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Sample of 1,10-Phenanthroline
Preferred IUPAC name
3D model (JSmol)
ECHA InfoCard 100.000.572
RTECS number
  • SF8300000
Molar mass 180.21 g/mol
Appearance colourless crystals
Density 1.31 g/cm3
Melting point 117 °C (243 °F; 390 K)
Solubility in other solvents acetone


Acidity (pKa) 4.86 (phenH+)[2]
Main hazards mild neurotoxin, strong nephrotoxin, and powerful diuretic
R-phrases (outdated) R25, R50/53
S-phrases (outdated) S45,S60,S61
Related compounds
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Phenanthroline (phen) is a heterocyclic organic compound. It is a white solid that is soluble in organic solvents. It is used as a ligand in coordination chemistry, forming strong complexes with most metal ions.[3][4] It is often sold as the monohydrate.


Phenanthroline may be prepared by two successive Skraup reactions of glycerol with o-phenylenediamine, catalyzed by sulfuric acid, and an oxidizing agent, traditionally aqueous arsenic acid or nitrobenzene.[5] Dehydration of glycerol gives acrolein which condenses with the amine followed by a cyclization.

Coordination chemistry[edit]

In terms of its coordination properties, phenanthroline is similar to 2,2'-bipyridine (bipy) but binds metals more tightly since the chelating nitrogen donors are preorganized. Phenanthroline is however a weaker donor than bipy.[6]

Many homoleptic complexes are known. Particularly well studied is [Fe(phen)3]2+, called "ferroin." It was used for the photometric determination of Fe(II).[7] It is used as a redox indicator with standard potential +1.06 V. The reduced ferrous form has a deep red colour and the oxidised form is light-blue.[8] The pink complex [Ni(phen)3]2+ has been resolved into its Δ and Λ isomers.[9] Copper(I) forms [Cu(phen)2]+, which is luminescent.[10][11]

Bioinorganic chemistry[edit]

The ferroin analogue [Ru(phen)3]2+ has long been known to be bioactive.[12]

1,10-Phenanthroline is an inhibitor of metallopeptidases, with one of the first observed instances reported in carboxypeptidase A.[13] Inhibition of the enzyme occurs by removal and chelation of the metal ion required for catalytic activity, leaving an inactive apoenzyme. 1,10-Phenanthroline targets mainly zinc metallopeptidases, with a much lower affinity for calcium.[14]

Related phen ligands[edit]

A variety of substituted derivatives of phen have been examined as ligands.[11] Substituents at the 2,9 positions confer protection for the attached metal, inhibiting the binding of multiple equivalents of the phenanthroline. Phen itself form complexes of the type [M(phen)3]Cl2 when treated with metal dihalides (M = Fe, Co, Ni). By contrast, neocuproine and bathocuproine form 1:1 complexes such as [Ni(neo/batho-cuproine)Cl2]2.[15]

Basicities of 1,10-Phenanthrolines and 2,2'-Bipyridine[16]
ligand pKa comment/alt. name illustration
1,10-phenanthroline 4.86 phen
Numbering for 1,10-phenanthroline derivatives.
2,2'-bipyridine 4.30 less basic than phen
5-nitro-1,10-phenanthroline 3.57
2,9-dimethyl-1,10-phenanthroline unknown neocuproine
4,7-dimethyl-1,10-phenanthroline 5.97
4,7-diphenyl-1,10-phenanthroline unknown bathophenanthroline
5,6-dimethyl-1,10-phenanthroline 5.20
3,4,7,8-tetramethylphenanthroline 6.31 3,4,7,8-Me4phen
4,7-dimethoxy-1,10‐phenanthroline 6.45 4,7-(MeO)2phen[17]

As an indicator for alkyllithium reagents[edit]

Alkyllithium reagents form deeply colored derivatives with phenanthroline. The alkyllithium content of solutions can be determined by treatment of such reagents with small amounts of phenanthroline (ca. 1 mg) followed by titration with alcohols to a colourless endpoint.[18] Grignard reagents may be similarly titrated.[19]


