Gas Phase Mass- and Mobility-Resolved Structures of Metalated Glyphosate Dimers

Metalated glyphosate dimers were investigated by using electrospray ionization ion mobility-mass spectrometry and tandem ion mobility-infrared multiple photon dissociation-mass spectrometry. [M(glyphosate)(glyphosate-H)]⁺ dimers where M = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Mn²⁺, Cu²⁺ and Zn²⁺ were mass-selected prior to mobility separation. Each possessed a single mobility resolved isomer, with measured collision cross sections (^N2CCS_exp) ranging from 165 to 175 Ų. The dimers were all of similar size, with size trends consistent with periodic differences in the incorporated metals' cationic radii, except M = Cu²⁺. Upon IR irradiation between 2700 and 3700 cm^-1, the experimental IR spectra of mass- and mobility-resolved [M(glyphosate)(glyphosate-H)]⁺ dimers revealed two significant absorption peaks at 3550 and 3660 cm^-1. These correspond to the O-H stretching on both the carboxylate and phosphonate groups of the substituent glyphosate molecules. A thorough isomer search using CREST-CENSO algorithms and DFT optimization predicted the energetically preferred gas-phase structures of [M(glyphosate)(glyphosate-H)]⁺ dimers. Comparing calculated collision cross sections (^N2CCS_calc) and predicted vibrational frequencies with experimental data confirmed the predicted structures of the [M(glyphosate)(glyphosate-H)]⁺ dimers, which all share a common structural motif. In all cases, the incorporated deprotonated glyphosate is deprotonated at the phosphonate group. The divalent metal cation coordinates the deprotonated phosphonate group in a bidentate fashion and is located in the center of the dimer. The neutral glyphosate molecule is wrapped around the metal cation in an octahedral coordination. As the metal cation increases in size, the coordination distance increases, thereby increasing the overall size of the dimer. The different bonding afforded by M = Cu²⁺ to the amine nitrogen center leads to the observed structural difference for this metal, through the modulation of a key hydrogen bond in the dimer.