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

Abstract

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.

Surface chemistry of porphyrins and phthalocyanines

“…Rothemund approach for the synthesis of meso-substituted porphyrins [56]. details and the general validity of this reaction mechanism have been debated for many years [69]. Experimental evidence for the SAT complex was provided by NMR, EXAFS [70] and other techniques, whereas theoretical treatment indicated that the EXAFS data are ambiguous [71] and that they can also be explained with a mixture of reactant and product complexes instead of the SAT complex [72].…”

“…Experimental evidence for the SAT complex was provided by NMR, EXAFS [70] and other techniques, whereas theoretical treatment indicated that the EXAFS data are ambiguous [71] and that they can also be explained with a mixture of reactant and product complexes instead of the SAT complex [72]. The debate continues with recent work, in which the SAT complex was traced with UV-Vis, FTIR, NMR and theoretical methods [69], although a final proof for its existence has remained elusive. At present, the complete mechanism of porphyrin metalation is believed to consist of at least six steps, which include the partial desolvatation of the metal ion, SAT formation, and deprotonation [73].…”

Density functional theory studies on the structures and electronic communication of meso-ferrocenylporphyrins: Long range orbital coupling via porphyrin core

“…This, however, has not yet been confirmed from the point of theoretical investigation because of the lack of theoretical calculations in this regard. On the other hand, density functional theory (DFT) and time dependant density functional theory (TD-DFT) methods have proved suitable for the calculation of the energy-minimized structures, electronic distribution, molecular orbitals, electron transfer, and electronic absorption spectra of a series of porphyrin and phthalocyanine derivatives [17][18][19][20][21][22][23] as well as ferrocene derivatives [24][25][26][27][28][29]. As a consequence, towards understanding the long range electronic coupling between ferrocenyl groups connected via porphyrin framework, theoretical studies over multi-component compounds composed of porphyrin and ferrocenyl units appear interesting.…”

Interaction of Fe3+meso-tetrakis (2,6-dichloro-3-sulfonatophenyl) porphyrin with cationic bilayers: magnetic switching of the porphyrin and magnetic induction at the interface

“…PP does not exhibit circular dichroism (CD) signal. The presence of a magnetic field results in the magnetic circular dichroism signal as resulting from orbital splitting (Faraday A term contribution) and mixing (Faraday B term contribution) [38,[66][67][68].…”

“…Considering that single reference Hartree-Fock wave function theory (WFT) quantum mechanics and local density approximation (LDA) and even the generalized gradient approximation (GGA) KS-DFT do not have the accuracy required for the calculation of weak van der Waals-dispersion interactions/forces, both WFT and KS-DFT needed to be extended and even reformulated to be applied for the dispersion forces and interactions that are responsible for the stability of the noble gas dimers and stacked aromatics compounds. Within KS-DFT theory, the so-called dispersion correction has been used [34], while in WFT, the methods commonly used are WFT electron correlation, that are known as correlated methods such as complete active space perturbation theory (CASPT2) [35] or multi-reference couple cluster (MR-CC) methods [36][37][38][39][40]. These are two of the most accurate WFT methods which are able to quantitatively treat the types of interactions and forces responsible for the stability of the structures in supramolecular chemistry, physics, and materials science.…”