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.