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Carbene analog

January 01, 1970

Carbene analogs in chemistry are carbenes with the carbon atom replaced by another chemical element. Just as regular carbenes they appear in chemical reactions as reactive intermediates and with special precautions they can be stabilized and isolated as chemical compounds. Carbenes have some practical utility in organic synthesis but carbene analogs are mostly laboratory curiosities only investigated in academia. Carbene analogs are known for elements of group 13, group 14, group 15 and group 16.

Group 13 carbene analogs edit

In group 13 elements the boron carbene analog is called a borylene or boranylidene.

Group 14 carbene analogs edit

The heavier group 14 carbenes are silylenes, R2Si:, germylenes R2Ge: (example diphosphagermylene), stannylenes R2Sn: and plumbylenes R2Pb:, collectively known as metallylenes and regarded as monomers for polymetallanes.[1] The oxidation state for these compounds is +2 and stability increases with principal quantum number (moving down a row in the periodic table). This makes dichloroplumbylene PbCl2 and dichlorostannylene SnCl2 stable ionic compounds although they exist as polymers or ion pairs.

Group 14 carbene analogs do not form hybrid orbitals but instead retain (ns)2(np)2 electron configuration due to the increasing s p gap for larger elements. Two electrons remain in an s-orbital and therefore their compounds have exclusively singlet ground states and not the triplet ground state which can be observed in carbenes depending on the substituents. The s-orbital (lone pair) is inert and the vacant p-orbital is very reactive. Stable group 14 carbenes require stabilization of this p-orbital which is usually accomplished by coordination of a Cp* ligand or coordination to nitrogen, oxygen or phosphorus containing ligands, although stabilization can be achieved through steric protection alone.

General methods for the synthesis of carbon-substituted (aryl or alkyl) metallylenes are reduction of M4+ species or substitution reactions at M2+ halides. Stable metallylenes require bulky substituents in order to prevent nucleophilic attack of the metal center at the p-orbital. Examples of these bulky substituents in R2M: are mesityl, Dis (di(trimethylsilyl)methyl) and adamantyl groups. With insufficient steric shielding the metallylene will form a dimer or a polymer. The first isolable dialkylgermylene was synthesised in 1991:[2]

Me5C5GeCl + LiCH(Si(Me3))2 → Me5C5GeCH(Si(Me3))2
Me5C5GeCH(Si(Me3))2 + LiC(Si(Me3))3 → (SiMe3)3CGeCH(Si(Me3))2

Stable diarylgermylenes also require bulky ligands:[3]

Ge[N(SiMe3)2]2 + 2 LiC5H3(C10H7)2 → Ge[LiC5H3(C10H7)2]2
compound C-M distance (pm) C-M-C angle (°) Absorbance (nm)
(Ardip)2Ge 203.3 204.8 113.8 608
(Ardip)2Sn 225.5 117.6 600
(Ardip)2Pb 239.0 238.0 121.5 586
Stable Ge, Sn, Pb metallylenes with Ardip = C6H3-2,6-[C6H3-2,6-(C3H7)2]2 [4]

The C-M-C bond angle in metallylenes is less than 120° confirming hybridization other than sp2. The higher p-character for the C-MII bond compared to the C-MIV bond is reflected in its slightly higher bond length.

N-heterocyclic silylenes are known to be stable for months and have been studied extensively.

Group 15 carbene analogs edit

In the group 15 elements the neutral nitrogen carbene analog (RN) is called a nitrene. The phosphorus analog is a phosphinidene. There are charged group 15 carbene analogs as well, most notably phosphenium ions (R2P+) which are isolobal with (hetero-)carbenes possessing a singlet ground state.

Group 16 carbene analogs edit

Carbene analogs of group 16 elements have been first reported in 2009.[5][6] Sulfur, selenium and tellurium dications have been found to be stabilized by the diiminopyridine ligand DIMPY. For example, the reaction product of triflate S(Otf)2 and (2,6-diisopropylphenyl)2DIMPY at -78 °C results in an air-stable dicationic sulfur compound with a naked S2+ atom coordinated by three nitrogen atoms by dative bonds.

