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

Abstract

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⁺ (3b^tt, pz = pyrazole; t = trigonal-bipyramidal) is oriented towards the axial active coordination site with the phenyl ring ca. 4.5 Å 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 3b^tt and [(bpaAc–Phe–OMe)Zn(N-Meim)]²⁺ (4b^ttt, 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 3b^oo and 4b^ooo (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.