Structure and basicity. Part IX. Triphenylphosphazenylcyclophosphazenes: examples of exo- and endo-cyclic protonations and the relation of these to the conformation of the triphenylphosphazenyl group

Abstract

The pK′_a,1 values of a series of triphenylphosphazenylcyclotriphosphazatrienes of known structure have been measured, viz. N₃P₃Cl₅(NPPh₃), N₃P₃Ph₄Cl(NPPh₃)₂(three isomers), N₃P₃PhCl₄(NPPh₃), N₃P₃Ph₂Cl₃(NPPh₃), N₃P₃Cl₄(NPPh₃), N₃P₃Cl₄(NMe₂)(NPPh₃)(two isomers), N₃P₃Cl₃(NMe₂)₂(NPPh₃), N₃P₃Cl₂(NMe₂)₃(NPPh₃), N₃P₃Cl₄(NC₅H₁₀)(NPPh₃)(two isomers), N₃P₃Cl₄(NH₂)(NPPh₃), N₃P₃Cl₄(NPPh₃)(OEt), N₃P₃Ph(NMe₂)₄(NPPh₃), and N₃P₃(NMe₂)₅(NPPh₃). In almost all the compounds an unambiguous assignment of the first sites of protonation can be made. These can be classified as (i) type (I), where the group on the same phosphorus atom as the Ph₃PN substituent is Cl or Ph₃PN and gives rise to ring protonation, or (ii) type (II), where the group is NH₂, NMe₂, or Ph and exocyclic protonation occurs. Type (I) and (II) behaviour is correlated with the conformation of the Ph₃PN group relative to the ring; this is borne out by X-ray crystallographic data. Basicity measurements are suggested as a tool to determine the conformation of phosphazenyl substituents. Values of ΔpK′_a(= pK′_a,1– PK′_a,2) enable the assignment of the two sites of protonation in doubly protonated species. Probable sites of protonation are adduced for N₄P₄Cl₆(NPPh₃)₂. In the ground state, the electron-releasing properties of the Ph₃PN group resemble those of NR′₂ and NHR′(R′= alkyl) groups; on protonation, however, the Ph₃PN group becomes by far the most powerful electron-releasing group observed to date in cyclophosphazene chemistry.