Dirhodium(II) carboxylate complexes as building blocks. cis-Chelating dicarboxylic acids designed to bridge the dinuclear core

The syntheses and crystal structures of a series of dicarboxylic acids of general structure HO2CCR2OArOCR2CO2H (LH2) designed to span a pair of cis sites on the dirhodium(II) tetracarboxylate framework are described. Monochelate complexes LRh2(OAc)(2) and bischelate complexes L2Rh2 were prepared by heating the diacids with Rh-2(OAc)(4) in N,N-dimethylaniline. Chelates were formed under kinetic control and the best yields were obtained from diacids with methyl groups on the ether side arms and bulky substituents on the aromatic ring next to the ether links. Effective molarities for chelation were measured under thermodynamic control by H-1 NMR in tetrachloroethane. The stability of chelates and the conformational properties of diacids were investigated by molecular mechanics and quantum chemical techniques.