The effect of counter ions on the far-infrared spectra of tris(triphenylphosphinegold)oxonium dimer salts

Abstract

Two tris(triphenylphosphinegold)oxonium dimer salts [{{Au(PPh₃)}₃(μ₃−O)}₂]²⁺(X⁻)₂ (X = BF⁻₄, MnO⁻₄) were investigated via synchrotron-based far-infrared vibrational spectroscopy and density functional theory modelled at the M06/LANL2DZ level of theory. The 50–800 cm⁻¹ region of both oxonium salts is presented, with the spectrum for [{{Au(PPh₃)}₃(μ₃−O)}₂]²⁺(BF⁻₄)₂ found to possess a large feature at 330.3 cm⁻¹, attributable to counter-ion vibrational modes, which is only predicted upon explicit inclusion of counter-ions in the calculation. A feature around 107 cm⁻¹ observed for the [{{Au(PPh₃)}₃(μ₃−O)}₂]²⁺(BF⁻₄)₂ infrared spectrum is assigned to 21 distinct vibrational modes arising from Au–Au bond stretching and other motions of the Au core. The same feature is predicted to be present within the [{{Au(PPh₃)}₃(μ₃−O)}₂]²⁺(MnO⁻₄)₂ spectrum but is masked by experimental noise. In the 50–400 cm⁻¹ region, the relative intensities of predicted vibrational modes is found to depend heavily on the presence and nature of the counter-ions, while within the 400–800 cm⁻¹ region, little dependence of the theoretical spectra on the type of counter-ion is predicted. Finally, the dimerization energies of both [{{Au(PPh₃)}₃(μ₃−O)}₂]²⁺(BF⁻₄)₂ and [{{Au(PPh₃)}₃(μ₃−O)}₂]²⁺(MnO⁻₄)₂ are calculated to be 3.06 eV and 3.20 eV, respectively, when the counter-ions are explicitly included within the calculation, and just 1.10 eV in their absence.