Solution Phase Cationic Niobium and Tantalum Alkoxide Clusters: Insights into Formation and Structure

Abstract

Electrospray mass spectrometry (ESMS) is used to investigate cationic niobium and tantalum alkoxide clusters generated in solution by adding equal molar amounts of B(C6F5)3 to the metal ethoxide, M(OEt)5. The dominant metal ethoxide cations that were observed in the electrospray mass spectrum correspond to M(OEt)4+, M2O(OEt)7+, M2(OEt)9+, and M3O(OEt)12+, where M is niobium or tantalum. In separate experiments heterometallic Nb/Ta ethoxide clusters were produced, and the heterometallic cations present in the electrospray mass spectrum are NbTaO(OEt)7+, NbTa(OEt)9+, Nb2TaO(OEt)12+, and NbTa2O(OEt)12+. Collision-induced dissociation (CID) experiments were completed to provide structural information about the observed metal ethoxide cations. At low collision energy (ECM < 2 eV), the CID fragmentation pattern of Nb3O(OEt)12+ favors loss of Nb2(OEt)9+ and neutral NbO(OEt)3, whereas the fragmentation pattern of Ta3O(OEt)12+ favors loss of Ta2O(OEt)7+ and neutral Ta(OEt)5. In addition, the major CID product of M2O(OEt)7+ and M2(OEt)9+ at low collision energy is M(OEt)4+. During CID experiments of M(OEt)4+, four consecutive ethylene molecules were released leaving M(OH)4+, which at higher collision energies (ECM > 2 eV) liberates two water molecules resulting in MO2+. The CID fragmentation pattern of crotyl alcohol (CH3CHCHCH2OH) substituted and n-butanol (CH3CH2CH2CH2OH) substituted metal alkoxides resembles the fragmentation pattern proposed for M(OEt)4+, in which four consecutive organic molecules are shed leaving M(OH)4+.