An investigation of species involved in the methanol carbonylation reaction by exafs and ultramicroelectrodes

Abstract

Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to study the mechanistic cycles involved in the rhodium, iridium or palladium catalysed carbonylation of methanol. The validity of using microelectrodes to determine iodide concentrations, in particular HI, in acetic acid mixtures was also investigated.

EXAFS was used to characterise a number of catalytic precursors, revealing bond lengths that closely matched those reported in the literature using X-ray crystallography. Bond angles were also determined for some structures, either by using multiple scattering calculations or using trigonometry from distances obtained by the refinement of both transition metal and iodine edges.

In situ EXAFS experiments were also carried out. Stopped flow results, using the X-STRIP detector, for the oxidative addition of CH₃OSO₂CF₃ to [Ir(CO)₂I₂]^- showed a classic S_N2 reaction, with the first step having occurred 1.2 ms into the reaction. [Pd(CO)I₃]^- was shown to be the dominant species in the palladium catalysed carbonylation cycle whilst the study of the [Bu₄N][Rh(CO)₂I₂] catalysed carbonylation reaction, carried out in a newly designed cell, showed that only the species [Rh(CO)₂I₂]^- was present during the timescale of the reaction; however little carbonylation occurred under these conditions. The crystal structure of [(CO₃CO)Rh(dppe)I₂], obtained from the catalytic reaction of [Rh(CO)I(dppe)], MeI and methanol demonstrated that the oxidative addition and methyl migration steps occurred.

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