Synthesis, crystal structure, magnetic study and magneto-structural correlation of three Cu(II) complexes formed via pyridine bis(hydrazone) based ligand

Abstract

This report describes the synthesis and characterization of three different complexes of molecular formulae {[Cu₆L₂(ClO₄)₄(μ-ClO₄)₂(H₂O)₉](ClO₄)₂·8H₂O}_n(1), [Cu₆L₂Cl₆(μ₂-Cl)₂(H₂O)₂]·3H₂O (2) and [Cu₉L₆](ClO₄)₆·6H₂O·2CH₃CN·CH₃OH (3), where, H₂L = bis[(2-pyridyl)methylene] pyridine 2,6 dicarbohydrazone. X-ray crystallography reveals that complex 1 exhibits 1D chain, complex 2 is a hexanuclear entity and an unsymmetrical [3 × 3] grid formation in complex 3. Variable temperature magnetic measurements were performed, they show that weak antiferromagnetic interactions exist among the metal centers in complex 1 and both ferro- and antiferromagnetic interactions coexist for complexes 2 and 3. Below 20 K antiferromagnetic interactions dominate for complex 2 whereas it is ferromagnetic for complex 3. Transformation from one cage to another is possible at mild reaction conditions, which results in a dramatic change in the magnetic properties.

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Introduction

Ligand directed self-assembly is a powerful synthetic tool for molecular fabrication as predetermined polymetallic architectures are developed when specifically designed ligands are coordinated to metal ions according to their coordination algorithms.^1–3 Polytopic hydrazone based ligands are of wide importance in this context owing to their design attributes and power of self-assembly, which account for their candidature for the synthesis of polymetallic systems and nanosized supramolecular objects.^2–4 Although employing polypyridyl ligands, significant contribution has already been documented in the literature,^1,2,4–7 yet the logical approaches for the inclusion of large numbers of metal ions in a small, single molecular entity with interesting functional properties is a synthetic challenge to the researchers and is of current research interest. Metal-mediated self-assemblies are largely dependent on the choice of solvent, pH, molar ratio etc.^8,9 Once the product cage is formed, their transformation to another cage is unusual and needs special mention. As a part of the ongoing program in designing suitable ligands for several metal ion coordination, we report the synthesis, transformation, structural and detailed magnetic property study of complexes 1–3. The comprehensive magneto-structural correlation has also been done for all three complexes. The chosen ligand bis[(2-pyridyl)methylene]pyridine,2,6-dicarbohydrazone (H₂L)¹⁰ consists of three pyridine groups connected by carbohydrazide linkages, thus forming three tridentate binding sites. The ligand on reaction with copper salts in CH₃CN/MeOH (1 : 1) at different pH generated three complexes having molecular formulae {[Cu₆L₂(ClO₄)₄(μ-ClO₄)₂(H₂O)₉] (ClO₄)₂·8H₂O}_n (1), [Cu₆L₂Cl₆ (μ₂-Cl)₂(H₂O)₂]·3H₂O (2) and [Cu₉L₆](ClO₄)₆·6H₂O·2CH₃CN·CH₃OH (3) respectively. We also report the transformation of one cage to another at mild reaction conditions. Magnetic studies show that at low temperature a weak antiferromagnetic interaction dominates for complex 1. For complexes 2 and 3 both ferromagnetic and antiferromagnetic interactions coexist.

