Space-filling model offerrocene, the archetypal sandwich compound
Inorganometallic chemistry, asandwich compound is achemical compound featuring ametal bound byhaptic,covalent bonds to twoarene (ring)ligands. The arenes have the formulaCnHn, substituted derivatives (for exampleCn(CH3)n) andheterocyclic derivatives (for exampleBCnHn+1). Because the metal is usually situated between the two rings, it is said to be "sandwiched". A special class of sandwich complexes are themetallocenes.
The termsandwich compound was introduced in organometallic nomenclature in 1956 in a report by J. D. Dunitz, L. E. Orgel and R. A. Rich, who confirmed the structure offerrocene byX-ray crystallography.[1] The correct structure, in which the molecule features an iron atomsandwiched between two parallelcyclopentadienyl rings, had been proposed several years previously byRobert Burns Woodward and, separately, byErnst Otto Fischer. The structure helped explain puzzles about ferrocene'sconformers. This result further demonstrated the power ofX-ray crystallography and accelerated the growth oforganometallic chemistry.[2][page needed]
The best known members are themetallocenes of the formulaM(C5H5)2 where M =Cr,Fe,Co,Ni,Pb,Zr,Ru,Rh,Os,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 areTi(C5H5)(C7H7) and (C60)Fe(C5H5Ph5) where thefullerene ligand is acting as a cyclopentadienyl analogue.
Bis(benzene) complexes:M(C6H6)2, the best known example beingbis(benzene)chromium.
Bis(cyclooctatetraene) complexes:M(C8H8)2, such asU(C8H8)2 andTh(C8H8)2 (bothactinocenes).
Metal–carborane complexes (metallacarboranes), a very large and diverse family in which main-group or transition metal ions are coordinated to carborane ligands to formpolyhedral cages ranging in size from 6 to 15 vertices. Examples include bis(dicarbollide) complexes,[4] such as[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 methyl or ethyl.[5][6]
Structure of(Me4N+)2[Fe(C2B9H11)2]+, showing only oneMe4N+.[4]
Closely related are the metal complexes containingH3C3B2R2 (diborolyl) ligands.[7]In addition to these, other sandwich complexes containing purely inorganic ligands are known, such asFe(C5Me5)(P5) and[(P5)2Ti]2−.[8]
Half sandwich complexes have only one facially-bound planar organicligand instead of two gives rise to a still larger family ofhalf-sandwich compounds. One well studied example is probablymethylcyclopentadienyl manganese tricarbonyl. Such species are occasionally referred to aspiano-stool compounds, at least when there are three diatomic ligands. In such cases, the facially-bound planar organic ligand comprises the "seat" of the piano stool.
The first isolated multidecker sandwich was the tris(cyclopentadienyl)dinickel triple-decker complex[Ni2Cp3]BF4, a highly air- and water-sensitive compound reported in 1972,[9] with X-ray crystallographic confirmation in 1974.[10]
In 1973 the electrically neutral air-stable triple-decker cobaltacarborane sandwiches 1,7,2,3- and 1,7,2,4-CpCo(RHC2B3H3)Cp (where R = H, Me) were isolated and characterized by multinuclearNMR andX-ray studies[11] (the structure of the 1,7,2,3 isomer is shown).
1,7,2,3-CpCo(MeC2B3H4)CoCp, the first structurally confirmed multidecker sandwich.[11]
Since then many three-, four-, five-, and six-decker sandwich complexes have been described.[12][13] The largest structurally characterized multidecker sandwich monomer is the hexadecker shown at lower right.[14]
A structurally characterized cobaltacarborane hexadecker.[14]
An extensive family of multidecker sandwiches incorporating planar(R2R′C3B2R″2)3− (diborolyl) ligands has also been prepared.[15]
Numerous multidecker sandwich compounds featuring hydrocarbon bridging rings have also been prepared, especially triple deckers.[16] A versatile method involves the attachment ofCp*Ru+ to preformed sandwich complexes.[17]
Monomeric 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 shown below, which contains a planarC16B8 macrocycle.[18]
In theseanti-bimetallic compounds, the metals are found to be bridged by a single carbocyclic ring. Examples include[(THF)3Ca]2(1,3,5-triphenylbenzene)[19] and[(Ar)Sn]2COT.
