Homoleptic chromium(II) aminopyridinates: Transoid vs cisoid coordination

Abstract

Rare homoleptic divalent chromium complexes have been isolated by reacting two equivalents of sterically bulky deprotonated 2–aminopyridine ligands, N–(2,4,6–trimethylphenyl)–[6–(2,4,6–trimethylphenyl)–pyridine–2–yl]–amine (1), N–(2,6–diisopropylphenyl)–[6–(2,6–dimethylphenyl)–pyridine–2–yl]–amine (2) and N–(2,6–diisopropylphenyl)–[6–(2,4,6–trimethylphenyl)–pyridine–2–yl]–amine (3) with CrCl₂ in tetrahydrofuran (THF). Reaction of deprotonated 1 proceeds smoothly at room temperature while that of sterically more bulky 2 and 3 at reflux temperature. The respective bis(aminopyridinate) Cr complexes (4–6) are monomeric and show different orientations of the coordinated ligands. For compound 5, two isomers were identified. For 4 and 5a the two ligands show head to tail arrangement while in 5b and 6 the two ligands adopt the unknown head to head arrangement of aminopyridinato ligands. All the complexes show distorted square planar geometries around the chromium center. For 4 and 5a in which the ligands are coordinated in transoid manner the pyridine rings and amido nitrogen atom lie in the coordination plane of chromium. Hirshfeld analyses showed that H∙∙∙H and H∙∙∙C/C∙∙∙H π–interactions were the main and at times the strongest contributions for the intermolecular interactions.