Bimolecular Reductive Elimination of Ethane from Pyridine(diimine) Iron Methyl Complexes: Mechanism, Electronic Structure, and Entry into [2+2] Cycloaddition Catalysis

Abstract

The application of bimolecular reductive elimination to the activation of iron catalysts for alkene–diene cycloaddition is described. Key to this approach was the synthesis, characterization, electronic structure determination, and ultimately solution stability of a family of pyridine(diimine) iron methyl complexes with diverse steric properties and electronic ground states. Both the aryl-substituted, (MePDI)FeCH3 and (EtPDI)FeCH3 (RPDI = 2,6-(2,6-R2-C6H3NCMe)2C5H3N), and the alkyl-substituted examples, (CyAPDI)FeCH3 (CyAPDI = 2,6-(C6H11NCMe)2C5H3N), have molecular structures significantly distorted from planarity and S = 3/2 ground states. The related N-arylated derivative bearing 2,6-di-isopropyl aryl substituents, (iPrPDI)FeCH3, has an idealized planar geometry and exhibits spin crossover behavior from S = 1/2 to S = 3/2 states. At 23 °C under an N2 atmosphere, both (MePDI)FeCH3 and (EtPDI)FeCH3 underwent reductive elimination of ethane to form the iron dinitrogen precatalysts, [(MePDI)Fe(N2)]2(μ-N2) and [(EtPDI)Fe(N2)]2(μ-N2), respectively, while (iPrPDI)FeCH3 proved inert to C–C bond formation. By contrast, addition of butadiene to all three iron methyl complexes induced ethane formation and generated the corresponding iron butadiene complexes, (RPDI)Fe(η4-C4H6) (R = Me, Et, iPr), known precatalysts for the [2+2] cycloaddition of olefins and dienes. Kinetic, crossover experiments, and structural studies were combined with magnetic measurements and Mössbauer spectroscopy to elucidate the electronic and steric features of the iron complexes that enable this unusual reductive elimination and precatalyst activation pathway. Transmetalation of methyl groups between iron centers was fast at ambient temperature and independent of steric environment or spin state, while the intermediate dimer underwent the sterically controlled rate-determining reaction with either N2 or butadiene to access a catalytically active iron compound.

CS-Symmetric Pyridine(diimine) Iron Methyl Complexes for Catalytic [2+2] Cycloaddition and Hydrovinylation: Metallacycle Geometry Determines Selectivity

“…Zero-field 57 Fe Mössbauer spectra were recorded in the solid state at 80 K and exhibit quadrupole doublets with isomer shifts (δ) of 0.52–0.53 mm/s and quadrupole splittings (Δ E Q ) between 1.39 and 1.45 mm/s (Figure ). Geometries distorted from planarity where the methyl group is lifted from the iron-chelate plane and the Mössbauer values measured are consistent with high-spin iron­(II) centers (S Fe = 2) antiferromagnetically coupled to bis­(imino)­pyridine radical anions (S PDI = 1/2) …”

“…The [2+2]-cycloaddition of ethylene and butadiene was selected to compare the series of C S -symmetric (PDI)­FeCH 3 complexes to the (bis)­arylated ( Me PDI)­FeCH 3 precatalysts. The latter provided VCB exclusively after 16 h (entry 1) . The addition of an equimolar mixture of butadiene and ethylene to a benzene- d 6 containing 5 mol % of 1-CH 3 resulted in a 48% conversion of butadiene after 6 h (entry 2) and complete consumption of the substrates over the course of 14 h. A 51:47 mixture of vinylcyclobutane to 1,4-hexadiene was obtained as judged by NMR spectroscopy.…”

“…Initial attempts to access C S -symmetric iron(0) catalysts through known reduction methods of the corresponding iron­(II) dihalides with sodium amalgam, sodium naphthalene, or magnesium­(butadiene) were unsuccessful. Attention was therefore directed to the synthesis of the C S -symmetric iron methyl complexes: ( Cy A Me PDI)­FeCH 3 ( 1-CH 3 ), ( Cy A iPr PDI)­FeCH 3 ( 2-CH 3 ), ( Cy A Me (Me,Et)­PDI)­FeCH 3 ( 3-CH 3 ), and ( Cy A Me (Et)­PDI)­FeCH 3 ( 4-CH 3 ) using the procedure reported for ( Me PDI)­FeCH 3 . Variations at the aniline and at the imine backbone substituents were selected to tune the solubility and steric environment of the resulting iron catalyst.…”

“…By comparison, few pyridine­(diimine) iron dinitrogen or butadiene complexes have been prepared (Scheme B), owing to the lack of general synthetic methods and the propensity to form catalytically inactive bis­(chelate) iron derivatives . A new synthetic route to [( Me PDI)­Fe­(N 2 )] 2 (μ-N 2 ) and ( Me PDI)­Fe­( trans - η 4 -C 4 H 6 ) was recently reported and relied on the bimolecular reductive elimination of ethane from the corresponding iron methyl derivative, ( Me PDI)­FeCH 3 (Scheme C) . This method enabled the direct entry into catalytic [2+2]-cycloaddition as ( Me PDI)­FeCH 3 underwent in situ activation to form the cycloaddition catalyst.…”

The Atmosphere Matters: The Effect of “Inert” Gas on the Catalytic Outcomes in Cobalt-Mediated Alkyne and Olefin Hydroboration

“…Nevertheless, samples of (P Pym DI)CoH 4 (generated under argon) were found to be reactive with N 2 and yielded (P Pym DI)Co(N 2 ) 3 as confirmed by IR spectroscopy. The reaction presumably proceeds via a Co–H bond homolysis reaction to yield H 2 alongside 3 . However, the timescale of the reaction ( t 1/2 = 120 min) rules out 4 as an intermediate in the reduction of 2 to 3 (full conversion 15 min, Scheme ), as confirmed by NMR spectroscopy…”