Experimental

Materials and methods

Infrared spectra were recorded in the solid state (KBr pellets) on a Perkin Elmer FTIR spectrometer in the range of 400–4000 cm⁻¹. Elemental analyses were performed on an Elementar vario Microcube elemental analyzer. PXRD were done on a PANalytical EMPYREAN instrument using Cu-Kα radiation. Variable temperature direct current (dc) and alternating current (ac) magnetic susceptibility data were collected on a quantum design SQUID-VSM magnetometer equipped with a 7 T magnet. The measured values were corrected for the experimentally measured contribution of the sample holder, while the derived susceptibilities were corrected for the diamagnetism of the samples, estimated from Pascal's tables.¹¹

X-Ray crystallography

X-ray crystallographic data were collected at 300 K on a Bruker Smart Apex2 CCD diffractometer using graphite monochromated Mo-Kα (λ = 0.71073) radiation. Data collections were performed using φ and ω scan. The structures were solved using direct methods followed by full matrix least square refinements against F² (all data HKLF 4 format) using SHELXTL. Anisotropic refinement was used for all non-hydrogen atoms. Since the structure contains solvent accessible voids, program “SQUEEZE” has been applied and necessary commands from the .SQF file are included in the final CIF. Organic hydrogen atoms were placed in appropriate calculated positions. X-ray crystallographic data in CIF format is available in CCDC 922322–922324.

Synthesis

All manipulations were performed under aerobic conditions using reagents and solvents as received. H₂L was prepared as reported in the literature.¹⁰ Caution! Although we encountered no problem, appropriate care should be taken in the use of the potentially explosive perchlorate salt.

{[(Cu₆L₂)(ClO₄)₄(μ-ClO₄)₂(H₂O)₉](ClO₄)₂·8H₂O}n(1). H₂L (75 mg, 0.2 mmol) and Cu(ClO₄)₂·6H₂O (222.3 mg, 0.6 mmol) were added to 10 mL CH₃CN/MeOH mixture (1 : 1) and stirred for 1 h. Final pH of the solution was measured to 7.6. The solution was then filtered and the filtrate kept for crystallization at room temperature. After one week block shaped dark green single crystals were formed by slow evaporation (yield, 156 mg, 70.1% based on Cu). Elemental analysis: calculated (found) for (C₃₈H₆₀Cu₆O₅₃N₁₄Cl₈)_n: C, 20.34 (20.91); H, 2.78 (2.09); N, 8.74 (8.67). Selected IR data (KBr pellet): 3436(s), 1615(s), 1549(s), 1392(m), 1298(s), 1374(s), 1143(s), 1087(s), 869(m), 785(s), 626(s) cm⁻¹.

[Cu₆L₂Cl₆(μ₂-Cl)₂(H₂O)₂]·3H2O (2). H₂L (75 mg, 0.2 mmol) was added to 10 mL CH₃CN/MeOH mixture (1 : 1) and pH of the solution was adjusted to 1.2 by addition of 6 M HCl solution. Cu(ClO₄)₂·6H₂O (148.2 mg, 0.4 mmol) was added to the solution in parts and the reaction mixture was stirred for 0.5 h until a green precipitate appeared. It was filtered and the resulting precipitate was washed with acetonitrile and dried in vacuum (yield, 67 mg, 67.10% based on Cu). The precipitate was dissolved in water and after one week, diamond shaped green single crystals were obtained at the bottom of the beaker container by slow evaporation. Elemental analysis (%): calculated (found) for C₃₈H₃₆Cu₆O₉N₁₄Cl₈: C, 30.47 (30.97); H, 2.42 (2.19); N, 13.09 (13.13). Selected IR data (KBr pellet): 3400(s), 1615(s), 1566(s), 1548(s), 1387(s), 1293(s), 1220(s), 786(m), 752(m), 678(s), 646(m) cm⁻¹

[Cu₉L₆](ClO₄)₆·6H2O·2CH₃CN·MeOH (3). H₂L (75 mg, 0.2 mmol) was added to 10 mL CH₃CN/MeOH mixture (1 : 1) and pH of the solution was adjusted to 9.8 by drop wise addition of NEt₃. Cu(ClO₄)₂·6H₂O (148.2 mg, 0.4 mmol) was added to the solution in parts and reaction mixture was stirred for 0.5 h. The solution was filtered and the filtrate was kept at room temperature. After 1 day, precipitate started forming at the bottom of the container. It was again filtered and the filtrate kept for crystallization. After one week diamond shaped green single crystals were formed by slow evaporation. The precipitate was also confirmed to be complex 3 by IR and PXRD (yield, 79 mg, 48.5% based on Cu). Elemental analysis (%): calculated (found) for C₁₁₉H₁₀₀Cu₉O₄₃N₄₄Cl₆: C, 39.49 (39.16); H, 2.78 (2.24); N, 17.02 (17.03). Selected IR data (KBr pellet): 3435(s), 1624(s), 1508(s), 1458(s), 1361(s), 1280(m), 1143(s), 1088(s), 786(s), 626(s) cm⁻¹.

