Schwesinger Bases - Phosphazene Bases Stabilized by Multiple Counterpoise Hyperconjugative Interactions

Electronic structure and proton affinity of Schwesinger bases (P-n bases where n=1-4 indicates the number of phosphorous atoms) were studied using quantum mechanical calculations carried out at M06/def2-TZVPP//BP86-D3(BJ)/def2-TZVPP level of theory. Each phosphorous atom is in the ligating environment of four tricoordinated/dicoordinated nitrogen atoms having one/two lone pairs. While the lone pairs on the tricoordinated/dicoordinated nitrogen atoms act as hyperconjugative donors, the P-N sigma* orbitals act as hyperconjugative acceptors. These phosphazene bases are highly stabilized by synergetic hyperconjugative interactions which can accordingly be termed as counterpoise hyperconjugation. The number and the extent of pseudo-pi-delocalization resulting from hyperconjugative interactions get enhanced as P-1 changes to P-4. Moreover, additional pseudo-pi-delocalization starts to appear as P-1 changes to P-4. The phosphazene P-2 has one N-di(sigma)-P-N-di(sigma)-P pseudo-pi-delocalization which is similar to that of classical pi-delocalization in 1,3-butadiene. On the other hand, phosphazene P-3 has two pseudo-pi-delocalization similar to that of 1,3-butadiene and one pseudo-pi-delocalization similar to that of 1,3-pentadienyl cation. There are three pseudo-pi-delocalization similar to that of 1,3-butadiene and another three pseudo-pi-delocalization similar to that in 1,3-pentadienyl cation in P-4. Consequently, pseudo-pi-delocalization is present in all the tetrahedral symmetry planes of P-4. Hence, P-4 can be considered as a three-dimensional pseudo-pi delocalized system. Even though the lone pair orbitals are stabilized by hyperconjugative interactions, they are susceptible to protonation. Among the dicoordinated and tricoordinated nitrogen atoms, the proton affinity of the dicoordinated nitrogen atom connected to the Bu-t group is much higher (257.4 kcal/mol-291.6 kcal/mol), and the proton affinity values increase on going from P-1 to P-4. The very high proton affinity of this dicoordinated nitrogen atom indicates its superbasic character.