MULTIMETALLIC ACTIVATION OF MOLECULAR-HYDROGEN, LEADING TO HYDROGENATION OF THE COORDINATED AZULENES IN DINUCLEAR, TRINUCLEAR, AND TETRANUCLEAR RUTHENIUM CARBONYL-COMPLEXES

Hydrogenation of di-, tri-, and tetranuclear ruthenium carbonyl comple xes bearing guaiazulene or 4,6,8-trimethylazulene as the bridging liga nd to bind the multimetallic framework was studied: [mu(2) :eta-guaiaz ulene)Ru-2(CO)(5) (1a)], [mu(2):eta-4, 6,8-trimethylazulene)Ru-2 (CO)( 5) (1b)], [mu(3):eta-guaiazulene)Ru-3 (CO)(7) (2a)], [mu(3):eta-4,6,8- trimethylazulene)Ru-3 (CO)(7) (2b)], [mu(3):eta-guaiazulene)Ru-4(CO)(9 ) (3a)], and [(mu(3):eta-4,6,8-trimethylazulene)Ru-4(CO)(9) (3b)]. Rea ctions of these di-, tri-, and tetraruthenium complexes with dihydroge n (P-H2 = 5-10 atm) at 100 degrees C resulted in cluster fragmentation and addition of five hydrogen atoms to the azulene ligands to form mo nonuclear ruthenium carbonyl hydride compounds, [(eta(5)-pentahydrogua iazulenyl)RuH(CO)(2) (4a)] or [(eta(5)-pentahydrotrimethylazulenyl)RuH (CO)(2) (4b)]. Despite potential formation of several stereoisomers de pendent on the addition modes of hydrogen atoms, only one isomer of 4a or 4b was obtained in the hydrogenation. The crystal structure of a d erivative of 4a revealed that the addition of hydrogen atoms occurred from the face of the azulene Ligand originally bonded with the rutheni um species. Hydrogenation of the di-, tri-, and tetranuclear ruthenium complexes below 100 degrees C revealed that only the triruthenium com pounds reacted with H-2 at 50 degrees C via triruthenium dihydride int ermediates: [(mu(2):eta-tetrahydroguaiazulene)Ru3H2(CO)(7) (6a)] or (m u(2):eta-tetrahydrotrimethylazulene)Ru3H2(CO)(7) (6b)]; this indicates that there exists a reaction pathway to achieve facile activation of dihydrogen by the triruthenium clusters.