(1) to (2) conversion. Complex 1 (222 mg, 0.1 mmol) was added to 10 mL CH₃CN/MeOH mixture (1 : 1) and pH of the solution was adjusted to 1.2 by 6 M HCl solution. The solution was refluxed for 1 h, allowed to cool to room temperature and a green precipitate was formed at the bottom. The solution was filtered and the precipitate was washed with methanol and dried in vacuum. Conversion of 1 to 2 was confirmed by single crystal XRD and PXRD of solid product.

(3) to (2) conversion. Complex 3 (366 mg, 0.1 mmol) was added to 10 mL CH₃CN/MeOH mixture (1 : 1) and the solution was refluxed for 1 h, allowed to cool to room temperature and a green precipitate was formed at the bottom. The solution was filtered and the precipitate was washed with methanol and dried in vacuum. Conversion of 3 to 2 was confirmed by single crystal XRD and PXRD of the solid product.

Results and discussion

Synthesis

The reaction of tritopic ligand H₂L and copper perchlorate salt generates complexes 1–3 under ambient conditions. Interaction of H₂L and Cu^II perchlorate in a 1 : 3 mole ratio resulted in the formation of complex 1 whereas 2 and 3 were obtained by the reaction of H₂L and Cu^II perchlorate in 1 : 2 mole ratio and at pH 1.2 and 9.8 respectively. Complex 2 was obtained from 1 and 3 by adjustment of pH to 1.2 under mild conditions. Conversions from 1 and 3 to 2 were confirmed by PXRD and single crystal structure determination. Because of the flexibility, three different conformations of the ligand are possible (Chart S1†), which can lead to distinct molecular structures. Conformation I may be favorable for getting the complexes 1 and 2 whereas conformations II and III seem to be responsible in the grid formation of complex 3. When the pH of the medium is high enough, it facilitates the enol formation and consequently prefers conformation II and in neutral or low pH the ligand exists in the keto form and subsequently favors conformation I. As evident from Fig. S1,† the intensity of –OH stretching is much lower than the conjugated carbonyl in complex 2, whereas, these frequencies are almost same for complex 1. This indicates that the keto form is dominant over the enol form at low pH. At pH = 9.8 for complex 3 in solution phase, the frequencies shift for the conjugated carbonyl group from 1616 and 1550 cm⁻¹ to 1624 and 1509 cm⁻¹ respectively and increase in intensity of –OH stretching (3436 cm⁻¹) confirms the presence of enol form of the ligand. As a result complex 3 was formed at pH = 9.8 and complex 1 and 2 were formed at pH = 7.6 and 1.2 respectively. The molar ratio of metal to ligand 3 : 1 could be considered as optimum for 1, a higher amount of metal results in some unidentified impurities.

Similarly for 2 and 3, a metal to ligand ratio less than 2 : 1 results in a lower yield of the complexes. Chloride and perchlorate ions are part of the molecules and therefore they have a definite role in obtaining the molecules.

TG analysis

Thermogravimetric analysis of the complexes 1–3 were carried out to get the information about the number of solvent molecules in the molecular formula. As shown in Fig. S2,† the weight loss in 1 from 40–130 °C was ∼13.1% (calc. 13.7%), corresponding to the loss of eight uncoordinated and nine coordinated water molecules. Similarly the weight loss in 2 from 40–150 °C was ∼3.3% (calc. 3.6%), corresponds to the loss of three non-coordinated water molecules and for 3, the weight loss of ∼23.2% (calc. 22.6%) in the temperature range of 40–110 °C, corresponds to the loss of six perchlorate ions, six water, one methanol and two acetonitrile molecules.

