X-ray structure, reactivity, and catalytic properties of a (cyclopentadienone)ruthenium dimer, [(C₄Ph₄CO)(CO)₂Ru]₂

The title complex 4a as well as several other isostructural complexes have been prepared. Single-crystal X-ray diffraction analysis of 4a indicates a centrosymmetric dimeric structure (Figure 1); the complex crystallizes in space group P1 (No. 2): a = 10.938 (1) Å, b = 11.990 (1) Å, c = 12.528 (1) Å, α = 68.62 (1)°, β = 77.99 (1)°, γ = 64.18 (1)°, V = 1374.8 (2) ų, Z = 2 (monomer units), 4835 unique reflections, R = 0.056, 4324 reflections, F > 4σ(F). The cyclopentadienone carbonyl is bent back from the diene plane at an angle of 6.7°, with a >C=0 bond length of 1.270 (7) Å. A crossover experiment with two structurally different dimer complexes indicates that the thermal dissociation of these dimers is a relatively low-energy process. Nucleophilic ligands, L (L = CO, phosphines, amines), react readily with 4a to give mononuclear complexes of the type (η⁴-C₄Ph₄CO)Ru(CO)₂L. Treatment of 4a with either dihydrogen or ethanol leads to oxidative addition of dihydrogen to Ru with the formation of (η⁵-C₄Ph₄COH)(CO)₂RuH. The catalytic activity of 4a and other related complexes is discussed in the light of the above results.