Polymorphism of [Cu<sub>15</sub>(PhCH<sub>2</sub>CH<sub>2</sub>S)<sub>13</sub>(PPh<sub>3</sub>)<sub>6</sub>][BF<sub>4</sub>]<sub>2</sub> and Double-Helical Assembly of [Cu<sub>18</sub>H(PhCH<sub>2</sub>CH<sub>2</sub>S)<sub>14</sub>(PPh<sub>3</sub>)<sub>6</sub>Cl<sub>3</sub>]: Origin of Two Chiral Nanoclusters with Triple-Helical Core from Intermediates
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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.
