The structural mobility of zinc complexes with tetradentate tripodal ligands derived from the amino acids glycine and phenylalanine

Crystallographic studies indicate that aromatic interactions favor the formation of trigonal-bipyramidal zinc(II) complexes with the tetradentate tripodal ligand bpaAc-Phe-OMe (N,N-bis(2-picolyl)aminoacyl-(S)-phenylalanine-methylester). The benzyl side chain in [(bpaAc-Phe-OMe)Zn (OTf)]+(1b, OTf = triflate), [(bpaAc-Phe-OMe)Zn(H2O)]2+(2b) and [(bpaAc-Phe-OMe)Zn(pz)]2+(3bt, pz = pyrazole; t = trigonal-bipyramidal) is oriented towards the axial active coordination site with the phenyl ring ca. 4.5 away from the metal center. Here we show that this conformation is retained in solutions of 1b, 2b and [(bpaAc-Phe-OMe)Zn(Cl)]+(8b). Temperature-dependent 1H NMR spectra reveal that the phenylalanine side chain is locked in its position as long as the coordination sphere stays trigonal-bipyramidal. Based on our crystallographic results we expected the pyrazole and N-methylimidazole complexes 3bt and [(bpaAc-Phe-OMe)Zn(N-Meim)]2+(4bt, N-Meim =N-methylimidazole) to be labile with respect to addition of a sixth ligand such as triflate, water, pyrazole or N-methylimidazole. 1H NMR studies confirm that the six-coordinate species 3bo and 4bo(o: octahedral) are formed in solution. This is presumably due to solvation effects and mobilizes the benzyl side chain which is flexible in octahedral complexes.