N-confused Tetraphenylporphyrin Research Articles

Well-defined asymmetric nitrogen/carbon-coordinated single metal sites for carbon dioxide conversion

Asymmetric nitrogen/carbon-coordinated single metal sites (M-NxC4-x) outperform symmetric M-N4 sites in carbon dioxide (CO2) electroreduction. However, the challenge of crafting well-defined M-NxC4-x sites complicates the understanding of their structure-catalytic performance relationship. In this study, we employ metallized N-confused tetraphenylporphyrin (M-NCTPP) to investigate CO2 conversion on M-N3C1 sites using both density functional theory and experimental methods. The optimal cobalt (Co)-N3C1 site (Co-NCTPP) achieves a current density of 500 mA cm−2 and a carbon monoxide Faraday efficiency exceeding 90 % at −1.25 V vs. the reversible hydrogen electrode, surpassing the performance of Co-N4 (Co-TPP). This research introduces a novel approach for designing and synthesizing high-activity heteroatom-anchored single metal sites, advancing fundamental understanding in the field.

Water-soluble anionic N-confused porphyrin for sensitive and selective detection of heavy metal pollutants in aqueous environment.

Efficient detection and quantification of metal ions in real time and in a cost-effective manner is a critical step in combating the increasing danger of heavy metal contamination of our biosphere. The potential of water-soluble anionic derivative of N-confused tetraphenylporphyrin (WS-NCTPP) has been investigated for quantitative detection of heavy metal ions. The results show that the photophysical properties of WS-NCTPP differ significantly in the presence of four metal ions, namely Hg(II), Zn(II), Co(II) and Cu(II). The variation in the spectral behaviour is driven by the formation of 1:1 complexes with all the four cations with varied degree of complexation. The selectivity of the sensing is studied through interference studies, indicating maximum selectivity for Hg(II) cations. Computational studies of the structural features of the metal complexes with WS-NCTPP help in establishing the geometry and binding interactions between the metal ions and the porphyrin nucleus. The results demonstrate the promising potential of the NCTPP probe which should be utilized for detection of heavy metal ions, especially mercury, in the near future.

Spectroscopic investigation of preferential solvation of N-confused tetraphenylporphyrinin binary mixtures of dichloromethane with organic cosolvents

Spectroscopic behaviour of porphyrin is known to be sensitive to the choice of the solvent and has been used to probe solvation phenomena. The UV–visible spectra of N-confused tetraphenylporphyrin are observed in a series of binary solvent mixtures – (dichloromethane + diethyl ether), (dichloromethane + chlorobenzene) and (dichloromethane + ethyl acetate). The resulting ET scale for NCTPP solvation indicates nonlinear behaviour and preferential solvation by a solvent–solvent complex in all the three solvents – with the dichloromethane + ethyl acetate system showing the largest deviation from ideality. The data is fitted to the model based on solvent exchange equilibria for determining the preferential solvation parameters which are specific to the probe as well as the identity of solvents in the binary mixture. Further analysis using polarity parameters from literature indicate that the solvation of NCTPP in the (dichloromethane + ethyl acetate) mixture is dependent on the hydrogen bond accepting capacity and polarizability of the medium. Excess spectra derived from ATR-FTIR measurement of dichloromethane + ethyl acetate solutions lead to important inferences about the structure and role of solvent–solvent interactions responsible for the preferential solvation.

Porphyriynes: 18-π-Conjugated Macrocycles Incorporating a Triple Bond.

Tetraphenylporphyriyne (Pyne1), a novel porphyrin analogue with a C≡C bond incorporated into an 18-π-conjugated system, has been created via cleavage of the N-confused pyrrolic ring in Ag(III) N-confused tetraphenylporphyrin. The structure of Pyne1 was confirmed by X-ray crystallography and 1H NMR, IR, and UV-vis spectroscopy. The mechanism of cleavage of the N-confused pyrrolic ring was investigated by theoretical calculations. The successful synthesis of other Pynes indicated the generality of this protocol.

One-pot synthesis of N-confused porphyrin-dipyrrin conjugates and their optical properties.

