Neocuproine

Neocuproine
IUPAC name 2,9-dimethyl-1,10-phenanthroline
Identifiers
CAS number	484-11-7
Properties
Molecular formula	C14H12N2
Molar mass	208.26 g/mol
Appearance	Pale Yellow solid
Melting point	162-164 °C
Solubility in water	Slightly Soluble
Solubility	Ethanol, Acetone, Ether, Benzene, Light Petroleum (slightly)[1]
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Neocuproine is a heterocyclic organic compound and chelating agent. Phenanthroline ligands were first published in the late 19th century, and the derivatives substituted at the 2 and 9 positions are among the most studied of the modified phenanthrolines.^[2][3]

Synthesis and structure

Neocuproine can be prepared by sequential Skraup reactions (Doebner-Miller reaction/condensation) of o-nitroaniline (2-Nitroaniline) with crotonaldehyde diacetate:

An alternate synthesis involves the condensation of o-phenylenediamine, m-nitrobenzenesulphonate, and crotonaldehyde diacetate. This method gives higher yields but is less economical.^[1]

Neocuproine crystalizes as a dihydrate and a hemihydrate as well as a hydrated hydrochloride salt. The structure of the dihydrate features a π-stacking motif, with additional hydrogen bonding between the basic nitrogen centers and water.^[4]

Coordination chemistry

In the early 1930s, phenanthroline derivatives became known for their use as colorimetric indicators for many transition metals. Neocuproine proved to be highly selective for copper(I). The resulting complex, Cu(neocuproine)₂⁺ has a deep orange-red color.^[1] The properties of copper(I) neocuproine complexes have been widely studied, e.g. for the preparation of catenane and rotaxane complexes.^[5] The copper-catalyzed release of NO⁺ (nitrosonium) from S-Nitrosothiols is inhibited by neocuproine.^[6]

Other metals

Platinum forms the square planar complexes [PtX2(2,9-dimethyl-1,10-phenanthroline)].^[7]

Neocuproine has also been discovered to have properties that cause fragmentation and disappearance of the melanin in adult zebrafish melanocytes. Those expressing eGFP also have been observed to lose eGFP fluorescence in the presence of neocuproine.^[8]

References

1. ^ ^{a b c} O'Reilly, E. J.; Plowman, R. A. Coordination Compounds of Substituted 1,10-Phenanthrolines and Related Dipyridyls. Australian Journal of Chemistry, 13(1) 1959, p.145-149. doi: 10.1071/CH9600145
2. ^ Eggleston, M. K., P. E. Fanwick, A. J. Pallenberg, D. R. McMillin "A Twist on the Copper Center in the Crystal Structure of [Cu(dnpp)₂]PF₆ and the Charge-Transfer Excited State? (dnpp = 2,9-Dineopentyl-1,10-phenanthroline)" Inorganic Chemistry, 1997, volume 36, p. 4007-4010.doi:10.1021/ic970135e
3. ^ Chandler, Christopher J.; Deady, Leslie W.; Reiss, James A. Synthesis of some 2,9-Disubstituted-1,10-phenanthrolines. Journal of Heterocyclic Chemistry 18, 1981, p.599-601. doi:10.1002/jhet.5570180332
4. ^ Koroglu, Ahmet; Bulut, Ahmet; Ucar, Ibrahim; Nichol, Gary; Harrington, Ross W.; Clegg, William "A second monoclinic polymorph of 2,9-dimethyl-1,10-phenanthroline dihydrate" Organic Papers, E61, 2005, p.3723-3725. doi:10.1107/S1600536805033179
5. ^ McCleverty, J; Meyer, T. J. "Phenanthroline Ligands" in Comprehensive Coordination Chemistry II, Vol. 1, 2004, p.25-39.
6. ^ Al-Sa’doni, H.H.; Megson, I.L.; Bisland, S.; Butler, A.R.; Flitney, F.W. Neocuproine, A Selective Cu(I) Chelator, and the relaxation of rat vascular smooth muscle by S-nitrosothiols. British Journal of Pharmacology, 121(6), 1997, p.1047-1050. doi: 10.1038/sj.bjp.0701218
7. ^ Fanizzi, Francesco P.; Margiotta, Nicola; Lanfranchi, Maurizio; Tiripicchio, Antonio; Pacchioni, Gianfranco; Natile, Giovanni "A Molecular Tool for Measuring the Electron-Acceptor Ability of Ligands from Crystallographic Data" European Journal of Inorganic Chemistry volume 8, 2004, p.1705-1713. doi:10.1002/ejic.200300888
8. ^ O’Reilly-Pol, Thomas; Johnson, Stephen L. "Neocuproine Ablates Melanocytes in Adult Zebrafish" Zebrafish 5(4). Mary Ann Liebert, Inc. 2008. doi: 10.1002/ejic.200300888

Appendix: NMR Shifts

The following figures contain information on the nuclear magnetic resonance spectroscopic data of neocuproine (from Chandler et al.):

¹H NMR

Substituent	Chemical Shift (δ ppm)
H-3,8	7.45
H-4,7	8.03
H-5,6	7.65

¹H Decoupled ¹³C NMR

Substituent	Chemical Shift (δ ppm)
C-2	159.2
C-10b	145.1
C-4	136.2
C-4a	126.7