STEREOSELECTIVITY IN CHIRAL FE(III) AND GA(III) TRIS(CATECHOLATE) COMPLEXES EFFECTED BY NONBONDED, WEAKLY POLAR INTERACTIONS

A series of Fe(III) and Ga(III) tris(catecholate) complexes of chiral terephthalamide and catecholamide ligands has been investigated in aqueous solution with respect to their stereoselective formation of the DELTA or LAMBDA coordination isomers. Stereoselectivity is effected by nonbonded, weakly polar interactions between the aryl and methyl substituents of the chiral moieties peripheral to the metal center. The diastereomeric distributions (DELTA:LAMBDA) in the complexes were studied in solution using circular dichroism (CD) spectroscopy and H-1 NMR spectroscopy. With the symmetric bidentate ligand N,N'-bis((S)-alpha-methylbenzyl)-2,3-dihydroxyterephthalamide (S-1), the Fe(III) and Ga(III) complexes, S-1Fe and S-1Ga, exist purely in a LAMBDA conformation in aqueous solution. The isomorphic crystal structures of LAMBDA-S-1Fe and LAMBDA-S-1Ga allow the weakly polar interactions responsible for the stereoselectivity to be identified. In addition, aryl/cation interactions between the tetramethylammonium cations and phenyl rings in the complex anions are observed in the crystal structures of LAMBDA-S-1Fe(Me4N3) and LAMBDA-S-1Ga(Me4N)3. The relevance of the aryl/cation interactions to the biological binding of acetylcholine is discussed. Crystal data for S-1Fe(Me4N)3.3MeOH: space group I23, a = 27.330(8) angstrom, V = 20414(13) angstrom3, Z = 8, R(R(w)) = 0.075(0.077). Crystal data for S-1Ga(Me4N)3.3MeOH: space group I23, a = 27.259(6) angstrom, V = 20255(12) angstrom3, Z = 8, R (R(w)) = 0.078 (0.076).