Iron Complexes with Terdentate Ligands: Preparation, Electronic Structure Determination, and Utility as Catalyst Precursors

A series of bis(imino)pyridine iron complexes bearing monoanionic ligands, (PDI)Fe-X (PDI = 2,6-(ArN=CMe)2C5H3N; X = halide, alkyl, alkoxide, carboxylate) has been prepared and the electronic structure of each compound investigated. Combining spectroscopic, X-ray crystallographic, and magnetic data, the electronic structure of the halide, alkoxide, and carboxylate complexes has been described as having a high-spin ferrous center that is antiferromagnetically coupled to a bis(imino)pyridine radical. Magnetic and M?ssbauer spectroscopic data collected on the carboxylate complexes revealed a lower degree of antiferromagnetic coupling for due to a weaker ligand field. The electronic structure of the alkyl complexes appeared highly dependent on field strength of the hydrocarbyl group; sp3-alkyls were found to have high-spin ferrous centers while acetylides had intermediate spin centers. The electronic structure of the alkyl complex, (iPrPDI)Fe-(?3-C3H5), was best described as having an intermediate-spin ferric center antiferromagnetically coupled to two chelate radicals. The preparation of bis(imino)pyridine iron alkyl complexes possessing ?-hydrogen atoms was accomplished from the substoichiometric addition of alkyl bromides to (iPrPDI)Fe(N2)2. Because the electronic structure of the alkyl and halide complexes was elucidated, this reaction has been described as a one-electron oxidative addition, where oxidation occurs at the bis(imino)pyridine ligand rather than the metal. For these alkyls, the kinetic stability of each complex at ambient temperature was inversely proportional to the number of ?-hydrogen atoms present. A series of deuterium labeling experiments confirmed fast and reversible ?-hydrogen elimination and that transfer dehydrogenation of chelate isopropyl groups was a main decomposition pathway. Additionally, the scope of (iPrPDI)Fe(N2)2 mediated olefin hydrogenation has been expanded to include amine, ether, ketone, and ester containing substrates. Conducting stoichiometric experiments between each substrate and (iPrPDI)Fe(N2)2 in the absence of 4 atmospheres of dihydrogen revealed important catalyst degradation pathways. The C-O bond cleavage of allylic and vinylic ethers was observed over the course of hours at ambient temperature while ester addition to (iPrPDI)Fe(N2)2 often resulted in C-O bond cleavage to form the corresponding alkyl and carboxylate complexes. Although the redox-active bis(imino)pyridine chelate is known to stabilize a reducing ferrous center, one electron processes have often resulted in catalyst decomposition

Carbon-Oxygen Bond Cleavage by Bis(imino)pyridine Iron Compounds:Catalyst Deactivation Pathways and Observation of Acyl C-O Bond Cleavage in Esters

Investigations into the substrate scope of bis(imino)pyridine iron-catalyzed hydrogenation and [2 pi + 2 pi]. diene cyclization reactions identified C-O bond cleavage as a principal deactivation pathway. Addition of diallyl or allyl ethyl ether to the bis(imino)pyridine iron dinitrogen complex, ((iPr)PDI)Fe(N(2))(2) ((iPr)PDI = 2,6-(2,6-(i)Pr-C(6)H(3)N=CMe)(2)C(5)H(3)N, 1-(N(2))(2)), under a dinitrogen atmosphere resulted in facile cleavage of the C-O bond and yielded a mixture of the corresponding paramagnetic iron allyl and alkoxide complexes. For ethyl vinyl ether, clean and selective formation of the iron ethoxide was observed with concomitant loss of the vinyl fragment. In situ monitoring of the catalytic hydrogenation of trans-methyl cinnamate established ester C-O bond cleavage as a competing process. Stoichiometric reactions between 1-(N(2))(2) and allyl and vinyl acetate also produced facile C-O oxidative addition. For the latter, a six coordinate diamagnetic bis(imino)pyridine acetatoxy iron vinyl compound was obtained and characterized by X-ray diffraction. Phenyl acetate undergoes exclusive acyl C-O bond cleavage, while alkyl-substituted esters such as ethyl, pentyl, benzyl, isopropyl, cyclohexyl, and tert-butyl acetate undergo competing ester and acyl C-O bond cleavage accompanied by iron-promoted decarbonylation. Deuterium labeling studies established that reversible C-H activation and chelate cyclometalation Occur prior to, but are not a prerequisite for, carbon-oxygen bond oxidative addition of ethyl acetate. The molecular and electronic structures of the ether and ester C-O bond cleavage products have been established and demonstrate that ligand- rather than metal-based oxidation accompanies substrate activation.</p

Asymmetric catalysis using iron complexes : 'ruthenium lite'?

A review of recent developments in the use of iron catalysts for asymmetric transformations, including hydrogenations, transfer hydrogenation, hydrosilylation and oxidation reactions