Chemistry:Metallacarbaboranes
Metallacarbaboranes (or metallacarboranes) are compounds that contain cluster polyhedra comprising carbon, boron, and metal atoms in various combinations.[1] Most of the structures of metallacarbaborane clusters derive from triangular-faced polyhedra. The most numerous examples are icosahedral and pentagonal bipyramidal cages.[2]
Classes of compound and geometrical aspects
Single Cages
In a single-cage metallacarbaborane, the cluster consists of one complete polyhedron or one polyhedral fragment. An example of closo metallacarbaborane is the dicarbollide [(OC)3MoC2B9H11]2−. Another example is the 11 vertex closo-1-(η6-MeC6H4-4-i-Pr)-2,3-Me2-1-Ru-2,3-2-C2B8H8. A large group of metallacarbaboranes adopts the closo-C2B4M core and each possesses a pentagonal bipyramidal structure.
Sandwich complexes
The open face of nido-[C2B9H11]2− often binds to metal in a similar manner to a cyclopentadienyl ring. commo-[Fe(C2B9H11)2]2- and CpFeC2B9H11 are analogs of ferrocene. In f-block metal complexes, the higher coordination numbers of the metal center lead to the addition of halide or solvent ligands and a concomitant tilting of the carboranyl ligands, for example, in [(C2B9H11)2UX2]2-. An actinide containing metallacarbaborane is [U(C2B9H11)2Cl2]2−.
Stacked compexes
Extended stacks containing small boron–carbon rings acting as spacers between metal centers are well established. nido-Carbaborane ligands such as [C2B4H6]2− can terminate a stack that has a central π-organometallic sandwich unit. Combined with a variety of organic π-ligands, metallacarbaborane units can be incorporated into linked stacks. The structural variation within the class of stacked complexes can be controlled by molecular precursors.
Slip distortion
In a sandwich complex, one might expect the two π-ligands on either side of a metal atom to lie directly over one another, and to be mutually eclipsed or staggered. However, the occurrence of commo-metallacarbaboranes with slipped geometries (a so-called slip distortion) is quite common, notably in cases where a d-block metal ion has a nearcomplete d-electron configuration: NiII, CuII, or AuIII. In single-cage closo metallacarbaboranes containing a main group metal atom, the slippage of the heteroatom with respect to the open face of a nido-[C2B9H11] or nido-[C2B4H6]2− ligand, or C-substituted analogs of these ligands, is commonly observed. The p-block atom may or may not carry a terminal substituent. In cases where a bare metal atom is incorporated into the metallacarbaborane cluster, it may function as a Lewis acid. For the donor–acceptor complexes, the trend is for an increase in the slip distortion and a decrease in the tilting of the donor ligand as the Lewis acid–base interaction becomes stronger. A number of theoretical studies have addressed the origins of slip distortion.
Ion pairing
In most metallacarbaboranes, the interaction between the metal atom and carbaborane cage is considered to be essentially covalent, and this is supported by the experimentally determined M–B and M–C distances. However, in [3-Tl-1,2-C2B9H11]−, the distances from the thallium(I) center to the C and B atoms in the open face of the C2B9 cage are particularly long, suggesting Tl+ [1,2-C2B9H11]2− ion pairing. Consistent with this is the fact that the TlI center is readily displaced by other metal ions.
σ-Bonded and edge-bridging metal fragments
In the extreme, the slip distortion described above leads to the C2B3 face of a dicarbollide ligand interacting with a metal atom through only three B atoms. The similarity to an organometallic allyl complex results in the term borallyl being used for this type of bonding mode. Copper(I), silver(I), gold(I), and mercury(II) fragments exhibit a tendency to bridge the edges of polyhedral clusters of various types. The preference is for the metal atom to bridge a B–B rather than a B–C or a C–C edge, and this reflects the observed preferences for bridging H atoms in borane and carbaborane clusters.
Exo-metallated clusters
In some metallacarbaborane clusters, a metal fragment is supported on the outside of the carbaborane skeleton, typically by B–H–M interactions, for example, complex involves exo interactions between the Tl(I) atom of one cage and two terminal boron–hydrogen bonds of a second cage. The exo descriptor illustrates the relationship of the metal fragment to the carbaborane and the nido descriptor classifies the carbaborane cage.
Metal-rich clusters
In almost all metallacarbaboranes, the total number of B and C atoms in the polyhedral cluster exceeds the number of metal atoms. A rare example of a metal-rich metallacarbaborane is closo-Fe3(CO)9BHCHCMe. The complex has also been described in terms of an η3-borirene ligand coordinating to an Fe3 unit.
Synthesis
If a metallacarbaborane is represented as {MCB}, then obvious methods of synthesis would be based on the following combinations {M}+{CB}, {MB}+{C}, {MC}+{B}, or aggregation of {M}, {C}, and {B}. The nonspecific nature of particle aggregation makes unattractive in most instances.
