Elsevier

Polyhedron

Volume 128, 28 May 2017, Pages 68-75
Polyhedron

Solvato- and thermochromism study in oxalato-bridged dinuclear copper(II) complexes of bidentate diamine ligands

https://doi.org/10.1016/j.poly.2017.02.042Get rights and content

Abstract

Two new oxalato complexes of the formula [Cu2(L)2(µ-ox)(MeOH)2](ClO4)2 (1) and [Cu2(L)2(µ-ox)](NO3)2 (2), where ox = oxalato and L = N,N′-dibenzylethane-1,2-diamine, were prepared and characterized by elemental analysis, IR and UV–vis spectroscopies and molar conductance measurements. The crystal structure of compound 1 was determined by single-crystal X-ray analysis. The compounds are solvatochromic and their solvatochromism was studied with different solvent parameter models using the stepwise multiple linear regression (SMLR) method with SPSS/PC software. It was demonstrated that the donor power of the solvent plays the most important role in the observed positive solvatochromism of the compounds. The complexes show irreversible thermochromism in solution due to the substitution of the weakly coordinated counter ions and coordinated solvents by solvent molecules.

Graphical abstract

Two dinuclear copper(II) complexes with oxalato-bridged ligand have been prepared. The compounds are soluble in organic solvents and exhibit positive solvatochromism and irreversible thermochromism. Their solvatochromism were analyzed by stepwise multiple linear regression method. The analysis showed donor number is responsible for solvatochromism phenomenon.

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Introduction

Bridged dinuclear and polynuclear copper(II) complexes have been the subject of current attention for chemists due to their ability to communicate electronic interactions between paramagnetic centers separated by connectors. It has been demonstrated that among the various types of bridging ligands, the oxalate anion (C2O42−) is an outstanding ligand for metal ions because it is a good chelating agent of 3d ions and also a good transmitter of magnetic and electronic interactions. The oxalato ligand can yield complexes having several kinds of nuclearity, for example mononuclear [1], [2], homodinuclear [3], [4], [5], heterodinuclear [6], homotrinuclear [7], [8], [9], [10], [11], [12] and heterotrinuclear [13] complexes.

Chromotropism is the change in color of a substance due to the physical and chemical properties of its ambient surrounding medium, such as temperature, solvent, pH and ion. Chromotropism properties of inorganic compounds have been studied during the past four decades [14], [15], [16], [17]. This phenomenon has many applications in thermo-sensitive, imaging, photo-switching sensor materials [18], [19], pollutant sensor [20] and as Lewis acid-base indicators [15]. Among the metal complexes whose color changes are due to d–d transitions, the copper(II) ion can be an excellent candidate due to its diverse coordination numbers, various geometrical structures and the presence of a strong Jahn–Teller effect. Mixed-chelate copper(II) complexes containing β-diketonate and diamine ligands have exhibited notable solvatochromism properties [21], [22], [14]. However, these complexes suffer from low color intensity because their colors originate from the d–d transitions of the copper(II) ions. Thus, the preparation of complexes with higher molar absorptivity values is extremely desirable. In this regard, mixed-chelate oxalato-bridged dinuclear complexes, as shown in Fig. 1, were synthesized and their chromotropism properties were studied. It was expected that the dinuclear copper(II) complexes would show higher molar absorptivity in comparison with their analogous mononuclear complexes.

Section snippets

Materials

The diamine ligand was prepared according to a published procedure [23], [24]. All chemicals were of reagent grade quality. The solvents used in the solvatochromism study were spectral-grade, purchased from commercial sources and used as received. These solvents are as follows: dichloromethane (DCM), nitromethane (NM), nitrobenzene (NB), benzonitrile (BN), acetonitrile (AN), propionitrile (PN), acetone (Ac), tetrahydrofuran (THF), ethanol (EtOH), methanol (MeOH), dimethylformamide (dmf),

Results and discussion

The complexes were synthesized by mixing the diamine ligand (L), CuX2 (X = ClO4, NO3) and K2C2O4·H2O with the molar ratios of 1:1:0.5, respectively, in a methanol–water mixture. The chemical identities of the complexes were established by elemental analysis, FT-IR spectroscopy and molar conductance data together with X-ray single crystallography, which are discussed below. All attempts to obtain X-ray quality crystal for compound 2 failed.

Conclusion

Two new dinuclear copper(II) complexes containing an oxalato-bridged group were prepared and characterized. The structure around the copper(II) ions with N2O4 cores is distorted octahedral, but with different coordination ligands. The dinuclear complexes are solvatochromic and their solvatochromism was studied with different solvent parameters models using the SMLR computational method. The statistical assessment of the data (R, S.E., F-test, Q2 and PRESS/SSY) showed that DN model of the

Acknowledgement

We are grateful for the financial support of the University of Mazandaran of the Islamic Republic of Iran.

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