具不同軸取代之脯胺醇配位基於含鎳超氧化物歧化酶擬態化合物之合成、鑑定及對超氧離子活性探討

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(1)國立台灣師範大學化學系碩士論文. 指導教授：李位仁博士. 具不同軸取代之脯胺醇配位基於含鎳超氧化物歧化酶擬態化合物之合成、鑑定及對超氧離子活性探討 Synthesis, Characterization and Superoxide Reactivity of Prolinol-based NiSOD Mimics with Various Axial Ligands. 研究生：陳虹伶. 中華民國一百零三年六月.

(2) 謝誌感謝指導教授李位仁老師這些年悉心的教導，時常的討論並指點我正確的方向，讓我在專業領域及待人處事上都有許多成長。老師對學問的熱誠更是我輩學習的典範。感謝口試委員廖文峯老師、王雲銘老師和洪政雄老師在百忙之中來參加口試，於期間提供許多寶貴的建議且不吝指正論文之疏漏處，讓我獲益良多，使本論文能更臻完備，在此致謝。感謝國科會的在研究經費上的支持，特別感謝師大貴儀郭頂審助教、清大貴儀陳若琪小姐、台大貴儀陸靖蔚小姐及林震煌老師的學生協助實驗數據的收集，你們的耐心使我的研究得以順利進行。感謝建緯學長自我專題生時期起便時常教導我諸多研究方法，感謝子立學長和皓晴學長在實驗上的指教，還有個別指導過我的惠文學姊、阿勇學長，以及其他幫助過我的學長姐們。一起奮鬥的于凡、俊傑和筑翔、延壕、少緯等學弟妹們，謝謝你們包容我、支持我花費時間在準備考試上。你們在實驗生活上的關懷及鼓勵，讓我留下許多美好的回憶。最後感謝我的父母願意支持我的學習，並資助我的生活，讓我可以參加國際研討會開拓眼界。感謝那些在我沮喪時聽我抱怨、陪伴我的朋友們，你們的扶持和鼓勵使我有繼續前進的動力。 “Pride goes before destruction, a haughty spirit before a fall.”. 謹致民國一百零三年六月于師大.

(3) CONTENTS Abstract ..................................................................................................... I Abstract (Chinese version) ..................................................................... II List of Figures ......................................................................................... III List of Tables............................................................................................ V List of Figures ......................................................................................... VI. CHAPTER ONE: INTRODUCTION ............................................................. 1 1.1. Significance and Specific Objectives of the This Work .................... 1. 1.2. Superoxide Dismutases (SODs) .......................................................... 2. 1.3. 1.2.1. Cu/ZnSOD ........................................................................... 2. 1.2.2. MnSOD. ................................................................................. 3. 1.2.3. FeSOD. .................................................................................. 4. 1.2.4. NiSOD ................................................................................. 5. Literature Survey of NiSOD Analogues ............................................ 9 1.2.1. Structural NiSODred Mimics .................................................. 9. 1.2.2. Structural NiSODox Mimics ................................................. 12. 1.2.3. Functional Models of NiSOD .............................................. 14. CHAPTER TWO: EXPERIMENTAL SECTION ................................... 18 2.1. Synthetic Materials and Methods ..................................................... 18. 2.2. Synthesis and Characterization of Ligands and Complexes .......... 20 2.2.1. Synthesis and Characterization of H2BDPMeP ..................... 20. 2.2.2. Synthesis and Characterization of Ni(BDPMeP) (1) ............. 24. 2.2.3. Synthesis and Characterization of [Ni(HBDPMeP)](ClO4)·(CH2Cl2)2 (2).................................. 25. 2.2.4. Synthesis and Characterization of H2BDPPA...................... 26.

(4) 2.2.5. Ni2(BDPPA)2 (3).................................................................. 30. 2.2.6. Synthesis and Characterization of 1,1’-Dibromoferrocene (FcBr2) ................................................................................. 31. 2.2.7. Synthesis and Characterization of [FcBr2](BF4) .................. 32. 2.2.8. Synthesis and Characterization of [NiIII(BDPMeP)](BF4) (4) ............................................................................................. 32. 2.2.9. Synthesis and Characterization of NiIII2(BDPPA)2(BF4)2 (5) ............................................................................................. 33. 2.3. Reaction of NiIII-complexes with KO2 .............................................. 34 2.3.1. [NiIII(BDPMeP)](BF4) (4) ..................................................... 34. 2.3.2. NiIII2(BDPPA)2(BF4)2 (5) ..................................................... 34. CHAPTER THREE: RESULTS AND DISCUSSION ............................ 36 3.1. Synthesis and Discussion of Ni(BDPMeP) (1) ................................... 39. 3.2. Synthesis and Discussion of [NiIII(BDPMeP)](BF4) (4) .................... 47. 3.3. Synthesis and Discussion of NiBDPPA (3) ....................................... 53. 3.4. Synthesis and Discussion of NiIII2(BDPPA)2(BF4)2 (5) .................... 57. 3.5. Discussion between Nickel-complexes and Superoxide .................. 62 3.5.1. [NiIII(BDPMeP)](BF4) (4) ..................................................... 62. 3.5.2. NiIII2(BDPPA)2(BF4)2 (5) ..................................................... 65. CHAPTER FOUR: CONCLUDING REMARKS .................................... 68 REFERENCE ......................................................................................................... 70. APPENDIX.

(5) Abstract Recently, the complex Ni(BDPP) was reported as a NiSOD model compound. In this study, we would like to improve the superoxide reactivity of the NiSOD mimics and to augment the ligand alterability. The. alternative. NiSOD. model. compounds,. Ni(BDPMeP). (1),. [Ni(HBDPMeP)](ClO4)·(CH2Cl2)2 (2), and Ni2(BDPPA)2 (3), were supported. by. a. new. ligand,. 2,6-bis(((S)-2-(bis(4-methylphenyl)-. hydroxymethyl)-1-pyrrolidinyl)methyl)-pyridine. (H2BDPMeP). or. (S,. S)-bis[2-(diphenylmethanol)- pyrrolidine]dipropylaniline (H2BDPPA), respectively. These NiSOD mimics were characterized by X-ray crystallography, UV-vis spectroscopy, and cyclic voltammetry. Complex 1, a derivative of Ni(BDPP), demonstrated a reversible NiIII/NiII redox couple at E1/2 = 249.5 mV versus Ag/AgCl in DCM (ΔE = 124 mV). In addition, complex 3 showed a quasi-reversible NiIII/NiII redox couple at E1/2 = 714 mV versus Ag/AgCl in DCM (ΔE = 192 mV). Complex 1 and 3 can be oxidized to nickel(III) species, [NiIII(BDPMeP)](BF4) (4) and NiIII2(BDPPA)2(BF4)2 (5), by ferrocenium salt and 1,1’-dibromoferrocenium salt respectively. Importantly, complexes 4 and 5 have shown the ability to convert O2– into O2 along with the formation of 1 and 3. Keywords: NiSOD, Ni-dimer complex. I.

(6) 摘要本實驗室之前發表了 Ni(BDPP)作為 NiSOD 的擬態化合物，而本研究為了改良擬態化合物對超氧離子的活性及提升配位基的可變性，利用 2,6-bis(((S)-2-(bis(4-methylphenyl)hydroxymethyl)-1-pyrrolidinyl)methyl)-pyridine. (H2BDPMeP) 及 (S,. S)-bis[2-(diphenylmethanol)-. pyrrolidine]dipropylaniline (H2BDPPA) 兩種配位基，分別得到了 Ni(BDPMeP) (1)、[Ni(HBDPMeP)](ClO4)·(CH2Cl2)2 (2)和 Ni2(BDPPA)2 (3) 等幾種不同的 NiSOD 擬態化合物。這些 NiSOD 擬態化合物以 X 光單晶繞射儀、紫外光可見光光譜儀及循環伏安法所鑑定。錯合物 1 為 Ni(BDPP) 的衍生物，其在 249.5 mV (versus Ag/AgCl in DCM, ΔE = 124 mV)有一可逆的 NiIII/NiII 氧化還原峰，而錯合物 3 在 714 mV (versus Ag/AgCl in DCM, ΔE = 192 mV)也顯示了一組准可逆的 NiIII/NiII 氧化還原峰，由錯合物 1 和 3 的電化學分析可知，他們可藉由二茂鐵離子或 1,1’-二溴二茂鐵離子氧化生成三價鎳錯合物 [NiIII(BDPMeP)](BF4) (4)及 NiIII2(BDPPA)2(BF4)2 (5)。更重要的是錯合物 4 和 5 能將超氧離子轉化為氧氣，並分別還原回二價鎳錯合物 1 和 3。. 關鍵字：含鎳超氧化物歧化酶、鎳二聚體錯合物. II.

(7) List of Figures Fig. 1-1. The active site of the four types of SOD................................................ 2. Fig. 1-2. The protein structure and active site of Cu/ZnSOD............................... 3. Fig. 1-3. The protein structure and active site of MnSOD ................................... 4. Fig. 1-4. The protein structure and active site of FeSOD ..................................... 5. Fig. 1-5. The protein structure and active site of NiSOD ..................................... 6. Fig. 1-6. The disproportionation of superoxide by NiSOD .................................. 7. Fig. 1-7. Ni(II)N2S2 complexes reported by Krüger and Holm ........................ 10. Fig. 1-8. EPR spectra of the Ni(III)N2S2 complexes reported by Krüger and Holm .................................................................................................... 10. Fig. 1-9. Structural NiSODred mimic by Shearer and Hegg and co-workers ...... 11. Fig. 1-10. Structural NiSODred mimics by Jesen and co-workers ........................ 11. Fig. 1-11. Structural NiSODred mimics by Harrop and co-workers ..................... 12. Fig. 1-12. EPR specrum of structural NiSODox mimics by adding excess pyridine .............................................................................................................. 13. Fig. 1-13. Structural NiSODox mimic by Duboc and co-workers ........................ 13. Fig. 1-14. Structures of the NiSOD maquette models based on [Ni(SODM1)] ..... 14. Fig. 1-15. Functional NiSODred model by Darensbourg and co-workers............. 15. Fig. 1-16. Nitroblue tetrazolium (NBT) reaction with superoxide to produce formazan .............................................................................................. 15. Fig. 1-17. The proposed coordination in Ni-NCC ................................................ 16. Fig. 1-18. Functional NiSODox model by Lee and co-workers ............................ 17. Fig. 3-1. ORTEP diagram of Ni(BDPMeP) (1).................................................... 40. Fig. 3-2. Chemdraw diagrams of 1 versus Ni(BDPP) ........................................ 41. Fig. 3-3. UV-vis spectrum of 1 (upper) and Ni(BDPP) (lower) ......................... 42. Fig. 3-4. Cyclic voltammogram of 1 in DCM .................................................... 43. Fig. 3-5. ORTEP diagram of Ni(HBDPMeP)·ClO4 (2) ....................................... 44 III.

