Elucidation of Heterocumulene Activation by a Nucleophilic-at-Metal Iridium(I) Carbene

A carbene complex supported by the (PNP)Ir framework is shown to facilitate sulfur-atom transfer from CS2 and PhNCS by an unusual multiple-bond metathesis pathway, and kinetically trapped intermediates are provided to support the proposed metal-initiated mechanism for heterocumulene activation. Experimental and theoretical studies on a series of (PNP)Ir−L complexes suggest that a high-lying, nucleophilic Ir(dz2) orbital mediates this unique reactivity. The combination of evidence indicates that square-planar Ir(I) carbenes of this type are best formulated as nucleophilic-at-metal carbenes, exhibiting reactivity initiated by a nucleophilic metal center rather than a nucleophilic or electrophilic carbene and providing products that are complementary to those typically observed for high-valent alkylidenes.