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

Abstract

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 ¹H and ¹³C 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 P2₁2₁2₁, a= 6.197(1), b= 12.772(1), c= 13.717(1)Å, Z= 4; 1b crystallizes in the monoclinic space group P2₁, a= 9.532(2), b= 6.371(1), c= 11.832(2)Å, β= 104.89(1)°, Z= 2; 1c crystallizes in the triclinic space group P, a= 8.015(2), b= 9.297(1), c= 9.401(2)Å, α= 80.47(1), β= 71.33(2), γ= 79.78(2)°, Z= 2. Preliminary solution spectroscopy of the copper(II) complexes indicates that the solution structures of a whole series of copper(II) 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.