(8) Fig. 3-6. UV-vis spectrum of 2 ........................................................................... 46. Fig. 3-7. UV-vis spectrum of 4 ........................................................................... 47. Fig. 3-8. Titration experiment for the oxidation of 1 to 4 .................................. 48. Fig. 3-9. The basic EPR spectra ......................................................................... 49. Fig. 3-10. The EPR spectra and simulate diagram of 4 ........................................ 49. Fig. 3-11. EPR spectrum of as-isolated NiSOD from S. coelicolr with selected g value ..................................................................................................... 50. Fig. 3-12. The spin density population of 4 by computation ................................ 51. Fig. 3-13. The spin density population of NiSODox by computation ................... 51. Fig. 3-14. ORTEP diagrams of Ni2(BDPPA)2 (3) ................................................ 54. Fig. 3-15. UV-vis spectrum of 3 ........................................................................... 56. Fig. 3-16. Cyclic voltammogram of 3 in DCM .................................................... 56. Fig. 3-17. UV-vis spectrum of the formation of 5 ................................................ 57. Fig. 3-18. The EPR spectra and simulate diagram of 5 ........................................ 58. Fig. 3-19. The spin density population of 5 by computation ................................ 60. Fig. 3-20. UV-vis spectrum of the reaction between 4 and KO2 .......................... 63. Fig. 3-21. GC spectrum of the reaction between 4 and KO2 ................................ 64. Fig. 3-22. UV-vis spectrum of the reaction between 5 and KO2 .......................... 66. Fig. 3-23. GC spectrum of the reaction between 5 and KO2 ................................ 67. IV.

(9) List of Tables Table. 3-1. Selected bond distances and angles for complex 1 ...................... 40. Table. 3-2. Selected bond distances for complex 1 and Ni(BDPP) ............... 41. Table. 3-3. The E1/2 values of 1 and other model compounds ....................... 44. Table. 3-4. Selected bond distances for complex 2 and Ni(HBDPP)·ClO4 ... 45. Table. 3-5. Selected bond distances and angles for complex 2 ...................... 45. Table. 3-6. Orbital and atomic contribution to the spin density of complex 4 ...................................................................................................... 52. Table. 3-7. Selected bond distances and angles for complex 3 ...................... 55. Table. 3-8. Orbital and atomic contribution to the spin density of complex 5 (Non-boding)................................................................................ 60. Table. 3-9. Orbital and atomic contribution to the spin density of complex 5 (Bonding) ..................................................................................... 61. V.

(10) List of Schemes Scheme 1-1. NiSOD catalytic mechanism. ......................................................... 8. Scheme 3-1. Proposed mechanism for superoxide dismutation reacted by Ni(BDPP) ..................................................................................... 36. Scheme 3-2. Synthetic procedure of H2BDPMeP .............................................. 39. Scheme 3-3. Synthetic procedure of the linker of H2BDPPA .......................... 53. Scheme 3-4. Synthetic procedure of H2BDPPA ............................................... 53. VI.

(11) CHAPTER ONE: INTRODUCTION 1.1 Significance and Specific Objectives of the This Work Reactive oxygen species (ROS), such as singlet oxygen, hydroxyl radical, and superoxide, are byproducts of aerobic metabolism. In general, it can inhibit cancer cell from growing1 and play an important role in cell signaling.2 However, excess ROS will be harmful for human body. For example, superoxide radical can cause oxidative damage of several biological systems. Oxidative stress resulting from the superoxide radical has been implicated in numerous neuronal degenerative diseases such as Parkinson’s disease,3 Alzheimer’s disease,4 amyotrophic lateral sclerosis (Lou Gehrig’s disease), 4a,5 complications from diabetes,6 cataract,7 certain cancers,8 and aging in general.9 Moreover, during times of environmental stress, UV light or heat exposure, ROS can increase dramatically.10 Fortunately, there are many kinds of enzymes that can reduce such damages. In this work, we focus on superoxide dismutase (SOD), which can catalyze the disproportionation of superoxide radical, and convert it into lower toxic hydrogen peroxide and oxygen.11 In order to discuss the reaction mechanism, we synthesize NiSOD model complexes that are able to perform the similar reactivity of NiSOD. We also believe that these complexes would be appealing to practitioners of medicinal chemistry in the future.. . 1.

(12) 1.2 Superoxide Dismutases (SODs) Superoxide dismutases (SODs), discovered by McCord and Fridovich at 1969, are enzymes that can catalyze the disproportionation of superoxide radical anion (O2•-) into oxygen and hydrogen peroxide to protect organisms from oxidative damage. Compare with other enzymes, SODs have the fastest turnover frequency as 7 × 109 M-1s-1.12 Three classes of superoxide dismutases: (i) CuZnSOD, (ii) FeSOD or MnSOD, and (iii) NiSOD, have evolved in various organisms. The dismutation of superoxide by SOD can be classified into two parts: Oxidation Phase: Mn+-SOD + O2·- + 2H+ →M(n+1)-SOD + H2O2 Reduction Phase: M(n+1)-SOD + O2·- → Mn+-SOD + O2 Overall: 2 O2·- + 2H+ → O2 + H2O2 The metal center Mn+ can for the reduced form of SODs be Cun+ (n = 1), Mnn+ (n = 2), Fen+ (n = 2), or Nin+ (n = 2).. I. J. Am. Chem. Soc. 2000, 122, 2193.; J. Am. Chem. Soc. 2002, 3769, 15064.. Fig. 1-1 The active site of the four types of SOD.. 1.2.1 Cu/ZnSOD Cu/ZnSOD was the ﬁrst enzyme to be characterized and is a copper and zinc-containing homodimer found almost exclusively in intracellular cytoplasmic spaces. The bovine Cu-Zn protein was the first SOD structure to be solved by. . 2.

(13) Richardson, in 1975.13. Fig. 1-2 The protein structure and active site of Cu/ZnSOD.14. Cu/ZnSOD are commonly used by eukaryotes. It is a homodimer with molecular weight of 32,500. The active site of Cu/ZnSOD contains CuII and ZnII in a histidine-rich environment. The CuII is bound tetragonally to four histidine ligands, with an axial water molecule as a distant fifth ligand. The ZnII is bound to three histidine and one aspartate ligand, in tetrahedral geometry. In Cu/ZnSOD, the copper ion is absolutely essential for the SOD activity, while the roles of imidazolate bridge and zinc ion remain less understood. The mechanism of Cu/ZnSOD is shown below:. 1.2.2 MnSOD MnSOD was first isolated from E. coli at 1970 by Keele and his co-worker,15 which is the major antioxidant enzyme in mitochondria and mainly existed in the . 3.

(14) mitochondrial prokaryotic cells and eukaryotic cells. The structure of human mitochondrial manganese superoxide dismutase reveals a novel tetrameric interface of two 4-helix bundles. The molecular weight of each unit in MnSOD is about 19-24 kDa, containing a manganese ion coordinated by three histidine side-chains, an aspartate side-chain and a water molecule or hydroxyl ligand, depending on the oxidation state of manganese.. Fig. 1-3 The protein structure and active site of MnSOD.16. The mechanism of MnSOD is shown below17:. 1.2.3 FeSOD FeSOD was purified from E. coli and found in the plastids of plants, its color is light yellow, distributed over algae and cell matrix of prokaryotic bacterial, however, it was rarely found in animal tissue. While the Mn- and Fe dependent SODs are homologous, they are unrelated to the other SODs. Mn- and FeSODs accomplish their. . 4.

(15) function through disproportionation of O2- to O2 and H2O2. Its molecular weight is about 8.5-22 kDa.. Fig. 1-4 The protein structure and active site of FeSOD.18. The mechanism of FeSOD is shown below:. 1.2.4 NiSOD Nickel superoxide dismutase (NiSOD) was isolated from Streptomyces species and cyanobacteria.19 The oligomeric state of NiSOD was first reported as a homotetramer composed of 13.4-kDa subunits as concluded by gel filtration experiments. However, analytical ultracentrifugation coupled with MS by Wuerges, which reveals that the enzyme is a homohexamer in solution. The hexamer exhibits a globular shape in which all protein atoms lie in a hollow sphere with outer diameter of 72 Å and inner diameter of 23 Å.20a The active site of NiSOD contains one N from the backbone amide of Cys2, one N from the N-terminal amine of His1, two cysteinate. . 5.

(16) (S−) residues from Cys2 and Cys6 on the equatorial plane, and the imidazole of His1 at the position proximal to the nickel center. (Fig. 1-5). Fig. 1-5 The protein structure and active site of NiSOD.20b. The reduced form and oxidized form of NiSOD were different, which were shown as Fig. 1-6. In reduced form of NiSOD (NiSODred), the X-ray structure reveals an active site composed of a NiII center and a square planar N2S2 ligand set in its first coordination sphere, one N from the backbone amide of Cys2, one N from the N-terminal amine of His1, and two cysteinate (S-) residues from Cys2 and Cys6 in a cis fashion to each other. In contrast, in the oxidized form of NiSOD (NiSODox), a NiIII center coordinated by the N2S2 ligand set on the equatorial plane and the nitrogen atom of His1 ligated to nickel center on the axial position to construct a NiIIIN3S2 square pyramidal structure. The disproportionation of superoxide radical •-. anion (O2 ) into oxygen and hydrogen peroxide was catalyzed by the transformation between NiSODred and NiSODox.. . 6.

(17) Fig. 1-6 The disproportionation of superoxide by NiSOD. (Right: reduced form; Left: oxidized form). The proposed mechanism of NiSOD was shown as Fig. 1-7 published by Barondeau et. al. in 2004. Superoxide binds to the substrate binding pocket of the nickel center of the active site, which opposite to the His1 side chain. The Ni-bound superoxide is stable by hydrogen-bonding with the amino acid residue around, such as Tyr9, Cys2, and Cys6 (Path A). After the binding of superoxide to the nickel ion, electron is transfered between the superoxide substrate and the active site nickel ion through inner sphere mechanisms. Electron transfer from the nickel(II) to superoxide would be coupled to proton transfer to generate the hydrogen peroxide product. Meanwhile, the active site is converted from four-coordinate square planar to five-coordinate square pyramidal geometry upon nickel oxidation (Path B). Subsequently, another superoxide again binds axially to the oxidized NiSOD center and the superoxide is stable by hydrogen-bonding with the amino acid residue (Path C), and electron transfer reduces Ni(III) to Ni(II), forming the square planer geometry of NiSODred and generates dioxygen to complete the catalytic cycle (Path D).. . 7.

