Classically closo-carborane anions, particularly
[HCB11H11]− and [HCB9H9]−, and their derivatives have primarily
been used as weakly coordinating anions to isolate reactive intermediates,
platforms for stoichiometric and catalytic functionalization, counteranions
for simple Lewis acid catalysis, and components of materials like
liquid crystals. The aim of this article is to educate the reader
on the contemporary nonclassical applications of these anions. Specifically,
this review will cover new directions in main group catalysis utilized
to achieve some of the most challenging catalytic reactions such as
C–F, C–H, and C–C functionalizations that are
difficult or impossible to realize with transition metals. In addition,
the review will cover the utilization of the clusters as dianionic
C σ-bound ligands for coordination chemistry, ligand substituents
for coordination chemistry and advanced catalyst design, and covalently
bound spectator substituents to stabilize radicals. Furthermore, their
applications as solution-based and solid-state electrolytes for Li,
Na, and Mg batteries will be discussed.
In this feature article we cover new directions in the fundamental and applied chemistry of the closo-carborane anions [HCB11H11]−1 and [HCB9H9]−1, including energy storage applications, ionic liquids, anionic carborane fused heterocycles/radicals, ligand substituents, and ligands for catalysis and coordination chemistry.
Here we report the surprising discovery that high-energy vinyl carbocations can be generated under strongly basic conditions, and that they engage in intramolecular sp3 C–H insertion reactions through the catalysis of weakly coordinating anion salts. This approach relies on the unconventional combination of lithium hexamethyldisilazide base and the commercially available catalyst, triphenylmethylium tetrakis(pentafluorophenyl)-borate. These reagents form a catalytically active lithium species that enables the application of vinyl cation C–H insertion reactions to heteroatom-containing substrates.
The syntheses of unsymmetrical N-heterocyclic carbenes (NHCs) that contain a single N-bound icosahedral carborane anion substituent are reported. Both anionic C-2 and doubly deprotonated dianionic C-2/C-5 NHC lithium complexes are isolated. The latter species is formed selectively, which reveals a surprising directing effect conveyed by icosahedral carborane anion substituents.
The syntheses of the first carboranyl N-heterocyclic carbene complexes with transition metals are reported. Both unsymmetrical mono-anionic and symmetrical dianionic NHCs readily react with ClAuSMe2 to afford unusual zwitterionic and anionic Au(i) dimethyl sulfide adducts. The compounds are characterized by NMR, mass spectrometry, and single crystal X-ray diffraction studies. Percent buried volume (%Vbur) calculations indicate that replacement of an adamantyl group by a hydride substituted icosahedral carborane anion results in a 3.7% increase in %Vbur.
Weakly coordinating anions (WCAs) are generally tailored
to act
as spectators with little or no function. Here we describe the implementation
of strongly coordinating dianionic carboranyl N-heterocyclic carbenes
(NHCs) to create organometallic -ate complexes of Au(I) that serve
both as WCAs and functional catalysts. These organometallic WCAs can
be utilized to form both heterobimetallic (Au(I)−/Ag(I)+; Au(I)−/Ir(I)+) and
organometallic/main group ion pairs (Au(I)−/(CPh3
+ or SiEt3
+). Because parent
unfunctionalized dianionic carboranyl NHC complex 3 is
unstable in most solvents when paired with CPh3
+, novel synthesis methodology was devised to create polyhalogenated
carboranyl NHCs, which show superior stability toward electrophilic
substitution and cyclometalation chemistry. Additionally, the WCAs
containing polyhalogenated carboranyl NHCs are among the most active
catalysts reported for the hydroamination of alkynes. This investigation
has also produced the first examples of a low-coordinate Au(III) center
with two cis accessible coordination sites and the first true dianionic
carbene. These studies pave the way for the design of functional ion
pairs that have the potential to participate in tandem or cooperative
small-molecule activation and catalysis.
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