Mechanistic insight into homogeneous catalytic reactions by ESI-MS
Date
2013-08-28
Authors
Ahmadi, Zohrab
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Abstract
For the study of homogeneous catalytic reaction mechanisms, the ideal technique would be capable of identifying and measuring in real time the abundances of all components of the reaction mixture, including reactants, products, byproducts, intermediates, and catalyst resting states. This thesis details the development of methodologies designed to transform electrospray ionization mass spectrometry into just such a tool.
Species of interests must be charged otherwise invisible in ESI-MS. Therefore, charge-tagged aryl iodide ([4-I-C6H4CH2PPh3]+[Br]-) and a terminal alkyne ([para-(HCC)C6H4CH2PPh3]+[Br]-) were synthesized as the ESI-active substrates for the homogeneous catalysis study. A method named PSI (pressurized sample infusion) was developed to introduce the air and moisture sensitive reaction mixtures to the ESI-MS. The analytical aspects of the method were investigated and optimized. Applicability of the technique was demonstrated through several organic and organometallic mechanism investigations.
The above developments were employed to the detailed study of the copper-free Sonogashira (Heck alkynylation) reaction and the hydrodehalogenation of the charged-tag aryl iodide. Simultaneous monitoring of the charged substrate, products and intermediates in the copper-free Sonogashira reaction by PSI-ESI-MS provided rich information about the kinetic and mechanism of this reaction. Kinetic isotope effect study shows a remarkable inverse kinetic isotope effect which is completely unexpected. Numerical models were constructed to simulate the mechanistic observation and to extract the rate constant of each step in the proposed mechanism cycle.
The same methodology (PSI technique) was used to the study of the hydrodehalogenation reaction. Key intermediates were detected under the typical reaction conditions. Kinetic isotope effect study was performed in CH3OD and CD3OD. A primary KIE was observed in both deuterated solvents. A revised mechanism cycle was suggested for this reaction based on KIE results, numerical modelling and other experiments. In the proposed cycle deprotonation of methanol occurs on the palladium metal centre instead of the conventional in solution deprotonation (off metal deprotonation).
The mechanism of the ligand substitution of charged-tag of a palladium aryl iodide [Pd(TMEDA)(Ar)(I)]+ (Ar = [C6H4CH2PPh3]+[PF6]-) complex against PPh3 was studied in methanol by PSI-ESI-MS. Results revealed that the pathway proceeds quite differently to what had been assumed by others; there was a very fast displacement of [I]– by PPh3 to form [Pd(TMEDA)(Ar)(PPh3)]2+ , followed by a much slower displacement of TMEDA and recoordination of [I]– to form the product [Pd(PPh3)2ArI]+.
We successfully integrated UV/Vis spectroscopy, as a complementary method with ESI-MS to shed light into the systems where ESI-MS only is unable to provide a full assignment to homogenous catalysis. The combination of the two fast and sensitive techniques provides a unique opportunity to study the composition of the organometallic reaction mixtures over time.
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Keywords
ESI-MS, Homogeneous catalysis, operando, simultaneous, numerical modelling, spectroscopy, organometallic, analytical chemistry, inorganic, catalysis