PHOTOCHEMISTRY OF THE CLUSTERS OS-3(CO)(10)(L) (L=2,2'-BIPYRIDINE, 2,2'-BIPYRIMIDINE, 2,3-DIPYRID-2-YLPYRAZINE, 2,3-DIPYRID-2-YLBENZOQUINOXALINE) - REVERSIBLE OPENING OF AN OS-OS BOND WITH FORMATION OF A ZWITTERION

The complexes Os-3(CO)(10)(L)(L = 2,2'bipyridine (bpy), 2,2'-bipyrimidine (bpym), 2,3-dipyrid-2-ylpyrazine (dpp), and 2,3-dipyrid-2-ylbenzoquinoxaline (dpb)) were synthesized, and their low-energy Os --> L (MLCT) transitions were characterized with resonance Raman spectroscopy. Continuous wave excitation into the MLCT transitions does not lead to product formation. However, rapid-scan TR and nano- and picosecond time-resolved absorption spectra show the formation of a transient species (for L = bpy, bpym, and dpp) which reacts back to give the parent complex. In toluene the transient has a lifetime of less than in nr and is assigned to an MLCT state of the cluster. In coordinating solvents a different transient species is observed with a lifetime that strongly depends on the coordinating ability of the solvent and the basic properties of L (200-900 ns in tetrahydrofuran (THF), 5-20 s in CH3CN). The transient species could even be obtained as a stable photoproduct by reaction in 2-MeTHF at 113 K and upon irradiation at room temperature in the presence of a halide salt, The transient species is proposed to be the zwitterion Os-(CO)(4)-Os(CO)(4)-Os+(CO)(2)(S)(L). The positive charge of the metal in the Os+(CO)(2)(S)(L) moiety is evident from the Raman and H-1-NMR spectral the coordination of the solvent from its large influence on the lifetime, and the absorption maximum of the transient intermediate, Since the zwitterion. is already formed within 50 ps after the laser pulse, it is most probably produced by reaction from the MLCT state(s). A coordinating solvent molecule may initiate this reaction by coordination to the metal center of the Os(CO)(2)(alpha-diimine) fragment in its MLCT state. The extra negative charge donated by the solvent may then cause a heterolytic splitting of an Os-Os bond with formation of thp zwitterion.