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 [Pd2 L 2 L′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 [Pd2 L 3(solvent)2]4+ and [Pd2 L 2(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‐[Pd2 L 2 L′2]4+ coordination cages, thus furthering the possibility to increase structural and functional complexity in supramolecular systems.

Resolution of minor size differences in a family of heteroleptic coordination cages by trapped ion mobility ESI-MS

“…We and others have therefore recently begun to invest effort in the rational design of multi-ligand heteroleptic coordination cages that form selectively from more than one kind of ligandmostly under thermodynamic control without generating statistical mixtures. 4 While some of our recently reported approaches make use of donor-site engineering to achieve the formation of heteroleptic assemblies, 5 the method forming the basis for the herein reported findings makes use of the size and shape complementarity of pairs of bis-monodentate ligands to selectively form a series of cisconfigured [Pd 2 L 2 L′ 2 ] cages with a bent appearance (Fig. 1).…”

Tunable Fullerene Affinity of Cages, Bowls and Rings Assembled by PdII Coordination Sphere Engineering

“…[10] Fujita and Yoshizawa used pyridine/lutidinep airs around cis-protectedP d II to constructp rismatic cages, [11] and we recently expandedthis con-cept to work on "naked" Pd II centers, using ac ombination of inward and outward pointing picolines to give cis-[Pd 2 L 2 L' 2 ] cages. [12] Crowley achieved similars tructures with 2-aminomodifiedp yridine ligands. [13] When considering other sterically more demanding donor functionalities, we recently started to substitute pyridines with quinolines, which carry an hydrogen atom on the annelated benzene ring that causes steric congestion aroundt he metal binding site.…”

“…Further strategies base on the non‐statistical construction of heteroleptic coordination environments by engineering the direct electronic or steric environment of the metal coordination site . Fujita and Yoshizawa used pyridine/lutidine pairs around cis ‐protected Pd II to construct prismatic cages, and we recently expanded this concept to work on “naked” Pd II centers, using a combination of inward and outward pointing picolines to give cis ‐[Pd 2 L 2 L′ 2 ] cages . Crowley achieved similar structures with 2‐amino‐modified pyridine ligands …”

Directional asymmetry over multiple length scales in reticular porous materials

“…More recently, Zhu et al combined endo and exo steric repulsion to directly form an asymmetric heteroleptic [Pd2(Lh)2(Li)2] 4+ cage. 27 Combining tripyridyl ligands with ortho methyl group of the terminal ligating pyridyl units pointing either inside (Lh) or outside (Li), leaded to a preferred cis 2in/2out configuration of the ligands in the cage as observed by ESI-MS, 2D NMR and DFT calculation.…”