The strength of hydrogen bonding to metal-bound ligands can contribute to changes in the redox behaviour of metal centres

Abstract

A series of nine tripodal tetradentate ligands based on tris(pyridyl-2-methyl)amine TPA with hydrogen bond donors R in one, two and three of the pyridine 6-positions (R = NH₂ amino, L^Am-1,2,3; NHCH₂^tBu neopentylamino, L^Np-1,2,3; NHCO^tBu pivaloylamido, L^Piv-1,2,3) and TPA are used to investigate the effect of different hydrogen bonding microenvironments on electrochemical properties of their LCuCl complexes. The hydrogen bond donors are rigidly preorganised and suitably oriented for intramolecular N–H⋯Cl–Cu hydrogen bonds. Cyclic voltammetry studies show that the reduction potential of the Cu(II)/Cu(I) couple as a function of the ligand follows the order TPA < L^Am-n < or ∼ L^Np-n < L^Piv-n, and that the magnitude of the effect increases with the number of hydrogen bonding groups. These trends could be explained in terms of the steric and electronic effects exerted by these groups stabilising the Cu(I) oxidation state. In fact, the X-ray structure of the air-stable [(L^Piv-3)Cu^ICl] complex is reported and shows elongated Cu–N and Cu–Cl bonds, presumably due to the combination of steric and electron withdrawing effects exerted by the three pivaloylamido groups. We reasoned that the strength of hydrogen bonding in the Cu(I) and Cu(II) oxidation states could differ and therefore contribute also to the aforementioned redox changes; this hypothesis is tested using IR and NMR spectroscopy. IR studies of the [(L^Piv-1,2,3)Cu^ICl] and [(L^Piv-1,2,3)Cu^IICl]⁺ complexes in acetonitrile show that the intramolecular N–H⋯Cl–Cu hydrogen bonding weakens in the order L^Piv-1 > L^Piv-2 > L^Piv-3, and that it is stronger in the Cu(I) complexes. The ¹H NMR spectra of the [(L^Piv1,2,3)Cu^ICl] complexes are in complete agreement with the IR data, and reveal that the stability of the Cu(I) complexes to oxidation in air increases in the order L^Piv-1 < L^Piv-2 ≪ L^Piv-3. The hydrogen bonds in the Cu(I) complexes are stronger because of the higher electron density on the Cl ligand, when compared to the Cu(II) complexes. This is consistent with ab initio MP2 calculations performed on the complexes [(L^Piv-3)Cu^ICl] and [(L^Piv-3)Cu^IICl]⁺. Thus, the electron density of a metal-bound ligand acting as hydrogen bond acceptor is revealed as the major factor in determining the strength of the hydrogen bonds formed. From the IR data the energies of the N–H⋯Cl–Cu hydrogen bonds is estimated, as is the contribution of changes in hydrogen bond strength with the oxidation state of the copper centre and number of interactions to stabilising the Cu(I) state. Some of the implications of this result in dioxygen activation chemistry are discussed.