Unimolecular reactivity of organotrifluoroborate anions, RBF , and their alkali metal cluster ions, M(RBF ) (M = Na, K; R = CH , CH CH , CH (CH ) , CH (CH ) , c-C H , C H , C H CH , CH CHCH , CH CH, C H CO).

RATIONALE

Potassium organotrifluoroborates (RBF 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 RBF (where R = CH , CH CH , CH (CH ) , CH (CH ) , c-C H , C H , C H CH , CH CHCH , CH CH, C H CO) and M(RBF ) (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 RBF 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(RBF ) , formation of RBF 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 RBF 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(RBF ) but not K(RBF ) is in agreement with the known fluoride affinities of Na and K and can be rationalized by Pearson's HSAB theory.