Multinuclear Ni(II), Cu(II) and Zn(II) complexes of chiral macrocyclic nonaazamine†

The chiral macrocyclic amines R-L and S-L derived from the 3 + 3 condensation of 2,6-diformylpyridine and (1R,2R)-1,2-diaminocyclohexane or (1S,2S)-1,2-diaminocyclohexane form enantiopure trinuclear Ni(II) and Cu(II) complexes [Ni3(L)(H2O)2Cl5]Cl and [Cu3(L)Cl4]Cl2 and form the dinuclear complex [Zn2(L)Cl2] (ZnCl4) with Zn(II). The X-ray crystal structures of these complexes indicate remarkably different conformations of the ligand and different binding modes of the chloride anions. The structure of the copper(II) derivative [Cu3(R-L)Cl4]Cl2·CH3CN·7.5(H2O) indicates unsymmetrical conformation of the macrocycle with three dissimilar pentacoordinate copper(II) ions bridged by chloride; the structure of [Ni3(R-L) (H2O)2Cl5]Cl·0.4CH3CN·4.2H2O is somewhat more symmetrical, with three Ni(II) ions of distorted octahedral geometry, also bridged by a common chloride anion. On the other hand, the macrocycle is highly folded in [Zn2(R-L)Cl2](ZnCl4)·CHCl3·0.8CH3OH·3.7H2O, forming a cleft where the third Zn(II) ion is held via electrostatic interactions as the ZnCl4 2− anion. The magnetic data for [Cu3(R-L)Cl4]Cl2 indicate the coexistence of antiferromagnetic and ferromagnetic interactions within the quasi isosceles tricopper(II) core (J = −85.6 cm, j = 77.1 cm). Compound [Ni3(R-L)(H2O)2Cl5]Cl shows the presence of weak antiferromagnetic coupling (J = −2.56 cm, j = −1.54 cm) between the three Ni(II) ions.


Introduction
Trinuclear and dinuclear complexes, including macrocylic complexes, are attracting attention due to their magnetic and catalytic properties and because they are models for multinuclear metalloenzymes. [1][2][3] For example, many nucleases and esterases utilize two or three metallic centers for cooperative catalysis, which has inspired the synthesis of multinuclear transition metal complexes 1 that mimic these enzymes. Research on trinuclear Cu(II) complexes has also aimed at mimicking the trinuclear metalloenzyme sites present in copper oxidases 1 such as ascorbate oxidase, ceruplasmin and laccase. It should be mentioned that one form of these copper oxidases corresponds to an antiferromagnetically coupled trinuclear Cu(II) cluster. This form has triggered interest in synthetic trinuclear Cu(II) complexes. In particular, the chiral 3 + 3 macrocycle R-L ( Fig. 1) forms an interesting trinuclear Cu(II) complex, [Cu 3 (R-L)(μ 3 -OH) 2 ]Cl 2 (ClO 4 ) 2 , which exhibits intramolecular ferromagnetic interactions. 3 In this complex, three Cu(II) ions of distorted trigonal bipyramidal geometry are bound in the interior of the macrocycle and are additionally bridged by two hydroxo anions; thus, a Cu 3 (μ 3 -OH) 2 cluster is bound by the macrocycle. For comparison, the same ligand forms mononuclear complexes with lanthanide(III) ions, 4 while the Schiff base analogue of the amine macrocycle of L forms dinuclear complexes with Cd(II) ions. 5 Trinuclear metal complexes are also formed by a similar chiral macrocyclic 3 + 3 amine, R-L′ (Fig. 1), which has three phenolic fragments instead of three pyridine fragments. The macrocycle L′ (or its Schiff base analogue) is able to form trinuclear complexes with Cu(II) ions, 6 Zn(II) ions, 7,8 and lanthanide(III) ions. 9 The cooperative action of zinc(II) ions in complexes with 3 + 3 macrocycles was utilized in catalytic cleavage of DNA 7 and the nitroaldol reaction. 10 indicates lower symmetry, in contrast to that observed for the previously reported trinuclear Cu(II) complex [Cu 3 (R-L)(μ 3 -OH) 2 ]Cl 2 (ClO 4 ) 2 . 3 The spectrum consists of 19 paramagnetically shifted lines, which are relatively narrow for Cu(II) complexes as a result of magnetic interactions between the Cu(II) ions (Fig. 6S †). On the other hand, the effective C 3 symmetry observed in solution is higher than the C 1 symmetry observed in the crystal structure (vide infra). It follows that axial ligand exchange also operates for complex [Cu 3 (R-L)Cl 4 ]Cl 2 or that the structure of the complex in solution is different from that observed in the solid. The temperature dependence of the chemical shifts of this complex (Fig. 7S †) is typical for paramagnetic species and does not exhibit anti-Curie behavior, which would be observed in the case of very strong antiferromagnetic interactions.
