Cyclotetraphosphinophosphonium ions: Synthesis, structures, and pseudorotation

The first derivatives of catenated cyclotetraphosphinophosphonium cations, [(PhP)(4)PPhMe](+) (8a), [(MeP)(4)PMe2](+) (8b), [(CyP)(4)PPh2](+) (8d), [(CyP)(4)PMe2](+) (8e), [(PhP)(4)PPh2](+) (8f), [(PhP)(4)PMe2](+) (8g), are synthesized as trifluoromethanesulfonate (triflate, OSO2CF3-) salts through the reaction of cyclopentaphosphines (PhP)(5) (4a) or (MeP)(5) (4b) with methyl triflate (MeOTf) or by a net phosphenium ion [PR2+, R) Ph, Me; from R2PCI and trimethylsilyltriflate (Me3SiOTf)] insertion into the P-P bond of either cyclotetraphosphine (CyP)(4) (3c) or cyclopentaphosphines (PhP)(5) (4a) or (MeP)(5) (4b). Although more conveniently prepared from 4a, compound 8a[OTf] can also be formed from (PhP)(4) (3a) and MeOTf, and derivatives 8f[OTf] and 8g[OTf] are also accessible through reactions of 3a and R2PCI/Me3SiOTf with R) Ph or Me, respectively. A tetrachlorogallate salt of [(PhP)(4)(PPhBu)-Bu-t](+) (8c) has been synthesized by alkylation of 4a with (BuCI)-Bu-t/GaCI3. P-31{H-1} NMR parameters for all derivatives of 8 have been determined by iterative simulation of experimental data. Derivatives 8a[OTf], 8b[OTf], 8c[GaCl4], 8e[OTf], 8f[OTf], and 8g[OTf] and have been characterized by X-ray crystallography, showing the most favorable all-trans configuration of substituents for the phosphine centers, thus minimizing steric interactions. Each derivative adopts a unique envelope or twist conformation of C-1 symmetry. The effective C-2 symmetry observed for 8b, d, e, f, and g in solution, signified by their P-31{H-1} NMR AA ' BB ' X spin systems, implies a rapid conformational exchange for derivatives of 8. The core frameworks of the cations in the solid state are viewed as snapshots of different conformational isomers within the solution-phase pseudorotation process.