Syntheses and characterization of neutral complexes of Zn(II) with mono- and dianionic pyrrole-2-imine ligands [(pyrrole-2-CHN) nR] n− (n = 1, 2)

Abstract

The reactions of the dianionic [(pyrrole-2-CHN) 2R] 2− ligands [(N′ 2N 2) 2−] (R = (R)(S)-1,2-cyclohexane or 1,2-ethane) with Zn(II) yield neutral dimeric [Zn 2(N′ 2N 2) 2] complexes. The dimeric nature of the complexes was established by field-desorption mass spectrometry. 1H NMR studies show that these complexes have dimeric structures in solution in which the (N′ 2N 2) 2− ligands act as di-bidentates. The metal centres have tetrahedral geometries and bot have Δ or Λ configurations. The complex with the (R)(S)-1,2-cyclohexanediyl bridges has a rigid structure in solution. Neither intermolecular nor intramolecular exchange processes are observed The 1H NMR spectrum of the complex with the 1,2-ethanediyl bridging groups shows that at 213 K in CDCl 3 a fast conformational movement is already taking place between two identical structures of the complex. It is not possible to determine whether in this complex intermolecular exchange processes are also taking place. The reactions of the anionic [pyrrole-2-CHNR′] − ligands [(N′N) −] (R′ = t-Bu, i-Pr, (S)-CHMePh or 2,6-xylyl) with Zn(II) yield the neutral Zn(N′N) 2 complexes. These complexes were synthesized to study the coordination properties of the [pyrrole-2-CHNR′] − moieties with Zn(II). A 1H NMR study established that the zinc centres in the complexes containing the prochiral i-Pr or chiral (S)-CHMePh substituents have tetrahedral geometries with Δ or Λ configurations in CDCl 3 at 213 K. These complexes undergo an intramolecular exchange process at higher temperatures (above 260 K when R′ = i-Pr) which involves inversion of the configuration of the zinc centre. A mechanism for this exchange process is proposed.