Generation, thermochemistry, and reactivity of novel transition metal containing ions in the gas phase

Steven Wayne Buckner, Purdue University

Abstract

The gas phase chemistry of some singly and doubly charged metal ions and metal ion complexes have been investigated using a Fourier transform ion cyclotron resonance mass spectrometer. The thermal ion-molecule reactions of singly charged niobium and tantalum ions with small alkanes and alkenes show these species to be the most reactive singly charged metal ions reported to date. Ta$\sp{+}$ is the first metal ion reported to dehydrogenate CH$\sb4$ exothermically. Collision-induced dissociation was used to probe the product ion structures for the Nb$\sp{+}$/alkane and Nb$\sp{+}$/alkene reactions. Using this information, mechanisms are proposed which include migratory insertion of alkene and alkyne ligands into Nb$\sp{+}$-C bonds, and dehydrocyclization of larger alkenes to form Nb(c-C$\sb5$H$\sb5)\sp{+}$ and Nb(benzene)$\sp{+}$. The formation of the first mononuclear niobocene complex was observed from the reaction of Nb$\sp{+}$ with two molecules of cyclopentane. The reactions of Nb$\sp{2+}$ with C$\sb{\rm n}$H$\sb{2\rm n+2}$ (n = 1 $-$ 4) were investigated. Nb$\sp{2+}$ is even more reactive toward C-H bonds than Nb$\sp{+}$, reacting with CH$\sb4$ up to six times via dehydrogenation. Charge transfer and hydride transfer are also prominent reaction pathways. An increase in % charge transfer occurs with increasing hydrocarbon chain length. These results are rationalized using a simple curve-crossing model. The kinetic energy released in the Nb$\sp{2+}$/benzene charge transfer reaction was investigated at thermal energies using kinetic energy ion cyclotron resonance spectrometry. The results indicate the benzene cation is produced almost exclusively in the ground electronic state. However, population of numerous low-lying excited electronic states ofNb$\sp{+}$ and ro-vibrational states of C$\sb6$H$\sb6\sp{+}$ is indicated. Primary amide complexes of Fe$\sp{+}$, Co$\sp{+}$, and Rh$\sp{+}$ were generated by thermal ion-molecule reactions, and their CID fragments were studied. Mechanisms were proposed for these reactions involving oxidative addition of an N-H bond to the metal center, followed by a reductive elimination of HX where X represents a $\sigma$-bonded ligand from the reactant metal-ion complex.

Degree

Ph.D.

Advisors

Freiser, Purdue University.

Subject Area

Analytical chemistry

COinS