PORPHYRINS AND METALLOPORPHYRINS - . Electronic Properties of Porphyrins and Metalloporphyrins ... hemoglobin,

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  • Chapter 1

    PORPHYRINS AND METALLOPORPHYRINS

    1.1. Introduction

    Porphyrins are one of the vital chemical units essential for several life

    processes on the earth. Many biological molecules function with prosthetic

    groups essentially made of these units. Chlorophylls of chloroplasts which

    drive photosynthesis, heme as a component of hemoglobin that transports

    oxygen to animal tissues and as the central unit of myoglobin ensures the

    storage of oxygen - all these have active sites essentially made of porphyrin core'". Over the years, a great deal of concerted efforts have brought to light

    substantial understanding of the structure-function relationship in these natural

    porphyrins"'O

    A large variety of synthetic porphyrins and their metalloderivatives were

    made over the years to study the porphyrin based natural systems. The search

    for anti-cancer drugs, useful catalysts, semiconductors and superconductors,

    electronic materials with novel properties has also made this synthetic

    porphyrin chemistry a very actively probed one by chemists, biologists and

    physicists alike. The synthetic meso-substituted porphyrins offer a great

    advantage to study the physical and chemical properties of the porphyrin

    nucleus quantitatively by a judicious choice of the substituents that may be

    attached on the periphery. Metalloporphyrins are widely and intensely

    investigated in the area of catalysis and also as models and mimics of enzymes

    l i e catalase, peroxidases, P450 cytochromes or as transmembrane electron 11-13 transport agents . They have also been used as NMR image enhancement

    agents14, Nonlinear optical materials" and DNA-binding or cleavage agent1" . 17 . Currently there is interest in using chelated radioactiv diagnostic imaging and therapeutic agents. In that con

    excellent compounds because of their extremely high s

    many metal ions.

  • 1.2. The Porphyrin System

    Porphyrins are basically cyclic tetrapyrrole derivatives with a highly

    delocalised planar ?r-framework having a core structure 1. They can exist in

    varied forms by having different peripheral substituents at all the eight pyrrole

    P-carbon atom and the four meso-carbon centres and also by undergoing

    certain structural variations.

    Porphyrin is an 18-n: electron system and hence exhibits aromaticity18.

    The simplest porphyrin is known as porphine which is the H-analogue (R1 -

    R12 = H) of 1. Besides many synthetic and naturally occurring ones with the

    core structure mentioned above, there are many biologically active systems

    which also have porphyrin-like structures which are given in 2-7.

    Porphin Chlorin Phorbin

    2 3 4

  • Bacteriochlorin Porphyrazine Phthalocyanine

    5 6 7

    The prophyrin ring provides a vacant site at its center, ideally suited for

    metal incorporation. The NH protons inside the ring of porphyrins possess

    acidic character and hence can get deprotonated to give porphyrinato ions.

    These dianion species with their electronically sensitive planar n-framework

    and central cavity with more or less rigid size exhibit remarkable ligation

    characteristic towards metal ions. Thus derivatives of porphyrins with almost

    all metals and semimetals have been synthesisedl. A crucial factor to form

    stable metalloporphyrins seems to be the compatibility of porphyrin ring size

    with the ionic radii of the metal cations". Hence stable complexes generally

    result when these two sizes match while their instability tends to increase when

    the size of the cation is too big or too small with very few exceptions. The

    porphyrinato dianion is ideally suited to act as a tetradenate ligand with metal

    ions2'. Thus the minimum coordination number of the metal ion possible in a

    metalloporphyrin is four. A size matching divalent metal ion would give

    neutral complex, while a higher valent cation would carry with it balancing

    anion@) mostly covalently bound with the metal, in addition to the

    porphyrinato ion. f i e normal coordination geometry around the metal ion in

    the former species would be square planar, while in the latter case the

    coordination geometry would be square pyramidal. Coordination number

    greater than four is also possible. The two ligands of the six coordinate

    metalloporphyrins are found on the opposite sides of the porphyrinato plane

    yielding complexes with tetragonal or octahedral geometries20. Ability to

    exhibit variable oxidation states of metals in their metalloporphyrins is another

  • important feature in this class of compounds. They are also capable of

    stabilizing metal ions in their unusual oxidation states, which have resulted in

    extensive studies revealing interesting chemistry.

