JOURNAL OF CHEMICAL RESEARCH 2016114 RESEARCH PAPER VOL. 40 FEBRUARY, 114–119

Binuclear transition metal complexes have received much interest in coordination chemistry due to their applicability in various fields, such as in catalytic processes in different organic reactions,1–3 models of the coordination sites in metallo–proteins or in enzymes in bioinorganic chemistry4–6 and in molecular devices.7–9 The electronically and sterically tunable feature of Schiff base ligands, along with their easy preparation, make these compounds promising candidate ligands in coordination chemistry for investigating the above mentioned properties.10–13 Binuclear copper(II) complexes of Schiff base ligands containing the Cu

2O

2 core, in which the oxygen bridges are phenoxido- or

hydroxido-groups, have been investigated due to their relevance to the active site behaviour of multinuclear copper oxidases14–16 and their unique role in molecular magnetism.17 There are only a few ferromagnetic binuclear copper(II) complexes containing diphenoxido-bridges, in which large substituents have been introduced in the ligand structure.18–28 The study of magnetochemistry in ferromagnetic complexes is an active area of research due to their attractive structural and magnetic behaviour as well as their technological applications.17,29–32 It has been demonstrated that the magnetic interaction between metal centres through superexchange coupling and its strength along with its nature in these compounds is affected by several factors such as the coordination geometry around copper(II) ions and the kind of bridging groups and structural properties of the Cu

2O

2 core, including Cu···Cu separation, Cu–O bond

lengths and Cu–O–Cu bond angles.33–39 Quantum calculations, in particular density functional theory (DFT) calculations combined with the broken symmetry (BS) approach,40,41 can be successfully used not only to offer a quantitative description of the magnetic interaction in a variety of binuclear complexes by computing the magnetic coupling constant (J), but also to predict the J value in new complexes.42–45 Synthesis of new binuclear copper(II) complexes containing diphenoxido-bridges in combination with the evaluation of their magnetic interaction by quantum calculation methods could be helpful in finding high performance ferromagnetic compounds for technological application purposes. Here, we report the synthesis, characterisation by FTIR, molar conductance

measurement and crystal structure of the diphenoxido-bridged binuclear copper(II) complex [Cu

2L

2] with the deprotonated

tetradentate Schiff base ligand H2L (Scheme 1) as well as the

prediction of its magnetic coupling constant by the broken symmetry approach based on density functional theory.

Experimental

Materials and physical measurements

All reagents were purchased from Merck and were used without further purification. CHN analysis was carried out using a Heraeus CHN-O-RAPID elemental analyser. Fourier transform infrared spectra were obtained on a PerkinElmer FTIR RX-I spectrometer in KBr pellets in the region 4000–400 cm–1. The UV-Vis spectrum was obtained in DMF solution using a PG T80+ UV–Vis spectrophotometer. Conductance measurement was performed using a Metrohm 712 conductometer.

X-ray crystallography

Dark mustard coloured single crystals of the binuclear copper(II) complex were obtained from a DMF solution of the complex by slow evaporation at 298 K. Single crystal X-ray diffraction data for the complex was collected using ω and φ scans on a Bruker D8 Venture diffractometer equipped with a CMOS PHOTON100 detector.46 Cell refinement and data reduction were carried out with the use of the program Bruker SAINT.47 The structure was solved by direct methods using the SIR92 program48 and then refined by full-matrix least

A binuclear copper(II) complex with diphenoxido-bridges: synthesis, characterisation, crystal structure and predicting the magnetic coupling constant Asmar Mashhuna, Seyed Amir Zareia*, Mohammad Piltana, Sylviane Chevreuxb and Emmanuel Guillonb

aDepartment of Chemistry, Faculty of Science, Sanandaj Branch, Islamic Azad University, Sanandaj, IranbInstitut de Chimie Moléculaire de Reims (ICMR, CNRS UMR 7312), Groupe Chimie de Coordination, Université de Reims Champagne-Ardenne, BP 1039, 51687 Reims Cedex 2, France

Reaction of copper(II) acetate with 1,1’-[(2,2-dimethylpropane-1,3-diyl)bis(nitrilomethylidyne)]-di-2-naphthol (H2L) gave the diphenoxido-

bridged binuclear copper(II) complex [Cu2L

2]. Its crystal structure shows a slightly distorted square pyramidal geometry around both

of the copper(II) ions. In the symmetric Cu2O

2 core, each of the phenoxido bridges occupies an equatorial position around one of the

copper(II) centres. A broken symmetry (BS) computation based on density functional theory (DFT) evaluated the magnetic interaction in the complex using the (U)B3LYP level of theory in association with the basis sets LANL2DZ for the copper atoms and 6-31G** for the other atoms. The calculations showed ferromagnetic interaction between the two magnetic copper(II) centres with a magnetic coupling constant (J) of 1.16 cm–1.