A conjugated N-confused porphyrin-dipyrrin system NCP-IN-X (X = H, Br, Cl, OMe, COOEt) has been reported via N-Confused tetraphenylporphyrin (NCTPP) as a π-enlarged pyrrole and substituted trimethylindole subunits in 75-82% yields by one pot. All the NCP-INs show red-shifted absorption and emission in the NIR range compared with NCTPP in a series nonpolar and polar solvents. Interesting, relative higher emission in DMF can be observed. The structures of dipyrrin-NCP conjugates are characterized by 1H-NMR and high resolution mass spectrum (HR-MS). Four frontier molecular orbital and simulated stick absorption spectra are calculated by DFT and TD-DFT calculation which is in good agreement with the experiments.

The selectivity between inner C-cyanation and alkylation in cobalt(II) complexes of N-confused porphyrin with an axial NCS− ligand

Some inner C-cyanation and C-alkylation complexes have been described. Reactions of 2-N-substituted N-confused tetraphenylporphyrin (2-NCH2YNCTPPH) with Co(SCN)2 afford C-alkylation (14) and C-cyanation (15–17) complexes. The crystal structures for the inner C-alkylation product of thiocyanato(2-aza-2-[p-methylbenzyl]-5,10,15,20-tetraphenyl-21-benzyl-21-carbaporphyrinato-N,N′,N″)cobalt(II)·0.5 octane solvate [Co(2-NCH2-p-CH3C6H4-21-CH2C6H5NCTPP)(NCS)·0.5C8H18; 14·0.5C8H18] and the inner C-cyanation products of thiocyanato(2-aza-2-ethoxycarbonylmethyl-5,10,15,20-tetraphenyl-21-cyano-21-carbaporphyrinato-N,N′,N″)cobalt(II) [Co(2-NCH2COOCH2CH3-21-CN NCTPP) (NCS); 15], thiocyanato(2-aza-2-methyl-5,10,15,20-tetraphenyl-21-cyano-21-carbaporphyrinato-N,N′,N″)cobalt(II)[Co(2-NCH3-21-CNNCTPP) (NCS); 16], and thiocyanato(2-aza-2-benzyl-5,10,15,20-tetraphenyl-21-cyano-21-carbaporphyrinato-N,N′,N″)cobalt(II) [Co(2-NCH2C6H5-21-CNNCTPP) (NCS); 17] have been reported. The geometry around Co2+ ion in these four complexes is a distorted tetrahedron with a C2v symmetry. Two possible mechanisms were proposed to explain the formation of inner C-alkylation (14·0.5C8H18) versus C-cyanation (15–17) products in these complexes with an axial *NCS− ligand.

Solvent Effects on the UV-Vis Absorption Properties and Tautomerism of N-Confused Tetraphenylporphyrin.

The two tautomeric forms of N-confused tetraphenylporphyrin (NCTPP) show distinctly different absorption spectra. The existence of each tautomer in solution has been shown to be strongly solvent-dependent. In the present work, we have studied the tautomerization using absorption spectroscopy in 15 different solvents. While changes in the two tautomers are not large in the Soret band region, the distinct spectral changes between the two tautomers in the Q-band region provide a convenient way to measure the concentration of each tautomer. The resulting data shows a strong correlation between the tautomer and the H-bond accepting ability of the solvent. The anomaly in this data is for the alcoholic solvents ethanol and methanol, for which we observe evidence for H-bonding, presumably between the exterior N2 nitrogen of the NCTPP and the O-H proton of the solvent.

Near-infrared luminescent Sn(IV) complexes of N-confused tetraphenylporphyrin: Effect of axial anion coordination

Novel tin(IV) halo complexes of an N-confused tetraphenylporphyrin with different axial ligands have been synthesized and characterized by various spectroscopic methods including X-ray crystallographic analysis. The molecular structures of the dichloro and dibromo derivatives possess perfect octahedral geometries, which are nearly comparable to the corresponding regular porphyrin complexes. In contrast, the iodide/triiodide complex obtained by a same reaction manner, demonstrated that the tin(IV) cation is slightly displaced towards axially coordinated iodide anion, giving rise to the different electronic structure due to the tautomeric form of N-confused porphyrin ligand. These structural differences reflected to the distinct photophysical and electrochemical properties. The Sn(IV) complexes are near IR luminescent, however the unsymmetrical axial coordination of iodide and triiodide anions in the tin(IV) N-confused porphyrin complex allows, in particular, the longer emission lifetimes and a smaller singlet-triplet energy gap, which were investigated by steady-state and time-resolved spectroscopies as well as theoretical calculations.