Structural description

The solid state structure of all the complexes reveal a bent conformation near the central pyridine system and an overall extended structure around the terminal pyridinyl moiety, where the aromatic units are in edge to face and face to face contact with the central pyridine ring (Scheme 1). Core structures of complexes 1–3 are presented in Scheme 1. Crystallographic parameters of complexes 1–3 are given in Table 1.

Scheme 1 Synthesis and interconversion of the complexes (1–3). Core structure of complexes are shown only after removing carbon and hydrogen atoms. Color code: N (blue), O (red), Cl (green), Cu (sky blue).

Table 1 Summary of crystallographic data for complexes 1–3

Molecule	1	2	3
a R1 = Σ||Fo| − |Fc||/Σ|Fo|.b wR2 = [Σw(Fo^2 − Fc^2)/Σ(Fo^2)2]^1/2.
Formula	(C38H60Cu6O53N14Cl8)n	C19H21Cu3O6N7Cl4	C119H100Cu9O43N44Cl6
Mr	2225.85	775.88	3618.98
Crystal system	Triclinic	Triclinic	Triclinic
SpaceGroup	P1̄	P1̄	P1̄
a/Å	14.6784 (4)	11.0301 (5)	15.4073 (9)
b/Å	15.7494 (5)	11.4682 (6)	18.0665 (10)
c/Å	17.5002 (5)	11.7195 (5)	30.6962 (18)
α/°	97.985 (1)	103.008 (4)	94.292 (3)
β/°	102.139 (1)	105.587 (3)	99.050 (3)
γ/°	103.572 (1)	99.950 (3)	114.692 (3)
V/Å^3	3768.53 (19)	1347.64 (12)	7571.9 (8)
Z	2	2	2
Dc/g cm^−3	1.961	1.907	1.584
μ (Mo-Kα)/cm^−1	0.71073	0.71073	0.71073
Reflection measured	16116	5582	25595
Unique refelctions	11666	3513	24603
R1^a	0.0592	0.0594	0.0868
wR2^b	0.1840	0.1946	0.2681

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X-ray crystallographic analysis reveals that complex 1 can be ascribed as a 1D chain where each motif constitutes of trinuclear Cu₃ units held together by the ligands and bridged by ClO₄⁻ anions (Fig. 1). The terminal Cu^II centres (Cu2, Cu4, Cu5, Cu6) of each trimeric unit experience N₂O₄ coordination whereas the middle one (Cu1, Cu3) observes N₃O₃ as its coordination environment. Each of the Cu^II ions resides in a slightly distorted octahedral geometry with equatorial and axial Cu–O bond lengths ranging between 1.927(1)–2.029(4) Å and 2.359(2)–2.624(7) Å respectively. The Cu–N bonds are equatorially positioned with distances within 1.936(7)–2.047(9) Å. The Cu–Cu distances within the Cu₃ unit (Cu1–Cu2, Cu1–Cu6, Cu3–Cu4, Cu3–Cu5) are in the range of 4.823(1) to 4.871(1) Å whereas it is 7.240(5) Å for the inter trinuclear (Cu₃) Cu^II centres (Cu1–Cu4) bridged by the perchlorate anions (Fig. 1). The packing diagram illustrates several H-bonding interactions (Table S1†).¹²

Fig. 1 Molecular structure of complex 1. Colour code: C (gray), N (blue), O (red), Cl (green), Cu (sky blue). Solvent molecules and hydrogen atoms are omitted for clarity.

