Dirhodium(II) carboxylates as building blocks. Macrocyclic dimers with vertically stacked Rh24+ units

Reaction of Rh2(OAc)4 and H2L (=2,7-di-tert-butyl-9,9-dimethyl-4,5-xanthenedicarboxylic acid) in N,N-dimethylaniline gave the singly bridged dimer (AcO)3Rh2LRh2(OAc)3 and three doubly bridged dimers, (AcO)2Rh2L2Rh2(OAc)2, (HL)(AcO)Rh2L2Rh2(OAc)2, and (HL)(AcO)Rh2L2Rh2(OAc)(HL). Crystal structures of the last two compounds showed a macrocyclic core with a trans arrangement of bridging dicarboxylates, with one or two of the four remaining acetate ligands replaced by a bridging ligand bound through one carboxylate only. The rhodium cages are separated by 4.5 Å in the direction of the Rh–Rh axes, and offset horizontally by 2.5 Å so that a rhodium atom of one cage lies over a carboxylate oxygen of the other, with Rh⋯O distances of 2.248–2.286 Å.