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), 3a, and (carbPNP)­Ir­(H)2, 3b, have been prepared and characterized.
The dihydride, 3b, reacts with ethylene to yield the cis-dihydride ethylene complex cis-(carbPNP)­Ir­(C2H4)­(H)2. Under
ethylene this complex reacts slowly at 70 °C to yield ethane
and the ethylene complex, 3a. 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 3a. Exposure of cis-(carbPNP)­Ir­(C2H4)­(H)2 to hydrogen results in very rapid formation of ethane and
dihydride, 3b. 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, 3b. Under catalytic hydrogenation conditions
(C2H4/H2), cis-(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.