Photocrystallographic studies of metastable states of transition metal complexes with small ligands In previous work we discovered photoinduced metastable linkage isomers of transition metal nitrosyl complexes in which the nitrosyl is bonded through the oxygen atom (η¹, isonitrosyl) or through both the N and O atoms in a side-on (η²) configuration^1,2, and established that nitrosyl linkage isomerism also takes place in biologically relevant five- and six-coordinate iron porphyrins^3,4.

Photoinduced linkage isomerization is not limited to diatomic ligands. We identified metastable η²-S,O-side-bound linkage isomers of several Ru-SO2 complexes, the isomerization being accompanied by a pronounced color change of the crystals (Fig. 1). The SO2 linkage isomers are stable to higher temperatures than the corresponding NO complexes⁵. In the search for ambient-temperature stability of the linkage isomers, required for their practical application as high-density storage media, we plan to extend the photocrystallographic work to linkage isomers of dimethylsulfoxide6 by synthesis of a series of simple inorganic complexes with this ligand and following characterization by spectroscopic, calorimetric and photocrystallographic techniques. As a second topic, we plan to explore the double linkage isomerism of complexes with both NO and NO2 ligands, for which spectroscopic evidence was recently obtained4. The spectroscopic work involves collaboration with Prof. Kim Bagley of Buffalo State College, where the photoinduced vibrational bands of the new species and their decay will be studied by low-temperature IR techniques. Synthesis, DSC measurements of the light-irradiated species and combined laser/diffraction experiments will be done at SUNY/Buffalo. The work is very well suited for training of undergraduates, as it involves a variety of experiments of relatively short duration.

References:

1. Coppens, P., D. V. Fomitchev, M. D. Carducci and K. Culp (1998). "Crystallography of Molecular Excited States: Transition Metal Nitrosyl Complexes and the Study of Transient Species." J. Chem. Soc., Dalton Trans. 6: 865-872.
2. Coppens, P., I. Novozhilova and A. Y. Kovalevsky (2002). "Photoinduced Linkage Isomers of Transition Metal Nitrosyl Porphyrins." Chem. Rev. 102: 861-884.
3. Cheng, L., I. Novozhilova, C. Kim, A. Kovalevsky, K. A. Bagley, P. Coppens and G. B. Richter-Addo (2000). "First Observation of Photoinduced Nitrosyl Linkage Isomers of Iron Nitrosyl Porphyrins." J. Am. Chem. Soc. 122: 7142-7143.
4. Lee, J., A. Y. Kovalevsky, I. V. Novozhilova, K. A. Bagley, P. Coppens and G. B. Richter-Addo (2004). "Single- and Double-Linkage Isomerism in a Six-Coordinate Iron Porphyrin Containing Nitrosyl and Nitro Ligands." J. Am. Chem. Soc. 126: 7180-7181.
5. Kovalevsky, A. Y., K. A. Bagley and P. Coppens (2002). "The First Photocrystallographic Evidence for Light-Induced Metastable Linkage Isomers of Ruthenium Sulfur Dioxide Complexes." J. Am. Chem. Soc. 124: 9241-9248.
6. Rack, J. J., J. R. Winkler and H. B. Gray (2003). "Phototriggered Ru(II)-Dimethylsulfoxide Linkage Isomerization in Crystals and Films." J. Am. Chem. Soc. 123: 2432-2433.