High nuclearity ruthenium carbonyl cluster chemistry III. Synthesis of [Ru₁₀(μ,-H)(μ₆-C)(CO)₂₄]^-, its reactivity towards triphenylphosphine and ligand dynamics of the resulting decaruthenium anionic clusters

Thermolysis of Ru₃(μ-H)(μ-NC₅H₄)(CO)₁₀ in refluxing chlorobenzene affords [Ru₂(μ-H)(μ-NC₅H₄)₂(CO)₄(NC₅H₅)₂][Ru₁₀(μ-H)(μ₆-C)(CO)₂₄] (1a) in excellent yield. The results of a structural study of 1a are consistent with the hydride occupying an edge-bridging site at an apex of the "giant tetrahedron". Thermolysis of Ru₃(μ-H)(μ-NC₅H₄)(CO)₁₀ with [PPh₄][BF₄] in chlorobenzene affords the cluster [PPh₄][Ru₁₀(μ-H)(μ₆-C)(CO)₂₄] (1b) in moderate yield. Compound 1a reacts immediately with 1 equiv. of triphenylphosphine at room temperature to afford the monosubstituted cluster [Ru₂(μ-H)(μ-NC₅H₄)₂(CO)₄(NC₅H₅)₂][Ru₁₀(μ-H)(μ₆-C)(CO)₂₃(PPh₃)] (2a) in high yield; the crystal structure of 2a and NMR studies show that ligand substitution occurs at the apical ruthenium associated with the hydride ligand. Further reaction with triphenylphosphine at room temperature affords successively the complexes [Ru₁₀(μ-H)(μ₆-C)(CO)_24-x(PPh₃)_x]^- with x = 2 (3a) and 3 (4a); the tetrasubstituted cluster (x = 4, 5c) is obtained as its [Ru₂(μ-H)(μ-NC₅H₄)₂(CO)₄(PPh₃)₂]⁺ salt following a short reflux in acetone. In each case, substitution occurs at the apical ruthenium atoms. Hydride and CO fluxionality in the five cluster anions was investigated by ¹³C EXSY experiments. Compound 1a exhibits complete hydride fluxionality between the four apices at low temperature, while 5c shows similar behaviour at room temperature. CO fluxionality increases with phosphine substitution, reaching a maximum at the bis-substituted cluster, and becoming less facile on the tris-and tetrakis-substituted cluster anions. Compound 1b reacts in a similar manner with triphenylphosphine to form an analogous series of complexes. Attempted metathesis of preformed 1a by reaction with [PPN]Cl was unsuccessful; instead, conversion to [PPN]₂[Ru₁₀(μ₆-C)(CO)₂₄] occurs.