Accessibility in Calix[8]arene-Bound Gold Nanoparticles: Crucial Role of Induced-Fit Binding

Abstract

Metal nanoparticles bound with polymeric and oligomeric ligands are commonly employed in areas where accessibility to the underlying metal is critical, such as in catalysis. Developing a fundamental understanding of molecular-scale factors that control accessibility to the metal surface in these systems enables approaches for ligand design. Here, we implement a comparative synthetic approach to investigate the role of an induced-fit binding mechanism on accessibility, which reduces to elucidating the correlation between ligand flexibility and accessibility. Four nm gold nanoparticles are bound with a calix[8]arene phosphine ligand in the comparative series 3a-c, and ligand molecular footprints of similar to 230 angstrom(2)/calix[8]arene are measured on the gold surface using UV-vis spectroscopy of the surface plasmon resonance absorption band. Ligand flexibility in CDCl3 solution is characterized using variable-temperature H-1 NMR spectroscopy, and results demonstrate 3c to be the most rigid of all three ligands. This is further reinforced by conformational analysis of 3a-c using molecular modeling calculations of the free ligand in the gas phase. Additional conformational analysis of bound 3a-c is modeled by using a constrained variant of the corresponding phosphine oxide 2a-c, in which the P=O bonds of the phosphine oxide are aligned, as they would be when bound to a surface underneath. Interestingly, the most rigid ligand in the bound state is 3a, a result that is in stark contrast to experiments in solution and simulations in the gas phase described above. The amount of accessible surface is measured using steady-state fluorescence of 2-naphthalenethiol (2NT) as a chennisorption probe. The most rigid ligand in the bound state (3a) synthesizes no accessible surface whereas the more flexible ligands in the bound state (both 3b and 3c) synthesize 0.5 2NT worth of accessible metal surface per calix[8larene ligand, corresponding to 5-7% of the gold surface of the nanoparticle being accessible. These results demonstrate a correlation between accessibility and ligand flexibility in the bound state, and suggest an induced-fit binding mechanism in which a flexible bound ligand subtly changes shape in order to accommodate adsorption of an incoming molecule on the metal surface.