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Cumulenylidene complexes of ruthenium: Highly delocalized organometallic chromophres


Cumulenylidene complexes (Scheme 1) constitute an intriguing class of organometallic chromophores comprising an extended cumulenic p-system with potential use in non-linear optics, molecular electronics and as catalysts for the processing of alkynes and alkenes. Their properties strongly depend on the substituents bonded to the terminal carbon atom and the electron density at the metal center. We have developed synthetic routes to novel allenylidene complexes of ruthenium that allow for previously unknown substitution patterns (Scheme 2). This allowed for a detailed anlysis of the substituent’s influence on their physical properties and the bonding.

scheme1
Scheme 1
Scheme2
Scheme2

Characteristic spectroscopic features include the intense IR stretch of the cumulenic ligand, the shift of the allenylidene carbon atoms in 13C NMR spectroscopy, the positions of the electronic transitions in the optical range and electrochemically measured redox potentials (see Figure 1). A detailed analysis reveals quantitative correlations between any of the different spectroscopic and electrochemical parameters. We are therefore able to rationally design allenylidene complexes with certain properties or to predict reliably any of these parameters if only a single one of them is known. The issue of substitutional effects on the properties of these complexes has also been addressed through derivatives incorporating electroactive substituents and ligands by monitoring the spectroscopic changes upon oxidation of these secondary redox tags.
Scheme3
Figure 1

Several amino substituted derivatives possess highly intriguing photophysical properties. Irradiation into either the HOMO-1 / LUMO or the HOMO / LUMO transitions produces intense long-lived emissions at ca. 600 and 800 nm with half-lives of up to 175 µs in frozen solution. The main emission is a rare example of an emission from a higher lying triplet state (Figure 2).
Scheme4
Figure 2

In order to identify the primary redox sites and to address the effect of electron transfer on the bonding within these systems, we have employed various combinations of electrochemical and spectroscopic techniques such as IR-, UV/Vis-Near Infrared (NIR) or ESR spectroscopy. These studies were mainly conducted under in situ conditions in home-built optically transparent thin layer electrolysis (OTTLE) cells. We have found that the oxidation occurs at the metal center while reduction involves the cumulenic ligand. This follows from the coupling patterns of the unpaired spin with other ESR active nuclei. Bonding changes within the unsaturated ligand can be monitored by IR-spectroelectrochemistry and involve a weakening of the C1-C2 bond upon oxidation and a strengthening of this bond upon reduction (Figures 3-5). All our conclusions based on experimental work were supported by quantum chemical calculations. (Cooperation with Prof. Stanislav Záli¨)

Scheme5
Figure 2: ESR Spectrum of the reduced form of above allenylidene complex

Scheme6
Figure 4: IR changes upon reduction of the above allenylidene complex under in situ conditions.

Scheme7
Figure 5: Schematic representation of the effect of electron transfer on the bonding

We have also initiated a programme aimed on studying the chemical reactivity of these systems following electron transfer. Future work in this area will be directed to utilizing the cumulated ligands for the construction of complex organometallic and organic molecules and to preparing novel allenylidene complexes with additional functionalities e. g. for applications as reporter molecules or redox active ligands.

 

References

Synthesis and properties of allenylidene complexes

R. F. Winter, F. M. Hornung,
The Aza-Cope Rearrangement in Transition Metal Complexes: Construction of an Unsaturated C7-Ligand from Butadiyne and an Allylic Amine
Organometallics 1997, 16, 4248.

R. F. Winter
The First Thioallenylidene Complexes from Ruthenium-Butatrienylidene Intermediates
Eur. J. Inorg. Chem. 1999, 2121.

R. F. Winter, K.-W. Klinkhammer, S. Záli¨
Ruthenium-Aminoallenylidene Complexes from Butatrienylidene Intermediates via Aza-Cope Rearrangement: Synthetical, Spectroscopic, Electrochemical, Spectroelectrochemical and Computational Studies
Organometallics 2001, 20, 1317.

R. F. Winter, S. Hartmann, S. Záli¨
Elektronentransfer von heteroatomsubstituierten Allenylidenkomplexen und seine Konsequenzen
in: Elektronenübertragung in Chemie und Biologie, Hrsgb. J. Russow, H. J. Schäfer, GDCh-Monographie Bd. 23, 2001, GDCh, Frankfurt/Main.

R. F. Winter, S. Hartmann, S. Záli¨, K.-W. Klinkhammer
Aminoallenylidene Complexes of Ruthenium(II) from the Regioselective Addition of Secondary Amines to Butatrienylidene Intermediates. A Combined Experimental and Theoretical Study of the Hindered Rotation around the CN-Bond
Dalton Trans. im Druck.

R. C. Harbort, S. Hartmann, R. F. Winter, K. W. Klinkhammer
Five-membered 5-methylene-4,5-dihydro heterocycles from ruthenium butatrienylidene intermediates and 2-dimethylaminomethyl substituted furans, thiophenes and selenophenes
Organometallics, im Druck.

R. F. Winter
Rutheniumkomplexe mit hoch ungesättigten C3- und C4-Liganden aus Diacetylen
(Habilitationsschrift), Verlag Grauer, Beuren, Stuttgart, 2002.


Photophysical Properties of allenylidene complexes

J. van Slageren, R. Winter, A. Klein, S. Hartmann
Long-Lived Higher Excited State Luminescence from New Ruthenium(II) Allenylidene Complexes
J. Organomet. Chem. 2003, 670, 137.

 

Allenylidene Complexes with redox active entities

R. F. Winter
A Ru-allenylidene complex with an appended redox-active substituent: spectroscopic characterization of three oxidation states
Chem. Commun. 1998, 2209.

S. Hartmann, R. F. Winter, T. Scheiring, M. Wanner
Allylferrocenylselenide and the synthesis of the first seleno-substituted allenylidene complex: Synthesis, spectroscopy, electrochemistry and the effect of electron transfer from the ferrocenyl subunit
J. Organomet. Chem. 2001, 637-639, 240.

S. Hartmann, R. F.Winter, B. M. Brunner, B. Sarkar, A. Knödler, I. Hartenbach
Ruthenium Allenylidene Complexes with Redox-Active Substituents and Ligands
Eur. J. Inorg. Chem., 2003, 876.






     
     
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