Another family of sandwich compound involves more than one metal sandwiched between two carbocyclic rings. Examples of the double sandwich includeV2(indenyl)2,[20]Ni2(COT)2[21] andCr2(pentalene)2. Depicted at right is an example of a multimetallic sandwich compound, which has fourpalladium atoms joined in a chain sandwiched between twoperylene units.[22] Thecounterions are bulkytetraarylborates.
^Zeinstra, J.D.; De Boer, J.L. (1973). "Structure of Cyclopentadienylcycloheptatrienyl-titanium".Journal of Organometallic Chemistry.54:207–211.doi:10.1016/S0022-328X(00)85010-X.
^abKang, H. C.; Lee, S. S.; Knobler, C. B.; Hawthorne, M. F. (1991). "Syntheses of Charge-Compensated Dicarbollide Ligand Precursors and Their Use in the Preparation of Novel Metallacarboranes".Inorganic Chemistry.30 (9):2024–2031.doi:10.1021/ic00009a015.
^Grimes, R. N. (1999). "Small Carborane Ligands as Spectators and as Players".Journal of Organometallic Chemistry.581 (1–2):1–12.doi:10.1016/S0022-328X(99)00050-9.
^Grimes, R. N. (2016). "13. Metallacarboranes of the Transition and Lanthanide Elements".Carboranes (3rd ed.). Oxford: Elsevier.ISBN9780128019054.
^Salzer, A.; Werner, H. (1972). "Studies on the Reactivity of Metal π-Complexes. 6. A New Route to Triple-Decker Sandwich Compounds".Angewandte Chemie International Edition.11 (10):930–932.doi:10.1002/anie.197209301.
^Dubler, E.; Textor, M.; Oswald, H.-R.; Salzer, A. (1974). "X-Ray Structure Analysis of the Triple-Decker Sandwich Complex Tris(η-cyclopentadienyl)dinickel Tetrafluoroborate".Angewandte Chemie International Edition.13 (2):135–136.doi:10.1002/anie.197401351.
^abGrimes, R. N.; Beer, D. C.; Sneddon, L. G.; Miller, V. R.; Weiss, R. (1974). "Small cobalt and nickel metallocarboranes from 2,3-dicarbahexaborane(8) and 1,6-dicarbahexaborane(6). Sandwich complexes of the cyclic C2B3H7(2^{-}) and C2B3H5(4^{-}) ligands".Inorganic Chemistry.13 (5):1138–1146.doi:10.1021/ic50135a025.
^Wang, X.; Sabat, M.; Grimes, R. N. (1995). "Organotransition-Metal Metallacarboranes. 43. Directed Synthesis of Carborane-End-Capped Multidecker Sandwiches".Journal of the American Chemical Society.117 (49):12218–12226.doi:10.1021/ja00154a023.
^abWang, X.; Sabat, M.; Grimes, R. N. (1995). "Organotransition-Metal Metallacarboranes. 44. Construction of Pentadecker and Hexadecker Sandwiches from Triple-Decker Building Blocks".Journal of the American Chemical Society.117 (49):12227–12234.doi:10.1021/ja00154a024.
^Yao, H.; Sabat, M.; Grimes, R. N.; Fabrizi de Biani, F.; Zanello, P. (2003). "Organotransition-Metal Metallacarboranes. 63. Metallacarborane-Based Nanostructures: A Carbon-Wired Planar Octagon".Angewandte Chemie International Edition.42 (9):1002–5.CiteSeerX10.1.1.615.6577.doi:10.1002/anie.200390255.PMID12616549.
^Krieck, S.; Gorls, H.; Yu, L.; Reiher, M.; Westerhausen, M. (2009). "Stable "Inverse" Sandwich Complex with Unprecedented Organocalcium(I): Crystal Structures of [(thf)2Mg(Br)\sC6H2\s2,4,6\-Ph3] and [(thf)3Ca{μ\-C6H3\s1,3,5\-Ph3}Ca(thf)3]".Journal of the American Chemical Society.131 (8):2977–2985.doi:10.1021/ja808524y.PMID19193100.
^Brauer, D. J.; Kruger, C. (1976). "The stereochemistry of transition metal cyclooctatetraenyl complexes: di-η3,η3′-cyclooctatetraenedinickel, a sandwich compound with two enveloped nickel atoms".Journal of Organometallic Chemistry.122:265–273.doi:10.1016/S0022-328X(00)80619-1.
