68S	Zn/Dy	P21	9	−262, −303, +382 a	13.44	19.40/51.82 g 20.48/51.72 h	1.23 × 10−8/3.75 × 10−9 8.97 × 10−9/3.55 × 10−9	[53]
69R/69S	Zn/Tb	P21	9	−261, −303, +386 a	-	-	-	[54]
71S	Zn/Nd	C2	9	-	-	-	-	[52]
73R	Zn/Er	P212121	9	-	-	8.1 i	5.3 × 10−7	[57]

^a the values correspond to the R-enantiomer, same values with opposite sings correspond to the S-enantiomer; ^b under 1.8 kOe external dc field, n = 5.49, C = 1.6 × 10⁻³ s⁻¹K^−5.49 (34R), n = 5.72, C = 7.6 × 10⁻⁴ s⁻¹K^−5.72 (37S); ^c under 2.0 kOe external dc field, n = 5.06, C = 1.2 × 10⁻² s⁻¹K^−5.06 (35R), n = 5.29, C = 1.9 × 10⁻² s⁻¹K^−5.29 (36R), n = 4.54, C = 3.9 × 10⁻² s⁻¹K^−4.54 for the slow relaxation and n = 5.98, C = 1.8 × 10⁻⁴ s⁻¹K^−5.98 for the fast relaxation (38S), n = 4.78, C = 5.4 × 10⁻² s⁻¹K^−4.78 (39S); ^d under 0.5 kOe external dc field; ^e under 1.0 kOe external dc field; ^f under 4.0 kOe external dc field, for the low- and high-temperature relaxation; ^g under 1.5 kOe external dc field for the R-enantiomer, for the low- and high-temperature relaxation; ^h under 1.5 kOe external dc field for the S-enantiomer, for the low- and high-temperature relaxation; ⁱ under 1.0 kOe external dc field.

Table 5. Ferroelectric and spectroscopic data for the chiral 4f and 3d/4f complexes.

Complex	Pr (μCcm−2)	Ec (kVcm−1)	UV-Vis (nm)	Fluorescence (nm)	MCD gmax (T−1)	CPL gPL	SHG, χR(2) (pmV−1)	THG, χR(3) (pm2V−2)	Ref.
6R	4.51	28.11	-	-	-	-	-	-	[35]
7S	7.98	22.12	-	-	-	-	-	-	[36]
32R	-	-	-	460	-	-	-	-	[40]
34R	-	-	-	-	-	-	0.04	581	[44]
35R	-	-	-	-	-	-	0.01	821	[44]
36R	-	-	-	-	-	-	0.07	848	[44]
37S	-	-	-	-	-	-	0.39	485	[44]
38S	-	-	-	-	-	-	0.02	703	[44]
39S	-	-	-	-	-	-	0.40	718	[44]
66R/66S	-	-	279, 372	490	6.7 × 10−2	±2.5 × 10−3	-	-	[51]
68R	9.1	17.0	-	-	-	-	-	-	[53]
70R	-	-	310	-	-	-	-	-	[55]

Table 5. Ferroelectric and spectroscopic data for the chiral 4f and 3d/4f complexes.

Complex	Pr (μCcm−2)	Ec (kVcm−1)	UV-Vis (nm)	Fluorescence (nm)	MCD gmax (T−1)	CPL gPL	SHG, χR(2) (pmV−1)	THG, χR(3) (pm2V−2)	Ref.
6R	4.51	28.11	-	-	-	-	-	-	[35]
7S	7.98	22.12	-	-	-	-	-	-	[36]
32R	-	-	-	460	-	-	-	-	[40]
34R	-	-	-	-	-	-	0.04	581	[44]
35R	-	-	-	-	-	-	0.01	821	[44]
36R	-	-	-	-	-	-	0.07	848	[44]
37S	-	-	-	-	-	-	0.39	485	[44]
38S	-	-	-	-	-	-	0.02	703	[44]
39S	-	-	-	-	-	-	0.40	718	[44]
66R/66S	-	-	279, 372	490	6.7 × 10−2	±2.5 × 10−3	-	-	[51]
68R	9.1	17.0	-	-	-	-	-	-	[53]
70R	-	-	310	-	-	-	-	-	[55]

