A Dynamic Silver(I) Nanocluster Holds Together a 3 × 3 Self-Assembled Grid

Abstract

Metal ions with well-defined coordination geometries can serve as fixed joints within self-assembled architectures, defining the relative orientations of ligands within higher-order superstructures. The exchange of ligands and metal ions between different positions is slow, involving disruption or distortion. Here we report a series of Ag^I₁₂X₄L₆ 3 × 3 metal-organic grid-like structures, where the core Ag^I₁₂X₄ nanocluster is in dynamic motion, with Ag^I ions moving between different binding sites, with concomitant conformational changes of the organic ligands, which continue to occupy well-defined positions nevertheless. The identity of the incorporated halide anion governs the activation barrier for silver ion exchange, thus enabling rate control in response to two distinct stimuli: by changing the temperature, and by exchanging one halide for another. The dynamic nanocluster within these grids thus provides a new mode of using metal ions in coordination-driven self-assembly, establishing that the mobile Ag^I ions behave in similar ways to Ag⁰ atoms in surface-bound clusters and in silver nanoparticles. The kinetic parameters determined in this work, and the techniques developed to measure them, could serve the scientific community to provide additional insight into dynamic metal nanoclusters.

1

Comparison between the original Ag^I ₉L₆ 3 × 3 metal–organic grids of Lehn et al., where each Ag^I ion defines a fixed junction between ligands, and the present work describing Ag^I ₁₂X₄L₆ 3 × 3 metal–organic grids where the Ag^I ions can move through the structure in either direction.

2

Synthesis and characterization of 1-I, [Ag^I ₁₂I₄L₆]­(NTf₂)₈. (a) Self-assembly of 1-I from subcomponents A and B, silver­(I) triflimide and tetra-n-butylammonium iodide. (b) X-ray crystal structure of 1-I with inset showing the Ag^I ₃I subcluster structure within the Ag^I ₁₂I₄ nanocluster core. (c) Partial ¹H–¹H NOESY NMR spectrum showing H_I···H_a correlations (500 MHz, CD₃CN, 298 K).

3

NMR characterization and molecular dynamics simulation of silver ion movement within 1-I. (a) Cartoon depicting rotation of silver ions within one Ag^I ₃I cluster within the Ag^I ₁₂I₄ nanocluster core of 1-I. Silver NMR chemical shifts were determined at 232 K. (b) Partial ¹H–¹⁰⁹Ag HSQC-EX NMR spectrum at 243 K with 10 ms mixing time, showing no ¹⁰⁹Ag exchange. (c) Partial ¹H–¹⁰⁹Ag HSQC-EX NMR spectrum at 243 K with 210 ms mixing time, showing maximal ¹⁰⁹Ag exchange between environments. (d) Plot of the imine-silver cross-peak signal volumes from HSQC-EX spectra at 243 K, as the mixing time increased from 10 to 1210 ms. Diagonal peaks I _AA and I _BB decreased in volume with increasing mixing time due to relaxation and exchange. Exchange peaks I _AB and I _BA initially increased in volume due to exchange, finding maxima at approximately 210 ms, then decreasing due to relaxation. Orange and violet lines correspond to the 10 and 210 ms HSQC-EX spectra shown in Figure b,c, respectively. (e) The starting configuration of a QM/MM MD simulation of 1-I at 243 K with explicit solvent. (f) The configuration at 1724 fs of this QM/MM MD simulation, showing the furthest displacement of the migrating Ag^I ion. (g) Plot of four key N···Ag distances over the course of a 4000 fs QM/MM MD simulation. Pink and gray lines indicate the 0 and 1724 fs configurations shown in Figure e,f, respectively.

4

Halide exchange enabled the transformation between grids, where halide identity determined silver cluster nuclearity and geometry. (a) Halide exchange transformed 1-Cl to 1-Br and finally 1-I. Total silver magnetization exchange-rates (k _1‑X ^T) increase as the halide component increases in softness, decreasing the Gibbs free activation energy (ΔG ^‡ _1‑X). (b) Ag^I ₂Cl and Ag^I ₄Cl clusters observed in the crystal structure of 1-Cl. (c) Ag^I ₂Br, Ag^I ₃Br and Ag^I ₄Br clusters observed in the crystal structure of 1-Br. (d) Ag^I ₃I cluster observed in 1-I.