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Investigation on transition metal-radical complexes for oxidative cleavage of aromatic CC bond, aliphatic CN bond and radical-to-metal electron transfer phenomena

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Investigation on Transition Metal-Radical Complexes for Oxidative Cleavage of Aromatic C  C Bond, Aliphatic C  N Bond and Radical-to-Metal Electron Transfer Phenomena” is an authentic account of the results obtained from the research work carried out by Mr. I. would like to thank many people who directly or indirectly contributed help, support and encouragement during the course of my research work.

Ganesh Chandra Paul Thesis Title

Date of Submission of Thesis

List of Publication

List of Conferences/Symposiums

Doctoral Committee

Abstract

General Introduction

Schematic representation of the ligands

Synthesis and Characterization of Non-Innocent Ligand, [H 2 Gan AP ] and its Corresponding Dinuclear  2 -oxo-Bridge Fe(III) and Mn(III) Complexes

Synthesis of Monoradical-Containing Four Coordinate Co-Complexes

Homolytic SS, SeSe Bonds Cleavage and Catalytic Conversion of Isocyanate to Urea under Sunlight

CONTENTS

General Introduction, Motivation, and Objectives

General Introduction, Motivation, and Objectives 3

Synthesis and Characterization of Non-Innocent Ligand, [H 2 Gan AP ] and its Corresponding Dinuclear  2 -oxo-Bridge Fe(III) and Mn(III)

Synthesis and Characterization of Five-coordinate Pyridine based Aminophenol Appended Non-innocent Ligand H 2 Gan AP

Synthesis and Characterization of Ligand H 4 L Piperazine(AP/AP)

Synthesis of Monoradical-Containing Four Coordinate Co- Complexes: Homolytic SS, SeSe Bonds Cleavage and Catalytic

Thesis Perspectives 164 Chapter VI

Equipment and Experimental Section

Appendices

LMCT: ligand-to-metal charge transfer ls: low spin MLCT: metal-to-ligand charge transfer I: nuclear spin. LLCT : ligand-to-ligand charge transfer HOMO : highest occupied molecular orbital IET : intramolecular electron transfer LUMO : lowest unoccupied molecular orbital SET : single electron transfer m/z : mass per charge.

Units

Symbols

Solvents and reagents

Techniques

Latin expressions

General Introduction, Motivation and Objectives

  • Introduction, Motivation, and Objectives
  • A synthetic model of the putative Fe(II)-iminobenzosemiquinonate intermediate in the catalytic cycle of o-Aminophenol Dioxygenases
  • Reaction of the iron(III)-aminophenolate complex with dioxygen
  • Electrocatalytic coupling of dibenzyl by Co(II)bis(o-phenylenediamine) complex

The role of the urea group has been implicated in influencing the catalytic reactivity (Scheme 1.4).12c. The active form of GOase consists of the Cu(II) ion, which is coordinated to the Tyr272 radical.

Synthesis and Characterization of Non-Innocent Ligand, [H 2 Gan AP ] and its Corresponding Dinuclear  2 -oxo-Bridge Fe(III) and Mn(III)

Complexes

Introduction

APD catalyzed CC bond cleavage of 2˗aminophenol

  • Synthesis and Characterization of Five-coordinate Pyridine based Aminophenol Appended Non-innocent Ligand H 2 Gan AP

A 1:1 reaction of 2-picolyl amine (I) with pyridinecarboxaldehyde (II) in CH3OH followed by NaBH4 reduction gave bis(2-pyridylmethyl)amine (III). 11 g of 2-[bis(2-pyridylmethyl) )aminomethyl]-nitrobenzene (V) was formed by the reaction of bis(2-pyridylmethyl)amine (III) with 2-nitrobenzyl bromide (IV) in dry CH3CN in the presence of K2CO3, which after reduction with Pd/C in the presence of NaBH4 provided 2-[bis(2-pyridylmethyl)aminomethyl)]aniline (VI).11h-i The H2GanAP ligand was obtained in 50% yield (Scheme 2.2) by the reaction of equimolar amounts of 2-[bis(2-pyridylmethyl)aminomethyl]aniline ( VI) and 3,5-di-tert-butyl catechol (VII) in hexane in the presence of Et3N in air. 2-[bis(2-pyridylmethyl)aminomethyl]aniline(VI) was then reacted with the generated 3,5-di-tert-butyl benzoquinone (BQ) to provide a compound (GanIBQ) that will be reduced by the same amount of 3,5 -di-tert-butyl catechol(VII) and consequently provided the ligand H2GanAP (Scheme 2.3).11j.