Similarly, the solid state structure of complex 2 comprises of two trinuclear Cu₃ units, bridged by two chloride ions thus generating a hexanuclear moeity (Fig. 2). The asymmetric unit contains one Cu₃ unit and generates the full molecule by an inversion centre. The coordination environment of all Cu^II centres in each Cu₃ unit are different. One terminal Cu^II (Cu3) has Square pyramidal geometry with an N₂O₂Cl coordination environment whereas other two (Cu1, Cu2) possess distorted Square pyramidal geometry with N₂Cl₂O and N₃Cl₂ in its surroundings respectively. The intra trinuclear Cu–Cu distances (Cu1–Cu2, Cu2–Cu3) range from 4.858(1) to 4.881(2) Å and is 4.174(9) Å for the Cu^II centers (Cu1–Cu2ⁱ, Cu1ⁱ–Cu2) bridged by the chloride ions. Each hexanuclear moeity forms intermolecular hydrogen bonds¹² (Table S2†) with its neighbours and is also involved in π–π interactions through pyridine rings (centriod–centriod distance 3.972 Å) (Fig. S3†). Competitive Lewis base interactions between coordinated anions and solvent molecules seem to restrict formation of extended structure in this case, thereby forming a bent conformation.

Fig. 2 Molecular structure of complex 2 where ‘i’ denotes atom generated by center of inversion (−x, −y, −z). Colour code: same as in 1. Solvent molecules and hydrogen atoms are omitted for clarity.

The structure of complex 3 can be described as a [3 × 3] grid of nine pseudo-octahedral Cu^II centres (Fig. 3). However, the grid is not a symmetrical square grid. So, a better way to describe the structure is as it is composed of a [3 × 2] grid that is linked with a trimeric unit (Scheme 2). Each metal centre in the [3 × 2] grid is surrounded by two ligands at perpendicular direction and have an N₄O₂ coordination environment. In a [3 × 2] grid the ligands lying along the width and have one coordination pocket available for binding. The flexible nature of the ligand made them puckered and bridged to the trimeric unit through the other face of the molecule. The average Cu–Cu distances between the Cu^II ions in unit a and b is 4.04(3) Å, whereas it is 5.21(2) Å for b and c. This feature makes the grid unsymmetrical and unique among the others. In general, Cu^II tends to form a square planar or square pyramidal geometry, but in this complex all the metal centers are in pseudo-octahedral geometry, which is also unusual.

Scheme 2 Grid formation of complex 3. Cu²⁺, rotation.

Fig. 3 Molecular structure of complex 3. Colour code: same as in 1. Solvent molecules and hydrogen atoms are omitted for clarity.

Magnetic study

DC magnetic susceptibility data for complexes 1–3 were collected on polycrystalline samples and the phase purity of the complexes was checked by the PXRD pattern (Fig. S15–S17†). Although for complex 2, the experimental PXRD pattern is slightly different than the simulated one that may be because of a partial loss of the solvents of crystallization. DC susceptibilities are shown in the form of χ_MT (χ_M is molecular magnetic susceptibility) as a function of temperature in the range of 1.8–300 K at 0.1 T field (Fig. 4). The data were fitted with the FITMART¹³ assuming isotropic spin Hamiltonian. For complex 1, χ_MT value is 2.20 cm³ K mol⁻¹ at room temperature, which is slightly lower than the expected value (2.25 cm³ K mol⁻¹ for six isolated Cu^II, g = 2) because of antiferromagnetic interactions. As the temperature is lowered from 300 K, χ_MT value decreases gradually to the value of 1.005 cm³ K mol⁻¹ at 56 K and then decreases quite slowly until it attains the value of 0.997 cm³ K mol⁻¹ at 1.8 K. The magnetic interactions were modeled (Fig. 5), with the isotropic spin Hamiltonian given in eqn (1)and matches well with the experimental one, giving the value, g = 2.279(7), J1 = −157.7 cm⁻¹, J2 = −0.3364 cm⁻¹ Where J1 is the intra-trinuclear coupling constant and J2 is the inter-trinuclear coupling constant. Since J2 corresponds to the perchlorate bridged Cu^II centres (Cu–Cu distance = 7.05(7) Å), its value is much lower than J1, which gives the interaction between the ions within trimers (average Cu–Cu distance is 4.84(5) Å). As, all the octahedral Cu^II centres in 1 show tetragonal elongation, hence the d_x²−y² orbital acts as an effective magnetic orbital and results in a much effective equitorial–equitorial interaction with the p-orbitals of the ligands through Cu–N–N–Cu pathway (Fig. 6). For complex 2, the room temperature χ_MT value is 2.24 cm³ K mol⁻¹ which is consistent with the expected value (2.25 cm³ K mol⁻¹, g = 2) for six isolated Cu^II ions. As the temperature decreases the χ_MT value decreases gradually to 1.12 cm³ K mol⁻¹ at 102.9 K and then plateaus with a value of 0.83 cm³ K mol⁻¹ at 19.9 K and finally reaches 0.51 cm³ K mol⁻¹ at 1.8 K (Fig. 4).

