Coordination Chemistry ofo‐Semiquinones

Abstract

o ‐Semiquinone (SQ) radicals are ideally suited to chelate with transition metal ions. Ligand paramagnetism results in spin‐coupling interactions with paramagnetic metal ions, and complexes containing multiple SQ ligands have magnetic properties resulting from inter‐ligand coupling. The unpaired SQ electron resides in a ligand‐localized π orbital close in energy with the valence d orbitals of many transition metal ions. Electron transfer between ligand and metal may be observed under equilibrium conditions by monitoring the temperature‐dependence of spectral and magnetic properties in a process known as valence tautomerism. The redox potentials of common SQ ligands overlap with the redox potentials of metal ions contributing to redox series that may involve reversible electron transfer steps at both the metal and SQ ligands. Multielectron redox reactions involving SQ ligands are useful in small molecule transformations that involve substrate oxidation or reduction over a narrow range in electrochemical potential. These properties are unique to complexes containing ligands capable of changing their charged state by either intramolecular electron transfer within the complex molecule or by an exchange of charge with an external redox component. Reactions and processes of this type will be the focus of this review on the coordination chemistry of o ‐semiquinonate ligands.

Charge and Spin-State Characterization of Cobalt Bis(o-dioxolene) Valence Tautomers Using Co Kβ X-ray Emission and L-Edge X-ray Absorption Spectroscopies

“…In order to explain the metal–ligand magnetic exchange interaction based on a hs-Co II center in valence tautomers at high- T , two alternate scenarios are possible. In the first and most probable scenario, the magnetic exchange interaction between cobalt and semiquinones is weak, on the order of a few millielectronvolts, such that the unpaired electrons on the ligands are weakly coupled to and oriented randomly with respect to the cobalt spin vector, an idea that was proposed by Pierpont and co-workers. − With a small-energy-level spacing between the S = 1 / 2 , 3 / 2 , 5 / 2 electronic states, all spin configurations will be thermally populated at room temperature. Alternatively, the high- T state can be described as having a strong antiferromagnetic interaction between the spins on the semiquinone ligands.…”

Macrocyclic Chromium(III) Catecholate Complexes

“…Redox-active ligands have been of recent interest due to their ability to serve as an electron reservoir for transition metal catalysts. , Catecholate and its derivatives are the classic example of such ligands, which may exist in three valence states depending on the coordination environment: catecholate (Cat), semiquinonate (SQ), and benzoquinone (BQ) (Scheme ). , Cat and SQ are known to readily coordinate to 3d transition metals, while BQ complexes remain elusive in the literature . Cat/SQ complexes containing Mn, Fe, and Cu have been examined for various biological applications, especially modeling the chemistry of catechol dioxygenases. − Co Cat/SQ complexes exhibit valence tautomerism, , making them a strong candidate for molecular switches and spin-crossover materials. , Metal–organic frameworks bridged by chloranilate (dianion of 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone) exhibited exceptionally high magnetic ordering temperature (M = Fe) and expedient metal transport (M = Mn) …”

“…3,4 Cat and SQ are known to readily coordinate to 3d transition metals, while BQ complexes remain elusive in the literature. 5 Cat/SQ complexes containing Mn, Fe, and Cu have been examined for various biological applications, 6 especially modeling the chemistry of catechol dioxygenases. 7−9 Co Cat/ SQ complexes exhibit valence tautomerism, 10,11 making them a strong candidate for molecular switches and spin-crossover materials.…”

Síntese e caracterização de complexos de rutênio com ligantes fenólicos

“…Os metais das séries 4d e 5d dos grupos 7 e 8, rênio, rutênio e ósmio, chamam atenção por causa do seu variável comportamento redox, diversos autores têm estudado extensivamente complexos rutênio-dioxoleno [15][16][17] .…”

“…O composto foi caracterizado por voltametria cíclica, espectroscopia de absorção na região do infravermelho e uv-visível. .L), estes dados estão condizentes com a literatura [16] , sendo atribuídas à transferência de carga ligante-metal (TCLM). As bandas em 354 nm e 422 nm foram atribuídas à transferência do ligante dppb( ) Ru(III) (eg) e a banda em 534 nm a transferência Cl(p ) Ru(III) (eg) [23] .…”