An Acidity Scale for Phosphorus-Containing Compounds Including Metal Hydrides and Dihydrogen Complexes in THF:  Toward the Unification of Acidity Scales

More than 70 equilibrium constants K between acids and bases, mainly phosphine derivatives, have been measured in tetrahydrofuran (THF) at 20 °C by 1 H and/or 31 P NMR. The acids were chosen or newly synthesized in order to cover the wide p K α THF range of 5−41 versus the anchor compound HPCy 3 BPh 4 at 9.7. These p K α THF values are approximations to absolute, free ion p K a THF and are obtained by crudely correcting the observed K for 1:1 ion-pairing effects by use of the Fuoss equation. The acid/base compounds include 14 phosphonium/phosphine couples, 17 cationic hydride/neutral hydride couples, 9 neutral polyhydride/anionic hydride couples, 14 dihydrogen/hydride couples, and 4 other nitrogen- and phosphorus-based acids. The effects on p K α of the counterions BAr‘ 4 - and BF 4 - vs BPh 4 - and K(2,2,2-crypt) + versus K(18-crown-6) + are found to be minor after correcting for differences in inter-ion distances in the ion-pairs involved. Correlations with ν(M−H) noted here for the first time suggest that destabilization of M−H bonding in the conjugate base hydride is an important contributor to hydride acidity. It appears that Re−H bonding in the anions ReH 6 (PR 3 ) 2 - is greatly weakened by small increases in the basicity of PR 3, resulting in a large increase in the p K α of the conjugate acid ReH 7 (PR 3 ) 2 . Correlations with other scales allow an estimate of the p K α THF values of more than 1000 inorganic and organic acids, 20 carbonyl hydride complexes, 46 cationic hydrides complexes, and dihydrogen gas. Therefore, many new acid−base reactions can be predicted and known reactions explained. THF, with its low dielectric constant, disfavors the ionization of neutral acids HA over HB +, and therefore separate lines are found for p K α THF (HA) and p K α THF (HB + ) when plotted against p K a DMSO or p K a MeCN . The crystal structure of Re(H) 2 (PMe 3 ) 5 BPh 4 is reported.