SPring-8, the large synchrotron radiation facility

As a part of the Core Research for Evolutional Science and Technology (CREST) [Japan Science and Technology Agency (JST)], a research group led by Hikaru Takaya (associate professor) and Masaharu Nakamura (professor) of Kyoto University succeeded in directly observing iron-catalyzed cross-coupling reactions^*2 in the reacting solution using the BL14B2 beamline of the large synchrotron radiation facility, SPring-8.^*1 On the basis of direct evidence obtained by the solution-phase XAFS measurement, they proposed a new mechanism of iron-catalyzed cross-coupling reactions, being undefined for 45 years.

This study aimed to identify the catalytically active organoiron intermediates formed in the iron bisphosphine complex-catalyzed Kumada–Tamao–Corriu type cross-coupling reaction (KTC cross-coupling), which are almost invisible to the conventional NMR^*3 and EPR spectroscopies. The effectiveness of solution-phase X-ray absorption fine structure (XAFS)^*4 analysis for investigating these organoiron species were successfully demonstrated. Under the KTC cross-coupling condition with Fe^IIX₂(SciOPP) and mesitylmagnesium borimide, oxidation states and coordination geometries of the resulting Fe^IIBrMes(SciOPP) and Fe^IIMes₂(SciOPP) complexes were able to determined using solution-phase X-ray absorption near-edge structure X-ray absorption near-edge structure (XANES) spectroscopy, and the solution-phase molecular structure of these organoiron species were also confirmed using extended X-ray absorption fine structure (EXAFS) spectroscopy.

Nowadays, iron-catalyzed cross-coupling reactions have greatly impacted organic synthesis because of non-classical reactivities, such as for sp2–sp3 and sp3–sp3 cross-coupling reactions, in comparison with the conventional Pd-catalyzed cross-coupling reactions. Despite the practical advantages of iron, for example, low cost, low toxicity, high abundance on the earth, and easy separation of metal residue due to the lower ionization energy, the catalytic use of iron has been overlooked for 45 years since Kochi’s first discovery of iron-catalyzed cross-coupling reaction in 1971. The principal factor responsible for this backward is the strong paramagnetic property of iron which interferes with conventional solution-phase NMR- and ESR-based mechanistic studies. On the other hand, the Novel Prize chemistry of Pd-catalyzed cross-coupling reaction, and also the Ru-catalyzed methasesis, hydrogenation, dramatic progress had been achieved along with the prevailing of high-field FT-NMR equipped with superconducting magnet in the 1980s through 1990s, because the commonly diamagnetic low-spin 4d-transition-metal complexes well facilitate for solution-phase NMR-based characterization in the organic reaction mixture. For XAS-based study of catalysis, it has been well-established and a mature technique for mechanistic study of heterogeneous catalysts. In 1981, Stults, Friedman, Knowles, Lytle, and coworkers demonstrated the effectiveness of solution-phase XAS technique for the investigation of catalytic intermediates in the Novel Prized-Rh-catalyzed hydrogenation; however, the extremely limited accessibility to the strong X-ray source required for solution-phase SAX and the infant methods of XAS spectrum analysis had hampered the widespread use of XAS among chemists.
In this decade, the situation has changed. The easy access to 3rd generation synchrotron facilities provides more opportunities for convenient use of XAS technology by all chemists. The high brilliant and coherent X-ray provides enables us to carry out high resolution and excellent SN ratio solution-phase XAS analysis for the investigation of homogeneous catalysts in organic media. So recently there have been an excellent pioneering studies of XAS-based investigation of homogeneous iron salts-catalyzed reactions by Bauer and co-workers. However the nature of iron complex, easy formation of clusters, a large equilibrium constant of ligand exchange along with disproportionation, and the instability of organoiron species toward oxygen, moisture, and heat, complicates XAFS spectrum and the XAFS-based solution-phase structure elucidation of organoiron species has been still a challenging issue in the area of oganometallic catalysis.
The key to success of this research is the combination of solution-phase XAS and rapid single crystal X-ray analysis which affords the atomic coordinates of unstable organoiron complexes. Accurate EXAFS fitting simulation and DFT-level core excitation calculation for XANES pre-edge assignment made it easy to analyze XAS spectrum of high spin state organoiron. We also confirmed that the DFT-based atomic coordinates, obtained by B3LYP* with 6-31G** calculation under considering spin state and electron configuration, can be used for accurate XAS analysis. This result suggests that structure elucidation of crystallographically undefined organoiron species in homogeneous reaction mixture can be carried out by the combination of solution-phase XAS and DFT calculation. We consider that the wide scope of observable nuclei in XAS spectroscopy provides most practical method to investigate other paramagnetic organometallic catalyst such as Mn, Co, Ni, and Cu complexes all of which are significantly important metal for element strategy.

This research was conducted in collaboration with Hideo Nagashima (professor) and Yusuke Sunada (assistant professor) of Kyushu University, Tetsuo Honma and Masafumi Takagaki of SPring-8, Osamu Takahashi (associate professor) of Hiroshima University, and Daisuke Hashizume of RIKEN.

The report about the achievements of this study was selected as the BCSJ Award Article in the Bulletin of the Chemical Society of Japan (English) and published in print and online on 15 March 2015.

The achievements of this study were obtained as part of the following project, research area, and research topic.
Core Research for Evolutional Science and Technology (CREST)
Research area: Creation of Innovative Functions of Intelligent Materials on the Basis of Element Strategy
Research supervisor: Kohei Tamao, Director of Green-forefront Materials Department, RIKEN
Research topic: Development of Iron Catalysts for Advanced Organic Synthesis
Researcher director: Hideo Nagashima (professor) of Kyushu University)
Term of project: From October 2012 to March 2017

Publication:
Bulletin of the Chemical Society of Japan
Title："Investigation of Organoiron Catalysis in Kumada–Tamao–Corriu-Type Cross-Coupling Reaction Assisted by Solution-Phase X-ray Absorption Spectroscopy"
DOI : 10.1246/bcsj.20140376