The NMR spectra of the Zn(II) complex [Zn 2 (R-L)Cl 2 ](ZnCl 4 ) (Fig. 8S †) indicates C 1 symmetry of the macrocycle in solution, in accord with the structure observed in the solid state (although the macrocycle is folded, it has no C s symmetry plane due to the presence of the chiral cyclohexane fragments, vide infra). Thus, the COSY and HMQC spectra indicate that the aromatic region of the 1 H NMR spectrum consists of six doublets coupled to three triplets (two of which are ideally overlapped, see ESI Fig. 11S †). This indicates the presence of three different unsymmetrical pyridine rings, in agreement with the crystal structure. Similarly, the C 1 symmetry is confirmed by the observation of six different methylene fragments, and six different >CHN positions are seen in the COSY, HMQC and 13 C NMR spectra ( Fig. 9S-14S †).
The titration of ligand L with ZnCl 2 , monitored with NMR spectroscopy, indicates initial formation of a presumably mononuclear species with signals broadened by chemical exchange, followed by formation of the dinuclear complex and formation of yet another species ( probably trinuclear complexes) in the presence of excess zinc(II) ions ( Fig. 15S and 16S †). The signals of the dinuclear complex appear when 1.25 equivalents of ZnCl 2 are used and seem to dominate the spectrum even when 3 equivalents are added. The positions of the lines vary slightly as the concentration of added ZnCl 2 increases, probably indicating fast chemical exchange corresponding to ion-pair formation between the cationic dinuclear macrocyclic complex and chloride anion or [ZnCl 4 ] 2− anion. , the ligand R-L binds each metal ion via the nitrogen atom of one of the pyridine rings and two nitrogen atoms of the adjacent diaminocyclohexane fragment (Fig. 2). In the Ni(II) complex [Ni 3 (R-L)(H 2 O) 2 Cl 5 ]Cl·0.4CH 3 CN·4.2H 2 O, the macrocycle R-L exhibits a relatively flat, helically twisted conformation ( Fig. 2 and 3). This conformation is similar to some extent to that reported previously for the trinuclear Cu(II) complex [Cu 3 (R-L)(μ 3 -OH) 2 ]Cl 2 (ClO 4 ) 2 ·5H 2 O. 3 The macrocycle R-L in the latter complex is, however, more compact and resembles the protonated form of the macrocycle. 5 The less compact form of [Ni 3 (R-L)(H 2 O) 2 Cl 5 ]Cl·0.4CH 3 CN·4.2H 2 O is reflected in the larger radius of the macrocycle and the smaller extent of the helical twist of the pyridine rings. In addition, the macrocycle is partly domed; therefore, its approximate symmetry is C 3 , in contrast to the approximate D 3 symmetry of this macrocycle in complex [Cu 3 (R-L) (μ 3 -OH) 2 ] Cl 2 (ClO 4 ) 2 ·5H 2 O. 3 In contrast, in complex [Cu 3 (R-L)Cl 4 ] Cl 2 ·CH 3 CN·7.5(H 2 O) the ligand R-L adopts a highly distorted, irregular saddle-type conformation (Fig. 3), reflecting both the helical twist and considerable folding of the macrocycle. This distortion towards an irregular conformation of R-L upon complexation is similar to the distortion of a similar macrocycle, R-L′, caused by formation of a mononuclear Eu(III) complex. 