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(Photo)ionization of tris(phenolato)iron(III) complexes - generation of phenoxyl radical as ligand
Citation key ISI:A1993ML59000022
Author Hockertz, J and Steenken, S and Wieghardt, K and Hildebrandt, P
Pages 11222-11230
Year 1993
ISSN 0002-7863
Address 1155 16TH ST, NW, WASHINGTON, DC 20036
Journal J. Am. Chem. Soc
Volume 115
Number 24
Month DEC 1
Publisher AMER CHEMICAL SOC
Abstract FeL(p) or FeL(o), the high-spin Fe(III) complexes of the hexadentate macrocyclic ligands 1,4,7-tris(3- or 5-tert-butyl-2-oxybenzyl)-1,4,7-triazacyclononane (L(o)3- or L(p)3-), which contain three o,o- or o,p-disubstituted phenolates, respectively, and which form a pseudo-octahedral environment (N3O3 donor set), have been photolyzed or chemically oxidized. The products were investigated employing (time resolved) UV/vis-detection and Mossbauer and resonance Raman (RR) spectroscopy. With light of almost-equal-to 250 nm (1B1u <– 1A1g (pi-pi*) transition of the ligand phenolates), in chlorinated hydrocarbon solvents, a monophotonic ionization takes place with quantum yields of 0.1 for FeL(o) and almost-equal-to 0.2 for FeL(p). In nonchlorinated solvents, FeL(p) and FeL(o) cannot be photoionized with almost-equal-to 250-nm light. However, with 193-nm light (1E1u <– 1A1g (pi-pi*) transition), ionization is possible also in CH3CN, but in the absence of an electron scavenger, the solvated electrons recombine rapidly with the ionized complexes FeL(o/p) inverted left perpendicular .+. By 254-nm photolysis of FeL(p) in CH2Cl2, it is possible to produce solutions containing greater-than-or-equal-to 90\% in the oxidized form Fe-than-or-equal-to p inverted left perpendicular .+. This species can also be produced by reaction with SO4.- (rate constant 3.5 X 10(9) M-1 s-1 in 3:1 (v:v) CH3CN/H2O), H2PO4., or O2.+. In dry and inert solvents and in the solid state, FeL(p) inverted left perpendicular .+ is stable at room temperature for months. Small quantities of water lead to a slow regeneration of FeL(p), accompanied by production of H+. On the basis of Mossbauer data, supported by information from UV/vis and RR, in FeL(p) inverted left perpendicular .+ the oxidation state of Fe(III) is retained; i.e., the missing electron (the hole) is located on (the phenolate part of) the ligand. RR investigations show that the electron deficiency is localized at an individual phenol moiety; i.e., it is present as a phenoxyl radical which remains coordinated to the high-spin Fe(III) core. FeL(o) and FeL(p) can be protonated in organic solvents. In the case of FeL(p), this leads to production of a dimer, Fe2(LpH)2 inverted left perpendicular 2+, in which the protons connect the monomers by forming two O...H...O hydrogen bridges between phenolates. The RR spectra of the protonated FeL(o) and FeL(p) complexes strongly resemble those of the oxidized species, indicating that protonation of a phenolate has an effect very similar to that of its one-electron oxidation and can in both cases be understood in terms of the resulting decrease in electron density at that site.
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