Pressure-dependent luminescence spectroscopy of molybdenum(IV) oxo complexes.

Luminescence spectra are reported for trans-[MoOCl(CN-t-Bu)(4)]BPh(4) and trans-[MoOF(pyridine)(4)]BPh(4). 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)]BPh(4), and a red-shift of -8 cm(-1) kbar(-1) is obtained for [MoOF(pyridine)(4)]BPh(4). At pressures above 25 kbar, the luminescence maximum of [MoOCl(CN-t-Bu)(4)]BPh(4) 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.