Synthesis,Electrochemistry And Spectroelectrochemistry Of Novel N-confused Porphyrins

Porphyrin molecules contain four pyrrole rings which connected by four –C= bridges to form a large ? conjugated aromatic system with 18 atoms and 18 ?-electrons.Porphyrin-like compounds have many important applications in the fields of photoelectrocatalysis,environmrntal protection and pollution treatment,novel medicine,energy source and materials.Researchers have synthesized various different porphyrins by changing the type and number of peripheral substituents on the meso-or ?-positions of porphyrin macrocycle or by expanding or contracting the conjugated system in different experimental conditions.Some porphyrin related derivatives having a different macrocycle to normal porphyrins are defined as porphyrinoids,sush as sapphyrins and N-confused porphyrins.The molecular structures of porphyrinoids have great similarity to the ‘normal' porphyrins,but they have many special properties due to a partial change in their structures.N-confused porphyrins are one kind of new porphyrinoids.Like “normal” porphyrins,they also have four meso-carbon atoms,but have an inverted pyrrole which leads to a N3 C core with a carbon inside the cavity and a nitrogen on the periphery of the ?-system.As compared to the‘normal' porphyrin,it was found that the inverted pyrrole ring of N-confused porphyrins leads to the outside N atom having a smaller steric hindrance and higher activity,which is beneficial to occurring methylation and other chemical reactions.In this dissertation,the synthesis and application progress of porphyrins,especially Nporphyrins,are summarized for a start.On the basis of that,several new types of N-confused porphyrins and related metalloporphyrins containing copper,nickel and cobalt central metal ions were synthesized and characterized.The structure of these compounds was characterized by the technologies of UV-visible spectroscopy,NMR and mass spectrometry.The oxidation/reduction behavior and the mechanism of electron transfer of the compounds were examined in detail using electrochemical and spectroelectrochemical methods.Influence of the substituents,metal ion and the inverted pyrrole on spectral and electrochemical properties was also discussed.In the light of structure features for the synthesized compounds,the main research contents of this dissertation are summarized in the chapters from two to seven.In the second Chapter,four N-methylated N-confused free-base tetraarylporphyrinscontaining different meso substituents were synthesized by Geier method.These N-confused porphyrins were then characterized by several analytical technologies.The research indicates that unlike the non-CH3 N-confused porphyrins which exist two different froms of in different solvents,the CH3 substituted derivatives exhibit the same form in different solvents.As shown by the electrochemical result,each N-methylated derivative undergoes three oxidations and two reductions in nonaqueous media.The products obtained from the first two electrooxidations are easier to occur protonation reactions.Therefore,the final products generated after the first and second oxidations are of the mono-protonated mono-cation and bis-protonated dications,respectively.The experimental results also indicated that the first two reductions are both the porphyrin ring-centered electron transfer processes,but the product generated after the first reduction is easier to occur a deprotonation reaction to give a mono-deprotonated mono-anion as the final product under the given experimental conditions.As compare to the non-CH3 compound,N-methylated derivatives are harder to occur the deprononation reactions.This is because the methyl on the inverted pyrrole of the compound is a strong electron-donating group which may stabilize the N-H bond in porphyrin core,lead to the deprotonation being harder to occur.Because of the existence of CH3,the N-methylated N-confused porphyrins are easier to be oxidized but harder to be reduced under the given solvent.However,the HOMO-LUMO gaps of the N-methylated compounds((35)E = 1.63 to 1.69 V)are very similar to that of non-methylated N-confused porphyrins((35)E = 1.66 to 1.68 V).In the third Chapter,three NO2-substituted N-methylated N-confused free-base tetraarylporphyrins with different meso-substituents were synthesized and characterized as to their electrochemical and spectroelectrochemical properties in dichloromethane and DMF.Each NO2 substituted derivatives undergoes three oxidations and two reductions in dichloromethane but only two oxidations and two reductions in DMF.As compare to N-methylated N-confused porphyrins,the first oxidation is harder to occur but the first reduction is easier for the compound containing an NO2-substituent.However,if