Tuning the Electronic Properties of Carbenes: A Systematic Comparison of Neighboring Amino versus Amido Groups

A related series of six-membered carbenes featuring adjoining amino and/or amido groups (i.e, a diaminocarbene, a monoamido-aminocarbene (3), and a a diamidocarbene (6)) were systematically compared using crystallographic, spectroscopic, electro-chemical, and density functional theory methods. The solid-state structure of 3 was found to exhibit inequivalent nitrogen carbon bond lengths (C-carbene-N-amide= 1.395(4) angstrom vs C-carbene-N-amine = 1.323(4) angstrom). Moreover, the C-carbene-N-amide distance was longer than that measured in the solid-state structure of 6 (1.371(3) angstrom), while the C-carbene-N-amine distance was similar to that measured in the solid-state structure of a cyclic alkyl-aminocarbene (1.315(3) angstrom). Iridium complexes of the aforementioned 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-1/2 potential of the carbene-ligated iridium I/II redox couple (+165 mV per carbonyl added) as well as the Tolman electronic parameter value of the corresponding carbene-Ir(CO)(2)Cl complex (ca. 7 cm(-1) per carbonyl added). Beyond attenuated ligand donicity, the introduction of carbonyl groups was found to broaden the chemical reactivity: unlike prototypical N-heterocyclic carbenes, including diaminocarbenes, the monoamido-aminocarbene was found to couple to isonitriles to form the respective ketenimines.