Nucleophilic additions to carbonyl ligands on tetracobalt clusters. Formation of polynuclear formyl and carbene complexes

Article abstract of DOI:10.1021/om00147a016

The tetracobalt cluster Co₄(CO)₁₀(μ₄-PPh)₂ (I) undergoes facile reductive decarbonylation to afford the bridged hydride Co₄(CO)₉(PPh)₂(μ-H)^- in quantitative yields upon the treatment with various types of borohydrides at 25°C. However, when the reductions are carried out at -78°C, a transient intermediate can be detected and its structure shown to be the corresponding formyl derivative Co₄(CO)₉(PPh)₂CHO^- (III) from its IR spectrum and ¹H and ¹³C NMR spectra. The formation of the formyl intermediate III is confirmed by deuterium-labeling studies, and its spontaneous decarbonylation to the μ-hydride is described. Nucleophilic addition to a CO ligand as a route to the formyl intermediate is demonstrated by the generation of the corresponding acyl analogues in excellent yields following the treatment of I with methyl- and n-butyllithium. The formyl and acetyl tetracobalt clusters are efficiently trapped by O-alkylation with methyl inflate to afford the carbene derivatives Co₄(CO)₉(PPh)₂[μ-C(OMe)H] and Co₄(CO)₉(PPh)₂[μ-C(OMe)Me], respectively. Nucleophilic addition to I followed by alkylation to afford these tetracobalt carbene derivatives is thus analogous to the behavior of the triosmium cluster Os₃(CO)₁₂ recently reported by Kaesz and co-workers. The acid-catalyzed 1,2-elimination of methanol from the carbene cluster Co₄(CO)₉(PPh)₂[μ-C(OMe)Me] yields the vinylidene cluster Co₄(CO)₉(PPh)₂(μ-C=CH₂), which is also produced in high yields when the acetyl cluster is treated with trifluoroacetic anhydride. The molecular structpres of Co₄(CO)₉(PPh)₂[μ-C(OMe)Me] and the vinylidene derivative are established by X-ray crystallography. Co₄(CO)₉(PPh)₂[μ-C(OMe)Me] crystallizes in the monoclinic space group P2₁/c with a = 16.465 (2) A?, b = 23.987 (3) A?, c = 15.775 (2) A?, β = 112.61 (1)°, V = 5752 A?³, and Z = 8. Co₄(CO)₉(PPh)₂(μ-C=CH₂) crystallizes in the monoclinic space group P2₁/n with a = 9.954 (1) A?, b = 12.865 (3) A?, c = 10.270 (1) A?, β = 98.88 (1)°, V = 1299 A?³, and Z = 2. Reactivity studies of the formyl and the various carbene ligands bound to the tetracobalt cluster are described.