Elsevier

Polyhedron

Volume 85, 8 January 2015, Pages 467-475
Polyhedron

Coordination complexes and polymers from the initial application of phenyl-2-pyridyl ketone azine in mercury chemistry

https://doi.org/10.1016/j.poly.2014.09.005Get rights and content

Abstract

A series of new mercury(II) azine Schiff base complexes have been synthesized and characterized from the initial use of phenyl-2-pyridyl ketone azine in HgII chemistry. The synthetic/crystallization technique utilized involved the use of a branched tube, where temperature differential allowed for the slow crystallization of the products. The synthesized compounds are the mononuclear [Hg(L)Cl2] (1), [Hg(L)Br2] (2), [Hg(L)(NO2)2] (3), and the dinuclear [Hg2(μ-L)(SCN)4] (4) coordination compounds, as well as a 1D coordination polymer [Hg(L)(μ-I)2HgI2]n (5) (L = phenyl-2-pyridyl ketone azine). From the X-ray data, it is evident that this versatile ligand functions as a bi- or tridentate chelate, and is also able to bridge two HgII centers. The crystal structures of 1 and 2 are similar, both containing two crystallographically independent HgII molecules, one tetrahedrally coordinated and one exhibiting trigonal bipyramidal geometry. The heptacoordinated HgII center in 3 adopts a distorted capped trigonal prismatic coordination sphere, while in the dinuclear complex 4, the metal ions are bridged via the bis(bidentate) L and each center is also bound to two S-bonded thiocyanate units. The one-dimensional coordination polymer in 5 consists of a tetrahedral HgI4 and a trigonal bipyramidal HgN3I2 chromophore unit, bridged by μ-I bridges. The thermal stability of the crystal lattice in 15 follows the pattern 3 > 1 > 2 > 5 > 4, as studied by TG/DTA, while the TG data of 1, 2, and 5 are similar, but different than the respective ones for 3 and 4, between which important similarities are observed. In the solid state, the ligand and compounds 15 exhibit intraligand π  π fluorescence at room temperature.

Graphical abstract

The initial use of phenyl-2-pyridyl ketone azine in mercury chemistry afforded a series of neutral monomeric and dimeric HgII coordination compounds and polymers. The synthesis of different products is dependent upon the identity of the ancillary ligands present in the reaction mixture, namely halides and pseudo-halides. All products were characterized using X-ray crystallography, thermal analyses, and fluorescence studies.

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Introduction

Mercury and its compounds [1], [2], [3], [4], [5], [6], [7], [8] are of immense importance in chemistry and related disciplines due to their potential applications in the paper industry and as preservatives, paints, cosmetics, fluorescent lamps, sensors and mercury batteries [9], [10], [11], [12]. Exploiting the diversity in coordination geometries around this 5d10 ion, different coordination frameworks may be accessed using a variety of organic ligands along with different inorganic/organic bridging units [1], [13], [14]. The coordination behavior of azines as organic spacers has spawned great interest in recent years due to the ease of their syntheses, their chelating abilities, and the various denticities; additionally, subtle steric and/or electronic control on their frameworks has proven to lead to different monometallic and homo- or heterobimetallic complexes with interesting properties [15], [16], [17]. Halides [1], [14](b), [14](c), [18], [19], thiocyanates, ambidentate pseudohalides [13](a), [14](a), [20], [21], and nitrites [22] are suitable terminal/bridging groups in mercury chemistry, and in combination with organic ligands often result in different molecular frameworks and crystalline networks through their versatile ligation modes and different non-covalent forces [23], [24].

The ligand of choice for this work was phenyl-2-pyridyl ketone azine (L; Scheme 1). This ligand has previously afforded a small suite of products, including mono and/or dinuclear Cu, Ag, Ni, Zn, Co (all 3d metals) and Ag (4d metal) complexes [25]; however, there is no report of 5d metal complexes with L. Thus, we ventured to study the coordination behavior of this ligand in HgII, and explore its coordination chemistry with a 5d metal ion. This work also focuses on the metal ion and its coordination geometry; along these lines we were particularly interested to investigate the synthesis of various complexes, and also structurally characterize the resulting products. Subtle changes to these geometries were also to be probed in this systematic study, and as such we varied the anionic terminal/bridging ancillary ligands (halides, pseudohalides, etc.). Finally, the chromophore qualities of the resulting systems were of interest, and as such we were able to investigate the fluorescence properties of the resulting compounds. Therefore, herein we report the successful syntheses, X-ray crystallography, thermal behavior, and luminescence properties of three mononuclear compounds [Hg(L)Cl2] (1), [Hg(L)Br2] (2) and [Hg(L)(NO2)2] (3), one dinuclear compound [Hg2(μ-L)(SCN)4] (4) and one 1D coordination polymer [Hg(L)(μ-I2)HgI2]n (5), all incorporating the aforementioned phenyl-2-pyridyl ketone azine ligand.

Section snippets

Materials and measurements

The Schiff base, phenyl-2-pyridyl ketone azine (L) was prepared following the reported method as described elsewhere [15] and used without further purification. All other reagents and solvents used for the syntheses and analyses were commercially available and used as received. FT-IR spectra were recorded on a Bruker Tensor 27 FT-IR spectrometer. Microanalyses were performed using a Heraeus CHN-O-Rapid analyzer. Melting points were measured on an Electrothermal 9100 apparatus and are

Synthesis and spectroscopic results

The apparatus used in this work is shown in Fig. 1, and consists of a glass tube fitted with a side branched arm, which forms an angle of ∼45° with the main reaction tube. The HgII salts are typically added to the main reaction tube, and the ligand is added as a methanolic solution (0.5 mmol of the ligand L in 25 ml of methanol). Alternately, all solid reactants may be added to the main reaction tube and neat methanol (25 ml) is added to fill the apparatus. Then, the main reaction tube is sealed

Conclusions

In summary, a series of new luminous mercury(II) coordination complexes of different nuclearities were isolated from the use of a versatile Schiff base ligand, namely phenyl-2-pyridyl ketone azine, combined with various anionic co-ligands such as chloride, bromide, iodide, nitrite and thiocyanate. Structural analyses reveal an interesting variation in the coordination behavior of the ligand system from bidentate to tridentate chelating, to binucleating bis(bidentate) coordination modes. This is

Acknowledgments

We are grateful to the University of Tabriz Research Council for the financial support of this research. BKG thanks the CSIR, New Delhi, India for financial support. KB is grateful to CSIR New Delhi, India for fellowship. Finally, CL acknowledges the Cottrell College Science Award (CCSA) sponsored by the Research Corporation for Science Advancement, for the financial support.

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