| 摘要: | CoII(HCTPFPP) (1a) 進行一系列的光譜分析，光譜結果可推斷CoII(HCTPFPP)為兩價的物質。氧化速率探討的結果發現，和一般CoII(HCTPP)相比，meso位改為氟苯取代的確會使氧化速率降低，主要原因是氟苯取代強拉電子基造成HOMO能階較低而不易給出電子的緣故。
除了鈷金屬外，我們也對鎳金屬做一系列的探討：我們成功的合成出三種不同的鎳金屬異位紫質錯合物，分別是Ni(3-CNHCTPP) (3a)、 Ni(HCTPFPP) (1c) 及 Ni(HCTPFPOP) (4a)，並進行NMR、UV-vis、CV光譜及晶體結構分析，在電化學的結果中發現C≡N取代的鎳金屬錯合物中心金屬較不容易氧化，和氟苯取代的鎳金屬錯合物有類似的現象，而氟苯取代的錯合物拉電子性質較顯著。除此之外，我們發現 Ni(HCTPFPOP) (4a)有三組連續的可逆氧化還原峰，這在紫質系統中相當罕見，因此我們對Ni(HCTPFPOP) (4a) 進行光譜電化學分析並推斷第一組可逆氧化還原波的電子密度分佈在金屬中心的軌域上。將Ni(HCTPP)加入氧化劑後得到了三明治結構的{[Ni(HCTPPH)·]2Ag}SbF6 (2)，經EPR及1H-NMR分析後推測反應物應為兩價自由基的物質。除此之外，2D-NMR鑑定中發現了環內碳上的氫訊號，且d8-d10鍵的結構在紫質或異位紫質系統中非常罕見。
我們發現氟苯取代的鎳金屬錯合物在有機溶劑中的溶解度比一般苯環取代的鎳金屬錯合物大很多，且氟苯取代缺電子的效應造成錯合物較容易還原。而電催化產氫的過程中，質子還原和催化劑的還原電位有很大的關係，因此具有強拉電子基的錯合物能夠降低產氫所需提供的偏壓，所以我們嘗試把Ni(HCTPFPP) (1c) 及 Ni(HCTPFPOP) (4a) 作為產氫的催化劑，初步測試的結果發現Ni(HCTPFPP) (1c) 及 Ni(HCTPFPOP) (4a) 在有醋酸及TFA作為質子來源的情況下皆具有產氫製備的活性，其中以Ni(HCTPFPOP) (4a) 為催化劑，TFA為質子源的產氫效果較好，轉換率有6.664 molH2/ molcatalyst。
;Using meso-tetrakis(pentafluorophenyl) N-confused porphyrin (NCTPFPP ) (1) as a ligand, CoII(HCTPFPP) (1a) was firstly synthesized and characterized by UV-vis spectrometer, 1H-NMR spectroscopy, EPR spectroscopy, and cyclic voltammetry. The spectroscopic data confirmed that Co(HCTPFPP) is Co(II). The electron withdrawing meso-substituents of 1a, shifted the first redox wave toward positive (22mV more positive in comparision with the first oxidation potential in Co(HCTPP) )and lower the energy level of HOMO, resulted in a 16% slower in reducing the oxidation rate compared with phenyl substitued CoII(HCTPP).
Besides cobalt complex, three new types of nickel N-confused porphyrin complex analogues, Ni(3-CNHCTPP) (3a), Ni(HCTPFPP) (1c) and Ni(HCTPFPOP) (4a) were also prepared and characterized by UV-vis spectrometer, 1H-NMR pectroscopy, cyclic voltammetry, and single crystal X-Ray structure determinations.
From the results of cyclic voltammetry, we found that the C≡N α position substitued- nickel N-confused porphyrin has the most positive oxidation potential with the potential close to the value in meso-pentapluoro phenyl substitued-nickel N-confused porphyrin. As for Ni(HCTPFPOP) (4a), which has three reversible oxidation processes, the first oxidation step occurs on a nickel ion centered molecular orbital as confirmed by spectroelectrochemical studies.
Besides, we demonstrate a new sandwich structure of nickel N-confused porphyrin complex, {[Ni(HCTPPH)·]2Ag}SbF6 (2), accroding to the result of EPR spectrum and 1H-NMR, we insist that (2) is Ni(II) radical spices. In addition, 2D-NMR spectra also show a singlet proton signal located at inner carbon of N-confused porphyrin ligand and confirms a dianionic NCP core under 2H tautomeric form.
We found that the replacement of meso-phenyl by meso-pentapluoro phenyl significantly increased the solubility and caused an anodic shift to the reduction waves. As electrocatalytic hydrogen evolution is closely correlated to the redox potential of catalysts, metalloporphyrins with strong electron-withdrawing substituents are considered to be able to reduce the energy inputs for generating H2, and thus lower the cost of hydrogen.Therefore, we use Ni(HCTPFPP) (1c) and Ni(HCTPFPOP) (4a) as the hydrogen-evolution-reaction (HER) catalyst. The results show that Ni(HCTPFPP) (1c) and Ni(HCTPFPOP) (4a) can catalyze H2 generation under electrochemical coditions in the presence of acetic acid or TFA as the exogeneous proton source, and Ni(HCTPFPOP) using TFA as proton source has the best HER reactivity with onset potential of 0.8V. |
