Review
Self-assembled coordination complexes from various palladium(II) components and bidentate or polydentate ligands

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Abstract

Palladium(II) has four coordination sites and forms square planar complexes. Discrete self-assemblies are generated by the combination of a variety of palladium(II) components and ligands ranging from bi- to polydentate. The Pd(II) components used are generally of two varieties: cis-protected Pd(II) and unprotected Pd(II). Most common cis-protecting units (X-X) such as ethylenediamine, 2,2′-bipyridine and 1,3-bis(diphenylphosphino)propane and a few other related chelating systems have been exploited for the complexation reactions. The self-assemblies formed are generally represented as [{cis-Pd(X-X)}x(L)y](monoanion)2x and [Pdm(L)n](monoanion)2m when generated from the complexation of a suitable ligand (L) with cis-protected Pd(II) and simple Pd(II) units, respectively. When Pd(solvent)2Cl2 is complexed with ligands, the solvent molecules are replaced with the incoming ligands, leading to complexes in which the trans positions are occupied by the chloride anions.

Highlights

► Self-assembly of a palladium(II) component (M) and a ligand (L) provides (M)x(L)y. ► Self-assembled (M)x(L)y(L′)z are achieved by employing two different ligands. ► cis-protected Pd(II), simple Pd(II) salts and PdCl2, are employed for complexation. ► Bidentate or polydentate ligands are engaged to construct the assemblies. ► A dynamic equilibrium of assemblies is observed in some cases.

Introduction

Self-assembly is one of the aspects of supramolecular chemistry that can be utilized to synthesize larger molecules out of small building blocks. This self-assembly of molecules can lead to the formation of highly fascinating and complex structures from very simple building blocks. Self-assembly is therefore an interesting alternative to the covalent synthesis of larger structures. The controlled self-assembly of small molecules with well-defined association properties is an easier and more economical method than the direct synthesis of a covalent structure of comparable shape and size. Self-assembled systems can differ significantly in their chemical and physical properties from the individual building blocks, thereby making them interesting for applications as new materials with tailor-made properties.

The “molecular library” approach [1] proposed by Stang et al. which is equivalent to the “directional-bonding” approach [2] proposed by Mirkin et al. is a general high-yielding method, which produces metal complexes with a wide range of shapes and sizes. The final structure of the complexes can be controlled via the rational choice of metals and ligands. This strategy involves the assembly of large metallosupramolecules in which the metal centers remain highly directional corners or side units in the resulting cages. Typically, the metals provide coordination sites for the incoming ligands with the appropriate angles to form the desired shapes; the target molecule's shape is predetermined by a careful selection of the metal component and bridging ligands.

One of the early examples of metal-driven self-assembly is a binuclear metallomacrocycle composed of two Cu(II) ions and two bis(β-diketonate) ligands [3]. Another early example is a spontaneously assembled double-stranded helicate obtained from an oligobipyridine ligand and a Cu(I) salt [4]. An interesting structure, 1, was obtained when two classical compounds, cis-Pd(en)(NO3)2 and 4,4′-bipyridine (L1), were combined in an aqueous-alcohol medium (Scheme 1) [5]. The quantitative formation of the molecular square, 1, was confirmed by an impressively simple NMR spectrum (only one set of pyridine ring signals in the aromatic region) and, later, by MS and X-ray studies [6]. The construction of palladium(II)-driven self-assemblies was first started by Fujita et al. and has attracted the attention of various research groups.

Various reviews have been published [1], [2], [7], [8], [9], [10], [11], [12], [13], [14] based on the metal-driven self-assembled molecular architectures. Recently, polynuclear self-assembled coordination cages were extensively reviewed by the Stang group [15]. Review articles of a more specific nature describing triangles [16], helicates [17], metallacrowns [18], capsules [19], host–guest complexes [20] and molecular reactors [21], [22], [23], [24], [25] have also been reported. Self-assembled palladium(II) complexes have been reviewed previously, including reports of binuclear complexes [26], tri- or higher-order nuclear complexes [27], squares [28], [29], polygons [30], polyhedrons [31], [32], [33], [34], [35], catenanes [36], [37], box-type complexes [38], tubes [39] and molecular reactors [40], [41]. The present review differs from the previous articles in that it includes a systematic collection of palladium(II) components and their complexation with a variety of ligands, and it is presented based on the geometrical similarities of the resulting assemblies.

