Structural and magnetic characterization of three tetranuclear Cu(II) complexes with face-sharing-dicubane/double-open-cubane like core framework

https://doi.org/10.1016/j.jssc.2017.02.009Get rights and content

Highlights

  • Novel tetranuclear copper(II) complexes have been structurally characterized.

  • Complexes possess face-sharing dicubane/double open cubane core structures.

  • Variable temperature magnetic measurements reveal antiferromagnetic coupling.

  • PHI program was used to explain the observed magnetic properties.

Abstract

Three novel tetranuclear copper(II) complexes namely [Cu4(L1)4]∙2(dmf) (1), [Cu4(L1)4] (2) and [Cu4(L2)2(HL2)2(H2O)2]∙2(ClO4)·6(H2O) (3) (H2L1, (E)−2-((1-hydroxybutan-2-ylimino)methyl)phenol; H2L2, (E)−2-((1-hydroxybutan-2-ylimino)methyl)−6-methoxyphenol)) were synthesized from the self-assembly of copper(II) perchlorate and the tridentate Schiff base ligands. The structural determination reveals that complex 1 crystallizes in the monoclinic system with space group C2/c, whereas both the complexes 2 and 3 crystallize in the triclinic system with space group P-1. Complexes 1 and 2 possess face-sharing dicubane core, on the other hand complex 3 has double open cubane core structure. The copper(II) ions in the cubane core are in distorted square planar geometries, and weak π…π and C–H…π interactions lead to formation of a 2D supramolecular architecture for complexes 1 and 2. At room temperature complexes 1, 2 and 3, exhibit fluorescence with a quantum yield (Φs) of 0.47, 0.49 and 0.38, respectively. Variable temperature magnetic susceptibility measurements in the range 2–300 K indicate an overall weak antiferromagnetic exchange coupling in all complexes. The PHI program was used to study their magnetic behaviour. In agreement with their face-sharing dicubane structure, a Hamiltonian of the type H =– J1(S1S2+S1S2’+S1'S2+S1'S2’) – J2S1S1’, where S1 = S1’ = S2 = S2’ = SCu =1/2, was used for studying complexes 1 and 2. Simulations performed suggest magnetic exchange constants with values close to J1 =−20 cm−1 and J2 =0 cm-1 for these complexes. On the other hand, the spin Hamiltonian H =– J1(S1S4+S2S3) – J2(S1S3+S2S4) – J3S1S2, where S1 = S2 = S3 = S4 = SCu =1/2, was used to study the magnetic behaviour of the double open cubane core of complex 3 and a good agreement between the experimental and simulated results was found by using the parameters g1 = g2 =2.20, g3 = g4 =2.18, J1 =−36 cm-1, J2 =−44 cm-1 and J3 =0 cm-1.

Graphical abstract

Tetranuclear Cu(II) complexes with face-sharing-dicubane / double-open-cubane like core frameworks were synthesized and characterized by crystal structure and magnetic analysis. Variable temperature magnetic properties corroborate with their structural arrangement.

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Introduction

High nuclear complexes containing paramagnetic transition metal ions represent important class of compounds for their potential application in the area of molecular magnetism [1] and their ability to mimic metalloprotein active sites [2]. Among the higher nuclear complexes, tetranuclear copper complexes of various structure e.g. dimeric, [3a] square planar [3b], cyclic [3c], pin-wheel [3d], face to face [3e], roof-shaped [3f] and cubane types [3g] are reported in the literature due to their potential application as in the area of magnetism [4], catalysis [5] and bioinorganic modelling [6]. Literature survey reveals that cubane-like copper complexes (Cu4O4) containing hydroxo, alkoxo and phenoxo bridges are studied implementing experimental and theoretical approaches with the aim of establishing magneto-structural correlation [7]. Depending on the arrangement of the copper and oxygen atoms in Cu4O4 units, various cubane geometries such as regular cubane [8], single-open cubane [9], double open cubane [10] and face-sharing dicubane have been reported [11].

