Reduced Bis(Imino)Pyridine Iron And Manganese Complexes: Electronic Structure, Reactivity Comparisons And Synthesis Of An Iron Carbene

摘要

The synthesis, electronic structures and reactivity of bis(imino)pyridine iron and manganese complexes were investigated. A series of dimeric bis(imino)pyridine iron dinitrogen compounds was prepared by reduction of the corresponding iron dihalide complexes with sodium naphthalenide. The dinitrogen compounds were shown to have electronic structures similar to the monomeric bis(imino)pyridine iron bis(dinitrogen) compound, (iPrPDI)Fe(N2). Evaluation of the catalytic olefin hydrogenation activity of the new dinitrogen compounds revealed a significant improvement in rate over (iPrPDI)Fe(N2), which was attributed to the smaller substituents on the bis(imino)pyridine aryl groups. The reactivity of the bis(imino)pyridine iron dinitrogen compounds with diazoalkanes was investigated. For all of the iron dinitrogen compounds, diazoalkane N-N bond cleavage was observed with monosubstituted diazoalkanes. Addition of diphenyldiazomethane to smaller bis(imino)pyridine iron dinitrogen compounds furnished bis(imino)pyridine iron carbene complexes. A combination of 1H NMR, Mössbauer and X-ray absorption spectroscopies, SQUID magnetometry and X-ray diffraction was used to propose several possible electronic structures for the bis(imino)pyridine iron carbene complexes. The spectroscopic data most strongly supported an electronic structure having an intermediate spin iron(III) center and a one-electron reduced bis(imino)pyridine chelate. The bis(imino)pyridine iron carbene complexes underwent carbene transfer to carbon monoxide and aryl azides, but not cyclopropanation or olefin metathesis reactions. The bis(imino)pyridine iron dinitrogen compounds were also found to be competent catalysts for carbon-carbon bond forming catalysis. Addition of ethylene to butadiene in the presence of 5 mol% iron catalyst resulted in a [2[pi] + 2[pi]] cycloaddition to furnish vinylcyclobutane, while addition of ethylene to isoprene under the same conditions resulted in 1,4hydrovinylation to yield 5-methyl-1,4-hexadiene. Under similar reaction conditions, [(MePDI)Fe(N2)]2([mu]2-N2) also catalyzed the cycloisomerization of 1,5-hexadiene. Finally, syntheses of variations of the bis(imino)pyridine iron dinitrogen compounds were investigated. Reduction of a bis(aldimino)pyridine iron dibromide did not afford an iron dinitrogen compound, but instead gave a diiron compound. Modeling variable temperature SQUID data for this compound provided experimental evidence for the redox activity of the bis(aldimino)pyridine ligand. The reduction of a bis(imino)pyridine manganese dichloride also did not afford a dinitrogen complex, but instead resulted in a bis(tetrahydrofuran) compound.