(18) Scheme. 1-1 NiSOD catalytic mechanism.. . 8.

(19) 1.3 Literature Survey of NiSOD Analogues According to the protein structure of NiSOD reported by literature, some researchers started to synthesize model compounds for investigating the spectroscopy and reactivity of NiSOD. The active site structure of NiSODred is a Ni(II) square planar geometry and NiSODox is a Ni(III) square pyramidal geometry. In synthetic chemistry, Ni(II) compounds could be stablized with four or six coordination environments, depending on its ligand donating ability. Several complexes with a Ni(III) center have been reported,21-24 but structurally characterized Ni(III) complexes with a square pyramidal geometry are rare. Hence, the analogues of NiSODred were much more than that of NiSODox. Herein, we will introduce some literature survey about NiSOD analogues, including structural NiSODred mimics, structural NiSODox mimics, and functional NiSOD models. 1.3.1 Structural NiSODred Mimics Because NiSODred demonstrates a N2S2 square planer coordination environment, most of structural NiSODred mimics were square planar Ni(II) complexes. Their structural and spectroscopic studies were investigated. In 1987, Krüger and Holm reported several complexes as hydrogenase mimics (Fig. 1-7). The nickel center of these complexes was coordinated by two nitrogen atoms and two sulfur atoms with cis arrangement. With this structural similarity, these nickel compounds were used for the some further investigations of NiSODred. In addition, because of the electron rich ligand environments, the nickel center of compounds can be oxidized to higher oxidation state by the addition of oxidants.. . 9.

(20) Fig. 1-7 Ni(II)N2S2 complexes reported by Krüger and Holm.. Fig. 1-8 EPR spectra of the Ni(III)N2S2 complexes reported by Krüger and Holm.. Shearer and Hegg reported a structural NiSODred mimic with a similar N2S2 ligand skeleton in 2010 and provided insight into the consequences of the different coordination environments on the properties of the Ni ions. They systematically examined two square-planar Ni(II)N2S2 complexes and discussed the spectroscopy and DFT results by comparing with the active center of NiSODred.25. . 10.

(21) Fig. 1-9 Structural NiSODred mimic by Sheare and Hegg and co-workers.. In 2008, Jensen and co-workers synthesized several pentadentate nickel(II) complexes, they prepared hydrotris(3-phenyl-5-methylpyrazoyl)- boratonickel(II) complexes with organoxanthate or dithiocarbamate coligands equilibrate between κ2and κ3-chelation modes of the scorpionate ligand in solution, connecting N2S2 square-planar and N3S2 pyramidal ligand fields and a spin crossover. The complexes also exhibit quasi-reversible oxidations at low anodic potentials, thus modeling the structure, dynamics, and redox reactivity of the reduced NiSOD active site.26. Fig. 1-10 Structural NiSODred mimics by Jesen and co-workers.. Moreover, Harrop reported several mimics, one of those mimics was synthesized by N3S2 ligand by modified from a N2S2 ligand, forming a square planar Ni(II) complex with an axial position pyrindinyl group. The structural and electoral property of this mimic is quite similar to that of NiSODred. However, oxidation of this complex provides a disulfide-linked dinuclear species, which is due to the formation of thiyl. . 11.

(22) radical during the redox process. The EPR spectrum revealed an isotropic signal at g = 2.00 that likely originates from an S-based (thiyl) radical. Also an anisotropic signal with a large g spread (g = [2.26, 2.17, 2.00]) is observed, indicating a Ni(III) species. Simulation of this data suggests that it is likely coincidental with the S-radical signal and the five coordinate Ni(III) intermediate can not be isolated.27. Fig. 1-11 Structural NiSODred mimics by Harrop and co-workers.. 1.3.2 Structural NiSODox Mimics In 1998, Hanss and Krüger found a nearly axial signal in EPR by adding excess pyridine to [Ni(phmi)]-, which is one of the series complexes published by Krüger and Holm in 1987. The splitting of the gz component due to super-hyperfine coupling of a single nitrogen donor atom is consistent with the formation of [Ni(phmi)(py)]-. The formation of a similar product upon addition of pyridine to a solution containing the. . 12.

(23) complex [Ni(emi)]- was also been reported.. Fig. 1-12 EPR specrum of structural NiSODox mimics by adding excess pyridine.. In 2010, a square pyramidal [NiIIN2S2] complex was generated by electrochemical oxidation in the presence of imidazole by Duboc and co-workers, mimicking the redox structural changes of NiSOD. In addition, EPR measurements coupled to DFT calculations demonstrate that the metal character in the redox active orbital increases drastically upon imidazole binding, implicating that these geometrical modifications are crucial for the stabilization of the Ni(III) state.23b. Fig. 1-13 Structural NiSODox mimic by Duboc and co-workers.. According to these results, an additional ligand, such as pyridine or imidazole, . 13.

(24) made it possible to oxidize the nickel center from Ni(II) to Ni(III), and the Ni(III) EPR signal was demonstrated. However, these salen-type or N2S2 nickel complexes could not execute the reactivity of superoxide disproportionation.. 1.3.3 Functional Models of NiSOD In functional models of NiSOD study, several model systems employed peptides maquettes, and some low-MW coordination complexes have been constructed. The first peptide analogue was synthesized by Shearer and co-workers, several derivatives were also constructed with electronically modification of the axial position histidine (Fig. 1-14).28. Fig. 1-14 Structures of the NiSOD maquette models based on [Ni(SODM1)] [SODM1 = H′CDLPCGVYDPA, where H′ = H (1), Me (1MeIm), 2,4-dinitrophenyl (1DNP), and tosyl (1Tos)].. On the other hand, the first low-MW NiSOD model complex that demonstrated reactivity toward superoxide radical was synthesized by Darensbourg and co-workers in 2009.29 The superoxide reactivity of these complexes was investigated by the nitroblue tetrazolium assay. This qualitative test based on the reduction of NBT by O2− is detected by the change of colorless NBT to the blue formazan (λmax =580 nm, ∼ 30 000 M-1cm-1; Fig. 1-16).30 This model implies that the mixed N/S-donors of. . 14.

(25) NiSOD provide O2− stability to the coordination unit, however, this complex could not stable with the addition of H2O2. Besides, The exact role of the nickel center in the SOD chemistry was not defined.. Fig. 1-15 Functional NiSODred model by Darensbourg and co-workers.. Fig. 1-16 Nitroblue tetrazolium (NBT) reaction with superoxide to produce formazan.. In 2010, Laurence and co-workers synthesized a model shown that the coordination sphere of Ni-SOD can be mimicked using the tripeptide asparaginecysteine-cysteine (NCC).31 A standard SOD activity assay using xanthine oxidase was performed,32 showing that Ni-NCC does exhibit SOD activity, but it is slower than NiSOD. The IC50 for Ni-NCC (4.1 × 10-5 M) is comparable to those values reported for other peptide mimics, particularly the maquette with bis-amide nitrogen coordination (3 × 10-5 M).33. . 15.

(26) Fig. 1-17 The proposed coordination in Ni-NCC.. The first five-coordinate analogue of NiSODox was described in 2012 by our group utilizing an N3O2 ligand. Because of the steric restrictions imposed by the ligand frame, the pyridine-N was forced to occupy the axial position upon coordination to the Ni center. Regardless of the oxidation state, a clever design strategy to impose a five-coordinate geometry, a Ni(II) complex, Ni(BDPP) was formed. Chemical oxidation of Ni(BDPP) cleanly yielded the Ni(III) complex [Ni(BDPP)](PF6) that was structurally characterized by X-ray crystallography. Furthermore, [Ni(BDPP)](PF6) was employed to react with excess KO2, O2 and Ni(BDPP) in stoichiometric yields occured. Unfortunately, Ni(BDPP) did not react with KO2 to produce H2O2. It appears that careful construction of the ligand frame to house five-coordinate and low-spin Ni(II) could be an additional requirement for a functional NiSOD model.22e. . 16.

(27) Fig. 1-18 Functional NiSODox model by Lee and co-workers. (Left: Ni(BDPP), Right: [Ni(BDPP)](PF6)). In this study, we design and synthesize several complexes based on Ni(BDPP) skeleton for the superoxide reactivity improvement and the ligand alterability. These complexes could provide us the opportunities to compare the differences of the coordination numbers and electronic environments of the nickel complexes with Ni(BDPP). Furthermore, we could oxidize these Ni(II) complexes to Ni(III) species and then investigate their reactivity towards superoxide, and gain insight into the role of structure and reactivity with the active center of NiSOD. . . . 17.

(28) CHAPTER TWO: EXPERIMENTAL SECTION 2.1 Synthetic Materials and Methods Unless otherwise noted, all manipulations were carried out at room temperature under an atmosphere of dinitrogen in a mBRAUN glove box (N2(g)-filled glove box maintained at or below 0.1 ppm of O2 and 1 ppm of H2O) or using high-vacuum Schlenk techniques. Dichloromethane (DCM), acetonitrile (ACN), hexanes, and diethyl. ether. were. dried. over. mBRAUN. solvent. purification. system. (MB-SPS-Compact), and purged with dinitrogen prior to use. Tetrahydrofuran (THF) and pentane were distilled from sodium/benzophenone ketyl. Sodium hydride (60 % in mineral oil) was washed with hexanes and dried by the vacuum, stored in the glove box. [Ni(CH3CN)6](ClO4)2 was synthesized from Ni(ClO4)2·6H2O, washed by ACN for three times and crystallized by the slow diffusion method of diethyl ether. Geduran® Silica gel 60 (EMD Millipore) was used for column chromatograph. Analytical thin layer chromatography was performed by using silica gel 60 GF254 (EMD Millipore). All other reagents and solvents were purchased from chemical suppliers. and. used. as. received.. 2,6-Bis(bromomethyl)pyridine,. N-benzylbis(3-bromopropyl)amine, (S)-2-(diphenylhydroxymethyl)pyrrolidine, and 1,1’-dibromoferrocene were prepared by following the literature procedures.34-37 NMR spectra were recorded on Bruker AVANCE 400 NMR as noted. Chemical shifts were reported in ppm and referenced to residual protonated solvent; coupling constants were reported in Hz. UV-vis spectra were recorded on Agilent 8453 Spectrophotometer and cyclic voltammetry spectra were recorded on Epsilon . 18.

