Coordination to metal centers: A tool to fix high energy conformations in organic molecules. Application to 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene and related macrocycles

The solid state conformational preferences of ligand 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene (L1) and its 9-methyl derivative (L2) in transition metal complexes have been determined by a probabilistic method using data retrieved from the Cambridge Structural Database. These macrocyclic compounds, as ligands, tend to adopt a preferential conformation (85% of cases). The ring containing the C=N bond adopts a distorted half-chair conformation, the ring defined by both the N-sp(3) shows a distorted envelope conformation, and the remaining ring exhibits a chair conformation. This conformation corresponds to the enantiomer pair RN5SN9SP/SN5RN9RP. Molecular mechanics calculations demonstrate that this is a high energy conformation for the organic molecule, far from the energy minimum. Two other enantiomer pairs are observed in experimental structures. The influence of the coordination on the conformation of the organic ligands has been studied by DFT calculations, and a clear correlation with the geometry of the coordination sphere has been found.