Synthesis and Characterization of Carbazolide-Based Iridium PNP Pincer Complexes. Mechanistic and Computational Investigation of Alkene Hydrogenation: Evidence for an Ir(III)/Ir(V)/Ir(III) Catalytic Cycle

New carbazolide-based iridium pincer complexes (^carbPNP)­Ir­(C₂H₄), <b>3a</b>, and (^carbPNP)­Ir­(H)₂, <b>3b</b>, have been prepared and characterized. The dihydride, <b>3b</b>, reacts with ethylene to yield the <i>cis</i>-dihydride ethylene complex <i>cis</i>-(^carbPNP)­Ir­(C₂H₄)­(H)₂. Under ethylene this complex reacts slowly at 70 °C to yield ethane and the ethylene complex, <b>3a</b>. Kinetic analysis establishes that the reaction rate is dependent on ethylene concentration and labeling studies show reversible migratory insertion to form an ethyl hydride complex prior to formation of <b>3a</b>. Exposure of <i>cis</i>-(^carbPNP)­Ir­(C₂H₄)­(H)₂ to hydrogen results in very rapid formation of ethane and dihydride, <b>3b</b>. DFT analysis suggests that ethane elimination from the ethyl hydride complex is assisted by ethylene through formation of (^carbPNP)­Ir­(H)­(Et)­(C₂H₄) and by H₂ through formation of (^carbPNP)­Ir­(H)­(Et)­(H₂). Elimination of ethane from Ir­(III) complex (^carbPNP)­Ir­(H)­(Et)­(H₂) is calculated to proceed through an Ir­(V) complex (^carbPNP)­Ir­(H)₃(Et) which reductively eliminates ethane with a very low barrier to return to the Ir­(III) dihydride, <b>3b</b>. Under catalytic hydrogenation conditions (C₂H₄/H₂), <i>cis</i>-(^carbPNP)­Ir­(C₂H₄)­(H)₂ is the catalyst resting state, and the catalysis proceeds via an Ir­(III)/Ir­(V)/Ir­(III) cycle. This is in sharp contrast to isoelectronic (PCP)Ir systems in which hydrogenation proceeds through an Ir­(III)/Ir­(I)/Ir­(III) cycle. The basis for this remarkable difference is discussed