Fig. 4 Plots of χ_MT vs. T for complexes 1–3 in the 1.8–300 K temperature range at a field of 0.1 T. The solid lines are the best fits obtained.

Fig. 5 Model used for the data fitting of complex 1. Balls represent metal centers and lines represent connectivity between two metal centers.

Fig. 6 Orbital orientation of the d_x²−y² orbital of the Cu²⁺ ions.

The decline of χ_MT can be attributed for the dominant antiferromagnetic interactions between the Cu^II centers whereas the plateau like behavior shall be accounted for by the net spin in one trimer due to the strong antiferromagnetic coupling between Cu(II) ions. However the χ_MT value is higher than the usual range of 0.375–0.5 cm³ K mol⁻¹ for the contribution of one Cu(II) ion. This might be due to the presence of some residual spin originating from canting of the spins of the adjacent metal centres at a definite angle (Fig. S4†). The magnetic data were simulated using the following model with the Hamiltonian given in eqn (2):

that gives, g = 2.279(7), J1 = −151.2 cm⁻¹, J2 = 9.8 cm⁻¹ and (Fig. 7). Where J1 is the intra-trinuclear coupling constant and J2 is the inter-trinuclear coupling constant. Similar to complex 1, the magnetic orbital of the copper centre and the ligand is in same dx²−y² plane, hence from the strong overlap originates antiferromagnetic interactions (Fig. 6) between the metal centers via J1. Although ferromagnetic interactions originate through chloride bridging via J2, yet at low temperatures strong antiferromagnetic interactions dominate over weak ferromagnetic exchange. Generally, for the chloro-bridged complex, if α/R (α = Cl–Cu–Cl angle and R = Cu–Cl axial bond distance) is in between 32.6 and 34.8, then the resulting interactions are ferromagnetic, otherwise antiferromagnetic.^14,15 In case of complex 2, α/R is 50.82 still it generates ferromagnetic exchange through this pathway. This can be explained by considering the bridging mode of Cu(II) centres.¹⁶ Both the Cu(II) centres through a chloride bridge have a distorted square pyramidal environment. Cu2 has equatorial-chloride bridging and Cu1 has axial chloride bridging. The equatorial bond lengths (2.33 Å) are shorter than the axial bond lengths (2.53 Å), which indicates that the unpaired electron is situated in the d_x²−y² orbital and that the d_z² orbital contains paired electrons. So the magnetic orbital of only one Cu(II) center interacts with the bridging chlorine via the d_x²−y² orbital, whereas the other interacts via dz². Because the dz² orbital does not have unpaired electrons, pairing of this orientation of the overlapping orbitals around each Cu(II) creates a ferromagnetic interaction (Fig. S5†). Although complex 1 is a 1D chain and complex 2 is a hexanuclear entity, the basic building blocks are the same, which is a trinuclear Cu₃ unit. So, the magnetic properties of complexes 1 and 2 should be comparable. At higher temperatures, the magnetic properties of complexes 1 and 2 are almost the same due to the unapparent interaction between Cu₃ units. In the low temperature range, the interaction between Cu₃ unit in complex 2 is stronger than that in 1, which is responsible for showing different nature of the plots.

