High Oxidation State, Late Transition Metals Featuring Metal-Ligand Multiple Bonds: Strategies for Sequential C-H Bond Activation and Functionalization

Reports: G3 48410-G3: High Oxidation State, Late Transition Metals Featuring Metal-Ligand Multiple Bonds: Strategies for Sequential C-H Bond Activation and Functionalization

Theodore A. Betley, Harvard University

The subject of our award was for the discovery of iron-based C-H bond functionalization catalysts. Towards this end, we have synthesized a family of semi-porphyrin ligands (dipyrromethenes) to form Fe(II) coordination compounds. This involves extensive organic and inorganic synthetic procedures. Routine characterization involves multi-nuclear NMR, 57Fe Mossbauer, EPR, SQUID magnetometry, cyclic voltammetry, and UV-Vis absorption spectroscopy. Reactive intermediates have been modeled using Gaussian 3.0 software packages purchased in the department.

Room temperature, catalytic C–H bond amination and olefin aziridination have been observed from the reaction of organic azides with a simple iron(II) coordination complex. Isolation of a reactive intermediate reveals the putative nitrene-delivery precursor to be a high-spin (S = 2) iron complex featuring a terminal imido ligand. Crystallographic, spectroscopic, and theoretical analyses suggest a formulation for this reactive species to be a high-spin iron(III) center antiferromagnetically coupled to an imido-based radical. We proposed the catalytic activity of the iron imido is directly attributable to its electronic structure. Our findings have contributed to the fields of organomellic, inorganic, and bioinorganic research in the following two ways: 1. We have identified a simple iron-based catalyst that rivals the efficacy of dirhodium catalysts for atom and group transfer reactions. During the funding period we hope to make our catalysts viable alternatives to expensive rhodium-based catalysts. 2. We have isolated a previously uncharacterized metal-ligand multiple bond formulation. Traditional iron-based catalysts are thought to be mediated by Fe(II)-Fe(IV) cycles, while we have established that the operativecatalytic species is Fe(III) with a ligand based-radical. Similar electronic formulations have been proposed for Fe(oxo) transferases in biochemical cycles (as ferric oxyl radicals) and for related Cu-based nitrene transfer catalysts (reported by Warren and coworkers). Our isolation of this reactive intermediate is a first in the field and suggests a reformulation of how catalytic group transfer processes operate.

As mentioned in the previous section, our understanding of the catalytic system we have developed mirrors many of the spectroscopic signatures found in biological O-atom transfer catalysts found in non-heme, iron-based systems. While the typical formulation of the reactive high-spin iron oxo complexes has been Fe(IV), the similarities between the reaction profile and spectroscopic signatures of the Fe(III)(imide radical) suggest similar electronic structures may be present.

Publications during or resulting from the funding period:

King, E. R.; Betley, T. A." Unusual Electronic Structure of First Row Transition Metal Complexes Featuring Redox-Active Dipyrromethane Ligands." J. Am. Chem. Soc. 2009, 131, 14374-14380.

King, E. R.; Betley, T. A. " C-H Bond Amination from a Ferrous Dipyrromethene Complex." Inorg. Chem. 2009, 48, 2361-2363

Austin B. Scharf, Theodore A. Betley, "Electronic perturbations of iron dipyrromethene complexes by variations in the ligand periphery." Abstracts of Papers, 240th ACS National Meeting, Boston, MA, United States, August 22-26, 2010 (2010), INOR-733, (2010).

Elisabeth T. Hennessy, Theodore A. Betley, "Catalytic C-H bond amination and aziridination using organoazides from a ferrous dipyrromethene platform." Abstracts of Papers, 240th ACS National Meeting, Boston, MA, United States, August 22-26, 2010 (2010), INOR-660, (2010).

Theodore Betley, Elisabeth Hennessy, Evan King, Guy Edouard, "Catalytic, iron-mediated C-H bond functionalization chemistry." Abstracts of Papers, 240th ACS National Meeting, Boston, MA, United States, August 22-26, 2010 (2010), INOR-654, (2010).

Evan R. King, Theodore A. Betley, "Dipyrromethene supported low-coordinate high-spin iron complexes and their reactivity as platformsfor C-H functionalization via oxidative group transfer." Abstracts of Papers, 240th ACS National Meeting, Boston, MA, United States, August 22-26, 2010 (2010), INOR-608, (2010).