The first metal complexes of 3,3′-bi-1,2,4-oxadiazole: A curiously ignored ligand

Abstract

The crystal structure of 3,3'-bi-1,2,4-oxadiazole (1) reveals a planar centrosymmetric structure for this molecule in the solid state. It forms a mononuclear complex (3) with palladium(II) chloride, which is shown by X-ray crystallography to contain the ligand chelated through the two N4 nitrogens. The X-ray structure of the complex (5) with silver(I) nitrate shows a one-dimensional coordination polymer in which the ligand acts in a bridging mode, again with coordination through the N4 nitrogen atoms.Keywords: 1,2,4-Oxadiazole; Palladium; Silver; N Ligands [11,12]. Within this context, we noted that 1,2,4-oxadiazoles have been curiously ignored as subcomponents within ligands, be they monodentate, chelating or bridging. Indeed, a search of the Cambridge Crystallographic Database revealed that only three papers reported metal complexes of ligands containing a 1,2,4-oxadiazole subunit [13][14][15].In continuation of our interest in the chemistry of biheterocycles [16], we realised that of the three isomeric bi-1,2,4-oxadiazoles, the 3,3'-isomer (1) represents a particularly interesting potential ligand. This compound has been known for over forty years [17] and could potentially chelate to a single metal in any of three modes (Fig. 1, (a)-(c)) and, furthermore, offers the possibility of bridging two metals in either of two ways ( Fig.1, (d)-(e)). Curiously however, it has never been used as a ligand in coordination or metallosupramolecular chemistry. We therefore set out to explore the use of this compound as a ligand and herein describe the synthesis and X-ray crystal structures of the free ligand, a mononuclear palladium(II) complex and a one-dimensional silver(I) coordination polymer.[ Fig. 1 here] 3,3'-Bi-1,2,4-oxadiazole (1) was prepared in 76% yield by a double cyclisation of oxamidoxime (Scheme 1), based on a literature procedure [18]. In order to confirm unambiguously its structure and to examine the conformation of the ligand in the solid state we determined the X-ray crystal structure of 1 [19]. It crystallises in the orthorhombic space group Cmca with a mere quarter of the molecule in the asymmetric unit; the whole molecule lies on a crystallographic mirror plane and also resides upon a crystallographic centre of inversion (Fig. 2). As such the molecule is perfectly planar and centrosymmetric, a situation that we have previously proposed to be favoured for symmetrical biheterocycles for the 3 reason that this eliminates any dipole moment and facilitates maximal π-π stacking of the molecules in the solid state [20].[Scheme 1 and Fig. 2 here] We next proceeded to explore the coordination chemistry of 1 by preparing a mononuclear complex. We chose palladium(II) as the metal, based on a long standing affection for this well-behaved, square-planar metal synthon [21]. Our first attempt to prepare a palladium complex of 1 involved reaction with PdCl 2 in 2M HCl solution. This reaction furnished crystals of a product that we were able to identify as the ring-opened hydrolysis derivative 2 (Scheme 1), o...

Maximum Compaction of Ionic Organic Explosives: Bis(hydroxylammonium) 5,5′‐Dinitromethyl‐3,3′‐bis(1,2,4‐oxadiazolate) and its Derivatives

“…The structure of the backbone compound 3,3'-bis-1,2,4-oxadiazole has been described in the literature. [22] Compound 2 crystallizes in the monoclinic space group P2 1 /n with two formula units in the unit cell and a density of 1.620 g cm À3 , which is higher than its unnitrated precursor 1 (1.480 g cm À3 ). The molecular structure is shown in Figure 2.…”

Copper(I) hydrazone complexes: Synthesis, structure, DNA binding, radical scavenging and computational studies

“…Interest in these studies stems not only from the different coordination modes but also from their chemical reactivity. Hydrazine derivatives containing N-heterocycles and their complexes exhibit strong antitumor and antivirus activities [8,9]. Metal complexes constitute an important subset of DNA-binding compounds, often DNA-intercalators and groove binders [10][11][12], the latter characterized by non-covalent sequence specific interactions [13].…”

Metal Complexes of Oxadiazole Ligands: An Overview

“…1,2,4-Oxadiazoles have been, by far, less explored as ligands for metal complexes compared to 1,3,4-oxadiazoles. In 2007, the group of Steel published a paper with the indicative title “The first metal complexes of 3,3′-bi-1,2,4-oxadiazole: A curiously ignored ligand” [18]. The authors stressed the multi-chelating potential of this bi-1,2,4-oxadiazole ligand and synthesized a mononuclear palladium(II) complex ( 12 ) and a one-dimensional silver(I) coordination polymer ( 13 ) (Figure 6).…”