Tetraphenylporphyrin
Encyclopedia
Tetraphenylporphyrin, abbreviated TPP or H2TPP, is a synthetic heterocyclic compound
that resembles naturally occurring porphyrin
s. Porphyrins are dyes and cofactors found in hemoglobin
and cytochrome
s and are related to chlorophyll
and vitamin B12
. The study of naturally occurring porphyrins is complicated by their low symmetry and the presence of polar substituents. Tetraphenylporphyrin is hydrophobic, symmetrically substituted, and easily synthesized. The compound is a dark purple solid that dissolves in nonpolar organic solvents such as chloroform
and benzene
.
and pyrrole
in refluxing propanoic acid, the so-called Rothemund method:
Despite low yields, the synthesis of H2TPP is a common experiment in university teaching labs.
The porphyrin belongs to the symmetry group
C2h, the low symmetry being due to the orientation of the two N-H bonds, which project out of the plane of the four pyrrole rings. Unlike natural porphyrins, H2TPP is substituted at the oxidatively sensitive "meso" carbon positions, and hence the compound is sometimes called meso-tetraphenylporphyrin. Another synthetic porphyrin, octaethylporphyrin (H2OEP) does have a substitution pattern that is biomimetic. Many derivatives of tetraphenylporphyrin are known, including those prepared from substituted benzaldehydes. One of the first functional analogues of myoglobin
was the ferrous derivative of the "Picket-Fence Porphyrin," which is structurally related to H2TPP, being derived via the condensation 2-nitrobenzaldehyde and pyrrole. H2TPP can be sulfonated to give water-soluble derivatives, e.g. tetraphenylporphine sulfonate
:
→ (HO3SC6H4C)4(C4H2N)2(C4H2NH)2 + 4 H2O
or EPR spectra. For example, Cu(TPP) has D4h symmetry. The corresponding iron complexes are more complex owing to variable oxidation states and coordination numbers. Well-studied derivatives include the ferric compounds, e.g. Fe(TPP)Cl and the oxide
[Fe(TPP)]2O, and ferrous compounds, e.g. Fe(TPP)CO(L) (L = imidazole, pyridine).
Heterocyclic compound
A heterocyclic compound is a cyclic compound which has atoms of at least two different elements as members of its ring. The counterparts of heterocyclic compounds are homocyclic compounds, the rings of which are made of a single element....
that resembles naturally occurring porphyrin
Porphyrin
Porphyrins are a group of organic compounds, many naturally occurring. One of the best-known porphyrins is heme, the pigment in red blood cells; heme is a cofactor of the protein hemoglobin. Porphyrins are heterocyclic macrocycles composed of four modified pyrrole subunits interconnected at...
s. Porphyrins are dyes and cofactors found in hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
and cytochrome
Cytochrome
Cytochromes are, in general, membrane-bound hemoproteins that contain heme groups and carry out electron transport.They are found either as monomeric proteins or as subunits of bigger enzymatic complexes that catalyze redox reactions....
s and are related to chlorophyll
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
and vitamin B12
Vitamin B12
Vitamin B12, vitamin B12 or vitamin B-12, also called cobalamin, is a water-soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood. It is one of the eight B vitamins...
. The study of naturally occurring porphyrins is complicated by their low symmetry and the presence of polar substituents. Tetraphenylporphyrin is hydrophobic, symmetrically substituted, and easily synthesized. The compound is a dark purple solid that dissolves in nonpolar organic solvents such as chloroform
Chloroform
Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...
and benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
.
Synthesis and structure
Tetraphenylporphyrin is prepared by the reaction of benzaldehydeBenzaldehyde
Benzaldehyde is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful. This colorless liquid has a characteristic pleasant almond-like odor...
and pyrrole
Pyrrole
Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3...
in refluxing propanoic acid, the so-called Rothemund method:
- 8 C4H4NH + 8 C6H5CHO + 3 O2 → 2 (C6H5C)4(C4H2N)2(C4H2NH)2 + 14 H2O
Despite low yields, the synthesis of H2TPP is a common experiment in university teaching labs.
The porphyrin belongs to the symmetry group
Symmetry group
The symmetry group of an object is the group of all isometries under which it is invariant with composition as the operation...
C2h, the low symmetry being due to the orientation of the two N-H bonds, which project out of the plane of the four pyrrole rings. Unlike natural porphyrins, H2TPP is substituted at the oxidatively sensitive "meso" carbon positions, and hence the compound is sometimes called meso-tetraphenylporphyrin. Another synthetic porphyrin, octaethylporphyrin (H2OEP) does have a substitution pattern that is biomimetic. Many derivatives of tetraphenylporphyrin are known, including those prepared from substituted benzaldehydes. One of the first functional analogues of myoglobin
Myoglobin
Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. The only time myoglobin is found in the...
was the ferrous derivative of the "Picket-Fence Porphyrin," which is structurally related to H2TPP, being derived via the condensation 2-nitrobenzaldehyde and pyrrole. H2TPP can be sulfonated to give water-soluble derivatives, e.g. tetraphenylporphine sulfonate
Tetraphenylporphine sulfonate
Tetraphenylporphine sulfonate is a trypanocidal agent....
:
- 4 SO3 + (C6H5C)4(C4H2N)2(C4H2NH)2
→ (HO3SC6H4C)4(C4H2N)2(C4H2NH)2 + 4 H2O
Complexes
Complexation can be thought of as proceeding via the conversion of H2TPP to TPP2-, with 4-fold symmetry. The metal insertion process proceeds via several step, not via the dianion. The resulting complexes are symmetrical with simple NMRNMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
or EPR spectra. For example, Cu(TPP) has D4h symmetry. The corresponding iron complexes are more complex owing to variable oxidation states and coordination numbers. Well-studied derivatives include the ferric compounds, e.g. Fe(TPP)Cl and the oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
[Fe(TPP)]2O, and ferrous compounds, e.g. Fe(TPP)CO(L) (L = imidazole, pyridine).