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 ¹H and/or ³¹P NMR. The acids were chosen or newly synthesized in order to cover the wide pK_α^THF range of 5−41 versus the anchor compound [HPCy₃]BPh₄ at 9.7. These pK_α^THF values are approximations to absolute, free ion pK_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 pK_α of the counterions BAr‘₄^- and BF₄^- vs BPh₄^- 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 [ReH₆(PR₃)₂]^- is greatly weakened by small increases in the basicity of PR₃, resulting in a large increase in the pK_α of the conjugate acid ReH₇(PR₃)₂. Correlations with other scales allow an estimate of the pK_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 pK_α^THF(HA) and pK_α^THF(HB⁺) when plotted against pK_a^DMSO or pK_a^MeCN. The crystal structure of [Re(H)₂(PMe₃)₅]BPh₄ is reported.