Linear Free Energy Relationships Associated with Hydride Transfer From [(6,6′-R₂-bpy)Re(CO)₃H]: A Cautionary Tale in Identifying Hydrogen Bonding Effects in the Secondary Coordination Sphere

Six rhenium hydride complexes, [(6,6'-R₂-bpy)Re(CO)₃H] (bpy = 2,2'-bipyridine, R = OEt, OMe, NHMe, Me, F, Br), were synthesized. These complexes insert CO₂ to form rhenium formate complexes of the type [(6,6'-R₂-bpy)Re(CO)₃{OC(O)H}]. All the rhenium formate species were characterized using X-ray crystallography, which revealed that the bpy ligand is not coplanar with the metal coordination plane containing the two nitrogen donors of the bpy ligand but tilted. A solid-state structure of [(6,6'-Me₂-bpy)Re(CO)₃H] determined using MicroED also featured a tilted bpy ligand. The kinetics of CO₂ insertion into complexes of the type [(6,6'-R₂-bpy)Re(CO)₃H] were measured experimentally and the thermodynamic hydricities of [(6,6'-R₂-bpy)Re(CO)₃H] species were determined using theoretical calculations. A Brønsted plot constructed using the experimentally determined rate constants for CO₂ insertion and the calculated thermodynamic hydricities for [(6,6'-R₂-bpy)Re(CO)₃H] revealed a linear free energy relationship (LFER) between thermodynamic and kinetic hydricity. This LFER is different to the previously determined relationship for CO₂ insertion into complexes of the type [(4,4'-R₂-bpy)Re(CO)₃H]. At a given thermodynamic hydricity, CO₂ insertion is faster for complexes containing a 6,6'-substituted bpy ligand. This is likely in part due to the tilting observed for systems with 6,6'-substituted bpy ligands. Notably, the 6,6'-(NHMe)₂-bpy ligand could in principle stabilize the transition state for CO₂ insertion via hydrogen bonding. This work shows that if only the rate of CO₂ insertion into [(6,6'-(NHMe)₂-bpy)Re(CO)₃H] is compared to [(4,4'-R₂-bpy)Re(CO)₃H] systems, the increase in rate could be easily attributed to hydrogen bonding, but in fact all 6,6'-substituted systems lead to faster than expected rates.