Fundamental Studies of Reactive Intermediates in Organometallic Chemistry

Citation

Stevens, Amy Elizabeth (1981) Fundamental Studies of Reactive Intermediates in Organometallic Chemistry. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/XKHS-2M38. https://resolver.caltech.edu/CaltechTHESIS:04262018-131940882

Abstract

The techniques of ion cyclotron resonance spectroscopy and photoionization-mass spectrometry are used to characterize the thermochemistry and reactivity of transition metal and organometallic species in the gas-phase. Chapter I gives an introduction emphasizing the need for physical studies of these compounds. An assessment of the differences in chemical properties and reactivity between the gas phase and solution is also made.

Chapter II details the properties and reactions of (CO)₅MnR (R = H, CH₃) determined using the techniques of ion cyclotron reso­nance spectroscopy. An examination of the products (CO)₅Mn(R)H⁺, (CO)₄Mn(R)H⁺, and (CO)₅Mn⁺ which result from proton transfer with varying exothermicity to (CO)₅MnR permits several thermochemical and mechanistic inferences. In particular the prcton affinities of these species are derived and the mechanism of reductive elimination of RH from the conjugate acids is detailed.

An examination of processes involving negative ions yields the heterolytic bond energies D[(CO)₅Mn^--R⁺]. The hydride is found to be an exceptionally strong acid in the gas phase.

Positive and negative ion mass spectra and ion-molecule reactions are reported briefly.

Chapter III presents the results of an ion cyclotron-resonance trapped ion study of the kinetics of proton transfer from MnH⁺ (formed as a fragment ion from HMn(CO)₅ by electron impact) to bases of varying strength. Deprotonation is rapid with bases whose proton affinity exceeds 196 ± 3 kcal mol^-1. Using this value for PA[Mn] yields the homolytic bond dissociation energy D[Mn⁺-H] = 53 ± 3 kcal mol^-1.

In Chapter IV the results of a photoionization mass­ spectrometric determination of the ionization potentials and selected fragment ion appearance potentials of (CO)₅MnR where R = H , CH₃, CH₂F, CHF₂ and CF₃ are presented. A comparison of the appearance potential of (CO)₅Mn⁺ from all five species yields the metal-carbon bond dissociation energies relative to the metal-hydrogen bond dissociation energy with no additional thermochemical data. Using the literature value D⁰[(CO)₅Mn-H ] = 57 kcal/mol gives D⁰[(CO)₅Mn-R] = 44, 32, 33, and 42 kcal/mol, respectively. Fragmentation thresholds for the metal carbene fragment ions (Co)₅MnCXY⁺ where X, Y = H or F are analyzed to yield the fluoride and hydride affinities of these species. Ion cyclotron resonance spectroscopy is used to examine hydride and fluoride transfer react ions involving these carbenes to corroborate the photoionization data. The carbene bond dissociation energies D⁰[(CO)₅Mn⁺-CXY] decrease from 104 to 98 to 82 kcal/moL with successive substitution of F for H .

In Chapter V the proton affinities of twenty organotransition metal complexes in the gas phase are reported. Combined with adiabatic ionization potentials, these data yield metal-hydrogen hemolytic bond energies for the sixteen species for which protonation occurs on the metal center. These bond energies range from 53 to 87 kcal/mol. Bond energies increase on going from a first-row complex to its second-row homologue, but no increase is seen on going to the third-row metal. The metal-hydrogen bond energy decreases markedly with increasing oxidation state of the same metal. Comparison to isoelectronic neutral complexes is made.

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