From Symmetry Breaking to Unraveling the Origin of the Chirality of Ligated Au13Cu2 Nanoclusters

A general method, using mixed ligands (here diphosphines and thiolates) is devised to turn an achiral metal cluster, Au13Cu2, into an enantiomeric pair by breaking (lowering) the overall molecular symmetry with the ligands. Using an achiral diphosphine, a racemic [Au13Cu2(DPPP)(3)(SPy)(6)](+) was prepared which crystallizes in centrosymmetric space groups. Using chiral diphosphines, enantioselective synthesis of an optically pure, enantiomeric pair of [Au13Cu2((2r,4r)/(2s,4s)-BDPP)(3)(SPy)(6)](+) was achieved in one pot. Their circular dichroism (CD) spectra give perfect mirror images in the range of 250-500nm with maximum anisotropy factors of 1.2x10(-3). DFT calculations provided good correlations with the observed CD spectra of the enantiomers and, more importantly, revealed the origin of the chirality. Racemization studies show high stability (no racemization at 70 degrees C) of these chiral nanoclusters, which hold great promise in applications such as asymmetry catalysis.