Exceptionally High O–H Bond Dissociation Free Energy of a Dicopper(II) μ‑Hydroxo Complex and Insights into the Geometric and Electronic Structure Origins Thereof

The strength of the relevant bonds in bond-making and bond-breaking processes can directly affect the overall efficiency of the process. Copper–oxygen sites are known to catalyze reactions with some of the most recalcitrant C–H bonds found in nature as quantified by the bond dissociation free energy (BDFE), yet only a handful of copper-bound O–H bond strengths have been defined. Equally important in the design of synthetic catalysts is an understanding of the geometric and electronic structure origins of these thermo­dynamic parameters. In this report, the BDFE­(OH) of two dicopper–hydroxo complexes, {[LCu]2-(μ-OH)}3+ and {[LCu]2-(μ-OH)}4+ (L = tris­(2-pyridylmethyl)­amine), were measured. Two key observations were made: (i) the BDFE­(OH)­s of these complexes were exceptionally high at 103.4 and 91.7 kcal/mol, respectively, which are the highest condensed phase MO-H BDFEs to date and (ii) that the higher oxidation state had a lower BDFE­(OH), which is counter to expectations based on known mononuclear BDFE­(OH)­s which increase with the oxidation state. To understand the origin of these thermo­dynamic values, the BDFE­(OH)­s were measured and analyzed for the mononuclear complexes [LCu­(OH2)]1+ and [LCu­(OH2)]2+ in the same ligand environment. This treatment revealed “dinuclear effects” that include contributions from rehybridization of the oxygen, mixed valency of the metals, magnetic exchange between the metals, and differences in solvation, which are general with respect to [M]2–OH complexes to varying degrees. These analyses are important because they provide a starting point for rationally tuning the thermo­dynamics of catalytic intermediates broadly and for understanding how copper active sites achieve activation of strong C–H bonds.