Stimuli-responsive metal-directed self-assembly of a ring-in-ring complex

Abstract

Concentration, temperature and/or solvent polarity control the speciation on the metal-directed self-assembly of a ditopic pyridyl ligand L with cis-protected Pd(ii) metal centers. This results into a controllable dynamic system, involving a [Pd2L2](6+) metallacycle and a [Pd4L4](12+) ring-in-ring complex.

Tuning of the Self‐Threading of Ring‐in‐Ring Structures in Aqueous Media

“…In this fashion, the 1 H NMR spectra of equimolar 10 m m solutions of L a – c ⋅NO 3 and (en)Pd(NO 3 ) 2 was recorded at different temperatures. For L a , we observed the conversion of the initially formed self‐threaded complex into the expected dinuclear metallacycle upon heating (Figure S2 in the Supporting Information), with the latter being the only species present above 323 K. The same behaviour was previously reported for [(en)Pd 4 L b 4 ] 12+ . However, as mentioned above, the self‐threaded species [(en)Pd 4 L c 4 ] 12+ remains as the main product of the self‐assembly even at 343 K, with no evidence of the presence of the corresponding metallacycle (Figure S8d in the Supporting Information).…”

“…According to our previously reported methodology, the self‐assembly processes were monitored by recording the corresponding 1 H and 13 C NMR spectra at room temperature, after mixing equimolar amounts of the ligands and (en)Pd(NO 3 ) 2 in D 2 O. Thereby, a 2.5 m m equimolar solution of (en)Pd(NO 3 ) 2 and L a ⋅NO 3 showed one predominant set of signals, with chemical shifts in good agreement with the self assembly of the expected dinuclear metallacycle (Figure b and Figure S1b).…”

“…However, [(en)Pd 4 L b 4 ] 12+ and [(en)Pd 4 L c 4 ] 12+ are present up to lower concentration thresholds, compared to [(en)Pd 4 L a 4 ] 12+ , in accordance with the increased hydrophobic nature of the corresponding ligand cores. Thus, the corresponding postulated ring‐in‐ring complexes formed from L b and L c assemble in the 10–1 m m and 10‐0.5 m m (at room temperature, see Figure S7 in the Supporting Information) ranges, respectively, and are present at lower concentrations than their [(en)Pd 4 L a 4 ] 12+ counterparts, which are not detected at ligand concentrations lower than 2.5 m m . Hence, for L c , the corresponding self‐threaded complex [(en)Pd 4 L c 4 ] 12+ remains as the major species at 2.5 m m at 343 K and at 1 m m at 298 K and can still be detected at 1 m m at 343 K and at 0.5 m m at 298 K (Figures S7 and S8 in the Supporting Information).…”

“…To validate to some extent this hypothesis, we studied the structure of the corresponding metallacycles, ring‐in‐ring complexes, and the hypothetical [2]catenanes derived from L a,c by conducting DFT theoretical calculations at the M06/6‐31G(d,p) level (Figure and the Supporting Information). From the optimised structures, we estimated the solvent‐accessible surface area (SASA) and the buried solvent‐accessible surface area (BSASA) (Table ) for the aryl units in each calculated structure within the different species potentially involved in the dynamic libraries, comparing the results obtained to those previously reported for L b …”

“…Following our ongoing interest in the metal‐directed self‐assembly of 4,4′‐bipyridinium‐based salts into metallacyclic molecular receptors, their implementation into novel metal‐organic mechanically interlocked molecules, and in the development of new constitutionally dynamic systems, in a previous communication, we reported the self‐assembly around metal centres (en) M (NO 3 ) 2 ( M =Pd II /Pt II ; en=ethylenediamine) of a biphenyl‐containing N ‐monoalkyl‐4,4′‐bipyridinium‐based ligand ( L b ; Scheme ). This approach yielded a dynamic system composed of the expected molecular rectangle [(en) M 2 L b 2 ] 6+ and its self‐threaded ring‐in‐ring dimer [(en) M 4 L b 4 ] 12+ (Scheme ), which was responsive to external stimuli such as temperature, concentration, solvent polarity or the addition of a chemical effector .…”