Steric and Hydrogen-Bonding Effects on the Stability of Copper Complexes with Small Molecules

A series of the copper(II) complexes with tripodal tetradentate tris(pyridyl 2-methyl)amine-based ligands possessing
the hydrogen-bonding 6-aminopyridine units (tapa, three amino groups; bapa, two amino groups; mapa, one amino
group) have been synthesized, and their copper(II) complexes with a small molecule such as dioxygen and azide
have been studied spectroscopically and structurally. The reaction of their Cu(II) complexes with NaN3 have given
the mononuclear copper complexes with azide in an end-on mode, [Cu(tapa)(N3)]ClO4 (1a), [Cu(bapa)(N3)]ClO4
(2a), [Cu(mapa)(N3)]ClO4 (3a), and [Cu(tpa)(N3)]ClO4 (4a) (tpa, no amino group). The crystal structures have revealed
that the coordination geometries around the metal centers are almost a trigonal-bipyramidal rather than a square-planar except for 1a with an intermediate between them. The UV−vis and ESR spectral data indicate that the
increase of NH2 groups of ligands causes the structural change from trigonal-bipyramidal to square-pyramidal
geometry, which is regulated by a combination of steric repulsion and hydrogen bond. The steric repulsion of
amino groups with the azide nitrogen gives rise to elongation of the Cu−Npy bonds, which leads to the positive shift
of the redox potentials of the complexes. The hydrogen bonds between the coordinated azide and amino nitrogens
(2.84−3.05 Å) contribute clearly to the fixation of azide. The Cu(I) complexes with bapa and mapa ligands have
been obtained as a precipitate, although that with tapa was not isolated. The reactions of the Cu(I) complexes with
dioxygen in MeOH at −75 °C have given the trans-μ-1,2 peroxo dinuclear Cu(II) complexes formulated as
[{(tapa)Cu}2(O2)]2+ (1c), [{(bapa)Cu}2(O2)]2+ (2c), and [{(mapa)Cu}2(O2)]2+ (3c), whose characterizations were
confirmed by UV−vis, ESR, and resonance Raman spectroscopies. UV−vis spectra of 1c, 2c, and 3c exhibited
intense bands assignable to π*(O22-)-to-d(Cu) charge transfer (CT) transitions at λmax/nm (ε/M-1cm-1) = 449
(4620), 474 (6860), and 500 (9680), respectively. The series of the peroxo adducts generated was ESR silent. The
resonance Raman spectra exhibited the enhanced features assignable to two stretching vibrations ν(16O−16O/18O−18O)/cm-1 and ν(Cu−16O/Cu−18O)/cm-1 at 853/807 (1c), 858/812 (2c), 847/800 (3c), and at 547/522 (2c), 544/518
(3c), respectively. The thermal stability of the peroxo-copper species has increased with increase in the number of
the hydrogen-bonding interactions between the peroxide and amino groups.