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Sandwich compound

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Space-filling model of ferrocene, the archetypal sandwich compound

In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic covalent bonds to two arene ligands. The arenes have the formula CnHn, substituted derivatives (for example Cn(CH3)n) and heterocyclic derivatives (for example BCnHn+1). Because the metal is usually situated between the two rings, it is said to be "sandwiched". A special class of sandwich complexes are the metallocenes.

The term sandwich compound was introduced in organometallic nomenclature in the mid-1950s in a report by J. D. Dunitz, L. E. Orgel and R. A. Rich, who confirmed the structure of ferrocene by X-ray crystallography.[1] The correct structure had been proposed several years previously by Robert Burns Woodward and, separately, by Ernst Otto Fischer. The structure helped explain puzzles about ferrocene's conformers, the molecule features an iron atom sandwiched between two parallel cyclopentadienyl rings. This result further demonstrated the power of X-ray crystallography and accelerated the growth of organometallic chemistry.[2]

Classes

The best known members are the metallocenes of the formula M(C5H5)2 where M = Cr, Fe, Co, Ni, Pb, Zr, Ru, Rh, Sm, Ti, V, Mo, W, Zn. These species are also called bis(cyclopentadienyl)metal complexes. Other arenes can serve as ligands as well.

  • Mixed cyclopentadienyl complexes: M(C5H5)(CnHn). Some examples are Ti(C5H5)(C7H7) and (C60)Fe(C5H5Ph5) where the fullerene ligand is acting as a cyclopentadienyl analogue.
  • Bis(benzene) complexes: M(C6H6)2, the best known example being bis(benzene)chromium.
  • Bis(cyclooctatetraenyl) complexes: M(C8H8)2, such as U(C8H8)2 and Th(C8H8)2 (both actinocenes).
  • Metal-carborane complexes (metallacarboranes), a very large and diverse family in which main-group or transition metal ions are coordinated to carborane ligands to form polyhedral cages ranging in size from 6 to 15 vertices. Examples include bis(dicarbollide) complexes[3], e.g., [M(C2B9H11)2]z and [Fe(C2B9H11)2]2-, and small-carborane sandwiches such as (R2C2B3H5)M(C2B4H6) and (R5C5)M(R'2)C2B4H4) where M is a transition metal and R and R' are Me or Et.[4],[5]

Structure of (Me4N+)2[Fe(C2B9H11)2]+, showing only one Me4N+.[6]
Space-filling model of ferrocene, the archetypal sandwich compound

Closely related are the metal complexes containing H3C3B2R2 (diborolyl) ligands.[7] In addition to these, other sandwich complexes containing purely inorganic ligands are known, such as Fe(C5Me5)(P5) and [(P5)2Ti]2−.[8]

Half-sandwich compounds

Main article: Half-sandwich compound

Monometallic half-sandwich compounds

Ball-and-stick model of methylcyclopentadienyl manganese tricarbonyl, a "piano stool" compound

Metallocenes including just one facially-bound planar organic ligand instead of two gives rise to a still larger family of half-sandwich compounds. The most famous example is probably methylcyclopentadienyl manganese tricarbonyl. Such species are occasionally referred to as piano-stool compounds, at least when there are three diatomic ligands in addition to the hydrocarbon "seat" of the piano stool. The name derives from the similarity of the structure to such a "stool" with the seat being a facial planar organic compound, e.g. benzene or cyclopentadiene, and the legs being ligands such as CO or allyl.[9][10]

Dimetallic half-sandwich compounds

Compounds such as the cyclopentadienyliron dicarbonyl dimer and cyclopentadienylmolybdenumtricarbonyl dimer can be considered a special case of half-sandwiches, except that they are dimetallic.[9] A structurally related species is [Ru(C6H6)Cl2]2.