(29) EC-V160 as noted. Single crystals were characterized by the Bruker Enraf-Nonius Kappa CCD Single-Crystal Diffractometer and Bruker Kappa Apex II Single-Crystal Diffractometer at the NSC Regional Instrumental Center at Nation Taiwan Normal University, Taipei, Taiwan. Elemental Analyses were performed on Heraeus varioIII-NCH Analyzer at the NSC Regional Instrumental Center at Nation Taiwan University, Taipei, Taiwan. EPR measurements were performed at X-band using a Bruker E580 spectrometer equipped with a Bruker ELEXSYS super high sensitivity cavity at the NSC Regional Instrumental Center at Nation Tsing Hua University, Hsinchu, Taiwan.. . 19.

(30) 2.2 Synthesis and Characterization of Ligands and Complexes 2.2.1 Synthesis and Characterization of H2BDPMeP OH OH N Br. N N. OH. N. Br. K 2CO3, EtOH. i) 2,6-Bis(bromomethyl)pyridine HBr N. N. OH. OH. Br. Br. 48 wt% HBr (100 mL) was gently added to 2,6-pyridinedimethanol (5.00 g, 35.9 mmol), refluxed for 5 h and then cooled to room temperature. The reaction solution was quenched and neutralized by adding NaOH(aq) (1 M). The resulting aqueous solution was extracted by DCM. The organic layers were combined and extracted with saturated NaCl(aq), dried over anhydrous MgSO4. The solution was evaporated under vacuum, and the residue was purified by flash column chromatography with the DCM as eluent to yield 2,6-bis(bromomethyl)pyridine (4.28 g, 46%) as a white solid. 1. H NMR (400MHz, CDCl3): δ 7.70 (t, 1H, Ph), 7.38 (d, 2H, Ph), 4.54 (s, 4H, CH2).. See Appendix A-1.. ii) (S)-Ν -Ethoxycarbonylproline methyl ester H N. . O OH. +. O. O Cl. K 2CO3 MeOH. OEt. 20. N. OEt O OMe.

(31) Ethyl chloroformate (17.54 mL, 184.31 mmol) was added dropwisely to a methanol solution of L-proline (7.00 g, 60.80 mmol) and K2CO3 (16.80 g, 121.55 mmol) under ice-bath and reacted for 16 h. After the reaction was completed, the reaction solution was evaporated under vacuum to form a white solid residue, which was dissolved in DCM and washed with water. The DCM solution was then dried over anhydrous MgSO4, and concentrated under reduced pressure. The colorless oil was formed in 95% yield (11.69 g). 1H NMR (400MHz, CDCl3): δ 4.32 (m, 1H, CH), 4.11 (m, 2H, CH2), 3.72 (d, 3H, CH3), 3.58-3.44 (m, 2H, CH2), 2.21-1.86 (m, 4H, CH2), 1.22 (dd, 3H, CH3). See Appendix A-2.. iii) (S)-2-(Bis(4-methylphenyl)hydroxymethyl)-N-ethoxycarbonylpyrrolidine. O N. MgBr. OEt O. OEt. O N. OMe. OH. THF, 0 oC. (S)-Ν-Ethoxycarbonylproline methyl ester (8.0488 g, 40.0 mmol) was dissolved in. dried. THF. under. nitrogen. atmosphere. and. 120. mL. of. 1. M. 4-methyl-phenylmagnesium bromide (120.0 mmol) was dropwisely added to the reaction solution at 0 °C. After 2 h, the solution was quenched by saturated NH4Cl(aq) to see the color of the solution changed from black to white. The reaction solution was evaporated under vacuum to form a white residue. The residue was dissolved in ethyl acetate and washed with water for 3 times. The organic layer was dried over. . 21.

(32) anhydrous MgSO4, and concentrated under reduced pressure. Purification of the residue was performed by flash column chromatography to afford a product of colorless oil in 79% yield (11.1959 g). 1H NMR (400MHz, CDCl3): δ 7.26 (dd, 4H, Ph), 7.10 (dd, 4H, Ph), 4.90 (d, 1H, CH), 4.12 (d, 2H, CH2), 3.40 (d, 1H, CH2), 2.94 (s, 1H, CH2), 2.33 (s, 6H, CH3), 2.05 (m, 1H, CH2), 1.92 (m, 1H, CH2), 1.47 (s, 1H, CH2), 1.26 (d, 3H, CH3). See Appendix A-3.. iv) (S)-2-(Bis(4-methylphenyl)hydroxymethyl)pyrrolidine OEt. O N. H N. OH. OH. KOH EtOH. A solution of (S)-2-(Bis(4-methylphenyl)hydroxymethyl)-N-ethoxycarbonylpyrrolidine (13.7514 g, 38.9 mmol) and KOH (65.4900 g, 1167.0 mmol) was refluxed in ethanol (200 mL) for 2 days. The solution was evaporated under vacuum to form a white solid residue, which was dissolved in DCM and washed with water. The DCM solution was then dried over anhydrous MgSO4, and concentrated under reduced pressure. A white powder was obtained in 83% yield (9.0988 g). 1H NMR (400MHz, CDCl3): δ 7.44 (d, 2H, Ph), 7.36 (d, 2H, Ph), 7.08 (t, 4H, Ph), 4.21 (t, 1H, CH), 3.72 (t, 1H, NH), 3.02-2.90 (m, 2H, CH2), 2.30 (s, 6H, CH3), 1.75-1.54 (m, 4H, CH2). See Appendix A-4.. . 22.

(33) v) 2,6-Bis(((S)-2-(bis(4-methylphenyl)hydroxymethyl)-1-pyrrolidinyl)methyl)pyridine (H2BDPMeP) H N. OH OH N Br. N N. N. OH. Br. K 2CO3, EtOH. A solution of (S)-2-(bis(4-methylphenyl)hydroxymethyl)pyrrolidine (2.8118 g, 10.0 mmol), 2,6-bis(bromomethyl)pyridine (1.3150 g, 5.0 mmol), and K2CO3 (1.3812 g, 10.0 mmol) was refluxed in ethanol (200 mL) for 3 days. The solution was evaporated under vacuum to form a light yellow residue, which was dissolved in DCM and washed with water. The DCM solution was then dried over anhydrous MgSO4, and concentrated under reduced pressure. The resulting residue was dissolved in DCM and precipitated by the addition of hexanes. The precipitate was washed by hexanes for 3 times, followed by evaporating under vacuum to form a white powder of H2BDPMeP in 82% yield (2.7207 g). 1H NMR (400MHz, CDCl3): δ 7.53 (d, 4H, Ph), 7.46 (t, 5H, Ph), 7.09 (d, 4H, Ph), 7.00 (d, 4H, Ph), 6.691 (d, 2H, Ph), 4.05(t, 2H, CH), 3.49 (d, 2H, CH2), 3.40 (d, 2H, CH2), 2.92 (q, 2H, CH2), 2.48 (dd, 2H, CH2), 2.28 (s, 6H, CH3), 2.19 (s, 6H, CH3), 1.94 (q, 2H, CH2), 1.77 (d, 2H, CH2) 1.74 (d, 4H, CH2); 13C NMR (400MHz, CDCl3): δ 158.8, 144.6, 136.4, 135.5, 128.6, 125.5, 120.3, 77.8, 70.6, 61.8, 55.5, 29.7, 24.3, 20.9, 20.8. See Appendix A-5, 6. Anal. Calcd for C45H51N3O2: C, 81.16; H, 7.72; N, 6.31. Found: C, 80.698; H, 7.847; N, 6.320.. . 23.

(34) 2.2.2 Synthesis and Characterization of Ni(BDPMeP) (1). N. N Ni NO. O. ACN (30 mL) was added to a mixture of H2BDPMeP (0.1330 g, 0.2 mmol), NaH (0.0012 g, 0.5 mmol) and [Ni(CH3CN)6](ClO4)2 (0.1008 g, 0.2 mmol) in a 100-mL Schlenk flask. The reaction solution was stirred at room temperature overnight, and a green precipitate was formed. The upper solution was removed by cannula, and the precipitate was dissolved in 5 mL of DCM. The DCM solution was washed with hexanes (3 × 30 mL) to precipitate the green powder, which was dissolved in 5 mL DCM for recrystallization by slow diffusion (DCM/pentane) at room temperature. Green crystals of NiBDPMeP (1) were obtained over one day in 58% yield (0.0837 g). UV-vis (DCM): λmax (ε, M−1 cm−1) 360 (900), 700 (20) nm, 1090 (30). Anal. Calcd for C45H49N3NiO2·0.5 THF: C, 74.41; H, 7.04; N, 5.54. Found: C, 74.07; H, 6.62; N, 5.83.. . 24.

(35) 2.2.3 Synthesis and Characterization of [Ni(HBDPMeP)](ClO4)·(CH2Cl2)2 (2). N. N Ni O. N HO. ACN (30 mL) was added to a mixture of H2BDPMeP (0.1330 g, 0.2 mmol), NaH (0.0048 g, 0.2 mmol) and [Ni(CH3CN)6](ClO4)2 (0.1008 g, 0.2 mmol) in a Schlenk flask. The reaction solution was stirred at room temperature overnight. The solvent of the resulting solution was removed under vacuum to afford a red solid residue, which was dissolved in 7 mL THF. The THF solution was washed with diethyl ether (3 × 35 mL) to precipitate a red powder. The red powder was dissolved in 10 mL THF for recrystallization by slow diffusion (THF/diethyl ether) at room temperature. Crystals of [NiHBDPMeP](ClO4)·(CH2Cl2)2 (2) was obtained over three days in 66% yield (0.1306 g). UV-vis (DCM): λmax (ε, M−1 cm−1) 340 (2130), 480 (200) nm. Anal. Calcd for C45H50ClN3NiO6·2.0 CH2Cl2: C, 56.85; H, 5.48; N, 4.23. Found: C, 57.52; H, 5.88; N, 4.45.. . 25.

