2022 Fiscal Year Annual Research Report
Construction of tetrahedral chiral-at-metal complexes towards asymmetric light-driven redox catalysis
| Project/Area Number |
22J12312
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| Allocation Type | Single-year Grants |
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| Research Institution | The University of Tokyo |
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Principal Investigator |
LIU Yuanfei 東京大学, 理学系研究科, 特別研究員(DC2)
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| Project Period (FY) |
2022-04-22 – 2024-03-31
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| Keywords | nickel / cobalt / chiral-at-metal / tetrahedral geometry / redox-active / spontaneous resolution |
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| Outline of Annual Research Achievements |
In one year research, I mainly focus on construction and resolution of redox-active chiral-at-nickel(II) and chiral-at-cobalt(II) complexes.
First, I constructed two tetrahedral chiral-at-nickel(II) complexes using an unsymmetrical, achiral tridentate ligand and characterized them by NMR, ESI-MS, single-crystal XRD and UV-vis absorption spectroscopy. The structural information was carefully elucidated. The complexes adopt a tetrahedral geometry and exhibit redox activity as well as visible-light absorption. Also, they are capable of ligand exchange for substrate activation.
Second, I succeeded in spontaneous resolution of S/R-[NiL(ClLi(thf)2)] complex to conglomerate crystals at low temperature. I optimized the crystallization conditions and elucidated the crystallographic structure. I found that hydrogen bondings are essential to achieve spontaneous resolution. Starting from achiral building blocks, the ligands and nickel ions assemble chiral-at-metal complexes, 1D chiral helices, 3D chiral suprastructure and finally homochiral crystals in a highly asymmetric manner. This is related to the origin of homochirality in biological systems in ancient earth and sheds light on the concept of crystal engineering.
I also successfully constructed a tetrahedral chiral-at-cobalt(II) complex and charactered it by NMR, ESI-MS, single-crystal XRD, UV-vis absorption spectroscopy. Together with silver salts, the complex was found to be capable for carbon-carbon bond cleavage of 1,2-diarylethylenediamine. The reaction condition is under optimization currently.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
First, I successfully constructed chiral-at-nickel(II)and cobalt(II)complexes and characterized them as I had planed. The complexes adopt a tetrahedral geometry and exhibit redox activity as well as visible-light absorption. Also, they are capable of ligand exchange for substrate activation.
Second, although diastereoselective synthesis and chiral chromatography separation didn't work with the nickel complex. During crystallization, I happened to discover that the complex tends to form conglomerate crystals at low temperature. This is called spontaneous resolution and can separate enantiomers without using chiral substances.
Finally, I found that together with silver salts, the cobalt complex was capable of carbon-carbon bond cleavage of 1,2-diarylethylenediamine. Optimization is ongoing.
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| Strategy for Future Research Activity |
First, I will examine configurational stability of the complex by circular dichroism and NMR with a chiral shift reagent. Also I will check electronic properties by cyclic voltammetry, EPR and DFT calculation.
Second, I will try crystallization-induced chiral transformation: Add one piece of conglomerate crystal to the racemic mixture solution. Under the optimized condition, same configured complexes tend to crystallize on the conglomerate crystal surface. By utilizing the equilibrium of enantiomers in solution phase, finally all crystals are of same configuration.
Finally, I will explore the asymmetric catalytic function of the nickel complex. The complex is expected to be applied to asymmetric photo-induced redox reactions, like reductive coupling of aldehydes and alkenes.
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