Selenourea

The compound was first synthesized in 1884 by Auguste Verneuil by the reaction of hydrogen selenide and cyanamide:{{cite journal|last=Hope |first=H. |title=Synthesis of selenourea |journal=Acta Chemica Scandinavica |date=1964 |volume=18 |page=1800 |doi=10.3891/acta.chem.scand.18-1800|doi-access=free}}

:{{chem2|H2Se + N\tC\sNH2 → Se\dC(NH2)2}}

While this reaction has even found use in industrial synthesis of selenourea,{{cite journal|last=Suvorov |first=V. |display-authors=etal |title=Production of selenourea of high purity |journal=Vysokochistye Veshchestva |date=1996 |volume=3 |pages=17–23}} more modern methods concern themselves with synthesis of substituted selenoureas. These can be synthesized using organic isoselenocyanates and secondary amines:

:{{chem2|R\sN\dC\dSe + NHR′R″ → Se\dC(\sNHR)(\sNR′R″)}}

Alternatively, isocyanides react with amines in the presence of elemental selenium:{{cite journal |doi=10.1021/jacs.6b11021 |title=A Library of Selenourea Precursors to PbSe Nanocrystals with Size Distributions near the Homogeneous Limit |date=2017 |last1=Campos |first1=Michael P. |last2=Hendricks |first2=Mark P. |last3=Beecher |first3=Alexander N. |last4=Walravens |first4=Willem |last5=Swain |first5=Robert A. |last6=Cleveland |first6=Gregory T. |last7=Hens |first7=Zeger |last8=Sfeir |first8=Matthew Y. |last9=Owen |first9=Jonathan S. |journal=Journal of the American Chemical Society |volume=139 |issue=6 |pages=2296–2305 |pmid=28103035 |osti=1437942 }}

:{{chem2|RN\tC + R'2NH + Se -> R(H)NC(Se)NR'2}}

X-ray crystallographic measurements on crystals 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 the O−H and S−H hydrogen bonding found in crystals of urea and thiourea—was confirmed.{{cite journal|last1=Rutherford |first1=J. S. |last2=Calvo |first2=C. |title=The crystal structure of selenourea |journal=Zeitschrift für Kristallographie |date=1969 |volume=128 |issue=3–6 |pages=229–258 |doi=10.1524/zkri.1969.128.3-6.229|bibcode=1969ZK....128..229R |s2cid=98443594}}

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 selenium atom, while the nitrogen's lone pair is drawn towards the carbonyl carbon. A similar effect is observed in urea and thiourea. 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 urea and thiourea 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).{{cite journal|last1=Hampson |first1=P. |last2=Mathias |first2=A. |title=Nitrogen-14 chemical shifts in ureas |journal=Journal of the Chemical Society B |date=1968 |volume=1968 |pages=673–675 |doi=10.1039/J29680000673}}. These minor differences suggest that the properties emergent from the delocalized nitrogen lone pair and destabilization of the C=S and C=Se π bond in thiourea and selenourea will also be similar.

:File:Resonance.gif

Unlike urea and thiourea, which have both been researched extensively, relatively few studies quantitatively characterize selenourea. While the selone tautomer (I) has been shown to be the more stable form,{{cite journal|last=Rostkowska |first=H. |display-authors=etal |title=Proton transfer processes in selenourea: UV-induced selenone → selenol photoreaction and ground state selenol → selone proton tunneling |journal=Chemical Physics |date=2004 |volume=298 |issue=1–3 |pages=223–232 |doi=10.1016/j.chemphys.2003.11.024|bibcode=2004CP....298..223R}} mainly qualitative and comparative information on selenourea's tautomerization is available.

In comparable manner to ketones, selones also tautomerize:

:File:Selenourea_Tautomers.gif

Since the greater delocalization of the lone pair electrons correlates with the selone 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 nonexistent at neutral pH.{{cite journal|last1=Pramanick |first1=D. |last2=Chatterjee |first2=A. K. |title=Thiourea as a transfer agent in the radical polymerization of methyl methacrylate in aqueous solution at 42° |journal=European Polymer Journal |date=1980 |volume=16 |issue=9 |pages=895–899 |doi=10.1016/0014-3057(80)90122-6|bibcode=1980EurPJ..16..895P }}

An important class of reactions of selenourea is the formation of heterocycles. Some selenium-containing heterocycles exhibit antiinflammatory and antitumor activity, among other medicinal uses. Using selenourea as a precursor is considered to be the most efficient means of selenium-containing heterocyclic synthesis.{{cite journal|last1=Ninomiya |first1=M. |display-authors=etal |title=Selenium-containing heterocycles using selenoamides, selenoureas, selenazadienes, and isoselenocyanates |journal=Heterocycles |date=2010 |volume=81 |issue=9 |pages=2027–2055 |doi=10.3987/REV-10-677|doi-access=free }}

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 selenium–metal π bond. In selenourea complexes only selenium–metal bonding has been observed, unlike in the urea and thiourea counterparts, which also bond through the nitrogen atom.{{cite journal|last1=Jones |first1=P. G. |last2=Thöne |first2=C. |title=Preparation, crystal structures and reactions of phosphine(selenourea)gold(I) complexes |journal=Chemische Berichte |date=1991 |volume=124 |pages=2725–2729 |doi=10.1002/cber.19911241213}}

{{Reflist|30em}}

Category:Organoselenium compounds

Category:Ureas

Category:Selenium(−II) compounds