Heavy metal complexes bearing mixed oxidation states: characterization & applications

ABU DARDAM.Sc.(polymer science and technology)

• Introduction• Classification• Conventional mixed valent complexes• Unconventional mixed valent complexes• High nuclearity complexes• Ruthenium versus Osmium• Conclusion and Application• References

Contents

• Mixed valence complexes are the complexes containing two or more metal atoms in variable oxidation states.

• extensively studied correlated with molecular and crystal structure.

• ranging from the smallest ionic molecule H2+ with a hydrogen

oxidation state of 0.5, to coordination polymers

• Eg- Prussian blue, Fe4[Fe(CN)6]3.

Introduction

• Naturally occurring mixed valent entities are evident from geosphere to biosphere as magnetite (Fe3O4) ore and [2Fe-2S]+ center proteins.

• Ruthenium and Osmium, the heavier homologue of iron have also been of great significance for the field of mixed valence chemistry owning to the thermodynamic and kinetic stability of the neighboring oxidation states and their complexes may give information about the process of electron exchange in bimolecular redox reactions.

Contd….

ClassificationRobin and Day classified mixed valent system in 3 classes :

Class INo M-M interaction.

Class IISmall but non-zero M-M interaction

Class IIIVery high M-M interaction; activationless e- transfer between equivalent metal centers.

• The variable oxidation states achieved by the metal centers via electron transfer reaction can lie under two systems.

• These two systems based on mixed oxidation states can be viewed as…

Contd…

Conventionalsystem

Unconventional system

• Usually comprises of dinuclear mixed valent complexes having most common oxidation states of the metal, ie RuII and RuIII [Lx RuIII (m-BL) RuII Lx]

• Here the bridging ligands (BL) and ancillary ligands (L) are innocent ligands.

Conventional system

• Have involvement of unusual oxidation states of the metal.• most unusual oxidation state of ruthenium is +IV (d4) i.e. RuIII/RuIV mixed valence is seen.• Requires strongly electron donating ligands such as oxides, acetylacetonate, cyanamides• bridging ligand involves non innocent ligand which are redox active.

Unconventional system

In fig 4- glyoxalbis(2-hydroxyanil) (gbha-2) act as a bidentate chelating ligand with [Ru(acac)2]+n

Example,

Combinations involving oxidation states lower than the classical +III/+II , such as

Ruthenium(I) typically require stabilization through π-acceptor ligands

• Number of higher nuclearity complexes have been reported with common RuIII/RuII pair involved.

High nuclearity complexes

Trinuclear Tetranuclear

Hexanuclear

OsIII is prefered over strongly π-donating OsII.

Kc value tend to be higher for acceptor bridged system but lower for donar bridged compounds.

Os complex give more complex optical spectra owing to spin-orbit enhanced intensities of normally forbidden transitions.

Large spin orbit coupling of OsIII (3000 cm-1) versus RuIII (1200 cm-1) leads to increased g-factor anisotropy in EPR spectra.

Heavy Metal effects : Ru vS Os

Study of a mixed valence complex of Ruthenium via various spectroscopic techniques.

Study of a mixed valence complex of Ruthenium via various spectroscopic techniques.

[Cp* RuII(indacene)RuII Cp*]

[Cp* RuII(indacene)RuIIICp*]+1

[Cp* RuIII(indacene)RuIII Cp*]+2

.

study the mixed valence system by electrochemical and spectroscopic techniques for its characterization.

Study of electron transfer reaction

Cyclic Voltametry

Electronic spectra

Electron paramagnetic

resonance

Crystallography

Characterisation techniques

Information about electrochemical redox potential.

Cyclic Voltametry

The UV-Vis-NIR spectra show presence of intervalence band, between 680 and 1150 nm, with λmax at 898 and 901 nm.

Electronic Spectra

Mixed valent complexes are recognized by intervalence charge transfer absorption band.

The EPR spectra of [{Cp*Ru}2Ic]+1 shows significantly reduced g-tensor anisotropy of Δg=|gII-g┴|= 0.28.

Electronic paramagnetic resonance

Since most mixed valent systems are paramagnetic EPR measurements can be used to determine physical oxidation states.

Crystallography examines the arrangement of atoms in a crystal. It depends on the analysis of the diffraction patterns of a sample targeted.

Crystallography

For mixed valent systems two possibilities can be attained

• Valence trapped system (combination of two distinct metal centers)

• Valence detraped system (having equivalent metal centers)

As suggested by the characterization techniques the mixed valent complexes show strong electronic communication between metal sites.

Hence mixed valence system is a delocalized one, i.e. belonging to Class III of the robin and Day classification.

Conclusion

To study intramolecular electron transfer between equivalent metal sites in a stable coordination compound.

electron transfer between organized functional atoms or molecules is an essential process of molecular electronics.

Concept of molecular computing, with binary information being coded in the molecular configuration; quantum-dot cellular automata were proposed on smaller or large mixed valent systems.

Application

Kaim, W.; Lahiri, G.K. Angew Chem..Int. Ed 2007

Carey, D.; Morales-Verdejo, C.; Abril, D.; Admas, C.; Molins, E. Pohyhedron..29 (2010)

F. Albert Cotton. Advanced inorganic Chemistry.

References