Sitting-atop Complexes Research Articles

Push‐Pull Effect at Formation of Sitting‐Atop Metal‐Porphyrin Complex in Solvating Media: H‐Bonding and Electrostatic Repulsion

AbstractThis work presents the results of experimental and theoretical studies of the solvation effects on the kinetics of the porphyrins reaction with zinc and cadmium acetate, the structure and characteristics of the intermediate ‐ sitting‐atop (SAT) complex. Kinetic rate constants of metal complexes of tetra‐p‐substituted tetraphenylporphins formation in binary solutions with a regulated solvating ability were obtained. Extreme dependences of the rate constants on the solvent composition were found. The acceleration of the reaction was shown to be observed with an enhancement of the electron‐donating properties of the substituent. These data indicate the formation of the SAT complex as limiting stage. The structure and energy parameters of porphin‐zinc SAT complexes and its solvates were studied by DFT method using B3LYP and long range corrected CAM‐B3LYP hybrid functionals. A significant distortion of the porphin structure during the formation of an intermediate and its solvation was ascertained. The influence of these processes on structural and energy changes was founded to be the consequence of push‐pull effect due to the repulsion of NH hydrogen atoms by a metal ion (push) and stretching of these bonds (pull) during the formation of H‐bonds.

Aggregation and metal-complexation behaviour of THPP porphyrin in ethanol/water solutions as function of pH

The effect of pH change on 5,10,15,20-Tetrakis(4-hydroxyphenyl)-21H,23H-porphine (THPP) with its aggregation as function of water-ethanol mixture was studied with UV–vis, fluorescence, Raman and computational analysis. In neutral pH, THPP was present as free-base and, increasing the water amount, aggregation occurred with the formation of H- and J-aggregates. The aggregation constant and the concentration of dimers were calculated, other information about the dimer aggregation were evaluated by computational study.In acidic pH, by the insertions of two hydrogens in the porphyrin rings, the porphyrin changed its geometry with a ring deformation confirmed by red-shifted spectrum and quenching in fluorescence; at this low pH, increasing the water amount, the acidic form (THPPH2)2+ resulted more stable due to a polar environment with stronger interaction by hydrogen bonding.In basic pH, reached by NH4OH, THPP porphyrin was able to react with alkali metals in order to form sitting-atop complex (M2THPP) confirmed by the typical absorption spectrum of metallo-porphyrin, Raman spectroscopy and by computational analysis.

Human Ferrochelatase: Insights for the Mechanism of Ferrous Iron Approaching Protoporphyrin IX by QM/MM and QTCP Free Energy Studies.

Ferrochelatase catalyzes the insertion of ferrous iron into protoporphyrin IX, the terminal step in heme biosynthesis. Some disputes in its mechanism remain unsolved, especially for human ferrochelatase. In this paper, high-level quantum mechanical/molecular mechanics (QM/MM) and free-energy studies were performed to address these controversial issues including the iron-binding site, the optimal reaction path, the substrate porphyrin distortion, and the presence of the sitting-atop (SAT) complex. Our results reveal that the ferrous iron is probably at the binding site coordinating with Met76, and His263 plays the role of proton acceptor. The rate-determining step is either the first proton removed by His263 or the proton transition within the porphyrin with an energy barrier of 14.99 or 14.87 kcal/mol by the quantum mechanical thermodynamic cycle perturbation (QTCP) calculations, respectively. The fast deprotonation step with the conservative residues rather than porphyrin deformation found in solution provides the driving force for biochelation. The SAT complex is not a necessity for the catalysis though it induces a modest distortion on the porphyrin ring.

Study of Multiporphyrin Compounds as Colorimetric Sitting-Atop Metal Complexes: Synthesis and Photophysical Studies.

