2,2'-Dipyrromethene, often called just dipyrromethene or dipyrrin, is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

with formula whose skeleton can be described as two pyrrole rings connected by a methyne bridge =CH– through their

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

-adjacent (position-2) carbons; the remaining bonds being satisfied by

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

atoms. It is an unstable compound that is readily attacked by nucleophylic compounds above −40 °C. 2,2'-Dipyrromethene and its more stable and easily prepared derivatives—formally obtained by replacing one or more

atoms by other functional groups—are important precursors for the family of

BODIPY BODIPY is the technical common name of a chemical compound with formula , whose molecule consists of a boron difluoride group joined to a dipyrromethene group ; specifically, the compound 4,4-difluoro-4-bora-3a,4a-diaza-''s''-indacene in the ...

fluorescent dies. The derivatives include salts of the dipyrrinato

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

and of the cation .

Preparation

2,2'-Dipyrromethene and its derivatives can be obtained from suitable pyrrole derivatives by several methods. The unsubstituted compound can be prepared by

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

of 2,2'-dipyrrolemethane with

2,3-dichloro-5,6-dicyano-1,4-benzoquinone 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (or DDQ) is the chemical reagent with formula C6Cl2(CN)2O2. This oxidant is useful for the dehydrogenation of alcohols, phenols, and steroid ketones. DDQ decomposes in water, but is stable in aqueous mine ...

(DDQ) at −78 °C in dry dichloromethane solution. An alternative synthesis that avoids the oxidation step is the condensation of 2-formyl pyrrole and pyrrole catalyzed by

trifluoroacetic acid Trifluoroacetic acid (TFA) is an organofluorine compound with the chemical formula CF3CO2H. It is a structural analogue of acetic acid with all three of the acetyl group's hydrogen atoms replaced by fluorine atoms and is a colorless liquid with ...

, followed by deprotonation with

N,N-diisopropylethylamine ''N'',''N''-Diisopropylethylamine, or Hünig's base, is an organic compound and an amine. It is named after the German chemist Siegfried Hünig. It is used in organic chemistry as a base. It is commonly abbreviated as DIPEA, DIEA, or ''i''-Pr2N ...

. More generally, one starts with a pyrrole with suitable substituents at positions 3, 4, or 5 (but not 2). Condensation of two such molecules at their 2 positions with a bridging compound gives the corresponding 2,2'-dipyrromethane. The condensation may use, for example, the

Knorr pyrrole synthesis The Knorr pyrrole synthesis is a widely used chemical reaction that synthesizes substituted pyrroles (3). The method involves the reaction of an α-amino-ketone (1) and a compound containing an electron-withdrawing group (e.g. an ester as shown) � ...

, with an aromatic aldehyde in the presence of TFA. The dipyrromethane core is then oxidized to dipyrromethene using a quinone oxidant such as DDQ or

p-chloranil Chloranil is a quinone with the molecular formula C6Cl4O2. Also known as tetrachloro-1,4-benzoquinone, it is a yellow solid. Like the parent benzoquinone, chloranil is a planar molecule J.-M. Lü, S. V. Rosokha, I. S. Neretin and J. K. Kochi, "Qui ...

Alternatively, one may use an activated carboxylic acid derivative, usually an

acyl In chemistry, an acyl group is a moiety derived by the removal of one or more hydroxyl groups from an oxoacid, including inorganic acids. It contains a double-bonded oxygen atom and an alkyl group (). In organic chemistry, the acyl group (IUPAC ...

chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride sa ...

. As another possibility, one may condense a substituted pyrroles with a 2-acylpyrrole; this route allows the synthesis of unsymmetrical dipyrromethenes.

Reactions

Dipyrrin is unstable above −40 °C. However, its acts as a base, and its

[] [] is sufficiently stable in solution. The so-called BODIPY, BODIPY dyes can be obtained by reacting 2,2'-dipyrromethene or its derivatives with boron trifluoride-diethyl ether complex (·) in the presence of

triethylamine Triethylamine is the chemical compound with the formula N(CH2CH3)3, commonly abbreviated Et3N. It is also abbreviated TEA, yet this abbreviation must be used carefully to avoid confusion with triethanolamine or tetraethylammonium, for which TEA ...

or 1,8-diazabicyclo .4.0ndec-7-ene (DBU). Dipyrrin and its derivatives for coordination complexes with transition metals. For example, the derivative anion 5-phenyl dipirrinato (pdp) forms the neutral

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

(III) complex (dark green monoclinic crystals, soluble in benzene, orange solution in dichloromethane), where the ion is coordinated to six nitrogen atoms of the dipyrrin cores in distorted octahedral geometry. A similar

cobalt Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, p ...

(III) complex has also been reported, as well as a complex with

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

(II)

References

Seth M Cohen and Sara R Halper (2002): "Dipyrromethene complexes of iron". ''Inorganica Chimica Acta'', volume 341, pages 12-16. Sara R. Halper, Mitchell R. Malachowski, Heather M. Delaney, and Seth M. Cohen (2004): "Heteroleptic copperdipyrromethene complexes: synthesis, structure,and coordination polymers". ''Inorganic Chemistry'', volume 43, pages 1242−1249 =K. Tram, H. Yan, H. A. Jenkins, S. Vassiliev, and D. Bruce (2009): "The synthesis and crystal structure of unsubstituted 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)". ''Dyes and Pigments'', volume 82, issue 3, pages = 392–395. A. Schmitt, B. Hinkeldey, M. Wild , and G. Jung (): "Synthesis of the core compound of the BODIPY dye class: 4,4′-difluoro-4-bora-(3a,4a)-diaza-s-indacene". ''Journal of Fluorescence'', volume = 19, issue = 4, pages = 755–759 Brandon R. Groves, Sarah M. Crawford,a Travis Lundrigan, Chérif F. Matta, Shahin Sowlati-Hashjin, and Alison Thompson (2013): "Synthesis and characterisation of the unsubstituted dipyrrin and 4,4-dichloro-4-bora-3a,4a-diaza-s-indacene: improved synthesis and functionalisation of the simplest BODIPY framework." ''Chemical Communications'', volume 49, pages 816-818. pyrroles {{DEFAULTSORT:Dipyrromethene, 2,2'-