Unimolecular reactivity of organotrifluoroborate anions, RBF3- , and their alkali metal cluster ions, M(RBF3 )2- (M = Na, K; R = CH3 , CH3 CH2 , CH3 (CH2 )3 , CH3 (CH2 )5 , c-C3 H5 , C6 H5 , C6 H5 CH2 , CH2 CHCH2 , CH2 CH, C6 H5 CO).

RATIONALE: Potassium organotrifluoroborates (RBF3 K) are important reagents used in organic synthesis. Although mass spectrometry is commonly used to confirm their molecular formulae, the gas-phase fragmentation reactions of organotrifluoroborates and their alkali metal cluster ions have not been previously reported. METHODS: Negative-ion mode electrospray ionization (ESI) together with collision-induced dissociation (CID) using a triple quadrupole mass spectrometer were used to examine the fragmentation pathways for RBF3- (where R = CH3 , CH3 CH2 , CH3 (CH2 )3 , CH3 (CH2 )5 , c-C3 H5 , C6 H5 , C6 H5 CH2 , CH2 CHCH2 , CH2 CH, C6 H5 CO) and M(RBF3 )2- (M = Na, K), while density functional theory (DFT) calculations at the M06/def2-TZVP level were used to examine the structures and energies associated with fragmentation reactions for R = Me and Ph. RESULTS: Upon CID, preferentially elimination of HF occurs for RBF3- ions for systems where R = an alkyl anion, whereas R- formation is favoured when R = a stabilized anion. At higher collision energies loss of F- and additional HF losses are sometimes observed. Upon CID of M(RBF3 )2- , formation of RBF3- is the preferred pathway with some fluoride transfer observed only when M = Na. The DFT-calculated relative thermochemistry for competing fragmentation pathways is consistent with the experiments. CONCLUSIONS: The main fragmentation pathways of RBF3- are HF elimination and/or R- loss. This contrasts with the fragmentation reactions of other organometallate anions, where reductive elimination, beta hydride transfer and bond homolysis are often observed. The presence of fluoride transfer upon CID of Na(RBF3 )2- but not K(RBF3 )2- is in agreement with the known fluoride affinities of Na+ and K+ and can be rationalized by Pearson's HSAB theory.

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.