Crystallographic evidence for a continuum and reversal of roles in primary-secondary interactions in antimony Lewis acids: applications in carbonyl activation.

Primary and secondary interactions form the basis of substrate activation in Lewis-acid mediated catalysis, with most substrate activations occurring at the secondary binding site. We explore two series of antimony cations, [(NMe 2 CH 2 C 6 H 4 )(mesityl)Sb] + (A) and [(NMe 2 C 6 H 4 )(mesityl)Sb] + (B), by coordinating ligands with varying nucleophilicity at the position trans to the N-donor. The decreased nucleophilicity of the incoming ligands leads to reversal from a primary bond to a secondary interaction in A, whereas a constrained N-coordination in B diminishes the border between primary and secondary bonding. Investigations on carbonyl olefin metathesis reactions and carbonyl reduction demonstrate increased reactivity of a Lewis acid when the substrate activation occurs at the primary binding site.