(36) 2.2.4 Synthesis and Characterization of H2BDPPA. i) N-benzylbis(3-hydroxylpropyl)amine. + Cl. OH. CaCO3 H 2O. NH 2. HO. N. OH. A mixture of aniline (1.8626 g, 20 mmol), 3-chloro-1-propanol (6.615 g, 70 mmol), and calcium carbonate (4.0032 g, 40 mmol) was refluxed in H2O for 24 h, and the reaction solution was alkalified by NaOH(aq) (2 M) to pH = 10 at 0 °C. The resulting solution was extracted by DCM for 3 times, and the organic layers were combined and dried over anhydrous MgSO4 to obtain yellow oil product in 90% yield (3.7752 g). 1H NMR (400MHz, CDCl3): δ 7.23 (t, 2H, Ph), 6.77 (dd, 3H, Ph), 3.73 (t, 4H, CH2), 3.43 (t, 4H, CH2), 1.83 (t, 4H, CH2). See Appendix A-7.. ii) N-benzylbis(3-bromopropyl)amine. N-benzylbis(3-hydroxylpropyl)amine (1.4 g, 6.4 mmol) was added to concentrated 48 wt% HBr (24 mL), and the mixture was refluxed for 36 h. When the reaction was completed, the solution was neutralized by NaOH (1 M) at 0 °C. DCM was added to the solution for extraction; the organic layer was washed with water for. . 26.

(37) 2 times and brine for 1 time. The organic and which was dried over anhydrous MgSO4 to obtain brown oil product in 82% yield (1.611g). 1H NMR (400MHz, CDCl3): δ 7.23 (d, 2H, Ph), 6.74 (d, 3H, Ph), 3.48 (m, 8H, CH2), 2.14 (t, 4H, CH2). See Appendix A-8.. iii) (S)-2-(Diphenylhydroxymethyl)-N-ethoxycarbonylpyrrolidine. O N. MgBr. OEt O. OEt. O N. OMe. OH. THF, 0 oC. (S)-Ν-Ethoxycarbonylproline methyl ester (2.012 g, 10.0 mmol) was dissolved in dried THF under nitrogen atmosphere. Phenylmagnesium bromide (1 M, 40.0 mL) was dropwisely added to the reaction solution at 0 °C. After 2 h, the solution was quenched by saturated NH4Cl(aq) and the color of the solution changed from black to white. The resulting solution was evaporated under vacuum to form a white residue. The residue was dissolved in ethyl acetate and washed with water for 3 times. The organic layer was dried over anhydrous MgSO4, and concentrated under reduced pressure.. Clear. crystals. of. (S)-2-(Diphenylhydroxymethyl)-N-ethoxycarbonyl-. pyrrolidine were obtained from the recrystallization in ethyl acetate in 80% yield (2.6052 g). 1H NMR (400MHz, CDCl3): δ 7.41-7.24 (m, 10H, Ph), 4.93 (dd, 1H, CH), 4.13 (t, 2H, CH2), 3.42 (d, 1H, CH2), 2.95 (s, 1H, CH2), 2.10 (dd, 1H, CH2), 1.96 (dd, 1H, CH2), 1.48 (q, 1H, CH2), 1.23 (t, 3H, CH3). See Appendix A-9.. . 27.

(38) iv) (S)-2-(Diphenylhydroxymethyl)pyrrolidine OEt. O N. H N. OH. OH. KOH EtOH. A solution of (S)-2-(Diphenylhydroxymethyl)-N-ethoxycarbonylpyrrolidine (1.4815 g, 4.6 mmol) and KOH (7.7432 g, 138.0 mmol) was refluxed in ethanol (100 mL) for 2 days. The solution was evaporated under vacuum to form a white residue, which was dissolved in DCM and washed with water. The DCM solution was then dried over anhydrous MgSO4, and concentrated under reduced pressure. The white powder of (S)-2-(Diphenylhydroxymethyl)pyrrolidine was obtained in 95% yield (1.108 g). 1H NMR (400MHz, CDCl3): δ 7.65 (dd, 4H, Ph), 7.36 (d, 4H, Ph), 7.24 (s, 2H, Ph), 4.31 (s, 1H, CH), 2.98 (t, 2H, CH2), 1.68 (m, 4H, CH2). See Appendix A-10.. v) (S, S)-bis[2-(diphenylmethanol)pyrrolidine]dipropylaniline (H2BDPPA). A solution of (S)-α, α-diphenyl-2-pyrrolidinemethanol (1.8455 g, 6.01 mmol), N-benzylbis(3-bromopropyl)amine (3.0456 g, 12.02 mmol), and K2CO3 (1.6613 g, 6.02 mmol) was refluxed in ACN (50 mL) for 3 days. The solution was evaporated under vacuum to form a yellow residue, which was dissolved in DCM and washed with water. The DCM solution was then dried over anhydrous MgSO4, and. . 28.

(39) concentrated under reduced pressure. Purification by flash column chromatography afforded colorless oil which was recrystallized in ethyl acetate. Colorless crystals of H2BDPPA were obtained in 43% yield (1.7659 g). 1H NMR (400MHz, CDCl3): δ 7.64 (d, 4H, Ph), 7.56 (d, 4H, Ph), 7.30 (q, 8H, Ph), 7.17 (m, 6H, Ph), 6.65 (t, 1H, Ph), 6.32(d, 2H, Ph), 4.75 (s, 2H, CH2), 3.81 (dd, 2H, OH), 3.23 (m, 2H, CH), 2.79 (m, 2H, CH2), 2.58 (m, 2H, CH2), 2.40 (m, 2H, CH2), 2.09 (m, 2H, CH2), 1.90 (m, 4H, CH2) 1.72 (m, 6H, CH2), 1.41 (m, 4H, CH2); 13C NMR (400MHz, CDCl3): δ 148.1, 147.5, 146.3, 129.0, 128.1, 126.2, 125.6, 115.3, 111.6, 77.7, 71.2, 64.3, 55.3, 53.9, 48.3, 46.6, 46.2, 29.4, 26.2, 26.0, 25.4, 24.4, 11.5. See Appendix A-11, 12. Anal. Calcd for C46H53N3O2: C, 81.26; H, 7.86; N, 6.18. Found: C, 81.32; H, 7.86; N, 6.22.. . 29.

(40) 2.2.5 Synthesis and Characterization of Ni2(BDPPA)2 (3). ACN (30 mL) was added to a mixture of H2BDPPA (0.3400 g, 0.5 mmol), NaH (0.0300 g, 1.25 mmol) and [Ni(CH3CN)6](ClO4)2 (0.2520 g, 0.5 mmol) in a Schlenk flask. The reaction solution was stirred at room temperature overnight. The solvent of the resulting solution was removed under vacuum to afford a red residue, which was dissolved in 10 mL of DCM. The DCM solution was washed with of hexanes (3 × 50 mL) to precipitate the red powder, which was dissolved in 7 mL DCM. Single crystals were obtained by layering the DCM solution of 3 with hexane at room temperature over two days in 31% yield (0.1195 g). UV-vis (DCM): λmax (ε, M−1 cm−1) 490 (160), 650 (25) nm. Anal. Calcd for C92H102N6Ni2O4·0.5CH2Cl2: C, 73.30; H, 6.85; N, 5.54. Found: C, 73.44; H, 6.95; N, 5.51.. . 30.

(41) 2.2.6 Synthesis and Characterization of 1,1’-Dibromoferrocene (FcBr2). FeII. i) nBuLi TMEDA. aq. FeCl 3. ii) C 2H 2Br 4. hexane. FeII. Br Br. Hexanes (30 mL) was added to a mixture of ferrocene (5.58g, 30.00 mmol) and TMEDA (10.5 mL, 69.9- mmol) in a 150-mL three-necked flask equipped with a pressure-equalizing dropping funnel. nBuLi (2.5 M in hex, 25.5 mL, 63.75 mmol) was added to the solution dropwisely at 0 °C. After nBuLi was completely added, the temperature of the solution was slowly raised to room temperature and the reaction solution was stirred overnight. Orange solid was suspended in the resulting brown solution, which was washed with hexanes (3 × 30). The resulting solution was evaporated under vacuum to afford an orange solid. Diethyl ether (90 mL) was added to the orange solid to form a suspension solution, which was cooled down to –70 °C. 1,1,2,2-tetrabromoethane (6.5 mL, 55.90 mmol) dissolved in diethyl ether (30 mL) was dropwisely added to the suspension solution with vigorous stirring. The temperature of the reaction solution was gradually raised to room temperature, and the solution was stirred overnight. The top dark-brown layer was decanted, and quenched with water (15 mL) and provided a dark brown solid after solvent removal. The residue was extracted by hexanes (15 mL) to form a suspension. The suspension was filtered through celite. The filtrate was washed by a saturated aqueous solution of FeCl3 (ca. 5 × 100 mL) to remove the FcH/FcBr contaminants (composition monitored by 1H NMR spectroscopy between washings). The organic layer was extracted with water until the washings were colorless. Then, the resulting solution . 31.

(42) was dried over MgSO4. The orange crystals of FcBr2 were obtained from the recrystallization in hexanes in 8% yield (0.8485 g). 1H NMR (400MHz, CDCl3): δ 4.42 (s, 4H, Ph), 4.17 (d, 4H, Ph). See Appendix A-13.. 2.2.7 Synthesis and Characterization of [FcBr2](BF4) BF 4FeII. Br Br. i) DDQ ii) HBF 4. FeIII. Br Br. 2,3-Dihloro-5,6-dicyano-1,4-benzoquinone (DDQ, 0.1135 g, 0.5 mmol) was added to 1,1’-dibromoferrocene (0.3418 g, 1.0 mmol) in diethyl ether (20 mL) at room temperature, and the reaction mixture was stirred for 10 mins. Then, HBF4 (32.5 wt% solution in H2O, 2 mmol) was added at 0 °C. A dark blue solid formed immediately, which was washed by cold diethyl ether until the solution became colorless. The solvent of the resulting solution was evaporated under vacuum to give a dark blue product. Anal. Calcd for C92H102N6Ni2O4: C, 27.89; H, 1.87; N, 0.00. Found: C, 28.09; H, 1.86; N, 0.11.. 2.2.8 Synthesis and Characterization of [NiIII(BDPMeP)](BF4) (4) ACN (15 mL) was added to a mixture of Ni(BDPMeP) (0.0723 g, 0.1 mmol) and [Fe(C5H5)2]BF4 (0.0273 g, 0.1 mmol) in a Schlenk flask at 0 °C. After 1 h, the solvent of the resulting solution was removed under vacuum to afford a brown solid, then dissolved in 3 mL DCM. The DCM solution was washed by diethyl (3 × 25 mL) ether to precipitate a brown powder of [NiIII(BDPMeP)](BF4). UV-vis (ACN): λmax (ε, M−1 . 32.

