Competitive Activation of N−C and C−H Bonds of the PNP Framework by Monovalent Rhodium and Iridium

This work describes how reactions of oxidative addition of N−C and C−H bonds are in
competition in the PNP-ligated Rh and Ir complexes. Iridium appears to have a higher
preference than Rh for the C−H activation over the N−C activation, and the Ir C−H activated
complexes are more kinetically stable than their Rh analogues. A new generation of a
diarylamido-based PNP pincer is presented, a “tied” PNP ligand 1c based on the iminodibenzyl substructure. This ligand is more definitively prearranged for binding to a metal
center in a meridional, anionic PNP fashion. As a result, its N−C cleavage reactions (that
lead to complexes of anionic PNP) are faster than for the “untied” ligands 1a,b. Structural
evidence indicates that the “tied” anionic PNP pincer ligand is bulkier than the “untied”
ligands when bearing the same substituents on the donor atoms because of the influence of
the conformation of the pincer backbone. The “tied” pincer ligand also allows for the
observation of the products of activation of the C−H bonds of the central N−CH3 group,
which are not detected with the “untied” ligands. The N−C oxidative addition reaction with
“untied” ligands proceeds in the solid state as well as in solution. A remarkable result is
reported where the solid-state N−C oxidative addition reaction displays superior selectivity
to the solution reaction. The mechanistic studies are augmented by the investigations of
the isotopically labeled (2H and 13C) ligands.