  1. ^ Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 211. doi:10.1039/9781849733069-FP001. ISBN 978-0-85404-182-4.
  2. ^ Durand J; et al. (2006). "Long-Lived Palladium Catalysts for Co/Vinyl Arene Polyketones Synthesis: A Solution to Deactivation Problems". Chemistry – A European Journal. 12 (29): 7639–7651. doi:10.1002/chem.200501047. PMID 16807965.
  3. ^ C.R. Luman, F.N. Castellano "Phenanthroline Ligands" in Comprehensive Coordination Chemistry II, 2003, Elsevier. ISBN 978-0-08-043748-4.
  4. ^ Sammes, Peter G.; Yahioglu, Gokhan (1994). "1,10-Phenanthroline: A versatile ligand". Chemical Society Reviews. 23 (5): 327. doi:10.1039/cs9942300327.
  5. ^ B. E. Halcrow; W. O. Kermack (1946). "43. Attempts to find new antimalarials. Part XXIV. Derivatives of o-phenanthroline (7 : 8 : 3′ : 2′-pyridoquinoline)". J. Chem. Soc.: 155–157. doi:10.1039/jr9460000155. PMID 20983293.
  6. ^ Teng, Qiaoqiao; Huynh, Han Vinh (2017). "A unified ligand electronic parameter based on C NMR spectroscopy of N-heterocyclic carbene complexes". Dalton Transactions. 46 (3): 614–627. doi:10.1039/C6DT04222H. PMID 27924321.
  7. ^ Belcher R (1973). "Application of chelate Compounds in Analytical Chemistry". Pure and Applied Chemistry. 34: 13–27. doi:10.1351/pac197334010013.
  8. ^ Bellér, G. B.; Lente, G. B.; Fábián, I. N. (2010). "Central Role of Phenanthroline Mono-N-oxide in the Decomposition Reactions of Tris(1,10-phenanthroline)iron(II) and -iron(III) Complexes". Inorganic Chemistry. 49 (9): 3968–3970. doi:10.1021/ic902554b. PMID 20415494.
  9. ^ George B. Kauffman, Lloyd T. Takahashi (1966). Resolution of the tris-(1,10-Phenanthroline)Nickel(II) Ion. Inorg. Synth. Inorganic Syntheses. 5. pp. 227–232. doi:10.1002/9780470132395.ch60. ISBN 9780470132395.
  10. ^ Armaroli N (2001). "Photoactive Mono- and Polynuclear Cu(I)-Phenanthrolines. A Viable Alternative to Ru(Ii)-Polypyridines?". Chemical Society Reviews. 30 (2): 113–124. doi:10.1039/b000703j.
  11. ^ a b Pallenberg A. J., Koenig K. S., Barnhart D. M. (1995). "Synthesis and Characterization of Some Copper(I) Phenanthroline Complexes". Inorg. Chemistry. 34 (11): 2833–2840. doi:10.1021/ic00115a009.CS1 maint: multiple names: authors list (link)
  12. ^ F. P. Dwyer; E. C. Gyarfas; W. P. Rogers; J. H. Koch (1952). "Biological Activity of Complex Ions". Nature. 170 (4318): 190–191. Bibcode:1952Natur.170..190D. doi:10.1038/170190a0. PMID 12982853.
  13. ^ Felber, JP, Coombs, TL & Vallee, BL (1962). "The mechanism of inhibition of carboxypeptidase A by 1,10-phenanthroline". Biochemistry. 1 (2): 231–238. doi:10.1021/bi00908a006. PMID 13892106.CS1 maint: multiple names: authors list (link)
  14. ^ Salvesen, GS & Nagase, H (2001). "Inhibition of proteolytic enzymes". Proteolytic Enzymes: A Practical Approach, 2 Edn. 1: 105–130.
  15. ^ Preston, H. S.; Kennard, C. H. L. (1969). "Crystal Structure of di-mu-Chloro-sym-trans-Dichloro-Bis-(2,9-Dimethyl-1,10-Phenanthroline)dinickel(II)-2-Chloroform". J. Chem. Soc. A: 2682–2685. doi:10.1039/J19690002682.
  16. ^ J. G. Leipoldt, G. J. Lamprecht, E. C.Steynberg (1991). "Kinetics of the substitution of acetylacetone in acetylactonato-1,5-cyclooctadienerhodium(I) by derivatives of 1,10-phenantrholine and 2,2′-dipyridyl". Journal of Organometallic Chemistry. 402 (2): 259–263. doi:10.1016/0022-328X(91)83069-G.CS1 maint: uses authors parameter (link)
  17. ^ Ryan A. Altman (2008). "1,10-Phenanthroline, 4,7-Dimethoxy". Encyclopedia of Reagents for Organic Synthesis. eEROS. doi:10.1002/047084289X.rn00918. ISBN 978-0471936237.
  18. ^ Paul J. Fagan and William A. Nugent (1998). "1-Phenyl-2,3,4,5-Tetramethylphosphole". Organic Syntheses.; Collective Volume, 9, p. 653
  19. ^ Ho-Shen Lin; Leo A. Paquette (1994). "A Convenient Method for Determining the Concentration of Grignard Reagents". Synth. Commun. 24 (17): 2503–2506. doi:10.1080/00397919408010560.