Intramolecular electron transfer

In chemistry, intramolecular electron transfer is a redox reaction that takes place via a covalent link (bridge) between oxidizing and reducing reactants. Because this requires the formation of bridged intermediates, intramolecular reactions only occur with complex compounds with small ligands. In intramolecular electron transfer (ISET), a ligand bridges two metal redox centers during the electron transfer process, in which one metal center as a donor donates an electron to the other metal center, which in turn acts as an acceptor. ISET is rare in biological systems where the redox centers are often shielded by misshapen proteins . It is usually used to describe reactions that take place in complexes of transition metals .

Scheme

• Bridging: ${\displaystyle \mathrm {{D^{\star }}+[X{-}A]\longrightarrow {[D^{\star }}{-}X{-}A]} }$

• Electron transfer: ${\displaystyle \mathrm {[{D^{\star }}{-}X{-}A]\longrightarrow [D{-}X{-}{A^{\star }]}} }$

• Product: ${\displaystyle \mathrm {[D{-}X{-}{A^{\star }]}\longrightarrow [D{-}X]+A^{\star }} }$

(Explanation: in this case, the asterisk should not describe the excited state, but the migrating electron, a point would be too difficult to see here.)

The bridging complex must form before the electron transfer takes place , and this process is often very reversible. Intramolecular electron transfer takes place via the ligand bridge as soon as it has formed. In some cases a stable bridged structure exists in the ground state, in other cases the bridging structure can be a transient intermediate or a transition state during the reaction. Intramolecular electron transfer proceeds according to the donor-bridge-acceptor scheme. The bridge may also be a molecular wire ( molecular wire form).

Suitability of complexes for electron transfer

Mixed valence substances contain an element that exists in more than one oxidation state: Well-known examples of this are the Creutz-Taube complex, Berlin blue and molybdenum blue. Many solids with mixed valences are indium-sulfur compounds. Mixed valences occur in organic metal complexes.

They can be divided into three groups according to the Robin Day classification:

Examples

class	Complex compound
I.	Lead (II, IV) oxide and antimony tetroxide
II	Berlin blue
III	Creutz Taube Complex

• I: There is almost no interaction between the reaction centers. The complex shows the same properties as the isolated metal centers.

• II: Weak electronic interactions change the properties of the two redox centers. A low activation energy is required to induce electron transfer between the metal centers across the ligand bridge. The electron transfer takes place via the cyanide bridge between the iron (II) and iron (III) reaction center.

• III: Strong interaction between the two identical reaction centers of the mixed-valent complex, which differs in its properties from those of the isolated metal ions. The metallic reaction centers are surrounded by the same ligands and the bridging ligand is well suited for intramolecular electron transfer, has a conjugated system and is easy to reduce. Intramolecular electron transfer was first found in the Creutz-Taube complex .

Deaf experiment

Henry Taube was awarded the Nobel Prize in Chemistry in 1983 for his discovery of the intramolecular mechanism . The publication can be summarized in the following equation:

${\displaystyle \mathrm {[CoCl(NH_{3})_{5}]^{2+}\ +\ [Cr(H_{2}O)_{6}]^{2+}\longrightarrow [Co(NH_{3})_{5}(H_{2}O)]^{2+}\ +\ [CrCl(H_{2}O)_{5}]^{2+}} }$

It is important that chloride is originally bound to cobalt (the oxidant), then to chromium (in the +3 oxidation state), which then forms an inert bond with the ligand. The observation proves the intermediate stage of a bimetal complex:

${\displaystyle \mathrm {[Co(NH_{3})(\mu -Cl)Cr(H_{2}O)_{5}]^{4+}} }$

where μ-Cl means that the chlorine ion bridges Cr and Co atoms by serving as a ligand for both. This chloride ion is used for the flow of electrons from Cr (II) to Co (III) with the formation of Cr (III) and Co (II).

A complex that serves as a model for investigating intramolecular electron transfer is the Creutz-Taube complex :

${\displaystyle \mathrm {[(NH_{3})_{5}RuNC_{4}H_{4}Ru(NH_{3})_{5}]^{5+}} }$

It contains two ruthenium cations with mixed valences.

There are also organic substances with mixed valences such as B. Tetrathiafulvalene and the radical cation of N , N , N ', N ' -Tetramethyl- p -phenylenediamine.

literature

• Bruce S. Brunschwig, Carol Creutz, Norman Sutin: Optical transitions of symmetrical mixed-valence systems in the Class II – III transition regime . In: Chemical Society Reviews . tape 31 , no. 3 , May 7, 2002, pp. 168-184 , doi : 10.1039 / B008034I .

Individual evidence

1. ↑ Entry on inner-sphere electron transfer . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.I03052 Version: 2.3.3.
2. ↑ Luisa G. Heinz, Oleksandr Yushchenko, Markus Neuburger, Eric Vauthey, Oliver S. Wenger: Tetramethoxybenzene is a Good Building Block for Molecular Wires: Insights from Photoinduced Electron Transfer . In: Journal of Physical Chemistry A . tape 119 , no. 22 , June 4, 2015, p. 5676-5684 , doi : 10.1021 / acs.jpca.5b03649 .
3. ↑ Mélina Gilbert, Bo Albinsson: Photoinduced charge and energy transfer in molecular wires . In: Chemical Society Reviews . tape 44 , no. 4 , February 10, 2015, p. 845-862 , doi : 10.1039 / C4CS00221K .
4. ↑ Mirco Natali, Sebastiano Campagna, Franco Scandola: Photoinduced electron transfer across molecular bridges: electron and hole transfer superexchange pathways . In: Chemical Society Reviews . tape 43 , no. 12 , May 27, 2014, p. 4005-4018 , doi : 10.1039 / C3CS60463B .
5. ↑ Melvin B. Robin, Peter Day: Mixed Valence Chemistry: Advances in Inorganic Chemistry and Radiochemistry . Vol. 10, 1967, pp. 247-422 ( limited preview in Google book search).
6. ^ Henry Taube, Howard Myers, Ronald L. Rich: The mechanism of Electron Transfer in Solution . In: Journal of the American Chemical Society . tape 75 , no. August 16 , 1953, p. 4118-4119 , doi : 10.1021 / ja01112a546 .
7. ↑ Jihane Hankache, Oliver S. Wenger: Organic Mixed Valence . In: Chemical Reviews . tape 111 , no. 8 , 10 August 2011, p. 5138-5178 , doi : 10.1021 / cr100441k .