Two-point cooperative binding of ketones by a metal and by a neighboring pendant NH group

An iridium complex having a 2-aminoquinolinate ligand can bind both lone pairs of ketones such as acetone and 2-hexanone in a two-point fashion via a coordinate Ir-O bond and an N-H ... O hydrogen bond. This two-point binding orients the ketone so that the fluxional exchange of the two distinct methyl groups in the acetone complex is strongly slowed relative to the situation where the pendant NH2 group is absent. In addition, certain substrates, such as 3-hexanone, can form complexes only when the H-bonding group is absent. We propose that H-bonding orients the substrate in the mirror plane of the molecule in such a way that the Et group of the substrate causes steric repulsion with the Ir-H group. In unsymmetrical cases such as PhCHO, the orientation occurs so that the aldehyde proton is located near the Ir-H group.