Mono-, di-, tri-, and hexaporphyrin derivatives were synthesized and their sensorial ability toward Na+ , Li+ , Ca2+ , Cu2+ , Ni2+ , Co2+ , Fe3+ , Zn2+ , Cd2+ , Hg2+ , Ag+ , Cr3+ , Pb2+ , and Al3+ was explored by absorption and emission spectroscopy and 1 H NMR spectroscopy. For all of the studied porphyrin derivatives the most significant spectral changes were observed upon addition of Zn2+ , Cd2+ , Cu2+ , Co2+ , Pb2+ , Hg2+ , and Fe3+ , which resulted in a color change from red to green. When the remaining metal ions were tested, no significant changes were observed. The results reveal, as expected, the formation of mononuclear complexes for porphyrin 4, dinuclear complexes for diporphyrin 7, and trinuclear complexes for triporphyrins 9 and 10. In addition, complexes of type M6 L were determined for hexaporphyrin 8. The NMR spectroscopy studies suggest that the interaction with the metals occurs with the formation of sitting-atop complexes. This study also shows that the sensitivity increases with increasing number of coordination sites, with the most pronounced case observed for Hg2+ . The lowest value of detectable concentration of 0.05 μm for Hg2+ was achieved for hexaporphyrin 8.

Synthesis, structure, and reactions of iridium(I) complexes with 5,10,15,20-tetraphenyl-21H,23H-porphine and 5,10,15,20-tetraphenyl-21H,23H-porphine dianion

One-step joint synthesis of two iridium porphyrin complexes, a donor-acceptor SAT (sitting a top) complex μ-(5,10,15,20-tetraphenylporphine)-bis-chloroiridium(I) and the covalent complex (5,10,15,20-tetraphenylporphinato)chloroiridium(III) by the reaction of free porphyrin and chloroiridic acid (H3O)2IrCl6 in boiling phenol was studied. The structure of complexes was confirmed by spectroscopy (UV/Vis, IR, 1H NMR) and TLC. The iridium(III) SAT complex with the hydride ligand in the first coordination sphere, (acetato)hydrido(5,10,15,20-tetraphenylporphine)iridium(III), was obtained by oxidative addition reaction, which is quite rare for porphyrin complexes. The thermodynamic stability of the complexes to oxidants (aerated acids) was studied by spectrophotometric titration.

Diffusion, Rotation, and Surface Chemical Bond of Individual 2H-Tetraphenylporphyrin Molecules on Cu(111)

We address the dynamic behavior and the surface chemical bond of 2H-tetraphenylporphyrin (2HTPP) on Cu(111) around room temperature by variable-temperature scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) in ultrahigh vacuum. Between 280 and 345 K, the molecules predominantly display unidirectional diffusion along one of the three densely packed substrate ⟨110⟩ directions, which is attributed to a high site selectivity of the adsorbate−substrate bond. Above 305 K, the diffusion direction is found to change occasionally by ±120°. The activation barriers for the unidirectional diffusion and for rotation of the diffusion direction are determined to 0.71 ± 0.08 and 1.28 ± 0.12 eV, respectively. XPS shows that the iminic nitrogen atoms of 2HTPP interact strongly with the Cu surface. It is postulated that the local bonding situation is similar as in the initial complex (sitting-atop complex), which has previously been observed during the surface-confined in situ metalation of porph...

A valuable visual colorimetric and electrochemical biosensor for porphyrin

Porphyrin is able to specifically combine with phosphorus, thus a novel bifunctional sensing platform for determination of porphyrin by visual colorimetry and electrochemistry was demonstrated. A pretreated gold sheet (or electrode) with 2-mercatpoethanol (2-ME) was chemically modified by POCl 3 to obtain the surface phosphate active sites. The different stages of modified electrode were characterized by electrochemical impedance spectroscopy (EIS). The 1:1 cationic sitting-atop (SAT) complex P(V)–porphyrin was formed due to the high affinity of the modified gold sheet (or electrode) towards the porphyrin, resulting in electron transfer resistance increase of the electrode surface. Meanwhile, a dramatic color changing from burgundy to dark green of porphyrin solution was observed with the naked-eye within 3 s. What's more, this was reflected by the notable change of the Soret band of porphyrin when using UV–vis. Two sensing systems provide different sensitivity for porphyrin analysis. With visual colorimetry, porphyrin can be detected at a level of 1.0 × 10 −6 M, whereas the detection limit of porphyrin is 3.0 × 10 −8 M using the EIS method. The practical application of the sensor to determination of pheophytin which was obtained from fresh spinach leaves has been accomplished. The results demonstrate the facility and effectivity of our introduced bifunctional biosensor for quantitative analysis of porphyrin.