 

References edit

  1. ^ Stable Heavier Carbene Analogues Yoshiyuki Mizuhata, Takahiro Sasamori, and Norihiro Tokitoh Chem. Rev. 2009, 109, 3479–3511 doi:10.1021/cr900093s
  2. ^ Synthesis and solid-state structure of (Me3Si)3CGeCH(SiMe3)2, a monomeric dialkylgermylene P. Jutzi, A. Becker, H. G. Stammler, B. Neumann Organometallics, 1991, 10 (6), pp 1647–1648 doi:10.1021/om00052a002
  3. ^ Ligand-Protected Strain-Free Diarylgermylenes Gerald L. Wegner, Raphael J. F. Berger, Annette Schier, and Hubert Schmidbaur Organometallics, 2001, 20 (3), pp 418–423 doi:10.1021/om000743t
  4. ^ Synthesis and Characterization of the Monomeric Sterically Encumbered Diaryls E{C6H3-2,6-(C6H3-2,6-Pri2)2}2 (E = Ge, Sn, or Pb) Geoffrey H. Spikes, Yang Peng, James C. Fettinger, Philip P. Power, Davis Zeitschrift für anorganische und allgemeine Chemie Volume 632 Issue 6, Pages 1005 - 1010 2006 doi:10.1002/zaac.200500532
  5. ^ Remarkably Stable Chalcogen(II) Dications Caleb D. Martin, Christine M. Le, and Paul J. Ragogna J. Am. Chem. Soc., Article ASAP 2009 doi:10.1021/ja9073968
  6. ^ Reactive Dications Tamed Main-Group Chemistry: Stabilizing positive charge with three nitrogen atoms yields stable group 16 complexes Steve Ritter Chemical & Engineering News October 12, 2009 Volume 87, Number 41 pp. 12 - 12 Link