(43) cm−1) 300 (6090), 375 (7020), 455 (3160) nm. Anal. Calcd for C45H49BF4N3NiO2·1.0 CH2Cl2: C, 61.78; H, 5.75; N, 4.70. Found: C, 64.04; H, 5.81; N, 5.09.. 2.2.9 Synthesis and Characterization of NiIII2(BDPPA)2(BF4)2 (5) THF (5 mL) was added to a mixture of Ni2(BDPPA)2 (0.0155 g, 0.01 mmol) and [FcBr2](BF4) (0.0086 g, 0.02 mmol) in a Schlenk flask at –40 °C. After 30 min, the reaction solution turned to dark purple color, which was washed by of diethyl ether (3 × 15 mL) to form a dark purple precipitate of NiIII2(BDPPA)2(BF4)2. UV-vis (THF): λmax (ε, M−1 cm−1) 300 (11035), 335 (5200), 570 (3255) nm.. . 33.

(44) 2.3 Reaction of NiIII-complexes with KO2 2.3.1 [NiIII(BDPMeP)](BF4) (4) i) Detection by UV-vis spectroscopy The. reaction. was. performed. by. adding. 3. mL. ACN. solution. of. [NiIII(BDPMeP)](BF4) (4) (1.5 × 10-4 M) to a solid of KO2 (0.0053 g, 50 equiv.) in a UV-vis cell. The reaction was detected by a UV-vis spectro meter.. ii) Detection by GC In order to detect the production of O2(g) from the reaction of 4 and KO2, two vials, each was filled control and sample experiments, were prepared. First, two vacuum vials, which contained with KO2 (0.0711 g, 1.00 mmol), 17 mL N2(g) and 1 mL ACN. In the sample experiment, 2 mL of [NiIII(BDPMeP)](BF4) (4) (0.0405 g, 0.05 mmol) ACN solution was added to the vial mentioned above. In the control experiment, 2 mL ACN was added to the other vial. Both solutions were stirred vigorously. The color of the sample experiment changed from brown to green. The containing O2(g) in both vials was detected by the Milli-Whistle GC38. The produced amount of O2(g) was calculated for comparison with the ideal produced amount of O2(g). The yield of the produced O2(g) is 92%.. 2.3.2 NiIII2(BDPPA)2(BF4)2 (5) i) Detection by UV-vis spectroscopy The . reaction. was. performed. by 34. adding. 3. mL. THF. solution. of.

(45) NiIII2(BDPPA)2(BF4)2 (5) (1.0 × 10-3 M) to a solid of KO2 (0.0053 g, 50 equiv.) in a UV-vis cell. The reaction was detected by a UV-vis spectro meter.. ii) Detection by GC In order to detect the production of O2(g) from the reaction of 5 and KO2, two vials, each was filled control and sample experiments, were prepared. First, two vacuum vials, which contained with KO2 (0.0711 g, 1.00 mmol), 17 mL N2(g) and 0.4 mL THF. In the sample experiment, 2 mL of NiIII2(BDPPA)2(BF4)2 (5) (0.0156 g, 0.010 mmol) THF solution was added to the vial mentioned above. In the control experiment, 2 mL THF was added to the other vial. Both solutions were stirred vigorously. The color of the sample experiment changed from brown to green. The containing O2(g) in both vials was detected by the Milli-Whistle GC. The produced amount of O2(g) was calculated for comparison with the ideal produced amount of O2(g). The yield of the produced O2(g) is 82%. . . . 35.

(46) CHAPTER THREE: RESULTS AND DISCUSSION NiSOD is an enzyme, which is capable to catalyze the dismutation of O2− into O2 and H2O2 through a cycle of NiII and NiIII oxidation states. In order to mimic the function of NiSOD, a functional model [Ni(H2BDPP)(CH3CN)](BF4)2 was synthesized by Lee and co-workers. [Ni(H2BDPP)(CH3CN)](BF4)2 could be oxidized to a nickel(III) species by the addition of KO2, and O2- could be reduced to H2O2 through a hydrogen atom abstraction. In addition, the nickel(III) species [NiIII(BDPP)](PF6) is able to promote the oxidation of O2- into O2, through the redction of NiIII to NiII.22e As far as we know, only peptide models such as Ni(SODM1) and Ni(SODM2),39 reported by Shearer and Ni-GGNCC and Ni-NCC,31 reported by Laurence. have. demonstrated. the. function. of. NiSOD.. Therefore,. [Ni(H2BDPP)(CH3CN)](BF4)2 is the first model which possesses the function of NiSOD. . . . . Scheme 3-1. Proposed mechanism for superoxide dismutation reacted by Ni(BDPP).40. . 36.

(47) However, as a enzyme model, [Ni(H2BDPP)(CH3CN)](BF4)2 still remains some weakness. For example, the structure of [Ni(H2BDPP)(CH3CN)](BF4)2 is different to the coordination sphere of the active site of NiSOD and the geometry translation of [Ni(H2BDPP)(CH3CN)](BF4)2 in the catalytic cycle also showed the difference with NiSOD. Reduced form of NiSOD (NiSODred) contains a square planar NiII geometry ligated in an N2S2 environment and has an imidazole residue of His1 on the proximal position of the nickel center. On the other hand, the oxidized form of NiSOD (NiSODox) formed a five-coordinate distorted square pyramidal NiIII-N3S2 geometry (NiSODox) via the coordination of His1-N. This could provide an evidence that the axial ligand may exist important influences between the reduction state and oxidation state. of. nickel. center;. however,. the. axial. pyridinyl. group. of. [Ni(H2BDPP)(CH3CN)](BF4)2 could not demonstrate the switch behavior like His1-N of NiSOD, the axial pyridinyl group binds to the nickel center through the whole cycle. In order to compare the reactivity of [Ni(H2BDPP)(CH3CN)](BF4)2 and get the insight into the the effect of axial ligand, we designed and synthesized two derivatives of H2BDPP in this study. First, we modulated the side-arms of H2BDPP to provide a chance to evaluate the reactivity difference toward superoxide, by altering phenyl group to the tolyl group, H2BDPMeP has been obtained. Moreover, we modified the linker of H2BDPP by introducing aniline as the axial coordination group to replace pyridinyl group, and H2BDPPA could be prepared. With the axial group modified and the flexible long CH2 chains of H2BDPPA, we expect the nitrogen donor of aniline . 37.

(48) can ligate on and off to the nickel center between the NiII and NiIII oxidation states, which could be able to possess the similar behavior of NiSOD. Hence, both of H2(BDPMeP) and H2BDPPA can be synthesized to NiII complexes by addition of Ni salts and oxidized to NiIII complexes by using a proper oxidant. The NiIII complexes could be expected to investigate the superoxide reactivity.. . 38.

(49) 3.1 Synthesis and Discussion of Ni(BDPMeP) (1) The synthetic procedure of H2BDPMeP is similar to H2BDPP. First, L-proline was used as an initiator, reacting with ethyl chloroformate to form an ester. Then, 4-methyl-phenylmagnesium bromide instead of phenylmagnesium bromide was employed to react with the ester to form a pyrrolidiniol derivative. The further procedure is the same as that of H2BDPP, shown in Scheme 3-2. H N. O N. O OH. +. MgBr. OEt O OMe. O. O Cl. K 2CO3 MeOH. OEt. OMe. OEt OEt. OO NN. H N. OH OH. OH. KOH EtOH. THF, 0 oC. H N. N. OEt O. OH OH N Br. N N. N. OH. Br. K 2CO3, EtOH. Scheme 3-2. Synthetic procedure of H2BDPMeP.. H2BDPMeP was deprotonated by 2.5 equiv of sodium hydride, and reacted with 1.0 equiv of [Ni(CH3CN)6](ClO4)2 in ACN, subsequently giving a green complex Ni(BDPMeP) (1). The molecular structure of 1 was determined by X-ray crystallography as seen in Fig. 3-1.. . 39.

(50) Fig. 3-1 ORTEP diagram of Ni(BDPMeP) (1). Thermal ellipsoid representation of 1 at probability level. Hydrogen atoms and solvent molecules are omitted for clarity.. The crystal of complex 1 is green, and its crystal system is belonged to orthorhombic, space group is P21. Its lattice constant are a = 19.9912(13) Å, b = 7.9938(4) Å, c = 13.1018(10) Å, α = 90º, β = 90º, γ = 90º, and every unit cell has two molecules (Z=2), R1 = 0.0589, wR2 = 0.1579. The selected bond distances and angles of 1 are listed in Table 3-1, see appendix B for other data. Table 3-1 Selected bond distances and angles for complex 1. Bond lengths [Å] N(1)-Ni(1). 2.119(4). Ni(1)-O(1)#1. 1.925(3). N(2)-Ni(1). 1.935(5). Ni(1)-N(1)#1. 2.119(4). O(1)-Ni(1). 1.925(3) Bond angles [º]. O(1)-Ni(1)-N(2). 104.80(10). O(1)-Ni(1)-O(1)#1. 150.4(2). O(1)#1-Ni(1)-N(2). 104.80(10). O(1)-Ni(1)-N(1)#1. 97.92(15). O(1)-Ni(1)-N(1). 85.85(14). O(1)#1-Ni(1)-N(1)#1. 85.85(14). O(1)#1-Ni(1)-N(1). 97.92(15). N(2)-Ni(1)-N(1)#1. 82.64(13). N(2)-Ni(1)-N(1). 82.64(13). N(1)-Ni(1)-N(1)#1. 165.3(3). The geometry of 1 is distorted square pyramidal (τ = 0.25), which has a greater . 40.

