Sodium complexes containing 2-iminopyrrolyl ligands: the influence of steric hindrance in the formation of coordination polymers

Abstract

Iminopyrrolyl complexes of sodium were prepared from the reaction of 2-arylformiminopyrrole ligand precursors (aryl = C₆H₅ (I); 2,6-Me₂C₆H₃ (II); 2,4,6-Me₃C₆H₂ (III); 2,6-ⁱPr₂C₆H₃ (IV)) with one equivalent of sodium hydride. The resulting corresponding compounds 1–4, [{Na(μ₂:κ²N,N′-iminopyrrolyl)}_2n(OEt₂)_2x] (n≥ 1; x = 0 or 1), were obtained in moderate to high yields and were characterised by NMR spectroscopy, high resolution mass spectrometry and X-ray diffraction, when suitable crystals were obtained. The X-ray structure of compound 1 (n≫ 1; x = 0) reveals the formation of a coordination polymer with repeating units consisting of dimers that contain two iminopyrrolyl ligands chelating two sodium atoms, where both pyrrolyl rings exhibit bridging σ + σ coordination to the Na atoms within the dimer; the self-assembling of the polymer is established by additional π-bonds (η⁵-coordination) of each of the pyrrolyl rings to the sodium atoms of the adjacent dimer units. Conversely, the structure of complex D^IV (n = x = 1) shows it as one of such dimers capped by two diethyl ether molecules, each coordinated to the sodium atoms (n = 2; x = 1). DFT calculations indicate that the differences between the structures of 1–4 arise from the increasing bulkiness imposed by the corresponding substituents of the iminic aryl groups.