Fig. 7 Model used for the data fitting of complex 2. Balls represent metal centers and lines signify connectivity between two metal centers. Distances between metal centers are given in angstrom (Å).

The room temperature χ_MT value for complex 3 is 3.61 cm³ K mol⁻¹ which is consistent with the expected value (3.375 cm³ K mol⁻¹, g = 2) for nine isolated Cu^II ions (Fig. 4) in which the ferromagnetic exchange process seems to dominate at low temperature. The data were fitted using the model (Fig. 8) with Hamiltonian given in eqn (3): assuming all the Cu–O–Cu exchange pathways and all the Cu–N–N–Cu exchange pathways are equal except for the interaction through Cu2–Cu5. All the Cu–O–Cu and the middle Cu–N–N–Cu interactions are axial–equatorial in nature whereas the terminal Cu–N–N–Cu interactions are equatorial–equatorial and it give the values g = 2.17, J1 = −210.1 cm⁻¹, J2 = 4.03 cm⁻¹ and J3 = 15 cm⁻¹. Small values of J2 and J3 indicates weak ferromagnetic interactions because of symmetrically less favored axial–equatorial interactions through all the Cu^II centers except that of the terminal Cu–N–N–Cu pathway, which undergoes strong antiferromagnetic interactions favoured by equatorial–equatorial interactions.¹⁷

Fig. 8 Model used for the data fitting of complex 3. Balls represent metal centers and lines signify connectivity between two metal centers. Distances between metal centers are given in angstrom (Å).

Isothermal magnetization (M/NμB vs. H) and M/NμB vs. H/T plots (Fig. S6–S11†) show almost isotropic magnetic interactions for complexes 1–3 and the magnetzation values reach up to 2.23 μB, 2.25 μB and 5.77 μB respectively at 7 T and 2 K (expected spin only value 3 μB for 1 and 2, 4.5 μB for 3) respectively for six Cu^II and nine Cu^II with g = 2. The magnetization data for complexes 1 and 2 do not show any saturation even at the highest field measured (Fig. S6 and S7†). This could be due to the presence of spin frustration of low lying ground states of the molecule, which get populated at a higher field.¹⁸ Upon diagonalization of the spin Hamiltonian of eqn (1)–(3), we have simulated experimental data with a decreased g value of 2.2, resulting in an S = 1 ground state for 1 whereas for 2 experimental data were fitted with g = 2.3 giving S = 0 for 0.1–3.9 T field and S = 1 for 3.9–7 T field. For 3, S = 2.5 at the field of 0.1–7 T for a g value of 2.3. Further analysis of the energy states can be seen in energy (E) vs. spin (S) spectra showing lowest energy value for the ground state spin (Fig. 9, see ESI† for complete spectra).

Fig. 9 Spin ladder (Energy vs. Spin plot) for complexes 1–3.

Conclusions

In summary, we have prepared a multidentate ligand that resulted in a family of three multinuclear Cu(II) complexes. Slight change in reaction conditions yielded vivid structural changes in the complexes. The structural diversity of the cages described here highlights the versatility of a metal-mediated self-assembly approach for making magnetically coupled cage complexes. Variable temperature magnetic measurements were performed, which show that weak antiferromagnetic interactions exist among the metal centers in complex 1 and both ferro and antiferromagnetic interactions coexist for complexes 2 and 3.

Acknowledgements

AA and JAS acknowledge CSIR for SRF fellowship. ADK and SK thank to DST, Government of India for financial support through project numbers SR/FT/CS-016/2010 and SR/FT/CS-025/2010 respectively. We thank IISER Bhopal for generous financial and infrastructural support. AA thanks to Mr Soumyabrata Goswami and Dr Himanshu S. Jena for helpful scientific discussion.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional synthetic and magnetic data for all complexes. CCDC 922322–922324. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ra45928d

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