Competition between π- and σ-based interactions in metal ion complexes of the phenyl radical

Hybrid density functional theory (B3-LYP and B3-PW91) and counterpoise-corrected ab initio (MP2) calculations have been performed for the adducts of the main-group metal ions Na⁺, Mg⁺, Al⁺, K⁺, and Ca⁺ with the phenyl radical C₆H₅. While the calculated bond dissociation energies (BDEs) and complexation geometries of the alkali metal ion adducts do not differ substantially from those determined for the analogous M⁺/benzene π-complexes, the alkaline earth ions exhibit a strong preference for σ-bond formation at the bare C atom of the phenyl ring, with calculated BDEs exceeding those determined for benzene complexation by approximately 80 and 150kJmol^-1 for Mg⁺ and Ca⁺, respectively. This σ-coordination is only feasible when the unpaired spins on the reactant Mg⁺ (or Ca⁺) and C₆H₅ radicals are opposed, thereby leading to a closed-shell (singlet state) adduct ion. When the unpaired spins are aligned, the triplet-state adduct thus produced adopts π-complex minimum energy geometry with a BDE significantly below that of the corresponding M⁺/benzene complex. Both π- and σ-coordination are also found to be local minima on the AlC₆H₅⁺ potential energy surface, with the BDE of the Al⁺/C₆H₅ σ-complex being approximately 50kJmol^-1 larger than that of the corresponding π-complex, and about 20kJmol^-1 above the BDE for Al⁺/benzene. The overall results of our study show that the phenyl radical exhibits a much greater degree of selectivity between the main-group metal ions represented here than is evident for benzene. The phenyl radical BDEs range from 59.7 (K⁺) to 264.7kJmol^-1 (Ca⁺) according to calculations at the B3-PW91/6-311+G(2df,p) level of theory, which contrasts with the 65.5 (K⁺) to 147.9kJmol^-1 (Al⁺) BDE values determined for benzene at the same level of theory.