Stannylene

{{Short description|Class of organotin(II) compounds}} right|120px|thumb|General structure of stannylene '''Stannylenes''' (R₂Sn:) are a class of organotin(II) compounds that are analogues of carbene. Unlike carbene, which usually has a triplet ground state, stannylenes have a singlet ground state since valence orbitals of tin (Sn) have less tendency to form hybrid orbitals and thus the electrons in 5s orbital are still paired up.<ref name="sasamori">{{cite book|last1=Sasamori|first1=T.|last2=Tokitoh|first2=N.|title=Encyclopedia of Inorganic Chemistry II|date=2005|publisher=John Wiley & Sons|location=Chichester, U.K.|pages=1698–1740}}</ref> Free stannylenes are stabilized by steric protection. Adducts with Lewis bases are also known.<ref name="rev heavy carbene analog">{{cite journal|last1=Mizuhata|first1=Yoshiyuki|last2=Sasamori|first2=Takahiro|last3=Tokitoh|first3=Norihiro|title=Stable Heavier Carbene Analogues|journal= Chemical Reviews|date=2009|volume=109|issue=8|pages=3479–3511|doi=10.1021/cr900093s|pmid=19630390}}</ref>

==History== The first persistent stannylene, [(Me₃Si)₂CH]₂Sn, was reported by Michael F. Lappert in 1973.<ref name="Lappert1973">{{cite journal|last1=Davidson|first1=Peter J.|last2=Lappert|first2=Michael F.|title=Stabilisation of Metals in a Low Co-ordinative Environment using the Bis(trimethylsilyl)methyl Ligand; Coloured SnII and PbII Alkyls, M[(Me₃Si)₂CH]₂|journal=Journal of the Chemical Society, Chemical Communications|date=1973|issue=9|page=317|doi=10.1039/C3973000317A}}</ref> Lappert used the same synthetic approach to synthesize the first diamidostannylene [(Me₃Si)₂N]₂Sn in 1974.<ref name="Lappert1974">{{cite journal|last1=Harris|first1=David H.|last2=Lappert|first2=Michael F.|title=Monomeric, volatile bivalent amides of group IV_B elements, M(NR¹₂)₂ and M(NR¹R²)₂(M=Ge, Sn, or Pb; R¹=Me₃Si, R²=Me₃C)|journal=Journal of the Chemical Society, Chemical Communications|date=1974|issue=21|pages=895–896|doi=10.1039/C39740000895}}</ref>

The short-lived, transient stannylene Me₂Sn has been generated by thermolysis of ''cyclo''-(Me₂Sn)₆.<ref name="Neumann">{{cite journal|last1=Bleckmann|first1=Paul|last2=Maly|first2=Hartwig|last3=Minkwitz|first3=Rolf|last4=Watta|first4=Barbel|last5=Neumann|first5=Wilhelm P.|title=Matrix Isolation and IR Spectroscopy of Stannylenes (CH₃)₂Sn and (CD₃)₂Sn|journal=Tetrahedron Letters|date=1982|volume=23|issue=45|pages=4655–4658|doi=10.1016/s0040-4039(00)85679-8 }}</ref>

==Synthesis and characterization== ===Persistent stannylene=== Most alkyl stannylenes have been synthesized by alkylation of tin(II) dihalides with organolithium reagents. For example, the first stannylene, [(Me₃Si)₂CH]₂Sn, was synthesized using (Me₃Si)₂CHLi and SnCl₂.<ref name="Lappert1973" /> 560x167px|Synthesis of stannylene using organolithium reagent

In some cases, stannylenes have been prepared by reduction of a tin(IV) compound by KC₈<ref>{{cite journal|last1=Kira|first1=Mitsuo|last2=Ishida|first2=Shintaro|last3=Iwamoto|first3=Takeaki|last4=Yauchibara|first4=Rika|last5=Sakurai|first5=Hideki|title=New synthesis of a stable dialkylstannylene and its reversible complexation with tetrahydrofuran|journal=Journal of Organometallic Chemistry|date=2001|volume=636|issue=1–2|pages=144–147|doi=10.1016/s0022-328x(01)00998-6}}</ref>

560x97px|Stannylene synthesis using organolithium and KC8

Amidostannylene can also be synthesized by using a tin(II) dihalide and the lithium amide.<ref name="Lappert1974" />

