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Molecular Mechanics (MM3*) Parameters for

Ruthenium(II)-Polypyridyl Complexes.

Peter Koralla, Björn Åkermarka, Per-Ola Norrbyb,* 

a) Department of Chemistry, Organic Chemistry, Royal Institute of Technology, S - 100 44  Stockholm, Sweden 

b) Department of Medicinal Chemistry, Royal Danish School of Pharmacy, Universitetsparken 2,  DK 2100 Copenhagen, Denmark 


Ru(II)-polypyridyl complexes are frequently utilized as photosensitizers and photoelectron donors. This is due to a unique combination of the chemical stability and the redox and excited state properties of these complexes.1 Polypyridyl complexes of Ru(II) have been used in a wide variety of applications. This includes use as photosensitizers or photoelectron donors in diads and triads, and for example use in metallodendrimers, use in chloride selective receptors, and in rotaxanes. They have also been utilized in metal assisted self-assembly of polypeptides and in DNA mediated electron transfer as DNA metallointercalators. 
Several x-ray determinations of structures have been reported for Ru(II)-polypyridine complexes. With this structural data as a basis, together with a DFT frequency calculation on a smaller model system, we have developed a molecular mechanics force field (MM3*). This force field will allow extensive studies of new or existent, but less thoroughly characterized, complexes.  

Most photoinduced processes are strongly distance dependent and most commonly also dependent on the geometries of the photoactivated species. Comprehensive knowledge about geometries and conformational energies will be of outmost importance in order to achieve a better understanding of the photophysics and photochemistry of these systems. This force field will hopefully lead to an improved design of supramolecular devices and may also be utilized in combination with molecular orbital calculations to predict properties of photosenitizers. 

Conformationally flexible systems, composed of photosensitizers covalently linked to electron donors or acceptors are an important subject for this force field. Understanding the conformational thermodynamics in these systems will be a valuable piece of information in the analysis of the photochemistry. For an example see figure 1.

Figure 1. The global minimum of a Ru(II)trisbipyridyl complex linked to a tyrosine. Mimicking the electron transfer from tyrosine to P680+ in PSII .2

The parametrization have been performed using automated routines recently developed.3 The parameter refinement has been performed using various least-squares optimization techniques, including Simplex and Newton-Raphson with numerically determined derivatives, the latter allowing an accurate assessment of the convergence. (See figure 2.) Two different types of substructure parameter sets have been developed. The second set has an additional parameter, treating the repulsive transeffect appearing in terpyidine complexes.  

Figure 2. An overlap of an optimized structure with an x-ray structure.4

1. Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85.  
2. Sun, L.; Magnusson, A.; Berglund, H.; Korall, P.; Styring, S.; Åkermark, B.; Hammarström, L. (to be submitted)  
3. Norrby, P.-O.; Liljefors, T. submitted.  
4. Ashby, M.T.;Govindan, G.N.; Grafton, A.K. Inorg.Chem., 32, 3803,1993.