Arsenic-, Antimony- and Bismuth-Oxygen Rings

Cycloarsoxanes (RAsO) were discovered 140 years ago and their oligomeric nature in solution was established in the 1930s.Subsequent NMR studies showed that the trimer n = 3) and tetramer n = 4) predominate in solution for alkyl derivatives (R = Me, Et), although smaller amounts of the dimer n = 2) and pentamer n = 5) are also present. 

Like their phosphorus(III) analogues, cycloarsoxanes (RAsO) undergo metal-mediated ring expansion to produce flexible macrocyclic ligands n = 5, 6, 8) that 

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