Carbaborane anion with metal halide
The most general synthetic strategy in the preparation of metallacarbaborane clusters is the addition of a metal fragment to an anionic carbaborane cluster, that is, {M}+{CB}. Many methods involve the metallic capping of a nido-carbaborane to generate a closo-metallacarbaborane. cases in which the electron count for the cluster remains unaltered during the reaction and hence the polyhedral skeleton of the closo product is related to that of the nido precursor. In each case, the metal fragment being added provides two electrons to cluster bonding:
[math]\ce{ nido-[C2B9H11]^2- + SmI_2(THF)_x -> closo-Sm(THF)4C2B9H11 + 2I^- }[/math],
[math]\ce{ nido-[C2Me2B9]^2- + SnCl2 -> nido-SnC2Me2B9H9 + 2Cl^- }[/math] – in the two electrons from the metal fragment replace those originally provided by the (2-) charge.
[math]\ce{ nido-[C2(SiMe3)2B4H5]^- + SnCl2 ->[THF] closo-Sn(THF)2C2(SiMe3)B4H4 ->[{100°C}] closo-SnC2(SiMe3)2B4H4 }[/math] – the monoanionic precursor loses H+ during the reaction.
The addition of a metal fragment to a nido-carbaborane anion may result in an increase in the number of clusterbonding electrons. The reaction of nido-carbaborane anions with metal halides is also a method of preparing commo-metallacarbaboranes.
Neutral carbaborane with a metal fragment
Reaction conditions often involve thermolysis or photolysis. Capping a nido cluster to form a skeletally related closo cluster proceeds with the extrusion of ligands if the cluster-electron count is to remain constant. Reaction of nido-C2B4H8 with Fe(CO)5 results in the addition of an Fe(CO)3 unit to the open face of the nido cluster. The Fe(CO)3 unit provides two electrons to cluster bonding; in order to retain the pentagonal bipyramidal skeleton, the carbaborane responds to the addition of the metal fragment by eliminating H2.
Metal fragment addition with nido to closo cluster change:
[math]\ce{ nido-C2B4H8 + Fe(CO)5 ->[{t°}] closo-Fe(CO)3C2B4H6 + H2 + 2CO }[/math].
Metal fragment insertion into a closo-cluster:
[math]\ce{ nido-C2B4H6 + Fe(CO)5 ->[{t°}] closo-Fe(CO)3C2B4H6 + 2CO }[/math].
Metal fragment-for-BH unit substitution in a closo-cluster:
[math]\ce{ closo-C2B10H12 + CpCo(CO)2 -> closo-CpCoC2B9H11 + 2CO }[/math].
Metal fragment insertion into a nido-cluster:
[math]\ce{ nido-C2B8H12 + Cp2Ni -> nido-CpNiC2B8H11 + C5H6 }[/math].
The insertion of a two-electron cluster fragment into a closo-carbaborane cluster can result in the expansion of the cluster framework.
Metallaborane with an alkyne
An important method for the synthesis of carbaboranes is the reaction of a borane cluster with an alkyne. However, an analogous reaction of a metallaborane cluster with an alkyne is not commonly used to prepare metallacarbaborane clusters:
[math]\ce{ nido-2-(CpCo)B4H8 + C2H2 -> nido-2-(CpCo)C2B3H7 }[/math].
Reactions between a metallaborane and alkyne are not always predictable. For example, nido-1,2-(Cp∗Rh)2B3H7 catalyzes the cyclotrimerization of alkynes rather than undergoing insertion of the alkyne into the cluster cage.
Aggregation of metal atoms, borane, and alkynes
Metal-atom vapor synthesis techniques have been successfully applied to the preparation of metallacarbaboranes, that is, reaction type {M}+{C}+{B}. A disadvantage is that product distribution tends to be nonspecific, for example, when Co metal is vaporized by electrical heating and cocondensed at −196°C with cyclopentadiene, B5H9, and the alkyne MeC≡CMe, 1-(CpCo)-2,3-Me2-2,3-C2B4H4, 1,7-(CpCo)2-2,3-Me2-2,3-C2B3H3, and 1,7-(CpCo)2-2,5-Me2-2,5-C2B5H5 are formed in low yield. If B6H10 is used instead of B5H9 as the borane precursor, the products are 3-(CpCo)-1,2-Me2-1,2-C2B5H5, 2,5-(CpCo)2-6,7-Me2-6,7-C2B6H8, and 2,4-(CpCo)2-6,7-Me2-2,4–6,7-C2B6H6.
Borane with an organometallic Complex
The method of metallacarbaborane synthesis that involves the combination of {MC}+{B} fragments is not commonly used. The reagent BH3·THF is a source of boron for the metallacarbaborane product, and is also able to abstract phosphine from the organometallic precursor.
References
- ↑ Housecroft, Catherine E. (15 December 2011). "Boron: Metallacarbaboranes". Encyclopedia of Inorganic and Bioinorganic Chemistry. Wiley Online Library. doi:10.1002/9781119951438.eibc0025.pub2. ISBN 9781119951438. https://onlinelibrary.wiley.com/doi/abs/10.1002/9781119951438.eibc0025.pub2. Retrieved 25 October 2021.
- ↑ Russell N. Grimes (2016). Carboranes, 3rd Edition. Elsevier. ISBN 9780128019054.
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