Dissecting Transmetalation Reactions at the Molecular Level: Phenyl Transfer in Metal Borate Complexes

Despite the wide use of transmetalation reactions in
organic and
organometallic synthesis, little is known about the involved intermediates
and mechanisms. Here, we use a combination of electrospray multistage-mass
spectrometry experiments and quantum chemical calculations to examine
the unimolecular gas-phase reactivity of [Mn(BPh4)n+1]− and [Mn(BPh4)n−1]+ ions (M = Li and Ag) as model
for transmetalation. This approach excludes any bimolecular equilibrium
processes occurring in solution and thus ensures exact control over
the composition of the probed species. Our findings show how the nature
of the metal M, the charge of the complex, and the oligomeric state
of the complex influence the occurrence of transmetalation. Efficient
transmetalation reactions only proceed for the silver-containing complexes
but not for their lithium-containing counterparts. This behavior reflects
the better stabilization of the soft Lewis base Ph– by the softer Lewis acid Ag+ than by the harder Lewis
acid Li+, in accordance with the predictions of HSAB theory.
The participation of a second silver ion in [Ag2(BPh4)]+ further facilitates the abstraction of the
phenyl anion from boron. The resulting product complex closely resembles
a transmetalation product isolated from solution and thus directly
demonstrates the implications of our gas-phase model for condensed-phase
chemistry.