From μ3- to μ- agostic methyl coordination: NMR and solid state study of donor ligands uptake by the triangular cluster anion [Re3(μ-H)3(μ3-CH3)(CO)9]-

[Re3(μ-H)3(μ3-CH3)(CO)9]- (1) reacts with CO at 273 K to quantitatively give the [Re3(μ-H)3(μ-CH3)(CO)10]- derivative (2) through the conversion of the methyl group from triply to doubly-bridging coordination. An NMR isotopic perturbation experiment demonstrates the unsymmetrical bridging of the methyl group on the Re-Re bond, and a fast exchange, in solution, between one agostic and two terminal CH bonds. This is in agreement with X-Ray diffraction analysis performed on single crystals of 2. In solution at room temperature 2 decomposes with CH4 elimination, which is also observed when 2 is treated with CO affording the [Re3(μ-H)2(CO)12]- anion. The opening of the triple bridging methyl coordination in 1 occurs also with other different L ligands, yielding to the syn and anti isomers of unsymmetrically bridged methyl complexes of general formula [Re3(μ-H)3(μ-CH3)(CO)9(L)]- (L = MeCN, PMePh2, THF). The kinetic and thermodynamic stabilities of these isomers vary with the nucleophilicity of the ligand and the polarity of the solvent. In particular, the strong nucleophile PMePh2 affords the quantitative conversion of anion 1 into the thermodynamic syn isomer, which is stable in THF at room temperature for several days. On the contrary, weak nucleophiles, such as MeCN or THF, give equilibrium mixtures containing residual amounts of anion 1. The adducts with MeCN are stable in solution at room temperature for several hours, while decompose fastly in neat MeCN affording mainly the cluster anion [Re3(μ-H)4(CO)9(MeCN)]-. This suggests that cluster stabilization after methane evolution follows a different path with respect to that observed in the reaction of 1 with CO.