| Abstract
| - The coordination chemistries of the triple tetradentate triplesalen ligands H6talen, H6talent-Bu2, and H6talenNO2 havebeen investigated with nickel(II). These triplesalen ligands provide three salen-like coordination environments bridgedin 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 compoundsare composed of neutral trinuclear complexes with square-planar coordinated NiII ions in a salen-like coordinationenvironment. 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 thecentral nickel−phenolate bond of [(talent-Bu2)NiII3], resulting in the formation of a chiral hemispherical pocket. Thedependence 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 electronicallyinteracting via the π system of the bridging phloroglucinol backbone. The strength of this interaction is mediatedby two opposing effects: the electron density at the terminal phenolates and the folding of the ligand at the centralphenolates. 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 Vversus Fc+/Fc, respectively. The oxidized species [(talent-Bu2)Ni3]+ and [(talenNO2)Ni3]+ undergo a valence-tautomerictransformation involving a NiIII and a phenoxyl radical species, as observed by EPR spectroscopy. Thus, theseoxidized forms exhibit the phenomena of valence tautomerism and mixed valence simultaneously. The extent ofdelocalization of the radical species and of the NiIII species is discussed.
- We have prepared a series of triple tetradentate triplesalen ligands that bridge three metal ions with a salen-like coordination environment in a meta-phenylene arrangement. The trinuclear NiII complexes undergo reversible one-electron oxidations to yield the respective monocations, which are inherently mixed valence as a result of the presence of three salen−nickel subunits. Spectroelectrochemical and EPR spectroscopic investigations reveal a temperature-dependent valence tautomerism between a ligand-oxidized radical form and a metal-oxidized NiIII form.
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