| Autor: |
Kovel CB; Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States., Darmon JM; Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States., Stieber SCE; Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States., Pombar G; Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States., Pabst TP; Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States., Theis B; Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States., Turner ZR; Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States., Üngör Ö; Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States., Shatruk M; Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States., DeBeer S; Max Planck Institute for Chemical Energy Conversion, Stiftstraße 34-36, 45470 Mülheim an der Ruhr, Germany., Chirik PJ; Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States. |
| Abstrakt: |
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, ( Me PDI)FeCH 3 and ( Et PDI)FeCH 3 ( R PDI = 2,6-(2,6-R 2 -C 6 H 3 N═CMe) 2 C 5 H 3 N), and the alkyl-substituted examples, ( Cy APDI)FeCH 3 ( Cy APDI = 2,6-(C 6 H 11 N═CMe) 2 C 5 H 3 N), 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, ( iPr PDI)FeCH 3 , has an idealized planar geometry and exhibits spin crossover behavior from S = 1/2 to S = 3/2 states. At 23 °C under an N 2 atmosphere, both ( Me PDI)FeCH 3 and ( Et PDI)FeCH 3 underwent reductive elimination of ethane to form the iron dinitrogen precatalysts, [( Me PDI)Fe(N 2 )] 2 (μ-N 2 ) and [( Et PDI)Fe(N 2 )] 2 (μ-N 2 ), respectively, while ( iPr PDI)FeCH 3 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, ( R PDI)Fe(η 4 -C 4 H 6 ) (R = Me, Et, i Pr), 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 N 2 or butadiene to access a catalytically active iron compound. |
