Despite worldwide interest from synthetic chemists, the rational design of catalytically active organoiron species remains problematic. While noble metal catalysis proceeds through diamagnetic low-spin intermediates, iron species are often in the high or intermediate spin states, which are paramagnetic and difficult to analyze. Possible spin change during catalysis also complicates the problem. This report describes two extremes for the catalyst design of iron complexes. One involves diamagnetic 14-electron iron(II) species useful for two-electron chemistry often seen in noble metal catalysis. The disilaferracyclic carbonyl complex 4 is a good catalyst precursor, and shows good catalytic performance for the hydrogenation and hydrosilylation of alkenes, and the hydrosilane reduction of carbonyl compounds. Based on DFT calculations, mechanisms involving ·-CAM (sigma-complex-assisted metathesis) for the hydrogenation and hydrosilane reduction are suggested. Further catalyst design inspired by the success of 4 led to the discovery of iron and cobalt catalyst systems composed of metal carboxylates and isocyanide ligands leading to a practical substitute for industrially useful platinum catalysts for hydrosilylation with hydrosiloxanes. The second approach involves paramagnetic 16-electron iron (II) catalyst species. A series of "(R3TACN)FeX2" complexes were prepared and found to be good catalysts for atom transfer radical polymerization, giving rise to well-controlled polymerization of styrene, methacrylates, and acrylates with high activity. Moreover, the catalyst could be easily removed from the polymer and was reusable. Mechanistic studies of iron-catalyzed crosscoupling reactions in collaboration with Nakamura and Takaya opened a new approach to the catalyst design of unknown spin states by using new analytical methods for paramagnetic species in the solution state.
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