4. Chiral Materials from Racemic Salen-Type Schiff Base Ligands

The condensation reaction of 2-hydroxy-benzaldehyde (salicylaldehyde) with 1,2-diamino-cyclohexane gave the Schiff base ligand H₂L1 (Scheme 1) which upon reaction with Ce(NO₃)₃·6H₂O in EtOH gave complex [Ce^IV(L1)₂] (74) which crystallizes in the chiral monoclinic space group P2₁ [34]. The absolute structure was determined by the Flack parameter, x = 0.53(4), which indicates racemic twinning. The absolute configuration of both chiral centers in L(1)²⁻ is R,S- and due to racemic twinning, the S,R- also exist in the crystal structure. The Ce^IV ion is eight-coordinated to the two O_phenoxo and the two N_imino atoms from two (L1)²⁻ ligands with Ce-O and Ce-N distances in range 2.148(7)-2.241(9) Å and 2.525(11)-2.619(9) Å, respectively. The best planes of the coordination spheres of the two ligands form a dihedral angle of 87.7(3)°. The coordination geometry is best described as snub diphenoid (JSD-8, CShM = 1.31927).

The zwitterion of racemic H₂L1 forms 3D coordination polymers [Ln^IIICl(NO₃)₂(H₂L1)]_n (Ln^III = La (75), Ce (76), Nd (77)) [59,60] which crystallize in the tetragonal chiral space group P4₃2₁2 with diamond (dia) network topology (Figure 20). The Ln^III ion is nine-coordinated to two chelate nitrato and one chlorido group, as well as to four O_phenoxo atoms from four different H₂L1 zwitterions. The coordination polyhedron around the Ln^III ion is a tricapped trigonal prism (JTCTPR-9). The 3D structure is built by [Ln^III(H₂L1)]³⁺ subunits, with the Ln^III ion and the H₂L1 serving as the head and tail, respectively, of a tadpole-like model, with all the tails being right-handed. In this head-to-tail ligation mode, the tadpole-like models link each other and form two 1D right-handed helical chains, each one directed along the a and b axis. Two such helixes interweave each other by sharing the Ln^III ions, thus forming the dia 3D network. Compounds 75–77 contain the enantiopure H₂L1_SS ligand due to the trans orientation of the ligand, which shows higher stability and lower steric hindrance, and also due to hydrogen bonds which stabilize the structure. Compounds 75–77 represent the first 3D chiral lanthanide MOFs with dia topology and opened new ways for the synthesis of enantiopure materials from racemate precursors. The chirality of these MOFs results in the SHG effects, and they are the first examples of NLO-active enantiopure Ln-salen frameworks.

The use of the zwitterion of racemic ligand H₂L2 (Scheme 1) gave the 3D polymer {[Ce^III(H₂L2)(NO₃)₂](PF₆)}_n (78), which crystallizes in the hexagonal chiral space group P6₄22 with quartz (qtz) topology [61]. The structure consists of right-handed helixes of [Ce^III(H₂L2)]_n. The Ce^III ion is eight-coordinated to four O_phenoxo atoms from four different H₂L2 ligands and to two chelate nitrato groups in a hexagonal bipyramid geometry (HB PY-8, CShM = 7.53410, Figure 21).