The 2-aminophenol unit of the synthesized ligand H 2 Gan AP might behave as non- innocent and thus might exist in different oxidation states in the presence of metal ions

  • Dioxygen Reactivity of an Iron Complex of 2-Aminophenol- Appended Ligand: Crystallographic Evidence of the Aromatic Ring

The 13C-NMR spectrum (Figure 2.3) for the H2GanAP ligand showed 23 distinct characteristic peaks for 23 different types of carbon atoms. The isotope distribution pattern of the observed mass peaks confirmed the composition as [C33H40N4O+H]+ for H2GanAP (Figure 2.4).

Synthetic route for the preparation of complex 2A

  • Mechanistic Investigation for the Formation of Complex 2A

FT-IR spectrum of complex 2A (Figure 2.6) showed a strong band at 1676 cm-1 due to the presence of (C=O) unit of the carboxylate group. Isotope distribution pattern indicated [C66H74Fe2N8O7]2+ composition for the observed positive mode peak at m/z = 601.22 and confirmed the formation of complex 2A.

Synthetic route for the preparation of complex 2B

  • Reactivity Study of the Species [Gan AP Fe III ] + with O 2
  • An Mono(  -Oxo)-Bridged Binuclear Mn(III,III) Complex Coordinated to Two Iminosemiquinone -Radical Anions

This was consistent with the formation of complex 2A as evidenced by ESI-MS analysis of the final solution (Figure 2.13A). To consolidate the proposal, the formation of complex 2A was carried out in the presence of H2O18 in CH3CN, and the positive mode ESI mass spectrum of the solution was recorded (Figure 2.14A). Furthermore, incorporation of three 18O atoms during the reaction consolidated the participation of the hydroxyl group in the formation of the furan derivative.

Synthesis, Structure, and Redox Properties

Synthesis route of dinuclear oxo-bridged manganese complex 2C

  • Conclusions

The cyclic voltammogram (CV) of complex 2C showed three successive one-electron oxidation waves at E1/2Ox. It is noteworthy that complex 2C is the first report of a mono(-oxo)-bridged binuclear Mn(III) complex with coordinated--radical anions. The CV of complex 2C in CH2Cl2 solution showed two one-electron reduction waves and three one-electron oxidation waves.

A New Piperazine-Based Non-Innocent Ligand, [H 4 L Piperazine(AP/AP)

Complexes: Synthesis and Characterization

Introduction

A growing interest in the field of metal radical complexes is caused by: biomimetic activity1a; redox isomerism;1b and the development of new catalytic systems.1c In addition, poly ® -semiquinone complexes exhibit various magnetic properties and thus can be used as potential building blocks for single-molecule-based magnetic devices.2a-c The specific features of transition metal complexes with paramagnetic metal ion- coordinated to redox-active ligands are known in the literature.1c While less attention has been given to the development of radical-containing metal complexes with metal ions of diamagnetic electronic configurations. Studying the magnetic coupling between ® -semiquinone ligands in diamagnetic metal complexes along with their structures would help reveal magnetic structural features, which in turn will allow designing metal complexes with desired magnetic properties. To investigate electronic structure and magnetic interactions between transition metal complexes and the metal-coordinated radical centers, ligand H4LPiperazine(AP/AP).

Synthesis and Characterization of Ligand [H 4 L Piperazine(AP/AP)

Furthermore, the probability of mononuclear complex formation with the boat shape of the piperazine backbone can be investigated. A single peak at  = 3.69 was attributed to the four basilic protons, and two broad peaks at  = 2.31 and  = 2.90 corresponded to each of the four aliphatic protons in the piperazine moiety.

