Synthesis, Characterization, and Preliminary Oxidative Reactivity Study of Two Novel Cu Complexes with a Tripodal Amidate Ligand Restricted; Files Only

Zhang, Yiran (Spring 2022)

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Oxidation reactions are an important class of chemical reactions. With the consideration that many traditional oxidants tend to generate much chemical waste, efforts have been devoted to developing catalysts that can activate O2 as a more sustainable oxidant. Cu-centered oxidases and oxygenases are crucial models for designing O2-activation catalysts. Among the O2-activating Cu centers found in nature, a specific type features a mononuclear Cu center with a non-planar, histidine-rich coordination environment, such as the active sites in peptidylglycine-alpha-hydroxylating monooxygenase (PHM) and Cu amine oxidase (CAO). To simulate the O2 reactivity of such Cu centers, many synthetic Cu complexes bearing tripodal ligands with four coordinating nitrogen atoms (tripodal-N4 Cu complexes) have been prepared and studied as modeling compounds. Despite the abundant investigations made in this field, all the reported tripodal-N4 ligands are neutral. Therefore, with the purpose to expand the scope of tripodal-N4 Cu complexes available as candidate catalysts for O2 utilization, a redox pair of Cu complexes supported by an anionic amidate ligand N(o-PhNHC(O)CF3)3 are prepared in the current study.

In chapter 2 of the thesis, synthesis, characterization, and preliminary oxidative reactivity of (PPh4)2[Cu(N(o-PhNC(O)CF3)3)] (1) and PPh4[Cu(N(o-PhNC(O)CF3)3)] (2) are presented. Both complexes are produced in high yields, and their solid-state characterization reveals unusual structural features which can be attributed to the charged ligand backbone. The solution-state behavior of the two complexes cannot be unambiguously described so far. The reaction of complex 1 with O2 at room temperature displays a reaction profile inconsistent with the previously established pathway where a Cu(II)-superoxo intermediate is involved, and several postulates are proposed.

In the same chapter, two unexpected discoveries made in this study are also included: first, a transformation of PPh4+ to P(O)Ph3 is detected while reacting complex 1 with Et4NCN, and kinetic experiments indicate the positive roles of O2 and Et4NCN; second, hydroxylation-defluorination of the ligand backbone in complex 2 is observed while optimizing the preparation of complex 2. Curiosity into these two findings and other unresolved results in the current study raises several questions for future research.

Table of Contents

Chapter 1: Development of Tripodal-N4 Cu Complexes Modelling Cu-centered Oxygenases and Oxidases Reactivity......1                  

Figure 1-1. Hydroxylation reactions catalyzed by PHM and DβM in nature……2                                                                                                                                                                                     

Figure 1-2. The oxidized catalytic core of PHM……3                                                                               

Figure 1-3. Proposed hydroxylation mechanisms for PHM/ DβM……4                                                                  

Figure 1-4. Oxidative deamination reaction catalyzed by CAO……5                                                                        

Figure 1-5. The oxidized resting state Cu center in the CAO active site……5                                                      

Figure 1-6. Proposed oxidative deamination mechanisms for CAO……7                                                             

Figure 1-7. Reported tripodal-N4 ligands……9                                                                                         

Figure 1-8. Mechanism for Cu(I) reacting with O2……10                                                                                                   

Figure 1-9. Oxidation of p-X-DTBP by [Cu(L)(O2)]+ via HAT……11                                                                                

Figure 1-10. Intramolecular C-H activation in [Cu(TMG3tren)(O2)]+……12                                                         

Figure 1-11. Intermolecular C-H Activation in BNAH mediated by [Cu(mppa)(O2)]+……12

Chapter 2: Synthesis, Characterization, and Preliminary Reactivity Study of Two Novel Cu Complexes with a Tripodal Amidate Ligand......19               

Scheme 2-1. Synthesis of the N(o-PhNHC(O)CF3)3 Ligand……20                                                                                  

Scheme 2-2. Syntheses of Complex 1 and 2……21                                                                                                                  

Figure 2-1. Solid-state molecular structures of complex 1 and 2……22                                                                 

Table 2-1. Selected Bond Lengths (Å) and Angles (°) for Complex 1 and 2……22                                             

Figure 2-2. UV-vis spectrum of complex 1 in acetonitrile and dichloromethane at 25℃……24                                                      

Figure 2-3. 1H NMR of complex 1 in deuterated acetonitrile and dichloromethane……25

Figure 2-4. Cyclic voltammogram of complex 1 at 25℃......26

Figure 2-5. 1H NMR of complex 2 in deuterated dichloromethane and acetonitrile......28

Figure 2-6. 1H NMR of complex 2 in deuterated dichloromethane and acetonitrile (zoomed-in)......29

Figure 2-7. UV-vis spectrum of complex 1 in acetonitrile with excess dioxygen at 25℃......30 

Figure 2-8. UV-vis spectrum of complex 2 in acetonitrile at 25℃......32                                                                                                                 

Scheme 2-3. Reaction of complex 1 with Et4NCN in Drybox......33

Figure 2-9. Formation of P(O)Ph3 in reaction between complex 1 and Et4NCN in acetonitrile at 25℃ in drybox and under an O2 atmosphere......33

Figure 2-10. Infrared spectrum of dried reaction mixture of complex 1 with Et4NCN in acetonitrile at 25℃ in drybox......34

Figure 2-11. Structure of [Cu2(tmpa)2CN]+…… 35                                 

Scheme 2-4. Synthesis of Complex 1’……35                                                                                                                                

Figure 2-12. Solid-state molecular structure of complex 1’……36                                                                             

Figure 2-13. Infrared spectrum of white precipitate in reaction mixture of complex 1 with Et4NCN in acetonitrile at 25℃ in drybox......37

Figure 2-14. Molecular structure of complex 3……38                                                                                                         

Table 2-2. Bond Lengths (Å) and Angles (°) for Complex 3……38                                                                                  

Figure 2-15. Hydroxylation-defluorination mechanism of nonheme iron 2-oxoglutarate dependent oxygenases……39                                                                                               

Experimental Section……42                                                                                                                                                                  

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