===Short-lived stannylene=== The isolation of a transient alkyl stannylene is more difficult. The first isolation of dimethylstannylene was believed to be done by thermolysing cyclostannane (Me₂Sn)₆, which was the product of the condensation of Me₂Sn(NEt₂)₂ and Me₂SnH₂. The evidence came from the vibrational frequencies of dimethylstannylene identified by infrared spectroscopy, which is consistent with the calculated value.<ref name="Neumann" /> The existence of this elusive SnMe₂ was further confirmed by the discovery of visible light absorption matching the calculated electronic transition of SnMe₂ in gas phase.<ref name="Walsh SnMe2">{{cite journal|last1=Walsh|first1=Robin|title=First Gas-Phase Detection of Dimethylstannylene and Time-Resolved Study of Some of Its Reactions|journal= Journal of the American Chemical Society|date=2002|volume=124|issue=25|pages=7555–7562|doi=10.1021/ja012691k |pmid=12071766}}</ref>

Another method to prepare short-lived stannylene is laser flash photolysis using tetraalkyltin(IV) compound (e.g. SnMe₄) as a precursor. The generation of stannylene can be monitored by transient UV-VIS spectroscopy.<ref name="Transient2005">{{cite journal|last1=Becerra|first1=Rosa|last2=Gaspar|first2=Peter P.|last3=Harrington|first3=Cameron|last4=Leigh|first4=William J.|last5=Vargas-Baca|first5=Ignacio|last6=Walsh|first6=Robin|last7=Zhou|first7=Dong|title=Direct Detection of Dimethylstannylene and Tetramethyldistannene in Solution and the Gas Phase by Laser Flash Photolysis of 1,1-Dimethylstannacyclopent-3-enes|journal= Journal of the American Chemical Society|date=2005|volume=127|issue=49|pages=17469–17478|doi=10.1021/ja052675d|pmid=16332099}}</ref>

==Structure and bonding== thumb|left|Structure of a stannylene from X-ray crystallography. Stannylenes can be viewed as sp²-hybridized with vacant 5p orbital and a lone pair. This gives rise to their red color from n to p transition.<ref name="Lappert1973" /><ref name="davies">{{cite book|last1=Davies|first1=Alwyn G.|title=Organotin Chemistry|date=2004|publisher=Wiley-VCH|location=Weinheim|isbn=3-527-31023-1|page=364}}</ref>

With specific type of ligands, the electron deficiency of monomeric stannylene is reduced by the agostic interaction from B-H bond. This concept was proved by Mark Kenyon and coworkers in 2006 when they synthesized the cyclic dialkylstannylene [{n-Pr₂P(BH₃)}(Me₃Si)CCH₂]₂Sn. The crystal structure of the synthesized compound showed the arrangement of one B-H bond toward the Sn atom with the B—H--Sn bond distance of 2.03 Å. The mitigation of Sn electron deficiency was proved by the spectroscopic data, especially the ¹¹⁹Sn NMR spectra which showed the drastically low chemical shift (587 and 787 ppm comparing to 2323 ppm in analogous dialkylstannylene) indicating more electron density around Sn in this case.<ref name="agostic monomeric stannylene">{{cite journal|last1=Izod|first1=Keith|title=Stabilization of a Dialkylstannylene by Unusual B-HâââSn ç-Agostic-Type Interactions. A Structural, Spectroscopic, and DFT Study|journal=Organometallics|date=2006|volume=25|pages=1135–1143|doi=10.1021/om0600036}}</ref> {{Clear}}

==Reactivity==

===Oligomerization=== Small, unstable stannylenes (e.g. dimethylstannylene) undergo self-oligomerization yielding cyclic oligostannanes, which can be used as stannylene sources.<ref name="Neumann" />

More bulky stannylenes (e.g. Lappert's stannylene), on the other hand, tend to form a dimer. The nature of the Sn-Sn bond in stannylene dimer is rather different from C-C bond in carbene dimer (i.e. alkene). As alkene develops a typical double bond character and the molecule has a planar geometry, stannylene dimer has a trans-bent geometry. The double bond in stannylene dimer can be considered as two donor-acceptor interactions. The electron localization function (ELF) analysis of stannylene dimer shows a disynaptic basin (electrons in bonding orbitals) on both Sn atom, indicating that the interaction between two Sn atom is two unusual bent dative bonds.<ref name="topological Sn dimer">{{cite journal|last1=Popelier|first1=Paul L. A.|last2=Malcolm|first2=Nathaniel O.J.|last3=Gillispie|first3=Ronald J.|title=A topological study of homonuclear multiple bonds between the elements of group 14|journal=Journal of the Chemical Society, Dalton Transactions|date=2002|issue=17|pages=3333–3341|doi=10.1039/B110610B}}</ref> Apart from that, the stability of stannylene dimer is also affected by the steric repulsion and dispersion attraction between bulky substituents.<ref name="Sn dimer calc.">{{cite journal|last1=Růžička|first1=Aleš|last2=Hobza|first2=Pavel|title=New Insight into the Nature of Bonding in the Dimers of Lappert's Stannylene and Its Ge Analogs: A Quantum Mechanical Study|journal= Journal of Chemical Theory and Computation|date=2016|volume=23|issue=4|pages=1696–1704 |doi=10.1021/acs.jctc.6b00065|pmid=26953594|hdl=11104/0259593|hdl-access=free}}</ref> 300x300px|thumb|Double donor-acceptor interaction diagram in dimethylstannylene dimer {{Clear}}