Keywords: binuclear copper(II) complex, Schiff base, crystal structure, hydrogen bond, density functional theory, broken symmetry, magnetic coupling constant

* Correspondent. E-mail: Seyedamirzarei@yahoo.com; sa_zarei@iausdj.ac.ir

OH HO

N N

Scheme 1 The structure of the H2L ligand.

JOURNAL OF CHEMICAL RESEARCH 2016 115

squares procedures on F2 using SHELXL97.49 Details of the X-ray experiment and crystal data are summarised in Table 1.

Synthesis of 1,1’-[(2,2-dimethylpropane-1,3-diyl)bis(nitrilomethyl-idyne)]-di-2-naphthol (H

2L)

The H2L ligand was synthesised by a condensation reaction between

2-hydroxy-naphthalene-1-carbaldehyde (6 mmol, 1.032 g) and 2,2-dimethyl-1,3-diaminopropane (3 mmol, 0.306 g) in 60 mL of absolute ethanol.50 The resulting mixture was refluxed for 12 h and then the required product recrystallised from ethanol. Selected IR bands (cm–1): 3325 (m, O–H), 3067 (w, C–H

aromatic), 2963 (w, C–H

aliphatic), 1598

(vs, C=N), 1245 (s, C–O).

Synthesis of [Cu2L

2]

A solution (20 mL) of H2L (2 mmol, 0.820 g) in ethanol was added

to a warm solution of [Cu(CH3COO)

2]·H

2O (2 mmol, 0.399 g) in

ethanol (30 mL). The mixture was then refluxed for 4 h. The volume

Table 1 X-ray experiment and crystal data for [Cu2L

2]

Chemical formula C54

H48

Cu2N

4O

4

Formula weight 944.04Radiation MoKα (λ = 0.71073)Absorption Coefficient/mm–1 1.097Crystal size (mm3) 0.45 × 0.36 × 0.26Crystal system TriclinicSpace group, Z P-1, 2Temperature (K) 100a/Å 8.369(5)b/Å 10.531(5)c/Å 12.285(5)

α/° 82.398(5)

β/° 72.470(5)

γ/° 89.674(5)V/Å3 1022.7(9)Reflections/Independent/Unique 53606/6573/299F (000) 490

θ/° 3.16–31.01Goodness-of-fit on F2 1.044

R1 [I > 2σ(I)] 0.0333/0.0824

Final R indexes [I > 2σ(I)] 0.0340wR2 (all data) 0.0843Largest difference in peak and hole (e Å–3) 0.598 and –0.623

of the resulting solution was then reduced using a rotary evaporator to 5 mL. The product was filtered off and then recrystallised from DMF: Yield 0.349 g, (74%); m.p. 212 °C (decomposed). Anal. calcd for C

54H

48Cu

2N

4O

4 (944.04): C, 68.70; H, 5.10; N, 5.93; found: C, 68.59;

H, 5.16; N, 5.85%. Selected IR bands (cm–1): 1622 (vs, C=N), 1315 (s, C–O), 602 (w, Cu–O), 434 (w, Cu–N). UV-Vis (DMF, in the range 400–900 nm): λ

max (ε) = 620 (112 mol–1 dm3 cm–1). Λ

m = 5 cm2 Ω–1 mol–1

in DMF.

Computational details

The magnetic properties were investigated using the Heisenberg Hamiltonian as Ĥ = –2 Σ Ј

abŜ

aŜ

b where Ј

ab is the orbital-averaged

effective exchange integral between a and b spin sites with total spin operators Ŝ

a and Ŝ

b respectively.51 The crystal structure of the

synthesised binuclear copper(II) complex without any optimisation was used in the calculations. The calculations of magnetic coupling constants J were carried out using density functional theory with the GAUSSIAN98W program52 by means of the broken symmetry approach at the (U)B3LYP level.53–57 The LANL2DZ and 6-31G** basis sets were used for copper and the other atoms respectively. Two calculations were performed for the complex. The Ј value was calculated using the following equation:

J = –2 (EHS

– EBS

) / (<S2>HS

– <S2>BS

)

where EHS

and EBS

are the energies of the triplet and the broken symmetry singlet states respectively and <S2>

HS and <S2>

BS are the

mean values of the square of total high spin and broken symmetry operators respectively.58 A positive value of J indicates a ferromagnetic interaction, whereas a negative value indicates an antiferromagnetic interaction.

