Use of the Imine-Enamine Equilibrium in Cooperative Ligand Design

The imine-phosphine ligands Ph2PC3H7NAr, where Ar = 2,6-(Pr2C6H3)-C-i, 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-(Pr2C6H3)-C-i, 4a; Ar = 2,6-Me2C6H3, 4b). These complexes were exposed to alcohols, H-2, 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 H-2 generates the dinuclear hexahydride [(Ph2PC5H7N-2,6-(Pr2C6H3)-C-i)IrH2](2) (mu-H)(2) (Sa), while the reaction with primary alcohols generates the dicarbonyl enamidophosphine complex (CO)(2)Ir(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.