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Trinuclear nickel complexes with triplesalen ligands: Simultaneous occurrence of mixed valence and valence tautomerism in the oxidized species
Citation key ISI:000230629900034
Author Glaser, T and Heidemeler, M and Frohlich, R and Hildebrandt, P and Bothe, E and Bill, E
Pages 5467-5482
Year 2005
ISSN 0020-1669
DOI 10.1021/ic050268x
Address 1155 16TH ST, NW, WASHINGTON, DC 20036 USA
Journal Inorg. Chem.
Volume 44
Number 15
Month JUL 25
Publisher AMER CHEMICAL SOC
Abstract The coordination chemistries of the triple tetradentate triplesalen ligands H(6)talen, H(6)talen(t-Bu2), and H(6)talen(NO2) 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)Ni-3(II)], [(talen(t-Bu2))Ni-3(II)], and [(talen(NO2))Ni-3(II)] have been determined by single-crystal X-ray diffraction, All three compounds are composed of neutral trinuclear complexes with square-planar coordinated Ni-II ions in a salen-like coordination environment. Whereas the overall molecular structure of [(talen(NO2))Ni-II3] is nearly planar, the structures of [(talen)Ni-II3] and [(talen(t-Bu2))Ni-3(II)] are bowl-shaped as a result of ligand folding. The strongest ligand folding occurs at the central nickel-phenolate bond of [(talen(t-Bu2))Ni-3(II)], 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 7, 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)Ni-3(II)] is irreversibly oxidized at 0.32 V versus ferrocenium/ferrocene (Fc(+)/ Fc), whereas [(talen(t-Bu2))Ni-3(II)] and [(talen(NO2))Ni-3(II)] exhibit reversible oxidations at 0.22 V versus Fc(+)/Fc and 0.52 V versus Fc(+)/Fc, respectively. The oxidized species [(talen(t-Bu2))Ni-3](+) and [(talen(NO2))Ni-3](+) undergo a valence-tautomeric transformation involving a Ni-II 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 Ni-II species is discussed.
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