Terdentate Iron Complexes: Catalytic Hydrosilylation Activity And The Determination Of Electronic Structure Of Redox-Active Bis(Imino)Pyridine Iron Complexes.

摘要

A three-electron series of four-coordinate bis(imino)pyridine iron nitrosyl complexes was synthesized. The electronic structure of this series of compounds was determined by the use of X-ray crystallography, Mössbauer, IR, NMR, and EPR spectroscopies, and corroborated by DFT calculations. (iPrPDI)Fe(NO) was determined to be an intermediate-spin ferric complex with a triplet two electron reduced bis(imino)pyridine chelate and a triplet NO-. The overall spin state of (iPrPDI)Fe(NO) is S = ½. The oxidation of (iPrPDI)Fe(NO) occurs at the bis(imino)pyridine chelate, and the electronic structure of the iron nucleus remains intermediate-spin ferric. The reduction of (iPrPDI)Fe(NO) also is bis(imino)pyridine chelate centered. This yields a three-electron reduced chelate, leaving the iron intermediate-spin ferric and the nitrosyl as NO-. The reduction and oxidation of compounds was carried out to yield two other three-electron series. The synthesis and characterization of [Li(OEt2)3][(iPrPDI)Fe(CH2CMe3)(N2)], (iPrPDI)Fe(CH2CMe3), and [(iPrPDI)Fe(CH2CMe3)][BPh4] allowed for the determination of the degree of chelate participation over a three-electron series pertinent to olefin polymerization. The redox events were shown to occur at the bis(imino)pyridine chelate, leaving the iron nucleus Fe(II) throughout the series. [Na15-Crown-5][(iPrPDI)Fe(CO)2], (iPrPDI)Fe(CO)2, and [(iPrPDI)Fe(CO)2][BArF24] were also synthesized. Analysis of [(iPrPDI)Fe(CO)2][BArF24] indicates that oxidation of the formally Fe(0) complex (iPrPDI)Fe(CO)2 results in the oxidation of the bis(imino)pyridine chelate to a neutral ligand, giving a formally Fe(I) species. [Na-15Crown-5][(iPrPDI)Fe(CO)2] was not structurally characterized, but EPR spectroscopy indicates that the reduction occurred at the ligand, and the iron is low spin Fe(II). Bis(imino)pyridine iron complexes also were utilized as catalysts for the hydrosilylation of ketones, aldehydes, and olefins. The hydrosilylation of ketones and aldehydes was performed with primary and secondary silanes using bis(imino)pyridine iron dialkyl complexes and pybox iron dialkyls. The hydrosilylation of olefins was performed with tertiary silanes utilizing several reduced bis(imino)pyridine iron complexes. The result was reactivity that proved to be competitive with platinum based catalysis. In several instances iron outperformed platinum in terms of selectivity and fewer side-products.