(51) tendency to square pyramidal than Ni(BDPP) (τ = 0.38). The π-π interaction between the aryl rings and the pyridyl ring of Ni(BDPMeP) is more significant than that of Ni(BDPP).. . 1. Ni(BDPP). Fig. 3-2 Chemdraw diagrams of 1 versus Ni(BDPP).. The average bond length between coordinate atoms and nickel center in Ni(BDPMeP) are shorter than those of Ni(BDPP), that is, the electron donating ability of Ni(BDPMeP) is stronger than Ni(BDPP). Therefore, when NiIII complex was formed, NiIII(BDPMeP) would be more stable than NiIII(BDPP) because the anticipate electron density on nickel center of NiIII(BDPMeP) will be higher, and the axial pyridinyl group, a π-acceptor, could receive the electron density from nickel center to stabilize the complex and to make nickel center remaining at nickel(III) oxidation state. As a result, the bond length between coordinate atoms and nickel center of Ni(BDPMeP) are more shorter than Ni(BDPP), especially the distance of axial pyridinyl-N to nickel center. Table 3-2 Selected bond distances for complex 1 and Ni(BDPP).. . Bond lengths [Å]. 1. Ni(BDPP). Avg. Namine-Ni. 2.119(4). 2.149(2). Npy-Ni. 1.935(5). 1.969(2). Avg. O-Ni. 1.925(3). 1.935(2). 41.

(52) Since the structure of Ni(BDPMeP) is very similar to Ni(BDPP), the UV-vis spectrum of Ni(BDPMeP) is nearly identical to that of Ni(BDPP). The UV-vis spectra of Ni(BDPMeP) displayed a characteristic absorption band at 360 (ε = 900 M-1cm-1) and two d-d transition bands at 700 and 1090 nm (ε = 20, 30 M-1cm-1). Ni(BDPP) has a similar characteristic absorption band at 350 (ε = 830 M-1cm-1) and two d-d transition bands at 690 and 1080 nm (ε = 70, 80 M-1cm-1). The comparison of UV-vis spectra is shown below in Fig. 3-3.. Fig. 3-3 UV-vis spectrum of 1 (upper) and Ni(BDPP) (lower). . 42.

(53) In the cyclic voltammogram, complex 1 possesses a reversible wave with an E1/2 value at 249.5 mV versus Ag/AgCl in DCM (Fig. 3-4), which is more negative than the E1/2 value of Ni(BDPP) (308 mV in DCM). From the CV data, it is obvious that the electron donating group such as methyl group was equipped on the aryl ring of complex 1, the nickel center could be more easier to oxidize to nickel(III) species. In addition, a series of Ni(BDPP) complexes used the deprotonated alkoxy group to bind to the nickel center, and the alkoxy group can provide more electron density to stabilize the high valent nickel center. As a result, Ni(BDPP) series complexes have more opportunities to form NiIII complexes.. Fig. 3-4 Cyclic voltammogram of 1 in DCM.. This result reveals that complex 1 could oxidize to nickel(III) oxidation state, easier than Ni(BDPP), which is consistent to our speculation in the structure about the bond length between the donating group and nickel center. Notably, the E1/2 value of complex 1 is in the range of wild-type NiSOD and other NiSOD functional mimics in the literature, exhibited that complex 1 has opportunity to disproportionate superoxide. . 43.

(54) like NiSOD. Table 3-3 The E1/2 values of 1 and other model compounds. Sample. E1/2 (mV vs Ag/AgCl). Ref.. WT-NiSOD. 93. 41. Y9F-NiSOD. 97. 41. D3A-NiSOD. 108. 41. Ni(BDPMeP). 250. This work. Ni(BDPP). 308. 22e. [NiII(SODM2)]. 520. 39. [NiII(SODM1)]. 705. 39. NiII-NCC. 720. 31. Furthermore, if H2BDPMeP was deprotonated by 1.0 equiv of sodium hydride, and subsequently reacted with 1 equiv of [Ni(CH3CN)6](ClO4)2 in ACN, a red complex Ni(HBDPMeP)·ClO4 (2) could be formed. The molecular structure of 2 was characterized by X-ray crystallography as Fig. 3-5. From the literature, complex 2 can be referred to an intermediate of the catalytic cycle of Ni(BDPP), the proposed mechanism for the superoxide dismutation by Ni(BDPP) was shown in Scheme 3-1.. Fig. 3-5 ORTEP diagram of Ni(HBDPMeP)·ClO4 (2). Thermal ellipsoid representation of 2 at probability level. Hydrogen atoms and counter ions are omitted for clarity. . 44.

(55) The crystals of complex 2 are red, and the crystal system belongs to monoclinic, space group is C2. Its lattice constants are a = 41.084(3) Å, b = 10.6063(8) Å, c = 10.8933(8) Å, α = 90º, β = 91.7430(10)º, γ = 90º. Every unit cell has four molecules (Z = 4), R1 = 0.0478, wR2 = 0.1354. The selected bond distances and angles of 2 are listed in Table 3-4, see appendix B for other data. The structure of 2 and Ni(HBDPP)·ClO4 are almost indistinguishable, the selected bonds are shown in Table 3-5.. Table 3-4 Selected bond distances for complex 2 and Ni(HBDPP)·ClO4.. Bond lengths [Å]. 2. Ni(HBDPP)·ClO4. Avg. Namine-Ni. 1.862(4). 1.860(4). Npy-Ni. 2.014(4). 2.000(4). O-Ni. 1.847(3). 1.844(3). O-H. 0.8400. 0.8400. Table 3-5 Selected bond distances and angles for complex 2. Bond lengths [Å] N(1)-Ni(1). 1.896(4). N(3)-Ni(1). 2.014(4). N(2)-Ni(1). 1.828(3). O(1)-Ni(1). 1.847(3). Bond angles [º] N(2)-Ni(1)-O(1). 165.08(13). N(2)-Ni(1)-N(3). 84.65(17). N(2)-Ni(1)-N(1). 83.95(18). O(1)-Ni(1)-N(3). 104.82(13). O(1)-Ni(1)-N(1). 87.23(14). N(1)-Ni(1)-N(3). 167.81(14). The UV-vis spectra of complex 2 and Ni(HBDPP)·ClO4 are very similar. . 45.

(56) Complex 2 displayed a characteristic absorption band at 340 (ε = 2130 M-1cm-1) and a d-d transition band at 480 nm (ε = 200 M-1cm-1), and Ni(HBDPP)·ClO4 has a characteristic absorption band at 330 nm (ε = 2900 M-1cm-1) and a d-d transition band at 490 nm (ε = 260 M-1cm-1).. Fig. 3-6 UV-vis spectrum of 2.. . 46.

(57) 3.2 Synthesis and Discussion of [NiIII(BDPMeP)](BF4) (4) According to previous discuss, the electrochemical result of complex 1 demonstrated that 1 can be oxidized to nickel(III) species. The redox potential of 1 is lower than that of ferrocenium salt (Fc+/Fc, 467 mV vs Ag/AgCl in DCM) suggesting that complex 1 can be oxidized by [Cp2Fe]PF6 to form a nickel(III) complex, [NiIII(BDPMeP)](BF4) (4). Then, from the characterization of UV-vis spectroscopy, EPR, and DFT calculation could gain insight into the geometric and electronic structure of this NiIII complex. The UV-vis spectrum of [NiIII(BDPMeP)](BF4) displayed three characteristic absorption bands at 300, 375, and 455 nm (ε = 6090, 7020, and 3160 M-1cm-1).. Fig. 3-7 UV-vis spectrum of 4.. Therefore, if the different equivalent of [Cp2Fe]PF6 were added to complex 1, we can found that the characteristic absorption bands changed from 360 nm to 300 and 375 nm. Besides, since one equivalent of [Cp2Fe]PF6 was added, the absorption bands reached the largest intensity, the spectrum could be keep even more [Cp2Fe]PF6 were . 47.

(58) added. Hence, the titration experiment has shown that only one equivalent [Cp2Fe]PF6 is needed for the complete formation of 1 to 4 (Fig. 3-8).. Fig. 3-8 Titration experiment for the oxidation of 1 to 4. (The inset was the absorbance of 375 nm). The electron configuration of NiIII is d7, which has an unpaired electron on the dz2 orbital while the complex geometry is square pyramidal, so in electron paramagnetic resonance spectroscopy (EPR) a NiIII signal will be found. The EPR spectra according to the symmetry of its ligand field exhibit three cases: 1. When gx = gy = gz. It is said to be isotropic and a single symmetric EPR absorption is obtained (Fig. 3-9 (a)). 2. When gx = gy < gz or gx = gy > gz. It exhibits a minor feature at low field (from gz) and a major feature at high field (from gx and gy), and vice versa. These spectra are said to be axial. The two common g values are often reffered to as g⊥ while the unique g value is called g∥(Fig. 3-9 (b) and (c)).. . 48.

(59) 3. When gx ≠ gy ≠ gz. The spectrum is said to be rhombic (Fig. 3-9 (d)).. Fig. 3-9 The basic EPR spectra.42. The EPR spectrum of 4 in DCM at 77K exhibited a rhombic signal (gx = 2.183, gy = 2.144, and gz = 1.992 ) with a super-hyperfine triplet (Azz = 26.4 G) (Fig. 3-10). This is almost identical to NiIII(BDPP)·PF6, which presented a rhombic signal (gx = 2.22, gy = 2.18, and gz = 2.02) with a triplet super-hyperfine triplet (Azz = 26.3 G), too.. Fig. 3-10 The EPR spectra and simulate diagram of 4.. This super-hyperfine coupling is due to the coupling between the unpaired electron on . 49.

(60) the nickel center and the nitrogen atom of pyridine. Besides, because the I value of nitrogen is 1, a triplet (2I + 1 = 3) super-hyperfine coupling could be found in the EPR spectrum. Interestingly, this spectrum has a similar splitting pattern and coupling constant to NiSODox (Fig. 3-11).. Fig. 3-11 EPR spectrum of as-isolated NiSOD from S. coelicolr with selected g value.20b. Density-functional theory (DFT) calculation of 4 was performed to gain insight into the geometric and electronic structure of the NiIII center. DFT calculations were performed on a 48CPU workstation (ALPS, Advanced Large-scale Parallel Supercluster). Theoretical calculation employed the Gaussian 09 (G09) program package43 in B3LYP level44 with a mixed basis set comprised of 6-31 g(d) for C, H; 6-31g+(d) for N, O and 6-31g++(d,p) for H atom interaction of the prolinol and the Hay-Wadt relativistic effective core potentials (LANL08)45 for the Ni atom was performed. The geometry for the optimized structure of 4 revealed a square pyramidal with τ = 0.25, which is represent more square pyramidal character than NiII complex (τ = 0.38), and it is identical to NiIII(BDPP)·PF6 with τ = 0.24. The spin density plot is . 50.

