2‑Azetidinones as Precursors of Pincer Ligands: Preparation, Structure, and Spectroscopic Properties of CC′N-Osmium Complexes

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%.