Dillip Kumar Chand

The simple combination of PdII with the tris-monodentate ligand bis(pyridin-3-ylmethyl) pyridine-3,5-dicarboxylate, L, at ratios of 1:2 and 3:4 demonstrated the stoichiometrically controlled exclusive formation of the "spiro-type" Pd1L2 macrocycle, 1, and the quadruple-stranded Pd3L4 cage, 2, respectively. The architecture of 2 is elaborated with two compartments that can accommodate two units of fluoride, chloride, or bromide ions, one in each of the enclosures. However, the entry of iodide is altogether restricted. Complexes 1 and 2 are interconvertible under suitable conditions.

Complexation of 1,4-phenylenebis(methylene) diisonicotinate, L1, with cis-protected PdII components, [Pd(L′)(NO3)2], in an equimolar ratio yielded binuclear complexes, 1 a-d of [Pd2(L′)2(L1)2](NO3)4 formulation where L′ stands for ethylenediamine (en), tetramethylethylenediamine (tmeda), 2,2′-bipyridine (bpy), and phenanthroline (phen). The combination of 4,4′-bipyridine, L2, with the cis-protected PdII units is known to yield molecular squares, 2 a-d. However, 2 b-d coexist with the corresponding molecular triangles, 3 b-d. Combination of an equivalent each of the ligands L1 and L2 with two equivalents of cis-protected PdII components in DMSO resulted in the D-shaped heteroligated complexes [Pd2(L′)2(L1)(L2)](NO3)4, 4 a-d. Two units of the D-shaped complexes interlock, in a concentration dependent fashion, to form the corresponding [2]catenanes [Pd2(L′)2(L1)(L2)]2(NO3)8, 5 a-d under aqueous conditions. Crystal structures of the macrocycle [Pd2(tmeda)2(L1)(L2)](PF6)4, 4 b′′, and the catenane [Pd2(bpy)2(L1)(L2)]2(NO3)8, 5 c, provide unequivocal support for the proposed molecular architectures. Interlock game: A series of D-shaped self-assembled molecules are prepared from a three-component mixture of cis-protected PdII, rigid rod-like and flexible C-shaped ligands in a 2:1:1 ratio in DMSO. These molecules are considered as molecular karabiners bearing coordination bond loaded gates. The D-shaped molecules are interlocked in aqueous conditions, forming the corresponding [2]catenanes (see figure).

Self-assembled molecular triangles [Pd3(phen)3(imidazolate)3](NO3)3, 1a and [Pd3(phen)3 (imidazolate)3](PF6)3, 1b are prepared by the combination of imidazole with Pd(phen)(NO3)2 and Pd(phen) (PF6)2, respectively. Imidazole was deprotonated during the complexation reactions and the imidazolate so formed acted as a bis-monodentate bridging ligand to form the bowl-shaped trinuclear architectures of 1a/b. Relative orientation of the imidazolate moieties can be best described as syn,anti,antias observed in the crystal structure of 1b. However, in solution state, slow conformational changes are assumed on the basis of 1HNMR spectral data. The molecular triangles are crafted with three peripheral phen units capable of π-π stacking interactions. Well-fashioned intermolecular π-π interactions are observed in the solid-state, wherein further self-assembly of already self-assembled triangle is observed.

A series of Pd2L4-type binuclear self-assembled coordination cages (1–4), where L stands for a nonchelating bidentate ligand, were prepared. The strategies adopted for the synthesis of the cages were: combination of PdIIwith 1) a selected ligand or 2) subcomponents of the ligand. Highly efficient cage-to-cage transformation reactions are demonstrated by suitable covalent modification (from 1 to 2 or 3 or 4) or ligand-exchange reactions (from 1 to 2 or 3 or 4; from 2 to 3 or 4). Thus, new cascade transformations (from 1 to 2 to 3; from 1 to 2 to 4) are achieved beautifully.

Supramolecular gels prepared from low-molecular-weight gelators have been extensively explored. However, the exploitation of discrete or polymeric metal complexes as gelators is a relatively recent trend. The synthesis of self-assembled coordination complexes from palladium(II) and selected ligands is well established, but the potential of these complexes as gelators is a less explored treasure. Herein we focus on the gelation abilities of some self-assembled palladium(II) complexes and the resulting unique properties. First, discrete complexes with PdL, PdL2, Pd2L, Pd2L2, Pd2L4, and Pd3L6 compositions are discussed. Second, gelation behavior promoted by coordination-polymer-like gelators formed in situ is explored. These gel samples have been employed in catalysis and the uptake of organic and dye molecules from the solution and gas phases. It is concluded that untapped unique properties can be realized by further exploration of designer palladium(II) complexes.

