Solid State and Solution Characterization of Chiral, Conformationally Mobile Tripodal Ligands

Abstract

The synthesis of the ligands N,N-bis[(2-pyridyl)methyl]-1-(2-pyridyl)ethanamine (1, α-MeTPA), N,N-bis[(6-phenyl-2-pyridyl)methyl]-1-(2-pyridyl)ethanamine (2, α-MePh2TPA), N,N-bis[(2-quinolyl)methyl]-1-(2-pyridyl)ethanamine, (3, α-MeBQPA), and N,N-bis[(2-pyridyl)]methyl(phenyl)(2-pyridyl)methanamine (4, α-PhTPA) is described. The ligands form chiral, pseudo C3-symmetric complexes with ZnII and CuII salts that possess an available electrophilic coordination site. X-ray crystallographic structures of the complexes [Zn(α-MeTPA)Cl](ClO4), [Zn(α-MeBQPA)Cl](ClO4), and [Zn(α-PhTPA)Cl](ClO4) show that the spatial arrangement of the three pyridyl groups resembles a propeller whose directional sense is controlled by a substituent on one methylene arm. Chiroptical measurements provide supporting information that the complexes maintain similar structures in solution. Monte Carlo/stochastic dynamics (MC/SD) simulations of the [Zn(L)Cl]+ complexes indicate that only two conformers are populated at normal temperatures, suggest the presence of a synchronicity in the movement of the heteroaromatic rings during conformer interconversions, and provide an estimate of the energy difference between the conformers.