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.