Exploration of the Building Blocks of Catalysis in the Gas Phase: Using Cryogenic Vibrational Spectroscopy to Understand Reactive Species in Organometallic Catalysis

摘要

Catalysts play a crucial role in the efforts to transform atmospheric carbon dioxide into transportable fuels. The molecular level mechanism of their action is, however, often poorly understood, making it difficult to optimize their performance through rational molecular design. The keys to the mechanism lie in identification of the reaction intermediates in the catalytic cycle, but these have proven difficult to isolate using tradition experimental tools such as FTIR and NMR. Cryogenic ion vibrational predissociation (CIVP) spectroscopy presents a unique opportunity to systematically capture and structurally characterize these transient species cooled close to their potential minima. This Dissertation extends the capabilities of CIVP spectroscopy specifically to generate active catalytic species in the gas phase and dock substrate molecules onto the active site so that their degree of activation can be spectroscopically determined. This work focuses on the homogenous catalytic reduction of CO 2 through a Ni(I) coordination compound, as well as two nitrogen heterocycles: imidazole and pyridine. In the case of the Ni system, importance of the oxidation state of the metal center is established by comparing the efficacies of Ni(I) and Ni(II) in the activation of CO2, CO, N2 and N 2O. This necessitated the implementation of a new method for the reduction of organometallic catalysts to their fragile active states using the unique capabilities of gas phase ion chemistry. After activation, the conversion of CO2 to transportable fuels depends on the introduction of protons, which is often facilitated by "water wires" that provide long range pathways for excess proton translocation. Following this process with vibrational spectroscopy is complicated by the fact that the vibrational signatures of "excess" or mobile protons are often very diffuse. This raises the important issue of the relative contributions of heterogeneous thermal broadening and homogeneous vibrationally excited state dynamics to the observed breadth of the bands. These contributions were unraveled in a comparative study of the "Eigen ion", in which the hydronium molecule is symmetrically hydrogen bonded to three water molecules in its first hydration shell, and the ion-molecule complex in which H3O+ is sequestered in a rigid (18-crown-6) crown ether host. The surprisingly diffuse bands of the latter system, despite being cooled close to its vibrational zero-point energy, clearly establish a dominant role of homogeneous excited state dynamics as the origin of the broadening. This behavior is traced to frustrated intramolecular proton transfer between the oxygen atom on the hydronium core and an oxygen atom on the ring. This gives rise to strong anharmonic coupling between soft modes of the hydronium and its local OH stretching frequencies. Treatment of this coupling in the context of a vibrational adiabatic model quantitatively accounts for the isotope dependence of the broadening. This effect persists even at 0 K as it reflects the displacements of the H3O+ ion in the zero-point vibrational level. To further understand and quantify the temperature of the ions in the trap, the rotational temperature of a small ion (I-- (HOD)) is obtained in two different trapping configurations: a 3D quadrupole trap and a linear octopole trap, which yield surprisingly similar temperatures that are about twice that of the metal electrodes at low temperature.