Cyclotetraphosphinophosphonium ions: synthesis, structures, and pseudorotation

The first derivatives of catenated cyclotetraphosphinophosphonium cations, [(PhP)4PPhMe]+ (8a), [(MeP)4PMe2]+ (8b), [(CyP)4PPh2]+ (8d), [(CyP)4PMe2]+ (8e), [(PhP)4PPh2]+ (8f), [(PhP)4PMe2]+ (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 R2PCl 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 R2PCl/Me3SiOTf with R = Ph or Me, respectively. A tetrachlorogallate salt of [(PhP)4PPhtBu]+ (8c) has been synthesized by alkylation of 4a with tBuCl/GaCl3. 31P[1H] 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 C1 symmetry. The effective C2 symmetry observed for 8b, d, e, f, and g in solution, signified by their 31P[1H] 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.