Investigating the effect of anion substitutions on the structure of silver-based coordination polymers

Highlights

• Non-covalent interactions CH⋯π and CH⋯O lead to formation of 2D structures.

• Nitrate anion acting as a monodentate terminal ligand.

• The W–S bands shifted to lower frequencies in WS₄ complexes with the silver(I) ion.

Abstract

Five silver(I) compounds namely, [Ag(tdmpp)(NO₃)]_n (1), [Ag₂(SeCN)₂(tdmpp)]_n (2), [Ag(tdmpp)]PF₆ (3), [WS₄Ag₃Cl (tdmpp)_1.5]·2CH₃CN (4) and [WS₄Ag₄I₂(tdmpp)]1.5CH₃CN (5) were prepared by the reactions of 1,1,3,3-tetrakis(3,5-dimethyl-1-pyrazolyl)propane (tdmpp) with various silver(I) salts in order to investigate the impact of the variation of the inorganic anions on the structure of these complexes. In the chain structures of 1 and 2, each tdmpp ligand acts as a bridge between a pair of adjacent silver(I) centers. In the structure of 1, the nitrate ion act as a terminal, monodentate ligand, while in compound 2 the SeCN anion functions as bidentate-bridging ligand between two neighboring Ag(I) centers. The parallel adjacent chains in 1 and 2 are linked together by means of non-covalent interactions to generate two-dimensional structures. In contrast to the distorted AgN₄O square–pyramidal structure of 1, in the structure of 2 each of the silver ions possesses a distorted tetrahedral with an AgN₃Se coordination geometry. Crystals of compounds 3–5 were not suitable for X-ray diffraction studies.

Introduction

The rational design and synthesis of silver(I) coordination polymers have been widely studied. This study was motivated not only by the possibility of their application as functional materials i.e. fluorescent materials, but also by the prospect of obtaining fascinating structures that can be accessed by the variable coordination numbers from 2 to 6 and different conformations adopted by the silver ions [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. The study of the coordination chemistry with the pyrazole-based ligands began in 1889 with a report of the polymeric [Ag(pz)]_n complex [20]. Much later, Trofimenko stimulated further research with the introduction of poly(pyrazol-1-yl)borate chelating ligands in coordination chemistry [21], [22], [23], [24]. Following the discovery that chelating poly(pyrazol-1-yl)borate ligands formed strong interactions with metal centers, the coordination chemistry of these ligands became the focus of considerable attention [25], [26], [27]. These ligands may be used as synthetic analogs of imidazole and mimic the coordinating sites found in metal enzymes or metalloproteins [28]. Unlike monodentate pyrazole and the rigid poly(pyrazol-1-yl)borate chelating ligands [29], [30], [31], [32], flexible pyrazole-based ligands, offer the prospect of conformations that will lead to variation in coordination geometries, influenced by the spacer length and the orientations of the donor atoms of the organic bridging ligands. The flexibility of these ligands can lead to the generation of structures with interesting properties. Architectures with specific structural motifs can be achieved by careful selection of organic ligands with the suitable coordinating groups, metal centers with preferred coordination geometries and variation of reaction conditions [33], [34], [35], [36]. As a part of our research devoted to the synthesis, structural and spectroscopic characterization of the coordination chemistry of pyrazole ligands, here we report the synthesis and structural characterization of five new silver(I) coordination polymers formed by the reaction of tdmpp ligand (Scheme 1) with appropriate silver(I) salts. It is anticipated that this investigation will provide insights into the effect of different inorganic anions on the structures of the silver(I) coordination polymers.