(61) shown in Fig. 3-12. According to this result, we can found that the unpaired electron was concentrated on the nickel center and coupled to the nitrogen atom of pyridine.. Fig. 3-12 The spin density population of 4 by computation.. Comparing this result with the reported DFT result of LUMO of NiSODox, we can notice that the electron population of 4 resembled to NiSODox (Fig. 3-13).. Fig. 3-13 The spin density population of NiSODox by computation46.. In addition, according to Table 3-6, the unpaired electron was mainly located on the dz2 orbital of NiIII, and this orbital has a strong overlap with the pz orbital of the . 51.

(62) axial nitrogen atom of the pyridine. That is the reason why a triplet super-hyperfine coupling could be observed at gz in EPR spectrum of 4. Meanwhile, the comparison of orbital and atomic contribution of 4 and NiIII(BDPP)·PF6 are also shown below. Notable, all the contribution value of 4 are larger than NiIII(BDPP)·PF6 except oxygen atoms. That is due to the ligand of 4, H2BDPMeP, having substituted tolyl groups, which equipped an electron-donating methyl group in the para-position. The substituted tolyl groups make the electron density more intense on nickel center and its coordination environment. It could be therefore an evidence for the complex 4 to activate the disproportionation of O2- to O2. Table 3-6 Orbital and atomic contribution to the spin density of complex 4.. 4. NiIII(BDPP)·PF6. d z2. 0.711. 0.697. dx2-y2. 0.230. 0.218. dxz. 0.019. 0.000. dyz. 0.015. 0.002. dxy. 0.011. -0.001. Npy. 0.272. 0.141. N1. 0.016. 0.008. N2. 0.017. 0.008. O1. -0.099. -0.045. O2. -0.099. -0.045. Ni. . 52.

(63) 3.3 Synthesis and Discussion of NiBDPPA (3) Complex 3 was designed for increasing the flexibility and alterability of axial group. The side-arm of H2BDPPA is the same as that of H2BDPP, but the linker of H2BDPPA is the dipropylanilinyl substituent instead of 2,6-pyridinedimethyl group. The preparation of dipropylanilinyl substituent could be separated to two steps. First, aniline. was. reacted. with. 3-chloro-1-propanol. to. form. a. long. chain. 3,3'-(phenylazanediyl)bis(propan-1-ol). Then, 3,3'-(phenylazanediyl)bis(propan-1-ol) could react with hydrobromic acid to form the linker, N-benzylbis(3-bromopropyl)amine (Scheme 3-3). Further, the overall synthetic procedure is shown in Scheme 3-4.. + Cl. CaCO3 H 2O. OH. NH 2. HO. N. OH. Scheme 3-3 Synthetic procedure of the linker of H2BDPPA.. H N. O N. O OH. +. MgBr. OEt O. Cl. OEt. THF, 0 oC. K 2CO3 MeOH. N. OEt O OMe. OEt. O N. OMe. O. O. H N. OH. OH. KOH EtOH. Scheme 3-4 Synthetic procedure of H2BDPPA.. H2BDPPA was deprotonated by 2.5 equiv of sodium hydride, and subsequently reacted with 1.0 equiv of [Ni(CH3CN)6](ClO4)2 in ACN subsequently to give a red . 53.

(64) complex of Ni2(BDPPA)2 (3). The molecular structure of 3 was determined by X-ray crystallography as Fig. 3-14.. Fig. 3-14 ORTEP diagrams of Ni2(BDPPA)2 (3). Thermal ellipsoid representation of 3 at probability level. Hydrogen atoms and solvent molecules are omitted for clarity.. The crystals of complex 3 are red, and the crystal system belongs to trigonal, space group is P32. Its lattice constants are a = 16.486(2) Å, b = 16.486(2) Å, c = 32.156(4) Å, α = 90º, β = 90º, γ = 120º. Every unit cell has three molecules (Z=3), R1 = 0.0828, wR2 = 0.1806. The selected bond distances and angles of 3 are listed in Table 3-7, see appendix B for other data.. . 54.

(65) Table 3-7 Selected bond distances and angles for complex 3. Bond lengths [Å] N(1)-Ni(1). 1.923(7). O(1)-Ni(1). 1.859(6). N(2)-Ni(1). 1.891(8). O(2)-Ni(1). 1.831(6). N(4)-Ni(2). 1.911(8). O(3)-Ni(2). 1.831(6). N(5)-Ni(2). 1.926(7). O(4)-Ni(2). 1.842(6). Bond angles [º] O(2)-Ni(1)-O(1). 175.2(3). O(3)-Ni(2)-O(4). 171.6(2). O(2)-Ni(1)-N(2). 87.1(3). O(3)-Ni(2)-N(4). 86.6(3). O(1)-Ni(1)-N(2). 92.3(3). O(4)-Ni(2)-N(4). 92.2(3). O(2)-Ni(1)-N(1). 94.7(3). O(3)-Ni(2)-N(5). 95.1(3). O(1)-Ni(1)-N(1). 86.5(3). O(4)-Ni(2)-N(5). 86.7(3). N(2)-Ni(1)-N(1). 172.9(3). N(4)-Ni(2)-N(5). 174.8(3). Due to the flexibility of (CH2)3 chains, the structure of complex 3 does have the rigid property of Ni(BDPP). Complex 3 shows a dimeric structure of four-coordinate NiII complex, which has two ligands and two nickel centers staggered to each other. Each nickel center is coordinated by one arm of each ligands, and it reveals that the geometry of 3 is square-planar. Because the structure of complex 3 is quite different to Ni(BDPP), the the UV-vis spectrum of 3 is totally different from Ni(BDPP). The UV-vis spectrum of 3 displayed a characteristic absorption band at 460 (ε = 160 M-1cm-1) and a d-d transition band at 650 (ε = 25 M-1cm-1). The UV-vis spectra of 3 is shown in Fig. 3-15.. . 55.

(66) Fig. 3-15 UV-vis spectrum of 3.. However, in the cyclic voltammogram, complex 3 possesses a quasi-reversible wave with an E1/2 value of 591 mV versus Ag/AgCl in DCM (Fig. 3-16). Moreover, this redox wave will be decreased with the scan times increasing, exposed that complex 3 could be oxidized to NiIII species, the NiIII species is considerably unstable.. Fig. 3-16 Cyclic voltammogram of 3 in DCM.. . 56.

(67) 3.4 Synthesis and Discussion of NiIII2(BDPPA)2(BF4)2 (5) From the electrochemical evidence of complex 3, which shows its potential of forming an nickel(III) species. The redox potential of 3, however, is higher than ferrocenium salt (Fc+/Fc, 467 mV vs Ag/AgCl in DCM). Thus, we synthesized a derivative of ferrocenium salt, 1,1’-dibromoferrocenium tetrafluoroborane (Fc+/Fc, 797 mV vs Ag/AgCl in DCM) as oxidant to oxidized complex 3 to [NiIII2(BDPPA)2](BF4)2 (5). Complex 5 can be characterized by UV-vis spectroscopy and electron paramagnetic resonance spectroscopy (EPR). In addition, DFT calculation results could provide some possibilities of the geometric and electronic structure of this NiIII complex. The UV-vis spectrum of the formation of NiIII2(BDPPA)2(BF4)2 can be shown by adding 2 equiv 1,1-dibromoferroceium tetrafluoroborate to complex 3 at –40 °C. Complex 5 displayed three characteristic absorption bands at 300, 335, and 570 nm (ε = 11035, 5200, and 3255 M-1cm-1) (Fig. 3-17).. Fig. 3-17 UV-vis spectrum of the formation of 5.. . 57.

(68) The electron configuration of the nickel center in complex 5 is d7, thus a NiIII signal will be found in EPR spectroscopy. The EPR spectrum of 5 in ACN at 77K exhibited a rhombic signal (gx = 2.182, gy = 2.136, and gz = 2.073) with several super-hyperfine triplet (Ayy = 67.8 G, Ayz = 26.04 G) (Fig. 3-18). The significant super-hyperfine coupling (from EPR simulation data) shows that the unpaired electrons of both nickel(III) center could overlap with the nitrogen atoms on the equatorial plane by the dyz and dz2 orbitals in each nickel center. These results inferred that the axial nitrogen might not be able to ligate to nickel center when the complex oxidized to a nickel(III) species.. Fig. 3-18 The EPR spectra and simulate diagram of 5.. DFT calculation of 5 was performed to gain insight into the geometric and electronic structure of the NiIII center. DFT calculations were performed on a 48CPU workstation (ALPS, Advanced Large-scale Parallel Supercluster). Theoretical. . 58.

(69) calculation employed the Gaussian 09 (G09) program package in B3LYP level with a mixed basis set comprised of 6-31 g for C, H; 6-31g+(d) for N, O and the Hay-Wadt relativistic effective core potentials (LANL08) for the Ni atom was performed. The geometry for the optimized structure of 5 has two hypotheses, either the axial donor has bound to nickel center to form a five coordinate square pyramidal when it oxidized to nickel (III) species, or the nickel(III) species remains a square planer geometry. The spin density plots are shown in Fig. 3-19. According to these results, we can found that in the non-bonding case, the unpaired electrons were complicatedly located on the dxz orbitals of each NiIII, and these orbitals have strong overlaps with the pz orbital of the equatorial nitrogen atoms of the pyrrolidine. But in the bonding case, the unpaired electrons were separated concentrated on two nickel centers—the dz2 orbital of NiIII and coupled to the pz orbital of the axial nitrogen atom of aniline and the dxz orbital of NiIII just like the nickel center of non-bonding case. From the DFT calculation results, we can observed the potential energy of the axial nitrogen non-bounding to nickel center case is lower than the bounding case for 23.2 kcal/mol. Thus, from the ERP and DFT results, we could conclude that the axial nitrogen atom could not bind to the nickel(III) center while it oxidized to nickel(III). The nickel(III) species with square planer geometry are quite rare in the literature.23c. . 59.

(70) Fig. 3-19 The spin density population of 5 by computation. (Left: Non-bonding, Right: Bonding.). Meanwhile, the comparison of orbital and atomic contribution of the non-bonding and bonding cases are also shown below. Table 3-8 Orbital and atomic contribution to the spin density of complex 5 (Non-bonding).. Ni1. Ni2. d z2. 0.032. 0.036. dx2-y2. 0.090. 0.093. dxz. -0.002. 0.603. dyz. 0.616. -0.016. dxy. 0.024. 0.015. N1. -0.042. -0.023. N2. -0.039. -0.024. N3 (Aniline). 0.000. 0.003. O1. 0.220. 0.214. O2. 0.209. 0.222. Ni. . 60.