Gas-phase nucleophilic reactions of Ge(OCH 3) 4: experimental and computational characterization of pentacoordinated Ge anions

Abstract

The gas-phase ion/molecule reactions of F − and CH 3O − with Ge(OCH 3) 4 have been investigated by Fourier transform ion cyclotron mass spectrometry. Both nucleophiles react preferentially by an addition mechanism to yield XGe(OCH 3) 4 − (X = F, OCH 3) complexes that are identified as typical pentacoordinated Ge species. Pentacoordinated Ge adducts formed with excess internal energy can undergo elimination of formaldehyde to yield HGe(OCH 3) 4 −, or further elimination processes that result in the formation of germyl anions like Ge(OCH 3) 3 −. Other minor product ions are also observed which can be attributed to the intermediacy of a pentacoordinated adduct. Dissociation of the XGe(OCH 3) 4 − anions induced by infrared multiphoton excitation leads to sequential losses of formaldehyde and gives rise to different germyl anions like Ge(OCH 3) 3 −, HGe(OCH 3) 2 −, and H 2GeOCH 3 −. The XGe(OCH 3) 4 − and germyl anions react readily with BF 3 through successive methoxide-fluoride exchange and this reaction provides a gas-phase synthetic pathway for multiply fluorinated Ge anions. Ab initio calculations performed on model pentacoordinated species F n+1 Ge(OH) 4− n − (n = 0–4) reveal that addition of a fluoride ion on hydroxygermanes occurs preferentially in the apical position of a trigonal bipyramid. The fluoride affinity of the prototype molecule Ge(OH) 4 is calculated to be 60.9 kcal mol −1, and fluoride affinity increases monotonically with increasing fluorine substitution. The fluoride affinity of GeF 4 is calculated to be 79 kcal mol −1. Similar calculations also predict an unusually high hydride affinity (60 kcal mol −1) for Ge(OH) 4 with the hydride occupying an equatorial position.