Geometric Complementarity in Assembly and Guest Recognition of a Bent Heteroleptic cis-[Pd₂L^A₂L^B₂] Coordination Cage

Abstract

Due to the inherent difficulties in achieving a defined and exclusive formation of multicomponent assemblies against entropic predisposition, we present the rational assembly of a heteroleptic [PdLL] coordination cage achieved through the geometric complementarity of two carefully designed ligands, L and L. With Pd(II) cations as rigid nodes, the pure distinctly angular components readily form homoleptic cages, a [PdL] strained helical assembly and a [PdL] box-like structure, both of which were characterized by X-ray analysis. Combined, however, the two ligands could be used to cleanly assemble a cis-[PdLL] cage with a bent architecture. The same self-sorted product was also obtained by a quantitative cage-to-cage transformation upon mixing of the two homoleptic cages revealing the [PdLL] assembly as the thermodynamic minimum. The structure of the heteroleptic cage was examined by ESI-MS, COSY, DOSY, and NOESY methods, the latter of which pointed toward a cis-conformation of ligands in the assembly. Indeed, DFT calculations revealed that the angular ligands and strict Pd(II) geometry strongly favor the cis-[PdLL] species. The robust nature of the cis-[PdLL] cage allowed us to probe the accessibility of its cavity, which could be utilized for shape recognition toward stereoisomeric guests. The ability to directly combine two different backbones in a controlled manner provides a powerful strategy for increasing complexity in the family of [PdL] cages and opens up possibilities of introducing multiple functionalities into a single self-assembled architecture.

A Truncated Molecular Star

“…It is appropriate to mention here that synthesis of self‐assembled heteroleptic coordination complexes has been a recent trend in supramolecular coordination chemistry . Fittingly, the present work describes a heteroleptic complex of exotic design.…”

“…The design principles and related coordination chemistry of such compositions are thus less understood. Even though the homoleptic Pd 2 L 4 type complexes are extensively explored, there have been only a handful of examples where two types of bidentate ligands are used to prepare heteroleptic complexes of Pd 2 L a 3 L b 1 , Pd 2 L a 2 L b 2 formulations. In the same line, two types of bidentate ligand are known to afford hexanuclear Pd 6 L a 6 L b 6 and dodecanuclear Pd 12 L a 12 L b 12 type complexes.…”

Coordination Assembly of the Water‐Soluble Ti4(embonate)6 Cages with Mn2+ Ions

“…Metal‐organic cages (MOCs) have attracted great interest because of their appealing structures and wide applications in catalysis, gas sequestration, photoreactions, drug delivery, molecule sensing, etc . Up to now, the search for novel MOCs has driven the exploration of different geometries .…”

Self-assembly of cucurbit[7]uril based triangular [4]molecular necklaces and their fluorescence properties

“…Next, we prepared the unthreaded SCCs 17 by heating ligand 1 (5.5 mM) with an aqueous solution of Pd(en)(NO 3 ) 2 (100 °C, 24 h). 1 H NMR displayed resonances for two SCC’s (ratio = 86:14) that were of different size as determined by diffusion ordered spectroscopy (DOSY) analysis (Fig.…”

“…We attribute the low magnitude of these shifts to the presence of the charged pyridinium substituent as previously observed by Quintela. 17c We hypothesised that two products are triangle 3 (most abundant, D = 3.02 × 10 −10 m 2 /s) and square 4 (D = 2.18 × 10 −10 m 2 /s). When [ 1 ] was reduced from 5.5 to 2.5 mM, the relative abundance of 3 with respect to 4 increased upon dilution from 86% to >99% ([ 4 ] is below NMR detection limit).…”