  • Synthesis and Characterization of an Octahedral Co(III) Complex(3A) of H 4 L Piperazine(AP/AP)
  • Synthesis and Characterization of a Dinuclear Cu(II) Complex(3B) of H 4 L Piperazine(AP/AP)
  • Synthesis and Characterization of a Cu(II) Complex (3C) of H 4 L Piperazine(AP/AP)
  • Probable Pathway to the Formation of Complex 3C from Ligand H 4 L Piperazine(AP/AP)
  • Conclusions

Electrospray ionization mass spectrum (ESI-MS) of a solution of complex 3A in CH3OH provided a 100% molecular ion peak at m/z = 761.44 (Figure 3.8) in positive mode. ORTEP diagram of the complex is shown in Figure 3.9 and selected bond distances and bond angles are given in Table 3.2. ORTEP diagram of complex 3B is shown in Figure 3.15 and selected bond distances and bond angles are given in Table 3.4.

Monoradical-containing Four-coordinate Co(III) Complexes

Homolytic SS, SeSe Bonds Cleavage and Catalytic Isocyanate to Urea Conversion under Sunlight

Introduction

Over the years, metal-coordinated redox-active organic ligands, known as non-innocent ligands, have been continuously investigated as electron acceptors and/or electron donors for catalytic oxidation and reduction reactions.1,2 For example: Chaudhuri, Wieghardt and co-workers Cu(II)-bis(iminosemiquinone) -complexes have been extensively studied as functional models of Galactose oxidase for the two-electron oxidation of primary alcohols to their corresponding aldehydes1h; A four-coordinate Co(III)-bis(amidophenolate) complex was successfully used as a catalyst for CC bond formation reactions by Soper and colleagues.1a Recently, Sarkar and colleagues described an electrocatalytic CC bond formation reaction which makes use of electrochemically in situ generating a four-coordinate Co(II)-bis(1,2-diamide) complex as the catalyst and benzyl bromide as the substrate.1e In 2015, van der Vlugt, de Bruin and co-workers reported the redox active nature utilized. of a coordinated-2-amidophenoate derivative in a four-coordinate Pd(II) complex for the one-electron homolytic SS bond cleavage of diphenyl disulfide.1i It has recently been demonstrated that H2 gas can be generated using make from Cu(II) )-bis(iminoquinone) complex and NaBH4 in dry acetonitrile.1g. We envisioned that the presence of an ortho substituent would exert steric crowding and consequently favor the possibility of four-coordinate cobalt complexation over six-coordinate. Notably, cobalt complexes with unsaturated coordination environment are essential for substrate activation and catalysis.

Synthesis and Characterization of Ligand H 2 L AP(Ph)

The ligand scaffolds were primarily based on bidentate 2-anilino-3,5-di-tert-butylphenol (H2LAP) backbones with phenyl substituents in the ortho-position of the aniline moiety. Ligand H2LAP(Ph) was characterized by FT-IR spectroscopy, NMR spectroscopy, and mass spectrometry techniques. 13C-NMR spectrum (Figure 4.3) for the ligand H2LAP(Ph) showed the seventeen distinct characteristic peaks for 17 different kinds of carbon atoms.

Synthesis and Characterization of Co(III) Complexes (4A and 4B) of Ligand H 2 L AP(Ph)

Synthetic route for the preparation of complex 4A and complex 4B

  • Homolytic (S–S) and (Se–Se) Bonds Activation and Synthesis and Characterization of Co(III) Complexes (4C and 4D) with

Crystals suitable for single-crystal X-ray diffraction analysis of complex 4B were grown by slow evaporation of a CH 2 Cl 2 /CH 3 CN (3:1) solvent mixture of the complex. The ORTEP diagram of complex 4B is shown in Figure 4.7 and selected connection distances and connection angles are given in Table 4.1. The electronic absorption spectra of complex 4A and 4B were recorded in HPLC-grade CH2Cl2 at room temperature and shown in Figure 4.8.