Difunctionalized N-Confused Porphyrins: Synthesis, Fluorescence, and Electrochemical Studies

Seven di-substituted N-confused porphyrins (NCPs) 9–15 bearing two aryl functional groups (cis-A2B2 type) were synthesized in 4–7 % yields via [3+1] approach. The corresponding five 5,10-diaryl-substituted symmetrical tripyrranes 1–5 were prepared and condensed with 2,4-bis(hydroxypentaflurophenyl)pyrrole 6. Two outer N-methyl type A2B2 NCPs 14 and 15 were also prepared via a similar approach using a new key precursor 8. All the porphyrins 9–15 were characterized by high-resolution mass spectrometry, NMR, infrared spectroscopy, UV–visible spectroscopy, fluorescence spectroscopy, and cyclic voltammetry. Fluorescence studies of 9–15 showed blue-shifted emission maxima and lower Stokes shifts values when compared with N-confused tetraphenylporphyrin (NCTPP). Electrochemical studies indicated easier oxidation of N-methyl NCPs 14 and 15 when compared with remaining NCPs 9–13.

Theoretical study on the electronic structure, formation and absorption spectra of lithium, sodium and potassium complexes of N-confused tetraphenylporphyrin

The present work is a theoretical study on lithium, sodium and potassium complexes of N-confused tetraphenylporphyrin (NCTPP). Its purpose is to determine the stability, binding, absorption spectra and formation energy of the complexes of NCTPP with sodium and potassium, studied here for the first time. All calculations were carried out employing density functional theory (DFT) and time-dependent DFT, using the B3LYP, PBE0 and M06-2X functionals in conjunction with the 6-31G(d,p) basis set. The results show that the energy ordering of different low-lying minimum energy structures of the three metal complexes is not the same, with the global minimum energy structures of the lithium and sodium resulting from the same tautomer of NCTPP while potassium complexes result from another tautomer of NCTPP. The insertion of Li, Na and K into NCTPP in the presence of THF is exothermic with reaction energies calculated to be −68, −56 and −50kcal/mol using the corresponding metal bis-(trimethylsilyl)amide reagents. The absorption vis–UV spectra are similar for the different metals and the Q and Soret bands are slightly red shifted as the metal changes from Li to K.

Sulfur-assisted interconversion between N-confused porphyrin and N-fused porphyrin

The ring-opening reaction of N-fused tetraphenylporphyrin (NFTPP-H, 2) to C3-substituted N-confused tetraphenylporphyrin (NCTPP-S-Ar, 4) proceeded efficiently in 85–95% yields. Furthermore, removal of the C3-arylthio-substituents in 4 was achieved by the two types of desulfurization reactions. The Ni2B-mediated desulfurization afforded C3-free N-confused tetraphenylporphyrin (NCTPP-H, 1) whereas the radical-mediated desulfurization with (n-Bu)3Sn-H and AIBN promoted the ring-fusion to afford 2.

Synthesis of Three Asymmetric N-Confused Tetraarylporphyrins

Two monosubstituted and one tetrasubstituted N-confused porphyrins (1-3) were prepared in ca. 3-5% yields using a [2 + 2] synthesis. The monosubstituted porphyrins have carbomethoxy (1) or nitro (2) substituents on one of the meso-phenyl groups, while the meso-phenyl groups of the third NCP (3) are substituted with nitro, bromo, and methyl groups in an AB(2)C pattern. The specific regiochemistry of the aryl rings around the macrocycle in each porphyrin was definitively determined using a combination of 1D ((1)H and (13)C) and 2D (gHMBC, gHSQC and ROESY) NMR spectroscopy. The absorption spectra of 1-3 in CH(2)Cl(2) are similar to those of N-confused tetraphenylporphyrin (NCTPP) but have Soret and Q bands that are shifted to lower energies with smaller extinction coefficients in comparison to those for NCTPP.

