Osme Bond: Geometric and Energetic Features in the Adducts between OsO₄ and Lewis Bases

Abstract

Adducts between OsO4 and Lewis bases exert a role in important oxidation processes such as epoxidation and dihydroxylation. It has been shown that the attractive interaction driving the formation of these adducts is a σ‐hole bond involving the metal as the electrophilic species; the term Osme Bond (OmB) was proposed for designating it. Here some new adducts between OsO4 and various bases have been characterized through single crystal x‐ray diffraction (XRD) and computational studies (density functional theory, DFT), confirming the existence of a robust correlation between σ‐hole interaction energy and deformation of the tetrahedral geometry of OsO4. Also some adducts formed by RuO4 with nucleophiles were investigated computationally.

Strengthening of Noncovalent Bonds Caused by Internal Deformations

“…These ideas are not purely theoretical. As an example, a recently published protein–ligand complex paired OsO 4 with a pyridine-oxide (PyO: C 5 H 5 NO) base, rather than the smaller NH 3 discussed above . The observed intermolecular Os···O contact distance was only 2.334 Å, which is surprisingly short.…”

“…This idea is particularly important within crystals where packing forces can cause significant deviations of the intermolecular geometries from what would occur in vacuo, which is a calculation model. A particular example arose recently in a combined crystal and computational study of the Osme bond between OsO 4 and a series of bases . Extending the earlier analysis of crystal studies, the optimized intermolecular Os···O distance when OsO 4 is paired with pyridine oxide is 2.503 Å, with an interaction energy of 16.38 kcal/mol.…”

“…This enhanced binding was attributed in part to a deepened σ-hole on the Os center, which was in turn caused by the deformation of the OsO 4 unit from the geometry it adopts within the fully optimized geometry. As a further insight into this bond strengthening, if the two subunits are held in their internal geometries within the crystal, then they are drawn closer together and the shorter X-ray intermolecular separation is thus a natural outcome of the internal deformations imposed by the crystal …”