Synthetic route for the preparation of complex 4C and complex 4D

  • Conversion of Isocyanate to Urea under Sunlight by Using Complexes (4A, 4C and 4E) as Catalyst

The diamagnetic character of the complexes was further supported by 1H-NMR analysis (Figure 4.14 and Figure 4.15). A similar phenomenon was observed for 4D (Figure 4.18A), where 4A was formed by cleavage of the Co-SePh bond. The appearance of a co-centered X-band EPR signal (Figure 4.17B and Figure 4.18B) in the processes further supported the formation of 4A.

Reaction scheme for the catalysis

  • Conclusions

General method for catalysis: A pressure tube was first evacuated and then filled with argon. The low TON (4-6) in the absence of sunlight (Table 4.6, entries 8 and 10) supported the possibility that a five-coordinate intermediate bound to the substrate was formed during catalysis (Figure 4.20) and a photostimulus was then essential to regenerate the four coordinates. Depending on the aforementioned results, a mechanistic proposal for the CN coupling between two isocyanate molecules to form the urea derivative is given in Figure 4.20.

Geometry Driven Iminosemiquinone Radical to Cu(II) Electron Transfer and Formation of an Elusive Five-Coordinate Cu(I)

Introduction

Thus, understanding metal-ligand interactions in the geometry-dependent electron transfer processes is one of the compelling objectives. To clarify the geometric dependence of the electron transfer process, one-electron oxidation of 5A was carried out using ferrocenium hexafluorophosphase (FcPF6) as the oxidant. The thus formed four-coordinate Cu(II) complex [CuII(LISQ(Ph))(LIBQ(Ph))]PF6 (5C) refrained from such electron transfer phenomenon.

Synthesis and Characterization of Ligand H 2 L AP(Ph)

The steric hindrance created by the phenyl rings will thus favor the approach of only one CuCl2 molecule along the axial position to the Cu(II) center and therefore one electron oxidation will lead to the expected complex. Interestingly, instead of the expected complex formation, a five-coordinate chloride-linked Cu(I)-bis(iminoquinone) complex [CuI(LIBQ(Ph))2Cl]0 (5B) was generated via ligand[iminosemiquinone]-to - metal[Cu(II)]. In addition, a four-coordinate [CuI(LIBQ(Ph))2]SbF6(5D) complex was synthesized from complex 5B to extend the investigation of the geometry-dependent metal-to-ligand electron transfer process.

Synthesis and Characterization of Copper Complexes (5A and 5B) of Ligand H 2 L AP(Ph)

Synthetic route for the preparation of complex 5A and complex 5B

  • Synthesis and Characterization of Complexes 5C and 5D

The ORTEP diagram of complex 5A is shown in Figure 5.3 and selected bond distances and bond angles are given in Table 5.1. The CV of complex 5B showed one one-electron oxidation and three one-electron reduction waves (Figure 5.7B). To support the formation of Cu(II) species, the X-band EPR spectrum of the oxidized solution was measured and shown in Figure 5.8B.

Synthetic route for the preparation of (A) complex 5C and (B) complex 5D

  • Reactivity Study of Complex 5B, 5C and 5D with KO 2
  • Conclusions

Single crystal of complex 5D suitable for X-ray diffraction measurement was obtained by the slow evaporation of a CH2Cl2/MeOH (3:1) solution of the complex. One one-electron oxidation and three one-electron reduction processes were observed in the CV of complex 5C (Figure 5.17A). The increase of ligand-based negative character may have favored the removal of the chloride ion.

Thesis Conclusion and Perspectives

Proposed ligands (L1 and L2) for synthesizing new complex

  • Methods and Equipments

1H, 13C NMR spectra were recorded on 'Varian Mercury plus 400 MHz' and on 'Bruker Ascend™ 600 MHz' nuclear magnetic resonance (NMR) spectrometer at 298 K. The determination of C, H, N was performed on 'FLASH EA 1112 series ' CHN Analyzer at SAIF, Mumbai, on 'Perkin – Elmer 2400 series II' CHN Analyzer at IACS, Kolkata and on 'EuroEA3000' Elemental Analyzer at Guwahati Biotech Park, Guwahati. The thermal electronic absorption spectrum of the complex (2A) was recorded on 'Perkin Elmer, Lamda 350, UV/Vis/NIR spectrometer' in HPLC grade toluene at a temperature range of 30–70 °C.

References

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