Methide Affinity Scale: Key Thermodynamic Data Underpinning Catalysis, Organic Synthesis, and Organometallic and...

Methide transfer reactions play important roles in many areas of chemistry, including the Grignard reaction, in the transmetalation steps of metal-catalyzed cross-coupling reactions, and in the generation of cationic metal polymerization catalysts. Methide affinities (MAs) are the key thermodynamic quantity that underpin such reactions, and yet comprehensive methide affinity scales are poorly developed. Here, B3LYP-D3BJ/def2-TZVP calculations are used to calculate the energy changes (MAs) for cations (MeZ → Z⁺ + Me^-), neutrals (MeY^- → Y + Me^-), and anions (MeX^2- → X^- + Me^-) derived from permethyl species of all group s and p elements. The MAs range from 2525.8 for the singlet cation F⁺ to -820.4 kJ/mol for the tetramethylborate anion, Me₄B^-. The cations show the clearest trends: the MAs in all cases decrease going down the group, while moving across a period, the MAs increase from group 1 to group 2 and then decrease for group 3, remaining about the same or with a modest increase moving to group 4, and then continue to increase across a period to a maximum for the halogens (group 17). The anions and dianions are sensitive to hypervalency; those elements that cannot expand the octet have very unfavorable MAs (e.g., MA of Me₄C requires the formation of Me₅C^- and of Me₄B^- requires the formation of Me₅B^2-). To address whether the anion MeY^- and dianion MeZ^2- are stable, the vertical detachment energies of the anions and dianions were calculated. All of the anions are thermodynamically stable with respect to electron loss, except for Me₄N^-, while the dianions are all thermodynamically unstable with respect to electron loss. The kinetic stability of the dianions with respect to methide and electron loss was also evaluated for the lowest MAs. The only dianions that might be kinetically stable and observable in the gas phase are Me₄Ca^2-, Me₄Sr^2-, and Me₄Ba^2-. The dianion CF₃CaF₃^2- is predicted to be both thermodynamically and kinetically stable in the gas phase.