Amido Rhenium Trioxides: Cases of Hindered Agostic C-H-M Interactions?

TL;DR

This study investigates the electronic factors hindering agostic interactions in amido rhenium trioxides, revealing that trans-influence suppresses beta-agostic interactions despite structural indications of their presence.

Contribution

It introduces the concept of hindered agostic interactions, showing how electronic effects like trans-influence control agostic bonding in rhenium and titanium complexes.

Findings

Trans-influence suppresses beta-agostic interactions.

Structural short Re-H-C contacts do not indicate true agostic bonds.

Ligand electronic properties can override trans-influence effects.

Abstract

The amido rhenium trioxides of composition (iPr2N)ReO3, (iPrCyN)ReO3 and (Cy2N)ReO3 (Cy = cyclohexyl) were synthesized in a one pot reaction starting from Re2O7, Me3SiCl and the corresponding amines (iPr)2NH, (iPr)(Cy)NH, and (Cy)2NH, respectively. In the solid state the amido ligands of all three complexes are asymmetrically coordinated to the ReO3 core allowing for one short Re-H-C contact in each case which might indicate the presence of beta-agostic interaction. However, analysis of the charge density distribution provided us clear-cut criteria that beta-agostic interactions are suppressed by the trans-influence of the oxo-groups. Comparison with structurally related tetra-coordinated d0 titanium amido complexes highlighted a systematic concept how the extent of beta-agostic interactions in these complexes can be controlled by reducing the trans-influence of the co-ligands. We therefore suggest to employ the expression "hindered agostic interactions" in cases where covalent M-H-C are in principle supported by vacant coordination sites at Lewis-acidic metal centers but are actually hindered for electronic reasons. We further point out that electronic hindrance originating from the transinfluence might be overruled by changing the s/p-donor characteristics of ligands.