11 An even more distorted conformation of macrocycle R-L is observed for the dinuclear Zn(II) complex [Zn 2 (R-L)Cl 2 ](ZnCl 4 ) CHCl 3 ·0.8CH 3 OH·3.7H 2 O (Fig. 4). This time, the ligand conformation is dominated by strong bending and is completely different to that observed for the free macrocycle. This complex also differs from complexes  Table 1S. † The equatorial plane for all three Ni atoms is created by three nitrogen atoms from the macrocyclic ligand and the bridging chloride atom, with Ni1-(μ 3 -Cl1), Ni2-(μ 3 -Cl1) and Ni3-(μ 3 -Cl1) bond distances equal to 2.745(3), 2.642(3) and 2.554(3) Å, respectively. The apical positions are occupied by two terminal chloride atoms in the case of Ni1 or one chloride atom and one water molecule in the cases of Ni2 and Ni3. The [Ni 3 ] unit is strictly a scalene triangle; however, it can be considered as an approximate isosceles triangle with Ni1⋯Ni2, Ni2⋯Ni3 and Ni3⋯Ni1 distances of 4.579 (2) Table 2S. † Two of the Cu(II) ions are closer to square-pyramidal geometry, while the third Cu(II) ion has distorted trigonal bipyramid geometry. The distortions from the ideal geometries are reflected in the values of the angular structural parameter τ (index of trigonality); 12 τ = (β − α)/60°, where α and β are the two largest angles in the coordination sphere. The values of τ for the Cu(II) ions are equal to 0.57, 0.19 and 0.15 for Cu1, Cu2 and Cu3, respectively. The limit value of τ = 0 corresponds to an ideal square pyramid (α = β ∼ 180°) and τ = 1 corresponds to an ideal trigonal bipyramid (α = 120°and β = 180°). For Cu1, the basal plane of the bipyramid consists of one terminal chloride atom and two nitrogen atoms, one from diaminocyclohexane and one from the pyridine ring. The apical positions are occupied by a second nitrogen atom from diaminocyclohexane and a bridging chloride ligand μ 3 -Cl1, with a Cu1-(μ 3 -Cl1) bond distance equal to 2.345(2) Å. The square-planar base of Cu2 is defined by three nitrogen atoms from the macrocyclic ligand and a terminal chloride atom. The apical position is occupied by the bridging chloride ligand μ 3 -Cl1, with a distance of 2.675(1) Å. In the case of Cu3, the axial position is occupied by the terminal chloride atom, while the equatorial plane consists of three nitrogen atoms from the macrocyclic ligand and the bridging chloride ligand μ 3 -Cl1, with a Cu3-(μ 3 -Cl1) bond distance equal to 2.438(1) Å. The [Cu 3 ] unit is strictly a scalene triangle but can be considered as an approximate isosceles triangle with Cu1⋯Cu2, Cu2⋯Cu3 and Cu3⋯Cu1 distances of 3.970 (3)   , the quasi-isosceles core of these compounds and the two J and j parameters were assumed by applying the derived Hamiltonian (eqn (1)):

X-ray crystal structures
However, the distances between the nickel ions are comparable (ESI Table 1S †). The experimental data were fitted using the PHI program, 18 including the ZFS parameter D of Ni(II) ion and a term zJ′ for intertrimer exchange. The best-fit parameters are: J = −2.56 cm −1 , j = −1.54 cm −1 , zJ′ = −0.09 cm −1 , g = 2.21 and R = 1.26 × 10 −4 , R = ∑(χ exp T − χ calc T ) 2 /∑(χ exp T ) 2 . The S = 0 ground state was found for complex [Ni 3 (R-L)(H 2 O) 2 Cl 5 ]Cl from a j/J ratio 13 equal to 0.6. The ground state S = 0 was proposed for ratios between 0.5 and 2.0 and the ground state S = 1 should be observed for ratios less than 0.5 and greater than 2.0. 