2‐Azetidinones as precursors of pincer ligands: Preparation, structure, and spectroscopic properties of CC´N‐osmium complexes

Abstract

A metal-promoted degradation of 2-azetidinones to afford CC′N-pincer ligands is reported. The hexahydride complex OsH6(PiPr3)2 (1) reacts with (±)-cis-1-(4-methoxyphenyl)-3-phenoxy-4-(pyridin-2-yl)azetidin-2-one (I), (±)-cis-1-(4-methoxyphenyl)-3-phenoxy-4-(isoquinolin-2-yl)azetidin-2-one (II), and (±)-cis-1-(4-methoxyphenyl)-3-phenoxy-4-(quinolin-2-yl)azetidin-2-one (III) to give the respective OsH2(PiPr3)2(CC′N) (2–4) complexes, which add HBF4·OEt2 to yield [OsH2(PiPr3)2(CC″N)]BF4 (5–7). These salts are the result of the addition of the proton of the acid to the dianionic CC′N-pincer ligand. The hydride ligands of these compounds undergo quantum mechanical exchange coupling, which has been experimentally quantified according to a two-dimensional harmonic oscillator model, where Jex is determined by the separation between the hydrides, their hard sphere radius, and a ν parameter describing the H–M–H vibrational wag mode allowing the movement along the H–H vector. The comparison of the results reveals that the phenomenon is particularly intense for 5–7. Furthermore, in these compounds, the separation between the hydrides is ∼0.1 Å shorter than in the respective neutral species 2–4, whereas the hydride hard sphere radius increases by ∼10%, and the ν value decreases by ∼20%.