January 01, 1970
carbene, analog, chemistry, carbenes, with, carbon, atom, replaced, another, chemical, element, just, regular, carbenes, they, appear, chemical, reactions, reactive, intermediates, with, special, precautions, they, stabilized, isolated, chemical, compounds, ca. Carbene analogs in chemistry are carbenes with the carbon atom replaced by another chemical element Just as regular carbenes they appear in chemical reactions as reactive intermediates and with special precautions they can be stabilized and isolated as chemical compounds Carbenes have some practical utility in organic synthesis but carbene analogs are mostly laboratory curiosities only investigated in academia Carbene analogs are known for elements of group 13 group 14 group 15 and group 16 Contents 1 Group 13 carbene analogs 2 Group 14 carbene analogs 3 Group 15 carbene analogs 4 Group 16 carbene analogs 5 ReferencesGroup 13 carbene analogs editIn group 13 elements the boron carbene analog is called a borylene or boranylidene Group 14 carbene analogs editThe heavier group 14 carbenes are silylenes R2Si germylenes R2Ge example diphosphagermylene stannylenes R2Sn and plumbylenes R2Pb collectively known as metallylenes and regarded as monomers for polymetallanes 1 The oxidation state for these compounds is 2 and stability increases with principal quantum number moving down a row in the periodic table This makes dichloroplumbylene PbCl2 and dichlorostannylene SnCl2 stable ionic compounds although they exist as polymers or ion pairs Group 14 carbene analogs do not form hybrid orbitals but instead retain ns 2 np 2 electron configuration due to the increasing s p gap for larger elements Two electrons remain in an s orbital and therefore their compounds have exclusively singlet ground states and not the triplet ground state which can be observed in carbenes depending on the substituents The s orbital lone pair is inert and the vacant p orbital is very reactive Stable group 14 carbenes require stabilization of this p orbital which is usually accomplished by coordination of a Cp ligand or coordination to nitrogen oxygen or phosphorus containing ligands although stabilization can be achieved through steric protection alone General methods for the synthesis of carbon substituted aryl or alkyl metallylenes are reduction of M4 species or substitution reactions at M2 halides Stable metallylenes require bulky substituents in order to prevent nucleophilic attack of the metal center at the p orbital Examples of these bulky substituents in R2M are mesityl Dis di trimethylsilyl methyl and adamantyl groups With insufficient steric shielding the metallylene will form a dimer or a polymer The first isolable dialkylgermylene was synthesised in 1991 2 Me5C5GeCl LiCH Si Me3 2 Me5C5GeCH Si Me3 2 Me5C5GeCH Si Me3 2 LiC Si Me3 3 SiMe3 3CGeCH Si Me3 2 Stable diarylgermylenes also require bulky ligands 3 Ge N SiMe3 2 2 2 LiC5H3 C10H7 2 Ge LiC5H3 C10H7 2 2 compound C M distance pm C M C angle Absorbance nm Ardip 2Ge 203 3 204 8 113 8 608 Ardip 2Sn 225 5 117 6 600 Ardip 2Pb 239 0 238 0 121 5 586 Stable Ge Sn Pb metallylenes with Ardip C6H3 2 6 C6H3 2 6 C3H7 2 2 4 The C M C bond angle in metallylenes is less than 120 confirming hybridization other than sp2 The higher p character for the C MII bond compared to the C MIV bond is reflected in its slightly higher bond length N heterocyclic silylenes are known to be stable for months and have been studied extensively Group 15 carbene analogs editIn the group 15 elements the neutral nitrogen carbene analog RN is called a nitrene The phosphorus analog is a phosphinidene There are charged group 15 carbene analogs as well most notably phosphenium ions R2P which are isolobal with hetero carbenes possessing a singlet ground state Group 16 carbene analogs editCarbene analogs of group 16 elements have been first reported in 2009 5 6 Sulfur selenium and tellurium dications have been found to be stabilized by the diiminopyridine ligand DIMPY For example the reaction product of triflate S Otf 2 and 2 6 diisopropylphenyl 2DIMPY at 78 C results in an air stable dicationic sulfur compound with a naked S2 atom coordinated by three nitrogen atoms by dative bonds nbsp References edit Stable Heavier Carbene Analogues Yoshiyuki Mizuhata Takahiro Sasamori and Norihiro Tokitoh Chem Rev 2009 109 3479 3511 doi 10 1021 cr900093s Synthesis and solid state structure of Me3Si 3CGeCH SiMe3 2 a monomeric dialkylgermylene P Jutzi A Becker H G Stammler B Neumann Organometallics 1991 10 6 pp 1647 1648 doi 10 1021 om00052a002 Ligand Protected Strain Free Diarylgermylenes Gerald L Wegner Raphael J F Berger Annette Schier and Hubert Schmidbaur Organometallics 2001 20 3 pp 418 423 doi 10 1021 om000743t Synthesis and Characterization of the Monomeric Sterically Encumbered Diaryls E C6H3 2 6 C6H3 2 6 Pri2 2 2 E Ge Sn or Pb Geoffrey H Spikes Yang Peng James C Fettinger Philip P Power Davis Zeitschrift fur anorganische und allgemeine Chemie Volume 632 Issue 6 Pages 1005 1010 2006 doi 10 1002 zaac 200500532 Remarkably Stable Chalcogen II Dications Caleb D Martin Christine M Le and Paul J Ragogna J Am Chem Soc Article ASAP 2009 doi 10 1021 ja9073968 Reactive Dications Tamed Main Group Chemistry Stabilizing positive charge with three nitrogen atoms yields stable group 16 complexes Steve Ritter Chemical amp Engineering News October 12 2009 Volume 87 Number 41 pp 12 12 Link Retrieved from https en wikipedia org w index php title Carbene analog amp oldid 1172564680, wikipedia, wiki, book, books, library,

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