Catalyst Design of Iron Complexes

Abstract

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 "(R 3 TACN)FeX 2 " 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.

Manipulating metal spin states for biomimetic, catalytic and molecular materials chemistry

“…58 For example hydrogenation and other catalytic double bond reductions proceed in two-electron oxidative addition/reductive elimination steps. 59 These processes are favoured for low-spin catalysts 60 or, rarely, for high-spin species that can access a spin-paired reaction pathway through twostate reactivity. 61 Alternatively, some base metal catalysts circumvent the requirement for two-electron metal redox with a non-innocent supporting ligand, which acts as an H atom acceptor during the catalytic cycle.…”

A Physical‐Inorganic Approach for the Elucidation of Active Iron Species and Mechanism in Iron‐Catalyzed Cross‐Coupling

“…While not covered in this review, Nakamura and and Nagashima have previously described the use of XAS in iron-catalyzed cross-coupling reactions. [78–79] …”

Iron-Catalyzed Homogeneous Hydrosilylation of Ketones and Aldehydes: Advances and Mechanistic Perspective

“…Due to its abundance, many low-price iron salts are commercially available for their use at laboratory or industry scale. Consequently, the use of iron-based catalysts for organic reactions has received considerable attention during the last two decades, and several reviews have summarized the advances in this field . Especially after the review by Bolm et al in 2004, some organic transformations have contributed to remarkable developments, reductions among them, as has been highlighted in the extensive recent review by Bauer and Knölker .…”

“…The use of iron as a core metal in homogeneous catalysis has become a research topic of interest during the last years . The reasons are clear: iron is the most abundant transition metal in Earth’s crust, is widely distributed across the world and it has been extracted and processed since the dawn of civilization.…”