The racemic ligand H₂L15 (Scheme 2) gave the chiral 1D coordination polymers [{CuLn(L15)(H₂O)₄}{M(CN)₈}]_n (M^V = Mo, Ln^III = Gd (79), Tb (80), Dy (81); M^V = W, Ln^III = La (82), Gd (83), Tb (84), Dy (85)) and [{Cu₂Ln(L15)₂(H₂O)₄}{Mo(CN)₈}] (Ln^III = La (86), Pr (87)) [58]. The polymeric structure of 81–85 is based on {M(CN)₈} units coordinated through one cyano ligand to the Cu^II ion of a dinuclear {CuLn} moiety. The {Cu^IILn^IIIM^V} fragment is further linked through a second cyano group to the Ln^III ion of an adjacent trimetallic fragment (Figure 22, Table 3). The (L15)²⁻ ligand is coordinated via the two O_phenoxo and the N_imino atoms to the Cu^II ion and through the two bridging O_phenoxo and the two O_methoxo to the Ln^III. The nine-coordination of the latter is completed by four aqua ligands and one nitrogen atom from the cyano bridge. The coordination geometry is distorted monocapped square antiprism. Intermolecular O···O and N···O type hydrogen bonds, involving the free cyano groups, the crystallization, and coordinated H₂O molecules, establish a 3D supramolecular network. The structure of 86 is based on trinuclear {Cu^II₂(L15)₂La^III} moieties linked by {Mo(CN)₈}³⁻ anions, resulting in 1D chains running along the crystallographic c axis. There are two C2 axes passing through La^III and Mo^V; thus, half of the [{CuLa(L15)(H₂O)₄}{Mo(CN)₈}] repeating unit is independent (Figure 23). The structure of the trinuclear {Cu^II₂(L15)₂La^III} moiety consists of a central La^III ion, which is bound to the (O_phenoxoO_methoxo)₂ donor set from the two (L15)²⁻ ligands, whereas each of the Cu^II ions is hosted in the (O_phenoxoN_imino)₂ pocket of each (L15)²⁻ ligand. The ten-coordination around the La^III ion is completed from cyano bridges and is described as distorted bicapped square antiprism. Each Cu^II ion is five-coordinated with one cyano bridge in the apical position of the square pyramid. The chains of 86 are linked through C-H···π intermolecular interactions and form a 3D supramolecular network.

5. Concluding Comments

The enantiopure Schiff base ligands depicted in Scheme 1, Scheme 2 and Scheme 3 gave homometallic 4f and heterometallic 3d-4f chiral complexes with diverse nuclearities, metal topologies, coordination geometries, and physical properties. In the homometallic 4f complexes 1–8, the Ln^III ion is coordinated to the O_phenoxo/N_imino atoms of the Schiff base ligand, whereas in complexes 9 and 10, which contain the ligand in the zwitterion form, the lanthanide is bound to the O_phenoxo and/or O_methoxo atoms. In the 4f complexes 1–10, the Ln^III ions display coordination numbers, which range from seven to ten. In the heterometallic 3d-4f complexes 11–73, the 3d metal ions are coordinated to the Schiff base ligands through the O_phenoxo/N_imino atoms in square planar geometry. In some cases, five- or six-coordination is observed from coordinated co-ligands, such as solvent molecules, organic or inorganic anions, and the 3d ions display square planar or octahedral geometries. In complexes 11–73, the lanthanide ions are coordinated via the O_phenoxo/O_alkoxo atoms of the ligands; the O_phenoxo atoms serve as bridging atoms between the 3d and 4f ions. The coordination environment of the Ln^III ions is completed by co-ligands, such as nitrates, azides, and carboxylates, and gives coordination numbers from six to eleven. In the chiral complexes 74–86 derived from racemic Schiff bases, the Ln^III ions are coordinated to the O_phenoxo/N_imino atoms, or to the O_phenoxo/O_alkoxo atoms, or to the O_phenoxo atoms, depending on the simultaneous presence of other metal ions, or the coordination diversity of the ligand which promotes the formation of network structures.

The chiral complexes discussed herein constitute multifunctional molecular materials that combine chirality with various physical properties. The majority of the complexes display field-induced slow magnetic relaxation compatible with single-ion magnetic behavior. This behavior was observed in complexes of oblate ions, such as Tb^III, Dy^III, and Ho^III, prolate ions, such as Yb^III, and of ‘uncommon magnetic lanthanides’, Ce^III, and Nd^III. Other physical properties, such as ferroelectric, optical, catalytic, and anticancer behavior, were reported.