Multidecker sandwiches

The first isolated multidecker sandwich complex was the triscyclopentadienyl dinickel complex [Ni2Cp3]BF4, prepared in 1972 as a very air- and water-sensitive salt[11], whose structure was confirmed by X-ray crystallography in 1974[12] The first air-stable multidecker sandwich complexes, which were also the first electrically neutral multideckers, were the isomeric triple-decker sandwiches 1,7,2,3- and 1,7,2,4-CpCo(RHC2B3H3)Cp (R = H, Me), isolated in 1973[13],[14] with X-ray characterization of the 1,7,2,4 isomer.[15] Since then a considerable number of triple-, tetra-, penta-, and hexadecker sandwiches featuring planar (R2C2B3R'3)4- ligands (isoelectronic with Cp-) have been prepared via stepwise assembly and incorporated into extended linked-chain systems.[16][17]

The largest structurally characterized multidecker sandwich monomer is the hexadecker shown at lower right[18]

A crystallographically established cobaltacarborane hexadecker (see ref. 19)

An extensive family of multidecker sandwiches incorporating planar (R2R'C3B2R"2)3- (diborolyl) ligands has also been prepared.[19]

Numerous multidecker sandwich compounds featuring hydrocarbon bridging rings have also been prepared, especially triple deckers.[20] A versatile method involves the attachment of Cp*Ru+ to preformed sandwich complexes.[21]

Linked sandwiches

Monnmeric double-decker and multidecker sandwiches have been used as building blocks for extended systems, some of which exhibit electron delocalization between metal centers. An example of a cyclic poly(metallacarborane) complex is the octahedral "carbon-wired" system which contains a planar C16B8 macrocycle.[22]

File:Carbon-wired tetracobaltacarborane1.jpg

Inverse sandwiches

In these anti-bimetallic compounds, the metals are found to be bridged by a single carbocyclic ring. Examples include {(THF)3Ca}2(1,3,5-triphenylbenzene)[23] and {(Ar)Sn}2COT.

Perylene–tetrapalladium sandwich complex

Double- and multimetallic sandwich compounds

Another family of sandwich compound involves more than one metal sandwiched between two carbocyclic rings. Examples of the double sandwich include V2(indenyl)2,[24] Ni2(COT)2[25] and Cr2(pentalene)2. Depicted at right is an example of a multimetallic sandwich compound, which has four palladium atoms joined in a chain sandwiched between two perylene units.[26] The counterions are bulky tetraarylborates.

Applications

Ferrocene and methylcyclopentadienyl manganese tricarbonyl have been used as antiknock agents. Certain bent metallocenes of zirconium and hafnium are effective precatalysts for the polymerization of propylene. Many half sandwich complexes of ruthenium, such as those derived from (cymene)ruthenium dichloride dimer catalyse transfer hydrogenation, a useful reaction in organic synthesis.[27]

[non-primary source needed]

