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.