Synthesis under solvent free conditions and photoluminescence study of ionic intermediate sitting-atop complexes of meso-tetraarylporphyrins and phosphorus oxychloride

The reaction of meso-tetraarylporphyrins with phosphorus oxychloride was studied. The reaction product is the so-called intermediate sitting-atop (i-SAT) complex where two pyrrolic nitrogen atoms of the porphyrin core coordinate to the phosphorus atom and two protons on the pyrrolic nitrogen atoms remain. Selection of solvent free conditions is caused that the reaction does not progress until deprotonation step of porphyrin and stopped in the intermediate step for formation of the sitting-atop complex, [POCl 2(H 2t(X)pp)]Cl. The sitting-atop complexes were characterized by ( 1H, 31P, 13C) NMR, FT-IR, UV–vis and photoluminescence spectroscopy (PL), elemental analysis and electrical conductometry. Photoluminescence study of the complexes indicates that their emission spectra are different from those of free base porphyrins.

Synthesis and Spectroscopic Characterization of the New Sitting-atop Complexes from Reaction of Zirconyl Nitrate and Free Base meso-Tetraarylporphyrins in Mild Conditions

A series of sitting-atop (SAT) complexes, [(ZrO)<TEX>$H_2t(X)pp(NO_3)_2$</TEX>], have been prepared via the reactions of free base meso-tetraarylporphyrins, <TEX>$H_2$</TEX>t(X)pp, with zirconyl nitrate hydrate, ZrO(N<TEX>$O_3)_2{\cdot}xH_2$</TEX>O. The products have been characterized by a variety of methods including <TEX>$^1H\;NMR,\;^{13}C$</TEX> NMR, IR and UV-Vis spectroscopies, elemental analysis and conductance measurements. The data indicate that the meso-tetraarylporphyrins coordinate with two pyrrolenine nitrogen atoms to the zirconyl cation located above the distorted porphyrin plane and two protons remain on the pyrrole nitrogens. Such half sandwich-type sitting-atop complexes may be considered as models for the initial steps of the metallation of the macrocycles.

Sitting-Atop Metallo-Porphyrin Complexes: Experimental and Theoretical Investigations on Such Elusive Species

The interaction between the sodium cation and two meso-aryl porphyrins (tetraphenylporphyrin, TPP, and tetra(4-methoxyphenyl)porphyrin, TMPP) leads to the formation of new species that have been identified as Sitting-Atop (SAT) complexes, where the metal ion interacts with the N atoms of the porphyrin core without the concomitant deprotonation of the N-H groups. These species have been attained in low polarity solvent through the interaction of the porphyrin free bases with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB), and investigated in situ through a combination of spectroscopic techniques, such as UV/vis absorption and fluorescence (static and time-resolved), resonance light scattering, FT-IR, and (1)H NMR. All spectroscopic evidence points to the occurrence of a single equilibrium between each parent compound and its SAT complex, ruling out the presence of other metallo-, protonated, or aggregated porphyrins in solution. The 1:1 stoichiometry of the adducts has been determined via continuous variation method (Job's plot), and an estimate of the corresponding association constants in CH(2)Cl(2) at 298 K have been obtained by UV/vis titration (K(eq) = (9 +/- 4) x 10(5) L mol(-1) and (5 +/- 2) x 10(6) L mol(-1) for TPP and TMPP, respectively). Density-functional theory (DFT) calculations on SAT model complexes, [NaTPP(PF(6))] and [NaTMPP(PF(6))], have provided information on the molecular structure of these elusive species and on the nature and strength of the sodium-porphyrin interaction. It is found that the sodium ion is bound to the four nitrogen atoms of the porphyrin core. The involvement of the pyrrolic N atoms results in a modest but not negligible elongation of the N-H bonds, pyramidalization of the hydrogen atoms, and blue shift of the N-H stretching frequencies. Electronic structure and energy decomposition analysis reveal that covalent interactions, mainly consisting of porphyrin to sodium charge transfer interactions, are an important component of the sodium-porphyrin bond. Time-dependent DFT (TDDFT) calculations of the lowest excited states of the model systems provide an unambiguous interpretation of the absorption and emission properties of the experimentally identified SAT complexes.

