Structures and Redox Reactivities of Copper Complexes of (2-Pyridyl)alkylamine Ligands. Effects of the Alkyl Linker Chain Length

Ligand effects on the structures and redox reactivities of copper complexes have been examined using (2-pyridyl)alkylamine derivatives as the supporting ligands, where particular attention has been focused on the effects of the
alkyl linker chain length connecting the tertiary amine nitrogen atom and the pyridine nucleus:  N−CH2−Py (Pym)
vs N−CH2CH2−Py (Pye). X-ray crystallographic analysis of the copper(I) complex of tridentate ligand PheLPym2 [N,N-di(2-pyridylmethyl)-2-phenylethylamine] (complex 1) has demonstrated that it possesses a trigonal pyramidal geometry
in which a d−π interaction with an η1-binding mode exists between the metal ion and one of the ortho carbons of
the phenyl ring of the ligand side arm (phenethyl). The result shows sharp contrast to the d−π interaction with an
η2-binding mode existing in the copper(I) complex of PheLPye2 {N,N-di[2-(2-pyridyl)ethyl]-2-phenethylamine} (complex
2). Such a d−π interaction has been shown to affect the stability of the copper(I) complex in CH2Cl2. Oxygenation
of copper(I) complex 1 supported by PheLPym2 produces a bis(μ-oxo)dicopper(III) complex, also being in sharp
contrast to the case of the copper(I) complex 2 with ligand PheLPye2, which preferentially affords a (μ-η2:η2-peroxo)dicopper(II) complex in the reaction with O2. Such an effect of the alkyl linker chain length of the metal binding site
has also been found to operate in the RSSR (disulfide)/2RS- (thiolate) redox system. Namely, ligand S2,RLPym1
(di{2-[(alkyl)(2-pyridinylmethyl)amino]ethyl} disulfide) with the methylene linker group (Pym) induced the reductive
disulfide bond cleavage in the reaction with copper(I) ion to give a bis(μ-thiolato)dicopper(II) complex, while the
ligand with the ethylene linker group (Pye), S2,BnLPye1 (di{2-[(benzyl)(2-(2-pyridinyl)ethyl)amino]ethyl} disulfide), gave
a disulfide−dicopper(I) complex. These ligand effects in the Cu2−O2 and Cu2−S2 systems have been discussed by
taking into account the difference in electron-donor ability of the pyridine nucleus between the Pym and Pye ligand
systems.