A gas -phase study of the structure, reactivity and thermochemistry of transition metal -ligand systems by Fourier transform ion cyclotron resonance mass spectrometry and density functional theory

Huiping Chen, Purdue University

Abstract

The gas-phase chemistry of several transition metal-ligand systems has been studied using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and density functional theory (DFT). ^ The reactions of M+-2,3-didehydropyrazine (M = Fe, Co) with small alkenes and alkynes were investigated. Overall, M+-2,3-didehydropyrazine has a richer and more complex reactivity than Fe+- o-benzyne with the unsaturated hydrocarbons. Collision-induced dissociation and sustained off-resonance irradiation were used to probe the product ion structures for all ion-molecule reactions. Using this information, metal-centered mechanisms were proposed. DFT calculations on Fe+-2,3-didehydropyrazine suggest a pyrazinometallacyclopropene structure and predict D°(Fe +-2,3-didehydropyrazine) to be 87 ± 10 kcal/mol. ^ β-H migration from an organic ligand to the transition metal center and the reverse process, insertion of the unsaturated organic ligand into the metal-hydrogen bond are ubiquitous processes for organometallic chemistry. MC2H3+ and MC2H3 + (M = Fe, Co) represent ideal model systems for the gas-phase studies of β-H migration where complexity is minimized while the relevant structural and electronic requirements are preserved. Our experimental results indicate M(C2H3)+ ↔ HM(C2H2)+ and M(C2H 5)+ ↔ HM(C2H4)+ interconversion are facile processes for activated MC2H3+ and MC2H5+ ions, as predicted by DFT calculations. ^ α-H migration for the MHNO+ (M = Fe, Co) system was also investigated. Optimized structures and energetics of three CoHNO + isomers were obtained by DFT calculations. CoHNO+ reacts with methane by dehydrogenation, an unusual reactivity not observed by many other cobalt complexes. Potential energy surface diagrams and proposed mechanism for selected ion-molecule reactions are presented. ^

Degree

Ph.D.

Advisors

Major Professors: Ben S. Freiser, Purdue University, R. G. Cooks, Purdue University.

Subject Area

Chemistry, Analytical|Chemistry, Inorganic

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