Synthesis, characterization, and reactivity of dicationic dihydrogen complexes of osmium and ruthenium

The dicationic complexes [Os(H-2)(PR3)(2)(bpy)(CO)](2+) [PR3 = PPh3, PMePh2 (2a, b)], [Os(H-2)(PPh3)(2)(phen)(CO)](2+) (2c), and [Ru(H-2)(PPh3)(2)(bpy)(CO)](2+) (4) (bpy = 2, 2'-bipyridine; phen = 1, 10-phenanthroline) have been prepared by the protonation of the corresponding monocationic hydrides using an excess of trifluoromethanesulfonic acid. The presence of a bound dihydrogen ligand is indicated by short T-1 minimum values consistent with H-H distances of 0.92-1.01 Angstrom. For the partially deuterated derivatives, J(HD) values of 25.1-31.0 Hz were observed. The dicationic complexes are strong acids, indicating that the bound Hz is substantially activated toward heterolytic cleavage. The Hz ligand is tightly bound to the metal center and does not undergo exchange with D-2 over the course of several weeks. The complex [Os(H-2)(PPh3)(2)(bpy)(CO)](2+) (2a) has been shown to be very stable in solution at room temperature. In contrast, the ruthenium analogue, [Ru(H-2)(PPh3)(2)(bpy)(CO)](2+) (4), decomposes in solution at room temperature but if relatively stable at temperatures less than 245 K.