Palladium(II) has four coordinating sites, and it forms square-planar complexes when combined with a suitable ligand, L. The Pd(II) units used to construct supramolecules are generally of two types: cis-protected Pd(II) and unprotected Pd(II) (Fig. 1). The self-assemblies so formed may be represented as MxLy-type [{cis-Pd(X-X)}x(L)y](monoanion)2x and MmLn-type [Pdm(L)n](monoanion)2m when created via the complexation of a suitable ligand, L, with cis-protected Pd(II) and simple Pd(II) units, respectively. In this representation, the ligand is considered neutral and the metal component is di-positive. Most common cis-protecting units (X-X) are bidentate chelating in nature, e.g., ethylenediamine, (en); 2,2′-bipyridine, (bpy); N,N,N′,N′-tetramethylethylenediamine, (tmeda); 1,3-bis(diphenylphosphino)propane, (dppp); 1,1′-bis(diphenylphosphino)ferrocene, (dppf); 1,2-bis(diphenylphosphino)ethane, (dppe); 1,1-bis(diphenylphosphino)methane, (dppm); 1,2-bis(diethylphosphino)ethane, (depe); 1,2-bis(diphenylphosphanyl)benzene, (dppbz); 4,4′-di-tert-butyl-2,2′-bipyridine, (bpy-t-butyl); 1,10-phenanthroline, (phen); 1,10-phenanthroline-crown ethers and related compounds. Chiral chelating cis-protecting units are also used, e.g., (R)-(+)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl, (BINAP). In some cases, dinuclear Pd(II) components in which each metal center is doubly or triply protected are also used. There are a few examples in which two units of triethylphosphine or triphenylphosphine are used as cis-protecting agents. When Pd(solvent)2Cl2 is complexed with ligands, the solvent molecules are replaced by the incoming ligands, leading to the formation of complexes in which the trans positions are commonly occupied by chloride anions.

Among the ligands used, pyridine-appended compounds are most often used in combination with Pd(II) components. The formation of the smallest possible entropically driven architecture is the expected product. However, in some cases, more than one product is observed, and decrease in concentration or increase in temperature favors the entropic product. The solubility of the complexes can be tuned by suitably changing the counter anion. Generally, the complexes containing nitrate anions are water soluble, and those containing other anions are more soluble in polar organic solvents. The careful selection of a solvent system or the guest molecule may yield completely different complexes. Once a suitable solvent is found, it may be possible to prepare the complex in the corresponding deuterated solvent, which, upon complexation, can be monitored directly using proton NMR. Generally, quantitative formation of the product is observed.

Section snippets

Self-assemblies using cis-protected Pd(II)

As mentioned above, the Pd(II) unit forms tetra-coordinated square-planar complexes with suitably chosen ligand moieties. One of the earliest approaches, called the directional bonding approach, involves the blockage of two of the four coordination sites that are cis to each other such that the remaining two sites are available for the incoming ligand (Fig. 1). The supporting ligand used for the cis-protection can be either monodentate or chelating bidentate in nature. Such a ligand is termed a

Self-assemblies using Pd(II)

This section deals entirely with the self-assemblies formed from Pd(II) in the absence of any cis-protection and using ligands of varying denticity. Palladium(II) can be used as such without any protecting unit attached to it; thus, all four accepting sites are available for further coordination with different ligands. The added complexity arising from the availability of four sites for ligand binding requires more imagination to predict the geometry of the final ensemble (Fig. 1). This

Self-assemblies using PdCl2

The construction of self-assemblies via the combination of PdCl2 with suitable ligands has not been well explored. When PdCl2 is dissolved in CH3CN or C6H5CN, the corresponding solvent adducts, Pd(solvent)2Cl2, are formed. Upon combination with ligands, such an adduct results in complexes in which the chlorides are trans to each other. A few examples are discussed below, and lists are provided in Table 20, Table 21. Such complexes may be considered to be assemblies containing trans-protected

Conclusion

In conclusion, a systematic collection of various types of palladium(II) components and their complexation with a variety of ligands is discussed. It can be seen that, although various examples are available in the literature, the discovery of many more such complexes is anticipated as a result of several additional groups showing research interest in this field.

Acknowledgment

We thank DST, New Delhi, for financial support.

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