Results and discussion

Synthesis and characterisationThe diphenoxido-bridged binuclear copper(II) complex was prepared from the reaction between equal molar equivalents of Cu(CH

3COO)

2·H

2O and H

2L in ethanol solution, as summarised

in Scheme 2. Elemental analysis, IR spectroscopy and UV-Vis spectroscopy, conductance measurement and single crystal X-ray analysis were used for characterisation of the complex. The elemental analysis data showed the ratio of metal–ligand is 1:1 in the complex. The measured molar conductance value of the complex for ca. 10–3 M solution in DMF at 25 °C is

OH

OH

N

NCu(CH3COO)2.H2O

Absolute EthanolO

O

N

N O

O

N

N

Cu Cu0.5

Scheme 2 Complex formation.

116 JOURNAL OF CHEMICAL RESEARCH 2016

very low which reflects the non-electrolyte behaviour for this complex.59 This implies that the H

2L must coordinate to the

copper(II) cation as a doubly negatively charged ligand through deprotonation of phenolic OH’s.60 This hypothesis is supported by the absence of the phenolic O–H stretching vibration in the IR spectrum of the complex in comparison with the IR spectrum of H

2L. Furthermore, other useful structural information can

be obtained from the infrared spectrum of the complex. The absorption bands at 1622 and 1315 cm–1 are attributed to C=N and C–O stretching vibrational modes respectively, which are shifted towards higher frequencies compared with the free H

2L

Schiff base ligand. These observations suggest the coordination of the azomethine nitrogen and phenolic oxygen atoms to the Cu(II) cation.60

Structural descriptionSingle crystal X-ray diffraction analysis reveals the formation of a neutral and centrosymmetric diphenoxido-bridged binuclear copper(II) complex, [Cu

2L

2], which crystallises in the triclinic

system with space group P –1. The unit cell of the [Cu2L

2]

complex contains one molecule. Figure 1 shows the molecular structure of the complex and the atomic labelling of its coordination environment. Selected bond lengths and angles of the complex are listed in Table 2. The molecular structure of the complex illustrates that both of the copper(II) ions are bridged by two phenoxido-oxygen atoms with a Cu···Cu intermetallic separation of 3.248(13) Å. The two copper and two phenoxido-oxygen atoms construct a perfect plane in which there is no deviation from the least-squares Cu

2O

2 plane. This planarity

is confirmed by the torsion angle of 0° in the Cu2O

2 unit and

the dihedral angle (δ = 0°) between the two basal planes. The value of the Addison parameter (τ = 0.0515) for both copper(II) centres indicates that the copper(II) ions are in slightly distorted square pyramidal geometries.61 In the coordination environment around each equivalent copper(II) ions in a homo-bridged di-μ-phenoxido dicopper(II) complex [Cu

2L

2], the basal positions

are defined by two phenolic oxygen atoms (O1 and O2 for Cu1 and O1’ and O2’ for Cu1’) and the two iminic nitrogen atoms (N1 and N2 for Cu1 and N1’ and N2’ for Cu1’) from one of the deprotonated Schiff base ligands (L2–), and the apical position is occupied by one phenolic oxygen atom (O1’ for Cu1 and O1 for Cu1’) from the other deprotonated Schiff base ligand (L2–).

The bond lengths of Cu–N [1.9518(16) and 1.9704(15) Ǻ] and Cu–O

basal [1.9338(13) and 1.9143(15) Ǻ] are comparable with the

corresponding reported values in square pyramidal geometry.14 The Cu–O

apicl bond lengths of 2.5921(14) Ǻ are significantly

longer than the Cu–Obasal

bond lengths. The interplanar distance between the two basal planes is 2.674 Å. Both of the bridging oxygen atoms occupy basal positions (equatorial–equatorial) of both of the copper centres with bridge angles of 90.51° [i.e. for Cu(1)–O(1)–Cu(1’) and Cu(1’)–O(1’)–Cu(1)] that is smaller than the crossover angle (97.5°) and can indicate the propagation of very strong ferromagnetic interaction.17 The dihedral angles between the basal plane [O1N1N2O2] with the planes through [C27C18C19C20C21C22C23C24C25C26] and [C11C10C9C8C7C6C5C4C3C2] naphthalene moieties are 27.36 and 32.47° respectively. Also, the dihedral angle between the planes through [C27C18C19C20C21C22C23C24C25C26] and [C11C10C9C8C7C6C5C4C3C2] naphthalene moieties is 7.70°. In the molecular structure of the [Cu

