Dissecting transmetalation reactions at the molecular level: C-B versus F-B bond activation in phenyltrifluoroborate silver complexes

The gas-phase unimolecular reactions of the silver(I) complex [Ag(PhBF₃)₂]⁻, formed via electrospray ionisation mass spectrometry of solutions containing the phenyltrifluoroborate salt and AgNO₃, are examined. Upon collision induced dissociation (CID) three major reaction channels were observed for [Ag(PhBF₃)₂]⁻: Ph⁻ group transfer via a transmetalation reaction to yield [PhAg(PhBF₃)]⁻; F⁻ transfer to produce [FAg(PhBF₃)]⁻; and release of PhBF₃⁻. The anionic silver product complexes of these reactions, [LAg(PhBF₃)]⁻ (where L = Ph and F), were then mass-selected and subjected to a further stage of CID. [PhAg(PhBF₃)]⁻ undergoes a Ph⁻ group transfer via transmetalation to yield [Ph₂Ag]⁻ with loss of BF₃. [FAg(PhBF₃)]⁻ solely fragments via loss of BF₄⁻, a reaction that involves Ph⁻ group transfer from B to Ag in conjunction with F⁻ transfer from Ag to B. Density functional theory (DFT) calculations on the various competing pathways reveal that: (i) the overall endothermicities govern the experimentally observed product ion abundances; (ii) the Ph⁻ group and F⁻ transfer reactions proceed via late transition states; and (iii) formation of BF₄⁻ from [FAg(PhBF₃)]⁻ is a multistep reaction in which Ph⁻ group transfer from B to Ag proceeds first to produce a [FAgPh(BF₃)]⁻ complex in which the BF₃ moiety is initially weakly bound to the ipso-carbon of the phenyl group and then migrates across the linear [FAgPh]⁻ moiety from C to Ag to F yielding [PhAg(BF₄)]⁻, which can then dissociate via loss of PhAg.