Linear Free\nEnergy Relationships Associated with Hydride\nTransfer From [(6,6′‑R<sub>2</sub>‑bpy)Re(CO)<sub>3</sub>H]: A Cautionary Tale in Identifying Hydrogen Bonding Effects\nin the Secondary Coordination Sphere

Six rhenium hydride complexes, [(6,6′-R<sub>2</sub>-bpy)Re(CO)<sub>3</sub>H] (bpy = 2,2′-bipyridine, R\n= OEt,\nOMe, NHMe, Me,\nF, Br), were synthesized. These complexes insert CO<sub>2</sub> to\nform rhenium formate complexes of the type [(6,6′-R<sub>2</sub>-bpy)Re(CO)<sub>3</sub>{OC(O)H}]. All the rhenium formate species\nwere characterized using X-ray crystallography, which revealed that\nthe bpy ligand is not coplanar with the metal coordination plane containing\nthe two nitrogen donors of the bpy ligand but tilted. A solid-state\nstructure of [(6,6′-Me<sub>2</sub>-bpy)Re(CO)<sub>3</sub>H]\ndetermined using MicroED also featured a tilted bpy ligand. The kinetics\nof CO<sub>2</sub> insertion into complexes of the type [(6,6′-R<sub>2</sub>-bpy)Re(CO)<sub>3</sub>H] were measured experimentally and\nthe thermodynamic hydricities of [(6,6′-R<sub>2</sub>-bpy)Re(CO)<sub>3</sub>H] species were determined using theoretical calculations.\nA Brønsted plot constructed using the experimentally determined\nrate constants for CO<sub>2</sub> insertion and the calculated thermodynamic\nhydricities for [(6,6′-R<sub>2</sub>-bpy)Re(CO)<sub>3</sub>H] revealed a linear free energy relationship (LFER) between thermodynamic\nand kinetic hydricity. This LFER is different to the previously determined\nrelationship for CO<sub>2</sub> insertion into complexes of the type\n[(4,4′-R<sub>2</sub>-bpy)Re(CO)<sub>3</sub>H]. At a given thermodynamic\nhydricity, CO<sub>2</sub> insertion is faster for complexes containing\na 6,6′-substituted bpy ligand. This is likely in part due to\nthe tilting observed for systems with 6,6′-substituted bpy\nligands. Notably, the 6,6′-(NHMe)<sub>2</sub>-bpy ligand could\nin principle stabilize the transition state for CO<sub>2</sub> insertion\nvia hydrogen bonding. This work shows that if only the rate of CO<sub>2</sub> insertion into [(6,6′-(NHMe)<sub>2</sub>-bpy)Re(CO)<sub>3</sub>H] is compared to [(4,4′-R<sub>2</sub>-bpy)Re(CO)<sub>3</sub>H] systems, the increase in rate could be easily attributed\nto hydrogen bonding, but in fact all 6,6′-substituted systems\nlead to faster than expected rates.