Design of a double-decker coordination cage revisited to make new cages and exemplify ligand isomerism

Abstract

The complexation study of cis-protected and bare palladium(II) components with a new tridentate ligand, i.e., pyridine-3,5-diylbis(methylene) dinicotinate (L1) is the focus of this work. Complexation of cis-Pd(tmeda)(NO3)2 with L1 at a 1:1 or 3:2 ratio produced [Pd(tmeda)(L1)](NO3)2 (1a). The reaction mixture obtained at 3:2 ratio upon prolonged heating, produced a small amount of [Pd3(tmeda)3(L1)2](NO3)6 (2a). Complexation of Pd(NO3)2 with L1 at a 1:2 or 3:4 ratios afforded [Pd(L1)2](NO3)2 (3a) and [(NO3)2@Pd3(L1)4](NO3)4 (4a), respectively. The encapsulated NO3 – ions of 4a undergo anion exchange with halides (F–, Cl– and Br– but not with I–) to form [(X)2@Pd3(L1)4](NO3)4 5a–7a. The coordination behaviour of ligand L1 and some dynamic properties of these complexes are compared with a set of known complexes prepared using the regioisomeric ligand bis(pyridin-3-ylmethyl)pyridine-3,5-dicarboxylate (L2). Importantly, a ligand isomerism phenomenon is claimed by considering complexes prepared from L1 and L2.

Configurational ligand isomerism in conjoined-cages

“…Complexation of Pd(II) with L4-L6 resulted in trinuclear cages of Pd 3 L 4 formulation. The architecture or overall configuration of already reported Pd 3 L 4 type cages resemble the double-decker shape [21][22][23][24][25][26][27][28][29] (Fig. 2); however, we disclose here Pd 3 L 4 type cages of unprecedented configuration resembling an hour-glass shape (Fig.…”

“…[9][10][11][12][13][14][15][16][17] Although the complexation of Pd(II) with tripodal tridentate ligands is well understood, 18,19 the use of E-shaped/W-shaped tridentate ligands is only sporadically explored. [20][21][22][23][24][25][26][27][28][29] Such ligands (flexible/rigid) afford Pd 3 L 4 21-29 type double-decker shaped molecules having two cavities that are conjoined through a common metal center (along with its immediate coordination environment). For some ligands (flexible), a PdL 2 type motif is also produced where the ligand adopts a distorted V-shape and connects to the cis-positions of Pd(II) in a bidentate chelating fashion, by means of only the two terminal coordination sites of a ligand unit.…”

Conjoined and non-conjoined coordination cages with palladium(ii) vertices: structural diversity, solution dynamics, and intermolecular interactions

“…9b), in which the symmetrical ester-functionalized tris-monodentate ligand L27 was designed as a positional isomer of the ligand L5 . 70 The complexes 32 and 33 can be considered to display “ligand isomerism”.…”

Self-Assembly of Metallo-Supramolecules under Kinetic or Thermodynamic Control: Characterization of Positional Isomers Using Scanning Tunneling Spectroscopy

“…In coordination-driven self-assembly of discrete metallo-supramolecules, more attention was focused on precise control over the shape and size of assemblies . Through careful design, a single metallo-supramolecule rather than a mixture of isomers was obtained in individual self-assembly . However, isomerism sometimes still occurred, for instance, positional isomers from the unbiased arrangement of dissymmetrical building blocks, configurational isomers from the asymmetric chelating ligand arrangements around the metal centers, social and constellation isomers through different arrangement/orientation of guests encapsulated in a confined host, and unidentical packing modes during the crystallization process .…”