Lithium 2,2,6,6-tetramethylpiperidinoselenolate: An unsymmetrical dimer with an unusually coordinated lithium

Lithium 2,2,6,6-tetramethylpiperidide inserts elemental selenium into its Li-N bond with the formation of lithium 2,2,6,6-tetramethylpiperidinoselenolate 1. In the crystal 1 is an unsymmetric dimer. One lithium atom is coordinated tetrahedrally by two molecules of tetrahydrofuran and two selenium atoms, the other lithium atom exhibits an approximately rectangular-planar coordination by two N,Se-chelating groups. The lithium atoms are bridged by two selenium atoms thus forming a planar Li2Se2 core. Ab initio Hartree-Fock calculations show 1 and the hypothetical symmetric dimer 2 to be of equal energy, whereas the non-chelated dimer 3 is of distinctly higher energy. Exclusive crystallization of 1 is most Likely a consequence of its large dipole moment of 6.18 D. Calculations also show that the deformation of the observed geometry around the uniquely coordinated Lithium atom in 1 towards distorted tetrahedral or complete planarity leads to an increase of energy. Excessive charge transfer from selenium to nitrogen explains the overwhelming tendency for the N,Se-chelating bonding.