Selenourea
Encyclopedia
Selenourea is the organoselenium compound with the formula SeC(NH2)2. It is a white solid. This compound features a rare example of a stable, unhindered carbon-selenium double bond. The compound is used in the synthesis of selenium heterocycles. Compared to urea
, the oxo-analog of selenourea, few studies have been done on the compound due to the instability and toxicity of selenium
compounds.
and cyanamide
:
While this reaction has even found use in industrial synthesis of selenourea, more modern methods concern themselves with synthesis of substituted selenoureas. These can be synthesized using organic isoselenocyanates and secondary amines:
Alternatively, a substituted carbodiimide could be used as follows:
s at -100 °C give average C=Se bond lengths of 1.86 Å, and 1.37 Å for C-N. Both the Se-C-N and N-C-N angles were measured at 120°, as expected for an sp2 hybridized carbon. Through these same studies, the existence of Se-H hydrogen bonding in the crystal lattice—suggested from to the O-H and S-H hydrogen bonding found in crystals of oxo- and thiourea—was confirmed.
Both the shortened length of the N-C bond and the longer Se=C bond suggest a delocalization of the lone pair on the amines; the Se=C π-bonding electrons are drawn towards the Se atom, while the N-lone pair is drawn towards the carbonyl carbon. A similar effect is observed in oxo- and thiourea analogs. In going from urea to thiourea to selenourea the double bond is more delocalized and longer, while the C-N σ bond is stronger and shorter. In terms of resonance structures, the selenol form (structures II, III) is more prevalent compared to oxo- and thione analogs; however, the lone pair the nitrogen of selenourea delocalizes only slightly more than the lone pair on thiourea (in contrast to a much greater delocalization in going from urea to thiourea). These minor differences suggest that the properties emergent from the delocalized N-lone pair and destabilization C=S and C=Se π bond in thiourea and selenourea will also be similar.
Unlike urea and thiourea
, which have both been researched extensively, relatively few studies quantitatively characterize selenourea. While the selenone tautomer (I) has been shown to be the more stable form, mainly qualitative and comparative information on selenourea’s tautomerization is available.
In comparable manner to ketones, selenones also tautomerize:
Since the greater delocalization of the lone pair electrons correlates with the selenone product, the equilibrium position of selenourea likely has an equilibrium position comparable to thiourea’s (which is lies more to the right that than urea’s). Thiourea has been shown to exist predominantly in its thione form at 42 °C in dilute methanol, with the thionol tautomer almost non-existent at neutral pH.
Another class of reactions is the complexation of selenourea with transition metals and metalloids. Its ability to act as an effective ligand is attributed to the electron-donating effect of the amino groups and consequent stabilization of the Se-M π-bond. In selenourea complexes only Se-M bonding has been observed, unlike in the oxo- and thiourea counterparts, which also bond through the nitrogen atom.
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
, the oxo-analog of selenourea, few studies have been done on the compound due to the instability and toxicity of selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
compounds.
Synthesis
The compound was first synthesized in 1884 by Auguste Verneuil by the reaction of hydrogen selenideHydrogen selenide
Hydrogen selenide is the inorganic compound with the formula H2Se. It is the simplest and virtually the only hydride of selenium. H2Se is a colorless, flammable gas under standard conditions. It is the most toxic selenium compound with an exposure limit: 0.05 ppm over an 8 hour period...
and cyanamide
Cyanamide
Cyanamide is an organic compound with the formula CN2H2. This white solid is widely used in agriculture and the production of pharmaceuticals and other organic compounds. It is also used as an alcohol deterrent drug in Canada, Europe and Japan. The molecule features a nitrile group attached to an...
:
- H2Se + NCNH2 → SeC(NH2)2
While this reaction has even found use in industrial synthesis of selenourea, more modern methods concern themselves with synthesis of substituted selenoureas. These can be synthesized using organic isoselenocyanates and secondary amines:
- RN=C=Se + NHR’R’’ → Se=C(NRH)(NR’R’’H)
Alternatively, a substituted carbodiimide could be used as follows:
- RN=C=NR’ Se=C(NRH)(NR’H)
Properties
X-ray crystallographic measurements on crystalCrystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s at -100 °C give average C=Se bond lengths of 1.86 Å, and 1.37 Å for C-N. Both the Se-C-N and N-C-N angles were measured at 120°, as expected for an sp2 hybridized carbon. Through these same studies, the existence of Se-H hydrogen bonding in the crystal lattice—suggested from to the O-H and S-H hydrogen bonding found in crystals of oxo- and thiourea—was confirmed.
Both the shortened length of the N-C bond and the longer Se=C bond suggest a delocalization of the lone pair on the amines; the Se=C π-bonding electrons are drawn towards the Se atom, while the N-lone pair is drawn towards the carbonyl carbon. A similar effect is observed in oxo- and thiourea analogs. In going from urea to thiourea to selenourea the double bond is more delocalized and longer, while the C-N σ bond is stronger and shorter. In terms of resonance structures, the selenol form (structures II, III) is more prevalent compared to oxo- and thione analogs; however, the lone pair the nitrogen of selenourea delocalizes only slightly more than the lone pair on thiourea (in contrast to a much greater delocalization in going from urea to thiourea). These minor differences suggest that the properties emergent from the delocalized N-lone pair and destabilization C=S and C=Se π bond in thiourea and selenourea will also be similar.
Unlike urea and thiourea
Thiourea
Thiourea is an organosulfur compound of with the formula SC2 . It is structurally similar to urea, except that the oxygen atom is replaced by a sulfur atom, but the properties of urea and thiourea differ significantly. Thiourea is a reagent in organic synthesis. "Thioureas" refers to a broad...
, which have both been researched extensively, relatively few studies quantitatively characterize selenourea. While the selenone tautomer (I) has been shown to be the more stable form, mainly qualitative and comparative information on selenourea’s tautomerization is available.
In comparable manner to ketones, selenones also tautomerize:
Since the greater delocalization of the lone pair electrons correlates with the selenone product, the equilibrium position of selenourea likely has an equilibrium position comparable to thiourea’s (which is lies more to the right that than urea’s). Thiourea has been shown to exist predominantly in its thione form at 42 °C in dilute methanol, with the thionol tautomer almost non-existent at neutral pH.
Reactivity
An important class of reactions of selenourea is the formation of heterocycles. Some selenium-containing heterocycles exhibit anti-inflammatory and anti-tumor activity, among other medicinal uses. Using selenourea as a precursor is considered to be the most efficient means of selenium-containing heterocyclic synthesis.Another class of reactions is the complexation of selenourea with transition metals and metalloids. Its ability to act as an effective ligand is attributed to the electron-donating effect of the amino groups and consequent stabilization of the Se-M π-bond. In selenourea complexes only Se-M bonding has been observed, unlike in the oxo- and thiourea counterparts, which also bond through the nitrogen atom.