Selective binding of chloride over the other most abundant anions in living organisms is pivotal due to its essential role in physiological functions. Herein, we report a template-free Pd2L4 cage exhibiting high selectivity for medium-sized halides (i. e., Cl−, Br−) in water owing to the size-discriminatory nature of the cage cavity. In pure water, this cage displays high selectivity and micromolar affinity for chloride. The cage shows no binding towards other biologically more abundant essential anions such as phosphates, carboxylates, or bicarbonate. This cage shows an unprecedented nanomolar affinity with 1 : 1 binding stoichiometry for chloride in aqueous-DMSO media. This high affinity was achieved with the best use of traditional hydrogen bonding and electrostatic interactions, as confirmed by single-crystal X-ray diffraction analysis. This well-defined cage sequestrates F− by cleaving a B−F bond in BF4− in a facile manner in a nonpolar solvent or in the presence of excess ligand. This cage also demonstrates capture of the sub-ppm chloride level that is present in commercial D2O samples.

A pentanuclear coordination complex assembled from any palladium(II) component and non-chelating ligands is hitherto unreported. The pentanuclear complex [Pd5(L1)5(L2)5](BF4)10, 1 reported here was prepared by the spontaneous complexation of [Pd(DMSO)4](BF4)2 with the non-chelating bidentate ligands 1,4-phenylenebis(methylene) diisonicotinate, L1 and 4,4′-bipyridine, L2 in a one-pot method at room temperature. The planar polycyclic complex 1 with outer diameters of ≈3 nm is termed as a “molecular star” owing to its resemblance with a pentagram shape. Interim paths leading to the star were also probed to decipher related dynamics of the system.

A rare variety of self-assembled molecular triangle [Pd3(bpy)3(imidazolate)3](NO3)3, 1 is prepared by the combination of Pd(bpy)(NO3)2 with imidazole, at 1:1 ratio, in acetonitrile-water. Deprotonation of imidazole happened during the course of the complexation reaction where upon the metallomacrocycle is formed. The bowl-shaped trinuclear architecture of 1 is crafted with three peripheral bpy units capable of π-π stacking interactions. While the solution state structure of 1can be best described as a trinuclear complex, in the solid-state well-fashioned intermolecular π-π and CH- π interactions are observed. Thus, in the solid-state further self-assembly of already self-assembled molecular triangle is witnessed. The triangular panels are arranged in a linear manner utilizing intermolecular π-π interactions where upon two out of three bpy units of each molecule participated in the chain formation.

Metal-driven self-assembly is one of the most effective approaches to lucidly design a large range of discrete 2D and 3D coordination architectures/complexes. Palladium(II)-based self-assembled coordination architectures are usually prepared by using suitable metal components, in either a partially protected form (PdL′) or typical form (Pd; charges are not shown), and designed ligand components. The self-assembled molecules prepared by using a metal component and only one type of bi- or polydentate ligand (L) can be classified in the homoleptic series of complexes. On the other hand, the less explored heteroleptic series of complexes are obtained by using a metal component and at least two different types of non-chelating bi- or polydentate ligands (such as La and Lb). Methods that allow the controlled generation of single, discrete heteroleptic complexes are less understood. A survey of palladium(II)-based self-assembled coordination cages that are heteroleptic has been made. This review article illustrates a systematic collection of such architectures and credible justification of their formation, along with reported functional aspects of the complexes. The collected heteroleptic assemblies are classified here into three sections: 1) [(PdL′)m(La)x(Lb)y]-type complexes, in which the denticity of La and Lb is equal; 2) [(PdL′)m(La)x(Lb)y]-type complexes, in which the denticity of La and Lb is different; and 3) [Pdm(La)x(Lb)y]-type complexes, in which the denticity of La and Lb is equal. Representative examples of some important homoleptic architectures are also provided, wherever possible, to set a background for a better understanding of the related heteroleptic versions. The purpose of this review is to pave the way for the construction of several unique heteroleptic coordination assemblies that might exhibit emergent supramolecular functions.