Guanidinophosphazenes: Design, Synthesis, and Basicity in THF and in the Gas Phase

A principle for creating a new generation of nonionic superbases is presented. It is based on
attachment of tetraalkylguanidino, 1,3-dimethylimidazolidine-2-imino, or bis(tetraalkylguanidino)carbimino
groups to the phosphorus atom of the iminophosphorane group using tetramethylguanidine or easily available
1,3-dimethylimidazolidine-2-imine. Seven new nonionic superbasic phosphazene bases, tetramethylguanidino-substituted at the P atom, have been synthesized. Their base strengths are established in
tetrahydrofuran (THF) solution by means of spectrophotometric titration and compared with those of eight
reference superbases designed specially for this study, P2- and P4-iminophosphoranes. The gas-phase
basicities of several guanidino- and N‘,N‘,N‘ ‘,N‘ ‘-tetramethylguanidino (tmg)-substituted phosphazenes and
their cyclic analogues are calculated, and the crystal structures of (tmg)3PN-t-Bu and (tmg)3PN-t-Bu·HBF4 are determined. The enormous basicity-increasing effect of this principle is experimentally verified
for the tetramethylguanidino groups in the THF medium:  the basicity increase when moving from (dma)3PN-t-Bu (pKα = 18.9) to (tmg)3PN-t-Bu (pKα = 29.1) is 10 orders of magnitude. A significantly larger basicity
increase (up to 20 powers of 10) is expected (based on the high-level density functional theory calculations)
to accompany the similar gas-phase transfer between the (dma)3PNH and (tmg)3PNH bases. Far stronger
basicities still are expected when, in the latter two compounds, all three dimethylamino (or tetramethylguanidino) fragments are replaced by methylated triguanide fragments, (tmg)2CN. The gas-phase
basicity (around 300−310 kcal/mol) of the resulting base, [(tmg)2CN]3PNH, having only one phosphorus
atom, is predicted to exceed the basicity of (dma)3PNH by more than 40 powers of 10 and to surpass
also the basicity of the widely used commercial [(dma)3PN]3PN-t-Bu (t-BuP4) superbase.