A Surprising “Folded-In” Conformation of a Self-Assembled Arsenic-Thiolate Macrocycle

The synthesis and crystal structures of two new macrocyclic compounds, As2(L2,5)2Cl2 and As2(L2,3)2Cl2, are described, where L2,3 and L2,5 are dithiolate ligands. For each compound, several diastereomeric isomers are possible (anti-“aligned”, syn-“aligned”, anti-“unaligned”, syn-“unaligned”), as shown by density functional theory (DFT) calculations, but only one is crystallized out of solution. For As2(L2,5)2Cl2, only the anti-aligned isomer crystallizes, as expected based on steric arguments and backed up by DFT calculations. Surprisingly, for As2(L2,3)2Cl2, an anti isomer crystallizes in which one As−Cl unit has folded into the macrocyclic cavity. While this goes against steric arguments and DFT calculations, this conformation best fills the space within the cavity, avoiding the entropic cost of a large void.