Computational Insight into the Electronic Structure and Absorption Spectra of Lithium Complexes of N-Confused Tetraphenylporphyrin

The present work is a theoretical investigation on lithium complexes of N-confused tetraphenylporphyrins (aka inverted) employing density functional theory (DFT) and time-dependent DFT, using the B3LYP, CAM-B3LYP, and M06-2X functionals in conjunction with the 6-31G(d,p) basis set. The purpose of the present study is to calculate the electronic structure and the bonding of the complexes to explain the unusual coordination environment in which Li is found experimentally and how the Li binding affects the Q and the Soret bands. The calculations show that, unlike a typical tetrahedral Li(+) cation, this Li forms a typical bond with one N and interacts with the remaining two N atoms, and it is located in the right place to form an agostic-like interaction with the internal C atom. The reaction energy, the enthalpy for the formation of the lithium complexes of N-confused porphyrins, and the effect of solvation are also calculated. The insertion of Li into N-confused porphyrin, in the presence of tetrahydrofuran, is exothermic with a reaction energy calculated to be as high as -72.4 kcal/mol using the lithium bis(trimethylsilyl)amide reagent. Finally, there is agreement in the general shape among the vis-UV spectra determined with different functionals and the experimentally available ones. The calculated geometries are in agreement with crystallographic data, where available.

Controllable and Reversible Inversion of the Electronic Structure in Nickel N-Confused Porphyrin: A Case When MCD Matters

Nickel N-confused tetraphenylporphyrin, 1, and nickel 2-N-methyl-N-confused tetraphenylporphyrin, 1-Me, exhibit unusual sign-reversed (positive-to-negative intensities in ascending energy) MCD spectra in the Q-type band region, suggesting a rare ΔHOMO < ΔLUMO relationship between π and π* MOs in the porphyrin core. Simple and reversible deprotonation of the external NH proton in 1 dramatically changes the electronic structure of the porphyrin core into the ΔHOMO > ΔLUMO combination characteristic for the meso-(tetraaryl)porphyrins. DFT, time-dependent DFT, and semiempirical ZINDO/S calculations on 1, 1-Me, and 1(-) confirm the experimental finding and successfully explain the MCD pattern in the target compounds.

Spectroscopy of Free-Base N-Confused Tetraphenylporphyrin Radical Anion and Radical Cation

The radical anions and radical cations of the two tautomers (1e and 1i) of 5,10,15,20-tetraphenyl N-confused free-base porphyrin have been studied using a combination of cyclic voltammetry, steady state absorption spectroscopy, and computational chemistry. N-Confused porphyrins (NCPs), alternatively called 2-aza-21-carba-5,10,15,20-tetraphenylporphyrins or inverted porphyrins, are of great interest for their potential as building blocks in assemblies designed for artificial photosynthesis, and understanding the absorption spectra of the corresponding radical ions is paramount to future studies in multicomponent arrays where electron-transfer reactions are involved. NCP 1e was shown to oxidize at a potential of E(ox) 0.65 V vs Fc(+)|Fc in DMF and reduce at E(red) -1.42 V, while the corresponding values for 1i in toluene were E(ox) 0.60 V and E(red) -1.64 V. The geometries of these radical ions were computed at the B3LYP/6-31+G(d)//B3LYP/6-31G(d) level in the gas phase and in solution using the polarizable continuum model (PCM). From these structures and that of H(2)TPP and its corresponding radical ions, the computed redox potentials for 1e and 1i were calculated using the Born-Haber cycle. While the computed reduction potentials and electron affinities were in excellent agreement with the experimental reduction potentials, the calculated oxidation potentials displayed a somewhat less ideal relationship with experiment. The absorption spectra of the four radical ions were also measured experimentally, with radical cations 1e(•+) and 1i(•+) displaying significant changes in the Soret and Q-band regions as well as new low energy absorption bands in the near-IR region. The changes in the absorption spectra of radical anions 1e(•-) and 1i(•-) were not as dramatic, with the changes occurring only in the Soret and Q-band regions. These results were favorably modeled using time-dependent density functional calculations at the TD-B3LYP/6-31+G(d)//B3LYP/6-31G(d) level. These results were also compared to the existing data of free base tetraphenylporphyrin and free base tetraphenylchlorin.

