Abstract

European Journal of Mass Spectrometry
Volume 10 Issue 6, Pages 921–930 (2004)
doi: 10.1255/ejms.701

Li⁺ vs Cu⁺ association to toluene, phenylsilane and phenylgermane. Conventional vs non-conventional π-complexes

Inés Corral, Otilia Mó and Manuel Yáñez
Departamento de Química, C-9, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain

The complexes between Li⁺ and Cu⁺ with toluene, phenylsilane and phenylgermane were investigated through the use of high-level density functional theory (DFT) methods. Both harmonic vibrational frequencies and optimized geometries were obtained at the B3LYP/6-311G(d,p) and B3LYP/6-311+G(2df,2p) levels of theory. Cu⁺ interactions have a non-negligible covalent character, which clearly differentiate them from Li⁺ interactions. As a consequence, the topology of the potential energy surfaces (PES) is richer for Cu⁺ than for Li⁺ complexes and Cu⁺ binding energies are 1.4 times higher than Li⁺ binding energies. For Li⁺, only conventional π-complexes should be expected in the gas-phase, while for Cu⁺, other complexes, in which the metal cation interacts specifically with only one pair of carbon atoms of the aromatic ring, are found to be as stable as the conventional π-complexes. Furthermore, for the particular cases of phenylsilane and phenylgermane, the global minima of the PES correspond to a non- conventional complex in which the metal ion interacts with the ortho carbon of the aromatic ring and with one of the hydrogen atoms of the XH₃ (X = Si, Ge) substituent group, through a typical agostic-type interaction. This specific interaction is followed by a large activation of the corresponding X–H bond, whose stretching frequency is significantly shifted to the red. Accordingly, the predicted infrared spectra for these non-conventional complexes markedly differ from those of the conventional π-complexes. Toluene is more basic than phenylsilane and phenylgermane when the reference acid is Li⁺, whereas the basicity of phenylgermane towards Cu⁺ metal cations is slightly higher than those of phenylsilane and toluene.

Keywords: toluene, phenyl-silane, phenyl-germane, Cu⁺ and Li⁺ complexes, non-conventional complexes, theoretical calculations

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