Salicylaldoxime

Salicylaldoxime is an organic compound described by the formula C6H4CH=NOH-2-OH. It is the oxime of salicylaldehyde. This crystalline, colorless solid is a chelator and sometimes used in the analysis of samples containing transition metal ions, with which it often forms brightly coloured coordination complexes.[1]

Salicylaldoxime
Names
IUPAC name
Salicylaldehyde oxime
Other names
Saldox; 2-[(E)-(hydroxyimino)methyl]phenol
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.002.140
Properties
C7H7NO2
Molar mass 137.138 g·mol−1
Appearance white to off-white crystals
Melting point 59 to 61 °C (138 to 142 °F; 332 to 334 K)
25 g L−1
Hazards
NFPA 704 (fire diamond)
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
2
0
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

Reactions

Salicylaldoxime is the conjugate acid of a bidentate ligand:

2 C6H4CH=NOH-2-OH + Cu2+ → Cu(C6H4CH=NOH-2-O)2 + 2 H+

In highly acidic media, the ligand decomplexes and the metal aqua complex is liberated. In this way the ligand is used as a recyclable extractant. It typically forms charge-neutral complexes with divalent metal ions.

Analytical chemistry

In the era when metals were analysed by spectrophotometry, many chelating ligands were developed that selectively formed brightly coloured complexes with particular metal ions. This methodology has been eclipsed with the introduction of inductively coupled plasma methodology. Salicylaldoxime can be used to selectively precipitate metal ions for gravimetric determination. It forms a greenish-yellow precipitate with copper at a pH of 2.6 in the presence of acetic acid. Under these conditions, this is the only metal that precipitates; at pH 3.3, nickel also precipitates. Iron (III) will interfere. [2] It has been used as an ionophore in ion selective electrodes, with good response to Pb2+ and Ni2+.[3]

Structure of the copper(II) complex of the conjugate base of salicylaldoxime. Notice that the hydroxyl groups form hydrogen bonds to the coordinated phenolate centers.

Extraction of metals

Saloximes are used in the extraction and separation of metals from their ores. In one application of hydrometallurgy, Cu2+ is extracted into organic solvents as its saloxime complex.[4]

References
  1. Smith, Andrew G.; Tasker, Peter A.; White, David J. "The structures of phenolic oximes and their complexes" Coordination Chemistry Reviews 2003, volume24, pp. 61-85. doi:10.1016/S0010-8545(02)00310-7
  2. SH Simonsen; HM Burnett (1955). "Spectrophotometric Determination Of Copper With Salicylaldoxime - Application To Analysis Of Aluminum Alloys". Analytical Chemistry. 27 (8): 1336–1339. doi:10.1021/ac60104a039.
  3. EK Quagraine; VPY Gadzekpo (December 1992). "Studies Of Spectrophotometric Reagents In Some Transition-Metal And Lead Ion-Selective Electrodes". Analyst. 117 (12): 1899–1903. doi:10.1039/an9921701899.
  4. Peter A. Tasker, Christine C. Tong, Arjan N. Westra "Co-extraction of cations and anions in base metal recovery" Coordination Chemistry Reviews 2007, vol. 251, pp. 1868–1877. doi:10.1016/j.ccr.2007.03.014
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