Samuel J. Kyran

Carbon monoxide induced reductive elimination of disulfide in an N-heterocyclic carbene (NHC)/thiolate dinitrosyl iron complex (DNIC).

Dinitrosyliron complexes (DNICs) are organometallic-like compounds of biological significance in that they appear in vivo as products of NO degradation of iron-sulfur clusters; synthetic analogues have potential as NO storage and releasing agents. Their reactivity is expected to depend on ancillary ligands and the redox level of the distinctive Fe(NO)2 unit: paramagnetic {Fe(NO)2}(9), diamagnetic dimerized forms of {Fe(NO)2}(9) and diamagnetic {Fe(NO)2}(10) DNICs (Enemark-Feltham notation). The typical biological ligands cysteine and glutathione themselves are subject to thiolate-disulfide redox processes, which when coupled to DNICs may lead to intricate redox processes involving iron, NO, and RS(-)/RS•. Making use of an N-heterocyclic carbene-stabilized DNIC, (NHC)(RS)Fe(NO)2, we have explored the DNIC-promoted RS(-)/RS• oxidation in the presence of added CO wherein oxidized {Fe(NO)2}(9) is reduced to {Fe(NO)2}(10) through carbon monoxide (CO)/RS• ligand substitution. Kinetic studies indicate a bimolecular process, rate = k [Fe(NO)2](1)[CO](1), and activation parameters derived from kobs dependence on temperature similarly indicate an associative mechanism. This mechanism is further defined by density functional theory computations. Computational results indicate a unique role for the delocalized frontier molecular orbitals of the Fe(NO)2 unit, permitting ligand exchange of RS• and CO through an initial side-on approach of CO to the electron-rich N-Fe-N site, ultimately resulting in a 5-coordinate, 19-electron intermediate with elongated Fe-SR bond and with the NO ligands accommodating the excess charge.

Mechanism of CO displacement from an unusually labile rhenium complex: an experimental and theoretical investigation.

The displacement of a CO ligand from an unusually labile rhenium carbonyl complex containing a bidentate carboxyaldehyde pyrrolyl ligand by PPh(3) and pyridine has been investigated. The reaction is found to proceed by an associative, preequilibrium mechanism. Theoretical calculations support the experimental data and provide a complete energetic profile for the reaction. While the Re-CO bond is found to be intrinsically weak in these complexes, it is postulated that the unusual lability of this species is due to the presence of a weak aldehyde Re-O link that can easily dissociate to open a coordination site on the metal center and accommodate an incoming ligand prior to CO loss. The resulting intermediate complex has been identified by IR spectroscopy. The presence of the hemilabile pyrrolyl ligand provides a lower-energy reaction channel for the release of CO and may be of relevance in the design of CO-releasing molecules.

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 η(2)-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 (3)Fe(CO)4 which undergoes rapid CO loss to afford the bis phosphine derivative. The activation enthalpies for dissociative loss of the deprotonated η(2)-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.

Hammett correlations as test of mechanism of CO-induced disulfide elimination from dinitrosyl iron complexes

The displacement of RS˙ from [(NHC)(SPh)Fe(NO)2] (NHC = N-heterocyclic carbene) by carbon monoxide follows associative kinetics, rate = k [CO]1 [(NHC)(SPh)Fe(NO)2]1, resulting in reduction of the oxidized form of the dinitrosyliron unit, {Fe(NO)2}9 (Enemark–Feltham notation) to {Fe(NO)2}10. Thermodynamically driven by the release of PhS–SPh concomitant with formation of [(NHC)(CO)Fe(NO)2], computational studies suggested the reactant dinitrosyliron unit serves as a nucleophile in the initial slanted interaction of the π* orbital of CO, shifting into normal linear Fe–CO with weakening of the Fe–SPh bond. The current study seeks to experimentally test this proposal. A series of analogous {Fe(NO)2}9 [(NHC)(p-S–C6H4X)Fe(NO)2] complexes, with systematic variation of the para-substituents X from electron donor to electron withdrawing groups was used to monitor variation in electron density at the Fe(NO)2 unit via Hammett analyses. Despite the presence of non-innocent NO ligands, data from ν(NO) IR spectroscopy and cyclic voltammetry showed consistent tracking of the electron density at the {Fe(NO)2} unit in response to the aryl substituent. The electronic modifications resulted in systematic changes in reaction rates when each derivative was exposed to CO. A plot of the rate constants and the Hammett parameter σp is linear with a negative slope and a ρ value of −0.831; such correlation is indicative of rate retardation by electron-withdrawing substituents, and provides experimental support for the unique role of the delocalized frontier molecular orbitals of the Fe(NO)2 unit.

Light-enhanced displacement of methyl acrylate from iron carbonyl: Investigation of the reactive intermediate via rapid-scan fourier transform infrared and computational studies

The thermal displacement of methyl acrylate from Fe(CO)4(η(2)-CH2=CHCOOMe) by phosphine ligands is a relatively slow reaction requiring several hours at elevated temperatures. In the present study, it is observed that photolysis of the tetracarbonyl complex with UV light activates the process such that the reaction is complete within a few seconds. This rate enhancement is due to the formation of an intermediate η(4) complex where the organic C=O and C=C units of methyl acrylate occupy axial and equatorial coordination sites on the Fe center, respectively, following photochemical CO loss. The displacement of methyl acrylate from this photolytically generated intermediate is facile with a remarkably low barrier of 8.7 kcal/mol. Density functional theory calculations support these experimental observations.