Concealed cyclotrimeric polymorph of lithium 2, 2, 6, 6- tetramethylpiperidide unconcealed: X-ray crystallographic and NMR spectroscopic studies

Lithium 2, 2, 6, 6-tetramethyl-piperidide (LiTMP), one of the most important polar organometallic reagents both in its own right and as a key component of ate compositions, has long been known for its classic cy-clotetrameric (LiTMP) solid-state structure. Made by a new approach through transmetalation of Zn(TMP) with tBuLi in n-hexane solution, a crystalline polymorph of LiTMP has been uncovered. X-ray crystallographic studies at 123(2) K revealed this polymorph crystallises in the hexagonal space group P6/m and exhibited a discrete cyclotrimeric (C) structure with a strictly planar (LiN) ring containing three symmetrically equivalent TMP chair-shaped ligands. The molecular structure of (LiTMP) was redeter-mined at 123(2) K, because its original crystallographic characterisation was done at ambient temperature. This improved redetermination confirmed a monoclinic C2/c space group with the planar (LiN) ring possessing pseudo (non-crystallographic) C symmetry. Investigation of both metalation and transmetalation routes to LiTMP under different conditions established that polymorph formation did not depend on the route employed but rather the temperature of crystallisation. Low-temperature (freezer at - 35°C) cooling of the reaction solution favoured (LiTMP); whereas high-temperature (bench) storage favoured (LiTMP). Routine H and C NMR spectroscopic studies in a variety of solvents showed that (LiTMP) and (LiTMP) exist in equilibrium, whereas H DOSY NMR studies gave diffusion coefficient results consistent with their relative sizes.