Semiquinone ligands

It can be seen that inside the octahedral coordination of the Ir(III) centre an unusual peroxide bond forms between the terminal oxygen atom (04) of coordinated dioxygen and the carbon atom (C7) of the semiquinone ligand. In fact, the 0-0 distance of the oxygen molecule (1.47 A) is within the typical bond distance of peroxide coordination (1.49 A). 

Metal Complexes Containing Catechol and Semiquinone ligands. 41 331... 

It was noted [229] that magnetic exchange between radical semiquinone ligands results in the near diamagnetism of M(DBSQ)2 (M = Pd, Pt). 

Transition metal complexes containing catecholate and semiquinone ligands have been reviewed by Pierpont and Buchanan.336 The syntheses of catecholate adducts of platinum group metals by oxidative addition of benzoquinone (BQ) have been reported (reaction 92).337 The coordinated catechol can then be oxidized to the semiquinone with silver ion (reaction 93). In reactions (92) and (93), L M = lr(Cl)(CO)(PPh3)2] and the benzoquinones (BQ) are shown in (146). The oxidative addition of phenanthrenequinone to [Ir(Cl)(CO)(PPh3)2] has also been cited.338... 

Pierpont, Conlandt G. and Lange, Christopher W., The Chemistry of Transition Metal Complexes Containing Catechol and Semiquinone Ligands. 

Scheme 2 Redox active ligand participation in catalytic water oxidation by a binuclear Ru catalyst containing ortho-quinone/semiquinone ligands...

Synthesis and molecular structure of two bis(zinc-dioxolene) complexes have been also described.186 The [ Zn(TpCum,Me) 2(L-H2)] derivative in Fig. 3.39 has two protonated catecholate ligands, whereas [ Zn(TpCum,Me) 2(L-H)] has one protonated catecholate and one semiquinone ligand. 

The complexes [Co(CO)2 LnCp]+ undergo radical-radical coupling and substitution reactions rather similar to those of [Fe(CO)3(PPh3)2] + (Section II,E). The o-quinone derivatives [Co(0-0)LCp] + (e.g., 0-0 = o-chloranil, L = PCys) (361) and the rhodium analogs (363) show ESR spectra consistent with the single electron confined mainly to the semiquinone ligand. 

The reaction of [Ru3(CO)i2] with 3,5-/crt-butyl-l,2-benzoquinone yields two isomers of [Ru4(CO)8 /<3-02C6H2(Bu ) 2], that differ in the arrangement of metal atoms (types 12 and 13) all contain /r3-semiquinone ligands coordinated via terminal and bridging oxygen atoms and a ring. The two isomers are not... 

On coordination to metals, the quinone-CO stretching frequency shifts at least 200-300cm", which infers a formal reduction to catechol with oxidation of the metal. The possibility of an intermediate radical anion coordination mode for o-benzoquinones has been demonstrated by the reversible chemical and electrochemical oxidation of reduced quinone complexes. ESR spectra of [Ir(Cl)(l,2-02QCl4)(C0)(PR3)2] (PR3 = PPhj, PMePh2) indicate that the unpaired electron is localized in the semiquinone ligand. The quinone coordination mode is stron y affected by the oxidizing ability of the quinone and the nature of the complex itself. 

Metal Complexes Containing Catechol and Semiquinone Ligands. 

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