Zwitterionic weak-link approach complexes based on anionic icosahedral monocarbaboranes

The anionic hemilabile phosphinothioether ligand, [1-(Ph ₂PCH₂CH₂S)-closo-1-CB₁₁H ₁₁ ]^-, which is functionalized with an anionic icosahedral monocarbaborane anion, was synthesized in three steps from [HNMe ₃][closo-CB₁₁H₁₂]. The ligand was used to synthesize a family of zwitterionic Weak-Link Approach (WLA) complexes that contain platinum(II), palladium(II), and rhodium(I). These complexes were characterized using multinuclear NMR spectroscopy, high-resolution mass spectrometry, and single-crystal X-ray diffraction analyses. Although the C-bound [closo-CB₁₁H₁₁]^- anion behaves as an electron-withdrawing moiety, hemilabile phosphinothioether ligands that are based on this unit are strongly chelating, as determined via the measurement of the chloride association constant. The chelating strength is comparable to that of hemilabile ligands that are functionalized with the very electron-rich B-bound closo-1,7-C₂B₁₀H₁₁ moiety, thus demonstrating the use of charge to influence ligand coordination strength. The anionic Rh(I) WLA complex that is synthesized using this ligand can act as the noncoordinating anion of a regular cationic Rh(I) WLA complex. Thus, an unprecedented type of salt, in which the anion and cation are mutually isostructural and isoelectronic WLA complexes, has been synthesized and characterized crystallographically.