References

  1. ^ Dunitz, J.; Orgel, L.; Rich, A. (1956). "The crystal structure of ferrocene". Acta Crystallogr. 9 (4): 373–5. doi:10.1107/S0365110X56001091.
  2. ^ Miessler, Gary L.; Tarr, Donald A. (2004). Inorganic Chemistry. Upper Saddle River, NJ: Pearson Education. ISBN 0-13-035471-6.
  3. ^ H. C. Kang, S. S. Lee,C. B. Knobler, and M. F. Hawthorne, Inorganic Chemistry 1991, 30, 2024
  4. ^ R. N. Grimes, "Small Carborane Ligands as Spectators and as Players", J. Organometal. Chem. 1999, 581, 1.
  5. ^ R. N. Grimes, Carboranes Third Edition, Elsevier, Oxford, 2016, Chapter 13
  6. ^ "Syntheses of Charge-Compensated Dicarbollide Ligand Precursors and Their Use in the Preparation of Novel Metallacarboranes". Inorganic Chemistry. 30: 2024–2031. 1991. doi:10.1021/ic00009a015. {{cite journal}}: Cite uses deprecated parameter |authors= (help)
  7. ^ Siebert, W. Pure Appl. Chem. 1988, 60, 1345.
  8. ^ Urnezius, E.; Brennessel, W. W.; Cramer, C. J.; Ellis, J. E.; Schleyer, P. von R. (2002). "A Carbon-Free Sandwich Complex [(P5)2Ti]2−". Science. 295 (5556): 832–834. Bibcode:2002Sci...295..832U. doi:10.1126/science.1067325. PMID 11823635.
  9. ^ a b Begley, Michael J.; Puntambekar, Shakher G.; Anthony H., Wright (1987). "A di-iron–anthracene complex via ultrasonics". J. Chem. Soc., Chem. Commun. 1987: 1251–1252. doi:10.1039/C39870001251.
  10. ^ Begley, Michael J.; Puntambekar, Shakher G.; Wright, Anthony H. (1989). "Synthesis and reactivity of a new class of half-sandwich arene–iron complex: structure of [C6H3Me3Fe(C3H5)(CO)]PF6". J. Organomet. Chem. 362 (1–2): C11–C14. doi:10.1016/0022-328X(89)85301-X.
  11. ^ Salzer, A.; Werner, H. Angew. Chem. Int. Ed. Engl. 1972, 11,
  12. ^ Dubler, E.; Textor, M.; Oswald, H.-R.; Salzer, A. Angew. Chem. Int. Ed. Engl. 1974, 13, 135.
  13. ^ Grimes, R. N.; Beer, D. C.; Sneddon, L. G.; Miller, V. R.; Weiss, R. Inorg. Chem. 1974, 13, 1138
  14. ^ Beer, D. C.; Miller, V. R.; Sneddon, L. G.; Grimes, R. N.; Mathew, M.; Palenik, G. J. J. Am. Chem. Soc. 1973, 95, 3046
  15. ^ Beer, D. C.; Miller, V. R.; Sneddon, L. G.; Grimes, R. N.; Mathew, M.; Palenik, G. J. J. Am. Chem. Soc. 1973, 95, 3046.
  16. ^ Wang, X.; Sabat, M.; Grimes, R. N. J. Am. Chem. Soc. 1995, 117, 12218
  17. ^ R. N. Grimes, "Boron-Containing Rings Ligated to Metals", in Comprehensive Organometallic Chemistry III, R. H. Crabtree and D. M. P. Mingos, Eds., Elsevier, Oxford, 2007, Chapter 3.01, pp. 1-48
  18. ^ Wang, X.; Sabat, M.; Grimes, R. N. J. Am. Chem. Soc. 1995, 117, 12227
  19. ^ Siebert, W. Adv. Organomet. Chem. 1993, 35, 187 [
  20. ^ Beck, V.; O'Hare, D. (2004). "Triple-decker transition metal complexes bridged by a single carbocyclic ring". J. Organomet. Chem. 689: 3920–3938. doi:10.1016/j.jorganchem.2004.06.011.
  21. ^ Fagan, P. J.; Ward, M. D.; Calabrese, J. C. (1989). "Molecular engineering of solid-state materials: organometallic building blocks". J. Am. Chem. Soc. 111 (5): 1698–719. doi:10.1021/ja00187a024.
  22. ^ Yao, H.; Sabat, M.; Grimes, R. N.; Fabrizi de Biani, F.; Zanello, P. Angew. Chem. Int. Ed. Engl. 2003, 42, 1002
  23. ^ Krieck, Sven; Gorls, Helmar; Yu, Lian; Reiher, Markus; Westerhausen, Matthias (2009). "Stable "Inverse" Sandwich Complex with Unprecedented Organocalcium(I): Crystal Structures of [(thf)2Mg(Br)–C6H2–2,4,6-Ph3] and [(thf)3Ca{μ-C6H3–1,3,5-Ph3}Ca(thf)3]". J. Am. Chem. Soc. 131: 2977–2985. doi:10.1021/ja808524y. {{cite journal}}: |format= requires |url= (help)
  24. ^ Jonas, Klaus; Rüsseler, Wolfgang; Krüger, Carl; Raabe, Eleonore (1986). "Synthesis of Diindenyldivanadium—a New Variant of the Reductive Degradation of Metallocenes and Related Compounds". Angew. Chem. Int. Ed. Engl. 25: 928–929. doi:10.1002/anie.198609281. {{cite journal}}: |format= requires |url= (help)
  25. ^ Brauer, D. J.; Kruger, C. (1976). "The stereochemistry of transition metal cyclooctatetraenyl complexes: di-h3,h3′-cyclooctatetraenedinickel, a sandwich compound with two enveloped nickel atoms". J. Organomet. Chem. 122: 265–273. doi:10.1016/S0022-328X(00)80619-1.
  26. ^ Murahashi, Tetsuro; Uemura, Tomohito; Kurosawa, Hideo (2003). "Perylene–Tetrapalladium Sandwich Complexes". J. Am. Chem. Soc. 125 (28): 8436–8437. doi:10.1021/ja0358246. PMID 12848540. {{cite journal}}: |format= requires |url= (help)
  27. ^ Ikariya, Takao; Hashiguchi, Shohei; Murata, Kunihiko; Noyori, Ryoji (2005). "Preparation of Optically Active (R,R)-Hydrobenzoin from Benzoin or Benzil". Organic Syntheses. 82: 10.
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