TEMPLATE SYNTHESES OF CHIRAL TETRADENTATE LIGANDS DERIVED FROM L-AMINO-ACIDS - STRUCTURAL AND SPECTROSCOPIC CHARACTERIZATION OF THE FREE LIGANDS AND OF THEIR COPPER(II) COMPLEXES

The copper(II)-directed condensation of amino acids with formaldehyde and nitroethane in basic methanol produces open-chain tetradentate ligands with a pendant nitro substituent in high yield. With optically pure L-amino acids full retention of configuration is achieved, and with racemic D/L-amino acids the condensation is fully selective, viz. only one out of three possible isomers is produced. These aspects are discussed based on H-1 and C-13 NMR data for the free ligands based on glycine, L-alanine, and D/L-alanine, and on the X-ray structures of the corresponding copper(II) template products, (5-methyl-5-nitro-3,7-diazanonanedioato)copper(II) 1a, [(2S,8S)-2,5,8-trimethyl-5-nitro-3,7-diazanonanedioato]copper(II) hydrate 1b and [(2R,8S)-5-methyl-5-nitro-3,7-diazanonanedioato)copper(II)] 1c respectively. Complex 1a crystallizes in the orthorhombic space group P2(1)2(1)2(1), a = 6.197(1), b = 12.772(1), c = 13.717(1) angstrom, Z = 4; 1b crystallizes in the monoclinic space group P2(1), a = 9.532(2), b = 6.371(1), c = 11.832(2) angstrom, beta = 104.89(1)-degrees, Z = 2; 1c crystallizes in the triclinic space group P1BAR, a = 8.015(2), b = 9.297(1), c = 9.401(2) angstrom, alpha = 80.47(1), beta = 71.33(2), gamma = 79.78(2)-degrees, Z = 2. Preliminary solution spectroscopy of the copper(ll) complexes indicates that the solution structures of a whole series of copper(ll) condensation products with amino acids are similar to the reported X-ray structures, and all are similar to each other except for 1a which in solution is predominantly dimeric.