13,17 The χ m T value of complex [Cu 3 (R-L)Cl 4 ]Cl 2 is equal to 1.05 cm 3 mol −1 K at room temperature, which is only slightly lower than the expected value for three uncoupled S = 1/2 spins (ca. 1.2 cm 3 mol −1 K). This value systematically decreases with decreasing temperature to 0.473 cm 3 mol −1 K at 60 K. This behavior is characteristic of a dominant antiferromagnetically coupled system. Between 10 and 60 K, a plateau in the χ m T vs. T relation is observed, with a value of ∼0.4 cm 3 mol −1 K, as expected for an isolated S = 1/2 ground state. The plateau has been observed for many other Cu(II) triangles; [19][20][21]23,26 it indicates that these compounds follow Curie's law, and only the ground spin doublet (or degenerate spin doublets) is thermally populated. The experimental χ m T data decreases below 10 K, which suggests that other kinds of antiferromagnetic interactions are operative; intermolecular interactions through hydrogen N-H⋯Cl bonds are observed in the crystal structure (Fig. 19S †). This observation is characteristic of equilateral as well as isosceles and scalene copper(II) triangles 19 and is a result of spin frustration. Spin frustration occurs when only two out of three spins achieve full spin compensation simultaneously. 13,22 Triangular, trinuclear Cu(II) complexes can be regarded as geometrically spin-frustrated systems 13 where an isotropic Heisenberg-Dirac-van Vleck (HDVV) Hamiltonian formalism is not sufficient to investigate the magnetic properties and an antisymmetric term should be added. [18][19][20][21][22][23][24][25][26][27][28][29] The experimental data were fitted using the PHI program. 18 Anisotropic, antisymmetric interactions were included in the program. The best fit to the experimental data of [Cu 3 (R-L)Cl 4 ] Cl 2 results in these parameters: J = −85.6 cm −1 , j = 77.1 cm −1 , zJ′ = −0.14 cm −1 .

Magnetostructural correlation
To our knowledge, the trinuclear compounds    24 where the spin delocalization mechanism was used. The relationships presented 23 between the magnetic coupling and structural features for trinuclear complexes with [Cu 3 O] cores, and the principal structural factors, are: (a) The major factor controlling the spin coupling between the metal centers in hydroxido, alkoxido or phenoxido bridged compounds is the bridging Cu-(μ 3 -O)-Cu angles. The magnetic coupling interaction is switched from ferromagnetic to antiferromagnetic as the Cu-(μ 3 -X)-Cu angle changes from 76 to 120°. [23][24][25] (b) A linear correlation was found between the coupling constant J and the deviation of the μ 3 -O atom from the centroid of the Cu 3 triangular motif. A smaller deviation determines strong antiferromagnetic coupling. 26,27 (c) A more flattened Cu 3 O(H) bridge favors stronger magnetic interaction. 28 The ions are also different, as is the arrangement of magnetic orbitals with respect to the central chloride bridge. According to the τ parameters, the geometries of Cu2 and Cu3 are square pyramidal (τ = 0.19 and 0.15, respectively), while Cu1 could be regarded as a distorted trigonal bipyramid (τ = 0.59). For that reason, in accordance with the orbital model for magnetic interactions, 13 different magnetic orbitals with interacting unpaired electrons could be engaged in magnetic interactions (d x 2 −y 2 in the cases of Cu2 and Cu3, and a part of orbital d z 2 in the case of Cu1). The spin delocalization between the p orbitals of the μ 3 -Cl bridging ligand and the Cu(II) centers contributes to both the ferromagnetic ( j ) and antiferromagnetic ( J) coupling interactions.
