Hydrogenation of di-, tri-, and tetranuclear ruthenium carbonyl complexes bearing guaiazulene or 4,6,8-trimethylazulene as the bridging ligand to bind the multimetallic framework was studied: [μ2 : η-guaiazuIene)Ru2(CO)5 (1a)], [μ2 : η4,6,8-trimethyIazulene)Ru2(CO)5 (1b)], [(/μ3 : η-guaiazuIene)Ru3(CO)7 (2a)], [(μ3 : η-4,6,8-trimethyIazulene)Ru3(CO)7 (2b)], [(μ3 : η-guaiazulene)Ru4(CO)9 (3a)], and [(μ3 : η-4,6,8-trimethylazulene)Ru4(CO)9 (3b)]. Reactions of these di-, tri-, and tetraruthenium complexes with dihydrogen (PH2 = 5-10 atm) at 100 °C resulted in cluster fragmentation and addition of five hydrogen atoms to the azulene ligands to form mononuclear ruthenium carbonyl hydride compounds, [(η5-pentahydroguaiazulenyl)RuH(CO)2 (4a)] or [(η5-pentahydrotrimethylazulenyl)RuH(CO)2 (4b)]. Despite potential formation of several stereoisomers dependent on the addition modes of hydrogen atoms, only one isomer of 4a or 4b was obtained in the hydrogenation. The crystal structure of a derivative of 4a revealed that the addition of hydrogen atoms occurred from the face of the azulene ligand originally bonded with the ruthenium species. Hydrogenation of the di-, tri-, and tetranuclear ruthenium complexes below 100 °C revealed that only the triruthenium compounds reacted with HZ at 50 °C via triruthenium dihydride intermediates: [(μ2 : η-tetrahydroguaiazulene)Ru3H2(CO)7 (6a)] or [(μ2 η-tetrahy-drotrimethylazulene)Ru3H2(CO)7 (6b)]; this indicates that there exists a reaction pathway to achieve facile activation of dihydrogen by the triruthenium clusters.
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