===Insertion reaction=== Alkylstannylenes can react with various reagents (e.g. alkyl halides, enones, dienes) in an oxidative addition (or insertion) fashion. The reaction between stannylene and 9,10-phenanthrolinedione produces an EPR signal that was identified to be 9,10-phenanthrenedione radical anion, indicating that this reaction proceeds ''via'' radical mechanism.<ref name="david smith">{{cite journal|last1=Eaborn|first1=Colin|last2=Smith|first2=J. David|title=Reactions of a Highly Crowded Cyclic Stannylene with Iodoalkanes, Enones, and Dienes. Inhibition of Nucleophilic Substitution at Tin(IV) Centers|journal=Organometallics|date=2002|volume=21|issue=12|pages=2430–2437|doi=10.1021/om020106y}}</ref>

left|560x160px|thumb|Oxidative addition of ethyl iodide to a stannylene.<ref name="david smith" /> {{clear}}

===Cycloaddition=== Although stannylenes are more stable than its lighter carbene analogs, they readily undergo [2+4] cycloaddition reaction with Z-alkenes. The addition of (CH(SiMe₃)₂)₂Sn, to 2,7-diphenylocta-2,3,5,6-tetraene proceeds in a cheletropic fashion, as allowed by Woodward-Hoffmann rules.<ref name="cycloaddition1">{{cite journal|last1=Neumann|first1=Wilhelm P.|title=[2+4] Cheletropic cycloaddition of stannylenes R₂Sn|journal=Tetrahedron Letters |date=1984|volume=25|issue=6|pages=625–628|doi=10.1016/S0040-4039(00)99955-6}}</ref><br> [[File:SnR2 cycloaddition.png|left|560x160px|thumb|[2+4] Chelotropic cycloaddition of stannylene to 2,7-diphenylocta-2,3,5,6-tetraene in disrotatory fashion]] {{Clear}}

===Metal center for oxidative addition and reductive elimination=== In terms of the Sn^II/Sn^IV couple, certain stannylenes resemble transition metals. The singlet-triplet energy separation is considered to have a strong effect on oxidative addition reactivity,<ref name="ST energy gap">{{cite journal|last1=Wang|first1=Yong|last2=Ma|first2=Jing|title=Silylenes and germylenes: The activation of H–H bond in hydrogen molecule|journal= Journal of Organometallic Chemistry|date=2009|volume=694|issue=16|pages=2567–2575|doi=10.1016/j.jorganchem.2009.03.051}}</ref> by utilizing a very strong σ-donor boryl (-BX₂) ligand. The results demonstrated that not only molecular hydrogen but also E-H bond (E = B, Si, O, N) can be oxidative added on Sn. In ammonia and water cases, the oxidative added product could also undergo reductive elimination, yielding O- or N-B bond formation.<ref name="Oxadd">{{cite journal|last1=Protchenko|first1=Andrey|last2=Aldridge|first2=Simon|title=Enabling and Probing Oxidative Addition and Reductive Elimination at a Group 14 Metal Center: Cleavage and Functionalization of E−H Bonds by a Bis(boryl)stannylene|journal= Journal of the American Chemical Society|date=2016|volume=138|issue=13|pages=4555–4564|doi=10.1021/jacs.6b00710|pmid=26981766|doi-access=free}}</ref><br> left|560x160px|thumb|The oxidative addition of ammonia on Sn center and subsequent reductive elimination to yield B-N bond. (Dipp = 2,6-C₆H₃iPr₂)<ref name="Oxadd" /> {{Clear}}

==See also==

*Carbene analogs *Silylene

==References== {{reflist}}

Category:Organotin compounds Category:Tin(II) compounds Category:Substances discovered in the 1970s Category:Octet-deficient functional groups