Vibrational spectroscopy of free di-manganese oxide cluster complexes with di-hydrogen

The reaction of free di-manganese oxide clusters Mn2Ox+ (x = 2-7) with hydrogen in a flow tube reactor results in a strongly cluster size dependent complex formation: Mn2O3+ and Mn2O4+ are most reactive and adsorb up to two H2 molecules, the reactivity is reduced for Mn2O2+/Mn2O5+, and Mn2O6+/Mn2O7+ appear to be non-reactive. Infrared multiple-photon dissociation (IR-MPD) spectra of the complexes Mn2O4H2+ and Mn2O5H2+ reveal one band that shifts upon isotopic labelling of hydrogen, whereas the spectra of Mn2O2H2+ and Mn2O3H2+ do not show any clear spectral shifts upon H2/D2 exchange. Detailed analysis of the IR-MPD spectra in conjunction with density functional theory (DFT) calculations strongly indicate the molecular η2-binding of H2 to one of the Mn atoms. H2 dissociation via hydroxylation of the cluster oxo bridges yielding H-Mn(MH)O(OH)O2+ or Mn2(OH)2O2+ appears to be thermodynamically favourable but kinetically hampered.