Research paperAssembly of anion-controlled cadmium(II) coordination polymers from the use of 2-acetyl-pyridyl-isonicotinoylhydrazone
Abstract
Four Cd(II) coordination polymers have been synthesized, utilizing a tetradentate pyridine-based Schiff base ligand, namely 2-acetyl-pyridyl-isonicotinoylhydrazone (HL), and different anions (CH3COO−, N3−, SCN−). The products {[Cd2(L)2(CH3COO)2]·C2H5OH}n (1), {[Cd1.5(L)(CH3COO)(N3)(H2O)]·C2H5OH}n (2), {[Cd(HL)(SCN)2]·CH3OH}n (3) and [Cd3(HL)2(SCN)6]n (4), were characterized with single-crystal X-ray diffraction, IR spectroscopy, and luminescence studies. The ligand in 1 and 2 is singly-deprotonated and coordinates to the cadmium centre in the enolic form (
N–NC–O) while in compounds 3 and 4 it coordinates in its neutral keto form (N–NH–CO). The tetradentate ligand in 2 acts as a linker and leads to the formation of a one-dimensional coordination polymer. The CH3COO− in 1 and SCN− in 3 and 4, further act as bridges, forming two-dimensional coordination polymers (1 and 3) and a three-dimensional network (4). Structural diversity is thus induced by the anions, due to the variable steric hindrance they impose as well as differing bridging capacity; metal-organic network formation is also evident via different Van der Waals forces.
Graphical abstract
The investigation of 2-acetyl-pyridine-isocotinoylhydrazone in Cd(II) chemistry is reported. Four different coordination polymers were produced from the anion dependence study. The coordination and bridging ability of the hydrazone ligand is crucial in the assembly of the resulting coordination networks. The chelate/bridging capacity of the ancillary ligands was also found to be a key factor for the structural diversity of the resulting materials.
Introduction
The rapidly growing field of crystal engineering of one, two-, and three-dimensional (1D, 2D, 3D) coordination polymers (CPs) has been attractive, not only for their structural and topological diversities [1], [2], [3] but also for their potential application as functional materials in catalysis [4], [5], optics [6], [7], magnetism [8], [9], [10], molecular architectures [11], materials chemistry, etc [12]. However, control of the complex structures in hydro-/solvo-thermal reactions remains a challenge, owing to the fact that the assembly of such complexes can be easily influenced by the geometrical and electronic properties of metal ions and ligands, the temperature, as well as the pH of the solution [13], [14], [15], [16]. Among the factors in the design of CPs, the counterion dependence has been actively investigated, due to a) the co-ligand effects these ions exhibit, and b) their ability to direct and/or template the formation of diverse assemblies [17], [18], [19], [20]. Furthermore, counterions such as carboxylate, nitrate, azide and thiocyanate exhibit different coordination modes (monodentate, chelate, and/or bridging) within the framework, which may further enrich the structural diversity [20]. Therefore, using organic/inorganic anions in the assembly of new functional materials has become a rapidly emerging field [21], [22], [23].
On the other hand, to meet the requirement of metal-ligand binding preference and the energetic consideration of overall crystal packing, a mixed-ligand assembling strategy was proven an effective approach. A variety of organic ligands, especially polycarboxylate, polyalcohols, and polypyridyl types, have been generally regarded as the most familiar and reliable candidates to construct the desired coordination architectures in recent years [24], [25], [26], [27], [28], [29]. Another class of flexible and versatile polydentate ligands are hydrazones, which show very high efficiency at chelating transition metal ions [30]. Hydrazones obtained from 4-pyridine carboxylic acid hydrazide can act as ditopic ligands via two different donor sites (a tridentate coordination pocket and a N-donor pyridine moiety); this makes them excellent ligand candidates for the formation of mono- and multinuclear structures [31], [32], [33].
The Cd(II) ion with its d10 electronic configuration exhibits great flexibility in its coordination environment/geometry [34]. In the literature, reports of the anion effect on the coordination mode of Cd(II) have been rare [35], and in this context, we used the potentially tetradentate ligand, 2-acetyl-pyridyl-isonicotinoylhydrazone (HL, Scheme 1) in a systematic investigation of the anion dependence of self-assembled Cd(II) CPs. The HL ligand has the potential to form different types of complexes due to the multiple coordination sites and the potential to adopt either enol and keto tautomeric forms (Scheme 1) [36], [37], [38]. From this systematic investigation, we report the synthesis and characterization of four different coordination polymers, exemplifying that diversity of anions may cause major changes in the resulting structures, which in turn is of interest in the design and construction of CPs.
Section snippets
Materials and measurements
All reagents were obtained from commercial sources and were used as received. The ligand HL was prepared as described previously [37]. In short, pure HL was obtained in >80% yield by the condensation of 2-acetylpyridine and isonicotinohydrazide in EtOH. Selected IR bands (KBr pellet, cm−1): 3290 (N
H), 1667(CO). The branched tube method for synthesis is described detail in our previous reports [39].
Elemental analyses were carried out using an Elementar Vario EL III instrument and FT-IR spectra
Synthesis and IR spectroscopy
For the syntheses of 1 and 2, typical wet chemistry techniques were used, whereas for 3 and 4, the branched tube apparatus was used, as we have previously discussed in detail [39]. The velocity of the convection current is proportional to the thermal gradient across the vessel, so we were careful not to make the gradient too large because the rapid convection inhibits crystal growth [36]. It is also noted here that reactions using various reactant ratios were performed; herein the reactions
Conclusions
Herein we report a series of Cd(II) coordination complexes and networks, featuring a pyridine-based tetradentate Sciff base ligand. The different complexes were the result of a systematic study of the anion influence on the resulting materials. The anions used in this study were CH3COO−, N3−, and SCN−. These ancillary ligands were proven to play a central role in the self assembly of the resulting structures, due to their variable ability to bridge metal centres. Cd(II) is a versatile metal
Acknowledgements
We are grateful to the University of Tabriz for the generous financial support of this research. C.L. acknowledges the Cottrell College Science Award by the Research Corporation for Science Advancement, the Camille and Henry Dreyfus foundation, and the US National Science Foundation (DMR-1429428, DMR-1626332) for support of this work.
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