Trinuclear Nickel Complexes with Triplesalen Ligands: Simultaneous Occurrence of Mixed Valence and Valence Tautomerism in the Oxidized Species

The coordination chemistries of the triple tetradentate triplesalen ligands H6talen, H6talent-Bu2, and H6talenNO2 have
been investigated with nickel(II). These triplesalen ligands provide three salen-like coordination environments bridged
in a meta-phenylene arrangement by a phloroglucinol backbone. The structures of the complexes [(talen)NiII3],
[(talent-Bu2)NiII3], and [(talenNO2)NiII3] have been determined by single-crystal X-ray diffraction. All three compounds
are composed of neutral trinuclear complexes with square-planar coordinated NiII ions in a salen-like coordination
environment. Whereas the overall molecular structure of [(talenNO2)NiII3] is nearly planar, the structures of [(talen)NiII3] and [(talent-Bu2)NiII3] are bowl-shaped as a result of ligand folding. The strongest ligand folding occurs at the
central nickel−phenolate bond of [(talent-Bu2)NiII3], resulting in the formation of a chiral hemispherical pocket. The
dependence of the physical properties by the substituents on the terminal phenolates has been studied by FTIR,
resonance Raman, UV−vis−NIR absorption, and electrochemistry. The three nickel−salen subunits are electronically
interacting via the π system of the bridging phloroglucinol backbone. The strength of this interaction is mediated
by two opposing effects:  the electron density at the terminal phenolates and the folding of the ligand at the central
phenolates. The parent complex [(talen)NiII3] is irreversibly oxidized at 0.32 V versus ferrocenium/ferrocene (Fc+/Fc), whereas [(talent-Bu2)NiII3] and [(talenNO2)NiII3] exhibit reversible oxidations at 0.22 V versus Fc+/Fc and 0.52 V
versus Fc+/Fc, respectively. The oxidized species [(talent-Bu2)Ni3]+ and [(talenNO2)Ni3]+ undergo a valence-tautomeric
transformation involving a NiIII and a phenoxyl radical species, as observed by EPR spectroscopy. Thus, these
oxidized forms exhibit the phenomena of valence tautomerism and mixed valence simultaneously. The extent of
delocalization of the radical species and of the NiIII species is discussed.