Only a few examples in the literature are concerned with similar Cu 3 Cl cores, which prevents further comparison and discussion of the data obtained in this work. The first triangular structural Cu 3 (μ 3 -Cl) 2 motif was presented by R. Boča et al.; 30 however, no experimental magnetic data were acquired for this compound. A triangular Cu(II) cluster, doubly capped by two μ 3 -X ligands (X = O(H), Cl, Br), and progression from strongly antiferro-to ferromagnetic exchange has been presented, 31 accompanying the change of the Cu-(μ 3 -X)-Cu angle (X = O, OH, halogen) from 120°to 80°. X-ray crystal geometries of these compounds were used in DFT-BS 24 calculations. In all the above cited compounds, equatorial ligands also play important roles in the magnetic interactions.
Analysis of the structural and magnetic data and DFT calculations of the triangle Ni 3 O(H) compounds indicates that the antiferromagnetic interaction also depends on the Ni-O-Ni angles. [32][33][34][35][36][37] Magnetostructural data of the complexes analyzed by us, with [Ni 3 Cl] and [Cu 3 Cl] cores, are presented in Table 1 (explanation of the signs is given in Scheme 1). The magnetic coupling parameters exert effects through the triply bridged μ 3 -Cl core, which has not been described to date.

Conclusions
Six enantiopure Ni(II), Cu(II) and Zn(II) complexes with chiral nonaazamine have been prepared, and three of the complexes were structurally characterized by X-ray crystallography. As was shown, depending on the metal used, different arrangements of the ligand R-L and different binding modes of the chloride anions were observed. X-ray crystal structures of the trinuclear complexes

Methods
The NMR spectra were acquired using a Bruker Advance 500 MHz spectrometer. The electrospray mass spectra ( Fig. 20S Corrections for the diamagnetism of the constituting atoms were calculated using Pascal's constants; 38 the value of 180 × 10 −6 cm 3 mol −1 was used to characterize the intermolecular interactions, χ tri is the magnetic susceptibility of the trinuclear cluster, and zJ′ is the intermolecular interaction parameter 18 used as the temperature-independent paramagnetism of trinuclear Cu(II) complex; the value of 300 × 10 −6 cm 3 mol −1 was used for the trinuclear Ni(II) complex. 13 The effective magnetic moments were calculated from the expression The fitting of the magnetic susceptibility and the simulation of the magnetization were carried out using PHI software. 18 The mean-field approximation was calculated as follows:  (2) K on an Xcalibur PX instrument (Oxford Diffraction) using Mo-Kα radiation (λ = 0.71073 Å) and CCD; computing cell refinements were performed using CrysAlis RED. 39 The X-ray data for [Zn 2 (R-L)Cl 2 ](ZnCl 4 )·CHCl 3 ·0.8CH 3 OH·3.7H 2 O were collected at 110(2) K using a KM4CCD instrument with Mo-Kα radiation (λ = 0.71073 Å). The images were indexed, integrated and scaled using the Oxford Diffraction data reduction package. 40 The structures were solved by direct methods using the SHELXS97 (Sheldrick, 1990) program 41 and refined by the full matrix least-squares technique using SHELXL2013 (Sheldrick, 2013). 42 All ordered non-hydrogen atoms were refined with anisotropic thermal parameters; the disordered atoms were isotropic. The H atoms attached to C and N atoms were added geometrically and were treated as riding on the concerned parent atoms. H atoms attached to O atoms were located from difference Fourier maps and included in the final refinement cycles on fixed positions. The known absolute configurations of chiral carbon atoms were confirmed on the basis of the value of the Flack parameter. All figures were made using the MERCURY programme. 43 Details of the data collection, refinement and crystallographic data are presented in Table 2.

Synthesis
Both enantiomers of macrocyclic ligand L, R-L and S-L, were prepared according to a literature procedure.  CuCl 2 ·2H 2 O (112.5 mg, 0.6600 mmol) were dissolved in 12 mL of acetonitrile, and the mixture was refluxed for 5 h. The obtained green precipitate was filtered, and the clear, dark green filtrate was allowed to stand at room temperature. After a few days, dark green crystals suitable for X-ray measurement