Donor-site-directed rational assembly of heteroleptic cis-[Pd2L2L'2] coordination cages from picolyl ligands

Abstract

A donor‐site engineering approach facilitates the formation of heteroleptic [Pd2L2L′2]4+ cage structures through a favored cis‐′in2/out2′ spatial configuration of the methyl groups of 5‐ and 3‐substituted bis‐monodentate picolyl ligands with flat acridone and bent phenothiazine backbones. The heteroleptic cages were confirmed by ESI‐MS and 2D NMR experiments as well as DFT calculations, which pointed toward a cis‐configuration being energetically favored. This was further supported by the synthesis and X‐ray structure of a previously unreported cis‐[Pd(2‐picoline)4]2+ complex. The formation of homoleptic structures, however, was met with considerable steric hindrance at the PdII centers, as observed by the formation of [Pd2L3(solvent)2]4+ and [Pd2L2(solvent)4]4+ species when only one type of acridone‐based ligand was offered. In contrast, bent phenothiazine ligands with outside‐pointing methyl groups showed the ability to form interpenetrated double‐cages, as revealed by X‐ray crystallography. The general route presented herein enables the assembly of uniform cis‐[Pd2L2L′2]4+ coordination cages, thus furthering the possibility to increase structural and functional complexity in supramolecular systems.