Anions of Tridentate SPS Ligands:  Syntheses, X-ray Structures and DFT Calculations

Reaction of 2,6-bis(diphenylphosphine sulfide)-3,5-diphenylphosphinine (SPS) (1) with alkyllithium reagents (MeLi, n-BuLi, Li−methyl pyridine) led to formation of the stable lithium salts [Li(THF)₂][SPS^(Me)] (2), [Li(THF)₂][SPS^(n-Bu)] (3), and [Li(THF)₂][SPS^(CH2Py)] (12) by a nucleophilic attack on the central phosphorus atom of the phosphinine ring, whereas treatment of 1 with t-BuLi led to the intermediate formation of [Li(THF)₂][SPS^(t-Bu)] (4) followed by elimination of isobutene and formation of [Li(THF)₂][SPS^(H)] (5). By addition of optically pure lithium (−)-mentholate or (−)-menthyl Grignard reagent to 1 the chiral derivatives [Li(THF)₂][SPS^(OMen)] (6) and [Li(THF)₂][SPS^(Men)] (9) were obtained, respectively. The potassium salt of the anionic SPS pincer ligand [K(THF)₂][SPS^(Me)] (10) was isolated from a salt exchange reaction of 2 with t-BuOK in diethyl ether. Coordination of BH₃ to the central phosphorus atom in compound [K([18]crown-6)THF)][SPS^(Me)·BH₃] (11) confirmed the phosphine character of the hypervalent λ⁴ phosphorus atom. The X-ray crystal structures of compounds 2a, 2b, 10, and 12 reveal the flexibility of the [SPS^(Me)] anion ligand. Finally, the difference in the electronic properties and structural features of compounds 2a and 2b were investigated by means of DFT calculations and a NBO analysis.