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).

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.

Abstract Palladium nanoparticles (PdNPs) were used as a catalyst for the one-pot synthesis of a variety of benzofurans by Sonogashira cross-coupling reactions under ambient conditions. The catalyst could be recycled without significant loss in its activity.

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.

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 facile one-pot synthesis of mono(hpp)- or di(hpp)-substituted N-heterocyclic ligands from halogenated N-heterocycles and 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (H-hpp) is presented. N,N-Bidentate and N,N,N-tridentate ligands incorporating electron-donating and electron-withdrawing substituents can also be readily synthesized using this method.

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.