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 now 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, P-2- and P-4-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)(3)P=N-t-Bu and (tmg)(3)P=N-t-Bu center dot 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)(3)P= N-t-Bu (pK(a) = 18.9) to (tmg)(3)P=N-t-Bu (pK(a) = 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)(3)P=NH and (tMg)(3)P=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)(2)C=N-. The gas-phase basicity (around 300-310 kcal/mol) of the resulting base, [(tMg)(2)C=N-](3)P=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)(3)P=N](3)P=N-t-Bu (t-BuP4) superbase.