2L

2] complex

(Fig. 2), four π–π interactions are observed. The centroid–

Table 2 Selected bond lengths (Å) and bond angles (°) of the [Cu2L

2]

complex

Bond lengthsCu(1)–O(1) 1.9338(13) Cu(1’)–O(1’) 1.9338(13)Cu(1)–O(2) 1.9143(15) Cu(1’)–O(2’) 1.9143(15)Cu(1)–N(1) 1.9518(16) Cu(1’)–N(1’) 1.9518(16)Cu(1)–N(2) 1.9704(15) Cu(1’)–N(2’) 1.9704(15)Cu(1)–O(1’) 2.5921(14) Cu(1’)–O(1) 2.5921(14)Bond anglesO(1)–Cu(1)–N(1) 89.29(5) O(1’)–Cu(1’)–N(1’) 89.29(5)N(1)–Cu(1)–N(2) 90.68(5) N(1’)–Cu(1’)–N(2’) 90.68(5)N(2)–Cu(1)–O(2) 91.26(5) N(2’)–Cu(1’)–O(2’) 91.26(5)O(2)–Cu(1)–O(1) 88.58(5) O(2’)–Cu(1’)–O(1’) 88.58(5)O(1)–Cu(1)–N(2) 177.79(5) O(1’)–Cu(1’)–N(2’) 177.79(5)O(2)–Cu(1)–N(1) 174.70(5) O(2’)–Cu(1’)–N(1’) 174.70(5)O(1)–Cu(1)–O(1’) 89.49(5) O(1’)–Cu(1’)–O(1) 89.49(5)N(1)–Cu(1)–O(1’) 91.48(5) N(1’)–Cu(1’)–O(1) 91.48(5)N(2)–Cu(1)–O(1’) 92.72(5) N(2’)–Cu(1’)–O(1) 92.72(5)O(2)–Cu(1)–O(1’) 93.00(5) O(2’)–Cu(1’)–O(1) 93.00(5)Cu(1)–O(1)–Cu(1’) 90.51(5) Cu(1’)–O(1’)–Cu(1) 90.51(5)

Fig. 1 Molecular structure of the [Cu2L

2] complex. Hydrogen atoms are omitted for clarity.

JOURNAL OF CHEMICAL RESEARCH 2016 117

centroid distance between the [C11C10C9C8C3C2] and [C27’C18’C19’C24’C25’C26’] benzene rings is 3.558 Å and the centroid–centroid distance between the [C8C7C6C5C4C3] and [C19’C20’C21’C22’C23’C24’] benzene rings is 3.558 Å. These reciprocal interactions lead to a larger separation for the peripheral atoms in comparison with those more centrally located, e.g. the basal donor atoms and hence they create binuclear molecules with a slightly saddle-shaped conformation. The π–π interaction between neighbouring binuclear molecules with an interplanar separation of 3.550 Å constructs the parallel layers (Fig. 3). Also, further stabilisation of the crystal is caused

by the C15–H15A···π (2.950 Å) interaction between the centroid of the [C27C26C25C24C19C18] benzene ring with the methyl hydrogen H15A (Fig. 4). This interaction connects the adjacent constructed layers by intermolecular π–π interactions and leads to a 3D network structure in the [Cu

2L

2] complex.

Broken symmetry DFT calculation of the magnetic coupling constant (J)The BS–DFT method was employed to calculate the magnetic coupling constant (J) in the [Cu

2L

2] complex using its crystal

structure geometry. The calculations were performed at the (U)

Fig. 2 Intramolecular π–π interactions between naphthalene moieties.

Fig. 3 Intermolecular π-π interactions between naphthalene moieties.

Fig. 4 The C–H···π interaction in the crystal structure of the [Cu2L

2] complex.

118 JOURNAL OF CHEMICAL RESEARCH 2016

B3LYP level of theory using the basis set LANL2DZ for the copper atoms and the basis set 6-31G** for the other atoms. The results of the theoretical studies are summarised in Table 3.

The ferromagnetic interaction can be attributed to the high electron delocalisation on two parallel conjugated moieties in the [Cu

2L

2] complex.62–64 This can be concluded from the singly

occupied molecular orbitals (SOMOs) that are shown in Fig. 5.

Conclusions

The diphenoxido-bridged binuclear copper(II) complex, [Cu

2L

2], has been prepared. The geometry around both of the

copper(II) ions is a slightly distorted square pyramidal one. The magnetic coupling constant was evaluated by the BS-DFT method that illustrated the ferromagnetic interaction in the complex.

CCDC1026759 contains the supplementary crystallographic data for the synthesized Schiff base compound. The data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

We are grateful to the Sanandaj Branch, Islamic Azad University Council for financial support of this work.

Received 28 November 2015; accepted 6 January 2016Paper 1503742 doi: 10.3184/174751916X14534008407571Published online: 27 January 2016

Table 3 The energies and <S2> of high spin (HS) and broken symmetry (BS) states along with the magnetic coupling constant (J) in the [Cu

2L

2]

complex

Energy/hartree <S2> J/cm–1

HS –2998.18578821 2.0059 –BS –2998.18579652 1.0058 1.158636936

Fig. 5 Singly occupied molecular orbitals in the [Cu

2L

2] complex.

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