Ring puckering in group 14 β-diketiminate chlorometallylenes is of quantum-mechanical origin

The ring in cyclic β-diketiminate group 14 chlorometallylenes puckers, pushing the metal out of the β-diketiminate plane, while the rings in metallylene cations are planar. Several hypotheses have been advanced in the past to explain this puckering, such as steric interactions, η5 bonding or cavity size. We have investigated the origin of this ring puckering by calculations at the B3LYP/LANL2DZdp level of theory using a variety of substituents on nitrogen, and found that the ring puckers even with hydrogen on the nitrogen atoms. Analysis of the resulting wavefunction by Natural Energy Decomposition reveals that the puckering is mainly due to an interplay between electrostatics and electronic exchange.

Group 14 chlorometallylenes are common synthetic intermediates in the production of carbene-analogs. The lack of planarity of these heterocycles has intrigued researchers for years, with the most accepted reason for the ring puckering being steric crowding by the N-substituents around the metal. Through DFT calculations, we found out that steric crowding plays only a minor role by amplifying puckering, with the most prominent factor being a tug-of-war between electrostatics and electronic exchange.Figure optionsDownload as PowerPoint slideHighlights
► Larger metals do not increase puckering.
► Puckering does not depend on size of nitrogen substituents.
► The metal pushed out of the ligand plane by electronic exchange.
► Electrostatics pulls the metal towards the ligand plane.