Multimetallic activation of molecular hydrogen, leading to hydrogenation of the coordinated azulenes in di-, tri-, and tetranuclear ruthenium carbonyl complexes

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 (P_H2 = 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.