Influence Of Ligand Modification And Electronic Structure In Homogeneous Catalysis With Reduced Iron Complexes
The synthesis of a family of para-substituted pyridine di(imine) ligands, and their corresponding iron dihalide, dicarbonyl and dinitrogen complexes are described. The influence of the substituents on the electronic structure of each derivative was investigated using a combination of X-ray diffraction, cyclic voltammetry, infrared, Mößbauer, and NMR spectroscopies as well as density functional theory. No dramatic changes to the electronic structure description were observed as a result of the para-substituent. However, significant differences in catalytic activity were observed for the hydrogenation of olefins and the [2[pi] + 2[pi]] cycloaddition of [alpha],[omega]-heptadienes, and were also found to be substrate dependent. The electronic structure and catalytic activity of a structurally related pyridine di(carbene) complex, (iPrCNC)Fe(N2)2 was investigated. The spectroscopic data establish that (iPrCNC)Fe(N2)2 and several of its derivatives are redox non-innocent. They are best described as hybrid structures with [(iPrCNC0)Fe0] and [(iPrCNC2-)FeII] resonance forms with the iPrCNC ligand acting as a [pi]-acceptor. The activity of (iPrCNC)Fe(N2)2 for the catalytic hydrogenation of olefins and [2[pi] + 2[pi]] cyclization of [alpha],[omega]-diolefins was also investigated. (iPrCNC)Fe(N2)2 exhibited hydrogenation activities that were consistent with the trends observed in pyridine di(imine) iron olefin hydrogenation, but exhibited no activity for the [2[pi] + 2[pi]] cycloaddition of [alpha],[omega]-heptadienes. Isotopic experiments indicated that a competitive, unproductive 2,1-insertion pathway or a vinylic C-H activation mechanism was responsible for the deviation in hydrogenation activity trends observed with 1,1-diphenylethylene and the electron-rich iron catalysts. A series of dialkyl dinitrogen complexes were prepared from treatment of (iPrCNC)FeBr2 with two equivalents of the corresponding alkyl lithium reagent. The iron dialkyl dinitrogen complexes were characterized by infrared, NMR and Mößbauer spectroscopies as low-spin, ferrous compounds. One member of the series, (iPrCNC)Fe(CH3)2(N2) underwent extrusion of ethane at 23 [MASCULINE ORDINAL INDICATOR]C. The mechanism of this transformation was investigated by isotopic cross-over experiments. The dimeric iron bis(dinitrogen) complex, [(MeBPDI)Fe(N2)]2([MICRO SIGN]2-N2) was prepared by sodium naphthalenide reduction of the corresponding iron dichloride. This complex was established to be more susceptible to contamination by THF complexes, and underwent the irreversible formation of [eta]6-arene species in solution. [(MeBPDI)Fe(N2)]2([MICRO SIGN]2-N2) and (iPrBPDI)Fe(N2)2 exhibited remarkably lower activity for the hydrogenation of ethyl 3,3dimethylacrylate relative to (iPrPDI)Fe(N2)2. NMR studies and stoichiometric experiments established that this was due to the more electrophilic iron center as engendered by the BPDI chelate, resulting in a greater affinity for carbonyl coordination. A family of bis(oxazoline) iron dialkyl complexes were prepared by treatment of their ferrous dihalide precursors with the appropriate alkyl lithium reagent and examined for the asymmetric hydrosilylation of ketones using PhSiH3 as the stoichiometric reductant. While these complexes expressed high levels of activity, low enantiomeric inductions were observed regardless of the identity of the auxiliary ligand or the alkyl substituent. Activation of these complexes in situ using B(C6F5)3 resulted in higher levels of enantiomeric induction; however, synthetically useful e.e.'s were not observed.
Homogeneous Catalysis; Iron; Hydrogenation
Ganem, Bruce; Wolczanski, Peter Thomas
Chemistry and Chemical Biology
Ph. D., Chemistry and Chemical Biology
Doctor of Philosophy
dissertation or thesis