Dirhodium(II) carboxylates as building blocks. Macrocyclic dimers with vertically stacked Rh-2(4+) units

Reaction of Rh-2(OAc)(4) and H2L (=2,7-di- tert-butyl-9,9-dimethyl-4,5-xant henedicarboxylic acid) in N,N-dimethylaniline gave the singly bridged dimer (AcO)(3)Rh2LRh2(OAc)(3) and three doubly bridged dimers, (AcO)(2)Rh2L2Rh2( 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 tran s arrangement of bridging dicarboxylates, with one or two of the four remai ning acetate ligands replaced by a bridging ligand bound through one carbox ylate only. The rhodium cages are separated by 4.5 Angstrom in the directio n of the Rh-Rh axes, and offset horizontally by 2.5 Angstrom so that a rhod ium atom of one cage lies over a carboxylate oxygen of the other, with Rh . . .O distances of 2.248-2.286 Angstrom.