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Vinyl ruthenium and osmium complexes

as components of

molecular wires, molecular switches and electroswitchible dyes


General assets of the vinyl complexes

Our general interest in redox-active, π-conjugated metal-organic chromophores has recently focused on vinyl ruthenium and osmium complexes. These are rather unique in that their anodic redox chemistry is strongly influenced or even dominated by the vinyl ligand. Experimental evidence for the strong ligand contribution to the oxidation processes comes from EPR spectra of their radical cations (Figure 1) and from the unusually small blue shift of the metal-CO stretch upon oxidation. The CO band shift systematically decreases with increasing π-conjugation within the vinyl ligand. Results from quantum chemical calculations parallel our experimental observations and pinpoint the non-innocent character of vinyl ligands (Figure 2).

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Figure 1. EPR spectrum of Ru(CO)Cl(PiPr3)2(CH=CHPh)•+ at room temperature (left) and at 77K (right).

Vibrationally structured low-energy absorption bands in the electronic spectra of the associated radical cations are also more characteristic of an organic radical cation than of an oxidized ruthenium complex (Figure 3).

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Figure 2. Calculated metal and vinyl ligand contributions to the HOMO orbital and experimental CO band shift upon one-electron oxidation.
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Figure 3. Spectroscopic changes in the UV/Vis/NIR upon oxidation of a pyrenylvinyl complex.

Divinylphenylene bridged diruthenium complexes thus resemble organometallic phenylene vinylenes. The extent of electron delocalization within divinylphenylene bridged diruthenium complexes depends surprisingly little on their topology. The 1,4- and 1,3 isomers show nearly identical splitting of half-wave potentials in cyclic voltammetry and full electron delocalization on the EPR timescale of about 10-9 s (Figure 4). The presence of just one CO IR band at the radical cation state indicates that the 1,4-isomers retain full electron delocalization even on the faster IR timescale of 10-12 s. Monooxidized 1,3-isomers, on the other hand, are valence trapped on the IR timescale as follows from the observation of two CO bands.

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Figure 4. Voltammograms of 1,4- and 1,3-isomers of divinylphenylene bridged diruthenium complexes and solution EPR spectra of their associated radical cations.

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Figure 5. IR spectra of isomeric neutral and monooxidized divinylphenylene bridged diruthenium complexes.


Towards vinyl-metal based molecular wires

Delocalized frontier orbitals, low oxidation potentials and reasonable stabilities of their oxidized forms make vinyl ruthenium complexes favourable building blocks for hole transporting, molecule based materials that relate to oligo- and polyphenylene vinylenes (PPVs). Vinylphenylene oligomers and polymers are amongst the most actively investigated functional polymers owing to their robustness and their electric conductivity in oxidized or reduced states. The transition from individual dinuclear complexes to a bulk material requires that a large number of individual and per se conducting repeat units are aligned in a fashion that preserves electric conductance across the entire system or a larger segment thereof. The coordinative unsaturation of five-coordinated, 16 valence electron vinyl complexes allows for addition a further donor ligand. Once this ligand bears a terminal alkyne function, the free alkyne group can be used to generate a further ruthenium vinyl bond upon hydroruthenation with an appropriate hydride ruthenium complex. In this manner we have prepared isomeric tetraruthenium complexes with a more electron rich divinylphenylene diruthenium core and less electron rich vinylpyridine moieties at the tips (Figure 6).

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Figure 6. Divinylphenylene and vinylpyridine bridged tetraruthenium complexes.

These systems show the same degree of electron delocalization across the central divinylphenylene diruthenium part of the molecule as their dinuclear precursors. The outer vinylpyridine appended moieties are, however electronically decoupled from the inner part of the molecule because the vinylpridine based orbitals are too low in energy compared to the central divinylphenylene diruthenium unit (Figure 7). We are presently evaluating different bridging ligands as for theor ability to provide efficient electronic coupling of the bridged vinyl metal sites.



Figure 7. Calculated HOMOs of the divinylphenylene/vinylpyridine bridged tetraruthenium complexes.

