Alkali metal complexes having sterically bulky bis-iminopyrrolyl ligands – control of dimeric to monomeric complexes

Abstract

We report the syntheses and structural diversity of three different bis-iminopyrrole ligands and their alkali metal (Li, Na, K) complexes featuring a shift from dimeric to monomeric nature by a simple tuning of the steric control of the imine nitrogen substitutions. The bis-iminopyrrole ligands having the molecular formula [(ArNCH)₂C₄H₂NH] [Ar = CHPh₂ (1-H), Ad (2-H), Ad = adamentyl, and CPh₃ (3-H)], were prepared through the reaction of 2,5-pyrrole-dicarbaldehyde with corresponding amines (ArNH₂) in a 1 : 2 molar ratio and under acidic conditions. Using ligand 1-H, the lithium [{Li(Ph₂CHNCH)₂C₄H₂N}₂] (1-Li), sodium [{Na(Ph₂CH–NCH)₂C₄H₂N}₂(THF)] (1-Na) and potassium [{κ³-(Ph₂CHNCH)₂C₄H₂N}-K(THF)₂] (1-K) complexes were obtained through either alkane elimination (for Li) or amine elimination (for Na, K). The alkali metal complexes [{κ²-(Ph₃CHNCH)₂C₄H₂N}M(THF)_n] [n = 2, M = Li (3-Li), Na (3-Na); n = 3, M = K (3-K)] were synthesized using the ligand 3-H. Molecular structures of ligands 1-3-H, and alkali metal complexes 1-Li, 1-Na, 1-K, 3-Li, 3-Na and 3-K, in the solid state were established. The complexes 1-Li and 1-Na were found to be dimeric in the solid state whereas the complexes 1-K, 3-Li, 3-Na and 3-K were all monomeric – either due to a larger ionic radius (for 1-K) or because of the presence of the bulky triphenylmethyl group on the imine nitrogen.