Use of the Imine–Enamine Equilibrium in Cooperative Ligand Design

The
imine–phosphine ligands Ph2PC5H7NAr, where Ar = 2,6-Pri2C6H3, 2,6-Me2C6H3, were deprotonated
using KH to generate the corresponding potassium
salts, which were reacted with [(COD)­IrCl]2 to generate
the enamidophosphine derivatives (COD)­Ir­(Ph2PC5H6NAr) (Ar = 2,6-Pri2C6H3, 4a; Ar = 2,6-Me2C6H3, 4b). These complexes were exposed to
alcohols, H2, and CO to generate a series of products,
some of which involve
protonation of the enamido unit to generate the imine tautomer. The
reaction of 4a with isopropyl alcohol or H2 generates the dinuclear hexahydride [(Ph2PC5H7N-2,6-Pri2C6H3)­IrH2]2(μ-H)2 (5a), while the reaction with primary alcohols generates the dicarbonyl
enamidophosphine complex (CO)2Ir­(Ph2PC5H6NAr) (6a). The reaction of the hexahydride 5a with CO generates 6a, for which a mechanism
is proposed on the basis of monitoring this reaction as a function
of time by NMR spectroscopy. On the basis of these experiments, cooperative
ligand effects can be replicated by imine–phosphine ligands
by proton transfer to and from the ligand backbone.