Introduction
In a textbook for high-school students, "chemical reaction" is defined to "the process by which one or more substances change to produce one or more different substances". This definition is correct; however, most of chemical reactions do not occur in a moment, and proceed via multistep processes called elementary reactions (Fig.1(a)). The transformation reactions catalyzed by coordination compounds are also no exceptions. For example, the well-known Pd-catalyzed coupling reaction shown in Fig.1(b) includes three elementary reactions.The transformation reactions catalyzed by coordination compounds have contributed for our fluitful life by providing numerous useful substances, and this research field is now heading toward the highly challenging transformation reactions. In this field, we are focusing on the development of coordination compounds for the transformation reactions that require extremely multi-step elementary processes.
Fig.1 : (a)A general mechanism of chemical reactions.(b)A coupling reaction catalyzed by a Pd complex and the mechanism.
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Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457–2483 (Link)
Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457–2483 (Link)
Concept
Our methodology to achieve the purpose described above is to use flexible ancillary ligands. The transformation reactions containing multi-step elementary processes form versatile intermediates. For example, during the catalytic ammonia synthesis from dinitrogen (Fig. 2(a)), the electronic environment around the molybdenum center is expected to change dramatically with the change of oxidation states. In such reactions, flexible ancillary ligands are expected to inhibit the decomposition of intermediates by tuning the electronic and steric environment around the metal center. Based on this strategy, we have focused on "indolyl", the anionic species obtained from the deprotonation of the N–H group of indole. As shown in Fig.2(b), indolyl exhibits a variety of coordination modes when coordinating to a metal center. If indolyl can maintain this coordination feature in the polydentate ligands, indolyl-based polydentate ligands potentially serve as ancillary ligands for the transformation reactions that require extremely multi-step elementary processes.
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Nishibayashi, Y. (Ed.) Transition Metal-Dinitrogen Complexes, Chap. 1 (Link)
Ohta, S. et al. Bull. Chem. Soc. Jpn. 2018, 91, 1570–1575 (Link)
Nishibayashi, Y. (Ed.) Transition Metal-Dinitrogen Complexes, Chap. 1 (Link)
Ohta, S. et al. Bull. Chem. Soc. Jpn. 2018, 91, 1570–1575 (Link)
Acievements
To initiate this study based on the concept described above, we explored the coordination compounds carrying the indoly-based polydentate ligands. As a result, we found an unsolved issue: "Does indolyl exhibit a variety of coordination modes even in the polydentate ligands?" Thus, we have tried to reveal the question for the coordination features of indolyl in the polydentate ligands. So far, we have revealed that the coordination geometry of the indolyl nitrogen atom in bis(indolyl) ligands coordinated to a titanium or zirconium center is affected by the steric impact of the substituents attached to the central aromatic ring (Fig.3). Moreover, some of these complexes were found to serve as catalyst precursors for alkyne hydroamination and ethylene polymerization.
Fig.3 : Structures of bis(indolyl)-coordianted titanium diamido complexes and the geometries around the N3 atoms.