Synthesis and Structural Characterization of Phosphine–Borane-Stabilized Dicarbanions with either Rigid or Flexible Spacers

The reaction between 1,2-C₆H₄(CH₂Cl)₂ and 2 equiv of in situ generated [R₂P­(BH₃)]Li in THF gives the corresponding <i>o</i>-phenylene-bridged bis­(phosphine–boranes) 1,2-C₆H₄{CH₂P­(BH₃)­R₂}₂ (R = <i>i</i>Pr (<b>1a</b>), Ph (<b>2a</b>), Cy (<b>3a</b>)). Treatment of <b>1a</b>–<b>3a</b> with 2 equiv of <i>n</i>BuLi and 2 equiv of tmeda yields the corresponding phosphine–borane-stabilized carbanion (PBC) complexes [1,2-C₆H₄{CHP­(BH₃)­R₂}₂]­[Li­(tmeda)]₂·<i>n</i>L (R = <i>i</i>Pr, <i>n</i> = 0 (<b>1b</b>); R = Ph, <i>n</i>L = THF (<b>2b</b>); R = Cy, <i>n</i>L = 2PhCH₃ (<b>3b</b>)). In contrast, treatment of <b>1a</b> with 2 equiv of MeK, followed by 2 equiv of pmdeta, yields the monodeprotonation product [1,2-C₆H₄{CHP­(BH₃)<i>i</i>Pr₂}­{CH₂P­(BH₃)<i>i</i>Pr₂}]­[K­(pmdeta)] (<b>1c</b>), due to a competing side reaction with the solvent. Treatment of <b>1a</b> and <b>3a</b> with the less aggressive metalating agent PhCH₂K gives the corresponding dipotassium salts, the latter of which was isolated as the adduct [1,2-C₆H₄{CHP­(BH₃)­Cy₂}₂]­[K­(pmdeta)]₂ (<b>3c</b>). X-ray crystallography reveals that <b>1b</b>–<b>3b</b> adopt similar structures in which the lithium ions are coordinated by the carbanion centers and the borane hydrogen atoms of the phosphine–borane-stabilized carbanions. The potassium ion in <b>1c</b> is coordinated by the carbanion center and by B–H···K contacts with both borane groups, whereas the two potassium ions in <b>3c</b> exhibit multihapto interactions with the aromatic ring of the PBC ligand, along with B–H···K contacts. The reaction between ClSiMe₂CH₂CH₂SiMe₂Cl and 2 equiv of in situ generated [R₂P­(BH₃)­CH₂]Li gives the bis­(phosphine–boranes) [CH₂SiMe₂CH₂P­(BH₃)­R₂]₂ (R = Me (<b>4a</b>), Ph (<b>5a</b>)). Treatment of <b>4a</b> or <b>5a</b> with 2 equiv of <i>n</i>BuLi in THF readily yields the 1,6-dicarbanion complexes [CH₂SiMe₂CHP­(BH₃)­R₂]₂[Li­(THF)₂]₂ (R = Me (<b>4b</b>), Ph (<b>5b</b>)). A similar reaction of <b>5a</b>, 2 equiv of PhCH₂K, and 2 equiv of pmdeta in THF gives the potassium complex [CH₂SiMe₂CHP­(BH₃)­Ph₂]₂[K­(pmdeta)]₂ (<b>5c</b>). Complex <b>5b</b> adopts a linear structure in the solid state, while <b>5c</b> adopts an unusual polycyclic structure by virtue of bridging K···H–B–H···K contacts