Template syntheses of chiral tetradentate ligands derived fromL-amino acids. Structural and spectroscopic characterization of the free ligands and of their copper(II) complexes

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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.

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