compare to the related non-CH3 nitroated porphyrins,an easier oxidation and harder reduction can be observed for the compound having an electron-donating CH3 group.In CH2Cl2,the HOMO-LUMO gap(1.57-1.66 V)of the NO2-substituted N-methylated N-confused porphyrins is slightly lower than that of N-methylated N-confused porphyrins(1.63-1.69 V),but it is almost identical as compared to the value ofHOMO-LUMO gap(1.58-1.68 V)for NO2-substituted N-confused porphyrins.As indicated by spectroelectrochemical measurements,the final products of the two oxidations for the NO2-substituted N-methylated N-confused porphyrin are the mono-protonated porphyrin mono-cations and dications,while the reductions generate the deprotonated mono-anion and dianions and bis-protonated two one-electron reductions and two one-electron oxidations to give ?-anion radicals and dianions upon reduction and ?-cation radicals and dications upon oxidation under the given solution conditions in CH2Cl2.In the fourth Chapter,four copper N-methylated N-confused porphyrins were synthesized.Electrochemical and spectroelectrochemical properties as well as the electron transfer mechanism of the porphyrins were examined in nonaqueous media.Two oxidations and two reductions were observed in dichloromethane and Ph CN.As compare to the ‘normal' copper porphyrins having the same meso-substituents,both the first oxidation and first reduction are easier to occur under the given solution conditions Therefore,the HOMO-LUMO gaps of the copper N-methylated N-confused porphyrins((35)E = 1.37-1.40 V in dichloromethane and1.41-1.46 V in Ph CN)are much smaller than those of copper tetraarylporphyrins((35)E = 2.29 V in dichloromethane and 2.30-2.37 V in Ph CN).The spectroelectrochemical data shown that both the first and second reductions of copper N-methylated N-confused porphyrins are porphyrin ring-centered electron transfer processes to give ?-cation radicals and dications,while both the reductions are also the porphyrin ring-centered to give ?-anion radicals and dianions under the two utilized solvents.In the fifth Chapter,three nickel N-methylated N-confused porphyrins were synthesized via a reaction between the nickel salt and free-base N-methylated N-confused porphyrins under the given experimental conditions.The electrochemical and spectroelectrochemical properties of the compounds were examined in detail by cyclic voltammetry and spectroelectrochemiscal thechnigues.Three oxidations and two reductions can be observed for each compound in CH2Cl2.Both the oxidation and reduction of nickel N-methylated N-confused porphyrins are harder than that of the related copper N-methylated N-confused porphyrins,leading to a much larger HOMO-LUMO gaps(1.77-1.83 V)for the nickel porphyrins than the copper derivatives(1.37-1.40 V).Based on the spectroelectrochemical data,the first two oxidations and two reductions are all porphyrin ring-centered electron transfer processes,the products of thesereactions are proposed to be the mono-?-cation radicals and dications for the oxidation,and the mono-?-anion radicals and dianions for the reduction under the given solvent.In the sixth Chapter,three cobalt N-methylated N-confused porphyrins containing different meso-substituents were firstly synthesized and characterized as to their electrochemical and spectroelectrochemical properties in dichloromethane.Two oxidations and one reduction were observed in dichloromethane.The oxidation mechanism of the cobalt porphyrins was the same as that of the corresponding copper and nickel N-methylated N-confused porphyrins,i.e.the oxidations of cobalt methylated N-confused porphyrins are the porphyrin ring-centered electron transfer processes to give a p-cation radical and dication as the products.However,the first reduction the compound is metal-centered,leading to formation of a Co(I)porphyrin which is easy to react with the CH2Cl2 solvent to generate [N-CH3-(RPh)Ncp Co III-CH2Cl] which contains a Co-C sigma bond.In the seventh Chapter,four aryl-substituted tripyrrinone copper complexes were synthesized and characterized as to their electrochemical and spectroelectrochemical properties.Three reductions were observed for the investigated compounds rather than two reductions as seen for the structurally related copper porphyrins and corroles.As compare to copper porphyrins,tripyrrinone copper complexes have a different macrocycle but a similar electron transfer mechanism which leads to a ?-anion radical and dianion upon reduction and a ?-cation radical and dication upon oxidation.The reversible half-wave potentials for the first reduction of tripyrrinone copper complexes are shifted positive by about 700 m V as compared to copper porphyrins.Therefore,tripyrrinone copper complexes have obvious smaller HOMO-LUMO gaps.