Electrochromism and cross-conjugated systems

Electrochromism means that a system can be switched between states which strongly differ by their absorbance in a particular spectral window. Such systems may, inter alia, be used as effect dyes. The intense low-energy electronic absorptions of some of the radical cations make some vinyl complexes electrochromic dyes. Tetrakis(styryl)ethene bridged tetraruthenium complexes undergo two sequential two-electron oxidations and can reversibly be switched between three different states at mild potentials. The associated dication displays intense absorption bands that cover the entire low energy part of the visible and the near infrared (NIR) while the neutral is transparent in this regime (Figure 8, left). The higher oxidized tetracation is another 5
strongly absorbing species with peaks at higher energies as the dication but lower ones as the neutral (Figure 8, right). Each of the three possible states (neutral, dication and tetracation) thus has distinctive absorption properties in the visible and in the near infrared.



Figure 8. Spectroscopic changes upon the oxidation of a tetra(styryl)ethane bridged tetraruthenium complex from the neutral to the dication (left) and from the dication to the tetracation (right).
π-Conjugation in tetrastyrylethene-bridged tetraruthenium complexes may occur by three different pathways - diagonal, lateral or geminal conjugation


(Figure 9) - and there has been a lot of debate which of them is more efficient. The dication of the above complex has two different CO bands. This means that only one of the three possible pathways of electron delocalization is effective on the fast IR timescale of 1 to 10 ps. We are presently trying to answer the question as to which of the three pathways is the more efficient one.



Figure 9. Delocalization pathways in cross-conjugated tetrastyrylethene-bridged tetraruthenium complexes.

Photoswitchable vinyl complexes

Azobenzenes constitute one of the most popular classes of optically addressable switches in laser physics. We are presently exploring how incorporation of vinyl metal moieties into modifies the properties of the parent organic systems. To these ends we have prepared various complexes that differ with respect to the number of coligands (five or six) and thus the valence electron count, and with respect to the electronic nature of the sixths ligand (weak or a strong π-acceptor).
The pure trans-isomers of the ground state can be excited to a photostationary mixture of cis and trans isomers by irradiation with a 460 nm LED lamp (collaboration with Bernhard Dick). The respective percentage of the cis-isomer depends on the coordination number at the metal and the electronic nature of the coligand. We are presently investigating the rates of the dark, thermal back-isomerization to the trans ground state and comparing them to their organic precursors (Figure 10).

   

Figure 10. Spectroscopic changes upon 1s irradiation of the pure trans-isomer with a 460 nm lamp and thermal back-reaction of the photostationary mixture of cis and trans isomers.

The configuration around the N=N double bond influences the electronic properties of distyryldiazenes. X-ray structural investigations and quantum chemical calculations confirm that the trans-isomer constitutes a fully delocalized system whose HOMO extends over the entire π-conjugated bridge and the terminal ruthenium moieties. In the cis-isomer, however, the individual styryl ruthenium subunits are electronically decoupled from each others. This is of possible relevance for the photomodulation of current flow over a molecule bridged gap between two solid electrodes.

Our publications in this field:
• J. Maurer, R. F. Winter, B. Sarkar, J. Fiedler, S. Záli¨, “Bridge dominated oxidation of a diruthenium 1,3-divinylphenylene complex”, Chem. Comm., 2004, 1900.
• J. Maurer, R. F. Winter, B. Sarkar, S. Záli¨, “Electron Delocalization in Mixed-Valent Butadienedienediyl-Bridged Diruthenium Complexes”, J. Solid State Electrochem., 2005, 9, 738.
• J. Maurer, B. Sarkar, B. Schwederski, W. Kaim, R. F. Winter, S. Záli¨, “Divinylphenylene Bridged Diruthenium Complexes Bearing Ru(CO)Cl(PiPr3)2 Entities”, Organometallics 2006, 25, 3701.
• J. Maurer, B. Sarkar, W. Kaim, R. F. Winter, S. Záli¨, “Towards New organometallic Wires? Tetraruthenium Complexes Bridged by Phenylenevinylene and Vinylpyridine Ligands”, Chem. Eur. J., 2007, 13, 10257-10272.
• J. Maurer, M. Linseis, B. Sarkar, B. Schwederski, M. Niemeyer, W. Kaim, S. Záli¨, C. Anson, M. Zabel, R. F. Winter, „Ruthenium Complexes with Vinyl, Styryl, and Vinylpyrenyl Ligands: A Case of Non-Innocence in Organometallic Chemistry“J. Am. Chem. Soc. 2008, 130, 259.
• M. Linseis, R. F. Winter, B. Sarkar, W. Kaim, S. Záli¨, “A tetranuclear complex of a tetradonor-substituted olefin: non-innocence resulting in multi-step electrochromic behaviour”, Organometallics 2008, 27, 3321.
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