Structural and Electronic Effects in the Metalation of Porphyrinoids. Theory and Experiment

Abstract

The structure-reactivity relationships in metalation reactions of porphyrinoids have been studied using experimental and theoretical methods. A series of eight porphyrinoic ligands, derivatives of chlorophylls, was prepared in which both the peripheral groups and the degrees of saturation of the macrocycle were systematically varied. To reveal the solvent and structural factors which control the interactions of these macroligands with metal centers, their interactions with reactive Zn(2+) and inert Pt(2+) ions were investigated using absorption spectroscopy. In parallel, quantum chemical calculations (density functional theory, DFT) were performed for the same set of molecules to examine the influence of structural and electronic factors on the energy of the frontier orbitals, the nucleophilicity/electronegativity of the macrocycle, its hardness, and conformation. These static descriptors of chemical reactivity, relevant to metalation reactions, were verified against the results obtained in the experimental model. The experimentally obtained kinetic data clearly show that the solvent has a crucial role in the activation of the incoming metal center. In terms of chelator structure, the largest effects concern the size of the delocalized pi-electron system and the presence of side groups. Both the DFT calculations and experimental results show the strong influence of the macrocycle rigidity and of the peripheral groups on the chelating ability of porphyrinoids. In particular, the peripheral functionalization of the macrocyclic system seems to drastically reduce its reactivity toward metal ions. The effect of peripheral groups is two-fold: (i) a lower electron density on the core nitrogens, and (ii) increased rigidity of the macrocycle. The outcomes of the theoretical and experimental analyses are discussed also in terms of their relevance to the mechanism of biological metal insertion in the biosynthesis of heme and chlorophyll.

Engineering of relevant photodynamic processes through structural modifications of metallotetrapyrrolic photosensitizers

“…They are generally less stable in solution than analogous porphyrins due to their lower oxidation potential [53]. Moreover, the decreased ligand nucleophilicity that accompanies saturation of the pyrrole rings poses serious challenges in the metallation of bacteriochlorins [89]. Their photostability can be increased by introduction of electron-withdrawing groups.…”

Fine tuning of copper(ii)–chlorophyll interactions in organic media. Metalation versus oxidation of the macrocycle

“…The spectra of most metallochlorophylls have been thoroughly assigned. 11,25,36 Since the UV-Vis spectra of Chls reflect mainly the electronic state of the macrocycle, they are also useful in investigations of other types of interactions with the metal ion, involving electron transfer processes. This was confirmed in studies on major Chls' catabolites that provided inter alia detailed spectroscopic characteristics of the products of both one-electron oxidation processes, [37][38][39][40] as well as decomposition of the macrocyclic system.…”

Factors controlling the reactivity of divalent metal ions towards pheophytin a

“…Pheo a was prepared from stereochemically pure chlorophyll a (Chl a ), extracted from the cyanobacterium Arthrospira maxima and purified following the methods described earlier [29]. Chl a was demetalated by a short treatment with glacial acetic acid, which was then evaporated under a stream of N 2 .…”

“…All-electron Gaussian type orbitals of def2-TZVP quality were used to define the atomic orbitals [50]. The electronic structures were additionally studied using Lowdin population analysis [51], as in our previous study [29]. The standard convergence criteria for geometry optimization were applied, i.e.…”

“…Such a model of “assisted” metal ion insertion shows the essence of the metalation of rigid macrocyclic ligands, such as hemes, Chls and BChls. Recently, we have shown that the reactivity of porphyrinoids in metalation reactions is determined by both the extended aromatic π-electronic system and the peripheral groups, whose interplay creates the energetic barrier to be overcome [29]. Moreover, the type of bonding between the central metal ion and porphyrinoids depends on the structural details of the chelating molecules [6, 8].…”