Strongly Coordinating Ligands To Form Weakly Coordinating Yet Functional Organometallic Anions

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.