Structural Properties of Gas-Phase Molybdenum Oxide Clusters [Mo(4)O(13)](2−), [HMo(4)O(13)](−), and [CH(3)Mo(4)O(13)](−) 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 [Mo(4)O(13)](2−), [HMo(4)O(13)](−), and [CH(3)Mo(4)O(13)](−) 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 MoO(2) 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 [Mo(4)O(13)](2−) and [HMo(4)O(13)](−), dissociation requires opening or rearrangement of the ring structure, which is quite facile for the doubly charged [Mo(4)O(13)](2−), but energetically more demanding for [HMo(4)O(13)](−). In the latter case, the hydrogen atom is found to stay preferentially with the negatively charged fragments [HMo(2)O(7)](−) or [HMoO(4)](−). The doubly charged species [Mo(4)O(13)](2−) loses one MoO(3) unit at low energies while Coulomb explosion into the complementary fragments [Mo(2)O(6)](−) and [Mo(2)O(7)](−) dominates at elevated collision energies. [CH(3)Mo(4)O(13)](−) affords rearrangements of the methyl group with low barriers, preferentially eliminating formaldehyde, while the ring structure remains intact. [CH(3)Mo(4)O(13)](−) also reacts efficiently with water, leading to methanol or formaldehyde elimination. [Image: see text] ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s13361-019-02294-4) contains supplementary material, which is available to authorized users.