From the structural point of view two classification for Cu4O4 cubane-like complexes have been proposed. Depending on the distribution of the long Cu-O bonds in the cube, Mergehenn and Hasse classified [12] the cubanes as type-I and type-II. In the copper cubane where there are four long Cu-O distances between two dinuclear sub-unit categories as type-I. On the contrary when long copper-oxygen distances are within each dinuclear sub-unit are classified as type-II [13].

Based on the Cu…Cu distances within the Cu4O4 cubane core Alvarez et al. classify [14] the copper cubane into three types: (i) (2+4), has two short and four long Cu…Cu distance, which is equivalent to type-I, (ii) (4+2), this class of cubane compound has two long and four short Cu…Cu distances, and when the Cu4O4 core symmetry is S4, it would be equivalent to type-II, and (iii) (6+0), where six Cu…Cu bonds of Cu4O4 cubane core are similar.

Alkoxo and phenoxo donor Schiff bases are potential ligands for the synthesis of cubane compounds. The structure of the cubane core can be tuned by slight modification of Schiff base and reaction conditions [15]. In the present contribution we report synthesis, crystal structure and magnetic properties of three tetranuclear copper complexes [Cu4(L1)4]∙2(dmf) (1), [Cu4(L1)4] (2) and {[Cu4(L2)2(HL2)2(H2O)2]∙2(ClO4)·6(H2O)} (3). Complexes 1 and 2 possess face sharing dicubane core structure. On the other hand, complex 3 has double open cubane structure. The different magnetic exchange coupling pathways existing within these compounds have been evaluated by means of variable temperature magnetic measurements and simulations, and the results obtained have been correlated with their corresponding structural features.

Section snippets

Materials and methods

High purity 2-amino-1-butanol (Aldrich Chemical Co. Inc.), 2-hydroxy benzaldehyde and 2-hydroxy-3-methoxybenzaldehyde (Spectrochem-India) were purchased and used as received. All other chemicals used were analytical grade. Solvents used for spectroscopic studies were purifies and dried by standard procedures before use [16].

Elemental analyses (C, H and N) were performed using a Perkin-Elmer 240 C elemental analyzer. IR spectra were recorded as KBr pellets on a Bruker Vector 22FT IR

Synthetic aspects

The multisite coordinating ligands, H2L1 and H2L2, were prepared by a one pot synthesis employing condensation of the 2-amino-1-butanol and corresponding aldehyde in methanol under reflux condition, and characterized by 1H and 13C NMR spectra (Figs. 1S and 2S). Using these ligands, complexes 13 were synthesized at room temperature.

Complexes [Cu4(L1)4] ∙2(dmf) (1) and [Cu4(L1)4] (2)

Single crystals of complex 1 were obtained after one week from a saturated methanol-DMF (1:1) solution. On the other hand the single crystals of complex 2 were

Conclusion

In summary, we report synthesis, crystal structures, and magnetic properties of tetranuclear [Cu4] copper(II) complexes. Single crystal X-ray diffraction analysis evidenced the {Cu4O4} cubane core of the complexes. Both the complexes 1 and 2 have been synthesized using same Schiff base H2L1, but the use of different solvents results complexes 1 and 2 with different crystal system and space group. Complexes 1 and 2 possess face-sharing dicubane core structure, whereas use of slightly different

Appendix A.Supplementary data

CCDC 1476822−1476824 contain the supplementary crystallographic data for this article and are deposited at the Cambridge Crystallographic Data Centre (CCDC). This data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Tables for C-H…π interactions; figures of NMR, IR and electronic spectrum; crystal structure are

Acknowledgements

Authors gratefully acknowledge the financial assistance given by the DST, Govt. of India, under the SERB Fast Track Scheme to Dr. Subal Chandra Manna (Grant No. SR/FT/CS-131/2010). A. F. acknowledges financial support from the Spanish Ministerio de Economía y Competitividad (MINECO) through CTQ2012-32247, CTQ2015-68370-P and for a Ramón y Cajal Fellowship (RYC-2010–05821), and from the Regional Generalitat de Catalunya Authority (2014SGR-129).

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