Self-Assembly of Cobaloximes and Rhodoximes with 3-Aminophenylboronic Acid: A Molecular Triangle and a Polymer

Abstract

The reactions of methylaquacobaloxime and ethylaquacobaloxime with 3-aminophenylboronic acid in water produce supramolecular triangles 3a and 3b respectively, through the reaction of boron with the oxime oxygens of the alkylcobaloxime and the coordination of pyridine to cobalt. The reaction is reversible and strongly pH dependent. The complexes 3a and 3b precipitate immediately from the solution at pH 7, but dissolve quickly at both acidic (below 4) and basic (above 9) pH values. The aggregates are re-formed by readjusting the pH to neutrality and the precipitation-dissolution cycle may be repeated several times without an apparent decrease in yield. The complexes were fully characterised by 1H and 13C NMR spectroscopy. The X-ray structure shows that the neutral chiral supramolecule 3a is a triangle of cobalt atoms with an approximate C3 symmetry. The side of the triangle is about 9.5 A. The ring closure is ensured by the torsion angles around the B−C and C−NH2 bonds, which vary in the range 25−30° and 97−109°, respectively. The reaction of methylaquarhodoxime with 3-aminophenylboronic acid in water at pH 7 results in the displacement of H2O by 3-aminophenylboronic acid only. The polymeric form 4 precipitates from CD3OD in the NMR tube with time. The X-ray structure shows that 4 is a polymeric species with a zig-zag chain arrangement, with Rh···Rh distances of 9.2 A. The chain arrangement is characterized by torsion angles around the B−C and C-NH2 bonds of 61° and 92°, respectively. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)