Acrylic Acid Derivatives of Group 8 Metal Carbonyls: A Structural and Kinetic Study

The
synthesis, spectroscopic, and X-ray structural studies of acrylic
acid complexes of iron and ruthenium tetracarbonyls are reported.
In addition, the deprotonated η2-olefin bound acrylic
acid derivative of iron as well as its alkylated species were fully
characterized by X-ray crystallography. Kinetic data were determined
for the replacement of acrylic acid, acrylate, and methylacrylate
for the group 8 metal carbonyls by triphenylphosphine. These processes
were found to be first-order in the concentration of metal complex
with the rates for dissociative loss of the olefinic ligands from
ruthenium being much faster than their iron analogues. However, the
ruthenium derivatives afforded formation of primarily mono-phosphine metal tetracarbonyls, whereas the iron complexes led largely
to trans-di-phosphine tricarbonyls.
This difference in behavior was ascribed to a more stable spin crossover
species 3Fe­(CO)4 which undergoes rapid CO loss
to afford the bis phosphine derivative. The activation
enthalpies for dissociative loss of the deprotonated η2-bound acrylic acid ligand were found to be larger than their corresponding
values in the protonated derivatives. For example, for dissociative
loss of the protonated and deprotonated acrylic acid derivatives of
iron(0) the ΔH⧧ values determined
were 28.0 ± 1.2 and 34.1 ± 1.5 kcal·mol–1, respectively. Density functional theory (DFT) computations of the
bond dissociation energies (BDEs) in these acrylic acids and closely
related complexes were in good agreement with enthalpies of activation
for these ligand substitution reactions, supportive of a dissociative
mechanism for olefin displacement. Processes related to catalytic
production of acrylic acid from CO2 and ethylene are considered.