Modification of N-heterocyclic carbene scaffolds : insights into reactivity and electronic properties

Starting from the initial efforts to prepare, study, and utilize carbenes, the choice of substituents has been recognized as a critical factor in determining their properties. Following the isolation of stable carbenes, a further aspect of design has become apparent; creation of a supporting scaffold to provide influence over the carbene’s reactivity and electronic properties as well as providing a protective envelope to ensure stability. Modification of previous N-heterocyclic carbene scaffolds to provide a new range of diamidocarbenes, which demonstrated enhance electrophilic properties. Further elaboration of these scaffolds has allowed the synthesis of novel carbenes with tailored properties. A series of six-membered carbenes featuring adjoining amino and/or amido groups were studied to determine the effect of substituent modification. A mono-amino/amido carbene (MAAC), a diamidocarbenes, and a diaminocarbene were systematically compared using crystallographic, spectroscopic, electrochemical, and density functional theory methods. Using single crystal X-ray diffraction analysis, the free MAAC was found to exhibit inequivalent nitrogen-carbon bond lengths Iridium complexes of the carbenes were also evaluated and the collected data revealed that the introduction of carbonyl groups to the carbene-containing scaffold had a nearly linear, additive effect on the E [subscript 1/2] potential of the carbene-ligated iridium I/II redox couple (+165 mV per carbonyl added) as well as the TEP value of the corresponding carbene-Ir(CO)₂Cl complex. Beyond attenuated ligand donicity, the introduction of carbonyl groups expanded the carbene’s reactivity: unlike prototypical NHCs, the MAAC was found to couple to isonitriles to form the respective ketenimines. Additionally, remotely substituted diamidocarbenes (DACs) were prepared and characterized. These carbenes were compared to their parent carbene via spectroscopic and density functional theory methods, and subjected to reactivity trials to determine the effects of remote substitution. Spectroscopic examination of the carbene-Ir(CO)₂Cl complexes demonstrated a difference in TEP value, indicating a small effect of substitution on the carbene’s electronic state. However, treatment of the differently substituted carbenes with substrates known to react with the parent DAC indicated no significant difference in reactivity scope; additionally, computational methodology demonstrated similarities in calculated geometries and orbital energy levels.