2,2'-Dipyrromethene
Chemical compound
From Wikipedia, the free encyclopedia
2,2'-Dipyrromethene, often called just dipyrromethene or dipyrrin, is a chemical compound with formula C
9H
8N
2 whose skeleton can be described as two pyrrole rings C
4N connected by a methyne bridge =CH– through their nitrogen-adjacent (position-2) carbons; the remaining bonds being satisfied by hydrogen atoms. It is an unstable compound that is readily attacked by nucleophilic compounds above −40 °C.[1]
Schematic formula of the dipyrromethene molecule. | |
 Schematic formula of dipyrromethene (fractional bonding view). | |
| Names | |
|---|---|
| Preferred IUPAC name
2-[(2H-Pyrrol-2-ylidene)methyl]-1H-pyrrole | |
| Other names
dipyrrin, pyrromethene, dipyrromethene | |
| Identifiers | |
3D model (JSmol) |
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PubChem CID |
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CompTox Dashboard (EPA) |
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| Properties | |
| C9H8N2 | |
| Molar mass | 144.177 g·mol−1 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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2,2'-Dipyrromethene and its more stable and easily prepared derivatives—formally obtained by replacing one or more hydrogen atoms by other functional groups—are important precursors for the family of BODIPY fluorescent dies.[1] The derivatives include salts of the dipyrrinato anion C
9H
7N–
2 and of the cation C
9H
9N+
2.
Preparation
2,2'-Dipyrromethene and its derivatives can be obtained from suitable pyrrole derivatives by several methods.[2]
The unsubstituted compound can be prepared by oxidation of 2,2'-dipyrrolemethane with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) at −78 °C in dry dichloromethane solution.[1] An alternative synthesis that avoids the oxidation step is the condensation of 2-formyl pyrrole and pyrrole catalyzed by trifluoroacetic acid, followed by deprotonation with N,N-diisopropylethylamine.[3]
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, 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.
Alternatively, one may use an activated carboxylic acid derivative, usually an acyl chloride. 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 chloride [C
9H
9N+
2] [Cl−
] is sufficiently stable in solution.[4]
The so-called BODIPY dyes can be obtained by reacting 2,2'-dipyrromethene or its derivatives with boron trifluoride-diethyl ether complex (BF
3·(C
2H
5)
2O) in the presence of triethylamine or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU).[1]
Dipyrrin and its derivatives for coordination complexes with transition metals. For example, the derivative anion 5-phenyl dipirrinato (pdp) forms the neutral iron(III) complex Fe(pdp)
3 (dark green monoclinic crystals, soluble in benzene, orange solution in dichloromethane), where the Fe3+
ion is coordinated to six nitrogen atoms of the dipyrrin cores in distorted octahedral geometry.[5] A similar cobalt(III) complex has also been reported, as well as a complex with copper(II) Cu(pdp)
2[6]