Insights into Racemization of Metal Nanoclusters and Strategic Spontaneous Resolution

Chiral metal nanoclusters stand as pivotal materials in contemporary scientific research, yet their enantiomers predominantly crystallize as racemic mixtures, posing substantial challenges for chirality-based applications. This study elucidates the racemization mechanism in metal nanoclusters and proposes a rational strategy for achieving spontaneous resolution. We demonstrate that racemic cluster assemblies are stabilized in the unit cell by intermolecular interactions between opposite enantiomers; eliminating or diminishing these interactions induces spontaneous resolution, yielding homochiral crystals of either enantiomer. Two illustrative cases are presented: the racemic assembly of Cu₂₅H₁₀(PhFS)₁₈(PPh₄)₃ (PhFS = 2-fluorobenzenethiolate) is stabilized by extensive C-H···F hydrogen bonds between enantiomers. Removing fluorine substituents to form Cu₂₅H₁₀(PhS)₁₈(PPh₄)₃ (PhS = benzenethiolate) disrupts these interactions and enables resolution. In Cu₄(PhS)₆(PPh₄)₂, substituting benzene with thiophene ligands (forming R/S-Cu₄(ThS)₆(PPh₄)₂, ThSH = 2-thiophenethiol) diminishes C-H···π interactions between enantiomers, facilitating spontaneous resolution. Successful resolution is unambiguously confirmed by mirror-image circular dichroism spectra of enantiopure crystals. This study demonstrates the critical role of intermolecular interactions in racemate formation and presents strategies for achieving spontaneous chiral resolution in metal nanoclusters.