title: Guanidinophosphazenes: design, synthesis, and basicity in THF and in the gas phase.
abstract: 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 x 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 (pKalpha = 18.9) to (tmg)3P=N-t-Bu (pKalpha = 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.
authors.affiliation: Institute of Inorganic & Physical Chemistry, University of Bremen, Leobener Strasse, D-28334 Bremen, Germany. kolomeit@chemie.uni-bremen.de
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Kolomeitsev
authors.forename: Ilmar A
authors.initials: IA
authors.lastname: Koppel
authors.forename: Toomas
authors.initials: T
authors.lastname: Rodima
authors.forename: Jan
authors.initials: J
authors.lastname: Barten
authors.forename: Enno
authors.initials: E
authors.lastname: Lork
authors.forename: Gerd-Volker
authors.initials: GV
authors.lastname: Röschenthaler
authors.forename: Ivari
authors.initials: I
authors.lastname: Kaljurand
authors.forename: Agnes
authors.initials: A
authors.lastname: Kütt
authors.forename: Ivar
authors.initials: I
authors.lastname: Koppel
authors.forename: Vahur
authors.initials: V
authors.lastname: Mäemets
authors.forename: Ivo
authors.initials: I
authors.lastname: Leito