Conformational dynamics of Cu(I) complexes of tripodal ligands: steric control of molecular motion

The dynamics of molecular conformational changes for Cu(I) complexes of two tripodal ligands were studied. Variable-temperature NMR and circular dichroism in combination with two-dimensional NMR experiments were employed to determine the structural and energetic details of a dynamic process in which one of three arms dissociates from coordination to Cu(I). Dissociation was triggered by addition of a strongly coordinating anion (SCN⁻). One-electron oxidation to the Cu(II) complex returned coordination of all three ligand arms. The observed phenomena were dependent upon steric crowding; addition of a single methyl group to one arm resulted in marked differences in behavior. The CuSCN and CuPF₆ complexes of tris(2-quinolylmethyl)amine (TQA) and the CuPF₆ complex of 1-(quinolin-2-yl)-N,N-bis(quinolin-2-ylmethyl)ethanamine (MeTQA) did not give any evidence for differences in coordination over the temperature range 0–35 °C, while Cu^I(MeTQA)NCS demonstrated marked differences in NMR but not CD spectra over this temperature range. In the latter complex, two diastereotopic arms displace each other with a transition energy of 14.0 kcal mol⁻¹. The structure of the arm-dissociated complex contains a higher degree of stereochemical complexity than the tetradentate complexes.