Can aromatic interactions control the coordination geometry of zinc complexes? Structural evidence and a possible mechanism for the conversion of trigonal-bipyramidal to octahedral compounds

Abstract

Aromatic interactions are well known to participate in the structural stabilization of biological macromolecules. We have utilized this motif as an unconventional new means to control the stereochemistry of synthetic zinc complexes. The aromatic amino acid L-phenylalanine has been applied as a building block in the chiral ligand bpaAc–Phe–OMe (N,N-bis(2-picolyl)aminoacyl-(S)-phenylalanine-methylester). Zinc(II) complexes of this ligand are always trigonal–bipyramidal in the solid state. This is confirmed by the X-ray structures of [(bpaAc–Phe–OMe)Zn(OTf)]+ (1b, OTf = Triflate), [(bpaAc–Phe–OMe)Zn(H2O)]2+ (2b), and [(bpaAc–Phe–OMe)Zn(pz)]2+ (3b). In contrast, the octahedral complexes [(bpaAc–Gly–OEt)Zn(pz)(OTf)]+ (3a) and [(bpaAc–Gly–OEt)Zn(N-Meim)(H2O)]2+ (4a) have been obtained with the related glycine derived ligand bpaAc–Gly–OEt. A comparison with the five-coordinate structures of [(bpaAc–Gly–OEt)Zn(OTf)]+ (1a) and [(bpaAc–Gly–OEt)Zn(Cl)]+ (5) allows an appreciation of effects brought about by (1) the coligand (chloride, triflate, pyrazole, N-methylimidazole) and (2) the non-coordinating amino acid side chain (benzyl in bpaAc–Phe–OMe). It is shown that the pyrazole complex 3b adopts an unusual tense five-coordinate geometry which is stabilized by weak aromatic interactions. The structure represents an analogue of the possible transition state between five-coordinate trigonal-bipyramidal and six-coordinate octahedral complexes in our series.