Synthesis of N-Confused Tetraphenylporphyrin Rhodium Complexes Having Versatile Metal Oxidation States

A variety of N-confused tetraphenylporphyrin rhodium complexes were synthesized, and their structures and physical properties were investigated. Depending on the reaction conditions, the rhodium(I), -(III), and -(IV) complexes were produced, which exemplified the versatile coordination mode of N-confused porphyrin ligands.

A Computational Study of the Ground and Excited State Structure and Absorption Spectra of Free-Base N-Confused Porphine and Free-Base N-Confused Tetraphenylporphyrin

Computational investigations into the ground and singlet excited-state structures and the experimental ground-state absorption spectra of N-confused tetraphenylporphyrin tautomers 1e and 1i and N-confused porphines (NCP) 2e and 2i have been performed. Structural data for the ground state, performed at the B3LYP/6-31G(d), B3LYP/6-31+G(d)//B3LYP/6-31G(d), and B3LYP/6-311+G(d)//B3LYP/6-31G(d) levels, are consistent with those performed at lower levels of theory. Calculations of the gas-phase, ground-state absorption spectrum are qualitatively consistent with condensed phase experiments for predicting the relative intensities of the Q(0,0) and Soret bands. Inclusion of implicit solvation in the calculations substantially improves the correlation of the energy of the Soret band with experiment for both tautomers (1e, 435 nm predicted, 442 nm observed in DMAc; 1i, 435 nm predicted, 437 nm observed in CH2Cl2). The x- and y-polarized Q-band transitions were qualitatively reproduced for 1e in both the gas phase and with solvation, although the low-energy absorption band in 1i was predicted at substantially higher energy (646 nm in the gas phase and 655 nm with solvation) than observed experimentally (724 nm in CH2Cl2). Franck-Condon state and equilibrated singlet excited-state geometries were calculated for unsubstituted NCP tautomers 2e and 2i at the TD-B3LYP/SVP and TD-B3LYP/TZVP//TD-B3LYP/SVP levels. Electronic difference density plots were calculated from these geometries, thereby indicating the change of electron density in the singlet excited states. Adiabatic S1 and S2 geometries of these compounds were also calculated at the TD-B3LYP/SVP level, and the results indicate that while 2i is a more stable ground-state molecule by approximately 7.0 kcal mol-1, the energy difference for the S1 excited states is only approximately 1.0 kcal mol-1 and is 6.1 kcal mol-1 for the S2 excited states.

Luminescent Au(iii) organometallic complex of N-confused tetraphenylporphyrin

An organometallic Au(III) complex of N-confused tetraphenylporphyrin has been synthesized and its electrochemical and photophysical properties investigated; unique emission is observed in solution at ambient temperature.

Endocyclic extension of porphyrin π-system in etheno-bridged N-confused tetraphenylporphyrin

Etheno-bridged N-confused tetraphenylporphyrin was synthesized through flipping of the confused pyrrole ring and endocyclic extension of [18]annulenic pi-conjugated system was exemplified by the absorption spectra as well as the calculated Kohn-Sham orbitals.

Continuous-Wave and Pulse EPR Study of the Copper(II) Complex of N-Confused Tetraphenylporphyrin: Direct Observation of a σ Metal−Carbon Bond

N-confused or inverted porphyrins, a family of porphyrin isomers that contain a confused pyrrole ring connected through its alpha and beta' positions in the macrocycle, exhibit unique physical and chemical properties, like, for instance, the ability to stabilize unusual oxidation states of metals due to the reactivity of the inverted pyrrole. In this Article, a combined multifrequency continuous-wave and pulse electron paramagnetic resonance (EPR) study of the copper(II) complex of N-confused tetraphenylporphyrin (TPP) is presented. By use of pulse EPR methods like ENDOR and HYSCORE, the magnetic interactions between the unpaired electron of the compound and the surrounding nitrogen nuclei were revealed. Through 13C labeling of the macrocycle, a detailed study of the carbon hyperfine interaction became possible and provided further insight into the character of the metal-carbon bond. The observed hyperfine couplings of the ligand atoms in the first coordination sphere showed the presence of a remarkably strong sigma Cu-C bond and allowed for a detailed analysis of the spin delocalization over the porphyrin macrocycle. Interestingly, it was found that the observed delocalization is approximately 11% larger than the corresponding one for CuTPP.