Elsevier

Journal of Molecular Structure

Volume 1150, 15 December 2017, Pages 196-205
Journal of Molecular Structure

Supramolecular structures of Ni(II) and Pt(II) based on the substituted 2, 2′: 6′, 2″-terpyridine: Synthesis, structural characterization, luminescence and thermal properties

https://doi.org/10.1016/j.molstruc.2017.08.081Get rights and content

Highlights

  • Two new supramolecular systems of Ni and Pt through noncovalent interactions.

  • Two structurally similar tpySH and tpyOH complexes of Pt(II) were synthesized.

  • A novel yellow polymorph of Pt(II)-terpyridine is reported.

  • The emissions properties of the complexes in solution and in the solid state.

  • The thermal properties of the complexes were investigated.

Abstract

Three new d8 transitions metal complexes containing substituted-2,2′:6′,2″-terpyridine ligands of [NiII(pytpy)2]Cl2. H2O (pytpy = 4′- (4-pyridyl)-2,2′:6′,2″-terpyridine) (1), [Pt(tpyOH)Cl]+Cl. 2H2O (2) (tpyOH = 4′-hydroxy-2,2′:6′,2″-terpyridine) and [Pt(tpySH)Cl]+Cl.2H2O (3) (tpySH = 4′-mercapto-2,2′:6′,2″-terpyridine) have been prepared. The crystal structure of 1 reveals that the nickel(II) is six-coordinated by six nitrogen atoms of pytpy in a distorted octahedral geometry NiN6, while the platinum complex (2) is four-coordinated by one Cl and three nitrogen atoms of tpyOH in a distorted square planar geometry PtClN3. The lattice crystal water molecule plays a significant structure directing role in the complexes 1 and 2. Many strong noncovalent interactions are present in the crystal structure of 1 and 2. For example, the supramolecular network of Csingle bondH⋯Cl, Osingle bondH⋯Cl and Cl⋯Cl interactions connected molecules and ions in the crystalline 1, while there are several Pt⋯Pt, Csingle bondH⋯Cl, H2O⋯H2O, Csingle bondOH⋯H2O, Cl⋯H2O and π-π interactions in 2. The solution luminescence properties of 2 and 3 have been investigated. The emissions of the platinum complexes 2 and 3 exhibit the high-energy intense π→π* intraligand and low-energy MLCT transitions in solution. The solid-state emissions of complexes 1–3 due to the MLCT and π-π interactions are also observed in the solid state. The thermal stability of all complexes reveals that the loss of terpyridine ligand is observed at higher temperatures due to the strong metal-nitrogen bonds of terpyridine ligands.

Introduction

The coordination chemistry of 2,2′:6′,2″-terpyridine (tpy) and its derivatives has experienced a rapid expansion in the past decades in a wide range of applications such as supramolecular chemistry, antitumor activity, DNA intercalation, luminescent devices and catalysis [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Most of the 4′-substituted derivatives of the tridentate terpyridine complexes are in [M(tpy)]n+ or [M(tpy)2]m+ forms which almost depends upon the central metal atom; however, the role of the substituent at the 4′-position of the terpyridine should be considered on the coordination number of metal [1], [7], [11]. The π-π stacking interactions in 4′-phenyl-2,2′:6′,2″-terpyridine derivatives of Mn, Fe, Co and Cu can control the self-assembly of these complexes [11]. Two excellent ligands for the design of supramolecular polymers self-assembled from a terpyridine ligand are functionalized ligand in the 4′-position with a hydroxyl group or a heterocycle, e.g. 4-pyridine [12], [13], [14], [15], [16]. Terpyridines were also found to form stable square-planar or octahedral complexes with d8 late transition metal ions such as Ni(II) and Pt(II) [13], [17], [18], [19], [20], [21]. There are several synthetic methods for the preparation of the terpyridine derivatives of platinum(II) which are involved [PtCl2(COD)] (COD = 1,5-cyclooctadiene), K2PtCl4, cis/trans-[PtCl2(SMe2)2], cis-[PtCl2(NCPh)2] or cis-[PtCl2(DMSO)2] [17]. The first example of the preparation of the Pt(II)-tpy complex has been reported in 1934 by Morgan et al. who described that the reaction of K2PtCl4 and terpyridine in H2O affords [Pt(tpy)Cl]Cl. 3H2O as a minor product as well as the double salt [Pt(tpy)Cl]2[PtCl4] as a major product [22].

Interestingly, the diimine and terpyridine complexes of platinum(II) reveal Pt…Pt and π-π interactions, resulting in the formation of the red and yellow polymorphs of platinum which would change the electronic properties of these complexes [23], [24], [25]. The linear chain platinum(II) complexes reveal the interesting spectroscopic properties due to the intermolecular stacking interactions [23]. However, a few linear chain platinum(II)-terpyridine complexes have been structurally characterized [25], [26], [27]. Two different stacking arrangements in the red and yellow forms of [Pt(tpyCl)Cl] depend on the solvent of crystallization. Therefore, the energy difference between these two structures is very small. The red form reveals Pt…Pt interaction in a linear chain but the yellow form exhibits only pairwise short Pt…Pt [25]. The identification of platinum-platinum interaction is easily achieved by structural characterization via single X-ray diffraction. However, indirect evidence of these interactions can be obtained using luminescence properties [28], [29], [30], [31]. It has been shown that the terpyridine complexes of platinum are able to be chromogenic and luminescent sensors for acids and bases [32], [33]. 4′-substituted terpyridines have been described in which 4′-substitution plays an important role in the structure determination. The role of metal center, anion and solvent should be considered in the crystal engineering design [34]. Note that the M-Cl moieties are among the good hydrogen bond acceptors, forming hydrogen bonds containing OH or NH fragments [35], [36]. Taking into account all of these information, we report the preparation and characterization of three new Ni(II) and Pt(II) complexes derived from 4′-substituted terpyridine ligands. In addition, the thermal and luminescence behaviors of these complexes were investigated.

Section snippets

General remarks

Elemental analyses were performed by Thermo Finnigan Flash Ea 111 elemental analyzer. IR spectra in the 4000-400 cm−1 were recorded on KBr pellets using ABB Bomem Model FTLA200-100 spectrophotometer. The luminescence study was performed using a Perkin-Elmer LS50 luminescence spectrometer. Experiments were carried out at room temperature. The excitation and emission band pass (slit) was 10 nm with the scan rate of 1000 nm/min. The solutions were placed in a 1 cm path-length quartz cell for the

Synthesis and structure of the nickel complex

The red single crystals of complex [Ni(pytpy)2]Cl2. H2O (1) were prepared by the reaction of NiCl2·6H2O and pytpy in 1:2 M ratio in water under hydrothermal conditions as shown in Scheme 1. We could obtain suitable single crystals only under hydrothermal conditions. The resulting product under normal conditions were not soluble in most of organic solvents. Therefore, we could not prepare suitable single crystals under normal conditions. The complex 1 is air stable which was characterized by IR,

Conclusions

In conclusion, the substituted terpyridine ligands of 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine) and 4′-hydroxy-2,2′:6′,2″-terpyridine are among the most suitable ligands for the construction of supramolecular chemistry. Easy synthetic access of all of the reported compounds opens new possibilities for the design of different types of 4′-functionalized terpyridine as building blocks in supramolecular chemistry. Two substituted terpyridine complexes of Ni(II) and Pt(II) gave single crystals suitable

Acknowledgements

We would like to thank the Iran National Science Foundation (INSF) for financial support (Grant No. 93051215). We also thank the Science Research Council of K.N. Toosi University of Technology for financial support.

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