Polymorphism of [Cu₁₅(PhCH₂CH₂S)₁₃(PPh₃)₆][BF₄]₂ and Double-Helical Assembly of [Cu₁₈H(PhCH₂CH₂S)₁₄(PPh₃)₆Cl₃]: Origin of Two Chiral Nanoclusters with Triple-Helical Core from Intermediates

Here, we report the solvent-induced polymorphism in [Cu₁₅(PET)₁₃(TPP)₆][BF₄]₂(Cu₁₅) (TPP = triphenylphosphine, PET = 2-phenylethanthiol), and double-helical assembly of the [Cu₁₈H(PET)₁₄(TPP)₆Cl₃] (Cu₁₈) nanocluster (NC) from reaction intermediates. Both copper NCs have an intrinsically chiral triple-stranded helicate metal core, unlike traditional copper NCs with a polyhedral-based kernel. The chiral structure of Cu₁₅ resembles an enantiomeric pair in the unit cell. Moreover, Cu₁₈ has a three-layered 3D chirality of a sandwich constructed of sulfur-bridged copper NCs aligned in a top-middle-down configuration. Furthermore, the Cu₁₈ NC self-hierarchically assembles into a complex double-stranded helix secondary structure sustained by noncovalent interactions. Electrospray ionization mass spectrometry (ESI-MS), density functional theory (DFT), and X-ray photoelectron spectroscopy (XPS) were utilized to validate the single-crystal X-ray diffraction (SCXRD) data. Overall, this study provides an interesting example of polymorphism, chirality, and hierarchical double-helical assembly of NCs, allowing for extensive understanding of complicated structures at the atomic level.