A spectrophotometric and thermodynamic study of the sitting-atop complex formation from reaction between free base meso-tetraarylporphyrins and zirconyl nitrate in chloroform solution

The sitting-atop complexation of meso-tetraarylporphyrins and its para-substituted derivatives (H 2t(4-X)pp, X:H, Br, Cl, CH(CH 3) 2, OCH 3, CH 3), as electron donors, with zirconyl, as an electron acceptor, have been investigated spectrophotometrically in chloroform. The mole ratio studies based on physicochemical techniques were employed clearly and revealed the formation of 1:1 sitting-atop complexes which was confirmed by UV–vis, 1H NMR and IR spectroscopic data. The value of the formation constant was estimated for each complex using a nonlinear optimization of the complex absorbance vs. mole ratio data by package KINFIT. The results showed that the stability of these complexes decreases with the temperature enhancement. Thermodynamic parameters, Δ G°, Δ H° and Δ S°, of the SAT complexes have been determined from the temperature dependence of formation constants by Van’t Hoff equation. Also, the influence of the substituents of the aryl rings in H 2t(4-X)pp on the stability of the SAT complexes is discussed.

Synthesis of new sandwich intermediate sitting-atop complexes between meso-tetraarylporphyrins and germanium(IV) chloride

GeCl 4 and meso-tetraarylporphyrins (H 2TAPP) react in chloroform solvent for formation sandwich intermediate sitting-atop (i-SAT) complexes, [GeCl 4(H 2TAP) 2]. The various spectral data ( 1H NMR, 13C NMR, UV–Vis, FT-IR and elemental analysis) were used for characterization of the i-SAT complexes. In the sandwich complexes, the pyrrole rings of two porphyrin macrocycles are tilted up and down and act as electron donors by lone pairs of pyrrolic nitrogens to germanium center of GeCl 4. The spectral results of 1H NMR and FT-IR showed that in the i-SAT complexes, the hydrogen atoms of two pyrrolic nitrogens remained still on the porphyrin macrocycles.

Synthesis of new ionic intermediate sitting-atop complexes of free base meso-tetraarylporphyrin and phosphorus(V) chloride under solvent free conditions

The intermediate sitting-atop (i-SAT) complexes are metalloporphyrins in which the metal ion is bonded to fewer than four nitrogen atoms in the distorted porphyrin plane. The i-SAT complexes may be considered as models for the initial steps of the metallation of the macrocycles. The new ionic intermediate sitting-atop complexes of [PCl 3(H 2t(X)pp)]Cl 2 containing phosphorus(V) was synthesized by reaction of PCl 5 and meso-tetraarylporphyrins (H 2t(X)pp) in solvent free conditions. The structure of the complexes was confirmed by ( 1H, 31P, 13C) NMR, FT-IR and UV–Vis spectroscopies, elemental analysis and electrical conductometry. These data proved that only two pyrrolenine nitrogen atoms of the porphyrin act as electron donors to one phosphorus(V) center and so two pyrrolic protons remained still on the macrocycle.

Spectrophotometric studies of the thermodynamics of sitting-atop complexation between free base meso-tetraarylporphyrins and titanium(IV) chloride

Titanium(IV) chloride reacts with free base meso-tetraarylporphyrin and its ortho, meta and para-substituted derivatives (H 2T(X)PP; X: OCH 3, CH 3 and Cl) for formation of sitting-atop (SAT) complexes, [TiCl 4(H 2T(X)PP)]. The computer fitting of the variation of the absorbance versus mole ratio by KINFIT program was used for calculation of the formation constants of these complexes in chloroform. Thermodynamic parameters, Δ G°, Δ H°, Δ S°, have been determined and the influence of the temperature and the substituted aryl groups (electronic and steric effects) in the free base porphyrins on the stability of the SAT complexes was studied.

Synthesis of intermediate sitting-atop complexes (i-SAT) from the reaction between free base meso-tetraarylporphyrins and phosphorus(III) chloride in solvent free media

The reaction of meso-tetraarylporphyrins (H 2tap) with phosphorus(III) chloride has been studied in the solid state. The reaction products are intermediate sitting-atop (i-SAT) complexes in which two pyrrolenine nitrogen atoms of the porphyrins act as electron donors to the phosphorus(III) atoms and two protons on the pyrrole nitrogen atoms remain. These new complexes have been characterized by ( 1H, 31P, 13C) NMR, FT-IR and UV–Vis spectroscopy, elemental analysis and electrical conductometry.