    The nature of bonding between a central metal and the porphyrin ligand

    is found to be originating essentially from the following two types of primary

    interactions: a-coordination of nitrogen lone pairs directed towards the central

    metal atoms and n-interaction of metal pn or dn orbitals with nitrogen-based n

    orbitals2'. The appropriate symmetry-adapted linear combinations of

    prophyrin-ligand orbitals involved in the bonding with metal orbitals are shown

    in 8.

    (b)

    8

    The symmetry adapted linear combi~tion ofpotphyin-Iigand orbitals involved in the

    bonding with metal obitals (a) me suitable for einteraclonsand (6) for z-interactions

    In the a-system the prophyrin is clearly a donor to the metal while in the

    x-system porphyrin has the appropriate orbitals to act both as a n-donor and as

    a n-acceptor.

  • The versatile characteristics of the ubiquitous porphyrin molecules can

    be attributed largely to the extensively delocalised 7r-system which is

    electronically very sensitive and tunable. A knowledge on all such crucial

    factors is often necessary before one tries to design and develop the ideal

    molecular system for any specific purpose. The studies on porphyrins so far

    indicate the following factors to be very significant.

    (i) The nature of peripheral substituents has great ability to tune the

    electronic levels of porphyrin and their metalloderivatives.

    (ii) The type of central metal ion has a very pronounced effect on the

    electronic property of the porphyrins. The nature of interaction between

    the metal ion and the porphyrinato moiety is such that both the species

    mutually influence their electronic levels.

    (iii) There is generally a thermodynamic drive for square planar complexes

    to add on axial ligands, if available. Most of the metal(II) porphyrin

    complexes thus tend to take octahedral geometry when exposed to

    coordinating species. Such a change in geometry affects both the metal

    electronic levels and porphyrinato orbital energies. Five coordinated

    species for metal(1I) porphyrins with one neutral axial ligand are

    generally unstable but have been suggested to occur as an intermediate

    in solution state. Electronically, five coordinate species are different

    from six coordinate complexes and are more reactive due to

    coordinative unsaturation.

    (iv) In most of the biological systems, the porphyrin moiety is often covered

    and buried at specific sites by the long chain of the protein residues.

    This steric crowding of protein chain around it can cause some tilt or

    puckering in the planar n-framework of the metalloporphyrin. Such a

    distortion then would cause a noticeable decrement in the extent of

    overlap of certain n-molecular orbitals of the macrocycle with symmetry

  • matching metal orbitals. The net effect of this would be an enforced

    strain on the molecule. The tendency of these entatic species would be to

    release the strain that is supposed to be the driving force for these

    species to behave as biological catalysts.

    1.3. Electronic Properties of Porphyrins and Metalloporphyrins

    The most useful spectroscopic technique for the study of porphyrin and

    their metalloderivatives is the electronic absorption spectroscopy. As the

    spectral absorptions are found to be sensitive to the nature of porphyrins and its

    surroundings, vital information could be obtained on the nature of chemical

    environment in which they exist and on the role these molecules play in key

    biological functions they take part, all by just monitoring the electronic spectra

    in respective conditions. Since the present study deals with some aspects of

    aggregation characteristics of porphyrins and environment effects on their

    electronic spectra provide a vital tool to study them. A brief description on the

    origin of the spectra of the porphyrins and their metalloderivatives are given

    below.

    The electronic heart of a porphyrin is the inner 16-membered ring with

    18-x electrons. The ring is structured with basic fourfold symmetry, including

    four nitrogen atoms directed towards the center. This electronic heart is

    responsible for the unique porphyrin-type optical spectra, which are then

    perturbed to a greater or lesser extent by various chemical modifications to the

    basic structure.

    Porphyrin and its metal derivatives are of considerable spectroscopic

    interest because of their simplicity and uniqueness. The optical absorptions of

    porphyrin are determined essentially by the nelectrons on the porphyrin rin