Synthesis and crystal structures of the carbido cluster anions [Re6C(µ-CO)(CO)18]2– and [Re6(µ-H)C(µ-CO)(CO)18]–

Abstract

The reaction of the carbido cluster anion [Re₇C(CO)₂₁]^3– with I₂, in MeCN, under CO, allows the removal of the Re(CO)₃⁺ capping moiety, affording the octahedral cluster dianion [Re₆C(µ-CO)(CO)₁₈]^2– almost quantitatively. The anion, characterized by single-crystal X-ray analysis as its [PPh₄]⁺ salt, has an idealized C_s symmetry and consists of a carbon-centred metal octahedron surrounded by 18 terminal (three for each metal atoms) and one doubly bridging carbonyl ligand. The same species is obtained by dissolving the monoanion [Re₇C(CO)₂₂]^– in MeCN. Variable-temperature ¹³C NMR spectra indicate that the bridging carbonyl, in solution, is completely delocalized on the cluster edges and interchanges with the terminal carbonyls. The ion [Re₆C(µ-CO)(CO)₁₈]^2– can be reversibly protonated, affording [Re₆(µ-H)C(µ-CO)(CO)₁₈]^–, characterized by single-crystal X-ray analysis as its [NEt₄]⁺ salt (CH₂Cl₂ clathrate). The carbonyl connectivity pattern resembles that of the parent anion, although the presence of the H ligand in a doubly briding location (detected by the atom-pair potential method) lowers the symmetry to C₁. The ion [Re₆C(µ-CO)(CO)₁₈]^2– is also able to react with Group 11 cations, as Ag⁺, Cu⁺ or Au(PPh₃)⁺, giving adducts which can be considered intermediate between molecular complexes and ion pairs, on the basis of the ν(CO) frequencies and of the ¹³C NMR spectrum of the silver derivative.