Synthesis, characterization and the thermodynamic study of intermediate sitting-atop (i-SAT) complexes of free base meso-tetraarylporphyrins with InCl 3

The reaction of free base para-substituted meso-tetraarylporphyrins (H 2T( p-X)PP, X = H, OMe, Me, and Cl) with indium(III) chloride in CHCl 3 and mild conditions produced intermediate sitting-atop (i-SAT) complexes, [InCl 2(H 2T( p-X)PP)]InCl 4, as sole products. In the proposed structures of these complexes, four pyrrole rings are tilted alternatively up and down the porphyrin plane. This distortion makes suitable orientation of lone pairs of two pyrrolenine nitrogens for electron donation to an indium center of InCl 2 + cation. The 1:2 (porphyrin:indium) formation constant of resulting i-SAT complexes were calculated by the computer fitting of the complexes absorbance versus mole ratio data based on appropriate equations. Thermodynamic parameters, Δ G 0, Δ H 0, and Δ S 0, have been determined and the influence of electron donation of the para-substituted aryl groups in the free base porphyrins on the stability of the complexes is discussed.

Synthesis and characterization of intermediate sitting-atop (i-SAT) complexes of free base meso-tetraarylporphyrins and tin(IV) chloride

The reaction of meso-tetraarylporphyrins (H 2T(X)PP) with SnCl 4 affords green intermediate sitting-atop (i-SAT) complexes, [(H 2T(X)PP)SnCl 4]. UV–Vis, 1H NMR and 13C NMR spectral data show that the porphyrin core of the complexes is distorted, thus two nitrogen atoms of the pyrrolenine groups on one side of the porphyrin plane act as electron donors to the tin center of SnCl 4. The intermediate sitting-atop (i-SAT) complex is formed each time during the incorporation of the metal center, where in the intermediate state the pyrrolic protons still remain on the porphyrin.

Thermodynamic studies of sitting-atop (SAT) complexation of uranyl and free base meso -tetraarylporphyrins

The interaction of uranyl with meso-tetraphenylporphyrin and its para-substituted derivatives (H2t(4-X)pp, X : H, Br, Cl, CH(CH3)2, OCH3, CH3) in chloroform produced 1 : 1 sitting-atop (SAT) complexes ((uranyl)H2t(4-X)pp). Formation constants were calculated by computer fitting of complex absorbance versus mole ratio data to appropriate equations and found to decrease with temperature increase. Thermodynamic parameters, ΔG 0, ΔH 0 and ΔS 0 were obtained. The formation constants vary with changing of the substituent on the aryl rings of H2t(4-X)pp in the following order: (uranyl)H2t(4-OCH3)pp > (uranyl)H2t(4-CH3)pp > (uranyl)H2t(4-CH(CH3)2)pp > (uranyl)H2tpp > (uranyl)H2t(4-Br)pp > (uranyl)H2t(4-Cl)pp.

Sitting-atop complex formation of free base meso-tetraarylporphyrins with zirconium(IV) chloride

We have succeeded in the preparation and spectroscopic characterization of sitting-atop (SAT) complexes of meso-tetraarylporphyrins (H 2t(X)pp) with zirconium(IV) chloride under mild conditions and at room temperature, where two pyrrolenine nitrogens in the SAT complexes, [(H 2t(X)pp)ZrCl 4], coordinate to a zirconium atom and two protons still remain on the pyrrole nitrogens. UV–Vis and NMR ( 1H and 13C) spectral data show that the porphyrin core of the SAT complexes is distorted and two pyrrolenine nitrogen atoms of the porphyrin act as electron donors to the zirconium atom of ZrCl 4.

Thermodynamic studies of sitting-atop complexation between free base meso-tetraarylporphyrins and antimony(III) chloride in chloroform

Interaction of para, meta and ortho-substituted meso-tetraarylporphyrins, (H2t(X)pp, X: OMe, Me, H and Cl) with SbCl3 in chloroform solution afforded 1 : 1 sitting-atop (SAT) complexes ([(SbCl3)(H2t(X)pp)]). The formation constants were calculated by KINFIT and found to decrease with increasing temperature. The thermodynamic parameters, ΔH°, ΔS° and ΔG°, were obtained. Formation constants of these complexes change with changing substituent (X) on the aryl rings of H2t(X)pp in the following order: (SbCl3)H2t(4-OMe)pp > (SbCl3)H2t(4-Me)pp > (SbCl3)H2tpp > (SbCl3)H2t(4-Cl)pp > (SbCl3)H2t(3-OMe)pp > (SbCl3)H2t(3-Me)pp> (SbCl3)H2t(2-OMe)pp > (SbCl3)H2t(2-Me)pp.