Li + vs Cu + Association to Toluene,Phenylsilane and Phenylgermane. Conventional vs Non-Conventional π-Complexes

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.