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 o-phenylene-bridged bis­(phosphine–boranes)
1,2-C6H4{CH2P­(BH3)­R2}2 (R = iPr (1a),
Ph (2a), Cy (3a)). Treatment of 1a–3a with 2 equiv of nBuLi and
2 equiv of tmeda yields the corresponding phosphine–borane-stabilized
carbanion (PBC) complexes [1,2-C6H4{CHP­(BH3)­R2}2]­[Li­(tmeda)]2·nL (R = iPr, n = 0 (1b); R = Ph, nL = THF (2b);
R = Cy, nL = 2PhCH3 (3b)).
In contrast, treatment of 1a with 2 equiv of MeK, followed
by 2 equiv of pmdeta, yields the monodeprotonation product [1,2-C6H4{CHP­(BH3)iPr2}­{CH2P­(BH3)iPr2}]­[K­(pmdeta)] (1c), due to a competing side reaction
with the solvent. Treatment of 1a and 3a 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 (3c). X-ray
crystallography reveals that 1b–3b 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 1c is coordinated by
the carbanion center and by B–H···K contacts
with both borane groups, whereas the two potassium ions in 3c 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 (4a), Ph (5a)). Treatment
of 4a or 5a with 2 equiv of nBuLi in THF readily yields the 1,6-dicarbanion complexes [CH2SiMe2CHP­(BH3)­R2]2[Li­(THF)2]2 (R = Me (4b), Ph (5b)). A similar reaction of 5a, 2 equiv of PhCH2K, and 2 equiv of pmdeta in THF gives the potassium complex
[CH2SiMe2CHP­(BH3)­Ph2]2[K­(pmdeta)]2 (5c). Complex 5b adopts a linear structure in the solid state, while 5c adopts an unusual polycyclic structure by virtue of bridging K···H–B–H···K
contacts.