Reaction of 1,3,4,5-tetramethylimidazole 2-ylidene with cyclopentadienyl cobalt carbonyl complexes : characterization of a novel dinuclear carbene adduct

The synthesis and characterization of an unusual dinuclear oganometallic complex derived from Cp₃Co₃(CO)₂ 1 is presented. Reaction of 1 with 1,3,4,5-tetramethyl- imidazole-2-ylidene 2 was anticipated to form a trinuclear carbene adduct. The actual product arises from fragmentation of the trinuclear cluster 1 to form the dinuclear monocarbene adduct Cp₂CO₂(CO)₂(C₇H₁₂N₂) 3. The x-ray crystal structure of 3 has been determined and an ORTEP illustration of the structure in the solid state is shown below.

The x-ray crystal structure of the adduct was solved (space group P-1; a = 8.8600 Å, b = 9.0374 Å, c = 12.1181 Å; α = 92.300; β = 103.979; γ = 109.514; 2403 reflections; R = 0.0220). As the illustration shows, one atom of the imidazole ring, the carbene carbon is coordinated to just one of the cobalt atoms at a distance of 1.899(5) Å. The tilt of the ligand towards the other cobalt atom is approximately 10° such that this center is 2.788 (5) Å away. The Co-Co bond (2.432(1) Å) is asymmetrically bridged by the two carbonyl ligands. The average Cco-Co distances are of 1.955 Å (C_CO-Co₁) and 1.602 Å (C_CO-Co₂). The dinuclear adduct 3 can also be made by adding the carbene to Cp₂Co₂(CO)₂. The complex is also formed by fragmentation of electronically saturated Cp₃Co₃(CO)₃ and Cp₄Co₄(CO)₂ in the presence of 2. The imidazole carbene adduct decomposes at room and low temperatures in solution. One of the decomposition products, CpCo(CO)(C₇H₁₂N₂) 4, will be discussed briefly.