UC Riverside

Carbenes are neutral, divalent, six electron carbon species. Although initially perceived as reaction intermediates, pioneering work from Bertrand showed that they can be isolated in the laboratory. Further, Arduengo isolated N-Heterocyclic Carbenes (NHCs), a stable variety of persistent carbenes usually flanked by bulky aryl groups. These compounds have become popular as strongly coordinating ligands for transition metals producing some of the most active catalysts so far. Since then, there has been a growing interest in the development of NHCs for transition metal catalysis. Over the past few years, our lab has been developing NHCs featuring weakly coordinating Carborane anions. Due to the inherent charge on these molecules, the resulting NHCs are overall mono or dianionic depending on the number of carboranes in the molecule.To impart more robust nature for reactivity investigations, the dianionic 12-vertex closo carboranyl NHCs were hexahalogenated. The monoanionic Au(I) complex of this NHC is itself a weakly coordinating anion allowing us to pair it with a variety of main-group and organometallic cations. The synthetic utility of these ion-pairs was demonstrated by a tandem hydroamination of alkynes followed by the hydrosilylation of the imines using a Au(I)/N-methyl benzothiazolium system. The scope of these carboranyl NHCs was then expanded to the 10-vertex closo carborane where stable NHCs bearing this carborane were synthesized using novel synthetic methods. Perchlorination of this carbene, including the backbone bearing the imidazolylidene ring rendered an unprecedented water stable carbene. Moreover, we show the 10-vertex carboranyl NHC is a strong ligand through synthesis of the corresponding Cu(I) and Au(I) complexes. The dicarbollide ligand, derived from the o-carborane was then functionalized to produce the bis(dicarbollide)iron appended NHCs, the isolobal equivalents of ferrocene. Despite the long-established analogy with ferrocenes, these complexes were seen to behave in a promiscuous way. An unusual transmetalation reaction with other transition metals was discovered to produce the first examples of metallacarborane appended NHCs. Coordination of these NHCs with Cu(I) produced bimetallic complexes with a rare coordination environment. The promiscuous nature of the dicarbollide-NHC ligands was further illustrated by their diverse reactivity with a Pd(II) precursor.