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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R-phrases (outdated) R25, R50/53
S-phrases (outdated) S45,S60,S61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]


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.[4] Dehydration of glycerol gives acrolein which condenses with the amine followed by a cyclization.

Coordination chemistry

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.[5]

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).[6] 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.[7] The pink complex [Ni(phen)3]2+ has been resolved into its Δ and Λ isomers.[8] Copper(I) forms [Cu(phen)2]+, which is luminescent.[9][10]

Bioinorganic chemistry

The ferroin analogue [Ru(phen)3]2+ has long been known to be bioactive.[11]

1,10-Phenanthroline is an inhibitor of metallopeptidases, with one of the first observed instances reported in carboxypeptidase A.[12] 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.[13]

Related phen ligands

Basicities of 1,10-Phenanthrolines and 2,2'-Bipyridine[14]
ligand pKa comment/ alt. name
1,10-phenanthroline 4.86 phen
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[15]

A variety of substituted derivatives of phen have been examined as ligands.[10] 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.[16]

Numbering for 1,10-phenanthroline derivatives.

As an indicator for alkyllithium reagents

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.[17] Grignard reagents may be similarly titrated.[18]


  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., "Long-Lived Palladium Catalysts for Co/Vinyl Arene Polyketones Synthesis: A Solution to Deactivation Problems", Chemistry – A European Journal 2006, volume 12, 7639-7651. doi:10.1002/chem.200501047
  3. ^ C.R. Luman, F.N. Castellano "Phenanthroline Ligands" in Comprehensive Coordination Chemistry II, 2003, Elsevier. ISBN 978-0-08-043748-4.
  4. ^ 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.
  5. ^ 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.
  6. ^ Belcher, R. "Application of chelate Compounds in Analytical Chemistry" Pure and Applied Chemistry, 1973, volume 34, pages 13-27.
  7. ^ 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: 3968–3970. doi:10.1021/ic902554b. PMID 20415494.
  8. ^ George B. Kauffman, Lloyd T. Takahashi (1966). "Resolution of the tris-(1,10-Phenanthroline)Nickel(II) Ion". Inorg. Synth. 5: 227–232. doi:10.1002/9780470132395.ch60.
  9. ^ Armaroli, N., "Photoactive Mono- and Polynuclear Cu(I)-Phenanthrolines. A Viable Alternative to Ru(Ii)-Polypyridines?", Chemical Society Reviews 2001, volume 30, 113-124.doi:10.1039/b000703j
  10. ^ a b Pallenberg, A. J.; Koenig, K. S.; Barnhart, D. M., "Synthesis and Characterization of Some Copper(I) Phenanthroline Complexes", Inorg. Chemistry 1995, volume 34, 2833-2840. doi:10.1021/ic00115a009
  11. ^ F. P. Dwyer; E. C. Gyarfas; W. P. Rogers; J. H. Koch (1952). "Biological Activity of Complex Ions". Nature. 170 (4318): 190–191. doi:10.1038/170190a0. PMID 12982853.
  12. ^ 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)
  13. ^ Salvesen, GS & Nagase, H (2001). "Inhibition of proteolytic enzymes". Proteolytic enzymes: a practical approach, 2 edn. 1: 105–130.
  14. ^ 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: 259–263. doi:10.1016/0022-328X(91)83069-G.CS1 maint: uses authors parameter (link)
  15. ^ Ryan A. Altman (2008). eEROS. doi:10.1002/047084289X.rn00918. Missing or empty |title= (help)
  16. ^ 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.
  17. ^ Paul J. Fagan and William A. Nugent (1998). "1-Phenyl-2,3,4,5-Tetramethylphosphole". Organic Syntheses.; Collective Volume, 9, p. 653
  18. ^ 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.