The structural mobility of zinc complexes with tetradentate tripodal ligands derived from the amino acids glycine and phenylalanineElectronic supplementary information (ESI) available: Fig. S1: 1H NMR spectra (CDCl3 solutions) of the bpaAc–Phe–OMe complexes 1b/2b, 3bo, 5b and 8b at room temperature. See http://www.rsc.org/suppdata/dt/b2/b212049f/

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+ (3btt, 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 3btt and [(bpaAc–Phe–OMe)Zn(N-Meim)]2+ (4bttt, 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 3boo and 4booo (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.