Pressure-dependent luminescence spectroscopy of molybdenum(iv) oxo complexes

Abstract

Luminescence spectra are reported for trans-[MoOCl(CN-t-Bu)₄]BPh₄ and trans-[MoOF(pyridine)₄]BPh₄. The maxima of the broad luminescence bands are at energies of 12000 and 13000 cm⁻¹, respectively. Resolved vibronic structure involves the metal-oxo stretching mode with a vibrational frequency of approximately 950 cm⁻¹ (Raman) in both compounds. The pressure behavior of the luminescence bands shows a distinct difference: a blue shift of +12 cm⁻¹ kbar⁻¹ is observed for [MoOCl(CN-t-Bu)₄]BPh₄, and a red-shift of -8 cm⁻¹ kbar⁻¹ is obtained for [MoOF(pyridine)₄]BPh₄. At pressures above 25 kbar, the luminescence maximum of [MoOCl(CN-t-Bu)₄]BPh₄ shows a change from a blue-shift to a red-shift and the Raman peak corresponding to the metal-oxo stretching mode at 954 cm⁻¹ becomes broad and its maximum decreases with increasing pressure, leading to a strong deviation from the linear increase of +0.24 cm⁻¹ kbar⁻¹ observed at pressures below 25 kbar. These unusual pressure effects are rationalized in terms of the influence of pressure on the metal–ligand bonds and oxo-metal–ligand angles using DFT calculations.