Experimental Gas-Phase Basicity Scale of Superbasic Phosphazenes Ivari Kaljurand, ² Ilmar A. Koppel, ² Agnes Ku 1 tt, ² Eva-Ingrid Ro ˜ o ˜ m, ² Toomas Rodima, ² Ivar Koppel, ² Masaaki Mishima, ‡ and Ivo Leito* ,² Department of Chemistry, UniVersity of Tartu, Jakobi 2 str, 51014 Tartu, Estonia, and Institute for Materials Chemistry and Engineering, Kyushu UniVersity, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan ReceiVed: September 21, 2006; In Final Form: December 2, 2006 Seventeen superbasic phosphazenes and two Verkade’s bases were used to supplement and extend the experimental gas-phase basicity scale in the superbasic region. For 19 strong bases the gas-phase basicity values (GB) were determined for the first time. Among them are such well-known bases as BEMP (1071.2 kJ/mol), Verkade’s Me-substituted base (1083.8 kJ/mol), Et-NdP(NMe 2 ) 2 -NdP(NMe 2 ) 3 (Et-P 2 phosphazene, 1106.9 kJ/mol), and t-Bu-NdP(NMe 2 ) 3 (t-Bu-P 1 phosphazene, 1058.0 kJ/mol). For the first time experimental GB values were determined for P 2 phosphazenes. Together with our previous results self-consistent experimental gas-phase basicity scale between 1020 and 1107 kJ/mol is now established. This way an important region of the gas-phase basicity scale, which was earlier dominated by metal hydroxide bases, is now covered also with organic bases making it more accessible for further studies. The GB values for several superbases were calculated using density functional theory at the B3LYP/6-311+G** level. For the phosphazene family the standard deviation of the correlation between the experimental and theoretical values was 6.5 kJ/mol. Introduction Superbasic phosphazene bases have been under extensive study during recent years. They are used in synthesis; 1-3 aryl phosphazenes have proved to be very suitable for building the superbasic region of self-consistent Brønsted basicity scales in acetonitrile 4 (AN) and THF. 5-8 The possibility to predictably fine-tune the basicities of aryl phosphazenes by insertion of various substituents into the phenyl ring, by varying the structure of the aliphatic periphery or using higher phosphazene homo- logues, makes this family very flexible for designing molecules with predictable basicity. Also the gas-phase basicity (GB) values for some phosphazene bases have been calculated 7-9 and experimentally determined. 6,10 In our previous work 6 a limited selection of P 1 phosphazenes was studied. The obtained results were in part surprising as they implied that the GB values of aryl substituted phosphazenes, compared to the basicities in THF, were rather insensitive toward the substituent in the aromatic nucleus. To further study this surprising result, a wider selection of aryl substituted P 1 phosphazenes was taken into the study described in this report. Also, several attempts to experimentally determine the basicity change when going from P 1 phosphazenes to P 2 phosphazenes for two homological series were made. For the sake of comparison two Verkade’s bases 12 (see Chart 1 for structures and designations) and three bicyclic guanidine bases (MTBD, ETBD, ITBD) were also included in the continuous basicity ladder. MTBD was used as the anchoring point for the measurements reported in this work. It has been suggested more than a decade ago that phosp- hazene superbases could be one of the most promising families of compounds for extending the continuous gas-phase basicity scale of organic compounds into the domain of very strong bases. 11 However, the practical success in realization of this promise has been very limited to date. To the best of our knowledge there have been only two works reporting experi- mental gas-phase basicity data for phosphazene bases, 6,10 both reporting data only for a very narrow selection of compounds. This situation is caused by the serious experimental complica- tions that arise when measuring the gas-phase basicities of phosphazene bases. The foremost among these is the very low volatility of most of these compounds making it very difficult to achieve and maintain suitable and constant vapor pressure in the mass spectrometer. Also, it is necessary to have on hand a range of bases with gradually changing basicities in order to be able to build a continuous “ladder”. In this work we have succeeded to overcome both of these complications (see the Supporting Information for experimental details), and we present a continuous phosphazene-based gas- phase basicity ladder with the span of ca. 87 kJ/mol from which around 40 kJ/mol is an extension of the so far existing continuous gas-phase basicity scale of organic bases. Experimental Section Chemicals. Compounds 1, 3, 7, 9, 12, 13, 18, and 23 were of commercial origin and were used in ICR experiments without additional purification. Synthesis, purification, and identification of 2, 4, 5, 8, 10, 11, 14-17, 19, 20, 21, and 24-30 has been described in previous works. 5-7,13 HCl salt of 6 was synthesized as described in ref 14. The salt was converted into tetrafluo- roborate salt by treatment with NaBF 4 in water. The free base was liberated from the HBF 4 salt by means of MeOK in MeOH. ETBD (22) was a kind donation from Prof. Reinhard Schwesing- er (University of Freiburg). Measurements of Gas-Phase Basicity (GB exp ). The instru- mentation, general experimental setup, and conditions have been previously described. 6 Detailed experimental conditions for introducing low-volatile bases into the ICR cell are given in * Corresponding author. Phone: +372 7 375 259. Fax: +372 7 375 264. E-mail: ivo.leito@ut.ee. ² University of Tartu. ‡ Kyushu University. 1245 J. Phys. Chem. A 2007, 111, 1245-1250 10.1021/jp066182m CCC: $37.00 © 2007 American Chemical Society Published on Web 02/01/2007