Structural Properties of Gas-Phase Molybdenum Oxide Clusters [Mo4O13]2-, [HMo4O13]-, and [CH3Mo4O13]- Studied by Collision-Induced Dissociation.

Molybdenum oxide-based catalysts are widely used for the ammoxidation of toluene, methanation of CO, or hydrodeoxygenation. As a first step towards a gas-phase model system, we investigate here structural properties of mass-selected [Mo4O13]2-, [HMo4O13]-, and [CH3Mo4O13]- by a combination of collision-induced dissociation (CID) experiments and quantum chemical calculations. According to calculations, the common structural motif is an eight-membered ring composed of four MoO2 units and four O atoms. The 13th O atom is located above the center of the ring and connects two to four Mo centers. For [Mo4O13]2- and [HMo4O13]-, dissociation requires opening or rearrangement of the ring structure, which is quite facile for the doubly charged [Mo4O13]2-, but energetically more demanding for [HMo4O13]-. In the latter case, the hydrogen atom is found to stay preferentially with the negatively charged fragments [HMo2O7]- or [HMoO4]-. The doubly charged species [Mo4O13]2- loses one MoO3 unit at low energies while Coulomb explosion into the complementary fragments [Mo2O6]- and [Mo2O7]- dominates at elevated collision energies. [CH3Mo4O13]- affords rearrangements of the methyl group with low barriers, preferentially eliminating formaldehyde, while the ring structure remains intact. [CH3Mo4O13]- also reacts efficiently with water, leading to methanol or formaldehyde elimination.